Green hydrogen production from proton exchange membrane water electrolyzers (PEMWEs) has the potential to help decarbonize pollution sources like the transportation and energy sectors. For PEMWEs to be more sustainable and widespread, the loading of iridium oxide catalyst used for these electrolyzers must be reduced by about an order of magnitude which can lead to thinner, discontinuous catalyst layers. To combat this, the porous transport layer (PTL), the layer that contacts the catalyst layer and bipolar plate, must utilize a microporous layer (MPL) to improve the interfacial contact and in-plane connectivity at the catalyst layer and MPL interface. Here, we use X-ray micro-computed tomography (CT) to investigate six novel PTLs with MPLs ex-situ to attain morphological data and operando to determine oxygen content at 0.5 and 1 A/cm2 for low-loaded membrane electrode assemblies (MEAs). These results are combined with polarization curves to relate ex-situ and operando findings to cell performance. With an MPL to provide improved contact with the catalyst layer, the base PTL can have a more porous structure to aid with oxygen transport through the PTL to the channel. The morphology and average pore size of the base PTL dictates the oxygen transport out to the flow field, which is seen to have an effect on the cell voltage at a current density of 3 A/cm2. Strategies to best design PTLs with MPLs are discussed to help enable the deployment of low iridium oxide loading catalyst layers.

Hydrogen produced with no greenhouse gas emissions is termed "green hydrogen" and will be essential to reaching decarbonization targets set forth by nearly every country as per the Paris Agreement. Proton exchange membrane water electrolyzers (PEMWEs) are expected to contribute substantially to the green hydrogen market. However, PEMWE market penetration is insignificant, accounting for less than a gigawatt of global capacity. Achieving substantive decarbonization via green hydrogen will require PEMWEs to reach capacities of hundreds of gigawatts by 2030. This paper serves as an overarching roadmap for cell-level improvements necessary for gigawatt-scale PEMWE deployment, with insights from three well-established hydrogen technology companies included. Analyses will be presented for economies of scale, renewable energy prices, government policies, accelerated stress tests, and component-specific improvements.

There is interest in valorization of existing natural gas infrastructure to facilitate the co-transportation of hydrogen via blending of hydrogen gas initially at limited concentrations of 1-20 vol% H-2 and to subsequently extract hydrogen at fuel cell quality standards (SAE J2719/ISO14687-2). High temperature proton exchange membrane electrochemical hydrogen pump (HT-PEM EHP) based on phosphoric acid doped polybenzimidazole (PA-PBI) exhibits good performance at elevated temperatures (>120 degrees C), which provides desirable tolerance to non-methane natural gas constituents that are problematic for lower temperature based EHP. To better understand the suitability of the HT-PEM EHP for such gas separation processes, a two-dimensional model of EHP based on PA-PBI was developed. The model is validated for several relevant operating conditions and across cells with differing amounts of phosphoric acid content in the electrodes. Operando micro x-ray computed tomography (CT) imaging of an HT-PEM EHP was used to further validate physical parameters and assumptions of the model. The impacts of pressure, relative humidity of the anode feed, and concentration of feed gases on separation performance are investigated. This study shows that a specific energy of separation of 5.1 kWh/kg H-2 at a hydrogen recovery factor (HRF) of 50 vol% can be achieved in a single stage with the EHP, producing fuel cell quality hydrogen purity of 99.99 vol% H-2 from a 2 vol% H-2/CH4 feed blend, while pressurizing the product H-2 at a pressure ratio of 1.3 relative to feed pressure.

