Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

Transcription

1 international journal of hydrogen energy 34 (2009) Available at journal homepage: Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions Guobao Chen a,b, Huamin Zhang a, *, Haipeng Ma a, Hexiang Zhong a a Key Material Laboratory for PEMFCs, Dalian Institute of Chemical Physics, CAS, Dalian , China b Graduate School of the Chinese Academy of Sciences, CAS, Beijing , China article info Article history: Received 24 April 2009 Received in revised form 31 May 2009 Accepted 23 July 2009 Available online 21 August 2009 Keywords: Gas diffusion layer Polymer electrolyte membrane fuel cell Electrochemical durability Carbon corrosion abstract An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.sce) in 0.5 mol L 1 H 2 SO 4. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.sce) for 96 h decreased 178 and 486 mw cm 2, respectively. The electrochemical impedance spectra measured at 1500 ma cm 2 are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction Enhancing the durability of fuel cells is one of the key research and development goals currently being pursued to promote wide-scale commercialization of polymer electrolyte membrane fuel cell (PEMFC) technology. The U.S. Department of Energy [1] showed that for commercial applications PEMFCs are required to demonstrate durability of about 6000 h under normal operating conditions. Durability is difficult to quantify and improve basically because of the quantity and duration (i.e., up to several thousand hours or more) of testing required, but also because the fuel cell stack is a system of components, electrocatalysts, membranes, gas diffusion layers (GDLs), and bipolar plates, for which the degradation mechanisms, component interactions and effects of operating conditions are not fully understood. Thus, further understanding of how the individual components degrade is much needed. Early studies have always focused on the durability and degradation of the catalyst layer and the membrane. Borup et al. [2] found that platinum particle size increased with rising relative humidity when other operating conditions were kept constant during load-cycling test. Iojoiu et al. [3] found enhanced stability and durability of the proton exchange membranes is critical to the lifetime and commercial viability of a PEMFC. However, the durability issue of the GDL has not attracted extensive attention in recent studies. A gas diffusion layer typically consists of a thin layer of carbon black mixed with polytetrafluoroethylene (PTFE) that is coated onto a sheet of macro-porous carbon backing paper. It allows the gaseous reactants to move towards the catalyst layer areas situated above the ribs between the gas channels. It also provides a path for electrons to flow between catalyst layers and bipolar plates and plays a critical role in water management within the cell. Although the GDL is a seemingly * Corresponding author. Tel.: þ ; fax: þ address: zhanghm@dicp.ac.cn (H. Zhang) /$ see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi: /j.ijhydene

2 8186 international journal of hydrogen energy 34 (2009) minor component in a fuel cell, it has been shown that altering the characteristics of the diffusion layer can lead to substantial improvements in the performance of the cell [4 8]. Previous researches have noted that the PEMFC operating environment gradually changes the GDL from hydrophobic to hydrophilic, which can degrade fuel cell operation. For example, gas convection and diffusion are hindered through a GDL after lifetime testing [9 12]. Besides, the primary constituent of the GDL is carbon, which can be oxidized in acid environment as follows: C þ 2H 2 O / CO 2 þ 4H þ þ 4e (0.207 V vs. RHE) (1) C þ H 2 O / CO þ 2H þ þ 2e (0.518 V vs. RHE) (2) The equations indicate that the carbon undergoes an oxidation reaction, albeit at a very slow rate, in parallel with the oxidation reduction reaction at the cathode even during ordinary power generation. This means that a higher electrode potential promotes greater carbon corrosion and causes irreversible damage to fuel cells. Typical examples include fuel starvation and start stop operation, which can induce a sharp increase in electrode potential [13 16]. In the case of vehicle applications in particular, the frequency of start stop operation is especially high, which has a huge impact on durability. That makes it extremely important to analyze degradation phenomena carefully and to find suitable countermeasures. However, the electrode reactions in porous electrodes, especially gas diffusion layers, involve complicated factors and it is difficult to evaluate the influence of one parameter separately with other properties being constant. For example, a change in the ionomer content affects gas permeability, catalytic activity and ionic resistance simultaneously [17 19]. Our understanding of how a GDL degrades during operation and the effects of its degradation on fuel cell performance is based on only a limited number of studies, research publications in these areas have not been identified and more and more evidence has demonstrated that further investigation is needed [20,21]. In this paper, an effective ex-situ method for characterizing the electrochemical durability of GDL under simulated PEMFC conditions is reported. The physical characteristics of the GDLs before and after corrosion tests are measured and compared. The experimental studies on the effect of the graded GDLs on the performance of PEMFCs are also studied in detail. 2. Experimental 2.1. Preparation of gas diffusion layers and their corrosion tests Gas diffusion layers were prepared using wet-proofed carbon papers (TGPH-060, Toray) as gas diffusion backings (GDBs). To form a micro-porous layer (MPL), an alcohol suspension of carbon powder XC-72 and 10 wt. % PTFE emulsion were stirred thoroughly by an ultrasonic machine and then was spread onto the GDB with a doctor blade to form the precursor for the MPL, then the precursor was baked at 240 C for 30 min, and finally it was sintered at 350 C for 40 min. The PTFE content in the MPL was 8 wt.% and carbon powder loading was 1.5 mg cm 2. The anode GDLs used in the experiments in this paper are the same, with TGPH-060 carbon papers as GDBs and XC-72 as MPL carbon blacks. The corrosion investigations of the as-prepared cathode GDLs were conducted in a homemade three-electrode cell setup as shown in Fig. 1a. The GDLs (4.0 cm 4.0 cm) were prepared as working electrode as seen in Fig. 1b. Before the samples were tested, they were pressed as what had done to membrane electrode assembly (MEA) to ensure the real pore structure at cell testing. A graphite board and a saturated calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. The prepared working electrode and the counter electrode were held vertically in a chamber filled with 0.5 M H 2 SO 4, which was kept in a water bath at 80 C. For investigation of the GDLs corrosion behavior, a constant potential was applied at the working electrode with a Model 263A potentiostat/galvanostat (EG&G Fig. 1 (a) Schematic illustration of the homemade three-electrode cell setup for the 96 h corrosion test; (b) Schematic illustration of the working electrode.

