Fuel Cell

How a fuel cell works

A fuel cell is a battery whose reactants never run out — you keep flowing fuel into one electrode and oxidizer into the other, and you keep getting electricity. The classic version burns hydrogen, but it doesn't actually burn anything. The two half-reactions happen on opposite sides of an electrolyte that lets ions through but blocks electrons, forcing the electrons to take the long way around through your circuit.

For a hydrogen PEM cell, the chemistry splits like this:

Anode (oxidation):    H₂ → 2 H⁺ + 2 e⁻
Cathode (reduction):  ½ O₂ + 2 H⁺ + 2 e⁻ → H₂O
Net:                  H₂ + ½ O₂ → H₂O      ΔG° = −237 kJ/mol

The electrolyte (a wet polymer like Nafion) carries protons across; the external wire carries electrons. Each H₂ molecule pushes 2 electrons through the load. The whole cell is the reverse of water electrolysis — same chemistry, opposite sign of free energy.

The maximum voltage is set by ΔG: E° = −ΔG/(nF) = 237 000 / (2 × 96 485) ≈ 1.23 V. Real cells deliver less, because three loss terms pile up as you draw current.

Energy flow through one cell

          H₂ in                   O₂ in
            │                       │
            ▼                       ▼
       ┌────────┐  ──H⁺──►   ┌────────┐
       │ ANODE  │            │CATHODE │
       │ Pt/C   │            │ Pt/C   │
       └────┬───┘            └────┬───┘
            │   2 e⁻ ──────►       │
            └────────────► load ◄──┘
                          ↓
                       work out
                          │
                       H₂O out (cathode)

The voltage actually delivered to the load follows the polarization curve:

V(I) = E°  −  η_act(I)  −  I·R_ohm  −  η_conc(I)
       ──────  ────────   ──────────   ──────────
       1.23 V   ~0.3 V     small       big near limit
                (cathode)  (membrane)  (mass transport)

Most of the voltage drop is the oxygen reduction reaction (ORR) at the cathode. Splitting O=O is hard: a four-electron, four-proton dance through three intermediates. That's why platinum is everywhere on the cathode side.

Worked numbers — the 60% claim

Take a PEM cell running at the standard automotive operating point: 0.7 V cell, 1.0 A/cm² current density.

• Voltage efficiency: 0.7 / 1.23 = 56.9%.
• Faradaic efficiency: ≈ 100% (every electron came from one H₂).
• Thermal efficiency (LHV basis): 0.7 / 1.25 = 56% — sometimes quoted as ~60% because vendors use the "thermoneutral" 1.25 V reference.
• Stack-level losses: parasitic compressors and humidifiers eat ~10%, leaving ~50% wheel-to-fuel for a passenger car.

For comparison, a Toyota Camry gasoline engine peaks near 35% thermal efficiency and averages 25% over a city cycle. A Toyota Mirai's 110 kW PEM stack averages closer to 50%. Per kWh of useful work, the Mirai burns about half the fuel energy.

Five major fuel-cell types compared

	PEM	SOFC	MCFC	Alkaline	DMFC
Electrolyte	Polymer (Nafion)	Yttria-stabilized ZrO₂	Molten Li/K carbonate	Aqueous KOH	Polymer (Nafion)
Charge carrier	H⁺	O²⁻	CO₃²⁻	OH⁻	H⁺
Operating temperature	60–80 °C	600–1000 °C	~650 °C	60–250 °C	60–90 °C
Fuel	Pure H₂	H₂, CH₄, CO directly	H₂, natural gas	Pure H₂	Methanol
Electrical efficiency	50–60%	50–60% (85% CHP)	45–55%	60% (Apollo)	20–30%
Catalyst	Pt (~0.1 mg/cm²)	Ni cermet, no PGM	Ni	Ni or Pt	Pt-Ru
Start-up time	Seconds	Hours	Hours	Minutes	Minutes
Where it shipped	Toyota Mirai, buses	Bloom Energy data centers	Power plants (FuelCell Energy)	Apollo, ISS, sub propulsion	Laptops, small portables

The split is mostly temperature-driven. Hot cells (SOFC, MCFC) are easy on catalysts and fuel-flexible but unbearably slow to start. Cold cells (PEM, AFC, DMFC) start quickly but demand pure H₂ and platinum.

