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Tailoring the Porosity and Microstructure of Printed Graphene Electrodes via Polymer Phase Inversion Ethan B. Secor, Manuel H Dos Santos, Shay G Wallace, Nathan P Bradshaw, and Mark C Hersam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00580 • Publication Date (Web): 17 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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The Journal of Physical Chemistry

Tailoring the Porosity and Microstructure of Printed Graphene Electrodes via Polymer Phase Inversion Ethan B. Secor†,‡, Manuel H. Dos Santos†, Shay G. Wallace†, Nathan P. Bradshaw†, and Mark C. Hersam†, §, * † Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, United States § Department of Chemistry, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, United States * Corresponding Author: [email protected]

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Abstract

Phase inversion is demonstrated as an effective method for engineering the microstructure of graphene films by exploiting the well-defined solubility characteristics of polymer dispersants. Drying of a tailored phase inversion ink containing a nonvolatile nonsolvent leads to gelation and subsequent pore formation, providing a promising strategy to tailor the porosity of the resulting graphene films. Graphene films with tunable porosity and electrical conductivity ranging from ~1,000 to ~22,000 S/m are fabricated by this method. Moreover, this dry phase inversion technique is compatible with conventional coating and printing methods, allowing direct ink writing of porous graphene microsupercapacitor electrodes for energy storage applications. Overall, this method provides a straightforward and versatile strategy for engineering the microstructure of solution-processed nanomaterials.


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Introduction Methods to tailor the microstructure and porosity of graphene films affect many key application areas for this promising nanomaterial.1–3 Furthermore, the high specific surface area of graphene drives interest in a wide range of technologies, in combination with its high thermal and electrical conductivities, mechanical strength and flexibility, and chemical stability.4 Recent work on porous graphene structures has focused on applications for electrochemical energy storage devices, including batteries and supercapacitors,5–7 pollutant adsorption,8,9 separations,10 and solar steam generation.11–14 For the practical realization of these applications, solution-processed graphene is desirable owing to its low cost and scalable production and manipulation.3,15 To date, efforts along these lines have largely focused on reduced graphene oxide due to its ease of manufacture and liquid-phase processing.16 However, pristine graphene derived from graphite by liquid-phase exfoliation can offer well-defined chemistry, improved electrical performance, and more environmentally benign production,17,18 motivating research focused on liquid-phase processing and manipulation of pristine graphene to achieve microstructural control.15 One promising method for liquid-phase exfoliation and processing of pristine graphene employs polymer dispersants to stabilize colloidal graphene flakes and prevent aggregation or sedimentation.15,19,20 The well-defined solubility and rheological characteristics of the polymer dispersant provide an effective means to formulate graphene inks for various printing methods.21 These same characteristics provide an opportunity for microstructural control, by adapting established methods for tailoring porosity in polymers. One such method is phase inversion, a liquid-phase processing method for engineering porosity that has been widely developed for polymer materials.22–24 For example, polymer membrane fabrication for separations commonly employs a phase inversion processing route.25 The underlying concept of phase inversion is to


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drive a system into a thermodynamically unstable state, leading to phase separation into a poreforming phase and a solid phase.26–28 Upon removal of the pore-forming phase, a porous microstructure is realized. The thermodynamic driving force behind phase inversion is commonly provided by temperature changes or the introduction of a nonsolvent. A dry phase inversion method, in which the nonsolvent is incorporated into the original dispersion, offers benefits for process integration with conventional printing and coating methods.29–32 In this case, the engineered ink is deposited, and preferential evaporation of the good solvent drives the system into an unstable state, leading to phase inversion. Here, we demonstrate a dry phase inversion processing method for producing graphene films with tailored microstructure, and leverage the compatibility of this technique with direct ink writing to fabricate graphene microsupercapacitors.

Experimental Section Graphene Production. Graphene is produced by high shear mixing using ethyl cellulose as a polymer dispersant, following previously established methods.33,34 In short, a mixture of ethanol, ethyl cellulose (EC), and graphite is processed by high shear mixing to exfoliate graphite particles. The resulting mixture is centrifuged to remove unexfoliated graphite, leaving a supernatant containing ethanol, EC, and graphene. The resulting suspension is mixed with salt water to flocculate the graphene/EC, which is collected by centrifugation, washed, and dried. This yields a fine black powder containing ~40% graphene and ~60% EC, with a typical graphene flake thickness of 1-5 nm and lateral size of 100-500 nm. More detailed descriptions of the experimental procedure are included in the Supporting Information. Ink Preparation. The inks for phase inversion processing are directly prepared from the graphene/EC powder. For a typical ink, graphene/EC powder (80 mg/mL), nitrocellulose powder


