High Hydrogen Evolution Reaction (HER) and Hydrogen Oxidation Reaction (HOR) Activity Rh_xS_y Catalyst Synthesized with Na₂S for Hydrogen-Bromine Fuel Cell

• Main Authors: Yuanchao Li, Trung Van Nguyen
• Format: Article
• Language: English
• Published: MDPI AG 2020-08-01
• Series: Energies
• Subjects: hydrogen-bromine fuel cell; rhodium sulfide catalyst; hydrogen evolution; hydrogen oxidation catalyst
• Online Access: https://www.mdpi.com/1996-1073/13/15/3971
• ISSN: 1996-1073

A Rh_xS_y/C catalyst with high mass-specific electrochemical surface area (ECSA/mass), high hydrogen oxidation reaction (HOR)/hydrogen evolution reaction (HER) activity, and high Nafion^® ionomer-affinity was synthesized and evaluated. A new sulfur source, Na₂S instead of (NH₄)₂S₂O₃, was applied to prepare the rhodium sulfide precursor Rh₂S₃ that resulted in a Rh_xS_y catalyst with higher HOR/HER catalytic activity after thermal treatment. The higher activity was attributed to the higher quantity formation of the more active phase Rh₃S_4, in addition to the other active Rh₁₇S₁₅ phase, in the Rh_xS_y catalyst. Using this new sulfur source, carbon substrate functionalization, and the mass-transfer-controlled nanoparticle growth process, the average particle size of this catalyst was reduced from 13.5 nm to 3.2 nm, and its ECSA/mass was increased from 9.3 m²/g-Rh to 43.0 m²/g-Rh. Finally, by applying the Baeyer–Villiger and ester hydrolysis process to convert the Nafion^® ionomer-unfriendly ketone group on the carbon support surface to the Nafion ionomer-friendly carboxylic group, which increases the Nafion^® affinity of this catalyst, its use in the hydrogen electrode of an H₂-Br₂ fuel cell resulted in a performance that is 2.5× higher than that of the fuel cell with a commercial Rh_xS_y catalyst.