GB2385332A

GB2385332A - A Treatment for Stainless Steel Plates for use in Electrochemical Cells

Info

Publication number: GB2385332A
Application number: GB0203328A
Authority: GB
Inventors: David Ronald Hodgson; Emma Farndon
Original assignee: Ineos Chlor Ltd
Current assignee: Inovyn Chlorvinyls Ltd
Priority date: 2002-02-13
Filing date: 2002-02-13
Publication date: 2003-08-20
Also published as: GB0203328D0

Abstract

A method of manufacturing an electrochemical cell assembly in which at least one component which is exposed to the chemical environment during operation of the cell comprises a stainless steel component. The method comprising treating a surface of the stainless steel, prior to incorporation in the assembly, with an electrical current while contacted to an electrolyte. The viscosity of the electrolyte is selected in conjunction with other conditions to secure a reduction in the interfacial resistance associated with said surface. A fuel cell plate formed with fluid flow channels, is also treated in this way and a fuel cell stack with said treated plates. This stack may comprise: a plurality of fuel cell units, each of which contains a proton-exchange membrane separating the cell into anolyte and catholyte chambers and is provided with an anode and a cathode on opposite sides thereof: a separator plate, flow field plate or bipolar plate disposed between adjacent cell units; current collecting means comprising a pair of plates located one at each end of the stack; means for feeding the fuel to the anolyte chambers of the stack; and means for feeding an oxygen-containing gas to the catholyte chambers to the stack.

Description

- 1 SUBSTRATE TREATMENT

This invention relates to substrate treatment, particularly treatment of s stainless steel components to be incorporated in electrochemical cells. Although the invention will be described in the context of components for fuel cells, it is to be understood that the invention also finds application in other types of electrochemical cells, for example electrolytic cells as used for organic synthesis of chemicals and for production of chlorine.

A fuel cell is an electrochemical device in which electricity is produced without combustion of fossil fuel. In a fuel cell a fuel, which is typically hydrogen, is oxidised at a fuel electrode (anode) and oxygen, typically from air, is reduced at a cathode to produce an electric current and form byproduct water. An electrolyte is required which 5 is in contact with both electrodes and which may be alkaline or acidic, liquid or solid. Heat and water are the only by-products ofthe electrochemical reaction in fuel cells wherein the i,, fuel is hydrogen. Accordingly, the use of such cells in power generation offers potential environmental benefits compared with power generation from combustion of fossil fuels - or by nuclear activity.

,., In proton-exchange membrane fuel cells, hereinafter referred to for convenience as "PEM" fuel cells, the electrolyte is a solid polymer membrane which allows transport of protons from the anode to the cathode and is typically based on perfluorosulphonic acid materials. The electrolyte must be maintained in a hydrated form 25 during operation in order to prevent loss of ionic conduction through the electrolyte.

A PEM fuel cell typically comprises two electrodes, an anode and a cathode, separated by a proton-exchange membrane electrolyte. At the anode, hydrogen fuel

- 2 catalytically dissociates into free electrons and protons. The free electrons are conducted in the boron of usable electric current through the external circuit with which the fuel cell is in electrical contact. The protons migrate through the membrane electrolyte to the cathode where they combine with oxygen from the air and electrons from the external 5 circuit to form water and generate heat. Individual fuel cells may be combined into assemblies which are often referred to in the art as stacks to provide the amount of power required. Electrochemical cells, such as fuel cells, often incorporate stainless steel lo components where possible for reasons of economy. In a fuel cell stack for instance, separator plates between adjacent fuel cells and/or current collecting end plates may comprise stainless steel. In electrolytical cells as used for example in the generation of chlorine, electrodes of the cell may comprise a stainless steel substrate.

