Regular durability testing of heavy duty fuel cell systems for transit bus application requires several thousand hours of operation, which is costly and time consuming. Alternatively, accelerated durability tests are able to generate failure modes observed in field operation in a compressed time period, by applying enhanced levels of stress. The objective of the present work is to design and validate an accelerated membrane durability test (AMDT) for heavy duty fuel cells under bus related conditions. The proposed AMDT generates bus relevant membrane failure modes in a few hundred hours, which is more than an order of magnitude faster than for regular duty cycle testing. Elevated voltage, temperature, and oxidant levels are used to accelerate membrane chemical stress, while relative humidity (RH) cycling is used to induce mechanical stress. RH cycling is found to significantly reduce membrane life-time compared to constant RH conditions. The role of a platinum band in the membrane is investigated and membranes with Pt bands demonstrate a considerable life-time extension under AMDT conditions, with minimal membrane degradation. Overall, this research serves to establish a benchmark AMDT that can rapidly and reliably evaluate membrane stability under simulated heavy duty fuel cell conditions.
Enhancing the durability of fuel cells for the transportation sector requires a better understanding of the fundamental processes that cause degradation. Field-operated PEMFCs have been shown to develop a thin parallel band of Pt inside the membrane. Reports on the effect of the Pt band on membrane durability are contradictory. Here, we examined the influence of the Pt band by performing in situ and ex situ membrane degradation tests. We report that the Pt band significantly decreases the rate of membrane degradation, thereby enhancing its longevity.Polymer electrolyte membrane (PEM) fuel cell technology is promising for zero-emission transportation, but lowering cost and improving durability remain challenges for widespread commercialization. The membrane-electrode-assembly (MEA), which consists of a polymer electrolyte membrane, catalyst layers, and gas diffusion layers, is prone to degradation under automotive fuel cell duty cycles consisting of rapid potential cycling, changes in temperature, changes in humidity, and start-up/shut-down events. Degradation of the polymer electrolyte membrane can be attributed to both chemical attack and mechanical stressors. It is believed oxidative radicals generated during fuel cell operation are the main source of chemical degradation of the polymer, leading to changes of the membranes' polymer structure. 1 These radicals may form as a result of decomposition of hydrogen peroxide formed during the ORR or from crossover of the gaseous reactants and reaction at the anode and cathode. Conditions such as low humidity, high temperature, and high cell voltage have been reported to increase the concentration of H 2 O 2 in the cell, thereby accelerating chemical degradation. 2-4 Moreover, swelling and shrinking of the membrane through changes in water content during fuel cell operation increases internal stress on the membrane and exacerbates degradation due to fatigue, pinhole, and/or crack formation. 5,6 Over the last decade, different strategies have been proposed to improve the durability of PEMs. These include removing transition metal impurities, chemical stabilization of the polymer ends, use of inorganic oxides, redesigning the side chain to reduce the number of ether bonds, and mechanical reinforcement. 7-10 The approach is to decrease the propensity of radical attack and reduce mechanical stresses during operational cycling.Membrane-electrode assemblies that have been extricated from long term, field-operated fuel cells have been shown to develop a thin parallel band of Pt inside the membrane. It is reported that Pt originally located in the cathode catalyst layer dissolves during voltage cycling and re-deposits inside the membrane, 11-14 promoted by the highly acidic fuel cell environment at voltages above 0.9 V. 15,16 Pt ions that migrate toward the anode are reduced by hydrogen crossing from the anode. 17 The precise location of the band is determined by the flux of hydrogen permeating the PEM as well as the position in the membrane where the cathode potential abruptly...
