Nesting Co₃Mo Binary Alloy Nanoparticles onto Molybdenum Oxide Nanosheet Arrays for Superior Hydrogen Evolution Reaction

Abstract

Transition-metal alloys have attracted a great deal of attention as an alternative to Pt-based catalysts for hydrogen evolution reaction (HER) in alkaline. Herein, a facile and convenient strategy to fabricate Co3Mo binary alloy nanoparticles nesting onto molybdenum oxide nanosheet arrays on nickel foam is developed. By modulating the annealing time and temperature, the Co3Mo alloy catalyst displays a superior HER performance. Owing to substantial active sites of nanoparticles on nanosheets as well as the intrinsic HER activity of Co3Mo alloy and no use of binders, the obtained catalyst requires an extremely low overpotential of only 68 mV at 10 mA cm–2 in alkaline, with a corresponding Tafel slope of 61 mV dec–1. At the same time, the catalyst demonstrates excellent stability during the long-term measurements. The density functional theory calculation provides a deeper insight into the HER mechanism, unveiling that the active sites on the Co3Mo-based catalyst are Mo atoms. This strategy of combining catalytic active species with hierarchical nanoscale materials can be extended to other applications and provides a candidate of nonnoble metal catalysts for practical electrochemical water splitting.

The Synergistic Effects of Alloying on the Performance and Stability of Co3Mo and Co7Mo6 for the Electrocatalytic Hydrogen Evolution Reaction

“…The alloys show overpotentials of 120 ± 5 mV (Co 3 Mo) and 160 ± 5 mV (Co 7 Mo 6 ) at 10 mA cm −2 . In comparison with recently reported Co 3 Mo on nickel foam, the observed overpotential seems high [20]. Chen et al reported the overpotentials of 78 mV (C DL = 15.8 mF cm −2 ) and 75 mV (C DL = 17.5 mF cm −2 ).…”

“…In other words, based on this approximation Co7Mo6 can deliver 2.7 times higher current densities, as demonstrated in Figures S10 and S11, which also leads to an improved overpotential of 123 mV at 10 mA cm -2 . This is an interesting result, especially given that recent DFT calculations on Co3Mo have shown that Mo-atoms are the catalytic sites in Co3Mo [20]. One can assume that increasing the amount of Mo would lead to improved catalytic performance.…”

“…However, the resulting Tafel slope values of 71 ± 7 mV dec -1 (Co3Mo) and 84 ± 5 mV dec -1 (Co7Mo6) are outside the values for defined reaction mechanisms, i.e., 40 mV dec -1 (Volmer-Heyrovsky) and 120 mV dec -1 (Volmer) [8,33]. The deviation from ideal Tafel slopes is a common situation with powdered materials and Co3Mo in particular [8,20,33], as they do not always follow perfect charge-transfer behaviour (Tafel behaviour) due to mass transport effects [34]. Proper accounting for mass transport effects requires comprehensive numerical simulation procedures that are beyond the scope of this manuscript.…”

“…The phase diagram of Co-Mo is well established [24][25][26][27]. Initial attempts were made to make Co 3 Mo from CoMoO 4 as described in [20]. However, only formation of Co 7 Mo 6 with Mo as an impurity (in line with the nominal Co:Mo = 1:1 composition in CoMoO 4 ) was observed within tested temperature ranges of 700-950 • C. Therefore, the reaction of mixtures of CoMoO 4 and Co(OH) 2 was used instead of a single CoMoO 4 precursor.…”

“…A growing interest in nanostructured materials appeals for a better understanding of the electrochemical properties of pure-phased polycrystalline alloys, and in this respect the Co-Mo system remains relatively unexplored. Coupled with the recent reports of impressive performance of Co 3 Mo towards HER in 1M KOH [20,21], the investigation of electrochemical properties of alloys in the Co-Mo system is key to understanding how cheaper and more efficient electrocatalysts could be designed and made.…”