Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
pubs.acs.org/cm
Silk-Derived Highly Active Oxygen Electrocatalysts for Flexible and Rechargeable Zn−Air Batteries Chunya Wang,†,‡,⊥ Nan-Hong Xie,†,⊥ Yelong Zhang,§,∥,⊥ Zhenghong Huang,† Kailun Xia,†,‡ Huimin Wang,†,‡ Shaojun Guo,*,§,∥ Bo-Qing Xu,*,† and Yingying Zhang*,†,‡ †
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Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, and ‡Center for Nano and Micro Mechanics, School of Aerospace Engineering, Tsinghua University, Beijing 100084, PR China § Department of Materials Science and Engineering and ∥Beijing Innovation Center for Engineering Science and Advanced Technology, College of Engineering, Peking University, Beijing 100871, PR China S Supporting Information *
ABSTRACT: Flexible and rechargeable Zn−air batteries, because of their high energy density, low cost, and environmental and human benignity, are one kind of the most attractive energy systems for future wearable electronics. The development of high-performance rechargeable Zn−air batteries depends on the synthesis of highly efficient and highly stable electrocatalysts for the oxygen reduction reaction/oxygen evolution reaction (ORR/OER). Herein, a silk-derived defect-rich and nitrogen-doped nanocarbon electrocatalyst [SilkNC/Ketjenblack (KB)] is reported. The SilkNC/KB is synthesized by pyrolyzing commercially available porous KB carbon impregnated with silk fibroin. It exhibits remarkable electrocatalytic activities and long-term stability for the 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, enabling its promising applications in wearable and energy-efficient batteries.
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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. Nonprecious metal catalysts, such as transition metal nanoparticles/oxides hybrid oxides,4−6 metal−nitrogen codoped carbon materials,7−10 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 © XXXX American Chemical Society
activity and stability. In the nonmetal catalytic system, heteroatom-doping or intrinsic edge and topological defects have been demonstrated as the activity origins for catalyzing the oxygen reaction.21,22 It is desirable if metal-free nanocarbon electrocatalysts with synergistic effect of heteroatomdoping, 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 attention 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 a simple thermal treatment.26 Carbonized silk materials have been investigated as wearable sensors,27−29 supercapacitors,30,31 sodium ion batteries,32 and electrocatalysts for the 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, Received: October 30, 2018 Revised: January 2, 2019 Published: January 3, 2019 A
DOI: 10.1021/acs.chemmater.8b04572 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials natural silk fibers can be processed into SF solution and regenerated into various formats. Particularly, regenerated SF films from SF solution possess a 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 a large surface area and abundant active sites for boosting electrocatalytic activities for the ORR/OER to achieve high-performance rechargeable and flexible Zn−air batteries. The nanoporous defect-rich and N-doped nanocarbon structure was made through pyrolyzing commercially available porous Ketjenblack (KB) carbon impregnated with SF solution. The combination of regenerated SF-derived defect-rich and N-doped carbon nanosheets with highly porous KB enables remarkable ORR catalytic performance by showing the onset potential of 0.95 V for the 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 (CV) cycles, as well as good catalytic activity toward the OER. On the basis of the superior electrocatalytic properties of the silk-derived defect-rich and Ndoped 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 that this work may open an avenue for the synthesis of large-scale, cost-effective, and high-efficient metal-free ORR/OER electrocatalysts toward the development of next generation energy storage and conversion systems.
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Electrochemical Evaluation. All electrochemical measurements were performed on a potentiostat/galvanostat model 263A equipped with a rotating disk electrode (RDE) system (PINE Research Instruments). ORR and OER performances of the electrocatalysts were evaluated in a three-electrode cell at room temperature, where an electrocatalyst-coated glassy carbon electrode, a Pt electrode, and a Ag/AgCl (3.5 M KCl) electrode were used as the working electrode, counter electrode, and reference electrode, respectively. The working electrode was fabricated as follows: 3 mg SilkNC/KB was dispersed in 450 μL deionized water and 50 μL Nafion solution (5.0 wt %) through ultrasonic treatment for 30 min, forming a homogeneous suspension. Then, 20 μL of the suspension was transferred onto a mechanically polished and ultrasonically washed rotating glassycarbon disk electrode (diameter of 5 mm) and baked for 10 min to get the final working electrode. CV and linear sweep voltammetry (LSV) were performed in O2 saturated or N2-saturated 0.10 M KOH solution with the scan rate of 100.0 mV s−1 for CV and 10.0 mV s−1 for LSV measurements, respectively. The accelerated durability test (ADT) was performed in O2-saturated 0.1 M KOH solution at the potential range of 0.6−1.0 V with a scan rate of 50 mV s−1. Liquid Zn−Air Battery Assembly. The liquid Zn−air battery was assembled with catalyst-coated carbon cloth as the air cathode, a polished Zn plate as the anode, and 6.0 M KOH containing 0.2 M zinc acetate aqueous solution as the electrolyte. The air cathode was prepared by drop-casting SilkNC/KB ink onto carbon cloth with the catalyst loading of 1 mg cm−2 and drying at 60 °C in vacuum overnight. For the assembly of a control battery, a commercial catalyst (20 wt % Pt/C: IrO2 = 1:1, mass ratio) was used with the same loading amount. The discharge/charge tests were conducted using an automatic battery testing system (LANHE CT2001A). The specific capacity and energy density were calculated from the galvanostatic discharge and normalized to the mass of consumed zinc. Flexible All-Solid-State Zn−Air Battery Assembly. The flexible all-solid-state Zn−air battery was assembled with the SilkNC/KB-coated carbon cloth as the air cathode, a polished Zn foil (0.10 mm thickness) as the anode, and the gel polymer electrolyte as the electrolyte. The air electrode was made by dropping a certain volume of SilkNC/KB ink onto a clean carbon cloth substrate with a catalyst loading of 2 mg cm−2. Note that a piece of pressed Ni foam was used as the current collector next to the air electrode. The gel polymer electrolyte was prepared as follows: 15 g of polyvinyl alcohol (PVA) powder was dissolved in 5.0 mL of deionized water at 95 °C under stirring for 3 h. Then, 1.0 mL of 7.0 M KOH filled with 0.10 M zinc acetate was added to the PVA solutions, followed by stirring at 95 °C to form a homogeneous mixture. Then, the solution was poured onto a glass plate to form a thin film, which was frozen at −3 °C over 8 h, and thawed at room temperature.
EXPERIMENTAL SECTION
Synthesis of Silk-Derived N-Doped Porous Carbon. 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 °C 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 (w/w) in this study; T indicates the pyrolysis temperature and T was set as 900, 1050, and 1100 °C), the obtained SF/KB composites were thermally treated under a mixed atmosphere of argon (100 sccm) and hydrogen (
