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Highly efficient electrodes with facile fabrication, enhanced reaction sites, low cost, and acceptable durability are of paramount significance for the development of gigawatt-scale proton exchange membrane electrolyzer cells (PEMECs).1-4 In this study, IrRuOx catalysts with high activity are firstly electroplated on titanium liquid/gas diffusion layers via a facile electroplating process at room temperature, serving as highly efficient integrated anode electrodes in PEMECs. When assembled with a PEMEC, low cell voltages of 1.620 and 1.867 V are achieved at 2 and 6 A/cm2, respectively, achieving high cell efficiency of above 91% at 2 A/cm2, superior to the commercial CCM. Moreover, the IrRuOx integrated electrode shows low-performance loss during a 500-h operation at a high current density of 5 A/cm2, and a degradation rate of less than 40 μV/h is observed in the last 250 h, as shown in the figure. The remarkable performance and stability of the IrRuOx integrated electrode are mainly due to the excellent electrode design, high intrinsic activity of amorphous IrRuOx catalysts exposing abundant active sites and a well-controlled Ir and Ru ratio. References Mo, Jingke, Zhenye Kang, Scott T. Retterer, David A. Cullen, Todd J. Toops, Johney B. Green Jr, Matthew M. Mench, and Feng-Yuan Zhang. "Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting." Science advances 2, no. 11 (2016): e1600690. Xie, Zhiqiang, Shule Yu, Xiaohan Ma, Kui Li, Lei Ding, Weitian Wang, David A. Cullen et al. "MoS2 nanosheet integrated electrodes with engineered 1T-2H phases and defects for efficient hydrogen production in practical PEM electrolysis." Applied Catalysis B: Environmental 313 (2022): 121458. Yang, Gaoqiang, Shule Yu, Zhenye Kang, Yifan Li, Guido Bender, Bryan S. Pivovar, Johney B. Green Jr, David A. Cullen, and Feng‐Yuan Zhang. "Building electron/proton nanohighways for full utilization of water splitting catalysts." Advanced Energy Materials 10, no. 16 (2020): 1903871. Kang, Zhenye, Jingke Mo, Gaoqiang Yang, Scott T. Retterer, David A. Cullen, Todd J. Toops, Johney B. Green Jr, Matthew M. Mench, and Feng-Yuan Zhang. "Investigation of thin/well-tunable liquid/gas diffusion layers exhibiting superior multifunctional performance in low-temperature electrolytic water splitting." Energy & Environmental Science 10, no. 1 (2017): 166-175. Figure 1

