Are you looking to purchase a new potentiostat for your research lab?  Whether you’re a new faculty, veteran electrochemist, graduate student, or industrial chemist, finding the right potentiostat can be difficult.  Understanding specifications such as potential and current range is reasonably straight forward, but what about compliance voltage, ADC inputs, input impedance, and CMRR?  Are these important?  How will you know the optimal potentiostat specifications for an experiment you’ve never performed?  Rest assured.  In this article, we hope to debunk some of these specifications and provide you with real information you need when deciding which potentiostat to purchase.

Before diving into the topic of potentiostat specifications, a brief mention on terminology.  You may notice that this electrochemical instrument has several general names:  electrochemical workstation, potentiostat, galvanostat, electrochemical instrument, etc.  Worry not – clasically, most refer to these instruments simply as “Potentiostats” as we do in this article.  Technically, potentiostat is just one thing that such an instrument can do, so really, calling an electrochemical workstation only by one of its features is a bit disingenusous.  Still, the term potentiostat is well understood to mean “an electrochemical instrument capable of applying and measuring current and voltage across electrodes.”  Without even referring to a potentiostat, electrochemistry itself is full of confusing jargon.  In describing the usefulness of certain specifications, this article will use the phrase “voltage” and “potential.”  A voltage refers to the difference between two electrodes, and we use the term “voltage” when describing energy storage devices like batteries and fuel cells.  We will use the phrase “potential” when referring to an individual electrode, redox reaction, and traditional 3-electrode solution phase electrochemistry experiments.

Michael Faraday – By Probably albumen carte-de-visite by John Watkins – Opposite p. 290 of Millikan and Gale’s Practical Physics (1922), Public Domain, https://commons.wikimedia.org/w/index.php?curid=2525521

Regarding potentiostat terminology, the name “potentiostat” actually describes its function. “Potentio” refers to electrical potential, and “stat” stems from the Greek word “stato,” which means standing or set.  Similarly, you use a “thermostat” to set the temperature in a room.  You may recall from analytical chemistry or an instrumentation course that a potentiostat is made up of several operational amplifiers with the sole purpose of maintaining a controlled potential through feedback loops.  However, performing electrochemical experiments require additional equipment.  A potentiostat, waveform generator, and chart recorder were some of the items used by researchers when potentiostats were first developed.  These early systems didn’t include controlled current techniques (galvanostat) or electrochemical impedance spectroscopy (EIS) and required separate circuitry.  A modern potentiostat, by contrast, contains most of these components to form a single “out-of-the-box” instrument for electrochemistry experiments.  Some potentiostats have multiple channels and are referred to as bi and multi-channel potentiostats.  Another phrase that is tossed around is “electrochemical workstation” or “potentio/galvanostat system.”  All of these terms make up what we call a “potentiostat” and they all refer to the same thing…a black box with a bunch of cables coming out of it that you hook up to your electrochemical system, and pray to the ghost of Michael Faraday that your experiment works.

Now that the terminology is understood, let’s start with the most important question when choosing a potentiostat (or any analytical instrument for that matter).  Will the instrument work for your application?  In the world of scientific research, you never know if an experiment is going to work.  Most of the time, it does not.  You may feel comforted to know this is somewhat “par for the course,” particularly in the field of electrochemistry (Michael Faraday frowns upon us).  However, an inadequate instrument should not be the reason why an experiment fails.  When purchasing a potentiostat, you need to make sure that it has the capability to measure what you are trying to study.  For example, if you are working in highly resistive solutions, your potentiostat needs a high compliance voltage to overcome the solution resistance.   As you read this article, keep in mind the kind of data you will require of your potentiostat based on your experimental design.

The potential range needed will depend on your application.  For electrochemistry at a solid-liquid interface, we need to recognize differences between using aqueous and non-aqueous solvents.  The electrolysis of water is your primary limiting factor when working in aqueous solvents.  The potential window for water is roughly -1 to +1 V vs. the normal hydrogen electrode (NHE).¹  This window can vary with supporting electrolyte, working electrode material, and pH, but as a ballpark estimate, your potentiostat does not likely need capabilities beyond -2 to +2 V vs. NHE for aqueous electrochemistry.

Non-aqueous solutions, on the other hand, carry a much wider potential range.  For example, acetonitrile has a potential window between -2.5 to +2.5 V vs. NHE.¹  Regarding non-aqueous solvents, there two things to note.

