Jan 3, 2019 - Silk Derived Highly Active Oxygen Electrocatalysts for Flexible .... Based on the superior electrocatalytic properties of the silk-derived defect-.
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Silk Derived Highly Active Oxygen Electrocatalysts for Flexible and Rechargeable Zn-Air Battery Chunya Wang, Nan-Hong Xie, Yelong Zhang, Zhenghong Huang, Kailun Xia, Huimin Wang, Shaojun Guo, Bo-Qing Xu, and Yingying Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04572 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019
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Chemistry of Materials
Silk Derived Highly Active Oxygen Electrocatalysts for Flexible and Rechargeable Zn-Air Battery Chunya Wang1,2‡, Nan-Hong Xie1‡, Yelong Zhang3,4‡, Zhenghong Huang1, Kailun Xia1,2, Huimin Wang1,2, Shaojun Guo3,4*, Bo-Qing Xu1* and Yingying Zhang1,2* 1 Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China 2 Center for Nano and Micro Mechanics, School of Aerospace Engineering, Tsinghua University, Beijing 100084, PR China 3 Department of Materials Science and Engineering, Peking University, Beijing 100871, PR China 4 Beijing Innovation Center for Engineering Science and Advanced Technology, College of Engineering, Peking University, Beijing 100871, PR China ABSTRACT: Flexible and rechargeable Zn-air batteries, due to their high energy density, low cost, as well as environmental and human benignity, are one kind of the most attractive energy systems for future wearable electronics. The development of highperformance rechargeable Zn-air batteries depends on the synthesis of high efficient and high stable electrocatalysts for oxygen reduction reaction/oxygen evolution reaction (ORR/OER). Herein, a silk-derived defect-rich and nitrogen-doped nanocarbon electrocatalyst (SilkNC/KB) is reported. The SilkNC/KB is synthesized by pyrolyzing commercially available porous Ketjenblack carbon impregnated with silk fibroin. It exhibits remarkable electrocatalytic activities and long-term stability for ORR/OER, enabling its applications in high-performance liquid and solid rechargeable Zn-air batteries. Particularly, the all-solid-state Zn-air battery based on SilkNC/KB exhibits good flexibility and remarkable charge/discharge stability, promising its applications as wearable and energyefficient batteries.
INTRODUCTION Flexible and wearable electronics have been explosively developed during the past decade, and been innovating the way of our daily life.1 Conventional power-supply systems are rigid and not suitable for wearable electronics, which raises the great demand for the development of flexible energy systems. Flexible Zn-air batteries, as one of the most attractive flexible energy systems, have attracted much attention for future wearable electronics because of their high energy density, low cost, resource-abundant, as well as environmental and human benignity.2 However, the performance (such as power density, energy efficiency and cycling stability ) of flexible Zn-air batteries, which is mainly hampered by the kinetically sluggish rate of the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) at the air cathode, needs to be improved for practical applications.3 It is significant to develop highly active bifunctional ORR/OER electrocatalysts with good stability. Non-precious metal catalysts such as transition metal nanoparticles/oxides hybrid oxides,4-6 metal-nitrogen co-doped carbon materials7-10 and metal-free heteroatom-doped carbon materials (heteroatom = N, P, B, S, O, etc.), 10-20 have been developed as promising alternatives of high-cost and scarce noble metal electrocatalysts (such as Pt, Ir, and Ru) because of their comparable or even superior electrocatalytic activity and stability. In the non-metal catalytic system, heteroatom-doping or intrinsic edge and topological defects have been demonstrated as the activity origins for catalyzing oxygen reaction.21, 22 It is desirable if metal-free nanocarbon electrocatalysts with synergistic effect of heteroatom-doping, edge sites, and topological defects can be prepared through a
cost-effective and scalable strategy. However, achieving such target remains a great challenge. Natural biomaterial-derived nanocarbon has received wide attentions owing to their intrinsically heteroatom-doped feature, low cost, large scalability and environmental benignity.23-25 Silk, especially silk fibroin (SF), as a kind of well-known and abundant natural protein biomaterials, can be transformed into electrically conductive N-doped graphitic carbon through simple thermal treatment.26 Carbonized silk materials have been investigated as wearable sensors,27-29 supercapacitors,30, 31 sodium ion batteries32 and electrocatalysts for ORR.23, 33 However, all the reported natural silk fiber-derived carbon material-based ORR electrocatalysts generally showed low specific surface area, and unfortunately inferior ORR performance due to their inadequate active sites. Actually, natural silk fibers can be processed into SF solution and regenerated into various formats. Particularly, regenerated SF films from SF solution possess lamellar-like nanometer-thick layer structure induced by the self-assembly of hydrophobic and hydrophilic blocks of SF,30 which may have great potential in additionally producing abundant edge defects for boosting the performance of ORR/OER-driven rechargeable and flexible Zn-air batteries. Herein, we report a silk-derived defect-rich and N-doped carbon with large surface area and abundant active sites for boosting electrocatalytic activities for ORR/OER to achieve highperformance rechargeable and flexible Zn-air batteries. The nanoporous defect-rich and N-doped nanocarbon structure was made through pyrolyzing commercially available porous Ketjenblack carbon (KB) impregnated with SF solution. The
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combination of regenerated SF-derived defect-rich and Ndoped carbon nanosheets with highly porous KB enables remarkable ORR catalytic performance by showing the onset potential of 0.95 V for ORR and high long-term stability with only 30 mV loss in its half-wave potential and 10% loss in its limiting current density after 50,000 cyclic voltammetry cycles, as well as good catalytic activity toward OER. Based on the superior electrocatalytic properties of the silk-derived defectrich and N-doped carbon, we further assembled rechargeable Zn-air batteries, including liquid batteries and flexible solid batteries, which showed good energy storage performance and charge-discharge cycling durability. We hope this work may open an avenue for the synthesis of large-scale, cost-effective and high-efficient metal-free ORR/OER electrocatalysts towards the development of next generation energy storage and conversion systems. EXPERIMENTAL SECTION Synthesis of Silk-Derived N-Doped Porous Carbon Silk fibroin (SF), which was extracted from Bombyx mori silkworm cocoons as reported,34 was dissolved in formic acid to obtain SF solution. A certain amount of KB was added into the SF solution, stirred for 3 h and then dried at 80 oC to obtain the SF/KB composites. To obtain the silk-derived N-doped porous carbon (named as SilkNC/KB (x, T), where x indicates the initial SF/KB weight ratio and x is set as 0.5, 1.0, and 2.0 (wt/wt) in this study; T indicates the pyrolysis temperature and T was set as 900 oC, 1050 oC, and 1100 oC), the obtained SF/KB composites were thermally treated under a mixed atmosphere of argon (100 sccm) and hydrogen (
