Synthesis of Superhydrides Using Pressure and Electrochemistry
Superhydrides of main group or rare earth elements have received a great deal of attention in recent years due to the prediction and observation of superconductivity at our near room temperature at pressures approaching two megabars (200 GPa). Compounds containing a large excess of hydrogen, such as LaH10, are a realization of the prediction that a metal hydride could exert a chemical “pecompression” and promote metallization and perhaps also superconductivity at pressures below the that required for the transition of pure hydrogen to the monoatomic, metallic, superconducting state.
Such experiments are especially challenging due to the synthesis pressures required, which approach the pressures at which the onset of superconductivity is observed. In new theoretical/computational work, CDAC Director Russell Hemley (University of Illinois – Chicago) and CDAC collaborators at Carnegie-Mellon University propose a synthesis method for superhydrides that combines high pressure with an electrochemical potential provided by an electrode inside the pressure cell. The electrode is designed to surpress the evolution of hydrogen and at the same time allow the loading of hydrogen by modulating the activity of mobile protons from the electrolyte.
As an example, the calculated activity required to synthesize palladium hydrides as a function of pressure is illustrated in a Pourbaix diagram (Fig. 1). Although current experimental work has shown that PdH10 would be difficult if not impossible to synthesize even at multimegabar pressures with laser heating, the pressure-potential method suggests that modest pressures of about 0.1- 1.0 GPa would be needed to synthesize this material in an electrochemical environment. Through a judicious choice of electrolyte that could suppress the hydrogen evolution reaction further, PdH10 could be stable at even lower pressures with a more negative electrode potential. This approach may be extended to other superhydride systems, such as La-H, Y-H and Mg-H.
Guan, P.-W., R. J. Hemley and V. Viswanathan, Combining pressure and electrochemistry to synthesize superhydrides. Proceedings of the National Academy of Sciences USA, 118, e2110470118 (2021).
Figure 1. Stability fields of palladium hydrides within the electrochemical potential - pressure space. The potential is defined relative to the reversible hydrogen electrode (RHE). The lower dashed line represents the potential of the hydrogen evolution reaction (HER) taking into account the overpotential related to absorption of hydrogen.