Fuel cell · Glossary

A fuel cell is an electrochemical energy converter. It converts the chemical energy of a supplied fuel directly into electrical energy and heat. Unlike an internal combustion engine or a boiler, it needs no flame, no explosion and no thermal engine for this. The conversion takes place through separate electrochemical partial reactions at two electrodes, which are separated from each other by an electrolyte. The fuel is supplied continuously, usually hydrogen. On the other side, an oxidant is available, as a rule oxygen from the air.

In a hydrogen fuel cell, hydrogen and oxygen react to form water. Electrons flow through an external circuit and perform electrical work. In theory, a single hydrogen-oxygen cell delivers a voltage of around 1.23 volts at 25 °C. Under load, because of internal losses, it is typically only about 0.6 to 0.85 volts, i.e. less than 1 volt. For this reason, many cells are connected in series to form a fuel cell stack, the so-called stack. A stack can consist of hundreds of individual cells. To turn it into a usable system, compressors, pumps, valves, cooling, water and heat management, controls, power electronics and safety technology are also needed. These auxiliary units are often called balance of plant. They help determine how efficient, durable and expensive the system is in practical operation.

The fuel cell is thus neither an electricity storage device nor a power plant in the classical sense. It does not store the energy itself, but converts an energy carrier into electricity and heat. The storage lies in the fuel, for example in a hydrogen tank, in ammonia, in methanol or in a gas grid. Anyone evaluating a fuel cell must therefore always consider the upstream energy carrier as well. Without a reliable, affordable and sufficiently clean fuel supply, it remains a converter without a suitable source.

Electrochemical function and technical parameters

The output of a fuel cell is stated in watts, kilowatts or megawatts. It describes how much electrical power the system can provide at a given moment. The amount of energy generated results from power times operating time and is measured in kilowatt-hours or megawatt-hours. An example: a fuel cell with 10 kilowatts of electrical output that runs at full power for ten hours generates 100 kilowatt-hours of electricity. This presupposes that it is technically available and receives enough fuel.

The electrical efficiency indicates what share of the fuel's chemical energy becomes usable as electrical energy. For hydrogen, it is important whether it refers to the lower or the higher heating value. The higher heating value additionally includes the heat of condensation of the water produced, the lower does not. For hydrogen, the lower heating value is around 33.3 kilowatt-hours per kilogram, the higher around 39.4 kilowatt-hours per kilogram. The same system therefore has an efficiency that is about one sixth lower when based on the higher heating value. The US Department of Energy gives as typical electrical efficiencies, based on the lower heating value, about 60 percent for PEM fuel cells with pure hydrogen and about 40 percent for PEM systems with reformed fuel. For solid oxide fuel cells it likewise states about 60 percent. Such values come from product and system data. In real operation, auxiliary units, partial load and ageing can lower them. Efficiency figures are therefore only comparable if the heating value and system boundary are clearly stated. In brochures and political debates this reference basis is not infrequently missing.

Besides the electrical efficiency, heat output counts. How hot the waste heat is depends strongly on the cell type. Low-temperature fuel cells supply heat that may be usable for space heating or hot water, but is not sufficient for every industrial process. High-temperature fuel cells give off considerably hotter waste heat and can thus serve further applications. If the heat is put to sensible use, the overall efficiency of the system rises. If it remains unused, only the electrical share counts for the assessment.

A fuel cell supplies direct current. For the public electricity grid, for buildings and for many industrial plants, this direct current must be converted into alternating current via power electronics. In vehicles, the direct current can supply an electric motor or a battery directly. Even there, however, voltage adjustment, control and safety technology are needed. In everyday use, the word fuel cell therefore often means the whole unit, although the electrochemical stack is only one part of it.

Distinction from battery, electrolyser and combustion

The fuel cell is most often confused with the battery. A battery is an electrochemical store: it contains the reactive substances inside it and releases stored energy when discharging. A fuel cell is an electrochemical converter: during operation it needs a steady flow of fuel and oxidant. In a battery, power and storage capacity are closely linked in the same device. In a fuel cell, converter power and amount of energy can be designed more separately. The power depends mainly on the stack, the available amount of energy on the tank or the fuel supply.

This separation is an important reason why fuel cells remain interesting for certain applications. A vehicle with long range, high daily mileage or short refuelling times can benefit from a large hydrogen tank and a smaller buffer battery. A stationary emergency power system can keep fuel in stock for many hours or days without needing a correspondingly large battery. A general superiority over batteries does not follow from this, however. In many applications, using electricity directly via a battery store is more efficient, simpler and cheaper, because fewer conversion steps are needed.

