Ruthenium-rhodium sulphides based gas diffusion electrode and method for making the same

Abstract

The Present invention relates to a gas diffusion electrode (GDE), preferably oxygen-depolarized cathode (ODC), comprising an inert conductive substrate and a catalytic coating, wherein the catalytic coating comprises ruthenium-rhodium sulphides, wherein rhodium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, wherein ruthenium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides. The present invention also relates to the use of the aforementioned oxygen-depolarized cathode (ODC) in a hydrochloric acid electrolysis cell (HCl-ODC).

Description

[0001] RUTHENIUM-RHODIUM SULPHIDES BASED GAS DIFFUSION

[0002] ELECTRODE AND METHOD FOR MAKING THE SAME

[0003] TECHNICAL FIELD

[0004] The present invention relates to ruthenium-rhodium sulphides-based gas diffusion electrode (GDE) , preferably oxygen-depolarized cathode (ODC) , method for making such electrode and the use thereof in hydrochloric acid electrolysis (HC1-ODC) .

[0005] PRIOR ART

[0006] Gas diffusion electrodes (GDE) are electrodes with a conjunction of a solid, liquid, and gaseous interface, and a catalytic coating supporting the electrochemical reactions. Gas diffusion electrodes are used in various applications such as oxygen-depolarized cathodes (ODC) in hydrochloric acid electrolysis.

[0007] Chlorine gas is widely utilized in various industrial processes which involve chlorination phases, thus producing chlorinated organic or inorganic chemicals and hydrochloric acid as a by-product. Produced hydrochloric acid can be used in different applications or, alternatively, can be oxidized back to chlorine, which can then be reused in the above chemical reaction.

[0008] Oxidation of hydrochloric acid using electrolysis is a widespread method for the recovery of chlorine atoms into valuable chlorine gas .

[0009] Platinum is recogni zed as the most ef fective electrocatalyst for the electroreduction of oxygen; the activation of gas di f fusion electrodes with platinumbased catalysts is well known in the art . However, the use of platinum in aqueous HC1 electrolysis poses some serious drawbacks . For instance , platinum based cathodic catalyst in gas di f fusion cathodes may interact with liquid electrolytes , which comprises chloride ion that can poison platinum atoms in the catalyst .

[0010] Rhodium sulphide has been used as an active catalyst in the oxygen-depolari zed cathode , particularly in the oxygen-depolari zed cathode used in hydrochloric acid electrolysis cells .

[0011] For example , patent application W02000073538 Al describes rhodium sulphide-based catalyst for the reduction of oxygen in industrial electrolyzers , which is highly resistant towards corrosion, thus proven to be particularly suitable for use in the electrolysis o f hydrochloric acid . W02002018675 A2 discloses a process for electrolyzing an aqueous solution of hydrochloric acid to chlorine in an electrochemical cell having an anodic compartment and a cathodic compartment comprising at least one gas di f fusion cathode , which comprises an electrically conductive web provided on at least on side with a rhodium sulphide for the electro-reduction of oxygen . However, Rhodium price has significantly raised in the last 5 years and shows frequent fluctuations. This high price is severely impacting the overall economics of HC1-ODC technology.

[0012] Thus, the Applicant is faced with the problem of finding an alternative coating to be used in a gas diffusion electrode (GDE) , preferably in an oxygen depolarized cathode, that is less expensive than Rhodium sulphide, and at the same times, possess similar activity and stability, especially in the HC1-ODC technology.

[0013] The need is thus apparent to identify an inexpensive yet efficient catalytic coating with respect to the formulations of the prior art to be used in a gas diffusion electrode (GDE) .

[0014] SUMMARY OF THE INVENTION

[0015] From what is stated above, the present application aims at providing a cost-effective and active electrocatalyst to be used in gas diffusion electrode (GDE) having cell voltage and shutdown tolerance analogous to those of Rhodium sulphide currently used in the industry.

[0016] This result is achieved using gas diffusion electrode (GDE) comprising an inert conductive substrate and a catalytic coating comprising ruthenium-rhodium sulphides, as specified in claim 1.

[0017] Advantageously, the gas diffusion electrode (GDE) is oxygen-depolarized cathode (ODC) , preferably a cathode in a hydrochloric acid electrolysis cell.

[0018] A massive reduction of cost can be achieved using ruthenium-rhodium sulphides as described above with respect to rhodium sulphide RhSx.

[0019] These and other objects and advantages of the invention will become obvious from the following detailed description .

