High Performance Nitrogen-Doped Hierarchical Nanostructured Carbon Electrode for Vanadium Redox Flow Batteries

Abstract

In recent years, we have seen an impressive progress in the deployment of renewable energy sources, and in particular of photovoltaics and wind turbines. However, renewable energies have some well-known drawbacks due to their intermittent nature and direct usage of the electricity generated would destabilize the grid. To overcome these problems large scale energy storage systems have drawn great attention in the last few years both from the academic and industrial world. Redox flow batteries (RFB) are emerging as one more promising electrochemical energy storage technologies. Notably, the Vanadium redox flow battery (VRFB) is currently the most widespread technology as it allows for the implementation of strategies to recover the capacity decay induced by ion cross-over. Despite the many advantages, VRFBs have long suffered from low power density, mainly caused by the usage of electrodes which are not optimized for this application. The use of heteroatom dopants, such as nitrogen, phosphor, and boron, is currently the most common method to enhance the performance of carbon electrodes. Nitrogen has been extensively studied as a catalyzer for Vanadium redox reactions, showing exceptional boosts in performance in several works. In a previous work we developed a two-step process based on a plasma enhanced nano-aerosol jet deposition source and a vacuum thermal treatment. Starting from a gas precursor we deposited a nanostructured carbonaceous material on a commercially available substrate, this process allows to increase the current density to 400 mAcm-2 with an energy efficiency of 80%, that is, 4 times greater than the commercial reference. The two-steps process permits a fine tuning of the properties and structure of the material leading to an exceptionally high surface area and a defect rich structure. The advantage of this process is that nitrogen can be added easily during the deposition phase by using a N2-C2H2 mixture, without the need for any other processing steps. In this work we aim to further increase the performance of our nanostructured electrode by adding nitrogen functionalities to the carbon nanostructure, thus combining the effects of an incredibly high surface area with the excellent catalytic properties of nitrogen. The morphology and chemistry of the synthesized material is analyzed by different techniques such as SEM, Raman spectroscopy, XPS, BET analysis. Nitrogen doping was confirmed by XPS analysis, in particular a high amount of graphitic nitrogen, which is considered to be the most active towards vanadium ions, is present. Moreover, BET analysis shows an incredibly high surface area, up to 756 m2/g. The properties of the material are then correlated to the electrochemical properties by testing the material in increasingly complex setups, starting from a three-electrode setup, then in symmetric cell and finally in a full-cell setup. The kinetic activity of the material is studied by cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammetry showed an impressive activity of the electrodes towards the active species, especially towards the V2+/V3+ couple where a peak-to-peak separation of 65 mV was obtained, which is among the lowest reported in literature. These results were confirmed by EIS, from which the reaction rate constants were calculated following the method proposed by Stimming et al. [1] The addition of nitrogen led to an order of magnitude increase of the reaction rate with respect to the un-doped material. The performance enhancement was then validated by the test in symmetric cell. Polarization curves show a significant decrease in the overpotential, especially in V2+/V3+. Thus, we propose a vertically integrated process, from the synthesis of the material to the final application, which is able to deliver a comprehensive overview on the material properties, its electrochemical performance and the correlation between the two.[1] Friedl, J., Bauer, C. M., Rinaldi, A., & Stimming, U. (2013). Electron transfer kinetics of the VO2+/VO2+–Reaction on multi-walled carbon nanotubes. Carbon, 63, 228-239.