Polymer Electrolyte Fuel Cell Cathode Research Articles

Study of Ruthenium-Contamination Effect on Oxygen Reduction Activity of Platinum-Based PEMFC and DMFC Cathode Catalyst

We outline a systematic experimental and theoretical study on the influence of ruthenium contamination on the oxygen reduction activity (ORR) of a Pt/C catalyst at potentials relevant to a polymer electrolyte fuel cell cathode. A commercial Pt/C catalyst was contaminated by different amounts of ruthenium, equivalent to 0.15–4 monolayers. The resulting ruthenium-contaminated Pt/C powders were characterized by energy–dispersive x–ray spectroscopy (EDS), x–ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) to verify ruthenium contamination. A rotating disk electrode (RDE) technique was used to study the influence of ruthenium on oxygen reduction kinetics. Density functional theory (DFT) calculations were performed to estimate the oxygen reduction activity of the platinum surface with increasing ruthenium coverage, simulating ruthenium-contaminated Pt/C. The binding energies of O and OH on the surfaces were used for activity estimations. It was found that the specific activity of the ORR at 0.85 V vs RHE exhibited an exponential-like decay with increased ruthenium contamination, decreasing by ∼45% already at 0.15 monolayer-equivalent contamination. The results of the DFT calculations were qualitatively in line with experimental findings, verifying the ability of the chosen approach to predict the effect of ruthenium contamination on ORR on platinum.

High Surface Area Pt/C Electrocatalyst Modification with Ionic Liquids for Improved Ionic Conductivity in Polymer Electrolyte Fuel Cell Catalyst Layer

Oxygen reduction reaction (ORR), specifically in technological case of platinum (Pt) electrocatalyst, is one of the determining factors for commercialization and mass production of affordable fuel cell electric vehicles. ORR takes place at the triple phase boundary of the Pt electrocatalyst, the ionomer (Nafion) or water, and oxygen gas. In order to maximize the electrochemical surface area (ECSA) of the Pt nanoparticles, they are dispersed on high surface area (HSA) carbon supports and a large portion of them are buried inside the micro (< 2nm) and mesopores (< 50nm). Because of the size exclusion, ionomer cannot penetrate pores below 20 nm and water acts as the proton conducting media in these pores1. In addition, the dense polytetrafluoroethylene (PTFE) backbone blocks the oxygen transport. Therefore, the interface design principle in this work for improving catalyst layer ORR performance is to infiltrate micropores with ionic liquids (ILs) to facilitate sufficient proton and oxygen delivery to the Pt active sites. Ionic strength of ILs is within the range of 2.77 – 5.3×103 mol.m- 3 which is up to 7 orders of magnitude higher than that of water1. Higher ionic strength of electrolyte in micropores will ensure that electric double layers are thin and they do not exclude protons from entering the pores as fuel cells typically operate at higher potentials than the potential of zero charge of Pt. We implemented three imidazolium-derived ILs including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim]+[NTf2]-), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]-), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim]+[NTf2]-) to modify 40 wt.% Pt on HSA Ketjenblack EC-300J catalyst (Pt/C). Our RDE prescreening showed improved proton conduction and accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion making them great candidates for catalyst modification2. After optimization of ink recipe including IL to carbon and water to IPA ratios, the MEAs with 5cm2 active area were fabricated via decal transfer of anode and cathode catalyst layers on Nafion XL membrane. Figure 1a represents the fuel cell polarization curves of the IL-modified catalysts compared to the baseline Pt/C. Pt/C-[C4mim]+[NTf2]- shows the best performance and highest kinetic activity compared to the baseline and other two IL-modified catalyst layers. The open circuit voltages (OCVs) were 0.942, 0.933, 0.948, and 0.929 V for baseline Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-), respectively. Electrochemical impedance spectroscopy (EIS) data was used for measuring cathode catalyst layer ionic conductivity (Figure 1b), as Nyquist spectra represents a 45° line in the high frequency range where real and imaginary parts are equal, followed by a straight 90° line. A physical impedance model obtained from Obermaier et al.3 was fitted to EIS data, specifically at high frequencies to get the catalyst layer’s ionic conductivity. All the modified catalysts showed improved ionic conduction compared to the baseline Pt/C. Conductivity values for catalyst layers of Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-) with approximately 10μm thickness were 1.93, 3.95, 2.34, 2.04 S/m, respectively. Our fuel cell results show that high conductivity of ([C4mim]+[NTf2]-) along with its optimum chemical structure is responsible for improved performance across kinetic and ohmic regions compared to the baseline Pt/C. References Avid, A. & Zenyuk, I. V. “Confinement effects for nano-electrocatalysts for oxygen reduction reaction”. Curr. Opin. Electrochem. 25, 100634 (2021). Avid, A. & Zenyuk, I. V. Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. ECS Meet. Abstr. MA2020-02, 2155 (2020). Obermaier, M., Bandarenka, A. S. & Lohri-Tymozhynsky, C. A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere. Sci. Rep. 8, 4933 (2018). Figure 1

Development of Polymer Electrolyte Fuel Cell (PEFC) Cathodes with High Oxygen Permeability Ionomer (HOPI) for High Performance and Durability

