Coating for Metal Bipolar Plates in PEM Water Electrolysers
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- Institutt for fysikk 
To increase the efficiency of hydrogen as an energy storage medium, this project's goal is to lower the cost of Proton Exchange Membrane Water Electrolysers (PEMWE) by decreasing the cost of the most expensive component: the bipolar plates (BPP). Given the corrosive environment if PEMWE cells, one method to prevent corrosion of BPP is to coat the plates with an appropriate agent, for example a conductive powder mixed with a binder. The conductivity of twelve powders was measured where four powders were considered good enough as a base for coating of the BPP: LSC, LSC-C, LSC-E and LSC-N (lanthanum-doped strontium cobalt oxide with different production methods) with an electric conductivity of 7.10, 6.0, 2.20 and 7.47 S/cm, respectively (measured at 14 bar (1.4 MPa)). The epoxy binder lowered the conductivity by 99.97%, while silica lowered it with 69.80%. The lowest initial interfacial contact resistance (ICR) measured for a coupon with LSC-powder was with 75 mass percent silica sol, and was measured as 137 mΩcm^2, while for LSC-C, LSC-E, LSC-N the lowest value was with 10 volume percent epoxy and measured as 237, 288 and 243 mΩcm^2. These four coatings were tested potentiodynamic and -static and after both tests the ICR values were 1003 mΩcm^2 (LSC), 124 mΩcm^2 (LSC-C), 164 mΩcm^2 (LSC-E) and 209 mΩcm^2 (LSC-N), whereas stainless steel 316 had a final ICR value of 121 mΩcm^2. All four coatings had a much higher corrosion potential than stainless steel, but the corrosion current, passivation current and passivation potential are approximately the same as for steel. This indicates that the electrolyte was in contact with the steel and that the coatings were too porous. The potentiostatic test shows a lower corrosion rate for LSC-C and LSC-N than pure stainless steel, while LSC and LSC-E has a higher corrosion rate than steel. The analysis of the coatings with secondary electrons indicates a morphology that contains spherical and porous particles in the case of the three powders LSC-C, LSC-E and LSC-N. These range in size from 3 to 200 µm, which were partly crushed under a pressure of 120 bar (12 MPa). In contrast, LSC had dense particles with sizes from 200 to 500 nm. X-ray analysis of the four coatings showed large intensity of iron and chromium (from the steel coupon) in the LSC and LSC-C coatings, which indicates porous or inhomogeneous coating. The mixing ratio of the LSC/silica coating were homogeneous distributed in the coating. From the test done in this master's thesis, the powders LSC, LSC-C, LSC-E and LSC-N appear to be good candidates as base for coatings for BPP in PEMWE, but when mixed with the binder, the conductivity falls rapidly. New binders need to be considered or a change of the morphology is required to prevent the binder from insulating the conductive powder particles.