Essays on Conceptual Electrochemistry: I. Bridging Open-Circuit Voltage of Electrochemical Cells and Charge Distribution at Electrode-Electrolyte Interfaces

We ponder over how an electrochemical cell conforms itself to the open-circuit voltage (OCV) given by the Nernst equation, where properties of the electrodes play no role. We first show, via a pedagogical derivation of the Nernst equation, how electrode properties are canceled and then take a closer look into the electrode-electrolyte interface at one electrode by linking charge and potential distributions. We obtain an equilibrium Poisson-Nernst equation that shows how the charge distribution across an electrode-electrolyte interface can be dictated by the chemical potentials of redox species. Taking a H-2/O-2 fuel cell as an example, we demystify the formal analysis by showing how the two electrodes delicately regulate their "electron tails " to abide by the Nernst equation. In this example, we run into a seemingly counterintuitive phenomenon that two electrodes made of the same transition metal display two distinct potentials of zero charge. This example indicates that the double layer at transition metals with chemisorption can display distinct behaviors compared to ideally polarizable double layers at sp metals.