Search

The increased use of fluctuating renewable energy calls for efficient, safe and environmentally friendly energy storage solutions. To meet the variable power-, capacity- and profitability requests of mobile and stationary applications, it is necessary to identify promising materials. Zinc-air batteries with specific energy density higher than lithium-ion and low cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also allows the independent scaling of capacity and power. The recharging of this system can be performed due to classical electrical charging or due to a hydraulic recharge by changing the used zinc-slurry with a fresh one. The cooperative project, ZnMobil, funded by the Federal Ministry of Economic Affairs and Energy in Germany combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a zinc-air flow battery with high performance and economic efficiency. Here we present strategies for the hydraulic recharging of zinc-air flow batteries. The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with a gas-diffusion layer (supplied by Covestro Deutschland AG). The zinc-slurry contains zinc particles (supplied by Grillo-Werke AG) suspended in alkaline solution (30 wt.-% KOH) and stabilized with polyacrylic acid. Depth-of-discharge and battery resistance were measured for zinc-slurries in a flow system under regulated conditions. The influence of the depth-of-discharge and several additives on the rheology of the zinc-slurries was evaluated. Depending on the current density and additive selection, it is possible to achieve energies of up to 300 Wh during discharge.

The reduction of the platinum amount for efficient PEM (polymer electrolyte membrane) fuel cells was achieved by the use of graphene/carbon composites as catalyst support. The influences of the carbon support type and also of the catalyst preparation method on the fuel cell performance were investigated with electrochemical, spectroscopic and microscopic techniques. Using pure graphene supports the final catalyst layer consists of a dense and well orientated roof tile structure which causes strong mass transport limitations for fuels and products. Thus the catalysts efficiency and finally the fuel cell performance were reduced. The addition of different carbon additives like carbon black particles or multi-walled carbon nanotubes (MWCNT) destroys this structure and forms a porous layer which is very efficient for the mass transport. The network structure of the catalyst layer and therefore the performance depends on the amount and on the morphology of the carbon additives. Due to optimizing these parameters the platinum amount could be reduced by 37% compared to a commercial standard system.

Polymer electrolyte membrane fuel cells PEMFC are the type of fuel cell with the broadest range of possible applications, ranging from power supply to electric drives to small scale grid independent power sources and residential CHP units. The technology has been intensively developed for more than two decades and the remaining R&D tasks focus on life time improvement and cost reduction. The following paper will give results on bipolar plate development for PEMFC.

The current energy system is subject to a fundamental transformation: A system that is oriented towards a constant energy supply by means of fossil fuels is now expected to integrate increasing amounts of renewable energy to achieve overall a more sustainable energy supply. The challenges arising from this paradigm shift are currently most obvious in the area of electric power supply. However, it affects all areas of the energy system, albeit with different results. Within the energy system, various independent grids fulfill the function of transporting and spatially distributing energy or energy carriers, and the demand-oriented supply ensures that energy demands are met at all times. However, renewable energy sources generally supply their energy independently from any specific energy demand. Their contribution to the overall energy system is expected to increase significantly. Energy storage technologies are one option for temporal matching of energy supply and demand. Energy storage systems have the ability to take up a certain amount of energy, store it in a storage medium for a suitable period of time, and release it in a controlled manner after a certain time delay. Energy storage systems can also be constructed as process chains by combining unit operations, each of which cover different aspects of these functions. Large-scale mechanical storage of electric power is currently almost exclusively achieved by pumped-storage hydroelectric power stations. These systems may be supplemented in the future by compressed-air energy storage and possibly air separation plants. In the area of electrochemical storage, various technologies are currently in various stages of research, development, and demonstration of their suitability for large-scale electrical energy storage. Thermal energy storage technologies are based on the storage of sensible heat, exploitation of phase transitions, adsorption/desorption processes, and chemical reactions. The latter offer the possibility of permanent and loss-free storage of heat. The storage of energy in chemical bonds involves compounds that can act as energy carriers or as chemical feedstocks. Thus, they are in direct economic competition with established (fossil fuel) supply routes. The key technology here – now and for the foreseeable future – is the electrolysis of water to produce hydrogen and oxygen. Hydrogen can be transformed by various processes into other energy carriers, which can be exploited in different sectors of the energy system and/or as raw materials for energy-intensive industrial processes. Some functions of energy storage systems can be taken over by industrial processes. Within the overall energy system, chemical energy storage technologies open up opportunities to link and interweave the various energy streams and sectors. Chemical energy storage not only offers means for greater integration of renewable energy outside the electric power sector, it also creates new opportunities for increased flexibility, novel synergies, and additional optimization. Several examples of specific energy utilization are discussed and evaluated with respect to energy storage applications. The article describes various technologies for energy storage and their potential applications in the context of Germany’s Energiewende, i.e. the transition towards a more sustainable energy system. Therefore, the existing legal framework defines some of the discussions and findings within the article, specifically the compensation for renewable electricity providers defined by the German Renewable Energy Sources Act, which is under constant reformation. While the article is written from a German perspective, the authors hope this article will be of general interest for anyone working in the areas of energy systems or energy technology.

