Electrolysis: processes – compositions used therein – and methods – Electroforming or composition therefor
Reexamination Certificate
2000-03-20
2003-01-07
Nguyen, Nam (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electroforming or composition therefor
C204S483000, C204S489000, C204S490000, C205S075000, C205S102000, C205S112000, C205S220000
Reexamination Certificate
active
06503382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to porous films, in particular porous films having a substantially regular structure and uniform pore size, and to a method of preparing porous films by electrodeposition.
BACKGROUND OF THE INVENTION
Porous films and membranes have found extensive applications as electrodes and solid electrolytes in electrochemical devices and sensors. Their open and interconnected microstructure maximises the area over which interaction and/or redox processes can occur, allows electrical conduction, and minimises distances over which mass transport has to occur in order to ensure efficient device operation.
Conventional processes for preparing porous films include the sintering of small particles, deposition from vapour phase reactants, chemical etching and electrodeposition from multicomponent plating solutions. These processes tend to produce materials with a variable pore size, generally in the macroporous range, and with variable thickness of the walls separating the pores. Consequently, these materials may not have sufficiently large specific surface areas, and their irregular structure does not allow for optimum mass transport or electrical conductivity, and may result in poor mechanical and chemical stability.
In the drive towards providing porous films showing improved properties, for use in for example batteries, fuel cells, electrochemical capacitors, light-to-electricity conversion, quantum confinement effect devices, sensors, magnetic devices, superconductors, electrosynthesis and electrocatalysis, to our knowledge no one has yet succeeded in developing an effective process for preparing at least mesoporous films of regular structure and uniform pore size, with the attendant advantages in terms of properties which such films might be expected to show.
For example, previous reported attempts to form polypyrrole films by electrodeposition, from thermotropic liquid crystalline phases, resulted in films of only weakly anisotropic structure.
Previously, we have shown that porous, non-film, materials such as ceramic oxide monoliths and metal powders can be crystallised, gelled or precipitated from lyotropic liquid crystalline phase media, whereby the liquid crystalline phase topology directs the synthesis of the material into a corresponding topology showing structural regularity and uniformity of pore size. However, it was not expected that this templating mechanism could be used to synthesise porous materials other than by simple crystallisation, gelation or precipitation.
What we have found, surprisingly, is that porous films can be prepared from an homogeneous lyotropic liquid crystalline phase by electrodeposition. Surfactants have previously been used as additives in electroplating mixtures in order to enhance the smoothness of electrodeposited films or to prevent hydrogen sheathing (see for example J. Yahalom, O. Zadok, J. Materials Science (1987), vol 22, 499-503). However, in all cases the surfactant was used at concentrations that are much lower than those required to form liquid crystalline phases. Indeed, in these applications high surfactant concentrations were hitherto regarded as undesirable because of the increased viscosities of the plating mixtures.
BRIEF SUMMARY OF THE INVENTION
The present invention in a first aspect provides a method of preparing a porous film which comprises electrodepositing material from a mixture onto a substrate to form a porous film, wherein the mixture comprises:
a source of metal, inorganic oxide, non-oxide semiconductor/conductor or organic polymer, or a combination thereof;
a solvent; and
a structure-directing agent in an amount sufficient to form an homogeneous lyotropic liquid crystalline phase in the mixture, and optionally removing the organic directing agent.
In a second aspect, the invention provides a porous film electrodeposited onto a substrate, wherein the film has a regular structure such that recognisable architecture or topological order is present in the spatial arrangement of the pores in the film, and a uniform pore size such that at least 75% of the pores have pore diameters to within 40% of the average pore diameter.
DETAILED DESCRIPTION OF THE INVENTION
According to the method of the invention, an homogeneous lyotropic liquid crystalline mixture is formed for electrodeposition onto a substrate. The deposition mixture comprises a source material for the film, dissolved in a solvent, and a sufficient amount of an organic structure-directing agent to provide an homogeneous lyotropic liquid crystalline phase for the mixture. A buffer may be included in the mixture to control the pH.
Any suitable source material capable of depositing the desired species onto the substrate by electrodeposition may be used. By “species” in this context is meant metal, inorganic oxide, including metal oxide, non-oxide semiconductor/conductor or organic polymer. Suitable source materials will be apparent to the person skilled in the art by reference to conventional electroplating or electrodeposition mixtures.
One or more source materials may be included in the mixture in order to deposit one or more species. Different species may be deposited simultaneously from the same mixture. Alternatively, different species may be deposited sequentially into layers from the same mixture, by varying the potential such that one or another species is preferentially deposited according to the potential selected.
Similarly, one or more source materials may be used in the mixture in order to deposit one or more materials selected from a particular species or combination of species, either simultaneously or sequentially. Thus, by appropriate selection of source material and electrodeposition regime, the composition of the deposited film can be controlled as desired.
Suitable metals include for example Group IIB, IIIA-VIA metals, in particular zinc, cadmium, aluminium, gallium, indium, thallium, tin, lead, antimony and bismuth, preferably indium, tin and lead; first, second and third row transition metals, in particular platinum, palladium, gold, rhodium, ruthenium, silver, nickel, cobalt, copper, iron, chromium and manganese, preferably platinum, palladium, gold, nickel, cobalt, copper and chromium, and most preferably platinum, palladium, nickel and cobalt; and lanthanide or actinide metals, for example praseodymium, samarium, gadolinium and uranium.
The metals may contain surface layers of, for example, oxides, sulphides or phosphides.
The metals may be deposited from their salts as single metals or as alloys.
Thus, the film may have a uniform alloy composition, for example Ni/Co, Ag/Cd, Sn/Cu, Sn/Ni, Pb/Mn, Ni/Fe or Sn/Li, or if deposited sequentially, a layered alloy structure, for example Co/Cu|Cu/Co, Fe/Co|Co/Fe or Fe/Cr|Cr/Fe, wherein “Co/Cu|Cu/Co” denotes a film containing alternate layers of cobalt-rich alloy and copper-rich alloy. Sequential electrodeposition of species can be achieved according to the method disclosed by Schwarzacher et al., Journal of Magnetism and Magnetic Materials (1997) vol 165, p23-39. For example, an hexagonal phase is prepared from an aqueous solution containing two metal salts A and B, where metal A is more noble than metal B (for example nickel (II) sulphate and copper (II) sulphate) and optionally a buffer (for example boric acid). The deposition potential is alternated from a value only sufficiently negative to reduce A, to one that is sufficiently negative to reduce both A and B. This gives and produces an alternating layered structure consisting of layers A alternating with layers A+B.
Suitable oxides include oxides of for example first, second and third row transition metals, lanthanides, actinides, Group IIB metals, Group IIIA-VIA elements, preferably oxides of titanium, vanadium, tungsten, manganese, nickel, lead and tin, in particular titanium dioxide, vanadium dioxide, vanadium pentoxide, manganese dioxide, lead dioxide and tin oxide.
In some cases, the oxides may contain a proportion of the hydrated oxide i.e. contain hydroxyl groups.
The
Attard George Simon
Bartlett Philip Nigel
Elliott Joanne
Owen John Robert
Leader William T.
Nguyen Nam
Price Heneveld Cooper DeWitt & Litton
University of Southampton
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