In an alkaline water electrolyser, two electrodes sit in hot, concentrated potassium hydroxide. Between them is the separator. It has four jobs, and each one appears somewhere in the cost of hydrogen.
1. Resistance is paid for on every kilogram
A cell needs a minimum voltage to split water, the reversible voltage of about 1.23 V at standard conditions. Everything above that is loss: the energy needed to drive the electrode reactions, and the ohmic loss of pushing current through the electrolyte and the separator.
The ohmic loss is simple arithmetic: voltage lost equals current density multiplied by area resistance. At 0.4 A/cm², every 0.1 Ω·cm² of resistance costs 40 mV. In a cell running near 1.9 V, that is about 2% of the electricity, on every kilogram of hydrogen, for the life of the plant. Electricity is the largest cost of green hydrogen, so a few tens of millivolts matter.
2. More current per square centimetre means a smaller stack
Run the comparison the other way. At the same cell voltage, a separator with lower resistance lets more current flow through each square centimetre. More current means more hydrogen from the same active area, or the same hydrogen from a smaller stack. If current at equal voltage rises by a fraction g, the active area needed falls to 1 / (1 + g).
In cell tests at IIT Bombay, Nerva H2Sep 500 carried up to 38% more current than a commercial 500 µm-class reference at 2.4 V. At equal voltage, that corresponds to about 27.5% less active area for the same hydrogen output: fewer cells, less nickel, smaller frames.
Measured at IIT Bombay against a commercial 500 µm-class reference. Typical values, not a specification.
3. Gas purity sets the safe operating window
A separator must stop hydrogen and oxygen from mixing. Its bubble point, the pressure needed to push gas through the wetted pores, is one measure of how well it does that. Conventional polymer fabric diaphragms (PPS) conduct well but have large open pores; published studies report bubble points of around 0.02 bar and significant gas crossover. Composite separators close the pores while keeping ions moving. Nerva H2Sep 500 has a bubble point of 2.5 ± 1 bar.
Crossover matters most at low load, when less gas is produced and any leakage is a larger share of it. As electrolysers follow solar and wind output, low-load operation becomes routine, and gas separation becomes an operating constraint, not just a specification line.
4. Strength decides how the stack ages
A stack is compressed at assembly and then heated, cooled and pressurised for years. A separator that creeps or tears changes the cell gap, the gas separation and eventually the stack's performance. Mechanical strength is therefore an economic property: it protects the efficiency you paid for. Nerva H2Sep 500 has about 2.7 times the tensile strength of the commercial reference and a Young's modulus above 600 MPa.
5. A small cost line with a large reach
The separator is about 1% of an electrolyser's cost. Every megawatt of alkaline electrolysis needs roughly 150 m² of it, and stacks are rebuilt every 7–10 years, so separators are bought again over the life of a plant. Because it sets resistance, current density, gas purity and durability at once, it influences far more than its own price: the power bill, the size of the stack and the uptime of the plant.
In short
- Lower resistance means less electricity per kilogram, for the life of the plant.
- More current at the same voltage means a smaller stack for the same output.
- Higher bubble point means cleaner gas and a wider safe operating window.
- Higher strength means the stack keeps its performance through assembly and cycling.
Sources. Nerva H2Sep 500 values: measured at IIT Bombay against a commercial 500 µm-class reference; typical values, not a specification. PPS fabric separators: Chem. Rev. 124 (2024), “Separators and Membranes for Advanced Alkaline Water Electrolysis”; ACS Omega (2024). Separator area and stack rebuild interval: Greenprastha estimates. Worked examples are illustrative.
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