Durable Platinum Thin Film on Silver Leaf, Formed by Self‐Terminating Electrodeposition

Porous Metal-Organic Frameworks: Promising Materials for Methane Storage

<p indent="0mm">Proton exchange membrane fuel cells (PEMFCs) operating at low temperatures and high-power densities are considered one of the most promising technologies for clean energy. However, some fundamental challenges remains to be addressed before PEMFCs can truly become the top contender, high performance with low Pt usage for example. In order to achieve high-performance and cost-effective in PEMFCs, Pt alloy are currently one of the most promising catalyst candidates. Electrochemical active surface area (ECSA) is a key indicator to gauge the performance of Pt catalysts and an important parameter in studying catalytic kinetic issues in PEMFCs. Therefore, the accurate estimation on the ECSA is exceptionally critical. Known and mature ECSA characterization methods for Pt/C catalysts have been established, which has been widely available for many decades. However, there are challenging issues remaining for ECSA characterization of Pt alloy catalysts due to the significant changes in physical structure (composition, particle size, shape, etc.) and chemical properties (active site, catalytic performance, etc.) compared with Pt catalysts. Therefore, the ECSA characterization approaches for Pt/C catalysts cannot be transplanted to Pt alloy catalysts since the characterization accuracy will no longer meet the requirement. In this review, the present methods of ECSA characterization for Pt alloy catalysts and their inaccuracy sources are described and explained, including physical methods and electrochemical methods. Physical methods mainly cover X-ray diffractometry (XRD) and transmission electron microscopy (TEM). hydrogen underpotential deposition (H-upd), carbon monoxide stripping (CO-stripping) and underpotential deposition of metals are classified as electrochemical methods. In XRD and TEM methods, the geometrical areas calculated based on the average particle size is regard as ECSA. In more cases, it is considered to be theoretical ECSA to evaluate the catalyst utilization. Therefore, the ECSA calculated from XRD and TEM is widely used as a reference. Owing to the changes in surface adsorption properties after alloying, it will be underestimated the ECSA of Pt alloy catalysts by H-upd and CO-stripping, which leads to exaggerate specific activity of when compared with Pt/C catalysts. It is generally accepted that sites from electrochemical methods are catalytically active, but it is still a controversy point. The surface area of different metal components of Pt alloy could be quantified by metal underpotential deposition method. Since the contribution of different metal components to the catalytic activity is different, the actual ECSA cannot be simply calculated by adding the respective metal surface area. The surface active of Pt alloy catalysts is affected by many factors, which there is currently not a universal method to characterize the ECSA accurately. To evaluate the performance of Pt alloy catalysts more objectively, it is necessary to clarify the relationship between the surface structure of different Pt alloys and adsorption energy and coverage of adsorbed species, then modifying the calculated equations of ECSA. In addition, to innovatively develop new electrochemical methods with high accuracy is also essential. Such knowledge is in urgent need in order to develop new high-performance Pt alloy catalysts.

Proton exchange membrane fuel cell (PEMFC) is an efficient electrochemical energy conversion device that directly generates electricity from the chemical energy of fuels without the emission of greenhouse gases. The most reliable catalyst in PEMFC is Platinum (Pt) metal nanoparticles (NPs) that exhibit excellent electrochemical activity and stability compared to other catalysts. However, using of Pt catalyst is limited due to its very high cost and low abundance on Earth. Therefore, it is important to use Pt catalyst efficiently for making the PEMFC economically viable. In this regard, several synthesis techniques have been developed to reduce the loading and uniform distribution of Pt NPs on carbon support with high electrochemically active surface area (ECSA). One of the most efficient techniques to uniformly deposit Pt NPs with a controllable size on carbon support is to use a fluidized bed reactor (FBR) atomic layer deposition (ALD). Our group recently demonstrated that FBR-ALD Pt/C catalysts can exhibit high fuel cell performance and high endurance even with low Pt NPs loading by optimizing the surface of carbon supports combined with proper ALD process parameters [1]. However, it is still challenging to further improve the fuel cell performance by rational designing the Pt NPs surfaces in order to make FBR-ALD into a viable commercial production.In this study, a unique way to improve the fuel cell performance was suggested to design and optimize atomic scale surface textures of Pt NPs. During the FBR-ALD of Pt NPs, in-situ surface modulation of Pt NPs was applied via a proper protective oxide deposition and etching. A careful surface studies was performed to analyze the surface morphology, distribution and uniformity of Pt NPs. Electrochemical performances were evaluated and optimized by measuring cyclic voltammetry (CV) and oxygen reduction reaction (ORR). Finally, a fuel cell performance was studied through membrane electrode assembly (MEA) characteristics.References W. J. Lee, S. Bera, H. C. Shin, W. P. Hong, S. J. Oh, Z. wan, S. H. Kwon, Adv. Mater. Interfaces 6, 1901210 (2019).

