Batteries and Fuel Cells
4. Fuel Cells
Learning outcomes
- I can describe how fuel cells generate electricity.
- I can explain the operation of a hydrogen fuel cell.
- I can compare fuel cells with conventional batteries.
- I can identify advantages and challenges of fuel-cell technology.
- I can evaluate fuel cells as an energy source.
What Is a Fuel Cell?
A fuel cell is an electrochemical device that converts the chemical energy of a fuel directly into:
electrical energy.
Like a battery, a fuel cell uses:
redox reactions
to produce an electric current.
However, there is an important difference.
A battery stores its chemical reactants inside the battery.
A fuel cell receives its reactants continuously from an:
external supply.
As long as fuel and an oxidizing agent continue to be supplied, the fuel cell can continue producing electricity.
Energy Conversion in a Fuel Cell
A fuel cell converts:
chemical energy → electrical energy
Some energy is also transferred as:
thermal energy.
In a hydrogen fuel cell, the overall reaction combines:
hydrogen and oxygen
to produce:
water.
The overall reaction is:
2H₂ + O₂ → 2H₂O
Energy is released during this reaction.
Instead of allowing all of the energy to be released directly as heat, the fuel cell directs electrons through an external circuit.
This produces:
useful electrical energy.
Hydrogen Fuel Cells
One of the most important types of fuel cell is the:
hydrogen fuel cell.
A common example is the proton exchange membrane fuel cell, often abbreviated:
PEM fuel cell.
It uses:
- hydrogen
- oxygen
- two electrodes
- a catalyst
- a proton exchange membrane
The products are primarily:
- water
- electrical energy
- heat
Structure of a Hydrogen Fuel Cell
A simplified hydrogen fuel cell contains:
Anode
The electrode where hydrogen enters and oxidation occurs.
Cathode
The electrode where oxygen enters and reduction occurs.
Electrolyte or proton exchange membrane
Allows hydrogen ions, H⁺, to move through the cell while separating the reactant gases and blocking electrons from crossing directly.
Catalyst
Helps the electrode reactions occur at useful rates.
External circuit
Provides a path for electrons to travel through the electrical device.
How a Hydrogen Fuel Cell Works
The operation of a PEM hydrogen fuel cell can be divided into several stages.
1. Hydrogen enters the anode.
2. Hydrogen molecules are oxidized.
3. Hydrogen ions move through the membrane.
4. Electrons travel through the external circuit.
5. Oxygen enters the cathode.
6. Oxygen, hydrogen ions, and electrons combine to form water.
This separation of electron flow from ion movement allows the cell to generate:
electricity.
Step 1: Hydrogen Enters the Anode
Hydrogen gas enters the:
anode side
of the fuel cell.
Hydrogen exists as molecules:
H₂
At the catalyst surface, hydrogen molecules undergo:
oxidation.
Remember:
Oxidation = loss of electrons
Step 2: Hydrogen Is Oxidized
At the anode:
H₂ → 2H⁺ + 2e⁻
Each hydrogen molecule produces:
- two hydrogen ions, H⁺
- two electrons, e⁻
The hydrogen ions and electrons then take:
different paths.
Step 3: Protons Cross the Membrane
The H⁺ ions are:
protons.
The proton exchange membrane allows these protons to pass through toward the:
cathode.
However, the membrane does not allow electrons to simply follow the same path.
The electrons must travel through the:
external circuit.
Step 4: Electrons Flow Through the Circuit
The electrons leave the anode and travel through:
an external electrical circuit.
Their movement produces an electric current that can power:
- an electric motor
- lights
- electronics
- other electrical devices
The electrons eventually reach the:
cathode.
This is how the fuel cell extracts useful electrical energy from the chemical reaction.
Step 5: Oxygen Enters the Cathode
Oxygen is supplied to the:
cathode.
In many fuel-cell systems, this oxygen comes from:
air.
At the cathode, oxygen participates in a reduction reaction.
Remember:
Reduction = gain of electrons
Step 6: Water Is Produced
At the cathode, oxygen combines with:
- hydrogen ions
- electrons
A simplified cathode reaction is:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
The product is:
water.
