Visible-light-driven oxygen reduction by an anisotropically crystallized CuBi2O4 photocathode fabricated using a mixed metal-imidazole casting method

An anisotropically crystallized CuBi2O4 (CuBi2O4(w)) film adhering rigidly on a fluorine doped tin oxide (FTO) electrode was prepared by a mixed metal-imidazole casting (MiMIC) method using 1-methylimidazole (MeIm). MeIm acts as a binder among metal oxide nanoparticles to result in tightly interconnected angular nanoparticles, as well as a structure-directing agent for growth of the anisotropically crystallized CuBi2O4 film. The photoelectrochemical (PEC) oxygen reduction reaction (ORR) at the CuBi2O4(w) electrode was investigated to compare with the electrode (CuBi2O4(w/o)) prepared without MeIm in a similar manner. The photocurrent for the ORR at the CuBi2O4(w) electrode under O2 in liner sweep voltammetry (LSV) was generated at an onset potential (Eon) of 0.96 V vs. RHE. The photocurrent decreased immediately for the CuBi2O4(w) electrode in chronoamperometry (CA) under Ar, being ascribable to reductive decomposition of the CuBi2O4(w) film. However, under O2, the photocurrent remained for 1 h (4% decrease) due to the ORR proceeding stably at the CuBi2O4(w) film. The high selectivity of the ORR to produce water was confirmed at the CuBi2O4(w) electrode, despite 8.0% hydrogen peroxide production after 1 h of photoelectrolysis under O2. The IPCE value (21.0%) at 440 nm and 0.41 V vs. RHE for the ORR at the CuBi2O4(w) electrode was 2.2-fold higher than that (9.6%) at the CuBi2O4(w/o) electrode. The photoelectrochemical impedance spectroscopic (PEIS) measurement suggested the faster ORR at the surface of the CuBi2O4(w) electrode, likely resulting from the anisotropic crystallization of tetragonal CuBi2O4, which is responsible for the high IPCE and stability for the PEC ORR at the CuBi2O4(w) electrode.