In Proton Exchange Membrane Fuel Cell (PEMFC), the membrane electrode assembly (MEA), especially the electrode, is considered 'the heart' and is designed to accommodate the need for efficient transport of electrons, reactants, and heat as well as constraints imposed by the platinum cost. Advances in PEMFC electrode design over the years, which range from using Pt black films with a loading of 10 gpt/cm2 in the 1970s to present-day Pt/ PGM (Platinum Group Metal) nanoparticle coated carbon-black particles (Pt/C) that use about 0.3 mgpt/cm2, have led to significant cost-reduction and performance enhancement. However, durability problems associated with corrosion of carbon support and subsequent loss of active surface area during start-up or shut down cycles have led to renewed interest in carbon-free nanostructured electrodes that employ a thin coating of Platinum or PGM-based catalytic layer on mesostructured conductive support ((Debe 2012), (Liu et al. 2012)).In this context, we carried out systematic ex-situ investigations on the durability of platinum thin films comprising of atomic layers formed by Self-Terminating platinum electrodeposition (Liu et al. 2012). We characterized the sequential growth of platinum film using XPS and AFM measurements; and analyzed the electrocatalytic performance as a function of the number of self-terminating pulses. We found that eight pulses of electrodeposition, corresponding to ~10 nm thick platinum film, can meet stringent ex-situ durability targets set, by the Department of Energy (DOE), USA, for a potential electrocatalyst in Fuel Cell Vehicles (FCVs) application. Our results also show that platinum layers formed using four pulses (< 4 µg/cm2 loading) and even one pulse (< 1 µg/cm2 loading) are electrochemically active. For platinum samples formed using eight or more pulses, we observed ~ 20% loss in Electrochemically Active Surface Area (ECSA) during the first 3000 cycles and 10-15% ECSA loss in the remaining 27000 cycles, indicating stabilization of platinum activity over time. We will present the results of our studies on the minimum number of platinum pulses needed to form a complete overlayer of electrochemically active platinum and discuss the platinum loss during the ex-situ durability tests using ICP-OES and Cyclic Voltammetry measurements. These studies pave the way for an additive, roll-to-roll, and cost-effective manufacturing process of ultra-low platinum loaded MEAs for PEMFCs by building on our earlier results on printing silver nanostructures using a simple inkjet printer by Print-Expose-Develop technique (Parmar and Santhanam 2014) in conjunction with the ability to controllably form atomic layers of platinum on metallic substrates by Self-terminating electrodeposition. Figure 1

On account of their low-cost, corrosion resistance, and lightweight, carbon-polymer composites (CPCs) are very promising for producing bipolar plates (BPs) for proton exchange membrane fuel cells (PEMFCs). However, the substantial conductive filler concentration that is required to give CPCs with the desired level of electrical conductivity for BP application often results in difficulty in processing and degradation in mechanical strength. In this study, the potential of polyethylene (PE)/epoxy/graphite composite to yield BPs with electrical and mechanical performance that meet the technical targets for commercial PEMFC BPs was explored. PE/epoxy/graphite composites with varying graphite contents were prepared by melt mixing followed by compression moulding. The morphology, flexural properties, and electrical conductivity of the composites were investigated. The electrical conductivity and flexural modulus of the composites increased with increase in graphite content while the flexural strength increased with graphite content up to a maximum of 42.1 MPa at 70 wt% graphite and then decreased with further increase in graphite content. With respect to the United States Department of Energy (DOE) targets, the PE/epoxy/80 wt% graphite composite has the most promising combination of electrical conductivity (72.96 Scm−1 for in- plane and 4.12 Scm-1 for through-plane) and flexural strength (39.15MPa). Although the electrical conductivities of the PE/epoxy/graphite composites are still below the DOE targets, achieving this level of conductivities with a single filler indicates that the composites have promising potential for CPC BP applications.

