Electrochemical Surface Treatment of Discontinuous Carbon

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Electrochemical Surface Treatment of Discontinuous Carbon Fibers Ngon Tran, Brendan Patterson, Alec G Kolodziejczyk, Vincent Wu, and Daniel Brainard Knorr Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01850 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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Electrochemical Surface Treatment of Discontinuous Carbon Fibers Ngon T. Tran, Brendan A. Patterson, Alec G. Kolodziejczyk, Vincent M. Wu, Daniel B. Knorr, Jr.* U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 Keywords: carbon fibers, discontinuous fibers, electrochemistry, surface modification

Abstract We developed an operationally simple electrolytic design for the surface treatment of short carbon fibers. Using X-ray photoelectron spectroscopy (XPS), we demonstrated that the electrochemical surface treatment of discontinuous fibers is highly reproducible, uniform, and tunable. Specifically, total amounts of surface oxygen and nitrogen contents (0 to 17 atomic %) as well as surface oxygen-to-nitrogen ratio (1:0 to 1:2) vary significantly over the ranges of each processing parameter: applied voltage (1.5–21 V), location of carbon fiber (i.e., anode, cathode, or mixed mode), initial temperature (3–70.5 °C), and ammonium bicarbonate concentration (0.005–0.75 M). Optimized processing conditions afforded carbon fibers that have similar surface compositions (86.3 ± 1.1 at. % C, 8.9 ± 0.8 at. % O, 4.7 ± 0.6 at. % N) as those of commercially available continuous fibers. In addition, these fibers retain their mechanical properties (tensile strength and tensile modulus) and exhibit no detectable surface damage based on single fiber tensile tests and scanning electron microscopy (SEM). We also performed a number of control experiments to develop a proposed mechanism for the surface functionalization of the carbon fiber. These mechanistic studies demonstrated that water splitting contributes significantly to the oxidation of carbon fibers and that other species in the chemical equilibria of ammonium bicarbonate (and not just its individual ions) play a significant role in functionalizing carbon fiber surfaces. Introduction It is both environmentally and economically practical to recycle carbon fiber reinforced composites (CFRCs). Over the past decade, production of CFRCs has increased rapidly while carbon fiber remains relatively expensive, and a significant amount of waste is produced throughout the life-cycle of CFRCs (Figure 1). 1-2 The manufacture of virgin carbon fibers has about 50% yield by mass and is energetically intensive (198–594 MJ kg−1). In addition, manufacturing cut offs during composite fabrication generate up to 40% CFRC waste and out-oflife/aged prepregs generate an additional 10% of CFRC waste.1, 3 Instead of landfilling or incinerating CFRC waste, the high-value carbon fibers can be reclaimed and re-used in lightweighting applications, including structural applications.1, 4-5 Recycling carbon fibers may also enable some markets to take advantage of CFRC lightweighting strategies because the sales price of recycled carbon fibers is lower than that of virgin carbon fibers due to the significantly lower manufacturing costs.2 Moreover, European initiatives and environmental awareness will most likely encourage more CFRC recycling and increase CFRC demand because the superior strength-to-weight properties of CFRCs can lead to more efficient wind turbines and more fuelefficient automobiles.6-8 Therefore, recycling can potentially bridge the gap between supply and demand of CFRCs as well as mitigate waste accumulation.

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Figure 1. Life-cycle phases of carbon fiber reinforced composites (CFRCs).2, 9

Consequently, mechanical and chemical and thermal processes for recycling CFRCs have been developed.2-3, 10-11 In order to be effective, a recycling method for CFRCs must meet 5 criteria: environmental impact, efficiency, commercial viability, good interfacial compatibility with various resins, and good retention of fiber properties.2 The most widely used technology to recover carbon fibers from CFRCs is pyrolysis (or the thermal degradation of organic molecules in an inert atmosphere), because it satisfies the 4 criteria above and offers additional advantages over other recycling processes. These advantages include its applicability to any organic polymer resins, no post-rinsing or cleaning, no use of chemicals (e.g. acids, bases, and catalysts), and reliance on existing infrastructure/technology.2, 10 In fact, pyrolysis has already been applied industrially and is commercially exploited for CFRC recycling.2 Most importantly, optimized pyrolytic reclamation processes can recover fibers with virtually no degradation of mechanical properties, 12-14 which implies that recycled carbon fibers can be re-used in structural applications.1, 4-5 In addition, pyrolysis removes all previously existing surface treatments. Removal of surface treatments, however, creates a major challenge to reintroducing pyrolytically recovered carbon fibers. Good fiber-matrix interfacial adhesion strength is usually required in order to have more efficient transfer of applied load from matrix to fiber, and CFRCs without surface treatments suffer from poor interlaminar shear strength.15 Enhanced fiber-matrix interfacial interactions can significantly increase tensile strength (range – 71.2%) interlaminar shear strength (24.7%), and interfacial shear strength (83.4%) in carbon-fiber reinforced composites.16-18. In general, surface treatments improve interfaces by removing a weak layer of amorphous carbon (e.g. contaminations and coking products), depositing functional groups for covalent interactions with subsequently added sizings or polymer matrices, and increasing surface roughness for better mechanical interlocking with the matrices.15 Because the majority of recycled carbon fibers are discontinuous and there is no relatively efficient and modular surface treatment for discontinuous carbon fibers, CFRCs that contain pyrolytically recovered carbon fibers may not have good mechanical properties. Many of the chemical modifications of continuous carbon fibers and other graphitic carbons (e.g. carbon black and carbon nanotubes) should also apply to discontinuous carbon fibers. These include: anodic oxidation,19-20 plasma treatment,21-23 solution oxidation,24-25 gas-phase oxidation,26-27 grafting of polymers and graphitic carbons,28-30 Friedel-Crafts acylation,31-32 Diels-Alder addition, 33 and diazonium coupling.34-36 To improve interfacial interactions further, the aforementioned “primary” surface treatments are combined with either coupling agents17, 37 or sizings.18, 38-39 These secondary treatments are not


