The chemistry underlying the activation of nickel hydroxide toward electrocatalytic oxygen evolution by incorporation of iron remains a subject of debate. We extract insights into the role of geometric strain on the electrochemical behavior in this class of materials by blending aluminum, gallium, or iron into a disordered nickel hydroxide lattice. The electrochemical behavior and electronic structure of the three binary composition series are found to be similarly influenced by each additive cation. Density functional theory models indicate that the additive cations asymmetrically impede the voltageinduced expansion and contraction of the nickel hydroxide host lattice, a feature that is supported by near-infrared spectroscopy. Reaction coordinate diagrams suggest that this distortion decreases the activation energy for electron transfer by decreasing the extent to which the lattice can expand and contract but that iron is unique in its ability to favor oxidation by distorting the shape of the potential energy surface of the oxidized state to lower the electron transfer coefficient. These results reveal that interference with voltage-induced structural changes by incorporation of suitably sized ions alters the electrochemical behavior but that overall electrocatalytic performance cannot be linearly controlled by such geometric distortions.

The ability of layered perovskites to accommodate both oxygen vacancies and hyperstoichiometry provides a dimension of tunability that makes them appealing for electrocatalytic applications, but the resulting ionic conductivity enables electron transfer reactions within bulk crystals. We report on the stability of La 2 CuO 4 in the voltage regimes relevant to oxygen reduction, hydrogen evolution and CO 2 reduction. Voltammetric experiments, X-ray photoelectron spectroscopy and X-ray diffraction reveal both surface and bulk electron transfer reactions. Application of anodic voltages results in expansion in the crystal c-axis, while cathodic voltages induce contraction. The ability to catalyze each of the three cathodic reactions is confirmed, but X-ray diffraction and electron microscopy reveal amorphization of the electrocatalyst at voltages below À 0.4 V that affects both the oxygen reduction and CO 2 reduction reactions. While the ionic conductivity of Ruddlesden Popper oxides introduces intriguing properties, it simultaneously introduces the risk of structural instability in catalytically relevant voltages.[a] A.

Heterogeneous electrocatalytic reactions are believed to occur at a minority of coordination sites through a series of elementary reactions that are balanced by minor equilibria. These features mask changes in reaction sites, making it challenging to directly identify and analyze reaction sites or intermediates while studying reaction mechanisms. Systematic perturbations of a reaction system often yield systematic changes in material properties and behavior. Correlations between measurable changes in parameters describing the structure and behavior, therefore, serve as powerful tools for distinguishing active reaction sites. This review explores structure–property correlations that have advanced understanding of behavior and reaction mechanisms in heterogeneous electrocatalysis. It covers correlations that have advanced understanding of the contributions of the local reaction environment to reactivity, of structure and bonding within solid-state materials, of geometric or mechanical strain in bonding environments, and of the impact of structural defects. Such correlations can assist researchers in developing next generation catalysts by establishing catalyst design principles and gaining control over reaction mechanisms.

Layered perovskites such as La2-xSrxCuO4 are active electrocatalysts for CO2 reduction, but they suffer from structural instability under catalytic conditions. This structural instability is found to arise from the reaction of CO2 with surface sites. Variable scan rate voltammetry shows the growth of a Cu-based redox couple when potentials cathodic of 0.6 V vs. RHE are applied in the presence of CO2. Electrochemical impedance spectroscopy identifies a redox active surface state at this voltage, whose concentration is increased by electrochemical reduction in the presence of CO2. In-situ spectroelectrochemical FTIR identifies surface bound carbonates as being involved formation of these surface sites. The orthorhombic lattice for La2-xSrxCuO4 is found to uniquely enable monodentate binding of (bi)carbonate ions from solution as well as bidentate carbonate ions through reaction with CO2. The incorporation of Sr(II) induces a transition to a tetragonal lattice, for which only monodentate carbonate ions are observed. It is proposed that the binding of carbonate ions in a bidentate fashion generates sufficient strain at the surface to result in amorphization at the surface, yielding the observed Cu(II)/Cu(I) redox couple.

The atomic proximity of two disparate structural motifs in interleaved structures such as that adopted by La2NiO4, which alternates rock salt and perovskite motifs, offers potential synergistic effects and increased tunability for electrocatalyst behavior. Structural and voltammetric analysis of the intersecting composition series LaSrNi1−yFeyO4+δ and La2−xSrxNi0.7Fe0.3O4+δ show that tuning different cation sites uniquely affects structure and behavior. Correlations between X‐ray diffraction and Raman spectroscopy results reveal that structural changes in the a‐b plane are uniformly matched by the c‐axis following La−Sr exchange, while Ni−Fe exchange distorts the structure by expanding the crystal c‐axis and generating oxygen defects with a distinct signature in Raman spectra. Correlations between rotating ring‐disk electrode voltammetry results and structural parameters show that only Fe substitution has a direct effect on the electrocatalytic oxygen reduction reaction. We link systematic changes observed for onset of electrocatalytic oxygen reduction reaction, Tafel slope, and selectivity to distortions in the B‐site environments. This analysis demonstrates that correlational analysis linking electrochemical behavior parameters to multiple structural characterization techniques is an effective means to probe the effect of individual compositional substitution on electrocatalytic behavior.