High Surface Area Carbon Support Research Articles

Selective transformation of carbohydrates to 5-ethoxymethylfurfural over tungstophosphoric acid supported on activated carbon derived from orange peel

A series of tungstophosphoric acid supported on activated carbon derived from left-over orange-peel catalysts (TPA/OAC) have been prepared. These catalysts were examined for one-pot catalytic transformation of fructose/glucose to 5-ethoxymethylfurfural. Physico-chemical properties of the catalytic materials were executed by using various characterization methods. Spectroscopic analysis results propose that TPA was finely distributed on the high surface area carbon support with persistent Keggin ion structure. The EMF yield is depended on the content of active component TPA on support and also on the reaction conditions. The catalyst with 25 wt% TPA on OAC presented highest activity towards EMF synthesis from fructose. Although, the catalyst 25 wt% TPA/OAC showed low activity towards EMF synthesis from glucose, higher yields of ethyl glucopyranoside could be formed from glucose in EtOH. Activated carbon was synthesized by carbonization of orange peel treated with H3PO4, which was used as a good support for HPAs. The catalysts are quite stable and recyclable for the dehydrative alkylation of fructose.

Insight into Oxygen Transport in Solid and High-Surface-Area Carbon Supports of Proton Exchange Membrane Fuel Cells

Understanding the oxygen transport mechanism through an ionomer film that covered the catalyst surface is essential for reducing local oxygen transport resistance and improving the low Pt-loading proton exchange membrane fuel cell performance. Besides the ionomer material, the carbon supports, upon which ionomers and catalyst particles are dispersed, also play a crucial role in local oxygen transport. Increasing attention has been paid to the effects of carbon supports on local transport, but the detailed mechanism is still unclear. Herein, the local oxygen transports based on conventional solid carbon (SC) and high-surface-area carbon (HSC) supports are investigated by molecular dynamics simulations. It is found that oxygen diffuses through the ionomer film that covered the SC supports via "effective diffusion" and "ineffective diffusion". The former denotes the process by which oxygen diffuses directly from the ionomer surface to the Pt upper surface through small and concentrated regions. In contrast, ineffective diffusion suffers more restrictions by both carbon- and Pt-dense layers, and thus, the oxygen pathways are long and tortuous. The HSC supports exhibit larger transport resistance relative to SC supports due to the existence of micropores. Also, the major transport resistance originates from the carbon-dense layer as it inhibits oxygen from diffusing downward and migrating toward the pore opening, while the oxygen transport inside the pore is facile along the pore's inner surface, which leads to a specific and short diffusion pathway. This work provides insight into oxygen transport behavior with SC and HSC supports, which is the basis for the development of high-performance electrodes with low local transport resistance.

Determination of the Electroactive Surface Area of Supported Ir-Based Oxygen Evolution Catalysts by Impedance Spectroscopy: Observed Anomalies with Respect to Catalyst Loading

Recently, several successive papers have proposed a method for determining the actual electrochemically active surface area (ECSA). The method is based on measurements of the impedance response within a suitable potential window and the use of a simple equivalent circuit for the extraction of the so-called adsorption capacity, which is related to the phenomenon of specific adsorption of intermediates in the electrochemical reaction of interest, in particular the oxygen evolution reaction (OER). In this paper, the applicability of the proposed method to the characterization of Ir-based nanostructured electrocatalysts is investigated in more detail. As a model catalyst, we use a commercial sample of Ir nanoparticles dispersed over a high surface area carbon support (Vulcan XC-72). We test the methodology-predicted linear scaling of adsorption capacity with increasing amount (and hence surface area) of catalyst. Systematic experiments are performed with different loadings of the same catalyst. The analysis shows significant deviations from the scaling predicted by the model. To further verify this unexpected result, we extend the analysis to the other model parameters, namely the so-called charge transfer and adsorption resistances and the empirically added double layer capacitance. Interestingly, the analysis shows scaling anomalies only for the first two parameters, which, like the adsorption capacitance, are related to catalytically active sites, but not for the double layer capacitance, which measures the total conductive surface of the studied material. Based on the results, we propose possible reasons for the anomalies and confirm the main hypotheses by impedance simulations using the full physical model - instead of the simplified equivalent circuit.

