Electrodeposition of niobium-doped amorphous carbon coating on stainless steel bipolar plates in citric acid-assisted deep eutectic solvent

Proton exchange membrane fuel cells (PEMFC/PEMWE) are important devices for the conversion and utilization of renewable hydrogen energy. By electrodeposition in deep eutectic solvent (DES) mixed with critic acid (CA), a Nb-doped amorphous carbon coating (ACC) with enhanced conductivity was successfully grown on the surface of 316L stainless steel bipolar plates. The addition of Nb significantly lowered contact resistance of ACC to 13.8 mΩ cm 2 , while the Nb-doped ACC still exhibits excellent corrosion resistance and high electrochemical stability up to 300 h. Theoretical calculation and characterization indicate that the solvent shell structure formed between CA and Nb 5+ ions in the electrolyte facilitates the migration of Nb 5+ to the cathode, thereby promoting more Nb deposition. With the help of CA, the nucleation process of the coating shifts to instantaneous nucleation, favoring the formation of a dense and smooth coating. Moreover, the chemical adsorption of CA on the stainless steel surface enhances the adhesion between the ACC and the substrate. The excellent electrochemical property, strong interfacial adhesion, and high stability in service of the Nb-doped ACC indicates that the proposed electrodeposition method is a promising approach for fabricating ACC-modified stainless steel serving as bipolar plates.