GB1566708A

GB1566708A - Electrodeposition of iron active mass

Info

Publication number: GB1566708A
Application number: GB50478/75A
Authority: GB
Original assignee: Inco Europe Ltd
Current assignee: Inco Europe Ltd
Priority date: 1975-12-09
Filing date: 1975-12-09
Publication date: 1980-05-08
Also published as: DK551476A; LU76346A1; NL7613631A; CA1072911A; BE849226A; AU503728B2; ES454048A1; AT354540B; FR2335059A1; IN145470B; DE2655577A1; ATA910776A; AU2026976A; SE7613783L; JPS5270340A; NO764158L; ZA767110B

Description

(54) ELECIlRODEPOSITION OF IRON ACTIVE MASS (711) We, INCO EUROPE LIMITED, formerly known as International Nickel Limited, a British company, of Thames House, Millbank, London, S.W.1, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the ,following statement: The present invention relates to a process for the production of iron electrodes suitable for,nickel-iron storage batteries.

It is desirable in any battery to make the ratio by weight of active mass to the metal which holds or supports it as high as possible, and also to reduce the total weight.

To this end, foil has been used as the supporting material in some nickel-cadmium and nickel-iron batteries. Problems have been experienced however in providing processes for the deposition of iron active mass on to smooth impermeable surfaces such as metal foil.

Processes have been disclosed and claimed in U;K Patent Specification 1,392,188 and UK Patent Application 54845/72 (Serial No.

1421y33B) in which electrolytes containing ferrous ions have been used in carefully controlled pH and current density conditions to deposit an adherent deposit of iron active mass on to supports such as metal foil.

Although these processes work adequately, ferrous electrolytes lack stability and deteriorate on standing. Moreover during the production of iron active mass from such ferrous electrolytes, particularly when copper is to be co-deposited with the iron, there is a considerable build-up of the iron content of the electrolyte arising from dissolution of the iron anode, so that when used continuously on a large scale it is essential to take steps to reduce the iron content of the electrolyte at regular intervals to maintain consistency of the deposit.

US Patent 3,527,613 discloses the use of ferric electrolytes in the electroprecipitation of an iron compound-sulphur active material in the pores of a porous electrode.

Iiawever attempts to deposit iron active mass on foil from ferric ammonium nitrate and ferric nitrate have been found to produce only very thin adherent deposits of ferric hydroxide and high current densities, > 100 mA/cm2, were necessary to produce even this. Once a monomolecular layer has been deposited, the poor conductivity of the deposited ferric hydroxide inhibits further deposition.

The present invention is based on the discovery that ferric electrolytes may be modified so that adherent, useful, deposits of iron active mass may be produced on smooth impermeable surfaces such as metal foil. By impermeable herein is meant nonporous.

According to the present invention a process for the deposition of iron active mass on to a smooth non-porous electrically conducting surface comprises electrically depositing active mass consisting of iron and iron oxide and/or hydroxide from an electrolyte made up from ferric ions, and soluble carboxylate ions as defined herein and having a bulk pH of less than 4.0 using a cathode current density of between 10 and 1000 mA/cm2. Normally ferric ions are provided in a molar ratio of at least 2:1 over carboxylate ions.

Preferably the electrolyte also contains a large molar concentration of ammonium ions.

By soluble carboxylates in the present specification is meant carboxylate ions of weak carboxylic acids having pKa values greater than the bulk pH and where both the ferric carboxylate and the ferrous carboxylate are soluble in aqueous solutions.

Such soluble carboxylates are typified by tartrate, acetate, lactate and citrate.

When the electrolyte is made up the carboxylate ions complex a stoichiometric amount of the ferric ion available, and thereby stabilise the electrolyte and bring about modification of the electrode reaction. This is because carboxylates form more stable complexes with ferric ions than with ferrous ions so that the cathode reaction, the reduction of Fe (III) to Fe (11) in the presence of carboxylate ions, is accompanied by an increase in pH at the electrode surface due to the increase in concentration of free carboxylate ions. In the acid conditions of the electrolyte these carboxylate ions become protonated to a large extent and the local increase in pH causes deposition of iron oxide and/or hydroxide on to the cathode. At current densities exceeding the diffusion ]imited value for reduction of Fe (III) to Fe (II), iron platihg also takes place so that the deposited active mass is a mixture of iron and iron oxide and/or hydroxide the relative proportions of the two being variable with the current density.

It has been found that the process of the present invention will produce deposits of high density active mass having good adhesion to smooth impermeable surfaces such as metal foil. The good adhesion may be attributable to the mildly corrosive nature of the electrolyte so that the surface of the metallic substrate is lightly etched before deposition starts.

