Full Length ArticleFacile synthetic approach to produce optimized molybdenum carbide catalyst for alkaline HER
Graphical abstract
Mo2C microspheres synthesis by facile, low cost, and scalable solvothermal route is presented using industrial by product cyclooctatetraene as a carbon precursor, recycled into functional materials for green energy. The microspherical Mo2C catalyst exhibited reduced overpotentials of only 138 mV with 85 mV dec−1 Tafel slope in 0.5 M KOH and 170 mV with 88 mV dec−1 Tafel slope in 0.5 M H2SO4.
Introduction
Developing renewable energy generation and storage technologies represents one of the major scientific challenge today. Hydrogen is one of the clean and eco-friendly source of energy [1], [2], produced by multiple pathways such as photocatalytic [3], electrochemical [4], and photoelectrochemical [5], [6]. Among these techniques, electrochemical hydrogen generation is one of the cleanest and most sustainable for large-scale hydrogen generation, which can fulfill the current and future energy requirements [7], [8]. Specifically, the number of efficient and inexpensive hydrogen evolution reaction (HER) catalysts for alkaline medium is much lower than those for acidic medium, although there is promising commercial viability for the design of efficient and stable catalysts for alkaline conditions. Currently, Pt-based electrocatalysts exhibit the best catalytic performances for the HER in acidic solution (2H+ + 2e− ⟶ H2), while in alkaline medium (2H2O + 2e− ⟶ H2 + 2OH−) their activity is more limited due to sluggish kinetic resulting from the energy barrier of the water dissociation step, H-–OH (and insufficient OH− interaction) [9]. This understanding initiated development of bifunctional catalysts such as Ni(OH)2/Pt [10], but the high cost of platinum-based catalysts and the limited resources of platinum maintained quite limited utilization as cathodes in hydrogen production cells [11].
Therefore, non-precious catalysts, in particular molybdenum-based materials such as MoS2 [8], [12], MoSe2 [13], MoSexS2-x [14], MoO3 [15] and Mo2C [16] have attracted significant attention as promising catalysts for HER in acidic media and also in alkaline media under certain structural modifications. Among these catalysts, Mo2C is a promising, low-cost alternative to the noble-metal catalysts due to its high conductivity, chemical stability and Pt-like electronic structure [7], [17]. Therefore, efforts has been devoted to enhance the HER activity of Mo2C by structural and phase modification [18], formation of composite structures with graphene, carbon nanotubes and by doping [19], [20]. Despite these efforts, the electrocatalytic activity of the pristine molybdenum carbide is still much lower than that of Pt and other noble metal catalysts [7], [21], [22]. Moreover, the structure–function relationship and the possible promotion of HER by the identification and realization of the more active phases, still remains to be deliberated. Previous works used two-step synthesis protocols, first producing molybdenum oxide and then making use of techniques such as CVD to subsequently convert it to Mo2C, C-Mo2C, N-Mo2C and C/N-Mo2C [23], [24], [25]. The presence of Mo3+/Mo2+ and their corresponding mole ratios within the molybdenum carbide were identified as key factors for tailoring the properties of the active centers for HER [26].
In this study, micro-spherical structures of Mo2C were synthesized by template-free solvothermal approach followed by thermal annealing. We chose cyclooctatetraene as the carbon precursor since it is found in the waste of the rubber and plastic industry and in various used tiers, thus allowing its recycling into environmentally friendly materials [27], [28], [29]. Varying the feed ratio of the precursors enabled us to control the valence states of Mo and achieve optimized Mo2C phase formation for electrocatalytic HER. Electrodes produced with the micro-spherical Mo2C catalyst were stable and exhibited reduced overpotentials of 138 mV in 0.5 M KOH and 170 mV in 0.5 M H2SO4 at geometrical current density of 10 mA cm−2. This study presents green synthetic approaches to achieve optimal Mo2C catalyst by using industrial waste byproduct as a carbon precursor.
Section snippets
Experimental section
The Mo2C samples were synthesized by a solvothermal technique. Detailed protocols for the synthesis and characterization are provided in the supporting information. In brief, ammonium heptamolybdate tetra hydrate (2 mmol) was dissolved in 30 ml of methanol and 25 ml of 30% hydrogen peroxide (H2O2) with constant stirring for 10 min until a clear solution was obtained. 1,3,5,7- Cyclooctatetraene (5 g, 48 mmol) was added to the solution, and diluted with water to a final volume of 150 ml. The
Results and discussion
The morphology and microstructure of the as-synthesized Mo2C products were characterized by SEM (Fig. 1). As seen in the SEM image (Fig. 1), the samples with 5 g and 2 g of carbon precursor in the feed have spherical morphology, app. 300 nm in size (Fig. 1a-d). Smaller amounts of carbon precursor in the feed (1 g and 0.5 g) resulted in larger structures of 2.0–2.5 µm in size. Small pores can be seen on the surfaces of the microspheres (Fig. 1g-h). Decorated particles on the microsphere were
Electrochemical hydrogen evolution reaction
The electrocatalytic HER activity of the as-prepared samples was examined in alkaline medium (0.5 M KOH) with a three-electrode system and using graphite rod as the counter electrode (all the potentials reported as is, i.e. without iR-compensated). From the linear sweep voltammetry curve (LSV) (Fig. 5a), the 2 g Mo2C exhibited the best catalytic activity within the sample set with remarkably low onset potential of −20 mV versus RHE. To drive a current density of 10 mAcm−2, the 2 g Mo2C required
Conclusions
In summary, we report synthesis of Mo2C with the use of cyclooctatetraene as the carbon precursor for the formation of Mo carbides. We choose cyclooctatetraene since it is found in the waste of the rubber and plastic industry, thus allowing its recycling into environmentally friendly compounds. By varying the feed ratio of the precursors, we controlled the valence states of Mo and achieved optimized Mo2C phase formation for the HER. The electrodes produced with the micro-spherical Mo2C catalyst
CRediT authorship contribution statement
Sunil R. Kadam: Investigation, Writing - original draft. Sirshendu Ghosh: Investigation. Ronen Bar-Ziv: Writing - review & editing, Supervision. Maya Bar-Sadan: Conceptualization, Resources, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This Centre of excellence was supported by The Israel Science foundation (grant No. 1212/21).
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2022, Applied Surface ScienceCitation Excerpt :Impressively, MM-NH4Cl affords the η10 and η100 of 126 and 187 mV, in addition to the Tafel slope of 60.97 mV dec−1. Moreover, it is superior to other control groups and many previously reported Mo2C-based catalysts[37–50] (Table 1, Fig. 6b). Although the catalytic performance of as-prepared MM-NH4Cl is better than the previously reported Mo2C-based materials, there is still a large space for improvement compared with the catalytic materials with superior performance (Table S1).
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2022, Journal of Solid State ChemistryCitation Excerpt :For example, Sunil R et al. synthesized Mo2C by ammonium heptamolybdate tetrahydrate, showing good HER activity [25]. Dai and Hu reported Mo2C@C with core-shell structure acquired by phosphomolybdic acid dispersed in polypyrrole and then calcined at high temperature, exhibiting remarkable hydrogen evolution performance [25]. Tungsten carbides is a kind of transition metal carbide with the electronic structure similar to platinum, possessing the advantages of stability high conductivity and abundant storage capacity [26].
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