Simultaneous accelerated stress testing of the proton exchange membrane, catalyst layer, and gas diffusion layer in polymer electrolyte fuel cells

doi:10.21203/rs.3.rs-4794816/v1

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Simultaneous accelerated stress testing of the proton exchange membrane, catalyst layer, and gas diffusion layer in polymer electrolyte fuel cells

https://doi.org/10.21203/rs.3.rs-4794816/v1

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The durability of polymer electrolyte fuel cells (PEFCs) in fuel cell electric vehicles (FCEVs) is important for the shift from passenger cars to heavy-duty vehicles. The components of a PEFC, namely the proton exchange membrane (PEM), catalyst layer (CL), and gas diffusion layer (GDL), contribute to the degradation of the fuel cell performance. These degradation studies were conducted independently and focused on PEMs and CLs. The degraded fuel cell stack in FCEVs is completely replaced. Therefore, it is necessary to counteract rapid material degradation in PEFCs. In this paper, we propose a method for simultaneously evaluating the degradation rates of these components by combining electrochemical characterization with operando synchrotron X-ray radiography. The open-circuit voltage, electrochemically active surface area, and water saturation were used as the degradation indicators for the PEMs, CLs, and GDLs, respectively. The results of two accelerated stress tests (loading and start-stop cycles) showed that the increase in water saturation owing to the loss of hydrophobicity due to carbon corrosion in the cathode GDL occurred on the same time scale as the degradation in the PEM and cathode CL. This suggests that more efforts should be devoted to studies on the durability of GDLs for heavy-duty applications.

Physical sciences/Materials science/Materials for energy and catalysis/Fuel cells

Physical sciences/Chemistry/Electrochemistry/Fuel cells

Physical sciences/Energy science and technology/Fuel cells

Physical sciences/Materials science/Materials for energy and catalysis/Corrosion

Since the launch of fuel cell electric vehicles (FCEVs) as commercial passenger cars in 2014, new materials have been developed to extend the driving range¹. The inventions of highly oxygen-permeable ionomers^2,3 and accessible porous carbon supports⁴ are game changers for recent FCEVs. Heavy-duty vehicles are currently being targeted for 2030⁵, and interest in next-generation FCEVs is shifting from performance to durability of polymer electrolyte fuel cells (PEFCs).

Past studies have focused on proton electrolyte membranes (PEMs) and catalyst layers (CLs) of the membrane assembly electrodes (MEAs) of PEFCs. Scientists have investigated the chemical and mechanical degradation of PEMs due to radical formation and dry/wet cycling, respectively⁶. Free-radical scavengers⁷ and polytetrafluoroethylene (PTFE)-reinforced membranes⁸ have been introduced as solutions to chemical and mechanical degradation, respectively. CLs have persistent problems of loss of catalytic activity and carbon corrosion owing to potential cycling during operation and jumping to high potentials at the start-stop cycles, respectively^9,10. Toyota MIRAI has a control system for avoiding such high potential swings to prevent carbon corrosion¹¹. Introduction of a protective layer, such as silica¹², carbon^13,14, and melamine¹⁵, on the surface of Pt-based catalysts, and immobilization of Pt-based catalysts via the incorporation of functional groups such as amino groups on the surface of the carbon support^16–18 have been proposed to enhance catalyst durability. The use of metal oxides to support corrosion resistance has been investigated^19–21. The replacement of carbon with tin oxide improves the oxygen reduction reaction activity and durability via a strong metal‒support interaction effect²².

In contrast, the durability of the gas diffusion layers (GDLs) in MEAs has often been neglected. A serious degradation phenomenon of GDLs is the cross-leakage of gases from the PEM, which penetrate the carbon fiber within the GDL owing to repeated compression²³. This is often resolved by the side effects of a powdered microporous layer (MPL), which plays a major role in the water management of PEFCs²⁴. The other degradation modes are carbon surface oxidation, carbon corrosion, and delamination of the waterproofing agent (typically PTFE)^25–28. Their degradation modes cause flooding in the high current density region (> 1 A cm^–2). The GDLs in MEAs degrade during operation under harsh conditions; however, researchers believe that their degradation is less likely to be a serious problem than that of PEMs or CLs.

If a fuel cell stack in an FCEV degrades, the basic solution is to replace it. However, previous studies have been limited to investigating the degradation individually, and the differences in the degradation rates of the components of MEAs have not been clearly presented. This study provides a method for simultaneously evaluating the degradation of PEMs, CLs, and GDLs in MEAs during accelerated stress tests (ASTs) to determine the components that should be prioritized.

