Periodic Reporting for period 1 - Double layer (Spectroscopic investigation of the electrochemical interface for sustainable electrocatalysis)
Periodo di rendicontazione: 2023-04-01 al 2025-03-31
Developing our tools to achieve a better future through well-defined science has often been the approach of our ever-developing society. Today, a better future is synonymous with favoring a more renewable and cleaner pathway toward large-scale energy production and consumption. This project is presented as one of the many ways toward this goal, that is, through an improved understanding of how to convert electricity into and from renewable feedstock. This understanding relies on our knowledge of the interface where electrons are transferred, typically from an electrode to a reactant, itself often contained in a liquid electrolyte. A principal objective of this project lies in developing a characterization method suitable for probing changes at this electrified solid-liquid interface. Upon optimization of the characterization tool, a subsequent objective of the project was to extract information about the physical and chemical interactions defining the dynamics of the interface. Ultimately, drawing from these results, the project aims to provide guidelines on accounting for those interactions to better steer electron transfers towards the desired reaction with improved efficiency.
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.
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.
The distinction between the role of the effective potential, the ions, the water molecules, and the electric field defined by the first three will be of utmost importance in the future of electrochemical interface design. The findings of this project strongly point towards the necessity to modulate the electrode-electrolyte interface as a whole. The cations can affect the reactant through their interactions with the steps of the electrode. The K+ cations can pack differently at potentials negative of the potential of zero charge in the presence of Cl- or ClO4- anions. For electrochemical conversion to occur efficiently, facilitating the charge transfer to and from the outer Helmholtz plane through an engineered local electrical field might be key to the future of electrocatalysts’ development. To this end, future work should focus on further functionalizing the electrode surface with charged molecules and purposefully modulate the surface and interfacial charge to maximize the rate of a desired reaction.