title: 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 (CoSx) catalyst. Ex situ measurements reveal that the as-deposited CoSx catalyst is composed of small clusters in which the cobalt is surrounded by both sulfur and oxygen. Operando experiments, performed while the CoSx 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.
authors.affiliation: †Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States.
authors.forename: Nikolay
authors.initials: N
authors.lastname: Kornienko
authors.affiliation: ‡Department of Chemical Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
authors.forename: Joaquin
authors.initials: J
authors.lastname: Resasco
authors.affiliation: †Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States.
authors.forename: Nigel
authors.initials: N
authors.lastname: Becknell
authors.affiliation: †Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States.
authors.forename: Chang-Ming
authors.initials: CM
authors.lastname: Jiang
authors.affiliation: §Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Yi-Sheng
authors.initials: YS
authors.lastname: Liu
authors.affiliation: §Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Kaiqi
authors.initials: K
authors.lastname: Nie
authors.forename: Xuhui
authors.initials: X
authors.lastname: Sun
authors.affiliation: §Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Jinghua
authors.initials: J
authors.lastname: Guo
authors.affiliation: †Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States.|⊥Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States.|#Department of Physics, University of California, Berkeley, Berkeley, California 94720, United States.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Leone
authors.affiliation: †Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States.|∇Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States.|○Kavli Energy Nanoscience Institute, Berkeley, California 94720, United States.
authors.forename: Peidong
authors.initials: P
authors.lastname: Yang