At the cathode of a CO2 electrolyzer, two reactions compete for the same electrons: CO2 reduction (CO2RR), which turns dissolved CO2 into CO, formate, or — on copper at strongly negative potentials — hydrocarbons and alcohols; and the hydrogen evolution reaction (HER), which just splits water into H2 and wastes charge. Both rates rise exponentially with how negative the applied potential is (a Butler-Volmer / Tafel relationship), but CO2RR is capped by how fast CO2 can diffuse to the surface — its mass-transport limit — set by CO2 partial pressure and electrolyte flow.
i_CO2RR = min( A·exp(k·|η|), i_lim(p_CO2, flow) )
i_HER = B·exp(k_H·|η|)
FE_CO2RR = i_CO2RR / (i_CO2RR + i_HER)
- Applied potential — more negative drives both reactions faster, but once CO2RR hits its diffusion ceiling, the extra current goes entirely into HER (falling FE — the classic "selectivity cliff").
- Catalyst — Cu supports multi-carbon products at high overpotential; Ag/Au strongly suppress HER for clean CO; Sn/In favor formate; Pt binds hydrogen so well that HER dominates almost everywhere.
- CO2 supply — higher partial pressure and faster electrolyte flow raise the mass-transport ceiling, letting CO2RR keep pace with the applied current before HER takes over.
This tug-of-war — kinetics pushing current up, mass transport capping how much of it CO2RR can absorb — is why real CO2 electrolyzers need gas-diffusion electrodes and pressurized CO2 feeds to run at industrially useful current densities without collapsing into a hydrogen generator.