TaS2, TaSe2, and Their Heterogeneous Films as Catalysts for the Hydrogen Evolution Reaction

Metallic two-dimensional transition-metal dichalcogenides (TMDs) of the group 5 metals are emerging as catalysts for an efficient hydrogen evolution reaction (HER). The HER activity of the group 5 TMDs originates from the unsaturated chalcogen edges and the highly active surface basal planes, whereas the HER activity of the widely studied group 6 TMDs originates solely from the chalcogen- or metal-unsaturated edges. However, the batch production of such nanomaterials and their scalable processing into high-performance electrocatalysts is still challenging. Herein, we report the liquid-phase exfoliation of the 2H-TaS2 crystals by using 2-propanol to produce single/few-layer (1H/2H) flakes, which are afterward deposited as catalytic films. A thermal treatment-aided texturization of the catalytic films is used to increase their porosity, promoting the ion access to the basal planes of the flakes, as well as the number of catalytic edges of the flakes. The hybridization of the H-TaS2 flakes and H-TaSe2 flakes tunes the Gibbs free energy of the adsorbed atomic hydrogen onto the H-TaS2 basal planes to the optimal thermo-neutral value. In 0.5 M H2SO4, the heterogeneous catalysts exhibit a low overpotential (versus RHE, reversible hydrogen electrode) at the cathodic current of 10 mA cm–2 (η10) of 120 mV and high mass activity of 314 A g–1 at an overpotential of 200 mV. In 1 M KOH, they show a η10 of 230 mV and a mass activity of 220 A g–1 at an overpotential of 300 mV. Our results provide new insight into the usage of the metallic group 5 TMDs for the HER through scalable material preparation and electrode processing.


S2. Double-layer capacitance measurements of H-TaS2 films
The double-layer capacitance (Cdl) of catalytic films deposited onto a glassy carbon (GC) substrate (catalyst mass loading = 0.1 mg cm -2 ) was estimated by cyclic voltammetry (CV) measurements in a non-Faradaic region at different potential scan rate (ranging from 10 to 400 mV s -1 ). Figure S1a,b show the CV curve of H-TaSe2 flakes films before and after thermal treatment in a H2-rich environment at 600 °C (samples herein named H-TaS2 and H-TaS2 -Ar/H2@600 °C, respectively). By plotting the difference between anodic and cathodic current densities (∆j = (ja-jc)) at 0.25 V vs. RHE as a function of the scan rate (SR) (Figure S1c), Cdl can be calculated by: Cdl = d(∆j)/d2 (SR). The calculated Cdl values are 0.18 mF cm -2 for H-TaS2 and 0.25 mF cm -2 for H-TaS2 -Ar/H2@600 °C. Therefore, Cdl of H-TaS2 -Ar/H2@600 °C increases by 39% compared to Cdl of as-produced H-TaS2. Since Cdl is proportional to the electrochemically accessible surface area, these result indicate an increase of the porosity of the thermally treated catalytic films, allowing for an optimal electrolyte ion accessibility. Figure S1. CV measurements at various potential SRs for the following catalytic films: a) asproduced H-TaS2 films deposited onto glassy carbon (H-TaS2); b) H-TaS2 films deposited onto GC after thermal treatment in a H2-rich environment at 600 °C (H-TaS2 -Ar/H2@600 °C). c) SR dependence of ∆j for H-TaS2 and H-TaS2 -Ar/H2@600 °C. The linear fit of the curves are also shown. Figure S2 shows the X-ray diffraction spectra (XRD) spectra of a H-TaS2 catalytic film on a glass substrate before and after thermal treatment at 600°C in H2-rich environment (sample herein named H-TaS2 and H-TaS2 -Ar/H2@600°C, respectively). The data reveal that the loss of chalcogens occurring during the thermal treatment (see details in main text, Figure 3), leads to the formation of elemental Ta, which subsequently undergoes to an oxidation when material is exposed to air. Since the flakes preserve their two-dimensional (2D) morphology (see SEM analysis in the main text,

S4. Scanning electron microscopy-coupled energy dispersive X-ray spectroscopy analysis of assynthetized 2H-TaSe2 crystals
The as-produced 2H-TaSe2 crystals were characterized by SEM-coupled EDS measurements ( Figure   S3a-c). Their analysis evidences a near-ideal stoichiometric phase (Se-to-Ta ratio = 2.2) ( Table S2), in agreement with previous studies. 1,2 The excess of Se could be ascribed to one dimensional (1D)like trigonal Se by-products of the synthesis of 2H-TaSe2 crystals. These by-products can be formed by the recrystallization of polycrystalline Se. [3][4][5] The layered structure of 2H-TaSe2 crystals is clearly visible on their edges, as shown by high-magnification SEM imaging ( Figure S3d). The significant atomic content of O and C is associated to the carbon tape used as the sample substrate, and is not attributed to impurities of the as-synthetized crystals (in agreement with complementary characterization shown in the manuscript).  Table S2. Elemental composition of the as-synthetized 2H-TaS2 crystals, as estimated by SEMcoupled EDS analysis.

