Electrolysis Activation of Fused Filament Fabrication 3D Printed

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Electrolysis Activation of Fused Filament Fabrication 3D Printed Electrodes for Electrochemical and Spectroelectrochemical Analysis Denise M Wirth, Marjorie J Sheaff, Julia V Waldman, Miranda P Symcox, Heather D Whitehead, James D Sharp, Jacob R Doerfler, Angus A Lamar, and Gabriel LeBlanc Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01331 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 31, 2019

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Analytical Chemistry

Electrolysis Activation of Fused Filament Fabrication 3D Printed Electrodes for Electrochemical and Spectroelectrochemical Analysis Denise M. Wirth, Marjorie J. Sheaff, Julia V. Waldman, Miranda P. Symcox, Heather D. Whitehead, James D. Sharp, Jacob R. Doerfler, Angus A. Lamar, Gabriel LeBlanc* Department of Chemistry and Biochemistry, The University of Tulsa, 800 S. Tucker Dr., Tulsa, Oklahoma 74104, United States ABSTRACT: Following the expiration of the patents on fused filament fabrication (FFF), the availability and uses of this 3D printing technology have exploded. Several recent reports describe how conductive composites can be used with FFF printers to generate 3D printed electrodes (3DEs) for energy storage and electrochemical analysis. As printed materials, these electrodes have very high impedance values due to the high content of insulating thermoplastic required for FFF printers. To overcome this challenge, deposition of metals or activation with harsh chemicals have previously been employed. Here, a benign post-printing process was developed using the electrolysis of water to selectively remove the insulating thermoplastic (polylactic acid) via saponification. Optimization of the hydroxide treatment process was found to reduce the impedance of 3DEs by three orders of magnitude in filaments from two manufacturers. This electrolysis activation strategy offers a safe, accessible, and affordable means for improving the electrochemical performance of 3DEs. Here, the ability for these modified 3DEs to be used for electrochemical analysis and integrated into complex electrochemical cells is demonstrated. Rapid prototyping techniques, especially 3D printing, have inspired a variety of new concepts for science and technology due to the ability to quickly design and manufacture customized parts that previously required a multitude of complex equipment and procedures to produce.1,2 Due to the versatility of these technologies, they have recently received interest from a wide variety of communities, including industry, research laboratories, and hobbyists. As the types of materials capable of being used with rapid prototyping techniques increases, the applications of these systems have amplified. This is particularly true for a 3D printing technique known as fused filament fabrication (FFF). FFF, also known as fused deposition modeling (FDM), was first developed in 1988 by S. Scott Crump and commercialized by Stratasys.3 This technique uses a heated nozzle to extrude a thermoplastic filament onto a print surface. The nozzle can be controlled in three directions, enabling a digital design file (.stl) to be printed precisely in layers. After the expiration of the Stratasys patent in 2009, the availability and use of FFF exploded thanks in large part to open design projects like RepRap.4 The increased affordability and access to this technology has led to a wide range of research regarding its applications. To expand this range of applications, different thermoplastics and thermoplastic composites have been developed. The most common thermoplastics used in FFF are polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS); however, dozens of other thermoplastic materials can be purchased from vendors to take advantage of different material properties.5,6 For some applications, the intrinsic properties of the thermoplastic are insufficient, leading to the development of thermoplastic composites.1 For example, carbon fiber can be added to either PLA or ABS to increase the strength and rigidity of a 3D printed component.7 More recently, conductive materials have been added to thermoplastic materials to enable the generation of FFF 3D printed components that are electrically conductive.8–10

3D printed conductive materials have been applied in systems ranging from touch sensors to wireless power transfer circuits.9 In the area of electrochemistry, electrodes have been 3D printed for the development of custom batteries11–15 and unique sensor technologies.16–22 For example, the Muñoz lab demonstrated the use of FFF 3D printed electrodes (3DEs) for the amperometric detection of several analytes and the determination of heterogeneous electron transfer constants.22 Using a graphene/PLA composite filament from the same company (BlackMagic), Foo and co-workers showed how a 3DE could be used to generate a solidstate supercapacitor and photoelectrochemical sensor.20 However, the authors noted that the insulating nature of the PLA required them to sputter gold on the filaments in order to decrease the resistance of the electrode. Earlier this year, the Pumera group described how 3DEs prepared using this BlackMagic composite could be studied using common redox probes and used for the detection of analytes with cyclic voltammetry (CV).17 However, to improve the conductivity of the electrodes for these CV experiments, the electrodes required “activation” using dimethylformamide (DMF). This activation strategy was attributed to the ability of DMF to remove the insulating PLA from the surface of the electrode through a swelling process and resulted in significant improvement in the electrochemical activity.23 While DMF is commonly found in chemistry laboratories, DMF can be more difficult to acquire and use in less formal settings (such as FabLabs) where many FFF 3D printers are found. Furthermore, DMF can be easily absorbed through the skin and is known to have several adverse health effects, including liver toxicity.24,25 Here, we describe a benign activation method for FFF 3DEs based on the saponification of the PLA insulating material using hydroxide. This method uses readily available materials and can even be applied using the electrolysis of water to prevent the generation of hazardous waste. We describe the mechanism behind the performance enhancement and detail how different PLA materials can be used in tandem with these conductive filaments to enable spectroelectrochemical experiments.

