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Authors: Lin Caishun, Wang Xindong, Wang Shuyan, Ma Lijun. With the rapid development of the electronics industry, wireless communication devices and various portable consumer electronics are emerging in increasing numbers, which leads to a growing demand for batteries. At the same time, higher requirements are placed on the performance of these batteries. Currently widely used batteries such as nickel-cadmium, nickel-metal hydride, and lithium-ion batteries, due to their relatively mature manufacturing technologies and limited room for performance improvement, can no longer meet the demands of the development of electronic products. Therefore, the electronics market urgently needs a new type of battery with higher capacity and better environmental compatibility to replace the batteries currently in widespread use. Micro direct methanol fuel cells (DMFCs) are proton exchange membrane fuel cells that utilize a methanol aqueous solution as fuel. They feature small size, light weight, a simple system structure, high energy density, an abundant and inexpensive fuel source, ease of storage and transportation, and high safety. Additionally, they can provide electrical power continuously for long periods, and the fuel can be replaced easily. As such, they can meet the increasing energy demands of portable electronic devices such as mobile phones, laptops, camcorders, personal digital assistants, and medical equipment systems. They can also serve as power sources for individual use in various applications, as well as for spacecraft and microelectromechanical systems. Therefore, they are likely to supplement and replace the batteries currently in widespread use, becoming ideal power sources. Therefore, the research and development of micro direct methanol fuel cells are becoming a major focus and trend in the fields of electrochemistry and energy science research. 1. Working principle and characteristics of DMFC. The working principle of a micro DMFC is as follows: In the anode region, the aqueous solution of methanol, which serves as the anode active material, is evenly distributed by the anode flow field plate. It then diffuses through the anode diffusion layer and enters the anode catalytic layer (i.e., the region where electrochemical reactions take place). There, under the action of the platinum-ruthenium catalyst supported on carbon, an electrochemical oxidation reaction occurs, resulting in the production of protons, electrons, and carbon dioxide. The protons generated migrate to the cathode through the perfluorosulfonic acid membrane polymer electrolyte, while electrons are transferred to the cathode via the external circuit. Carbon dioxide is expelled from the anode outlet with the help of the acidic electrolyte. In the cathode region, the positive electrode active material, oxygen or air, is evenly distributed by the cathode flow field plate, then diffuses through the cathode diffusion layer and enters the cathode catalytic layer (i.e., the region where electrochemical reactions occur). There, under the action of the platinum-ruthenium electrocatalyst supported on carbon, it undergoes an electrochemical reduction reaction with the protons that have migrated from the anode, resulting in the formation of water, which is then expelled from the cathode outlet along with the reaction exhaust gases. Its electrode reactions are as follows: Anodic reaction: CH3OH + H2O → CO2 + 6H+ + 6e; Cathodic reaction: 3/2 O2 + 6H+ + 6e → 3H2O; Overall reaction: CH3OH + 3/2 O2 → CO2 + 3H2O. Unlike secondary batteries, micro direct methanol fuel cells can generate electricity by continuously supplying methanol fuel and an oxidant, allowing electrons to flow from the anode to the cathode through an external circuit, thereby supplying power externally. Micro direct methanol fuel cells are characterized by: 1) high energy conversion efficiency, ranging from 60% to 80%, without being constrained by the \"Carnot cycle\"; 2) environmental friendliness. The products of fuel cells are mainly water and a small amount of carbon dioxide, with low noise levels; 3) high specific energy. The specific energy and specific volume energy of DMFC reach 6000 Wh/kg and 4800 Wh/L respectively, which are much higher than those of batteries. 