Method of enhancing efficiency of carbon felts in flow battery through sonication

Abstract

A method is provided to enhance efficiency of carbon felts in a flow battery. The carbon felts are directly immersed in a mixed acid solution. The carbon felts with the solution are heated at a low temperature and processed through sonication. On surface defects of the carbon felts, OH and CO functional groups are efficiently generated. The functional groups catalyze the redox reaction of vanadium ions. More active positions are obtained on the carbon felts through the activation treatment. Both of valence exchange and redox velocity of the vanadium ions are enhanced. Thus, the present invention has simple and fast processes with easily regulated experimental parameters for good modification without high temperature treatment but low cost.

Claims

1. A method of enhancing efficiency of carbon felts in a flow battery through sonication, comprising steps of: (a) immersing a plurality of carbon felts in a mixed acid solution; processing with heating and sonication; and, then, washing said carbon felts with ultrapure water and drying said carbon felts; (b) obtaining said carbon felts after being processed with said sonication to be made into a positive electrode and a negative electrode; and assembling a cell stack with said positive electrode and said negative electrode; (c) filling an electrolyte into said cell stack by two pumps, said electrolyte comprising 1 mole (M) to 3M of vanadyl sulfate and an aqueous solution of 1M to 5M of inorganic acid; under a flow rate controlled between 20 and 100 milliliters per minute (mL/min), flowing said electrolyte through an anode inlet and a cathode inlet to be uniformly in contact with said positive electrode and said negative electrode which are made of said carbon felts; flowing said electrolyte through an anode outlet and a cathode outlet and said anode inlet and said cathode inlet to be collected in a positive electrolyte container and a negative electrolyte container; recycling said electrolyte to be pumped into said cell stack again through said anode inlet and said cathode inlet until air in said cell stack is completely expelled; and (d) connecting an external power supply to said positive electrode and said negative electrode; conducting a current of 12 amperes (A) under a constant-current mode with a cut-off voltage of 1.6 volts (V); processing charging and discharging operations in said cell stack; and repeating said charging and discharging operations until charge ends.

2. The method according to claim 1, wherein, in step (a), said mixed acid solution has a volume ratio of 3:1 of sulfuric acid to nitric acid.

3. The method according to claim 1, wherein, in step (a), said heating is processed to a temperature of 5585 C. and, then, said sonication is processed for 1090 min.

4. The method according to claim 1, wherein, in step (b), said cell stack comprises a proton exchange membrane; two gaskets, said two gaskets clipping said proton exchange membrane; two electrodes, said two electrodes clipping said two gaskets, said two electrodes being said positive electrode and said negative electrode, said two electrodes being made of said carbon felt; two flow plates, said two flow plate clipping said two electrodes, one of said two flow plates having an anode inlet and a cathode inlet, another one of said two flow plates having an anode outlet and a cathode outlet; and two end plates, said two end plates clipping said two flow plates.

5. The method according to claim 1, wherein, in step (c), said inorganic acid is selected from a group consisting of sulfuric acid, hydrochloric acid, nitric acid and phosphoric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood from the following detailed description of the preferred embodiment according to the present invention, taken in conjunction with the accompanying drawings, in which

(2) FIG. 1 is the flow view showing the preferred embodiment according to the present invention;

(3) FIG. 2 is the structural view showing the cell stack;

(4) FIG. 3 is the view showing the architecture using the cell stack.

(5) FIG. 4 is the SEM view showing the surfaces of the carbon felts; and

(6) FIG. 5 is the view of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) The following description of the preferred embodiment is provided to understand the features and the structures of the present invention.

(8) Please refer to FIG. 1FIG. 4, which are a flow view showing a preferred embodiment according to the present invention; a structural view showing a cell stack; a view showing an architecture using the cell stack; and a SEM view showing surfaces of the carbon felts. As shown in the figures, the present invention is a method of enhancing efficiency of carbon felts in a flow battery through sonication, comprising the following steps:

(9) (a) Fabricating carbon felts 11: A plurality of carbon felts (5 centimeters (cm)5 cm) are immersed into a mixed acid solution of sulfuric acid and nitric acid. The solution is heated to 5585 celsius degrees ( C.) and sonication is processed for 1090 minutes (min). Then, the carbon felts are washed with ultrapure water and dried. Therein, the mixed acid solution has a volume ratio of 3:1 of sulfuric acid to nitric acid.

(10) (b) Assembling cell stack 12: The carbon felts obtained after being processed through the sonication is made into a plurality of electrodes; and a cell stack is assembled with the electrodes. As shown in FIG. 2, the cell stack 2 comprises a proton exchange membrane 21, two gaskets 22,23, two electrodes 24,25, two flow plates 26,27 and two end plates 28,29. Therein, the two gaskets 22,23 clips the proton exchange membrane 21; the two electrodes 24,25 clips the two gaskets 22,23; the two electrodes 24,25 are a positive electrode 24 and a negative electrode 25; the two electrodes 24,25 are made of the carbon felts; the two flow plate 26,27 clips the two electrodes 24,25; one of the two flow plates 26 has an anode inlet 261 and a cathode inlet 262; another one of the two flow plates 27 have an anode outlet 271 and a cathode outlet 272; and, the two end plates 28,29 clips the two flow plates.

