James B. Gerken

Electrochemical Water Oxidation with Cobalt-Based Electrocatalysts from pH 0–14: The Thermodynamic Basis for Catalyst Structure, Stability, and Activity

Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.

Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe4+ by Mössbauer Spectroscopy

Nickel-iron oxides/hydroxides are among the most active electrocatalysts for the oxygen evolution reaction. In an effort to gain insight into the role of Fe in these catalysts, we have performed operando Mössbauer spectroscopic studies of a 3:1 Ni:Fe layered hydroxide and a hydrous Fe oxide electrocatalyst. The catalysts were prepared by a hydrothermal precipitation method that enabled catalyst growth directly on carbon paper electrodes. Fe(4+) species were detected in the NiFe hydroxide catalyst during steady-state water oxidation, accounting for up to 21% of the total Fe. In contrast, no Fe(4+) was detected in the Fe oxide catalyst. The observed Fe(4+) species are not kinetically competent to serve as the active site in water oxidation; however, their presence has important implications for the role of Fe in NiFe oxide electrocatalysts.

A survey of diverse earth abundant oxygen evolution electrocatalysts showing enhanced activity from Ni–Fe oxides containing a third metal

Mixed metal oxides comprise a diverse class of materials that are appealing as potential water oxidation electrocatalysts. Here we report combinatorial screening of nearly 3500 trimetallic AxByCzOq mixed metal oxide compositions that led to the discovery of electrocatalysts with enhanced activity relative to, inter alia, the well-studied pure oxides, ABO3, and AB2O4 stoichiometries of those metals. Using a fluorescence-based parallel screening method, we directly detect electrolytic oxygen-evolution activity of catalyst arrays under alkaline conditions. From these data, composition–activity relationships amongst mixed oxides composed of earth-abundant elements have been determined. Significant sustained activity is observed only in the presence of Co or Ni, and the data draw attention to synergistic interactions between these redox-active ions and Lewis-acidic cations, such as Fe, Al, Ga, and Cr. The best activities are observed with oxides composed of Ni and Fe, together with another element.

Inverse spinel NiFeAlO4 as a highly active oxygen evolution electrocatalyst: promotion of activity by a redox-inert metal ion

Ni:Fe:Al mixed oxides were identified as highly active water oxidation electrocatalysts. A systematic investigation of these materials has led to the characterization of a well-defined NiFeAlO4 inverse spinel catalyst. Electrochemical characterization of NiFeAlO4 shows activity exceeding previously reported catalysts of similar composition and/or structure, including NiO, NiFe (9 : 1), and NiFe2O4.

Development of an O2‐Sensitive Fluorescence‐Quenching Assay for the Combinatorial Discovery of Electrocatalysts for Water Oxidation

High kinetic barriers associated with the oxidation of water to O2 [4,5] and the common use of high-cost electrocatalytic materials are among the challenges that limit the utility of photoelectrochemical energy storage. Improved electrocatalysts that operate at lower overpotential and avoid the use of expensive precious-metal or rare-earth elements are needed. While valuable progress is being made toward this goal, the rational design of optimal catalysts from first principles remains infeasible, and the number of possible catalyst compositions, even those with well-defined metal stoichiometry, far exceeds the number that can be tested in a traditional sequential fashion. Combinatorial methods can play an important role in the discovery of new electrocatalysts, and we describe herein a fluorescence-based assay for spatially resolved, direct detection of O2 across an array of metal-oxide electrocatalysts. Initial implementation of this technique has led to the identification of new electrocatalysts, which are composed entirely of earth-abundant elements (e.g., Ni/Al/ Fe) and warrant further investigation. Combinatorial methods for the discovery of electrocatalysts have been pursued previously. For example, a soluble fluorescent pH indicator has been used to screen electrocatalysts for reactions that consume or generate protons, including water oxidation mediated by platinum-group-metal electrocatalysts. Potential complications with this method include the use of poorly buffered electrolytes to ensure sensitivity to pH changes, and the instability of organic pHsensitive fluorophores under conditions required for water oxidation. Various combinatorial approaches have been used to probe photoelectrocatalytic performance of mixed-metaloxide materials. In most of these assays, the oxides are required to act simultaneously as a light-harvesting semiconductor and as the electrocatalyst for one or both watersplitting half-reactions. The most efficient PECs, however, will probably integrate separate photovoltaic (PV) and catalytic materials. Therefore, we targeted an assay that would enable rapid assessment of electrocatalysts for water oxidation independent of the other PEC functions. Catalysts discovered by such methods could then be used in indirect PECs (Figure 1A) or developed further for integration with PV semiconductors in direct PECs (Figure 1B). The essential feature of electrocatalytic water oxidation is O2 production, and an ideal catalyst-screening assay would directly monitor O2 evolution. In addition, a fluorescencebased assay seemed appealing because such methods are often compatible with parallel, rather than serial, analysis of activity, and they avoid the need for costly specialized analytical instrumentation. Fluorescent pressure-sensitive paints are well suited to meet these criteria. These paints are used in the automobile and aerospace industries to study aerodynamics in wind tunnels, and their utility arises from the sensitivity of their fluorescence intensity to the partial pressure of O2 (pO2). [23] Quantitative measurements are improved by incorporating two fluorophores into the paint, one that is insensitive to O2 as a background reference, and another that exhibits fluorescence quenching in proportion to the pO2. Our assay takes advantage of a commercially Figure 1. Schematic representations of indirect (A) and direct (B) PEC configurations for water splitting. The former employs a PV solar cell coupled to an electrolysis cell, whereas the latter features direct integration of the electrocatalysts with the charge-separating PV semiconductor.

