Hierarchically Designed Electron Paths in 3D Printed Energy Storage

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Hierarchically Designed Electron Paths in 3D Printed Energy Storage Devices Seonghyeon Park, Manpreet Kaur, Dongwon Yun, and Woo Soo Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02404 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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For Table of Contents Use Only

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Manuscript Title: Hierarchically Designed Electron Paths in 3D Printed Energy Storage Devices

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Authors: Seong Hyeon Park, Manpreet Kaur, Dongwon Yun, and Woo Soo Kim

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ACS Paragon Plus Environment

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Hierarchically Designed Electron Paths in 3D Printed Energy Storage Devices

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Seong Hyeon Park1, Manpreet Kaur1, Dongwon Yun2* and Woo Soo Kim1* 1

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School of Mechatronic Systems Engineering, Simon Fraser University, Surrey, B.C. Canada V3T 0A3 2

Department of Robotics Engineering, Daegu Gyeongbuk Institute of Science and Technology, South Korea

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Title Running Head: Hierarchically Designed Electron Paths in 3D Printed Energy Storage Devices

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*

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E-mail: [email protected], [email protected]

To whom correspondence should be addressed. Phone: +1-778-782-8635, Fax: +1-778-782-7514,

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Abstract

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Three-dimensional (3D) micro-supercapacitors (MSC) have been spotlighted, since they overcome

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limited areal capacitance of two-dimensional planar MSCs. Specially, 3D printing technology offers

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numerous advantages to generate 3D electrodes for MSC, which includes time-saving, cost-effective

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manufacturing and realization of tailorable complex electrode designs. In this paper, we report novel

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hierarchical 3D designs of conductive 3D electrodes for MSC by digital light processing (DLP) based

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3D printing. Photocurable composite resin with silver nanowires was optimized for DLP printing for

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the hierarchical design of high aspect ratio in 3D electrodes. The hierarchical 3D electrodes showed

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unique patterns on the structure corresponding to stacking of layers in the direction of 3D printing. The

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fabricated 3D MSC demonstrated low electrical resistance to be used as a feasible MSC electrodes.

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Energy storage from silver redox reaction were demonstrated in hierarchical 3D electrodes designed

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with mechanically durable 3D octet trusses.

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.

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Keywords: 3D printing, 3D electrode, Micro-supercapacitor.

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Introduction

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Miniature electronic storage devices have been rapidly developed for the demand of required energy

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autonomy of the compact devices. As a candidate of energy/power source, micro-supercapacitors

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(MSCs) have been selected because of its high-power densities, high rate capability, long

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charge/discharge cycles and fast charge/discharge rates1–3. However, conventional 2D planar MSCs

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have showed limited areal capacitance due to low-volume loading of active materials per unit area. To

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enhance electrochemical performance while keeping low footprint of MSCs, more active material

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needs to be loaded per unit area of electrode4.

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Therefore, 3D electrodes having high aspect ratio with porous architecture has been adapted to

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achieve higher electrochemical performance. Several fabrication technologies have been introduced to

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develop 3D electrodes such as graphene aerogel5, MEMS6 (and C-CMEMS7), and templated synthesis

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(self-assembly method)8. However, these technologies have disadvantages like high cost, complex

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fabrication, time-consuming, and use of toxic materials for etching process6 or removal of template8

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after self-assembly.

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As a novel fabrication method, 3D printing has been employed to fabricate 3D electrodes9–12 to

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overcome those disadvantages. The interdigitated micro-battery by Direct Ink Writing (DIW), where

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for the active materials, Li4Ti5O12 (LTO) and LiFePO4 (LFP) were reported as anode and cathode

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materials respectively9. Also, the graphene oxide based interdigitated electrode by DIW was reported10.

