Hydrogen storage properties of Mg–Cr2O3 nanocomposites: The role of catalyst distribution and grain size

Nanocrystalline Mg–x wt.% Cr2O3 (x = 5–20) composites were prepared by ball milling Mg and nanocrystalline Cr2O3 under toluene. The Cr2O3 used (particle/crystallite size of 80 nm) was prepared by combustion synthesis of chromium nitrate and sucrose. Grain sizes of Mg and Cr2O3 in the composites, based on XRD peaks, were in the range of 32–37 and 76–90 nm, respectively.Absorption/desorption kinetics of these composites at 100, 200 and 300 °C were measured. In all cases absorption rates increased with Cr2O3 content at each temperature studied and absorption capacity decreased with decrease in temperature. Maximum hydrogen absorption at 300 and 200 °C was in Mg–5 wt.% Cr2O3 composite whereas at 100 °C it was in Mg–15 wt.% Cr2O3 composite. The higher absorption capacity of Mg–15 wt.% composite at 100 °C was probably due to smaller interparticle distance of Cr2O3 on Mg surface/grain boundary. All the composites exhibited a large absorption in the initial few seconds at all the temperatures. This has been attributed to adsorption aided by the catalyst, and subsequent surface and grain boundary diffusion. Desorption was incomplete in all the composites at 300 °C and only marginal at 200 and 100 °C. Desorption rates at 300 °C increased with increase in Cr2O3 content in all the compositions, while the desorption capacity increased with increase in Cr2O3 content till 15 wt.% and decreased with further increase in Cr2O3 content. The catalytic effect of Cr2O3 was only in absorption process and does not seem to have as large an influence on desorption process.