Element
Atomic content (%) Ta 8.     Figure S8 shows the XRD spectra of H-TaSe2 films on glass substrates before and after thermal treatment at 600°C in H2-rich environment (sample herein named H-TaSe2 and H-TaSe2 -Ar/H2@600°C, respectively). The data evidence that the loss of chalcogens occurring during the thermal treatment (see details in the main text, Figure 3) leads to the formation of elemental Ta, which subsequently undergoes to an oxidation when the catalytic film is exposed to air. Similar effects have been observed and discussed for the case of H-TaS2 catalytic films (see Section S3).   Figure S10 shows the chronoamperometry measurements for the thermally treated heterogeneous electrodes (named (H-TaS2:H-TaSe2 -Ar/H2@600°C in the main text) at a fixed potential corresponding to an initial cathodic current density of 80 mA cm -2 . In 0.5 M H2SO4, the electrode retains 97% of the initial current density after 12 h, thus promising a durable HER-activity. In alkaline condition, the electrode degrades during the first 4 h, thenceforth its current density is progressively stabilized (current density equal to 81% of the initial one after 12 h). Figure S10. Chronoamperometry measurements (j-t curves) at a fixed potential corresponding to an initial cathodic current of 80 mA cm -2 for H-TaS2:H-TaSe2 -Ar/H2@600°C electrodes in acidic (0.5 M H2SO4) and alkaline (1 M KOH) solutions. The percentage current density degradation after 12 h is also indicated in the plot. S16 S12. Scanning electron microscopy analysis of H-TaS2 electrodes before and after cyclic voltammetry cycling Figure S11 reports the image of the untreated and the thermally treated H-TaS2 electrodes (i.e., H-TaS2 and H-TaS2 -Ar/H2@600°C) before and after the CV cycling (1000 cycles). Figure S11a,b show that H-TaS2 significantly changes its morphology after CV cycling, resulting in a fragmented surface, in agreement with previous studies on H-TaS2 electrodes. [6][7][8] Differently, H-TaS2 -Ar/H2@600°C does not show any significant morphology change before and after CV cycling ( Figure S11c,d). This indicates that the initial porosity of the thermo-texturized electrode is enough to allow the evolved H2 to escape from the electrode surface without altering its morphology.  As shown in Figure S13, XRD measurements of H-TaS2:H-TaSe2 -CV@1000 cycles further evidence chemical changes on its surface. In particular, after the electrochemical treatment, the intensity of the XRD peaks attributed to the oxides (namely Ta2O5) increases relatively to the peaks of H-TaS2 and H-TaSe2. S18 Figure S13. XRD spectra of SWCNTs (substrate), H-TaS2:H-TaSe2 and H-TaS2:H-TaSe2 -CV@1000 cycles.

S11. Electrochemical stability tests of heterogeneous H-TaS2:H-TaSe2 electrodes
These results partially contradict those previously reported for H-TaS2 electrodes in literature, where it is claimed that H-TaS2 catalysts preserve their chemical integrity. [6][7][8] At current stage, more specific studies on electrodes with catalyst mass loadings similar to those of our electrodes are still needed to provide definitive understandings regarding to possible chemical changes of these catalysts during HER. S19

S14. Electrochemical stability tests in alkaline condition using polytetrafluoroethylene cell
The dissolution of the quartz of the cell in alkaline media could alter the electrolyte composition, affecting the HER-activity of the electrodes. In order to exclude these effects, the stability of the heterogeneous electrode was also tested in an alkaline resistant polytetrafluoroethylene (PTFE) cell.
As shown in Figure S14, the data confirm an initial degradation of the electrodes (-8% after 1.9 h).
Subsequently, the HER-activity of the electrode progressively increases over time (+16% after 12 h), suggesting an evolution toward a progressive electrochemical equilibrium, which was also observed in the quartz cell. Interestingly, our durable HER-activity was achieved without using any binder, such as Nafion, which could prospectively improve the mechanical stability of the electrodes during HER. In fact, the mechanical stresses originated by the gas evolution have been demonstrated to be the cause of the self-optimizing fragmentation of the initial catalytic group-5 TMDs. [6][7][8] However, these effects could cause significant material losses, that have to be controlled for practical applications.