Patent Application
20250052007
Not applicable.
Not applicable.
Carbon-fiber paper (CFP) is an inexpensive, nontoxic, robust, scalable, and chemically inert material with high electrical conductivity and high surface area. These beneficial properties make CFP desirable for use in many electrochemical, fuel cell,1,2 electrolyzer,3,4 supercapacitor,5-10 sensing,11, 12 water desalination13 and wastewater treatment 14′ applications. Another advantage is the excellent biocompatibility of CFP, which renders it useful for in-vitro and in-vivo studies,16 for use as a three-dimensional scaffold for tissue engineering,17 and as a biomaterial with applications in regenerative medicine and cancer treatment.18 Globally scalable clean energy19-21 and water purification22, 23 technologies as well as biocompatible systems operate in aqueous media and, therefore, use electrode support materials that are hydrophilic for long periods of time.
One obstacle to widespread use of CFP in aqueous systems is its high hydrophobicity. Water is repelled at the outer surface of CFP and forms droplets because water does not wet individual carbon fibers and thus cannot penetrate the carbon fiber network and make contact with inner carbon fibers. Such contact is needed to take advantage of the high internal surface area of CFP in aqueous media. Hydrophilicity resulting from carbon surface modification is required in applications that rely on close electrical contact of CFP carbon with catalysts or the environment. Hydrophilic lithium-ion conducting thin film polyelectrolyte coatings, which were prepared by initiated chemical vapor deposition, have been reported,24 but such polymeric overlayers impedes efficient electron transfer that is needed for CFP to serve as high surface area electrode support. Widespread use of CFP has been impeded, however, by that lack of hydrophilicity-imparting treatments that leave the carbon fiber network structurally intact, so that specifically designed carbon network architectures and pore sizes, which can critically affect mass transport characteristics, are not destroyed or undesirably altered during the process.
A practical way to monitor hydrophilicity is the sink-or-float test in deionized water. When dry CFP is dropped into water, hydrophilic CFP immediately sinks because hydrophilic carbon surfaces are sufficiently wetted by water to displace all air trapped in dry CFP. In contrast, hydrophobic CFP floats because its carbon repels water so that trapped air cannot escape, rendering the CFP buoyant. Floating CFP can be agitated in water, e.g., by pushing it under water, to monitor sinking with agitation or resurfacing. In this patent specification, time of sustained hydrophilicity refers to the time passed until a CFP piece stopped sinking altogether, even upon agitation.
Popular methods to increase hydrophilicity of CFP are plasma or chemical etching techniques, sometimes in combination with heat treatments. Ammonia,14 nitrogen,25 oxygen13,26-28 or air29 plasma etching protocols have been reported. Such plasma etches can work reasonably well on a laboratory scale, where electrode sizes are on the order of square centimeters or less. But even on that small scale, plasma methods lead to significant embrittlement of CFP, precluding the production of structurally stable, large area CFP sheets that can be needed for industrial scale electrolyzers and other applications. For example, oxygen plasma treatment of each side of a 2.8 cm2 CFP piece at 150 W power for 5 min, followed by annealing in air at 800° C. for 5 min, resulted in CFP that was very brittle, but no longer floated on water after some agitation.26 Ozone treatment also oxidizes carbon surfaces for improved hydrophilicity.30,31 An additional disadvantage is that plasma and ozone generators require large capital expense, especially for large area substrates.
Solution-processable modifications are a more cost-effective alternative. Reported acid treatments are immersion in a 3:1 (v/v) mixture of concentrated sulfuric and nitric acids at 60° C. for 1 hour7,10,32 or 5 hours,33 sonication in the same acid mixture for 15 min at room temperature,34 immersion in concentrated sulfuric acid at 60° C. for 2 hours,35 ultrasonication in 5 M aqueous HCl, absolute ethanol and water for 15 min each,36 soaking in concentrated nitric acid at 115° C. for 80 min and followed by soaking in 1 M NaOH,12 or immersion in concentrated sulfuric acid with sodium nitrite, potassium permanganate and hydrogen peroxide at 0° C. for a total of 5 hours.37 These tedious acid etch surface treatments raised hydrophilicity, but materials were further functionalized immediately, indicating that hydrophilicity of carbon surfaces did not last.
A hydrothermal process using sodium citrate in water-ethanol solution in a Teflon-lined stainless-steel autoclave at 180° C. for 12 h was reported for CFP activation for further impregnation by salicylic acid.11 Hydrothermal methods require expensive autoclaves, whose scaleup for large area CFP sheets is inherently difficult and limited by the safety and design of reactors. The long preparation time of this hydrothermal process is another obstacle for many applications.
Thermal oxidations have also been used to increase hydrophilicity. Washing CFP with water and ethanol three times, drying the CFP at 60° C. for 12 h and baking it at 500° C. in air for 3 h after a temperature ramp of 5° C. per min was reported to lead to long-term durability,38 but hydrophilicity of CFP stored in air lasted only for 2 days when following the procedure described in Zhang et al., which takes in total more than 16 h. This long preparation time can preclude most applications and is not conducive for commercial viability.
Another approach to enhance CFP hydrophilicity is electro-etching. Two reports on electro-etched CFP in the literature were found.39, 40 One described the electrooxidation of CFP in a mixed H2SO4—HNO3 acid (1:1 v/v) electrolyte at an applied potential of +2 V vs. the saturated calomel electrode for 10 min; the counter electrode was Pt wire. Subsequently, the CFP was washed with deionized water and dried at 60° C. Photographs and scanning electron microscopy (SEM) images before and after treatment reported by Wang et al. indicate that the carbon fibers were significantly damaged upon electrooxidation in the harsh acid electrolyte.39 The other procedure is a combination of an acid etch and electrooxidation. The acid-etch consisted of immersion in 1 M aqueous HNO3, followed by washing with deionized water and drying at 80° C. for 2 h. The electrooxidation was performed in 1 M aqueous sulfuric acid electrolyte at +2.1 V vs. Ag/AgCl for 10 min; a Pt wire served as counter electrode. This acid-bath-electro-etch preparation also led to marked damage to the carbon fibers, as evidenced by SEM images before and after treatment, as provided by Kazemi et al.40
It would be desirable to have additional methods of rendering CFP hydrophilic, without embrittlement of the CFP, without damaging the structure of the carbon fibers and without the expense and capital investment required by the techniques currently available in the art. Surprisingly, the present invention fills these and other needs.
The present invention provides methods for rendering hydrophobic carbon fiber paper (CFP) hydrophilic without damaging the internal structure of the paper or the carbon fibers that compose the paper, and provides hydrophilic CFP that is composed of carbon fibers with a high edge density and high hydroxyl content, but that are undamaged when imaged by scanning electron microscopy.
In one group of embodiments, the invention provides methods of providing hydrophilic carbon fiber paper (CFP) with undamaged carbon fibers. The methods comprise, in the following order: (a) subjecting to sonication in an aqueous surfactant solution hydrophobic CFP comprising a plurality of carbon fibers in an arrangement, which fibers have exteriors; and, (b) subjecting said sonicated CFP to electrooxidation in an aqueous electrolyte that does not damage the exteriors of the fibers or the arrangement of the fibers in the CFP; thereby providing hydrophilic CFP with carbon fibers that are undamaged. In some embodiments, the CFP serves as a working electrode in the electrooxidation in the aqueous electrolyte. In some embodiments, the sonication in the aqueous surfactant solution is for a time of 10 to 240 minutes. In some embodiments, the sonication in the aqueous surfactant solution is for a time of 10 to 60 minutes. In some embodiments, the sonication in the aqueous surfactant solution is for a time of 20±5 minutes. In some embodiments, the aqueous surfactant solution is environmentally benign. In some embodiments, the aqueous surfactant solution does not comprise organic solvents. In some embodiments, the method further comprises step (c) storing said CFP in water following the electrooxidation. In some embodiments, the method further comprises step (a1), rinsing the sonicated CFP with water between steps (a) and (b). In some embodiments, the method further comprises step (b1), rinsing the CFP with water following step (b). In some embodiments, the method further comprises step (b2), drying said CFP with dry nitrogen gas following the rinsing of step (b1). In some embodiments, the method further comprises step (c), storing said CFP in water until use. In some embodiments, the aqueous surfactant is sodium dodecyl sulfate, cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate, cetyltrimethylammonium bromide, citrate, or polyethylene glycol tert-octylphenyl ether, or a combination of two or more of these. In some embodiments, the aqueous surfactant is sodium dodecyl sulfate, cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate, cetyltrimethylammonium bromide, citrate, or polyethylene glycol tert-octylphenyl ether. In some embodiments, the aqueous surfactant solution is sodium dodecyl sulfate or cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate. In some embodiments, the aqueous surfactant is sodium dodecyl sulfate. In some embodiments, the aqueous surfactant is present at a concentration of 0.5 M to 3 M. In some embodiments, the aqueous surfactant is present at a concentration of 0.5 M to 2.5 M. In some embodiments, the aqueous surfactant is present at a concentration of 0.5 M to 2 M. In some embodiments, the aqueous surfactant is present at a concentration of 0.5 M to 1.75 M. In some embodiments, the aqueous surfactant is present at a concentration of 0.75 M to 1.25 M. In some embodiments, the aqueous surfactant is present at a concentration of 1 M±0.10 M. In some embodiments, the aqueous surfactant is present at a concentration of 1 M±0.02 M. In some embodiments, the aqueous electrolyte is potassium hydroxide, potassium nitrate, lithium perchlorate, or potassium bicarbonate, or a combination of two or more of these. In some embodiments, the aqueous electrolyte is potassium hydroxide, potassium nitrate, lithium perchlorate, or potassium bicarbonate. In some embodiments, the aqueous electrolyte is potassium hydroxide. In some embodiments, the aqueous electrolyte is potassium nitrate. In some embodiments, the aqueous electrolyte is lithium perchlorate. In some embodiments, the aqueous electrolyte is potassium bicarbonate. In some embodiments, the aqueous electrolyte is present at a concentration of from 0.01M to 1 M. In some embodiments, the aqueous electrolyte or combination thereof is present at a concentration of from 0.05M to 0.5 M. In some embodiments, the aqueous electrolyte or combination thereof is present at a concentration of 0.1 M±0.05 M. In some embodiments, the aqueous electrolyte or combination thereof is present at a concentration of 0.1 M±0.02 M. In some embodiments, the aqueous surfactant is sodium dodecyl sulfate and said aqueous electrolyte is potassium hydroxide. In some embodiments, the sodium dodecyl sulfate is present at a concentration of from 0.5 M to 1.5 M and said potassium hydroxide is present at a concentration of from 0.01M to 0.5 M. In some embodiments, the sodium dodecyl sulfate is present at a concentration of 1 M±0.1 M and said potassium hydroxide is present at a concentration of 0.1 M±0.02 M.
