PROCEDURE FOR PROTECTING ELECTROLYTIC CELLS EQUIPPED WITH GAS-DIFFUSION ELECTRODES IN SHUT-DOWN CONDITIONS.

DESCRIPTION OF THE INVENTION

The present invention relates to a shut-down procedure of an electrochemical cell, in particular of an electrolysis cell equipped with a gas-diffusion cathode. By electrolysis cell reference is made to an elementary electrochemical reactor used in an electrolytic process, either alone (single electrolysis cell) or as modular element of a stack of equivalent cells (electrolyser).

The use of gas-diffusion electrodes in the field of electrolytic cells is fundamentally associated with the depolarisation of the relevant electrolysis processes. Gas- diffusion electrodes, in particular, are used in the most common of cases to suppress the cathodic hydrogen evolution reaction, replacing the same with oxygen reduction to water, thereby obtaining a net saving in terms of electrolysis voltage and hence of energy consumption. In this case, reference is thus made to gas-diffusion cathodes fed on oxygen, either pure or in admixture (for instance air). The two main depolarised electrolysis processes making use of gas-diffusion electrodes are chlor-alkali (particularly chlorine-caustic soda) electrolysis and aqueous hydrochloric acid electrolysis.

In the former case, the principle on which is based the depolarisation of the process carried out with the oxygen-diffusion cathode derives from the substitution of the overall electrolysis reaction, as indicated hereafter: [traditional process] 2 NaCI + 2 H ₂ O → Cl ₂ + 2 NaOH + H ₂ [process with oxygen-diffusion cathode] 2 NaCI + 14 O ₂ + H ₂ O → Cl ₂ + 2 NaOH

The two reactions are sensibly different under an energy standpoint, the reaction typical of the depolarised process in particular requiring a substantially reduced amount of energy, with a theoretical saving of 1.23 Volts.

Practically, due to unavoidable energy dissipation phenomena, such as ohmic drops and overvoltage, the achievable cell voltage is 1.9 - 2.1 Volts at current densities of 4000 - 5000 A/m ² .

The case of depolarised aqueous hydrochloric acid electrolysis is analogous, as summarised by the following reactions:

[traditional process] 2 HCI → Cl ₂ + H ₂

[process with oxygen-diffusion cathode] 2 HCI + 14 O ₂ → Cl ₂ + H ₂ O

Also in this case, the theoretical voltage decrease brought about by suppressing the hydrogen evolution reaction is 1.23 V, and the cell voltage achievable in real process conditions is about 1.3 í 1.4 V at 4 kA/m ² .

Both of the above electrochemical processes are characterised by the high aggressiveness of the reaction environment, which makes the selection of construction materials particularly delicate, in particular as concerns the gas- diffusion cathode. In the case of chlor-alkali electrolysis, the gas-diffusion cathode normally consists of a support which may either be metallic, for instance a silver or silver-plated nickel net, or non-metallic, for instance a carbon cloth, in both cases activated with a suitable catalyst, in general containing silver, optionally in combination with platinum or other metals. Although this kind of materials is very resistant to the corrosive action of the process fluids during electrolysis, the situation is remarkably worse during shut-downs, when the electrical power supply is discontinued and the cathodic potential climbs to very high values, namely those corresponding to the rest potential of the relevant catalyst under an oxygen stream (above 1 V/RHE). These potential levels may bring silver or carbonaceous materials to critical conditions, triggering corrosion phenomena which may have serious consequences on catalyst lifetime and on the stability of the relative supports.

The high cathodic potential in shut-down conditions is even more dangerous in the case of oxygen-diffusion cathodes for aqueous hydrochloric acid electrolysis: the electrolyte corrosive action (hydrochloric acid containing a high amount of dissolved chlorine) is very strong per se, such that it imposes the utilisation of expensive catalysts like oxides or sulphides of noble metals such as rhodium, as disclosed for instance in EP 0 931 857 and EP 1 181 397. Both cited documents, and in particular EP 0 931 857, precisely highlight the difficulty in confining the

cathodic catalyst corrosion phenomena to an acceptable extent in conditions of power supply interruption; the use of special catalysts must be coupled to accurately controlled shut-down procedures, in order to minimise the loss of noble metal due to leaching induced by the process electrolyte. Regarding this matter, it must be also kept into account that the shut-down of an electrolyser is not always a foreseeable and scheduled event, as occurs for instance in the case of periodical maintenance procedures; it happens quite often that a shut-down is forced by fortuitous anomalies occurring in the operation of the cell or of the relevant plant, associated with increased difficulties in following appropriate procedures for catalyst protection, especially if they involve a certain degree of complexity. In this case, although the corrosion phenomena can be curbed to some extent, it is not possible to eliminate them completely in the medium-long range by resorting to the teachings of the prior art.

