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Methanol fuel cells are divided into two types: direct methanol fuel cells (DMFC) and recombinant methanol fuel cells (RMFC), which are mainly developed for use in portable devices such as mobile phones and laptops. Although the energy density of methanol fuel cells is not as high as other types of fuel cells, they have the advantages of easy portability and storage of methanol fuel, making them more suitable for portable devices.
So far, there have been no actual methanol fuel cell products entering the civilian market, and the development of methanol fuel cells is still ongoing. The research direction is mostly focused on improving the miniaturization, service life, energy density, power efficiency, and other aspects of the cells. It will take time for methanol fuel cells to truly enter mass production. Anyway, most people believe that methanol fuel cells will replace traditional batteries as the main power source for portable devices. It can be seen that in recent years, various manufacturers have successively launched their own prototypes/prototypes. This article roughly describes the principle of DMFC, and introduces RMFC from its defects. Finally, it focuses on listing several prototype and products of RMFC.
1 DMFC
In terms of technical principles, DMFC is mature. It can immediately generate stable energy without cooling the battery body during the reaction process; The methanol fuel used is easy to store in liquid form and does not condense in cold environments; DMFC is also easy to achieve in terms of reducing volume, weight, and safety. All these advantages make DMFC more advantageous than other types of fuel cells in certain application fields, such as in household appliances such as lawn mowers and chainsaws, as well as in passenger ships and shops using DMFC as a backup energy source. Micro DMFC has become an alternative energy source for portable devices. Figure 1 shows Toshiba's music player based on DMFC.
DMFC typically includes a permeable electrolyte membrane, with methanol passing through the anode of the DMFC and air passing through the cathode of the DMFC. Using methanol and air for chemical reaction to generate electricity, this process does not require combustion and only produces CO2 and water. Methanol is decomposed into hydrogen atoms and CO2, while protons react with O2 in the air to form H2O, and electrons pass through an external circuit to reach the negative electrode of the film. The equation for a chemical reaction is as follows:
Full reaction equation:
CH3OH+3/2O2＝CO2+2H2O
Anode:
CH3OH+H2O＝CO2+6H++6e-
Cathode:
3/2O2+6H++6e-＝3H2O
2. Defects of DMFC
The electrolyte membrane of methanol water solution fuel cells (DMFC) mostly uses Perfluorosulfone acid based materials. Due to the formation of clusters inside this material, protons surrounded by water molecules can form channels for proton hydrates, resulting in high proton conductivity. However, methanol, which is combined with proton hydrates, will cross the membrane and reduce the utilization rate of methanol. This is commonly known as methanol crossover phenomenon. Once methanol penetrates, it will react with oxygen at the cathode catalyst, causing problems such as voltage drop. The use of high concentration methanol aqueous solution is very effective in increasing battery capacity. However, high concentration methanol aqueous solution can also easily cause methanol penetration phenomenon. Therefore, the electrolyte membrane requires high proton conductivity and also needs to control the penetration problem of methanol. This is actually a fatal flaw of DMFC, as hydrogen ions need to be carried through the polymer film by water. To avoid this situation, researchers have adopted various other methods to prevent the penetration of methanol, such as adding an isolation layer to separate methanol from the polymer film, using a water repellent gradient, and so on.
Another issue faced by DMFC is the emission of CO2. Although methanol can be supplied passively (i.e. without using a pump), the accumulation of CO2 in the catalyst can lead to a decrease in the utilization rate of the catalyst. The use of pumps in the system will increase the complexity and volume of the system. Finally, while carbon atoms and oxygen atoms produce CO2, CO is also produced incidentally. When using a platinum catalyst system, CO temporarily poisons the platinum catalyst. Although ruthenium atoms can be added to the electrode to cause the CO on the poisoned catalyst to react and detach from the catalyst, when the CO concentration is too high, the fuel cell has to increase the ruthenium content of the electrode. This is also why the electrode active area of DMFC is 10 times that of PEMFC.
A DMFC system developed for electric bicycles. Figure 4 shows the structure of this system and the names of each component. In Figure 5, the principle of power generation and the structure of the battery theme can also be seen. This system claims to have a rated output power of 500W, a rated voltage of 24V, and a weight of 20kg. This system includes a fuel tank and a water tank, which stores a 50% concentration of methanol solution in the fuel tank. The function of the water tank is to ensure that the concentration of the methanol water solution supplied to the battery body remains constant at 1M/L (3.2% mass). In the battery body, the aqueous solution contains CO2 bubbles generated by chemical reactions, which are transported back to the water tank through pipeline circuits and isolated from the bubbles. Yamaha has developed specialized concentration sensors and control circuits to monitor the concentration of methanol. The working principle of this system is as follows: when the concentration of methanol in the solution transmitted to the battery body drops to a certain level, the system will generate a control signal to transfer high concentration methanol solution from a methanol tank to the solution to be reacted to increase their concentration. In addition, Yamaha has also developed an efficient air pump to pump air onto the cathode of the battery body, which includes a screening program. Finally, these air pass through vapor devices and heat exchange devices, where the heat is used to accelerate the concentration of the solution, and finally they are transported out of the system. In a low concentration solution tank, the moisture content of the used solution is controlled, and excess moisture will be discharged outside the system. In order to integrate this system into the electric bike, the structure of the battery should be adjusted according to the shape of the electric bike to achieve weight balance.
