Porous Catalyst Layer Research Articles

Porous electrodes from self-assembled 3D jointed Pd polyhedra for direct formic acid fuel cells

The direct formic acid fuel cell (DFAFC) is recognized as a promising power source for its high electromotive force, easy fuel storage and transport. However, its power performance is low and the cost is high, which are known to originate from the sluggish formic acid oxidation (FAO) kinetics and poor mass transport within the DFAFC anode. Here, we present a new DFAFC anode design with a thin, porous, 3D-structured catalyst layer based on self-assembled jointed Pd polyhedra in-situ grown on the gas diffusion layer surface. The Pd polyhedra provide highly active jointed interfaces and high-index facets, boosting their catalytic activity towards FAO. The porous 3D catalyst layer facilitates the transport of reactants and products at the large current density region. Consequently, the present anode design exhibits a previously unachieved power density of 202 mW cm−2 at a Pd loading of 1.0 mg cm−2 in the HCOOH/air DFAFC test.

Influence of the catalyst layer thickness on the determination of the OER activity of Fe3O4@CoFe2O4 core-shell nanoparticles

Transition metal oxides-based catalytic layers often present a complex 3D porous architecture affecting the evaluation of their intrinsic electrocatalytic activity. In this work the oxygen evolution reaction activity of core-shell Fe3O4@CoFe2O4 nanoparticles combining a conductive magnetite core and a catalytically active cobalt ferrite shell was studied at different loading and thickness of the catalytic layer. It was observed that their apparent activity is decreasing and that the Tafel slopes are becoming convex when the loading increases. The activity decay could be attributed to the significant resistance to charge transport in the thick porous catalyst layer. This resistance could be estimated by fitting the electrochemical impedance spectra using the transmission line model. The influence of the layer thickness on the experimental current-potential curves and on their Tafel slopes could be simulated using a simple model based on the Telegrapher's equations. It is concluded that in order to measure accurately the activity and Tafel slopes of an electrocatalyst, thin layers must be used, notably for catalyst layers that are not highly conductive.

Modeling and Analysis of Mass Transport Losses of Proton Exchange Membrane Water Electrolyzer

Proton exchange membrane water electrolyzers (PEMWEs) coupled with renewable energy resources are considered to be a key technology for producing green hydrogen. However, the high current density PEMWE operation features remarkable voltage losses. A significant part of these losses is due to the mass transport resistance in the PEMWE. Even though the importance of mass transport resistance is widely recognized, it is still poorly understood. Currently, the two-phase transport through the anode porous transport layer (PTL) and catalyst layer is considered to be the main cause of the mass transport losses. In this work, a dynamic macroscopic mathematical model, coupling electrochemical reaction with mass transport through the PTL and flow channels, has been developed to study the two-phase flow in the PTL and mass transport losses of a PEMWE. The influence of the current density, inlet water flow rate, PTL structural parameters, and capillary pressure curve was evaluated. The existence of the critical current density was observed, as well as its dependence on the operating parameters and PTL structure. Even though the results show that the PTL structure has a significant influence on the PEMWE performance, they indicate that a better mathematical description of the two-phase flow in the PTL is necessary.

(PEFC&E 2022 Student Poster Award Winner - 2nd Place) Investigation of a Correlation Between IV Performance and Cathode Structure for MEAs Using Mesoporous Carbon Supports

