The electrochemical performance test mainly includes polarization curve, power density and electrochemical impedance. Physical performance test mainly includes water contact angle, SEM, roughness and in-plane resistance test.
4.1 Analysis of polarization curve test results
We have prepared three types of anode GDL to tested the influence of anode GDL on fuel cell performance when the temperature is 80°C, and the humidity is 40%, 60% and 100%. The polarization curve test of GDL29BC at the temperature of 80°C and humidity of 40% was used as reference group.
The polarization curves of GDL 2:8,1:9 and GDL 3:7,2:8 basically coincide. The reason may be that the PTFE content of the double MPL in GDL 2:8,1:9 and 3:7,2:8 is not much different, so it does not have good self-humidification effect, and finally the performance of GDL 2:8,1:9 and GDL 3:7,2:8 are close to the same when humidity is 40%. According to the polarization curve test result, GDL 3:7,1:9 can show good performance regardless of low or high current density when running humidity is 40%. We believes that when PEMFC running at small current density, GDL 3:7,1:9 can store amount of water in MPL near the anode CL side to achieve self-humidification. Therefore, the performance is significantly different during operation at small current density. The test results in figure 1 is comparison diagram of GDL 3:7,1:9 and 2:8,1:9. Although the ratio of PTFE to carbon black in MPL near anode CL side is 1:9, GDL show significant differences in performance. The reason for this phenomenon is that the content of PTFE in MPL near carbon paper side of GDL 3:7,1:9 is higher, the higher the content of PTFE, the higher the efficiency of water vapor condensing into water. Therefore, GDL 3:7,1:9 can not only improve the coagulation efficiency of water, moreover, due to the large difference in PTFE in the double microporous layer, the condensed water is easier to transfer from hydrophobic pores to hydrophilic pores. Finally, the water content in MPL near the anode CL side of GDL 3:7,1:9 is relatively high, which is more conducive to improving the self-humidification effect.
When fuel cell is running at high current density, GDL 3:7,1:9 can still get good performance. We believes that the produces more water under high current density, part of water flows out of the fuel cell through cathode GDL, and the other part flows from cathode to anode through penetration. The penetration rate of water is affected by hydrophilicity and hydrophobicity of the cathode and anode, and the hydrophilicity of anode is proportional to the penetration rate of water. The lower the content of PTFE in the MPL near the anode CL side, the more hydrophilic pores in the MPL, and the faster the water penetration rate. When PEMFC is running at high current density, more H+ are required to participate in the reaction, and more water is needed to assist H+ transmission. Therefore, GDL 3:7,1:9 can increase water penetration rate of PEMFC under high current to provide more water for the transmission of H+, thereby enabling the PEMFC to maintain good performance.
Then compared the polarization curve when temperature is 80°C and the humidity is 60%. It is found that with the humidity increases, the impact of the three anode GDL performance has not changed greatly, especially when the current density is between 0-3A/cm2, there is almost no difference in performance of the three anode GDLs. It can be inferred that when PEMFC is running under high humidity, there is basically no need for self-humidification treatment. With the increase of current density, the voltage value of GDL 2:8,1:9 and GDL 3:7,2:8 under the same current density have different. It can be seen from figure 1 that the difference is not very large, this phenomenon may be caused by the manufacturing deviation of the membrane electrode and cathode GDL. By comparing the polarization curve when fuel cell temperature is 80°C and the humidity is 100%, the change trend of the three GDL when humidity is 100% is basically the same as the change trend when the humidity is 60%, and the three polarization curves basically coincide at low current density. This means that improving the hydrophilicity and hydrophobicity of anode GDL under high humidity will not affect the performance of the fuel cell in the activated polarization zone and the ohmic control zone, it is further proved that the change of polarization curve in the ohmic control zone under low humidity is caused by proton transmission efficiency. With the increase of current density, the polarization curves of GDL 2:8,1:9 and GDL 3:7,1:9 basically coincide, but the performance of GDL 3:7, 2:8 has decreased. The reason is that the lower the content of PFTE in MPL near the anode CL side, the more water permeability can be increased to share the water generated on the cathode side. To certain extent, it can improve the water management ability of PEMFC under high current density, thereby reducing the voltage loss under the same current density. The MPL near anode CL side consists of two layers and the ratio of PTFE to carbon black in the two layers is different, which can improve the performance of the fuel cell under low humidity. We also compared the polarization curves of GDL 3:7,1:9 and GDL29BC when the PEMFC temperature is 80°C and the humidity is 40%. With the increase of current density, GDL 3:7,1:9 show good performance, which shows that the performance of GDL 3:7,1:9 is better than GDL29BC under low humidity.