15 An important factor in securing good cell efficiency is the interracial c resistance between the surfaces of stainless steel substrates and the surfaces of other en.; components to which the steel substrates are directly or indirectly electroconductively coupled. À 20 While the use of metal plates in fuel cells is seen as having many advantages .. - over other materials, there are concerns over the issue of corrosion which can lead to increased cell resistances. Maligns et al (J. Power Sources, 86 (2000) 274) for instance has reported that the anode side of the fuel cell (i.e. the hydrogen side) containing stainless bipolar plates, gives rise to a greater corrosion of the plate than the cathode (i.e. air) side.

25 Makkus et al also refers to metal passivation resulting in increased electrical as well as corrosion resistance.

In our prior International Patent Application No. PCT/GBO1/033 19, there is disclosed a method of manufacturing an electrochemical cell assembly in which at least one component, e.g. a plate, which is exposed to the chemical environment during operation ofthe cell comprises a stainless steel, said method comprising treating a surface 5 of the stainless steel, prior to incorporation in the assembly, with an electrical current while contacted by an electrolyte, preferably an acidic electrolyte, under conditions which reduce the interracial resistance associated with said surface.

The present invention seeks to provide modifications and improvements in 0 the method disclosed in said prior International Patent Application No. PCT/GBO1/033 19 (the entire disclosure of which is incorporated herein by this reference).

According to one aspect ofthe present invention there is provided a method of manufacturing an electrochemical cell assembly in which at least one component, e.g. 15 a plate, which is exposed to the chemical environment during operation of the cell comprises a stainless steel, said method comprising treating a surface of the stainless steel, prior to incorporation in the assembly, with an electrical current while contacted by an electrolyte, preferably an acidic electrolyte, the viscosity of which is selected in -i conjunction with other conditions to secure a reduction in the interracial resistance (:, i 20 associated with said surface.

The treatment is typically carried out with the stainless steel as the anode (anodic treatment) using do current.

-4 - A feature of the invention is that it is possible to use a relatively inexpensive stainless steel. We have surprisingly found that it is possible to reduce the interracial resistance associated with such a stainless steel by a process which would normally be expected to increase it. Conventional wisdom leads to an expectation that subjecting a 5 stainless steel surface to anodic treatment would normally result in the growth of oxides on the surface and hence result in an increased interracial resistance. We have found that controlling the viscosity of the electrolyte can play a key role in ensuring a reduction in interracial resistance.

lo In this specification, interracial resistance is measured by the method as

described hereinbelow.

Control of viscosity may be effected by the addition of a viscosityenhancing agent but we do not exclude the possibility of controlling the concentration of the acid ... 0.

- ^;'15 component of the electrolyte, particularly when the acid component is one, such as phosphoric acid, having significant viscosity compared with for example sulphuric acid.

The viscosity enhancing agent may be organic. Typically the viscosity enhancing agent is a polyol, ea. a glycol such as polyethylene glycol, or a glycerol. The use 20 of a viscosity enhancing agent has been found to be particularly advantageous when used in conjunction with an electrolyte containing sulphuric acid, e.g. an electrolyte in which sulphuric acid is the main or only acidic component present.

Reduction in interracial resistance may be obtained by subjecting the stainless 25 steel to an electrical potential or current of substantially constant or variable magnitude.

c _ A _ For example, during at least major part of the treatment, the current density or applied voltage may be maintained substantially constant.

The treatment of the stainless steel surface may involve modification of the 5 surface composition of the stainless steel and/or the surface morphology thereof.

The treatment may be such that the ratio of iron content to the chromium within the surface region of the stainless steel is reduced compared with the iron to chromium ratio prevailing prior to such treatment; the reduction in the iron to chromium lo ratio may be to such an extent that prior to treatment the chromium content at the surface of the stainless steel is less than the iron content and in which following treatment the chromium content at the surface of the stainless steel exceeds the iron content.

4 ' " (; Prior to such treatment, the surface or surfaces of the stainless steel may be 15 roughened by physical techniques which are known in the art, e.g. by grit blasting.

The treatment may be applied to two or more surfaces of the stainless steel.

. The same treatment may be applied at least to each major surface of the stainless steel or the treatment applied to one surface of the stainless steel may be different from that applied 20 to one or more other surfaces of the stainless steel...DTD: The cell assembly may comprise two or more stainless steel components, e. g. plates, treated as aforesaid.