Online state classification of PEM fuel cell systems is a challenging task. While electrochemical impedance spectroscopy (EIS), the measurement of the 1kHz impedance, cyclic voltammetry, and the polarisation curve are the state-of-the-art in laboratory testing, it is hardly possible to install these analytic methods in application-oriented fuel cell systems. The necessary technical equipment is expensive, and hybridisation would be essential to create artificial operating conditions necessary during the measurement. A more economic solution for online state classification is the integration of a real-time model-based state classifier. A model, operating in parallel to the fuel cell, is able to provide extra information about the fuel cell such as the humidity of the membrane, which is not measurable in situ. On the other hand, online comparison of measurement data with the theoretical calculation allows us to detect malfunction of components. Based on a detailed model and a complex classification database, including error states of the system, it is possible to classify the state of a fuel cell or detect system errors. The voltage drift between classical physical models and a real fuel cell caused by irreversible degradation (platinum diversion, membrane thinning, carbon corrosion) and reversible degradation (contamination) is not yet completely understood, and therefore modelling so far remains imprecise. To avoid the misinterpretation caused by these constantly growing differences between the model and the reality, the described modelling approach has been focused on dynamic fuel cell reactions. Dynamic step responses of the fuel cell for abruptly changing operation conditions are being simulated in real time. The changing operation conditions may result from a real load cycle or, as for theses analyses, by forced stoichiometric steps. The comparison of the real and the simulated step responses is the basis for the state classifier and the controller to react. A conclusion of the series of measurement is shown, the model and its parameters are presented, and the opportunities of the state classifier and controller are discussed in this paper.

A single-channel DMFC is constructed that allows for flow measurements at the anode side as well as detailed time–resolved cell-voltage measurement. The coherence between flow and bubble clogging and slug movement can be investigated without parasitic effects like flow shortcuts through the gas diffusion layer (GDL) between neighbouring channels, as in serpentine or parallel-channel configurations. Optical access is granted to the anode side by a transparent foil, which is necessary for the application of the laser-optical velocity measurement technique (micro–particle image velocimetry, μPIV). Small fluorescent particles are added to the fluid, which are illuminated by a laser. The particle movement can be optically detected using a microscope, and transferred to a planar velocity field. Hence, the appearance and evolution of CO2 bubbles can be qualitatively and quantitatively investigated. The analysis of the velocity structure around a CO2 bubble or a moving slug allows a deeper understanding of the coherence of fluid motion, channel blockage, and cell performance. In addition to the flow analysis, a time-resolved measurement of the cell voltage is performed. The results clearly indicate that the cell power increases when huge bubbles reduce the free cross-section area of the channel. Methanol is forced into the GDL, i.e. methanol is continuously convected to the catalyst layer and is oxidised to CO2. Hence, the fuel consumption increases and the cell performance rises. When the huge bubble is released from the GDL and forms a moving slug, the moving slug effectively cleans the channel from CO2 bubbles on its way downstream. Since the channel cross-section is not severely diminished by the bubbles at this stage, the methanol flow is no longer forced into the GDL. The remaining amount of methanol in the GDL is oxidised. The cell power decreases until enough CO2 is produced to eventually form bubbles again that significantly reduce the free cross-section of the channel, and the process starts again.

An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800 °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100–1200 °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting. Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks. This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process.

Brennstoffzellen als elektrochemische Energiewandler haben durch gezielte Materialentwicklung Anwendungsreife erlangt. Fur die Membranbrennstoffzelle sind das insbesondere der ionenleitende, polymere Festelektrolyt, Edelmetall-Nanokatalysatoren auf korrosionsstabilen Kohlenstofftragermaterialien, Gasdiffusionsschichten mit ausgeklugeltem Porensystem sowie Werkstoffe fur die Bipolarplatte, die fur die Serienfertigung geeignet sind. Die Fortschritte der letzten Jahre auf dem Gebiet der Materialentwicklung sind beeindruckend und haben zu Brennstoffzellen mit hoher Leistungsdichte, einer guten Lebensdauer und mit robustem Betriebsverhalten gefuhrt. Fuel cells as electrochemical conversion devices are nowadays ready for use thanks to targeted material development. For membrane fuel cells, the most important materials are the ionic conducting polymer electrolyte, the noble metal nanostructured electrocatalyst and its corrosion resistant carbon carrier material, the gas diffusion layer with its highly sophisticated pore system, as well as suitable construction materials for bipolar plates, which make a mass production possible. The progress of the past years has led to fuel cells with high specific power density, good durability and also with robust operational behavior.

The cogeneration of heat and power by means of a fuel cell based CHP unit is a promising option for efficient residential power supply. For most applications natural gas is used as fuel. One main component of such a CHP unit is a fuel processor in order to generate hydrogen from the natural gas with hydrogen thermal power output of about 6 kW. Usually the steam reforming process is used for hydrogen production. In order to meet the heat demand of the endothermic steam reforming process the fuel processor is equipped with a burner, which has to work with natural gas during start up phase and mainly with the low calorific anodic off gas of the fuel cell stack during normal operation. The presented work is focused on aspects of the main pollutant emissions (carbon monoxide and nitrogen oxide) of burners integrated into the reformer. Experimental investigations of two different burners, which were developed and adapted to the steam reformer requirements, in a real fuel processor environment show, that it is possible to operate both burner concepts with high and low calorific gases with very low pollutant emissions in order to compete with emissions of current heating boilers, which are in the range of 15 mg kWh−1 for CO and of 20 mg kWh−1 for NOx by adjusting suitable excess air ratios in the range of 1.2–1.4. But it is also demonstrated, that the efficiency of the fuel processor is influenced by the excess air ratio. An increase of the air ratio from 1.05 to 1.45 leads to an decrease of the efficiency from 80% to 76%. This results in a conflict of objectives between low pollutant emissions and high system efficiencies. The choice of a suitable burner concept and the definition of a suitable operation strategy can be based on the presented results. Additionally, aspects like fuel processor geometry, flame monitoring, pressure drop in the burner feed gas line as well as in the flue gas duct, investment costs and safety items have also to be considered for the burner selection.

