ELECTRODE FOR GAS EVOLUTION IN ELECTROLYTIC PROCESSES

Abstract

An electrode for gas evolution in electrolytic processes and a method for the production of such an electrode, the electrode having a metal substrate and a coating formed on the substrate, wherein the coating has at least a highly porous catalytic outer layer containing nickel oxide and nickel hydroxide, the porous outer layer having a surface area of at least 40 m.sup.2/g (BET). The catalytic layer is prepared from a Ni oxide/V oxide initial coating with subsequent leaching of V.

Claims

1. An electrode for gas evolution in electrolytic processes comprising a metal substrate and a coating formed on said substrate, wherein said coating comprises at least a catalytic porous outer layer containing nickel oxide and nickel hydroxide, said porous outer layer having a surface area of at least 40 m.sup.2/g BET, wherein said porous outer layer is obtained by leaching vanadium oxide from a thermally treated gel-like precursor coating containing nickel salts and vanadium salts.

2. The electrode according to claim 1, wherein said metal substrate is a substrate selected from the group consisting of nickel-based substrates, titanium-based substrates and iron-based substrates.

3. The electrode according to claim 1, wherein said porous outer layer consists of nickel oxide and nickel hydroxide.

4. The electrode according to claim 1, wherein said porous outer layer has a surface area comprised between 40 and 120 m.sup.2/g BET.

5. (canceled)

6. The electrode according to claim 1, wherein said coating comprises an interlayer deposited between said metal substrate and said catalytic porous outer layer, the interlayer comprising nickel and/or nickel oxide.

7. The electrode according to claim 1, wherein said porous outer layer has thickness in a range from 5 to 40 μm.

8. The electrode according to claim 1, wherein said porous outer layer has a nickel loading in a range from 5 to 50 g/m.sup.2 referred to the metal element.

9. The electrode according to claim 6, wherein said interlayer has a nickel loading in a range from 100 to 3000 g/m.sup.2 referred to the metal element.

10. The electrode according to claim 6, wherein said interlayer has a porosity of less than about 1 m.sup.2/g BET.

11. The electrode according to claim 5, wherein said interlayer has an electric double layer capacitance, normalized by the metal loading, in a range of from about 1.0 to about 10.0 mF/g.

12. The electrode according to claim 6, wherein said coating consisting of the porous outer layer and the interlayer has an overall thickness in a range from 30 to 300 μm.

13. The electrode according to claim 6, wherein said nickel interlayer is obtained by thermal spraying, laser cladding or electroplating.

14. The electrode according to claim 13, wherein said thermal spraying, is wire-arc spraying or plasma spraying.

15. The electrode according to claim 1, wherein said substrate is a nickel mesh.

16. An electrochemical cell for electrolytic processes comprising an anode for oxygen evolution and a cathode, wherein said anode is an electrode according to claim 1.

17. A method for the production of the electrode according to claim 1 comprising the following steps: a) applying to a metal substrate a coating solution comprising a nickel salt, a vanadium salt and a gelling agent; b) drying at a temperature in the range of 80-150° C.; c) calcining at a temperature in the range of 300-500° C.; d) repeating steps a) to c) until a coating having a desired specific load of nickel is obtained; e) finally, thermally treating at a temperature in the range from 300-500° C.; and f) carrying out leaching of vanadium from said coating in an alkaline bath.

18. The method according to claim 17, wherein said coating solution comprises a solvent comprising water and/or an alcohol, and an acid.

19. The method according to claim 17, wherein said gelling agent comprises ethylene glycol and citric acid.

20. The method according to claim 17, wherein said nickel salts are nickel halides, and said vanadium salts are vanadium halides.

21. The method according to claim 17, wherein step f) is carried out in an aqueous alkaline hydroxide solution at a temperature in the range from 60 and 100° C. for a time period between 12 and 36 hours.

22. The method according to claim 16 further comprising an intermediate step a0) preceding step a), wherein step a0) comprises forming an interlayer of nickel and nickel oxide on the metal substrate via thermal spraying, laser cladding or electroplating, the interlayer having a porosity of less than about 1 m.sup.2/g BET.

23. The method according to claim 22 wherein the interlayer in step a0) is formed via thermal spraying by electric wire or by plasma spraying nickel powder on the metal substrate in ambient air.

24. The method according to claim 23 wherein said nickel powder is plasma sprayed onto the metal substrate and has a mean particle size of from about 10 μm to about 150 μm, or from about 45 μm to about 90 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The invention will now be described in connection with certain preferred embodiments and corresponding figures in more detail.

