Some brief introduction of methanol fuel cell

Methanol fuel cell is divided into direct methanol fuel cell (DMFC) and recombinant methanol fuel cell (RMFC), which are mainly used in mobile phones, laptops and other portable devices. Although the energy density of methanol fuel cell is not as high as that of other types of fuel cells, it has the advantages of being easy to carry and store methanol fuel, so methanol fuel cell is more suitable for portable devices.

So far, there has been no actual methanol fuel cell products to enter the civilian market, for the development of methanol fuel cell is still in progress, the direction of the research are mostly concentrated in the miniaturization, the service life of the battery, energy density and power efficiency improvements, methanol fuel cells to real production will take time. In any case, most people believe that methanol fuel cells will replace traditional batteries as the main power source for portable devices. It can be seen that various manufacturers have launched their own prototypes/prototypes in recent years. This paper roughly describes the principle of DMFC, and DMFC defects lead to RMFC, finally highlighted several RMFC prototypes and products.

A, DMFC

In terms of technical principles, the DMFC is mature. It immediately produces a stable energy source and does not require cooling of the battery body during the reaction. The methanol fuel used is easily stored in liquid form and does not condense in cold environments. DMFCS are also easy to do in terms of reducing size and weight and safety. All these advantages, make DMFCS in some application fields have more advantages than other types of fuel cells, for example we'll be in the lawnmower, saws and other household appliances device can see the figure of DMFC, also can see the ferry, shops use DMFCS as backup power, and the micro DMFC become more carry equipment, alternative energy sources. Figure 1 shows a dmfc-based music player by Toshiba.

DMFC usually consists of a permeable electrolyte membrane, methanol passing through the anode of DMFC, and air passing through the cathode of DMFC. Using methanol to react with air to produce electricity, the process does not burn and produces only CO2 and water. Methanol is broken down into hydrogen and CO2, protons form H2O with O2 in the air, and electrons travel through an external circuit to the negative pole of the film. The equation of the chemical reaction is as follows:

Full reaction equation:

CH3OH + o2 = 3/2 CO2 + 2 h2o

Anode:

CH3OH + H2O = CO2 + 6 h + + e -

Cathode:

3/2 o2 + 6 h + + e - = 3 h2o

2. Defects of DMFC

Methanol aqueous electrolyte membrane fuel cell (DMFC), mostly using Perfluorosulfone acid material, because this material will be formed in the internal cluster, surrounded by water molecules (Proton) can form Proton hydrate of the channel, so the Proton conductivity is very high, however, the same as the Proton hydrate combination of methanol can through the membrane and reduce the utilization of methanol, which is called methanol through (crossover) phenomenon, once methanol penetration will be at the cathode catalyst react with oxygen, which brings the problems such as lower voltage. It is very effective to use high-concentration methanol aqueous solution to increase battery capacity, but high-concentration methanol aqueous solution is also easy to cause methanol penetration, so the electrolyte membrane requires high proton conductivity, and at the same time the methanol penetration needs to be controlled. In fact, this is also the fatal defect of DMFC. Hydrogen ions need to be carried by water through the polymer film. In order to avoid this situation, researchers currently adopt various other methods to prevent the penetration of methanol, such as increasing the separation layer between methanol and the polymer film, using the water-repellent gradient and other measures.

Another problem facing DMFC is CO2 emission. Although methanol can be supplied passively (that is, without the use of pumps), the accumulation of CO2 in the catalyst will lead to the reduction of the utilization rate of the catalyst. And the use of pump system will increase the complexity of the system and volume increase. Finally, as carbon and oxygen atoms produce CO2, they also produce CO. In systems using platinum catalysts, CO will temporarily poison the platinum catalyst. Although the electrode can be added such as ruthenium atom to make the CO on the poisoned catalyst react and break away from the catalyst, when the CO concentration is too high, the fuel cell has to increase the ruthenium content of the electrode, which is also the reason that the electrode active area of DMFC is 10 times that of PEMFC.