- 6 The assembly may include bipolar plates, separator plates, flow held plates and/or current collecting plates, at least one of which comprises stainless steel treated as aforesaid. 5 The acid(s) present in the electrolyte may be selected from one or more of the group comprising sulphuric acid, a halogenic acid such as hydrochloric acid, nitric acid, chromic acid, oxalic acid and phosphoric acid. The acid component or primary acid component of the electrolyte will usually be one which is non-halogenic. Good results have been obtained using sulphuric acid as the acid component or primary acid component of lo the electrolyte.

The treatment may be carried out at ambient temperature (e.g. with the electrolyte initially at substantially room temperature). Nevertheless, we do not exclude the possibility of the treatment being carried out at elevated temperatures. Usually, however, - 15 the temperature at least initially will not be greater than about 100 C, typically not greater than about 90 C and more usually not greater than about 70 C.

- The electrolyte may typically have a pH of up to about 6 or less, e.g. less than - 5. The treatment may be such that the interracial resistance of the stainless steel is reduced by a factor of at least 5%, preferably at least 10% and more preferably at least 15%, less than would otherwise prevail in the case ofthe untreated surface.

Typically said factor is at least 25%, e.g. at least 40% or even at least 50% (ea. at least 75%), less than would otherwise prevail would otherwise prevail in the case of the untreated surface.

5 The so treated surface ofthe stainless steel maybe coated with an electrically conductive material following treatment to reduce its interracial resistance, e.g. with a coating of titanium nitride or chromium nitride or an electrocatalytically active material.

Examples of the electrocatalytical coating treatment that may be applied are 0 disclosed in our prior International Patent Application No. WO 00/22689, the entire contents of which are where the context admits incorporated herein by this reference. For example, the electrocatalytically-active material may be selected fom the group comprising: one or more platinum group metals or oxides thereof, cerium or an oxide thereof; ruthenium or an oxide thereof; ruthenium oxide and a non-noble metal oxide; ;..- i 15 mixtures of RuO2 with at least one of TiO2, SnO2, IrO2, PtO, Sb2O3, Ta2Os, PdO, CeO2, CO304 . -a; The stainless steel is preferably an austenitic stainless steel although we do - not exclude other types such as ferritic.

The stainless steel may be a 300 series stainless steel such as 304, 316 or 316L stainless steel. Other stainless steels such as 400 series stainless steels are not excluded.

- 8 The stainless steel may be one with a high nickel content, e.g. one having a nickel content of at least 15% by weight, e.g. at least 17% by weight and even as much as 20% or more by weight (e.g. 904 stainless steel).

s The treatment may be carried out while the stainless steel is in the form of a sheet, the treated sheet subsequently being divided to form a number of plates for incorporation in one or more electrochemical cell assemblies.

Alternatively, the stainless steel may initially be in the form of a sheet which lo is then divided to form a number of plates before the treatment is applied to the individual plates. Fluid flow channels may be formed in the sheet before such division into plates is effected. Alternatively the fluid flow channels may be formed after division has 5 been effected.

The electrochemical cell may comprise a fuel cell, e.g. a PEM fuel cell, an alkaline fuel cell, a phosphoric acid fuel cell, a direct methanol fuel cell, a molten carbonate fuel cell or a solid oxide fuel cell.

According to another aspect of the present invention there is provided a filet cell stack comprising: a) a plurality of fuel cell units each of which contains a proton exchange membrane separating the cell into anolyte and catholyte chambers and provided with an anode and a cathode on opposite sides thereof; b) a separator plate, flow field plate

25 or bipolar plate disposed between adjacent cell units; c) currentcollecting means

- 9 - comprising a pair of plates located one at each end of the stack; d) means for feeding fuel, e.g. hydrogen or methanol, to the anolyte chambers of the stack; and e) means for feeding an oxygen-containing gas to the catholyte chambers ofthe stack; at least one of said plates being a stainless steel plate, e.g. a 300 series stainless steel such as a 304, 316 or 316L or 5 a 400 series stainless steel, a surface of which has been treated with an electrical current while contacted by an acidic electrolyte containing a viscosity enhancing agent such as polyethylene glycol or glycerol under conditions which reduce the interracial resistance associated with said surface.

