The cell, layer by layer
A PEM cell is a sandwich. In the middle sits a thin solid polymer membrane. Catalyst layers are bonded to each face, and porous transport layers plus current collectors sit outside those. Water is fed to the anode side; direct current is applied across the stack.
At the anode, water is split, releasing oxygen, electrons and protons. The protons — hydrogen ions — travel through the membrane itself, which is engineered to conduct them while blocking gas crossover. At the cathode they recombine with electrons to form hydrogen gas.
Why the membrane replaces the electrolyte
Traditional alkaline electrolysis needs a conductive liquid, typically potassium hydroxide, to carry ions between the electrodes. That liquid is caustic, has to be maintained, and sits in the same vessel as both gases.
The polymer membrane does the ion-carrying job as a solid. That removes the caustic solution from the product entirely, allows the cell to run on distilled water, and — the decisive part for consumer devices — keeps the two gas streams on opposite sides of a physical barrier.
SPE and PEM are the same thing
You will see both acronyms used, sometimes in the same spec sheet. SPE stands for solid polymer electrolyte and PEM for proton exchange membrane; they describe the same technology from two angles — the material and its function.
Treat any distinction drawn between them in marketing copy as a red flag rather than a specification.
What PEM does not do
A PEM cell does not purify the feed water. Minerals and dissolved solids in the input will foul the stack over time, which is why distilled water is a requirement rather than a suggestion in inhalation systems.
It also does not guarantee any particular purity by itself. Separation makes a defined-purity stream achievable; only measurement demonstrates one, which is why third-party purity testing exists (QTT reports LH2025050017 and LH2025050018 cover our inhalation systems).
Maintenance implications
The practical consequences of a PEM design are pleasant: no electrolyte to handle, no caustic refills, and maintenance that mostly reduces to using the right water and keeping the reservoir clean.
Cell life is finite in any technology. Water discipline, run hours and thermal management are what separate a stack that lasts from one that degrades early.