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Boron and Nitrogen Co-doped Molybdenum Carbide Nanoparticles Imbedded in BCN Network as a Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution Reactions Mohsin Ali Raza Anjum, Min Hee Lee, and Jae Sung Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01794 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Boron and Nitrogen Co-doped Molybdenum Carbide Nanoparticles Imbedded in BCN Network as a Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution Reactions Mohsin Ali Raza Anjum,†,‡Min Hee Lee† and Jae Sung Lee*† †

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan

National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919 Republic of Korea. ‡

Chemistry Division, Directorate of Science, Pakistan Institute of Nuclear Science and

Technology (PINSTECH), P.O. Nilore, Islamabad, 45650 Pakistan.

Abstract

Boron and nitrogen co-doped molybdenum carbide nanoparticles imbedded in a B, N-doped carbon networks (B,N:Mo2C@BCN) are synthesized as a noble metal-free hybrid electrocatalyst via an eco-friendly organometallic complex of Mo-imidazole and boric acid. When used as a bifunctional electrocatalyst for the hydrogen evolution (HER) and oxygen evolution (OER) reactions in an aqueous alkaline solution, the B,N:Mo2C/BCN catalyst displays high activity and

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stability in basic electrolytes, better than noble metal-based Pt/C, IrO2, and previously reported transition metal carbides-based electrocatalysts. The mechanistic study reveals that the enhanced performance of the hybrid material is attributable to the improved charge transfer characteristics as well as increased active surface areas owing to its modified electronic structure by B and N co-doping and formation of tiny nanoparticles imbedded in BCN networks. The synthesis approach employed in this study could also be suitable for tuning properties of other transition metal carbides for use as electrocatalysts.

KEYWORDS: Molybdenum carbide; B and N co-doping; reversibility; bifunctional electrocatalysts; water electrolysis; organomettalic complex.

The water electrolysis has attracted a great attention recently as a key technology for sustainable hydrogen production by using electricity generated from a renewable energy.1-3 A critical challenge in practical water electrolysis is to replace the precious Pt-based catalysts for hydrogen evolution reaction (HER) and Ir/Ru-based catalysts for oxygen evolution reaction (OER) with non-precious metals having similar performance and stability.4 To achieve this goal, much effort has been exercised to reduce overpotentials of HER kinetics using cheaper earthabundant materials such as metal carbides, sulfides, borides, nitrides, and phosphides,4-7 and of OER with metal phosphates, perovskites, chalcogenides, oxides/hydroxides and phosphides.1, 8-9 Although commercially available alkaline electrolyzers offer higher system efficiency than acidic proton exchange membrane electrolyzers, the intrinsic rate of HER in alkaline solutions is 2-3 orders of magnitude lower than that in acidic solutions.7, 10 The development of efficient and


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durable non-noble metal HER and OER electrocatalysts for alkaline electrolyzer has a great fundamental and practical significance. Transition metal carbides have received a great research interest as active materials for electrochemical applications for HER,11-13 OER14 and ORR15 owing to their noble metal-like surface

reactivity,16

excellent

electrical

and

thermal

conductivities,

mechanical

strength/hardness, and their chemical stabilities in corrosive environment.17 In particular, the molybdenum carbides are extensively investigated especially for HER in both acidic and basic solutions.18-19 Attempts have been made to tune their electrochemical properties for improved catalytic performance by; i) making nanoporous structure,20-23 ii) doping with heteroatoms such as N, S, P or dual doping,17, 24-25 iii) MoS2/Mo2C heterostructures26-27 and iv) decorating with transition metals (Ni, Co, W or Pt).28-30 Most of these efforts were directed to enhanced HER performance, yet molybdenum carbides have not received much attention as HER/OER bifunctional water splitting catalysts until recently.14, 31 Hence, there is much room to improve their OER activity to the level of Ru/Ir oxides. Some reports have shown that coupling Mo2C with other transition metals (Co or Ni)32-33 or making bimetallic (Co, Mo) carbide-based nanocomposites34 improves significantly the OER performance. In fact, Mo2C-based catalysts are converted to actual OER-active Mo-oxides by in-situ electro-oxidation during OER,32 which already have been demonstrated as a good OER catalyst.35 The presence of heteroatoms (e.g. B and N) in Mo-oxides structure could increase its OER activity and stability upon in-situ oxidation heteroatom-doped Mo2C. Thus, the same doping strategy is equally effective to improve both OER and HER performance. Herein, we report a simple strategy to synthesize B and N co-doped molybdenum carbide nanoparticles imbedded in a B, N-doped carbon (BCN) network (B,N:Mo2C@BCN), which


