CN110863213A - Method for improving corrosion resistance of nickel electrode in molten salt system - Google Patents
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Abstract
The invention relates to a method for improving the corrosion resistance of a nickel electrode in a molten salt system, which comprises the following steps: (1) carrying out preoxidation film forming treatment on the nickel electrode by a molten salt in-situ film forming or air heating film forming method to obtain a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer; (2) and (2) using the modified nickel electrode obtained in the step (1) as an anode in a molten salt system. The method of the invention carries out preoxidation film forming treatment on the nickel electrode by a fused salt in-situ film forming or air heating film forming method, changes the surface structure composition of the nickel electrode, slows down the oxidation and corrosion of the metal nickel electrode in a fused salt system, protects the internal metal by virtue of a nickel oxide protective layer on the surface of the modified nickel electrode, reduces the loss of a nickel anode, prolongs the time of electrochemical reaction, and improves the continuity of the electrochemical reaction and the current efficiency of gas products.
Description
Technical Field
The invention belongs to high-temperature molten salt driven CO2/H2The technical field of O hydrocarbon production systems, in particular to a method for improving the corrosion resistance of a nickel electrode in a molten salt system.
Background
CO2Is one of the most prominent greenhouse gases causing global climate change. CO 22The large amount of emissions has become an international problem that has a great influence on future changes in world patterns, how to control CO2The emission is already listed as the first issue by governments and united nations meetings, and becomes a strategic issue to be solved urgently in a plurality of global major problems. In addition, carbon dioxide is a potential carbon resource, and the development of a corresponding carbon dioxide recycling technology has important strategic significance. Compared with the chemical method which requires harsh reaction conditions such as high temperature and high pressure, the electrochemical conversion of CO has mild conditions and easy operation in recent years2The technology has become CO2One of the hot spots of research in the field of recycling.
Chinese patent application CN104562073A provides a high-temperature electrolysis method for CO2/H2O production hydrocarbon system capable of realizing CO at low electrolysis voltage and relatively low temperature2/H2The co-electrolysis conversion of O is used for preparing hydrocarbon, the electrolysis reaction is relatively simple, and the selectivity is good in short time; this patent application has been on CO2The transformation mechanism in a molten salt system and key control factors of the electrolysis process are clearly explored, but the problem of the electrode is not well solved all the time; this is because the electrochemical oxidation of conventional metal anodes and the physical dissolution process of surface oxide films do not maintain stability in molten salt systems for long periods of time. For example, in chinese patent application CN104562073A, metallic nickel is selected as the high temperature molten salt to drive CO2/H2When the anode of the hydrocarbon-rich carbon-based fuel gas system is prepared by the synergistic conversion of O, certain corrosion resistance can be kept in a short time, but along with the prolonging of reaction time, the nickel anode is gradually oxidized into nickel oxide, and a relatively serious corrosion phenomenon occurs, so that the current efficiency of a gas product is reduced and the energy is lost,even electrode breakage occurs, and the long-time continuous proceeding of the electrochemical reaction cannot be guaranteed.
In summary, it is very necessary to provide a method capable of prolonging the service life of the nickel anode, improving the energy conversion efficiency, avoiding the pollution of molten salt, and reducing the corrosion rate of the nickel anode in the molten salt system.
Disclosure of Invention
In order to solve the technical problems in the prior art, the invention provides a method for improving the corrosion resistance of a nickel electrode in a molten salt system, the method carries out preoxidation film forming treatment on the nickel electrode through molten salt in-situ film forming or air heating film forming, changes the surface structure composition of the nickel electrode, slows down the oxidation and corrosion of the metal nickel electrode in the molten salt system, protects internal metal by means of a nickel oxide protective layer on the surface of the modified nickel electrode, reduces the loss of a nickel anode, prolongs the time of electrochemical reaction, and improves the continuity of the electrochemical reaction and the current efficiency of gas products.
In order to achieve the above object, the present invention provides in a first aspect a method for improving corrosion resistance of a nickel electrode in a molten salt system, the method comprising the steps of:
(1) carrying out preoxidation film forming treatment on the nickel electrode by a molten salt in-situ film forming or air heating film forming method to obtain a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer;
(2) and (2) using the modified nickel electrode obtained in the step (1) as an anode in a molten salt system.
Preferably, the method further comprises adding a metal oxide to the molten salt system.
Preferably, the metal oxide is one or more of calcium oxide, barium oxide, magnesium oxide, strontium oxide and zinc oxide; preferably, the metal oxide is calcium oxide and/or barium oxide; and/or the molar percentage of the metal oxide in a molten salt system is 0.5-10 mol%.
