If your measured cell voltage is higher than the datasheet, match the test conditions before you suspect the electrode. The conditions behind each published figure, half-cell versus cell voltage, seven checks and the usual causes, based on our HXP electrodes.
Laboratories testing our electrodes or using our test cell often ask why they cannot reproduce the performance shown in the datasheet. This article answers that question.
When we go through these enquiries, the cause is usually not the electrode but the test conditions and the assembly. A datasheet curve is the result of one particular set of conditions, and the same electrode gives a different cell voltage at a different temperature or electrolyte concentration. So when someone tells us their numbers differ, we match the conditions together before anyone replaces an electrode. This is the order we follow.
- Find the test conditions behind the figure you are comparing first. One supplier's published figures use different conditions from chart to chart.
- Do not compare half-cell values directly with cell voltages. A cell voltage contains the equilibrium voltage, the overpotentials of both electrodes and the resistive losses in the membrane and contacts.
- Match the temperature the cell actually reached, not the setpoint. Do not compare a room-temperature measurement directly with datasheet results obtained at elevated temperature.
- Change one condition at a time. Change several together and you cannot tell which one was the cause.
The conditions behind each published figure
Figures published by the same supplier are measured under different conditions from chart to chart. These are the conditions behind ours.
| Published figure | What was measured | Test conditions |
|---|---|---|
| HXP-an datasheet performance chart | Anode catalyst activity (compared with the bare nickel-foam substrate) | Three-electrode half-cell · 0.3 M KOH · room temperature · Tafel without iR correction |
| HXP-ca datasheet Tafel slope | Slope of overpotential against log current density for the cathode reaction (mV/dec) | Three-electrode · 0.3 M KOH · room temperature · N₂ purge |
| HXP-ca datasheet single-cell curves | Pt/C and PtRu/C cathodes compared in a single cell (anode HXP-an) | 0.3 M KOH · 60 °C |
| HXS-0 catalogue i-V curve | Single-cell performance | 4 cm² active area · 1 M KOH · 80 °C |
| HXS-2 and HXS-30 stack curves | Performance of stacks built with HXP electrodes | 0.1 M KOH · 50 °C · ambient pressure |
If you test in a neutral or chloride-containing electrolyte, treat datasheet values as a reference point, not a prediction.
Once you have chosen the curve to compare with, record its document revision and test conditions first. Where the source does not state a condition, record it as unknown rather than assuming it matches your own experiment.
Half-cell values and cell voltages are not compared directly
A three-electrode half-cell test measures the potential of one working electrode against a reference electrode. A cell voltage, by contrast, contains the equilibrium voltage for splitting water, the overpotentials of both anode and cathode, and the resistive losses in the membrane, ionomer, contacts and electrolyte. A half-cell Tafel slope or potential therefore cannot be compared directly with a cell voltage. Uncorrected solution resistance can still affect a half-cell measurement.
- Half-cell values: for comparing catalysts on the same substrate under the same conditions.
- Single-cell and stack values: for the voltage needed under real operating conditions. The membrane and cell construction are part of the result.
When you compare with a stack curve, check the cell count. Dividing the total stack voltage by the number of cells in series gives the average voltage per cell; if the chart already shows voltage per cell, do not divide again. An average cannot show the voltage differences between cells. If you compare efficiency, also apply the same basis on both sides: stack or system, and higher heating value (HHV) or lower heating value (LHV).
Seven things to check before comparing
With a different temperature or electrolyte concentration, the same electrode gives a different cell voltage at the same current density. Check these seven before you compare with a datasheet.
- Settling: measured after at least 30 minutes at rated current, once the cell voltage had settled.
- Temperature: the temperature the cell actually reached, not the setpoint.
- Electrolyte concentration: the same as in the chart you are comparing with.
- Clamping: the torque given in the assembly guide.
- Gaskets: still in their seats.
- Membrane and ionomer: what you assembled with.
- Method and area basis: galvanostatic or potentiostatic, scan rate, the area used for current density, and whether iR correction was applied.
The same cell gives a different curve depending on the area used to calculate current density. Record the measured current and the area basis, not only the calculated current density. Record the electrode condition too: a fresh electrode and one with operating history can give different results.
- Download the test-condition worksheet (CSV)→Reference experiment and your experiment, side by side. Opens in Excel or another spreadsheet app.
How we measure our electrodes
These are the current reference conditions and sequence we use to measure a single cell built with HXP electrodes. All voltages in the tables are cell voltages between anode and cathode. Some datasheet charts were taken under earlier conditions, so read this together with the first table.
| Electrolyte | 0.1 M KOH (deionised water) |
|---|---|
| Temperature | 50 °C |
| Pressure | Ambient |
| Electrolyte flow | About 5 mL/min per cm² of active area |
| Cathode | Dry cathode (electrolyte circulates on the anode side only) |
There is no separate activation step for the electrodes. We heat up, run at rated current straight away for about 30 minutes, and start measuring once the cell voltage has settled.
