Page 172 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Chapter 6

Anodes

Augustin McEvoy

6.1 Introduction

Like the cathode, the anode must combine catalytic activity for fuel oxidation
with electrical conductivity, Catalytic properties of the anode are necessary for
the kinetics of the fuel oxidation with the oxide ions coming through the solid
electrolyte. Ionic conductivity allows the anode to spread the oxide ions across a
broader region of anode/electrolyte interface, and electronic conductivity is
necessary to convey the electrons resulting from the electrode reaction out into
the external circuit.
Early in the twentieth century, many candidate anode materials were tested,
including precious metals like platinum and gold, and transition metals such as
iron and nickel, as described in Chapter 2. But platinum does not last long in an
operating solid oxide fuel cell (SOFC), peeling off after a few hours, and nickel
aggregates at high temperatures inhibiting access of the fuel. Spacil [l] first
recognised that the nickel aggregation problem could be solved by mixing
yttria-stabilised zirconia (YSZ) electrolyte particles in with the nickel matrix to
form a composite anode. Such nickel cermet anodes can provide adequate
performance under certain conditions but do exhibit problems such as carbon
fouling from carbonaceous fuels. However, nickel cermet is the material which
has been most successful in SOFC development until now and so is emphasised
in this chapter.
This chapter first considers the complex mix of attributes required of SOFC
anodes, including matching of thermal expansion coefficients, chemical
compatibility with the electrolyte and the interconnect, porous structure to
allow gas permeation, and corrosion resistance to the fuel and impurities
therein. Then the nickel cermet anode is described in detail, especially its
fabrication processes. Steady-state anode reactions of hydrogen and carbon
monoxide are analysed, followed by a description of transient effects. Finally,
behaviour under current load and operation on different fuels are discussed. The
details of the anode reactions and polarisations are described in Chapter 9.