Degradation indicators for the PEMs, CLs, and GDLs in the MEAs

The degradation of PEMs and CLs can be detected by an electrochemical reaction; PEMs and CLs are indexed by their open-circuit voltage (OCV) and electrochemically active surface area (ECSA) values, respectively. In contrast, GDL degradation is generally difficult to assess separately based on the electrochemical properties. This study identifies the degradation rate of GDLs via water saturation using operando synchrotron X-ray radiography²⁹. Operando cells equipped with resin-impregnated graphite block separators³⁰ are typically used for rib/channel identification for radiographic measurements. In this study, metal separators were designed because carbon separators are not suitable for research involving carbon corrosion. The rib/channel geometry of the metal separator was not identified because of insufficient X-ray transmission, and only the morphology of the MEA sandwiched between the separators was visualized (Fig. 1a). The rib/channel positions were determined by referring to the markers placed on the metal separator. The liquid water in the MEAs could be visualized during power generation by recording the difference between the wet and dry states (Fig. 1b). Figures 1c and 1d show the transmitted X-ray intensity profiles along the x- and y-axes shown in Figs. 1a and 1b, respectively. The transmitted X-rays in the GDL exhibited a relatively high intensity and intensity differences within the GDL, including the components of the carbon fiber, interfacial layer, and MPL (Fig. 1c). The interfacial layer is defined as an MPL intrusion layer in the carbon fiber, which is typically formed during the coating process³¹. The transmitted X-rays in the GDL, except for the MPL, are attenuated at the cathode by the water produced during power generation. Therefore, the water thickness in the GDL can be quantified during power generation. In contrast, the incident X-rays were attenuated in the metal separator, CL, and PEM (Fig. 1c). The water saturation in the CL and the swelling ratio of the PEM were difficult to quantify because of insufficient X-ray transmittance. Recently, a carbon separator with a super-shortened X-ray path length was proposed for water visualization in the PEM and cathode CL³²; however, metal separators with low X-ray transmittance could not be applied. The transmitted X-rays under the ribs were attenuated by the produced water (Fig. 1d); these results are consistent with those widely reported in previous studies^30,32–36. The obtained water thickness in the GDL was converted to water saturation using the GDL porosity (0.61) calculated by considering the compression state³⁷ (compressed to 130 µm from the initial thickness of 215 µm from the transmission image) from the initial porosity (0.77³¹). To simplify the discussion, the water saturation in the cathode GDL under the ribs was calculated by averaging over three components (carbon fiber, interfacial layer, and MPL) and was used as the degradation indicator.

Load cycle AST

AST simulating load cycles was recorded by applying rectangular potential cycles between 0.6 and 0.95 V (Fig. 2a). Figures 2b and 2c show the cyclic voltammograms (CVs) during the AST and the I–V curves at the beginning-of-life (BOL) and end-of-life (EOL), respectively. The OCV, ECSA, current (0.2 V), and water saturation in the cathode GDL (under the ribs) normalized by their maximum values are shown in Fig. 2d. The OCV was almost constant throughout the measurements, and the PEM did not degrade under these conditions. The CVs curves showed typical butterfly shapes with hydrogen adsorption/desorption peaks (below 0.35 V) and Pt redox peaks (above 0.55 V), as shown in Fig. 2b. The ECSA, determined from the hydrogen adsorption peaks, decreased by 26% during the AST (Fig. 2d). In general, this parameter is affected by three phenomena: electrochemical Ostwald ripening, which is explained by the dissolution of Pt nanoparticles during oxidation and their redeposition on larger particles during reduction; particle coalescence induced by particle migration on the surface of the carbon support; and particle detachment from the carbon support^38,39. For carbon-supported Pt nanoparticles, particle detachment is caused by the potential-driven oxidation of the carbon support. In the potential range of the AST (0.6–0.95 V), hydroxyl groups are adsorbed onto the surfaces of Pt at the cathode CL, producing carbon dioxide via carbon defects⁴⁰.

