AEM water electrolyser
Single cell crash course · Chemical Engineering, KMUTT
Anion exchange membrane water electrolysis
Two half reactions, a membrane between them, and eleven more layers whose job is to get water in, gas out and current through. Turn the model, take it apart, and click any layer for what it does and how thick it really is.
The two half reactions
Water is reduced at the cathode and hydroxide is oxidised at the anode. OH⁻ moves through the membrane, electrons go the long way round through the supply.
Cathode · negative
Two electrons per H2. Fast on platinum: i0 near 10−3 A cm−2.
Membrane
Fixed positive groups on the polymer pass the anion and block electrons. About 50 µm thick.
Anode · positive
Four electrons and one O to O bond. Slow: i0 six to eight decades lower than the cathode.
The hardware, as delivered
Step one
Five layers, about 0.8 mm thick in total. Build this first, on the bench, before anything is bolted together.
Step two
The MEA goes between two gaskets, then two flow field plates, then two end plates with their heaters. Eight bolts, torqued in a star pattern.
Drag to turn it. Slider takes it apart.
arrows turn · + − zoom · [ ] apart · R reset
Run the cell feeds KOH into the lower port of each plate and brings gas out of the upper one, with the hardware solid. Cut away makes everything except the membrane assembly transparent, so the hydroxide crossing the membrane is visible as well.
Layer by layer
Bipolar plate and flow field
A single cell has two of these, one each side. In a stack, one plate carries the anode field on its front face and the cathode field of the next cell on its back face. That is what bipolar means.
The anode sits above 1.5 V in hot alkali, which oxidises carbon. Titanium survives by growing a passive oxide, at the cost of contact resistance, so the interface is often coated with platinum or gold. The cathode is reducing, so graphite is safe there and cheaper to machine.
Six channel geometries
Same plate, same 5 cm² window, six ways of cutting it. Turn the plate and switch the pattern.
The same six, flat
Cross section
Under a rib there is electrical contact but no gas path. Over a channel there is a gas path but no contact. The porous layer bridges the two, so the ratio of rib width to channel width sets how far current travels sideways through the PTL before it reaches metal.
Most designs land near equal widths, typically 1 to 2 mm each on a cell this size.
Theory
Four terms, each one measurable on your own cell. Move the sliders and watch which term grows.
1 · Thermodynamics
Erev is the minimum voltage set by ΔG. It falls with temperature, because part of the energy can come from the surroundings as heat, and rises with product pressure. The thermoneutral voltage Etn is set by ΔH and does not move; above it the cell heats itself.
2 · Kinetics
At equilibrium the reaction still runs, forwards and backwards at equal rate. That rate per unit area is the exchange current density i0, and it is the number that measures a catalyst. Above it, overpotential grows with the logarithm of current: the Tafel line. Where a line meets zero overpotential is that electrode's i0.
3 and 4 · Resistance and transport
Ohmic loss is linear in current and comes mostly from the membrane. Concentration overpotential is flat until the current approaches iL, then rises steeply. Added to Erev and the two activation terms, they give the polarisation curve.
Starting values. αa = 1.65, αc = 0.73 and i0,c = 10−3 A cm−2 are the alkaline defaults in the Aspen Plus Electrolyzer block, so this page and the simulation start from the same numbers.
Gas fills the pores of the transport layer faster than liquid can refill them, the catalyst runs dry, and no extra voltage buys more current. On the curve it appears as a steep upturn.
It is a transport problem, so the fix is flow rate, temperature, gasket thickness or PTL structure, not a better catalyst.
The thermoneutral voltage divided by the voltage paid. Quote it with the current density and the temperature, because a cell at 1.7 V and 0.2 A cm⁻² and a cell at 1.7 V and 2 A cm⁻² are not the same machine.
At the default settings this cell runs at 1.93 V and 77 % on that definition.
Aspen Plus Electrolyzer block
The block asks for about thirty numbers. Each one describes a specific layer of the cell you have just taken apart, and each one comes from somewhere: a supplier data sheet, a measurement, or a fit to your own curve.
Click a part name to jump to that layer in the 3D cell. Where two values are shown they are anode / cathode.
Where i₀ and α come from
A full cell gives the sum of both electrodes plus the ohmic drop, so it cannot separate them. Three electrode mode measures the working electrode against a reference that carries no current.
Reading the two numbers off
Sweep slowly, correct the potential for the uncompensated resistance, plot η against log i, and fit the straight part. The slope gives α, the intercept at η = 0 gives i₀. Repeat at three or four temperatures for the activation energy.
Notice how far the fit is extrapolated to reach η = 0. A 5 % error in the slope moves i₀ by a factor of two, which is why published values scatter so widely. Quote the slope alongside it.
Step the current, wait for the voltage to settle, record. Gives the curve on the theory page and, with an iR correction, the Tafel region.
The high frequency intercept on the real axis is the ohmic resistance: membrane, contacts and plates together. That is the number the membrane parameters have to reproduce.
Hold the voltage and watch the current over hours. A slow decay is degradation; a noisy one is usually gas blocking a channel.
Next
The hardware, the theory, the Aspen map and the half cell are in. What is left is the measurement side.
A Nyquist plot you can drag, with the equivalent circuit next to it: which arc is charge transfer, which is transport, and where the ohmic resistance is read off.
Real data from your own cell, with the iR correction and the Tafel fit done on screen, so the students see the arithmetic and not just the answer.
The same cell repeated, bipolar plates shared between neighbours, and what changes: voltage adds, current does not, and the manifolds become the hard part.
Tell me which of the three you want first, and anything on this page that reads wrong for your students.