In another group of embodiments, the invention provides hydrophilic carbon fiber paper (CFP) that (a) remains hydrophilic when stored under water for 30 days and, (b) is composed of carbon fibers that are undamaged. In some embodiments, whether the carbon fibers are undamaged is determined by imaging said carbon fibers by scanning electron microscopy (SEM). In some embodiments, the CFP remains hydrophilic when stored under water for 60 days. In some embodiments, the CFP has both a high edge density and a high density of hydroxyl groups. In some embodiments, the CFP is not brittle. In some embodiments, the hydrophilic CFP is made by the process of subjecting hydrophobic CFP, which hydrophobic CFP is composed of a network of carbon fibers, to sonication in an aqueous surfactant solution followed by electrooxidation in an aqueous electrolyte. In some embodiments, the network of carbon fibers is undamaged after said process of sonification and electrooxidation compared to said network of carbon fibers of said hydrophobic CFP prior to said process. In some embodiments, whether the network of carbon fibers of the hydrophilic CFP is undamaged compared to said network of carbon fibers of the hydrophobic CFP is determined by SEM imaging the carbon fibers of the hydrophobic CFP, thereby creating a first SEM image, and SEM imaging the carbon fibers of the hydrophilic CFP, thereby creating a second SEM image, and comparing the first and the second SEM images for damage to the carbon fibers, wherein if there is no visible fraying of said carbon fibers of the second image compared to the carbon fibers in the first image, the carbon fibers are undamaged.
In still another group of embodiments, the invention provides hydrophilic carbon fiber paper (CFP) that (a) remains hydrophilic for at least 30 days when stored under water and, (b) is not brittle. In some embodiments, the CFP is composed of carbon fibers that are undamaged. In some embodiments, the determination of whether the carbon fibers are undamaged is made by imaging them by scanning electron microscopy. In some embodiments, the CFP remains hydrophilic when stored in ambient air having a relative humidity of 78%±2.5% for 15 days. In some embodiments, the CFP remains hydrophilic when stored in ambient air having a relative humidity of 78%±2.5% for 30 days. In some embodiments, the CFP has both a high edge density and a high density of hydroxyl groups. In some embodiments, the CFP is made by the process of subjecting hydrophobic CFP comprising a network of carbon fibers with an architecture to sonication in an aqueous surfactant solution followed by electrooxidation in an aqueous electrolyte. In some of these embodiments, the CFP is remains hydrophilic for at least four weeks after said sonication and electrooxidation, and the network of carbon fibers with an architecture is the same after said sonification and electrooxidation as that of said starting hydrophobic CFP.
In a further group of embodiments, the invention provides water purification systems, capacitors, flow batteries, aqueous electrolyzers, and sensors comprising hydrophilic carbon fiber paper that (a) remains hydrophilic when stored under water for 30 days and, (b) is composed of carbon fibers that are undamaged. In some embodiments, the hydrophilic CFP is not brittle. In some embodiments, the CFP has both a high edge density and a high density of hydroxyl groups.
As set forth in the Background, hydrophilic carbon fiber paper, or “CFP,” is an inexpensive, nontoxic, robust, scalable, biocompatible, and chemically inert material with high electrical conductivity and high surface area. These beneficial properties make CFP desirable for use in electrochemical, fuel cell, electrolyzer, supercapacitor, sensing, water desalination, tissue engineering, and wastewater treatment applications. Unfortunately, unmodified CFP is hydrophobic, and the techniques available to date to make CFP hydrophilic have required either expensive equipment or harsh conditions, such as high concentrations of strong acids, that damage the internal structure of the network of carbon fibers comprising the CFP. This is a problem, because the internal carbon network architecture and pore sizes between the fibers critically affect the mass transport characteristics of the CFP that are needed for many of its intended applications.
Surprisingly, the present invention overcomes these problems. It provides CFP that not only remains hydrophilic for long periods, but also does not damage the internal structure of the carbon fiber network that forms the hydrophilic CFP. Further, while some previous techniques have produced hydrophilic CFP, but by methods that cannot be readily scaled, are time consuming, that require expensive equipment, or some combination of these, the inventive methods are inexpensive, scalable, and fast. Thus, the inventive methods not only provide CFP which has a highly desirable combination of properties, but also do so in a manner that solves problems that have up to now made the adoption of hydrophilic CFP impractical or too expensive for use at commercial scale in the applications listed above. The solution of these problems by the present invention is expected to make hydrophilic CFP made by the inventive methods useful for at least the applications noted above.
This patent specification reports a surprising new approach—an acid-free CFP treatment that imparts hydrophilicity by carbon surface modification using mild conditions and common, inexpensive reagents. As reported in more detail below, CFP treated by some embodiments of the invention retained hydrophilicity for over two months when stored in ambient air having relatively low humidity and more than 60 weeks (so far) when stored under deionized water. The inventive methods are not only fast—in typical embodiments, they take less than 30 min of preparation time—but are also are amenable to large-scale manufacturing and do not contaminate the CFP with iron or other transition metals. Further, the inventive methods do not destroy the carbon fiber network architectures of the treated CFP and does not lower the flexural strength of the CFP. Scanning electron microscopy (SEM) further revealed that the treatment does not damage the carbon fibers composing the CFP in rendering the CFP hydrophilic, as does treatment with the concentrated, harsh acids used in some prior methods. SEM imaging established that the carbon fibers of CFP treated by exemplar embodiments of the inventive methods appeared virtually identical before and after treatment: the carbon fiber network remained structurally intact.
Studies in the course of the present invention discovered, for the first time, that hydrophilicity in CFP depends on both the presence of surface hydroxyl groups and high edge density. Our studies further revealed that treatment of CFP by embodiments of the inventive methods resulted in CFP that had both high surface hydroxyl content and high edge density. Without wishing to be bound by theory, it is believed that the high edge density of the graphitic surfaces of the CFP treated by the inventive methods results not only in more hydroxyls on the treated CFP, but also hydroxyls that are clustered together. Again, without wishing to be bound by theory, it is believed that this clustering of hydroxyls helps maintain the presence of hydroxyl groups on the carbon fibers of the CFP. Hydrophilicity persisted because selectively generated surface hydroxyls were bound to graphitic edge sites that kept a threshold amount of adsorbed water of 0.5% with respect to total surface carbon content in place, explaining why storage under water enabled unprecedentedly long-lasting hydrophilicity. Inhibition of overoxidation of carbon beyond hydroxylation was achieved by the mild treatment conditions.
In contrast, CFP treated by a similar method, but with an exemplar strong acid, and then examined, revealed under examination that CFP treated with the strong acid had edges that were smoothed. CFP thus treated had a high surface hydroxyl content, but a low edge density. It is thus believed that CFP rendered hydrophilic by the inventive methods is physically different that CFP that has been made by prior techniques (including treatment with strong acids) and, without wishing to be bound by theory, that this physical difference is responsible in whole or in part for the ability of CFP treated by embodiments of the present invention to remain hydrophilic even when stored exposed to ambient air for days to months, depending on the particular treatment used and the humidity of the ambient air.
