A Hard X-Ray Laboratory XES and XAFS Spectrometer Implementing Asymmetric Bragg Diffractions for High-Resolution Operando Measurements of Ni₂P Nanoparticle Electrocatalysts

X-ray spectroscopy techniques typically reserved for synchrotrons, namely x-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES), have been undergoing a renaissance in the laboratory over the past decade. Advancements in detector technologies and Bragg optic manufacture has enabled high resolution (~1 eV) XAFS and XES spectrometers to be built in the lab. Spherically bent crystal analyzers (SBCAs) are the dominant high-resolution hard x-ray optic both in the ongoing rebirth of laboratory-based and in synchrotron hard x-ray photon-in/photon-out methods. In almost all cases, SBCAs are implemented in a ‘symmetric’ configuration on the Rowland circle, wherein the diffracting crystal plane is nominally coincident with the analyzer wafer surface. The laboratory spectrometer described is a powerful characterization tool capable of <i>in situ</i> and <i>operando</i> measurements across many disciplines while implementing asymmetric Rowland geometries with many benefits.<br/><br/>This work details the following. First, the construction and operation of a laboratory spectrometer specialized in operating with asymmetric Rowland geometries, wherein the diffracting crystal plane is not coincident with the optical surface of the analyzer. Results indicate several benefits of such an instrument, demonstrating increased energy resolution compared to the symmetric counterpart and a massively extended energy range of a single SBCA, enabling a technique we refer to as ‘<i>hkl</i> hopping’. Second, the design and implementation of a low-absorption electrolytic cell optimized for transmission XAFS measurements. This cell enables <i>operando</i> x-ray measurements of catalytic materials in the laboratory due to a clear line of sight to the sample window and very thin low-Z materials to minimize unwanted x-ray absorption outside the sample. Third, the use of such a cell and the laboratory spectrometer for <i>operando</i> measurements of a Ni₂P nanoparticle electrocatalyst. Ni₂P is an earth-abundant material proposed for use as an electrocatalyst for hydrogen evolution, nitrate reduction, CO₂ reduction, and alcohol oxidation. However, Ni₂P is susceptible to oxidation. Several studies on the material and operating cell were conducted with the laboratory spectrometer, measuring the XAFS of the Ni K-edge to monitor changes in oxidation state with respect to changes in pH, air exposure, and applied potential as the cell operates and is measured simultaneously. Measured in the laboratory, synchrotron quality data of nanoparticle stability and assessment of cell performance is made possible, and through a sweeping series of experiments controlling pHs and applied potentials, a pseudo-Pourbaix diagram of nanoparticle Ni₂P can be constructed.