Synergic effects between carbon nanofluids and redox couples dispersed in O2 reduction electrolytes

Abstract

Carbon materials have a high electronic conductivity and low density, which make them essential additives in electrodes for electrochemical storage devices[1]. However, the use of nanocarbon material suspensions in the electrolyte of such devices is also attracting for two aspects: increasing total electrode active surface area and/or building percolation networks for electrons[2]. For these reasons nanocarbons have been considered as components in semi-solid electrolytes, which can be considered one class of nanofluids. Furthermore, the introduction of multiredox couples dispersed in the electrolyte can also lead to remarkable improvements, as seen in Li/air batteries[3]. These redox couples can act both as redox mediators and catalysts for the oxygen reduction or evolution reactions (ORR, OER) that take place during the battery cycling. Polyoxometalate clusters or other nanoparticles such as IrOx are already known by their catalytic activity in O2 reduction and reoxidation processes[5].

In this work we show the influence of carbon black nanocarbons and redox couples in organic media for the cathode of metal-air batteries, i.e. O2 reduction electrolytes. The addition of nanocarbons or mediators separately does not result on an impedance decrease or a clear effect on terms of current densities and overpotentials. Nevertheless, we observe a synergy between the redox and the conducting components added simultaneously, giving rise to enhanced currents, lower overpotentials and lower impedances for the charge transfer and diffusion processes. The relevant parameters and hypothesis for a mechanism will be discussed based on impedance analysis.