The three-electrode cell and reference electrodes

Why three electrodes

Every electrochemical measurement asks two things of the cell. We want to control, or know, the potential of one electrode, and we want to measure the current that flows through it. With only two electrodes, the same pair has to do both jobs. The current then shifts the potential of the second electrode and adds an ohmic drop through the solution, so the potential we think we apply is not the one the surface sees.

The three-electrode cell splits the jobs. The current flows between the working electrode (WE) and the counter electrode (CE). The potential of the WE is measured against a reference electrode (RE) that carries almost no current, through a high-impedance input of the potentiostat. Because the RE draws almost no current, its potential stays put, and the potentiostat can adjust the CE until the WE sits exactly where we asked.

The working electrode

The working electrode is where the chemistry you care about happens: your catalyst film, a polished glassy carbon disk, a Pt foil, a carbon paper or a screen-printed electrode. A few habits make results from the WE more trustworthy.

• Know the area. Current is reported as current density, so the geometric area has to be defined, for example by a disk of known diameter, a mask or epoxy around a foil. A loose wire touching the electrolyte adds area you did not count.
• Start clean. Polish glassy carbon before each new film. A quick CV of a known redox couple such as ferro/ferricyanide shows whether the surface is clean (Lesson 2).
• Record the loading. For drop-cast catalysts, write down the mass per area and the ink recipe. Activity per mass and the binder content both depend on it.
• Mind the substrate. Some supports react at the potentials you use. Carbon oxidises at high anodic potentials, and Ni foam can contribute OER activity of its own, so measure a bare substrate as a control.

The counter electrode

The counter electrode only has to carry the current, but it can still spoil a measurement. Whatever reaction happens at the WE, the opposite reaction happens at the CE at the same rate. If the WE oxidises water, the CE reduces it to hydrogen, and the reverse.

• Make it larger than the WE so that it never limits the current. A coiled wire or a mesh works well.
• Watch for dissolution. A Pt counter electrode can dissolve slowly under some conditions, and the Pt can redeposit on the working electrode. Pt is an excellent HER catalyst, so a few nanograms can make a poor HER catalyst look good. For HER studies, many groups use a graphite rod or put the CE behind a frit or membrane.
• Separate compartments (an H-cell or a fritted tube) keep CE products, such as O₂ during HER, away from the WE. This matters most for product analysis, where a product formed at one electrode could be consumed at the other.

The reference electrode

A good reference electrode has a well-defined, stable potential because it contains both forms of a redox couple at fixed activity. In the common Ag/AgCl electrode, a silver wire coated with AgCl sits in a KCl solution of fixed concentration, and its potential depends on the chloride activity. Changing the filling solution therefore changes the potential, which is why the concentration is always part of the name.

Pick a reference that suits the electrolyte. Long use of Ag/AgCl in strong alkali can damage it, and its chloride can leak into the cell, where it may adsorb on or corrode some catalysts. A double-junction electrode, with a second salt bridge, slows that leakage. Hg/HgO is the usual choice for concentrated KOH, and Hg/Hg₂SO₄ for acid where chloride must be avoided.

Converting potentials and the RHE

The standard hydrogen electrode (SHE) is the formal zero of the potential scale. For water splitting and many other reactions that involve protons, results are easier to compare on the reversible hydrogen electrode (RHE) scale. The RHE moves with pH by 59 mV per pH unit at 25 °C, the same amount as the equilibrium potentials of the HER and OER themselves. On the RHE scale, the HER equilibrium is always at 0 V and the OER at 1.23 V, whatever the pH.

Try it below. The second part of the calculator gives the value to type into the potentiostat when a paper or protocol asks for a potential vs RHE.

The pH of concentrated alkali is not exactly 14 for 1 M KOH, because activity differs from concentration, and the pH of a CO₂-saturated bicarbonate solution differs from that of the fresh solution. Use the pH you measured, or better, calibrate.

Calibrating against RHE yourself

Literature values assume a new, well-kept reference electrode. Many groups therefore calibrate their reference against a real hydrogen electrode in the same electrolyte. One common approach, described for example by Wei and co-workers (2019), is as follows.

1. Use a clean Pt wire or foil as the working electrode in the electrolyte you will measure in.
2. Bubble high-purity H₂ through the solution for 20 to 30 minutes and keep a gentle flow during the measurement.
3. Record a slow CV (about 1 mV/s) or linear sweeps across the hydrogen evolution and oxidation region.
4. The potential at zero current, averaged from the two scan directions, is the RHE potential on your reference scale. Then E_RHE = E_measured − E_{zero current}.

Repeat it regularly and whenever the reference has been refilled, and report the calibrated value in your methods.

The resistance between WE and RE

Even in a three-electrode cell, a slice of electrolyte sits between the tip of the reference and the working electrode. Current through that slice causes an ohmic drop, i·R_u, which the potentiostat cannot see. R_u is called the uncompensated resistance. It is larger in dilute electrolytes and when the reference is far from the WE. A Luggin capillary brings the reference close, but not so close that it shields the electrode. Lesson 3 shows how to measure R_u and correct for it, and the CV simulator shows how it distorts a voltammogram.

Common mistakes

• Quoting potentials with no reference electrode named, or converting with the wrong filling concentration.
• Using a Pt counter electrode in the same compartment as an HER working electrode without a control experiment.
• An air bubble trapped at the reference frit or in the Luggin capillary. The potential then drifts or the potentiostat oscillates.
• Letting the reference dry out, or storing it in water. Both shift its potential.
• Assuming pH 14 for 1 M KOH without measuring, then reporting overpotentials to the nearest millivolt.

Check your understanding

References and further reading

1. N. Elgrishi, K. J. Rountree, B. D. McCarthy, E. S. Rountree, T. T. Eisenhart, J. L. Dempsey, A practical beginner's guide to cyclic voltammetry, J. Chem. Educ. 95 (2018) 197–206. doi:10.1021/acs.jchemed.7b00361
2. C. Wei, R. R. Rao, J. Peng, B. Huang, I. E. L. Stephens, M. Risch, Z. J. Xu, Y. Shao-Horn, Recommended practices and benchmark activity for hydrogen and oxygen electrocatalysis in water splitting and fuel cells, Adv. Mater. 31 (2019) 1806296. doi:10.1002/adma.201806296
3. A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, 2001, chapters 1 and 15.
4. Metrohm Autolab, application note AN-EC-002, on reference electrodes and their potentials.
5. Pine Research Instrumentation, guidance on reference electrode care and storage, document DRP10026.