1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 DOI: 10.1002/elan.201800089 Electrocatalytic Oxygen Reduction by Dopant-Free, Porous Graphene Aerogel Woojun Choi + , [a] Uday Pratap Azad + , [a] Jai-Pil Choi,* [a, b] and Dongil Lee* [a] Abstract: Porous graphene aerogel (GA) was prepared by the hydrothermal reduction of graphene oxide (GO). The prepared GA was characterized with the various analytical techniques. Its morphological and electrochemical proper- ties were investigated and compared with those of GO and electrochemically reduced graphene oxide (erGO). Three glassy carbon (GC) electrodes modified with GO, erGO, and GA were employed to study the electro- catalytic oxygen reduction reaction (ORR). The influence of GA porosity on the kinetic parameters of the ORR was also investigated. Whereas two-electron transfer ORR was observed at the erGO- and GO-modified electrodes, the GA-modified electrode exhibited drastically enhanced electrocatalytic ORR activity. The remarkable electro- catalytic activity of GA can be attributed to the unique porous effect of GA. The porous GA offers confined spaces for ORR that would drastically increase the residence time of oxygen and thus enhance the contact frequency for the reduction of oxygen. Additional rotating disk electrode voltammetry corroborates that the porous GA facilitates efficient ORR. Keywords: Electrocatalysis · Graphene Aerogel · Oxygen Reduction Reaction · Porosity, Kinetic Parameters 1 Introduction Over the last decade, graphene and its derivatives have been extensively used in many applications: fabrication of energy storage materials, sensor, fuel cells etc. [1]. In addition, porous carbon materials, such as ordered meso- porous carbon, activated carbon, carbon nanotubes and graphene aerogels (GAs), have been also exploited for the same purpose [2]. Of them, the three-dimensional (3D) GA is one of the most prevailing materials, which have the macroscopic architectures of graphene with various extraordinary properties, such as very low density, high roughness, large porosity, high surface area, con- ductivity and excellent mechanical strength [3]. A wide variety of efforts have been made on the assembly of graphene into 3D foams or gels for practical applications. GAs have great potential in many fields due to the combined respective characteristics of the graphene sheets and aerogels. For example, Shi and co-workers prepared GA which revealed its promising use as a chemical supercapacitors [4] and Wang and co-workers exploited metal ion-promoted assembly to obtain noble metal hybridized GA and used it as a catalyst [5]. Various metal nanoparticles and metal oxide doped GAs have also been used for electrocatalysis. Wu, et al. reported Fe 3 O 4 -doped GA as very efficient electrocatalyst for the oxygen reduction reaction (ORR) [6]. The cathodic ORR is an active research area because of its crucial role in electro- chemical energy conversion in fuel cells. Many papers describing the kinetics of the ORR on metal oxide, metal nanoparticles or carbon surfaces have been published [7]. However, there is a lack of information concerning the kinetics of the ORR on dopant-free porous GAs. The main objective of this work is to explore the role of chemical and morphological effect of dopant-free GA on the kinetic parameters of the ORR. In order to understand the chemical and morphological effects on the ORR, we prepared chemically and morphologically differ- ent graphene oxide (GO) derivatives, such as GA by the hydrothermal reduction of GO and electrochemically reduced graphene oxide (erGO). With the electrodes modified with GO, erGO, and GA, we clearly observed the different catalytic effects on the ORR depending on the chemical and morphological conditions of GO deriva- tives. Here, we report the influence of oxygen-containing functional groups (chemical effect) and porosity (morpho- logical effect) found in GO, erGO, and GA on the ORR in alkaline media. [a] W. Choi, + U. P. Azad, + J.-P. Choi, D. Lee Department of Chemistry, Yonsei University, Seoul 03722, Korea E-mail: dongil@yonsei.ac.kr jchoi@csufresno.edu [b] J.-P. Choi Department of Chemistry, California State University–Fresno, 2555 E. San Ramon Avenue, Fresno, CA 93740, USA [ + ] These authors contributed equally to this work. Supporting information for this article is available on the WWW under https://doi.org/10.1002/elan.201800089 Full Paper www.electroanalysis.wiley-vch.de  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2018, 30, 1 – 8 1 These are not the final page numbers! ÞÞ