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Highly Active NiO Photocathodes for H2O2 Production Enabled via Outer-Sphere Electron Transfer

Jung, Onyu, Pegis, Michael L., Wang, Zixuan, Banerjee, Gourab, Nemes, Coleen T., Hoffeditz, William L., Hupp, Joseph T., Schmuttenmaer, Charles A., Brudvig, Gary W., Mayer, James M.
Journal of the American Chemical Society 2018 v.140 no.11 pp. 4079-4084
cathodes, coumarin, dyes, electric current, electron transfer, hydrogen peroxide, nickel oxide, oxygen, porphyrins, ruthenium, solar energy, thermodynamics
Tandem dye-sensitized photoelectrosynthesis cells are promising architectures for the production of solar fuels and commodity chemicals. A key bottleneck in the development of these architectures is the low efficiency of the photocathodes, leading to small current densities. Herein, we report a new design principle for highly active photocathodes that relies on the outer-sphere reduction of a substrate from the dye, generating an unstable radical that proceeds to the desired product. We show that the direct reduction of dioxygen from dye-sensitized nickel oxide (NiO) leads to the production of H₂O₂. In the presence of oxygen and visible light, NiO photocathodes sensitized with commercially available porphyrin, coumarin, and ruthenium dyes exhibit large photocurrents (up to 400 μA/cm²) near the thermodynamic potential for O₂/H₂O₂ in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of H₂O₂ with essentially 100% faradaic efficiency. To our knowledge, these are among the most active NiO photocathodes reported for multiproton/multielectron transformations. The photoelectrosynthesis proceeds by initial formation of superoxide, which disproportionates to H₂O₂. This disproportionation-driven charge separation circumvents the inherent challenges in separating electron–hole pairs for photocathodes tethered to inner sphere electrocatalysts and enables new applications for photoelectrosynthesis cells.