Moving towards lower iridium oxide (IrOx) catalyst loadings for the anode side of polymer electrolyte membrane water electrolyzers (PEMWE) requires tuning of the catalyst layer-porous transport layer (PTL) interface. To see widespread deployment, PEMWE anodes need to lower the loadings from 1-2 mg/cm2 IrOx down to 0.1 mg/cm 2 [1]. The catalyst layer can become less uniform and more disconnected at these lower loadings, so the addition and optimization of a microporous layer (MPL) onto the PTL to improve interfacial contact and in-plane connectivity is essential to move toward the more sustainable lower loadings [2]. First, a set of PTLs with MPLs are made such that morphological properties are varied between them. Then, the PTLs with MPLs are combined with anode catalyst layers with IrOx loadings of 0.7, 0.5, 0.3, and 0.1 mg/cm 2 to create cells to record polarization curves, stability tests, and catalyst degradation rates. The morphology of the PTLs and MPLs is quantified using X-ray computed tomography (CT). The important morphological characteristics that can be varied and measured with X-ray CT include porosity, pore size distribution, tortuosity, and thickness for each the MPL and base PTL. Using operando X-ray CT and radiography, oxygen transport through the PTL and channel can be observed to provide further information to the performance of the novel PTLs and MPLs. By using a two-dimensional continuum PEMWE model, insights into the effects on the cell performance and the contact resistance present at the MPL-catalyst layer brought about from PTL and MPL morphology alterations can be extracted to help guide the optimization efforts. Through combining these results, an optimal set of manufacturing/morphological parameters can be found to tune PTL and MPL production for the low-loaded catalyst layers that are needed for the future of PEMWEs. References: [1] M. Clapp, C. M. Zalitis, and M. Ryan, “Perspectives on current and future iridium demand and iridium oxide catalysts for PEM water electrolysis,” Catalysis Today , vol. 420, p. 114140, 2023. doi:10.1016/j.cattod.2023.114140 [2] M. Bernt, A. Siebel, and H. A. Gasteiger, “Analysis of voltage losses in PEM water electrolyzers with low platinum group metal loadings,” Journal of The Electrochemical Society , vol. 165, no. 5, 2018. doi:10.1149/2.0641805jes

To enable gigawatt-scale deployment of proton exchange membrane water electrolysers (PEMWEs), drastic reductions from current iridium loadings of 2–3 mgIr cm−2 to less than 0.4 mgIr cm−2 must occur due to iridium's high cost and scarcity. State-of-the-art systems use these high loadings to compensate for degradation experienced over prolonged operation. Thus, to attain low loadings while meeting commercial lifetime targets, factors such as ink formulation, MEA fabrication, catalyst layer–porous transport layer (CL–PTL) contact, and catalyst durability must be optimised. This review paper discusses the fundamentals of PEMWE technology and the modifications/improvements necessary for effective low iridium-loading design. Important milestones for future research include developing durable catalyst layers at low loadings, optimising the CL–PTL interface, and improving roll-to-roll production processes.

The deployment of NEL’s next-gen PEM water electrolyzer stacks and systems is very much dependent on our ability to evaluate the degradation of individual components and thereby predict 50000 hr. operability of the stack. In addition, Nel Hydrogen is constantly working to unveil degradation mechanisms from our long run (< 100.000 hours) fielded stacks, so that we can make the developing of accelerated stress tests (ASTs) less challenging. Typically, performance losses in PEM electrolyzer stacks are the result of electrode, membrane, and/or PTL degradation during different operation strategies. Here we accelerated the performance losses of electrodes used in PEM water electrolyzer stacks using an in-house developed voltage cycling protocol. Membrane degradation was also accelerated through cation-doped catalyst coated membranes and is assessed through ex-situ monitoring of fluoride emission rates during the stack operation. We also leveraged a fielded Nel Hydrogen legacy PEM stack that ran for over 50000 hr. and other R&D stacks that ran for intermediate life span of 5000 hr. at steady state. A correlation of the degradation mechanism to different cell components observed from that of the AST protocols is then here reported.

