Anode Materials for Mitigating Hydrogen Starvation Effects in PEM Fuel Cells

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability—milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

ABSTRACT: A new synthesis method for the preparation of Pt electrocatalysts on carbon nanofibers (CNFs) isreported. In this method, Pt electrocatalysts are loaded onto 2-naphthalenethiol (NT) functionalizedCNFs. The noncovalent functionalization of CNFs by NT is the effective way for better distributionof Pt particles and higher electrocatalytic activity in polymer electrolyte membrane fuel cells. It wasfound that the presence of NT acts as a poison to catalysts. Therefore, it is necessary to remove NTthrough the heat treatment at 400 o C.Keywords : 2-naphthalenethiol, Graphitized carbon, Non-covalent functionalization, PEM fuel cell Received December 13, 2010 : Accepted December 27, 2010 1. Introduction Carbon black (CB) is the typical catalyst supportmaterial for polymer electrolyte membrane (PEM) fuelcells. However, the intrinsic drawback in use of CBas catalyst support is electrochemical carbon corrosion.Many studies reported that electrochemical carbon cor-rosion occurring during fuel cell start-up and shut-downprocess is one of the major contributors to performancedegradation of PEM fuel cells.

This paper is the outcome of a small scale fuel cell project. Fuel cell is an electrochemical device that converts energy from chemical reaction to electrical work. Proton Exchange Membrane Fuel Cell (PEMFC) is one of the different types of fuel cell, which is more efficient, having low operational temperature and fast start up capability results in high energy density. In this study, a mathematical model of 1.2 W PEMFC is developed and simulated using MATLAB software. This model describes the PEMFC behaviour under steady-state condition. This mathematical modeling of PEMFC determines the polarization curve, power generated, and the efficiency of the fuel cell. Simulation results were validated by comparing with experimental results obtained from the test of a single PEMFC with a 3 V motor. The performance of experimental PEMFC is little lower compared to simulated PEMFC, however both results were found in good agreement. Experiments on hydrogen flow rate also been conducted to obtain the amount of hydrogen consumed to produce electrical work on PEMFC.

Proton-Exchange Membrane (PEM) Fuel Cells have the potential to contribute to the future of emission-free, efficient power systems for a wide range of applications. However, operational lifetime of the fuel cells is limited due to failure of cell components, with the membrane being one of the key ones. In PEFCs, chemical decomposition of the membrane due to the attacks of radicals formed during operation along with formation of pinholes, cracks, and delamination influenced by mechanical stressors leads to increased crossover of reactant gases and reduced power loss. Since the chemical and mechanical stressors are largely influenced by the temperature and humidity, it is difficult to determine what mechanisms dominate the membrane lifetime in operation. To understand underlying mechanisms of failure under chemical, mechanical, and combined chemical-mechanical degradation modes, and assess membrane lifetime in shorter time periods, accelerated stress testing (AST) methods are commonly employed. Swelling (or hydration) of the membrane is a key property in fuel cells since it controls both conductivity and stability of the membrane, which could be related to the cell performance and durability, respectively. Swelling-induced operational stresses in the membrane trigger failure mechanisms in the form of cracks or delamination, the impact of which could be further accelerated in the presence of chemical degradation.In this talk, the mechanical aspects of the durability of the polymer fuel cell membranes will be discussed at multiple length-scales. First, membrane's mechanical response during cell operation will be discussed along with the effects of swelling and mechanical properties. We will elaborate on how membrane deformation could be related to the mechanical failure modes ranging from fatigue behavior to the growth of pinholes and cracks, depending on the operating conditions and stress-states. Strategies that could potentially improve membrane durability by tuning its properties will also be discussed. The results will be demonstrated using experimental data as well as modeling methodologies.We will then show how the swelling and properties of membranes change with degradation by focusing on perfluorosulfonic-acid (PFSA) ionomers, the most widely studied membrane in PEM fuel cells. Changes in structure-property relationship of PFSA membranes due to degradation are highlighted based on the investigations on the morphology, mechanical properties and water uptake of degraded membranes. By studying the correlations between measured properties and structural features one can better understand the mechanical aspects of degradation. We will also elaborate how chemical and mechanical stressors could influence each other and control the overall membrane failure. Lastly, mechanistic approaches will be introduced to identify key membrane properties for mechanical durability, to simulate membrane deformation and failure, and to explore models for lifetime prediction. Acknowledgements The authors would like to thank the Los Alamos Fuel Cell Team for their help preparing the degraded membrane samples and for helpful discussions. This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02-05CH11231.

