Dynamics and Hysteresis of Hydrogen Intercalation and Deintercalation in Palladium Electrodes: A Multimodal In Situ X-ray Diffraction, Coulometry, and Computational Study

Abstract

Understanding hydrogen intercalation and deintercalation in palladium is the key to utilizing palladium-based materials for hydrogen storage, hydrogen separations, and electrochemical hydrogen evolution and CO2 reduction catalysis. Here, we combine in situ synchrotron X-ray diffraction and coulometry measurements with density functional theory calculations to provide complementary insights on the dynamics of hydrogen intercalation and deintercalation under electrochemical conditions. By employing multimodal in situ characterization, we demonstrate that the interplanar d-spacing and the hydrogen/palladium ratio are decorrelated under certain conditions. Additionally, there is a clear hysteresis in the electrode potentials where the β-phase of palladium hydride forms and disappears. Computed energetics of hydrogen intercalation and deintercalation predict this hysteresis. These calculations indicate that the potential-driven absorption of subsurface hydrogen during intercalation and oxidation of surface hydrogen during deintercalation could contribute to the observed hysteresis. These results suggest that surface processes during hydrogen intercalation and deintercalation are important, providing additional mechanistic understanding that is complementary to bulk phase transition theory. This multimodal in situ characterization and computational study provides new insights into hydrogen intercalation and deintercalation in palladium electrodes, which could lead to improvements in palladium-based materials needed in a sustainable energy economy.

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

“…We hypothesize that these inconsistencies largely stem from the corresponding differences in measurement conditions used in those studies and this work, which we have summarized in Table S3. Beyond the discrepancies in the morphology and size of the nanomaterials already discussed above, the varying electrolyte concentrations (ranging from 0.1 to 1 M) and counter-cations (Na + or K + ) used in those works and herein could also result in differences in the interfacial proton activity that may in turn affect the potential- and time-dependence of the PdH x -formation process . This effect may be aggravated by the convective properties specific to the geometry of the cells used for such studies, which determine the homogeneity and thickness of the diffusion boundary layers, along with the corresponding surface concentration of reactant and products .…”

“…Beyond the discrepancies in the morphology and size of the nanomaterials already discussed above, the varying electrolyte concentrations (ranging from 0.1 to 1 M) and counter-cations (Na + or K + ) used in those works and herein could also result in differences in the interfacial proton activity 82 that may in turn affect the potential-and time-dependence of the PdH x -formation process. 28 This effect may be aggravated by the convective properties specific to the geometry of the cells used for such studies, which determine the homogeneity and thickness of the diffusion boundary layers, along with the corresponding surface concentration of reactant and products. 83 Additionally, while references 7−9 did not provide detailed descriptions of their XAS acquisition procedures, Chang et al 35 used relatively short (150 s) potential holds before XAS acquisition over 450 s, and Lee et al 34 performed their XAS measurements in the course of a linear potential sweep at a scan rate of 0.05 mV s −1 .…”

“…In this context, the main focus has been on the study of the extent of palladium hydride (PdH x ) formation under CO 2 -electroreduction conditions (i.e., at potentials negative of 0 V RHE and in CO 2 -saturated, near-neutral electrolytes). Palladium’s ability to form hydrides at room temperature is well-known, − and the investigation thereof requires measuring the hydride stoichiometry (i.e., the x in PdH x ) as it is commonly done in heterogeneous catalysis − and electrochemistry − using in situ X-ray diffraction (XRD) ,− or X-ray absorption spectroscopy (XAS). ,,,,, Notably, at room temperature, complete β-phase hydride formation in Pd-nanostructures is generally ascribed to hydrogen-to-palladium atomic ratios of x ≥ 0.5, with the exact β-phase stoichiometry strongly depending on particle size. ,, Meanwhile, the pure α-hydride corresponds to a H:Pd content of ∼0.03, and a combination of these α- and β-phases is found for all hydride stoichiometries in between these two extremes. ,,,,, …”

Chemical Limits on X-ray Nanobeam Studies in Water

“…It is therefore reasonable to interpret the observed phase transition as such. The hydriding phase transition has been studied with in situ X-rays previously, both using BCDI during the metal–gas reaction , and in electrochemical environments using less focused X-ray beams. , Hydrogen atoms initially randomly occupy octahedrally coordinated sites in the host lattice, forming the low-concentration phase α-PdH x with x < 0.02 at room temperature. As occupancy increases, the β phase with x > 0.6 becomes stable, coexisting with the α phase to accommodate the total amount of absorbed hydrogen.…”

Chemical limits on X-ray nanobeam studies in water

“…The hydriding phase transition has been studied with in situ X-rays previously, both using BCDI during the metal-gas reaction, 47,48 and in electrochemical environments using less focused X-ray beams. 49,50 Hydrogen atoms initially randomly occupy octahedrally coordinates sites in the host lattice, forming the low-concentration phase α-PdH x with x < 0.02 at room temperature. As occupancy increases, the β phase with x > 0.6 becomes stable, coexisting with the α phase to accommodate the total amount of absorbed hydrogen.…”