Capacitance characteristics of carbon-based electrochemical capacitors exposed to heteropolytungstic acid electrolyte

A Keggin-type heteropolytungstic acid, H3PW12O40, is evaluated as new high ionic conductivity aqueous electrolyte of potential interest for electrochemical capacitors, with enhanced safety and low environmental concern. For the performance evaluation, two types of electrode materials have been selected, i.e. mesoporous multi-walled carbon nanotubes and microporous activated carbons. To address the influence of changes in the porous structure and surface properties on capacitors performance, the materials have been subjected to additional oxidation treatment under liquid or gas phase conditions. The electrochemical parameters of two-electrode cells were evaluated from a series of voltammetric, galvanostatic, constant power, and AC impedance measurements and compared with those obtained for conventional H2SO4 electrolyte. It has been shown that due to the size of the heteropolytungstate anion (exceeding 1 nm), the selection of carbon electrode materials in terms of appropriate porous structure and wettability is a key factor in avoiding capacitance limitations. As a result, the specific capacitance, energy density as well as the rate capability of H3PW12O40 supported capacitors may approach the performance of conventional H2SO4-based systems. Furthermore, the potentiodynamic anodic polarization tests and post-mortem scanning electron microscopy analysis confirmed a higher corrosion resistance of 316 L type steel in the presence of H3PW12O40 solution, in comparison to the conventional H2SO4 electrolyte. It enables stable capacitor performance upon utilization of low cost non-noble current collectors which was evaluated during galvanostatic cycling (13 000 cycles) and accelerated ageing by potentiostatic floating during 120 h.