Microwave‐Induced Structural Engineering and Pt Trapping in 6R‐TaS₂ for the Hydrogen Evolution Reaction

Abstract

of using molecular hydrogen (H 2) as carbon-free fuel produced by renewable energy sources. [2] To embrace H 2 as game changer, academic research is struggling to develop advanced electrolysers, while reducing their investment and operational costs. [3] In this context, proton exchange membrane (PEM) water electrolysers overcome several operational drawbacks of commercial alkaline ones, [4] for example, low maximum achievable current density (between 200 and 400 mA cm −2), low operating pressure (<30 bar), inefficient dynamic operation (acceptable part-load operation between 10 and 40% of the nominal load), and gas crossover phenomena (typical gas purity <99.9%). [3,5] Nonetheless, the costs and the scarcity of their most effective catalysts, for example, Pt-group elements for the hydrogen evolution reactions (HER) at the cathode, [6,7] and RuO 2 /IrO 2 for the oxygen evolution reactions at the anode, [8,9] hinder massive commercial products. [10] To face the cost-related barriers of the PEM electrolysers, it is mandatory to search for alternative nonprecious catalysts, [11,12] or at least to reduce the content of precious metals, while still maintaining the electrochemical The nanoengineering of the structure of transition metal dichalcogenides (TMDs) is widely pursued to develop viable catalysts for the hydrogen evolution reaction (HER) alternative to the precious metallic ones. Metallic group-5 TMDs have been demonstrated to be effective catalysts for the HER in acidic media, making affordable real proton exchange membrane water electrolysers. Their key-plus relies on the fact that both their basal planes and edges are catalytically active for the HER. In this work, the 6R phase of TaS 2 is "rediscovered" and engineered. A liquid-phase microwave treatment is used to modify the structural properties of the 6R-TaS 2 nanoflakes produced by liquid-phase exfoliation. The fragmentation of the nanoflakes and their evolution from monocrystalline to partly polycrystalline structures improve the HER-activity, lowering the overpotential at cathodic current of 10 mA cm −2 from 0.377 to 0.119 V. Furthermore, 6R-TaS 2 nanoflakes act as ideal support to firmly trap Pt species, which achieve a mass activity (MA) up 10 000 A g Pt −1 at overpotential of 50 mV (20 000 A g Pt −1 at overpotentials of 72 mV), representing a 20-fold increase of the MA of Pt measured for the Pt/C reference, and approaching the state-of-the-art of the Pt mass activity.

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“…AE 0.2, which is similar to the values previously measured for TaS 2 produced with similar protocols 42,44,46,47. …”

“…Both the XRD patterns shows reections which match those of the 6R phase ((ICSD-52117), although a secondary 2H phase (ICSD-68488) coexists with a marginal contribution, as observed by previous studies on 6R-TaS 2 polytypes. 42 Fig. S3b † shows the Raman spectra of both bulk and exfoliated TaS 2 samples, further conrming their crystal structure.…”

“…Synthesis and exfoliation of 6R-TaS 2 crystals 6R-TaS 2 crystals were prepared through direct synthesis from elements, i.e., Ta and S, following protocols previously reported in literature. 42 Experimentally, amounts of Ta (99.999%, <6 mm) and S (99.9%, <100 mm) powders (Strem Chemicals, Inc.), with a S : Ta stoichiometry of 2 : 1, were placed in a quartz glass ampoule (20 mm  120 mm). Once evacuated until a pressure of 1  10 À3 Pa, the ampoule was sealed using an oxygenhydrogen welding torch.…”

“…The ampoule was heated to 450 C for 12 h, and subsequently heated to 600 C for 48 h, and up to 900 C for 48 h. Aerwards, the ampoule was cooled down to room temperature over 24 h. The heating rate was +5 C min À1 for all the heating steps. The TaS 2 akes were produced by ultrasonication-assisted LPE of fragments of 6R-TaS 2 crystals, [42][43][44] followed by sedimentation-based separation (SBS), 45 in anhydrous isopropanol (IPA). Briey, 50 mg of crystal fragments were inserted in 50 mL of anhydrous IPA.…”

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“…[ 94–96 ] This possibility enables large‐scale and cost‐effective solution processing, [ 97,98 ] through spray coating, [ 99 ] vacuum filtration, [ 90 ] ink‐jet printing, [ 100,101 ] and drop‐casting methods. [ 102 ] Finally, 2D TMDs have been widely established as ECs for the HER, [ 9,91,103,104 ] which means that they can stably operate under cathodic HER operation.…”