Before preparing for a long trip, we always don't forget to check whether we have a credit card or wallet, and of course, spare batteries and chargers for mobile phones or laptops. With people's dependence on mobile phones and laptops, their importance is becoming more and more obvious. The crux of the problem is that the working time of the battery is limited. At present, lithium-ion batteries used in portable electronic products can no longer meet the needs of long-term operation. An ordinary mobile phone lithium battery can only be used for a few days, and the laptop battery can only be used for a few hours. With the increasing popularity of wireless technology and audio and video functions, the requirements for battery working time are increasing. The traditional secondary battery (including lithium battery and nickel battery) has become a bottleneck, which hinders the development of portable products in a more functional direction.
Compared with traditional secondary batteries, the energy of fuel cells is at least 10 times higher. Lithium-ion batteries can provide a power density of 300 Whr/L, while the power density of methanol fuel cells is as high as 4,800 WHR/L, and 10ml of methanol can guarantee a talk time of 13.5 hours or a standby time of 642 hours. Therefore, Toshiba, IBM, NEC and many other internationally renowned electronic companies have put their energy and financial resources into the research of fuel cells. At present, the top ten companies in the world, except Wal-Mart, have invested in hydrogen energy or fuel cell industry.
DMFC focuses on portable applications.
Theoretically, a fuel cell is not a battery, but a device that makes fuel (such as hydrogen) react with oxidant through electrodes to directly generate current. Because its product is water, it has considerable environmental advantages. The typical structure of a fuel cell is a stack of stacked battery packs, and a stack can contain multiple individual fuel cells (Figure 1). The basic structure of each unit is similar to an electrolytic water device, including two positive and negative electrodes (anode and cathode), electrolyte and catalyst. The anode is a hydrogen electrode and the cathode is an oxygen electrode. Both anode and cathode contain a certain amount of catalyst to accelerate the electrochemical reaction on the electrode. Taking the hydrogen-oxygen reaction as an example, under the action of cathode catalyst, a hydrogen molecule is decomposed into two hydrogen ions and two electrons are released at the same time. Because of the filtering effect of the barrier film on electrons, electrons can't pass through the electrolyte, so they can only bypass, thus forming a current. However, hydrogen ions can smoothly pass through the electrolyte to reach the cathode and react with oxygen atoms in the air to generate water (Figure 2).
Figure 1 Basic structure of fuel cell
Fig. 2 Basic working principle of fuel cell
It is not difficult to see from the working principle that catalyst, electrode, diaphragm and electrolyte are the main materials of fuel cells. The working principles of various fuel cells are basically similar, and their classification is determined by the material of electrolyte. At present, the widely developed fuel cells include proton exchange membrane fuel cell (PEMFC), direct methanol fuel cell (DMFC), alkaline fuel cell (AFC), phosphate fuel cell (PAFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC) and so on. In addition, due to the difference of working temperature and power generation (Table 1), the application fields of fuel cells can also be divided into four categories: portable electronic products, including notebook computers, digital cameras, mobile phones, PDA and so on. Residential power generation, residential or standby power supply; Transport vehicles, cars, buses, etc. ; Large power generation buildings, small and large power plants.
Among them, PEMFC has the advantage of high energy conversion efficiency (generally 40%~60%, while internal combustion engine is only 65,438+08% ~ 24%) because it can directly convert chemical energy in fuel and oxidant into electric energy through electrochemical reaction without combustion, especially in automobile fuel, the application of PEMFC is close to 65,438+000% of this market.
In addition, DMFC, which belongs to PEMFC, uses polymer barrier membrane, while DMFC uses liquid methanol as fuel. Compared with hydrogen fuel cells, DMFC has certain advantages in battery system structure, fuel source and many other aspects. Its anode catalyst can directly extract hydrogen molecules from liquid methanol without fuel reformer, so high-purity methanol can be directly used as fuel for batteries. At the same time, it can effectively reduce the battery volume and simplify the system structure, so it is more suitable as a portable power supply for civil and military industries, such as electric cars, electric bicycles, mobile phones, notebook computers and so on.
Compared with the current secondary batteries, DMFC has the consistent advantages of fuel cells. The theoretical power density of DMFC is 4780 Whr/L, which is much higher than 200 Whr/L of Ni-MH and 3 10 Whr/L of Li-ion, so it can support longer working hours. In addition, different from the storage/discharge working mechanism of secondary batteries, fuel cells can be said to be energy converters. As long as fuel is continuously supplied, power generation can continue, and there will be no consideration of power failure or battery replacement. Moreover, DMFC is also considering gradually making people accept fuel cells through hybrid power supply, which has been actively verified in hybrid vehicles. Hybrid power supply is a combination of fuel cell and energy storage device (such as super capacitor or battery). The fuel cell will provide constant power, and the capacitor or battery will meet the peak power requirements.