The electrolyser is functionally the counterpart to the hydrogen fuel cell. It uses electrical energy to split water into hydrogen and oxygen. The fuel cell turns hydrogen and oxygen back into electricity and water. If electricity is first converted into hydrogen by electrolysis, then stored, transported and later converted back into electricity in a fuel cell, losses occur at every step. This chain can make sense for seasonal storage, emergency supply or applications that are hard to electrify directly. For short-term storage in the electricity grid, a battery is usually more efficient, provided that power, duration, space requirements and raw material issues suit the application.

The distinction from combustion is also important. In a fuel cell, the fuel is oxidised electrochemically, not burned. With pure hydrogen, therefore, no carbon dioxide arises on site and considerably fewer classic air pollutants than in combustion. With natural gas, methanol or other carbon-containing fuels, this is different. Carbon dioxide then arises directly in the system or in an upstream reformer, which produces a hydrogen-rich gas from the fuel, for example by steam reforming. The statement that a fuel cell is emission-free is therefore true only for local operation with a carbon-free fuel. Even then, the climate balance depends on the origin of the fuel.

Fuel cell types and their fields of application

Fuel cells differ in electrolyte, operating temperature, tolerance of different fuels, dynamics and application. The polymer electrolyte membrane fuel cell, PEM fuel cell for short, typically operates at about 80 °C and usually below 120 °C. It starts quickly and follows load changes well. It is therefore relevant for vehicles, smaller stationary systems and emergency power systems. Its electrodes generally contain platinum or platinum alloys as catalyst. This affects costs and raw material demand. It also needs very pure hydrogen, because even small amounts of carbon monoxide poison the platinum catalyst and reduce performance.

Solid oxide fuel cells, see Solid Oxide Fuel Cell, use a ceramic electrolyte and, depending on design, operate at about 500 to 1,000 °C. Because of the high temperature, they manage without a noble metal catalyst. Depending on the design, they can use processed carbon-containing fuels in addition to hydrogen. Especially in stationary continuous operation, they achieve high electrical efficiencies. Their hot waste heat makes them interesting for combined use of electricity and heat. At the same time, the demands on materials increase, start-up times are long, and the number of shutdowns and temperature cycles is limited. They are therefore less suitable for applications with frequent rapid switching on and off.

Other types serve niches and special cases. These include phosphoric acid fuel cells at about 150 to 200 °C, used mainly for decentralised generation with heat utilisation, and molten carbonate fuel cells at about 600 to 700 °C for larger stationary systems. There are also alkaline fuel cells, which were used for example in spaceflight, and direct methanol fuel cells for small mobile devices. For the energy-economic classification, the list of types matters less than the question of which mode of operation a technology tolerates. A system designed for steady continuous operation is a poor fit for many rapid load changes. A system with expensive peripherals and limited lifetime must reach enough full-load hours for the investment to pay off.

Significance in the power system

In the power system, a fuel cell can take on several tasks. It can generate electricity in a decentralised way, for example in buildings, data centres, industrial plants or critical infrastructure. Together with a hydrogen store, it can serve as emergency power supply. In combined heat and power, it supplies electricity and usable heat. In vehicles, it provides electricity for the electric drive. It can also be part of reconversion to electricity: surplus renewable electricity is then first converted into hydrogen and later back into electricity.

For security of supply, the installed capacity of the fuel cell is not the only thing that counts. Also needed are fuel stocks, supply chains, technical availability, maintenance, permits, suitable sites and integration into grid or building technology. A fuel cell contributes to firm capacity only if it is actually ready for operation when needed and the fuel is not scarcer or more valuable in another sector at that moment. This organisational question is often underestimated, because the system is visible but the fuel logistics remain in the background.

In a power system with a lot of wind and solar energy, the fuel cell is of interest above all where a stored chemical energy carrier is to become electricity again. The reconversion of hydrogen to electricity can bridge a longer Dunkelflaute (prolonged period of low wind and solar output), seasonal differences or regional bottlenecks. The low overall efficiency of the chain, however, limits its use. Electricity that can be used directly should as a rule not first be converted into hydrogen and then reconverted. Economically, the role of the fuel cell therefore lies more in rare, longer-lasting or particularly valuable uses than in everyday short-term storage.

For reconversion to electricity, the battery is not the only alternative to the fuel cell. Hydrogen can also be used in gas turbines, gas engines or other thermal power plants. These plants achieve high outputs and can in part build on existing power plant concepts. Fuel cells are electrically more efficient than small engines and simple gas turbines without a steam process and have lower emissions on site. Large combined-cycle gas turbine plants, however, achieve electrical efficiencies of a similar order of magnitude as high-temperature fuel cells. A general efficiency advantage of the fuel cell over every thermal plant therefore does not exist. Which technology fits depends on power class, running time, fuel quality, investment costs, start-up behaviour and the value of the heat generated.

