New kinetic rate theory explains slow-down of hydriding kinetics near critical points and phase transformations
Researchers at Hamburg University of Technology and at Helmholtz-Zentrum Hereon have developed a new theoretical framework that explains a long-standing puzzle in hydrogen-solid interactions: why hydrogen uptake in metals can slow down dramatically near phase transformations, even when diffusion inside the material is extremely fast. The results have been published in Physical Review Letters.
Hydrogen absorption in solids is a key process in energy storage, electrochemical actuation, and adaptive materials. Hydrogen typically enters the crystal lattice as an interstitial solute. While the equilibrium thermodynamics of this phenomenon is well understood, theory struggles to explain why the kinetics of hydrogen uptake can slow down by orders of magnitude near critical points or phase transformations. Rationalizing the observation by conventional kinetic rate laws, based on the Butler-Volmer equation, faces multiple challenges. Ultimately, one touches on the question whether the established rate laws are inherently compatible with the principle of microscopic reversibility, a cornerstone of statistical mechanics.
The study in Physical Review Letters derives a physically consistent rate equation for hydrogen insertion. The theory describes each phase as an internally equilibrated, ergodic ensemble of particles, and it derives the insertion flux from the statistical-mechanics-based probability of finding the ensemble in a transition configuration. The insertion rate than naturally exhibits microscopic reversibility.
As a key result, the hydrogen chemical potentials in the two phases enter the rate law explicitly and separately, rather than only through their difference. This makes the theory naturally compatible with established equations of state for hydrides that undergo phase transformations. On that basis, the theory explains experimental observations, including the slow-down in hydrogen charging and the pronounced asymmetry of hydrogen uptake kinetics on different sides of a critical composition.
 |
Beyond resolving a fundamental question in materials physics, the results are highly relevant to the Cluster of Excellence BlueMat: Water-Driven Materials. Hydrogen electrosorption from aqueous electrolytes is a prime example of how water-mediated chemical processes can actively control materials properties. Hydrogen insertion into nanoporous metals is known to induce strong changes in elastic moduli, enabling reversible switching between stiff and highly compliant states.
By identifying the kinetic bottleneck that governs hydrogen exchange near phase boundaries, the new framework provides a crucial link between hydrogen chemistry, phase stability, and mechanical response. This insight supports BlueMat’s overarching goal: to understand and exploit water-driven chemical processes as control parameters for designing adaptive, responsive materials.
The work was carried out as preparatory research within the context of the Cluster of Excellence BlueMat, funded by the Deutsche Forschungsgemeinschaft (DFG) under Germany’s Excellence Strategy.