Aging of a Vanadium Precursor Solution: Influencing Material Properties and Photoelectrochemical Water Oxidation Performance of Solution‐Processed BiVO₄ Photoanodes

Abstract

Metal-organic decomposition is an easy way to fabricate BiVO 4 (BVO) photoanodes; however, it often experiences a reproducibility issue. Here, the aging duration of a vanadium precursor solution, vanadyl acetylacetonate in methanol, is identified as a factor that profoundly affects reproducibility. Substantial changes in structural, optical, and electrical properties of BVO films are observed upon varying aging time of vanadium precursor solutions, which subsequently impacts photoelectrochemical (PEC) water oxidation and sulfite oxidation reactions. With the optimum number of aging days (3 d), some deficiency of oxygen is observed, which is accompanied by an increase in carrier concentration and a reduced charge transfer resistance in the PEC device, which produces the highest PEC performance that is comparable to the state-of-the-art undoped BVO photoanodes. The findings point to the importance of understanding solution chemistry and demonstrate that utilization of the understanding of fine adjustment of the composition of BVO films can produce highly reproducible and efficient BiVO 4 photoanodes.

Improving Uniformity and Reproducibility of Photoelectrochemical Water Oxidation Performance of BiVO4 Photoanodes via Selective Removal of Excess V2O5 by Electrochemical Etching

“…Because of the much faster kinetics of sulfite oxidation compared to water oxidation, charge transfer efficiency at the electrode/electrolyte interface during SOR is almost 100%. Therefore, it allowed us to predict the ultimate water oxidation photocurrent density of BiVO 4 if it had ideal water oxidation catalysts on it. , As shown in Figure a, the photocurrent density of the chemically etched samples exhibited the best PEC performance at 10 min of etching and further etching degraded the performance because of the damage to the BiVO 4 confirmed by SEM images (Figure d). The lower PEC performance of NaOH­(3 min) and NaOH­(5 min) compared to NaOH­(10 min) must be due to the remaining V 2 O 5 on the surface of BiVO 4 (Figure S1a, b).…”

“…Although batch-to-batch variation existed even in the EC etched devices (Figure. b, d , f), which must have originated from fluctuations in the two-step process such as BiVO 4 thickness, stoichiometry, amount of oxygen content, the overall variation was again much reduced when compared to the chemical etching. The improved uniformity of SOR performance was also observed from oxygen evolution reaction (Figure S11).…”

“…This may cause unintended modifications to BiVO 4 formed in the previous coating steps. 9 Hence, reproducibility is often an issue for the MOD method. Another common synthesis method that has become recently popular is a two-step method that has produced highly efficient BiVO 4 photoanodes.…”

“…Photoelectrochemical (PEC) water splitting has garnered a lot of attention as a promising ecofriendly solution for the intermittency of solar energy by converting it into storable chemical energy such as hydrogen. − BiVO 4 is one of the promising materials for a photoanode in PEC water splitting cells because of its direct bandgap of around 2.4 eV, suitable valence band edge position for the water oxidation reaction, and high chemical stability under neutral and mild basic electrolyte condition. − However, low charge transport efficiency through the bulk and poor charge transfer efficiency across the electrode/electrolyte interfaces are main obstacles to reaching the theoretical maximum photocurrent density of 7.6 mA cm –2 . , To overcome these limitations, researchers have used various strategies such as introducing external dopants, ,, catalyst loading, − formation of a heterojunction, − nanostructuring − and the control of crystal orientations. , …”

“…In the MOD, a solution mixture consisting of Bi and V precursors is spin-coated, spray-coated, or drop-cast onto a substrate, which is then converted to a BiVO 4 film after an annealing step at above 400 °C. ,, However, the MOD method needs several repeated coating and annealing processes to obtain a desired thickness that is thick enough for sufficient light absorption. This may cause unintended modifications to BiVO 4 formed in the previous coating steps . Hence, reproducibility is often an issue for the MOD method.…”