Fact-checked by Grok 2 weeks ago

Overpotential

Overpotential in electrochemistry is defined as the additional potential required beyond the thermodynamic equilibrium potential to drive an electrochemical reaction at a finite rate.^[1] This excess voltage, often denoted by η, accounts for the deviations from ideal behavior due to kinetic barriers, mass transport limitations, and resistive losses in the system.^[2] It is a fundamental concept that quantifies the energy inefficiency in electrochemical processes, directly impacting the performance and viability of devices like batteries and fuel cells.^[3] Overpotential can be decomposed into three primary types: activation overpotential, which stems from the energy required to surmount the activation barrier for electron transfer at the electrode surface; concentration overpotential, arising from gradients in reactant concentration near the electrode due to diffusion limitations; and ohmic overpotential, caused by the voltage drop across the electrolyte and electrodes due to their electrical resistance.^[4] Activation overpotential dominates at low current densities and is described by the Butler-Volmer equation, while concentration overpotential becomes significant at high currents, often modeled using the Nernst diffusion layer approximation.^[5] Ohmic overpotential, in contrast, is linearly proportional to current and can be minimized through improved conductivity of materials. The study and reduction of overpotential are essential for enhancing the efficiency of electrochemical systems, particularly in applications such as hydrogen evolution via electrolysis, where high overpotentials lead to substantial energy losses.^[6] In fuel cells and metal-air batteries, low overpotential catalysts—often based on platinum or advanced nanomaterials—are critical for achieving practical power densities and longevity.^[7] Ongoing research focuses on computational modeling and experimental techniques to predict and mitigate these effects, enabling more sustainable energy conversion technologies.^[8]

Thermodynamic Basis

Reversible Potential

The reversible potential, also known as the equilibrium potential, represents the electromotive force (EMF) of an electrochemical cell under conditions of thermodynamic equilibrium, where no net current flows and the system is at rest. It serves as the theoretical voltage at which the forward and reverse reactions occur at equal rates, with no net change in the concentrations of species involved. This potential is calculated using the Nernst equation, which adjusts the standard electrode potential for non-standard conditions.^[9] The Nernst equation is given by

E = E^\circ - \frac{RT}{nF} \ln Q

where E is the reversible potential, E^\circ is the standard electrode potential, R is the gas constant (8.314 J/mol·K), T is the absolute temperature in Kelvin, n is the number of moles of electrons transferred in the balanced half-reaction, F is the Faraday constant (96,485 C/mol), and Q is the reaction quotient expressing the activities or concentrations of reactants and products. This equation originates from the principles of chemical thermodynamics and allows prediction of the potential for any electrochemical half-cell or full cell under varying conditions.^[10] The reversible potential is fundamentally linked to the Gibbs free energy change (\Delta G) of the electrochemical reaction through the relation

\Delta G = -nFE

At equilibrium, \Delta G = 0, so E = 0, but under standard conditions, \Delta G^\circ = -nFE^\circ, establishing the connection between the spontaneity of the reaction and its electrical driving force. This thermodynamic foundation underscores that the reversible potential quantifies the maximum useful work extractable from the reaction as electrical energy, bridging classical thermodynamics with electrochemical processes.^[11] Representative examples illustrate the application of reversible potentials in common half-cells. The standard hydrogen electrode (SHE), defined by the half-reaction $2H^+ + 2e^- \rightleftharpoons H_2, has a reversible potential of 0 V by international convention under standard conditions of 1 M H^+ activity, 1 bar H_2 partial pressure, and 25°C. For the oxygen evolution reaction, $2H_2O \rightleftharpoons O_2 + 4H^+ + 4e^-, the standard reversible potential is 1.23 V versus SHE at 25°C and pH 0, reflecting the thermodynamic requirement for water oxidation.^[12]^[13] Several factors influence the reversible potential, primarily through their effects on the terms in the Nernst equation, derived from thermodynamic principles. Temperature T appears explicitly in the RT/nF factor, causing the potential to vary logarithmically with thermal energy, as higher temperatures increase entropy contributions to \Delta G. Concentration enters via the reaction quotient Q, where deviations from standard activities (e.g., 1 M for solutes or 1 bar for gases) shift E according to \ln Q; for instance, increasing reactant concentration raises the potential for reduction reactions. Pressure affects gaseous species in Q, such as partial pressures of H_2 or O_2, altering the equilibrium as per the ideal gas law integrated into thermodynamic derivations. These dependencies arise from the general expression for \Delta G = \Delta G^\circ + RT \ln Q, combined with \Delta G = -nFE and \Delta G^\circ = -nFE^\circ, yielding the Nernst form directly from the temperature and composition dependence of free energy.^[10]^[9]

