Electrochemical cell

Abstract: The invention relates to an electrochemical cell comprising a negative electrode (6) of a stable hydride-forming material, in which a large loading capacity at low temperature is obtained by the addition of a catalytic material of the type DE₃, wherein D is one or more of the elements selected from Or, Mo and W, and wherein E represents Ni and/or Co. The catalytic material can be provided in the form of a eutectic segregation or it can be provided at the surface of powdered hydride-forming material by means of an electrochemical reaction.

Description: The invention relates to an electrochemical cell comprising a negative electrode whose electrochemically active material consists of an intermetallic compound which forms a hydride with hydrogen and which has the CaCu₅-structure, and of a catalytic material at the surface of which hydrogen exhibits a large electrochemical activity. The invention also relates to an electrochemically active material for use in such a cell and to a method of manufacturing such a material. The electrochemical cell may be in open communication with the atmosphere or may be sealed from the atmosphere. A cell which is sealed from the atmosphere may comprise a valve which is dimensioned such that it becomes operative at a predetermined pressure. In a rechargeable cell of the sealed type, the electrochemically active part of the positive electrode consists of, for example, nickel hydroxide, silver oxide or manganese oxide, nickel hydroxide being generally preferred for practical reasons. An electrolyte is used in the cell, which generally consists of an aqueous solution of one or more alkali hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, having a pH in excess of 7. The cell may further comprise a separator which separates the electrodes electrically, but permits transport of ions and gas. The separator may consist of synthetic resin fibres (woven or non woven), for example, of polyamide fibres or polypropylene fibres. Such an electrochemical cell is described in European Patent Application EP-A1-0251384. In said Application, one or more of the metals Pd, Pt, Ir and Rh are added to the electrochemically active material to improve the loadability at low temperature and the activation speed of the cell. A disadvantage of the known cell is that it requires the use of a relatively large quantity of a noble metal. When the noble metal is used in the form of a monolayer on the grains of the hydride-forming material, as is also described in EP-A1-0251384, a smaller quantity of the noble metal is required, but in this case, however, a labour-intensive electrochemical treatment is necessary to apply the monolayer. Moreover, for some applications it is desirable to further increase the loadability of the cell, in particular at low temperatures. One of the objects of the invention is to provide an electrochemical cell and an electrochemically active material having a large loadability also at a low temperature, for use in such a cell. The aim of the invention is to achieve this by adding a material which has a catalytic effect on the formation and conversion of atomic hydrogen. The capacity of the cell to be manufactured should be as large as possible while preserving the other favourable properties of the cell. For this reason, it is desirable to achieve said purpose with the smallest possible quantity of the additive if the catalytic material is of a type in which no hydrogen is stored. A further object of the invention is to provide an electrochemical cell and an electrochemically active material which can meet the requirements without using noble metals. An additional object of the invention is to provide a method of manufacturing the electrochemically active material in which the catalytic material is optimally distributed. These objects are achieved according to the invention by an electrochemical cell and an electrochemically active material as described in the opening paragraph, which are characterized in that the catalytic material is of the type DE₃, wherein D is one or more of the elements selected from Mo and W, and wherein E is one or more of the elements selected from Ni and Co. In a preferred embodiment of the electrochemical cell according to the invention, the catalytic material comprises the compound Mo(Co, Ni)₃, wherein Co and Ni may be present separately or together in any ratio. The catalytic material is very effective, both in the electrochemical cell and in the electrochemically active material, if it is located on the grain boundaries of the intermetallic compound having the CaCu₅-structure. This is the case, in particular, when the average crystallite size of the intermetallic compound is less than 30 µm. The use of the catalytic material is very advantageous with stable hydride-forming materials as described in, for example, United States Patent Specification US 4487817. In said Specification, the intermetallic compound having the CaCu₅-structure is of the type having the compositional formula: AB m C n , wherein m + n ranges between 4.8 and 5.4, wherein n has a value from 0 to 0.6, wherein A consists of Mischmetall or of one or more elements selected from the group consisting of Y, Ti, Hf, Zr, Ca, Th, La and the other rare earth metals, wherein B consists of two or more elements selected from the group consisting of Ni, Co, Cu, Fe and Mn, and wherein C consists of one or more elements selected from the group consisting of Al, Cr and Si. To obtain a compound within the existence range of the intermetallic compound having the CaCu₅-structure, the overall atomic quantity of the elements Y, Ti, Hf and Zr preferably amounts to less than 40% of A. In order to obtain an electrochemically stable material, the maximum atomic quantity per gram atom of A is for Ni : 3.5, for Co : 3.5, for Cu : 3.5, for Fe : 2.0 and for Mn : 1.0. In order to obtain a corrosion-resistant material, n preferably has a value of at least 0.05. In this case, C consists of one or more of the elements selected from the group consisting of Al, Cr and Si in the atomic quantities indicated, Al : 0.05-0.6, Cr : 0.05-0.5 and Si : 0.05-0.5. According to the invention, the object of providing a method of manufacturing an electrochemical material is achieved by a method in which the intermetallic compound having the CaCu₅-structure is ground to obtain a powder, after which the grains thus formed are provided with a layer of the catalytic material by a galvanic, a currentless deposition or an exchange method. Electrochemical methods are to be understood to include a galvanic method, a currentless deposition method and an exchange method. A particular advantage of this method is that it permits a maximum quantity of catalytic material to be applied to the powder surface. In an optimum situation, the layer of the catalytic material is not thicker than a monolayer, so that the overall quantity of the hydride-forming intermetallic compound is as large as possible and the hydrogen can diffuse to the intermetallic compound via a short path. A simple and little labour-intensive alternative method of manufacturing an electrochemical material, in which the catalytic material is located on the grain boundaries of the intermetallic compound having the CaCu₅-structure is characterized according to the invention in that a non-stoichiometric melt is made of the elements A, B and C, the composition being selected such that it is outside the existence range of the compound AB m C n , and a part of the elements B being replaced by one or more elements D, the melt being cooled while forming a two-phase material consisting of the compound having the CaCu₅-structure and of the catalytic material. In this case, an excess of the elements B is used for the quantity of element E which is necessary for the catalytic material DE₃. This means that in the preferred compositions indicated the composition of the melt corresponds to the compositional formula AB p C n , wherein p + n is larger than 5.4. In the above-described stable hydride-forming materials which are known per se , an increased corrosion resistance and electrochemical stability is attained (during a large number of charging and discharging cycles) by means of substitutions and additions to the generic LaNi₅-compound, which is partly at the expense of the capacity of the cell to be manufactured. The sometimes adverse consequences hereof can be overcome partly by the larger loadability attained according to the invention. The catalytic activity for the formation of hydrogen can be measured by means of the exchange current density J o which is higher for the noble metals Pt, Pd, Rh and Ir than for Co, Ni, W and Mo, see an article by S. Trasatti in J. Electroanal. Chem. 39 , pages 163 to 184 (1972), in particular Figs. 3 and 4 on pages 173 and 175. Consequently, it is surprising that compounds of Co and/or Ni with W and/or Mo render an at least equally good result, all the more because these compounds, unlike Pt and Pd, do not form hydrides with hydrogen themselves. In an article by M.M. Jaksic in Electrochimica Acta, 29 (11), pages 1539 to 1550 (1984) the catalytic activity of compounds such as MoCo₃, WNi₃ and MoNi₃ is attributed to a synergetic effect. With respect to LaNi₅, it has been established in experiments leading to the invention that the life of the electrochemical cell and the corrosion resistance of the electrochemically active material are insufficient for practical use. The use of said catalytic materials in an electrochemical cell and an electrochemically active material according to the invention has the additional advantage that few new elements are introduced into the overall composition. According to the invention, a small addition of Mo and/or W and an increase of the quantity of Ni and/or Co relative to La and the other rare earth metals is sufficient. The invention will be explained in greater detail by means of exemplary embodiments and with reference to a drawing, in which the only. Figure is a partly sectional and partly elevational view of a sealed rechargeable electrochemical cell according to the invention. Example of a rechargeable cell construction. The cell (as shown in the Figure), which is sealed from the air and which is manufactured by means of a suitable housing 1 of metal, such as stainless steel, having a cover 2 in which apertures are formed for conductors 3 and 4. Said conductors are insulated from the metal housing 1, 2 by means of synthetic resin rings 5. The outside diameter of the house may be, for example, 22 mm and the height may be 41 mm. A roll of a negative electrode 6, a separator 7 and a positive electrode 8 are accommodated in the space inside the housing, the assembly being surrounded by an electrically insulating synthetic resin foil 9 of, for example, polyvinyl chloride, and supported by a disc 10 of an electrically insulating material such as polyvinyl chloride. The negative electrode 6 consists of a hydride-forming intermetallic compound as described hereinbefore and is connected to the conductor 3. The negative electrode 6 is manufactured by fusing suitable quantities of the relevant elements, pulverizing the intermetallic compound thus formed and applying it to a nickel carrier, for example, by means of a polymer binder material such as polyvinyl alcohol. The intermetallic compound can be pulverized, for example by grinding, to a grain size of approximately 40 µm. When the cell is activated by repeated hydrogen adsorption and desorption the grain growth is further reduced to the order of magnitude of 1 to 5 µm. The positive electrode 8 is a nickel hydroxide electrode of the conventional