CH686788A5 - A method of electrolytic reduction of carbon dioxide. - Google Patents

Description

CH 686 788 A5

2

Description

The present invention relates to an improved process for electrolytic reduction of CO2.

Carbon dioxide (CO2) is, in the long term, an essential raw material for the chemical industry and the manufacture of liquid (methanol, ethanol) and gaseous (methane) fuels. The conversion of CO2 into fuels or intermediate chemicals can be carried out by reduction with hydrogen in the gas phase (high temperature and pressure) or directly by electrolytic reduction in an aqueous medium (ambient conditions). In both cases, such an operation requires the supply of electrical energy either to manufacture the hydrogen intended for the reduction process in the gas phase, or to conduct a direct electrolysis of CO2 in an aqueous medium. The direct electrolytic process, on condition of being effective, on the other hand has an obvious advantage of simplicity and of considerably lower operating cost.

In the shorter term, the CO2 reduction process by electrolysis can also be a good means of storing electrical energy whatever its origin. Indeed, methanol or methane are easier to store than hydrogen, the use of which for this purpose has often been envisaged. This is why much research has been undertaken to try to improve this electrolytic reduction process, more particularly with the aim of increasing its yield.

On the other hand, we already know in such a process the activity of copper electrodes for CO2 reduction, which was first described in 1985 (Hori et al., Chemistry Letters, 1695, 1985) and which , since that date, has been the subject of numerous publications. This interest is explained by the fact that copper currently remains the only electrode material making it possible to obtain the most advantageous products for reducing CO2, namely methane, ethylene and pethanol. However, it soon became apparent that the yield (relative to current) of the CCte reduction products, high in the initial period (30 min. To 1 hour) of electrolysis, then dropped rapidly due to poisoning of the copper electrode, hydrogen becoming the predominant product of electrolysis and hydrocarbons tending to disappear completely. Despite various attempts, no effective remedy against blockage of the copper electrode has been described so far.

Thus the aim of the present invention is to provide a process for the electrolytic reduction of CO2 in aqueous saline solution saturated with CO2 and in the presence of a copper or silver cathode, which makes it possible to remedy the aforementioned drawbacks of the known processes, and which more particularly provide an improved yield and which prevents poisoning and thereby blockage of the cathode.

The above object is achieved by the method according to the invention, in which the copper or silver cathode is subjected to an electrochemical activation consisting in the application of at least one scan of the potential of the cathode comprising a linear increase in this potential up to a positive potential compared to the initial electrolysis potential, then the linear return to it.

According to a first variant of the invention, the cathode is made of copper and the activation can then comprise from 1 to 5 cycles, preferably 3 cycles, succeeding each other every 3 to 15 minutes, preferably every 5 minutes approximately. The actual scanning consists of a linear increase in the cathode potential up to 0.5 to 1.5 V, preferably up to about 1 V, compared to a normal calomel reference electrode (ENC), at a speed of 0.5 to 20 V / sec., or better still from 1 to 10 V / sec., and preferably about 5 V / sec., the potential then returning to its usual original value at a speed similar.

The reduction of CO2 is in principle carried out at ambient temperature and at atmospheric pressure, in aqueous saline solution saturated with CO2 (for example acid carbonates, chlorides, perchlorates or sulphates of alkali metals), at a neutral to weakly alkaline pH. Electrolysis is generally carried out at a potential of -2 V (compared to an ENC reference electrode).

As regards the temperature and the pressure of the electrolysis, this can also be carried out between 0 ° and 60 ° C. and at a pressure higher than atmospheric pressure, for example up to about 5 atm.

As electrolyte, it is advantageous to use, for example, bicarbonate of Na or K in a concentration of 0.1 to 1 M, preferably from 0.3 to 0.5 M.

According to a second variant of the method, the copper cathode can be replaced by a silver cathode; in this case the solution will contain, in addition to a perchlorate, sulfate or alkali chloride, a small amount of a soluble copper salt, for example CUSO4, in a concentration of 10-6 to 5 x 10 "3 mole / l, preferably between 10 ~ 5 and 5 x 10 ~ 4 mole / l. The electrolytic reduction of the CO2 dissolved in the solution will then be accompanied by a deposit of metallic copper on the silver cathode. silver, scanning consists of a linear increase in the cathode potential up to about 0.4 V, preferably about 0.3 V, relative to the ENC, at a speed of 0.1 to 1 V / sec, preferably at about 0.2 V / sec, the potential then returning to its initial value at a similar speed.

The method according to the invention will now be described in more detail and with reference to the following two nonlimiting examples.

Figs. 1 and 2 are graphs representing the faradic yields (in%) relative to the duration of the electrolysis for two experiments carried out with the method according to the invention.

Figs. 1A and 2A are graphs corresponding to those of FIGS. 1 and 2, but representing the results of corresponding experiments carried out in the usual way, that is to say without activation.

Example 1

Electrolysis is carried out in a cell

5

10

15

20

25

30

35

40

45

50

55

60

65

2

3

CH 686 788 A5

4

in glass, with two compartments separated by a Nafion membrane. The catholyte (30 ml) is continuously traversed by a CO2 flow of approximately 15 ml / min. The working electrode here is a copper rod 6 mm in diameter (surface = 0.28 cm2), polished with alumina 0.3 n, then pickled for 15 seconds in 10% HCl, just before the start of the 'electrolysis. Instead of this chemical pickling, an electrochemical method of electrode preparation can be used, which has the advantage of giving better reproducible surfaces, and which consists in applying an anode current of 100 mA for 5 minutes and in an 85% H3PO4 solution.