(11) (c) Filling electrolyte 13: An electrolyte is filled into the cell stack by two pumps 31,32, where the electrolyte comprise 1 mole (M) to 3M of vanadyl sulfate and an aqueous solution of 1M to 5M of inorganic acid. Under a flow rate controlled between 20 and 100 milliliters per minute (mL/min), the electrolyte is flown through the anode inlet 261 and the cathode inlet 262 to be uniformly in contact with the carbon felts (i.e. the positive electrode 24 and the negative electrode 25). The electrolyte flows through an anode outlet 271 and a cathode outlet 272 along with the anode inlet 261 and the cathode inlet 262 to be collected in a positive electrolyte container 33 and a negative electrolyte container 34. The electrolyte is recycled to be pumped into the cell stack 2 again through the anode inlet 261 and the cathode inlet 262 until air in the cell stack 2 is completely expelled. Therein, the inorganic acid is sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid

(12) (d) Charging cell stack 14: An external power supply 35 is connected to the positive electrode 24 and the negative electrode 25. A current of 12 amperes (A) is conducted under a constant-current mode with a cut-off voltage of 1.6 volts (V). Charging and discharging operations are processed in the cell stack 2 of a flow battery 3 and the charging and discharging operations are repeated until charge ends.

(13) The present invention modifies carbon felts through sonication to efficiently generate OH and CO functional groups on surface defects of the carbon felts. The OH and CO functional groups catalyze redox reaction of vanadium ions. It means that, through an activation treatment, the carbon felts obtains more active positions to enhance valence exchange of vanadium ions and increase efficiency of the redox reaction. The general method of acid treatment needs to set up a reflux condenser with reaction time up to several hours or more. In contrast, the present invention is simple and uses sonication with no need for a high pressure environment. A plurality of 5 cm5 cm carbon felts are immersed into a mixed acid solution having a volume ratio of 3:1 of sulfuric acid to nitric acid. Under a temperature of 70 C., the sonication is processed for 30, 60 and 90 min, respectively, followed by washing with ultrapure water and drying to be used for subsequent detection and cell testing.

(14) A scanning electron microscope (SEM) is used to detect surfaces of the carbon felts. As shown in FIG. 4, the picture of notation 41 shows an unmodified carbon felt; and the other pictures of notation 42, 43 and 44 show modified carbon felts obtained through 30, 60 and 90 minutes of sonication, respectively. From the results, it is known that the surface of the unmodified carbon felt is smooth and the surfaces of the modified carbon felts become rougher and more uneven as the reaction time increases. Defects are generated on the surfaces of the modified carbon felts.

(15) The above four carbon felts are examined. Nafion 117 is obtained as a proton exchange membrane. An industrial-grade vanadyl sulfate is obtained as an electrolyte, whose concentration is 1.6M. Under a flow rate of 50 mL/min, a current of 2 A is conducted in a constant-current mode with a cutoff voltage of 1.6V for charging and discharging a flow battery with the electrolyte. The charging and discharging operations keep repeating until charge ends. Table 1 shows a result obtained after 50 rounds of charging and discharging operations. It shows that the carbon felt modified through sonication for 60 min has the best voltage efficiency and energy efficiency, which obtains about 3% improvement as compared to the unmodified carbon felt. The results show that sonication with acid treatment can effectively modify carbon felts and significantly shorten reaction time, and thus enhance the overall energy efficiency of single cell.

(16) TABLE-US-00001 TABLE 1 Coulomb Energy Voltage Time efficiency efficiency efficiency unmodified 94.89% 76.30% 80.41% carbon felt Sonication 30 min 95.80% 78.08% 81.51% Sonication 60 min 95.96% 80.14% 83.51% Sonication 90 min 96.40% 77.38% 80.28%

(17) Thus, the present invention directly immerses carbon felts in a mixed acid solution to be heated at a low temperature and processed through sonication for efficiently generating OH and CO functional groups on surface defects of the carbon felts. The OH and CO functional groups catalyze redox reaction of vanadium ions for obtaining more active positions on the carbon felts to enhance valence exchange of vanadium ions and increase efficiency of the redox reaction. Thereby, the present invention has simple and fast processes with easily regulated experimental parameters for good modification without high temperature treatment but with low cost.

(18) To sum up, the present invention is a method of enhancing efficiency of carbon felts in a flow battery through sonication, where carbon felts are modified through sonication to generate OH and CO functional groups on surface defects of the carbon felts; and the present invention has simple and fast processes with easily regulated experimental parameters for good modification without high temperature treatment but with low cost.

(19) The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.