Nanocrystalline lithium peroxide (Li2O2) is considered to play a critical role in the redox chemi... more Nanocrystalline lithium peroxide (Li2O2) is considered to play a critical role in the redox chemistry during the discharge–charge cycling of the Li–O2 batteries. In this report, a spatially resolved, real‐time synchrotron X‐ray diffraction technique was applied to study the cyclic formation/decomposition of Li2O2 crystallites in an operating Li–O2 cell. The evaluation of Li2O2 grain size, concentration, and spatial distribution inside the cathode is demonstrated under the actual cycling conditions. The study not only unambiguously proved the reversibility of the Li2O2 redox reaction during reduction and evolution of O2, but also allowed for the concentration and dimension growths of the peroxide nanocrystallites to be accurately measured at different regions within the cathode. The results provide important insights for future investigation on mass and charge transport properties in Li2O2 and improvement in cathode structure and material design.
Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction... more Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.
Li-O2 battery has attracted a great deal of attentions recently due to its high theoretical energ... more Li-O2 battery has attracted a great deal of attentions recently due to its high theoretical energy density.[1] Many studies were reported on the investigation of catalysts and cathode structures to the battery performance.[2] They revealed that the actual redox mechanism in Li-O2 battery could be significantly more complicated than originally thought. During the discharge-charge cycling, cathodic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occur in Li-O2 battery. Various catalysts were disclosed in attempts to promote ORR and OER processes at lower overpotentials therefore higher overall round-trip efficiency. However, little information is available on the catalytic mechanism. For example, it is commonly accepted that lithium peroxide, Li2O2, serves as a redox intermediate during cycling reactions. Li2O2 is nearly insoluble in the organic electrolytes and precipitates on the surface of the cathode once formed. How a solid-state catalyst could promote not only Li2O2 formation during ORR, but also its dissipation during OER is completely unclear. Such catalytic reactions would require a solid-solid interaction with limited or no mass-transport. How the catalytic transition intermediates can be formed under such condition remains a mystery. Without a better understanding of the catalytic mechanism, it is difficult to rationally design and improve the next generation Li-O2battery. At Argonne National Laboratory, our team has explored a low-cost, high surface area Fe/N/C composite as the cathode catalyst for Li-O2 battery application, inspired by the non-precious metal catalyst study for the proton exchange membrane fuel cell.[3] The catalyst demonstrated reduced ORR and OER overpotentials in the discharge-charge cycle and minimized the electrolyte decomposition. We also developed a holistic approach in studying electrochemical processes in Li-O2 battery using various characterization tools, such as SEM, TEM, FTIR, XRD, etc..[4] Particularly, we introduced a microfocused synchrotron X-ray diffraction (m-XRD) and a micro-tomographic techniques (µ-CT) for the spatiotemporal study on the phase and structural change in Li-O2 battery under actual cell cycling condition.[5] The m-XRD has a spatial resolution at micron level with the complete penetration and sampling from cell’s radial direction, rendering it possible to probe battery’s composition layer-by-layer under in situconditions. The data collection at any given position usually takes a few seconds, making the method particularly suitable to study the dynamic change inside the battery in real time. In this presentation, we will focus on our recent operando, spatiotemporal investigation on the catalytic processes at the Li-O2 battery cathode using the m-XRD technique. We prepared an in situ Li-O2 battery using the representative material and cell design and studied