Design of Carbon-based Oxygen Reduction Reaction Electrode for Fuel Cell

As a new non-noble metal catalyst for oxygen reduction reactions (ORRs) in acidic media, N-doped carbon has attracted much attention due to its low price and high stability in acidic media compared with commercial noble metal catalysts (e.g., Pt or Pd). However, alternatives of noble metal catalysts for N-doped carbon in commercial markets have limitations due to the relatively low ORR activity in acidic media. Therefore, various state-of-the-art techniques have been developed recently in order to modify the carbon-based catalysts. Herein, various approach to advance the nature of carbon-based materials as ORR catalysts are introduced. The first strategy is heteroatom-doping method. Nitrogen, phosphorous and boron are simultaneously doped into carbon with heat-treatment. B, P, N-doped carbon exhibits efficiently enhanced ORR performance.[1] The second strategy is modification of morphology. N-doped graphene-CNT self-assembly (NGCA) catalyst shows 2.13 mA mg-1 of ORR activity at 0.75 V in acid media, which is more than six-fold higher than that of NGr. In addition to high activity, NGCA displays high stability in acid media with an enhanced 4-electron pathway in ORRs.[2] Thirdly, graphene-derived catalysts are developed for application in oxygen reduction reactions (ORRs) in acidic media. Among the carbon-based materials, graphene, which is a two-dimensional layer structure of sp2-hybridized carbon, has been highlighted recently as a promising material for energy conversion. Heat-treatment and addition of the metal and DCDA caused a restacking of the graphene layers and the catalyst exhibited a high ORR activity in acidic media:~0.9 V (vs. RHE) onset potentials and 1.28 mA mg-1 mass activity at 0.75 V (vs. RHE) with low H2O2 formation.[3] Finally, as a fundamental study related to graphene, we synthesized size-reduced graphene which elevates the electrode potential, and thereby the energy of electron, which directly promotes the electron transfer rate even from the basal plane. Through the physical and electrochemical characterization, we demonstrate that the initial electron transfer occurs mainly into O2 molecules in the outer Helmholtz plane without necessarily involving O2 adsorption onto the graphene surface.[4]References

1. C. H. Choi, S. H. Park and S. I. Woo, ACS Nano, 6 (2012) 7084
2. C. H. Choi, M. W. Chung, H. C. Kwon, J. H. Chung and S. I. Woo, Appl. Catal. B: Environ., 144 (2013) 760
3. C. H. Choi, M. W. Chung, S. H. Park and S. I. Woo, RSC Advances, 3 (2013) 4246
4. C. H. Choi, H. K. Lim, M. W. Chung, J. C. Park, H. Shin, H. Kim and S.I. Woo, Nat. Chem., submitted.