Small-Angle Neutron Scattering Characterization of SrCl₂-ENG Composites for Thermochemical Heat Storage

This work presents an in situ nanoscale structural characterization of a SrCl₂-expanded natural graphite (ENG) composite during ammonia absorption and desorption using small-angle neutron scattering (SANS) together with X-ray powder diffraction and sorption measurements. For the processing of the composite material SANS patterns, we developed and implemented two methods, which showed comparable results. The study allowed following the evolution of the SrCl₂ particles and the nanopores inside the particles during five sorption cycles. The structural changes were compared to the absorption and desorption kinetic measurements, allowing us to make qualitative analysis of the impact of the structural changes on the material properties, such as thermal conductivity and permeability. It was shown that the structural evolution of the composite material did not affect the desorption rate but significantly influenced the absorption rate after the first cycle. We also observed a significant improvement of the absorption kinetics due to the formation of nanopores in the fully deammoniated sample. In addition, the ENG matrix was shown to hinder the agglomeration of the SrCl₂ particles during sorption processes, which is in contrast to literature findings reported for a nonsupported metal halide. The findings presented in this study can be of great interest in the research areas where SrCl₂-ENG composites are widely studied, i.e., heat storage, heat pumps/refrigerators, deNO_x removal, and solid-state ammonia storage.