PROCESS FOR THE PREPARATION OF POROUS METAL

Abstract: In a process of producing a porous layer of electrolytically depositable metal a support material (1) is immersed in an electrolytic medium (2) comprising ions of the metal and the metal is electrolytically deposited onto the support material (1) in the presence of an oxidising species which reacts with the metal to form a product which is reducible under the deposition conditions. In one form of process, a metal is deposited onto a support material (1) from an aqueous medium comprising ions of the metal, oxygen being introduced into the medium to pass in the vicinity of the support material. The process enables highly porous metal to be manufactured. The porous metal obtained is useful as catalytic material or as electrode material.

Description: Process for the preparation of porous metal The present invention relates to a process for producing porous metal and to articles comprising porous metal. The invention also relates to a method for forming metal oxides. Porous metal is used in a wide variety of applications including, for example, in battery plates, catalytic supports, catalytic electrodes, filters and catalytic converters. In such applications the higher surface area of porous metal in comparison to non-porous metal provides increased efficiency. Porous metal has previously been manufactured by powder metallurgical techniques. Fine metal powder is compacted and sintered at elevated temperatures in a reducing atmosphere. A pore-forming agent, for example, ammonium carbonate, is generally added to the powder before compacting in order to increase the number of pores present after sintering. During sintering, however, grain growth occurs and the mean grain size of porous metal formed by sintering is typically greater than 5u, the pore size being of a similar order. Porous electrodes have been made by pasting " metallic oxides or hydroxides onto a metallic screen, followed by electrochemical reduction _in situ. The grain size of the porous metal so formed may be in the region of from 3u upwards. It is frequently necessary, however, to incorporate inert conducting powders, for example, graphite into the electrode in order to obtain satisfactory electrical conductivity. When porous metal products made by the above methods are used in batteries the actual energy density obtainable is significantly lower than the theoretical value. Electrodeposition is a well-known method of producing metallic layers of low or negligible porosity. The metal atoms are deposited to form dense. ordered layers of atoms. In electrodeposition applications, for example electroplating, it will be appreciated that it is highly desirable to minimise any interference with the ordered formation of layers of atoms and thus to minimise the porosity of the metallic layer produced. The present invention provides a process of producing a porous layer of an electrolytically depositable metal on a support material, comprising immersing the support material in an electrolytic medium comprising ions of the metal and electrolytically depositing the metal onto the support material, there being provided in the medium in the vicinity of the support material an oxidising species which reacts with the metal to form a product which is reducible under the deposition conditions. The process of the invention enables highly porous metal to be produced. The porous metal so produced may have, for example, a mean grain size of less than 3u, especially less than 1u, the grains being separated by pores of similar size. As a result of the higher porosity of porous metal layers produced according to the invention, as compared with prior art methods, a significantly larger surface area of the metal is available for utilisation. For example, when the metal is used in a battery electrode, the performance of the battery is significantly improved in comparison to prior art batteries; in particular, the large surface area means that the electrode has a high maximum current density and hence high instantaneous power density is obtainable. Further, the high porosity enhances the catalytic properties of metals when used as catalysts. The term "oxidising species" as used herein is used in the broad sense of a species which increases the oxidation state of the metal with which it reacts. It is believed that, in the process of the invention, the oxidising species reacts with the deposited metal to form a metal compound which is subsequently reduced back to the metal. The temporary presence of the metal compound prevents the build up of dense metal layers during deposition, so enabling a porous product to be formed. The oxidising species is advantageously supplied or generated continuously during deposition of the metal. The oxidising species is advantageously an oxygen- containing oxidising * species. "Oxygen-containing" as used herein means containing one or more oxygen atoms and includes molecular oxygen. The oxygen-containing oxidising species may be, for example, oxygen or an oxidising species generated from oxygen under the deposition conditions. Thus, the oxidising species may be, or may be generated from, gaseous oxygen which is introduced into the electrolytic medium so that it passes in the vicinity of the support material. Oxygen-containing oxidising species generated from oxygen under the deposition conditions may be, for example, hydrogen peroxide, ozone, oxyhalide anions, especially hypochlorite anions. Other oxidising species which may be used include, for example, the following substances, or oxidising species generated from those substances under the deposition conditions: hydrogen sulphide, halogens, especially chlorine. Advantageously, at least two oxidising species are used, at least one oxidising species then preferably being an oxygen-containing species. As indicated above, it is a requirement that at least one oxidising species reacts with the metal to form a product which is reducible under the deposition conditions. It will of course be appreciated that in some cases a material which is in some circumstances incapable of acting as, or generating, an oxidising species which meets that requirement may, if the conditions are adjusted in an appropriate manner, be capable of doing so. Thus, for example, whereas it is possible to use chloride ions as an oxidising species or a source of an oxidising species when another oxidising species, especially oxygen or an oxidising species generated therefrom under the deposition conditions, is present, chloride ions would not, in the absence of such a further oxidising species, react with the metal in the