Amorphous molybdenum sulfide (MoSx) is a promising material for hydrogen evolution reaction (HER)... more Amorphous molybdenum sulfide (MoSx) is a promising material for hydrogen evolution reaction (HER) due to its nearly zero hydrogen adsorption free energy at the sulfur (S) edge-sites. To prepare more efficient MoSx-based electrocatalysts, new attempts are required to increase the exposure of the MoSx lateral size and, therefore, increase the S atom’s contents. The majority of studies reported in the literature investigate MoSx over conductive substrates. However, MoSx can be electrodeposited over inexpensive and chemically stable platforms, such as semiconductors. This work presents the semiconductor substrate morphology effect for prepared sulfur-rich MoSx for electrochemical hydrogen evolution reaction. The electrodes are prepared by cyclic voltammetry with 25 cycles over TiO2 film and TiO2 nanotubes (TiO2NT) substrates. The MoSx deposit on TiO2NT presents an increase S atoms contents and exhibits excellent HER activity with a low overpotential of 93 ± 7.5 mV to reach −10 mA cm−2 a...
Abstract Co–Mo materials have been reported as electrocatalysts that present good performance in ... more Abstract Co–Mo materials have been reported as electrocatalysts that present good performance in alkaline electrolytes. In this paper, the addition of copper into Co–Mo catalysts was evaluated for the hydrogen evolution reaction (HER). It was observed that the electrochemical activity of the Co–Mo for the HER benefited from the addition of copper. The overpotentials required to reach a current density of −10 mA cm−2 were of −156 mV and −119 mV for Co67Mo33 and Co56Mo21Cu23, respectively. Besides the increased surface area resulting from the addition of copper, it was observed that the improved intrinsic activity for Co61Mo32Cu7, compared to Co67Mo33, is related to a thermodynamic favoring of the hydrogen adsorption and desorption stages. Large quantities of copper do not favor the HER; therefore, the increased catalytic activity depends on a balance between the intrinsic catalytic activity and the increase of the electroactive area.
Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) t... more Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrite such as Cu(In, Ga)(Se,S)2 (CIGS) and CuIn(Se,S)2 (CIS) presents excellent semiconductor characteristics for this purpose but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demands improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present review highlights the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.
Hydrogen is seen as a future energy carrier that will assist the global energy fuel demand. Howev... more Hydrogen is seen as a future energy carrier that will assist the global energy fuel demand. However, it is still necessary to study efficient and inexpensive catalysts that can be used on a large scale to make this technology practical. In this paper the electrodeposition of molybdenum disulfide (MoS 2) film on titanium (Ti) substrate was performed by cyclic voltammetry and the film was evaluated for hydrogen evolution reaction (HER). The deposition optimized conditions was 25 cycles and 4 mmol L-1 (NH 4) 2 [MoS 4 ] in 0.1 mol L-1 NaClO 4 solution. The film was characterized from X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and they showed an amorphous structure and high catalytic activity for HER. The Tafel slope was 85 mV dec-1 and overpotential at −10 mA cm-2 was −139 mV showing that Ti substrate compared with fluorine-tin doped oxide (FTO) substrate favored the HER and electroactivity of MoS 2 films.
Sb 2 Se 3 is considered a promising photocathode material for hydrogen production via solar water... more Sb 2 Se 3 is considered a promising photocathode material for hydrogen production via solar water splitting. Nevertheless, losses caused by photocorrosion and slow charge transfer at the semiconductor/electrolyte interface require the presence of a cocatalyst to improve the kinetic factors. In this work, the activity of all-electrochemically grown Sb 2 Se 3 /amorphous-MoS x photocathodes was studied in function of the number of cocatalyst deposition cycles. MoS x , a noble-metal-free electrocatalyst, has shown high activity toward the hydrogen evolution reaction. It promoted an ∼70-time higher improvement in the photocurrent of the Sb 2 Se 3-absorber. An antagonistic effect was observed as the number of cycles increased: thin MoS x layers promoted the maximum enhancement in photocurrent and the decrease in surface recombination, but the photocorrosion inhibition was compromised. In the counterpart, thick MoS x layers allowed the material to be less susceptible to photocorrosion, but the photocurrent was inhibited. Therefore, the relationship between cocatalyst thickness and photocurrent enhancement/inhibition, variation in surface recombination, and photocorrosion stability was evaluated.
