Carbon coated electrodes

Abstract

An electrode for electrochemical applications is coated with a layer of a-C, wherein the layer of a-C comprises at least 10 each of first and second sub-layers, being(i) first sub-layers having high conductivity with a sp2 content of 60-95%, alternating with(ii) second sub-layers having high corrosion resistance with a sp2 content of 50-90%, wherein the sp2 content of the first sub-layers is at least 3% greater than the sp2 content of the second sub-layers. A method of making such electrodes comprises: a) depositing a first sub-layer comprising a-C, b) depositing a second sub-layer comprising a-C wherein the sp2 content of the first sub-layer is at least 3% greater than the sp2 content of the second sub-layer, andc) repeating the steps above to deposit at least 10 first sub-layers alternating with 10 second sub-layers, so as to produce the electrodes.

Claims

1. An electrode, coated with a layer of a-C, wherein the layer of a-C comprises at least 10 each of first and second sub-layers, being (i) first sub-layers having high conductivity with a sp2 content of 60-95%, alternating with (ii) second sub-layers having high corrosion resistance with a sp2 content of 50-90%, wherein the sp2 content of the first sub-layers is at least 3% greater than the sp2 content of the second sub-layers.

2. The electrode of claim 1, wherein the sp2 content of the first sub-layers is at least 7% greater than the sp2 content of the second sub-layers.

3. The electrode of claim 1, wherein the first sub-layers have a sp2 content of 70-85% and the second sub-layers have a sp2 content of 60-80%.

4. The electrode of claim 1, wherein the sub-layers each have thicknesses up to 5 nm.

5. The electrode of claim 4, wherein the sub-layers each have thicknesses up to 3 nm.

6. The electrode of claim 5, wherein the sub-layers each have thicknesses in the range 0.2 to 2 nm.

7. The electrode of claim 1, wherein the electrode is a bipolar plate for a PEM hydrogen fuel cell.

8. The electrode of claim 1, wherein the electrode is an electrode for hydrogen generation from water.

9. The electrode of claim 1, wherein there are at least 20 each of the first and second sub-layers.

10. A The electrode of claim 1, wherein the layer of a-C comprises at least 30 of each of the alternating first and second sub-layers, the sub-layers each have thicknesses of about 0.8-2 nm, the first sub-layers have a sp2 content of 70-85% and a sp3 content of 15-30%, and the second sub-layers have a sp2 content of 65-80% and a sp3 content of 20-35%.

11. The electrode of claim 1, wherein the layer of a-C has a molar hydrogen content of 1% or less.

12. The electrode of claim 1, wherein the layer of a-C has an oxygen content of 1% or less.

13. The electrode of claim 1, wherein the layer of a-C has a thickness of 2 m or less.

14. The electrode of claim 13, wherein the layer of a-C has a thickness of 1 m or less.

15. The electrode of claim 1, wherein the coating is deposited by Filtered Cathodic Vacuum Arc.

16. The electrode of claim 1, wherein the sp.sup.2 content of the coating as a whole is at least 55%.

17. A method of coating an electrode for electrochemical applications with a carbon-containing coating, the method comprising: a) depositing a first sub-layer comprising a-C, b) depositing a second sub-layer comprising a-C wherein the sp2 content of the first sub-layer is at least 3% greater than the sp2 content of the second sub-layer, and c) repeating the steps above to deposit at least 10 first sub-layers alternating with 10 second sub-layers, so as to produce the electrode of claim 1.

18. A method of coating an electrode for use in electrochemical applications with a carbon-containing coating, the method comprising: d) depositing a first sub-layer comprising a-C and having a sp2 content of 65-90% and a sp3 content of 10-35%, e) depositing a second sub-layer comprising a-C and having a sp2 content of 45-80% and a sp3 content of 20-55%, and f) repeating the steps above to deposit at least 10 first sub-layers alternating with 10 second sub-layers, wherein the sp2 content of the first sub-layers is at least 3% greater than the sp2 content of the second sub-layers, and wherein the sub-layers each have thicknesses in the range 0.3 to 5 nm.

19. The method of claim 18, comprising depositing at least 30 alternating first and second sub-layers.

20. The method of claim 18, for coating a bipolar plate, comprising: depositing the first sub-layers by Filtered Cathodic Vacuum Arc while applying a first biasing regimen to the bipolar plate; and depositing the second sub-layers by Filtered Cathodic Vacuum Arc while applying a second biasing regimen to the bipolar plate, wherein the second biasing regimen is adjusted compared to the first so as to result in sp2 content in the a-C being decreased by at least 3% and sp3 content in the a-C being increased by at least 3%.

21. The method of claim 20, wherein the biasing regimen comprises applying a biasing voltage according to a duty cycle, and the method comprises periodically adjusting the duty cycle, whereby adjusted duty cycle deposits the second-sub-layers having increased sp3 content and decreased sp2 content.

