(Invited) Understanding the Effects of Metal Nanoparticle Exsolution from La 0.3 Ca 0.7 Fe 0.7 Cr 0.3 O 3-δ Perovskites on CO 2 -CO Electrocatalysis

Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs) are highly useful devices, capable of generating and storing large amounts of energy, respectively. 1 They can do this by catalyzing CO oxidation at a SOFC anode and CO 2 reduction (CO 2 RR) at a SOEC cathode, while the other electrode catalyzes oxygen reduction or evolution. The conduction of oxide ions through the electrolyte completes the circuit, making Solid Oxide Cells (SOCs) an excellent choice for clean energy production and use since there are no undesired byproducts. 1 While traditional SOC electrodes are composed of oxide-conducting ceramics mixed with electronically conducting metals, 1 a newer category of catalysts are Mixed Ionic Electronic Conductors (MIECs), with one example being the perovskite La 0.3 M 0.7 Fe 0.7 Cr 0.3 O 3-δ (M = Sr, Ca) (LMFCr), investigated heavily by our group. 2,3,4 As an MIEC, the full surface of LMFCr is electrochemically active, giving excellent activity at both the cathode and anode, including for CO 2 RR and CO oxidation. 2,3 However, efforts are being made to further improve the CO 2 RR-CO oxidation kinetics and durability, with the Ca analogue having a better chemical match with standard electrolytes. 4 One approach used recently to improve MIEC oxide performance is B-site doping with transition metals (TMs) while also creating an A-site deficiency, resulting in nanoparticle (NP) formation (exsolution) when the perovskite is subjected to reducing conditions. 2 For instance, Fe-Ni NPs of a ~20 nm average size can be exsolved from (La 0.3 Ca 0.7 ) 0.95 Fe 0.7 Cr 0.25 Ni 0.05 O 3-δ (LCFCrNi), even at 600 °C in 70CO 2 :30CO (pO 2 ~10 -18 atm). Furthermore, NP features can easily be tailored by changing the reducing conditions or dopant used. 2 In general, higher temperatures and lower pO 2 lead to larger NPs over time, and more easily reducible metals tend to exsolve first under less harshly reducing conditions. 2 Recent studies have suggested that NP formation enhances electrocatalytic activity by creating additional sites of reactivity, suggesting that strong NP-substrate interactions are important to catalysis. 5 However, there is no clear understanding of the role played by exsolved NPs in catalyzing SOC reactions. To gain further insights, detailed electrochemical studies of LCFCrNi electrodes were done on 1-inch diameter cells constructed using Samarium-Doped Ceria buffered Scandia-Stabilized Zirconia electrolyte substrates. LCFCrNi wa