Patent Application
20110236769
Microbial fuel cells (MFCs) convert chemical energy into electrical energy by the catalytic activity of microorganisms. Promising applications of MFCs include energy recovery from wastewater, marine sediment, and human excrement in space. The basic operation of MFCs is similar to that of other fuel cells, the oxidation of an electron donor at an anode releases electrons that pass through an external circuit to a cathode where an oxidant, such as oxygen, is reduced. In MFCs, however, oxidation at the anode is mediated by “exoelectrogens,” microorganisms that transfer electrons to an electrode. The transfer of electrons may occur by direct contact between redox-active membrane-bound proteins and the electrode surface, by the diffusion of redox-active molecules that ferry electrons between the electrode surface and the cell, or by conduction through microbially generated nanowires (or a solid matrix) that link cells to the electrode surface.
Prior studies have investigated various carbon-based electrodes in MFCs, including carbon cloth, carbon paper, carbon foam, and reticulated vitrified carbon. A graphite fiber brush anode has also been employed, as well as carbon-based anodes and composite anodes.
To date, electrode performance remains one of the most important factors limiting the power density of MFCs for practical application. The present invention addresses this and other needs.
Embodiments of the present invention provides an electrode for a microbial fuel cell, a method for making the electrode, as well as microbial fuel cells and methods of their use for generating electricity. The electrode for the microbial fuel cell can be configured as an anode or a cathode. The electrode comprises a porous substrate and a conductive nanostructure coating. In the case of the anode, the porous substrate supports internal colonization of bacteria and high density biofilm formation. The conductive nanostructure coating supports tight mechanical contact between microorganisms and the anode.
In one aspect, an embodiment of the present invention is directed to an electrode for a microbial fuel cell. The electrode comprises a porous substrate, which has a conductive nanostructure coating. The pores may be macroporous or microporous, and may have cross-sections that vary within the range of about 10 μm to about 100 mm, or about 20 μm to about 100 mm. For example, the substrate may have a median or mean pore cross-section in the range of about 50 μm to about 10 mm.
The porous substrate may be selected from any suitable material, and need not be conductive. For example, the porous substrate may be formed or fabricated from cotton, paper, textile, rubber, wood, synthetic polymer, copper, stainless steel, nickel, ceramic, sponge (natural or synthetic) or glass. In certain embodiments, the porous substrate is ceramic (natural or synthetic), and is formed from one or more of coral, silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum oxide (Al2O3), kaolinite (Al2Si2O5(OH)4), silicon carbide, tungsten carbide and zinc oxide.
The electrode has a conductive nanostructure coating and may be appropriately selected to enhance microbial contact (anode) or to provide high specific catalyst-electrolyte interfacial surface area (cathode), and may be selected from one or more of single wall carbon nanotubes, multiwalled carbon nanotubes, metal nanoparticles, transparent and conductive oxide (TCO) nanoparticles, metal nanowires, and graphene.
In another aspect, an embodiment of the invention provides a microbial fuel cell employing the electrode of the invention, either as a cathode or anode, or both. In some embodiments, the microbial fuel cell of the invention comprises both an anode and a cathode, each comprising the porous substrate conformally coated with the nanostructure material. In embodiments where the porous substrate is used for both the anode and cathode, the substrate may be the same or different. Likewise, the nanostructure coating may be the same or different.
In one embodiment, the microbial fuel cell comprises an anode compartment comprising the anode, and a cathode compartment comprising a cathode, for example, a cathode comprising the porous substrate, e.g., a macroporous substrate, coated with the nanostructure material. The anode and cathode compartments are separated by an ion exchange membrane, for example, a proton exchange membrane, and the anode and cathode are electrically connected. In a further embodiment, the cathode comprises a metallic catalyst, for example, a platinum catalyst. The catalyst can be deposited in the form of nanoparticles. As described herein, the microbial fuel cell employing the anodes and cathodes of embodiment of the invention provide improved power density and improved energy production.
In certain embodiments, the microbial fuel cell comprises a plurality of anodes, where at least one of the anodes comprises the porous substrate having the conductive nanostructure coating. In other embodiments, the microbial fuel cell comprises a plurality of cathodes, where at least one of the cathodes comprises the porous substrate having a conductive nanostructure coating. Each of the cathodes is electrically connected to at least one anode.
Also provided herein are methods for fabricating the highly conductive three dimensional electrode for use in a microbial fuel cell. In one embodiment, the method comprises coating at least one surface of a porous substrate with a nanostructure dispersion, and drying the coated substrate. For example, the coating step may comprise dip coating the substrate in the nanostructure dispersion.
Yet another aspect of the invention is a method for generating an electrical current with the microbial fuel cell. The method comprises, in one embodiment, introducing a feedstock solution comprising a carbon source and biofilm forming microorganism into the anode compartment of the microbial fuel cell. Biofilm formation is allowed to form on the surface of the anode, which then provides for an electric current as microbes within the biofilm oxidize substrate material present in the anode compartment, and transmit electrons to the cathode compartment through the electrical connection.
Embodiments of the present invention provide three-dimensional electrodes for microbial fuel cells. The electrodes provided herein each harness a porous substrate conformally coated with a nanostructure material. When configured as an anode, this configuration design combines an open macroscale porous structure for internal microbial colonization by diverse microflora, and an affinitive anode surface for improved electron transfer. The anode is conductive, stable and biocompatible. When configured as a cathode, the open and porous substrate provides a highly specific catalyst-electrolyte interfacial surface area.
Microbial fuel cells (MFCs) can recover energy from waste, e.g., wastewater, due to the catalytic activity of microorganisms, and are promising for alleviating both energy and environmental problems. The basic operation of MFCs is similar to that of other fuel cells (
A schematic of one cathode embodiment is also provided in
The electrode provided herein comprises a macroscale porous substrate (e.g., textile) conformally coated with a nanostructure material, e.g., single wall or multi-wall carbon nanotubes, and is designed to be configured as one or more components of a microbial fuel cell. The nanostructure layer, in one embodiment, when configured as an anode, promotes active surface interaction with the microbial biofilm and facilitates electron transfer from exoelectrogens to the anode, thus resulting in high-power operation of the microbial fuel cell to which the anode is associated.
The electrode provided herein, in some embodiments, affords a two-scale porous structure—a porous nanostructure layer (e.g., carbon nanotube layer) coated onto a porous substrate, e.g., a textile. Without being bound by theory, when configured as an anode, the macroscale porous substrate provides an open three dimensional space accessible to microbial growth and the microscale nanostructure layer strongly interacts with the microbial biofilm. Similarly, the macroscale porous substrate, when configured as a cathode, provides highly specific catalyst-electrolyte interfacial surface area.