required manner. Thus, in a preferred form of process according to the invention, an oxygen-containing oxidising species, for example oxygen or an oxidising species generated from oxygen under the deposition conditions, is provided together with a second oxidising species. The second oxidising species may be, but is not necessarily, an oxygen-containing species. Preferably the second oxidising species is, or is generated from, a halide ion, especially a chloride ion, or an oxyhalide ion, especially a hypochlorite anion. The presence of a second oxidising species will in many cases enhance the porosity of the porous metal produced. In an especially important form of process according to the invention porous metal is produced on a support material by a process comprising immersing the support material in an electrolytic medium comprising ions of the metal and electrolytically depositing the metal onto the support material, there being introduced into the electrolytic medium gaseous oxygen in such a manner that it passes close to the surface of the support material. The electrolytic medium then preferably contains halide ions or gaseous halogen. Gaseous halogen may be provided in the medium by arranging the conditions such that a halogen gas is evolved at the counterelectrode or by bubbling the gas into the medium, the oxygen gas and chlorine gas then being introduced separately during deposition in such a manner that each passes in the vicinity of the support material. In the latter case it will of course be understood that precautionary measures will be required for control of the explosive oxygen/chlorine mixture leaving the medium. The use of an oxidising species in accordance with the invention offers the possibility of obtaining porous metal consisting essentially of a mixture of fine and coarse grains in a network of fine and coarse pores of sizes similar to the grain sizes. Such a fine and coarse pore network is particularly advantageous, for example, in air electrodes, which are widely used in metal-air batteries and fuel cells, enabling rechargeable electrodes to be made. In the oxygen evolution mode the coarse pores will be emptied and allow efficient escape of the oxygen generated inside the fine pores. In the oxygen reduction mode sufficient air pressure is applied to the gas-supply side of the electrode to denude the coarse pores of electrolyte. Provided the air pressure is not high enough to expel the electrolyte from the fine pores, efficient oxygen reduction will take place. In previously proposed porous electrodes only the outer regions of the electrode can function because when gas is evolved inside the electrode the electrolyte inside the pores is expelled by the gas bubbles. In an attempt to avoid this problem Teflon- bonded porous electrodes have previously been used in order to allow the gas to escape via the dry Teflon channels so permitting greater utilisation of the available electrocatalyst surface. The presence of non¬ conducting particles, however, leads to a higher electrode resistance. In porous metal formed by the process of the present invention the coarse pores act in a similar manner to the dry Teflon pores but the electrode has an improved conductivity, which is of importance in electrochemical applications requiring high currents (e.g. 0.1 amp/cm-^) . Whereas the performance of electrodes made of porous metal formed according to the invention is comparable to that of Teflon bonded electrodes, for gas evolution reactions the measured iR losses are significantly lower. It is considered that the nature of the mixed fine and coarse grain structure can be varied by adjusting one or more of a number of process variables which are believed to influence porosity. Factors which are believed to influence porosity include the nature of the first oxidising species, the presence of a second oxidising species and the nature of that species if present, the rate of supply of one or each of the oxidising species, the concentration of the electrolytic medium and temperature, the pressure of the atmosphere above the electrolyte and, where the oxidising species or the source thereof is a gas, the pressure of that gas. The rate of supply of an oxidising species may be changed by, for example, changing the rate of introduction or generation of the oxidising species or by changing the rate of rotation of a rotatable support material. Thus, for example, in the case of cobalt, porous cobalt prepared in accordance with the invention from a cobalt chloride solution, oxygen gas being bubbled through the solution to pass in the vicinity of the support material, has significantly higher porosity than cobalt similarly prepared from cobalt acetate solution or cobalt sulphate solution. Moreover, replacement of oxygen by air at the same flow rate (which contains approximately 20 per cent oxygen) lowers the porosity significantly. In the case of iron prepared on a rotating support material from a ferrous chloride solution with oxygen, the porosity was found to increase with the concentration of the solution over the range of 3wt% to 15wt%, and an optimum rate of rotation was found to be 300 rpm, the porosity of iron prepared at higher and lower rotation rates being less than that obtained at 300 rpm. The electrolytic medium may comprise ions of two or more metals, the porous metal so produced then comprising at least two metals. Instead, the electrolytic medium may comprise ions of two or more metals, one of which reacts with the oxidising species to form a product which is not reducible under the deposition conditions. Thus, the invention makes possible the manufacture of a porous material comprising a first metal and an oxide of a second metal. Advantageously the support material is rotated during deposition of the metal. In an alternative preferred process, electrolyte may be circulated by a pump, the flow of electrolyte, where gaseous oxygen is introduced into the medium, being in the opposite direction to the oxygen flow. This increases the residence time of the gas bubbles on the surface of the electrode and enables higher loadings of the metal to be plated onto the support material. Further, where the electrolytic medium is replenished during, deposition or where the volume of the electrolytic medium is sufficiently large, an electrolyte circulation process makes it possible to maintain a relatively constant composition of the electrolyte. Advantageously, the counterelectrode is of the same metal as the metal to be deposited. This also helps to maintain a relatively constant concentration of the metal ions in the medium. Particularly advantageous