H-Cell vs Gas Diffusion Flow Cell for CO₂ Reduction
Use an H-cell for fast, low-cost screening of catalysts. Move to a gas diffusion flow cell when you need high current density, because CO₂ dissolves poorly in water and limits an H-cell. Use a zero-gap electrolyzer to test how a catalyst behaves in a device.
The three cell types compared
| H-cell | Gas diffusion flow cell | Zero-gap electrolyzer | |
|---|---|---|---|
| How CO₂ arrives | Dissolved in electrolyte | As gas, through a gas diffusion electrode | As gas, through a gas diffusion electrode |
| Current density | Low, limited by dissolved CO₂ | High | High |
| Best for | Catalyst screening | Testing catalysts at practical rates | Testing as a device |
| Setup effort | Lowest | Moderate | Highest |
| Main problem | Low current | Electrode flooding | Salt build-up, water balance |
Why an H-cell runs out of CO₂
CO₂ dissolves in water at only about 33 millimoles per litre at 25 °C and 1 atmosphere. In an H-cell, the catalyst can only use CO₂ that reaches it through the liquid, so the current levels off at a low value however good the catalyst is.
A gas diffusion electrode solves this by bringing the gas right up to the catalyst through a porous layer, leaving only a very thin film of liquid to cross.
A sensible order of work
- Screen catalysts in an H-cell, where runs are quick and cheap.
- Move the best ones to a gas diffusion flow cell, to see how they behave at high current.
- Test the winner in a zero-gap electrolyzer with an anion exchange membrane.
Frequently asked questions
What membrane goes in a CO₂ reduction H-cell?
Proton exchange and anion exchange membranes are both used. Choose by your electrolyte, and by which products must not cross to the other side.
What is flooding in a gas diffusion electrode?
Electrolyte seeping into the pores of the electrode and blocking the gas, which cuts performance. It is the commonest failure in flow cells.