Ammonium and Tartrate Salts as Alternatives to Neutral Aqueous Electrolytes for Supercapacitors

Supercapacitors are promising energy storage devices in terms of power density and lifetime. Organic electrolytes are frequently applied in commercial supercapacitor devices. However, their water-based counterparts are much safer, more sustainable and cost-effective. In this study we therefore present, for the first time, aqueous tartrate-based electrolytes (sodium tartrate / ammonium tartrate) for supercapacitor applications, and relate them to well-known inorganic aqueous electrolytes like Na2SO4. Additionally, the influence of the cation on the electrochemical performance of supercapacitors is investigated using sodium and ammonium cations for comparison. We demonstrate the electrochemical performance and physicochemical properties of ammonium tartrate / sulfate and sodium tartrate / sulfate. An improvement of the conductivity in the range of 40-60 % was achieved by the exchange of sodium cation with ammonium cation. Carbon electrodes in newly introduced aqueous tartrate-based electrolytes deliver high specific capacitances up to 117 Fg-1. Furthermore, electrical double layer capacitors (EDLCs) containing 1 M ammonium tartrate display a high energy density at 0.1 Ag-1 and at 10 Ag-1 (9.88 Whkg-1 and 1.14 Whkg-1, respectively). Floating tests show excellent long-term performance. Tartrate-based EDLCs retain >80 % of their initial capacitance at 1.6 V cell voltage (120 h floating time). In the case of ammonium tartrate electrolyte, a novel metal-free and non-toxic concept for an eco-friendly supercapacitor device is proposed.