CARBON COATED HYDROGEN FUEL CELL BIPOLAR PLATES

Abstract

A bipolar plate for a PEM hydrogen fuel cell is coated with a carbon-containing coating, the carbon-containing coating comprising in order: a) a titanium seed layer; b) a titanium nitride interfacial layer; and c) a a-C top layer, and wherein the bipolar plate is formed from stainless steel. Methods for making such coated plates are described. The a-C has a density of greater than 2.0 g/cm3, a molar hydrogen content of 5% or less, an sp2 carbon content of 40% to 80% and an sp3 carbon content of 20% to 60%. The coated plates possess good electrical conductivity and are resistant to corrosion.

Claims

1.-56. (canceled)

57. A bipolar plate for a hydrogen fuel cell (e.g. a PEM hydrogen fuel cell), coated with a carbon-containing coating, the carbon-containing coating comprising in order: a) a titanium seed layer; b) a titanium nitride interfacial layer; and c) a a-C top layer, and wherein the coating has a thickness of less than 1.5 μm and the bipolar plate is formed from stainless steel.

58. A bipolar plate according to claim 57, wherein the layer of a-C has a volumetric density of greater than 2.5 g/cm.sup.3, for example greater than 3.0 g/cm.sup.3, preferably 3.5 g/cm.sup.3 or greater.

59. A coated bipolar plate according to claim 57, wherein the carbon-containing coating consists of in order: a) a titanium seed layer; b) a titanium nitride interfacial layer; and c) a a-C top layer.

60. A bipolar plate according to claim 57, wherein the layer of a-C has a molar hydrogen content of 5% or less and/or a molar nitrogen content of 5% or less.

61. A bipolar plate according to claim 57, wherein the layer of a-C has an sp.sup.2 carbon content of 50% to 80% (e.g. an average sp.sup.2 carbon content of about 55%).

62. A bipolar plate according to claim 57, wherein the layer of a-C has an sp.sup.a carbon content of 30% to 60% (e.g. an average sp.sup.a carbon content of about 45%).

63. A bipolar plate according to claim 57, wherein the layer of a-C has a hardness of 1500 HV to 2000 HV.

64. A bipolar plate according to claim 57, wherein the carbon-containing coating comprises in order: a) a titanium seed layer with a thickness of 0.5 μm or less; b) a titanium nitride interfacial layer with a thickness of 0.5 μm or less; and c) a a-C top layer with a thickness of 1.0 μm or less, and wherein the bipolar plate is formed from stainless steel and has a thickness of 0.5 mm or less.

65. A bipolar plate according to claim 57, wherein the a-C layer has a molar hydrogen content of 5% or less, an sp.sup.2 carbon content of 50% to 70%, and an sp.sup.3 carbon content of 30% to 60%.

66. A bipolar plate for a hydrogen fuel cell, coated with a carbon-containing coating, the carbon-containing coating comprising in order: a) a seed layer comprising a metal or alloy (e.g. titanium); b) an interfacial layer comprising a carbide or nitride of the metal/alloy in the seed layer; and c) a top layer comprising a-C (e.g. consisting of a-C); wherein the a-C has a density of greater than 3.0 g/cm.sup.3, a molar hydrogen content of 5% or less, an sp.sup.2 carbon content of 40% to 70%, and an sp.sup.3 carbon content of 30% to 60%.

67. A bipolar plate according to claim 66, wherein the layer of a-C has a molar hydrogen content of 2% or less.

68. A bipolar plate according to claim 66, wherein the layer of a-C has an average sp.sup.2 carbon content of about 55% and/or wherein the layer of a-C has an average sp.sup.3 carbon content of about 45%.

69. A bipolar plate according to claim 66, wherein the bipolar plate is formed from stainless steel.

70. A bipolar plate according to claim 66, wherein the bipolar plate and seed layer are adjacent, the seed layer and interfacial layer are adjacent, and interfacial layer and top layer comprising a-C layer are adjacent.

