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

Abstract

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 H2/O2 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.

Ce4+/Ce3+ Redox-Promoted Electron Transfer for Efficient Neutral H2O2 Electrosynthesis from Two-Electron Oxygen Reduction

“…The OCP of the CeO x /CNT(6) catalyst increased by 107 mV, while that of the CNT increased by 60 mV. When O 2 gas is supplied into the electrolyte, the electron pullout effect by interfacial O 2 results in a positive shift of OCP . Given the similar ECSA of CeO x /CNT(6) and CNT, the larger OCP change in CeO x /CNT(6) implies that the interaction between Ce 3+ and O 2 generates more positively charged Ce 4+ species.…”

“…When O 2 gas is supplied into the electrolyte, the electron pullout effect by interfacial O 2 results in a positive shift of OCP. 64 Given the similar ECSA of CeO x /CNT (6) and CNT, the larger OCP change in CeO x / CNT (6) implies that the interaction between Ce 3+ and O 2 generates more positively charged Ce 4+ species. The partial oxidation of Ce 3+ in CeO x /CNT(6) under OCP potential could also be confirmed by a lower Ce 3+ fraction (12.8 ± 1.1%) at the OCP compared to its powder state (31.5 ± 1.7%, Figure 1b and Table S2).…”

Correlated surface-charging behaviors of two electrodes in an electrochemical cell

“…Following a previous discussion on the open-circuit voltage (OCV) ( 30 ), we stick to the “thought experiment” of a hydrogen–oxygen fuel cell with two identical platinum electrodes. Herein, we close the electrical circuit and pull the cell voltage below the OCV ( Fig.…”

“…1 A ). It is important to note that both electrodes, assumed to be platinum in this case, are charged under the open-circuit condition ( 30 ). Consider the cathodic platinum; a small amount of electrons must have been consumed in the ORR until equilibrium is attained.…”