Effect of composition and crystal structure of CoRe alloys on electrocatalytic properties and hydrogen interaction

This study investigates the codeposition of cobalt and rhenium from pyrophosphate-ammonia electrolytes. The results show that, depending on the deposition current density and the concentration of components in the solution, the coatings contain 17.7 to 43.8 at.% Re, with a high current efficiency reaching up to 76 %. The formation of a solid solution of rhenium in hexagonal close-packed (hcp) cobalt leads to an increase in lattice parameters and interplanar spacings along the (100), (101), and (110) planes compared to pure cobalt. CoRe alloys absorb a significant amount of hydrogen, which, during electrodeposition, promotes the formation of stressed coatings prone to cracking. During hydrogen evolution on the alloy surfaces in KOH solution, the expanded crystal lattice absorbs hydrogen atoms and facilitates hydride formation, which is characteristic of intermetallic compounds. The coatings that interact with hydrogen via both mechanisms (25 to 30 at.% Re) exhibit the highest electrocatalytic activity in the hydrogen evolution reaction. A further increase in rhenium content results in the formation of nano­crystalline textured coatings with a predominant (002) orientation, exhibiting lower electro­catalytic activity and reduced hydrogen absorption capacity.