Hydriding kinetic mechanisms of Mg thin films resolved by operando optical microscopy

The kinetic analysis of metal hydride systems under operando conditions is essential for evaluating their performance and stability in a hydrogen-based energy system. In this work, we use operando optical microscopy in combination with theoretical solid-state reaction kinetic models to unravel hydrogen sorption kinetics in a straightforward, accessible, and cost-effective manner. Optical microscopy methods can directly monitor reaction progress by leveraging the pronounced optical contrast arising from the phase transitions exhibited by many metal hydrides. The archetypal system of Mg thin films of 100 nm capped with Pd is employed to validate the correlation between the intensity of transmitted light during hydrogenation and solid-state kinetic models, thereby revealing the rate-limiting steps of the process. Comparison of the obtained results with structural measurements confirms the accuracy of this methodology, allowing reliable determination of activation energies and the time evolution of the rate-limiting mechanism. This work highlights operando optical microscopy as a readily available technique for determining H-sorption mechanisms, offering a straightforward alternative to conventional, indirect, or complex methods while maintaining accuracy and accessibility.