Water management strategies during cold starts in polymer electrolyte fuel cells: insights from operando synchrotron X-ray imaging

Water management strategies during cold starts in polymer electrolyte fuel cells: insights from operando synchrotron X-ray imaging | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Water management strategies during cold starts in polymer electrolyte fuel cells: insights from operando synchrotron X-ray imaging Wataru Yoshimune, Satoshi Yamaguchi, Akihiko Kato, Yoriko Matsuoka, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6019201/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Jul, 2025 Read the published version in Scientific Reports → Version 1 posted 11 You are reading this latest preprint version Abstract Due to the vapor-liquid-ice phase transitions, polymer electrolyte fuel cells (PEFCs) encounter significant challenges during cold starts. This study employs operando synchrotron X-ray computed tomography to visualize water distribution in PEFCs during cold starts, where temperature fluctuations surpass the freezing threshold of 0°C. The results suggest a potential risk of condensation due to a circulating coolant system in fuel cell stacks during cold starts, which can adversely affect cold start performance. By comparing transient water saturation across PEFC components, we find that flooding near the cathode catalyst layer is most detrimental to fuel cell operation. These findings provide insights into the interplay between water distribution and cold start performance, underscoring the necessity of optimized water management strategies for next-generation fuel cell electric vehicles. Physical sciences/Energy science and technology/Fuel cells Physical sciences/Chemistry/Energy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Polymer electrolyte fuel cells (PEFCs) are clean energy devices that use hydrogen as fuel and produce only heat and water as byproducts 1 . Their core component, the polymer electrolyte membrane (PEM), serves as a proton conductor, electron insulator, and gas barrier (Fig. 1 ). The catalyst-coated membrane (CCM) consists of catalyst layers (CLs) on both sides of the PEM, with a mixture of ionomer and Pt-based nanoparticles on carbon supports. The membrane electrode assembly (MEA) is formed by sandwiching the CCM with gas diffusion layers (GDLs), comprising a gas diffusion substrate (GDS) and microporous layer (MPL). The cathode CL produces water as a result of the electrochemical oxygen reduction reaction. The vapor-liquid-ice phase transitions significantly impact fuel cell performance and material degradation 2 . At high temperatures (typically above 60°C), vapor transport facilitates effective water removal from the MEA, reducing the risk of performance deterioration 3 . In contrast, at lower temperatures, condensation within the MEA impedes gas transport 4 . During subzero temperatures, ice formation at the cathode electrode can damage the MEA 5 . Numerous studies, too many to list here, have examined water behavior in PEFCs using operando techniques. Especially, operando imaging experiments have provided insight into the water management challenges posed by water accumulation in MEAs. For example, operando synchrotron X-ray radiography has highlighted the effects of back-diffusion 6 , 7 and oversaturation 8 on the water transport mechanism in PEFCs and revealed high water saturation of the cathode CL 9 and GDL 10 , 11 during performance drops at high current densities. Studies have further demonstrated that water saturation of GDLs depends on the polytetrafluoroethylene (PTFE) content used as a waterproofing agent 12 . Accelerated stress testing during operando experiments have also shown that hydrophobicity loss in GDLs increases the water saturation and causes performance deterioration 13 , 14 . Moreover, these techniques have identified specific water drainage pathways, such as perforations 15 , 16 , cracks 17 , large pores 18 , and percolation networks 19 within GDLs. Flow field design using operando neutron radiography has also shown promise for improved water management in PEFCs 20−22 . Cold start research remains limited due to the challenges of establishing operando measurement systems under cryogenic conditions. With the global commercialization of fuel cell electric vehicles (FCEVs), improving cold start capability and performance has become increasingly critical. Recent studies using fast operando synchrotron X-ray computed tomography (CT) have revealed that the presence of supercooled water has a significant impact on cold start capability during cold starts 23 , 24 , and pre-purging improves cold start capability by increasing the allowable water content in the MEA 25 . Operando neutron imaging studies have shown that freezing events in PEFCs occur randomly, influenced by GDL properties 26 , cell size 27 , mechanical stress 28 , or residual water 29 , resulting in a high pressure drop at the cathode electrode 30 . Additionally, operando neutron studies have identified cathode CL freezing as a key factor in cold start performance during rapid cold starts 31 . Neutron wavelength analysis can differentiate between water and ice phases 32−36 . Energy-resolved operando neutron imaging has demonstrated that partial freezing can trigger total freezing in a practical cell of 280 square centimeters 37 . Another critical factor in cold start performance is temperature management. A commercial fuel cell stack incorporates a circulating coolant system to regulate stack temperature 38 , but during cold starts, the coolant temperature oscillates due to the coexistence of heating (power generation) and cooling (external freezing environment) 39−41 . While thermostatically controlled systems have been shown to migrate the temperature oscillation 42 , the effect on water distribution remains unclear. Here, this study aims to gain insight into transient water management strategies during cold