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displays excellent electrocatalytic HER and OER performances in alkaline media. The hybrid electrocatalyst is prepared by single-step annealing of the mixture of Mo-imidazole complex with an optimized amount of boric acid (H3BO4), leading to restrained in-situ carburization/doping reactions between Mo-imidazole complex and H3BO4. The unique B,N:Mo2C@BCN hybrid structure exhibits excellent HER/OER performance in alkaline electrolytes better than noble mental-based Pt/C, IrO2 and most of molybdenum carbides reported in the literature. The exceptional performance is related to formation of tiny nanoparticles, low charge transfer resistances and enhanced wetting properties due to dual doping and the B,N:Mo2C@BCN hybrid structure.

RESULTS AND DISCUSSION Mo-imidazole-assisted synthesis of B, N co-doped Mo2C nanoparticles imbedded in BCN network.. The molybdenum complexes with imidazole (ImH) and its substituted derivatives (MoL3X3, L=imidazole, X=Cl-, Br-, NCS-) have been widely studied due to their several biologically important aspects.36 It is proposed that the tertiary nitrogen (-N=C-) of imidazole binds metal (Mo, Co) ions rather than the secondary nitrogen (-N-H).36-37 These Mo-complexes could be used as precursors for synthesis of molybdenum carbides according to the process illustrated in Scheme 1, and described in detail in Experimental Section. Thus, a brownish molybdenum-imidazole complex [Mo(ImH)1-xClx] was synthesized by aging the imidazole monomer with MoCl5 at 80 oC for 24 h according to Eqn. 1. Then the complex was annealed along with boric acid (H3BO4) at ramping temperatures to 900 ⁰C under Ar atmosphere to synthesize in a single step B and N co-doped Mo2C nanoparticles (NPs) imbedded in a B, Ndoped

carbon

(BCN)

network

denoted

as

B,N:Mo2C@BCN.


The

simultaneous

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carburization/doping process forms Mo2C NPs imbedded in carbon network doped with N and B. For comparison, N-doped (N:Mo2C@NC) and undoped Mo2C@C NPs were also synthesized by just annealing [Mo(ImH)1-xClx] complex under Ar or H2 environment, respectively. ~

1

Scheme 1: Schematic illustration of [Mo(ImH)1-xClx] complex-assisted synthesis of B and N codoped Mo2C nanoparticles imbedded in a B, N-doped carbon network (B,N:Mo2C@BCN).

To identify chemical bonding in Mo(ImH)1-xClx, Fourier transform infrared (FTIR) spectra were recorded in Figure 1a. The characteristic bands at 742, 968 and 1068 cm−1 are assigned to single and doubly bonded Mo with O, and bands at 1631 cm−1 (–C=C), 3143 cm−1 (–C–H) and 3415 cm−1 (−N−H) are related to the imidazole functional groups. After formation of the [Mo(ImH)1-xClx] complex, the C−N band at 1056 cm−1 disappears, indicating that the imidazole has made a bond to Mo through its tertiary nitrogen as reported in literature.36-37 The X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure 1b) confirms the presence of Mo, N, C and Cl species. A doublet at 231.4 and 234.6 eV in high resolution scan of Mo 3d and two peaks at 198.1 and 199.7 eV in chlorine 2p spectra indicate bonding between Mo-Cl as shown in Figure S1a,b of Supporting Information (SI). The other doublet at 232.5 and 235.5 eV in Mo 3d


