A General Strategy to Boost Electrocatalytic Nitrogen Reduction on Perovskite Oxides via the Oxygen Vacancies Derived from A‐Site Deficiency

Abstract

pressures (10-30 MPa), but also demands a large deal of H 2 mainly from natural gas reforming which causes abundant consumption of fossil fuels and huge CO 2 emissions. [3-6] As a result, for energy conservation and environmental protection, it is of crucial importance to explore a clean and sustainable route for NH 3 production. Electrocatalytic N 2 reduction reaction (NRR) under ambient conditions has been emerged recently as an attractive strategy for the green synthesis of NH 3 through utilizing inexhaustible N 2 and H 2 O. [7-12] However, due to the competition with H 2 evolution reaction (HER) and ultra-stable NN covalent triple bond, NRR suffers from unsatisfactory Faradaic efficiency (FE) and sluggish reaction kinetics. [13] It is of eager desire, but very challenging to develop selective and efficient electrocatalysts toward NRR that can inhibit the competitive HER and activate the N 2 molecule. Among various developed electrocatalysts, perovskite oxides have proven their great potential toward NRR because of their low cost, good adjustability in intriguing physicochemical properties, as well as economic and environmental friendliness. Typical examples include a few simple perovskite oxides, such as LaCoO 3 , LaCrO 3 , LaFeO 3 , La 2 Ti 2 O 7 , and Ce 1/3 NbO 3. [14-19] Although significant progress has been made, their NRR performance is still far from the requirements of commercial NH 3 synthesis. Most recently, several studies have shown that the electrocatalytic The electrocatalytic N 2 reduction reaction (NRR) under ambient conditions is an attractive strategy for green synthesis of NH 3. Due to the ultra-stable NN covalent triple bond, it is very challenging to develop highly selective and efficient electrocatalysts toward NRR. Here a general strategy to enhance the NRR activity through modulating A-site-deficiency-induced oxygen vacancies of perovskite oxides is reported. One successful example is La x FeO 3−δ (L x F, x = 1, 0.95, and 0.9) perovskite oxides with tunable oxygen vacancies that are directly proportional to the La-site deficiencies. As compared to the pristine LF, the L 0.95 F and L 0.9 F exhibit significantly improved NRR activities, which are positively correlated with the La-site deficiency and the amount of oxygen vacancies. Among them, the L 0.9 F delivers the best activity, with an NH 3 yield rate of 22.1 µg•h −1 •mg −1 cat. at −0.5 V and a Faradaic efficiency of 25.6% at −0.3 V, which are 2.2 and 1.6 times those of the pristine LF, respectively. Both experimental characterizations and theoretical calculations suggest that the enhanced NRR activity can be mainly attributed to the favorable merits produced by the oxygen vacancies: the promoted adsorption/activation of reaction species, and thus optimized reaction pathways. Application of this strategy to other perovskite oxides generates similarly successful results.

Cation‐Deficiency‐Dependent CO2 Electroreduction over Copper‐Based Ruddlesden–Popper Perovskite Oxides

“…Angewandte Chemie XPS and iodometric titration were performed to elucidate the correlation between the oxygen vacancies and La-site deficiencies in these four samples.Figure 2a shows O1sXPS spectra of the four samples.T he peaks at about 533.1, 531.6, 530.4, and 528.9 eV were indexed to surface-adsorbed H 2 O, surface-adsorbed O 2 or hydroxyl groups,h ighly oxidative oxygen species (O 2 2À /O À ,c orresponding to oxygen vacancies), and lattice O 2À ,respectively. [27] According to the relative areas of their peaks (Table S3), the concentrations of oxygen vacancies of the L 2Àx Cs amples were calculated to be 0, 36.6 %, 33.8 %, and 35.7 %, respectively (Figure 2b). In the iodometric titration experiments,w eo bserved as imilar change trend of oxygen vacancy concentrations to that in the XPS analysis,w ith the values of 0.9 %( L 2 C), 10.4 % (L 1.9 C), 11.3 %( L 1.8 C), and 11.8 %( L 1.7 C), respectively (Figure 2b).…”

An Efficient Symmetric Electrolyzer Based On Bifunctional Perovskite Catalyst for Ammonia Electrolysis

“…Figure 5e shows the high‐resolution XPS spectra of the O 1 s core levels of LNCO55‐Air, LNCO55‐Ar, LNCO55‐Ar anode, and LNCO55‐Ar cathode, which can be divided into five characteristic peaks of lanthanum‐oxygen bond (≈528.4 eV for La—O), lattice oxygen species (≈529.8 eV for O 2− ), hydroxyl groups or the surface adsorbed oxygen (≈531.6 eV for —OH or O 2 ), carbon—oxygen bond (≈532.9 eV for C—O), and adsorbed molecular water (≈534.7 eV for H 2 O). [ 44 , 60 ] As the O 1 s spectra are complicated in quaternary perovskites, it is difficult to distinguish the oxygen vacancy concentration directly by XPS. The XPS results of La 4d and Cu 2p in Figure S15 , Supporting Information, showed that all samples only contained Cu 2+ and La 3+ .…”

Perovskite Oxides for Cathodic Electrocatalysis of Energy‐Related Gases: From O2 to CO2 and N2

“…These materials only presented inferior performance when compared with La 0.8 Cs 0.2 Fe 0.8 Ni 0.2 O 3− δ and LaFeO 3 . Beyond the cation doping/substitution strategy, post‐treatment by annealing in a reductive atmosphere (5% H 2 /Ar) [ 121 ] or A‐site deficiency [ 26 ] can also create oxygen vacancies in LaFeO 3 and induce efficient NRR catalysis. Further theoretical studies demonstrated that the introduction of oxygen vacancies into LaFeO 3 could not only facilitate the adsorption and activation of the N 2 molecule, but also optimize the reaction pathways through intensifying the interactions between adsorbed species and unsaturated Fe sites.…”

“…Moreover, the creation of oxygen vacancies can be a versatile strategy for developing effective perovskite oxide‐based NRR catalysts with other A or B‐site elements. [ 26 ]…”