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EXPERIMENTAL SECTION

Materials. Sodium hydroxide, potassium ferricyanide, and potassium chloride were purchased from Fisher Chemical. pH test universal indicator solution was purchased from General Hydroponics and used at a 1:50 dilution with 1M KCl electrolyte. Natural PLA and ABS filaments were purchased from MatterHackers. Conductive PLA composite filaments were purchased from ProtoPasta and BlackMagic3D. 3D Printing. All 3D printed objects were designed with SketchUp Make 2017, available free from Google (http://www.sketchup.com/). All CAD and .stl files described in this work are freely available under a creative commons license (https://www.thingiverse.com/LeBlanc-ResearchGroup/designs). Designs were sliced using Cura for Lulzbot (https://www.lulzbot.com/cura), a free software that is based off of the more universal Cura software from Ultimaker (https://ultimaker.com/en/products/cura-software). Objects were printed using with a LulzBot Mini or LulzBot Taz 6 FFF printer. Note that the Taz 6 printer was equipped with a v3 dual extruder tool head and used to print the spectroelectrochemical cells in a single print. PLA based filaments were printed at 220 °C onto a print bed heated to 60 °C using 0.1 mm layer heights and 100% infill. Parts in ABS (namely electrolysis cells and the electrochemical cells) were printed at 240 °C onto a print bed heated to 115 °C using 0.3 mm layer heights and 100% infill. Electrochemistry. All electrochemical experiments, with the exception of most hydrolysis experiments, were performed using a Biologic SAS SP-300 potentiostat. A Ag/AgCl and platinum wire were used as the reference and counter electrodes, respectively. A 3D printed electrochemical cell and cap were used for electrochemical experiments. Additional information regarding the parameters for electrochemical impedance spectroscopy can be found in the supporting information. An electrolyte volume of 10 mL was used for voltammetry and impedance experiments. Electrolysis experiments were performed in a 3D printed divided electrochemical cell with 15 mL of electrolyte. Electrochemical connection between the compartments was maintained by using an approximately 1 cm2 section of paper towel dipped in the electrolyte of both compartments. Unless otherwise stated, electrolysis was performed using a 9V battery. Connection from the battery to the electrodes was accomplished using commercial 9V battery snap connectors with alligator clips. Note that the negative terminal (typically the black wire) will connect to the compartment that becomes more alkaline. Conductivity and Spectroscopy. Conductivity was measured using a Van Der Pauw controller (MMR Technologies, CA). UV-vis spectra were collected using a Shimadzu UV-1800 UVvis spectrometer using a 3D printed cuvette adapter based on a previously published design from our group.26 RESULTS AND DISCUSSION The selective removal of a material to enhance performance is a common practice in several fields. For example, block copolymers27 and polystyrene microspheres28 have often been incorporated into composite materials and then later removed to increase the surface area and improve performance. In the area of 3D printing, polyvinyl alcohol (PVA) filament is commonly employed as a support material during the printing of challenging structures (e.g. large overhangs) and subsequently removed using