Origin of Overpotential

Overpotential, denoted as \eta, is defined as the difference between the applied electrode potential E_\text{applied} and the reversible (equilibrium) potential E_\text{rev} for a given electrochemical reaction: \eta = E_\text{applied} - E_\text{rev}. This excess potential is required to drive the reaction at a measurable rate beyond the infinitesimal currents observed at equilibrium. In practical terms, E_\text{applied} represents the actual voltage imposed on the electrochemical cell, while E_\text{rev} is the thermodynamically determined equilibrium potential under the prevailing conditions, derived from the Nernst equation.^[3] From a thermodynamic perspective, overpotential originates in the inherent irreversibility of real electrochemical processes operating far from equilibrium. At finite current densities, the system experiences entropy production due to dissipative processes, such as heat generation and non-ideal interfacial behaviors, which deviate from the reversible pathway assumed in ideal thermodynamics. This overpotential serves to balance the Gibbs free energy change, compensating for these losses to sustain the reaction kinetics and maintain charge balance across the electrode-electrolyte interface. Nonequilibrium thermodynamics frameworks, including those based on proton-coupled electron transfer theories, further elucidate how these deviations manifest as additional energy barriers in reactions like hydrogen evolution.^[14]^[15] The concept of overpotential was first systematically quantified by Julius Tafel in 1905, who measured significant deviations during cathodic hydrogen evolution on various metals, revealing overpotentials far exceeding predictions from ideal thermodynamics. Tafel's empirical observations, documented in his seminal paper, established the logarithmic relationship between overpotential and current density (now known as the Tafel equation), highlighting the practical limitations of electrochemical systems even under controlled conditions. This work laid the foundation for understanding overpotential as a universal phenomenon in electrode kinetics, influencing subsequent research in corrosion, batteries, and electrocatalysis.^[16] In electrochemical cells, the total overpotential \eta_\text{total} arises as the additive contribution from distinct loss mechanisms and is expressed conceptually as \eta_\text{total} = \eta_\text{act} + \eta_\text{conc} + \eta_\text{ohmic}, where each term accounts for specific inefficiencies without implying independent operation. Overpotential becomes pronounced at finite current rates, where the need to overcome kinetic barriers and transport limitations results in reduced energy efficiency compared to theoretical minima. For instance, water electrolysis to produce hydrogen and oxygen requires an applied voltage exceeding the reversible potential of 1.23 V (at standard conditions of 25°C and 1 atm), often by 0.5–1 V or more, due to these cumulative overpotentials that diminish the process's thermodynamic efficiency.^[17]

Types of Overpotential

Activation Overpotential

Activation overpotential, also known as charge-transfer overpotential, refers to the additional voltage required beyond the reversible potential to overcome the activation energy barrier associated with slow electron transfer kinetics at the electrode-electrolyte interface. This kinetic limitation arises primarily from the energy needed to reorganize the solvation shell around reacting species and to facilitate the charge transfer step itself, distinct from thermodynamic driving forces.^[18]^[19] The quantitative description of activation overpotential stems from the Butler-Volmer equation, derived from transition state theory (TST). In TST, the rate constant for an elementary reaction step is given by k = \frac{k_B T}{h} \exp\left( -\frac{\Delta G^\ddagger}{RT} \right), where k_B is Boltzmann's constant, h is Planck's constant, T is temperature, R is the gas constant, and \Delta G^\ddagger is the Gibbs free energy of activation for the transition state. For an electrochemical charge transfer reaction, such as \ce{Ox + n e^- ⇌ Red}, the electrode potential modulates the activation barriers asymmetrically. The overpotential \eta = E - E_\text{eq} (where E is the applied potential and E_\text{eq} is the equilibrium potential) lowers the cathodic barrier by (1 - \alpha) n F |\eta| and raises the anodic barrier by \alpha n F \eta, with \alpha (0 < \alpha < 1) as the transfer coefficient representing the symmetry of this effect, often near 0.5 for symmetric barriers, and F as Faraday's constant.^[20]^[21] This leads to the forward (anodic) rate constant k_a = k^\circ \exp\left( \frac{\alpha n F \eta}{RT} \right) and the backward (cathodic) rate constant k_c = k^\circ \exp\left( -\frac{(1 - \alpha) n F \eta}{RT} \right), where k^\circ is the standard rate constant at equilibrium. The net current density i is then i = n F A \left( k_a c_\text{Ox} - k_c c_\text{Red} \right), with A as the electrode area and c as concentrations. At equilibrium (\eta = 0, i = 0), the exchange current density i_0 = n F A k^\circ c_\text{Ox}^\text{eq} emerges, where c_\text{Ox}^\text{eq} / c_\text{Red}^\text{eq} = \exp(n F E_\text{eq} / RT). Substituting yields the Butler-Volmer equation:

i = i_0 \left[ \exp\left( \frac{\alpha n F \eta}{RT} \right) - \exp\left( -\frac{(1 - \alpha) n F \eta}{RT} \right) \right],

