Operando Spectroscopic Analysis of an Amorphous Cobalt Sulfide Hydrogen Evolution Electrocatalyst

Abstract

The generation of chemical fuel in the form of molecular H2 via the electrolysis of water is regarded to be a promising approach to convert incident solar power into an energy storage medium. Highly efficient and cost-effective catalysts are required to make such an approach practical on a large scale. Recently, a number of amorphous hydrogen evolution reaction (HER) catalysts have emerged that show promise in terms of scalability and reactivity, yet remain poorly understood. In this work, we utilize Raman spectroscopy and X-ray absorption spectroscopy (XAS) as a tool to elucidate the structure and function of an amorphous cobalt sulfide (CoS x ) catalyst. Ex situ measurements reveal that the as-deposited CoS x catalyst is composed of small clusters in which the cobalt is surrounded by both sulfur and oxygen. Operando experiments, performed while the CoS x is catalyzing the HER, yield a molecular model in which cobalt is in an octahedral CoS2-like state where the cobalt center is predominantly surrounded by a first shell of sulfur atoms, which, in turn, are preferentially exposed to electrolyte relative to bulk CoS2. We surmise that these CoS2-like clusters form under cathodic polarization and expose a high density of catalytically active sulfur sites for the HER.

Electrodeposited cobalt-copper sulfide counter electrodes for highly efficient quantum dot sensitized solar cells

“…The CoS peak shows a broad Raman feature which covers the range of 240-370 cm -1 and is deconvoluted to two broad lines which peak at 285 and 340 cm -1 as well as a second band which covers the 400-520 cm -1 range also well fitted by two lines at 445 and 485 cm -1 . The low frequency Raman feature resembles that observed in the amorphous CoS2 film and is tentatively attributed to Co−S vibrations, while the high frequency one can be correspondingly assigned to S-S vibrations [40].…”

Electrocatalysts for hydrogen oxidation and evolution reactions

“…However, some powerful surface observation techniques, such as XPS, cannot be adapted to electrochemical reactions because of the presence of the electrolytes. A few spectroscopic techniques, such as infrared spectroscopy (IR) [108], Raman spectroscopy [109,110], and extened X-ray absorption fine structure spectroscopy (EXAFS) [111] have been used to evaluate electrochemical reactions. Recently, in situ grazing incident X ray diffraction (GIXRD) has also been utilized to monitor phase transitions during electrochemical processes [112].…”

Exceptionally Long-Lived Charge Separated State in Zeolitic Imidazolate Framework: Implication for Photocatalytic Applications

“…The XANES spectrum of Co(NO3)2·6H2O ,which is the starting material known for octahedral (Oh) coordination at Co center, is also shown in Figure 1d for comparison. The prominent pre-edge feature (quadrupole allowed 1s-3d transition) in ZIF-67, characteristic of tetrahedral (Td) coordination, 31,32 is distinct from the negligible 1s-3d transition in the Oh coordinated Co(NO3)2·6H2O. Furthermore, the main absorption edge in the XANES spectrum of ZIF-67, arising from dipole allowed 1s-4p transition, shows a less sharp peak than that of Co(NO3)2·6H2O, which can be ascribed to the decreased multiple scattering for a Td coordination in the former compared with Oh geometry in the latter.…”