Carbon Nanotube-Supported MoSe2 Holey Flake:Mo2C Ball Hybrids

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Carbon Nanotube-Supported MoSe2 Holey Flake:Mo2C Ball Hybrids for Bifunctional pH-Universal Water Splitting Leyla Najafi, Sebastiano Bellani, Reinier Oropesa-Nunez, Mirko Prato, Beatriz Martin-Garcia, Rosaria Brescia, and Francesco Bonaccorso ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08670 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Carbon Nanotube-Supported MoSe2 Holey Flake:Mo2C Ball Hybrids for Bifunctional pHUniversal Water Splitting Leyla Najafi,†,‡ Sebastiano Bellani,†,‡ Reinier Oropesa-Nuñez,†,∥ Mirko Prato,# Beatriz Martín-García,† Rosaria Brescia⊥ and Francesco Bonaccorso†,∥*

† Graphene Labs, Istituto Italiano di Tecnologia, via Morego 30, 16163, Genova, Italy.

∥ BeDimensional Spa., Via Albisola 121, 16163 Genova, Italy

# Materials Characterization Facility, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy.

⊥ Electron Microscopy Facility, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy.

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KEYWORDS: hydrogen evolution reaction (HER), electrocatalysts, oxygen evolution reaction (OER), molybdenum diselenide (MoSe2), molybdenum carbide (Mo2C), water splitting, pH.

ABSTRACT

The design of cost-effective and efficient electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is pivotal for the molecular hydrogen (H2) production from electrochemical water splitting as a future energy source. Herein, we show that the hybridization between multiple HER- and OER-active components is effective for the design and realization of bifunctional electrocatalysts for universal water splitting, i.e., in both acidic and alkaline media. Our strategy relies on the production and characterization of MoSe2 holey flake:Mo2C ball hybrids supported by single-walled carbon nanotubes (SWCNT) electrocatalysts. Flakes of MoSe2 are produced through hydrogen peroxide (H2O2)-aided liquid phase exfoliation (LPE), which promote both the


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exfoliation of the materials and the formation of nanopores in the flakes via chemical etching. The amount of H2O2 in the solvent used for the exfoliation process is optimized to obtain ideal high-ratio between edge and basal sites ratio, i.e., high-number of electrocatalytic sites. The hybridization of MoSe2 flakes with commercial ball-like shaped Mo2C crystals facilitates the Volmer reaction, which works in both acidic and alkaline media. In addition, the electrochemical coupling between SWCNTs (as support) and MoSe2:Mo2C hybrids synergistically enhance both HER- and OER-activity of the native components, reaching high ƞ10 in acidic and alkaline media (0.049 and 0.089 V for HER in 0.5 M H2SO4 and 1 M KOH, respectively; 0.197 V and 0.241 V for OER in 0.5 M H2SO4, and 1 M KOH, respectively). The exploitation of the synergistic effects occurring between multi-component electrocatalysts, coupled with the production of the electrocatalysts themselves through scalable and cost-effective solution-processed manufacturing techniques, is promising to scale-up the production of H2 via efficient water splitting for the future energy portfolio.


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The production of molecular hydrogen (H2) from electrochemical water splitting is an ideal high-energy density source (between 120-140 MJ kg-1)1 because both its production from renewable sources2 and its consumption are sustainable and environmentally friendly.3 Electrocatalysts, which are highly active and durable, are essential to accelerate the kinetics of water splitting reactions.4 The best-known effective electrocatalysts for hydrogen evolution reaction (HER) (4H+ + 4e-  2H2 in acidic media; 4H2O + 4e-  2H2 + 4OH- in alkaline media)5 are Pt-group elements,6 while RuO2 and IrO2 are the benchmark for oxygen evolution-reaction (OER)7,8 (2H2O  O2 + 4H+ + 4e- in acidic media; 4OH-  2O2 + 2H2O + 4e- in alkaline media).5,9 However, the cost10 and the scarcity11 of the aforementioned noble metal-based electrocatalysts hinder their massive use. Therefore, recent advances have focused on developing Earth-abundant electrocatalysts as prospective practical and sustainable electrochemical water splitting.12–14 In this regard, transition metal dichalcogenides (TMDs),15–17 composed of covalently bonded X-M-X blocks (M = transition metal; X = S, Se, Te),18,19 and transition metal carbides (TMCs),20–22 consisting of MxCy crystals,23 have attracted huge interest for HER. Meanwhile, non-metallic carbon based-electrocatalysts,9,24–26 such as carbon


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nanotubes (CNTs),27 graphene,28,29 graphene derivatives30,31 and graphitic carbon nitride32 are emerging for OER, competing with state-of-the-art performance. Theoretical33–35 and experimental investigations36–39 have shown that the HER-active sites of the TMDs are the unsaturated X-edges, since they have a nearly zero Gibbs free energy of adsorbed atomic H (ΔGH0), while TMCs exhibit HER-catalytic properties that are similar to Pt-group metals due to their d-band electronic structure.40,41 In this context, the controllable synthesis of nanostructured TMDs35,37,42–44 and TMCs45–48 is pursued for maximizing the number of the catalytic edges, showing the possibility to reach overpotential at 10 mA cm-2 absolute current density (ƞ10) approaching those of noble metal-based electrocatalysts (i.e., < 0.1 V).49 However, the majority of the studies focused on the HER-activity in acidic media. This precludes the development of TMDs and TMCs for commercially viable overall water splitting, because the most promising non-noble metal OER-electrocatalysts studied to date are active in alkaline media.9,50 Actually, the HER in acid solution is assumed to proceed by an initial discharge of the hydronium ion (H3O+) and the formation of intermediated, i.e. atomic hydrogen adsorbed on the electrocatalyst surface (Hads), in the so-named Volmer step (H3O+ + e- ⇄ Hads + H2O),


