Method for generating hydrogen and oxygen by steam electrolysis The present invention relates to a method for generating highly reactive hydrogen and oxygen by steam electrolysis by means of a proton- 5 conducting membrane. Conductive ceramic membranes are today the subject of widespread research to increase their performances; in particular, said membranes find particularly interesting applications in fields such as the electrolysis of water at high temperature for the production of hydrogen or 10 the treatment of carbon gases (C02, GO) by electrochemicai hydrogenation. Patent applications W02008152317 and W02009150352 describe examples of such methods. Hydrogen (Hz) appears today as a very interesting energy vector, which is likely to take more and more importance for treating among others 15 petroleum products, and which could, in the longer term, advantageously act as a substitute for oil and fossil fuels, the reserves of which are going to decrease considerably over coming decades. In this perspective, it is nevertheless necessary to develop efficient methods of producing - hydrogen. 20 Numerous methods for producing hydrogen, from different sources, have certainly been described but many of said methods prove to be unsuitable for massive industrial production of hydrogen. In this context, the synthesis of hydrogen from the steam reforming of hydrocarbons may be cited for example. One of the major problems of 25 this synthesis route is that it generates, as sub-products, important quantities of greenhouse gases of C02 type. In fact, 8 to 10 tonnes of C02 are released to produce 1 tonne of hydrogen. Two challenges thus arise for future years: searching for a novel energy vector that can be used without danger for our environment, such 30 as hydrogen, and reducing the quantity of carbon dioxide. Technical-economic estimations of industrial methods now take this latter piece of information into account. However, it mainly involves sequestration, in particular underground sequestration in anfractuosities that do not necessarily correspond to former oil reservoirs, which in the 5 long run may not be without danger. A promising way forward for the industrial production of hydrogen is the technique known as steam electrolysis, for example at high temperature (HTE), at moderate temperature, typically above 200°C, or instead at intermediate temperature comprised between 200°C and 10 1000°C. At the present time, two steam electrolysis production methods are known: According to a first method illustrated in figure 1, an electrolyte is used that is capable of conducting 02-io ns and operating at temperatures 15 generally comprised between 750°C and 1000°C. More specifically, figure 1 schematically represents an electrolyser 1 comprising a ceramic membrane 2, conducting 02- ions, assuring the function of electrolyte separating an anode 3 and a cathode 4. The application of a potential difference between the anode 3 and 20 the cathode 4 leads to a reduction of the steam Hz0 on the side of the cathode 4. This reduction forms hydrogen H2 and 02- ions (0: in the Kroger-Vink notation) at the surface of the cathode 4 according to the reaction: The 02- ions, more specifically the oxygen vacancies (V;), migrate through the electrolyte 2 to form oxygen 0 2 at the surface of the anode 3, electrons e' being released according to the oxidation reaction: 30 Thus, this first method makes it possible to generate at the outlet of the electrolyser 1 oxygen - anodic compartment - and hydrogen mixed 5 with steam - cathodic compartment. According to a second method illustrated in figure 2, an electrolyte is used that is capable of conducting protons and operating at lower temperatures than those required by the first method described above, generally comprised between 200°C and 800°C. 10 More specifically, this figure 2 schematically represents an electrolyser 10 comprising a proton-conducting ceramic membrane 11 assuring the function of electrolyte separating an anode 12 and a cathode 13. The application of a potential difference between the anode 12 and 15 the cathode 13 causes an oxidation of the steam Hz0 on the side of the anode 12. The steam fed into the anode 12 is thus oxidised to form oxygen 0 2 and H+ ions (or OH; in the Kroger-Vink notation), this reaction releasing electrons e- according to the equation: 20 1 H,0 + 20; + 20H, +-0, + 2e' 2 The H+ ions (or OH; in the Kroger-Vink notation) migrate through the electrolyte 11, to form hydrogen Hz at the surface of the cathode 13 according to the equation: Thus, this method provides at the outlet of the electrolyser 10 pure hydrogen - cathodic compartment - and oxygen mixed with steam - anodic combartment. More specifically, the formation of H2 goes through the formation of intermediate compounds, which are atoms of hydrogen adsorbed at the surface of the cathode with variable energies and degrees of interaction andlor radical hydrogen atoms n iH,,., H;(or the Kroger-Vink notation). 