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Also, 3D interdigitated Ti6Al4V electrodes were introduced with Selective Laser Melting (SLM)

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technique11. Recently, the graphene-based polylactic acid filament (graphene/PLA) electrode by Fused

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Deposition Modeling (FDM) printing was also reported12. Recent 3D printing driven MSCs relatively

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offers not only facile, scalable and fast fabrication, but also the freedom of designing a complex

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structure with high aspect ratio for 3D electrodes. However, among other 3D printing methods, Digital

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Light Processing (DLP) printing has not been explored much for the development of 3D electrodes, ACS Paragon Plus Environment

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which has capability of higher resolution and complex designs. It is demonstrated that highly complex

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3D architectures with micro to mesoscales can be fabricated by DLP printing13. Moreover, it is suitable

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to generate porous or hollow structures since DLP printing doesn’t require supports in structures as this

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photocuring takes place in a liquid resin14, 15. This technique offers advantages including relatively low-

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cost of printer, compared with other 3D printers, simple printing method, cost effective and high

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throughput fabrication. Since the printing speed of DLP relies on height of object to print in Z axis, the

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size of object in X-Y axis direction does not affect printing speed. Also, multiple objects can be printed

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on the same bed simultaneously without the need of additional printing time. With these benefits, DLP

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printing proved to be a promising method to fabricate 3D electrodes. One of the main reason why DLP

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printing is not reported much for producing 3D electrode is the limitation of material, which is related

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to mechanism of DLP printing. DLP printing utilizes UV light to cure liquid resin consisting of

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monomer, oligomer and photo initiator, and the irradiated pattern is polymerized, which convert shorter

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chain (monomer) into longer-crosslinked chain (polymer) forming a 3D network by free radical

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polymerization like Figure 1A, B and C. However, these resin materials are not usually conductive and

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the final samples are non-conductive polymer. As an approach to develop a conductive 3D structure,

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composite material which has combination of resin and conductive filler has been introduced. Silver

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nitrate (AgNO3) with polyethylene glycol diacrylate (PEGDA) based composite resin16 (500 KΩ),

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multiwall carbon nanotube (MWCNT) with acrylic resin (2.7x 10-2 S/m) 17, (4x10-6 S/m) 18 have been

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reported. Despite all these efforts, these electrical resistances are too high, or the conductivities are too

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low to use it as 3D electrode for supercapacitor.

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In this paper, we report a 3D printed hierarchical 3D design of MSC electrodes, composed of

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mechanically durable octet micro-trusses designs. After DLP printing, pyrolysis in relatively low

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temperature was performed to achieve char-silver 3D electrodes, resulted in relatively low electrical

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resistance as 40.2 Ω. The present work includes design and optimization of high precision photocurable


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composite resin, which is suitable for DLP printing, to produce well-defined interdigitated electrodes.

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And the hierarchical 3D electrodes are designed to have high surface area as open spaces for ion

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diffusion between the electrodes for MSC application.

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Experimental Section

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Photocurable composite resin

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Methacrylic acid monomer from Sigma–Aldrich, urethane tri-acrylate based oligomer, EBCRYL 265,

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Allnex, and phenyl bis(2,4,6-trimethylbenzoyl) phosphine oxide, Irgacure 819, Sigma–Aldrich, as a

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photo-initiator were used for DLP printable resin. Silver nanowires were synthesized using following

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chemicals: anhydrous ethylene glycol (99.8 %, Sigma–Aldrich), silver nitrate (AgNO3) (Sigma–

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Aldrich) and polyvinylpyrrolidone (PVP) (Mw ≈ 55,000, Sigma–Aldrich), potassium chloride (KCl)

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(99 %, Sigma–Aldrich), Isopropyl alcohol (IPA) (Sigma–Aldrich). For a gel type electrolyte, polyvinyl

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alcohol (PVA) (Mw ≈ 205,000, Sigma–Aldrich) and LiClO4 (Sigma–Aldrich) were used. The

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preparation of photocurable composite resin includes the following steps: AgNWs were synthesized by

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large-scale polyol method with dimensions of AgNW as 50 nm in diameter and 10 µm in length

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respectively19. An optical microscope image of as-synthesized AgNWs was shown in supporting

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information S1. The concentration of AgNW was decided as 1.9 vol% initially based on previous

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research in our research group20,21, which demonstrated high conductivity. However, 3D printing

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process of DLP with 1.9 vol% (15.5 wt%) of AgNW was not successful with numerous defects in

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printed structures due to insufficiency of polymerization caused by light absorption of AgNWs.

