Platinum Loading Research Articles

Drying Controlled Synthesis of Catalytic Metal Nanocrystals Within 2D‐Material Nanoconfinements

AbstractThe synthesis of low‐dimensional metal nanocrystals with precise atom‐to‐nanoscale structure control is crucial for modulating their physicochemical properties. Traditional synthetic routes encounter challenges due to isotropic metallic bonding, which leads to limited control over metal nanostructures. Herein, a versatile approach is developed using various 2D material (2DM) nanoconfinements to produce a wide range of metal nanocrystals with controllable morphologies. Utilizing graphene oxide (GO) and Ti3C2Tx MXene nanosheets, thin multilayer films are assembled through vacuum filtration and are crosslinked with tetraammineplatinum(II) nitrate (TPtN), followed by in situ thermal reduction. By controlling the concentration of TPtN solution, precise loadings of platinum (Pt) are attained while preserving the nanoconfinement integrity. Two water removal techniques, air‐drying and freeze‐drying, are investigated to assess their impacts on resultant morphologies of Pt nanocrystals. Transmission electron microscopy and molecular dynamics simulations demonstrate high‐aspect‐ratio Pt nanosheets on MXene substrates and few‐atom Pt nanoclusters on GO substrates. A decrease in size distribution is observed upon the use of freeze‐drying. In the semihydrogenation reaction of phenylacetylene, freeze‐dried Pt–MXene heterostructures achieve a high turnover frequency of 2.93 s−1. This comprehensive study highlights the potential of utilizing 2DM nanoconfinement to synthesize metal nanostructures for catalysts and beyond.

Low-valence platinum single atoms in sulfur-containing covalent organic frameworks for photocatalytic hydrogen evolution

This study focuses on optimizing catalytic activity in photocatalytic hydrogen evolution reaction by precisely designing and modulating the electronic structure of metal single atoms. The catalyst, denoted as PtSA@S-TFPT, integrates low-valence platinum single atoms into sulfur-containing covalent organic frameworks. The robust asymmetric four-coordination between sulfur and platinum within the framework enables a high platinum loading of 12.1 wt%, resulting in efficient photocatalytic hydrogen production activity of 11.4 mmol g−1 h−1 and stable performance under visible light. These outcomes are attributed to a reduced hydrogen desorption barrier and enhanced photogenerated charge separation, as indicated by density functional theory calculations and dynamic carrier analysis. This work challenges traditional notions and opens an avenue for developing low-valence metal single atom-loaded covalent organic framework catalysts to advance photocatalytic hydrogen evolution.

Effects of hydrogen dilution on performance and in-plane uniformity of large-scale PEM fuel cell with low anode catalyst loading

Enhancing hydrogen utilization is crucial for improving the efficiency of Proton Exchange Membrane (PEM) fuel cells. However, the widespread implementation of ultra-thin PEMs introduces a challenging objective: balancing hydrogen utilization with hydrogen dilution, which can adversely affect performance. To achieve this balance, understanding the impact of hydrogen dilution on fuel cell performance is critical, particularly for commercial large-scale fuel cells with low anode catalyst loadings, where significant research gaps remain. This study aims to fill these research gaps by investigating the performance and current distribution of a 291 cm2 fuel cell with an anode platinum loading of 0.1 mg/cm2 under varying hydrogen molar fractions (HMFs) and hydrogen stoichiometric ratios (HSRs). The results reveal that hydrogen dilution affects performance through three primary mechanisms: decreasing anode hydrogen partial pressure, exacerbating hydrogen supply non-uniformity, and altering the water balance. Notably, the latter two factors interact and collectively affect in-plane uniformity, leading to complex performance characteristics under hydrogen dilution conditions. Furthermore, the performance loss due to hydrogen dilution observed in this study is more pronounced than previously reported, primarily due to low catalyst loading and in-plane non-uniformity resulting from scale expansion. Nevertheless, at medium to low current densities and high HSR conditions, where the impact of hydrogen dilution is diminished, moderate hydrogen dilution can be permitted to enhance hydrogen utilization. Based on the data collected, this study maps the boundary for hydrogen dilution constrained by performance loss, offering valuable insights into the design and optimization of future control strategies.

Evaluation of Reaction and Mass Transport Properties of Fuel Cell Catalyst Layers

Recently, polymer electrolyte fuel cell (PEFC), which are expected to be applied to heavy-duty vehicles with high environmental load, are required to further improve their output power. the output characteristics of PEFC depend on the structure of the catalyst layer, including the aggregation form of the carbon support and the ionomer coating state. The structure of the catalyst layer is also important. The catalyst layer structure is also determined by the composition of the catalyst ink that forms the structure and the agglomeration form of the particles. However, the correlation between each ink condition and output characteristics is difficult to understand directly because it involves complex factors of structure formation and transport reaction distribution. In this study, we fabricated various catalyst layer structures under various ink conditions with different compositions and fabrication processes, analyzed their output characteristics, and evaluated the correlation using machine learning as a learning model.In the cathode catalyst layer, oxygen, protons, and electrons move through voids, ionomer (electrolyte polymer), and carbon black (CB), respectively, and oxygen reduction reaction occurs on the Pt surface. Since the anodic reaction is sufficiently rapid compared to the cathodic reaction, the anodic reaction was neglected, and the inside of the battery was assumed to be isothermal and steady state. A 300 nm substrate aggregate was formed and filled into the computational domain, and the catalyst layer structure was fabricated by arranging the Pt catalyst and coating it with ionomer. The structure was simulated by varying the spatial probability density of the support arrangement. The structure was subdivided into blocks of primary particle size and transport calculations 1) were performed between adjacent blocks.The I/C ratio (weight ratio of ionomer to CB), platinum loading, and spatial probability density related to the arrangement of the CB support were varied, and the battery characteristics were determined by transport calculations for each structure using 100 structures that reproduced the coarse and dense structure of the catalyst layer. From the results of the transport calculations, the voltage at a current density of 0 A/cm2 and the current density for each 0.05 V between 0.6 and 0.85 V were associated with the images of the catalyst layer, and a dataset consisting of 399 cross-sectional images of the catalyst layer, each voltage and current density was created.CNN 2) was used for the machine learning model.Four hundred images were prepared for the study, of which 240 were used for training, 80 for validation, and 80 for testing. Adam was used for the optimization method, RMSE for the loss function, and Keras for implementation.Transport calculations were performed under conditions similar to current fuel cell operating conditions: temperature 80 ℃, RH 80 %, and supply oxygen partial pressure 20 kPa. In order to reproduce differences in the fabrication process, the spatial probability density associated with the arrangement of CB carriers was varied to fabricate structures with different aggregation morphologies.This calculation confirmed the voltage drop associated with the formation of aggregates, especially in the high current density region where mass transport is the rate-limiting factor. This is because oxygen transport was inhibited as the aggregates became larger in the aggregation area, and the high current density region inside the catalyst layer became localized.The voltage at a current density of 0 A/cm2 and the coefficient of determination (R2) between the production value of the transport calculation and the machine learning prediction at the current density at each voltage were all better than 0.8, confirming that the machine learning model we constructed can predict battery characteristics. In addition, the machine learning model was able to predict differences in battery characteristics where the I/C ratio and platinum loading were equal and only the coarseness and density of the structure differed.As described above, it is suggested that differences in the fabrication process can cause differences in battery characteristics, and a machine learning model was constructed to predict battery characteristics based on catalyst layer structure images where process differences are assumed.AcknowledgmentThis study was supported by NEDO program, Japan.

