Is Your Counter Electrode Distorting Your Results? How to Choose Material, Size and Setup
The counter electrode is the least-discussed part of a three-electrode cell. It rarely appears in a figure, gets half a sentence in the methods section, and is often whatever platinum wire was already in the drawer.
Yet it carries the full cell current, its potential is not controlled by the potentiostat, and under some conditions it can release species that reach the working electrode. When that happens, part of the activity you measure belongs to the counter electrode rather than to your sample.
The counter electrode does not have to be perfect. It has to stay out of your data.
This guide covers the decisions that determine whether it does: material, size and cell configuration, plus the controls that confirm the choice. The recommendations are starting points. They do not replace a compatibility check for your specific electrolyte, reaction and test duration.
Key facts
- Role: completes the current path by running a reaction opposite to the working electrode
- Potential: not independently controlled; the potentiostat drives it wherever needed to pass the current, within its compliance voltage
- Size: usually several times larger than the working electrode; ratios of about 6–10× appear in parts of the literature as starting guidelines, not universal requirements [2], [3]
- Best-documented risk: Pt dissolution and redeposition onto the working electrode during acidic HER testing [1], [2]
- Common reporting gap: not stating counter-electrode material, approximate area and whether it was separated
1. What the counter electrode does
In a three-electrode cell, the potentiostat controls the working-electrode potential relative to the reference electrode, and almost no current flows through the reference. The current therefore flows between the working electrode and the counter electrode (also called the auxiliary electrode).
There is no other path, so the same current passes through both [10]. Whenever reduction occurs at the working electrode, a corresponding oxidation carries that same current at the counter electrode, and vice versa. The molar reaction rates can differ, because the two reactions may involve different numbers of electrons.
| Working-electrode reaction | Counter electrode must | Typical counter-electrode reaction (aqueous) |
|---|---|---|
| HER | Oxidise | Oxygen evolution, or oxidation of the electrode itself |
| OER | Reduce | Hydrogen evolution |
| CO₂ reduction | Oxidise | Oxygen evolution |
| CV of a redox couple | Alternate | Whichever reaction is available at that moment |
The counter electrode does not choose a “clean” reaction. It runs whatever delivers the required current, and that can include oxidation of its own surface.
2. The potential nobody is controlling
The potentiostat adjusts the voltage between working and counter electrode until the working electrode sits at the set potential. The counter electrode goes wherever that requires.
One study measured this directly, using a second potentiostat channel to track a Pt counter electrode while the working electrode ran HER in 0.5 M H₂SO₄ [3]:
- the counter electrode reached about 2.0 V vs RHE, rising to about 2.6 V vs RHE as the HER current increased
- with the working electrode swept at 50 mV/s, the counter electrode experienced roughly 1000–3000 mV/s
That study was designed to deposit Pt from the counter electrode deliberately, so its conditions are not a general benchmark. It does show how different the counter electrode’s experience can be. Pt dissolution is a transient process that occurs as Pt oxide forms and is reduced during potential cycling, above roughly 1.1 V vs RHE in acidic media [9], which is well within the range recorded above.
Your protocol is written for the working electrode. The counter electrode may experience a different, harsher one.
3. How Pt counter electrodes can distort HER results
Platinum is chemically durable under many common electrochemical conditions, conductive and kinetically fast, which is why it became the default. It is not universally inert. For HER catalyst testing in particular, its weakness is well documented: Pt can dissolve at the counter electrode and redeposit on the working electrode. Because Pt is also the benchmark HER catalyst, trace deposits can make a non-precious catalyst appear more active than it is.
What the studies show
An early study found that electrochemically activating carbon cathodes for HER worked efficiently when Pt wire was the counter electrode, and Pt transfer from anode to cathode was observed [6]. A follow-up analysis noted that work on non-precious HER catalysts had routinely used Pt counter electrodes without an ion-exchange membrane [1].
