Electrolysis of Water

How splitting water works

Water electrolysis is the reverse of a hydrogen fuel cell: instead of letting H₂ + ½O₂ → H₂O run downhill and harvesting electricity, you push electricity in to drive the reaction uphill. Same atoms, same electrolyte, opposite arrow.

The two half-reactions depend on whether your electrolyte is acidic or alkaline:

ACID (PEM electrolyzer)
Cathode (HER):    2 H⁺ + 2 e⁻ → H₂
Anode  (OER):     H₂O → ½ O₂ + 2 H⁺ + 2 e⁻
Net:              H₂O → H₂ + ½ O₂            E°cell = 1.23 V

ALKALINE
Cathode (HER):    2 H₂O + 2 e⁻ → H₂ + 2 OH⁻
Anode  (OER):     2 OH⁻ → ½ O₂ + H₂O + 2 e⁻
Net:              H₂O → H₂ + ½ O₂            E°cell = 1.23 V

Either way, the net cell reaction is the same and so is the 1.23 V thermodynamic minimum at 25 °C and 1 bar. That's just ΔG° / (nF) = 237 000 / (2 × 96 485). Below this voltage nothing happens; above it, you start getting bubbles.

Cell anatomy

     ┌──────────────────────────────────┐
     │           power supply           │
     │           +    │    −            │
     └─────┬──────────┴──────────┬──────┘
           │                     │
       ╔═══▼═══╗   electrolyte  ╔═══▼═══╗
       ║ ANODE ║◄────H⁺ or────► ║CATHODE║
       ║ (OER) ║   OH⁻ ions     ║ (HER) ║
       ╚═══╤═══╝                ╚═══╤═══╝
           │                        │
        ½O₂ ↑                     H₂ ↑
           ╲                        ╱
            ╲   never let mix —    ╱
             diaphragm or membrane
             (avoids 4–94% explosive H₂/air)

The diaphragm or membrane is critical: H₂ in air is flammable from 4% and detonable above ~18%. Industrial designs keep gas crossover under 2% to stay safely outside the explosive envelope.

Why real cells need 1.7–2.0 V

The 1.23 V is just the floor. Real cells dissipate three additional voltage drops as heat:

1. Activation overpotential. The oxygen evolution reaction (OER) is the slow step — making one O₂ takes 4 electrons and 4 protons through several intermediates. OER eats 0.3–0.5 V on every type of electrode known. The cathode HER, by contrast, takes only ~0.05 V on Pt.
2. Ohmic loss. i·R drop across the electrolyte, the membrane, and the contact resistance. Scales linearly with current — a cell happily at 1.6 V at 0.5 A/cm² climbs past 1.9 V at 2 A/cm².
3. Bubble shielding. H₂ and O₂ bubbles cling to electrodes and reduce their active area, raising local current density and pushing into the activation regime. Forced-flow electrolytes and roughened electrodes mitigate this.

So a healthy industrial PEM cell at 2 A/cm² lives at ~1.8 V, gives 1.23/1.80 = 68% voltage efficiency, and dissipates the rest as ~30 °C of heat the cooling loop has to remove.

Worked example: 1 kg of green hydrogen

To make 1 kg of H₂ (= 500 mol):

• Charge required: Q = nF = 2 × 500 × 96 485 = 9.65 × 10⁷ coulombs.
• Energy at 1.23 V (theoretical): E = QV = 9.65 × 10⁷ × 1.23 = 1.19 × 10⁸ J = 33.0 kWh. Add the latent heat of water and you get 39.4 kWh-HHV — the textbook minimum.
• Energy at 1.80 V (real cell): 9.65 × 10⁷ × 1.80 = 48 kWh, plus parasitic loads = ~52 kWh per kg.
• Cost at $30/MWh: $1.56 of electricity per kg H₂ — the floor of green-H₂ economics today.
• By volume: 1 kg H₂ at STP = 11.1 m³. At 700 bar (vehicle storage): ~16 L.