X-ray computed tomography (CT) is a nondestructive three-dimensional(3D) imaging technique used for studying morphological propertiesof porous and nonporous materials. In the field of electrocatalysis,X-ray CT is mainly used to quantify the morphology of electrodes andextract information such as porosity, tortuosity, pore-size distribution,and other relevant properties. For electrochemical systems such asfuel cells, electrolyzers, and redox flow batteries, X-ray CT givesthe ability to study evolution of critical features of interest inex situ, in situ, and operando environments. These include catalystdegradation, interface evolution under real conditions, formationof new phases (water and oxygen), and dynamics of transport processes.These studies enable more efficient device and electrode designs thatwill ultimately contribute to widespread decarbonization efforts.

The efficient and cost-effective production of green hydrogen is essential to decarbonize heavily polluting sectors such as transportation and heavy manufacturing industries such as metal refining. Polymer electrolyte membrane water electrolysis (PEMWE) is the most promising and rapidly maturing technology for producing green hydrogen at a scale and on demand. However, substantial cost reduction by lowering precious metal catalyst loadings and efficiency improvement is necessary to lower the cost of the produced hydrogen. Porous transport layers (PTLs) play a major role in influencing the PEMWE efficiency and catalyst utilization. Several studies have projected that the use of microporous layers (MPLs) on PTLs can improve the efficiency of PEMWEs, but very limited literature exists on how MPLs affect anodic interfacial properties and oxygen transport in PTLs. In this study, for the first time, we use X-ray microtomography and innovative image processing techniques to elucidate the oxygen flow patterns in PTLs with varying MPL thicknesses. We used stained water to improve contrast of oxygen in PTLs and demonstrate visualization of time averaged oxygen flow patterns. The results show that PTLs with MPLs significantly improve interfacial contact by almost 20% as compared to single layer sintered PTL. For the single layer PTL without MPL, the pore volume utilization for oxygen flow is low and the oxygen follows a viscous fingering flow regime. With MPLs, the pore volume utilization is higher, and the number of oxygen transport pathways is increased significantly. MPLs were also shown to suppress capillary fingering and transition oxygen flow to the viscous fingering regime, which has been proven to decrease site masking effects. Finally, durability tests showed the least voltage degradation for thin MPL and thicker MPLs run into mass transport limitations. Based on these findings, PTL/MPL design optimization strategies are proposed for enabling low catalyst loadings and improving durability.

Porous transport layers (PTLs) serve many important functions for proton exchange membrane water electrolyzers. PTLs facilitate fluid transport towards and away from the anode catalyst layer, act as a mechanical support for the membrane, and provide electrical contact with the anode catalyst layer [1]. As a result, PTLs can greatly impact cell performance. However, while there has been an effort to improve similar gas diffusion layers in PEM fuel cells, there has not been a significant effort to optimize the overall form factor and design of PTLs for PEM water electrolyzers. One of the primary concerns about the current design of PEM water electrolyzer PTLs is how they interact with the anode catalyst layer. In order to ensure proper fluid transport through the PTL, there is significant porosity throughout the PTL including at the anode interface. The large porosity and particle sizes of the PTL can cause heterogeneous contact of the PTL and catalyst layer thus reducing the catalyst utilization. Therefore, high catalyst loadings are required to obtain acceptable performance and durability. One method to address this concern would be to develop a metal microporous layer (MPL) that can be integrated onto a PTL at the anode interface that can withstand the high potentials at the anode while maintaining sufficient fluid transport. The MPL is fabricated using smaller metal particle sizes compared to the bulk PTL, which results in smaller pores creating a more uniform surface. The uniform surface and small pore sizes of the MPL provide a much higher interfacial contact area at the anode interface compared to a traditional PTL and would allow for a more uniform contact pressure across the anode interface. Higher interfacial contact will improve catalyst utilization and facilitate the reduction in anode catalyst loading, leading to reductions in overall electrolyzer capital cost [2]. In this work, prototype PTLs with metal MPLs are developed and tuned for optimal PEM water electrolyzer cell performance. The prototype PTLs with MPLs are characterized to understand how specific properties (thickness, porosity, tortuosity, etc.) influence cell performance. Electrochemical testing shows that adding an MPL at the anode/PTL interface can allow for acceptable cell performance with 90% lower anode catalyst loading compared to when using a baseline PTL. References [1] X.-Z. Yuan, N. Shaigan, C. Song, M. Aujla, V. Neburchilov, J.T.H. Kwan, D. P. Wilkinson, A. Bazylak, K. Fatih. The porous transport layer in proton exchange membrane water electrolysis: perspectives on a complex component. Sustainable Energy & Fuels, 6, (2022), 1824-1853. [2] T. Schuler, J.M. Ciccone, B. Krentcher, F. Marone, C. Peter, T.J. Schmidt, F.N. Büchi. Hierarchically Structured Porous Transport Layers for Polymer Electrolyte Water Electrolysis. Advanced Energy Materials, 10, (2019), 1903216

