Interface Engineering and Characterization of BiVO₄ Photoanodes Boosted with 2D Interlayers and Co-Catalysts

<br/>Photoelectrochemical conversion of water using solar energy offers a clean solution to the world energy requirements of a sustainable future. Achieving its full potential depends on developing inexpensive photoanodes that can efficiently absorb solar light and drive the difficult oxygen evolution reaction (OER), which requires 4 electrons per molecule, and currently hampers reaching high solar to hydrogen efficiencies.<br/><br/>In this talk, I will first present recent developments we have achieved in my group in the preparation of inexpensive BiVO₄ photoanodes functionalized with 2D bismuthene. Partially oxidized two-dimensional (2D) bismuthene is prepared by reduction of BiCl₃ and demonstrated to be an effective, stable, functional interlayer between BiVO₄ and the archetypal NiFeOOH co-catalyst. Comprehensive (photo)electrochemical and surface photovoltage characterizations show that NiFeOOH passivates hole trap states of BiVO₄; however, it is limited in influencing electron trap states related to oxygen vacancies (V_O). Loading bismuthene on BiVO₄ photoanodes fills the V_O-related electron traps, leading to more efficient charge extraction. This is confirmed by kelvin probe measurements. Moreover, bismuthene increases adsorption sites for reaction intermediates and increases interfacial band bending boosting hole charge flux to the electrolyte. With the synergistic interaction of bismuthene and NiFeOOH on BiVO₄, this composite photoanode achieves a 5.8-fold increase in photocurrent compared to bare BiVO₄ reaching a stable 3.4 (±0.2) mA cm^–2 at a low bias of +0.8 V_RHE or 4.7(±0.2) mA cm^–2 at +1.23 V_RHE. The use of 2D bismuthene also boosts other photoanodes such as hematite, demonstrating its wide potential to boost the performance of photoelectrodes for energy conversion applications.<br/><br/>In a second part of my talk, I will present our study on the role of composition and interactions between BiVO₄ and core-shell structured bimetallic nickel-cobalt phosphide co-catalysts with varying metal ratios to improve their photoanodic performance The best performance obtained from BiVO₄/Ni_1.5Co_0.5P and BiVO₄/Ni_0.5Co_1.5P photoanodes achieved a photocurrent of 3.2 (±0.14) mA cm-2 at +1.23 V_RHE, a 3.5-fold increase in photocurrent compared with bare BiVO₄. Through an extensive characterization considering interfacial energetics, hole storage, and catalytic ability, we have discovered that the enhanced photoelectrochemical performance cannot be solely attributed to the catalytic activity of the co-catalysts. Instead, it arises from a synergistic interplay involving effective band bending, catalytic activity, and capacitive ability. The contact between the core-shell metal phosphides, BiVO₄, and the electrolyte creates three pathways for hole injection into the electrolyte, all of which are notably enhanced by the presence of a second metal cation in the co-catalysts. Kinetic studies demonstrate that the BiVO₄/metal phosphide photoanodes show significantly improved interfacial charge injection due to a lower charge-transfer resistance, enhanced OER kinetics, and larger surface hole concentrations. Moreover, by analysing the distribution of relaxation times, we can distinguish the kinetics of various processes, including bulk charge carrier transport, hole injection through different pathways, and surface recombination. This analysis provides deeper insights into the carrier dynamics within these photoanode/cocatalyst systems.<br/><br/>To conclude, these results provide key insights into the preparation of BiVO₄ photoanodes boosted with interlayers and co-catalysts to achieves its full potential and pave the way to rationally design other complementary photoelectrodes.<br/><br/>Cui, J.; Daboczi, M.; Regue, M.; Chin, Y.; Pagano, K.; Zhang, J.; Isaacs, M.A.; Kerherve, G.; Mornto, A.; West, J.; Gimenez, S.; Kim, J.; Eslava, S. <i>Adv. Funct. Mate</i>r. 2022, 32, 1-12<br/><br/>Cui, J.; Daboczi, M.; Cui, Z.; Gong, M.; Flitcroft, J.; Skelton, J.; Parker, S.C. Eslava, S. <i>Small</i> 2023, 2306757