Choosing an AEM test cell: start with your power supply, then the area

We make HXS-0, a research test cell for anion exchange membrane (AEM) water electrolysis, and we use the same cell in our own laboratory every day. Research institutes and universities in Korea have also used it for single-cell measurements.

Three questions come up again and again: can the cell be as small as 1 × 1 cm, will a 25 cm² cell do, and what else is needed besides the cell. When we get them, the first thing we ask about is not the active area but the power supply in your laboratory. This article explains why, and what to settle before you order.

• Size the active area from the maximum current of your instrument. At 5 A, a 25 cm² cell reaches only 0.2 A/cm².
• Do not choose a gasket by matching the electrode thickness. Substrates compress differently; settle the thickness by clamping trials.
• A cell alone is not a test. Circulation pumps, electrolyte tanks, heating and gas measurement have to be specified with it.
• For a reliable comparison, reassemble and retest the reference configuration. Check that reassembly scatter is smaller than the improvement you are claiming.

Look at the power supply before the active area

The first number people think of is the active area. But the current needed to reach a given current density grows in proportion to the area, so the number to check first is the maximum current your potentiostat or power supply can deliver.

Put your target current density next to that maximum current. The table divides a 5 A maximum by the active area.

To measure up to 1 A/cm² on a 25 cm² cell you need 25 A, which a 5 A instrument cannot supply. That is why the instrument comes first. The usable range can be narrower still, depending on the voltage range and the materials.

Check the maximum current by model name. Instruments from the same maker can reach tens of amperes with a current booster. Send us the model and booster specification and we will check it with you.

Our standard answer to small-area enquiries is a 4 cm² (2 × 2 cm) cell with gaskets that reduce the area. With gaskets for 4, 2 and 1 cm², one cell covers three areas. The flow field stays the same when you change the gasket, so the assembly guide explains how each gasket and electrode size go together. If you will only ever run 1 cm², we machine the flow field to that size; HXS-0 flow fields can be made from 1 to 25 cm².

Cell build and sample dimensions

When we build a single cell with our HXP electrodes, a gasket goes on the anode-side hardware at the bottom and the anode sits inside its opening. The membrane goes on top, then a second gasket with the cathode inside it, then the cathode-side hardware, and the cell is clamped. The gasket is a frame around the electrode, and the membrane is sealed by being pressed between the two gaskets. HXP-ca is laid with its catalyst-layer face towards the membrane; HXP-an has no orientation.

The specified active area of HXS-0 is the area of the gasket opening, and the electrode is cut to that opening. An electrode larger than the opening rides up onto the gasket and the clamping becomes uneven. Current density is calculated from the specified active area, so check during assembly that the electrode fills the opening.

How large should the membrane be? A membrane only slightly larger than the electrode can be assembled, but it leaves a narrow seal and the leak risk goes up. Cutting it to the gasket size works; cutting it to the outline of the bipolar plate, without the terminal tab, is best.

This describes electrodes made by coating catalyst on a substrate and placing them on each side of the membrane. With a catalyst-coated membrane (CCM) the catalyst layers are on the membrane and a porous transport layer goes outside each, so tell us the membrane outline, the coated area on each side and the transport layer dimensions separately.

When you order, give us these alongside the active area:

The same area can come in different shapes. A 4 cm² opening can be 2 × 2 cm or 1 × 4 cm, and those are different gaskets. A drawing is more precise than a single number.

Do not choose the gasket thickness by matching the electrode thickness. Nickel foam, carbon paper and carbon cloth compress differently when clamped, and nickel foam compresses a great deal, so the nominal electrode thickness alone does not settle the gasket. Our standard electrodes use nickel foam for the anode and carbon paper for the cathode, and HXS-0 also takes carbon cloth electrodes. The reliable way is to assemble with a few candidate thicknesses and decide by leak tightness and cell performance.

For the same reason, do not carry over the tightening torque from another cell. Sequence and torque depend on the cell and the materials; for HXS-0 they are in the assembly guide.

What the cell needs around it

The cell holds the membrane electrode assembly (MEA). Supplying current, circulating electrolyte, holding temperature and measuring gas are done by equipment outside the cell, so ordering the cell alone and ordering a full setup are very different supply scopes.

