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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.