Combining pressure and electrochemistry to synthesize superhydrides

Recently, superhydrides have been computationally identified and subsequently synthesized with a variety of metals at very high pressures. In this work, we evaluate the possibility of synthesizing superhydrides by uniquely combining electrochemistry and applied pressure. We perform computational searches using density functional theory and particle swarm optimization calculations over a broad range of pressures and electrode potentials. Using a thermodynamic analysis, we construct pressure–potential phase diagrams and provide an alternate synthesis concept, pressure–potential (P2), to access phases having high hydrogen content. Palladium–hydrogen is a widely studied material system with the highest hydride phase being Pd₃H₄. Most strikingly for this system, at potentials above hydrogen evolution and ∼ 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH₁₀). We predict the generalizability of this approach for La-H, Y-H, and Mg-H with 10- to 100-fold reduction in required pressure for stabilizing phases. In addition, the P2 strategy allows stabilizing additional phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.

Hydrides are a large class of materials containing hydrogen, the lightest and most abundant element in the universe. They have attracted much research interest due to their scientific significance and numerous applications. As important hydrogen storage media (1), they are able to store hydrogen at densities higher than that of liquid hydrogen (2). They also find applications in hydrogen compressors (3), refrigeration (4), heat storage (5), thermal engines (6), batteries (7), fuel cells (8), actuators (9), gas sensors (10), smart windows (11), H₂ purification (12), isotope separation (13), alloy processing (14), catalysis (15), semiconductors (16), neutron moderators (17), low-energy nuclear reactions (18), and recently possible high-temperature superconductors with a critical superconducting temperature T_c in the vicinity of room temperature in hydrogen-rich materials under pressure (19–38).In the late 1960s, Neil Ashcroft (19) and Vitaly Ginzburg (20) independently considered the possibility of high-temperature superconductivity in metallic solid hydrogen at high pressure. Later, the idea of chemical precompression was proposed in which chemical “pressure” is exerted to form hydrogen dominant metal hydrides stable at lower pressures (21). Following the successful prediction (22, 23) and confirmation (24) of very high T_c superconductivity in H₃S, near–room-temperature superconductivity was predicted (25, 26), synthesized (27), and discovered (28) in the superhydrides (defined as MH_n, for n > 6) in the La-H system. Later, comparable T_c values were observed experimentally for other La-H (29), Y-H (30–32), and La-Y-H (33) superhydrides, and room-temperature superconductivity was also reported in the C-S-H system (34). In addition, even higher T_c s have been theoretically predicted, such as Li₂MgH₁₆ with T_c as high as ∼ 470 K at 250 GPa (35).High pressures are needed to synthesize superhydrides (38). One major reason is that at lower pressures, the thermodynamic stability of superhydrides is weakened or no longer exists. To overcome such a challenge, it is obvious that more processing variables need to be introduced in addition to chemical composition and pressure. A processing variable that has been largely hidden is the electrical potential when utilizing electrochemistry for synthesis, which has been used in synthesizing palladium hydride at ambient pressure (39). In the present work we show that the synergetic use of pressure and electrical potential can dramatically extend the thermodynamic stability regime of superhydrides to modest pressures, an approach we term P2. This approach opens more opportunities for the creation of superhydrides and other materials by combining pressure and electrochemical loading techniques. We begin by outlining the general thermodynamic framework. We then apply the approach to the Pd-H system, where we also present density functional theory (DFT) predictions of palladium hydrides under pressure. This is followed by predictions for other metal hydride systems and then a discussion of the broad implications.