Tailoring the electrochemical charge storage properties of carbonaceous support by redox properties of heteropoly acids: where does the synergy come from?

Abstract

The synergistic effects between two Keggin-type heteropoly acids (HPAs) and carbon surface were examined and elucidated. An improved high rate capability (and potential high capacitor electrode for supercapacitors) of the hybrid materials, obtained by anchoring of α-dodecamolybdophosphoric (MoPA), α-dodecatungstophosphoric (WPA), and their mixture to activated carbon (AC), was achieved through the different mechanism of interaction. In order to elaborate this, a detailed analysis of AC-HPA composites has been performed by scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, temperature-programmed desorption (TPD), Fourier-transform infrared spectroscopy (FTIR), micro Raman spectroscopy, and zeta potential measurements. The zeta potential measurements revealed positive charge of carbon surface thus indicating attractive interactions with negatively charged Keggin anion. The surface analysis has shown that WPA spontaneously reduces the carbon surface, while interaction with MoPA leads to its oxidation. As the consequence of the tailoring of the functional groups at carbon surface through HPAs’ action, the distortion of cyclic voltammograms (CVs) decreased in the following order: AC-MoPA, AC-MoPA-WPA, and AC-WPA. A prominent rectangular shape of AC-WPA, even at an extremely high scan rate of 400 mVs−1, was measured, which is rarely demonstrated for carbon-based composites. By applying the theory of electrode potentials, the HPA-AC synergistic effect was explained and discussed in terms of charge storage improvement of HPA-modified carbon.