Research on air cathode for vertical methanol fuel-powered lithium battery -Lithium - Ion Battery Equipment
The vertical methanol fuel-powered lithium battery (DMFC) is a power generation device that converts the chemical energy stored in the fuel methanol solution and the oxidant (oxygen or air) vertically into electrical energy. Its significant advantages are: abundant sources of methanol fuel and low cost. The energy density is high. When the battery is working, the fuel is fed straightly without reforming. The structure is simple, the reaction time is short, the operation is convenient, and it is easy to carry and store. It is an ideal power source for portable electronic devices, mobile phones, cameras and electric vehicles. , is considered to be the battery most likely to be used commercially, and thus has received great attention.
According to the working principle of DMFC, water is generated at the air cathode, and its reaction formula is:
If the water generated on the cathode side of the air and the anode methanol solution diffuse to the cathode through the Nafion membrane are not enough to make up for the moisture brought out by the large amount of air in the cathode, the cathode water balance will be destroyed, causing the proton exchange membrane on the air electrode side to lose When the water dries out, the internal resistance of the battery increases significantly, the battery performance decreases rapidly, and finally the battery becomes difficult to operate normally. To this end, we focused on studying the electrochemical effects of linear methanol fuel-powered lithium batteries under normal temperature and pressure conditions, using methanol liquid as anode fuel and air as oxidant. Different operating process parameters, such as air humidification, air flow and air humidification temperature, Performance impact.(Lithium - Ion Battery Equipment)
1. Experimental part
1.1 Reagents, materials and instruments
The anode and cathode catalysts are respectively pt-Ru/C (mass fraction 90%) and pt/C (mass fraction 40%) processed by Johnson Matthey Co. in the United Kingdom, Nafion117 membrane processed by DuPont Company in the United States, and Nafion with a mass fraction of 10%. Solution, carbon paper processed by Japan Toray Company, pTFE solution with a mass fraction of 60%, carbon black (VulcanXC-72), isopropyl alcohol (chemical reagent), saturated calomel electrode, bT01-100 peristaltic pump, LZb liquid Rotor flow meter, XMTb digital display temperature control box, REX-C700 digital control heater, ACO-318 air pump and VMp2 electrochemical comprehensive detector (Princeton Company, USA).
1.2 Production of flow field plate
Both cathode and anode flow field plates are made of graphite plates, and their flow field size is 20mm×25mm. Use flow field engraving equipment to make a single-channel serpentine flow field and sealing groove on the graphite plate. The groove depth, groove width and ridge width of the single-channel serpentine flow field are all 1mm. The sealing material is silicone resin or glass glue.
1.3 Preparation of membrane electrode (MEA)
1) Pre-solution of Nafion117 membrane: boil it in a hydrogen peroxide solution with a volume fraction of 3% for 0.5h, take it out, rinse it with deionized water three times, put it into a 2mol/L sulfuric acid solution and boil it for 1h to protonate it. Then rinse with deionized water several times and leave it in deionized water for later use.
2) Preparation of the diffusion layer: Mix a certain amount of pTFE emulsion, carbon black, Nafion solution and isopropyl alcohol aqueous solution and dissolve it with ultrasonic waves for 30 minutes, then drop-coat it on two pieces of carbon paper with an area of 20mm×25mm, dry it and set aside. .
3) Preparation of the catalytic layer: Take a certain amount of pt-Ru/C and pt/C, add a certain proportion of Nafion solution and isopropyl alcohol aqueous solution respectively, solve it with ultrasonic waves 200 times, and then coat it on the diffusion layer that has been solved in advance. And dried in a vacuum drying oven for 12h.
4) Hot pressing molding of MEA: Place the above two pieces of carbon paper containing the diffusion layer and the catalytic layer on both sides of the resolved Nafion117 membrane, and hot press it for 3 minutes at 135°C and 1Mpa to obtain a methanol electrode. MEA composed of air electrode and electrolyte membrane.
1.4 Single cell assembly and performance detection
Put the above MEA into two self-made graphite flow field plates with an effective area of 5cm2, add current collecting plates, insulating sheets and end plates on both sides, clamp and seal, and assemble into a single cell. The battery is heated with a hot rod and the temperature is measured with a thermocouple. Its performance was measured on the electrochemical integrated detection system VMp2 (princetonAppliedReseach). The reactants are methanol and air, and the reaction conditions are normal temperature and normal pressure.