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followed by either the electrochemical Heyrovsky step (Hads + H3O+ + e- ⇄ H2 + H2O) or the chemical Tafel recombination step (2Hads ⇄ H2).51,52 In alkaline conditions, the Hads is formed by the discharge of H2O (H2O + e- ⇄ Hads + OH-). Then, HER proceeds with the Heyrovsky step (H2O + Hads + e- ⇄ H2 + OH-) or the chemical Tafel recombination step (2Hads ⇄ H2).51,52 Based on the above HER-pathways, it is clear that the HER-activity is ruled by different electrocatalyst design depending on the operating pH. It is just only recently that HER-activity in alkaline conditions (ƞ10 < 0.4 V) have been reported for MoS2,53–55,56 MoSe254,57 and MoxC.58–61 Moreover, recent studies also reported that TMDs have OER-activity,62–66 which has been long disregarded. As a successful modus

operandi to enhance/achieve both HER- and OER-activity, the hybridization between multiple HER- and OER-active components is effective to design/produce bifunctional electrocatalysts, having performances exceeding the ones of their single counterparts.67– 74

However, the rationale explaining the HER-activity enhancement is not fully clarified

yet. Moreover, hybrids based on HER-electrocatalysts have also been found to increase the catalytic activity of the native components by either changing the surface morphology to expose more active sites,75–78 or tuning the electrochemical properties by synergistic


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effects.78–80 In fact, the catalytic properties of the single components towards different HER-pathways, such as Volmer and Heyrowsky/Tafel reactions,52,57,81 synergistically increase the HER-activity of the overall hybrid.77–80 Herein, we report MoSe2 holey flake:Mo2C ball hybrids supported by single-walled CNTs (SWCNTs) substrates as solution-processed flexible electrocatalyst for HER and OER in pH-universal media. The rationale behind the choice of the materials composing the hybrid electrocatalyst is shown in Scheme 1, which illustrates the synergistic effects on HER- and OER-pathways (see also Supporting Information –S.I.–, Scheme S1).

Scheme 1. Illustration of the synergistic electrocatalytic effects in MoSe2 flake/Mo2C ball hybrids deposited on SWCNTs. On the left: HER-activity enhancement as a consequence of the hybridization between MoSe2 fakes and Mo2C ball with catalytic properties towards Heyrowsky/Tafel and Volmer reactions, respectively. On the right: Increase of HER- and OER- activity in alkaline and acidic media, respectively, as a result of the favourable electrochemical coupling between MoSe2:Mo2C hybrids and SWCNTs for the realization of bifunctional water splitting electrocatalysts.


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Among the HER-active TMDs, we opted for MoSe2 because it has: a) low ΔGH0 (< 0.1 eV),82,83 which promotes the Volmer step in acidic media, as well the subsequent Heyrovsky/Tafel reaction; and b) high electrical conductivity (~10-1 Ω-1 cm-1),84 superior to the one of the most investigated HER-active TMDs85 (e.g., ~10-2 Ω-1 cm-1 for MoS284), which enables optimal electron accessibility to the catalytic sites even in presence of high mass loading materials (> 1 mg cm-2).49 More in detail, we focused on MoSe2 flakes that were produced through a modified liquid phase exfoliation (LPE) method,86–91 exploiting hydrogen peroxide (H2O2) as oxidant to promote bulk MoSe2 exfoliation and nanopores formation in the basal plane of the exfoliated flakes.92–94 The as-produced flakes have shown enhanced edge-to-basal (active-to-inert) sites ratio in comparison to the flakes


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obtained from LPE in absence of H2O2. Mo2C was specifically selected as hybridizing agent because it has: a) strong hydrogen binding capability (i.e. ΔGH0 < 0);47,95,96 and b) HER-activity in alkaline condition.47,97 The hydrogen binding capability facilitates the Volmer reaction in acidic media at the interface with MoSe2, whose edges promote the Heyrovsky/Tafel reaction (Scheme 1, left-top side). The HER-activity of Mo2C in alkaline condition promotes the H2O discharge (Volmer reaction), whereas MoSe2 still provide site for desorbing hydrogen, favouring the subsequent HER-pathways (Scheme 1, left-bottom side). Therefore, MoSe2/Mo2C interfaces were initially expected to be highly active for HER. Lastly, SWCNTs were intended to provide flexible, conductive solution-processed substrates98,99 and OER-activity.100–102 In particular, the latter is crucial for the activation of the catalytic sites in MoSe2:Mo2C hybrids in alkaline condition by initiating the H2O discharge, i.e. the Volmer reaction (Scheme 1, right-bottom side). Similarly, the HERactivity of the MoSe2:Mo2C hybrid promote the H2O discharge in acidic media, activating their OER-activity and improving the OER-activity of SWCNTs (Scheme 1, right-top side).67–71,73,74,103,104 To summarize, here we report an advanced bifunctional electrocatalyst, providing insight for producing and engineering noble metal-free hybrid


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materials compatible with scalable solution-based manufacturing for viable and sustainable electrochemical water splitting. By optimizing the electrocatalysts material formulation and design, we achieved relevant ƞ10 for HER and OER in both acidic and alkaline media (0.049 and 0.089 V for HER in 0.5 M H2SO4 and 1M KOH, respectively; 0.197 and 0.241 V for OER in 0.5 M H2SO4, and 1M KOH, respectively).

RESULTS AND DISCUSSION

MoSe2 flakes production and material dispersion formulation

MoSe2 flakes were produced in form of dispersion in 2-Propanol (IPA) through a modified LPE approach of the MoSe2 bulk crystal. Specifically, a mixed-solvent strategy,105,106 consisting of the use of an IPA/hydrogen peroxide (H2O2) mixture as dispersion solvent, was adopted (see Methods section).92–94 This method avoids typical problems related to the LPE of TMDs in high boiling point solvents, i.e., N-Methyl-2pyrrolidone (NMP) and dimethylformamide (DMF),86,88 such as the annealing processing at high temperature for the solvent removal.86,88,107 As previously reported, the use of