Three electrolyzer technologies compared

	Alkaline (AEL)	PEM (PEMEL)	Solid-oxide (SOEC)
Electrolyte	30% KOH (aq)	Nafion / PFSA membrane	Yttria-stabilized zirconia ceramic
Charge carrier	OH⁻	H⁺	O²⁻
Operating temperature	60–90 °C	50–80 °C	700–850 °C
Cell voltage @ design point	1.8–2.0 V	1.7–2.0 V	1.0–1.3 V (steam)
Catalyst	Ni, Ni-Mo (no PGM)	Ir at anode, Pt at cathode	Ni cermet (no PGM)
Current density	0.2–0.4 A/cm²	1–2 A/cm²	0.5–1 A/cm²
Stack efficiency (LHV)	60–70%	60–70%	~80% (uses heat)
Dynamic response	Minutes (KOH thermal mass)	Seconds — tracks PV/wind	Slow ramp, runs steady
Lifetime (h)	80 000+	50 000–80 000	20 000 (research)
Capex (2026 estimate)	~$700/kW	~$900/kW	~$2500/kW (early)
Where it ships	NEL, ThyssenKrupp at GW scale	Siemens, ITM, Plug	Sunfire, Topsoe pilots

SOEC's 80% efficiency is the killer feature, but it depends on having waste heat to feed (industrial steam, nuclear). For pure-electricity inputs, PEM and alkaline are within a few points of each other and the choice comes down to dynamic response and capex.

Real-world plants

• NEOM (Saudi Arabia, 2026 commissioning). 2 GW alkaline electrolyzer fleet, ~640 t H₂/day, dedicated solar+wind feed. Largest single green-H₂ project to date.
• REFHYNE II (Rhineland, Shell). 100 MW PEM electrolyzer at the Wesseling refinery, brought online 2025 — a step up from 10 MW REFHYNE I (2021).
• Hystock (Netherlands). 20 MW PEM at the Eemshaven, supplying H₂ injection into the existing natural-gas grid.
• ITER pre-cooling tritium plant. Uses high-purity electrolyzers with Pt catalyst loadings around 0.5 mg/cm² — research-grade hardware, not GW-class.
• Norsk Hydro Notodden (1928). The original industrial electrolyzer site, feeding the Birkeland-Eyde process for fertilizer N₂ fixation. Replaced by Haber-Bosch but still demonstrates that "green H₂" predates the term by a century.
• Submarines. US and Russian nuclear subs run small alkaline electrolyzers (~25 kW) on seawater + KOH to keep crew O₂ supply, scrubbing H₂ overboard.

Variants

• Anion-exchange membrane (AEM). Polymer membrane like PEM, but conducting OH⁻ — lets you run on cheap nickel catalysts. Promising for the 2030s; lifetimes still trail PEM.
• Photoelectrochemical (PEC) cells. Fuse a photovoltaic and an electrolyzer into one slab — light hits a semiconductor that directly drives water splitting. Lab-scale, ~10% solar-to-H₂ today.
• Molten-carbonate electrolysis cells. Co-electrolysis of H₂O and CO₂ to make syngas (H₂ + CO). The route to "e-fuels" — synthetic kerosene from electricity, water, and air-captured CO₂.
• Chlor-alkali plant H₂. ~2% of world H₂ supply is a by-product of NaCl(aq) electrolysis. Almost free, but only as much as Cl₂ demand drags along with it.
• Direct-seawater electrolysis. Tempting (~70% of Earth's water) but Cl⁻ oxidizes preferentially to O₂ at the anode under acidic conditions. Membranes that exclude Cl⁻ are an active research area.

Common pitfalls and misconceptions

• "You can't run an electrolyzer below 1.23 V." Correct in principle, but in practice cells start drawing real current only above ~1.5 V because OER kinetics are sluggish — the threshold isn't crisp.
• "Higher current = more H₂ per kWh." The opposite: efficiency drops as you push current density higher because i²R losses scale faster than output. There's an economic sweet spot, not a thermodynamic one.
• "Pure water gives the cleanest H₂." You still need an electrolyte. Modern PEM systems use the membrane itself as the electrolyte and feed deionized water on both sides; alkaline systems circulate KOH that has to be filtered and replenished.
• "Electrolyzers are simple — just hook up a battery." Industrial stacks are 100+ cells in series, with deionizers, gas-liquid separators, demisters, dryers, and a power-electronics rectifier converting grid AC. The stack is ~50% of system cost; balance-of-plant is the rest.
• "Hydrogen storage is solved." Compression to 350–700 bar costs ~10% of the energy in the H₂ itself. Liquefaction at 20 K costs ~30%. Nobody has a free option.
• "Electrolyzers are zero-carbon." Only if the electricity is. A grid mix at 400 g CO₂/kWh times ~52 kWh/kg H₂ = 21 kg CO₂ per kg H₂ — worse than steam-methane reforming.