Authors: Luque-Centeno, JM; Martinez-Huerta, MV; Sebastian, D; Lemes, G; Pastor, E; Lazaro, MJ

Article.
Renew. Energy. vol: 125. page: 0960-1481.
Date: SEP. 2018.
Doi: 10.1016/j.renene.2018.02.073.

Abstract:
Developing efficient, durable, and low cost catalysts based on earth-abundant elements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. We report herein a simple one-pot procedure for the synthesis of non-precious metals N-doped graphene composites employing urea as nitrogen source, and their application as bifunctional catalysts for both the ORR and OER in alkaline environment. In this study, the effects of the addition of Ti and Co on the structure and performance of the N-doped graphene composites are investigated. The incorporation of Ti leads to a composite with both anatase and rutile TiO2 crystalline phases as well as Ti3+ species stabilized upon hybridization with N-doped reduced graphene oxide. The ORR onset potential for the Ti-based composite is 0.85 V (vs. RHE) and the number of electrons transferred is 3.5, showing superior stability than Pt/C after accelerated potential cycling in alkaline solution. However, this composite shows low activity and stability for the OER. On the other hand, the composite consisting of metallic Co and Co3O4 nanocrystals grown on N-doped reduced graphene oxide exhibits the best performance as bifunctional catalyst, with ORR and OER onset potentials of 0.95 V and 1.51 V (vs. RHE), respectively, and a number of electrons transferred of 3.6 (ORR). The results reveal the presence of important structural features such as metallic Co as the predominant crystalline component, amorphous Co3O4 phase and the coordination of Co-N-doped graphene. All of them seem to be fundamental for the high activity and stability towards ORR and OER. (C) 2018 Elsevier Ltd. All rights reserved..