Electrodes for the electrochemical evolution of hydrogen

Abstract

This invention relates to electrodes, and more particularly to electrodes suitable for use as cathodes for generating hydrogen in industrial electrolysis processes, equipped with a catalytic coating comprising an outer layer containing ruthenium and selenium; and to a method for preparing the same.

Description

Technical Field

[0001] The present invention relates to electrodes, and in particular to electrodes suitable for use as cathodes for generating hydrogen in industrial electrolytic processes, and to methods for their preparation. Background Art

[0002] In the field of electrolytic processes, such as the electrolysis of alkaline brines for the simultaneous production of chlorine and alkali and the electrolysis of water for the production of hydrogen and oxygen, competitiveness is linked to several factors, the main of which is the reduction of energy consumption, which is directly related to the current voltage of the entire process.

[0003] Such reductions in current and voltage may be achieved, as in the case of hydrogen, chlorine or oxygen evolution, by using anodes and cathodes having catalytic coatings designed to promote the desired electrochemical processes.

[0004] In membrane processes for electrolysis of alkaline brines, for example, the global voltage depends on factors related to ohmic drop and mass transport, the resistance of the ion exchange membrane and electrolyte and the anodic and cathodic overvoltages involving the gas evolution reactions of chlorine and hydrogen, respectively.

[0005] The cathodic overvoltages achievable with electrodes of chemically resistant materials but lacking good catalytic activity, such as in the case of carbon, have long been considered acceptable.

[0006] However, in the specific case, today's market demands focus on obtaining alkali products with increasingly higher concentrations, which makes the use of carbon steel cathodes impractical due to significant corrosion problems, and in addition to reducing energy consumption and the resulting costs, attention is focused on the research of catalytic coatings capable of continuously reducing the overvoltage of the hydrogen evolution reaction.

[0007] The use of activated electrodes having a catalytic coating capable of reducing the cathodic overvoltage of hydrogen has therefore become widespread; good results have been obtained by using metal substrates such as nickel, copper or steel itself provided with a catalytic coating based on ruthenium or platinum oxide.

[0008] The use of activated electrodes with similar catalytic coatings has resulted in energy savings, for example capable of offsetting the costs resulting from the use of catalysts based on precious metals.

[0009] Another fundamental factor affecting the economic convenience of using electrodes activated by catalytic coatings based on noble metals relates to the operating life of the electrodes at high current densities.

[0010] However, most catalytic coatings based on noble metals tend to be severely damaged by current reversal, which can occur inefficiently on industrial equipment; switching to anodic current and the subsequent increase in the electrode potential to high values ​​can cause uncontrolled dissolution of platinum or ruthenium oxides.

[0011] A partial solution to this problem has been obtained by adding rare earth elements to the formulation of the catalytic coatings. Cathodes with these coatings have proven to be sufficiently resistant under normal operating conditions of the device.

[0012] Further improvements in resistance to the reversal of the current are obtained by applying to the metal substrate a catalytic coating consisting of different phases including platinum, rhodium and / or palladium. However, this type of coating requires high loadings of platinum and rhodium in the catalytic phase, which determines rather high production costs.

[0013] Therefore, there is a clear need for a new cathode composition for industrial electrolysis processes, in particular for electrolysis processes with cathodic hydrogen evolution, characterized by improved catalytic activity at high current densities compared to prior art formulations, with equal or lower overall cost in terms of raw materials, and with a duration equal to or greater than usual operating conditions and resistance to accidental current reversals.

[0014] US 4,300,992 A describes an electrode for hydrogen evolution in an electrolysis process, comprising a metal substrate provided with a catalytic coating comprising an inner layer containing ruthenium and an outer layer of selenium. US 4,975,161 A describes a hydrogen evolution electrode having a single catalytic coating comprising ruthenium and selenium. US 2014 / 008215 A1 describes an electrode for hydrogen evolution, comprising a metal substrate provided with a catalytic coating made of a platinum layer followed by a layer containing ruthenium and praseodymium. US 3,990,957 A describes an electrode for hydrogen evolution, comprising a metal substrate provided with a catalytic coating comprising ruthenium and strontium.

[0015] The present invention has the object of solving the above described problems and relates to a cathode characterized by low hydrogen overvoltage and excellent resistance to unexpected current reversal when electrolysis is interrupted. The invention also relates to a method for its production and an electrolyzer containing it. Summary of the invention

[0016] In the electrolytic process industry, the main driver is the quest for competitiveness in terms of voltage reduction. Cathode coatings represent an important element in achieving such voltage reductions, however all changes leading to voltage improvements often result in coatings that are weak to current reversals that can occur unexpectedly in the event of a malfunction in industrial equipment.

