Investigation on pH/temperature-manipulated hydrothermally reduced graphene oxide aerogel impregnated with MgCl2 hydrates for low-temperature thermochemical heat storage

Abstract

A novel reduced graphene oxide aerogel (rGOA)-MgCl2 composite sorbent is developed by impregnation for low-temperature thermochemical heat storage. Salt particles are dispersed on tunable rGOAs via pH values and hydrothermal temperatures. Parameters of salt particles including salt content, size, and crystallinity strongly depend on reduction degree of the rGOAs. High reduction degree of the rGOAs can improve salt content in composite sorbents and the highest salt content reaches up to 97.3 wt%. Moreover, lower reduction degree of the rGOAs is conducive to formation of salt particles with smaller size and lower crystallinity. Therefore, the composite sorbents with lower salt contents possess better sorption kinetics owing to weaker mass transfer barrier and the maximum water uptake is up to 1.16 g/g at 95% humidity under 25 °C. Dehydration temperature of the composite sorbents decreases to varying degrees compared with that of pure MgCl2⸱6H2O. Mass energy density (Qm) and volume energy density (Qv) of the composite sorbents increase with the rise of salt contents. The largest Qm of 2225.7 J/g is 216.5 J/g higher than that of MgCl2⸱6H2O and the largest Qv reaches up to 0.709 GJ/m3. Heat storage capacity of the composite sorbent after 5 dehydration-hydration cycles remains above 95% of the original. All the results indicate that the composite sorbent is a potential candidate for long-term heat storage.