Understanding Platinum Ionomer Interface Properties of Polymer Electrolyte Fuel Cells

A well-designed cathode catalyst layer with optimal ionomer distribution is critical to minimizing amount of Platinum (Pt) content in polymer electrolyte fuel cells (PEFCs). The impact of Pt loading, ionomer content and carbon support types on the catalyst/ionomer interface were also investigated at dry and wet conditions. Higher Pt loadings resulted in higher double layer capacity (Cdl) and similar electrochemical surface area (ECSA) due to well dispersed ionic phase material. Higher ionomer content resulted in higher ionic conductivity but also showed similar group coverage. High surface area (HSA) carbon support had larger ECSA and Cdl at both dry and wet conditions, as less agglomerated Pt was well dispersed in the meso-pores of the support. Lower group coverages were observed for HSA carbon than for Vulcan carbon due to Pt particles being buried within the porous HSA carbon support. The effect of cell conditioning and voltage recovery on the PEFC cathode catalyst layer was shown to have minimal impact on group coverage despite a decrease in Cdl and ECSA due to the size increase of Pt particles. At dry condition, a significant increases in group coverage were observed for all MEAs due to higher adsorptivity of ionomer in dry conditions.

Electron-Conductive Three-Dimensional Inorganic Nanocrystals As a New Catalyst Support for PEM Fuel Cell Cathode

State-of-the-art polymer electrolyte membrane fuel cells (PEMFC) cathode catalysts are platinum or platinum alloy nanoparticles loaded on a high surface area carbon support. Although these catalysts have been successfully implemented in passenger fuel-cell electric vehicles (FCEVs) (e.g., the Toyota Mirai), these catalysts are facing many performance and durability challenges that limit their applications in medium-duty and heavy-duty transportation. Among these challenges is the corrosion of the carbon support, which represents a source of efficiency loss (i.e., maximum voltages must be “clipped” to prevent catalyst degradation) and limitation of PEMFC lifetime. Corrosion of the carbon support affects catalyst utilization and the transport properties of the cathode as it can result in loss of electrical connection to the platinum alloy nanoparticles and destruction of electrode porosity.1-4 This talk introduces a new class of PEM fuel cell cathode catalyst supports called electron-conductive three-dimensional inorganic nanocrystals. The highly-conductive nanocrystals are fabricated by a chemical vapor deposition (CVD) assisted route using a home-built vertical plug-flow fused-quartz reactor. The developed synthesis method can produce gram scale amounts and operates in batch mode. Platinum nanoparticles have been introduced to the surface of newly- developed supports using different synthetic routes in order to demonstrate the excellent support durability after being subjected to different synthetic treatments. The ORR activity, catalyst performance durability, and catalyst support durability have been evaluated by means of the thin-film rotating disk electrode (TF-RDE) technique. The platinum content in the prepared catalysts has been measured using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF). The morphology, crystal structure, and spatial distribution of the metal nanoparticles have been characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS), respectively. The electronic structure of the conductive 3-D inorganic nanocrystals has been identified by means of electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA) to be graphene-like with sp2 hybridized bonds.AcknowledgementsThis work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and 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.References F. 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).Cullen, David A., K. C. Neyerlin, Rajesh K. Ahluwalia, Rangachary Mukundan, Karren L. More, Rodney L. Borup, Adam Z. Weber, Deborah J. Myers, and Ahmet Kusoglu. "New roads and challenges for fuel cells in heavy-duty transportation." Nature Energy (2021): 1-13.

Investigation of cathode catalyst layer interfaces evolution during accelerated stress tests for polymer electrolyte fuel cells

To elucidate the evolution of ionomer-Pt and ionomer-carbon interfaces during life cycle of polymer electrolyte fuel cells (PEFCs), electrodes with high surface area (HSA) and durable carbon supports underwent catalyst and carbon corrosion accelerated stress tests (ASTs) under stoichiometric and sub-stoichiometric gas flow conditions. Electrochemical characterizations, as well as X-ray photoelectron spectroscopy (XPS) were utilized to assess degree of components degradation. Catalyst AST results revealed that Pt nanoparticles dispersed within the micro- and meso-pores and not in contact with ionomer are the main contributor to the 50% electrochemical surface area (ECSA) loss observed after 30k cycles resulting in significant polarization loss, which is much more notable in the case of sub-stoichiometric operating condition. Support AST results for HSA carbon showed severe ionomer degradation and 55% ECSA loss within the first 100 cycles, while cell with durable support exhibited negligible polarization and ECSA decay during the first 2500 cycles attributed to higher ionomer coverage, preservation of proton pathways preventing Pt from detachment.