which relates overpotential to current density, capturing the exponential increase in reaction rate with applied potential. This equation assumes a single rate-determining charge transfer step and negligible mass transport effects.^[20]^[21]^[18] For sufficiently large overpotentials (typically |\eta| > 0.1 V), one exponential term dominates, leading to the Tafel approximation. In the anodic regime (\eta \gg 0), the cathodic term becomes negligible, simplifying to i \approx i_0 \exp\left( \frac{\alpha n F \eta}{[RT](/page/RT)} \right), or rearranged as \eta = \frac{[RT](/page/RT)}{\alpha n F} \ln\left( \frac{i}{i_0} \right) + \text{constant}. Similarly, for cathodic processes (\eta \ll 0), \eta = -\frac{[RT](/page/RT)}{(1 - \alpha) n F} \ln\left( \frac{|i|}{i_0} \right) + \text{constant}. Plotting \eta versus \log |i| yields a straight line (Tafel plot) with slope b = \frac{2.303 [RT](/page/RT)}{\alpha n F} (Tafel slope, often ~120 mV/decade for \alpha = 0.5, n=1 at 298 K), allowing extraction of kinetic parameters like \alpha and i_0. This approximation highlights the non-linear, potential-dependent nature of activation losses./02%3A_Physical_and_Thermal_Analysis/2.07%3A_Electrochemistry)^[18] Several factors influence activation overpotential magnitude. The exchange current density i_0, a measure of intrinsic kinetics, varies greatly with electrode material; for instance, platinum exhibits high i_0 (~1 mA/cm² for hydrogen evolution) due to favorable d-band alignment facilitating charge transfer, while mercury shows low i_0 (~10^{-12} mA/cm²) from weak metal-hydrogen bonding. Catalysts reduce overpotential by lowering activation barriers, often through adsorption site optimization or ensemble effects. Solution pH also plays a role, as it alters proton availability and surface speciation, increasing overpotential in alkaline media for pH-sensitive reactions due to slower water dissociation.^[18]^[22]^[23] In practice, activation overpotential manifests in key reactions like the hydrogen evolution reaction (HER: $2\ce{H+} + 2e^- \to \ce{H2}) and oxygen reduction reaction (ORR: \ce{O2 + 4H+ + 4e^- \to 2H2O}). For HER on platinum in acidic media, typical overpotentials are low, around 0.03-0.13 V at 10 mA/cm² current density, reflecting platinum's benchmark activity. For ORR on platinum, activation overpotentials are higher, typically 0.2-0.3 V at practical currents (e.g., 10 mA/cm²), due to the multi-electron pathway's inherent kinetic barriers. These values underscore the need for optimized catalysts to minimize losses.^[24]^[25]^[26] Activation overpotential in multi-step mechanisms, often termed reaction overpotential, arises when sequential elementary steps exhibit varying rates, with the rate-determining step (RDS) dictating overall kinetics and thus the required overpotential. For HER, common mechanisms include Volmer (H adsorption: \ce{H+ + e^- \to H_{ads}}), Heyrovsky (electrochemical desorption: \ce{H_{ads} + H+ + e^- \to H2}), and Tafel (recombination: $2\ce{H_{ads}} \to \ce{H2}); the RDS may shift from Volmer at low overpotential (high barrier for adsorption) to Heyrovsky at higher overpotential, leading to Tafel slopes of 120 mV/dec (Volmer RDS) or 40 mV/dec (Heyrovsky RDS). In ORR, the multi-proton/multi-electron pathway involves intermediates like \ce{OOH_{ads}} and \ce{OH_{ads}}, with the first electron transfer (\ce{O2 + e^- \to O2^-}) or \ce{O-OH} bond formation often as RDS, contributing ~0.2 V overpotential even on platinum due to strong O-binding energies. Identifying the RDS via Tafel analysis or microkinetic modeling guides catalyst design to balance adsorption energies per Sabatier principle.^[27]^[28]