The electrolyte-catalyst interface in the cathodes of polymer electrolyte fuel cells (PEFCs) has a significant impact on the utilization, performance, and durability of fuel cell systems. The ionomer plays a critical role in providing a facile proton conduction pathway from the membrane to the Pt-based catalyst and creating an acidic local environment for the oxygen reduction reaction with high specific activity. However, the commonly used perfluorosulfonic acid (PFSAs) ionomers also negatively impact performance by hindering oxygen transport and partial poisoning of the catalyst by the anionic acid groups. There is a growing consensus that the densification of the PFSA ionomers at the Pt-alloy catalyst greatly increases the local oxygen transport resistance relative to permeation through bulk PFSA films. To address this issue, new high oxygen permeability ionomers (HOPIs) that resist the densification are being developed. This presentation will present our development of membrane electrode assemblies (MEAs) with HOPI in the cathode. The results include a model-based analysis, ink and coating optimization, and experimental fuel cell and transport characterization. The results highlight the significant increase in voltage efficiency and maximum power density when fabricating cathodes with HOPI as well as the significant positive impact on performance durability.This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office, Award Number DE-EE0008822.

PEFCs Using Metallic Ti and Sn Electrocatalyst Supports

Introduction Currently, Pt-based catalyst on carbon black support (Pt/C) is widely used as the electrocatalyst for polymer electrolyte fuel cells (PEFCs). Carbon black support has high electronic conductivity and high specific surface area for supporting highly-dispersed Pt catalyst nanoparticles. However, under cathode condition of PEFCs e.g. during start-stop cycles, carbon black support can be electrochemically oxidized, causing detachment and aggregation of Pt catalyst nanoparticles (1). In order to solve this issue, we are developing SnO2 and TiO2 supports, which are relatively stable even under cathode conditions, coated on carbon-based materials as the conductive backbone, exhibiting high durability (2, 3). However, as far as we use carbon materials, carbon corrosion problem may still be remained. Here in this study, porous metallic Ti and Sn sheets are applied as catalyst support. Such electrodes may exhibit high durability under PEFC cathode conditions due to the formation of stable TiO2 and SnO2 layers on the outermost surface of Ti and Sn metals. Furthermore, metallic sheets with porous structures also have a potential to act as gas diffusion layers (GDL). This study aims to prepare such electrodes integrating electrocatalysts with high conductivity and high durability. Experimental For preparing metallic Ti supported electrodes, porous Ti sintered sheets were used. First, the porous Ti sintered sheets were chemically etched in NaOHaq to increase their surface area. The etched Ti sheets were then heat-treated at 400 °C in 5%H2-N2. Pt catalyst nanoparticles were then decorated on the Ti sheets by the arc plasma deposition (APD) or the acac method (4). We also prepared Nb-doped Ti sheets to improve their electronic conductivity. For preparing metallic Sn supported electrodes, cellulose-based filter paper was used as a template. First, SnO2 sol was impregnated into the filter papers in vacuum, followed by heat treatment in oxidizing atmosphere to remove the cellulose-based templates. After that, porous metallic Sn sheets were obtained by reducing heat-treatment. The Pt catalyst nanoparticles were decorated on the porous Sn sheets by the acac method (4). Microstructures of the porous Pt/Ti and Pt/Sn sheets were observed by FE-SEM and STEM-EDS. Their electrochemical activities were measured in a half-cell setup in HClO4 electrolyte. Results and Discussion Figure 1 shows an FE-SEM image of the porous Ti sheet after the surface treatment by NaOHaq. The surface of the Ti sheets after the surface treatment has needle-like TiO2 nanostructures with a diameter of several nm, with high surface area. Figure 2 shows the linear sweep voltammograms by the half-cell tests for the Pt/Ti sheet electrodes. The Pt/Ti sheet electrode prepared by the acac method may have higher contact resistance than the Pt/Ti sheet electrode by the APD method. However, such contact resistance may be reduced by Nb-doping the Ti sheets. Figure 3 shows an FE-SEM image of the prepared Sn sheet for catalyst support. We could prepare metallic Sn sheets with porous structures derived from the filter paper template. However, optimization of heat-treatment conditions is further required. It is because the reduction of SnO2 is difficult if the heat-treatment temperature is too low, but Sn will be melted if the temperature is too high. Acknowledgement Financial support from New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged (Contract No. 20001214-0). References L. M. Roen, C. H. Pail, and T. D. Jarvi, Electrochem. Solid-State Lett., 7 (1), A19 (2004). S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), 1165 (2018). Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 80 (8), 897 (2017).A. Hayashi, H. Notsu, K. Kijima, J. Miyamoto, and I. Yagi, Electrochim. Acta, 53 (21), 6117 (2008). Figure 1

