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.

Nitrate (NO3-) is one of the most harmful contaminants in the groundwater, and it causes various health problems. Bimetallic catalysts, usually palladium (Pd) coupled with secondary metallic catalyst, are found to properly treat nitrate-containing wastewaters; however, the selectivity toward N2 production over ammonia (NH3) production still requires further improvement. Because the N2 selectivity is determined at the nitrite (NO2-) reduction step on the Pd surface, which occurs after NO3- is decomposed into NO2- on the secondary metallic catalyst, we here performed density functional theory (DFT) calculations and experiments to investigate the NO2- reduction pathway on the Pd surface activated by hydrogen. Based on extensive DFT calculations on the relative energetics among ∼100 possible intermediates, we found that NO2- is easily reduced to NO* on the Pd surface, followed by either sequential hydrogenation steps to yield NH3 or a decomposition step to N* and O* (an adsorbate on Pd is denoted using an asterisk). Based on the calculated high migration barrier of N*, we further discussed that the direct combination of two N* to yield N2 is kinetically less favorable than the combination of a highly mobile H* with N* to yield NH3. Instead, the reduction of NO2- in the vicinity of the N* can yield N2O* that can be preferentially transformed into N2 via diverse reaction pathways. Our DFT results suggest that enhancing the likelihood of N* encountering NO2- in the solution phase before combination with surface H* is important for maximizing the N2 selectivity. This is further supported by our experiments on NO2- reduction by Pd/TiO2, showing that both a decreased H2 flow rate and an increased NO2- concentration increased the N2 selectivity (78.6-93.6% and 57.8-90.9%, respectively).