Concentration Overpotential

Concentration overpotential, also known as concentration polarization, refers to the voltage loss resulting from differences in reactant or product concentrations at the electrode-electrolyte interface compared to the bulk solution. This deviation occurs due to mass transport limitations, where the rate of species diffusion or migration cannot keep pace with the reaction rate, depleting reactants or accumulating products near the electrode surface.^[29] In electrochemical systems, it becomes prominent at higher current densities when transport processes dominate over kinetics.^[30] The phenomenon is fundamentally tied to Fick's laws of diffusion. Under steady-state conditions in the Nernst diffusion layer approximation, the flux of a reactant to the electrode is given by Fick's first law: J = -D \frac{\partial C}{\partial x} \approx D \frac{C_b - C_s}{\delta}, where D is the diffusion coefficient, C_b is the bulk concentration, C_s is the surface concentration, and \delta is the diffusion layer thickness. At the limiting current density i_L, the surface concentration drops to zero (C_s = 0) for a cathodic process, yielding i_L = n F \frac{D C_b}{\delta}, where n is the number of electrons transferred and F is Faraday's constant. This expression highlights how i_L scales with bulk concentration and diffusion properties while inversely depending on the diffusion layer thickness.^[31] The concentration overpotential can be quantified using a Nernst-based adjustment to account for the altered surface concentration. For a cathodic reaction, the overpotential is \eta_\text{conc} = \frac{RT}{nF} \ln \left(1 - \frac{i}{i_L}\right), where i is the applied current density, R is the gas constant, and T is temperature; the negative sign indicates a shift in potential to more negative values to drive the reaction. This logarithmic form arises because the electrode potential follows the Nernst equation, E = E^\circ + \frac{RT}{nF} \ln \left( \frac{C_\text{ox}}{C_\text{red}} \right), with C_s = C_b \left(1 - \frac{i}{i_L}\right) substituting for the surface oxidized species concentration. As i approaches i_L, \eta_\text{conc} diverges, limiting the maximum achievable current.^[29] Hydrodynamics plays a critical role in modulating concentration overpotential by controlling the diffusion layer thickness \delta. In quiescent solutions, natural convection may yield \delta on the order of 0.1–1 mm, but forced convection—such as stirring, electrolyte flow, or electrode rotation—thins the layer, enhancing mass transport and elevating i_L. For instance, in the rotating disk electrode (RDE) setup, the Levich equation describes i_L = 0.62 n F D^{2/3} \omega^{1/2} \nu^{-1/6} C_b, where \omega is the rotation speed and \nu is kinematic viscosity; this demonstrates how increasing rotation reduces \delta \propto \omega^{-1/2}, thereby minimizing \eta_\text{conc}. Such hydrodynamic enhancements are essential in practical systems to sustain high currents without excessive polarization.^[32] In industrial applications like chlor-alkali electrolysis, concentration overpotential becomes significant at high current densities (e.g., >200 mA cm⁻²), often exceeding 0.2 V due to depleted chloride ions at the anode and hydroxide accumulation at the cathode. Gas bubbles from chlorine and hydrogen evolution further hinder mass transport by adsorbing on the electrode surface, blocking active sites and effectively increasing \delta by reducing the accessible area for diffusion; this bubble-induced effect can amplify \eta_\text{conc} by altering local hydrodynamics and species flux.^[30] To mitigate these losses, strategies include employing flow cells to promote convection and thin \delta, or adding surfactants and polymers as bubble suppressants and diffusion enhancers, which can reduce \eta_\text{conc} by 10–30% in gas-evolving systems.^[33]

Ohmic Overpotential

Ohmic overpotential, often referred to as the ohmic drop or IR loss, represents the voltage deviation in an electrochemical cell arising from resistive losses during current flow. It is mathematically expressed as \eta_{\text{ohmic}} = i R, where i is the current density and R is the total cell resistance, encompassing contributions from the electrolyte, electrodes, external leads, and interfaces.^[34]^[35] This linear relationship with current distinguishes it as a direct ohmic process, independent of reaction kinetics or mass transport limitations. The primary sources of this resistance include ionic conduction in the electrolyte, electronic conduction in the electrode materials and cell hardware, and contact resistances at electrode-electrolyte boundaries. Electrolyte resistance is governed by its ionic conductivity \kappa = 1/\rho, where \rho is the specific resistivity; low \kappa values lead to significant voltage drops, particularly in systems with poorly conducting media. Electrode resistivity arises from the material properties of conductors like carbon or metals, while contact resistance stems from imperfect interfaces that impede charge transfer.^[35]^[36] In polymer electrolyte membrane fuel cells, for instance, the membrane's ionic resistance dominates under humid conditions. The value of R is typically determined through electrochemical impedance spectroscopy (EIS), where the real-axis intercept at high frequencies in the Nyquist plot yields the ohmic resistance R_s. This method isolates the resistive component by probing the cell at frequencies where capacitive and inductive effects are minimal.^[37]^[38] Key factors influencing ohmic overpotential include temperature, electrolyte concentration, and cell geometry. Elevated temperatures enhance ion mobility, thereby increasing \kappa and reducing R; for example, a rise from 25°C to 80°C can halve the resistance in aqueous systems. Higher electrolyte concentrations generally lower resistance up to an optimal point, as seen in 1 M KOH solutions with a conductivity of approximately 0.26 S/cm, which minimizes ionic losses in alkaline electrolyzers. Cell design affects the effective path length for charge carriers; shorter distances between electrodes reduce the iR drop, particularly in thin-layer configurations.^[34]^[39]^[40] In practical applications, ohmic overpotential manifests as modest but cumulative losses. For proton exchange membrane fuel cells operating at 1 A/cm², it typically ranges from 0.05 to 0.1 V, primarily due to membrane and bipolar plate resistances. In lithium-ion batteries, internal ohmic resistance causes observable voltage sag during high-rate discharge, limiting power delivery and efficiency. To mitigate these effects, strategies focus on employing highly conductive electrolytes, such as those with optimized salt concentrations, or utilizing thin electrolyte films to shorten ion pathways and minimize R.^[41]^[42]^[43]

