Composite air electrode and associated manufacturing method

Abstract

A method for manufacturing a composite electrode for a metal-air electrochemical cell with a liquid electrolyte of basic pH. A liquid solution comprising a fluoropolymer suspended in a solvent is synthesized, then deposited on the outer surface of a porous structure forming an air electrode. The fluoropolymer comprises SO2N groups suitable for conducting hydroxyl ions and is capable of forming a membrane impermeable to at least the liquid electrolyte of basic pH. When the liquid solution is applied to the porous structure, the solvent flows through the porous structure and the fluoropolymer is deposited by aggregating into a layer on the outer surface of the porous structure.

Description

TECHNICAL AREA[0001]The invention relates to the field of protecting the air electrode of a metal-air electrochemical cell against the corrosive effects of a liquid electrolyte of basic pH. It may have applications in zinc-air batteries.BACKGROUND[0002]Electrochemical cells are generally composed of a negative electrode, a positive electrode, and an electrolyte for the transit of charge carriers from one electrode to the other.[0003]Metal-air electrochemical cells generally comprise a liquid electrolyte. The negative electrode, typically formed from a metal compound M, decomposes into Mn+ ions during discharge while oxygen from the air is reduced at the positive electrode, called the air electrode, in the following reactions:Discharge at the negative electrode: M→Mn++ne−Discharge at the positive electrode: O2+2H2O+4e−→4OH−[0004]One of the advantages of metal-air systems is the use of a positive electrode, also called an air electrode, of infinite capacity. The oxygen consumed at the...

AI Technical Summary

Benefits of technology

[0024]Furthermore, the method of the present invention allows depositing a thinner polymer layer than methods of the prior art, without losing impermeability to the liquid electrolyte. Typically, the thickness of a protective layer for the electrode has a thickness chosen as a compromise between acting as a barrier to the liquid electrolyte and good conductance. A protective layer according to the prior art is typically with a sufficient thickness to avoid holes and to form an effective barrier against the liquid electrolyte, although this is at the expense of conductance. The resistivity of a protective layer increases with thickness. The method of the invention facilitates obtaining a hole-free layer of low thickness, thus forming an impermeable barrier to the liquid electrolyte without increasing the resistivity of the protective layer in comparison to methods of the prior art.

[0054]By combining the hydroxyl ion conduction properties of the polymer membrane with an optimized conduction of hydroxyl ions by the porous structure of the air electrode, the hydroxyl ions can move more easily from the electrolyte to the porous structure and occupy as much of the volume of the air electrode as possible. It is thus possible to obtain an air electrode providing particularly efficient oxygen reduction, exploiting the full volume of the electrode.

Problems solved by technology

The triple contact interface of an air electrode presents several technical challenges.

In particular, air electrodes degrade quickly, even when not in operation, in particular because of the corrosive effect of the liquid electrolyte of basic pH of the electrochemical cells.

The cations from the dissolved catalyst may contribute to degrading the performance of a metal-air cell, by facilitating an undesirable water reduction reaction likely to interfere with the deposition of metal on the negative electrode, reduce the Coulomb and energy efficiency, and consume the water.

An overvoltage at the zinc negative electrode facilitates the zincate reduction reaction over the water reduction, but is insufficient to prevent the water reduction, particularly in the presence of ion impurities from a deteriorated air electrode.

However, peroxide is known to degrade the polymer electrolytes likely to be used in a fuel cell.

In addition to the corrosive effect of the basic compound of the liquid electrolyte, the simple progressive wetting of the porous structure of an air electrode until it is flooded eventually renders such an electrode inoperative.

The low solubility of the formed carbonates leads to progressive carbonatation of the electrolyte.

In addition, the precipitation of carbonates in the pores of the air electrode gradually destroys the air electrode and renders it inoperative.

Images