Exploring alternative sources for platinum catalyst carbon support material is vital for the further development of polymer electrolyte membrane fuel cells (PEMFC).1 The support material and the method of depositing Pt nanoparticles are crucial for defining the catalytic properties of the catalyst. In this study, carbon support material, derived from coffee grounds was used in the preparation of Pt catalysts for PEMFC application. The carbon material was synthesized using ZnCl2 activated pyrolysis method2 and the Pt nanoparticles were deposited onto carbon support by two methods: using ethylene glycol3 or hydrogen4 as a reducing agent.The physical properties of the support material, as well as catalysts were analysed through thermogravimetric analysis (TGA), dynamic light scattering, X-ray diffraction (XRD), N2 sorption analysis and high-resolution scanning electron microscopy (HR-SEM) methods. The electrochemical activity, as well as the electrochemically active surface area (ECSA) of the catalysts were determined in a completed PEMFC. The properties and performance of catalysts synthesised were compared with the catalyst which was deposited onto commercial carbon (Ketjenblack, EC-300J).TGA displayed that carbon derived from coffee grounds contained 6.9 % of impurities by mass in the form of metal oxides and that the coffee carbon Pt catalyst contained 60 wt% of Pt. The Pt crystallite size was determined through XRD and it was 1.4 nm. HR-SEM analysis displayed that the average particle size of Pt nanoparticles on the coffee carbon catalyst was 4.7±2.5 nm. Results from N2 sorption analysis showed that all materials were micro-mesoporous, with coffee ground carbon having a specific surface area of 590 m2 g-1. The coffee carbon Pt catalyst produced smaller results of 370 m2 g-1 compared to the carbon support, due to the Pt nanoparticles depositing in the micropores, however still had a larger specific surface area than the commercial carbon catalyst with a specific surface area of 300 m2 g-1.Catalysts deposited on coffee ground carbon produced comparable electrochemical results to catalysts deposited on commercially available carbon. The coffee carbon catalyst produced a higher ECSA of 64 mPt 2 gPt -1, compared to the commercial carbon catalyst with an ECSA of 51 mPt 2 gPt -1. The current density of the Pt catalyst deposited onto coffee carbon was 0.68 A cm-2 at 670 mV, whilst in the same conditions commercial carbon peaked at 0.70 A cm-2. Both coffee and commercial carbon Pt catalysts produced similar power density maximums with the coffee Pt catalyst producing 0.56 W cm-2 and the commercial Pt catalyst producing 0.60 W cm-2.Physical and electrochemical characterizations displayed that higher ECSA values were strongly correlated to smaller Pt crystallite size. Other techniques to increase fuel cell performance were also experimented with, such as covering the gas diffusion layer with catalyst instead of Nafion membrane and adding additional Nafion layers, with both methods producing promising results.This study displays that there is a lot of potential in improving the catalytic performance of Pt catalysts by depositing on support materials derived from alternative sources. The methods of boosting PEMFC performance could be further investigated for catalysts deposited on different carbon supports. Acknowledgements This work was supported by the EU through the European Regional Development Fund TK141 “Advanced materials and high-technology devices for energy recuperation systems” (2014-2020.4.01.15-0011), Personal Research Grant PRG676 and by private limited company AuVe Tech LLTKT20148 “Production of Polymer Electrolyte Membrane Fuel Cells”. The SEM measurements were conducted using the NAMUR+ core facility funded by the Estonian Research Council (TT 13). References S. Shahgaldi and J. Hamelin, Carbon, 94, 705-728 (2015). M. Härmas, PhD thesis, University of Tartu, Tartu, Estonia (2020). Y. Shao, S. Zhang, R. Kou, X. Wang, C. Wang, S. Dai, V. Viswanathan, J. Liu, Y. Wang, Y. Lin, J. Power Sources 195, 1805-1811 (2010). Y. Zheng, Z. Dou, Y. Fang, M. Li, X. Wu, J. Zeng, Z. Hou, S. Liao, J. Power Sources, 306, 448-453 (2015).