Noble Metal-Added Titanate Nanosheets for PEFC Cathode

A large overpotential of oxygen reduction reaction (ORR) at a cathode in polymer electrolyte fuel cells (PEFCs) is one of the serious problems to decrease the energy conversion efficiency. Nørskov’s group reported that even Pt which is currently used as an ORR catalyst cannot reach the theoretical oxygen electrode potential of 1.23 V at 298 K [1]. On the other hand, Yamamoto et al. recently suggested that noble metal doped titanium dioxides (TiO2) can reach the theoretical potential by changing the absorption energies of the ORR intermediates according to the first-principle calculations [2]. However, the first-principle calculations did not consider the continuous electron supply to proceed the ORR. It is difficult to form the sufficient electron conduction path from supports to active sites on the TiO2 because TiO2 has low conductivity due to a large band gap. Therefore, we focused on the formation of the active sites on the TiO2 surface by doping noble metal such as Rh and Pd and the electron conduction path from carbon supports to the active sites by preparing the titanate nanosheets.Na2Ti3O7 was synthesized as precursor of anionic titanate nanosheets([Ti3O7]2- ;TiNS) according to the previous paper [3]. Na2Ti3O7 was treated through ion-exchange reaction with hydrochloric acid and tetramethylammonium aqueous solution, finally TiNS were obtained. A TEM observation revealed that the formation of very large sheet-like materials with a size of approximately upwards of ten micrometers, resulting that TiNS were completely synthesized.Next, we focused on the formation of the electron conduction path through nanosheets from supports using the electron tunneling effect. When the nanosheets were directly supported on the glassy carbon electrode, the ORR activity was the same as that of GC, and almost no ORR current flowed even low potential region. Then, vapor grown carbon fibers (VGCFs) as purchased, with immersion of mixed acid (MA; H2SO4 and HNO3) at 170 oC for 24 h and PDDA (Poly(diallyl dimethylammonium chloride)) as polycation were attempted to use as supports. The TiNS were immersed in the distilled water dispersed with the treated VGCFs to be supported by them. These catalysts were designated as TiNS/VGCF, TiNS/VGCF(MA), and TiNS/VGCF(MA-PDDA).The ORR current of TiNS/VGCF(MA-PDDA) was larger than that of TiNS/VGCF, TiNS/VGCF(MA). In addition, the ORR onset potential of the TiNS/VGCF(MA-PDDA) increased by 0.3 V compared to that of TiNS/VGCF. These results suggest that surface treatment of VGCFs was effective to form the electron conduction path from VGCF to active sites of nanosheet surface. The electrostatic interaction between VGCF surface positively charged by PDDA and anionic TiNS contributed the formation of the electron conduction path. However, according to the TEM observation, TiNS were partially contacted with VGCF and some TiNS were agglomerated. It is necessary to obtain higher ORR activity that a control of size of TiNS and contact with supports such as VGCF.On the other hand, we attempted to dope Rh and Pd to TiNS through solid-state reaction. Rh-,Pd-doped TiNS (designated as Rh-TiNS and Pd-TiNS) were synthesized the same way as previous paper [3]. XRD patterns of the precursors revealed that Rh2O3 or PdO were existed with Na2Ti3O7 above the addition of 5 atomic %. Rh-TiNS and Pd-TiNS were supported by VGCF(MA-PDDA) after Rh-TiNS and Pd-TiNS were treated TMAOH aqueous solution. The ORR activities of the samples with the addition of 5at% Rh or 10at% Pd were higher than those of the samples which Rh2O3 or PdO were not observed in XRD patterns. The ORR active sites were estimated by Rh2O3 or PdO deposited on the surface of the TiNS. This result showed that an amount of doped Rh or Pd was limited to Na2Ti3O7. The limitation of doped Rh or Pd was 5at% or 10at%, respectively. Therefore, the doped amounts of noble metals such as Rh or Pd to TiNS should be appropriately controlled.AcknowledgementThe authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.Reference(1) A. Kulkarni et al., Chem. Rev., 118, 2302 (2018).(2) Y. Yamamoto et al., J. Phys. Chem. C., 123, 19486 (2019).(3) W. Soontornchaiyakul et al., RSC Adv., 7, 21790 (2017). Figure 1

Noble Metal-Added Titanate Nanosheets for PEFC Cathode

Improvement of the activity of the oxygen reduction reaction (ORR) at a polymer electrolyte fuel cell (PEFC) cathode is one of the serious problems to widespread PEFCs. We focused on Rh- or Pd-doped titanate nanosheets (TiNSs) to form the appropriate electron conduction path using tunneling effect of electrons and high quality active sites with Rh or Pd doping. To apply the tunneling effect of electrons to TiNSs, TiNSs were attempted to be supported on vapor growth carbon fiber (VGCF) with a large diameter. The surface of the VGCF was modified by poly (​diallyl dimethylammonium) chloride (PDDA) as polycation because the TiNSs were anionic. The electrostatic interaction improved slightly the contact between the PDDA modified VGCF and the TiNSs to increase the ORR activity of the TiNSs. Rh- or Pd-added TiNSs were successfully synthesized via the solid phase method and ionic exchange reaction processes.1 atomic % addition of Rh and Pd was found to be effective to enhance the ORR activity of the TiNSs.