5 These species being highly reactive, they usually recombine to form hydrogen H2 according to the equation: 10 The patent application W02008152317 has shown that the insertion of steam under pressure makes it possible to remain at moderate operating temperatures (of the order of 500 to 600°C) while obtaining conductivity values assured by the movement of relatively high H+ protons. Nevertheless, this type of proton-conducting electrolysis is above all 15 studied and developed at the laboratory level with low current levels. Some fear, as in the case of 02- conducting electrolysis, phenomena of delamination of the electrode, which can Induce decohesion between said electrode and the electrolyte during use with higher current densities. - ., In fact, unlike the field of anionic-conducting electrolyses, the 20 chargecarriers (protons) are not intrinsic to the structure of the membrane and are thus consequently more limited in the structure than the charge carriers of an anionic conddion, which are formed by the vacancies of the structure. Consequently, it is known to use in the field of anionic-conducting 25 electrolyses current densities at the terminals of the electrodes greater than the current densities used in the field of proton-conducting electrolyses. Nevertheless, the application of such current densities at the terminals of the electrodes of a proton-conducting electrolyser in electrolysers of the prior art would cause localised over-voltages which would lead to phenomena of delamination of the electrodes. In this context, the present invention aims to propose a method for generating highly reactive hydrogen and oxygen adsorbates by steam 5 electrolysis using an electrolysis cell comprising a solid proton-conducting electrolyte, said method being able to be industrialised while limiting the risks of delamination of the electrodes. To this end, the invention proposes a method for generating hydrogen and oxygen adsorbates by steam electrolysis at 200"G to 800°C 10 using an electrolysis cell comprising a solid electrolyte, which is made of a proton-conducting ceramic, said electrolyte being arranged between an anode and a cathode, said anode and cathode each comprising a protonconducting ceramic and the ratio of the electroactive surface to the geometric surface of each of which is equal to at least 10, said method 15 comprising the following steps: - circulating a current between the anode and the cathode, wherein the density of the current is no less than 500 mNcm2; - inserting water in the form of steam, which is fed under pressure to the anode; 20 - oxidizing said water in the form of steam at the anode; - generating highly reactive oxygen at the anode after said oxidation; - generating protonated species in the electrolyte after said oxidation; - migrating said protonated species in the electrolyte; - reducing said protonated species at the surface of the cathode in 25 the form of reactive hydrogen atoms. It will be noted that the current may be continuous or pulsed; in the case of a pulsed current, current density is taken to mean the current density corresponding to the maximum value of the current intensity reached during the pulse. 30 The generation of the current may be obtained by different means: - a generator imposing a voltage at the terminals of the assembly (i.e. a potential difference between the electrodes) may be used; - a current source impos~nga current between the electrodes may be used; 5 - operation in potentiostatic mode may also be used; in other words, in addition to the two cathode and anode electrodes, at least one third electrode known as reference is used. The working electrode (preferentially the cathode) is then going to be taken to a given potential with respect to the reference electrode (in which one 10 avoids passing too much current so as not to modify its potential that serves as reference). The generator making it possible to maintain automatically the potential of the working electrode, even under current, is known as a potentiostat. As explained above, reactive hydrogen atoms are taken to mean 15 hydrogen atoms adsorbed at the surface of the cathode andlor radical hydrogen atoms H. (or H&c,,o, in the Kroger-Vink notation). Geometric surface of an electrode is taken to mean its flat external surface and electroactive surface is taken to mean the surface constituted of the internal surface of the pores of the electrode wherein takes place 20 the electrochemical reaction; in other words, it is the internal surface on which the reaction takes place: 2e' + 20H; + 20; + H, . The electrodes according to the invention thus have a large number of triple points, namely points or contact surfaces between an ionic conductor, an electron conductor and a gas phase. 25 The invention results from the finding made by the applicant that the use of electrodes (cathode and anode) comprising a proton-conducting ceramic (typically electrodes formed of a cermet including a mixture of said ceramic of perovskite type and a metal alloy andlor a perovskite doped with a ianthanide with one or more degrees of oxidation) surrounding a 30 proton-conducting electrolyte and having a sufficiently high electroactive surface I geometric surface ratio makes it possible to work at much higher current densities than those provided in the prior art without risk of delamination of said electrodes. In fact, the consequent increase of the ratio between the 5 electroactive surface and the geometric surface of the electrodes compared to the ratio of electrodes of the prior art makes it possible to reduce local over-voltages, which are responsible for phenomena of delamination of the electrodes. The method according to the invention generates highly reactive 10 hydrogen at the cathode of the electrolyser (particularly hydrogen atoms adsorbed on the surface of the electrode andlor radical hydrogen atoms). These highly reactive hydrogen atoms H&,,o, are formed at the surface of the cathode according to the reaction: 15 i + OH; -+ 0: + H&c,,o, These highly reactive hydrogen atoms may be used as such for the production of hydrogen or for other applications that will be detailed hereafter. 