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Due to its ability to perform clean, efficient processes, hydrogen has long been promoted as the energy carrier of the future, but recent activity and advances suggest a tipping point has been reached as hydrogen is now being recognized by much of the world for its unique capabilities to strongly support global efforts in achieving climate neutrality. The coming decades will require a massive increase in the amount of hydrogen used in the energy system as well as a transition from largely thermochemical to predominantly electrochemical processes involving hydrogen. For this to happen, hydrogen will need to globally approach the gigaton-scale on an annual basis. Thus, before hydrogen can fully achieve its potential, challenges that need to be addressed include achieving scale and further research and development (R&D) advances.

The transformation of energy systems towards high shares of renewable energy production while phasing out the use of fossile carbon-based energy carriers requires both a highly resilient energy system and an energy carrier which couples the electricity sector with energy consuming sectors like transportation, industry and chemistry. This is why we now see the advent of large-scale water electrolyzers for the production of sustainable hydrogen. Hydrogen will be an important feedstock for fuel cell cars, busses, trucks and trains, but also an important basic chemical for refineries, for the steel production and for various kinds of sustainable chemicals like fertilizers or polymers. This talk will adress the status quo of electrolyzer technologies and techno-economic analyses of power-to-hydrogen plants with high shares of intermittent power supply. The whole value chain from sustainable power generation to the production of liquid energy carriers will be described. An overview of the research activities of the Fraunhofer institute for Solar Energy Systems (ISE) for the realization of an integrated and sustainable energy system will be given as well. Figure 1

A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell.

The isothermal operation of single PEM fuel cells below 0 °C is investigated by potentiostatic experiments and analyzed by physical modeling. Current decay is observed during potentiostatic operation, which is caused by the freezing of product water in the porous cathode layers. This paper describes the implementation of the ohmic losses and changes of the active cathode area as a function of the charge transfer density in a physical PEM fuel cell model. The model is comprised of a set of differential equations accounting for the electrochemical reaction and diffusion processes in the cathode of the fuel cell. The model describes the time-resolved current decay during freezing. Thus a better understanding of freezing processes at sub-zero conditions in PEM fuel cells is gained and mechanisms proposed from earlier experimental data and statistical based interpretations are confirmed. Furthermore, the transient water uptake behavior of the membrane is studied in situ at different temperatures.

A hydrogen fuelled, 30 W proton exchange membrane fuel-cell (PEMFC) system is presented that is able to operate at an ambient temperature between −20 and 40 °C. The system, which comprises the fuel-cell stack, pumps, humidifier, valves and blowers is fully characterized in a climatic chamber under various ambient temperatures. Successful cold start-up and stable operation at −20 °C are reported as well as the system behaviour during long-term at 40 °C. A simple thermal model of the stack is developed and validated, and accounts for heat losses by radiation and convection. Condensation of steam is addressed as well as reaction gas depletion. The stack is regarded as a uniform heat source. The electrochemical reaction is not resolved. General design rules for the cold start-up of a portable fuel-cell stack are deduced by the thermal model and are taken into consideration for the design. The model is used for a comparison between active-assisted cold start-up procedures with a passive cold start-up from temperatures below 0 °C. It is found that a passive cold start-up may not be the most efficient strategy. Additionally, the influence of different stack concepts on the start-up behaviour is analysed by the thermal model. Three power classes of PEMFC stacks are compared: a Ballard Mk902 module for automotive applications with 85 kW, the forerunner stack Ballard Mk5 (5 kW) for medium power applications, and the developed OutdoorFC stack (30 W), for portable applications.