results may be obtained if the concentration in the electrolytic medium of the metal to be deposited is low. When the medium is a solution, the ionic conductivity of the solution is then preferably provided in part by another electrolyte which does not give rise to deposition on the support material under the deposition conditions. Thus, in a process according to the invention, the electrolytic medium is an aqueous electrolytic medium, which preferably comprises a first electrolyte which contains ions of the metal to be deposited and a second electrolyte which does not give rise to deposition on the support material under the deposition conditions, a substantial proportion of the ionic conductivity of the medium being provided by the second electrolyte. For example, a cobalt electrode may be prepared by deposition from a dilute (for example 0.05 to 0.1M) cobalt chloride solution containing potassium chloride in a concentration of 0.5M. When such a dilute solution is used, it is particularly advantageous to use as the counter electrode an electrode comprising the same metal as the metal to be deposited, in order to maintain the bath composition relatively constant. When a dilute solution of the metal to be deposited is used, rotation of the support material may be unnecessary. The support material is preferably of generally laminar form and the support may be, for example, a nickel screen. Other support materials that can be used include, for example, titanium foil. The composition of the electrolytic medium may be varied during deposition by the addition of one or more substances for example by the addition of solvent, a salt of the metal, or a salt of a second metal to the medium, the porous metal layer so produced then having a plurality of regions of differing compositions. Any electrolytically depositable metal may be used in the process of the invention, including for example lead which is widely used as the electrode material in lead-acid batteries. The metal may be selected from the transition metals and is advantageously cobalt, iron, nickel or cadmium. Other preferred transition metals include for example Zn, Ag, Mn and precious metals, for example, Pt, Pd, Ir. Porous transition metal products according to the invention have excellent properties for use in, for example, air batteries, fuel cells, and catalytic materials. Thus, for example, in Teflon-bonded cobalt oxide/graphite air electrodes, the cobalt oxide/graphite catalysts possess high activity for the reduction of oxygen in alkaline media. Cobalt oxides, however, are slightly soluble in alkaline solution and in applications where there is a continual replacement of an alkaline electrolyte, for example in aluminium- air batteries or chlor-alkaline plants, the cobalt oxides will be gradually leached out, so reducing the activity of the electrode. Depositing porous cobalt onto the electrode in accordance with the process of the invention enables the electrode to be regenerated in situ so that performance can be maintained at a satisfactory level over a longer period of time. The present invention therefore makes possible the use of cobalt/graphite air electrodes as the oxygen-reducing cathodes in chlor-alkali cells instead of hydrogen- evolving cathodes, giving a reduction of up to 0.8V in cell voltage, that is, a reduction of approximately 25% in power consumption. A cobalt electrode can also be used, for example, as an anode in Ni-Co batteries. An iron electrode can be used as an anode in Ni-Fe batteries. An iron electrode produced in accordance with the process of the invention will have a significantly higher maximum current density than porous iron electrodes made by sintering and, therefore, a high power density. NiOOH electrodes are used as cathodes in alkaline rechargeable batteries principally because of their robustness, reliability and ability to undergo many thousand charge and discharge cycles. The conductivity of NiOOH is relatively poor, however, and, for example, the conductivity of electrodes in which NiOOH is deposited on a nickel screen is poor. NiOOH is frequently incorporated in a porous nickel sinter to obtain better conductivity. NiOOH material made from porous nickel manufactured according to the present invention has excellent conductivity and allows an improved utilisation of the NiOOH material to be achieved. Thus, the conductivity and, in particular, the energy density and the power density of electrodes comprising the NiOOH material on a nickel screen is significantly improved by using NiOOH prepared using the process of the invention. Nickel electrodes prepared in accordance with the invention may already contain some NiOOH as a result of the nickel product formed under the deposition conditions being only partially reduced to nickel. The nickel electrodes may then be used without further treatment. Preferably, however, they are subjected to potentiostatic cycling before use. Porous cadmium anodes are used in Ni-Cd cells. The improved conductivity and efficiency of utilisation of material that is obtainable in porous cadmium produced in the process of the invention improves the performance of the cells. The electrolytic medium may be an aqueous electrolytic medium. In a preferred process the medium is an aqueous medium which contains ions of a transition metal, the deposition current being not 2 greater than 300 mA/cm , more especially not greater than 100 mA/cm 2 . In another process according to the invention, the electrolytic medium is a molten salt and the metal is a refractory or rare earth metal, for example La, Nb, W, Mo. In a further process according to the invention, the electrolytic medium is an organic electrolyte and the metal is an alkali metal. Porous alkali metal products, for example porous lithium products, are used in organic electrolyte batteries, which require a high instantaneous power density. The high porosity obtainable in accordance with the present invention will permit increased instantaneous power density over prior art batteries to be attained. Thus, the process of the invention may be used, for example, to manufacture electrodes. Advantage¬ ously, after preparing the metal layer on a support to form the electrode, the resulting electrode is subjected to potentiostatic cycling which has the effect of increasing the anodic peak current obtainable in use of the electrode. If desired, in the manufacture of electrodes, inorganic or organic extenders may be suspended in the electrolytic medium and codeposited with the metal. Electrodes having a porous metal layer including an extender may have enhanced porosity and/or enhanced conductivity. The