1
1
Electrochemical Technologies for Energy Storage
and Conversion
Neelu Chouhan and Ru-Shi Liu
1.1
Introduction
In this chapter, authors review the contemporary demand, challenges and future
prospective of energy resources and discuss the relevant socioeconomical and
environmental issues with their impact on global energy status. A sincere effort
has been made to explore the better energy options of clean and sustainable
energy sources such as hydro, biomass, wind, solar, geothermal, and biofuel as an
alternative to the conventional energy sources. Electrolysis, photoelectrochemical,
and photocatalytic water-splitting techniques were adopted for green and light fuel
generation. Advancement in electrochemical technology for energy storage and
conversion devices such as rechargeable batteries, supercapacitors, and fuel cells
are also briefed.
1.2
Global Energy Status: Demands, Challenges, and Future Perspectives
World’s economy revolves around the axis of energy prices, which are primarily
governed by the political consequences, environmental impact, social acceptance, availability, and demand. Nation-wise world’s energy consumption plot
(1980–2050) is depicted in Figure 1.1, which rated the United States, China, Russia, South Korea, and India as potential energy consumers. Energy consumption
rate of our planet in 2007 was 16%, which would be accelerated to an alarming
rate of 34% by 2050 (Figure 1.2) [1]. Our severe dependency on oil and electricity
makes energy a vital component of our daily life [2]. Soaring prices of oil (starting
from $42 per barrel in 2008 to $79 per barrel in 2010, to $108 per barrel in 2020
and $133 per barrel in 2035) as projected in Figure 1.3 [3] and other associated
necessary commodities along various burning environmental issues resulted from
industrial revolution compel us to give a careful thought on this serious issue.
Figure 1.4 assesses the geographical region-wise oil reserve that projects the oil
assets and capacities of the different regions [4]. The current global energy scenario
Electrochemical Technologies for Energy Storage and Conversion, First Edition. Edited by Lei Zhang, Ru-Shi Liu,
Hansan Liu, Andy Sun, and Jiujun Zhang.
2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.
1 Electrochemical Technologies for Energy Storage and Conversion
World energy consumption
900.0
800.0
Other central and S. America
Africa
Middle
Other Asia
India
China
Other Europe and Eurasia
Russia
Australia/New Zealand
South Korea
Japan
Mexico
Canada
United States
Quadrillion BTU
700.0
600.0
500.0
400.0
300.0
200.0
100.0
2050
2040
2035
2030
2025
2020
2015
2010
2003
2002
1980
0.0
Figure 1.1 Nation-wise world energy consumption in
the time interval of 1980–2050. (Energy Information
Administration Annual Energy Review, 2007.)
Change in energy consumption rates
(conservative projection)
40
2007
2050
35
Terawats
30
25
20
15
10
5
ld
W
or
a
ric
Ea
e
M
id
dl
Af
st
a
er
S.
&C
.A
m
Pa
As
pe
ro
Eu
ia
ur
,E
m
.A
ic
fic
ci
as
er
ic
a
ia
0
N
2
Figure 1.2 Comparative change in energy consumption
rates of different zones against the world (actual reported
for 2007 vs projected for 2050). (Renewable in global energy
supply: an IEA fact sheet, January 2007.)
is full of uncertainty and faces three major energy challenges in the form of energy
demand/energy supply ratio and security and their impact on the environment.
The present worldwide population of 6.9 billion needs 14 TW annual energy [5] to
sustain the current standard of life. Of the total energy production, 45% is required
for industries, 30% for transport, 20% for residential and commercial buildings,
1.2 Global Energy Status: Demands, Challenges, and Future Perspectives
250
History
Projections
High
oil price
$210
Reference
$133
Low
oil price
$51
200
150
100
50
0
1990
2000
2009 2015
2025
2035
Figure 1.3 World oil prices in three oil price cases
on the timescale of 1990–2035 ($2007 per barrel).
(1980–2035: EIA, Annual Energy Outlook 2010,
DOE/EIA-0383(2010) (Washington, DC, April 2010), web
site: www.eia.gov/oiaf/aeo.)
753
Middle East
194
North America
135
Central & South America
Africa
119
Eurasia
100
40
Asia
Europe 12
0
World total:
1,354 billion barrels
200
400
600
800
Figure 1.4 World’s proved oil reserves by geographic region as of
1 January 2010 [4].
and the rest for services such as education, health, finance, government, and
social services. Electricity is the world’s fastest growing form of end-user energy
consumption. Coal provides the largest share in the world’s electricity generation,
accounting for 42% in 2007, and its share will be largely unchanged through 2035.
Rest share of the world’s electricity generation is contributed by water, natural gas,
nuclear power, hydropower, wind, and solar power. Economic trends and population growth drive the commercial sector activities and the resulting energy use.
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1 Electrochemical Technologies for Energy Storage and Conversion
The need for services (health, educational, financial, and governmental) increases
as population increases. Slower expansion of gross domestic product (GDP) and
declining population growth rate in many organization for economic cooperation
and development (OECD membership) nations contribute to slower anticipated
rates of increase in commercial energy demand. In addition, continued efficiency
improvements moderate the growth of energy demand over time, as energy-using
equipment is replaced with newer and more efficient stock. World’s projected
population would be quadrupled by 2050, the energy use doubled and electricity consumption tripled to our present energy demand. According to Hubbert’s
bell-shaped curve [6] of the worldwide oil production projection, we have already attained the peak and now observe a downfall and finally, the oil will last for 200 years
(Figure 1.5) [7]. Lord Ron Oxburgh, former chairman of Shell, gave the statement
on oil production possibilities and price, ‘‘It is pretty clear that there is not much
chance of finding any significant quantity of new cheap oil. Any new or unconventional oil is going to be expensive.’’ Despite the greenhouse gas concentrations
approaching twice as those in the preindustrial period, coal and gasoline are still
the major energy sources (34.3% oil, 25.9% coal, 20.9% gas, 13.1% renewables
(10.4% combustion renewables and waste, 2.2% hydro, and 1.5% other renewables).
Furthermore, alternative sustainable energy sources are still in the experimental
stage; for example, some recent studies suggest that biofuels may not be as effective
in reducing greenhouse gas emissions as previously thought. As a result, many
countries have relaxed or postponed renewal of their mandates [8]. For example,
Germany reduced its biofuel quota for 2009 from 6.25 to 5.25%. Therefore, governments, industrialists, and researchers have put their heads together on this leading
energy issue with their concerns about the environmental challenges and renewed
14
Production (109 bbls/year)
4
12
Proven reserves
250 × 109 bbls
10
8
6
4
Cumulative
production
90 × 109 bbls
Future discoveries
910 × 109 bbls
2
0
1850
1900
1950
2000 2050
Year
2100
2150
Figure 1.5 Hubbert’s bell-shaped curve for time versus production of any exhaustible resources projection plot for the
time interval 1850–2200 AC [7].
2200
1.3 Driving Forces behind Clean and Sustainable Energy Sources
the interest in development of alternatives to fossil fuels, specifically, nuclear power,
and renewable energy sources (wind, solar, biofuel, geothermal, tidal, hydro) using
breakthrough concepts (catalysis by design, multielectron transfer) and accelerated
application of cutting-edge scientific, engineering, and analytical tools. There are
three major options of getting clean energy including carbon neutral energy (fossil
fuel in conjunction with carbon sequestration), nuclear, and renewable energy. To
satisfy the 10 TW no-carbon energy demands [9], a 38% conservation of energy
for the next 50 years via combustion of fossil fuel is required, but the challenge of
disposing 25 billion metric ton of CO2 annually needs to be conquered. The need
for nuclear-powered energy required the establishment of 365 GW electric nuclear
fission plants per year for 50 years. The amount of annual renewable trappable
energy from resources is as follows: the most viable and abundantly sourced solar
energy with a capacity of 12 000 TW; integrated overall geothermal energy, 12
TW; globally extractable wind power, 2–4 TW; tidal/ocean current, 2 TW; and
hydroelectric energy, 0.5 TW. Among all sources, obviously solar energy stands out
as a promising choice of renewable energy, and currently, we are exploiting it only
for the satisfaction of 0.1% of the demand. Therefore, by reducing energy demand
and emissions accompanied with the use of the diversifying energy sources, we
should be able to meet our energy target.
1.3
Driving Forces behind Clean and Sustainable Energy Sources
Our atmosphere is in a constant state of turmoil, and it is never being static.
Relatively, internal and external changes in the earth’s atmosphere, made by either
Nature or man, bring changes in weather and climate. Scientific evidences pointed
out the role of man in environmental degradation by insanely exploiting Mother
Nature, which causes a disturbance in the delicate balance of Nature by accelerating
global warming and associated climate changes, increasing ocean temperature, and
bringing out changes in terrestrial geography, rain fall ratio, temperature, and type
of soil. These changes fuel the growing consensus about the eminent need for a
more pervasive action for environmental protection. Technological advancement
attained during the past two decades has provided us a comfortable lifestyle full
of facilities on a very high cost of resources consumption and degradation of our
environment. The effect of the world’s economic development on the environment was defined in the words of Elsa Reichmanis, the former president of the
American Chemical Society, ‘‘We are past the days when we can trade environmental contamination for economical prosperity that is only a temporary bargain
and the cost of pollution both economically and on human health is too high’’
[10]. However, it has disturbed ecological balance and damaged the environment,
which has been proven disastrous for global life and has resulted in tremendous
critical issues such as extinction of rare species of flora and fauna from earth,
various incurable or semicurable diseases, global warming, acid rain, ozone layer
depletion, excessive pollution, nuclear winter, and photochemical smog, especially
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1 Electrochemical Technologies for Energy Storage and Conversion
in and around the urban areas. But still we do not wish to quit the comfortable
lifestyle, and we simply cannot afford to continue along this path. Therefore, it
is high time we shake hands with Nature to satisfy our energy demands in an
eco-friendly manner by utilizing decentralized renewable energy sources such as
solar power, wind, geothermal energy, biofuel and biomass, tidal power, wave, and
hydropower. Furthermore, these sources are more efficient, abundant, and affordable (available free of cost) and are an environmentally benign solution for getting
clean and green energy but on the condition that people master the technology.
Both industrialized and developing countries should adopt the above-mentioned
sustainable resources to build their energy capacity and improve their regulatory
for clean, safe, and renewable energy. Therefore, large-scale transformation in
energy policies should be executed with strong willpower along the necessary
course of action toward clean and sustainable energy. Volatile energy prices of
fossil fuels and increasingly scarce natural resources impend the government
legislation that a growing trend of higher corporate social responsibility (CSR) and
consumer sentiment favor environmentally friendly products and services. They
induce the businesses, industries, and governments to respond in the innovative
ways that might have been unimaginable just a few years ago. Executing relatively
new wireless technology, networked sensors, management dashboard reporting,
and automated alarm management is one way for businesses to reduce waste and
optimize their position as environmental stewards in multiple domains (company,
government, and geography). Reducing waste, managing scarce natural resources,
saving energy, and following efficient operating conditions have always been good
business tenets. The vital driving force behind the search for the most powerful,
clean, and renewable energy sources is energy security for the future in energy
policies of the governments all around the world, assuming renewable energy
sources as a guaranteed growth sector [11]. Modern renewable energy industry has
been hailed by many analysts because of the global trends and drivers underlying its expansion during the past decade. Without widespread improvements
in environmental stewardship, impacts from the fundamental drivers will lead
to adverse consequences around the world. Among these consequences, strict
government legislations, climate changes, water stress, natural resource and raw
material scarcity, public pressure, market risk and national security, and safety
concerns are highlighted below.
