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Ionomer migration within PEMFC catalyst layers induced by humidity changes
Journal Pre-proofs Ionomer migration within PEMFC catalyst layers induced by humidity changes Yan Yin, Ruitao Li, Fuqiang Bai, Weikang Zhu, Yanzhou Qin, Yafei Chang, Junfeng Zhang, Michael D. Guiver PII: DOI: Reference:
S1388-2481(19)30253-X https://doi.org/10.1016/j.elecom.2019.106590 ELECOM 106590
To appear in:
Electrochemistry Communications
Received Date: Revised Date: Accepted Date:
27 September 2019 29 October 2019 29 October 2019
Please cite this article as: Y. Yin, R. Li, F. Bai, W. Zhu, Y. Qin, Y. Chang, J. Zhang, M.D. Guiver, Ionomer migration within PEMFC catalyst layers induced by humidity changes, Electrochemistry Communications (2019), doi: https:// doi.org/10.1016/j.elecom.2019.106590
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 The Author(s). Published by Elsevier B.V.
Ionomer migration within PEMFC catalyst layers induced by humidity changes
Yan Yin a, Ruitao Li a, Fuqiang Bai b, Weikang Zhu a, Yanzhou Qin a, Yafei Chang a,
a
Junfeng Zhang a*, Michael D. Guiver a
State Key Laboratory of Engines, Department of Mechanical Engineering, Tianjin
University 135 Yaguan Road, Tianjin Haihe Education Park, 300350, P. R. China. b
Tianjin Internal Combustion Engine Research Institute, Tianjin, P. R. China
* E-mail: [email protected]
Abstract The degradation of the membrane electrode assembly, originating from microstructural changes in the catalyst layer, inhibits the commercialization of polymer electrolyte membrane fuel cell. In particular, changes in relative humidity during starting/working conditions cause crack growth and propagation within the catalyst layer, but the reason is still not clear. Here, accelerated stress tests are designed from starting conditions (25 oC, 45% RH) to working conditions with different RHs (20%, 45% and 99%) at 85 oC for different cycles. For low working RH of 20%, no obvious change can be observed, while the accelerated stress test to high working RH induces apparent changes in the catalyst layer structure, with ionomer aggregation and migration leading to crack generation. The obtained results indicated that ionomer migration plays an important role in the structure changes of catalyst layer, suggesting that the design of
ionomer with high stability and reliable water retention is necessary to improve the structural stability of the catalyst layer.
Keywords: Catalyst layer degradation; Ionomer migration; Polymer electrolyte membrane fuel cell; Membrane electrode assembly
1. Introduction Polymer electrolyte membrane fuel cells are promising clean energy devices that transform chemical energy directly into electrical energy through chemical reactions occurring in the catalyst layers (CL). The CL is composed of carbon supports, catalysts, ionomer and voids; it dominates the performance and also accounts for the majority of the cost of fuel cell stacks [1-2]. Carbon supported catalysts and voids provide a suitable environment for accelerating chemical reactions, conducting electrons, and providing gas and water diffusion channels [3]. The proton conducting ionomer, which is sensitive to hydration, links the CL to the membrane and also binds the catalyst particles [4]. Although high water content promotes proton conductivity of the ionomer [5], repetitive hydration and dehydration cycles of the ionomer lead to irreversible viscoelastic strain [6-9], resulting in the swelling and shrinking of ionomer volume. The non-homogenous distribution of ionomer has an adverse effect on the properties of the catalyst layer (e.g. ion conductivity, porosity and pore diameter) [10-12], resulting in an increase in resistance [13, 14]. The initial power attenuation of the fuel cell, originating from degradation of CLs, affects fuel cell performance even earlier than membrane degradation
[15]. Microstructural changes like cracks, corrosion, catalyst loss and ionomer migration are contributory factors in fuel cell performance loss [16]. Therefore, the study of microstructural changes occurring in the CLs during dynamic operating conditions, in particular frequent start/stop switches [17], is beneficial to gaining an understanding of the degradation mechanism of the CLs, thus providing insights into the design of robust CLs with higher durability for fuel cells [10, 18, 19]. Microstructural changes in CLs have been studied in previous work to investigate the influence of fuel cell working conditions [4, 20-24]. An assumptive elastically dissipated energy was introduced to simulate crack propagation, proving that crack expansion is initiated from crack tips [21]. A modeling study in the literature [22] considers that the amplitude of the humidity change is the main influence on the rate of crack propagation. However, both the viscoelastic behavior of Nafion ionomer and the relationship between the ionomer and the CL microstructural changes were not included in the modeling. Tracking observations of the crack evolution under wet-dry cycling conditions found that the water intrusion-evaporation process significantly influences the growth of cracks [4]. Furthermore, our previous work [23, 24] on the effect of humidity and/or thermal cycles on the structural changes of CL indicates that relative humidity (RH) changes play a more important role in crack propagation and growth than temperature changes, but the reason for this is still not clear. Accelerated stress test (AST) are desirable method to probe the evolution of specific factors and to investigate the degradation of catalyst layers experimentally [25, 26]. Here,
we attempt to further study the mechanism of CL structural evolution during humidity changes using accelerated stress tests (ASTs), which is from a starting condition (25 oC, 45% RH) to working conditions with different RHs (20%, 45%, 99%) at 85 oC for different cycles. Based on the analysis of electrochemical data, SEM images and AFM adhesion force mappings, the migration of ionomer during humidity cycling is found to reconstruct the catalyst layer, thus having significant effects on the structural changes in CL and the resulting fuel cell performance.
