Activated carbons for flow electrode capacitive deionization (FCDI) – Morphological, electrochemical and rheological analysis

Abstract

Flow electrode capacitive deionization (FCDI) is a desalination technology employing flowable carbon slurries to remove salt from an influent through the electro-sorption of ions at the surface of pores of activated carbon particles. This study presents an extensive morphological, electrochemical and rheological analysis of flow electrodes prepared using commercial (YP50F, YP80F, Norit, PAC) and lab-synthesized (KUA, PAC-OX) activated carbons. Simultaneous optimization of particle size, surface area, and surface chemistry of activated carbons is essential to enhance desalination efficiency in FCDI applications. The lab-made highly microporous activated carbon (KUA), prepared from a Spanish anthracite, exhibited a remarkably high specific surface area (~2800 m²/g) but required first a particle size reduction through ball milling (from 56 μm to 12 μm) for the respective flow electrodes to achieve flowability. The slurry of milled fine KUA (designated as KUAF) shows a specific capacitance of 55 F/g, a 38-fold increase compared to its pristine form. The KUA-F flow electrode also achieved a maximum salt adsorption capacity of 185 mg/g, outperforming the leading commercial alternative (YP80F) by 26 %. The FCDI cell with the KUA-F flow electrode exhibited a desalination efficiency of 79 % at 15 wt% loading, surpassing YP80F by 29 %. In contrast, using PAC-OX (oxidized form of PAC), despite increasing oxygen functional groups and with relatively higher specific surface area, led only to a 2 % improvement in desalination performance, highlighting that oxidation alone at larger particle sizes and broader distribution is insufficient.