Inverted Capacitive Deionization with Highly Enhanced Stability Performance Utilizing Ionic Liquid-Functionalized Carbon Electrodes

Capacitive deionization (CDI) has attracted widespread attention as an emerging desalination technology. The co-ion repulsion of the anode has a serious impact on the cycle stability of CDI performance. Here, we proposed a new strategy to prevent co-ion repulsion by developing an inverted capacitive deionization (i-CDI) cell using carbon electrodes with completely different charged functional groups. Commercial activated carbon (AC) was treated with nitric acid (N-AC) to introduce the negatively charged COO- on the N-AC electrode. The positively charged AC (P-AC) was obtained by further modification of N-AC with ionic liquid (1-aminopropyl-3-methylimidazolium bromide) (APMIB) to graft positively charged imidazolium groups on the surface of P-AC. Through multiple characterizations, compared with the pristine AC electrode, the charged functional group layers were grafted on the modified electrode surface, which greatly enhanced the wettability and ion transportation performance of the electrodes. An asymmetric i-CDI cell with N-AC as the anode and P-AC as the cathode, where the anode can spontaneously adsorb cations and the cathode can spontaneously adsorb anions in the salt solution, was obtained. The i-CDI cell exhibits a great salt adsorption capacity (SAC) of 10.3 mg g(-1) at similar to 1.4/0 V in 500 mg L-1 NaCl solution, which is comparable to that of a conventional CDI cell with AC as the anode and cathode (11.2 mg g(-1)). However, the i-CDI cell displays better cycle stability and charge efficiency. The present work provides a novel method of preparing i-CDI cell electrodes, which enlarges the use of the i-CDI cell for stable and efficient desalination applications.