2. Results and discussion
2.1 Single battery performance
Activation experimental conditions: The membrane electrode is placed in the battery detection device, the anode is passed through the methanol solution, and then the peristaltic pump is stopped to allow the static methanol solution to slowly diffuse; the cathode uses air to diffuse naturally, and then operates at 35°C with a small current density discharge for 9 hours. The experimental conditions for performance detection are: low temperature, normal pressure, methanol concentration 1.5mol/L, battery temperature 35°C, methanol flow rate 2.5mL/min, and the cathode is fed with natural air.
The cathode air is transported by a small air pump, passes through the air flow meter, and enters the first humidifier. After the air is humidified, it then enters the second humidifier, and finally enters the battery cathode. The temperatures of the two humidifiers are controlled by a constant temperature water bath. The first humidifier is mainly used for air humidification and temperature control. The second humidifier is mainly used to prevent the humidified air from bringing water into the battery cathode, causing flooding of the cathode, affecting battery performance, and serving as buffer and temperature control. use. As can be seen from Figure 1, when the output voltage of a single cell is 0.277V, its output current density and peak power density reach 142.6mA/cm2 and 39.5mW/cm2 respectively.
2.2 Effect of air humidification on battery performance
Cathode air humidification has a significant impact on the steady-state current-voltage polarization curve of the battery. The battery performance after the cathode air is humidified is obviously better than that without humidification. The main reason is that the water balance of the air cathode is imbalanced, which leads to difficulty in proton transmission in the membrane and reduced battery performance. If the water generated on the cathode side of the air and the anode methanol solution diffuse to the cathode through the Nafion membrane are not enough to make up for the moisture brought out by the large amount of air in the cathode, the cathode water balance will be destroyed, causing the proton exchange membrane on the air electrode side to lose The water dries out, causing difficulty in proton transmission in the membrane and structural changes in the membrane electrode (for example, shrinkage of the membrane due to loss of water will cause the contact between the catalytic layer and the membrane to loosen, etc.), resulting in a decrease in battery performance.
2.3 Effect of air humidification temperature on battery performance
Since DMFC uses methanol solution, relative to pEMFC, it can better maintain the water balance of the Na-fion117 membrane and improve the conductivity of the membrane. There are not many literature reports on the impact of cathode air humidification temperature on battery performance. Experiments have found that air humidification temperature has a greater impact on battery performance.
As the air humidification temperature increases, the battery performance improves greatly. Under the same conditions with other process parameters, when the air humidification temperature is 30°C, the battery open circuit voltage is 0.581V and the battery peak power is 10.319mW/cm2; when the air humidification temperature is increased to 60°C, the battery open circuit voltage is 0.721V, the peak power of the battery can reach 12.869mW/cm2. The increase in humidification temperature, on the one hand, increases the temperature of the battery and accelerates the rate of cathode electrochemical reaction; on the other hand, it also causes the air to obtain more moisture, thereby making up for the loss of moisture brought out of the battery by the air, to a certain extent. This ensures the water balance of the membrane electrode and prevents the Nafion117 membrane from drying out and causing the membrane resistance to rise sharply due to excessive water loss. At the same time, the experiment also showed that the air humidification temperature is too high, causing the air humidity to be too high, bringing in too much water, and when the battery is discharged at a larger current density, a large amount of cathode reaction product water will be added, causing the air to have no time to remove it. The purging and discharge of moisture from the cathode can easily cause "electrode flooding" in the cathode flow field, leading to a decline in battery performance. Therefore, the air humidification temperature is generally controlled between 40 and 60°C.
2.4 Effect of air flow on battery performance
If the air flow is too low, the oxygen concentration of the cathode reactant will be reduced, and the battery performance will be reduced; if the air flow is too high, although the amount of oxygen in the cathode reactant will be increased, when the oxygen is enough to satisfy the cathode reaction, just adding oxygen will not be beneficial to the battery. The improvement of performance will, on the contrary, cause a large amount of water from the cathode to be taken away, causing the cathode water balance to become unbalanced, the internal resistance of the membrane electrode to rise, and the battery performance to decline.
3.Conclusion
Using pt-Ru/C and pt/C as anode and cathode catalysts, self-made membrane electrodes were assembled, and a DMFC single cell and detection system were assembled. Using the steady-state current-voltage polarization curve method, the effects of air humidification, air humidification temperature and air flow on the electrochemical performance of DMFC were studied. The research results state that the performance of batteries with air humidification is significantly better than that of batteries without air humidification. The optimal operating process parameters of air humidification temperature and air flow are 40~60℃ and 670mL/min respectively. Under 35℃ and normal pressure conditions, when the DMFC output voltage is 0.277V, its output current density and peak power density can reach 142.6mA/cm2 and 39.5mW/cm2 respectively.