Effect of Formic Acid on Fuel Cell Performance for Automobile Applications

Introduction The allowable concentration of formic acid is defined as 0.2 ppm in Grade D of ISO 14687: 2019, which is the hydrogen quality specification for fuel cell vehicles. The effect of formic acid in hydrogen on polymer electrolyte fuel cell (PEFC) performance has been reported by some researchers [1-4], but the reaction mechanism of formic acid in the PEFC is not well understood. In this study, the reaction behavior of formic acid in the PEFC and its impact on voltage decrease were investigated. Experimental The effect of formic acid in hydrogen was evaluated using a single cell. A “JARI Cell 2” with a 25 cm2 electrode area and double serpentine flow channels was used in the experiment. The anode and cathode catalysts were Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo). The platinum loadings on the anode and cathode were 0.05 mg cm-2 and 0.30 mg cm-2, respectively. The electrolyte membrane was a fluorine-based material with a thickness of 12 μm. The 22BB (SGL Carbon) gas diffusion layers were used on both the anode and cathode.The single-cell tests were performed at a cell temperature of 60°C and without external humidification. This is because formic acid easily dissolves in water and is likely to be exhausted from the cell with water at high humidification, so no humidification is considered the most severe condition. Before formic acid addition, the preconditioning was conducted at 1.0 A cm-2 of current density using high-purity hydrogen as a fuel for more than 10 hours. Then, formic acid (15-300 ppm) was started to supply the anode. After a period of time, the supply of formic acid to the anode was stopped and the voltage recovery was evaluated. Results and discussion Figure 1 shows the voltage drop over time due to formic acid in hydrogen at a current density of 1.0 A cm-2 and cell temperature of 60˚C. The voltage dropped quickly after the start of the formic acid supply, but the amount was small even at high formic acid concentrations. The voltage drop due to formic acid was 14 mV after 25 hours at 60 ppm and 20 mV after 5 hours at 300 ppm. The voltage was recovered after stopping the formic acid addition.To understand the reaction behavior of formic acid during the fuel cell operation, the anode and cathode exhaust gas were analyzed by gas chromatograph-mass spectrometer. The result showed that no formic acid was detected from the anode or cathode while the fuel cell was operated at a current density of 1.0 A cm-2 with 15 ppm formic acid addition to the anode. Meanwhile, the amount of CO2 equivalent to that supplied to the anode as formic acid was detected from the cathode exhaust gas. The results suggest that formic acid supplied to the anode permeated through the electrolyte membrane as shown in Fig. 2 and was oxidized at the cathode. Acknowledgment This work is based on results obtained from a project, JPNP18011, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Assessing the Performance of Gas Diffusion Electrodes: Varying Pt Loading and Manufacturing Techniques

It has been reported that the amount of platinum in a fuel cell electrode can govern performance, cost and durability of the fuel cell stack. This study focuses on utilizing platinum black as the electrochemically active catalyst in an 8-cell miniature fuel cell stack and assessing the minimal electrode loading required to meet specific targeted performance requirements. Unlike typical catalyst, platinum black catalyst is unsupported (i.e. platinum is not decorated on to carbon substrates). A catalyst ink solution is prepared and is dispersed onto a gas diffusion layer (GDL) using either a spray coating instrument or slot die coater as the manufacturing technique to form the gas diffusion electrodes (GDEs). XRF elemental analysis is used to confirm the platinum content. The FC stacks are all assembled with identical materials and components, excluding the GDEs at the cathode. The cathodes were manufactured with different platinum loadings. The stacks are operated in dead-end mode, which means there is no outlet for the fuel and oxidant to exit. Finally, each stack is subjected to an identical test protocol in galvanostatic mode, and the corresponding voltage outputs are compared. The assessment of these results will aid in establishing large scale manufacturing of GDEs.

Impedance Based Comparison of PEMFC Cathode Catalyst Layer Materials

The performance of Polymer Electrolyte Membrane Fuel Cells (PEMFC) is strongly dependent on the oxygen reduction reaction and therefore the cathode catalyst layer. Different catalyst layers with the same platinum loading can achieve vastly different performances. The exact composition of ionomer, carbon support and catalyst has a big impact on important electrode characteristic. These characteristics define the contributions of the loss processes that occur in the electrode. By improving one of the performance defining features of the catalyst layer, another one is oftentimes worsened. This creates a trade-off between the loss contributions and the catalyst design parameters.Impedance spectroscopy (EIS) is a powerful tool to make these trade-offs visible. By calculating the distribution of relaxation times (DRT) the different loss processes can be deconvoluted and by fitting the impedance spectra to a physicochemical meaningful transmission line model, the loss processes can be quantified. In addition the electrochemical active surface area (ECSA) can be determined by cyclic voltammetry (CV) and correlated to the charge transfer losses.In this contribution PEMFCs with different cathode catalyst layers and the same anodes are compared in a wide spectrum of operating conditions by means of EIS and DRT. On the basis of these measurements a transmission line model (TLM) is fitted to the impedance spectra and the loss contributions are quantified. The results enable a direct comparison of the cathode catalyst specific loss processes in different operating conditions. By means of CV and Hydrogen Underpotential Desorption, the ECSA is estimated and correlated with the loss contributions.