Introduction Polymer Electrolyte Fuel Cell (PEFC) is a promising energy technology for a future decarbonized society. PEFC is composed of porous electrode catalyst layers, which largely determines current-voltage (IV) performance. However, the optimal structure of porous catalyst layers is still unknown. Carbon supports are the main component to build up such porous structure. Mesoporous carbon (MC) supports are recently paid more attention to since they are used in Toyota’s fuel cell vehicles, MIRAI. We are also studying MC whose diameter is about 10 nm. Therefore, we are interested in a correlation between IV performance and the structure of catalyst layers based on MC supports. The aim of this study is to clarify the optimal structure of cathode catalyst layers for high IV performance. In this study, we synthesized two types of MC, MC bulk and fibers, and evaluated the IV performance and cathode structure of MC based cathode catalyst layers. Experimental MC bulk was prepared based on self-organization of Pluronic F127 and carbon precursors (resorcinol, formaldehyde, and triethyl orthoacetate). They were mixed in the acid solution, and then the supernatant was removed after centrifugation. The remaining portion was heated in the oven for polymerization of carbon precursors. The resulting material was heated by stepwise in the oven under the nitrogen atmosphere in order to decompose Pluronic F127 and carbonized precursors. MC fibers were made by electrospinning of the similar precursor solution mentioned in the above, but for electrospinning phloroglucinol instead of resorcinol was in use, and PVA was added. Then, after MC bulk and fibers were milled by hands or zirconia balls, Pt nanoparticles were deposited on MC supports by using Pt(acac)2 as a precursor. MEAs with 1cm2 electrodes were fabricated by spray-printing of catalyst dispersions on the Nafion film. Prepared MC based catalysts were used for the cathode, and 46.5% Pt/KB (TEC10E50E) was used for the anode. Results and discussion IV performance of MEAs using MC based cathodes was evaluated and compared to that of a standard MEA. MEA with MC bulk support showed slightly lower IV performance than that of standard MEA. By analyzing each overvoltage separately, ohmic overvoltage was found to be a biggest problem, and low concentration overvoltage was realized as an advantage. In order to find out the reason of low concentration overvoltage, the structure of cathode layers was evaluated using FIB-SEM. The 300 cross-sectional SEM images were obtained by slicing cathodes every 10 nm, and the cathode was reconstructed into a three dimensional image. As a result, the size of MC support was found to be much larger than that of KB. When pore diameter distribution was evaluated based on the reconstructed image, as shown in Figure 1, rather large pores expressed in red in the figure was observed for MC bulk based cathode probably owing to large MC particles. The low concentration most likely comes from the existence of such large pores. In this structural analysis, the analysis of mesopores is not included, and so in reality mesopores might have contributed to lowering concentration overvoltage. Similar analyses were done for MC fiber based cathodes. We have found that the IV performance and structure of cathodes is rather influenced by the milling processes, which will be discussed in detail. Figure 1

Nitrogen-Doped Carbon Nanosphere Supports for Platinum-Decorated Oxygen Reduction Reaction Catalysts

Carbonaceous materials are widely used as supports for platinum-based oxygen reduction reaction (ORR) catalysts that are prevalent in proton exchange membrane fuel cells (PEMFC). In particular, a variety of carbon blacks are popular due to factors such as commercial availability and high surface area. However, as applications of PEMFCs shift towards heavy-duty vehicle deployment, durability of the supports has become an important criterion when evaluating materials. Carbon support corrosion can lead to Pt nanoparticle detachment and agglomeration, both of which decrease the electrochemically active surface area of the catalyst. Recently, nitrogen-doping of carbon supports has shown promising results towards improving stability in fuel cell operating conditions1. Graphitization is another strategy that mitigates carbon corrosion, as graphitic carbon is more resistant to oxidation than amorphous carbon2. In this work, we present a set of nitrogen-doped carbon nanosphere supports obtained via a one-pot colloidal synthesis and then decorated with Pt nanoparticles for ORR catalysis (Pt/NCS). The nitrogen-doped supports are monodispersed, contain a mesoporous pore structure, and show a higher level of bulk graphitization than commercial carbon blacks. Our work also elucidates how differences in certain nitrogen moieties on the carbon support’s surface impact catalytic activity. The Pt/NCS catalysts exhibit similar mass activity and improved specific activity in rotating disk electrode measurements compared to a commercial Pt/C catalyst at similar Pt weight loadings.(1) Ott, S.; Orfanidi, A.; Schmies, H.; Anke, B.; Nong, H. N.; Hübner, J.; Gernert, U.; Gliech, M.; Lerch, M.; Strasser, P. Ionomer Distribution Control in Porous Carbon-Supported Catalyst Layers for High-Power and Low Pt-Loaded Proton Exchange Membrane Fuel Cells. Nat. Mater. 2020, 19 (1), 77–85.(2) Yu, P. T.; Gu, W.; Zhang, J.; Makharia, R.; Wagner, F. T.; Gasteiger, H. A. Carbon-Support Requirements for Highly Durable Fuel Cell Operation. In Polymer Electrolyte Fuel Cell Durability; Büchi, F. N., Inaba, M., Schmidt, T. J., Eds.; Springer New York: New York, NY, 2009; pp 29–53.