1.3.1
Local Governmental Policies as a Potential Thrust
In 1960s, the slogan Think Globally and act Locally was coined by David Ross Brower,
a prominent environmentalist and the founder of Sierra Club Foundation, John
Muir Institute for Environmental Studies, and Friends of the Earth (1969), and many
others that work actively for environment and initiate the worldwide consensus
and awareness about environmental issues. In many countries, great progress
has been made through awareness programs, proactive guiding principles and
policies, new legislation, and government incentives in the form of tax relaxation
1.3 Driving Forces behind Clean and Sustainable Energy Sources
and subsidies. Environmental risks force the people to go green in a significant way
and promote a joint effort at governmental, enterprise, and even individual level
to contribute in the awareness programs, proactive guiding principles, and policy
making, all together. Government policies for renewable energy are a diverse and
growing segment. Hundreds of local governments are setting future targets and
adopting a broad array of proactive planning and promotion policies, including new
legislation, government incentives for local feed-in tariffs and renewable electricity
generation and heating, and mandates for buildings and businesses. Government
regulations can play an efficient role in achieving effective changes in favor of
environment, but it’s only one out of several forces that will drive the needful
changes into the future.
1.3.2
Greenhouse Gases Emission and the Associated Climate Changes
Emission of greenhouse gases significantly banks on industries, refrigeration,
and transportation because fossil fuels, which are still a primary energy source
and responsible for CO2 emission, are accounted as one of the greenhouse
gas. Major driving forces and science behind the green movement are focusing
primarily on suppressing greenhouse gas emissions and their unwanted effects
on global warming and associated climate changes, natural resource scarcity
(oil, gasoline, and minerals in particular), and eventual ozone layer depletion,
consequences of unabated human-driven pollution. New legislation, community
pressure, customer safety concerns encourage proactive actions and strengthen
the corporate environmental activities. Environmental improvement is one area of
new business activity whose driving forces are so strong, responsibly compelling,
and widely appreciated that raise the call to action and can appeal to every
industry, enterprise, and organization, from the senior-most executives to the
newest entry-level employees.
1.3.3
Public Awareness about Environmental Protection Rose around the World
Baton of the public awareness lights the world to see the real picture of mesmerization of environment done by humans, which push the government and
industries to enact on the holistic green strategies to make this world more
livable. Layman protesting by chaining themselves to trees or lying in front of
bulldozers against deforestation and by organizing boycotts are the most visible
environmental stewardships in 1980–1990s against the activities of big business,
which enforce driving forces behind environmental sustainability. But nowadays
the clashes between the corporate world and the environmentalists have become
a thing of past. Businesses are adapting and applying suitable methodology to
sustain the benefits to business operations without going against Nature. Moreover, they started working in favor of Nature by cross-company, cross-industry, and
cross-geography cooperation, which will enable a clear and sustainable focus on
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1 Electrochemical Technologies for Energy Storage and Conversion
improving environment by applying technology in new ways to achieve granular
understanding of their operation and their impact on the planet. Public who wants
to ride on green wings becomes a major positive driving force for environmental
protection. However, public pressure could shift from a positive driving force to a
more negative one. Public pressure in the form of unrest is certainly more likely
when food production declines while population rises, basic natural resources
become scarce, water supplies become more stressed, and there is occurrence of
more frequent and more severe natural disasters. Public sympathy is also with endangered species of flora and fauna, an important part of the food chain that might
perish because of global warming and associated climate changes. One survey
on average American attitude toward the environment found out that more than
three-quarter US workers want to have an employer, who is well informed about
green movement on a daily basis and contributes accordingly [12]. Furthermore,
developments in renewable energy have enough potential to create new industries
and generate millions of new jobs around the world. The European Commission
published statistics on Europeans’ attitudes toward climate change and found that
73% of the Brits feel they were well informed about the causes of climate change
and methods to tackle them, placing them in the same group as the citizens of
Sweden, the Netherlands, and Finland. Despite all this, only 46% Brits and 50%
Europeans but 82% Swedish ranked climate change as the most alarming global
problem.
1.3.4
Population Growth and Industrialization
World population continues to grow, and statistical analysis predicts that by the
year 2050, it will be increased by 30% of today’s population, which will accelerate
scarcity of natural resources such as oil, fossil fuel, water, minerals, agricultural
land, and clean air. Industrialization further demands additional resources that can
create environmental risk and jeopardizes the economy. The Intergovernmental
Panel on Climate Change (IPCC) reported in 2007 [13] that the future climate
change attributable to global warming is expected to put 50 million extra people at
risk of hunger by 2020, which might be increased to 132 and 266 million by 2050
and 2080, respectively, because rising air temperatures could decrease rain-fed rice
yields by 5–12% only in China and net cereal production in South Asian countries
could decline by 4–10% by the end of this century, making it unable to meet the
food demands of that time. Pressure of high consumption of resources because of
population explosion, requiring more amount of drinkable water without drought
or climate change. Furthermore, the threats such as oil and chemical spills,
unmitigated waste, resources exploitation, residential and commercial real estate
development, unsafe living conditions, and drainage of polluted industrial waste
into river system pose a clear risk to public health and degrade the environment,
enhancing the ultimate depletion or extreme scarcity, which would generate high
risk to business and Nature.
1.3 Driving Forces behind Clean and Sustainable Energy Sources
1.3.5
Security and Safety Concerns Arising from Scarcity of Resources
National security became more relevant when the impact of climate or weather
change crosses the country borders and countries rich in natural resources start
dominating other countries. Public unrest develops as resource supplies become
scarce and global conditions become adverse enough to result in riots, corruption,
and military action and is a sufficient cause for disturbing national or international
security. Safety for all living organisms and properties is another issue that
prominently arises after any natural disaster, which is a result of global warming
and associated climate changes, such as severe weather patterns, floods, fires, and
hurricanes. Generous cooperation in the form of human help and financial support
among countries with proactive measurements can make the affected country or
region to be able to overcome the situation effectively. The above-mentioned risks
are also a driving force behind environmental protection to mitigate before any
of the direst predictions unfold and the current course of global cooperation
changes.
1.3.6
Platforms Advocating in Favor of Sustainable and Renewable Resources
Environmental degradation, soaring prices and high consumption of conventional
energy sources, perpetual resource wars, catastrophic effect of greenhouse gases
on climate change, inextricable link between nuclear weapons and nuclear power,
high cost of nuclear plant establishment and nuclear fuel, and problematic disposal
of nuclear waste have fostered the international agencies to establish a platform
such as United Nations Environmental Program (UNEP) that advocates renewable
energy sources to take the place of fossil fuels without resorting to nuclear-powered
energy. In 2000, a global network for the elimination of nuclear weapons lobbied
nations (accounted 142 in 2009) around the world to institute the International
Renewable Energy Agency (IRENA). IRENA opened its headquarters in Abu Dhabi
and branch offices in Bonn and Vienna, and it is committed to becoming a
principal driving force in promoting a rapid transition toward the sustainable use
of renewable energy on a global scale. It included all forms of renewable energy
produced in a sustainable manner, including solar power, wind, geothermal energy,
hydropower, ocean, and appropriate bioenergy. Another example of such agencies
is the Forest Stewardship Council (FSC) certifies some wood as sustainable
when it meets the established criteria [14]. The U.S. Green Building Council
(USGBC) has created the Leadership in Energy and Environmental Design (LEED,
founded in 1993 by Robert K. Watson), a third-party certification program for
the design, construction, and operation of green buildings. All agencies working
together under the same theme to benefit the environment are a positive driving
force.
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1.3.7
Economic Risk Generated from Price Pressure of Natural Resources
World economy is a grand driving force to regulate the energy and raw material
prices. Speculation on long-term demand, current availability, and cost of resources
suggest some substitute raw materials, alternative or renewable energy sources, and
conservation of traditional fossil fuels. As long as energy and commodity prices for
scarce raw materials remain unpredictable and long-term global demand sustains
the heightened levels, the drive for initiatives that reduce energy consumption and
raw materials waste or shift to a less risky source with a more stable long-term cost
has become stronger. For example, in 2004, crude oil was trading at $40 a barrel and
the estimated fair value was to be only $27 a barrel [15]. Risk premium has varied
up to 30% and potentially higher over time. On the other hand, the price for water
in many areas of the world is still much lower than its actual economic value, which
is a notable exception for water management. Price pressure alone is a powerful
driving force, and as result of this, the world would be destined to experience
an endless ebb and flow of cyclical activity without new taxation or cap-and-trade
mechanisms to sustain the current green movement. That’s why some enterprises
install straightforward technology devices, such as motion-sensitive light switches,
implement new technology to monitor and optimize energy consumption, train
employees on energy-saving practices, or appoint new roles in the organization
with accountability for achieving business and environmental benefits.
1.3.8
Regulatory Risk from Governmental Action and Legislation
Regulatory risk from new governmental legislations and global agreements is a
powerful driving force that can accelerate the trend of environmental sustainability
if applied effectively. The United Nations held a conference in 1997 at Kyoto,
Japan, on climate change that resulted in an international agreement. The Kyoto
Protocol took effect from 2005 to reduce greenhouse gas emissions worldwide, but
the protocol’s obligations are limited to monitoring and reporting, without actual
provisions for enforcement and penalties if reductions are not achieved. Another
prominent meeting of leading industrial nations (G8) held at Tokyo, Japan, 8 July
2008, endorsed halving world emissions of greenhouse gases by 2050 but set no
near-term targets. The UK Climate Change Act (2008) aims to move the United
Kingdom to a low-carbon economy and society, with an 80% cut in emissions by
2050 from a 1990 baseline [16]. The California Energy Commission recognizes the
energy efficiency standards for real estate business and made future legislation
that would require zero net energy homes and commercial buildings by 2020 and
2050, respectively [17]. In this series, the U.S. Army Energy Strategy set five tenets
in 2005 to the strategy: eliminate waste, increase efficiency, reduce dependence
on fossil fuels, conserve water resources, and increase energy security. Regulatory
proposals being actively developed at the industrial, state, and local levels are more
1.4 Hydro, Biomass, Wind, Solar, Geothermal, and Biofuel
effective than the legislations at global and country levels. Therefore, it is high time
for nations to follow international regulations strictly.
1.3.9
Fear of Reputational Risk to Strengthen Corporate Social Responsibility
Reputational risk at CSR is a vital driving force for companies to improve their
status as environmental friendly, and it is viewed as an investment that brings
financial returns and an opportunity or platform for growth that would increase
the visibility in action. To reduce environmental impact, businesses adopted ethical
standards to win customers’ loyalty and market share and lowered their business
risk that led to higher profitability through increased sales or decreased costs,
which were often maintained during the adverse environmental events. A large
number of companies are enrolled under the CSR network and few of them are
(i) Catalyst Paper Corporation (Canadian) uses its own by-products (biomass) to
power its operations; (ii) Tesco is a retail chain holder in grocery and industries
(>2800 stores in central Europe, Asia, and North America), runs 75% of its delivery
fleet on biodiesel fuel, had labeled 70 000 of its products with carbon counts
(carbon labeling articulates the total carbon emissions from bringing a product
to the store shelf) by 2008, and runs many straw-powered stores with heat and
solar photovoltaic (PV) power plant for carbon-neutral electricity; and (iii) Wal-Mart
(Latin America) has installed the largest sun-operated PV installation to satisfy 20%
of the energy needs of its store. In California, new cars being sold are required to
include labeling with global warming scores.
