PL218729B1 - Negative electrode for Ni-MH batteries - Google Patents

Description

The present invention relates to a negative electrode for nickel metal hydride batteries.

Nickel metal hydride (Ni-MH) batteries, where the positive electrode is a redox transformable NiOOH / Ni (OH) 2 system and the negative electrode is the M / MH system, are considered to be one of the best sources of electricity for hybrid vehicles (HEVs) and as an alternative source of power for electric vehicles.

Known negative electrodes of nickel metal hydride batteries are made of multi-component metal alloys based on the LaNi5 compound. The materials used for MH electrodes can be generally described by the formula ABx, which denotes the mutual ratio of elements from group A to elements from group B. Most commonly, the AB5 type alloys are used in Ni-MH cells, where the group A most often includes such metals such as lanthanum, cerium, praseodymium and neodymium, while group B includes: nickel, cobalt, manganese and aluminum. Similar properties have non-equilibrium alloys of the A2B7 type, where group A includes: lanthanum, cerium, praseodymium and neodymium, while group B includes: nickel, cobalt, manganese, aluminum and magnesium (MA Fetcenko, SR Ovshinsky, B. Reichman, K Young, C. Fierro, J. Koch, A. Zallen, W. Mays, T. Ouchi, J. Power Sources 165 (2007) 544-551 and Y. Li, D. Han, S. Han, X. Zhu. , t, Hu, Z. Zhang, Y. Liu, Int. J. Hydrogen Energy 34 (2009) 1399-1404). The metal hydride electrode can absorb hydrogen with a volume 1000 times greater than its own volume. Even small changes in the composition of multicomponent alloys, when the stoichiometric ratio of elements or the type of components in groups A and B changes, cause significant changes in the amount of hydrogen absorbed and the stability of hydrides formed (J. Kleperis, G. Wójcik, A. Czerwiński, JMSkowroński M.Kopczyk, M.Bełtowska-Brzezińska, J. Solid State Electrochem., 5 (2001) 229 and JM Skowroński, A. Sierczyńska, B. Olejnik, J. New Materials for Electrochemical Systems 9 (2006) 395-399).

A hydrogen-absorbing paste electrode is known in which the active material film is made of a mixture of a hydrophilic alloy powder and a composite powder containing carbon particles and rare earth elements therein to partially cover the surface of the carbon particles.

It is also known from US Patent No. 7,534,529 a negative electrode of a nickel metal hydride system battery having an active material film formed on a conductive substrate, wherein the film comprises a hydrogen storage alloy powder and pulverized carbon. The carbon powder contains a plurality of carbon particles, each containing at least one metal selected from the group consisting of Ni, Co, Ca, Fe, Mg, Mn, Ti and V.

The aim of the invention is to develop a negative electrode for Ni-MH cells, using a composite electrode material that absorbs greater amounts of hydrogen than an electrode made of a pure metal alloy of the AB5 type.

The essence of the invention, which is the negative electrode for nickel-metal hydride batteries, consists in the fact that the AB5 type hydrophilic alloy is covered on the surface with carbon nanomaterial deposited as a result of thermal decomposition of acetylene, and the amount of carbon nanomaterial is 0.1-25% by weight.

It was found that the negative electrode according to the invention is characterized by an improved process of hydrogen electrosorption as a result of sorption into the structure of the composite, with a simultaneous limitation of the spontaneous desorption process (limitation of self-discharge).

Due to the deposition of carbon nanostructures on the alloy surface, the ohmic drop between the alloy grains is reduced and the efficiency of hydrogen sorption reaction on the alloy grains is significantly increased.

The properties of the electrode according to the invention, generated by the use of the M / MH / C type composite material, allow to achieve higher specific energy values per unit volume of the electrode and thus improve the energy parameters of nickel-metal hydride batteries.

The negative electrodes according to the invention operate in an alkaline solution of lithium, sodium or potassium ranging from 0.1 to 7 mol / L.

The electrode according to the invention can be prepared as follows. An AB5 metal alloy is treated with a nitrogen / acetylene mixture at 700 ° C to form, by thermal decomposition of acetylene, a carbon nanodeposit on the alloy surface.

PL 218 729 B1

The composite material obtained in this way, after mixing with a binder, constitutes the active layer, which was applied by paste to the degreased support. After the introduction of the paste, the electrodes were dried, pressed and activated.

Electrochemical tests of the electrodes were carried out.

Immediately after activation, the electrodes were transferred to an electrolytic vessel filled with 6M KOH. Electrochemical half-cells were built, consisting of a test electrode made of a composite (AB5 alloy - carbon nanomaterial), an excess Ni (OH) 2 / NiOOH counter electrode made on a sintered nickel support and a reference electrode Hg / HgO / 6M KOH.

The half-cell was subjected to electrochemical tests using the cyclic voltammetry method. In the conducted voltammetric tests, the signal was excited linearly with a triangular impulse from the equilibrium potential E0 in the anodic direction to the value of -0.4 V in relation to the reference electrode, and then cathodically, to the potential equal to -1.1 or -1.15 V. The polarity was changed automatically using a generator, pulses. The next cycles were carried out in the potential range E ₀ V -0.4 V -1.1 (-1.15) VE ₀ V. The potential scanning speed was 25 mV / s.

The accompanying figure shows cyclic voltamograms; for an AB5 type electrode - gray line, for an AB5 type electrode with carbon nanodeposit formed on the surface - black line.

P r z k ł a d

A powdered alloy of MnNi3.55Al0.3Mn0.4Co0.75 in an amount of 3 g was placed in a tube furnace under a nitrogen atmosphere with a gas flow of 10 L / h and heated to 700 ° C. Acetylene was supplied additionally at 700 ° C. The gas flows were respectively 10 l / h nitrogen and 5 l / h acetylene. The process was carried out for 5 minutes in order to create a carbon nanodeposit on the surface of the alloy. The obtained composite was characterized by a carbon content of about 6% by weight in relation to the total weight of the sample. The furnace with its contents was cooled under nitrogen atmosphere. Then, electrodes were prepared from the obtained composite anode material. For this purpose, 0.4 g of the composite was taken and mixed with the addition of 10% by weight, based on the weight of the composite, of an aqueous solution of 3% PVA (polyvinyl alcohol) until a homogeneous paste was obtained. Then, the paste is applied by _two double-sided polishing to the media area of 1 cm ^2, made of foam than the claw ₃ with a thickness of 1.6 mm and a density of 500 g / m ^3, previously degreased in boiling distilled water. After the paste was introduced, the electrodes were dried at 70 ° C until constant weight. After drying, the electrodes were pressed in a press under a pressure of 40 MPa. Then, the electrodes were activated, which consisted of conditioning in 6M KOH solution for 1 hour. The electrodes were obtained with the electrochemical properties shown in the attached drawing.

Claims (1)

Negative electrode for nickel metal hydride batteries having an active layer containing a hydrophilic alloy type AB5 and carbon material, where this layer is applied to a conductive carrier, characterized in that the hydrophilic alloy of type AB5 is covered on the surface with carbon nanomaterial deposited as a result of thermal decomposition of acetylene, and the amount of carbon nanomaterial is 0.1-25 wt%.