Types of Hydrogen Fuel Cells
There are many common types of hydrogen fuel cells, which are usually differentiated according to the type of electrolyte used in the cell: Alkaline Auel Cell (AFC), Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC), and Proton Exchange Membrane Fuel Cell (PEMFC).
The fuel cells of different types have different operating temperatures.
Different types of fuel cells have different operating temperatures, so they can also be classified into different categories according to their operating temperatures.
AFC and PEMFC are low temperature fuel cells; PAFC is a medium temperature fuel cell (operating temperature between 200 and 750°C); SOFC is a high temperature fuel cell (operating temperature over 750°C).
1.Alkaline Fuel Cell，AFC
When the electrolyte of the hydrogen fuel cell is alkaline, the permeability of the fuel will be lower and the current density of the electrolyte will be higher. The electrolyte is usually KOH solution. AFC generally uses the metal element platinum (Pt) as a catalyst. For alloy catalysts, the stability and reactivity of catalysts are affected to varying degrees by the differences in carrier type, dispersion degree, loading capacity and other factors. The catalyst performance can be better improved by doping modifiers. Generally, the positive and negative reactions of alkaline fuel cells are as follows:
Anodes : 2H2+4OH-=4H2O+4e- Cathode : O2+2H2O+4e-=4OH-
Generally, AFC has relatively good working performance in an environment of 80°C, and has the characteristics of very fast start-up response, but its energy density is only one tenth of that of PEMFC. The AFC electrolyte is alkaline, so in actual work, the oxidant must use pure oxygen. If air is used as the oxidant, the actual service life will be greatly reduced due to the CO in the air, which will greatly increase the cost of commercial application. AFC is currently only used in the military field, and other application fields are relatively rare, and its commercial application rate is not high.
2.Phosphoric Acid Fuel Cell，PAFC
The PAFC electrolyte and catalyst are concentrated phosphoric acid and platinum respectively, and usually work at around 200°C, which belongs to medium temperature fuel cells. PAFC can not only use hydrogen as fuel, but also directly use cheap fuels such as natural gas, methanol, and natural gas. Compared with alkaline fuel cells, its biggest advantage is that it does not require special equipment for CO treatment, so the reaction gas can directly use air. PAFC uses reformed gas as fuel, and its application in fixed power stations and other related fields has great advantages and potential.
The principle of the PAFC reaction is: fuel gas is introduced into the reformer, and the fuel undergoes a chemical reaction [CxHy + XH2O→XCO+(X+Y/2)H2] at a reaction temperature of 800°C and is converted into a mixture of H, CO and water vapor . At the same time, HO and CO are catalyzed by the catalyst to generate HO and CO in the shift reactor. Finally, the fuel is processed and enters the fuel stack at the negative electrode, while the O in the air undergoes a chemical reaction at the positive electrode (air electrode) of the fuel stack, and is catalyzed by the catalyst to generate electricity and heat. The electrode reaction is shown in the figure below. The reaction process is:
Anodes : 2H2+4OH-=4H2O+4e- Cathode : O2+2H2O+4e-=4OH- Chemical Reaction : 1/2O2+H2→H2O
The initial research and development of PAFC was to control the power balance of the power grid. At the end of the 20th century, its focus was on providing combined heat and power services to citizens’ residences, hospitals, shopping malls, hotels, etc. In addition, PAFC can also be used for vehicle power supply and portable power supply.
3.Molten Carbonate Fuel Cell，MCFC
Carbonate mixtures of metals Li, K, Na, and Cs are usually used as the electrolyte of MCFC. Diaphragm material (porous ceramic electrolyte) LiAiO porous ceramic cathode (nickel oxide), porous metal anode (porous nickel), metal plate and other structures together with the electrolyte form the basic structure of MCFC. MCFC has a higher reaction speed due to its higher working temperature (650-700°C). Moreover, liquid electrolyte is used, and the operation is simple. What is important is that the fuel cell requires relatively low purity of the fuel and does not require noble metal catalysts, which greatly reduces the cost. MCFC is generally used in regional power supply.