In sum, the inventive methods are environmentally benign, use inexpensive and scalable equipment and mild reagents at room temperatures, and do not contaminate the CFP with iron or other transition metals. The carbon fibers of the CFP rendered hydrophilic by the inventive methods are not visibly damaged, and thus the hydrophilic CFP is believed to retain the mass transfer properties of the original hydrophobic CFP. Thus, the inventive methods are surprisingly more useful than the techniques previously known in the art for rendering CFP hydrophilic, and result in hydrophilic CFP that is physically different from, and more useful than, CFP that has been rendered hydrophilic by previously known techniques.
We now turn to a general description of the inventive methods, followed by a more detailed description of particular aspects and embodiments.
In studies underlying the present invention, it was surprisingly found that sonication of CFP in an aqueous surfactant solution, followed by electrooxidation in an aqueous electrolyte, resulted in conferring hydrophilicity on the treated CFP that lasted for substantial periods when the CFP was stored in ambient air.
A study was conducted to investigate whether other surfactants or detergents could be used to render hydrophobic CFP hydrophilic for a sustained period in place of the SDS used in the initial studies. This subsequent study was conducted when the ambient air had a relative humidity of ˜21%, rather than the ˜78% relative humidity of the ambient air during the study discussed above. Treated CFP exposed to ambient air with a relative humidity of ˜21% remained hydrophilic for a shorter time than did treated CFP exposed to ambient air with a relative humidity of ˜78%. This allowed a faster comparison of the durability of the hydrophilicity imparted to the CFP samples by the respective treatments, compared to the studies conducted when the ambient air had a relative humidity of ˜78%. The detergents tested were, in addition to SDS: citrate, cetyltrimethylammonium bromide, sometimes referred to as “CTAB,” polyethylene glycol tert-octylphenyl ether, usually referred to by the brand name Triton™-X, 3-[(3-Cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate, commonly referred to as “CHAPS”, and polyethylene glycol sorbitan monooleate, usually referred to by its brand name, Tween® 80.
The results of the study are reported in more detail in Example 3. In brief, however, SDS, the surfactant tested in the original embodiments of the inventive methods, proved to provide treated CFP with the most durable hydrophilicity in the methods. CHAPS and CTAB also resulted in significantly more hydrophilicity than did the worst of the surfactants tested (Tween® 80), while citrate and Triton™-X also increased the hydrophilicity of the CFP compared to the worst-performing surfactant in the study. It is contemplated that two or more of the surfactants or detergents (other than Tween® 80) used in the study reported in Table 2 can be mixed together to form the solution in which the sonication and later electrooxidation is performed. Any particular combination of surfactants of interest can be readily tested for its suitability for use as a surfactant in some embodiments of the inventive methods by using it as the surfactant in place of SDS in the assay reported in Example 3.
In addition to the surfactants tested in the study reported in Example 3, any particular surfactant of interest can be readily tested for its suitability for use as a surfactant in some embodiments of the inventive methods by using it as the surfactant in place of SDS in the assay reported in Example 3. Further, while all of the surfactants were tested at the same molarity for ease of comparison, it is contemplated that sonication with the surfactant can be conducted with the surfactant present at other concentrations. Finally, it is noted that the surfactant is in an aqueous solution. No organic solvents are needed, and they are preferably omitted. Accordingly, the aqueous solution is more environmentally benign than that used in some prior techniques and creates less hazardous waste than previously reported procedures.
A study was performed to determine how long hydrophobic CFP needed to be sonicated with the surfactant to result in sustained hydrophilicity after the subsequent electrooxidation. As
Accordingly, in some embodiments, the CFP is sonicated in the aqueous surfactant solution for 10 minutes to 240 minutes. In some embodiments, the CFP is sonicated in the aqueous surfactant solution for 10 minutes to 180 minutes, for 10 minutes to 150 minutes, for 10 minutes to 120 minutes, for 10 minutes to 90 minutes, for 10 minutes to 60 minutes, for 10 minutes to 40 minutes, for 10 minutes to about 30 minutes, or for 10 minutes to about 20 minutes, with “about” in this sentence meaning±5 minutes, with each successive recitation being preferred to the one before it. In some embodiments, the CFP is sonicated in the aqueous surfactant solution for about 20 minutes, with “about” in this sentence meaning±5 minutes, ±4 minutes, ±3 minutes, ±2 minutes, ±1 minute, ±30 seconds, or ±15 seconds, with each successive recitation being preferred to the one before it. In some embodiments, the CFP is sonicated in the aqueous surfactant solution for 20 minutes.
A study was conducted to investigate whether other aqueous electrolytes could be used to render hydrophobic CFP hydrophilic for a sustained period in place of the potassium bicarbonate used in the initial studies. As with the study discussed above regarding the testing of alternative surfactants, this study was conducted when the ambient air had a relative humidity of ˜21%, rather than the ˜78% relative humidity of the ambient air during the study discussed above, and allowed a faster comparison of the durability of the hydrophilicity imparted to the CFP samples by the respective treatments, compared to the studies conducted when the ambient air had a relative humidity of ˜78%. The aqueous electrolytes tested were, in addition to potassium bicarbonate: potassium hydroxide (KOH), potassium acetate (CH3CO2K), potassium nitrate (KNO3), lithium perchlorate (LiClO4), and sodium sulfate (Na2SO4).
The study, and its results, are reported in more detail in Example 4 and Table 3, below. To our surprise, however, electrooxidation of CFP in several of the aqueous electrolytes tested resulted in CFP which retained hydrophilicity for much longer periods, even in ambient air with relatively low humidity, than did CFP subjected to electrooxidation in the presence of the original exemplar aqueous electrolyte, potassium bicarbonate. Strikingly, CFP that had been sonicated in SDS and then subjected to electrooxidation in aqueous KOH remained hydrophilic for over two months when maintained in air at ˜19.5° C. with a relative humidity of ˜21% (as the CFP was still hydrophilic subjected to this treatment when this patent disclosure had to be prepared, the outer limit of how long CFP subjected to this treatment remains hydrophilic cannot be reported here). CFP subjected to electrooxidation in aqueous potassium nitrate as the aqueous electrolyte remained hydrophilic for 27 days when maintained in air with a relative humidity of ˜21%, while CFP subjected to electrooxidation in aqueous lithium perchlorate as the aqueous electrolyte remained hydrophilic for 12 days. As shown in Table 3, CFP subjected to electrooxidation with potassium bicarbonate, the original aqueous electrolyte we employed in our studies, remained hydrophilic for 5 days. It is contemplated that mixtures of two or more of the aqueous electrolytes shown in Table 3 that resulted in CFP that retained hydrophilicity can be used.
In contrast, CFP subjected to electrooxidation in aqueous potassium acetate as the aqueous electrolyte did not become hydrophilic. Thus, use of aqueous electrolytes of metal acetates are not preferred.
In addition to the aqueous electrolytes tested in the study reported in Table 3, any particular electrolyte of interest can be readily tested for its suitability for use as an aqueous electrolyte in some embodiments of the inventive methods by using it as the aqueous electrolyte in the assay reported in Example 4. Further, while all of the aqueous electrolytes were tested at the same molarity for ease of comparison, it is contemplated that the electrooxidation can be conducted with the aqueous electrolyte present other concentrations, such as from and including 0.01 M to and including 1 M, or from and including 0.05 M to and including 0.5 M. Any particular concentration of any particular electrolyte of interest can be readily tested for its suitability for use in embodiments of the inventive methods by using the aqueous electrolyte at the concentration of interest in the assay reported in Example 4.
Hydrophilic CFP enables the deposition of nanoparticles from aqueous colloids because the water that surrounds the nanoparticles can penetrate the three-dimensional network of the carbon fibers, thus taking full advantage of the high internal surface area of CFP architectures for nanoparticle immobilization. Such nanoparticle-CFP assemblies have many applications, including serving as electrodes in aqueous electrocatalysis, toward the development of climate-friendly successor technologies for sustainable energy.41-54 Proof of principle experiments demonstrated the successful implementation of commercial, aqueous colloid of citrate-capped gold nanoparticles with CFP electrode supports for carbon dioxide reduction electrocatalysis in aqueous electrolyte.
The inventive methods may further include one or more of the following: (a) the electrooxidation can be in aqueous electrolyte having pH in the range of 2 to 14; (b) the CFP can serve as a working electrode in said electrooxidation is in aqueous electrolyte; (c) the sonication in aqueous surfactant solution preferably is for a time in the range of 1 to 120 minutes; (d) said sonication can be in aqueous surfactant solution that is environmentally benign; (e) said sonication can be in aqueous surfactant solution excluding organic solvents; (f) said sonication can be in aqueous surfactant solution comprising sodium dodecyl sulfate; (g) said sonication can be in aqueous surfactant solution comprising 1.0 M aqueous sodium dodecyl sulfate or other detergent; (h) said electrooxidation can be in aqueous electrolyte comprising any of several aqueous electrolytes; (i) the electrooxidation in aqueous electrolyte can comprise electrooxidation in 0.1 M aqueous potassium bicarbonate; and (j) decontaminating the CFP with a stream of nitrogen before said sonication, rinsing the CFP with water after said sonication but before said electrooxidation, and storing the CFP immersed in water after the electrooxidation.