This work presents a three-dimensional, steady-state, non-isothermal model of a high-temperature polymer-electrolyte-membrane fuel cell (HTPEMFC) using a phosphoric acid-doped polybenzimidazole (PBI/H3PO4) sol–gel membrane. The model accounts for the gold-plated copper current collector plates, the bipolar plates, all gas flow channels (flow-field), the gas diffusion layers, the reaction layers, and the membrane. Electrochemical reactions are modeled using an agglomerate approach and include the gas diffusivity and the gas solubility. The conductivity of the membrane is modeled using the Arrhenius equation to describe the temperature dependence. Finite elements are used to discretize all computational subdomains, and a commercially available code is used to solve the problem. The predicted values are compared to typical operating conditions, and a good agreement is found. The current density, the solid- and fluid-(gas)-phase temperatures and other quantities are analyzed throughout the computational subdomains. It was observed that the Arrhenius approach is valid in a certain temperature range and may overpredict the PBI/H3PO4 sol–gel membrane conductivity at higher solid-phase temperatures. Moreover, it is shown how the fluid-(gas)-phase temperature influences the solid-phase temperature and the current density distribution. Concrete values are deduced from the simulations and discussed according to experimental test.

Wenn regenerativ erzeugter Strom, biogene Energietrager und Wasserstoff die fossilen Energietrager nach und nach ablosen, ist eine der wichtigsten Zukunftstechnologien die effiziente Wandlung von Wasserstoff in elektrische Energie und Warme. Mittlerweile sind in verschiedensten Anwendungen Polymer-Elektrolyt-Membran (PEM)-Brennstoffzellensysteme in der Erprobung. Dank gezielter und kontinuierlicher staatlicher Forderung kann es gelingen, diese neue umweltfreundliche Technologie auch in Konkurrenz zu existierenden Techniken im Markt zu etablieren. Die Arbeit gibt einen Uberblick uber die Entwicklung der Brennstoffzellentechnologie.

The thermal conductivity of commercially available thermally conductive polymer-compounds is usually between 1 W/mK and 20 W/mK what means that they are already 10 to 100 times higher than the conventional unfilled polymer. During the development at ZBT the values for thermal conductivity of polymer-compounds could be increased to more than 30 W/mK. In order to achieve such conductivities, the compound materials generally consist of a polymer, which functions as a binding matrix, and a high content (up to 80 wt.%) of conductive filler material. In various series of investigations the thermal conductivity but also the mechanical properties and the processing by extrusion and injection molding of the highly filled materials were evaluated. An orientation of filler particles by injection molding could be observed. Images by scanning-electron-microscopy of injection molded samples showed a structure of anisotropic layers which has a significant influence on the values of thermal conductivity. That’s why a dif...

Portable fuel cells are under development at present for remote off-grid applications, the first niche markets having notable numbers of pieces are military and leisure applications of the direct methanol fuel cell DMFC in the power range up to several hundreds of watts. Hydrogen-fuelled cells are not applicable yet due to missing refueling infrastructure. Different storage options are evaluated at present. Specially developed micro-fuel cells are an interesting alternative to batteries, particularly if extreme long and autarkic operation is required, but in fact the breakthrough of a marketable product has not yet been achieved.

An LPG-based auxiliary power unit (APU) is currently being developed at the Centre for Fuel Cell Technology (ZBT). Due to the modular design of the APU, the reformer/burner module and the CO purification module, combined in the mobile gas processor (MGP), and the low temperature PEM fuel cell (PEM FC) can be developed and characterized simultaneously. While the separate characterizations of the MGP and the PEM FC are satisfactory during the initial coupling tests, a condensation problem occurs in the PEM FC, because of the high water content in the reformate gas. This paper presents two possible solutions for the prevention of condensation inside the PEM FC, when coupled with a gas processor. Analogous testing is carried out by supplying synthesized reformate gas to the PEM FC, resulting in a 360 W maximum electrical power output at 85 °C. New membrane electrode assemblies (MEAs), which are suitable for reformate gas operation at temperatures up to 90 °C, are integrated in this PEM FC. Furthermore, the impact of CO on PEM FC performance can be minimized by adding an airbleed of 1.0 mol-% of the complete anode gas. Thus, stable operation is achieved with the separate PEM FC, by supplying a synthesized anode gas, which includes a CO concentration constantly between 30 and 50 ppm. The coupling of the MGP with the PEM FC, under the improved operating conditions, leads to an electrical power output of 351 W. The overall APU efficiency is 28.5%.

ZBT can look back on 10 years of history in successful development of hydrogen generators for residential power supply using PEM fuel cells. ZBT has always focused on efficient steam reforming and heat integration. All hydrogen generators contain a steam reformer, a shift unit, a CO-purification unit, evaporator, heat exchanger and burner. The core technology is protected by international patents (1). Especially the requirements obtained from industrial partners were used to optimise the hydrogen generator to meet the demands of residential applications. Sometimes not all of these demands can be fulfilled and compromises are necessary. For example high efficiency requires heat integration which leads to high complexity of the system, higher volume/weight and finally elevated production costs or short start-up time requires a radical temperature gradient which has a negative impact on life time (2). All these correlations have to be discussed between product management and development to identify reasonable compromises.