The material of the electrolyte separator is LiAiO2, the lithium-added nickel oxide is used as the positive electrode, and the porous nickel is used as the negative electrode. This material will undergo a phase transition at 650°C to produce CO2-3. And combined with H2 to produce H2O, CO2 and electrons. The chemical reaction is as follows:
Anodes : CO2-3+H2=H2O+CO2+2e- Cathode : CO2+O2+4e-=2CO2-3 Chemical Reaction : 2H2+O2=2H2O+Electricity Energy
It can be known from the chemical reaction of MCFC that the conductive ion is CO2-3, and CO2 is a reactant at the cathode and a product at the anode. The stable three-phase interface of its MCFC is established by the pressure balance of the capillary inside the porous electrode. When MCFC works, CO2 is constantly circulating. The CO2 produced by the anode is returned to the cathode, thus ensuring the normal and continuous operation of the battery. The working principle diagram of MCFC is shown in the figure. MCFC can be used as an independent power generation system, but the fuel cell cannot convert all the energy of the fuel at present, and there are certain factors such as loss. Therefore, the efficiency of MCFC as an independent power generation system is low. Under normal circumstances, MCFC can be combined with gas turbines to generate electricity, and its power generation efficiency and fuel utilization can be increased by about 60%, and the cost can be reduced.
4.Solid Oxide Fuel Cell，SOFC
Among several common fuel cells, solid oxide fuel cell (SOFC) has the highest theoretical energy density. The electrolyte of SOFC is a solid ceramic, and the single cell consists of two porous electrodes and a tight electrolyte layer sandwiched between them. The operating temperature of SOFC is very high, and the maximum operating temperature can reach 800-1000°C, so its electrolyte has the function of transferring O2- and separating oxidant and fuel. Oxygen molecules undergo a reduction reaction at the cathode to produce O2-. Under the influence of the potential difference and poor oxygen concentration on both sides of the diaphragm, O2- will directional transition to the anode side for oxidation reaction with the fuel.
When SOFC is working, the oxygen on the cathode side is reduced to oxygen ions due to gaining electrons. The oxygen ions are transported to the anode side through the oxygen vacancies in the electrolyte layer due to the partial pressure and pressure difference, and oxidize with the fuel to lose electrons. Its working principle is shown in the figure.
Anodes : H2+O2-=H2O+2e- Cathode : 1/2O2+2e-=O2- Chemical Reaction : H2+1/2O2=H2O
Due to the limited actual power of the SOFC single battery, it can only generate a voltage of about 1V. Therefore, it is necessary to form a battery pack by connecting several single batteries in series, parallel, and hybrid to greatly increase the power to meet the power demand of practical applications. may have practical application conditions. SOFC is usually used for small and medium-sized stationary combined heat and power generation. The continuous progress and innovation of thermoelectric materials have also greatly improved its economic benefits. Because the operating temperature of SOFC is usually 650-1000°C, too fast heating rate will cause damage to the battery components. Therefore, there are still key technical problems such as slow start-up time (65-200min).
5.Proton Exchange Membrane Fuel Cell，PEMFC
PEMFC uses a polymer membrane as an electrolyte, also known as a polymer electrolyte fuel cell, and is composed of a cathode, an anode and an external circuit. At present, the application of PEMFC in the field of electric vehicles and material handling is the most potential. Inside the fuel cell, protons pass through the exchange membrane from the anode to the cathode, thereby forming a loop with the electrons in the external circuit to supply power to the external load. Compared with other batteries, the operating temperature of PEMFC is lower (generally lower than 100°C), and the output power can be flexibly adjusted according to actual work requirements. At the same time, the fuel cell emissions are water and water vapor, which can achieve zero pollution; the energy conversion efficiency is as high as 60% to 70%; no vibration and noise will be generated during the working process. In addition, PEMFC also has the advantages of fast start-up speed, high specific power, simple structure, and convenient operation.
When the PEMFC is working, the anode catalyst promotes the ionization of the fuel (H2) into hydrogen ions (H+), and then the hydrogen ions (H+) pass through the proton exchange membrane to the cathode and release electrons (e–), doing work on the external circuit. The catalyst reduces the oxidant, and H+and e–produce water. The working principle diagram of PEMFC is shown in the figure.
Anodes : 4H++4e-=2H2 Cathode : O2+4H++4e-=2H2O Chemical Reaction : 2H2+O2=2H2O+Heat
PEMFC is already being used in buses, light vehicles and forklifts. In recent years, fuel cell vehicles have made significant breakthroughs and advances in performance, with range and top speed comparable to that of conventional petrol vehicles.
How To Choose
If you are new to this field. I would recommend PEMFC. the product is mature, easy to get started and simple to use. Easy to do basic research. We also produce and customise smaller PEMFC, so please contact us if you are interested.
