NiMH No.7 battery.Research progress of passive direct methanol fuel cells

　　Passive direct methanol fuel cell (passive DMFC) is a fuel cell that directly uses methanol aqueous solution or methanol vapor as fuel, and oxygen or air as oxidant. In addition to the advantages of direct methanol fuel cells such as no pollution, high energy density, high efficiency, no noise and continuous operation, passive DMFC also has the ability to simply utilize gravity, capillary action and natural convection, cathode air, anode air, etc. Methanol solution or methanol vapor is naturally inhaled into the membrane electrode assembly (MEA). It does not use a power pump to feed the electrode. It completely abandons the features of external methanol peristaltic pump and air pump, greatly reducing battery power consumption and improving efficiency. Energy utilization. Moreover, passive DMFC generally works in room temperature environment and smaller current density, and does not require a separate temperature control system and water management system, further simplifying the battery structure and reducing system power consumption. Therefore, passive DMFC is considered to be one of the most promising technologies to replace lithium-ion batteries as the power source for a new generation of portable devices.

　　Passive DMFC generally consists of three parts: a three-in-one membrane electrode assembly (MEA), anode feeding system, and cathode air supply system. In addition, the structural design of the battery and stack also has a great impact on the performance and portability of the battery. In recent years, many researchers have devoted themselves to the research of passive DMFC, hoping to solve the problem of low power density of passive DMFC due to the passive feeding of cathodes and anodes and the low-temperature working environment. The author of this article will introduce the research progress of passive DMFC technology at home and abroad from the above four aspects.

　　1 Research progress of membrane electrode assembly (MEA)

　　Membrane electrode assembly (MEA) is the core component of DMFC, which usually consists of a pair of gas diffusion electrodes (GDL) and a proton exchange membrane through hot pressing. Many researchers have conducted systematic research on the MEA technology of active DMFC. However, because passive DMFC uses different methanol feeding methods and air supply methods, there are different influencing factors compared to active DMFC. Therefore, in recent years, many Researchers have improved and developed the MEA structure based on the characteristics of passive DMFC. Due to the slower mass transfer rate and lower operating temperature of passive DMFC, the activation process of membrane electrodes takes longer than that of active DMFC to fully open up the reactant and product channels of MEA and obtain higher battery performance.

　　Kho et al. [1] studied the activation process of passive DMFC membrane electrode in detail. The results showed that compared with the pretreatment process of simply using water, the pretreatment method of MEA using methanol aqueous solution has a better activation effect and can shorten the pretreatment time. Processing time. The special feeding method of passive DMFC puts forward higher requirements on the diffusion rate of reactants and products, and the gas diffusion layer (GDL) structure in MEA has a great impact on the diffusion rate of substances, so how to improve the diffusion rate of GDL Structure to achieve better mass transfer effect is a research hotspot of passive DMFC membrane electrodes. Lin et al. [2] studied the effect of polytetrafluoroethylene (PTFE) content in the diffusion layer on performance. The results show that the battery open circuit voltage increases with the increase in the PTFE load in GDL, and the battery internal resistance also increases with the increase in the PTFE load in GDL.

　　At the anode, battery performance decreased with increasing PTFE loading in GDL; at the passive DMFC cathode, carbon cloth treated with 10% PTFE emulsion had higher performance. More researchers have focused their attention on new MEA structures and GDL structures. Chen et al. [3] designed a new MEA structure for passive DMFC, as shown in Figure 1.

　　Using porous media instead of GDL for the cathode, this new structure increases the contact resistance of the battery, but greatly improves the performance and long-term operation stability of the battery due to improved oxygen transmission. Liu et al. [4] used stainless steel fiber as the anode GDL substrate instead of traditional carbon paper. Stainless steel has high electronic conductivity and high hydrophilicity, which greatly reduces the resistance of the electrode and promotes the smooth passage of the methanol solution through the GDL to the electrode catalytic layer for electrochemical reaction.

　　The maximum power density of batteries using this new structure can reach 24mW/cm2. At the same time, the research group used this technology to successfully assemble a 12-cell battery pack to charge mobile phones.