Catalyst Layer Resistance, Utilization, and Degradation in PEM Electrolysis

Proton exchange membrane (PEM) electrolysis is a promising route for renewable hydrogen production. However, to enable widespread, low-cost hydrogen generation, PEM electrolyzers must make advances in performance and durability with greatly decreased loadings of iridium as the anode oxygen evolution catalyst. The ionic and electronic resistance of the anode catalyst layer is an important consideration, as high internal resistance lowers catalyst utilization and cell performance while accelerating degradation. Catalyst layer resistance (CLR) is relatively well-understood in fuel cells and other porous electrode systems. However, characterization of CLR is not routinely used in the PEM electrolysis community, and the impacts of CLR for PEM electrolysis are not widely understood.Here we will present in-situ methods for measuring CLR in electrolysis cells using a non-faradaic H2/H2O condition as well as methods for calculating the associated voltage losses. These methods are applied to anode catalyst layers based on IrO2 nanoparticles as well as dispersed nano-structured thin film (NSTF) Ir catalysts. Trends of CLR, performance, and durability with electrode properties such as loading and interactions between the porous transport layer and catalyst layer will be discussed. We will also present investigation of characteristic uneven degradation of the catalyst layer caused by CLR and strategies for mitigating this degradation mechanism.

(Digital Presentation) Micro-Scale Simulation of PEFC Catalyst Layer with Dynamic Structure Change

In order to develop high performance Polymer Electrolyte Fuel Cells (PEFCs), the innovative electrode materials are needed and utilized in optimal catalyst structure from the viewpoint of high output density, low amount of Pt and durability. In present PEFC, the resistance of the oxygen reduction reaction (ORR) at the cathode are dominant factors, and a key requirement is the smooth, rapid, and uniform supply of protons, electrons, and oxygen to the Pt surface. Moreover, an improved mass transport flux in the catalyst layer offers the possibility of increasing current densities, which in turn allows the reduction of cell areas, enabling further cost reductions. In our past studies, we focused on the actual heterogeneous porous structure of catalyst layer. From these results, it was found that mass transport performance of oxygen and proton in heterogeneous porous catalyst layer, which consists of carbon support, ionomer and void space, is much lower than that of theoretical prediction in homogeneous porous media [1-5]. In addition, it depends on the structure of carbon black, the coating structure of ionomer and the local distribution of these materials. Accordingly, optimization of porous structure of catalyst layer is needed, and the mechanism of mass transport resistance and reaction distribution have to be understood in detail. The numerical simulation is effective to understand it because the difficulty of experimental measurement directly. In this study, to understand the relationship between heterogeneous porous structure and cell performance, a three-dimensional heterogeneous CL structure consisting of a simulated CB aggregate and an ionomer coating model was developed using numerical analysis. Moreover, this simulation was applied to actual experimental cell test condition.Firstly, we simulated CB aggregate structure. Previously, our research group examined a numerical model for actual CB aggregate structures (Vulcan and Ketjen black) [6, 7]. These structures were reconstructed with a probability density distribution that was a function of the distance between particles. And various structural properties, such as aggregate size, anisotropy, surface volume and surface weight were evaluated with experimental data. With this model, actual CB aggregate structure was simulated which was same as that in experimental cell test. The structure of catalyst layer was simulated by random packing these CB structure and ionomer coating model [8]. Pt particles were placed on the CB surface and in CB pore. In the calculation of cathode electrochemical reaction, the kinetic activity (exchange current density) of exterior and interior Pt surface were set different value by considering effect of ionomer coating [9]. The electrode reaction was calculated using the Butler–Volmer equation for the formation of Pt oxide on the Pt surface. This oxide layer inhibits the absorption of oxygen, and the coverage depends on the local potential. The resistance of oxygen diffusion and dissolution in ionomer was included. The mass balance equations for oxygen, vapor, protons, and electrons in catalyst layer was coupled with local reaction [10]. The current density distribution was found to depend on the carbon black structure and ionomer adhesion shape. From the viewpoint of increasing both Pt utilization and mass transport performance, an adequate heterogeneous pore structure is necessary in the catalyst layer. Furthermore, this simulation was applied the evaluation of structure change of catalyst layer after long-time durability test. The information of the diameter of CB primary particle and Pt particle of BOL and EOL that obtained by 3D TEM was used. The effect of this dynamic structure change on cell output performance will be reported in this presentation.Acknowledgements This research was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, grant number P20003-20001327-0.

