Comparison of Two Models for Temperature Observation of Miniature PEM Fuel Cells Under Dry Conditions

Water and thermal management have been identified as technical hurdles to the successful implementation of low-temperature polymer electrolyte membrane (PEM) fuel cell (PEMFC) power systems. In low-power applications, miniature PEMFCs show significant promise as a competitor to lithium-ion batteries. Significant design work is underway to improve the specific power and energy densities of these fuel cells. However, little attention has been given to characterizing transient response in these miniature applications to enable gains in system design, optimization, and control. This work develops, calibrates, and experimentally validates two different dynamic control-oriented models for open-loop temperature state observation in miniature PEMFCs. Of critical importance, these estimators target operation under dry conditions with no reactant pretreatment. Operational conditions are then identified for which each model architecture is more suitable, specifically targeting minimal model complexity. A sensitivity analysis was completed that indicates necessary sensor measurements with sensor frugality in mind. The dynamic responses under changes in load and fuel stoichiometry are well captured over a range of operating conditions

Assessment and development of mitigation strategies for membrane durability in fuel cells

Fuel cell membranes undergo simultaneous or individual chemical and mechanical degradation under dynamic fuel cell operating conditions. This combined stress development effect compromises the functionality of the membrane and ultimately, the overall durability of the fuel cell system. Therefore, it is critical to understand the underlying degradation mechanisms and failure modes under operational conditions. In this thesis, an extensive research methodology including accelerated stress tests, visualization techniques, and finite element modeling is adopted in order to understand and mitigate membrane degradation. The membrane characterization is facilitated using a non-invasive laboratory-based X-ray computed tomography (XCT) system for 3D visualization of membrane damage progression over the lifetime of the fuel cell. The 3D XCT approach is first applied to understand the degradation mechanism responsible for combined chemical and mechanical membrane degradation. The XCT approach is further expanded to 4D in situ visualisation through periodic same location tracking within a miniature operational fuel cell. Fuel cell membranes with mechanical reinforcements and chemical additives are tested as existing mitigation strategies for the isolated degradation stressors. Under pure chemical degradation, the chemically and mechanically reinforced membrane does not show membrane thinning or shorting sites and exceeds the lifetime of the non-reinforced membrane by 2x. The reinforced membrane also mitigated/delayed the crack development during pure mechanical degradation as compared to the non-reinforced membrane. However, significant membrane degradation is still observed and attributed to buckling and delamination mechanisms within the membrane electrode assembly (MEA). Mitigation of these mechanisms is demonstrated through two novel approaches proposed in this thesis: i) reduced surface roughness gas diffusion layers (GDLs); and ii) bonded MEAs. Both mitigation strategies are tested using the same experimental workflow and shown to provide substantial mitigation against fatigue driven mechanical membrane degradation via reduced membrane buckling, resulting in a doubling of the test lifetime in each case. Complementary finite element simulations corroborate the experimental findings and further estimate the critical GDL void sizes to prevent membrane buckling and the required interfacial MEA adhesion quality to stabilize the MEA for improved membrane durability

Two-phase flow dynamics in a micro hydrophilic channel: A theoretical and experimental study

In this paper, two-phase flow dynamics in a micro hydrophilic channel are experimentally and theoretically investigated. Flow patterns of annulus, wavy, and slug are observed in the range of operating condition. A set of empirical models based on the Lockhart-Martinelli parameter and a two-fluid model using several correlations of the relative permeability are adopted; and their predictions are compared with experimental data. It shows that for low liquid flow rates most model predictions show acceptable agreement with experimental data, while in the regime of high liquid flow rate only a few of them exhibit a good match. Correlation optimization is conducted for individual flow pattern. Through theoretical analysis of flows in a circular and 2-D channel, respectively, we obtain correlations close to the experimental observation. Real-time pressure measurement shows that different flow patterns yield different pressure evolutions. © 2013 Elsevier Ltd. All rights reserved

Modeling, diagnosis, and control of fuel-cell-based technologies and their integration in smart grids and automotive systems

Society is gradually becoming aware that the current energy system based on the use of fossil fuels is inefficient, highly polluting, and finite supply. Within the scientific community and industry stakeholders, there is a unified agreement that indicates that hydrogen (H2), as an energy vector, combined with other sources of alternative energy, represents a safe and viable option to mitigate the problems associated with hydrocarbon combustion because the entire system can be developed as an efficient, clean, and sustainable energy source. In this context, the change from the current energy system to a new system with a stronger involvement of H2 relentlessly involves the introduction of fuel cells as elements of efficient energy conversion.Peer Reviewe

