Feb
28
New Understanding for Oxygen Fuel Cells and Zirconia
February 28, 2013 | Leave a Comment
K. L. Ngai, from the University of Pisa in Italy, and colleagues from the Complutense University in Madrid, Spain, devised a model of the oxygen-ion dynamics that contribute to the conductivity of Yttria Stabilized Zirconia (YSZ). This is significant because of the movement of oxygen within a fuel cell and for devices we are already using made by mass production such as the oxygen sensors in your car.
YSZ is a material of great interest because of its relatively high oxygen-ion based conductivity. The Italian and Spanish collaboration’s work on the high-conductivity material demonstrates the role of oxygen ions in enhancing device capability. The team’s study has been published in The European Physical Journal B.
Fuel cells are of particular interest due to the problems of their operation. Commercial models operate above 700 ºC (1,292ºF), which is a major limitation for their use. Understanding oxygen-ion diffusion is key to helping lower operating temperatures down to room temperature.
Previous attempts to do so were done with the so-called coupling model (CM), describing simple physical concepts related to ion-ion interaction. This helped uncover the importance of ion-ion correlation in limiting long-range ion mobility, and thus conductivity.
The trouble is that experiments show that ionic conductivity in YSZ requires an activation energy that is much higher than that supplied by computer simulations describing independent ion hopping. Relying on the CM model, the authors first established a quantitative description of the ion dynamics in YSZ. Then they compared the predictions published in the last ten years of the CM with experimental results and with simulations, particularly those of nanometric-scale thin films.
Thus, in their model, the team established the connection between the level of the energy barrier for independent ion-hopping simulations and the level of activation energy measured experimentally for long-range movement of oxygen ions. In addition, they attributed an increase of the conductivity in nanometers-thick YSZ films to a decrease in the ion-ion correlations. This model could also be used to study the conductivity relaxation of so-called molten, glassy and crystalline ionic conductors and ambient temperature ionic liquids.
Thus might all seem esoteric, but the motion of oxygen over YSZ has been crucial for automotive emissions controls for nearly 40 years. Estimating what automotive pollution controls over hundreds of million of vehicles during four decades without the seemingly mundane oxygen sensor would have cost would be a large and quite complex problem with a huge a result.
YSZ applied to oxygen a technology of significant importance today and should be even more so in the future.
Getting the YSZ properties to work in fuel cells at commercially viable rates hasn’t been successful just yet. The Italian and Spanish team has gotten us closer to figuring out how. Whether or not fuel cells can work at room temperature may be an optimist’s goal, but coming down from 700ºC or 1,292ºF even by half would be a huge improvement.
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