which results in multiple coupled or decoupled pathways, as shown in Figure . In practice, the required proton (H + ) is transferred from an aqueous solution, while electron (e -) is obtained from an electrode. In order to assess the redox reaction (Eq. ( )), it is necessary to calculate Gibbs free energy change ∆G at each ET/PT step: ∆G leads to the measurable reduction potential E red. and acidity constant pK a for the ET and PT reactions, respectively. One can also predict the number of transferred electrons and protons at the given solution pH and electrode potential using these quantities.