Preferably, the pre-oxidation film-forming treatment of the nickel electrode by the molten salt in-situ film-forming method comprises the following steps: taking a mixed molten salt formed by sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, taking a nickel electrode as an anode and a cathode respectively, and carrying out pre-oxidation film-forming treatment for 1-3 h under the conditions of an electrolysis voltage of 0.5-1.5V and an electrolysis temperature of 650-750 ℃ in an argon atmosphere; preferably, the mass ratio of the sodium carbonate to the potassium carbonate is 1:1, and the molar ratio of the total amount of the sodium carbonate and the potassium carbonate to the lithium hydroxide is 1: 0.2.
Preferably, the pre-oxidation film forming treatment of the nickel electrode by the air heating film forming method comprises the following steps: roasting the nickel electrode for 1-3 h at 500-900 ℃ in air atmosphere; preferably, the nickel electrode is roasted for 2 hours at 800-900 ℃ in the air atmosphere.
Preferably, the molten salt system takes mixed molten salt formed by carbonate and hydroxide as electrolyte, and takes nickel, platinum, titanium, iridium, ruthenium, palladium, iron, tungsten, chromium, copper, gold, graphite or stainless steel as cathode material; preferably, the carbonate is one or more of lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate and rubidium carbonate, and the hydroxide is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, calcium hydroxide, magnesium hydroxide, strontium hydroxide, barium hydroxide and zinc hydroxide; more preferably, the carbonate is formed by mixing lithium carbonate, sodium carbonate and potassium carbonate in a mass ratio of 1:1:1, and the hydroxide is lithium hydroxide.
Preferably, the molar ratio of the carbonate to the hydroxide is 1: (0.1 to 0.4), preferably 1: 0.2.
The invention provides a preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer in a second aspect, which comprises the following steps: taking a mixed molten salt formed by sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, taking a nickel electrode as an anode and a cathode respectively, and carrying out pre-oxidation film-forming treatment for 1-3 h under the conditions that the electrolysis voltage is 0.5-1.5V and the electrolysis temperature is 650-750 ℃ in an argon atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer at the anode.
The invention provides a preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer in a third aspect, which comprises the following steps: and roasting the nickel electrode for 1-3 hours at 500-900 ℃ in an air atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer.
The present invention provides, in a fourth aspect, a modified nickel electrode coated with a nickel-nickel oxide composite oxide layer produced by the production method according to the second aspect of the present invention or the production method according to the third aspect of the present invention.
Compared with the prior art, the invention at least has the following beneficial effects:
(1) the invention carries out preoxidation film forming treatment on the nickel electrode for the first time by a method of in-situ film forming of molten salt or film forming by air heating, changes the surface structure composition of the nickel electrode, forms a layer of nickel-nickel oxide composite protective film with a protective effect on the surface of the nickel anode, and the protective film can prevent oxygen anions in the molten salt from corroding metals in the anode, slow down the oxidation and corrosion of the metal nickel electrode in a molten salt system, remarkably improve the corrosion resistance of the nickel anode in the molten salt system, reduce the loss of the nickel anode, prolong the time of electrochemical reaction, and improve the continuity of the electrochemical reaction and the current efficiency of gas products.
(2) In some preferred embodiments of the present invention, the solubility of nickel oxide can be significantly reduced by adding a metal oxide such as calcium oxide and/or barium oxide to the molten salt system, the corrosion of the modified nickel electrode can be further inhibited by controlling the solubility of nickel oxide in the molten salt system, and the electrolysis product CH can be increased4Selectivity and current efficiency.
(3) The nickel anode formed by heating air into a film shows better corrosion resistance, can be stably used for 4 hours in a molten salt system, and prolongs the service life of the nickel anode in a molten salt medium from 30min to 240 min; the solubility of NiO in the molten salt is gradually reduced along with the temperature rise, and is 0.145 wt% at about 600 ℃; when the addition amount of CaO is 1.5 mol%, the solubility of NiO is reduced from 0.145 wt% to 0.107 wt%; when the addition amount of BaO is 3 mol%, the NiO solubility is reduced to 0.118 wt%; when the addition amount of CaO reaches 5 mol%, CH in the gas product4The selectivity and the current efficiency are both obviously improved.
Drawings
FIG. 1 is a photograph of a pre-oxidized film-formed nickel electrode prepared by a method of forming a film by heating air at different temperatures according to the present invention.
