Hydrogen Electrode Potentials as a Descriptor of Catalyst Reactivity for Sustainable Redox Chemistry: A Tutorial Review

Redox catalysts play a critical role in advancing sustainability by enabling cleaner, more efficient chemical transformations. The redox potential of a catalyst determines the reaction direction thermodynamically, as electrons are transferred from the substrate through the catalyst to the product. The accurate assessment of redox catalyst potential by in situ methods is fundamental for developing effective strategies to optimize the redox reactivity of these materials. Typically, the catalyst potential measured in situ is relative to the reference electrode in the same solution. The standard potential of the catalyst, or metal center, is commonly reported relative to the standard hydrogen electrode (SHE). In electrochemical measurements, the potential recorded in situ versus a reference electrode is converted to scale by adding the known potential of that reference electrode relative to the SHE. The potential measured with the reversible hydrogen electrode (RHE), one of the reference electrodes, depends on the fugacity of hydrogen (H2), the activity of hydrogen ions (pH) and the temperature. In this review, RHE is introduced as a descriptor of the redox catalyst activity for sustainable redox chemistry.