2. Current research status of DMFC at home and abroad 2.1 Research status abroad In the United States, research on micro direct methanol fuel cells has yielded numerous patents related to the development of certain key technologies, and prototypes of these micro fuel cells have been presented to the public. Manhattan Sciences was the first company to propose micro direct methanol fuel cells for mobile phone power supplies, with its core technology lying in the non-pressure-filtered structure of the stack. A technology similar to that of printed circuit boards is used to embed membrane electrodes on an insulating plate, and then the various membrane electrodes are connected in series within the same planar structure to form a battery pack. The area of this micro DMFC battery is 5 cm×13 cm; when methanol solution is used as fuel and air as an oxidizer, it enables the phone to remain in conversation for 20 hours, which is 10 times the talk time of current lithium-ion batteries. The American company MMT MicroFuelCells has achieved remarkable results in the research of micro-proton exchange membrane fuel cells. It released three generations of micro-DMFC prototypes in 2001, 2002, and 2003 respectively. On February 14, 2003, U.S. President Bush even used a third-generation fuel cell phone from MMT to make a call. Its core technology is the MobionTM technology based on the DMFC principle; by using this technology, methanol penetration can be effectively overcome, allowing the battery to accept high-concentration methanol and thereby increasing the specific energy of the system. Currently, the company has received funding from a U.S. ** agency to supply micro DMFÇ portable power sources with a capacity of around 5W for use in military soldier systems and ** communication equipment. In the second half of 2004, the company also showcased a miniature DMFC that could deliver a normal power output of 5W, with a peak power output of 35W; its total energy output exceeded 50W·h, and its specific energy was 900W·h/L. This is more than twice the energy capacity of the military batteries widely used by the U.S. military (the BA5590 battery). Leveraging its strengths in the development of microsensors using microelectromechanical systems technology, Case Western Reserve University received funding from the U.S. military to focus on research and development of MEMS microfuel cells designed to power MEMS sensors and actuators in precision-guided missiles; these fuel cells have a power output ranging from 10 to 100 mW. Its core technology involves manufacturing fuel cells on the surface of single-crystal silicon substrates; the anode uses metal gold sputtered on the silicon surface as a current collector, and the silicon’s flow field is utilized to ensure even distribution and transport of the methanol solution. The cathode uses a porous gold structure as the current collector, which facilitates electrical conductivity and the mass transfer of oxygen in the air. In the field of civil communications, the U.S. Los Alamos National Laboratory has developed a miniature DMFDC battery that can convert lower voltages into higher operating voltages. The battery has a pressure-filter type structure, with an electrode area of 45 cm2 and a thickness of only 2 mm per cell. At an operating voltage of 14 V, its peak power is 80 W, its specific power is 300 W/L, and its specific energy is 200 W·h/kg h/kg, and is expected to replace the BA5590 lithium-ion battery used in the U.S. military’s communication systems. JetPopulationLaboratory replaces the traditional bipolar plate design with a \"flatpack\" design; in this approach, each cell forms part of a fuel cell stack through electron conductors that run across the membrane plane, and every 2 cells combine to form 3 pairs of \"twinpacks\" using a back-to-back configuration. Methanol fuel enters from the common anode plate, while air enters from the outer cathode plate. By connecting 6 batteries in series in this way, a continuous power supply of 150 mW per battery is sufficient to ensure normal operation. On January 30, 2001, the Japanese company Toshiba demonstrated on-site its first micro DMFC for PDAs. It has a peak power of 8 W and an average power of 3–5 W; it is composed of 5 individual cells, with dimensions of 105 mm × 127 mm × 25 mm and a weight of 500 g. The fuel tank has a volume of 10 mL. The use of high-concentration (90%) methanol allows PDA to operate continuously for 40 hours. On March 5, 2003, the company successfully developed a small fuel cell suitable for use in laptops. This new type of fuel cell uses methanol as fuel; compared to laptops that use lithium batteries, 50 mL of methanol can be used for 5 hours continuously, which is more than twice as long as the runtime provided by lithium batteries. Toshiba’s fuel cell is 27.5 cm long, 7.5 cm wide, and 4 cm high; it generates an average power of 12 W, with a maximum output of 20 W. On June 24, 2004, Toshiba unveiled the world’s smallest DMCF to date, with dimensions of 22 mm × 56 mm × 4.5 mm, a weight of about 8.5 g, and an output power of 100 mW. Using 2 mL of high-concentration methanol allows MP3 to play continuously for 20 hours. The newly developed fuel cell by NEC has an average output power of 14 W, a maximum output power of 24 W, a voltage of 12 V. The total weight of the cell is 900 g, of which 300 g is the weight of the fuel. Its dimensions are 270 mm × 270 mm × 40 mm. When using methanol with a concentration of about 10% at 300 cm3, a laptop can run continuously for approximately 5 hours. In February 2004, the renowned Fujitsu Research Institute announced the fuel cell it had developed for laptops. With a maximum output power of 15 W and using a 300 mL methanol solution as fuel, it can keep the company’s laptop running