Three-dimensional graphene network supported ultrathin CeO2 nanoflakes for oxygen reduction reaction and rechargeable metal-air batteries
Graphical abstract
Introduction
With the ever increasing demand on energy supplies and concerns over environmental issues associated with burning fossil fuels over the past decade, the development and application of clean energy is becoming more and more important. Although renewable power sources such as solar energy and wind energy are ubiquitous, energy storage devices are generally needed to complement these systems for a smooth and continuous power generation, with lithium-ion batteries (LIBs) being the most widely used electrochemical devices in consumer electronics at the moment. However, the maximum theoretical energy density of LIBs is only about 360 Wh kg−1 due to the limitation posed by electrode materials with intercalation chemistry and thus not satisfactory to power long-range electric vehicles or conduct heavy-duty energy storage [1], [2]. Consequently, new electrochemical devices with higher energy densities are in great needs. The rechargeable metal-air batteries (MABs) are considered as promising alternatives. For example, zinc-air batteries (ZABs), aluminum-air batteries (AABs) and lithium-oxygen batteries (LOBs) have high theoretical energy densities of 1,086, 2796 and 3500 Wh kg−1, respectively [3], [4], [5], [6]. Unfortunately, they generally face the same challenge in searching for an efficient air electrode to facilitate the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) by decreasing the excessive charge-discharge overpotential and reducing side-reactions in MABs [7]. In general, catalysts are needed to alleviate the large activation energy for the ORR and the structure of the air electrode is also important in determining the performance of the final battery. To date, depositions of various heterogeneous electrocatalysts such as noble metals (Pt and PtAu) [5], [8], [9], [10], metal oxides (RuO2, Co3O4 and ZnCo2O4) [11], [12], [13] and carbonaceous materials (carbon nanotube and graphene) [14] on the air cathode remain as the prevalent approach to promote the ORR and OER for a reduced overpotential in MABs. Therefore, developing highly efficient catalysts with low costs in MABs is still challenging but highly desirable, particularly with materials based on noble-metal-free alternatives such as CeO2. As a ubiquitous rare earth metal oxide, CeO2 features a fluorite structure capable of switching between the +3 and + 4 oxidation states, making it a popular catalyst in solid oxide fuel cells (SOFCs) and some types of MABs. For instance, Shao et al. fabricated the Co3O4CeO2/C as a high-performance ORR catalyst for AABs and the device exhibited a higher discharge voltage plateau (1.27 V) than Co3O4/C (1.23 V) and CeO2/C (1.12 V) in the full cell tests [15]. Chi and coworkers synthesized CeO2 nanoparticles on N-doped reduced graphene oxide as the cathode and the LOBs delivered a high capacity of 11,900 mAhh g−1 at current density of 400 mAhh g−1 and can cycle up to 40 cycles with a capacity of 1000 mAhh g−1, with the formation and decomposition of Li2O2 on the CeO2 surface nicely elucidated by the density function theory (DFT) calculations [16], [17]. Despite of these progresses, there are no reports on the CeO2-catalyzed ZABs and the performance of the CeO2-catalyzed AABs needs further improvement in a more in-depth study. Herein, we develop of an efficient CeO2-based catalyst for MABs by loading ultrathin CeO2 nanoflakes (UCNFs) on a three-dimensional graphene (3DG) network to form a hybrid catalyst of UCNFs@3DG. As a result, the UCNFs@3DG cathode in the ZAB delivers a capacity of 613 mAhh gZn−1 at current density of 2 mA cm−2 (97 mA gZn−1) and the performance is stable over 80 cycles without obvious degradation. Additionally, it shows a capacity of 839 mAhh gAl−1 in the AAB at a current density of 2 mA cm−2 (204 mA gAl−1) and the device can be cycled for at least 100 cycles. Finally, the UCNFs@3DG cathode in the LOB achieves a high discharge capacity of 21,166 mAhh gc−1 at 0.2 A gc−1 and over 15,000 mAhh gc−1 under elevated current densities up to 2 A gc−1. The above results show that the UCNFs@3DG is a powerful catalyst in different MABs and the unique nanoarchitecture reported in this work can be easily extended to the formation of many other 3DG-based materials for developing high-performance electrodes in electrochemical energy storage devices.
Section snippets
Reagents
Cerium nitrate hexahydrate [Ce(NO3)3·6H2O] was purchased from aladdin. Ammonia solution and sodium hydroxide were purchase from Fuchen Chemical Reagents Factory (Tianjin, China). Lithium bis(trifluoromethane)sulfonimide (LiTFSI) and N,N-dimethylacetamide (DMA) were purchased from Aldrich. All materials were used as received without any further purification.
Synthesis of UCNFs
To obtain ultrathin CeO2 nanoflakes, 0.86 g Ce(NO3)3·6H2O (0.5 mmol) was dissolved in a flask with 80 mL of a water/ethanol mixed solution
Results and discussion
Scheme 1 summarizes the synthetic process of the UCNFs@3DG. Briefly, UCNFs were first prepared by heating an aqueous solution of cerium (III) nitrate and alkali. As the hydrolyzed cerium (III) hydroxide is extremely sensitive to oxygen, it was instantaneously converted to CeO2 [18]. The as-obtained UCNFs were added to a solution of graphene oxide (GO) under vigorous stirring and then dried by lyopilization before a subsequent annealing at 600 °C to give the final product of the UCNFs@3DG.
The
Conclusion
In summary, a convenient solution-processed method has been developed to synthesize the UCNFs@3DG. The as-prepared UCNFs@3DG composite is of high electrocatalytic activity in the ORR and can be used as a general efficient air cathode in a range of metal-air batteries including ZABs, AABs and LOBs. The current method can be easily extended to prepare many other 3DG-based materials for applications in catalysis and electrochemical devices towards energy conversion and storage.
Acknowledgements
This work was supported by NSFC (Grant No. 21774015) and the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201601).
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