method of the invention may also be used to regenerate an electrode _ir situ, so prolonging its operating life. Thus, the present invention further provides a method of regenerating an electrode in a cell, comprising forming a porous layer of a metal on the electrode by any process of the invention described above. In some cases it will be possible to regenerate the electrode without replacing the electrolyte of the cell. For example, where the cell electrolyte contains ions of the metal to be deposited on the electrode, oxygen may be bubbled through the cell electrolyte and a small amount of an active species which may act as a second oxidising species and/or is a suitable source for generating a second oxidising species, for example a metal halide, may be introduced. In other cases the cell electrolyte may be replaced by an electrolyte containing ions of the metal to be deposited on the electrode. Thus, the present invention enables the electrodes of, for example, fuel cells and metal-air batteries to be regenerated _in_ situ. Hitherto the regeneration of air electrodes in situ has not been achieved. The present invention further provides a process of producing a metal oxide on a support material, comprising producing a porous layer of a metal on the support material by any process of the invention described above and, following production of the porous layer, anodising the said layer to form an oxide of the metal. The porous metal may comprise two or more metals, the metals being selected so that on anodisation, substantially all of a first metal is oxidised and substantially all of a second metal remains in elemental form. Such a combination of an oxide and a metal is particularly useful as a catalytic material. Instead, the porous layer may comprise two or more metals which are oxidised during anodisation to form a combined oxide. Advantageously the metals are so selected that, on anodisation the metals are conver¬ ted to a ceramic material. The ceramic material may be for example, a perovskite oxide e.g. lanthanum barium copper oxide. Perovskite oxides are of interest as superconductors. The support material may be of copper. The invention further provides a process of form¬ ing a continuous ceramic coating on tubing, for example, copper tubing, which comprises passing the tubing through an electrolytic medium in which porous metal is produced on the tubing by any process of the invention described above and thereafter passing the tubing into a region in which the porous metal is anodised to form a ceramic material. The invention thus enables ceramic coated tubing to be manufactured cheaply in a continuous process. For example, copper tubing having a continuous coating of a perovskite oxide, more especially of lanthanum barium copper oxide, may be manufactured in accordance with the invention. In use, liquid nitrogen or another suitable low-temperature fluid can be passed through the copper tube to maintain the oxide material at a temperature at which it is superconducting. The present invention also provides porous metal having a mean grain size of less than 3u and an article having a porous metal portion of mean grain size of less than 3 . The porous metal portion preferably comprises a mixture of fine and coarse grains in the range of from 0.05u to 1 . The article is advantage¬ ously a battery plate, a filter, a catalytic support or a catalytic electrode. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows partly in section and partly in diagrammatic form one type of cell assembly suitable for preparing a porous metal according to the invention; Fig. 2 shows partly in section and partly in diagrammatic form an alternative cell assembly for preparing a porous metal in accordance with the invention. Fig. 3 shows a circulating electrolyte system suitable for preparing porous metal in accordance with the invention; Fig. 4 shows a similar assembly to that of Fig. 3 with a different arrangement of the support material; Fig. 5 shows voltage-current curves of porous iron anodes prepared in accordance with the invention, with and without NiCl 2 as an additive in the plating solution; Fig. 6 shows a voltage-current curve of a porous iron electrode prepared in accordance with the invention; Fig. 7 shows a comparison of anodic performance between cobalt electrodes prepared in the presence of oxygen gas in accordance with the invention and for comparison in the absence of such gas; Fig. 8 shows a comparison of cobalt electrodes of higher loading, a first electrode being prepared by a process of the invention and a second electrode being prepared by sintering; Fig. 9 shows the effect of potentiostatic cycling on the anodic peak current of a porous cobalt electrode prepared in accordance with the invention; Fig. 10 shows steady charge and discharge voltage- current curves for a porous cobalt electrode prepared in accordance with the invention; Fig. 11 shows the cathodic polarisation curve of a porous nickel electrode prepared in accordance with the invention; Fig. 12 shows the anodic polarisation curve of a porous cadmium electrode prepared in accordance with the invention; Fig. 13 shows voltage-time curves for porous iron electrodes prepared in accordance with the invention discharged at different temperatures; Fig. 14 shows voltage-time curves for sintered iron electrodes for comparison with Fig. 13; Fig. 15 shows the current density obtainable from porous cobalt electrodes of various loadings prepared in accordance with the invention compared with the values, corrected for iR losses, obtained from Teflon- bonded and non-Teflon-bonded porous NiCθ2θΛ elec¬ trodes; and Fig. 16 illustrates the performance of a cobalt electrode prepared according to the invention and a Teflon-bonded electrode, neither being corrected for iR losses. Fig. 17 illustrates the performance of two cobalt electrodes prepared according to the invention, one of the electrodes being prepared from a more dilute cobalt solution; and Figs. 18a and 18b illustrate the discharge performances of two cobalt electrodes prepared in accordance with the invention from solutions of different cobalt concentrations. Preparation of substrates 60 mesh nickel screens were used as the support materials. Nickel wires were spot welded on the substrates as current leads and protected by Canning Laco it F65441 lacquer. The substrates were first soaked in dilute alkaline solution placed in an ultrasonic bath, followed by rinsing in distilled, deionised water. The one square centimeter substrates were either circular or square. Preparation of electrodes Porous metal layers were prepared on support materials using an electrochemical cell of any of the types sfio n in Figs. 1 to 3. In each case the support material 1 was suspended in an electrolytic medium 2 in a container 3. Oxygen was introduced into the medium through a sintered glass plug 4 in the vicinity of the support material. The support material 1 and the counter electrode 5 were electrically connected in known manner. The electrodes were arranged in horizontally spaced or vertically spaced relationship. The vertical positioning of the working electrode (that is, when the electrodes are horizontally spaced) has the advantage of making the cell more compact (Fig. 3) whilst posi¬ tioning the electrode horizontally (Fi . 