　　Some researchers have also studied the MEA structure suitable for passive DMFC systems from the perspective of electrolyte membranes. Kim et al. [5] used a synthetic methanol permeation-resistant electrolyte membrane to design a membrane electrode structure that can improve water transport and effectively discharge CO2. The author added a mixture of nano-sized silicon and polyvinylidene fluoride to the traditional MEA cathode and anode GDL to create a hydrophilic water storage layer. This new MEA structure greatly improves battery performance. Under room temperature conditions, the maximum power density obtained when the battery voltage is 0.3V is 48mW/cm2.

　　Jewett et al. [6] examined the use of different types of electrolyte membranes in passive DMFC from the aspects of water management, fuel utilization and power density. The study showed that passive DMFC using Nafion 117 electrolyte membrane has better performance and higher fuel utilization. The author also prepared a water management layer composed of hydrophobic substances to ensure long-term stable operation of the battery in order to solve the problem that water from the anode will penetrate into the cathode through the electrolyte membrane, causing changes in the concentration of the methanol solution and flooding of the cathode. Liu et al. [7] believe that a thick electrolyte membrane is beneficial to improving the utilization of methanol. In the low current density region, thick electrolyte membranes have higher performance due to lower methanol permeability. In the high current density region, batteries using thin electrolyte membranes perform better due to higher temperatures. In addition, Hong et al. [8] studied the impact of changes in catalyst loading on the performance of passive DMFC. At room temperature, both the cathode and the anode use a metal loading of 8mg/cm2, and the maximum power density reaches 45mW/cm2.

　　2 Research progress of anode feeding system

　　The passive DMFC anode feed is completely through the natural inhalation of methanol solution or methanol vapor into the membrane electrode assembly (MEA) without using a peristaltic pump. Although the system volume is simplified and the system power consumption is reduced, the slow natural diffusion rate of methanol also leads to a reduction in battery performance. Therefore, it is necessary to conduct detailed research on the anode methanol feeding system. At present, most researchers believe that higher methanol concentration is more conducive to improving the performance of passive DMFC. Kim et al. [9] believe that the battery performance is best when using 4mol/L methanol solution, which can reach 37mW/cm2 at room temperature.

　　And by monitoring the battery temperature changes when using different methanol concentrations, it was found that when using high concentrations of methanol, the battery temperature was higher. The same reports can also be seen in other literature [10-11]. Kho et al. [12] conducted a detailed study on the temperature and open circuit voltage changes of passive DMFC under different methanol concentrations. The study confirmed that the increase in battery temperature was caused by methanol penetrating into the cathode and oxidation reaction exotherm under the action of the cathode catalyst. . Since the passive DMFC anode adopts passive feeding, it is necessary to use a high-concentration methanol solution to increase the diffusion rate of the reactants. However, the high-concentration methanol solution causes more serious permeation problems, poisons the cathode catalyst, and reduces the fuel efficiency of the battery. Utilization.

　　Chu et al. [13] analyzed the use of different methanol concentrations in passive DMFC from the perspective of fuel utilization. In the case of using high-concentration methanol solutions, fuel utilization is significantly reduced due to severe methanol penetration. When the methanol concentration is 3mol/L, the fuel utilization rate is only 72.9%, while when using 0.5mol/L methanol solution, the fuel utilization rate can reach 80.8%. Zeng [14] and others used gas chromatography detection method to detect the change of methanol concentration in the fuel chamber of passive DMFC during long-term discharge process, and determined that the Faradaic efficiency of passive DMFC can reach 44%. Lai et al. conducted a detailed study on the fuel utilization of passive DMFC using different methanol concentrations [15]. High-concentration methanol solutions are suitable for long-term discharges of large currents due to their faster methanol diffusion rates, while in small In the case of current discharge, the severe methanol penetration problem leads to lower fuel utilization, while changes in the total amount of methanol will not affect the fuel utilization of the battery.

