Enhancing the Performance of a Robust Sol–Gel‐Processed p‐Type Delafossite CuFeO₂ Photocathode for Solar Water Reduction

Abstract

Delafossite CuFeO2 is a promising material for solar hydrogen production, but is limited by poor photocurrent. Strategies are demonstrated herein to improve the performance of CuFeO2 electrodes prepared directly on transparent conductive substrates by using a simple sol-gel technique. Optimizing the delafossite layer thickness and increasing the majority carrier concentration (through the thermal intercalation of oxygen) give insights into the limitations of photogenerated charge extraction and enable performance improvements. In oxygen-saturated electrolyte, (sacrificial) photocurrents (1 sun illumination) up to 1.51 mA cm(-2) at +0.35 V versus a reversible hydrogen electrode (RHE) are observed. Water photoreduction with bare delafossite is limited by poor hydrogen evolution catalysis, but employing methyl viologen as an electron acceptor verifies that photogenerated electrons can be extracted from the conduction band before recombination into mid-gap trap states identified by electrochemical impedance spectroscopy. Through the use of suitable oxide overlayers and a platinum catalyst, sustained solar hydrogen production photocurrents of 0.4 mA cm(-2) at 0 V versus RHE (0.8 mA cm(-2) at -0.2 V) are demonstrated. Importantly, bare CuFeO2 is highly stable at potentials at which photocurrent is generated. No degradation is observed after 40 h under operating conditions in oxygen-saturated electrolyte.