Heat is also released during fuel-cell operation.
The Complete Reaction
At the anode:
2H₂ → 4H⁺ + 4e⁻
At the cathode:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Add the reactions together:
2H₂ + O₂ → 2H₂O
Notice that the electrons appear in both half-equations and cancel from the overall equation.
They are transferred from:
hydrogen to oxygen.
Oxidation and Reduction
The hydrogen fuel cell is a:
redox system.
At the anode:
hydrogen is oxidized.
At the cathode:
oxygen is reduced.
Therefore:
Anode = oxidation
Cathode = reduction
This is the same fundamental electrochemical principle found in:
galvanic cells and batteries.
Electron Flow vs Proton Flow
This distinction is very important.
Electrons:
travel through the external circuit.
Protons (H⁺):
travel through the proton exchange membrane.
The two paths eventually meet again at the:
cathode.
A common mistake is to say that electrons travel through the membrane.
They do not.
Why Does the Fuel Cell Produce Electricity?
Hydrogen and oxygen can react to form water.
The overall reaction releases energy.
A fuel cell separates the oxidation and reduction processes so that electrons cannot transfer directly between the reactants.
Instead, electrons must travel through an:
external circuit.
This controlled electron flow produces:
electric current.
The Role of the Catalyst
Fuel-cell reactions need to occur quickly enough to provide useful power.
A catalyst increases the rate of the electrochemical reactions without being consumed in the overall reaction.
PEM fuel cells commonly use catalysts containing:
platinum.
Platinum is effective but expensive.
Reducing the amount of expensive catalyst material is therefore an important area of:
fuel-cell research.
Fuel Cell Stacks
A single fuel cell produces a relatively small voltage.
To provide greater voltage and power, many individual cells can be connected together.
This arrangement is called a:
fuel-cell stack.
Fuel-cell vehicles and larger power systems typically use stacks containing many:
individual cells.
Fuel Cells and Batteries
Fuel cells and batteries have several similarities.
Both:
- are electrochemical devices
- involve redox reactions
- contain an anode and cathode
- use an electrolyte
- produce an electric current
- convert chemical energy into electrical energy
However, they differ in how their:
reactants are supplied.
Batteries Store Their Reactants
A conventional battery contains its active chemical materials:
inside the battery.
As the battery discharges, these materials undergo chemical changes.
A primary battery eventually needs to be:
replaced.
A secondary battery can be:
recharged.
Fuel Cells Receive Fuel Continuously
A fuel cell does not depend only on a fixed quantity of reactants stored inside the electrochemical cell itself.
Instead:
fuel enters continuously
and:
products leave continuously.
For a hydrogen fuel cell:
hydrogen enters
oxygen enters
water leaves
As long as suitable reactants are supplied, the fuel cell can continue:
generating electricity.
Fuel Cell vs Primary Battery
A primary battery:
- stores reactants internally
- operates until its reactants are depleted
- is normally replaced after discharge
A fuel cell:
- receives reactants externally
- can continue operating while fuel is supplied
- is refueled rather than electrically recharged in the same way as a secondary battery
Fuel Cell vs Rechargeable Battery
A rechargeable battery stores energy chemically inside the battery.
When discharged, it is connected to an electrical source and:
recharged.
A hydrogen fuel-cell system is instead supplied with more:
hydrogen fuel.
Therefore:
battery → recharge
fuel cell → refuel
Comparing Fuel Cells and Batteries
| Feature | Fuel Cell | Rechargeable Battery |
|---|---|---|
| Energy source | External fuel | Stored internally |
| Produces electricity by | Redox reactions | Redox reactions |
| Main energy conversion | Chemical → electrical | Chemical → electrical |
| Restoring energy | Refueling | Electrical charging |
| Operation | Continues while fuel is supplied | Limited by stored charge |
| Example | Hydrogen PEM fuel cell | Lithium-ion battery |
| Main exhaust/product at PEM fuel cell | Water | No continuous exhaust stream |
Hydrogen as an Energy Carrier
Hydrogen is often described as an:
energy carrier
rather than a primary energy source.