Applications and Measurement

Role in Electrochemical Devices

Overpotential significantly impacts the efficiency of electrochemical devices by necessitating additional voltage beyond the thermodynamic reversible potential, resulting in energy dissipation primarily as heat. The cell efficiency can be expressed as ε = ΔG / (n F E_applied), where ΔG is the Gibbs free energy change, n is the number of electrons transferred, F is the Faraday constant, and E_applied is the applied voltage, which exceeds the reversible potential due to overpotential, yielding ε < 100%. In water electrolysis, for instance, total overpotentials typically range from 0.5 to 1 V, leading to 30-50% energy loss as heat and reducing overall system efficiency to 50-70%.^[44] In practical applications, overpotential manifests across various electrochemical devices, limiting performance and output. In lithium-ion batteries during discharge, overpotentials around 0.1 V arise mainly from activation and ohmic contributions, contributing to voltage drops and reduced energy density under load. In proton exchange membrane fuel cells (PEMFCs), the oxygen reduction reaction (ORR) at the cathode incurs overpotentials exceeding 0.3 V, which dominates losses and lowers the open-circuit voltage from the theoretical 1.23 V to practical values below 0.9 V. For electrolyzers, combined hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials often surpass 0.4 V, elevating the required cell voltage to 1.8-2.0 V and hindering scalable hydrogen production.^[45]^[46]^[47] To mitigate these losses, design strategies target specific overpotential components through material and system optimizations. Catalyst optimization lowers activation overpotential (η_act); for example, IrO2-based electrocatalysts for OER achieve overpotentials as low as 0.2 V at 10 mA/cm², outperforming alternatives like RuO2 in acidic conditions due to enhanced stability and activity. Concentration overpotential (η_conc) is addressed via flow systems in electrolyzers, which improve mass transport and reduce polarization at high currents by up to 20%. Ohmic overpotential (η_ohmic) is minimized with conductive additives, such as carbon nanotubes in electrodes, decreasing internal resistance and voltage drops in batteries and fuel cells.^[48] High overpotentials elevate operational costs and environmental footprints in energy systems. In green hydrogen production via electrolysis, overpotentials contribute to energy demands of 50-60 kWh/kg H₂ for typical PEM and alkaline electrolyzers, while advanced solid oxide electrolysis cells (SOEC) achieve 40-45 kWh/kg as of 2025, compared to the theoretical minimum of 39.4 kWh/kg based on the higher heating value; this results in energy inefficiencies of 10-50% depending on technology, increasing electricity costs by 10-35% relative to the theoretical minimum and limiting competitiveness with fossil-based gray hydrogen (typically 1-2 €/kg).^[49]^[50] This inefficiency amplifies carbon emissions indirectly through higher renewable energy consumption and strains grid infrastructure for large-scale deployment. Recent advancements since 2020 have focused on nanostructured electrodes to reduce total overpotential in alkaline electrolyzers. For instance, nickel-iron layered double hydroxides on porous substrates have lowered cell overpotentials by 20-30% at industrial currents (e.g., 500 mA/cm²), enabling efficiencies above 75% through improved active site exposure and bubble management. As of 2025, SOEC technologies have further advanced, achieving stack-level efficiencies of 34 kWh/kg H₂, supporting global targets for cost reductions toward 2 €/kg H₂ by 2030. These developments, including 3D nanostructured Ni foams, enhance durability and scalability.^[51]^[52]^[49]^[53]

Experimental Measurement Techniques

Polarization curves provide a fundamental method for measuring total overpotential in electrochemical systems by plotting the electrode potential E against current density i. These curves are obtained through techniques such as linear sweep voltammetry or galvanostatic methods, where the applied potential or current is varied while monitoring the response, allowing the total overpotential \eta_{\text{total}} to be extracted by subtracting the reversible potential E_{\text{rev}} from the measured potential at each current. This approach captures the combined effects of all overpotential components and is widely used to assess overall cell performance.^[54] Tafel analysis builds on polarization data to isolate activation overpotential \eta_{\text{act}} by fitting the linear region of the overpotential versus the logarithm of current density, \eta vs. \log i. In this regime, the slope of the linear fit yields the Tafel slope b, which relates to the charge transfer coefficient \alpha via b = 2.303 RT / ((1-\alpha) n F), while extrapolation to \log i = 0 provides the exchange current density i_0, a key kinetic parameter. This method is particularly effective for low to moderate overpotentials where activation dominates, enabling quantification of reaction kinetics without interference from other components when combined with corrections.^[55]^[56] Electrochemical impedance spectroscopy (EIS) offers a frequency-domain approach to separate overpotential components by applying a small sinusoidal perturbation and analyzing the impedance response, often visualized in Nyquist plots of imaginary versus real impedance. The high-frequency intercept on the real axis corresponds to the ohmic overpotential \eta_{\text{ohmic}} via the solution resistance R_s, the mid-frequency semicircle diameter reflects charge transfer resistance R_{ct} associated with activation overpotential, and the low-frequency tail or Warburg element indicates concentration overpotential \eta_{\text{conc}} due to mass transport limitations. Equivalent circuit modeling of the spectra allows precise deconvolution of these contributions, making EIS valuable for identifying rate-limiting processes in complex systems.^[36]^[57] Limiting current methods, particularly using the rotating disk electrode (RDE), quantify concentration overpotential by varying the rotation speed \omega to measure the limiting current density i_L, which is related to mass transport via the Levich equation: i_L = 0.620 n F A D^{2/3} \nu^{-1/6} C \omega^{1/2}, where D is the diffusion coefficient, \nu the kinematic viscosity, C the bulk concentration, n the number of electrons, F Faraday's constant, and A the electrode area. By plotting i_L against \omega^{1/2} (Levich plot) and fitting the slope, diffusion parameters are obtained, enabling calculation of \eta_{\text{conc}} as the deviation from i_L at operating currents. This technique is essential for convective mass transport studies, providing insights into reactant depletion effects.^[58]^[59] Accurate measurement of overpotential relies on the three-electrode setup, comprising a working electrode (where the reaction occurs), a counter electrode (to complete the circuit), and a reference electrode (to maintain a stable potential benchmark). This configuration isolates the working electrode potential, minimizing contributions from counter electrode processes and uncompensated resistance, thus providing precise \eta values relative to the reversible potential. Without it, two-electrode systems can introduce errors from asymmetric drops, underscoring the three-electrode cell's role in standard electrochemical experimentation.^[60] Challenges in these measurements include compensating for ohmic drops (iR) and minimizing error sources such as temperature fluctuations. The current interrupt method addresses iR by briefly halting the current and measuring the instantaneous voltage relaxation, which equals the uncompensated resistance drop iR_u, allowing subtraction from recorded potentials; this technique is simple and non-perturbing for DC measurements. Positive feedback or hardware compensation can also apply 85-95% correction automatically, but requires careful tuning to avoid oscillations. Temperature variations affect kinetics and diffusion rates, potentially skewing \eta by 1-2 mV/°C, necessitating controlled environments like thermostated cells to ensure reproducibility.^[61]^[62]

