Li-air batteries have recently captured scientific interest for the much higher theoretical energy density than conventional Li ion batteries. However, some major problems still limit the practical realization of Li-O₂ batteries, including the reversible capacity (and energy density), side reactions occurred on cycling, low energy efficiency due to the hysteresis between charge and discharge (up to 2V). The use of proper electrocatalysts on porous cathodes is regarded as the key to reduce the overpotential of the charge process, including precious metals [1-3], metal oxides [4-6] and MOFs [7]. Bruce et al. [8] demonstrated that porous gold cathodes in dimethyl sulfoxide (DMSO) electrolyte were able to maintain the high ratio of Li₂O₂ in the charge products, resulted in only a 5 % capacity loss after 100 charge cycles. However, the high cost of gold limits the application of porous gold in Li-O₂batteries.

In this work, a facile method was used to synthesize the Au/MWNT nanocomposite was used instead of porous gold cathode in the DMSO electrolyte with 0.1 M LiClO₄. The MWNTs were degassed before the immersion in H₂O₂, resulted in hydrophilic surface with –OH and –COOH groups. Then, the reaction between 1.5 mM HAuCl₄ and 1.5 mM tetrakis(hydroxylmethyl)phosphornium chloride alkaline solution was allowed to take place in the presence of the treated MWNTs, led to Au/MWNT nanocomposite (d = 2.1 nm) with the Au loading of 29 wt. %. The catalytic activity of the nanocomposite on oxygen reduction reaction (ORR) was investigated by cyclic voltammetry and RRDE experiments, which demonstrated that the nanocatalyst exhibits a stronger ORR catalytic performance than those reduced by ethylene glycol, and the MWNTs substrate is also better than Vulcan XC72R. Moreover, the number of electron for the ORR on the nanocatalyst was determined as 2.4, corresponding to the reduction product of H₂O₂, which also suggests that Li₂O₂ is probably the major ORR product in the Li-O₂battery if the nanocatalyst is used.

A nickel foam cathode sprayed with Au/MWNTs catalyst (1.6 mg/cm²) was assembled in a Swagelok cell, together with a Li disk and a Nylon 66 filter membrane. The Li-O₂ battery was cycled between the final discharge potential of 2.2 V and charge potential of 4.0 V vs. Li/Li⁺ in 0.1 M LiClO₄DMSO electrolyte, from which the specific capacity for the Au/MWNT nanocatalyst was determined as 1293 mAh/g, larger than the porous gold reported by Bruce et al. [8], indicating that the nanogold catalysts possess much stronger catalytic property than bulk gold, and is expected to be used in Li-air batteries at low cost.

ACKNOWLEDGEMENTS

The work was supported by the National Natural Science Foundation of China (Grant No. 21163004) and Guangxi Natural Science Foundation (Grant No. 2013GXNSFAA 019029).

REFERENCES

1. Y. C. Lu, et al. Electrochem. Solid-State Lett., 13 (6): A69 (2010).
2. H. D. Lim, et al. Science, 6: 3570 ( 2013).
3. X. J. Li, et al. ACS Appl. Mater. Interf., 6, 12409 (2014).
4. C. W. Sun, et al. J. Mater. Chem. A, 2, 7188(2014).
5. Z. L. Jian, Angew. Chem. Int. Ed., 126, 452 (2014).
6. J. J. Xu, et al. Angew. Chem. Int. Ed., 125, 3979 (2013).
7. D. F. Wu, et al. Adv. Mater. 26, 3258(2014).
8. Z. Q. Peng, Science, 337, 563(2012).