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Therefore, the concentration of AgNW in resin was optimized as 13 wt%, which demonstrated well-

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defined structures. This will be more discussed in following result and discuss section with light

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absorbing behaviour of conductive composite resin. We used this percentage (13 wt%) to formulate


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photocurable composite material for the all the experiment in this paper. Prepared AgNWs (13 wt% of

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resin) were added to an aluminum foil-covered vial containing monomer (35 wt%) and mixed by

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vortex mixer and magnetic stirring for 30 mins and by ultrasonicator for 1 minute to ensure uniform

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distribution of AgNWs in monomer. Then, initiator (3 wt%) and oligomer (65 wt%) were added and

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mixed thoroughly by vortex mixer and magnetic stirring for 10 mins and 1hr respectively followed by

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degas processing by ultrasonication for 3 mins to eliminate bubbles capped in the composite resin.

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Finally, the formulated resin was gently mixed by spatula and poured into cartridge of DLP printer and

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stabilized for 10 mins to remove bubbles which can be formed while composite resin is transferred into

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the cartridge.

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DLP printing

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DLP printer (ProJet 1200) from 3D Systems has been used for all the printing experiments in this

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research. The reported resolution of the 3D printer is 585 x 585 dpi and each layer height is 30 µm

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using a LED projector at 405 nm wavelength22. The 3D structures were printed by DLP printer and

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rinsed by isopropyl alcohol to remove unreacted resin residues and dried in the air overnight. Post-

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curing process was conducted by built in UV lamp (Osram Dulux L BL UVA 18W/78 2G11) inside of

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DLP printer, wavelength about 400 nm, irradiance 1350 µw/cm2 to increase polymerization conversion

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rate and to terminate polymerization of activated resin, which results in higher mechanical properties23.

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Pyrolysis and fabrication of micro-supercapacitor

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Low temperature baking and high temperature pyrolysis were conducted at 150 °C for 30 min and

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430 °C for 30 min respectively in box furnace under oxygen containing ambient condition (Thermo

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Scientific Lindberg/Blue M™ Moldatherm™ box furnace). Interdigitated MSC device was fabricated

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as depicted in Figure 5A. The gel type electrolyte was prepared using PVA (1 g) and LiClO4 (0.34 g)

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dissolved in deionized water (10 ml) and heated to 90 °C with vigorous stirring for 24 hrs. MSC cases

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were also printed by DLP printer for packaging. Filter paper, Whatman™ 1001-150, was used as a ACS Paragon Plus Environment

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separator to prevent possible electrical short between two electrodes caused by capillary force of PVA

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and silver electrodes24. Finally, copper wires were connected to each electrode with carbon-glue to

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create electrical contacts for electrochemical characterization. Fabricated MSCs were dried in ambient

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air condition and stabilized for 24 hrs to make electrolyte fully diffuse in electrodes.

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Characterization

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To characterize the morphology of printed electrodes, digital optical microscopy and Scanning

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Electron Microscopy (SEM) analysis were carried out. Also, Energy-dispersive X-ray spectroscopy

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(EDS) analysis was performed to see relative composition of 3D printed electrodes before and after

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pyrolysis. Electrical resistance values of printed electrodes before and after pyrolysis were measured by

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a sourcemeter (Keithley 2400) with two-wire DC resistance measurement method. Viscosity of

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photocurable composite resin was measured by viscometer (microVISC™, RheoSense). Cyclic

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Voltammetry (CV) and Galvano Charge/Discharge (GCD) tests were performed to evaluate

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electrochemical characteristics with electrochemical analyzer (CHI 1205B, CH Instruments, Inc.) and

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with CT2001a battery tester (Landt Instruments) respectively.

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Results and Discussion

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Viscosity design of photocurable composite resin

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In the preparation of the 3D printable resin, viscosity of composite resin is an important parameter. If

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the viscosity is too high, the resin cannot fill in between the printed layer and the bottom of cartridge in

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time, making an empty space. This phenomenon causes missed layers or entire printing failure while

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the object is printing. However, if the viscosity is too low, AgNW fillers could be segmented during

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printing, which induce gradient concentration of AgNWs inside resin, due to the gravitational force


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against the buoyancy of resin and friction force25. To calculate the velocity of segmentation of silver

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fillers, Stokes’ law can be used, which is defined as follows:

=

2(ρ − ρ) (1) 9

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where ρ is density of particle, ρ is resin density, particle diameter, solution viscosity, and

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gravitational acceleration. However, as stokes law can analyze segmentation of ‘spherical particles’,

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only silver nanoparticles, as a minor product during AgNWs synthesis, are applicable with this

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equation. On the other hand, segmentation velocity of AgNWs can be different from that of spherical

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particle due to high aspect ratio of AgNWs which has anisotropic behavior in resin. Another model for

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the calculation of segmentation velocity of nanowires is reported like below26,

= g × (ρ − ρ)V

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ln 2 − 0.81 2 !