Origin of hydrogen and climate impact

The climate impact of a fuel cell does not depend on the cell alone, but on the entire fuel chain. Hydrogen can come from natural gas, coal, biomass, electrolysis or other processes. Grey hydrogen from fossil sources causes considerable carbon dioxide emissions, even if the fuel cell emits only water on site. Blue hydrogen reduces emissions significantly only if the CO₂ capture is effective, methane emissions in the upstream chain remain limited and the captured carbon dioxide is stored permanently. Green hydrogen from renewable electricity is more favourable for the climate, but competes with the direct use of the same electricity.

This distinction is central to the assessment of fuel cells. A hydrogen fuel cell in a vehicle, building or emergency generator shifts emissions from the exhaust or chimney into the upstream chain. Locally this can be very useful, for example for air quality, noise protection or operation in enclosed spaces. For climate protection and energy security, however, this local view is not sufficient. Also relevant are the origin of the electricity, the efficiency of electrolysis, transport, compression, liquefaction and storage losses. And it matters whether the renewable electricity would otherwise have been curtailed or would have displaced fossil generation elsewhere.

With imported hydrogen, the balance becomes even more complex. The energy carrier can be transported in gaseous or liquefied form, as ammonia, as methanol or in other carrier substances. Each pathway has its own losses, safety requirements and conversion steps. A fuel cell at the end of this chain can work cleanly in technical terms and still be part of an expensive or inefficient supply if producing and transporting the fuel has cost a lot of energy. Conversely, a less efficient local use can make sense in an individual case if it avoids grid bottlenecks, secures supply or makes high-grade use of the waste heat.

Economic viability, lifetime and mode of operation

Whether a fuel cell pays off depends on investment costs, fuel costs, maintenance, lifetime, utilisation and revenues. In many applications, fuel costs over the entire operating period weigh more heavily than the purchase price of the system. Hydrogen remains expensive as an energy carrier as long as production, transport, storage and distribution do not take place on a large scale and with inexpensive renewable energy. A high electrical efficiency lowers these costs, but does not eliminate them.

The lifetime of the stack is a cost factor in its own right, because fuel cells age in operation. Start-stop cycles, load changes, impurities, temperature cycles and unfavourable water management can accelerate ageing. The US Department of Energy explicitly names durability under realistic operating conditions as a central research goal. In vehicles, therefore, it is not only the rated power that matters, but also durability under real driving profiles. In stationary systems, what counts is whether the fuel cell can run for many years with predictable maintenance. A system with good laboratory performance may still fail to pay off if replacement stacks are needed too early or the peripherals fail.

The mode of operation determines the benefit. A fuel cell that generates electricity and heat around the clock in a building needs a heat demand that matches the electricity generation. Otherwise heat is dissipated unused or the system has to be throttled. A fuel cell for emergency power may run only a few hours a year, but must start reliably in an emergency. A fuel cell in a lorry must fit refuelling infrastructure, payload, range, driving times and fleet logistics. The same technical characteristics lead to very different assessments in these applications.

For operators, market rules and remuneration also matter. Stationary fuel cells can combine self-consumption, grid feed-in, heat supply, balancing energy or the securing of critical loads. For each of these roles, different metering concepts, levies, grid connection rules, emission requirements and funding conditions apply, depending on the country and the size of the system. Many economic viability problems therefore lie not in the technology alone, but in whether technical capability, fuel price and legal framework fit together.

Electromobility and the role in transport

In electromobility, the fuel cell is one form of electric drive. An electric motor drives the vehicle, and the fuel cell generates electricity for it from hydrogen, usually supplemented by a buffer battery. This battery absorbs braking energy, covers power peaks and lets the fuel cell run at a more favourable operating point. A fuel cell vehicle is therefore not a counter-model to the electric drive, but a particular design within electric drives.

The comparison with battery-electric vehicles is often made too generally. For passenger cars, high energy efficiency, existing electricity connections, falling battery costs and a growing charging network clearly favour direct electrification. Battery-electric vehicles need considerably less electricity per kilometre. The chain electricity, battery, electric motor has fewer conversion losses than the chain electricity, electrolysis, processing, transport, refuelling, fuel cell, electric motor. For heavy commercial vehicles, long-distance fleets, certain buses, special vehicles or very short refuelling windows, hydrogen can nevertheless be practically relevant if infrastructure and vehicle utilisation fit together.

In transport, the bottleneck lies not only in the vehicle. A hydrogen fleet needs filling stations with sufficient dispensing capacity, suitable pressure levels, compressors, storage, safety concepts and reliable delivery. This infrastructure pays off more readily where demand is bundled, for example in depots, ports, industrial parks or along logistics corridors. For scattered individual applications, the cost per user is high. The fuel cell in transport is therefore plausible above all where vehicles run on predictable schedules, achieve high mileages and refuel at few points.

Heat, buildings and decentralised generation

In buildings, fuel cells are often regarded as small combined heat and power units. They generate electricity for self-consumption and supply heat for heating or hot water. If the heat is put to sensible use over many hours, such systems achieve high overall efficiencies. Their electrical efficiency is often above that of small combined heat and power units with an internal combustion engine, and they can be quieter and lower in pollutants.