starts. Results Electrochemical Performance. Figure 2 shows the fuel cell performance measured during operando X-ray CT experiments. The operando cell was operated in a potentiostat mode with a cell voltage of 0.1 V, where the current density increased as the cell was ramped from 5°C at a rate of 1°C/6 s. Upon reaching 40°C, the cell was subsequently cooled to 5°C at the same rate, leading to a corresponding current density decrease. High-frequency resistance (HFR) remained constant (~ 200 mΩ cm 2 ) throughout the measurements, with fluctuations attributed to the periodic rotation of the operando cell for CT scans. When the CT scans were paused at 40°C, these values stabilized. The temperature dependence of the current density followed a concave trend during heating and a convex trend during cooling. Some have attributed the performance hysteresis to the slow change in ohmic resistance caused by the gradual water uptake of the PEM or the ionomer in the CL, as well as to the oxygen transport resistance originating from the ionomer adsorption at the ionomer/Pt interface 43 . Park et al. have claimed that water accumulation, particularly under high current densities, plays a dominant role in the performance hysteresis 44 . Since the gradual water uptake of the PEM and CL occurs over several minutes, the second scale hysteresis observed here is likely due to the rapid water accumulation in the MEA. To validate this hypothesis, we performed water behavior analyses. Water Saturation. In an example of the vertically sliced MEA image (Fig. 1 ), liquid water was widely observed from the MPL to the flow channel. Water within the CL was not visible due to the strong X-ray absorption of Pt 9 . Figure 3 shows an example of water distribution in the stacking direction of the MEA at a representative moment (120 s). Some cracks in the MPL contained water, while the rest appeared in a dry state (Fig. 3 a). Water accumulation in the GDS was observed under regions compressed by the flow field (Fig. 3 b), and water transport in the flow channels was visualized (Fig. 3 c). Figure 4 shows the water saturation, defined as the percentage of water filled in the void of each component. The calculations here assume that the MPL contains a porosity of 70% due to invisible nanopores 45 . During heating, the water saturation increased from the core to the outside. Due to the presence of intervals in the CT scans, it was tough to judge whether the detected water droplets were formed by condensation or migrated from the core side. At the cell temperature of 25–35°C in the heating process, vapor transport contributes to the decrease of water saturation with increasing the cell temperature in the following order: MPL cracks, GDS, and flow channels. At 40°C, water completely disappeared, except for residual water accumulation in the GDS under the flow field (Fig. 3 b). During cooling, water saturation increased in the GDS, followed by the MPL cracks, due to the suppressed vapor transport. Eventually, the water saturation increased in the flow channels, obstructing the airflow. The water saturation of the MPL was negligible compared to the other components, regardless of the temperature profile. Variations in water saturation affect fuel cell performance by altering gas transport properties. To investigate these relationships, we performed a multivariate analysis (Fig. 5 ). The results showed a positive correlation between the current density and water saturation of the GDS and flow channels during heating, and a negative correlation during cooling. This suggests that there is no universal correlation. A strong negative correlation was observed between the current density and water saturation of the MPL including cracks throughout the measurements. This fact indicates that flooding close to the heart of the PEFC components has a negative impact on the fuel cell performance. Whether condensation occurred in the MPL or whether water migrated from the cathode CL was not presented in our observations, but it is inferred that flooding occurred at the cathode CL because oxygen diffusion was maintained through the dry MPL. Discussion The use of an operando synchrotron X-ray CT system with a customized forced cooling unit enables water distribution visualization in an MEA for PEFCs during a rapid temperature response as well as cold starts. The present study offers new insight into the correlation between fuel cell performance and water saturation under transient temperature responses. Hwang et al. have suggested that in partially saturated GDLs, water accumulation obstructs diffusion pathways, resulting in a reduction of the effective gas diffusivity 46 . Some have claimed that MPL cracks promote water transport away from the cathode CL, thereby preventing flooding in the cathode CL 15−19 . These explanations sound reasonable, but our observation showed that fuel cell performance deteriorated especially when water saturation increased in the MPL. The results suggest that although MPL cracks can facilitate water transport away from the cathode CL, this drainage route is the result of serious flooding in the cathode CL. Therefore, the most important factor in maintaining fuel cell performance is to prevent flooding in the cathode CL by ensuring an environment suitable for vapor transport. Our findings shed light on water management strategies during cold starts. Researchers tend to focus only on the cold start capability, but even above the freezing point, serious performance deterioration may happen due to flooding. Since cold start protocols prioritize heat generation over power generation 38 , the cell temperature is higher than the surrounding temperature due to heat generation from the MEA and the thermal insulation provided by the GDLs 47 . This allows safe cold starts from subzero temperatures. In a fuel cell stack, the circulating coolant system works to lower the stack temperature, so condensation may occur