Search

The development of new proton exchange membranes for PEM technology in fuel cells and electrolysers with increased durability is paramount to system´s lifetime and scalability. In this work, new modified Nafion membranes are proposed with increased resilience to chemical degradation by H2O2 /Fe2+, mimicking ex-situ radical attack to membrane structure. N/A

In this work, a compact and low-cost electrochemical laboratory prototype for the storage and production of hydrogen, based on metallic hydrides, with high reversibility in the charge/discharge process is demonstrated, using electricity either from the grid or by direct coupling to renewable energies as power source. The reactor is a 316 L stainless steel vessel with a capacity up to 15 bar internal pressure. It includes working electrodes of alloy LaNi4.3Co0.4Al0.3 and counter-electrodes of Ni foam in an electrolyte solution of 35% KOH. The reactor uses unicellular/multicellular configurations, so that the overall capacity of the system can be extended by increasing the number of working electrodes, resulting in a highly modular system. Results show excellent linearity, reversibility, and stability under cycling at room temperature and pressure, demonstrated either when powered by the grid or by off-grid renewable energy. Furthermore, criteria were established for the quantification of the state of full charge and full discharge. The system was integrated with a custom electronic system, developed inhouse for monitoring and control the reactor and to optimize the performance and energy efficiency of the hydrogen storage and discharge processes. info:eu-repo/semantics/publishedVersion

info:eu-repo/semantics/publishedVersion

Current US DOE 2020 targets for electrocatalysts aim for a total platinum group metals (PGM) loading of 0.125 mgcm-2 on membrane-electrode assemblies. The catalysts are expected to endure an elevated number of load cycles in order to comply with the expected 5000 h stack lifetime. At the stated PGM low loading attributes of performance, power and durability, as well as costs, are simultaneously required. The present paper deals with performance assessment of new catalyst using Pt nanoparticles and a surface functionalized carbon support. Due to its relevance to fuel cell operation, the stability and durability were assessed in half cell mode, through the application of demanding aging cycling protocols (30000 cycles). Full fuel cell assessments were also conducted entailing power density curves and cell electrochemical impedance. Results denote significant improvements in cell performance as well as in catalyst stability, regarding a commercial catalyst tested in the same conditions, at Pt loadings of 0.1 mgcm-2. N/A

The integration of new cleaner, renewable and environment-friendly sources and energy vectors for sustainable energy systems are a key challenge for 21st century society. Fuel cells are among the clean energy conversion technologies with vast applications and scope, introducing hydrogen as a flexible and storage energy vector and presenting a viable alternative to fossil fuels. Proton exchange membrane fuel cells (PEMFCs) are considered promising power sources, but their performance depends crucially on the properties of their proton exchange membranes (PEM). N/A

info:eu-repo/semantics/publishedVersion

The performance of PEM fuel cells critically depends on their proton exchange membrane structural and chemical stabilities as well as on their proton conductivity. Limitations of commercially available Nafion membranes to operating at temperatures above 80 ºC have fostered the interest in research and development of new membranes [1,2]. The aim of this work is the preparation of new modified Nafion composite membranes, with a bisphosphonic acid moiety, a promising proton carrier exhibiting good proton donating/accepting properties and thermal stability. Synthesis and characterization were undertaken of a series of bisphosphonic acid derivatives and their incorporation into a Nafion matrix, by casting. The new membranes were characterized by ATR-FTIR and SEM along with their ion exchange capacity and water-uptake. The evaluation of their proton conductivity was carried out by electrochemical impedance spectroscopy, at various temperature and relative humidity (RH) conditions. The incorporation of BPs dopants enhances the proton conductivity, with all membranes exhibiting higher values than Nafion N-115, tested in the same experimental conditions. Selected membranes were integrated into a fuel cell MEA, using a single cell assembly, with an active area of 2.5x2.5 cm2 and a catalyst loading of 0.5 mgPtcm-2. Performance was evaluated, using an air fed cathode, at temperatures from 30 ºC to 80 ºC. Membrane doped with BP2 showed the best performance, with higher power density outputs than Nafion N-115 shown at all temperatures. info:eu-repo/semantics/publishedVersion

Herein we present the synthesis and characterization of new phosphonate-, bisphosphonate- and hydroxybisphosphonatebenzimidazole derivatives substituted at the N-1 position and new regioisomers phosphonate-, bisphosphonate-, and hydroxybisphosphonatebenzotriazole derivatives substituted at N-1 or N-2 positions. The compounds were characterized by NMR and IR spectroscopies, and mass spectrometry (low and high resolution) allowing the assignment of their structure, including the identification of regioisomers. These new azole monomers will be precursors for a mesoporous silica host to produce novel membrane materials with high proton conductivity for intermediate temperature proton exchange membrane fuel cells.

