Insights into Charge-Redistribution in Double Layer Capacitors

Abstract

Charge-redistribution (CR) in electrochemical double layer capacitors (EDLCs) manifests as voltage drop during open-circuit relaxation (OCR) after galvanostatic (GS) charging, and voltage recovery during OCR after GS discharging. The complex porous structure of electrodes causes a capacitor to charge/discharge non-uniformly which causes charge-redistribution in OCR. In this work, we have used a macro-homogeneous transport model which incorporates electrolyte concentration dependent conductivity and constant capacitance to analyze CR. The model makes a new prediction that CR occurs over two time scales, a short one, of a couple of seconds, driven by potential gradient across an electrode at the termination of the preceding galvanostatic operation, and a long one, of hundreds of seconds, driven by electrolyte concentration gradients inside and outside pores. The extent of CR at two electrodes differs dramatically. The model also modifies other predictions of constant conductivity based models developed earlier in view of depletion/accumulation of electrolyte in pores. The linear variation of cell potential with log(t), used as a test of self-discharge due to faradaic reaction, is predicted by our transport models for CR itself. The scaling is different for short and long time CR, and sensitively depends on electrolyte conductivity in pores.