Other characteristics of the electrodes provided herein include high conductivity, chemical stability, resistance to decomposition, mechanical stability and biocompatibility. Additionally, the lightweight nature of the anodes and the ease of preparation may allow for large-scale applications.
The electrodes, i.e., anodes and cathodes, provided herein employ a porous substrate for conformal coating with a nanostructure material. The large surface area of the porous substrate allows for maximal interaction with a conductive nanostructure material, e.g., carbon nanotubes (CNTs).
Rapid microbial growth can clog the pores of certain substrates used for microbial fuel cell anodes and therefore hinder the diffusion of the substrate, making the inner anode surface unfavorable for microbial colonization. Embodiments of the present invention are directed in part, to the finding that a porous substrate, e.g., a macroporous substrate, provides an open three dimensional space accessible for microbial film formation. In the context of a cathode, the substrate, when coated with a nanostructure provides a high specific catalyst-electrolyte interfacial surface area (cathode).
The pores in the electrode substrate may be of uniform shape, or may be non-uniform. Further, the aspect ratio of the pores may be uniform or non-uniform.
The size of the substrate's pores is sufficient so that the pores are not clogged (or are substantially unclogged) when a microbial film starts to form therein, and provides for high surface area. Generally, the mean or median cross-section of the pore size ranges from about 10 μm to about 500 mm, or from about 10 μm to about 400 mm, or from about 10 μm to about 300 mm, or from about 10 μm to about 200 mm, or from about 10 μm to about 100 mm, or from about 10 μm to about 10 mm, or from about 10 μm to about 1 mm.
In another embodiment, the mean or median cross-section of the substrate pores is from about 1 mm to about 300 mm, or from about 1 mm to about 200 mm, or from about 1 mm to about 100 mm, or from about 1 mm to about 10 mm. In yet other embodiments, the mean or median pore size ranges from about 10 mm to about 200 mm, or from about 10 mm to about 100 mm, or from about 10 mm to about 50 mm.
In another embodiment, the mean or median pore size is at least about at least about 15 μm, or at least about 25 μm, or at least about 50 μm, or at least about 100 μm, or at least 200 μm, or at least about 300 μm, or at least about 400 μm, or at least about 500 μm, or at least about 600 μm, or at least about 700 μm, or at least about 800 μm, or at least about 900 μm, or at least about 1 mm, or at least about 2 mm, or at least about 3 mm, or at least about 4 mm, or at least about 5 mm, or at least about 10 mm.
In another embodiment, the present invention is directed to an electrode for a microbial fuel cell having a porous substrate comprising a conductive nanostructure coating, wherein the porous substrate has a porosity ranging from about 70% to about 99%, or from about 70% to about 98%, or from about 70% to about 97%, or from about 70% to about 96%, or from about 70% to about 94%, or from about 70% to about 92%, or from about 70% to about 90%, or from about 70% to about 88%, or from about 70% to about 86%, or from about 70% to about 84%, or from about 70% to about 82%, or from about 70% to about 80%, or from about 70% to about 78%, or from about 70% to about 76%, or from about 80% to about 99%, or from about 80% to about 98%, or from about 82% to about 98%, or from about 84% to about 98%, or from about 86% to about 98%, or from about 88% to about 98%, or from about 90% to about 98%, or from about 92% to about 98%, or from about 94% to about 98%, or from about 94% to about 99%.
In one embodiment, the porous substrate is selected from one or more of cotton, paper, textile, rubber, wood, synthetic polymer, copper, stainless steel, nickel, ceramic, graphite felt, sponge (natural or synthetic) (see
As stated above, the porous substrate, in one embodiment, is ceramic. In a further embodiment, the ceramic (natural or synthetic) substrate is selected from one or more of coral, silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum oxide (Al2O3), kaolinite (Al2Si2O5(OH)4), silicon carbide, tungsten carbide and zinc oxide.
The macroscale porous substrate in one embodiment, is a porous textile comprising randomly intertwined polymeric (e.g., polyester) fibers with diameters of approximately 10 μm to about 50 μm. In a further embodiment, the porous textile is conformally coated with CNTs to form a CNT-textile anode. In even a further embodiment, the CNTs are single-wall carbon nanotubes.
In one embodiment, the porous substrate comprises silicon. For example, the porous substrate can be a borosilicate fritted disk. For example, as commercially available from R&H Filter Company, Georgetown, Del. (RH1000). Borosilicate glass composition is typically about 73% silica SiO2, 10% boron oxide B2O3, 8% sodium oxide Na2O, 8% potassium oxide K2O, and 1% calcium oxide CaO.
High surface area carbon mat nonwovens can also be used as microbial fuel cell electrode substrates. These substrates can either be electrospun or solution blown. For example, three-dimensional porous carbon fibers, produced by gas-assisted electrospinning can be employed as the electrode substrate. In another embodiment, electrospun carbon fibers are used as the electrode substrate. In even another embodiment, solution-blown carbon fibers are used as the electrode substrate. In one embodiment, electrospun carbon fibers or solution-blown carbon fibers are modified to incorporate 15% carbon black to increase porosity of the nonwovens, before use as the electrode substrate.
A substrate that is not sufficiently porous can be made so by techniques known to those of ordinary skill in the art. For example, sol-gel (Davis (2002). Nature 417, p. 813) and chemical vapor deposition (CVD) (Ying et al. (1999). Angew. Chem. Int. Edn. 38, p. 56), methods to produce silica membranes, hydrothermal synthesis to derive zeolite substrates (Nomura et al. (2005). J. Membr. Sci. 251, p. 151), anodization of aluminum (alloys) for pore patterning in alumina membranes (Masuda and Fukuda (1995). Science 268, p. 1466) and dealloying techniques to create nanoporous gold structures (Erlebacher et al. (2001). Nature 410, p. 450) can all be employed to arrive at a sufficiently porous substrate.
In addition to the above techniques, electrochemical material removal techniques can be used for fabricating porous electrode structures. For example, see Mukherji et al. (2008). Nanotechnology 19, pp. 1-8. Traditional lithographic approaches with etching can also produce nanopores; alternatively, substances can be selectively leached out of a solid, leaving pores of different shapes and sizes. In another embodiment, electrochemical phase dissolution techniques can be employed to produce meso-porous structures from simple two-phase metallic alloys.
The electrode of embodiments of the present invention, comprises a porous substrate as discussed, which in one embodiment, is macroporous, and coated with a nanostructure material. Various nanostructure materials can be used to coat the porous substrate. Non-limiting examples are described below. Additionally, the conductive materials provided in PCT application PCT/US2010/54776 are incorporated by reference herein.
