Hexagonal Co₉S₈: Experimental and Mechanistic Study of Enhanced Electrocatalytic Hydrogen Evolution of a New Crystallographic Phase

Abstract

The crystallographic phase is one of the most important parameters in determining the physicochemical properties of an electrocatalyst. However, existing understanding of phase‐performance relationship is still very limited, especially for unconventional phases. Herein, the experimental discovery of the hexagonal close‐packed (hcp) phase of Co9S8 is presented. This is the first demonstration of the hexagonal phase of Co9S8, and through correlated experimental and computational data, the first to elucidate the origin of enhanced catalytic performance from this new phase. The synthesized Fe doped Co9S8‐hcp (Fe@Co9S8‐hcp) catalyst, compared to its face‐centered cubic (fcc) phase, exhibits small overpotentials of 44.1 and 298 mV at 10 mA and 500 mA cm−2 for the hydrogen evolution reaction (HER), respectively. Mass activity is enhanced by 64.9 folds compared to the conventional Co9S8‐fcc at 300 mV, which is the best among all Co9S8‐based catalysts ever reported. Density functional theory calculations reveal that the enhanced HER of Fe@Co9S8‐hcp mainly occurs at the Co sites, which synergizes with the doped Fe playing the role of coordination to strengthen H2O adsorption and dissociation. This study opens a new avenue for designing high‐performance electrocatalysts with unconventional phases for energy and environmental applications.

S-Scheme Co9S8 Nanoflower/Red Phosphorus Nanosheet Heterojunctions for Enhanced Photocatalytic H2 Evolution

“…In Figure c, it is observed that the signals at 779.23 (Co 2p 3/2 ) and 794.43 eV (Co 2p 1/2 ) originate from Co 3+ , while the peaks at 781.86 (Co 2p 3/2 ) and 798.55 eV (Co 2p 1/2 ) belong to Co 2+ . The S 2p spectra of samples reveal three different chemical species (Figure d), including S 2– (162.17 and 163.20 eV), S n 2– (164.22 and 165.42 eV), and SO x 2– (169.33 eV) . The binding energy of P 2p shifts negative, and the binding energies of Co 2p and S 2p shift positive, indicating a strong interaction between Co 9 S 8 and RP, making the transfer of electrons from Co 9 S 8 to RP.…”

“…45 The S 2p spectra of samples reveal three different chemical species (Figure 3d), including S 2− (162.17 and 163.20 eV), S n 2− (164.22 and 165.42 eV), and SO x 2− (169.33 eV). 46 The binding energy of P 2p shifts negative, and the binding energies of Co 2p and S 2p shift positive, indicating a strong interaction between Co 9 S 8 and RP, making the transfer of electrons from Co 9 S 8 to RP. .…”

Strain and Surface Engineering of Multicomponent Metallic Nanomaterials with Unconventional Phases

“…In another study, Pd-doped 1T MoS 2 with sulfur vacancies was successfully prepared via a spontaneous interfacial redox reaction between MoS 2 and Pd­(II), where Pd substitutes the Mo site simultaneously inducing the phase change of MoS 2 from the original 2H to 1T (Figure c). Very recently, Ho et al demonstrated the unconventional hcp Co 9 S 8 and successfully doped Fe into this novel phase structure . Besides the metal dopant, some nonmetallic species have also been adopted as dopants to be incorporated into unconventional nanomaterials, such as B and Se .…”

“…Very recently, Ho et al demonstrated the unconventional hcp Co 9 S 8 and successfully doped Fe into this novel phase structure. 180 Besides the metal dopant, some nonmetallic species have also been adopted as dopants to be incorporated into unconventional nanomaterials, such as B 181 and Se. 182 As demonstrated by the structure model in Figure 6d, our group reported boron-incorporated amorphous IrO x (IrO x -B) by a boric acid assisted method.…”

Materials Horizons Emerging Investigator Series: Professor Derek Ho, City University of Hong Kong, China

“…My main research interests are in electronic materials and devices, emphasizing two application areas: sensing as well as energy storage and conversion. My group and I build advanced electronic devices through optimizing a materials structure-property relationship and device architecture, for example, synthesizing new crystallographic phases, 1 studying crystal growth and nucleation, 2 engineering the interlayer space in 2D materials, 3 molecular design, 4 building hierarchical structures across multi length-scales, 5 and creating new device architectures. 6 In our effort, it dawned on us that there is a recurring theme, especially true for sensors and electrocatalysts, which is the idea of balancing order against disorder.…”