New Advances in Proton Conducting Polymer Electrolytes and their Applications in Ultrahigh Rate Solid Electrochemical Capacitors

Keryn Lian - Associate Professor at the Department of Materials Science and Engineering, University of Toronto. \n \nRate and power capabilities are key characteristics of electrochemical capacitors (EC). The ultra-high rate capabilities of up to 200 V/s charge/discharge rate as reported by Pech et al.[1] and 120 Hz filtering as reported by Miller et al. [2] have further broadened their application area in electronics. Both reports attributed the achieved ultrahigh rates to novel electrode materials such as onion-like nanocarbon (OLC) and graphene, while relying on conventional liquid electrolytes. We believe that solid-polymer electrolytes can contribute to high rate energy storage devices and have demonstrated solid EC devices with very promising results. In addition to improving device energy density and power density by reducing packaging, solid-polymer electrolytes acting as separator and ionic conductor can also enable thin, flexible, and light form factors and can provide better environmental safety and stability compared to their liquid counterparts. \n \nSolid-polymer electrolytes comprised of silicotungstic acid (SiWA) in a polyvinyl alcohol (PVA) matrix were developed for ECs. Addition of phosphoric acid has further enhanced the ionic conductivity of SiWA-PVA electrolytes. The proton conductivity of the phosphoric acid modified electrolyte reached >0.02 S/cm with very good stability. Both SiWA-PVA and H3PO4-modified SiWA-PVA electrolytes conducted protons via a hopping mechanism. All-solid ECs utilizing our polymer electrolytes with double layer or pseudocapacitive electrodes have demonstrated ultra-high rate performance. While the solid EC with planer stainless steel electrodes reached 5000 V/s, solid ECs with graphite and pseudocapacitive MoN were able to charge and discharge up to 100 V/s in cyclic voltammetry. These are, to our knowledge, the fastest ever been reported. Thermal and structural characterizations revealed that several measures, such as additives or polymer cross-linking, can further enhance the proton conductivity and stability. For example, the addition of phosphoric acid enhanced the thermal stability of the polymer electrolyte and promoted the retention of crystallized water without altering the bonding structure of the original SiWA-PVA network.

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