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Oxygen Vacancy-Induced Structural, Optical, and Enhanced Supercapacitive Performance of Zinc Oxide Anchored Graphitic Carbon Nanofiber Hybrid Electrodes Gowra Raghupathy Dillip,† Arghya Narayan Banerjee,*,† Veettikkunnu Chandran Anitha,† Borelli Deva Prasad Raju,‡ Sang Woo Joo,*,† and Bong Ki Min§ †

School of Mechanical Engineering and §Center for Research Facilities, Yeungnam University, Gyeongsan 712 749, South Korea ‡ Department of Future Studies, Sri Venkateswara University, Tirupati 517 502, India S Supporting Information *

ABSTRACT: Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile coprecipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The nonstoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300 °C), followed by diffusion into the gaseous phase with further increase in the annealing temperature (600 °C) in an oxygen-deficient atmosphere. This induction of oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g−1, the hybrid electrode exhibited higher energy density (119.85 Wh kg−1) and power density (19.225 kW kg−1) compared to a control ZnO electrode (48.01 Wh kg−1 and 17.687 kW kg−1). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids. KEYWORDS: zinc oxide/carbon nanofiber hybrid, oxygen deficiency, bandgap narrowing, supercapacitor capacitance and abundance.7 Among the various metal oxides, capacitors based on ruthenium oxide (RuO2) show remarkably high specific capacitance and power.8 However, because of high toxicity and cost, the use of RuO2-based capacitors in practical large-scale production is limited.9 Therefore, much effort has been devoted to identifying inexpensive and low-toxicity metal oxide electrode materials with reasonable electrochemical properties as alternatives to RuO2.10 In practice, the most important characteristics required for using a metal oxide as a capacitor electrode are pseudocapacitive behavior, large surface area, high conductivity, and high electrochemical stability.9 Zinc

1. INTRODUCTION Supercapacitors (SCs) exhibit many outstanding properties compared to conventional dielectric capacitors and batteries, such as higher energy and power density, fast charging and discharging, and long cycle life for a wide range of applications, such as consumer electronics, medical electronics, memory backup systems, hybrid electric vehicles, transportation, and military defense systems.1−4 However, to satisfy demands in the rapidly growing field of energy applications, more efforts have to be expended to the development of new electrodes and electrolytes without sacrificing the power density and cycle life.5,6 Over the past few decades, transition metal oxides/ hydroxides have been explored for use as high energy-density pseudocapacitor electrodes because of their theoretical © 2016 American Chemical Society

Received: December 17, 2015 Accepted: February 2, 2016 Published: February 2, 2016 5025

DOI: 10.1021/acsami.5b12322 ACS Appl. Mater. Interfaces 2016, 8, 5025−5039

Research Article

ACS Applied Materials & Interfaces

performance of the microelectrodes and (ii) a systematic study of the sample properties in terms of oxygen deficiency. The deficiency was studied in correlation with the surface morphology, structural/microstructural properties, and optical properties of the nanohybrid with the enhanced electrochemical properties for use in SC applications. The results could lead to the cost-effective fabrication of novel composite electrode materials for superior electrochemical supercapacitors.

oxide nanostructures are one of the promising candidates for supercapacitors because of their high specific energy density, improved biocompatibility, nontoxicity, good electrochemical activity, low cost, chemical stability, abundant availability, and environmental friendliness compared to other transition metal oxides.11−13 Additionally, nanostructured ZnO possesses some unique physicochemical properties due to the unique spatial architecture and large aspect ratio (so also higher active surface area) compared to their bulk counterpart to meet some specific device-related demands for supercapacitor applications. Studies on the suitability of ZnO as a promising candidate for supercapacitors are limited, and there is a need to better understand the behavior of this material in order to improve its electrochemical properties. To exploit the power density of available metal oxide supercapacitors, several groups have recently fabricated composite/hybrid electrodes via the modification of carbonaceous materials with metal oxides.14,15 Carbon-based materials in the form of powders, fibers, aerogels, composites, sheets, monoliths, and tubes have been widely used as electrodes because of their low cost, variety of morphology/structure, easy processing, high electrical conductivity, improved chemical stability, relatively inert electrochemistry, extremely high mechanical strength, controllable porosity, and electrocatalytic active sites for a wide range of redox reactions.10,11,16−18 Among these materials, carbon nanofibers (CNFs) are attractive electrode additive materials for improving the performance of metal oxide supercapacitors. CNFs have highspecific-surface area, well-defined hollow cores, and a high aspect ratio greater than 1 × 106 .19 Thus, a hybrid nanostructure exploiting the electric double layer capacitance of CNF and faradic pseudocapacitance of zinc oxide could be a suitable candidate for electrochemical capacitor with high specific capacitance and energy density. Although ZnO has been combined with additives like carbon nanotubes, fibers, aerogel, and graphene to enhance the pseudocapacitance of composites,20,21 there are several issues involved in these types of hybrid/composite materials. For example, the energy and power density of the related devices are far from satisfactory and do not meet current market demands. A majority of these issues are related to the presence of defects in the carbon nanostructures, surface/interfacial states within the nanocomposites, and poor crystallinity of the metal oxides. These issues lead to deterioration of the electrical transport properties of the hybrid nanomaterials. Therefore, novel composite/hybrid nanomaterials are needed to overcome the present obstacles. Generally, postsynthesis heat treatment of hybrid electrodes in a vacuum or in a controlled inert atmosphere is expected to improve the structure of the carbon nanomaterials or metal oxides by the removal of defects/surface states, leading to improved electrical characteristics of the nanocomposites.22 Very few reports describe the role of vacuum-annealing treatment of metal oxide/CNT composites (such as MnO2/ CNT, RuO2/MWNT, and SnO2/MWNT) to improve cycle ability and energy density.23,24 To the best of our knowledge, there is no report on the electrochemical properties of ZnO/ CNF nanohybrid electrodes that have been heat treated in an oxygen-deficient atmosphere for use in supercapacitors. In the present investigation, ZnO NPs that are well-attached to CNF walls were synthesized via a precipitation process, followed by heat-treatment in a vacuum furnace. The novelty of the present work includes (i) improved specific capacitance and cyclic

2. EXPERIMENTAL PROCEDURE 2.1. Materials. Commercially available graphitized CNFs ( CNFZnO-600. The CNFZnO-600 hybrid electrode had the lowest ESR because the CNF reduces the aggregation of ZnO particles during the formation of the hybrid, and increases the contact area, and reduces the electrical resistance. Second, the conductivity of ZnO increases due to the effect of high-temperature vacuum annealing, and the defects in the grain boundary of the ZnO NPs and CNFs are minimized compared to the other samples. These provide a highly conductive path to the electrodes that reduces the electrical resistance of the CNFZnO-600 hybrid sample. As shown in the table, the CPE values decrease in the following order: CNFZnO-600 > CNFZnO-300 > ZnO-600. Therefore, we have successfully demonstrated that adding carbon materials with nonstoichiometric oxygen deficiency in metal oxides could affect the electrochemical properties greatly.

ACKNOWLEDGMENTS This work was funded by the grant NRF-2015002423 of the National Research Foundation of Korea. REFERENCES

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4. CONCLUSION ZnO/CNF hybrid was prepared by a chemical precipitation method. Oxygen vacancies were induced by annealing in a vacuum to enhance the electrochemical properties of the hybrid. The induced oxygen vacancies and good interfacial contact in the hybrids remarkably enhanced the charge transport during cycling and provided a large reaction surface area to improve the specific capacitance compared to the 5037


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