4.2Test results of power density
Figure 2 shows the power density test results of anode GDL with double-layer MPL structure and GDL29BC under the fuel cell temperature of 80°C and humidity of 40%, 60% and 100% respectively, From figure 2, we can see that when humidity is 40%, the maximum power density of GDL 3:7,1:9 is 1.74W/cm2, GDL 2:8,1:9 and GDL 3:7,2:8 have the same power density of 1.45W/cm2, and the difference in power density is 0.29W/cm2. There are two reasons for this phenomenon, On the one hand, under relatively low humidity, anode GDL can show better self-humidification effect, which provides more water to assist H+ from anode CL to cathode CL, the increase of the H+ transmission efficiency will reduce the ohmic loss, thereby increasing the peak power of PEMFC; On the other hand, part of the water generated on the side of the cathode CL flows out of the PEMFC, and the other part flows from the cathode CL to the anode through the PEM through osmosis. Since the MPL close to anode CL side has different hydrophilicity and hydrophobicity, the water penetration rate is also different. The better hydrophilicity of MPL or the more hydrophilic pores in MPL will help increase the water permeability, the water management pressure of the cathode GDL is relieved, so that the PEMFC maintains a good water vapor transmission capacity under large current density, thereby reducing the material transmission impedance.By comparing the power density when the running temperature is 80°C and the humidity is 60% and 100% respectively, it is found that when humidity is 60%, the self-made GDL have almost no big difference in power density. When humidity is 100%, the power density of GDL 2:8,1:9 and GDL 3:7,1:9 is basically the same, while the peak power of GDL 3:7,2:8 is relatively low. The reason may be that the high content of PTFE in the MPL near the anode CL side (compared to the other two GDLs) caused a decrease in the rate of water penetration from cathode to anode, which ultimately reduced the peak power density. By comparing GDL 3:7,1:9 and GDL29BC, it can be seen that the self-humidifying GDL has larger power peak, which is 0.23W/cm2 higher than that of GDL29BC. It is verified that the anode GDL adopts double-layer MPL with different hydrophilicity and hydrophobicity can improve the performance of the PEMFC under low humidity.
4.3 Analysis of electrochemical impedance spectroscopy test results
Figure 3 is a schematic diagram of the electrochemical impedance spectroscopy (EIS) test results of anode GDL and GDL29BC at running temperature of 80°C and humidity of 40%, 60% and 100% respectively. From figure 3, we can obtain that when humidity is 40%, GDL 3:7,2:8 have larger ohmic impedance.The reason is that the H+ conduction rate of GDL under low humidity is relatively low, resulting in relatively high ohmic impedance of the GDL. According to the EIS under the humidity of 60% and 100%, the ohmic impedance of the three GDLs is basically the same. This means that when humidity is relatively high, the transmission efficiency of H+ is hardly affected. By comparing with the polarization curve test results in figure 1, we found that at low and medium current density, the polarization curves of the above three GDLs basically coincide when the humidity is 60% and 100%,this verifies that higher humidity will reduce the influence of anode GDL on PEMFC performance. The material transmission impedance of the above three GDLs changes when the humidity is 40%, 60%, and 100% respectively. The reason is that, on the one hand, it is caused by the manufacturing deviation of the commercial GDL29BC. On the other hand, because the anode GDL has different water penetration rates, its ability to share the water management of the cathode GDL is different, resulting in a difference in material transmission impedance. By comparing GDL 3:7,1:9 with GDL29BC, the ohmic impedance and material transmission impedance of GDL29BC are relatively large. This is because the self-humidification ability of GDL 3:7,1:9 can reduce the transmission resistance of H+ in PEM, thereby reducing the ohmic impedance of PEMFC. At the same time, GDL 3:7,1:9 can promote the penetration of the water produced by cathode to anode through PEM, to alleviate the water management ability of the cathode GDL, thereby reducing the material transmission loss of the PEMFC.
4.4Analysis of water contact angle test results
Figure 4 shows the water contact angle measurement results of GDL. The average water contact angles of self-made GDL are 142.9°, 145.2°, and 143.3°, respectively. The difference in water contact angle of the above three GDLs is 0.4-2.3°, the difference in water contact angle is small, and it can be considered that the hydrophobicity of the GDL surface is basically the same. Although the content of PTFE to carbon black in GDL 3 near the anode CL side is 2:8, we did not obtain a larger water contact angle. This is because the hydrophobicity of the GDL is largely determined by nature of the hydrophobic material itself. Since hydrophobic material used in the preparation of GDL is PTFE, the hydrophobicity of the GDL surface will not change much.