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(20 mg/mL), glycerol (10% v/v), and ethyl lactate (90% v/v) are mixed and ultrasonicated in a bath sonicator for 6 hours to disperse the graphene. The inks contain varying ratios of glycerol to ethyl lactate: 0:100, 5:95, 10:90, and 15:85 v/v. Sample Fabrication. Thin films of graphene are prepared for characterization by stencil printing to produce a 2 cm diameter circle, using a 120 µm thick mask. The same ink is utilized for direct ink writing to prepare microsupercapacitor patterns, using a Hyrel System 30M 3D printer equipped with a 210 µm diameter extrusion tip. After casting the samples, they are dried on a hotplate at 70 °C for 10 minutes to remove ethyl lactate, dried in a vacuum oven at 100 °C for 10 hours to remove glycerol,32 and annealed on a hotplate at 325 °C for 30 minutes to decompose the polymer dispersants. In some cases, explicitly denoted in the text, the vacuum drying step is replaced with either water immersion and drying, or drying at atmospheric pressure and elevated temperature. Characterization. Electrical conductivity is calculated from the sheet resistance, measured by four point probe, and the thickness, measured by optical profilometry. Cross-sectional scanning electron microscopy (SEM) imaging is performed with drop-cast samples on Si substrates. Supercapacitor Fabrication and Testing. To fabricate microsupercapacitors, graphene interdigitated electrodes are printed and processed as described above. The remaining microsupercapacitor fabrication steps follow previously established methods.35 In particular, the samples are treated with oxygen plasma (2 minutes, low power), followed by deposition of the electrolyte solution containing 100 mg/mL PVA in 1:3:6 H3PO4/IPA/H2O. The electrolyte is then dried overnight. Copper tape is finally used to electrically contact the devices, which are measured using a CHI potentiostat.


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Results and Discussion For phase inversion processing of graphene, the ethyl cellulose dispersant provides well-defined solubility characteristics that streamline the development of suitable solvent/nonsolvent pairs.36,37 Ethyl cellulose is hydrophobic, and a small amount of nitrocellulose is also added to the ink to enhance hydrophobicity and mechanical properties (Supporting Information, Figure S1) such that each ink contains 80 mg/mL graphene/ethyl cellulose (~40% graphene, 60% ethyl cellulose) and 20 mg/mL nitrocellulose.38 For the primary solvent, ethyl lactate is selected for its moderate boiling point (~154 °C) and strong solvation character.39 A film cast with a solvent of only ethyl lactate leads to a dense, aligned microstructure of graphene flakes upon drying, as shown in Figure 1a. This case is defined as the control, denoted by the red labels in the ternary phase diagram provided in Figure 1b. For dry phase inversion driven by evaporation of the primary solvent, the nonsolvent must have lower volatility. Consequently, glycerol is selected for the nonsolvent due to its high boiling point of 290 °C and hydrophilic character. Inks with 5, 10, and 15% v/v glycerol were prepared to study the impact of nonsolvent content. For the ink with 15% glycerol, some inhomogeneity is observed, indicating that this composition is only partially stable at room temperature, thus precluding the study of inks with higher glycerol content. These three phase inversion inks are denoted by blue labels in Figure 1b. As the ethyl lactate evaporates, these inks move into an unstable region of the phase diagram characterized by gelation of the graphene/EC and NC solid components. When the glycerol is removed by vacuum annealing (Supporting Information, Figure S2),32 the resulting films have the same material composition as the control ink, but exhibit a porous microstructure, as shown in Figure 1c. Thus, phase inversion with cellulosic polymer dispersants is effective for tailoring the microstructure of the final graphene film.


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Figure 1. Overview of phase inversion processing of graphene inks. (a) Cross-sectional SEM image of a graphene film cast from a standard graphene ink, showing a dense and aligned microstructure. (b) Schematic phase diagram showing the fundamental phase inversion process, in which solvent engineering drives gel formation prior to drying. (c) Cross-sectional SEM image of a graphene film cast from a phase inversion ink, showing a porous microstructure of graphene flakes.

To better characterize the microstructure and properties resulting from the different phase inversion inks, more detailed analysis was performed. Cross-sectional SEM imaging, shown in Figure 2a-d, illustrates the porous microstructure resulting from phase inversion processing. The control sample with 0% glycerol exhibits a dense graphene film with limited porosity. With 5 or 10% glycerol, the resulting film still contains aligned graphene flakes, but the characteristic pore size is increased. For the ink containing 15% glycerol, the alignment of graphene flakes with the substrate is less evident (Figure 2e), and the flakes form a more disordered network with significant exposed surface area.


