Effect of Cu/Bi ratio on the photoelectrochemical performance of CuBi ₂ O ₄ /CuO nanocomposite films for CO ₂ reduction

Abstract Solar energy is free, clean, and virtually limitless; however, its conversion into a storable form presents technological challenges. One avenue towards solar energy utilization is the photoelectrochemical (PEC) reduction of CO 2 to one- or two-carbon fuels, employing a semiconductor configured as an electrode. A potential material for this application is the p-type copper bismuth oxide (CuBi 2 O 4 ) with a band gap capable of visible light absorption and a conduction band edge position suitable for CO 2 reduction. In this study, CuBi 2 O 4 nanocomposite films of varying Cu/Bi ratios (0.25, 0.51, 0.68, 0.94, 2.04) were prepared via an electrodeposition-spray deposition-annealing route. Where the Cu/Bi ratio exceeded the stoichiometric value of 0.5, a bilayered film composed of a copper (II) oxide (CuO) phase on top of CuBi 2 O 4 was formed, creating a planar heterojunction between the two oxide layers. With increasing Cu/Bi ratio, the light absorption range of the films broadened due to the CuO phase. Analysis of the photocurrent–potential behavior of the films under visible-light illumination showed a 4–7-fold increase in the photocurrent from an inert electrolyte to a CO 2 -saturated electrolyte, confirming potential activity for CO 2 reduction of the CuBi 2 O 4 /CuO films. A higher Cu/Bi ratio resulted to an improved charge separation efficiency, enhancing the photocurrent generation. However, the transient photocurrent response of the films showed a 70%–80% decrease in the photocurrent after only 15 min of testing. When tested in an electrolyte with an electron scavenger, the percent decrease was lowered to <10%, indicating that the instability of the films resulted from poor interfacial kinetics. While the CuBi 2 O 4 /CuO nanocomposite films can accomplish CO 2 reduction, further strategies to improve their efficiency and stability are needed to realize practical application.