RESUMEN
Highly active catalysts for the oxygen evolution reaction (OER) are required for the development of photoelectrochemical devices that generate hydrogen efficiently from water using solar energy. Here, we identify the origin of a 500-fold OER activity enhancement that can be achieved with mixed (Ni,Fe)oxyhydroxides (Ni(1-x)Fe(x)OOH) over their pure Ni and Fe parent compounds, resulting in one of the most active currently known OER catalysts in alkaline electrolyte. Operando X-ray absorption spectroscopy (XAS) using high energy resolution fluorescence detection (HERFD) reveals that Fe(3+) in Ni(1-x)Fe(x)OOH occupies octahedral sites with unusually short Fe-O bond distances, induced by edge-sharing with surrounding [NiO6] octahedra. Using computational methods, we establish that this structural motif results in near optimal adsorption energies of OER intermediates and low overpotentials at Fe sites. By contrast, Ni sites in Ni(1-x)Fe(x)OOH are not active sites for the oxidation of water.
RESUMEN
Resonant inelastic X-ray scattering and high-resolution X-ray absorption spectroscopy were used to identify the chemical state of a Co electrocatalyst in situ during the oxygen evolution reaction. After anodic electrodeposition onto Au(111) from a Co(2+)-containing electrolyte, the chemical environment of Co can be identified to be almost identical to CoOOH. With increasing potentials, a subtle increase of the Co oxidation state is observed, indicating a non-stoichiometric composition of the working OER catalyst containing a small fraction of Co(4+) sites. In order to confirm this interpretation, we used density functional theory with a Hubbard-U correction approach to compute X-ray absorption spectra of model compounds, which agree well with the experimental spectra. In situ monitoring of catalyst local structure and bonding is essential in the development of structure-activity relationships that can guide the discovery of efficient and earth abundant water splitting catalysts.
RESUMEN
A detailed investigation has been carried out of the structure and electrochemical activity of electrodeposited Ni-Fe films for the oxygen evolution reaction (OER) in alkaline electrolytes. Ni-Fe films with a bulk and surface composition of 40% Fe exhibit OER activities that are roughly 2 orders of magnitude higher than that of a freshly deposited Ni film and about 3 orders of magnitude higher than that of an Fe film. The freshly deposited Ni film increases in activity by as much as 20-fold during exposure to the electrolyte (KOH); however, all films containing Fe are stable as deposited. The oxidation of Ni(OH)2 to NiOOH in Ni films occurs at potentials below the onset of the OER. Incorporation of Fe into the film increases the potential at which Ni(OH)2/NiOOH redox occurs and decreases the average oxidation state of Ni in NiOOH. The Tafel slope (40 mV dec(-1)) and reaction order in OH(-) (1) for the mixed Ni-Fe films (containing up to 95% Fe) are the same as those for aged Ni films. In situ Raman spectra acquired in 0.1 M KOH at OER potentials show two bands characteristic of NiOOH. The relative intensities of these bands vary with Fe content, indicating a change in the local environment of Ni-O. Similar changes in the relative intensities of the bands and an increase in OER activity are observed when pure Ni films are aged. These observations suggest that the OER is catalyzed by Ni in Ni-Fe films and that the presence of Fe alters the redox properties of Ni, causing a positive shift in the potential at which Ni(OH)2/NiOOH redox occurs, a decrease in the average oxidation state of the Ni sites, and a concurrent increase in the activity of Ni cations for the OER.
RESUMEN
We quantitatively characterized oxygen reduction kinetics at the nanoscale Ptmid R:CsHSO(4) interface at approximately 150 degrees C in humidified air using conducting atomic force microscopy (AFM) in conjunction with AC impedance spectroscopy and cyclic voltammetry. From the impedance measurements, oxygen reduction at Ptmid R:CsHSO(4) was found to comprise two processes, one displaying an exponential dependence on overpotential and the other only weakly dependent on overpotential. Both interfacial processes displayed near-ideal capacitive behavior, indicating a minimal distribution in the associated relaxation time. Such a feature is taken to be characteristic of a nanoscale interface in which spatial averaging effects are absent and, furthermore, allows for the rigorous separation of multiple processes that would otherwise be convoluted in measurements using conventional macroscale electrode geometries. The complete current-voltage characteristics of the Ptmid R:CsHSO(4) interface were measured at various points across the electrolyte surface and reveal a variation of the oxygen reduction kinetics with position. The overpotential-activated process, which dominates at voltages below -1 V, was interpreted as a charge-transfer reaction. Analysis of six different sets of Ptmid R:CsHSO(4) experiments, within the Butler-Volmer framework, yielded exchange coefficients (alpha) for charge transfer ranging from 0.1 to 0.6 and exchange currents (i(0)) spanning 5 orders of magnitude. The observed counter-correlation between the exchange current and exchange coefficient indicates that the extent to which the activation barrier decreases under bias (as reflected in the value of alpha) depends on the initial magnitude of that barrier under open circuit conditions (as reflected in the value of i(0)). The clear correlation across six independent sets of measurements further indicates the suitability of conducting AFM approaches for careful and comprehensive study of electrochemical reactions at electrolyte-metal-gas boundaries.