Triple-Phase Pore Network for Oxygen and Proton Transport Model in the Cathode Catalyst Layer of Proton Exchange Membrane Fuel Cells

Improvement in the mass transport overpotentials of the oxygen reduction reaction provides a pathway for reducing the high cost associated to platinum group metal loading as well as enhancing the lifetime of electrodes in proton exchange membrane fuel cells (PEMFCs). State-of-the-art PEMFCs typically have Pt loadings of the order of 0.3 mgPtcm-2, which is the main part of the estimated stack cost of $55/kW. Understanding transport phenomena at triple phase boundaries that consist of a (solid) electrocatalyst, ionomer, and (gas-phase) oxygen, where the electrochemical reactions occur has gained importance to improve electrocatalyst accessibility. Mathematical models that address transport of electrons, protons, and oxygen in the cathode is critical for an effective catalyst layer design. Modeling of this multiphysics phenomena is further complicated due to multiscale effects. Pore network model (PNM) is a promising technique to tackle these challenges because of its capability of resolving porous networks locally in greater detail. Moreover, improvements in microscopic imaging techniques (e.g., SEM, TEM, and nano CT) enable the generation of realistic pore networks of the catalyst layers that can be directly integrated into PNM. Most modeling studies to date focus on single networks for oxygen transport in the open pore space, or transport of oxygen and water. A pore network that describes oxygen, proton, and electron transport simultaneously remains uncharted. Here, we construct a triple-phase pore network model using nano CT images of heterogeneous porous catalyst layers. Binary images of solid-open pore space and ionomer-catalyst support are generated, which are then overlapped to create ternary images. Pores and throats are then fit to theseimages to create pore-networks that describe open-pores for oxygen and water transport, ionomer pores for proton transport, and a solid network for electron transport. A schematic of the approach is shown in Figure 1.The study offers a new method that constructs a triple pore network for a porous material in which two levels of open space porosity (micro and macro) exist in connection with solid and ionomer networks. Therefore, arealistic pore network is obtained to be further modeled through the approach presented. Figure 1

Cryo ion polishing and high-resolution electron microscopy on a layered PEM composite of an automotive fuel cell

Abstract Gas and ion diffusivity within the membrane electrode assembly (MEA) are of central importance for the functioning of a proton exchange membrane (PEM) fuel cell. These properties are, among other things, directly impacted by the microstructure of the layering and the morphology of the individual layers. Different types of preparation were used to prepare the inner layer composite of the MEA/PEM of a fuel cell. The preparation quality provided for high resolution electron microscopy and the microstructure quantification were compared. The conventional mechanical preparation is compared to planar ion polishing, and cryo ion polishing at -80 °C. Advantages and disadvantages of the methods are discussed by way of examples related to the microstructure such as porosity, elemental distribution, and the measurement of the layer thickness. It is shown that cryo ion polishing facilitates a largely artifact-free preparation of the highly thermo-sensitive, partially highly porous layered PTFE composites, and the porous catalyst layers. Thus, not only easily ascertainable layer thicknesses but also qualitative features and important quantitative microstructural parameters, such as nanometer-sized pore structures and the porosity of the various composite layers can be revealed.