Preferred electrolytes for use in processes of the present invention contain a large molar concentration of ammonium ions i.e.

up to saturation point. Preferably the molar ratio of ammonium ions to ferric ions is of the order of 4:1. This high ammonium ion content further stabilises the electrolyte and minimises or prevents the precipitation of ferric hydroxide on standing. Furthermore the ammonium ions compete with the ferrous and ferric ions to react with the free hydroxyl ions which brings about an increase in the density and metallic content of the deposited active mass for a given current density.

The preferred electrolytes for use in the present invention may be made up from any convenient starting material. It is preferred not to use ferric or ammonium halides, since any residual halide ions left after activation can causes corrosion when the iron electrode is eventually incorporated in a battery. Moreover it is preferred that any anion which will preferentially reduce, such as nitrate, should not be used in the electrolyte. Generally it is preferred to use ferric sulphate, ammonia, ammonium sulphase, and/or ferric ammonium sulphate in the electrolytes because these are cheap and readily available. The carboxylate ion may be added as any suitable soluble salt, e.g.

citrate as sodium citrate, and pH control is normally achieved by sodium hydroxide addition.

The pH of the electrolyte used in processes of the present invention must not exceed 4 since at higher pH the solution becomes too unstable to be commercially useful.

It will be observed however that at pH's P below 4 there may be a slight precipitation. When the ammonium : ferric content of the electrolyte is below about 2.5:1 this is normally ferric hydroxide. In the preferred electrolytes having high ammonium ion contents, it is found that basic ferric sulphate is precipitated. Although this precipitation does not affect activation adversely it does lower the ferric content of the solution and it is usual to make consequential changes to the current density and duration of the activation process as the ferric content diminishes.

If the pH is too low the metal content of the deposit becomes high and the solution becomes excesively corrosive. The pH varies critically with the carboxylate ion used and the amount of excess ammonium ion available but may be readily determined experimentally for any electrolyte for use in the present invention Thus where an electro lyte is to be made up using X mal/1 ferric ion, Y mol/l carboxylate ion and Z mol/l ammonium ion, one makees up a similar solution except that it contains X mol/1 ferrous ion in place of ferric. Then this is titrated with a standard solution of caustic soda and the pH recorded during titration. It is observed that the p!H rises to a constant level at which precipitation occurs.

The amount of caustic soda added as the curve levels out is noted and an identical amount added to an aliquot of the ferric electrolyte. The resultant pH of the ferric solution is the minimum pH at which this electrolyte can be operated. This is because below this pH there can be no ferrous hydroxide formation on reduction. For the preferred electrolytes for use in the present invention the pH should generally be maintained in the range 2.2 to 3.5, or preferably 2.5 to 3.0.

Processes of the present invention are carried out in conditions in which the selected current density is between 10 and 1000 mA/cm2 and is correlated with the ferric content of the electrolyte to produce the desired deposit. As a rough guide, the current density is between 20C and 200C mA/ cm3 where C is the molar ferric content of the electrolyte so that at a typical ferric content of 0.5M current densities in the range 10 to 100 mA/cm2 are used. Preferably the current density is in the range 25 to 40 mA/cm2. Generally it has been found that at low current densities the electrode reaction Fe (III)9Fe (II) is favoured whereas there is less of the reaction Fe (II)9Fe The process is normally operated at about room temperature.

Processes of the present invention are normally carried out using an inert anode since if an iron, or other reactive metal anode is used there is a possibility of reaction between the ferric ion in solution and the anode to produce a deleterious amount of ferrous ion in the electrolyte. Preferred anodes include ruthenium dioxide on titanium and platinised titanium.

Preferred eletcrolytes for use in processes of the present invention contain citrate ions, preferably in a concentration such that the molar ratio Fe (III) : citrate is between 10:1 and 2:1, preferably about 3:1.

As is known from the prior art, iron active mass is improved by the introduction of a small quantity of sulphur into the deposit, either as elemental sulphur or as a sulphide, to increase the initial capacity and cycle life of the electrode. A preferred route for incorporating such sulphur may be to dissolve suitable sulphur-containing compounds such as sodium thiosulphate, thiourea or naphthalene disulphonate disodium salt in the electrolyte from which deposition takes place in concentrations such that a suitable quantity of sulphur, norm- ally in the range of from 0.01 to 0.5% and preferably 0.01 to 0.1% by weight of the active mass, is incorporated into the deposit. Alternatively however the process of the resent invention may be followed by a further step in which the electrode produced by the deposition process is dipped in a solution of a sulphur-containing substance, for example a sulphide dissolved in potassium hydroxide, or is initially charged in an electrolyte containing a suitable source of sulphur. One suitable electrolyte for carrying out the initial charging consists of a solution of potassium hydroxide and may contain aLkali metal sulphide or polysulphide at a concentration of from 10-3 to 10-1 molar.