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are assigned to MoO3. The signals at 284.7, 285.8 and 287.9 eV in C 1s (Figure S1c) are ascribed to –C=C, –C=N and –C–N–X of imidazole. Similarly, the peaks at 395.9, 397.9, 400.3 and 402 eV in Figure S1d correspond to Mo–N, pyridinic, pyrolic and graphitic N, respectively.38 Thermogravimetric analysis (TGA) in Figure 1c indicates that the whole quantity of imidazole is decomposed at ~200 °C while two decomposition steps are observed for Mo(ImH)1-xClx. The first degradation from 250 to ~700 °C represents the thermal decomposition of imidazole ligand to the carbon nitride matrix along with removal of H2O and CO2 as shown in Figure S1e. The second stage from 800 to 1000 °C can be attributed to the formation of nitrogen-doped Mo-carbides through reaction between Mo species with the carbon nitride matrix and the graphitization of remaining excess carbon materials, which is also doped with B and N forming a BCN matrix. Therefore, the temperature of 900 °C was chosen to synthesize all catalysts in this study. The Mo content was found to be 4.1 wt.%, 2.1 At. % and 9.2 wt. % by TGA in air, XPS and ICP, respectively as displayed in Table S1 of Supporting Information (SI). The crystal structure of Mo(ImH)1-xClx was also determined by powder X-ray diffraction (XRD) in Figure 1d. The complex displays amorphous-like structure with wide peaks at low angles of 7.7, 9.0 and 23.5 degrees indicating the organic-inorganic hybrids between imidazole and Mo. The orthorhombic β-phase of Mo2C NPs (space group: Pbcn, ICDD No. 01-071-0242) in both N:Mo2C@NC and B,N:Mo2C@BCN catalysts, is confirmed by XRD patterns after thermal decomposition of Mo(ImH)1-xClx at 900 °C. The B,N-doping does not significantly alter the bulk crystal structure of Mo2C. But B,N-doping decreased the crystal size of Mo2C NPs from 29.1 nm (pure Mo2C@C) to 17.6 nm (N:Mo2C@NC) and 8.6 nm for B,N:Mo2C@BCN as calculated by applying the Scherrer equation to X-ray line broadening in Figure S2. Conversely, the hydrogen treatment sintered Mo2C to bulk material by removal of nitrogen species as


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displayed in Figure S3. Scanning electron microscopy (SEM) image of the as-synthesized B,N:Mo2C@BCN catalyst (Figure S4) displays particles with a grain size of ~50 nm forming a closely interconnected porous B and N doped carbon network (BCN). The grain size increases without B-doping for N:Mo2C@NC (70 nm) and pure Mo2C@C (100 nm) as displayed in Figure S5.

Figure 1: : FTIR spectra (a), XPS survey (b) and TGA analysis in N2 and air (c) of Mo(ImH)1xClx. (d) XRD of Mo(ImH)1-xClx, N:Mo2C@NC and B,N:Mo2C@BCN.

Structural evolution of catalysts during HER/OER. The nature of chemical bonding and oxidation states of B,N:Mo2C@BCN NPs were investigated before and after 20 h of HER or OER by XPS in Figure 2 and Figure S6. Two prominent Mo 3d peaks at 228.2 and 232.4 eV for the fresh catalyst in Figure 2a can be assigned to 64 % of surface Mo2C species along with 21 % MoO2 (229.6, 233.6 eV) and 15% MoO3 (232.7, 235.1 eV) due to surface oxidation.39 The shift


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of Mo2C 3d5/2 and 3d3/2 peaks towards lower binding energy close to elemental Mo (228.0 eV)39 relative to pure β-Mo2C@C (228.9, 232.5 eV)40 (Figure S7) indicates more electronic clouds around Mo atoms, which provides a strong evidence of N and B doping into the β-Mo2C structure. The companionable role of B and N in Mo2C structure to dissociate water will be discussed later. The post HER Mo3d spectrum confirms that there is no significant alteration of the catalyst surface during HER from that of the fresh catalyst. In contrast, post-OER analysis of B,N:Mo2C@BCN catalyst suggests that around 64 % of Mo species are converted to Mo-oxides by the in-situ electro-oxidation of Mo2C during OER, which are actually responsible for OER activity as reported by previous reports for MoO2.32,

35

This in-situ oxidation of

B,N:Mo2C@BCN under OER conditions is further confirmed by the presence of an oxide peak at 529.9 eV along with prominent metal-carbonates and B-O peaks in the O1s spectra (Figure S6a). The peak positions and surface compositions calculated by integrating XPS peak areas are summarized in Table S2. The surface and bulk elemental composition of the B,N:Mo2C@BCN catalyst are derived from XPS and elemental anaylysis (EA), respectively. As XPS is a surface sensitive tecnique, the surface composition by XPS is naturally different from bulk composition determined by EA. The Table S3 shows that there is not much change in atomic ratio of N/B after HER/OER. However, post-OER catalyst displays a pronounced increment in O/Mo ratio from 1.6 (fresh) to 6.3 (after OER) due to dissociation of Mo-C bond and formation of OER-active MoOx species. The N1s XPS spectra (Figure 2b) give four peaks of Mo-N (396 eV), pyridinic N (398.2 eV), pyrrolic N (400 eV), graphitic N (402 eV) and one extra peak of N-O (404.2 eV) only for the post-OER catalyst. B1s spectra (Figure 2c) confirm that these species remain in the catalyst throughout HER and OER in the form of –B-C, –Mo-B, –B-O and –B-N. The –B-C, –N-C and –Mo-C