Graphene synthesized from the exfoliation of graphite has a great potential in polymer electrolyte membrane fuel cell (PEMFC) applications.[1] The highly graphitic structure composed of the conjugated carbons endows graphene with extremely high electric conductivity and chemical/electrochemical stability. Improved stability is required for PEMFC catalyst support materials as the stability plays a critical role in determining the overall durability of PEMFC systems. While graphene is an attractive material, there are three major barriers when using graphene-supported Pt-based catalysts in PEMFCs: 1) the lack of bonding sites for catalyst landing on the basal plane of graphene causes the migration/aggregations of Pt nanoparticles when subjected to harsh accelerated stress tests (ASTs), 2) the highly hydrophobic surface of graphene is difficult to wet when mixed with the Nafion ionomer particles, leading to a poor catalyst/ionomer interface, 3) 2D graphene sheets tends to restack back to graphite structure through π-π interactions, which can severely block the O2 diffusion and retard the catalytic reactions, in particular when PEMFCs are operated at high current density (>1.5 A/cm2),and 4) the exfoliation of natural graphite using wet chemistry method usually yield graphene that contains some defects and oxygenated functional groups, which can de-stabilizes its conjugated electronic structure which negatively affects electronic conductivity and stability. Furthermore, when subjected to harsh potential cycling (e.g. 1.0-1.5 V; potentials which can be observed during start-up/shut-down or local fuel starvation), these defect sites are vulnerable to carbon corrosion. To overcome these challenges, we use a novel approach to make the 2D graphene sheets into 3D composite materials with many channels and pores to facilitate the facile mass transport by developing the highly stable hierarchical polybenzimidazole (PBI) -grafted graphene hybrids supported Pt catalysts for PEMFCs. PBI- functionalization is found to homogenize the chemical environment of graphene surface, resulting in a uniform dispersion of Pt nanoparticles that exposes more active sites for electro-catalytic reactions. In order to construct appropriate pore structures in the catalyst layers, spacers were introduced using graphitized carbon materials or metal oxides. The surface charge was imposed on the surfaces of these spacer-particles as opposed to PBI-graphene, so that the columbic attraction forces drive them to anchor onto graphene sheets. As a result, the secondary pore volume (characterized by mercury porosimetry) significantly increases, leading to the improved mass transport of reactants (H2 and O2) and water, and consequently an improvement in the PEMFCs performance at high current density (i.e. 2.0 A/cm2). Additionally, we prepared graphene nanoplatelet with smaller dimensions as the catalyst supports, which leads to extra voltage gains at the high current density. Covalent grafting polymers onto graphene sheets containing semi-flexible backbones on graphene defects can help to re-build the conjugated carbons and de-localize the electron, as well seal the dangling bond resulting from graphene synthesis. The PEMFC durability studies carried out by our research group has shown significant improvement on support stability when they were cycled between 1.0 V to 1.5 V for 5000 cycles compared to traditional carbon support materials,[RB1]during which the mass activity is only decreased by 47% for Pt/PBI-graphene and 25% for Pt/PBI-nanographene. The reported results are very close to DOE 2020 targets on catalyst support stability [2]. In addition, these materials show improved Pt stability. Strengthened interaction between functional groups and Pt particles, as characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) greatly improves the catalyst stability. Our developed PBI-graphene supported Pt demonstrated a 27% decrease in mass activity and 15% loss of ECSA after 10,000 cycles between 0.6 V and 1.0 V. This exceeds the DOE 2020 targets for automotive PEM fuel cell electro-catalysts.[2] The unique advantages of the hierarchical PBI-functionalized graphene/nanographene hybrids demonstrates the potential for these materials to be the next generation of catalyst supports for PEMFCs. [1]. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183 [2]. U.S. DRIVE FuelCell Tech Team Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells (2013)