Polymer Composite Bipolar Plates (PCBP) with carbon fillers are emerging as a lightweight and compact alternative to pure graphite bipolar plate for Proton Exchange Membrane Fuel Cells (PEMFC). PCBP can reduce the weight and volume occupied by PEMFC assembly, enhancing the payload capacity of hydrogen-powered electric vehicles. This study focuses on the development of PCBP using Multi-Walled Carbon Nanotube (MWCNT) as tertiary filler, Carbon Black (CB) as secondary filler, Natural Flake Graphite (NFG) as primary filler and bisphenol A Epoxy resin (EPR) as binder. The fabrication is done by compression molding. The study aims to examine the effect of filler concentration of MWCNT on the mechanical, electrical, morphological, chemical and thermal properties of the PCBP. The optimized PCBP with 3.0 vol% MWCNT achieved a flexural strength of 46 MPa, in-plane electrical conductivity of 182 S cm−1 and a corrosion current density of 0.221 μA cm−2, satisfying 2025 US Department of Energy targets. Additionally, it demonstrated a bulk density of 1.46 g cm−3, shore D hardness of 70, thermal conductivity of 5.45 W mK−1 and a water contact angle of 86.78°, which are desirable values for PEMFC application. These results underline the potential of hybrid nanocomposite PCBPs to enhance the desired properties of PEMFC systems, paving the way for advancements in hydrogen fuel cell vehicles.

In this project, the objective was to develop new oxygen reduction reaction (ORR) electrocatalysts for proton exchange membrane fuel cells (PEMFCs) which could exceed all of the Department of Energy (DOE) 2020 targets listed in DE-FOA-0001224, Subtopic 1b, Table I. The expected outcome was development of one or more electrocatalysts which are substantially improved over the current state-of-the-art in terms of overall activity, durability, and cost, and are suitable for automotive traction and stationary fuel cell applications. This report summarizes this project’s progress towards meeting the stated objectives and includes a summary of key findings and conclusions. The development activities towards new highly active and durable thin film electrocatalysts has led to the discovery of several electrocatalysts which approached or exceeded several DOE 2020 targets for activity, durability, and performance in proton exchange membrane fuel cell (membrane electrode assemblies (MEAs). In this project, electrocatalyst development focused on systematic physical and electrochemical characterization of electrocatalyst activity, durability, and performance in MEAs as a function of electrocatalyst fabrication, compositional and structural variables. The project approach was to develop relationships between the catalyst functional responses (activity, durability and performance) with catalyst physical properties and catalyst fabrication methods. Additionally, the thin film catalysts were integrated onto the unique and durable 3M Nanostructured Thin Film (NSTF) support, consisting of arrays of self-assembled organic crystalline whiskers. The catalysts were evaluated against the project targets via extensive electrochemical characterization in MEA format at 3M, advanced structural and compositional microscopy at Oak Ridge National Laboratory (ORNL), and atomic structure analysis via XAFS at Argonne National Laboratory (ANL). The extensive electrochemical and physical characterization resulted in development of several trends which correlate the catalysts’ electrochemical properties to their physical properties, including composition, structure, and method of fabrication. The development was guided by density functional theory (DFT) modeling at Purdue University and kinetic Monte Carlo (kMC) modeling at Johns Hopkins University. The catalyst simulations provided key insights into the observed experimental activity and durability trends, and additionally were utilized to assess new electrocatalyst concepts prior to or coincident with physical catalyst development. This development has led to several electrocatalyst candidates with activity and durability which approach or exceed DOE targets. One class of catalysts, based on nm-scale thin layers of Pt on Ir, met or exceeded 6 of the 6 DOE targets the project addressed.