This distinction is important.
Hydrogen must normally be produced using another source of:
energy.
Energy is used to produce hydrogen.
The hydrogen can then be:
- stored
- transported
- supplied to a fuel cell
The fuel cell converts its chemical energy into:
electrical energy.
Producing Hydrogen
Hydrogen can be produced in several ways.
One method is:
electrolysis of water.
Electrical energy is used to split water into hydrogen and oxygen.
In simplified form:
water + electrical energy → hydrogen + oxygen
The hydrogen can later be used in a fuel cell.
Electrolysis and Fuel Cells
Electrolysis and hydrogen fuel cells can be viewed as related but different processes.
During electrolysis:
electrical energy → chemical energy stored in hydrogen
During fuel-cell operation:
chemical energy in hydrogen → electrical energy
This creates a possible energy-storage pathway:
electricity → hydrogen → electricity
Energy is lost at each conversion stage, so the complete cycle is:
not 100% efficient.
Where Does Hydrogen Come From?
The environmental impact of hydrogen depends strongly on:
how the hydrogen is produced.
Hydrogen can be produced from:
- water using electrolysis
- natural gas
- other hydrocarbons
- biomass
- other industrial processes
Different production methods can have very different:
greenhouse-gas emissions.
Hydrogen Produced from Fossil Fuels
Much hydrogen has historically been produced from:
natural gas and other fossil fuels.
These processes can release:
carbon dioxide.
Therefore, using hydrogen in a fuel cell does not automatically mean that the entire energy system has:
zero greenhouse-gas emissions.
The complete production pathway must be considered.
Hydrogen from Renewable Electricity
Hydrogen can also be produced by electrolysis using electricity generated from sources such as:
- solar power
- wind power
- hydroelectric power
When low-carbon electricity is used, the overall greenhouse-gas emissions associated with hydrogen production can be substantially reduced.
This is often called:
renewable hydrogen or green hydrogen.
Advantages of Hydrogen Fuel Cells
Hydrogen fuel cells offer several potential advantages.
They can provide:
- electrical energy without combustion at the point of use
- water as the main reaction product in a PEM fuel cell
- relatively quiet operation
- continuous operation while fuel is supplied
- relatively rapid refueling
- useful energy storage for some applications
- high electrochemical conversion efficiency compared with many heat-engine systems
No Carbon Dioxide at the Fuel Cell
A hydrogen fuel cell does not contain carbon in its fuel.
Therefore the fuel-cell reaction itself does not produce:
carbon dioxide.
The main chemical product is:
water.
However, this describes the:
point of use.
The production and transportation of the hydrogen may still create greenhouse-gas emissions.
Fuel Cells Do Not Require Combustion
A fuel cell does not need to burn hydrogen to produce electricity.
Instead, it uses:
electrochemical reactions.
This is important because combustion converts chemical energy first into thermal energy.
A fuel cell converts chemical energy more directly into:
electrical energy.
Efficiency
Fuel cells can convert a substantial fraction of the fuel's chemical energy directly into electrical energy.
Some energy is still converted into:
heat.
In certain applications, this heat can also be used.
Systems that produce electricity while also using the waste heat are called:
combined heat and power systems.
Rapid Refueling
One potential advantage of hydrogen vehicles is that hydrogen tanks can be:
refilled.
This differs from battery-electric vehicles, which must transfer electrical energy into their batteries during:
charging.
For applications requiring high utilization and quick turnaround, refueling time can be an important consideration.
Long-Duration Energy Storage
Hydrogen can potentially store energy for relatively long periods.
For example, excess renewable electricity could be used to produce hydrogen.
The hydrogen could then be stored and later used in:
- fuel cells
- industrial processes
- other applications
This may be useful where electricity must be stored for:
long periods or in very large quantities.
Challenges of Fuel Cells
Fuel-cell technology also faces significant challenges.
These include:
- hydrogen production
- hydrogen storage
- hydrogen transportation
- infrastructure
- cost
- catalyst materials
- overall energy efficiency
- durability
These factors must be considered when evaluating fuel cells as an:
energy technology.