2. Materials and methods 2.1 Experimental method for the accelerated stress test (AST) on GDE A gas diffusion electrode (GDE, Shanghai Hesen Electric Co. Ltd.) with 0.5 mg cm Pt loading and thickness of the CL around 10 μm was used as the pristine sample and placed in a controlled environment (Espec, SH-222) to undergo experiments. The RH and temperature of thermostat were changed from the starting condition to operating condition back and forth for 500 cycles. Each cycle was about 40 mins, including maintaining at the starting and working conditions for 3 min, which follows a similar method reported in our previous study [23]. With the aim of exploring the influence of actual working conditions, AST experiments were designed from the starting conditions (25 oC, 45% RH) to the working conditions at 85 oC with different RHs (20%, 45%, 99%), and are denoted as C-20%RH, C-45%RH, C-99%RH, respectively, according to the literature [22, 23].
2.2 Analysis of CL microstructure on GDE The morphology changes before and after humidity cycling AST were analyzed at the same locations by scanning electron microscopy (SEM, Hitachi, S-4800) and optical microscopy (OM, Keyence, VHX-S550E) following the method described in the previous literature [23]. Atomic force microscopy (AFM, Park, NX10) measurements were performed in quantitative nano-mechanical mode (QNM-mode) to obtain the adhesion force mapping images. The mechanical information was recorded when samples were touched by the contractive probe scanning in sinusoidal direction back and forth with frequency of 0.5 Hz [27]. VTI-SA+ vapor sorption analyzer (TA Instruments, USA) was used to measure the water vapor sorption of the pristine GDE at a constant temperature or RH. The samples were dried at 60 oC for 3 h to achieve an equilibrium state. The water vapor sorption of samples were tested using set procedures of 60 oC with different RHs (20%, 30%, 40%, 50%, 60%) as well as at different temperatures (25, 35, 45, 55, 60 oC) at 20% RH. The weight of absorbed vapor was recorded once the fluctuation is less than 0.0005 wt% in each step. 2.3 Fuel cell performance measurements The membrane electrode assemblies (MEAs, 2 × 2 cm2) were prepared by sandwiching an as-received Nafion 212 membrane between two electrodes (a pristine GDE as anode and the experimental GDE as cathode), followed by hot pressing at 130 oC
for 2 min. Prior to the MEA fabrication process, Nafion 212 membrane was cleaned
in 3 wt% H2O2, and acidified in 0.5 M H2SO4 at 80 oC, following the method reported in the literature [28]. The single cell was evaluated on a Greenlight G60 station at 80 oC with hydrogen (100 sccm) and oxygen (200 sccm) flowing through the anode and cathode, respectively. The backpressure for both electrodes were 50 kPa under fully hydrated conditions. All MEAs were potentiostatically activated at set voltages 0.8 V, 0.75 V, 0.7 V and 0.65 V each for 15 minutes, respectively, and followed by 45 minutes activation in the sequence at 0.6 V, 0.5 V, 0.45 V, 0.5V, 0.45 V, 0.5 V and 0.6 V respectively [29]. The final polarization curve is obtained by averaging three polarization curves and the standard deviation is less than 5.0%. 2.4 Electrochemical measurements Cyclic voltammetry was performed with 100 sccm H2 and 100 sccm N2 flowing through the anode and cathode at 25 oC at a scan rate of 0.02 V s-1, respectively. Electrochemical impedance spectroscopy (EIS) measurement was performed on an electrochemical workstation (Zahner, Zennium-E) at frequency of 100 kHz ~ 100 mHz at 50 mA cm-2 and 1800 mA cm-2 with amplitude of 50 mA and 500 mA, respectively.