[0048] In the drawings,

[0049] FIG. 1 depicts SEM-photographs of the surface and of a cross-sectional image of the catalytic outer layer of the electrode of example 2 without nickel interlayer;

[0050] FIG. 2 depicts the results of a BET surface area measurement of the outer surface of the electrode of example 2;

[0051] FIG. 3 depicts a diffraction pattern of the electrode of example 2;

[0052] FIG. 4 shows the results of an accelerated life time test of an electrode of example 2 compared with prior art electrodes;

[0053] FIG. 5 depicts SEM-photographs of the surface and of a cross-sectional image of the catalytic outer layer of the electrode of example 3 with nickel interlayer;

[0054] FIG. 6 shows the results of shutdown tests of an electrode of example 3 compared with a bare nickel electrode of prior art; and

[0055] FIG. 7 shows the results of shutdown tests of an electrode of example 3 compared with an iridium-based electrode of prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Example 1: Preparation of Coating Solution

[0056] For preparing one litre (l) of coating solution, 0.4 l of demineralized water, 0.4 l of ethylene glycol and 0.2 l of 37% hydrochloric acid were mixed in a flask and stirred for 10 minutes. 300 g of VCl.sub.3 were added to the solution and dissolved under stirring for 30 minutes. Subsequently, 450 g NiCl.sub.2 6H.sub.2O were added to the solution and dissolved under stirring for 30 minutes. 300 g of citric acid were added to the solution and dissolved under continuous stirring for 45 minutes.

Example 2: Preparation of an HP—NiO.SUB.x .Coated Nickel Mesh Electrode without Interlayer

[0057] For preparing 1 m.sup.2 of coated mesh, a nickel rhombic mesh with a 0.5 mm thickness was sandblasted and etched in a hydrochloric acid solution. 4 ml of the coating solution of Example 1 were deposited by brushing on each side of the mesh, dried at 130° C. for 30 minutes and calcinated at 400° C. for 10 minutes resulting in a nickel loading for one cycle of 1 g/m.sup.2 projected area. The deposition, drying and calcination steps were repeated for a total of 10 cycles to obtain a final nickel loading of 10 g/m.sup.2 projected area. Subsequently, the coated electrode was post-baked at 400° C. for 2 hours. Finally, the electrode was leached in an alkaline NaOH bath for vanadium removal at a temperature of 80° C. for a total time of 24 hours.

Example 3: Preparation of a HP—NiO.SUB.x .Coated Nickel Mesh Electrode with Nickel Interlayer

[0058] A nickel rhombic mesh, with a 0.5 mm thickness, was plasma sprayed with 99.9% purity nickel powder with a particle size of 45±10 μm (Fe<0.5, 0<0.4, C<0.02, S<0.01 in ambient air on both sides in an amount of 4.8±0.5 g/dm.sup.2 and with a target thickness of 50 μm on each side). Afterwards, the sprayed wire mesh was heated in an oven at 350° C. for 15 minutes in air. The plasma-sprayed woven mesh was allowed to cool and then was coated with a precursor composition, by means of a brush, in a series of coating, heating and cooling steps. For preparing 1 m.sup.2 of coated mesh provided with the nickel interlayer, 14 ml of the coating solution of Example 1 were deposited by brushing on each side of the mesh, dried at 130° C. for 30 minutes and calcinated at 400° C. for 10 minutes resulting in a nickel loading for one cycle of 3 g/m.sup.2 projected area. The deposition, drying and calcination steps were repeated for a total of 7 cycles to obtain a final nickel loading of 21 g/m.sup.2 projected area. Subsequently, the coated electrode was post-baked at 400° C. for 2 hours. Finally, the electrode was leached in an alkaline NaOH bath for vanadium removal at a temperature of 80° C. for a total time of 24 hours.

Counterexample 4

[0059] A nickel rhombic mesh with a 0.5 mm thickness comprising a three-layer coating made of a LiNiO base layer, a NiCoO.sub.x interlayer and a IrO.sub.x top layer was obtained by sequentially applying via brushing and thermally decomposing each corresponding precursor solution onto the mesh substrate (or the respective underlying layer).

Counterexample 5

[0060] A nickel rhombic mesh with a 0.5 mm thickness comprising a two-layer coating made of a LiNiO base layer, a LiNiIrO.sub.x top layer was obtained by sequentially applying via brushing and thermally decomposing each corresponding precursor solution onto the mesh substrate (or the respective previous layer).

[0061] The electrodes of Examples 2 and 3 according to the present invention have been characterized using different techniques and compared with Counterexamples 4 and 5.