A 2022 study quantified the effect with a Pt foil counter electrode in 0.5 M H₂SO₄ [2]:
| Activation protocol (300 cycles) | Apparent improvement at −10 mA/cm² | Pt observed on working electrode |
|---|---|---|
| Narrow range, 0.06 to −0.20 V vs RHE | 170 mV | A few small Pt nanoparticles |
| Wide range, 0.06 to −0.34 V vs RHE | 400 mV | Surface heavily covered with Pt nanoparticles |
Data from Hasan and Allam, Scientific Reports 2022 [2]. Values apply to that system and protocol.
Under the same acidic HER conditions, the study also reported [2]:
- Larger working electrode, more Pt. With the counter electrode unchanged, increasing working-electrode area from 0.25 cm² to 1 cm² lowered the apparent overpotential sharply, with a stronger Pt signal in EDX.
- Gold also dissolved and redeposited, so it is not a straightforward substitute.
- No Ti was detected on the working electrode when titanium mesh was the counter electrode.
A separate study found that the working-to-counter area ratio influences how much Pt dissolves [4].
Separation lowers the risk; it does not prove its absence
Pt and Au contamination has been reported during extended testing even with a Nafion membrane between the compartments [5]. A divided cell is good practice, but it is not by itself evidence that the working electrode is clean.
Dissolved gas changes the picture
A 2024 review describes a comparison in which HER activity stayed essentially unchanged over two hours of cycling in H₂-saturated electrolyte, whether the counter electrode was graphite or Pt. Under N₂ saturation, activity improved dramatically only with Pt, whose potential rose into the oxygen-evolution region [7]. With H₂ available, the counter electrode can oxidise hydrogen at a much lower potential. Electrolyte composition, not only electrode choice, decides how hard the counter electrode is driven.
Most of this evidence comes from acidic HER. That is where the risk is best documented, not the only place it can occur, so the controls in Section 7 are worth running in other electrolytes too.
4. Sizing the counter electrode
A larger counter electrode lowers its current density and therefore the overpotential it needs [10]. If the counter reaction cannot keep up, it can limit the current or push the cell toward the instrument’s compliance limit.
As a practical starting point, the counter electrode is often made several times larger than the working electrode. Ratios of about 6–7× [3] and about 10× [2] are reported in parts of the literature, but they are guidelines rather than requirements. The size you actually need depends on:
- total expected current
- the counter reaction and its kinetics
- electrolyte composition and conductivity
- how much counter-electrode polarisation is acceptable
- instrument compliance voltage
- electrode shape, effective wetted area and gas coverage
- whether the counter electrode sits in a divided compartment
The final check is practical: can the counter electrode pass the required current without excessive polarisation, contamination or compliance errors? Foams, gas diffusion electrodes and high-loading catalyst layers deliver far more current per geometric cm² than a polished disk, so size to the current, not only to geometric area.
Illustrative calculation: wire area
The wetted area of a straight wire is approximately A ≈ π × d × L. The table below compares Beyond Battery Pt wires with a 5 mm glassy carbon disk (0.196 cm²).
| Wire (listed dimensions) | Approximate area | Ratio to 5 mm disk |
|---|---|---|
| PT0537, φ0.5 × 37 mm | 0.58 cm² | ≈ 3× |
| PT1037, φ1.0 × 37 mm | 1.16 cm² | ≈ 6× |
| PT0523, φ0.5 × 230 mm | 3.61 cm² | ≈ 18× |
This calculation assumes the listed length is exposed Pt, fully immersed, with no part covered by the PTFE sheath, and ignores the tip and attachment. Confirm the exposed length for your electrode, and count only what is immersed.
For mesh, the rectangular outline is not the active area. Wire diameter, weave, exposed lead and wetting all change it, so treat mesh size as a relative guide rather than an area value.