FIG. 2 is an XRD spectrum of a pre-oxidized film-formed nickel electrode prepared by the method of heating the air to form a film at 800 ℃.
FIG. 3 is a current-time curve diagram monitored when an electrolysis experiment was performed using a pure nickel anode that was not subjected to pre-oxidation film formation, a molten salt in-situ pre-oxidation film formation nickel anode for 3 hours, and a pre-oxidation film formation nickel anode heated by air at 800 ℃ for 2 hours, respectively, in example 1 of the present invention. In the figure, the abscissa Time represents Time in minutes (min); the ordinate Current represents the Current in amperes (a).
FIG. 4 is a photograph of the nickel anode as a film formed by pre-oxidation in example 1 of the present invention after the electrode reaction. In the figure, a represents a nickel anode which is pre-oxidized into a film by air heating; in the figure, b represents the nickel anode formed by in-situ preoxidation of molten salt.
FIG. 5 is a graph showing the change in solubility of nickel oxide according to the amount of lithium oxide added in example 2 of the present invention. On the abscissa of the graph, Li2Molar addition amount of O (Li)2O addition in mol%), and the ordinate is the Solubility of NiO (Solubility in wt%).
FIG. 6 is a graph showing the solubility of nickel oxide according to the amount of calcium oxide or barium oxide added in example 2 of the present invention. In the figure, the abscissa represents the molar addition amount of CaO (CaO addition in mol%) and the ordinate represents the Solubility of NiO (solubilty in wt%); in the figure, the abscissa of b represents the molar addition amount of BaO (BaO addition in mol%) and the ordinate represents the Solubility of NiO (Solubility in wt%).
FIG. 7 shows the amount of CaO added to CH in a gaseous product in accordance with example 3 of the present invention4Selectivity and current efficiency. The abscissa of the graph represents the molar addition amount of CaO (CaO addition in mol%) and the left ordinate represents CH4Selectivity of Methane in%, the right ordinate is the Current efficiency in%.
FIG. 8 shows BaO addition in example 3 of the present inventionAddition amount to CH in gas product4Selectivity and current efficiency. The abscissa of the graph represents the molar addition amount of BaO (BaO addition in mol%) and the ordinate on the left side represents CH4Selectivity of Methane in%, the right ordinate is the Current efficiency in%.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
The present invention provides in a first aspect a method of improving the corrosion resistance of a nickel electrode in a molten salt system, the method comprising the steps of:
(1) pre-oxidizing the nickel electrode to form a film by a molten salt in-situ film forming method or an air heating film forming method to obtain a modified nickel electrode (a pre-formed film Ni-NiO composite modified nickel electrode or a pre-oxidized film nickel electrode) coated by a nickel-nickel oxide composite oxide layer;
(2) and (2) using the modified nickel electrode obtained in the step (1) as an anode in a molten salt system.
The invention carries out preoxidation film forming treatment on the nickel electrode for the first time by a method of in-situ film forming of molten salt or film forming by air heating, changes the surface structure composition of the nickel electrode, forms a layer of nickel-nickel oxide composite protective film with a protective effect on the surface of the nickel anode, and the protective film can prevent oxygen anions in the molten salt from corroding metals in the anode, slow down the oxidation and corrosion of the metal nickel electrode in a molten salt system, remarkably improve the corrosion resistance of the nickel anode in the molten salt system, reduce the loss of the nickel anode, prolong the time of electrochemical reaction, and improve the continuity of the electrochemical reaction and the current efficiency of gas products.
According to some preferred embodiments, the method further comprises adding a metal oxide to the molten salt system.
According to some preferred embodiments, the metal oxide is one or more of calcium oxide, barium oxide, magnesium oxide, strontium oxide, and zinc oxide; preferably, the metal oxide is calcium oxide and/or barium oxide; in the present invention, it is preferable that the solubility of nickel oxide can be significantly reduced by adding a metal oxide such as calcium oxide and/or barium oxide to the molten salt system, corrosion of the modified nickel electrode can be further suppressed by controlling the solubility of nickel oxide in the molten salt system, and the electrolysis product CH can be increased4Selectivity and current efficiency.
According to some preferred embodiments, the method further comprises the step of pre-treating the metal oxide: grinding metal oxide (such as barium oxide, calcium oxide) into fine powder, placing in a muffle furnace, drying at 250 deg.C for 24 hr in argon atmosphere, removing water mixed with the metal oxide, cooling in argon atmosphere, transferring into a sealed tank, and placing in a dryer for use.