Developed a DMFC system for motorcycle. Figure 4 shows the structure of the system and the names of each part of the device. Figure 5 also shows the principle of power generation and the structure of the battery theme. The system claims a rated output of 500W, a rated voltage of 24V, and a weight of 20kg. In this system, the fuel tank and a water tank are included. In the fuel tank, the methanol solution with a concentration of 50% is stored. The role of the water tank is to ensure that the methanol-water solution supplied to the battery body is maintained at a constant concentration of 1M/L (3.2% mass). In the battery body, the aqueous solution contains CO2 bubbles generated by the chemical reaction, which are sent back to the tank through a pipe loop and the bubbles are isolated. Yamaha has developed special concentration sensor and control circuit used for monitoring the concentration of methanol, the working principle of this system is that when the battery main body in a solution of methanol to low concentration to a certain extent, the system will generate a control signal, from a methanol tank transfer to react in a solution of high concentration of methanol solution in order to improve their concentration. In addition, yamaha has developed its own highly efficient air pump that pumps air to the cathode of the battery body, including a screening program. Finally the air passes through the vapor devices through the heat exchange devices, where the heat is used to speed up the concentration of the solution, and finally they are transferred out of the system. In low-concentration solution tanks, the moisture content of used solutions is controlled, and excess moisture is discharged out of the system. In order to integrate this system into the bike, the battery structure should be adjusted according to the shape of the bike to achieve weight balance.

A DMFC developed for military use. The fuel cell displayed at the fuel cell show in November 2006 shows the "MOBION1M" portable fuel cell developed by MTI for military use. It USES 100% methanol as fuel, rated power is 0.7w, and dimensions are 34mm x 95mm x 153mm. The fuel box is built in, with an energy density of 150Wh per charge. By using MIT's mobion technology, 100% methanol can be injected directly into the anode of the DMFC, thus avoiding the problem of injecting methanol into the battery body with water required by other DMFCS, as well as the subsystem of adding micro-pump and micro-catheter into the system. Its principle can be referred to in the MTI technology by controlling to maintain a constant supply of 100% concentration of methanol, and making them evenly distributed through the battery body without using the pump.

Second, the RMFC

RMFC is actually a recombinant methanol PEMFC, again using only methanol as the main raw material. The difference is that an external recombinator is used, usually a micro methanol recombinator. In RMFC, methanol does not directly enter the battery body for chemical reaction, thus avoiding the defects of DMFC described above, and it can also make up for the lack of DMFC output power. According to casio and Hitachi's research last year, the methanol fuel cell could increase its output energy density to 200mW/cm2 or more, which would mean its output power could exceed 10 watts to drive portable devices.

1, the introduction

In order to maintain energy density of PEMFC and avoid power attenuation caused by external recombination; In addition, since a certain temperature environment is required in the recombination process, increasing the recombination temperature will contribute to increasing the hydrogen-oxygen conversion rate of methanol. Therefore, the expected concentration of hydrogen and oxygen can be obtained with proper control of temperature and chemical dose. The steam restructuring or self heating temperature can be as low as 200-300 ℃. Another advantage of using external recombination is that the recombined gas can qualitatively oxidize CO, thus reducing CO problems and reducing the amount of catalyst. However, high-temperature fuel cells that are resistant to CO poisoning can also be used.

Due to the micro restructuring of methanol fuel cell operating temperature as high as 200-300 ℃, and the problems faced by current RMFC is start time and start temperature, so the micro RMFC in order to speed up the start, usually keep catalyst combustion in restructuring start temperature of the upper to quickly reach restructuring. Both DMFC and RMFC need to add micro rechargeable batteries to cope with the sudden power demand, and the power demand of fuel cells can also be reduced by means of hybrid fuel cells and secondary cells.

2. RMFC prototype developed by Casio

A recombinant methanol fuel cell prototype was demonstrated in November 2006 for Casio, in which the system could power a digital camera. Prototype will restructure (Reformer), fuel cell body (CellStack) and two fuel box (Fuelcartridge) compact together, the fuel pipe is installed at the bottom. The composition of other device includes two liquid pump is used to supply battery main methanol, a liquid flow sensor is used to measure the rate of flow of methanol, on/off switch valve used to control the supply of methanol, pump provides air and hydrogen, two different valve regulates the flow of air, two used to measure the air flow sensor as auxiliary device. As can be seen from the prototype, the control circuit is not integrated together. The DC/DC circuit and the control circuit are peripheral circuits, which are not shown in figure 8.

(1) structure of Casio prototype. The system USES 60% mass methanol as fuel. Methanol is pumped by two liquid pumps from two 8mL fuel tanks (18mm in diameter, 10mm in length) to the recombinator, and the flow is controlled by a liquid sensor. The liquid pump was developed jointly by Casio and FraunhoferIZM, a German research organization. The recombinator produces hydrogen from methanol by steam recombination. The resulting hydrogen is transferred to the fuel cell body or burned in a recombinator to keep the catalyst at the right temperature for start-up. For this reason, on/off valves are used to control flow in different flow paths.