Relative Humidity Distribution along the Gas Flow Channel in a Wide Temperature Range

Introduction This study focuses on analyzing the distribution of relative humidity within a parallel flow channel while correlating it with temperature, T, and relative humidity, RH. The gas composition profile inside the cell is determined by space time, defined as a ratio of the active area, A, to the volumetric gas flow rate, v 0, at the cell inlet. By increasing the gas flow rate and hence decreasing the space time, the profile is elongated and the gas composition inside the cell at a longer space time can be measured at the outlet at a shorter space time. Measuring the relative humidity at the outlet of the cell with varying the active area or the gas flow rate for varying the space time yields the relative humidity distribution inside the cell. The measured relative humidity distributions are then compared with theoretical distributions calculated with the assumptions of no water permeation and quick permeation (equal RH on both sides) through the membrane. Experimental A membrane electrode assembly (MEA) with an active area of 5.0×5.0 cm2 was constructed using a Nafion membrane (50.8 µm thick) with a 10.3 µm thick catalyst layer (CL) for anode and one for cathode. The platinum loading was 0.30 mg/cm2, employing 50 wt% Pt/carbon black catalyst particles with an ionomer. The ionomer/carbon weight ratio was 1.0.RH measurements were facilitated by tracking water flow rates at both the anode and cathode outlets. RH at the cell inlet was measured at the outlet of the cell with a non-permeable polytetrafluoroethylene (PTFE) sheet utilized instead of an MEA under identical conditions for maintaining the total pressure in the humidifier equal. To assess various oxygen conversions, the oxygen molar flow rate at the inlet, F O0, was adjusted at each temperature under atmospheric pressure conditions at 0.7 V. Results and Discussion The relationship between oxygen conversion and space time was investigated (Fig. 1). The space time is varied by varying the active area [1] and varying the gas flow rate. The oxygen conversion is higher when the space time is longer. The oxygen conversion is determined by the space time regardless of the active area or the gas flow rate. Varying the active area and the gas flow rate are equivalent. The oxygen conversion changed monotonically with change in the space time. The oxygen conversion can represent the space time and therefore the position in the cell.RH on the anode and cathode side are plotted against the oxygen conversion, which represents the RH distribution in the cell (Fig. 2). The RH increases from the inlet to the outlet as water is generated in the electrochemical reaction (higher increase at cathode due to water generation and water not permeating quickly from the cathode to the anode side). RH distribution calculated by assuming no permeation (Eqs. 1 and 2) of water is shown by dotted lines. On the anode, hydrogen consumption raised the RH downstream; on the cathode side, oxygen consumption and water generation raised the RH downstream. Solid lines represent RH distributions calculated by assuming the quick permeation (Eq. 3) of water yielding equal RHs on both sides which is located between the two lines calculated with the no-permeation model. At higher temperatures, the two models result in similar RH distributions because the water permeation rate is sufficiently low compared with the water flow rate in the channels. The actual RH distribution is located between RH distributions calculated with the two models, though data on the anode side at 80 oC include some experimental errors.RH[A]=(RH0 [A]/(1-2F g0 [C] y O0 x O/F g0 [A])(P [A]/P 0 [A]) (1)RH[C]=(RH0 [C]+2p O0 x O/p S sat)/(1+y O0 x O)(P [C]/P 0 [C]) (2)RH=((RH0 [A] P/P 0 [A])+(RH0 [C] P/P 0 [C])(1+2y O0 x O/y S0 [C])(F g0 [C]/F g0 [A]))/(1+(1-y O0 x O)(F g0 [C]/F g0 [A])) (3)At higher temperatures, RH becomes low unless more humid gas is supplied. Therefore, performance of PEFC can be adjusted by varying inlet RH. When the average RH at inlet and outlet of the cell is equal, higher temperature showed higher performance (Fig. 3). Conclusions RH distribution in the cell was investigated by varying oxygen conversion. The space time which determines oxygen conversion was varied by varying the gas flow rate in this study. At higher temperatures, more water generation is required to attain an equal RH because the saturated water vapor strongly depends on temperature. When the average RH at inlet and outlet of the cell is equal, higher temperature showed higher performance. Reference(s) [1] Ma Y., Kageyama M., and Kawase M. ECS Transactions, (2021) 104(8). Figure 1

Doping Effects of Metal-Doped Tin Oxide Nanoparticles Used As Carbon-Free ORR Catalysts for Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) are an important energy device for the realization of a hydrogen society. However, a longer service life is essential for the further spread of PEFCs. Currently, catalyst ( platinum supported on carbon, Pt/C) is the most mainstream cathode one. But, it has durability issues due to carbon degradation during high potential sweeps. As an alternative material to carbon, tin oxide (SnO2) is attracting attention due to its electrical conductivity and chemical stability. It has already been reported that Pt/SnO2 catalysts using SnO2 doped with antimony (Sb), niobium (Nb), tantalum (Ta), etc., and they exhibit high durability. However, they have a lower Pt electrochemical surface area (ECSA) and oxygen reduction mass activity (MA) compared to Pt/C. Therefore, further improvement of ECSA and MA is required for the practical use of Pt/SnO2. Other dopants may improve catalyst activity, so it’s worth checking the effects for doping other elements. In this study, a total of 20 metal-doped tin oxides (M-doped SnO2) were synthesized. Pt metal-doped tin oxide (Pt/ M-SnO2) was prepared by loading platinum onto the synthesized M-doped SnO2, and its catalytic performance was evaluated. M-doped SnO2 nanoparticles were synthesized by the solvothermal method. Tin chloride pentahydrate, metal chlorides as doping materials, methanol, and tetramethylammonium hydroxide solution were mixed and stirred to form a precursor. The solution was placed in a Teflon-coated stainless steel autoclave and heat treated. After 12 hours, the reactants were washed and dried to obtain M-doped SnO2. Pt/M-SnO2 was prepared using the polyol method. The prepared M-doped SnO2 powder was mixed and dispersed in a mixed solvent of ethylene glycol and water, then platinum chloride hexahydrate was added and stirred overnight. The solution was then heated at 120°C for 2 hours to produce the platinum nanoparticles on the SnO2 surface. The activity evaluation of the prepared catalysts was carried out at a rotating disk electrode in a three-electrodes cell system. The electrolyte was 0.1 M HClO4, the counter electrode was a platinum wire electrode, the reference electrode was a reversible hydrogen electrode, and the working electrode was a glassy carbon disk (GCD) electrode. Catalyst ink was dripped onto the working electrode and dry by spinning at 300 rpm for 1 hour. Measurements were performed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). From the obtained peak, ECSA and MA values were calculated. Among the Pt/M-doped SnO2, Pt/Mo-SnO2 and Pt/Sb-SnO2 showed specific peaks. In the CV, the peak area of the lower left part corresponding to the H2 adsorption was greatly enlarged in Pt/Mo-SnO2. As a result, Pt/Mo-SnO2 recorded the highest ECSA value (74.4 m2g-1 -Pt), exceeding that of Pt/pure-SnO2 (24.1 m2g-1 -Pt). In the LSV, the onset potential of the Pt/Sb-SnO2 peak shifted to the higher side than Pt/pure-SnO2. As a result, Pt/Sb-SnO2 recorded the highest value of Mass Activity (93.5 A g-1 -Pt), which is much higher than that of Pt/pure-SnO2 (17.1 A g-1 -Pt). The catalytic performance is significantly changed by doping metallic elements into tin oxide crystal.

