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3.1.3 Phosphoric Acid Fuel Cell
Concentrated phosphoric acid adsorbed in a ca. 40 μm thick SiC matrix at a temperature of about 180–220 °C is used in PAFCs. Cooling to room temperature of the
essentially water-free acid leads to crystallization of H 3 PO 4 , causing mechanical
stresses that lead to failure over a large number of cooling cycles. Therefore, PAFCs
have only been used for stationary applications. With a few shutdown per year,
PAFCs have demonstrated field lifetimes in excess of 40,000 h [2].
Carbon-supported Pt catalysts are used for both the cathode and anode, at loading levels of 0.3 mgPt/cm
2
[3]. In PAFCs, catalysts are bonded with PTFE to graphitized carbon fiber paper gas diffusion layers. Instead of phosphoric acid adsorbed in
a SiC matrix, a more recent variant of the PAFC technology utilizes membranes
made by incorporating phosphoric acid into a polybenzimidazole (PBI)-based
membrane. The operating temperature of PBI-based fuel cells is typically
150–180 °C.
PAFC systems have already been commercially available worldwide, over 400
PAFC power plants have been installed. The largest PAFC system in the world was
an 11 MW plant manufactured by the International Fuel Cells and Toshiba, which
was operated for >23,000 h.
3.1.4 Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) use metal oxide and ceramic compound such as zirconium oxides as electrolyte. Cell operating temperatures are about 900–1000 °C
and cell efficiency is about 60%. An SOFC essentially consists of two porous electrodes separated by an oxide ion-conducting electrolyte. In such a cell, oxygen is
supplied at the cathode and reacts with incoming electrons from the external circuit
to form oxide ions, which migrate to the anode (fuel electrode) through the oxide
ion-conducting electrolyte. At the anode, oxide ions combine with H 2 (and/or CO)
in the fuel to form H 2 O (and/or CO 2 ), liberating electrons. Electrons flow from the
anode through the external circuit to the cathode. Stability of materials at high temperatures is a challenge. In addition, they have to have suitable electrical conductivity. Reduction of the operating temperature by 200 °C would simplify many material
property requirements. These are the topics of the current research. Yttria-doped
zirconia (YSZ) is the most widely used material for the electrolyte in SOFCs
because of its many good properties. Ceria based materials are successfully being
used as electrolyte.
3 Electrochemical Energy Conversion in Fuel Cells
3.1.3 Phosphoric Acid Fuel Cell
Concentrated phosphoric acid adsorbed in a ca. 40 μm thick SiC matrix at a temperature of about 180–220 °C is used in PAFCs. Cooling to room temperature of the
essentially water-free acid leads to crystallization of H 3 PO 4 , causing mechanical
stresses that lead to failure over a large number of cooling cycles. Therefore, PAFCs
have only been used for stationary applications. With a few shutdown per year,
PAFCs have demonstrated field lifetimes in excess of 40,000 h [2].
Carbon-supported Pt catalysts are used for both the cathode and anode, at loading levels of 0.3 mgPt/cm
2
[3]. In PAFCs, catalysts are bonded with PTFE to graphitized carbon fiber paper gas diffusion layers. Instead of phosphoric acid adsorbed in
a SiC matrix, a more recent variant of the PAFC technology utilizes membranes
made by incorporating phosphoric acid into a polybenzimidazole (PBI)-based
membrane. The operating temperature of PBI-based fuel cells is typically
150–180 °C.
PAFC systems have already been commercially available worldwide, over 400
PAFC power plants have been installed. The largest PAFC system in the world was
an 11 MW plant manufactured by the International Fuel Cells and Toshiba, which
was operated for >23,000 h.
3.1.4 Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) use metal oxide and ceramic compound such as zirconium oxides as electrolyte. Cell operating temperatures are about 900–1000 °C
and cell efficiency is about 60%. An SOFC essentially consists of two porous electrodes separated by an oxide ion-conducting electrolyte. In such a cell, oxygen is
supplied at the cathode and reacts with incoming electrons from the external circuit
to form oxide ions, which migrate to the anode (fuel electrode) through the oxide
ion-conducting electrolyte. At the anode, oxide ions combine with H 2 (and/or CO)
in the fuel to form H 2 O (and/or CO 2 ), liberating electrons. Electrons flow from the
anode through the external circuit to the cathode. Stability of materials at high temperatures is a challenge. In addition, they have to have suitable electrical conductivity. Reduction of the operating temperature by 200 °C would simplify many material
property requirements. These are the topics of the current research. Yttria-doped
zirconia (YSZ) is the most widely used material for the electrolyte in SOFCs
because of its many good properties. Ceria based materials are successfully being
used as electrolyte.
3 Electrochemical Energy Conversion in Fuel Cells