Proton exchange membrane electrolyzer cells (PEMECs) are electrochemical energy conversion devices, which can provide a pathway to store electrical energy in the form of hydrogen through a water splitting process [1, 2]. Numerous studies in this research area focused on improving cell performance and durability, and lowering cost [3]. The porous transport layers (PTLs), that are key components within these devices, facilitate mass transport, thermal and electrical conduction, and are required to sustain a good interfacial contact with adjacent components [4, 5]. The interfacial contact properties between adjacent components in PEMECs have great influence on cell performance and durability [6, 7]. In this study, the effects of interfacial contacts between PTLs and catalyst layers (CLs) are comprehensively investigated by adopting various operating protocols, including step scan, dynamic sweep scan, galvanostatic and potentiostatic control. We also compare wet/dry and wet/wet operating modes on cells by using PTLs with and without protective coatings. The impacts of current and voltage perturbation amplitudes for conducting EIS diagnostics are also studied. Figure 1 highlights the performance effects of adding an Ir coating onto a Ti based PTL. The data indicate that the performance is significantly improved due to the Ir coating. Identical improvement is accomplished when coating the PTL with platinum. Therefore, we assign the observed improvement to the reduction of the electrical contact resistances, and assume that the Ir coating does not significantly contribute to the electrochemical reaction as discussed by Liu et al. [8]. In addition, the current dependence of the cell resistance that is observed for operation with uncoated PTL materials disappears for cells with Ir coated PTLs. This effect is assigned to the improved heat conduction properties at the PTL/CL interface as depicted in Figure 2. Results will be discussed that focus on the optimization of performance with regards to the impact of the PTL/CL interface and operating strategy. References: [1] B. Pivovar, N. Rustagi, S. Satyapal, Hydrogen at Scale (H2@ Scale): Key to a Clean, Economic, and Sustainable Energy System, The Electrochemical Society Interface, 27 (2018) 47-52. [2] G. Yang, S. Yu, Z. Kang, Y. Li, G. Bender, B.S. Pivovar, J.B. Green Jr, D.A. Cullen, F.Y. Zhang, Building Electron/Proton Nanohighways for Full Utilization of Water Splitting Catalysts, Advanced Energy Materials, (2020) 1903871. [3] G. Bender, M. Carmo, T. Smolinka, A. Gago, N. Danilovic, M. Mueller, F. Ganci, A. Fallisch, P. Lettenmeier, K. Friedrich, Initial approaches in benchmarking and round robin testing for proton exchange membrane water electrolyzers, International Journal of Hydrogen Energy, 44 (2019) 9174-9187. [4] Z. Kang, Development of Novel Thin Membrane Electrode Assemblies (MEAs) for High-Efficiency Energy Storage, (2018). [5] Z. Kang, J. Mo, G. Yang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr, M.M. Mench, F.-Y. Zhang, Investigation of thin/well-tunable liquid/gas diffusion layers exhibiting superior multifunctional performance in low-temperature electrolytic water splitting, Energy & Environmental Science, 10 (2017) 166-175. [6] P. Lettenmeier, S. Kolb, F. Burggraf, A. Gago, K. Friedrich, Towards developing a backing layer for proton exchange membrane electrolyzers, Journal of Power Sources, 311 (2016) 153-158. [7] J. Lopata, Z. Kang, J. Young, G. Bender, J. Weidner, S. Shimpalee, Effects of the Transport/Catalyst Layer Interface and Catalyst Loading on Mass and Charge Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysis Devices, Journal of The Electrochemical Society, 167 (2020) 064507. [8] C. Liu, M. Carmo, G. Bender, A. Everwand, T. Lickert, J.L. Young, T. Smolinka, D. Stolten, W. Lehnert, Performance enhancement of PEM electrolyzers through iridium-coated titanium porous transport layers, Electrochemistry Communications, 97 (2018) 96-99. Figure 1

Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chemical sectors via power-to-liquid or power-to-gas in a future sustainable energy system [1]. While proton exchange membrane electrolyzer cells (PEMEC) offer several advantages over traditional electrolysis technologies, the high cost of catalysts and associated components remain a barrier for their wide industrial commercialization [2]. Fundamental understanding of the PEMEC degradation mechanisms is crucial to elucidate the key parameters for developing new high-performance, low-cost and durable materials. This work aims to reveal degradation mechanisms of two novel material systems as investigation models. Electron microscopy methods, including transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM) and energy dispersive X-ray spectroscopy (EDS) are applied to quantify the degradation of iridium- and platinum-based electrocatalysts in membrane electrode assemblies (MEAs). The first material system involves an ultra-low catalyst loaded MEA with Pt loading of 0.3 mg cm−2 and Ir loading of 0.08 mg cm−2. A long-term test of ~4500 h at 1.8 A cm−2 and 2.76 MPa differential pressure was performed on the MEA. A mechanism of cathode degradation is proposed using established physical models from two aspects: (1) direct Pt dissolution from nanoparticles due to the Gibbs-Thomson relation and (2) transient Pt dissolution due to rapid Pt oxide reduction during start/stop of operation [3]. Furthermore, as Ir dissolves and migrates through the membrane, Pt-Ir precipitates are formed in the membrane. The degradation mechanisms and the distribution of platinum and iridium across the MEA are provided from this study. The second material system involves a catalyst-coated liquid/gas diffusion layers (CCLGDLs) to reduce the cost of traditional Ti MEAs [4]. The CCLGDLs are significantly thinner than commercial catalyst-coated membranes (CCMs) and PTLs where the LGDL is less than 100 µm thick and the catalyst layer (CL) is less than 0.3 µm thick. The catalyst coated LGDLs demonstrated excellent PEMEC performance with cell potential less than 1.85 V at 2 A/cm2. This work examines and compares as-prepared and post-test CCLGDLs to investigate their morphology, composition, and stability relative to traditional electrode layers. In summary, quantitative analysis from microscopic imaging (TEM/STEM) and chemical analysis (EDS) is an effective tool for analyzing catalyst morphology, dissolution, and re-distribution in degraded MEAs. Quantitative microscopic insights into catalyst degradation mechanisms, such as those presented here, help to accelerate research into improved PEMEC performance and durability. References [1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454. [2] Spori, C., Kwan, J.T.H., Bonakdarpour, A., et al., Angew. Chem. Int. Ed., 2017, 56, 5994-6021. [3] Yu, H., Bonville, L., Jankovic, J., Maric, R., Appl. Catal. B: Environ., 2020, 260, 118194. [4] Kang, Z., Mo, J., Yang, F., et al., Energy. Environ. Sci., 2017, 10, 166-175. [5] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat) and Award DE-EE0008426. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

A liquid/gas diffusion layer in a proton exchange membrane electrolyzer cell is located between the catalyst layer and flow field/current distributors, which is expected to transport electrons, heat, and reactants/products to and from the catalyst layers. In addition, conventional materials, which are typically used in PEM fuel cells, are unsuitable due to the high ohmic potential and highly oxidative environment of the oxygen electrode. In this study, a set of well-tunable and titanium thin-film liquid/gas diffusion layer with different thicknesses and porosities are designed and fabricated by using photolithography and mask-patterned wet etching techniques. The effect of mask pore size as well as different etchants on the etch rate and pore morphology of titanium is investigated. Ex-situ analysis is also performed to characterize the morphological features of the liquid/gas diffusion layer post etching. From the results of the fabrication and analysis, new methods for the fabrication of well-tunable thin film liquid/gas diffusion layers is detailed.

In a PEMWE the oxygen electrode is in a harsh environment during electrolysis. This paper highlights recent efforts to investigate the material corrosion and interfacial effects that occur in the electrode interface between gas diffusion layers (GDLs) and catalyst layers of the PEMWE. The in-situ performance and impedance were studied with different GDLs. Both SEM microscopy and EDX spectroscopy were utilized as ex-situ tools to examine and detail the GDLs used before and after testing. A significant interfacial degradation was discovered due to the electrode corrosion with stainless steel.

Hydrogen is now considered the most preferred medium for storing energyout of renewable, intermittent power sources because of its great advantages such as high energy density, low weight, environmental friendliness, etc. Water electrolysis is the most accessible hydrogen production method. Proton exchange membrane (PEM) water electrolysis is attracting much attention with its high efficiency. In a PEM electrolyzer cell (PEMEC), liquid/gas diffusion layers (LGDLs) are porous layers whose main functions are conducting electrons and heat and transporting reactants/products to and from the reaction sites. To achieve high efficiency, a PEMEC demands LGDLs with minimum intrinsic ohmic, thermal, and mass diffusion losses and also low interfacial contact resistances with other components. Thus, the morphologies and surface properties of LGDLs can greatly affect the cell performance. In this research, anode LGDLs featuring pore sizes of approximately ten microns are fabricated by pulsed-laser micromachining (PLM). The effects of pore morphologies, including porosities and pore sizes, are characterized comprehensively ex-situ and in-situ. Electrochemical impedance spectroscopy (EIS) analysis is also carried out to investigate the impact of LGDL pore morphologies on the PEMEC performance. Results from this research can help the community better understand the relationship between LGDL morphology and PEMEC performance. A prediction of the performance at the design stage should also be available based on the morphology of an anode LGDL. Furthermore, an optimal design for anode LGDLs may be acquired.

