Enhancing capacitive deionization performance with charged structural polysaccharide electrode binders

Capacitive deionization (CDI) performance, as measured by salt adsorption capacity (SAC) and energy normalized adsorption of salt (ENAS), is frequently limited by anion repulsion at the positive electrode. In this work, we investigate the ability to prevent co-ion repulsion by increasing complementary fixed charged within the electrode macropores by binding composite CDI electrodes with the ionically charged structural polysaccharides chitosan and carboxymethyl cellulose. When employing asymmetrically charged electrode binders, co-ion repulsion was prevented, resulting in SAC and ENAS values that were three times greater than composite electrodes bound with polyvinylidene fluoride (PVDF) and similar to CDI electrodes composed of chemically modified carbon. Polysaccharide binders did not modify the charge balance in the carbon micropores but did shift the discharge voltage of maximum adsorption, enabling a shift in operating voltage that prolonged cycle lifetime without a significant loss in performance. The mechanism of improved salt accumulation with polysaccharide binders was explored with a one-dimensional model that integrated CDI and ion-exchange membrane covered (MCDI) sub-units. Model simulations indicate that carbon macropores covered with thin layers of charged polysaccharides increase adsorption by a sequential accumulation and release of salt to depleted uncovered pores.