For a full setup around a 4 cm² cell, this is what we propose:

1. One HXS-0 single cell (4 cm² active area, inlets and outlets on both anode and cathode)
2. Three gasket sets (for 4, 2 and 1 cm²)
3. Two electrolyte tanks (2 L each, anode and cathode, doubling as gas–liquid separators)
4. Two circulation pumps (alkali-resistant, rated for 80 °C)
5. Two hotplate stirrers
6. Tubing and fittings (including hydrogen and oxygen outlets and vents)
7. One MEA and a spare membrane (if you order the MEA with the cell)
8. One assembly and operating guide

There is a reason for two pumps: the same cell can then run with a dry cathode (anode circulation only) or with both sides circulating. Piping and switching between the two are in the assembly and operating guide. Our reference data for HXP electrodes is taken with a dry cathode, so run the cathode dry when you compare against it. With cathode circulation, hydrogen leaves together with electrolyte, which is why the cathode side needs its own tank and heater.

If you plan to measure gas, check the outlets. HXS-0 has separate hydrogen and oxygen outlets, so the gases can be collected without modifying the cell.

A mass flow meter (MFM) on the hydrogen line gives the Faradaic efficiency. Convert the meter reading (SCCM or similar) to a molar flow (mol/s) using its reference temperature and pressure, then divide by the theoretical hydrogen molar flow I/(2F) over a period when current and flow are steady; I is the current and F the Faraday constant. Separate the gas from the electrolyte first, and remove condensate and moisture before the meter — even with a dry cathode the outlet gas carries water. Moisture or other gases in the stream can make the efficiency read high, so calculate from the hydrogen molar flow, taking the gas composition and the meter's gas setting into account.

Measuring both gases at once takes two meters. With one meter used in turn, choose a model that handles both gases, change the gas setting each time and purge thoroughly. Ventilation and hydrogen safety measures follow your laboratory's rules.

Start a comparison from a reference build

If a new membrane lowers the cell voltage, the change can only be attributed to the membrane when everything else is the same: electrodes, active area, electrolyte concentration and temperature, with current density calculated on the same active area.

Gaskets are a little different. When the membrane thickness changes, the same gasket may not give the same compression. Rather than keeping the gasket fixed at all costs, record what you changed and why. Writing down the sample lot, conditioning, gasket and assembly sequence for every run gives you something to stand on when you interpret the results.

Record the measured cell voltage and the resistance-corrected voltage separately. The corrected voltage is the measured voltage minus the current times the resistance used for the correction (I × R), so also record which resistance you used and how you obtained it. When comparing membranes, the difference in membrane resistance is part of the performance, so look at both values.

The most important check is to build the reference configuration again and rerun it. If the scatter from reassembly is similar to the improvement from the new material, a single polarisation curve cannot show that the material is better. That is why it matters how easily a cell lets you swap samples and reassemble.

The European Commission's Joint Research Centre has published a harmonised polarisation-curve test method for low-temperature water electrolysis, which is a useful reference when setting test conditions.

SourceJRC: EU harmonised polarisation curve test method for low-temperature water electrolysis (2018)

Start with a single cell, then a short stack

Material comparisons are best started on a single cell: the build is simpler than a multi-cell one, and samples are easier to swap.

A short stack answers a different question. Several cells share the fluid distribution and are assembled differently, so the test is whether the same material behaves uniformly across cells. Total stack voltage can hide differences between cells, so decide how you will measure each cell before adding more. Keep the single-cell result as the reference and record what changed in area, flow, compression and measurement.

What to specify for HXS-0

HXS-0 is built to order against the points above. Active area, cell count and flow-field configuration are agreed before manufacture, and the standard lead time is four weeks after the order is accepted and the specification is fixed.

80 °C is the limit of the cell. The membrane and electrodes under test may need a lower temperature, so check the limits of your materials as well.

For laboratories new to the cell, we recommend coming to our site to assemble and run it yourselves. Installing and training in one visit at delivery rarely leaves enough time for hands-on practice. At our site we show the build and operation on real equipment, from a small single cell up to a stack, and then provide technical support through installation and operation.

What to send with an enquiry

With these details we can check that your samples and equipment fit before we quote:

1. Power supply model and maximum current.
2. The active area you want, and whether you will use one area or compare several.
3. Membrane and electrode substrate, dimensions, thickness and conditioning state.
4. Operating conditions such as electrolyte, temperature and whether the cathode circulates.
5. Which materials you will supply, and whether you want the MEA and the surrounding equipment with the cell.