High-energy density aqueous supercapacitors: The role of electrolyte pH and KI redox additive

Supercapacitors (SCs), including the most established electrochemical double layer capacitors (EDLCs), are energy storage systems that can be charged in the second timescale, while sustaining a great number of re-charge cycles without losing efficiency. Undoubtedly, their major drawback is their insufficient energy density compared to batteries. Meanwhile, the reduction of the SC costs using cheap and sustainable electrolytes is also a trivial criterion to be considered in the competition race of the energy storage technologies. In this work, we report an extended characterization of aqueous SCs, screening acidic, neutral and alkaline electrolytes, as well as the addition of KI as a prototypical redox additive, and performing both two- and three-electrode configuration measurements. By using near-neutral electrolytes, our aqueous EDLCs can reach a maximum cell voltage superior to 2 V, enabling energy densities higher than 18 W h kg−1 (comparable or approaching those of lead acid and Ni–Cd batteries) at a power density up to almost 7 kW kg−1 (significantly superior to those of competing battery technologies). The introduction of redox additives can significantly increase the capacity of the SCs. However, compared to EDLCs, both the cell voltage and the energy efficiency of the SCs decrease because of partially irreversible faradaic redox reactions and overpotentials of kinetically limited redox reactions. While debunking the myth that aqueous SCs exhibit low energy density, our study also remarks the importance of adequately assessing aqueous SCs, showing the current challenges of advanced SC architectures alternative to EDLCs.