(Invited) In Situ and Operando Synchrotron X-Ray Spectroscopy and Scattering Characterization of PEFC Cathode Catalysts

With wavelengths on the order of atomic dimensions and the ability to penetrate through low atomic number materials (e.g., Nafion® and carbon), hard X-rays, particularly those arising from synchrotron sources with high brilliance, are ideal for the atomic-level characterization of polymer electrolyte fuel cell (PEFC) catalysts in situ, in aqueous environments that mimic PEFC conditions, and operando, in the membrane-electrode assembly (MEA) environment. Two particularly valuable X-ray techniques for this purpose are X-ray absorption spectroscopy (XAS) and small angle X-ray scattering (SAXS) or anomalous SAXS (ASAXS), which allows discrimination of scattering from the absorbing element. Analysis of the X-ray spectrum near the absorption edge of interest (i.e., the near edge or XANES region) provides information on the oxidation state and electronic structure (e.g., d-band vacancy) of the material, properties that have been associated with catalyst activity. The extended region of the XAS spectrum (EXAFS) yields information on the atomic structure and coordination environment of the absorbing element (i.e., number and identity of nearest neighbors and distances between atoms). Unlike X-ray diffraction, XAS is particularly useful for characterization of platinum nanoparticle catalysts and of single-site platinum-group metal-free catalysts, such as Fe-N-C catalysts. However, EXAFS can be limited in the information it can provide regarding the bonding geometries, but absorption in the pre-edge region, which is sensitive to bonding geometry, can provide this essential information. SAXS and ASAXS are well-established techniques for probing particle sizes and particle size distributions on the nanometer level. While not as widely-used in the fuel cell community as XAS, SAXS and ASAXS are well-suited to examining nanoparticle fuel cell catalysts, which typically have particle diameters of 2-5 nm.This presentation will discuss the information that can be obtained from X-ray spectroscopy and scattering techniques and provide examples of application of these techniques to the study of platinum and platinum alloy nanoparticle catalysts and atomically-dispersed Fe-N-C and Co-N-C PEFC cathode catalysts. The utility of using a combination of XAS and ASAXS to study platinum alloy catalyst degradation and of the combination of information from XANES, EXAFS, and XAS pre-edge data to an Fe-N-C catalyst will be presented.Acknowledgements:This research was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO), through the Electrocatalysis Consortium (ElectroCat). This research used resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. The authors thank Hoon Chung of Los Alamos National Laboratory for providing the Fe-N-C catalyst and Johnson Matthey Fuel Cells for providing the Pt and PtCo catalysts. This work was conducted at Argonne National Laboratory, a U.S. DOE Office of Science Laboratory operated for the U.S. DOE by UChicago Argonne, LLC, under Contract DE-AC02-06CH11357.

Non-precious metal cathodes for anion exchange membrane fuel cells from ball-milled iron and nitrogen doped carbide-derived carbons

Iron and nitrogen doping of carbon materials is one of the promising pathways towards replacing Pt/C in polymer electrolyte fuel cell cathodes. Here, we show a synthesis method to produce highly active non-precious metal catalysts and study the effect of synthesis parameters on the oxygen reduction reaction (ORR) activity in high-pH conditions. The electrocatalysts are prepared by functionalizing silicon carbide-derived carbon (SiCDC) with 1,10-phenanthroline, iron(II)acetate and, optionally polyvinylpyrrolidone, by ball-milling with ZrO2 in dry or wet conditions, followed by pyrolysis at 800 °C. The catalysts are characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, N2 physisorption and inductively coupled plasma mass spectrometry. By optimizing the ball-milling conditions, we achieved a reduction in the size of SiCDC grains from >1 μm to 200 nm without negatively affecting the high BET area of catalysts derived from SiCDC. This resulted in increased ORR activity in both rotating disk electrode and anion exchange membrane fuel cell (AEMFC) environments, and improved mass-transport properties of the cathode layer in fuel cell. The ORR activity at 0.9 V in AEMFC of the optimized iron and nitrogen-doped SiCDC reaches 52 mA cm−2, exceeding that of a Pt/C cathode at 36.5 mA cm−2.

(PEFC&E 2nd Place Best Poster Winner) Simulation of Reaction and Mass Transport in PEFC Cathode Catalyst Layer Fabricated by Inkjet Printing Method