3 international journal of hydrogen energy 34 (2009) Instruments Corp.). After corrosion test, the GDLs were washed with ultrapurewater several times to remove the H 2 SO 4 solution for further physical characterization and fuel cell test. The GDLs hold at 1.0 V (vs. SCE), 1.2 V (vs. SCE) and 1.4 V (vs. SCE) after 96 h were named as GDL1, GDL2 and GDL3, respectively Physical characteristics of GDLs The surfaces of GDLs were observed by means of a color laser microscope (VK8550, Keyence, Japan). The cross-section SEM images of the GDLs were taken on a JSM6360LV instrument. In-plane resistances of the GDLs were measured using a fourpoint method. Surface contact angles were measured using the sessile drop method [22]. Through-plane permeability was measured by flowing nitrogen through GDL and then relating the flow rate to the applied pressure drop by means of a differential manometer. The size of each sample was 4.0 cm 4.0 cm. The permeability coefficient was calculated based on Darcy s law: k ¼ nm DX DP where k is the permeability coefficient of a porous substrate (m 2 ); n is the superficial velocity, calculated from nitrogen flow rate divided by the area of sample (m 2 s 1 ); m is the fluid viscosity, Pa s for nitrogen at 23 C; DX is the thickness of a substrate (m); DP is the pressure drop across a substrate (Pa), the pressure drop was controlled to lower than 30 Pa by adjusting the flow rate during the permeability test Evaluation of single cells After physical characteristics, these GDLs were used as the cathode GDLs for the evaluation of fuel cells. MEAs (with 5 cm 2 active area) were prepared as described in our previous paper [23]. The fuel cells with different GDLs were operated at 80 C with H 2 /O 2 at pressures of 0.2 MPa. Pure hydrogen and oxygen both were externally humidified before entering the cell by bubbling them through the water at 90 C and 85 C and flow rates are 50 ml min 1 and 100 ml min 1, respectively. The polarization curves of the fuel cells were galvanostatically measured using a constant current supply. 3. Results and discussion 3.1. The corrosion tests of gas diffusion layers under simulated PEMFC conditions For the electrochemical oxidation experiments, the GDL samples were immersed in 0.5 M H 2 SO 4 at constant potentials. The oxidation currents recorded at different fixed potentials for 96 h are shown in Fig. 2. It is evident that the rate of the carbon corrosion reaction is highly dependent on potential. For the potential 1.2 and 1.4 V (vs. SCE), two current steps of oxidation of gas diffusion layers can be observed, while for 1.0 V (vs. SCE), the corrosion current was very low and tended to be constant as seen in Fig. 2. As mentioned above, the GDL typically consists of two layers bonded together: a GDB made of conductive carbon fibers and a MPL made of carbon particles. Although the PTFE used in the GDL is stable against chemical corrosion and large fluctuations in potential or temperature, the carbon can be oxidized at high potentials as Eqs. (1) and (2). Besides, Maass et al. [24] used non-dispersive infrared spectrometry to analyze fuel cell exhaust, which presumably contains gaseous products of carbon degradation. These results showed that carbon was directly oxidized to carbon dioxide above 1.0 V (vs. RHE). Therefore, a higher electrode potential promotes greater carbon corrosion and results in more carbon loss. In our tests, the corrosion rates of the carbon fiber and carbon powder might be different at high potentials, so the layer easier to corrode will degrade more quickly. As shown in Fig. 2, when the corrosion times were 40 h and 66 h for 1.2 V (vs. SCE) and 1.4 V (vs. SCE) respectively, the corrosion currents of the GDLs dropped noticeably, which may indicate that one layer in the GDL may erode almost entirely Physical analysis of the GDLs In order to study the variations in the surface of the GDLs preoxidized and oxidized at different potentials, optical micrographs and contours of the GDLs were obtained with the help 2.4. Electrochemical measurements Electrochemical impedance spectra (EIS) of fuel cells were measured by the KFM2030 impedance meter (Kikusui, Japan) which connected to the Fuel cell-load&impedance Meter software (Kikusui, Japan) for in situ EIS measurement. They were measured at 1500 ma cm 2 under the same operation conditions of the evaluation of single cells. The maximal measured ac current for the sine signal was 165 ma over a frequency range of 10 mhz 10 khz. The cathodes were used as the working electrode, while the anodes of the single cells were used as the reference electrode and counter electrode, respectively. The impedance data were modeled by using ZSimpWin software. Fig. 2 The electrochemical oxidation currents of the gas diffusion layers hold at different constant potentials at 80 8C in 0.5 M H 2 SO 4. The in-set graph was the magnified graph of the corrosion current curves recorded at 1.0 and 1.2 V (vs. SCE).