0 The electrolyte may be one in which sulphuric acid is present, typically as the main or only acidic component present.

The invention will now be described further with reference to the tabulated examples and the following description.

:;, G it' r: it. ": Interfacial resistance as referred to in this specification is measured using the

measurement system illustrated in, and the procedure described with reference to, Figure : 1 of International Patent Application No. PCT/GB01/03319 to which reference should be made for further details.

Our prior International Patent Application No. PCT/GB01/03319 represents typical plots (see Figure 2 thereof) for untreated stainless steel plate and a treated stainless steel plate and similar plots may be obtained when, in accordance with one example of the present invention, a stainless steel plate is subjected to anodic treatment involving a current 25 density of 25 mA.cm-2 for 30 minutes with the plate irunersed in an aqueous electrolyte

- 10 of 0.5M sulphuric acid and a viscosity enhancing agent such as PEG, initially at room temperature. Typically stainless steel plates to be treated for use in an electrochemical cell s in accordance with the invention are subjected to the following treatment steps. The stainless steel workplace is degreased using a suitable solvent (e.g. acetone, isopropyl alcohol, trichloroethylene, caustic agents etc.) and immersed in a bath of electrolyte having a predetermined viscosity. The electrolyte is acid based containing one or more of the following: sulphuric acid, hydrochloric acid, nitric acid, chromic acid, oxalic acid and to phosphoric acid. Using appropriate counter electrodes (e.g. platinised Ti), a do current is passed between the workplace (as the anode) and the counter electrodes (as the cathode).

This current is calculated as a current density and may be from 0.01 mA/cm2 to SOO mA/cm2, preferably in the range 1 to 50 mA/cm2. The current is controlled at the required level for a time between 0.5 and 180 minutes, preferably between 1 and 60 minutes and 15 more preferably between 2 and 7 minutes. At the start of the anodic treatment, the electrolyte is at room temperature (and may increases during the course of the treatment).

The particular conditions necessary to secure a reduction in interracial resistance while maintaining the corrosion resistance properties of the stainless steel substantially unchanged or enhancing the same may be determined experimentally.

. in;,.. The workplace is removed and rinsed in deionised water or deionised water made slightly alkaline to remove excess acid, and dried, e.g. in air or by passing a warm air current over the workpiece. The workplace is then installed as a bipolar plate, separator plate, flow field plate and/or a current collecting plate in the electrochemical cell, e.g. a

25 fuel cell.

- Optimised conditions, including viscosity of the electrolyte, appropriate for anodic treatment of the stainless steel in order to reduce interracial resistance may be established for a given electrolyte and temperature by configuring the sample of the stainless steel to be treated as the anode in a bath of electrolyte, establishing a potential 5 difference between the anode and a counter electrode (as cathode) so as to pass a dc current of substantially constant current density through the electrolyte between the anode and electrode and using a range of current densities and treatment times. From measurements of the interracial resistance, the corrosion potential Ecorr and corrosion current Icorr, a suitable operating regime can be determined. In practice, a suitable current 0 density can be established by conducting experiments to determine current densities at which the interracial resistance (as measured using the technique described above) is significantly reduced. For 316 stainless steel for instance, we have found that a marked reduction in interracial resistance can be achieved using current densities of 20 mA.cm 2 and upwards. For efficient energy usage, it is of course desirable to carry out the treatment 15 at as low a current density as can be achieved while securing a significant reduction in - interracial resistance.

. For a given electrolyte, viscosity and temperature, a suitable treatment time may be established by reference to graphical representations similar to those illustrated in . 20 Figure 3 of our prior International Patent Application No. PCT/GB01/033 19.