[0017] As described in the present invention, electrodes provided with a catalytic coating solve the above problems by providing improved cell voltage relative to the prior art while maintaining optimized robustness against reversal.

[0018] In a first aspect, the present invention relates to an electrode for gas evolution in an electrochemical process, comprising a metal substrate provided with a catalytic coating, the catalytic coating comprising an inner layer and an outer layer, the inner layer containing an element selected from at least one noble metal, and optionally one or more elements selected from rare earth metals and / or elements selected from alkaline earth metals (in the form of metal or its oxides), and the outer layer containing 80-99.5% by weight of ruthenium and 0.5-20% by weight of selenium, calculated as the metal elements in the form of metal or its oxides.

[0019] The use of selenium allows to significantly reduce the hydrogen cathode overvoltage and to reach stable cell voltage values ​​in a short time, comparable to the voltages obtained by using catalytic compositions with much higher amounts of noble metals. The use of ruthenium in the outer layer improves the resistance of the coating to current reversals.

[0020] The composition as described in the present invention allows further obtaining an excellent robustness of the cathode coating towards switching-off events, equal to the best robustness obtained with large amounts of ruthenium stabilized by the presence of rare earths in the catalytic coating.

[0021] According to the present invention, the metal substrate can be any metal suitable for use as an electrode support for an electrochemical process. In particular, it is a metal substrate for a cathode used in chlor-alkali electrolysis and water electrolysis processes. In this case, the most commonly used metal substrate can be selected from nickel, nickel alloys, copper and steel.

[0022] According to an embodiment of the electrode of the present invention, the at least one element selected from the group consisting of noble metals of the inner layer is platinum and / or ruthenium.

[0023] According to an embodiment of the electrode according to the invention, the inner layer of the catalytic coating comprises 60-100 wt. % of ruthenium and 0-40 wt. % of a metal selected from rare earths.

[0024] The presence of one or more elements belonging to the rare earth group has the purpose of stabilizing the noble metal matrix, making it more resistant to the current reversals that can occur in the electrolyzer during uncontrolled stopping events.

[0025] According to an embodiment of the electrode according to the invention, said outer layer of said catalytic coating comprises 90-99.5% by weight of ruthenium and 0.5-10% by weight of selenium, calculated as metal.

[0026] The inventors have surprisingly observed that the addition of selenium, even in small amounts, to a catalytic coating comprising predominantly ruthenium allows obtaining unexpectedly improved performance in terms of the catalytic activity of the hydrogen evolution reaction.

[0027] In further embodiments, the outer layer of the catalytic coating has a composition, by weight of metal, comprising 95-99.5% ruthenium and 0.5-5% selenium.

[0028] In yet further embodiments, the outer layer of the catalytic coating has a composition, by weight of metal, comprising 90-99% ruthenium and 1-10% selenium.

[0029] Selenium constitutes the active component responsible for improving the overvoltage of the hydrogen evolution reaction, allowing a significant improvement in the catalytic activity even when present in low concentrations.

[0030] Furthermore, the presence of selenium in a matrix consisting essentially of ruthenium, together with the presence of a metal from the rare earth group, has the advantage of increasing the operating life of the electrode, allowing the consumption of the catalytic coating expressed as a percentage of the amount of precious metal consumed to be reduced.

[0031] According to a further embodiment, the invention relates to an electrode, wherein the inner layer contains one or more elements selected from the group consisting of rare earths, and wherein the metal selected from the group consisting of rare earths is praseodymium, cerium and / or lanthanum.

[0032] Experiments carried out by the inventors have shown that praseodymium provides better results than other elements belonging to the rare earth group, but the invention can also be successfully implemented with cerium and / or lanthanum.

[0033] In one embodiment, the inner layer of the catalytic coating has a composition, by weight of metal, comprising 60-90% ruthenium and 10-40% praseodymium, and the outer layer has a composition, by weight of metal, comprising 80-99.5% ruthenium and 0.5-20% selenium.

[0034] The inventors have found that the weight composition represented can impart high catalytic activity and excellent resistance to current reversal.

[0035] In a further embodiment, the inner layer of the catalytic coating comprises ruthenium and at least one other element selected from alkaline earth metals, in particular selected from strontium, calcium and barium. Experiments carried out by the inventors have shown that such formulations provide further improved hydrogen overvoltages than are usually observed using other formulations, also allowing steady-state cell performance to be achieved in an improved time.

[0036] In one embodiment, the inner layer of the catalytic coating has a composition, by weight of metal, comprising 90-99% ruthenium and 1-10% of a metal selected from alkaline earth metals and the outer layer has a composition, by weight of metal, comprising 80-99.5% ruthenium and 0.5-20% selenium.

