Understanding Nickel Nanoparticle Exsolution and Growth in Lcfcr-Based Perovskite Anodes Operating on Fully Reformed Natural Gas

Solid oxide fuel cells (SOFCs) are efficient combined heat and power devices that can flexibly operate on a variety of fuels, such as H 2 , CO, or hydrocarbons. Shifting to these alternative fuels as a primary energy source can contribute significantly to the decarbonization of the energy sector, as electrochemical oxidation of these fuels produces H 2 O and capture-ready CO 2 . Additionally, power production from hydrocarbons, for example, could serve as an effective approach to allow for continued development of the oil and gas sector, while still achieving significantly reduced emissions during energy production. The long-term stability of SOFCs is still a major challenge that is hindering widespread commercialization. The prominent reason for this is the high operating temperatures, which greatly accelerate degradation phenomena. Particle sintering, phase segregation, cation migration, and microstructural changes are some of the major challenges to overcome when it comes to electrode design. 1,2,3 Traditional anode materials currently employed in SOFC systems are composite cermets, such as Ni- yttria-stabilized zirconia or Ni- gadolinium-doped ceria. 4,5 These materials are well known for suffering deactivation due to the aforementioned causes of degradation, including morphology change, carbon deposition in hydrocarbon fuels and sulfur poisoning. 6 Therefore, perovskite systems, such as La-based La x A 1-x BO 3-δ (A = Sr, Zr, Y; B = Cr, Fe, Ni, Co, Mn), have emerged as next generation anode materials due to their high redox stability at intermediate and high operating temperatures (500-800 o C). 7,8,9,10 Our group has been developing and modifying a perovskite oxide (ABO 3 ) catalyst with the specific formula of La 0.3 Ca 0.7 Fe 0.7 Cr 0.3-x M x O 3-δ (M = Ni, Co, Cu). Investigations of this material and its analogue, La 0.3 Sr 0.7 Fe 0.7 Cr 0.3 O 3-δ , have established a stability window in mixtures of CO 2 -CO, showing that the perovskite phase remains intact under a pO 2 window of 0.21-10 -21 atm. 11 Consistent with this, these catalysts have displayed high activity towards many reactions, such as H 2 oxidation, steam reduction, oxygen reduction, oxygen evolution, CO oxidation, and CO 2 reduction. 11,12,13,14 Further enhancement in conversion kinetics has been achieved by decorating the catalyst surface with transition metal nanoparticles (NPs) though exsolution. For example, we have demonstrated that exsolved Ni NPs can enhance the kinetics of CO 2 reduction at 800 o C by ~15% and CO oxidation by ~75%. In addition, these NPs display no carbon formation in 90:10 CO:CO 2 for 25 hours at 800 o C. 15 Here, we have expanded on prior work with La 0.3 Ca 0.7 Fe 0.7 Cr 0.3-x Ni x O 3-δ (LCFCrNi) and its parent materials by studying the long-term performance and durability in a simulated 100% fully reformed gas mixture of 1:2 CH 4 :H 2 O, which would generate 4:1 H 2 :CO 2 upon complete steam methane reforming. Material phase, morphology, and structure changes as a function of long-term constant current experiments are also being explored. LCFCr and its doped analogues were prepared by combustion synthesis and characterized by SEM/EDX, TEM, and XRD, to confirm morphology, crystal structure, and composition as a function of temperature and gas environment. 15 Symmetrical electrolyte-supported SOFCs were constructed using our catalyst, with LCFCrNi at the anode (0.5 cm 2 ) and LCFCr at the cathode (1 cm 2 ), with the catalyst layers screen-printed on both sides of a samaria-doped ceria (SDC)-buffered scandia-stabilized zirconia (ScSZ) electrolyte (ca. 150 μm). This was followed by sintering at 1100 °C for 2 h in air, with porous Au current collectors used. Electrochemical evaluation was conducted at 800 o C under 4:1 H 2 :CO 2 with the gas flow rates and composition controlled by mass flow controllers. Under both open circuit and polarized conditions, durability was measured in-situ for 100-250 h, with electrochemical impedance spectroscopy carried out every 20 h to track changes in resistance and capacitance. Additionally, gas analysis was carried out by on-line mass spectrometry to verify outlet gas composition and fuel utilization. Figure 1a shows the current-voltage data obtained from a SOFC with a LCFCrNi anode in 4:1 H 2 :CO 2 and 4:1 H 2 :N 2 at 800 °C. Under 4:1 H 2 :N 2 , a maximum power density of 0.43 W/cm 2 was achieved at 0.92 A/cm 2 (0.20 V overpotential vs. LCFCr reference in air). The reformed gas mixture (4:1 H 2 :CO 2 ) has a 22% lower performance, achieving a maximum power output of 0.35 W/cm 2 at the same overpotential. This is explained by the reverse water-gas shift reaction producing CO, which could have more sluggish oxidation kinetics compared to H 2 . 16 Reverse water-gas shift reaction also consumes H2 to produce H2O, thereby decreasing the H2 concentration at the anode. The OCV in 4:1 H 2 :CO 2 is ca 1.0 V and significantly lower than the OCV value of 1.15 V in 4:1 H 2 :N 2 at 800 °C. The difference in OCV values suggests that pO2 in 4:1 H 2 :CO 2 is significantly higher than in 4:1 H 2 :N 2 , thereby reducing the driving force. Chronopotentiometry experiments were carried out at 0.35 A/cm 2 at an anode overpotential of 0.16 V (vs. a

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