Heavy duty fuel cells used in transportation system applications such as transit buses expose the fuel cell membranes to conditions that can lead to lifetime-limiting membrane failure via combined chemical and mechanical degradation. Highly durable membranes and reliable predictive models are therefore needed in order to achieve the heavy duty fuel cell lifetime target of 18,000 h. In the present work, an empirical membrane lifetime model was developed based on laboratory data from a suite of accelerated membrane durability tests. The model considers the effects of cell voltage, temperature, oxygen concentration, humidity cycling, humidity level, and platinum in the membrane using inverse power law and exponential relationships within the framework of a general log- Labs where all the TEM testing was done. List of Tables List of Acronyms Chapter 1. IntroductionFuel cells are electrochemical devices that convert chemical energy in fuels into electrical energy directly, providing power generation with high efficiency and low environmental impact [1]. In a fuel cell system unit cells are stacked up next to each other creating an electrically connected stack according to the desired output capacity.The feed stream conditioning, thermal management, and electric power conditioning of the stack is provided by components that belong to the balance of plant.The main components of a fuel cell unit are an anode (negative electrode), cathode (positive electrode) and the electrolyte. Additional components are necessary for assembly of a fuel cell stack such as bipolar plates, flow fields and balance of plant components such as blowers and compressors for fuel supply and product removal, water and temperature management devices, converters, etc. Fuel is fed to the anode, and oxidant is fed to the cathode continuously at the same time. Hydrogen oxidation and oxygen reduction are the electrochemical reactions necessary for splitting the fuel and oxidant into ions and electrons. These reactions take place at electrode triple phase boundaries, which are catalytically active regions where the electrode particles, electrolyte phase, and gas pores intersect [2]. Highly porous electrode surfaces allow efficient diffusion of reactant gases to catalyst sites, and product removal from the fuel cell. Fuel cells are classified according to their electrolyte and fuel. The electrolyte also determines the electrode reactions and the type of ions that pass through the electrolyte.The electrolyte is thin in order to avoid losses caused by ion diffusion due to electrolyte material resistance. The electrolyte also acts as a physical barrier to prevent mixing of fuel and oxidant gas streams [1]. Some common types of fuel cells are the polymer electrolyte fuel cell (PEMFC), solid oxide fuel cell (SOFC), alkaline fuel cell (AFC), phosphoric acid fuel cell (PAFC), and molten carbonate fuel cell (MCFC) [1], [3]. SOFCs offer fuel flexibility, since they are capable of fuel reforming conventional hydrocarbon 2 Commercial Confidential fue...
Monodispersed Li 4 Ti 5 O 12 (LTO) nanoparticles with controlled microstructure were successfully synthesized by a combination of hydrolysis and hydrothermal method followed by a post-annealing process. The scanning electron microscopy images showed that particles with a size of 30-50 nm were precisely controlled throughout the synthesis process. The electrochemical tests of the as-prepared LTO electrodes in a half-cell proved its high rate performance and outstanding cyclability which benefits from the preserved well-controlled nanoparticle size and morphology. LTO electrodes were also tested in a full cell configuration in pairing with LiFePO 4 cathodes, which demonstrated a capacity of 147.3 mA h g À1. In addition, we have also demonstrated that LTO materials prepared using lithium salts separated from geothermal brine solutions had good cyclability. These demonstrations provide a promising way for making low-cost, large-scale LTO electrode materials for energy storage applications.With a fast growth of the renewable energy market, the demand for high-performance energy storage increases rapidly.As one of the most important energy-storage devices in the past decades, lithium-ion batteries (LIBs) have been widely applied in our daily life from powering cell phones, laptops and other portable electronic devices due to their relative high energy density and long cycling life.[1] However, as limited by the material functions of carbon-based anodes (mostly graphite) for fast lithium ion intercalation and deintercalation, the current commercial LIBs suffer from significant capacity degradation when working under larger-current conditions for their potential application in electric vehicles and hybrid electronic vehicles.[2] In the meantime, lithium is thermodynamically unstable in contact with the conventional organic electrolytes under % 1.0 V. This electrochemical behavior always causes the formation of an insoluble Li-ion salts passivating layer on the surface of anode materials, referred to as the solid electrolyte interphase (SEI).[3] This SEI film not only limits the capacity and dynamic response of the batteries by consuming lithium and lowering the lithium ion conduction, but also facilitates the formation of lithium dendrite that could create internal short circuits in the cells.[4] Therefore, the development of low-cost anode materials with robust recycling stability to sustain fast lithium ions insertion and extraction, as well as with reduced or eliminated SEI formation is crucial for the high power application of LIBs.To avoid the formation of SEI, extensive studies were focused on titanium-based anodes including various polymorphs of TiO 2[5] and lithium titania. [6] Among these possible alternate anode materials for high-power LIBs, spinel Li 4 Ti 5 O 12 (LTO) with a structure of Fd3 m space group has attracted significant attention owing to its unique electrochemical properties. [7] Unlike the commonly used graphite, LTO is a zero-strain insertion material. Each LTO structure unit is able t...
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