The coatings presented herein can vary in thickness, and can be adjusted according to methods known to those of ordinary skill in the art, for example, by varying the times in coating protocols. In one embodiment, the electrode is coated with a nanostructure coating that has a thickness in the range of about 10 nm to about 500 nm, or in certain embodiments, from 50 nm to 300 nm. For example, in various embodiments the nanostructure coating is about 100 nm thick, or about 150 nm thick, or about 200 nm thick, or about 250 nm thick, or about 300 nm thick, or about 350 nm thick, or about 400 nm thick, or about 450 nm thick, or about 500 nm thick. Additionally, the coating need not be uniform, i.e., the thickness can vary from one portion of the substrate to another portion.
In one embodiment, the nanostructure material is carbon nanotubes (CNTs). When employed, CNTs can be single wall, multi-wall, or a combination thereof. The nanotubes can be fabricated by methods known to those of ordinary skill in the art, or are commercially available. For example, single-wall carbon nanotubes (SWCNTs) are available from Carbon Solutions, Inc. (Riverside, Calif.) and Unidym (Sunnyvale, Calif.). Carbon nanotubes from these companies are amenable for use with embodiments of the present invention.
Multi-wall carbon nanotubes are also available commercially, for example, from Nanocyl, Inc. (Belgium), and can be employed as the nanostructure material for the electrode of embodiments of the present invention.
In some embodiments, the CNTs have a mean or median diameter (e.g., outer diameter) in the range of about 1 nm to about 1 μm, about 1 nm to about 100 nm, about 10 nm to about 20 nm, about 10 nm to about 50 nm, about 10 nm to about 80 nm, and about 30 nm to about 70 nm, and a mean or median length in the range of about 10 nm to about 100 μm, about 100 nm to about 100 μm, about 500 nm to about 50 μm, or about 5 μm to about 50 μm.
CNTs can be dispersed in an appropriate solvent, and the dispersion can be used to coat the porous substrate. For example, the CNTs can be dispersed by sonication, in a concentrated sulfuric and nitric acid mixture for about 1 hr, about 2 hr., about 3 hr. about 4 hr. or about 5 hr., for example by the procedure outlined by Tsai et al. (Tsai et al. (2009). J. Power Sources 194, p. 199). The sulfuric acid and nitric acid, in one embodiment, is mixed in a ratio of 1:1, 2:1, 3:1, 4:1 or 5:1. The mixture can then be set still, for example, for 1 hr., before diluting with deionized water. Large particles and agglomerates can be removed from the solution by centrifuging. The centrifuged liquid can then be decanted, and filtered through a polycarbonate membrane, and washed with de-ionized water until slurry with a pH 6-7 is obtained. The slurry can then be added to a small amount of ethanol, which can then be sonicated to disperse the CNTs, resulting in a stable, dispersed product of the multi-wall carbon nanotube ink.
Carbon-nanotube ink can also be prepared by dispersing the CNTs (multi-wall or single-wall) in water with a surfactant. In one embodiment, sodium dodecylbenzene sulfonate (SDBS, Sigma-Aldrich, St. Louis, Mo.) is used as a surfactant. The amount of surfactant used, in one embodiment, is about 0.1%, or about 0.5%, or about 1.0%, or about 1.5%, or about 2%, or about 3%, or about 4%, or about 5%.
In one embodiment, the CNTs are present in the dispersion at about 0.05% by weight, or about 0.1% by weight, or about 0.15% by weight, or about 0.20% by weight, or about 0.25% by weight. In another embodiment, the CNTs range from about 0.05% to about 0.2% by weight in the dispersion.
Typically, when carbon nanotubes are dispersed in solution, sonication is employed. For example, in one embodiment, the carbon nanotube solution is dispersed with a probe-sonicator for about 10 minutes, or for about 15 minutes, or for about 20 minutes, or for about 25 minutes, or for about 30 minutes, or for about 45 minutes or for about 60 minutes at 150 W, or 160 W, or 170 W, or 180 W, or 190 W, or 200 W, or 210 W, or 220 W, to form a carbon nanotube dispersed ink. For example, the VC 505 (Sonics, Inc., Milpitas, Calif.) can be used as the sonicator-probe.
Nanotube dispersants are also available commercially, for example from Nano Lab (Waltham, Mass.), and are amenable for use with embodiments of the present invention.
In one embodiment, carbon nanotubes are used to coat a sponge substrate. For example, see
In one embodiment, conductive nanoparticles are used to coat the porous substrate of the invention. For example, nanoparticle diameters can range from as few as ten nanometers to a few microns, such as with a mean or median diameter in the range of about 1 nm to about 10 μm, about 10 nm to about 1 μm, about 10 nm to about 500 nm, or about 10 nm to about 100 nm. In other embodiments, the nanoparticles can be monodisperse in size or shape. Alternatively, the nanoparticles can be polydisperse in size or shape.
In some embodiments, the nanoparticles can have any desirable shapes, such as spherical, oblong, prismatic, ellipsoidal, irregular objects, or in the form of nanorods. When in the form of a nanorod the diameter of the rods can range from a few nanometers to several tens of nanometers, or several hundreds of nanometers. In some embodiments, nanorods can have a mean or median diameter (e.g., outer diameter) in the range of about 1 nm to about 1 μm, about 1 nm to about 100 nm, about 10 nm to about 20 nm, or about 10 nm to about 50 nm, about 10 nm to about 80 nm, about 10 nm to about 90 nm, or about 30 nm to about 70 nm, and a mean or median length in the range of about 10 nm to about 100 μm, about 100 nm to about 100 μm, about 500 nm to about 50 μm, or about 5 μm to about 50 μm. Their aspect ratio can be as low as 5 to a few thousand.
In one embodiment, the nanoparticles can be gold, silver, titanium (e.g., titanium carbide particles available from Nanostructured & Amorphous Materials, Inc., Houston, Tex.), or a combination thereof. Gold nanoparticles are available commercially, for example, from Nanocs (NY, N.Y.).
Alternatively, metal nanoparticles, for example, gold or titanium nanoparticles, can be fabricated according to methods known to those of ordinary skill in the art.
Metal nanoparticles, in general, can be fabricated by both bottom-up and top-down approaches. For example, in a top-down approach, a macroscopic precursor is used as the starting material, and is divided into smaller particles by milling or through lithographic processes. Sputtering, laser ablation, vapor phase deposition and lithography all are top-down approaches, and can be employed to arrive at a nanostructure material for use as an electrode coating.