1.3.10
Operational and Supply Chain Risks from Inefficiencies and Environmental Changes
Operational and supply chain risks are another driving force generated from
extreme adverse weather patterns, environmental hazards, and inefficiencies that
will push the businesses to invest in innovative and sustainable energy sources
and high-impact renewables, to favor upstream suppliers, and to set examples for
other business partners. Although opportunities have been available since the past
decades, only recently have all the driving forces aligned in the right direction to
prompt the worldwide call to action.
1.4
Green and Sustainable Energy Sources and Their Conversion: Hydro, Biomass, Wind,
Solar, Geothermal, and Biofuel
Environmental degradation and soaring prices and high consumption of conventional energy sources (34.3% oil, 25.9% coal, 20.9% gas, 13.1% renewables
(10.4% combustion renewable and waste, 2.2% hydro, and 0.5% other renewables),
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Non
renewable
waste (0.2%)
6.5%
1
ene
34
wab
Gas
le
al
r Co
a
cle
Nu
%
3.1 R
Oil
.3%
%
.9
20
25.1%
Figure 1.6 World total energy (conventional and renewable) consumption plotted against their percentage contribution (total primary energy, 410 EJ/year)
(http://www.iea.org/papers/2006/renewable_factsheet.pdf).
6.5% nuclear, and 0.2% nonrenewables) are illustrated in the world energy consumption chart (Figure 1.6). The perpetual resource wars, catastrophic effect of
greenhouse gases on climate change, inextricable link between nuclear weapons
and nuclear power, high cost of nuclear plant establishment and nuclear fuel,
and problematic disposal of nuclear waste all foster the international agencies to
develop the sustainable energy sources to take the place of conventional energy
sources without resorting to nuclear power. All time free availability and huge
amount of decentralized renewable energy are the principal driving force against
the promotion and rapid transition toward the sustainable renewable energy on
the global scale. Renewable energy includes a wide spectrum of sustainable and
powerful sources of natural energy such as solar, wind, geothermal, hydropower,
ocean, and appropriate bioenergy. One can estimate the power of these sources as
a single day of sunlight can supply enough energy to satisfy the world’s electricity
demand for 8 years, whereas wind can meet the world’s electricity needs 40 times
over and is capable of fulfilling all the global energy demands five times over,
and the geothermal energy stored in the top 6 miles of the earth’s crust contains
50 000 times the world’s energy storage in oil and gas resources. Tidal, wave,
and small hydropower can also provide vast stores of energy, available everywhere
on earth. Both industrialized and developing countries should start adopting the
above-mentioned sustainable resources to build their energy capacity and improve
their regulatory for clean, safe, and renewable energy. By the year 2008, the top six
countries rated by their total amount of renewable power capacity in use were China
(76 GW), US (40 GW), Germany (34 GW), Spain (22 GW), India (13 GW), and
Japan (8 GW) (9 September 2009, by Eric Martinot and Janet Sawin, London, UK,
Renewables Global Status Report 2009 and update of the Renewable Energy Policy
Network for 21st Century (REN21) annual report). Renewable energy capacity in
developing countries grew to 119 GW in 2009, or a 43% share (out of which, 18% is
1.4 Hydro, Biomass, Wind, Solar, Geothermal, and Biofuel
Table 1.1
Renewable energy added and existing capabilities, 2008 (estimated).
Added during 2008
Existing at the end of 2008
25–30
27
6–8
2
5.4
0.4
0.06
0
860
121
85
52
13
10
0.5
0.3
n/a
19
n/a
250
145
50
Power generation (GW) electric
Large hydro power
Wind power
Small hydro power
Biomass power
Solar PV grid connected
Geothermal
Concentrating solar power
Ocean (tidal) power
Hot water or heating (GW) (thermal)
Biomass heating
Solar for hot water/space heating
Geothermal heating
Transport fuels (billion liters/year)
Ethanol production
Biodiesel production
17
3
67
12
Global Energy Report-2009 REN21.
global electricity supply) of the total global energy capacity. Even though affected by
the global economic downturn, the years 2008 and 2009 were remarkably the best
for renewable, which is clear from Table 1.1, exhibiting the existing sources, and by
2008, renewable energy was added. The Global Status Report of 2010 on renewables
conducted by REN21 shows that all the forms of grid-tied solar PV plants grew
annually by 60% from the past decade. On average, the past 5 years’ annual growth
of wind power was 27%, solar hot water was 19%, and the ethanol and biodiesel
production expanded by 34% [18]. Heat and power from biomass and geothermal
sources continued to grow, and small hydropower increased by 8%. Globally, the
approximate technology share of $120 billion (¤85 billion) as renewable capital
investment was divided into wind power (42%), solar PV (32%), biofuels (13%),
biomass and geothermal power (6%), solar hot water (6%), and small hydropower
(5%). Renewable capacity is discussed individually in the following sections.
1.4.1
Solar PV Plants
PV power generation employs solar panels comprising a number of cells containing a PV material, which include monocrystalline silicon, polycrystalline silicon,
amorphous silicon, cadmium telluride, and copper indium selenide/sulfide. The
cost of PV’s has declined steadily since the first solar cells were manufactured,
because of the advancement in technology and large-scale manufacturing units
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[19]. More than 1800 solar PV plants of 16 GW existed worldwide by the end
of 2008; Spain was leading with 2.6 GW of new capacity added, followed by the
former PV leader Germany (added 1.5 GW), the United States (310 MW added),
South Korea (200–270 MW), Japan (240 MW), and Italy (200–300 MW). Solar
PV markets in Australia, Canada, China, France, and India have also continued
to grow. Their additions reached a record high of 7 GW in 2009, when Germany
topped the market with 3.8 GW added capacity and captured more than half of
the global market. Other large markets were Italy, Japan, the United States, Czech
Republic, and Belgium.
1.4.2
Wind Power
Wind power is the conversion of wind energy into a useful form of energy, such
as using wind turbines to make electricity, wind mills for mechanical power, wind
pumps for pumping water or drainage, and sails to propel ships. Around the
world, >80 countries had installed commercial wind power by the year 2008 for
their energy demand. The global wind power leader since mid-1990s, Germany
(24 GW), has handed over its top position to the United States (25 GW), followed by
Spain (18 GW), China (12 GW), and India (8 GW). Wind power additions reached
a record high of 38 GW, that is, around 60% of the total global energy capacity
(80 GW) on renewables utility scale investment in 2009 (excluding small projects).
China tops the market with 13.8 GW, the United States was second, with 10 GW
added. The share of wind power generation in several countries reached record
highs, including 6.5% in Germany and 4% in Spain.
1.4.3
Geothermal Power
Geothermal wells release greenhouse gases trapped deep within the earth, but
these emissions yield much lower energy per unit than those of fossil fuels. As a
result, geothermal power has the potential to help mitigate global warming if widely
deployed in place of fossil fuels. The United States remained the world leader in
geothermal power development, with more than 120 projects underdevelopment,
representing at least 5 GW. Geothermal projects were underway in over 40
countries, with another 3 GW in the pipeline. Globally, geothermal power capacity
reached over 10 GW in 2008 and is increasing yearly [20].
1.4.4
Concentrating Solar Thermal Power (CSP) Plants
Concentrating solar thermal power (CSP) plants employ sunlight concentrated onto
PV surfaces for the purpose of electrical power production. The United States and
Spain are the leading figures in this field. New projects are also underdevelopment
in Abu Dhabi, Algeria, Egypt, Israel, Italy, Portugal, Spain, and Morocco. One
1.5 Electrochemistry: a Technological Overview
of the key trends is that a growing number of these CSP plants will include
thermal storage in daytime, allowing power generation in the evening hours. The
recently completed Andasol 1 plant in Spain has more than 7 h of full-load thermal
storage capability. Overall, it is clear that parabolic trough plants are the most
economic, most mature, and efficient thermal storage plants and a promise to solar
thermal technology available today, although there are still significant areas for
improvement and cost cutting in the near future.
1.4.5
Biomass
Biomass production contributes the energy equivalent of 5% of world gasoline
output. Many countries evident the record use of biomass, notably, Sweden, where
biomass accounted for a larger share of energy supply than oil for the first time in
2009, which was followed by Brazil.
1.4.6
Biofuel
The United States scored top in biofuels with 31 new ethanol refineries of 40
billion l per year production strength along with an additional 8 billion l per year
capacity under-construction plants established by the year 2009. In transport fuels,
ethanol production in Brazil ramped up dramatically in 2008 to 27 billion l in 400
ethanol mills and 60 biodiesel mills, after being maintained constant for a number
of years, for the first time ever; more than half of Brazilian nondiesel vehicle
fuel consumption came from ethanol. Notwithstanding Brazil’s achievement, the
United States remained the leading ethanol producer, with 34 billion l produced in
2008. Other ethanol fuel–producing countries include Australia, Canada, China,
Colombia, Costa Rica, Cuba, the Dominican Republic, France, Germany, India,
Jamaica, Malawi, Poland, South Africa, Spain, Sweden, Thailand, and Zambia. The
European Union (EU) alone is responsible for about two-thirds of world biodiesel
production, with Germany, France, Italy, and Spain with a biodiesel production
capacity of 16 billion l per year in more than 200 biodiesel production units, and
an additional ethanol production plant with a capacity of over 3 billion l per year is
under construction.
1.5
Electrochemistry: a Technological Overview
Electrochemistry serves to illustrate the fundamentals related to the existence
and movement of electrons present in bulk, as well as the interfaces between
ionics, electronics, semiconductors, photonics, and dielectric materials and their
consequences on various fields of science, that is, chemistry, engineering, biology,
materials, and environmental [21–23]. It also accomplishes the reverse of above,
15
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1 Electrochemical Technologies for Energy Storage and Conversion
that is, withdrawal of electricity from energetic chemicals by electrolysis. Electrons
are inexpensive redox reagents, as the cost of a mole of electrons is <$0.01 compared to $0.03–$3.00 for common redox reagents [24]. John O’M Bockris described
electrochemistry as a subject that deals with the making of substances by means
of electricity or making of electricity by consuming substances. In traditional
electrolytic techniques, the electric current directly passes between the electrodes
(anode and cathode) in contact with the electrolytic phase that contains ions. Since
1972 [25], when the use of semiconductor materials as electrodes came into much
closer focus, it widely extended the realm of subjects that can be treated under
electrochemistry. Electrodes (anode and cathode) bring about specific chemical
changes (oxidation and reduction, respectively), usually in conditions close to ambient temperature and pressure, without the use of any toxic reagents. Electrolysis
can be a selective, an easily computer controllable, a convenient, and a cost-effective
technology for synthesis, separation, characterization, and pollution control. Sophisticated electrochemical cells and cell components are readily available in market
to assist us with high technical expertise. Suitable electrolytic cells are available off
the shelf and are capable of being combined with other necessary processor units to
construct fully integrated and compact production systems, which may be in a batch
or continuous process. Electrochemical phenomenon plays a fundamental role in
providing essential materials and devices that contribute significantly to the area of
importance in national security and well-being of the mankind. Moreover, humans
themselves are bioelectrochemical machines, converting solar energy stored in
food via electrochemical reaction into muscle power. On the basis of widely spread
occurrence of the electrolytic phenomenon in technology and devices, the arena of
electrochemistry is categorized as follows:
• Materials of interest includes concrete, ceramic, catalytic materials, composites,
colloids, semiconductors, surfactants, inhibitors, biomaterials such as proteins
and enzymes, emulsion and foams, metal and alloys, ionic solids, dielectric, polymers, membrane and coating, and aqueous and nonaqueous solvent solutions.