Description

EXAMPLES

(1) The invention is now illustrated in the following examples.

Example 1Preparation of a Coated Bipolar Plate

(2) A 316L stainless steel bipolar plate was coated as follows.

(3) Step 1) Sample Preparation.

(4) The bipolar plate was cleaned according to the following process: a. A weak alkaline solution was used for ultrasonic cleaning to remove oil stains on the surface and in the flow channels. b. An acidic acid solution was used to remove the oxide layer and any rust on the substrate. c. The substrate was rinse with pure water under ultrasonic conditions. d. The substrate was then dried under vacuum conditions for 0.5 hours.

(5) Step 2) Sample Coating

(6) Coating equipment: FCVA coating machine that also includes ion etching capabilities and magnetron sputtering sources.

(7) Process: a. The cleaned bipolar plate to be coated is placed into the coating chamber and the pressure within the coating chamber is reduced to 5.010.sup.5 Torr (6.6 mPa) and the temperature was increased to 130 C. b. Ion beam cleaning takes place (using convention ion beam cleaning methodology). c. The pressure within the chamber is reduced further to 210.sup.5 Torr (2.6 mPa), and the Ti seed layer is deposited under magnetron sputtering conditions, for a time period sufficient to deposit a Ti layer having a thickness of 0.06 m. d. After the seed layer has been deposited, deposition of the interfacial layer begins. Acetylene gas is let into the chamber until the vacuum is 410.sup.3 Torr. A TiC interfacial layer is deposited using a sputtering deposition method using a titanium target at a power of 12 kW in the presence of acetylene gas. This deposition step is conducted for a time period sufficient to deposit a TiC layer having a thickness of 0.1 m. e. After the interfacial layer has been deposited, a layer of 0.3 m of a-C having multiple sub-layers is deposited using filtered cathodic vacuum arc (FCVA) technology with periodic variation of biasing parameters. Deposition parameters and method were as described below. f. After deposition is complete, the vacuum chamber is brought to room temperature and pressure and the coated substrate is removed from the coating chamber.

(8) The finished coated substrate has the following structure:

(9) TABLE-US-00002 Multi sub-layered sp2/sp3 a-C layer (0.3 m) TiC interfacial layer (0.1 m) Ti seed layer (0.06 m) Substrate - 316L stainless steel bipolar plate

(10) Step 2(e)a-C Layer Deposition

(11) Bipolar plate substrates were mounted on a rotating carousel inside the FCVA deposition chamber. Arc current and substrate bias were fixed with the substrate bias having a duty cycle switching between sp2 and sp3 bias. Internal chamber temperature was measured at approximately 100 degrees Celsius. Coating continued for 280 sub-layers in total, 140 each of sp2 and sp3 content sub-layers, total thickness approximately 300 nm. Sp2 content of alternating sub-layers was 60-70% (sp2 sub-layers) and 50-40% (sp3 sub-layers) respectively.

Example 2Coatings

(12) 316L steel plates were prepared and given seed and interfacial layers as per Example 1. The following specific coatings were deposited onto individual 316L steel plates (with reference only to the a-C coating component):

(13) TABLE-US-00003 Number of repeats first sub-layers second sub-layers of each sub-layer sp2 thick- sp2 thick- (total multi-layer Coating content ness content ness coating thickness) A1 78% 2 nm 70% 2 nm 60 (240 nm = 0.240 m) A2 78% 1.5 nm 74% 1.5 nm 100 (300 nm = 0.3 m) B1 80% 1.8 nm 72% 1.8 nm 80 (288 nm = 0.288 m) B2 76% 1.5 nm 70% 2 nm 70 (245 nm = 0.245 m)

Example 3Testing the Properties of Coatings of the Invention

(14) Coated stainless steel bipolar plates were prepared following the method described above, however in this example the sp2 content of alternating sub-layers was 70-85% (higher sp2 sub-layers) and 60-80% (lower sp2 sub-layers), respectively. The hardness, interfacial contact resistance (ICR) before and after testing* and corrosion current density (I.sub.corr) of the coated bipolar plates was measured. The results are shown in the table below and are represented graphically in FIGS. 1 and 2.

(15) * The testing conditions were 1.8V vs SHE at 0.1 ppm Fluorine+H.sub.2SO.sub.4, pH 3, for 10 hours.

(16) TABLE-US-00004 ICR before ICR after test test Icorr sp2 sp3 Hardness m .Math. cm.sup.2 m .Math. cm.sup.2 A .Math. cm.sup.2 Sample Sub-layer Average % HV Test condition (1.8 V vs SHE) 1.sup.# Layer A Layer A 78 22 743 2.67 5.79 1.05E04 2 Layer A Layer B 76 24 987 2.92 6.39 7.35E05 3 Layer A Layer C 74 26 1171 3.16 6.88 9.93E06 4.sup.# Layer C Layer C 70 30 1201 3.4 10.09 7.12E06 5 Layer D Layer E 79 21 1000 1.7 6.31 3.75E06 .sup.#coatings 1 and 4 are comparative

(17) The sp2 and sp3 values in the above table are an average for the whole a-C layer of each coating (including any sub-layers present), individual sp2 and sp3 values for each of the sub-layers Layer A, Layer B, Layer C, Layer D and Layer E, are provided in the table below.