The thin liquid/gas diffusion layers (LGDLs) with well-tunable pore morphologies are developed by employing advanced manufacturing and show significant performance improvements in proton exchange membrane electrolyzer cells (PEMECs) for hydrogen/oxygen production and renewable energy storage. In addition, the LGDL thickness reduction from hundreds of µm for conventional LGDLs to tens of µm will decrease the weight and volume of the PEMEC stack. More importantly, by taking advantage of the novel material development coupled with a transparent PEMEC and a high-speed micro visualization system, the electrochemical reactions are revealed to occur only on the catalyst layer adjacent to good electrical conductors. Based on these findings, new fabrications of thinfilm catalyst depositions on the novel LGDLs exhibit much higher mass activity than conventional catalyst-coated membranes. This discovery presents an opportunity to enhance the multiphase interfacial effects, maximizing the use of the catalysts and significantly reducing the cost of these electrochemical devices. Furthermore, the well-tunable features of thin electrodes, including pore size, pore shape, pore distribution, and thus porosity and permeability, will be very valuable for developing models and validating simulations of electrochemical energy devices.

The electrochemical reactions and two-phase flow that occur in energy devices could be a very important part in promoting energy efficiency of both engineering and sciences. The electrochemical reactions only occur at a micro scale at the center part of the electrochemical devices like fuel cells and electrolyzers. The objective of this research is to visualize the bubble generation and growth process inside a proton exchange membrane electrolyzer cell (PEMEC) by taking advantage of the thin/well tunable liquid gas diffusion layer (LGDL) and high-speed visualization system. The effects of temperature, current density, and flow rate on bubble dynamics in pore scale and multiphase flow in channel scale, including bubble nucleation, growth, and detachment, are investigated. The reaction sites and rapid bubble evolution phenomena are captured under various operating conditions. The number of reaction sites and bubble growth rates increases with the operating current density and temperature, while the flow rate in the channel has limited effects. The gas bubbles are observed to emerge at the edge of the LGDL pores and gradually detach from the reaction sites. This work will provide a pathway to further investigate the electrochemical reaction in energy devices.

One of the chief difficulties associated with low temperature (

Novel Liquid/Gas Diffusion Layers with Micro/nano Surface Modifications for High-efficiency Water Electrolysis Zhenye Kang[1], Jingke Mo1, Gaoqiang Yang1, Scott T. Retterer[2], David A. Cullen2, and Feng-Yuan Zhang*1 Abstract Proton exchange membrane (PEM) water electrolysis coupled with renewable sources is considered one of the most practical and high-efficiency methods to produce pure hydrogen. The liquid/gas diffusion layers (LGDLs), which located between the catalyst layer (CL) and bipolar plate, are expected to transport electrons, heat, and reactants/products with minimum losses. Due to the harsh environment at anode of PEMECs, most materials, including carbon and stainless steel, can be easily corroded, which significantly affects the performance and durability of PEMECs. Titanium based thin/well-tunable LGDLs have many advantages, such as high corrosion resistance, small thickness (25 µm), well-controlled pore shape, pore size and pore distribution. Most importantly, it can achieve superior multifunctional performance in low temperature PEMECs, which significantly improves the PEMEC efficiency. In this study, the novel thin/well-tunable LGDLs with different pore morphologies and surface modifications are comprehensively investigated in order to enhance the PEMEC performance. Gold (Au) is deposited on the surface of the thin/well-tunable LGDLs by different micro/nano surface modification methods, and the surfaces of the LGDLs are both ex-situ characterized and in-situ investigated. In addition, the performance and electrochemical impedance spectroscopy (EIS) of the PEMECs with different LGDLs are experimentally investigated and analyzed. [1] Nano HELP, Department of Mechanical, Aerospace & Biomedical Engineering, UT Space Institute, University of Tennessee, Knoxville, USA. [2] Oak Ridge National Lab, USA * Contacting Author: E-mail: fzhang@utk.edu; Tel: +1-(931)-393-7428

