ELECTRICALLY CONDUCTIVE NANOFIBERS FOR POLYMER MEMBRANE-BASED ELECTROLYSIS

Abstract

The invention preferably relates to an electrolytic cell for generating hydrogen and oxygen with a layer system comprising at least one pair of catalytically active layers between which a polymer membrane is arranged, wherein the layer system comprises electrically conductive ceramic or metallic nanofibers. In particular, the layer system comprises a pair of catalytically active layers, as well as transport layers close to the anode and/or close to the cathode, wherein the pair of catalytically active layers comprises catalytically active nanoparticles, and wherein, in order to increase in-plane conductivity or connectivity of the catalytically active nanoparticles, an intermediate layer comprising ceramic or metallic nanofibers is present between one of the catalytically active layers and one of the transport layers, or metallic or ceramic nanofibers are present within one of the catalytically active layers in addition to the catalytically active nanoparticles. The nanofibers can themselves be catalytically active or catalytically inactive.

Claims

1. Electrolytic cell for generating hydrogen and oxygen with a layer system comprising at least one pair of catalytically active layers between which a polymer membrane is arranged, wherein the layer system comprises the following layers: a pair of catalytically active layers to form an anode and a cathode, and an anode-side transport layer and/or a cathode-side transport layer, wherein the pair of catalytically active layers comprises catalytically active nanoparticles and wherein to improve connectivity of the catalytically active nanoparticles: (i) an intermediate layer comprising ceramic or metallic nanofibers is present between one of the catalytically active layers and a transport layer, or (ii) wherein ceramic or metallic nanofibers are present within one of the catalytically active layers in addition to the catalytically active nanoparticles, wherein the nanoparticles exhibit a maximum dimension of 1 nm-1000 nm and a sphericity of more than 0.5, and the nanofibers exhibit a diameter of 10 nm-1000 nm and an aspect ratio of 5-1000, and wherein the metallic or ceramic nanofibers are produced as a coherent network of nanofibers by means of a spinning process, thermally post-treated, comminuted as a dispersion for further processing and subsequently incorporated as a dispersion into the layer system by means of a coating process.

2. Electrolytic cell according to claim 1, wherein the nanofibers are electrically conductive ceramic or metallic nanofibers, the nanofibers exhibit a diameter of 50 nm-400 nm and/or the nanofibers exhibit an aspect ratio of 5-250.

3. Electrolytic cell according to claim 1, wherein the nanoparticles exhibit a sphericity of more 0.7.

4. Electrolytic cell according to claim 1, wherein the nanofibers exhibit a diameter of less than 1000 nm and a length of less than 20 μm and have been applied in a coating process.

5. (canceled)

6. Electrolytic cell according to claim 1, wherein a loading of the catalytically active layer with catalytically active material has been selected in a range which, when the metallic or ceramic nanofibers are provided, ensures sufficient in-plane conductivity for operation of the electrolytic cell and, without provision of the metallic or ceramic nanofibers, does not ensure sufficient in-plane conductivity for operation of the electrolytic cell.

7. Electrolytic cell according to claim 1, wherein the ceramic or metallic nanofibers are formed from a catalytically active material or are formed from a non-catalytically active material and are coated with a catalytically active material.

8. Electrolytic cell according to claim 1, wherein the metallic or ceramic nanofibers are formed from a non-catalytically active material.

9. Electrolytic cell according to claim 1, wherein the polymer membrane consists of a proton-conductive polymer, and the electrolytic cell is configured for acid electrolysis, or the polymer membrane consists of an anion-conductive polymer, and the electrolytic cell is configured for alkaline electrolysis.

10. Electrolytic cell according to claim 1, wherein the electrolytic cell is an acidic electrolytic cell with a proton-conductive polymer membrane and the nanofibers consist of a catalytically active material and/or are coated with a catalytically active material.

11. Electrolytic cell according to claim 1, wherein the electrolytic cell is an acidic electrolytic cell having a proton-conductive polymer membrane and the nanofibers consist of a catalytically non-active material.

12. Electrolytic cell according to claim 1, wherein the electrolytic cell is an alkaline electrolytic cell having a polymer membrane permeable to hydroxide ions (OH.sup.−) and the nanofibers consist of a catalytically active material.