Effects of interfacial contact under different operating conditions in proton exchange membrane water electrolysis

Proton exchange membrane water electrolysis (PEMWE) cells have made significant progress with regards to their performance, durability and cost reduction in the past few decades. Although the interfacial contacts between each PEMWE cell component have a significant impact on cell performance and durability, their effects are still not fully investigated. In this study, the interfacial contact between porous transport layers (PTLs) and catalyst layers (CLs) is systematically studied utilizing various test protocols and conditions, including step scan, dynamic sweep scan, galvanostatic and potentiostatic control. Poorly designed electrical contact at the anode PTL/CL interface will lead to an increased electrical resistance, and results in a high frequency resistance (HFR) and dependency on current density. In addition, the data suggest that an uncoated PTL/CL interfacial contact causes an inhomogeneous temperature distribution on the micro-scale, which may accelerate cell degradation. Improving the interfacial contact by applying platinum group metal coatings, such as thin film Ir coatings, reduces the cell ohmic resistance and improves the heat management. This study gives insights into the effects of interfacial contacts, test protocols and impact on operation conditions. The impact of PTL microstructure as function of PTL thickness is elucidated. It emphasizes the importance of an optimized interfacial contact and provides guidelines for PEM electrolysis measurement protocols.

Multi-Scale Modeling of Hydrogen Transport in a Porous Fuel Cell Anode

Proton-exchange-membrane fuel cells (PEMFC) are a clean energy conversion alternative to traditional fossil-fuel combustion; however, transport resistances in the electrode pose a lower-limit to catalyst loading and commercialization of PEMFCs. PEMFCs consist of simultaneous hydrogen (H2) oxidation and oxygen (O2) reduction at the anode and cathode, respectively. While oxygen transport resistances in PEMFCs have been widely studied both experimentally and analytically, hydrogen transport resistances are less understood. Herein, we present a physics-based model that encompasses multi-scale transport within the anode side of the PEMFC. The O2 in the cathode here is omitted and replaced with H2 to deconvolute O2 transport resistance contributions, similar to that of a hydrogen pump. Replication of the hydrogen pump setup allows for comparison of the model against experimental analysis of H2 gas-transport resistance in H2 limiting-current experiments, which can also inform gas transport (including oxygen) in general. Herein, we present a multi-scale analytical model of the porous anode catalyst layers and individual catalyst agglomerates that enables determination of the effects of electrode morphology such as agglomerate size, catalyst loading, etc. on H2 transport resistance through the porous electrode to complement and better understand H2 limiting current experiments and deconvolute local H2 transport resistances.Electrodes within PEMFCs are heterogeneous porous structures formed by agglomerates of carbon particles containing Pt nanoparticles, held together by an ion-conducting polymer (ionomer). The gas transport hence involves transport through multiple phases and length-scales: (i) gas-phase transport through electrode pores at the length scale of electrode thickness (micrometer) and (ii) diffusion into the agglomerate through an ionomer film at the length scale of agglomerate radius (nanometer). Based on previous modeling efforts of varying length-scales of the catalyst layer, our analytical model adequately and simultaneously captures reaction-diffusion at all of the aforementioned scales (i.e., the electrode thickness and agglomerate radius) to better describe porous anode performance. A sensitivity analysis is subsequently performed to compare current generated against transport parameters for both scales. This work ultimately provides insight that will guide the design and engineering of future porous anodes for applications in hydrogen pumping and PEMFCs.

(Digital Presentation) Fabrication and Electrochemical Characterization of Inkjet Printed IrO2 Electrodes for Water Electrolysis

