Degradation Of Carbon Support Research Articles

Development of Accelerated Durability Test Protocols for Polymer Electrolyte Membrane Fuel Cell Stacks Under Realistic Operating Conditions

ABSTRACTPolymer electrolyte membrane fuel cell durability is still a major challenge. To overcome time‐consuming durability tests, so‐called accelerated durability tests (ADTs) are of urgent need. This work presents our recent results in developing ADT protocols in the context of realistic operating conditions, especially voltage clipping at 0.85 V. A 5500 h long‐term test was carried out as a reference applying a realistic automotive drive cycle. Focusing on different stressors such as temperature, relative humidity (RH), and load profile four different ADT protocols of 1200 h duration were derived. Seven‐cell short stacks with 240 cm2 active area were used. Comparing cell voltage as a key indicator, an acceleration factor of 3–7 could be achieved. In‐situ characterization techniques such as spatially resolved current measurement, cyclic voltammetry, and electrochemical impedance spectra were employed to investigate the influences of individual stressors on specific degradation mechanisms and components. The highest acceleration was observed in the mass transport region of ADTs addressing RH as a stressor, suggesting that RH cycling leads to increased degradation of hydrophobic surfaces. Increased temperature was found to accelerate primarily carbon support degradation. Accelerated catalyst aging seems to be low, demonstrating the effectiveness of voltage‐clipping conditions. Our most promising ADT shows quite a homogeneous acceleration of voltage degradation across all current regions.

Microscale Simulation of Carbon Support Structure Degradation in Polymer Electrolyte Fuel Cell

Understanding the mechanism of Polymer electrolyte fuel cell (PEFC) degradation plays an important role in improving fuel cell performance and durability. For the PEFC system, the structure of catalyst layers (CLs) is the critical component that determines cell performance. The PEFC degradation due to the reduction of carbon on the CLs has been investigated in previous studies [1]. In our past studies, we focused on the actual heterogeneous porous structure of catalyst layer. From these results, it was found that mass transport performance of oxygen and proton in heterogeneous porous catalyst layer, which consists of carbon support, ionomer and void space, is much lower than that of theoretical prediction in homogeneous porous media [2-5]. In addition, it depends on the structure of carbon black, the coating structure of ionomer and the local distribution of these materials. Thus, it is important to understand and to predict the carbon support structure change by degradation because this structural degradation strongly affect the mass transport performance. In this work, the effect of CLs porous carbon support degradation on cell performance is investigated by microscale simulation.The CLs structures, consisting of porous carbon, ionomer, and Pt, are created by our previous study multi-block method [2,6]. In our simulation, we focus on studying the effect of CLs' beginning-of-life (BOL) and end-of-life (EOL) structure in the degradation process. The CLs creation procedure is illustrated in Fig. 1. Firstly, CLs structure are created by packing CB and covered ionomer and Pt. The amount of ionomer and Pt are assumed as constant during the degradation process. Therefore, the amount of carbon is reduced in EOL structure. In this study, various CLs structures with decreasing of carbon volume are constructed. Furthermore, the mechanical stress distribution under compression condition was simulated in each structure to understand the effect of framework of carbon on local porosity in catalyst layer. The effect of carbon corrosion in CLs is investigated by calculating the cell performance of PEFC of each CLs structure by ORR mass transport and electrochemical reaction calculation with our previous method [6].The cell performance is significantly decreased by reducing carbon. The degradation mechanism of PFEC is explained by degradation of not only effective surface area but also the porous structure change which affect oxygen diffusion performance. Moreover, compared with BOL structure the apparant thickness of covered ionomer layer is higher in EOL structures because of carbon corrosion and constancy of ionomer. Hence, the higher oxygen diffusion resistance in secondary pore and ionomer layer occur in CLs degradation structures. The effect of this dynamic structure change on cell output performance will be reported in this presentation.Acknowledgements:This research was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan, grant number P20003-20001327-0.

Segmented tomographic evaluation of structural degradation of carbon support in proton exchange membrane fuel cells

The variation of the three-dimensional (3D) structure of the membrane electrode of a fuel cell during proton exchange cycling involves the corrosion/compaction of the carbon support. The increasing degradation of the carbon structure continuously reduces the electrocatalytic performance of proton exchange membrane fuel cells (PEM-FCs). This phenomenon can be explained by performing 3D tomographic analysis at the nanoscale. However, conventional tomographic approaches which present limited experimental feasibility, cannot perform such evaluation and have not provided sufficient structural information with statistical significance thus far. Therefore, a reliable methodology is required for the 3D geometrical evaluation of the carbon structure. Here, we propose a segmented tomographic approach which employs pore network analysis that enables the visualization of the geometrical parameters corresponding to the porous carbon structure at a high resolution. This approach can be utilized to evaluate the 3D structural degradation of the porous carbon structure after cycling in terms of local surface area, pore size distribution, and their 3D networking. These geometrical parameters of the carbon body were demonstrated to be substantially reduced owing to the cycling-induced degradation. This information enables a deeper understanding of the degradation phenomenon of carbon supports and can contribute to the development of stable PEM-FC electrodes.

