Fuel cell technology depends on the type of fuel cell electrolyte, which can be classified into six main fuel cells: proton exchange membrane (PEM) or polymer exchange membrane fuel cells (PEMFCs), alkaline fuel cells (AF CS), phosphoric acid fuel cells (PAFCS), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs) and direct methanol fuel cells (DMFCs).
The advantages of fuel cells are as follows:
(1) Clean and safe power generation device, harmful gases SO2 and No. And low noise emission; The scale and installation location are flexible, the floor area of fuel cell power station is small, and the construction cycle is short.
(2) The energy conversion efficiency is high, which directly converts the chemical energy of fuel into electric energy without combustion process.
At present, the fuel electric energy conversion efficiency of fuel cell system is 45% ~ 60%, while the efficiency of thermal power generation and nuclear power is 30% ~ 40%.
(3) For multi fuel system, the most suitable fuel can be selected according to the use and conditions of various fuel cells.
(4) With fast load response and high operation quality, the fuel cell can change from the lowest power to the rated power in a few seconds.
A single fuel cell stack cannot generate electricity and be used in automobiles. It must form a fuel cell power generation system with fuel supply and circulation system, oxidant supply system, water / heat management system and a control system that can coordinate the above systems, which is referred to as fuel cell system for short.
The fuel cell system is mainly composed of fuel cell stack, auxiliary devices and key equipment, including:
(1) Fuel and fuel storage (including reformer for hydrocarbon conversion).
(2) Oxidant and oxidant storage.
(3) Supply pipeline system and regulation system (including gas transfer pump, heat exchanger, gas separation and purification device).
(4) Water and heat management system. The working principle of different fuel cells is introduced below, and other auxiliary systems are discussed in the following chapters.
- Proton exchange membrane fuel cell
The key materials and components of proton exchange membrane fuel cell are: ① electrocatalyst; ② Electrodes (cathode and anode);
③ Proton exchange membrane; ④ Bipolar plate.
Its working principle is as follows:
(1) Hydrogen reaches the anode through a pipe or gas guide plate.
(2) Under the action of anode catalyst, one hydrogen molecule dissociates into two hydrogen protons and releases two electrons. The anode reaction is
2H2 → 4H+ + 4e-
(3) At the other end of the battery, oxygen (or air) reaches the cathode through the pipeline or air guide plate. Under the action of cathode catalyst, oxygen molecules and hydrogen ions react with electrons reaching the cathode through the external circuit to generate water. The cathode reaction is
O2 + 4H+ +4e- → 2H2O
The total chemical reaction is
2H2 + O2 → 2H2O
Electrons form direct current in the external circuit. Therefore, as long as hydrogen and oxygen are continuously supplied to the anode and cathode of the fuel cell, electric energy can be continuously output to the load of the external circuit.
The working temperature of proton exchange membrane fuel cell is about 80 ℃. At such low temperatures, electrochemical reactions can normally proceed slowly, usually catalyzed by a thin layer of platinum on each electrode.
Each battery can produce about 0.7V, which is enough for a lighting bulb. Driving a car requires about 300V power. In order to obtain higher voltage, multiple single cells can be connected to form what people call fuel cell memory.
Proton exchange membrane fuel cell uses solid polymer membrane as electrolyte. The polymer membrane is a perfluorosulfonic acid membrane, also known as Nafion (DuPont), which is acidic, so the migrated ions are hydrogen ions H + or protons. Proton exchange membrane fuel cell is fueled by pure hydrogen and oxygen or air as oxidant.
The polymer electrolyte membrane is covered with carbon based catalyst, which is in direct contact with the diffusion layer and electrolyte in order to achieve the maximum interaction surface. The catalyst forms an electrode on which a diffusion layer is directly formed. The combination of electrolyte, catalyst layer and gas diffusion layer is called diaphragm electrode assembly.
The catalyst in proton exchange membrane fuel cell is the key focus. In early practice, considerable platinum loading was required for the specific operation of fuel cells. Great progress has been made in catalyst technology, reducing the platinum loading from 28 mg / cm2 to 0.2 mg / cm2. Due to the low operating temperature of fuel cells and the acidic nature of electrolytes, noble metals are required for the applied catalyst layer. Because the catalytic reduction of oxygen is more difficult than the catalytic oxidation of hydrogen, the cathode is the most critical electrode.
