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There are many factors affecting the performance of PEM (proton exchange membrane) fuel cells, such as external factors such as temperature, pressure, gas flow of fuel and oxidant, and internal factors such as mass transfer heat transfer. One of the more important factors is internal water management. The quality of water management will directly affect its performance, and water is a double-edged sword inside the PEM fuel cell. There should be no water shortage in the membrane. The gas diffusion layer cannot be blocked by water, and excessive moisture should be considered. Drain and so on.
Many people have studied the water management inside the PEM fuel cell, and most of them simulate the transfer process in the form of a model. Hua Meng [1] established a three-dimensional model to simulate the continuous water transfer process in MEA (membrane electrode assembly). B. Carnes et al. [2] used a one-dimensional and two-dimensional model to analyze the protons and water inside the PEM fuel cell. Wei-Mon Yan et al. [3] used a one-dimensional model to analyze the hydrothermal management of PEM fuel cell membranes by coupling temperature gradient and mass transfer.
Trung Van Nguyen et al. [4] analyzed the water management inside the PEM fuel cell stack from the aspect of gas transfer and distribution. The experiment done by N. Rajalaks hmi et al. [5] is to measure the amount of water produced by the dry gas passing through the battery and change the shape of the flow channel to analyze the water transmission. JJ Baschuk et al. [6] used mathematical models to simulate the effect of battery flooding on performance. The existing two-dimensional and three-dimensional mathematical models simulate the performance of PEM fuel cells, internal water transfer and distribution. Basically, gravity is not considered, and some are only mentioned in the model. There is no experimental method to verify the effect of gravity on water.
This paper mainly focuses on the influence of experimental gravity on the cathode liquid water transfer of PEM fuel cells under different humidification conditions of PEM fuel cell anode and cathode, thus affecting the performance of PEM fuel cell. Some experimental schemes are designed to change the cathode and anode. Position, highlighting the effect of gravity on the current density of PEM fuel cells.
1 Theoretical analysis In the process of establishing mathematical models, most people always use the three conservation equations of mass conservation, momentum conservation and energy conservation. Most of the mathematical models that have been established now use these three equations. However, in the process of using the momentum equation, many people ignore the influence of the gravity term.
In the PEM fuel cell momentum equation, the fluids to be analyzed are mainly fuel gas, oxidant gas and water, etc. They belong to Newtonian fluid, incompressible fluid, and the vector form of the momentum equation after simplification is equation (1) is the momentum conservation equation. Referred to as the momentum equation, also known as the Navier-Stokes equation. It can be seen from the equation that there is a gravity term, and we believe that the role of the gravity term in this equation cannot be ignored. Therefore, an experiment was conducted on the effect of gravity on the performance of PEM fuel cells.
2 Experimental system In the experiment, a single cell of 2.24 cm × 2.23 cm area was used. The membrane electrode assembly was composed of a Nafion 115 membrane and an electrode with a platinum loading of 0.4 mg/cm 2 . The diffusion layer was made of carbon paper, and the outside of the MEA was Two graphite plates, clamped with two gold-plated copper plates. The graphite plate is a three-row serpentine flow path.
The tester used in the experiment was an MTS150 tester manufactured by American Electrochemical Company, which can measure the temperature of the battery, display flow and back pressure. In this test system, the reaction gas is humidified by an external gas humidifier. The temperature at which the gas is humidified is adjusted to control the humidity of the gas. The humidification temperature is controlled by a temperature control table on the humidifier. The back pressure is controlled by a back pressure valve. The battery is heated by two heating plates mounted on both sides of the battery. The battery temperature is measured by measuring the temperature of the electrode plate near the center of the battery. The external electronic load of the battery is measured by changing the electronic load. The experimental data of voltage and current, according to the experimental data, draw a polarization curve and analyze its performance. The test system is as shown. The geometry of the PEM fuel cell is listed in Table 1.
3 Experimental results and discussion The main factors affecting the distribution of water inside the battery: (1) Electromigration. During the transfer of the proton from the anode to the cathode, a part of the water is taken away from the anode side of the proton exchange membrane in the form of hydronium ions, thus changing the distribution of water in the proton exchange membrane; (2) anti-diffusion. Since the molar concentration of the cathode water is higher than that of the anode, water will diffuse from the cathode side of the proton exchange membrane to the anode side of the proton exchange membrane. This action on water is exactly the opposite of electromigration, and the general condition is electromigration to water. The effect is greater than the effect of back diffusion; (3) the water generated at the cathode. The electrochemical reaction also produces water continuously at the cathode, and the amount of water is proportional to the current; (4) the water content of the reaction gas. Both the fuel and oxidant gases are humidified gases with water vapor, which also brings a portion of the water to the battery.
In a large number of related literatures, the positional pendulum amplification of the proton exchange membrane fuel cell single anode is mostly placed in parallel and vertical, and most people focus on the observation of temperature, pressure, humidification temperature, etc. The effect of conditions on the performance of proton exchange membrane fuel cells. In order to test the effect of gravity on the discharge of cathode liquid water inside the battery, we can derive the effect of gravity on the performance of the battery. The following experimental scheme was designed in which the anode and cathode were placed upside down, as shown in Fig. 2, or the anode was as shown. The effects of gravity on the performance of the proton exchange membrane fuel cell were observed by the change of the relative positions of the cathode and the anode. Corresponding to the position of the cathode on the upper and the anode, several experiments were carried out. The experimental conditions were humidification temperature and battery temperature. Synchronous changes, other conditions and experimental results differ from output voltage, current cell density at different battery temperatures. It is a comparison of performance under different conditions at E=0.85V. 0.85V is closer to the open circuit voltage, and the polarization effect is electrochemical polarization. From the point of view, at 40 ° C and 50 ° C, the performance of the battery is staggered; at 60 ° C and 70 ° C, it seems irregular, but there are rules to follow, the cathode is not humidified (anode humidification) The current density is better than the current density of the anode without humidification (cathode humidification). The reason for this is that the anode gas does not humidify, causing less moisture on the anode side, affecting the resistivity, thus affecting the current density, and because of continuous testing, a large amount of moisture is brought into the battery with an increase in temperature. The excessive moisture on the cathode side causes the above results.
