Electron transfer (ET) is a key process in all redox reactions in (bio-) chemistry, from natural photosynthesis to cellular respiration to electricity storage and conversion technologies. The importance of this area was highlighted by 1992 Nobel Prize in Chemistry awarded for Rudolph Marcus "for his contributions to the theory of electron transfer reactions in chemical systems". Understanding electron transfer reactions is also of crucial importance in selecting materials for flow batteries, a key candidate for stationary energy storage for storing wind and solar energy. In flow batteries, redox active molecules typically dissolved in aqueous electrolyte solutions undergo electron transfer reactions with the electrodes in the electrochemical cell to store or discharge electricity. As the electrolytes are stored in large tanks, and pumped through electrochemical cell, the energy capacity and power density are decoupled. In other words, instead of storing electricity within the electrode itself, like conventional rechargeable batteries, energy is stored into solution inside the storage tanks, while power depends on the surface area of the electrochemical cell.