Conductive Coatings for Metal Bipolar Plates in Proton Exchange Membrane Water Electrolyzers
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Due to the use of expensive materials, only 4 % of hydrogen production today is from water electrolysis. The most efficient low-temperature electrolyzer is the proton exchange membrane electrolyzer (PEMWE), where the bipolar plates are resposible for 48 % of the total stack cost, along with the current collectors. As the proton exchange membrane in the electrolyzer causes an acidic environment, finding suitable and inexpensive materials is a challenge. Stainless steel has gained signifant attention as bipolar plate material due to good mechanical and electrical properties. However, the corrosive conditions will cause the stainless steel to corrode and thus it needs to be protected by a coating. Coatings with lanthanum strontium cobaltite (LSC) were investigated for depostion on metal bipolar plates. Three LSC powders calcined at different temperatures were characterized with respect to electrical conductivity, microstructure and phase composition. The coatings were studied regarding the effect of binder amount, viscosity, mixing method, pressing and temperature. Further, the coatings were characterized by measuring the contact resistance, performing corrosion testing and by investigating the coating surface. The contact resistance was observed to be most affected by the amount of binder added to the coating suspension. An amount of 2 wt% of both PVB and EC was found to be the most optimal with respect to contact resistance and adhesion to the substrate. An amount of isopropanol between 92 and 95 vol% resultated in the best suspension viscosity regarding the coating thickness. Mixing with milling was found to be most effective, as the method provided smaller agglomerates and thus lower contact resistance, compared to mixing by sonication. The effect of pressing provided lower contact resistance, compared to no pressing. However, the pressing temperature should be kept below the glass transition temperature of PVB. Above this temperature, PVB melts and separates the metallic agglomerates, which results in an increasing contact resistance. The coating which provided lowest contact resistance, while still adhering to the substrate, was prepared with 2 wt% of both PVB and EC, which were mixed into the suspension by milling. The viscosity of the suspension was 2.36 cP and the coating was pressed at room temperature. The value of the contact resistance was 130 mΩ cm^2 . Corrosion testing of the coating showed that the coating exhibited poor density and did not cover the metal substrate completely. As a consequence, the metal substrate corroded due to galvanic corrosion.