Electrolyzers are a key component in transitioning the world’s reliance on fossil fuels for energy. Accordingly, a substantial influence on electrolyzer costs is operational efficiency. The means of quantifying this efficiency is necessary for sustaining the continuous improvement of this technology. Polarization curves are a critical tool in understanding the behavior and mechanisms of electrochemical devices such as electrolyzers and fuel cells. In this work, a semi-empirical voltage-breakdown method is used to model and identify a series of resistance levels and voltage losses. This model presents the modes of change in electrolyzer performance. Polarization curves are taken at several temperature and time intervals throughout steady state operation. The unique benefit of this method is that no potentiostat is required and that key parameters, such as the Tafel slope and ohmic resistance, are compared to more direct measurements. Presented here, is a user-friendly technique of evaluating the degradation of an electrolyzer over the course of its lifetime. More specifically, it provides insight on the durability of cell components such as membrane, gas diffusion layers, and catalyst layers.

Producing green hydrogen efficiently via proton exchange membrane water electrolysis (PEMWE) is the key for achieving decarbonization targets. Iridium catalyst is expensive, and it is important to minimize its use and to optimize interface between Ir and ionomer or water for higher utilization of catalyst in oxygen evolution reaction. In this paper, x-ray computed tomography along with electrochemical and modeling techniques are used to characterize the interface for two different porous transport layers (PTLs) and catalyst layers at various loadings. We show that low porosity sintered PTLs exhibit higher interfacial contact with the catalyst and the membrane that results in improved kinetics. Radiography and modeling results indicate that oxygen taking multiple transport pathways through the PTL results in slug flow through the channels that reduces mass transport overpotential. Based on the results, we suggest design guidelines for high efficiency and durable PEMWE and their components.

Demand for high purity hydrogen production using renewable energy sources is growing to meet the clean energy demands. Polymer electrolyte membrane water electrolyzer (PEMWE) is one of the viable options for H 2 production, but its high capital cost and operational expenditures increase the cost of H 2 . Improving the interface between the catalyst layer (CL) and the porous transport layer (PTL) is critical to increasing the efficiency of PEMWEs and thereby lowering the cost of H 2 . Increased contact between the CL and PTL improves catalyst utilization, and the optimal structure of the PTL reduces the mass transport issues related to O 2 bubble removal.(1-3) Improved understanding of the PTL microstructure is necessary to improve the performance and efficiency of the PEMWE. This work presents a systematic study to elucidate the effect of PTL properties (morphology, thickness, and porosity) and their impact on PEMWE performance under different operating conditions. Polarization curves with different anode PTL (felt, sinter and pore graded hierarchical PTL) are presented in Figure 1a. The separation of mass transport resistance and the contract resistance for the different PTLs will be elucidated to show the impact of water management and interfacial contact. Mass transport in an operating electrolyzer is also studied by estimating the water content using neutron imaging. Figure 1b shows water thickness across a membrane electrode assembly (MEA) with a pore graded hierarchical PTL in anode at different current densities. Water content across the MEA with different PTL is also studied. The cells with these PTLs were evaluated in operando using micro x-ray computed tomography (CT) and x-ray radiography. The x-ray techniques revealed oxygen distribution within the PTLs on the pore-scale at varied current densities, complementing neutron imaging water thickness studies and providing micro-scale insight into transport. Acknowledgment This research is supported by the U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office, through the H2NEW consortium. References J. K. Lee, C. Lee, K. F. Fahy, B. Zhao, J. M. LaManna, E. Baltic, D. L. Jacobson, D. S. Hussey and A. Bazylak, Cell Reports Physical Science , 1 , 100147 (2020). T. Schuler, J. M. Ciccone, B. Krentscher, F. Marone, C. Peter, T. J. Schmidt and F. N. Büchi, Advanced Energy Materials , 10 , 1903216 (2020). P. Lettenmeier, S. Kolb, F. Burggraf, A. S. Gago and K. A. Friedrich, Journal of Power Sources , 311 , 153 (2016). Figure 1

