Coconut shell derived activated biochar–manganese dioxide nanocomposites for high performance capacitive deionization

Abstract

In this study, a Faradaic/electric double-layer hybrid capacitor using coconut shell derived activated biochar (AB) and MnO2 nanocomposite was fabricated for capacitive deionization (CDI) applications. Three different co-precipitation methods including indirect, direct, and acid-assisted grafting oxidation methods were explored to produce the AB-MnO2 nanocomposites. The α-MnO2 with an excellent ion intercalation ability was successfully deposited onto the AB, making the AB-MnO2 an excellent electrode material for CDI application in the presence of neutral Na2SO4 electrolytes. Notably, the AB-MnO2 nanocomposites prepared by indirect co-precipitation method (AB-MnO2-indirect) has shown a high specific surface area of 304 m2 g−1 with high mesopore volume ratio, high capacitance retention, good hydrophilicity and suitable pore texture to shorten the diffusion distance of ions. Cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) spectra showed that the AB-MnO2 yields a quasi-rectangular and symmetric CV plot, and specific capacitances of 410–523 F g−1 at 5 mV s−1 were observed. Moreover, the AB-MnO2-indirect exhibits excellent CDI performance at 1.2 V, and the specific electrosorption capacity of 33.9–68.4 mg g−1 was obtained. Results of this study demonstrate that AB-MnO2 nanocomposites are superior electrochemical materials for CDI application, which can serve as alternative materials for designing an innovative electrochemical technology for water purification, desalination, and energy storage.