(Energy Technology Division Graduate Student Award) Designing Polyoxometalate-Carbon Hybrid Materials for Supercapacitor Electrodes

Abstract

This talk will provide an overview of our recent work in the development of supercapacitor electrode materials based on the modification of carbon substrates (MWCNTs and Activated Carbons) with custom designed polyoxometalate (POM) molecular coatings. POMs undergo fast reversible multi-electron transfer reactions, an ideal characteristic for electrochemical energy storage. When immobilized on a carbon substrate these POM molecules provide enhanced charge storage capacity due to their faradaic reactions, while the carbon material contributes mechanical support, improved conductivity, and physical capacitive effects. Different POM molecules such as PMo12O40 3- (PMo12) and PW12O40 3- (PW12) demonstrate characteristic electrochemical activity related to their chemical composition. However, we have discovered that when these molecules are combined in aqueous solutions they do not just physically mix, but instead react spontaneously to form PMo12-xWx mixed addenda chemistries with unique electrochemical properties different from that of individual PMo12 or PW12. We have also demonstrated that this mixture behaviour can be extended to additional POM combinations such as SiMo12O40 4- (SiMo12) and SiW12O40 4- (SiW12). These novel POM combinations have electrochemical properties that can be easily tuned based upon the compositions of the mixtures. This control over POM redox behavior can be used along with layer-by-layer (LbL) deposition to design molecular coatings that demonstrate desired pseudocapacitive characteristics. The best performance was achieved with a coating that superimposed a 3:1 PMo12O40 3−-PW12O40 3− mixed layer on a 1:1 GeMo12O40 4−-SiMo12O40 4− mixed layer, which resulted in a 5X capacitance enhancement over unmodified MWCNT. This dual layer electrode also demonstrated a close to rectangular CV profile due to the overlapping redox features of the POM combination (Figure 1). In addition to our work on the POM active layers, our current research on the development of alterative carbon substrates will also be discussed. While MWCNT is an effective substrate for POM modification, we have recently focused on the fabrication of less expensive activated carbon materials based on waste biomass precursors. We have developed a nanostructured carbon material based on corn-cob biomass via a simple high temperature exfoliation procedure. This exfoliated corn biochar had excellent energy storage performance, demonstrating a capacitance over 100 times higher than natural corn biochar produced without exfoliation treatment. Additionally, high surface area activated carbons from pine cone biomass were synthesized via a two-step carbonization and KOH activation method. These high surface area substrates result in carbon-POM hybrids with further enhanced specific capacitance compared to nanocarbon based materials. The capacitive performance of these different carbon-POM composites will be compared along with a discussion of how our approach can be leveraged to design the optimal hybrid material for different energy storage applications. Figure 1