Originating from the demand for obtaining depth‐resolved magnetization profiles from thin films and heterostructures, polarized neutron reflectometry (PNR) has developed into a unique research tool, which also finds application in the analysis of superconducting or soft matter thin films. While certain in situ sample environments such as gas‐loading or humidity cells were quickly realized after PNR first emerged, preparing and growing thin magnetic films directly in the neutron beam could only be realized in recent years. Herein, a dedicated insight is given on the history and development of in situ thin film growth capabilities for PNR, from early pioneering experiments to the present day. The scientific and technological challenges as well as the advances of neutron sources, neutronics, and data treatment that have led to its realization are highlighted together with the unique research opportunities that it provides and recently obtained experimental results.

We present a study on the effect of externally applied elastic strain on the catalytic activity of metal films in the context of hydrogen evolution reaction (HER) and Oxygen evolution reaction (OER). Thin metal films supported on elastic substrates are uniaxially strained in compression and tension while they participate in the HER and their catalytic activity is measured through shifts in the cyclic voltammograms. We show that elastic strain tunes the catalytic activity in a controlled and predictable way; for each metal considered here, compressive and tensile strains have the opposite effect on the catalytic activity; also, the changes in the catalytic activity scale with the strain magnitude within the range of strain values accessed in our experiments. The experimental results show that Pt and Ni films show increased HER under compressive strain; while Cu's HER activity is retarded by compressive strain. The opposite was observed under tensile strain. The experimental observations are understood by considering the influence of elastic strain on hydrogen binding energy, which has been calculated through density functional theory (DFT). Compressive strain increases the hydrogen binding on Ni, shifting it towards the volcano peak, while tensile strain has the opposite effect. However, the same strains have the opposite effect on Cu since it is located on the other side of the volcano peak. By isolating elastic strain from the ligand effect, this study provides a better understanding of the processes that control electrocatalytic activity towards HER and can guide design of strained core-shell nano-particle catalysts.