Impact of catalyst loading, ionomer content, and carbon support on the performance of direct isopropanol fuel cells

Liquid Organic Hydrogen Carriers (LOHC) offer a promising solution for hydrogen storage in the existing infrastructure for conventional fuels. Within this framework, the isopropanol/acetone couple as a light-LOHC system is used to generate electricity in a direct isopropanol fuel cell (DIFC). This work focuses on the impact of catalyst loading, ionomer content and catalyst support on the performance of DIFCs. We achieve a performance rise from 95 mW cm -2 to 219 mW cm -2 under air operation by increasing the anode catalyst loading from 0.5 mg cm -2 to 4 mg cm -2 , which can be attributed to the increased abundance of active catalyst sites with higher loadings. In contrast, we find that the cathode loading for the oxygen reduction reaction (ORR) plays a minor role in the performance of DIFCs. Therefore, the cathode loading can be minimized to decrease the total amount of platinum-group metals and, consequently, to save cost. It was also found that an ionomer content of 30% on the anode side is optimal. Additionally, different carbon supports were investigated, where advanced high surface area carbon support showed superior performance to Vulcan with an increase of 20% in power density, motivating the development of new carbon supports for DIFCs. • PtRu catalyst loading on the anode is critical for the performance of direct isopropanol fuel cells. • Cathode Pt loading plays a minor role and can be reduced or even exchanged with PGM-free ORR catalysts. • An ionomer content of 30 wt% (I/C 1.07) is optimal for the anode catalyst layer. • Advanced high surface area carbon support leads to record-high power densities with 259 mW cm −2 at 0.55 V and air operation. • Dynamic operation reveals the potential of isopropanol oxidation without catalyst poisoning.

Interpreting Ionic Conductivity for Polymer Electrolyte Fuel Cell Catalyst Layers with Electrochemical Impedance Spectroscopy and Transmission Line Modeling

A cathode catalyst layer containing optimally distributed ionomer is critical to reduce the platinum loading and increase its utilization in polymer electrolyte fuel cells. Here, electrochemical impedance spectroscopy (EIS) was used to measure effective ionic conductivity of pseudo catalyst layers (PCLs) at a relative humidity (RH) range of 50%–120%. These results are compared to previous work using the hydrogen pump (HP) method. EIS effective ionic conductivity results reported here are higher than those from the HP because in the HP set-up ionic pathways must be effectively connected through the PCL to be counted, whereas in the EIS measurement, ionomer segments that are in contact with the membrane but are not effectively connected all the way through the PCL can be detected. Double layer capacitances and effective ionic conductivities of Pt/C catalyst layers with various supports and ionomer to carbon (I/C) ratios were studied. High surface area carbon support resulted in a lower effective ionic conductivity compared to the graphitized carbon support due to worse ionomer dispersion. Effective ionic conductivities of Pt/C layers were compared to that of PCLs. On average, effective ionic conductivities of Pt/C layers were higher than PCLs because of possible carbon agglomeration within the PCLs.

Fuel Cell Automotive Drive-Cycle Cathode Catalyst (Pt/C) Layer Durability: Dependence on Low Vs. High Carbon Supports and Ionomer-to-Carbon-Ratio