in the fuel cell stack above the freezing point and eventually lead to the performance deterioration. Possible countermeasures include controlling the coolant supply 42 or heating the coolant 48 . Looking ahead, the push for next generation PEFCs with higher fuel cell performance involves the development of thinner GDLs to reduce oxygen diffusion resistance. For example, metal-based GDLs have been proposed as a promising solution 49 . However, their high thermal conductive GDLs may exacerbate the risk of freezing and flooding in the cathode CL during cold starts, underscoring the need for further research into water management strategies. This study demonstrates progress in understanding the cold start capability and performance. With continued improvements in water visualization technologies, we will address cold start challenges and guide the design of next generation FCEVs. Methods Materials. A 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 were used to prepare a catalyst ink. Pt/C (300 mg) and 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 this mixture was placed in an ultrasonic bath filled with cold water and sonicated for 10 min 50 . The ink was applied to a PTFE sheet and dried under vacuum at 120°C for 10 min. The obtained catalyst layers were transferred to both sides of the Nafion membrane by annealing at 120°C for 10 min 51 . For both the cathode and anode, the Pt loading in the CCM was 1.0 mg/cm 2 . A GDS with an MPL was used for the GDL (Sigracet 22BB, SGL Carbon, Germany). The MEA with ethylene-propylene-dienemonomer rubber gaskets was compressed using two electrically conductive flow fields to fabricate an operando cell. The flow fields had two straight parallel flow channels with depths, lengths, and widths of 0.3, 1.0, and 0.8 mm, respectively. Operando Synchrotron X-ray CT. Our previous operando synchrotron X-ray CT setup 18 was updated prior to the present work with an additional function for studying PEFCs (Fig. 6 ) 24 , 25 . A high-speed rotary stage (Kohzu Precision, EM200-11) was used for X-ray CT scanning. For operando measurements, slip rings and rotary joints were used for wiring and gas piping, respectively. These features enabled high-speed X-ray CT measurements with a rotational speed and accuracy of 1800°/s and 1 µm, respectively. The fuel cell was surrounded by a resin chamber to prevent condensation and increase cooling efficiency. Dry air supplied from a compressor was cooled down to − 30°C using a Peltier device and blown into the chamber (Fig. 7 ). The cell temperature was monitored at the top and bottom positions. The average cell temperature was successfully reduced to a minimum of − 20°C using our forced cooling unit. The fabricated cell was conditioned at 40°C and RH 30% under 200 cm 3 /min in an air environment at the cathode and 100 cm 3 /min in a hydrogen environment at the anode without back pressure. The details of the break-in process are described in our previous study 18 . Following the break-in procedure, the fuel cell was cooled to 5°C under the same gas flow rate. Subsequently, the fuel cell was operated at a constant voltage of 0.1 V with the temperature control for operando measurements. In this work, low-dewpoint gas was supplied by mixing wet gas at 18°C dew point and dry gas with a controlled mixing ratio depending on the cell temperature, so that the condensation did not occur in the operando cell (Fig. 2 ). The operando synchrotron X-ray CT measurements were performed on a Toyota beamline (BL33XU) at the SPring-8 facility 52 with an X-ray energy of 20 keV and a pixel size of 2.96 µm. The offset scan in the phase contrast mode was acquired using the on-the-fly method with 1,500 projections during a 360° continuous rotation in 2 s. Post-processing for Imaging Data. Image reconstruction was performed using a software package provided by the Japan Synchrotron Radiation Research Institute 53 , and rotation correction was performed using ImageJ software 54 . The obtained phase contrast X-ray CT images were processed using Amira software. Ring artifact reduction is first executed. Next, the flow field region was extracted using a watershed segmentation method, and the CCM region was identified using a threshold segmentation method. The GDS was then determined using a top-hat segmentation method. Finally, the MPL was indexed using a watershed segmentation method. The remaining portion was recognized as the flow channel. An additional water region was assigned to the dry image by comparing the images before and after power generation. As a result, six-phase segmentation was achieved under operando conditions. The segmentated images were visualized using GeoDict software 55 . The correlations between the current density and water saturation of each component were calculated in R software (version 4.4.2). Declarations Acknowledgements We thank Mr. K. Yaegashi (Toyota Central R&D Labs, Inc.) for designing and fabricating the forced cooling experimental setup. Synchrotron radiation experiments were performed at the BL33XU beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal nos. 2022A7032, 2022B7032, and 2024B7032). We received technical support from the Paul Scherrer Institute and Thermo Fisher Scientific Inc. to introduce the forced cooling unit into the X-ray CT setup and the segmentation procedure, respectively. Author contributions statement W.Y. and S. K. conceived the study, W.Y., S.Y., A.K., and S.K. conducted the operando X-ray CT measurements, S.Y. performed the image reconstruction, Y.M. segmented the CT images, and S.K. analyzed 4D water distribution images, and W.Y. examined the results and wrote the initial draft. All authors reviewed the manuscript. Data availability Data are available upon reasonable request from the corresponding author. Funding There is no funding for this research work. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6019201","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":415799355,"identity":"2488e707-0f1c-4613-9559-be257385ab77","order_by":0,"name":"Wataru Yoshimune","email":"data:image/png;base64,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","orcid":"","institution":"Toyota Central R\u0026D Labs., Inc.","correspondingAuthor":true,"prefix":"","firstName":"Wataru","middleName":"","lastName":"Yoshimune","suffix":""},{"id":415799356,"identity":"4ed2e97f-c3a7-47ae-9488-cde0ef0eb95c","order_by":1,"name":"Satoshi Yamaguchi","email":"","orcid":"","institution":"Toyota Central R\u0026D Labs., Inc.","correspondingAuthor":false,"prefix":"","firstName":"Satoshi","middleName":"","lastName":"Yamaguchi","suffix":""},{"id":415799357,"identity":"7c9175d0-cd4f-45f7-bdd1-2edc64c2a783","order_by":2,"name":"Akihiko Kato","email":"","orcid":"","institution":"Toyota Central R\u0026D Labs., Inc.","correspondingAuthor":false,"prefix":"","firstName":"Akihiko","middleName":"","lastName":"Kato","suffix":""},{"id":415799358,"identity":"18e2825b-490d-4f85-bcc9-93fc1f658d13","order_by":3,"name":"Yoriko Matsuoka","email":"","orcid":"","institution":"Toyota Central R\u0026D Labs., Inc.","correspondingAuthor":false,"prefix":"","firstName":"Yoriko","middleName":"","lastName":"Matsuoka","suffix":""},{"id":415799359,"identity":"671c7130-9e21-40ec-bb80-5aafc3e0a5dd","order_by":4,"name":"Satoru Kato","email":"","orcid":"","institution":"Toyota Central R\u0026D Labs., Inc.","correspondingAuthor":false,"prefix":"","firstName":"Satoru","middleName":"","lastName":"Kato","suffix":""}],"badges":[],"createdAt":"2025-02-13 03:38:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6019201/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6019201/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-025-08939-7","type":"published","date":"2025-07-02T15:57:57+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":76578186,"identity":"6ac29f60-9c07-44e8-9198-17c63ef43329","added_by":"auto","created_at":"2025-02-18 14:36:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":16872,"visible":true,"origin":"","legend":"\u003cp\u003eVisualization in PEFCs obtained from \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray CT imaging.\u003c/p\u003e","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/95e7547828d558a635e6f577.png"},{"id":76576668,"identity":"67b8de04-c32b-453b-8379-fae80ba9878f","added_by":"auto","created_at":"2025-02-18 14:20:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":36976,"visible":true,"origin":"","legend":"\u003cp\u003eFuel cell performance during \u003cem\u003eoperando\u003c/em\u003esynchrotron X-ray CT imaging.\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/5fefdc39f98fc237f104b3c0.png"},{"id":76577971,"identity":"ca3bb9db-0dbc-4f68-8d9f-7c1801df1f4e","added_by":"auto","created_at":"2025-02-18 14:28:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":43998,"visible":true,"origin":"","legend":"\u003cp\u003eWater visualization in PEFC components obtained from \u003cem\u003eoperando\u003c/em\u003esynchrotron X-ray CT imaging: (a) MPL, (b) GDS, and (c) flow channels.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/eed32d9878ca9db3bf9a68ae.png"},{"id":76576670,"identity":"6b62ba8a-8dbf-4c56-9f24-ee55e287e09a","added_by":"auto","created_at":"2025-02-18 14:20:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":28161,"visible":true,"origin":"","legend":"\u003cp\u003eWater saturation of each component during power generation.\u003c/p\u003e","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/1604675d0696480f0d61416e.png"},{"id":76578187,"identity":"02ed5e6e-aab0-435c-a313-3dc07b63252f","added_by":"auto","created_at":"2025-02-18 14:36:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":56621,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation coefficient matrix between current density and water saturations.\u003c/p\u003e","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/5283ecea7e6a3343b89a85a9.png"},{"id":76577974,"identity":"37dd08b7-15ff-4962-a4ab-bfe314ab8446","added_by":"auto","created_at":"2025-02-18 14:28:23","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":52790,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eOperando\u003c/em\u003e cell for synchrotron X-ray CT.\u003c/p\u003e","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/ef942dfaed348ffaeeb18b27.png"},{"id":76576677,"identity":"e2e18845-0a1e-4a67-a950-8b285ee06d69","added_by":"auto","created_at":"2025-02-18 14:20:23","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":41481,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eOperando\u003c/em\u003esynchrotron X-ray CT system with a forced cooling unit.\u003c/p\u003e","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/1bd2268926ab1bb358357ae5.png"},{"id":86179066,"identity":"7acc04cb-0626-4e11-9039-e9a1185d984e","added_by":"auto","created_at":"2025-07-07 16:15:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":975459,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6019201/v1/138dc66a-cabd-4021-8d96-9cfcb9e15305.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Water management strategies during cold starts in polymer electrolyte fuel cells: insights from operando synchrotron X-ray imaging","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePolymer electrolyte fuel cells (PEFCs) are clean energy devices that use hydrogen as fuel and produce only heat and water as byproducts\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Their core component, the polymer electrolyte membrane (PEM), serves as a proton conductor, electron insulator, and gas barrier (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The catalyst-coated membrane (CCM) consists of catalyst layers (CLs) on both sides of the PEM, with a mixture of ionomer and Pt-based nanoparticles on carbon supports. The membrane electrode assembly (MEA) is formed by sandwiching the CCM with gas diffusion layers (GDLs), comprising a gas diffusion substrate (GDS) and microporous layer (MPL). The