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Enhanced hydrogen evolution reaction on hybrids of cobalt phosphide and molybdenum phosphide

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ABSTRACT

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention owing to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer–Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP2 crystal phase is formed which enhances the electrochemical activity.

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X-ray diffraction patterns corresponding to the CoφMoτP catalysts annealed at 800°C as well as the single-phase MoP and CoP powder. The diffraction peaks are assigned to specific crystalline phases as follows. Black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2.
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RSOS161016F2: X-ray diffraction patterns corresponding to the CoφMoτP catalysts annealed at 800°C as well as the single-phase MoP and CoP powder. The diffraction peaks are assigned to specific crystalline phases as follows. Black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2.

Mentions: To understand how the composition affects the HER activity, XRD patterns were collected from these catalysts. The powder XRD studies in figure 2 evidence the presence of cobalt phosphide and molybdenum phosphide, and their relative peak intensities differ with the various ratios of Co/Mo. For φ from 0 to 0.3, the patterns suggest that the major crystal phase is WC-type MoP (ICSD No: 186 874), with a = 3.25 Å, b = 3.25 Å, c = 3.24 Å, and space group symmetry P6m2 (#187). For φ = 0.1, new diffraction peaks appeared at 2θ = 31.3, 39.7, 42.0, and 45.2° (marked as asterisk) with a very low intensity. The intensity of these peaks increases as φ increases and reaches maximum at φ = 0.5. These peaks were confirmed as hexagonal CoMoP2 structure (hereinafter h-CoMoP2 is used to represent this crystal phase in order to distinguish the difference from the hybrid composition Co0.5Mo0.5P) with a = 3.33 Å, c = 11.22 Å, and a space group P63/mmc (ICSD No: 624219, see the electronic supplementary material, figure S1). In addition to MoP and h-CoMoP2 phases at φ = 0.5, minor signals related to MnP-type orthorhombic CoP (ICSD No. 43249) also exist. For spectra with φ = 0.5–1.0, the intensity of CoP peaks increase, but h-CoMoP2 peaks decrease and MoP peaks disappear completely. The XRD pattern of the sample with φ = 0.9 is not shown here because it showed a similar pattern to CoP. The formation of ternary h-CoMoP2 phase was previously explained by the composition of trigonal-prismatic MoP6 and octahedral CoP6 prism to form intermetallic linear –Mo–Co–Mo– chains along the c-axis by Guérin & Sergent [37]. Interestingly, the h-CoMoP2 crystal possesses a similar trigonal-prismatic MoP6 prism to MoP crystal. X-ray photoelectron spectroscopy (XPS) was used to probe the surface electronic properties in the Co0.5Mo0.5P hybrid. The Co 2p spectrum (see the electronic supplementary material, figure S2a) showed two dominating peaks appeared at 777.0 and 791.8 eV corresponding to the Co 2p3/2 and Co 2p1/2 signals, respectively. The minor broad peak at 780 eV indicates the presence of surface oxidized Co species resulting from the contact with air. The Mo 3d spectrum, as shown in electronic supplementary material, figure S2b, showed a strong 3d5/2 peak at 227.8 eV and 3d3/2 peak at 231.1 eV. These peaks are assigned to zero valence state metallic Mo which indicates no or very little oxidation occurs on the Mo site. In the P 2p XPS spectrum shown in the electronic supplementary material, figure S2c, the strong peak at 129.4 eV is assigned to negative charged (metal-Pδ−) phosphide. The minor peak at 133.4 eV is referred to the P–O species. The crystal field model describes that the strong P ligands split the Co 3d states in the octahedral CoP6 prism into high-spin t2g5eg2 ground state. Kibsgaard et al. [11] have shown that smaller differential hydrogen adsorption-free energies ΔGH of the Co-bridge phosphorus site on CoP surface at a low coverage than that of the P site on MoP surface at a high coverage makes the CoP6 prism a high turnover-frequency towards the HER.Figure 2.