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE the subject of this work. In any process to chemically modify the carbon fiber, it is important that the surface treatments do not modify the internal crystal structure of the carbon fiber otherwise fiber and composite strength and modulus are compromised. The most widely used “primary” surface treatment for continuous carbon fibers in industry is the roll-to-roll electrochemical oxidation because it is relatively fast, uniform, suited for mass production, and modular in terms of surface functionalization due to wide electrolyte selections.1920, 34, 40-42 Unfortunately, this roll-to-roll setup cannot be applied to discontinuous carbon fibers. Previous studies43-45 have shown successful electroplating of small carbon filaments (similar to milled carbon fiber), but these involved agitation of a slurry of fibers, which results in fiber attrition, fiber damage and a reduction in fiber aspect ratio (i.e., length over diameter), which is crucial for maintaining high mechanical properties in CFRCs Reported methods for electrochemical treatments of short carbon fibers are unsuitable/inefficient for treating long discontinuous carbon fibers because they would could not simultaneously treat more than a single fiber tow.38 Herein, we developed an electrolytic method for the surface treatment of multiple discontinuous carbon fibers without damaging surface topology and mechanical properties. In order to have a good comparison to commercially-available and electrochemically-treated continuous carbon fibers, we employed ammonium bicarbonate and DIALEAD chopped carbon fibers (K223HE). Ammonium bicarbonate is a commonly used electrolyte in industry,46-47 and the DIALEAD fibers (K223HE) are a perfect model system for pyrolytically reclaimed carbon fibers because the DIALEAD fibers are available “de-sized”. That is, they have experienced a high temperature treatment process to remove surface functional groups and thus contain very low surface heteroatoms (non-carbon atoms = 0.4–1.3 atomic %) (vide infra). Surface characterization, surface topography, single fiber fragment tension, and the effects of processing conditions (i.e., applied voltage, electrode polarity, initial temperature, and electrolyte concentration) were studied. 2. Experimental 2.1 Materials Untreated, de-sized discontinuous carbon fibers (pitch-based, K223HE) with lengths 6 mm and 25 mm were obtained from Mitsubishi Chemical Carbon Fiber and Composites. Platinum mesh (2.54 cm x 2.54 cm) electrodes were purchased from Ametek Scientific Instruments, Mueller BU60C alligator clips were obtained from McMaster-Carr, and carbon felt was purchased from Metaullics Systems Company, L.P. Electrolyte solutions were prepared from Milli-Q water (18 MΩ·cm−1) and ammonium bicarbonate (≥99.5 %, Sigma Aldrich), ammonium chloride (≥99.5 %, Sigma Aldrich), ammonium hydroxide solution (28 wt % ammonia in water, Sigma Aldrich), ammonium carbamate (99%, Sigma Aldrich) or sodium bicarbonate (≥99.7 %, Sigma Aldrich). In order to obtain a snug fitting mesh for specific chemical glasswares and have greater flexibility over open areas (or pore size) in the mesh, the PTFE meshes used in this study were made by rolling up and weaving polytetrafluroethylene thread seal tape (ULINE, 12.7 mm width x 0.07 μm thickness). However, commercially available PTFE meshes also worked. 2.2 Surface Treatment of Discontinuous Carbon Fibers Electrochemical surface treatments were performed in the undivided cell as illustrated in Figure 2, and a Laboratory DC Power Supply Model 4025 was used to apply voltage. To maximize surface contact, chopped carbon fiber tows (0.050−0.150 g) were disassembled into individual fibers by applying gentle pressure with a spatula. To distribute the chopped fibers as evenly as possible, a spatula full of disassembled carbon fibers was gently tapped (which allowed small


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE amounts of the fibers to fall onto the felt) over different locations of carbon felt (65 mm diameter, 2−3 mm thick). The carbon felt was placed over a 90° bent platinum mesh electrode in a flatbottom Pyrex® crystallizing dish (180 mL capacity, 65 mm inner diameter, 70 mm outer diameter x 50 mm height). After securing the fibers in place with the PTFE mesh (sheet diameter = 65 mm, thread thickness = 1−1.75 mm, mesh opening ≈ 2 mm x 3 mm), an ammonium bicarbonate solution (100 mL) was added. A platinum counter electrode was placed above the center of the PTFE mesh and immersed as far as possible below the electrolyte solution surface without short circuiting. Experiments were performed for 10 min. under varying temperatures (3.0−70.5 °C), ammonium bicarbonate concentrations (0.005−0.750 M), and applied voltages (1.5−18 V). These experiments were non-isothermal. After each experiment, carbon felts were rinsed thoroughly with Milli-Q water and reused until they become too thin (0.75 mm) or damaged (e.g. ripped). Thinning of carbon felts is probably a result of their participation in electrolysis, which was confirmed by XPS. The resulting fibers were washed thrice with highly purified water (18 MΩ·cm−1, Milli-Q system) and filtered, followed by drying under vacuum overnight at 100 °C. 2.3 Spectroscopic Measurements X-ray photoelectron spectroscopy (XPS) data was collected on a Physical Electronics Versa Probe II instrument with an Al Kα source. Compositions were obtained from high-resolution scans with a pass energy of 23.5 eV and a step size of 0.05 eV for the C 1s, O 1s, and N 1s regions. CasaXPS software was used to process XPS data as described in the Supporting Information. For each experiment, a minimum of three randomly selected carbon fiber bundles were analyzed. Scanning electron microscopy (SEM) was performed on a Hitachi S-4700 at an accelerating voltage of 5 kV. Prior to imaging, samples were sputtered with an approximately 2 nm thick coating of iridium. 2.4. Mechanical Measurements Single fiber tensile tests were performed on 25 mm long carbon fibers using an automatic singlefiber test system called a Favimat (Textechno H. Stein GmbH & Co. KG, Germany). The load cell capacity was 210 cN, the force resolution was 0.0001 cN, the maximum possible travel was 100 mm, and the elongation resolution was 0.1 μm. The Favimat performed an independent measurement of the fiber linear density using an acoustic resonance technique prior to each individual tensile test. The diameter of the fiber being tested was obtained from this measurement by assuming that the fiber has a cylindrical shape. The experiments were performed using grips supplied with the Favimat that are designed to grip carbon fibers between surfaces of hard and soft rubber. Testing parameters include a pretension of 0.5 cN/tex, a displacement rate of 0.9 mm/min, and a gauge length of 15 mm. Single fibers were loaded directly into the Favimat grips without the assistance of paper or cardstock tabbing. Single fiber tests that showed slipping of the fibers in the grips (as evidenced by a long slow decrease in stress without fiber breakage) were discarded. Thirty tests (with no slipping) were reported each for untreated and treated fibers. Physical properties, including diameter, break strength, and initial modulus, were collected from the software program included with the Favimat, Textechno35 (Version 3.0.3547). 2.5 Other Measurements Conductivity and pH measurements were performed on the OaktonTM PC 700 instruments. The pH probe was calibrated no more than a week prior to measurement, the conductivity probe was calibrated according to the manufacturer’s specifications using standard calibration solutions purchased from Oakton. The pH values and conductivities of the most commonly used ammonium bicarbonate solutions used in this work are given in Table 1 below.