cathode CL produces water as a result of the electrochemical oxygen reduction reaction. The vapor-liquid-ice phase transitions significantly impact fuel cell performance and material degradation\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. At high temperatures (typically above 60\u0026deg;C), vapor transport facilitates effective water removal from the MEA, reducing the risk of performance deterioration\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. In contrast, at lower temperatures, condensation within the MEA impedes gas transport\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. During subzero temperatures, ice formation at the cathode electrode can damage the MEA\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNumerous studies, too many to list here, have examined water behavior in PEFCs using \u003cem\u003eoperando\u003c/em\u003e techniques. Especially, \u003cem\u003eoperando\u003c/em\u003e imaging experiments have provided insight into the water management challenges posed by water accumulation in MEAs. For example, \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray radiography has highlighted the effects of back-diffusion\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e and oversaturation\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e on the water transport mechanism in PEFCs and revealed high water saturation of the cathode CL\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e and GDL\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e during performance drops at high current densities. Studies have further demonstrated that water saturation of GDLs depends on the polytetrafluoroethylene (PTFE) content used as a waterproofing agent\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Accelerated stress testing during \u003cem\u003eoperando\u003c/em\u003e experiments have also shown that hydrophobicity loss in GDLs increases the water saturation and causes performance deterioration\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Moreover, these techniques have identified specific water drainage pathways, such as perforations\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, cracks\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, large pores\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, and percolation networks\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e within GDLs. Flow field design using \u003cem\u003eoperando\u003c/em\u003e neutron radiography has also shown promise for improved water management in PEFCs\u003csup\u003e20\u0026minus;22\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCold start research remains limited due to the challenges of establishing \u003cem\u003eoperando\u003c/em\u003e measurement systems under cryogenic conditions. With the global commercialization of fuel cell electric vehicles (FCEVs), improving cold start capability and performance has become increasingly critical. Recent studies using fast \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray computed tomography (CT) have revealed that the presence of supercooled water has a significant impact on cold start capability during cold starts\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, and pre-purging improves cold start capability by increasing the allowable water content in the MEA\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. \u003cem\u003eOperando\u003c/em\u003e neutron imaging studies have shown that freezing events in PEFCs occur randomly, influenced by GDL properties\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, cell size\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, mechanical stress\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e, or residual water\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, resulting in a high pressure drop at the cathode electrode\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. Additionally, \u003cem\u003eoperando\u003c/em\u003e neutron studies have identified cathode CL freezing as a key factor in cold start performance during rapid cold starts\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. Neutron wavelength analysis can differentiate between water and ice phases\u003csup\u003e32\u0026minus;36\u003c/sup\u003e. Energy-resolved \u003cem\u003eoperando\u003c/em\u003e neutron imaging has demonstrated that partial freezing can trigger total freezing in a practical cell of 280 square centimeters\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Another critical factor in cold start performance is temperature management. A commercial fuel cell stack incorporates a circulating coolant system to regulate stack temperature\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, but during cold starts, the coolant temperature oscillates due to the coexistence of heating (power generation) and cooling (external freezing environment)\u003csup\u003e39\u0026minus;41\u003c/sup\u003e. While thermostatically controlled systems have been shown to migrate the temperature oscillation\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e, the effect on water distribution remains unclear. Here, this study aims to gain insight into transient water management strategies during cold starts.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003eElectrochemical Performance.