Processes of the present invention may be applied to the deposition of iron active mass to any appropriate substrate but for the purpose of obtaining the best ratio of capacity to weight it is desirable to have a metal foil substrate, the thickness of the foil being compatible with the current likely to be drawn from the battery. The foil may, advantageously, be perforated, and a stack of the foils bearing the deposits may then be formed into a battery plate as described in UK Patent No. 1,246,048.

Some examples of activation processes of the present invention will now be described.

EXAMPLE I An electrolyte was prepared containing 0.5 M ferric ammonium sulphate, 0.17 M trisodium citrate and 0.75 M ammonium sulphate, the pH adjusted to 2.8 with sodium hydroxide. A nickel foil, of dimensions 5 cm X S cm and 4 microns thick, was used as cathode in an electrolysis at room temperature, an inert counter electrode serving as anode, the cathode current density being 30 mA/cm2 for 25 mins. The gain in weight of the cathode was 0.2 + 0.04g the density of the deposit being 2.8+0.5g/cm . The electrode was initially cycled in an electrolyte consisting of sodium sulphide in 30% potassium hydroxide solution the sulphur content of the electrolyte being 4% of the weight of active mass. The surface capacity after 20 cycles of charge and discharge to 0.79V was measured against a mercurymercuric oxide reference electrode, and was found to be 1.3 +0.2mAh/cm2. The utilisa- tion i.e. the capacity as a percentage of the theoretical capacity was 39+5%.

EXAMPLE 2 An electrolyte was prepared containing 0.4M ferric ammonium sulphate and 0.4 M sodium lactate the pH adjusted to 3.0 with sodium hydroxide. A nickel foil of dimen- sions 5 cm > c 5 cm and 4 microns thick was activated for 15 mins. at a cathode current density of 50 mA/cm2. A deposit was obtained of total thickness 40 m, and having a density of 3.5 g/cm3, the iron content being 86%. The surface capacity after 20 cycles was 1.9 mAh/cm2 and utilisation 32not EXAMPLE 3 An electrolyte was prepared containing 0.5M ferric ammonium sulphate, 0.17 M trisodium citrate, 0.75 M ammonium sulphate and 7.5x10-S sodium thiosuiphate and the pH adjusted to 2.8 with sodium hydroxide.

A 5 cmx5 cmx4 nickel foil was used as cathode, platinised titanium as the counter electrode and activation carried out at 30 mA/cm2 at room temperature, 20"C, for 20 mins. The density of the deposit was found to be 2.0 g/cm3 and the active mass contained 0.04-0.05% S based on the weight of the active mass. The surface capacity after 5 cycles of charge and discharge was found to be 1.0 mAh/cm2 the utilisation being 35%.

WHAT WE CLAIM IS:- 1. A process for the deposition of iron active mass on to a smooth non-porous electrically conducting ssurface comprising electrolytically depositing active mass consisting of iron and iron oxide and/or hydroxide from an electrolyte made up from ferric ions, and soluble carboxylate ions as hereinbefore defined and having a bulk pH of less than 4.00 using a cathode current density of between 10 and 1000 mA/cm2.

2. A process as claimed in claim 1 wherein ferric ions are provided in the electrolyte in a molar ratio of at least 2:1 over carboxylate ions.