[0020] BRIEF DESCRIPTION OF THE FIGURES

[0021] Figure 1 is a graph which demonstrates the cell voltage as a function of Rh content in ruthenium-rhodium sulphides in a catalytic coating deposited on a substrate forming gas diffusion electrode (GDE) in a hydrochloric acid electrolysis cell as defined in the present invention .

[0022] Figure 2 shows Voltage-time graph for a comparative gas diffusion electrodes and gas diffusion electrodes according to the present invention.

[0023] Figure 3 is a rotating disk electrode (RDE) graph for a comparative (GDE) and (GDE) according to the present invention .

[0024] DETAILED DESCRIPTION OF THE INVENTION In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even i f not expressly declared .

[0025] For the purposes of the present discus sion the term " to comprise" or " to include" also comprises the term " to consist in" or " essentially consisting of" .

[0026] For the purposes of the present discussion the definitions of the ranges always comprise the extreme values unless otherwise speci fied .

[0027] In the present invention, an inert conductive substrate means a chemically inactive conductive substrate .

[0028] A first obj ect of the invention therefore relates to a gas di f fusion electrode ( GDE ) , which comprises an inert conductive substrate and a catalytic coating, wherein the catalytic coating comprises ruthenium-rhodium sulphides , wherein rhodium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , and ruthenium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , wherein said ruthenium-rhodium sulphides comprises crystalline phases of Rhl 7S 15 , RuS2 and RuRh .

[0029] Preferably, rhodium sulphide content is comprised between 40% and 60% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , and ruthenium sulphide content is comprised between 40% and 60% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides .

[0030] More preferably rhodium sulphide content is 50% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , and ruthenium sulphide content is 50% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides .

[0031] Advantageously, the catalytic coating comprise carbon to support the ruthenium-rhodium sulphides , preferably active carbon, more preferably active carbon having a surface area higher than 120 m2 / g . Active carbon, such as carbon black, is a conductive and inert in nature , thus can facilitate the Incorporation of the catalyst of the invention into gas di f fusion electrode structures without interacting with the solution or negatively influencing the performance of the catalyst .

[0032] Preferably, the catalytic coating further comprises an inert polymeric binder . For instance , perfluorinated, partially fluorinated or non- f luorinated binders can be used, perfluorinated binders are preferable binders . The catalyst / binder mixture can be applied on a conductive web directly, to obtain the so called " f low-through" gas di f fusion electrode . According to a preferred embodiment, the catalyst can be subjected to a sintering step under hydrogen H2. For instance, hydrogen gas may have a concentration of 1-4% in Ar, and the sintering step is preferably conducted under a temperature less than 100°C.

[0033] Sintering of the catalyst can improve the long-term stability of the overall gas diffusion electrode structure. Additionally, sintering under Hydrogen H2has demonstrated a surprising effect in the present invention in that it allows to improve the performance in term of cell voltage as demonstrated in the examples below. The ruthenium-rhodium sulphides obtained by the method comprising a sintering step under hydrogen comprises crystalline phases of Rh17Si5, RuS2and RuRh.

[0034] Advantageously, the gas diffusion electrode (GDE) of the present invention has a total metal load of ruthenium and rhodium in the catalytic coating comprised between 0.5 and 25 g / m2, preferably comprised between 1 and 10 g / m2referred to metals.

[0035] In a preferred embodiment of the present invention, the gas diffusion electrode (GDE) of the present invention is an oxygen-depolarized cathode (ODC) .

[0036] A second object of the invention relates to a method for making the gas diffusion electrode (GDE) of the present invention, wherein the method comprises the steps of: - providing an inert conductive substrate ,

[0037] - reacting a solution of precursors of ruthenium and rhodium with a sul fur precursor such as to obtain a catalytic coating,

[0038] - depositing the catalytic coating on the inert conductive substrate , wherein the method further comprises a step of sintering the catalytic coating under hydrogen H2.

[0039] Advantageously, the solution of precursors of ruthenium and rhodium comprises carbon suspension . carbon suspension is used in the method so as the final catalyst can be supported on active carbon .

[0040] According to a particularly preferred embodiment , ruthenium-rhodium sulphides can be obtained by reacting a solution of precursors of ruthenium and rhodium with an aqueous solution comprising thionic species . The thionic species can preferably be selected from the group consisting of thiosulphates , dithionates , trithionates , tetrathionates , pentathionates , heptathionates and noble or non-noble metal thionates .