• Phenomena that arise in the materials include conduction process, mass transfer
by convection, ion exchange, potential field effect, adsorption, electron and
ion disorders, colloidal and interfacial activity, wetting, membrane transport,
sintering, dendrite formation, electrokinetics, electrocatalysis, passivity, bubbles
evolution, and gaseous discharge (plasma) effect.
• Processes that critically depend on phenomenon include energy conversion and
storage, chlor-alkali industry, pulp and paper, corrosion and corrosion control,
membrane separation, surface reactions, desalination, deposition and etching
by electrolytic and plasma processes, mining and metallurgy, environmental
protection and control, water and wastewater treatment, processing and fabrication, electrochemical synthesis of inorganic and organic chemicals, and pollution
detoxification and recovery.
• Products resulting from these processes include batteries and fuel cells, microelectronic devices, devices in information technology, ceramics, sensors,
membranes, metals, gases, coatings and films, chemicals, pharmaceuticals, and
microelectronics
1.6 Electrochemical Rechargeable Batteries and Supercapacitors
This multidisciplinary field identifies new technological opportunities in widely
diverse applications and underpins many technologies. In addition, cutting-edge
applications in new areas, including in situ characterization, interfacial structures,
surface reactions, and plasma, also hold great promise for advancement in the
field. Aluminum for building and aircraft and titanium for supersonic aircraft
and tanks are made of electrochemical processes. Highly sensitive microsensors
implanted in human body can precisely report about the biochemical changes
in the body. Electrochemical knowledge has been made feasible to accelerate the
healing of tissue and to simulate the action of nerves that have been damaged.
Electrochemical life-lasting batteries for pacemakers are also available in market.
Coatings for car that would not change in appearance after years of service along
with propulsion system for electric vehicles and methods to remove toxic materials
selectively from streams of waters have also been made available. The electronics
industry underwent a rapid evolution from thick to thin films during the last
decade and often played an important and decisive role in the plating through mask
technology, plating for thin film heads, plating for high density magnetic thin film,
selective etching technology, and so on. New electrochemical approaches have also
been playing the prominent roles in the electronics industry, and their activities
touch almost all industrial sectors.
While all these technologies are based on the same fundamental principles
but their practical manifestations may be quite different with, for example, cell
configurations, electrode materials and sizes, electrolytes and separators, each
designed to meet the particular demands of the application. Electrolysis should
be selected as the preferred method over other competitive chemical routes
because redox species are always recycled; hence, only small amounts of redox
reagents are used without any stoichiometric by-products. High solubility of
reactants required for viable current density is a major limitation of electrochemical
technology.
1.6
Electrochemical Rechargeable Batteries and Supercapacitors (Li Ion Batteries,
Lead-Acid Batteries, NiMH Batteries, Zinc–Air Batteries, Liquid Redox Batteries)
Electrochemical rechargeable batteries or secondary batteries [26] are energy storage
devices, receive electricity that is produced elsewhere, and utilize electricity to derive
electrochemical reactions (uphill, a positive G) at both electrodes and become
ready to release this energy downhill in a spontaneous manner. A handsome
variety of rechargeable batteries is available in commercial market (including
lead-acid, nickel cadmium (Ni-Cd), nickel metal hydride (Ni-MH), zinc–air, liquid
redox, lithium ion (Li ion), and lithium ion polymer batteries) (Figure 1.7); they
come in different shapes and sizes with different energy to weight and energy
to volume ratios connected to stabilize an electrical distribution network and can
be used several times. Batteries are good at providing high power levels, but the
amount of energy they can store per unit weight (50–1000 W kg1 ) is not greater
17
1 Electrochemical Technologies for Energy Storage and Conversion
−250
Watt-hours/kilogram
18
Lithium polymer
prismatic
−200
Lithium phosphate
150
Lithium ion
cylindrical
Aluminum cans
Prismatic
Nickel cadmium
cylindrical
100
Prismatic
50
Lead-Acid
50
Nickel metal hydride
cylindrical
Prismatic
100
150
−250
300
−200
Watt-hours/litre
350
400
450
Figure 1.7 Different kinds of batteries rated on the scale
of their energy to weight and energy to volume ratios.
(http://en.wikipedia.org/wiki/Rechargeable_battery).
than that of fuel cells because they can use up, at best, all the material on their
plates, whereas fuel cells simply convert all the available chemical fuel into energy.
Most of the present day rechargeable batteries have to be cycled about 100 times
when the depth of discharge (DOD) is 90%; moreover, the batteries beyond 1000
recharges with a high DOD are also available. Furthermore, the systems with
50 000 recharges are possible at a 40% DOD. The calendar life of batteries depends
on their mode of use. They have made a revolutionary impact on our lives; for
example, they can be used in applications such as automobile starters, portable
consumer devices, light vehicles (such as motorized wheelchairs, golf carts, electric
bicycles, and electric forklifts), tools, providing long-term power to internal artificial
organs (heart pace makers, hearing aids, etc.), and providing uninterruptible power
supplies. Grid energy storage plants also use industrial rechargeable batteries for
load leveling, where they are used to store electric energy for the peak loading
periods. Emerging applications in hybrid electric vehicles/electric vehicles are
driving the technology to increase the lifetime of vehicles by reducing the cost and
weight. Rechargeable batteries have higher initial cost, but the total cost of use and
environmental impact are lower than disposable primary batteries, although they
are accounted as pseudopollution contributors because the electricity consumed
by them for charging is produced from the combustion of fossils or oil. Normally,
new rechargeable batteries have to be charged before their use, but the newer low
self-discharge batteries hold their charge for many months and are able to supply
charges to up to 70% of their rated capacity. The energy used to charge rechargeable
batteries usually comes from a battery charger that uses AC mains electricity, and
it will take a few minutes (rapid chargers) to several hours to charge a battery. Since
the end of twentieth century, in the United States, Japan, and Europe, the market
of rechargeable batteries became vibrant and extremely attractive, as the demand
for rechargeable batteries is growing twice faster than that for nonrechargeable
1.6 Electrochemical Rechargeable Batteries and Supercapacitors
ones. Still there is no best battery in the market, but there are a handsome variety
of batteries in the market for different situations, likewise, batteries for torpedoes
must be stable during storage and must give high power for a short time, whereas
batteries for submarine need giant rechargeable ones when submerged.
Unlike batteries, which store energy chemically, capacitors store energy as an
electrostatic field. A typical battery is known for storing a lot of energy and little
power, whereas a capacitor can provide large amounts of power, but low amounts
of energy. A capacitor is made of two conducting plates and an insulator called the
dielectric, which conducts ionically but not electrically. Few important batteries are
described below.
1.6.1
Lead-Acid Batteries
Lead-acid batteries revolutionize the portable power and fall into the classical
category invented by French physicist Gaston Planté in 1959 [27]. They consist
of six cells of 2 V nominal voltage, and each cell is composed of a lead dioxide
cathode, a sponge metallic lead anode, and about 37% w/w sulfuric acid solution
as electrolyte. Its main discharge reaction at anode is
Pb C HSO4 ! PbSO4 C HC C 2e
(1.1)
And the corresponding discharge reaction at cathode is
PbO2 C 3HC C HSO4 C 2e ! PbSO4 C 2H2 O
(1.2)
The thermodynamic reversible potential for the overall cell reaction is 1.93 V,
meaning that less number of cells is used to attain a given potential. The optimum
operating temperature for the lead-acid battery is 25 Ž C. For higher power applications, lead-acid batteries with intermittent loads are generally too big and heavy,
suffer from a shorter cycle life, and typical usable power down to only 50% DOD.
These batteries become the technology of choice for automotive starting, lighting,
and ignition (SLI) applications because they are robust, tolerant to abuse, and of
low cost. Lead-acid batteries have a huge market as the starter battery for internal
combustion engines. Although they have one of the worst energy to weight ratios
(35–40 Wh kg1 ) but quite good power to weight and energy to volume ratios,
their life seldom exceeds 4 years and can be recharged for 300–400 cycles. In a
valve regulated lead-acid (VRLA) batteries, electrolytes avoid spilling out, and the
hydrogen and oxygen produced in the cells largely recombine into water. Since the
1950s, chemical additives such as EDTA and Epsom salts [28] have been used to
reduce lead sulfate buildup on plates and improve battery condition when added
to the electrolyte of a vented lead-acid battery. EDTA can be used to dissolve the
sulfate deposits of heavily discharged plates. Residual EDTA in the lead-acid cell
forms organic acids that will accelerate corrosion of the lead plates and internal
connectors. Epsom salts reduce the internal resistance in a weak or damaged battery
and may allow a small amount of extended life. Heavy metal elements used in their
19
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1 Electrochemical Technologies for Energy Storage and Conversion
fabrication makes them toxic, and their improper disposal can be hazardous to the
environment.
1.6.2
NiMH Batteries
An NiMH battery is similar to the nickel-cadmium cell and was invented in 1967
[29]. The Ni-MH battery uses a hydrogen-absorbing alloy (sintered Ti2 Ni C TiNi C x
or the presently used AB5 , where A is a rare earth mixture of lanthanum, cerium,
neodymium, praseodymium and B is nickel, cobalt, manganese, and/or aluminum,
and AB2 compounds, where A is titanium and/or vanadium and B is zirconium
or nickel, modified with chromium, cobalt, iron, and/or manganese) as the
negative electrode (negative electrode of Ti–Ni alloy hydride phases, US patent US
3,669,745 (13 June 1972), inventor: K. D. Beccu of Battelle, Geneva R&D Center);
nickel oxyhydroxide (NiOOH) as the positive electrode developed by Dr Masahiko
Oshitani; and usually, 28% potassium hydroxide as the alkaline electrolyte. For
separation, hydrophilic polyolefin nonwovens are used. Respective cathodic and
anodic reactions of the Ni-MH batteries can be written as follows:
Cathode:
NiMAlloy H ! NiMAlloy C HC C e
(1.3)
Anode:
NiOOH C H2 O C e ! Ni(OH)2 C OH
(1.4)
Ni-MH batteries can possess 2–3 factors higher capacity (1100–3100 mAh at
1.2 V), the same as an equivalent size nickel-cadmium battery. Its volumetric energy
density (140–300 Wh l1 ) is similar to that of the lithium ion cell (250–360 Wh l1 ),
significantly better than that of nickel-cadmium battery at 50–150 Wh l1 , but
its self-discharge is higher (30% per month). It can retain specific energy of
approximately 30–80 Wh kg1 and a specific power of around 250–1000 W kg1
with a reasonable deep life cycle of 500–1000 cycles (DOD D 100%), and this
has led to the new environmentally friendly high-energy NiMH cells [30]. Low
internal resistance allows Ni-MH cells to deliver a near-constant voltage until
they are almost completely discharged. Modern Ni-MH cells contain catalysts
to immediately deal with gases developed as a result of overcharging, without
being harmed (2 H2 C O2 C catalyst ! 2 H2 O C catalyst). However, this works
only with overcharging currents of up to 0.1 C and is used to detect the safe
end-of-discharge voltage of the series cells and autoshutdown. Ni-MH cells are
used to power the devices such as digital cameras, GPS receivers and personal
digital assistants (PDAs), flashlights, and some toys or video games. Improper
disposal of Ni-MH batteries poses less environmental hazard than that of Ni-Cd
cells because of the absence of toxic cadmium. Although lithium ion batteries
(LIBs) have a higher specific energy than NiMH batteries, they also have a much
lower shelf life and are significantly more expensive to produce. Currently, more
than 2 million hybrid cars worldwide are running with Ni-MH batteries [31], for
1.6 Electrochemical Rechargeable Batteries and Supercapacitors
example, Prius, Lexus (Toyota), Civic, Insight (Honda), and Fusion (Ford). Many
of these batteries are manufactured by Panasonic (PEVE) and Sanyo.
