The direct energy conversion of sunlight into hydrogen by using photoelectrochemical (PEC) tandem cells has emerged as a leading technology to tackle the growing demand of hydrogen in an environmentally-friendly and potentially cost-effective way. Basically, the device comprises a photocathode and a photoanode wired together, which under illumination delivers an overall photovoltage above 1.23 V, enough to drive the reduction and oxidation of water on the respective surfaces of the electrodes with reasonable currents. Despite the encouraging progress in the development of a wide-variety of low-cost and high-performing photocathodes by solution-based approaches, the rather short family of available photoanodes for water oxidation (Fe₂O₃, WO₃, BiVO₄, TaON) and their low-performance is one major bottlenecks in the overall solar-to-hydrogen conversion of these devices. Exploring new materials for water photo-oxidation may help the development of promising photoanodes with favorable optical and electronic properties enlarging the family of photoanodes available to combine with the vast library of photocathodes.

The quest for new materials for water photo-oxidation has recently found in the vast family of spinel ferrites (MFe₂O_4, M = Ba, Ca, Cu, Co, Mg, Mn, Ni, Zn) a promising candidate, leveraging a remarkable chemical stability together with a broad optoelectronic tunability (band gap between 1.4-2.7 eV by selecting the M cation) [1]. However, very little is known about the potential of these materials as photoelectrodes for solar energy conversion or on the main limiting factors on the performance. Herein, thin-film electrodes of three representative spinels, namely CuFe₂O₄, MgFe₂O₄ and ZnFe₂O₄, were fabricated by a solution-based approach and their PEC properties were extensively characterized. Annealing post-treatments together with the deposition of a well-known electrocatalyst (NiFeO_x) demonstrated to effectively improve the native n-type response, although a strong bulk recombination (especially in MgFe₂O₄) limits the saturation photocurrents (below 0.4 mA cm^-2 at 1.23 V vs RHE). Apart from the drawbacks related to bulk properties, a strong Fermi level pinning detected at around 0.9 V vs RHE in all the cases appears to dictate the low photovoltages delivered by the electrodes (~ 300 mV for ZnFe₂O₄), which evidences the presence of a high-density of surface states. Although recent studies support the role of NiFeO_x as an effective passivating agent capable of mitigating the Fermi level pinning in Fe₂O₃ [2], the combination of photocurrent and open circuit potential experiments proved unambiguously that here NiFeO_x is incapable to alleviate the pinning, but participates as an electrocatalyst to improve the overall performance. Detailed analysis of the surface energetics by fast-scan cyclic voltammetry, revealed hole accumulation at the potential where Fermi level pinning occurs and next to the turn-on voltage. This provides direct evidence for the location of surface states, inherent to the oxide electrodes, and/or for intermediates involved in the water oxidation reaction. In more general vein, these results draw a complete picture of the PEC behavior of the spinel ferrites evidencing the current intrinsic limitations, and establishes a roadmap for the design of the next generation of photoanodes based on spinel ferrites.

[1] Dillert, R.; Taffa, D. H.; Wark, M.; Bredow, T.; Bahnemann, D. W. APL Mater. 2015, 3 (10), 104001.

[2] Jang, J.-W.; Du, C.; Ye, Y.; Lin, Y.; Yao, X.; Thorne, J.; Liu, E.; McMahon, G.; Zhu, J.; Javey, A.; Guo, J.; Wang, D. Nat. Commun. 2015, 6, 7447.