Compliance voltage in potentiostats: what it is and why it can limit your experiment

Compliance voltage is one of those specifications that many electrochemists do not check until it causes a problem. However, it can make a potentiostat lose control of the cell even when the potential between the working and reference electrodes looks completely normal.

Let us look at what it is, why it matters and when we need to consider it.

What is compliance voltage?

In a three-electrode cell, the potentiostat controls the potential of the working electrode (WE) relative to the reference electrode (RE). Current, however, flows between the WE and the counter electrode (CE).

The same current flows through the WE and CE in absolute terms. The reactions at their surfaces are complementary: if oxidation occurs at one electrode, a reduction must occur at the other to complete the circuit.

To maintain the programmed WE–RE potential, the control amplifier adjusts the CE potential. The maximum voltage the instrument can establish between WE and CE before losing control is the compliance voltage.

The CE does not simply adopt the WE potential with the opposite sign. It polarizes to the potential required to sustain its own reaction.

This shift can make the actual voltage between WE and CE much greater than the potential programmed between WE and RE.

An example with platinum and carbon CEs

As an idealized example, suppose the WE is at +1.5 V vs RHE and is driving an oxidation. The corresponding reduction must occur at the CE.

In an acidic aqueous electrolyte, a sufficiently large platinum CE may sustain the hydrogen evolution reaction (HER) close to 0 V vs RHE if the current density is not very high. The voltage between WE and CE would be approximately:

+1.5 V − 0 V = 1.5 V

Now replace the platinum CE with a carbon CE. In this example, carbon is less active for the HER and must reach a more negative potential to sustain the same current. Suppose it reaches −1.0 V vs RHE.

The WE remains at +1.5 V vs RHE, but the voltage between WE and CE becomes:

+1.5 V − (−1.0 V) = 2.5 V

The experiment still controls the WE at +1.5 V vs RHE, but it now demands 2.5 V between WE and CE. This 2.5 V value is the cell-voltage demand in the example, not the instrument's compliance voltage. Compliance voltage is the maximum the potentiostat can provide.

If the instrument has 10 V of compliance, a demand of 2.5 V is not a problem. If the cell demand exceeds 10 V, the instrument will no longer be able to maintain the programmed potential correctly.

Comparison of the WE–CE voltage required with platinum and carbon CEs while maintaining the same WE–RE potential.

Compliance voltage is not the same as potential range

When comparing potentiostats, it is important to distinguish three specifications:

• Applied potential range: the potential the instrument can program or measure between WE and RE.
• Compliance voltage: the maximum voltage available between WE and CE to maintain the programmed potential.
• Maximum current: the highest current the instrument can source, sink or measure under specified conditions.

For example, an instrument can have an applied potential range of ±10 V and a compliance voltage of ±20 V. The additional voltage is not necessarily applied to the WE. It is the headroom available to polarize the CE and overcome the other voltage drops in the cell.

What increases the compliance-voltage requirement?

Organic and nonaqueous systems

Compliance voltage can be particularly relevant in organic electrochemistry and nonaqueous systems. Many organic solvents have low conductivity without a supporting electrolyte, so salts are added to reduce solution resistance.

This does not mean that every organic system requires high compliance voltage. Current, supporting-electrolyte concentration, electrode spacing and cell geometry remain decisive. A flow cell with a narrow interelectrode gap, for example, can greatly reduce the ohmic drop.

High current

The ohmic contribution follows the iR relationship. If resistance remains constant, doubling the current doubles the resistive voltage drop.

This specification is therefore also important in electrolysis, electrosynthesis, electrodeposition, electrocatalysis, large electrodes and scaled experiments. In addition, if the CE area remains unchanged, higher current increases its current density and may shift its potential even further.

Membranes, separators and porous frits

Separating the CE can reduce contamination of the WE and limit product crossover between compartments, but it also introduces additional resistance.

The voltage drop associated with a membrane or frit increases with current. A configuration that works at low current may therefore reach the compliance limit when current increases, even if nothing else changes.

An undersized or unsuitable CE

The CE must sustain the reaction that balances the WE current. If it is too small, insufficiently active for that reaction, passivated, contaminated or partly covered by bubbles, it will need to reach a more extreme potential.

Increasing its area or choosing a more suitable material can reduce the voltage demand. Its stability, reaction products and risk of contaminating the WE must also be evaluated. CE selection is not just a matter of area or conductivity. Its chemistry matters too, as discussed in our article on platinum and carbon counter electrodes.

