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      An Isolated Zinc–Cobalt Atomic Pair for Highly Active and Durable Oxygen Reduction

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          Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction

          Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.
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            Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction.

            The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen reduction reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of -80 millivolts and a current density of 4.1 milliamps per square centimeter at -0.22 volts, compared with -85 millivolts and 1.1 milliamps per square centimeter at -0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
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              A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions.

              The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out with noble metals (such as Pt) and metal oxides (such as RuO₂ and MnO₂) as catalysts, respectively. However, these metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor stability and detrimental environmental effects. Here, we describe a mesoporous carbon foam co-doped with nitrogen and phosphorus that has a large surface area of ∼1,663 m(2) g(-1) and good electrocatalytic properties for both ORR and OER. This material was fabricated using a scalable, one-step process involving the pyrolysis of a polyaniline aerogel synthesized in the presence of phytic acid. We then tested the suitability of this N,P-doped carbon foam as an air electrode for primary and rechargeable Zn-air batteries. Primary batteries demonstrated an open-circuit potential of 1.48 V, a specific capacity of 735 mAh gZn(-1) (corresponding to an energy density of 835 Wh kgZn(-1)), a peak power density of 55 mW cm(-2), and stable operation for 240 h after mechanical recharging. Two-electrode rechargeable batteries could be cycled stably for 180 cycles at 2 mA cm(-2). We also examine the activity of our carbon foam for both OER and ORR independently, in a three-electrode configuration, and discuss ways in which the Zn-air battery can be further improved. Finally, our density functional theory calculations reveal that the N,P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity of our material.
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                Author and article information

                Contributors
                Journal
                Angewandte Chemie
                Angew. Chem.
                Wiley
                0044-8249
                1521-3757
                February 25 2019
                February 25 2019
                : 131
                : 9
                : 2648-2652
                Affiliations
                [1 ]Key Laboratory of Applied Chemistry in Hebei ProvinceYanshan University Qinhuangdao 066004 China
                [2 ]Canadian Light Source 44 Innovation Boulevard Saskatoon SK S7N 2 V3 Canada
                [3 ]Center for Electron Microscopy and Tianjin Key Lab of Advanced Functional Porous Materials, Institute for New Energy Materials, School of MaterialsTianjin University of Technology Tianjin 300384 China
                [4 ]Institute of Sustainable Energy/College of ScienceShanghai University Shanghai 200444 P. R. China
                [5 ]Shanghai Synchrotron Radiation FacilityChinese Academy of Sciences Shanghai 201204 China
                [6 ]The Key Laboratory of Fuel Cell Technology of Guangdong ProvinceSouth China University of Technology Guangzhou 510641 China
                [7 ]Department of Mechanical and Materials EngineeringUniversity of Western Ontario London Ontario N6A 5B9 Canada
                [8 ]Key Laboratory of Advanced Technology for Materials Synthesis and ProcessingWuhan University of Technology Wuhan 430070 China
                Article
                10.1002/ange.201810175
                55b405cc-7344-462f-8f7e-38a366d6ca03
                © 2019

                http://onlinelibrary.wiley.com/termsAndConditions#vor

                http://doi.wiley.com/10.1002/tdm_license_1.1

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