Aging of a vanadium precursor solution and its influence on the photoelectrochemical water oxidation performance of BiVO₄ photoanodes

The aging duration of a vanadium precursor solution, vanadyl acetylacetonate in methanol, is identified as a factor that affects the reproducibility of BiVO₄ photoanode fabrication. The findings point to the importance of the composition of BiVO₄ films for highly reproducible and efficient BiVO₄-based photoanodes.

Article | Fall 2019

Photoelectrochemical (PEC) water splitting technology is an attractive hydrogen generation technology that can replace conventional hydrogen production methods such as the reforming of fossil fuels. PEC water splitting technology converts solar energy into chemical bonding energy by activating a water splitting reaction with photo-generated electrical carriers at the surface of a semiconductor photo-electrode. The configuration of PEC water splitting can be divided into photocathode and photoanode which generate hydrogen and oxygen, respectively. Unlike photocathode, the performance of photoanode still needs to be further improved due to slow kinetics of oxygen evolution reactions.

Among the various materials for photoanodes, BiVO₄ is an attractive candidate due to its high chemical stability under electrolyte condition, suitable band position for an oxygen evolution reaction, and relatively small band gap energy compared with other oxide materials. Due to its simplicity, various solution-based methods have been used to fabricate BiVO₄ thin film such as metal-organic decomposition (MOD), electrodeposition, and hydrothermal deposition. Normally, bismuth nitrate (Bi(NO₃)₃) and vanadyl acetylacetonate (VO(acac)₂) are widely used for Bi- and V-precursor in MOD process, and a mixture of Bi- and V- precursor solution is coated on a transparent conducting substrate. Subsequently, coated film changed to oxide film during the high-temperature annealing process, and organic solvent is decomposed at the same time. However, the MOD process often incurs reproducibility issues by introducing unwanted modification of BiVO₄ film during the repeated coating and annealing process to obtain optimal thickness of film for light absorption.

A research team of Associate Professor Byungha Shin from the Department of Material Science and Engineering at the Korea Advanced Institute of Science and technology (KAIST), and Professor Mario Lanza from the Institute of Functional Nano & Soft Materials of Soochow University developed a modified MOD protocol to produce BiVO₄ films by replacing the solvent for V-precursor from acetylacetone to methanol. They successfully fabricated a highly efficient BiVO₄-based photoanode through the modified MOD protocol with fewer of coating and annealing processes. They also found significant change in the absorption characteristics of the V-precursor solution (i.e., VO(acac)₂ dissolved in methanol) developed under ambient storage condition, and this changes affect the chemical composition of the resulting BiVO₄ films, which influenced the performance of BiVO₄-b based photoanodes.

The research team prepared five different BiVO₄-b based photoanodes using V-precursor solutions with methanol aged for different durations (as-prepared, 4 h, 1 d, 3 d and 5 d) and analyzed the samples. Firstly, they found structural changes of BiVO₄ film corresponding to different aging durations. As-prepared BiVO₄ film exhibited the largest crystallite size and surface roughness, which explains high reflectance due to the diffuse scattering. Next, they found chemical composition changes (Bi/V ratio) corresponding to different aging durations. Based on the previous work by Associate Professor Byungha Shin’s group, they found that the 3-day and 5-day samples exhibited the optimal Bi/V ratio for highly efficient BiVO₄-based photoanodes. Lastly, they measured the photocurrent of all aged samples. Higher photocurrent means that more oxygen molecules are generated by photoanode devices. Unlike with the research team’s expectation, the 3-day sample exhibited the highest performance but the 5-day sample showed quite low PEC performance.

To find out the performance gap between the 3-day and 5-day samples, the research team measured the oxygen content in the BiVO₄ films. The oxygen content in the oxide film is an important material parameter because the optimal amount of oxygen loss in oxide film improves the electrical conductivity of the oxide film. Due to the poor electrical conductivity of BiVO₄ films, enhancement of electrical conductivity is one of the main strategies for highly efficient BiVO₄-based -photoanode fabrication. The 3-day sample was revealed to have optimal oxygen content in the film and it has the highest electrical conductivity among the other aged samples. Based on those results, the research team identified a clear correlation between the chemical composition (Bi:V:O) and PEC performance and highlights the importance of fine control of the composition for highly efficient BiVO₄- based photoanode fabrication.

This research was published in Advanced Functional Materials (DOI: 10.1002/adfm.20180662, 2019).