Next Generation Cathode Catalyst Layer Design with High Oxygen Permeable Ionomer – Benefits, Challenges, and Prospects for Heavy Duty Fuel Cell Applications

Proton Exchange Membrane Fuel Cell (PEMFC) has a growing demand to power zero emission fuel cell applications. Ballard is the fuel cell industry leader, developing the next generation core technology to fulfill such demand and giving continuous efforts on cost reduction. Key requirements in fuel cell market applications include high power density and long-life operation, while maintaining high efficiencies and reducing costs. This places a high demand on improving catalyst performance and electrode layer transport, which can be sustained over the duration of the product lifetime. To reduce fuel cell membrane electrode assembly (MEA) cost, the push for high performance catalyst layers (CLs) with low platinum (Pt) loadings results in new challenges for the cathode material selection and electrode design. The increased oxygen transport resistance that occurs at lower catalyst loadings with reduced available catalyst electrochemical active surface area (ECSA), causes two very closely related challenges: 1) greater performance losses at high current density increases the difficulty to achieve the power requirements for the heavy duty market, and 2) the loss of catalyst ECSA via platinum dissolution over time puts additional demands on the design to achieve the end of life (EOL) performance requirement. Therefore, a combination of material solution, design choice and processing parameters are the key to optimize next generation cathode design for fuel cell applications1.One material solution to achieve high performance while reducing catalyst loadings is the development of high oxygen permeable ionomer (HOPI) that mitigates oxygen transport limitations at the Pt surface. Chemours has developed a HOPI with a bulky, sterically rigid monomer group in the ionomer backbone, which disrupts the packing density, improving oxygen permeability on the catalyst surface2. One of the key challenges of HOPI integration and the scale-able fabrication of the catalyst layer is high crack density of CL due to the rigid nature of the ionomer. Ballard has integrated HOPI in the cathode CL overcoming scale-able fabrication challenges through appropriate process parameter selection and evaluated CL performance. The use of accelerated testing and modeling approach developed by numerous product iteration, has been applied for lifetime estimation for HOPI technology introduction. In this talk, we will discuss details about benefit of High Oxygen Permeable Ionomer (HOPI) in next generation cathode CL design.Recently, key requirements in heavy-duty market applications are placing more importance on high efficiency rather than high power density to reduce fuel cost rather catalyst cost. Reaching the target lifetime, catalyst overloading to 0.45 mg/cm2 total Pt in anode and cathode, and 44% active membrane area oversizing are suggested3. Therefore, to achieve high efficiency target, high catalyst activity is more desired. Interestingly, HOPI also showed high specific activity due to the mitigation of catalyst poisoning by sulfonate anion groups4. We will also discuss the catalyst layer design prospect with HOPI for such transition on heavy duty market application.Acknowledgement: US Department of Energy (DOE) funded heavy duty MEA development project (DE-EE0008822).References M. Dutta, A. Young, V. Colbow, J. Bellerive and S. Knights 2020 Examining Catalyst Layer Design Strategies for Improved Fuel Cell Performance and Durability. ECS PRiME 2020S. Litster et al. 2020 Durable High-Power Density Fuel Cell Cathodes for Heavy-Duty Vehicle. DOE AMR 2020K. Ahluwalia and X. Wang 2024 Performance and Durability of Hybrid Fuel Cell Systems for Class-8 Long Haul Trucks. J. Electrochem. Soc. 171 034507R. Jinnouchi, K. Kudo and K. Kodama 2021 The role of oxygen-permeable ionomer for polymer electrolyte fuel cells. Nat Commun 12, 4956.

Engineered Carbon Supports for Heavy-Duty Fuel Cell Vehicle Applications

Proton Exchange Membrane (PEM) fuel cells are a suitable electrochemical power source for heavy duty vehicle (HDV) applications due to their high performance, efficiency, and durability. The cathode of the fuel cell uses a higher geometric loading of platinum (~0.2 to 0.4 mgPt/cm2) for the electrocatalysis of the kinetically sluggish Oxygen Reduction Reaction (ORR), which requires higher weight percent loading of the metal (~50%) on the carbon support to decrease the catalyst layer thickness and hence, the reactant transport losses. Conventional supports for platinum catalysts, such as the KetjenBlackTM type high surface area carbon (HSC), feature limited mesopore area for the dispersion of Pt nanoparticles leading to increased aggregation and poor durability. Here, we show a new class of carbon materials developed by Pajarito Powder known as the Engineered Catalyst Support (ECS) with higher mesopore fraction for the dispersion of higher weight percentage of Pt nanoparticles. ECS materials can disperse up to 50% Pt by weight of the catalyst, thereby enabling lower catalyst layer thickness with higher performance retained after durability test. ECS materials are tunable in terms of their mesopore structure, graphitization levels and the use of metal-based additives as dopants on the carbon support to anchor catalyst nanoparticles. A comprehensive set of physico-chemical and electrochemical studies in membrane electrode assembly (MEA) are reported to understand the performance and durability of Pt/ECS catalysts. Acknowledgements: This work is partially supported by U.S Department of Energy, Hydrogen and Fuel Cell Technologies Office under grant DE-EE0008821 and DE-EE0010749.

Ultra-Low Loaded Platinum Via Plasma-Enhanced Atomic Layer Deposition for the Fuel Electrode Active Layer in Steam Reversible Solid Oxide Cells

Reversible solid oxide cells (rSOCs) are crucial energy devices in sustainable energy systems, offering efficient energy conversion and storage solutions through hydrogen. Conventional SOFC/SOEC operating temperatures are above 700°C; at these temperatures, the cells benefit from high energy conversion efficiencies due to facilitated electrochemical reactions and effective waste heat utilization. Despite their advantages, commercial rSOCs face hurdles including suboptimal performance in the intermediate temperature range of 500-700°C, limited lifespan, and elevated operational costs. Addressing these issues, significant progress has been made in electrode fabrication, particularly through advanced semiconductor fabrication methods such as atomic layer deposition (ALD).In this study, we explore the application of plasma-enhanced ALD (PEALD) for depositing exceptionally low loadings of platinum on Ni-yttria stabilized zirconia (Ni-YSZ) fuel electrodes, aiming to enhance both performance and stability. Employing this novel approach, we achieved precise control over the Pt nanoparticle deposition process, resulting in a catalyst loading of only 0.2 µg/cm2.Experimental results demonstrated that rSOC samples with 10 cycles of PEALD Pt exhibited a 20% increase in maximum power density (548 vs. 479 mW/cm2 at 700°C) and an 18% increase in current density (1260 vs. 1070 mA/cm2 at 1.5 V) compared to the bare samples. Notably, these improvements were achieved without any observable performance degradation over 50 hours of operation at 700°C with 50% humidified hydrogen. Density functional theory (DFT) simulations were employed to elucidate the mechanisms contributing to the enhanced activity, revealing that the nano-scale Pt deposition promotes efficient hydrogen oxidation and evolution reactions.These findings highlight the potential of PEALD for rSOC manufacturing, enabling the creation of highly active and stable electrodes with minimal precious metal loading. The outcomes of this study suggest that such advancements could lead to more economically viable and sustainable hydrogen energy solutions, aligning with broader goals of energy security and environmental sustainability. This work provides a compelling case for the wider application of nano-engineered catalysts in high-performance electrochemical systems at intermediate temperature ranges.

