Proton-Exchange Membrane Fuel Cells

Introduction

A proton-exchange membrane fuel cell, or PEMFC, converts hydrogen and oxygen into water and electrical power at comparatively modest temperature. Its central membrane transports protons, while electrons go through the external circuit. Catalysts, ionomer and porous gas pathways must meet within thin electrode layers. Water is both a reaction product and a requirement for good proton conduction, making water management central to performance.

Core explanation

At the hydrogen electrode, H₂ → 2H⁺ + 2e⁻ . Protons pass through the membrane; electrons flow through a load to the oxygen electrode. There, ½O₂ + 2H⁺ + 2e⁻ → H₂O . Adding the equations yields H₂ + ½O₂ → H₂O. A membrane such as a perfluorosulfonic-acid polymer in the Nafion family contains acid groups and hydrated ionic pathways that support proton motion. It must also separate gases and block electronic conduction. Membrane thickness trades lower ohmic resistance against gas crossover and mechanical durability.

Within each catalyst layer, electrons must reach the catalyst through carbon or another conductor, protons through ionomer, and gas molecules through pores. Reaction is effective where all required pathways overlap. The phrase three-phase boundary is a useful shorthand for this contact region, though actual catalyst layers have complex hydrated, nanostructured interfaces rather than a perfectly sharp line. Too much ionomer can cover gas-accessible catalyst; too little leaves electrically connected particles without proton access.

Platinum-based catalysts are used because they efficiently catalyse hydrogen oxidation and oxygen reduction under acidic PEM conditions, though the cathodic oxygen reduction remains kinetically demanding. Platinum is costly, so high utilisation and durable support structures matter. Carbon monoxide can bind strongly to anode platinum and block hydrogen oxidation sites, making fuel purity important. Catalyst nanoparticles can dissolve, migrate, agglomerate or lose support contact during potential cycling, reducing active area.

The membrane and ionomer generally need adequate hydration for proton conductivity. If they dry, ohmic resistance rises. If liquid water accumulates in pores, oxygen delivery falls and high-current voltage drops. Gas humidification, temperature, flow fields and porous diffusion layers help balance these effects. A change in cell voltage when humidity changes could reflect both conductivity and flooding, so water diagnostics are important.

At stack level, bipolar plates distribute gases and carry electrons between cells. Seals restrict hydrogen and oxygen crossover; cooling removes heat. The cell is therefore an integrated transport and reaction system, not just a membrane with catalyst painted on each face.

Step-by-step reasoning

Write both electrode half-reactions and trace H⁺, e⁻, H₂, O₂ and H₂O through the cell. Mark where gas, ionomer and electron-conducting catalyst support must meet. If voltage falls, ask whether activation, membrane resistance or gas transport is responsible. Test humidity and gas-flow effects and consider catalyst poisoning or area loss. Evaluate membrane choices together with crossover and durability.

Visual explanation

Draw hydrogen channels, anode catalyst layer, membrane, cathode catalyst layer and oxygen channels in a row. Use one arrow for protons through the membrane and another for electrons around an external load. Draw water at the cathode, with one icon for a dry membrane and another for flooded pores. Zoom into a catalyst particle touching carbon, ionomer and a gas pore to depict the active contact region.

Real-world analogy

A workshop needs three deliveries at one bench: raw gas, an electrical connection and a proton-conducting path. If any delivery is missing, the worker at that bench cannot perform the full reaction. This explains why merely having large platinum surface area is insufficient without access. The analogy does not describe electrochemical overpotential or the membrane's molecular water channels.

Real-world example

A PEMFC shows a steep high-current voltage drop on a humid day. Raising gas flow improves performance, consistent with clearing liquid water from oxygen pathways. In another test, drying the incoming gas raises high-frequency resistance and worsens the middle of the polarisation curve. These opposite responses illustrate why “more water” is neither always good nor always bad.

Why?

Why does the membrane pass protons but not electrons? It preserves charge separation so electrons do work externally while ionic charge moves internally. Why is water essential yet troublesome? Hydration creates proton-conducting pathways, but excess liquid blocks gas pores. Why use supported platinum nanoparticles? Dispersing catalytic metal exposes more active area per mass, provided it remains connected to gas, ions and electrons.

Common misconception

The membrane does not conduct the useful external electron current. Nor is every platinum atom necessarily active: buried particles or sites lacking gas or proton access contribute little. A membrane that stays wet is not automatically optimally managed if its cathode catalyst layer is flooded.

Worked example

Question: A PEMFC consumes 0.010 mol of H₂. What amount of electrons moves through the external circuit, and what charge does that represent using F=96,485 C mol⁻¹?

Reasoning: Each H₂ gives two electrons, so n(e⁻)=0.020 mol. The charge is 0.020×96,485=1,930 C to three significant figures, assuming complete electrochemical use. This charge is separate from the rate of delivery, which would require a time interval.

Answer: 0.020 mol electrons and about 1.93×10³ C.

Quick check

1. What happens to proton conduction if a typical PEM ionomer becomes too dry? Answer: Proton conductivity falls and membrane or ionomer ohmic resistance rises.

Exam focus

Label electron and proton paths separately and balance the half-reactions. Define the three contact requirements at a catalyst site. Discuss membrane hydration and flooding as opposing water-management limits. Relate CO poisoning specifically to adsorption on catalyst sites rather than to a change in hydrogen thermodynamics.

Advanced insight

The catalyst layer may contain a distribution of pore sizes and ionomer-film thicknesses. Local oxygen concentration, water activity and proton potential can vary across its depth, causing only part of the nominal platinum area to carry current at high load. A thin membrane can lower ohmic loss yet allow more gas crossover or experience mechanical stress during hydration cycles. Materials and morphology must therefore be optimised together rather than maximising one property.

Summary

PEM fuel cells route protons through a hydrated polymer membrane and electrons through an external circuit. Gas, ionomer and electron-conducting catalyst must meet at active sites. Platinum enables rapid reactions but can be poisoned or lose area. Water management balances proton conductivity against gas-pore flooding, and the full stack adds gas distribution, sealing and cooling requirements.

Practice questions

1. Write the acidic cathode half-reaction for a hydrogen–oxygen PEMFC per H₂ molecule. Answer: ½O₂ + 2H⁺ + 2e⁻ → H₂O.

2. Why might adding more ionomer to a catalyst layer eventually reduce power? Answer: Excess ionomer can impede gas access even while improving proton pathways.

3. What does CO do to anode platinum in a PEMFC? Answer: It can bind strongly to catalytic sites and inhibit hydrogen oxidation.

4. Why can excess water lower high-current voltage? Answer: Liquid water may flood pores and restrict oxygen transport to the cathode catalyst.

Sources: US DOE, PEM fuel-cell operation; US DOE, fuel-cell types and catalyst poisoning; US DOE NETL Fuel Cell Handbook.