Materials Engineering and Fuel Cell Development
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Introduction The direct conversion of chemical energy in a fuel cell has been one of the most challenging technological problems since a simple hydrogen/oxygen fuel cell w a s first d e m o n s t r a t e d 150 years ago by Groves at the London Institution. A q u e o u s electrolyte fuel cells have been successfully used in space to power electrical equipment in the Apollo spacecraft and in the space shuttle, but terrestrial applications have been principally limited to a few specialized military and telecommunication applications. In the original concept of a fuel cell, a primary fuel (e.g., hydrogen, hydrocar-. bon) is reacted with oxygen in an electrochemical cell incorporating an acid electrolyte, namely: CH 4 + 2 H 2 0 -* C 0 2 + 8H + + 8e" (anode) 2 0 2 + 8H
+
+ 8e" - * 4 H 2 0 (cathode)
so as to convert the free energy of oxidation (AG0) of the fuel to carbon dioxide and water directly into electrical energy. Fuel cells are attractive in that theoretical thermodynamic energy/fuel conversion ratios given by the appropriate value of AG°/AH° (free energy change/ enthalpy change) can be very high (e.g., - 9 5 % for H 2 /0 2 reaction at 298 K) compared to the limitations imposed on heat engines by the Carnot cycle efficiency, (T2—Ti)/T2. However, in practice it has proved difficult to realize the high electrical/fuel conversion efficiencies promised by fuel cell systems. This is caused by problems associated with the
MRS BULLETIN/JUNE 1989
electrochemical kinetics a n d / o r t h e requirement that in the Faradaic reaction both mass and electron transfer m u s t occur in a spatially restricted region where an electrode, an electrolyte, and one of the reacting phases (usually a gas) are in intimate contact. This region is often referred to as the three-phase boundary. Various types of fuel cells have common features,1"3 which are illustrated in Figure 1. The characteristics of the interfacial t h r e e - p h a s e b o u n d a r y play a major role in the electrochemical performance of a fuel cell, particularly in those s y s t e m s i n c o r p o r a t i n g liquid electrolytes. A delicate balance m u s t be maintained within the porous electrode structure, and materials technology has contributed significantly to improvements in the performance and longterm stability of these components. A fuel cell stack usually consists of individual cells connected in electrical series. While a number of different stack configurations can be designed, in practice most fuel stack s y s t e m s u n d e r development have a planar arrangement, shown schematically in Figure 2. The individual cells in Figure 2 are connected by a ribbed bipolar plate that provides a low electronic resistance between adjacent p o r o u s a n o d e and cathode structures and also functions as a gas barrier between the fuel and oxidants streams in adjacent cells. The bipolar plate must therefore be a good electronic conductor and impermeable to gases. The ribbed channels provide more uniform gas distribution and help preserve the mechanical integri
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