(18) TABLE-US-00005 Sub-layer sp2 % sp3 % Layer A 78 22 Layer B 74 26 Layer C 70 30 Layer D 85 15 Layer E 75 25

(19) Samples 1 and 4 comprise a single a-C layer, since the alternating sub-layers are the same. Therefore, samples 1 and 4 are present as comparative examples, being homogenous with constant sp2 levels throughout the whole coating. Samples 2, 3 and 5 comprise coatings of the invention with alternating different first and second sub-layers within the a-C layer.

(20) For sample 2, Layer A is the first sub-layer having high conductivity as a result of the higher sp2 percentage, and Layer B is the second sub-layer having high corrosion resistance as a result of the higher sp3 percentage.

(21) For sample 3, Layer A is the first sub-layer having high conductivity as a result of the higher sp2 percentage, and Layer C is the second sub-layer having corrosion resistance as a result of the higher sp3 percentage.

(22) For sample 5, Layer D is the first sub-layer having high conductivity as a result of the higher sp2 percentage, and Layer E is the second sub-layer having corrosion resistance as a result of the higher sp3 percentage.

(23) The individual thicknesses for each sub layer in samples 2, 3 and 5 are provided in the table below.

(24) TABLE-US-00006 Thickness Average sample layer sp2 (%) (nm) sp2 (%) 2 Layer A 78 0.5 76 Layer B 74 0.5 3 Layer A 78 0.5 74 Layer C 70 0.5 5 Layer D 85 0.4 79 Layer E 75 0.6

(25) The total thickness of the multilayer a-C coating is approximately 250 nm for the samples.

(26) Samples 2 and 3 comprise first and second sub-layers each with a thickness of 0.5 nm, therefore the average sp2 content of samples 2 and 3 is the mean average of the two sub-layers.

(27) Sample 5, however, comprises a first sub-layer having a thickness of 0.4 nm and a second sublayer having a thickness of 0.6 nm, therefore the average sp2 content is not equal to the mean average of the two layers, it is in fact slightly lower.

(28) It can be seen from the data that samples 2 and 3 have higher conductivity (lower ICR) and higher corrosion resistance (lower I.sub.corr) than would be expected for a homogenous coating with the same average sp2 and sp3 values but without alternating sub-layers. This is further demonstrated by the graphs in FIGS. 1 and 2, discussed below.

(29) Sample 5 has even higher conductivity (even lower ICR) than samples 2 and 3. Before the test (1.8V vs SHE at 0.1 ppm Fluorine+H2SO4, pH 3, for 10 hours), the ICR of sample 5 was lower than that of sample 2 or sample 3. After the test, the ICR of sample 5 has increased (as you would expect), but it is still lower than that of sample 3 while it is comparable to sample 2. The corrosion current density of sample 5 is also lower than that of samples 1 to 4, and sample 5 therefore shows the best performance.

(30) FIG. 1 shows a graph of ICR against sp2 content both before and after testing. A line has been drawn on the graph between comparative samples 1 and 4 for each data set. It is believed that this line demonstrates the expected ICR for a coating with an a-C layer with uniform sp2 content (i.e. without alternating first and second sub-layers). It can be seen that the ICR of samples 2 and 3 is approximately the same as would be expected for a coating of uniform sp2 content before testing, since the data points for samples 2 and 3 are approximately on the line. However, after testing samples 2 and 3 have better ICR than expected, since the ICR for both samples 2 and 3 is significantly below the line, demonstrating lower interfacial contact resistance (and therefore higher conductivity) than expected for a coating without alternating sub-layers. FIG. 1 therefore demonstrates the improved conductivity of bipolar plates coated with coatings of the invention compared to coatings with uniform sp2 content in the a-C layer.

(31) Turning to FIG. 2, again a line has been drawn between comparative samples 1 and 4 to demonstrate the expected I.sub.corr for a coating with an a-C layer of uniform sp2 content (i.e. without alternating first and second sub-layers). It is clearly seen in the figure that the I.sub.corr of samples 2 and 3 (comprising coatings of the invention), is better than expected, since the data points for these samples are below the expected line. Lower I.sub.corr indicates greater corrosion resistance and therefore FIG. 2 demonstrates the improved properties of samples 2 and 3 coated with coatings of the invention, compared to comparative coatings 1 and 4.

(32) Thus, the invention provides plates for use in electrochemical applications (such as electrodes, and bipolar plates for fuel cells), having a-C layers made up of multiple alternating first and second sub-layers, and methods of making the same.