Durability test and research of proton exchange membrane for fuel cells

Abstract: Proton exchange membrane (PEM) is the core component of fuel cells. In order to study the effect of the coupling of chemical and mechanical stress on PEM, a cyclic open circuit voltage (COCV) accelerated stress test (AST) is proposed in this paper. The durability of PEM was tested by open circuit voltage (OCV), wet-dry cycle (RHC) and COCV. The hydrogen permeation current density and open circuit voltage performance of PEM were analyzed, and the failed PEM was characterized by infrared temperature measurement and scanning electron microscopy (SEM). The attenuation of PEM under three working conditions was investigated. The results show that the open circuit voltage of the single cell decreased by 5.3% after 504h of COCV operation, while the open circuit voltage attenuation rates of the single cell after OCV and RHC conditions were 1.0% and 1.1%, respectively, indicating that COCV conditions accelerated the degradation of membrane electrode. The analysis shows that the hydrogen permeation flux of PEM increased and the thickness decreased. Therefore, this working condition can be used as a supplementary solution for OCV and RHC, and the coupling effect of chemical and mechanical degradation is comprehensively studied for PEM.

0. Introduction

Currently, fuel cells are developing rapidly around the world and have been applied in many fields such as transportation, fixed power supply and portable devices. In the automotive field, proton exchange membrane fuel cells (PEMFC) have attracted more and more attention due to their advantages such as zero emissions, high efficiency and fast start-up. However, the cost and durability of PEMFC are still the main obstacles to its large-scale commercialization. As the core component of fuel cells, proton exchange membrane (PEM) mainly plays the role of conducting protons and separating anode and cathode gases. Its durability directly affects the durability of fuel cells. Therefore, in-depth research on the durability of PEM is of great significance to improving the performance of fuel cells.

PEM is a thin film material with ion selective permeability. Its durability is divided into two aspects: chemical durability and mechanical durability. Its chemical durability refers to the ability of PEM to resist chemical corrosion, oxidation and reduction reactions during the operation of the fuel cell; mechanical durability refers to the ability of PEM to maintain its structural integrity and performance stability when subjected to external forces such as pressure and tension. Similarly, the degradation mechanism of PEM during fuel cell operation is also divided into chemical degradation and mechanical degradation. The chemical degradation of PEM is caused by free radical attack. Hydroxyl (HO·), hydrogen peroxide (HOO·) and hydrogen (H·) free radicals have been considered to be potentially harmful to the membrane. At the intersection of hydrogen and oxygen at the anode or cathode of the fuel cell, H2O2 is easily reacted to generate H2O2. When H2O2 encounters metal ions (㎡+) such as Fe2+ and Cu2+, it decomposes to generate free radicals. The free radicals attack the main chain and side chain of the proton exchange membrane, thereby causing the degradation of the membrane. Studies have shown that open circuit voltage (OCV) conditions can lead to a high degree of chemical degradation, which is specifically manifested as local thinning of the proton exchange membrane and the release of fluoride in the wastewater. Mechanical degradation of PEMs is caused by changes in the water content of the membrane due to changes in temperature and humidity in the fuel cell. Changes in temperature and humidity cause cyclic expansion and contraction of the membrane, which causes creep and fatigue of the proton exchange membrane and forms cracks, tears and pinholes on the surface of the membrane.

The United States Department of Energy (DOE) has developed a standard accelerated stress test (AST) for proton exchange membrane degradation to accelerate the chemical degradation and mechanical degradation of the membrane. Although this test scheme is helpful for screening and optimizing PEMs, they cannot evaluate the combined effects of the conditions encountered by the PEM during fuel cell operation. Because chemical degradation and mechanical degradation exist simultaneously, the coupling of chemical and mechanical stresses will aggravate membrane degradation. In order to evaluate the resistance of PEM under the coupling of chemical stress and mechanical stress, this paper proposes a cyclic open circuit voltage (COCV) AST condition. The durability of the proton exchange membrane was tested under this condition and compared with the test results of the proton exchange membrane after OCV and relative humidity cycling (RHC) accelerated tests. The attenuation of the proton exchange membrane under three AST conditions was investigated by hydrogen permeation current density and open circuit voltage tests, as well as infrared temperature measurement, scanning electron microscopy and other characterization methods, and the effects of chemical, mechanical degradation and their coupling on the durability of the proton exchange membrane were studied.

1. Experiment

1.1 Single cell assembly

The single cell consists of a membrane electrode, a sealing wire, a graphite plate, a current collector and an end plate. The membrane electrode consists of a catalyst-coated PEM and carbon paper. The catalyst is a Pt/C catalyst with an effective active area of 44 cm2. The graphite plate flow field is a parallel flow field. Three single cells were assembled using the same process and materials for parallel testing.

1.2AST working conditions

The working conditions of the OCV and RHC tests in this experiment refer to the DOE test plan, and the specific test conditions are shown in Table 1. During the OCV test, the hydrogen permeation current density was tested every 48 hours until the open circuit was maintained for 500 hours; during the RHC test, the single cell ran 2 minutes of dry gas and 2 minutes of wet gas for one cycle, and the hydrogen permeation current density and open circuit voltage tests were performed after every 2000 cycles, for a total of 20,000 cycles.