[0037] In one embodiment, the catalytic coating also comprises an additional layer in direct contact with the metal substrate comprising platinum and / or palladium in the form of the metal or its oxide. This can result in an additional improved resistance of the electrode to current reversal, making it surprisingly similar or even better than those of electrodes characterized by only using a large amount of platinum activation.

[0038] It will be appreciated that the elements present in the catalytic coating may be in metallic form or in oxide form.

[0039] In other embodiments, the catalytic coating has a relative humidity between 4 and 15 g / m 2 The inventors have found that, in the case of the catalytic coating shown, the reduced ruthenium loading is more than sufficient to impart good resistance to current reversal and excellent catalytic activity not found in the prior art of ruthenium-based catalytic coatings.

[0040] In a further embodiment of the electrode according to the invention, the preferred metal substrate is nickel or a nickel alloy.

[0041] In a further aspect, the invention relates to a method for preparing an electrode for the evolution of gaseous products in an electrolyser, for example for the evolution of hydrogen in an alkaline brine electrolysis or water electrolyser, comprising the following stages:

[0042] a) applying a solution containing a precursor of the inner layer component to a metal substrate;

[0043] b) optionally drying at 30-100° C. for a period of 5 to 60 minutes;

[0044] c) decomposing the solution containing the precursor of the inner layer component by heat treatment at 400-600° C.;

[0045] d) optionally repeating steps ac or one or more times until the desired loading amount is reached;

[0046] e) applying a solution containing precursors of said outer layer components;

[0047] f) optionally drying at 30-100° C. for a period of 5 to 60 minutes;

[0048] g) decomposing the solution containing the precursor of the outer layer component by heat treatment at 400-600° C.;

[0049] h) Optionally repeating stage eg one or more times until the desired loading is reached.

[0050] In one embodiment, the solution containing the precursor of the inner layer component contains precursors of ruthenium and an element selected from the rare earths, and the solution containing the precursor of the outer layer component contains precursors of ruthenium and selenium.

[0051] In further embodiments, the solution containing the precursor of the inner layer component contains precursors of ruthenium and praseodymium, and the solution containing the precursor of the outer layer component contains precursors of ruthenium and selenium.

[0052] In one embodiment, the solution containing the precursor of the inner layer component contains precursors of ruthenium and an element selected from the group consisting of alkaline earth metals, and the solution containing the precursor of the outer layer component contains precursors of ruthenium and selenium.

[0053] According to an embodiment of the above method, the method includes an additional step before step (a), including applying the platinum-containing solution in direct contact with the metal substrate, optionally drying at 30-100°C for a period of 5 to 60 minutes, and decomposing the platinum-containing solution by heat treatment at 400-600°C.

[0054] In a further aspect, the invention relates to an electrolysis cell for electrolysis of alkaline chloride solutions, comprising an anode chamber and a cathode chamber separated by an ion exchange membrane or diaphragm, wherein the cathode chamber is equipped with an electrode of one of the forms described above for use as a cathode for hydrogen evolution.

[0055] In a further aspect, the invention relates to an electrolyser for producing chlorine and alkali starting from alkaline brine, comprising a modular configuration of an electrolytic cell having anode and cathode chambers separated by an ion exchange membrane or diaphragm, wherein the cathode chamber comprises an electrode of one of the forms described above serving as cathode.

[0056] In a further aspect, the invention relates to an electrolyser for producing hydrogen by electrolysis of water, comprising an anode chamber and a cathode chamber separated by a membrane, wherein the cathode chamber is provided with an electrode of one of the forms described above.

[0057] The following examples are included to illustrate specific embodiments of the invention, which have been extensively tested for their usefulness over the range of values ​​claimed. It will be apparent to those skilled in the art that the compositions and techniques described in the following examples represent compositions and techniques that the inventors have discovered to work well in the practice of the invention; however, those skilled in the art will also appreciate that, in light of this description, various changes may be made to the various embodiments described and still produce the same or similar results without departing from the scope of the invention.

[0058] Example 1

[0059] Make it have 100mm×100mm×0.89mm 2 A nickel mesh of size was subjected to a process of sandblasting with corundum, pickling in HCl and stress relieved by heat treatment according to procedures known in the art.

[0060] A solution containing a precursor of ruthenium and strontium was prepared having a composition expressed in weight percentages equal to 95% Ru and 5% Sr.

[0061] A second solution containing a precursor of ruthenium and selenium was prepared having a composition expressed in weight percentages equal to 92% Ru and 8% Se.