High-Potential Electrocatalytic O2 Reduction with Nitroxyl/NOx Mediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis

Efficient reduction of O2 to water is a central challenge in energy conversion and many aerobic oxidation reactions. Here, we show that the electrochemical oxygen reduction reaction (ORR) can be achieved at high potentials by using soluble organic nitroxyl and nitrogen oxide (NOx) mediators. When used alone, neither organic nitroxyls, such as 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl (TEMPO), nor NOx species, such as sodium nitrite, are effective ORR mediators. The combination of nitroxyl/NOx species, however, mediates sustained O2 reduction with overpotentials as low as 300 mV in acetonitrile containing trifluoroacetic acid. Mechanistic analysis of the coupled redox reactions supports a process in which the nitrogen oxide catalyst drives aerobic oxidation of a nitroxyl mediator to an oxoammonium species, which then is reduced back to the nitroxyl at the cathode. The electrolysis potential is dictated by the oxoammonium/nitroxyl reduction potential. The overpotentials accessible with this ORR system are significantly lower than widely studied molecular metal-macrocycle ORR catalysts and benefit from the mechanism-based specificity for four-electron reduction of oxygen to water mediated by NOx species, together with kinetically efficient reduction of oxidized NOx species by TEMPO and other organic nitroxyls.

Modular Synthesis of Alkyne-Substituted Ruthenium Polypyridyl Complexes Suitable for “Click” Coupling

A modular synthetic method has been developed for the preparation of Ru polypyridyl complexes bearing a terminal alkyne. This method proceeds through a readily accessible intermediate with a silyl-protected alkyne and allows access to a variety of five- and six-coordinate Ru complexes. These complexes can be easily attached to azide-functionalized electrode surfaces with only slight perturbation of the redox properties of the parent complex.

Characterization of NiFe oxyhydroxide electrocatalysts by integrated electronic structure calculations and spectroelectrochemistry

Significance The conversion of water to oxygen and hydrogen molecules is essential for a variety of renewable energy technologies. Nickel–iron (NiFe) oxyhydroxide is an important, earth-abundant electrocatalyst for the oxygen evolution reaction. A combined experimental and computational study of pure Ni oxyhydroxide and mixed NiFe oxyhydroxide thin films elucidates the chemistry governing their different electrochemical and optical properties. The Ni and Fe oxidation states in each system are assigned as a function of applied potential based on quantum-mechanical calculations, cyclic voltammetry, and UV-visible spectroscopy. In the more catalytically active NiFe system, oxidation to Fe4+ coincides with the onset of oxygen evolution. Synergy between experiment and theory provides a detailed, atomistic understanding of this robust catalyst. NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni2+ to Ni3+, followed by oxidation to a mixed Ni3+/4+ state at a potential coincident with the onset of OER activity. Calculations on the 25% Fe-doped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe4+ and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixed-metal oxidation states in heterogeneous catalysts.

Conformational preferences of 3-(dimethylazinoyl)propanoic acid as a function of pH and solvent; intermolecular versus intramolecular hydrogen bonding.

The conformational equilibrium of 3-(dimethylazinoyl)propanoic acid (DMAPA, azinoyl = N(+)(O(-)) has a weak pH-dependence in D(2)O, with a slight preference for trans in alkaline solutions. The acid ionization constants of the protonated amine oxide and carboxylic functional groups as determined by NMR spectroscopy were 7.9 x 10(-4) and 6.3 x 10(-6), respectively. The corresponding value of K(1)/K(2) of 1.3 x 10(2) is not deemed large enough to provide experimental NMR evidence for a significant degree of intramolecular hydrogen bonding in D(2)O. Conformational preferences of DMAPA are mostly close to statistical (gauche/trans = 2/1) in other protic solvents, e.g., alcohols. However, the un-ionized form of DMAPA appears to be strongly intramolecularly hydrogen-bonded and gauche in aprotic solvents.

Structural Effects on the pH-Dependent Redox Properties of Organic Nitroxyls: Pourbaix Diagrams for TEMPO, ABNO, and Three TEMPO Analogs

Electrochemical studies of the reduction and oxidation reactions of five different organic nitroxyls have been performed across a wide pH range (0-13). The resulting Pourbaix diagrams illustrate structural effects on their various redox potentials and on the p Ka values of the corresponding hydroxylamine and hydroxylammonium ions. Evidence is also given for the reversible formation of a hydroxylamine N-oxide when nitroxyls are oxidized in alkaline media. Structural effects on the thermodynamics of this reaction are assessed.