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H2O2 is intended to promote bulk MoSe2 exfoliation and nanopores formation in the basal plane of the exfoliated flakes via an etching process.92–94 By using different H2O2 volume fraction (0%, 0.1%, 0.2%, 0.3%) samples with different morphology, i.e., thickness and lateral size (named MoSe2-0, MoSe2-1, MoSe2-2 and MoSe2-3, respectively), were selected through sedimentation-based separation (SBS)108,109 (see Methods for additional details). Figure 1 reports the morphological analysis of the various MoSe2 samples. The thickness and the lateral size of the exfoliated structures were evaluated by atomic force microscopy (AFM) (Figure 1a-d) and bright field transmission electron microscopy (BFTEM) (Figure 1e-h), respectively. The AFM and BFTEM statistical analysis are shown in Figure 1i,j, respectively. The AFM analysis of MoSe2-0 (Figure 1a,i) shows the presence of single- (the thickness of a monolayer of MoSe2 is in the 0.6-1.0 nm range)110 to a few-layer MoSe2 flakes (average thickness of 5.4 ± 2.6 nm). MoSe2-1 and MoSe2-2 consisted of flakes with thickness of 2.3 ± 1.3 nm and 1.7 ± 1.1 nm, respectively. The thickness of the latter two samples is significantly reduced if compared to the one of the MoSe2-0 flakes. Moreover, the flake structures of MoSe2-1 and MoSe2-2 were tailored to provide holey structure having irregular lateral edges. By increasing the concentration


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of H2O2 up to 0.3%, MoSe2-3 turned out to be composed by nanostructure aggregates, which lost the 2D topological structure. The different morphology of the latter sample is ascribed to an excessive H2O2-induced etching of MoSe2, which altered the native LPE process by creating defective oxide clusters.111,112 A representative BFTEM image of MoSe2-0 (Figure 1e) displayed flakes with a crumpled paper-like structure. Similar structure is also observed for MoSe2-1 and MoSe2-2 (Figure 1f,g). Differently, the MoSe23 shows aggregated nanostructure (Figure 1h), in agreement with the AFM analysis (Figure 1d). The BFTEM statistical analysis (Figure 1j) indicates that MoSe2-1 is composed by flakes with the largest lateral dimension (average value of 109.3 nm vs. 52.0 nm for MoSe2-0 and 59.4 57.4 for MoSe2-2). Figure 1k reports the lateral size-tothickness aspect ratio (lateral size/thickness) and the reciprocal of the product between the lateral dimension and the thickness (1/(lateral size × thickness)) of the nanostructures (flakes or aggregates) in the different samples. The first parameter qualitatively evaluates the effectiveness of the exfoliation process of the layered materials into 2D form.88,113 Instead, the second parameter qualitatively provides a prediction of the HER-activity. In particular, low value indicates a high number of edge sites, which are also HER-active


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sites. MoSe2-1 shows the highest lateral size-to-thickness aspect ratio (~47.5), while MoSe2-2 has the highest reciprocal of the product between the lateral dimension and the thickness (0.004 nm-2). Such results are explained by taking into account the concurrence of H2O2-prompted exfoliation, which reduced the flakes thickness,92,93 and etching, which limited the lateral dimension.92,93


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Figure 1. Morphological characterization of MoSe2-0, MoSe2-1, MoSe2-2 and MoSe2-3 samples. a-d) AFM images of the samples placed onto a mica sheet. The z-scale bar is: 3 nm for (a), 5 nm for (b) and (c), 50 nm for (d). e-h) BFTEM images of the MoSe2 samples; i) Statistical analysis of the thickness and j) the lateral dimension of the nanostructures (flakes or aggregates) observed in the different MoSe2 samples (calculated on 100 nanostructures); k) Lateral size-to-thickness aspect ratio (blue) and the reciprocal of the product between the lateral dimension and the thickness (red) of the nanostructures in the different MoSe2 samples.

The MoSe2 crystal structure was investigated through X-ray powder diffraction (XRD)114 and Raman spectroscopy.115 Figure 2a shows the XRD patterns obtained for the MoSe2 samples, together with the one of the native MoSe2 bulk crystal. The latter is indexed with the JCPDS Card No. 29-0914 of the hexagonal phase of MoSe2 (i.e. 2H-MoSe2).92 For the exfoliated samples, the (002) peak is broader (full width half maximum –FWHM– equal to 1.95, 0.71, 0.97 and 0.62 for MoSe2-0, MoSe2-1, MoSe2-2 and MoSe2-3, respectively) compared to the MoSe2 bulk crystal (FWHM = 0.25), while the other peaks are strongly reduced in intensity, although retaining their original position. This indicates that the MoSe2 samples consist of flakes oriented with the c-axis perpendicular to the substrate and retain their native crystal structure.44,116 In fact, the presence of (002) peak indicates


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a high degree of crystallinity, differently from what observed in TMDs produced by chemical methods (e.g., colloidal or one-pot hydrothermal synthesis),117,118 in which selfassembling effects also result in nanoflower-like morphology for the case of MoSe2. Figure 2b reports representative Raman spectra of both MoSe2 bulk crystal and the exfoliated samples. Taking into account the group theory analysis, MoSe2 bulk crystal is a member of D6h point group.119 Consequently, it is characterized by four Raman active modes, i.e. three in-plane E1g, E12g, and E22g, and one out-of-plane A1g.119 The A1g and E12g modes are located at ~240 cm-1 and ~287 cm-1, respectively. The E1g mode, which is activated by resonance-induced symmetry breaking effect,120 is observed at ~167 cm1.

The E22g mode, which is typically located at low frequency (~30 cm-1),121 is not

experimentally accessible. For the exfoliated samples the A1g mode is still located at ~240 cm-1, in agreement with previous studies on few-layer MoSe2 flakes.122–124 For MoSe2-1, the so-called Davydov splitting of the A1g mode123 is resolved because of the more effective flakes exfoliation (i.e. higher lateral size/thickness value, ~47.5) compared to other samples (~10.6, ~34.9, ~9.8 for MoSe2-0, MoSe2-2 and MoSe2-3, respectively), as reported by the morphological analysis (Figure 1k). The position of the E1g mode is


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independent from the number of layers,125 therefore, it does not change between the MoSe2 bulk crystal and the exfoliated samples. No Raman signature related to cristalline oxides, such as MoO3, MoO2 and Mo4O11 are detected. However, X-ray photoelectron spectroscopy (XPS) measurements (Figure 2c,d) of the samples exfoliated with H2O2 revealed oxidized states of both Mo and Se, which indicated the formation of amorphous oxides as by-products when H2O2 is used.