In proton exchange membrane fuel cell, another key problem is water management. For the specific operation of fuel cells, the polymer membrane must be kept wet. In fact, the conductivity of ions in the polymer membrane needs humidity. If the polymer membrane is too dry, there are not enough acid ions to carry protons; If the polymer film is too wet (stained), the pores of the diffusion layer will be blocked, so that the reaction gas cannot expand and touch the catalyst.
Water is generated in the cathode of proton exchange membrane fuel cell. By keeping the fuel cell at a certain temperature and flowing enough to evaporate the water, it can be migrated and removed from the fuel cell in the form of water vapor. However, due to the narrow error range, this method is difficult. Some fuel cell stacks operate in a state of far excess air, which should normally dry the fuel cell while using an external humidifier to supply water from the anode.
The last key problem in proton exchange membrane fuel cell is its poisoning. Platinum catalysts are highly active and thus provide excellent performance. The high activity of the catalyst is restricted by its high affinity for carbon monoxide and sulfur products compared with oxygen. The toxic effect strongly constrains the catalyst and hinders the expansion of hydrogen or oxygen into it. Thus, the electrode reaction cannot occur at the toxic site, and the performance of the fuel cell decreases. If hydrogen is supplied by the reformer, the gas stream will contain some carbon monoxide; Similarly, if the inhaled air comes from the atmosphere in the polluted city, carbon monoxide can also enter the fuel cell from the air stream. The poisoning caused by carbon monoxide is reversible, but it increases the cost and each fuel cell needs to be treated separately.
In 1960, the first proton exchange membrane fuel cell was successfully developed and applied to the manned space program of the United States. At present, most of the technologies to study the application of fuel cells in automobiles come from manufacturers such as Ballard. The product operates at 60 ℃ ~ 100 ℃, and can provide density of 0.35w/cm2 ~ 0.6w/cm2. In its applications supporting electric vehicles and hybrid electric vehicles, proton exchange membrane fuel cells have some certain advantages and can operate at low temperature. Therefore, for electric vehicles and hybrid electric vehicles, fast start performance can be expected; Secondly, among all available fuel cell types, its power density is the highest. Obviously, the higher the power density, the smaller the volume of fuel cells to be installed to meet the power demand; Thirdly, the solid electrolyte does not change, migrate or gasify from the fuel cell; Finally, in fuel cells, since the only liquid is water, the possibility of any corrosion has been essentially limited. However, it also has some disadvantages, such as the need for expensive precious metals, expensive polymer membranes, and easy to poison catalysts and polymer membranes.
- Alkaline fuel cell
Alkaline fuel cell uses ancient water potassium hydroxide (KOH) solvent as electrolyte to conduct ions between electrodes. Potassium hydroxide is alkaline. Because the electrolyte is alkaline, the ion conduction mechanism is different from that of proton exchange membrane fuel cell. The ions migrated by the alkaline electrolyte are hydrogen and oxygen ions (OH -), which have an impact on several other aspects of the fuel cell.
The semi reaction formula is as follows:
Anode: 2h2 + 4OH – — 4H2O + 4e-
Cathode: O2 + 4E – + 2H2O – 4OH-
Unlike acid fuel cells, water is generated at the hydrogen electrode. In addition, at the cathode, water is required due to the reduction of oxygen. The problem of water management is often decomposed according to the waterproof of electrode and the need to maintain water content in electrolyte. The cathodic reaction consumes water from the electrolyte, and the anodic reaction discharges its aquatic products. Excess water (2mol per reaction) is gasified outside the fuel cell stack.
Alkaline fuel cells can operate at a wide temperature (80 ℃ ~ 230 ℃) and pressure (2.2atm ~ 45atm)
Within the scope. High temperature alkaline fuel cells can also use high concentration electrolyte, which causes the ion migration mechanism to change from aqueous solvent to molten salt state.
Due to the fast kinetic effect provided by hydrogen oxygen electrolyte, alkaline fuel cell can obtain high efficiency. In particular, the reaction of oxygen (O2 → Oh -) is much easier than the reduction reaction of oxygen in acid fuel cell. Therefore, the activity loss is very low. The rapid kinetic effect in alkaline fuel cells makes silver or nickel can be used as a catalyst instead of platinum, which significantly reduces the cost of alkaline fuel cell stacks.
Through the complete circulation of electrolyte, the dynamic characteristics of alkaline fuel cell have been further improved. When the electrolyte circulates, the fuel cell is called “dynamic electrolyte fuel cell”.