4) ensures longer residence time of the oxygen bubbles on the •*> surface of the electrode. 4 cm * ' Platinum foil was used as the counter electrode and where appropriate a saturated Calomel electrode 6 was used as the reference electrode. The working electrode (support material 1 ) could in each case be rotated. During deposition the cell is thermostatted at constant temperature and a constant plating current was passed through the cell. Figs. 3 and 4 illustrate a cell in which the electrolyte can be circulated using a pump 7. The direction of flow of the electrolyte is opposite to the direction of flow of the oxygen. For comparison purposes electrodes were prepared by sintering metal powders in hydrogen with ammonium carbonate incorporated in the powder to increase porosity. Example 1 (a) 70 ml of 10wt% aqueous CoCl 2 solution was placed in a cell of the type illustrated in Fig. 1 or Fig. 2. The cell was thermostatted at 25°C and a constant plating current of 40mA/cm 2 was passed through the cell for 30 mins. Oxygen was passed into the cell through the sintered plug, the flow rate being controlled at 40cm 3 /min. The support material was rotated at a speed of 300 rpm. The porous cobalt layer so produced was dull black in colour; cobalt deposited under the same conditions but in the absence of bubbling oxygen is, in contrast, shiny and reflective. (b) 70ml of 10wt% aqueous FeCl, solution was placed in a cell of the type illustrated in Fig. 1 or Fig. 2. The cell was thermostatted at 25°C and a constant plating current of 60mA/cm 2 was passed through the cell for a period of 40 minutes. The oxygen flow rate was controlled at 40cm 3 /min. The support material was rotated at 300 rpm. (c) 70 ml of 1wt% aqueous NiCl2 solution was placed in a cell of the type illustrated in any of Figs. 1 to 3. The cell was thermostatted at 25°C and a constant plating current of 10mA/cm'' was passed through the cell for 60 minutes. The oxygen flow rate 3 wwaass ccoonnttrroolllleedd ;at 50cm / in. The working electrode was stationary. (d) 70ml of 6wt% aqueous CdC^ solution was placed in a cell of the type illustrated in any one of Figs. 1 to 3. The cell was thermostatted at 25°C and a constant plating current of 6mA/cm 2 was passed through the cell for two hours. Oxygen was passed into the cell at a flow rate of 10cm 3 /min. The working electrode was stationary. Example 2 (a) The effect on the anodic peak current of the resulting electrodes of using different electrolyte solutions in the preparation of cobalt and cadmium electrodes is illustrated by the results summarised in Table 1. The conditions for the preparation of the metal electrodes from metal chloride solutions were as specified above in Example 1 (a) and (d) respectively. For the preparation of electrodes from metal sulphate solutions and metal acetate solutions the metal chloride solution was replaced by a metal sulphate solution or a metal acetate solution, respectively, of the same concentration as that of the metal chloride solution. The peak anodic current was measured in 7M aqueous KOH at 25°C, sweep rate lOmV/s. As shown in Table 1 , the performance, as measured by the anodic peak current of the electrodes prepared from metal chloride solution, was in each case better than that obtained from metal sulphate and metal acetate solutions. TABLE 1 Anodic peak current (lap) in mA/cm 2 Metal (M) MCI. MAc. MSO. Co 620 30 50 Cd 220 196 56 (b) The performance of nickel electrodes prepared as described in Example 1(c) from a nickel chloride (NiCl2) solution was better than that of electrodes deposited using the same method but replacing the NiCl2 by a nickel acetate solution. Improved performance as compared with electrodes prepared from nickel acetate solution was obtained from electrodes prepared from nickel acetate solutions to which some nickel chloride had been added. The best performance was given by electrodes obtained from a solution of nickel acetate, nickel chloride and cobalt acetate; performance in the case of the nickel electrodes was evaluated by measur¬ ing the number of charge-discharge cycles to which the electrodes could be subjected before noticeable deterioration of the electrode occurred (see Table 2) . The charge-discharge cycles were performed in 7M aqueous KOH at 20°c with a sweep rate of 80mV/s. TABLE 2 Process variables Performance of deposits Method Electrolyte Current Time No. of Cycles density before noticeable (mA/cm ) (min.) deterioration 2a 1%(CH 3 COO) 2 Ni 10 120 200 2b 1% (CH 3 COO) 2 Ni 10 120 1000* 0.028% NiCl. (No change) 2c 1% (CH 3 COO) 2 i 10 120 1500 0.028% NiCl. (10-15) (0.028-0.056%) 0.1% (CH 3 COO) 2 Co (0.1-0.145%) Example 3 Cobalt, iron, nickel and cadmium electrodes were prepared as described in Example 1 above. In order to provide a basis for comparison with the process of the invention a further set of electrodes was prepared under identical conditions with the exception that no gas was passed through the cell. In a further process according to the invention a cobalt electrode was prepared under the conditions of Example 1 (a) except that the oxygen was replaced by air. The anodic peak current of each was measured in 7M aqueous KOH, at 25°C with a sweep rate of lOmV/sec. The results are shown in Table 3. As will be apparent from Table 3, the performance, as measured by the anodic peak current, of the electrodes prepared in the presence of bubbling oxygen was significantly better than that of electrodes prepared in the absence of bubbling gas. In the case of the cobalt electrode prepared in the presence of bubbling air, the performance was better than that of the cobalt electrode prepared in the absence of bubbling gas, but was less good than that of the cobalt electrode prepared in the presence of bubbling oxygen. TABLE 3 Example 4 (a) A number of cobalt electrodes were pepared as described in Example 1 (a) , except that each electrode was prepared using a different deposition current. As shown by the