In Situ Investigation of Reversible Exsolution/Dissolution of CoFe Alloy Nanoparticles in a Co‐Doped Sr₂Fe_1.5Mo_0.5O_6−δ Cathode for CO₂ Electrolysis

Abstract

CO 2 electrolysis via solid oxide electrolysis cell (SOEC) has shown promising practical applications in CO 2 conversion and renewable electricity storage due to low overpotential, large current density, high Faradaic efficiency, and energy efficiency facilitated by high-temperature operation. [1] Perovskites have been extensively investigated as cathode materials for direct CO 2 electrolysis in SOEC in the absence of protective gas [2] ; however, the perovskites still suffer from insufficient CO 2 electrolysis performance. [3] In situ exsolving metal nanoparticles on the perovskite surface have been explored as an efficient strategy to improve CO 2 electrolysis performance due to the exsolution of highly active metal nanoparticles and the simultaneous generation of oxygen vacancies within perovskite, where abundant metal-oxide interfaces are generated for highly efficient CO 2 electrolysis. [4] In addition, reversible exsolution and dissolution of metal nanoparticles in perovskite have been proposed as vital properties for resolving the possible particle agglomeration and coke formation during a long-term operation of SOECs. [2b,5] To date, although some perovskites have demonstrated redox reversibility with exsolution and dissolution of metal nanoparticles in reducing and oxidizing atmosphere, [6] fundamental understanding of these phenomena is still scarce. [7] Ex situ scanning/transmission electron microscopy (SEM/ TEM) and X-ray diffraction (XRD) techniques have been employed to investigate the morphology and crystalline evolution after reducing and oxidizing treatments. [8] Irvine et al. found that a decrease in the stoichiometry of perovskite from A/B = 1 to A/B<1 could break the bottleneck of exsolution level and facilitate high-population exsolution of metal nanoparticles. [7,9] Kim et al. investigated the reducibility of different cations in perovskite using co-segregation energy as a descriptor. The co-segregation energy of B-site dopant and oxygen vacancies plays a critical role in the exsolution. [10] Furthermore, Luo et al. used in situ TEM to investigate the exsolution of Co nano particles in Pr 0.5 Ba 0.5 Mn 0.9 Co 0.1 O x (PBMCo) perovskite, excluding the possibility of metal nanoparticles Reversible exsolution and dissolution of metal nanoparticles in perovskite has been investigated as an efficient strategy to improve CO 2 electrolysis performance. However, fundamental understanding with regard to the reversible exsolution and dissolution of metal nanoparticles in perovskite is still scarce. Herein, in situ exsolution and dissolution of CoFe alloy nanoparticles in Co-doped Sr 2 Fe 1.5 Mo 0.5 O 6-δ (SFMC) revealed by in situ X-ray diffraction, scanning transmission electron microscopy, environmental scanning electron microscopy, and density functional theory calculations are reported. Under a reducing atmosphere, facile exsolution of Co promotes reduction of the Fe cation to generate CoFe alloy nanoparticles in SFMC, accompanied by structure transformation from double perovskite to layer...

Atomic‐Scale Insight into Exsolution of CoFe Alloy Nanoparticles in La0.4Sr0.6Co0.2Fe0.7Mo0.1O3−δ with Efficient CO2 Electrolysis

“…DFT calculations reveal that the segregation energy of Co and Fe ions with oxygen vacancies in LSCFM are 0.70 and 1.79 eV, respectively (Figure e), indicating that Co ions are more prone to segregate onto the surface of LSCFM than Fe ions under the reducing atmosphere . The segregation energy of Fe ion could be decreased to 0.97 eV after introduction of Co and oxygen vacancies by Co exsolution, which is beneficial to the exsolution of CoFe alloy nanoparticles, in agreement with the in situ STEM results (Figure j–m).…”

“…With the help of Bader charge results as listed in the Supporting Information, Table S4, we can quantitatively acquire the charge gain and loss of the target atoms. The CoFe cluster donates abundant electron to bind the CO 2 molecule, and improves the interaction between the metal–oxide interface and CO 2 molecule, which is beneficial for CO 2 electrolysis . As shown in Figure c, the partial density of states (DOS) of Fe and Co around Fermi level in CoFe@LSCFM is much richer than that of O and La in LSCFM in Figure d, suggesting that the CoFe@LSCFM is more active in CO 2 adsorption and activation, which could convincingly account for the high CO 2 electrolysis performance of CoFe@LSCFM …”

“…In contrast, the adsorption energy is −0.10 eV on the LSCFM surface, indicative of a weak physisorption. Therefore, CoFe@LSCFM shows an improved CO 2 adsorption at the interface between the CoFe and LSCFM . The electronic structure of CoFe@LSCFM interface and LSCFM is shown in Figure b.…”

“…DFT calculations were performed to investigate the effect of Mo doping on the structural stability of LSCFM (Figure c–e, more details in the Supporting Information, Methods and Figures S2, S3). The formation energy of oxygen vacancy ( ) and co‐segregation energy of B‐site transition metals (V M , M=Co and Fe) accompanying oxygen vacancies were calculated . The formation energies of , V Co , and V Fe are 1.08, 0.52, and 0.58 eV for LSCF (La 0.4 Sr 0.6 Co 0.2 Fe 0.8 O 3− δ ), and 2.68, 0.70, and 1.79 eV for LSCFM (Figure e; Supporting Information, Figure S2), suggesting that the Mo doping could suppress excessive exsolution of CoFe alloy nanoparticles and thus improve the structural stability, as shown by SEM images (Supporting Information, Figure S4), ex situ X‐ray diffraction (XRD; Supporting Information, Figure S5) and temperature‐programmed reduction results (Supporting Information, Figure S6).…”

“…The polarization resistance ( R p ) of CoFe@LSCFM‐GDC cell is 0.076 Ω cm 2 (800 °C and 1.6 V), only 76 % of LSCFM‐GDC cell (0.1 Ω cm 2 ). All EIS spectra can be fitted with the three‐element equivalent circuit (inset in Figure S20), the high‐frequency process represents for the ion migration process (R1), the mid‐frequency process represents for the oxygen evolution reaction at the anode (R2), and the low‐frequency process represents for the electrochemical adsorption and activation processes (R3) . The sub‐process resistances are all reduced at different temperatures after in situ exsolution of CoFe alloy nanoparticles in LSCFM (Supporting Information, Tables S2 and S3).…”