Overpotential Calculator

Find the activation overpotential of an electrode reaction directly, or from the Tafel equation using current density.

⚡ Overpotential Calculator
V
V
mA/cm²
mA/cm²
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K
Overpotential (η)
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Classification
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Step-by-step working
Overpotential (η)
—
Step-by-step working

⚡ What is the Overpotential Calculator?

This overpotential calculator finds η, the extra electrode potential beyond the thermodynamic equilibrium value that is needed to drive an electrochemical reaction at a given rate. Use Direct mode when you already know both the applied (measured) electrode potential and the equilibrium (reversible) potential. Use Tafel mode when you instead know the current density, the exchange current density, and basic kinetic parameters for an activation-controlled reaction.

Overpotential is central to battery design, electroplating, corrosion science, and fuel cell engineering. A battery's real-world efficiency is limited by the overpotential at each electrode during charge and discharge, an electroplating bath needs enough overpotential to deposit metal at a usable rate, and a fuel cell's power output is throttled by the overpotential needed to drive oxygen reduction at the cathode.

A common misconception is that overpotential is a fixed property of a metal or reaction. In reality it depends on current density, electrode surface, electrolyte composition, and temperature, which is exactly why the Tafel equation expresses η as a function of the current you are trying to drive, not as a single constant.

This calculator is useful for electrochemistry students, battery and fuel cell engineers, and corrosion scientists who need a fast, correct overpotential value without manually working through the Tafel equation or hunting down a sign convention by hand.

📐 Formula

η  =  Eapplied − Eeq
Eapplied = measured or applied electrode potential, V
Eeq = equilibrium (reversible) potential, V
Example: E=−0.85 V, Eeq=−0.76 V: η = −0.09 V (cathodic)
η  =  (2.303RT) / (αnF) × log10(i / i0)
R = 8.314 J/(mol·K), T = temperature, K
α = transfer coefficient (0-1), n = electrons transferred
F = 96,485 C/mol (Faraday constant)
i = current density, i0 = exchange current density (same units)
Example: i=10, i0=0.5 mA/cm², α=0.5, n=2, T=298.15K: η ≈ 0.077 V (Tafel equation, per standard electrochemical kinetics)

📖 How to Use This Calculator

Steps

1
Choose Direct or Tafel mode - Use Direct mode if you know the applied and equilibrium potentials. Use Tafel mode if you know current density, exchange current density, and kinetic parameters instead.
2
Enter the required inputs - Direct mode needs applied potential and equilibrium potential. Tafel mode needs current density, exchange current density, transfer coefficient, electrons transferred, and temperature.
3
Read the overpotential - See the overpotential value in volts and whether it is anodic or cathodic.
4
Use the Tafel plot to see the trend - In Tafel mode, see how overpotential changes with current density on a log scale, with your operating point marked.

💡 Example Calculations

Example 1 - Direct mode, cathodic overpotential

1
E applied = −0.85 V, Eeq = −0.76 V
2
η = −0.85 − (−0.76) = −0.09 V
3
Negative η means this is a cathodic overpotential, driving a reduction reaction
η = −0.09 V (Cathodic)
Try this example →

Example 2 - Tafel mode, moderate current density

1
i = 10 mA/cm², i0 = 0.5 mA/cm², α = 0.5, n = 2, T = 298.15 K
2
η = (2.303 × 8.314 × 298.15) / (0.5 × 2 × 96485) × log10(10/0.5) = 0.077 V
3
Positive η means this is an anodic overpotential
η = 0.077 V (Anodic)
Try this example →

Example 3 - Tafel mode, high current density, single-electron transfer

1
i = 50 mA/cm², i0 = 1 mA/cm², α = 0.5, n = 1, T = 298.15 K
2
η = (2.303 × 8.314 × 298.15) / (0.5 × 1 × 96485) × log10(50/1) = 0.201 V
3
A lower n and higher current ratio both increase the required overpotential
η = 0.201 V (Anodic)
Try this example →

