Process for electrolysis of alkali metal chlorides with oxygen-consuming electrodes having orifices

Abstract

An oxygen-consuming electrode for use in chloralkali electrolysis, having a novel coating, the production thereof, an electrolysis cell comprising the oxygen-consuming electrode and parameters for the startup and shutdown of the electrolysis apparatus, compliance with which prevents damage to the cell.

Description

[0001]The invention relates to an oxygen-consuming electrode, especially for use in chloralkali electrolysis, and to the production thereof, to an electrolysis apparatus and to a process for electrolysis of aqueous solutions of alkali metal chlorides, complying with particular operating parameters. The invention further relates to the use of this oxygen-consuming electrode in chloralkali electrolysis or fuel cell technology.BACKGROUND OF THE INVENTION[0002]The invention proceeds from electrolysis processes known per se for electrolysis of aqueous alkali metal chloride solutions by means of oxygen-consuming electrodes which take the form of gas diffusion electrodes and typically comprise an electrically conductive carrier and a gas diffusion layer comprising a catalytically active component.[0003]Various proposals for operation of the oxygen-consuming electrodes in electrolysis cells on the industrial scale are known in principle from the prior art. The basic idea is to replace the h...

AI Technical Summary

Benefits of technology

The text claims that the electrolysis unit performed well during a shutdown period, with the voltage levels remaining stable. This shows that the unit was not negatively impacted by the shutdown and even improved slightly.

Problems solved by technology

A problem in the case of arrangement of an OCE in a cathode element where an electrolyte gap is present between membrane and OCE arises from the fact that, on the catholyte side, the hydrostatic pressure forms a pressure gradient over the height of the electrode, which is opposed on the gas side by a constant pressure over the height.

Both effects reduce the performance of the OCE.

However, the construction described in WO2001 / 57290A1 is very complex.

A disadvantage here is that the alkali metal hydroxide solution which forms has to be passed through the OCE to the gas side and then flows downwards at the OCE.

It has been found that a very high alkali metal hydroxide concentration can arise here, but it is stated that the ion exchange membrane at these high concentrations lacks long-term stability (Lipp et al., J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).

However, the construction in the embodiments shown is complex.

The alkali metal hydroxide solution then runs downwards on the gas side of the OCE, but this hinders access of oxygen to the OCE.

Disadvantages of the embodiments described in U.S. Pat. No. 4,332,662A1 include the relatively high loss of electrode area and the aforementioned hindrance of oxygen supply to the OCE.

The hydrophobic constituents make it difficult for electrolyte to penetrate through and thus keep the corresponding pores in the OCE unblocked for the transport of the oxygen to the catalytically active sites.

Due to the high costs of platinum, it is used exclusively in supported form.

However, stability of carbon-supported, platinum-based electrodes in long-term operation is inadequate, probably because platinum also catalyses the oxidation of the support material.

Carbon additionally promotes the unwanted formation of H2O2, which likewise causes oxidation.

Even though the carbon-supported silver catalysts are more durable than the corresponding platinum catalysts, the long-term stability thereof under the conditions in oxygen-consuming electrodes, especially in the case of use for chloralkali electrolysis, is also limited.

The oxidation can result in rearrangements in the catalyst structure, which have adverse effects on the activity of the catalyst and hence on the performance of the OCE.

The ion exchange membranes in electrolysis cells are subject to severe stress: They have to be stable towards chlorine on the anode side and to severe alkaline stress on the cathode side at temperatures around 90° C. Perfluorinated polymers such as PTFE typically do not withstand these stresses.

When the electrolyte concentrations increase, the membrane releases water and shrinks as a result; in the extreme case, withdrawal of water can cause precipitation of solids in the membrane.

Concentration changes can thus cause disruption and damage at the membrane.

The result may be delamination of the layer structure (blister formation), as a result of which the mass transfer through the membrane deteriorates.

In addition, pinholes and, in the extreme case, cracks can occur, which can result in mixing of anolyte and catholyte.

Due to variation in demand volumes and faults in production sectors upstream and downstream of the electrolysis, electrolysis cells in production plants, however, inevitably have to be repeatedly run down and back up again.

On shutdown and restart of the electrolysis cells, there occur conditions which can lead to damage to the cell elements and considerably reduce the lifetime thereof More particularly, oxidative damage has been found in the cathode space, as have damage to the OCE and damage to the membrane.

In the cathode, strongly oxidative conditions exist as a result of the oxygen, and these can no longer be compensated for by the electrolysis current on shutdown.

The result is damage to the electrode and also to the entire cathode space.

On restart, in turn, an excessively low water content hinders mass transfer through the membrane.

Inhomogeneities in the water and / or ion distribution in the membrane and / or the OCE, on restart, lead to local spikes in the current and mass transfer, which subsequently result in damage to the membrane or the OCE.

Problems are also presented by the precipitation of alkali metal chloride salts on the anode side.

The result is precipitation of alkali metal chloride salts, especially at the boundary region to the membrane, which lead to damage to the membrane.

However, in the case of use of OCEs, this measure alone is insufficient to prevent oxidative damage to OCEs in the course of startup and shutdown.

The flooding of the gas space with sodium hydroxide solution accordingly protects the cathode space from corrosion, but gives inadequate protection from damage to the electrode and the membrane on shutdown and startup, or during shutdown periods.

This procedure reduces the risk of damage to membrane and OCE during startup, but does not give any protection from damage during shutdown and shutdown periods.

The methods mentioned, however, are complex, especially from a safety point of view, and entail the installation of additional equipment for nitrogen and hydrogen supply.

The process also does not give any protection from damage to the ion exchange membrane and places high safety requirements for avoidance of explosive gas mixtures.

On reduction of the NaOH concentration as a result of increased water supply on the cathode side, with maintenance of the NaCl concentration on the anode side, there is an increase in diffusion of chloride ions into the cathode space, with the correspondingly adverse effects with regard to corrosion in the cathode space and the deactivation of the OCE.

On restart, pure water is initially charged on the cathode side and concentrated brine on the anode side, and so an even higher chloride burden is to be expected on the cathode side here too.

On replacement of the anolyte with water at 90° C. during the running-down operation, swelling and expansion of the membrane are to be expected, which increases the risk of cracks and other damage.

The procedure described is very complex; more particularly, for industrial electrolysis processes, a very high level of complexity is required.

It should be stated that the techniques described to date for startup and shutdown of an OCE are disadvantageous and give only inadequate protection from damage.

More particularly, the processes described do not give sufficient protection in the case of an OCE with micro-gap arrangement.

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