Anatomy of a PEM cell

• Bipolar plates. Graphite or coated steel, milled with serpentine flow channels. Distribute reactants and conduct electrons between adjacent cells in a stack.
• Gas diffusion layer (GDL). Carbon paper, 200 µm. Lets H₂ and O₂ reach the catalyst, drains water away, conducts electrons laterally.
• Catalyst layer. ~10 µm. Pt nanoparticles on carbon, mixed with ionomer. The "triple phase boundary" (gas + electron + ion path all meeting) is where the chemistry actually happens.
• Membrane. 10–25 µm of perfluorosulfonic acid polymer. Hydrated, it conducts H⁺ at ~0.1 S/cm. Dry it out, conductivity collapses; flood it, gas can't reach the catalyst.
• Stack. 200–400 cells in series at 0.7 V each. Mirai's stack is 370 cells in 24 L delivering 110 kW peak.

Real-world stacks

• Toyota Mirai (2nd gen, 2021). 128 kW PEM stack (some specs say 110 kW continuous), 1.6 kg-H₂/100 km, 5 min refuel, 650 km EPA range. Sold ~25 000 units globally — niche, but it works.
• Hyundai Nexo. 95 kW PEM, 6.33 kg H₂ at 700 bar, similar range. Hyundai has shipped more buses than passenger cars under this platform.
• Ballard FCmove HD+. 100 kW heavy-duty stack for buses and trucks. 30 000 hour design life vs ~5 000 hour for early automotive cells.
• Bloom Energy SOFCs. 250 kW "Energy Servers" running on natural gas at data centers — eBay, Apple, Google. ~60% electrical efficiency, no Pt.
• Apollo program AFCs. Pratt & Whitney's alkaline fuel cells flew on every Apollo mission. Crew drank the product water — that's how clean the output was.
• ISS regenerative life support. Pt loadings on the Orbiter shuttle's AFCs: ~10 mg/cm² in 1980. Modern PEM auto cells: ~0.1 mg/cm² — a 100× reduction in 40 years.

Variants and edge cases

• Reversible (regenerative) fuel cells. Run forward as a fuel cell, backward as an electrolyzer. NASA prototypes for lunar surface power; round-trip efficiency ~40%.
• Direct ammonia fuel cells. NH₃ has 1.7× the volumetric H₂ density of liquid hydrogen and an existing global supply chain. SOFC variants reach 50% LHV efficiency on cracked NH₃.
• Microbial fuel cells. Bacteria oxidize organic matter at the anode; current densities are 1000× lower but the fuel is sewage. Wastewater plants pilot these.
• Phosphoric acid (PAFC). The forgotten 5th type, 200 °C, ~40% efficient, very mature for stationary CHP. ~400 commercial units shipped (UTC PureCell).
• Solid-acid fuel cells (SAFC). CsH₂PO₄ electrolyte, 250 °C — pure-H₂ tolerant of CO at 100× the level a PEM survives. Still a research frontier.

Common pitfalls and misconceptions

• "Fuel cells beat the second law." They don't. They beat Carnot, because they never make heat in the first place. The thermodynamic limit is still ΔG/ΔH ≈ 83% at room temperature.
• "Hydrogen is clean energy." Hydrogen is an energy carrier, not a source. ~95% of global H₂ today is grey (from CH₄ + H₂O at 700–1000 °C, releasing 9 kg CO₂ per kg H₂). The fuel cell is only as clean as its supply.
• "PEM cells are CO-poisoned forever." CO at >10 ppm blocks Pt sites at 80 °C, but it desorbs above 150 °C and tolerance can be raised to 1000 ppm with Pt-Ru alloys or with brief air bleeds.
• "Higher voltage means more power." The polarization curve is downward-sloping. Maximum power is at intermediate voltage (~0.6 V), not open-circuit (1.0–1.1 V). Vendors quote stack power at ~0.6 V.
• "Fuel cells don't degrade like batteries." They do, just differently. Pt dissolves and reprecipitates as larger particles, membranes thin from radical attack, GDLs flood. Automotive targets: <10% performance loss over 5 000 hours.