sintered type which is connected to the conductor 4. A 6 mol/l (N) potassium hydroxide solution in water is used as the electrolyte. The electrolyte is absorbed in the separator 7 and is in wetting contact with the electrochemically active material of the two electrodes. The separator 7 is in the form of a non-woven membrane of polyamide fibres. The free gas space in the cell is approximately 5 cm³. A sealed cell of this type has an EMF of between 1.2 and 1.4 V. The cells according to the invention can be assembled in a conventional manner to form batteries comprising, for example, several series-arranged cells. Comparative example A, not according to the invention. An electrochemically active material for the negative electrode, having the composition La 0.8 Nd 0.2 Ni 2.5 CO 2.4 Si 0.1 is prepared by mixing the necessary quantities of the various components, after which they are fused, cooled, ground and pulverized by repeated hydrogen adsorption and desorption. The powder formed has a surface area of approximately 0.25 m²/g. Subsequently, an electrode is formed and incorporated in a cell, for example, as described hereinabove. The loadability of the negative electrode is determined at 25°C by measuring the storage capacity of the cell in charging and discharging cycles at a rate of 2.0 C, i.e. at a rate of charging and discharging which would supply to or extract from the cell 2.0 times the nominal capacity of the cell in 1 hour. The nominal capacity of a cell is the capacity at a low charging and discharging rate. The loadability after 10 charging and discharging cycles is 30% of the maximum value, after 20 cycles it is 90%, after 30 cycles it is 100% and after 300 cycles it is 95%. At 0°C, the loadability after a great number of cycles is only 50% of the value at 25°C. If the discharging rate is increased to 3.0 C, the loadability at 0°C is only 35% of the value at 25°C. The exchange current density J o is measured after more than 100 cycles and at a degree of loading of 15%, and is 190 mA/g. Comparative example B, not according to the invention. An electrochemical cell is manufactured as described in comparative example A, using a composition of the melt according to the formula La 0.8 Nd 0.2 Ni 3.0 Co 2.4 Si 0.1 . This composition is outside the existence range of the intermetallic compound having the CaCu₅-structure. For this reason, a second phase is formed, during cooling, on the grain boundaries in addition to the intermetallic compound. Said second phase consists of Ni and Co, having a composition near NiCo 2.8 . This material, in which no hydrogen is stored, acts catalytically on the formation of hydrogen. When the melt is cooled slowly, crystallites having dimensions of 60 to 100 µm are formed. In the electrode manufactured, the loadability at 3.0 C and 0°C is 50% of the value at 25° C. The exchange current density is 287 mA/g. When the melt is cooled rapidly, crystallites are formed having dimensions of 10 to 30 µm. In this case, the loadability at 3.0 C and 0°C is 70% of the value at 25°C. The exchange current density is 338 mA/g. Comparative example C, not according to the invention. An electrochemical cell is manufactured as described in example A, the composition of the electrochemically active material for the negative electrode being represented by the compositional formula La 0.8 Nd 0.2 Ni 2.5 Co 2.0 Pd 0.4 Si 0.1 . In the cell manufactured, the loadability at 3.0 C and 0°C is more than 90% of the value at 25°C. The exchange current density is approximately 500 mA/g. Example 1. An electrochemical cell is manufactured as described in example A, the composition in the melt corresponding to the formula La 0.8 Nd 0.2 Ni 3.0-x Mo x Co 2.4 Si 0.1 . An optimum composition is attained when x has a value of approximately 0.1. In conformity with example B, a second phase is formed during cooling. The dimensions of these eutectic segregations is in the order of magnitude of 1 to 2 µm, at an average crystallite size of the intermetallic compound having the CaCu₅-structure of 30 µm. The second phase consists of MoCo₃ with a small quantity of Ni. The composition can be represented by the formula Mo(Co, Ni)₃. MoCo₃ and MoNi₃ can be mixed in wide ratios and each form compounds having a small existence range, i.e., the ratio Mo to Co and/or Ni is always fairly accurately 1 : 3. In conformity with example B, the crystallite size can be influenced advantageously as well as the dimension of the eutectic segregations by the choice of the cooling rate. When the material is pulverized by means of repeated hydrogen adsorption and desorption, particles having dimensions of approximately 1 µm are formed. To obtain a proper distribution of the catalytic material in the electrochemically active material it is useful to cool the melt rapidly so that a small average crystallite size is obtained, preferably smaller than 30 µm. In the electrochemical cell manufactured, the loadability at 3.0 C and 0°C is 90% of the value at 25°C. The exchange current density is 585 mA/g. The electrochemically active material which is used according to the present example is sufficiently resistant to corrosion. After more than 100 charging and discharging cycles the capacity of the electrochemical cell was 91% of the maximum value. Example 2. An electrochemical cell is manufactured as described in example A, the composition of the electrochemically active material also being the same. After the melt has cooled, the material is ground to a grain size of approximately 40 µm. By means of electrodeposition a layer of MoNi₃ is formed at the surface. Instead of MoNi₃, WNi₃ can also suitably be used. The results obtained are at least just as good as when Pd-containing materials are used having a Pd content of 0.4 in the compositional formula of the intermetallic compound having the CaCu₅-structure.