Proton exchange membrane fuel cell vehicles using hydrogen as fuel has become one of the key development directions in the field of new energy vehicles. However, hydrogen is flammable, it is easy to trigger serious safety accidents once hydrogen leak occurs. Therefore, the application of hydrogen leakage diagnosis methods is essential for the safe operation of proton exchange membrane fuel cell system. This paper reviews the existing methods of diagnosing hydrogen leakage for proton exchange membrane fuel cell system. The principles, research status and application scopes of different methods are elaborated, and these methods are analyzed based on three aspects of on-line diagnosis, real-time performance and calculation complexity. Furthermore, suggestions are provided for the selection of on-line hydrogen leakage diagnosis methods for fuel cell vehicles. Based on this review, readers can easily understand the current status of hydrogen leakage diagnosis methods for proton exchange membrane fuel cell system, and it can provide references for the application selection of the hydrogen leakage diagnosis methods for fuel cell vehicles.

Copper Plates are widely used as the current collectors in Polymer Electrolyte Membrane (PEM) Fuel cells. With exposure to the atmosphere, copper oxidizes and causes bright copper surfaces to tarnish. This corrosion results in decreasing the current produced. This paper aims to examine the effects of copper corrosion in the performance of PEM Fuel cell. The experiments were conducted on a single PEM Fuel cell with working area of 25cm2 without external cell heating. The feed velocities of hydrogen and oxygen were fixed at 0.5 and 0.75 L min-1, respectively. To bring out the effect of corrosion, a set of copper plates were kept open to atmosphere in the temperature range of 30 – 40oC for 2 months. Then the corroded copper plates were used as current collectors in the PEM Fuel Cell and the performance was monitored. It was observed that ohmic losses increased drastically and the maximum voltage drop was 0.4V. Figure 1

A two-phase flow, multi-component model has been optimized for a PEM (Proton Exchange Membrane) Fuel Cell. The modeling domain consists of the membrane, two catalyst layers, two diffusion layers, and two channels. Both liquid and gas phases are considered in the entire cathode and anode, including the channel, the diffusion layer and the catalyst layer. The Gravity effect on liquid water was considered in channels. Typical two-phase flow distributions in the cathode gas channel, gas diffuser and catalyst layer are presented. Source term and porosity term were optimized. Based on the simulation results, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures. Water mass fraction for the fuel cell with anode upward is higher than that the case with cathode-upward. Liquid water with the case of cathode-upward blocks pores in the gas diffuser layer leading to increasing the concentration polarization. Gravity of liquid water exerts the effect on the water mass fraction in the cathode.

Water management is a critical design issue for polymer electrolyte membrane (PEM) fuel cells, because the PEM must be maintained at the proper water content to remain ionically conducting without flooding the electrodes. Furthermore, portable PEM power systems should operate at water balance to minimize weight. This paper presents the basic design relationships that govern water balance in a PEM fuel cell. Specific calculations are presented based on data from hydrogen/air and direct methanol fuel cells currently under development for portable power systems. We will show how the water balance operating point depends on the cell operating parameters and show the sensitivity to off-design conditions.

Fuel cell and hydrogen (FCH) technologies are expected to add to decarbonisation of energy and transport sector. One thing that prevents FCH commercialization is lack of recycling and dismantling strategies, due to not available lists of critical materials. One of the considered FCH technologies within the EU funded HyTechCycling project is polymer electrolyte membrane fuel cell (PEMFC), which is the focus of this paper. One of the goals of EU project HyTechCycling is to carry out a Life Cycle Assessment (LCA) of PEMFC technology. Main focus of this paper is to gather and properly set all the data needed for LCA study of the PEMFC system. First the PEMFC was broken down to core components to identify all materials constituting the PEMFC components. For PEMFC the LCA numerical model was done in GaBi Thinkstep software. In first part of the paper the list of common PEMFC materials were identified. Furthermore with this list of materials a LCA study for 1kW PEMFC system was done. The scope of the study was “cradle to gate” with four different scenarios of hydrogen production and 20,000 h of operation. For environmental impact assessment the CML2001 life cycle impact assessment (LCIA) methodology was used. LCA results of 1kW PEMFC system shows that in manufacturing phase the stack contributes 60-90% in total environmental impacts and the BoP components are in range of 10-20%. In operation phase the hydrogen production with electrolysis using hydroelectricity and natural gas reforming are scenarios with lowest environmental impacts. The platinum group metals (PGM's), among which Pt is standing out the most, are the biggest contributors in the total environmental impact in the manufacturing phase of 1 kW PEMFC system. Therefore, on average 61% reduction in environmental impacts can be achieved with an assumption of 95% recycling of Pt.

