Poly-p-phenylene as a novel pseudocapacitive anode or cathode material for hybrid capacitive deionization

Hybrid capacitive deionization (HCDI), which consists of one Faradaic and one carbon electrode, has recently developed as a high-efficiency and environmentally friendly water desalination technique. The advancement of HCDI, however, is hampered by a scarcity of high quality Faradaic materials. Here we demonstrated that con-ducting poly-p-phenylene (PPP) with a coplanar molecular structure of the extended pi-conjugated skeleton can be used as a reversible electrode material to capture either cations (Na+, K+, Ca2+) or anions (Cl-, F-, NO3-, and SO42-) from various salt solutions. This is benefited from its broad potential window and n-/p-doping charac-teristics. The desalination performance of PPP was evaluated systematically with the constructed HCDI cells in different electrolytes, which can deliver excellent salt adsorption capacity, charge efficiency, and long-term cycling stability. Further, based on complex capacitance analysis, the kinetic features and mechanism for the ion insertion-adsorption processes were further elucidated, which offers both a new perspective for under-standing the salt removal performance of PPP and a practical method for investigating other pseudocapacitive intercalation materials.