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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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(a) X-ray diffraction patterns and (b) HER polarization curves of the Co0.5Mo0.5P samples annealed at 650°C, 800°C and 1000°C. The assignments of the diffraction peaks are included: black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2; green hollow triangle, orthorhombic CoMoP. The HER polarization curves were collected in hydrogen-purged 0.1 M HClO4 solution.
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RSOS161016F4: (a) X-ray diffraction patterns and (b) HER polarization curves of the Co0.5Mo0.5P samples annealed at 650°C, 800°C and 1000°C. The assignments of the diffraction peaks are included: black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2; green hollow triangle, orthorhombic CoMoP. The HER polarization curves were collected in hydrogen-purged 0.1 M HClO4 solution.

Mentions: The effect of annealing temperature on the crystal structure and the HER activity of the Co0.5Mo0.5P catalyst are studied as shown in figure 4. The Co0.5Mo0.5P catalyst annealed at 650oC (Co0.5Mo0.5P-650, pink line in figure 4a) presents diffraction signals composed of h-CoMoP2 (black asterisk) and CoP (red hollow square) crystals. After annealed at 1000°C (Co0.5Mo0.5P-1000, black line in figure 4a), a new orthorhombic CoMoP with a Pnma space group (ICSD No. 2421) is formed (marked as green hollow triangle). The result indicates that increasing annealing temperature to 1000°C induced a phase transformation from h-CoMoP2 to CoMoP, and the excess phosphorus was evaporated. The polarization curves for the HER of Co0.5Mo0.5P-650, Co0.5Mo0.5P-800 and Co0.5Mo0.5P-1000 are compared in figure 4b. The HER activities of these catalysts are in series of Co0.5Mo0.5P-1000 < Co0.5Mo0.5P-650 < Co0.5Mo0.5P-800. The Co0.5Mo0.5P-1000 catalyst showed a poor HER activity with η10 of 338 mV, which indicates that CoMoP structure is less active than the P-rich h-CoMoP2. This can be explained by the higher phosphorus content in the h-CoMoP2 (28.6%) than that in CoMoP (16.7%). Callejas et al. [15] reported that CoP showed significantly lower overpotential than Co2P to produce the same current density. Xiao et al. [17] compared the HER activity between bulk MoP and Mo3P and concluded that MoP is more active than Mo3P. Thus, in this study, the h-CoMoP2 with higher P content showed better HER activity.Figure 4


Enhanced hydrogen evolution reaction on hybrids of cobalt phosphide and molybdenum phosphide
(a) X-ray diffraction patterns and (b) HER polarization curves of the Co0.5Mo0.5P samples annealed at 650°C, 800°C and 1000°C. The assignments of the diffraction peaks are included: black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2; green hollow triangle, orthorhombic CoMoP. The HER polarization curves were collected in hydrogen-purged 0.1 M HClO4 solution.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5383849&req=5

RSOS161016F4: (a) X-ray diffraction patterns and (b) HER polarization curves of the Co0.5Mo0.5P samples annealed at 650°C, 800°C and 1000°C. The assignments of the diffraction peaks are included: black hollow circle, hexagonal MoP; red hollow square, orthorhombic CoP; black asterisk, hexagonal CoMoP2; green hollow triangle, orthorhombic CoMoP. The HER polarization curves were collected in hydrogen-purged 0.1 M HClO4 solution.
Mentions: The effect of annealing temperature on the crystal structure and the HER activity of the Co0.5Mo0.5P catalyst are studied as shown in figure 4. The Co0.5Mo0.5P catalyst annealed at 650oC (Co0.5Mo0.5P-650, pink line in figure 4a) presents diffraction signals composed of h-CoMoP2 (black asterisk) and CoP (red hollow square) crystals. After annealed at 1000°C (Co0.5Mo0.5P-1000, black line in figure 4a), a new orthorhombic CoMoP with a Pnma space group (ICSD No. 2421) is formed (marked as green hollow triangle). The result indicates that increasing annealing temperature to 1000°C induced a phase transformation from h-CoMoP2 to CoMoP, and the excess phosphorus was evaporated. The polarization curves for the HER of Co0.5Mo0.5P-650, Co0.5Mo0.5P-800 and Co0.5Mo0.5P-1000 are compared in figure 4b. The HER activities of these catalysts are in series of Co0.5Mo0.5P-1000 < Co0.5Mo0.5P-650 < Co0.5Mo0.5P-800. The Co0.5Mo0.5P-1000 catalyst showed a poor HER activity with η10 of 338 mV, which indicates that CoMoP structure is less active than the P-rich h-CoMoP2. This can be explained by the higher phosphorus content in the h-CoMoP2 (28.6%) than that in CoMoP (16.7%). Callejas et al. [15] reported that CoP showed significantly lower overpotential than Co2P to produce the same current density. Xiao et al. [17] compared the HER activity between bulk MoP and Mo3P and concluded that MoP is more active than Mo3P. Thus, in this study, the h-CoMoP2 with higher P content showed better HER activity.Figure 4

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&thinsp;=&thinsp;1. Tafel analysis indicates a change in the dominating step of Volmer&ndash;Hyrovsk&yacute; 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.