Comparison of performance under different conditions. Generally this voltage is the operating voltage of the battery. It can be seen that the performance of the anode without humidification (cathode humidification) is obviously not as good as that of the cathode without humidification (anode humidification), mainly because the water balance inside the battery is different. Under normal circumstances, the integrated water transfer direction inside the battery is transmitted from the anode side to the cathode side. When the anode is not humidified, the water generated by the reaction of the battery on the cathode side and the moisture brought into the battery interior by the cathode gas are combined, causing a significant excess of moisture on the cathode side, and is not discharged to the outside of the battery in time, thereby causing accumulation of liquid water, thereby Blocking the porous gas diffusion layer, affecting the transfer of the oxidant, and ultimately causing the battery performance to decrease; while the cathode is not humidified, the anode is humidified, the effect of the water on the mass transfer is obviously small, because there is humidified gas on the anode side, It is easy to cause dehydration on the anode side, and since there is no humidified gas on the cathode side, the main source of moisture is the water generated by the battery from the anode and the reaction of the battery, which is basically discharged to the outside of the battery with the cathode gas in time. It can also be seen that as the temperature increases, the performance of the battery decreases. This is mainly because the experiment is continuous, causing the internal moisture of the battery to accumulate with the increase of temperature, thus affecting the performance behind. It can also be seen that the current density at the anode is significantly better than the current density at the cathode. When the anode is on the bottom and the cathode is on the bottom, the bottom of the drainage channel is a smooth graphite material. As shown in Fig. 5, the resistance between the liquid water and the flow channel is small, and the remaining gas of the cathode flows out to the outside of the battery; the cathode is Above, when the anode is down, since the cathode is mainly drained, the bottom of the drainage channel formed on the cathode side is a gas diffusion layer. As shown in Fig. 6, there are many micropores, which increase the resistance of liquid water with gas flow. And it is easy to form a water film on the gas diffusion layer, hindering gas mass transfer. Thereby affecting its performance.
The maximum output power of the battery under different conditions. As can be seen from the figure, only in the case where the anode is not humidified at 40 ° C and 70 ° C, the performance of the cathode is better than that of the anode, and the effect is not too obvious. Especially in the case where the anode at 70 ° C is not humidified, the maximum output power is very small. At 40 ° C, the anode does not humidify the power is higher, because the initial experiment, the internal cathode moisture of the battery is not much, and because the temperature is relatively low, the saturated vapor pressure of water is also relatively low, the water brought into the cathode is relatively small The cathode water input and output are well balanced, and the cathode gas diffusion layer is not blocked, so that the gas mass transfer of the cathode is not affected, so the battery exhibits good performance. The point where the maximum power is relatively high is 40 ° C, 50 ° C and 60 ° C under the condition that the cathode is not humidified (anode humidification). Under such conditions, the cathode does not humidify to solve the problem of excess moisture in the cathode. The anode humidification solves the problem of anode dehydration, so the performance is relatively good, but at 70 ° C, the saturated vapor pressure of water is very high, and it is brought into the interior of the battery. The excess moisture causes the anode to submerge, which affects its performance. The performance of the anode is better than that of the cathode.
4 Conclusions (1) Under the same conditions, when the anode is placed on top, the current density of the PEM fuel cell is greater than that when the cathode is placed on the same voltage.
(2) Under the same experimental conditions, the PEM fuel cell current density is greater than the cathode humidification (anode non-humidification) current density at the same voltage and the anode is humidified (the cathode is not humidified).
(3) In the case where the anode is on the upper side (the cathode is on the bottom), excess liquid water in the cathode is easily discharged to the outside of the PEM fuel cell when the cathode is on the upper side (the anode is on the bottom).
(4) In the application of PEM fuel cells, the upper and lower placement positions of the anode and cathode of the PEM fuel cell should be considered.
(5) Gravity should not be ignored when establishing a mathematical model of a PEM fuel cell.
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Effect of gravity on cathode water management of PEM fuel cells
This paper tests the effect of gravity on the discharge of cathode water from a PEM fuel cell. By changing the position of the anode and cathode, the method of changing the output voltage and current by changing the electronic load, corresponding to different humidification conditions, the cathode is on the upper side and the anode is on, and the polarization curve is drawn by using voltage/current density/temperature. Corresponding to the position of the anode and cathode placed up and down, the battery temperature, anode gas humidification temperature and cathode gas humidification temperature are synchronously changed between 40 °C and 70 °C, and four sets of experimental data are obtained. The experimental results show that gravity is beneficial to the discharge of cathode liquid water of PEM fuel cell when the anode of the PEM fuel cell is placed on the anode. When the cathode of the PEM fuel cell is placed on the cathode, gravity is not conducive to the cathode liquid water of the PEM fuel cell. discharge.