When PTFE can be evenly spread on MPL, the hydrophobicity of MPL will not be affected by the thickness of PTFE.When the content of PTFE is relatively small, although the surface of the MPL may have the same water contact angle, the interior of the MPL is composed of many pores, less PTFE will produce more hydrophilic pores to participate in the transmission of water and gas.
The size of the water contact angle has a great relationship with the surface roughness of the GDL. When the surface roughness is relatively large, the water contact angle under the same conditions will decrease. When the surface roughness of the GDL is relatively small, the water contact angle under the same conditions will increase.Therefore, according to the test results of the water contact angle on the MPL surface, the main reason for the change in performance is that the ratio of hydrophilic pores and hydrophobic pores in different GDLs is different,this leads to differences in the self-humidification effect of GDL.
4.5 Analysis of SEM test results
The SEM test results of anode GDL are shown in figure 5, the surface of GDL 1, 2, and 3 prepared by spraying has many pores of different sizes, while the surface of GDL29BC is relatively flat, the difference in surface morphology of the two GDLs is caused by the different preparation processes. Under low humidity (40%), the performance of GDL 2 is better than GDL29BC. This is because the surface of GDL prepared by spraying is porous, which is conducive to the condensation of water vapor into water to produce self-humidifying effect for H2. The fuel cell generates more water under high current density. Part of the water is discharged from the fuel cell through the cathode GDL, and the other part of the water is transferred from cathode to anode under the action of permeation. After the water is transferred to the contact between the anode CL and the anode GDL, since the surface of the GDL prepared by the spraying process has more pores, the water will more easily enter MPL of anode GDL, which reduces the probability of the anode Pt being covered. The surface of GDL29BC is relatively tight, and the water that penetrates from cathode is difficult to enter the anode GDL, therefore, part of the anode CL is covered by water, which reduces the active sites of the anode Pt, and the reaction appears on the polarization curve as the performance of PEMFC under high current density. Through the analysis of surface morphology of the two GDLs, the anode GDL prepared by the spraying method not only helps to reduce the probability of the anode Pt being covered by water, but also the GDL with the dual MPL structure with different hydrophilicity and hydrophobicity is more conducive to achieving part of the water storage in MPL, to make fuel cell produce self-humidification effect when running under low humidity.
4.6 Analysis of Roughness Test Results
The in-plane roughness test results of GDL are shown in figure 6. The roughnesses of GDL 1, 2, 3 and 4 are 4.12 μm, 4.24 μm, 3.9 8μm and 0.3 μm, respectively. The in-plane roughness of the self made GDL is not much different, the roughness of GDL will affect contact area between GDL and CL, resulting in an increase in ohmic loss. Since the roughness of self made GDL is not much different, the effect of roughness on ohmic impedance of PEMFC is no longer considered when comparing the performance of GDL 1, 2, and 3. According to the roughness comparison between self made GDL and GDL29BC, the roughness of the two GDL is quite different. This is due to the different preparation process, resulting in a big difference in roughness. From the SEM diagrams in figure 6, we can also see that the surface of GDL29BC is relatively flat while the self-made GDL has obvious particle stacking. Although the roughness of self-made GDLs is larger than GDL29BC, it can be seen from the test results of polarization curve and EIS that the self-made GDL with dual MPL structure is better than GDL29BC in performance. We believe that self-made GDL have more pores on the surface, that can improve the gas transmission efficiency of GDL, which results in better performance of self-made GDL. From polarization curve and power density test results, we can conclude that the impact of pores on GDL is greater than the impact of surface roughness on GDL.
4.7 Analysis of in-plane resistance test results
The in-plane resistance test result of GDL is shown in figure 7. Figure 7 shows that the resistance is related to the content of PTFE in GDL. The higher the content of PTFE, the greater the resistivity. By comparing the self-made GDL, although their resistivity is different, the difference is not big. By comparing the self-made GDL and GDL29BC, the resistivity of GDL29BC is larger, this phenomenon is caused by the different preparation processes of the two GDLs. The process of preparing GDL29BC by coating method will make the dispersion of PTFE and carbon black more uniform, which affects the contact between carbon black and carbon black, so that the resistivity of GDL29BC is relatively large. Larger resistivity will increase the ohmic impedance, so we try to reduce the resistivity of GDL to reduce the voltage loss.