Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of Photo-Oxidation Reactions of a WO₃ Photoanode

Abstract

A bare WO 3 electrode and a WO 3 electrode coupled with a layer of Co-Pi oxygen evolution catalyst (OEC) were prepared to investigate the effect of Co-Pi OEC on the selectivity of photo-oxidation reactions and photostabilities of WO 3 photoanodes. WO 3 photoanodes have been reported to produce peroxo species as well as O 2 during photooxidation reactions, and the accumulation of peroxo species on the surface is known to cause a gradual loss of photoactivity of WO 3 . The photocurrent to O 2 conversion efficiencies of the WO 3 and WO 3 /Co-Pi OEC electrodes were obtained by simultaneously measuring the photocurrent and O 2 gas generated during illumination at 0.8 V vs Ag/AgCl. The result shows that the presence of OEC increases the photocurrent to O 2 conversion efficiency from approximately 61% to approximately 100%. The complete suppression of peroxo formation provided the WO 3 /Co-Pi OEC photoelectrode with long-term photostability. The photocurrent-potential characteristics show that the presence of OEC effectively reduces the electron-hole recombination near the flat band potential region and shifts the onset potential of photocurrent by 0.17 V to the negative direction. However, when the applied potential became more positive than approximately 0.35 V vs Ag/AgCl, the WO 3 /Co-Pi OEC electrode produced less initial photocurrent than the bare WO 3 electrode. Mott-Schottky plots reveal the presence of interface states at the WO 3 /OEC junction that induce more electron-hole recombination when the Fermi level moves below these states. Regardless of the adverse effect on recombination present at 0.8 V vs Ag/AgCl, the WO 3 /Co-Pi OEC achieved a more efficient and sustainable solar to O 2 conversion owing to the ability of Co-Pi OEC to significantly increase the photocurrent to O 2 conversion efficiency and prevent the photocurrent decay of the WO 3 electrode.

Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency

“…Especially for the photoanode, modifying the surface with an appropriate oxygen evolution electrocatalyst (co-catalyst) has an important role in lowering the overpotential and stabilizing the surface. The Co-Pi oxygen-evolving complex has, since its discovery 32 , raised wide interest in the water-splitting field, and has been used to modify various photoanodes to improve their activity and stability 20,21,[33][34][35][36] . In our work, Co-Pi was employed as a cocatalyst for the Ba-Ta 3 N 5 nanorod photoanode.…”

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

“…Researchers have demonstrated enhanced photostability of WO 3 near neutral pH by annealing in a hydrogen environment 34 and by adding thick coatings of an oxygen evolution catalyst (OEC) to reduce the required oxygen evolution overpotential and kinetically favor water oxidation over peroxo-species formation. 21 The selectivity for water oxidation of WO 3 photoanodes without a co-catalyst depends strongly on the electrolyte species and pH. No O 2 formation is observed in HCl or acetic acid solutions and O 2 yields decrease with decreasing pH in H 3 PO 4 and H 2 SO 4 .…”

“…Tungsten trioxide, WO 3 , an earth-abundant, oxidatively stable semiconductor, is one such material that could fulfill the role of photoanode. [14][15][16][17][18][19][20][21][22][23][24] In most deposition methods, the presence of oxygen vacancies serve as shallow electron donors and naturally dope the WO 3 n-type. 25 Its band gap of 2.6 eV is higher than ideal for the large band gap absorber in a tandem cell.…”

Optical properties of tungsten trioxide from first-principles calculations

“…However its optical gap (3.0 eV [7]) is larger than desired to absorb visible light. Tungsten trioxide (WO 3 ) is another promising oxide for use in photo-electrochemical water-splitting systems [8][9][10][11][12]: it is stable against photocorrosion; its optical gap (2.6-2.7 eV) is smaller than that of TiO 2 and it may absorb sufficient visible light to generate modest photocurrents. Furthermore WO 3 electrochromism has additional applications, in e.g.…”