formation, dissipation and distribution of the lithium redox intermediate under the multiple discharge-charge cycling condition in the entire cathode region. We demonstrated, for the first time, the evolutions of the grain size, concentration and spatial distribution of lithium peroxide as the functions of ORR/OER potentials and capacity. A new “self-propagating” catalytic mechanism was derived from the experimental data and compared to the similar electrocatalytic processes in fuel cell. The new finding could clarify some of the on-going controversies on the redox mechanism. More importantly, it shed lights on the new structural and material research directions for better cycloability and durability of the Li-O2battery. Acknowledgement: The work performed at Argonne is supported by DOE under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. References: [1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193. [2] a) K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1; b) A. Débart, A. J. Paterson, J. Bao, P. G. Bruce, Angew. Chem. Int. Ed. 2008, 47, 4521; c) B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, A. C. Luntz, J. Phys. Chem. Lett. 2011, 2, 1161. [3] Jiang–Lan Shui, Naba K. Karan, Mahalingam Balasubramanian, Shu–You Li and Di–Jia Liu, J. Am. Chem. Soc. 2012 , 134 (40), 16654 [4] Jiang-Lan Shui, John S. Okasinski, Dan Zhao, Jonathan D. Almer and Di-Jia Liu, ChemSusChem, 2012, 5, 2421 [5] a) Jiang-Lan Shui, John S. Okasinski, Peter Kenesei, Howard A. Dobbs, Dan Zhao, Jonathan D. Almer, and Di-Jia Liu, Nature Comm. 2013 4, 2255; b) Jiang-Lan Shui, John S. Okasinski, Chen Chen, Jonathan D. Almer and Di-Jia Liu, ChemSusChem 2014 , 7, 543
Carbon dioxide emission from fossil fuel combustion has generated an on-going debate on its impac... more Carbon dioxide emission from fossil fuel combustion has generated an on-going debate on its impact to the global environmental and ecological systems. New carbon capture, sequestration and conversion technologies are widely pursued as potential solutions to mitigate the concerns. The electrochemical CO2 reduction reaction (CO2RR) to hydrocarbon fuels and chemicals using renewable electricity offers an attractive “carbon-neutral” or even “carbon-negative” mitigation strategy. CO2 is an inexpensive carbon source and can be used as a feedstock for producing high value chemicals. Key challenges facing the current CO2RR electrocatalysis include improving energy efficiency by reducing the overpotentials; increasing the process selectivity by enhancing a single-product Faradaic efficiency (FE); and lowering the system operating cost by prolonging the catalyst stability. The U.S. produced 5.1 billion metric tons of CO2 in 2018 from industrial processes, posing a major concern on its impact ...
$^{1}$ B.D. Rehfuss, D.-J. Liu, B.M. Dinelli, M.-F. Jagod, M.W. Crofton, and T. Oka, WG2Author In... more $^{1}$ B.D. Rehfuss, D.-J. Liu, B.M. Dinelli, M.-F. Jagod, M.W. Crofton, and T. Oka, WG2Author Institution: Department of Chemistry and Department of Astronomy and Astrophysics, The University of ChicagoFollowing the high resolution IR spectroscopy of the (0,0) and (1,1) bands in the electronic transition $A^{2}\Pi_{**} X^{2}\Sigma^{+}_{g}$ of $C_{2}^{-}$ using a difference frequency spectrometer1, we studied the (0,1) band using a tunable diode laser spectromter. $C_{2}^{-}$ was generated in an ac discharge of $C_{2}H_{2}$ and He and was detected using velocity modulation technique. We observed and assigned 25 lines in the $4.7 \mu m$ region. These lines were included in the non-linear least squares fit with the (0,0) and (1,1) bands1. this improved the molecular constants of both electronic states. The following molecular constants have been obtained for the (0,1) band (in $cm^{-1}$:[FIGURE] Numbers in parentheses are $3\sigma$
$^{1}$ N.N. Haese and T. Oka, J. Chem. Phys. 80 572 (1984). $^{2}$ P.R. Bunker, W.P. Kraemer, and... more $^{1}$ N.N. Haese and T. Oka, J. Chem. Phys. 80 572 (1984). $^{2}$ P.R. Bunker, W.P. Kraemer, and V. Spirko, J. Mol. Spec. 101 180 (1983) $^{3}$ P. Botschwina, P. Rosmus, and E.A. Reinsch, Chem. Phys. Lett. (in press).