According to some preferred embodiments, the metal oxide is present in the molten salt system in a molar percentage of 0.5 to 10 mol% (e.g., 0.5 mol%, 1 mol%, 1.5 mol%, 2 mol%, 2.5 mol%, 3 mol%, 3.5 mol%, 4 mol%, 4.5 mol%, 5 mol%, 5.5 mol%, 6 mol%, 6.5 mol%, 7 mol%, 7.5 mol%, 8 mol%, 8.5 mol%, 9 mol%, 9.5 mol%, or 10 mol%).
According to some preferred embodiments, the metal oxide is calcium oxide, and the molar percentage of the calcium oxide in the molten salt system is 0.5 to 8 mol%, and more preferably 5 mol%.
According to some more preferred embodiments, the pre-oxidation film forming treatment of the nickel electrode by the molten salt in-situ film forming method comprises the following steps: using a mixed molten salt of sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, using a nickel electrode as an anode and a cathode, respectively, and pre-treating the electrolyte in an argon atmosphere at an electrolysis voltage of 0.5 to 1.5V (e.g., 0.5, 1.0 or 1.5V) and an electrolysis temperature of 650 to 750 ℃ (e.g., 650 ℃, 700 ℃ or 750 ℃)The oxidation film forming treatment is carried out for 1-3 h (for example, 1, 1.5, 2, 2.5 or 3h), a composite oxide protective layer containing nickel oxide and nickel is attached to the surface of the nickel electrode subjected to the pre-oxidation film forming treatment in a molten salt system, and the inventor finds that the content of nickel oxide in the composite oxide protective layer is obviously increased along with the prolonging of the pre-oxidation film forming treatment time in the molten salt, and the content of nickel is in a descending trend along with the prolonging of the treatment time; in some more preferred embodiments, the mass ratio of the sodium carbonate to the potassium carbonate is 1:1, and the molar ratio of the total amount of the sodium carbonate and the potassium carbonate to the lithium hydroxide is 1: 0.2; in the invention, in the molten salt in-situ film-forming molten salt system of the molten salt, the selected mixed molten salt does not contain lithium because the pure nickel anode can not keep inert and stable work in the lithium-containing molten salt system for a long time, therefore, the invention uses Na which does not contain lithium2CO3-K2CO3And as an electrolyte system, carrying out molten salt in-situ preoxidation film forming treatment on the nickel anode. In the molten salt in-situ film-forming molten salt system, when the mass ratio of the sodium carbonate to the potassium carbonate in the mixed molten salt is 1:1, and the molar ratio of the total amount of the sodium carbonate and the potassium carbonate to the lithium hydroxide is 1:0.2, the molten salt system (electrolyte system) is also marked as Na2CO3-K2CO3+0.2LiOH。
According to some embodiments, Na is2CO3-K2CO3+0.2LiOH mixed molten salt 50g in total is put into an alumina crucible with the operation temperature of 700 ℃ and the surface area of 20cm2The nickel anode and the nickel cathode are inserted into the molten salt medium; in an argon atmosphere, a pre-oxidation film formation treatment was performed at a constant voltage of 1.0V for 3 hours.
According to some preferred embodiments, the pre-oxidation film forming treatment of the nickel electrode by the air heating film forming method comprises the following steps: roasting the nickel electrode in an air atmosphere at 500-900 ℃ (e.g. 500 ℃, 600 ℃, 700 ℃, 800 ℃ or 900 ℃) for 1-3 h (e.g. 1, 1.5, 2, 2.5 or 3 h); preferably, the nickel electrode is roasted for 2 hours at the temperature of 800-900 ℃ in the air atmosphere; in some specific embodiments, the invention pre-oxidizes clean nickel electrodes into films in air at 500 ℃, 600 ℃, 700 ℃, 800 ℃ and 900 ℃ for 2h, the photographs of the electrodes after film formation are shown in fig. 1, and as can be seen from fig. 1, the clean nickel sheets show bright silvery-white luster, and the electrodes after pre-oxidation film formation lose the silvery-white luster; with the increase of the preoxidation film forming temperature, the color of the surface of the nickel sheet is gradually deepened; when the temperature is lower than 700 ℃, the oxide film on the surface of the nickel sheet has obvious non-uniformity and is not completely covered; when the temperature reaches above 800 ℃, a uniform film is observed on the surface of the nickel, and an XRD spectrogram (shown in figure 2) shows that the film formed by heating and pre-oxidizing in air at 800 ℃ is a composite oxide protective layer with coexistence of nickel oxide and nickel; therefore, in the present invention, it is more preferable that the nickel electrode is baked for 2 hours at 800 to 900 ℃ in an air atmosphere to perform a pre-oxidation film formation treatment.
