Elsevier

Journal of Power Sources

Volume 370, 1 December 2017, Pages 1-13
Journal of Power Sources

Ag, Co/graphene interactions and its effect on electrocatalytic oxygen reduction in alkaline media

https://doi.org/10.1016/j.jpowsour.2017.10.004Get rights and content

Highlights

  • Ag nanoparticles on Co, N embedded graphene are prepared by hydrothermal route.

  • Ag develops into crystalline morphology while cobalt forms amorphous oxide.

  • As-synthesized material show superior catalytic activity and stability during ORR.

  • Particle size refinement and d-band center shift of Ag improves its ORR activity.

  • Ag/Co-NGr is methanol tolerant and functions with stable profile in Zn-air battery.

Abstract

Silver nanoparticles supported on cobalt and nitrogen embedded reduced graphene oxide, Ag/Co-NGr, are synthesized by one-step hydrothermal route with remarkable catalytic activity for oxygen reduction reaction (ORR). As-synthesized electrocatalyst exhibits half-wave potential (0.82 V) comparable to commercial Pt/C (0.85 V), specific activity (0.45 mA cm−2) better than commercial Pt/C (0.35 mA cm−2) along with superior stability in alkaline environment (≈95% activity retention after 5000s compared to 80% for Pt/C). Moreover, Ag/Co-NGr is highly tolerant to methanol poisoning during ORR and delivers an excellent specific capacity of 789 mAh.g−1Zn with energy density of 947 Wh. kg−1 at a current density of 20 mA cm−2 in a Zn-air battery. For the first time, it is proven that particle size refinement and electronic perturbation of Ag nanoparticles take place due to metal-support interactions between Ag and Co/NGr. d-band center of Ag in Ag/Co-NGr upshifts toward the Fermi level with respect to the Ag/NGr as a result of charge transfer between Ag and Co/NGr. The superior catalytic activity and excellent stability of Ag/Co-NGr is attributed to the structural and electronic modification of Ag nanoparticles by Co and N elements on graphene.

Introduction

Rising global energy demand accompanied with widespread awareness concerning the damaging effects of burning fossil fuels on environment have brought a renewed interest in the development of green energy systems. Fuel cells and metal air battery are clean and sustainable high energy density systems which consume air from environment and provide useful energy for work aimed at stationary and mobile applications [1], [2], [3], [4]. Fuel cells provide electrical energy with efficiencies up to 60% which is better than internal combustion engines. These devices are benign towards environment since they discharge water during operation and are free from smog causing particulate matter air pollutants. Fuel cells operating on bio-fuels (such as methanol and ethanol) reduce the dangers of fuel transportation/storage which are generally associated with hydrogen [5]. Metal air battery such as Li-air battery offers high energy density i.e. >11000 Wh. kg−1 which is comparable to the practical energy density from a gasoline combustion engine [6], [7], [8]. However, the commercialization of both fuel cells and metal air battery is largely limited due to fundamental issues on the catalyst side such as, 1) high cost and scarcity of commercial platinum used at the cathode 2) instability of commercial Pt/C in alkaline media and 3) methanol poisoning of commercial Pt/C catalysts [9], [10]. Therefore the development of non-precious metal electrocatalysts for ORR with low cost and good activity/stability is highly desirable.

Silver is a potential candidate for oxygen reduction reaction (ORR) in alkaline media because of its catalytic activity, low cost and good stability. In contrast to Pt, for which the strong binding of oxygen reduction intermediates limit the kinetics of ORR, in Ag-based electrocatalysts, a weak binding of ORR intermediates is suggested as the reason behind their low catalytic activity [6], [11]. The meager ORR activity of pure silver can be improved by modulation of its electronic structure by alloying and metal-support interaction (MSI) [6]. Supporting silver nanoparticles (NPs) onto conductive carbon frameworks can improve their ORR activity. Silver nanoparticles supported on reduced graphene oxide (Ag/RGO) with an average size of 10 nm have been synthesized by chemical reduction with NaBH4 for ORR [12]. A larger electrochemical surface area (ECSA) and improved catalytic activity was observed for Ag NPs supported on RGO compared to Ag NPs deposited on Vulcan carbon. Similarly, supporting Ag NPs on 3D graphene oxide/Vulcan carbon resulted in 3D composite (Ag/GO/C) with improved mass transfer and electronic conductivity [13]. Ag/GO/C was observed to have better ORR activity relative to Ag/GO because of the 3D architecture which allows better mass and charge flow. Improvement in catalytic activity of Ag by chemical interaction between Ag NPs and nitrogen doped reduced graphene oxide (NGr) is reported for Ag/NGr [14]. Ag/NGr displayed better ORR activity compared to Ag nanoparticles supported on either chemically reduced graphene oxide (Ag/c-rGO) or electrochemically reduced graphene oxide (Ag/e-rGO) due to interaction between silver nanoparticles and NGr. Silver nanoparticles incorporated on nitrogen doped reduced graphene oxide for ORR are also synthesized by high temperature solid state reaction [15]. Ag nanoparticles deposited on nitrogen doped reduced graphene oxide exhibited better ORR activity than Ag NPs deposited on reduced graphene oxide. More recently, 3D graphene aerogel made of silver nanowires/nanocrystals dispersed in graphene aerogel (AgNW–GA) is reported for a Zn-air battery [16]. The macroporous graphene sponge structure with Ag distribution was observed to be active for ORR. Although the Zn-air battery operated with a similar discharge behavior with respect to commercial Pt/C, intrinsic ORR activity of AgNW–GA (in terms limiting current density and half wave potential) was still inferior for AgNW–GA relative to Pt/C. Nevertheless, considering these results, regardless of significant efforts, there still exist some challenges and issues in the field of Ag/graphene hybrid catalysts: 1) most of the Ag/reduced graphene oxide hybrids display meager ORR activity with half-wave potential ‘E1/2’ lower relative to commercial Pt/C by 100–150 mV. 2) effect of support on the d-band center of Ag is not studied in these Ag/graphene hybrids which is an important parameter in regulating the electrocatalytic activity of Ag NPs. 3) preparation techniques for these silver nanoparticles/carbon matrix hybrids require tedious multi-step processes often involving toxic reducing agents/high temperatures which is not viable for scalable production.

