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This paper presents an experimental study dealing with operation and degradation during an electrical short circuit of a proton exchange membrane fuel cell stack. The physical quantities in the fuel cell (electrical voltage and current, gas stoichiometry, pressures, temperatures and gas humidity) are studied before, during and after the failure. After a short circuit occurs, a high peak of current appears but decreases to stabilize in a much lower value. The voltage drops in all the cells and even some cells presents reversal potentials. The degradation is quantified by using electrochemical impedance spectroscopy.

The NASA Glenn Research Center is developing both a novel cell design (BSC) and a novel ceramic fabrication technique to produce fuel cells predicted to exceed a specific power density of 1.0 kW/kg. The NASA Glenn cell design has taken a completely different approach among planar designs by removing the metal interconnect and returning to the use of a thin, doped LaCrO3 interconnect. The cell is structurally symmetrical. Both electrodes support the thin electrolyte and contain micro-channels for gas flow-- a geometry referred to as a bi-electrode supported cell or BSC. The cell characteristics have been demonstrated under both SOFC and SOE conditions. Electrolysis tests verify that this cell design operates at very high electrochemical voltage efficiencies (EVE) and high H2O conversion percentages, even at the low flow rates predicted for closed loop systems encountered in unmanned aerial vehicle (UAV) applications. For UAVs the volume, weight and the efficiency are critical as they determine the size of the water tank, the solar panel size, and other system requirements. For UAVs, regenerative solid oxide fuel cell stacks (RSOFC) use solar panels during daylight to generate power for electrolysis and then operate in fuel cell mode during the night to power the UAV and electronics. Recent studies, performed by NASA for a more electric commercial aircraft, evaluated SOFCs for auxiliary power units (APUs). System studies were also conducted for regenerative RSOFC systems. One common requirement for aerospace SOFCs and RSOFCs, determined independently in each application study, was the need for high specific power density and volume density, on the order of 1.0 kW/kg and greater than 1.0 kW/L. Until recently the best reported performance for SOFCs was 0.2 kW/kg or less for stacks. NASA Glenn is working to prototype the light weight, low volume BSC design for such high specific power aerospace applications.

A novel micro planar fuel cell power supplier, in which a six-cell PEM unitised regenerative fuel cell (URFC) stack is used as the power generator, was designed and fabricated. Six membrane electrode assemblies were prepared and integrated on one piece of membrane by spraying catalyst slurry on both sides of the membrane. Each cell was made by sandwiching a membrane electrode assembly (MEA) between two graphite monopolar plates and six cell units were mechanically fixed in two organic glass endplates. When the stack was operated in an electrolysis mode, hydrogen was generated from the splitting of water and stored using a hydrogen storage alloy; conversely, when the stack was operated in fuel cell mode, hydrogen was supplied by the hydrogen storage alloy and oxygen was supplied from air by self-breathing of the cathode. At room temperature and standard atmospheric pressure, the open-circuit voltage (OCV) of the system reached 4.9 V, the system could be discharged at a constant current density of 20 mA cm–2 for about 40 min, and the work voltage was ∼2.9 V. The system showed good stability for 10 charge–discharge cycles.

Electrolysis of water produces high-purity hydrogen directly, with no need for additional clean-up process. Efficiency and cost of electrolysis are favorable with high-temperature steam electrolyzers where a portion of the required energy can be supplied as thermal energy. With the similarity in requirements, much of the materials and designs aspects for the high-temperature electrolyzers have been derived from solid oxide fuel cell development. Lanthanum gallate electrolyte-based cells were evaluated in fuel cell and electrolysis modes of operation. A modified nickel anode is shown to lower the reactivity with the gallate.

The mixed-reactant fuel cell (MRFC) is a new concept, in which a mixture of aqueous fuel and gaseous oxygen (or air) flows directly through a porous anode-electrolyte-cathode structure or through a strip-cell with an anode-electrolyte-cathode configuration. These structures can be single cells or parallel stacks of cells and may be in a planar, tubular or any other geometry. Selectivity in the electrocatalysts for MRFCs is mandatory to minimize mixed-potential at the electrodes, which otherwise would reduce the available cell voltage and compromise the fuel efficiency. MRFC offers a cost effective solution in fuel cell design, since there is no need for gas-tight structure within the stack and, as a consequence, considerable reduction in sealing, manifolding and reactants delivery structure is possible. In recent years, significant advances have been made in MRFCs, using methanol as a fuel. This paper reviews the status of mixed reactant fuel cells and reports some recent experimental data for methanol fuel cell systems.

