skip to main content


Title: Controlling the Co–S coordination environment in Co-doped WS 2 nanosheets for electrochemical oxygen reduction
Cobalt sulfide nanomaterials are among the most active and stable catalysts for the electrocatalytic oxygen reduction reaction in pH 7 electrolyte. However, due to the complexity and dynamism of the catalytic surfaces in cobalt sulfide bulk materials, it is challenging to identify and tune the active site structure in order to achieve low overpotential oxygen reduction reactivity. In this work, we synthesize isolated Co sites supported on colloidal WS 2 nanosheets and develop a synthetic strategy to rationally control the first-shell coordination environment surrounding the adsorbed Co active sites. By studying Co–WS 2 materials with a range of Co–S coordination numbers, we are able to identify the optimal active site for pH 7 oxygen reduction catalysis, which comprises cobalt atoms bound to the WS 2 support with a Co–S coordination number of 3–4. The optimized Co–WS 2 material exhibits an oxygen reduction onset potential of 0.798 V vs. RHE, which is comparable to the most active bulk phases of cobalt sulfide in neutral electrolyte conditions.  more » « less
Award ID(s):
2106450
NSF-PAR ID:
10334487
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Journal of Materials Chemistry A
Volume:
9
Issue:
35
ISSN:
2050-7488
Page Range / eLocation ID:
19865 to 19873
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Many metal coordination compounds catalyze CO2electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure–reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X‐ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc‐catalyzed CO2reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi‐electron CO2reduction. CO, the key intermediate in the CO2‐to‐methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza‐N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X‐ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non‐centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co−CO adduct during the catalysis.

     
    more » « less
  2. Abstract

    Many metal coordination compounds catalyze CO2electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure–reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X‐ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc‐catalyzed CO2reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi‐electron CO2reduction. CO, the key intermediate in the CO2‐to‐methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO2for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza‐N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X‐ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non‐centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co−CO adduct during the catalysis.

     
    more » « less
  3. Polymer-encapsulated cobalt phthalocyanine (CoPc) is a model system for studying how polymer-catalyst interactions in the electrocatalytic systems influence performance for the CO2 reduction reaction. In particular, understanding how bulk electrolyte and proton concentration influences polymer protonation, and in turn how the extent of polymer protonation influences catalytic activity and selectivity, is crucial to understanding polymer-catalyst composite materials. We report a study of the dependence of bulk pH and electrolyte concentration on the fractional protonation of poly-4-vinylpyridine and related polymers with both electrochemical and spectroscopic evidence. In addition, we show that the fractional protonation of the polymer is directly related to both the activity of the catalyst and the reaction selectivity for the CO2 reduction reaction over the competitive hydrogen evolution reaction. Of particular note is that the fractional protonation of the film is related to electrolyte concentration, which suggests that the transport of counterions plays an important role in regulating proton transport within the polymer film. These insights suggest that electrolyte concentration and pH play an important in the electrocatalytic performance for polymer-catalyst composite systems, and these influences should be considered in both experimental preparation and analysis. 
    more » « less
  4. Non-noble metal based electrocatalysts for the hydrogen evolution reaction (HER) hold great potential for commercial applications. However, effective design strategies are greatly needed to manipulate the catalyst structures to achieve high activity and stability comparable to those of noble-metal based electrocatalysts. Herein, we present a facile route to synthesize layered Co 9 S 8 intercalated with Co cations (Co 2+ -Co 9 S 8 ) (with interlayer distance up to 1.08 nm) via a one-step solvothermal method. Benefiting from a large interlayer distance and efficient electron transfer between layers, the Co 2+ -Co 9 S 8 hybrid shows outstanding electrocatalytic hydrogen evolution performance in an acid electrolyte. The electrocatalytic performance is even better than that of 20% Pt/C at the <−0.54 V region with an overpotential of 86 mV at a current density of 10 mA cm −2 in 0.5 mol L −1 H 2 SO 4 . More importantly, the system can maintain excellent stability for more than 12 h without obvious decay. This study not only presents a novel and efficient approach to synthesize cobalt sulfide intercalated with Co cations for stable electrocatalytic HER but also provides an avenue for the design of intercalated materials used in other energy applications. 
    more » « less
  5. Abstract

    Carbon‐supported nitrogen‐coordinated single‐metal site catalysts (i.e., M−N−C, M: Fe, Co, or Ni) are active for the electrochemical CO2reduction reaction (CO2RR) to CO. Further improving their intrinsic activity and selectivity by tuning their N−M bond structures and coordination is limited. Herein, we expand the coordination environments of M−N−C catalysts by designing dual‐metal active sites. The Ni‐Fe catalyst exhibited the most efficient CO2RR activity and promising stability compared to other combinations. Advanced structural characterization and theoretical prediction suggest that the most active N‐coordinated dual‐metal site configurations are 2N‐bridged (Fe‐Ni)N6, in which FeN4and NiN4moieties are shared with two N atoms. Two metals (i.e., Fe and Ni) in the dual‐metal site likely generate a synergy to enable more optimal *COOH adsorption and *CO desorption than single‐metal sites (FeN4or NiN4) with improved intrinsic catalytic activity and selectivity.

     
    more » « less