Hydrogen Storage
Hydrogen has a very low density as a gas under ordinary conditions.
To store useful quantities, it may be:
- compressed to high pressure
- cooled to very low temperatures and liquefied
- stored using specialized materials or chemical carriers
Each approach requires:
energy and specialized equipment.
High-Pressure Storage
Fuel-cell vehicles commonly store hydrogen as:
compressed gas.
The storage tanks must withstand very high pressures.
This requires:
- strong materials
- careful engineering
- safety systems
- specialized refueling equipment
These requirements add:
cost and complexity.
Hydrogen Has Low Volumetric Energy Density
Hydrogen contains a large amount of energy relative to its:
mass.
However, gaseous hydrogen occupies a large volume.
This creates an important engineering challenge.
Hydrogen can have:
high specific energy by mass
but:
low volumetric energy density unless compressed or liquefied.
Transporting Hydrogen
Hydrogen must be transported from where it is produced to where it is used unless it is produced on site.
Possible approaches include:
- pipelines
- compressed-gas transport
- liquid hydrogen transport
- conversion into other hydrogen-containing materials
Each method has:
costs, energy losses, and infrastructure requirements.
Infrastructure
Gasoline and electricity already have extensive distribution infrastructure in many regions.
Hydrogen requires infrastructure such as:
- production facilities
- storage facilities
- pipelines or transport systems
- refueling stations
Building this infrastructure can require substantial:
investment.
Catalyst Cost
PEM fuel cells commonly use:
platinum-group catalysts.
These materials are:
- effective
- relatively scarce
- expensive
Researchers work to:
- reduce catalyst quantities
- improve catalyst durability
- develop alternative catalysts
Reducing catalyst cost could make fuel-cell systems more:
economically competitive.
Durability
Fuel cells must operate reliably for long periods.
Performance can decline because of:
- catalyst degradation
- membrane degradation
- contamination
- repeated operating cycles
- temperature and humidity changes
Increasing fuel-cell durability remains an important:
engineering challenge.
Fuel-Cell Vehicles
Hydrogen fuel cells can be used to power:
electric vehicles.
The fuel cell generates electricity.
That electricity powers an:
electric motor.
The basic energy pathway is:
chemical energy in hydrogen → electrical energy → kinetic energy
Fuel-Cell Vehicles Often Have Batteries Too
A fuel-cell vehicle may contain both:
a fuel-cell stack and a rechargeable battery.
The fuel cell provides electrical energy from hydrogen.
The battery can:
- store energy from regenerative braking
- provide additional power during acceleration
- smooth changes in power demand
Therefore fuel cells and batteries do not always have to be:
competing technologies.
They can work together.
Regenerative Braking
When a fuel-cell vehicle slows down, its electric motor can operate as a generator.
Some kinetic energy can be converted into:
electrical energy.
That energy can be stored in the vehicle's:
battery.
The energy pathway is:
kinetic → electrical → chemical
This improves overall vehicle efficiency.
Stationary Fuel Cells
Fuel cells can also generate electricity for:
- buildings
- hospitals
- data centers
- telecommunications equipment
- remote locations
- backup power systems
Stationary fuel cells can be especially useful when reliable:
continuous electrical power
is required.
Fuel Cells in Space
Fuel cells have also been used in:
spacecraft.
Hydrogen and oxygen can generate electricity while producing:
water.
This combination has made fuel cells useful in some space missions.
Fuel Cells vs Internal Combustion Engines
An internal combustion engine generally follows an energy pathway such as:
chemical → thermal → mechanical
A fuel-cell vehicle follows approximately:
chemical → electrical → mechanical
Fuel cells avoid the combustion step.
This can provide efficiency and emissions advantages at the:
point of use.
Fuel Cells vs Battery-Electric Vehicles
Both fuel-cell vehicles and battery-electric vehicles use:
electric motors.
The major difference is how the electrical energy is supplied.
Battery-electric vehicle:
electricity → battery → motor
Fuel-cell vehicle:
hydrogen → fuel cell → electricity → motor
Often a small battery is also included in the fuel-cell vehicle.
