How To Demonstrate Overpotential Contributions Using Electrochemical Pilot Plants: A Guide For Labs

The key is strategic placement of reference electrodes. In an electrochemical unit operations pilot plant, instructors can separate the overall cell voltage into its constituent losses—ohmic drop, concentration overpotential, and surface overpotential—by positioning reference electrodes at carefully chosen locations. With the cell operating under load, students measure potential differences against these probes to map the entire potential landscape, directly observing each inefficiency that deviates from the thermodynamic ideal. This turns abstract textbook concepts into a quantitative, visual diagnosis of where and why energy is lost.

The core insight: By treating the pilot plant as a potential-mapping tool, reference electrodes placed within the diffusion layer, at the electrode surface, and across the bulk solution reveal the distinct signatures of mass-transfer, kinetic, and resistive losses. The overall cell voltage is the sum of these measurable drops, and each can be isolated to teach the real-world limitations of electrochemical processes.

Deconstructing Cell Voltage: The Three Fundamental Losses

Every real electrochemical cell operates away from its equilibrium potential. The extra voltage beyond the thermodynamic requirement—the overpotential—has three distinct roots that a well-designed pilot plant can expose.

The Ohmic Drop: Pure Resistance in the Solution

The ohmic drop is the simplest to visualize. It arises from the electrical resistance of the ionic solution between the anode and cathode.

This loss follows Ohm’s law and scales linearly with current. In a pilot plant, it is the voltage needed simply to push ions through the electrolyte.

By placing two reference electrodes far from the electrode surfaces—deep in the bulk solution—students can measure the potential difference caused solely by solution resistance. This reading, when subtracted from the total cell voltage, isolates one part of the energy penalty.

Concentration Overpotential: When Mass Transfer Cannot Keep Up

Concentration overpotential stems from thermodynamic shifts. As a reaction proceeds, the local concentration of reactants at the electrode surface drops below the bulk concentration, while products accumulate.

This concentration difference alters the Nernstian equilibrium potential at that electrode. The overpotential is the extra voltage demanded to maintain current despite the depleted surface.

Instructors can demonstrate this by placing a reference electrode at the electrode surface and another just outside the diffusion layer. The measured potential difference reflects the mass-transfer limitation, and students can manipulate stirring or flow rates to see it shrink or grow in real time.

Surface Overpotential: The Kinetic Hurdle

The surface overpotential, often called activation overpotential, represents the driving force required to overcome the activation energy barrier of the electron-transfer reaction itself.

Even if reactants are plentiful, the reaction needs a finite voltage push to proceed at an appreciable rate. This is a purely kinetic constraint, governed by the Butler-Volmer equation.

By measuring the potential at the electrode surface with a reference electrode and correcting for the concentration overpotential (by operating under fast mass transfer), the remaining deviation from equilibrium is the surface overpotential. Switching electrode materials in the pilot plant—say from graphite to platinum—immediately demonstrates how the kinetic barrier changes.

The Pilot Plant as a Potential-Mapping Instrument

An electrochemical unit operations pilot plant becomes a teaching instrument when equipped with a set of precisely placed reference electrodes. This approach works for both galvanic cells (such as fuel cell modules) and electrolytic cells (such as electroplating or electrolysis units).

Positioning Reference Electrodes Inside the Diffusion Layer

A miniature reference electrode, like a Luggin capillary, can be maneuvered to within microns of the working electrode. This probe, combined with a reference in the bulk solution, captures the potential drop across the entire diffusion layer.

Students record the voltage difference at varying current densities. The data clearly show the onset of a limiting current, where the concentration overpotential spikes—a direct thermodynamic consequence of mass-transfer starvation.

Capturing the Ohmic Signature with Bulk Probes

Two reference electrodes placed a known distance apart in the bulk electrolyte, far from both electrodes, measure the ohmic drop exclusively. The slope of voltage versus current gives the solution resistance.

When this slope is compared to measurements made at the same current but with a different electrode spacing, students can verify that ohmic loss is directly proportional to the path length. This demonstration solidifies the concept of conductivity as a bulk material property.