In some embodiments, the invention provides a process of making hydrophilic CFP by sonicating the CFP in an aqueous solution comprising a surfactant, rinsing the CFP with water, and then subjecting the sonicated CFP to electrooxidation in a mild aqueous electrolyte solution (the “inventive treatment”), without damaging the carbon fibers of which the CFP is composed. As used in this section, “mild aqueous solution” means that the electrolyte solution contains an aqueous electrolyte but does not also contain a strong acid in an amount that affects the electrooxidation of the CFP compared to electrooxidation in a like sample of sonicated CFP in the same electrolyte solution but without the strong acid. As used in this section, “hydrophilic” means that the CFP sinks when placed in a container of water, which container is large enough to permit the entire surface of the CFP to contact the water therein and containing enough water to permit the CFP to sink below the surface of the water if it is permeable to the water. As used in this section, “without damaging the carbon fibers” means that the carbon fibers of which the CFP is composed look undamaged compared to the carbon fibers like sample of the same CFP that has not been subjected to the inventive treatment. In some embodiments, whether or not the carbon fibers look undamaged as recited in the preceding sentence is determined by visualizing the carbon fibers by scanning electron microscopy. In some embodiments, the hydrophilic CFP remains hydrophilic for 30 days or more when maintained exposed to ambient air at 19.5° C.±2.5° C., and 78%±2.5% relative humidity. In some embodiments, the hydrophilic CFP remains hydrophilic for 30 days or more when stored immersed in deionized water. In some embodiments, the invention provides hydrophilic CFP that has a mesostructure that has not been damaged by the process of making the CFP hydrophilic. In some embodiments, the invention provides hydrophilic CFP which has the flexural strength of the initial hydrophobic CFP prior to its being treated to render it hydrophilic by the embodiments of the inventive methods presented in this disclosure. As stated in Wikipedia, “flexural strength,” also known as “bend strength” is a “material property, defined as the stress in a material just before it yields in a flexure test,” and thus is a measure of the material's ability to bend or deform without damage. As least some previous methods for making CFP hydrophilic rendered it brittle.
It is expected that the hydrophilic CFP provided by the inventive methods will be inexpensive and available in sizes that will allow it to be used in, among other things, water purification systems, capacitors, flow batteries, aqueous electrolyzers, and sensors.
This Example describes materials and methods used in studies reported in later Examples.
All chemicals were used as received. Deionized water was obtained from a Thermo Scientific Barnstead™ Smart2Pure™ Pro UV/UF 15 LPH Water Purification System and had a resistivity of >17.5 MΩ·cm. All experiments were performed at room temperature. All dry nitrogen gas used came from oil-free liquid nitrogen boil-off to prevent transfer of compression oil, which most compressed gases contain, onto the carbon fiber paper (CFP). Data analysis and graphing were performed with Igor Pro 8.04 (Wavemetrics).
CFP Surface Treatment. Uncoated, initially hydrophobic CFP (AvCarb MGL190, FuelCellStore) was cut into pieces for use in a CO2 reduction electrochemical cell. The electrodes were ca. 23 mm wide and 39 mm high (referred to herein as the “rectangular part”). They also exhibited a tab for electrical contacting on one short side that was ca. 8 mm wide and 15 mm high (referred to herein as the “tab”). A few samples were strips with ca. 1.5 cm width and ca. 4.0 cm height. Clean plastic tweezers were used to handle CFP to prevent metal contaminations. Each entire CFP piece was first held for ca. 10 s in a dry nitrogen stream to remove loose crud. In the studies reported in Example 2, acid-free treatment consisted of sonication of the rectangular part of CFP for 5 min in a 1.0 M aqueous sodium dodecyl sulfate (“SDS,” AG Scientific, >99%) solution; using a 1210 Branson bath sonicator, delivering 80 watts. After that, the rectangular part was subjected to electrooxidation in 0.1 M aqueous potassium bicarbonate (KHCO3, Alfa Aesar, 99.7-100.5%) electrolyte in a standard three-electrode cell; the pH was 8.72, as determined by a Thermo Scientific Accumet Excel XL20 pH meter. The pretreated CFP piece served as working electrode. The counter electrode was also CFP, to prevent metal contaminations, and the distance between working and counter electrode was ca. 15 mm. The reference electrode was Ag/AgCl (3 M aqueous NaCl, BaSi). The potential was set to +1.63 V vs. Ag/AgCl for 20 min. After that, the entire CFP piece was first rinsed with and then dipped in deionized water for ca. 30 seconds. Finally, the entire CFP piece was dried in a dry nitrogen stream.
For use in comparison tests, “piranha treatment” consisted of immersion of the rectangular part for 5 min in a 4:1 (v/v) mixture of concentrated sulfuric acid (Fisher, ACS Plus) and 30% aqueous hydrogen peroxide solution (Fisher), after the initial dry nitrogen stream exposure. After this piranha treatment, the entire CFP piece was first rinsed with and then dipped for ca. 30 s in deionized water. Excess water was shaken off mechanically. On some samples a second chemical etch was employed after the piranha treatment: either (1) the rectangular part was immersed and sonicated for 5 min in a 1.0 M aqueous sodium dodecyl sulfate (SDS, AG Scientific, 99%) solution; a 1210 Branson bath sonicator was used, delivering delivered 80 watts, or (2) the second etch consisted of soaking for 5 min in concentrated formic acid (J. T. Baker, 90%) or in 6.0 M aqueous KOH (Sigma-Aldrich, 99.99%). After the second etch, the entire CFP piece was first rinsed with and then dipped for ca. 30 s in deionized water. Then, the entire carbon fiber paper piece was dried in a dry nitrogen stream to prevent the transfer of additional water into the next step. Some samples that were treated with this second chemical etch were subsequently electrooxidized, using the conditions described above. Select CFP samples were thermally treated in ambient air at 500 or 800° C., using a Thermolyne Model 48000 furnace vented to an exhaust, and ceramic boats.
Several triplicates were subjected to a preparation that consisted of soaking in piranha solution, sonication in 1 M aqueous SDS solution, followed by electrooxidation, using the conditions described above. This process had good reproducibility. Samples were stored in ambient air or under deionized water. The average relative humidity of ambient air was ca. 78% during summer in Rochester, New York.
Long-term time trials to assess the durability of hydrophilicity upon storage under deionized water were performed with CFP treated by acid-free SDS-electrooxidation or acid-containing piranha-SDS-electrooxidation procedures. Ten samples of each preparation were made and immediately stored under deionized water in new Falcon tubes at room temperature. Hydrophilicity was assessed every 7 days up to 10 weeks via the sink-or-float test after drying in an oil-free nitrogen stream. Pieces of that CFP sample were cut off for physical characterization; samples were discarded after a particular week's testing. After 10 weeks, the samples that had been under water for 10 weeks already were re-immersed and their hydrophilicity further re-tested every 7 days.
Hydrophilicity assessment. The hydrophilicity of CFP surfaces was tested by the sink-or-float test in deionized water. Dry CFP pieces were dropped from ca. 5 cm above the water surface into a beaker halfway filled with deionized water. Visual inspection detected sinking or floating. This way, time of sinking without agitation was measured. Pieces that floated were additionally agitated with plastic tweezers and pushed under water to monitor sinking with agitation or resurfacing. Time of sustained hydrophilicity refers to the time passed until a CFP piece stopped sinking altogether, even upon agitation.
Physical Characterization. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5900LV SEM instrument equipped with a thermionic tungsten electron gun, operated at 25 kV with a working distance of 10 mm. CFP samples were mounted on 1-inch diameter aluminum SEM stubs (Ted Pella) with carbon tape (Electron Microscopy Sciences). Images of gold on carbon were taken in backscatter mode to enhance elemental contrast. Energy-dispersive X-ray (EDX) spectroscopy was used to identify chemical elements present at CFP samples. A liquid-nitrogen cooled EDAX CUD LEAP detector was used in conjunction with the EDAX Genesis software package.