According to some preferred embodiments, the molten salt system uses a mixed molten salt formed by carbonate (molten carbonate) and hydroxide (molten hydroxide) as an electrolyte, and nickel, platinum, titanium, iridium, ruthenium, palladium, iron, tungsten, chromium, copper, gold, graphite, or stainless steel as a cathode material; preferably, the carbonate is one or more of lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate and rubidium carbonate, and the hydroxide is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, calcium hydroxide, magnesium hydroxide, strontium hydroxide, barium hydroxide and zinc hydroxide; more preferably, the carbonate is formed by mixing lithium carbonate, sodium carbonate and potassium carbonate in a mass ratio of 1:1:1, and the hydroxide is lithium hydroxide.
According to some preferred embodiments, the molar ratio of the carbonate to the hydroxide is 1: (0.1 to 0.4), preferably 1: 0.2. In step (2) of the present invention, when the electrolyte of the molten salt system is composed of lithium carbonate, carbonate obtained by mixing sodium carbonate and potassium carbonate in a mass ratio of 1:1:1, and lithium hydroxide, and the molar ratio of the carbonate to the lithium hydroxide is 1:0.2, the molten salt system is also referred to as Li0.85Na0.61K0.54CO3+0.2LiOH molten salt system.
The invention provides a preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer in a second aspect, which comprises the following steps: taking a mixed molten salt formed by sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, taking a nickel electrode as an anode and a cathode respectively, and carrying out pre-oxidation film-forming treatment for 1-3 h under the conditions that the electrolysis voltage is 0.5-1.5V and the electrolysis temperature is 650-750 ℃ in an argon atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer at the anode.
The invention provides a preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer in a third aspect, which comprises the following steps: and roasting the nickel electrode for 1-3 hours at 500-900 ℃ in an air atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer.
The present invention provides, in a fourth aspect, a modified nickel electrode coated with a nickel-nickel oxide composite oxide layer produced by the production method according to the second aspect or the third aspect of the present invention.
The present invention will be further described with reference to the following examples. These examples are merely illustrative of preferred embodiments of the present invention and the scope of the present invention should not be construed as being limited to these examples.
Example 1: corrosion resistance (durability) test of pre-oxidized film-formed nickel electrode
① mixing Na2CO3-K2CO3+0.2LiOH mixed molten salt 50g in total is put into an alumina crucible with the operation temperature of 700 ℃ and the surface area of 20cm2The nickel anode and the nickel cathode are inserted into the molten salt medium; and in an argon atmosphere, carrying out preoxidation film-forming treatment for 3h at a constant voltage of 1.0V to prepare the molten salt in-situ preoxidation film-forming nickel anode.
② surface area of 20cm2The nickel electrode is heated for 2 hours in air at 800 ℃ to prepare the air-heating preoxidized film-forming nickel anode.
The nickel electrode prepared by the two methods and pre-oxidized into a film is used as an anode, and iron with the same surface area is used as a cathodeWith Li0.85Na0.61K0.54CO3+0.2LiOH mixed molten salt as electrolyte, under argon atmosphere at 2.0V constant voltage 575 deg.C, CO is carried out2/H2Experiment of preparing hydrocarbon-rich carbon-based fuel gas by co-reforming O.
In this example, the corrosion resistance of the modified nickel electrode coated with the nickel-nickel oxide composite oxide layer prepared by the two pre-oxidation film-forming methods in a molten salt system was evaluated by monitoring the current change with time during the constant voltage electrolysis, and the result is shown in fig. 3; FIG. 3 is a current-time curve monitored during an electrolysis experiment using a pure nickel anode which is not subjected to pre-oxidation film formation, a nickel anode which is subjected to in-situ pre-oxidation film formation for 3 hours by molten salt, and a nickel anode which is subjected to pre-oxidation film formation by heating air at 800 ℃ for 2 hours, respectively.
