Combining battery-like and pseudocapacitive charge storage in 3D MnOx@carbon electrode architectures for zinc-ion cells

Abstract

We demonstrate that electrodes comprising nanoscale, birnessite-type manganese oxide affixed to carbon nanofoam paper (MnOx@CNF) exhibit two distinct charge-storage mechanisms—battery-like Zn²⁺ insertion/de-insertion and pseudocapacitance—when electrochemically cycled in aqueous electrolytes that include both Na⁺ and Zn²⁺ salts. When the mixed-electrolyte composition is 0.75 M Na₂SO₄ + 0.25 M ZnSO₄ (i.e., “6[Na⁺] : 1[Zn²⁺]”), the MnOx@CNF electrode delivers high specific capacity at low rates, approaching theoretical capacity for Zn²⁺ insertion/de-insertion at MnOx. At high rates (>10C) the Na⁺-supported pseudocapacitance mechanism maintains charge-storage capacity well above that observed with electrolytes that contain only ZnSO₄. Impedance analysis was performed to discriminate between these distinct charge-storage mechanisms by visualizing the frequency- and potential-dependent capacitance as 3D Bode plots. In the 6[Na⁺] : 1[Zn²⁺] electrolyte, the potential-independent pseudocapacitance is augmented by reversible Zn²⁺-based redox processes between 1.4 and 1.8 V vs. Zn/Zn²⁺. Galvanostatic testing with two-electrode zinc-ion cells that pair MnOx@CNF with a zinc foil negative electrode proves the practical performance advantages of combining pseudocapacitance and Zn²⁺-insertion mechanisms: higher energy efficiency and greater specific power in the 6[Na⁺] : 1[Zn²⁺] electrolyte compared to 1 M ZnSO₄.