Electrolysis cell of alkali solutions

Abstract

The invention relates to an electrochemical cell partitioned by a cation-exchange membrane suitable for production of high purity hydrogen and oxygen by electrolysis of alkaline solutions comprising a cathode in form of porous web including a platinum or palladium catalyst. The cell can be used as an element of a modular filter-press electrolyzer.

Claims

1. An electrolysis cell comprising: a cation-exchange membrane, an anodic compartment, a cathodic compartment partitioned by the cation-exchange membrane, and an electrolyte consisting of an aqueous solution of caustic soda comprising 8-45% by weight of concentration, said anodic compartment consisting of a liquid chamber delimited by an anodic wall and by said cation-exchange membrane and filled with the electrolyte, said anodic compartment containing an anode suitable for oxygen evolution, the anode comprising a nickel mesh activated with a thin layer of catalyst containing a mixture of oxides of lanthanum, cobalt and nickel assembled in direct contact with said cation-exchange membrane, said anodic compartment further comprising a feeding inlet and a discharging outlet for discharging the electrolyte, said cathodic compartment delimited by a cathodic wall and by said cation-exchange membrane, said cathodic compartment containing a gas-diffusion cathode suitable for hydrogen evolution, the gas-diffusion cathode comprising a carbon cloth having a hydrophilic layer consisting of 20% by weight of a catalytically-activated layer consisting of platinum and/or palladium, supported on high surface area carbon black, soaked with sulphonated perfluorinated ionomer dispersion and deposited upon the carbon cloth, and in direct contact with said cation-exchange membrane, and said gas-diffusion cathode further comprising an external hydrophobic layer suitable for facilitating the release of hydrogen to the cathodic compartment, the hydrophobic layer being a mixture of low surface area carbon black and polytetrafluoroethylene (PTFE) in a 1:1 weight proportion, wherein a purity of product hydrogen determined in terms of concentration of oxygen in a dried cathodic product ranges between 0.1-1 ppm of O.sub.2.

2. The electrolysis cell according to claim 1 wherein said cation-exchange membrane is a non-reinforced monolayer sulphonic membrane.

3. The electrolysis cell according to claim 1, wherein said catalytically-activated layer of said hydrophilic layer of the gas-diffusion cathode contains platinum.

4. The electrolysis cell according to claim 1, wherein said gas-diffusion cathode and said cathodic wall are put in electrical contact by means of a current collector consisting of a porous metal structure comprising distributed points of electrical contacts, wherein the porous metal structure is a nickel or steel foam.

5. The electrolysis cell according to claim 1, wherein said anode suitable for oxygen evolution and said anodic wall are put in electrical contact by means of a current collector consisting of a porous metal structure, wherein the porous metal structure is a nickel or steel foam or mat.

6. An electrolyzer of alkali solutions consisting of a modular arrangement of plural electrolysis cells according to claim 1, electrically connected through said anodic and cathodic walls according to a bipolar or monopolar configuration.

7. A process of electrolysis in the electrolysis cell according to claim 1 comprising the steps of: feeding the electrolyte consisting of the aqueous solution of caustic soda comprising 8-45% by weight of concentration to said anodic and cathodic compartments; connecting said cathodic compartment to a negative pole and said anodic compartment to a positive pole of a power unit, with subsequent supply of direct electrical current; carrying out cathodic evolution of hydrogen within said catalytically-activated layer and discharging said hydrogen from said cathodic compartment; carrying out evolution of oxygen on a surface of said anode; and obtaining the purity of product hydrogen determined in terms of concentration of oxygen in the dried cathodic product ranging between 0.1-1 ppm of O.sub.2.

8. The process according to claim 7 wherein the aqueous solution of caustic soda is 10 to 20% by weight concentration.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The FIGURE shows a side sectional view of an electrolysis cell according to the invention.

