Invited: Water Based AC/AC Supercapacitors Reaching the Performance of Systems in Organic Electrolyte

Abstract

It has been long time accepted that capacitors with activated carbon (AC) electrodes in aqueous electrolytes do not deliver a high energy due to their low voltage, e.g., 0.7 – 0.8 V in KOH and H2SO4. Recently, we have demonstrated that the voltage window can be dramatically enhanced by using salt solutions with pH close to 7 [1,2]. In that case, the over-potential for di-hydrogen evolution at the negative electrode is very high [3], and the voltage window can be extended up to 1.5 V with stainless steel current collectors [4]. In this presentation, we introduce a number of strategies to optimize the performance of these systems.First, the corrosion of the positive current collector has been reduced by adding an inhibitor (sodium molybdate) to the electrolyte. Besides the capacitance of the positive electrode is slightly enhanced, allowing its maximum potential to be reduced. In these conditions, the system displays an excellent cyclability, up to 1.6 V, without remarkable oxidation of the positive AC electrode [4].The second improvement is based on using the advantages of the pseudo-capacitance provided by the carbon/iodine interface, while keeping the large potential window allowed by alkali salt solutions. An AC/AC capacitor in the KI – Li2SO4 mixture displays capacitance values up to 400 F g-1 at voltage of 1.6 V [5]. These values lead to an energy density which is comparable to the one reached with the same carbon in Et4NBF4/acetonitrile at 2.3 V.Finally, we have explored various routes to extend the operational temperature range to low values. We have designed a formulation of aqueous lithium sulfate electrolyte allowing the AC/AC capacitor system to operate down to -40°C with excellent charge propagation (Figure 1). At this temperature, the pseudo-faradic contribution due to hydrogen storage is kinetically blocked, and the system consequently works as a typical EDL one.In summary, AC/AC capacitors in aqueous alkali sulfate solutions with appropriate additives are now able to reach the performance of organic systems, while using environmental friendly and cost effective components. Acknowledgements: The Foundation for Polish Science is acknowledged for supporting the ECOLCAP Project realized within the WELCOME Program, co-financed from European Union Regional Development Fund.