Switching Operational Modes: Galvanic vs. Electrolytic

Supplementary references show that pilot plants can be configured for both galvanic and electrolytic operation. In a galvanic cell (fuel cell), the overall cell voltage is the difference between the cathode and anode equilibrium potentials minus all losses; here, the overpotentials directly reduce the usable output.

In an electrolytic cell, an external power supply forces the reaction, and the overpotentials add to the minimum thermodynamic voltage required. By running the same pilot plant in both modes, instructors highlight that the same physical loss mechanisms steal energy in opposite directions—one reducing generation, the other increasing consumption.

Understanding the Trade-offs and Practical Pitfalls

No measurement technique is without its challenges. To produce clean educational data, instructors must anticipate and manage real-world complications.

Reference Electrode Placement Artifacts

Positioning a reference electrode too close to an electrode surface can itself distort the current distribution. A poorly shielded capillary creates a “shadow” effect, altering the very potential it is meant to measure.

Pilot plants must use thin, properly shielded Luggin capillaries. Even then, the measured potential contains an uncompensated ohmic component from the small gap between the capillary tip and the electrode.

Separating Concentration and Surface Overpotentials Is Difficult

At high current densities, concentration and surface overpotentials become intertwined. The surface kinetics depend on the local concentration, and the concentration profile is shaped by the reaction rate.

A clean separation requires operating under conditions where mass transfer is fast (e.g., high electrolyte flow, low current) to isolate the surface overpotential. Only then can the remaining loss be confidently attributed to kinetics.

Dynamic vs. Steady-State Measurements

Electrochemical systems have time constants. Changing the current demands that new concentration profiles be established, which can take minutes.

Instructors must build in sufficient wait times during experiments. A measurement taken prematurely will confuse transient mass-transfer lag with a true steady-state concentration overpotential.

Making the Right Choice for Your Teaching Lab

The specific goal of your course determines how you configure the pilot plant and which measurements you emphasize.

If your primary focus is visualizing the potential landscape: Use multiple reference electrodes with Luggin capillaries and map the potential versus distance from electrode surfaces at different currents. This creates a powerful, V-shaped potential profile that students can directly annotate with each overpotential region.
If your primary focus is quantifying energy efficiency losses: Operate the plant as an electrolytic cell under controlled current steps. Separate the total cell voltage into ohmic, concentration, and surface contributions at each step, then calculate the overpotential share as a percentage of the total energy input. Compare different electrode materials to emphasize the surface overpotential’s role.
If your primary focus is linking theory to industrial practice: Run the pilot plant in both fuel-cell (galvanic) and electroplating (electrolytic) modes. Have students measure the same losses under both spontaneity directions, reinforcing that ohmic drop, mass transfer, and kinetics are universal process constraints, not just academic constructs.

A pilot plant transforms overpotentials from invisible abstractions into measurable, improvable parameters—giving students the diagnostic language they will use to design and troubleshoot real-world electrochemical systems.

Summary Table:

Overpotential Type	Root Cause	Measurement Method	Demonstration Variable
Ohmic Drop	Solution resistance	Bulk reference electrodes	Adjust electrode spacing
Concentration	Mass-transfer limitation	Surface vs. bulk electrodes	Vary flow rates or stirring
Surface (Activation)	Kinetic reaction barrier	Surface electrode (fast mass transfer)	Change electrode materials

Elevate Your Engineering Lab with LABPARK

Are you looking to provide your students and researchers with hands-on, industry-relevant training? LABPARK designs and manufactures premium Educational and Vocational Unit Operations Pilot Plants in chemical engineering, bioprocess & biotech, and environmental & water treatment.

Our advanced electrochemical pilot plants allow instructors to easily demonstrate complex concepts like potential mapping, overpotential losses, and cell efficiency, helping students bridge the gap between textbook theory and real-world process constraints.

Contact LABPARK today to request a catalog or discuss a customized pilot plant solution for your university, research institute, or enterprise!