3. Results and discussion MEA performance of the samples were measured before and after AST experiments. The trend of optimal power density for different cycling number is shown in Figure 1a. The optimal power density of pristine sample (847 mW cm-2), which is close to our previous study [23], changes with cycle number. For C-20%RH, the optimal power
density show a smallest decline and decrease continuously from 847.9 mW cm-2 to 783.5 mW cm-2 to 775.8 mW cm-2 with increasing cycles. The degradation rate is around 9 μV h-1, which is much lower than that of DOE standards (15 μV h-1) [30, 31], indicating that the low working RH induces little attenuation on fuel cell performance. A similar trend is also observed for C-45%RH with a lower power density of 759.7 mW cm-2. For C99%RH, the optimal power density decreased faster initially to 749.1 mW cm-2 after 300 cycles, while recover obviously after 500 cycles (808.8 mW cm-2). The experiments were repeated three times and similar trends were observed. The electrochemically active surface area (ECSA) was measured using a cyclic voltammetry method. The ECSA was estimated by QH/210 μC cm-2, where QH is the integration of the hydrogen desorption charge, and 210 μC cm-2 is assumed as the electric charge of monolayer hydrogen adsorption/desorption at the Pt surface. The microstructural changes also lead to a slight reduction in the ECSA from 51.5 m2 gPt-1 for the pristine to 45.3 m2 gPt-1 for C-45%RH and 42.1 m2 gPt-1 for C-99%RH after 500 cycles due to the decrease of triple phase boundary (TPB) regions. EIS analysis can provide additional information on the CL electrochemical properties. Charge transfer resistance and ohmic resistance related to the reaction kinetics and electron/ion transportation dominate the performance loss at low current density region. The mass transfer resistance starts occurring with the current density increase which relates to gas diffusion and water management [32]. Figures 1b, c and d show the Nyquist plots of different MEA samples. The impedance spectra were analyzed using the equivalent circuits (the insets in Figure
1), whereas Ws=0 for that at 50 mA cm-2. As shown in Table 1, the charge transfer resistances decrease remarkably when current density increases from 50 mA cm-2 to 1800 mA cm-2, showing a good dependence of the charge transfer resistance on the current density. At a high current density of 1800 mA cm-2, mass transfer resistances become the dominated factor [32]. After AST experiment, a higher resistance can be observed compared to the pristine sample. At 50 mA cm-2, charge transfer resistance increases from 276.4 mΩ to 287.4 mΩ for C-20%RH, and it continues to increase to 296.1 mΩ for C45%RH. In C-99%RH, the resistance increases even more rapidly, suggesting hindered electron/ions pathways and reduced TPB regions are the result of structure changes in the CL (Figures 1c and 1d). At 1800 mA cm-2, mass transfer resistances present a gradual increase for C-20%RH and C-45%RH after 500 cycles (from 80.0 mΩ to 90.2 mΩ and 121.1 mΩ, respectively), but rises dramatically to 144.1 mΩ after 300 cycles for C99%RH, indicating a deterioration in mass transfer. The resistance of C-99%RH drops to 73.6 mΩ after 500 cycles as shown in Figure 1d, implying that structural changes may facilitate the water discharge and reactant diffusion through the propagating cracks, thus benefiting mass transfer and fuel cell performance in high current density regions [33, 34]. This result can explain that the power density for the sample C-99%RH with 500 cycles increased, as shown in Figure 1a. To track the microstructural changes in CLs for the simulated conditions at different working RHs, typical SEM and OM images at three selected locations are analyzed (Figure 2). The changes in height as a function of location is also shown in Figure 3. In
the case of C-20%RH, the CL microstructure exhibits no obvious change as shown in Figures 2a and 2b, indicating that an increase in temperature at low RH does not induce any changes in CL structure. With an increase in working RH from 20% to 99%, the microstructure of CL exhibits a gradual deformation with different cycles as shown in Figure 2 (a, c, and e). In the case of C-45%RH, a slight lengthwise change in the cracks is observed (Figure 2c), while the widthwise change is not obvious. 3D OM images (Figure 2d) show a subtle fluctuation on the CL surface, indicating the initiation of microstructural changes. At 99%RH, apparent deformation occurs on the CL with dramatic variations in surface topography (Figure 2f). Three typical microstructural changes can be observed on C-99%RH in Figure 2e, including the extension of existing cracks both widthwise and lengthwise (location 1, 2), the generation of new cracks (location 2), as well as the shrinkage of cracks (location 2, 3). In order to further investigate the dominating factors that influence the water uptake of CL during ASTs, dynamic water vapor sorption at constant temperature or RH was performed on a pristine GDE as shown in Figure 4. At 20% RH, when the temperature increases from 25 oC to 60 oC (Figure 4a), the absolute humidity increases from 3.93 g m-3 to 25.99 g m-3 and the water uptake does not show obvious changes, which is related to the balance between sorption and evaporation of water and the water loss at low humidity environment of Nafion [35, 36]. On the contrary, with RH increasing from 20% up to 60% at 60 oC (Figure 4b), the water uptake exhibits a dramatic increase after an absolute humidity of around 30 g m-3. It should be noted that changes of RH at the