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signals in C1s spectra (Figure S6b) further confirm that all species are chemically bonded with each other to form B,N:Mo2C@BCN hybrid catalyst.

Figure 2: High-resolution XPS (a) Mo 3d, (b) N 1s and (c) B 1s spectra of fresh (bottom), postHER (middle) and post-OER (top). The B,N:Mo2C@BCN NPs were further characterized by high resolution transmission electron microscopy (HR-TEM) before and after OER for 20 h. In the fresh catalyst, Mo2C NPs with an average size of ~5 nm (range: 3-10 nm) are embedded in BCN network as displayed in Figure 3a. Figure 3b-d exhibits lattice fringes of 0.228 and 0.226 nm assigned to (102) and (002) facets and corresponding fast Fourier transform (FFT) of orthorhombic β-Mo2C. Elemental mapping by energy-dispersive X-ray spectroscopy (EDS)-STEM of as-prepared B,N:Mo2C@BCN NPs (Figure S8) clearly display uniform distribution of Mo, C, B and N throughout the nanoparticles. All the characterization data corroborate the morphology of fresh and post-HER B,N:Mo2C@BCN, i.e. B,N-doped Mo2C NPs imbedded in B, N-doped carbon (BCN) network. The electro-oxidation of B,N:Mo2C@BCN NPs during OER is also observed by HR-TEM images in Figure 3e-h. The clear lattice fringes are observed with an interplanar distance from


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0.34 nm for (011) plane of MoO2 (PDF#: 01-086-0135, space group: P21/c) and 0.27 nm for (002) facet of MoO3 (PDF#: 00-047-1081, space group: P21/c).

Figure 3. (a) TEM image of Mo2C NPs (~5 nm) imbedded in BCN network, (b) HR-TEM image. (c) Magnified HR-TEM images showing (102) and (002) planes, and (d) corresponding FFT patterns. (e) TEM image, and (f) HR-TEM image of in-situ oxidized B,N:Mo2C@BCN after 20h OER. Magnified HR-TEM images of (g) MoO2 and (h) MoO3 showing (011) and (002) planes and corresponding FFT patterns (insets). Electrochemical HER and OER over B,N:Mo2C@BCN catalysts. The HER performances of as-prepared Mo2C-based electrocatalysts measured in 1.0M KOH aqueous solution using a threeelectrode electrochemical cell are displayed in Figure 4a as iR-corrected polarization curves. Undoped Mo2C@C requires overpotentials of η10~270 mV and η100~600 mV to obtain cathodic currents densities of 10 and 100 mA/cm2, respectively, similar to Mo2C-based catalysts reported in the literature.31,

41

Doping with nitrogen (N:Mo2C@NC) improves HER performance to

η10~150 mV and η100~294 mV, which is further improved when both B and N are introduced into Mo2C to form B,N:Mo2C@BCN by lowering η10 and η100 values to 100 and 198 mV, respectively. In particular, η100 value is surprisingly much smaller than that of conventional stateof-the-art Pt/C catalyst (239 mV). The insignificant performance BCN network is a clear indication that Mo species are active sites. The promotional effect of B remains most effective