(2)

where g is the gravitational acceleration constant, ρ and ρ are the densities of nanowire and fluid, is the drag correction factor, V is the volume of an NW, is the diameter of the NW, is

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respectively,

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the length of NW, ! is viscosity of solution. In both the equations (1) and (2), the viscosity is inversely

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proportional to segmentation velocity of NWs. In other words, high viscosity materials can impede

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segmentation of NWs during printing process. For this reason, the photocurable resin was designed to

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have relatively high viscosity: 1,300 mPa.s of viscosity at 25 °C. Although this viscosity is much

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higher than that of conventional resin with poly (ethylene glycol) diacrylate (PEDGA) for DLP printing

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(~100 mPa.s)27, the printed 3D architecture with the developed resin did not affect its printing quality.

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And there’s no noticeale segmentation of AgNWs in the resin while printing.

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Light absorbing sensitivity of conductive composite resin

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Another important component for better printing quality, especially for a 3D-complex structure, is the

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photo absorber, which is widely used in DLP printing to adjust curing depth and width by inhibiting

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polymerization accordingly and resulting in printing detailed features28, 29. However, without photo

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absorber, printed micro-truss structure (octet unit-cell 4 × 4 × 4) made by the developed composite

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resin showed well-defined structure (S2) as designed by 3D modeling. It is attributed to the light

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scattering effect30 of AgNWs serving as photo absorber to prevent over-curing of octet structure.

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Similarly, as a supporting reference, TiO2 nanoparticle-based photo curable composite having high

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refractive index has been introduced to adjust Z-axis resolution and the surface details31. Similar

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phenomenon with silver nanoparticle decorated Pb(Zr,Ti)O3 micro-particle has also been reported32.

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This can be explained in detail by Jacob’s equation33 like below. %

"# = $ ln , $ = %

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&

#() *) + ,-∆*+

(3)

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where Cd is the curing depth, Dp is the resin sensitivity, E is the energy density of incident light, Ec is

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the critical energy density, and d50 is the average particle size, Q is particle loading, n0 is refractive

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index of resin, ∆n is the refractive index difference between the particle and the resin. From this

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equation, curing depth can be decided by the particle size, refractive index of the particle and resin. In

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other words, use of particles having high refractive index and small size in composite decrease curing

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depth. Also, it has been reported that nanowire can enhance printing quality of object by physical

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interaction with resin34,

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structure by controlling sensitivity of the polymerization of composite resin resulting from light

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absorption, scattering and refractive properties of AgNWs while the resin without AgNWs (S4. A, B)

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showed over-cured and not defined structure in the core part.

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. Thus, composite resin with AgNWs (S4. C, D) showed well defined


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Structural design of electrode

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Three structures were designed to have different surface area; (1) Solid, (2) Octet-Thick, and (3)

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Octet-Thin, as shown in Figure 2A, 2B, and 2C. The specific surface area (per gram) of each sample

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like this: (1) Solid = 39.6 mm2/g, (2) Octet-Thick = 2931 mm2/g, (3) Octet-Thin = 4183 mm2/g based

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on computational 3D models. More importantly, the electrodes are specifically designed using cellular

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structures, which allows the material to have good mechanical properties at low weight. One of the

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well-known stretching-dominant structures is octet, whose elastic modulus and strength change linearly

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with relative density, making them stiffer than bending–dominated structures like foam36. Many natural

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biological materials such as wood, bone, cork, plant stems, etc. demonstrates exceptional resilience and

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robustness which is drawn from their intricate mechanical network with many levels of hierarchy37.

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Even at smaller scales, it has been shown that introducing hierarchy into the architecture of 3D cellular

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structures, we can attain unique combination of properties including lightweight, and linear scaling of

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strength and stiffness with density 38. Therefore, adding hierarchy into the octet structures can be very

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advantageous for our experiment where we use pyrolysis to generate 3D electrodes. Although the

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mechanism of pyrolysis in absence of oxygen is different from the case in presence in oxygen, it is

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clear that octet has higher relative elastic stiffness and shear modulus36, 39, 40, which is an inversely

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proportional to deformation of structure, contributing to rigid structure during pyrolysis41.