The fuel cell industry chain includes four parts: materials, components, subsystems and systems. Most famous consumer electronics companies are engaged in the research of DMFC fuel cell system to ensure the competitiveness of their electronic products in the future, such as Sanyo, Sony, Toshiba and Fujitsu in Japan, Samsung and LG in South Korea and BYD in China. There are also some companies specializing in system development, such as MTI Micro Fuel Cell Company of the United States, Angstrom Power Company, Antig Company of Taiwan Province Province, Tekion Company of Canada invested by Motorola, etc. Most of these companies develop together through cooperation with large electronic companies. For example, MTI Micro has formed an exclusive alliance with South Korea's Samsung, and MTI Micro will use DMFC technology named "Mobion" to develop the next generation fuel cell prototype for Samsung's mobile phone business.
Table 1 Performance comparison of various fuel cells
Source: Fuelcelltoday.com.
Upstream of the industrial chain are companies specializing in the development of electrolyte membranes and other materials, such as the famous DuPont Company and intel capital's PolyFuel Company. With the efforts of the whole industrial chain, fuel cells are rapidly entering the commercial scale application stage from the military and aviation fields. North America, Japan, Europe and Taiwan Province Province have been in the forefront. Although China has made some progress in the research and development of fuel cells, there is still a gap in R&D investment and technical research depth compared with the above countries and regions. This problem has attracted the attention of our country, and now it is one of the most important topics in the energy and power industries, and it is also an emerging energy industry supported by national policies.
Obstacles that DMFC urgently needs to break through
The core components of DMFC are a stack of cathodes, anodes and polymer electrolyte membranes with a thickness of only 1 mm, so that the catalyst in the electrodes can contact the proton exchange membrane as effectively as possible, thus improving the conversion efficiency and reducing the battery volume. Proton exchange membrane has the function of isolating methanol and oxygen, preventing them from directly reacting to exchange protons and isolating electrons. It is a polymer membrane with selective permeability. It works in harsh environments such as strong acid and strong oxidation of batteries, so it needs extremely high corrosion resistance. In addition, electrodynamics and thermal conductivity are also required, and the requirements for material properties are very strict.
Polymer electrolyte membrane has been a big problem that puzzles the development of DMFC for many years. Hydrogen ions need to be carried by water through the polymer membrane separating the anode and cathode, but methanol and water have similar characteristics, so it is easy to accompany methanol in this process. At present, researchers are trying to solve this problem from two different angles. One is to control the concentration of methanol, or to increase the catalyst isolation layer that separates methanol from polymer membrane. Another method is to rely on electrolyte membrane, which can reduce the mixing of methanol and water. Several companies have developed this product. No one thinks that there will be a film that can completely isolate methanol. Moreover, in some designs, slight methanol miscibility is beneficial, and methanol is oxidized at the cathode, releasing a small amount of heat, which can improve the reaction rate of the whole fuel cell. In 2002, Tel Aviv University in Israel successfully developed a direct methanol fuel cell for mobile phones for the first time. The electrolyte membrane used is different from Nafion produced by DuPont. The latter is composed of fluorocarbon, while the former is mainly composed of polyvinylidene fluoride (PVDF) and silicon dioxide, which reduces the methanol permeability to single digit. The new generation electrolyte membrane manufactured by American PolyFuel Company controls the methanol permeability to117 of DuPont Company. And the latest PolyFuel 20 mm, the maximum power density is increased to 190 Ma/cm2. Jim Balcom, CEO of PolyFuel, said that increasing the power density of electrolyte membrane can reduce the volume of battery cells. In addition, PolyFuel 20mm can reduce the size and complexity of the system by improving the back diffusion of water from the air electrode to the fuel electrode.
Fujitsu's notebook computers use DFMC as fuel cells.
Image source: Fuelcelltoday.com
In addition, from the comparison of table 1, it can be seen that the power density of DMFC is the lowest among several technologies. This is because the internal methanol compound hydrogen production inevitably reduces the output power of the original fuel cell due to internal consumption. For example, the power density of PEMFC can reach 250~ 1000mW/cm2 (depending on fuel composition and operating conditions), while that of DMFC is only about 25 ~1000 MW/cm2, with a difference of nearly 65,438.
Using Hitachi's PDA and fuel cell
Image source: Fuelcelltoday.com
Another obstacle to the use of methanol is safety regulations. At present, methanol is still banned from commercial flights because there is no organization or standard to manage the methanol carried by passengers. However, in 2005, ICAO has proposed to lift the ban on passengers carrying methanol on board. Recently, Peng Lim, CEO of MTI Micro, told reporters that ICAO has agreed to cancel this regulation, and the US Department of Transportation plans to implement it in June+10 next year. Peng Lim said that once the ban is lifted, the advantages of fuel cells will be fully reflected in long-distance travel. At the same time, consumers do not need to worry about the safety of fuel boxes, because the design and manufacture of fuel boxes need to be certified by international organizations.
label
Like CDMA, GPS and other popular technologies, fuel cells have also gone through the process from military or aviation to civil use. The development history of fuel cells has exceeded 100 years, which has been verified in terms of technology and safety. What researchers need to consider now is how to make it smoothly enter people's daily life. Most companies engaged in fuel cell research and development believe that portable consumer electronics products are an excellent breakthrough.