Herein, we report one-step green synthesis of highly active and stable Ag nanoparticles supported onto cobalt and nitrogen co-doped reduced graphene oxide (Ag/Co-NGr) for ORR in alkaline media. As-synthesized Ag/Co-NGr catalysts are observed to have similar catalytic activity along with superior stability with respect to commercial Pt/C. The specific activity of Ag/Co-NGr is better than the Pt/C, Ag/NGr and CoxOy/NGr catalysts. Structural analysis by electron microscopy and X-ray photoelectron spectroscopy reveals uniform deposition of Ag NPs onto Co-NGr support in which Co cation coordinate with oxygen forming amorphous oxide. Simultaneous reduction of Ag nanoparticles with doping of Co in oxide and N atoms in atomic form on reduced graphene oxide leads to particle size refinement of Ag, as confirmed by the transmission electron microscopy. Finally, Ag/Co-NGr electrocatalyst is observed to be highly tolerant to methanol poisoning and displays good discharge characteristics in a primary Zn-air battery, indicating its suitability to be used as an ORR electrocatalyst for alkaline fuel cells and metal-air battery.

Section snippets

Chemicals

Cobalt (II) acetate tetrahydrate (Co(CH3COO)2·4H2O, > 99.5%) and potassium hydroxide (KOH) procured from Guangdong chemicals, Silver acetate (CH3COOAg, ≥ 99.5%) from Shanghai SJX, Ammonia solution (25–27%) from Tianjin Fuchen, Platinum on carbon (Pt/C, 20%) from Johnson Matthey fuel cells, Nafion ionomer solution (5%) from Dupont, Graphene oxide (GO) solution with concentration 5 mg mL−1 from Tanfeng technology Inc. and methanol/ethanol (C2H5OH, >99.7%) from Tianjin Fuyu chemicals. All

Results and discussion

Ag/Co-NGr electrocatalyst is prepared by a simple and scalable hydrothermal method. Fig. 1 shows the XRD patterns of the as synthesized Ag/Co-NGr and reference catalysts. The peak present in Fig. 1d and all other patterns at about 2θ value of 25° corresponds to the nitrogen doped reduced graphene oxide support (NGr) [25]. For the Ag/Co-NGr and Ag/NGr catalysts, several clear peaks present at 38.1°, 44.2°, 64.4° and 77.4° correspond to the (111), (200), (220) and (311) reflections of Ag

Conclusion

Highly active and stable Ag nanoparticles supported on Co, N embedded reduced graphene oxide are synthesized by one-step hydrothermal route for ORR. Ag/Co-NGr is observed to have specific activity better than commercial Pt/C at 0.85 V vs. RHE in alkaline media. Durability test reveals superior stability of Ag/Co-NGr with only 23 mV shift in E1/2 after 5000 potential cycles. During hydrothermal treatment Ag develops into nanocrystalline morphology while cobalt coordinates with oxygen to form

Acknowledgments

This study was supported by the National Natural Science Foundation of China (grant nos. 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 150-ZH-2016), the Aeronautic Science Foundation Program of China (grant no. 2012ZF53073), the Science and Technology Innovation Fund of Western Metal Materials (grant no. XBCL-2-11) and the Doctoral Fund of Ministry of Education of China (grant no. 20136102110013).

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