Introduction Ethanol has been extensively blended in gasoline as fuel to improve the combustion e... more Introduction Ethanol has been extensively blended in gasoline as fuel to improve the combustion efficiency. Electrochemical CO2 reduction reaction (CO2RR) to ethanol and other value-added chemicals offers a promising “carbon-neutral” or even “carbon-negative” strategy1 when combined with renewable electricity, making the approach not only environmentally sound but also economically attractive. A key challenge for CO2RR is the lack of electrocatalyst that is capable of converting CO2 to a single product with high current efficiency, i.e. Faradaic efficiency (FE), low overpotentials and high durability2. Here, we report the preparation of commercial carbon-supported Cu SA catalysts by an amalgamated Cu–Li method. The catalysts demonstrated highly selective CO2-to-ethanol conversion with FE reaching ~91% at -0.7 V versus RHE and an onset potential as low as -0.4 V with FE of ~15%. The FE of the catalysts maintained at roughly 90% without degradation for 16 h. The CO2-to-ethanol FE is favored by high initial dispersion of single Cu atoms. Operando synchrotron X-ray absorption spectroscopy revealed that such dispersion played a critical role in a reversible transformation between Cu SA and Cu n (n= 3 and 4) as the active sites during the electrocatalytic reaction. Materials and Methods Preparing single atom catalyst starts with melting metal in liquid lithium in an Ar-filled glovebox. Lithium was heated to 200 °C, then the target metal (Cu in this case) was simultaneously added and kept for 4 h. The Li-Cu solid solutions were removed from the glovebox. The prepared amalgamated Cu-Li was oxidized to LiOH and mixed with carbon support materials before the LiOH was leached out with copious amounts of DI water, leaving the metal single atoms imbedded in the carbon support. Results and Discussion High-angle annular dark-field and aberration-corrected scanning transmission electron microscopy (HAADF-STEM), and X-ray absorption spectrum (XAS) are used to characterize the resulting samples. Figure. 1a shows a HAADF-STEM image of the atomically disperse Cu in sample Cu/C-0. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) are used to characterize the coordination structures and the electronic structure of the Cu atoms. Based on these XAS techniques, the Cu atoms are identified as being coordinated with four oxygens (Figure 1b). Figure 1c shows the FEs and CO2RR product distributions as the function of applied potentials from -0.4 V to -1.2 V over Cu/C-0.4. We detected the active potential for ethanol formation as low as -0.4 V with a decent FE of ~ 15%. The peak FE for ethanol is ~91% at a low potential of only -0.6 V and -0.7 V. To our knowledge, this value represents the highest FE for direct electrocatalytic CO2-to-ethanol conversion among Cu-based catalysts. The Cu/C-0.4 catalyst over a 16-hour span at -0.7 V showed an excellent stability in both current density and FE of CO2-to-ethanol over Cu/C-0.4 (Figure. 1d). Figure. 1e shows the FEs of the ethanol formation at different potentials obtained from individual catalyst studied. For Cu/C-0.1, Cu/C-0.4 and Cu/C-0.8, the FE for ethanol formation followed nearly identical profile with the active potential starting at as low as -0.4 V and reaching peak value > 90% at -0.7 V. Once the Cu loading reached to 1.6 wt.% or higher, however, the ethanol FE profile underwent a drastic reduction. Operando XAS study under the reaction conditions revealed a crucial dynamic catalytic mechanism, a reversible transformation between atomically dispersed Cu to Cun (n=3 or 4) clusters (Figure. 1f). Figure 1. (a) Representative HAADF-STEM images of Cu/C-0.4 showing the presence of isolated Cu species; (b) Fourier transform of k2-weighted R-space χ EXAFS data of the catalysts plus Cu(AcAc)2 as reference; (c) FE and the product distribution at different polarization potentials. The data were averaged over three repeated measurements with the standard deviations marked by black error bar for ethanol and red error bar for the total products; (d) FE and current density as the function of time during chronoamperometric electrolysis at -0.6 V; (e) FE of CO2-to-ethanol at different potentials over catalysts of different Cu loadings. (f) The hypothesised reaction mechanism suggested by the operando measurements. Significance Understanding the role of metal single atoms on the CO2 electrochemical reduction reaction is fundamentally important in designing new CO2RR catalyst for the large-scale production of value-added liquid fuels and chemicals. References 1. Xu, H. P. et al.; Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically-dispersed copper, Nature Energy, 2020, 5, 623-632. 2. Zhou, Y. S. et al. Dopant-induced electron localization drives CO2 reduction to C2 hydrocarbons. Nature Chemistry, 2018, 10, 974–980. Figure 1
Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy con... more Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy conversion technology. The sluggish oxygen reduction reaction (ORR) at the fuel cell cathode requires 3 to 5 times more catalysts than the hydrogen oxidation reaction at the anode.(1) Currently, platinum is still the catalyst material of choice. However, the heavy use of scarce Pt in the electrodes represents a major cost barrier to the commercialization of PEMFCs. Tremendous efforts have been dedicated to reduce or remove Pt usage through the development of Pt-transition metal catalysts, as highlighted recently by dealloyed PtNi (2), Mo-doped PtNi(3), and ordered PtCo(4); or through the development of transition metals-nitrogen-carbon based PGM-free catalysts (5, 6). In the fuel cell, catalysts should be highly dispersed over the electrode surface to be easily accessible by the reactants, especially at the high fuel cell current density where a large influx of O2 must be converted. For the...