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE Table 1. Values of pH and conductivity for concentrations. Concentration (M) 0.005 0.100 0.250 0.500 0.750

aqueous ammonium bicarbonate solutions of various pH 8.01 7.99 7.92 7.84 7.83

Conductivity (mS cm−1) 0.7 9.3 21.4 38.9 55.3

Results and Discussion 3.1 Electrochemical Design Previous studies on electrochemical oxidation of carbon fiber tows and electroplating carbon filaments demonstrated the successful surface treatment of non-continuous carbon fibers in electrochemical cells.38, 43, 45, 48-49 Esfandeh and Andideh demonstrated that high oxidation of short carbon fiber tows can be achieved by nitric acid electrolysis, and some research groups demonstrated that relatively thick layers of nickel and copper coatings can be deposited on carbon filaments through electrolysis with nickel and copper salt solutions. Unfortunately, the former method can only treat one or a few carbon fiber tows at a time, and the latter method would damage carbon fibers during agitation of the solution (i.e. fibers would be broken up during stirring). To develop a reproducible electrochemical surface treatment for discontinuous carbon fiber that addresses the above two issues, we sought designs that satisfied the following criteria: (1) excellent contact between electrode and multiple carbon fibers (e.g. tows), (2) ample fiber exposure to electrolytes, (3) minimal obstruction of gas evolution from both electrodes, and (4) prevention of free floating fibers to minimize fiber damage and improve uniform treatment. We discovered that having discontinuous fibers sandwiched between a carbon felt and PTFE mesh as shown in Figure 2 satisfied the above criteria. Higher reproducibility in surface treatment was observed when carbon fibers were placed above the carbon felt compared to that of when the fibers were placed below the carbon felt. This difference is likely a result of unobstructed exposure to the electrolyte and to the intermediates that are generated from both electrodes. In addition, it is significantly easier for gaseous byproducts to escape from the fiber surfaces. We next establish proof of concept and then vary processing conditions (i.e., applied voltage, bias, temperature, and electrolyte concentration) to understand their effects on the surface treatment of carbon fibers. 3.2 Surface Properties of CFs Proof-of-concept for the electrochemical treatment of discontinuous fibers was confirmed by XPS analysis. Because the surface of as-received Dialead fibers consisted of 98.2–99.6 at% (atomic %) carbon and 0.4–1.3 at% oxygen, either an increase in surface oxygen or the presence of surface nitrogen would indicate successful electrochemical oxidation with ammonium bicarbonate. Using our preliminary electrochemical parameters (i.e., applied voltage = 16 V, 10 min. anodic oxidation, starting temperature ≈ 23 °C, and electrolyte concentration = 0.50 M), the surface oxygen content increased to 6.5±0.4 at%, and the surface nitrogen content was 6.7±0.3 at.% (Figure 3). Encouraged by these results, we next attempted to optimize electrochemical processing conditions to achieve surface functionalization similar to commercially available electrochemically treated continuous fibers. These electrochemical parameters include applied voltage, oxidation time, temperature, and electrolyte concentration. They were chosen due to


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE ease of experimental setup and because they are known to significantly affect the surface functionalization of carbon fibers.19-20, 41, 50

Figure 2. Schematic representation of electrochemical cell for treatment of discontinuous carbon fibers in this study.

Carbon (C 1s)

Oxygen (O 1s)

Nitrogen (N 1s)

Untreated

Survey

Treated

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Figure 3. XPS survey scans and fitting curves of high-resolution XPS spectra of carbon, oxygen and nitrogen on electrochemically treated discontinuous carbon fibers at different applied voltages with the same parameters (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). 3.3. Effects of Applied Voltage on Surface Chemistry and Surface Energy To determine which applied voltage leads to the highest loading of surface functional groups without damaging the carbon fibers, we performed 10 min. anodic oxidation with varying applied voltages in 0.5 M ammonium bicarbonate (aq.) at an initial temperature of ~23 °C. As shown in Figure 4, the at. % of surface nitrogen increases steadily with increasing applied voltage, while that of oxygen remains relatively constant above a bias of 1.5 V. Hence, the oxygen-tonitrogen (O:N) ratio changes over the 1.5–21 V range from more oxygen than nitrogen to equal


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE amounts and then to more nitrogen than oxygen. At higher applied voltages (> 14 V), the atomic % of surface heteroatoms (or non-carbon and non-hydrogen atoms) does not change.

Figure 4. Effects of applied voltage on atomic concentration based on high-resolution XPS analysis (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.46, 50

Based on the SEM micrographs (Figure 5), no obvious surface damage was observed until 18−21 V at 35 k magnification (Figure 5e). That is, no pitting or changes in longitudinal grooves were observed for fibers treated with applied voltages of 1.5 to 18 V when other parameters (i.e., starting temperature ≈ 23 °C, electrolyte concentration = 0.50 M, and 10 min. at anodic oxidation only) were held constant. Interestingly, long anodic oxidation (2 h) with relatively low voltage (i.e., 1.5 V) causes severe pitting and striations that penetrate deep into the internal structure of the carbon fibers (see supporting information). Fibers treated at 18 V exhibited less defined longitudinal grooves compared to the untreated fibers. At higher voltages (e.g., 21 V), pitting and roughening/deepening of longitudinal groves on fiber surface were observed. These surface flaws are evidence of electrochemical etching and indicate a damaged surface as has been observed previously in electrochemical surface treatments that employed bases and ammonium salts in the literature.19-20, 38, 41, 51 Because good surface functionalization without obvious fiber damage were observed by XPS and SEM (Figures 4 and 5d) at 16 V, this applied voltage was used in further optimizations discussed below.


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Figure 5. SEM micrographs (x 10 k left, x 35 k right) of discontinuous carbon fibers (a) without treatment and (b)−(e) electrochemically treated with ammonium bicarbonate (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). 3.4. Effect of Electrode Polarity on Surface Functionalization and Surface Energy Carbon fibers are capable of both anodic oxidation and cathodic reduction, but the former is more common and is the subject of multiple publications.19-20, 34, 40-42, 52 Because our electrolytic setup is a batch process, it is possible change the bias during an experiment. Figure 6 shows that broad control of the nitrogen and oxygen contents can be achieved by changing electrode polarity


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE and polarity time. The fibers are subjected to an applied voltage of 16 V, an initial temperature of ~23 °C, and ammonium bicarbonate concentration of 0.5 M. In all cases in Figure 6, the anodic treatment (if any) was conducted before the cathodic treatment. Treatments at both the anode and the cathode increase surface oxygen and nitrogen. Slightly more nitrogen is observed under only anodic oxidation (Figure 6, 100% anode), and the opposite is true for cathodic reduction only (Figure 6, 0 % anode). Total amount of heteroatoms peaked at 80 % anode (or 8 min at the anode followed by 2 min at cathode). Similar to the case for applied voltage (above), the O:N ratio decreases with increasing time at the anode. However, the surface never contained significantly more nitrogen than oxygen. When ammonium bicarbonate is the only electrolyte, the electrochemical procedure that best resembles the commercially available fibers is 5 min at anodic oxidation, followed by 5 min at cathodic reduction for discontinuous carbon fibers (Table 2).46, 50 This mix-biased procedure was used for the remainder of the ammonium bicarbonate studies.