5. Choosing the material
There is no universally inert counter electrode. The better question is which reaction products and released species your experiment can tolerate.
| Material | Strengths | Limitations | Consider it when |
|---|---|---|---|
| Platinum | Conductive, kinetically fast, durable under many common conditions | Not universally inert; can dissolve under oxidising potentials and potential cycling [9] and contaminate HER measurements [2] | Pt dissolution and counter products will not interfere with the chemistry under study |
| Graphite | No precious-metal release | Carbon can oxidise at high anodic potential; particulates and soluble carbon products should be assessed | Metal contamination must be avoided, provided carbon oxidation and crossover are checked |
| Glassy carbon | No metal release | Corrodes electrochemically under anodic conditions [8]; thin rods may be too small [2] | Short experiments where the area is sufficient |
| Gold | Conductive, noble | Dissolved and redeposited during acidic HER testing [2] | Not a direct Pt replacement for HER work |
| Titanium mesh | No Ti detected on the working electrode in one acidic HER study [2] | Single-study evidence; passivation may cause significant counter-electrode polarisation | Promising for acidic HER; verify suitability and compliance voltage for your electrolyte and current |
| Lithium metal | Defined Li/Li⁺ couple in lithium cells | In a two-electrode half-cell it is both counter and reference, so its contribution is included in the measurement | Lithium half-cells; use a three-electrode setup when contributions must be separated [11] |
Form factor. Wire and coil are compact and easy to clean; a longer wire adds area without a larger body. Mesh gives high area in a small outline and lets gas escape. Sheet gives a defined, flat geometry for parallel-plate arrangements. Rod is mechanically robust and simple to mount.
6. Cell configuration: when to separate the counter electrode
Placing the counter electrode in its own compartment, behind a frit, membrane or salt bridge, is the most direct way to keep its products away from the working electrode.
- Bulk electrolysis and synthesis: a divided cell is standard, with working and reference electrodes in one chamber and the counter electrode in another [12].
- Product-sensitive work, such as CO₂ reduction with product analysis: use a divided cell.
- Long activation, stability tests and acidic HER on new materials: separate the counter electrode and still run the controls in Section 7.
- Short routine CV: a single compartment is often acceptable when neither counter products nor dissolution interfere with the chemistry.
The trade-off. A separator adds resistance, so the potentiostat needs more cell voltage for the same current. Check that the required voltage stays within its compliance limit, especially at high current.
Matching the configuration to the experiment
| Experiment | Starting recommendation |
|---|---|
| Short routine CV | Large-area Pt may be suitable if neither Pt dissolution nor counter products interfere with the chemistry |
| Non-precious HER catalyst screening | A sufficiently large carbon-based or otherwise validated non-precious counter electrode, isolated, with controls |
| OER testing | A large-area Pt counter electrode may be suitable because it normally performs HER, subject to electrolyte compatibility |
| CO₂ reduction and product analysis | Divided cell; choose the anode by electrolyte, anode reaction, contamination tolerance and stability, and check for Pt on the cathode if trace metals could affect selectivity |
| Bulk electrolysis and synthesis | Divided cell with a large-area electrode whose products cannot react with the desired product |
| Long stability tests | Validate the counter material experimentally; separation alone is not proof of a clean working electrode |
7. Controls that confirm your choice
A contaminated working electrode still produces clean, convincing curves. These checks help reveal problems:
- Run a blank. Test the bare substrate with the same counter electrode, cell and activation protocol. If the blank “activates”, investigate the counter electrode.
- Change the counter electrode. Repeat a key measurement with a chemically different, appropriately sized counter electrode. A material change in working-electrode performance should trigger further investigation. On its own it does not prove Pt contamination, because the counter reaction, cell voltage, released species, local pH and gas production can also change.
- Question steady improvement. HER activity that keeps rising with cycle number during activation deserves a closer look [2].
- Analyse the surface and electrolyte. EDX or XPS can show Pt deposited on the working-electrode surface [2]. ICP-MS can quantify Pt in the electrolyte or, after appropriate digestion, Pt associated with an electrode [3].
- Record the counter-electrode potential against the reference with a second channel, where available [3].
- Report the configuration: counter-electrode material, form, approximate area, and whether and how it was separated.