X-ray photoelectron spectra (XPS) were collected using a Kratos Axis Ultra XPS instrument (Kratos Analytical Limited, Manchester, UK) equipped with a monochromatized Al Kα radiation source, operated in high-power mode at 200 W and 15 kV, with a base chamber pressure of 3.0×10−8 mbar. Samples were mounted on double-sided adhesive copper tape. Survey scans were obtained between 0 and 1200 eV with a step size of 1 eV, a dwell time of 200 ms, and an analyzer pass energy of 140 eV averaged over 5 scans. Core-level region scans for C 1s, O 1s, Na 1s, S 2p, K 2p, and Fe 2p were obtained at the corresponding binding energy ranges with a step size of 0.1 eV, an average dwell time of 260 ms, and an analyzer pass energy of 20 eV averaged over 5 scans. Binding energies were referenced to the C 1s peak arising from adventitious carbon, taken to have a binding energy of 284.8 eV.55 Binding energies and quantitative peak areas were derived after Shirley background subtraction56 and Gaussian/Lorentzian envelope peak fitting. For the quantification of different components, instrument-specific atomic sensitivity factors determined from standard materials were used. Quantitative XPS analysis was performed with CasaXPS (Version 2.3.24, Casa Software Ltd.).
Electrocatalysis. A custom-made polycarbonate electrolysis cell provided by the Jaramillo group was used for electrochemistry experiments.57 In this cell, the working and counter electrode compartments each had electrolyte volumes of 8 mL and gas headspaces of ca. 3 mL. Both compartments of the electrochemical cell were filled with CO2-saturated 0.1 M pH 6.8 aqueous KHCO3 buffer, and the cell was continuously sparged with humidified CO2 (Airgas, 99.99%) at room temperature for 20 min to ensure that the electrolyte remained saturated with CO2 prior to electrochemical data collection. Throughout electrocatalysis, calibrated mass flow controllers (Aalborg Instruments & Controls, Inc., Orangeburg, NY) maintained the flow of CO2 at 20 mL min−1 through the cell. Gas diffusion frits (SP Wilmad-Labglass, Vineland, NJ) were used to enhance CO2 saturation in the electrolyte. The working and counter electrodes (5.8 cm2 geometric surface area) were separated by a Selemion anion exchange membrane (stored overnight in 0.1 M aqueous KHCO3, ACS grade, 99.7-100.5%, Alfa Aeasar, Tewksbury, MA). The counter electrode was a Pt foil (Aldrich, 0.025 mm thick, 99.9%) and the reference electrode was an Ag/AgCl electrode (BASI, stored in 3 M NaCl from Fisher Chemical, USP grade, 99.0-100.5%). The working electrode was SDS-electrooxidation treated CFP that was used neat or with immobilized gold nanoparticles (20 nm gold nanospheres, 0.05 mg mL-1 in 2 mM aqueous sodium citrate, nanoComposix, San Diego, CA). Gold nanoparticles were immobilized on treated CFP by placing the CFP on the bottom of a custom-made Teflon® tub, which was filled with 2 mL of gold nanoparticle colloid such that all CFP was evenly covered. This Teflon® tub was placed under a heat lamp at ca. 60° C. for 20 min. The resulting gold loading was ca. 10 μg cm−2geo. Ultra-Clean™ Supremium aluminum foil (VWR International, LLC, Radnor, PA) was used to electrically contact the working and counter electrodes to the leads of the potentiostat (BioLogic, SP-150-EIS). Solution resistances were determined from impedance measurements and compensated using automatic 85% iR compensation; the remaining 15% was corrected manually after data collection, following a published procedure.57 Each electrochemical experiment started with a measurement of solution resistance, followed by the collection of two cyclic voltammograms from +2.3 V to −1.2 V vs. the reversible hydrogen electrode (RHE) at a scan rate of 50 mV s−1, taking chronoamperometry data at −0.68 V vs. RHE for 2 hours, and concluding with the collection of two more cyclic voltammograms under virtually the same conditions as the initial ones. Carbon dioxide reduction products were detected by an in-line gas chromatograph (GC, SRI Instruments, Multi-Gas #5 configuration) that was connected to the headspace of the working electrode compartment of the electrochemical cell. Hydrogen was measured by a thermal conductivity detector, while CO and other hydrocarbon products were identified by a flame ionization detector equipped with a methanizer. Following a published procedure,57 the gas chromatograph was programmed to measure produced gases every 20 min, starting 5 min after the start of the chronoamperometry experiment. A certified standard calibration gas (Airgas) was used to calibrate the gas chromatograph and obtain molar concentrations of produced gases by quantifying peak areas. Faradaic efficiencies (FE) were calculated from these molar concentrations in combination with the average charge transferred during the gas chromatograph sampling, using the following equation:
In this equation, Nproduct is the number of electrons transferred to make gas-phase product, Ntotal is the total number of electrons transferred, cproduct is the molar concentration of gaseous product in units of ppm, ne is the number of electrons required to reduce one molecule of CO2 to one molecule of product, and Iaverage is average current during gas chromatography sampling in units of A. The constant parameters in the equation led to the number 1.327 in units of C s−1; this number was derived from multiplying the number of moles sampled (4.126×10−5 mol) with Avogadro's number and the elementary charge, and then dividing by the time it took to fill the sample loop of the gas chromatograph (3 s).
This Example presents the results from the studies whose materials and methods are set forth in Example 1.
A major goal of the studies undertaken in the course of the present was to develop a method that imparts long-lasting hydrophilicity to CFP without damaging the carbon fiber network of the CFP. To this end, an acid-free approach was developed that comprised sonication of CFP in a detergent (in the studies reported in this Example, 1 M sodium dodecyl sulfate (SDS)) solution, followed by electrooxidation in an aqueous buffer ((in the studies reported in this Example, 0.1 M pH 8.7 aqueous KHCO3) electrolyte at +1.63 V vs. Ag/AgCl for 20 min; this acid-free treatment resulted in sustained hydrophilicity for 31 days in ambient air with high (over 70%) relative humidity. The durability of hydrophilicity was assessed by storing treated CFP in ambient air and testing the dry CFP regularly with regard to its ability to sink in deionized water. Untreated CFP never sank because of its high hydrophobicity.
The known, previously described chemical procedures to make CFP hydrophilic include the use of strong acids at high concentrations. The new, acid-free approach, described in this patent specification paves the way for creating large areas of hydrophilic CFP via a benign, green treatment, without the need for harsh acids and concomitant environmental and safety issues. Importantly, carbon fibers remained intact and smooth upon our acid-free treatment; no fraying of or damage to carbon fibers was observed in SEM images.
Referring to
Fraying and breaking of carbon fibers leads to a darkened appearance of CFP because of increased internal scattering of ambient light at the rough carbon surfaces within the fiber network (
Hydrophilicity of carbon surfaces results from oxygenated surface functional groups.31 These groups can be installed by oxidation of surface carbon. Therefore, oxidizing treatments with acids or plasma bombardment are popular methods to impart hydrophilicity to CFP. In fact, most of the published treatments we found for inducing hydrophilicity in CFP included the use of acid, while the remaining quarter were based on plasma treatments.7, 10, 12-16, 25-34, 36-40 Because plasma treatments of CFP lead to embrittlement issues, require large capital expense for the plasma generators, and are overall not amenable to large-scale applications as outlined above, we sought new techniques that did not rely on plasma treatments to impart hydrophilicity.
Thermal oxidations in ambient air are another technique that has been used in the art to impart hydrophilicity. A preparation reported by Zhang et al. was duplicated, which took a total of 16 hours and included heat treatment at 500° C.38 The resulting hydrophilicity of CFP stored in air persisted for only two days, and SEM images showed significant damage to the carbon fibers. A few more thermal oxidations in ambient air were attempted by annealing CFP, which was either untreated or pretreated with piranha and SDS solutions, at 500° C. for 5 or 10 min or at 800° C. for 1.5, 3 or 5 min. All CFP samples that were annealed at 500° C. were hydrophobic, whereas pretreated CFP that was annealed at 800° C. for 5 min remained hydrophilic for four days when stored in ambient air. All heat treatments led to extremely brittle CFP; in fact, some CFP pieces disintegrated altogether upon heat treatment. SEM images revealed significant damage to the carbon fibers. In contrast, the new approach this patent specification describes employs chemical surface modifications to impart hydrophilicity.
