A H+-inhibiting anion exchange membrane as separator for a pH-decoupling Zn-ion hybrid supercapacitor

Abstract

The pH-decoupling strategy has shown enormous advantages on extending the voltage windows of aqueous electrochemical energy storage devices. Compared to neutral and basic electrolytes, acid electrolytes as catholytes make higher demands on the ion selective permeability of anion exchange membranes for Zn-ion hybrid supercapacitors, which refers to not only the anionic flux but also the inhibiting effect of H⁺ migration. We propose a H⁺-inhibiting anion exchange membrane (AEM-2) as separator for a pH-decoupling Zn-ion hybrid supercapacitor. AEM-1 and AEM-2 can be prepared by the copolymerization of 4-vinyl pyridine and different cross-linking agents (ethylene glycol dimethacrylate and p-divinylbenzene). The chemical structure difference between AEM-1 and AEM-2 accounts for the hydrophilicity of AEM-1 and the hydrophobicity of AEM-2, which are further confirmed by the contact angle measurements. The differences between the aqueous PyNH//Zn (AEM-1) supercapacitor and the aqueous PPyNH//Zn (AEM-2) supercapacitor on cycling stability, reflect the better effect of AEM-2 than that of AEM-1 on inhibiting the electromigration and diffusion of the hydrated H⁺ ions through the AEM. The soft-pack PPyNH//Zn (AEM-2) supercapacitor can provide a gravimetric specific capacitance of 218 F g⁻¹ at 1 A g⁻¹ and an energy density of 51.1 Wh kg⁻¹ at the power density of 0.65 kW kg⁻¹. This novel anion exchange membrane for suppressing H⁺ migration, opens up a new avenue for developing high-performance aqueous electrochemical energy storage devices.