The measurement sequence:
| Step | Technique | Conditions |
|---|---|---|
| Initial settling | Galvanostatic (CP) | Rated current · 30 min |
| Current hold | Galvanostatic (CP) | 0.1 A/cm² · 30 s |
| Polarisation curve | Linear sweep voltammetry (LSV) | Open-circuit voltage (OCV) → 1.70–1.75 V · 5 mV/s |
| Repeat | Return to the first current hold and repeat the current hold and LSV twice | |
| Cyclic voltammetry | CV | 1.0 → 0.4 → 1.8 → 1.0 V · 4 cycles · 50 mV/s |
| Open-circuit voltage | OCV | 30 s |
| Current hold | Galvanostatic (CP) | 0.1 A/cm² · 30 s |
| Impedance | Potentiostatic EIS (PEIS) | DC bias 1.5 / 1.6 / 1.7 / 1.75 / 1.8 V · 100 kHz → 100 mHz · 10 mV amplitude |
With good electrodes, the instrument often reaches its current limit before the cell reaches 1.8 V. If the current stops rising, first check the instrument's current-limit indication and the actual current. If it has reached the limit, lower the upper voltage to 1.70–1.75 V and check that this range also stays within the limit. The 1.8 V points in CV and PEIS have the same constraint.
- Electrode operation note→The full measurement sequence and the conditions for each technique.
Symptoms and what to check first
| Symptom | Check first | Action |
|---|---|---|
| Cell voltage higher than the datasheet at the same current density | HXP-ca assembled upside down (carbon-paper face to the membrane), temperature not reached, different concentration, too little settling, under-clamping, different area basis | First check that the HXP-ca catalyst-layer face points at the membrane, then work through the seven checks and measure again |
| Room-temperature values do not match the datasheet | Different test conditions | The datasheet single-cell curves were measured at elevated temperature. Compare again under the same conditions |
| Current stops rising before the cell reaches 1.8 V | The instrument's current limit | Check the current-limit indication and the actual current; if it is the limit, lower the upper voltage and measure |
| Values differ in a neutral or chloride electrolyte | Published figures are for alkaline conditions | Use the datasheet values as a reference only |
| Electrolyte leaks from the assembled cell | A gasket out of its seat or shrunk, debris on a sealing face | Take the cell apart, reseat the gaskets and wipe the sealing faces. If it keeps leaking, choose the gasket thickness again |
When a cell voltage reads high, the first thing we check is the HXP-ca orientation. HXP-ca must be assembled with its catalyst-layer face towards the membrane and its carbon-paper gas diffusion layer (GDL) face towards the cell hardware. Upside down, the catalyst layer does not touch the membrane and the electrode cannot perform. HXP-an is the same on both faces, so its orientation does not matter.
When you adjust test conditions or the assembly, change one thing at a time. Change the temperature and the gasket together and you cannot tell which was the cause. Change one, measure again, then move on to the next.
If the difference remains, what to send us
If your values still differ after matching the conditions, send us the following. An engineer will compare them under the same conditions and work out the cause with you.
| Electrode | Anode or cathode, format and size, cathode catalyst (Pt/C or PtRu/C), date received |
|---|---|
| Test and assembly conditions | Temperature reached, electrolyte concentration, settling time, clamping torque, membrane and ionomer |
| Measurement | Method and scan rate, area basis for current density, iR correction, raw data |
| Photos | Electrodes, gaskets and the cell taken apart |
'The cell voltage is too high' on its own is not enough; with the test and assembly conditions and the raw data we can see where the results diverge. Datasheet values come from our test conditions, so a different cell construction, membrane and ionomer, or instrument may not reproduce the same value. Even then, comparing the conditions item by item shows what differs and narrows down what still needs checking. How much each difference contributes to the cell voltage has to be confirmed separately.
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)
- Request a review of your results→Send the reference curve, your conditions and the raw data.
- HXP electrode handbook · performance→Which condition each published figure belongs to.
- Choosing an AEM test cell→The hardware side of the same comparison.
- Cell voltage distribution→Why the spread between cells matters.
- Specifying electrodes for an MEA→Area, thickness and catalyst loading, in the order the cell sets them.
- Electrolyzer efficiency explained→Heating value, boundary, load and age: the four things that move a figure.
- AEM electrolysis: technology and measured data→How AEM electrolysis works, AEM vs PEM vs alkaline, and the measured data behind our stacks.
Frequently asked questions
Why is my cell voltage higher than the datasheet with the same electrode?
Usually because the test conditions differ. A different temperature or electrolyte concentration changes the cell voltage, so first match the temperature actually reached and the concentration to the conditions of the datasheet chart.
Can I compare the datasheet Tafel slope with my measured cell voltage?
Not directly. The Tafel slope in this datasheet expresses the relationship between overpotential and the logarithm of current density from a three-electrode test (mV/dec); it is a different quantity from a cell voltage (V).
Do the electrodes need an activation procedure?
No. HXP electrodes have no separate activation step. Heat up, run at rated current for about 30 minutes, and measure once the cell voltage has settled.
I want to measure up to 1.8 V, but the current stops rising.
Often the instrument has reached its current limit. Check the current-limit indication and the actual current, and if it is the limit, lower the upper voltage. Our guide value is 1.70–1.75 V, but check that this range also stays within the limit.
Sources
- EU harmonised polarisation curve test method for low-temperature water electrolysis — European Commission Joint Research Centre, 2018
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