Ionomer immobilized onto nitrogen-doped carbon black as efficient and durable electrode binder and electrolyte for polymer electrolyte fuel cells

An implementation of the polymer electrolyte fuel cell (PEFC) electrode, composed of the perfluorinated sulfonic acid (PFSA) ionomer with unprecedented uniform distribution, has recently gained a great attention due to its positive impact on the cell performance as well as durability. However, an issue with the conventional electrodes is a stronger adsorption of sulfonate terminal groups specifically onto Pt catalyst than carbon black (CB) support, that accelerates an inhomogeneous distribution of the PFSA ionomer. In this work, we report a novel PEFC electrode by introducing the ionomer immobilized onto nitrogen-doped carbon black (I/NCB) as electrode binder as well as electrolyte. The carbon atoms adjacent to nitrogen allow a considerable attraction with sulfonated groups over the NCB surfaces, which creates more homogeneous distribution of ionomer when compared with Pt/CB in the conventional electrode. As compared with the conventional electrode, the electrode employing I/NCB presents a superior activity towards oxygen reduction reaction (Pt mass activity increased from 91.3 to 139.6 A g−1) and mass-transport characteristics (mass-transport resistance decreased from 65.9 to 45.5 mΩ cm2), with the cell current density enhanced by 23% at 0.6 V. The enhanced cell durability is confirmed by less degradation in the electrochemically available surface area after the accelerated stress test of carbon support, mainly attributed to the promoted stability of NCB over CB. Given the advances in both performance and durability, the proposed electrode architecture based on I/NCB may provide an alternative pathway in obtaining better electrode kinetics and O2 transport, along with an exceptional resistivity towards the carbon support degradation.

Electrochemical Stability of Platinum Nanoparticles Supported on N‐Doped Hydrothermal Carbon Aerogels as Electrocatalysts for the Oxygen Reduction Reaction

AbstractSustainable N‐doped carbon aerogels were synthesized by a scalable hydrothermal approach using low‐cost and abundant precursors such as glucose and ovalbumin. By adjusting the pyrolysis temperature (900–1500 °C), the surface chemistry, porosity and conductivity of these aerogels could be optimized for the design of Pt‐based oxygen reduction reaction (ORR) catalysts with high Pt loading (40 wt % Pt) and improved stability. Pt nanoparticle deposition was realized by wet impregnation followed by thermal reduction and their size and distribution was found to strongly depend on the surface chemistry of the carbon aerogels. The catalysts’ activities and stabilities, determined by rotating disc electrode measurements in HClO4, were found to strongly depend on the pyrolysis temperature of the N‐doped carbon aerogel supports. While the mass activity decreased with increasing temperature, in line with a decreasing ECSA related to an increase in Pt nanoparticle size, the long‐term stability of the catalysts, as revealed by accelerated stress tests for carbon support degradation (10,000 cycles), increased with increasing pyrolysis temperature, in line with increasing Pt nanoparticle sizes and increasing graphitization of the carbon aerogel supports. Most importantly, the catalyst derived from aerogels pyrolyzed at 1000 °C achieved a good compromise between activity and stability and revealed a superior ORR activity after the accelerated stress test in comparison to a commercially available Pt/C reference catalyst (40 wt % Pt).

Electrochemical Investigation of Pt/Fe-N-C Catalysts as a Promising Low Pt-Loading Catalyst for Polymer Electrolyte Membrane Fuel Cells