A typical protective measure known in the art for minimising the corrosion phenomena in electrolytic cells consists for instance of applying a small protecting current, in order to polarise the cell and decrease the cathode potential below the corrosion zone. Such measure, much valuable for traditional type electrolysers equipped with hydrogen-evolving cathode, proves totally inadequate for protecting oxygen-fed gas-diffusion cathodes, since the variation of the cathodic potential in the presence of oxygen upon application of currents of small entity is negligible. The own nature of gas-diffusion electrodes makes on the other hand very difficult the elimination of oxygen traces in the cathodic compartment even after evacuating the same: the porosity and the high degree of hydrophobicity typical of these electrodes cause a relevant amount of oxygen to be trapped inside the structure, favouring the persistence of a high potential level for a prolonged time. In the electrolysers equipped with oxygen-diffusion cathodes of the prior art, shutdown procedures thus comprise, following the power supply discontinuation, not only the compartment evacuation but also the injection of nitrogen or other inert gas, in order to proceed with a complete replacement of the cathodic atmosphere with consequent decrease of the potential below a critical value; for example, the attainment of a cathodic potential lower than 0.3 V/RHE is normally taken as an

indication of the achieved drainage of the cathodic compartment. Once more, however, the particular three-dimensional structure comprising long and tortuous pores with a prevailing hydrophobic character, typical of oxygen-diffusion cathodes, makes the drainage process rather lengthy and laborious, with times which may reach 30 to 60 minutes; during this transient phase, the corrosion of the catalyst and of the relevant support proceeds relatively undisturbed.

Under one aspect the present invention is directed to a novel procedure for the shut-down of an electrolytic cell equipped with an oxygen-fed gas-diffusion cathode overcoming the limitations of the prior art, particularly as regards lowering the cathodic potential below the corrosion zone in a very fast time.

This and other objects will be made clear by the following description, which shall not be intended as a limitation the present invention.

The scope of the invention is achieved by the procedure detailed in the annexed claims.

The invention consists of a shut-down procedure simultaneously or sequentially comprising the steps of:

- interrupting the power supply and

- draining the cathodic compartment by replacing the process feed, consisting of oxygen either pure or in admixture, with a reducing gas feed, preferably consisting of hydrogen mixed with an inert diluting agent.

The replacement may be advantageously carried out by means of an initial evacuation of the cathodic compartment by venting to the atmosphere and a subsequent feeding of a mixture containing hydrogen or a different gaseous reducing agent. In one preferred embodiment, hydrogen mixed with nitrogen is used as the reducing agent. Feeding a reducing gas feed to the cathode favours the chemical elimination of oxygen, which in most of the cases is very fast being promoted by the same cathodic catalyst which is to be protected. The utilisation of hydrogen as the reducing agent is particularly preferred, not only due to the high reactivity thereof towards oxygen on almost any metal catalyst, but also for its high diffusion rate in porous structures and because the reaction product in this case simply consists of water. The initial evacuation of the cathodic compartment at

atmospheric pressure guarantees that no explosive mixture can be formed during the replacement operation, in view of the limited amount of oxygen left in the compartment; for a better safety, when hydrogen is used it is nevertheless preferable to proceed to its preliminary dilution with an inert medium, for instance nitrogen, argon or other noble gas. Nitrogen is the preferred inert medium for this purpose also in view of its limited cost. Hydrogen dilution, besides preventing the formation of explosive mixtures, allows adjusting the recombination rate of oxygen with hydrogen to give water, which on one hand must be high enough to allow a quick decrease of the cathodic potential, but on the other hand should be limited in order to prevent a temperature increase up to dangerous levels for the integrity of the catalyst and the support. The preferred hydrogen to nitrogen ratio ranges between 0.02 and 0.2 depending on whether the cathodic compartment is vented or not before feeding the mixture to the cell. In one preferred embodiment, the cathodic potential is monitored either directly, by means of a suitable probe, or indirectly, for instance by deducing its value from the total cell voltage, during the drainage operation; when the cathodic potential goes down to sufficiently low values, typically 0.3 V/RHE, the cathodic compartment may be advantageously sealed and the feeding of the reducing agent discontinued. By a suitable selection of the reducing agent, of the concentration and of the flow-rate thereof it is possible to complete the operation in a very short time, even within 5 minutes. The procedure of the invention is suitable for protecting oxygen-diffusion cathodes with catalysts of different nature, including silver-based catalysts typical of the depolarised chlor-alkali process and noble metal oxides or sulphides used in depolarised hydrochloric acid electrolysis. Especially in the latter case, the use of a reducing agent such as hydrogen may further contribute to the catalyst stabilisation by destroying the chlorine and hypochlorite traces dissolved in the electrolyte, thereby eliminating an additional corrosion factor. The invention will be further clarified by the following examples but not limited thereto.