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Figure 2. Structural and chemical characterization of MoSe2 samples. a) XRD spectra of the bulk and as-produced MoSe2 samples. In the XRD spectra of the MoSe2 bulk are also indicated the diffraction peaks of the hexagonal phase of MoSe2 (2H-MoSe2). b) Raman spectra of the bulk and as-produced MoSe2 samples deposited onto the Si/SiO2 substrates. In the graph are named the main peaks, such as the in-plane modes E1g, E12g, and E22g, the out-of-plane mode A1g and the breathing mode B12g. c) Mo 3d and d) Se 3d XPS spectra for both the bulk and as-produced MoSe2 samples. Specifically, the two peaks positioned at ~229.3 eV and ~232.4 eV in the Mo 3d XPS spectra of all samples (Figure 2c) are assigned to the Mo 3d5/2 and Mo 3d3/2 peaks,


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respectively, of the Mo4+ state in MoSe2. These data are is in agreement with previous literature on MoS2126 and MoSe2.127 The additional peaks observed for MoSe2-1, MoSe2-2 and MoSe2-3 are assigned to oxidized forms of Mo (i.e., Mo5+ and Mo6+).128 The peaks located at ~54.9 eV and ~55.7 eV in the Se 3d spectra are ascribed to the Se 3d5/2 and Se 3d3/2 peaks of the diselenide moiety of MoSe2, respectively.49,129 For the samples exfoliated with H2O2, the additional peaks at higher binding energy are ascribed to the oxidized forms of Se (Se4+ and Se6+).130 A further Se component was present in all the samples, in a position consistent with Se0.130 Although XPS analysis revealed the presence of oxidized species, chemical mapping exploiting energy dispersive X-ray spectroscopy in the scanning transmission electron microscopy (STEM-EDS) of the flakes in MoSe2-2 (Figure 3a-d) excluded a predominant presence of oxide domains localized onto the flakes. This can be explained based on the different depth probed by the two techniques: being XPS much more surface-sensitive than any TEM-based technique, the oxidized Se species must be localized within the surface layers of the flakes. In addition, high-resolution transmission electron microscopy (HRTEM) (Figure 3e) and the corresponding fast Fourier transforms (FFTs) in different


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regions (Figure 3f,g) also reveal that the whole area of the flakes has a crystal hexagonal structure (2H-MoSe2), in agreement with XRD and Raman analysis (Figure 2a,b). It is also worth to notice that it has been recently demonstrated that the presence of metal oxides (or hydroxides) on TMD surface can also increase the HER-activity of the pristine TMDs in alkaline conditions,57,131 similarly to what observed on noble metal-based electrocatalysts.70,71,72


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Figure 3. X-ray spectroscopy in scanning transmission electron microscopy of flakes in the MoSe2-2 sample. a) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of a representative flake in the MoSe2-2 sample and the corresponding STEM-EDS maps for b) Mo (blue), c) Se (green) and d) O (red). e) HRTEM at the edge of a MoSe2 flake in the sample MoSe2-2 and f,g) FFTs corresponding to the regions in the frame, in agreement with the hexagonal phase of MoSe2. In


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particular, region 1 shows [001] orientation of the flake and region 2 a folded area at its edge, showing the characteristic {002} planes spacing of hexagonal MoSe2. Mo2C and SWCNTs powders are used as-purchased without any further purification. Hybrid MoSe2:Mo2C dispersions in IPA are obtained by adding Mo2C powder to each MoSe2 dispersions in 1:2 material weight ratio. Mo2C dispersions are also produced by dispersing Mo2C powder in IPA. The SWCNTs dispersions are obtained by dispersing commercial SWCNTs by means of ultrasonication for the de-bundling process.132 Additional details regarding the dispersion preparation, which follows protocols previously reported,44,49,57,133 are described in the Methods. As shown in our recent work by combing Raman spectroscopy and TEM measurements,81 we assessed that the length of the SWCNTs is in the 5-30 m range, while their average diameter is < 1 nm.

Fabrication of solution-processed HER-electrodes

Two different electrode typologies are investigated. The first was obtained by drop casting the as-produced MoSe2, Mo2C and MoSe2:Mo2C dispersions onto glassy carbon (GC) substrates (mass loading of the active material of 0.2 mg cm-2). The as-obtained


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electrodes are herein named GC/active material. The second was obtained by depositing in sequence the as-produced SWCNTs and MoSe2-2 or MoSe2-2:Mo2C dispersions onto nylon membranes through vacuum filtration. MoSe2-2 and MoSe2-2:Mo2C were specifically selected because of their better electrocatalytic activity on GC substrates compared to the other cases. The as-produced electrodes are herein named SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C, respectively. The mass loading is 1.31 mg cm-2 for SWCNTs and 0.78 mg cm-2 for MoSe2-2 and MoSe2-2:Mo2C. We also produced electrodes made of SWCNTs only (henceforth named SWCNTs) as reference. The details of the fabrication of the electrodes are reported in the Methods. The surface morphology of the as-prepared electrodes was characterized by scanning electron microscopy (SEM) and AFM. Figure 4 shows SEM images of the SWCNT/MoSe2-2 and SWCNT/MoSe2-2:Mo2C, which represents the two electrode typologies. Top-view SEM image of the bare SWCNTs is reported in S.I. (Figure S1), showing a mesoporous network having a bundle-like morphology. Differently, the SWCNT/MoSe2-2 surface is uniformly covered by the flakes of MoSe2 (Figure 4a), while the surface of SWCNT/MoSe2-2:Mo2C is composed by m-sized ball-like shaped Mo2C


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(Figure 4b) covered by MoSe2 flakes (Figure 4c). Figure S2 shows the AFM images of the electrode surfaces, which evidence that the morphologies are similar to those obtained by the SEM characterization. The average roughness (Ra) value is ~103 nm for SWCNT electrodes.

