Bimetallic Tungstate Nanoparticle-decorated-lignin Electrodes for Flexible Supercapacitors

Bimetallic tungstates (M2M1WO4; M = transition metal) are promising candidates for electrochemical energy applications. However, the effects of the secondary metal (M2) on the pseudocapacitance behavior of bimetallic tungstates have not been well understood. We report, for the first time, the effect of a secondary metal (Ni) on the pseudocapacitance of a bimetallic tungstate (NiWO4/CoWO4 composite called as NiCoWO4 hereafter) when used in a quasi-solid-state supercapacitor. Nanoparticles of NiCoWO4 and CoWO4 were prepared using a wet chemical synthesis technique and characterized using SEM, XPS, XRD, EDX, and XPS. Lignin/NiCoWO4//AC and lignin/CoWO4//AC supercapacitors were electrochemically tested using Electrochemical Impedance Spectroscopy, cyclic charge–discharge, and cyclic voltammetry. After 2000 charge–discharge cycles, the bimetallic tungstate (NiCoWO4) functionalized lignin supercapacitor shows a specific capacitance (862.26 mF cm−2, 96.12% retention) that is 141 times that of the monometallic tungstate (CoWO4) functionalized lignin supercapacitor. The lignin/NiCoWO4 supercapacitor has very high power and energy densities of 854.76 kW kg−1 and 5.75 W h kg−1, respectively, due to a synergistic effect of bimetallic tungstate nanoparticles encapsulated in lignin. The supercapacitor design explored detailed aspects of composite electrode constituent optimization, and the effects of discharge time, lignin carbonization, and cathode material on the supercapacitor performance. For an optimal mass ratio of lignin : NiCoWO4 : PVDF (15 : 75 : 10), the retention was 100% even after 2000 cycles. In addition to the cathode material's permittivity and surface area, the supercapacitor's electrochemical performance heavily depended on the dominant charge storage regime: an electric double-layered capacitor or pseudocapacitor dominant regime. This work provides new knowledge to design bimetallic tungstate based high-performance bioelectronics for advanced green technology.