working condition of 85 oC can introduce higher absolute humidity resulting in higher water adsorption of CLs. Because the catalyst is surrounded by hydrophilic ionomer in CL, the ionomer swelling and shrinking, along with the hydration and dehydration level, is of great importance to the structural changes. AFM is employed to investigate the relationship between ionomer and crack after AST experiments (Figure 5). The crack region and the ionomer distribution can be distinguished from respective topography and adhesion force mapping images [7]. In adhesion force mapping images, the visible dark areas present Pt/C agglomerates of low adhesion and the bright regions indicates the Pt/C particles well covered with ionomer [37]. Figure 5a and 5c shows a topography of crack-free and crack locations for pristine CL surface. Pt/C agglomerates appear to be visibly enclosed by film-like or aggregated ionomer clusters (Figure 5b and 5d). The distribution of ionomer appears to be relatively homogeneous, even inside the crack, which is largely defined by the spontaneous adsorption of Nafion onto Pt/C particles and self-assembly [38]. For C-20%RH (Figure 5f), the aggregation of ionomer can be observed on non-crack region. In Figure 4h of the crack region of C-45%RH, there appears to be a lesser distribution of ionomer area within the crack, and the ionomer seems to aggregate along the direction of crack propagation. In Figure 4j of C-99%RH, aggregation of ionomer is more serious and almost no ionomer can be detected in the crack. As reported in literature [39], the water uptake property of ionomer has been observed to vary obviously and non-monotonically with ionomer thickness, which in CLs, varies from several to hundreds of nanometer [6]. Irreversible
elastic-plastic deformation [9] during AST leads to ionomer migration, resulting in the detachment of catalyst/ionomer interface and inhibition of mass transfer [39]. Furthermore, the mechanical strainstress introduced by ionomer migration will further accelerate the surface microstructural change of CL. As the ionomer layer provides the ions transport pathway within MEA, the separation of ionomer will lead to lager ohmic resistance, with even small changes in thickness leading to significant variations in the average ion conductivity because of anisotropic transport limitations of the ionomer layer [40]. Detached Pt/C particles from ionomer reduce the TPB region, causing higher charge transfer resistance (see EIS analysis in Figure 1). Therefore, the changes in CL induced by the migration of ionomer show an adverse influence on fuel cell performance. On the other hand, the reconstruction of CLs induced by ionomer migration also benefit the water management, improving the fuel cell performance (Figures 1d). According to the results above, as the absolute humidity periodically changes during cycling between the starting and working conditions in ASTs (Figure 4), the ionomer hydrates and dehydrates periodically. The periodic swelling and shrinking induces ionomer migration due to irreversible viscoelastic strain (Figure 5), thus leading to surface microstructural changes, which appear as crack propagation and bulges (Figure 2). In the case of C-20%RH, as the fitted resistance values increase with the number of cycles, the performance loss is due mainly to the decrease in the triple phase boundary (e.g. influence of ion conductivity) caused by ionomer migration, even though no obvious changes are observed on the surface. In the case of C-99%RH, the increased resistance
introduced by microstructural changes leads to a more severe performance decrease, since crack propagation decreases the TPB regions, inhibiting the pathways of electron transfer and water management. However, the drop in mass transfer resistance after 500 cycles in C-99%RH indicates improved gas diffusion and mass transfer for the propagated cracks, which may provide an optimized pathway. In addition, the Pt loading in this work (0.5 mg cm-2) is relatively higher than the typical loading of 0.3~0.4 mg cm-2 used in the state of the art systems [41,42]. The higher loading increases the thickness of the catalyst layer, and thus probably physically influences the formation and propagation of cracks during the AST experiment, which will be examined in more detail in future work.
4. Conclusion In order to investigate the degradation mechanism of the catalyst layer during frequent start/stop cycles, the AST experiments were designed with 25 oC and 45% RH as the start condition and working conditions of 85 oC and (20%, 45%, 99%) RH. Three typical phenomena of crack generation, extension of existing cracks, and interaction of cracks were observed over AST on the CL surface. Microstructural changes inhibit the electron/ion transfer and reduce triple phase boundary region; on the other hand, the deteriorative cracks are possibly beneficial to water discharge and gas diffusion and then improve the water management. Based on the water vapor sorption curves and adhesion force mapping images from AFM analysis, it is found that the microstructural change in CL is related to ionomer migration, in particular under conditions of high absolute
humidity. Thus, the design of ionomer with a high stability and consistent water retention is of great importance for fabricating MEA with improved durability and good performance.
Acknowledgement This work was supported by the Natural Science Foundation of Tianjin (No. 18JCQNJC07100 and 17JCZDJC31000)
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Figure Captions Figure 1. (a) Optimal power density curves for samples before and after cycles. Electrochemistry impedance spectra at 50 mA cm-2 and 1800 mA cm-2 for (b) C-20%RH, (c) C-45%RH and (d) C-99%RH. Inset shows the equivalent circuit. Figure 2. SEM images of the cracks before and after 300, 500 cycles for (a) C20%RH, (c) C-45%RH, and (e) C-99%RH; CL surface profile before and after 500 cycles and variation of the surface height along the diagonal line for (b) C-20%RH, (d) C45%RH, and (f) C-99%RH. Figure 3. The changes in height of cracks as a function of location. Figure 4. Water vapor sorption isotherms of a pristine GDE at (a) constant relative humidity of 20% RH and (b) constant temperature of 60 oC. Figure 5. AFM topography and corresponding adhesion force mapping images of: (a), (b) crack-free pristine sample and (c), (d) crack region of pristine sample; (e), (f) crack region of C-20%RH; (g), (h) crack region of C-45%RH; (i), (j) crack region of C99%RH. Table Caption Table 1. Performance and electrochemical measurement value of different samples.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Table 1. Performance and electrochemical measurement value of different samples. Optimum
Charge transfer resistance
Mass transfer resistance
power density
(mΩ)
(mΩ) @ 1800 mA cm-2
（m2 g-1）
(mW cm-2)
@50 mA cm-2
Before cycling
847.9
276.4
12.8
80.0
51.5
C-20%RH 300c
783.5
281.1
13.8
80.7
46.5
C-20%RH 500c
775.8
287.4
14.5
90.2
46.2
C-45%RH 300c
775.1
293.4
14.9
96.2
45.5
C-45%RH 500c
759.7
296.1
15.1
112.1
45.3
C-99%RH 300c
749.1
308.0
16.2
144.1
45.1
C-99%RH 500c
808.8
308.2
16.7
73.6
42.1
1.
@1800 mA cm-2
ECSA
Influence of humidity changes on catalyst layers (CL) studied by accelerated stress tests (AST).
2.
