Platinum and Carbon as Counter Electrodes: Limits of Inertness

The CE (aka auxiliary electrode) has one job: to balance the charge generated at the working electrode (WE). To do this effectively, it must behave as a non-limiting source of current and without forcing the potentiostat to reach its compliance voltage.

While Platinum (Pt) and Carbon are the industry standards due to their perceived inertness, the truth is more nuanced: no electrode is 100% inert.

Here is what you need to know about Pt and C when acting as CE.

1. Platinum (Pt): The Standard

Platinum is favoured for its exceptional catalytic activity and high conductivity. In many cases, it behaves perfectly.

When is Pt truly inert?

In many cases, such as in low-current applications, for example, a Cyclic Voltammetry (CV) of 1 mM Ferrocene using a 1.5 mm WE and a large Pt mesh (e.g., 5 cm²).

Why? The current demand is so low that the double-layer charging and simple H⁺ adsorption on the Pt surface are enough to balance the charge. No new chemical species are created, and the Pt remains physically intact.

When does Pt fail?

The "Platinum trap" may occurs during positive high-current density measurements in the WE, for example in Hydrogen Evolution Reaction (HER) studies.

The Problem: If your WE is performing a reduction, your CE must perform an oxidation. At high potentials, Pt doesn't just evolve Oxygen; if you keep it long enough under relatively high currents, it can also undergo dissolution.
The Consequence: Pt ions can migrate through the electrolyte and deposit onto your WE. This can lead to "ghost" catalytic activity, giving you a false sense of stability or performance that actually comes from the contaminated CE.

Explore platinum counter electrodes for your research.

2. Graphite and Carbon-Based

Carbon (in the form of graphite rods or glassy carbon) is another of the most commonly used materials. It is also usually considered inert and generally cheaper. But it also has its "dark side".

When is Carbon truly inert?

When used with a high surface area (relative to the WE) and at moderate currents. Its high capacitive load often allows it to handle the counter-reaction without requiring a high compliance voltage from the potentiostat.

Sometimes it generates carbon oxides which in many cases do not affect the measurement, or it oxidizes water to O₂, which may also have no impact.

When does Carbon fail?

CO Poisoning: Carbon oxidation doesn't just produce CO₂; it can generate Carbon Monoxide (CO). If your Working Electrode is made of a CO-sensitive material (like Pt), these molecules can "poison" your catalyst, killing your reaction.
Mechanical Degradation: Under high current stress, graphite rods can physically disintegrate, shedding carbon micro-particles into the solution, which can interfere with optical measurements or block membranes.

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How to Protect Your Measurements

If your experimental conditions push the limits of your counter electrode, you don't necessarily need a new material. You need a better setup:

The H-Cell (Separated Compartments): You can use a cell with two compartments separated by a porous glass frit or an ion-exchange membrane. This allows ions to flow (closing the circuit) but prevents dissolved Pt ions or CO molecules from reaching your WE.
Area Ratio Rule: Always ensure the geometric area of your CE is at least 10 to 100 times larger than your WE. Among other improvements to your experiment, this keeps the current density at the CE low, minimizing the risk of unwanted side reactions.
Regular Cleaning: For Pt, flame-annealing or acid cleaning is essential to remove traces of previous experiments. For Graphite, consider replacing the rod frequently or cleaning it as well.

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Conclusion

There is no such thing as a "universal" counter electrode. Understanding the chemistry happening at the "other" electrode is the first step toward reproducible and honest electrochemical data.

Need the right tools for your next experiment? Compare counter electrodes, or contact us for help selecting the right one.

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Bibliography:

ACS Energy Lett. 2024, 9, 4581−4586
Nat Rev Methods Primers 2, 84 (2022)
ACS Energy Lett. 2017, 2, 5, 1070–1075
ACS Catal. 2022, 12, 4, 2661–2670