13. Electrolytic cell according to claim 1, wherein the electrolytic cell is an alkaline electrolytic cell having a polymer membrane permeable to hydroxide ions (OH.sup.−) and the nanofibers consist of a non-catalytically active material.

14. Electrolytic cell according to claim 1, wherein the layer system comprises the following layers: a cathode-side transport layer, a first catalytically active layer, a polymer membrane, a second catalytically active layer, and an anode-side transport layer, wherein an intermediate layer comprising ceramic or metallic nanofibers is present between the first catalytically active layer and the cathode-side transport layer and/or between the second catalytically active layer and the anode-side transport layer.

15. Electrolytic cell according to claim 1, wherein the intermediate layer is formed by nanofibers based on compounds containing iridium oxide.

16. An electrolytic cell stack comprising a plurality of electrolytic cells according to claim 1, which are stacked on top of each other and/or side by side.

17. A method of producing hydrogen and oxygen from water while providing electrical energy comprising using an electrolytic cell according to claim 1.

18. A method of manufacturing an electrolytic cell, according to claim 1, comprising the following steps: a. obtaining a polymer membrane b. applying a pair of catalytically active layers encapsulating the polymer membrane, the pair of catalytically active layers comprising catalytically active nanoparticles, and c. providing an anode-side transport layer and/or a cathode-side transport layer, wherein the method comprises providing ceramic or metallic nanofibers to improve electrical connectivity of the catalytically active nanoparticles, (i) wherein the ceramic or metallic nanofibers are applied as an additional intermediate layer onto one or both of the catalytically active layers, or (ii) wherein the application of the catalytically active layers comprises applying a mixture of catalytically active nanoparticles and ceramic or metallic nanofibers wherein the nanoparticles exhibit a maximum dimension of 1 nm-1000 nm and a sphericity of more than 0.5, and the nanofibers exhibit a diameter of 10 nm-1000 nm and an aspect ratio of 5-1000, and wherein the metallic or ceramic nanofibers are produced as a coherent network of nanofibers by means of a spinning process, thermally post-treated, comminuted as a dispersion for further processing and subsequently incorporated as a dispersion into the layer system by means of a coating process.

19. A method of manufacturing an electrolytic cell, according to claim 1, comprising the following steps: a. obtaining an anode-side transport layer or a cathode-side transport layer, b. applying a catalytically active layer onto the anode-side or cathode-side transport layer, wherein the pair of catalytically active layers comprises catalytically active nanoparticles c. applying a polymer membrane onto the catalytically active layer, wherein the method comprises providing ceramic and metallic nanofibers to enhance connectivity of the catalytically active nanoparticles, (i) wherein the ceramic and metallic nanofibers are applied as an additional intermediate layer to the transport layer, or (ii) wherein the application of the catalytically active layer comprises applying a mixture of catalytically active nanoparticles and ceramic or metallic nanofibers, wherein the nanoparticles exhibit a maximum dimension of 1 nm-1000 nm and a sphericity of more than 0.5, and the nanofibers exhibit a diameter of 10 nm-1000 nm and an aspect ratio of 5-1000, and wherein the metallic or ceramic nanofibers are produced as a coherent network of nanofibers by means of a spinning process, thermally post-treated, comminuted as a dispersion for further processing and subsequently incorporated as a dispersion into the layer system by means of a coating process.

20. The method according to claim 18, wherein the nanofibers exhibit a diameter of less than 1000 nm and a length of less than 20 μm and are applied in a coating process.

21. (canceled)

Description

DESCRIPTION OF THE ILLUSTRATIONS

[0182] FIG. 1 Schematic illustration of the structure and function of an electrolytic cell: Left: acid electrolysis, right: alkaline electrolysis

[0183] FIG. 2 Electron pathways from the porous transport layer to the catalyst layer in a PEM water electrolytic cell. At high catalyst loading, in-plane electron transport is enabled by a comparatively thick catalytic layer (CL, left). The catalytic layer consists substantially of catalytically active nanoparticles. At low loading with catalytically active material, the in-plane electrical resistance increases and some parts of the catalytic layer (CL) lose electrical connection, reducing the active surface area (middle). For example, clusters of nanoparticles may form which are no longer in contact with the transport layer. In a preferred embodiment of the invention, an intermediate layer with nanofibers is incorporated to increase the in-plane conductivity and to connect the entire catalytic layer (CL) (right).

