Effect of Electrolytes on the Selectivity and Stability of n-type WO₃ Photoelectrodes for Use in Solar Water Oxidation

Abstract

High surface area porous n-type WO3 electrodes were prepared electrochemically and used as a photoanode in a water-splitting photoelectrochemical cell. Various electrolytes containing different anions (acetate, chloride, phosphate, perchlorate, and sulfate) and cations (Li+, Na+, and K+) were prepared at pH 1, 3, and 5 to investigate how these anions and cations affect the efficiency of the photo-oxidation reaction of water on the WO3 surface. To quantitatively compare their effects on O2 evolution, photocurrent to oxygen conversion efficiency was obtained for each electrolyte by simultaneously measuring photocurrent and O2 produced in a sealed divided cell. The type of competing reactions present in each electrolyte and their effects on water oxidation to O2 or to hydrogen peroxide, which is known to accumulate on the WO3 surface and cause photocorrosion, were investigated. The results reported in this study will be useful in formulating optimum electrolyte compositions to enhance desired photo-oxidation reactions for various photoelectrochemical cells.

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

“…As reported in previous work, 14 the bare WO 3 photocurrent decayed over time; however, in many cases the photocurrent recovered after the measurement of a cyclic voltammogram (Fig. S15a, ESI †).…”

“…The WO 3 photoelectrochemical behavior observed herein is consistent with results reported by other groups in sulfuric acid electrolyte. 14,35 For one layer of spin-coated, sintered IrO 2 (B1 nm thick) on WO 3 (Fig. 2a), the back-illuminated photocurrent at 1.5 V vs. RHE was comparable to that for bare WO 3 ; however, under front illumination the photocurrent was reduced due to parasitic light absorption in the catalyst layer (Fig.…”

“…14,35 Thus the faradaic efficiency for O 2 production for these photoanodes was directly measured to determine the efficacy of each OEC attachment strategy. A fluorescence-based oxygen sensor (see Experimental) was employed to unambiguously separate water oxidation from sulfate oxidation.…”

“…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.…”

“…[22][23][24]32 Although peroxide formation and the oxidation of most acid counterion species are reactions that are thermodynamically less feasible than water oxidation, the kinetics of these reactions are often more favorable than oxygen evolution. 14 The photogenerated minority-carrier holes in the valence band of WO 3 have a potential of B2.97 V vs. NHE at pH 0. 33 This potential is sufficient to drive many competing reactions, thus limiting the faradaic oxygen evolution efficiency on bare WO 3 .…”