Plasma-Coating of Ti and Ta on Graphite Composite As Corrosion-Improved Bipolar Plate in PEM Electrolysis

In Proton Exchange Membrane (PEM) Electrolysis bipolar plates have to be more corrosion resistant than bipolar plates in PEM Fuel Cells. Several materials have been evaluated as cost-efficient material in PEM Electrolyzers. Since the electrochemical conditions in PEM Electrolyzers are quite harsh, cost-efficient materials like stainless steel or graphite composites are not stable. Coating the material with corrosion resistant material by magnetron sputtering is one possibility to enhance the stability. Gago et al.[1] have been coating stainless steel for example with Ti by vacuum plasma spraying. However, Ti is only useable if the oxidation potential is not too high, otherwise it will form an insulating TiO₂ layer. Tantal has good corrosion protective characteristics and is not forming an insulating layer.[2]

We developed a plasma process to deposit protective coatings on graphite composite bipolar plates. Therefore we used unbalanced magnetron sputtering of Ti and Ta. A new magnetron sputtering process was applied to keep the temperature of the graphite composites constant. Furthermore a combined plasma-process was used to enhance the stability of the deposited layer by activating the graphite composite. The Ti and Ta coatings were tested in a heated (T = 353 K) electrochemical cell amperometric at 2 V vs. RHE and by cyclic voltammetry in oxygen saturated 0.5 M H₂SO₄. We figured out the layer quality with the 4-Point Probe before and after each experiment. Furthermore we evaluated the film characteristics by microscope (SEM, light microscope), XRD and XPS.
We could demonstrate that the coating is protecting the graphite composite as bipolar plate material in harsh electrochemical environment which was choosen close to real conditions in PEM Electrolyzers. Thereby it should be possible to lower the cost of bipolar plates in PEM electrolyzers substantial.

[1]      A. Gago, A. Ansar, N. Wagner, J. Arnold, K.A. Friedrich, Titanium coatings thermal spraying bipolar plates PEM electrolyzer, in: 64th Annu. Meet. Int. Soc. Electrochem., 2013.

[2]      O.E. Kongstein, N. Guillet, A. Ødegård, Rapport technique DEHT-DR-10/051, 2010.