The typical urban automotive drive-cycle for proton exchange membrane fuel cells (PEMFC) consists of frequent idle-to-peak power cathode voltage transients, with extremes ranging from 1.2 V to 0.6 V, respectively. Understanding the platinum electrochemical surface area (ECSA) loss mechanisms dependence on material and operational conditions have been extensively investigated with the objective of minimizing performance losses over the lifetime of PEMFC powered automobiles. Commonly, automotive accelerated stress tests (AST) are performed ex-situ on thin catalyst films. The standard catalyst consists of carbon supported platinum nanoparticles (Pt/C). This study compares triangle waveform (TW) ASTs monitoring ECSA loss for ex-situ (thin film in 0.1 HClO4) and in-situ (catalyst layers bond with ionomeric electrolyte in a complete membrane electrode assembly (MEA)). The ionomer-to-carbon ratios (I/C) of the MEAs is varied from 0.6 to 1.5 on either low surface area carbon (LSAC) or high surface area carbon (HSAC) supports. The AST profiles included 25k cycles at 10 sec intervals, between the upper potential limit of either 0.9 V or 1.2 V and the lower potential limit of 0.6V, with intermediate ECSA evaluations from the H-desorption current (0.02 V → 0.4 V).The electrolyte environment alters, liquid vs. ionomer, the dominant AST decay mechanism, Figure 1A. The decay in the liquid electrolyte is most sever upon initiation of the AST and the rate of decay decrease after the first 2000 cycles. While the MEA decay is less initial, but maintains the same rate of initial decay over 5000 cycles resulting in a higher overall loss of ECSA. ECSA decay is exacerbated at temperatures of 70°C. Increasing the I/C increases the interfacial acidity at the surface of the Pt/C and results in exacerbated ECSA loss, Figure 1B. In accordance with the Pourbaix diagram for Pt, it is less stable in a more acidic environment. The relative rate of ECSA loss is reduced on HSAC supports. Figure 1

Studying Pt-based fuel cell electrode degradation with nanoscale X-ray computed tomography

Improvements in Pt-based catalysts and support durability are important to enable mass-market penetration of proton exchange membrane fuel cells (PEMFCs) in the automotive sector. This work focuses on a membrane electrode assembly (MEA) scale analysis of cathode catalyst layer (CCL) degradation processes occurring during catalyst accelerated stability testing (AST). We examined the degradation of the carbon support, decrease in electrochemically active surface area (ECSA), and formation of a platinum (Pt) band. We utilized a combined electrochemical and ex-situ nanoscale X-ray computed tomography (nano-CT) imaging technique to characterize these important degradation processes in state-of-the-art MEAs and examined the effects of AST cycling on two carbon supports, Vulcan (Vu) and a KetjenBlack high surface area carbon (HSC), and two Pt-based catalysts, pure Pt and PtCo. We found that neither support nor catalyst type completely suppressed the formation of a Pt band, but we did find that the PtCo catalyst and HSC support combination was able to limit the ECSA loss and Pt band formation better than the pure Pt and Vu combination. Our results also showed some loss of macroporosity in the HSC support after catalyst AST cycling, helping to explain the reduced performance seen with HSC-supported catalysts at high current densities.

(Invited) Pt/Metal Oxide/Ti and Pt/Metal Oxide/Carbon Composite Films for Ethanol Oxidation

Metal oxides prepared by thermal decomposition of metal acetylacetonate (acac) complexes have a significant support effect on platinum catalyzed oxidation of ethanol. Ru and Sn oxides and mixed Ru-Sn oxides deposited onto Ti foils and high surface area carbon support materials were drop coated with preformed Pt nanoparticles. Cyclic voltammetry in H2SO4(aq) and polarization experiments in proton exchange membrane cells have demonstrated great versatility for screening the co-catalytic effects of oxide layers and scale-up to fuel cell testing. A strong synergistic effect has been observed between Ru and Sn oxide in both environments, with a 2:1 ratio of Ru(acac)3 to Sn(acac)2 ratio in the oxide precursor solution producing the most active catalyst. Electrodes prepared by using Pb(acac)2, In(acac)3 and MoO2(acac)2 also provided substantially enhanced activities for ethanol oxidation.

Stable and Efficient PtRu Electrocatalysts Supported on Zn-BTC MOF Derived Microporous Carbon for Formic Acid Fuel Cells Application.