3D printed bipolar plate for water electrolysis Gaoqiang Yang1, Jingke Mo1, Zhenye Kang[1], Frederick A. List III2, Johney B. Green Jr3, Sudarsanam S. Babu2,4, Feng-Yuan Zhang*1 Abstract A proton exchange membrane electrolyzer is considered as a potential route to the hydrogen production in recent years. It offers a series of advantages, including high product purity, high working current and pressure. As a novel solution for manufacturing process, 3D printing technology is capable of fast and cost-saving prototyping with complex structure from digital models. By employing 3D printing technology, a stainless-steel (SS) component, which is functioned as both of bipolar plate and current distributor, was rapidly produced with selective laser melting (SLM) technique, and was then comprehensively investigated in a proton exchange membrane electrolyzer cell (PEMEC). The experimental results indicated the PEMEC with an 3D printed stainless steel bipolar plate provided an excellent performance, which reached 1.78 V at 2.0 A/cm2 and 80 °C. Furthermore, the 3D printed SS cathode bipolar plate was also ex-situ characterized before and after test by SEM and EDS. This research of 3D printed stainless steel bipolar plate not only provides a route to the quick development of light weight and cheap electrolyzer, but also lead to improvements for other energy related devices. [1] Nano HELP, Department of Mechanical, Aerospace & Biomedical Engineering, UT Space Institute, University of Tennessee, Knoxville, USA. 2 Oak Ridge National Lab, USA 3 National Renewable Energy Laboratory * Contacting Author: E-mail: fzhang@utk.edu; Tel: +1-(931)-393-7428

In-situ Investigation of Triple-phase Boundary Electrochemical Reactions in PEM Electrolyzer Cells Jingke Mo, Zhenye Kang, Gaoqiang Yang, Derrick Talley, and Feng-Yuan Zhang* Nano HELP : Nanodynamics and High Efficiency Lab for Propulsion & Power Department of Mechanical, Aerospace and Biomedical Engineering, UT Space Institute, University of Tennessee, Knoxville *fzhang@utk.edu Abstract Polymer electrolyte membrane electrolyzer cells (PEMECs) have been considered as one of the most effective and environment-friendly way for energy storage when coupled with renewable energy sources. In PEMECs, water is electrochemically split into oxygen and hydrogen. At the anode side, the circled water flows over the liquid/gas diffusion layer (LGDL) and diffuses to the triple-phase boundaries (TPBs) - the catalyst layers, where it is decomposed into oxygen, electrons, and protons. The proton will transport through the proton exchange membrane (PEM), reach the cathode catalyst layer, and form hydrogen at the cathode side. Due to limitations of the designs of conventional PEMECs and LGDLs, the microspatial and microtemporal scales of the electrochemical reactions, the phenomena of TPB electrochemical reactions has never been explored in operating PEMECs in-situ. In this research, an innovatively designed transparent PEMEC was developed, and the thin and well-tuned titanium LGDLs with straight-throughout pores were fabricated. A high-speed and micro-scale visualization system (HMVS) was built for in-situ characterizations. The TPB electrochemical reactions only occurred at the edges adjacent to the good conductive materials. Visualization results reveal the real phenomena inside an operating PEMEC, which can not only help maximize the utilization of catalyst, but also help to optimize the fabrication of catalyst layer. The discovery will have a lot application on improving the electrochemical devices.