3. A process as claimed in claim 1 or claim 2 wherein the electrolyte also contains ammonium ions.

4. A process as claimed in claim 3 wherein the molar ratio of ammonium ions to ferric ions is 4:1.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of ferrous ion in the electrolyte. Preferred anodes include ruthenium dioxide on titanium and platinised titanium. Preferred eletcrolytes for use in processes of the present invention contain citrate ions, preferably in a concentration such that the molar ratio Fe (III) : citrate is between 10:1 and 2:1, preferably about 3:1. As is known from the prior art, iron active mass is improved by the introduction of a small quantity of sulphur into the deposit, either as elemental sulphur or as a sulphide, to increase the initial capacity and cycle life of the electrode. A preferred route for incorporating such sulphur may be to dissolve suitable sulphur-containing compounds such as sodium thiosulphate, thiourea or naphthalene disulphonate disodium salt in the electrolyte from which deposition takes place in concentrations such that a suitable quantity of sulphur, norm- ally in the range of from 0.01 to 0.5% and preferably 0.01 to 0.1% by weight of the active mass, is incorporated into the deposit. Alternatively however the process of the resent invention may be followed by a further step in which the electrode produced by the deposition process is dipped in a solution of a sulphur-containing substance, for example a sulphide dissolved in potassium hydroxide, or is initially charged in an electrolyte containing a suitable source of sulphur. One suitable electrolyte for carrying out the initial charging consists of a solution of potassium hydroxide and may contain aLkali metal sulphide or polysulphide at a concentration of from 10-3 to 10-1 molar. Processes of the present invention may be applied to the deposition of iron active mass to any appropriate substrate but for the purpose of obtaining the best ratio of capacity to weight it is desirable to have a metal foil substrate, the thickness of the foil being compatible with the current likely to be drawn from the battery. The foil may, advantageously, be perforated, and a stack of the foils bearing the deposits may then be formed into a battery plate as described in UK Patent No. 1,246,048. Some examples of activation processes of the present invention will now be described. EXAMPLE I An electrolyte was prepared containing 0.5 M ferric ammonium sulphate, 0.17 M trisodium citrate and 0.75 M ammonium sulphate, the pH adjusted to 2.8 with sodium hydroxide. A nickel foil, of dimensions 5 cm X S cm and 4 microns thick, was used as cathode in an electrolysis at room temperature, an inert counter electrode serving as anode, the cathode current density being 30 mA/cm2 for 25 mins. The gain in weight of the cathode was 0.2 + 0.04g the density of the deposit being 2.8+0.5g/cm . The electrode was initially cycled in an electrolyte consisting of sodium sulphide in 30% potassium hydroxide solution the sulphur content of the electrolyte being 4% of the weight of active mass. The surface capacity after 20 cycles of charge and discharge to 0.79V was measured against a mercurymercuric oxide reference electrode, and was found to be 1.3 +0.2mAh/cm2. The utilisa- tion i.e. the capacity as a percentage of the theoretical capacity was 39+5%. EXAMPLE 2 An electrolyte was prepared containing 0.4M ferric ammonium sulphate and 0.4 M sodium lactate the pH adjusted to 3.0 with sodium hydroxide. A nickel foil of dimen- sions 5 cm > c 5 cm and 4 microns thick was activated for 15 mins. at a cathode current density of 50 mA/cm2. A deposit was obtained of total thickness 40 m, and having a density of 3.5 g/cm3, the iron content being 86%. The surface capacity after 20 cycles was 1.9 mAh/cm2 and utilisation 32not EXAMPLE 3 An electrolyte was prepared containing 0.5M ferric ammonium sulphate, 0.17 M trisodium citrate, 0.75 M ammonium sulphate and 7.5x10-S sodium thiosuiphate and the pH adjusted to 2.8 with sodium hydroxide. A 5 cmx5 cmx4 nickel foil was used as cathode, platinised titanium as the counter electrode and activation carried out at 30 mA/cm2 at room temperature, 20"C, for 20 mins. The density of the deposit was found to be 2.0 g/cm3 and the active mass contained 0.04-0.05% S based on the weight of the active mass. The surface capacity after 5 cycles of charge and discharge was found to be 1.0 mAh/cm2 the utilisation being 35%. WHAT WE CLAIM IS:-

1. A process for the deposition of iron active mass on to a smooth non-porous electrically conducting ssurface comprising electrolytically depositing active mass consisting of iron and iron oxide and/or hydroxide from an electrolyte made up from ferric ions, and soluble carboxylate ions as hereinbefore defined and having a bulk pH of less than 4.00 using a cathode current density of between 10 and 1000 mA/cm2.

5. A process as claimed in any one of

the preceding claims wherein the carboxylate is one or more of tartrate, acetate, lactate and citrate.

6. A process as claimed in any preceding claim wherein the electrolyte contains ferric ammonium sulphate, ammonium sulphate, sodium citrate and the pH is adjusted with sodium hydroxide.

7. A process as claimed in claim 6 wherein the molar ratio of ferric ion: citrate is between 10:1 and 2:1.

8. A process as claimed in claim 7 wherein the ratio of ferric ion-oitrate is about 3:1.

9. A process as claimed in any one of claims 3 to 8 wherein the pH is maintained in the range 2.2 to 3.5.

10. A process as claimed in any preceding claim wherein the cathode current density is in the range 10 to 100 mA/cm2.

11. A process as claimed in any preceding claim wherein sodium thiosulphate, thiourea or naphthalene disuiphonate disodium salt is incorporated in the electrolyte, in such an amount that the active mass produced contains 0.01 to 0.5% S by weight.

12. An electrode consisting of a nickel foil bearing iron active mass consisting of iron and iron oxide and/or hydroxide produced by a process as claimed in any one of claims 1 to 11.