With focus on mobile applications a fuel cell auxiliary power unit (APU) using liquefied petroleum gas (LPG) is currently being developed at the Centre for Fuel Cell Technology (Zentrum fur BrennstoffzellenTechnik, ZBT gGmbH). The system is consisting of an integrated compact and lightweight fuel processor and a low temperature PEM fuel cell for an electric power output of 300 W. This article is presenting the current status of development of the fuel processor which is designed for a nominal hydrogen output of 1 k W th , H 2 within a load range from 50 to 120%. A modular setup was chosen defining a reformer/burner module and a CO-purification module. Based on the performance specifications, thermodynamic simulations, benchmarking and selection of catalysts the modules have been developed and characterised simultaneously and then assembled to the complete fuel processor. Automated operation results in a cold startup time of about 25 min for nominal load and carbon monoxide output concentrations below 50 ppm for steady state and dynamic operation. Also fast transient response of the fuel processor at load changes with low fluctuations of the reformate gas composition have been achieved. Beside the development of the main reactors the transfer of the fuel processor to an autonomous system is of major concern. Hence, concepts for packaging have been developed resulting in a volume of 7 l and a weight of 3 kg. Further a selection of peripheral components has been tested and evaluated regarding to the substitution of the laboratory equipment.

At the Centre for Fuel Cell Technology (Zentrum fur BrennstoffzellenTechnik/ZBT GmbH) in Duisburg/Germany a 300 W fuel cell system of liquid gas-powered operation is currently being developed with focus on leisure boats. An excellent infrastructure in the domain of recreational activities is available for this fuel and users are familiar with the safe handling of liquid gas. On sailing yachts the consumption of electrical power is very restricted during long cruises because of low battery capacities. In this case an additional power supply based on the noiseless fuel cell technology promises an essential comfort increase without disturbing emissions. ZBT's engineers combined their experience in system simulation, reactor design for the gas process and fuel cell technology with the aim of developing a user friendly fuel cell product.

The University of Duisburg-Essen and the Center for Fuel Cell Technology (ZBT Duisburg GmbH) have developed a compact multi-fuel steam reformer suitable for natural gas, propane and butane. Fuel processor prototypes based on this concept were built up in the power range from 2.5 to 12.5 kW thermal hydrogen power for different applications and different industrial partners. The fuel processor concept contains all the necessary elements, a prereformer step, a primary reformer, water gas shift reactors, a steam generator, internal heat exchangers, in order to achieve an optimised heat integration and an external burner for heat supply as well as a preferential oxidation step (PrOx) as CO purification. One of the built fuel processors is designed to deliver a thermal hydrogen power output of 2.5 kW according to a PEM fuel cell stack providing about 1 kW electrical power and achieves a thermal efficiency of about 75% (LHV basis after PrOx), while the CO content of the product gas is below 20 ppm. This steam reformer has been combined with a 1 kW PEM fuel cell. Recirculating the anodic offgas results in a significant efficiency increase for the fuel processor. The gross efficiency of the combined system was already clearly above 30% during the first tests. Further improvements are currently investigated and developed at the ZBT.

The development of bipolar plates that can be produced by standard mass production techniques is a main issue for the commercialization of PEM fuel cells, as bipolar plates contribute significantly to the cost structure of PEM stacks. In recent years, the University of Duisburg-Essen together with the Zentrum fur BrennstoffzellenTechnik GmbH (ZBT) has identified a number of carbon–polymer composites with densities of 1.6 g/cm3, specific bulk conductivities between 5 and 150 S/cm and material prices between 2 and 10 €/kg. Standard composite mixtures consist of a thermoplast and a carbon compound mixture with additional additives to increase the conductivity of the compound material. The composites generally show high corrosion resistance in the PEM fuel cell environment. Composite material samples proved to be absolutely stable in immersion tests in sulphuric acid and deionized water under pure oxygen atmosphere for several thousand hours. ZBT has successfully demonstrated the production of bipolar plates by injection moulding with cycle times of 30–60 s. With the help of tailored moulds injection moulding of bipolar plates becomes price competitive even for comparatively small series in the range of several thousand plates. PEM stacks with injection moulded bipolar plates of 2.5–4 mm thickness and an electrical power of up to 200 W have been constructed and successfully operated.

The University of Duisburg–Essen has developed a compact multi-fuel steam reformer suitable for natural gas, propane and butane. This steam reformer was combined with a polymer electrolyte membrane fuel cell (PEM FC) and a system test of the process chain was performed. The fuel processor comprises a prereformer step, a primary reformer, water gas shift reactors, a steam generator, internal heat exchangers in order to achieve an optimised heat integration and an external burner for heat supply as well as a preferential oxidation step (PROX) as CO purification. The fuel processor is designed to deliver a thermal hydrogen power output from 500 W to 2.5 kW. The PEM fuel cell stack provides about 1 kW electrical power. In the following paper experimental results of measurements of the single components PEM fuel cell and fuel processor as well as results of the coupling of both to form a process chain are presented.

In the recent years, rechargeable lithium-ion batteries have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of lithium-ion batteries. Especially for high energy and power densities, the electrode materials must possess high specific storage capacities and coulometric efficiencies. However, state-of-the-art anode and cathode electrode materials, e.g. graphite and LiFePO4 exhibit high coulometric efficiencies but rather low theoretical storage capacities (372 and 170 mAh/g, respectively). In the last decade silicon has become a promising anode material due to its high theoretical specific capacity of 3579 mAh/g at ambient temperature. However, this high specific storage capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 280 % during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nano-sized active material or carbon coated silicon composite material. In addition to the materials science the electrode structure is of particular importance for the electrochemical performance. Electrode composition, binding mechanism due to the use of suitable binder polymers or particle size distribution of the active material are some exemplary parameters to stabilize the electrode structure and to handle such high mechanical stress during lithiation/delithiation. Here we present electrochemical investigations of high capacity and high efficiency graphene coated silicon nanocomposite based electrodes prepared by using a wet chemical doctor blade manufacturing process. This active material provides a capacity of >2000 mAh/g with efficiencies >99% over more than 500 cycles. Investigations focus on influence of the stability of the electrode structure by using impedance spectroscopy, scanning electron microscopy, confocal microscopy and estimation of the coating adhesion strength. Cyclic voltammetry and galvanostatic cycling will show the applicability of improved Si/C composite based electrodes compared to conventional graphite based electrodes for lithium-ion batteries both in half cells as well as full cells in combination with commercially available cathode material.

