Positive Effect of Induced Hydrophobicity in 3D N-Doped Porous Graphene Towards ORR Activity Under Acidic Condition
Authors: Singh Santosh , Takeyasu Kotaro , Furukawa Moeko , Nakamura Junji 
Published: February 2020

The less activity and durability of non-noble transition metal based ORR catalysts are bringing the metal-free catalysts derived from carbon allotropes on high demand. The pristine graphene is an electroneutral material and inactive towards the catalytic ORR reaction however the alteration of its electronic property with heteroatom doping makes it active towards electrocatalysis.[
1
,
2
] In addition, nitrogen doping is reported to be the most effective way to enhance the catalytic properties of nitrogen-doped graphene (NGr) which creates ORR active center at a nearby carbon atom. Previously reported experimental and theoretical results prove that the N-doped graphene with 3D morphology and doped pyridinic-N are the primary reason for the higher activity of N-doped graphene.[
1
,
3
] Taking into account the importance of nitrogen doping in graphene, here we have prepared the 3D N-doped reduced graphene oxide by using NaCl crystals as a structure directing agent. In addition, the fundamental understanding of the ORR at the catalyst surface is a major confront which is hindering the improvement of catalyst performance under operating conditions of fuel cell.[4] Moreover, the catalyst surface wettability is considered to be the most significant factor, which directly reflects from the O2 storage and activation by the active biological molecules such as hemoglobin, laccase, etc.

Here, we introduced the role of catalysts surface hydrophobicity created by induced defects towards the ORR reaction in acidic medium. We control the hydrophobicity of the catalyst surface by using NaCl crystals as a spacer and defect creator during the preparation of 3D N-doped porous graphene. The physical and electrochemical characterizations of the prepared catalysts revealed the role of surface hydrophobicity towards the triggering of ORR reaction at low overpotential by capturing O2 near to the reaction centers. The catalyst with underwater Wenzel-Cassie (UWC) coexistent state of surface hydrophobicity has started ORR at lower overpotential (onset potential 0.85 VRHE) than the catalyst with underwater Wenzel (UW) state (0.57 VRHE) in acidic medium. The adopted method for the synthesis of 3D porous N-doped graphene catalyst involves NaCl crystals acts as a structure directing agent to get 3D NrGO morphology (Fig. 1a). The used NaCl crystals also help in the introduction of localized defects in the graphene framework during the synthesis process. The prepared catalyst NrGO/NaCl shows a low overpotential and high current density compared to the NrGO, mainly due to the induced surface hydrophobicity involved in O2 storage, high concentration of doped pyridinic-N and the 3D graphene morphology. Fig. 1b shows the comparative Tafel analysis of NrGO and NrGO-NaCl samples displaying comparable ORR activity of the sample prepared by using NaCl as a template with the state-of-the-art (Pt/C) catalyst. Furthermore, we are now trying to demonstrate the effect of catalyst surface hydrophobicity of metal-free N-doped graphene in the real time PEMFCs application. It is believed that the induced catalyst surface hydrophobicity will be involved in prevention of water flooding problem faced in the PEMFCs operation.

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[2] Z. Hou, X. Wang, T. Ikeda, K. Terakura, M. Oshima, M.-a. Kakimoto, Physical Review B
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[3] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science
2016, 351, 361.

[4] S. K. Singh, K. Takeyasu, J. Nakamura, Advanced Materials, 2018, 1804297, DOI: 10.1002/adma.201804297.

Figure 1