Polymer electrolyte fuel cell (PEFCs) is a potential power source for new-generation automobiles like fuel cell vehicles that run on hydrogen energy, it has been studied a lot for last decades in order to use as the power sources of automotive. The most important characteristics of PEFCs are low operation temperature and quick start and shutdown. However, the properties of PEFCs are still limited due to its high cost and insufficient durability and performance.Since oxygen reduction reaction (ORR) at the cathode is the dominant reaction in PEFCs, oxygen proton and electron have to be transferred smoothly in porous catalyst layer. Under normal operating conditions, oxygen transfer is the dominant factor of limiting current density, therefore, oxygen transfer must be increased from the gas channel to the Pt surface through various porous media and various components.On the other hand, inkjet printing has been considered to be an economical and easy scale-up technology for the micro-scale patterning and fabrication as one of the on-demand methods in a variety of field because it allows for the patterning of various ink. In addition, it can reduce the process time, cost and the toxic waste created during the manufacturing process. Also, this method can be easily applied to perform ink composition studies. Especially for fuel cell applications, inkjet printing resolves many of the problems associated with previous methods of catalyst deposition by allowing a uniform distribution of catalyst ink onto the surface of GDL or membrane and by loading a very low amount of catalyst.So in this research, in order to increase the performance of polymer electrolyte cells, the oxygen diffusion resistance was tested based on the difference of surface of catalyst layer in cathode by using inkjet method. Firstly, we mixed carbon supported Pt catalyst, distilled water and 20wt% Nafion solution and NPA together and then use homogenizer to disperse the liquid, then we got the catalyst ink. The Second step was fabrication of cathode catalyst layer (CCL) by using inkjet method. Third step was to observe the morphology for CCL with catalyst ink, in order to know the length and the height of a drop of ink, we made a sample, and then we took this sample to observe through laser microscope. When we got the basic information about morphology of CCL, we fabricated catalyst layer by inkjet, and took it to do the MEA evaluation and characterization to investigate their electrochemical properties, such as I-V curves, limiting current densities and impedance. And in order to compare with the experimental results, a theoretical model of the 3D structure was also circulated and discussed. From this model and measured data, it was confirmed that the better electrochemical properties results from 3D structure of catalyst layer in cathode.

P-Doped SnO2 Powder as a Support for PEFC Cathode

Polymer electrolyte fuel cells (PEFCs) are used as power sources for residential and transportation use. In the present PEFCs, a large amount of platinum supported carbon is used as a catalyst in the cathode electrode. However, a platinum catalyst has some problems such as high cost, small resources, low durability, and large overvoltage for oxygen reduction reaction (ORR)considering the future widespread use of PEFC2. In addition, the durability of carbon support has become a serious problem especially under future driving conditions3 (high temperature and high voltage) for automobiles. In order to solve the problem, there is a strong demand for substitution of highly durable materials, that is, a non-platinum catalyst and a non-carbon support. Therefore, we have developed platinum substitution catalysts using group 4,5 metal oxides4.Regarding non-carbon supports, we have focused on Nb-doped TiO2 because of its high stability in acidic media. However, the specific surface area and the electro-conductivity of Nb-doped TiO2 are insufficient to support an oxide catalyst. Therefore, in this study, we focused on P-doped SnO2 (Mitsubishi Materials), which has excellent specific surface area, electronic conductivity, and stability under oxidative atmosphere, and examined its applicability as a cathode support.On the other hand, regarding the active sites, even carbon was used as a support, the oxygen reduction activities of group 4 and 5 oxide cathodes were much lower than that of platinum. This is because both quality and quantity of the active sites of oxide-based cathodes are lower than that of platinum. Recently, according to the first-principles calculation, Sugino et al, found that when Pd is doped into TiO2, the onset potential for ORR exceeds platinum (Because the adsorption Gibbs energies of the reaction intermediates of ORR are adjusted, the energy barrier along the reaction path become lower. The quality of active sites is improved by Pd doping into TiO26). Although no calculations have been performed for tin oxide, we focused on Pd doping into SnO2 to form the active sites in this study. Therefore, the purpose of this study was to modify P-doped SnO2 with Pd and investigate its catalytic activity.A 2 mM H2PdCl4 aqueous solution was mixed with ethanol, and P-doped SnO2 (Mitsubishi Materials) as a support was further added. An electron beam irradiation was carried out at room temperature. The conditions of the electron beam irradiation were as follows; electron beam energy of 2 MGy, absorbed dose to water of 500 kGy, and irradiation time of 50 min. Sample A was prepared by mixing 20 mL of a 2 mM H2PdCl4 aqueous solution and 1 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 0.05. Sample B was prepared by mixing 3.4 mL of 2 mM H2PdCl4 aqueous solution and 17 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 5. A loading Pd was adjusted to 1 wt% with respect to P-doped SnO2 in both samples.Figure 1 shows the comparison of potential-current curves for the ORR of Pd-modified P-doped SnO2. Sample A had higher catalytic activity than sample B. It was found that the ORR activity was improved by reducing the volume ratio of (ethanol) / (H2PdCl4) at the catalyst preparation.AcknowledgmentThis research is supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Strategic International Joint Research Program (SICORP) of the Japan Science and Technology Agency (JST). In addition, the authors wish to thank the support of JSPS grants-in-aid for scientific research, Suzuki Foundation and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation.

PEFC Cathode Catalysts of ZrO2/C Prepared By RF Magnetron Sputtering and Crosslinked Polymer Methods

