On the mechanisms of electrochemical transport in Polymer Electrolyte Fuel Cells

The Polymer Electrolyte Fuel Cell (PEFC) is well-poised to play a key role in the portfolio of future energy technologies for civil and military applications. Principally, the PEFC converts part of the chemical energy released during hydrogenoxidation and oxygen-reduction into electrical energy, generating water a bi-product. It is potentially a zero-emissions technology which can operate silently due to the absence of any moving parts, has quick start-up characteristics and can achieve high thermodynamic efficiency. In order to ensure that the PEFC emerges as a viable option for all applications, it is necessary to ensure that the technology is reliable, capable of delivering performance and cost-effective throughout its life-cycle. To achieve these objectives, a better fundamental understanding of the mechanisms of electrochemical transport in the PEFC is required than is presently available. The literature identifies that multi-component electrochemical transport within the PEFC plays a central role in fuel cell operation and longevity. Water transport is one of these. It is well-understood that excessive amounts of water within the porous electrodes of the cell can cause flooding, which impedes the supply of reactant gases. It is also well-understood that insufficient water can cause the polymer electrolyte membrane (PEM) to dehydrate, thereby reducing its proton conductivity. Both of these processes can undermine cell performance. Repetitive hydration cycles are also known to precipitate degradation mechanisms which can undermine reliability. However, the mechanisms of multi-component and potentially two-phase transport across the PEFC as a multi-layered assembly which includes the porous electrodes and the PEM are not understood as well: the mechanisms of contaminant transport, fuel crossover and liquid water infiltration particularly through the PEM are important examples. The modelling literature demonstrates that electrochemical transport in the PEFC is treated either through the use of dilute solution theory or concentrated solution theory. The modelling literature also demonstrates a wide spectrum in the application of modelling assumptions and the formulation of electrochemical equations to simulate transport in the different layers of the PEFC. This thesis describes research aimed at reconciling the different modelling approaches and philosophies in the literature by developing and applying a unified mechanistic electrochemical treatment to describe multi-component, two-phase transport across the layers of the PEFC. The approach adopted here is first to construct a multi-component zerodimensional model for multi-component input gases which is merged with a multilayer PEFC model to correctly predict the boundary conditions in the gas channels based on the cross-flow of components through the cell. The model is validated using data from the open literature and applied to understand contaminant crossover from anode to cathode. The second step is to develop a unified mechanistic electrochemical treatment to describe multi-component transport across the layers of the PEFC: the general transport equation. This is central to the contribution of this thesis. It is theoretically validated by deriving the key transport equations used in the benchmark fuel cell modelling literature. It is then implemented with the multi-component input model developed previously and validated using data from the open literature. The model is subsequently applied to understand fuel crossover characteristics in the cell. The third and final step is to further-develop the application of the general transport equation to account for two-phase transport across the layers of the PEFC. The resulting model is validated against three different sets of data from the open literature and subsequently applied to understand the effects of PEM thickness, anode gas humidification, cell compression and PEM structural reinforcement on liquid infiltration and two-phase transport across the PEM. It is demonstrated that the general transport equation developed in this thesis establishes a backbone understanding of the modelling and simulation of transport across the layers of the PEFC. The study successfully reconciles the different modelling philosophies in the fuel cell literature. The progressive validation and application of the general transport equation demonstrates the potential to enhance the scientific understanding of factors affecting PEFC performance and demonstrates its value as a tool for computationally-based cell design, optimisation and diagnostics.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

A review of performance degradation and failure modes for hydrogen-fuelled polymer electrolyte fuel cells

A qualitative account of the causes and effects of performance degradation and failure in hydrogen-fuelled polymer electrolyte fuel cells (PEFCs) is given in the present review. The purpose of the review is to establish a backbone understanding of the phenomenological processes that occur within the PEFC, how they interact, how they are influenced through elements of design, manufacturing and operation, and ultimately how they result in performance degradation and cell failure. In the current work, 22 common faults are identified which are induced by 48 frequent causes. The major PEFC components considered here that are susceptible to faults are the polymer electrolyte-based membrane, the anode and cathode catalyst layers, gas diffusion and microporous layers, seals and the bipolar plate. Faults pertaining to these components can cause irreversible increases in activation, mass transportation, ohmic and fuel efficiency losses, or indeed cause catastrophic cell failure

A polymer electrolyte membrane fuel cell model with multi-species input

With the emerging realization that low temperature, low pressure polymer electrolyte membrane fuel cell (PEMFC) technologies can realistically serve for power-generation of any scale, the value of comprehensive simulation models becomes equally evident. Many models have been successfully developed over the last two decades. One of the fundamental limitations among these models is that up to only three constituent species have been considered in the dry pre-humidified anode and cathode inlet gases, namely oxygen and nitrogen for the cathode and hydrogen, carbon dioxide, and carbon monoxide for the anode. In order to extend the potential of theoretical study and to bring the simulation closer towards reality, in this research, a 1D steady-state, low temperature, isothermal, isobaric PEMFC model has been developed. The model accommodates multi-component diffusion in the porous electrodes and therefore offers the potential to further investigate the effects of contaminants such as carbon monoxide on cell performance. The simulated model polarizations agree well with published experimental data. It opens a wider scope to address the remaining limitations in the future with further developments

Polymer electrolyte fuel cell transport mechanisms: simulation study of hydrogen crossover and water content

