Gravity of the PEM fuel cell cathode of water management
This paper tested the gravity of the PEM fuel cell cathode in the water discharged from the impact of. By changing the placement of anode and cathode, the use of electronic load changes the output voltage and current measurement methods, corresponding to different wetting conditions, the cathode and the anode in the last, using the voltage / current density / temperature polarization curve drawn. Anode corresponds to the different upper and lower placement, cell temperature, anode and cathode gas humidification temperature, gas humidification temperature of 40 deg ~ 70 deg synchronize changes between the obtained results of four sets of experimental data. Experimental results show that the PEM fuel cell anode electrode placed in the last, the gravity is conducive to liquid water in the PEM fuel cell cathode discharge; in the PEM fuel cell electrodes placed on the cathode, the gravity is not conducive to liquid water in PEM fuel cell cathode discharge.
Of PEM (proton exchange membrane) fuel cell performance factors, such as temperature, pressure, fuel and oxidizer in the gas flow and other external factors, but also internal factors such as heat and mass transfer. One of the more significant factor is the impact of the internal water management. The quality of water management will directly affect its performance is good or bad, and the water in the PEM fuel cell inside is a double membrane can not be short of water, gas diffusion layer in the water not be blocked, excess water should also be considered discharge and so on.
Many people inside the PEM fuel cell water management research, most are in the form of model to simulate the transfer process. Hua Meng  established a three-dimensional model to simulate the MEA (membrane electrode assembly) within the continuous water transfer process. B. Carnes et al  using one-dimensional and two-dimensional model of the PEM fuel cell inside the proton and water were analyzed. Wei-Mon Yan et al  coupled with a one-dimensional model of the temperature gradient through a detailed analysis of mass transfer and PEM fuel cell membrane of water and heat management.
Trung Van Nguyen et al  from the gas transmission and distribution analysis of the PEM fuel cell stack within the water management. N. Rajalaks hmi et al  have done experiments to measure the battery leads to produce dry gas flow of water and change the shape of the water transport case. JJ Baschuk et al , with a mathematical model of the battery level on the performance of flooding. The existing two-dimensional and three-dimensional mathematical model of PEM fuel cell performance, the internal water transport and distribution of the simulation analysis. No gravity to be considered basic, and some only mentioned in the model, there is no way to verify experimentally the influence of gravity on the water.
This article is a PEM fuel cell anode and cathode humidification under different experimental PEM fuel cell cathode of gravity on the transfer of liquid water, thus affecting the performance of PEM fuel cells, designed a number of experimental programs, and down by changing the cathode and the anode placed position, highlighting the gravity of the PEM fuel cell current density.
A theoretical analysis
In the mathematical model of the process, most people always use the conservation of mass, momentum and energy conservation of the three conservation equations. The mathematical model has been established now, most have used these three equations. However, the momentum equation used in the process, many people have overlooked the influence of gravity term.
PEM fuel cells in the momentum equation, the fluid is mainly to analyze the fuel gas, oxidant gas and water, they are Newtonian fluid, incompressible fluid, the simplified form of the momentum equation for the vector
Type (1) is momentum conservation equation, referred to as momentum equation, also known as the Navier - Stokes equations. As can be seen from the equation of gravity term, we believe that the gravity term in this equation is the role not be ignored. So the design on the gravity of the PEM fuel cell performance of the experiment.
2 Experimental System
Used in the experiment area is 2.24cm x 2.23cm single cell, membrane electrode assembly is Nafion115 film and platinum electrode loading of 0.4mg/cm2 composition of the diffusion layer is carbon paper used, MEA out of the two graphite plate, with two gold-plated copper clamp. Board is three rows of serpentine graphite flow channel.
In experiments using electrochemical tester is produced by the United States MTS150 tester, it can be battery temperature measurement and control, display flow rate and back pressure. In this test system, the reaction gas is conducted through an external humidification of the gas humidifier. Adjusting the temperature of the gas humidification to control the gas humidity. Humidification temperature is controlled by humidifier temperature on the table control. Back pressure by the back pressure valve control, battery heating is installed in a single cell by the two sides of heating plate, the battery temperature by measuring the center of the electrodes near the battery temperature, battery external electronic load, electronic load measured by changing the different voltage and current experimental data, the experimental data to draw the polarization curve of its performance. Test system as shown. PEM fuel cell geometry are listed in Table 1.
3 Results and discussion
The distribution of water affect the internal battery main factors: (1) electromigration effect. From the anode to the cathode of proton transfer in the process of proton exchange membrane from the anode side to take the form of hydronium part of the water, hence a change in the distribution of water proton exchange membrane; (2) Anti-proliferation effect. Molar concentration of water higher than the cathode anode, the water will be from the cathode of proton exchange membrane proton exchange membrane lateral diffusion anode side, this is precisely the role of water and electricity Transfer the contrary, in general, the electric migration of water stronger than the effect of counter-proliferation; (3) the water produced in the cathode. Electrochemical reaction at the cathode will continue to produce water, water and current is proportional to; (4) The reaction gas water content. Fuel and oxidizer gases are processed through the wet gas, with water vapor, which will bring the battery part of the water.
