Basic knowledge of fuel cell
Fuel cell is a power generation device that directly converts the chemical energy in fuel and oxidant into electrical energy. It looks like a battery with positive and negative poles and electrolyte from the outside, but in fact it cannot store electricity but is two “power plants”. It is the fourth power generation technology after hydropower, thermal power and atomic power. A car that uses fuel cell technology is called a fuel cell car. Because it needs a drive motor to drive the vehicle, a fuel cell car is a type of electric car.
(1) Working principle
The unit fuel cell is composed of a positive electrode, a negative electrode, an electrolyte, a diaphragm, and accessories. Its working principle is equivalent to the principle of water combination. The fuel is oxidized at the negative electrode (fuel electrode), and the oxidant is reduced at the positive electrode [air electrode (oxygen electrode)]. If gaseous fuel is continuously supplied on the negative electrode and oxygen (or air) is continuously supplied on the positive electrode, an electrochemical reaction can occur continuously on the electrode and an electromotive force can be generated. If a load is connected, a current will be generated. The fuel cell is different from other battery generators. Its fuel and oxidant are not stored in the battery, but stored in a storage tank outside the battery. It needs to continuously input fuel and oxidant into the battery during operation, and at the same time The reaction product is discharged.
The function of the fuel electrode of the fuel cell is to provide a common interface between fuel and electrolyte, and to catalyze the oxidation reaction of the fuel. The electrons generated in the reaction are transmitted to the external circuit or first to the collector plate and then to the external circuit. The role of the fuel cell oxygen electrode is to provide a common interface for oxygen and electrolyte, catalyze the reduction reaction of oxygen, and transfer electrons from the external circuit to the reaction site of the oxygen electrode. Since the reaction on the electrode is a multiphase interface reaction, the electrode generally adopts porous materials (such as nickel oxide electrode of solid oxide fuel cell and molten carbonate fuel cell, platinum electrode of phosphoric acid fuel cell and proton exchange membrane fuel cell, etc.) And coated with precious metal platinum as a catalyst. The role of the electrolyte is to transport the ions produced by the fuel electrode and the oxygen electrode in the electric mode reaction, and to prevent the electrode from passing electrons indirectly. The function of the diaphragm is to conduct ions (H+), hinder the direct transfer of electrons between the electrodes and separate the oxidant and the reducing agent. The diaphragm must be resistant to electrolyte corrosion and insulation, and have good wettability.
The output voltage of the fuel cell is the potential difference between the positive and negative electrodes. When the external circuit current I=0, it is called the open circuit voltage; when I ≠ 0, it is called the terminal voltage. The phenomenon that the terminal voltage is lower than the open circuit voltage is called polarization. The loss of the positive electrode potential when the battery outputs current is called the positive polarization. Similarly, the loss of the negative electrode potential is called the negative polarization. The polarization of a battery includes three parts: positive polarization, negative polarization, and Euro mother polarization.
(2) Features
Fuel cells are very complex, involving related theories of chemical thermodynamics, electrochemistry, electrocatalysis, materials science, power systems, and automatic control. They have the advantages of high power generation efficiency and less environmental pollution. In general, fuel cells have the following characteristics.
①High energy conversion efficiency. It directly converts the chemical energy of the fuel into electrical energy without going through the combustion process. At present, the fuel-electric energy conversion efficiency of the fuel cell system is 45% to 60%, while the efficiency of thermal power and nuclear power is 30% to 40%.
②Small environmental pollution. The harmful gases SOx, NOx and noise emissions are very low, CO2 emissions are greatly reduced due to the high energy conversion efficiency, and there is no mechanical vibration.
③The fuel has a wide range of applications.
④Easy to use. The installation location is flexible, the fuel cell power station occupies a small area, and the construction period is short. The power of the power station can be assembled from the battery stack as needed, which is very convenient.
⑤The load response is fast and the running quality is high. The fuel cell can be converted from the lowest power to the rated power within a few seconds, which is very suitable as a vehicle power.
⑥Greater than energy. For example, the specific energy of a hydrogen fuel cell reaches 700W·h/kg.
⑦Long life. For example, the life of a hydrogen fuel cell can reach 10 years.
(3) Type
There are many types of fuel cells. According to the different ways in which the battery obtains the reactive fuel, they are divided into three types: direct, indirect and regenerative.
① Direct type. Its fuel (such as hydrogen and methanol) directly interacts with the oxidant.
Direct fuel cells can be divided into three categories: high, medium, and low according to their operating temperature. The operating temperature is above 750°C for high temperature fuel cell; the temperature between 200°C and 750°C is for medium temperature fuel cell; the temperature below 200°C is for low temperature fuel cell. There are also four categories according to temperature, 25~100℃, 100~300℃, 500~1000℃, and above 1000℃.
②Indirect type. The fuel is not direct hydrogen or methanol, but a certain method of converting a hydrogen-rich compound into hydrogen before supplying the battery.
③ Regenerative, in which the water produced by the fuel cell reaction is decomposed into hydrogen and oxygen through a certain method, and then the reaction is restarted.
According to the different electrolyte types, fuel cells can be divided into alkaline fuel cells (AFC), solid polymer fuel cell fuel cells (ESPFC, also known as “proton exchange membrane fuel cell” PEMFC), phosphate fuel cells (PAFC), melting Carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), etc.