Water electrolysis is believed to be one of the most promising technologies to produce green hydrogen. However, the current challenge in commercialising this technology is the high relative cost. According to a recent report on cost breakdown of Polymer Electrolyte Membrane (PEM) electrolyser [1], the cell stack contributes to 35% - 45% of the total cost. Catalyst materials on the electrodes are believed to be the primary areas for innovation and cost reduction in PEM electrolysers. Several approaches have been applied to reduce the stack cost, with a focus on reducing material usage while enhancing the performance of PEM electrolysers. This project aims to reduce catalyst loading on the anode of PEM water electrolyser while maintaining high performance, through developing an inkjet printing technique to create a thin, porous and active catalyst layer. Conventional catalyst fabrication techniques such as spray coating produce large amount of material waste and non-homogenous distribution on the surface [2]. Inkjet printing will provide controlled catalyst loading, ensuring well catalyst distribution, creating porous structure, and having the ability to create patterned electrodes [3].Due to its high activity, Iridium Oxide (IrO2) is selected as the catalyst. The ink was formulated to be compatible with the printing requirement, with 7.5 wt% of IrO2 powder, 7.9 wt% of Nafion, and 84.5 wt% of solvents mixture of Isopropanol and Propylene Glycol. Full coverage of the catalyst on the Titanium substrate was proven by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX). The effect on varying the number of printed layers was analysed in terms of catalyst loading and activity to show a linear increase in activity with the number of layers. Moreover, electrochemical performance was inspected using cyclic voltammetry in a 3-electrodes cell. The printed electrodes showed comparable stability to conventional electrodes. Electrochemical surface area (ECSA) was higher for the 10-layered electrodes as compared to the 2-layered electrodes indicating higher catalyst utilisation. Printed electrodes showed activity of 300 mA/mg IrO2 with ECSA of 225 cm2 at reduced catalyst loading of 0.8 mg IrO2/cm2. These results outperform conventionally fabricated electrodes by spray and decal transfer coatings [4].

Oxygen Reduction Reaction in Alkaline Media Causes Iron Leaching from Fe-N-C Electrocatalysts.

The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells (FCs), however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe-N-C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in FCs, that is, with a porous catalyst layer (CL) and at current densities up to -125 mA·cm-2. Varying the rate of the oxygen reduction reaction (ORR), we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in the ORR, such as FeNxCy and Fe3C@N-C. Moreover, such a linear correlation allows the application of a simple metric─S-number─to report the material's stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand a more advanced mechanistic understanding of Fe-N-C degradation in CLs.

Multiphysics modeling of proton exchange membrane water electrolysis: From steady to dynamic behavior

AbstractProton exchange membrane water electrolysis (PEMWE) is currently developed for the design of mature industrial‐scale manufactures with commercialization. It needs reducing hydrogen production cost by lowering material cost and increasing operating current density. In engineering perspectives, the study of electrolytic performance during dynamic operation is crucial for PEMWE system management and process control. However, there is few multiphysics models of PEMWE considering transient behavior. The one‐dimensional (1D) comprehensive dynamic multiphysics model allows to explore temporal transport phenomena in the PEMWE, and predict electrolytic performance. The 1D model is endorsed by the spatially lumped model from the literature. Changing values of structural and physical properties of porous transport layers (PTLs) and catalyst layers (CLs) allows the observation of their effects on the electrolytic performance and transport phenomena in two‐phase flow regime. It suggests that the appropriate PTL properties, and CL fabrication method can lower the cost and remain high electrolytic performance.

COFs-confined multifunctional Sulfur-host design towards High-performance Lithium-sulfur batteries

Herein, a covalent organic framework (COF) confined multifunctional sulfur-host design strategy, with built-in porous catalyst and external microporous COF layer, is proposed to deal with the notorious lithium polysulfides (LiPSs) shuttle effect in lithium-sulfur (Li-S) batteries. The built-in hollow catalyst, consists of carbon supported cobalt nanodots (Co-NC) derived from ZIF-67 metal–organic framework, servers as sulfur accommodation and catalytic center for LiPSs trapping and conversion. The Co-NC is wrapped by a thin microporous TpBD-Me2 COF layer with polar functional groups, providing additional physical and chemical confinement for polysulfides shuttling. With these merits, the cell with S/Co-NC@TpBD-Me2 cathode exhibits a high initial capacity of 1178.15 mAh g−1 at 0.2C, and average 0.04% decay per cycle at 1C in 500 cycles. The cell also delivers a high rate performance (543.13 mAh g−1) up to 5C. Even at raised sulfur loading (5.71 mg cm−2) and lean electrolyte condition (E/S ratio 6.10 μL mg−1), a 4.53 mAh cm−2 high area capacity is achieved after 50 cycles. The Li-S pouch cell also achieves a specific capacity of 860.35 mAh g−1 at 0.1C after 50 cycles. This COF-confined multifunctional composite design provides an alternative strategy for the fabrication of high-performance Li-S batteries.