High volumetric and gravimetric efficiency are key to potential hydrogen energy carriers. Sodium borohydride emerges as such potentiality and a storage capacity well within DOE targets for 2015. Limitations exist due to the fact that hydrolysis is restricted by available water and due to the lack of low cost re-usable catalysts. An extensive amount of work has been done in our laboratories on Ni and Ru based catalysts, including synthesis and characterization and solutions have been found for durability, stability and reutilization under operating conditions in small volume batch reactors. Results showed that the Langmuir-Hinshelwood model described fairly well the reaction kinetics for all tested temperatures up to 60ºC and up to reactant exhaustion. In this work, issues such as self-hydrolysis, stability of solutions for storage, water management, some aspects of the catalyzed hydrolysis as well as gas conditioning are studied in order to associate a storage solution with sodium borohydride to a low power air breathing cathode PEM fuel cell.

Ionic liquid (IL) functionalized carbon supported nanocatalysts, including platinum-ruthenium catalysts, have been under investigation to achieve better catalytic performance on polymer electrolyte membrane fuel cells. Ionic liquids have particular physical properties, which usually improve the stabillity and dispersion of metal nanosized particles, providing an increase of the electrochemical active surface area. In this work, the synthesis and the electrochemical behaviour of two new ionic liquid functionalizated carbon supported Pt-Ru nanocatalysts is investigated regarding the methanol oxidation reaction. Preliminary obtained results show that IL functionalization of carbon can produce supported catalysts with improved performances depending on the ionic liquid and on the reduction process used to produce the catalyst nanosized particles.

The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices and the MicroDMFC is therefore an emergent technology. In this work, a set of experiences with a MicroDMFC of 2.25 cm2 active area are performed in order to investigate the effect of important operating parameters. The cell is presented in Fig. 1. The experimental rig consists in a rotameter for air flow, a peristaltic pump (Control CO) for methanol flow and an electrochemical station (Zahner) to obtain polarization curves. The effect of the methanol concentration on the cell performance, between 3 and 5 M was studied. All the experiments were performed at room temperature, a condition of special interest with view to portable applications. Maximum power density achieved was 32.6 m /cm2 using a 4 M methanol concentration at room temperature. Polarization curves are compared with mathematical model simulations in order to achieve a better understanding of how parameters affect performance. The one-dimensional model used in this work takes in account coupled heat and mass transfer, along with the electrochemical reactions occurring in a direct methanol fuel cell and was already developed and validated for DMFC in previous work [1-3]. The model is also used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and methanol crossover. This easy to implement simplified model is suitable for use in real-time MicroDMFC simulations.

In this work, the effect of chloride as a catalyst contaminant was studied on the performance and durability of a low power open-cathode fuel cell, intended for passive management of water. In an ex-situ study, cyclic voltammetry was used to assess the redox behaviour of platinum in chloride contaminated solutions at room temperature.The cell was operated integrating a range of relative humidity (RH) from ~30 to 80% and temperatures from 5 to 55 ºC. Results indicated that 60% RH is associated to maximum fuel cell performance over the studied temperature range. An examination of the fuel cell components after 100 h of operation revealed that chloride contamination has produced cathode failure associated to catalyst migration favored by operation conditions that allowed platinum particles to break free from their carbon backing and migrate toward the polymer electrolyte. Migration resulted in precipitation with larger mean particle size distribution within the solid electrolyte when compared to the original catalyst layer, rendering a very significant loss of thickness in the cathode material.