Enhanced hydrogen evolution reaction on hybrids of cobalt phosphide and molybdenum phosphide
X-ray diffraction patterns corresponding to the CoφMoτP catalysts annealed at 800°C as well as the single-phase MoP and CoP powder. The diffraction peaks are assigned to specific crystalline phases as follows. Black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2.
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Related In: Results  -  Collection

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RSOS161016F2: X-ray diffraction patterns corresponding to the CoφMoτP catalysts annealed at 800°C as well as the single-phase MoP and CoP powder. The diffraction peaks are assigned to specific crystalline phases as follows. Black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2.
Mentions: To understand how the composition affects the HER activity, XRD patterns were collected from these catalysts. The powder XRD studies in figure 2 evidence the presence of cobalt phosphide and molybdenum phosphide, and their relative peak intensities differ with the various ratios of Co/Mo. For φ from 0 to 0.3, the patterns suggest that the major crystal phase is WC-type MoP (ICSD No: 186 874), with a = 3.25 Å, b = 3.25 Å, c = 3.24 Å, and space group symmetry P6m2 (#187). For φ = 0.1, new diffraction peaks appeared at 2θ = 31.3, 39.7, 42.0, and 45.2° (marked as asterisk) with a very low intensity. The intensity of these peaks increases as φ increases and reaches maximum at φ = 0.5. These peaks were confirmed as hexagonal CoMoP2 structure (hereinafter h-CoMoP2 is used to represent this crystal phase in order to distinguish the difference from the hybrid composition Co0.5Mo0.5P) with a = 3.33 Å, c = 11.22 Å, and a space group P63/mmc (ICSD No: 624219, see the electronic supplementary material, figure S1). In addition to MoP and h-CoMoP2 phases at φ = 0.5, minor signals related to MnP-type orthorhombic CoP (ICSD No. 43249) also exist. For spectra with φ = 0.5–1.0, the intensity of CoP peaks increase, but h-CoMoP2 peaks decrease and MoP peaks disappear completely. The XRD pattern of the sample with φ = 0.9 is not shown here because it showed a similar pattern to CoP. The formation of ternary h-CoMoP2 phase was previously explained by the composition of trigonal-prismatic MoP6 and octahedral CoP6 prism to form intermetallic linear –Mo–Co–Mo– chains along the c-axis by Guérin & Sergent [37]. Interestingly, the h-CoMoP2 crystal possesses a similar trigonal-prismatic MoP6 prism to MoP crystal. X-ray photoelectron spectroscopy (XPS) was used to probe the surface electronic properties in the Co0.5Mo0.5P hybrid. The Co 2p spectrum (see the electronic supplementary material, figure S2a) showed two dominating peaks appeared at 777.0 and 791.8 eV corresponding to the Co 2p3/2 and Co 2p1/2 signals, respectively. The minor broad peak at 780 eV indicates the presence of surface oxidized Co species resulting from the contact with air. The Mo 3d spectrum, as shown in electronic supplementary material, figure S2b, showed a strong 3d5/2 peak at 227.8 eV and 3d3/2 peak at 231.1 eV. These peaks are assigned to zero valence state metallic Mo which indicates no or very little oxidation occurs on the Mo site. In the P 2p XPS spectrum shown in the electronic supplementary material, figure S2c, the strong peak at 129.4 eV is assigned to negative charged (metal-Pδ−) phosphide. The minor peak at 133.4 eV is referred to the P–O species. The crystal field model describes that the strong P ligands split the Co 3d states in the octahedral CoP6 prism into high-spin t2g5eg2 ground state. Kibsgaard et al. [11] have shown that smaller differential hydrogen adsorption-free energies ΔGH of the Co-bridge phosphorus site on CoP surface at a low coverage than that of the P site on MoP surface at a high coverage makes the CoP6 prism a high turnover-frequency towards the HER.Figure 2.

View Article: PubMed Central - PubMed

ABSTRACT

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention owing to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer–Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP2 crystal phase is formed which enhances the electrochemical activity.

No MeSH data available.