• The potential range of the solvent is highly dependent on contamination.  Techniques such as cyclic voltammetry are very sensitive to trace contaminants.  Small quantities of water will begin to electrolyze at extreme potentials (both positive and negative), effectively limiting your potential window in a non-aqueous solvent.
• The potential range may be characterized as bipolar and symmetric.  “Bipolar” refers to the accessibility of positive and negative potentials, and “symmetric” means the potential range is centered around zero.

In the previous example, the -1 to +1 V potential range for water is both bipolar and symmetric.  The non-aqueous solvent dimethylformamide, on the other hand, has a potential window between -2.75 to +1.5 V vs. NHE.¹  Dimethylformamide is bipolar (positive and negative potentials are accessible), but not symmetric (the range is not centered around zero).  As another example, ammonia has a potential range from -3 to 0 V vs. NHE.¹  In this case, ammonia is neither symmetric nor bipolar, since only the negative potential range is accessible and it is not centered around zero.  If you are working with non-aqueous solvents, we recommend your potentiostat should have minimum potential range capabilities of +4 to -4 V.

Research in energy storage devices like batteries and fuel cells have different voltage range requirements.  Often in this research area, the voltage range does not necessarily need to be bipolar or symmetric.  Energy storage devices are a source of power; this means it inherently carries a positive voltage, hence your potentiostat does not require negative voltages.  For batteries, the selection of potentiostat will depend on the kind of battery you are studying (i.e., chemistry, form factor, half or full cells) and whether you are stacking them in various series or parallel configurations.  Li-ion batteries typically have a cell voltage around +3.0 V.²  The cell voltage for zinc oxide under alkaline conditions is around +1.65 V,³ and lead-acid batteries around +2.05 V.⁴  When working with coin cells, the potentiostat should be able to supply at least +5 V.  When moving to larger cylindrical and pouch cells where stacked series/parallel combinations are often used, you’ll need a minimum of +10 V. For large battery packs, up to + 30 V can sometimes be necessary.

Fuel cells that use hydrogen and oxygen (e.g., proton-exchange membrane fuel cells, or PEMFCs) operate in a relatively narrow voltage window up to +1 V for a single cell.  The maximum achievable operating voltage for a PEMFCs is +1.23 V, because the 4-electron oxygen reduction reaction (ORR) at the cathode is +1.23 V and the hydrogen oxidation reaction (HOR) has an equilibrium potential of 0 V .⁵  In practice, PEMFC cell voltage is much lower than +1.23 V (unless you are Michael Faraday).  Whether you are working with a PEMFC, solid oxide fuel cell (SOFC), or an alkaline-exchange membrane fuel cell (AEMFC), the cell voltage will likely be around +1 V or slightly higher.⁶  Similar to batteries, you might be stacking fuel cells to get more voltage and/or current.  Keep that in mind when selecting a potentiostat.

It can be deceiving how much current your potentiostat needs to supply or measure because researchers often report in current density, which is the current response divided by the electrode area, instead of the absolute current value.  Additionally, chemists tend to think in terms of moles and molecules, which doesn’t readily translate into current.  When converting between moles and current, remember that current (i) is equal to:

where q is the charge in coulombs passed through the electrode, and t is time.  Current is the amount of charge passed per unit time.  And thanks to Michael Faraday, we can convert the charge in coulombs to moles via Faraday’s constant (96,485.34 C/mol).  In addition to this straight forward conversion, electrochemists use several equations to relate the current to parameters like the electrode area, the diffusion coefficient, scan rate, and temperature.  The measured current for a technique like cyclic voltammetry is dependent on the scan rate.  For example, faster scan rates typically cause a higher background and faradaic current, hence requiring in a larger current range.  We’ve seen a lot of electrochemistry experiments, and while it is difficult to predict the current you will need for your experiment, below are some ballpark numbers for expected current values to help you choose the correct current range for your potentiostat.