......DTD: ./ Table 1 below gives details of experimental work carried out to show the effect of viscosity control. The experimental work was carried out in an electrochemical 25 cell using the stated electrolyte with S x S cm stainless steel anodes at a current density of 25 mA cm2 for 6 min. Contact resistance measurements were made using the method specified in International Patent Application No. PCTMB01/03319. The method for determining corrosion potentials and currents was the same as that specified in

- 12 International Patent Application No. PCT/GBO1/03319 except that a potential scan rate of lmV so was used.

The contact resistance and corrosion potential of the particular stainless 316 s used, after degreasing but prior to anodic treatment, was determined to be 60 my cm 200 N cm2 and -280 mV vs SCE respectively. The corrosion potentials of samples anodised in all the electrolytes showed improvement over untreated stainless steel.

Table 1

Electrolyte Viscosity Ecorr Icorr centistokes (mohm.cm2' (mV vs (A.c2) H2SO4 (GEM) - 1.124 16.5 +67 4.7 x 10-' it. no additive ; 15 14.5% PEG in 2.596 12.6 +74 6.7 x 10-7 H2SO4 (O.SM)

10% glycerol in 1.681 35.8 -64 1.3 x 10-6 O.5M H2SO4

20% glycerol in N/A 11 -61 3.6 x 10-6 20 O.5M H2SO4

30% glycerol in N/A 15.3 0 1.25 x 10-6 O.5M H2SO4

Viscosity was determined by timing the flow of liquid through a 25 capillary viscometer at a controlled temperature. The method is described in BS188:1957 Determination of Viscocity of Liquids.

- 13 From Table 1, it will be seen that viscosity adjustment has an effect on interracial resistance and Ear. In all cases, the interracial resistance is significantly lower than the degreased but untreated stainless steel. The experiments involving the use of 5 glycerol indicate that the amount of additive employed should be optimised to obtain the desired effect As previously mentioned, the use of metal bipolar plates in fuel cells is seen as having many advantages over other materials, for instance: thin plates leading to low 0 volume stacks; ease of pressing a flow field into the plate; and low cost metals and alloys

are readily available in large quantities. One concern over the use of metals plates however is the issue of corrosion which can result in metal ions from the metal plates blocking active sites in the membrane in a PEM type fuel cell leading to increased cell resistances.

Makkus et al (J. Power Sources, 86 (2000) 274) reported that the anode side of the fuel I- 15 cell (i.e. the hydrogen side) containing metal bipolar plates, gives rise to a greater corrosion of the plate than the cathode (i.e. air) side. Maldives et al also refers to passivation resulting in increased electrical as well as corrosion resistance.

The anodic treatment of the stainless steel in accordance with the present 20 invention involves enrichment of the surface layer of the stainless steel in such a way that the iron to chromium content in the surface region of the treated steel (as measured using X ray photoelectron spectroscopy) is reduced in the manner described with reference to Figure 4 of our prior International Patent Application No. PCT/GB01/03319 to which reference should be made for further details.

- 14 In a typical production process, a number of sheets of stainless steel, e.g. 316 or 316L stainless steel, each has sets of features introduced into one or both major surfaces (e.g. by etching or pressing) in such a way that each set of the features serve as flow distribution channels for fluids in an electrochemical cell assembly such as a fuel cell 5 stack. The fluids may be (but are not limited to) hydrogen, air, oxygen, water and/or methanol. An example of such a flow pattern is shown in Figure 7 of prior International Patent Application No. PCT/GB01/033 19. Once the sheet has been treated in accordance with the invention, it can be cut into smaller pieces each forming a plate provided with a set of fluid flow channels on one or each major face, each plate being dimensioned and 0 configured for use in the fuel cell. In accordance with the invention, the stainless steel sheet is treated in such a way as to enhance the surface conductivity, preferably without reducing the corrosion resistance and, in some cases, increasing the corrosion resistance of the metal.

