Phosphorylated carbon cathodes achieve perfect monovalent cation selectivity in capacitive deionization

Selective sodium removal from water sources is crucial for irrigation and other applications, yet challenging due to the abundance of sodium ions compared to essential minerals like calcium and magnesium. Capacitive deionization (CDI) is an emerging electrochemical technique for water treatment and desalination which uses a small electrical potential to adsorb ions in the double layers of microporous electrodes. Given the micropore size is of the same order of magnitude as hydrated ions, CDI offers potential for ion differentiation based on properties such as charge, radius, diffusion constant, and more. Herein, the development of monovalent selective CDI through surface modification of activated carbon cathodes with phosphoric acid groups is described. Cathode phosphorylation induces negative surface charges within the micropores, resulting in enhanced absorption of smaller sodium ions and no calcium uptake during CDI. By applying short, alternating charge–discharge cycles, this membraneless system achieves perfect monovalent cation selectivity with low energy consumption (as low as 0.28 kWh m⁻³), removing sodium from the water without reducing the calcium concentration.