The assessment depends strongly on the fuel. Many household units available today run on natural gas, which is processed internally or in an upstream reformer into hydrogen-rich gas. CO₂ continues to arise in the process. Such a system can be more efficient than grid electricity plus an old heating system, but it is not a climate-neutral solution. If it is later to run on hydrogen, the unit, grid, safety concept and fuel quality must be suitable for this. Anyone who assumes that every gas-based fuel cell is automatically hydrogen-ready risks misinvestments.

In the building sector, the fuel cell competes with heat pumps, district heating, solar thermal energy, battery storage, photovoltaics and various forms of load shifting. A heat pump uses electricity to lift ambient heat to a higher temperature level and can provide several kilowatt-hours of heat from one kilowatt-hour of electricity. A fuel cell, by contrast, generates electricity and heat from a chemical energy carrier. The two technologies therefore answer different questions. If renewable fuels are scarce, using electricity directly for heat is often more efficient, provided that the building, grid connection and temperature requirements allow it.

Typical misunderstandings

Fuel cells generate energy: Technically they only convert it. The chemical energy of the fuel becomes electrical energy and heat. This is more than linguistic precision, because it makes the fuel question visible. Without hydrogen, methanol, natural gas or another suitable energy carrier, the fuel cell delivers no energy.
Fuel cells are emission-free: With pure hydrogen, no carbon dioxide arises on site. For the climate balance, however, the production of the hydrogen counts. Locally low-emission is not the same as climate-neutral. The difference is practically important in cities, ports or indoor spaces, but must not be confused with the overall balance.
High cell efficiency means an efficient chain: The fuel cell can be electrically more efficient than a small internal combustion engine. The entire hydrogen chain can nevertheless be less efficient than direct use of electricity. The two statements do not contradict each other, because they have different system boundaries. Anyone looking only at the cell is assessing the converter. Anyone including electrolysis, storage, transport and reconversion to electricity is assessing the energy chain.
Modular means arbitrarily scalable: Fuel cells can be built modularly, but modularity does not replace an industrial supply chain. Materials, manufacturing, quality assurance, service, hydrogen infrastructure and safety rules must grow at the same pace. A technology can convince in a demonstration project and nevertheless fail in broad deployment because of costs, fuel availability or maintenance structures.
Fuel cell equals hydrogen economy: Fuel cells are only one of many uses of hydrogen. Others lie in the steel industry, chemicals, high-temperature heat, synthetic fuels, electricity storage or hydrogen-ready power plants. If hydrogen is scarce and expensive, these applications compete with one another. The fuel cell makes sense where its particular properties provide great benefit: quiet electric operation, low local emissions, long range, fast refuelling, high availability or usable heat.

Classification for energy policy and planning

For energy policy and infrastructure planning, the fuel cell is a touchstone for clear concepts. It links electricity, gas, heat, transport and industry and cannot be understood in any one of these areas alone. Anyone who promotes fuel cell vehicles is at the same time deciding on filling stations, hydrogen production, electricity demand for electrolysis, land for renewable generation and import dependencies. A stationary fuel cell in a building touches electricity tariffs, gas grids, heat strategy, metering concepts and emission rules.

The fuel cell plays out its technical strengths where chemical energy carriers are needed anyway or where batteries reach limits because of mass, storage duration, charging time or usage profile. Its weaknesses are the conversion losses of the hydrogen chain, the costs of providing the fuel, the complex peripherals and the dependence on an infrastructure that is still incomplete. A robust assessment therefore always names the application, fuel source, operating profile and comparison alternative.

Fuel cells make an important distinction visible: an energy converter can work cleanly, quietly and efficiently without the entire energy chain automatically being inexpensive, climate-neutral or resource-saving. The term denotes the electrochemical conversion in the device. It becomes significant in energy-economic terms only through the fuel, the use of heat, the mode of operation, the infrastructure and the alternative with which the application is compared.

Sources

Comparison of Fuel Cell Technologies — U.S. Department of Energy; 2026-06-02 (letzte Änderung; Erstveröffentlichung 2014-04-24). Vergleichstabelle samt Fußnoten a–f
Types of Fuel Cells — U.S. Department of Energy; unbekannt. Abschnitte PEM, PAFC, MCFC und SOFC
NETL Demonstrating Fuel Cell Stack Cost, Performance and Durability — National Energy Technology Laboratory (U.S. DOE); 2023-03-29. Einleitung sowie Abschnitte Performance und Durability
Electrochemistry Encyclopedia: Fuel cells — The Electrochemical Society; unbekannt. Abschnitt Fuel-cell stacks and power plants

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Edited by eCMS on 11.11.2026 16:24