In a large number of relevant literature, that the proton exchange membrane fuel cell anode position monomer mostly placed vertically parallel to the plates, and the focus of most of the test is to observe the temperature, pressure, temperature and other external humidification conditions on the proton exchange membrane fuel cell performance. We test the gravity of the battery to the internal cathode discharge of liquid water, so as to arrive gravity on the battery performance. Experimental program designed as follows, from top to bottom relative anode placed, as shown in Figure 2, the cathode, or anode as shown in the last. Using the relative placement of the anode and cathode changes, observed gravity on the proton exchange membrane fuel cell performance
Corresponds to the cathode and the anode in the placement on and do a few experiments, experimental conditions of temperature and humidity changes in battery temperature simultaneously, other conditions and experimental results
Different output voltage, different battery temperature, current density and contrast. When in E = 0.85V performance comparison under different conditions. Relatively close to open circuit voltage of 0.85V, the polarization effect by the electrochemical polarization. From the view, at 40 deg and 50 deg circumstances, the merits of battery performance interspersed; at 60 deg and 70 deg cases, seemingly irregular, but still rule-based, non-humidification cathode (anode humidification ) current density than non-humidification anode (cathode humidification) of the current density is better. Reasons for this situation is caused by the anode gas humidification anode side is not less water, affecting the resistivity, thereby affecting the current density, and because the test is continuous, there are a lot of water as the temperature rises into the cell interior, the cathode side of the water caused by too much, causing the consolidated results.
Comparison of performance under different conditions. This voltage is generally the work of the battery voltage. Seen from the anode is not wetting (cathode humidification) performance was not without humidification cathode (anode humidification) performance is good, mainly because of the water balance inside the battery is different. Under normal circumstances, the battery passed the direction of integrated water inside the anode side from the cathode side of the pass. Humidification at the anode is not the case, the cathode side of the battery cathode reaction gas into the water and the water inside the battery integrated, resulting in significant excess of the cathode side of the water, but not be discharged into the battery externally, resulting in the accumulation of liquid water, which plug the porous gas diffusion layer, the transfer of oxidizing agents, eventually leading to a decline in battery performance; and no humidification at the cathode, anode humidification cases, this water mass transfer was significantly lower, due to the anode side of humidified gas, not easily lead to dehydration of the anode side, and the absence of the cathode gas humidification, water is the main source of batteries and battery from the anode reaction is passed over the water, basically the water in time with the cathode gas discharge to the battery externally. From which can be seen that with increasing temperature, battery performance has declined, mainly carried out experiments in a row, causing the battery internal moisture accumulation as the temperature rises, so the performance of the back. Anode can be seen clearly in the current density than the cathode current density on the better. Anode, the cathode in the next, the bottom of the drainage flow is smooth graphite material, as shown in Figure 5, between liquid water and the flow resistance, with the cathode residual gas flow to the battery discharged outside; cathode , the anode in the next, due mainly cathode drainage, so the formation of the cathode side of the drainage channel, the bottom is the gas diffusion layer, as shown in Figure 6, there are many pores, increasing the resistance of liquid water as the gas flow , and it is easy to form a layer of gas diffusion layer of water film, impede gas transport. Thus affecting its performance.
Cells under different conditions the maximum output power. It can be seen from the figure, only at 40 deg and 70 deg without wetting the case of the anode, the cathode than the anode in the performance a little better in the performance, the effect is not too obvious. Especially non-humidification anode 70 deg, the maximum output power is very small. But not at 40 deg, anode humidification power is relatively high, because the beginning of the experiment, the battery is not a lot of water inside the cathode, while the temperature is relatively low, the water vapor pressure is relatively low, less water into the cathode so that the cathode water input and output reached a good balance, there is no blocking the cathode gas diffusion layer, which did not affect the cathode gas transport, so the cells showed a good performance. Higher maximum power point is 40 deg, 50 deg and 60 deg, no humidification cathode (anode humidification) conditions. Under these conditions, the cathode is not humidified to solve the problem of excess moisture cathode, anode humidification anode solve the problem of dehydration, so the performance is quite good, but at 70 deg, the water saturation vapor pressure is very high, into the internal battery excess moisture, resulting in anode submerged, thus affecting its performance, the performance of the anode than the cathode in the better performance in the last few.
(1) Under the same conditions, the anode in place, the same voltage, PEM fuel cell cathode current density than when placed in the large.
(2) in the other under the same experimental conditions, the same voltage, anode humidification (cathode without humidification), the PEM fuel cell humidification cathode current density (anode without wetting) current density.
(3) on the anode (cathode in the next) case, the excess liquid water within the cathode than the cathode in the last (the anode in the next) is easily discharged to the PEM fuel cell external.
(4) in the PEM fuel cell applications, consider the PEM fuel cell anode and cathode of the upper and lower placement.
(5) in the establishment of mathematical model of PEM fuel cell when the gravity should not be ignored.
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