The effect of graphene as a support for non-PGM bifunctional oxygen catalyst in rechargeable metal/air batteries

Miguel A. Garcia-Contreras a,b, Pooya Hosseini Benhangi a, Amin Taheri Najafabadi a,

ElÅ?d Gyenge a

a) Department of Chemical and Biological Engineering & Clean Energy Research Center, The

University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3

b) Department of Chemistry, National Institute of Nuclear Research, Carretera México-Toluca s/n, La Marquesa Ocoyoacac, Estado de México, México 52750

Graphene, as the rising star in the world of advanced materials, offers game-changing prospects for many key areas of research including energy storage and conversion, biotechnology, novel electronic devices and so forth [1]. Bifunctional oxygen cathodes which can catalyze both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) are the backbone of rechargeable metal-air batteries as well as regenerative fuel cells [2-4]. Manganese oxides have been vastly employed as a robust cost-effective multifunctional and environmental friendly electrode material in battery industry, from primary to rechargeable metal-air batteries, as well as alkaline fuel cells and capacitors [4]. The electrolytic manganese dioxide (γ-MnO2) is known as the most electrochemically active crystallographic form of MnO2 for ORR in alkaline media with Tafel slope of 40 mV dec-1 and a low overpotential of -375 mV [2, 5]. However, poor OER electrocatalytic activity of MnOx in alkaline media diminish the hope of finding an exclusive bi- functional catalyst for both ORR and OER [2]. This could be avoided by adding another class of catalyst active for OER to the MnO2. LaCoO3 , also known as a perovskite-type oxide, has been reported to demonstrate distinguished electrocatalytic activity for OER in alkaline electrolytes in comparison to Pt or MnOx, i.e. up to 1 mA cm-2 at 1600 mV (vs. RHE) as well as a Tafel slope of
60 mV decade-1 with excellent durability over 100 cycles in 0.1 M KOH [2, 6]. A thorough
comparison with literature regarding other bifunctional electrodes shows that the MnO2-perovskite catalytic system is very competitive with other catalysts presented in the literature, while having the advantage of cost efficiency and easy preparation methodology [2].
Our research is aimed at investigating the effect support on the performance of MnO2-based catalysts for both ORR and OER with long cycle life. The mechanisms for OER and ORR of the mixed catalysts and the role of support are investigated by a combination of surface characterization methods and electrochemical techniques.
In the present work, LaCoO3 was synthesized via co-precipitation methods. While different methods for graphene synthesis has been proposed in the literature, a novel approach toward electrochemical exfoliation of graphite assisted by ionic liquids (ILs) was used to synthesize both graphene and N-doped graphene. This approach provides three main features: cost efficiency due to low IL content, extended electrochemical stability in a non-aqueous electrolyte, and high exfoliation yield by effective anionic intercalation within the graphitic layers by IL/acetonitrile
electrolyte with dramatically lower loads of ionic liquids (â?¼1:50 IL/acetonitrile vol. ratio) [1]. The
catalyst layer consisted of MnO2:LaCoO3:Support:Nafion with a weight ratio of 1:1:1:0.67. The
support varies from the electrochemically synthesized graphene and N-doped graphene to graphitized carbon. Cyclic voltammetry tests were performed in O2 and N2 saturated 6 M KOH at
293 K to investigate the electrocatalytic activity of the MnO2-LaCoO3 catalyst for both OER and ORR on different supports. The longer-term durability of the electrodes was also investigated by performing 100 repeated OER-ORR voltammetric cycles. Combination of Raman and X-ray photoelectron spectroscopy (XPS) as well as SEM analysis were employed to further characterize the catalyst layer.

References:

[1] A.T. Najafabadi, E. Gyenge, Carbon, 71 (2014) 58-69.
[2] P.H. Benhangi, A. Alfantazi, E. Gyenge, Electrochimica Acta, 123 (2014) 42-50. [3] J. Ludwig, J. Power Sources, 155 (2006) 23-32.
[4] A. Serov, A. Aziznia, P.H. Benhangi, K. Artyushkova, P. Atanassov, E. Gyenge, Journal of
Materials Chemistry A, (2013).
[5] E.L. Gyenge, J.-F. Drillet, J. Electrochem. Soc., 159 (2012) F23-F34.
[6] M. Risch, A. Grimaud, K.J. May, K.A. Stoerzinger, T.J. Chen, A.N. Mansour, Y. Shao-Horn, The Journal of Physical Chemistry C, 117 (2013) 8628-8635.