4 8188 international journal of hydrogen energy 34 (2009) of an optical microscope. The micrographs in Fig. 3 reveal some interesting observations. After aged 96 h, the surface of the GDL hold at 1.0 V (vs. SCE) was nearly the same with the fresh GDL, suggesting that the carbon in the GDL oxidized slowly at low potential. However, when the corrosion potentials came to 1.2 and 1.4 V (vs. SCE), the carbon fibers in the GDB were both shown up in the micrographs. It is seemed that the micro-porous layer has much faster degradation at high corrosion potentials. In order to display the surface structure of the GDLs after electrochemical oxidation at greater detail, the contour maps were also presented in Fig. 3. It can be readily detected that the surface of the GDL became more rough after oxidized at high corrosion potentials. The altitude difference between the highest position and the lowest Fig. 3 Optical micrographs and contours of the surface of the different GDLs before and after electrochemical oxidation experiments.

5 international journal of hydrogen energy 34 (2009) position was mm in the GDL3, which was nearly 75% larger than that in the fresh GDL. It confirms that the microporous layer will be corroded faster at 1.4 V (vs. SCE), which can be seen in the optical micrograph. It is worth noting that the GDL eroded more severely at 1.4 V (vs. SCE) than at 1.2 V (vs. SCE) from the contour maps, as the altitude difference between the two opposite verges in the GDL3 was about 16 mm larger than that in the GDL2. For further understanding the corrosion of carbon in the oxidized GDLs, the cross-section SEM images of the fresh GDL and the GDLs after corrosion tests were also shown in Fig. 4. Looking at the graphs it is very obvious that comparing to the fresh GDL and GDL1, the inner GDLs eroded at 1.2 and 1.4 V (vs. SCE) show much looser. More macropores appeared in the GDL2 and GDL3. The right section of the GDLs in the pictures seems to be oxidized more evidently. As mentioned above, it can be inferred that the micro-porous layer eroded almost entirely in GDL2 and GDL3. It is also interesting to note that the carbon fibers in the gas diffusion backing of the GDL3 were obviously thinner than those in the other GDLs. The diameter of the carbon fiber in the GDB of GDL3 was only about one half of that in other GDLs. This confirms that the GDL eroded most severely at 1.4 V (vs.sce). The in-plane electrical resistances were also measured. As shown in Table 1, it is clearly that the in-plane resistance increases in the order of corrosion potential. This may be due to the more serious carbon loss in the GDL at higher electrochemical potential. The electrical resistance of GDL3 achieved 48.4 mu cm, more than five times with the other GDLs. As we know, carbon is the main electron conductor for the GDL, the loss of carbon in the GDL will surely increase the electrical resistance. Consequently, the in-plane electrical resistances increased according to the reduction of the carbon. Through-plane gas permeability is expected to correlate with the through holes which serve as mass transport paths when cell working. The gas permeabilities of various GDLs were also recorded in Table 1. It is evident that the oxidation of gas diffusion layers increases the though-plane gas permeability significantly, especially at high corrosion potentials. The though-plane gas permeabilities of the GDL2 and GDL3 were more than eight times than that of the fresh GDL. It is accidental to learn that though GDL3 was proved to be encountering the most carbon loss mentioned above, it had the lower gas permeability than GDL2. As we know, PTFE, the other component of the GDL, will remain after carbon corrosion. It has been learned that the carbon in the inner GDL3 eroded most seriously from Fig. 4. Therefore, the most PTFE will remain in the GDL3, which may fill into the pores of the GDL and block the gas permeation. Images of water droplets on the surface of different GDLs are shown in Fig. 5. By fitting a tangent to three-phase point where liquid surface touches the solid surface the contact angles were measured and the results were also presented in Table 1. It is obvious that the plain gas diffusion layer exhibits hydrophobic behavior from Fig. 5. Once aged after electrochemical oxidation, the contact angles of the GDLs sharply decreased from to nearly 130. However, it is worth noting that, though the aggressiveness of the aging conditions was elevated, as the potential of the electrochemical oxidation increasing, the contact angles measured at the end of each experiment changed slightly. The reason for the phenomenon might be ascribed that even though the carbon materials in Fig. 4 The cross-section SEM images of (a) the fresh GDL and the GDLs after oxidized 96 h hold at: (b) 1.0 V vs. SCE, (c) 1.2 V vs. SCE and (d) 1.4 V vs. SCE. The gas diffusion backings of the GDLs were on the left while the micro-porous layers were on the other side.

