Co-Modified MnWO₄ Nanorods Coupled with H₂O₂-Treated Carbon Nanotubes for a Charge-Balanced Aqueous Hybrid...

A charge-balanced aqueous hybrid supercapacitor was constructed by coupling Co-modified MnWO₄ nanorods with H₂O₂-treated carbon nanotubes (OH-CNTs) in 1 M KOH. The samples were designated by the nominal Co/(Mn + Co) precursor molar fraction; ICP-OES measured bulk Co fractions of 0.09 ± 0.01, 0.47 ± 0.03, and 0.75 ± 0.04 mol% for the nominal 0.1, 0.5, and 0.8 mol% samples, respectively. Rietveld refinement confirmed retention of the monoclinic P2/c MnWO₄ phase with only small composition-dependent lattice changes. At an active-material loading of 2.00 ± 0.05 mg cm^-2, the optimized nominal 0.5 mol% sample delivered 429.5 ± 11.0 C g^-1 (119.3 ± 3.1 mAh g^-1) at 1 A g^-1 and retained 280.5 ± 8.5 C g^-1 (77.9 ± 2.4 mAh g^-1) at 15 A g^-1. Because the positive electrode exhibits battery-type behavior, specific charge and specific capacity are used as the primary performance descriptors. Over -0.9-0 V vs. SCE, pristine CNT and OH-CNT electrodes delivered of 189.0 ± 7.2 and 246.6 ± 8.1 C g^-1, corresponding to electrode-level apparent specific capacitances of 215 ± 8 and 280 ± 9 F g^-1 at 1 A g^-1 after correction for the measured IR drop. These values apply to the stated 90:5:5 CNT (or OH-CNT)/acetylene-black/PVDF formulation and CNT-active-mass normalization and should not be interpreted as intrinsic capacitances of isolated commercial MWCNT powders. Charge matching based on the measured gave m⁺/m^- = 0.574; integration at the actual device loadings yielded q⁺ = 0.861 ± 0.015 C and q^- = 0.854 ± 0.015 C (q⁺/q^- = 1.008). The device operated over 0-1.6 V and delivered 97.9 ± 3.1 F g^-1 at 0.51 A g^-1, corresponding to 34.8 Wh kg^-1 at 408 W kg^-1 when normalized to the combined active mass of both electrodes. Three independently assembled devices retained 96.0 ± 0.7% of the initial capacitance and showed a coulombic efficiency of 98.8 ± 0.1% at the 10,000th cycle at 5.13 A g^-1. Device metrics are normalized to the combined active mass of both electrodes and exclude current collectors, separator, electrolyte, and packaging.