71. A bipolar plate according to claim 66, wherein the carbon-containing coating comprises in order: a) a seed layer comprising a metal or alloy with a thickness of 0.5 μm or less; b) an interfacial layer comprising a carbide or nitride of the metal/alloy in the seed layer with a thickness of 0.5 μm or less; and c) a top layer comprising a-C with a thickness of 1.0 μm or less; wherein the a-C has a density of greater than 2.0 g/cm.sup.3, a molar hydrogen content of 5% or less, an sp.sup.2 carbon content of 40% to 80%, preferably 40% to 70% and an sp.sup.3 carbon content of 20% to 60%, preferably 30% to 60%.

72. A hydrogen fuel cell comprising one or more bipolar plates according to claim 57.

73. A method of coating a bipolar plate for a PEM hydrogen fuel cell with a carbon-containing coating, the method comprising: a) applying onto the plate a seed layer comprising a metal (e.g. titanium); b) applying onto the seed layer an interfacial layer comprising a nitride or carbide of the metal; and c) applying onto the interfacial layer a layer comprising a-C, wherein the a-C has a density of greater than 3.0 g/cm.sup.3, a molar hydrogen content of 5% or less, an sp.sup.2 carbon content of 40% to 80%, preferably 40% to 70% and an sp.sup.a carbon content of 20% to 60%, preferably 30% to 60%.

74. A method of coating a bipolar plate for a PEM hydrogen fuel cell with a carbon-containing coating according to claim 73, wherein the method comprises: a) applying onto the plate a seed layer comprising a metal via a HIPIMS, DC pulse or metallic FCVA deposition process; b) applying onto the seed layer an interfacial layer comprising a nitride or carbide of the metal via a sputtering process; and c) applying onto the interfacial layer a layer comprising a-C via an FCVA process.

75. A method of coating a bipolar plate for a PEM hydrogen fuel cell according to claim 74, wherein the seed layer is titanium and the interfacial layer is titanium nitride.

Description

EXAMPLES

[0107] The invention is now illustrated in the following examples.

Example 1—Preparation of a Coated Bipolar Plate

[0108] A 316L stainless steel bipolar plate was coated as follows.

[0109] Step 1) Sample Preparation The bipolar plate was cleaned according to the following process:

[0110] a. A weak alkaline solution was used for ultrasonic cleaning to remove oil stains on the surface and in the flow channels.

[0111] b. An acidic acid solution was used to remove the oxide layer and any rust on the substrate.

[0112] c. The substrate was rinse with pure water under ultrasonic conditions.

[0113] d. The substrate was then dried under vacuum conditions for 0.5 hours.

[0114] Step 2) Sample Coating

[0115] Coating equipment: FCVA coating machine that also includes ion etching capabilities and magnetron sputtering sources.

[0116] Process:

[0117] a. The cleaned bipolar plate to be coated is placed into the coating chamber and the pressure within the coating chamber is reduced to 5.0×10.sup.−5 Torr (6.6 mPa) and the temperature was increased to 130° C.

[0118] b. Ion beam cleaning takes place (using convention ion beam cleaning methodology).

[0119] c. The pressure within the chamber is reduced further to 2×10.sup.−5 (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 0.3 μm.

[0120] d. After the seed layer has been deposited, deposition of the interfacial layer begins. A TiN interfacial layer is deposited using a sputtering deposition method using a titanium target in the presence of nitrogen case. This deposition step is conducted for a time period sufficient to deposit a TiN layer having a thickness of 0.2 um

[0121] e. After the interfacial layer has been deposited, a layer of 0.2 μm of a-C is deposited using filtered cathode vacuum arc (FCVA) technology. a-C hardness was approximately 1300 HV. Sp.sup.2 content was not accurately measured but believed to be in the range 40-70%.

[0122] 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.

