Overestimated solar water splitting performance on oxide semiconductor anodes

Abstract

Water oxidation is a 4-electron uphill energy process that largely limits the overall water splitting reaction thermodynamically and kinetically [1]. Photocurrent measurement is one of the most important approaches to evaluate the performance of a photoanode. It has been well realized that photocurrent may render exaggerated performance when photocorrosion of electrode occurs, such as the case of ZnO and CdS or when sacrificial reagents are present in the electrolyte. However, the choice of electrolyte could be another very important but easily overlooked issue for the valid interpretation of photocurrent.Pioneering findings by Choi [2,3] and Mi [4,5] have demonstrated the existence of competing side reactions in various aqueous and non-aqueous electrolytes using WO3 as the photoanode, where the actual O2 evolution amount was measured and compared with photocurrent. It has been shown that the side reaction is more of a problem when 1) acidic or neutral aqueous electrolyte is used; 2) the photoanode semiconductor possesses a low valence band (VB) edge that generates highly oxidative holes. When acidic anions, such as Cl− exist in the aqueous electrolyte, the oxidation of these anions will compete with the oxygen evolution reaction (OER) for photogenerated holes. The oxidation of many acidic anions is thermodynamically less favourable than the OER, thus these competing side reactions are rarely of a concern for an electrolysis system in the dark. Under illumination, however, photogenerated holes would have enough energy to overcome the overpotential for the oxidation of acidic anions when the anode semiconductors possess the VB edges low enough to generate highly oxidative holes [3]. In addition, these competing side reactions are kinetically more favourable than the sluggish 4-electron O2 evolution process. For TiO2 photoanode, lower O2 evolution amount compared with the amount calculated from photocurrent was also reported with O2 Faradaic efficiencies of 9%-35% in 0.2 mol L −1 H2SO4 under Xe arc lamp illumination [6]. In this consideration, it might be misleading to evaluate the performance of a photocatalytic water oxidation system without measuring the actual O2 evolution rate.Recently, Niederberger and colleagues [7] have clearly pointed out this important yet easily overlooked issue for photoelectrochemical water splitting, i.e., photocurrent alone is insufficient to fully evaluate the performance of a photoanode. In their experiments, four aqueous electrolytes (1 mol L −1 CH3SO3H, 1 mol L −1 H2SO4, 0.1 mol L −1 Na2SO4 at pH 3, and 0.1 mol L −1 Na2SO4 at pH 5.5) were used to explore the influence of the competing reactions over the OER on WO3 photoanodes. The WO3 photoanodes prepared from the same condition exhibited noticeably different photocurrent density-potential characteristics in these four aqueous electrolytes under AM 1.5G illumination. When 1 mol L −1 CH3SO3H was used as the electrolyte, a photocurrent density of 3.5 mA cm −2 at 1.23 V vs. reversible hydrogen electrode (RHE) was achieved, followed...

Promoting charge carrier utilization by integrating layered double hydroxide nanosheet arrays with porous BiVO4 photoanode for efficient photoelectrochemical water splitting

“…The instability of BiVO4/NiFe-LDH photoanode in KOH may originate from the instability of pristine BiVO4 photoanode. On the other hand, owing to the water oxidation reaction process involving the removal of protons, it is imaginable that slow oxidation reaction kinetics may be not only blocked by the inappropriate modification of OECs, but also closely interrelated with the poor proton-accepting capability of the supporting electrolyte [54,55]. In K2SO4 neutral electrolyte, the diffusion process of protons is slow so that the photogenerated holes accumulation and the slow water oxidation kinetics of photoanodes are unavoidable [10,56,57].…”

Host/Guest Nanostructured Photoanodes Integrated with Targeted Enhancement Strategies for Photoelectrochemical Water Splitting

“…[67,68] When measuring photocurrent, side reactions or photocorrosion of semiconductors may also contribute to the total value of the photocurrent, resulting in the overestimation of PEC performance. [69] Thus, Faradaic efficiency, incident photon-tocurrent efficiency (IPCE), and STH efficiency are frequently used as evaluation parameters. Faradaic efficiency can reveal the percentage of the charge carriers that are genuinely used for O 2 or H 2 production.…”