An High Performance Carbon-Nano Onion Electrode for Vanadium Redox Flow Battery

The green energy transition is mandatory process. After the Glasgow COP 26 most of the major countries have defined a path to follow and milestones to reach by 2050. This process consists in the decarbonization process and utilization of renewable source for power and energy production. But renewable energies are intermittent and cannot fully guarantee the stability of the electrical grid. This limitation reflects in the necessity of an energy storage system which can be coupled with renewable energy for grid stabilization. For this task the redox flow batteries (RFB) seem to be the best solution thanks to their scalability, long life cycle and good coupling with renewable energy [Shigematsu, T. Sei technical review 73 (2011)]. RFB’s consists in a voltaic cell where the electrolytes are stored in tanks outside the system. Whilst the chemistries of the anodic and cathodic electrolytes define the energy density of a RFB system, its power density is determined primarily by its electrodes. These are typically porous carbon media on the surface of which the redox reaction occur. They are a critical component since it requires a delicate engineering of electrocatalytically active surface area, electron and mass transport. Still today, though, they remain together with the proton conducting membrane, the main cause of RFB low power density. Many authors dedicated to improve the reaction kinetic on the surface of the typical carbon fibers used in carbon felt and paper, yet only few reported remarkable results [Zhao, T.S. et al. Energy Storage Material 24, 529-540 (2020)]. In our work we propose a prototypal deposition method of high-throughput carbon allotropes known as carbon nano onion (CNO). By mean of this deposition technique and a vacuum annealing process, is possible to maximize the electrical conductivity and catalytic activity made by the hierarchical assembly of turbostratic CNO. The CNO kinetic properties has been tested by cyclic voltammetry in a three-electrode setup and with a rotating disk analysis. From the cyclic voltammetry performed at different scan rate, a net increasing catalytic activity has been observed in both the chemical reaction. At the negative site, which is known to be the limiting one, a separation peak of 80mV at 10mV/sec has been reported, giving a clear indication of quasi-reversibility reaction mechanism. Moreover, not a net hydrogen evolution can be appreciated. From the rotating disk analysis the calculation of the kinetic rate constant given a value within the same order of magnitude in literature, confirming the quasi-reversibility reaction kinetic. After the electrochemical film characterization, a thin CNO film (several microns) has been deposited over a Carbon Paper 39AA produced by Sigracet and used in a vanadium redox flow battery, VRFB. The adoption of the carbon paper instead of the more commercial graphite felt lean on the strategic decision to increase the current volume density while decreasing cell’s thickness. Moreover, the adoption of a thinner electrode helps the mass transport limiting the auxiliary power consumption and mass transport losses [Guarnieri, M. Journal of Power Sources 440 (2019)]. The deposited electrode has been tested with different deposition thicknesses and annealing temperatures in a symmetric cell to evaluate systematically the overpotentials of each configuration towards the V2+/V3+ and VO(2+)/VO2(+) reactions. The negative side, which is kinetically limited requires a thicker film to exploit all the active sites, while the positive side needs a thinner one for avoiding mass transport limitations. After symmetric cell test, a full cell test has been assembled. From charge/discharge curves, the system efficiencies have been calculated: a current density as high as 600mA/cm2 with 70.8% energy efficiency was obtained, with a peak current density of 1 A/cm2. These figures and the reduced thickness are calculated to allow a potential reduction to 1/10 in stack volume with respect the commercial benchmark graphite felt, while at the same time decreasing system costs to less than 150 $/kWh by diminishing materials costs directly connected with the aerial current density (i.e. membrane, current collectors, gaskets, etc.).