The science behind hydrogen fuel cells

01 A REDOX REACTION WITH A DIVIDED PATH

A fuel cell is an electrochemical cell that continuously converts a supplied fuel and oxidant into electricity. In a hydrogen PEM cell, the overall reaction is familiar: hydrogen and oxygen become water. The important scientific move is to separate the oxidation and reduction half-reactions so their electrons must pass through a load.

At the anode, H₂ → 2H⁺ + 2e⁻. The membrane carries H⁺; the external circuit carries e⁻. At the cathode, oxygen combines with protons and electrons to form water. The cell therefore has a chemical side, an ionic side, an electronic side, and a thermal side operating simultaneously.

02 THERMODYNAMICS SETS THE CEILING

The maximum reversible electrical work is related to the change in Gibbs free energy, ΔG, rather than the total enthalpy released by the reaction. The ideal reversible cell voltage is E = −ΔG/(nF), where n is the number of transferred electrons and F is Faraday’s constant. Under standard conditions, the familiar ideal voltage is around 1.23 volts per cell, before real operating losses.

That ceiling shifts with temperature, pressure, and product-water activity. Engineers use the Nernst equation to describe these changes. Higher reactant pressure can raise reversible potential, while operating conditions that change water activity alter the chemical potential. Thermodynamics tells us what is possible; kinetics and transport decide how much is accessible at useful speed.

03 CATALYSTS LOWER THE KINETIC BARRIER

The hydrogen oxidation reaction at the anode is comparatively fast on platinum, but oxygen reduction at the cathode is sluggish. The cathode therefore creates a large activation overpotential: extra voltage is needed to push the reaction at a practical rate. Catalyst particles increase the electrochemically active surface, while ionomer and pore structure determine whether reactants can reach those particles.

Catalyst science is a balancing act. Smaller particles expose more surface but can dissolve, migrate, or agglomerate during operation. Higher loading may improve performance but raises cost and material intensity. Alloying, protective supports, improved ionomers, and better electrode architecture aim to reduce platinum-group-metal use without losing durability.

04 THE POLARIZATION CURVE IS A DIAGNOSTIC

A cell’s voltage falls as current density rises. At low current, activation losses dominate. In the middle region, ohmic losses from proton, electron, and contact resistance create an approximately linear decline. At high current, mass transport becomes limiting: reactants cannot arrive or product water cannot leave quickly enough.

This curve is more than a performance plot. Its shape helps identify what to fix. A large low-current drop points toward catalyst or electrode kinetics. A steep middle slope suggests resistance or compression problems. A sharp high-current collapse suggests flooding, drying, poor gas distribution, or insufficient air supply.

05 PROTONS, WATER, AND GAS MOVE THROUGH POROUS MEDIA

The membrane must conduct protons while resisting electronic conduction and gas crossover. Proton conductivity depends strongly on hydration and temperature. The catalyst layer is a multiphase maze where gas, ionomer, liquid water, electrons, and catalyst surfaces must meet. Too much ionomer blocks pores; too little breaks ionic pathways.

Transport is governed by diffusion, convection, capillary pressure, and electro-osmotic drag. Water is generated at the cathode, but some is pulled toward the anode with moving protons. The result is a dynamic water balance, not a one-way drain. During transients, the stack can move between dry-out and flooding faster than a simple steady-state model predicts.

06 EFFICIENCY IS NOT THE SAME AS CLIMATE IMPACT

A fuel cell can be efficient at the point of use and still have a large upstream footprint. Hydrogen made by reforming natural gas carries emissions from the feedstock and process; adding carbon capture changes, but does not automatically erase, that balance. Electrolysis can be low-carbon when powered by genuinely low-carbon electricity, but electrolysers also require energy, water, equipment, and capacity.

The Real Engineering video, “The Truth about Hydrogen,” is a useful companion because it keeps the molecule connected to the production pathway. A fair comparison tracks the whole chain: primary energy, conversion losses, compression or liquefaction, transport, storage, fuel-cell conversion, and the service delivered.

07 THE SCIENCE IS A SET OF TRADE-OFFS

Fuel cells are neither magic batteries nor miniature combustion engines. They are open electrochemical reactors with continuously supplied reactants. Their quiet output, modularity, and potential for useful heat follow from that architecture; their cost, sensitivity to impurities, water-management needs, and upstream energy losses follow from it too.

The scientific question is therefore conditional: for which duty cycle, fuel pathway, and infrastructure does the chain beat alternatives? Answering it requires electrochemical measurements, lifecycle accounting, and honest boundary conditions. The same cell can look compelling in a long-duty fleet and inefficient in a lightly used passenger car, depending on what is being compared.