Elsevier

Journal of Alloys and Compounds

Volume 707, 15 June 2017, Pages 275-280
Journal of Alloys and Compounds

Electrocatalytic performance of CuO/graphene nanocomposites for Li–O2 batteries

https://doi.org/10.1016/j.jallcom.2016.11.317Get rights and content

Highlights

  • A porous CuO nanowire/leaf mixture anchored onto graphene was synthesized.

  • CuO/graphene catalysts are studied as electrocatalysts for an oxygen electrode for Li–O2 cells.

  • CuO/graphene catalysts exhibited enhanced ORR/OER kinetics and superior cycle reversibility.

Abstract

A hybrid catalyst system of a porous CuO nanowire/leaf mixture anchored onto graphene (p-CuO/G hybrid) was prepared as an oxygen-electrode electrocatalyst for lithium–oxygen (Li–O2) batteries. The p-CuO/G hybrid was prepared via the hybridization of a porous 1D/2D CuO mixture with 2D graphene. As an oxygen-electrode electrocatalyst for Li–O2 cells, the p-CuO/G hybrid exhibited high reversibility with a low voltage gap compared with a graphene electrode during 120 discharge–charge cycles under a fixed capacity regime of 1000 mA h g−1. We demonstrated the oxygen reduction and evolution kinetics of the p-CuO/G hybrid using electrochemical impedance spectroscopy.

Introduction

Climate change, caused by excessive carbon emissions resulting from overuse of fossil fuels, has innumerable adverse consequences. Governments are attempting to reduce our undue dependency on fossil fuels by subsidizing the development of technologies for renewable energy sources and energy storage systems [1], [2].

For more efficient energy use, rechargeable batteries as an energy storage system are an essential enabling technology. However, the major issue confronting rechargeable batteries is performance, specifically, the development of batteries with a safe and long life cycle, good cost efficiency, and a high specific energy and energy density [2], [3], [4]. The performance requirements for rechargeable batteries include a high energy density, especially in the case of batteries intended for use in energy storage systems of smart grids and in electric vehicles. Rechargeable lithium–oxygen (Li–O2) and lithium–sulfur battery systems are representative advanced rechargeable batteries with high energy densities [5]. Li–O2 batteries, in particular, have attracted worldwide attention because of their extremely high theoretical energy density, which is similar to the available energy density of gasoline [6], [7], [8], [9].

The components of Li–O2 batteries are similar to those of conventional Li-ion batteries: electrolyte, separator, anode, and cathode. The major differences are that, in Li–O2 batteries, the anode is lithium metal, while the cathode is an oxygen electrode with an electrocatalyst. Li-ion batteries typically operate on the basis of Li-ion intercalation/deintercalation into electrode materials, while Li–O2 batteries operate via the reversible adsorption/desorption of oxygen species (i.e., 2Li + O2 + 2e ↔ Li2O2, E0 = 2.96 V vs. Li/Li+) at the surface of an oxygen electrode, thereby benefitting from other technologies.

Electrocatalysts improve the efficiency of discharge–charge cycling through the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The ORR and OER activities of electrocatalysts depend strongly on their morphologies and surface areas and on the physical characteristics of their crystal phase during discharge–charge cycling.

Various strategies have been proposed to achieve efficient catalytic activity toward the reverse reaction, thereby enabling cycle reversibility. In one such strategy, nanostructuring enables effective transport of electrons and Li+ ions because of the high surface-to-volume ratios of nanostructured materials [10], [11]. In another strategy, the hybridization of electrode materials with carbon materials such as graphene [12], [13], [14] or carbon nanotubes [15], [16], [17] improves the charge transfer rate. To date, the most effective electrocatalysts have been graphene-supported nanostructures, wherein graphene imparts the electrodes with a high surface area, high electrical conductivity, and good chemical stability [10], [11], [18], [19].

Therefore, we herein describe our development of porous CuO nanowire/leaf mixtures anchored onto graphene (p-CuO/G hybrids) as electrocatalysts for oxygen electrodes. The resulting electrocatalysts improve the ORR and OER activities as well as the cycle reversibility of Li–O2 batteries.

Section snippets

Synthesis of p-CuO/G hybrids

Graphene (0.051 g) was dispersed into 75 ml of ethanol under sonication for 30 min. Cu(CH3COO)2·H2O (0.55 g) and 0.5 g of cetyltrimethylammonium bromide (CTAB) were dissolved in 175 ml distilled water over a period of 5 min. The as-prepared Cu precursor and CTAB solution were added to the solution of graphene dispersed in ethanol, and the temperature of the resulting mixture was maintained below 5 °C using an ice bath. A 1 M NaOH solution (8 ml) was slowly added into the aforementioned aqueous

Results and discussion

The morphologies of the graphene, Cu(OH)2/G hybrid, and p-CuO/G hybrid were examined by FESEM. As shown in Fig. 1a, the graphene exhibited a sheet-like morphology with a thin and smooth surface. In the first step, we synthesized the Cu(OH)2/G hybrid in CTAB solution; this material is shown in Fig. 1b and c. The as-prepared Cu(OH)2/G hybrid appeared to deposit onto the graphene surface without aggregating (Fig. 1c). In the second step, the p-CuO/G hybrid was obtained from the Cu(OH)2/G hybrid

Conclusion

We developed porous CuO nanowire/leaf mixtures anchored onto graphene for Li–O2 batteries; these hybrid materials offer structural advantages as electrocatalysts for the oxygen electrode. First, the graphene improved the electrical conductivity between the mixture of porous CuO nanowires and leaf-like structures. Second, the 3D hybridization of the p-CuO/G hybrid material combined with porous 1D/2D nanostructures and 2D graphene resulted in an increase in the number of catalytically active

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT, and Future Planning (NRF-2016R1A2B2012728, NRF-2016M3A7B4909318) and by the institutional research program of the Korea Institute of Science and Technology (2E26081-16-054).

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    These authors contributed equally to this article.

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