Mechanism of Cu2+ selective adsorption by copper sulfide in hybrid capacitive deionization with hydrophobic binder

To address the demands for heavy metal wastewater treatment and copper resource recycling, hybrid capacitive deionization (HCDI) technology offers advantages of low energy consumption and ion selectivity. However, the practical application of HCDI is limited by the insufficient adsorption performance and stability of electrode materials. In this study, flower-like copper sulfide (CuS) was synthesized via a hydrothermal method and used as CDI electrodes, with polyvinylidene fluoride (PVDF) and polyvinyl butyral/polyvinylpyrrolidone (PVB/PVP) as binders. The effects of the two binders on electrode structure and performance were investigated, focusing on elucidating the selective adsorption mechanism of CuS as an HCDI cathode. The CuS-PVDF electrodes exhibited superior conductivity and higher Cu 2+ adsorption capacity (551.49 mg·g −1 at 1.0 V), attributed to the polar stabilization effect of rigid molecular chains and low surface energy of PVDF, which facilitated efficient utilization of CuS active sites. In contrast, the PVB-PVP binder caused shielding of active sites on CuS, and delayed charge transport due to encapsulation by flexible polymer chains. Moreover, the CuS electrodes exhibited selective adsorption of Cu 2+ in the HCDI system. The selectivity of Cu 2+ exceeded 70 % in a mixture of multiple heavy metal ions. This selective mechanism can be attributed to pK sp (CuS) = 40.3 and redox-mediated phase transformation (CuS→Cu 2 S/Cu 2 O/Cu).