In order to optimize the electron transfer between the Li4Ti5O12-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li4Ti5O12 mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li4Ti5O12 electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector.

A robust and electrochemically stable 3D nanoheterostructure consisting of Si nanoparticles (NPs), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) is developed as an anode material (Si–CNT/rGO) for lithium-ion batteries (LIBs). It integrates the benefits from its three building blocks of Si NPs, CNTs, and rGO; Si NPs offer high capacity, CNTs act as a mechanical, electrically conductive support to connect Si NPs, and highly electrically conductive and flexible rGO provides a robust matrix with enough void space to accommodate the volume changes of Si NPs upon lithiation/delithiation and to simultaneously assure good electric contact. The composite material shows a high reversible capacity of 1665 mAh g−1 with good capacity retention of 88.6 % over 500 cycles when cycled at 0.5 C, that is, a 0.02 % capacity decay per cycle. The high-power capability is demonstrated at 10 C (16.2 A g−1) where 755 mAh g−1 are delivered, thus indicating promising characteristics of this material for high-performance LIBs.

Das Energiesystem steht vor einem grundlegenden Wandel: Ein System, das auf die konstante Bereitstellung durch fossile Energieträger ausgerichtet ist, soll durch die umfangreiche Integration erneuerbarer Energien eine nachhaltigere Energieversorgung gewährleisten. Die Herausforderung des Systemwechsels macht sich gegenwärtig in der Stromversorgung am deutlichsten bemerkbar, betrifft aber alle Bereiche des Energiesystems, wenn auch mit unterschiedlichen Auswirkungen. Im Energiesystem werden Energie und/oder Energieträger räumlich von Energieversorgungsnetzen verteilt, während die bedarfsgerechte Bereitstellung gegenwärtig dafür sorgt, dass der Energiebedarf zu jeder Zeit gedeckt wird. Energie aus erneuerbaren Quellen wird in der Regel nicht bedarfsgerecht bereitgestellt; ihr Anteil steigt. Energiespeicher sind eine Möglichkeit, das zeitlich versetzte Angebot mit der Nachfrage zur Deckung zu bringen. Energiespeicher sind Systeme, die eine Energiemenge kontrolliert aufnehmen, sie über einen im Kontext relevanten Zeitraum in einem Speichermedium zurückhalten und mit zeitlicher Verzögerung wieder kontrolliert abgeben können. Zu den Energiespeichern gehören nach dieser Definition auch Verfahrensketten, die diese Aspekte durch eine Kombination verschiedener Technologien abbilden. Als mechanische Großspeicher für elektrischen Strom dienen heute fast ausschließlich Pumpwasserspeicherkraftwerke, die zukünftig durch Druckluftspeicherkraftwerke und eventuell Luftzerlegungsanlagen ergänzt werden könnten. Im Bereich der elektrochemischen Energiespeicher befinden sich verschiedene Technologien im Forschungs-, Entwicklungs- und/oder Demonstrationsstadium für einen Einsatz in der stationären großtechnischen Stromspeicherung. Thermische Speichertechnologien beruhen auf der Speicherung von sensibler Wärme, der Ausnutzung von Phasenübergängen, Adsorption-/Desorptionsprozessen oder chemischen Reaktionen, die prinzipiell eine dauerhafte und verlustfreie Speicherung von Wärme ermöglichen können. Die Speicherung von Energie in Form chemischer Bindungen in stofflichen Speichern verläuft über Substanzen, die selbst als Energieträger oder Chemikalien verwendet werden können; sie befinden sich damit in direkter Konkurrenz zu alternativen Bereitstellungs- und Nutzungsvarianten. Die Schlüsseltechnologie hierbei ist auf absehbare Zeit die Elektrolyse von Wasser zu Wasserstoff und Sauerstoff. Wasserstoff kann wiederum durch verschiedene Verfahren in andere Energieträger umgewandelt werden. So lässt er sich in verschiedenen Sektoren des Energiesystems und/oder in energieintensiven Industrieprozessen stofflich nutzen. Teilfunktionen von Energiespeichern können auch von Industrieprozessen wahrgenommen werden. Dem Energiesystem in seiner Gesamtheit eröffnen sich neue Optionen, die bisher weitgehend getrennten Energieversorgungsströme zu verknüpfen und zu vernetzen. Neben der Möglichkeit, verstärkt erneuerbare Energien außerhalb des Stromsektors zu nutzen, ergeben sich auch neue Bedingungen für eine verstärkte Flexibilisierung, neuartige Synergieeffekte und zusätzliche Optimierungsmöglichkeiten. Anhand verschiedener Referenzfälle wird der mögliche Einsatz von Speichertechnologien aufgezeigt und bewertet. OA hybrid

A facile photothermal procedure for direct functionalization of carbon/polymer bipolar plate materials is demonstrated. Through irradiation with a microfocused beam of an Ar + -laser at λ = 514 nm in gaseous bromine and distinct laser powers and pulse lengths local bromination of the carbon/polymer material takes place. At a 1/ e spot diameter of 2.1 μm, functionalized surface areas with diameters down to 5 μm are fabricated. In complementary experiments large-area bromination is investigated using an ordinary tungsten lamp. For characterization contact angle goniometry, X-ray photoelectron spectroscopy and electron microscopy in conjunction with labeling techniques are employed. After irradiation bromine groups can easily be substituted by other chemical functionalities, e.g. azide and amine groups. This provides a facile approach in order to fabricate surface patterns and gradient structures with varying wetting characteristics. Mechanistic aspects and prospects of photothermal routines in micropatterning of carbon/polymer materials are discussed.