　　Because the development goal of passive DMFC is for portable power supply, system miniaturization is the focus of battery design. Under the condition that the volume of the methanol storage chamber is certain, the use of high-concentration methanol solution can provide longer discharge time and higher volume energy density. However, higher methanol concentration will inevitably cause serious methanol penetration problems, which not only affects the battery's Performance also reduces fuel utilization. How to resolve this contradiction is a difficult problem facing scientific researchers. Guo et al. [16-17] designed and developed an anode feeding device that can use high-concentration methanol solution, as shown in Figure 2. The capillary force of porous materials (ceramics, glass fiber, carbon fiber, polymer or cotton, etc.) is used to suck pure methanol into the methanol solution tank to achieve fuel replenishment. The advantage is that fuel and water can be transported separately and mixed at any time during battery operation. Therefore, applying this supply system to small mobile power sources can achieve higher energy density, energy efficiency and reliability.

　　Ye et al. [18] proposed a passive replenishment system. The CO2 generated by the reaction is used to create a solution density difference in the flow channel to achieve self-circulation. Its battery performance is equivalent to that of an active battery, but performance fluctuations will occur in low current density areas. This is because at low current densities, less CO2 is produced, causing fluctuations in the refueling speed. Abdelkareem et al. [19] placed a porous carbon plate between a high concentration of methanol and a membrane electrode. Using this device, they successfully used a 22.0 mol/L methanol solution for continuous battery operation, with a power density of up to 30 mW/ at room temperature. cm2. The alcohol-blocking effects of different types of carbon plates were also studied, and the results showed that hydrophilic carbon plates are more suitable for use in this device.

　　Yang et al. [20] took advantage of the different surface tension characteristics of methanol and water on a polytetrafluoroethylene (PTFE) film and successfully applied the PTFE film to the anode methanol transmission device. The membrane electrode is in direct contact with pure water, and the pure water is in contact with methanol through the PTFE membrane. Taking advantage of the fact that methanol can penetrate the PTFE membrane but water cannot, methanol can be continuously replenished to the electrode surface to perform continuous electrochemical reactions. At the same time, many researchers use methanol vapor as an anode reactant to improve the permeation phenomenon of methanol [21-23], as shown in Figure 3. Kim [22] used the methanol vapor feed operation method to enable the maximum power density of passive DMFC to reach 25mW/cm2, which is already equivalent to the performance when using liquid methanol.

　　As a component of the anode methanol feed system, there are currently relatively few studies on the anode current collector. Yang et al. [24] found through visual research that using a parallel groove flow field on the anode is more conducive to the discharge of CO2, the methanol oxidation product, and is more suitable as an anode current collector for passive DMFC than a point-like flow field. Litterst et al. [25] designed a tilted current collector structure, which is more conducive to the smooth discharge of CO2. Chuang et al. [26] used a high-speed camera to observe the generation, growth and separation of anode CO2 from the current collector for the first time. The whole process.

　　3 Research progress of cathode air supply system

　　The cathode of passive DMFC is completely exposed to the environment, and the supply of oxygen completely depends on the diffusion and convection of air. During actual use, surrounding environmental factors will inevitably affect the performance and long-term operation of the battery. Liu et al. [27] studied the impact of factors such as humidity and temperature on battery performance under different air environmental conditions. DMFC is not sensitive to changes in ambient humidity, and ambient temperature has a significant promotion effect on battery performance. When the ambient temperature is 40°C, the maximum power density of a single cell is close to 30mW/cm2.

　　Chen et al. [28] studied the impact of different placement methods on the performance of passive DMFC and found that when the battery was placed vertically, due to smooth air convection, more methanol penetrated into the cathode to undergo an oxidation reaction, causing the battery temperature to rise. , improves battery performance. Lai et al. [29] further studied the impact of oxygen transport on the long-term discharge performance of passive DMFC. Passive DMFC has the best long-term discharge performance when the anode is placed upward. The main factor affecting the long-term discharge performance when the anode is placed vertically is that the cathode is flooded, which makes oxygen transmission difficult. The reason why the long-term discharge performance when the anode is placed downward is due to the generation of anode products, which hinders the diffusion of methanol.