To reach Department of Energy (DOE) target of 1$ cost for 1 kg of H2 production in 1 decade the cost of green hydrogen generation needs to decrease. IrOx is a used as an electrocatalyst in anode electrode of proton exchange membrane water electrolyzers (PEMWE) and its loading must decrease below 2 mg/cm2 to reach the cost targets for electrolysis. One of the hurdles of catalyst layers with low IrOx loading is the inplane electric conductivity, which will be low as IrOx is not supported by carbon or other conductive additive. Therefore, the design of porous transport layers (PTLs) becomes critical to ensure a good interface contact area with the catalyst but at the same time optimal oxygen removal from the catalyst. In our earlier study we showed that PTLs with intermediate porosities of ~60 % achieved this goal [1]. In this work we use operando x-ray computed tomography to visualize oxygen transport inside the PTLs for the cells having various catalyst loadings and various PTLs (sintered and fiber). Using a staining agent of NaI we were able to differentiate between water and oxygen as shown by Figure 1. Oxygen transport pathways and oxygen content within the PTLs was studied using various cell configurations and current densities. This work will identify a path forward for designing PTLs for PEMWE. References: Peng, X., Satjaritanun, P., Taie, Z., Wiles, L., Keane, A., Capuano, C., Zenyuk, I. V., Danilovic, N., Sci.2021, 8, 2102950. Figure 1

A key component of the emerging hydrogen energy economy is the distribution system which facilitates the movement of hydrogen energy between production and end-use. There is interest in valorization of existing gas systems, which today are dedicated to natural gas movement, to facilitate the co-transportation of hydrogen via blending of hydrogen gas into these gas systems at concentrations up to 20% H2 by volume [1]. Fuel cell quality hydrogen, at a purity of 99.97%> H2, is required for the most valuable energy end-uses of hydrogen gas and thus the ability to efficiently extract H2 gas at such high purity is critical to the adoption of natural gas infrastructure as a means of co-transportation and distribution. Electrochemical hydrogen pump (EHP) utilizes a proton conducting membrane to separate hydrogen from mixtures by driving the electrochemical process of oxidation of hydrogen at an anode and subsequent evolution of hydrogen gas at a cathode, while other gaseous impurities are ideally unable to permeate through the membrane. HT-PEM EHP based on polybenzimidazole (PBI) membranes doped with phosphoric acid exhibit high proton conductivities at temperatures in the range of 160C Celsius while also exhibiting characteristics that are complimentary to their application in gas separation processes. These characteristics include a limited tolerance to common sources of platinum catalyst inhibition such as CO [1] and the presence of an aqueous phosphoric acid phase inhibits gas cross-over, providing better product purity although at the cost of performance in the form of catalyst inhibition. A two-dimensional model based on an HT-PEM system employing a phosphoric acid doped PBI membrane and free phosphoric acid as the proton conducting phase of the catalyst layer is developed to better understand the underlying processes governing the performance of the EHP. The model is validated with experimental measurements with mixtures as low as 2% H2 by volume in methane. In-operando micro-CT imaging of an HT-PEM EHP cell taken at the LBNL Advanced Light Source (ALS) is used to further validate physical parameters and assumptions of the model. The influence of differential pressure, relative humidity of the feed, and concentration of feed gases on separation performance are investigated. Losses due to back-permeation of hydrogen under pressure are accounted for. Power-loss voltage breakdown analysis indicates appreciable losses due to proton transport in the relatively thick catalyst layer, and the dependence of these losses on the volume fraction of acid present (Figure 1). The influence of CO2 as a catalyst inhibiting contaminant as was previously observed in low temperature EHP [3] is examined. The model and experiments show that a specific energy of separation of 5.1 kWh/kg H2 at a hydrogen recovery factor (HRF) of 50% can be achieved in a single stage with the EHP, producing 99.995% > H2 from a 2% H2 in CH4 blend, while pressurizing the product H2 at a product pressure ratio of 1.3 relative to feed pressure. Bibliography [1] C. J. Quarton and S. Samsatli, "Power-to-gas for injection into the gas grid: What can we learn from real-life projects, economic assessments and systems modelling?," Renewable and Sustainable Energy Reviews, vol. 98, pp. 302-316, 2018. [2] K. Perry and B. B. Eisman G.A., "Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane," Journal of Power Sources, pp. 478-484, 2008. [3] N. e. al., "Effect of CO2 on the performance of an electrochemical hydrogen compressor," Chemical Engineering Journal, vol. 329, 2020. Figure 1