for 8 to 10 hours. The methanol solution used in the fuel cells introduced this time has a concentration increased to 30%, and there was no issue of reduced output power due to osmosis. Since high-concentration methanol can be utilized, this **increases the energy density per unit mass or per unit volume. The Japanese company Toshiba has developed a miniature DMFC for use in personal digital assistants: it is 25 mm thick, weighs 500 g, has an output power of 8 W, and can operate continuously for 40 hours. In March 2003, the company introduced the world’s first micro DMFC system that could be connected directly to laptops. This system had dimensions of 275 mm × 75 mm, a weight of 900 g, and an average output power of 12 W. In October, the company developed a highly integrated micro fuel cell designed for portable consumer electronics such as PDAs and mobile phones: with a weight of 130 g, a volume of 140 mL, an average output power of 1 W, a methanol capacity of 25 mL, and a operating time of 20 hours. In June 2002, Samsung’s Institute of Comprehensive Technology in South Korea developed an integrated miniature DMFC for use in mobile phones, with dimensions of 60 mm × 80 mm × 10 mm and an output power of 2 W. The German company SmartFuelCell was founded in 2000, focusing on the research and development of DMDFC systems with an output power of 10–1000 W. The company’s first product is a remote power system designed as an independent power source for transportation systems, remote sensors, lighting, and outdoor equipment, with a peak power output of 80 W. This system can provide 3 to 5 times more energy than traditional batteries. In August 2003, SmartFuelCell introduced the world’s first commercial DMFC system for end-users, the SFCA25. The average output power of the SFCA25 is 25 W, with a maximum output power of up to 80 W. Its dimensions are 150 mm × 112 mm × 65 mm, and its weight is 1.1 kg. Its 2.5L methanol fuel tank is sufficient to keep the system running at full capacity for 70–80 hours. 2.2 Current research status in China: China started relatively late in the research of micro direct methanol fuel cells. The Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has developed miniature DMFC experimental demonstration prototypes for use in small fans, PDAs, toy cars, and mobile phones. The institute also uses physical vapor deposition to deposit a metal composite layer on the surface of silicon wafers as a current collector, effectively reducing the internal resistance of MEMS micro fuel cells. The Institute of Microelectronics at Tsinghua University has conducted in-depth research on MEMS micro-fuel cells based on porous silicon. The Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, has also made good progress in research on aspects such as the cell structure, packaging, and system integration of micro fuel cells. 3. Problems existing in the development of DMFCs. At present, although micro direct methanol fuel cells are the most likely to supplement and replace the batteries currently in widespread use, there are still many problems: 1) Technologically, two key issues—low catalyst activity and methanol permeation—hinder the development and application of micro direct methanol fuel cells; in particular, the performance of methanol anode catalysts at low temperatures needs to be improved urgently. 2) In terms of manufacturing, the development trend of micro direct methanol fuel cells is toward miniaturization, integration, and higher energy efficiency. However, since fuel cells have not yet been industrialized, the processing of various battery components sometimes fails to meet the required precision levels, and large-scale production is even impossible. At the same time, the miniaturization and integration of batteries inevitably lead to a decrease in specific energy, which contradicts the goal of increasing the specific power density of batteries. 3) In terms of cost, the materials required for micro direct methanol fuel cells, such as catalysts, electrolyte membranes, and electrodes, are expensive, resulting in high production and processing costs. Therefore, to commercialize and make micro direct methanol fuel cells competitive, it is necessary to reduce the production cost of these batteries to the level of the prices of batteries currently in use, or even lower. 4. Conclusion The micro direct methanol fuel cell is a new type of fuel cell. Thanks to its small size, simple structure, high energy density, ease of storage and transportation, as well as high safety levels, it holds great potential for application in the vast electronics consumer market. It is most likely to supplement and replace the batteries currently in widespread use, thus becoming an ideal power source. Therefore, in light of the many technical, manufacturing, and cost-related challenges facing the development of current micro direct methanol fuel cells, efforts should be focused on intensifying research and development so as to achieve commercialization and market availability at an early date.