What is an intermediate temperature fuel cell

There are five major fuel cell electrolytes in prototype stage or use today. Each has a specific operating temperature range and associated benefits and shortcomings. High temperature fuel cells (MCFC and SOFC) have developed slowly because of inherent materials problems. In polymer electrolyte membrane fuel cells (PEMFCs), the operating temperature is limited to about 175 o F (115 o C) by the dehydration and chemical instability of the polymer electrolyte. At these low temperatures, a high loading of precious metal catalysts is needed. In addition, a bulky and expensive fuel processing unit is needed to remove traces of carbon monoxide (CO) from reformed hydrogen fuel. NuVant is developing a new technology that exploits the wide temperature gap between high and low temperature fuel cells. The table below illustrates NuVant's positioning among current fuel cell technologies:

Fuel Cell Electrolyte	Temperature	Applications	Development Stage	Comments
SOFC : Solid Oxide	1830° F	Stationary	Laboratory demonstrations	Hot! Start-up and materials problems
MCFC : Molten Carbonate	1110° F	Stationary and Marine applications	Field demonstrations	Very hot, slow start up
NuVant Composite	450°F-950°F	Stationary, Marine and Portable	Prototype under development	Simple, with full scalability
PAFC : Phosphoric Acid	410°F	Stationary	Approximately 200 units deployed worldwide	Low power density due to liquid electrolyte
AFC : Alkaline hydroxides	180 ° F	Auxiliary power and transportation	Military and space missions	Requires pure H 2 and pure O 2
PEMFC : Polymer Membranes	80°F - 175°F	Stationary and portable	Early prototypes being tested	Needs highly purified H 2

The temperature gap in fuel cells. One of the most important technological problems with fuel cells is the need to run at higher temperatures. NuVant Systems is developing inorganic composite membranes that will enable the operation of hydrogen and methanol fuel cells above 450°F (270 o C). At these higher temperatures, dramatic improvements in cost and performance could be achieved by using inexpensive, non-precious metal catalysts and directly reformed hydrogen fuel. Because system cost is the primary barrier to implementation of fuel cells in both premium and commodity power markets, intermediate temperature fuel cells have a strong potential to acquire an early market share and to emerge as the dominant fuel cell technology in a broad range of applications.

The hybrid membrane concept. The temperature gap exists because no single material has sufficiently high ionic conductivity and mechanical strength in the temperature range of interest. There are a number of inorganic materials (alkali hydrogen phosphates, silica-polyphosphate, alkaline earth niobates) that retain water at low relative humidity and have high protonic conductivity. However, these are brittle ceramic materials, so membranes made from them need to be thick in order to be strong. The hybrid membrane solves this problem by supporting a thin layer of the proton conducting ceramic on a strong, flexible, proton conducting metallic foil. By itself the foil does not make a useful electrolyte layer because it is electronically conducting. The foil/ceramic composite, however, functions as an electronically insulating proton conductor.

NuVant has applied for international patent protection in over 70 countries for this hybrid membrane concept, independent of the choice of the proton conducting metallic and ceramic components. This gives NuVant a strong competitive advantage in intermediate temperature fuel cells, even if other companies develop improved components. NuVant has already demonstrated the hybrid membrane in fuel cells running at 480-570 o F (250-300 o C) using a 75 micron thick silica-polyphosphate film on a metallic foil, and has achieved a current density of 30 mA/cm 2 at 400 mV cell voltage. NuVant has also demonstrated that the intermediate temperature fuel cell is insensitive to CO poisoning, and that methanol can be used as a fuel by introducing a reforming catalyst into the anode flow fields. A factor of 10 higher current density will be needed for commercial applications, but this should be achievable within 1-2 years with continued work on thinning and improvement of the composition of the ceramic layer.

Competitive advantages of intermediate temperature fuel cells. Fuel cells are expected to penetrate both premium (back-up, remote residential, and portable) and commodity (automotive and domestic) power markets increasingly over the coming decade. Premium power will likely be where initial fuel cell market injection occurs because the cost per kilowatt is not a figure of merit. Several fuel cell companies already sell uninterrupted power supplies (UPS) and are developing fuel cells for commercial portable power applications. The primary figures of merit for backup power are system cost, reliability, convenience, and running time. System cost, energy density and convenience are the key issues for portable power. System cost – particularly the cost of precious metal catalysts and fuel processing - is a more severe hurdle for the huge automotive power market. Nevertheless, the higher energy conversion efficiency of fuel cells has motivated an aggressive development program by automobile manufacturers. The U.S. Department of Energy (DOE) predicts significant penetration of the automotive energy market by 2010, and nearly complete displacement of the internal combustion engine in cars and light trucks by 2040.

A cost analysis of the intermediate temperature fuel cell system has been made on the basis of results of a DOE-funded assessment of a 50 kW fuel cell system for transportation. This system included a multi-fuel capable reformer, a PEM fuel cell, and balance of-plant components. The results of this model should only be considered in conjunction with the assumptions used in selecting and scaling the system components. The components have been scaled to production volumes of 500,000 units. Based on this assumption, the overall cost of intermediate temperature system is $281/kW.

The key advantages of the intermediate temperature fuel cell, as illustrated in the cost analysis, are dramatic reduction of the cost of membrane electrode assembly and fuel processing components. NuVant composite membranes will enable fuel cell manufacturers to deliver a 50-kW unit at half the size of a PEM fuel cell stack, with significantly less weight and higher efficiency. In lower power ranges (200 W to several kW), NuVant composite fuel cells also have substantial potential for portable and off-grid home power generating applications.

The streamlined fuel cell design resulting from the use of the NuVant composite membranes reduces not only the manufacturing cost but, more importantly, the operating and maintenance costs of the entire fuel cell system. Furthermore, the NuVant composite fuel cells are scalable, allowing for the generation of power from 200 W to 10 kW with internal methanol reforming, and up to megawatts with external reforming of hydrocarbon fuel.

DOE draft report, Hydrogen, Fuel Cells & Infrastructure Technologies Program Multi-Year Research, Development and Demonstration Plan, www.eere.energy.gov

Cost Analysis of Fuel Cell System for Transportation, Arthur D. Little, Inc., March 2000, Ref 49739, SFAA No. DE-SCO2-98EE50526. www.eere.energy.gov/

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