Energy Efficiency of the Whole System
When evaluating fuel cells, it is important to consider more than the fuel cell itself.
For hydrogen produced using electricity, the complete pathway may involve:
electricity
↓
↓
hydrogen
↓
compression or liquefaction
↓
transport and storage
↓
fuel cell
↓
electricity
Each stage involves some:
energy loss.
Why Batteries Can Be More Efficient in Some Applications
A battery-electric system can use electricity more directly:
electricity → battery → electricity → motor
A hydrogen system may require additional energy conversions.
Therefore battery systems can have higher overall electricity-to-wheel efficiency in many light-duty vehicle applications.
However, fuel cells may offer other advantages, such as:
- rapid refueling
- lower fuel-storage mass in some long-range applications
- potential long-duration energy storage
The best technology depends on the:
application.
Evaluating an Energy Technology
A good evaluation should not focus on only one advantage or disadvantage.
Important factors include:
- efficiency
- cost
- environmental impact
- fuel availability
- infrastructure
- storage
- safety
- reliability
- lifetime
- energy density
- refueling or charging time
Different applications may give different importance to each:
factor.
Environmental Impact
Hydrogen fuel cells can have very low emissions at the point of use.
For a PEM fuel cell, the main chemical product is:
water.
However, a complete environmental assessment should consider:
- hydrogen production
- electricity source
- transportation
- compression
- storage
- fuel-cell manufacturing
- catalyst materials
- end-of-life recycling
This is called a:
life-cycle assessment.
Fuel Cells and Renewable Energy
Renewable electricity is sometimes produced when electricity demand is relatively low.
Some of this electricity could be used to produce:
hydrogen through electrolysis.
The hydrogen could then be stored.
Later:
hydrogen → fuel cell → electricity
This provides one possible method for storing renewable energy.
Advantages of Fuel Cells
Important potential advantages include:
- continuous electricity production while fuel is supplied
- no carbon dioxide from hydrogen fuel-cell operation itself
- water as the main chemical product of PEM hydrogen fuel cells
- relatively quiet operation
- no combustion required
- rapid refueling in some applications
- potential use for long-duration energy storage
- useful efficiency
- modular design
- potential integration with renewable energy
Challenges of Fuel Cells
Important challenges include:
- hydrogen must first be produced
- low-carbon hydrogen can be expensive
- hydrogen storage is difficult
- compression requires energy
- hydrogen infrastructure is limited in many regions
- fuel-cell catalysts can be expensive
- fuel cells can degrade
- hydrogen must be handled carefully
- the complete hydrogen energy pathway involves efficiency losses
Worked Example 1
What energy conversion occurs in a hydrogen fuel cell?
Chemical energy → electrical energy + thermal energy
The chemical energy comes from the reaction between:
hydrogen and oxygen.
Worked Example 2
Write the overall hydrogen fuel-cell reaction.
2H₂ + O₂ → 2H₂O
Hydrogen and oxygen react to form:
water.
Worked Example 3
What happens at the anode?
Hydrogen is oxidized:
H₂ → 2H⁺ + 2e⁻
Hydrogen loses:
electrons.
Worked Example 4
What happens at the cathode?
Oxygen gains electrons and combines with hydrogen ions.
A balanced cathode half-equation is:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Therefore:
reduction occurs at the cathode.
Worked Example 5
A student says:
"The electrons travel through the membrane from the anode to the cathode."
Is this correct?
No.
The protons travel through the proton exchange membrane.
The electrons travel through the:
external circuit.
Their movement through the external circuit produces useful electrical current.
Worked Example 6
Why doesn't a fuel cell need to be recharged like a lithium-ion battery?
The fuel cell receives fresh reactants from:
external supplies.
Instead of recharging the electrochemical cell, the system is:
refueled.
Worked Example 7
Why is it misleading to say that hydrogen fuel cells are always "zero-emission energy"?
The fuel cell itself produces water rather than carbon dioxide.
However, producing, compressing, transporting, and storing hydrogen can require energy and may produce emissions.