　　Due to the phenomenon of water flooding in the cathode, whether the structural design of the cathode current collector can smoothly discharge the reaction products out of the electrode has become an important issue. Yang et al. [24] studied in detail the differences between point current collectors and parallel groove current collectors in passive DMFC cathodes. The article found through visualization methods that point-shaped current collectors are more suitable as passive DMFC cathode current collectors. When the cathode uses a parallel groove current collector, there is a serious flooding phenomenon, which affects the performance of the battery. Chen et al. [30] used Ni2Cr alloy foam materials to make cathode point-like current collectors, which greatly improved the performance of passive DMFC. The main reasons are: (1) The larger specific surface area of porous materials improves oxygen transmission; (2) Compared with The low thermal conductivity ensures the temperature of the battery; (3) The internal mesh structure facilitates the rapid elimination of water.

　　Hwang et al. [31] used the finite element method to simulate the oxygen transmission when the cathode uses a point current collector. The study shows that larger vents can effectively improve oxygen transmission, but it also leads to an increase in battery resistance. The optimal vent diameter is calculated to be 2.1mm.

　　4Research progress on batteries and stack structures

　　Since the operating voltage of a single cell of passive DMFC is low, multiple cells must be connected in series to form a cell stack during use. In order to meet the requirements of miniaturization and portability, passive DThe structural design of MFC is crucial. Martin et al. [32] used organic glass as the casing of the stack to reduce the weight of the system, and used stainless steel mesh as the current collector to reduce the cost of the stack. They successfully assembled a planar three-unit stack with a maximum power density of 8.6 mW/cm2. Due to the large resistance of the stainless steel mesh, the performance of the stack is not high. The author believes that it is necessary to improve the material of the current collector to improve the performance of the stack.

　　Chan et al. [33] assembled a 6-cell passive DMFC stack using a high-concentration methanol solution of 6 mol/L. Since the use of a higher methanol solution increases the temperature of the stack, increases the activity of the electrochemical reaction, accelerates the volatilization of cathode water, and reduces cathode flooding, the maximum power density of the battery can reach 10.3mW/cm2.

　　Recently, many researchers use printed circuit boards (PCB) as the casing of the stack [34] to reduce the weight of the stack, reduce the processing cost of the stack, and improve the mechanical strength of the current collector. However, there is serious corrosion in the gold-plated layer. Phenomenon. Guo et al. [35] studied in detail the corrosion phenomenon and corrosion mechanism of a gold-plated PCB board when used as a passive DMFC current collector. Due to the presence of F- in the solution, the nickel base layer was corroded first, and the corrosion products increased the current collector. The contact resistance is severe, which may cause the gold plating layer to peel off.

　　5 existing problems

　　How to improve energy density and power density, reduce costs, and improve portability are the focus of passive DMFC research. Problems that need to be solved urgently include: how to prepare high-performance membrane electrodes suitable for the working conditions of passive DMFC. It can ensure that the electrolyte membrane has high conductivity while having good anti-alcohol permeation ability and ensure that high-concentration methanol solutions can be used. At the same time, improve the utilization rate of fuel; Anode Methanol Battery Industry Chinese Battery Industry Yin Geping, et al.: How the feeding system of passive direct methanol fuel cell research progress can ensure the uninterrupted replenishment of high-concentration methanol solution and reduce the consumption of methanol solution Concentration fluctuations ensure the stable operation of the battery, and a cheap and lightweight methanol sensor should be developed as soon as possible; the cathode air supply system should ensure that the cathode does not flood, reduce the loss of cathode water, and look for current collector materials to replace graphite plates; the overall stack The design should consider further reducing the weight and volume of the system, allowing for better electrical connections between individual cells and ensuring uniform performance of each individual battery.

　　6 Outlook

　　Volume, weight, and cost are three major factors that determine whether designers and users of portable devices choose passive DMFC to provide energy. What the market needs is batteries that are similar in size and weight to existing rechargeable batteries or even smaller and lighter, with better performance and longer use time. At present, there are still certain difficulties in realizing the industrialization of passive DMFC, which requires the continuous efforts of researchers and continuous breakthroughs in key technologies.

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