Os eletrocatalisadores Pt/C e Pt-Terras Raras/C (terras raras = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm, and Lu) foram preparados (20% em massa e razão atômica Pt-TR de 50:50) pelo método de redução por álcool, usando H2PtCl6.6H2O (Aldrich) e Terras Raras Cl3.xH2O (Aldrich) como fonte de metais, etileno glicol como solvente e agente redutor e, o carbono Vulcan XC72, como suporte. Os catalisadores foram caracterizados por espectroscopia de energia dispersiva de raios X (EDX), análises de difração de raios X (DRX) e microscopia de transmissão eletrônica (TEM). As análises por EDX mostraram que as razões atômicas dos diferentes eletrocatalisadores Pt-TR/C preparados foram similares às composições nominais de partida. Em todos os difratogramas, observa-se um pico largo em aproximadamente 2 = 25o o qual foi associado ao suporte de carbono Vulcan XC72 e quatro outros picos de difração em aproximadamente 2 = 40o, 47o, 67o e 82o os quais são associados aos planos (111), (200), (220) e (311), respectivamente, da estrutura cúbica de face centrada (CFC) de platina e suas ligas. Para os eletrocatalisadores Pt-TR/C também foram observadas fases nos difratogramas de raios X referentes aos óxidos de terras raras. Foram preparados eletrocatalisadores Pt-La/C com diferentes razões atômicas. Micrografias de transmissão eletrônica apresentaram uma razoável distribuição das partículas de Pt no suporte de carbono com algumas aglomerações...

Este trabalho tem por objetivo estudar as regiões dos canais catódicos de uma célula a combustível de membrana polimérica condutora de prótons PEM unitária, em que há acúmulo de água e os padrões de escoamento desta água nos canais, bem como as condições de operação em que isto ocorre. Esta água acumulada nos canais catódicos tem duas origens distintas, a saber: 1. água produzida na reação de redução do oxigênio no sítio catalítico do cátodo, 2. água de condensação formada a partir do vapor de água proveniente do umidificador de oxigênio. O arranjo experimental desenvolvido permitiu a perfeita visualização dos fenômenos; a saber: iniciando-se com gotículas que emergem da camada de difusão gasosa do cátodo, passando estas gotículas a se aglutinarem por um processo de coalescimento aumentando de tamanho até formarem um filme nas paredes dos canais. Em continuidade a este processo há um adensamento do filme com a formação de bolsões (slugs) de água líquida que ocupam a área de passagem do oxigênio nos canais. O bloqueio da passagem do oxigênio pelo bolsão de água líquida no canal impede que o oxigênio alcance os sítios catalíticos da camada catalítica do cátodo onde ocorre a reação de redução do oxigênio...

Due to their high power density, low operating temperature and high power-to-weight ratio, PEM fuel cells are considered promising power sources. As the technology matures and the timescale for commercialization continues to decrease, durability, reliability and cost are amongst the most critical issues to be tackled. The mechanisms of fuel cell degradation are not well understood. Even though the numbers of installed units around the world continue to increase and dominate the pre-markets, the present lifetime requirements for fuel cells cannot be guarantee, creating the need for a more comprehensive knowledge of material’s ageing mechanism. In this work, failure modes and mechanism in PEM fuel cells are reviewed including those related to thermal, chemical or mechanical issues that may constrain stability, power and lifetime. PEM fuel cell operates under very aggressive conditions in both anode and cathode. Increase in cell voltage leading to higher efficiencies may lead to surface oxidation of the catalyst, decreasing reaction activity and accelerating catalyst degradation. In the case of fuel starvation, the anode potential may rise to levels compatible with the oxidization of water. If water is not available, oxidation of the carbon support will accelerate catalyst sintering. Water management has a major impact on PEM fuel cell performance. Inappropriate relative humidity conditions lower performance and efficiency and may lead to irreversible degradation of catalyst and membrane. Diagnostics methods and tools used for in-situ and ex-situ analysis of PEM fuel cells will be discussed in order to better categorize irreversible changes in the kinetic and/or transport properties of the cell. Data regarding membrane electrode assembly (MEA) degradation obtained during and after fuel cell ageing in extreme testing conditions will be discussed. Electrochemical Impedance Spectroscopy (EIS) is found instrumental in the identification of fuel cell flooding conditions and membrane dehydration associated to mass transport limitations / reactant starvation and protonic conductivity decrease...

