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

Abstract

Nanoscale MnO x expressed in 3D carbon nanofoam electrodes exhibits both pseudocapacitive and battery-like charge-storage using a mixed Na ⁺ : Zn ²⁺ aqueous electrolyte.

Abstract

We demonstrate that electrodes comprising nanoscale, birnessite-type manganese oxide affixed to carbon nanofoam paper (MnO x@CNF) exhibit two distinct charge-storage mechanisms—battery-like Zn ²⁺ insertion/de-insertion and pseudocapacitance—when electrochemically cycled in aqueous electrolytes that include both Na ⁺ and Zn ²⁺ salts. When the mixed-electrolyte composition is 0.75 M Na ₂SO ₄ + 0.25 M ZnSO ₄ ( i.e., “6[Na ⁺] : 1[Zn ²⁺]”), the MnO x@CNF electrode delivers high specific capacity at low rates, approaching theoretical capacity for Zn ²⁺ insertion/de-insertion at MnO x. At high rates (>10C) the Na ⁺-supported pseudocapacitance mechanism maintains charge-storage capacity well above that observed with electrolytes that contain only ZnSO ₄. 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[Zn ²⁺] electrolyte, the potential-independent pseudocapacitance is augmented by reversible Zn ²⁺-based redox processes between 1.4 and 1.8 V vs. Zn/Zn ²⁺. Galvanostatic testing with two-electrode zinc-ion cells that pair MnO x@CNF with a zinc foil negative electrode proves the practical performance advantages of combining pseudocapacitance and Zn ²⁺-insertion mechanisms: higher energy efficiency and greater specific power in the 6[Na ⁺] : 1[Zn ²⁺] electrolyte compared to 1 M ZnSO ₄.