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Figure 2. Cross-sectional SEM images of graphene films cast from different phase inversion inks containing 0, 5, 10, and 15% v/v glycerol in (a)-(d), respectively. (e) Distributions of flake angle with respect to the substrate, showing enhanced alignment for low glycerol loading (n>100 for each histogram).

While SEM provides compelling visual evidence of the graphene film microstructure following the phase inversion process, more quantitative characterization is provided by measurements of thickness, roughness, and electrical conductivity. Specifically, samples are prepared from each of the inks by stencil printing with a common mask such that the wet film thickness, and hence the amount of graphene, remains constant. Figure 3a shows the thickness of the resulting films following complete removal of the solvent (As Cast) and subsequent decomposition of the polymer dispersant (Annealed). It is clear that the film thickness increases with an increased glycerol content in the original ink. Because the experiment is designed to produce films with a nominally equivalent areal mass density, the thickness scales inversely with density, leading to porosity estimates of 51, 74, 81, and 92% for samples with 0, 5, 10, and 15% glycerol, respectively (Supporting Information, Figure S3). Moreover, the thickness reduction resulting from annealing


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is consistent with the SEM imaging results. Samples with 0, 5, and 10% glycerol exhibit thickness reductions of 63, 58, and 59%, respectively, while the sample with 15% glycerol undergoes a thickness reduction of only 39%. This difference in thickness reduction is likely a result of the aligned nature of graphene flakes in the former samples, which is not the case for the ink with a high glycerol content. The increased porosity and disruption of the dense, aligned graphene film are also expected to impact electrical conductivity. As shown in Figure 3b, the control sample, with the most dense, aligned graphene flake network, has a high electrical conductivity of ~16,000 S/m. However, as the ink glycerol content increases, the conductivity decreases to ~6,500, ~3,700, and ~1,000 S/m for 5, 10, and 15% glycerol, respectively. While the electrical conductivity spans a wide range, even the most porous films possess favorable electrical conductivity for many applications. The surface roughness, shown in Figure 3c, also corroborates the prior observations. In particular, films with a low glycerol content have a relatively smooth surface. The sample with 15% glycerol, however, possesses a high surface roughness of ~10 µm, resulting from the dispersion quality of the corresponding ink (Figure 3d). While the addition of glycerol strongly influences the morphology and electrical characteristics of the graphene films, it does not alter the chemical composition as determined by Raman spectroscopy (Supporting Information, Figure S4).


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Figure 3. Summary of morphological and electrical characteristics for graphene films produced by phase inversion with different glycerol content. (a) Film thickness as a function of glycerol content before and after annealing for inks with 0, 5, 10, and 15% glycerol v/v. (b) Electrical conductivity as a function of glycerol content. (c) Surface roughness measured by optical profilometry as a function of glycerol content. (d) Optical profilometry images showing the topdown surface profiles of the graphene films.

For practical applications, the methods used to achieve phase inversion should be favorable from the perspectives of process integration, throughput, and cost. To better understand the processing space compatible with these inks, several different methods to remove glycerol were studied. The first method uses a drying step in air at 150 °C for 4 hours. However, the resulting thickness and conductivity do not exhibit a variation as consistent and reliable as the control method of drying


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under vacuum at 100 °C (Figure 4). One possible explanation for this difference is the softening and collapse of the graphene/polymer gel at higher temperatures in the presence of glycerol. To circumvent this issue, a room temperature method entailing immersion in water was employed to leach out glycerol. This step is performed following drying of the ethyl lactate at 70 °C for 10 minutes. This process is similar to traditional nonsolvent-induced phase separation methods, in which polymer films or fibers are extruded into a bath of nonsolvent to drive phase inversion.26 As shown in Figure 4, this approach results in film thickness comparable to vacuum drying, along with somewhat improved conductivity, thus establishing this method as an effective alternative to drying under vacuum. These conclusions are confirmed by the observation of similar porous microstructure following vacuum drying and water immersion (Figure 4c). As a rapid, ambient process, this strategy is particularly well-suited for large-scale manufacturing.

Figure 4. Exploration of processing conditions for phase inversion graphene inks, comparing glycerol removal by heating (150 °C), vacuum drying (100 °C/Vac), and water immersion (H2O). The efficacy of phase inversion using these different methods is assessed by (a) thickness, (b) conductivity, and (c) cross-sectional SEM imaging (for 10% glycerol-containing inks).


