Fig. 1.

Concepts applied in this article. (A) Steady-state electrochemical kinetics visualized by cyclic voltammetry. When both the oxidized and reduced forms of a redox-active species are present, a reversible electrochemical reaction (one with a large exchange current density) produces a single sigmoidal wave (blue) that cuts (without inflection) through the zero-current axis at the equilibrium potential (E_eq) and achieves a potential-independent limiting current in either direction at relatively low overpotential. Conversely, if the exchange current density is low, the current is negligible around E_eq and two sigmoidal waves (red), one for either direction, are separated in potential, emerging from the baseline with an exponential dependence on potential: A substantial overpotential is required to match the current produced by the reversible system. (B) Cartoon showing an adsorbed enzyme functioning as a molecular electrocatalyst.