❓ Frequently Asked Questions

What is overpotential in electrochemistry?+
Overpotential (η) is the difference between the actual electrode potential during current flow and the thermodynamic equilibrium potential for that reaction. It represents the extra energy needed to overcome reaction kinetics, ohmic resistance, or mass transport limits.
What is the Tafel equation used for?+
The Tafel equation, η = (2.303RT)/(αnF) log10(i/i0), relates activation overpotential to current density for an electrode reaction dominated by charge-transfer kinetics. It is used to extract kinetic parameters like exchange current density from experimental polarization data.
What is exchange current density?+
Exchange current density (i0) is the rate of the forward and reverse reactions at equilibrium, expressed as a current density, even though the net current is zero. A high i0 means the reaction is kinetically fast and needs little overpotential to proceed at a given rate.
What does a positive versus negative overpotential mean?+
A positive (anodic) overpotential drives an oxidation reaction faster than its equilibrium rate. A negative (cathodic) overpotential drives a reduction reaction faster. The sign always shows which direction is being pushed beyond equilibrium.
What is the transfer coefficient alpha in the Tafel equation?+
The transfer coefficient (alpha, typically 0.3-0.7, often approximated as 0.5) describes how the applied potential difference splits between speeding up the forward reaction and slowing the reverse reaction at the electrode. It is an empirical symmetry factor from Butler-Volmer kinetics.
Why does overpotential increase with current density?+
Higher current density means the reaction must proceed faster, which requires driving the electrode further from equilibrium. Because the Tafel equation has a logarithmic form, overpotential grows with the log of current density, not linearly.
What is a typical overpotential value?+
Typical activation overpotentials range from about 50 mV to 500 mV depending on the reaction and electrode material. Hydrogen evolution on platinum has very low overpotential (tens of mV), while oxygen evolution on most electrodes needs several hundred mV.
How is overpotential different from cell voltage loss?+
Overpotential specifically refers to activation, concentration, or ohmic losses at a single electrode relative to its own equilibrium potential. Total cell voltage loss sums the overpotentials at both electrodes plus any solution resistance (IR) drop between them.
Can overpotential be zero?+
Overpotential is zero only when the electrode is at true equilibrium with no net current flowing. Any nonzero current, however small, requires some overpotential to drive the net reaction rate, per the Butler-Volmer and Tafel relationships.
Why does the Tafel equation use log base 10 instead of natural log?+
The Tafel equation is traditionally written with log10 and the constant 2.303 (which equals ln 10) folded in, because early electrochemists plotted overpotential against log10 of current on Tafel plots. It is mathematically identical to writing it with ln and dropping the 2.303 factor.
Tags: Overpotential Calculator Tafel Equation Calculator Activation Overpotential Electrochemistry Calculator Formula Free Online

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What is overpotential in electrochemistry?

Overpotential (η) is the difference between the actual electrode potential during current flow and the thermodynamic equilibrium potential for that reaction. It represents the extra energy needed to overcome reaction kinetics, ohmic resistance, or mass transport limits.

What is the Tafel equation used for?

The Tafel equation, η = (2.303RT)/(αnF) log10(i/i0), relates activation overpotential to current density for an electrode reaction dominated by charge-transfer kinetics. It is used to extract kinetic parameters like exchange current density from experimental polarization data.

What is exchange current density?

Exchange current density (i0) is the rate of the forward and reverse reactions at equilibrium, expressed as a current density, even though the net current is zero. A high i0 means the reaction is kinetically fast and needs little overpotential to proceed at a given rate.

What does a positive versus negative overpotential mean?

A positive (anodic) overpotential drives an oxidation reaction faster than its equilibrium rate. A negative (cathodic) overpotential drives a reduction reaction faster. The sign always shows which direction is being pushed beyond equilibrium.

What is the transfer coefficient alpha in the Tafel equation?

The transfer coefficient (alpha, typically 0.3-0.7, often approximated as 0.5) describes how the applied potential difference splits between speeding up the forward reaction and slowing the reverse reaction at the electrode. It is an empirical symmetry factor from Butler-Volmer kinetics.

Why does overpotential increase with current density?

Higher current density means the reaction must proceed faster, which requires driving the electrode further from equilibrium. Because the Tafel equation has a logarithmic form, overpotential grows with the log of current density, not linearly.

What is a typical overpotential value?

Typical activation overpotentials range from about 50 mV to 500 mV depending on the reaction and electrode material. Hydrogen evolution on platinum has very low overpotential (tens of mV), while oxygen evolution on most electrodes needs several hundred mV.

How is overpotential different from cell voltage loss?

Overpotential specifically refers to activation, concentration, or ohmic losses at a single electrode relative to its own equilibrium potential. Total cell voltage loss sums the overpotentials at both electrodes plus any solution resistance (IR) drop between them.

Can overpotential be zero?

Overpotential is zero only when the electrode is at true equilibrium with no net current flowing. Any nonzero current, however small, requires some overpotential to drive the net reaction rate, per the Butler-Volmer and Tafel relationships.

Why does the Tafel equation use log base 10 instead of natural log?

The Tafel equation is traditionally written with log10 and the constant 2.303 (which equals ln 10) folded in, because early electrochemists plotted overpotential against log10 of current on Tafel plots. It is mathematically identical to writing it with ln and dropping the 2.303 factor.

📌 Quick Tips

💡Overpotential is the extra potential, beyond the thermodynamic equilibrium value, needed to drive a reaction at a given rate. It is always an energy penalty.
💡A positive (anodic) overpotential means oxidation is being driven; a negative (cathodic) overpotential means reduction is being driven.
💡The exchange current density i0 measures how kinetically fast a reaction is at equilibrium. A large i0 (like Pt for hydrogen evolution) needs very little overpotential to drive noticeable current.
💡The Tafel equation only holds for activation-controlled kinetics at moderate to high overpotential (typically |η| > 50-100 mV). At very low overpotential, use the full Butler-Volmer equation instead.
💡Total cell overpotential in real systems also includes ohmic (IR drop) and concentration (mass-transport) contributions, which this calculator does not include.