Highly efficient, well-dispersed PtRu alloy nanoparticles supported on high surface area microporous carbon (MPC) electrocatalysts, are prepared and tested for formic acid oxidation reaction (FAOR). The MPC is obtained by controlled carbonization of a zinc-benzenetricarboxylate metal-organic framework (Zn-BTC MOF) precursor at 950°C, and PtRu (30 wt.%) nanoparticles (NPs) are prepared and deposited via a polyol chemical reduction method. The structural and morphological characterization of the synthesized electrocatalysts is carried out using powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), an energy dispersive X-ray (EDX) technique, and gas adsorption analysis (BET). The FAOR performance of the catalysts is investigated through cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). A correlation between high electrochemical surface area (ECSA) and high FAOR performance of the catalysts is observed. Among the materials employed, Pt1Ru2/MPC 950 with a high electrochemical surface area (25.3 m2 g−1) consequently showed superior activity of the FAOR (Ir = 9.50 mA cm−2 and Jm = 2,403 mA ) at room temperature, with improved tolerance and stability toward carbonaceous species. The superior electrochemical performance, and tolerance to CO-poisoning and long-term stability is attributed to the high surface area carbon support (1,455 m2 g−1) and high percentage loading of ruthenium (20 wt.%). The addition of Ru promotes the efficiency of electrocatalyst by offering FAOR via a bifunctional mechanism.

Pt/Ru–Sn Oxide/Carbon Catalysts for Ethanol Oxidation

Mixed Ru–Sn oxides have been deposited onto a high surface area carbon support by thermal decomposition of Ru and Sn acetylacetonate (acac) complexes. Adsorption of preformed Pt nanoparticles produced catalysts with enhanced low potential activity for the oxidation of ethanol in aqueous sulfuric acid at ambient temperature and in a proton exchange membrane (PEM) cell at 80 °C. Varying the oxide composition between Ru0.38Sn0.62O2 and Ru0.67Sn0.33O2 did not influence the catalyst’s activity greatly but did increase stability in the sulfuric acid solution. Higher stability was observed in the PEM cell, where a Pt/Ru0.55Sn0.45O2/C anode provided much higher currents than a commercial Pt/C catalyst for ethanol oxidation at low potentials. Anodes for direct ethanol fuel cells can be fabricated by coating a carbon fibre paper backing layer consecutively with carbon black, Ru(acac)3 + Sn(acac)2, and Pt nanoparticles, with appropriate thermal processing.

Enhancement of the oxygen reduction reaction electrocatalytic activity of metallo-corroles using contracted cobalt(iii) CF3-corrole incorporated in a high surface area carbon support.

Molecular oxygen reduction reaction catalysts based on metallo-corrole with the smallest meso-substituent reported to-date, Co(iii)CF3-corrole, was synthesized and compared to the well-studied Co(iii)tpfcorrole when adsorbed on a high surface area carbon support. This study shows the improved electrocatalytic performance with the new corrole, attributed to its unique compact structure, which enables surface interactions in favor of lowering the reaction overpotential by 70 mV.

The relationship of morphology and catalytic activity: A case study of iron corrole incorporated in high surface area carbon supports

Metallo-corroles have shown to have unique catalytic properties for oxygen reduction when adsorbed on a high surface area carbon such as BP2000. The nature of the interaction between carbon surfaces and metallo-corroles has yet to be identified. This work suggests that the catalytic activity lays not in a bond between the carbon surface and the corrole molecule, but rather in the arrangement of corrole molecules within the porous structure of the carbon surface. This hypothesis was studied here using Electron Paramagnetic Resonance (EPR), X-ray diffraction (XRD), UV-VIS spectroscopy, elemental analysis, N2 adsorption and several electrochemical methods. BP2000 carbon was compared to four other carbons with different properties (oxygen surface groups, surface area, porous structure and crystallinity). All of these show that the Fe-corrole that was incorporated in BP2000 tends to adsorb in pores of 20–25 nm in diameter, which allow synergistic effects to take place, and enhance the electrocatalysis of oxygen reduction reaction.

