Question

Describe how a hydrogen fuel cell works, and write balanced half-reactions for its operation in acidic media. What other types of fuel cells exist?

Short answer

A hydrogen fuel cell converts hydrogen and oxygen into water and electricity through separate half-reactions. Other fuel cell types include alkaline, phosphoric acid, proton exchange membrane, molten carbonate, and solid oxide fuel cells.

Step-by-step solution

Understanding the Hydrogen Fuel Cell Process

A hydrogen fuel cell generates electricity through an electrochemical reaction between hydrogen and oxygen. In this process, hydrogen gas (H₂) is fed into the anode side of the fuel cell, where it is split into protons (H⁺) and electrons (e⁻) catalytically. These electrons move through an external circuit, providing electrical power, while the protons move across an electrolyte membrane.

Reaction at the Anode

At the anode, the following oxidation reaction takes place:\[ ext{Anode reaction: } ext{H}_2 ightarrow 2 ext{H}^+ + 2 ext{e}^-\]In this reaction, hydrogen molecules release electrons and form protons.

Proton Exchange and Electron Flow

The membrane in a hydrogen fuel cell allows only protons to pass through it, directing them towards the cathode. Electrons, however, flow through the external circuit to the cathode, creating an electric current that can be used to do work.

Reaction at the Cathode

At the cathode, the protons combine with electrons and oxygen from the air to form water. The cathode reaction is as follows:\[ ext{Cathode reaction: } rac{1}{2} ext{O}_2 + 2 ext{H}^+ + 2 ext{e}^- ightarrow ext{H}_2 ext{O}\]

Overall Cell Reaction

Combining the anode and cathode reactions, we get the overall balanced reaction for the hydrogen fuel cell:\[ ext{Overall reaction: } ext{H}_2 + rac{1}{2} ext{O}_2 ightarrow ext{H}_2 ext{O}\]This reaction represents the complete conversion of hydrogen and oxygen into water, releasing energy in the form of electricity.

Types of Fuel Cells

Besides hydrogen fuel cells in acidic media, there are several other types of fuel cells, such as: 1. **Alkaline Fuel Cells (AFC):** Use potassium hydroxide as the electrolyte. 2. **Phosphoric Acid Fuel Cells (PAFC):** Use liquid phosphoric acid as the electrolyte. 3. **Proton Exchange Membrane Fuel Cells (PEMFC):** Use a solid polymer electrolyte. 4. **Molten Carbonate Fuel Cells (MCFC):** Use molten carbonate salt as the electrolyte. 5. **Solid Oxide Fuel Cells (SOFC):** Use a solid ceramic as the electrolyte.

Key concepts

Electrochemical Reaction

An electrochemical reaction is at the heart of how hydrogen fuel cells generate energy. In these cells, electricity is produced through a chemical reaction between hydrogen and oxygen, rather than by combustion. This process involves converting chemical energy into electrical energy effectively and cleanly.
Hydrogen molecules (H₂) are introduced into the fuel cell at the anode, while oxygen (O₂) is introduced at the cathode. Through a design that only allows certain particles to move between the anode and cathode, the reaction proceeds creating electricity without producing harmful emissions.
The beauty of electrochemical reactions is that they can be incredibly efficient, often more so than traditional combustion-based energy generation methods. They produce a small ecological footprint as the only byproduct is water.

Anode and Cathode Reactions

In a hydrogen fuel cell, distinct reactions occur at the anode and cathode which are crucial to the system's operation.
At the anode, hydrogen molecules are split into protons (H⁺) and electrons (e⁻). This happens through a catalyst which aids in breaking the chemical bonds of hydrogen gas, releasing electrons that begin moving through an external circuit. This movement of electrons generates an electric current, which is then used to power devices. This anode reaction is represented by:

• Anode reaction: \[ \text{H}_2 \rightarrow 2\text{H}^+ + 2\text{e}^- \]

Simultaneously, at the cathode, these electrons combine with protons that have passed through the membrane, and with oxygen taken from air to form water. This essential reaction is shown as:

• Cathode reaction: \[ \frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2\text{O} \]

Together, these reactions drive the continuous flow of electrons in the circuit, powering the intended applications.

Proton Exchange Membrane

The proton exchange membrane (PEM) is a critical component of the fuel cell. Its main role is to act as an electrolyte, allowing protons to pass from the anode to the cathode while simultaneously blocking electrons. This selectivity is crucial because it forces electrons to travel through an external circuit rather than passing directly through the membrane.
This movement through the circuit produces electrical power as the hydrogen fuel cell operates. PEM also aids in water management and deals with heat because proper hydration is essential for maintaining conductivity.
Made from a special kind of plastic, PEMs are known for their efficiency and durability, though they require careful handling to maintain performance. Ensuring the membrane stays hydrated is paramount, as dry membranes lose their proton conductivity.
Because of their role, PEMs are used in various types of fuel cells, particularly in the well-known Proton Exchange Membrane Fuel Cells (PEMFC).

Types of Fuel Cells

There are several types of fuel cells, each with its unique features and applications. Here are some common examples:

• Alkaline Fuel Cells (AFC): These use an alkaline electrolyte such as potassium hydroxide. They are known for having high efficiencies but are sensitive to carbon dioxide, requiring purified oxygen and hydrogen.
• Phosphoric Acid Fuel Cells (PAFC): Using liquid phosphoric acid as the electrolyte, PAFCs provide efficiency better than a conventional electric generator and are typically used for medium to large-scale applications such as powering hospitals.
• Proton Exchange Membrane Fuel Cells (PEMFC): These fuel cells work at relatively low temperatures and are known for fast start-ups. They are popular in vehicles and portable applications.
• Molten Carbonate Fuel Cells (MCFC): Operating at high temperatures, they use molten carbonate salts as electrolytes and are suitable for industrial and electric utility use. They can also share waste heat for heating and power applications.
• Solid Oxide Fuel Cells (SOFC): Utilizes a solid ceramic electrolyte and can achieve high efficiency and excellent stability, making them suitable for large-scale stationary power generation.

Each type has different applications based on cost, efficiency, and material choices. Their advancements help drive sustainable energy solutions.

Balanced Chemical Reactions in Fuel Cells

Understanding the balanced chemical reactions in fuel cells is key to grasping how they function efficiently. The entire process in a hydrogen fuel cell is driven by two half-reactions occurring separately at each electrode (anode and cathode).
The reactions are summed to give the overall balanced chemical reaction of the hydrogen fuel cell as follows:

• Overall reaction: \[ \text{H}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O} \]

This equation highlights the fundamental conversion of hydrogen and oxygen into water, showcasing the elegant simplicity of the fuel cell process. The absence of pollutants like carbon dioxide contrasts with traditional fossil fuel usage, underscoring hydrogen's potential for clean energy.
Balanced reactions ensure that the fuel cell outputs are maximized while consuming the inputs fully, maintaining efficiency and sustainability. These reactions demonstrate how fuel cells offer a green alternative by leveraging the basic principles of chemistry for large-scale practical applications.