Nonmetallic Inorganic Materials

Anodes for Solid Oxide Fuel Cells

Contact: Michael Jörger

The main function of the SOFC anode is to provide reaction sites for the electrochemical oxidation of the fuel. The mostly used fuel is hydrogen that is generated externally by steam reforming of hydrocarbons. Other requirements for the anode are electronic conductivity, compatibility with the adjacent components and stability under operating and processing conditions. Today a ceramic-metal composite (cermet) consisting of nickel and yttria-stabilized zirconia meets the requirements the best.

One possibility to reduce the high SOFC system costs is the avoidance of external reforming. Due to the high operating temperature direct operation with hydrocarbons is possible.

Partial oxidation: CH4 + O2- ==> CO + 2 H2 + 2 e-

Full Oxidation: CH4 + 4 O2- ==> CO + 2 H2O + 8 e-

However, with standard Ni-containing anodes this is problematic since carbon deposition occurs easily. Nickel is a very good dehydrogenation catalyst, which decomposes hydrocarbons into carbon and hydrogen:

CxH2x+2 ==> x C + (x+1) H2

As a consequence carbon deposits may deteriorate the anode microstructure and the electrochemical performance.

The goal of this project is to develop an anode that can be operated directly with methane or propane without coking.

A promising alternative material composition might be a Cu-ceria cermet. Copper provides a high electronical conductivity and is stable under operating conditions. It has no catalytic activity for the decomposition of hydrocarbons.
Ceria or doped ceria solid solutions (e.g. cerium-gadolinium-oxide CGO) function as framework material to hinder the copper from coarsening, provide mixed ionic-electronic conductivity and have some catalytic activity for hydrocarbon oxidation.

Fabrication of Cu-containing anodes requires a processing temperature below 1100°C to avoid the melting of CuO. Two fabrication strategies are investigated.
One is the direct processing via powder mixing, screen-printing and sintering. Proper mixing of nanosized powder enable a processing via screen-printing and sintering at 1050°C without massive CuO coarsening [Figure 1]. Homogeneous microstructures with 50vol% copper and a stable conductivity of more than 400 Scm-1 could be produced.

The second path starts with the fabrication of a highly porous CGO framework and the anode is completed by infiltration of a metal ion solution. The advantage for this approach is the unlimited sintering temperature for the ceramic. But it is difficult to generate a percolating metal network throughout the anode [Figure 2].


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