A sustainable and robust electrochemical energy storage and/or a hybrid system to accommodate daily, even hourly changes of renewable resources becomes more critical. An advanced polymer electrolyte membrane water electrolyzer (PEMWE) has been shown to be effective energy storage medium by producing hydrogen/oxygen from water with electricity from renewable sources. When the renewable resources are available, hydrogen will be produced and stored, such that it can later provide a constant power supply with a PEM fuel cell, which is a reverse device of the electrolyzer. This entire portfolio will make renewable and hybrid energy systems effective to provide reliable and multiscale energy whenever needed. In proton exchange membrane water electrolyzers (PEMWEs), water is electrochemically splitting into oxygen and hydrogen. The oxygen generated at the anode side and the circled water flowing over the liquid/gas diffusion layer yield two-phase transport conditions in micro-channels and pores of LGDL, which significantly impact the performance. Due to limitations of the design of conventional PEMWEs and LGDLs, the in-situ phenomena of two-phase flow has few been explored in operating PEMWEs. In this research, an innovative design of a transparent PEMWE is developed coupled with the thin and well-tuned titanium LGDLs with straight pores. The high-speed and micro-scale visualization system (HMVS), and electrochemical impedance spectroscopy will be used for in-situ characterizations. Visualization results are used for better understanding the behavior of two-phase flow and its effects on the PEMWE performance. Patterns of gas-bubble formation, evolution, departure and transport are identified. The effects of property of materials, including LGDL pore sizes, channel dimensions, are also investigated.

Understanding multi-phase and multi-component transport in a porous electrode plays a crucial role in optimizing the actual electrode structures and maintaining the performance of a polymer electrolyte membrane (PEM) water electrolyzer, which has attracted more attention for renewable energy storage and hydrogen production due to its higher efficiency/density and a more compact design. Since the electrode microstructure and transport mechanisms of a PEM water electrolyzer is quite complex, until recently, it has been very difficult to provide a comprehensive understanding of these mechanisms. Although some experiments and few simple numerical models have been carried out for investigating the two-phase transport phenomena in a PEM water electrolyzer, many fundamental issues still remain, particularly concerning the effects of gas-liquid two-phase transport inside porous liquid/gas diffusion layer (LGDL) with various surface wetting properties, pore structures on the electrolyzer performance. In this study, a comprehensive two-phase transport model has been developed as an efficient numerical scheme for direct modeling the effects of two-phase transport on the electrolyzer performance, particularly for liquid water and oxygen transport in the anode porous gas diffusion layer. The liquid water saturations are found to decrease significantly along the LGDL thickness direction with larger contact angles. But for smaller contact angles, the saturation values along the LGDL thickness direction are almost identical. Further numerical results for the gas-liquid two-phase transport behaviors and electrolyzer performance with effects of LGDL pore morphology will also be presented. Results obtained from the present model will provide fundamental understandings of gas-liquid transport behaviors in a porous electrode and can be employed as the useful information to design and optimize the electrode microstructure and improve the performance of PEM water electrolyzers.

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An advanced polymer electrolyte membrane water electrolyzer (PEMWE), which is a reverse PEM fuel cell (PEMFC), has been an effective energy storage medium by producing hydrogen/oxygen from water with electricity from renewable energy sources because of its distinguished efficiency, high energy density, compact and large capacity. In a PEMWE, a gas diffusion layer (GDL) is located between the catalyst layer and the current collecting flow field. The purpose of the GDL is to transport electrons, heat and reactants/products to and from the catalyst layer with minimum voltage, current, thermal, interfacial, and fluidic loss. Carbon materials (carbon paper or carbon cloth), which are typically used in PEMFCs, are unsuitable for PEMWEs due to high potential of the oxygen electrode. The carbon corrosion and consumption will result in poor interfacial contacts which will degrade performance and efficiency. This presentation will highlight recent efforts to optimize the GDL properties at the anode side for high-efficiency PEMWEs. A set of GDLs with different thicknesses and a range of porosities from 30% to 80% are designed and examined with a PEMWE that was developed in lab. Both electropotential performance and impedance measurements are conducted on the different GDLs in order to evaluate the effectiveness of altering the aforementioned parameters. The microscale characterizations of all the GDLs are detailed. From the results and metrics, an optimized set of parameters for an anode-side GDL is created. Acknowledgements This research is supported by the U.S. Department of Energy under Grant DE-FE0011585.