Developed DMFC for special purposes. The fuel cell displayed at the Fuel Cell Exhibition in November 2006 is the "MOBION1M" portable fuel cell developed by MTI for special purposes. It uses 100% concentration methanol as fuel, with a rated power of 0.7W and dimensions of 34mm × 95mm × 153mm. The fuel tank is built-in, with an energy density of 150Wh per filling. Through MIT's MOBION technology, methanol with a concentration of 100% can be directly injected into the anode of DMFC, thus avoiding the problem of other types of DMFC requiring water to inject methanol into the battery body and eliminating the need for subsystems such as micro pumps and micro conduits in the system. Its principle can be referred to in MTI's technology, which maintains a constant supply of 100% concentration of methanol and evenly distributes them through the battery body without using a pump.
II RMFC
RMFC is actually a PEMFC that recombines methanol, using only methanol as the main raw material. The difference is that an external recombiner, usually a miniature methanol recombiner, is required. In RMFC, methanol does not directly enter the battery body for chemical reactions, which avoids the defects of DMFC described earlier and can also compensate for the insufficient output power of DMFC. Based on the research results of Casio and Hitachi last year, the output energy density of methanol fuel cells can be increased to 200mW/cm2 or higher, which means its output power can exceed 10W to drive portable devices.
1. Introduction
In order to maintain the energy density of PEMFC and avoid power attenuation caused by external recombination; In addition, due to the need for a certain temperature environment during the recombination process, increasing the recombination temperature will help increase the hydrogen oxygen conversion rate of methanol. Therefore, appropriate control of temperature and chemical dosage can achieve the expected concentration of hydrogen and oxygen. At present, the temperature for steam reforming or self heating reforming can be as low as 200-300 ℃. Another advantage of using external recombination is that the recombined gas can qualitatively oxidize CO, thereby reducing the problem of CO and reducing the amount of catalyst used. However, high-temperature fuel cells that are resistant to CO poisoning can also be used.
Due to the high working temperature of micro recombined methanol fuel cells reaching 200-300 ℃, and the current challenges faced by RMFCs being start-up time and start-up temperature, micro RMFCs usually install catalyst combustion on the upper layer of the recombiner to quickly reach the start-up temperature of the recombiner in order to accelerate start-up. Both DMFC and RMFC require the addition of micro rechargeable batteries to meet sudden power demands, and hybrid fuel cells and secondary batteries can also be used to reduce the power demand of fuel cells.
2. RMFC prototype developed by Casio
A prototype of a recombinant methanol fuel cell was demonstrated by Casio in November 2006, which could drive a digital camera during the demonstration. The prototype compactly assembles the Reformer, CellStack, and two fuel cartridges together, with fuel pipes installed at the bottom. The other components include two liquid pumps for supplying methanol to the battery body, a liquid flow sensor for measuring the flow rate of methanol, an on/off valve for controlling the supply switch of methanol, an air pump for providing air and hydrogen, two different valves for controlling the flow rate of air, and two flow sensors for measuring the flow rate of air as auxiliary devices. In the prototype, it can be seen that the control circuit is not integrated together. The DC/DC circuit and control circuit are peripheral circuits, which are not shown in Figure 8.
(1) The structure of the Casio prototype. This system uses methanol with a concentration of 60% mass as fuel. Methanol is transported from two 8mL fuel tanks (18mm diameter, 10mm length) to the recombiner by two liquid pumps, while the flow rate is controlled by liquid sensors. The liquid pump was jointly developed by Casio and Fraunhofer IZM (a research institution in Germany). The recombiner produces hydrogen from methanol through steam reforming. The final generated hydrogen is transported to the fuel cell body or burned in the recombiner to ensure the temperature required by the catalyst during start-up. For this reason, on/off valves should be used to control flow on different flow channels.
In addition to supplying air to the main body of the fuel cell, the air pump must inject air into the recombiner to remove the CO produced incidentally. In addition, supplying air is also for the combustion of hydrogen to promote the reaction rate of the catalyst in the recombiner. Air is directly injected into the fuel cell body by a pump without the need for valves. Air flow sensors and different types of valves are installed in each channel of the recombiner to accurately control the air flow rate. The power generated by the fuel cell is then used to provide independent voltage through a DC/DC converter circuit to drive the digital camera. When the fuel cell body seems to use four batteries to drive a digital camera in the way demonstrated, Casio claims that 20 batteries can drive a laptop. In order to achieve commercialization in 2008, the company plans to release fuel cell samples after upgrading the prototype.
