Organic Non-Nucleophilic Electrolyte Resists Carbonation during Selective CO2 Electroreduction.

The spontaneous reaction of CO2 with water and hydroxide to form (bi)carbonates in alkaline aqueous electrolytes compromises the energy and carbon efficiency of CO2 electrolyzers. We hypothesized that electrolyte carbonation could be mitigated by operating the reaction in an aprotic solvent with low water content, while also employing an exogenous non-nucleophilic acid as the proton donor to prevent parasitic capture of CO2 by its conjugate base. However, it is unclear whether such an electrolyte design could simultaneously engender high CO2 reduction selectivity and low electrolyte carbonation. We herein report selective CO2 electroreduction with low carbonate formation on a polycrystalline Au catalyst using dimethyl sulfoxide as the solvent and acetic acid/acetate as the proton-donating medium. CO2 is reduced to CO with over 90% faradaic efficiency at potentials relative to the reversible hydrogen electrode that are comparable to those in neutral aqueous electrolytes. 1H and 13C NMR studies demonstrate that only millimolar concentrations of bicarbonates are reversibly formed, that the proton activity of the medium is largely unaffected by exposure to CO2, and that low carbonation is maintained upon addition of 1 M water. This work demonstrates that electrolyte carbonation can be attenuated and decoupled from efficient CO2 reduction in an aprotic solvent, offering new electrolyte design principles for low-temperature CO2 electroreduction systems.

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

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.