Investigation of the Active Sites of Rhodium Sulfide for Hydrogen Evolution/Oxidation Using Carbon Monoxide as a Probe

Abstract

Carbon monoxide (CO) was observed to decrease the activity for hydrogen evolution, hydrogen oxidation, and H2-D2 exchange on rhodium sulfide, platinum, and rhodium metal. The temperature at which the CO was desorbed from the catalyst surface (detected by recovery in the H2-D2 exchange activity of the catalyst) was used as a descriptor for the CO binding energy to the active site. The differences in the CO desorption temperature between the different catalysts showed that the rhodium sulfide active site is not metallic rhodium. Using density functional theory, the binding energy of CO to the Rh sites in rhodium sulfide is found comparable to the binding energy on Pt. Coupled with experiment this supports the proposition that rhodium rather than sulfur atoms in the rhodium sulfide are the active site for the hydrogen reaction. This would indicate the active sites for hydrogen evolution/oxidation as well as oxygen reduction (determined by other groups using X-ray absorption spectroscopy) may be the same.

Doped rhodium sulfide and thiospinels hydrogen evolution and oxidation electrocatalysts in strong acid electrolytes

“…Thus, it is possible that FeRh 2 S 4 and CoRh 2 S 4 are not as good electrocatalysts for oxidation and reduction reactions as NiRh 2 S 4 because of their semiconducting behavior. Although the metal-to-sulfur ratio of the thiospinels is the same as that of Rh 3 S 4 (i.e., a phase believed to be active for HER/HOR) [8][9][10], the thiospinels do not contain the six-Rh atom cluster believed to be the active site of Rh 3 S 4 [8]. This site is believed to have a moderate hydrogen binding energy, similar to active metals such as Pt, Rh, Ir [8].…”

“…To be an active electrocatalyst for both oxidation and reduction reactions, it is preferable that the material has metallic conductivity. Rh 17 S 15 [8] and NiRh 2 S 4 [29,30] have metallic conductivity, while FeRh 2 S 4 [31] and CoRh 2 S 4 [31,32] are semiconducting. Thus, it is possible that FeRh 2 S 4 and CoRh 2 S 4 are not as good electrocatalysts for oxidation and reduction reactions as NiRh 2 S 4 because of their semiconducting behavior.…”

“…Although the metal-to-sulfur ratio of the thiospinels is the same as that of Rh 3 S 4 (i.e., a phase believed to be active for HER/HOR) [8][9][10], the thiospinels do not contain the six-Rh atom cluster believed to be the active site of Rh 3 S 4 [8]. This site is believed to have a moderate hydrogen binding energy, similar to active metals such as Pt, Rh, Ir [8]. The 'moderate' hydrogen binding energy (near 0 eV) results in low activation barriers for HER and HOR, based on the Sabatier principle [33].…”

“…Their activity can be improved through substitutional doping of metal atoms [5,7]. Of the metal sulfides, rhodium sulfide (Rh x S y /C) is the best performing electrocatalyst for hydrogen evolution and oxidation [1], with activity believed to be from the Rh sites in the Rh 17 S 15 and Rh 3 S 4 phases [8][9][10]. Rhodium sulfide is also used for oxygen-depolarized cathodes in hydrochloric acid [11,12].…”

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

“…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. There are three rhodium sulfide crystal phases, Rh 2 S 3 , Rh 3 S 4 , and Rh 17 S 15 [26][27][28]. Among these crystal structures, Rh 2 S 3 is a semiconductor and has no catalytic activity while Rh 3 S 4 and Rh 17 S 15 show high catalytic activity for the HOR/HER.…”