In the studies reported in this Example, the longest-lasting hydrophilicity was achieved by treating CFP by sonication in 1 M aqueous SDS solution, followed by electrooxidation in 0.1 M pH 8.7 aqueous KHCO3 electrolyte at +1.63 V vs. Ag/AgCl for 20 min. In keeping with many literature reports that make use of strong acids to install hydrophilic surface functional groups on CFP, acid treatments were employed as control experiments to find out if the new acid-free procedure was superior to acid-containing treatments in terms of durability of sustained hydrophilicity (
Acid-containing treatments started with a chemical etch in strongly oxidizing “piranha” solution, a 4:1 (v/v) mixture of concentrated sulfuric acid and 30% aqueous hydrogen peroxide solution, which in some cases was followed by a second chemical etch (1 M aqueous SDS or 6 M aqueous KOH solution, or concentrated formic acid, or none) and electrooxidation in 0.1 M aqueous KHCO3 electrolyte at +1.63 V vs. Ag/AgCl for 20 min The piranha methods are diagrammed in
Through systematic iterations, it was discovered that electrooxidation in an aqueous electrolyte (in the studies reported in this Example, 0.1 M aqueous KHCO3) was important for sustained hydrophilicity. A mild aqueous electrolyte was deliberately chosen, with the aim to retain the structural integrity of CFP. All known chemical procedures that did not contain an electrooxidation step, i.e., one-step piranha or SDS treatments, or two-step piranha-SDS, piranha-KOH, or piranha-HCOOH treatments, or three-step piranha-SDS-KOH or piranha-SDS-HCOOH treatments, led to CFP that became hydrophobic within only two days (
Chemical treatments that started with immersion in piranha solution and concluded with electrooxidation led to hydrophilicity that lasted for about a week (
The rationale for choosing these three chemicals was that formic acid might induce hydrophilic carboxylic surface functional groups, KOH might facilitate the formation of hydroxyl surface functional groups, and the detergent SDS would eliminate organic or otherwise labile contaminants from the carbon surface before electrooxidation. Another reason was the wide pH range that these three chemicals cover. It was hypothesized that another acid treatment might further oxidize surface carbon, whereas the strong base would lead to surface reactions complementary to the preceding strong acid treatment. In contrast, it was thought the more dilute 1 M aqueous SDS solution might be advantageous for applications of treated CFP in environments that are sensitive to strong acids or bases, which may be left in small quantities in the porous three-dimensional network of carbon fibers. Such residual acids or bases could be washed away with water if the CFP surface is sufficiently hydrophilic for water to fully penetrate into the three-dimensional carbon fiber network. Three-step treatments of CFP, i.e., piranha-SDS-electrooxidation, piranha-HCOOH-electrooxidation or piranha-KOH-electrooxidation, gave rise to sustained hydrophilicity of 6, 7, or 9 days, respectively.
SEM images showed that all thirteen chemical treatments employed to impart hydrophilicity to CFP left the carbon fibers and their network architectures intact (
Hydrophilicity of carbon surfaces results from oxygenated surface species, such as C—O, C═O and O—C═O functional groups.31 Work on the new approach described in this patent specification identified and quantified surface species from XPS data, collected on the day of treatment, and correlated these surface species to the observed durability of hydrophilicity of CFP that was stored in ambient air. Survey XP spectra revealed that only carbon and oxygen were present at untreated and treated CFP surfaces (
High-resolution XP spectra in the C 1s and O 1s regions (
Oxygenated carbon species were detected in the high-resolution C 1s and O 1s spectra and made assignments based on both elemental regions (Table 1 below). The O 1s spectra required two peaks to match the experimental data. These peaks had central binding energies of 531.6 to 532.3 eV, attributable to C═O functional groups, such as in carboxyls, aldehydes, carbonyls, esters, and ketones, and 533.0 to 533.7 eV, attributable to C—O species, such as in carboxyls, hydroxyls, and ethers.65-68 Accordingly, the central binding energies of the remaining three peaks in the C 1s spectra were constrained to the following ranges: 286.4 to 287.0 eV (C—O, hydroxyls, esters, and ethers), 287.4 to 288.0 eV (C═O, aldehydes, carbonyls, and ketones), and 288.7 to 289.2 eV (O—C═O, carboxyls and esters).65-71
C—O
C═O
The multi-peak fits matched the measured high-resolution XP spectra in the C 1s and O 1s regions well (
High-resolution XP spectra in the C 1s and O 1s regions showed significant changes of surface characteristics between untreated and treated CFP (
Oxygen content in C—O species, generally attributable to carboxyls, hydroxyls, and ethers,65-68 correlated best with the observed trends in durability of hydrophilicity (
The nature of the chemical treatment did matter for the observed surface C—O content of CFP. Strikingly, treatment in 1 M aqueous SDS led to higher surface C—O content than immersion in harsh piranha acid; however, hydrophilicity lasted only 3 hours for both preparations (
To understand which surface functional groups were responsible for long-lasting hydrophilicity of CFP, XPS was chosen to assess CFP surfaces because XPS is a surface-sensitive technique and well suited to assess oxygenated carbon species on graphitic carbon. However, small chemical shifts between carboxyl, hydroxyl, and ether groups on carbon65-68 resulted in a single C—O peak for all three surface functional groups. Likewise, C═O peaks included carboxyls, aldehydes, carbonyls, esters, and ketones, which have similar central binding energies65-68 and therefore cannot be differentiated by XPS. Similarly, each C 1s peak was comprised of multiple functional groups as outlined in Table 1. Nevertheless, it was possible to disentangle contributions from individual surface functional groups to sustained hydrophilicity. Aprotic oxygenated functional groups, i.e., carbonyls, esters, ethers, and ketones, induce surface dipole moments but lack hydrogen that is needed to form strong hydrogen bonding networks to water molecules at the solid-liquid interface; thus, such aprotic groups contribute less to sustained hydrophilicity than protic oxygenated functional groups, i.e., carboxyls, aldehydes, and hydroxyls. Since surface C—O content was predictive of the observed durability of hydrophilicity (
Interpretation of XPS signals arising from C═O (carboxyls, aldehydes, carbonyls, esters, and ketones) and C═O (aldehydes, carbonyls, and ketones) is less straightforward, but may be useful in explaining the observation of equally high surface C—O contents of 4.4% in piranha-SDS-electrooxidation treated vs. SDS-electrooxidation treated CFP, although these treatments led to markedly different durability of hydrophilicity of 6 vs. 31 days, respectively. Surface C═O and C═O contents did not correlate well with the observed durability of hydrophilicity (
Without wishing to be bound by theory, identification of surface hydroxyls as being most beneficial for sustained hydrophilicity explains why harsher treatments with strong acids or high temperatures did not generate CFP with long-lived hydrophilicity. More potent oxidants are capable of oxidizing carbon all the way to carboxyls, whereas the new acid-free, more benign detergent-electrooxidation treatments afforded by the present invention appear to have prevented this detrimental over-oxidation and resulted in the largest amounts of less oxidized surface hydroxyl groups. Inhibition of over-oxidation of carbon is notoriously difficult because the first oxidation step is thermodynamically the most difficult and subsequent oxidations are much easier. The discovery of a CFP treatment that provides selectivity for less oxidized hydroxyls over carboxyls has the potential to transform the utility of carbon surfaces and CFP.
XPS data of CFP stored in ambient air whose hydrophilicity lasted for a month upon SDS-electrooxidation treatment or six days upon piranha-SDS-electrooxidation treatment indicate that surface C—O content diminished over time but remained above the critical threshold of 2.3% as long as the CFP was hydrophilic (
Interestingly, the surface C—O content increased after one week of storage in ambient air to 6.4%, after which it steadily decreased. It can be surmised that adsorption of water from ambient air, which had an average relative humidity of ca. 78%, on hydrophilic carbon fibers led to surface reactions that produced species that contributed to the surface C—O content, which complicated the interpretation of the evolution of surface C—O content over time and its correlation to wettability of CFP by water. We found that after storage for a week or more in ambient air had a third peak needed to be included with central binding energies of 535.3 to 536.0 eV, attributable to adsorbed water, to match the O 1s signal. Adsorbed water has been observed before on carbon surfaces, with central binding energies of 535.3 to 536.1 eV.58, 72 Quantification of adsorbed water showed that the surface content of chemisorbed water was ca. 0.6 to 0.7% upon storage of SDS-electrooxidation treated CFP for one to four weeks in ambient air, while the CFP was still hydrophilic, and decreased to 0.4% after hydrophilicity was lost (
XPS data in the Na Is, S 2p, K 2s, or Fe 2p core-level regions was collected (
Electrooxidation time was adjusted in the acid-free treatment methods with respect to durability of hydrophilicity and time of the procedure. CFP which had been sonicated in 1 M aqueous SDS solution was used and the electrochemical treatment time was varied from 5 to 60 min. The time samples sank in deionized water without agitation. Electrooxidation times longer than 20 min did not improve the durability of hydrophilicity (
For practical applications of CFP, hydrophilicity is preferably sustained during operation in water or aqueous media, and not necessarily with intermittent storage in ambient air. Therefore, long-term time trials were conducted of select preparations of CFP that were stored under deionized water. CFP was tested that was treated by the acid-free SDS-electrooxidation or acid-containing piranha-SDS-electrooxidation procedures (the acid-containing piranha-electrooxidation method is diagrammed in
Although hydrophilic CFP has many uses, it was employed in the experiments described herein as proof-of-principle as inert supporting electrode for metal nanoparticles in aqueous CO2 reduction electrocatalysis. Ca. 20 nm gold nanoparticles were immobilized on CFP by soaking SDS-electrooxidation treated CFP electrodes in commercial aqueous citrate-capped gold colloid and drying under a heat lamp, resulting in a gold loading of ca. 10 μg cm2geo. Interestingly, the aqueous gold nanoparticle colloid did not wet the electrode tab of treated CFP; the tab had remained hydrophobic because it was deliberately not immersed in the treatment solutions (
The gold-nanoparticle-CFP assemblies were used as working electrodes for carbon dioxide reduction electrocatalysis in CO2-saturated aqueous 0.1 M pH 6.8 KHCO3 electrolyte. Referring to
This Example describes studies that tested the effect of substituting other surfactants in place of the SDS used in the initial studies in developing acid-free solutions for providing hydrophilic CFP.