Fuel cells are expected to be clean power generation devices in the future energy society. Various types of fuel cells, which are different in electrocatalysts, electrolytes, and operating temperatures, are available for practical use in various places depending on their characteristic. Among them polymer electrolyte fuel cells (PEFC) have been applied to fuel cell vehicles (FCV), household cogeneration systems and etc. PEFC uses large amounts of Pt for electrocatalysts, which is one of the serious problems for large scale commercialization. The worldwide production of Pt is about 180 ton/year, and about 60 g of platinum is practically used in one FCV. If all of the world production of Pt is used for FCV, only 3 million of FCV can be manufactured in a year, although the annual production of cars in the world is about 100 million. Therefore, the development of electrocatalysts with non-Pt materials is indispensable for the broad spread of PEFC. The cathode catalysts significantly affect to the cell voltage, current density, and durability, so that the developments of alternative cathode materials to conventional Pt-based catalysts are expected. Compounds of oxides of group 4 and 5 elements transition metals such as Zr, Ta, Nb, and Ti, chalcogenides such as Ru and Mo, and carbon materials with N-containing complexes of Fe and Co, carbon materials doped with B and N are intensively investigated in the past decade. In this study, ZrO2/C catalysts were investigated for oxygen reduction reaction (ORR) activity in acidic media. Two types of preparation methods have been examined; radio-frequency magnetron sputtering and crosslinked polymer methods.In magnetron sputtering method, carbon black (CB) powders were stirred in a rotary drum chamber with milling balls and cubes. A Zr metal target was attached to a sputtering gun placed in the stirring drum. Sputtering was performed in 10 Pa of Ar. In crosslinked polymer method, ammonium zirconium carbonate (AZC) was used as a crosslink reagent for polyvinyl alcohol (PVA). Addition of citric acid (CA) was found to be effective. Mixture of AZC, PVA, and CA were well stirred at 80ºC, and CB was immersed in the mixture. The mixture was dried at 300ºC in air and them heat treated 600~1200ºC in N2 or H2. The catalyst ink was obtained by ultrasonically dispersing 300 µL of isopropanol (IPA), 4 mg of catalyst and 12 µL of 5% Nafion-dispersed solution. The sample electrode was prepared by dropping 150 µL of the catalyst ink on glassy carbon (GC) and drying. ORR activity was measured in 0.1 M H2SO4 with O2 bubbling.ORR activity of ZrO2/C catalysts which were prepared by sputtering method was shown in the attached figure. This figure shows ORR properties of ZrO2/C catalysts prepared at various sputtering times in 0.1 M H2SO4. ZrO2/C was pretreated in H2 flow at 1000˚C for 3 h. ORR current was difference between current in O2 and N2 saturated solutions. ORR Current appeared around 0.8 VR HE for the catalysts deposited longer than 40 min. This result indicate ZrO2/C have obvious ORR activity. X ray diffraction (XRD) of ZrO2/C showed the formation of both monoclinic and tetragonal ZrO2 crystals. ZrO2 particles with diameter in several nm scale were observed by transmission electron microscopic (TEM) images, and formation of these particles were concluded to contribute the obvious ORR activity. On the other hand, ZrO2/C catalysts which were prepared by crosslinked polymer methods also showed similar ORR activity to those by sputtering method. The details were discussed in the presentation.This work was supported by ESICB, Kyoto University and New Energy and Industrial Technology Development Organization (NEDO) of Japan. Figure 1

Evolution of Ionomer Coverage during Accelerated Stress Tests in Polymer Electrolyte Fuel Cells

Despite the current readiness level and the market availability of the polymer electrolyte fuel cells (PEFCs) durability is still a technical challenge that the technology, has to resolve to become competitive with internal combustion engines (ICE). In the automotive sector, the ICE engines typically reach lifetimes of more than a decade. In the nearest future PEFCs can become competitive in the medium to heavy-duty vehicles sector, where the space constraints and the power density requirements are more relaxed, but cell lifetime becomes of critical importance for such vehicles, that drive for up to 100,000 miles per year. Accelerated stress tests (ASTs) are used to assess the lifetime of a PEFC in a limited amount of time. the US Drive Fuel Cell Technical Team of the US DOE adopted several protocols targeting specific components of the cell or ageing phenomena (platinum dissolution or carbon corrosion).In the PEFC cathode electrode ionomer is added to enable proton transport to the electrocatalyst. Ionomer forms thin films around Pt/C agglomerates. The SO3 - groups (from the side chains of PFSA-based polymer ionomers) adsorb onto the platinum surface and reduce its overall specific activity. Additionally, when Nafion ionomer that has crystalline backbone aligns parallel to the catalyst surface it impedes local oxygen transport1 . Recent study did not show direct correlation between SO3 - group coverage and specific mass activity, when the coverage was varied from 0.16 to 0.262 . This may be because even with 0.16 coverage Pt has already been ‘poisoned’ and further addition of ionomer can actually have a positive effect enhancing proton accessibility. Understanding the behavior of the ionomer in the electrode and its evolution during the cell lifetime is of great importance, but at the present the literature does not show durability studies focused on this aspect.In this contribution, we aim to investigate and understand the coverage of the ionomer on Pt and carbon and SO3- groups adsorption in a PEFC during ASTs. CO stripping and displacement were used to probe the ionomer coverage as described by Garrick et al.3 Two types of cells were investigated using carbon corrosion AST: the first one with traditional carbon material and the second one with start/stop tolerant catalyst support. While the tolerant support led to small performance decrease and ECSA loss, the cell without start/stop tolerant support saw a sharp decrease in the ionomer coverage in the electrode between 100 and 500 cycles, that corresponded to a considerable loss in polarization. The ECSA was not changed between 100 to 500 cycles indicating that Pt loss was not a reason for polarization decrease and therefore we attribute the cause of the cell failure mainly to the loss of ionomer.Catalyst AST was also investigated, and the ionomer coverage was measured both in dry and wet conditions to separate the contribution of the interfaces between platinum/ionomer and platinum/water as in Iden and Ohma work4. By comparing the ECSA in dry and wet conditions, we see that while ECSA in wet condition decreases (that corresponds to the total ECSA reachable by protons) the dry ECSA (that considers only the Pt portion contacted by ionomer) remains constant. This behavior, coupled with an ionomer coverage that remains rather constant during the cycling, suggests that Pt dissolution is accelerated in the regions where Pt is not covered by ionomer. This work highlights the importance of the ionomer coverage and its evolution during aging. It suggests the role of ionomer is critical for catalyst stability in both carbon corrosion and catalyst ASTs.

