One-pot synthesis of La_0.7Sr_0.3MnO₃ supported on flower-like CeO₂ as electrocatalyst for oxygen reduction reaction in aluminum-air batteries

Highlights

• A novel LSM-CeO₂ hybrid catalyst has been synthesized by a facile one-pot method.

• The LSM particles about 150 nm are well distributed on the flower-like CeO₂.

• The flower-like CeO₂ particles are formed from nanosheets, approx. 40 nm thick.

• Hybrid material shows the remarkable catalytic activity and stability during ORR.

• P_max of the Al-air battery with LSM-CeO₂ hybrid material reaches 238 mW cm⁻².

Abstract

A novel La_0.7Sr_0.3MnO₃-CeO₂ (LSM-CeO₂) hybrid catalyst for oxygen reduction reaction (ORR) has been synthesized by a facile one-pot method. The flower-like CeO₂ with the diameter of about 3 μm is formed by the agglomeration of nanosheets with the thickness of about 40 nm. The LSM particles with the diameter of about 150 nm are well distributed on the flower-like CeO₂, thus the interaction between LSM and CeO₂ is built. Therefore, the LSM-CeO₂ composite catalyst exhibits the much higher catalytic activity toward ORR with the direct four-electron transfer mechanism in alkaline solution than LSM or CeO₂. Furthermore, the stability of LSM-CeO₂ is superior to that of Pt/C, and the current retention is 93% after 100000 s. The maximum power density of the aluminum-air battery using LSM-CeO₂ as the ORRC can reach 238 mW cm^-2, which is about 29% higher than that with LSM (184 mW cm^-2). It indicates that LSM-CeO₂ composite material is a promising cathodic electrocatalyst for metal-air batteries.

Introduction

Aluminum-air (Al-air) battery is very attractive due to its high theoretical energy density (8.1 kWh kg⁻¹) and low cost [1]. The Al-air battery is mainly composed of the aluminum metal anode, alkaline aqueous electrolyte and air-breathing cathode electrode, which is generally composed of the gas diffusion layer, current collecting layer and oxygen reduction reaction catalyst (ORRC) [2], [3]. The low power density seriously limits the commercial application of the Al-air batteries due to the sluggish oxygen reduction reaction (ORR) [4], [5], [6]. Therefore, ORRCs for the metal-air batteries with high catalytic activity still need to be further studied.

So far, a variety of ORRCs with high catalytic activity including noble metals and their alloys, transition-metal oxides, carbon materials, and organometallic macrocycles have been developed in metal-air batteries [7], [8], [9], [10], [11], [12], [13]. The Pt/C is the state-of-the-art catalyst for ORR at room temperature in the alkaline solution. However, the scarcity and high price greatly inhibit the practical application of Pt/C. The LaMnO₃ is one of the most frequently-used ORRCs in the fuel cells and metal-air batteries [14], [15], [16], [17], [18], [19], and its intrinsic ORR activity can be comparable to that of the state-of-the-art Pt/C [11]. In general, the manganese valence state of the La_1-xSr_xMnO₃ (LSM) perovskite which can be tailored by the substitution of La with Sr was proposed as an important factor for their ORR activities [15], [16], [17], [18], [19], [20], [21], [22].

Cerium oxide (CeO₂) is a very important metal oxide and it has been widely used in solid oxide fuel cells [23], [24], [25], [26], proton exchange membrane fuel cell [27], [28], [29], [30], [31], [32], [33], [34], [35], direct methanol fuel cell [36], polymer electrolyte fuel cells [37], anion exchange membrane fuel cells [38] and microbial fuel cells [39], [40], due to its redox properties, transport properties and high oxygen storage capacity [41], [42], [43]. Recently, some reports have indicated that the CeO₂ could effectively improve the ORR catalytic activity of Pt [29], [30], [32], [33], [34], [35], [40], Pd [37], [38], and Au [44]. Chen et al. [45] has reported that CeO₂ can facilitate oxygen transfer to MnO_x nanoparticles, resulting in much better ORR activity when CeO₂ nanoparticles are locates in the vicinity of MnO_x nanoparticles. Zhu et al. [46] also has found that MnO_x decorated CeO₂ nanorods can be used as a cathode catalyst for lithium-air batteries with the high activity.

In this work, the La_0.7Sr_0.3MnO₃-CeO₂ (LSM-CeO₂) composite is synthetized by a facile one-pot synthesis method. Then, the ORR catalytic activity and long-term stability of the synthesized LSM-CeO₂ hybrid are studied in alkaline solution. Lastly, the power densities of the aluminum-air batteries using the different catalysts are tested. The maximum power density (P_max) of the aluminum-air battery using LSM-CeO₂ as ORRC can reach 238 mW cm^-2.