Using in Situ X-Ray Diffraction and Coulometry to Understand the Intercalation of Hydrogen into Palladium Thin Films

Because of its ability to readily intercalate hydrogen, palladium-based materials are widely studied as hydrogen-selective membranes, hydrogen-storage materials, and catalysts for various hydrogenation reactions. Because the intercalation is reversible, in situ characterization is necessary to understand the physical and chemical properties of palladium hydride. However, most previous studies of palladium hydride focus on a relatively narrow potential range, and the behavior of palladium hydride at more negative potentials relevant to reactions such as CO2 reduction or N2 reduction remains relatively unexplored. Here, we use the combination of synchrotron X-ray diffraction and coulometry measurements to study the absorption of hydrogen into palladium at significantly more negative potentials. X-ray diffraction measurements indicate three phases: palladium, α-PdHx with the lattice expanded by less than one percent, and β-PdHx with the lattice expanded by more than four percent. Concurrent coulometry experiments allow for a quantitative measure of how much hydrogen is stored in palladium in each of these phases. While these two measurements agree quite well near the transition from α-PdHx to β-PdHx, at the most negative potentials we studied, we observe a contraction in the lattice of the film by X-ray diffraction without a corresponding change in the coulometry measurements. This result highlights the need to employ multimodal characterization techniques when studying dynamic phenomena in electrodes, as a single technique may be insufficient to fully elucidate the electrodes’ behavior. Finally, we combine X-ray diffraction with cyclic voltammetry to explore the behavior of palladium hydride with greater time resolution. This measurement reveals clear hysteresis between hydrogen absorption and desorption, emphasizing how the history of the sample affects the degree of hydrogen intercalation. These results provide insight into hydrogen intercalation in palladium electrodes which could lead to improvements in palladium-based materials as electrocatalysts and hydrogen-storage media. Furthermore, this technique can also be applied to study other systems under in situ conditions, such as electrocatalysts for renewable energy transformations.