Proton-Conducting Polymer Electrolytes for Solid Flexible Supercapacitors

Polymer electrolytes, acting as separator and ionic conductor, are key enablers for the next generation flexible supercapacitors. An ideal high-performance polymer electrolyte should exhibit: (i) high ionic conductivity; (ii) good ion accessibility at the electrode\electrolyte interface; (iii) wide electrochemical stability window; and (iv) high environmental and temperature stability. Although the conductivity of polymer electrolytes is typically a few orders of magnitude lower than that of their liquid counterparts, deploying them in the form of thin film can mitigate this issue and provide high rate and power performance in a supercapacitor device.

A typical solid polymer electrolyte consists of an ionic conductor, a polymer matrix, and additives. Heteropolyacids (HPAs) are excellent solid-state proton conductors at room temperature [1]. One of the common HPAs is silicotungstic acid (SiWA, H₄SiW₁₂O₄₀•nH₂O).

We have developed a polymer-in-salt electrolyte system for supercapacitors using SiWA and polyvinyl alcohol (PVA). A systematic approach has been used to improve the performance of SiWA-PVA polymer electrolytes through additives and polymer structural modifications. The electrolytes showed excellent proton conductivity, stability, and film flexibility that enables thin and light weight solid supercapacitors (Fig. 1). They also outperformed Nafion^® in terms of environmental stability at ambient conditions [2].

These SiWA-based polymer electrolytes have demonstrated high rate performance in both electrochemical double layer capacitors and pseudo-capacitors [3-5]. The solid polymer electrolyte-based devices were able to charge and discharge up to 100 Vs^-1, with a time constant of 10 ms [3-5]. To further expand the potential window of the SiWA-based electrolyte, and thus increase the energy density, alternative HPAs have been synthesized. In this talk, proton conductivity and structural properties of the developed polymer electrolytes will be discussed. Device performance of different HPA-based solid supercapacitors will be presented and compared.

References:

1. U. B. Mioè et al., Solid State Ionics, 176, 3005-3017 (2005).

2. H. Gao et al., Electrochem. Commun., 17, 48-51 (2012).

3. H. Gao and K. Lian, J. Mater. Chem., 22, 21272-21278 (2012).

4. H. Gao and K. Lian, J. Power Sources, 196, 8855-8857 (2011).

5. H. Gao et al., J. Power Sources, 222, 301-304 (2013).

  Fig. 1. Photographs of (a) a HPA-based polymer electrolyte film and (b) an assembled flexible solid supercapacitor.