Nowadays polymer electrolyte membrane fuel cells (PEMFC) become commercially established systems used in automobile, stationary and portable power generation industry which provide an improved kinetic, high efficiency, zero emission and high tolerance to the impurities. A standard catalyst used in PEMFC is based on Pt nanoparticles on carbon support materials. However, high cost of this critical raw material stimulates interest in the development of platinum group metals-free electrocatalysts (1). Fe-N-C materials are one of the promising non-precious metal electrocatalysts for these fuel cells which have recently reached outstanding performance in terms of oxygen reduction reaction (ORR) activity (2). However, the volumetric activity and durability of Fe-N-C catalyst is still significantly lower compared to Pt/C in PEMFC mainly due to their lower turnover frequency and active sites density (3).Therefore, in this work, a Black Pearl (BP) based Fe-N-C catalyst is used as ORR active support material for Pt-nanoparticles to investigate synergetic effects of active Pt and Fe-N sites towards stability and volumetric activity. We present a comparative electrochemical characterization of Pt/Fe-N-C and Pt/BP electrocatalysts with loadings of 0.01, 0.1 and 1 wt.% Pt. The measurements were done in terms of rotating ring-disk electrode experiments in 0.1 M HClO4 electrolyte under accelerated stress test (AST) during 5000 cycles in potential range 0.6 – 1.5 V vs RHE in N2-saturated electrolyte, in order to provoke Pt as well as carbon support degradation. The electrochemical surface area (ECSA) of the Pt was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after AST for Pt/BP catalysts. However, for Pt/Fe-N-C the ECSA determination was impossible because for metal-oxide supported Pt catalysts, a straightforward analysis of HUPD and CO methods fails to give meaningful values. The RRDE experiments revealed that ORR mass and specific activities were significantly decreased after AST especially for Pt/BP (Fig. 1) which was attributed to the platinum dissolution as well as carbon support degradation during cycling.1. E. Eren, N. Özkan, Y. Devrim, Int J Hydrogen 2020, 45 (58), 33957-33967.2. Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev., 2020, 11, 3484–3524.3. T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem Commun 2020, 118, 106795.Figure 1. Cathodic scan of ORR curves of 0.1 wt.% Pt/BP catalyst with 40 µg cm-2 Pt loading in O2-saturated 0.1 M HClO4 at scan rate 5 mV s-1 by 1600 rpm before and after AST. Figure 1

Investigation and Quantification of Irreversible Degradations and Their Sensitivities during the Start-up Phases of PEMFC By an Accelerated Emulation Approach

In automotive application, frequent Start-up/Shut-down (SU/SD) phases of PEMFC system are prone to cause severe degrading conditions. Many studies have been achieved over the past decades to better understand and to mitigate MEA aging and performance decay due to the well-identified reverse current mechanism1. Indeed, the H2|Air front during H2 injection at the air-filled anode creates two sub-cells: H2/Air sub-cell operating in Fuel Cell mode and Air/Air sub-cell operating in Electrolysis Cell mode. This second sub-cell is subjected to high potential at cathodic side (about 1.6 V vs RHE), leading to harsh corrosion of both Pt nanoparticles and carbon support within the cathode catalyst layer. Two main mitigating solutions are generally proposed in the literature: (i) development of more stable materials and (ii) improvement of system control 2,3. Experimental DOE AST4 is usually performed only on small single cell to evaluate MEA component durability. Nevertheless, these tests cannot be fully correlated with real automotive operating conditions to predict PEMFC stack lifetime.In this work, more realistic AST protocols in regard to PEMFC stack/system configuration during start-up phase are proposed. Our experimental approach consists in emulating a representative transient cathodic potential profile (Figure a and b) during the H2|Air front formation and propagation along anodic channel using a 100 cm² single cell with a real stack active area design5. A comprehensive investigation and quantification of degradations caused by reverse current was carried out under SU cycling. Additional sensitivity study on the main operating parameters (potential profile, relative humidity and temperature) was performed to evaluate their respective impact on the cathode degradation and the performance losses. Periodic measurements such as polarization curves and cyclic voltammetry (Figure c) are coupled with CO/CO2 gas emission analysis (Figure d) during cycling at cathode to monitor and quantify the physical and electrochemical modifications within the MEA during our aging SU cycles.On the whole, this original experimental study proposes an overall degradation mechanism and these results are compared with model prediction to validate the most impactful parameters during reverse current phases at the real cell size and design. Ultimately, it will provide key-data to propose and optimize finely different mitigation strategies to be used in PEMFC stack and systems. References Reiser, C. A. et al. A reverse-current decay mechanism for fuel cells. Electrochemical and Solid-State Letters 8, A273–A276 (2005).Zhang, T., Wang, P., Chen, H. & Pei, P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Applied Energy 223, 249–262 (2018).Yu, Y. et al. A review on performance degradation of proton exchange membrane fuel cells during startup and shutdown processes: Causes, consequences, and mitigation strategies. Journal of Power Sources 205, 10–23 (2012).Fuel Cell Tech Team Accelerated Stress Test and Polarization Curve Protocols for PEM Fuel Cells. Energy.gov https://www.energy.gov/eere/fuelcells/downloads/fuel-cell-tech-team-accelerated-stress-test-and-polarization-curve.Randrianarizafy, B. Multi-physics modeling of startup and shutdown of a PEM fuel cell and study of the carbon support degradation : mitigation strategies and design optimization. (Université Grenoble Alpes, 2018). Figure 1

Advanced Nanocarbons for Enhanced Performance and Durability of Platinum Catalysts in Proton Exchange Membrane Fuel Cells.