8. Troubleshooting
| Observation | Possible causes | What to check |
|---|---|---|
| HER activity keeps improving with cycling | Pt deposition from the counter electrode; genuine surface restructuring | Blank on bare substrate; surface analysis for Pt; repeat with an isolated or non-Pt counter electrode |
| Results change when the counter electrode is changed | Contamination; different counter reaction, cell voltage, local pH or released species | Use blanks, electrolyte analysis and post-test surface characterisation to identify the cause before adopting a validated configuration |
| Compliance or overload error | Counter electrode too small; high separator resistance; low electrolyte conductivity | Cell voltage; counter-electrode area; frit or membrane condition; ionic path length |
| Current plateaus unexpectedly | Working-electrode mass transport or reactant depletion; gas blockage; uncompensated resistance; counter-reaction limitation; instrument limits | Cell voltage, counter-electrode potential, mixing, gas coverage and reactant supply before increasing counter-electrode area |
| Noisy current during electrolysis | Gas bubbles on electrodes or drifting toward the working or reference electrode | Electrode positioning; mesh instead of wire; separation |
| Unexpected products in CO₂RR analysis | Anode products or crossover; working-electrode contamination | Membrane integrity; blank electrolysis; cathode surface analysis |
9. Building a validated configuration with Beyond Battery
A counter electrode works as part of a configuration: the electrode, the way it is isolated, and the cell that holds it. Choose all three together.
| Component | Options | Consider it for |
|---|---|---|
| Graphite Rod Counter Electrode | Customisable length | Experiments where precious-metal release must be avoided, such as non-precious HER screening |
| Platinum Wire (99.99% Pt, PTFE sheath) | PT0537 (φ0.5 × 37 mm), PT1037 (φ1.0 × 37 mm), PT0523 (φ0.5 × 230 mm) | Routine voltammetry where Pt is compatible; the longer wire for more area |
| Salt Bridge Platinum Wire | PT052300 (φ0.5 × 230 mm) | Separating the Pt counter electrode within a single-compartment cell |
| Platinum Mesh (99.99% Pt, #52 mesh) | 5 × 10 mm to 30 × 30 mm, or Ø20 mm | Higher-current work in a divided cell where Pt is compatible |
| Platinum Sheet / Platinum Rod | Flat sheet; solid rod | Defined geometry; robust general use where Pt is compatible |
| Electrochemical cells | Single-compartment and divided cells | Matching isolation to the experiment |
When cells and electrodes are ordered together, electrode dimensions are matched to the cell. Custom sizes are available on request, and our team can help you work through the configuration for your chemistry.
Browse counter electrodes → Browse electrochemical cells →
Frequently asked questions
What is the difference between a counter electrode and an auxiliary electrode?
None. Both terms describe the same electrode.
How large should a counter electrode be?
Usually several times larger than the working electrode, with ratios of about 6–10× used as starting guidelines. The deciding test is whether it passes the required current without excessive polarisation, contamination or compliance errors.
Can a platinum counter electrode contaminate my working electrode?
Yes. It is best documented in acidic HER testing, where dissolved Pt can redeposit and make non-precious catalysts appear more active.
Does a membrane prevent contamination?
It reduces the risk, but contamination has been reported during extended testing even with a Nafion membrane. Confirm with blanks and surface analysis.
Is graphite always a better choice than platinum?
No. Graphite avoids precious-metal release, but carbon can oxidise at high anodic potential. Choose by what your experiment can tolerate.
Why does my potentiostat report a compliance error?
It cannot supply enough cell voltage for the requested current. Common causes are a small counter electrode, a resistive separator and low electrolyte conductivity.
The electrode your potentiostat does not control
The reference electrode defines your potential scale and the working electrode is your sample. The counter electrode is the one component whose potential nobody sets.
Choose a material whose products your experiment can tolerate, size it to the current, separate it when the chemistry or duration calls for it, and confirm with controls. That keeps the counter electrode in the circuit and out of the data.
