Enhanced Catalytic Activity Based on “Oxygen Self-Breathing” Ce-Doped Perovskite Oxygen Electrode

Abstract

The rechargeable metal-air batteries have attracted extensive interests due to their high energy density and environmental friendliness. However, the poor stability, high cost of the oxygen electrodes, and especially the sluggish kinetics hinder the large-scale applications of metal-air batteries. To enhance the oxygen reaction rate, the high-efficient catalyst, which can accelerate oxygen evolution reaction (OER: 4OH-→O2+2H2O+4e-) and oxygen reduction reaction (ORR: O2+2H2O+4e-→4OH-) is urgently required. Among the oxygen electrode catalysts, perovskite oxides with the general formula ABO3-δ, in which A is rare-earth or alkaline earth element and B is transition metal, have received significant attention due to high ionic and electronic conductivity as well as the good stability. In recent years, considerable efforts have been devoted to understanding the catalytic mechanism of ABO3-δ perovskite oxides to achieve high efficient OER and ORR. It is widely accepted that the OER and ORR processes involve a sequence of intermediates (OH*, O* and OOH*) on the active atoms, which is intimately related to electronic structure (various electron and orbital distribution) and geometric structure of active atoms. The electronic structure is sensitive to the component, valence state of A and B site and particularly the concentration of oxygen vacancies. The oxygen molecules produced during OER process could fill the oxygen vacancies, which could further supply oxygen loss in ORR process. Therefore, the presence of oxygen vacancies can significantly affect the catalytic activity. Besides the electronic structure, the geometric structure of active atom can directly affect the band length and band angle of active atom-intermediates, which could modify the adsorption energy of intermediates. Therefore, the optimization of electron structure and geometric structure plays crucial role in achieving the enhanced electrocatalytic activity of perovskite oxides. Following this strategy, in this work, cerium (Ce) ions are successfully doped into LaCoO3-δ, causing the improved catalytic activity by modifying the electronic and geometric structures. The cerium oxides with co-existed +3 and +4 valence state are capable in storing and releasing oxygen (“oxygen self-breathing”) into LaCoO3-δ, which is beneficial to improve the electrocatalytic activity. The XPS analysis of Ce 3d shows the characteristic peaks due to the coexistence of Ce3+ and Ce4+ ions (Figure 1a), confirming the doping of Ce ions in LaCoO3-δ. To further verify the introduction of oxygen vacancies in LaCoO3-δ, the O 1s core-level spectrum is shown in Figure 1b. As can be seen, the peak at 528.5 eV corresponds to the lattice oxygen species (O2-). Notably, Ce-doped LaCoO3- δ(La0.9Ce0.1CoO3-δ) catalyst possesses the higher O2- percentage than that of LaCoO3-δ. The electrocatalytic characteristics of the perovskite oxides have been systematically investigated with RDE system in the O2-saturated 0.1 M KOH solution. The La0.9Ce0.1CoO3-δ shows the enhanced ORR and OER activities in comparison with LaCoO3-δ. As shown in Figure 1(c), the La0.9Ce0.1CoO3-δ shows better ORR performance with the half-wave potential (0.677V) at -3 mA cm-2 and the onset potential (0.835V) at -2 mA cm-2. Meanwhile, La0.9Ce0.1CoO3-δ catalyst also displays better OER performance (Figure 1(d)). Compared with LaCoO3-δ catalyst, the over-potential of La0.9Ce0.1CoO3-δ catalyst shifts negatively by 76.8 mV at 10 mA cm-2. In conclusion, the Ce ions have been successfully doped into LaCoO3-δ, offering large amount of lattice oxygen species in perovskite structure and thus leading to the enhanced electrocatalytic activities. Figure 1