As can be seen from FIG. 3, when the reaction time is short, the currents of the three electrode systems are stable, and the anode surface is not strongly corroded; however, when the reaction time exceeds 60min, the current of the nickel anode system without pre-oxidation film forming treatment starts to fluctuate sharply and decrease gradually, because a layer of nickel oxide film is gradually formed on the surface of the pure nickel anode in a high-temperature oxygen-containing environment, and the film is not generated at one time, but is generated and dropped repeatedly in the electrolytic process, so that the anode is in a continuous oxidation state, and the anode is seriously corroded; the surface of the nickel anode subjected to the fused salt in-situ preoxidation film-forming treatment is covered with a protective film, but the density of the film is lower, and the current becomes unstable after 160min electrolysis; at the moment, the oxide film covered on the surface of the electrode falls off, so that the metal nickel inside is directly corroded by the molten salt medium, and the current obviously changes. Among the three nickel anodes, the nickel electrode formed by heating air into a film shows excellent molten salt corrosion resistance, and the current value is slightly reduced only at the later stage of the experiment; compared with a pure nickel anode, the air heating pre-oxidation film-forming nickel anode improves the service life of the nickel anode in a molten salt medium from 30min to 240 min.
While monitoring the current-time curve in the electrochemical reaction process, the current efficiency of the gas products generated by the above three electrode systems was calculated, and the results are shown in table 1; from the results of table 1, it can be seen that the corrosion resistance of the pure nickel anode is the worst, and a large amount of electric energy is consumed in the oxidation reaction of the nickel anode during the electrolysis process, resulting in extremely low current efficiency; the nickel anode formed by in-situ molten salt preoxidation film shows slightly better corrosion resistance, but the current efficiency is only 64.31%, and a more serious corrosion situation also occurs (figure 4 b). Among the three anodes, the nickel anode heated and pre-oxidized by air to form a film is covered with a composite nickel oxide film, so that the nickel anode can resist molten salt corrosion for a long time, has the optimal molten salt corrosion resistance (figure 4a), has the gas product current efficiency of 80.12 percent, and still has the surface film instability condition only in the later period of the experiment.
Table 1: current efficiency of the gas product for 240min for the different nickel anodes.
| Kind of nickel anode | Current efficiency of gaseous products |
| Pure nickel anode | 35.43％ |
| Molten salt in-situ preoxidation film-forming nickel anode | 64.31％ |
| Air heating preoxidation film-forming nickel anode | 80.12％ |
Example 2: experiment of influence of metal oxide on solubility of nickel oxide in molten salt system
To Li0.85Na0.61K0.54CO3+0.2LiOH molten salt system without additionLi of the same mass2O, CaO and BaO, the influence of NiO solubility of three metal oxides at 575 ℃ is respectively explored.
Lithium oxide (Li)2O) influence on NiO solubility in molten salt system is shown in FIG. 5: in the molten salt system, the solubility of NiO is dependent on Li2The increase in the amount of O added shows a tendency to gradually increase. The reasons for this may be: the addition of lithium oxide provides more positively charged Li in the molten salt system+Ions can be combined with nickel ions to generate eutectic crystal LiNiO which can be dissolved in a molten salt system in a large amount2The solubility of NiO increases. Thus, it was confirmed that Li2The addition of O is not favorable for slowing down the corrosion of the nickel anode.
The influence of calcium oxide (CaO) and barium oxide (BaO) on the NiO solubility in the molten salt system is shown in FIG. 6: because NiO is difficult to form a composite oxide with CaO or BaO, the solubility of NiO in a molten salt medium can be effectively reduced by adding CaO and BaO into a molten salt system. However, the solubility of NiO is not decreased infinitely, and when the amount of CaO added to the system reaches 1.5 mol%, the solubility of NiO is decreased to 0.106 wt%. Thereafter, even if the amount of CaO added continues to increase, the solubility of NiO does not decrease. In the case of BaO, when the addition amount reached 4 mol%, the rate of decrease in NiO solubility became remarkably slow, and the NiO solubility at this time was 0.117 wt%. In the invention, the solubility of NiO can be effectively reduced by adding CaO or BaO into a molten salt system. The combination with the air heating pre-filming treatment can more obviously improve the corrosion resistance and oxidation resistance of the nickel anode in a high-temperature molten salt system.
