AEM water electrolysis glossary

Water electrolysis with a membrane that conducts hydroxide ions (OH⁻). The cell therefore runs alkaline, which is what makes platinum-group-free anode catalysts possible.

The first-generation approach, using concentrated potassium hydroxide and a porous diaphragm. The longest industrial track record, with limits on load following and differential-pressure operation.

Uses a membrane that conducts protons (H⁺). The cell runs acidic, which requires platinum-group metals such as iridium on the anode.

Splits steam above roughly 700 °C. Supplying heat lowers the electricity demand, at the cost of start-stop handling and materials durability.

The two half reactions: oxygen at the anode, hydrogen at the cathode. Oxygen evolution is the slow half and carries most of the kinetic loss, which is why catalyst work concentrates on the anode.

The membrane bonded with the catalyst layers on both sides. It sets most of the cell's performance and life, and it is the part replaced when a stack is serviced.

The ion-conducting polymer inside the catalyst layer. It connects catalyst particles to the membrane; more of it improves ion transport but blocks the path gas needs to leave.

A series assembly of cells: cells, bipolar plates, porous transport layers, seals and end plates, held together under compression.

The porous layer between catalyst layer and bipolar plate. It spreads current, lets water in and gas out at the same time. In AEM cells the anode side is nickel and the cathode side is carbon-based.

The plate that carries current between adjacent cells and forms the flow channels. HydroXpand's HXS series uses nickel-based metallic plates, with SUS316L end plates.

The clamping pressure that holds a stack together. Too little raises contact resistance and risks leaks; too much crushes the porous transport layer and the membrane.

Disassembly and reassembly therefore need equipment. An HXS-2 can be opened because it is a compression-assembled stack, but reassembly means compressing to 5 tonnes on the gauge, bolting in two torque steps of 5 Nm and 7 Nm, and measuring compressed length in four directions. A hydraulic press, compression blocks, a torque wrench and callipers all have to be on hand.

The most common failure is the gaskets. The O-ring lifts out of its seat and the MEA gasket shrinks inward, so both have to be pushed back after seating.

A construction that uses no platinum-group metal. The HydroXpand HXP-an anode grows NiFe LDH directly on nickel foam and is PGM-free. The HXP-ca cathode uses Pt/C or PtRu/C, so the full cell is not PGM-free.

Painting catalyst particles onto a substrate, versus growing the active phase directly on it. A coated layer can start with the better number; keeping that surface attached is a different problem, and it decides how long the cell runs.

The electrode area where the reaction actually happens. Cell current is current density times this area.

When reading performance data, separate the test area from the product area: a curve measured on a small area does not transfer unchanged to a large one. HydroXpand's HXS-0 is built to order once active area, cell count and flow-field design are fixed.

Current per square centimetre of active area (A/cm²). Raising it increases hydrogen output and lowers efficiency in the same cell, so an efficiency figure only means something alongside its current density.

Cell voltage recorded while current density is increased. It shows performance across the whole operating range rather than at one point, which is why it is the first dataset to ask a stack supplier for. When you get one, check which active area and cell configuration it was measured on.

Holding current constant and watching the voltage that is needed. If the same hydrogen output starts to demand a higher voltage, losses have grown, which is why durability testing is usually galvanostatic.

Running with outlet pressure above ambient. It can reduce downstream compression, at the cost of higher gas crossover and tougher sealing. barg is gauge pressure, with ambient as zero.

HXB-V1 supports both 0 barg and 10 barg operation, and continuous operation at 10 barg has been verified. Control is galvanostatic, so current holds at its setpoint as pressure rises; the voltage increase measured in verification was below 1%. Direct output above 30 bar is not a current sales specification.

When placing pressurised and ambient data side by side, read the electrolyte concentration with them. HydroXpand's pressurised testing runs in 0.1 M KOH, while the published 2,281-hour ambient run used 0.3 M KOH.

How far below rated load the unit can run (turndown) and how fast output can change (ramp rate). For renewable-coupled installations these often matter more than rated capacity.

AEM cells run with a dilute potassium hydroxide solution in circulation. The flow does more than carry ions: it cools the stack and sweeps product gas out.

Raising the flow rate at will is therefore not safe. The 4 L/min standard circulation on HXB-V1 was set for stack cooling and anode pressure control rather than to match water consumption, and the bottleneck is the internal flow field and manifold cross-section, not the external piping. Higher flow raises differential pressure and widens the gap between anode and cathode.

KOH is not consumed the way water is, so it is replaced on condition and performance trend rather than on a fixed interval.

Everything around the stack: power conversion, circulation, gas-liquid separation, purification, control and safety. The consumption here is why stack efficiency and system efficiency are different numbers.

The vessel that separates circulating electrolyte from product gas. Incomplete separation loads the downstream purification and drying stages.

Removes water carried in the hydrogen stream. Whether it is needed depends on the purity you require. It is not part of the standard HXB-V1 configuration; with a dryer fitted, HydroXpand confirmed hydrogen purity at or above 99.999% under its verification conditions.

The hydrogen mole fraction, written as figures such as 99.99%. What has to be removed is residual oxygen and moisture, so the required grade decides whether deoxo and drying stages are needed. Product-level figures are on the product pages and in the FAQ.

The standard for water fed to the cell. Residual ions accumulate in the membrane and catalyst layer and degrade performance. HydroXpand recommends ASTM D1193 Type II, at or below 1.0 µS/cm at 25 °C, and has operated at around 2.0 µS/cm. Whether a given supply works is confirmed against the project conditions.

Converts AC supply into the DC the stack needs. Its conversion loss lands directly in system efficiency.

Two ways of stating output. One normal cubic metre of hydrogen is 0.0899 kg, so specifications written in different units have to be converted before they can be compared.

The energy in the hydrogen divided by the electricity put in. Choosing higher heating value (39.4 kWh/kg) or lower heating value (33.3 kWh/kg) makes the same performance look about 18% different.

An efficiency quoted without its basis therefore cannot be compared. Ask whether it is a stack or a system figure as well: the gap between the two is what the balance of plant consumes.

How fast the voltage of a single cell rises under galvanostatic operation. Cell counts differ between stacks, so a change in stack voltage is not a fair comparison and the figure is normalised per cell.

When you receive one, check the fitting window first. Segment slopes can differ, so a full-run fit should not be extrapolated into future voltage. HydroXpand's HXS-2 23-cell stack figure of 80.2 µV/h per cell is a full-run linear fit over the published 2,281 hours in 0.3 M KOH at 40 °C.

Product gas permeating through the membrane to the other side. The two directions have to be read separately. The safety threshold applies to hydrogen in oxygen (H₂-in-O₂); HydroXpand manages this below 2% in long runs and recommends stopping for inspection above it. Oxygen in hydrogen is a purity question instead, handled by the purification stage the required grade calls for.

How far the individual cell voltages in a stack spread apart. A mean voltage hides a single failing cell. In the published HXS-2 data, cell-to-cell standard deviation doubled from 0.010 V to 0.020 V over 2,281 hours, separately from the rise in the mean. That trend warns earlier than the mean does.

A test build with one cell, and one with a small number of cells in series. Materials and MEA screening happen in a single cell; cell-to-cell interaction and uniformity only appear in a short stack. HydroXpand's HXS-0 builds from a single cell up to 20.

Servicing by replacing the consumed MEA while reusing the stack hardware. HydroXpand handles this as a refurbishment: the stack comes back, the MEA is replaced, and the engineer who designed and built that stack does the work. Field disassembly is not the default because the clamping conditions have to be controlled.