[0062] The first solution was applied to the nickel mesh by brushing in 6 passes.

[0063] After each coat, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated for 10 minutes at 500° C. The web was air-cooled each time before the next coat was applied.

[0064] Repeat this process until it reaches 8g / m 2 The total Ru load.

[0065] Subsequently, the second solution was applied by brushing in 6 passes. After each pass, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated at 500° C. for 10 minutes. The web was air-cooled each time before the next pass was applied.

[0066] Repeat this process until it reaches 15g / m 2 The total Ru load.

[0067] The electrode thus obtained was designated as Sample E1.

[0068] Example 2

[0069] Make it have 100mm×100mm×0.89mm 2 A nickel mesh of size was subjected to a process of sandblasting with corundum, pickling in HCl and stress relieved by heat treatment according to procedures known in the art.

[0070] A solution containing a precursor of ruthenium and praseodymium was prepared having a composition expressed in weight percentages equal to 83% Ru and 17% Pr.

[0071] A second solution containing precursors of ruthenium and selenium was prepared having a composition expressed in weight percentages equal to 95% Ru and 5% Se.

[0072] The first solution was applied to the nickel mesh by brushing in 5 passes.

[0073] After each coat, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated for 10 minutes at 500° C. The web was air-cooled each time before the next coat was applied.

[0074] Repeat this process until it reaches 5g / m 2 The total Ru load.

[0075] Subsequently, the second solution was applied by brushing in 6 passes. After each pass, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated at 500° C. for 10 minutes. The web was air-cooled each time before the next pass was applied.

[0076] Repeat this process until it reaches 11g / m 2 The total Ru load.

[0077] The electrode thus obtained was designated as Sample E2.

[0078] Example 3

[0079] Make it have 100mm×100mm×0.89mm 2 A nickel mesh of size was subjected to a process of sandblasting with corundum, pickling in HCl and stress relieved by heat treatment according to procedures known in the art.

[0080] A solution containing a precursor of ruthenium and praseodymium was prepared having a composition expressed in weight percentages equal to 84% Ru and 16% Pr.

[0081] A second solution containing ruthenium and selenium precursors was prepared having a composition expressed in weight percentages equal to 98.5% Ru and 1.5% Se.

[0082] The first solution was applied to the nickel mesh by brushing in 5 passes.

[0083] After each coat, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated for 10 minutes at 500° C. The web was air-cooled each time before the next coat was applied.

[0084] Repeat this process until it reaches 5g / m 2 The Ru loading capacity.

[0085] Subsequently, the second solution was applied by brushing in 5 passes. After each pass, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated at 500° C. for 10 minutes. The web was air-cooled each time before the next pass was applied.

[0086] Repeat this process until it reaches 10.5 g / m 2 The total Ru load.

[0087] The electrode thus obtained was designated as Sample E3.

[0088] Comparative Example 1

[0089] Make it have 100mm×100mm×0.89mm 2 A nickel mesh of size was subjected to a process of sandblasting with corundum, pickling in HCl and stress relieved by heat treatment according to procedures known in the art.

[0090] 100 ml of a solution containing a precursor of ruthenium and praseodymium was prepared, having a composition expressed in weight percentages equal to 83% Ru and 17% Pr.

[0091] The solution was then applied to the nickel mesh by brushing in 8 passes.

[0092] After each coat, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated for 10 minutes at 500° C. The web was air-cooled each time before the next coat was applied.

[0093] Repeat this process until it reaches 11g / m 2 The total Ru load.

[0094] The electrode thus obtained was identified as sample CE1.

[0095] Comparative Example 2

[0096] Make it have 100mm×100mm×0.89mm 2 A nickel mesh of size was subjected to a process of sandblasting with corundum, pickling in HCl and stress relieved by heat treatment according to procedures known in the art.

[0097] 100 ml of a solution containing a precursor of ruthenium and strontium was prepared, having a composition expressed in weight percentage equal to 97% Ru and 3% Sr.

[0098] The solution was then applied to the nickel mesh by brushing in 6 passes.

[0099] After each coat, drying was carried out at 40-60° C. for about 10 minutes and then heat-treated for 10 minutes at 500° C. The web was air-cooled each time before the next coat was applied.

[0100] Repeat this process until it reaches 11g / m 2 The total Ru load.

[0101] The electrode thus obtained was identified as sample CE2.

[0102] The samples of the examples described above were subjected to performance testing in a laboratory cell filled with 32% NaOH at 90°C under hydrogen evolution, and some samples were subsequently subjected to cyclic voltammetry testing in the potential range from -1 to +0.5 V / NHE using a scan rate of 10 mV / s.