EXAMPLE 1

A lab cell for depolarised hydrochloric acid electrolysis of 64 cm ² active area was equipped with a Nafion ^® 324 membrane commercialised by DuPont/USA in direct contact on one side with a titanium anode activated with a Ruθ2-based catalyst and on the other side with a gas-diffusion cathode obtained from a carbon cloth coated with a rhodium sulphide-based catalyst supported on active carbon particles. The gas-diffusion cathode was fed with oxygen largely in excess with respect to the stoichiometric requirement, at a relative pressure of 50 mbar, while the anodic compartment was fed with a hydrochloric acid aqueous solution, stored in a suitable recycle tank at a concentration of 184 g/l, at 40 ml/min flow-rate and 120 mbar relative pressure. A periodical reintegration of fresh electrolyte was carried out whenever the hydrochloric acid concentration, measured by a probe introduced in the recycling tank, indicated a 20% depletion. The cell was operated at a current density of 3 kA/m ² at a temperature of 55°C controlled by means of a suitable heating tape applied to the walls. The cathodic potential, detected by a suitable probe, was stabilised at regime at a value of 0.4 V/RHE. After 10 hours of continuous operation, the electrical power supply was discontinued and at the same time, by acting on a three-way valve, the oxygen feed was replaced with a gaseous feed consisting of 10% by volume hydrogen in nitrogen, following a preliminary atmospheric venting of the cathodic compartment. The cathodic potential measured value showed a sudden peak at 1.05 V/RHE, quickly beginning to decrease until stabilising around a value below 0.3 V/RHE in the course of about four minutes. The cathodic potential remained consistently below 0.3 V when the reductant feeding was discontinued. Upon completion of the test, an analysis of rhodium carried out on the outlet condensate of the cathodic compartment could not detect any significant traces.

COUNTEREXAMPLE 1

The previous test was repeated after replacing the electrodes in the same conditions except that during the shut-down, the oxygen feed was replaced with pure nitrogen. The cathodic potential showed a sudden peak at 1.1 V/RHE, then decreasing more slowly until reaching a value of 0.3 V after 35 minutes. The

feeding was then discontinued after sealing the cathodic compartment outlet. The cathodic potential increased again up to a value of 0.6 V, probably under the effect of oxygen trapped in the cathode porosity. The nitrogen feed was then restored and again discontinued after checking that the cathodic potential remained below 0.3 V for 30 minutes. Upon completion of the test, an analysis of rhodium in the outlet condensate of the cathodic compartment was carried out and a 30 ppm concentration was detected.

EXAMPLE 2

A 100 cm high and 10 cm wide single cell for depolarised chlor-alkali electrolysis provided with percolator element consisting of a high density polypropylene foam, according to the disclosure of EP 1 446 515, was equipped with a Nafion ^® N2010WX membrane commercialised by DuPont/USA, a titanium anode activated with a Ti, Ir and Ru oxide-based catalyst and an oxygen-diffusion cathode obtained from a silver net activated with a 20% by weight carbon-supported Ag catalyst. The anodic compartment was fed with a 20% by weight sodium chloride brine, while the percolator interposed between oxygen-diffusion cathode and membrane was crossed by a 25 l/h downward flow of 32% caustic soda solution. The oxygen-diffusion cathode was fed from the bottom with an oxygen flow, with a 20% excess than the stoichiometric requirement.

The cell was operated at a current density of 4 kA/m ² , at a temperature of 85°C. The cathodic potential, detected by a suitable probe, was stabilised at regime at a value of 0.73 V/RHE.

After 8 hours of continuous operation, the electrical power supply was discontinued and the oxygen feed was immediately replaced, by acting on a three- way valve, with a gaseous feed consisting of 10% by volume hydrogen in nitrogen, after a preliminary atmospheric venting of the cathodic compartment. The cathodic potential showed a sudden peak at 1.12 V/RHE, quickly decreasing until stabilising at a value below 0.3 V/RHE in the course of about five minutes. The feeding was then discontinued after sealing the cathodic compartment outlet. The cathodic potential remained consistently below 0.3 V after discontinuing the reductant feed.

Upon completion of the test, an analysis of silver carried out on the outlet condensate of the cathodic compartment could not detect any significant traces.

COUNTEREXAMPLE 2

The previous test was repeated after replacing the electrodes in the same conditions except that the oxygen feed was replaced with pure nitrogen. The cathodic potential showed a sudden peak at 1.16 V/RHE, then decreasing more slowly until reaching a value of 0.3 V after 30 minutes. The feeding was then discontinued after sealing the cathodic compartment outlet. The cathodic potential increased again up to a value of 0.49 V, probably under the effect of oxygen trapped in the cathode porosity. The nitrogen feed was then restored and again discontinued after checking that the cathodic potential remained below 0.3 V for 30 minutes. Upon completion of the test, an analysis of silver in the outlet condensate of the cathodic compartment was carried out and a 65 ppm concentration was detected. The previous description shall not be intended as limiting the invention, which may be practised according to different embodiments without departing from the scopes thereof, and whose extent is exclusively defined by the appended claims.

Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other additives, components, integers or steps.

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.