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

Abstract

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.

Water‐based Supercapacitors with Amino Acid Electrolytes: A Green Perspective for Capacitance Enhancement

“…[1] Importantly, EDLC electrolytes must also ensure a wide electrochemical stability window (ESW) to exhibit purely capacitive behavior at high operating voltages, [2,3,4] being the energy density proportional to square of the applied voltage. Extended ESWs are typically achieved with organic electrolytes, or highly concentrated aqueous (e. g., salt-in-water) electrolytes, [5,6] which hamper parasitic water electrolysis reac-tions. Despite organic electrolytes are commonly established in commercially available EDLCs, their aprotic polar solvents (e. g., acetonitrile) are moderately toxic and flammable materials.…”

“…[14] To enhance the ESW of traditional EDLCs, ionic liquid-based electrolytes (e. g., EMI-TFSI, EMI-BF 4 and BMI-PF 6 ) and deep eutectic solvent-based electrolytes have been reported as effective options, even though their cost and/or toxicity still challenge their practical implementation. [5,6,15] The use of a highly concentrated aqueous electrolyte (e. g., salt-inwater) reduces the number of free water molecules, lowering the water activity and, thus, increasing the potential of the oxygen evolution reaction (OER). [16] Despite this strategy increases the ESW of EDLCs, water dipoles already engaged in the hydration shell of the ions are less reactive to the external electric field, resulting in aqueous electrolytes with low e r .…”

“…In fact, these molecules have excellent water solubility (up to 1 : 5 molar ratio [24,25] ) even in presence of additional salts, with a documented salting-in effect up to 3 M. [26] As electrolyte, we first investigated an aqueous solution of 10 m of L-proline + 2 M NaNO 3 , which was compared with a 2 M NaNO 3 water solution as reference nearly pH-neutral aqueous electrolyte. [5] As a second option, we tested zwitterionic L-lysine at 10 m concentration in water without additional salts. L-lysine is a peculiar case among the 20 proteogenic amino acids because at its isoelectric point (pH � 9.7) a relevant fraction of molecules is ionized (12 % Lys + and 12 % Lys À ) and only 76 % of the species are in the zwitterionic neutral form.…”

“…In depth electrochemical characterization was carried out on aqueous EDCLs based on this electrolytes and electrodes made of graphene:activated carbon active materials. [5,27] A theoretical model is then proposed to interpret the electrochemical results, providing insight on the design of novel types of high-e r aqueous electrolytes.…”

Overview of the oxygen vacancy effect in bimetallic spinel and perovskite oxide electrode materials for high-performance supercapacitors

“…13,14 Traditionally, activated carbon has been employed as the default active material in designing SC electrodes and can show electrochemical double-layer capacitive behavior (EDLC) based on the physical absorption of ions. [15][16][17][18][19][20][21] The general utilization of binders in the production of electrodes suppresses the growth of the specific surface area of the active materials, often limiting SC performance in energy storage systems. 7,22 The main properties (i.e., E d and P d ) used to distinguish traditional capacitors, SCs, batteries, and fuels are shown in the Ragone plot (Fig.…”