Claims: An electrochemical cell comprising a negative electrode whose electrochemically active material consists of an intermetallic compound which forms a hydride with hydrogen and which has the CaCu₅-structure, and of a catalytic material at the surface of which hydrogen exhibits a large electrochemical activity, characterized in that the catalytic material is of the type DE₃, wherein D is one or more of the elements selected from Mo and W, and wherein E is one or more of the elements selected from Ni and Co. An electrochemical cell as claimed in Claim 1, characterized in that the catalytic material comprises the compound Mo(Co, Ni)₃. An electrochemical cell as claimed in Claim 1 or 2, characterized in that the catalytic material is located at the grain boundaries of the intermetallic compound having the CaCu₅-structure. An electrochemically active material consisting of an intermetallic compound which forms a hydride with hydrogen and which has the CaCu₅-structure, and of a catalytic material at the surface of which hydrogen exhibits a large electrochemical activity, characterized in that the catalytic material is of the type DE₃, wherein D is one or more of the elements selected from Mo and W, and wherein E is one or more of the elements selected from Ni and Co. An electrochemically active material as claimed in Claim 4, characterized in that the catalytic material is located on the grain boundaries of the intermetallic compound having the CaCu₅-structure. An electrochemically active material as claimed in Claim 5, characterized in that the average crystallite size of the intermetallic compound is less than 30 µm. An electrochemically active material as claimed in any one of the Claims 4 up to and including 6, characterized in that the intermetallic compound having the CaCu₅-structure is of the type having the compositional formula AB m C n , wherein m + n ranges between 4.8 and 5.4, wherein n has a value of 0 to 0.6, wherein A consists of Mischmetall or of one or more of the elements selected from the group consisting of Y, Ti, Hf, Zr, Ca, Th, La and the other rare earth metals, wherein B consists of two or more elements selected from the group consisting of Ni, Co, Cu, Fe and Mn, and wherein C consists of one or more of the elements selected from the group consisting of Al, Cr and Si. An electrochemically active material as claimed in Claim 7, characterized in that the overall atomic quantity of the elements Y, Ti, Hf Zr is less than 40% of A. An electrochemically active material as claimed in Claim 7 or 8, characterized in that per gram atom of A the maximum atomic quantity is for Ni : 3.5, for Co : 3.5, for Cu : 3.5, for Fe : 2.0 and for Mn : 1.0. An electrochemically active material as claimed in any one of the Claims 7 up to and including 9, characterized in that the value of n is at least 0.05. An electrochemically active material as claimed in Claim 10, characterized in that C consists of one or more of the elements selected from the group consisting of Al, Cr and Si in the indicated atomic quantities Al : 0.05-0.6, Cr : 0.05-0.5 and Si : 0.05-0.5. A method of manufacturing the electrochemically active material of any one of the Claims 4 up to and including 11, characterized in that the intermetallic compound having the CaCu₅-structure is ground to form a powder, after which the grains thus formed are provided with a layer of the catalytic material by a galvanic, a currentless deposition or an exchange method. A method of manufacturing the electrochemically active material of any one of the claims 7 up to and including 11, in which the catalytic material DE₃ is located on the grain boundaries of the intermetallic compound AB m C n having the CaCu₅-structure, characterized in that a non-stoichiometric melt is produced of the elements A, B, C and D, the melt having the compositional formula AB p C n , wherein p+n is larger than 5.4 and a portion of the elements B is replaced by at least one of the elements D, the composition of the melt being selected such that it is outside the existence range of the compound AB m C n , said melt being cooled while forming a two-phase material consisting of the compound having the CaCu₅-structure and the catalytic material DE₃, the element E corresponding to element B.

Inventors: NOTTEN PETRUS HENDRICUS LAUREN, NOTTEN, PETRUS HENDRICUS LAURENTIUS
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01M4
IPC codes: H01M10, B01J23, H01M4