Soren Primdahl

Ph.D. thesis

Summary

This thesis deals with the porous Ni/yttria-stabilized zirconia (YSZ) cermet anode on a YSZ electrolyte for solid oxide fuel cells (SOFC). Such anodes are predominantly operated in moist hydrogen at 700°C to 1000°C, and the most important technological parameters are the polarization resistance and the long-term stability.

The polarization resistance can be measured by a number of techniques, in the present work impedance spectroscopy has been used extensively. By impedance spectroscopy limiting processes in the anode polarization resistance may often be separated and characterized individually, provided they have a reasonable separation in time constants.

Three limiting processes are recognized in impedance spectra obtained on technological Ni/YSZ cermet anodes characterized against a stable reference electrode atmosphere. By parameter studies and illustrative experiments, the two contributions at low and medium frequency have been identified as gas conversion and diffusion limitations, respectively. Both of these effects are concentration limitations relating to the inefficient exchange of fuel gas in the test setup outside the porous cermet. A test setup geometry where these concentration effects are avoided for high-performance electrodes is recommended.

The high frequency limitation is demonstrated to relate to the cermet structure. The dependence on gas composition, temperature, adsorbed species (sulfur), isotopes (H/D), sintering temperature and cermet thickness is investigated. Despite these studies and several similar studies by others, the exact chemical or physical nature of the limiting step has not been incontestably identified. However, there is a general consensus in literature about the hydrogen oxidation process taking place on or near to the triple phase boundary (TPB) line, where open gas-filled pores, the continuous electrolyte phase (oxide ion conductor) and the continuous Ni phase (electronic conductor) meet.

The physical thickness of a cermet anode that contains active TPB sites contributing to the current density of an SOFC is demonstrated to he about 10 µm. This result has technological importance, as any overlying structure serves primarily as a current collector and needs not be optimized for transport of oxide ions.

The anode durability is investigated over 1300 to 1900 hrs at 300 mA/cm² and shown to be stable at 1050°C. At 1000°C a degradation rate of about 10 mcm²/1000 hrs is observed, and at 850°C a degradation rate of about 100 mcm² /1000 hrs is found. The degradation during thermal cycling from 1000°C to below 100°C is demonstrated to be negligible after a 30 mcm² loss of performance over the first 2 cycles.

By adding Mn to the Ni/YSZ cermet anode the polarization resistance is reduced by a factor of 2, yielding down to 60 and 30 mcm² in hydrogen with 3% water at 850°C and 1000°C, respectively.