C + H₂O ↔ C–O_ad + 2H⁺ + 2e^– (1)

Pt + H₂O ↔ Pt–OH_ad + H⁺ + e^– (2)

C–O_ad + Pt–OH_ad → CO₂ + Pt + 2H⁺ + 2e^– (3)

Along with a reduction in the ECSA, the I–V performance deteriorated at low current densities, which is related to the catalytic activity (Fig. 2c). In contrast, the deterioration of the I–V performance at high current densities was only 5.8% at 0.2 V (Figs. 2c and 2d). However, water saturation in the cathode GDL increased as the number of potential cycles increased (Fig. 2d), even though the current decreased (i.e., the amount of water produced decreased). Recently, Zenyuk et al. investigated the effect of the GDL on the CL durability^41,42. They claimed that a higher liquid water flux due to the presence of MPL cracks in the GDL resulted in a more direct loss of dissolved Pt ions from the cathode CL during the durability test. The observed increase in water saturation did not have a significant negative impact on the I–V performance at high current densities; however, this water accumulation tendency may indirectly adversely affect water-mediated CL degradation.

Start-stop cycle AST

The durability of MEAs under high-potential conditions was tested by sweeping the potential between 1.0 and 1.5 V under an inert gas condition (Fig. 3a). Figures 3b and 3c show the CVs during the AST and the I–V curves at the BOL and EOL, respectively. The ECSA, OCV, current (0.2 V), and water saturation in the cathode GDL (under the ribs) normalized by their maximum values are shown in Fig. 3d. A decrease in the ECSA was observed up to 54% till 6,000 potential cycles (Fig. 3c). In addition, the electrical double layer was compressed during the AST. This is because of carbon corrosion occurring at the cathode CL. Depending on the type of carbon support, both the compression and expansion of the electrical double layer have been reported^40,43. In many cases, the nonporous carbon black (Vulcan) (used in this study) reduces the electrical double layer under high-potential conditions owing to carbon corrosion. In the potential range of the AST (1.0–1.5 V), carbon corrosion was primarily caused by the chemical reaction with water at the cathode:

C + 2H₂O → CO₂ + 4H⁺ + 4e^– (4)

After 7,000 potential cycles, the CVs were not properly recorded because of hydrogen leakage to the cathode through the PEM. The stability of the OCV was also affected by the leakage (Fig. 3d). A significant deterioration in I–V performance at both low and high current densities was observed after 10,000 potential cycles (Fig. 3c). Water saturation in the cathode GDL decreased as the number of potential cycles increased. This trend originated from the deteriorated I–V performance (i.e., less produced water). Eller et al.³⁴ have reported that the current and water saturation in GDLs exhibit a linear relationship in the low-current-density region up to 2.25 A cm^–2. The rate of decrease in water saturation was low compared to that in current (Fig. 3d), indicating that the produced water was more likely to accumulate in the cathode GDL during the AST, which in turn can be attributed to the reduced hydrophobicity of the aged GDL. The hydrophobicity loss in GDLs is closely associated with the loss of both carbon materials and PTFE in GDLs under oxidizing conditions²⁵, chemically aged conditions^26,27, and after real-time operations using a prototype vehicle²⁸.

To verify the above assumption that higher liquid water saturation in the cathode GDL is derived from the loss of hydrophobicity, time-resolved liquid water distribution images are shown in Fig. 4. Sequential images captured at the BOL and EOL are shown in the Movies (Movies 1 and 2: load cycle AST at the BOL and EOL, respectively; Movies 3 and 4: start-stop cycle AST at the BOL and EOL, respectively). During the start-stop cycle AST, the absolute amount of water at the EOL was lower than that at the BOL because of the lower current value owing to degradation (Figs. 3d and 4a). In contrast, the dynamic behavior of the water clusters wetted in the cathode MPL, as indicated by the arrows in Fig. 4b, was observed at the EOL. During the operation of PEFCs, water produced and condensed at the cathode CL migrates to the carbon fiber through large pores or cracks in the hydrophobic MPL^33–35. Water forms spherical droplets in carbon fibers⁴⁴ and MPLs⁴⁵ with a hydrophobic nature, wetted clusters⁴⁶ in MPLs with hydrophilic additives, and a widespread wetted region⁴⁷ in carbon fibers with a lower amount of PTFE. The wetted water clusters oscillated inside the cathode MPL with the hydrophobicity loss because of the reduced ability of the capillary force²⁴ to discharge water from the MPL into the carbon fiber and eventually into the channels of the metal separator.