ASTs at 99% relative humidity induce apparent changes in CL structure.
3.
No obvious changes were observed for AST at 20% RH.
4.
Structural changes of CL are related to ionomer migration during humidity changes.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1388-2481(19)30253-X https://doi.org/10.1016/j.elecom.2019.106590 ELECOM 106590
To appear in:
Electrochemistry Communications
Received Date: Revised Date: Accepted Date:
27 September 2019 29 October 2019 29 October 2019
Please cite this article as: Y. Yin, R. Li, F. Bai, W. Zhu, Y. Qin, Y. Chang, J. Zhang, M.D. Guiver, Ionomer migration within PEMFC catalyst layers induced by humidity changes, Electrochemistry Communications (2019), doi: https:// doi.org/10.1016/j.elecom.2019.106590
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 The Author(s). Published by Elsevier B.V.
Ionomer migration within PEMFC catalyst layers induced by humidity changes
Yan Yin a, Ruitao Li a, Fuqiang Bai b, Weikang Zhu a, Yanzhou Qin a, Yafei Chang a,
a
Junfeng Zhang a*, Michael D. Guiver a
State Key Laboratory of Engines, Department of Mechanical Engineering, Tianjin
University 135 Yaguan Road, Tianjin Haihe Education Park, 300350, P. R. China. b
Tianjin Internal Combustion Engine Research Institute, Tianjin, P. R. China
* E-mail: [email protected]
Abstract The degradation of the membrane electrode assembly, originating from microstructural changes in the catalyst layer, inhibits the commercialization of polymer electrolyte membrane fuel cell. In particular, changes in relative humidity during starting/working conditions cause crack growth and propagation within the catalyst layer, but the reason is still not clear. Here, accelerated stress tests are designed from starting conditions (25 oC, 45% RH) to working conditions with different RHs (20%, 45% and 99%) at 85 oC for different cycles. For low working RH of 20%, no obvious change can be observed, while the accelerated stress test to high working RH induces apparent changes in the catalyst layer structure, with ionomer aggregation and migration leading to crack generation. The obtained results indicated that ionomer migration plays an important role in the structure changes of catalyst layer, suggesting that the design of
ionomer with high stability and reliable water retention is necessary to improve the structural stability of the catalyst layer.
Keywords: Catalyst layer degradation; Ionomer migration; Polymer electrolyte membrane fuel cell; Membrane electrode assembly
1. Introduction Polymer electrolyte membrane fuel cells are promising clean energy devices that transform chemical energy directly into electrical energy through chemical reactions occurring in the catalyst layers (CL). The CL is composed of carbon supports, catalysts, ionomer and voids; it dominates the performance and also accounts for the majority of the cost of fuel cell stacks [1-2]. Carbon supported catalysts and voids provide a suitable environment for accelerating chemical reactions, conducting electrons, and providing gas and water diffusion channels [3]. The proton conducting ionomer, which is sensitive to hydration, links the CL to the membrane and also binds the catalyst particles [4]. Although high water content promotes proton conductivity of the ionomer [5], repetitive hydration and dehydration cycles of the ionomer lead to irreversible viscoelastic strain [6-9], resulting in the swelling and shrinking of ionomer volume. The non-homogenous distribution of ionomer has an adverse effect on the properties of the catalyst layer (e.g. ion conductivity, porosity and pore diameter) [10-12], resulting in an increase in resistance [13, 14]. The initial power attenuation of the fuel cell, originating from degradation of CLs, affects fuel cell performance even earlier than membrane degradation
[15]. Microstructural changes like cracks, corrosion, catalyst loss and ionomer migration are contributory factors in fuel cell performance loss [16]. Therefore, the study of microstructural changes occurring in the CLs during dynamic operating conditions, in particular frequent start/stop switches [17], is beneficial to gaining an understanding of the degradation mechanism of the CLs, thus providing insights into the design of robust CLs with higher durability for fuel cells [10, 18, 19]. Microstructural changes in CLs have been studied in previous work to investigate the influence of fuel cell working conditions [4, 20-24]. An assumptive elastically dissipated energy was introduced to simulate crack propagation, proving that crack expansion is initiated from crack tips [21]. A modeling study in the literature [22] considers that the amplitude of the humidity change is the main influence on the rate of crack propagation. However, both the viscoelastic behavior of Nafion ionomer and the relationship between the ionomer and the CL microstructural changes were not included in the modeling. Tracking observations of the crack evolution under wet-dry cycling conditions found that the water intrusion-evaporation process significantly influences the growth of cracks [4]. Furthermore, our previous work [23, 24] on the effect of humidity and/or thermal cycles on the structural changes of CL indicates that relative humidity (RH) changes play a more important role in crack propagation and growth than temperature changes, but the reason for this is still not clear. Accelerated stress test (AST) are desirable method to probe the evolution of specific factors and to investigate the degradation of catalyst layers experimentally [25, 26]. Here,
we attempt to further study the mechanism of CL structural evolution during humidity changes using accelerated stress tests (ASTs), which is from a starting condition (25 oC, 45% RH) to working conditions with different RHs (20%, 45%, 99%) at 85 oC for different cycles. Based on the analysis of electrochemical data, SEM images and AFM adhesion force mappings, the migration of ionomer during humidity cycling is found to reconstruct the catalyst layer, thus having significant effects on the structural changes in CL and the resulting fuel cell performance.