15 The treatment process usually comprises the following steps: i; 1. The sheet to be treated is first 'cleaned' using a degreasing solvent such as acetone, iso-propanol, trichloroethylene, or an alkaline based aqueous system.

2. The sheet is introduced into an aqueous treatment bath containing sulphuric acid, e.g. 1 mol dm3, and a viscosity enhancing agent such as a glycol (e.g. PEG) at room 20 temperature or other desired temperature. The treatment bath is equipped with counter electrodes made of suitable material (e.g. titanium coated with iridium dioxide or platinum) which may be in sheet or mesh form and the sheet to be treated is located between the two counter electrodes.

3. Electrical connections are made with both the stainless steel sheet workplace and to 25 the counter electrodes and a substantially constant do current is passed between the counter electrodes and the workpiece. The current density is usually between 1 and 100

- 15 mA cm2 and is applied for a suitable time (usually between 1 minute to 2 hours), the current density and the time of application being determined experimentally in the light of treating samples of the stainless steel in the manner described previously.

4. After the current has been applied for the required time, the workpiece is s disconnected from the electrical circuitry, removed from the bath and excess acid removed for example by suspending the workpiece over the bath for a time to allow excess acid to drain from the surface into the bath.

S. The treated sheet is then transported to a rinse bath containing deionised water, or deionised water made slightly alkaline with sodium carbonate or sodium hydroxide and lo after rinsing the sheet is dried in a clean environment. Drying may be performed by natural evaporation or by circulation of air at room or elevated temperature.

6.The dried sheet is cut into smaller plates compatible in size with the dimensions and configuration ofthe fuel stack into which the plates are to be incorporated as flow field

plates and/or bipolar plates for instance.

- In an alternative method, instead of dividing the stainless steel sheet into the individual plates after the treatment process, after being formed with the sets of fluid flow channel features, the stainless steel sheet may instead initially be separated (e.g. by cutting - or etching) into plates of suitable dimensions and configuration for incorporation in the 20 electrochemical cell. The individual plates so formed can then be individually treated in - accordance with steps 1 to 5 above.

In another variation, the opposite faces of the stainless steel sheet or individual plates may be subjected to the treatment to differing extents. For example, each face may 25 be treated with regard to the conditions to which it will be exposed in operation of the electrochemical cell. For example, in the case of a bipolar plate for a fuel cell, one face

- 16 might be exposed to a fluid comprising hydrogen or methanol (the anode side) and the opposite face may be exposed to a fluid comprising oxygen (the cathode side). By treating the two faces differently, e.g. for different lengths of time, each face may be optimised according to the conditions that it will be exposed to in operation ofthe fuel cell. Thus, the 5 above process may be modified at step 4 by terminating current flow between one face and the associated counter electrode after a first predetermined time interval appropriate for that face (e.g. the hydrogen or anode side). At that point, that side is disconnected and isolated whilst current flow between the opposite face and its associated counter electrode is continued until lapse of a second predetermined time interval.

One application in which stainless steel plates treated in accordance with the present invention may be used is described in Figure 7 of prior International Patent Application No. PCT/GB01/033 19, the disclosure of which is incorporated herein by this reference.

:,;

Claims

- 17 CLAIMS

1. A method of manufacturing an electrochemical cell assembly in which at least one component which is exposed to the chemical environment during operation of the cell 5 comprises a stainless steel component, said method comprising treating a surface of the stainless steel, prior to incorporation in the assembly, with an electrical current while contacted by an electrolyte, the viscosity of which is selected in conjunction with other conditions to secure a reduction in the interracial resistance associated with said surface.

lo 2. A method as claimed in Claim 1 in which a viscosity enhancing agent is

incorporated in the electrolyte.

3. A method as claimed in Claim 1 or 2 in which the treatment is effective to reduce the ratio of iron to chromium content within the surface region of the stainless steel 15 compared with the iron to chromium ratio prevailing prior to such treatment, e.g. such that prior to treatment the chromium content at the surface ofthe stainless steel is less than the iron content and following treatment the chromium content at the surface of the stainless . À steel exceeds the iron content.