Figure 6. Effect of electrode polarity on surface atomic concentrations of discontinuous fibers based on high resolution XPS analysis (16 V, Ti ≈ 23 °C, 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.46, 50

Table 2. Atomic % based on XPS analyses of commercially available continuous fibers and the tanode=5 min followed by tcathode=5 min in this work. Data for other fibers from 46 and 50. Element AS IM7 this studya C 86.6 ± 0.7 84.5 85.7 ± 1.3 O 12.1 ± 0.7 10.2 9.4 ± 0.9 N 4.3 ± 0.3 5.3 5.0 ± 0.6 a Surface composition of untreated fibers were 98.2–99.6% carbon and 0.4–1.3% oxygen

To determine the reproducibility of the electrochemical cell design, we performed four separate experiments and conducted XPS on 3−12 randomly chosen fibers in each experiment


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE (see Tables S7 and S8 in supporting information). Low standard deviations in the overall atomic concentrations were observed in each of the four separate experiments (Table S7, rows 1-4 in supporting information) as well as in the combined 21 replications (Table S7, row 5 in supporting information). Furthermore, we performed peak fitting for the C 1s region (as described in the supplemental information) and noted that there was good consistency in these elemental speciation results (Table S8 in supporting information). From these results, it is likely that surface treatment is not exclusively on one side of the fiber because the entire fiber has ample access to ammonium bicarbonate electrolyte due to the relatively high permeability of the carbon felt and the location of fibers (i.e., on top of the carbon felt). Moreover, rapid formation of bubbles from the flexible carbon felt probably caused individual fibers to move slightly but continuously during electrolysis. Therefore, our electrolytic method is highly reproducible and relatively uniform for the surface treatment of short carbon fibers.

3.5. Temperature Effects on Surface Chemistry and Surface Energy Broad control over surface loading of nitrogen and oxygen can also be achieved by changing the initial temperature of electrochemical reaction. These experiments were nonisothermal, and temperatures can increase by 40 °C over the course of a run. Because the thermal decomposition of ammonium bicarbonate solution could not be controlled in an opened electrochemical setup, electrolysis was performed immediately after reaching the desired temperature in order to maintain relatively constant concentration. Maintaining constant concentration is also facilitated by the relatively high concentration of ammonium bicarbonate (i.e., 0.5 M) and the relatively fast electrochemical treatment (i.e., 10 min). When fibers were subjected to 5 min at anodic oxidation followed by 5 min at cathodic reduction with an applied voltage of 16 V in an ammonium bicarbonate solution (aq. 0.5 M), nitrogen content varied linearly with initial temperature (Figure 7). On the other hand, the oxygen content did not change much, and only a small decrease was observed at initial temperature of 70.5 °C. Similar to the case for applied voltage (above), the carbon fiber surface changed from oxygen-rich to nitrogen-rich. This change occurs around the initial temperature of 50 °C, which is also the initial temperature that leads to the highest total amount of heteroatoms (Figure 7).


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Figure 7. Effects of initial temperature on oxygen and nitrogen atomic concentrations of electrochemically treated discontinuous carbon fibers based on high-resolution XPS analyses (16 V, tanode = 5 min. followed by tcathode = 5 min., 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.46, 50

3.6. Concentration Effects on Surface Chemistry and Surface Energy Another processing variable that significantly affects the surface loading of nitrogen and oxygen is electrolyte concentration. When discontinuous fibers were subjected to 5 min anodic oxidation followed by 5 min cathodic reduction with an applied voltage of 16 V and an initial temperature of ~23 °C, both nitrogen and oxygen loadings increased steadily with electrolyte concentration. At 0.005M, significant surface O content but negligible N content were observed. Unlike the cases for applied voltage and initial temperature, the surface remains oxygen-rich at all concentrations. At 0.75 M, the oxygen and nitrogen contents are within the standard deviations of commercially available, electrochemically treated continuous carbon fibers.

Figure 8. Effects of electrolyte concentration on surface oxygen and nitrogen atomic % of treated discontinuous carbon fibers based on high-resolution XPS analyses (16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.46, 50


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3.7. Tensile Measurements To confirm that our electrochemical method does not adversely affect tensile properties of carbon fibers, single fiber tensile tests were performed. The tensile strengths and moduli are reported in Figure 9. Here, treated fibers were treated for 5 min at the anode followed by 5 min at the cathode with an applied voltage of 16 V, an ammonium bicarbonate concentration of 0.5 M, and an initial temperature of ~22 °C.) While it seems that the treated fibers showed increased strength (3920 ± 840 MPa for untreated and 4440 ± 1290 MPa for treated), this increase is not outside of a single standard deviation and is therefore not conclusive. Furthermore, the difference in diameter (10.2 ± 0.7 µm for untreated and 10.0 ± 0.7 µm for treated) was not statistically significant. Similarly, the tensile moduli of the untreated and treated carbon fibers are also very similar (800 ± 190 and 840 ± 90 GPa, respectively). Therefore, we reach the modest conclusion that the surface treatment did not significantly influence the fiber strength, modulus or diameter. The relatively high standard deviations observed for the tensile strength can be attributed to handling of the fibers (which may have induced defects) and the relatively short nature of the fibers. Based on the manufacturer’s specifications sheet,53 the as-received chopped carbon fiber tows have a tensile strength of 3800 MPa and a tensile modulus of 900 GPa. These mechanical data are similar to our results, and the differences are probably due to the manufacturer’s data being based on fiber tow tests, while our tensile tests are based on single fibers. However, the manufacturer’s data are still within the statistical error of our tensile results.


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Figure 9. (a) Tensile strength and (b) modulus of untreated and treated carbon fibers (16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). Dashed lines are based on the manufacturer’s specifications.53

3.8. Development of a possible mechanism for surface treatment Understanding the functional groups on carbon fiber surfaces is important for enhancing interfacial interactions and mechanical properties of CFRCs. Because a variety of oxygen- and nitrogen-containing functionalities (e.g. amine, pyrrole, pyridine, amide, lactam, urea, imide, imine, carbamate, pyridine, and nitrile) have the same XPS signals peak positions,54-62 (see supplemental information Table S1) we performed mechanistic studies to help elucidate the identities of the surface functional groups on the carbon fibers that were treated with ammonium bicarbonate. Along these lines, we obtained data for various electrolytes that correspond to the equilibria of ammonium bicarbonate presented in Scheme 1. As shown in Table 3, these include water, ammonium chloride, ammonium hydroxide, sodium bicarbonate, and ammonium carbamate. These alternative cases were chosen, respectively, to test the following effects: (i) water splitting only, (ii) lack of bicarbonate ions under acidic conditions (i.e., essentially no free NH3) (iii) lack of bicarbonate ions under alkaline conditions (i.e., significant amounts of free NH3), (iv) lack of ammonium or ammonia, and (v) a different starting place in the ammonium bicarbonate equilibria (i.e., beginning at the center of Scheme 1 rather than the left side). Since we did perform


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE experiments with different polarities (i.e., the carbon fiber was either the anode or the cathode) the proposed mechanism must account for those differences. Experiments were conducted for each of these different cases at anode-only (10 min, 16V), cathode-only (10 min, 16V) and anode/cathode (5 min anode followed by 5 min cathode, 16V). These experiments were used to develop a possible mechanism for the generation of functional groups on the carbon fiber surface.

