In-situ hydrothermal synthesis of a bifunctional composite material multiwalled carbon nanotubes/cobalt phosphate for heavy-metal ion removal and supercapacitor electrode

Aiming at the drawbacks of existing bifunctional materials for adsorption-supercapacitor applications, including limited adsorption capacity and weak synergistic effect, a hydroxylated multi-walled carbon nanotubes/cobalt-phosphate (MWCNTs/CoTEDA-DAP) bifunctional composite was facilely fabricated via an in-situ hydrothermal strategy for aqueous Hg(II) (divalent mercury ion)/Pb(II) (divalent lead ion) removal and for use as supercapacitor electrodes. The adsorption behaviours towards Hg(II) and Pb(II), including kinetics, isotherms, thermodynamics and selectivity were systematically investigated. Box-Behnken response surface methodology (RSM) was employed to optimise adsorption parameters. Meanwhile, the electrochemical performance of MWCNTs/CoTEDA-DAP (hydroxylated multi-walled carbon nanotubes/cobalt-triethylenediamine-diammonium phosphate coordination phosphate) as supercapacitor electrode material was evaluated. The adsorption processes of Hg(II) and Pb(II) followed the Elovich and pseudo-second-order kinetic models, which demonstrates that chemisorption dominated the adsorption mechanism. The Langmuir isotherm model well described the monolayer adsorption behaviour. Thermodynamic parameters (ΔG < 0, Gibbs free-energy change, ΔH > 0, enthalpy change, ΔS > 0, entropy change) verified that the adsorption was spontaneous, endothermic and entropy-increasing. Under the optimised RSM conditions, the maximum adsorption capacities attained 1345.24 mg/g for Hg(II) and 1521.83 mg/g for Pb(II). When deployed as a supercapacitor electrode, the composite achieved a specific capacitance of 181 F/g at 1 A/g. The synergistic effect originating from chelation of -OH, -NH- and phosphate groups as well as the conductive network of MWCNTs (hydroxylated multi-walled carbon nanotubes) contributes to the desirable bifunctional performance. This study provides a feasible strategy for designing integrated materials toward environmental remediation and energy-storage applications.