Cement is the most widely used manmade materials today, about one ton is produced every year per person on Earth (1). Production process contributes to about 5% of industrial energy consumption and about 8% (8.2Gton/year) of global CO2 emissions [ref]. In conventional manufacturing process, calcination of CaCO3 (limestone) is obtained in a kiln using energy input from non-renewable sources producing CaO. Carbon dioxide emission come from both the decomposition reaction and from the combustion of the fuel necessary to provide heat to the system (calcination and sintering reactions happen at high temperature, between 900 and 1500°C). As a result, about one ton of CO2 is released for every ton of cement produced.

Polymer electrolyte water electrolyzers (PEWEs) are a promising technology to produce green hydrogen with high efficiencies at low temperature [1]. The widespread deployment of this technology is hindered by the catalyst cost and durability issues [2]. Higher anode catalyst loadings are currently used to enable longer operating time of the PEWEs. Interface between catalyst layer and porous transport layer (PTL) is not well understood, especially for novel PTLs with varied morphological properties, such as sintered vs. fiber. [3] The PTLs’ bulk properties, interface, catalyst loading, and catalyst distribution all impact cell performance and possible correlation between these properties needs further investigation. This study focuses on catalyst layers with different loadings, and the resulting catalyst layer-PTL interfaces with two different types of PTLs (sintered Ti and fiber Ti). For a given loading, two different electrode configurations have been compared namely the catalyst coated membrane (CCM) and gas diffusion electrode (GDE). The current density was applied from 1 to 5 A/cm 2 with a constant liquid water flow rate and temperature of 60 o C. We categorized the PEWEs into three categories according to catalyst loading, (1) high loadings: 1.75– 2.20 mg/cm 2 , (2) medium loadings: 1.00– 1.26 mg/cm 2 , and (3) low loadings: 0.50– 0.65 mg/cm 2 . Operando x-ray computed tomography (CT) and radiography were used to observe the catalyst distribution and oxygen transport in all the samples. The interfacial analysis using the tomography data and electrochemical testing gives insight into the percentage triple phase contact area (%TPCA) and the cell performance, respectively. Herein, we present a comprehensive comparative analysis of the catalyst loadings as well as the correlate cell performance with the %TPCA and the double layer capacitance. We observed a Tafel slope difference of over 10 mVdec -1 between sintered CCMs and sintered GDEs at low loadings suggesting proton or electron transport limitations at the interface. We then tested the cells at steady state conditions for 90 hours and compared the initial and final performances. Over 600 mV increase in the overpotential for low loaded fiber CCMs was observed, compared to < 200 mV increase for its high loaded counterpart at 1.5 A/cm 2 after the steady state holds (Figure 1c-d). Computational fluid dynamics (CFD) with the Lattice Boltzmann Method (LBM) was used to simulate the oxygen transport in the PTLs with varying catalyst loadings, which showed underutilization of transport pathways for lower loaded fiber PTLs and showing that sintered PTLs have more uniform oxygen distribution due to more uniform through-plane morphological properties. Additionally, with the x-ray radiography study, we showed that having large oxygen slugs in the channel is preferred, as long as oxygen content in the channels is below 80 %. References: [1] P. Patel and K. Ayers, Electrolysis for hydrogen production , vol. 44, no. 9. 2019. [2] M. Bernt et al. , “Current Challenges in Catalyst Development for PEM Water Electrolyzers,” Chemie-Ingenieur-Technik , vol. 92, no. 1–2, pp. 31–39, 2020, doi: 10.1002/cite.201900101. [3] E. Leonard et al. , “Interfacial analysis of a PEM electrolyzer using X-ray computed tomography,” Sustain. Energy Fuels , pp. 921–931, 2020, doi: 10.1039/c9se00364a. Figure 1