[0041] Using thionic species such as thiosul fate precursor in the preparation of ruthenium-rhodium sulphides is advantageous as it eliminates the use of dangerous gaseous H2S . Patent application W02005075071 Al incorporated herein by reference describes a method for producing a noble metal sulphide catalytic coating by reacting a precursor of a noble metal with an aqueous solution comprising a thionic species .

[0042] The method further comprises a step of sintering the catalyst under hydrogen H2. For example , hydrogen H2can have a concentration of 1-4 % in Ar, and the sintering step can be conducted under a temperature less than 100 ° C . As mentioned above , sintering under hydrogen H2is particularly preferred in the present invention as it allows to improve the performance in term of cell voltage .

[0043] A third obj ect of the invention relates to a use of the gas di f fusion electrode ( GDE ) of the present invention as described above as an oxygen-depolari zed cathode in a hydrochloric acid electrolysis cell (HC1-ODC ) .

[0044] A fourth obj ect of the invention relates to a hydrochloric acid electrolysis cell , the cell comprises at least one ion exchange membrane , the at least one ion exchange membrane dividing the electrolysis cell into at least one cathodic compartment and at least one anodic compartment , the at least one anodic compartment comprising connections for feeding aqueous hydrochloric acid and withdrawing residual acid and produced chlorine , the at least one cathodic compartment comprising connections for feeding oxygen or an oxygencontaining gas and withdrawing residual oxygen and reaction water, wherein the at least one cathodic compartment , comprises at least one oxygen-depolari zed cathode ( ODC ) of the present invention . The hydrochloric acid electrolysis cell can be operated by the standard methods known to the person skilled in the art . For example , aqueous hydrochloric acid is fed to the anodic compartment and oxygen, or oxygencontaining gas is feed to cathodic compartment , then electric current is applied, and residual acid and chlorine are withdrawn from the anodic compartment and residual oxygen and water are withdrawn from the cathodic compartment . The density of the electric current is preferably in the ranges comprised between 2 and 10 kA / m2.

[0045] Ruthenium is signi ficantly less expensive than rhodium . Nonetheless , Ruthenium-only formulations do not show neither enough activity nor stability to challenge Rhodium sulphide . Thus , ruthenium-rhodium sulphides would have not been considered a good candidate to replace rhodium-only formulations by the person skilled in the art .

[0046] Prior art demonstrates that RhSx may comprise a crystalline phase Rhl 7S 15 , which is a conductive and catalytically active phase . On the other hand, RuSx may comprise a crystalline phase RuS2 , which shows low activity towards ORR as it has a cell voltage of about 1 . 52 volt . This value can be seen in figure 1 , when Rh concentration is zero in the catalytic coating .

[0047] The behavior of catalysts comprising RhSx and RuSx as two different components characterized by different catalytic activity and different conductivity could be analyzed in terms of percolation theory applied to a particle mixture of different properties in terms of conductivity and catalytic activity. In this case the prediction of the behavior leads to the conclusion that the target to realize industrially applicable cells with cell voltages of 1.3 ± 0.03 can be obtained only with catalysts having Rh % of at least 70% in the rutheniumrhodium sulphides referred to metals. The present invention surprisingly found that the target of cell voltage and stability can be achieved with Rh % as low as 30% as demonstrated in figure 1, where cell voltage is reported as a function of Rh percent in a catalyst comprising ruthenium-rhodium sulphides.

[0048] The surprising technical effect of ruthenium-rhodium sulphides having cell voltage analogous to those of Rhodium sulphide is not entirely understood. While not wishing to be bound by any theory, the Applicant hypothesizes the following.

[0049] XRD patterns of a catalyst composed of Rh 50% and Ru 50% by weight referred to metals and subjected to a step of sintering under hydrogen according to the invention, as conducted by the Applicant, show that it is constituted by separated phases Rhl7S15 and RuS2 already described in the prior art. Additionally, a phase of Rhodium and Ruthenium RuRh was observed. The Applicant believes that RuRh phase leads to an increase in the effective catalytic material that allow to obtain cell voltages in line with the target at surprisingly low Rh content .

[0050] It is also believed that H2 treatment before the final heat treatment af fects the Ru incorporation in the crystal phase of the catalyst , eliminating a portion o f sul fur present in the precipitates as H2S .

[0051] The following examples are provided for illustrative purposes only and must not be understood as limiting to the scope of protection defined by the appended claims .

[0052] EXAMPLES

[0053] Catalysts for gas di f fusion electrodes ( GDE ) which comprise ruthenium-rhodium sulphides at di f ferent ruthenium and rhodium weight content were prepared as follows .