For a 1 mM solution of a redox species on an electrode between 2-5 mm in diameter, the expected current will be in the 1 to 100 μA range.  Using the same 1 mM solution and an ultra-microelectrode (UME), of 10 to 50 μm diameter, the expected current would be in the 10 to 100 nA range.  Current in an electrochemistry experiment changes linearly with concentration and electrode area.  An order of magnitude change in area or concentration will result in the same order of magnitude change in current.  Keep the above rule in mind if you are scaling up an electrolysis experiment.  If you are performing an electrocatalysis experiment, you should expect at least a 10-fold to 100-fold increase in current, compared to the non-catalytic case.  Remember, these are ballpark estimates to give you a sense of how much current you need for your experiments.

With energy storage devices like batteries and fuel cells, the current requirements are significantly different.  When working with large area anodes and cathodes in batteries, the current density might be small, while the actual current passed could be quite large.  This relationship can work to your advantage.  If you work with smaller batteries or coin cells where the area is small, you can report high current densities with a potentiostat that doesn’t have a large current range.   Approximately 1 mA of current is normally sufficient for small batteries and coin cells.  However, the required current range quickly ramps up when you move to cylindrical/pouch cells, where you may need at least -5 to +5 A.  Sometimes, stacking cells in parallel can lead to higher current requirements in the -20 to +20 A range.  As the current range increases, so does the cost of the potentiostat.  High currents require bigger cables, larger heat sinks, and more rigorous electronic safety standards, all of which get factored into the price of your potentiostat.  This is one reason why a current booster is sold as a separate accessory.

Let’s review two potentiostat fundamentals.

• The potential of the working electrode is measured with respect to the potential of the reference electrode.
• The current is driven between the counter electrode and the working electrode.

When the potentiostat changes the potential of the working electrode with respect to the reference electrode, it is the result of adjusting the voltage between the counter and working electrode.  The maximum voltage that can be applied between the counter and working electrode is called the compliance voltage.

The counter electrode supplies the cell current which, in turn, provides the extra voltage needed to drive the working electrode to the desired potential with respect to the reference electrode.  Below is a small diagram to help explain the cell compliance.

Compliance Voltage Diagram with Current Flow

Based in the diagram above:

(1)

(2)

(3)

The green, white, and red circles represent the counter, reference, and working electrode leads on our potentiostat respectively.   represents resistive elements between the counter and reference electrode, and represents resistive elements between the working and reference electrode.  The resistive elements can vary depending on your electrochemical system.  For example, in solution phase electrochemistry, the  consists of the double layer capacitance, charge transfer resistance, and uncompensated solution resistance.  If your solution is highly resistive, the potentiostat must apply a higher voltage to create the desired potential difference between the working and reference electrode.

If is 1 kΩ, applying 1 V between the working and reference electrode would result in a measured current of 1 mA based on Ohm's Law. This is equivalent to performing a bulk electrolysis experiment by applying 1 V and measuring 1 mA.  If is 1 kΩ, there is now two 1 kΩ resistors in series between the counter and the working electrode.  To achieve the measured current of 1 mA, the voltage between the counter and working electrode must be 2 V.  An additional 1 V is needed to meet the measured result of 1 mA.  The maximum voltage a potentiostat can supply to meet the desired potential and current response is the compliance voltage.  For you physics and circuit buffs out there, the diagram above should look a lot like a voltage divider….because it is.

Now that you have a better understanding of cell compliance, what is the minimum compliance voltage your electrochemical system needs?  Well…it’s difficult to say, only Michael Faraday really knows.  An electrochemical system may suddenly require double the compliance voltage if the measured current suddenly doubles.  Thinking back to our diagram, if the measured current doubles, then the voltage also doubles.  This also applies to an increase in the resistance .  It’s not uncommon that the addition of a membrane or fritted tube to separate the counter and working electrode results in a compliance voltage issue for an otherwise normal electrochemical system.

We recommend that your potentiostat has a larger compliance voltage than your present research requires.  For example, your future research may involve scaling up a reaction (requiring more current) or adding a membrane in-between your working and counter electrode.  Specifically, we recommend you double or quadruple the intended voltage range you plan to operate in.  For example, if you plan on working in aqueous solutions with a +1 to -1 V range (a span of 2 V), your potentiostat should have a compliance voltage of 4 to 8 V.

For energy storage devices such as batteries, compliance voltage is less of an issue because you are working in a two-electrode configuration.  In a two-electrode configuration, the counter and reference electrode are shorted, which bypasses  from the diagram above.  If your research involves a two electrode configuration, the applied and measured voltage range of the potentiostat will be roughly equal to the minimum requirement in compliance voltage.