Polymer electrolyte water electrolyzer (PEWE) is one of the most appealing options to produce hydrogen and oxygen from renewable resources[1]. Understanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the cell performance of PEWE. In order to probe this more fully, operando x-ray computed tomography (CT)[2], machine learning, and LBM simulation[3] were performed on a model electrolyzer at different water flowrates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. X-ray CT was employed to create 3D images (Figure 1A) and visualize oxygen content in the model electrolyzer (Figure 1C and 1D). Machine learning was used to quantify the oxygen content in the electrolyzer geometry, as well as any pathways or patterns the oxygen took as it exited the electrolyzer through the PTL. Computational fluid dynamics (CFD) was used to investigate the characteristics of oxygen transport in the PTL under different operating conditions. Our work has demonstrated new findings in oxygen transport behavior in the PTL. We report a direct observation of oxygen taking preferential pathways through the PTL regardless of the water flowrate or current density (1-4 A/cm2). The spatially periodic oxygen front has been observed in this study for the first time. Oxygen distribution in the PTL had a periodic behavior with period of 400 μm[4]. CFD model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer. A conceptual schematic showing transport of oxygen in the PTL is shown in Figure 1B. Once the transport pathway through the thickness of the PTL is established, oxygen bubbles will preferentially follow the pathway and no new pathways will be introduced within the current densities range studied here. These findings should be useful in improving mass transport and enhancing our understanding of processes in PEWE. References: [1] U. Babic, M. Suermann, F. N. Büchi, L. Gubler, and T. J. Schmidt, “Critical Review—Identifying Critical Gaps for Polymer Electrolyte Water Electrolysis Development,” J. Electrochem. Soc., vol. 164, no. 4, pp. F387–F399, 2017, doi: 10.1149/2.1441704jes. [2] D. Kulkarni, S. J. Normile, L. G. Connolly, and I. V Zenyuk, “ Development of low temperature fuel cell holders for Operando x-ray micro and nano computed tomography to visualize water distribution ,” J. Phys. Energy, vol. 2, no. 4, p. 044005, 2020, doi: 10.1088/2515-7655/abb783. [3] P. Satjaritanun, S. Hirano, I. V. Zenyuk, J. W. Weidner, N. Tippayawong, and S. Shimpalee, “Numerical Study of Electrochemical Kinetics and Mass Transport inside Nano-Structural Catalyst Layer of PEMFC Using Lattice Boltzmann Agglomeration Method,” J. Electrochem. Soc., vol. 167, no. 1, p. 013516, 2020, doi: 10.1149/2.0162001jes. [4] P. Satjaritanun, M. O’Brien, D. Kulkarni, S. Shimpalee, C. Capuano, K. E. Ayers, N. Danilovic, D. Y. Parkinson, and I. V. Zenyuk, “Observation of Preferential Pathways for Oxygen Removal through Porous Transport Layers of Polymer Electrolyte Water Electrolyzers,” iScience, vol. 23, no. 12, p. 101783, 2020, doi: 10.1016/j.isci.2020.101783. Figure 1