Cell geometry and connections

Long current paths, narrow channels, low electrolyte volume, partially immersed electrodes, poor contacts or inadequately wetted components can increase the effective cell resistance.

These issues should be ruled out before concluding that a potentiostat with higher compliance voltage is required.

How can you tell whether compliance voltage may be a problem?

Predicting the exact WE–CE voltage in advance is difficult because it depends on the complete experiment, not only on the potentiostat. The current, electrolyte conductivity, cell geometry, CE, membranes and other components can all affect the voltage demand, which may also change during the experiment.

Depending on the instrument and software, reaching the limit may trigger a control overload, voltage compliance error or cell voltage warning, or appear as clipped current or a distorted response. These signs are not exclusive to compliance voltage, so the connections, RE and current range should also be checked.

Whenever possible, I recommend recording the WE–CE voltage, CE potential or control-amplifier output. Not every instrument and software package provides the same channels, but this information can show how much output voltage the experiment is actually using. Even if you do not need it during a routine measurement, it can save considerable time when something behaves unexpectedly.

If you already have a potentiostat, the most useful approach is to monitor this voltage under representative conditions using your actual experimental setup. If you are still selecting an instrument, reviewing similar applications can provide a useful starting point, although an exact prediction may still be difficult.

Not sure whether compliance voltage could limit your experiment? Tell us about your application, cell configuration, electrolyte and expected current. We will help you assess your requirements, free of charge and with no obligation.

Is more compliance voltage always better?

Not necessarily. Higher compliance voltage can provide additional headroom for demanding cells, but it does not automatically improve a measurement and may add cost, size or complexity that your application does not need.

The voltage figure should not be considered on its own either. Compliance voltage, current and output power are related, and an instrument may not provide its maximum voltage and maximum current simultaneously. The relevant question is how much voltage it can deliver at the current your experiment is likely to require.

The current electrochemical workstation catalog includes instruments rated below 10 V, others rated at 10 V or more and some rated at 25 V or more. This range reflects the variety of electrochemical applications, not a universal recommendation.

What changes in a two-electrode configuration?

In a two-electrode cell, the RE and CE functions are combined at the second electrode. The controlled cell voltage and the actual voltage between the two terminals then refer to the same electrode pair.

This makes the voltage requirement more direct to interpret, but the compliance, current and power limits still apply. A cell stack or a highly resistive device may require much more voltage than a single cell.

The key takeaway

Compliance voltage is the voltage limit available to the potentiostat for maintaining control of the cell.

The required voltage increases with current, current-path resistance and the potential the CE must reach to sustain its reaction. When the demand exceeds the instrument limit, the programmed potential may no longer be the potential actually maintained at the WE.

Whenever possible, record the WE–CE voltage, review the complete setup and check the voltage and current available simultaneously before selecting an instrument.

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References

• Colburn, A. W.; Levey, K. J.; O'Hare, D.; Macpherson, J. V. “Lifting the lid on the potentiostat: a beginner's guide to understanding electrochemical circuitry and practical operation”. Physical Chemistry Chemical Physics 23 (2021), 8100–8117.
• Siegl, I.; Sailer, N.; Steffan, C. “A Holistic Presentation of 3-Electrode Potentiostat Measurement Data: The Hexa-Region Plot”. Journal of The Electrochemical Society 170 (2023), 073507.
• Zheng, J.; Du, X. “Applicability of Graphite as Anodic Counter Electrode for Electrocatalyst Evaluation”. ACS Energy Letters 9 (2024), 4581–4586.
• Wu, T.; Wang, G.; Zhang, Y.; Kang, S.; Zhang, H. “Electrochemical deposition of Pt on carbon fiber cloth utilizing Pt mesh counter electrode during hydrogen evolution reaction for electrocatalytic hydrogenation reduction of p-nitrophenol”. New Journal of Chemistry 41 (2017), 7012–7019.
• Elgrishi, N.; Rountree, K. J.; McCarthy, B. D.; Rountree, E. S.; Eisenhart, T. T.; Dempsey, J. L. “A Practical Beginner's Guide to Cyclic Voltammetry”. Journal of Chemical Education 95 (2018), 197–206.
• Noël, T.; Cao, Y.; Laudadio, G. “The Fundamentals Behind the Use of Flow Reactors in Electrochemistry”. Accounts of Chemical Research 52 (2019), 2858–2869.