[0103] Table 1 reports the initial cathode potentials, corrected for ohmic drop values, at 9 kA / m 2 Current density measurement.

[0104] Table 1:

[0105] sample mV vs NHE E1 932 E2 934 E3 937 CE1 952 CE2 942

[0106] Table 2 reports the initial cathodic potential and the cathodic potential after 10 and 25 cycles of cyclic voltammetry (10CV, 25CV), the exponential of the resistance reversal, at 6 kA / m 2 Current density measurement.

[0107] Table 2:

[0108] sample mV vs NHE mV vs NHE(10CV) mV vs NHE(25CV) E1 924 1100 1141 E2 924 1040 1094 E3 927 1027 1103 CE1 941 1052 1093 CE2 933 1112 1148

[0109] Table 3 shows the time to reach steady-state cell performance and the specific consumption of the electrodes, expressed as a percentage of residual noble metal and additionally as an index of resistance to current reversal. 2 The laboratory membrane diaphragm tank is 8kA / m 2 Data were obtained after 8000 hours of activation (HOL).Tests were performed at T = 89°C using NaCl 210 g / l anolyte, 32 wt% NaOH catholyte.

[0110] Table 3:

[0111] sample % Residual precious metal loading Time to reach steady state E1 65 <3 days E2 75 <3 days E3 75 < 4 days CE1 75 <7 days CE2 65 <5 days

[0112] The preceding description is not intended to limit the invention, which can be used according to different embodiments without departing from its purpose, and the scope of the invention is clearly defined by the appended claims.

[0113] In the description and claims of the present application, the terms "comprises" and "comprising" and variations thereof such as "includes" and "including" are not intended to exclude the presence of other additional elements, components or process steps.

[0114] The discussion of documents, documents, materials, instruments, articles and the like is included in the text for the sole purpose of providing a context for the present invention; however, it should not be assumed that the content or part of it constituted the general knowledge in the field relevant to the present invention before the priority date of each claim attached to this application.

Claims

1. An electrode for gas evolution in an electrolytic process, comprising a metal substrate provided with a catalytic coating, the catalytic coating comprising an inner layer and an outer layer, the inner layer containing at least one element selected from noble metals, and optionally one or more elements selected from rare earth metal elements and / or from alkaline earth metals, which are in the form of metals or their oxides, the noble metal being in the form of metals or their oxides, the outer layer comprising 80-99.5% by weight of ruthenium and 0.5-20% by weight of selenium, relative to the metal element in the form of metals or their oxides. 2 . The electrode according to claim 1 , wherein the at least one element selected from noble metals of the inner layer is platinum and / or ruthenium.

3. An electrode according to claim 1 or 2, wherein the outer layer comprises 90-99% by weight of ruthenium and 1-10% by weight of selenium, calculated as metal elements.

4. The electrode according to claim 1, wherein the inner layer contains one or more elements selected from the group consisting of rare earths, wherein the one or more elements selected from the group consisting of rare earths of the inner layer are praseodymium, cerium and / or lanthanum.

5. The electrode according to any one of claims 1 to 3, wherein the inner layer contains ruthenium and an element selected from the group consisting of alkaline earth metals.

6. An electrode according to any one of the preceding claims, wherein the catalytic coating comprises a further layer in direct contact with the metal substrate comprising platinum and / or palladium in the form of the metal or its oxide.

7. A method for preparing an electrode as defined in any one of the preceding claims, comprising the steps of: a. applying a solution containing a precursor of the components of the inner layer to a metal substrate; b. Optional drying at 30-100 ℃ for 5 to 60 minutes; c. decomposing the solution by heat treatment at 400-600°C; d. Optionally repeat steps ac one or more times until the desired loading amount is reached; e. applying a solution containing a precursor of the components of the outer layer; f. Optional drying at 30-100 ℃ for 5 to 60 minutes; g. decomposing the solution by heat treatment at 400-600 ℃; h. Optionally repeat stage eg one or more times until the desired loading is reached.

8. An electrolysis cell for the electrolysis of an alkaline chloride solution, comprising an anode chamber and a cathode chamber separated by an ion exchange membrane or by a diaphragm, wherein the cathode chamber is provided with an electrode according to any one of claims 1 to 6.

9. An electrolyser for producing chlorine and alkali from alkaline brines, comprising a modular arrangement of electrolytic cells, wherein each electrolytic cell is an electrolytic cell according to claim 8.

10. An electrolyser for producing hydrogen by electrolysis of water, comprising an anode chamber and a cathode chamber separated by a diaphragm, wherein the cathode chamber is provided with an electrode according to any one of claims 1 to 6.