Fuel cells are electrochemical devices that convert the chemical energy stored in a fuel, directly and efficiently into electrical energy and are a promising and clean alternative to traditional energy fuels [1]. The roton-exchange membrane fuel cells (PEMFCs) are considered a promising type of electrochemical device for power generation. The proton exchange membrane (PEM) is a key part for the operation of PEMFC. Usually, the proton exchange membranes are made of organic polymers containing acidic functionalities (ex. Nafion®), but the proton transport properties of these membranes strongly depend on their water content and, consequently, limit their operation temperatures up to 90ºC. These limitations have fostered the interest in research and development of new alternative membranes [1]. Phosphonic acids are considered to be promising proton carriers due to their good proton donating and accepting properties, furthermore they present better thermal stabilities than sulfonic acids [2]. This work reports on the synthesis and characterization of a series of mono-, bis- and trisbenzimidazole phosphonates derivatives, prepared from the new diaminebenzene diphosphonate, by cyclization with diferents reagents. These benzimidazole phosphonates derivatives were prepared by a new strategy which involves nickel-catalyzed Arbuzov reaction of 4,7-dibromo-2,1,3-benzothiadiazole with triethyl phosphite, followed by reductive sulfur extrusion to afford the new diaminebenzene diphosphonate [3], followed by cyclization with different reagents, such as acyl chlorides or orthoformate derivatives. The synthesized compounds have different spacers with different lengths between benzimidazole rings (Fig. 1). All compounds have been fully characterized by 1H, 31P and 13C NMR, IR spectroscopy and mass spectrometry (low and high resolution).

Catalytic hydrolysis of sodium borohydride (NaBH4) has been investigated as a method to generate hydrogen for fuel cell applications. The high purity of the generated hydrogen makes this process a potential source of hydrogen for polymer electrolyte membrane fuel cells (PEMFCs). In this paper, a PEMFC power system employing a NaBH4 hydrogen generator is designed to supply continuous power to residential power applications as stand-alone loads and simulated using Matlab/Simulink software package. The overall system is sized to meet a real end-use load, representative of standard European domestic medium electric energy consumption, over a 1-week period. Supervisory control strategies are proposed to manage the hydrogen generation and storage, and the power flow. Simulation results show that the proposed supervisory control strategies are effective and the NaBH4–PEMFC power system is a technologically feasible solution for stand-alone residential applications.

This paper proposes a new approach to determine the distribution of relaxation times (DRT) directly from the electro-chemical impedance spectroscopy (EIS) data, i.e. without the use of an equivalent electrical circuit model. The method uses a generalized fractional-order Laguerre basis to represent EIS where both the parameters of the basis and their co-efficients are estimated by solving a nonconvex minimization problem. Furthermore, the DRT confidence region is de-termined to assess the accuracy and precision of the DRT estimate. The approach is applied to analyze the dominant dynamic properties of an open-cathode hydrogen fuel-cell under different current and air-flow conditions. Results showed that the estimated DRT closely reconstructs EIS data even when there is a higher variance at smaller relaxation times.

This work reports the synthesis and characterization of new azaheterocyclic aromatic phosphonates derivated from benzimidazole and benzotriazole by nickel-catalyzed Arbuzov reaction of 4,7-dibromo-2,1,3-benzothiadiazole with triethyl phosphite, followed by reductive sulfur extrusion reaction and cyclization. The compounds were characterized by NMR and mass spectrometry (low and high resolution). These azoles will be incorporated into proton conductive inorganic-organic hybrid membranes of mesoporous silica to produce novel membranes materials with high proton conductivity for intermediate temperature PEMFCs. info:eu-repo/semantics/publishedVersion

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.

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.

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, was revealed by the 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 is made using two different approaches: 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) -avoiding the ambiguity of the a priori equivalent circuit and the need for provision of the initial fitting parameters. A resistance and two RQ elements connected in series are identified as describing the impedance response of the cell during normal functioning. A constant phase element (CPE) was chosen to describe the impedance observed behavior. The quality of the fit was evaluated by analysis of the residuals between the fit result and the measured data at every single point. Consistency and quality of the impedance data were established by Kramers-Kronning validation. With continuous operation, using a reduced hydrogen flow, an inversion of polarity was observed in the 16th cell of the stack, evident in the potential measurement of individual cells as a result of insufficient hydrogen to reach the last cells. EIS data analyses suggest that water electrolysis happens at the anode judging by the appearance of an intermediate semicircle associated to a marked change in resistance and capacitance values. The presence of an inductive loop at low frequencies is now evident, which cannot be explained by the relaxation of reaction intermediates involved in the oxygen reduction reaction [1]. It is to be noticed that when the incursion into the negative potential values is not too marked the phenomenon is partially reversible, so it is suggested that the relaxation is due to intermediates in the water electrolysis process. The anode potential rose to levels compatible with the oxidation of water. Once the phenomenon is made irreversible and when water is no longer available, oxidation of the carbon support is favored accelerating catalyst sintering. Ex-situ MEA cross section analysis, under a scanning electron microscope, confirmed it. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation are analyzed, according to evidence by SEM.