Figure 4. Morphology characterization of the as-prepared electrodes. Top-view SEM images of a) SWCNTs/MoSe2-2 and b) SWCNTs/MoSe2-2:Mo2C. c) Enlarged top-view


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SEM image of SWCNTs/MoSe2-2:Mo2C showing Mo2C crystal covered by MoSe2 flakes. Cross-section SEM image of the d) SWCNTs/MoSe2-2 and e) SWCNTs/MoSe2-2:Mo2C.

This value decreased to ~11.6 nm and 78.5 nm for the SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C, which indicates how the MoSe2 flakes deposition flattened the electrode surfaces. The latter effect is, however, reduced in presence of m-sized Mo2C crystals, as expected by the SEM analysis (Figure 4b,c). Figure 4d,e show cross-sectional SEM images of the SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2/Mo2C. For both the electrodes we observed a bilayer-like structure. In particular, for the SWCNTs/MoSe2-2, the MoSe2 flakes form a homogeneous overlayer with a thickness of ~0.8 m. For the SWCNTs/MoSe2-2:Mo2C, the MoSe2:Mo2C hybrids form a porous m-thick overlayer. The morphological analysis of representative GC-based electrodes revealed similar MoSe2 and MoSe2:Mo2C films to those obtained by vacuum filtration onto SWCNTs.

Electrochemical characterization


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We evaluated the HER- activity of the as-produced electrodes in both acidic (0.5 M H2SO4) and alkaline (1 M KOH) media. Figure 5 displays the iR-corrected polarization curves for the electrodes with GC substrate. In acidic condition (Figure 5a), the GC/MoSe2-2 shows higher HER-activity compared to the other MoSe2-based electrodes. In particular, η10 decreased from 0.309 V for GC/MoSe2-0 to 0.290 and 0.236 V for GC/MoSe2-1 and GC/MoSe2-2. For the GC/MoSe2-3, a decrease of the HER-activity is observed (η10 of 0.384 V). We attributed the trend of the HER-activity within the MoSe2 electrodes to the different morphology of the MoSe2 flakes, as discussed in the previous section (Figure 1). In fact, the peculiar holey structure (Figure 1c), low thickness (1.7 ± 1.1 nm) (Figure 1i) and small lateral dimension (59.4 ± 57.4) (Figure 1j) of the flakes in MoSe2-2 are beneficial for the HER-activity. The hybridization of MoSe2 with Mo2C also promoted the HER by further reducing the η10 to 0.204 V for the most HER-active electrode, i.e. GC/MoSe2-2:Mo2C. The enhancement of the HER activity observed in acidic medium of the hybrids is ascribed to the strong hydrogen binding energy of Mo2C,47,48,96,134 which facilitated the Volmer reaction at the interface with MoSe2, whose edges promoted the subsequent Heyrovsky/Tafel reaction (Scheme 1, left-top side). In


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alkaline condition, the MoSe2 electrodes show a poor HER-activity (η10 > 0.4 V) (Figure 5b). The sluggish HER kinetics of the MoSe2 in alkaline conditions is attributed to the ineffective adsorption of hydroxyl species on its surface,53,74,135 which resulted to be inefficient to discharge H2O (i.e. to promote the Volmer reaction in alkaline media).70–72,136 Interestingly, the hybridization of MoSe2 with Mo2C increased the HER-activity because of the synergistic effects of Mo2C, which promoted H2O discharge (Volmer step),47,97 whereas MoSe2 still provided sites for adsorbing the vacating hydrogen, which favoured the subsequent HER-pathways (Scheme 1, left-bottom side). Thus, η10 of 0.231 V is achieved for the most active case, i.e., GC/MoSe2-2:Mo2C. The OER-activity of the electrodes with GC substrate is also studied in acidic (0.5 M H2SO4) as well as alkaline (1 M KOH) media (Figure S3). All the electrodes show insignificant OER-activity in 0.5 M H2SO4 (ƞ10 > 0.8), while they held OER-catalytic properties in 1 M KOH (ƞ10 increase from 0.548 V for GC/MoSe2-0 to 0.461 and 0.403 V for GC/MoSe2-2 and GC/MoSe2-2:Mo2C, respectively). These results indicate that the peculiar morphology of the flakes in MoSe2-2 and the hybridization between MoSe2 and Mo2C have a strong effect on the OER-activity of the MoSe2 electrodes.


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Figure 5. HER-activity of the electrodes with GC substrates: iR-corrected polarization curves in a) acidic (0.5 M H2SO4) and b) alkaline (1 M KOH) media.

Once experimentally established the electrocatalytic activity of the electrodes using GC as substrate, SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C were investigated as flexible self-standing electrocatalytic electrodes. Electrodes based on SWCNTs were also tested as reference, since they have been also reported as OER-electrocatalyst.100,101,137– 139

Actually, we designed the heterostructures with the composing materials having


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different electrocatalytic properties to synergistically create an advanced bifunctional water splitting electrocatalysts, in agreement with Scheme 1, right side.67–74,103,104 The electrode were produced by depositing the SWCNTs and MoSe2 or MoSe2:Mo2C dispersions onto the nylon membranes through vacuum filtration process. Nylon membranes were used as catalyst support since their microporosity allows the materials dispersions to be deposited by facile vacuum filtration methods. In agreement with our previous work, such approach is especially effective for one/two dimensional (1D/2D) materials, since they are filtered without any material losses (differently from noble metal nanopowders used as electrocatalysts for HER and/or OER).71,81,88 Figure 6 shows the iR-corrected polarization curves obtained for the SWCNTs, SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C in both 0.5 M H2SO4 and 1 M KOH. The curves measured for commercial RuO2 and Pt/C are also plotted as benchmark for OER and HER, respectively. A very rigorous kinetic analysis of the HER including the founding of the Tafel slope and the exchange current (j0), was not carried out in this work because unambiguous results can originate by the presence of the SWCNTs. In fact, SWCNTs have a high surface area that leads to a significant capacitive current density (i.e., ~1 mA


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cm-2) even at a low LSV sweep voltage rate (i.e., ≤5 mV s-1).57 This can determine misleading interpretations on the estimation of the kinetic parameters (also in agreement with refs. 69,140).

