Two principal branches of experiments were carried out for the duration of the project. On the one hand, sample characterization and electrochemical studies were done in the host institution, the Leiden Institute of Chemistry. This included the electrochemical analysis of single crystal electrodes and electrochemical impedance spectroscopy (EIS) measurements. On the other hand, the work was taken to synchrotron facilities where more specialized experiments were run during pre-proposed beamtimes. These beamtimes were necessary to run X-ray photoelectron spectroscopy (XPS), total electron yield X-ray absorption spectroscopy (TEY-XAS), and surface X-ray diffraction (SXRD) spectroscopy.
EIS study was carried out in alkaline media for a broad range of Pt single crystals. Comparison of flat and stepped Pt surfaces in the hydrogen underpotentially deposited (HUPD) region revealed a step-dependent double layer capacitance CDL. In addition, the CDL was only affected by a change in cation concentration if in the presence of steps. Correlated to a lack of cation effect on the rate of the alkaline hydrogen evolution reaction (HER), we identified the cooperative role of steps and cations as a beneficial effect on the stabilization of adsorbed water toward its further reduction to H2. Further study of the Pt(111) at the onset of the HER indicated that the cation played an indirect role on the HER. While the CDL was insensitive to cation concentration changes, the charge transfer resistance representative of the kinetics of the hydrogen atom adsorption and desorption was found to be significantly faster in the presence of more cations. Such a distinct response to cations from the rate of HER suggested that as previously theorized, the HUPD is not an active intermediate for the formation of H2. Instead, the overpotential deposited hydrogen (HOPD) is the key reactant in the HER. Parallel work using SXRD to compare the least (Pt(111)) and most (Pt(110)) stepped surfaces in alkaline media further supported the hypotheses formulated from EIS measurements. The crystal truncation rod (CTR) analysis of the SXRD results indicated that a change in cation concentration or the applied potential does not lead to any further increase of cation occupancy at the electrolyte-electrode interface for Pt(111). Meanwhile, Pt(110) displayed a drastic change in its diffraction pattern. Compared and fit with a CTR model, these potential-dependent changes of diffraction seem to correspond to an increase in the cation occupancy near the Pt surface. In addition, cations appeared to sit much closer to the Pt(110) surface than Pt(111). This apparent affinity between the cations and the two different crystal surfaces confirmed the likelihood of a cooperative effect previously posited. Confirmation of a lack of cation occupancy for the Pt(111) suggests that the change in the kinetics of the hydrogen adsorption observed with different cation concentrations is principally a result of a change outside of the outer Helmholtz plane.
The dip-and-pull geometry was combined with XPS and TEY-XAS on a polycrystalline Au foil in different aqueous electrolytes. The optimization of this method was recorded in a comprehensive report providing the necessary guidelines for the scientific community to use it in their study. Of importance, the type of sample was identified as a key limit for the application of the method. For the dip-and-pull approach to be successful, very special care had to be taken in the formation, monitoring, and stabilization of the electrolyte thin film. This project led to identifying, through modeling and data treatment, the key indicators to achieve a measurement where the obtained spectra are indeed representative of the double layer composition. Similarly, the optimization of the conditions and parameters for the TEY-XAS to be viable is compiled into a technical report that will allow the field to not only obtain information about the quantitative changes in the cation concentration in the double layer, but also to distinguish meaningful changes in the coordination environment of the ions present in the double layer. The characterization of electrolyte species with both XPS and TEY-XAS is a significant achievement of this project.
