Defective Nitrogen-Doped Graphene Foam: A Non-Precious Electrocatalyst for the Oxygen Reduction Reaction in Alkaline Medium

Recently, graphene has received attention due to its large surface area, conductivity, and strength, and wide electrochemical potential window. We have developed a method to synthesize graphene foam (GF) with large surface area (>1500 m²/g), at extremely low cost [3].  Due to the unique large surface area, and high conductivity, this material is ideal for electrochemistry. For example we obtained high mass activity for oxygen reduction on platinum-decorated GF in acid medium. [4]

               Nitrogen-doped graphene has received attention as a cathode catalyst in AAEMFCs. [5] We have already developed nitrogen-doped graphene foam (GF_N) for use as a cathode catalyst in PEMFCs [6]. Here we apply these GF_Ncatalysts in alkaline media, and measure durability under potential cycling, compared with 20 wt% Pt-decorated carbon black (Pt/CB).

               GF_N was synthesized by combustion of nitrogen-containing sodium alkoxide, followed by washing, 1000^oC heat treatment in N₂ and H₂, and graphitization at 1400^oC. This is a 3D carbon with micron-scale pores encapsulated by thin defective graphene walls with a thickness of around 2 nm, with a surface area of > 700 m²/g and a nitrogen content of around 0.5 at.% (Fig. 1). The material was confirmed to be Fe-free by ICP analysis.

                Working electrodes were prepared according to Fuel Cell Commercialization Conference of Japan (FCCJ) guidelines [7]. GF_N was dispersed in deionized water, ethanol and 5 wt.% Nafion solution, in a volume ratio of 3:3:1. 8 μl ink was deposited onto a glassy carbon-disk platinum-ring electrode (0.196 cm², Hokuto Denko Corp; 580 μg_catcm^-2 GF_N / 17.3 μg_Ptcm^-2Pt/CB). We used an automatic polarization system; rotating ring-disk electrode apparatus; and a three-electrode electrochemical cell (Hokuto Denko Corp.; HZ-5000, HR-500, and HX-107). There are few rigorous durability studies published in alkaline media, therefore we propose a modified load potential cycling test, between -0.4 V and 0 V.

               Fig. 2 (a) shows LSVs of GF_N, with comparable activity to Pt/CB. The onset potential is -0.02 V for GF_N compared with 0.05 V for Pt/CB. Figure 2 (b) shows mass activity retention over 60,000 potential cycles. Pt/CB retains just 20% if it’s initial activity, however GF_N retains an astonishing 64% of its initial activity over the same number of cycles. This clearly demonstrates that GF_Nelectrocatalysts have sufficient activity and durability to be used in commercial alkaline fuel cells.

               The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.

Fig.1 (a) SEM image and (b) XPS N1s peak of GFN. (c) Linear sweep voltammograms, and (d) retention of mass activity in GFN and Pt/CB.

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4. J Liu, K Sasaki, SM Lyth, ECS Transactions 58 (1), 1751 (2013).5. M. Vikkisk, I. Kruusenberg, U. Joost, E. Shulga, I. Kink, K. Tammeveski, Applied Catalysis B: Environmental, 147, 369 (2014).

6. J. Liu, D. Takeshi, D. Orejon, K. Sasaki, S. M. Lyth, Journal of the Electrochemical Society, 161(4), F544, (2014).

7. Fuel Cell Commercialization Conference of Japan (FCCJ), Proposals of the development targets, research and development challenges and evaluation methods concerning PEFCs, http://www.fccj.jp/.