Water accumulation in the anode GDL under the ribs was also observed during the start-stop cycle AST (Fig. 4b, Movie 4). However, this trend was not observed during the load cycle AST (Movie 2). The loss of hydrophobicity in the aged anode GDL may have resulted in the retention of water under the ribs. During the operation of PEFCs, water produces at the cathode and a concentration difference occurs between the cathode and anode, causing the water back-diffusion from the cathode to the anode^36,48. Yang et al.⁴⁹ have reported that insufficient water management of the anode GDL leads to water accumulation at the anode, which inhibits hydrogen transport to the catalyst (hydrogen starvation), thereby causing carbon corrosion at the anode (Eq. 4)⁵⁰. Water accumulation in the aged anode GDL has the potential risk of carbon corrosion, leading to loss of elasticity, which is an important function of anode GDLs in PEFCs⁵¹.

In conclusion, we presented a diagnostic method for simultaneously evaluating the degradation rates of MEA components in PEFCs using operando synchrotron X-ray radiography. The proposed method was applied to degradation studies via two different ASTs: load cycling with rectangular potential cycles between 0.6 and 0.95 V and start-stop cycling with linear sweep potential cycles between 1.0 and 1.5 V. During the load cycle AST, the cathode CL degraded the most in the MEA components, with a 26% reduction in the ECSA after 10,000 cycles; no degradation was observed in the PEM, whereas the cathode GDL showed a 10% increase in water saturation, indicating a decrease in hydrophobicity due to degradation. During the start-stop cycle AST, all the MEA components deteriorated at the same timescale. The PEM caused hydrogen cross-leakage after 7,000 cycles. The cathode CL exhibited significant carbon corrosion and catalyst degradation. The cathode GDL exhibited increased water saturation relative to the amount of water produced, and wetting behavior was observed in the cathode MPL. Furthermore, water accumulation was also observed in the anode GDL. Considering the cost of fuel cell stacks, GDLs comprising carbon without precious metals are by far the least expensive components of the MEAs. This indicates that stack replacement owing to GDL degradation is unacceptable. Our findings indicate the importance of GDL degradation studies vis-à-vis the current trend of focusing on PEM and CL degradation studies for PEFCs.

Fuel cell materials

A catalyst-coated membrane (CCM) was fabricated by hot-pressing the electrode decals onto both sides of a Nafion membrane (NR211, Chemours, USA). Pt nanoparticles supported by Vulcan carbon (Pt/C, TEC10V30E, 29.3 wt% Pt, Tanaka Kikinzoku Kogyo, Japan), Nafion dispersion (D2020CS, Chemours, USA), ethanol (99.5%, FUJIFILM Wako Pure Chemical Corp., Japan), and deionized water (resistivity > 18.2 MΩ cm, total organic carbon < 5 ppb) produced by a Milli-Q system (Millipore, USA) were used for the catalyst-ink formulation. Pt/C (300 mg) and the Nafion ionomer (767 mg) were dispersed in ethanol:deionized water (1:3, v/v, 43.5 mL) to obtain a catalyst ink with a solid content of 10 wt%. A glass vial containing the ink was placed in an ultrasonic bath filled with cold water (< 5°C) and sonicated for 10 min to break down the carbon agglomerates⁵². The prepared catalyst ink was applied onto PTFE sheets and dried under vacuum at 120°C for 10 min. The obtained catalyst layers were transferred onto the Nafion membranes by annealing at 120°C for 10 min. The active CCM area was 0.24 cm², and the resultant Pt loading was 1.0 mg cm^–2 for each electrode. The CCM was sandwiched between two GDLs (22BB, SGL Carbon, Germany) and two current collectors to fabricate an operando cell (Fig. 5a). The current collector comprised an oxygen-free copper with a gold plating layer (5 ± 1 µm) as the electronic contact layer and nickel plating layers (1 µm) as the interfacial layers between the oxygen-free copper and the gold plating layer. The metallic current collector contained three gas channels. The cathode and anode GDLs were surrounded by an ethylene‒propylene‒diene monomer rubber gasket to control GDL compression. The cell side was sealed with an adhesive to prevent hydrogen leakage.