In addition to supplying air to the fuel cell body, the air pump must supply air to the recombinator to remove the associated CO. In addition, air is also supplied for the combustion of hydrogen to facilitate the reaction rate of the catalyst in the recombinator. Air is pumped directly into the fuel cell body without the need for a valve. Air flow sensors and different types of valves are installed in each channel of the recombinator to accurately control the air flow. The power generated by the fuel cell is fed through a DC/DC converter circuit to provide a separate voltage to drive the digital camera. While the fuel cell body seems to use four batteries to power a digital camera in the way shown in the demo, Casio claims that 20 batteries can power a laptop. To be commercialized in 2008, the company plans to release fuel cell samples after upgrading the prototype.

(2) several important components of the Casio prototype. Among the prototypes is the electronic osmosis (EO) pump, which was launched on November 29th last year. The device accurately allocates methanol fuel at high pressure in a 0.5cc compression cell. It is made from materials produced by nanofu-sion technologies. Casio's successful experience in RMFC includes other key components, such as thermal-insulated recombiners, used to extract hydrogen from methanol, and fuel cell bodies, among others, as shown in figure 9. The so-called EO pump is a small fuel pump that consists of an electroosmotic material, a silicon-like dielectric that generates an electric potential when it comes into contact with a liquid. When a voltage is applied to it, the liquid inside flows. It distributes liquids at high pressure regardless of size, does not use motor drive, and more importantly operates without noise and eliminates problems such as vibration. Casio combined its patented technology with NanoFusion's electro-osmotic materials (1mm in diameter and 1mm in thickness) to develop the liquid fuel pump, which is primarily used in RMFC mobile devices. Casio has fixed problems inherent in EO pumps, such as magnetization changes in the electroosmotic material due to collisions, or the buildup of vapor bubbles in liquid electrolysis. Eventually the EO pump can be concentrated in a 0.5cc container and maintain a flow rate of 90 L/min even at a pressure of 100kPa.

Another important restructuring machine main device by using the theory of water vapor, heated to 280 ℃ and the methanol extract hydrogen. Its structure is shown in figure 10. In fact, the recombiner has been modified several times, and it's currently said to solve problems with insulation, long start-up times, and producing too much CO, and casio claims to ship samples of the recombiner for laptops in 2007. In terms of internal structure, the main components of the recombinator are two glass substrates, and they utilize vacuum insulation to coat the inner surface of the substrates with a thin film of gold to minimize thermal radiation. According to reports, the working condition of the restructuring of surface temperature of 40 ℃, 20 ℃ or higher than the room temperature. The recombinator comprises three channels. One is a hydrogen combustion channel used to provide heat for the recombination of methanol into hydrogen. The recombination pathway, where the fuel and the water vapor react; CO elimination channels are used to eliminate CO by-products.

3, Ultracell25

Back in 2005, Ultracell launched an RMFC that claims to have twice the energy density of a normal lithium-ion battery, which at about 40 ounces is the size of a flat paper novel. Through ultracell's technology, used waste fuel can be "hotswap" and reused to ensure continuous power supply. Originally developed by ultracell for military use, the RMFC model XX90 provides 45 watts of power. The commercial ultracell25 was released in 2006. It can be used in enterprise, industrial and mobile devices. Its military counterpart is XX25. Figure 11 shows Ultracell's RMFC XX25 product for military use, which is said to be able to power production equipment for 72 hours without interruption.

Iii. Comparison of several fuel cells

Other fuel cells include molten carbonate FC (MCFC), solid oxygen FC (SOFC), and phosphate FC (PAFC), which are also used in power and heat generation. MCFCS typically run on natural gas. SOFC USES hydrocarbonic compounds or H2 as fuel. MCFC and SOFC working under high temperature (> 650 ℃ respectively, and 800-1000 ℃), SOFC can provide the highest power efficiency (44% 50%), and the symbiosis (co - generation) mode can be more than 80%. In addition, polymer electrolyte thin film FC (PEMFC) is also commonly used in electric vehicles, but also can be used for fixed electricity generation. In order not to emit harmful substances, PEMFC needs pure H2 input and cannot produce CO2 in the reaction process. They work at low temperatures and provide a conversion efficiency of 35 to 40 percent. Fuel cell vehicles mostly run on PEMFC, which also has a 70-80% share of 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 has a 15-20% share of this segment. Thousands of FCS are produced worldwide each year, 80% for fixed and mobile devices, and the rest for fuel-cell vehicle demonstration projects.

If the costs of H2 and fuel cells are significantly reduced, and rules to limit CO2 emissions are in place and effectively enforced, then FC could see significant market growth in the next 10 years (reaching 30% market share by 2050). The potential for growth in fixed FC distribution depends on raw material pricing rules, that is, on the price of electronic materials and natural gas. SOFC and MCFC, which use natural gas as their primary fuel, will have 5% of the global fuel cell market by 2050.

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