Compared to the studies of new electrolyte and electrode chemistries aimed to push the energy and power density of battery systems, investigations of self-discharge reactions contributing to capacity fading are still very limited, especially at the molecular level. Herein, we present a computational study of oxidation-reduction reactions between vanadium ions in solution leading to battery self discharge due to the crossover of vanadium species through the membrane in all-vanadium redox flow batteries (RFB). By utilizing Car-Parrinello molecular dynamics (CPMD) based metadynamics simulations in combination with the Marcus electron transfer theory, we examine the energetics of condensation reactions between aqueous vanadium ions to form dimers and their subsequent dissociation into vanadium species of different oxidation states after electron transfer has occurred. Our results suggest that multiple self-discharge reaction pathways could be possible under the vanadium RFB operation conditions. The study underscores the complexity of vanadium polymerization reactions in aqueous solutions with coupled electron and proton transfer processes that can lead to the formation of various mixed-valence vanadium polymeric structures.