Pseudocapacitive Behaviors of Polypyrrole Grafted Activated Carbon and MnO₂ Electrodes to Enable Fast and Efficient Membrane-Free Capacitive Deionization

Abstract

Capacitive deionization (CDI) has emerged as a promising technique for brackish water desalination. Here, composites of polypyrrole grafted activated carbon (Ppy/AC) were prepared via <i>in situ</i> chemical oxidative polymerization of pyrrole on AC particles. The Ppy/AC cathode was then coupled with a MnO₂ anode for desalination in a membrane-free CDI cell. Both the Ppy/AC and MnO₂ electrodes exhibited pseudocapacitive behaviors, which can selectively and reversibly intercalate Cl^- (Ppy/AC) and Na⁺ (MnO₂) ions. Compared to AC electrodes, the specific capacitances of Ppy/AC electrodes increased concurrently with the pyrrole ratios from 0 to 10%, while the charge transfer and ionic diffusion resistances decreased. As a result, the 10%Ppy/AC-MnO₂ cell showed a maximum salt removal capacity of 52.93 mg g^-1 (total mass of active materials) and 34.15 mg g^-1 (total mass of electrodes), which was higher than those of conventional, membrane, and hybrid CDI cells. More notably, the salt removal rate of the 10%Ppy/AC-MnO₂ cell (max 0.46 mg g^-1 s^-1 to the total mass of active materials and 0.30 mg g^-1 s^-1 to the total mass of electrodes) was nearly 1 order of magnitude higher than those in most previous CDI studies, and this fast and efficient desalination performance was stabilized over 50 cycles.