According to the working principle of DMFC, water is generated at the air cathode, and its reaction formula is:
If the water generated on the cathode side of the air and the anode methanol solution diffuse to the cathode through the Nafion membrane are not enough to make up for the moisture brought out by the large amount of air in the cathode, the cathode water balance will be destroyed, causing the proton exchange membrane on the air electrode side to lose When the water dries out, the internal resistance of the battery increases significantly, the battery performance decreases rapidly, and finally the battery becomes difficult to operate normally. To this end, we focused on studying the electrochemical effects of linear methanol fuel-powered lithium batteries under normal temperature and pressure conditions, using methanol liquid as anode fuel and air as oxidant. Different operating process parameters, such as air humidification, air flow and air humidification temperature, Performance impact.(Lithium - Ion Battery Equipment)
1. Experimental part
1.1 Reagents, materials and instruments
The anode and cathode catalysts are respectively pt-Ru/C (mass fraction 90%) and pt/C (mass fraction 40%) processed by Johnson Matthey Co. in the United Kingdom, Nafion117 membrane processed by DuPont Company in the United States, and Nafion with a mass fraction of 10%. Solution, carbon paper processed by Japan Toray Company, pTFE solution with a mass fraction of 60%, carbon black (VulcanXC-72), isopropyl alcohol (chemical reagent), saturated calomel electrode, bT01-100 peristaltic pump, LZb liquid Rotor flow meter, XMTb digital display temperature control box, REX-C700 digital control heater, ACO-318 air pump and VMp2 electrochemical comprehensive detector (Princeton Company, USA).
1.2 Production of flow field plate
Both cathode and anode flow field plates are made of graphite plates, and their flow field size is 20mm×25mm. Use flow field engraving equipment to make a single-channel serpentine flow field and sealing groove on the graphite plate. The groove depth, groove width and ridge width of the single-channel serpentine flow field are all 1mm. The sealing material is silicone resin or glass glue.
1.3 Preparation of membrane electrode (MEA)
1) Pre-solution of Nafion117 membrane: boil it in a hydrogen peroxide solution with a volume fraction of 3% for 0.5h, take it out, rinse it with deionized water three times, put it into a 2mol/L sulfuric acid solution and boil it for 1h to protonate it. Then rinse with deionized water several times and leave it in deionized water for later use.
2) Preparation of the diffusion layer: Mix a certain amount of pTFE emulsion, carbon black, Nafion solution and isopropyl alcohol aqueous solution and dissolve it with ultrasonic waves for 30 minutes, then drop-coat it on two pieces of carbon paper with an area of 20mm×25mm, dry it and set aside. .
3) Preparation of the catalytic layer: Take a certain amount of pt-Ru/C and pt/C, add a certain proportion of Nafion solution and isopropyl alcohol aqueous solution respectively, solve it with ultrasonic waves 200 times, and then coat it on the diffusion layer that has been solved in advance. And dried in a vacuum drying oven for 12h.
4) Hot pressing molding of MEA: Place the above two pieces of carbon paper containing the diffusion layer and the catalytic layer on both sides of the resolved Nafion117 membrane, and hot press it for 3 minutes at 135°C and 1Mpa to obtain a methanol electrode. MEA composed of air electrode and electrolyte membrane.
1.4 Single cell assembly and performance detection
Put the above MEA into two self-made graphite flow field plates with an effective area of 5cm2, add current collecting plates, insulating sheets and end plates on both sides, clamp and seal, and assemble into a single cell. The battery is heated with a hot rod and the temperature is measured with a thermocouple. Its performance was measured on the electrochemical integrated detection system VMp2 (princetonAppliedReseach). The reactants are methanol and air, and the reaction conditions are normal temperature and normal pressure.
2. Results and discussion
2.1 Single battery performance
Activation experimental conditions: The membrane electrode is placed in the battery detection device, the anode is passed through the methanol solution, and then the peristaltic pump is stopped to allow the static methanol solution to slowly diffuse; the cathode uses air to diffuse naturally, and then operates at 35°C with a small current density discharge for 9 hours. The experimental conditions for performance detection are: low temperature, normal pressure, methanol concentration 1.5mol/L, battery temperature 35°C, methanol flow rate 2.5mL/min, and the cathode is fed with natural air.