In contrast, bottom-up approaches use atomic and molecular precursors as the starting material. Self assembly and/or chemical reactions can be used to arrive at the desired nanoparticle.
In one embodiment, metal nanoparticles can be produced by liquid chemical methods by reduction of chlorauric acid or hexachlorplatinate (HAuCl4 or H2PtCl6). Once the desired precursor is dissolved, a reducing agent is added, which causes metal ions to be reduced to neutral metal atoms. As this process continues, the solution becomes supersaturated, and the metal precipitates into sub-nanometer and nanometer size particles. The sub-nanometer particles can then grow by agglomeration, coalescence, ripening, etc. Growth can also be stabilized to limit the size of particles, by methods known to those skilled in the art.
Graphene is another nanostructure material that can be used as a conductive coating for the porous substrate, either for the cathode or anode, or both. As with metal nanoparticles, and carbon nanotubes, graphene is available commercially, e.g., from Angstrom Materials; Vorbeck Materials (Jessup, Md.) and XG Sciences (East Lansing, Mich.). Graphene coatings, without wishing to be bound by theory, may provide films of lower resistances because of the material's planar geometry, and highly accessible surface area. For example, see Segal (2009). Nature Nanotechnology 4, pp. 612-615.
Conductive nanowires, in one embodiment, are employed as a conductive coating for the porous substrate. For example, nanowires fabricated from gold, chromium, aluminum, titanium, niobium, platinum, silver and nickel can all be used with the anodes and methods described herein. The nanowires may be fabricated by known methods, such as lift off procedures and electron beam lithography. In certain embodiments, the nanowires are nanowire networks.
In another embodiment, silver nanowires are used as the nanostructure coating. For example, the silver nanowires can be produced in solution phase. Table 1, below, provides a recipe for silver nanowire synthesis, in accordance with one embodiment. In the first step, a mixture of 0.668 g poly-vinylpyrrolidone (PVP) and 20 mL ethylene glycol is heated in a flask at 170° C. Once the temperature is stabilized, 0.050 g of silver chloride (AgCl) is ground finely and added to the flask for initial nucleation. After three minutes, 0.22 g of silver nitrate (AgNO3) is titrated for ten minutes. The flask is kept at the same temperature for about another thirty minutes. After the reaction is completed, the solution is cooled down, and centrifuged three times to remove solvent, PVP, and other impurities.
In one embodiment, the nanostructure material for the electrodes provided herein comprise one or more transparent conductive oxide particles, which in one embodiment, form a film on the porous substrate. Non-limiting suitable transparent conductive oxides include indium tin oxide (ITO), ZnO, Cd2SnO4 and ZnSnO3, In2O3:Sn, ZnO:F, Cd2SnO4, ZnO:AI, SnO2:F, ZnO:Ga, ZnO:B, SnO2:Sb, ZnO:In. See, for example, R. G. Gordon, MRS Bulletin, August 2000. Mixtures of different transparent conductive oxides may be used, in accordance with one embodiment.
In one embodiment, TCO particle diameters can range from a few tens of nanometers to few microns, such as with a mean or median diameter in the range of about 1 nm to about 10 μm, about 10 nm to about 1 μm, about 10 nm to about 500 nm, or about 10 nm to about 100 nm. In other embodiments, the TCO particles can be monodisperse in size or shape. Alternatively, the TCO particles can be polydisperse in size or shape.
In some embodiments, the TCO particles can have any desirable shapes, such as spherical, oblong, prismatic, ellipsoidal, irregular objects, or in the form of nanorods. In the form of a nanorod the diameter of the rods can range from a few nanometers to several tens of nanometers, to several hundreds of nanometers. Their aspect ratio can be as low as 5 to few thousand.
The TCO particles can be formed in the form of oxide sols by the hydrolysis of the corresponding metal-organic precursors. The mean diameter, size dispersity, and aspect ratios of the TCO particles can be controlled by various factors like concentrations, temperature and duration of the reaction.
In one embodiment, the microbial fuel cell electrode is fabricated by dip-coating the porous substrate into a nanostructure dispersion (for example single wall or multi wall carbon nanotubes), followed by drying of the substrate. For example, the substrate can be dried for about 2 hours at ≦100° C., or for about 90 minutes at ≦100° C. In another embodiment, the substrate is dried for about 30 minutes at 150° C. Drying time and temperature will depend on the substrate employed.
This process can be repeated multiple times, for example, two to ten times. For example, two, three, four, five, six, seven, eight, nine or ten times. Repetition may ensure the substrate, including the substrate pores, is fully coated with the nanostructure, or combination of nanostructures.
Although the examples provided below employ dip-coating of a substrate in a nanostructure dispersion, the present invention is not limited thereto. For example, physical vapor deposition, e.g., pulsed laser deposition can be employed to coat the porous substrates. Chemical vapor deposition may also be performed.
Vapor deposition of graphene can also be employed, for example in a single step chemical vapor deposition process.
Spray application coating can also be employed to coat the porous substrate, for example, by an aerosol or non-aerosol spray.
When spray coating, a nanostructure precursor or the nanostructure material itself may be sprayed onto the substrate. If the precursor is used, it is reacted on the substrate to form the nanostructure material. Typically, the number of layers, optical absorbance, and electrical conductance of the nanocoatings can be controlled by the speed of deposition, and diameter of the spray nozzle.
In another embodiment, ultrasonic atomization of nanostructure solutions is used to apply the nanostructure onto the desired substrate. In these systems, the nanostructure ink is atomized at the nozzle by pressure or ultrasound and then directed toward the substrate by a gas.
Another method for coating the porous substrate with the nanostructure material is spin coating. Single or multiple layers of nanostructure can be applied to the substrate with drying steps in between the spin coating steps.
Once the porous substrate is coated, it can be configured as either the cathode or the anode. If configured as the anode, it can be installed directly into the microbial fuel cell. If a cathode is desired, the coated substrate can then be derivatized with a metallic substrate.
If the coated nanostructure substrate is configured as a cathode, at this point, a metallic catalyst is added to the coated substrate, for example, in one embodiment, a metallic catalyst is added by electrochemical deposition. In a further embodiment, the metallic catalyst is platinum.
In one embodiment, electrochemical deposition process can be carried out in a flask, e.g., a four-neck flask, containing chloroplatinic acid (H2PtCl6) and hydrochloric acid (HCl) as electrolyte. For example, see, Saminathan (2009). International Journal of Hydrogen Energy 34, pp. 3838-3844. The substrate coated with the nanostructure is used as the working electrode, and two Pt meshes, one on each side of the coated substrate, can be used as counter electrodes in order to optimize the current lines and have a uniform Pt distribution on the whole current collector. The deposition, in one embodiment, is performed via a potentiostatic technique, fixing the potential at −0.6 V vs. a double junction Ag|AgCl|KCl (3.5 M) reference electrode (RE). For example, see, Saminathan (2009). International Journal of Hydrogen Energy 34, pp. 3838-3844.
