Insights into the interfacial carrier behaviour of copper ferrite (CuFe2O4) photoanodes for solar water oxidation

Abstract

Designing efficient yet robust photoanodes for water oxidation stands out as a major bottleneck in the realization of a feasible photoelectrochemical tandem cell for solar water splitting. Spinel copper ferrite (CuFe₂O₄) has been recently reported as a potential candidate photoanode, exhibiting an extended light absorption (band gap of 1.9 eV) with respect to traditional metal oxides. However, limiting factors dictating the poor performance (0.5 mA cm⁻² at 1.6 V vs. RHE) remain unclear. Here, CuFe₂O₄ thin-film photoanodes were examined using frequency-dependent electrochemical techniques, namely photoelectrochemical impedance spectroscopy (PEIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), to provide a detailed description of the photogenerated charge carrier behaviour under operational conditions. Results evidenced a strong Fermi level pinning during oxygen evolution caused by the accumulation of surface intermediates and a relatively slow rate of charge transfer (k_tran ∼ 5 s⁻¹). Moreover, the short hole diffusion length (L_p ∼ 4 nm) and the low charge collection efficiency (below 10%) further prevent efficient charge extraction. Overall, these findings point towards the need of both film nanostructuring and surface engineering to further advance this photoanode.