20 The method according to the invention may also have one or more of the characteristics below, considered individually or according to any technically possible combinations thereof: - in a particularly advantageous manner, said ratio between the electroactive surface and the geometric surface of said cathode and 25 anode is no less than 100; such a ratio makes it possible to further improve the resistance of the electrodes at high current densities without risk of delamination; - said density of the current is no less than 1 Alcm2; - the partial and relative steam pressure is advantageously no less 30 than 1 bar and preferentially no less than 10 bars; - the circulation of the current takes place between an anode and a cathode, each made of a cermet constituted of a mixture of a proton-conducting ceramic and a conducting material; - said conducting material is a passivable material with high melting point being able to comprise at least 40% of chromium; - the circulation of the current takes place between an anode and a cathode, each comprising a proton-conducting ceramic formed of a perovskite doped with a lanthanide with one or more degrees of oxidation, said ceramic being doped with a complementary doping element taken from the following group: niobium, tantalum, vanadium, phosphorous, arsenic, antimony, bismuth; - the method according to the invention comprises the following steps: o introducing carbon dioxide C02 andlor carbon monoxide CO at the cathode of the electrolysis cell; o reducing the C02 andlor CO introduced at the cathode from said generated reactive hydrogen atoms; o forming CxHyOz type compounds, where x>l,Ol, y2O and 220). 15 The cell 30 of figure 3 further comprises means 36 making it possible to insert under pressure NOx type compounds (x.52) into the cathodic compartment 33. The problem consists in enabling the reduction by electro-catalytic hydrogenation of the NOx content of effluents produced for example during 20 the combustion of hydrocarbons or gas. The production of these molecules is 60% due to urban transport and 40% due to boilers and thermal power plants. These molecules easily penetrate the bronchioles and affect respiration, causing hyper reactivity of the bronchial tubes in asthmatics, as well as increased vulnerability of the bronchial tubes to 25 microbes, at least in children. Consequently, the regulations in force require industries to limit their NOx discharges. It is known to those skilled in the art to reduce NOx by two types of method: selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). Whatever the solution retained (i.e. with or without 30 catalyst), the latter is based on the use of ammonia to reduce the NOx into N2. These solutions have the drawback of using ammonia as hydrogen vector whereas it would be more interesting to directly treat the NOx with ~~~ hydrogen. The production of ammonia assumes in fact using a method of steam reforming of hydrocarbons generating COz. This method furthermore implies the use of a second reactor for the production of- 5 ammonia. The method of using the cell 30 according to figure 5 is based on the following principle: pressurised steam is introduced at the anodic compartment 32 and the NOX is fed under pressure at the cathodic compartment 33. The incorporation under pressure of steam is going to 10 lead to oxidation of this water in the form of steam at the surface of the anode so as to generate protonated species in the membrane which, after migration within the membrane, are reduced at the surface of the cathode into very reactive hydrogen capable of reducing by hydrogenation the NOX introduced into the cathodic compartment, such that the NOx are reduced 15 into less oxidised NOy (where y, react with the nitrogen containing compounds at the cathode 33 to give reduced compounds of nitrogen oxides of NtOyHz type, where x>_l and yXl and z>_O according to the 25 reaction: fNOr+(2ts-2y+z)H,",; dr + ( t . ~ - y)H20+N,0,,Hz. As an example, these compounds are either NOy less oxidised than the NOx compounds fed in under pressure, nitrogen N2, or ammonia NH3. The overall reactions at the electrodes are written: 1 H,0 + 20," + 20H; + -0, + 2e' (anode 32) 2 tN0, + (2tr - 2y + z)e + (21% - 2y + z)OH; + (2t.t - 2y + z)Oa + (t.t - y)H,O + N,O,,H, (cathode 33). The solution according to the invention makes it possible to reduce 5 the number of reactors required for the reduction of NOx to a single and unique reactor accommodating the electro-hydrogenation. Figure 6 illustrates a third example in which the electrolysis cell 30 is used to produce ammonia by electro-catalytic hydrogenation of Nz. It should be noted that, according to this embodiment, it is also possible to 10 produce other NxHy type compounds where x2l and y2O before resulting in the formation of NH3. The cell 30 of figure 3 further comprises means 36 making it possible to insert nitrogen N2 under pressure into the cathodic compartment 33. 