Besides platinum, other metal catalysts can also be employed in the cathodes of the invention. In one embodiment, the catalyst is a late transition metal, and can be, for example, palladium (e.g., tetrachloropalladinate solution in H2SO4), gold, ruthenium (e.g., with a solution of Ru(NO)(NO3)3 in H2SO4), rhodium, platinum or iridium (e.g., with a solution of Na2IrCl6 in H2SO4), or a combination thereof. Regardless of the catalyst chosen, it can be deposited on the electrode to form the cathode, for example, by electrochemical deposition, as described above.
Although certain embodiments of the present invention have been described primarily with biofilms coating the anode, in some embodiments, a biofilm comprising a nitrate, chlorate or perchlorate reducing microbial community is present on the cathode of the fuel cell. In such “biocathode” embodiments, i.e., where a microbial biofilm is present on the cathode, the microbial community accepts electrons from the solid cathode. In microbial fuel cell embodiments incorporating a biocathode, a microbial biofilm may or may not be present on the anode. Additionally, a metallic catalyst may or may not be present on the biocathode. However, the cathode comprises the porous substrate and nanostructure coating, as described above. In one embodiment, biocathodes are inoculated with a chlorate-reducing enrichment from late sediment. The pH of the cathode compartment can be optimized to improved perchlorate reduction.
The basic structure of one embodiment of a microbial fuel cell (MFC) is provided in
In the embodiment shown in
According to embodiments of the present invention, the cathode or the anode, or both, comprise the porous substrate conformally coated with a nanostructure coating, as described above. In one embodiment, as detailed above, the nanostructure coating comprises carbon nanotubes, for example, single wall carbon nanotubes, multi wall carbon nanotubes, or a combination thereof.
Although the embodiment shown in
The domestic wastewater can have one or more microbes present in it, and the biofilm formed on the anode can comprise one or more of such microbes. For example, one or more of the following microbes may be present in the biofilm formed on the anode: Geobacter, Shewanella, Rhodopseudomonas, Ochrobactrum, Enterobacter, Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Acidiphilium cryptum, Acidiphilium multivorum, Acidiphilium symbioticum, Acidiphilium angustum, Acidocella aminolytica, Acidocella facilis, Sulfobacillus thermosulfidooxidans, Ferroplasma acidarmanus, Metallosphaera sedula, Sulfolobus acidocaldarius, Sulfolobus solfataricus, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans, Escherichia coli, Shewanella oneidensis and Acidithiobacillus ferrooxidans. However, any bacteria that can transfer electrons is amenable for use with embodiments of the present invention. Preferably, the bacteria is already present in wastewater, sewage, marine sediment, or some other waste stream.
One advantage of embodiments of the present invention is that the anode allows for internal colonization of the substrate, and strong interaction between the microbes and the anode, including affinitive mechanical contact and higher electrical conductivity between the anode surface and microbial biofilms. Without being bound by theory, this interaction facilitates efficient extracellular electron transfer from the microbes to the anode.
Without wishing to be bound by theory, three pathways of electron transfer from exoelectrogens to the nanostructure coated anode are likely facilitated by the particular electrode structure and properties of the nanostructure layer (
As described above, the MFC of the invention may have one or both electrodes conformally coated with a nanostructure material, e.g., a layer of carbon nanotubes (single-wall or multi-wall, or a combination thereof). When the cathode employs such a structure, as described above, a metallic catalyst may also be present on the electrode, and is deposited by methods such as electrochemical deposition.
As described above, in one embodiment, the catalyst is applied to the cathode by an electrochemical deposition method, which provides the electronic pathway to most or all of the catalyst, while the open and macroscale porous substrate provides highly specific catalyst-electrolyte interfacial surface area (
The MFC embodiment shown in
The electrodes provided herein can also be configured for use in other types of electrochemical cells. For example, in one embodiment, the electrode of the invention is configured as one or more components of a microbial electrolysis cell. Microbial electrolysis cells, in one embodiment, generate hydrogen gas or methane from organic material. In one microbial electrolysis cell embodiment, Pseudomonas spp. and/or Shewanella spp. are present on the anode. In one embodiment, soil or wastewater is used as the anolyte. The electrode can also be configured as one or more components of a microbial desalination cell. In one embodiment, the microbial desalination cell includes an anion exchange membrane adjacent to the anode, and a cation exchange membrane positioned next to the anode, creating a middle chamber between the anode compartment and the cathode compartment. The middle chamber is filled with water. Current is produced by bacteria as described above, and ionic species in the middle chamber are transferred to the two electrode chambers, thereby desalinating the water in the middle chamber. A plurality of anodes and/or cathodes may be present in the respective anode and cathode compartments of the microbial electrolysis cell and the microbial desalination cell. The electrodes described herein can be configured as one or more of the plurality of anodes and/or cathodes.
As described above, the basic configuration of a MFC is provided in
In another embodiment, the plurality of anodes are in electrical communication with one or more cathodes, as described herein, i.e., a porous substrate conformally coated with a nanostructure material with a metallic catalyst (e.g., in the form of nanoparticles) deposited thereon. The nanostructure material can be any of the materials described herein, for example, carbon nanotubes. In one embodiment, the catalyst is platinum, for example, platinum nanoparticles.
Another embodiment is directed to an MFC with a plurality of cathodes in the cathode compartment. In this embodiment, each of the cathodes is in electrical communication with an anode. In one embodiment, at least one of the plurality of cathodes comprises the porous substrate conformally coated with the nanostructure material, and a metallic catalyst deposited thereon, as described above. In a further embodiment, the nanostructure material comprises carbon nanotubes (single wall, multi wall, or a combination thereof).
In another embodiment, multiple MFCs as described herein are run in series or in parallel. Additionally, the anolyte and the catholyte can be recirculated in any one particular MFC.
Embodiments of the present invention is further illustrated by reference to the following Examples. However, it should be noted that these Examples, like the embodiments described above, are illustrative and are not to be construed as restricting the scope of the invention in any way.
Unless otherwise indicated, the materials used in the following examples were prepared according to the following methods and procedures.
The CNT-textile electrode was fabricated by dipping and drying of a piece of textile cloth in aqueous CNT ink (Hu et al. (2010). Stretchable, porous and conductive energy textiles. Nano Lett 120, pp. 708-714).
