Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They can exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher efficiency catalysts, challenging. We report a series of procedures and measurement techniques that we have adopted or developed to assess each of the above challenges in understanding materials for the OER. These include the targeted synthesis of hydrated oxyhydroxide phases, the assessment and elimination of electrolyte impurities, the use of a quartz crystal microbalance to monitor film loading and dissolution, and the use of an in situ conductivity measurement to understand the flow of electrons from the catalyst active sites to the conductive support electrode. We end with a recipe for the synthesis and characterization of a "standard" Ni(Fe)O x H y catalyst that can be performed in any laboratory with a basic electrochemical setup and used as a quantitative comparison to aid the development of new OER catalyst systems.

Anion exchange membrane (AEM) electrolysis is a promising technology to produce hydrogen through the splitting of pure water. In contrast to proton-exchange-membrane (PEM) technology, which requires precious-metal oxide anodes, AEM systems allow for the use of earth-abundant anode catalysts. Here we report a study of first-row transition-metal (oxy)­hydroxide/oxide catalyst powders for application in AEM devices and compare physical properties and performance to benchmark IrO x catalysts as well as typical catalysts for alkaline electrolyzers. We show that the catalysts’ oxygen-evolution activity measured in alkaline electrolyte using a typical three-electrode cell is a poor indicator of performance in the AEM system. The best oxygen-evolution-reaction (OER) catalysts in alkaline electrolyte, NiFeO x H y oxyhydroxides, are the worst in AEM electrolysis devices where a solid alkaline electrolyte is used along with a pure water feed. NiCoO x -based catalysts show the best performance in AEM electrolysis. The performance can be further improved by adding Fe species to the particle surface. We attribute the differences in performance in part to differences in the electrical conductivity of the catalyst phases, which are also measured and reported.

Higher energy densities in rechargeable batteries can be achieved using thicker cathode films, though it is a challenging endeavor since the electrochemical performance of thick electrodes is substantially worse than that of the conventional thin electrodes due to a variety of transport limitations, which are thus far poorly understood. Operando synchrotron studies have been, for the first time, applied to thick film samples to determine the depth-dependent state of charge (SOC) distribution inside 170 micron thick Li­(Ni0.8Mn0.1Co0.1)­O2 cathode films using an unconventional radial diffraction experiment geometry, allowing the SOC to be probed with both high spatial resolution (20 microns) and high temporal resolution (hundreds of time steps) in a single experiment. The resulting data allow the evolution of vertical inhomogeneity within these thick cathode films to be determined during cycling and they reveal a number of unexpected phenomena, such as the continuation of charging at some heights within the cathode during the discharge cycle of the cell. The new availability of comprehensive depth-dependent SOC data will drive the parameterization and advancement of whole-cell models, leading to an improved understanding of large-scale transport phenomena and enhanced capabilities for the rational design of thick electrodes with improved performance.

The kinetics and thermodynamics of solid-state ion exchange of Li + into Na 2 Mg 2 P 3 O 9 N were investigated using in situ synchrotron X-ray diffraction methods for a series of Li-containing salts, allowing systematic trends with general applicability to ion exchange reactions to be elucidated. The collection of data for a wide range of reaction conditions was enabled through the use of a novel multiwell sample environment which leverages the ability of the synchrotron beamline to rapidly (∼1 s) collect high-quality data suitable for monitoring reaction progress. From these data, it was possible to resolve the influence of salt choice and salt concentration on the ion exchange reaction kinetics. Furthermore, by carrying out experiments at different temperatures, initial estimates of activation energies were obtained. It was generally found that the ion exchange reaction kinetics strongly depended on the salt chosen as the ion source rather than being primarily determined by the ion mobility of the ceramic.

