Design of polyimide-based separators for effective suppression of self-discharge in non-aqueous electrical double layer capacitors

Abstract

The self-discharge phenomenon is an unavoidable response of electrical double layer capacitors (EDLCs) when the applied voltage is interrupted. How to alleviate this natural behavior is a challenge in the practical usage of EDLCs, especially such a self-discharge phenomenon is demonstrated to be induced by the presence of trace water in the organic electrolyte. This research focuses on the alleviation of the water-induced self-discharge process in non-aqueous EDLCs through the utilization of a polyimide (PI) separator coated with the polyurethane-poly(acrylic acid) (PUPAA) copolymer (denoted as PI@PUPAA). The trace water in the non-aqueous electrolytes is effectively absorbed by the copolymer, leading to the obvious suppression of self-discharge in the EDLC cells. The introduction of Al 2 O 3 nanoparticles into PI to form a composite separator (PI + Al 2 O 3 @PUPAA) substantially improves the electrochemical responses of EDLCs though the ionic conductivity of the PI@PUPAA separator is reduced by the PUPAA coating. This PI + Al 2 O 3 @PUPAA composite has been demonstrated to be a promising separator significantly decelerating the self-discharge process of an EDLC with a comparable charge/discharge performance in comparison with the same cell utilizing the commercial TF4030 separator. • The higher water content of electrolyte is, the more self-discharge of EDLCs is found. • PUPAA absorbing H 2 O from electrolytes suppresses water-induced self-discharge of EDLCs. • PI + Al 2 O 3 @PUPAA is a cost-effective separator reducing self-discharge of EDLCs.