Abstract
Multi-walled carbon nanotube (MWCNT) electrodes modified with mixtures of PMo12O403– (PMo12) and PW12O403− (PW12) Keggin ions are investigated for electrochemical capacitor (EC) applications. We have discovered that when these polyoxometalate (POM) ions are combined in solution, they do not just mix physically, but instead react to form the PMo12-xWx mixed addenda chemistries. By simply mixing PMo12 and PW12 in different ratios, a variety of mixed addenda ions, each with unique electrochemical properties, can be easily synthesized. This control over POM redox behavior afforded by the novel mixed addenda synthesis is 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 PMo12O403−-PW12O403− mixed layer on a 1:1 GeMo12O404−-SiMo12O404− mixed layer, which resulted in an 11X capacitance enhancement over unmodified CNT. This dual layer electrode also demonstrated a close to rectangular CV profile due to the overlapping redox features of the POM combination. The design of custom POM coatings to engineer the electrode surface represents a promising strategy for the development of high performance pseudocapacitive electrodes.
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