[0184] FIG. 3 Different configuration (1 to 3) for the preferred use of the electrically conductive nanofibers in a layer system of an electrolytic cell. Configuration 1 illustrates a preferred embodiment in which the electrically conductive nanofibers form an intermediate layer between a catalytically active layer and a transport layer. Configuration 2 illustrates a preferred embodiment in which a transport layer comprises electrically conductive nanofibers. The schematic view illustrates a particularly preferred embodiment in which the transport layer consists substantially of nanofibers. Configuration 3 illustrates a preferred embodiment in which one of the catalytically active layers comprises a mixture of catalytically active nanoparticles and nanofibers.

[0185] FIG. 4A: Scanning electron microscopy images of the PVA/iridium(III) chloride nanofibers, B: Scanning electron microscopy images of the iridium(IV) oxide nanofibers.

[0186] FIG. 5 Sheet resistance of a sample of IrOx nanoparticles compared to a sample with an intermediate layer of IrOx nanofibers on IrOx nanoparticles measured in ambient air.

[0187] FIG. 6 Measurement graph 1: Measurement results for a voltage-current density curve of the inventive electrolytic cells compared to the reference electrolytic cells.

[0188] FIG. 7 Measurement graph 2: Measurement results for a resistance-current density curve of the inventive electrolytic cells compared to the reference electrolytic cells.

[0189] FIG. 8 Measurement graph 3: Measurement results for a resistance-current density curve of the inventive electrolytic cells compared to reference electrolytic cells before and after aging by accelerated stress tests.

[0190] Example—Use of Iridium(IV) Oxide Nanofibers as an Additional Intermediate Layer (Configuration 1) and Use of a Mixture of Iridium(IV) Oxide Nanofibers and Iridium(IV) Oxide Nanoparticles as a Catalyst Layer (Configuration 3).

[0191] In the following, an experiment is described for configuration 1 of FIG. 3, where iridium(IV) oxide nanofibers are applied as an additional intermediate layer on top of a catalytically active layer of nanoparticles. Here, commercially available iridium(IV) oxide particles were sprayed on the anode side of a commercially available half CCM (Pt/C cathode on N115 membrane) by spray coating. In a second step, IrOx nanofibers were sprayed on. The membrane electrode assembly (MEA), prepared in this manner, was subsequently electrochemically characterized in an electrolysis stand and compared to conventionally constructed MEAs with the same and significantly higher IrOx loading. The same configuration can also be applied to even more active material systems (e.g. IrRuOx, IrNiOx, etc.) to further enhance the performance. However, since the experiment is initially intended to demonstrate only the performance advantage due to the modified structure, IrOx was used as a proven catalyst material.

[0192] Preparation of Iridium(IV) Oxide Nanofibers

[0193] Polyvinyl alcohol (PVA; M.sub.w>80,000 g/mol; 10.0 wt %) and iridium(III) chloride hydrate (3.0 wt %) were stirred in N,N-dimethylacetamide for 24 h at 100° C. The electrospinning process was carried out in a controlled environment of 30% relative humidity and a temperature of 30° C. The distance between the nozzle and the collecting or counter electrode was 15 cm with an applied voltage of 15 kV. The PVA/IrCl.sub.3 solution was injected into the electric field at a flow rate of 100 μl/h through a nozzle with a diameter of 0.6 mm. The nanofibers were deposited on a PTFE film from which the nanofiber mat could be detached using tweezers after successful fabrication of the nanofibers.

[0194] FIG. 4A shows a scanning electron micrograph of the PVA/iridium(III) chloride nanofibers.

[0195] The PVA/iridium(III) chloride nanofibers were thermally posttreated in air for 4 h in an oven at 350-500° C. (heating rate 1.3 K/min). In this process, both the PVA carrier polymer was oxidized to volatile products (mainly CO.sub.2 and H.sub.2O) and iridium(III) chloride was oxidized to iridium(IV) oxide.