The COCV test is a combination of OCV and RHC tests. According to the conditions shown in Table 1, the OCV test was first performed for 5 hours, and then the RHC test was performed for 1 hour, including 40 minutes of dry gas test and 20 minutes of wet gas test. The completion of OCV and RHC is 1 COCV cycle. The hydrogen permeation current density and open circuit voltage test were performed after every 4 COCV cycles. The test was stopped when the open circuit voltage of the single cell dropped to 20% of the initial value or dropped sharply suddenly.

1.3 Material Characterization

After the single cell durability test, an infrared thermometer was used to inspect the failed membrane electrode. The two sides of the membrane electrode were hydrogen and air respectively. If the proton exchange membrane was damaged or had pinholes, the temperature at that location would be higher than other locations. A scanning electron microscope was used to observe and analyze the cross-section of the failed proton exchange membrane.

2. Results and Discussion

2.1 Attenuation of Open Circuit Voltage

Figure 1 is a graph showing the change of the open circuit voltage of a single cell with the number of cycles and time after the COCV cycle test. As shown in Figure 1, before the first 80 cycles of the COCV test, the open circuit voltage of the single cell fluctuated between 0.936V and 0.960V, indicating that the battery performance was basically stable; after 80 cycles of the COCV test, the open circuit voltage of the single cell suddenly decayed severely, indicating that the proton exchange membrane was damaged, with tears or pinholes, resulting in a sudden increase in the amount of hydrogen permeation. In order to avoid the open circuit voltage being too low and the hydrogen permeation being serious during the subsequent tests, which would lead to the direct reaction between hydrogen and oxygen, the COCV test was conducted for a total of 88 cycles, or 528 hours.

Figure 2 shows the change in the open circuit voltage of the single cell before and after the OCV, RHC and COCV tests. As shown in Figure 2, the open circuit voltage decay rates of the single cell after the complete OCV test for 500 hours and the RHC test for 1333 hours were 1.0% and 1.1%, respectively, and the voltage decay was not obvious; while the open circuit voltage decay rate after the COCV test for 504 hours reached 5.3%, indicating that the scheme further accelerated the degradation of the membrane electrode after combining the chemical degradation of the steady-state OCV and the mechanical degradation of the periodic dry-wet cycle, and that there was an obvious coupling phenomenon between chemical degradation and mechanical degradation. After the chemical degradation of PEM, its molecular chain breaks, resulting in changes in its physical structure, which further accelerates the decay of mechanical properties; and the decline in mechanical properties will lead to an increase in the permeation of hydrogen, thereby generating more free radicals and further accelerating the chemical degradation of PEM. It can be seen that although PEM can meet the requirements of chemical durability and mechanical durability respectively, its durability remains to be verified in practical applications.

2.2 Analysis of hydrogen permeation flux

The hydrogen permeation current density change curve of a single cell during operation under different working conditions is shown in Figure 3. During the OCV and RHC tests of PEM, the hydrogen permeation current density did not change much; during the COCV test, the hydrogen permeation current density increased from the initial value of 5.4mA/cm to 14.4mA/cm at 504h. According to Faraday's law, the hydrogen permeation flux of the membrane electrode can be calculated according to the formula J---. Among them, dJ. is the hydrogen permeation flux, 1. is the hydrogen permeation current, A is the active area of the membrane electrode, F is the Faraday constant, and n is the number of electrons gained or lost in the reaction. The hydrogen permeation flux at 504h is 7.44x10-8mol/cm'·s. The significant increase in hydrogen permeation indicates that the gas barrier performance of PEM has decreased and small holes have formed in PEM.

2.3 Material Characterization Analysis

The membrane electrode after COCV test was subjected to infrared temperature measurement analysis, and the results are shown in Figure 4. As can be seen from Figure 4, the temperature of the membrane electrode near the hydrogen inlet side is significantly higher than that of other areas, indicating that the hydrogen permeation in this area is large, that is, the degradation of the PEM is more serious. Figures 5 (a) and (b) show the cross-sectional SEM images of the PEM before and after the COCV working condition test. As can be seen from the figure, the thickness of the PEM has been reduced from 15μm to 11μm after the COCV working condition operation, especially the cathode resin layer of the membrane has been thinned more seriously, thinning by about 40%. It can be seen that the main reason for the failure of the membrane electrode is the chemical degradation during the working condition operation, which leads to the thinning of the PEM, especially the cathode resin layer. This is because the pressure at the hydrogen inlet is higher than that at other parts of the membrane electrode, and the concentration of hydrogen permeating from the anode to the cathode is higher, which produces more free radicals on the cathode side of the membrane electrode, thereby accelerating the chemical decay of the PEM cathode resin layer. At the same time, during the dry and wet gas cycle, the dry and wet degree at the hydrogen inlet varies greatly, resulting in the maximum mechanical stress at the inlet, further aggravating the decay of the PEM. Under the action of chemical and mechanical coupling factors, the PEM at the hydrogen inlet eventually fails.

3. Conclusion

This paper uses COCV conditions to test the durability of PEM, and compares the test results of PEM after OCV and RHC accelerated tests. After 504h of operation under COCV conditions, the open circuit voltage of the single cell decreased by 5.3%, while the open circuit voltage attenuation rates of the single cell after complete OCV and RHC tests were 1.0% and 1.1%, respectively, indicating that COCV conditions accelerated the degradation of the membrane electrode. Hydrogen permeation current density and SEM analysis show that the hydrogen flux of PEM increases and the thickness decreases. Therefore, this COCV condition can be used as a supplementary solution to OCV and RHC conditions, and the coupling of chemical and mechanical degradation is integrated to conduct accelerated stress test research on proton exchange membranes.