Combining battery-like and pseudocapacitive charge storage in 3D MnO: X @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 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 2 SO 4 + 0.25 M ZnSO 4 (i.e., "6[Na + ]:1[Zn 2+ ]"), the MnOx@CNF electrode delivers high specific capacity at low rates, approaching theoretical capacity for Zn 2+ 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 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 MnOx@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 .