Insufficient stability of current carbon supported Pt and Pt alloy catalysts is a significant barrier for proton-exchange membrane fuel cells (PEMFCs). As a primary degradation cause to trigger Pt nanoparticle migration, dissolution, and aggregation, carbon corrosion remains a significant challenge. Compared with enhancing Pt and PtM alloy particle stability, improving support stability is rather challenging due to carbon's thermodynamic instability under fuel cell operation. In recent years, significant efforts have been made to develop highly durable carbon-based supports concerning innovative nanostructure design and synthesis along with mechanistic understanding. This review critically discusses recent progress in developing carbon-based materials for Pt catalysts and provides synthesis-structure-performance correlations to elucidate underlying stability enhancement mechanisms. The mechanisms and impacts of carbon support degradation on Pt catalyst performance are first discussed. The general strategies are summarized to tailor the carbon structures and strengthen the metal-support interactions, followed by discussions on how these designs lead to enhanced support stability. Based on current experimental and theoretical studies, the critical features of carbon supports are analyzed concerning their impacts on the performance and durability of Pt catalysts in fuel cells. Finally, the perspectives are shared on future directions to develop advanced carbon materials with favorable morphologies and nanostructures to increase Pt utilization, strengthen metal-support interactions, facilitate mass/charge transfer, and enhance corrosion resistance.

On the origin of deactivation of reversal-tolerant fuel cell anodes under voltage reversal conditions

Over the past decade, extensive efforts have been carried out to increase the durability of polymer electrolyte fuel cells (PEFCs) under voltage reversal conditions primarily caused by hydrogen starvation. One common strategy is the use of reversal tolerant anodes (RTAs) that incorporate oxygen evolution reaction (OER) catalysts (e.g., IrO2) to promote harmless water electrolysis over carbon oxidation and delay severe carbon support degradation. The present study investigates the RTA catalyst deactivation mechanism under hydrogen starvation conditions and the relative humidity (RH) dependence of the OER and carbon oxidation reactions to understand the relatively quick failure of an RTA at a high RH. The comparison of the OER durability of membrane electrode assemblies with and without carbon under hydrogen starvation conditions shows that the presence of carbon deactivates the OER catalyst through carbon oxidation species poisoning. The catalytic activity of IrO2 measured by the current density response shows a linear dependence on RH, while the rate of carbon oxidation reactions increases exponentially with RH, suggesting that the decreased durability of RTAs is related to the exponentially increased rate of carbon oxidation reactions. These findings can be used to design more robust electrodes for automotive fuel cells under hydrogen starvation conditions.

Multi-Physics Simulation Framework of Carbon Support Corrosion during Dynamic Startup and Shutdown: Optimization and Mitigation Strategies

The estimation and increase of the lifetime of PEM fuel cell under dynamic conditions is one of a major challenge. Increasing the durability of the fuel cell must be treated by both the development of new material and design but also by optimal strategies and management of the operating conditions of the fuel cell. The startup and shut down phase are important to optimize to reduce the carbon support corrosion [1].x Based on previous development of a multi-physics modeling framework, combining complex transport such as multicomponent transport in porous media and electrochemistry with the use of local conditions for MEA and channel design optimization [2], two 2D multi-physics models (2D model along the channel and 2D model at the rib/channel scale) are updated. The different reactions (hydrogen oxidation reaction, oxygen reduction reaction and the oxidation of the carbon support) are written in the general form [3-4]. The linking of the two models allows to simulate the transient potentials during startup and shutdown phase in two direction (along the channel and in the section of the MEA). In particular, during the injection of hydrogen in the channel, the reverse current mechanisms that accelerate the carbon support corrosion, is directly simulated without hypothesis. The validated models provide in-silico characterization to better explain the reverse current mechanisms and the interactions between the operating conditions of the cell and the local conditions in the catalyst layer. The CO2 concentration at the outlet of the channel is used as an observer to quantify the degradation of the carbon support. In a second step, different mitigation strategies are proposed. In particular, some strategies are studied to limit the high potential during the startup and shutdown phase (influence of the catalyst loading in the anode, external electrical resistance). Other strategies decrease the time during the reverse current mechanism (sensitivity of the hydrogen flow rate during the startup (Fig. sensitivity study of the hydrogen flow rate during startup on the cathodic potential and the CO2 production), design optimization of the rib/channel patern). These different strategies are explained and compared. An improvement of the carbon support corrosion is quantified and can be decreased by 50%. [1] Qiang Shen, Ming Hou, Dong Liang, Zhimin Zhou, Xiaojin Li, Zhigang Shao, and Baolian Yi. Study on the processes of start-up and shutdown in proton exchange membrane fuel cells. Journal of Power Sources, 189(2):1114–1119, 2009. [2] Bolahaga Randrianarizafy, Pascal Schott, Marion Chandesris, Mathias Gerard, and Yann Bultel. Design optimization of rib/channel patterns in a pemfc through performance heterogeneities modelling. International Journal of Hydrogen Energy, 43(18):8907 – 8926, 2018. [3] G. Maranzana, A. Lamibrac, J. Dillet, S. Abbou, S. Didierjean, and O. Lottin. Startup (and shutdown) model for polymer electrolyte membrane fuel cells. Journal of the Electrochemical Society, 162(7):F694–F706, 2015. [4] B. Randrianarizafy. Multi-physics modeling of startup and shutdown of a PEM fuel cell and study of the carbon support degradation: mitigation strategies and design optimization. PhD thesis, Communauté Université Grenoble Alpes, 2018. Figure 1