[0054] - comparative example 1 (RhS standard) rhodium sulphide RhS , wherein the preparation method does not comprise a step of sintering under hydrogen .

[0055] - comparative example 2 (RuRhS 90 : 10 ) ruthenium-rhodium sulphide , wherein rhodium sulphide content is 10% by weight and ruthenium sulphide content is 90% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , wherein the preparation method does not comprise a step of sintering under hydrogen .

[0056] - comparative example 3 (RuRhS 80 : 20 ) ruthenium-rhodium sulphide , wherein rhodium sulphide content is 20% by weight and ruthenium sulphide content is 80% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , wherein the preparation method does not comprise a step of sintering under hydrogen .

[0057] - comparative example 4 (RuRhS 90 : 10 H2 ) ruthenium-rhodium sulphide , wherein rhodium sulphide content is 10% by weight and ruthenium sulphide content is 90% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , wherein the preparation method comprises a step of sintering under hydrogen .

[0058] - comparative example 5 (RuRhS 80 : 20 H2 ) ruthenium-rhodium sulphide , wherein rhodium sulphide content is 20% by weight and ruthenium sulphide content is 80% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides , wherein the preparation method comprises a step of sintering under hydrogen .

[0059] - comparative example 6 (RuRhS 50 : 50 ) ruthenium-rhodium sulphide , wherein rhodium sulphide content is 50% by weight and ruthenium sulphide content is 50% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, wherein the preparation method does not comprise a step of sintering under hydrogen.

[0060] - example 1 according to the invention (RuRhS 50:50 H2 ) ruthenium-rhodium sulphide, wherein rhodium sulphide content is 50% by weight and ruthenium sulphide content is 50% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, wherein the preparation method comprises a step of sintering under hydrogen.

[0061] Preparation of the catalyst of comparative example 6

[0062] Preparation of carbon suspension

[0063] 350 ml of DI water was added to 3.5 g of Vulcan carbon powder in a glass beaker. The combination was then stirred for 15 min.

[0064] Preparation of metal precursors solutions

[0065] A solution of rhodium salt RhC13.H20 was prepared by dissolving the salt in water to obtain a solution having a concentration of 1.2 g of Rh / 1. A separate solution of ruthenium salt RUCI3.H2O was prepared by dissolving the salt in water to obtain a solution having a concentration of 1.2 g of Ru / 1.

[0066] Preparation of ammonium thiosulfate solution

[0067] 20 ml of DI water was added into a 200 ml glass beaker. 4.3 g of ammonium thiosulfate (NH4)2S2O3 was added into the beaker under stirring with a magnetic stirrer and the solution was then Filtered .

[0068] Rh on carbon catalyst slurry preparation

[0069] 100 ml of 1 . 2 gRh / 1 solution was added to the carbon solution prepared in step 1 and then 100 ml of 1 . 2 gRu / 1 solution was added to the carbon solution prepared in step 1 forming a slurry at a temperature between 60- 100 for 60- 90 minutes .

[0070] The ammonium thiosul fate salt is added to the slurry according to the method described in example 1 of patent application W02005075071 Al .

[0071] Filtration and rinse of RhS on carbon catalyst

[0072] After stopping stirring, the hot slurry was filtered removing all the reaction water .

[0073] The catalyst cake was rinsed with hot DI water .

[0074] The catalyst cake was then dried in air at 120 ° C overnight .

[0075] Dried catalyst grinding

[0076] The overnight dried catalyst was grinded .

[0077] The grinded material was sieved with 300-500 pm mesh sieve before catalyst storage .

[0078] Preparation of 100 g catalyst of example 1 according to the invention

[0079] The catalyst of example 1 according to the invention was prepared in the same way described above for preparing the catalyst of comparative example 6 with the di f ference of further subj ecting the grinded catalyst to a sintering step under hydrogen which is described as follows .

[0080] Dried catalyst thermal treatment and sintering

[0081] After grinding, the catalyst was sintered by heating the catalyst to 60- 80 ° C and kept for 2 hours under 2 % H2-in- Ar gas and then heated to 575 ° C and kept for 2 hours under Argon .

[0082] Comparative examples 1-5 were prepared in the same methods described above while adj usting ruthenium and rhodium content and the presence / absence of the sintering step under hydrogen .

[0083] Performance in hydrochloric acid electrolysis cells (HC1-ODC )

[0084] As shown in figure 2 , the performance of the GDE prepared with the catalysts of the comparative examples and the example according to the invention were tested in 1 dm2cell for HC1-ODC . The dotted line at 1 . 30 V marks an excellent threshold of GDE .

