No abstract

This study is to complement an early report (the manuscript is attached for review purpose) on the role of interlayer on activity and performance stability in praseodymium nickelates. The aforementioned report showed a remarkable 48% increase in power density while switching from common GDC interlayer to a new interlayer chemistry (PGCO). Furthermore, a stable long-term performance was linked with suppressed reaction between the cathode and PGCO interlayer. In this article, we report in situ studies of the phase evolution. The high energy XRD studies at a synchrotron source showed fully suppressed phase transition in praseodymium nickelates with PGCO interlayer, while the electrodes on the GDC interlayer undergo substantial phase transformation. Furthermore, in operando and post-test XRD analyses shown fully suppressed structural changes in electrodes operated in full cells at 750 • C and 0.80 V for 500 hours. SEM-EDS analysis showed that the formation of PrO x at the cathode-interlayer interface may play a role in a decrease of mechanical integrity of the interfaces, due to thermal expansion mismatch, leading to a local stress between the two phases. Consequently, phase evolution at a narrow interface may propagate toward the electrode bulk, leading to structural changes and performance degradation. The interest for Pr 2 NiO 4 (PNO) electrode stems from the necessity to develop active and stable oxygen electrodes 1-6 for solid oxide fuel cells (SOFCs).7-9 PNO is known for its highly active nature, 7,8,10 originated from its superior oxygen ion diffusion, surface exchange coeffocient 2,7,9-11 and structural flexibility over a wide temperature region (from 500 to 900• C). 3,12 PNO electrode, however, does undergo the structural evolution to form a higher order phase (Pr 3 Ni 2 O 7 ) and Pr 6 O 11 (PrO x ). 8 The structural change has been a major concern because it possibly links with the performance degradation over a long-term operation.7,8 Conventional X-ray diffraction (XRD) has been extensively used to investigate the structural evolution in nickelates in the form of powders or planar electrodes. 8,10 This method has two major limitations due to its low flux and low resolution: (1) it might overlook the presence of additional phases in the system, which is especially true for praseodymium nickelates where XRD diffraction patterns of higher order phase(s) (e.x. Pr 3 Ni 2 O 7 ) may overlap with the parent PNO phase, making quantification challenging; 8 and (2) the quantification of phase evolution in electrochemically operated PNO electrode may show major structural change with almost 100% of the parent phase transition from the conventional XRD analysis, while the transmission electron microscopy (TEM) studies clearly show the regions of preserved PNO phase. 7On the other hand, the high energy and high flux XRD (obtained at a synchrotron source) allows us to detect and resolve the presence of secondary phase(s), and in combination with high resolution capability provides the most accurate measurement regardin...

The purpose of this study is to investigate how CO 2 as a fluidizing agent affects the reactivity of a Cu−Mn mixed oxide as an oxygen carrier for chemical looping combustion (CLC) of CH 4 . The length of the reduction period of the oxygen carrier was found to be an important parameter while determining the effect of CO 2 . If the reduction period is short, which occurs in chemical looping combustion with oxygen uncoupling, no significant effect was observed. However, if the oxygen carrier is exposed for a longer reduction period (characteristic of CLC) to partially or fully reduce it to Cu and MnO phases, these reduced phases catalyze CH 4 dry reforming and reverse water−gas shift reactions. Consequently, a significant amount of CO and comparatively small amount of H 2 are produced. These side reactions should be avoided to achieve complete CH 4 combustion to CO 2 by adjusting the residence time of the oxygen carrier particles in the fuel reactor.

Exsolution is the process in which cations that make up the lattice of a mixed ionic/electronic conductor (MIEC) are reduced to metals and migrate to the surface of the host ceramic. These materials have become quite attractive for solid oxide fuel cells (SOFCs) because they can facilitate oxygen and electron transport through their bulk and act as electrocatalysts on the surface of SOFC anodes. Also, because of the nature of the exsolution process, the highly dispersed nano-catalysts have strong particle-host interactions which have shown resistance to common SOFC failure mechanisms such as agglomeration and coking. However, the practical application of such materials has not been well studied since most research focuses on the electrolyte-supported cells, which offer a quick method for screening electrochemical activity but the large ohmic resistance makes this cell unrealistic for real energy conversion devices, especially at lower temperatures. Here, we use a scalable ceramic processing technique to fabricate a ceramic-anode-supported SOFC that exsolves nano-catalysts for low temperature operation (600°C). SrFe0.2Ni0.4Mo0.4O3- δ (SFNM) is used as the anode material due to its high conductivity and exsolution ability. By shifting the mechanically supporting layer from the electrolyte to this anode, we are able to drastically reduce the overall ohmic resistance of the cell. Additionally, we avoid extra processing steps for conventional catalyst deposition by taking advantage of the in-situ catalyst exsolution. During the exsolution process, we show that our cell performance increases over time, up to a peak power density of 300 mW/cm2 at 600°C with H2 fuel. Further, we discuss two major challenges facing exsolution-anode-supported SOFCs along the way and we show our efforts to identify and address the root causes of these problems.

While ceramic anodes provide a path to overcome the obstacles of the current state of the art cermet-based solid oxide fuel cells (SOFCs), they are limited by processing challenges, low conductivity, poor catalytic activity, and instability. Here, we address these challenges by fabricating a ceramic-anode-supported SOFC composed of Sr2Fe0.4Ni0.8Mo0.8O6−δ (SFNM) which is highly conductive and spontaneously exsolves nano-catalysts for low temperature operation (600 °C). Using a simple and scalable procedure, we manufacture these SOFCs to have 60% lower ohmic loss than their electrolyte-supported analogues and we take advantage of the in-situ catalyst exsolution to circumvent additional catalyst deposition steps. We show that the catalyst exsolution triples the peak power density up to ∼200 mW cm−2 at 600 °C with H2 fuel, which is comparable to other ceramic anode SOFCs operating at considerably higher temperatures (>700 °C). Further, we discuss two major challenges facing exsolution-anode supported SOFCs, show our efforts to identify and address the root causes of such problems, and describe a potential path forward for next-generation anodes.