Hydrogen Fuel Cells Explained

How Fuel Cells Work

A proton exchange membrane (PEM) fuel cell, the most common type for transportation and portable applications, consists of a membrane electrode assembly sandwiched between bipolar plates that distribute reactant gases and collect electrical current. Hydrogen gas enters the anode side, where a platinum-based catalyst splits each hydrogen molecule into two protons and two electrons. The protons pass through the polymer electrolyte membrane (typically Nafion, a sulfonated fluoropolymer) while the electrons are forced through an external circuit, generating electrical current. At the cathode, the protons, electrons, and oxygen from air combine to form water, which exits the cell as liquid or vapor.

Individual fuel cells produce roughly 0.6 to 0.7 volts under load, so cells are stacked in series (30 to 400 cells per stack) to produce useful voltages of 100 to 400 volts DC. Power electronics convert this to the appropriate voltage and frequency for the application. PEM fuel cells achieve electrical efficiencies of 40 to 60%, considerably higher than internal combustion engines (20 to 35%) but lower than battery electric drivetrains (85 to 95%). When waste heat is captured for useful purposes (combined heat and power), total system efficiency can reach 80 to 90%.

Other fuel cell types serve different applications. Solid oxide fuel cells (SOFCs) operate at 700 to 1,000 degrees Celsius and can run on hydrogen, natural gas, or biogas with internal reforming. Their high operating temperature enables excellent electrical efficiency (50 to 65%) and high-quality waste heat suitable for industrial processes or building heating. Molten carbonate fuel cells (MCFCs) operate at approximately 650 degrees Celsius and are used for stationary power generation. Alkaline fuel cells (AFCs), the oldest type, were used in the Apollo and Space Shuttle programs and are being revived for stationary applications using improved electrolyte management.

Hydrogen Production Methods

Hydrogen production methods are commonly categorized by color labels indicating their carbon intensity. Gray hydrogen, produced by steam methane reforming (SMR) of natural gas, accounts for approximately 95% of current global hydrogen production. SMR heats methane with steam at 700 to 1,000 degrees Celsius over a nickel catalyst to produce hydrogen and carbon monoxide, then shifts the CO to CO2 and hydrogen in a second reactor. This process emits roughly 9 to 12 kg of CO2 per kg of hydrogen produced. Blue hydrogen adds carbon capture and storage (CCS) to the SMR process, potentially capturing 85 to 95% of CO2 emissions, though upstream methane leakage from natural gas supply chains reduces the actual climate benefit.

Green hydrogen, produced by electrolysis of water powered by renewable electricity, is the only production pathway compatible with deep decarbonization. Electrolyzers split water into hydrogen and oxygen using electrical energy. Alkaline electrolyzers (the most mature technology) use a potassium hydroxide electrolyte and nickel-based electrodes, achieving efficiencies of 60 to 70% and costs of $1,000 to $1,500 per kilowatt. PEM electrolyzers use a solid polymer membrane and precious metal catalysts (platinum and iridium), offering faster response times and higher current densities suited to coupling with variable renewable generation, at slightly higher capital costs. Solid oxide electrolyzers operate at high temperatures (700 to 850 degrees Celsius), achieving the highest efficiencies (80 to 90%) when supplied with waste heat from industrial processes or nuclear reactors.

The cost of green hydrogen depends primarily on the cost of renewable electricity (60 to 70% of production cost), electrolyzer capital cost (20 to 30%), and capacity factor (hours of operation per year). At current renewable electricity costs of $0.02 to $0.04/kWh and electrolyzer costs of roughly $700 to $1,400/kW, green hydrogen costs approximately $3 to $6 per kilogram. Industry projections suggest costs could fall to $1 to $2/kg by 2030 as electrolyzer manufacturing scales up, renewable electricity costs continue to decline, and larger systems benefit from economies of scale. At $2/kg, green hydrogen becomes competitive with gray hydrogen in many markets.

Transportation Applications

Fuel cell electric vehicles (FCEVs) use hydrogen fuel cells to generate electricity that powers electric motors, offering zero-emission driving with refueling times of 3 to 5 minutes and driving ranges of 300 to 400 miles, characteristics similar to conventional gasoline vehicles. The Toyota Mirai and Hyundai Nexo are the primary passenger FCEVs commercially available, though limited hydrogen refueling infrastructure restricts their market to regions with developed station networks (primarily California, Japan, South Korea, and parts of Germany). Approximately 1,000 hydrogen refueling stations operate globally as of 2025.

Heavy-duty transportation is where hydrogen fuel cells offer the clearest advantage over battery electric alternatives. Long-haul trucks, buses, trains, ships, and potentially aircraft face weight and range constraints that make large battery packs impractical or uneconomic. A fuel cell truck carrying 30 to 80 kg of compressed hydrogen can achieve 500 to 1,000 km range with a refueling time of 10 to 15 minutes, compared to the several tonnes of batteries and multi-hour charging times that would be needed for equivalent battery-electric range. Hyundai, Nikola, Daimler Truck, and several Chinese manufacturers are deploying fuel cell trucks in commercial service, with Hyundai's XCIENT fleet accumulating millions of kilometers of real-world operation in Switzerland and South Korea.

Hydrogen-powered trains are entering commercial service on rail lines where electrification (installing overhead catenary wires) is too expensive relative to traffic volume. Alstom's Coradia iLint hydrogen train began regular passenger service in Germany in 2022, replacing diesel trains on non-electrified regional lines. Maritime applications are progressing with fuel cell ferries and port vessels in Norway, Belgium, and California, while several companies are developing hydrogen and ammonia propulsion for ocean-going cargo ships. Aviation applications remain further from commercialization, though Airbus has announced plans for hydrogen-powered commercial aircraft by 2035.

Industrial and Stationary Applications

Industrial hydrogen demand currently totals approximately 95 million tonnes per year, primarily for petroleum refining (removing sulfur from fuels through hydrodesulfurization), ammonia production (the Haber-Bosch process combining hydrogen and nitrogen at high temperature and pressure for fertilizer manufacturing), and methanol synthesis. Replacing this existing gray hydrogen demand with green hydrogen would eliminate roughly 830 million tonnes of annual CO2 emissions, approximately 2% of global emissions, without requiring any new end-use technology.

Steelmaking presents one of the most important industrial decarbonization opportunities for green hydrogen. The conventional blast furnace process uses coke (processed coal) as both a fuel and a reducing agent, with carbon chemically stripping oxygen from iron ore while simultaneously providing the heat needed for the reaction. Direct reduction of iron using green hydrogen replaces carbon with hydrogen as the reducing agent, producing water instead of CO2. SSAB's HYBRIT project in Sweden has produced the first commercial batches of fossil-free steel using hydrogen direct reduction, and several other steelmakers are building hydrogen-based production facilities.

Stationary fuel cell systems provide combined heat and power for buildings, data centers, and critical infrastructure. Bloom Energy's solid oxide fuel cell servers generate electricity from natural gas or biogas at approximately 60% electrical efficiency, with total system efficiency exceeding 80% when waste heat is captured. In Japan, the ENE-FARM program has deployed over 400,000 residential fuel cell units that generate electricity and hot water from natural gas. As green hydrogen becomes available, these systems can transition to zero-emission operation without equipment replacement, since the fuel cells themselves are hydrogen-compatible.