Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2002-03-11
2003-12-02
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S233000, C429S210000, C429S209000, C029S623300, C029S623500, C029S623100
Reexamination Certificate
active
06656639
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrodes, particularly bipolar electrodes, particularly bipolar composite electrodes characterised by an electrochemically active layer bonded to a substrate material. The electrodes can be used in electrochemical/redox cells and other electrochemical applications.
BACKGROUND OF THE INVENTION
Electrodes are commercially applied in a wide variety of applications including being used in redox flow cells, in battery stacks and other electrochemical applications such as electrodepositing and electrowinning.
End electrodes are a type of electrode used in batteries. Typically, the end electrode in a battery stack acts as either an anode or as a cathode and has a current collector on one side. End electrodes, by their nature are connected to a power supply. One form of end electrode is conventionally characterised by an electrode substrate material, onto one side of which is bonded a thin layer of electrochemically active material such as graphite felt or carbon felt, with the other side having bonded to it a layer of electrically conductive material which acts as a current collector.
Bipolar electrodes are a form of electrode which have no direct electrical busbar connection to a power supply and act as an anode on one side and a cathode on the other side. There exist a multitude of different forms of bipolar electrodes, several of which are disclosed for example in U.S. Pat. Nos. 4,214,969, 4,339,332 and European patent application publication No. 0 268 397 A1. One particular form of bipolar electrode which is widely used in redox batteries, is conventionally characterised by an electrode substrate material, onto each side of which is bonded a thin layer of electrochemically active material such as graphite felt or carbon felt. It is an important function of the substrate material to provide a conductive pathway between the facing layers of electrochemically active material.
To date, a wide variety of materials have been used as the conducting electrode substrate material, to which the graphite or carbon felt is then bonded. Traditionally, carbon or graphite plates were used, but due to the expense of such plates coupled with their weight and their size which rendered them too bulky for large battery stack construction, these have long since been replaced. Conductive polymers or plastics have appeared in the last decade as ideal electrode materials, offering a low cost, low weight alternative to the traditional carbon or graphite. They are easy to shape, have good electrical conductivity, do not corrode, have low density and a high strength/weight ratio and are readily available. Given that most conventional polymers eg polyethylene, are not conductive (particularly when compared with the traditional metal conductors) due to the presence of saturated covalent bonds along the molecular backbone, then in order to produce conduction in polymers, it has been necessary to produce unsaturation at regular intervals along the polymer chain (such as seen in graphite). Therefore, by introducing conjugation structures (ie unsaturation) at regular intervals in conventional polymers, conducting polymers have been obtained.
However, due to the fact that conductive polymers can degrade due to thermal or electrical field environments and can be easily attacked by acidic and oxidative electrolytes, it has become apparent that in some applications, the conductive polymers are therefore unsuitable for use as bipolar or end electrodes. Accordingly, in respect of such applications, research has focussed on modified polymers in the form of polymer composites which have been successfully used as the conducting electrode substrate material in bipolar and end electrodes.
Polymer composites or conducting plastic composites (as they are also known) generally comprise polymer mixed with one or more forms of carbon and/or graphite and then pressure moulded to form the electrode. Alternatively, silver or aluminium particles can be added to ordinary polymers. Detailed research has been carried out on a variety of carbon/graphite composite plastics in order to determine the size and type of carbon particles, the form of graphite powder or fibres as well as the influence of different polymers on the conductivity behaviour of the conducting composites. Conventional bipolar electrodes which use carbon black polymer composites have relatively high (20-30%) conductive filler loadings in order to achieve acceptable conductivity through the composite material. Examples of composite plastic electrode materials which have been used as bipolar and end electrodes in fuel cell and redox battery cell applications include cross-linked polymer composite electrodes containing carbon black, a carbon fibre-carbon black thermosetting resin composite bipolar electrode; carbon-fibre-mat/resin electrode materials, carbon plastic electrodes based on polyvinylidene difluoride, Teflon® and graphite, and conducting graphite/polyethylene composite electrodes. HDPE, carbon black, graphite powder and graphite fibre can also be used to prepare a conducting plastic composite electrode.
WO 94/06164 discloses a conducting plastic composite electrode comprising a thermoplastic polymer, an elastomeric polymer and a conductive filler. In one embodiment, the electrode material comprises high density polyethylene (HDPE) cross linked with styrene-ethene-butene-styrene rubber (SEBS) block copolymer, carbon black and graphite fibre. In another embodiment, the electrode material comprises high density polyethylene (HDPE) cross linked with SEBS block copolymer, and graphite fibre.
In the manufacturing of such conducting plastic composite electrode materials, the polymer/polymer blend is mixed with the chosen form of carbon/graphite at moderately high temperature (>100° C.) and then heat and pressure moulded to form a thin, smooth conducting plastic composite sheet with good electrical conductivity and therefore low resistivity. By heat pressing a copper mesh on one side and heat-bonding a graphite felt on the other end, an end-electrode for a redox battery is made. By heat bonding a graphite/carbon felt on both sides of such a sheet, a bipolar electrode for a redox battery is completed. In the preparation of such a bipolar electrode, a window (having a border typically of about 1-8 cm around the electrode edges) is placed on either side of the conducting plastic composite sheet, and then an electrochemically active layer, typically a graphite felt layer is placed into each of the windows. This sandwich is then placed into a mould. Typically, pressure is applied to the mould for 10-15 minutes at high temperature and then cooled to obtain a conducting carbon composite plastic and graphite felt bipolar electrode.
Recently [C. M. Hagg, J. O. Besenhard and M. Skyllas-Kazacos, “Novel Carbon Polymer Electrodes for Battery Applications” in 1999 Fourteenth Annual Battery Conference on Applications and Advances (January 1998, Proceedings in press, IEEE Catalog Number: 99TH8371)], a new method for preparing low carbon black loaded polymer composite has been described as an alternative to the conventional mixing and compression moulding process described above. In the new method, fine polymer powder is first coated with a carbon black layer and subsequently heat pressed into conductive sheets. The coating of the polymer particles with the carbon black is performed using substrate induced coagulation (SIC) which results in enhanced mechanical properties and electrical conductivity of the electrodes made from such material. The composite material in the shape of flat sheets with a thickness of 1 mm was then subsequently used for the preparation of bipolar electrodes by heat bonding graphite felt to both sides of the sheets in the centre. The composite material in the shape of flat sheets with a thickness of 1 mm could also be used for the preparation of end electrodes.
It is well known that the conductivity of the composite plastics is affected by the type of polymer as well as by the type and particle si
Hagg Christoph M.
Skyllas-Kazacos Maria
Martin Angela J.
Ryan Patrick
Townsend and Townsend and Crew
Unisearch Limited
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