One of the important factors determining the lifetime of polymer electrolyte membrane fuel cells (PEMFCs) is membrane electrode assembly (MEA) degradation and failure. The lack of effective mitigation methods is largely due to the currently very limited understanding of the underlying mechanisms for mechanical and chemical degradations of fuel cell MEAs. This work reports on the effect of 1500 h operation of an eight-cell stack Portuguese prototype low power fuel cell. A performance decrease of 34%, in terms of maximum power, was found at the end of testing period. A post-mortem analysis by SEM and TEM was done for most cells of the fuel cell. Loss of the PTFE ionomer in the anode and cathode catalytic layers; morphological changes in the catalyst surfaces such as loss of porosity and platinum aggregation, deformation on the MEA components (anode, cathode and membrane) were identified. Others, like delamination and cracking were also detected. Catalyst migration and agglomeration on the interface of the electrodes was observed at cells 2, 4, 6 and 7. A platinum band was also detected on the membrane at 2 μm apart from the anode of cell 4. In some cases, dissolution occurred with re-deposition of the platinum particles with facet

The mechanisms of fuel cell degradation are multiple and not well understood. Irreversible changes in the kinetic and/or transport properties of the cell are fostered by thermal, chemical and mechanical issues which constrain stability, power and fuel cell lifetime. Within the in-situ diagnostics methods and tools available, in-situ electrochemical impedance spectroscopy (EIS) is within the most promising to better understand and categorize changes during fuel cell ageing. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 MEA (membrane-electrode assembly) –low power PEM fuel cell (15 W nominal power) is reported using EIS as a base technique. A frequency response analyzer from Solartron Model 1250 was used connected to an electrochemical interface also from Solartron, Model 1286. The range of covered frequencies spans from 37000 Hz to 0.01Hz. Hydrogen is supplied from a metallic hydride small reactor with a capacity of 50 NL H2 at a pressure of 0.2 bar. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack...

In this work, a low power PEM fuel cell intended for passive management of water was operated integrating a range of relative humidity (RH) from 30 to 80% and temperatures from 5 to 55ºC. An open air cathode, provided with an excess air stoichiometry condition, was designed for easy water removal and stack cooling. The 4 cell stack was fed with pure hydrogen and uses own design flow field drawn on graphite plates from Schunk and a commercial MEA with carbon supported catalyst containing 0.3 mgcm-2 Pt. Full stack characterization was made using a purpose-built test station and a climatic chamber with temperature and RH control. Results indicated that 60% RH is associated to maximum performance on the fuel cell under study over the studied temperature range. While water management is done in a passive fashion, heat management is accomplished on the basis of the injection of air at the cathode with the fuel cell showing good performances at relatively low currents where back diffusion towards the anode is favored.

As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials’ aging mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane electrode assembly (MEA) degradation of an eight-cell PEM low power stack was evaluated, during and after fuel cell aging in specified testing conditions of load-cycling that may compromise the durability of the catalyst. The stack degradation analysis comprised observation of catalytic layers, morphology and composition. Examination of the MEAs cross sections, in a joint SEM and TEM study, revealed thickness variation of catalytic layer (up to 47% for the cathode layers), and cracking, delamination, and catalyst migration were observed even though catalyst sintering and consequent loss of electrochemical active area seem to be predominant together with F loss from the ionomer used as binder in the catalytic layers.

Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base technique. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data was made using an a priori equivalent circuit describing the transfer function of the system in question –equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT). Results and interpretation of cell polarity reversal due to hydrogen starvation were complemented with ex-situ MEA cross section analysis...