20 4. A method as claimed in Claim 1, 2 or 3 in which the current density to which the stainless steel surface is subjected during at least a major part of said treatment is substantially constant.

5. A method as claimed in any one of Claims 1 to 3 in which the stainless steel is an 25 austenitic steel.

- 18 6. A method as claimed in any one of Claims 1 to 4 in which the stainless steel comprises a 300 or 400 series stainless steel, e.g. a 316 stainless steel.

5 7. A method as claimed in any one of Claims 1 to 4 in which the substrate comprises a 316L stainless steel.

8. A method as claimed in any one of Claims 1 to 4 in which the stainless steel is one in which nickel is present in an amount of at least 15% by weight.

9. A method as claimed in any one of Claims 1 to 4 in which the stainless steel is one in which nickel is present in an amount of at least 17% by weight.

- 10. A method as claimed in any one of Claims 1 to 4 in which the stainless steel is one 5 in which nickel is present in an amount of at least 18% by weight.

11. A method as claimed in any one of Claims 1 to 4 in which the stainless steel is one in which nickel is present in an amount of at least 20% by weight.

20 12. A method as claimed in any one of the preceding claims in which the electrolyte includes at least one acid selected from the group comprising sulphuric acid, a halogenic acid, nitric acid, chromic acid, oxalic acid and phosphoric acid.

- 19 13. A method as claimed in any one of Claims 1 to 11 in which the acid comprises sulphuric acid.

14. A method as claimed in Claim 12 or 13 in which the interfacial resistance of the stainless steel is reduced by a factor of at least 5% compared with the interfacial resistance prevailing prior to said treatment.

15. A method as claimed in Claim 12 or 13 in which the interfacial resistance of the stainless steel is reduced by a factor of at least 15% compared with the interfacial lo resistance prevailing prior to said treatment.

16. A method as claimed in Claim 12 or 13 in which the interfacial resistance of the stainless steel is reduced by a factor of at least 25% compared with the interfacial resistance prevailing prior to said treatment.

,. . .. 17. A method as claimed in Claim 12 or 13 in which the interfacial resistance of the stainless steel is reduced by a factor of at least 40%, ea. at least 50%, compared with the :: interfacial resistance prevailing prior to said treatment.

20 18. A method as claimed in any one of the preceding claims in which the treated surface of the stainless steel is coated with an electrically conductivity enhancing material following said treatment and prior to incorporation in the electrochemical cell assembly.

-.,,

- 20 19. A method as claimed in any one of Claims 1 to 17 in which the treated surface of the stainless steel is coated with titanium nitride, chromium nitride or an electrocatalytically active material.

5 20. A method as claimed in any one of the preceding claims in which two or more surfaces of the stainless steel are so treated.

21. A method as claimed in Claim 20 in which the same treatment is applied at least to each major surface of the stainless steel.

22. A method as claimed in Claim 20 or 21 in which the treatment applied to one surface ofthe stainless steel is different from that applied to one or more other surfaces of the stainless steel.

ma, c ,15 23. A method as claimed in any one of the preceding claims in which the viscosity of c the electrolyte is controlled by the incorporation therein of an organic viscosity enhancing .. i< agents e.g. a polyol such as polyethylene glycol or glycerol.

t:. C . O . TV 24. A method as claimed in any one of the preceding claims in which the cell assembly 20 comprises two or more stainless steel components treated as aforesaid.

25. A method as claimed in any one of the preceding claims, the assembly comprising bipolar plates, separator plates, flow field plates and/or current collecting plates, at least

one of which comprises stainless steel plate treated as aforesaid.

- 21 26. A method as claimed in any one of the preceding claims in which the treatment is carried out using a current density in the range from about 1 to about 100 mA.cm2.

27. A method as claimed in any one of the preceding claims in which the treatment is s carried out using a current density of at least about 20 mA.cm2.