Figure 6. Water splitting-activity of the hybrid heterostructures (SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C): iR-corrected polarization curves for HER in a) acidic (0.5 M H2SO4) and b) alkaline (1 M KOH) media. iR-corrected polarization curves for OER in c) acidic (0.5 M H2SO4) and d) alkaline (1 M KOH) media. The polarization curve of SWCNTs is also shown for comparison. The curves measured for Pt/C and RuO2 are also shown as benchmark for HER and OER, respectively.


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The HER-activity of the heterostructures (Figure 6a,b) increased compared to the one of the GC-based electrode (Figure 5). In particular, SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C exhibited ƞ10 of 0.103 and 0.047 V, respectively, in 0.5 M H2SO4, and 0.219 and 0.085 V, respectively in 1 M KOH. To the best of our knowledge, the obtained results compare with the ones achieved with the best-performance HERelectrocatalysts reported in literature (see Table S1), including those based on noble metals141,142,143,144 (for example Pt/C benchmark, also shown in Figure 6a,b). In addition, the proposed heterostructures held bifunctional electrocatalytic properties, since they are also effective for the OER process (Figure 6c,d). More in detail, SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C exhibited ƞ10 of 0.289 and 0.197 V, respectively, in 0.5 M H2SO4, and 0.295 and 0.241 V, respectively, in 1 M KOH. As far as we know, these values are superior or, at least, comparable to the state-of-the-art reached by RuO2 (also shown in Figure 6c,d) and IrO2, which are typically considered as benchmark electrocatalysts for OER working in both acidic and alkaline conditions,5,9,145 as well as Ru- and Ir-based alloy.143,146


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Beyond the electrocatalytic activity, the durability is another important standard for the commercial use of electrocatalysts. Figure 7 shows the chronoamperometry measurements (j–t curves) for the as-produced heterostructures. For all the electrocatalysts tested, a constant overpotential was applied to provide the same initial current density of -30 mA cm-2 for HER, and 20 mA cm-2 for OER. For HER (Figure 7a,b), the electrodes retained a steady activity over a period longer than 8 h. In particular, a decrease of the current density was observed for the SWCNTs/MoSe2-2 after 8 h (-12% in 0.5 M H2SO4, -22% in 1 M KOH), while an improvement of the HER-activity is shown for SWCNTs/MoSe2-2:Mo2C (+5% in 0.5 M H2SO4, +21% in 1 M KOH). Our understanding is that degradation evidenced by SWCNTs/MoSe2-2 is not due to electrochemical instability, but it is rather caused by mechanical stresses due to H2 bubbling through the layered structure of the electrode. These effects are reduced in SWCNTs/MoSe2-2:Mo2C, possibly because of its lowdensity morphology (see SEM analysis, Figure 4e), which facilitated the transport of the evolved H2 towards the electrode/electrolyte interface. Notably, XPS analysis revealed no chemical degradation of the MoSe2 flakes during chronoamperometry measurements,


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carried out both in acidic and alkaline solutions (Figure S4), while the electrochemical stability of Mo2C nanomaterials has been successfully reported in previous studies.147,148

For OER (Figure 7c,d), the current density decreased for both SWCNTs/MoSe2-2 (50% in 0.5 M H2SO4, -46% in 1 M KOH) and SWCNTs/MoSe2-2:Mo2C (-81% in 0.5 M H2SO4, -64% in 1 M KOH) after 30 min. In this case, electrode disruption is observed for both the heterostructures during testing.


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Figure 7. Chronoamperometry measurements (j-t curves) of the SWCNTs/MoSe2-2 and SWCNTs/MoSe2-2:Mo2C for: a,b) HER and c,d) OER in acidic (0.5 M H2SO4) (a,c) and alkaline (1 M KOH) media (b,d). A constant overpotential is applied to provide the same initial current density of -30 mA cm-2 for HER, and 20 mA cm-2 for OER.

This effect is tentatively attributed to both the OER from SWCNTs100,101,137–139,149–151 and the electrochemical oxidation of SWCNTs towards gaseous product (i.e. CO2 and CO),152,153 which might induce a detachment of the MoSe2 or MoSe2:Mo2C overlays. In fact, although carbon corrosion has been mostly reported for fuel cells,154,155 carbonbased anodes also degrade during OER.152,156 Prospectively, the SWCNTs stability could be improved by tuning their morphology and chemical quality, since their electrochemical activity is mostly originated by open ends and defect sites.152,153,156 For example, highly graphitized, rolled sheet-like structure of CNTs has been proved for preventing carbon corrosion.152,153,156 Therefore, SWCNTs still represent an attracting choice as OERelectrocatalysts. In particular, their electrical and mechanical properties allow selfstanding flexible electrodes to be produced through solution-processed manufacturing techniques, which are pivotal for a scalable high-throughput-implemented technology.


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Despite further efforts are needed to overcome the instability issues, these results demonstrated that the hybridization between carbon-based supports, i.e. SWCNTs, and HER-electrocatalysts, i.e., MoSe2 and MoSe2:Mo2C hybrids, is strongly effective for the developments of advanced bifunctional electrocatalysts exploited for water splitting in both acidic and alkaline media. Moreover, our approach could be extended to the other classes of electrocatalytic nanomaterials more stable for OER.

