Utilization of Fe-Porphyrin Sensitized NiO Photocathodes for O2 Reduction Electrocatalysis

The widespread commercialization of artificial photosynthesis technologies will require efficient photocathodes capable of catalyzing reactions such as H⁺/H₂ and CO₂/CO. Sensitized photocathodes operate in an analogous manner to DSSCs, where photoexcitation provides a driving force for electrocatalytic transformations. Geminate recombination is a major free energy loss in p-type photocathodes, which have a large thermodynamic driving force for electron-hole recombination that inhibit energy storing half reactions such as CO₂ and H⁺ reductions. In this work, a series of iron-(III) porphyrin complexes were sensitized on p-type NiO to examine photoelectrochemical activity for O₂ reduction. The thermodynamic potential alignment of the NiO valence band edge and the O₂/H₂O couple suggest improved opportunity for activity that could provide substantial insights on improving the efficiency of p-type photocathodes. Light-driven oxygen reduction reaction was achieved by a sensitized NiO photocathode, constructed by covalent attachment of iron-(III) porphyrins on the surface of porous NiO deposited onto FTO. The iron-(III) porphyrin plays a dual role as both the chromophore and electrocatalyst. Electrochemical responses obtained from CV, OPE, and CPE experiments indicate at a current density of 1 mA/cm², the oxygen reduction onset overpotential increases from -0.926 V to -0.906 V under the exposure to light. In this work, it is shown for the first time that NiO electrodes sensitized with iron-(III) porphyrins are able to photocatalytically drive the reduction of O₂. The shift in the current density for O₂ reduction under illumination confirms the possibility of utilizing photocatalysts that have better thermodynamic potential alignments.