A hydrogen fuel cell transforms the chemical energy in hydrogen and oxygen into electrical energy through an electrochemical reaction, generating electricity, heat, and water. Unlike batteries, fuel cells require a continuous supply of fuel and an oxidant to produce power. This article explains how these devices function, detailing their components, electrochemical reactions, construction principles, and safety considerations.
Key Components
A hydrogen fuel cell relies on several specific components. At its core are two electrodes: the anode, where hydrogen gas is introduced, and the cathode, where oxygen is supplied. Separating these electrodes is the Proton Exchange Membrane (PEM), a thin polymer film that selectively allows only protons (positively charged hydrogen ions) to pass through, blocking electrons and gases.
Catalyst layers, usually platinum, are coated onto both sides of the PEM. These layers initiate the splitting of hydrogen molecules at the anode and water formation at the cathode. Adjacent are Gas Diffusion Layers (GDLs), often porous carbon, which allow gases to reach the catalyst surfaces and conduct electrons away. Bipolar plates, or flow field plates, distribute reactant gases evenly, collect electrical current, and provide structural support.
The Electrochemical Process
Electricity generation in a hydrogen fuel cell is a continuous, multi-step electrochemical process. It begins at the anode, where hydrogen gas (H₂) contacts the catalyst layer. Here, the catalyst separates each hydrogen molecule into two protons (H⁺) and two electrons (e⁻).
The protons pass directly through the Proton Exchange Membrane to the cathode. Electrons, however, cannot traverse the membrane and are forced through an external electrical circuit. This flow of electrons constitutes the electrical current.
At the cathode, these electrons and protons combine with oxygen gas (O₂) from the air. With the cathode’s catalyst, they react to form water (H₂O), the only byproduct. This continuous cycle of reactant supply, electron flow, and water formation allows the fuel cell to generate electricity.
Basic Construction Principles
Assembling a functional hydrogen fuel cell involves a layered approach. The core is the Membrane Electrode Assembly (MEA), consisting of the proton exchange membrane sandwiched between two catalyst layers and two gas diffusion layers. This central assembly is where electrochemical reactions occur.
The MEA is placed between two bipolar plates. These plates have intricate channels to distribute hydrogen and oxygen gases uniformly across the gas diffusion layers. They also provide pathways for product water removal and function as current collectors, directing electricity to external circuits.
An airtight seal around the membrane and gas channels prevents leaks of hydrogen and oxygen, which could compromise efficiency and safety. Gaskets are commonly used for this purpose. Electrical connections are integrated, often through the bipolar plates, to draw current from the cell.
Safe Handling and Operation
Working with hydrogen fuel cells requires careful adherence to safety measures due to hydrogen’s properties and electricity. Hydrogen gas is highly flammable and ignites easily, with a wide flammability range (4% to 75% by volume in air) and low ignition energy. Hydrogen flames are nearly invisible, making leaks and fires difficult to detect. All work areas must be well-ventilated to prevent hydrogen accumulation, and all potential ignition sources avoided.
Electrical safety also requires attention, as fuel cells generate direct current. Even low-voltage cells can pose a shock hazard, and larger systems produce significant voltages. Exercise caution when accessing electrical components and consider wearing appropriate personal protective equipment (PPE), such as safety glasses and gloves.
Material compatibility is another consideration; hydrogen can cause some metals to become brittle, and the acidic environment of certain PEMs requires resistant materials. Proper pressure regulation is also necessary for hydrogen storage, ensuring tanks and lines handle the gas safely. While fuel cell technology offers many benefits, construction and operation should prioritize safety through informed practices and engineering controls.