Transition metal-nitrogen-carbon materials (M-N-C catalysts) are promising electrocatalysts in polymer electrolyte fuel cells (PEFCs) and electrolyzer applications. High temperature treatment in inert atmosphere (pyrolysis) is the essential, most common method for the synthesis of M-N-C catalysts and critical to achieve high electrocatalytic activity and electronic conductivity. To this day, despite many uses and successful implementations in materials manufacturing, pyrolysis has been an entirely empirical technology, with process control and optimization relying exclusively on "Edisonian" approach. The knowledge gap in the mechanism about how the precursor is being transformed into catalysts hinders further development of the M-N-C catalysts regardless of the precursor class and processing protocols. Herein, we probed the morphological evolution and chemical transformation of a nitrogen-containing charge transfer organic salt, mixed with transition metal (iron) salt and amorphous silica powder (precursor) during the pyrolysis process via a combination of in situ synchrotron and laboratory-based diagnostic techniques. The pyrolysis process is found to be divided into three stages. During a controlled temperature ramp, the selected organic N-C precursor (nicarbazin) began melting and decomposing just below 400 degrees C, forming a certain number of micrometer-scale pores and pathways. With increase in temperature from 400 degrees C to 900 degrees C, amorphous carbon domains started forming, and reduced (metallic) iron nanoclusters appeared, being dispersed uniformly throughout the carbonaceous matrix. When temperature advanced above 900 degrees C, graphitization of carbon commenced, associated with appearance and evolution of atomically dispersed metal-nitrogen moieties in the carbonaceous matrix. As the graphitization advanced further, a secondary process of agglomeration of metal nanoparticles occurred. Multi-analytical technique observations conducted here provide a base for rational design and optimization of M-N-C electrocatalysts via pyrolysis.

Synchrotron x-ray imaging techniques, like x-ray computed tomography (CT) and radiography have proven instrumental in expanding the communities knowledge of complex transport and reaction kinetics in electrochemical devices such as fuel cells and electrolyzers. This work presents the development of novel x-ray CT imaging techniques for operando visualization of water within low temperature fuel cells at spatial resolutions spanning the micro and nano scales. The design of operando sample holders, for both micro x-ray CT and nano CT experiments is described in depth, and prototypes of these sample holders were evaluated across a set of requirements, the most important of which are x-ray transmissibility, electrical conductivity and mechanical stability. Water segmentation from micro x-ray CT data was enabled by an image subtraction method, where the image without water is subtracted from the one with water. Through iterative experimentations, the operando nano CT cell was developed to optimize mechanical compression, electric conductivity and gas flow. While three-dimensional fuel cell reconstructions were shown possible, there remain challenges to overcome at typical lower energies (8 keV) due to beam damage, whereas it is not as significant for higher energies (>17.5 keV).

Understanding the relationships between porous transport layer (PTL) morphology and oxygen removal is essential to improve the polymer electrolyte water electrolyzer (PEWE) performance. Operando X-ray computed tomography and machine learning were performed on a model electrolyzer at different water flow rates and current densities to determine how these operating conditions alter oxygen transport in the PTLs. We report a direct observation of oxygen taking preferential pathways through the PTL, regardless of the water flow rate or current density (1-4 A/cm2). Oxygen distribution in the PTL had a periodic behavior with period of 400 μm. A computational fluid dynamics model was used to predict oxygen distribution in the PTL showing periodic oxygen front. Observed oxygen distribution is due to low in-plane PTL tortuosity and high porosity enabling merging of oxygen bubbles in the middle of the PTL and also due to aerophobicity of the layer.