ISSN 1998-0124 CN 11-5974/O4 2019, 12(5): 1093–1098 https://doi.org/10.1007/s12274-019-2352-5 Research Article Mn 3 O 4 nanoparticles@reduced graphene oxide composite: An efficient electrocatalyst for artificial N 2 fixation to NH 3 at ambient conditions Hong Huang 1,§ , Feng Gong 3,§ , Yuan Wang 1 , Huanbo Wang 2 , Xiufeng Wu 1,4 , Wenbo Lu 4 , Runbo Zhao 1 , Hongyu Chen 1 , Xifeng Shi 5 , Abdullah M. Asiri 6 , Tingshuai Li 3 , Qian Liu 3 , and Xuping Sun 1 (  ) 1 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China 2 School of Environment and Resource, Southwest University of Science and Technology, Mianyang 621010, China 3 School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, China 4 Key Laboratory of Magnetic Molecules and Magnetic Information Materials (Ministry of Education), School of Chemistry and Material Science, Shanxi Normal University, Linfen 041004, China 5 College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China 6 Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia § Hong Huang and Feng Gong contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 3 January 2019 / Revised: 1 February 2019 / Accepted: 18 February 2019 ABSTRACT Currently, industrial-scale NH 3 production almost relies on energy-intensive Haber-Bosch process from atmospheric N 2 with large amount of CO 2 emission, while low-cost and high-efficient catalysts are demanded for the N 2 reduction reaction (NRR). In this study, Mn 3 O 4 nanoparticles@reduced graphene oxide (Mn 3 O 4 @rGO) composite is reported as an efficient NRR electrocatalyst with excellent selectivity for NH 3 formation. In 0.1 M Na 2 SO 4 solution, such catalyst obtains a NH 3 yield of 17.4 µg·h −1 ·mg −1 cat. and a Faradaic efficiency of 3.52% at −0.85 V vs. reversible hydrogen electrode. Notably, it also shows high electrochemical stability during electrolysis process. Density functional theory (DFT) calculations also demonstrate that the (112) planes of Mn 3 O 4 possess superior NRR activity. KEYWORDS Mn 3 O 4 @rGO composite, electrocatalyst, NH 3 synthesis, N 2 reduction reaction, ambient conditions 1 Introduction NH3 is a significant substance to produce agricultural fertilizers, plastic and pharmaceuticals, and it is also a fascinating carbon-free energy carrier with high energy density [1–4]. Although N2 is the most abundant molecule in the atmosphere, the thermodynamic stability and strong N≡N triple bond (941 kJ·mol −1 ) make it chemically inert, and the N2 reduction into NH3 is an extremely difficult process. In Haber-Bosch process, this kinetic limitation is overcome by using high temperature and elevated pressure with large amount of CO2 emission [5]. So, it is imperative to develop an environmentally friendly process for sustainable N2 fixation. Electrochemical NH3 synthesis is a promising candidate for artificial N2 fixation at ambient conditions due to its environmental-friendly, convenient, and low-cost characteristics [6–8]. However, efficient catalysts are still needed for N2 reduction reaction (NRR). Noble- metal catalysts exhibit favorable NRR activity [9–11], but the high cost restricts their widespread use. It is thus highly desired to design and develop non-precious metal alternatives [12–23]. As an earth-abundant transition metal oxide, Mn3O4 shows many merits, such as low cost, natural abundance, and environmental friendliness. Our recent work suggests Mn3O4 nanocube is an efficient catalyst for NRR [24]. Unfortunately, Mn3O4 has poor electrical conductivity (10 −7 –10 −8 S∙cm –1 ) [25, 26]. Reduced graphene oxide (rGO) is deemed to be an attractive carbon material due to its good electrochemical stability, high surface area, and superior mechanical as well as electronic conductivity [27–30]. Study indicates that rGO anchored Mn3O4 composite exhibits improved conductivity and large active surface area [31], providing us a possible catalyst for electrochemical N2 reduction, which, however, has not been explored before. In this study, for the first time, we present our recent finding that Mn3O4 nanoparticles@rGO (Mn3O4@rGO) composite acts as an efficient catalyst for electrochemical NRR (Scheme 1). In 0.1 M Na2SO4, such Mn3O4@rGO catalyst exhibits a Faradaic efficiency (FE) of 3.52% and a NH3 yield of 17.4 μg h −1 ·mg −1 cat. at −0.85 V vs. reversible hydrogen electrode (RHE) with excellent selectivity. Moreover, it also shows high electrochemical stability for electrolysis process. Density functional theory (DFT) calculations suggest the superior NRR activity on the (112) planes of Mn3O4. Scheme 1 A schematic diagram to illustrate the preparation process of Mn3O4@rGO. Address correspondence to xpsun@uestc.edu.cn