[0085] It is clear from figure 2 that the GDEs prepared with the catalysts according to the invention have smaller cell voltages and consequently better performance with respect to the comparative examples . It can be further seen that the sintering step under hydrogen allows to improve the performance in term of cell voltage, and that the catalyst (RuRhS 50:50 H2 ) , when treated with H2, allows to match the GV target required for industrial application, i.e. less than 1.3. The catalyst RuRhS 50:50 H2 also shows resistance to SD (two shutdowns applied and marked as vertical lines in the graph) .

[0086] Initial cathodic drain of the catalyst (RuRhS 50:50

[0087] H2 ) shows that metal loss is still in the expected range and did not show an increase after shutdown.

[0088] Rotating disk electrode (RDE) analysis

[0089] As shown in figure 3, the RDE graph is reported for the (GDE) comprising a catalyst according to the comparative example 1 (RhS standard) and (GDE) comprising a catalyst according to comparative example 6 (RuRhS 50:50) .

[0090] In figure 3, the RDE curve recorded at 3600 rpm in HC1 5% wt, O2 purge, on Glassy Carbon GC tip 5 mm diameter. Catalyst load 0.025 mg / cm2of catalyst powder) . the RDE graph shows that the two catalysts have the same activity towards ORR reaction.

Claims

CLAIMS1. Gas diffusion electrode (GDE) comprising an inert conductive substrate and a catalytic coating, wherein said catalytic coating comprises ruthenium-rhodium sulphides, wherein rhodium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, wherein ruthenium sulphide content is comprised between 30% and 70% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, wherein said ruthenium-rhodium sulphides comprises crystalline phases of Rhl7S15, RuS2 and RuRh.

2. The gas diffusion electrode (GDE) according to claim 1, wherein rhodium sulphide content is comprised between 40% and 60% and ruthenium sulphide content is comprised between 40% and 60% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides, preferably rhodium sulphide content is 50% and ruthenium sulphide content is 50% by weight referred to metals with respect to the total weight of said ruthenium-rhodium sulphides.

3. The gas diffusion electrode (GDE) according to claims 1 or 2, wherein said catalytic coating comprises carbon, preferably having a surface area higher than 120 m2 / g .

4. The gas diffusion electrode (GDE) according to any one of the preceding claims, wherein said catalytic coating comprises an inert polymeric binder.

5. The gas diffusion electrode (GDE) according to any one of the preceding claims having a total metal load of ruthenium and rhodium in the catalytic coating comprised between 0.5 and 25 g / m2, preferably comprised between 1 and 10 g / m2referred to metals.

6. Method for making the gas diffusion electrode (GDE) according to any one of the preceding claims 1-5, said method comprising the steps of:- providing an inert conductive substrate,- reacting a solution of precursors of ruthenium and rhodium with a sulfur precursor such as to obtain a catalytic coating,- depositing said catalytic coating on said inert conductive substrate, wherein the method further comprises a step of sintering the catalytic coating under hydrogen H2.

7. Method according to claim 6 for making the gas diffusion electrode (GDE) of claim 3, wherein said solution of precursors of ruthenium and rhodium comprises a carbon suspension.

8. Method for making the gas diffusion electrode (GDE) according to claim 6 or 7, wherein said hydrogen H2presents in a concentration comprised between 1 and 4% in Ar, said sintering step is conducted under a temperature equal to or less than 100°C.

9. Method for making the gas diffusion electrode (GDE) according to any one of claims 6 to 8, wherein said sulfur precursor is an aqueous solution comprising thionic species.

10. Method for making the gas diffusion electrode (GDE) according to claim 9, wherein said thionic species is selected from a group consisting of thiosulphates, dithionates, trithionates , tetrathionates, pentathionates, heptathionates and noble or non-noble metal thionates.

11. Use of the gas diffusion electrode (GDE) according to any one of the preceding claims 1 to 5 in a hydrochloric acid electrolysis cell (HC1-ODC) .

12. A hydrochloric acid electrolysis cell comprising at least one ion exchange membrane, said at least one ion exchange membrane dividing said electrolysis cell to at least one cathodic compartment and at least one anodic compartment, said at least one anodic compartment comprising connections for feeding aqueous hydrochloric acid and withdrawing residual acid and produced chlorine, said at least one cathodiccompartment comprising connections for feeding oxygen or an oxygen-containing gas and withdrawing residual oxygen and reaction water, wherein said at least one cathodic compartment comprises at least one oxygen- depolari zed cathode ( ODC ) according to any of the preceding claims 1-5 .

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