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3D printing and pyrolysis process.

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Three interdigitated electrodes consisting of solid structures (IDE-Solid) with different high aspect

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ratio (3, 6, 12) were designed to examine dependence of height as shown in Figure 2A. Figure 2B and

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C represent IDE composed of thick octet (strut width: 480 µm; IDE-Thick) and thin octet (strut width:

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300 µm; IDE-Thin) with 12 mm height were designed to compare them with the IDE-Solid having a

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same aspect ratio as 12 to study relationship between specific surface area and electrochemical


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performance in a same aspect ratio. Figure 2D, E, and F depict photographs of 3D printed IDE based

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on 3D modeling of Figure 2A, B, and C respectively. The 3D printed IDEs showed well-defined

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structures as shown in digital microscope images (in inset of Figure 2E, F). Further, IDEs were heated

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in the furnace for 30 mins at 150 °C as pre-baking to remove possible organic residues such as

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isopropyl alcohol used in cleaning process of printed IDE. The printed IDEs after baking process

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showed too high electric resistance to be measured by a sourcemeter (resistance range; < 200 MΩ).

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Then, all samples were pyrolyzed in the furnace at 430 °C for 30 mins (no gas inlet) and cooled to

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room temperature. For the pyrolysis process of printed samples, pyrolysis temperature was decided

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based on reported thermogravimetric analysis (TGA) data of acrylate polymer

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. The TGA graph

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explains that there is significant weight reduction due to degradation of the polymer around

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400~500 °C. We performed the pyrolysis experiment at 300 °C, 400 °C and 500 °C for 30 min and 1

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hour. At 500 °C, the pyrolyzed structure even in IDC-octet showed significant structural collapses due

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to large amount of decomposition. At 300 °C, the pyrolyzed IDC-octet didn’t show any significant

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defects such as structural distortion or collapse, however, the sample was non-conductive with high

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resistance over 200 MΩ. Therefore, the furnace temperature was optimized as 430 °C for 30 min by

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considering high electrical conductivity with mechanical sustainability as 3D structures. We observed

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significant dimensional change after pyrolysis: 40 % of size reduction, and 88 % of weight loss. And

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IDC-Thick and IDC-Thin remarkably showed light gray color, sustaining its structures even in ambient

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condition while the structure of IDC-solid collapsed partially and numerous cracks were observed. This

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structural stability of IDC-Thick and IDC-Thin after pyrolysis accounts for superior mechanical

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property of octet micro-trusses with stretching-dominant behavior. Moreover, in case of the periodic

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cellular structure in IDC-Thick and Thin, the flawless connectivity and continuity by 3D design and 3D

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printing allow the stress applied to be transferred and distributed to each unit cell during pyrolysis

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process. Electrical resistance of pyrolyzed IDEs was measured by a sourcemeter. Contact points of

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resistance measurement are described in S3. Measured resistance was 1.65 KΩ in IDC-Thick with ACS Paragon Plus Environment

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standard deviation: 59.91. Also, IDC-Thin showed resistance values as 40.2 Ω with standard deviation

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as 35.4 in average. Higher resistance of IDC-Thick results from insufficient decomposition of the

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polymer inside and less sintering of silver to produce interconnected conductive structures. To identify

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these phenomena, SEM and EDS analysis were performed. The morphology of 3D printed IDE-Thin as

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printed and after pyrolysis was exhibited in Figure 3. IDE-Thin (as printed) illustrates layer by layer

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laminar structures indicating typical DLP printing patterns corresponding to the layer thickness of

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resolution information (30 µm) shown in Figure 3A, B. SEM images of IDE-Thin after pyrolysis in

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Figure 3D, E clearly show hollow structure as yellow arrows and more bright color patterns on the

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surface of structure corresponding to silver, based on EDS profile in Figure 3C, F in the profiles of

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yellow square region of Figure 3B, and E. Moreover, the atomic weigh ratio of carbon: silver before

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and after pyrolysis indicates significant atomic weight (at %) change from 18:1 to 2:1, which would be

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main cause of high electrical conductivity of IDE octet after pyrolysis. Highly concentrated silver

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network in IDE-Thin is mainly attributed to more decomposition of acrylates units in the oligomer and

