Aqueous Solution-Gel Deposition of Copper-Based Photo-Electrodes for Multipurpose Photo-Electrochemical Systems

Abstract

Efficient valorization of solar energy is expected to contribute significantly to a sustainable supply of energy and fuel sources, as well as the reduction of atmospheric pollution. To this end, photo-electrochemical systems show encouraging potential by converting light into chemical energy. These devices rely on semiconducting photoanodes and/or -cathodes that absorb light efficiently and generate electron-hole pairs with sufficient energy for initiating redox reactions. Several copper-based oxides exhibit appropriate band edges for both photo-electrochemical water splitting and CO2 reduction [1]. This, in combination with their bandgap values, low toxicity, and earth-abundance, has recently increased the attention for copper-based photo-electrodes for multipurpose photo-electrochemical systems. Substantial photocorrosion of CuO and Cu2O is the main issue restricting their application as functional photo-electrodes. In contrast, copper-based ternary oxides are found to be significantly less susceptible to photocorrosion. The citrate-based aqueous solution-gel method has proven to be a versatile approach for the formulation of stable aqueous metal ion precursor solutions, and the subsequent synthesis of (doped) multi-metal oxides by this method has been demonstrated numerously. [2,3] Still, due to the flexible redox chemistry of copper, the solution-gel synthesis of phase-pure copper-based ternary oxides remains challenging. This work describes the synthesis of copper-based photo-electrode materials by focusing on copper ferrite (CFO), copper tungstate (CWO), copper niobate (CNO), and copper bismuth oxide (CBO). The formulation of the respective precursor solutions is described, and their thermal decomposition is investigated. Thin-film photo-electrodes are deposited on transparent conductive substrates by spin coating. The phase purity of the resulting copper-based oxides is determined by X-ray diffraction and Raman spectroscopy. Using UV-Vis spectroscopy, the optical bandgap of the oxide semiconductors is calculated. Furthermore, the photo-electrodes are subjected to linear sweep voltammetry in dark and under illumination to evaluate their photo-electrochemical performance. [1] Wang et al. “Insights into the development of Cu-based photocathodes for carbon dioxide conversion” Green Chem. (2021) 23, 3207[2] Van den Rul et al. “Aqueous Solution-Based Synthesis of Nanostructured Metal Oxides” in “Handbook of Nanoceramics and Their Based Nanodevices” (2009) Ed. T-Y Tseng and H. S. Nalwa[3] Marchal et al. “Precursor Design Strategies for the Low-Temperature Synthesis of Functional Oxides: It’s All in the Chemistry” Chem. Eur. J. (2020) 26, 9070 This work has received funding from the Belgian federal government through the ETF project T-REX and from the VLAIO network “Flanders Innovation & Entrepreneurship” through the Catalisti Moonshot project SYN-CAT. Part of this work was financially supported by TNO in the framework of the “Energy Conversion and Storage” Early Research Program.