Dual metal–organic frameworks (MOFs, i.e., MIL‐100(Fe) and ZIF‐8) are thermally converted into Fe... more Dual metal–organic frameworks (MOFs, i.e., MIL‐100(Fe) and ZIF‐8) are thermally converted into Fe–Fe3C‐embedded Fe–N‐codoped carbon as platinum group metal (PGM)‐free oxygen reduction reaction (ORR) electrocatalysts. Pyrolysis enables imidazolate in ZIF‐8 rearranged into highly N‐doped carbon, while Fe from MIL‐100(Fe) into N‐ligated atomic sites concurrently with a few Fe–Fe3C nanoparticles. Upon precise control of MOF compositions, the optimal catalyst is highly active for the ORR in half‐cells (0.88 V in base and 0.79 V versus RHE in acid in half‐wave potential), a proton exchange membrane fuel cell (0.76 W cm−2 in peak power density) and an aprotic Li–O2 battery (8749 mAh g−1 in discharge capacity), representing a state‐of‐the‐art PGM‐free ORR catalyst. In the material, amorphous carbon with partial graphitization ensures high active site exposure and fast charge transfer simultaneously. Macropores facilitate mass transport to the catalyst surface, followed by oxygen penetration...
Sodium-ion batteries (SIBs) have been considered as one of the promising power source candidates ... more Sodium-ion batteries (SIBs) have been considered as one of the promising power source candidates for the stationary storage industries owing to the much lower cost of sodium than lithium. It is well-known that the electrode materials largely determine the energy density of the battery systems. However, recent discoveries on the electrode materials showed that most of them present distinct lithium and sodium storage performance, which is not yet well understood. In this work, we performed a comparative understanding on the structural changes of porous cobalt oxide during its electrochemical lithiation and sodiation process by in operando synchrotron small angel X-ray scattering, X-ray diffraction, and X-ray absorption spectroscopy. It was found that compared to the lithiation process, the porous cobalt oxide undergoes less pore structure changes, oxidation state, and local structure changes as well as crystal structure evolution during its sodiation process, which is attributed to th...
Nanocrystalline lithium peroxide (Li2O2) is considered to play a critical role in the redox chemi... more Nanocrystalline lithium peroxide (Li2O2) is considered to play a critical role in the redox chemistry during the discharge–charge cycling of the Li–O2 batteries. In this report, a spatially resolved, real‐time synchrotron X‐ray diffraction technique was applied to study the cyclic formation/decomposition of Li2O2 crystallites in an operating Li–O2 cell. The evaluation of Li2O2 grain size, concentration, and spatial distribution inside the cathode is demonstrated under the actual cycling conditions. The study not only unambiguously proved the reversibility of the Li2O2 redox reaction during reduction and evolution of O2, but also allowed for the concentration and dimension growths of the peroxide nanocrystallites to be accurately measured at different regions within the cathode. The results provide important insights for future investigation on mass and charge transport properties in Li2O2 and improvement in cathode structure and material design.
Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction... more Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.
Li-O2 battery has attracted a great deal of attentions recently due to its high theoretical energ... more Li-O2 battery has attracted a great deal of attentions recently due to its high theoretical energy density.[1] Many studies were reported on the investigation of catalysts and cathode structures to the battery performance.[2] They revealed that the actual redox mechanism in Li-O2 battery could be significantly more complicated than originally thought. During the discharge-charge cycling, cathodic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occur in Li-O2 battery. Various catalysts were disclosed in attempts to promote ORR and OER processes at lower overpotentials therefore higher overall round-trip efficiency. However, little information is available on the catalytic mechanism. For example, it is commonly accepted that lithium peroxide, Li2O2, serves as a redox intermediate during cycling reactions. Li2O2 is nearly insoluble in the organic electrolytes and precipitates on the surface of the cathode once formed. How a solid-state catalyst could promote not only Li2O2 formation during ORR, but also its dissipation during OER is completely unclear. Such catalytic reactions would require a solid-solid interaction with limited or no mass-transport. How the catalytic transition intermediates can be formed under such condition remains a mystery. Without a better understanding of the catalytic mechanism, it is difficult to rationally design and improve the next generation Li-O2battery. At Argonne National Laboratory, our team has explored a low-cost, high surface area Fe/N/C composite as the cathode catalyst for Li-O2 battery application, inspired by the non-precious metal catalyst study for the proton exchange membrane fuel cell.