As reported in Examples 1 and 2, the first acid-free solutions used in our initial method of providing CFP with durable hydrophilicity used SDS as the surfactant and potassium bicarbonate (KHCO3) as the aqueous electrolyte, followed by electrooxidation (sometimes referred to herein as “EO”). A study was conducted to investigate whether other surfactants or detergents could be substituted for SDS in providing durable hydrophilicity. The study was conducted using essentially the materials and method described in the preceding Examples, with the modification that the study was conducted during the late fall, in which the ambient air had a relative humidity of 21%, rather than the 78% relative humidity of the ambient air during the summer during the studies reported in Examples 1 and 2. The lower relative humidity of the ambient air during this study shortened the time during which the CFP remained hydrophilic, and thus allowed a faster comparison of the durability of the hydrophilicity imparted to the CFP samples by each of the detergents compared to the studies conducted when the ambient air had 78% relative humidity. The detergents tested were, in addition to SDS: citrate, cetyltrimethylammonium bromide, sometimes referred to as “CTAB,” polyethylene glycol tert-octylphenyl ether, usually referred to by the brand name Triton™-X, 3-[(3-Cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate, commonly referred to as “CHAPS”, and polyethylene glycol sorbitan monooleate, usually referred to by its brand name, Tween® 80.
The results are shown in Table 2, below.
Thus, while SDS, the surfactant or detergent tested in the original embodiments of the inventive methods, proved to provide treated CFP with the most durable hydrophilicity in the methods, CHAPS and CTAB also resulted in significantly more hydrophilicity than did the worst detergent tested, while citrate and Triton™-X also increased the hydrophilicity of the CFP compared to the worst-performing detergent in the study. It is contemplated that two or more of the surfactants or detergents (other than Tween® 80) used in the study reported in Table 2 can be mixed together to form the solution in which the sonication and later electrooxidation is performed.
In addition to the surfactants tested in the study reported in Table 3, any particular surfactant of interest can be readily tested for its suitability for use as a surfactant in some embodiments of the inventive methods by using it as the surfactant in place of SDS in the assay reported in this Example. Further, while all of the surfactants were tested at the same molarity for ease of comparison, it is contemplated that the electrooxidation can be conducted with the surfactant present other concentrations, such as from and including 0.01 M to and including 1 M, or from and including 0.05 M to and including 0.5 M.
This Example describes studies that tested the effect of substituting solutions of any of six different aqueous electrolytes on the durability of hydrophilicity of CFP subjected to electrooxidation in the respective solutions. To facilitate the comparisons, the same detergent, SDS, was used in each preparation.
As reported in Examples 1 and 2, the first acid-free solutions used in our initial method of providing CFP with durable hydrophilicity used SDS as the detergent and potassium bicarbonate (KHCO3) as the aqueous electrolyte, followed by electrooxidation (sometimes referred to herein as “EO”). A study was conducted to investigate whether other aqueous electrolytes could be substituted for potassium bicarbonate in providing durable hydrophilicity. The study was conducted using essentially the materials and method described in the preceding Examples, with the modification that the study was conducted during the late fall, in which the ambient air had a relative humidity of 21%, rather than the 78% relative humidity of the ambient air during the summer during the studies reported in Examples 1 and 2. The lower relative humidity of the ambient air shortened the time during which the CFP remained hydrophilic, and thus allowed a faster comparison of the durability of the hydrophilicity imparted to the CFP samples by each of the aqueous electrolytes. The aqueous electrolytes tested were, in addition to potassium bicarbonate: potassium hydroxide (KOH), potassium acetate (CH3CO2K), potassium nitrate (KNO3), lithium perchlorate (LiClO4), and sodium sulfate (Na2SO4). The results are presented in Table 3.
Electrooxidation of CFP in several of the aqueous electrolytes tested resulted in CFP which retained hydrophilicity for surprisingly longer periods, even when maintained in relatively low humidity, than did CFP subjected to electrooxidation in the presence of the original exemplar aqueous electrolyte, potassium bicarbonate. Strikingly, CFP subjected to electrooxidation in aqueous potassium hydroxide remained hydrophilic for over two months when maintained in air at ˜19.5° C. with a relative humidity of ˜21% (as the CFP was still hydrophilic subjected to this treatment when this patent disclosure had to be prepared, the outer limit of how long CFP subjected to this treatment remains hydrophilic cannot be reported here). CFP subjected to electrooxidation in aqueous potassium nitrate as the aqueous electrolyte remained hydrophilic for 27 days when maintained in air with a relative humidity of ˜21%, while CFP subjected to electrooxidation in aqueous lithium perchlorate as the aqueous electrolyte remained hydrophilic for 12 days. As shown in Table 3, CFP subjected to electrooxidation with potassium bicarbonate, the original aqueous electrolyte we employed in our studies, remained hydrophilic for 5 days. It is contemplated that mixtures of two or more of the aqueous electrolytes shown in Table 3 that resulted in CFP that retained hydrophilicity can be used.
In contrast, CFP subjected to electrooxidation in aqueous potassium acetate as the aqueous electrolyte did not become hydrophilic. Thus, potassium acetate does not appear to be suitable for use as an aqueous electrolyte for rendering CFP hydrophilic.
In addition to the aqueous electrolytes tested in the study reported in Table 3, any particular potential aqueous electrolyte of interest can be readily tested for its suitability for use as an aqueous electrolyte in an embodiment of the inventive methods by substituting it as the aqueous electrolyte in the assay reported in this Example. Further, while all of the aqueous electrolytes were tested at the same molarity for ease of comparison, it is contemplated that the electrooxidation can be conducted with the aqueous electrolyte present other concentrations, such as from and including 0.01 M to and including 1 M, or from and including 0.05 M to and including 0.5 M.
We wanted to understand why OH-group-containing surface oxygenates and concomitantly adsorbed water remained longer on CFP that was treated by our acid-free SDS-electrooxidation process compared to CFP treated by the acid-containing piranha-SDS-electrooxidation procedure. To this end, we employed SEM imaging to examine the nanostructure of carbon fibers as a function of treatment. We found that CFP treated by the acid-free SDS-electrooxidation process exhibited higher surface roughness than CFP treated by the acid-containing piranha-SDS-electrooxidation procedure or untreated CFP, as shown in
In CFP, the surface orientation of graphitic crystallites occurs with their basal plane parallel to the surface of carbon fibers. Ergo, the carbon fiber surface roughness observed in the SEM images shown in
Computations have demonstrated that oxygenates at the basal sites of graphite are thermodynamically unfavorable. Therefore, long-lasting oxygenates must bind at edge sites. We established in studies reported in Example 2 that the content of adsorbed water on CFP must be above the critical threshold of 0.5% for hydrophilicity. Molecular simulations have shown that water adsorbs as 3D water clusters and networks on graphite because of cooperative effects involving both fluid-fluid interactions and fluid-solid interactions with oxygenated graphite sites. Water is a poor adsorbate on graphite without surface oxygenates. We established in a study reported in Example 2 that the content of surface hydroxyls was predictive of durability of hydrophilicity.
We found that long-lasting hydrophilicity of CFP required a combination of high graphitic edge density and high surface hydroxyl content, as shown graphically in
We collected Raman and X-ray diffraction (XRD) data to provide additional evidence that untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP consisted indeed of graphite and to confirm our results regarding graphitic edge densities and amounts of surface oxygenates from SEM and XPS data. Raman spectra of untreated CFP (hydrophobic) and CFP that was treated by acid-containing piranha-SDS-electrooxidation (hydrophilic for 6 d) or acid-free SDS-electrooxidation (hydrophilic for 31 d) corroborated that untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP consisted of graphite, consistent with reported data. In single-crystal graphite, two E2g modes are Raman-active as fundamentals because the unit cell of graphite has the symmetry of the D6h space group. These two E2g bands arise because adjacent graphite planes can vibrate in phase or with opposite phase. Because graphite exhibits relatively weak interlayer forces, these two E2g bands exhibit an energy separation that cannot be resolved experimentally and appear as one band that is centered at 1575 cm−1 with 488 nm excitation. In graphitic materials that are not single-crystalline, a single fundamental E2g band appears at ≈1580-1600 cm−1 (G band); a 2D vibrational analysis of the in-plane modes of the hexagonal honeycomb network has been found to be more appropriate in non-single-crystalline graphitic materials because of weak interlayer forces and disorder. In addition, non-single-crystalline graphite exhibits a fundamental weak disorder band at ≈1350 cm−1 (D band) attributed to graphitic edges in pure graphite that consisted only of carbon. The frequency of this D band shifts to higher energy with increasing excitation energy. The intensity of the D band was found to be inversely proportional to the effective basal-plane crystallite diameter in pure graphite that consisted only of carbon. Also observable is a strong overtone 2D band at ≈2700 cm−1, caused by a double-resonance effect similar to the D band, but now due to two inelastic phonon scattering processes; no defects or disorder are necessary for this 2D band, which has also been observed in highly ordered graphitic materials. The G and 2D bands have been attributed to C—C bond stretching modes of in-plane sp2 carbons. With green laser excitation, as used here, a weak mode is present as a shoulder of the G band at ≈1620 cm−1 (D′ band). In addition, a weak nondispersive second-order peak is visible at ≈2450 cm−1, whose exact origin is still debated.