In this work a low power 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 55 ºC. The stack was fed with pure hydrogen. An open air cathode was designed for easy water removal and stack cooling. The stack 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. Polarization curves were registered for a full stack characterization using a purpose-built test station and a climatic chamber with temperature and RH control. Results indicated that 60% RH is associated to maximum fuel cell performance over the studied temperature range. While water management is done in a passive fashion, heat management is done 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. The loss of performance with temperature increase was related to an increase in the membrane resistance which may correspond to loss of water on the anode side. Performances at temperatures lower that room temperature showed only slight decrease in power. An examination of the fuel cell components after 100 h of operation revealed that chloride contamination has produced cathode failure associated to catalyst migration anomalies favored by operation conditions that allowed platinum particles to break free from their carbon backing and migrate toward the polymer electrolyte. Migration resulted in precipitation with larger mean particle size distribution within the solid electrolyte when compared to the original catalyst layer, rendering a very significant loss of thickness in the cathode material. Coarsening of platinum particles occurs at nano and micro-scale. The mechanism for the lost of catalyst by dissolution and growth is discussed on the basis of a joint electrochemical and SEM/TEM study.

A non-uniform utilization of the active area due to inhomogeneous current density distribution is one of the main factors for poor fuel cell performance. Furthermore, it leads to hot points which can be responsible for thermal stress in the membrane electrode assemble (MEA). Therefore, it became extremely important to have a consistent technic to visualize in real time the current density and temperature distribution on the active area. For this purpose a printed circuit board (current scan lin® form S++) was used to measure the current density and temperature distribution. With this equipment it was possible to generate high resolution counters for these two variables. With these results the effect on the current density distribution by different flow fields design, oxygen stoichiometry, and temperature were analysed. Besides, these results can be used to provide crucial data for simulation work, in particular for validation purpose.

Catalytic hydrolysis of sodium borohydride (NaBH4) has been investigated as a method to generate hydrogen for fuel cell applications. The high purity of the generated hydrogen makes this process an ideal source of hydrogen for polymer electrolyte membrane fuel cells (PEMFCs). In this paper, the possibility of using a NaBH4-based hydrogen generator with a PEMFC for stand-alone residential use is examined. A complete model of the system is developed, based on models taken from literature with appropriate modifications and improvements. Supervisory control strategies are also developed to manage the hydrogen generation and storage and the power flow. The operation and performance of the integrated system over a one-week period under real loading conditions is analyzed through simulation. Finally, results of the analysis are summarized and the limitations/further scope are indicated.

This work reports the synthesis and characterization of phosphonate-, hydroxybisphosphonate- and aminobisphosphonatebenzimidazole derivatives substituted at N-1 position and new regioisomers phosphonateand aminobisphosphonatebenzotriazole derivatives substituted at N-1 or N-2 positions. The compounds were characterized by NMR, IR spectroscopy and mass spectrometry (low and high resolution) allowing the assignment of their structure, including the identification of regioisomers. These azoles will be precursors of mesoporous silica host to produce novel membranes materials with high proton conductivity for intermediate temperature PEMFCs.