References

[1]
Determining the Overpotential for a Molecular Electrocatalyst
Dec 30, 2013 · Overpotential is the difference between the equilibrium potential and the catalytic potential. 2. We recommend defining the catalytic potential ...
[2]
[PDF] Lecture 2: Basic Physics of Galvanic Cells & Electrochemical Energy ...
The overpotential refers to the potential difference between the thermodynamically determined reduction potential and experimentally observed value of a redox ...
[3]
[PDF] Determining the Overpotential of Electrochemical Fuel ... - OSTI.GOV
Overpotential (η) is defined as the additional potential (in excess of the thermodynamic requirement) needed to drive reaction at a specific current density or ...
[4]
[PDF] MITLibraries DISCLAIMER OF QUALITY - DSpace@MIT
May 6, 2004 · The total overpotential can be subdivided into three sections: activation overpotential, Mass transfer overpotential, and ohmic overpotential.
[5]
[PDF] 10.626 Lecture Notes, Butler-Volmer equation - MIT OpenCourseWare
Overpotential is. This is the “extra voltage” which drives a Faradaic current. In terms of overpotential, the reaction rate is .
[6]
A comprehensive review on the electrochemical parameters and ...
There are different types of overpotential in water splitting i.e., activation overpotential, ohmic overpotential, concentration overpotential, bubble ...
[7]
[PDF] fuel cell system - OSTI.GOV
Specifically, concentration overpotential, activation. 9 overpotential, and ohmic overpotential were all separated; bulk liquid ohmic drop was also. 10.
[8]
Prediction of overpotential and concentration profiles in solid oxide ...
The accuracy of prediction of overpotential and relevant concentration distributions is important for understanding fundamentals and electrochemical reaction
[9]
Nernst Equation - Chemistry LibreTexts
Aug 29, 2023 · The Nernst Equation enables the determination of cell potential under non-standard conditions. It relates the measured cell potential to the reaction quotient.
[10]
[PDF] Relation between Free Energy and Electrode Potential, and
Derivation of the Nernst Equation. We can define a value ∆Go for any ... Eo = standard reversible electrode potential. F = Faraday's constant. And so,. +.
[11]
17.2: The Gibbs Free Energy and Cell Voltage - Chemistry LibreTexts
Apr 12, 2020 · The Relationship between Cell Potential & Gibbs Energy. Electrochemical cells convert chemical energy to electrical energy and vice versa. The ...
[12]
6.2: Standard Electrode Potentials - Chemistry LibreTexts
May 8, 2021 · The potential of the standard hydrogen electrode (SHE) is defined as 0 V under standard conditions. The potential of a half-reaction measured ...
[13]
A comprehensive review on the recent developments in transition ...
Dec 1, 2021 · The theoretical potential value of the OER is 1.23 V at 25 °C (vs. reversible hydrogen electrode (RHE)) in all reaction medium [16]. Since, OER ...
[14]
Chapter 12: Electrochemical Energy Conversion - Books
The entropy production is actively used to find the electric potential profile under reversible conditions and to define the overpotential.
[15]
Insight into the overpotentials of electrocatalytic hydrogen evolution ...
Oct 20, 2020 · The origin of thermodynamic overpotential in HER is studied systemically based on the proton-coupled electron transfer (PCET) theory.
[16]
A hundred years of Tafel's Equation: 1905–2005 - ResearchGate
Aug 6, 2025 · Even after 100 years, Tafel's law remains highly relevant, evidenced by its widespread use in electrochemistry [45] , corrosion science [46], ...
[17]
An overview of water electrolysis technologies for green hydrogen ...
Aug 23, 2022 · (1)) requires 1.23 V theoretical thermodynamic cell voltage to split the water into hydrogen and oxygen at room temperature. However ...
[18]
[PDF] ACTIVATION OVERPOTENTIAL
Hence we see that activation overpotential consists of two components: • Electrical energy associated due to the Interaction between ion & E. 一о • Activation ...
[19]
Effects of activation overpotential in photoelectrochemical cells ... - NIH
The overpotential is the excess potential applied to an electrode to initiate electron transfer from the electrode to the electrolytic solution in an ...
[20]
The Butler-Volmer equation in electrochemical theory: Origins, value ...
Sep 1, 2020 · The Butler-Volmer equation describes the relation between electrode potential and current density, used to describe electrode kinetics in ...<|control11|><|separator|>