1.6.3
Li-Ion Batteries
A LIB was first proposed by M. S. Whittingham of Binghamton University, Exxon,
in the 1970s using titanium(II) sulfide as the cathode and lithium metal as the
anode, and it belongs to the rechargeable type of batteries. The primary functional
components of LIBs are anode, cathode, and electrolyte [32]. Lithium ion cell uses
an intercalated lithium compound (Li in porous carbon or graphite) as the anodic
material instead of metallic lithium. A layered oxide (such as lithium cobalt oxide)
or a polyanion (such as lithium iron phosphate), or a spinel (such as lithium
manganese oxide) material is used as cathode. Pure lithium is niche industrial
material of very reactive nature. It reacts vigorously with water to form lithium
hydroxide and liberate hydrogen gas. Thus, nonaqueous electrolytes are required
for LIBs. A typical mixture of nonaqueous organic carbonates such as ethylene
carbonate or diethyl carbonate containing the complex of lithium ions (lithium
hexafluorophosphate (LiPF6 ), lithium hexafluoroarsenate monohydrate (LiAsF6 ),
lithium perchlorate (LiClO4 ), lithium tetrafluoroborate (LiBF4 ), or lithium triflate
(LiCF3 SO3 )) is used as the electrolyte. Lithium ions that move from the negative
electrode to the positive electrode during the discharge process through the
nonaqueous electrolyte and separator diaphragm move back when charging.
The cathodic half reaction of LIB is
LiCoO2 $ Li1x CoO2 C xLiC C xe
(1.5)
And another anodic half reaction is
xLiC C xe C 6C $ Lix C6
(1.6)
Recently, novel architectures fabricated using nanotechnology have been employed
to improve the performance of LIBs. Chemistry, performance, cost, and safety
are the characteristics that are basic to the electrode material used. LIBs became
technological of today’s choice because of the qualities of the best energy to weight
ratios (specific energy density, 150–250 Wh kg1 ; volumetric energy density,
250–530 Wh l1 ; specific power density, 300–1500 W kg1 ), no memory effect, and a
slow loss of charge when not in use. Beyond their popularity for portable electronics,
LIBs are also the growing interest in military, electric vehicles, and aerospace
applications because of their high energy density. There are few disadvantages
of LIBs, that is, its internal resistance is higher than other rechargeables such
as Ni-MH and Ni-Cd batteries and increases with both cycling and age thus
reducing the cell’s ability to deliver current. Charging forms electrolyte deposits
that inhibit ion transport; high charging levels with elevated temperatures (whether
from charging or ambient air) also hasten their capacity. LIBs with a lithium iron
phosphate cathode and graphite anode have a nominal open-circuit voltage of 3.2 V
and a typical charging voltage of 3.6 V. LIBs could not be charged fast and needed
21
22
1 Electrochemical Technologies for Energy Storage and Conversion
at least 45 min to 2 h to fully charge. In 2007, Cui and colleagues at Stanford
University’s Department of Materials Science and Engineering discovered that
using Si/Ge nanowires as the anode of an LIB increases the volumetric charge
density of the anode up to a factor of 10 [33, 34].
Further advancement in LIB is made by replacing lithium salt electrolyte in
an organic solvent with a solid polymer composite such as polyethylene oxide
or polyacrylonitrile [35]. The advantages of Li ion polymer over the LIBs include
potentially lower manufacturing cost, adaptability to a wide variety of packaging (in
various shapes), increased life cycle, slow degradation rate, and ruggedness. Lithium
ion polymer batteries started appearing in consumer electronics around 1996 [36].
However, in recent years, manufacturers have been declaring upward of 50010 000
charge–discharge cycles before the capacity drops to 80%. Li poly batteries are
gaining favor of the world in radio-controlled aircraft and radio-controlled cars,
airsoft gun, PDAs, and laptop computers, such as Apple’s MacBook family,
Amazon’s Kindle, Lenovo’s Thinkpad X300 and Ultrabay Batteries, the OQO series
of palmtops, the HP Mini, and Dell products featuring D-Bay batteries, where
the advantages of both lightweightness and greatly increased run times can be
sufficient justification for their price. They can also be found in small digital music
devices such as iPods and Zune and other MP3 players, the Apple iPhone, gaming
equipment such as Sony’s Playstation 3, and wireless controllers.
1.6.4
Zinc–Air Batteries
Zinc–air batteries [37] have some properties resembling the fuel cells, such as
zinc electrode consumed as fuel, and the reaction rate can be controlled by varying
the air flow. Commercial production began in 1932, when George W. Heise and
Erwin A. Schumacher of the National Carbon Company (US patent 1899615(A)
dated: 28 January 1933) built these cells. Zinc–air battery consists of a porous
air electrode (cathode) made of carbon that draws in atmospheric oxygen, which
gets absorbed into the electrolyte (aqueous KOH) through a gas-permeable and
liquid-tight membrane (also acts as catalyst) to the zinc electrode (anode), where
oxidation of zinc releases electrons to generate a current with ZnO production.
Cathode (porous air-carbon electrode):
O2 C 2H2 O C 4e ! 4OH (E0 D 0.4 V)
(1.7)
Anode (Zn electrode):
Zn C 4OH ! Zn(OH)4 2 C 2e (E0 D 1.25 V)
(1.8)
Fluid reaction:
Zn(OH)4 2 ! ZnO C H2 O C 2OH
(1.9)
Overall reaction:
2Zn C O2 ! 2ZnO (E0 D 1.65 V)
(1.10)
1.6 Electrochemical Rechargeable Batteries and Supercapacitors
The cell voltage for the above theoretical chemistry is 1.65 V; however, almost
all designs are optimized for <1.4 or 1.3 V in order to achieve longer lifetimes.
Recharging process is the reversal of the above reactions: zinc oxide is converted
back to zinc at negative anode and oxygen is released at the positive air electrode.
During discharge, Zn is converted to ZnO, which is replaced through mechanical
charging in which discharged zinc cartridges are replaced with fresh zinc cartridges.
Repeated charge–discharge cycles deactivate (slowing or stopping the oxygen
reactions) the air electrode and gradually tend to fill its pores with the liquid
electrolyte. The battery can also fail if it dries out or if zinc builds up unevenly,
forming branchlike structures that create a short circuit between the electrodes.
Like all other battery technology involving Ni-MH, lead-acid, and zebra batteries,
zinc–air batteries are safer than LIBs (requires much more extensive electronic
monitoring) because they are short-circuit proof, and they do not contain volatile
materials, hence, they do not catch fire or explode during a car crash. Zinc–air
batteries require filters to remove carbon dioxide from the air for their use; the
removal is via passing air through the inexpensive hydroxide scrubbers where it
is fixed to carbonate as a support to purify the air. Zinc–air batteries can beat
petrol-dependent road transportation for electric propulsion with their unique
capability of anthropogenic CO2 removal. A new breed of rechargeable zinc–air
batteries (Figure 1.8, ReVolt, Staefa, Switzerland) will soon to be available in market
and may replace LIBs in cell phones, laptops, and other consumer items. LIBs
store only one-third of the energy and cost around twice as much as the new breed
of rechargeable zinc–air batteries [37]. Ni-MH and LIBs are very expensive ($500
k Wh1 ), whereas the relative cost of zinc–air batteries is only $60 k Wh1 but
still are a favorite technology for battery industry. The zinc–air systems are quite
popular among their category because of the use of nontoxic, inexpensive, and
abundantly available raw materials; the high energy density (1480–9780 Wh l1 );
the high energy per unit of weight (470 (practical), 1370 (theoretical) Wh kg1 ); and
an excellent shelf life with a self-discharge rate of only 2% per year when sealed, but
they have a low specific power (100 W kg1 ). Their few disadvantages are sensitivity
to extreme temperature and humid conditions, chemicals tend to dry out, high
self-discharge, high internal resistance, and low recycle lifes. In the light of the low
cost, material availability, safety, environmental aspects, and financial constraints
faced by the world today, governments should prioritize zinc–air batteries to power
Battery cover, which lets in air
Porous air electrode
Interface between electrodes
Zinc electrode
Casing
Figure 1.8 Illustration of the multilayered structure of an
unpacked ReVolt rechargeable zinc–air battery. (ReVolt,
Staefa, Switzerland.)
23
24
1 Electrochemical Technologies for Energy Storage and Conversion
automobiles [38]. Zinc–air button cells are commonly used for watches and hearing
aids. Larger types are employed as prismatic or cylindrical cells for telecoms and
railway remote signaling and for safety lamps at road and rail construction sites, as
power sources for electric fences, and in film cameras that previously used mercury
batteries. Very large batteries are used for electric vehicle propulsion.
1.6.5
Liquid Redox Batteries
Liquid-metal batteries are fabricated by molten anode and cathode materials
separated by solid electrolyte (a beta-aluminum oxide ceramic ion conductor). They
are of low manufacturing cost, of extremely high power density, and are suitable
for grid-scale storage. A classic example of this category is sodium–sulfur batteries
(Figure 1.9) [39], discovered in 1980 and consisting of molten sodium anode
and molten sulfur cathode, separated by conductive ceramic electrolyte sodium
beta-alumina (approximately 11 parts of aluminum oxide Al2 O3 and 1 part of
sodium oxide NaO, with a melting point of 2100 Ž C). Sodium melts at 98 Ž C and
sulfur at 113 Ž C, but the operating temperature is around 350 Ž C. During discharge,
electrons are striped off the sodium atoms and flow through the external load to the
sulfur cathode. The positively charged sodium ions move through the electrolyte
where they react with the sulfur and the electrons to produce sodium polysulfide.
During recharge, the applied voltage strips electrons from the sodium polysulfide
returning it back to sodium ions. The sodium ions now cross the electrolyte into
the sodium where they are reunited with their missing electrons to form sodium
atoms. The sodium–sulfur battery can have very high energy (50–200 Wh kg1 )
and power densities (100–200 W kg1 ) and a limited shelf life, typically of the
order of 2–5 years. One of the key advantages of batteries with liquid electroactive
materials is that there are no morphological changes on cycling that can offer
infinite life cycle to the active materials, but the solid electrolyte is susceptible to
mechanical degradation by the phase change that occurs as the sodium ions are
reduced to the metallic state. In the new self-assembling liquid-metal batteries,
(Cell seal)
Aluminum case (+)
Molten S (cathode)
Molter Na (anode)
Typical sodium−sulfur battery
Figure 1.9 Schematic representation of the typical
sodium–sulfur battery, as liquid redox battery.
1.7 Light Fuel Generation and Storage
molten salt serves as both the separator and electrolyte of molten electrodes. Density
differences allow two of the three liquid layers to float on the heaviest layer. The
very high ionic conductivity of the electrolyte allows for extreme power density, well
suited for grid-scale power storage. The safety issues implicit in molten materials
and high operating temperature made them not practically attractive for use in
electric vehicles, but this problem may be resolved in time. In the proof-of-concept
cell by Donald Sadoway’s research group at MIT, the three layers, from bottom to
top, antimony, sodium sulfide, and magnesium, are expected to be able to handle
10 times the current of typical batteries used in power plants. During discharge,
the top and bottom layers are consumed to form magnesium antimonide, which
dissolves in the electrolyte. Upon recharge, the metal layers are reformed. New
patent pending liquid batteries are designed by Sadoway and his team, including
graduate student David Bradwell, using low-cost and abundant materials. The
three materials are chosen so that they have different densities that allow them to
separate naturally into three distinct layers, with the salt (Mg3 Sb2 ) in the middle
separating the two metal layers (Mg (top) and Sb (bottom)) and are operated at
about 700 Ž C; if corrosion issues for the electrodes and container can be resolved,
the cycle life of these batteries might be nearly infinite. Because of its high energy
density, the Na–S battery has been proposed for power grid [40], transport and
heavy machinery, and space applications [41].