[3] The catalyst demonstrated reduced ORR and OER overpotentials in the discharge-charge cycle and minimized the electrolyte decomposition. We also developed a holistic approach in studying electrochemical processes in Li-O2 battery using various characterization tools, such as SEM, TEM, FTIR, XRD, etc..[4] Particularly, we introduced a microfocused synchrotron X-ray diffraction (m-XRD) and a micro-tomographic techniques (µ-CT) for the spatiotemporal study on the phase and structural change in Li-O2 battery under actual cell cycling condition.[5] The m-XRD has a spatial resolution at micron level with the complete penetration and sampling from cell’s radial direction, rendering it possible to probe battery’s composition layer-by-layer under in situconditions. The data collection at any given position usually takes a few seconds, making the method particularly suitable to study the dynamic change inside the battery in real time. In this presentation, we will focus on our recent operando, spatiotemporal investigation on the catalytic processes at the Li-O2 battery cathode using the m-XRD technique. We prepared an in situ Li-O2 battery using the representative material and cell design and studied formation, dissipation and distribution of the lithium redox intermediate under the multiple discharge-charge cycling condition in the entire cathode region. We demonstrated, for the first time, the evolutions of the grain size, concentration and spatial distribution of lithium peroxide as the functions of ORR/OER potentials and capacity. A new “self-propagating” catalytic mechanism was derived from the experimental data and compared to the similar electrocatalytic processes in fuel cell. The new finding could clarify some of the on-going controversies on the redox mechanism. More importantly, it shed lights on the new structural and material research directions for better cycloability and durability of the Li-O2battery. Acknowledgement: The work performed at Argonne is supported by DOE under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC. References: [1] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke, J. Phys. Chem. Lett. 2010, 1, 2193. [2] a) K. M. Abraham, Z. Jiang, J. Electrochem. Soc. 1996, 143, 1; b) A. Débart, A. J. Paterson, J. Bao, P. G. Bruce, Angew. Chem. Int. Ed. 2008, 47, 4521; c) B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, A. C. Luntz, J. Phys. Chem. Lett. 2011, 2, 1161. [3] Jiang–Lan Shui, Naba K. Karan, Mahalingam Balasubramanian, Shu–You Li and Di–Jia Liu, J. Am. Chem. Soc. 2012 , 134 (40), 16654 [4] Jiang-Lan Shui, John S. Okasinski, Dan Zhao, Jonathan D. Almer and Di-Jia Liu, ChemSusChem, 2012, 5, 2421 [5] a) Jiang-Lan Shui, John S. Okasinski, Peter Kenesei, Howard A. Dobbs, Dan Zhao, Jonathan D. Almer, and Di-Jia Liu, Nature Comm. 2013 4, 2255; b) Jiang-Lan Shui, John S. Okasinski, Chen Chen, Jonathan D. Almer and Di-Jia Liu, ChemSusChem 2014 , 7, 543
Carbon dioxide emission from fossil fuel combustion has generated an on-going debate on its impac... more Carbon dioxide emission from fossil fuel combustion has generated an on-going debate on its impact to the global environmental and ecological systems. New carbon capture, sequestration and conversion technologies are widely pursued as potential solutions to mitigate the concerns. The electrochemical CO2 reduction reaction (CO2RR) to hydrocarbon fuels and chemicals using renewable electricity offers an attractive “carbon-neutral” or even “carbon-negative” mitigation strategy. CO2 is an inexpensive carbon source and can be used as a feedstock for producing high value chemicals. Key challenges facing the current CO2RR electrocatalysis include improving energy efficiency by reducing the overpotentials; increasing the process selectivity by enhancing a single-product Faradaic efficiency (FE); and lowering the system operating cost by prolonging the catalyst stability. The U.S. produced 5.1 billion metric tons of CO2 in 2018 from industrial processes, posing a major concern on its impact ...