6 8190 international journal of hydrogen energy 34 (2009) Table 1 Physical characteristics of the various gas diffusion layers. GDL type Oxidized potential (V vs. SCE) Oxidized time (h) In-plane resistivity (mu cm) Through-plane permeability (10 13 m 2 ) Contact angle ( ) Fresh GDL GDL GDL GDL the GDL suffered electrochemical corrosion, the other component PTFE is quite stable and hydrophobic and it might be still exist after corrosion experiment Effect of the oxidized GDLs on cell performance Fig. 6 shows the effect of the oxidized gas diffusion layers on the cell performance. The cell performance was almost similar at low current density region. However, when the current density increasing, the performance of GDL2 and GDL3 exhibited much higher mass transport hindrance than the fresh GDL. The cell performance of the GDL1 showed different with those of two other GDLs after corrosion tests and almost stayed the same with the fresh GDL. At 1000 ma cm 2, the performance of the fuel cell with the GDL1 achieves V while those with GDL2 and GDL3 were V and V, respectively. Besides, the maximum power densities of the fresh GDL and the GDL1 were both achieved about 795 mw cm 2 at 1680 ma cm 2, which is 178 and 486 mw cm 2 larger than that of the GDL2 and GDL3, respectively. The results might be due to the fact that the microporous layers in the GDL2 and GDL3 had serious carbon loss, which made the gas diffusion layer vulnerable to water flooding. It is easily observed that H 2 /O 2 cell performance with the GDL3 spread out much more performance degradation after aged 96 h than the others. This might be ascribed to the largest change in the electrochemical resistivity of GDL3, which induced by the most serious carbon loss as shown in Table 1. It is also interesting to note that even though the contact angles were similar with the GDLs after electrochemical oxidation, their fuel cell performance varied noticeably. It can be inferred that the hydrophobicity loss of the GDL is not the main reason to the degradation of fuel cell performance in these test EIS characterization of different cells Impedance analyses were performed to examine the effect of carbon corrosion on resistance changes in fuel cells. The EIS spectra obtained for the single cells with the different GDLs are shown in Fig. 7a. The measurements were conducted at a constant current density of 1500 ma cm 2 under the same operation conditions of the evaluation of single cells. The EIS of GDL3 cannot be measured at 1500 ma cm 2 because of insufficient cell performance. It was found from the spectra that there are two loops at the middle high frequency and the Fig. 5 Contact angles on the surface layer of (a) the fresh GDL and the GDLs after oxidized 96 h hold at: (b)1.0 V vs. SCE, (c) 1.2 V vs. SCE and (d) 1.4 V vs. SCE.