1.7
Light Fuel Generation and Storage: Water Electrolysis, Chloro-Alkaline Electrolysis,
Photoelectrochemical and Photocatalytic H2 Generation, and Electroreduction of CO2
Light fuel hydrogen has attracted great attention from environmentalists, scientists,
and industrialists as a benign fuel of future because of its capability to produce
pollution-free energy and because it has one of the highest energy density, that is,
values per mass of 140 MJ kg1 . The majority of hydrogen used in industries is
derived from fossil fuels or cleavage of water. Currently, a majority of industrial
hydrogen need is satisfied from conventional sources (coal, oil, and natural gas),
which contains about 10% CO2 with hydrogen gas, and only 4% of H2 comes
from electrolysis, which is the cheapest method to generate hydrogen ($3.51 per
kg). Central to the success of hydrogen technology is the efficient generation of
hydrogen from renewable sources (such as solar/wind) powered by water cleavage.
Water splitting by electrolysis, uses the most expensive proton exchange membrane
(PEM) and liquid electrolyte (KOH) or thermochemical process (need a temperature
of 700–1000 Ž C). Wind energy, the most cost-effective renewable energy source,
is also used to electrolyze water (costs about $6.64 per kg per H2 if grid back up
used). Biomass with solar energy costs $7.05 per kg H2 production. Solar energy
either by electrolysis (PV) or using photoelectrochemical cell now costs $2.82 per
kg, but solar energy is available only 20% of the time. Following are the advance
techniques to produce and use light fuels (H2 , CO2 ).
25
1 Electrochemical Technologies for Energy Storage and Conversion
1.7.1
Water Electrolysis
In 1789, Jan Rudolph Deiman and Adriaan Paets van Troostwijk produced electricity
from water using an electrostatic machine by discharge on gold electrodes in a
Leyden jar [42]. In 1800, the voltaic pile was invented for the electrolysis of water
by Alessandro Volta. Finally, the electrolysis of water became a cheap method of
hydrogen generation using the Gramme machine invented by Zénobe Gramme in
1869. A method of industrial synthesis of hydrogen and oxygen through electrolysis
of water was developed by Dmitry Lachinov in 1888 [43]. Decomposition of pure
water into hydrogen and oxygen at standard temperature and pressure is not
favorable in thermodynamic terms. Water electrolysis does not convert 100% of
the electrical energy into the chemical energy of hydrogen. For a well-designed cell
(Figure 1.10), the largest overpotential required for the four-electron oxidation of
water to oxygen at the anode. An effective and cheap electrocatalyst to facilitate
this reaction has not yet been developed. Platinum alloys are the default state of
the art for this oxidation. The simpler two-electron reaction to produce hydrogen
at the cathode can be electrocatalyzed with almost no reaction overpotential
by platinum or hydrogenase enzyme. In 2008, Kanan et al. [44] announced a
potentially efficient electrocatalyst (anode) for artificial photosynthesis, composed
of a Co3C /Co2C metal and phosphate electrolyte. Other researchers are pursuing
carbon-based catalysts for the same. Efficiency of converting electrical energy into
hydrogen’s chemical energy of water electrolysis varies optimally between 50 and
80% [45].
Output electric energy
∆G = 237.13 KJ mol−1
Input fuel energy
∆G = 285.83 KJ mol−1
H2
Anode : H2 → 2H+ + 2e−
H+
us
us
H+
Po
Po
ro
H2
an
od
e
ca
th
od
e
Load
e−
e−
O2
ro
H2
Electrolyte
26
O2
O2
Output heat energy
∆G = 48.7 KJ mol−1
Cathode : O2 + 4H+ + 4e− → 2H2O
Water
Migration across electrolyte
Hydrogen−oxygen fuel cell operation through electrolyte
Figure 1.10 Water electrolysis illustrated in
hydrogen–oxygen fuel cell with chemical and
energetic electrode operations using electrolyte.
1.7 Light Fuel Generation and Storage
1.7.2
Electrochemistry of Water Splitting
Pure water is considerably a bad conductor of current, 0.055 µS Ð cm1 (one
millionth of that of seawater) because it has a low autoionization coefficient,
Kw D 10 ð 1014 at room temperature that leads electrolysis of pure water very
slowly or not at all. Thus electrolysis of pure water requires excess energy in the
form of overpotential to overcome various activation barriers, which enhance the
rate of water autoionization. In a properly designed water-electrolysis cell, two
electrodes (anode and cathode) are placed in the water and connected by external
electrical power source. Hydrogen appeared at the cathode (the negatively charged
electrode, where electrons reduce the water), and oxygen appeared at the anode (the
positively charged electrode, where oxidation takes place) in 2 : 1 ratio. The efficacy
of electrolysis is increased by the addition of an electrolyte (such as solid polymer
membrane, Nafion; strong acids, H2 SO4 ; or a strong base, NaOH, KOH) and
electrocatalysts. A solid polymer electrolyte can also be used, such as Nafion, and
when applied with a special catalyst on each side of the membrane can efficiently
split the water molecule with as little as 1.8 V power supply.
Anode reaction:
2H2 O(l) ! 4HC (aq) C 4e C O2 (g) " (EA D 1.23 eV)
(1.11)
Cathode reaction:
2HC (aq) C 2e ! H2 (g) " (Ec D 0.00 V)
(1.12)
Ž
Thus, the standard potential of the water-electrolysis cell is 1.23 V at 25 C at pH 0
(HC D 1.0 M). It retains 1.23 V at 25 Ž C even for pH 7 (HC D 1.0 ð 107 M) based
on the Nernst equation. This endothermic reaction involves a change in Gibbs free
energy G D C2.46 eV or 237.2 kJ mol1 . The negative voltage indicates that the
Gibbs free energy for electrolysis of water is greater than zero. The system must
be provided with sufficient energy for the dissociation of water plus the energy to
expand the produced gases.
1.7.3
Chlor-Alkaline Electrolysis
Chlorine, one of the most important bulk chemicals in the world, is produced
by the electrolysis of brine (highly conducting sea water) [46]. Chlorine is used
in the day-to-day life as water purifier and is also an essential part in the chemical building block, resulting in a myriad of reactions and products in the major
plastic, pharmaceutical, inorganic, and fine chemical and specialty industries [45].
In chlor-alkali electrolysis (Figure 1.11), an air–oxygen gas-diffusion electrode or
traditional hydrogen-evolving electrode is used as a cathode, a nickel wire mesh
as a counter electrode in 8 M NaOH, and Hg/HgO as reference electrode. Coating of polytetraflouroethylene (PTFE) suspension was performed on gas-diffusion
electrode, shielded by an active layer made up of catalysts [46, 47] containing the
27
28
1 Electrochemical Technologies for Energy Storage and Conversion
Ion-selective membrane
Cl2 & depleted
brine
32−35% NaOH
O2
Cl2
+
OH−
OH
Cl−
Cl−
DSA
anode
OH−
Na(H2O)x
−
Cathode
O2
Brine
H2O
Figure 1.11
Graphical illustration of chlor-alkaline cell.
mixture of perovskite (La0.1 Ca0.9 MnO3 ) and the pyrolysis product of cobalt tetramethoxyphenylporphyrin (CoTMPP) with less proofed carbon. In the electrolytic
process, during the following overall reaction for every ton of chlorine produced,
about 1.1 tons of caustic is generated and 28 kg hydrogen is evolved as a by-product.
Four percent of total worldwide hydrogen production is created by electrolysis.
The majority of this hydrogen is produced by chlor-alkali electrolysis, where, the
chloride ions are oxidized to chlorine and water is reduced to hydrogen as shown
in the following reaction.
Overall reaction:
2NaCl C 2H2 O ! Cl2 C H2 C 2NaOH
(1.13)
It is an energy intensive process, with an electrical power consumption between
2100 and 3300 kWh per unit time. The amount of Cl2 generation depends on
the operating parameters and the type of the process. There are three major
processes based on mercury, diaphragm, and membrane cells in use for the
electrolysis. In the year 2001, the total world production of chlorine was about 43.3
million tons, and 18% of this production was met by the mercury cell technology,
whereas the remaining 82% included diaphragm (49%) and membrane cell (28%)
technologies, and others (5%). Hofmann voltametric, high-pressure (120–200 bar),
high-temperature electrolyses used for chlor-alkaline electrolysis are also in practice
[48, 49].
1.7.4
Photoelectrochemical and Photocatalytic H2 Generation
Global energy needs for sustainable development demands 50% increase by 2030.
The alternative energy source the sun is able to obtain the target by using solar
energy to split water into hydrogen and oxygen as a cost-effective storage of solar energy in chemical energy for its large-scale utilization. Nature provides us
a blueprint of water-splitting reaction in the form of photosynthesis for storing
1.7 Light Fuel Generation and Storage
sunlight in form of chemical energy. Laboratory-designed artificial photosynthesis
required photocatalyst, water, and sunlight to split the water into hydrogen and
oxygen in 2 : 1 ratio. A range of metal oxides, sulfide, nitrides, (oxy)sulfides, and
(oxy)nitrides containing either transition metal cations with d0 electronic configuration (e.g., Ti 4C , Nb5C , and Ta5C ) or typical cations with d10 electronic configuration
(e.g., Ga4C , Ge4C , and Sn4C ) as principal cation components have been reported
as active photocatalysts for overall water splitting. Photocatalytic reaction involves
three basic steps: (i) the photocatalyst absorbs more photon energy (sunlight) than
the band gap energy of the material and generates photoexcited electron–hole
pairs in the material bulk; (ii) the photoexcited charges separate and migrate to the
different sites of the photocatalyst’s surface without recombination; and (iii) water
is reduced and oxidized by the photogenerated electrons and holes to produce H2
and O2 , respectively. The first two steps are strongly dependent on the structural
and electronic properties of the photocatalyst, whereas the third step is promoted
by the presence of a solid cocatalyst (Figure 1.12). The cocatalyst is typically a
noble metal or metal oxide or a combination of both, loaded onto the surface of
photocatalyst as a dispersion of nanoparticles to produce active sites and reduce
the activation energy for gas evolution. To date, various transition metal oxides,
including NiOx , RuO2 , RhOx , IrO2 , and RhCr2 O3 , have been applied as a cocatalyst
for photocatalytic overall water splitting. Despite a plentiful library of photocatalytic
materials mostly active in UV light (5% of sunlight spectrum), visible light–driven
photocatalyst with high efficiency is still one of the holy grails of material chemistry.
Few photocatalysts reported with remarkable efficiency are (AgIn)x Zn2(1x) S2 [50],
ZnS–CuInS2 –AgInS2 [51], La-doped NaTaO3 [52], Ni-doped InTaO4 modified with
RuO2 or NiO as the cocatalyst [53], (Ga1x Znx )(N1x Ox ) [54], SrTiO3 , BaO-doped
·O2−
Visible light
Rh
O2
e−
2 nm
(a)
Cr2O3
h+
H2
Rh
H+
H2O
2 nm
(b)
O2
Rh2−yCryO3
nanoparticle
(~20 nm)
(Ga1−x Znx )(N1−x Ox )
(c)
Figure 1.12 Loading of cocatalyst on the
surface of the basic photocatalyst and its
role in controlling recombination reaction
of electrons and protons. HR-TEM images
of (Ga1x Znx N1x Ox ) (x D 0.12) with
photodeposited Rh (a) before and (b) after
further photodeposition of a Cr2 O3 shell. (c)
Schematic view of photocatalytic oxidation
and reduction sites [54]. (Copyright American
Chemical Society Publications.)