15 The problem is here to produce in massive quantity, at low cost and without emission of C02, ammonia, by electro-catalytic hydrogenation of N2. At present, ammonia is produced by catalytic hydrogenation reaction of N2 during steam reforming of hydrocarbons. The synthesis of 20 this product thus indirectly emits CO2. In addition, the synthesis method induces a very great volatility ~ of the production price of NH3. In fact 80 % . ~. - -~... . ~. ~ of the price of NH3 is directly dependent on the price of the gas from which is produced the hydrogen required for the synthesis. Thus, the volatility of the price of ammonia is very high and dependent on that of the gas. 25 Moreover, according to known techniques, even when hydrogen is produced by electrolysis, it is necessary to use two reactors, one for the production of hydrogen and the other for the catalytic reaction. The solution implemented in the cell 30 of figure 6 aims to produce ammonia using a single reactor. As previously, the hydrogenated monoatomic compounds are formed at the surface of the cathode according to the reaction: e' + OH; + 0: + H:~~,,~. ,,, Consequently, in the presence of N2 on the cathodic side 33, very 5 reactive hydrogen H&<,,,, reacts with the hydrogenated compounds on the electrode 33 to give NH3 according to the reaction: - N, + 6H& + 2NH,. The overall reactions at the electrodes are written: 1 H,O+ 20: --, 20Hb + -0, f2e' 2 10 N, + 6eS + 60H; + 60: + 2NH, The solution of figure 6 makes it possible to reduce the number of reactors required for the production of NH3 (which serves as HZ vector) to a single and unique reactor accommodating the electro-hydrogenation. In the solution proposed, the hydrogen required for the reduction of 15 the nitrogen is no longer produced from fossil energy; the method according to the invention is "cleaner" in so far as it does not generate C02. Moreover, such a method makes it possible to do without the use of catalyst, which it is necessary to change and to recycle on account of its 20 deactivation by the water produced during the catalytic reduction reaction. Finally, the solution proposed makes it possible to avoid a storage of Hz since the reactions of production of reactive hydrogen and reduction by hydrogenation take place in the same reactor. As it has been possible to see with reference to figures 4 to 6, the 25 highly reactive hydrogen produced by the cell 30 of figure 3 may be used industrially for very different applications. Obviously, the invention is not limited to the embodiments that have been described. Thus, the hydrogenation by the highly reactive hydrogen atoms may also be used in the petrochemical industry, for example to convert aromatic compounds 30 into saturated alkanes (paraffins) and into cycloalkanes (naphthenes). The method according to the invention may also be used to carry out hydrocracking making it possible to convert, under hydrogen pressure and at sufficiently high temperature, heavy petroleum products into light products: typically, hydrocracking makes it possible to obtain products 5 such as diesel oil or kerosene from heavy residues. CLAIMS -1. Method for generating hydrogen and oxygen adsorbates by steam electrolysis at 200°C to 800°C using an electrolysis cell (30) 5 comprising a solid electrolyte (31) which is made of a protonconducting ceramic, said electrolyte (31) being arranged between an anode (32) and a cathode (33), said anode and cathode each comprising a proton-conducting ceramic and the ratio of the electroactive surface to the geometric surface of each of which is 10 equal to at least 10, said method comprising the following steps: - circulating a current between the anode (32) and the cathode (33)' wherein the density of the current is no less than 500 mAlcm2; - inserting water in the form of steam, which is fed under 15 pressure to the anode (32); - oxidizing said water in the form of steam at the anode (32); - generating highly reactive oxygen at the anode (32) after said oxidation; - generating protonated species in the electiolyte (31) after 20 said oxidation; - migrating said protonated species in the electrolyte (31); ., : : . -. ~ ~ - reducing said protonated species at the sufface of the cathode (33) in the form of reactive hydrogen atoms. 2. Method according to the preceding claim characterised in that said 25 ratio between the electroactive surface and the geometric surface of said cathode and anode is no less than 100. 3. Method according to one of the preceding claims characterised in that said density of the current is no less than 1 A/cm2. 30 4. Method according to one of the preceding claims characterised in that the partial and relative pressure of the steam is advantageously no less than 1 bar and preferentially no less than 10 bars. 5 5. Method according to one of the preceding claims characterised in that the circulation of the current takes place between an anode and a cathode each made of a cermet constituted of a mixture of a proton-conducting ceramic and a conducting material. 10 6. Method according to one of the preceding claims characterised in that said conducting material is a passivable material with high melting point being able to contain at least 40% of chromium. 7. Method according to one of the preceding claims characterised in 15 that the circulation of the current takes place between an anode and a cathode each comprising a proton-conducting ceramic formed of a perovskite doped with a lanthanide with one or more degrees of oxidation. 20 8. Method according to one of the preceding claims characterised in that it comprises the following steps: ~. . ., , ~ , ~ .... .. .~ , ~ -. . . . ; ... .. .., - introducing carbon dioxide CO2 andlor carbon monoxide CO at the cathode of the electrolysis cell; - reducing the C0z andlor CO introduced at the cathode from said 25 generated reactive hydrogen atoms; - forming compounds of CxHyOz type, where x21, O