Electrochemical impedance spectroscopy of catalyst and carbon degradations in proton exchange membrane fuel cells

Wide–scale commercialisation of proton exchange membrane fuel cell (PEMFC) is hindered by the degradation of cathode catalyst and carbon support, making it crucial to understand these mechanisms. This study compares the cathode catalyst and carbon support degradations mechanisms using commercial gas diffusion electrode in operating PEMFCs. The impact of the accelerated stress tests (ASTs) is monitored by recording the polarisation, electrochemical surface area (ECSA) and mass activity. Furthermore, the electrochemical impedance spectroscopy (EIS) is monitored at seven operating points from 0.05 to 1 A cm−2. For both phenomena, the charge transfer resistance increases at high current densities, which is attributed to the losses in activity observed with the ECSA and the mass activity degradations. In the carbon–corroded PEMFC, the mass transport resistance first reduces by 45% (0–100 cycles), suggesting an increase in electrode porosity due to carbon corrosion. At a later stage (200–5000 cycles), the mass transport resistance increases by 225% as the electrode collapses. On the other hand, in the catalyst–degraded PEMFC, the mass transport resistance uniformly reduces while the charge transfer resistance increases by 30%. Altogether, EIS provides additional sensitivity to differentiate catalyst and carbon support degradation within PEMFCs.

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

Water Activity Dependence of Oxygen Evolution Reaction Catalysts and Carbon Corrosion of Reversal Tolerant Fuel Cell Anodes during Hydrogen Starvation Conditions

Before electric vehicles using polymer electrolyte fuel cells (PEFCs) can occupy a significant part of the vehicle fleet, one of their critical vulnerabilities must be resolved: rapid failure that occurs under hydrogen starvation conditions. Local hydrogen starvation within fuel cell system can occur when the hydrogen inlet flow channel is blocked by water and ice, or during the startup/shutdown process [1]. During hydrogen starvation, the fuel-starved anode requires another source of electrons and protons other than the usual hydrogen gas since hydrogen oxidation reaction (HOR) alone cannot support the current imposed by adjacent cells. The starvation eventually increases the anode half-cell potential relative to the cathode potential, and the cell potential is reversed. The high anode half-cell potential will lead to oxidation of the carbon support in the anode. The loss of carbon support in the electrode will decrease the performance of the cell, and the entire stack eventually becomes irreversibly inoperable. One popular mitigation strategy is the use of reversal tolerant anodes (RTAs), which integrates an oxygen evolution reaction (OER) catalyst, such as IrO2, to promote harmless water electrolysis over carbon corrosion [2]. The OER prevents the anode half-cell potential from further increasing, delaying the carbon support from significantly corroding. However, our group’s prior work has shown that RTA durability is significantly impaired under high relative humidity (RH) conditions [2]. Since the hydrogen starvations commonly arise in high RH conditions, it is important to clarify the exact mechanism that leads to the relatively quick RTA deactivation at high RH. The present work investigates the RTA catalyst deactivation mechanism during hydrogen starvation and the water activity dependence of water electrolysis and carbon corrosion reaction to better understand the relatively quick failure of RTA at high RH. Figure 1a shows the effect of carbon on the OER catalyst during hydrogen starvation, which is simulated by providing humidified nitrogen and applying 1 A cm-2 of current density to the cell. In contrast to the stable anode half-cell potential at 2.2 V by IrO2 alone, the presence of carbon leads to a quick increase of its half-cell potential and then the failure of the cell. This suggests that carbon deactivates the OER catalyst. There are three possible RTA failure mechanisms: (1) the loss of electronic connectivity of the catalysts due to the degradation of the carbon support (2) the isolation of the catalyst from ionomer after severe carbon corrosion (3) the poisoning of the catalyst by carbon oxidation species. To determine which mechanism causes the RTA failure, carbon and IrO2 were separated into two layers to avoid the loss of electronic connectivity and poor ionomer coverage after the carbon support degradation, and tested under hydrogen starvation conditions. As shown in Figure 1b, the carbon-separated IrO2 quickly increases the anode half-cell potential. This indicates that the catalyst deactivates even if the ionomer coverage and electric connectivity are maintained, and the deactivation mechanism is more likely to be the poisoning of the catalyst by carbon oxidation species rather than the loss of electronic or ionic connectivity. To investigate the water activity dependence of water electrolysis and carbon corrosion reactions, IrO2 and carbon black based membrane electrode assemblies (MEAs) were separately fabricated and tested under hydrogen starvation conditions. Potentials of 1.6, 1.7, and 1.8V, which are relevant to the OER catalyst function in RTAs during cell reversal, were applied to the MEAs under various RH conditions. As shown in Figure 1c and 1d, the catalytic activity of IrO2 measured by the current density response shows a substantially linear dependence on RH, while the rate of carbon corrosion increases exponentially with RH. This linear versus exponential relationship suggests that higher rate of catalyst poisoning at high RHs is correlated with the increased rate of carbon oxidation, resulting in the impaired durability of RTAs.

