Electrochemical Cell for Operando Grazing-Incidence X-ray Absorption Spectroscopic Studies of Low-Loaded Electrodes.

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X-ray absorption spectroscopy (XAS) is a powerful technique that provides information about the electronic and local geometric structural properties of newly developed electrocatalysts, especially when it is performed under operating conditions (i.e., operando). However, the large amounts of catalyst typically needed to achieve sufficiently high spectral quality and temporal resolution can result in working electrodes of several micrometers in thickness. This can in turn lead to an inhomogeneous potential distribution across the electrode, delamination, and/or incomplete utilization of the catalyst layer (CL), as well as to the (partial) shielding of the CL with electrochemically evolved bubbles trapped within its pores. These limitations can be tackled by performing such spectrochemical measurements with low-loaded (and thus thin) electrodes, which call for the acquisition of XAS spectra in fluorescence mode and using an X-ray beam incidence angle of ≤0.1° with regards to the working electrode's substrate plane in a grazing-incidence (GI) configuration. Thus, in this work, we introduce a new spectroelectrochemical flow cell that allows one to perform such measurements in this GI mode and verify its functionality by tracking the potential-induced formation of palladium hydride (PdHx) in a Pd nanoparticle-based electrocatalyst. A time resolution of 10 s per spectrum was achieved with a very low Pd-loading of only 30 μgPd/cm2. Moreover, the implementation of an ion-conductive membrane to separate the working- and counter-electrode compartments enables the quantification of reaction products, which, in the case of gaseous species, can be detected in a time-resolved manner by means of mass spectrometry. Chiefly, this allows us to determine the electrocatalytic activity and selectivity of a given material in the same cell configuration used for the spectroscopic measurements and assures a reliable comparison among the results derived from both techniques.