Highly Efficient B-Site Exsolution Assisted by Co Doping in Lanthanum Ferrite toward High-Performance Electrocatalysts for Oxygen Evolution and Oxygen Reduction

Abstract

Alloy/perovskite composites prepared by exsolution of Fe-based perovskite have attracted wide attention due to their embedded and well-anchored structure, which have broad applications in heterogeneous catalysis and energy conversion. Herein, we use Co-doped lanthanum ferrite as a model to study the effect of doping on the B-site exsolution of Fe-based perovskite. CoFe alloy can be exsolved from La0.9Fe0.9Co0.1O3 (LFCO) after heat treatment at 500 °C in a reduced atmosphere, whereas Fe will not be exsolved from La0.9FeO3 (LFO). Density functional theory calculations revealed that the stability of LFCO decreased after Co is doped into the lanthanum ferrite perovskite lattice and the formation energy of the Co–Fe bond on the surface of LFCO is lower than that of Fe–Fe in LFO, which promises an easier exsolution of CoFe alloy than the pristine Fe cluster. In addition, owing to the strong interaction and charge transfer between the exsolved CoFe alloy and parent perovskite, as well as the longer Fe–O bond after exsolution, the exsolved composite can act as an excellent bifunctional electrocatalyst for oxygen evolution and oxygen reduction reactions. Our work not only reveals the mechanism of the alloy exsolution in Fe-based perovskites but also provides a potential route to prepare the highly efficient electrocatalysts.

Trends and Prospects of Bimetallic Exsolution

“…This is because in mixed cation systems, the Gibbs energy of reduction is a function of the strength of the metal‐oxygen bonds of both substituted metals and hence the energy can be decreased by introducing more reducible ions (Figure 2 d). [11c–e] Besides, it is also reported that doping of Co increases the total energy of the perovskite system and the Co−Fe bond would form more easily than the Fe−Fe bond due to the lower formation energy, which also accounts for the promoting effects of Co on the Fe exsolution [11f] …”

Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules

“…As shown in Table 6 , exsolved metal systems include Ag, [ 95,109 ] Ru, [ 88 ] Ni, [ 130,131 ] Co, [ 138 ] Co/CoO x , [ 78 ] Pt 3 Ni alloy, [ 103 ] Ni–Co alloy, [ 151 ] Fe–Ni alloy, [ 77 ] and Co–Fe alloy. [ 70,158 ] Overall, single factors predicting low overpotentials universally across OER, HER, and ORR were not readily deducible, though Co doping of oxide matrices was frequently observed in most studied reaction systems. Nevertheless, such Co composition was a highly consistent predictor of multifunctional activity over all studied materials, likely due to the multiple stable electronic valence states Co ions can assume.…”

“…Several oxides with exsolved metal nanoparticles were reported to exhibit bi‐functional [ 130,158 ] or tri‐functional [ 78,138 ] activity for several reactions. Hua et al.…”

Progress and Opportunities for Exsolution in Electrochemistry

“…Although most all ceramic backbones (save SrRuO 3 ) are incapable of sufficient electrical conductivity at room temperature, the use of exsolution electrodes for room temperature applications should not be ignored. Jian et al [82] recently showed that the exsolution of Co from La 0.9 Fe 0.9 Co 0.1 O 3 (LFCO) could be used at room temperature if mixed with a conductive carbon black ink. This composite electrode showed enhancement for both oxygen evolution and oxygen reduction reactions.…”