The two solutions used are 0.1 M sodium bicarbonate, respectively 0.5 M potassium bicarbonate. The reference electrode is the ECN, the potentials mentioned below relating to it. The electrolyses were carried out at a potential of -2 V. The activation, which makes it possible to extend the life of the cathode, consisted of a succession of three cycles of scanning of the potential up to an anodic limit lying between + 1.5 and +0.5 V and at the speed of approximately 5 V / sec. These cycles were repeated every five minutes. Corresponding control electrolyses were carried out under the same conditions, but without the activation cycles.

The analysis of reduction products (here methane and ethylene), both in the gas phase, during an electrolysis, and in the solution, at the end of an experiment, was carried out using '' a gas chromatograph fitted with an FID detector.

The two experiments which have been carried out are therefore distinguished by the use of different electrolytes.

The first was carried out in 0.5 M KHCO3 and the results obtained are presented in FIGS. 1 and 1A. With activation according to the invention (fig. 1), the faradaic yields are approximately 40 to 50% for methane and 10% for ethylene, and remain stable, even over an electrolysis period of 24 hours. During this experiment, a few percent of ethanol was detected, as well as only fractions of a percent of CO and formates. On the contrary, the results obtained without activation (fig. 1A) confirm that very quickly (less than 2 hours for ethylene and approximately 3 hours for methane) the faradaic yields fell to negligible values close to zero. The cathodic current progresses during an electrolysis, in this solution, of approximately 40 to 60 mA / cm2. By way of comparison, the current densities used during the electrolysis of water on an industrial scale are of the order of 200 mA / cm2.

With 0.1 M NaHC03 as electrolyte, the second experiment was carried out under conditions identical to those of the first, but the faradaic yields obtained are slightly lower, namely approximately 40% of CH4 and 10-20% of C2H4 ( fig. 2). The much weaker currents (10 to 20 mA / cm2) make it possible to extend the reduction over more than 50 hours while keeping the yields constant. On the other hand, as before, it is noted that without activation the yield drops to insignificant values very quickly, after 1 h 30 to about 2 hours (fig. 2A).

Example 2

For this second example, the copper cathode has been replaced by a silver cathode. As in the case of the copper cathode alone, the proper functioning of the CO2 electolysis required periodic activation of the silver cathode. Such an activation here consisted of a succession of three cycles of a linear scan of the potential of the cathode between the usual potential of electrolysis, for example -2 V, and +0.3 V. This series of 3 cycles of scanning was carried out at a speed of 0.2 V / sec. and repeated every 5 min. during electrolysis. During these activation cycles, a more or less significant part of the copper deposited at the cathode during the electrolysis of CO2 is oxidized and passes into the solution in the form of Cu2 + ions.

This method makes it possible to obtain faradic yields of the main products comparable, or even higher compared to the copper electrode, while also making it possible to avoid deactivation of the cathode. For example, during an electrolysis conducted in 0.1 M NaCI04, saturated with CO2, containing 3.3 x 10 -4 mole / l of CUSO4, with the silver cathode polarized at -2 V (versus ENC) and activated every 5 min., the faradic yields after 5 h of electrolysis were approximately 35-40% of CH4, 30% of C2H4, 9% of C2H5OH and 3% of CO.

It is therefore clear from the above experiments illustrating the invention by way of examples that the activation which is the subject of this invention makes it possible to remove the blocking of the cathode, and thus to obtain high faradaic yields, even on electrolysis times greater than 24 hours. Such an improvement in yields and the possible duration of the process therefore makes it possible to envisage the use of electrolytic reduction of CO2 on an industrial scale.

Claims (7)

Claims 1. Process for the electrolytic reduction of carbon dioxide in an aqueous saline solution at neutral to weak alkaline pH saturated with CO2 and in the presence of a copper or silver cathode, in which this cathode is subjected to an electrochemical activation consisting of l application of at least one cycle of a scanning of the potential of the cathode comprising a linear increase in this potential up to a positive potential relative to the initial electrolysis potential, then the linear return thereto. 2. Method according to claim 1, in which the copper cathode is subjected to a succession of 2 to 5 cycles of a linear scan of the potential of the cathode which consists in an increase up to from 0.5 to 1.5 V of said potential with respect to a calomel reference electrode, at a speed of 0.5 to 20 V / s, then at the return to the initial electrolysis potential at a similar speed. 5 10 15 20 25 30 35 40 45 50 55 60 65 3 5 CH 686 788 A5 3. Method according to claim 2, in which the linear scanning of the cathode potential is carried out up to approximately 1 V, and the speed of this scanning from 1 to 10 V / sec., Preferably approximately 5 V / s. 4. The method of claim 1, wherein the silver cathode is subjected to a succession of 2 to 5 cycles of a linear scan of the potential of the cathode which consists in increasing up to about 0.4 V of said potential by relative to a calomel reference electrode, at a speed of 0.1 to 1 V / s, then to the return to the initial electrolysis potential at a similar speed, and in which the electrolyte contains a soluble copper salt in a concentration of 10 "6 to 5-10 ~ 3 mole / lt. 5. The method of claim 4, wherein the linear scanning of the cathode potential is carried out up to about 0.3 V, and the speed of this scanning is about 0.2 V / s. 6. Method according to one of claims 2 to 5, wherein the frequency of the scanning cycles is 3 to 15 minutes, preferably about 5 minutes. 7. Method according to one of claims 1 to 6, in which the electrolyte is chosen from acid carbonates, chlorides, perchlorates and sulfates of alkali metals, in a concentration of 0.1 to 1 M. 5 10 15 20 25 30 35 40 45 50 55 60 65 4