Towards more Durable Electrochemical Capacitors by Elucidating the Ageing Mechanisms under Different Testing Procedures

Abstract

Electrical double‐layer capacitors (EDLCs) commonly denoted supercapacitors are rechargeable energy storage devices with excellent power and energy delivery metrics intermediate to conventional capacitors and batteries. High‐voltage aqueous electrolyte based EDLCs are particularly attractive due to their high‐power capability, facile production, and environmental advantages. EDLCs should last for thousands of cycles and evaluation of future cell chemistries require long‐term and costly galvanostatic cycling. Voltage holding tests have been proposed to shorten evaluation time by accelerating cell degradation processes. Whether voltage holding can replace cycling completely remains undemonstrated. In this work, a systematic investigation of the influence of testing procedure on cell performance is presented. The state‐of‐the‐art post‐mortem and operando experimental techniques are implemented to elucidate ageing mechanisms and kinetics inside EDLC cells under different testing procedures. Carbon corrosion occurring on the positively polarized electrode leads to the lower active surface area and higher oxygen content. On the contrary, an increase of surface area and micropore volume are observed on the negatively polarized electrode. Repeated galvanostatic cycles at U <1.6 V appears to facilitate the depletion of oxygen species on the positively polarized electrode in comparison with voltage holding, which indicates a more complex degradation mechanism during cycling. Caution is advised when comparing results from different test procedures.

Combined electrochemical and DFT investigations of iron selenide: a mechanically bendable solid-state symmetric supercapacitor

“…He et al described the fading phenomena associated with voltage floating test extensively in his article. 68 There are similar kind deteriorates in cell performance shown in previous articles. 69,70 The stability of iron selenide electrode was examined by repeated cyclic curves for 4000 cycles recorded through constant 100 mV/s scan rate.…”

Redox Activity of Bromides in Carbon‐Based Electrochemical Capacitors

“…The motivation for this study was the relatively high oxidation potential of the Br À specimen, which might be beneficial for the total cell voltage and capacitance but harmful for the carbon electrode during long-term operation, as it has been reported that carbon oxidation might occur even below the water decomposition voltage. [23][24]61] In fact, initial work demonstrating the application of KBr as a redox-active agent confirmed that the redox activity of Br À is observed at rather elevated capacitor voltages. [62] This might cause the carbon surface to be prone to surficial oxidation and capacitance fade.…”

“…In this paper, we report on the electrochemical performance of a carbon‐based electrochemical capacitor operating with a redox active electrolyte based on a bromide/bromate redox couple in an aqueous electrolyte. The motivation for this study was the relatively high oxidation potential of the Br − specimen, which might be beneficial for the total cell voltage and capacitance but harmful for the carbon electrode during long‐term operation, as it has been reported that carbon oxidation might occur even below the water decomposition voltage . In fact, initial work demonstrating the application of KBr as a redox‐active agent confirmed that the redox activity of Br − is observed at rather elevated capacitor voltages .…”

“…The recent development of electrochemical capacitors is oriented towards novel electrode material design, [6][7][8][9][10][11][12][13] electrolyte formulation, [9,11,[14][15][16][17][18][19][20][21][22] or identification of the ageing mechanisms aimed at their prevention or inhibition. [23][24][25][26][27][28][29][30][31][32] To date, activated carbons, with their well-developed surface area, versatile porosity and tunable properties, seem to be the optimal electrode materials in electrochemical applications. As the specific capacitance depends linearly on the electrode/ electrolyte interface (of course, until the pore saturation point), the high specific surface area seems to be one of the most crucial criteria.…”

“…The recent development of electrochemical capacitors is oriented towards novel electrode material design, electrolyte formulation, or identification of the ageing mechanisms aimed at their prevention or inhibition …”