DETAILED DESCRIPTION OF THE FIGURE

(2) The FIGURE shows a side sectional view of an electrolysis cell subdivided by means of a cation-exchange membrane 100 into an anodic compartment and a cathodic compartment; the anodic compartment consists of a chamber delimited at the side opposite membrane 100 by an anodic wall 200; inside the anodic compartment, an anode 300 consisting of a substrate made of a mesh or other porous metal structure is present in direct contact with membrane 100 or spaced apart therefrom at most by a very small predefined gap, in the order of magnitude of a few millimetres. The electrical contact between anode 300 and the corresponding anodic wall 200 is achieved through an anodic current collector 600 consisting of a porous metal structure, for instance a nickel or steel foam or mat. The anodic compartment is equipped with feed 400 and discharge 401 means of process anolyte, for instance caustic soda or potash. The FIGURE shows electrolyte feed from the top and discharge from the bottom, but the cell may be operated also by feeding the electrolyte bottom up. At the anodic compartment, oxygen 500 is produced and discharged in form of bubbles within the electrolyte phase. The cathodic compartment consists of a chamber delimited at the side opposite membrane 100 by a cathodic wall 210; a cathode 310 consisting of a porous web provided with a layer 311 catalytically activated with platinum and/or palladium is arranged in intimate contact with membrane 100, for instance by hot pressing or other known technique. The cathode can also be provided with a second layer 312 consisting of carbon or metal powders and polymer binders adjacent the catalytically activated layer on the side opposite to the membrane and having less hydrophilic characteristics with respect to the catalytically activated layer. The electrical contact between cathode 310 and cathodic wall 210 is achieved through a cathodic current collector 610 consisting of a porous metal structure, preferably a nickel or steel foam. The cathodic compartment is equipped with feed 410 and discharge 411 means of process catholyte, which in one embodiment has the same composition of process anolyte but is separately circulated; the cathodic product consists of hydrogen 510 discharged as bubbles inside the electrolyte phase. The illustrated cell also comprises a gasketing system (not shown) and tightening means, for instance tie-rods distributed along the perimeter of the anodic and cathodic walls (not shown). It will be clear to a person skilled in the art how a multiplicity of cells as hereinbefore described is suitable for being employed as modular elements of an electrolyzer. By way of example, an electrolyzer in bipolar configuration, consisting of a stack of cells connected in electrical series, can be obtained by assembling the cells so that each of the intermediate cell walls acts at the same time as the anodic wall of one cell and as the cathodic wall of an adjacent cell, according to a filter-press design widely known in the art.

(3) The following examples are included to demonstrate particular embodiments of the invention, whose practicability has been largely verified in the claimed range of values.

(4) It should be appreciated by those of skill in the art that the compositions and techniques disclosed in the examples which follow represent compositions and techniques discovered by the inventors to function well in the practice of the invention; however, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the scope of the invention.

Example

(5) Two electrolyzers were assembled, one comprised of eight and the other of four cells of the type illustrated in the FIGURE having an electrode area of 63 cm.sup.2, mutually connected in electrical series and assembled in a filter-press bipolar configuration. The walls delimiting the different cell compartments were obtained out of a nickel sheet. As the anodic current collector, a nickel mat made of four layers of interlaced and superposed wires with an uncompressed thickness of 2 mm and as the cathodic current collector a 1 mm thick foam were used. The anodes were made of a nickel mesh activated with a thin layer of catalyst containing a mixtures of oxides of lanthanum, cobalt and nickel, assembled in intimate contact with the membrane. The cathode was made of a carbon cloth activated with a hydrophilic layer consisting of a 20% by weight platinum-based catalyst supported on high surface area carbon black, soaked with a Nafion® sulphonated perfluorinated ionomer dispersion from DuPont, deposited upon the carbon cloth by spraying, at a total Pt loading of 0.5 mg/cm.sup.2. On the hydrophilic layer side opposite the membrane a hydrophobic layer was deposited also by spraying, obtained from a mixture of low surface area carbon black and PTFE, in a 1:1 weight proportion. The cathode was overlaid to a monolayer sulphonic Nafion® membrane manufactured by DuPont and cold-pressed under the effect of cell tightening. To reach equilibrium conditions sooner, inventors have also verified the possibility of hot pressing the cathode and the membrane previously to the cell assemblage.

(6) The electrolyzers were operated in two test campaigns of 3000 hours, one on caustic potash and the other on caustic soda, varying electrolyte concentration (up to 45% by weight of alkali), current density (up to 9.5 kA/m.sup.2) and cathodic pressure (1 to 2 bar absolute). In all tests, hydrogen of higher purity with respect to that typical of PEM/SPE pure water electrolyzers was produced. Performances in terms of cell voltage were completely aligned to the expectations also at atmospheric pressure and moderate electrolyte concentration: by operating with 20% caustic soda at atmospheric conditions and at an average temperature of 73° C. in both compartments for instance, a stable voltage of 1.92 V was obtained at 9.5 kA/m.sup.2.

(7) Purity of product hydrogen was determined in terms of concentration of oxygen in the dried cathodic product: the different tests gave values within the range 0.1-1 ppm of O.sub.2.

Counterexample

(8) A four cell electrolyzer was assembled similar to the one of the above example except for the replacement of the cathode with a nickel mesh activated with a 5 g/m.sup.2 platinum galvanic coating, assembled in intimate contact with the membrane. The test campaign of the previous example was repeated operating at atmospheric pressure only, since pressurisation of cells with two metal meshes in contact with the two faces of the membrane was considered too hazardous for the integrity of the latter. By operating on 20% caustic soda at 73° C., a stable voltage of 2.34 V was obtained at 9.5 kA/m.sup.2. The maximum hydrogen purity detected during this test campaign corresponded to 400 ppm of O.sub.2 in the dried cathodic product.

(9) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(10) Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps.

(11) The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.