water afterwards.29 The solubility of ABS in acetone is also commonly employed as a post-printing process to improve the appearance and strength of a 3D printed part.30 As described in the introduction, recent work from the Pumera group demonstrated how the solubility of PLA in DMF could be used to improve the performance of 3D printed graphene/PLA composite electrodes.17,23 Exploration of other post-printing solutions to improve the performance of FFF 3DEs led us to the use of alkaline solutions. As an aliphatic polyester, PLA is susceptible to saponification. To explore if this strategy would be applicable to 3D printed PLA composite electrodes, printed electrodes were soaked in a strong hydroxide (4M NaOH) solution and evaluated using a variety of methods (see supporting information). Electrochemical performance of hydroxide soaked electrodes was found to be superior to DMF activation, likely due to the difference in the PLA removal mechanism. This was further confirmed using four point probe conductivity measurements. Spectroscopic analysis of the electrodes and the hydroxide soaking solution confirmed that the saponification process is taking place rather than the removal of the PLA through a physical process, as is the case in DMF activation. Evaluation of the hydroxide activation of different vendors of PLA composite filaments indicated that the more conductive BlackMagic filament undergoes a much more rapid decrease in mass and resistance than the less expensive ProtoPasta, which uses carbon black rather than graphene as the added conductive material. This is likely due to a second polymer present in the ProtoPasta, likely an elastomer, which may slow down the saponification process. This second polymer also provides additional structural support following exposure to the hydroxide activation process.

The use of hydroxide solutions rather than organic solvents certainly makes the activation of FFF 3DEs more accessible due to the low cost and availability of NaOH. However, the need to prepare, store, and dispose of chemical waste still presents a significant barrier in the application of this strategy. Thankfully, chemical preparation is not the only means of generating a strong alkaline solution. In one of the earliest demonstrations of electrochemistry, Nicholson and Carlisle observed the electrolysis of water.31 The electrolysis process proceeds through the reduction of water (or excess hydrogen cations) at the cathode to generate hydrogen gas and the oxidation of water (or excess hydroxide anions) at the anode to generate oxygen gas. Less obvious from these equations is the dramatic change in pH that occurs at each electrode. To demonstrate this phenomenon, we added universal indicator to a 1M KCl electrolyte solution and used a 9V battery to cause the electrolysis of water using FFF 3DEs in a separated electrochemical cell (Figure 1). As seen in the photographs, the pH of the electrolyte around the cathode quickly increases while the pH of the electrolyte around the anode decreases. Using a pH meter, the electrolyte in the cathode compartment was measured to be 12.4±0.1 and the anode compartment was measured to be 1.2±0.1. Importantly, at the conclusion of the electrolysis process, the two electrolysis compartments can be mixed to partially neutralize the solutions. Therefore, the hydroxide soaking process can be applied to a FFF 3DE using electrochemistry, with the waste solution safe enough to pour down the sink or potentially reuse in future experiments.

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Analytical Chemistry

Figure 1. Effect of electrolysis on the pH of the surrounding electrolyte using 3DEs. Photographs of a divided electrochemical cell with pH indicator (A) before and (B) after 60 minutes of electrolysis with a 9V battery. Panel (C) shows the system after mixing the compartments. Note that the pH indicator turns red in acidic conditions and blue/green in alkaline solutions. Relevant electrolysis equations are included below the photographs. To demonstrate that this electrolysis treatment is effective as an activation process for 3DEs, a divided 3D printed electrolysis cell was developed and a 24 hour electrolysis treatment was performed on both filament types (Figure 2A inset). Importantly, the electrolysis cell is printed using ABS, which is not sensitive to the drastic pH changes that occur during the electrolysis experiment. Additionally, we note that the two compartments of the electrolysis cell are connected by a simple salt bridge made by soaking a paper towel in the 1M KCl electrolyte. The 3DEs were designed in such a way that the entire electrode to be analyzed afterwards would be in contact with the electrolysis solution, while the connection to the 9V battery could be easily removed after electrolysis if desired. After 24 hours, the impedance of the treated electrodes is reduced in a similar fashion to the soaking process, as observed in the Bode and CV plots (Figure 2). While a 9V battery is easily accessible and modified with alligator clips to perform the electrolysis treatment of FFF 3DEs, the voltage required to perform this process is significantly less. CVs of the as-printed electrodes were used to demonstrate that the electrolysis process begins at approximately 2V vs Ag/AgCl for the ProtoPasta filament and approximately 1.5V vs Ag/AgCl for the BlackMagic filament. For applications of these 3DEs in electrochemical systems, the use of a potentiostat may offer a more cost sustainable strategy than the use of nonrechargeable 9V batteries if the equipment is readily available, though the use of 9V batteries can enable a significant number of electrodes to be activated simultaneously (see supporting information).