results summarised in Table 4a below, the optimum deposition current was 40mA/cm . The anodic peak current (lap) was measured in 7M aqueous KOH at 25°C with a sweep rate of 10mV/s. (b) A number of iron electrodes were prepared at different deposition currents the conditions otherwise being as specified in Example 1 (b) . The results are summarised in Table 4b. lap was measured in 7M aqueous KOH at 25°C with a sweep rate of 10mV/s. TABLE 4a Deposition current : 10 20 40 80 200 mA/cm 2 lap of electrode 375 520 620 510 70 mA/cm 2 TABLE 4b Deposition current : 40 60 80 100 mA/cm 2 lap of electrode 280 90 310 310 mA/cm 2 (c) Using similar experimental techniques to those described in Examples 4(a) and (b) above, the optimum plating current density for nickel in a 1wt% aqueous solution of NiCl 2 was found to be 1 OmA/cm . Example 5 A number of porous iron electrodes were prepared as described in Example 1 (b) with the exception that each was prepared at a different speed of rotation of the working electrode. The anodic peak current (lap) of each, measured in 7M aqueous KOH at 25°c with a sweep rate of lOmV/s, is given in Table 5. TABLE 5 Speed of rotation 200 300 400 600 900 1200 rpm lap: mA/cm 195 220 270 223 198 210 219 Example 6 (a) A number of iron electrodes were prepared at different concentrations of electrolyte (aqueous FeCl 2 solution) in the range of from 2.5wt% to 15wt% aqueous solution. The conditions were otherwise as specified in Example 1(b) . The anodic peak current of the resulting electrodes was found to increase with the concentration of the deposition electrolyte from a value of 148mA/cm 2 at 2.5wt% to 235mA/cm 2 at 15wt% solution. (b) A number of cadmium electrodes were prepared at different concentrations of electrolyte (aqueous CdCl solution) in the range of from 1.5wt% to 10wt%. The conditions were otherwise as specified in Example 1(d). The anodic peak current of the resulting electrodes was found to decrease with the concentration of the deposition electrolyte. (c) A number of nickel electrodes were prepared at different concentrations of electrolyte (aqueous NiCl 2 solution) in the range of from 0.7wt% to 5wt%. The performance as demonstrated by the anodic peak current was evaluated and the results are summarised in Table 6a. The best performance was obtained from the electrode prepared from a 1wt% solution. The effects of depositing the metal at different deposition current density and oxygen flow rates on the performance of a nickel electrode, the conditions otherwise being as specified in Example 1 (c) , are illustrated by the results summarised in Table 6b. TABLE 6a Process variables Performance of deposits Electrolyte Current Time 0_ flow Anodic peak density rate current (mA/cm 2) (min.) (cc/Min) (mA/cm2) 5% NiCl 2 10 180 (pH=3) 3% NiCl 2 10 215 (pH=4) 1% NiCl 2 10 310 (PH=5) .7% NiCl 2 10 60 50 145 (pH=5) TABLE 6b Process variables Performance of deposits Electrolyte Current Time 0_ flow Anodic peak density . rate current (mA/cm ) (min.) (cc/Min) (mA/cm ) A number of iron electrodes were prepared in ferrous chloride solutions (7g FeCl 2 in 100 ml distilled water) . The conditions were as specified in Example 1(b) but a variety of different additives were used as shown in Table 7. The concentrations of additives were also varied. Table 7 shows the anodic peak current (lap) of electrodes so prepared. The values given in the Table 7 are the maximum peak current obtained for the respective additive and the corresponding value of the additive concentration at maximum peak current. The performance of iron electrodes prepared in the absence and in the presence of nickel chloride (NiCl,,) as additive is compared in Fig. 5, which shows voltage-current curves of the electrodes measured in 7M aqueous KOH at 40°C with a sweep rate of lOmV/s. Example 8 Table 8 shows that, in the preparation of cadmium electrodes, increasing the acidity of the plating solution by the addition of HCI has a deleterious effect on the anodic peak current (lap) of the electrode obtained. TABLE pH lap mA/cm 2 , V=80mV/S (6% CdCl 2 , O (1.5% CdCl 2 , 0 2 1 Occ/min, 6mA/cm* 1 20cc/min, 2hr) 6mA/cm 2 , 2hr) 3.5 220 670 (no HCI) 32 275 2.0 HCI (added) Example 9 An iron electrode was prepared in a cell of the type shown in Fig. 2, a piece of nickel foil being used instead of platinum foil as the counterelectrode. The volume of the plating solution was 100 ml. The plating solution composition was as follows: 3wt% NiCl 2 0.03wt% N 2 H 4 .2HC1 10wt% FeCl 2 The deposition current density used was lOOmA/cm 2 . Oxygen gas was bubbled into the solution through the sintered plug 4 (see Fig. 2.) Since Ni has a high chlorine evolution potential, no chlorine was produced and oxygen was the only product in the anodic reaction. This maintains the pH at a relatively constant level in comparison to the method of Example 1 and it is therefore possible to obtain a larger loading of the iron. The electric- electric efficiency of the electrode was over 80%. Fig. 6 shows the voltage-current curves for an iron electrode containing nickel, loading 375mg/cm 2 (measured in 7M KOH, 10~ 3 M Na 2 S at 40°C with a sweep rate at lOmV/s) and shows that the electrode gives very high currents at 40°C, for example, 700mA/cm at -0.8V against an Hg/HgO reference electrode. Example 10 Fig. 7 shows a comparison of the anodic performance, as shown by the current density of a cobalt electrode prepared in accordance with the invention in the presence of bubbling oxygen and one prepared without bubbling oxygen. (Performance evaluated in 7M aqueous KOH at 25°C with a sweep rate of 10mV/s; the cobalt loading was 60mg.) Fig. 8 shows a similar comparison for cobalt electrodes of heavier loading, a first electrode 2 (loading 153mg/cm ) being prepared in accordance with the invention and a second electrode by sintering 2 (loading 486mg/cm ) (Performance evaluated in 7M aqueous KOH at 25°C with a sweep rate of 2.5mV/s.) The electrode of the invention performs significantly better in spite of the higher loading of the sintered electrode. Example 11 A cobalt electrode prepared in accordance with the invention was subjected to potentiostatic cycling. As illustrated in Fig. 9 the anodic peak current (lap) improved with the number of charge and discharge cycles, indicating that repeated cycling helped to improve the electrode performance. The anodic peak current was measured in 7M aqueous KOH at 25°c, with a sweep rate of 80mV/s. Similar results were observed for iron and nickel electrodes. Example 12 A cobalt electrode prepared in accordance with the invention having a cobalt loading of 249mg ^/cm was charged and discharged