A Rh_xS_y/C Catalyst for the Hydrogen Oxidation and Hydrogen Evolution Reactions in HBr

Abstract

Rhodium sulfide (Rh 2 S 3 ) on carbon support was synthesized by refluxing rhodium chloride with ammonium thiosulfate. Thermal treatment of Rh 2 S 3 at high temperatures (600 • C to 850 • C) in presence of argon resulted in the transformation of Rh 2 S 3 into Rh 3 S 4 , Rh 17 S 15 and Rh which were characterized by TGA/DTA, XRD, EDX, and deconvolved XPS analyses. The catalyst particle size distribution ranged from 3 to 12 nm. Cyclic voltammetry and rotating disk electrode measurements were used to evaluate the catalytic activity for hydrogen oxidation and evolution reactions in H 2 SO 4 and HBr solutions. The thermally treated catalysts show high activity for the hydrogen reactions. The exchange current densities (i o ) of the synthesized Rh x S y catalysts in H 2 -saturated 1M H 2 SO 4 and 1M HBr for HER and HOR were 0.9 mA/cm 2 to 1.0 mA/cm 2 and 0.8 to 0.9 mA/cm 2 , respectively. The intermittent availability of renewable electricity from solar and wind sources has increased significantly.1 To match their often unpredictable power generation cycles with demand, low cost electrical energy storage systems are required.A flow battery/regenerative fuel cell is an electrochemical storage device that stores the electrical energy as chemical energy in a fuel and converts the chemical energy of the fuel directly back to electrical energy. The potentially low cost and relatively rapid process of electrical-to-chemical or chemical-to-electrical energy, in comparison to the slower mechanical-to-electrical energy processes used with flywheel and compressed air storage, offers unique advantages. The discharge cycle of the flow battery utilizes a fuel cell which has proven to be efficient and clean devices for energy conversion. Fuel cells are regarded by some as the energy conversion devices of the future and provide a faster, cleaner, more efficient, and possibly more flexible chemical-to-electrical energy conversion platform than present combustion based systems. [2][3][4] In comparison to other flow battery systems, the regenerative hydrogen-bromine fuel cell has advantages including high round-trip energy conversion efficiency, high power density and storage capacity, fast kinetics of both hydrogen and bromine electrode reactions, low cost active materials, simplicity and reliability. [5][6] The core component of a H 2 /Br 2 regenerative fuel cell system is an acid-based H 2 /Br 2 fuel cell. The charge and discharge reactions occurring in the fuel cell are as follows. At the bromine electrode, during the charge cycle, bromide ions in an HBr solution are oxidized to form bromine and two electrons; require a catalyst that is highly active, to keep the performance high and the cost low, stable and durable in the highly corrosive HBr/Br 2 environment of the cell as required by the extended life of this application. During the operation of a H 2 -Br 2 fuel cell, HBr and Br 2 could cross from the bromine side through the proton conducting membrane to the hydrogen side potentially leading to the corrosion and poisoning of th...

A Review of Hydrogen/Halogen Flow Cells

“…Another strategy is to develop alternative materials that are tolerant to bromine and bromide. PtIrN x is dramatically more stable than Pt in the presence of Br, without sacrificing HOR/HER catalysis, similar to various transition‐metal sulfide compounds …”

“…The only material that proved satisfactory for technical application was graphite, and the high hydrogen overvoltage of graphite could be reduced to a large extent by adding small amounts of palladium or platinum salts directly to the electrolyte. Recent efforts typically use carbon for the bromine electrode and Pt/C for the hydrogen electrode, although it is recognized that the development of bromine‐tolerant alternatives to platinum is still required …”

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].…”

“…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 impact of the polymer coating on the hydrogen reaction rate is an issue that must be addressed [19,20]. Our previous studies suggest that the active Rh x S y phases are suitable catalyst candidates for the HOR/HER in the Br − /Br 2 solution because they show reasonable catalytic activity for the HOR/HER and outstanding stability in this corrosive environment [21][22][23].…”