Electrochemical impedance spectroscopy (EIS) is an analysis technique that is commonly used as a base diagnostics technique for the in-situ analysis of the kinetic and transport properties of proton exchange membrane (PEM) fuel cells. This work proposes to use the distribution of relaxation times (DRT) as a complementary analysis technique for the interpretation of EIS data. For this purpose, the DRT is deduced for a modified Randles electric circuit composed of a constant phase element (CPE) connected in parallel with a resistance in series with a finite diffusion Warburg element. The experimental EIS data collected from an eight cell PEMFC with an open-air cathode was modeled through the use of two modified Randles circuit representing the fuel-cell electrodes. The analysis of the DRT allows to identify further characteristics of the individual processes that occur at both electrodes, while also being instrumental in detecting the effect on the fuel cell performance of some operating conditions, namely hydrogen flow-rate and current.

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

A three-dimensional isothermal model of a high temperature polymer membrane fuel cell equipped with polybenzimidazole (PBI) membrane is described. All major transport phenomena were taken into account except the species cross-over thought the membrane. The cathode catalyst layer was treated as spherical catalyst agglomerates with porous inter-agglomerate spaces. The inter-agglomerate spaces were filled with a mixture of electrolyte (hot phosphoric acid) and polytetrafluoroethylene (PTFE). This approach proved to be an essential requirement for accurate simulation. In this particular paper the influence of different flow field designs and dimensions on performance was intensely study. Traditional configurations were tested (straight, serpentine, pin-in and interdigitated), and a new designs were proposed. With these new designs we tried to maximize performance by providing homogeneous reactants distribution over the active area keeping low pressure drop and relatively high velocity. The dimension and position of the inlet and outlet manifolds were also analysed. From the obtained results was observed a massive influence of the manifolds position and dimension on performance. This fact leaded to an optimization of the manifolds which can give important guidelines for future bipolar plates production.

Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. A steady state, one-dimensional, multi-component and thermal model is described and applied to simulate the operation of a passive direct methanol fuel cell. The model takes into consideration the thermal and mass transfer effects, along with the electrochemical reactions occurring in the passive DMFC. The model can be used to predict the methanol, oxygen and water concentration profiles in the anode, cathode and membrane as well as to estimate the methanol and water crossover and the temperature profile across the cell. Polarization curves are numerically simulated and successfully compared with experiments for different methanol feed concentrations. The model predicts with accuracy the influence of the methanol feed concentration on the cell performance and the correct trends of the current density and methanol feed concentration, on methanol and water crossover. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations. Due to its simplicity the model can be used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of the design parameters such as membrane thickness, catalyst loading, diffusion layers type and thicknesses.

Pt-Ru electrocatalysts supported on carbon xerogels and ordered mesoporous carbons were synthesized by reduction with formate ions (SFM method). Chemical and heat treatments were applied to modified the surface chemistry of original carbon supports. Physical characterization of the catalysts was performed using X-ray dispersive energy (EDX) and X-ray diffraction (XRD) techniques, while the electrochemical activity towards methanol oxidation was studied by cyclic voltammetry (CV). Pt-Ru catalysts with nominal metal content (20 wt.%) and atomic ratios (Pt:Ru 1:1) were successfully synthesized on the different supports. Higher methanol oxidation current densities were obtained for those supports with a higher content of surface oxygen groups. Gas diffusion electrode and membrane-electrode-assembly preparation allowed an in-house built of a direct methanol fuel monocell for the evaluation of the catalysts performance. Polarization curves were measured confirming the results obtained in a three electrodes electrochemical cell by CV. Normalized power curves per weight of Pt are discussed in terms of the significant impact on noble metal loading and attained cell maximum power, in comparison with results obtained with a commercial catalyst.

As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials ageing 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 ageing in extreme testing conditions. The stack degradation analysis comprised observation of catalytic layer, morphology and composition. Cross sections examination of the MEAs revealed thickness variation of catalytic layer and membrane. Other modes of degradation such as cracking

In this work a study of the performance of a low power fuel cell at sub-freezing temperatures has been undertaken. Knowledge in this area is still scarce. After global characterization of the stack on a wide range of temperatures and relative humidity’s the behaviour at negative temperatures (-5ºC -10ºC, -15ºC) has been established. Furthermore, performance was evaluated after the cell was submitted to cycles from -25ºC to + 25ºC. At the end of 10 cycles only marginal loss in performance was registered, when testing at + 2.5ºC and + 25ºC. On the basis of the obtained results a strategy for start-up and shut-down has been designed in order to be implemented for operation at low temperatures. A failure analysis of the membrane and catalyst layers and GDLs is under way in order to evaluate material degradation