The effective thermal conductivity of gas diffusion layers (GDL) is an important parameter for the analysis of polymer electrolyte membrane (PEM) fuel cells as thermal conductivity strongly influences fuel cell performance. The accuracy of modeling heat transfer ¿ and therefore also performance - in a PEM fuel cell relies on the accurate estimation of effective thermal conductivity. Commercially available gas diffusion layers were investigated by 3D x-ray computed tomography (CT). Based on the 3D structure reconstructed from tomography data, the macroscopic effective thermal conductivity of the gas diffusion layers was calculated by solving the energy equation considering a pure thermal conduction problem.; JRC.F.2-Cleaner energy

Water management is essential for PEM (Proton Exchange Membrane) fuel cell performance. Nafion® based membranes in fuel cells operating up to 90°C demand high hydration levels to exhibit good proton conductivity. However, too much of water present in fuel cell electrodes and GDLs (Gas Diffusion Layer) may lead to pore-blockage preventing efficient gas and water transport to and from reaction sites. Experimentally, water transport is difficult to quantify. In-situ visualization techniques usually do not distinguish regions of water production, condensation, and blockage during fuel cell operation. CFD (Computational Fluid Dynamics) modelling however can overcome this drawback because at the same time water and gas transport, electrochemical reactions and electric conduction can be simulated. In this paper a water transport model for PEM fuel cells is described and the effect of water transport for different flow field designs and different operation conditions (e.g. inlet gas humidity) on fuel cell performance are studied.; JRC.DDG.F.2-Cleaner energy

PEM fuel cell assembly pressure is known to cause large strains in the gas diffusion layer (GDL), which results in significant changes in its mechanical, electrical and thermal properties. These changes affect the rates of mass, charge, and heat transport through the GDL, thus impacting fuel cell performance and lifetime. The appropriate modeling of the inhomogeneous GDL compression process associated with the repetitive channel rib pattern is therefore essential for a detailed description of the physical chemical processes that take place in the cell. In this context, the mechanical characterization of the GDL is of special relevance, since its microstructure based on carbon fibers has strongly nonlinear orthotropic properties. The present study describes a new finite element model which fully incorporates the nonlinear orthotropic characteristics of the GDL, thereby improving the prediction of the inhomogeneous compression effects in this key element of the cell. Among other conclusions, the numerical results show that the linear isotropic models widely reported in the literature tend to overestimate the porosity and the partial intrusion of the GDL in the channel region, and may lead to incorrect predictions in terms of interfacial contact pressure distributions.; This work was supported by Project ENE2008 06683 C03 02 of the Spanish Ministerio de Ciencia e Innovación (GS1).

Mathematical modeling helps researchers to understand the transport and kinetic phenomena within fuel cells and their effects on fuel cell performance that may not be evident from experimental work. In this thesis, a 2-D steady-state cathode model of a proton-exchange-membrane fuel cell (PEMFC) is developed. The kinetics of the cathode half-reaction were investigated, specifically the reaction order with respect to oxygen concentration. It is unknown whether this reaction order is one or one half. First- and half-order reaction models were simulated and their influence on the predicted fuel cell performance was examined. At low overpotentials near 0.3 V, the half-order model predicted smaller current densities (approximately half that of the first-order model). At higher overpotentials above 0.5 V, the predicted current density of the half-order model is slightly higher than that of the first-order model. The effect of oxygen concentration at the channel/porous transport layer boundary was also simulated and it was shown the predicted current density of the first-order model experienced a larger decrease (~10-15% difference at low overpotentials) than the half-order model. Several other phenomena in the cathode model were also examined. The kinetic parameters (exchange current density and cathode transfer coefficient) were adjusted to assume a single Tafel slope...