The proton exchange membrane fuel cells (PEMFCs) are expected to be one of the most suitable power sources for electric vehicles to replace the traditional energy,1 but its commercialization is restricted by some problems, such as high cost, low durability and inadequate facilities and so on.2 Among these problems, the high cost, unsatisfactory activity and durability of Pt catalyst as the key part in fuel cells become particularly important.3 It is one of promising solutions to design and prepare low Pt loading catalysts4 with high efficiency and low cost. Recently, it is found that the (111) facets possess the highest oxygen reduction reaction (ORR) activity in the three low index facets of Pt alloys,5 and thus Pt-based octahedral catalysts with maximal (111) surface area have been paid more attention. However, in methods commonly employed, the capping agents difficult to remove not only block the active sites but also hinder the electron transfer between catalyst and support. Therefore, surfactant-free method is employed, and the growth of specific facet is synergistically regulated by suitable organic reducing agent and organic ligands of precursors to replace capping agents.6 The reaction conditions have important influence on the crystal growth process, crystal size, bulk and surface atom arrangement. In this work, impacts of reaction temperature, reaction time and ratio of metal elements on the formation of PtNi octahedral nanocrystals were investigated to optimize preparation conditions. The results demonstrate that reaction temperatures can affect crystallization rates, and lower temperature leads to excessively slow reduction and crystallization rate, which will result in small particles and incomplete (111) facet growth, whereas a higher temperature will cause a faster crystallization rate and larger particles, which will result in loss of active sites and decreased ORR performance. The influence of reaction time on electrochemical performance mainly depends on the synergistic effect between crystal growth and metal atom migration caused by concentration gradients. For one thing, crystal sizes are increased with the reaction time prolonged, which will result in the decrease of active area; for another, the atom concentration gradients resulted from different electrochemical deposition potentials impel Pt atom to migrate towards the surface, which is beneficial to the increase of active sites. The mechanism is more complex for impact of different atom ratios. We suggest that the ORR activities of PtNi octahedral nanocrystals with different Pt/Ni atom ratios are determined by the delicate balance between the thickness of Pt surface skin, the Ni content near the surface and the remaining (111) facets. Accordingly, reaction temperature of 140 °C, reaction time of 42 h and Pt/Ni atom ratio of 1: 1 were selected to prepare the PtNi/C octahedral nanocrystal catalyst in order to ensure optimal ORR performance. Under the optimized reaction conditions, PtNi/C octahedral nanocrystal catalyst with highly active (111) facets as surface was successfully prepared by a facile one-pot surfactant-free solvothermal method using acetylacetonate salts as precursors and dimethylfomamide (DMF) as reducing agent, and it exhibits well-defined structure and enhanced ORR performance compared with commercial Pt/C catalyst. Its mass activity and specific activity are 224.5 mA mgPt -1 and 618.5 μA cmPt -2, which are about 2.6 times and 3.8 times of Pt/C (Johnson Matthey, JM), respectively. The electrochemically active surface areas (ECSAs) of Pt/C (JM) decrease from 53.2 m2 gPt -1 to 32.5 m2 gPt -1 after 2000 cycles accelerated durability testing (ADT), indicating a loss of 38.9%, whereas the attenuation rate in ECSA of PtNi/C octahedral nanocrystal catalyst is only 23.1%. The mass activity of Pt/C (JM) at 0.9 V vs. RHE is reduced from 86.4 mA mgPt -1 to 30.6 mA mgPt -1 after durability test, and the attenuation rate is up to 64.6%; nevertheless, the mass activity of PtNi/C octahedral nanocrystal catalyst is dropped by 46.1%, and its mass activity after 2000 cycles is yet higher than the initial value of Pt/C (JM). Our work provides some valuable insights in the preparation condition optimization of PtNi octahedral catalyst, and has a certain guiding significance for further research on the preparation of shape-selective catalysts.