The project aims at the development of a portable power system as standard equipment for defence use, and is being developed by a Portuguese consortium leaded by SRE, a fuel cell developer, LNEG (Portuguese National Laboratory for Energy and Geology) and EID (Electronics and Telecommunications for Army) through a SRE/ Ministry of Defence contract. The system uses a SRE H2 PEM Fuel Cell and a Li battery buffer (30Wh). The fuel is provided by an on board H2 generator based on chemical hydrides, aiming to provide 72h autonomy to the dismounted soldier. Main project constraints and challenges are the power density (both gravimetric and volumetric) and the operation under severe atmospheric conditions. A field test pre-series shall take place by the end of 2010 with probable commercialization foreseen by early 2012.

A study was undertaken in order to investigate the potential of hydrogen generation by hydrolysis of sodium borohydride in batch reactors, operating at moderate pressures, in the presence of a reused nickel-ruthenium based catalyst, to feed on-demand a proton exchange membrane fuel cell. The effect of the buffer pressure is explored and hydrogen generation rates are evaluated by changing catalyst amount, operating pressure and successive refueling

Pt-Ru electrocatalysts supported on xerogels and CMK-3 ordered mesoporous carbons were synthesized by reduction with formate ions (SFM method). Some of the carbon supports were chemically treated with HNO3 in order to generate oxygen groups on the surface, while other supports were heat treated. Physical characterization of the catalyst was obtained using X-ray dispersive energy (EDX) and X-ray diffraction (XRD) techniques. Results showed that Pt-Ru catalysts with similar metal content (20%) and atomic ratios (Pt:Ru 1:1) were obtained. The electrochemical activity was studied by cyclic voltammetry and chronoamperometry. Higher methanol oxidation current densities were found for catalyst deposited on chemically treated supports. Electrode preparation and MEA assembly allowed an in-house built direct methanol fuel to be fitted with the synthesized catalysts and supports in order to assess their performance. Cell and reactants were conditioned by a direct methanol test station. Polarisation curves were measured and confirmed data obtained by voltammetry, regarding the effect of heat treatment of the carbon support. Normalised power curves per weight of catalyst are discussed in terms of the significant impact on noble metal loading and attained cell maximum power, in comparison with results obtained with a commercial catalyst.

The potential of fuel cells for clean and efficient energy conversion is generally recognized. Proton-exchange membrane (PEM) Fuel Cells are among the different types of fuel cells one of the most promising. The water management is a critical problem to overcome in the PEM fuel cell technology. Despite several studies on this topic effective water management is still elusive. Models play an important role in fuel cell development since they enable the understanding of the influence of different parameters on the cell performance allowing a systematic simulation, design and optimization of fuel cells systems. In this work, a model previously developed and validated [1], is used to predict the water transport through the cell. The model takes into account heat and mass transport effects The influence of membrane thickness and transport properties, GDL thickness and structure, reactants pressure and humidification temperatures, on the water content through the membrane and on the cell performance was studied. All these parameters have an important impact in the cell water management. The model predicts the membrane water contents and water concentration profiles along the MEA. This work represents a useful tool to set-up suitable operating conditions and optimized tailored MEAs to produce a better performance of PEM fuel cells.

LUCIS, a demonstration project co-financed by the Innovation Agency in Portugal (AdI), was carried out in the framework of the DEMTEC Programme (Incentives to Technologically Innovative Pilot Systems). Its main goals were: Validate the reliability of proton exchange membrane fuel cells (PEMFC) when used in practical situations and the competitive advantages that these solutions can represent compared to conventional solutions; Evaluate impacts associated with the use of hydrogen and the benefits to business competitiveness. This project allowed a learning experience in real applications of low power PEMFC. Demonstrations were grouped in two large categories that covered several applications of PEMFC. The prototypes used were produced by SRE (Portugal) and were specifically designed to be used in small power applications, portable, traction or stationary. In this work, the technological validation was carried out for different stacks with power from 10 to 100W. Hydrogen was supplied by compressed gas bottles and metallic hydrides. All the fuel cells were previously characterized in specialized laboratories. Recommendations were drawn for every application in order to improve fuel cell lifetime and operating strategies.

