Electrochemical Loading of Palladium with Hydrogen Is Governed by Ambient Gas Species

We report how gases impact the hydrogen concentration in the palladium metal lattice during electrochemical hydrogen loading. We built a unique <i>in situ</i> X-ray diffraction cell, where one surface of a palladium membrane is electrochemically loaded with hydrogen and the other surface faces a gas flow. Under N₂ and CO₂ gas, rapid phase transformation from α-Pd to β-PdH occurred with moderate H/Pd ratios of 0.63 ± 0.02 and 0.64 ± 0.01, respectively. Under CO gas, the α → β phase transformation was also fast, but the H/Pd ratio increased to 0.752 ± 0.001. In contrast, the O₂ gas induced a more gradual α → β phase transformation, achieving the maximum H/Pd ratio of 0.66 ± 0.03, followed by the reverse β → α phase transformation. Gas chromatography confirmed that the increased H/Pd ratio under CO originates from the suppressed recombination of hydrogen atoms into H₂ gas. Additionally, we found that O₂ reacts with hydrogen on the Pd surface to form water and hydrogen peroxide, which together promote hydrogen removal. These findings demonstrate that electrochemical hydrogen loading of Pd is governed not only by the applied electrochemical potential but also by gas-surface interactions.