The bipolar plate is the most important and most costly component of proton exchange membrane fuel cells. The development of a suitable low density bipolar plate is scientifically and technically challenging due to the need to maintain high electrical conductivity and mechanical properties. Here, bipolar plates were developed from different particle sizes of natural and expanded graphite with phenolic resin as a polymeric matrix. It was observed that the particle size of the reinforcement significantly influences the mechanical and electrical properties of a composite bipolar plate. The composite bipolar plate based on expanded graphite gives the desired mechanical and electrical properties as per the US Department of Energy target, with a bulk density of 1.55 <TEX>$g.cm^{-3}$</TEX> as compared to that of ~1.87 <TEX>$g.cm^{-3}$</TEX> for a composite plate based on natural graphite (NG). Although the bulk density of the expanded-graphite-based composite plate is ~20% less than that of the NG-based plate, the I-V performance of the expanded graphite plate is superior to that of the NG plate as a consequence of the higher conductivity. The expanded graphite plate can thus be used as an electromagnetic interference shielding material.

Tailoring a Three-Phase Microenvironment for High-Performance Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells

Introduction Commercializing polymer electrolyte membrane fuel cells (PEMFC) is ultimately a matter of achieving the necessary cost for broad market penetration. Cost is a function of materials, manufacturability, performance, and durability. Performance and durability targets have been set by the DOE [1] to provide guidance and focus to the PEMFC industry for specific applications such as the automotive sector. To cascade these challenging targets down to material and transport requirements we employ a fully integrated performance model to conduct parameter optimization studies in tandem with experimental validation. Design curves are utilized as inputs to the model to provide the current state-of-the-art capability regarding catalyst activity and ionomer proton conductivity. High performing commercial membrane and GDL components were selected with a standard catalyst (Figure 1) to provide a baseline for this evaluation. The gap between modelled MEA performance and DOE targets will be highlighted and a recommended path forward will be provided. Results & Discussion Aside from catalyst activity and available surface area, the cathode catalyst layer performance is dictated by the mass transport of protons (proton conductivity), oxygen (gas diffusivity), and water (gas & liquid permeability) [2-4]. Several material sets have been evaluated experimentally in-situ to provide a range in catalyst activity, proton conductivity, and effective layer diffusivity as shown in figures 1 and 2 (diffusivity not shown). During operation the distribution of current through the porous three dimensional catalyst layer structure [6] is dictated by the catalyst activity and layer transport properties. We will describe in this work the relationship between proton conductivity and voltage performance. Based on these results we can set relevant conductivity targets, which can be used for both ionomer development and material down selection. These parameters will be utilized in the Ballard/DOE funded FC-Apollo performance model to provide the current status toward achieving the DOE’s 2020 automotive targets listed in Table 1. Further, technology gaps will be identified in conjunction with a strategy toward meeting these objectives. Acknowledgement The authors would like to acknowledge Joey Jickain for his testing support and National Resources Canada (NRCan), National Research Council of Canada (NRC-IRAP), and the US Department of Energy (DOE) for funding various aspects of this work. Reference 1. http://energy.gov/eere/fuelcells/doe-technical-targets-polymer-electrolyte-membrane-fuel-cell-components2. Egushi, M., Baba, K., Onuma, T., Yoshida, K., Iwasawa, K., Kobayashi, Y., Uno, K., Komatsu, K., Kobori, M., Nishitani-Gamo, M., Ando, T., Polymers, 4, p. 1645 (2012)3. Xie, J., Xu, F., Wood, D.L., More, K.L., Zawodzinski, T.A., Smith, W.H., Electrochimica Acta 55 p. 7404 (2010)4. Gode, P., Jaouen, F., Lindbergh, G., Lundblad, A., Sundholm, G., Electrochimica Acta 48 p. 4175 (2003)5. Young, AP., Gyenge, E., Stumper, J., J. Electrochem. Soc., 156, B913 (2009) 6. Young, AP., Knights, S., Gyenge, E., Stumper, J., J. Electrochem. Soc., 157, B425 (2010) Figure 1