The advantages of such a structure are: since the electrolyte is used as the cooling medium, it is easy to heat management; The more uniform concentration of electrolyte solves the problem of electrolyte concentration distribution around the cathode; The possibility of using electrolyte for water management is provided; If the electrolyte has been excessively polluted by carbon dioxide, it is possible to replace the electrolyte; Finally, when the fuel cell stack is closed and it has the potential to significantly prolong the service life, it provides the possibility of removing the electrolyte from the fuel cell.
The recycling of electrolyte raises some problems. The most prominent problem is that it increases the risk of leakage: potassium hydroxide is highly corrosive and has the possibility of natural leakage, even through sealing. In addition, the structure of circulating pump and heat exchanger and the final gasifier are more complex. Another problem is that if the electrolyte is circulated too vigorously or the unit battery is not well insulated, there will be a risk of internal electrolyte short circuit between the two unit batteries.
The biggest problem of alkaline fuel cell is the poisoning of carbon dioxide. Carbon dioxide and reaction gas enter the cell together. Alkaline electrolyte has significant chemical force on carbon dioxide. They work together to form carbonate ions (CO2 -). The product is K2CO3. Because the conductivity of K2CO3 aqueous solution is much lower than that of KOH solution, it will increase the ohmic polarization and degrade the performance of the cell. Moreover, the vapor pressure of K2CO3 aqueous solution is high, and the formation of K2CO3 will lead to water loss and salt crystallization of the diaphragm. In serious cases, the diaphragm will lose its gas barrier performance, and hydrogen and oxygen cross each other, resulting in battery failure. That is, the deposition of carbonic acid and blocking the electrode will also be a possible risk, but this problem can be treated through the circulation of electrolyte. Using a carbon dioxide degasser is a solution that adds cost and complexity by removing carbon dioxide gas from the air stream.
The advantage of alkaline fuel cell is that it needs cheap catalyst, electrolyte, high efficiency and low temperature operation. However, it also has some disadvantages, for example, due to the corrosive electrolyte, water is generated on its electrode, and the continuous working time of the battery is damaged due to the poisoning of carbon dioxide.
- Phosphoric acid fuel cell
Like alkaline fuel cells, phosphoric acid fuel cells rely on acidic electrolyte to conduct hydrogen ions. The reaction of anode and cathode is the same as that of alkaline fuel cell. Phosphoric acid (H3PO4) is a kind of viscous liquid, which is stored in the fuel cell through the capillary in the porous silicon carbide matrix.
Phosphoric acid fuel cell is the first fuel cell technology to become a commodity. Many hospitals, hotels and military bases use phosphoric acid fuel cells to cover part or all of the required power and heat supply. Mostly due to its temperature problem, this technology is rarely used in vehicles.
The temperature of phosphoric acid electrolyte must be kept above 42 ℃ (its freezing point). Frozen and thawed acids will be difficult to excite the fuel cell stack. Keeping the fuel cell stack above this temperature requires additional equipment, which increases cost, complexity, weight and volume. Most problems are secondary for fixed applications, but incompatible for vehicle applications. Another problem due to high operating temperature (above 150 ℃) is the energy loss associated with the heating up of fuel cell stack. Whenever the fuel cell is started, some energy (i.e. fuel) must be consumed to heat the fuel cell until its operating temperature, and when the fuel cell is shut down, the corresponding heat (i.e. energy) is consumed. This loss is significant for the short-term operation corresponding to the normal situation of driving in urban areas. However, in the case of public transport, such as buses, this problem seems to be secondary.
The advantages of phosphoric acid fuel cell are its application of cheap electrolyte, low-temperature operation and reasonable start-up time. Its disadvantages are the use of expensive catalyst (Platinum), the corrosiveness of acidic electrolyte, the poisoning of carbon dioxide and low efficiency.
- Molten carbonate fuel cell
Molten carbonate fuel cell is a high-temperature fuel cell (500 ℃ ~ 800 ℃), which relies on molten carbonate (usually lithium potassium carbonate or lithium sodium carbonate) to conduct ions. The ion to be conducted is carbonate ion (CO32 -). The ion conduction mechanism is similar to that of molten salt in phosphoric acid fuel cell or high concentration alkaline fuel cell.
