The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoO x ) with adsorbed Fe 3+ ions. The CoO x was electrodeposited onto glassy-carbon-disk electrodes, while Fe 3+ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system's OER activity. The addition of Fe 3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm 2 O 2 current from 378 to 309 mV. The Tafel slope of the CoO x + Fe 3+ catalyst also decreased from 59.5 (pure CoO x ) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm 2 O 2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoO x , from Co 2+ to Co 3+ occurred at a more positive potential when Fe 3+ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoO x , CoO x + Fe 3+ , and control Fe 48 Co 52 O x catalysts were similar during the OER, which indicates that the improved OER performance of the CoO x + Fe 3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe 3+ adsorbed on CoO x did not further oxidize under OER conditions. However, Fe's coordination number was notably reduced from 6 in pure FeO x to 3.7 when it was adsorbed on CoO x . No change in the Fe−O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO x are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO x + Fe 3+ catalyst.

The large-scale deployment of efficient artificial photosynthesis systems to convert carbon dioxide (CO2) into carbon-based fuels and chemical feedstocks holds great promise as a way to ensure a carbon neutral cycle. While catalysts have been developed for the pertinent half-reaction of CO2 reduction to C2 molecules, an integrated system for this purpose has never been designed and built. In this work, we demonstrate an energetically efficient formation of ethylene directly from CO2 and water (H2O) using solar energy at room temperature and pressure. A two-electrode cell (electrolyzer) was designed, and cell parameters such as electrolyte and voltage were optimized. Oxide-derived copper (Cu) and iridium oxide (IrO x ) were used as electrocatalysts respectively in the cathode and anode. Coupling this electrolyzer with silicon solar panels under laboratory 1 sun illumination (100 mW/cm2), we show that CO2 could be facilely reduced to ethylene with a faradaic efficiency of 31.9%, partial current density of 6.5 mA/cm2, and a solar-to-ethylene energy efficiency of 1.5%. When liquid fuels such as ethanol and n-propanol were included, the total solar-to-fuel efficiency was 2.9%. These outstanding figures-of-merits are the state-of-the-art. We also introduced insoluble chelating agents in the electrolyte to capture contaminants such as dissolved iridium ions, and thus significantly improved the longevity of the electrolyzer. Compared to previously reported solar-to-fuel setups which were only tested under simulated sunlight, our system, when coupled with a rechargeable battery, could run and produce ethylene continuously using only intermittent natural sunlight.

A comprehensive study of the synergy between Fe and six transition metals (M=Ti, Co, Ni, Cu, Ag, Au), and how their M-Fe oxides electrocatalyze the oxygen evolution reaction (OER) was undertaken. Measurements were made using metal disks as the working electrodes and the addition of Fe ions to the 1 m KOH electrolyte. The surfaces of the metal disks were oxidized after the OER. Interestingly, Fe interacted synergistically with all metal oxide layers except for that of Ti, resulting in enhanced catalytic activity for the OER. At an overpotential (η) of 400 mV, the current densities of the Ni and Ag disks in the Fe ions-spiked electrolyte increased by 253 and 132 times, respectively, whereas it was only 20-30 times for the Co and Cu disks (compared with the OER in pure KOH at η=400 mV). The Tafel slopes of the Fe, Co, Ni, Cu, and Ag disks in 1 m KOH+Fe electrolyte were in the range of 29-42 mV dec . The surface morphology and post-OER concentration of Fe in the catalysts could not be used to account for differences in the OER activities. Cyclic voltammetry showed that improvements in the OER performance were accompanied by changes in the redox features of the metal disk electrodes, which indicated the presence of electronic interactions between them and the Fe . Strikingly, this was not observed between Ti and the Fe ions, which could explain the lack of synergy between Ti and Fe towards the OER catalysis. Electrochemical impedance spectroscopy indicated that the charge-transfer resistances of all the electrodes (except Ti) decreased after the addition of Fe ions. Fe plays an important role in all these observed phenomena and we propose that the surface-adsorbed Fe species serve as the main active sites for OER in these synergistic M-Fe combinations.

Metal oxides have been extensively explored as catalysts for the electrochemical oxygen evolution reaction (OER). Here, we present an excellent OER catalytic system consisting of pulse-electrodeposited Ir/Ni oxides in Fe(3+)-spiked 1 M KOH. In pure 1 M KOH electrolyte, the optimized catalyst, which had an Ir:Ni atom ratio of 1:1.49, could catalyze 10 mA/cm(2) of O2 production at a small overpotential (η) of 264 mV. Remarkably, we found that its OER performance could be significantly improved by adding 0.3 mM Fe(3+) into the electrolyte. At an η of just 343 ± 3 mV, a huge current of 500 mA/cm(2) was achieved. Furthermore, this catalytic system exhibited a small Tafel slope of 31 mV/dec and a large iridium mass-normalized current of 1260 mA/mgIr at η = 280 mV. We also discovered that the durability of the Ir/Ni oxide catalyst during OER (at 10 mA/cm(2) with η < 280 mV) could be maintained for more than 4.5 days by simply spiking Fe(3+), Ir(3+), and Ni(2+) into the KOH electrolyte. The figures-of-merit in this work, in terms of both activity and stability, compare favorably against values from several state-of-the-art catalysts. Hypotheses for the outstanding performance of the Ir/Ni catalyst are proposed and discussed.

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