7 international journal of hydrogen energy 34 (2009) Table 2 Parameters evaluated from fit of EIS at j [ 1500 ma cm L2 with the equivalent circuit shown in Fig. 7b. Catalyst layer type j ¼ 1500 ma cm 2 R U (mu cm 2 ) R 1 (mu cm 2 ) R 2 (mu cm 2 ) Fresh GDL GDL GDL Fig. 6 The polarization curves of the cells using different gas diffusion layers; the anode humidifier temperature 90 8C, cathode humidifier temperature 85 8C, cell temperature 80 8C, H 2 /O 2 [ 0.2 MPa/0.2 Mpa, pure hydrogen and oxygen are put into the cell. Flow rates of H 2 and O 2 are 50 ml min L1 and 100 ml min L1, respectively. low frequency. The large loop at intermediate frequencies is attributed to the charge-transfer resistance and the double layer capacity and the relaxations of the intermediate species. The second loop at low frequencies is not commonly reported in literature because it falls below the frequency range usually employed in the experimental investigation for fuel cell. In this work it is proposed that the phenomenon is related to the mass transport characteristics in the membrane electrode assembly. The equivalent circuit for the single cells is shown in Fig. 7b. R U represents the impedance at the intersection of the high frequency curve with the real axis. It is attributed to the internal resistance of the cell including the total ohmic resistance of the cell, which can be expressed as a sum of the contributions from uncompensated contact resistance and ohmic resistance of the cell components such as membrane, catalyst layer, backing, end plate, and that between each of them [25]. R 1 and Q 1 represent the resistance and constant phase element (CPE) that arose by the charge-transfer resistance for oxygen reduction. R 2 represents the mass-transfer resistance for oxygen reduction, and Q 2 represents the CPE associated by the mass transport properties. Table 2 shows the simulated data using the equivalent circuit shown in Fig. 7b by ZSimpWin. There is worth noting that the R U, R 1 and R 2 of fuel cell with GDL2 are all larger than those with the fresh GDL and GDL1. For example, the ohmic resistance of the cell is 12.8 mu cm 2 and 10.9 mu cm 2 larger than that with the fresh GDL and GDL1 respectively. It mainly caused by the increase of electrical resistivity in the GDL2 as seen in Table 1. From Table 2, it also can be noted that the R 1 and R 2 of the fuel cell with GDL2 were about 40% and 22% larger than those with the fresh GDL, respectively. The high R 1 and R 2 may have a great contribution to the decline of the cell performance. It is interesting to learn that R 1 of the GDL1 was slightly larger than that of the fresh GDL while R 2 of the GDL1 was lower than that of the fresh GDL reversely. As we know, the GDLs have the role of reactant permeation from flow fields to catalytic sites by the gas access, product permeation from the catalytic layer to flow fields, and electronic and thermal conductivity. At high current densities, the electrochemical reaction rate is faster than the amount of reactants supplied. Therefore, the reaction rate is limited by the transport rate of oxygen to the catalytic sites. Since a lower carbon loading was obtained after electrochemical oxidation, water passage will become easier as a result [26]. This also can be confirmed by the reduction of hydrophobicity as mentioned above. For high current densities, when carbon eroded slightly, such as the GDL1, a bit more water in the GDL may be beneficial for the solution of the reactant gas, which caused the mass-transfer resistance decreasing. However, when carbon loss was severe, like the GDL2, then the mass water will permeate into the large pores of the inner GDL and influence the gas access, which resulted in the mass-transfer resistance of the fuel cell increasing. 4. Conclusion Fig. 7 (a) Nyquist plots of single cells with the different GDLs at 1500 ma cm L2 under the same operation conditions mentioned in the polarization curves measurement; (b) equivalent circuit used in the simulation. An ex-situ method was successfully used to study the gas diffusion layer degradation under simulated PEMFC conditions. From the micrographs, the carbon loss in the GDL is observed to be more substantial with the increasing corrosion

8 8192 international journal of hydrogen energy 34 (2009) potential. After electrochemical oxidation, there is an increase in the electronic resistivity of the GDL. It is also found that the fuel cell performance degraded evidently with the oxidized GDL hold at 1.2 and 1.4 V (vs. SCE) for 96 h. The maximum power densities of the fuel cells with the GDL aged 96 h at 1.2 and 1.4 V (vs. SCE) decreased 178 mw cm 2 and 486 mw cm 2, respectively. The electrochemical impedance spectra reveal that, after the corrosion test, ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell with GDL2 are all increased. The charge-transfer and mass-transfer resistances of the cell with the oxidized GDL at 1.2 V (vs. SCE) were about 40% and 22% larger than those of the cell with the fresh GDL, respectively. Further study about the influence of carbon loss on the water management of fuel cell will be necessary. 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