Fuel cells are under discussion for a large variety of applications as efficient and clean power generators today. In general, fuel cells are easily scalable due to their modular design and furthermore, the storage capacity and the output power can be combined independently. This enables the integration into portable devices or even in micro systems, which is favourable in those cases when batteries do not meet the respective demands. In particular, fuel cells offer advantages like variable geometry, high storage capacity, no self discharge and the potential for low production costs which make them interesting candidates for an environmentally friendly energy source [1].

The reforming of natural gas to produce hydrogen for fuel cells is described, including the basic concepts (steam reforming or autothermal reforming) and the mechanisms of the chemical reactions. Experimental work has been done with a compact steam reformer, and a prototype of an experimental reactor for autothermal reforming was tested, both containing a Pt-catalyst on metallic substrate. Experimental results on the steam reforming system and a comparison of the steam reforming process with the autothermal process are given.

Direct methanol fuel cells (DMFCs) are attractive for several applications, however, at present, methanol crossover from the anode to the cathode appears to be a major limitation. For this reason, as one of the alternatives, membrane technology tries to obtain new methanol-impermeable polymer electrolytes. In this sense, it would be interesting to develop easy methods to check the new materials in relation to the methanol transport. In this work, a simple model is presented which permits to estimate easily the methanol diffusion coefficient of the membrane of a DMFC from open circuit voltage (OCV) measurements. The model has been checked in a DMFC using Nafion membranes as electrolyte.

In the recent years, rechargeable lithium-ion batteries (LIBs) have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of LIBs. Currently, commercially available graphite anodes with a specific capacity of 372 mAh g–1 are used, but these cannot satisfy the abovementioned demands. Silicon is a very promising candidate as an anode material due to its high theoretical capacity of 3579 mAh g–1 at room temperature (according to the formation of the Li15Si4alloy). However, this high specific capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 300% during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nanosized active material or carbon-based silicon composites. Nanosized structures can shorten the diffusion pathways of the lithium-ions, thus facilitating rapid lithiation and delithiation processes. In addition, smaller sized structures can also significantly reduce the inner-mechanical stress of the materials during lithiation/delithiation of the active material. Carbon-based additives, e.g. carbon nanotubes (CNTs) for forming nanocomposites with silicon nanostructures can not only improve the electrical conductivity of the silicon anodes, but can also effectively accommodate the large volume changes of silicon nanoparticles (Si NPs) during the electrochemical reactions. ln this study, we propose a new strategy using reduced graphene oxide (rGO) in combination with a stabilized Si-CNT hybrid to generate a robust, highly conducting nanoheterostructure to further enhance the stability and the electrochemical performance of silicon-based electrode materials. Si NPs were synthesized in the gas phase by decomposition of monosilane in a hot-wall reactor. Si NPs and CNTs were functionalized and reacted to form the Si-CNT hybrids, utilizing the formation of peptide bonds between amine-modified Si NPs and the carboxyl-functionalized CNTs. This chemical linking is the first step to improve the binding strength between the Si NPs and an electrically conducting network. The Si-CNT hybrid was combined with GO to self-assemble to form a robust nanoheterostructure, which is subsequently reduced. Si NPs, CNTs, and rGO sheets play important roles in this unique nanoheterostructure as LIB anode. Si NPs provide high capacity, rGO ensures high electrical conductivity for the entire structure and provides sufficient void spaces to buffer the volume changes of the Si NPs, and CNTs act as the scaffolding to bind the Si NPs and provide additional electrically-conductive channels. Here we present electrochemical investigations of silicon anodes based on Si-CNT/rGO nanoheterostructure nanocomposite material. The electrode preparation is based on a well-established wet chemical doctor blade manufacturing process using a water based binder. The nanocomposite material shows a high reversible capacity of 1665 mAh g–1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5 C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2 Ag-1) where 755 mAh g–1 are delivered. Full cell experiments demonstrate the applicability of improved silicon anodes for lithium-ion batteries.

In recent years, zinc-air flow batteries have regained importance as promising energy storage system for mobile and stationary applications. This revived importance is driven by the increased demand of high-performance, efficient, safe and environmentally friendly energy storage solutions to meet the challenges of modern energy supply systems with high amounts of renewable energy. These energy storage systems need to address variable power, capacity and profitability requests. Zinc-air batteries with a specific energy density higher than lithium-ion and low cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with oxygen reduction electrodes developed for the chloralkali process allows the development of high-power zinc-air batteries. This setup also enables the independent scaling of capacity and power. The cooperative research project ZnMobil, supported by the Federal Ministry of Economic Affairs and Energy in Germany, combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; The fuel cell research center ZBT GmbH) to develop a low-cost zinc-air flow battery with high performance. Here we present the electrochemical performance of the new zinc-air flow battery with respect to different zinc-slurry compositions. The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with gas-diffusion layer (supplied by Covestro). The zinc-slurry contains zinc particles (supplied by Grillo) suspended in alkaline solution (30 wt-% KOH) and stabilized with polyacrylic acid. Current voltage curves and depth of discharge were measured as a function of slurry composition (e.g. zinc content, additives) and different flow velocities of the zinc slurry. Corresponding zinc-slurry conductivities were evaluated in a flow-through conductivity measurement setup. Depending on the selected parameters, it is possible to achieve discharge current densities higher than 4 kA/m².