Catalyst Particle Scale Analysis By Transport and Reaction Simulation on 3D Nano-Scale Images for PEMFC

Optimizing the structure of the catalyst layer is critical to improving the proton exchange membrane fuel cell (PEMFC) performance at low Platinum (Pt) loading. A cell with reduced cathode Pt-loading for the cost reduction suffers from a limited performance at high current density, and it is a barrier to practical applications of PEMFC [1]. In addition to Pt-loadings, the types of carbon supports affect cell performance [2]. In the prior work, the nano-scale scanning transmission electron microscopy computed tomography (STEM-CT) was applied to visualize the individual Pt particle distributions in 3D at two Pt-loadings on two types of carbon supports [3]. This imaging showed that with medium surface area, low porosity carbon support (e.g., Vulcan), a large majority of Pt particles are on the surface of the carbon support. In contrast, STEM-CT showed that with a high-surface-area carbon (HSC), a majority of Pt particles are within the porous carbon support. These internal Pt particles are not in contact with the acidic electrolyte, therefore it is unclear how protons are transported to the Pt surface and how the oxygen reduction reaction (ORR) occurs on these Pt particles. In addition, prior experimental results suggest that the utilization of inner Pt particles is improved at high relative humidity with HSC since the micropores in the carbon support are likely to be filled with condensed water [3]. Here, to investigate the characteristics of those catalyst materials, we developed a transport and ORR model on 3D structures obtained from the STEM-CT images. We solve the Poisson-Nernst-Planck equations to model the electromigration and surface charge effects on the proton concentration in the porous carbon supports with a continuum approximation. Figure 1 shows the ORR current density on the Pt particles in the HSC support at the electrode potential, 0.7 V. The simulation result suggests that negative surface charge on Pt particles may be necessary for a high proton concentration and activity of inner particles in water-filled micropores at high relative humidity. This model can be used to characterize the performance of the catalyst layers in different configurations. AcknowledgementThis work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Towards High-Performance Proton Exchange Membrane Fuel Cells Using Polymer Coated Heteroatom Doped Partially Unzipped Carbon Nanotubes As Catalyst Support

Enhancing the durability and reducing the cost of Proton Exchange Membrane Fuel Cells (PEMFC) are the bottlenecks for the commercialization. Platinum is reported as the best electrocatalyst for both anode and cathode due to its optimum binding energy towards hydrogen, oxygen atoms and products during the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). The electrocatalyst loading at the anode and the cathode can be minimized by dispersing Pt nanoparticles (NPs) on high surface area and conductive carbon support material. The durability and performance of PEMFC depend on the interaction between the support and Pt NPs as well as the number of active sites available for the reactions, which can be enhanced by selective coating of proton conducting polymer on the hetroatom doped supporting material. In the present work, we have synthesized highly proton conducting polymer polyvinyl phosphonic acid (PVPA) coated heteroatom doped partially unzipped carbon nanotubes (H-PECNT) and used as the catalyst support material for Pt NPs. The heteroatom doping increases the ORR activity of the support material and the PVPA coating adhere Pt NPs to the support during cycling and also increases the active sites. Moreover, we have coated PVPA on H-PECNT by varying different ratios of PVPA to H-PECNT and to find the optimum PVPA percentage on H-PECNT, which enhances the performance of PEMFC. The single cell measurements deliver a high power density with 6% of PVPA than the commercial Pt/C as well as other PVPA ratios wherein both electronic conductivity of the support material and proton conductivity of PVPA contribute significantly to the performance.

Effect of Carbon Support on Fe-N3/C Model Active Site for the Oxygen Reduction Reaction

We report on an investigation of the role carbon support has on a model non-precious metal catalyst for the oxygen reduction reaction (ORR) prepared through molecularly defined terpyridine moiety covalently embedded into high surface area carbon support (Black Pearls 2000) is reported. A terpyridine modified catalyst was previously prepared and allowed for the controlled deposition of one specific and unique N3/C active site on the surface of the support. The effect of changing the porosity of the carbon was analyzed for its oxygen reduction activity and characterized using thermogravimetric analysis, pore size determination, and rotating disk measurements. This system showed that by using a more microporous support, the activity for the oxygen reduction reaction decreased due to different active sites forming on the high surface area support.

Modeling Water Uptake and Pt Utilization in High Surface Area Carbon

Local transport losses limit high-current density performance of fuel-cells, especially at low Pt-loadings. Optimizing catalyst-layer structure and material properties to mitigate these losses is critical for wide-scale fuel-cell commercialization. Carbon supports form the primary porous structure of catalyst layers in which Pt particles are deposited and ionomer films are distributed. Unlike non-porous carbon supports, high surface area carbon support is unique in its relative humidity dependent Pt utilization and performance due to presence of Pt particles in the micropores interior of carbon particles. We model water uptake and Pt utilization in catalyst layers with high-surface-area carbon support. Interactions between functional groups on the carbon pore walls and water play a vital role in controlling water uptake from water vapor by catalyst layers. Ionomer is shown to influence proton access in micropores, even though it may not deposit into the micropores.