Performance analysis of PEM fuel cell in mobile application under real traffic and environmental conditions

Proton exchange membrane fuel cells (PEMFC) technology is already in commercialization phase for automotive applications. However, the biggest challenges for its successful mass commercialization are cost and durability. Current PEMFC technology can meet the lifetime targets but the cost to achieve it can be very high due to material’s cost and system level mitigation complexities. Understanding of the membrane electrode assemblies (MEA) degradations using relevant durability accelerated tests at early stages of development is essential. PEMFCs for automotive applications goes through thousands of cycles during normal operation of the vehicle over the expected life span of 10 years. These cycles include a combination of load or voltage cycles, start and stop cycles, humidity cycles and temperature cycles. Study have shown that start-stop cycling accounts for 44% of operation mode while load cycling and idling for 28% each (1). Under these cycles, fuel cells suffer from severe degradations of its main components; cathode, anode, membrane and bipolar plates. The PEMFC cathode can degrade during start-stop operation via carbon corrosion (2). Cathode degradation can be mitigated by using stable materials or by applying system mitigations. System mitigation by a voltage limiting control (VLC) can be one of the effective measures (3). The VLC can, however, lead to voltage spikes (high positive potentials) at the anode during fuel cell start-up and shut-down. Moreover, during fuel (H2) starvation conditions, the potential at the anode can give rise to high positive potentials leading to anode catalyst layer degradation. During idling operation mode, PEMFC is at or near open circuit conditions which is known to accelerate proton exchange membrane (PEM) decomposition especially under low humidification (4). Peroxy-radicals originating from hydrogen peroxide produced at the anode catalyst surface cause the PEM degradation (5). Pt band formation in the membrane is also known to promote peroxide formation leading to membrane decomposition (6-7). Several accelerated protocols are currently being used to evaluate the durability of PEMFC. At Nissan, our durability protocols are focused on understanding the material degradation under relevant automotive cycles and correlating single cell durability to the stack level. Start-stop accelerated protocol to evaluate the cathode support durability consist of potential cycling from 1.0 V to 1.5 V using triangular waveform for 1000 cycles with diagnostics conducted intermittently after certain number of cycles. Cathode catalyst durability is evaluated using load cycling accelerated protocol wherein the potential is cycled between 0.6 V to 0.95 V to simulate peak load and OCV/idle using a square wave profile with 3 second hold at each potential. The upper potential is known to have significant impact on catalyst degradation (8). Load cycling is performed for 10,000 cycles with diagnostics conducted intermittently. Anode degradation under cell reversal condition is evaluated using a protocol designed to simulate high voltage spikes at the anode. Cell reversal cycling is performed by applying a load of 0.2 A/cm2 for 1 sec while flowing N2 at the anode and air at the cathode while measuring the cell voltage under the applied load for a total of 200 cycles. Membrane durability is evaluated using humidity cycling (mechanical durability) and potential hold at open circuit (chemical durability). Membrane’s chemical durability is evaluated at 90 °C and 30% RH under hydrogen and oxygen by operating MEA at open circuit. The target of OCV hold test is 500 hrs while maintaining potential above 0.8 V. Mechanical durability of membrane is evaluated by cycling RH to simulate dry condition (0%) and wet condition (100%) for target of 20,000 cycles. All these accelerated tests are performed at ambient pressure and summarized in the Table 1 below. Acknowledgments: The author would like to gratefully acknowledge Nissan Technical Center North America Fuel Cell Research team members (Dr. Amod Kumar, Dr. Ramesh Yadav and Dr. Cenk Gumeci) and Nissan Motor Co. Ltd Japan Fuel Cell research team members (Dr. Atsushi Ohma, Dr. Yoshihisa Furuya) for technical discussions and data.