Synchrotron X-ray radiography

Operando experiments were performed using the Toyota Beamline (BL33XU) at the SPring-8 facility⁵³. The cell hardware (Fig. 5b) was fixed such that the incident X-ray beam was parallel to the gas channels (Fig. 5c). The transmitted X-rays were detected using a camera system that included a LuAG:Ce single-crystal scintillator with a 10 µm-thick phosphor screen, a custom-made AA 50 imaging unit with a 5× objective lens, and a CMOS sensor (ORCA-Flash 4.0 sCMOS sensor, Hamamatsu Photonics, Japan). The energy of the incident X-rays was set to 11.4 keV using a liquid-nitrogen-cooled Si(111) double-crystal monochromator. Higher-order harmonic X-rays were reduced by using Rh-coated double total-reflection mirrors. The X-ray beam size was set to 2.66 mm × 2.66 mm (width × height). Each pixel was equal to 1.3 µm. The exposure time was set to 1 s, and the interval between exposures was set to 0.5 s, enabling a time resolution of 1.5 s.

Fuel cell operation

The cell potential, current density, high-frequency resistance, and transmitted X-ray images were simultaneously recorded. The cell potential was controlled using a potentiostat (HZ-7000, Hokuto Denko, Co., Ltd., Japan), and the high-frequency (10 kHz) resistance was measured using a resistance meter (356E, Tsuruga Denki, Japan). The flow rates of the supplied gases were controlled using mass-flow controllers (FCST1000ML, Fujikin, Japan) and temperature-controlled bubblers were used for gas humidification. The carbon corrosion rate exhibits an Arrhenius-type temperature dependence⁵⁴ and involves water in the chemical reaction⁵⁵. All the fuel cell operations were performed at 60°C and a relative humidity (RH) of 100% to ensure that the degradation is sufficiently accelerated by the 10,000 cycles of ASTs. Protocols for the loading and start-stop cycle ASTs were based on the AST protocols provided by the Fuel Cell Commercialization Conference of Japan.

For the break-in test, a cell voltage sweep between 0 and 1.0 V at a scan rate of 100 mV s^–1 was repeated 100 times, with a supply of synthetic air (300 ccm) to the cathode and hydrogen (200 ccm) to the anode. The CVs for the fifth scan were recorded under gas stop conditions at the cathode and under 200 ccm H₂ at the anode between 0.05 and 1.0 V versus RHE, the potential of which was determined from the partial pressure of hydrogen at the anode using the Nernst equation. The OCVs, I–V polarization curves, and chronoamperometry (CA) data were recorded using a supply of synthetic air (300 ccm) to the cathode and hydrogen (200 ccm) to the anode. I–V polarization curves were obtained by sweeping the cell voltage between 0 and 1.0 V at a scan rate of 10 mV s^–1. The high-frequency resistances were simultaneously collected using the AC resistance meter. The chronoamperometry measurements were performed while holding the cell potential constant (0.2 V) for 10 min.

Figure 6 summarizes the procedures for ASTs for simultaneously measuring the electrochemical properties and water saturation in the GDLs. In total, it required approximately 40 h for the AST measurements. These procedures were performed manually and monitored constantly to ensure that no unintended potential fluctuations occurred during the measurement.

Analysis of the water thickness in the GDLs

The water distribution in the GDLs was determined by dividing the intensity of the transmitted X-rays during power generation by that in the absence of liquid water (at the OCV) using the ImageJ software⁵⁶. The amount of accumulated liquid water in the GDLs, represented by the equivalent water thickness, was quantified using the Beer–Lambert law. The attenuation coefficient for water was 0.344 mm^–1 at an X-ray energy of 11.4 keV³⁰.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Author contributions

W.Y. outlined the research, performed the operando synchrotron X-ray radiography measurements, analyzed the data, and wrote the manuscript. A.K., T.H., S.Y., and S.K. performed the operando synchrotron X-ray radiography measurements. S.K. was the supervisor. All the authors have reviewed and revised the manuscript. All the authors approved the final version of the manuscript.

Acknowledgements

Synchrotron radiation experiments were performed using the Toyota Beamline (BL33XU) at the SPring-8 facility with the approval of the Japan Synchrotron Radiation Research Institute (proposal no. 2024A7032).

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You are reading this latest preprint version

Simultaneous accelerated stress testing of the proton exchange membrane, catalyst layer, and gas diffusion layer in polymer electrolyte fuel cells

Status:

Version 1

Abstract

Figures

Introduction

Results and discussion

Degradation indicators for the PEMs, CLs, and GDLs in the MEAs

Load cycle AST

Start-stop cycle AST

Methods

Fuel cell materials

Synchrotron X-ray radiography

Fuel cell operation

Analysis of the water thickness in the GDLs

Declarations

Data availability

Competing interests

Author contributions

Acknowledgements

References

Additional Declarations

Supplementary Files

Status:

Version 1