Engineering topics
Publications and source records attributed to Nicollet, Clement.
The ability to recover the oxygen reduction reaction of poisoned metal oxide surfaces, central to many energy related applications, is demonstrated by controlling relative surface acidity.
Improving the kinetics of O 2 reduction on oxide surfaces is critical in many energy and fuel conversion technologies. Here we show that the acidity scale for binary oxides is a powerful descriptor for tuning and predicting oxygen surface exchange kinetics on mixed conducting oxides. By infiltrating a selection of binary oxides from strongly basic (Li 2 O) to strongly acidic (SiO 2 ) onto the surface of Pr 0.1 Ce 0.9 O 2-δ samples, it was possible to vary the chemical surface exchange coefficient k chem by 6 orders of magnitude, with basic oxides such as Li 2 O increasing k chem by nearly 1,000 times, with surface concentrations as low as 50 ppm impacting k chem . Strikingly, although the pre-exponential of k chem scales linearly with the acidity of the infiltrated binary oxide, there is nearly no change in the activation energy. Here, the origin of these dramatic changes is proposed to arise from the systematic increase in electron concentration at the Pr 0.1 Ce 0.9 O 2-δ surface with the decreasing acidity of the infiltrated binary oxide.
In this project we explored the possibility of mitigating silicon poisoning of Pr doped ceria (PCO) electrodes for oxygen reduction in SOFC. The approach was to dope PCO with various amount of lanthanum and study the new compositions in terms of their mixed conduction properties, their oxygen surface exchange kinetics, as well as their resistance to silicon poisoning. The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of La x (Ce 0.9 Pr 0.1 ) 1-x O 2-δ (x=0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted La x Ce 1-x O 2-δ , with a maximum for a La content of x = 0.1, reaching σ = 9.5×10 -3 S·cm -1 at 600 °C and a decrease in activation energy from ≈ 1 eV for Ce 0.9 Pr 0.1 O 2-δ down to ≈ 0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity.
Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). Impurities that block active reaction sites on the electrode surface can markedly degrade the oxygen exchange rate, and lead to a reduction in long term durability of such devices. In this work, we propose to develop electrodes that are tolerant to Si which is one of the most prevalent cathode impurities. This work draws on our recent method to fully recover oxygen exchange kinetics following Si induced aging of ceria containing cathodes. We take advantage of elements, which over time during SOFC operation, are released by exsolution to actively trap (i.e. getter) Si impurities in a self-regulating chemical fashion. The objective is to evaluate impurity scavenging by added reactive elements that are intentionally exsolved from the SOFC cathode electrode during operation. In this study, we chose to focus our effort on one cathode material: Pr doped ceria (PCO). This material has the advantage of not containing strontium, an element known to segregate to the surface of the well-known perovkite based cathodes materials, thereby degrading their performance. As for the Si getter, we choose to focus on lanthanum, which is added in PCO in solid solution.