What are the primary components of operational potential drop (overpotential) in fuel cell and electrolyzer cell testing? Unlock the Keys to Better Cell Performance

Operational overpotential is the gap between the thermodynamic cell potential and the potential actually required or delivered during operation. Its primary components are activation overpotential at the anode and cathode, ohmic potential drop through the electrolyte and conductive interfaces, and mass-transport or concentration overpotential. In practical testing, contact resistance and other assembly-related losses are usually treated as part of the ohmic contribution.

The measured cell polarization is not caused by one loss mechanism. It is the combined effect of reaction kinetics, ionic and electronic resistance, and reactant or product transport limitations.

The Main Components of Operational Potential Drop

Activation overpotential at the electrodes

Activation overpotential is the extra potential required to drive an electrode reaction at a specified rate. It reflects the intrinsic kinetics of charge transfer and the effectiveness of the catalyst, electrode structure, temperature, and operating conditions.

Both electrodes contribute:

• Anode activation overpotential
• Cathode activation overpotential

The dominant electrode can differ between fuel cells and electrolyzers. For example, oxygen-related reactions are often kinetically demanding because they involve multiple electron-transfer steps and O–O bond formation or cleavage.

Oxygen-reaction kinetics

Oxygen electrode reactions can create a particularly large activation loss. The primary reference notes that molecular oxygen adsorption and O=O bond cleavage can account for approximately 300 mV of loss even on platinum under relevant conditions.

This loss is influenced by catalyst composition, active surface area, catalyst utilization, temperature, and electrode microstructure.

Ohmic potential drop

Ohmic overpotential is the voltage lost when ionic or electronic current passes through resistive components of the cell. It generally increases approximately in proportion to current density.

The main contributors are:

• Electrolyte or membrane resistance
• Electrode and current-collector resistance
• Contact resistance between layers and components
• Interfacial resistance within the cell assembly

Contact resistance is especially important because imperfect compression, contamination, poor surface conformity, or unsuitable materials can create a significant voltage drop that may be incorrectly attributed to the catalyst.

Mass-transport or concentration overpotential

Mass-transport overpotential arises when reactants cannot reach the active reaction sites, or products cannot be removed, quickly enough to sustain the imposed current.

It can result from:

• Limited gas or liquid supply
• Diffusion through porous electrodes
• Flooding or drying
• Product accumulation
• Inadequate flow-field design
• Operation near the limiting current

This contribution is often small at low current density but becomes increasingly important at high load. It should therefore be separated from activation and purely ohmic losses when interpreting polarization curves.

How the Losses Appear in Testing

Fuel cell operation

In a fuel cell, irreversible losses reduce the operating voltage below the reversible thermodynamic voltage. The measured cell voltage can be understood as the equilibrium voltage minus activation, ohmic, and mass-transport losses, with additional corrections where relevant.

At low current density, activation losses commonly dominate. In the intermediate region, ohmic losses become prominent, while mass-transport losses often dominate at high current density.

Electrolyzer operation

In an electrolyzer, the applied voltage must exceed the thermodynamic decomposition voltage. The required excess voltage consists primarily of electrode activation losses, ohmic losses, and mass-transport losses.

The same physical mechanisms are present, but the voltage interpretation is reversed: losses increase the required input voltage rather than reducing an output voltage.

Separating the contributions

A polarization curve alone shows the combined effect of the loss mechanisms. It does not, by itself, uniquely identify how much voltage is associated with each component.

Useful diagnostic methods include:

• Electrochemical impedance spectroscopy for separating resistive and kinetic processes
• High-frequency resistance measurements for estimating ohmic contributions
• Reference electrodes for resolving anode and cathode potentials
• Gas-flow, pressure, and humidity variation for identifying transport limitations
• Temperature and catalyst-loading studies for evaluating kinetic effects

Why Component Identification Matters

Improving catalyst and electrode design

If activation losses dominate, improvements should focus on catalyst activity, catalyst utilization, active surface area, electrode ionomer distribution, and reaction-layer structure.

Increasing catalyst loading alone may not solve the problem if poor transport or inadequate electronic and ionic connectivity prevents the additional catalyst from being used effectively.

Improving cell assembly

If the main loss is ohmic, attention should shift to membrane or electrolyte thickness, conductivity, current collectors, compression, interfacial contact, and assembly tolerances.

This is why measuring contact resistance separately can be important: an apparently poor electrochemical performance may originate from the hardware rather than the catalyst.

Improving operating conditions

If mass transport dominates, the relevant changes may involve flow rate, pressure, humidity, temperature, porous-layer structure, flow-field design, or product removal.

The correct intervention depends on the limiting mechanism; changing catalyst chemistry will have limited effect if the electrode is flooded or reactant-starved.

Understanding the Trade-offs

Losses are coupled

The three major loss categories are conceptually distinct but not always experimentally independent. For example, changing electrode porosity can improve reactant transport while reducing electrical contact or increasing ionic path length.

Consequently, a fitted loss model should be checked against operating-condition changes rather than accepted solely because it matches one polarization curve.

High catalyst loading is not automatically better

More catalyst can reduce activation losses only when the added material is accessible to reactants, electronically connected, and ionically connected.

Excessive loading can increase thickness, transport resistance, water-management problems, and material cost.

Apparent activation losses can be misleading

Uncompensated resistance, gas-transport limitations, temperature gradients, and contact resistance can distort the apparent kinetic slope. These effects should be corrected or independently measured before extracting intrinsic kinetic parameters.

Making the Right Choice for Your Goal

Use the dominant loss mechanism to guide both testing and design decisions:

• If your primary focus is electrode kinetics: Separate anode and cathode potentials, control temperature and reactant conditions, and evaluate activation overpotential before changing catalyst loading.
• If your primary focus is cell efficiency: Quantify electrolyte, electronic, and contact resistance using resistance measurements and impedance methods.
• If your primary focus is high-current operation: Investigate mass transport, flooding, drying, reactant supply, and product removal.
• If your primary focus is assembly quality: Measure interfacial and contact resistance independently rather than attributing every voltage loss to electrochemical activity.

Understanding which component causes the potential drop turns a polarization measurement into a practical guide for improving the catalyst, electrode, operating conditions, and cell assembly.

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