The cathode air is transported by a small air pump, passes through the air flow meter, and enters the first humidifier. After the air is humidified, it then enters the second humidifier, and finally enters the battery cathode. The temperatures of the two humidifiers are controlled by a constant temperature water bath. The first humidifier is mainly used for air humidification and temperature control. The second humidifier is mainly used to prevent the humidified air from bringing water into the battery cathode, causing flooding of the cathode, affecting battery performance, and serving as buffer and temperature control. use. As can be seen from Figure 1, when the output voltage of a single cell is 0.277V, its output current density and peak power density reach 142.6mA/cm2 and 39.5mW/cm2 respectively.
2.2 Effect of air humidification on battery performance
Cathode air humidification has a significant impact on the steady-state current-voltage polarization curve of the battery. The battery performance after the cathode air is humidified is obviously better than that without humidification. The main reason is that the water balance of the air cathode is imbalanced, which leads to difficulty in proton transmission in the membrane and reduced battery performance. If the water generated on the cathode side of the air and the anode methanol solution diffuse to the cathode through the Nafion membrane are not enough to make up for the moisture brought out by the large amount of air in the cathode, the cathode water balance will be destroyed, causing the proton exchange membrane on the air electrode side to lose The water dries out, causing difficulty in proton transmission in the membrane and structural changes in the membrane electrode (for example, shrinkage of the membrane due to loss of water will cause the contact between the catalytic layer and the membrane to loosen, etc.), resulting in a decrease in battery performance.
2.3 Effect of air humidification temperature on battery performance
Since DMFC uses methanol solution, relative to pEMFC, it can better maintain the water balance of the Na-fion117 membrane and improve the conductivity of the membrane. There are not many literature reports on the impact of cathode air humidification temperature on battery performance. Experiments have found that air humidification temperature has a greater impact on battery performance.
As the air humidification temperature increases, the battery performance improves greatly. Under the same conditions with other process parameters, when the air humidification temperature is 30°C, the battery open circuit voltage is 0.581V and the battery peak power is 10.319mW/cm2; when the air humidification temperature is increased to 60°C, the battery open circuit voltage is 0.721V, the peak power of the battery can reach 12.869mW/cm2. The increase in humidification temperature, on the one hand, increases the temperature of the battery and accelerates the rate of cathode electrochemical reaction; on the other hand, it also causes the air to obtain more moisture, thereby making up for the loss of moisture brought out of the battery by the air, to a certain extent. This ensures the water balance of the membrane electrode and prevents the Nafion117 membrane from drying out and causing the membrane resistance to rise sharply due to excessive water loss. At the same time, the experiment also showed that the air humidification temperature is too high, causing the air humidity to be too high, bringing in too much water, and when the battery is discharged at a larger current density, a large amount of cathode reaction product water will be added, causing the air to have no time to remove it. The purging and discharge of moisture from the cathode can easily cause "electrode flooding" in the cathode flow field, leading to a decline in battery performance. Therefore, the air humidification temperature is generally controlled between 40 and 60°C.
2.4 Effect of air flow on battery performance
If the air flow is too low, the oxygen concentration of the cathode reactant will be reduced, and the battery performance will be reduced; if the air flow is too high, although the amount of oxygen in the cathode reactant will be increased, when the oxygen is enough to satisfy the cathode reaction, just adding oxygen will not be beneficial to the battery. The improvement of performance will, on the contrary, cause a large amount of water from the cathode to be taken away, causing the cathode water balance to become unbalanced, the internal resistance of the membrane electrode to rise, and the battery performance to decline.
3.Conclusion
Using pt-Ru/C and pt/C as anode and cathode catalysts, self-made membrane electrodes were assembled, and a DMFC single cell and detection system were assembled. Using the steady-state current-voltage polarization curve method, the effects of air humidification, air humidification temperature and air flow on the electrochemical performance of DMFC were studied. The research results state that the performance of batteries with air humidification is significantly better than that of batteries without air humidification. The optimal operating process parameters of air humidification temperature and air flow are 40~60℃ and 670mL/min respectively. Under 35℃ and normal pressure conditions, when the DMFC output voltage is 0.277V, its output current density and peak power density can reach 142.6mA/cm2 and 39.5mW/cm2 respectively.
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