Amorphous molybdenum sulfide (MoSx) is a promising material for hydrogen evolution reaction (HER)... more Amorphous molybdenum sulfide (MoSx) is a promising material for hydrogen evolution reaction (HER) due to its nearly zero hydrogen adsorption free energy at the sulfur (S) edge-sites. To prepare more efficient MoSx-based electrocatalysts, new attempts are required to increase the exposure of the MoSx lateral size and, therefore, increase the S atom’s contents. The majority of studies reported in the literature investigate MoSx over conductive substrates. However, MoSx can be electrodeposited over inexpensive and chemically stable platforms, such as semiconductors. This work presents the semiconductor substrate morphology effect for prepared sulfur-rich MoSx for electrochemical hydrogen evolution reaction. The electrodes are prepared by cyclic voltammetry with 25 cycles over TiO2 film and TiO2 nanotubes (TiO2NT) substrates. The MoSx deposit on TiO2NT presents an increase S atoms contents and exhibits excellent HER activity with a low overpotential of 93 ± 7.5 mV to reach −10 mA cm−2 a...
Abstract Co–Mo materials have been reported as electrocatalysts that present good performance in ... more Abstract Co–Mo materials have been reported as electrocatalysts that present good performance in alkaline electrolytes. In this paper, the addition of copper into Co–Mo catalysts was evaluated for the hydrogen evolution reaction (HER). It was observed that the electrochemical activity of the Co–Mo for the HER benefited from the addition of copper. The overpotentials required to reach a current density of −10 mA cm−2 were of −156 mV and −119 mV for Co67Mo33 and Co56Mo21Cu23, respectively. Besides the increased surface area resulting from the addition of copper, it was observed that the improved intrinsic activity for Co61Mo32Cu7, compared to Co67Mo33, is related to a thermodynamic favoring of the hydrogen adsorption and desorption stages. Large quantities of copper do not favor the HER; therefore, the increased catalytic activity depends on a balance between the intrinsic catalytic activity and the increase of the electroactive area.
Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) t... more Solar radiation is a renewable and clean energy source used in photoelectrochemical cells (PEC) to produce hydrogen gas as a powerful alternative to carbon-based fuels. Semiconductors play a vital role in this approach, absorbing the incident solar photons and converting them into electrons and holes. The hydrogen evolution reaction (HER) occurs in the interface of the p-type semiconductor that works as a photocathode in the PEC. Cu-chalcopyrite such as Cu(In, Ga)(Se,S)2 (CIGS) and CuIn(Se,S)2 (CIS) presents excellent semiconductor characteristics for this purpose but drawbacks as charge recombination, deficient chemical stability, and slow charge transfer kinetics, demands improvements like the use of n-type buffer layer, a protective layer, and a cocatalyst material. Concerning the last one, platinum (Pt) is the most efficient and stable material but the high price due to its scarcity imposes the search for inexpensive and abundant alternative cocatalyst. The present review highlights the use of metal alloys, transition metal chalcogenides, and inorganic carbon-based nanostructures as efficient alternative cocatalysts for HER in PEC.
Hydrogen is seen as a future energy carrier that will assist the global energy fuel demand. Howev... more Hydrogen is seen as a future energy carrier that will assist the global energy fuel demand. However, it is still necessary to study efficient and inexpensive catalysts that can be used on a large scale to make this technology practical. In this paper the electrodeposition of molybdenum disulfide (MoS 2) film on titanium (Ti) substrate was performed by cyclic voltammetry and the film was evaluated for hydrogen evolution reaction (HER). The deposition optimized conditions was 25 cycles and 4 mmol L-1 (NH 4) 2 [MoS 4 ] in 0.1 mol L-1 NaClO 4 solution. The film was characterized from X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and they showed an amorphous structure and high catalytic activity for HER. The Tafel slope was 85 mV dec-1 and overpotential at −10 mA cm-2 was −139 mV showing that Ti substrate compared with fluorine-tin doped oxide (FTO) substrate favored the HER and electroactivity of MoS 2 films.
Sb 2 Se 3 is considered a promising photocathode material for hydrogen production via solar water... more Sb 2 Se 3 is considered a promising photocathode material for hydrogen production via solar water splitting. Nevertheless, losses caused by photocorrosion and slow charge transfer at the semiconductor/electrolyte interface require the presence of a cocatalyst to improve the kinetic factors. In this work, the activity of all-electrochemically grown Sb 2 Se 3 /amorphous-MoS x photocathodes was studied in function of the number of cocatalyst deposition cycles. MoS x , a noble-metal-free electrocatalyst, has shown high activity toward the hydrogen evolution reaction. It promoted an ∼70-time higher improvement in the photocurrent of the Sb 2 Se 3-absorber. An antagonistic effect was observed as the number of cycles increased: thin MoS x layers promoted the maximum enhancement in photocurrent and the decrease in surface recombination, but the photocorrosion inhibition was compromised. In the counterpart, thick MoS x layers allowed the material to be less susceptible to photocorrosion, but the photocurrent was inhibited. Therefore, the relationship between cocatalyst thickness and photocurrent enhancement/inhibition, variation in surface recombination, and photocorrosion stability was evaluated.
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Papers by Marina Medina