Low Temperature Performance of MnOx@Carbon Nanofoam-Based Ecs with Neutral pH-Based Aqueous Electrolytes

Abstract

Electrochemical capacitors (ECs) comprising high-surface-area carbon electrodes and nonaqueous electrolytes offer many attractive performance attributes including long cycle life and the ability to operate at low temperatures (–40⁰C), yet their energy density is ultimately limited by reliance on the double-layer charge-storage mechanism. Asymmetric EC configurations that include pseudocapacitive materials (e.g., transition metal oxides) offer the opportunity to increase energy density while also using safer and cheaper aqueous electrolytes. We have shown that with the appropriate electrode structure and electrolyte additives, the additional charge stored via pseudocapacitance is delivered at EC-like rates over 1000 cycles with minimal capacitance fade. To further prove the practical feasibility of such aqueous-electrolyte ECs, we turned our attention to low-temperature performance, focusing on aqueous electrolyte compositions containing lithium or sodium cations and sulfate or nitrate anions. We characterized pertinent bulk properties (e.g., freezing point and ionic conductivity) of down-selected electrolytes and found that the anion dominates the freezing point, with sulfate-based electrolytes having a freezing point of –45oC, while nitrate-based electrolytes have a freezing point of –35oC. The ionic conductivities ranged from 63 to 136 mS cm-1 at 20oC, conductive enough to support high-rate operation. To assess the impact of temperature on performance as a function of electrolyte composition, we fabricated symmetric pouch cells comprised of MnOx@carbon nanofoam electrodes, evaluating the resulting devices via cyclic voltammetry and electrochemical impedance spectroscopy over a wide temperature range (25⁰C to –45⁰C). All ECs exhibited a gradual decrease in capacitance as the temperature decreased; however, capacitance was recovered upon warming to 25°C for all electrolyte compositions. We found that ECs with Li2SO4-based electrolytes (neat or mixed composition) were operational at temperatures as low as –35°C, revealing that these electrolytes are competitive with the nonaqueous electrolytes used in conventional double layer capacitors.