\u003c/b\u003e Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the fuel cell performance measured during \u003cem\u003eoperando\u003c/em\u003e X-ray CT experiments. The \u003cem\u003eoperando\u003c/em\u003e cell was operated in a potentiostat mode with a cell voltage of 0.1 V, where the current density increased as the cell was ramped from 5\u0026deg;C at a rate of 1\u0026deg;C/6 s. Upon reaching 40\u0026deg;C, the cell was subsequently cooled to 5\u0026deg;C at the same rate, leading to a corresponding current density decrease. High-frequency resistance (HFR) remained constant (~\u0026thinsp;200 mΩ cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) throughout the measurements, with fluctuations attributed to the periodic rotation of the \u003cem\u003eoperando\u003c/em\u003e cell for CT scans. When the CT scans were paused at 40\u0026deg;C, these values stabilized. The temperature dependence of the current density followed a concave trend during heating and a convex trend during cooling. Some have attributed the performance hysteresis to the slow change in ohmic resistance caused by the gradual water uptake of the PEM or the ionomer in the CL, as well as to the oxygen transport resistance originating from the ionomer adsorption at the ionomer/Pt interface\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Park et al. have claimed that water accumulation, particularly under high current densities, plays a dominant role in the performance hysteresis\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Since the gradual water uptake of the PEM and CL occurs over several minutes, the second scale hysteresis observed here is likely due to the rapid water accumulation in the MEA. To validate this hypothesis, we performed water behavior analyses.\u003c/p\u003e \u003cp\u003e \u003cb\u003eWater Saturation.\u003c/b\u003e In an example of the vertically sliced MEA image (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), liquid water was widely observed from the MPL to the flow channel. Water within the CL was not visible due to the strong X-ray absorption of Pt\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows an example of water distribution in the stacking direction of the MEA at a representative moment (120 s). Some cracks in the MPL contained water, while the rest appeared in a dry state (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Water accumulation in the GDS was observed under regions compressed by the flow field (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), and water transport in the flow channels was visualized (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the water saturation, defined as the percentage of water filled in the void of each component. The calculations here assume that the MPL contains a porosity of 70% due to invisible nanopores\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e. During heating, the water saturation increased from the core to the outside. Due to the presence of intervals in the CT scans, it was tough to judge whether the detected water droplets were formed by condensation or migrated from the core side. At the cell temperature of 25\u0026ndash;35\u0026deg;C in the heating process, vapor transport contributes to the decrease of water saturation with increasing the cell temperature in the following order: MPL cracks, GDS, and flow channels. At 40\u0026deg;C, water completely disappeared, except for residual water accumulation in the GDS under the flow field (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). During cooling, water saturation increased in the GDS, followed by the MPL cracks, due to the suppressed vapor transport. Eventually, the water saturation increased in the flow channels, obstructing the airflow. The water saturation of the MPL was negligible compared to the other components, regardless of the temperature profile.\u003c/p\u003e \u003cp\u003eVariations in water saturation affect fuel cell performance by altering gas transport properties. To investigate these relationships, we performed a multivariate analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The results showed a positive correlation between the current density and water saturation of the GDS and flow channels during heating, and a negative correlation during cooling. This suggests that there is no universal correlation. A strong negative correlation was observed between the current density and water saturation of the MPL including cracks throughout the measurements. This fact indicates that flooding close to the heart of the PEFC components has a negative impact on the fuel cell performance. Whether condensation occurred in the MPL or whether water migrated from the cathode CL was not presented in our observations, but it is inferred that flooding occurred at the cathode CL because oxygen diffusion was maintained through the dry MPL.\u003c/p\u003e "},{"header":"Discussion","content":"\u003cp\u003eThe use of an \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray CT system with a customized forced cooling unit enables water distribution visualization in an MEA for PEFCs during a rapid temperature response as well as cold starts. The present study offers new insight into the correlation between fuel cell performance and water saturation under transient temperature responses. Hwang et al. have suggested that in partially saturated GDLs, water accumulation obstructs diffusion pathways, resulting in a reduction of the effective gas diffusivity\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. Some have claimed that MPL cracks promote water transport away from the cathode CL, thereby preventing flooding in the cathode CL\u003csup\u003e15\u0026minus;19\u003c/sup\u003e. These explanations sound reasonable, but our observation showed that fuel cell performance deteriorated especially when water saturation increased in the MPL. The results suggest that although MPL cracks can facilitate water transport away from the cathode CL, this drainage route is the result of serious flooding in the cathode CL. Therefore, the most important factor in maintaining fuel cell performance is to prevent flooding in the cathode CL by ensuring an environment suitable for vapor transport. Our findings shed light on water management strategies during cold starts.