Proton-electron transfer (PET) is a fundamental reaction in electrochemistry, biochemistry, 4,5 material science, and in some other fields. To give one example, we can consider the charging and discharging processes in aqueous redox flow batteries, in which the two elementary steps, i.e., electron transfer (ET) and proton transfer (PT), are taken either competitively or jointly to interconvert electricity and chemical energy. Therefore, understanding the pathways from reactants to products through PET reactions is crucial to understand flow battery performance.