In situ X-ray diffraction investigation of NiS_x -formation on Ni-foam using chemical vapor deposition with H₂S

Nickel foam treated via chemical vapor deposition (CVD) with H₂S has demonstrated potential in applications such as supercapacitors and catalysis for alkaline water electrolysis. However, the formation mechanism of the nickel sulfide surface layer remains poorly understood. In this study, in situ powder X-ray diffraction (PXRD) was employed to identify the crystalline phase transformations and the reaction mechanism and assess its kinetics. Ni₃S₂ formation was investigated under industrially relevant conditions by passing 3% H₂S/Ar through Ni foam and tracking the growth of the sulfided layer in relation to thickness and time. The reduced sulfidation rate observed at low flow, extended time, and greater depths indicated strong mass transfer limitations, whereas the pronounced increase between 90 and 170 °C revealed the high activation energy of the sulfidation process. A diffusion-reaction model is proposed to describe the spatial and time evolution of the Ni₃S₂ layer growth, assuming that H2S diffuses through the newly formed Ni₃S₂ layer before reacting at the Ni interface. The modelling results indicate that both the reaction and diffusion occur at fast rates and compete in the temperature range of 130-170 °C. Post-synthesis SEM and tomography analysis confirmed improved uniformity in nickel-sulfide layer thickness and extrusion coverage when the process is reaction limited rather than diffusion limited: either at synthesis temperatures below 130 °C or a higher flow rate at 130 °C. On the other hand, higher temperatures promote the formation of large NiS_x extrusions. These results provide insight into the effect of the synthesis parameters on the microstructure and the formation of Ni₃S₂ and NiS_x , providing fundamental physico-chemical and transport properties for process optimization and upscaling.