Viessmann is developing a PEM fuel cell system for residential applications. The uncharged PEM fuel cell system has a 2 kW electrical and 3 kW thermal power output. The Viessmann Fuel Processor is characterized by a steam-reformer/burner combination in which the burner supplies the required heat to the steam reformer unit and the burner exhaust gas is used to heat water. Natural gas is used as fuel, which is fed into the reforming reactor after passing an integrated desulphurisation unit. The low temperature (600 °C) fuel processor is designed on the basis of steam reforming technology. For carbon monoxide removal, a single shift reactor and selective methanisation is used with noble metal catalysts on monoliths. In the shift reactor, carbon monoxide is converted into hydrogen by the water gas shift reaction. The low level of carbon monoxide at the outlet of the shift reactor is further reduced, to approximately 20 ppm, downstream in the methanisation reactor, to meet PEM fuel cell requirements. Since both catalysts work at the same temperature (240 °C), there is no requirement for an additional heat exchanger in the fuel processor. Start up time is less than 30 min. In addition, Viessmann has developed a 2 kW class PEFC stack, without humidification. Reformate and dry air are fed straight to the stack. Due to the dry operation, water produced by the cell reaction rapidly diffuses through the electrolyte membrane. This was achieved by optimising the MEA, the gas flow pattern and the operating conditions. The cathode is operated by an air blower.

Experimental results on the performance of a Ballard 5 kW proton exchange membrane fuel cell stack for different oxygen contents in the oxidant are presented. A description of the experimental setup is given. Polarization, power, and efficiency curves as a function of the current density, for different oxygen concentrations are presented. This detailed characterization of the fuel cell stack behavior is required in order to evaluate the effects of oxygen enrichment on the net power output of the stack. This investigation is done in the framework of a project on stand-alone power generation systems using renewable energy sources, and based on hydrogen production and storage. An electrolyzer, powered by the excess electrical energy from renewable energy sources, produces hydrogen. The stored hydrogen could then be used to feed an energy conversion device, such as a fuel cell stack, which acts as a secondary power source in periods of high demand. Therefore, a second objective is to evaluate the possibility of using the oxygen produced by the electrolyzer for the enrichment. Other oxygen enrichment techniques such as membrane gas separation and pressure swing adsorption are also discussed. Net available power and system efficiency are used as comparison factors.

The niche or early entry market penetration by ONSI and its phosphoric acid fuel cell technology has proven that fuel cells are reliable and suitable for premium power and other opportunity fuel niche market applications. Now, new fuel cell technologies ‐ solid oxide fuel cells, molten carbonate fuel cells, and polymer electrolyte fuel cells ‐ are being developed for near-term distributed generation shortly after 2003. Some of the evolving fuel cell systems are incorporating gas turbines in hybrid configurations. The combination of the gas turbine with the fuel cell promises to lower system costs and increase efficiency to enhance market penetration. Market estimates indicate that significant early entry markets exist to sustain the initially high cost of some distributed generation technologies. However, distributed generation technologies must have low introductory first cost, low installation cost, and high system reliability to be viable options in competitive commercial and industrial markets. In the long-term, solid state fuel cell technology with stack costs under $100/kilowatt (kW) promises deeper and wider market penetration in a range of applications including a residential, auxillary power, and the mature distributed generation markets. The Solid State Energy Conversion Alliance (SECA) with its vision for fuel cells in 2010 was recently formed to commercialize solid state fuel cells and realize the full potential of the fuel cell technology. Ultimately, the SECA concept could lead to megawatt-size fuel-cell systems for commercial and industrial applications and Vision 21 fuel cell turbine hybrid energy plants in 2015.

Fuel cells will make a valuable contribution to future power generation facilities. They improve the flexibility and increase the options for many applications, such as distributed power, vehicle propulsion, and portable devices. Their main property is the high electrical efficiency compared to other energy conversion devices. Both the low-temperature and the high-temperature fuel cells have their advantages and disadvantages depending on the application. Sometimes, they can both be implemented in similar applications. The modularity of fuel cells makes them quite flexible as the power needed can easily be attained by changing the number of modules. Twenty years ago, mainly universities and research institutions but only a few companies conducted fuel cell research working on the fundamentals of fuel cells. Nowadays due to the commercial interest innumerable research groups and companies have fuel cell activities ranging from the investigation of catalysts (both improving existing catalyst systems as searching for new catalysts), the development of novel membranes for PEMFCs and DMFCs, to the development of other components for fuel cells. Optimisation of flow field structures, backing layers, and other components of the single cells have intensified. (orig.)

The sections in this article are Introduction Principles of Fuel Cells Functions Setup Efficiency Classification of Fuel Cell Systems Low Temperature Fuel Cells Alkaline Fuel Cells (AFC) Proton Exchange Membrane Fuel Cells (PEMFCs) Phosphoric Acid Fuel Cells (PAFC) High-temperature Fuel Cells Molten Carbonate Fuel Cells (MCFCs) Solid Oxide Fuel Cells (SOFC)

A fuel cell is a primary voltaic cell that directly converts the chemical energy of a fuel into electricity. Not being a heat engine its efficiency is not subject to the Carnot limitation and in suitable circumstances very high efficiencies can be attained, although progress has been limited by a number of constraints. In this article low temperature fuel cells operating with acid and alkaline electrolytes are described as well as high temperature cells with both fused carbonate and solid oxide electrolytes. In all cases catalysis of the reactions at both fuel and oxygen (air) electrodes is crucial to satisfactory performance. Keywords: fuel cells; electrodes; fuels reformers; electrode structures; hydrogen; hydrocarbon; methanol