Operando synchrotron X-ray diffraction (XRD) studies have not previously been used to directly study Li metal in standard batteries due to the extremely weak scattering from Li atoms. In this work, it is demonstrated the stripping and plating of Li metal can be effectively quantified during battery cycling in appropriately designed synchrotron XRD experiments that utilize an anode-free battery configuration in which a Li-containing cathode material of LiNi0.6Mn0.2Co0.2O2 (NMC622) is paired with a bare anode current collector consisting of either Cu metal (Cu/NMC) or Mo metal (Mo/NMC). In this configuration, it is possible to probe local variations in the deposition and stripping of Li metal with sufficient spatial sensitivity to map the inhomogeneity in pouch cells and to follow Li deposition and stripping with sufficient time resolution to track state of charge dependent variations in the rate of Li usage at a single point. For the Cu/NMC and Mo/NMC batteries, it was observed that the initial plating of Li occurred in a very homogeneous manner but severe macroscopic inhomogeneity arose on a mm-scale during the subsequent stripping of Li, contrasting with the conventional wisdom that the greatest challenges in Li metal batteries are associated with Li deposition.

Operando studies that probe how electrochemical reactions propagate through a battery provide valuable feedback for optimizing the electrode architecture and for mitigating reaction heterogeneity. Transmission-geometry depthprofiling measurements carried out with the beam directed parallel to the battery layers -in a radial geometry -can provide quantitative structural insights that resolve depth-dependent reaction heterogeneity which are not accessible from conventional transmission measurements that traverse all battery layers. However, these spatially resolved measurements are susceptible to aberrations that do not affect conventional perpendicular-beam studies. Key practical considerations that can impact the interpretation of synchrotron depthprofiling studies, which are related to the signal-to-noise ratio, cell alignment and lateral heterogeneity, are described. Strategies to enable accurate quantification of state of charge during rapid depth-profiling studies are presented.

Solid state ionic conduction plays a central role in the functionality of many energy materials, including the cathodes being used in the present generation of battery technologies and the solid state electrolytes being considered for the next generation of batteries. Solid state ion exchange of Li+ into Na2Mg2P3O9N was investigated using in situ synchrotron powder X-ray diffraction. By using a 2D area detector many samples were studied simultaneously in novel high throughput studies of ion exchange reactions. Kinetic rate constants were extracted from the timedependent evolution of lattice parameters. Reactions were followed using a novel on-the-fly Rietveld refinement tool which enabled real time monitoring of reaction progress. The ion exchange rates were found to be limited by the ion transport in the salt rather than the host ceramic. From this data, it was seen that reaction rates substantially varied with salt concentration in a manner than appears to follow a universal scaling relationship. By carrying out experiments at different temperatures, activation energies for reactions could be precisely determined. The origin of the experimentally observed activation energies is being investigated through DFT studies.

Although halide salts such as LiCl and LiBr are routinely used as a source of Li ions during ion exchange reactions, a detailed understanding of the processes controlling the rates of these reactions is presently lacking. Recently, we discovered that the rate-limiting barriers for ion exchange are commonly associated with these salts rather than the ceramic target of ion exchange, making it important to quantitatively understand salt processes. Here, it is demonstrated that in situ synchrotron studies of ion exchange reactions can be used to precisely quantify the thermodynamic activation energies associated with these solid-state reactions in a manner that can be directly compared with predictions from density functional theory (DFT). While the temperature dependence of the LiCl reaction rate is found to be set by a barrier associated with ion hopping, it was discovered that for LiBr, the rate is also affected by the defect formation energy�an energy found to be substantially lower than predicted by DFT. Furthermore, it is shown that by varying the relative amounts of reactants, the resulting change in reaction rate can be used to identify the rate-limiting reagent and to elucidate an overall scaling relationship that controls the concentration dependence of the reaction rate. Also, it is demonstrated that global fits across doped and undoped salts can be used to probe both intrinsic and extrinsic vacancy concentrations. This improved understanding of ion exchange mechanisms can be used to predict reaction conditions that can accelerate ion exchange reaction rates by orders of magnitude. The techniques demonstrated here can be broadly applied to probe the kinetics and thermodynamics of solid-state reactions.