Air-breathing PEMFCs consist of an open cathodic side to allow an entirely passive supply of oxygen by diffusion. Furthermore, a large fraction of the produced water is removed by evaporation from the open cathode. Gas diffusion layers (GDLs) and the opening size of the cathode have a crucial influence on the performance of an air-breathing PEMFC. In order to assure an unobstructed supply of oxygen the water has to be removed efficiently and condensation in the GDL has to be avoided. On the other hand good humidification of the membrane has to be achieved to obtain high protonic conductivity. In this paper the influence of varying cathodic opening sizes (33%, 50% and 80% opening ratios) and of GDLs with different wetting properties are analysed. GDLs with hydrophobic and hydrophilic properties are prepared by coating of untreated GDLs (Toray ® carbon paper TGP-H-120, thickness of 350 μm). The air-breathing PEMFC test samples are realised using printed circuit board (PCB) technology. The cell samples were characterised over the entire potential range (0–0.95 V) by extensive measurements of the current density, the temperature and the cell impedance at 1 kHz. Additionally, measurements of the water balance were carried out at distinct operation points. The best cell performance was achieved with the largest opening ratio (80%) and an untreated GDL. At the maximum power point, this cell sample achieved a power density of 100 mW cm −2 at a moderate cell temperature of 43 °C. Furthermore, it could be shown that GDLs with hydrophilic or intense hydrophobic properties do not improve the performance of an air-breathing PEMFC. Based on the extensive characterisations, two design rules for air-breathing PEMFCs could be formulated. Firstly, it is crucial to maximise the cathode opening as far as an appropriate compression pressure of the cell assembly and therewith low contact resistance can be assured. Secondly, it is advantageous to use an untreated, slightly hydrophobic GDL.

A measurement set-up is presented containing a multichannel frequency response analyser system that allows the measurements of impedance spectra of single cells in a fuel cell stack simultaneously. As a proof of reliability, the sum of the impedances of the single cells is compared to the measured impedance of the whole stack. A good correlation was found. The absolute deviation is less than 2.5%. A measurement of the stack during low load operation is presented. Differences in the individual cell voltages are investigated. A flooding event in a single cell is observed by means of the impedance measurement minutes before the polarisation shows a voltage drop.

To date, the available equipment for characterising the microstructure of membrane electrode assembly (MEA) is still not well developed. For example, applying the normal scanning electron microscope (SEM) only provides information on the dry structure of MEAs. This paper presents a microstructure analysis method of MEAs in proton exchange membrane fuel cells (PEMFC). The microstructure analysis in this paper utilises the environmental scanning electron microscope (ESEM), which shows its advantage on the sample microstructure analysis in wet mode. When water is present, the characteristics of the MEA, especially the hydrophobic and/or hydrophilic properties, are distinguishable on the ESEM images. With proper temperature and pressure control, the water distribution within both the membrane and the catalyst layer can be viewed by ESEM. Based on ESEM measurement and mercury porosity measurement, the distributions of hydrophobic and hydrophilic pores in MEA have been analyzed. By means of ESEM and energy dispersive X-ray (EDX), a degraded MEA is characterized. The microstructure change of the degraded MEA has been discussed. The results provide helpful information for the understanding of MEAs in PEMFC.

A measurement set-up is presented containing a multichannel frequency response analyser system together with a multichannel potentiostat that allows measurements of the local electrical impedance spectra of segmented fuel cells simultaneously. For evaluation the current and impedance distribution is combined with optical spatially resolved measurement and numerical simulation. Furthermore the multichannel frequency response analyser system is used for the measurement of impedance spectra of single cells in a fuel cell stack simultaneously.

The flow-field design can have great influence on the operating performance of both proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs). Inhomogeneous transport of reactants to and products from the active area of these low-temperature fuel cells result in loss of power. Newly designed fractal structures are tested as flow-fields in PEMFCs and DMFCs for portable applications. To achieve a uniform fluid distribution and simultaneously minimize energy demand for mass transport (pressure loss), a computer algorithm is developed to provide a given area with a multiple ramified fluid network. By virtue of the self-similarity, the structures of such a network are called fractals. These are investigated and compared with common serpentine and parallel flow-fields. For both PEMFCs and DMFCs fractal flow-fields show similar performance to parallel designs. The most stable and highest power output is reached with the serpentine flow-field.

A fuel cell has been designed that combines three methods of spatially resolved measurements. The experimental setup allows to perform a simultaneous evaluation of current, temperature and water distribution in a polymer electrolyte fuel cell (PEFC) under operation. The test cell has a segmented anode flow field for current distribution measurement. The back plate of the cathode flow field is made of an optical window, which is transparent for infrared (IR) as well as for visible wavelengths. This allows infrared thermography and optical surveillance of water droplets and flow field flooding.

