Graphene-based aerogels with carbon nanotubes as ultrahigh-performing mesoporous capacitive deionization electrodes for brackish and seawater desalination

Abstract

As water scarcity has become a serious global issue, capacitive deionization (CDI) with high energy efficiency and cost effectiveness is considered a promising desalination technique to address this problem and produce potable freshwater. In this work, we rationally designed nanomaterials with three-dimensional porous architectures that have been an urgent need for CDI application. We strategically prepared graphene-based novel ‘hybrid’ aerogels interspersed with multi-walled carbon nanotubes (Gr-MWCNT) and their nitrogenated analogs (N-Gr-MWCNT) as mesoporous CDI electrodes using a facile hydrothermal synthesis method. The three-dimensional monolith aerogels show (a) topologically interconnected network of graphene nanosheets (GNS) with, (b) hierarchical in-plane porosity with uniform strut size having, (c) higher specific surface area, (d) good electrical conductivity, and (e) wettability, resulting in efficient salt ion electrosorption capacity and pathways for ion transportation which allowed achieving ultrahigh desalination performance during CDI process. We also studied CDI electrodes made from holey graphene (hG) of narrow pore size and precursor graphene oxide (GOfilm) for comparison. As compared with other nanocarbons, threedimensional graphene aerogels and hG feature a homogeneous nanoscale porosity, reasonable defects density and carbon edge sites besides moderate specific capacitance. The significant electrosorption capacity determined from isotherms ranged 20.1–22.5 mg g–1 at cell potential 1.8 V in 0.5 g L–1. This could be attributed to the interconnected mesopores within the graphene sheet and carbon nanotube network that allows larger accessibility for ion adsorption. The results elucidate the interplay of mesoscale and nanoscale porosity, high chemical surface charge, oxygen and nitrogenated surface functional groups that are favorable for significantly improved CDI performance, by buffering ions to reduce the diffusion distance from saline water to the interior surfaces/interfaces of these ‘hybrid’ aerogels.