Comparison of electrolysis conditions and role of specific cell components in batch and flow CO2 electrolyzers.
| Batch (H-type) cell | Flow cell | |
|---|---|---|
| Achievable CO2RR current density20,21,24,27 | < 50 mA cm−2 | >1 A cm−2 |
| Reactant | Dissolved CO2/bicarbonate | Gas-phase/dissolved CO2 |
| Diffusion length of CO224,43,44,47 | ∼50 μm | ∼50 nm |
| Catholyte | MHCO3a | MOH, MHCO3, none |
| Local pH at the cathode48–53 | Near-neutral or alkaline | Alkaline |
| pH gradient48–53 | pHsurface > pHbulk (up to 5 pH unit difference) | Bicarbonate electrolyte: pHsurface > pHbulk, alkaline electrolyte: pHsurface < pHbulk |
| Catalyst support6,54–57 | Flat (glassy carbon, metal foil) or porous (carbon paper, metal foam) | Porous (carbon paper, metal foam, PTFE) |
| Ionomer16,29,58 | Not mandatory, mainly catalyst binder, but can change morphology, surface chemistry | Microfluidic: same role as for H-cell; zero-gap: ensures ionic conductance |
| Membrane59 | Type: CEM/AEM; role: product separation; does not limit cell performance | Type: CEM/AEM/BPMb/none; role: product separation, in zero-gap-cells it determines catalyst surface chemistry; can limit cell performance |
| Surface concentration of reactants/intermediates24 | Lower | Higher |
| Electrode configuration | 3-Electrode, cathode potential | 2 or 3-electrode, full cell voltage/cathode potential |
a
M: alkali metal cation.
b
CEM: cation exchange membrane, AEM: anion exchange membrane, BPM: bipolar membrane.