Nanoparticles Coated Electrodes for Vanadium-Based Non-Aqueous Redox Flow Battery

Abstract

Redox flow batteries (RFBs) are a promising technology for large-scale energy storage. However, their main limitation concerns their inherent lower energy density compared to other batteries such as Li-ion, Ni-metal hydride, Ni-Cd, etc. The research community is addressing this challenge with the introduction of the non-aqueous RFB (NAqRFB).The electrodes composing the NAqRFB play a vital role in the redox reaction mechanism as their microstructure is ultimately responsible for the ohmic polarisation of the cell and mainly for the activation polarisation. Pristine carbon-felts are typically used in Vanadium based redox flow batteries. These show good electrical conductivity and excellent mechanical stability. Still, they fail both to assure fast kinetics and exemplary reactants diffusion due to poor electrochemical surface area and low permeability of fibers to the electrolyte, which is reflected through high charge transfer resistances. Hence, it is crucial to develop high-performance, robust and tailored-made electrodes that improve the electrochemical activity of redox species in non-aqueous electrolytes while ensuring a blockage-free path for the electrolyte to reach these active electrochemical sites.Several thermal and/or chemical treatments (e.g., nitrogen doping, sulfuric acid leaching) or even surface decoration with particles (SnO2/TiO2, Pt, graphite) had been applied to carbon felts to ensure higher wettability and to tune their surface chemistry to reach more excellent reversibility of vanadium redox reactions at the electrodes. These attempts had been explained for aqueous systems. However, there is a gap in the literature to have a systematic study on the modified/coated electrodes with several carbon nanoparticles of different shapes and sizes, surface areas, conductivity, and their effect on the overall performance and stability of a non-aqueous Vanadium RFB. Moreover, tuning the electrode with an ionomer to the carbon particles should also be assessed as it may contribute to generating a longer ionic conductive path onto the 3D microstructure of carbon felts.The present work acknowledges the impact of adding nanoparticles such as reduced graphene oxide (rGO), graphene (G), multi-walled carbon nanotubes (CNT), carbon black as Vulcan XC72 (V), and carbon black as Ketjen (K) to carbon felts. The electrodes were prepared by a facile and fast drop-casting coating technique. Nafion ionomer was used as a binder, and its concentration was optimized to increase the number of triple-phase boundaries (TPBs). More TPBs consider a simultaneous boosted interaction between the ion conduction phase (electrolyte/ionomer), electron conduction phase (carbon felt fibers), and the 3D porous network for better electrolyte transport, which results in higher energy efficiencies.After extensive cyclic voltammetry, SEM and EDS analysis, it was observed that the addition of CNT or Graphene coatings at both the cathode and anode pose a better bargain between increased electroactivity and enhanced stability towards both redox couple reactions. The adhesion of the nanoparticles using the drop-casting method was highly influential, mainly for rGO and CNT coated electrodes, as they demonstrate widespread dispersion of nanoparticles on their fibers. The enhancement of surface defects suggests an increment in the number of active sites for the redox reactions is a possibility, confirmed by the higher electrochemical surface areas attained. Addition of the ionomer in a ratio of I/C = 15 wt. % is effective and offers good binding of the carbon nanoparticles.Charge Discharge Cycling (CDC) was used to assess the performance of the redox flow cell. The cell was assembled using each prepared sample of coated carbon felts and operated for 25 cycles at a C-rate = C/2, i.e., equivalent to the current density of 4.2 mA⋅cm-2. Post-mortem analyses reveal that the nanoparticles generally remain attached to the fibers after intense CDC. The carbon felt containing CNT nanoparticles depicts the most similar structure compared to its beginning of life. Nonetheless, an optimized suggested configuration for operating a single NAqRFB-cell may include using graphene-coated carbon felts at the negative side and CNT-coated carbon felts at the positive side of the NAqRFB. Advancements in this domain are attractive since it paves the way for designing stacks of reduced size and lowers the overall capital expenditure of installing the NAqRFB after encouraging the pilot level commercialization and screening. The results are summarized in Table 1, and this work is still in progress for an in-depth study covering other aspects. Figure 1