Nano Research DOI 10.1007/s12274-015-0960-2 Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction Ruguang Ma1,2, Xiaodong Ren1,2, Bao Yu Xia3, Yao Zhou1,2, Chi Sun1,2, Qian Liu1,2 (*), Jianjun Liu1,2 (*), and Jiacheng Wang1,2 (*) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0960-2 http://www.thenanoresearch.com on November. 30, 2015 © Tsinghua University Press 2015 Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. 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Nano Res. 64 TABLE OF CONTENTS (TOC) Novel Synthesis of N-doped Porous Graphene as An Efficient Electrocatalyst toward Oxygen Reduction Reaction Ruguang Ma,1,2 Xiaodong Ren,1,2 Bao Yu Xia,3 Yao Zhou,1,2 Chi Sun,1,2 Qian Liu, *1,2 Jianjun Liu, *1,2 and Jiacheng Wang*1,2 1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, China 2 Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai, 200444, China 3 School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore Nitrogen doping into graphene is achieved via a one-pot solvothermal method under a mild reaction condition. The following annealing at 600 oC initiates a conversion of N-containing species in the graphene, i.e. from pyrrolic N to pyridinic N, thus boosting the catalytic efficiency toward ORR with a good selectivity of four-electron pathway in alkaline and acidic electrolytes, respectively. Provide the authors’ webside if possible. Author 1, webside 1 Author 2, webside 2 www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Research DOI (automatically inserted by the publisher) Research Article Please choose one Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction 1,2 1,2 3 1,2 1,2 1,2 1,2 Ruguang Ma , Xiaodong Ren , Bao Yu Xia , Yao Zhou , Chi Sun , Qian Liu (*), Jianjun Liu (*), 1,2 and Jiacheng Wang (*) 1 State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai, 200050, China 2 Shanghai Institute of Materials Genome, 99 Shangda Road, Shanghai, 200444, China 3 School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore Received: day month year ABSTRACT Revised: day month year Nitrogen-doped graphene (NG) is successfully synthesized by a novel, facile Accepted: day month year and scalable bottom-up method. The annealed NG (NG-A), possessing high (automatically inserted by specific surface and hierarchical porous texture, exhibits remarkably improved the publisher) electrocatalytic activity towards ORR in both alkaline and acidic media, respectively. Ab initio molecular dynamic (MD) simulation indicates that fast H-transfer and thermodynamic stability of six-membered N structure promote the transformation of N-containing species from pyrrolic N to pyridinic N at 600 oC. In O2-staturated 0.1 M KOH solution, the half-wave potential (E1/2) of © Tsinghua University Press NG-A is only 62 mV lower than that of commercial Pt/C catalyst and the and Springer-Verlag Berlin limiting current density of NG-A is 0.5 mA cm-2 larger than that of Pt/C. Both Heidelberg 2014 the Koutecky-Levich (K-L) plots and rotating ring-disk electrode (RRDE) measurement reveal the four-electron-transfer pathway of NG-A, which could be ascribed to the large percentage of pyridinic N with higher catalytic activity than that of pyrrolic N. KEYWORDS Nitrogen doping, graphene, molecular dynamic simulation, oxygen reduction reaction Nano Res. 2 1 Introduction improved understanding on mechanism of N-doped graphene, the mentioned synthesis Nowadays, various clean and renewable energy approaches are still not satisfactory enough, either technologies, such as fuel cells and metal-air employing complicated steps (e.g. graphite→GO→ batteries, have stepped into the spotlight in the reduced GO (rGO)) or involving in contamination arena responding to environmental issues and of transition metal catalysts (e. g. Co, Ni or Mn) energy crisis [1, 2]. However, the sluggish oxygen [21]. Moreover, the catalytically active site in reduction reaction (ORR) at the cathode, arising N-doped graphene remains a controversial topic in from the strong chemical bond of oxygen in both the literature. The role of N species, such as alkali and acidic electrolytes, notoriously hinders pyrrolic N, pyridinic N and graphitic N, needs to the widespread implementation and be further clarified in the preparation process as commercialization of these technologies [3]. well as catalysis process. Although precious metals, e.g. platinum-based Herein we report a versatile and scalable strategy electrocatalysts, possess excellent catalysis to to synthesize porous NG via a facile solvothermal overcome the high overpotential of ORR, the method followed by a mild heat treatment scarceness along with high cost is an unavoidable (denoted as NG-A), as shown in Scheme 1. In both limitation [4, 5]. As an alternative, transition alkaline and acidic environments, the as-prepared metal-based electrocatalysts, such as metal N-doped graphene demonstrates excellent oxides/sulfides, have been demonstrated promising electrocatalytic activity toward ORR, in terms of electrochemical activities for ORR, however, low half-wave potential, electron-transfer pathway, electrical conductivity and poor stability in acid kinetic current density, long-term stability, and and alkaline electrolytes are primary problems to tolerance to crossover effect. The transformation be solved [6-9]. Therefore, it is imperative to mechanism of N-containing species from pyrrolic develop efficient and cost-effective electrocatalysts N to pyridinic N is elucidated by Ab initio with good activity and durability for ORR. molecular dynamic (MD) simulation. Recently, graphene (G) has initiated an enormous interest in the field of electrochemistry as a highly promising electrode candidate because of many advantages including immense specific area, remarkable mechanical flexibility, and good electrical conductivity [10-12]. In particular, N-doped graphene (NG) sheets have shown high catalytic activity toward the reduction of oxygen, resulting from the altered charge distribution on the carbon atoms [13-15]. For example, N-doped graphene was prepared by either chemical vapor deposition (CVD) of methane in the presence of ammonia (NH3) or thermal annealing graphene oxide (GO) using melamine or polypyrrole as nitrogen source, exhibiting excellent electrocatalysis for ORR [16-18]. Furthermore, N-doped or N, B-doped graphene quantum dots were synthesized by electrochemically reducing GO, electrocatalysis of which towards ORR was Scheme 1 Schematic illustration of fabrication procedure of N-doped graphene and the oxygen reduction reaction investigated experimentally and theoretically.[19] catalysed by the annealed N-doped graphene. Very recently, freestanding 3D N-doped graphene with high catalytic efficiency has been reported by Moreover, the density functional theory (DFT) Ito et al., which was fabricated by a nanoporous calculations and experimental results indicate that Ni-based chemical vapor deposition (CVD) method the pyridinic N plays a more important role in [20]. improving catalytic efficiency than pyrrolic N and Despite the harvest of good catalytic efficiency and www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research Nano Res. 3 graphitic N. The pyridinic N not only facilitates size analyzer at liquid nitrogen temperature reductive O2 adsorption, but also eliminates H2O2 (-196 °C). Prior to measurement, the powders were formation by raising the density of p states near the dehydrated under vacuum at 120 °C overnight. The Fermi level and lowering the work function [22-24]. specific surface areas were calculated by the In addition, the hierarchically porous structure is Brunauer-Emmett-Teller (BET) method. The pore also favourable for the increase of catalytically size distribution curves were calculated based on active sites and the exchange of reactants, due to the analysis of the desorption branch of the the enhanced interactions of oxygen molecule with isotherm using the Barrett-Joyner-Halenda (BJH) catalytically active sites at a low overpotential [20]. model. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co.,