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monomer than decomposition in IDE-Thick. And sintering of AgNWs generates silver-interconnected

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3D structure sustaining its structure with help of characteristics of octet structure. This can be observed

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in Figure 3C more clearly where the AgNWs are embedded in photocured resin in accordance with

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layer pattern of 3D printing, while the morphology of AgNWs is changed after pyrolysis in Figure 3G

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and eventually spheroidized43. The mechanism of instable morphology in AgNWs is originated by

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Rayleigh or Plateau-Rayleigh instability44,45 where atomic diffusion is enhanced as temperature is

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increased. At higher temperature as 430 °C, AgNWs tend to lower their surface energy by making

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spherical shape. This spheroidized AgNWs with reduction of distance between each AgNWs driven by

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decomposition of polymer portion and sintering of Ag generate 3D network structures. Additionally,

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the hollow structures can be related to catalyst reaction of silver, increasing the gas yield and reduce the

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amount of charcoal yield46. Therefore, the acrylate starts to decompose in the center of strut, making


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voids and those voids are emerged in the centers. AgNWs migrate into outer shell and those are

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aggregated and sintered, making an interconnected silver.

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Energy storage of MSC

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Energy storage of MSC can be explained by two possible mechanisms: First, electrostatic double

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layer capacitance (EDLC) can be generated on the IDC electrodes by electrostatic accumulation of

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surface charge47. Because the capacitance of EDLC highly depends on surface area of electrode,

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hierarchically designed 3D electrodes having higher surface area increase the capacitance. However,

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CV curve in Figure 5B indicates that the both graphs are not pseudo-rectangular. This explains the

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generated capacitance from EDLC will be minor. Therefore, the predominant energy storage of 3D

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printed MSC is mainly attributed to redox reactions in silver 3D structure: Ag0 ↔ Ag+ + e-, 2Ag +

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2OH-→ Ag2O + H2O48,49. This pseudocapacitive behavior can increase total capacitance of MSC

12

coupled with EDLC by overcoming relatively low specific capacitance of EDLC. The schematics of

13

the electrode in Figure 4C, D, which correspond to SEM image of figure 4A, B respectively, describe

14

possible advantage of fabricated IDE. The high conductive 3D interconnected silver framework offers

15

pathway for fast electron transfer shown in Figure 4D (yellow arrows) resulting in increasing efficiency

16

and performance of MSC. The highly ordered 3D truss structure allows electrolyte to penetrate pores,

17

gap between struts, and increase accessibility of ions, which increases capacitance of MSC.

18

Electrochemical characterization

19

Cyclic Voltammetry (CV) and Galvano Charge/Discharge (GCD) tests were performed to evaluate

20

electrochemical characteristics. The capacitance of the MSCs was calculated from CV data using the

21

equations (4).


14

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"0122 = 3

1

4(5) 5 (4) 25

where 4 is the current, V is the voltage, and is the scan rate.

2

CV test result, measured by two electrode system, like Figure 5B depicts the performance of two (IDC-

3

thin and IDC-thick) different devices at 200 mV/s scan rate in 0 to 1 voltage range. The specific

4

capacitance of IDC-Thin is 3.01 mF/g (0.301 mF/cm2) while that of IDC-Thick is 1.41 mF/g (0.206

5

mF/cm2). This can be explained by the higher specific surface area of IDC- Thin, as it is designed, and

6

higher conductivity50 of IDC-Thin than that of IDC- Thick, contributing higher capacitance. Both

7

graphs exhibited sigmoidal behavior resulting from silver redox reaction48,

8

charge/discharge (GCD) curves of IDC-thin supercapacitor at constant current of 0.2 mA is shown in

9

Figure 5C. Nearly linear charge/discharge profiles were obtained as an indicative of good capacitor

10

behavior. However, there was high IR drop at the beginning of discharge curve attributed to the internal

11

resistance i.e. electrode resistance, interfacial resistance and the electrolyte ionic resistance 51–53.