[21]
[PDF] Lecture #19 of 20+ - UCI Department of Chemistry
Kinetic overpotential is often just called overpotential, but can also be called activation overpotential… … okay, that summarizes this topic nicely… there ...
[22]
Disentangling Multiple pH-Dependent Factors on the Hydrogen ...
Understanding how the electrolyte pH affects electrocatalytic activity is a topic of crucial importance in a large variety of systems.
[23]
Effects of activation overpotential in photoelectrochemical cells ...
The overpotential is the excess potential applied to an electrode to initiate ... Here, the total activation overpotential is η a = η a , c + η a , a .Missing: E_applied - E_rev
[24]
Platinum Electrodes for Hydrogen Evolution in Alkaline Media
Thereby, the overpotential at 10 mA cm−2 was approximately 128 mV, comparable to 130 mV reported by Feliu et al.19 and 124 mV reported by Yu et al.20 The ...
[25]
Electrocatalysts for the Oxygen Reduction Reaction: From Bimetallic ...
In acidic media, the best know catalyst for the ORR is platinum (Pt), but even on Pt, the overpotential for the ORR is around 0.3 V [8,13,14]. Pt is also one ...<|separator|>
[26]
Deciphering the high overpotential of the oxygen reduction reaction ...
Feb 28, 2024 · The onset potential of the oxygen reduction reaction (ORR) maintains an activation overpotential exceeding ∼0.20 V, even with the most efficient catalysts.
[27]
Molecular Catalysis of Electrochemical Reactions. Overpotential and ...
The import of the notion of overpotential from the domain of electrocatalysis is a source of confusion.
[28]
A review of oxygen reduction mechanisms for metal-free carbon ...
Jul 19, 2019 · The overpotential of ORR can be determined by examining the reaction free energy of the different elementary steps, based on the assumption ...Missing: HER | Show results with:HER
[29]
[PDF] Lecture #19 of 20+ - UCI Chemistry
Dec 8, 2023 · An overpotential that is derived from rate-limiting mass transfer alone is called a concentration overpotential, ηconc… … it is also called ...
[30]
Impact of Gas Bubble Evolution Dynamics on Electrochemical ...
Feb 25, 2025 · This study establishes a theoretical framework to elucidate the impact of gas bubble evolution dynamics on the reaction overpotentials in electrolytic hydrogen ...
[31]
[PDF] Lecture #14 of 18(?)
how tiny (or large) is this diffusion-limited current (density)?. Let's ... What's the value of the limiting current? i = −nFAD. ΔC. 𝛿. i = 1 eq ...
[32]
[PDF] Chapter 16 Electrochemical Processes This chapter considers some ...
Mar 31, 2005 · The overpotential for each reaction must also be calculated or measured with respect to its own equilibrium potential. If β = 0.5, the Butler- ...
[33]
Influence of Bubbles on the Energy Conversion Efficiency of ...
Mar 18, 2020 · This review synthesizes our current understanding on the relationship between bubble evolution and energy losses in electrochemical reactors.Missing: thickness | Show results with:thickness
[34]
[PDF] Lecture 9 Fuel Cells at Finite Current Ahmed Ghoniem
Mar 2, 2020 · cathodic concentration overpotential, cathodic activation is more sluggish and hence the fast drop. Ong and Ghoniem, overpotential, etc.
[35]
Ohmic Resistance - an overview | ScienceDirect Topics
... cell hardware and other inter connections. Three main sources of ohmic resistances are: (a) ion migration resistance within the electrolyte, (b) resistance ...
[36]
Electrochemical Impedance Spectroscopy A Tutorial
In practice, this ohmic resistance also includes the ohmic resistance of the connection cables and the ohmic resistance of the working electrode, which in ...
[37]
[PDF] Electrochemical Impedance Spectroscopy Primer
The solution resistance can found by reading the real axis value at the high frequency intercept. This is the intercept near the origin of the plot.
[38]
[PDF] Impedance Basics - University of Washington
The high-frequency intercept yields Rs, the value of frequency-independent contributions, most commonly the ohmic resistance of the electrolyte. The low ...
[39]
A review of specific conductivities of potassium hydroxide solutions ...
Aug 9, 2025 · The conductivity of the KOH electrolyte increases steadily from 0.2592 S/cm at 1 M to 0.8117 S/cm at 7 M but decreases from 0.7950 S/cm to ...
[40]
Macro-level Electrochemical Modeling Strategies of Low ...
Jun 3, 2025 · The key factor is decreased ohmic resistance at higher temperatures, which lowers operating cell voltage. At higher temperatures, the ...
