0.57
V
0.47
V
0.1
V
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0.57
V
0.47
V
0.1
V
—
The Overpotential Calculator determines the excess voltage beyond the thermodynamic equilibrium potential required to drive an electrochemical reaction at a given rate. Overpotential (η) is the difference between the applied potential and the equilibrium potential, and it represents energy losses in the electrochemical process. These losses arise from three sources: activation overpotential (kinetic barrier at the electrode surface), concentration overpotential (mass transport limitations), and ohmic overpotential (resistance of electrolyte, leads, and interfaces). Understanding and minimizing overpotential is critical for fuel cell optimization, battery efficiency, electrolysis energy costs, corrosion engineering, and electrocatalyst development.
The total overpotential is defined as:
$$\eta_{total} = |E_{applied}| - |E_{equilibrium}|$$
This total overpotential can be decomposed into components:
$$\eta_{total} = \eta_{activation} + \eta_{concentration} + \eta_{ohmic}$$
The activation overpotential (η_act) is described by the Butler-Volmer or Tafel equation and depends on electrode kinetics. The concentration overpotential (η_conc) arises when mass transport limits the reaction rate:
$$\eta_{conc} = \frac{RT}{nF} \ln\left(1 - \frac{i}{i_L}\right)$$
where i_L is the limiting current density. The ohmic overpotential follows Ohm's law:
$$\eta_{ohmic} = I \times R$$
The electrode overpotential (activation + concentration) is obtained by subtracting the IR drop from the total: η_electrode = η_total − IR.
If electrode overpotential dominates (η_electrode >> IR), the process is kinetically limited — a better electrocatalyst would help. If ohmic drop dominates (IR >> η_electrode), the process is resistance-limited — reducing electrode gap, increasing conductivity, or using membranes would help. The classification output suggests which improvement strategy would be most effective. In fuel cells, activation overpotential at the oxygen cathode is typically the largest loss. In industrial electrolysis, IR drop through the electrolyte is often dominant. Reducing total overpotential directly improves energy efficiency and reduces operating costs.
Inputs
Results
Total η = 1.80 − 1.23 = 0.570 V. After subtracting IR = 0.10 V, the electrode overpotential is 0.470 V. This is activation-dominated — the oxygen evolution reaction requires significant kinetic overpotential. A better OER catalyst would reduce operating voltage.
Inputs
Results
Total η = 4.50 − 2.20 = 2.30 V. IR drop = 1.50 V is the dominant loss (65%). Electrode η = 0.80 V. This cell is ohmic-dominated — reducing electrolyte resistance or electrode spacing would be the most effective improvement.
Overpotential is the excess voltage beyond the thermodynamic equilibrium potential required to drive an electrode reaction at a finite rate. It represents energy that is dissipated as heat rather than performing useful electrochemical work.
Activation overpotential (kinetic barrier for electron transfer), concentration overpotential (mass transport limitation of reactants to the electrode surface), and ohmic overpotential (electrical resistance of the electrolyte and cell components).
The oxygen evolution reaction (OER) involves a 4-electron transfer mechanism with multiple intermediate steps. Each step has an activation barrier. Even the best catalysts (IrO₂, RuO₂) require 200–400 mV overpotential. This is the main efficiency bottleneck in water electrolysis.
During charging, overpotential means you must apply more voltage than the thermodynamic potential (energy wasted as heat). During discharging, overpotential means you get less voltage out. Total round-trip efficiency decreases with increasing overpotential.
The exchange current density (i₀) is the rate of both forward and reverse electrode reactions at equilibrium. High i₀ means fast kinetics and low activation overpotential. Platinum has high i₀ for hydrogen evolution (~10⁻³ A/cm²), while lead has very low i₀ (~10⁻¹³ A/cm²).
Electrocatalysts provide alternative reaction pathways with lower activation energies. They increase the exchange current density, reducing the overpotential needed to achieve a given current density. Platinum group metals are the most effective but also most expensive catalysts.
IR compensation (or iR correction) is an experimental technique to remove the ohmic drop from measured potentials. This is done using current interrupt or positive feedback methods in potentiostats, allowing measurement of the true electrode overpotential.
Higher temperature generally decreases activation overpotential (faster kinetics via Arrhenius equation), decreases ohmic overpotential (better electrolyte conductivity), and may increase or decrease concentration overpotential depending on the system.
When the electrode reaction is fast enough to deplete reactants at the surface faster than diffusion can supply them, the surface concentration drops below the bulk value. This creates an additional potential loss described by η_conc = (RT/nF)ln(1 − i/i_L).
Using a three-electrode setup: working electrode, counter electrode, and reference electrode. The reference electrode measures the potential at the working electrode surface. A potentiostat controls and measures the overpotential while monitoring current response.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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