\u003c/p\u003e \u003cp\u003eResearchers tend to focus only on the cold start capability, but even above the freezing point, serious performance deterioration may happen due to flooding. Since cold start protocols prioritize heat generation over power generation\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, the cell temperature is higher than the surrounding temperature due to heat generation from the MEA and the thermal insulation provided by the GDLs\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. This allows safe cold starts from subzero temperatures. In a fuel cell stack, the circulating coolant system works to lower the stack temperature, so condensation may occur in the fuel cell stack above the freezing point and eventually lead to the performance deterioration. Possible countermeasures include controlling the coolant supply\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e or heating the coolant\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eLooking ahead, the push for next generation PEFCs with higher fuel cell performance involves the development of thinner GDLs to reduce oxygen diffusion resistance. For example, metal-based GDLs have been proposed as a promising solution\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. However, their high thermal conductive GDLs may exacerbate the risk of freezing and flooding in the cathode CL during cold starts, underscoring the need for further research into water management strategies. This study demonstrates progress in understanding the cold start capability and performance. With continued improvements in water visualization technologies, we will address cold start challenges and guide the design of next generation FCEVs.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cb\u003eMaterials.\u003c/b\u003e A 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 were used to prepare a catalyst ink. Pt/C (300 mg) and Nafion ionomer (767 mg) were dispersed in ethanol/deionized water (1/3, \u003cem\u003ev\u003c/em\u003e/\u003cem\u003ev\u003c/em\u003e, 43.5 mL) to obtain a catalyst ink with a solid content of 10 wt%. A glass vial containing this mixture was placed in an ultrasonic bath filled with cold water and sonicated for 10 min\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. The ink was applied to a PTFE sheet and dried under vacuum at 120\u0026deg;C for 10 min. The obtained catalyst layers were transferred to both sides of the Nafion membrane by annealing at 120\u0026deg;C for 10 min\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. For both the cathode and anode, the Pt loading in the CCM was 1.0 mg/cm\u003csup\u003e2\u003c/sup\u003e. A GDS with an MPL was used for the GDL (Sigracet 22BB, SGL Carbon, Germany). The MEA with ethylene-propylene-dienemonomer rubber gaskets was compressed using two electrically conductive flow fields to fabricate an operando cell. The flow fields had two straight parallel flow channels with depths, lengths, and widths of 0.3, 1.0, and 0.8 mm, respectively.\u003c/p\u003e \u003cp\u003e \u003cb\u003eOperando\u003c/b\u003e \u003cb\u003eSynchrotron X-ray CT.\u003c/b\u003e Our previous \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray CT setup\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e was updated prior to the present work with an additional function for studying PEFCs (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e)\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. A high-speed rotary stage (Kohzu Precision, EM200-11) was used for X-ray CT scanning. For \u003cem\u003eoperando\u003c/em\u003e measurements, slip rings and rotary joints were used for wiring and gas piping, respectively. These features enabled high-speed X-ray CT measurements with a rotational speed and accuracy of 1800\u0026deg;/s and 1 \u0026micro;m, respectively. The fuel cell was surrounded by a resin chamber to prevent condensation and increase cooling efficiency. Dry air supplied from a compressor was cooled down to \u0026minus;\u0026thinsp;30\u0026deg;C using a Peltier device and blown into the chamber (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The cell temperature was monitored at the top and bottom positions. The average cell temperature was successfully reduced to a minimum of \u0026minus;\u0026thinsp;20\u0026deg;C using our forced cooling unit. The fabricated cell was conditioned at 40\u0026deg;C and RH 30% under 200 cm\u003csup\u003e3\u003c/sup\u003e/min in an air environment at the cathode and 100 cm\u003csup\u003e3\u003c/sup\u003e/min in a hydrogen environment at the anode without back pressure. The details of the break-in process are described in our previous study\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. Following the break-in procedure, the fuel cell was cooled to 5\u0026deg;C under the same gas flow rate. Subsequently, the fuel cell was operated at a constant voltage of 0.1 V with the temperature control for \u003cem\u003eoperando\u003c/em\u003e measurements. In this work, low-dewpoint gas was supplied by mixing wet gas at 18\u0026deg;C dew point and dry gas with a controlled mixing ratio depending on the cell temperature, so that the condensation did not occur in the \u003cem\u003eoperando\u003c/em\u003e cell (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray CT measurements were performed on a Toyota beamline (BL33XU) at the SPring-8 facility\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e with an X-ray energy of 20 keV and a pixel size of 2.96 \u0026micro;m. The offset scan in the phase contrast mode was acquired using the on-the-fly method with 1,500 projections during a 360\u0026deg; continuous rotation in 2 s.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePost-processing for Imaging Data.