The complete:
life cycle
must therefore be considered.
Worked Example 8
A wind farm produces excess electricity during the night.
Explain how hydrogen could be used to store this energy.
Step 1: Electricity powers electrolysis.
Step 2: Water is split to produce hydrogen.
Step 3: Hydrogen is stored.
Step 4: Later, hydrogen enters a fuel cell.
Step 5: The fuel cell produces electricity.
The pathway is:
electrical → chemical → electrical
Worked Example 9
Why might fuel cells be attractive for vehicles that must operate for long periods with short stops?
A hydrogen fuel-cell system can potentially be:
refueled relatively quickly.
This may be useful where long charging periods would interfere with vehicle operation.
The overall suitability still depends on fuel availability, infrastructure, efficiency, and cost.
Worked Example 10
Why might batteries be preferred for some smaller vehicles?
Batteries can use electricity more directly and avoid several energy-conversion stages associated with producing, storing, and reconverting hydrogen.
They can therefore provide high overall:
energy efficiency.
Common Mistake: Fuel Cells Store Electricity
Fuel cells do not primarily store electricity.
They convert the chemical energy of continuously supplied fuel into:
electrical energy.
Common Mistake: Fuel Cells Are Batteries
Fuel cells and batteries are both electrochemical devices, but they operate differently.
A battery stores its reactants internally.
A fuel cell receives:
reactants from outside the cell.
Common Mistake: Hydrogen Is a Source of Energy Like Sunlight
Hydrogen is better described as an:
energy carrier.
Energy must first be used to produce usable hydrogen.
Common Mistake: Hydrogen Fuel Cells Burn Hydrogen
Fuel cells do not normally produce electricity by combustion.
They use:
electrochemical redox reactions.
Common Mistake: Electrons Cross the Proton Exchange Membrane
The membrane allows:
H⁺ ions
to move through it.
Electrons travel through the:
external circuit.
Common Mistake: Fuel Cells Produce Only Electricity
Fuel cells also produce:
thermal energy.
Hydrogen PEM fuel cells additionally produce:
water.
Common Mistake: Hydrogen Is Automatically Green
The environmental impact depends strongly on:
how the hydrogen was produced.
Hydrogen produced using fossil fuels can be associated with significant greenhouse-gas emissions.
Common Mistake: Fuel Cells and Batteries Cannot Work Together
Some systems combine:
fuel cells and rechargeable batteries.
Fuel-cell vehicles are an important example.
The technologies can perform different roles within the same system.
Check Your Understanding
- Define a fuel cell.
- What energy conversion occurs in a fuel cell?
- What fuel is used in a hydrogen fuel cell?
- What substance provides oxygen to many hydrogen fuel cells?
- Write the overall reaction for a hydrogen fuel cell.
- What happens at the anode?
- Write the hydrogen oxidation half-equation.
- What happens at the cathode?
- Write the oxygen reduction half-equation.
- Where does oxidation occur?
- Where does reduction occur?
- What travels through the external circuit?
- What travels through the proton exchange membrane?
- What is the purpose of the catalyst?
- Why is platinum useful in PEM fuel cells?
- What is a fuel-cell stack?
- Why are several fuel cells connected together?
- Give three similarities between batteries and fuel cells.
- What is the major difference between a fuel cell and a battery?
- Why can a fuel cell continue operating while fuel is supplied?
- Why is a fuel cell refueled rather than recharged in the same way as a secondary battery?
- Why is hydrogen described as an energy carrier?
- What is electrolysis?
- Explain how electrolysis and fuel-cell operation can form an energy-storage cycle.
- Why is this cycle not 100% efficient?
- Give four advantages of hydrogen fuel cells.
- Give four challenges associated with hydrogen fuel cells.
- Why is hydrogen difficult to store?
- How can hydrogen gas be stored for vehicle use?
- Why does hydrogen infrastructure present a challenge?
- Why is catalyst cost important?
- What is the main chemical product of a PEM hydrogen fuel cell?
- Does the operation of a hydrogen PEM fuel cell produce carbon dioxide?