Scheme 1. Nitrogen-containing species in the chemical equilibria of ammonium bicarbonate.63-65

Our first control study investigated the influence of water without any added electrolyte. When fibers were subjected to only to water at an applied voltage of 16 V for ten minutes, a significant increase in oxygen content was observed in treatments at either the anode (4.4±0.8%) or the cathode (5.2±1.7%) and at the anode followed by the cathode (2.5±0.7%) (Table 3, trials 2, 8, and 14). Only the single bonded C–O moiety was observed based on C 1s peak fitting (Tables 4‒6), which implies that electrolysis in water gives rise to reactive intermediates H·, HO·, HOO·, and ·O2 reactive intermediates (Scheme 2). These reacted with the carbon fiber to form alcohols, ethers, phenols, and/or epoxies by mechanisms highlighted in blue in the proposed scheme of the anodic reactions (Scheme 3) and in the proposed scheme of the cathodic reactions (Scheme 4). These results suggest that water splitting contributes significantly to the overall surface oxygen content on carbon fibers during the electrolytic reaction whether aqueous ammonium bicarbonate (or any other electrolyte) is present or not. We now focus the discussion on providing a possible mechanism for the electrolyte reactions on the carbon fiber surface that fit the data that we have presented above (Tables 3‒6). Mechanistic studies with ammonium chloride, ammonium hydroxide, and sodium bicarbonate suggest the importance of acidity and bias in obtaining nitrogen-containing functional groups and carbonyl moieties. When subjected to anodic oxidation at an applied voltage of 16 V and an electrolyte concentration of 0.5 M, surface nitrogen was only observed for carbon fibers that were treated with ammonium hydroxide (pH = 13.6) but not with ammonium chloride (pH = 4.9) (Table 3, rows 3 and 4). These results suggest that fairly acidic conditions discourage the formation of nitrogen-containing functionalities on carbon fiber surfaces in electrolysis with ammonium salts. Bias is also an important factor in obtaining nitrogen-containing functional groups, because surface nitrogen was observed in carbon fibers that were treated at the anode but not at the cathode (Table 3, rows 4 and 10). Similarly, some carbonyl moieties (2.8 ± 0.7 %) were observed


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE on carbon fibers that were treated with aqueous sodium bicarbonate at the anode (Table 4) but not at the cathode (Table 5).


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE Table 3. Mechanistic study for discontinuous fibers based on high resolution XPS analysis (applied voltage = 16 V, Ti ≈ 23 °C, 0.5 M electrolyte). Trial Anode 1 2 3 4 5 6 Cathode 7 8 9 10 11 12 Anode/Cathode 13 14 15 16 17 18

Electrolyte (0.5 M)

Conductivity (mS cm‒1)

pH

T range (°C)

%C

%O

%N

%Cl

%Na

NH4HCO3 H2O NH4Cl

38.9 --58.6

7.8 --4.9

22.0‒73.0 23.0‒23.0 22.0‒92.0

86.7 ± 0.5 95.6 ± 0.8 91.6 ± 1.5

6.5 ± 0.4 4.4 ± 0.8 8.0 ± 1.4

6.7 ± 0.3 --0

-----

NH4OH NaHCO3 NH2CO2NH4

0.42 30.2 43.8

13.6 8.2 9.3

17.0 to 13.0 23.0‒71.0 20.4‒73.0

92.6 ± 0.4 85.7 ± 0.5 87.1 ± 1.4

4.3 ± 0.8 12.7 ± 0.5 5.9 ± 0.8

3.1 ± 0.3 --7.1 ± 0.8

----Trac e -------

--1.6 ± 0.1 ---

NH4HCO3 H2O NH4Cl NH4OH NaHCO3 NH2CO2NH4

38.9 --58.6 0.42 30.2 43.8

7.8 --4.9 13.6 8.2 9.3

21.0‒62.5 21.8‒22.0 22.0‒91.0 19.5 to 14.0 22.0‒67.0 21.5‒73.0

93.5 ± 2.5 94.8 ± 1.7 93.8 ± 1.3 95.8 ± 4.0 95.1 ± 0.8 90.4 ± 2.2

5.4 ± 1.8 5.2 ± 1.7 5.7 ± 1.1 4.0 ± 0.8 4.9 ± 0.8 6.8 ± 1.9

1.1 ± 0.7 --bd 0 --2.8 ± 0.4

----trace -------

--------bd ---

NH4HCO3 H2O NH4Cl NH4OH NaHCO3 NH2CO2NH4

38.9 --58.6 0.42 30.2 43.8

7.8 --4.9 13.6 8.2 9.3

22.0‒65.5 21.0‒21.0 22.2‒91.0 21.0 to 14.0 22.0‒64.0 21.5‒75.0

85.7 ± 1.3 97.5 ± 0.7 95.0 ± 0.9 92.4 ± 0.8 88.3 ± 1.4 82.3 ± 1.1

9.4 ± 0.9 2.5 ± 0.7 4.8 ± 0.8 5.7 ± 0.7 11.2 ± 1.4 10.8 ± 0.6

5.0 ± 0.6 --trace 1.9 ± 0.3 --7.0 ± 0.6

----Bd -------

--------0.5 ± 0.2 ---

*bd = below detection


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE Table 4. Surface chemical compositions (at. %) of mechanistic studies (Table 3) at the anode based on high resolution XPS analyses. Binding energies (BE) and full width at half maximum (FWHM) are in eV. (Applied voltage = 16 V, tanode = 10 min, Ti ≈ 23 °C, 0.5 M electrolyte) Element

NH4HCO3 BE at. % (fwhm)

BE (fwhm)

H2O

284.6 (0.9) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.8)

284.6 (0.7) 286.2 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.3)

at. %

NH4Cl BE at. % (fwhm)

NH4OH BE at. % (fwhm)

NaHCO3 BE at. % (fwhm)

NH2CO2NH4 BE at. % (fwhm)

284.6 (0.7) 286.2 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.2)

284.6 (0.7) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.2)

284.6 (0.7) 286.1 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.3)

284.6 (0.9) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.3)