Mapping Localized Peroxyl Radical Generation on a PEM Fuel Cell Catalyst Using Integrated Scanning Electrochemical Cell Microspectroscopy.

Understanding molecular-level transformations resulting from electrochemical reactions is important in designing efficient and reliable energy technologies. In this work, a novel integrated scanning electrochemical cell microspectroscopy (iSECCMS) capability is developed by combining a high spatial resolution electrochemical scanning probe with in situ fluorescence spectroscopy. Using 6-carboxyfluorescein as a fluorescent probe, the iSECCMS platform is employed to measure the effect of the detrimental generation of reactive oxygen species (ROS) formed at the active sites of oxygen reduction reaction (ORR) catalysts. Carbon-supported tantalum-doped titanium oxide (TaTiOx) catalysts, a potential Pt-group-metal-free (PGM-free) cathode material explored for low temperature polymer electrolyte fuel cells (PEFCs), is used as a representative model ORR system, where generation of intermediate H2O2 instead of fully oxidized H2O is a major concern. We establish that the iSECCMS platform provides a novel and versatile capability for spatially resolved mapping of in situ ROS generation and activity during the kinetically-limited ORR and may, therefore, aid the future characterization and development of high-performance PGM-free PEFC cathodes.

Efficient Phosphorus Doping into the Surface Oxide Layers on TiN to Enhance Oxygen Reduction Reaction Activity in Acidic Media

Oxide catalysts have been developed in polymer electrolyte fuel cell cathodes to replace platinum-group-metal catalysts while retaining moderate oxygen reduction reaction (ORR) activity. Recently, ...

Simulation of Reaction and Mass Transport in PEFC Cathode Catalyst Layer Fabricated by Inkjet Printing Method

The structure of cathode catalyst layer (CCL) is critically important for improved performance, durability, and stability of polymer electrolyte fuel cells (PEFCs). In this research, two types of CCL are designed to do comparison, one is flat structure, and the other is 3D structure. Inkjet printing method was used in this research to fabricate the structures of cathode catalyst layer, then the MEA was taken to do electrochemical evaluation. The results showed that electrochemical properties of CCL with 3D structure were better than the flat one through I-V curve and impedance. Overall, it is demonstrated that with a 3D structure, it is possible to achieve favorable morphological and microstructural characteristics.

Durability evaluation of a Fe–N–C catalyst in polymer electrolyte fuel cell environment via accelerated stress tests

Abstract An “atomically-dispersed” iron-nitrogen-carbon (Fe–N–C) catalyst was used to provide a systematic comparison of platinum group metal (PGM)-free electrocatalyst degradation as a function of accelerated stress tests (ASTs) in an acidic polymer electrolyte fuel cell (PEFC). It was determined that the majority of catalyst degradation was caused by cell operation in presence of O2. In contrast, potential cycling of the Fe–N–C-containing cathode under inert atmosphere over typical PEFC cathode operation from 0.95 to 0.6 V had little to no effect. The increase in kinetic overpotential is shown to be the major source of the PEFC performance decrease during the ASTs. These results continue to showcase the need for development of robust PGM-free electrocatalysts in concert with improved electrochemical performance.

Meso-Structured Polymer Electrolyte Fuel Cell Electrode

Increasing the utilization of Pt and Pt alloy catalysts in polymer electrolyte fuel cell cathodes is critical to improving the high power density operation, particularly at low Pt loadings. State of the art electrodes are fabricated in an ink deposition process that leads to uncontrolled electrode architecture with random aggregates of functional domains (catalyst, ionomer, and pore volume) (1). The randomness in the domains induces high tortuosity transport pathways for ions and fluids, which cause severe transport resistance during high current density operation. Thin ionomer films cause additional transport resistance and poisoning of the Pt catalyst, which becomes more significant at low Pt loadings. Reducing the amount of ionomer in the catalyst domain without affecting the ionic transport resistance is key to improving the utilization of the Pt and reducing the transport resistance at low Pt loading.Rational design of the electrode structure with controlled low tortuous ionic transport pathways could improve performance. The introduction of the ionomer pathways could also enable reduction of the ionomer volume in the catalyst domain, reducing the transport resistance. Middelmen et al. proposed electrode structures consisting of aligned components in a low tortuosity configuration to improve performance (2). In this work, we present an alternative electrode structure based on a vertically aligned array of Nafion pillars in the cathode catalyst layer, as shown in Figure 1a. Figure 1b shows the SEM image of the Nafion pillars. Pt supported on carbon catalyst was deposited on the Nafion pillars to fabricate a meso-structured electrode. Nafion pillars provide high conductive and low tortuous pathways for protons, reducing the effective transport distance, and enabling reduction of the ionomer binder in the catalyst domain. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

Enhancement of service life of polymer electrolyte fuel cells through application of nanodispersed ionomer.

