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PEM explained: one thin membrane, two jobs

Both halves of the system, the electrolyser and the fuel cell, are built around the same idea: a proton exchange membrane, or PEM.

What the membrane is

A proton exchange membrane is a thin sheet of a special polymer that conducts hydrogen ions, which are simply protons, while blocking electrons and gases. It is sandwiched between catalyst layers and electrodes to make a membrane electrode assembly, and many of those are stacked to build a working unit. The membrane is the reason the hydrogen and oxygen sides stay apart, which is central to both performance and safety.

What it would keep running

  • 25 kW DC PEM electrolyser stack
  • PEM fuel cell stack at 45 to 55% efficiency
  • De-ionised water feed
  • Filtered air feed to the fuel cell
  • Quick response to changing wind input

Why PEM suits a home

PEM technology runs at relatively low temperatures compared with some other fuel cell and electrolyser types, so it can start and respond quickly rather than needing hours to warm up. It is compact, uses a solid membrane rather than a liquid electrolyte, and works well with varying input. That last point matters when the power source is a gusty wind turbine or night-rate charging that starts and stops.

The trade-off is that PEM membranes are sensitive to impurities. Minerals in the water or contaminants in the air feed can damage them, which is why water treatment and air filtering are taken seriously in the design. Looking after those two inputs is most of what keeps a PEM stack healthy over its working life.

How it works →
The H2OME loop: an electrolyser splits water into hydrogen, the hydrogen is stored in a tank for weeks, a fuel cell turns it into electricity for your home, and the water returns to the loop.

Catalysts and cost

PEM stacks rely on precious metal catalysts to drive the reactions at practical rates. That is one reason hydrogen equipment is not cheap, and a reason we have no price to quote yet. Research worldwide is aimed at using less catalyst and making membranes last longer. For a household, the practical points are simple: keep the water pure, keep the intakes clear, and follow the maintenance schedule.

Questions

Is PEM the only kind of fuel cell?

No. There are alkaline, solid oxide, phosphoric acid and other types, each with different temperatures and uses. PEM is common in vehicles and smaller stationary systems because it is compact and quick to respond.

Does the membrane need replacing?

Membranes degrade over time, so stacks are expected to need service or replacement eventually. How often in real home use is one of the things the pilot stage is designed to find out.

Why does PEM need such pure water?

Dissolved minerals and ions can contaminate the membrane and catalyst, cutting performance and shortening life. De-ionising the water protects the most expensive part of the system, which is why the treatment stage is monitored rather than left to chance.

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