In this blog post, we’ll take a natural look at the structure and operating principles of fuel cells—a clean energy source—as well as their advantages and limitations.
Would you believe it if someone told you that you could collect and drink the exhaust from a fuel-cell-powered car? Most people would think that’s a dirty and dangerous idea. However, when it comes to fuel-cell vehicles, this isn’t an impossible scenario at all. Unlike gasoline-powered internal combustion engine vehicles, cars powered by fuel cells and electric motors emit pure water instead of various environmental pollutants.
It’s easier to understand a fuel cell if you think of it as a type of battery that also functions like a small power plant. While ordinary batteries generate electricity using chemical energy stored inside them, fuel cells produce electrical energy by continuously receiving an external supply of fuel. This device can be used as an energy source in a wide range of fields, from small electronic devices to automobiles and spacecraft. It uses hydrogen and oxygen as fuel. Hydrogen is the lightest gas on Earth, and oxygen is the gas we inhale when we breathe. When these two gases are supplied to a fuel cell, it produces electrical energy.
First, let’s examine the structure of a fuel cell. A fuel cell consists of three thin layers. The anode is on the left, the cathode is on the right, and the electrolyte is located between them. An electrolyte is a substance that can conduct electricity when dissolved in a solvent such as water or when in a liquid state. For example, when salt is dissolved in water, electricity flows, and in this case, the salt acts as an electrolyte. The electrolyte used in a fuel cell takes the form of a very thin membrane that has the property of allowing ions to move when it absorbs water. Inside the fuel cell, maintaining an appropriate level of moisture in this membrane creates an environment where electric current can flow. Additionally, fuel supply systems are connected so that hydrogen is supplied to the outside of the anode and oxygen is supplied to the outside of the cathode. Finally, the anode and cathode are connected to an external electrical circuit, allowing electricity to be used.
Now, let’s examine how these three components generate electrical energy. The hydrogen supplied to the anode is split into electrons (e-) and hydrogen ions (H+) through the action of a catalyst. Here, it is necessary to first understand the concepts of electrons and ions. Electrons are one of the fundamental particles that make up matter, along with protons and neutrons. For example, a hydrogen atom consists of one proton and one electron, while an oxygen atom consists of eight protons, eight neutrons, and eight electrons. When an atom loses or gains an electron, it becomes charged; this state is called an ion. For example, a hydrogen ion is a hydrogen atom that has lost one electron, while an oxygen ion is an oxygen atom that has gained an additional electron.
The reason hydrogen separates into electrons and hydrogen ions is that the separated state is more stable. Here, “stable” means that the energy level after separation is lower than before separation. This is similar to how a person’s body temperature rises and their body tenses up when excited, but returns to a calm state once they settle down. Since all substances tend to move toward a more stable state, hydrogen also seeks to reach its dissociated state. So why isn’t hydrogen in its natural state always dissociated? The reason is that dissociation requires a certain amount of energy to be supplied first. The minimum amount of energy required to initiate a reaction in this way is called activation energy.
For example, let’s assume the energy level before dissociation is 100 and the energy level after dissociation is 50. In this case, an activation energy of 30 must be supplied to raise the energy level to 130 before the reaction begins. As the reaction proceeds, the energy level drops to 50, and 80 units of energy are released in the process. Since hydrogen in its natural state cannot obtain sufficient activation energy, it does not dissociate easily. This can be likened to a student who must study to achieve their desired English test score.
Just as effort is required until the target score is reached, but once the goal is achieved, a stable state is attained.
However, because fuel cells contain a catalyst, the activation energy required for the hydrogen decomposition reaction is significantly reduced. A catalyst is a substance that accelerates or regulates the rate of a chemical reaction; it participates in the reaction process but remains unchanged before and after the reaction. In other words, a catalyst is neither a reactant nor a product. As we saw earlier, energy is released when hydrogen decomposes. Since the catalyst lowers the activation energy, even a portion of the released energy is sufficient to continue decomposing other hydrogen molecules in the vicinity. Therefore, the reaction proceeds continuously. Using the student analogy, the catalyst plays the role of significantly reducing the effort required to reach the target score.
In this way, hydrogen is split into electrons and hydrogen ions with the help of a catalyst. Hydrogen ions pass through the electrolyte, while electrons travel along the electrical circuit. The movement of electrons is the electric current, and this current performs various electrical tasks, such as lighting a bulb or powering a motor. The electrons and hydrogen ions, which have traveled along different paths, meet again at the anode, where oxygen combines with them to form water (H₂O). The resulting water is discharged as a liquid or water vapor.
To summarize, hydrogen and oxygen are supplied to the anode and cathode, respectively, and the electrons separated at the anode travel through the external circuit, providing electrical energy. Subsequently, at the cathode, hydrogen ions, electrons, and oxygen combine to form water. Using a human analogy, food represents hydrogen, breathing represents oxygen, the process of performing work represents the movement of electrons, and the process of discharging the produced water is similar to the body’s excretory functions.
Fuel cells offer the major advantage of being a highly eco-friendly energy source, and they are currently being commercialized not only in automobiles but also in power generation facilities and various industrial sectors. However, challenges that still need to be addressed include high manufacturing costs, the use of precious metals in catalysts, issues related to hydrogen production, storage, and transportation, and improving durability. Furthermore, if the water generated inside the fuel cell is not properly managed, reaction efficiency may decline, making control technologies crucial. To address these issues, the field of chemical engineering is developing new electrolytes and catalysts, while the field of mechanical engineering is actively conducting research to enhance the performance and stability of the entire fuel cell system, including hydrogen storage devices and product handling systems. If these technologies continue to advance, we can look forward to a cleaner urban environment alongside eco-friendly vehicles that emit nothing but water.