In polymer electrolyte fuel cells (PEFCs), protons from the anode are transferred to the cathode through the ionomer membrane. By impregnating the ionomer into the electrodes, proton pathways are extended and high proton transfer efficiency can be achieved. Because the impregnated ionomer mechanically binds the catalysts within the electrode, the ionomer is also called a binder. To yield good electrochemical performance, the binder should be homogeneously dispersed in the electrode and maintain stable interfaces with other catalyst components and the membrane. However, conventional binder materials do not have good dispersion properties. In this study, a facile approach based on using a supercritical fluid is introduced to prepare a homogeneous nanoscale dispersion of the binder material in aqueous alcohol. The prepared binder exhibited high dispersion characteristics, crystallinity, and proton conductivity. High performance and durability were confirmed when the binder material was applied to a PEFC cathode electrode.

Effects of Nitrogen-Doped Carbon Support on Oxygen Reduction Reaction Activity of Pt–Ni Nanoframe

Polymer electrolyte fuel cells (PEFCs) are clean, sustainable devices and can produce electricity without the emission of CO2 gas. The oxygen reduction reaction (ORR) catalysts are required for the cathode in PEFCs because the ORR is sluggish. Pt shows the highest ORR activity in the metals. The ORR activity of Pt has been improved by alloying with 3d transition metals such as Ni,1) and Pt–Ni alloys have been used for the cathode in PEFCs. The ORR activity was also improved by using nitrogen-doped carbon (NC) as supports for PGM nanoparticles.2) This is because platinum–nitrogen interaction finely tunes the electronic structure of catalyst surface. Herein, we studied Pt–Ni nanoframe3) supported on NC (Pt–Ni NF/NC) or non-doped carbon black (Pt–Ni NF/C) for the ORR.4) In order to clarify support effects on the Pt–Ni NF, we performed in situ Pt L3-edege X-ray absorption fine structure (XAFS) measurements of Pt–Ni NF/NC and Pt–Ni NF/C. The ORR activity of Pt­–Ni NF/NC was higher than that of Pt­–Ni NF/C. In situ Pt L3-edge XAFS spectroscopy of Pt­–Ni NF/NC showed almost no potential dependence of the intensity of the white line region, whereas Pt­–Ni NF/C showed some changes. This result suggests that the formation of Pt surface oxide was suppressed for Pt­–Ni NF/NC under electrochemical conditions, leading to the improvement of the ORR activity. Acknowledgments This work was supported by NEDO. References 1) V. Stamenkovic et al., Angew. Chem., 118, 2963-2967 (2006)2) Y. Zhou et al., Energy Environ Sci., 3, 1437 (2010)3) C. Chen et al., Science, 343, 1339-1343 (2014).4) M. Kato et al., ACS Omega, 3, 9052‒9059 (2018).

Sol-Gel Assisted Synthesis of Encased Polymer Electrolyte Fuel Cell Cathode Catalysts

State-of-the-art polymer electrolyte fuel cell (PEFC) cathode catalyst layers (CCLs) are composites of proton-conducting perfluorosulfonic acid (PFSA) ionomer and platinum alloy catalyst particles. The PFSA is in the form of agglomerates dispersed throughout the CCL and also as a thin film coating the carbon support and platinum alloy particle surface.1 The thin PFSA film on the catalyst surface has been shown to both decrease oxygen reduction reaction (ORR) activity and cause high oxygen transport resistance.2-4 Such ionomer-catalyst surface interactions can negatively affect both PEFC performance and durability. This talk will describe an approach to addressing the issues of ionomer poisoning of ORR activity, local oxygen transport resistance, and the lack of PEFC cathode performance durability by encasing Pt and Pt alloy catalyst particles in the cavity of a 3D porous inorganic framework. Platinum (Pt) and platinum alloys (PtxMy) encased in 3D porous inorganic nanocrystal catalysts have been prepared by a sol-gel process combined with chemical reduction. Catalysts with 5, 10 and 20 Pt/PtxMy wt% have been prepared and their ORR activity has been evaluated using the thin-film rotating disk electrode (TF-RDE) technique. The platinum content in the encased catalysts has been determined using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF). The morphology, crystal structure, and spatial distributed of the metal nanoparticles have been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy, respectively. References C. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7, 1127 (2016).Subbaraman, D. Strmcnik, A. P. Paulikas, V. R. Stamenkovic, and N. M. Markovic, Chemphyschem, 11, 2825 (2010).Subbaraman, D. Strmcnik, V. Stamenkovic, and N. M. Markovic, J. Phys. Chem. C, 114 8414 (2010). Acknowledgements This work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.