[0123] The finished coated substrate has the following structure:

TABLE-US-00002 a-C layer (0.2 μm) TiN interfacial layer (0.2 μm) Ti seed layer (0.3 μm) Substrate - 316L stainless steel bipolar plate

Example 2—Ion Leaching Resistance

[0124] Ion-leaching electrochemical corrosion testing is carried out in a sealed electrochemical corrosion cell with 100 ml of H.sub.2SO.sub.4 at pH3 and 0.1 ppm HF as electrolytic solution. The working electrode (WE) used is a coated foil of 316L stainless steel as substrate having no visible defects. The area to which the WE is exposed in electrolyte is approximately 7 cm.sup.2. The experiment is performed with saturated Ag/AgCl as the reference electrode (RE), and a 3 cm by 3 cm Pt net as counter electrode (CE). An electrochemical potential of 1.6V vs the standard normal hydrogen electrode (NHE) is applied for 10 hours under 80° C., and the experiment is uncontaminated by outside metal ion content. After the test, the corrosion solution is stored in a PTFE container then tested by a calibrated inductively coupled plasma mass spectrometry (ICP-MS). An Fe ion concentration of less than or equal to 50 ppb is acceptable.

[0125] When subjected to the above test conditions, the bipolar plate of Example 1 had an Fe ion concentration of less than 40 ppb.

[0126] We compared ion leaching for the bipolar plate of Example 1 with a commercially available gold-plated bipolar plate. The results were:

TABLE-US-00003 Coating Example 1 Gold-plated Ion-leaching 37 ppb 70~80 ppb

[0127] The coating of Example 1 demonstrated improved performance in direct comparison with the industry standard coating.

Example 3— Interfacial Contact Resistance (ICR) and Corrosion Current Density

[0128] The ICR test is carried out using electrochemical corrosion cell containing 500 ml of H.sub.2SO.sub.4 at pH3 and 0.1 ppm HF as electrolytic solution. The working electrode (WE) is a coated 316L foil having no visible defects. The area to which the WE is exposed in electrolyte is approximately 1.9 cm.sup.2. The experiment uses saturated Ag/AgCl as reference electrode (RE), and a 3 cm×3 cm Pt net as counter electrode (CE). After three parallel electrochemical tests, the average of ICR measurements under 0.6 MPa should satisfy both of the final contact resistance requirements in the table below.

TABLE-US-00004 Test Potential Corrosion Time ICR Requirement  1.6 V  5 hours ≤5 mΩcm.sup.2 0.84 V 96 hours ≤5 mΩcm.sup.2

[0129] When subjected to the above test conditions, the bipolar plate of Example 1 met these two requirements.

[0130] In addition, during testing at a potential of 1.6 Vm the corrosion current density was measured to be less than 1 μA/cm.sup.2.

Example 4—Conductivity

[0131] Conductivity of bipolar plates obtained according to the invention was tested, using the following methods and protocols: [0132] A modified Wang's method for measuring the contact resistance between stainless steel and carbon paper was used. [0133] All the samples (including carbon paper and tested stainless steel sample) were prepared as wafers. [0134] The sample diameter was 60 mm and had the same size as the copper plates in the device. The mating anvils were machined to a flatness of 5-15 μm. [0135] Toray carbon paper (TGPH-060, non-teflonized) was used as gas diffusion layer in these experiments. Stainless steel bipolar plate thickness is determined according to the actual scheme.

[0136] The tested stainless-steel sample, coated as per the invention or prior art, was sandwiched by two pieces of carbon paper TGP-H-060, and the carbon paper TGP-H-060 was then sandwiched by the copper plates plated by gold, respectively.