Este trabalho desenvolve um modelo computacional para simulação do comportamento de uma célula a combustível do tipo PEM. O modelo foi desenvolvido usando equações físico-químicas e ajuste de parâmetros baseados em resultados experimentais - os quais foram tirados da literatura, pois não foram conduzidos testes práticos em laboratório - sendo capaz de determinar o valor de variáveis de saída importantes como potencia elétrica, tensão de saída e rendimento energético, assim com outros parâmetros de desempenho, como a razão de excesso de oxigênio, a partir de variáveis de entrada. O sistema completo foi dividido em diversos subsistemas, permitindo a análise dinâmica dos principais estados internos. O sistema de controle proposto e desenvolvido via inversão do modelo matemático. da célula. Dada a complexidade do modelo completo, sua divisão em diversos sub-sistemas permite a inversão de cada bloco isoladamente, de acordo com princípios da representação energética macroscópica (energetic macroscopic representation, EMR) e a construção de um controlador completo. No caso de subsistemas que possuem equacionamento não linear, o processo de inversão e obtido iterativamente via um algoritmo de busca repetitiva usando uma função Lyapunovcandidata construída a partir da equação que descreve o subsistema. Este controlador mostra excelentes resultados em simulação frente as não linearidades inerentes do sistema.; In this work we develop a computational model for the dynamical behavior simulation of a PEM fuel cell. The model was developed using physical and chemical equations and parameter fitting based on experimental data - which were obtained from literature. - being capable of predicting the value of important output variables as power...

In this paper an ethanol reformer based on catalytic steam reforming with a catalytic honeycomb loaded with RhPd/CeO2 and palladium separation membranes with an area of 30.4 cm2 has been used to generate a pure hydrogen stream of up to 100 ml/min to feed a PEM fuel cell with an active area of 5 cm2. The fuel reformer behavior has been extensively studied under different temperature, ethanolewater flow rate and gas pressure at a fixed S/C ratio of 1.6 (molar). The hydrogen yield has been controlled by acting upon the ethanol-water fuel flow and gas pressure. A mathematical model of the ethanol reformer has been developed and an adaptive and predictive control has been implemented on a real time system to take account of its nonlinear behavior. With this control the response time of the reformer can be reduced by a factor of 7 down to 8 s. The improved dynamics of the controlled reformer match better the quickly changing hydrogen demands of fuel cells. They reached a magnitude where costly hydrogen buffers between the reformer and the fuel cell can be omitted and an electric buffer at the output of the fuel cell is sufficient.; Fil: Koch, Reinhold. Universitat Technical Zu Munich;; Fil: Lopez, Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Bahia Blanca. Planta Piloto de Ingenieria Quimica (i); Argentina;; Fil: Divins...

Proton exchange membrane (PEM) fuel cells have emerged as a potential alternative to internal combustion engines in order to curb dependency on fossil fuels and reduce harmful CO₂ emissions. Water management has been identified as a key research area for the advancement of PEM fuel cell technology, especially as it affects the purge protocol prior to cell shutdown. The presence of water in the cell is necessary to sustain membrane hydration, but the accumulation of excess liquid water, referred to as flooding, can lead to increased mass transport losses and reductions in performance and durability. In this work, a technique was developed to characterize the two-phase flow in the anode and cathode flow field channels simultaneously using a transparent fuel cell with dual-visualization capability. The transparent fuel cell used in this work was designed to represent actual full scale automotive fuel cell geometry. A video processing algorithm was developed to automatically detect dynamic and static liquid water present in the gas channels and generate relevant quantitative information. The water coverage ratio is introduced as a parameter to capture the time-averaged flow field water content information through recorded video sequences. The algorithm also yields information pertaining to the distribution of water among different two-phase flow structures. The water coverage ratio and distribution metrics were employed in comparing the performance of Freudenberg and Toray gas diffusion layers (GDLs) from a water management perspective...