- Why can hydrogen production still produce carbon dioxide emissions?
- What is meant by renewable or green hydrogen?
- How can fuel cells be used in vehicles?
- Why might a fuel-cell vehicle also contain a battery?
- How can regenerative braking help a fuel-cell vehicle?
- Give three stationary applications of fuel cells.
- Explain how hydrogen could be used to store excess renewable energy.
- Compare the energy pathways of a battery-electric vehicle and a fuel-cell vehicle.
- Why can battery-electric systems have higher overall energy efficiency in some applications?
- Why might rapid refueling make fuel cells attractive in some applications?
- Explain why point-of-use emissions do not tell us the complete environmental impact of hydrogen.
- What factors should be considered in a life-cycle assessment?
- Explain why fuel cells do not require combustion.
- Compare a primary battery, secondary battery, and fuel cell.
- Explain why no single energy-storage technology is ideal for every application.
- Evaluate the advantages and disadvantages of hydrogen fuel cells for transportation.
- Evaluate whether hydrogen fuel cells could play a role in a future low-carbon energy system. Support your answer using efficiency, hydrogen production, storage, infrastructure, and environmental impact.
Key Terms
Fuel cell: Electrochemical device that converts the chemical energy of externally supplied fuel into electrical energy.
Hydrogen fuel cell: Fuel cell that uses hydrogen as a fuel and oxygen as an oxidizing agent.
PEM fuel cell: Proton exchange membrane fuel cell in which protons move through a specialized membrane.
Anode: Electrode where oxidation occurs.
Cathode: Electrode where reduction occurs.
Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Redox reaction: Reaction involving both oxidation and reduction.
Proton exchange membrane: Material that allows H⁺ ions to pass while preventing electrons from directly crossing between electrodes.
Catalyst: Substance that increases reaction rate without being consumed in the overall reaction.
Fuel-cell stack: Group of individual fuel cells connected to provide greater voltage and power.
Hydrogen: Energy carrier that can be used as fuel in certain fuel cells.
Electrolysis: Process that uses electrical energy to drive a nonspontaneous chemical reaction, such as splitting water to produce hydrogen and oxygen.
Green hydrogen: Hydrogen produced using electrolysis powered by renewable or other qualifying low-carbon electricity, depending on the definition being used.
Energy carrier: Substance or system used to store and transport energy obtained from another source.
Life-cycle assessment: Evaluation of environmental impacts across production, transportation, use, and end-of-life stages.
Fuel-cell electric vehicle: Electric vehicle in which a fuel-cell system supplies electrical energy to the drivetrain.
Regenerative braking: Recovery of some kinetic energy during braking and conversion into stored energy.
Key Takeaways
- A fuel cell converts chemical energy directly into electrical energy through electrochemical reactions.
- Hydrogen fuel cells use hydrogen and oxygen to produce electricity, water, and heat.
- The overall reaction is 2H₂ + O₂ → 2H₂O.
- Hydrogen is oxidized at the anode.
- Oxygen is reduced at the cathode.
- Protons travel through the proton exchange membrane.
- Electrons travel through the external circuit, producing useful electric current.
- Fuel cells and batteries both use redox reactions, but fuel cells receive their reactants from external supplies.
- Batteries are generally recharged or replaced; fuel-cell systems are refueled.
- Hydrogen is an energy carrier, meaning energy must first be used to produce it.
- Hydrogen can be produced through electrolysis using renewable electricity.
- Fuel cells can provide low point-of-use emissions, quiet operation, and relatively rapid refueling.
- Important challenges include hydrogen production, storage, transportation, infrastructure, catalyst cost, durability, and overall efficiency.
- Hydrogen fuel cells produce no carbon dioxide at the point of use, but the complete environmental impact depends strongly on how the hydrogen is produced.
- Fuel cells can be used in transportation, stationary power, backup systems, and some energy-storage applications.
- Fuel cells and batteries can work together in the same system.
- Evaluating fuel cells requires considering the entire energy pathway rather than only the fuel cell itself.
- Fuel cells are one possible component of a lower-carbon energy system, but their suitability depends strongly on the particular application.