Nitrogen Doped Short-Length Carbon Nanofiber Supported Cobalt Oxides for Oxygen Reduction Reaction and Evolution Reaction Catalysts

High demand of energy, environmental problems, and energy crisis have brought great attentions in renewable and environment-friendly energy resources(1). Among various candidate energy sources, electrochemistry based energy storage and harvesting devices such as rechargeable batteries, supercapacitors, utilized regenerative fuel cells(URFCs), and solar cells have received great interests (1, 2). Particularly, URFCs which include fuel cells and water electrolyzers are getting a big spotlight in the renewable energy fields due to their high efficiency and clean properties(3, 4). The key technology for URFCs is to develop the cost-effective, durable, and high catalytic active catalysts for oxygen reduction(ORR) and evolution reactions(OER) in order to decrease the overpotential and improve efficiency of cells (5). Ideal bifunctional ORR and OER catalysts are Pt, RuO, and IrO for URFCs(6-8). However, precious metal-based catalysts are inadequate in terms of price competitiveness and limited durability to commercialize the URFCs(9). Recently, modification of cobalt oxides nanostructures with various dopants have reported to reduce the overpotential with high OER and ORR performances (10). Furthermore, nitrogen-doped graphene oxide is used to further enhance the electronic conductivity with durable mechanical property, because nitrogen sites can function as electrochemical active sites(11). In this work, several short-length N-doped carbon nanofibers(N-CNF) are introduced to maximize electrochemical active sites of catalysts. The shortened N-CNF catalysts are synthesized by the hydrothermal method. The physicochemical properties of the N-CNF-supported cobalt oxides nanostructures are investigated by various tools such as X-ray diffraction, scanning electron microscope, Brunauer Emmett Teller, and transmission electron microscope. To analyze electrochemical activities, rotating ring disk electrode system is used with an Hg/HgO, a platinum wire, and a 0.1M KOH solution for reference, counter, and electrolyte respectively. The electrochemical characteristics are measured by linear sweep voltammetry at a scan rate of 5 mVs -1 for OER (1.2V–1.7V) and ORR (0.2V–1.2V). To analyze the long-term stability for OER and ORR, the accelerated degradation tests are performed at a scan rate of 200 mVs -1 . References Wang H-F, Tang C, Zhang Q. Template growth of nitrogen-doped mesoporous graphene on metal oxides and its use as a metal-free bifunctional electrocatalyst for oxygen reduction and evolution reactions. Catalysis Today. 2017. Kushwaha HS, Halder A, Thomas P, Vaish R. CaCu3Ti4O12: A Bifunctional Perovskite Electrocatalyst for Oxygen Evolution and Reduction Reaction in Alkaline Medium. Electrochimica Acta. 2017;252(Supplement C):532-40. Bau VM, Bo XJ, Guo LP. Nitrogen-doped cobalt nanoparticles/nitrogen-doped plate-like ordered mesoporous carbons composites as noble-metal free electrocatalysts for oxygen reduction reaction. J Energy Chem. 2017;26(1):63-71. Zhang X, Ding P, Sun Y, Li X, Li H, Guo J. CoMoS3.13 nanosheets grafted on B, N co-doped graphene nanotubes as bifunctional catalyst for efficient water electrolysis. Journal of Alloys and Compounds. 2018;731(Supplement C):403-10. Yang J, Fujigaya T, Nakashima N. Decorating unoxidized-carbon nanotubes with homogeneous Ni-Co spinel nanocrystals show superior performance for oxygen evolution/reduction reactions. Scientific Reports. 2017;7. Song W, Ren Z, Chen SY, Meng Y, Biswas S, Nandi P, et al. Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction. ACS Applied Materials and Interfaces. 2016;8(32):20802-13. Elumeeva K, Masa J, Tietz F, Yang F, Xia W, Muhler M, et al. A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts. ChemElectroChem. 2016;3(1):138-43. Lee DU, Park MG, Park HW, Seo MH, Ismayilov V, Ahmed R, et al. Highly active Co-doped LaMnO3 perovskite oxide and N-doped carbon nanotube hybrid bi-functional catalyst for rechargeable zinc-air batteries. Electrochem Commun. 2015;60:38-41. Zhao BT, Zhang L, Zhen DX, Yoo S, Ding Y, Chen DC, et al. A tailored double perovskite nanofiber catalyst enables ultrafast oxygen evolution. Nat Commun. 2017;8. So IS, Kim NI, Cho SH, Kim YR, Yoo J, Seo Y, et al. Enhancement of Bifunctional Activity of the Hybrid Catalyst of Hollow-Net Structure Co3O4 and Carbon Nanotubes. J Electrochem Soc. 2016;163(11):F3041-F50. Kim NI, Afzal RA, Choi SR, Lee SW, Ahn D, Bhattacharjee S, et al. Highly active and durable nitrogen doped-reduced graphene oxide/double perovskite bifunctional hybrid catalysts. Journal of Materials Chemistry A. 2017;5(25):13019-31.