$^{1}$ B.D. Rehfuss, D.-J. Liu, B.M. Dinelli, M.-F. Jagod, M.W. Crofton, and T. Oka, WG2Author In... more $^{1}$ B.D. Rehfuss, D.-J. Liu, B.M. Dinelli, M.-F. Jagod, M.W. Crofton, and T. Oka, WG2Author Institution: Department of Chemistry and Department of Astronomy and Astrophysics, The University of ChicagoFollowing the high resolution IR spectroscopy of the (0,0) and (1,1) bands in the electronic transition $A^{2}\Pi_{**} X^{2}\Sigma^{+}_{g}$ of $C_{2}^{-}$ using a difference frequency spectrometer1, we studied the (0,1) band using a tunable diode laser spectromter. $C_{2}^{-}$ was generated in an ac discharge of $C_{2}H_{2}$ and He and was detected using velocity modulation technique. We observed and assigned 25 lines in the $4.7 \mu m$ region. These lines were included in the non-linear least squares fit with the (0,0) and (1,1) bands1. this improved the molecular constants of both electronic states. The following molecular constants have been obtained for the (0,1) band (in $cm^{-1}$:[FIGURE] Numbers in parentheses are $3\sigma$
$^{1}$ N.N. Haese and T. Oka, J. Chem. Phys. 80 572 (1984). $^{2}$ P.R. Bunker, W.P. Kraemer, and... more $^{1}$ N.N. Haese and T. Oka, J. Chem. Phys. 80 572 (1984). $^{2}$ P.R. Bunker, W.P. Kraemer, and V. Spirko, J. Mol. Spec. 101 180 (1983) $^{3}$ P. Botschwina, P. Rosmus, and E.A. Reinsch, Chem. Phys. Lett. (in press).
Introduction Ethanol has been extensively blended in gasoline as fuel to improve the combustion e... more Introduction Ethanol has been extensively blended in gasoline as fuel to improve the combustion efficiency. Electrochemical CO2 reduction reaction (CO2RR) to ethanol and other value-added chemicals offers a promising “carbon-neutral” or even “carbon-negative” strategy1 when combined with renewable electricity, making the approach not only environmentally sound but also economically attractive. A key challenge for CO2RR is the lack of electrocatalyst that is capable of converting CO2 to a single product with high current efficiency, i.e. Faradaic efficiency (FE), low overpotentials and high durability2. Here, we report the preparation of commercial carbon-supported Cu SA catalysts by an amalgamated Cu–Li method. The catalysts demonstrated highly selective CO2-to-ethanol conversion with FE reaching ~91% at -0.7 V versus RHE and an onset potential as low as -0.4 V with FE of ~15%. The FE of the catalysts maintained at roughly 90% without degradation for 16 h. The CO2-to-ethanol FE is favored by high initial dispersion of single Cu atoms. Operando synchrotron X-ray absorption spectroscopy revealed that such dispersion played a critical role in a reversible transformation between Cu SA and Cu n (n= 3 and 4) as the active sites during the electrocatalytic reaction. Materials and Methods Preparing single atom catalyst starts with melting metal in liquid lithium in an Ar-filled glovebox. Lithium was heated to 200 °C, then the target metal (Cu in this case) was simultaneously added and kept for 4 h. The Li-Cu solid solutions were removed from the glovebox. The prepared amalgamated Cu-Li was oxidized to LiOH and mixed with carbon support materials before the LiOH was leached out with copious amounts of DI water, leaving the metal single atoms imbedded in the carbon support. Results and Discussion High-angle annular dark-field and aberration-corrected scanning transmission electron microscopy (HAADF-STEM), and X-ray absorption spectrum (XAS) are used to characterize the resulting samples. Figure. 1a shows a HAADF-STEM image of the atomically disperse Cu in sample Cu/C-0. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) are used to characterize the coordination structures and the electronic structure of the Cu atoms. Based on these XAS techniques, the Cu atoms are identified as being coordinated with four oxygens (Figure 1b). Figure 1c shows the FEs and CO2RR product distributions as the function of applied potentials from -0.4 V to -1.2 V over Cu/C-0.4. We detected the active potential for ethanol formation as low as -0.4 V with a decent FE of ~ 15%. The peak FE for ethanol is ~91% at a low potential of only -0.6 V and -0.7 V. To our knowledge, this value represents the highest FE for direct electrocatalytic CO2-to-ethanol conversion among Cu-based catalysts. The Cu/C-0.4 catalyst over a 16-hour span at -0.7 V showed an excellent stability in both current density and FE of CO2-to-ethanol over Cu/C-0.4 (Figure. 1d). Figure. 1e shows the FEs of the ethanol formation at different potentials obtained from individual catalyst studied. For Cu/C-0.1, Cu/C-0.4 and Cu/C-0.8, the FE for ethanol formation followed nearly identical profile with the active potential starting at as low as -0.4 V and reaching peak value > 90% at -0.7 V. Once the Cu loading reached to 1.6 wt.