Two worlds, one goal
The story behind our most recent paper in the Proceedings of the National Academy of Sciences, begins on the Memorial Day weekend of 2019, when a perspective paper with the title, Revisiting the cold case of cold fusion, was published. My close collaborator, Yet-Ming Chiang, was part of a team that re-investigated conditions for increasing hydrogen content in palladium. Incidentally, a week earlier, in Nature, a group of researchers had demonstrated superconductivity at 250K in lanthanum hydride under extreme pressures. Rus Hemley reached out to the author group of the cold-fusion perspective and informed us of the developments in superhydrides and their role in superconductivity, and this sparked our wonderful collaboration.
We began with the following curiosity-driven question: given the interest in loading hydrogen into palladium, could we make superhydrides in palladium? Superhydrides have an unusually high content of hydrogen (typically over 6:1 ratio) and possess many interesting properties as a result. I discussed this idea with my postdoc, Pin-Wen Guan, who had extensive expertise on modeling states of materials under various conditions. The initial search was to find out how much hydrogen could be loaded into palladium by applying an electrochemical driving force, “electrode potential” and whether such superhydrides could be stablized in palladium, the cathode of the Fleischmann–Pons experiments.
This proved to be very challenging to realize, since hydrogen has a very limited solubility in metals under normal conditions (pressure), and adding too much hydrogen makes the hydride thermodynamically unstable and decompose. In the Pd-H system, the H/Pd ratio is typically not larger than 1. Previously, Yet worked on electrochemically loading hydrogen into Pd achieving a H/Pd ratio close to unity. However, we found that, it was tricky to be able to synthesize superhydrides with the competing hydrogen evolution reaction. The question then naturally arose: could we combine electrochemistry and pressure?
Having this general idea in mind, we explored the field of high-pressure electrochemistry. This field had been largely unexplored with very limited theoretical and experimental exploration. The two fields of electrochemistry and high pressure to synthesize hydride materials, had developed almost in parallel with little overlap. We believed merging these two fields could bring even more fascinating results, and set out to pioneer some theoretical works that could drive more experimental efforts towards this challenging but promising field. Combining pressure and electrochemistry to synthesize superhydrides would be a great proof-point for pressure-electrochemistry. In October 2020, Ranga Dias and his team demonstrated room-temperature superconductivity in superhydride with carbon and sulfur at several hundred GPa pressure. Given the confluence of these two worlds, combining pressure and electrochemistry offered the huge promise and a new pathway to stabilizing superhydrides at moderate pressures and electrode potential.
We then started our march into such a vast, untamed but attractive hinterland behind two perpendicular coastal lines called “electrochemistry” and “pressure” respectively, with our professional guide in pressure, Rus Hemley. The adventure was interesting but full of difficulties. In some cases, there were no existing ways and we needed to remove the obstacles and dig a way to pass through. There were many unknowns, chief among them being the candidate crystal structures of hydrides stable at these conditions of pressure and electrochemistry. Pin-Wen overcame these technical difficulties successfully, by utilizing his expertise and creating or adapting necessary tools. Finally, we found something shining in front! We found that a palladium superhydride can be thermodynamically stabilized at several hundreds of MPa at a suitable electrode potential by kinetically suppressing the competing hydrogen evolution reaction, often referred to as the leaky bucket problem. Such a pressure is still thousands of times higher than atmospheric pressure, but orders of magnitudes lower than the GPa pressures previously used for synthesizing superhydrides. We found similar results in several other metal superhydrides, confirming the generalizability of our approach, which we called 𝒫² (pressure-potential).
The prospects shown by the 𝒫² approach are very exciting. This allows a new method for controlling material synthesis, extending the stability range of some known materials requiring extreme conditions previously, but also making it possible to synthesize new materials that cannot be made using pressure or electrode potential alone. These “hidden” phases may have interesting physics and useful properties to be discovered, with low-energy nuclear reactions and superconductivity being only two examples. The trio consisting of composition, pressure and electrochemistry will bring numerous exciting discoveries and applications. Some exotic phenomena requiring extreme conditions previously such as superconductivity can be realized in more moderate conditions. In the future, room-temperature superconductors at modest pressures may be created along this pathway. The sciences of mechanics and electricity have been established for hundreds of years, but the synergetic use of the great forces from these two basic fields to design and create matter has been largely ignored. Now it is time to combine them.
[This article was co-written with Dr. Pin-Wen Guan.]