Influence of SnO2 Orientation on Electrocatalytic Activities of Pt/SnO2 Model Electrodes for Methanol Oxidation

As an electrocatalyst in fuel cells, carbon-supported platinum (Pt/C) catalysts are widely used, but there are some problems in the degradation of carbon support and CO poisoning of Pt surface. To mitigate these problems, oxides supports are considered to be an alternative support for Pt catalysts. Oxide supports have some advantages to improve the long-term stability and the CO tolerance of electrocatalysts, but the detailed mechanism of oxide supports has not yet been unveiled. In this study, we fabricated Pt/SnO2 model electrodes with controlling SnO2 crystalline orientation.Pulsed laser deposition (PLD) technique was used for fabricating the SnO2 support oxides. Single crystals of aluminum and titanium oxides were used as a support for SnO2 thin film fabrication. Platinum nanoparticles were deposited on SnO2 film with the arc plasma deposition. Fabricated model electrodes were characterized with X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electrochemical measurements were performed in an aqueous solution with methanol and perchloric acid. CO stripping voltammetry was also carried out to investigate CO tolerance of model electrodes.XRD patters show SnO2 thin films had surface orientation of (101), (100), and (110). XPS spectra also show the deposited Pt nanoparticles were metallic and had identical positive shift (~ 0.3 eV). The activities for methanol oxidation were in the following order: Pt/SnO2 (101) > Pt/SnO2 (100) > Pt/SnO2 (110). CO stripping voltammograms also showed similar activities for CO oxidation. Detailed influence and mechanism of surface orientation on catalytic activities will be discussed in the meeting.

Corrosion of Carbon Supports, Induced by High-Temperature PEMFC Operating Conditions

In high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC) one of the detrimental factors that lead to loss of performance is the corrosion of the electrocatalyst support. In both anode and cathode, platinum (Pt) supported on carbon is generally the selected electrocatalyst. Platinum significantly increases the speed of the anode and cathode reactions, but also causes carbon oxidation (corrosion of the support) and the consequent degradation of the electrode materials. In a HT-PEMFC, corrosion is influenced by temperature. For this reason, this paper will present a detailed evaluation of the carbon support degradation under HT-PEMFC conditions. The carbon oxidation of two types of carbon supports will be compared.

Analysis of the Corrosion Kinetic of Pt/C Catalysts Prepared on Different Carbon Supports Under the “Start-Stop” Cycling

