UNDERSTANDING OVERPOTENTIALS IN BIOMASS ELECTROCHEMISTRY THROUGH FREE-ENERGY LANDSCAPES: A GRAPHICAL EDUCATIONAL APPROACH

This article takes an educational perspective on a central limitation in biomass electrochemistry: the impossibility of driving multielectron reactions at the global thermodynamic potential. Standard electrode potentials are defined solely by the free-energy difference between global reactants and products (including electrons), whereas experimental operation depends on the stability of adsorbed intermediates formed along the reaction pathway. When free energy must be distributed across many elementary steps, large overpotentials usually arise as an inevitable thermodynamic consequence of multielectron reactions, even before kinetic effects are considered. To make this point clear, a graphical rather than mathematical approach is used to illustrate the thermodynamic origin of overpotentials, first revisiting simple twoand four-electron reactions such as the H+/H2 and O2/H2O couples. These examples highlight how adsorption energies determine the potential-determining step and why even apparently simple and well-understood processes require overpotentials. This framework is then extended to the interconversion of organic functional groups, specifically the alcohol/carbonyl and aldehyde/acid transformations, which represent key steps in the electro-oxidation of biomass-derived molecules.