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

Abstract

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 HO. 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/cm) near the thermodynamic potential for O/HO in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of HO 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 HO. 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.

An Efficient Strategy for Boosting Photogenerated Charge Separation by Using Porphyrins as Interfacial Charge Mediators

“…Clear characteristic peaks of porphyrins (see Figure S6 a, inset) and the Q-band of porphyrins in solution ( Figure S6 b) prove that the porphyrins were successfully dispersed onto the BV surface (see Table S1 in the Supporting Information). [32] FN-H was chosen as an OEC with enhanced electrocatalytic activity (see Figures S7 and S8) and deposited on the FTO by electrodeposition. In PEC measurements,t he optimum conditions were found to be the use of BV/CoPy and BV/FN-H (60 s) photoanodes (see Figure S9).…”

An Efficient Strategy for Boosting Photogenerated Charge Separation by Using Porphyrins as Interfacial Charge Mediators

“…Next, different porphyrins, including CoPy, ZnPy, and Py (Py=5,10,15,20‐tetrakis(4‐carboxyphenyl)porphyrin), were homogeneously distributed on the different surfaces of BV photoanodes by the spin‐coating method, as further confirmed by the EDX profiles (see Figures S3–S5) and UV/Vis diffuse reflectance spectroscopy (DRS; see Figure S6). Clear characteristic peaks of porphyrins (see Figure S6 a, inset) and the Q‐band of porphyrins in solution (Figure S6 b) prove that the porphyrins were successfully dispersed onto the BV surface (see Table S1 in the Supporting Information) …”

Synthetic Organic Design for Solar Fuel Systems

“…Another P2H (13, Figure 7) is capable of photo-catalyzing the production of H 2 O 2 from O 2 when immobilized on NiO. [33] Flavins are organic electro-and photo-catalysts studied toward chemical oxidation chemistry that commonly use O 2 as the final electron acceptor in order to produce H 2 O 2 . [34] Similar to quinone,f lavin-derivatives exist in three redox states:n eutral (flavin-quinone FMN,F igure 7), 1e À -reduced, and 2e À -reduced (flavin-hydroquinone, FMNH 2 ).…”