A series of 50 wt% Pt/C catalysts based on carbon blacks with different specific surface areas and morphologies was synthesized and investigated using “start-stop” protocol (triangular scan in the 1.0–1.5 V vs. reversible hydrogen electrode (RHE) potential range with a scan rate of 500 mV s−1). The commercial 40 wt% Pt/Vulcan XC-72 and 50 wt% Pt/TEC were used for comparison. The oxygen reduction reaction (ORR) activities and electrochemically active surface area (ECSA) of Pt were obtained at 25 °C in 0.1 M HClO4 electrolyte after every 4000 cycles. All synthesized Pt/C catalysts were characterized by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), low-temperature nitrogen sorption, CO pulse chemisorption methods, and cycling voltammetry including a rotating disc electrode (RDE) method for analysis of the ORR kinetics. Dependences of the ORR activities, ECSA, and values of Tafel slopes on the number of oxidative cycles were obtained and analyzed in detail. It was shown that the initial values of Tafel slopes of the Pt/C catalysts decrease with increase of the surface area of carbon blacks. The increase in the values of Tafel slopes in the high-current density region with cycling was associated with the degradation of carbon support. The reciprocal ratios of the ECSA to initial values were found to grow linearly on the number of cycles. The ORR surface activity was significantly decreased in the first 8000–10,000 oxidation cycles as the result of changes of the active component and carbon support degradation. The following slight increase in the ORR surface activities with cycling was related to the increase in the Pt particle sizes in the Pt/C catalysts. Modification of Ketjen black EC 600 DJ through pyrocarbon deposition (KB600-C sample) led to appreciable increase of the stability of the Pt/KB600-C sample.

Electrocatalyst Support Durability

While commercialization of PEFCs has been started in residential application and is planned in automotive application, detailed understanding of degradation phenomena is essential. A large part of PEFC degradation, especially in automotive applications, is known to be related to carbon support degradation, mainly caused by carbon corrosion and related phenomena. During the fuel cell start-stop cycles, the potential of the cathode can reach up to 1.5 V (versus reversible hydrogen electrode, RHE). Fluctuation of cell voltage up to such higher potentials can cause oxidation-induced carbon support corrosion especially for cathode electrocatalysts [1-3]. In this tutorial talk, after summarizing typical degradation mechanisms with the state-of-the-art carbon black support, various efforts to develop alternative electrocatalyst support materials are reviewed. Table 1 compiles possible electrocatalyst support materials considered recently. There are three possible strategic directions in developing alternative durable supports as shown in Fig. 1: (1) graphitized carbon black, (2) alternative carbon support, and (3) alternative non-carbon support. (1) Graphitized carbon black While carbon is thermochemical unstable under the PEFC cathode condition, carbon corrosion can be kinetically slow for graphitized carbon black [4]. As graphite is relatively stable, so that higher temperature heat-treatment to increase the degree of graphitization is a useful procedure to ensure longer-term durability. (2) Alternative carbon support Various nanostructured carbon-based materials, such as mesoporous carbon [5] and graphene [6] may be applicable to design nanostructure of electrocatalysts. Generally speaking, graphene-like surface is stable against carbon corrosion, but Pt particles on the graphene surface tend to agglomerate each other [7]. More defective surfaces are suitable to stabilize Pt particles but such surface defects could be a site where carbon corrosion may start. (3) Alternative non-carbon support such as oxides As alternatives to the conventional carbon black electrocatalyst support, conductive oxides are mainly considered as carbon-free catalyst support materials [8-10]. Thermochemical calculations have revealed, as shown in Fig. 2 [8], that several oxides could be stable under the PEFC cathode conditions. As an example, Figure 3 shows the I-V curves before and after such start-stop cycle tests for an MEA with the cathode electrocatalyst layer consisting of the Pt/Sn0.98Nb0.02O2 mixed with 5wt % vapor grown carbon fibers (VGCF-H) as the electron-conductive filler. Start-stop cycle degradation was almost negligible up to 60,000 cycles. This result confirms that Nb-doped SnO2 can be an alternative electrocatalyst support material for e.g. fuel cell vehicles (FCVs) which are suffered from the voltage cycling up to a high potential. Materials design principles for PEFC electrocatalyst supports will be discussed. Acknowledgement Financial support by the Grant-in-Aid for Scientific Research (S) (No. 23226015), JSPS Japan, is gratefully acknowledged.

Application of the Transmission Line EIS Model to Fuel Cell Catalyst Layer Durability

