Mechanistic insights into the use of oxide nanoparticles coated asymmetric electrodes for capacitive deionization

Capacitive deionization (CDI) is an emerging water desalination method, which employs high surface area porous electrode materials for electro-sorption of ions. We used an asymmetric CDI cell constructed with alumina and silica nanoparticle (NP) coated electrodes and KCl as a probe electrolyte to gain insights into electro-sorption behavior and elucidate underlying process mechanisms. This CDI system is efficient for use in desalination and up to 15 to 60 mu mol/g (total electrode) sorption capacity was achieved. Higher removal of K+ compared to Cl- was obtained attributable to competition between OH- and Cl-. The presence of NPs not only creates highly accessible surface area but also increases the charge efficiency by shifting the applied potential to a high efficiency range due to protonation/deprotonation occurring on metal oxide surfaces. Data were described using both mechanistic electrical double layer (EDL) based Gouy-Chapman-Stern (GCS) formulation and empirical Freundlich equations. Our results suggest that the presence of metal oxide NPs can effectively modify the isoelectric points and an increase in planar charge efficiency of up to 20% could be achieved. However, global charge efficiency was still severely constrained by backward thermal diffusion and mass transfer limitations. EDL over-lapping effect plays an important role in determining critical pore size for electro-sorption. The GCS model revealed that only BJH associated (pore size: 1.5-50 nm) surface area is effective for electro-sorption. Though the Freundlich equation adequately described the sorption data, we attribute this sorption behavior to reduction in micropore overlapping effects and an increase in mass transfer caused by higher concentration gradients. Our results are expected to guide the design and development of appropriate electrode materials for CDI. (c) 2012 Elsevier Ltd. All rights reserved.