Cell water balance and anode to cathode water transport were investigated using high resolution neutron radiography in the through-plane direction. Induced pressure and relative humidity gradients were imposed across the membrane electrode assembly to determine how these gradients alter saturation profiles in the diffusion media. The asymmetric anode/cathode channel pitch used in this study resulted in large anode water saturation changes with applied gradients while cathode saturation remained more constant. A new low-cost core-shell diffusion media is introduced as a potential replacement of current diffusion media. This new material was found to reduce overall cell water content, but suffers poor overall performance due to excessive ohmic losses, and more work is needed to develop this concept.

The catalyst layer CL in the polymer electrolyte membrane fuel cell PEMFC has not been explored in detail due to its complexities and difficulties associated with accessing it. It is important to understand water ingress and egress at the CL as it offers potential for water management by manipulating the evaporation rate. A technique for in situ visual characterization of the CL is presented. This is accomplished by designing and fabricating a catalyst-visible operational fuel cell and developing a microvisualization system. The dynamics of microdroplets on the CL surface including formation, growth, coalescence, and removal were visualized in an operating PEMFC. The liquid water behavior at the interface of the CL and the gas diffusion layer GDL were shown to promote the periodic droplet reemergence on the GDL surface in the flow channels. Mechanisms of water condensation and transport within the CL pores are discussed with respect to pore architecture and wetting properties. It has been shown that reduction of pore size and CL thickness alleviates flooding therein and promotes better catalyst utilization. Evaporation was identified as one of the distinguishing mechanisms of the CL, and one of the future challenges will be to control this mechanism. The experimental results should prove useful in clarifying the role of the CL in water management, and in refining models used to optimize PEMFC performance.

Liquid water transport and removal from the gas diffusion layer GDL and gas channel of a polymer electrolyte fuel cell PEFC are studied experimentally and theoretically. In situ observations of the liquid water distribution on the GDL surface and inside the gas channel were made in an operating transparent PEFC. Liquid droplet formation and emergence from the GDL surface are characterized and two modes of liquid water removal from the GDL surface identified: one through droplet detachment by the shear force of the core gas flow followed by a mist flow in the gas channel, and the other by capillary wicking onto the more hydrophilic channel walls followed by the annular film flow and/or liquid slug flow in the channel. In the former regime, typical of high gas flow rates, the droplet detachment diameter is correlated well with the mean gas velocity in the channel. In the latter regime characteristic of low gas flow rates, liquid spreading over hydrophilic channel surfaces and drainage via corner flow were observed and analyzed. A theory is developed to determine what operating parameters and channel surface contact angles lead to sufficient liquid drainage from the fuel cell via corner flow. Under these conditions, the fuel cell could operate stably under a low flow rate or stoichiometry with only a minimum pressure drop required to drive the oxidizer flow. However, when the corner flow is insufficient to remove liquid water from the gas channel, it was observed that the annular film flow occurs, often followed by film instability and channel clogging. Channel clogging shuts down an entire channel and hence reduces the cell’s active area and overall performance.

Using an optical /air polymer electrolyte fuel cell (PEFC), the mechanics of liquid water transport, starting from droplet emergence on the gas diffusion layer (GDL) surface, droplet growth and departure, to the two-phase flow in gas channels, is characterized under automotive conditions of 0.82 A/cm2, 70°C, and 2 atm. It is observed that water droplets emerge from the GDL surface under oversaturation of water vapor in the gas phase, appear only at preferential locations, and can grow to a size comparable to the channel dimension under the influence of surface adhesion. Liquid film formation on more hydrophilic channel walls and channel clogging are also revealed and analyzed. © 2004 The Electrochemical Society. All rights reserved.