Fuel Cell Electrode Inks with Pre-Formed Particle/Binder Fibers

When fuel cell electrodes are prepared by a conventional spraying or slot-die coating method, there is little or no control over the multi-scale organization of catalyst particles and polymer binder. Electrode structural features such as particle and binder interconnectivity, macro-porosity, and micro-porosity are difficult to control but are critically important when high-performance nanocatalysts are used. Nanofiber electrospinning is an electrode fabrication technique that can address fuel cell electrode morphology issues for next-generation MEAs. As a commercial fabrication method, electrospinning has been shown to be scalable, robust, and cost-effective, especially for the creation of non-woven mats of sub-micron-diameter polymer fibers. Electrospinning can also be used to prepare particle/polymer fiber networks with intra- and inter-fiber porosity. Such fiber mats have been used as the electrode material in Li-ion batteries, H2/Br2 redox flow batteries, and H2/air proton-exchange membrane fuel cells [1-4].Recent work on nanofiber electrodes at Vanderbilt has focused on inks containing pre-formed electrospun catalyst/binder fibers. With such inks, nanofiber electrode MEAs can be made using commercial roll-to-roll fabrication lines. For this poster presentation, experimental results will be presented on nanofiber mat electrode MEAs with Pt/C catalyst and Nafion™ binder. Procedures for preparing inks with nanofibers and then using the inks to create CCMs or GDEs will be shown. Fuel cell performance and durability results with the resulting MEAs will be contrasted with fuel cell data where the electrodes are made directly from electrospun mats, i.e., by the direct transfer of a mat from the electrospinner to a GDL or membrane, and made using a conventional catalyst powder + binder ink. The effects of catalyst loading and fuel cell operating conditions on fuel cell power output before and after a metal dissolution accelerated stress tests will also be presented. For example, an MEA with a Nafion 211 membrane and electrodes made using an ink with pre-formed fibers (TKK Pt/C catalyst at 0.13 mg/cm2 for the anode and 0.20 mg/cm2 for the cathode) generated 0.99 W/cm2 at 0.7 V, at 100% RH, 80oC, and 3 atmabs total pressure with gas feeds of H2 and air.AcknowledgmentThis work was funded by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, Fuel Cell Performance and Durability Project, under contract No. DE-EE0007653.References C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk,G. Liu, J. Nanda, and P. N. Pintauro, “High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats”, ChemSusChem, 10, 1823 – 1831 (2017).Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, “Ultralow Platinum Loading for Redox-Flow Battery by Electrospinning the Electrocatalyst and the Ionomer in Core-Shell Fibers”, Journal of Energy Storage, 59, 106430-106435 (2023).Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, “Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells”, Journal of the Electrochemical Society, 167, 054517 (2020).Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. Shawn Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, “Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation”, Journal of the Electrochemical Society, 170, 024507 (2023).

Fuel Cell Meas Made Using Electrode Inks with Pre-Formed Particle/Binder Fibers

When fuel cell electrodes are prepared by a conventional spraying or slot-die coating method, there is little or no control over the multi-scale organization of catalyst particles and polymer binder. Electrode structural features such as particle and binder interconnectivity, macro-porosity, and micro-porosity are difficult to control, but are critically important when high-performance nanocatalysts are used. Nanofiber electrospinning is an electrode fabrication technique that can address fuel cell electrode morphology issues for next-generation MEAs. As a commercial fabrication method, electrospinning has been shown to be scalable, robust, and cost-effective, especially for the creation of non-woven mats of sub-micron-diameter polymer fibers. Electrospinning can also be used to prepare particle/polymer fiber networks with intra- and inter-fiber porosity. Such fiber mats have been used as the electrode material in Li-ion batteries, H2/Br2 redox flow batteries, and H2/air proton-exchange membrane fuel cells [1-4].Recent work on nanofiber electrodes at Vanderbilt has focused on inks containing pre-formed electrospun catalyst/binder fibers. With such inks, nanofiber electrode MEAs can be made using commercial roll-to-roll fabrication lines. In this talk, experimental results will be presented on nanofiber mat electrode MEAs with Pt/C catalyst and Nafion™ binder. Procedures for preparing inks with nanofibers and then using the inks to create CCMs or GDEs will be presented. Fuel cell performance and durability results with the resulting MEAs will be contrasted with fuel cell data where the electrodes are made directly from electrospun mats, i.e., by the direct transfer of a mat from the electrospinner to a GDL or membrane. The effects of catalyst loading and fuel cell operating conditions (backpressure, feed gas flow rate, and feed gas relative humidity) on fuel cell power output before and after metal dissolution and carbon corrosion accelerated stress tests will also be discussed. As an example of nanofiber electrode performance: an MEA with a Nafion 211 membrane and TKK Pt/C anode and cathode catalyst (0.1 mg/cm2 anode and 0.22 mg/cm2 cathode) operating at 100% RH, 80oC, and 3 atm. total pressure generated 1.0 W/cm2 at 0.7 V. Acknowledgment This work was funded by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, for the Fuel Cell Performance and Durability Project, contract No. DE-EE0007653. References C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk,G. Liu, J. Nanda, and P. N. Pintauro, “High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats”, ChemSusChem, 10, 1823 – 1831 (2017).Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, “Ultralow Platinum Loading for Redox-Flow Battery by Electrospinning the Electrocatalyst and the Ionomer in Core-Shell Fibers”, Journal of Energy Storage, 59, 106430-106435 (2023).Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, “Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells”, Journal of the Electrochemical Society, 167, 054517 (2020).Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. Shawn Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, “Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation”, Journal of the Electrochemical Society, 170, 024507 (2023).