Aqueous CNT ink was prepared by dispersing single-wall CNTs in water with sodium dodecylbenzene sulfonate (SDBS) as a surfactant. The concentration was 0.16% for CNT and 1% for SDBS by weight. The dispersion was bath sonicated for 5 minutes followed by probe sonication for 30 minutes. A piece of textile made of randomly intertwined polyester fibers (Cloud 9 dream fleece, Wal-Mart Inc) was then dipped into CNT ink, removed and dried. The dipping-drying process was repeated for 4 times to increase the CNT loading in textile. The CNT-textile was functionalized in nitric acid (4 mol L−1) before installed as the anode for MFCs.
H-shaped two-chamber MFCs were constructed by connecting two 200 mL media bottles with a 40 mm-diameter tube. The anode was CNT-textile (1 cm×1 cm, projected area of 2 cm2) or carbon cloth (1 cm×1 cm, projected area of 2 cm2, Fuel Cell Earth LLC, MA). The cathode was carbon cloth (2 cm×5 cm, projected area of 20 cm2, Fuel Cell Earth LLC, MA) with a catalyst layer (0.5 mg cm−2 10 wt % Pt on XC-72). The anode and the cathode were connected to external circuit with titanium wire, and all exposed metal surfaces were sealed with a nonconductive epoxy (Dexter, N.J.). Anode and cathode compartments were separated by an anion exchange membrane (AMI-7001, Membranes International Inc., NJ). The distance between the anode and the cathode was about 11 cm. Domestic wastewater was used as the inoculum. The anode chamber was filled with artificial wastewater (pH ˜7), containing glucose (1.0 g L−1), NaH2PO4.H2O (4.90 g L−1), Na2HPO4 (9.15 g L−1), KCl (0.26 g L−1), NH4Cl (0.62 g L−1), mineral solution (12.5 mL L−1) and vitamin solution (5 mL L−1).
During normal operation, the MFCs underwent a semi-batched feeding regime. Additional glucose (0.15 g for 150 mL anolyte) was fed intermittently when the voltage generation is lower than 0.02 V. The cathode chamber was filled with the same media, but without glucose, mineral solution, and vitamin solution. The anolyte was mixed using a magnetic stirrer (200 rpm). The cathode chamber was continuously sparged with air using a diffusion stone (0.1 L min−1). The voltage across a 1 kΩ external resistor was recorded. All experiments were conducted at room temperature (˜20° C.).
The electrochemical characterization was carried out using a BioLogic VMP3 potentiostat-galvanostat multichannel equipped with electrochemical impedance spectroscopy (EIS) board. A double junction Ag|AgCl|KCl (3.5 M) reference electrode (RE) was used in the measurement. Cyclic voltammetries were performed in the potential range −0.5 V to 0.5 V vs. RE under a sweep rate of 10 mV s−1. Linear staircase voltammetries were applied by increase the anode potential from −0.5 to 0 V vs. RE by 25 mV each time and recording the current after 3 minutes for equilibrium. Polarization curves were measured under a step-sweep rate of 30 mV per 5 minutes starting from the OCV value. EIS was conducted at the OCV in the frequency range of 105-0.1 Hz with a 10 mV peak-to-peak sinusoidal potential perturbation. The results were reported as Nyquist plots.
The pretreatment process of all the samples with microorganisms was as follows: (1) small pieces of the anode were primarily fixed overnight in the fixative containing 0.1 M sodium cacodylate buffer (Ph 7.3), 2% glutaraldehyde and 4% paraformaldehyde at 4° C., then washed with the same buffer for 5 minutes; (2) secondary fixation was performed in 1% osmium tetroxide at 4° C. for 1-2 hours, followed by washing with Milli-Q water for 10 minutes; (3) the samples were dehydrated in increasing concentrations of ethanol solution (50, 70, 90 and 100%), and critical point dried in 100% ethanol with liquid CO2; (4) the samples were finally sputter coated with 10 nm of gold. All the SEM images were taken by a field emission scanning electron microscope (FEI XL30 Sirion SEM).
After 50 days of operation, genomic DNA was extracted from a carbon nanotube-textile (CNT-textile) anode sample (0.1 cm2) in duplicate using the FastDNA Spin Kit for soil (MP Biomedicals, Solon, Ohio) according to the manufacturer's protocol, except for the initial bead-beating step. A Vortex Adapter (MO BIO laboratories, Inc., Carlsbad, Calif.) with the Vortex Genie 2T (Scientific Industries, Inc., Bohemia, N.Y.) was used to physically disrupt cells in lysing matrix at maximum speed for 15 minutes.
Bacterial 16S rRNA genes were PCR amplified from the genomic DNA using the bacteria-specific forward primer 8F (5′-AGA GTT TGA TCM TGG CTC AG-3′) and the universal reverse primer 1492R (5′-TAC GGY TAC CTT GTT ACG ACT T-3′) (Lane (1991). 16S rRNA sequencing. In Nucleic Acid Techniques in Bacterial Systematics, eds Stackebrandt E, Goodfellow M (John Wiley & Sons, New York) pp. 115-175).
Each 25 μL PCR mixture comprised 0.25 μM of each primer, 1× Fail-Safe PCR buffer F (Epicentre, Madison, Wis.), 1.25 units of AmpliTaq LD Taq polymerase (Applied Biosystems, Inc., Foster City, Calif.), and 100-140 ng of genomic template DNA. The PCR temperature profile was as follows: an initial melting step at 94° C. for 5 minutes, followed by 35 cycles consisting of 94° C. for 45 seconds, 55° C. for 30 seconds, and 72° C. for 90 seconds, with a final extension at 72° C. for 10 minutes. Presence or absence of the expected amplicon was checked via agarose gel electrophoresis.
For 16S rRNA gene cloning and sequencing, quadruplicate PCR products were pooled and purified via gel electrophoresis using the QIAquick gel extraction kit (Qiagen Inc., Valencia, Calif.). Purified PCR products were cloned using the pGEM-T Easy Vector System and transformed into E. coli JM109 competent cells (Promega, Madison, Wis.), as per the manufacturer's protocol. To confirm the presence of ˜1500-bp 16S rRNA gene inserts, Escherichia coli transformants were grown overnight at 37° C. and used as PCR templates with T7 and SP6 sequencing primers. Forty eight clones were randomly selected and sequenced from both the T7 and SP6 priming sites on ABI 3730x1 automated sequencers by Elim Biopharmaceuticals, Inc. (Hayward, Calif.), generating a total of 48 bacterial 16S rRNA gene sequences.