C 1s C–C/C–H C–O/C–N/C=N C=O/N–C–O O–C=O/N–C=O π→π*

85.5±1.1 7.8±0.8 2.3±0.3 2.7±0.4 1.8±0.2

94.5±1.1 4.3±1.0 0 0 1.3±0.3

90.6±1.7 8.0±1.2 0 0 1.2±0.9

91.9±0.4 6.3±0.8 0.4±0.4 0.4±0.2 1.0±0.2

85.3±1.1 11.5±0.7 0 2.1±0.7 0.7±0.3

84.5±3.0 8.4±0.8 2.2±0.7 3.1±0.9 1.8±01.5

O 1s O–C/O=C

532.0. (2.6)

100

532.4 (2.1)

100

532.7 (2.1)

100

399.5 (2.7)

100

---

0

---

0

---

0

---

0

---

0

532.1 (2.3)

100

532.5 (3.0)

100

532.0 (2.8)

100

83.0±6.9

---

0

399.7 (2.8)

100

17.0±6.9

---

0

---

0

N 1s R2NH2/RNH2/RCONH2 R2NH3+

399.4 (2.5) 401.9 (2.5)

Table 5. Surface chemical compositions (at. %) of mechanistic studies (Table 3) at the cathode based on high resolution XPS analyses. Binding energies (BE) and full width at half maximum (FWHM) are in eV. (Applied voltage = 16 V, tcathode = 10 min., Ti ≈ 23 °C, 0.5 M electrolyte) element


BE (fwhm)

H2O

284.6 (0.8) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.4)

284.6 (0.8) 285.9 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.7)

at. %





284.6 (0.7) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.6)

284.6 (0.8) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.1)

284.6 (0.7) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.3)

284.6 (0.8) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.0)


92.2±1.8 4.9±1.4 0.6±0.5 0.9±0.5 1.4±0.7

93.0±1.2 4.4±1.0 0 0 2.4±1.8

91.8±1.3 6.5±1.5 0 0 1.6±0.7

95.0±0.5 4.5±0.5 0 0 0.6±0.4

93.4±1.8 4.7±0.6 0 0.3±0.5 1.4±1.1

88.6±1.0 9.8±0.5 0.4±0.1 0.9±0.5 0.6±0.3

O 1s O–C/O=C

532.4 (2.4)

100

532.4 (1.9)

100

532.5 (2.1)

100

532.4 (2.0)

100

532.5 (2.2)

100

532.4 (2.2)

100

399.9 (2.4)

100

---

0

---

0

---

0

---

0

399.7 (2.3)

100

N 1s R2NH2/RNH2/RCONH2


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Table 6. Surface chemical compositions (at. %) of mechanistic studies (Table 3) for the mix-bias experiments based on high resolution XPS analyses. Binding energies (BE) and full width at half maximum (FWHM) are in eV. (Applied voltage = 16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C, 0.5 M ammonium bicarbonate) element


BE (fwhm)

H2O

284.6 (0.9) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.3)

284.6 (0.7) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (2.0)

at. %





284.6 (0.7) 286.1 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (1.7)

284.6 (0.8) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (0.7)

284.6 (0.8) 286.1 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (0.7)

284.6 (0.9) 286.0 (1.5) 287.1 (1.5) 288.6 (1.5) 291.1 (0.9)


85.5±1.5 9.2±1.0 1.1±0.4 3.3±0.6 0.8±0.4

94.9±0.8 2.1±0.5 0 0 3.0±1.3

93.0±1.5 4.9±1.0 0 0 2.0±0.7

92.7±1.2 7.1±1.3 0 0 0.2±0.2

90.0±1.2 10.5±1.5 0.5±0.5 0.8±0.2 0.3±0.4

81.9±1.3 13.1±0.2 1.9±0.1 2.8±0.7 0.3±0.3

O 1s O–C/O=C

532.1. (2.6)

100

532.3 (1.8)

100

532.5 (2.0)

100

532.4 (2.1)

100

532.6 (2.5)

100

532.3 (2.3)

100

399.8 (2.4)

100

---

0

---

0

399.7 (2.8)

100

---

0

399.6 (2.5)

100

N 1s R2NH2/RNH2/RCONH2

Using XPS and time-of-flight secondary ion mass spectrometry (TOF SIMS), Alexander and Jones proposed that amide formation via pathway B (Scheme 3) is the main nitrogen-containing functionality following electrochemical oxidation of continuous carbon fibers with aqueous ammonium bicarbonate.46, 66 If pathway B is the only route to nitrogen-containing functionality under oxidizing environments (e.g. electrolysis at the anode), then there could never be more nitrogen than oxygen because the theoretical O:N ratio is 1:1 for the amide-only functionalization. In addition, a C‒X:X‒C=O ratio (where X is O or N) of 0:1 would be expected for amide-only functionalization. Hence, other pathways toward nitrogen functionalities must also occur because Alexander and Jones’ hypothesis could not explain how there could be more nitrogen than oxygen on carbon fiber surfaces and why there are always significantly more C‒N/C‒O moieties compared to X‒C=O moieties (Tables 7) even for cases were the O:N ratio is approximately 1:1 (Table 3, rows 1 and 6).