(2) Several important components of the Casio prototype. The prototype includes the Electronic Osmosis (EO) pump, which was launched on November 29th last year. This device can accurately distribute methanol fuel while maintaining high pressure in a compressed 0.5cc unit. It is made of materials manufactured by NANOFu silicon technology company. Casio's successful experience in the RMFC field also includes other key components, such as thermal insulation recombiners for extracting hydrogen from methanol, and fuel cell bodies, as shown in Figure 9. The so-called "EO pump" mentioned here is a small fuel pump composed of an electro permeable material, which is a dielectric similar to silicon that can generate an electric potential when in contact with a liquid. When a voltage is applied to it, the liquid inside will flow. No matter the size, it can distribute liquids under high pressure without using a motor drive. More importantly, it operates without noise and eliminates issues such as vibration. Casio combined its patented technology with NanoFusion's electroosmotic material (diameter 1mm, thickness 1mm) to develop this liquid fuel pump, mainly used in RMFC for mobile devices. Casio has solved the inherent problems in EO pumps, such as the magnetization changes caused by collisions in electroosmotic materials and the accumulation of vapor bubbles generated during liquid electrolysis. The final EO pump can be concentrated in a 0.5cc container and maintain a flow rate of 90 μ L/min even at a pressure of 100kPa.
Another important device, the recombiner, mainly utilizes the principle of water vapor to heat methanol to 280 ℃ and extract hydrogen from it. Its structure is shown in Figure 10. In fact, this recombiner has undergone multiple improvements and is said to have solved problems such as insulation, long startup time, and excessive CO production. Casio claims to ship recombiner samples for laptops in 2007. In terms of internal structure, the main components of the recombiner are two glass substrates, which utilize vacuum insulation and are covered with a thin film of gold on the inner surface of the substrate to minimize heat radiation. According to reports, the surface temperature of this recombiner during operation is 40 ℃, or 20 ℃ higher than room temperature. The recombiner includes three channels, one of which is a hydrogen combustion channel used to provide heat for the recombination of methanol into hydrogen; The channel for recombination reaction, where the reaction between fuel and water vapor takes place; Eliminating CO channels is used to eliminate CO by-products.
3 Ultracell25
As early as 2005, Ultracell launched an RMFC and claimed that its energy density was twice that of a regular lithium-ion battery. At around 40 ounces, the size of this battery is comparable to a flat paper novel. Through ultracell technology, used waste fuel can be "hot swapped" and reused to ensure continuous power supply. RMFC was initially developed by Ultracell for special purposes, with the model XX90, which provides 45 watts of power. The ultracell25 for commercial use was released in 2006 and can be used in the fields of enterprise, industrial, and mobile devices. Its corresponding special version is XX25. Figure 11 shows the RMFC product XX25 launched by Ultracell for special purposes, which is claimed to provide uninterrupted power supply for production equipment for 72 hours.
3、 Comparison of Several Fuel Cells
Other types of fuel cells include fused carbonate fuel cells (MCFC), solid oxygen fuel cells (SOFC), and phosphoric acid fuel cells (PAFC), all of which are also used for electricity and heat generation. MCFC typically uses natural gas as fuel. SOFC uses hydrogen carbon compounds or H2 as fuel. MCFC and SOFC operate at high temperatures (>650 ℃ and 800-1000 ℃ respectively), with SOFC providing the highest power efficiency (44% -50%) and exceeding 80% in co generation mode. In addition, polymer electrolyte film FC (PEMFC) is often used in electric vehicles and can also be used for fixed power generation. In order to avoid emitting harmful substances, PEMFC requires pure H2 input and cannot produce CO2 during the reaction process. They can provide a conversion efficiency of 35% -40% when working at low temperatures. Most fuel cell vehicles use PEMFC, which also holds a 70% -80% market share in the small solid-state fuel cell market. In the medium to long term, MCFC and SOFC are expected to dominate the large-sized solid-state fuel cell market. SOFC currently holds a 15% -20% market share in this segment. Thousands of FCs are produced globally each year, with 80% used for fixed and mobile devices and the remaining for fuel cell vehicle demonstration projects.
If the costs of H2 and fuel cells are significantly reduced, and rules to limit CO2 emissions are introduced and effectively implemented, FC may experience significant market growth in the next 10 years (reaching a market share of 30% by 2050). The potential for fixed FC distribution growth depends on the rules of the raw material price list, which are determined by the prices of electronic materials and natural gas. SOFC and MCFC use natural gas as their main fuel, and by 2050, they will occupy 5% of the global fuel cell market share.

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