Bacterial 16S rRNA gene sequences (˜1500 bp) were compared to all available sequences in Genbank using the NCBI BLAST utility (ncbi.nlm.nih.gov/blast/Blast.cgi). Sequences were subsequently aligned with the GreenGenes NAST utility and imported in the ARB software package to a database of 236,469 16S rRNA sequences included in the Nov. 18, 2008 release of GreenGenes (greengenes.lbl.gov) (DeSantis et al. (2006). NAST: a multiple sequence alignment server for comparative analysis of 16S rRNA genes. Nucleic Acids Res 34, pp. W394-W399; DeSantis et al. (2006). Greengenes, a chimera-checked 16S rRNA gene database and workbench compatible with ARB. Appl Environ Microbiol 72, pp. 5069-5072; Ludwig et al. (2004). ARB: a software environment for sequence data. Nucleic Acids Res 32, pp. 1363-1371).
A neighbor-joining phylogenetic tree with the Jukes-Cantor correction was generated in ARB based on a multiple alignment of cloned 16S rRNA gene sequences and closely related database samples.
A CNT-textile anode was prepared as described above.
In the CNT-textile, the intertwined macroscale textile fibers (
The anode comprised a ˜200-nm-thick CNT coating (
In order to determine whether the CNT-textile satisfied the desired three-dimensional anode configuration and improved the extracellular electron-transfer efficiency, the CNT-textile was installed as the anode in a classic H-shaped two-chambered MFC.
The MFC was inoculated with domestic wastewater and fed glucose. The uncolonized CNT-textile was initially inactive (
The cyclic voltammogram (
For the CNT-textile anode, although the textile fiber provided mechanical support, the sole electroactive material present was CNT. Therefore, the results confirmed that CNT is biocompatible in MFCs and can function as an anode.
The microbial community structure of the biofilm associated with the CNT-textile anode was analyzed via a bacterial 16S rRNA gene clone library. The results revealed a diverse community (
To evaluate the performance of the CNT-textile anode, an MFC with an identical configuration but with a commercial carbon cloth anode was operated in parallel with a CNT-textile-equipped MFC. A schematic of each electrode is provided in
The carbon cloth is made of regularly woven graphite fibers (
After 55 days of operation, both anodes were sampled and the cross sections were characterized under SEM (
Assuming that all of the CNT-textile fiber surfaces are occupied by biofilms, the anolyte-biofilm-anode interfacial area was calculated to be 10-fold larger than the projective surface of the anode.
In the case of the carbon cloth anode, however, microbial colonization was largely restricted to the outer surface of the anode, with few microorganisms present on the interior fibers, as evidenced in
Besides the increase in the anolyte-biofilm-anode interfacial area, the CNT-textile fiber surface revealed excellent interaction with the microbial biofilm. First, the CNT coating made the surface of CNT-textile fibers rough (
Second, the coated CNTs themselves formed a secondary microscale porous structure. This provided high surface area with active functional groups to collect electrons from electron mediators or shuttles in the electrolyte.
Finally, the CNT layer displayed effective interaction with microbial nanowires. A great number of microbial nanowires were observed under SEM (
The MFC equipped with a CNT-textile anode achieved better performance than that prepared with a carbon cloth anode. Anode comparisons were made after 2 months of operation when both of the MFCs achieved repeatable power generation cycles with a 1 kΩ external resistor (
With the 1 kΩ loading, the potential of the CNT-textile and the carbon cloth dropped to −0.34 and −0.25 V versus Ag|AgCl, respectively. The CNT-textile showed a 0.09 V less anodic potential loss than the carbon cloth (0.11 vs 0.20 V). Maximum current density and power density measurements were applied 30 h after the replacement of fresh glucose media, when the power generation returned to steady state and the glucose concentration was still close to the original level (1 g L−1).
Both the current density and power density results were normalized to the projective surface area of the anode. As shown in
The total electric energy generation of the MFCs was calculated by integrating the power-time curve (
Electrochemical impedance spectroscopy (EIS) tests were carried out to investigate the internal resistance of both MFCs. The charge-transfer resistance was also calculated for both anodes. This parameter is indicated by the diameter of the first semicycle in the Nyquist curve.
As shown in
In conclusion, compared with a widely used commercial carbon cloth anode, the CNT-textile achieved significantly improved MFC performance.
The strength of the mechanical binding of microbial biofilms to the CNT-textile anode was assessed. Anode samples with associated mature microbial biofilms (
A CNT-textile sheet with thickness of ˜1 mm was cut into a 1 cm×3 cm piece, and then treated with nitric acid (4 M, 2 hours) and glacial acetic acid (2 hours) successively before the Pt deposition. The acid treatment process increased the sample's hydrophilicity and produced oxygen-rich functional groups on the originally inert CNT surface to act as nucleating sites for Pt deposition.
The electrochemical deposition process was performed in a four-neck flask containing chloroplatinic acid (H2PtCl6, 0.019 M) and hydrochloric acid (HCl, 0.6 M) as electrolyte (Saminathan (2009). International Journal of Hydrogen Energy 34, pp. 3838-3844). The CNT-textile was the working electrode with only 1 cm×1 cm dipped into the electrolyte. Two Pt meshes, one on each side of the CNT-textile, were used as counter electrodes in order to optimize the current lines and have a uniform Pt distribution on the whole current collector. The deposition was performed via a potentiostatic technique, fixing the potential at −0.6 V vs. a double junction Ag|AgCl|KCl (3.5M) reference electrode (RE) (Saminathan (2009). International Journal of Hydrogen Energy 34, pp. 3838-3844).
The morphologies of the electrode surfaces were investigated using a field emission scanning electron microscope (SEM, FEI XL30 Sirion). The Pt nanoparticles were also examined by a transmission electron microscopy (TEM, FEI Tecnai G2 F20 X-TWIN 200 kV). Pt loadings were measured by an IRIS advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES) system.
The average sizes of Pt particles on different electrodes were calculated by the Debye Scherrer equation, based on the X-ray Diffraction (XRD) test results.
ECAS area was characterized using an electrochemical method, in which the ECAS area was proportional to the hydrogen adsorption-desorption capability of the electrode.
Oxygen reduction efficiencies were tested by performing linear staircase voltammetries (LSVs) at step-sweep rates of 10 mV per 10 seconds from 0.3 to −0.3 V vs. a double junction Ag|AgCl|KCl (3.5M) reference electrode and cyclic voltammetries (CVs) at scan rates of 10 mV s−1 between −0.5 to 0.5 V vs. a double junction Ag|AgCl|KCl (3.5M) reference electrode. The counter electrode was Pt foil, and the electrolyte was a phosphate buffer solution simulating the working condition in aqueous-cathode MFCs. The electrolyte was saturated with oxygen under ambient pressure and temperature.