From Catalyst Ink to PEM Fuel Cell Stack: First Full-Size Screen Printed Catalyst Layers Tested within a Generic Stack

The PEM fuel cell industry is currently facing the challenge of upscaling its production, trying to reduce costs, allowing the fuel cell an easier entry in the mobility sector. Therefore, research on developing production processes, increasing throughput rates and quality control along the value chain is of special interest. Often, new materials, which show enhanced performance or durability are developed on very small scale. However, their key properties need to be investigated from a perspective of scalability, starting from catalyst ink to stack-level.Fraunhofer ISE and Center for Solar Energy and Hydrogen Research (ZSW) had the unique opportunity to investigate the production parameters along the whole value chain of a PEM fuel cell: starting with the catalyst ink at Fraunhofer ISE and ending with a generic stack at ZSW [1]. This generic stack is the approach to have a standardized open access stack design for the industry and research community with the size (active area 283cm²) and power output corresponding to the current automotive market, developed by ZSW and EKPO [2].In this study, the catalyst ink volume and catalyst layer area have been upscaled in a sheet-to-sheet process to meet the requirements of the generic stack automotive size membrane electrode assemblies (MEAs) with focus on heavy duty vehicles, hence aiming for platinum loadings of 0.4 mg/cm² (cathode) and 0.1 mg/cm² (anode). With on small scale proven ink recipes, specifically developed for screen printing cathode and anode electrodes [3], more than 50 catalyst layers have been produced with an upscaled area of 378cm² by a semi-automatic industrial screen printer. Flatbed screen-printing is a well-developed high throughput printing technology, known from printed electronics and solar cell metallization industries. Its benefit lies in the printability of high layer thicknesses with less solvent mass (high viscous pastes) and its degree of freedom to manufacture any structure, enabling platinum reduction in the electrodes by catalyst layer design.The catalyst coated membranes (CCMs) have been manufactured by a decal transfer on state of the art membranes with a roll-calendar. Special focus lies on this challenging transfer process for larger active areas. Two different membrane types, two different decal foils and different transfer process parameters like velocity and temperature have been investigated for all. The optical and gravimetrical transfer yields are the most important quality parameters during transfer because they strongly affect the costs of CCM production, especially when platinum material is not transferred, sticking to the decal foil. The optimal transfer parameters have been chosen to further produce MEAs towards stack-level. Afterwards, the CCMs were cut to fit the generic stack format and sent to ZSW in Ulm, Germany.At ZSW, the sub gaskets and gas diffusion layers (GDLs) from SIGRACET® SGL 22BB have been applied to form the 7-layer MEA. 15 CCMs have been stacked with bipolar plates and sealed to form a short stack. In addition, 15 commercially available CCMs were added as reference to the same stack. The whole manufacturing process chain is seen in Figure 1. After conditioning, the polarization curves have been measured at two different gas pressure settings. All other operating conditions were kept the same.As can be seen in Figure 2, the screen printed CCMs from Fraunhofer ISE (red) show very similar performance in comparison to the commercial reference (black) within the stack. In addition, the voltage deviation between the CCMs is slightly smaller for the ISE-CCMs, indicating a very good reproducibility in production. This study showed for the first time that screen printed catalyst layers can be upscaled to industrial active areas, overcoming challenges in printing, drying and transfer, while reaching similar performance as commercial references in a short stack. In future experiments many more iterations along the production chain will follow, to further improve the stack performance and investigate the effect of production pathways, process parameters and new materials.[1] “HyFab-BW - HyFab-Baden-Württemberg”, https://www.ise.fraunhofer.de/en/research-projects/hyfab-bw.html; Fraunhofer ISE, accessed April 2024[2] “Manufacturer-independent fuel cell stack: an innovative development platform for industry and science”, https://www.zsw-bw.de/en/research/fuel-cells/topics/stack-technology.html; ZSW; accessed April 2024[3] L. Ney, J. Hog, R. Singh, N. Göttlicher, P. Schneider, S. Tepner, M. Klingele, R. Keding, F. Clement, U. Groos, J Coat Technol Res 20 (2023) 73–86. https://doi.org/10.1007/s11998-022-00710-1. Figure 1

Durable Ion-Pair High-Temperature Proton Exchange Membrane Fuel Cells with a Low Platinum Loading Intermetallic Catalyst

Conventional high-temperature proton exchange membrane fuel cells (HT-PEMFCs) often necessitate a relatively high platinum cathode loading, primarily due to the undesirable issue of phosphate poisoning. Previous research has suggested that employing ordered Pt-alloy catalysts could mitigate this problem [1, 2]. Our study demonstrates that an ion-pair proton exchange membrane (PEM) based on quaternary ammonium functionalized membrane can further alleviate phosphate poisoning by reducing the concentration of phosphoric acid in the electrode [3, 4].In this study, we integrate two strategies: the utilization of intermetallic catalysts and the ion-pair PEM platform, aiming to decrease the Pt loading of the catalyst without compromising performance and durability. Carbon-supported intermetallic PtCoNiCrN catalysts were synthesized via a single thermal annealing process [5, 6]. Their crystallographic structure was confirmed through X-ray diffraction, atomic-resolution transmission electron microscopy, and in-/ex-situ X-ray absorption spectroscopy.Rotating disk electrode (RDE) experiments revealed that the half-wave potential of ORR of the intermetallic PtCoNiCrN/C in 0.1 M H3PO4 with 0.1 M HClO4 was 876 mV, surpassing that of disordered PtCoNiCrN/C (829 mV) and commercial Pt/C (TEC10E20E) catalysts (769 mV). In membrane electrode assembly (MEA), the MEA utilizing the intermetallic catalyst with a platinum loading of 0.3 mg cm-2 exhibited a current density of 0.212 A cm-2 at 0.7 V and the peak power density (PPD) of 713 mW cm-2 under H2/air conditions, significantly outperforming the MEA employing the commercial Pt/C catalyst with optimized electrode composition (0.103 A cm-2 at 0.7 V and PPD of 625 mW cm-2).Remarkably, the intermetallic catalyst demonstrated stable operation for 1,000 hours at 160 °C under anhydrous conditions without notable performance degradation. This study presents a pathway to reduce the catalyst loading of HT-PEMFCs, emphasizing the crucial role of the crystallographic structure of nanoparticles in enhancing the performance of HT-PEMFCs. Acknowledgments This work was supported by the US Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (L’Innovator program).

Electrochemical Characterisation and Fuel Cell Testing of Bespoke, Low Loading Platinum on Carbon Black Catalysts Fabricated By Physical Vapour Deposition.

