Designing Polyoxometalate Thin Films on Carbon Nanomaterials for Pseudocapacitive Electrodes

Multi-Walled Carbon Nanotubes (MWCNTs) modified with mixtures of polyoxometalate (POM) ions were investigated as pseudocapacitive electrodes for electrochemical capacitors.  POMs undergo fast reversible multi-electron transfer reactions, and 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, the resulting mixture shows unique electrochemical properties leading to cyclic voltammograms (CVs) very different from those of the constituent molecules.  Further analyses revealed that these POM ions do not just mix physically in aqueous solutions, but instead react spontaneously to form PMo_12-xW_x 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 (Figure 1).  In this talk we will discuss the unique electrochemistry of various POM mixtures, the mechanisms of the novel mixed addenda synthesis, and our approach for using these molecules to design high performance composite EC electrodes.