All cathode samples were investigated in an H-shaped two-chamber MFC with a CNT-textile anode (1 cm×1 cm). The MFC was inoculated with domestic wastewater and was operated for 6 months to obtain a mature biofilm on the anode. An anion exchange membrane (AMI-7001, Membranes International Inc., NJ) was used as the separator. The catholyte was a phosphate buffer solution (100 mM, pH 7) comprised of NaH2PO4.H2O (4.90 g L−1), Na2HPO4 (9.15 g L−1), KCl (0.26 g L−1), and NH4Cl (0.62 g L−1). The anolyte contained glucose (1 g L−1), mineral solution (12.5 mL L−1), vitamin solution (5 mL L−1), and the same PBS as that used for the catholyte (Oh et al. (2004).
The cathode chamber was continuously purged with air using a diffusion stone (flow rate ˜0.1 L min−1). To evaluate the MFC performance, polarization curves were measured under a step-sweep rate of 10 mV per 10 seconds starting from the OCV value. The anode was the working electrode and the cathode was both counter electrode and reference electrode. Power and current densities were normalized by the projected surface area of the cathode (2 cm2). All experiments were conducted at room temperature.
The scanning electron microscopy (SEM) image of the plain surface of CNT-textile is shown in
The performance of the CNT-textile-Pt composite as an aqueous cathode was compared with a commercial carbon cloth cathode with Pt catalyst (CC-Pt), which was prepared by a general painting method. Images of the commercial cathode are provided in
The oxygen reduction activity of the cathode samples (1 cm×1 cm) was characterized by applying linear staircase voltammetry (LSV) at a step-sweep rate of 10 mV per 10 seconds from 0.3 to −0.3 V vs. a double junction Ag|AgCl|KCl (3.5 M) RE. The electrolyte was a phosphate buffer solution saturated with oxygen under ambient pressure and temperature, in order to simulate the working condition in aqueous-cathode MFCs. As shown in
While oxygen reduction by the plain CC without Pt was almost negligible, the plain CNT-textile without Pt also reveals certain oxygen reduction activity. However, the catalytic activity of CNTs alone is lower than that of Pt.
The long term stability of the CNT-textile-Pt cathode was investigated by performing cyclic voltammetry (CV) measurements at a scan rate of 10 mV s−1 between −0.5 and 0.5 V vs. a double junction Ag|AgCl|KCl (3.5M) RE in the same electrolyte as that applied in the LSV tests. The result showed that the oxygen reduction activity of the CNT-textile-Pt did not decay after 2000 cycles (
Cathode samples were investigated in an H-shaped two-chamber MFC with a CNT-textile anode (1 cm×1 cm).
The MFC was inoculated with domestic wastewater and had been operated for 6 months to obtain mature biofilms on the anode. Maximum power densities of the MFCs equipped with different cathodes were determined from the polarization curves measured under a step-sweep rate of 10 mV per 10 seconds starting from the OCV value.
As shown in
The maximum power density of the MFC with the CNT-textile-Pt cathode was 837 mW m−2, 2.14 times of that achieved by the MFC with the carbon cloth-Pt cathode (391 mW m−2). The MFC prepared with the plain CNT-textile cathode also generated a maximum power density of 177 mW m−2.
Additionally, after removing this contribution of the CNTs, the CNT-textile-Pt cathode showed 1.7 times better performance than the carbon cloth-Pt cathode, which is indicative of greater catalytic activity of the Pt in CNT-textile-Pt.
The maximum current density achieved by the CNT-textile-Pt cathode was 5.2 A m−2, which was greater than previously reported values (0.1 A m−2).
Pt loadings were determined by an IRIS advantage inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The Pt loading of CNT-textile-Pt was controlled by the charge applied during electrochemical deposition. As shown in Table 2, the Pt loading of the CNT-textile-Pt cathode is five times less than a commercial carbon cloth-Pt cathode (0.048 mg cm−2 vs. 0.249 mg cm−2). Decreasing Pt loading in the CNT-textile-Pt provides a direct approach to reduce the capital cost of the cathode.
Two more CNT-textile-Pt samples with lower Pt loadings (0.008 mg cm−2 and 0.002 mg cm−2) were synthesized by further decreasing the applied charges to 200 mC and 100 mC, respectively. The maximum power densities of the MFCs equipped with these two cathodes were reduced from 837 mW m2 to 559 mW m−2 and 205 mW m−2, respectively (
The average sizes of Pt particles on different electrodes were calculated by Debye Scherrer equation, based on the X-ray Diffraction (XRD) test results (
With the average particle sizes and total Pt loading, the theoretical overall Pt surface area was estimated. As shown in Table 2, the CNT-textile-Pt has less theoretical Pt surface area (14.5 cm2 Pt per cm2 electrode) than the carbon cloth-Pt (262.5 cm2 Pt per cm2 electrode) due to the lower mass loading and larger particle sizes. However, the CNT-textile-Pt had larger ECAS area (7.79 cm2 Pt per cm2 electrode, vs. 0.92 cm2 Pt per cm2 electrode for CC-Pt), determined by an electrochemical method, in which the ECAS area is proportional to the hydrogen adsorption-desorption capability of the electrode (
Comparing the surface area utilization efficiency calculated from the ratio of ECAS area to the theoretical surface area, CNT-textile-Pt was two orders better than the carbon cloth-Pt (53.6% vs. 0.4%). Without wishing to be bound by theory, the significant improvement is thought to be due to several reasons: (1) CNT-textile-Pt provided direct electronic pathways for all the Pt particles while CC-Pt has isolated Pt particles where no electronic pathways are formed; (2) CNT-textile-Pt provided macroscale pores for the fast access of electrolyte although CC-Pt has some Pt particles located in small or closed-end pores that are not accessible for the electrolyte; (3) glacial acetic acid treatment in CNT-textile-Pt made the surface hydrophilic for electrolyte wetting while the hydrophobic surface of the CC-Pt hinders the electrode-electrolyte contact.
Pt electrochemical deposition was applied on commercial carbon cloth electrodes (Fuel Cell Earth, USA) following the same procedure as described above. The SEM image (
aCalculated from Pt loading and average particle size.
bObtained from electrochemical measurement.
cRatio of electrochemically active surface area to theoretical surface area.
All, document, patents, patent applications, publications, product descriptions, and protocols which are cited throughout this application are incorporated herein by reference in their entireties for all purposes.
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use certain embodiments of the invention. Modifications and variation of the above-described embodiments of the invention are possible without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/316,641, filed Mar. 23, 2010, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61316641 | Mar 2010 | US |