49

. Galvanostatic

12 13 14

Conclusion

15

In summary, we demonstrated a novel approach to fabricate 3D hierarchical design of MSC

16

electrodes by DLP printing. A photocurable composite resin with AgNWs was optimized to be suitable

17

for DLP printing for well-defined 3D hierarchical complex truss structures. Pyrolyzed 3D electrodes

18

demonstrated unique silver pattern on the surface of hollow structure sustaining octet shape. And by

19

achieving low electrical resistance, which is attributed to structural octet effect and inter-connected

20

silver patterns, a packaged MSC was fabricated by using a gel type electrolyte. Authors believe that

21

this demonstrated approach can be utilized for designing next generation 3D electrodes by 3D printing

22

for the following reasons: 1) The interconnected silver materials offer a 3D continuous electron ACS Paragon Plus Environment

15

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Page 16 of 28

1

pathway. And the ordered pores allow the electrolyte to penetrate and diffuse evenly between

2

electrodes. 2) Freestanding electrodes without current collector are capable with 3D printed electrodes,

3

offering facile and low-cost fabrication. 3) It has freedom of geometrical designs in specific

4

applications for the need of scale-up. 4) It is possible to make 3D electrodes with higher resolution by

5

high resolution 3D printing, generating hierarchical macroporous framework which can have higher

6

surface area and ideal size for electrolyte diffusion. Also, 3D hierarchical electrodes’ performance can

7

be significantly enhanced by deposition of electrochemical active materials.

8 9 10

Acknowledgements

11 12

This work received financial support from the Discovery Accelerator Supplement Grant 493028-

13

2016, funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), and

14

International Collaborative Research Grant N0002117 by Korea Institute for Advancement of

15

Technology and the Ministry of Trade, Industry & Energy of the Republic of Korea.

16 17

Supporting Information Available

18

Supporting information includes photographs of DLP printed octet structure (with and without silver

19

nanowires), the optical microscope images of as-synthesizd AgNWs and printed samples as printed and

20

after pyrolysis, 3D model describing contact points for electrical resistant measurement, SEM images

21

and EDS spectra of IDE after pyrolysis.


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1

FIGURE CAPTIONS

2

Figure 1. (A) Schematics of 3D printable photocurable composite resin; i) Urethane tri-acrylate, ii)

3

Methacrylic Acid, iii) Phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (Irgacure 819), and iv)

4

Silver nanowire(AgNW), (B) Scheme of Digital Light Processing based 3D printing, (C) Scheme of

5

light induced curing process as radical polymerization.

6

Figure 2. (A), (B), and (C) 3D model images of interdigitated electrode (IDE); A: IDE-Solid with

7

different aspect ratio (3 mm ,6 mm and 12 mm from left), B: IDE-Octet with thick (480 µm) strut size,

8

C: IDE-Octet with thin (300μm) strut size. (Inset: a unit cell of octet, scale bar: 500 µm); (D), (E), and

9

(F) Photographs of 3D printed IDE based on 3D modeling (A, B, C) respectively (Inset: digital

10

microscope image of the printed electrode, scale bar: 500 µm); (G), (H), and (I) Photographs of 3D

11

printed IDE after pyrolysis at 430°C with samples from (D), (E), and (F) respectively. (Inset: digital

12

microscope image of the electrode after pyrolysis, scale bar: 500 µm).

13

Figure 3. Scanning Electron Microscope images of 3D printed IDE; (A), (B) and (C) as printed; (E),

14

(F), and (G) after pyrolysis. EDS spectra of (D) IDE as printed in selected yellow area of (B) and (H)

15

IDE after pyrolysis in selected yellow area of (F).

16

Figure 4. (A) and (B) Scanning Electron Microscope images of 3D printed IDE after pyrolysis with

17

different magnification. (C) Schematic of IDE unit cell (octet) after pyrolysis. (D) schematic of

18

electrochemical mechanism of the electrode in the gel-type electrolyte.

19

Figure 5. (A) Schematics of packaging process of interdigitated MSCs with upper case, gel-electrolyte,

20

separator, IDE, and bottom case, Inset shows photographs of fabrication process of MSCs, scale bar:10

21

mm; (B) Cyclic Voltammetry result of IDE-Thin vs IDE-Thick MSCs at 200 mV/s scan rate; (C)

22

Galvanostatic charge/discharge result of IDE-Thin at 0.2 mA.


23

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Figure 1.

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1 2 3

Figure 2.

4 5 6 7 8 9 10 11 12 13


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Figure 3.

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Figure 4.

4 5 6 7 8 9 10 11 12 ACS Paragon Plus Environment

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Hierarchically Designed Electron Paths in 3D Printed Energy Storage

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