29
30
1 Electrochemical Technologies for Energy Storage and Conversion
H2O → 2H+ + ½O2 + 2e−
2e− + 2M+ → 2M
2e− + 2H+ → H2
2M → 2M+ + 2e−
hν
hν
O2
H2
H2O
Figure 1.13
Schematic of a dual-bed photocatalytic water-splitting system [60].
La2 Ti2 O7 [55], and K3 Ta3 B2 O12 [56]. Photoelectrochemical water-splitting technology is economically superior to electrolysis of water for electricity production
because of the single-plant massive photoproduction of clean hydrogen fuel. Titania
(TiO2 ) was the first material described as a photoelectrochemical water-splitting
catalyst, but because of its wide band gap, 3.2 eV, it is able to work in UV light.
Fujishima and Honda, in 1972, described an electrochemical cell consisting of an
n-type TiO2 (rutile) anode and a Pt black cathode for photoelectrochemical water
splitting. Much attention has been given to photoelectrocatalytic splitting of water
using metal oxide or nitride semiconductors, for example, ZnO, GaN, TiO2 , WO3 ,
and Fe2 O3 , to supply clean and recyclable hydrogen energy. Advancements such
as introduction of nanostructures; elemental doping with N, C, P, S, and dyes
(Ru complexes); and quantum dot (CdS, CdSe, InP, CdTe, Bi2 S3 ) sensitization of
based material are also in practice. Bockris and Reddy [57] made their effort to
produce faceted TiO2 (about 1000-fold increase in surface area) and decorated it
with dye with a suitable photoreceptor (bipyridyl complexes of ruthenium), then
they became capable of harvesting visible light. Key point is the use of traces of
electrocatalyst added to the surface of both photoanode and photocathode [58, 59]
to an appropriate extent. Use of a perylene diimide derivative as the O2 -evolving
photocatalyst and copper phthalocyanine as the H2 -evolving photocatalyst in the
IO3 /I redox electrolyte demonstrates the feasibility of continuous closed-cycle
dual-bed photocatalytic water splitting (Figure 1.13) [60]. Recently, artificial inorganic leafs have been developed as biotemplates [61] by using catalysis modules
(Pt/N-TiO2 ) for enhanced light-harvesting, and photocatalytic water-splitting activities stem from the reproduction of the leafs complex structures and self-doping of
nitrogen during synthesis (Figure 1.14).
1.7.5
Carbon Dioxide Reduction
Before the industrial revolution, CO2 concentration in the atmosphere was about
290 ppm. By 1995, it had reached 360 ppm. Today, it is about 410 ppm, and
1.7 Light Fuel Generation and Storage
2H2O
hn,catal.
2H2 + O2
Figure 1.14 Artificial inorganic leafs for efficient photochemical hydrogen production inspired by natural
photosynthesis [61].
extrapolation of the present data raised it up to 560 ppm in 2060, accompanied with
rise in temperature to about 1.5 K at the equator and a corresponding estimated
increase in the sea level by 50 cm and 3.2 K temperature at poles will reduce the
reflectivity of the region (water is dark and ice is bright) eventually increasing the
earth’s average temperature [62]. The situation is becoming more and more critical
day by day. One way to deal practically with this problem is to fix this undesirable
CO2 into hydrogen carrier liquid, that is, methanol, to be used in fuel cells [63].
Fisher and Prziza [64] were first to observe the 100% electrochemical reduction
of CO2 to HCOOH. Photoelectrochemical conversion of CO2 to oxalic acid was
performed using InP-electrode (cathode) in two-photon cell having LaCrO3 -TiO2 as
photoanode [65]. An 18-crown-6-ethers-decorated-CdTe electrode in the presence
of tetraalkylammonium ions reduces CO2 mainly to CO with a small amount of
CH3 OH [66]. A major breakthrough would be the discovery of a suitable effective
catalyst or photocatalyst in CO2 fixation for the reduction of the major greenhouse
gas CO2 into a useful chemical, that is, methanol (MeOH), using H2 gas supply.
CO2 C 3H2 C catalyst ! CH3 OH C H2 O C catalyst
(1.14)
Inspired by photosynthetic reactions in plants, now a reasonable goal of electrochemical science is to fix planetary CO2 by photochemical/photoelectrochemical
reaction to form MeOH, and the further goal would be based on the production of
synthetic food and textile from water, solar light, atmospheric CO2 , and nitrogen
(bacteria). However, it is also possible to use CO2 in electrochemical synthesis of
organic compounds that exemplify in the formation of phenyl acetic acid by benzyl
chloride [67].
31
32
1 Electrochemical Technologies for Energy Storage and Conversion
1.8
Fuel Cells: Fundamentals to Systems (Phosphoric Acid Fuel Cells, PEM Fuel Cells,
Direct Methanol Fuel Cells, Molten Carbon Fuel Cells, and Solid Oxide Fuel Cells)
Fuel cells are compact electrical energy producer devices that consume fuels
(chemical energy) via fuel (e.g., hydrogen, methanol, and hydrocarbon) oxidation
at anode and reduce oxygen (usually comes from air and triggered into solution)
as oxidant at counter cathode on immerging into electrolyte. The electrolyte is
specifically designed so that ions can pass through it, but the electrons cannot.
The free electrons travel through a wire, creating the electrical current. The
revolutionary advantage of fuel cells over other energy-producing devices is that
the free highly dense energy directly comes out as pollution-free electrical energy
without planetary warming. As no combustion reactions are involved in fuel
cells, they do not produce any of the undesirable products (CO2 , SO2 , oxides of
nitrogen, or particulate matters), which are normally associated with the oxidation
of fossil fuels in conventional energy conversion systems. Thus, fuel cells are
environmentally friendly. However, fuel cells offer the additional advantages of
lighter weight, noiselessness, nonpolluting, and vibration-free operation than
batteries that require recharging after use. For these reasons, they are beloved
candidates to be used as battery’s supplements. Formal discovery of fuel cell
principle is attributed to Sir William Grove (February 1839) [68] as he passed current
through the connecting wire between anode that was bubbled with hydrogen and
its counter electrode was blown with oxygen from air, the device was named as gas
voltaic battery [2] (Figure 1.15). Francis Thomas Bacon made pioneer contribution
in the development of fuel cells in 1959. NASA uses Bacon’s cells for auxiliary
power in space vehicles, as they are three times as effective as any other method
of providing energy on board. Fuel cells have the potential to replace the internal
combustion engine in vehicles and to provide power in stationary and portable
power applications because they are energy efficient, clean, and fuel flexible. Direct
fuel cells can feed hydrocarbons directly to the fuel cell stack, without requiring
an external reformer to generate hydrogen [69]. Fuel cells are thermodynamically
open systems that work spontaneously and go down the free energy gradient by
Water
ox
ox hy
ox hy
hy
ox hy
ox hy
Sulfuric acid solution
Figure 1.15 Sketch of the first hydrogen–oxygen
Grove’s fuel cell invented in 1839. The dark lines in the
central of the tubes are platinized-foil electrodes [68b].
1.8 Fuel Cells: Fundamentals to Systems
consuming reactant from an external source, which must be replenished. There are
two governing factors in fuel cells. First, is the exchange current density of cathode
that causes low overpotential (higher limiting current density i0 ¾ 103 A cm2 ),
resulting in high efficiency of chemical energy conversion. Second, is the limiting
current density of cell that depends on the type of electrode. Flat-type electrodes
show maximum power density, <1 mW cm2 ; on the other hand, porous electrodes
make the limiting current 100–1000 times greater, with power densities of up to
1 W cm2 . Fuel cells are categorized on the basis of the electrolyte employed and
their operating temperature range, that is, low-temperature fuel cells (alkaline,
proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC))
and high-temperature fuel cells (phosphoric acid fuel cell (PAFC), molten salt,
and solid oxide fuel cells). Low-temperature fuel cells use a platinum catalyst.
Therefore, these fuel cells are prone to catalyst poisoning by impurities. Although,
there are sufficient platinum resources for the future, soaring prices of precious
platinum has led recent research to replace platinum with a material that may
be less susceptible to poisoning, economic by cost, safe from an environmental
point of view with improved lifetime. Considerably, gold and palladium coated [70]
iron and sulfur instead of pure platinum through an intermediate conversion by
bacteria would lower the cost of a fuel cell substantially from $1500 to $1.50.
1.8.1
Alkaline Fuel Cells
Alkaline fuel cells (AFCs) are economically qualified for providing a power of
10–100 kW, can work optimally at 80 Ž C with a cell efficiency of 60–70%, and use
basic solutions as electrolytes, hence, a wide range of electrode catalysts (relatively
inexpensive materials) are available for them, whereas the cells using acid solution
or high temperature can use only a noble metal as electrode material. At the time
of being switched on, they can produce almost one-fourth of the power that is
produced at optimal temperature, whereas intermediate- and high-temperature
cells need auxiliary power source to start them and warm them up. However, AFCs
have a disadvantage: air contains CO2 with O2 and forms carbonates in alkaline
solution and blocks the pores of porous electrodes, thus CO2 -free air is used in
cells. Bacon cells used in Apollo moon project are AFCs run on pure H2 and O2 .
1.8.2
Direct Methanol Fuel Cells
DMFCs are qualified for providing a power of 100 mW to 1 kW and 20–30%
efficiency and can produce a small amount of power over a long period. Low-volume,
lightweight packaging, zero-emission power system, low operating temperature
(50–120 Ž C), and no requirement for a fuel reformer unit made the DMFCs an
excellent candidate for very small to mid-sized applications [71]. DMFC systems
are used to power portable applications (smaller vehicles such as forklifts and
tuggers, small portable power supply units for household use, cell phones, digital
33
34
1 Electrochemical Technologies for Energy Storage and Conversion
camera, laptop, music systems, soldier-carried tactical equipment, battery chargers,
and autonomous power for test and training instrumentation) and also as auxiliary
power units (APUs) for some niche transport sectors such as marine and submarine
vehicles, scooters, and motorbikes [72, 73]. Surya Prakash and Nobel laureate
George A. Olah invented the present breed of fuel cells that would directly convert
methanol to electricity using a Nafion membrane and platinum electrodes for both
half-reactions, in 1992 [74]. DMFC is similar to the PEMFCs in that the electrolyte is
a polymer and the charge carrier is the hydrogen ion (proton). However, the liquid
methanol (CH3 OH) is oxidized in the presence of water at the anode, generating
CO2 , hydrogen ions, and electrons that travel through the external circuit as the
electric output of the fuel cell. The hydrogen ions travel through the electrolyte and
react with oxygen from the air and the electrons from the external circuit to form
water at the cathode, thus completing the circuit.