The effect of the diffusion layer on the performance and mass transport in a direct methanol fuel cell at ambient conditions is reported. Carbon cloths with variable Teflon contents and pore sizes, carbon paper and a metal wire cloth were investigated. Membrane-electrode-assemblies (MEAs) for direct methanol fuel cells (DMFCs) prepared after an in-house receipt are used, giving reproducible results after a pre-treatment involving polarisation with hydrogen and air. Long-term effects and methanol crossover were also briefly investigated. Adding Teflon to the diffusion layer leads to better gas transport, as gas and liquid transport takes place in different paths. Thus, the fuel cell power output is more stable. The same effect was seen with increasing pore size. Carbon paper is found not suitable as a diffusion layer for low temperature DMFC. The metal wire cloth yielded best performance giving 15.8 mW cm−2.

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 United Nations Framework Convention on Climate Change (COP21) in Paris last year has underlined the urgency of a fast and consequent transformation of the various energy systems from a fossil based energy feedstock towards renewable energies like photovoltaics, wind- and hydropower, in order to drastically decrease the greenhouse gas emissions (GHG). Such a transformation requires a thorough restructuring of the energy systems with all energy carriers and consumer sectors including its intersectoral dependencies, grid structures and energy storage needs. At Fraunhofer ISE we performed time-resolved studies on how to reach the main goal of the German energy transformation to decrease its GHG emissions by at least 80% until 2050 and thus, to widely decarbonize all energy related sectors. Potential transformation pathways based on various scenarios were studied and the required costs were modelled for each scenario. The results of this modelling made very obvious that the ambitious GHG reduction targets can only be fulfilled by a strong rise in electricity generation by fluctuating renewable energies and on the other hand by the installation of large plants for producing synthetic energy carriers from renewable energies. The most versatile energy carrier for such large amounts of energy is hydrogen produced from water electrolysis. Hydrogen can be stored in large amounts in gas tube fields or salt caverns and can be used directly either in fuel cell cars and buses as a fuel or in stationary combined heat and power fuel cell systems. Furthermore the hydrogen can be used in order to hydrogenate CO2 and thus to generate green liquid fuels such as oxymehylenether (OME) or other chemicals in order to replace oil based products. The conversion of electricity into hydrogen is called Power-to-Gas, the further processing of the hydrogen and CO2 towards liquid energy carriers and chemicals is called Power-to-Liquid. With the advent of various Power-to-Gas demonstration projects it becomes clear how important this technology will be in the future energy system. Distributed water electrolyzers in the Megawatt power range will stabilize the grid frequency and voltage or will deliver other ancillary services and thus, will contribute to the secondary balancing market. Alkaline water electrolyzers are applied for more than 100 years in industrial applications and are nowadays complemented by proton exchange membrane (PEM) electrolysers in order to cover the needs for a highly dynamic load in the grid. However, main challenges of the PEM technology are high costs associated with expensive materials and the durability of cells and stacks. With ongoing technological development it is expected that PEM electrolysis systems become a competitive alternative to alkaline systems in the next years. In this talk, the technological base of water electrolyzers will be described and the related performance data, life-time expectation and cost reduction potential will be discussed. Furthermore the impact of distributed water electrolyzers in the Megawatt power range as part of an increasingly renewable energy system in terms of Power-to-Gas modelling scenarios will be given. Figure 1

This paper presents a model-based analysis of a proton exchange membrane fuel cell (PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self-breathing, operation. This planar fuel cell is fabricated using printed circuit board (PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components (O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB (COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water.

The characterisation of a proton exchange membrane (PEM) fuel cell with a straight channel flow field design is performed. Spatially resolved current distribution measurements at different air flow rates are compared to numerical simulation results. The numerical model is validated by agreement of measured and simulated current distribution. The test cell is segmented. It is operated at steady state conditions and the gas flow rates and cell temperature are controlled. The numerical simulation is realised with a PEM fuel cell model based on the FLUENT T M computational fluid dynamics (CFD) software. It accounts for mass transport in the gaseous phase, heat transfer, electrical potential field and the electrochemical reaction. It provides three-dimensional distributions of, for example, current densities, reactant concentrations and temperature.

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