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 (E_on) 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. [ABSTRACT FROM AUTHOR]