Development and Scale-up of High Performance Electrodes for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow battery (VRFB) is a promising technology for energy storage because of its independent energy to power ratio and long cycle life. However, VRFB commercialization is still hindered by some technological issues, among which the limited power density. This results in increased cell area and system costs.In order to tackle this issue is fundamental to boost the sluggish electrode kinetic. In the literature different wet processes, such as acid treatments, have been widely employed to add catalytic functionalities to commercial carbon electrodes [1,2]. Also plasma etching or plasma doping have been adopted for the improvement of electrode performance [3]. Recently, the introduction of heteroatoms is becoming one of the most common method to enhance the catalytic activity of carbon electrodes. Zhao et al. [21] performed thermal treatment of graphite felt combined with electrochemical deposition of Bismuth nanoparticles, obtaining 80% energy efficiency at 600 mA cm-2. Li et al. [22] developed a hierarchical nitrogen doped electrode, reaching 76.8% energy efficiency at 400 mA cm-2.In this work it is proposed a prototypal deposition method, called NanoJeD. It is a high throughput, dry and easily scalable process starting from gas phase. It consists in radiofrequency (RF) plasma operating in dusty mode, through which a non-flammable mixture of Acetylene and Argon is fluxed and the RF plasma ignites the decomposition and clasterization of the Acetylene. The former nanoparticles, following the gas streamline are dragged to a high aspect ratio orifice and ejected in a low-pressure chamber where a substrate is placed. Kinetic energy of the nanoparticles ejected in the chamber defines their impact energy with the substrate and the consequent collection mechanism. Subsequently, a vacuum thermal treatment is performed and this two-steps process allows a fine tuning of the material properties and the formation of a carbon allotropes with catalytic peculiarities known as Carbon Nano Onions (CNOs).Deposited CNOs were firstly characterized with Raman spectroscopy, Scanning Electrode Microscopy (SEM) and Transmission Electrode Microscopy (TEM). Electrochemical characterization was performed in a three-electrode setup with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CNOs exploit their unique properties: high conductivity, high chemical-mechanical stability, and catalytic activity towards redox reactions. The peculiarities of CNO reside in the edge of graphene sheet, which is ascertained to share a higher catalytic activity with respect to the basal plane of the same graphene sheet [6].Afterwards CNOs were deposited on a commercial carbon paper electrode, Sigracet® SGL 39 AA (nominal thickness of ∼290 μm), that was tested in 4 cm2 symmetric cell with Nafion® 211, permitting to evaluate the effect of CNOs on both positive and negative electrode. Subsequently, full cell test at different current densities was performed in cycles with fixed cut-off voltages (1-1.65 V), obtaining an energy efficiency of 81% and 74% at 400 mA cm-2 and 600 mA cm-2, respectively. The corresponding electrolyte utilization was 70% and 59%. Moreover, electrode stability has been evaluated during 1’000 cycles operation.Finally, the process has been scaled-up permitting to obtain a 100 cm2 electrode. The corresponding full cell test with Nafion® 212 resulted in 75% energy efficiency at 0.3 A cm-2.