Engineering oxygen vacancy in nickel oxalate by Mn-doping for hybrid supercapacitors

Transition-metal oxalates as promising coordination compounds have gained recent attention in supercapacitors. Nevertheless, the unsatisfied charge-storage performance prevents the practical application. Herein, the heteroatom doping strategy was proposed to construct Mn-doped NiC₂O₄ (MnNiC₂O₄) with moderate oxygen vacancies as the positive material towards high-performance supercapacitors. Multiple characterizations revealed that the Mn ions were doped into NiC₂O₄ lattice via the partial substitution of Mn ions at Ni sites together with the generation of oxygen vacancies, where the oxygen vacancy content was tuned by adjusting Mn-doping amount. It was found that the markedly improved charge-storage performance was mainly due to the suitable oxygen vacancies and enriched complementary multi-oxidation states (Ni²⁺/Ni³⁺; Mn²⁺/Mn³⁺). Density functional theory (DFT) calculations confirmed that Mn-doping could narrow the bandgap and promote the Faradaic redox reactions. As a result, the optimal Mn_(0.25)NiC₂O₄ with suitable oxygen vacancies exhibited a high specific charge (634.3C g^-1 at 1 A g^-1) and satisfying cycling stability. The assembled hybrid supercapacitor (MnNiC₂O₄//AC) with the Mn_(0.25)NiC₂O₄ as the positive electrode delivered a high energy density of 36.2 Wh kg^-1 at 800 W kg^-1. This work provides a feasible strategy to apply transition-metal oxalates as electrode materials in energy storage devices.