% or higher, however, the ethanol FE profile underwent a drastic reduction. Operando XAS study under the reaction conditions revealed a crucial dynamic catalytic mechanism, a reversible transformation between atomically dispersed Cu to Cun (n=3 or 4) clusters (Figure. 1f). Figure 1. (a) Representative HAADF-STEM images of Cu/C-0.4 showing the presence of isolated Cu species; (b) Fourier transform of k2-weighted R-space χ EXAFS data of the catalysts plus Cu(AcAc)2 as reference; (c) FE and the product distribution at different polarization potentials. The data were averaged over three repeated measurements with the standard deviations marked by black error bar for ethanol and red error bar for the total products; (d) FE and current density as the function of time during chronoamperometric electrolysis at -0.6 V; (e) FE of CO2-to-ethanol at different potentials over catalysts of different Cu loadings. (f) The hypothesised reaction mechanism suggested by the operando measurements. Significance Understanding the role of metal single atoms on the CO2 electrochemical reduction reaction is fundamentally important in designing new CO2RR catalyst for the large-scale production of value-added liquid fuels and chemicals. References 1. Xu, H. P. et al.; Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically-dispersed copper, Nature Energy, 2020, 5, 623-632. 2. Zhou, Y. S. et al. Dopant-induced electron localization drives CO2 reduction to C2 hydrocarbons. Nature Chemistry, 2018, 10, 974–980. Figure 1
Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy con... more Proton exchange membrane fuel cell (PEMFC) has been demonstrated as a highly efficient energy conversion technology. The sluggish oxygen reduction reaction (ORR) at the fuel cell cathode requires 3 to 5 times more catalysts than the hydrogen oxidation reaction at the anode.(1) Currently, platinum is still the catalyst material of choice. However, the heavy use of scarce Pt in the electrodes represents a major cost barrier to the commercialization of PEMFCs. Tremendous efforts have been dedicated to reduce or remove Pt usage through the development of Pt-transition metal catalysts, as highlighted recently by dealloyed PtNi (2), Mo-doped PtNi(3), and ordered PtCo(4); or through the development of transition metals-nitrogen-carbon based PGM-free catalysts (5, 6). In the fuel cell, catalysts should be highly dispersed over the electrode surface to be easily accessible by the reactants, especially at the high fuel cell current density where a large influx of O2 must be converted. For the...
Dual metal–organic frameworks (MOFs, i.e., MIL‐100(Fe) and ZIF‐8) are thermally converted into Fe... more Dual metal–organic frameworks (MOFs, i.e., MIL‐100(Fe) and ZIF‐8) are thermally converted into Fe–Fe3C‐embedded Fe–N‐codoped carbon as platinum group metal (PGM)‐free oxygen reduction reaction (ORR) electrocatalysts. Pyrolysis enables imidazolate in ZIF‐8 rearranged into highly N‐doped carbon, while Fe from MIL‐100(Fe) into N‐ligated atomic sites concurrently with a few Fe–Fe3C nanoparticles. Upon precise control of MOF compositions, the optimal catalyst is highly active for the ORR in half‐cells (0.88 V in base and 0.79 V versus RHE in acid in half‐wave potential), a proton exchange membrane fuel cell (0.76 W cm−2 in peak power density) and an aprotic Li–O2 battery (8749 mAh g−1 in discharge capacity), representing a state‐of‐the‐art PGM‐free ORR catalyst. In the material, amorphous carbon with partial graphitization ensures high active site exposure and fast charge transfer simultaneously. Macropores facilitate mass transport to the catalyst surface, followed by oxygen penetration...
Sodium-ion batteries (SIBs) have been considered as one of the promising power source candidates ... more Sodium-ion batteries (SIBs) have been considered as one of the promising power source candidates for the stationary storage industries owing to the much lower cost of sodium than lithium. It is well-known that the electrode materials largely determine the energy density of the battery systems. However, recent discoveries on the electrode materials showed that most of them present distinct lithium and sodium storage performance, which is not yet well understood. In this work, we performed a comparative understanding on the structural changes of porous cobalt oxide during its electrochemical lithiation and sodiation process by in operando synchrotron small angel X-ray scattering, X-ray diffraction, and X-ray absorption spectroscopy. It was found that compared to the lithiation process, the porous cobalt oxide undergoes less pore structure changes, oxidation state, and local structure changes as well as crystal structure evolution during its sodiation process, which is attributed to th...
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Papers by Di-Jia Liu