Synthesis and Evaluation of Rh_xS_y Catalyst with Functionalized Carbon for HOR/HER in H₂-Br₂ Reversible Fuel Cell

Abstract

Synthesis and characterization of a Rh x S y catalyst with high mass specific electrochemical active surface area (ECSA/mass) for the H 2 -Br 2 fuel cell are discussed in this paper. Supporting carbon materials were functionalized to create more suitable nucleation sites for the precipitation of rhodium sulfide nano-particles resulting in a catalyst with a smaller particle size, better particle distribution, and higher hydrogen oxidation and evolution reaction performance. XPS and FT-IR were used to analyze the amounts and the types of surface functional groups on the carbon substrate. The results confirm that the dominant functional group on the carbon is the ketone group. TEM and cyclic voltammetry results confirm that the catalysts with treated carbon have smaller average particle size (7.2 nm vs. 13.2 nm) and higher mass specific ECSA (21.8 m 2 /g-Rh vs. 9.1 m 2 /g-Rh) than those with the untreated carbon. The H 2 -Br 2 fuel cell test results show that the catalysts with functionalized carbon have better performance in the kinetic region but poorer performance in the mass transfer dominant region. The issue was identified to be due to the poor affinity of the ketone group on the carbon surface with the Nafion ionomer in the catalyst ink and is currently being addressed.

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

“…The carbon substrate was functionalized by the previously reported method [25]. The activated carbon, XC72R, was functionalized by three steps: Oxidation in the 70% w/w HNO 3 solution, reduction in H 2 at 300 • C, and overnight air exposure at room temperature.…”

“…The activated carbon, XC72R, was functionalized by three steps: Oxidation in the 70% w/w HNO 3 solution, reduction in H 2 at 300 • C, and overnight air exposure at room temperature. The three-step functionalization was used to create a widely distributed ketone-dominated carbon surface that would be needed to attract the rhodium cations and precipitate the rhodium sulfide precursor Rh 3 S 4 phase uniformely on the carbon support surface [25]. The application of these processes is expected to create nanoparticles that are smaller and more dispersed.…”

“…The precursor material, Rh 2 S 3 /C, was centrifuged and washed by DI water and dried at 90 • sulfur atoms were released. The thermal treatment was done at the previously reported temperature and temperature ramping rates [25].…”

“…The functionalization of the carbon support and the employment of the mass-transfer-controlled nanoparticle (NP) growth process resolve the low mass-specific ECSA caused by the large and broad distribution of NP size. Using these approaches, the Rh x S y catalyst nanoparticle size was reduced from~13 nm to~5 nm [24,25]. The low HOR/HER activity of the current Rh x S y catalyst is related to the composition of the crystal phases in the catalyst.…”

“…In this study, Na 2 S was evaluated as an alternative sulfur source for synthesizing rhodium sulfide catalyst that contains a higher level of the more active Rh 3 S 4 phase, instead of the ammonium thiosulfate source used in our previous studies and in the synthesis of some commercial rhodium sulfide catalysts [22][23][24][25][33][34][35]. To the authors' best knowledge, this is the first reported work to use different sulfur sources to adjust the crystal phases of Rh x S y .…”