[0062] A. Characterization of the Electrode of Example 2 (Electrode with HP—NiO.sub.x Catalytic Layer but without Nickel Interlayer)

[0063] A.1 Scanning Electron Microscopy (SEM) was employed to evaluate the morphology of the coating both on surface and cross-section, respectively. The analysis has been performed on fresh and used samples to qualitatively estimate properties as stability, adhesion and consumption of the coating. FIG. 1 shows SEM images of surface view (a) and of a cross-sectional view (b) of an electrode of the present invention prepared according to Example 2. The morphological surface analysis shows the flat “dry mud” morphology of the HPNiO.sub.x coating while the cross section shows the porosity of the coating. In addition, in the cross section it is possible to see the phase homogeneity of the coating. The images, especially the cross-sectional view (b) show that the bulk nickel substrate 10 exhibits a certain roughness after sandblasting and etching which benefits the adhesion/anchoring of the catalytic porous outer layer 11 on the substrate. However, the outer surface of the catalytic outer layer 11 applied according to the method of the present invention is smooth, thus preventing damage to a delicate membrane when assembled into an electrolysis cell.

[0064] A.2 A Corrected Impedance Single Electrode Potential (CISEP) test was employed to characterize the electrochemical performance of the electrode of the invention compared to prior art anodes used in alkaline water electrolysis. To determine the oxygen overvoltage of the electrode of the present invention, it has been tested as an anode in a three-electrode beaker-cell. The testing conditions are summarized in Table 1.

TABLE-US-00001 TABLE 1 Electrolyte 25 wt % KOH in ultrapure H.sub.2O (1.5 l) Temperature 80° C. Cathode Nickel mesh (projected area 12 cm.sup.2) Working anode 1 cm.sup.2 projected area electrolysis area Reference electrode Saturated Calomel Electrode (SCE)

[0065] At first, the sample undergoes 2 hours of pre-electrolysis (conditioning) at 10 kA/m.sup.2 to stabilise the oxygen overvoltage (OOV). Then, several chronopotentiometry steps are applied to the sample. Final output of the CISEP test is the average of the three steps performed at 10 kA/m.sup.2, corrected by the resistance of the electrolyte.

[0066] Table 2 summarizes a comparison between a bare nickel anode (Base Ni), the iridium-based anode of Counterexample 4 (CEx 4), a Raney nickel anode (Ni Raney), and the electrode of Example 2 (HP—NiO.sub.x):

TABLE-US-00002 TABLE 2 OOV vs NHE [mV] @ 10 kA/m.sup.2 Bare Ni 340 CEx 4 260 Ni Raney 240 HP-NiO.sub.x 200

[0067] The energetic saving (140 mV lower 00V than Bare Ni) obtainable with the anode of the present invention solves the problem of the high operational costs given by the sluggish kinetic of the anodic reaction of an uncoated nickel mesh without involving costly noble metals or hazardous manufacturing processes.

[0068] A.3 BET measurements were performed to determine the surface area of the electrode of Example 2 as compared to the electrode of Counterexample 5 (CEx 5) which is also suitable for alkaline water electrolysis. The results shown in FIG. 2 indicate that the electrode of Example 2 has a surface area which is considerably higher than the prior art electrode.

[0069] A.4 X-Ray diffraction (XRD) techniques were used to evaluate the type of formed oxides and their crystalline structure. A typical diffraction pattern resulting from an electrode according to Example 2 is shown in FIG. 3. The x-axis denotes the diffraction angle 2θ and the y-axis denotes the diffraction intensity in arbitrary units (for instance in counts per scan). Strong peaks (1), (2) and (3) correspond to the Ni substrate at crystallographic planes (111), (200) and (220), respectively. The weaker peaks (4), (5) and (6) correspond to a NiO phase of the highly porous catalytic outer layer at crystallographic planes (111), (200) and (220), respectively. Even weaker peaks (7), (8), (9) and (10) correspond to a Ni(OH).sub.2 phase of the highly porous outer catalytic coating, corresponding at crystallographic planes (001), (100), (101) and (110), respectively. Accordingly, it was determined that the catalytic coating is composed of nickel oxide (NiO) and nickel hydroxide (Ni(OH).sub.2). Moreover, as can be clearly taken from the diffraction pattern of FIG. 3, the highly porous catalytic coating of the present invention clearly does not contain any iridium or other rare/expensive metals. Accordingly, the cost and supply problems associated with prior art electrodes can be avoided with the electrode of the present invention.

[0070] A.5 An Accelerated Lifetime Test (ALT) was employed to estimate the lifetime of the catalytic coating. The test consists of long term electrolysis in a beaker cell with a two-electrode set up and a continued electrolysis current directly applied to them. The applied conditions are harsher compared to the one of the CISEP test and are above typical operating conditions in order to accelerate the consumption process. The conditions implied in the accelerated lifetime test are summarized in Table 3 below:

TABLE-US-00003 TABLE 3 Electrolyte 30 wt % KOH in ultrapure H.sub.2O Current density 20-40 kA/m.sup.2 Temperature 88° C. Counter electrode Nickel mesh Working electrode 1 cm.sup.2 projected area electrolysis area

