A hydrogen fuel cell is an electrochemical device that converts chemical energy from hydrogen and oxygen directly into electricity and heat, with water as the only byproduct. Unlike a combustion engine, there is no intermediate conversion to heat and mechanical motion, making fuel cells fundamentally more efficient. The most common type is the Proton Exchange Membrane (PEM) fuel cell, which operates at 60–80°C and uses a solid polymer membrane (Nafion) as the electrolyte, conducting protons while blocking electrons.
At the anode, hydrogen gas is split into protons and electrons by the platinum catalyst: H₂ → 2H⁺ + 2e⁻. Protons migrate through the membrane to the cathode while electrons flow through the external circuit (generating useful electrical current). At the cathode, protons, electrons, and oxygen combine to form water: ½O₂ + 2H⁺ + 2e⁻ → H₂O. The theoretical maximum efficiency (Carnot-unlimited) is ~83% at standard conditions, though practical PEM fuel cells achieve 50–60% electrical efficiency—roughly double that of internal combustion engines.
This simulator models ion transport, current-voltage (polarization) curves, and the losses due to activation overpotential, ohmic resistance, and mass-transport limitation at different current densities. You can observe how temperature, pressure, and hydrogen/oxygen partial pressures affect cell voltage, and explore the three regions of the polarization curve to understand where efficiency gains are possible in real fuel cell stacks.
How is a fuel cell different from a battery?
A battery stores chemical energy in its electrodes and gradually depletes those materials as it discharges, requiring recharging to restore them. A fuel cell is an open system that continuously consumes externally supplied hydrogen and oxygen, producing electricity as long as fuel is supplied—it does not deplete or need recharging in the same sense. Fuel cells can be refueled in minutes (like filling a gas tank), while battery recharging may take hours. However, batteries have simpler infrastructure and higher round-trip efficiency for short-duration storage.
What is the role of the platinum catalyst in a PEM fuel cell?
Platinum catalyzes the hydrogen oxidation reaction (HOR) at the anode and the oxygen reduction reaction (ORR) at the cathode. Both reactions have high activation energies without a catalyst—the ORR in particular is kinetically sluggish and accounts for most of the efficiency loss in PEM fuel cells. Platinum provides active surface sites that adsorb and dissociate H₂ and O₂ molecules, lowering the activation energy. Platinum's high cost (~$30,000/kg) and scarcity are major barriers to fuel cell commercialization, driving research into platinum-group-metal-free catalysts.
What are the three regions of a fuel cell polarization curve?
The voltage-current (polarization) curve shows how cell voltage decreases as current increases. At low current, the steep drop is dominated by activation losses—the energy needed to start electrode reactions. At intermediate current, voltage decreases linearly due to ohmic losses (resistance of membrane and electrodes). At high current, voltage drops sharply again due to mass-transport limitations—insufficient delivery of hydrogen or oxygen to the catalyst layer. Understanding these regions guides engineers to improve catalysts (activation), membrane conductivity (ohmic), and gas diffusion layers (mass transport).
Hydrogen is green only if produced from renewable electricity via water electrolysis (green hydrogen). Currently, 95% of hydrogen is produced by steam methane reforming (grey hydrogen), releasing CO₂. Green hydrogen production costs are falling rapidly as electrolyzer costs decline and renewable electricity becomes cheaper. Challenges include storage (hydrogen is the lightest element: low volumetric energy density requires high-pressure tanks, cryogenic storage, or solid-state hydrides), distribution infrastructure, and membrane durability under start-stop cycling and contamination by CO or sulfur compounds.
Individual PEM fuel cells produce ~0.6–0.7 V at useful current densities. For automotive applications, 300–400 cells are stacked in series to reach the 150–300 V needed for the electric drive motor, with bipolar plates managing gas distribution and cooling. The Toyota Mirai fuel cell stack produces ~128 kW from 370 cells in a package the size of a suitcase. For stationary power, molten carbonate or solid oxide fuel cells (operating at 600–900°C) offer higher efficiency and can use natural gas or biogas, making them suitable for combined heat and power (CHP) plants in buildings and industry.