Semiconductor photocathodes either for the reduction of water or CO₂ have received much less attention than their photoanode counterparts. In part, this is due to a lack of p-type materials having good minority carrier dynamics. However, the photostability of p-type materials is also often problematic. These issues are particularly challenging for metal oxide based materials, and thus few examples of p-type metal oxide based photocathodes having any level of practical functionality are available. Nonetheless, such materials are likely to have electrocatalytic activity of interest for the reactions noted above. We have therefore undertaken a search for photostable p-type metal oxides having advantageous electrode properties. To that end, we have focused our investigation on delafossite structures of the form ABO₂ where A = Cu⁺ or Ag⁺ and B= Fe³⁺ or Rh³⁺.

Initial studies, focused on p-CuFeO₂ evaluating both its solid state properties dopant response and photoelectrochemical capabilities.¹ We reported that this material had a conduction band edge well placed for the reduction of either water or CO₂ and a band gap (E_g) well matched to the solar spectrum (1.36 eV). Photoelectrochemical stability was found to be passable, but we did observe a slow reduction of Cu(I) to Cu(0), which degraded the electrode over time. In the absence of CO_2(aq), this material, under illumination, was found to reduce water to hydrogen. Introduction of aqueous CO₂ into the cell caused the photoelectrochemical formation of formate with good faradaic efficiency.

We elected to compare the response of this material to that of p-CuRhO₂, since we predicted a more negative conduction band edge for the rhodium system, and therefore, anticipated improved CO₂ reduction kinetics.² Surprisingly, this material was highly efficient at water reduction, but incapable of reducing aqueous CO₂. However, we did note that the band edge positions of p-CuRhO₂ straddled the water oxidation and reduction redox potentials making p-CuRhO₂ thermodynamically capable of splitting water without application of an external bias (Eg = 1.9 eV). And, though this is observed, a low optical conversion efficiency was obtained. Photochemical stability in this system was found to be unusual, in that when the electrode was utilized at a basic pH in the presence of air, it was very stable. However, at neutral to acidic pHs or in the absence of O₂ (at 3% or higher levels) the system degraded via Cu(0) formation.

Based on these results, we have now developed and explored a p-AgRhO₂ electrode. Like the Cu counterpart, this material is also excellent at water reduction and inept at CO₂ reduction chemistry. The system is quite stable toward photoreductive decomposition at all aqueous pH values. Water reduction conversion efficiency exceeds that reported for the Cu-based system. A measured Eg of 1.7 eV is found. However, the band edges of this material do not straddle both of the water redox potentials.

These observations have suggested to us that CO₂ reduction only proceeds when an ABO₂ surface lacks the ability to efficiently carry out water reduction. To test that hypothesis, we have synthesized a p-AgFeO₂, making the prediction that this material will reduce CO₂. In preliminary studies, this appears to be the case.

The stability and electrocatalytic properties of the delafossite systems studied have led to an interest in p-type chalcopyrite materials in the family
[(Cu_xAg_(1-x))(In_yGa_(1-y))(S_zSe_(1-z))₂] as photocathodes for water reduction.³ These systems have band gaps fairly well matched to the solar spectrum and have been studies as thin film electrodes, however, we are focusing on bulk electrode materials where solid state synthesis allows one to carefully tailor the band edge positions and the bulk electronic properties of the materials. Though we are early in this work, our results point to unusual couplings between the bulk optical and electronic properties of these materials, and the observed electrochemistry. Optimization of these interactions is anticipated to lead to efficient water splitting capabilities under solar irradiation.

References

1. (a) Gu, J.; Wuttig, A.; Krizan, J. W.; Hu, Y.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B., The Journal of Physical Chemistry C 2013, 117 (24), 12415-12422; (b) Wuttig, A.; Krizan, J. W.; Gu, J.; Frick, J. J.; Cava, R. J.; Bocarsly, A. B., Journal of Materials Chemistry A 2017, 5 (1), 165-171.
2. Gu, J.; Yan, Y.; Krizan, J. W.; Gibson, Q. D.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B., J Am Chem Soc 2014, 136 (3), 830-3.
3. Frick, J. J.; Kushwaha, S. K.; Cava, R. J.; Bocarsly, A. B., The Journal of Physical Chemistry C 2017, 121 (32), 17046-17052.