CONCLUSIONS

Herein, we rationalized the hybridization between multiple HER- and OER-active components to design and produce bifunctional electrocatalysts for water splitting, with superior performances compared to the ones of the single counterparts. More in detail, we first produced MoSe2 flakes through a modified liquid phase exfoliation method that utilizes H2O2 as oxidant to promote bulk MoSe2 exfoliation and nanopores formation in the basal plane of the exfoliated flakes. The control of H2O2 amount in the solvent used for the exfoliation process is crucial to obtain ideal high-ratio between edge and basal plane sites ratio, i.e., high-number of electrocatalytic sites. Then, MoSe2 flakes were


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hybridized with ball-like shaped Mo2C crystals to improve the electrocatalytic performance. In fact, Mo2C plays a dual functional role: 1) it facilitates the Volmer reaction at the interface with MoSe2 in acidic media, since it has strong hydrogen binding capability (i.e. ΔGH0 < 0);48,95,96,134 2) it promotes the H2O discharge (Volmer reaction) in alkaline media, whereas MoSe2 active sites favour the subsequent HER-pathways. Lastly, the electrochemical coupling between SWCNTs (as support) and MoSe2:Mo2C hybrids synergistically enhanced both HER- and OER-activity of the native components, reaching high ƞ10 in both acidic and alkaline media (0.049 and 0.089 V for HER in 0.5 M H2SO4 and 1M KOH, respectively; 0.197 and 0.241 V for OER in 0.5 M H2SO4, and 1M KOH, respectively). These results confirm the presence of synergistic effects occurring between HER- and OER-electrocatalysts, as often unwittingly reported when HER-electrocatalyst are hybridized with nanostructured carbon allotropes. Furthermore, we also pointed out that our electrocatalysts were produced as self-standing flexible electrodes through solution-processed manufacturing techniques, which are promising for their scalable high-throughput implementation.


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Overall, our work demonstrated the realization of synergistic bifunctional advanced electrocatalysts, providing key-guideline for their design. We believe that the rationalization of the synergistic electrocatalytic effects provides an approach towards the realization of advanced hybrid electrocatalysts for water splitting.

EXPERIMENTAL METHODS

Production and processing of 2D materials MoSe2 samples were produced in form of dispersion in IPA through a modified LPE of the MoSe2 bulk crystal (Smart element®). A solvent-mixed strategy, using a IPA/H2O2 mixture as dispersion solvent, was adopted.92–94 MoSe2-0, MoSe2-1, MoSe2-2 and MoSe2-3 were obtained using a H2O2 volume fraction of 0%, 0.1%, 0.2%, 0.3%, respectively. Subsequently, SBS was used to control the morphology of the samples.108,109,157 Experimentally, 30 mg of MoSe2 bulk were added to 50 mL of dispersion solvent and ultrasonicated by means of a bath sonicator (Branson® 5800 cleaner, Branson Ultrasonics) for a time of 6 h. The obtained dispersion was


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ultracentrifuged at 2700 g (Optima™ XE-90 having a SW32Ti rotor, Beckman Coulter) for 60 min at 15 °C. The ultracentrifugation process allows separating un-exfoliated bulk MoSe2 (collected as sediment) from the MoSe2 that remains in the supernatant. Then, 80% of the supernatant is collected, obtaining the MoSe2 sample. The concentration of MoSe2 dispersions was tuned to 0.4 g L-1 by evaporating the excess of solvent. Hybrid MoSe2:Mo2C dispersions were obtained by adding Mo2C powder (Sigma Aldrich, −325 mesh, 99.5%) to each MoSe2 dispersion in 1:2 MoSe2:Mo2C weight ratio. Mo2C dispersions were also produced by dissolving Mo2C powder in IPA at a concentration of 0.2 g L-1. The SWCNTs dispersions were produced by dispersing SWCNTs (> 90% purity, Cheap Tubes) in NMP with a concentration of 0.2 g L-1 by means of ultrasonication-based debundling.132,158 In particular, 10 mg of SWCNTs powder was added to 50 mL of NMP. The dispersion was then sonicated for 30 min by using a sonic tip (Vibra-cell 75185, Sonics) with vibration amplitude set to 45%. The sonic tip was pulsed for 5s on and 2 s off to reduce the solvent heating, which was also mitigated by an ice bath around the beaker.


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The dispersion of Pt/C was produced by dissolving 4 mg of Pt/C (10% wt%, Sigma Aldrich) and 80 µL of 5 wt% Nafion solution (Sigma Aldrich) in 1 mL of 1:4 v/v ethanol/water. The dispersion of RuO2 was produced by dissolving 1 mg of RuO2 powder (99.9% purity, Sigma Aldrich) and 20 µL of 5 wt% Nafion solution (Sigma Aldrich) in 380 µL of ethanol. Both the Pt/C and RuO2 dispersions were subsequently sonicated for 60 min before to be used. Materials characterization The BFTEM images were acquired with a JEM 1011 (JEOL) TEM (thermionic W filament), operating at 100 kV. The morphological and statistical analysis was done by using ImageJ software (NIH) and OriginPro 9.1 software (OriginLab), respectively. The lateral dimension of a flake was estimated as its maximum lateral dimension. Samples for the TEM measurements were prepared by drop casting the as-prepared MoSe2 dispersions onto ultrathin C-on-holey C-coated Cu grids and rinsed with deionized water and subsequently dried under vacuum for one night.


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The HRTEM images were acquired with a JEM-2200FS (JEOL) TEM (Schottky emitter), which operates at 200 kV, equipped with a CEOS image aberration corrector, an incolumn image filter (Ω-type) and an EDS spectrometer (Bruker, XFlash 5060 detector). Samples for STEM-EDS and HRTEM were prepared by casting the as-prepared MoSe2 samples onto an ultrathin carbon-on-holey carbon film/Cu grid (UHC/Cu). The samples were dried under vacuum overnight before the measurements. The presented STEMEDS elemental maps were obtained by integration of the Kα peaks for Mo, Se and O. The AFM images were acquired with a Nanowizard III (JPK Instruments, Germany), which is mounted on an Axio Observer D1 (Carl Zeiss, Germany) inverted optical microscope. The AFM measurements were carried out by means of PPP-NCHR cantilevers (Nanosensors, USA) having a nominal tip diameter of 10 nm. We used a drive frequency of ~295 kHz. We collected intermittent contact mode AFM images (512×512 data points) of 2.5×2.5 µm2 by keeping the working set point above 70% of the free oscillation amplitude. We used a scan rate of 0.7 Hz for the acquisition of the images. JPK Data Processing software (JPK Instruments, Germany) was exploited to process the height profiles, while the data were analysed by using OriginPro 9.1 software. The latter