Necessity to Avoid Titanium Oxide As Catalyst Support in PEM Fuel Cells

Although fuel cell electric vehicles (FCEVs) has become a commercial reality, overall cost reduction as well as durability improvement continue to remain as the key challenges facing fuel cell developers. One of the area of durability concern in proton exchange membrane (PEM) fuel cells relates to the degradation of carbon supported Pt-based electro catalysts. [1] Carbon-supported (e.g., Vulcan or Ketjen black) Pt or Pt-alloy (e.g., PtCo) catalysts are used in PEM fuel cells to catalyse the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR). In an automotive environment, the catalysts must survive highly dynamic operations. State-of-the-art high surface area carbon support suffer from severe corrosion at potentials above 1.1 V. [1] A few transient operations that contribute to the corrosion of carbon support including vehicle start-up/shutdown, cell reversal due to fuel starvation, as well as load cycling. The degradation of carbon support and the electrocatalyst can induce significant performance decay. Therefore, material that is inherently more corrosion resistant has been actively investigated as alternative catalyst support to carbon in PEM fuel cell applications. In recent years, transition metal oxides are of interest as an alternate to carbon electro catalyst support due to their improved stability at higher potentials [2-3]. Furthermore, it is also believed that metal-oxide supports can possibly influence the intrinsic activity of the supported Pt (Pt-alloy) catalyst toward ORR due to strong metal-support interactions(SMSI). Titanium oxide has been demonstrated to be one of the suitable metal oxide, and was frequently reported as effective ORR catalyst support showing both significantly improved stability at high potential as well as improved ORR activity [3]. However, no long-term durability and possible negative effects of oxide supports on PEM fuel cell durability has been reported. In this study, for the first time we report that the use of titanium oxide (TiO2) as catalyst support in the PEM fuel cell electrode can lead to significant degradation of the polymeric membrane, causing membrane thinning and increased gas crossover. Experiments were conducted using Pt supported on titanium oxide (TiO2) as either the anode HOR catalyst or the cathode ORR catalyst. The impact on membrane chemical degradation is evaluated at open circuit voltage (OCV) condition at 115 oC and 25% RH for 100 hours. The exhaust water from both the anode and cathode of the fuel cell during the OCV hold was collected and analyzed using ion chromatography (IC) to quantify the concentration of fluoride and determine the fluoride release rate (FRR) which is a measure of the rate of membrane chemical degradation. With measured FRR, a total fluorine inventory loss of the membrane is also calculated. It was found that the total fluorine inventory loss is 26% and 15% for Pt/TiO2 used as anode and cathode catalyst, respectively, after 100hrs of OCV test. A comparable electrode with carbon supported Pt catalyst exhibit only about 0.3% total fluorine inventory loss at identical OCV conditions, which is more than 50X lower. Additional experiments were designed to confirm the impact of titanium oxide by adding titanium oxide only (without Pt) to either anode or cathode electrode while using Pt/C as anode or cathode catalyst. One reduced form of TiO2, i.e., Magneli phase (Ti4O7) which is shown to have improved electrical conductivity and has been investigated as Pt support is also tested in this experiment. The results show that by adding TiO2 or Ti4O7 to anode or cathode both increase the FRR compared titanium oxide free electrodes. The FRR release rate data with titanium oxide added in the cathode is shown in Fig. 1. In this presentation, we will discuss the details of the experimental study on the impact of titanium oxide on polymeric membrane chemical degradation, including fluoride release rate, fluorine inventory loss, as well as the change in hydrogen crossover rate. Additional analysis on the location of chemical degradation (membrane close to anode vs. membrane close to the cathode) has also been conducted which may shed additional light on the fundamental mechanisms of TiOx induced membrane degradation. Finally, the necessity to find alternative metal oxide support rather than TiOx in PEM fuel cell application is highlighted Figure 1