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PROCESS FOR THE PREPARATION OF CARBON FELT ELECTRODES FOR REDOX FLOW BATTERIES AND PROCESS FOR PRODUCING REDOX FLOW BATTERIES

20180127895 ยท 2018-05-10

    Inventors

    Cpc classification

    International classification

    Abstract

    A process prepares metal-doped felt fabric made from carbon fibers. A textile structure of pre-oxidized polyacrylonitrile fibers is carbonized at temperatures of up to 1500 C. and wherein polyacrylonitrile and/or oxidized polyacrylonitrile as precursor fibers are functionalized with a metal precursor.

    Claims

    1. A method for producing metal-doped felt from carbon fibers, which comprises: carbonizing a textile structure composed of peroxidized polyacrylonitrile fibers at temperatures of up to 1500 C. and polyacrylonitrile as precursor fibers being functionalized with a metal precursor.

    2. The method according to claim 1, which further comprises forming the metal-doped felt to have a thickness of from 0.5 to 10 mm.

    3. The method according to claim 1, which further comprises forming the metal-doped felt to have a weight per unit area of from 100 to 1,000 g/m.sup.2.

    4. The method according to claim 1, which further comprises forming the metal-doped felt to have a BET surface area of from 0.4 to 10 m.sup.2/g.

    5. The method according to claim 1, which further comprises forming the metal-doped felt to have a specific electrical resistance perpendicular to a felt direction of from 0.5 to 5 ohm mm.

    6. The method according to claim 1, which further comprises forming the metal-doped felt to have a carbon content of from 90 to 99%.

    7. The method according to claim 1, which further comprises forming the metal-doped felt to have a proportion of nitrogen of from 0.2 to 5%.

    8. The method according to claim 1, which further comprises forming the metal-doped felt to have an interplanar spacing of from 3.40 to 3.55 angstrom.

    9. The method according to claim 1, which further comprises forming the metal-doped felt with proportions of tin, bismuth, manganese, indium, phosphorus and/or antimony in each case from 200 to 5000 ppm.

    10. A method of producing a battery, which comprises the steps of: producing a metal-doped felt from carbon fibers by carbonizing a textile structure composed of peroxidized polyacrylonitrile fibers at temperatures of up to 1500 C. and polyacrylonitrile as precursor fibers being functionalized with a metal precursor; and using the metal doped felt in a redox flow battery.

    Description

    BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

    [0045] FIG. 1 is a graph showing voltage efficiency (in %) of an individual vanadium redox flow battery cell as a function of a current density (in mA/cm.sup.2) using two electrodes; and

    [0046] FIG. 2 is a graph showing a charging efficiency (in %) of the individual vanadium redox flow battery cell as a function of the current density (in mA/cm.sup.2) using two electrodes.

    DETAILED DESCRIPTION OF THE INVENTION

    Embodiment 1

    [0047] Dispersion 1A:

    A solution, or dispersion, is produced from 1 wt. % bismuth(III) isopropoxide in water/isopropanol (9:1)

    [0048] Dispersion 1B:

    A solution, or dispersion, is produced from 0.5 wt. % bismuth(III) isopropoxide, 0.5 wt. % bismuth hexanoate and 0.4 wt. % tin isopropylate in water/isopropanol (9:1)

    [0049] Dispersion 1C:

    A solution, or dispersion, is produced from 1 wt. % bismuth hexanoate, 0.5 wt. % indium(III) isopropylate and 0.3 wt. % antimony(III) isopropylate in water/isopropanol (9:1).

    [0050] Carbon precursor fibers made of polyacrylonitrile (1.7 dtex or 2.2 dtex) are in each case impregnated with the described dispersions (1A, 1B, 1C), dried and stabilized by means of thermal oxidation under normal atmospheric conditions at 240-280 C. The fibers thus obtained are processed into curled staple fibers (62 mm fiber length). After combing/carding, the fibers are laid to form a single-layer or multi-layer web and processed into a felt (mass per unit area of from 200 to 800 g/m.sup.2) by needle punching on one or both sides. Subsequently, carbonization takes place in an inert atmosphere in a continuous furnace at a temperature of 1480 C.

    [0051] A reference sample without the addition of metal compounds was carbonized in the same manner (control sample 2). A commercial, graphitized carbon fiber Sigracell GFD 4.6 (SGL Carbon GmbH, Meitingen) was used as another reference material (control sample 1).