Choosing a reference electrode as well? Read Ag/AgCl Reference Electrode: Potential, Conversion and Selection and the Reference Electrodes Guide: Ag/AgCl, SCE, Hg/HgO & RHE.
References
- R. Chen, C. Yang, W. Cai, H.-Y. Wang, J. Miao, L. Zhang, S. Chen and B. Liu, “Use of platinum as the counter electrode to study the activity of nonprecious metal catalysts for the hydrogen evolution reaction,” ACS Energy Letters, 2, 1070–1075 (2017). https://doi.org/10.1021/acsenergylett.7b00219
- M. M. Hasan and N. K. Allam, “An alternative, low-dissolution counter electrode to prevent deceptive enhancement of HER overpotential,” Scientific Reports, 12, 9368 (2022). https://doi.org/10.1038/s41598-022-13385-w
- R. R. Srivastava, D. Gautam, R. Sahu, P. K. Shukla, B. Mukherjee and A. Srivastava, “Mechanistic insights on bi-potentiodynamic control towards atomistic synthesis of electrocatalysts for hydrogen evolution reaction,” Scientific Reports, 13, 16433 (2023). https://doi.org/10.1038/s41598-023-43301-9
- M. Tian, C. Cousins, D. Beauchemin, Y. Furuya, A. Ohma and G. Jerkiewicz, “Influence of the working and counter electrode surface area ratios on the dissolution of platinum under electrochemical conditions,” ACS Catalysis, 6, 5108–5116 (2016). https://doi.org/10.1021/acscatal.6b00200
- M. A. Bird, S. E. Goodwin and D. A. Walsh, “Best practice for evaluating electrocatalysts for hydrogen economy,” ACS Applied Materials & Interfaces, 12, 20500–20506 (2020). https://doi.org/10.1021/acsami.0c03307
- G. Dong, M. Fang, H. Wang, S. Yip, H.-Y. Cheung, F. Wang, C.-Y. Wong, S. T. Chu and J. C. Ho, “Insight into the electrochemical activation of carbon-based cathodes for hydrogen evolution reaction,” Journal of Materials Chemistry A, 3, 13080–13086 (2015). https://doi.org/10.1039/C5TA02551F
- A. A. Feidenhans’l, Y. N. Regmi, C. Wei, D. Xia, J. Kibsgaard and L. A. King, “Precious metal free hydrogen evolution catalyst design and application,” Chemical Reviews, 124(9), 5617–5667 (2024). https://doi.org/10.1021/acs.chemrev.3c00712
- Y. Yi, G. Weinberg, M. Prenzel, M. Greiner, S. Heumann, S. Becker and R. Schlögl, “Electrochemical corrosion of a glassy carbon electrode,” Catalysis Today, 295, 32–40 (2017). https://doi.org/10.1016/j.cattod.2017.07.013
- A. A. Topalov, S. Cherevko, A. R. Zeradjanin, J. C. Meier, I. Katsounaros and K. J. J. Mayrhofer, “Towards a comprehensive understanding of platinum dissolution in acidic media,” Chemical Science, 5, 631–638 (2014). https://doi.org/10.1039/C3SC52411F
- N. Watanabe (Laboratory of Research & Development, BAS Inc.), “Counter electrode,” ALS Co., Ltd. technical note. https://www.als-japan.com/1787.html
- J. Costard, M. Ender, M. Weiss and E. Ivers-Tiffée, “Three-electrode setups for lithium-ion batteries: II. Experimental study of different reference electrode designs and their implications for half-cell impedance spectra,” Journal of The Electrochemical Society, 164(2), A80–A87 (2017). https://doi.org/10.1149/2.0241702jes
- Pine Research Instrumentation, “Bulk Electrolysis (BE),” support article. https://pineresearch.com/support-article/bulk-electrolysis-be/
Next in the Beyond Battery electrode series: Working Electrode Selection and Preparation — Glassy Carbon, Platinum and Gold Disks.