During operational lifetime, proton exchange membrane fuel cells (PEMFCs) suffer from high performance losses. For the year 2025, the U.S. Department of Energy (DoE) set the durability target for PEMFC light-duty transportation applications at 5,000 hours.1 To meet the automotive target, different aging protocols were established in order to simulate load cycle variations.2 It is well known that load or voltage cycling induces substantial catalyst degradation due to an increasing loss of electrochemically active surface area (ECSA), which is known to be a main driver for the resulting performance penalties.3 Thus, an increasing interest exists in developing accelerated stress test (AST) protocols that can sufficiently describe the degradation of the different components of a membrane electrode assembly (MEA) during automotive application using shorter measurement times.In this study, we report on a strategy to predict the performance degradation of an MEA, making use of a strongly accelerated voltage cycling based ASTs. For this, voltage cycling based ASTs were performed using 5 cm2 MEAs with a 0.1 mgPt cmMEA -2 loaded Pt/C catalyst (TEC10V20E, Tanaka) for both cathode and anode. Voltage cycling was done under H2/N2 (200/75 nccm) at 80 °C, 95% RH, and ambient pressure using square wave profiles with a constant lower potential limit (LPL) of 0.6 V and different upper potential limits (UPLs) of 0.85, 0.95, and 1.0 V, with LPL/UPL hold times of 1, 2, or 8 s. Full characterization of the MEA at beginning-of-life and after each set of voltage cycling intervals was performed by: i) measuring H2/O2 and H2/air polarization curves; ii) determining the ECSA by cyclic voltammetry and CO stripping; iii) conducting limiting current measurement to calculate the O2 transport resistance (R O2 total); and, iv) conducting electrochemical impedance spectroscopy (EIS) measurements under blocking conditions to quantify the proton conduction resistance in the cathode catalyst layer (R H+ cath).When the ECSA loss ranges between ≈20-85%, it was found that the ECSA decreases approximately linearly when plotted against the logarithm of the number of cycles (see fig. 1), exhibiting higher slopes for procedures with higher UPLs and longer hold times, in agreement with the results from previous studies.3, 4 In addition, a direct and AST protocol independent correlation between the loss in total available surface area, i.e., the roughness factor (rf ≡ ECSA × Pt-loading), and the individual loss contributions in H2/air performance curves, namely kinetic (mass/specific activity for the oxygen reduction reaction (ORR)) and O2 transport resistance contributions, was shown. This is due to the fact, that these individual voltage losses are not only highly affected by the cathode rf,3, 5 but that the Pt dissolution/redeposition mechanism seems to be identical for all of the here investigated voltage cycling ASTs, despite the largely varying UPLs and hold times. As a result, a universal correlation between the H2/air performance losses and the rf deterioration was found throughout all ASTs. The important corollary of this finding is that the data from quickly degrading voltage cycling ASTs (i.e., with high UPL) can be used to project the H2/air performance losses when cycling under less degrading conditions.

Over the past several decades, planar proton exchange membrane fuel cells (PEMFCs) received considerable attention due to their simple structure, manufacturing, and easy integration. However, current PEMFC electrode designs are disadvantaged by high cost, insufficient mass transport, nonuniform reactant, current and temperature distribution, and limited water removal. The development of a novel tubular fuel cell design could address these challenges. Tubular-shaped PEMFC offers several advantages over planar one, including lower pressure drop, efficient water removal, reduced mass transport losses, and cost reduction owing to removing one of gas diffusion layer/bipolar plate (GDL/BPP) side.This work developed a carbon nanofiber (CNF)/Pt-based cathode for a tubular fuel cell. The tubular CNF support for Pt catalyst was fabricated employing the electrospinning method. Polyacrylonitrile (PAN) was used as the precursor. The electroless deposition of Pt using the chloroplatinic acid solutions was applied to produce well-dispersed low Pt loading nanowires. The tubular CNF/Pt electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), ex-situ cyclic voltammetry to determine the CF and Pt morphology and feasibility of the prepared layer in PEMFC application. SEM images indicated that increasing the Pt precursor concentration in the plating bath leads to higher Pt loading and larger Pt clusters on the CNF surface. The calculated electrochemical active surface area (ECSA) trend imparts that high Pt loading is less favorable as thick Pt layers lead to low Pt utilization and increased material costs. ECSA results agreed with SEM images, suggesting that the medium Pt concentration (1.5 g L-1 Pt precursor) developed a homogenous Pt distribution and comparable Pt surface area of 24 m2 gPt -1 for the Pt loading of 0.046 mg cm-2, proving the feasibility of the proposed process.

High performance and durability of order-structured cathode catalyst layer based on TiO2@PANI core-shell nanowire arrays