2. Materials and methods 2.1 Experimental method for the accelerated stress test (AST) on GDE A gas diffusion electrode (GDE, Shanghai Hesen Electric Co. Ltd.) with 0.5 mg cm Pt loading and thickness of the CL around 10 μm was used as the pristine sample and placed in a controlled environment (Espec, SH-222) to undergo experiments. The RH and temperature of thermostat were changed from the starting condition to operating condition back and forth for 500 cycles. Each cycle was about 40 mins, including maintaining at the starting and working conditions for 3 min, which follows a similar method reported in our previous study [23]. With the aim of exploring the influence of actual working conditions, AST experiments were designed from the starting conditions (25 oC, 45% RH) to the working conditions at 85 oC with different RHs (20%, 45%, 99%), and are denoted as C-20%RH, C-45%RH, C-99%RH, respectively, according to the literature [22, 23].
2.2 Analysis of CL microstructure on GDE The morphology changes before and after humidity cycling AST were analyzed at the same locations by scanning electron microscopy (SEM, Hitachi, S-4800) and optical microscopy (OM, Keyence, VHX-S550E) following the method described in the previous literature [23]. Atomic force microscopy (AFM, Park, NX10) measurements were performed in quantitative nano-mechanical mode (QNM-mode) to obtain the adhesion force mapping images. The mechanical information was recorded when samples were touched by the contractive probe scanning in sinusoidal direction back and forth with frequency of 0.5 Hz [27]. VTI-SA+ vapor sorption analyzer (TA Instruments, USA) was used to measure the water vapor sorption of the pristine GDE at a constant temperature or RH. The samples were dried at 60 oC for 3 h to achieve an equilibrium state. The water vapor sorption of samples were tested using set procedures of 60 oC with different RHs (20%, 30%, 40%, 50%, 60%) as well as at different temperatures (25, 35, 45, 55, 60 oC) at 20% RH. The weight of absorbed vapor was recorded once the fluctuation is less than 0.0005 wt% in each step. 2.3 Fuel cell performance measurements The membrane electrode assemblies (MEAs, 2 × 2 cm2) were prepared by sandwiching an as-received Nafion 212 membrane between two electrodes (a pristine GDE as anode and the experimental GDE as cathode), followed by hot pressing at 130 oC
for 2 min. Prior to the MEA fabrication process, Nafion 212 membrane was cleaned
in 3 wt% H2O2, and acidified in 0.5 M H2SO4 at 80 oC, following the method reported in the literature [28]. The single cell was evaluated on a Greenlight G60 station at 80 oC with hydrogen (100 sccm) and oxygen (200 sccm) flowing through the anode and cathode, respectively. The backpressure for both electrodes were 50 kPa under fully hydrated conditions. All MEAs were potentiostatically activated at set voltages 0.8 V, 0.75 V, 0.7 V and 0.65 V each for 15 minutes, respectively, and followed by 45 minutes activation in the sequence at 0.6 V, 0.5 V, 0.45 V, 0.5V, 0.45 V, 0.5 V and 0.6 V respectively [29]. The final polarization curve is obtained by averaging three polarization curves and the standard deviation is less than 5.0%. 2.4 Electrochemical measurements Cyclic voltammetry was performed with 100 sccm H2 and 100 sccm N2 flowing through the anode and cathode at 25 oC at a scan rate of 0.02 V s-1, respectively. Electrochemical impedance spectroscopy (EIS) measurement was performed on an electrochemical workstation (Zahner, Zennium-E) at frequency of 100 kHz ~ 100 mHz at 50 mA cm-2 and 1800 mA cm-2 with amplitude of 50 mA and 500 mA, respectively.
