How far are we from fuel cells?

The international energy community predicts that hydrogen energy will be widely used in this century, and fuel cells will become an important way to use hydrogen energy. Fuel cells are the fourth generation of power generation devices after hydropower, thermal power, and nuclear power. They are power devices that can replace internal combustion engines. Fuel cells are safe, efficient, pollution-free, widely applicable, and noise-free, and have become a hot spot for development in the world's energy field.

1 Basic principles

Ordinary batteries convert chemical energy inside the battery into electrical energy , while fuel cells convert the energy released by the fuel (hydrogen and oxygen) outside the battery into electrical energy output through chemical reactions. The fuel storage system outside the fuel cell is a mobile device that can easily replace and replenish the fuel.

The basic principle of a fuel cell is the reverse reaction of water electrolysis. It consists of a positive electrode, a negative electrode, and an electrolyte sandwiched between the positive and negative electrodes. When working, fuel (hydrogen) is supplied to the negative electrode, and an oxidant (air) is supplied to the positive electrode. A catalyst (such as platinum) is often used on the electrode to accelerate the electrochemical reaction. Hydrogen decomposes into positive ions H+ and electrons e at the negative electrode. The hydrogen ions enter the electrolyte, while the electrons move to the positive electrode along the external circuit. The load that uses electricity is connected to the external circuit. At the positive electrode, oxygen in the air and hydrogen ions in the electrolyte absorb the electrons that reach the positive electrode to form water.

2 Types and characteristics of fuel cells

2.1 Proton Exchange Membrane Fuel Cells (PEMFC)

The electrolyte of the battery is an ion exchange membrane, the surface of which is coated with a catalyst (such as platinum) that can accelerate the reaction, and hydrogen and oxygen are supplied on both sides. Since the only liquid in the PEM fuel cell is water, the corrosion problem is small, and the operating temperature is between 80℃ and 100℃, so the safety concerns are low; the disadvantage is that platinum as a catalyst is expensive. PEMFC is an ideal power source for light vehicles and home applications, and it can replace rechargeable batteries.

2.2 Alkaline Fuel Cells (AFC)

The design of alkaline fuel cells is similar to that of proton exchange membrane fuel cells, but the electrolyte is a stable potassium hydroxide matrix. The temperature required for operation is not high, the conversion efficiency is good, and a variety of catalysts can be used and they are cheap, such as silver and nickel. However, in the recent development of fuel cells in various countries, it has not become the main development target, the reason is that the electrolyte must be liquid and the fuel must be high-purity hydrogen. At present, this type of battery is too expensive for commercial applications, and it mainly serves space research, including providing power and drinking water for space shuttles.

2.3 Phosphoric Acid Fuel Cells ( PAFC )

It is named because the electrolyte it uses is 100% phosphoric acid. The operating temperature is about 150℃～220℃, and the waste heat can be recycled due to the high temperature. Its catalyst is platinum, so it also faces the problem of high platinum prices. So far, this fuel cell is mostly used in large-scale power generation units and has been commercially produced, but the high cost is the main reason why it has not been rapidly popularized.

2.4 Molten Carbonate Fuel Cells ( MCFC )

Its electrolyte is alkaline carbonate such as lithium carbonate or potassium carbonate. In terms of electrodes, both fuel electrodes and air electrodes use porous nickel with gas permeability. The operating temperature is about 600℃~700℃. Because the temperature is quite high, the carbonate that appears as a white solid at room temperature melts into a transparent liquid. This type of fuel cell does not require precious metals as catalysts. Because of the high operating temperature, waste heat can be recycled and reused, and its power generation efficiency is as high as 75%~80%, which is suitable for centralized power plants. It has been used in Japan and Italy.

2.5 Solid Oxide Fuel Cells (SOFC)

The electrolyte is zirconium oxide, which contains a small amount of calcium oxide and yttrium oxide, so it is highly stable and does not require a catalyst. Generally speaking, the operating temperature of this type of fuel cell is about 1000°C, and the waste heat can be recycled and reused. Solid oxide fuel cells have the greatest tolerance to sulfur pollution that all current fuel cells have. Due to the use of solid electrolytes, this type of battery is more stable than molten carbonate fuel cells. Its efficiency is about 60%, which can be used by the industry for power generation and heating, and it also has the potential to provide backup power for vehicles. The disadvantage is that the high-temperature resistant materials used to construct this type of battery are expensive.

2.6 Direct Methanol Fuel Cells (DMFC)

A direct methanol fuel cell is a variant of a proton exchange membrane fuel cell that uses methanol directly at the anode to convert it into carbon dioxide and hydrogen, which then reacts with oxygen as in a standard proton exchange membrane fuel cell. The cell operates at 120°C, slightly higher than a standard proton exchange membrane fuel cell, and has an efficiency of around 40%. The technology used is still in the development stage, but has been successfully shown to be a power source for mobile phones and laptops . The disadvantage is that more platinum catalyst is required than in a conventional proton exchange membrane fuel cell when methanol is converted to hydrogen and carbon dioxide at low temperatures.

2.7 Regenerative Fuel Cells — RFC

The concept of regenerative fuel cells is relatively new, but there are many research groups around the world working on it. This type of cell is a closed system that does not require external hydrogen generation, but instead sends water generated in the fuel cell back to a solar- powered electrolyzer to break it down into hydrogen and oxygen, which is then sent back to the fuel cell. Currently, there are still many issues to be resolved before the commercial development of this cell, such as cost, stability of solar energy utilization, etc. NASA is working on this type of cell.