For low power fuel cells, it is paramount that management of reactants, water and heat, be realized in a passive fashion in order to minimize parasitic losses. Effective fuel, oxygen supply and water management for reliable performance are also greatly affected by cell geometry and materials. Fuel cells are complex systems to optimize on a mere experimental basis. As an aid to this goal, data-driven analysis techniques, requiring no mathematical model to be fixed a priori, are gaining a reputation in other fields of work, where a phenomenological modeling approach might be intractable. This work presents a characterization study of a 12W PEMFC series stack by means of a new data-driven technique, M-NMF. The stack was developed for low temperature operation, uses own designed flow field plates, integrated in a series configuration, and is operated for 12 combinations of hydrogen/air flowrate ratios, generating as many polarization curves. M-NMF is applied, in combination with an alternating least squares algorithm, to the analysis of the overvoltage data matrix derived from the original experimental polarization data. From this analysis, it is possible to group and differentiate data according to similar overvoltage patterns and gain insight into their relative contribution to fuel cell performance immunization.

In: Proceedings HYPOTHESIS VIII- Hydrogen Systems and Materials for Sustainability, HYP_45, Lisbon, 1-3 April 2009, 5p. 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 of the membrane-electrode assembly (MEA) in PEM fuel cells are studied stressing the issues that may constrain stability, power and lifetime. Diagnostics methods and tools used for in-situ and ex-situ analysis of PEM fuel cells were used in order to better categorize irreversible changes in the kinetic and/or transport properties of the cell. Data for 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, respectively. Cross sections of the membrane catalyst and gas diffusion layers examined by scanning electron microscopy indicate electrode thickness reduction as a result of ageing. Catalyst particles are found to migrate outwards and located on carbon backings. Nafion degradation in fuel cell environment is analysed in terms of the mechanism for fluoride release which is considered an early predictor of membrane degradation. The application of a full range of diagnostic techniques and modelling is felt necessary in order to aid design and operating strategies in PEM fuel cells.

Hydrogen is a valuable alternative for long-term energy storage, particularly for renewable energy based stand-alone systems. The described stand-alone system has been developed and installed at the INETI facilities. The exceeding renewable energy (provided by sun and wind) is used to generate hydrogen, which accumulated as an energy buffer, while the fuel cell uses this stored hydrogen to produce electrical energy when there is insufficient solar/wind energy. To provide the stand-alone system with a reliable energy storage it was designed a system for storing hydrogen based on metal hydrides. In order to supply sudden power demands two options were considered: a standard DC battery bank and a supercapacitor bank. Experimental and simulation results are presented in order to show the installation obtained performance.

The direct methanol fuel cell (DMFC) with proton exchange membrane (PEM) as electrolyte and liquid methanol/water as the energy carrier is a promising power source for micro and various portable electronic devices (mobile phones, PDA’s, laptops and multimedia equipment). However a number of issues need to be resolved before DMFC can be commercially viable such as the methanol crossover and water crossover which must be minimised in portable DMFC’s. In the present work, a detailed experimental study on the performance of an «in-house» developed DMFC with 25cm2 of active membrane area, working near ambient conditions (ambient temperature and pressure) is described. Tailored MEAS (membrane electrode assemblies), with different structures and combinations of gas diffusion layers (GDL), were designed and tested in order to select optimal working conditions at relatively high methanol concentration levels without sacrificing performance. The experimental polarization curves were successfully compared with the predictions of a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC recently developed by the same authors. The influence of the anode gas diffusion layer media, the membrane thickness and the MEA properties on the cell performance is explained under the light of the predicted methanol crossover rate across the membrane