Hydrogen generation for fuel cell systems by reforming technologies from various fuels is one of the main fields of investigation of the Fraunhofer ISE. Suitable fuels are, on the one hand, gaseous hydrocarbons like methane, propane but also, on the other hand, liquid hydrocarbons like gasoline and alcohols, e.g., ethanol as biogenic fuel. The goal is to develop compact systems for generation of hydrogen from fuel being suitable for small-scale membrane fuel cells. The most recent work is related to reforming according to the autothermal principle — fuel, air and steam is supplied to the reactor. Possible applications of such small-scale autothermal reformers are mobile systems and also miniature fuel cell as co-generation plant for decentralised electricity and heat generation. For small stand-alone systems without a connection to the natural gas grid liquid gas, a mixture of propane and butane is an appropriate fuel.

The low power range can be an interesting application market for the PEM fuel cell in the near future. With a possible function as battery in portable devices the banded structure PEM fuel cell will be an advantageous alternative to the conventional stack. The new system provides high output voltages, a flat cell geometry and can be optimized with respect to smaller cell volumes. The article describes the principle function of such a system and the necessary parameter for optimizing the cell geometry. A comparison with a conventional stack design is given and experimental results with an eight cell banded structure stack are presented.

Oil and gas are raw materials—the availability of which is prognosticated to run short in the near future. The peak oil discussion is an example generally perceived as proof of this development to come.

Fuel cell hybrid vehicles offer a high-efficiency and low-emission substitute for their internal combustion engine counterparts. The hybridization significantly improves the fuel economy of the vehicle; however, exploiting the hybridization requires a well-designed power management strategy that optimally shares the power demand between the power sources. This paper deals with the optimization of power management strategy of a fuel cell/battery hybrid vehicle, both off-line and in real-time. A new formulation of the optimization problem for the real-time strategy is presented. The new approach allows the optimization of the controller over a set of driving cycles at once, which improves the robustness of the designed strategy. The real-time optimization is applied to two forms of real-time controllers: a PI controller based on Pontryagin's Minimum Principle with three parameters and a fuzzy controller with ten parameters. The results show that the PI controller can outperform the fuzzy controller, even though it has fewer parameters. The real-time controllers are designed by simulation and then validated by experiment.

A simple coating routine in order to tune the wettability of carbon/polymer bipolar plate materials is presented. Standard carbon/polypropylene composite materials as used for commercial bipolar plates for polymer-electrolyte-membrane fuel cell application are chemically modified via oxygen plasma activation and subsequent silanization using distinct precursor molecules including perfluorodecyltrichlorosilane and aminopropyltrimethoxysilane. For characterization of the samples contact angle measurements, infrared and Auger electron spectroscopy and scanning electron and atomic force microscopy are employed. Spectroscopic data provides direct evidence for successful functionalization of the substrates. Microscopic data reveals the inherent roughness of the micro-/nanostructured substrate surfaces. Depending on the particular silane precursor, the coating procedure yields hydrophilic and hydrophobic surfaces with static water contact angles ranging from 55° to 160°. The wettability of these substrates remains unchanged upon storage in clean air over a period of one year and more. Prospects of the coating procedure targeting the optimization of the water management in fuel cell applications are discussed.

In the recent years, rechargeable lithium-ion batteries (LIBs) have gained in importance for electronic devices and electric vehicles. Thus, research and development focuses on improving energy and power densities as well as durability of LIBs. Especially for high energy and power densities, the electrode materials must possess a high specific storage capacity and a high coulombic efficiency. However, state-of-the-art anode and cathode electrode materials, e.g. graphite and LiFePO4 exhibit a high coulombic efficiency but a rather low theoretical storage capacity (372 and 170 mAh⋅g-1, respectively). In the last decade silicon has become a promising anode material due to its high theoretical specific capacity of 3579 mAh⋅g-1at ambient temperature. However, this high specific storage capacity owing to host up to 3.75 lithium atoms per silicon atom leads to extreme volume expansion up to 300 % during lithiation, which results in pulverization and delamination of the electrode material after few cycles. Various approaches have been conducted to overcome these issues e.g. by using nano-sized active material or carbon coated silicon composite material. In addition to the materials science the electrode structure is of particular importance for the electrochemical performance. Electrode composition, binding mechanism due to the use of suitable binder polymers, particle size distribution of the active material or the modification of the SEI are some exemplary parameters to stabilize the electrode structure and to handle such high mechanical stress during lithiation/delithiation. Finally, the developed silicon anode must be implemented into a full cell by combining the anode with a suitable cathode. Because in a commercial LIB only the cell voltage is controllable, the electrode balancing of the full cell, that means the capacity ratio of the negative to positive electrode (N/P ratio) and the electrochemical voltage window in which the full cell is operated, is practically important to achieve a long cycle life and a high coulombic efficiency for the battery. For example, it can be ensured by adjusting the electrochemical potential window and choosing a well-balanced electrode design (normally N/P>1) that irreversible capacity losses can be prevented, which are correlated to lithium plating on the anodes surface during the charging process. Here we present electrochemical investigations of high capacity and high efficiency graphene coated silicon nanocomposite based electrodes prepared by using a water based wet chemical doctor blade manufacturing process. The commercially available Si/C-composite is mixed with graphite to obtain a Si/C-anode that provides a capacity of 1000 mAh⋅g-1 with an average coulombic efficiency >99 % over more than 500 cycles in half cells. The developed Si/C-anode is combined with a LiFePO4-cathode to build high-energy full cells. Investigations focus on the influence of the N/P ratio and the voltage window on the electrochemical performance of Si/C-LiFePO4 full cells. It will be shown that the developed Si/C-anode improves the energy density of the full cell regarding to a comparable C6-LiFePO4 full cell over more than 200 cycles.

