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Chapters / Chapter 7, Fuel cells / Lesson 7.1

7.1 How fuel cells work

How a fuel cell turns hydrogen into electricity, its voltage and efficiency, the main types of fuel cells, and the parts of a membrane electrode assembly.

About 25 minutesOpen to everyoneQuiz at the end

A battery that is refuelled

A fuel cell turns the chemical energy of a fuel, most often hydrogen, directly into electricity, as long as fuel and oxygen are supplied. It does not store energy itself, so its power is set by the size of the cell and its energy by the size of the fuel tank.

A proton exchange membrane fuel cell: hydrogen is oxidised at the anode, protons cross the membrane, and oxygen is reduced at the cathodeAnodeCathodeMembraneH₂ inair, O₂ inH₂O outH⁺ through the membraneH₂ → 2H⁺ + 2e⁻½O₂ + 2H⁺ + 2e⁻ → H₂Oe⁻ through the load
A proton exchange membrane fuel cell. Hydrogen is oxidised at the anode, protons cross the membrane, and oxygen from the air is reduced at the cathode to water.

Voltage and efficiency

H2 + ½O2 → H2O,   Erev = −ΔG / 2F ≈ 1.23 V at 25 °CAt 80 °C it is about 1.18 V. Using the enthalpy instead gives the thermoneutral voltage, about 1.48 V, often used as the reference for efficiency on the higher heating value.
  • The highest possible efficiency is ΔG/ΔH, about 83% at 25 °C on the higher heating value.
  • In practice a cell runs at 0.6 to 0.8 V, so its voltage efficiency is about 40 to 55% on the higher heating value. The rest of the energy becomes heat.
  • Even with no load, the open-circuit voltage is lower than Erev, typically about 0.9 to 1.0 V, mostly because a little hydrogen crosses the membrane and reacts directly at the cathode.

Kinds of fuel cells

TypeElectrolyteTemperatureNotes
Proton exchange membrane (PEMFC)Perfluorosulfonic acid membrane, such as Nafion60 to 90 °CCars and portable power, needs Pt catalysts
Anion exchange membrane (AEMFC)Hydroxide-conducting polymer60 to 80 °CAllows non-precious catalysts, membranes still maturing
Phosphoric acid (PAFC)Phosphoric acid in a matrixabout 200 °CStationary power
Solid oxide (SOFC)Oxide-ion conducting ceramic, such as yttria-stabilised zirconia600 to 1000 °CHigh efficiency, can run on methane, slow start-up

Inside the membrane electrode assembly

  • Membrane, typically 10 to 50 µm thick, conducts protons and keeps the gases apart. It must stay humid to conduct well.
  • Catalyst layers, a few µm thick, contain Pt nanoparticles on carbon mixed with ionomer. The reaction can only happen where catalyst, ionomer, gas and electron path meet.
  • Gas diffusion layers of carbon paper or cloth spread the gas, carry electrons and remove water.
  • Bipolar plates with flow fields supply gas to the whole area and connect cells in a stack.

The oxygen reduction reaction (ORR) at the cathode is much slower than hydrogen oxidation at the anode, so most of the platinum in a PEMFC sits at the cathode, and most of the research on catalysts targets the ORR.

Hydrogen and other fuels

Fuel cells are only as clean as their hydrogen. Hydrogen from water electrolysis powered by renewable electricity links fuel cells to the electrocatalysis in chapter 2. Direct methanol, ethanol and formic acid fuel cells use liquid fuels, at the cost of slower anode reactions and fuel crossover.

Check your understanding

Quiz for lesson 7.1

Pick one answer per question. You see the explanation straight away.

References and further reading

  1. J. Larminie, A. Dicks, Fuel Cell Systems Explained, 2nd ed., Wiley, 2003.
  2. R. O'Hayre, S.-W. Cha, W. G. Colella, F. B. Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016.
  3. H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs, Appl. Catal. B 56 (2005) 9–35.