We observed five bands in the Raman spectra of untreated, piranha-SDS-electrooxidation treated and SDS-electrooxidation treated CFP, which had Raman shifts of 1351 cm−1 (assigned to the D band), 1581 cm−1 (assigned to the G band), 1620 cm−1 (assigned to the D′ band), 2450 cm−1 (assigned to the 2450 cm−1 band), and 2702 cm−1 (assigned to the 2D band), in keeping with published reports. Bands were broad, as expected for disordered graphite. Generally, more disordered graphite exhibits broader G and D bands, and the D band intensity increases relative to the G band intensity. We analyzed peak area ratios of the D bands with respect to the G bands, to gain information about structural disorder in untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP. The disorder-induced D bands had Gaussian line shape because of disorder, and the G bands, which arise from E2g C—C bond stretching modes of in-plane sp2 carbons and are less affected by disorder, exhibited Voigt line shape, consistent with published reports; we used the respective line shapes for peak fitting and integrations to deduce peak area ratios. We found relative D band peak areas of 0.14, 0.19, and 0.21 with respect to G band peak areas, for untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP, respectively. We observed the highest D to G band area ratio for CFP treated by our acid-free SDS-electrooxidation process, corroborating the above discussed findings of highest graphitic edge density (from SEM data) and highest amount of surface hydroxyls with respect to total surface carbon (from XPS data) in SDS-electrooxidation treated CFP. Untreated CFP had the lowest D to G band area ratio. Nevertheless, a more detailed interpretation of these peak area ratios is not feasible because both graphitic edges and surface oxygenates increase amorphization (or structural disorder) in graphite, concomitant with an increase of the fraction of sp3 carbons, which induces significant changes in the G and D bands. As a result, a differentiation of contributions from graphitic edges versus those from surface oxygenates is impossible from Raman band intensities and widths. An additional, often underappreciated, complication in the analysis of G and D bands of (oxygenated) graphite arises from polarization effects. The frequencies, intensities, and shapes of the D, G, and 2D bands depend on the polarization angle between the incident and scattered light during Raman processes. Graphene can serve as a plasmonic material whose photonic response depends on the geometry and structure of graphene flakes. Since different nanostructures of the same plasmonic material exhibit different polarization patterns of light, the polarization-based frequencies, intensities, and shapes of the 2D bands reflect nanoscale changes in the graphite surface structure, presumably explaining the different peak shapes we observed for the 2D bands of untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP. We were unable to find literature on the exact details of how CFP nanostructuring affects the polarization-based frequencies, intensities, and shapes of Raman bands. Strikingly, piranha-SDS-electrooxidation treated CFP, which exhibited the smoothest graphite surfaces according to our SEM data, differed the most from the observed line shape of untreated CFP. The line shapes of the 2D bands of untreated CFP and SDS-electrooxidation treated CFP were similar and appeared like what has been observed for disordered graphite. We did not detect any Raman signals attributable to carbon-oxygen modes; the Raman spectra of untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP showed only bands arising from graphitic carbon. The absorption depth for 532 nm light in bulk graphite is ≈0.2 μm, resulting in a Raman probing depth of CFP of ≈200 nm with 532 nm excitation, as used here. We detected only a maximum amount of 8.5% total surface oxygen with respect to total surface carbon by XPS (in SDS-electrooxidation treated CFP, see above); XPS probes graphite to 8.7 nm depth and is thus more surface sensitive. Ergo, our Raman spectra of CFP were dominated by graphitic carbon modes, irrespective of surface treatment, and any Raman-active carbon-oxygen modes were unobservable. Likewise, oxidized carbon groups were undetectable in attenuated total reflectance Fourier transform infrared spectra of untreated, piranha-SDS-electrooxidation treated, and SDS-electrooxidation treated CFP, consistent with reported findings that strong skeletal in-plane and aromatic vibrations effectively obscure any oxidized carbon groups in CFP.
Powder XRD data showed that untreated CFP and CFP that was treated by acid-containing piranha-SDS-electrooxidation or acid-free SDS-electrooxidation exhibited virtually the same powder XRD patterns and that graphite was the only crystalline phase present, in accordance with published crystallographic data of graphite. We detected diffraction peaks with maxima at 2θ values of 26.5° and 54.5°), attributable to the (002) and (004) reflections of graphite (JCPDS file no. 12-0212). We acquired powder XRD data of intact CFP sheets and deliberately did not pulverize the CFP samples to prevent surface contaminations or modifications by grinding processes. We obtained asymmetric peak shapes for both reflections, as has previously been observed for CFP sheets. We surmise that Bragg reflections from different depths of the porous CFP, which was 0.2 mm thick, caused the asymmetric peak shapes. Reported selected area diffraction data of the (002) reflection that probed single carbon fibers showed symmetric peak shapes, albeit at the expense of signal intensity. In Bragg-Brentano reflection diffractometers, deviations in sample height lead to a shift of the position of the reflections; moving a powder sample downwards shifts the 2θ value of a particular reflection to lower angles. The powder diffractometer we used (Bruker D8 ADVANCE) had Bragg-Brentano reflection geometry and an X-ray line focus of 0.3×3 mm2; thus, multiple carbon fibers at multiple depths were probed simultaneously, which caused unavoidable underground sample contributions in the powder XRD patterns of CFP sheets, manifesting as lower-angle shoulders of the (002) and (004) reflections.
This Example reports the results of studies in which treated CFP was maintained under water.
We were able to preserve the hydrophilicity of chemically treated CFP for more than one year (62 weeks, so far) by storage under water, corroborating the key role of adsorbed water for sustained hydrophilicity. For practical applications of CFP, hydrophilicity must be sustained during operation in water or aqueous media, and not necessarily with intermittent storage in ambient air. Therefore, we conducted long-term time trials of select preparations of CFP that were stored under deionized water. The hydrophilicity of SDS-electrooxidation and piranha-SDS-electrooxidation treated CFP lasted for more than one year so far upon storage under water. These two treatments, however, led to CFP with markedly different nanostructures (compare
This Example discusses the findings and implications of some of the studies reported in Examples 5 and 6.
Selective carbon surface hydroxylation of initially hydrophobic CFP by an acid-free, transition-metal-free, environmentally benign, solution-processable method, comprised of treatment in 1 m aqueous SDS solution, followed by electrooxidation in 0.1 m aqueous bicarbonate electrolyte, imparted sustained hydrophilicity for more than one year (62 weeks) so far when CFP was stored under water. The procedure takes less than 30 min of preparation time and is amenable to large-scale manufacturing. Importantly, our work solves the long-standing challenge of making initially hydrophobic carbon fiber paper hydrophilic for long periods of time without destroying the mesostructure of carbon fiber network architectures and without lowering the flexural strength of CFP, suggesting that our surface modification process provided a substantial advantage over previously reported procedures. Hydrophilicity persisted because selectively generated surface hydroxyls were bound to graphitic edge sites that kept a threshold amount of adsorbed water of 0.5% with respect to total surface carbon content in place, explaining why storage under water enabled unprecedentedly long-lasting hydrophilicity. Inhibition of overoxidation of carbon beyond hydroxylation was achieved by the mild treatment conditions. ICP-MS data showed with ppb sensitivity that the chemical SDS-electrooxidation treatment did not leave residues on CFP surfaces. Application of structurally intact, hydrophilic CFP as high-surface-area electrode support in alkaline water oxidation catalyzed by [NiFe]-layered double hydroxide nanosheets showed greatly enhanced mass activity on hydrophilic high-surface-area CFP compared to that on flat highly ordered pyrolytic graphite (HOPG). Development of a mild and rapid surface functionalization process and understanding what drives sustained hydrophilicity transform the utility of inexpensive, high-surface-area carbon for clean energy technologies, such as aqueous electrolyzers, flow batteries, and supercapacitors, as well as water purification, electronics, biomedical, and sensing, especially biosensing, applications.
The following references are referred to in the text above by the number preceding the reference in the list set forth below.
It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.
This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63/294,638, filed Dec. 29, 2021, the contents of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/054297 | 12/29/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63294638 | Dec 2021 | US |
