Article pubs.acs.org/Langmuir
Highly Controlled Nanoporous Ag Electrode by Vaporization Control of 2‑Ethoxyethanol for a Flexible Supercapacitor Application Jinwoo Lee,† Jaehak Lee,‡ Jinhyeong Kwon,† Habeom Lee,† Hyeonjin Eom,§ Yeosang Yoon,† Inho Ha,† Minyang Yang,*,‡ and Seung Hwan Ko*,†,∥ †
Applied Nano and Thermal Science (ANTS) Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ‡ Agile Technology Lab, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Korea § Surface R&D Group, Korea Institute of Industrial Technology (KITECH), 156, Gaetbeol-ro, Yeounsu-gu, Incheon 21999, Korea ∥ Institute of Advanced Machines and Design (IAMD), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea S Supporting Information *
ABSTRACT: Controlling the surface morphology of the electrode on the nanoscale has been studied extensively because the surface morphology of a material directly leads to the functionalization in various fields of studies. In this study, we designed a simple and cost-effective method to fine-tune the surface morphology and create controlled nanopores on the silver electrode by utilizing 2-ethoxyethanol and two successive heat treatments. High electrical conductivity and mechanical robustness of nanoporous silver corroborate its prospect to be employed in various applications requiring a certain degree of flexibility. As a proof-of-concept, a high-performance supercapacitor was fabricated by electrodepositing MnO2. This method is expected to be useful in various electronic applications as well as energy storage devices.
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INTRODUCTION Manipulating a surface morphology on the nanoscale has been drawing much attention due to its diverse usage in various applications such as optics,1 material sensing,2,3 electrocatalysis,4 and energy storage device.5−8 Among various structures of nanomaterials, noble-metal nanoporous networks are extensively studied owing to their outstanding mechanical, thermal, and chemical stability9 as well as their ability to adjust the size of pores.10 Various metal nanoporous networks such as copper, nickel, palladium, platinum, and gold nanoporous networks have been reported.11−16 These metals possess superior electrical conductivity; therefore, provision of electrons does not become a limiting factor for the potential electronics application, yet metals such as copper and nickel suffer from oxidation and thus pose a durability issue when exposed to the ambient atmosphere for a certain amount of time, while less abundancy and high price of noble metals like platinum, palladium, and gold make scalability of device application impractical. On the contrary, silver suffices most of the conditions aforementioned. Its outstanding electrical conductivity, invulnerability to oxidation, and relatively higher abundancy than other noble metals in nature make it an attractive candidate as a material to build a nanoporous network. In fact, the studies about the nanoporous network using silver by resorting to various fabrication methods have been © XXXX American Chemical Society
reported. Most of them fabricated nanoporous metal electrodes depend on dealloying to selectively remove one or more metal from an alloy. However, dealloying requires sophisticated steps as well as a relatively larger expense.17−21 In addition, many of the cases consist a process to prepare alloy ingots, which result in additional cost. Other methods to create a nanoporous electrode include using colloidal bijels,22 aluminum mask,2,23 colloidal lithography,24 furnace sintering nanoparticle,25,26 thermal evaporator,27 or hydrothermal reaction,28 but these methods either utilize complicated means to realize such structure or lack a capability to tune the size of nanopores. Here we report a simple and cost-effective method to fabricate a nanoporous silver (NPS) electrode by carefully designing a vaporization point of solvent, and a supercapacitor has been fabricated as one of the practical applications by depositing MnO2 upon it to make use of the high-surface-area electrode. To analyze the surface morphology, we utilized SEM and AFM characterization, and XRD confirmed the absence of oxidation in NPS. After electrodepositing MnO2 on NPS, various electrochemical examinations such as cyclic voltammetry, galvanostatic charge−discharge measurement, and electroReceived: December 26, 2016 Revised: February 3, 2017
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DOI: 10.1021/acs.langmuir.6b04625 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
process at temperature higher than boiling point of 2ethoxyethanol removes the solvent on the substrate (iii), thereby producing the Ag thin film with highly populated nanopores (iv). These nanopores emerge from the space that has been occupied by 2-ethoxyethanol, and evaporation of solvent leaves empty nanopores behind. In detail, the chemical composition of Ag organometallic ink, which was used in this study to fabricate NPS, is composed of the organic group and positive Ag ion bound by chelate groups. If a certain amount of thermal energy was introduced, then a bond between the organic group and positive Ag ion would be broken, and it would result in a conductive film of silver after evaporation of organic groups. The principle of the reaction is represented in eq 1.
chemical impedance spectroscopy were conducted to investigate the practicality of device.
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EXPERIMENTAL SECTION
Fabrication of Nanoporous Silver. Different amount of 2ethoxyethanol (10, 20, 30, 40 vol %, Sigma-Aldrich) is added to Ag organometallic ink (InkTec), and the mixed solution is kept in a refrigerator after it is sonicated for 10 min for complete dissolution of two solutions. Then, the solution is deposited on the polyimide film by spin-coating and heated to 100 °C for 1 min to extract Ag from the organometallic ink. The nanoporous structure is fabricated by heating the sample at different temperature (150, 200, 250 °C) for 10 min to remove 2-ethoxyethanol on the substrate. For the sample utilized to fabricate the supercapacitor, 40 vol % of 2-ethoxyethanol was used with post annealing temperature of 200 °C. Deposition of MnO2. Electrodeposition is conducted upon NPS using an aqueous plating solution containing 0.1 M manganese acetate and 0.1 Na2SO4. The three-electrode system, which consists of a platinum plate counter electrode and Ag/AgCl reference electrode, is employed to supply constant current of 0.1 mA applied for 20 min. After deposition, the electrode was subsequently rinsed with deionized water thoroughly and dried with N2 blower. Material Characterization and Electrochemical Measurement. AFM (atomic force microscopy) and SEM analysis were performed with NANOStationII and JEOL JSM-7600F, respectively. XRD measurements were conducted using D8-Advance. Mass of the electrode consisting Ag and MnO2 was measured with inductively coupled plasma mass spectrometer (Agilent ICP-MS 7700S). To test the electrochemical properties of the supercapacitor in a threeelectrode system with aqueous solution of 1.0 M Na2SO4, the device was prepared in a sandwich structure with a separator in between electrodes. Cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy comprise electrochemical measurements using a VersaSTAT 3 potentiostat galvanostat.
SEM and AFM results of metal organic thin-film morphology are measured in Figure 2. Two experimental variables, the amount of 2-ethoxyethanol added to Ag organometallic ink and postannealing temperature, are employed to fine-tune the size and morphology of nanopores. Apparently, the concentration of 2-ethoxyethanol has a major effect on controlling the size of nanopores. For instance, the pore size varies from
