Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg−1 based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30–50 Wh kg−1 is possible with optimization.
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机译：电化学双层电容器具有高功率和长循环寿命，但与电池相比具有较低的比能，从而限制了应用。氧化还原增强型电容器通过使用氧化还原活性电解质来提高比能，该电解质在充电过程中在正极被氧化，在负极被还原。在这里，我们报告几种氧化还原电解质的特性，以说明操作/自放电机理和高性能设计规则。我们发现基于电极和电解质的质量，无需使用离子选择膜分离器即可产生约14 Wh kg -1 的高比能的甲基紫精（MV）/溴化物电解质。用庚基紫精代替MV可提高稳定性，在20,000个循环内不会降解。由于带电状态的氧化还原对吸附在活性炭上，因此自放电率低，与具有惰性电解质的电池相当。电化学模型可以再现实验结果，并预测通过优化可以达到30–50 Wh kg -1 。
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