[41]
[PDF] Fuel Cell Handbook (Seventh Edition)
Page 1. Fuel Cell Handbook. (Seventh Edition). By. EG&G Technical Services, Inc ... (A/cm2). C e ll V o lta g e. (V. ) '91, d, 5 atm, 95°C. '01, b, 3 atm, 70°C.
[42]
[PDF] Electrochemistry-based Battery Modeling for Prognostics
The concentration overpotential is the difference in voltage between the two ... equation. The equation is given as. Ji = Ji0. ( exp. ((1 - α)F. RT. Vη,i. ).
[43]
Highly conductive and stable electrolytes for solid oxide electrolysis ...
Nov 27, 2024 · 22,23 Besides, the electrolyte should be as thin as possible with good mechanical properties to decrease the ohmic overpotential, and the ...
[44]
Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for ...
Jun 24, 2024 · Therefore, highly efficient electrocatalytic electrodes require careful consideration of three key factors: 1) High intrinsic activity catalyst, ...
[45]
Capturing the Current-Overpotential Nonlinearity of Lithium-Ion ...
The overpotentials of the cathode are strongly dependent on the state of charge of the lithium-ion battery and generally increase with decreasing cell voltage ( ...Theory · Measurement Methods · Results · Conclusion
[46]
Theoretical Studies of Potential-Dependent and Competing ...
Aug 7, 2025 · Industrial research on ORR is focused on determining the cause of significant kinetic overpotential (over 0.3 V) that reduces the cell voltage ...
[47]
Hydrogen production from water electrolysis: role of catalysts
Feb 11, 2021 · For electrochemical water splitting reaction, the thermodynamic potential is 1.23 V at 25 °C and 1 atm. ... overpotential vs. kinetic ...
[48]
Iridium metallene oxide for acidic oxygen evolution catalysis - Nature
Oct 14, 2021 · This material demonstrates high activity for oxygen evolution reaction with a low overpotential of 197 millivolt in acidic electrolyte.
[49]
Alkaline Water Electrolysis for Green Hydrogen Production
These modified electrodes outperform any other reported electrocatalysts tested under similar conditions. Moreover, the effect of Ni to Fe ratio, cluster ...
[50]
Recent progress of enhanced bubble separation in alkaline water ...
Several studies have shown that nickel foam (NF) electrodes with porous structures reduce overpotential and increase current densities in the electrolyzer.<|control11|><|separator|>
[51]
Polarization Curves: setup, recording, processing and features
Polarization curves are made by making a linear sweep of the sample's potential over time. In analytical research fields of electrochemistry this technique is ...
[52]
Tafel Slope Plot as a Tool to Analyze Electrocatalytic Reactions
The Tafel slope is then obtained from the linear fit of the overpotential vs log J or from the overpotential vs log 1/Rct for LSV (CA/CP) and impedance ...
[53]
How to extract kinetic information from Tafel analysis in ...
Dec 8, 2023 · Tafel analysis is based on the ln-linear relationship between current and overpotential to experimentally investigate the electrode kinetic parameters.
[54]
[PDF] Ohmic Drop Part 2 – Measurement - Metrohm
Current interrupt and positive feedback are fast methods, but care is necessary for their use in order to avoid data misinterpretation or damage to the setup.<|control11|><|separator|>
[55]
Rotating Ring Disk Electrode Fundamentals
Sep 19, 2024 · ... rotating disk electrode is given by the Levich equation,2,9. {i_{LC} = 0.620 n F A D_O^{2/3} (10). in terms of the concentration of the ...
[56]
Levich and Koutecký-Levich analysis tools: Electrochemical reaction ...
Jan 31, 2024 · This note describes these analysis tools that can be used to analyse data obtained with a Rotating Disk Electrode (RDE).
[57]
Precautions of Using Three-Electrode Configuration to Measure ...
Aug 6, 2020 · Three-electrode configuration is a standard scheme to measure the overpotential of a specific electrode in electrochemical cells involving ...
[58]
Navigating iR Compensation: Practical Considerations for Accurate ...
Sep 22, 2023 · The DC technique-based current interruption method is typically used to measure the series resistance in an electrochemical system, which ...Supporting Information · Author Information · References
[59]
iR Compensation for Electrocatalysis Studies: Considerations and ...
Mar 28, 2023 · iR compensation is developed to correct for the voltage loss (ie, iR drop) caused by the electrolyte solution between the working electrode and the reference ...Supporting Information · Author Information · References