National University of Singapore researchers devise membrane-based supercapacitors; possible new route to high-performance supercapacitive energy storage
|(a) Chemical structure of the PEDT:PSSH polymer blend. (b) Formation of hydrated ionic conduction channels in the PEDT:PSSH film network due to the presence of the hydrophilizing SO3− groups. Xie et al. Click to enlarge.|
A team from the National University of Singapore's Nanoscience and Nanotechnology Initiative (NUSNNI), led by principle investigator Dr. Xian Ning Xie, has developed a polystyrene membrane-based supercapacitor that they say will be easier to scale up than the current alternatives. Unlike more conventional supercapacitor electrode materials with large surface areas and high porosities, the new hydrophilized polymer network uses ion-conducting channels for fast ion transport and charge storage.
When sandwiched between and charged by two metal plates, the membrane can store charge at 0.2 farads per square centimeter, well above the typical upper limit of 1 microfarad per square centimeter for a standard capacitor. They reported on their work in a paper published earlier this summer in the Journal of Polymer Science Part B.
Conventional electrode materials for supercapacitors are based on nanoscaled structures with large surface areas or porosities. This work presents a new electrode material, the so-called hydrophilized polymer network. The network has two unique features: 1) it allows for high capacitance (up to 400 F/g) energy storage in a simple film configuration without the need of high-surface-area nanostructures; 2) it is unstable in water, but becomes extremely stable in electrolyte with high ionic strength.
The above features are related to the hydrophilizing groups in the network which not only generate hydrated ionic conduction channels, but also enable the cross-linking of the network in electrolyte. Because of its practical advantages such as easy preparation and intrinsic stability in electrolyte, the hydrophilized network may provide a new route to high-performance supercapacitive energy storage.—Xie et al.
The polymer membrane includes PSSH (poly(styrene sulfonic acid)). PSSH is an excellent hydrophilizer due to its high density of sulfonic SO3 hydrophilizing groups, the team notes in their paper. The incorporation of SO3 groups allows for the formation of hydrated paths for enhanced ionic conduction in proton-conducting fuel cell membranes.
In the PEDT:PSSH network presented...the hydrophobic PEDT forms a water-insoluble framework, while the superhydrophilic PSSH provides SO3 groups for hydration channel formation throughout the framework. Due to the ion-conducting channels, the network facilitates fast ionic transport and charge storage, and thus is a promising electrode material for supercapacitors.—Xie et al.
Use of the membrane could also reduce the cost. With existing technologies based on liquid electrolytes, it costs about US$7 to store each farad. With the advanced energy storage membrane, the cost to store each farad falls to US$0.62. This translates to an energy cost of 10-20 Wh per US dollar for the membrane, as compared to just 2.5 Wh per US dollar for lithium ion batteries.
Compared to rechargeable batteries and supercapacitors, the proprietary membrane allows for very simple device configuration and low fabrication cost. Moreover, the performance of the membrane surpasses those of rechargeable batteries, such as lithium ion and lead-acid batteries, and supercapacitors.—Xian Ning Xie
The research is supported by grants from the Singapore-MIT Alliance for Research & Technology (SMART) (Ignition Grant ING10022-ENG(IGN)), and National Research Foundation. Dr Xie and his team started work on the membrane early last year and took about 1.5 years to reach their current status, and have successfully filed a US patent for this invention.
The team is currently exploring opportunities to work with venture capitalists to commercialize the membrane.
Xian Ning Xie, Junzhong Wang, Kian Keat Lee, Kian Ping Loh (2011) Supercapacitive energy storage based on ion-conducting channels in hydrophilized organic network. Journal of Polymer Science Part B: Polymer Physics Volume 49, Issue 17, pages 1234–1240 DOI: 10.1002/polb.22295