Enhanced Oxygen Evolution Reaction Performance on NiSx@Co3O4/Nickel Foam Electrocatalysts with Their Photothermal Property.

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiSx-coupled nanourchin-like Co3O4 was prepared on nickel foam (NiSx@Co3O4/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiSx@Co3O4/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co3O4/NF and NiSx/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiSx@Co3O4/NF in 1.0 M KOH owing to the NiSx/Co3O4 synergetic effect. In addition, the oxygen evolution reaction performance of NiSx@Co3O4/NF can be obviously enhanced under near-infrared light irradiation, since NiSx@Co3O4 can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiSx@Co3O4/NF at 50 mA cm-2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy.

Insight into the Kinetic Influence of Oxygen Vacancies on the WO3 Photoanodes for Solar Water Oxidation.

Improvements to solar water oxidation performance for WO3 photoanodes due to oxygen vacancies have in general been ascribed to thermodynamic effects. Detailed insights into the water oxidation kinetics for WO3 photoanodes with oxygen vacancies are still lacking. Here, our experimental and computational investigations revealed that the water oxidation pathway on WO3 photoanodes with oxygen vacancies is more inclined to follow the four-hole pathway. This finding reasonably explained the common observations of higher faradaic efficiency for oxygen evolution, better stability, and faster kinetics for water oxidation usually achieved on the WO3 photoanodes with oxygen vacancies.

Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets.

The crystal facet of the BiVO4 photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO4 polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi2O3 template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO4 film with a {121} monohigh-index facet, the BiVO4 film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm2 was achieved on the multihigh-index facet BiVO4 photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na2SO4, which is about 200% improved over the normal BiVO4 photoanode (0.61 mA/cm2 at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO4 photoanode (0.22 V vs RHE) relative to the normal BiVO4 photoanode (0.52 V vs RHE) in 0.1 M Na2SO4. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO4 film were boosted obviously. Theory calculations reveal that the adsorption of H2O molecules on BiVO4{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO4 is changed to the step of two protons reacting with •O to form •OOH species (•O + H2O(l) + 2H+ + 2e- → •OOH + 3H+ + 3e-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H2O to •OH (2H2O(l) + • → •OH + H2O(l) + H+ + e-). In addition, the overpotential of water oxidation limiting step on BiVO4{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.

Enhanced Photoelectrochemical Water Oxidation Performance on BiVO4 by Coupling of CoMoO4 as a Hole-Transfer and Conversion Cocatalyst.

Manipulation of interfacial charge separation and transfer is one of the primary breakthroughs to improve the water oxidation activity and stability of BiVO4 photoanode. In the present work, a CoMoO4-coupled BiVO4 (BiVO4/CoMoO4) film was designed and prepared as the photoanode for photoelectrochemical (PEC) water oxidation. Compared with the bare BiVO4 film, obviously improved PEC water oxidation performance was observed on the BiVO4/CoMoO4 film. Specifically, a higher water oxidation photocurrent density of 3.04 mA/cm2 at 1.23 V versus RHE was achieved on the BiVO4/CoMoO4 photoanode, which is of about 220% improvement over bare BiVO4 photoanode (1.34 mA/cm2 at 1.23 V vs RHE). In addition, the BiVO4/CoMoO4 film photoanode was of better stability and faster hole-to-oxygen kinetics for water oxidation, without significant activity attenuation for 6 h of reaction at 0.65 V versus RHE. The enhanced water oxidation performance on the BiVO4/CoMoO4 film photoanode can be ascribed to the synergistic effect of the following factors: (i) thermodynamically, the photogenerated holes of BiVO4 are directionally transferred to CoMoO4 through their physical coupling interface and valance band potential matching; and (ii) kinetically, the transferred holes induce the formation of Co3+-active sites on CoMoO4 that could synergistically oxidize H2O to molecular O2 with stable activity.