Direct methanol fuel cells (DMFCś) are promising candidates as portable power sources due to their lower weight, volume and high-energy density. They can potentially provide an energy content that exceeds current battery technology, with the possibility of instantaneous recharge. However, DMFCś have several serious drawbacks such as slow methanol oxidation kinetics, poisoning by CO of the catalyst surface, the high methanol crossover through the polymer membrane, the high costs of the Nafion membrane and catalyst. The flowing streams of fuel and oxidizer need to be evenly distributed over the entire surface of the catalyst layer in order to achieve a good performance together with an efficient removal of reaction products. This is achieved through the optimal design of the flow field. In this work the effect of flow field design on cell performance is studied, using polarization curves. A testing fuel cell was designed and implemented consisting of two stainless steel end plates, two sets of graphite collector plates, two carbon cloth diffusion layers, two catalyst layers of platinum black (0.70 mg.cm-2 and 0.75 mg.cm-2 Pt on cathode and anode, respectively) and an electrolyte polymer membrane (Nafion 117). Pt-Ru catalyst on the anode with a load of 4 mgcm-2 of catalyst was also studied. The active surface area of the cell is 25 cm2. An own built fuel cell and a Lyntech testing station were used in this work. The effects of temperature, methanol concentration, feeding flow rates of fuel and flow field design on the cell performance were studied. Results showed that the performance of the fuel cell increases with increase in cell temperature due to improvement in methanol oxidation kinetics and cathode kinetics, as indicated by polarization curves. The effect of temperature is complex; the oxygen partial pressure decreases with temperature increase due to the increase in vapor partial pressure, which causes decreases in the open-cell voltage and increases the concentration overpotential; the rate of methanol crossover increases with temperature, so the cell performance decreases; water transfer from anode to cathode through the membrane increases with temperature and the additional water increases the liquid water fraction in both the cathode catalyst and diffusion layers, thus causing an increase in concentration polarization. Increase in methanol concentration affected the performance of the DMFC, as expected due to a higher methanol crossover with higher concentrations. The methanol transferred from the anode to the cathode through the polymer electrolyte membrane is oxidized at the platinum electrocatalyst and causes a mixed potential, which lowers the cathode performance and thus the overall cell voltage output. The effect of the flow field design, single serpentine and parallel, on cell performance was also investigated. The serpentine flow field was better than the parallel design. The obtained results point towards an enhancement in the mass transfer of methanol improving the cell performance. Another important issue is the eventual blocking of the flow channels, by CO2, at high current densities. A reduction of the effective contact area between the fuel (methanol) and the gas diffusion layer is expected, as CO2 gas bubbles accumulate in the channels, bearing implications in the continuous supply of methanol to the catalyst sites through the gas diffusion layer. This is less likely to occur with the serpentine design. Cathode kinetics indicates slightly lower methanol crossover and better voltage efficiency at low current densities. Comparison is made with results obtained for the two different catalyst layers used emphasizing the effect of Ru.

In this work, hydrogen is produced by a hydrolysis process that uses sodium borohydride as a hydrogen carrier and storage media. High purity hydrogen is obtained at low temperatures with high volumetric and gravimetric storage efficiencies; reaction products are non-toxic. The produced hydrogen can be supply on-demand at specified flow by tailor made developed catalyst. Hydrogen feeding to a low power fuel cell was accomplished. According to experimental conditions conversion rates of 100% are possible. Catalyst is demonstrated to be reusable.

As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials ageing mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane-electrode assembly (MEA) degradation of a 5 cell PEM stack was evaluated, during and after fuel cell ageing in extreme testing conditions. Cross sections of the membrane catalyst layers and gas diffusion layers were examined indicating that cathode thickness is considerably reduced as a result of ageing. Catalyst particles were found to migrate outwards and located on carbon backings. Fluoride release was considered an indicator of membrane degradation, quantified using an ion selective electrode at gases outlet. MEA degradation mechanisms are discussed.

In this paper are presented, the recent advances of a demonstration project that focuses on the implementation of a hybrid system for a wheel chair incorporating a fuel cell and a metal hydride hydrogen storage system. Advantages regarding weight reduction and greater autonomy are emphasized, apart from the use of clean energy and the drastic reduction in charging time, when compared with the system before conversion.

This paper describes the control strategy for a Stand-Alone Energy System Supported by Totally Renewable Hydrogen Production. The basic control strategy considers the pressurized hydrogen gas storage as the energy buffer. The basic logic is that the exceeding renewable energy (solar and wind) is used to accumulate hydrogen, while the fuel cell uses this hydrogen to produce electrical energy within insufficient solar/wind energy. The system is completely autonomous, conceived for off-grid operation. It is composed by solar panels and a wind turbine as primary energy converters, hydrogen tanks as energy buffer and a fuel-cell and an electrolyzer as secondary energy converters.