\u003c/b\u003e Image reconstruction was performed using a software package provided by the Japan Synchrotron Radiation Research Institute\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, and rotation correction was performed using ImageJ software\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. The obtained phase contrast X-ray CT images were processed using Amira software. Ring artifact reduction is first executed. Next, the flow field region was extracted using a watershed segmentation method, and the CCM region was identified using a threshold segmentation method. The GDS was then determined using a top-hat segmentation method. Finally, the MPL was indexed using a watershed segmentation method. The remaining portion was recognized as the flow channel. An additional water region was assigned to the dry image by comparing the images before and after power generation. As a result, six-phase segmentation was achieved under \u003cem\u003eoperando\u003c/em\u003e conditions. The segmentated images were visualized using GeoDict software\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. The correlations between the current density and water saturation of each component were calculated in R software (version 4.4.2).\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eWe thank Mr. K. Yaegashi (Toyota Central R\u0026amp;D Labs, Inc.) for designing and fabricating the forced cooling experimental setup. Synchrotron radiation experiments were performed at the BL33XU beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal nos. 2022A7032, 2022B7032, and 2024B7032). We received technical support from the Paul Scherrer Institute and Thermo Fisher Scientific Inc. to introduce the forced cooling unit into the X-ray CT setup and the segmentation procedure, respectively.\u003c/p\u003e\n\u003ch2\u003eAuthor contributions statement\u003c/h2\u003e\n\u003cp\u003eW.Y. and S. K. conceived the study, W.Y., S.Y., A.K., and S.K. conducted the \u003cem\u003eoperando\u003c/em\u003e X-ray CT measurements, S.Y. performed the image reconstruction, Y.M. segmented the CT images, and S.K. analyzed 4D water distribution images, and W.Y. examined the results and wrote the initial draft. All authors reviewed the manuscript.\u003c/p\u003e\n\u003ch2\u003eData availability\u003c/h2\u003e\n\u003cp\u003eData are available upon reasonable request from the corresponding author.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThere is no funding for this research work.\u003c/p\u003e\n\u003ch2\u003eAdditional information\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting\u0026nbsp;interests\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e The authors declare no competing interests.\u003cstrong\u003e\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKodama, K., Nagai, T., Kuwaki, A., Jinnouchi, R. \u0026amp; Morimoto, Y. Challenges in applying highly active Pt-based nanostructured catalysts for oxygen reduction reactions to fuel cell vehicles. \u003cem\u003eNat. 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Jan (2025). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.geodict.com/\u003c/span\u003e\u003cspan address=\"https://www.geodict.com/\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e accessed 16.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6019201/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6019201/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDue to the vapor-liquid-ice phase transitions, polymer electrolyte fuel cells (PEFCs) encounter significant challenges during cold starts. This study employs \u003cem\u003eoperando\u003c/em\u003e synchrotron X-ray computed tomography to visualize water distribution in PEFCs during cold starts, where temperature fluctuations surpass the freezing threshold of 0\u0026deg;C. The results suggest a potential risk of condensation due to a circulating coolant system in fuel cell stacks during cold starts, which can adversely affect cold start performance. By comparing transient water saturation across PEFC components, we find that flooding near the cathode catalyst layer is most detrimental to fuel cell operation. These findings provide insights into the interplay between water distribution and cold start performance, underscoring the necessity of optimized water management strategies for next-generation fuel cell electric vehicles.\u003c/p\u003e","manuscriptTitle":"Water management strategies during cold starts in polymer electrolyte fuel cells: insights from operando synchrotron X-ray imaging","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-18 14:20:18","doi":"10.21203/rs.3.rs-6019201/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-03-19T08:26:41+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-03-05T11:53:11+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-02-28T01:18:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"118596026804188039823427499606260281404","date":"2025-02-24T07:34:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"228425663844524561232000388698682632881","date":"2025-02-21T17:38:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"240482723106682610784814423749856485727","date":"2025-02-18T20:30:53+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-02-18T13:51:34+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-02-18T13:45:40+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-02-18T01:42:59+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-02-14T11:13:10+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-02-13T03:22:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"de102e74-0ed9-4bbc-bfa6-74c25c686a4c","owner":[],"postedDate":"February 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":44340678,"name":"Physical sciences/Energy science and technology/Fuel cells"},{"id":44340679,"name":"Physical sciences/Chemistry/Energy"}],"tags":[],"updatedAt":"2025-07-07T16:04:13+00:00","versionOfRecord":{"articleIdentity":"rs-6019201","link":"https://doi.org/10.1038/s41598-025-08939-7","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-07-02 15:57:57","publishedOnDateReadable":"July 2nd, 2025"},"versionCreatedAt":"2025-02-18 14:20:18","video":"","vorDoi":"10.1038/s41598-025-08939-7","vorDoiUrl":"https://doi.org/10.1038/s41598-025-08939-7","workflowStages":[]},"version":"v1","identity":"rs-6019201","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6019201","identity":"rs-6019201","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}