Elucidating effects of catalyst loadings and porous transport layer morphologies on operation of proton exchange membrane water electrolyzers

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.

Overall design of novel 3D-ordered MEA with drastically enhanced mass transport for alkaline electrolyzers

A novel three-dimensionally (3D)-ordered membrane electrode assembly (MEA) based on highly porous catalyst layers with unique vertical channels, an ultrathin membrane layer and 3D interface structures was nanoengineered for alkaline electrolyzers.

Understanding Transports Dynamics inside PEMFC Using Combination between Direct Simulation and Operando X-Ray Computed Tomography

Computational Fluid Dynamics (CFD) with the Lattice Boltzmann Method (LBM) was used to simulate the temperature distribution and water saturation inside the porous gas diffusion and catalyst layers of polymer electrolyte membrane fuel cell (PEMFC) [1-3]. The model was validated with the images of liquid water saturation in the three dimensional micro-structure of porous layers taken by the in-operando X-ray computed tomography (CT) [4]. Meanwhile, the ultra-fine thermocouple was introduced into the operando PEMFC to obtain the temperature distribution at the various site in the porous layers, including at the interface of GDL under the channel and channel land, at the micro porous layer (MPL) and catalyst layer (CL) [5]. For this time, the model was validated with the temperature distribution data and the local saturation of water, heat transfer, species transport, and electrochemical kinetics were explored under different cell operating conditions. Figure 1 shows a comparison of simulation and experimental data (in operando X-ray CT and ultra-fine thermocouples) for the water saturation in the detailed structure of porous layer as well as the temperature distribution. The model was well validated. Implication between water saturation and cell over potentials will be presented and discussed.

Ru2P nanofibers for high-performance anion exchange membrane water electrolyzer

The development of advanced electrodes with highly active electrocatalysts and appropriate structures is essential for sustainable hydrogen production via water electrolysis. Moreover, there is significant demand for an affordable electrocatalyst that exhibits comparable activity to that of Pt. Herein, we report Ru2P nanofibers (NFs) as an efficient electrode material for a high-performance anion exchange membrane water electrolyzer (AEMWE). The electrospinning method enables the formation of a porous catalyst layer that comprises tangled NFs, which exhibit a three-dimensional structure with abundant empty space. In a half cell test, the Ru2P NFs exhibit a high catalytic activity for the hydrogen evolution reaction, which is comparable to the activity of a commercial Pt/C. In a single cell test, an AEMWE with Ru2P NFs demonstrates a higher performance than that with a commercial Pt/C, especially in the high current density region; this is attributed to the structural advantage of the porous catalyst layer, which enhances the mass transfer of the reactant, as well as the product.

Single-Step Fabrication of a Multiscale Porous Catalyst Layer by the Emulsion Template Method for Low Pt-Loaded Proton Exchange Membrane Fuel Cells

Multiscale porosity assures better mass transport for a low Pt-loaded fuel cell; however, achieving a complex pore structure in a simple manner presents a major challenge. In this work, by using the emulsion template method, a multiscale porous catalyst layer with macro- and mesopores was fabricated in a single step. A stable high internal phase emulsion is achieved with catalyst layer components by exploiting a Nafion ionomer as a surfactant. The oil phase of the emulsion catalyst slurry induces the macropores, and the Pt/C and ionomer mixture in the hydrophilic solvent forms a mesoporous framework in the catalyst layer. Scanning electron microscopy and micro X-ray computed tomography confirmed the formation of uniform and well-interconnected macropores. The performance of the low Pt-loaded catalyst layer derived from the emulsion template method considerably improved the mass transport property due to the mitigation of water flooding. The fabrication of an emulsion-based catalyst layer provides an efficient platform to design high-performance low Pt-loaded fuel cells.