Hydrogen crossover and membrane hydration are significant issues for polymer electrolyte fuel cells (PEFC). Hydrogen crossover amounts to a quantity of unspent fuel, thereby reducing the fuel efficiency of the cell, but more significantly it also gives rise to the formation of hydrogen peroxide in the cathode catalyst layer which acts to irreversibly degenerate the polymer electrolyte. Membrane hydration not only strongly governs the performance of the cell, most noticeable through its effect on the ionic conductivity of the membrane, it also influences the onset and propagation of internal degradation and failure mechanisms that curtail the reliability and safety of PEFCs. This paper focuses on how hydrogen crossover and membrane hydration are affected by; (a) characteristic cell geometries, and (b) operating conditions relevant to automotive fuel cells. The numerical study is based on the application of a general transport equation developed previously to model multi-species transport through discontinuous materials. The results quantify (1) the effectiveness of different practical mechanisms which can be applied to curtail the effects of hydrogen crossover in automotive fuel cells and (2) the implications on water content within the membrane

Failure analysis of Polymer Electrolyte Fuel Cells (PEFC)

A qualitative FMEA study of Polymer Electrolyte Fuel Cell (PEFC) technology is established and presented in the current work through a literature survey of mechanisms that govern performance degradation and failure. The literature findings are translated into Fault Tree (FT) diagrams that depict how basic events can develop into performance degradation or failure in the context of the following top events; (1) activation losses; (2) mass transportation losses; (3) Ohmic losses; (4) efficiency losses and (5) catastrophic cell failure. Twenty-two identified faults and forty-seven frequent causes are translated into fifty-two basic events and a system of FTs with twenty-one reoccurring dominant mechanisms. The four most dominant mechanisms discussed that currently curtail sustained fuel cell performance relate to membrane durability, liquid water formation, flow-field design, and manufacturing practices

Polymer electrolyte fuel cell transport mechanisms: a universal modelling framework from fundamental theory

A mathematical multi-species modelling framework for polymer electrolyte fuel cells (PEFCs) is presented on the basis of fundamental molecular theory. Characteristically, the resulting general transport equation describes transport in concentrated solutions and also explicitly accommodates for multi-species electro-osmotic drag. The multi-species nature of the general transport equation allows for cross-interactions to be considered, rather than relying upon the superimposition of Fick’s law to account for the transport of any secondary species in the membrane region such as hydrogen. The presented general transport equation is also used to derive the key transport equations used by the historically prominent PEFC models. Thus, this work bridges the gap that exists between the different modelling philosophies for membrane transport in the literature. The general transport equation is then used in the electrode and membrane regions of the PEFC with available membrane properties from the literature to compare simulated one-dimensional water content curves, which are compared with published data under isobaric and isothermal operating conditions. Previous work is used to determine the composition of the humidified air and fuel supply streams in the gas channels. Finally, the general transport equation is used to simulate the crossover of hydrogen across the membrane for different membrane thicknesses and current densities. The results show that at 353 K, 1 atm, and 1 A/cm2, the nominal membrane thickness for less than 5 mA/cm2 equivalent crossover current density is 30 mm. At 3 atm and 353 K, the nominal membrane thickness for the same equivalent crossover current density is about 150 mm and increases further to 175 mm at 383 K with the same pressure. Thin membranes exhibit consistently higher crossover at all practical current densities compared with thicker membranes. At least a 50 per cent decrease in crossover is achieved at all practical current densities, when the membrane thickness is doubled from 50 to 100 mm

Study of current distribution and oxygen diffusion in the fuel cell cathode catalyst layer through electrochemical impedance spectroscopy

In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCLeGas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements

Impedance study on oxygen diffusion through fuel cell cathode catalyst layer at high current

A mathematical model to simulate the electrochemical impedance spectrum in the frequency domain and the current distribution in the time domain of polymer electrolyte fuel cell cathode catalyst layer CCL operated at high currents has been developed. In the model, Fick’s second law in the frequency domain is solved to define oxygen distribution through CCL. The rate of oxygen transportation and proton conductivity are related to the current distribution equation reported in the authors’ previous study for low current operations. The model, compared against the frequency response of an experimental impedance spectrum, is then converted into the time domain using the inverse Laplace transform method. The results show the nonsteady oxygen diffusion in the CCL which allows equilibrium to be established between the bulk concentration supplied at the gas diffusion layer boundary and the surface concentration of the oxygen within the CCL. The developed model can be applied to unveil the effect of kinetic, ohmic, and mass transport mechanisms on current distribution through the thickness of the CCL from the measured impedance results

The low current electrochemical mechanisms of the fuel cell cathode catalyst layer through an impedance study

Based on the fundamental electrode theory and the impedance experimental study, a numerical model to simulate the low current distribution in the time domain and the electrochemical impedance spectra of the cathode catalyst layer (CCL) of polymer electrolyte fuel cells (PEFCs) has been developed in this study. The model development consists of two stages: to establish the fundamental equations for the low current distribution in the CCL in the time domain and to resolve the fundamental theory in the frequency domain. It was validated by comparing the simulated impedance of the CCL directly against the impedance data measured from an operational test cell. The simulated frequency response agrees well with the experimental data. The model was applied in the time domain to simulate the effects of the proton resistance and the double-layer capacitance across the CCL on the transitory and steady-state current distribution. The results showed that the model has established a backbone understanding of how the low current electrochemical mechanisms relate to the electrochemical impedance spectra of the CCL. It establishes a wider scope to relate the electrochemical impedance data to the fundamental theory of PEFCs