    Embodiment 2

    [0052] Bismuth(III) oxide (nanoscale 80-200 nm), in an amount of 3 wt. %, and indium isopropoxide, in an amount of 1 wt. %, are added to a spinning solution of polyacrylonitrile and solvent (DMF) and from this polymer fibers are produced by means of wet-spinning. After thermal oxidation of the fibers under normal atmospheric conditions at 280 C., the fibers are processed into curled staple fibers (62 mm fiber length). After combing/carding, the fibers are laid to form a single-layer or multi-layer web and processed into a felt (mass per unit area of from 400 to 700 g/m.sup.2) by means of needle punching on one or both sides. Subsequently, carbonization takes place in an inert atmosphere in a continuous furnace at a temperature of 1480 C.

    [0053] Material Analyses

    [0054] The specific surface area (BET) was determined by means of krypton sorption (DIN-ISO 9277). The interplanar spacing (d.sub.002) and the crystallite height (L.sub.a) were determined by X-ray diffraction from the (002) diffraction maximum (DIN EN 13925). The specific electrical resistance perpendicular to the felt plane (z) was determined by means of two-point measurement using gold contacts during compression of the felt of 80% of the initial thickness. Parameters were obtained for the materials:

    TABLE-US-00001 d.sub.002 L.sub.c BET Electrical (nm) (nm) (m.sup.2/g) resistance (z) Control sample 1 0.3466 4.7 0.41 2.4 Control sample 2 0.3517 2.4 0.58 2.9 Embodiment 1 0.3512 2.5 0.55 2.7 Embodiment 2 0.3501 2.4 0.54 2.8

    [0055] Electrochemical Testing

    [0056] In order to determine the electrode properties, the felts and the reference material in an individual vanadium redox flow battery cell having an electrode surface area of 20 cm.sup.2 were analyzed. The materials, compressed to 75% of their initial thickness, were applied to the anode and cathode, respectively. A partially fluorinated anion exchange membrane (Fumasep FAP 450, Fumatech GmbH, Bietigheim-Bissingen) was used as the separator and graphite compound plates were used as the current collector. All cell tests were carried out using 0.8 M vanadium/4M sulphate and electrolyte flow rates of 80 mL/min.

    [0057] For each test, the cells were conditioned by full charging of the electrolyte. In order to determine the electrochemical characteristics of the felts, three successive charging/discharging cycles (end-of-charging voltage 1.65 V, end-of-discharging voltage 0.9 V) were carried out in each case at current densities of from 20 to 60 mA/cm.sup.2.

    [0058] The following were determined in each case as characteristic variables for the cell tests:

    [00001] Voltage .Math. .Math. efficiency .Math. .Math. v ( % ) = average .Math. .Math. discharge .Math. .Math. voltage .Math. .Math. ( V ) average .Math. .Math. charging .Math. .Math. voltage .Math. .Math. ( V ) .Math. 100 Charging .Math. .Math. efficiency .Math. .Math. L ( % ) = discharge .Math. .Math. capacitance .Math. .Math. ( Ah ) charging .Math. .Math. capacitance .Math. .Math. ( Ah ) .Math. 100 Cycle .Math. .Math. resistance .Math. .Math. R Z ( .Math. cm 2 ) = 1.38 .Math. .Math. V current .Math. .Math. density .Math. .Math. ( A .Math. / .Math. cm 2 ) .Math. ( 100 - v 100 + v )

    [0059] The embodiments show a clearly higher voltage efficiency (FIG. 1) and a lower cell resistance (discernible from the lower decrease in voltage efficiency with rising current density).

    [0060] The cycle resistances were determined as 2.9 ohmcm.sup.2 (control sample 1), 2.3 ohmcm.sup.2 (control sample 2), 2.0 ohmcm.sup.2 (embodiment 1, dispersion 1A) and 2.1 ohmcm.sup.2 (embodiment 2).

    [0061] Moreover, the charging efficiency (FIG. 2) is higher than in the control samples above all at low current density, at which a high charge state (>99%) is achieved as a result of the end-of-charging voltage of 1.65 V. This indicates lower parasitic hydrogen evolution during use of the felts according to the invention.

    LEGEND FOR THE FIGURES

    [0062] FIG. 1

    [0063] (A): Voltage efficiency (in %) of an individual vanadium redox flow battery cell as a function of the current density (in mA/cm.sup.2) using two electrodes of the type from control sample 1

    [0064] (B): Control sample 2

    [0065] (C): Embodiment 1, dispersion 1A

    [0066] (D): Embodiment 2

    [0067] FIG. 2

    [0068] (A): Charging efficiency (in %) of an individual vanadium redox flow battery cell as a function of the current density (in mA/cm.sup.2) using two electrodes of the type from control sample 1

    [0069] (B): Control sample 2

    [0070] (C): Embodiment 1, dispersion 1A

    [0071] (D): Embodiment 2