Unleashing the power of capacitive deionization: Advancing ion removal with biomass-derived porous carbonaceous electrodes

Capacitive deionization (CDI) is a promising electrochemical technique for the removal and recycling of ions from micro-polluted wastewater but is still hindered by the co-ion expulsion effect and anode oxidation. In this study, these issues are addressed through optimization of both materials and electrochemical systems. A diverse set of porous carbons are prepared using biomass as a precursor and KOH as the activation agent. It is found that direct carbonization and/or KOH activation induce a negative surface charge, whereas intense nitrogen-doping results in an inverse surface charge for all biomass-derived carbons, characterized by the potential of zero charge (Epzc). Density functional theory calculations suggest that the carboxyl group and quaternary N contribute most among other functional groups to the negative and positive charges, respectively. A Epzc-matching asymmetric CDI system is constructed employing negatively charged and positively charged carbons as the cathode and anode, respectively. This configuration, coupled with precise optimization of the cathode-to-anode mass ratio (m-/m+), unlocks a high adsorption capacity of 17.2 mg g−1 for NaCl, surpassing the symmetric system by 84.7 %. Further fine tuning of the m-/m+ ratio results in a removal capacity of 167.4 mg g−1 for Cu2+ ions, which is the highest reported for carbonaceous materials to date.