WO2014109957A1

WO2014109957A1 - Improved bipolar plate for flow batteries

Info

Publication number: WO2014109957A1
Application number: PCT/US2014/010165
Authority: WO
Inventors: Haiming XIAO
Original assignee: Graftech International Holdings Inc.
Priority date: 2013-01-11
Filing date: 2014-01-03
Publication date: 2014-07-17

Abstract

A flow battery includes a proton exchange membrane and a pair of porous electrodes positioned on each side thereof. A bipolar plate is sized and positioned to engage the porous electrode. At least one of the surfaces of the bipolar plate has an arithmetic mean roughness of at least 100 μ-in.

Description

IMPROVED BIPOLAR PLATE FOR FLOW BATTERIES

Background of the Invention

[0001] A flow battery is a type of fuel cell in which an electrolyte flows through an electrochemical cell. The cell reversibly converts chemical energy to electricity. The electrolyte is generally a liquid which is stored in tanks, and is pumped through the cells of the battery. Recharging can be performed by simply replacing the electrolyte fluid if no power source is available for charging.

[0002] The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy. The vanadium redox battery operates based on the capability of vanadium to exist in solution in four different oxidation states. One advantage of the vanadium redox battery design is that it can offer very large capacity through the use of large storage tanks that can be in a discharged state for long periods without the common ill effects found in other battery technologies.

[0003] A vanadium redox battery commonly includes an assembly of power cells wherein two electrolytes are separated by a proton exchange membrane. Both electrolytes are vanadium based, the electrolyte in the positive cells contain V(¾⁺ and V0²⁺ ions and the electrolyte in the negative cells contain V³⁺ and V²⁺ ions. The electrolytes may be prepared by any number of processes. For example, one approach includes electrolytically dissolving vanadium pentoxide (V₂O₅) in sulfuric acid (H₂SO₄). The resulting solution is typically strongly acidic.

[0004] In vanadium flow batteries, each half-cell is connected to a storage tank and pump so that large volumes of the electrolytes can be circulated through the cell. When a vanadium battery is charged, the VO₂+ ions in the positive cell are converted to V0²+ ions as electrons are removed from the positive terminal of the battery. Likewise, in the negative cell, electrons are introduced converting the V³⁺ ions into V²⁺.

Summary of the Invention

[0005] According to one aspect, a bipolar plate for a flow battery is disclosed. The flow battery has a porous electrode positioned on each side of a proton exchange membrane. The bipolar plate includes a resin impregnated compressed exfoliated natural graphite plate including two opposed major surfaces. At least one of the opposed major surfaces is sized to engage at least one of the porous electrodes. At least one of the opposed major surfaces of the bipolar plate has an arithmetic mean roughness of at least 100 μ-ίη.

[0006] According to another aspect, a flow battery includes a proton exchange membrane having two opposed major surfaces. A porous electrode has two opposed major surfaces and each porous electrode is positioned to contact one of the major surfaces of the proton exchange membrane. A resin impregnated compressed exfoliated natural graphite plate includes two opposed major surfaces. At least one of the opposed major surfaces of the resin impregnated compressed exfoliated natural graphite plates is sized to engage the porous electrode. At least one of the opposed major surfaces of the resin impregnated compressed exfoliated natural graphite plate has an arithmetic mean roughness of at least 100 μ-ίη.

Brief Description of the Drawings

[0007] Figure 1 is a partially schematic view of a flow battery in accordance with the present invention.

[0008] Figure 2 is a partially schematic view of a multi-cell flow battery in accordance with the present invention.

[0009] Figure 3 is an exemplary graph showing a comparison of electrical resistance of surface treated vs. untreated bi-polar plates.

Detailed Description

[0010] A single-cell vanadium redox battery is shown and described in Figure 1 and generally indicated by the numeral 10. Each cell 12 includes a proton exchange membrane 14 positioned between opposing electrodes 16. On the opposed side of each electrode 16 from the membrane 14 is a plate 18 (also commonly referred to as a bipolar plate). A first tank 20 includes a first electrolyte fluid and a second tank 22 includes a second electrolyte fluid. First tank 20 is in fluid connection with the first electrode 16a and second tank 22 is in fluid connection with the second electrode 16b. A pump 24 selectively draws the electrolyte fluid from the tanks 20/22 and through electrodes 16 to create an operating current.

Likewise, a current may be applied to the cell 12 while pumping electrolyte fluid there through to "charge" the battery.