Proton exchange membrane fuel cells have developed rapidly due to their low operating temperature, high power density, fast start-up, long service life, and simple structure. It is predicted that the proton exchange membrane fuel cell may be the first fuel cell to be commercialized after the phosphate fuel cell.
(4) Proton exchange membrane fuel cell
Proton exchange membrane fuel cell refers to a fuel cell that uses proton exchange as an electrolyte and separator. The working temperature of the proton exchange membrane fuel cell is lower than 100°C, which is an ideal power source for electric vehicles.
The proton exchange membrane fuel cell is the first fuel cell used in space flight tests. The early development of proton exchange membrane fuel cells has always been plagued by the high prices of necessary structural materials and high-content platinum catalysts, and the research is difficult. Later, led by Canada’s Ballard, Chrysler, Ford, General Motors, Honda, Toyota, Nissan, Volkswagen, and Volkswagen all invested heavily in the research of this type of fuel cell.
① Composition. The proton exchange membrane fuel cell is composed of fuel electrode (negative electrode), air electrode (oxygen electrode, positive electrode), electrolyte membrane (proton exchange membrane) and separator. The fuel is hydrogen and the oxidant is oxygen. The role of the proton exchange membrane is twofold: as an electrolyte, it provides a channel for hydrogen ions; as an isolation membrane, it separates the two-polar reaction gas. Dehydration of the proton membrane will make it more difficult for hydrogen ions to form hydrates, and the resistance of the fuel cell will increase; too much water will flood the electrodes. Both of these conditions will lead to a decrease in battery performance, so optimizing the proton transport performance of the proton exchange membrane and proper moisture management are the keys to ensuring battery performance.
②Working principle. Hydrogen passes through a pipe or a gas guide plate to reach the fuel electrode. Under the catalysis of the catalyst, hydrogen molecules dissociate into positively charged hydrogen ions (H+) and release negatively charged electrons (e﹣). H+ passes through the electrolyte to the oxygen electrode in the form of hydrate H3O+, e– reaches the positive electrode through the external circuit, and e– forms a current in the external circuit.
The oxygen reaches the oxygen electrode through the pipe or the gas guide plate. Under the catalytic action of the catalyst, the oxygen reacts with H+ and e– to produce water. In the proton exchange membrane fuel cell, the solid acid electrolyte is saturated with water and contains free H+, so it can complete the task of transferring H+ from the fuel electrode to the oxygen electrode, but e– cannot pass through the electrolyte membrane. H+ is also called proton, hence the name “Polymer Proton Exchange Membrane” (PEM). Hydrogen fuel flows into the bipolar plate near the fuel electrode side, and oxygen flows into the bipolar plate flow channel near the oxygen electrode side.
At the fuel electrode, hydrogen molecules first contact the Pt surface of the catalyst on the electrode surface, and are split and bonded to the Pt surface to form a weak H-P t bond. After the hydrogen molecules split, an oxidation reaction takes place. Each hydrogen atom releases its e–, which moves along the external circuit and reaches the negative electrode, forming a current in the external circuit. The remaining H+ adheres to the water molecules on the surface of the membrane to form hydronium ions H3O+. These hydronium ions leave the Pt catalyst and travel through the membrane material to reach the oxygen electrode. The Pt catalyst is free again and can “receive” the next batch of hydrogen. molecular.
At the oxygen electrode, the oxygen molecules entering the fuel cell also first contact the catalyst Pt on the electrode surface. The oxygen molecules are split and bonded to the Pt surface to form a weak O-Pt bond, allowing the reduction reaction to occur. Then each oxygen atom leaves the catalyst Pt and combines with the two e﹣ from the external circuit and the two H+ passing through the exchange membrane to form a water molecule. At this point, the redox reaction is complete, and the catalyst on the oxygen electrode is free again, waiting for the arrival of the next batch of oxygen molecules.
Hydrogen and oxygen undergo two “half reactions” at the same time in the fuel cell: one is the oxidation reaction at the fuel electrode (losing e﹣); the other is the reduction reaction at the oxygen electrode (getting e﹣). These two half reactions constitute a total redox reaction, and the reaction product is water.
The reaction process of the fuel electrode releases e﹣ and produces H+, and at the same time releases energy; and in the process of the oxygen electrode reaction, oxygen is with the fuel electrode. And H+ from the electrolyte forms water. To make these two processes happen continuously, it is necessary to make the e﹣ generated by the fuel electrode reach the negative electrode through an external circuit, and at the same time, H+ must pass through the electrolyte membrane to reach the oxygen electrode.The battery cell voltage is only about 0.7V. In order to obtain a sufficiently high working voltage, multiple fuel cell cells need to be connected in series to form a fuel cell stack.
(5) Fuel cell system
The fuel cell system is mainly composed of battery stack, hydrogen and oxygen supply device, humidifier and deionized water supply device, cooling device, exhaust gas and product discharge device. Among them, the hydrogen is provided by a high-pressure gas cylinder, and the oxygen (air) is provided by a blower. After the hydrogen and oxygen are decompressed by a pressure reducing valve and then humidified by a humidifier, they enter the fuel electrode and oxygen electrode of the battery stack for reactive power generation. The reactant is discharged with the tail gas, and the water is collected and discharged. The battery temperature is adjusted by the circulating water volume.