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was also used to carry out the statistical analysis on multiple AFM images for all the tested samples. The latter were prepared by drop-casting the MoSe2 dispersions onto mica sheets (G250-1, Agar Scientific Ltd.) and dried under vacuum. The X-ray photoelectron spectroscopy characterization was performed with a Kratos Axis UltraDLD spectrometer, having a monochromatic Al Kα source (15 kV, 20 mA). The spectra were acquired onto a 300×700 µm2 area. We used constant pass energy of 160 eV and energy step of 1 eV to collect wide scans. High-resolution spectra were acquired at constant pass energy of 10 eV with energy step of 0.1 eV. We referenced the binding energy scale to the C 1s peak at 284.8 eV. The spectra were then analysed using the CasaXPS software (version 2.3.17). The samples were prepared by casting the MoSe2 dispersions onto Si/SiO2 substrate (LDB Technologies Ltd) followed by a drying process under vacuum. Raman spectroscopy measurements were performed by using a Renishaw microRaman invia 1000 mounting a 50× objective, with an excitation wavelength of 532 nm and an incident power of 1 mW on the samples. For each sample, we collected 50


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spectra. The samples were prepared by drop casting the MoSe2 dispersions onto Si/SiO2 substrates and subsequently dried under vacuum. Electrodes fabrication The electrodes on GC were produced by drop casting the MoSe2, Mo2C and MoSe2:Mo2C dispersions onto the GC substrates (Sigma Aldrich). The active material mass loading of the electrodes was 0.2 mg cm-2. The heterostructures (SWCNTs/MoSe22 and SWCNTs/MoSe2-2:Mo2C) were produced by depositing the SWCNTs and MoSe2 or MoSe2:Mo2C dispersions onto the nylon membranes (Whatman® membrane filters nylon, 0.2 μm pore size) through vacuum filtration process. The measured mass loading was 1.31 mg cm-2 for SWCNTs and 0.78 mg cm-2 for MoSe2-2 and MoSe2-2:Mo2C. The electrode area was 3.14 cm2. Before the electrochemical characterization, the asprepared electrodes were dried overnight at room temperature. Electrodes made entirely of SWCNTs were also produced as reference with a mass loading of 1.31 mg cm-2. Electrodes of Pt/C and RuO2 were produced as benchmark for HER and OER, respectively, by depositing the corresponding dispersion onto GC substrates (Sigma


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Aldrich). The active material mass loading of the electrodes was 0.262 mg cm-2 for Pt/C and 0.1 mg cm-2 for RuO2, similar to standard protocols.159 Electrodes characterization The SEM analysis was carried out using a Helios Nanolab® 600 DualBeam microscope (FEI Company), with measurement conditions of 10kV and 0.2 nA. The samples were characterized without any metal coating process or pre-treatment. The electrochemical measurements were performed at room temperature in a flatbottom fused silica cell using the three-electrode configuration of the CompactStat potentiostat/galvanostat station (Ivium), controlled via Ivium's own IviumSoft. A Pt wire and a saturated KCl Ag/AgCl were employed as counter-electrode and reference electrode, respectively. The measurements were carried out in 200 mL of 0.5 M H2SO4 (99.999% purity, Sigma Aldrich) or 1 M KOH (≥ 85% purity, ACS reagent, pellets, Sigma Aldrich). 30 min before starting the measurements, the oxygen was purged from electrolyte by flowing N2 gas throughout the liquid volume using a porous frit. The Nernst equation: ERHE = EAg/AgCl + 0.059×pH + E0Ag/AgCl, where ERHE is the converted potential vs. RHE, EAg/AgCl is the experimental potential measured against the Ag/AgCl reference


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electrode, and E0Ag/AgCl is the standard potential of Ag/AgCl at 25 °C (0.1976 V vs. RHE), was used to converte the potential difference between the working electrode and the Ag/AgCl reference electrode to the reversible hydrogen electrode (RHE) scale. The polarization curves were acquired at 5 mV s-1 scan rate and were iR-corrected by considering i as the measured working electrode current and R as the series resistance arising from the working electrode substrate and electrolyte resistances. Electrochemical impedance spectroscopy (EIS), at open circuit potential and at frequency of 10 kHz, was exploited to measure R. The stability tests were performed by chronoamperometry measurements (j-t curves), i.e., by measuring the current in potentiostatic mode at fixed overpotential over time.

ASSOCIATED CONTENT

Supporting Information. Water splitting mechanism; SEM and AFM of electrodes (Figure S1 and Figure S2); OER-activity for GC-based electrodes (Figure S3); ƞ10 for


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HER of the MoS2:Mo2C-based electrocatalysts and other relevant noble metal-free electrocatalysts reported in literature.

AUTHOR INFORMATION

Corresponding Author * Tel: +39 01071781795. E-mail: [email protected]

Author Contributions The manuscript has been written with the contributions of all authors, which have given approval to the final version of the manuscript. ‡These authors contributed equally.

Funding Sources

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no.785219-GrapheneCore2.

ACKNOWLEDGMENT


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This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 785219—GrapheneCore2. We thank Sergio Marras (Materials Characterization Facility – Istituto Italiano di Tecnologia) for support in XRD data acquisition and analysis; and the Electron Microscopy facility – Istituto Italiano di Tecnologia for support in TEM data acquisition.

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TOC


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TOC description Carbon nanotube-supported MoSe2 holey flake:Mo2C ball hybrids are reported as efficient bifunctional pH-universal water splitting electrocatalysts. The aware exploitation of the synergistic effects occurring between multi-component electrocatalysts, coupled with the production of our electrocatalysts through scalable and cost-effective solutionprocessed manufacturing techniques is promising to scale-up the production of hydrogen

via efficient water splitting.


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Carbon Nanotube-Supported MoSe2 Holey Flake:Mo2C Ball Hybrids

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