The RSPFC is a battery-like hydrogen/oxygen system which offers the possibility of splitting water by electrolysis, storing the gases hydrogen and oxygen and regenerating electricity by the fuel cell process. The most outstanding feature is the use of only one energy conversion device for both processes, electrolysis and fuel cell operation. Membrane technology is the most suitable system for realization of this concept. Basic investigations concerning the best catalysts for the electrode reactions at electrode/membrane units have been carried out. The preparation of these units and the electrochemical characterization by half cell measurements and impedance measurements are reported as well as first experiences in reversibly operating small single cells.

Olivine, LiFePO4 is a promising cathode material for lithium-ion batteries due to its low cost, environmental acceptability and high stability. Its low electric conductivity prevented it for a long time from being used in large-scale applications. Decreasing its particle size along with carbon coating significantly improves electronic conductivity and lithium diffusion. With respect to the controlled formation of very small particles with large specific surface, gas-phase synthesis opens an economic and flexible route towards high-quality battery materials. Amorphous FePO4 was synthesized as precursor material for LiFePO4 by flame spray pyrolysis of a solution of iron acetylacetonate and tributyl phosphate in toluene. The pristine FePO4 with a specific surface from 126–218 m2 g−1 was post-processed to LiFePO4/C composite material via a solid-state reaction using Li2CO3 and glucose. The final olivine LiFePO4/C particles still showed a large specific surface of 24 m2 g−1 and were characterized using X-ray diffraction (XRD), electron microscopy, X-ray photoelectron spectrocopy (XPS) and elemental analysis. Electrochemical investigations of the final LiFePO4/C composites show reversible capacities of more than 145 mAh g−1 (about 115 mAh g−1 with respect to the total coating mass). The material supports high drain rates at 16 C while delivering 40 mAh g−1 and causes excellent cycle stability.

During the past decade several important development steps, such as the 700 bar hydrogen storage or the freeze start capability, have brought fuel cell electric vehicles close to market introduction. Further drive system cost reduction by e.g. simplification of the fuel cell system architecture are intended for future fuel cell vehicle generations. Removing the anode recirculation loop and operating the fuel cell stack in the so-called near-dead-ended1 mode is one promising concept. Key experiments focussed on the fuel cell stack's anode side under vehicular load conditions and temperature levels have been performed successfully while maintaining fuel consumption constraints. The impact of cathode operating conditions on the liquid water accumulation and hydrogen concentration on the anode side has been investigated by simulation in order to optimize the operation of this lean anode concept.

Segmented temperature measurements were performed to better understand the thermal behaviour and thermal interactions between the fluid-(gas)-phase and solid-phase temperature within a working high temperature polymer electrolyte membrane (HTPEM) fuel cell. Three types of flow-fields were studied, and the influence of temperature for no-load and load operating conditions was investigated. Tests were performed under various operating conditions, and the results demonstrate the utility of segmented temperature measurements. A significant difference in the temperature distribution was observed when the HTPEM fuel cell was operated with pure hydrogen and with hydrogen containing carbon monoxide. The findings may lead to improved HTPEM fuel cells and future middle temperature polymer electrolyte membrane (MTPEM) fuel cell designs.

Significant advances have been reported in building and testing of high-temperature polymer electrolyte membrane (HTPEM) fuel cells and stacks during recent years. Quantity distribution measurement techniques (e.g. current density, temperature and electrochemical impedance spectroscopy (EIS)) using segmented cells are commonly used to characterise low-temperature PEM (LTPEM) fuel cells. Performing these measurements at higher temperatures is more difficult and a relatively new process. For this study, a fully operational segmented HTPEM fuel cell using a straight flow-field configuration was designed, constructed and tested. The cathode side bipolar half-plate consisted of 36 exchangeable segments, whereas, the anode side bipolar half-plate was not segmented. The cell was operated at various operating temperatures with various anode gas compositions and air (no backpressure). In addition to the experimental results, a simple computational fluid dynamics model based on COMSOL Multiphysics® 3.5a was used to support the observed behaviour during segmented measurements. The computational domain consisted of the cathode side gas channels and the porous media. All of the boundary conditions and gas properties were defined in a manner similar to the experimental investigations. Some of the theoretical results were compared to the experimental results and conclusions were drawn.

In the current work, three types of flow field structures were analysed: i) Fluid flow patterns such as uniformity in gas distribution in various channels of these flow fields were studied using particle image velocimetry (PIV) technique, ii) Pressure drop across each flow field structure was measured using standard anemometers, iii) Impedance of 3 single cells built with these three types of flow fields, each with a commercially available Celtec-P 1000 based membrane electrode assembly (MEA) was examined, to ensure, adequate contact between various cell components using electrochemical impedance spectroscopy (EIS) technique, iv) 3Dcomputational fluid dynamics (CFD) simulations were performed using a commercially available software to ascertain oxidant distribution in these flow fields. Finally, v) Experiments were performed in a test stand with these three types of single cells, (with hydrogen and air as reactant gases) to gauge each cell's overall performance. The active area of each cell was 27.6 cm². High temperature stable graphite compound based bipolar plates (FU 4369) from Schunk Kohlenstofftechnik GmbH, Germany were used in each cell.

High temperature polymer electrolyte membrane fuel cells (HT PEMFCs) offer tremendous flexibility when used as energy converters in stationary as well as mobile power devices. Coupling HT PEMFC stacks with fuel processors that use liquid as well as gaseous fuels to generate hydrogen rich gas is a promising prospect, which paves the way for a possible hydrogen economy. The current paper deals with the performance aspects of a 150 Wel HT PEMFC stack, which potentially could be coupled to (i) a natural gas reformer, (ii) a propane reformer, or (iii) a methanol reformer. A 12 cell HT PEMFC stack with a total active area of about 600 cm2 was operated in a test rack, and the results show that HT PEMFCs are principally suited for operation with reformates.