[0011] The membrane 14 is a proton exchange membrane. Membrane 14 separates the two electrolytes (creating a positive and negative side) but allows the transfer of H+ to keep the cell conductive. Membrane 14 may advantageously be made from, for example, Nafion, though any other suitable membrane material may be employed.

[0012] The electrode 16 is a generally porous material that allows the electrolyte fluid to flow through while enabling the electrochemical reaction that creates the working charge from the battery. In one particularly preferred embodiment, electrode 16 is a porous carbon material. In particular, a carbon fiber matrix or felt has been shown to be particularly suitable to this application given the chemical resistance and relatively high electrical conductivity. Fibers may advantageously be carbonized rayon- based fibers. In other embodiments, the fibers may be carbonized PAN-based fibers. In still other embodiments, the fibers may be derived from carbonized pitch fibers or carbonized fibers from other petroleum based products. In still further embodiments, the carbonized fibers may be derived from plant derived cellulose fibrils such as lignen.

[0013] Plates 18 are advantageously formed of a graphite sheet material. A common method for manufacturing graphite sheet is described by Shane et al. in U.S. Patent

3,404,061, the disclosure of which is incorporated herein by reference. In one method, natural graphite flakes are intercalated by dispersing the flakes in an intercalation solution. The intercalation solution contains oxidizing and other intercalating agents known in the art such as nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid.

[0014] The thus treated particles of graphite are sometimes referred to as "particles of intercalated graphite". Upon exposure to high temperature, e.g. about 700°C to 1000°C and higher, the particles of intercalated graphite expand as much as about 80 to 1000 or more times its original volume in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the constituent graphite particles. The expanded, i.e. exfoliated graphite particles are vermiform in appearance, and are therefore commonly referred to as worms. The worms may be compressed together into flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes.

[0015] The graphite sheet is coherent, with good handling strength, and are suitably compressed, e.g. by roll-pressing, to a thickness of about 0.075 mm to 3.75 mm, more advantageously from between about 0.2 to about 1.5 mm, and still more advantageously from between about 0.4 mm and about 1.0 mm. The graphite sheet for use in the bipolar plate advantageously has a density of about 1.0 to 2.0 grams per cubic centimeter, more advantageously from between about 1.5 to 2.0 grams per cubic centimeter. In still further embodiments, the graphite sheet advantageously has a density greater than about 1.5 grams per cubic centimeter and even more advantageously greater than about 1.8 grams per cubic centimeter.

[0016] The graphite sheet is advantageously treated with resin and the absorbed resin, after curing, enhances the moisture resistance and handling strength, i.e. stiffness of the flexible graphite sheet. Suitable resin systems may be, for example, epoxy based or polyimide based. Suitable resin content is preferably from between about 5 to 50% by weight, and more preferably between about 10 and about 40% by weight. In this or other embodiments, the resin content may be up to about 60% by weight. The resin density may be from between 1.0 g/cc to 1.5g/cc depending upon the resin. The density of the graphite sheet after resin impregnation may be from between about 1.5 g/cc to about 2 g/cc. In still other embodiment the graphite sheet after resin impregnation may be from between about 1.6 g/cc to about 1.8 g/cc.

[0017] With reference now to Fig. 2, a multi-cell vanadium redox battery is shown and generally indicated by the numeral 100, and where like numerals indicate like elements. The multi-cell battery functions substantially the same as the single cell variant, except that multiple cells 12a, 12b, and 12c are positioned in a stacked arrangement. In this manner, the current generated through operation of the battery 100 is produced in series by cells 12, which allows higher power applications.

[0018] When in use, as the electrolyte fluid flows through electrodes 16, an electrical potential is created across the electrodes 16. The electrodes 16 are operatively electrically connected to an operating circuit. In practice, the electrical current generated in electrodes 16 is passed to the adjacent plates 18 at an electrode interface 26. Advantageously, the opposed major surfaces of the electrode 16 and of the plate 18 are substantially the same size and profile. In other embodiments, advantageously at least 70 percent, more advantageously 80 percent, and still more advantageous at least 95 percent of the surface area of the plates 18 engages the surface of the electrode 16. The plate 18 may in turn be electrically connected to the operating circuit directly, through current collectors at the end of a stack of cells (not shown), or to another electrode in the case of stacked multi-cell batteries.