[0196] FIG. 4B shows a scanning electron microscopy of the iridium(IV) oxide nanofibers obtained in this manner.

[0197] Manufacture of the Membrane Electrode Assembly (MEA)

[0198] Five electrolytic cells were fabricated, two references comprising a catalytically active layer with nanoparticles on the anode side as is common in the prior art: [0199] Reference sample 1—NP 1.2 mg/cm.sup.2 with catalyst loading of 1.2 mg/cm.sup.2 IrO.sub.x (black) [0200] Reference sample 2—NP 0.2 mg/cm.sup.2 with catalyst loading of 0.2 mg/cm.sup.2 IrO.sub.x (red)

[0201] In addition, as a further reference, a membrane electrode assembly was fabricated using only IrOx nanofibers as the catalyst material in the anode. [0202] Reference sample 3—NF 0.2 mg/cm.sup.2 with catalyst loading of 0.2 mg/cm.sup.2 IrOx (blue)

[0203] Moreover, an electrolytic cell according to configuration 1 of the invention was fabricated comprising, in addition to a catalytically active layer comprising nanoparticles, an intermediate layer comprising IrOx nanofibers: [0204] Inventive sample—Conf 1—0.2 mg/cm.sup.2 with catalyst loading of 0.2 mg/cm.sup.2 IrO.sub.x (blue)

[0205] Lastly, an electrolytic cell according to configuration 3 of the invention was fabricated comprising a mixture of IrO.sub.x nanoparticles and IrOx nanofibers in the catalytically active layer: [0206] Inventive sample—Conf 3—0.2 mg/cm.sup.2 with catalyst loading of 0.2 mg/cm.sup.2 IrO.sub.x (olive)

[0207] First, a dispersion containing 1% iridium(IV) oxide, 0.4% Nafion D520 dispersion, 49.3% isopropanol and 49.3% (both percentages by mass) deionized water was prepared. The dispersions were treated in an ultrasonic bath for 30 min immediately before spray coating.

[0208] To prepare the MEAs, the dispersion was sprayed by spray coating on the anode side on 5 cm.sup.2 with 0.5 mg/cm.sup.2 Pt/C cathode and N115 membrane. For Reference Sample 1—NP 1.2 mg/cm.sup.2, 1.2 mg/cm.sup.2 IrO.sub.x was sprayed as nanoparticles, for Reference Sample 2—NP 0.2 mg/cm.sup.2, 0.2 mg/cm.sup.2 IrO.sub.x was sprayed as nanoparticles and for Inventive Sample—Conf 1—0.2 mg/cm.sup.2, only 0.1 mg/cm.sup.2 IrO.sub.x was sprayed as nanoparticles in this step. For the Reference Sample 3—NF 0.2 mg/cm.sup.2 no IrO.sub.x nanoparticles were sprayed onto the membrane.

[0209] For the Inventive Sample—Conf 1—0.2 mg/cm.sup.2 and Reference Sample 3—NF 0.2 mg/cm.sup.2, a further dispersion containing 1% previously prepared iridium(IV) oxide nanofibers, 0.4% Nafion D520 dispersion, 49.3% isopropanol and 49.3% (respectively percentages by mass) deionized water was prepared and treated in an ultrasonic bath for 30 min immediately prior to spray coating. The dispersion was subsequently spray coated onto the pre-existing anode with 0.1 mg/cm.sup.2 IrO.sub.x in the case of Inventive Sample—Conf 1—0.2 mg/cm.sup.2, resulting in a total loading of 0.2 mg/cm.sup.2 IrO.sub.x. In the case of Reference Sample 3—NF 0.2 mg/cm.sup.2, a total of 0.2 mg/cm.sup.2 IrO.sub.x nanofibers was applied in a single spray. For the Inventive Sample—Conf 3—0.2 mg/cm.sup.2, a dispersion containing 0.5% iridium(IV) oxide nanoparticles, 0.5% iridium(IV) oxide nanofibers, 0.4% Nafion D520 dispersion, 49.3% isopropanol, and 49.3% (respectively percentages by mass) deionized water was prepared. A total of 0.2 mg/cm.sup.2 of the dispersion was sprayed to prepare the MEA. All loadings refer to the pure precious metal.

