Investigating the Electrochemical Absorption of Hydrogen into Palladium Nanoparticles

Abstract

The apparent simplicity of palladium hydride systems hides complex interactions between hydrogen atoms and the palladium crystalline lattice. Tailoring these interactions is primordial for many industrial and research applications, as the properties of the hydride vary greatly with the concentration of hydrogen. In the field of electrocatalysis, the fine control of the hydrogen concentration may open the way to higher selectivity towards reactions of interest such as carbon dioxide reduction or nitrogen reduction reactions. Recently, our group investigated the electrochemical insertion of hydrogen into Pd nanoparticles (NPs), and more specifically the influence of the insertion potential, insertion duration, temperature and Pd NPs diameter on the hydrogen concentration, reported as H/Pd ratio. Electrochemical hydrogen stripping appeared as a very efficient method to study the Pd hydride composition, and our study is supported by an original model to separate the contributions coming from hydrogen absorption, hydrogen adsorption and hydrogen oxidation, often neglected in the literature. Among the main results, it was found that (i) hydrogen absorption reaches a plateau at about -0.1 VRHE; (ii) an equilibrium between H insertion and desorption is reached after a few seconds for Pd NPs and longer insertion durations do not lead to increase in the insertion rate; (iii) temperatures around 30 – 40 °C are optimal for maximizing the H insertion in Pd NPs, likely due to an optimal balance between the H adsorption rate, and the H diffusion rate into Pd; and (iv) the maximum concentration of H inserted into Pd varies linearly with the Pd NPs diameter in the range of 3.8 – 21.5 nm. Our investigation was carried out in both a three-electrode setup (RDE) with an acidic electrolyte (0.5 M H2SO4) and a proton pump setup (PP) with a proton exchange membrane. The latter, despite being rarely employed in the literature, allows unique investigations in a broad voltage range, i.e. <-0.5 V, and brings insight into the behaviour of catalyst materials in a representative real application device. Figure 1