Core-Shell Rhodium Sulfide Catalyst for HER/HOR in HBr Solution

2018

J. Electrochem. Soc.

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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.

Section: Resultsmentioning

confidence: 99%

Section: Synthesis Of Rhodium Sulfide Catalyst-the Synthesis Process ...mentioning

confidence: 99%

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

2018

J. Electrochem. Soc.

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“…The carbon paper was pretreated in 2 M HNO 3 solution to make it hydrophilic to make it easier for the electrolyte to penetrate into the electrode. The catalyst loading process can be found in our previous work [23][24][25]31].…”

Section: Electrode Preparationmentioning

confidence: 99%

“…The reference electrode was placed adjacent to the working electrode. Details of the working electrode set-up can be found in our previous publications [23,31]. Unstirred nitrogen-saturated 1 M H 2 SO 4 served as the electrolyte.…”

Section: Cyclic Voltammetrymentioning

confidence: 99%

“…If the treatment temperature is either too low or too high, one may end up with either too much of the unconverted non-active Rh 2 S 3 phase or Rh metal. Rhodium metal is an unwanted phase because it is unstable in HBr/Br 2 like platinum [23,31]. A recent study has shown that different sulfur sources can lead to different metal sulfide monomers (building blocks of nanoparticles) and final compositions.…”

Section: Introductionmentioning

confidence: 99%

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

2020

Energies

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A RhxSy/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 RhxSy 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 Rh3S4, in addition to the other active Rh17S15 phase, in the RhxSy 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 RhxSy catalyst.

Synthesis and Evaluation of Rh_xS_y Catalyst with High Affinity for Nafion Ionomer for HOR/HER in a H₂-Br₂ Regenerative Fuel Cell

2019

J. Electrochem. Soc.

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Synthesis and evaluation of RhxSy with high mass-specific electrochemical surface area (ECSA/mass) and high affinity to Nafion polymer are reported. Besides the previously reported supporting carbon functionalization, the nanoparticle size was optimized by a diffusion-controlled growth process to control Rh2S3 monomer concentration, which reduced the average particle size from 13.2 to 5.2 nm and increased the ECSA/mass from 9.1 to 36.5 m2/g-Rh. The affinity issue between the surface ketone functional group and the Nafion ionomer in the catalyst ink was resolved by using the Baeyer-Villiger reaction and ester hydrolysis to convert it to the Nafion-friendly carboxylic group with no adverse effect on the precipitated catalysts. The H2-Br2 fuel cell tests confirmed that the high mass transfer issue observed with the RhxSy catalysts on ketone-dominated carbon substrate was resolved with the new catalysts, validating the hypothesis that the low affinity to Nafion polymer of the surface ketone group was the cause for the formation of thick Nafion polymer layer on the catalyst surface and high mass transportation resistance. Using the new catalyst, the discharge performance of the H2-Br2 fuel cell improved by 5.6 times (0.15 vs. 0.027 A/cm2 at 0.2 V below OCV) over that of the fuel cell with the RhxSy/untreated carbon catalyst.