The electrode reaction of molten carbonate fuel cell is different from other fuel cells, i.e
H2 + CO32- → H2O + CO2 + 2e-
O2 + CO2 + 2e- → CO32-
The main difference is that carbon dioxide must be supplied at the cathode. Since carbon dioxide can be recovered from the anode, there is no need for an external carbon dioxide supply source. Molten carbonate fuel cells never use pure hydrogen, but use hydrocarbons. In fact, the main advantage of high temperature fuel cell is its ability to process hydrocarbon fuel almost directly. This is because the high operating temperature enables the decomposition of hydrocarbons at the electrode to produce hydrogen. Therefore, this should be a great advantage for cars, because hydrocarbon fuels are effectively used today. In addition, the high operating temperature enhances the kinetics to the extent that cheap catalysts can be used.
However, molten carbonate fuel cell has many problems due to the nature of its electrolyte and required operating temperature. Carbonate is an alkaline substance, which is highly corrosive especially at high temperature. This is not only unsafe, but also a corrosion problem to the electrode. It is obviously unsafe to install a large equipment with a temperature of 500 ℃ ~ 800 ℃ under the vehicle shell. On the other hand, it is true that the temperature in the internal combustion engine reaches more than 1000 ℃, which is constrained by the gas itself, and most of the engine remains slightly cold (about 100 ℃) with the help of the cooling system. Fuel consumption associated with fuel cell temperature rise is also a problem, which is exacerbated by the high operating temperature and the latent heat necessary for molten electrolyte. These problems may restrict the application of molten carbonate fuel cells in fixed or constant power demand, such as marine applications.
The main advantages of molten carbonate fuel cell are filling hydrocarbon fuel, low price catalyst, perfect efficiency due to rapid kinetic effect and low sensitivity to poisoning. Its main disadvantages are slow start-up, reduced material selectivity due to high temperature, complexity of fuel cell system due to CO2 cycle, electrode corrosion and slow power response.
- Solid chloride fuel cell
Solid oxide fuel cells conduct ions in ceramic membranes at high temperatures (1000 ℃ ~ 1200C). Generally, the Amoy ceramic material is stabilized double oxide (YSZ), which will conduct oxygen ions (02 -), while other ceramic materials conduct hydrogen ions. Its conduction mechanism is similar to the mechanism observed in semiconductors. It is often called solid-state device, and the name of fuel cell is derived from this similarity. The semi reaction formula is as follows:
H2 + O2 → H2O + 2e-
O2 +2e- → O2-
Here, water is also generated at the fuel electrode. The biggest advantage of solid oxide fuel cell is that its static electrolyte has no migration effect except in auxiliary equipment. The very high operating temperature makes it possible to use hydrocarbon fuels like molten carbonate fuel cells. At the same time, it should be noted that the solid oxide fuel cell will not be poisoned by carbon monoxide, and its treatment of carbon monoxide is roughly as effective as that of hydrogen, so the anodic reaction is
CO + O2 → CO2 + 2e-
Solid oxide fuel cells also benefit from reducing activity loss due to their high operating temperature. This loss is mainly ohmic loss. Solid oxide fuel cells can be divided into two types: planar or tubular. Planar solid oxide fuel cell is similar to other fuel cell technologies, which is a bipolar combination.
- Direct methanol fuel cell
Instead of hydrogen, methanol can be directly used as fuel of fuel cell, which is commonly referred to as direct methanol fuel cell. For direct methanol fuel cells used in vehicles, there are some definite motives. First, methanol is a liquid fuel, which is easy to store, distribute and sell in vehicle applications. Therefore, the current infrastructure of fuel supply can be applied without too much reinvestment; Secondly, methanol is the single organic fuel, which can be produced on a large scale from relatively abundant fossil fuels, i.e. coal and natural gas, most economically and effectively; In addition, methanol can be produced from agricultural products, such as sugarcane.
In direct methanol fuel cell, platinum or platinum alloy is used as electrocatalyst for both anode and cathode. The electrolyte can be trifluoromethane sulfonic acid or proton exchange membrane. The chemical reaction formula is as follows:
CH3OH + H2O → CO2 + 6H+ +6e-
O2 +6H+ + 6e- → 3H2O
CH3OH + O2 → CO2 + 2H2O
Among the aforementioned fuel cells, direct methanol fuel cell is a relatively immature technology. In terms of the current technical state of the fuel cell, it generally operates at 50 ℃ ~ 100 ℃. Compared with direct hydrogen fuel cells, direct methanol fuel cells have low power density, slow power response and low efficiency.