Specifically, in this example, the NiO solubility was measured by ① using carbonate and hydroxide Li0.85Na0.61K0.54CO3And +0.2LiOH electrolyte accounting for 100g and 1g are uniformly mixed with metal oxide powder, then the mixture is transferred into a corundum crucible, the mixture is heated to 550 ℃ in the argon atmosphere, after the mixed molten salt completely reaches a molten state and is stable, an aluminum oxide rod is used for slowly stirring to uniformly disperse the metal oxide powder in a molten salt system, and then the mixture is stood for dissolving for 4 hours. Taking a certain amount of mixed molten salt electrolyte after the mixed molten salt electrolyte reaches a saturated stateThe method comprises the following steps of transferring an upper layer of liquid molten salt into a mortar, grinding the upper layer of liquid molten salt into powder, weighing the powder, recording the mass as m (mg), taking a molten salt sample to be measured by the same method at 600 ℃ and 650 ℃ respectively, dissolving the powder into 50mL of nitric acid (the concentration is 3mol/L), heating to completely dissolve mixed molten salt powder, evaporating unreacted excessive nitric acid, measuring the solution to be close to neutrality by using a pH test paper, transferring the residual liquid into a 100mL volumetric flask, fixing the volume, and sealing for detection, ② detecting the concentration of metal cations in the sample to be measured by using inductively coupled plasma emission spectrometry (ICP), recording the concentration as C (mg/L), and when the detector is provided with a vacuum Czerny-Turner spectrometer and a high-frequency emitter, carrying out qualitative and quantitative analysis on metal elements such as Na, Li, K, Ni, Ca, Ba and the like 72 in an inorganic aqueous solution close to neutrality, adopting a method of stepwise dilution to respectively configure a standard metal cation concentration gradient formula when the concentration of metal cations in the sample to be measured is measured by using an inductively coupled plasma emission spectrometry, and drawing a standard oxide dissolution ratio (③) for calculating the molten salt dissolution ratio:
in the formula: C-ICP test concentration results, unit mg/L;
m1-the relative molecular mass of the metal cation;
m2-the relative molecular mass of the oxides;
m is molten salt sampling mass, unit mg.
Example 3: experiment of influence of metal oxide on selectivity and current efficiency of gas product
The surface area is 20cm2The nickel electrode is heated for 2 hours in air at 800 ℃ to prepare the air-heating preoxidized film-forming nickel anode.
The air heating preoxidation film-forming nickel electrode is used as an anode, iron with the same surface area is used as a cathode, and Li is used0.85Na0.61K0.54CO3+0.2LiOH mixed molten salt as electrolyte, under argon atmosphere at 2.0V constant voltage 575 deg.C, CO is carried out2/H2Experiment for preparing hydrocarbon-rich carbon-based fuel gas by co-transformation of O and conversion to Li0.85Na0.61K0.54CO3The calcium oxide and the barium oxide with different contents are respectively added into the +0.2LiOH mixed molten salt, the influence of the addition of the metal oxide on the methane selectivity and the current efficiency in the gas product after the reaction for 240min is researched, and the results are respectively shown in FIG. 7 and FIG. 8. In the present invention, methane selectivity in the gas product refers to the potential conversion of carbon dioxide and water to methane, ethane, propane, hydrogen, carbon monoxide, and the volume fraction of methane in the fuel gas product is numerically equal to the methane selectivity.
FIG. 7 is a graph showing the amount of CaO added versus CH in the gaseous product4The influence of selectivity and current efficiency; as can be seen from FIG. 7, the product obtained after adding CaO contains CH4The selectivity of (A) is obviously increased, which indicates that the addition of CaO is to CO2The reduction of (A) plays a promoting role; when the amount of CaO added is 5 mol%, CH in the electrolysis product4The selectivity reaches 62.8 percent, and the current efficiency is 94.5 percent; but when the addition of CaO continues to increase, the CH in the product4The selectivity of (A) is significantly reduced, indicating that excessive CaO addition will block CH4And (4) generating. Therefore, the addition of a proper amount of CaO in the electrolyte system can improve the CH content in the product4The selectivity of the method is improved to a certain extent, and the current efficiency is improved to a certain extent.
FIG. 8 shows the amount of BaO added versus CH in the gaseous product4Selectivity and current efficiency. Unlike the effect of CaO, the addition of BaO to the CH in the gaseous product4The accelerating effect is not significant, and when the addition amount reaches 2 mol%, CH is present4The selectivity begins to decrease; the stability of the electrode is obviously improved by adding BaO, and the maximum stability reaches 97.5%. Therefore, consider CH comprehensively4Yield and current efficiency, in the present invention, CaO is added in an amount of 5 mol% as an optimal electrolyte additive.
Comparative example 1
Using a surface area of 20cm2The pure nickel electrode is used as an anode and is in phaseIron with the same surface area as the cathode and Li0.85Na0.61K0.54CO3+0.2LiOH mixed molten salt as electrolyte, under argon atmosphere at 2.0V constant voltage 575 deg.C, CO is carried out2/H2The experiment of preparing hydrocarbon-rich carbon-based fuel gas by the CO-transformation of O measures CO in the comparative example2/H2The selectivity of the system for preparing the hydrocarbon-rich carbon-based fuel gas by the O-concerted conversion to methane in the gas product reaches 48.41%, the current efficiency of the gas product after 30min of electrode reaction reaches 90.32%, but the long-term current efficiency cannot be guaranteed.
