Open-Circuit Voltage Comes from Non-Equilibrium Thermodynamics

Abstract

Originally derived by Walther Nernst more than a century ago, the Nernst equation for the open-circuit voltage is a cornerstone in the analysis of electrochemical systems. Unfortunately, the assumptions behind its derivation are often overlooked in the literature, leading to incorrect forms of the equation when applied to complex systems (for example, those with ion-exchange membranes or involving mixed potentials). Such flaws can be avoided by applying a correct thermodynamic derivation independently of the form in which the electrochemical reactions are written. The proper derivation of the Nernst equation becomes important, for instance, in modeling of vanadium redox flow batteries or zinc-air batteries. The rigorous path towards the Nernst equation derivation starts in non-equilibrium thermodynamics.

Variable-valence ion and heterointerface accelerated electron transfer kinetics of electrochemical water splitting

“…When the electrode is at open-circuit state, the V ocp represents the quasi-Fermi level of the catalyst ( E cat ) against the reference electrode. 32 The V ocp decay results from the E cat upshift, equivalent to changes in the average oxidation state of the Ni 2+/ Ni 3+ redox couple. When the energy difference (Δ E ) between E cat and the water oxidation energy level ( E O2/OH − ) is small, the chemical water oxidation no longer occurs (inset in Fig.…”

Estimation of activity coefficients for aqueous organic redox flow batteries: Theoretical basis and equations

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“…Activity coefficients impact the OCV of the cell and typically introduce a few tens of mV of discrepancy with the same equations parametrized with concentrations ( Hayer and Kohns, 2020 ; del Olmo et al., 2021 ; Mourouga et al., 2022 ). Although this value may sound negligible for performance models over a single cycle, modeling a flow battery over several thousands of charge-discharge cycles requires accurate transport models.…”

Thermodynamic Analysis of LiNi0.6Mn0.2Co0.2O2 (NMC622) Voltage Hysteresis Induced through High Voltage Charge

“…In GITT tests, the open-circuit state where the loaded current is briefly paused should not be considered as the strictly defined thermodynamic equilibrium state. 28 The OCP may deviate from the equilibrium potential after the charge or discharge to a certain SoC, which imposes a OCP hysteresis, as shown in 4b) and C/ 2 (Figure 4c) was small. An inconsequential OCP hysteresis suggests the deviation from equilibrium in either charge or discharge is small.…”

“…Because the OCP is temperature-dependent, cells for GITT tests were also enclosed in a temperature-controlled chamber at 30 °C. In GITT tests, the open-circuit state where the loaded current is briefly paused should not be considered as the strictly defined thermodynamic equilibrium state . The OCP may deviate from the equilibrium potential after the charge or discharge to a certain SoC, which imposes a OCP hysteresis, as shown in areas enclosed by OCP-postcharge and OCP-postdischarge curves in Figure b–e.…”