PEM Fuel Cell performance at sub-zero temperatures

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 degradatio

Electrochemical performance of organic electroactive materials for application in rechargeable batteries

Rechargeable batteries consisting of organic base electrodes are increasingly being studied as an alternative to conventional inorganic electrodes. The approach adopted in this work involves the development of new cathode organic materials based on polyimide derivatives with significant redox potential or capacity in order to increase stability and improve the energy density of current lithium ion cells. Preparation of organic polymeric polyimides was carried out, followed by their integration in an electrode structure that uses carbon nanofibers, as a support and as a means to increase conductivity. An electrochemical study based on cyclic voltammetry was conducted in order to evaluate the performance and redox capacity of the electrodes. The implemented strategy is based on the fact that the aromatic imide group may be electrochemically reduced and oxidized in a reversible manner, with each molecule of polyimide being able to transfer four electrons in two steps, accounting for a high specific capacity of the electrodes. Some of the composite electrodes studied in this work demonstrated good redox capacity, stability under potential cycling and higher current densities, 10-20X, than those reported in the literatures for similar compounds

Influence of the electrolyte on the electrochemical performance of a polyimide material for electrodes in rechargeable batteries

The charge storage on organic polymers has led to increasing application of these new materials such as active electrodes in rechargeable batteries [1, 2]. Taking advantage of the redox properties of aromatic polyimide imide group [3], which allows it to be electrochemically oxidized and reduced reversibly, we will present the advances made on the electrochemical studies carried out with a polyimide derivative electrode material towards the constitution of a new battery. Previous studies by the authors with a poliimide derivative [4]using cyclic voltammetry have shown their significant redox capacity to be applied as an alternative material for energy storage in rechargeable batteries. Furthermore, the use of polyimide is considered safe owing to this type of polymer not being flammable and having an excellent thermal stability and high mechanical strength. Since the polyimide is hydrolyzable, its use can be considered environmentally favorable. Attempts to use composite poliimides through solvent blending methodologies lead to poor dispersion of the polymer in the composite mixture. In situ polymerization techniques were well succeed with added incorporation of carbon fibre with the poliimide precursor. Research is in progress with studies with different electrolytes and polyimide composite in order to ensure an efficient lithium ion exchange and obtain batteries with satisfying energy performance. Advances made will be presented herein

Determining factors in becoming a sustainable smart city: an empirical study in Europe

This paper aims firstly to identify differences and similarities between cities according to the activities which are necessary to become a smart city and secondly to identify relevant factors that could influence positions in the smart cities ranking. To achieve these goals, this study uses a quantitative analysis applied to 73 European cities to identify the factors that influence differences between smart cities. The results suggest the formation of clusters around European smart cities and also that cities located geographically in the west of Europe and with governance by women present higher levels in the European Smart Cities rankings.info:eu-repo/semantics/publishedVersio

Platinum instability in PEM fuel cells MEA’s subjected to chloride contamination

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

Materials degradation mechanisms in an open cathode low power PEM Fuel Cell

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

PEM fuel cells: materials ageing mechanisms and performance impact

Polymer exchange membrane (PEM) fuel cells are considered promising power sources, with a vast application domain that includes consumer electronics, automotive and residential applications. As the technology matures, durability, reliability and cost are amongst the most critical issues, so creating the need for a more comprehensive knowledge of material’s ageing mechanism. In this work, the Membrane-Electrode Assembly, MEA, is considered a key component subject to material’s ageing with considerable impact on fuel cell performance. As it contains the polymer electrolyte membrane, the active catalysts and the gas diffusion layers (GDL), the mechanisms of degradation are complex. Furthermore, performance is also link to components such as gas distributor plates, since the used design and flow channels dimensions (channel width, channel depth, rib width) allow minimization of the diffusion pathway for gases. Effective oxidant supply and water management is greatly affected by cell geometry and materials. In-situ and ex-situ evaluation of MEA degradation were conducted after fuel cell ageing in extreme testing conditions. Humidified and dry gas feeds were also examined and the effect on cell performance and membrane conductivity examined. Variations of membrane conductivity with temperature and water content were considered critical: drying during operation as a result of dragging of water by protons or over saturated conditions cause condensation at the electrodes causing flooding with the consequent voltage degradation. Electrochemical Impedance Spectroscopy was found instrumental in the identification of flooding conditions using an equivalent circuit to model the interfaces at critical current densities, according to the location of identified irreversibility’s in the voltage-current domain of the fuel cell. Electrocatalyst surface area loss due to growth of catalyst particle size and particle agglomeration with the number of load cycles is suggested when using cyclic voltammetry of electrodes, this is thought to be due to a mechanism involving catalyst dissolution/precipitation. Cross sections of the membrane catalyst layers and GDLμs were examined under a FEG-SEM 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 as an early predictor of membrane degradation, quantified using an ion selective at gases outlet. MEA degradation mechanisms are discussed together with contributions that might aid design and operating strategies in PEM fuel Cells

Characterisation and performance studies of a LiFePO4 cathode material synthesized by microwave heating

Lithium iron phosphate with incorporated carbon, LiFePO4-C, was synthesized by the microwave-assisted method. X-ray diffraction analyses showed higher crystallization degrees for samples submitted to higher irradiation times. A particle-agglomerated morphology was associated as revealed by scanning electron microscopy. The electrochemical character-istics of a composite cathode containing the synthesized product were evaluated. The two-phase electrochemical process between FePO4 and LiFePO4 was evidenced in the cycling voltammogram profile and its reversibility and stability were demonstrated. An additional redox reversible reaction was revealed and assigned to another phosphate present in the synthesized product. The charge/discharge performance study indicated a good capacity retention after the initial cy-cles where capacity fading was associated to the resistance of a SEI film that forms and grows on the cathode’s surface. Results obtained by electrochemical impedance analysis before and after cell’s cycling are discussed

PEM Fuel Cells: materials ageing and degradation

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