Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm −2 per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoO x , CoPi, CoFeO x , NiO x , NiCeO x , NiCoO x , NiCuO x , NiFeO x , and NiLaO x . The oxygen-evolving activity of an electrodeposited IrO x catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated:(1) in alkaline solution, every non-noble metal system achieved 10 mA cm −2 current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrO x was unstable under oxidative conditions in acidic solutions.

Section S1: Materials and Electrolysis Cell Design Materials. Materials were purchased in the grade indicated and used as received. Ammonium carbonate ((NH4)2CO3, 99.999%), ammonium chloride (NH4Cl. BioUltra), ammonium hydroxide (NH4OH, BioUltra), ammonium perchlorate (NH4ClO4, 99.999%), ammonium sulfate ((NH4)2SO4, 99.999%), ammonium tetrathiomolybdate ((NH4)[MoS4], 99.97%), boric acid (H3BO3, BioUltra), chromium(III) sulfate monohydrate (Cr2(SO4)3•H2O, 99.999%), cobalt(II) acetate tetrahydrate (Co(OAc)2•4H2O, 99.999%), cobalt(II) chloride hexahydrate (CoCl2•6H2O, ACS 98%), cobalt(II) nitrate hexahydrate (Co(NO3)2•6H2O, 99.999%), copper(II) sulfate heptahydrate (CuSO4•5H2O, 99.995%), glycine (NH2CH2COOH, BioUltra), iridium(III) acetylacetonate (Ir(acac)3, 97%), iron(II) chloride tetrahydrate (FeCl2•4H2O, 99.99%), iron(III) chloride heptahydrate (FeCl3•6H2O, ACS 97%), iron(III) nitrate nonahydrate (Fe(NO3)3•9H2O, 99.99%), iron(II) sulfate heptahydrate (FeSO4•7H2O, ACS 99%), manganese(II) sulfate hydrate (MnSO4•H2O, 99.99%), nickel(II) nitrate hexahydrate (Ni(NO3)2•6 H2O, 99.999%), nickel(II) sulfate hexahydrate (NiSO4•6H2O, 99.99%),nickel(II) chloride hexahydrate (

16 crystalline metal oxide nanoparticulate systems are adhered to an electrode surface using a conventional drop-casting method and measured for their activity for the oxygen evolution reaction.

A series of water-soluble molecular cobalt complexes of tetraazamacrocyclic ligands are reported for the electrocatalytic production of H2 from pH 2.2 aqueous solutions. The comparative data reported for this family of complexes shed light on their relative efficiencies for hydrogen evolution in water. Rotating disk electrode voltammetry data are presented for each of the complexes discussed, as are data concerning their respective pH-dependent electrocatalytic activity. In particular, two diimine–dioxime complexes were identified as exhibiting catalytic onset at comparatively low overpotentials relative to other reported homogeneous cobalt and nickel electrocatalysts in aqueous solution. These complexes are stable at pH 2.2 and produce hydrogen with high Faradaic efficiency in bulk electrolysis experiments over time intervals ranging from 2 to 24 h.