2.8 Zinc-air Fuel Cells (ZAFC)

The chemical reaction of zinc and air in the electrolyte generates electricity. The biggest advantage of zinc-air fuel cells is their high energy. Compared with other fuel cells, zinc-air batteries can run longer for the same weight. In addition, the abundant zinc resources on the earth make the raw materials of zinc-air batteries very cheap. It can be used in electric vehicles, consumer electronics , and military fields, and has broad prospects. Currently, Metallic Power and Power Zinc are committed to the research and commercialization of zinc-air fuel cells.

2.9 Protonic Ceramic Fuel Cells (PCFC)

The mechanism of this new type of fuel cell is that the ceramic electrolyte material has a high proton conductivity at high temperature. Protonetics International Inc. is working on this battery.

3. Current status of fuel cell research and development and application

The development of fuel cell technology is extremely active around the world. Thousands of companies and institutions in more than 20 countries around the world have invested huge amounts of money in the research and commercialization of fuel cells. Currently, more than 2,500 fuel cell systems have been installed around the world, providing basic and backup power supply for hospitals, nurseries, hotels, office buildings, schools, airports and power plants.

The United States is the earliest country to study fuel cells and is in a leading position in this field. As early as the early 1960s, NASA began to study new power devices to solve the problem of excessive weight of ordinary batteries in the space shuttle. In the following decades, the Department of Energy (DOE), the Electric Power Research Institute (EPRI) and the Gas Research Institute (GRI) and other departments have invested a lot of manpower and financial resources in research and development. At present, alkaline batteries have been used by NASA for a long time; phosphoric acid battery technology is also quite mature and has been widely used commercially. A 2MW molten carbonate battery has been put into operation, and Westinghouse's 100kW solid oxide battery has also been installed in the Netherlands.

Japan started research on fuel cells more than 30 years ago, and has made remarkable achievements in recent years. The development focus is on three major types: phosphoric acid type, molten carbonate type, and solid oxide type. A phosphate power generation device with a capacity of 11MW has been put into operation by Tokyo Electric Power Company, with an efficiency of 43.6%. A molten carbonate type device with a 2MW level has been put into operation. In addition, many 100kW phosphoric acid type on-site power generation battery systems have been established for use in hotels and hospitals.

European countries developed fuel cells later than the United States and Japan. In the early years, the main interest was in alkaline batteries. With the development of fuel cell technology, its superior characteristics were gradually recognized by people, and European countries also accelerated the introduction and development of fuel cell technology. The Netherlands, Italy, Germany, Spain and other countries have completed the development of 10kW, 100kW, and 280kW carbonate batteries respectively, and Germany and Switzerland have developed 7kW and 10kW solid oxide batteries respectively; Italy put into operation a 1MW phosphoric acid battery device made in the United States in 1991.

Due to the increasing environmental problems such as oil shortage and automobile exhaust pollution, an important direction of fuel cell research and development is to provide power for automobiles. Almost all major automobile manufacturers are developing electric vehicles using fuel cells and have demonstration models. At present, Toyota and Honda have started electric vehicle rental business in Japan and the United States. There are some electric vehicles using rechargeable batteries, but the market for electric vehicles using fuel cells is still in the development stage. Experts predict that commercialization will be achieved around 2010. The competition for the development of micro fuel cells for portable devices (mobile phones, laptops, PDAs, etc.) is also fierce.

my country started the research and development of fuel cells not too late, but the development was slow. In the 1970s, we made some progress in the field of alkaline fuel cells to support the development of the aerospace industry, but in the 1980s, the research and development slowed down due to funding reasons. It was not until the late 1990s that a new round of research and development and commercialization attempts began.

The Dalian Institute of Chemical Physics, which is a representative institution in the research and development of fuel cells in China, has been engaged in the research of fuel cells for nearly 50 years. In the early years, it successfully developed a 500W alkaline fuel cell. In recent years, it has been committed to the research of proton membrane, molten carbonate and solid oxide batteries. From 2001 to 2003, the institute successfully demonstrated the use of 30kW proton membrane battery packs in small cars and large buses, and established Xinyuan Power Company to start the commercialization process of products. In the spring of 2003, the institute cooperated with Tsinghua University to apply 75kW proton membrane stacks to buses. In terms of direct methanol fuel cells, the Dalian Institute of Chemical Physics, Samsung Corporation of South Korea, and Nanfu Battery Company have established a cooperative laboratory. At present, the Institute of Inorganic Membranes of the University of Science and Technology of China has successfully developed a new medium-temperature solid oxide fuel cell. The application and technical status of the six types of fuel cells are shown in Table 1, and some domestic institutions engaged in fuel cell research and development are shown in Table 2 [5].

Table 1 Application and technical status of six types of fuel cells

4 Conclusion

Due to the high cost of fuel cells, they are still in the research and development and demonstration application stage. However, their safety, reliability, high efficiency, pollution-free energy storage and supply and their broad application prospects have led to a research and development competition in this field around the world.

The research and development in the field of fuel cells is a large system engineering, involving materials, components , research and development, manufacturing, integrated applications, distribution and end users. Therefore, the combination of "government, industry and research" is a significant feature of the development of this field and is also the only way. We are also fully aware of the difficulty of industrialization of fuel cells, a high-tech in the energy basic industry. This process will go through three stages: the achievement stage focusing on technical level; the product stage focusing on practical application; and the commodity stage focusing on sales price and production cost.

Table 2 Domestic companies and institutions engaged in fuel cell research and development