Electrically-Driven Ion Transmission Through Two-Dimensional Nanomaterials

Two-dimensional nanomaterials such as graphene and hexagonal boron nitride are being intensively studied as selective barriers in separation technology owing to their unique subatomic selectivity. In their pristine forms, they are impermeable to atoms, molecules, and ions except for thermal protons. Graphene, with its angstrom-scale thickness, is regarded as the thinnest membrane so its transport selectivity comes from the selectivity of active sites at which permeant transmission occurs. This dissertation tested the hypothesis that the selectivity ratio of hydrogen isotopes (protium, Deuterium, and tritium) through membrane could be improved by incorporating graphene and related 2D materials in the membrane electrode assembly of a polymer electrolyte membrane electrolysis cell. The mechanism by which protons or deuterons traverse the energy barrier of 2D materials was also investigated with a focus on the temperature dependence of isotopic selectivity in crossing rates. By carefully positioning a 2D material within the ionomer membranes of a membrane electrode assembly, the isotopic ion filtering functionalities of graphene and analogs were enhanced. Proton transmission through graphene was found to occur at a very high rate (1.0 A cm-2 achieved at a potential bias of \u3c 200 mV) with a selectivity ratio of at least 10 compared to deuteron transmission. The transmission rates of Protons and deuterons across single-layer graphene were measured as a function of temperature. An electrochemical model based on charge-transfer resistance was invoked to estimate standard heterogeneous ion-transfer rate constants. An encounter pre-equilibrium model for the ion-transfer step was used to estimate rate constants which provide values for activation energies and exponential pre-factors for proton (or deuteron) transmission across graphene. Activation energies of 48 ± 2 kJ mole-1 (0.50 ± 0.02 eV) and 53 ± 5 kJ mole-1 (0.55 ± 0.05 eV) were obtained for protons and deuterons respectively, through single-layer graphene. The difference of 50 meV is in good agreement with the expected difference in vibrational zero-point energies for O-H and O-D bonds. This work is an important harbinger for the prospects of developing graphene-based PEM electrochemical cell ion filters for fuel cells, electrolysis cells, gas separation and purification, and desalination applications

Modeling, diagnosis and control of fuel-cell-based technologies and their integration in smart-grids and automotive systems

The main objective of the current Special Section is to collect, formally present and discuss the most recent and relevant advances in control-oriented modeling and validation, system diagnosis and advanced control design of complex energy conversion systems based on fuel cells. Moreover, the Special Section is also focused on providing the researchers and engineers with the state-of-art research and guidelines in these important fields for the next years. In total, the Special Session is composed by 17 contributions covering the research in theoretical aspects related to modelling, diagnosis and control applied to energy management systems based on fuel cells or considering fuel cells as part of overall hybrid systems.Peer ReviewedPostprint (author's final draft

In situ measurement, characterization, and modeling of two-phase pressure drop incorporating local water saturation in PEMFC gas channels

Proton Exchange Membrane Fuel Cells (PEMFCs) have been an area of focus as an alternative for internal combustion engines in the transportation sector. Water and thermal management techniques remain as one of the key roadblocks in PEMFC development. The ability to model two-phase flow and pressure drop in PEMFCs is of significant importance to the performance and optimization of PEMFCs. This work provides a perspective on the numerous factors that affect the two-phase flow in the gas channels and presents a comprehensive pressure drop model through an extensive in situ fuel cell investigation. The study focused on low current density and low temperature operation of the cell, as these conditions present the most challenging scenario for water transport in the PEMFC reactant channels. Tests were conducted using two PEMFCs that were representative of the actual full scale commercial automotive geometry. The design of the flow fields allowed visual access to both cathode and anode sides for correlating the visual observations to the two-phase flow patterns and pressure drop. A total of 198 tests were conducted varying gas diffusion layer (GDL), inlet humidity, current density, and stoichiometry; this generated over 1500 average pressure drop measurements to develop and validate two-phase models. A two-phase 1+1 D modeling scheme is proposed that incorporates an elemental approach and control volume analysis to provide a comprehensive methodology and correlation for predicting two-phase pressure drop in PEMFC conditions. Key considerations, such as condensation within the channel, consumption of reactant gases, water transport across the membrane, and thermal gradients within the fuel cell, are reviewed and their relative importance illustrated. The modeling scheme is shown to predict channel pressure drop with a mean error of 10% over the full range of conditions and with a mean error of 5% for the primary conditions of interest. The model provides a unique and comprehensive basis for developing a fundamental adiabatic two-phase flow pressure drop predictive scheme for PEMFC reactant channels

The formation of water droplets in an air-breathing PEMFC

Air-Breathing Proton Exchange Membrane Fuel Cells (AB-PEMFC) have the potential to supersede lithium-ion batteries in portable electronics. However, their water management issue has yet to be resolved to ensure optimum cell performance and safe system operation. In this paper, the formation of water droplets and their aggregation in the cathode flow channels of an operating AB-PEMFC is investigated by direct visualisation under various operating conditions. The developed optical set-up enables observation of droplet formation on the surface of the membrane from the top and side view of the channels simultaneously. The two orthogonal views reveal that during formation the receding and advancing droplet contact angles are almost identical with values that increase, in a similar trend to the droplet height, with increasing droplet diameter. Water films were able to develop and maintain direct contact with the side wall of the channels even under the effect of gravitational force. The aggregation of water droplets in the channels was strongly influenced by the change in the air and hydrogen stoichiometry conditions. However, these operating parameters appear to have no significant effect on the water extraction from the channels contrary to load and temperature, where temperature has proved to be the most effective water removal mechanism with minimum reduction in the current density of AB-PEMFC