at a discharge current density 2 of 100mA/cm . Fig. 10 shows the steady state charge and discharge voltage current curves (measured in 7M aqueous KOH at 25°C) and indicates that the overall electric-electric efficiency of the electrode is high under the specified conditions. Example 13 A nickel electrode was prepared in accordance with the invention from a solution of nickel acetate, nickel chloride and cobalt acetate (see Method 2 of Table 2) . The cathodic polarisation curve is shown in Fig 11 (measured in 7M aqueous KOH at 20°C and a sweep rate of lOmV/s; loading of electrode 73mg/cm 2 ). Since the main use of the nickel electrode is as a cathode in battery applications, it is interesting to note from Fig. 11 that an electrode according to the invention is very active, with very little polarization up to a current density 400 mA/cm . There was a relatively sharp drop in operating voltage ("0.2V drop) thereafter and the electrode proceeded to give another 400 mA/cm 2 with very little polarisation, indicating that there are two cathodic steps. The last step may involve the reduction of Ni(OH) 2 back to Ni and the first step may be reduction of NiOOH to Ni(OH) 2 . In conven¬ tional nickel electrodes because of the size of the individual grains and the poor conductivity, the two stage reduction process is never observed. This fact may have practical advantages in rechargeable batteries using nickel cathodes since it could provide a con¬ venient way of detecting the end of charge. Example 1 4 The anodic polarisation curve of a porous cadmium electrode prepared in accordance with the invention is shown in Fig. 12 (measured in 7M aqueous KOH at 25°C with a sweep rate of TOmV/s). Fig. 12 indicates that the anodic performance of the cadmium anode is subject to very low polarisation, indicating high reactivity and good conductivity. Example 15 The change of anode voltage as a function of time for an iron electrode according to the invention having an 2 iron loading of 375 mg/cm was monitored in 7M aqueous KOH/10 M Na 2 s at a discharge current density of lOOmA/cm and at discharge temperatures of from 25 to 50°C. The results are shown in Fig. 13 which indicates a very high degree of utilisation. Fig. 14 shows the performance of a sintered iron electrode having almost ten times the loading of the electrode of Fig. 13, the aqueous electrode being discharged at 25mA/cm measured in a solution of 30% KOH and 5% LiOH. The high loading sintered electrode shows poor utilisation and, despite the lower discharge current density the polarisation was found to be much greater than that of the electrode according to the invention. Example 16 The current density obtainable from cobalt electrodes of various loadings prepared in accordance with the invention was measured. Figure 15 shows the results obtained, together, for comparison, with results for a Teflon-bonded and non-Teflon-bonded porous iCo 2 0^ electrodes prepared by the thermal decomposition of nickel-cobalt nitrates. (The current density was measured in 5M aqueous KOH at 25°C at a potential of 1720mV vs. DHE (dynamic hydrogen electrode).) The results for the cobalt electrode have not been corrected for iP losses, whereas the results for the two NiCo 2 0 4 electrodes have been corrected. Fig. 16 shows that, when no iR correction is made, a cobalt electrode according to the invention having a loading of 7.5mg/cm performs significantly better at a current density of 1.1A cm "2 than the Teflon-bonded NiCo 2 0 4 electrode. Example 17 A cobalt electrode (17A) was prepared in accordance with the invention by deposition from a 0.77M cobalt chloride solution, at a deposition current 2 density of 40m A/cm for a period of 1 hour. A piece of platinum was used as the counter electrode. The support material was rotated at 500 cps. Oxygen was supplied to the support material at a rate of 100 cm /min. A second cobalt electrode (17B) was prepared in accordance with the invention by deposition of cobalt onto a stationary support material from an aqueous solution containing cobalt chloride in a concentration of 0.05M and potassium chloride in a concentration of 0.5M, at a deposition current density 2 of 20m A/cm , for a period of 2.2 hours. A piece of cobalt foil was used as the counter electrode. Oxygen was supplied to the support material at a rate of 3 100 cm /min. Fig. 17 shows that the anodic peak current of electrode 17B is significantly higher than that of electrode 17A. The use of solid cobalt as the counter electrode has the benefits that no chlorine is evolved in the process and that there is no need for close control of the bath composition. Example 18 The depth of discharge of electrode 17A was measured at different C rates. (C rate means the rate of discharge where C is the theoretical capacity; for example C/3 means that the discharge current is adjusted to a value which will discharge the electrode in 3 hours based on its theoretical capacity) . The discharge performance of electrode 17A is illustrated in Fig. 18a and the discharge performance of a cobalt electrode prepared from a dilute cobalt chloride solution is illustrated in Fig. 18b. The electrode of Fig. 18b was prepared on a stationary support material from a dilute aqueous cobalt chloride solution (0.1M cobalt chloride, 0.5M potassium chloride) using a 2 deposition current density of 40m A/cm , an oxygen 3 flow rate of 100 cm /min and with a cobalt foil as counter electrode. The electrode loading was 219mg (surface area 6cm 2) . Figs. 18a and 18b show that an improved discharge performance is obtained by depositing the metal from a dilute solution of cobalt, potassium chloride being present as another electrolyte to maintain a relatively high level of ionic conductivity. For the purposes of Figs. 18a and 18b, the depth . of discharge was measured in 7N KOH at 20°C. The currents referred to in Fig. 18b are total currents. It is thought that the improved properties obtainable by depositing cobalt from a dilute aσueous solution may result from the deposition of the metal from coordinated cobalt compounds as opposed to cobalt ions. In particular, at lower cobalt concentrations most of the cobalt ions form coordination compounds (for example [Co (0H 2 ) 4 C1 2 ]) and the potential at which cobalt metal is deposited from such compounds is more negative than that at which it is deposited from Co 2+ ions. Thus, it is thought, more oxygen can be reduced, leading to the production of more hydroxyl ions which will oxidize a greater proportion of the cobalt metal, so increasing the porosity of the deposited metal and perhaps also reducing the size of the individual grains.

