Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu(III) and/or Cu(IV) , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu(II) in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu(II) and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.

Chloride oxidation to chlorine is a potential alternative to water oxidation to oxygen as a solar fuels half-reaction. Ag(I) is potentially an oxidative catalyst but is inhibited by the high potentials for accessing the Ag(II/I) and Ag(III/II) couples. We report here that the complex ions AgCl2(-) and AgCl3(2-) form in concentrated Cl(-) solutions, avoiding AgCl precipitation and providing access to the higher oxidation states by delocalizing the oxidative charge over the Cl(-) ligands. Catalysis is homogeneous and occurs at high rates and low overpotentials (10 mV at the onset) with μM Ag(I). Catalysis is enhanced in D2O as solvent, with a significant H2O/D2O inverse kinetic isotope effect of 0.25. The results of computational studies suggest that Cl(-) oxidation occurs by 1e(-) oxidation of AgCl3(2-) to AgCl3(-) at a decreased potential, followed by Cl(-) coordination, presumably to form AgCl4(2-) as an intermediate. Adding a second Cl(-) results in "redox potential leveling", with further oxidation to {AgCl2(Cl2)}(-) followed by Cl2 release.

Simply mixing a Cu­(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu­(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu­(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu­(II) oxidation to Cu­(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu­(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu­(II) phosphate film during electrolyses. As the pH is decreased, en, with pK b ≈ 6.7, becomes less coordinating and the precipitation of the Cu­(II) film inhibits water oxidation. The Cu­(II)-based reactivity toward water oxidation is shared by Cu­(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range.

Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu III and/or Cu IV , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu II in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu II and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.

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