Y. Liu and N. Guijarro shine new light on copper ferrite photoanodes

In general, 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. In this work, 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 complete description of the photogenerated charge carrier behavior 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 ~ 6 nm) and the low charge collection efficiency (below 10%) further prevent efficient charge extraction. Overall, these findings point towards the need of both nanostructuring the thin film and implemementing surface engineering routes to further advance on this photoanode.