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE We hypothesized that the chemical equilibria of ammonium bicarbonate (Scheme 1) and the associated radical/ionic reactive intermediates (Scheme 2) during electrolysis significantly affect the surface functionalization of carbon fibers (Schemes 3 and 4). In general, the complex chemical equilibria on the top row of Scheme 1 are applicable whenever ammonium carbonate, ammonium bicarbonate, ammonium carbamate, or a mixture of ammonium chloride and sodium bicarbonate are dissolved in water.63 At room temperature and high concentration, there is about 84 % ammonium bicarbonate and ≤16 % ammonium carbamate/carbonate. But, this equilibrium shifts to the right towards ammonia/ammonium hydroxide when the temperature increases.64 While the Raman spectroscopic studies by Brooker and Wen showed that no urea was detected in an ammonium carbamate solution under basic conditions, water splitting during electrolysis may generate a sufficient amount of protons to facilitate the formation of urea in the complex equilibrium of ammonium bicarbonate.67 Moreover, the Bazarov synthesis method demonstrates that urea can be prepared by heating ammonium carbamate.68 In addition, computational studies by Tsipis and Karipidis showed that the activation barriers for this conversion would be significantly reduced in polar environments (e.g. water and ammonia).65 They also proposed that isocyanate/isocyanic acid as a feasible intermediate in the conversion from carbamate/carbamic acid to urea. We will focus first on a possible mechanism occurring when the carbon fiber is at the anode, and our discussion follows Scheme 3. We first note that water-only electrolysis produces a significant oxygen content of 4.4 ± 0.8 at. % (Table 3), while showing no evidence of either carbonyl or carboxylic acid character in the C 1s or O 1s speciation fits (Table 4). These results imply that water-only electrolysis results in surfaces with hydroxyl or epoxy groups (1, 2, 7, and 12) via the reactive species present due to water splitting or via Kolbe electrolysis (Scheme 3, blue box).69 The addition of ammonium chloride (low pH, essentially no free NH3), results in an increase in oxygen content to 8.0 ± 1.4 at. % but still does not provide any nitrogen (Table 3) and again no carbonyl or carboxyl character (Table 4). This result implies that the same mechanism is in play as for water-only electrolysis, but that the low pH facilitates the addition of oxygen at the surface.70 A very different result was observed for the addition of ammonium hydroxide (high pH, significant amounts of free NH3), wherein we see an oxygen content similar to that of water only (i.e., 4.3 ± 0.8 at. %), and the addition of a significant amount of surface nitrogen (3.1 ± 0.3 at. %). The speciation fits suggest that the majority of the nitrogen-containing functionalities that are generated from ammonia do not contain carbonyl moieties (Table 4). These results suggest that the presence of NH3 opens up routes to 8, 10, 11 and 13, but 8 and/or 13 are preferred. Next, we note that the use of sodium bicarbonate as the electrolyte results in no nitrogen deposition (not surprisingly since there was no nitrogen in the electrolyte) and an increase in oxygen content, along with observed carboxyl/ester character in both the C 1s and O 1s fits. This suggests that paths to 4 (e.g., via 3) and 16 (via Kolbe-Schmitt reaction with 12 and carbon dioxide) are available under basic conditions.70 When electrolysis was performed in the presence of both ammonium and bicarbonate ions, additional routes that generate amide 11 and isocyanates 9 and 14 are possible via isocyanide or isocyanate radical as the heteroatom sources. Finally, we note that the use of ammonium carbamate results in oxygen and nitrogen contents that are very similar to those obtained from ammonium bicarbonate, except that the nitrogen content (7.1 ± 0.8 at. %) is somewhat higher than that of the oxygen (5.9 ± 0.8 at. %). This requires pathways to species that have no oxygen (e.g., 10 and 13) or those that are high in nitrogen (e.g., 15), which may be accessible through a pathway involving Kolbe electrolysis.69, 71 We stress here that this


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE mechanism is not proven nor comprehensive, but is a reasonable proposal based on the literature and our experimental observations herein. We now turn our attention to when the carbon fiber was used as the cathode (Table 3), which corresponds to a separate proposed reaction mechanism shown in Scheme 4. Again the same electrolytes were used to determine the surface chemistry of the resulting carbon fiber. We first note that, rather surprisingly, the oxygen content and speciation obtained using water-only electrolysis was about the same as that when the carbon fiber was at the anode (Table 3, rows 2 and 8). This suggests that the water splitting that occurs generates species that react with the carbon fiber surface independent of electrode polarity. Second, we note that electrolysis with the acidic ammonium chloride solution and the basic sodium bicarbonate solution produced a similar oxygen content as that of the water-only electrolysis (Table 3). These results are different compared to those of the anode, which showed a significant increase in oxygen content compared to the water-only electrolysis. In addition, no nitrogen was observed following electrolysis at the cathode with ammonium hydroxide but some nitrogen was observed for that at the anode (Table 3, rows 4 and 10). In addition, significantly less nitrogen was observed for electrolysis with ammonium bicarbonate at the cathode compared to that at the anode. Differences in the contents of oxygen and nitrogen between electrolysis at the anode and cathode may be explained by differences in the reactivity of intermediates 5b and 6b compared to those of 5a and 6a, because intermediates 5b and 6b could not react with hydroxyl radical (or hydroxide), hydroperoxyl radical (or hydroperoxide ion), amine radical (or ammonia), or abstract oxygen from water due to their negative charge, while the positively charged or neutral intermediates 5a and 6a can readily do so. Conversely, intermediates 5a and 6a are not as reactive as intermediates 5b and 6b towards carbon dioxide, which would form carboxylic acids 4 and 18. Electrolysis with ammonium carbamate generated similar amounts and species of oxygen and nitrogen as those of the electrolysis with ammonium bicarbonate, except slightly higher surface oxygen and nitrogen contents were observed for ammonium carbamate (Tables 3 and 5).

Scheme 2. Depiction of the multiple species in ammonium bicarbonate chemical equilibria (Scheme 1) and reactive intermediates, including those generated at the auxiliary/counter electrode during the surface functionalization of carbon fibers.


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Scheme 3. Possible pathways toward surface functionalization of carbon fibers during anodic oxidation in aqueous ammonium bicarbonate. Blue highlighted reactions can occur in the absence of electrolytes (or water-only electrolysis).66, 70-71


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Scheme 4. Possible pathways toward surface functionalization of carbon fibers during cathodic treatment in aqueous ammonium bicarbonate. Blue highlighted reactions can occur in the absence of electrolytes (or water-only electrolysis).66, 70-71

Our proposed mechanisms in Schemes 3 and 4 also explain other trends observed in the previous sections on varying processing conditions (i.e., applied voltage, mixed electrode bias, initial temperature, and electrolyte concentration). More nitrogen was observed on carbon fibers that had longer treatments at the anode in the mixed-bias studies (Figure 6). These experiments have very similar temperature changes over the entire range (i.e., 21.0‒67.5 °C), and this similarity in temperature means that the relative concentrations of species in the chemical equilibria of ammonium bicarbonate, including ammonium/ammonia, in Scheme 2 should be similar for all mixed-biased experiments. These results are consistent with Schemes 3 and 4, because differences in surface functionalizations must result from differences in chemical transformations at the electrodes, and Schemes 3 and 4 show that the cathode has fewer pathways toward nitrogen-containing functionalities. Interestingly, more nitrogen compared to oxygen was observed on carbon fiber surfaces whenever the temperature rose above ~80 °C over the course of the electrochemical treatment including in the following cases: (1) 18 V, tanode = 10 min, Ti ≈ 23 °C, 0.5 M NH4HCO3, (2) 21 V, tanode = 10 min, Ti ≈ 23 °C, 0.5 M NH4HCO3, (3) 16 V, tanode = 5 min. followed by tcathode = 5 min., Ti = 50.0 °C, 0.5 M NH4HCO3, and (4) 16 V, tanode