[0019] Performance is advantageously improved if reduced electrical contact resistance at the electrode interface 26 is achieved. Indeed, reduced contact resistance leads to lower voltage loss across the interface 26, which in turn leads to higher battery power output. Advantageously, the plate 18 is generally sheet or plate shaped, and thus includes opposed major surfaces. Advantageously, at least one major surface of plate 18 receives a surface treatment which reduces electrical resistance at the interface. The thus treated surface being adjacent to and in contact with an electrode 16 to form the electrode interface 26. In other embodiments, both major surfaces of the plate 18 include a surface treatment.

[0020] According to one aspect of the present invention the surface treated plate 18 is made according to the following method. A resin impregnated compressed exfoliated natural graphite sheet is made in accordance with the description above. Thereafter, a surface treatment is performed on the surface of the plate 18 that contacts the electrode 16. The surface treatment preferably increases contact surface area and consequentially reduces interfacial surface energy at interface 26. Surface treatment advantageously includes a roughening operation. Examples of roughening operations may include one or more of, low power sand-blasting, abrasive paper treatment, falling sand brush, and chemical etching treatments. [0021] For purposes of this disclosure, surface roughness measurements (mean max height and arithmetic mean roughness) are performed using a Mahr Meter having a traversing detector distance of 0.224 in. In one embodiment, the surface treatment results a greater surface roughness of the resin impregnated graphite sheet. Resin impregnated, compressed exfoliated natural graphite plates 18, prior to surface treatment, may exhibit an arithmetic mean roughness of from between about 5 μ-in to about 50 μ-in, more preferably from between about 15 μ-in to about 40 μ-in, and still more preferably from between about 20 μ- in to about 35 μ-in. Resin impregnated, compressed exfoliated natural graphite plates, prior to surface treatment may exhibit an arithmetic mean roughness of less than 100 μ-in, more preferably less than about 50 μ-in, still more preferably less than about 30 μ-ίη. [0022] Still further, resin impregnated, compressed exfoliated natural graphite plates, prior to surface treatment, exhibit a mean max height of profile from between about 100 μ-ίη to about 200 μ-in, more preferably from between about 120 μ-in to about 180 μ-in, and still more preferably from between about 130 μ-in to about 170 μ-in. Resin impregnated, compressed exfoliated natural graphite plates, prior to surface treatment may exhibit a mean maximum height of profile less than 200 μ-in, more preferably less than about 180 μ-in, still more preferably less than about 170 μ-ίη. [0023] Resin impregnated, compressed exfoliated natural graphite plates, after surface treatment, may exhibit an arithmetic mean roughness of from between about 100 μ-in to about 500 μ-in, more preferably from between about 200 μ-in to about 400 μ-in, and still more preferably from between about 250 μ-in to about 350 μ-in. Resin impregnated, compressed exfoliated natural graphite plates, after surface treatment may exhibit an arithmetic mean roughness of greater than 100 μ-in, more preferably greater than about 200 μ-in, still more preferably greater than about 300 μ-ίη.

[0024] Still further, resin impregnated, compressed exfoliated natural graphite plates, after surface treatment, exhibit an mean max height of profile from between about 1000 μ-ίη to about 2500 μ-in, more preferably from between about 1200 μ-in to about 2000 μ-in, and still more preferably from between about 1300 μ-in to about 1700 μ-in. Resin impregnated, compressed exfoliated natural graphite plates, after surface treatment may exhibit a mean maximum height of profile greater than 1000 μ-in, more preferably greater than about 1250 μ-in, still more preferably greater than about 1500 μ-ίη.

[0025] Advantageously, the ratio of arithmetic mean roughness after vs prior to surface treatment (roughness after treatment / roughness before treatment) is preferably at least 3 or more, more preferably at least 5 and still more preferably at least 10. Likewise, the ratio of mean maximum height of profile after vs prior to surface treatment (mean max height after treatment / mean max height before treatment) is preferably at least 3 or more, more preferably at least 5 and still more preferably at least 10. Example

[0026] A test setup was prepared to simulate the series electrical resistance of an eight (8) cell flow battery. Alternating electrodes and bipolar plates were arranged in an alternating stacked arrangement. For purposes of the experiment, the proton exchange membranes were not included in the stack. Electrodes were positioned on each end of the stack so that the stack consisted of nine (9) electrodes and eight (8) bipolar plates. The bipolar plates were 0.6 mm thick and the electrodes were carbon felt approximately 5.5 mm thick. The electrical resistance across the stack was measured at various levels of compression. In a first test setup, bipolar plates without a surface treatment were used. In a second test setup, the bipolar plates each received a surface treatment in the form of a light sandblast on both major surfaces. The non-surface treated bipolar plates had an arithmetic mean roughness of approximately 28 μ-in and a mean maximum profile height of approximately 155 μ-in. The surface treated bipolar plates had an arithmetic mean roughness of approximately 315 μ-in and a mean maximum profile height of approximately 1575 μ-ίη.