Figure 2. Effect of electrolysis treatment on 3DEs. A) Bode plot of ProtoPasta (PP) and BlackMagic (BM) electrodes before and after 24 hour electrolysis treatment in a 1M KCl electrolyte. Inset is a photograph of the 3D printed divided electrolysis cell system. The color in the cell comes from universal indicator that was added to demonstrate the effectiveness of the cell. B) CV of untreated (lighter curves) and electrolysis treated (darker curves) FFF 3DEs prepared from ProtoPasta conductive filament (left, red) and BlackMagic conductive filament (right, black). CV performed in an electrolyte consisting of 10mM ferricyanide and 1M KCl at a scan rate of 10mV/s. The utility of these 3DEs was demonstrated using a 3D printed spectroelectrochemical system (Figure 3). The spectroelectrochemical cell is a modification of our previously published design,26 where a porous working electrode has been included in the print process. This illustrates the unique advantages of FFF 3D printing, where multiple material types can be simultaneously incorporated into a single design with patterns that would be complex to manufacture using traditional manufacturing technologies. By using a PLA based conductive composite, material adhesion with non-conductive PLA filament is enhanced. Leveraging the different saponification kinetics observed with different PLA filament manufacturers, we are able to perform the hydroxide treatment (via hydrolysis) without compromising the integrity of the overall spectroelectrochemical cell. Performing spectroelectrochemical experiments in an electrolyte containing universal indicator (Figure 3A) allows for observation of pH changes in the electrochemical cell during the hydrolysis treatment process. Note that the absorbance at 600nm increases under alkaline conditions and decreases under acidic conditions relative to neutral conditions. From these experiments, it is clear that the hydrolysis process begins immediately following application of sufficient voltage. Experiments performed in an electrolyte containing ferricyanide (Figure 3B) show the expected changes in absorbance as a function of oxidation state during a CV experiment. These experiments demonstrate the versatility and utility of these 3DE systems. Furthermore, the integration of these 3DEs into a

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larger 3D printed device illustrates the value of a hydroxide treatment strategy that is benign compared with other organic solvent treatment systems that would damage the non-electrode components present in the device.

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printer, activated using hydrolysis, and tested using two different electrolyte solutions to demonstrate the advantages of using these materials with a hydroxide activation process. By providing an accessible means for creating electrochemically active 3DEs, we believe that new electrochemical applications will be explored by a greater variety of researchers. More broadly, the concept of activating 3D printed composite systems using electrochemical treatment offers a unique strategy to alter the properties of 3D printed materials post-printing.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional data, photographs, and a table of the various designs that were 3D printed as a part of this work and how to download the designs (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 1-918-631-2528.

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 interest.

ACKNOWLEDGMENT

Figure 3. Spectroelectrochemical experiments performed using a 3D printed electrochemical cell. A) Spectroelectrochemical experiments using an untreated 3DE printed as part of a larger electrochemical cell in an electrolyte consisting of universal indicator and 1M KCl with application of a 9V battery beginning at 30s. The solid line represents the connection of the 3DE to the cathode, while the dashed line represents the connection to the anode. Inset is a photograph of a cell printed using natural PLA for contrast (experiments performed on cells using black PLA to minimize light scatter). D) Spectroelectrochemistry in an electrolyte consisting of 10mM ferricyanide and 1M KCl at a scan rate of 1mV/s using a hydrolysis treated 3DE.

We gratefully acknowledge the financial support from The University of Tulsa. Specifically, this work was supported through both startup funds and a Faculty Development Summer Fellowship Program. Authors J.D.S. and M.J.S. were supported in part through the Student Research Grant Program through the Office of Research and Sponsored Programs at The University of Tulsa. Authors M.J.S., J.V.W., and J.R.D. were supported through both the Chemistry Summer Undergraduate Program and the Tulsa Undergraduate Research Challenge offered through The University of Tulsa. The authors would also like to acknowledge useful conversations with Laura Waldman and Isabella Turner.

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CONCLUSION FFF can be used to generate custom electrodes with commercial carbon-based filaments; however, the presence of insulating PLA significantly hinders the conductivity of the printed electrodes. We have demonstrated that saponification of the PLA can be used to decrease the resistance of these printed electrodes by three orders of magnitude. This saponification process can be applied through either a hydroxide soaking process or through the use of water electrolysis with near equal efficacy. The use of electrolysis significantly improves the accessibility of this electrode activation process while minimizing exposure to hazardous or caustic solutions. We believe that this will make the use of 3DEs easier to adopt for a wide variety of unique applications. As a proof-ofconcept, a 3D printed spectroelectrochemical cell complete with a honeycomb electrode was printed using a dual extrusion FFF

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