1 Supporting information Seawater Battery Performance Enhancement Enabled by Defect/Edge-rich, Oxygen Self- doped Porous Carbon Electrocatalyst S. T. Senthilkumar a , Sung O Park a , Junsoo Kim a , Soo Min Hwang a , Sang Kyu Kwak a,* and Youngsik Kim a,b,* a School of Energy & Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea b Energy Materials and Devices Lab, 4TOONE Corporation, UNIST-gil 50, Ulsan 689-798, Republic of Korea E-mail: ykim@unist.ac.kr (Y. Kim); skkwak@unist.ac.kr (S.K.Kwak) S1. Experimental Preparation of porous carbon catalyst To prepare the porous carbon, a bio-waste, grapefruit (Citrus paradise) peel was used as a precursor for carbon sources. First, the grapefruit peel was cut into small species and washed with distilled water. Then, the washed grapefruit peel waste was placed into a Teflon-lined stainless steel autoclave with 80 mL of distilled water and sealed, heated at 180 °C for 24 h, and allowed to cool to room temperature. The resulting sample was washed with distilled water and dried at 100 °C overnight. After that, the hydrothermally treated sample was activated with NaOH and carbonized in a tube furnace at 800 °C for 3 h under N 2 atmosphere. Afterward, the carbonized sample was thoroughly washed with the desired amount of HCl and distilled water. Then, the washed sample was dried at 80 °C overnight. Characterizations of catalyst Powder X-ray diffraction (XRD) patterns were collected (Rigaku D/MAX 2500V/PC) using an X-ray diffractometer equipped with a Cu Kα (at 40 KV and 200 mA) radiation over the 2θ range of 10-80°. The Raman spectra were obtained using a WITec alpha300R couple with a He-Ne laser of 532 nm at 1.0 mW. The morphologies of the samples were observed using scanning electron microscopy (SEM, Quanta 200, FEI). The microstructure of the catalyst was performed using a transmission electron microscope, HR-TEM (JEOL, JEM 2100F) with an accelerating voltage of 200 kV. Specific surface area was calculated from the results of N 2 physisorption at 77 K (Micromeritics ASAP 2020) by using the BET (Brunauer–Emmet–Teller) equation and pore size distributions of the sample were calculated using the BJH (Barrett–Joyner–Halenda) method. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCLAB 250Xi equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Evaluation of electrocatalytic OER/ORR activity The OER/ORR activities were measured using a rotating disk electrode (RDE) and a three-electrode electrochemical cell. A Pt wire, Ag/AgCl, and glassy carbon rotating disk electrode were used as a counter, reference and working electrode. The electrolyte used in OER/ORR was natural seawater 1 . The catalyst inks in this work were prepared using the following formulation. 8 mg of catalyst (PC, 20% Pt/C, IrO, Vulcan X76 and re-heat-treated PC) was mixed in a glass vial with 0.25 ml of Nafion 5 wt% dispersion solution (Sigma-Aldrich), 0.75 ml of ethanol and 40µl of deionized water. The inks were sonicated for 1 h and then coated onto a glassy rotating disk electrode with a 3 mm diameter, and dried naturally. The working electrode was cycled at least 15 times before the data were recorded at a scan rate of 50 mV s -1 . To examine Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A. This journal is © The Royal Society of Chemistry 2017