[0137] An electrical current of 28.26 A (1 A/cm.sup.2) was provided from a PSP-2010 Programmable power supply. During the experiments, the compacting force was controlled by a WDW Electromechanical Universal Testing Machine developed by Changchun Kexin Tester Institute. The compacting force was increased with the step of 5 N.Math.s.sup.−1 until a final compression pressure of 60 Ncm.sup.−2 (0.6 MPa) was applied over circular electrode area to simulate stack conditions. By measuring the total voltage drop with an EDM-3150 multi display multimeter, the total resistance can be calculated as:

[00001]$\begin{array}{cc}{R}_{\mathrm{total}}=\frac{V_{\mathrm{total}}{A}_o}{I}& \left(1\right)\end{array}$

[0138] where R.sub.total is the total electrical resistance, V.sub.total is the total voltage drop through the setup, I is the current applied to the samples and A.sub.0 is the surface area (28.26 cm.sup.2).

[0139] The total resistance is a sum of four interface resistances and three bulk resistances:

[0140] two carbon paper/gold interfaces (R.sub.c/Au), two carbon paper/tested stainless steel sample interfaces (R.sub.c/ss), two bulk resistance of carbon paper (R.sub.c) and one stainless steel sample bulk electrical resistance (R.sub.ss). That is:

R.sub.total=.sup.2R.sub.C/An+2R.sub.C/ss+2R.sub.c+R.sub.ss  (2)

[0141] The resistance (R.sub.c/Au) can be expressed as the following equation:

[00002]$\begin{array}{cc}{R}_{c/\mathrm{Au}}=\frac{V_{c/\mathrm{Au}}{A}_o}{I}& \left(3\right)\end{array}$

[0142] where V.sub.C/Au is the voltage drop.

[0143] R.sub.c/Au may also be expressed as follows:

R.sub.c/Au=2R.sub.c/Au+2R.sub.c  (4)

[0144] According to equations (2)-(4), the equation below can be obtained:

[00003]$\begin{array}{cc}R=\frac{\left(V_{\mathrm{total}}-V_{c/\mathrm{Au}}\right)A_o}{2I}-\frac{R_{\mathrm{ss}}}{2}\approx \frac{\left(V_{\mathrm{total}}-V_{c/\mathrm{Au}}\right)A_o}{2I}& \left(5\right)\end{array}$

[0145] in which, R is the sum of the contact resistance between the carbon paper and the tested stainless-steel sample and part of the bulk resistance of the carbon paper. Due to the high conductivity of tested stainless steel, the bulk electrical resistance (R.sub.ss) was ignored. Thus, R can be calculated according to equation (5).

[0146] Area specific resistance measurements were performed at 12 different locations on the plate and were recorded and averaged.

TABLE-US-00005 Results Contact Corrosion current Coating Thickness resistance density Gold-plating 1.1 um 9 mΩ .Math. cm.sup.2 8.23 × 10.sup.−8 A Example 1 0.8 um 2 mΩ .Math. cm.sup.2 1.27 × 10.sup.−7 A

[0147] These results show the performance of the coating of the invention against the prior art electrode. The Example 1 results are improved, as increased contact resistance increases the electrical efficiency loss of the fuel cell and reduces the performance.

[0148] Note that in preparing the comparison experiments, the above gold-plating results are all from the following literature: Qin Ziwei, Mi B S, Chen Zhuo, Wang Hongbin, Research on properties of coating of stainless-steel bipolar plate, Shanghai metals, 2017, 5, vol 39: 5-10.

[0149] Thus, the invention provides electrodes, e.g. bipolar plates for fuel cells, and methods of making the same.

Example 5—Preparation of a Coated Bipolar Plate

[0150] A further 316L stainless steel bipolar plate was coated largely according to the method described in example 1 above. Substrate bias was modified to increase the hardness, determined to be approximately a hardness of 2000 HV and the average sp2 content of the a-C was approximately 50%— 60%. Layer thicknesses and composition were as follows:

TABLE-US-00006 a-C layer (0.3 μm) TiN interfacial layer (0.2 μm) Ti seed layer (0.3 μm) Substrate - 316L stainless steel bipolar plate.

[0151] This coated bipolar plate had good properties, in particular it was found to exhibit desirable low ion leaching and to be relatively resistant to corrosion.