(192at) First-Principles Studies of the Interactions Between Chemical Species inside Vanadium Redox Flow Batteries

The operation of a large scale energy storage Redox Flow Battery (RFB)[i] is driven by reversible redox reactions of electro-active electrolyte species inside the two half-cells. The reversible redox reactions that happen during the charge – discharge cycles reuse all of the components of the system and thus eliminate all wastes. Therefore, RFBs should provide a solution to an environmentally clean energy alternative as all of the chemical wastes produced during the energy consumption process can be reversed by the charging process. One of the most studied system of RFBs is an all-Vanadium Redox Flow Battery (VRFB)[ii]. VRFB exploits the vast range of oxidation states that can be reached by vanadium. VRFB employs the redox couple of V²⁺/V³⁺ in the negative half-cell and VO²⁺/VO₂⁺in the positive half-cell. The introduction of Nafion as the membrane in the system of VRFB leads to the formation of all sorts of complexes between vanadium of all oxidation states and Nafion. Vanadium – Nafion complexes block the pores on the Nafion, which reduces the performance of the battery, and leads to VRFB eventual degradation. Molecular level understanding of these interactions would provide insights into the chemistry of RFBs at the real operation temperature to aid the further improvement of RFBs.

Our computational studies[iii] from the DFT based gas phase calculations of the enthalpy of formation for the vanadium - Nafion complexes indicate that all four vanadium species have driving force to covalently bind to sulfonic acid group of Nafion ionomer unit via the formation of a single V-O bond. From the Car-Parrinello molecular dynamics based metadynamics simulations of the aqueous solution at 298 K we find that the formation of covalently bonded vanadium-Nafion complexes is spontaneous for V²⁺ and V³⁺, while V⁴⁺ and V⁵⁺ have a sizable activation barrier to attach to Nafion and diffuse away from SO₃^-group.

In addition, we analyze the calculated IR spectra for the most energetically favorable vanadium complexes with Nafion.

[iii] Intan, N., et al. "Interactions between Vanadium Ions and Nafion Membrane: Insights from First - Principal Calculations" (In Preparation)