Mechanical and Electrical Properties of Ceramic Electrodes for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) operate efficiently on both hydrogen and hydrocarbon fuels.  Their fuel flexibility makes them lucrative energy conversion devices; they can integrate immediately into the current hydrocarbon infrastructure and can utilize renewable fuels in the future once that infrastructure exists. Carbon deposition, or coking, due to the CO disproportionation reaction (Eq. 1) or CH₄cracking (Eq. 2) can occur when using hydrocarbon fuels, which blocks the anode gas channels.

2CO -> CO₂+ C(s)     (1)

CH₄ -> 2H₂+ C(s)     (2)

Typical nickel-based cermets used for anode-supported cells exacerbate coking problems because Ni surfaces catalyze solid carbon deposition.  Furthermore the ~60% expansion of Ni upon reoxidation complicates its use as supporting material.  Many researchers have suggested and demonstrated conductive ceramic anodes as supports for SOFCs to avoid the coking and mechanical problems of conventional cermet anode supports.  The anodes must be >30% porous to allow for effective gas transfer to the electrochemically active triple phase boundary (TPB).  Unfortunately, the properties of porous materials for this purpose are not as well characterized as the dense materials used for electrolytes or cermets for supporting anodes.  To employ these materials in anode-supported cells, it is imperative to better understand the relationship between porosity and the electronic and mechanical properties of anode materials.  General relationships between the elastic modulus and flexural strength of ceramics are described by Eq. 3 and 4, respectively, where n is an empirical fitting parameter and P is porosity.

E=E_o(1-1.9P+0.9P²)  (3)

σ_fs = σ_oe^-nP(4)

Original work in preparing and characterizing porous ceramics of gadolinia doped ceria (GDC), yttria-stabilized zirconia (YSZ), and niobium-doped strontium titanate (SNT) for electrodes is presented.  The empirical parameters that relate flexural strength of the materials to their porosity are determined as shown in Fig. 1 for GDC10 and YSZ3.  The effect of pore former on pore geometry and strength is discussed.  Finally, the electrical properties of the anodes are measured, both in their native state and with the addition of conductive and catalytic materials to the scaffolds.