In particular, in the present invention, the gas product current efficiency can be calculated according to faraday's law; where the electrochemical equivalent q represents the mass of the product per unit of charge, often expressed in kilograms of product formed a · h (per ampere hour), the faraday constant F is 96485C/mol, and 1A · h is 3600C. Therefore, when 1F is 26.8A · h, the current efficiency is calculated as follows:
η -Current efficiency,%;
q-electrochemical equivalent, kg. A-1·h-1；
G-the mass of electrolysis product actually produced during the electrolysis time, kg;
i-intensity of current through the cell, A;
t is electrolysis time, h;
n-number of transferred electrons.
Finally, the description is as follows: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the embodiments can still be modified, or some technical features can be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the present invention in its spirit and scope.
Claims (10)
1. A method of increasing the corrosion resistance of a nickel electrode in a molten salt system, the method comprising the steps of:
(1) carrying out preoxidation film forming treatment on the nickel electrode by a molten salt in-situ film forming or air heating film forming method to obtain a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer;
(2) and (2) using the modified nickel electrode obtained in the step (1) as an anode in a molten salt system.
2. The method of claim 1, further comprising adding a metal oxide to the molten salt system.
3. The method of claim 2, wherein:
the metal oxide is one or more of calcium oxide, barium oxide, magnesium oxide, strontium oxide and zinc oxide;
preferably, the metal oxide is calcium oxide and/or barium oxide; and/or
The metal oxide accounts for 0.5-10 mol% in the molten salt system.
4. The method according to any one of claims 1 to 3, wherein the pre-oxidation film forming treatment of the nickel electrode by the molten salt in-situ film forming method comprises the following steps:
taking a mixed molten salt formed by sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, taking a nickel electrode as an anode and a cathode respectively, and carrying out pre-oxidation film-forming treatment for 1-3 h under the conditions of an electrolysis voltage of 0.5-1.5V and an electrolysis temperature of 650-750 ℃ in an argon atmosphere; preferably, the mass ratio of the sodium carbonate to the potassium carbonate is 1:1, and the molar ratio of the total amount of the sodium carbonate and the potassium carbonate to the lithium hydroxide is 1: 0.2.
5. The method according to any one of claims 1 to 3, wherein the pre-oxidation film formation treatment of the nickel electrode by the air heating film formation method comprises: roasting the nickel electrode for 1-3 h at 500-900 ℃ in air atmosphere; preferably, the nickel electrode is roasted for 2 hours at 800-900 ℃ in the air atmosphere.
6. A method according to any one of claims 1 to 3, characterized in that:
the molten salt system takes mixed molten salt formed by carbonate and hydroxide as electrolyte, and takes nickel, platinum, titanium, iridium, ruthenium, palladium, iron, tungsten, chromium, copper, gold, graphite or stainless steel as cathode material;
preferably, the carbonate is one or more of lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate and rubidium carbonate, and the hydroxide is one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, calcium hydroxide, magnesium hydroxide, strontium hydroxide, barium hydroxide and zinc hydroxide;
more preferably, the carbonate is formed by mixing lithium carbonate, sodium carbonate and potassium carbonate in a mass ratio of 1:1:1, and the hydroxide is lithium hydroxide.
7. The method of claim 6, wherein:
the molar ratio of the carbonate to the hydroxide is 1: (0.1 to 0.4), preferably 1: 0.2.
8. A preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer is characterized by comprising the following steps:
taking a mixed molten salt formed by sodium carbonate, potassium carbonate and lithium hydroxide as an electrolyte, taking a nickel electrode as an anode and a cathode respectively, and carrying out pre-oxidation film-forming treatment for 1-3 h under the conditions that the electrolysis voltage is 0.5-1.5V and the electrolysis temperature is 650-750 ℃ in an argon atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer at the anode.
9. A preparation method of a modified nickel electrode coated by a nickel-nickel oxide composite oxide layer is characterized by comprising the following steps:
and roasting the nickel electrode for 1-3 hours at 500-900 ℃ in an air atmosphere to obtain the modified nickel electrode coated by the nickel-nickel oxide composite oxide layer.
10. The modified nickel electrode coated with a nickel-nickel oxide composite oxide layer produced by the production method according to claim 8 or claim 9.
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