The durability of proton exchange membranes (PEMs) is a critical factor influencing the reliability and longevity of PEM fuel cells (PEMFCs). In the quest for durable fuel cell systems, expanded polytetrafluoroethylene (ePTFE) reinforced ionomer membranes stand as a promising avenue for designing membrane electrode assembly (MEA) compared to plain perfluorinated sulfonic acid (PFSA) membranes. The incorporation of ePTFE reinforcement introduces a complex microstructure that influences the membrane's ability to withstand chemical attack, mechanical stress, and oxidative damage during operation1,2. Despite several recent studies demonstrating enhanced durability in accelerated stress testing (AST)3–5, the intricate relationship between the membrane's microstructure, ePTFE reinforcement, and chemo-mechanical degradation in fuel cell operation has not been explored. In the present work, the durability of MEAs composed with ePTFE-reinforced ionomer membranes of 15 µm nominal thickness in PEMFCs was investigated using a combined chemical and mechanical accelerated stress test (CM-AST). The chemo-mechanical degradation of the ePTFE-reinforced membranes was characterized using X-ray computed tomography (XCT). The cell exhibited two degradation stages under CM-AST. The initial phase of 25 AST cycles was marked by a notable decline in cell performance. Subsequently, in the second phase, the cell maintained consistent operation until the conclusion of testing at the 65th cycle. The cell degradation rate for the first stage was 62 µV h-1 and the overall degradation rate was 58 µV h-1. The hydrogen crossover current density slowly increased from 3.3 mA cm−2 to 5.1 mA cm-2 at 80 °C under atmospheric pressure. A comprehensive analysis of cells using XCT showed significant decay of the catalyst layers and membrane thinning. 3D visualization by XCT scans of the entire active surface area confirmed the absence of pinholes or cracks in the membrane that explained the observed low hydrogen crossover. Chemical degradation was observed in the ionomer-rich layers, while no signs of ePTFE reinforcement degradation were found. Therefore, the reinforcement layer plays a key role in mitigating the chemical and mechanical degradation by limiting the gas crossover rather than altering the degradation process. The results concluded that by optimizing the microstructure of ePTFE, reinforced membranes can pave the way for more reliable and durable PEMFC systems.Keywords: Composite ionomer membranes, ePTFE reinforcement, PEMFC, chemo-mechanical degradation, accelerated stress testing, X-ray computed tomography References (1) Liu, W.; Suzuki, T.; Mao, H.; Schmiedel, T. Development of Thin, Reinforced PEMFC Membranes through Understanding of Structure-Property-Performance Relationships. ECS Trans. 2013, 50 (2), 51–64. https://doi.org/10.1149/05002.0051ecst.(2) Shi, S.; Weber, A. Z.; Kusoglu, A. Structure/Property Relationship of Nafion XL Composite Membranes. Journal of Membrane Science 2016, 516, 123–134. https://doi.org/10.1016/j.memsci.2016.06.004.(3) Ramani, D.; Singh, Y.; White, R. T.; Wegener, M.; Orfino, F. P.; Dutta, M.; Kjeang, E. 4D in Situ Visualization of Mechanical Degradation Evolution in Reinforced Fuel Cell Membranes. International Journal of Hydrogen Energy 2020, 45 (16), 10089–10103. https://doi.org/10.1016/j.ijhydene.2020.02.013.(4) Sadeghi Alavijeh, A.; Bhattacharya, S.; Thomas, O.; Chuy, C.; Kjeang, E. A Rapid Mechanical Durability Test for Reinforced Fuel Cell Membranes. Journal of Power Sources Advances 2020, 2, 100010. https://doi.org/10.1016/j.powera.2020.100010.(5) Tang, H.; Pan, M.; Wang, F.; Shen, P. K.; Jiang, S. P. Highly Durable Proton Exchange Membranes for Low Temperature Fuel Cells. J. Phys. Chem. B 2007, 111 (30), 8684–8690. https://doi.org/10.1021/jp073136t.

Construction and operation of hydrogen energy utilization system for a zero emission building