28. A method as claimed in any one of the preceding claims in which the treatment is carried out using a current density of up to about SO mA.cm2.

lo 29. A method as claimed in any one of the preceding claims in which the treatment is carried out for a time interval of between 0.5 and 180 minutes.

30. A method as claimed in any one of the preceding claims in which the treatment is I:, carried out for a time interval of at least about 1 minute, preferably at least about 2 5 minutes. L e c 31. A method as claimed in any one of the preceding claims in which the treatment is carried out for a time interval of up to about 60 minutes, e.g. up to about 10 minutes.

. 20 32. A method as claimed in any one of the preceding claims in which the treatment is carried out for a time interval of between about 3 and about 7 minutes.

- 22 33. A method as claimed in any one of the preceding claims in which the treatment is carried out while the stainless steel is in the form of a sheet and in which the treated sheet is subsequently divided to form a number of plates for incorporation in one or more electrochemical cell assemblies.

34. A method as claimed in any one of Claims 1 to 33 in which the stainless steel is initially in the form of a sheet and in which the sheet is divided to form a number of plates before the treatment is applied to the individual plates.

lo 35. A method as claimed in Claim 33 or 34 in which fluid flow channels are formed in the sheet before such division into plates is effected.

36. A method of manufacturing an electrochemical cell assembly in which at least one plate which is exposed to the chemical environment during operation ofthe cell comprises - 15 a stainless steel plate, optionally of 316 or 316L stainless steel, said method comprising treating a surface of the stainless steel, prior to incorporation in the assembly, with an electrical current having a current density in the range of about 20 mA. cm2 to about 100 mA.cm2, preferably in the range from about 20 mA.cm2 to about 50 mA.cm2, for a time interval of about 1 to about 10 minutes, preferably from about 3 to about 7 minutes, while 20 in contact with an electrolyte containing sulphuric acid and an organic viscosity enhancing agent under conditions which reduce the interracial resistance associated with said surface and reduce the ratio of iron content to chromium content at the surface of the stainless steel, the stainless steel thereafter being incorporated in the cell assembly in the form of a plate.

- 23 37. A method as claimed in Claim 36 in which the treatment results in an increase in the ratio of chromium content to the iron content at the surface of the stainless steel, ea.

such that, prior to treatment, the chromium content at the surface is less than the iron content and, following treatment, the chromium content at the surface of the stainless steel 5 exceeds the iron content.

38. An electrochemical cell assembly manufactured by the method of any one of Claims 1 to 37.

lo 39. A feel cell assembly manufactured by the method of any one of Claims 1 to 37.

40. A fuel cell assembly as claimed in Claim 39, being a PEM fuel cell, an alkaline fuel cell, a phosphoric acid fuel cell, a direct methanol fuel cell, a molten carbonate fuel cell or a solid oxide fuel cell.

41. An electrochemical cell including bipolar plates, separator plates, flow field plates

and/or current collecting plates, at least one of which is constituted by a plate of stainless r A; steel treated using the method of any one of Claims 1 to 37.

20 42. A fuel cell plate formed with fluid flow channels, the plate comprising a stainless steel which, prior to incorporation in the fuel cell, has been treated with an electrical current while in contact with an electrolyte containing a viscosity-modifying additive to reduce the interracial resistance associated with the surface.

- 24 43. A fuel cell stack comprising: a) a plurality of fuel cell units each of which contains a proton-exchange membrane separating the cell into anolyte and catholyte chambers and provided with an anode and a cathode on opposite sides thereof; 5 b) a separator plate, flow field plate or bipolar plate disposed between adjacent cell units;

c) current-collecting means comprising a pair of plates located one at each end ofthe stack; d) means for feeding fuel to the anolyte chambers of the stack; and e) means for feeding an oxygen-containing gas to the catholyte chambers of the stack; at least one of said plates being a stainless steel plate a surface of which has been treated lo with an electrical current while contacted by an acidic electrolyte, the viscosity of which is selected in conjunction with other conditions to secure a reduction in the interracial resistance associated with said surface.

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