A critical component of achieving net zero emissions by 2050 is the continuing research and development of improved methods of producing, as well as using, green hydrogen. Hydrogen is a key component of many industrial and chemical processes, but here we focus on its role in the energy transition, specifically proton exchange membrane (PEM) fuel cells. A key part of the PEM fuel cell is the catalyst material and structure, and this work is part of a wider, multi-institutional collaboration concerning metal atoms on surfaces and interfaces (MASI).MASI’s goal is to increase the surface area to volume ratio of metal particle on support catalysts, exploring the sub-nanometre metal particle size domain (referred to henceforth as nanoclusters) by fabricating catalysts utilising physical vapour deposition (PVD.) In contrast to wet chemical methods, the size of metal particle depositing onto the surface can be better controlled. This versatile method has been applied to deposit many different metals onto a variety of morphologies of surface to date. The chemical and physical properties of these nanoclusters can then be studied and exploited where an improvement in activity on nanoparticle catalysts is observed, as well as reducing the amount of ‘wasted’ atoms buried below the active surface of the catalyst, reducing the unit cost of, for example, a fuel cell stack.In this study, platinum is deposited onto a carbon black (XC-72R) support at four different weight loadings (1.1wt%, 2.3wt%, 5.0wt% and 9.7wt% Pt/XC-72R carbon named Pt-1, Pt-2, Pt-5 and Pt-10 respectively,) and is compared to two commercially available equivalents (46.2wt% Pt/HSC TKK and 10wt% Pt/XC-72 carbon E-TEC.) The catalyst loadings were determined using ICP-OES. Imaging of the catalyst surface was performed using aberration-corrected scanning transmission electron microscopy (AC-STEM), including the generation of a size distribution of the platinum particles present. The electrochemically active surface area (ECSA) of the platinum catalyst was obtained using carbon monoxide stripping voltammetry. The kinetic properties of the catalysts were then probed using the oxygen reduction reaction (ORR) via rotating disc electrode (RDE) voltammetry, and a discussion of the factors impacting the observed onset potential (specifically kinetics and catalyst coverage) is presented. The selectivity of the ORR with respect to hydrogen peroxide production is also studied using rotating ring disc electrode (RRDE) voltammetry. Finally, single cell testing to study fuel cell performance, specifically on the anode side of the cell, is performed and analysed for future optimisation. Accelerated stress testing (AST) was also performed; ORR activity and fuel cell performance were probed as a function of number of break in cycles, up to 4000 cycles.STEM imaging revealed significant differences in the size of platinum particle across the samples studied. For example, the 2.3wt% Pt/C catalyst exhibited a platinum particle size of 0.6 ± 0.3 nm, far smaller than the 2.2 ± 0.5 nm found for TKK.Further, our study found that all PVD fabricated catalysts showed a significantly higher ECSA than the commercial equivalents, along with a shift in onset towards a higher overpotential for the ORR which is explained by kinetic differences due to size of platinum particle – platinum catalysts become worse at catalysing the ORR as the size of particle decreases. This is therefore used as further evidence of our nanoclusters’ catalytic activity. RRDE voltammetry revealed a general trend of decreasing peroxide production as weight loading increasing, explained by probable decreasing interparticle distance as well as increased particle size. AST showed all catalysts are relatively stable; increases in overpotential between initial performance and after 4000 cycles were in the region of 16 mV.Single cell testing revealed exceptional performance for the PVD catalysts tested. The 5.0wt% and 9.7wt% catalysts showed superior initial performance to TKK at a fraction of the platinum loading in all regions of the IV curve, with an associated improved peak power density. AST revealed that peak power density decreased in percentage terms more for the PVD catalysts, and the decrease was larger as weight loading increased. After 4000 cycles, the 9.7wt% catalyst still showed comparable performance to TKK, with the 5wt% just below.Future work will focus on trying to decrease the mean size of platinum nanocluster further, optimising catalyst loading and enhancing stability of the catalyst with respect to degradation. Figure 1

Modeling The Hydrogen Crossover Through Membrane With And Without Catalyst Layers

Introduction At elevated temperatures under low humidification, the failure of the Nafion membrane is unavoidable and it leads to fuel/oxidant passing through the membrane yielding loss of fuel, undesired reaction of fuel and oxidant, and further damage to the membrane. The reaction of hydrogen and oxygen possibly results in formation of hydrogen peroxide. That causes degradation and pinhole formation and thus, membrane durability is affected at elevated temperatures. In the current study, the hydrogen crossover rate was measured under wide range of conditions.The hydrogen crossover flux, N H (M) [mol/(m2 s)], is expressed by Eq. (1) as a function of the difference in the partial pressure of hydrogen between the supply and permeate sides, Δp H, and the total pressure difference, ΔP, with the hydrogen permeance, k pH (M) [mol/(Pa m2 s)], and the gas permeation coefficient, k G (M) [mol/(Pa m2 s)], which are both functions of temperature, moisture content, and film thickness. N H (M)=k pH (M)Δp H+y H k G (M)ΔP (1)where y H represents the hydrogen mole fraction on the supply side. Experimental In the experiments, we used Nafion membranes of various thicknesses, NR-211 (25.4 µm), NR-212 (50.8 µm), and N-115 (127 µm), without any pre-treatment. Both the proton exchange membrane (PEM) and the membrane electrode assembly (MEA) samples were used in the testing cell. The catalyst layer thickness was 10 µm with a platinum loading of 0.35 mg/cm2. Our design featured a serpentine gas channel. Hydrogen diluted with nitrogen (H2:N2) and pure N2 were supplied to the supply side and the permeate side, respectively. To measure hydrogen permeation, micro gas chromatograph (Agilent 990 micro GC) connected to the outlet of the permeate side was employed. Results and Discussion Fig. 1(a) shows the N H (M) vs. Δp H profiles measured with NR-212. They displayed a linear tendency at ΔP = 0.5, 25, 50, and 75 kPa. k pH (M) was represented by the slope of N H (M) vs. Δp H. ΔP did not influence the total permeation flux up to 50 kPa as shown in Fig. 1(b). When ΔP increased above 50 kPa, N H (M) increased by approximately 10 %, which resulted from increase in the contribution of the total pressure difference, y H k G (M)ΔP, shown in Fig. 1(c). It means the flux was determined by the partial pressure difference; the total pressure difference did not remarkably contribute to the permeation flux.By modeling the membrane as a bulk layer sandwiched with skin layers, k pH (M) of PEM is expressed by Eq. (2),1/k pH (M)=δ(B)/P H (B)+2δ(S)/P H (S) (2)where the bulk layer thickness and the skin layer thickness are denoted by δ (B) and δ (S), respectively. P H (B) and P H (S) respectively represent the hydrogen permeability through bulk and skin layers. As shown in Fig. 2(a), PEM had the permeation resistance even at zero membrane thickness, which represents the permeation resistance of the thin skin layers. As shown in Fig. 2(b), however, MEA had no skin layers, and the permeation resistance, 1/ k pH (M), of PEM was approximately 1.5 times higher than that of MEA. It suggested the skin layers of PEM were fused with ionomer in catalyst layers (CLs) during hot pressing, which annihilated the skin layers.The slope in Fig. 2(a) gives P H (B) while the intercept gives the H2 permeance of the skin layer, k pH (S) = P H (S)/δ (S). They were found to be power-law functions of the moisture content at each temperature as shown in Figs. 3(a) and 3(b). P H (B) of PEM with and without CLs were substantially equal as shown in Fig. 3(a). The measured P H (B) was formulated as, P H (B) = (1.72×10– 14 mol Pa– 1m– 1 s– 1×λ 0.339) exp[–(22.8 kJ mol– 1/ R){1/T – 1/(353 K)}]Recalculated P H (B) is also shown in Fig. 3(a).The H2 permeance of the skin layer, k pH (S), was formulated as, k pH (S)= (2.71×10– 9 mol Pa– 1 m– 2s– 1×λ 0.858) exp[–(12.3 kJ mol– 1/R (1/T – 1/(353K)}]These equations reproduce the measured data accurately as shown in Figs. 3(a) and 3(b). Conclusions Hydrogen permeance through the bulk layer and the skin layer of PEM was obtained by measuring the permeation flux. The equations estimating them were determined as functions of temperature and moisture content of the membrane. Whereas the bulk layer showed similar behaviors both in PEM and MEA, the skin layer of MEA was not identified. To estimate the H2 crossover in an actual PEFC, the H2 permeance of the PEM should be measured using an MEA instead of a standalone PEM. Figure 1