Anode reaction:
CH3 OH C H2 O ! CO2 C 6HC C 6e
Cathode reaction:
3
O2 C 6HC C 6e ! 3H2 O
2
Overall cell reaction:
3
CH3 OH C O2 $ CO2 C 2H2 O
2
(1.15)
(1.16)
(1.17)
Low concentration of methanol (1–3 M) is preferred in DMFCs because methanol
has the tendency to diffuse in high concentrations through the membrane to the
cathode, without reacting with anode (methanol cross over), where its concentration
becomes zero because of the rapid consumption by oxygen. It limits the maximum
attainable current and loses almost half of the methanol. Oxidization of methanol
at cathode contributes to the loss of cell voltage potential and produces carbon
monoxide that strongly adsorbs onto the platinum catalyst and reduces the active
surface area, thus reducing the performance of the cell. Addition of ruthenium
or gold to the catalyst tends to ameliorate this problem because according to
the most well-established theory in the field, these catalysts oxidize water to
yield OHž radicals: H2 O ! OHž C HC C e . These OHž species oxidizes CO
to CO2 : CO C OHž ! CO2 C HC C e . Carbon supported Pt-Cr electroanodes
also applied to enhance the activity of DMFCs [75]. Low operating temperatures
requiring a high noble metal loading to enhance the kinetics of the methanol
electro-oxidation reaction and low efficiency and toxic nature of methanol are the
prominent factors that often go against its progress.
1.8.3
Phosphoric Acid Fuel Cells (PAFCs)
Molten phosphoric acid (H3 PO4 ) fuel cells are able to produce <100 MW power
at a working temperature of 150–220 Ž C with a cell efficiency of 80% and cost
1.8 Fuel Cells: Fundamentals to Systems
at $4–4.5 per W. Pyrophosphoric acid (H4 P2 O7 ), a polymer of phosphoric acid
usually synthesized at <150 Ž C, is used as the electrolyte in PAFCs and forms
ionic solution of considerably high conductivity than the parent acid. The charge
carrier in this type of fuel cell is the hydrogen ion (HC , proton). This is similar
to the PEMFCs where the hydrogen introduced at the anode is split into protons
and electrons. The protons migrate through the electrolyte to cathode and combine
with the oxygen (which usually comes from air) to produce water. And electrons
are routed through an external circuit to cathode where they can perform useful
work. This set of reactions in the fuel cell produces electricity and the by-product
heat as written below.
Anode reaction:
2H2 ! 4HC C 4e
(1.18)
Cathode reaction:
O2 (g) C 4HC C 4e ! 2H2 O
(1.19)
Overall cell reaction:
2H2 C O2 ! 2H2 O
(1.20)
In addition, CO2 in air does not affect the electrolyte or cell performance. Therefore,
it can easily be operated with fossil fuel reformer. Simple construction, low
electrolyte volatility, and long-term stability are additional advantages associated
with PAFCs. But high concentration of H3 PO4 creates freezing problem when the
cell is turned off, requiring auxiliary heating to maintain a temperature of >40 Ž C.
Alloys of Pt with Ti, Cr, V, and so on, of small particle size [76] and non-noble metal
(hence cheap) electrocatalyst were preferred in PAFCs to achieve high performance
in hot acidic solution [77]. Graphite from porphyrine ash applied to keep the central
atom apart by preventing the aggregation of particles gives better catalysis [78].
PAFCs are increasingly used in big buildings for light and heat supply that utilize
80% of overall energy instead of 40% of others. Some international brands, which
bring PAFCs into limelight, are Plug Power (the United States), International
Fuel Cells Corporation (the United States), and Japan’s Fuji Electric Corporation,
Toshiba Corporation, and Mitsubishi Electric Corporation.
1.8.4
Proton Exchange Membrane Fuel Cells
In the archetypal PEMFC design, a proton-conducting, thin, permeable polymer
membrane (the electrolyte) separates the anode and cathode sides and was called
a solid polymer electrolyte fuel cell (SPEFC) in the early 1970s till the proton
exchange mechanism was well understood. In a typical membrane electrode
assembly (MEA), the bipolar electrode plates are usually made of noble metals,
nickel, or carbon nanotubes and coated with a catalyst (such as platinum, nano
iron powders, or palladium) for higher efficiency [79]. Carbon papers are used
to separate them from the electrolyte. PEMFCs possess a cell efficiency of about
35
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1 Electrochemical Technologies for Energy Storage and Conversion
40–70%, are qualified for providing 100–500 kW power with suitable operating
temperature of 50–120 Ž C (Nafion) and 120–220 Ž C (polybenzimidazole (PBI)),
and are cost worthy for $30–35 per W. Ethanol, butanol, hydrogen, and methanol
presently made from either natural gas or biomass are utilized as fuel. PEM units
are still considered to be the most prevalent alternative because of its operating
on stored hydrogen gas. The first major breakthrough for PEM systems came as
part of NASA’s space program, Project Gemini. Compact design, light-weighted
unit with rapid start up, and the use of solid electrolyte rather than a liquid
makes the sealing of the anode and cathode gases far easier and cheaper unit
to manufacture and can lead to a longer cell and stack life as it is less prone to
corrosion than some other electrolyte materials. However, major disadvantages
associated with PEM are low operation temperatures, that is, 80 Ž C, which are not
high enough to perform useful cogeneration. Furthermore, in order to achieve
the most effective operation of the electrolyte, it must be saturated with water,
otherwise the membrane would be cracked causing the breakdown of the cell.
Therefore, control of the moisture at the anode and cathode streams becomes an
important consideration. The future of PEMFC systems is certainly developing as
a promising technology to be utilized in several key market sectors such as cars,
buildings, and smaller stationary applications, which span growth opportunities
over the short-, medium-, and long-term time frame. Commercially viable PEMFC
vehicles are the need to lower the platinum loading of the MEA without loss of
performance and to improve the stability of the catalyst with respect to platinum
dissolution and carbon-support corrosion [80].
1.8.5
High-Temperature Molten Carbonate Fuel Cells
Molten carbonate fuel cells (MCFCs) are high-temperature fuel cells, which operate
at the temperatures of 600 Ž C or above and are composed of a molten carbonate
salt (potassium carbonate, K2 CO3 ) mixture as electrolyte, suspended in a porous
and chemically inert ceramic matrix of beta-alumina solid electrolyte (BASE)
that transports carbonate ions rather than protons (Figure 1.16). The operating
temperature of 600–650 Ž C is lower than that of solid oxide fuel cells (SOFCs)
and could be worked up to 50–60% efficiency with an operating power of 100
MW [81]. On utilization of waste heat, overall fuel efficiencies can become as
high as 85%. Improved efficiency of MCFCs over PAFCs is a considerable cause
for significant reduction in cost. Unlike alkaline, phosphoric acid, and polymer
electrolyte membrane fuel cells, MCFCs do not require an external reformer to
convert more energy-dense fuels to hydrogen because MCFCs operate at high
temperatures that convert these fuels into hydrogen within the fuel cell itself by a
process called internal reforming, which also reduces cost. MCFCs are not prone to
carbon monoxide or carbon dioxide poisoning because the oxidation reactions that
occur at anode at high temperature produce steam that was utilized by CO/CO2
in reforming hydrocarbon fuel inside the anode [82]. Furthermore, they can even
use carbon oxides as fuel, making them more attractive for fueling with gases
1.8 Fuel Cells: Fundamentals to Systems
Molten carbonate fuel cell
H 2O
Anode reaction:
H2 + CO3
e−
Load
CO32− Ions
Embedded on
LiAIO2
2− ∆
→ H2O + CO2 + 2e−
Porous conducting
cathode (NiO)
H2
Porous conducting
anode (Ni)
e−
O2 (air)
CO2
Cathode reaction:
1/2O2 + CO2 + 2e− → CO32−
Anode internal reformer reactions:
∆
CO + H2O → H2 + CO2
∆
CH4 + H2O → 3H2 + CO
∆
CO2 + 2H2O → CH4 + 2O2
Figure 1.16 Conceptual diagrammatic presentation of
the molten carbonate fuel cell, which includes anodic and
cathodic chemical reactions with anode internal reformer
reactions.
made from coal, and they are highly resistive toward impurities than other fuel
cells. Primary disadvantage of current MCFC technology is low durability because
the high operating temperatures and corrosive nature of electrolyte accelerate
component breakdown that decreases cell life. Scientists are currently exploring
corrosion-resistant materials for components as well as fuel cell designs that
increase cell life without decreasing performance.
1.8.6
Solid Oxide Fuel Cells
SOFCs were first systematically described by Grubb in 1957. SOFCs work at high
temperatures, and ionic conductor or aqueous liquid electrolytes are replaced with
well-humidified thin membranes (0.1 mm thick) of perfluorosulfonic acid polymer
(Nafion), yttria (Y2 O3 )-stabilized zirconia (ZrO2 ), and other such materials between
anode and cathode, which can transport protons and oxygen ions, respectively.
Their electrode reactions are as follows:
Anode reaction:
CH3 OH C H2 O ! CO2 C 6HC C 6e
(1.21)
Internal reformer reaction:
CO2 C 2H2 O C heat ! CH4 C 2O2
(1.22)
37
38
1 Electrochemical Technologies for Energy Storage and Conversion
Cathode reaction:
3
O2 C 6HC C 6e ! 3H2 O
2
(1.23)
Overall reaction:
3
CH3 OH C O2 ! CO2 C 2H2 O C electrical energy
2
(1.24)
Standard operating temperature of SOFCs is >250 Ž C. SOFCs are extremely
advantageous because of the possibility of using a wide variety of fuels [83] such as
hydrogen, butane, methanol, and other petroleum products. Nickel and nickel oxide
are used as catalyst instead of the costlier platinum, and they also do not experience
catalyst poisoning by carbon monoxide, so they do not require high-purity hydrogen
fuel. High temperature decreases i0 in η D RT/F ln i0 /i reduces the overpotential
[62] required for given current densities. High-performance cathodes obtained high
power density of SOFCs. For larger stationary applications at the level of 10–100
MW, SOFCs offer the additional advantage over conventional power plants of
cogeneration of both electric power and low-grade heating. A major disadvantage
of the SOFCs is that as a result of the high operating temperature, a considerable
constraint on the choice of the electrode materials and unwanted reactions may
occur inside the fuel cell [84]. It is very common for SOFCs to build up carbon dust
or graphite on the anode, preventing the fuel from reaching the catalyst. SOFCs
with nonfluorine membrane (polystyrenesulfonic acid) was used in the Gemini
spacecraft. High-temperature cells are envisaged largely for stationary power plants
and electrochemical engines in cars.
1.9
Summary
Global energy-need-projections (based on sound scientific facts) for sustainable
development suggest a 50% increase in world energy requirement by 2030.
Energy demand accompanied by downfall in conventional energy sources, which
is associated with a myriad of environmental issues and their very impact on life,
grew a worldwide consensus on clean energy that became the highly witted driving
force behind the current trends of transition from conventional to renewable energy
resources, that is, hydro, biomass, wind, solar, geothermal, and biofuels. In the
foreseeable future, the prospect of using renewables made the strong commitments
to expand them as alternate energy sources because of their abundance and vast
potential that decrease the cost and hold a strong promise to revolutionize the
present energy technology to meet future energy demands. After getting the
crystal clear picture of past, present, and future energy scenario, we focus on
electrochemical technologies for energy storage and conversion devices. This essay
reviews the current energy status and applications of advanced technologies for
energy generation and concludes with a discussion of the prospects of a future
global-scale energy storage and conversion systems based on electrolysis, PV and
References
photocatalytic hydrogen generation, rechargeable batteries, supercapacitors, and
fuel cells. Fundamental issues of electrochemical technologies were scaled-up
here with their related safety problems, which have been well understood, and
their relevant solutions were short listed, which attracted the world’s focus toward
sustainable green energy sources.
Acknowledgments
The authors would like to thank the Institute of Atomic & Molecular Sciences,
Academia Sinica (Contract No. AS-98-TP-A05) and National Science Council of
Taiwan (Contract Nos. NSC 97-2113-M-002-012-MY3 and NSC 99-2120-M-002-012)
for financial support.
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