3. Results and discussion MEA performance of the samples were measured before and after AST experiments. The trend of optimal power density for different cycling number is shown in Figure 1a. The optimal power density of pristine sample (847 mW cm-2), which is close to our previous study [23], changes with cycle number. For C-20%RH, the optimal power
density show a smallest decline and decrease continuously from 847.9 mW cm-2 to 783.5 mW cm-2 to 775.8 mW cm-2 with increasing cycles. The degradation rate is around 9 μV h-1, which is much lower than that of DOE standards (15 μV h-1) [30, 31], indicating that the low working RH induces little attenuation on fuel cell performance. A similar trend is also observed for C-45%RH with a lower power density of 759.7 mW cm-2. For C99%RH, the optimal power density decreased faster initially to 749.1 mW cm-2 after 300 cycles, while recover obviously after 500 cycles (808.8 mW cm-2). The experiments were repeated three times and similar trends were observed. The electrochemically active surface area (ECSA) was measured using a cyclic voltammetry method. The ECSA was estimated by QH/210 μC cm-2, where QH is the integration of the hydrogen desorption charge, and 210 μC cm-2 is assumed as the electric charge of monolayer hydrogen adsorption/desorption at the Pt surface. The microstructural changes also lead to a slight reduction in the ECSA from 51.5 m2 gPt-1 for the pristine to 45.3 m2 gPt-1 for C-45%RH and 42.1 m2 gPt-1 for C-99%RH after 500 cycles due to the decrease of triple phase boundary (TPB) regions. EIS analysis can provide additional information on the CL electrochemical properties. Charge transfer resistance and ohmic resistance related to the reaction kinetics and electron/ion transportation dominate the performance loss at low current density region. The mass transfer resistance starts occurring with the current density increase which relates to gas diffusion and water management [32]. Figures 1b, c and d show the Nyquist plots of different MEA samples. The impedance spectra were analyzed using the equivalent circuits (the insets in Figure
1), whereas Ws=0 for that at 50 mA cm-2. As shown in Table 1, the charge transfer resistances decrease remarkably when current density increases from 50 mA cm-2 to 1800 mA cm-2, showing a good dependence of the charge transfer resistance on the current density. At a high current density of 1800 mA cm-2, mass transfer resistances become the dominated factor [32]. After AST experiment, a higher resistance can be observed compared to the pristine sample. At 50 mA cm-2, charge transfer resistance increases from 276.4 mΩ to 287.4 mΩ for C-20%RH, and it continues to increase to 296.1 mΩ for C45%RH. In C-99%RH, the resistance increases even more rapidly, suggesting hindered electron/ions pathways and reduced TPB regions are the result of structure changes in the CL (Figures 1c and 1d). At 1800 mA cm-2, mass transfer resistances present a gradual increase for C-20%RH and C-45%RH after 500 cycles (from 80.0 mΩ to 90.2 mΩ and 121.1 mΩ, respectively), but rises dramatically to 144.1 mΩ after 300 cycles for C99%RH, indicating a deterioration in mass transfer. The resistance of C-99%RH drops to 73.6 mΩ after 500 cycles as shown in Figure 1d, implying that structural changes may facilitate the water discharge and reactant diffusion through the propagating cracks, thus benefiting mass transfer and fuel cell performance in high current density regions [33, 34]. This result can explain that the power density for the sample C-99%RH with 500 cycles increased, as shown in Figure 1a. To track the microstructural changes in CLs for the simulated conditions at different working RHs, typical SEM and OM images at three selected locations are analyzed (Figure 2). The changes in height as a function of location is also shown in Figure 3. In
the case of C-20%RH, the CL microstructure exhibits no obvious change as shown in Figures 2a and 2b, indicating that an increase in temperature at low RH does not induce any changes in CL structure. With an increase in working RH from 20% to 99%, the microstructure of CL exhibits a gradual deformation with different cycles as shown in Figure 2 (a, c, and e). In the case of C-45%RH, a slight lengthwise change in the cracks is observed (Figure 2c), while the widthwise change is not obvious. 3D OM images (Figure 2d) show a subtle fluctuation on the CL surface, indicating the initiation of microstructural changes. At 99%RH, apparent deformation occurs on the CL with dramatic variations in surface topography (Figure 2f). Three typical microstructural changes can be observed on C-99%RH in Figure 2e, including the extension of existing cracks both widthwise and lengthwise (location 1, 2), the generation of new cracks (location 2), as well as the shrinkage of cracks (location 2, 3). In order to further investigate the dominating factors that influence the water uptake of CL during ASTs, dynamic water vapor sorption at constant temperature or RH was performed on a pristine GDE as shown in Figure 4. At 20% RH, when the temperature increases from 25 oC to 60 oC (Figure 4a), the absolute humidity increases from 3.93 g m-3 to 25.99 g m-3 and the water uptake does not show obvious changes, which is related to the balance between sorption and evaporation of water and the water loss at low humidity environment of Nafion [35, 36]. On the contrary, with RH increasing from 20% up to 60% at 60 oC (Figure 4b), the water uptake exhibits a dramatic increase after an absolute humidity of around 30 g m-3. It should be noted that changes of RH at the
working condition of 85 oC can introduce higher absolute humidity resulting in higher water adsorption of CLs. Because the catalyst is surrounded by hydrophilic ionomer in CL, the ionomer swelling and shrinking, along with the hydration and dehydration level, is of great importance to the structural changes. AFM is employed to investigate the relationship between ionomer and crack after AST experiments (Figure 5). The crack region and the ionomer distribution can be distinguished from respective topography and adhesion force mapping images [7]. In adhesion force mapping images, the visible dark areas present Pt/C agglomerates of low adhesion and the bright regions indicates the Pt/C particles well covered with ionomer [37]. Figure 5a and 5c shows a topography of crack-free and crack locations for pristine CL surface. Pt/C agglomerates appear to be visibly enclosed by film-like or aggregated ionomer clusters (Figure 5b and 5d). The distribution of ionomer appears to be relatively homogeneous, even inside the crack, which is largely defined by the spontaneous adsorption of Nafion onto Pt/C particles and self-assembly [38]. For C-20%RH (Figure 5f), the aggregation of ionomer can be observed on non-crack region. In Figure 4h of the crack region of C-45%RH, there appears to be a lesser distribution of ionomer area within the crack, and the ionomer seems to aggregate along the direction of crack propagation. In Figure 4j of C-99%RH, aggregation of ionomer is more serious and almost no ionomer can be detected in the crack. As reported in literature [39], the water uptake property of ionomer has been observed to vary obviously and non-monotonically with ionomer thickness, which in CLs, varies from several to hundreds of nanometer [6]. Irreversible