Proton Exchange Membrane (PEM) Fuel Cells convert hydrogen into water by causing electrochemical reaction with oxygen, producing an electric current which can be used to power electric motors. This is seen as a viable alternative to the Internal Combustion Engine which operates on fossil fuels and is often blamed for contribution to the global climate change. Due to the low temperature operation, compared to other forms of fuel cells, it is possible to adapt the PEM Fuel Cell for automotive application. By running on hydrogen, the PEM Fuel Cell promises to enable a clean mode of transport. Water vapor transport inside the fuel cell takes place by two primary mechanisms: diffusion and permeability. Diffusion is important in the through-plane direction, whereas permeability is most important in the in-plane direction. Some work has been done to measure the permeability; it has been correlated with the porosity. However, the work has focused on the permeability at room temperature for ease of measurement. The PEM fuel cell works most efficiently between 60⁰C and 95⁰C. In this work, we direct our efforts at verifying whether there is any change in permeability in the in-plane direction with change of temperature. The in-plane permeability has been measured at 25⁰C...

A PEM fuel cell assembly requires the simultaneous delivery of reactants to the electrodes of multiple cells in order to produce electrical power. The mass transport of reactants to the electrode surfaces is complicated by the presence of liquid water during operation in low temperature environments. The water transport characteristics of an operational fuel cell were experimentally determined to build a low ambient temperature control strategy for a full scale automotive application. Two key topics for fuel cell control are covered: (a) purge of the fuel cell at shutdown, and (b) the fuel cell stack power management during freeze-start. After a PEMFC system is shut down, excess liquid water is often present in the MEA and flow field. Upon exposure to sub-freezing temperatures, this water can block the flow of reactants to the cell and prevent the fuel cell system from operating. To mitigate the effects of residual water in the fuel cell, this study tests the effects of various parameters during shutdown purge on freeze-start reliability. Both cell orientation and water content in the membrane-electrode assembly (as inferred by measurement of high frequency resistance) were shown to influence freeze-start reliability. The transport of product water away from the cell electrodes during start-up is significantly limited at lower temperatures (below about 45 °C)...

Prasad, Ajay; Advani, Suresh; As the demand for clean energy grows rapidly, proton exchange membrane (PEM) fuel cell technology is seen as a viable candidate for an alternative energy conversion device. Despite much research progress in the past two decades, several remaining technical challenges must be overcome for the successful commercialization of PEM fuel cells. In particular, computational models play an important role in PEM fuel cell research by decreasing the dependence on expensive and time-consuming experimental approaches to develop a cost-effective and efficient PEM fuel cell. The goal of this research study is to formulate multiscale catalyst layer (CL) modeling strategies to provide more accurate predictions for the effects of variations in the loading of platinum, carbon and ionomer, as well as porosity, within the catalyst layer. The ultimate goal is to provide guidelines for the design of an improved CL with better performance and lower cost. The well-known agglomerate approach was chosen to model the CL. First, we conducted micro and macroscale numerical studies to investigate the shortcomings of the classical agglomerate approach which lead to unrealistic predictions of the effects of catalyst loading on performance. The investigation showed that the agglomerate model predictions can be improved either by employing relations between agglomerate parameters and compositional variables...

This paper presents the design, implementation, and experimental validation of a digitally-controlled emulator of proton exchange membrane (PEM) fuel cells for static and dynamic behavior. The emulator is a low cost, easy to use, and portable device designed to evaluate power systems and control strategies for fuel cell-based generation systems. For the implementation of this emulator, an appropriate mathematical model is chosen, parameterized, and experimentally validated. The resulting model is processed digitally by the emulator, which generates the appropriate electrical behavior to a load. The emulator power stage is implemented by using a two-inductor step-down DC/DC switching converter, which is controlled directly by the digital processing system. Later, the electrical scheme of the power stage and the block diagram of the system are presented, and the behavior of the emulator is illustrated with a simulation. Finally, the emulator is validated using experimental data.

This paper presents a non linear state space model and a linear control system for a Polymer Electrolyte Membrane fuel cell. The dynamics modeled are the temperature of the stack and the air flow compressor, and their main feature is the reproduction of the oxygen excess ratio behavior. The linear control system is a linear quadratic state feedback regulator and a Kalman filter, where the control objective is to avoid oxygen starvation and to minimize fuel consumption, through the tracking of an optimal load power profile. The Kalman filter is designed in order to obtain information from some non-measurable states.