[0071] ALT data are shown in FIG. 4. The x-axis denotes the duration of the test in hours and the y-axis denotes the cell voltage in volt. Data points (1) indicate the results for a non-coated Ni substrate showing an increase of the cell voltage from 2.5 V to 2.7 V after only a couple of hours of operation. The cell voltage remains stable at 2.7 V indicating that no further deterioration occurred. Data points (2) indicate the electrode of Example 2, which maintains a lower cell voltage of 2.5 V for approximately 250 hours until an increase of cell voltage and subsequent failure of the electrode occurred. This indicates that the electrode of Example 2 having a highly porous outer catalytic nickel oxide layer (without interlayer) has superior performance in terms of cell voltage compared to the bare nickel substrate, but is not suitable for prolonged operation under the harsh conditions of the ALT. As indicated above, the electrode of Example 2 is particularly suitable for operation under lower current densities. Data points (3) and (4) will be described in detail in connection with the characterization of the electrode of Example 3 below.

[0072] B) Characterization of the Electrode of Example 3 (Electrode with HPNiO.sub.x Catalytic Layer with Nickel Interlayer)

[0073] B.1 Again, Scanning Electron Microscopy (SEM) was employed to evaluate the morphology of the coating both on surface and cross-section, respectively. The analysis has also been performed on fresh and used samples to qualitatively estimate properties as stability, adhesion and consumption of the coating. FIG. 5 shows SEM images of surface (a) and of a cross-section (b) of an electrode of the present invention prepared according to Example 3 (note that the images of FIG. 5 are obtained at a lower resolution/magnification then the images of FIG. 1). Again, especially the cross-sectional view (b) shows that the while the bulk nickel substrate 10 exhibits a certain roughness after sandblasting and etching, the application of a nickel interlayer 12 by plasma spraying and a catalytic outer layer 11 using the method of the present invention result in a smooth surface.

[0074] B.2 An Accelerated Lifetime Test (ALT) as described in section A.5 above has also been conducted with the electrode of Example 3. The corresponding results are also depicted in FIG. 4. Data points (3) indicate a nickel substrate with plasma-sprayed NiO interlayer, i.e. without additional HP—NiO.sub.x catalytic outer layer. The mere interlayer-electrode exhibits a lower cell voltage than the bare nickel substrate, but still at least 100 mV higher than the electrode of Example 2 with a further continuous increase throughout the electrode lifetime. Data points (4) show the electrode of Example 3, i.e. a nickel substrate with a plasma-sprayed nickel interlayer and a highly porous catalytic outer layer. Electrode 3 shows the best performance in the accelerated lifetime test, having a similar low initial cell voltage of 2.5 V with a very slow continuous increase over an operational lifetime of nearly 1,500 hours.

[0075] B.3 In order to assess the resistance of the electrode of Example 3 to inversion of polarity and to estimate its' resistance to simulated plant shutdowns, shutdown tests have been performed under the operational conditions, as summarized in Table 4 below:

TABLE-US-00004 TABLE 4 Temperature 80° C. Electrolyte 30 wt % KOH in ultrapure H.sub.2O Current density 10 kA/m.sup.2

[0076] The following test protocol was carried out: After a grate-in period of 48 hours, a 6-hour shutdown was simulated by shortening the electrolysis cell with pumps staying on and letting the temperature drop to room temperature. After shutdown, electrolysis was continued for 6 hours at the operating conditions of Table 4. The shutdown cycle was repeated until failure of the electrode.

[0077] FIG. 6 shows the results of an electrode of Example 3 (data points (1)) and a bare nickel electrode (data points (2)). On the x-axis, the number of shutdowns is depicted, while the y-axis shows the cell voltage. The results indicate that the bare nickel electrode while operating at a higher cell voltage was only capable of withstanding 40 shutdowns, while the electrode of Example 3 maintained its' low cell voltage for up to 55 shutdowns. In FIG. 7, a comparison of an electrode of Example 3 (data points (1)) with the electrode of Counterexample 4 (data points (2)) is shown. On the x-axis, the number of shutdowns is depicted, while the y-axis shows the deviation from a normalized cell voltage to eliminate the constitution of cathode and separator. As can be taken from FIG. 7, the highly porous nickel oxide outer catalytic layer on a plasma-sprayed nickel interlayer can withstand more than 50 shutdowns without increase of the cell voltage. In contrast, the cell voltage of the electrode of Counterexample 4 starts to increase after 20 shutdowns already.

[0078] The preceding description is not intended to limit the invention, which may be used according to various embodiments without however deviating from the objectives and whose scope is uniquely defined by the appended claims.

[0079] In the description and in the claims of the present application, the terms “comprising”, “including” and “containing” are not intended to exclude the presence of other additional elements, components or process steps.

[0080] The discussion of documents, items, materials, devices, articles and the like is included in this description solely with the aim of providing a context for the present invention. It is not suggested or represented that any or all of these topics formed part of the prior art or formed a common general knowledge in the field relevant to the present invention before the priority date for each claim of this application.