elastic-plastic deformation [9] during AST leads to ionomer migration, resulting in the detachment of catalyst/ionomer interface and inhibition of mass transfer [39]. Furthermore, the mechanical strainstress introduced by ionomer migration will further accelerate the surface microstructural change of CL. As the ionomer layer provides the ions transport pathway within MEA, the separation of ionomer will lead to lager ohmic resistance, with even small changes in thickness leading to significant variations in the average ion conductivity because of anisotropic transport limitations of the ionomer layer [40]. Detached Pt/C particles from ionomer reduce the TPB region, causing higher charge transfer resistance (see EIS analysis in Figure 1). Therefore, the changes in CL induced by the migration of ionomer show an adverse influence on fuel cell performance. On the other hand, the reconstruction of CLs induced by ionomer migration also benefit the water management, improving the fuel cell performance (Figures 1d). According to the results above, as the absolute humidity periodically changes during cycling between the starting and working conditions in ASTs (Figure 4), the ionomer hydrates and dehydrates periodically. The periodic swelling and shrinking induces ionomer migration due to irreversible viscoelastic strain (Figure 5), thus leading to surface microstructural changes, which appear as crack propagation and bulges (Figure 2). In the case of C-20%RH, as the fitted resistance values increase with the number of cycles, the performance loss is due mainly to the decrease in the triple phase boundary (e.g. influence of ion conductivity) caused by ionomer migration, even though no obvious changes are observed on the surface. In the case of C-99%RH, the increased resistance
introduced by microstructural changes leads to a more severe performance decrease, since crack propagation decreases the TPB regions, inhibiting the pathways of electron transfer and water management. However, the drop in mass transfer resistance after 500 cycles in C-99%RH indicates improved gas diffusion and mass transfer for the propagated cracks, which may provide an optimized pathway. In addition, the Pt loading in this work (0.5 mg cm-2) is relatively higher than the typical loading of 0.3~0.4 mg cm-2 used in the state of the art systems [41,42]. The higher loading increases the thickness of the catalyst layer, and thus probably physically influences the formation and propagation of cracks during the AST experiment, which will be examined in more detail in future work.
4. Conclusion In order to investigate the degradation mechanism of the catalyst layer during frequent start/stop cycles, the AST experiments were designed with 25 oC and 45% RH as the start condition and working conditions of 85 oC and (20%, 45%, 99%) RH. Three typical phenomena of crack generation, extension of existing cracks, and interaction of cracks were observed over AST on the CL surface. Microstructural changes inhibit the electron/ion transfer and reduce triple phase boundary region; on the other hand, the deteriorative cracks are possibly beneficial to water discharge and gas diffusion and then improve the water management. Based on the water vapor sorption curves and adhesion force mapping images from AFM analysis, it is found that the microstructural change in CL is related to ionomer migration, in particular under conditions of high absolute
humidity. Thus, the design of ionomer with a high stability and consistent water retention is of great importance for fabricating MEA with improved durability and good performance.
Acknowledgement This work was supported by the Natural Science Foundation of Tianjin (No. 18JCQNJC07100 and 17JCZDJC31000)
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Figure Captions Figure 1. (a) Optimal power density curves for samples before and after cycles. Electrochemistry impedance spectra at 50 mA cm-2 and 1800 mA cm-2 for (b) C-20%RH, (c) C-45%RH and (d) C-99%RH. Inset shows the equivalent circuit. Figure 2. SEM images of the cracks before and after 300, 500 cycles for (a) C20%RH, (c) C-45%RH, and (e) C-99%RH; CL surface profile before and after 500 cycles and variation of the surface height along the diagonal line for (b) C-20%RH, (d) C45%RH, and (f) C-99%RH. Figure 3. The changes in height of cracks as a function of location. Figure 4. Water vapor sorption isotherms of a pristine GDE at (a) constant relative humidity of 20% RH and (b) constant temperature of 60 oC. Figure 5. AFM topography and corresponding adhesion force mapping images of: (a), (b) crack-free pristine sample and (c), (d) crack region of pristine sample; (e), (f) crack region of C-20%RH; (g), (h) crack region of C-45%RH; (i), (j) crack region of C99%RH. Table Caption Table 1. Performance and electrochemical measurement value of different samples.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Table 1. Performance and electrochemical measurement value of different samples. Optimum
Charge transfer resistance
Mass transfer resistance
power density
(mΩ)
(mΩ) @ 1800 mA cm-2
（m2 g-1）
(mW cm-2)
@50 mA cm-2
Before cycling
847.9
276.4
12.8
80.0
51.5
C-20%RH 300c
783.5
281.1
13.8
80.7
46.5
C-20%RH 500c
775.8
287.4
14.5
90.2
46.2
C-45%RH 300c
775.1
293.4
14.9
96.2
45.5
C-45%RH 500c
759.7
296.1
15.1
112.1
45.3
C-99%RH 300c
749.1
308.0
16.2
144.1
45.1
C-99%RH 500c
808.8
308.2
16.7
73.6
42.1
1.
@1800 mA cm-2
ECSA
Influence of humidity changes on catalyst layers (CL) studied by accelerated stress tests (AST).
2.
ASTs at 99% relative humidity induce apparent changes in CL structure.
3.
No obvious changes were observed for AST at 20% RH.
4.
Structural changes of CL are related to ionomer migration during humidity changes.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: