Valuable insight into the degradation mechanisms, lifetimes and state of health of supercapacitor devices can be obtained through the understanding and quantification of gas evolution during use. In typical commercial cylindrical devices, the hard casing can withstand the pressure build up resulting from the evolution of gases to pressures as high as 8-12 bar, but with detrimental effects on ESR and capacitance observed through the lifetime of device. In typical pouch cells, a small increase in pressure can result in the ballooning of cells and significant loss of performance. This is managed in some cases by the inclusion of pressure release valves and the like.

Although supercapacitors typically produce gas throughout their lifetime, the rate of gas production varies significantly depending on the point in their lifetime and the potentials and charge/discharge rates that they experience. In order to maximise the lifetime and performance of supercapacitor devices, a “burn-in“ period is usually applied, during which time the evolution of gases is likely to be at its highest. After such a burn-in for pouch cells, this gas can be removed with ease before performing a final seal on cells, as is common with battery devices, and can have a significant positive effect on the cell lifetime.

The evolution (and consumption) of gas in supercapacitors can be attributed to various processes including decomposition of carbon-surface functional groups, solvent splitting (hydrogen and oxygen evolution reactions, including decomposition of trace water) and chemical/electrochemical gas consumption^[1,2]. In this work, the rate and amount of gas evolved in pouch cell supercapacitors and its effect on device performance has been studied through the use of calibrated mass load cells. Small format pouch cells (70x50mm) constructed in-house, consisting of YP50-F (Kuraray, Japan) activated carbon electrodes and TEABF4 in acetonitrile electrolyte have been suspended from calibrated load cells into oil and have been subjected to various charge–discharge conditions and floating at different potentials over different temperatures. The buoyancy of the cells as derived from the load cell has been used to calculate the gas evolved at various potentials / conditions. Through observation of the rate and quantity of gas evolved, insight into the optimal burn-in conditions (float potential, time and temperature) for new cells and impact on performance in operation have been observed. Furthermore, the effect of contaminant water concentration in electrolyte on gas production has been investigated at various points in the cell lifetime.

[1] S. Phadke, S. Amara and M. Anouti, Chem Phys Chem., 18, 2364, 2017

[2] M. He, K. Fic. E. Frackowiak, P. Novak and E. J. Berg, Energy Environ. Sci., 9, 623, 2016