[0027] As shown in a dry stack of plate/felt correlating to an 8 -cell stack, the total electrical resistance is reduced by about 40 -50% using surface enhanced bipolar plates. The electrical resistance of the stack that included surface treated bipolar plates was from between about 40 - 50% less than the stack that included non-surface treated bipolar plates. One can further see that the electrical resistance is also reduced with increasing felt compression. It can also be seen that resistance is reduced as compression increases. However, even at higher compressions, improved resistance characteristics are seen in the stack with bipolar plates receiving a surface treatment. Further, there is a limit to the amount of compression a stack may be put under as it restricts the ability of the carbon felt electrodes to allow the electrolyte to flow through. Higher flow resistance of the felt is detrimental to the flow battery operation. Thus, through surface modification, increased pressures (and thus increased flow resistance) are not necessary to achieve optimal electrical resistance. [0028] It should be appreciated that, though the above disclosure is principally directed towards vanadium redox flow batteries, the concepts and techniques disclosed herein are applicable to other flow batteries that include an interface between a carbon matrix or carbon felt electrode and bipolar plates.

[0029] The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence that is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.

Claims

What is Claimed is:

1. A bipolar plate for a flow battery having a porous electrode positioned on opposed sides of a proton exchange membrane, the bipolar plate comprising: a resin impregnated compressed exfoliated natural graphite plate including two opposed major surfaces, at least one of said opposed major surface sized to engage at least one of the porous electrodes, said at least one of said opposed major surfaces has an arithmetic mean roughness of at least about 100 μ-ίη.

2. The bipolar plate according to claim 1 wherein said at least one major surface has an arithmetic mean roughness of at least about 300 μ-ίη.

3. The bipolar plate according to claim 1 wherein said at least one major surface has an arithmetic mean roughness of from between about 100 μ-in and about 500 μ-ίη.

4. The bipolar plate according to claim 1 wherein said at least one of said opposed major surfaces has a mean max height of profile from between about 1000 μ-in to about 2500 μ-ίη.

5. The bipolar plate according to claim 1 wherein said resin impregnated compressed exfoliated natural graphite plate has a density of from between about 1.5 g/cc to about 2 g/cc.

6. The bipolar plate according to claim 1 wherein said resin is an epoxy or a polyimide.

7. A flow battery comprising: a proton exchange membrane having two opposed major surfaces; a porous electrode having two opposed major surfaces, each said porous electrode positioned to contact one of said major surfaces of said proton exchange membrane; a resin impregnated compressed exfoliated natural graphite plate including two opposed major surfaces, at least one of said opposed major surfaces sized to engage said porous electrode, said at least one of said opposed major surfaces of said resin impregnated compressed exfoliated natural graphite plate has an arithmetic mean roughness of at least about 100 μ-ίη.

8. The flow battery according to claim 7 wherein said at least one major surface of said resin impregnated compressed exfoliated natural graphite plate has an arithmetic mean roughness of at least about 300 μ-ίη.

9. The flow battery according to claim 7 wherein said at least one major surface of said resin impregnated compressed exfoliated natural graphite plate has an arithmetic mean roughness of from between about 100 μ-in and about 500 μ-ίη.

10. The flow battery according to claim 7 wherein said at least one major surface of said resin impregnated compressed exfoliated natural graphite plate has a mean max height of profile from between about 1000 μ-in to about 2500 μ-ίη.

11. The flow battery according to claim 7 wherein said at least one major surface of said resin impregnated compressed exfoliated natural graphite plate has a density of from between about 1.5 g/cc to about 2 g/cc.

12. The flow battery according to claim 7 wherein said resin is an epoxy or a polyimide.

13. The flow battery according to claim 7 wherein said porous electrode is a carbon felt.

14. The flow battery according to claim 13 wherein said carbon felt is derived from carbonizing or graphitizing one or more of PAN, rayon, lignin, pitch fibers or petroleum based fibers.