[0210] The MEAs lay on a plate heated to 90° C. during the spraying process. This causes isopropanol and water to evaporate between the individual passes, creating a porous layer of IrOx and Nafion.

[0211] Measurement of Layer Resistance

[0212] To determine the extent to which the provision of nanofibers can increase the in-plane conductivity of a layer of nanoparticles, the in-plane resistivity for catalyst layers of IrOx nanoparticles was determined in comparison to a combination of IrOx nanoparticles with IrOx nanofibers. The layer resistance of the catalyst layers was measured by a transfer line method using a similar setup as explained in more detail in Ahadi et al. 2019 (Ahadi, M.; Tam, M.; Stumper, J.; Bahrami, M. Electronic conductivity of catalyst layers of polymer electrolyte membrane fuel cells: Through-plane vs. in-plane. International Journal of Hydrogen Energy 2019, 44, 3603-3614.22).

[0213] The catalyst layers for the measurement of layer resistance were prepared with a width of 1 cm by spray coating on an insulating glass substrate. For a reference sample (IrOx NP, 0.2 mg.sub.IR/cm.sup.2), only IrOx nanoparticles with a loading of 0.2 mg.sub.IR/cm.sup.2 were applied to the insulating glass substrate. In one inventive sample, an intermediate layer of IrOx nanofibers was deposited in addition to a layer of IrOx nanoparticles (IrOx hybrid, 0.2 mg.sub.IR/cm.sup.2). Then, carbon paper with a microporous layer (MPL) (Freudenberg H24C5) and a width of 5 mm was pressed onto the film with 0.7 N, first MPL side, to electronically connect the whole width of the film. Resistance was measured for contact distances of 1, 2, 3, and 4 cm at ambient conditions (25° C., 50% RH) using a FLUKE 175 multimeter. By plotting the resistance against the distance between the measurement points and a linear fit, the layer resistance was determined from the slope. The electrical layer resistance (in plane resistivity) can thus be defined as R.sub.sheet=dR/dx−w, where R is the measured ohmic resistance, w is the width of the catalyst layer and x is the measurement distance between the contact points of the test sample.

[0214] As can be seen in FIG. 5, the layer resistance of a catalyst layer in which an intermediate layer of IrOx nanofibers is applied in addition to the IrOx nanoparticles (IrOx hybrid, 0.2 mg.sub.IR/cm.sup.2) is significantly lower than is the case for a catalyst layer consisting solely of nanoparticles (IrOx NP, 0.2 mg.sub.IR/cm.sup.2). Accordingly, the in-plane conductivity is higher when using IrOx nanofibers.

[0215] Electrochemical Characterization

[0216] All MEA tests were performed in a single cell with gold-coated titanium flowfields. A Freudenberg H24C5 (30% compression) was used on the cathode side and a Bekaert Ti fiber transport layer on the anode side. To run-in the cell, 15 polarization curves from 1.4 to 2.2 V were measured before starting the measurements. In order to estimate the lifetime of the inventive sample compared to the reference samples, accelerated stress tests (AST) were performed in addition to the polarization measurements. For this, a test protocol was used which allows the MEAs to degrade to a similar extent, by applying an AC voltage within a short period of time, as regular operation over a much longer period of time (Angewandte Chemie, International ed. in English, 56, 5994-6021, 2017). After 40000 AST cycles, polarization measurements were again taken and compared with measurements of the unaged samples.

[0217] Results: Measurement Data

[0218] The electrolytic cells with the layers produced according to configuration 1 (“Electrolytic cell Conf-1”) and configuration 3 (“Electrolytic cell Conf-3”) are now compared with electrolytic cells according to the prior art (“Reference Samples 1-3”). In the graph, the Y-axis denotes the voltage in V (E_Stack) and the X-axis denotes the current density or current per area in mA/cm.sup.2 (I). The hydrogen production rate is proportional to the current density, i.e. the higher the current density, the more hydrogen is produced. The lower the voltage at the same current density, the more efficient the electrolytic cell, since the energy to be used per quantity of hydrogen produced is directly proportional to the voltage.

