Oxygen‐Tolerant Two‐Electron Storage with Viologens for Aqueous Organic Flow Batteries

Flow batteries are a promising technology for safe, low‐cost energy storage but suffer from poor energy density. Widely used viologen anolytes can accept two electrons; however, their doubly reduced form is rarely accessed due to stability and solubility issues in aqueous solutions. Here, it is shown that an alternative supporting electrolyte cation, tetrabutylammonium (TBA ⁺ ), leads to remarkably stable two‐electron, high‐voltage cycling demonstrated with two highly promising symmetrically substituted viologen anolytes. Promotion of solubility and robustness against reactive oxygen species by TBA ⁺ enables the lowest two‐electron capacity fade rate (<1% _2e d ⁻¹ ) for viologens to date without cycling in a glovebox.