Typical catalyst layers (CL) employed in PEM fuel cells are composed of a carbon supported Pt catalyst bound together with an ionomer, most often Nafion. Both the carbon support and ionomer play a crucial role in optimizing the catalyst utilization by proving electronic and ionic conductive pathways, respectively, without hindering gas transport. Upon fuel cell operation, the catalyst layer can degrade by one of 3 primary pathways:Loss of Pt surfaces area through Pt dissolution/Ostwald ripening Corrosion of the carbon supportDegradation of the ionomer network The contribution of each of these pathways depends upon CL compositions as well as the cell operating conditions.The extent of degradation is normally monitored in situby cyclic voltammetry (CV) by measuring changes in the electrochemically active surface area (ECSA) of Pt. However, CV is limited in that it can only conclusively confirm the presence of the first pathway. Contributions from the other 2 pathways are typically confirmed by post mortem analysis of the CL.We have recently reported that the addition of periodic electrochemical impedance spectroscopy (EIS) measurements to an accelerated degradation testing protocol (ADTP) allows one to clearly diagnose the presence of the other 2 degradation pathways. By performing EIS under conditions where the transmission line model is valid, the presence of carbon corrosion can be clearly detected by a characteristic shift in the EIS response over time [1], as shown in Figure 1. Likewise, the loss of ionic conductivity can also be observed through a unique change in a typical capacitance plot [2].Our methodology has since been extended and further validated by studying the stability of numerous commercial and in-house prepared catalysts using with different carbon supports. In this presentation, we will describe how EIS can be incorporated into almost any ADTP protocol and how changes in the EIS response can be used for in situdiagnosis of degradation pathways that can be attributed to CL components. In particular we will describe how the presence and absence of both Pt and C impacts the degradation processes and subsequently the EIS response [3]. Furthermore, we will also demonstrate how by carefully selecting the DC bias potential one can quantitatively monitor ECSA degradation (pathway 1) with EIS (instead of CV) by using monitoring the pseudo-capacitance associated with H-adsorption (Figure 2) [4].ReferencesF. S. Saleh and E. B. Easton, J. Electrochem. Soc., 159, B546 (2012).J. I. Eastcott and E. B. Easton, J. Power Sources, 245, 487 (2014).F. S. Saleh and E. B. Easton, J. Power Sources, 246, 392 (2014).O. Reid, F. S. Saleh, and E. B. Easton, Electrochim. Acta, 114, 278 (2013)

Promising anode candidates for direct ethanol fuel cell: Carbon supported PtSn-based trimetallic catalysts prepared by Bönnemann method

To find an efficient anode catalyst for ethanol electrooxidation, several trimetallic PtSnM/C (M = Ni, Co, Rh, Pd) and their corresponding bimetallic PtX/C (X = Sn, Ni, Co, Rh, Pd) catalysts were synthesized by Bönnemann's colloidal precursor method and evaluated by comparing their electrocatalytic activity using conventional electrochemical techniques. For better understanding of the catalyst deactivation during the ethanol electrooxidation, chronoamperometric test was also combined to X-ray photoelectron spectroscopy (XPS) analysis. A significant finding is that trimetallic compositions PtSnCo/C and PtSnNi/C have enhanced activity compared to that of PtSn/C, with lower onset potential for ethanol electrooxidation and notably improved peak current densities. Thus the presence of Ni and Co heteroatom seems to promote C–C bond cleavage and facilitate the removal from the catalyst surface of adsorbed intermediates. These trends are satisfactorily confirmed by testing in a direct ethanol fuel cell (DEFC), since trimetallic PtSnNi/C and PtSnCo/C anode catalysts have significantly higher overall performance and peak power density than Pt/C, PtSn/C or other trimetallic catalyst compositions PtSnRh/C or PtSnPd/C. Furthermore, the presence of Ni or Co helps to improve the weak stability of PtSn/C by providing a stronger Pt–carbon support interaction. XPS results revealed that the surface Pt/Sn atomic ratio of PtSnNi/C catalyst only slightly decreased even after 12 h at 500 mV. On the other hand, a higher concentration of oxide species appeared on the treated PtSn/C surface as a result of a high degradation of carbon support.

The influence of the membrane thickness on the performance and durability of PEFC during dynamic aging

The influence of the membrane thickness on the performance and durability of 25 cm2 membrane electrode assembly (MEA) toward dynamic aging test was investigated. The tested MEAs consist of chemically stabilized membranes (AQUIVION™) with thicknesses of 30 and 50 μm, electrocatalyst – 46 %Pt/C (Tanaka) with Pt loadings of 0.25 (anode), 0.45 mg cm−2 (cathode) and gas diffusion layers 25 BC (SGL Group). The applied dynamic aging procedure is repetitive current cycling between 0.12 A cm−2 for 40 s and 0.6 A cm−2 for 20 s. The testing conditions were 80 °C, fully saturated hydrogen and air, total pressure of 2.5 atm abs. The aging procedure was regularly interrupted for evaluating the MEAs' “health” via electrochemical methods and mass spectrometry. The carbon support degradation as a function of the electrode potential, current cycling and supplied gas was studied. The effects of the Pt particles agglomeration and Pt physical loss in the active layer of the cathode on the MEAs performance degradation were individually assessed. The effect of the membrane thicknesses on the performance and durability of the PEFC was established. The reasons and stages of MEAs performance degradation were analyzed.