Rodney D. L. Smith

Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis

Amorphous and More Active The electrochemical generation of hydrogen from water could help in the storage of energy generated by renewable resources at off-peak times. However, catalysts for the slow step of this reaction, the oxygen evolution reaction (OER), are based on oxides of noble metals (iridium and ruthenium) that have limited abundance. A strategy for improving the performance of earth-abundant elements is to explore mixed-metal oxides and to prepare these as amorphous phases. Smith et al. (p. 60, published online 28 March) developed a general method for preparing amorphous oxides, based on photodecomposition of organometallic precursors. Amorphous mixed-metal oxides of iron, nickel, and cobalt were more active than comparable crystalline materials and provided OER performance comparable to noble metal oxides. Amorphous oxides of earth-abundant metals catalyze water oxidation with performance approaching that of noble metal catalysts. Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe100-y-zCoyNizOx are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.

Water oxidation catalysis: electrocatalytic response to metal stoichiometry in amorphous metal oxide films containing iron, cobalt, and nickel.

Photochemical metal-organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations.

Accounting for the Dynamic Oxidative Behavior of Nickel Anodes

The dynamic behavior of the anodic peak for amorphous nickel oxy/hydroxide (a-NiOx) films in basic media was investigated. Chronocoulometry of films with known nickel concentrations reveals that a total of four electrons per nickel site comprise the signature anodic peak at 1.32 V during the first oxidative scan, and two electrons are passed through the associated cathodic peak on the reverse scan. The anodic and cathodic signals each contain two electrons on the successive scans. Catalytic oxygen evolution reaction (OER) was detected within the anodic peak, which is at a lower potential than is widely assumed. In order to rationalize these experimental results, we propose that the four-electron oxidation event is the conversion of the film from nickel(II) hydroxide ([Ni(II)-OH](-)) to a higher valent nickel peroxide species (e.g., Ni(IV)-OO or Ni(III)-OO·). The subsequent reduction of the nickel peroxide species is confined by a chemical step resulting in the accumulation of [Ni(II)-OOH](-), which is then oxidized by two electrons to form Ni(IV)-OO during the subsequent oxidative scan on the time scale of a cyclic voltammetric experiment. Our proposed mechanism and the experimental determination that each nickel site is oxidized by four electrons helps link the myriad of seemingly disparate literature data related to OER catalysis by nickel electrodes. The faster catalysis that occurs at higher oxidative potentials is derived from a minority species and is not elaborated here.

Mapping the performance of amorphous ternary metal oxide water oxidation catalysts containing aluminium

An extensive series of ternary amorphous catalysts for the oxygen evolution reaction are synthesized by a facile photochemical metal–organic deposition method. The electrocatalytic activity of catalyst films containing aluminium with a combination of iron, nickel, and/or cobalt are mapped out to determine how composition affects reactivity in these novel ternary amorphous metal-oxide oxygen-evolving catalysts. An important outcome of this work is the finding that the Al/Fe/Ni series of materials delivered strikingly low Tafel slopes of 11–27 mV dec−1, which are among the lowest values reported to date.

Spectroscopic identification of active sites for the oxygen evolution reaction on iron-cobalt oxides

The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-μ-oxo bridged cobalt–cobalt, di-μ-oxo bridged cobalt–iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-μ-oxo bridged cobalt–cobalt and di-μ-oxo bridged iron–cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.Optimization of electrocatalysts requires an understanding of all active reaction sites. Here, the authors combine X-ray absorption spectroscopy and electrochemistry to identify cobalt atoms with different coordination geometries and probe their contribution to electrocatalytic water oxidation.

Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions

The dramatic change in electrochemical behavior of nickel (oxy)hydroxide films upon incorporation of Fe ions provides an opportunity to establish effective electrocatalyst design principles. We characterize a photochemically deposited series of Fe–Ni (oxy)hydroxides by X-ray absorption spectroscopy and track the voltage- and composition-dependence of structural motifs. We observe a trigonal distortion in di-μ-hydroxo bridged NiII–NiII motifs that is preserved following a symmetric contraction of Ni–O bond lengths when oxidized to di-μ-oxo NiIV–NiIV. Incorporation of Fe ions into the structure generates di-μ-hydroxo NiII–FeIII motifs in which Ni–Fe distances are dependent on nickel oxidation state, but Fe–O bond lengths are not. This asymmetry minimizes the trigonal distortion in di-μ-hydroxo NiII–FeIII motifs and neighboring di-μ-hydroxo NiII–NiII sites in the reduced state, but exacerbates it in the oxidized state. We attribute both the Fe-induced anodic shift in nickel-based redox peaks and the improved ability to catalyze the oxygen evolution reaction to this inversion in geometric distortions. Spectroelectrochemical experiments reveal a previously unreported change in optical absorbance at ca. 1.5 V vs. RHE in Fe-containing samples. We attribute this feature to oxidation of nickel ions in di-μ-hydroxo NiII–FeIII motifs, which we propose is the process relevant to catalytic oxygen evolution.