[0219] From measurement graph 1 of the voltage-current density curve (FIG. 6), it can be concluded: [0220] 1. Reference sample 2—NP 0.2 mg/cm.sup.2 has a higher cell voltage (and thus a lower conversion efficiency) than reference sample 1—NP 1.2 mg/cm.sup.2. This is to be expected as the catalyst loading was significantly reduced. [0221] 2. Reference sample 3—NF 0.2 mg/cm.sup.2, has a lower cell voltage than reference sample 2—NP 0.2 mg/cm.sup.2 due to the better connection of the catalyst material at higher current densities. At low current densities, however, the disadvantage of lower catalyst activity becomes apparent in comparison with reference sample 2—NP 0.2 mg/cm.sup.2. Here the overvoltage of reference sample 3—NF 0.2 mg/cm.sup.2 is lower than that of reference sample 2—NP 0.2 mg/cm.sup.2. [0222] 3. The electrolytic cell conf-1-0.2, on the other hand, combines the advantages of nanoparticles and nanofibers and, despite low catalyst loading, has a comparable voltage to reference sample 1—NP 1.2 mg/cm.sup.2 with 6 times the catalyst loading. [0223] 4. The electrolytic cell conf-3-0.2, similarly combines the advantages of nanoparticles and nanofibers but has a higher voltage than the electrolytic cell conf-1-0.2 in this test.

[0224] In another measurement graph (FIG. 7), the high frequency resistance (HFR) is plotted against the current density. The high frequency resistance was determined by in-situ impedance measurements at 1 kHz during the polarization measurement. The HFR includes both ionic and electrical resistances in the cell. For the same electrolytic cells, it can be concluded from the graph: [0225] 1. Reference sample 2—NP 0.2 mg/cm.sup.2 has a higher HFR than reference sample 1—NP 1.2 mg/cm.sup.2. This is to be expected as the catalyst loading was significantly reduced. [0226] 2. The electrolytic cell-NF-0.2, has a significantly lower HFR than reference sample 2—NP 0.2 mg/cm.sup.2. [0227] 3. The electrolytic cell Conf-1-0.2, despite low catalyst loading, has a very low HFR, which is comparable to the reference sample 1—NP 1.2 mg/cm.sup.2 with 6-fold catalyst loading. [0228] 4. The electrolytic cell conf-3-0.2, achieves a significantly lower HFR than the equally loaded prior art reference cell (reference sample 2—NP 0.2 mg/cm.sup.2). However, the HFR is higher than the HFR of the electrolytic cell conf-1-0.2, the highly loaded reference sample 1—NP 1.2 mg/cm.sup.2 and slightly higher than the HFR of the low loaded pure nanofiber layer.

[0229] In another measurement graph (FIG. 8), the polarization curves of reference sample 2—NP 0.2 mg/cm.sup.2 and the inventive electrolytic cell conf-1-0.2 are compared before and after aging by accelerated stress test (AST). After 40000 ASTs, the overpotential of the electrolytic cell conf-1-0.2 is significantly less increased than the overpotential of reference sample 2—NP 0.2 mg/cm.sup.2. Thus, the performance of the electrolytic cell conf-1-0.2 is better preserved. The lower overpotential indicates an increased lifetime of the inventive sample compared to the reference samples.

[0230] Overall, the measurements show that by using nanofibers, especially in configurations 1 and 3, the catalyst loading can be significantly reduced without increasing the cell voltage or the HFR. This means that by using nanofibers, for example as an intermediate layer or a mixture of nanofibers and nanoparticles, catalyst material can be saved and the performance of the cell can be improved at the same time. The results of the accelerated stress tests indicate an increased service life of the inventive electrolytic cells.

REFERENCE LIST

[0231] 4 Cathode-side transport layer

[0232] 5 Catalytically active layer (cathode side)

[0233] 7 Polymer membrane

[0234] 9 Catalytically active layer (anode side)

[0235] 11 Anode-side transport layer

[0236] 13 Nanofibers

[0237] 15 Nanoparticles