Claims: CLAIMS 1. A process of producing a porous layer of an electrolytically depositable metal on a support material, comprising immersing the support material in an electrolytic medium comprising ions of the metal and electrolytically depositing the metal onto the support material, there being provided in the medium in the vicinity of the support material an oxidising species which reacts with the metal to form a product which is reducible under the deposition conditions. 2. A process as claimed in claim 1, wherein the oxidising species is supplied or generated continuously during deposition of the metal. 3. A process as claimed in claim 1 or claim 2, wherein the oxidising species is an oxygen-containing oxidising species. 4. A process as claimed in claim 3, wherein the oxygen-containing oxidising species is oxygen or an oxidising species generated from oxygen under the deposition conditions. 5. A process as claimed in claim 4, wherein the oxygen-containing oxidising species is, or is generated from, gaseous oxygen which is introduced into the electrolytic medium so that it passes in the vicinity of the support material. 6. A process as claimed in any one of claims 3 to 5, wherein there is provided in the medium in the vicinity of the support material a further oxidising species that reacts with the metal to form a product which is reducible under the deposition conditions. 7. A process as claimed in claim 6, wherein the further oxidising species is selected from halide ions, halogens and anions which can be formed from halogens or halide ions under the deposition conditions. 8. A process of producing a porous layer of an electrolytically depositable metal on a support material, comprising immersing the support material in an electrolytic medium comprising ions of the metal and electrolytically depositing the metal onto the support material, there being introduced into the electrolytic medium gaseous oxygen in such a manner that it passes close to the surface of the support material. 9. A process as claimed in claim 8, wherein the electrolytic medium comprises halide ions. 10. A process as claimed in claim 8 or claim 9, wherein the electrolytic medium comprises a halogen. 11. A process as claimed in any one of claims 8 to 10, wherein the electrolytic medium comprises oxyhalide ions. 12. A process as claimed in any one of claims 1 to 11 , wherein the electrolytic medium comprises ions of two or more metals, the porous layer so produced then comprising at least two metals. 13. A process as claimed in any one of claims 1 to 11 , wherein the electrolytic medium comprises ions of two or more metals, one of which reacts with the oxidising species to form a product which is not reducible under the deposition conditions. 14. A process as claimed in any one of claims 1 to 13, wherein the support material is rotated during deposition of the metal. 15. A process as claimed in any of claims 1 to 14, wherein the composition of the electrolytic medium is varied during deposition by the addition of one or more substances to the medium, the porous metal layer so produced then having a plurality of regions of differing grain structure. 16. A process as claimed in any one of claims 1 to 15, wherein the porous layer comprises a metal selected from the transition metals. 17. A process as claimed in claim 16, wherein the metal is selected from cobalt, iron, nickel and cadmium. 18. A process as claimed in any one of claims 1 to 17, wherein the electrolytic medium is an aqueous electrolytic medium. 19. A process as claimed in claim 18, wherein the metal is a transition metal and the deposition current is not 2 greater than 300 mA/cm . 20. A process as claimed in claim 19, wherein the 2 deposition current is not greater than 100 mA/cm . 21. A process as claimed in claim 18, wherein the electrolytic medium comprises a first electrolyte which contains ions of the metal to be deposited and a second electrolyte which does not give rise to deposition on the support material under the deposition conditions, a substantial proportion of the ionic conductivity of the medium being provided by the second electrolyte. 22. A process as claimed in claim 21 wherein as counter electrode there is used an electrode comprising the same metal as the metal to be deposited. 23. A process as claimed in any one of claims 1 to 15, wherein the electrolytic medium is a molten salt and the metal is a refractory metal. 24. A process as claimed in any one of claims 1 to 15, wherein the electrolytic medium is an organic electrolytic medium and the metal is an alkali metal. 25. A process of producing a metal oxide on a support material, comprising producing a porous layer of the metal on the support material by a process according to any one of claims 1 to 22 and, following production of the porous metal layer, anodising the porous metal to form an oxide of the metal. 26. A process as claimed in claim 25, wherein the porous layer comprises two or more metals, the metals being selected so that on anodisation, substantially all of a first metal is oxidised and substantially all. of a second metal remains in elemental form. 27. A process as claimed in claim 25 wherein the porous layer comprises two or more metals which are oxidised during anodisation to form a combined oxide. 28. A process as claimed in claim 27, wherein the metals are selected such that, on anodisation, the metals are converted to a ceramic material. 29. A process as claimed in any one of claims 25 to 28, wherein the support material is made of copper. 30. A process of forming a continuous ceramic coating on tubing, comprising passing the tubing through an electrolytic medium in which a porous metal layer is produced on the tubing by a method according to any one of claims 1 to 24, and thereafter passing the tubing into a region in which the porous metal layer is anodised to form a ceramic material. 31. A process of regenerating an electrode in a cell, which comprises forming a porous metal layer on the electrode by a process according to any one of claims 1 to 24. 32. A process of preparing an electrode, which comprises preparing a porous metal layer on a support material by a method according to any one of claims 1 to 24 to form an electrode and subjecting the electrode to potentiostatic cycling. 33. A process of producing a porous layer of metal substantially as described in any one of Examples 1 to 9, 17 and 18 herein. 34. Porous metal whenever produced by a process according to any one of claims 1 to 24 or ' 33. 35. An article whenever manufactured by a process according to any one of claims 1 to 33. 36. An article having a porous metal portion having a mean grain size of less than 3u. 37. An article as claimed in claim 36, comprising coarse and fine grains within the size range of from 0.05u to 1μ. 38. An article as claimed in any one of claims 35 to 37, wherein the article is a battery plate, a filter, a catalytic support or a catalytic electrode. 39. A battery including an electrode produced by a process according to any one of claims 1 to 27 or 30 to 33.

Inventors: CHEN YAN ZHEN, JIANG SAN PING, TSEUNG ALFRED CHAN CHUNG, YOU JIN KUA, CHEN, YAN, ZHEN, JIANG, SAN, PING, TSEUNG, ALFRED, CHAN, CHUNG, YOU, JIN, KUA
Applicants: UNIV CITY, UNIV ESSEX, THE CITY UNIVERSITY, UNIVERSITY OF ESSEX
Application year: 1989
Main classification: C25D5
IPC codes: C25D5, H01M4