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A clear advantage of the dry phase inversion method developed here is the compatibility with conventional liquid-phase deposition and patterning methods.32,35 For instance, due to the high colloidal stability of the polymer-stabilized graphene and phase inversion solvents, the inks developed here are compatible with direct ink writing,40 which is a digital, extrusion-based printing method. A key property for printing is the ink rheology, shown in Figure 5a for each of the four inks studied. It is evident that the addition of glycerol leads to a substantial increase in viscosity, particularly at low shear rates. In addition, the shear thinning nature of the inks is enhanced with a greater glycerol content. As shown in Figure 5b, the graphene inks are printed to form interdigitated electrodes. The thickness of the electrodes can be readily controlled by varying the number of printing passes, with a consistent increase in thickness upon each pass indicating stable and reliable deposition (Figure 5c, Supporting Information Figure S5).

Figure 5. Direct ink writing of graphene phase inversion inks for microsupercapacitors. (a) Viscosity of the inks as a function of shear rate, showing an increased viscosity with increased


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glycerol content. (b) Photograph showing direct ink writing of the graphene phase inversion ink. (c) Thickness characterization of printed graphene lines as a function of printing passes. (d) Representative cyclic voltammetry data with varying voltage scan rate. (e) Representative charge/discharge cycling data for different current densities. (f) Capacitance as a function of current density for devices with one and four printed layers of graphene.

The

printed,

porous

graphene

interdigitated

electrodes

are

used

to

fabricate

microsupercapacitors upon application of a suitable electrolyte. The ink containing 10% glycerol was selected following electrochemical testing of stencil-printed, sandwich-structured supercapacitors (Supporting Information Figure S6). Microsupercapacitors employing coplanar thin-film electrodes are promising for on-chip energy storage, in that they can be fabricated in-line with additional printed electronics components.41,42 These devices store energy as charges at the interface of the graphene electrode and the electrolyte, in this case a phosphoric acid/polyvinyl alcohol gel. The amount of charge or energy stored is related to the active surface area of the electrode in contact with the electrolyte, thus motivating the use of porous, conductive graphene electrodes. Representative cyclic voltammetry (CV) curves are shown in Figure 5d, while charge/discharge curves at various currents are shown in Figure 5e. From these data, the capacitance can be calculated, as is shown in Figure 5f for devices with one and four printing passes. Importantly, the capacitance increases substantially for the thicker device, indicating that this method is scalable to thicker, and hence higher capacitance, electrodes. At a current of 0.2 mA/cm2, the four-layer device has a capacitance of 0.81 mF/cm2, which is a value comparable to or better than other printed graphene microsupercapacitors (Supporting Information Figure S7).42 These results demonstrate the value of microstructural control afforded by phase inversion, and its


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beneficial application to printable graphene inks. It should further be noted that the demand for conductive graphene electrodes with high surface area spans many applications beyond energy storage such as sensing and catalysis.

Summary and Conclusions We have demonstrated here a phase inversion processing method to tailor the microstructure and porosity of printed graphene thin films. Polymer-stabilized graphene inks provide a suitable system for phase inversion due to their well-defined solubility characteristics and versatile liquid-phase processing compatibility. By engineering the solvent system to drive phase inversion, graphene films with a porous microstructure are realized, with the ability to tailor resulting electrical and structural properties. Furthermore, the phase inversion inks are compatible with direct ink writing, which enables the fabrication of printed microsupercapacitors and related high surface area applications. Overall, this demonstration of phase inversion processing for graphene can likely be generalized to related nanomaterial inks, thereby providing a general platform for tailoring microstructure in printed electronics, sensors, and energy storage technologies.

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedure; SEM imaging of samples for ink optimization; Raman spectroscopy; and additional electrochemical characterization and discussion can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Mark C. Hersam: [email protected] ORCID Mark C. Hersam: 0000-0003-4120-1426 Ethan B. Secor: 0000-0003-2324-1686 Shay G. Wallace: 0000-0002-7535-6138

Present Address Sandia National Laboratories, Albuquerque, NM 87185, United States

‡

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the Air Force Research Laboratory under agreement number FA8650-15-2-5518. EBS was further supported by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate (NDSEG) Fellowship Program, the Ryan Fellowship administered through the Northwestern University International Institute for


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Nanotechnology, and the Cabell Terminal Year Fellowship. NPB also acknowledges a National Science Foundation (NSF) Graduate Research Fellowship. This work made use of the EPIC and Keck-II facilities within the Northwestern University NUANCE Center, which has received support from the NSF Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (ECCS1542205); the NSF MRSEC program (DMR-1720139); the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois. Rheometry was performed in the Materials Characterization and Imaging facility at Northwestern University, which receives support from the NSF MRSEC (DMR-1720139). The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the sponsors.

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