Electrospun Fuel Cell Cathode Catalyst Layers Under Low Humidity Conditions

Electrospinning has emerged as a promising approach to prepare porous cathode catalyst layers (CCLs) with low loading of platinum group metals (PGM) for PEM fuel cells. In particular, electrospun nanometer-sized fibers can be used as a backbone to increase the utilization and accessibility of catalytically active sites at low PGM loading as well as improve water management at high current density. [1, 2] Beside the low PGM loading, the absence of external humidifier is important for practical applications, because it can further decrease the parasitic power losses as well as complexity, weight, volume, and cost of the PEMFC system. [3] In this context, electrospun nanofiber CCLs show a pronounced humidity-dependent performance loss, which hinders their operation range compared to conventional CCLs prepared by decal transfer process. For example, Brodt et al. show a performance decrease for electrospun CCL of around 75 % at relative humidity (RH) of 40 % compared to 100 % RH. [4] In contrast, the electrosprayed CCL exhibits only a loss of around 30 % under the same conditions. [4] Therefore, the development of highly active and robust CCLs operating at high current density under low humidity remain a great challenge to date.In this work, we systematically investigated the effects of the humidity on the performance of the nanofiber-based CCLs. To prepare the CCLs with a geometric surface area of 50 cm2, a nozzle-free electrospinning machine by Elmarco S.R.O. (Liberec, Czech Republic) was employed. The ink containing Pt/C catalyst, ionomer, and polyacrylic acid (PAA) was mixed using a disperser. Thereby, several highly homogeneous CCLs deposited on a gas diffusion layer (GDL) with controlled platinum loading of up to 0.1 mg cm-2 geo were prepared and characterized using micro X-ray fluorescence spectroscopy (μXRF). The morphology and chemical distribution of the as-prepared CCLs were evaluated from the scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). We then correlated the structural information (Pt and Nafion distribution, size of the nanofibers, etc.) of the CCLs with the electrochemical performance (U-I curve, ECSA, HFR, proton conductivity, etc.) at single cell level. Additionally, the impact of the humidity variations (20 – 100 % RH) for the nanofiber-based CCLs was compared to the conventional CCLs prepared by decal-transfer method. Moreover, high resolution (S)TEM-EDX was employed to uncover the distribution of the PFSA ionomer, Pt/C catalyst and PAA. These data are correlated with the ECSA and indicate the utilization and accessibility of Pt nanoparticles linked with the spatial distribution of PFSA ionomer.Altogether, we provide fundamental insights into the relationship between humidity and performance for nanofiber-based PEMFC CCLs prepared by electrospinning. New strategies will be presented to overcome the structure-humidity-sensitivity behavior of these novel three-dimensional nanofiber-based CCLs. [5] References [1] Brodt M, Wycisk R, Pintauro P N: Nanofiber Electrodes for High Power PEM Fuel Cells. J. Electrochem. Soc. 2013, 160 (8), F744-F749. DOI: https://doi.org/10.1149/2.008308jes.[2] Zhang W, Pintauro P N: High-Performance Nanofiber Fuel Cell Electrodes. ChemSusChem 2011, 4 (12), 1753-1757. DOI: https://doi.org/10.1002/cssc.201100245.[3] Ren G, Qu Z, Wang X et al.: Electrospun fabrication and experimental characterization of highly porous microporous layers for PEM fuel cells. International Journal of Hydrogen Energy 2024, 55, 455 - 463. DOI: https://doi.org/10.1016/j.ijhydene.2023.11.226.[4] Brodt M, Han T, Dale N et al.: Fabrication, In-Situ Performance, and Durability of Nanofiber Fuel Cell Electrodes. J Electrochem Soc 2015, 162, F84-F91. DOI: https://doi.org/10.1149/2.0651501jes.[5] Kallina V, Hasché F, Oezaslan M: Advanced Design of Electrospun Nanofiber Cathode Catalyst Layers for PEM Fuel Cells at Low Humidity. Current Opinion in Electrochemistry 2024, submitted.

Construction of directional electron transfer from Pt to MoO2-x in macroporous structure for efficient hydrogen oxidation

Hydrogen oxidation reaction (HOR) as the anode reaction in proton exchange membrane fuel cell, usually suffers from the high loading of platinum (Pt) and subsequent CO poisoning especially by using industrial crude hydrogen as fuel. In this work, we propose a directional electron transfer route from Pt to MoO2-x in the macroporous structure to significantly enhance the HOR activity as well as the CO tolerance, which is constructed by interface engineering and defect strategy to anchor highly dispersed Pt nanoparticles onto the three-dimensional MoO2-x-C framework. The optimized 2Pt-MoO2-x-C with 1.02 wt% Pt demonstrates higher HOR peak current density (3.57 mA cm−2) and nearly 25 times higher mass activity than 20 wt% Pt/C. The excellent HOR performance is attributed to the synergistic effect between Pt and MoO2-x species, in which the charge transfer from Pt to MoO2-x improves H2 adsorption ability of Pt and accelerates the activation of H2 due to the reduced hydrogen binding energy of MoO2-x caused by Pt-O construction, leading to the release of H* thereby the enhancement of HOR activity. The construction of three-dimensional macroporous structure enhances the HOR dynamics by promoting the conductivity, mass transfer and the exposure of active sites. Moreover, the formed Mo-OH in Pt-MoO2-x-C can effectively react with CO species to remove the CO poisoning of Pt, endowing the excellent CO tolerance.