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE = 5 min. followed by tcathode = 5 min., Ti = 70.5 °C, 0.5 M NH4HCO3. This trend seems more or less independent of the applied voltage, percentage of time at anode, and initial temperature. We can explain this observation by noting that pathways C, E, H and I should be more readily accessible for two reasons: (1) because most (if not all) electrochemical transformations should have enough energy to proceed at these relatively high applied voltages and (2) the high temperature should shift the chemical equilibria of ammonium bicarbonate (Scheme 1) to give higher relative concentration of urea (in addition to ammonia) 63. These reactions can help explain how high temperature could increase the relative surface contents of nitrogen over that of oxygen while always having more C‒X moieties than X‒C=O/X=C=O moieties (see supporting information). To our knowledge, no one has proposed that nitrogen species other than ammonium/ammonia in the chemical equilibria of ammonium bicarbonate can also participate in functionalizing carbon fiber surface. Further support for this hypothesis is provided by the electrolysis with ammonium carbamate, because ammonium carbamate exists in ammonium bicarbonate chemical equilibria (Scheme 1) and gives nearly identical oxygen and nitrogen speciations as those of ammonium bicarbonate. In other words, these data suggest ammonium bicarbonate and ammonium carbamate share the same reactive intermediates in electrolysis. Slightly higher nitrogen is observed for ammonium carbamate electrolysis over that of ammonium bicarbonate, and this difference probably results from faster conversion of carbamate (due to its position in the equilibria in Scheme 2) to the reactive intermediates (i.e., ammonia, isocyanate, and urea). Conclusions Here, we have described a method for electrochemically treating multiple discontinuous carbon fibers without surface damage or loss of mechanical properties based on XPS, SEM and single fiber tensile tests. Our electrochemical surface treatment of discontinuous carbon fibers is relatively rapid (i.e., 10 min) and gives surface element loadings that are similar to commercially treated continuous fibers. Our operationally simple electrochemical design is highly reproducible and provides uniform surface treatment even for experiments wherein we switched the bias between electrodes during the experiment. The latter capability is unique to batch processes and cannot be used in the roll-to-roll continuous processes. Because these studies were performed on DIALEAD short carbon fiber tows that had been de-sized and have negligible heteroatoms (i.e., 0.4–1.3 at % oxygen) to begin with, the conditions optimized herein are directly applicable to pyrolytically reclaimed fibers. Broad control over surface functionalization was achieved by varying applied voltage, bias, initial temperature, and concentration. In general, total loading of surface heteroatoms (or elements that are not carbon or hydrogen) increases with increasing applied voltage, initial temperature, and electrolyte concentration. The oxygen-to-nitrogen ratio also changes when varying each of the four parameters, and there are usually more surface oxygen atoms than surface nitrogen atoms, but more nitrogen is observed at relatively high voltages (>18 V) and at high initial temperatures (>50 °C). We conducted various control experiments using different electrolytes to develop proposed mechanisms for the surface treatment when the carbon fiber is either the cathode or the anode. These mechanisms account for the surprisingly high oxidation of carbon fibers due to water splitting at near neutral pH as well as the participation of multiple species in the aqueous ammonium bicarbonate chemical equilibria. Specifically, our mechanisms expanded the mechanism of amide formation, which was proposed by Alexander and Jones,46, 66 to include


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DISTRIBUTION A: APPROVED FOR PUBLIC RELEASE multiple pathways toward N-containing and O-containing functionalities and are able explain how there could be more nitrogen than oxygen on surfaces. To our knowledge, no one has previously proposed that other nitrogen species in the chemical equilibria of ammonium bicarbonate can also participate in functionalizing carbon fiber surface. We believe that our method will provide a critical technology needed for the surface treatment of recycled carbon fibers that have been reclaimed via pyrolysis. Supporting information XPS fitting details and high resolution XPS spectra of C 1s, O 1s, and N 1s are provided in the supporting information. Acknowledgements The research reported in this document was performed in connection with contracts/instruments W911QX-16-D-0014 and W911NF-18-2-0304 with the U.S. Army Research Laboratory. Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the use thereof. This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government.

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Figure 1. Life-cycle phases of carbon fiber reinforced composites (CFRCs). 338x190mm (96 x 96 DPI)


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Figure 2. Schematic representation of electrochemical cell for treatment of discontinuous carbon fibers in this study. 338x190mm (96 x 96 DPI)


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Figure 3. XPS survey scans and fitting curves of high-resolution XPS spectra of carbon, oxygen and nitrogen on electrochemically treated discontinuous carbon fibers at different applied voltages with the same parameters (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). 338x190mm (96 x 96 DPI)


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Figure 4. Effects of applied voltage on atomic concentration based on high-resolution XPS analysis (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.[46, 50] 82x69mm (300 x 300 DPI)


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Figure 5. SEM micrographs (x 10 k left, x 35 k right) of discontinuous carbon fibers (a) without treatment and (b)−(e) electrochemically treated with ammonium bicarbonate (tanode = 10 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). 298x536mm (150 x 150 DPI)


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Figure 6. Effect of electrode polarity on surface atomic concentrations of discontinuous fibers based on high resolution XPS analysis (16 V, Ti ≈ 23 °C, 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.[46, 50] 82x69mm (300 x 300 DPI)


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Figure 7. Effects of initial temperature on oxygen and nitrogen atomic concentrations of electrochemically treated discontinuous carbon fibers based on high-resolution XPS analyses (16 V, tanode = 5 min. followed by tcathode = 5 min., 0.5 M NH4HCO3). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.[46, 50] 82x70mm (300 x 300 DPI)


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Figure 8. Effects of electrolyte concentration on surface oxygen and nitrogen atomic % of treated discontinuous carbon fibers based on high-resolution XPS analyses (16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C). Shaded regions correspond to reported surface contents of commercially available, electrochemically treated continuous carbon fibers.[46, 50] 82x65mm (300 x 300 DPI)


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Figure 9. (a) Tensile strength and (b) modulus of untreated and treated carbon fibers (16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). Dashed lines are based on the manufacturer’s specifications.[53] 82x63mm (300 x 300 DPI)


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Figure 9. (a) Tensile strength and (b) modulus of untreated and treated carbon fibers (16 V, tanode = 5 min. followed by tcathode = 5 min., Ti ≈ 23 °C, 0.5 M NH4HCO3). Dashed lines are based on the manufacturer’s specifications.[53] 82x64mm (300 x 300 DPI)


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Scheme 1. Nitrogen-containing species in the chemical equilibria of ammonium bicarbonate.[61-63] 169x49mm (300 x 300 DPI)


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Scheme 2. Depiction of the multiple species in ammonium bicarbonate chemical equilibria (Scheme 1) and reactive intermediates, including those generated at the auxiliary/counter electrode during the surface functionalization of carbon fibers. 122x78mm (300 x 300 DPI)


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Scheme 3. Possible pathways toward surface functionalization of carbon fibers during anodic oxidation in aqueous ammonium bicarbonate. Blue highlighted reactions can occur in the absence of electrolytes (or water-only electrolysis).[66, 70, 71] 186x209mm (300 x 300 DPI)


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Scheme 4. Possible pathways toward surface functionalization of carbon fibers during cathodic treatment in aqueous ammonium bicarbonate. Blue highlighted reactions can occur in the absence of electrolytes (or water-only electrolysis).[66, 70, 71] 193x150mm (300 x 300 DPI)


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Table of Contents Graphic 301x73mm (96 x 96 DPI)


Electrochemical Surface Treatment of Discontinuous Carbon

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