Direct seawater electrolysis for green hydrogen has one hard problem: seawater is ~0.55 M in Cl⁻, and at the anode the chlorine evolution reaction (CER) competes directly with the wanted oxygen evolution reaction (OER):
OER: 2H₂O → O₂ + 4H⁺ + 4e⁻ E° = 1.23 V vs SHE
CER: 2Cl⁻ → Cl₂ + 2e⁻ E° = 1.36 V vs SHE (thermodynamic)
but CER is kinetically much faster (2 e⁻, low overpotential)
so it wins on real catalysts unless OER is deliberately favoured
Even though OER is thermodynamically preferred by 130 mV, its 4-electron mechanism is kinetically sluggish, so on a generic anode CER dominates and the cell produces toxic, corrosive Cl₂ instead of O₂. This lab renders the anode surface in 2D cross-section, nucleating and releasing green O₂ bubbles and red Cl₂ bubbles from a Butler–Volmer-style kinetic competition: each reaction's rate depends on its overpotential η = Eapplied − E°, and the observed Faradaic split follows
i_OER / i_CER ≈ (k_OER / k_CER) · exp[(η_OER − η_CER)·F / (2RT)]
FE(Cl₂) = i_CER / (i_CER + i_OER)
Selective catalysts (Ni-Fe layered double hydroxides, MnOx, or Co-oxide films) raise the OER exchange-current density k_OER by orders of magnitude without touching CER kinetics — that's the "η(OER) − η(CER)" readout going positive (favouring O₂) as you move the catalyst slider toward the engineered end. Raising local pH also helps: OH⁻ availability accelerates OER while a local pH rise near the anode (from H⁺ release) suppresses hypochlorite formation, which is why real systems co-design a bicarbonate/borate buffer layer alongside the catalyst.
- Chloride slider — 0 M (pure water) to 1.0 M (concentrated brine); more Cl⁻ pushes the split toward chlorine.
- pH slider — alkaline conditions favour OER kinetics and destabilize free Cl₂/HOCl.
- Current density — higher j increases both overpotentials, widening the kinetic gap the selective catalyst must overcome.
- Catalyst slider — bare Pt/IrO₂ (chlorine-prone) → mixed oxide → Ni-Fe LDH → engineered high-selectivity OER catalyst.
- Best-case setpoint — jumps to the parameter combination (best catalyst, high pH, moderate j) that current seawater-electrolysis research uses to push Cl₂ Faradaic efficiency below 1%.
- Chamber view — drag to pan, scroll or pinch to zoom into the bubble nucleation sites; the strip chart below tracks the live Faradaic split over the last 30 seconds.
Real-world relevance: this selectivity gap is the reason direct (membrane-free) seawater splitting is not yet standard for green-hydrogen production — most industrial plants still desalinate first. Solving it is one of the most active research fronts in electrochemical climate technology.