Simulating the random motion of molecules at an electrode can help determine when an electrochemical reaction becomes kinetically controlled. In common electrochemical usage, a "kinetically controlled reaction" is one in which the transport of redox molecules is fast relative to the rate of electron transfer at the electrode-electrolyte interface. Increasing diffusion rates to reach the kinetic limit is a central concept of many electroanalytical techniques used to investigate electron-transfer kinetics (1, 2). For example, in cyclic voltammetry experiments of freely diffusing redox species, the electrode reaction rate becomes controlled by electron-transfer kinetics at sufficiently high potential sweep rates, a consequence of increased diffusion rates as the sweep rate is increased (3-5). In this article, we discuss the role of collisional encounters between the redox molecule and electrode in determining when an electrochemical reaction becomes kinetically controlled. Brownian dynamics simulations of the random motion of redox molecules provide insights into the physical factors that determine the kinetic limit in steady-state voltammetric measurements using electrodes with nanometer dimensions.
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