Strategies for tuning hierarchical porosity of 3D rGO to optimize ion electrosorption

In order to further understand the impact of porous characteristics of the electrode materials on their electrosorption performance, we have conducted a systematic investigation on the cause effect relationship among various parameters, such as precursor concentrations, 3D rGO packing densities and out-of-plane porosities. Firstly, when the precursor concentrations for 3D rGO were changed from low to high concentrations, sample rGO-4 with medium GO concentration showed a dual pore size distribution with higher numbers of mesopores as well as the less stacking of the rGO sheets, demonstrated better electrosorption specific capacitance of 236 F g^-1 than the low and high GO concentrations. Secondly, when the out-of-plane hierarchal pores were created by employing sacrificing nanocrystal-template crystals α-Fe₂O₃, sample T-rGO displayed higher micropore volume and the more active sites of the hierarchal pores, while its 3D graphene structure remained highly interconnected, both facilitated better ion access and achieved a specific capacitance of 409 F g^-1. In capacitive deionization (CDI) experiments, sample T-rGO demonstrated a fast salt adsorption and desorption cycle and achieved 100% complete desorption/regeneration within an average of 59 min and a higher electrosorption capacity of 7.3 mg g^-1. This study not only provided insights on the impact of rGO packing density on the electrosorption behavior, but also confirmed that creating additional out-of-plane nanopores by partial removal of template crystals post hydrothermal synthesis was a promising strategy to prepare porous electrode material for CDI.