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when the N/B ratio is controlled at 1.5-1.8 by adjusting boric acid to Mo(ImH)1-xClx precursor ratio (X=4 in B,N:Mo2C@BCN-X) as in Figure S9. The reduction in current density with increasing of B content above the optimum is due to the formation of boride species by annealing at 900 oC, which is considered as less active and unstable in alkaline media relative to Mo2C as reported.19 As shown in Figure 4b, our B,N:Mo2C@BCN catalyst needs much lower overpotential (η10) to generate current density of 10 mA cm–2 in 1.0M KOH as compared to other reported Mo2C-based electrocatalysts in Table S4. The Tafel plots in Figure 4c and Figure S9b give the Tafel slopes; Mo2C@C (144 mV/dec), N:Mo2C@NC (97 mV/dec), B,N:Mo2C@BCN-3 (62 mV/dec), and Pt/C (40 mV/dec). The results suggest that HER on B,N:Mo2C@BCN follows the Volmer−Heyrovsky mechanism in alkaline media. The exchange current densities (J0) at zero overpotential and overpotential (η100) required for 100 mA cm-2 (Figure 4d) on different B,N:Mo2C@BCN-X (X=1-4) are also compared with Pt/C, pure Mo2C@C and N:Mo2C@NC catalysts. The comparable J0 values of B,N:Mo2C@BCN catalyst to Pt/C confirm that the HER kinetics over B,N:Mo2C@BCN become faster due to electronic or chemical promotional effects of B and N-doping into Mo2C lattice. The electrochemical surface areas (ECSA) were calculated from double layer capacitances (Cdl) measured at the electrolyte-electrode interface with cyclic voltammetry (CV) at different scan rates (20 to 100 mV s-1). As shown in Figure S10, Cdl and ECSA values of B,N:Mo2C@BCN-2 (20.51 mF/cm2, 512.8 cm2) are higher than other B,N:Mo2C@BCN-X, and N:Mo2C@NC (5.08 mF/cm2, 127 cm2). The results indicate that B and N co-doping modifies not only electronic properties but also physical properties like Cdl and ECSA by forming ultra-small Mo2C NPs, which are related with the number of exposed active sites. Electrochemical impedance spectroscopy (EIS) analysis at an overpotential of 160 mV (Figure S11) indicates the relatively


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smaller charge transfer resistance (Rct) values of B,N:Mo2C@BCN-X catalysts than that of N:Mo2C@NC, suggesting an improved charge transfer process at electrode-electrolyte interface due to dual B, N-doping. The electrochemical HER data of all prepared catalysts are summarized in Table S5 of SI. Another important performance factor, electrochemical stability was determined by the chronoamperometry (CA) test for 20 h at a static overpotential of 160 mV for B,N:Mo2C@BCN2, N:Mo2C@NC, and Pt/C. As displayed in Figure 4e, the B,N:Mo2C@BCN electrode exhibits excellent stability with a negligible decrease in current densities per hour (∆J/∆t) of 0.15 mA.cm2 -1

h as compared to Pt/C (1.5 mA.cm-2h-1). The η100 values decrease for B,N:Mo2C@BCN (198

to 184 mV) and N:Mo2C@NC (294 to 284 mV) while it increases for Pt/C from 239 to 318 mV after the 20 h stability test as in Figure 4f (inset). The reduced overpotential is related to the insitu reduction of surface oxides on Mo2C-based catalysts during HER. The linear sweep voltammetry curves in Figure 4f further confirm excellent stability of B,N:Mo2C@BCN catalyst even after a long period of 20 h operation, which is superior to commercial Pt/C.


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Figure 4: (a) iR-corrected polarization curves of HER in 1.0 M KOH over undoped Mo2C@C, N:Mo2C@NC, and B,N:Mo2C@BCN. (b) Overpotentials required for 10 mA cm-2 over reported Mo2C-based electrocatalysts listed in Table S4. (c) Tafel slopes. (d) Exchange current densities (right) and η100 values (left) of all as-prepared catalysts. (e) Chronoamperometry (CA) and (f) polarization curves before and after 20 h CA. Now, electrochemical OER performances of B,N:Mo2C@BCN catalysts are compared in 1.0M KOH with pure Mo2C@C, N:Mo2C@NC and a commercial IrO2 electrode for the same mass loading (~1.0 mg/cm2) on a nickel foam (NF) substrate. As in Figure 5a, B,N:Mo2C@BCN electrode requires a lower overpotential (η100~360 mV) to reach 100 mA/cm2 as compared to N:Mo2C@NC (380 mV), Mo2C@C (464 mV) and commercial IrO2 (435 mV). The OER performance is further compared with different levels of B, N-doping as shown in Figure S12. The overpotentials η10 are also compared with other recently reported Mo2C and Mo-based electrocatalysts in Figure 5b derived from Table S6, which demonstrate a remarable effect of B, N co-doping in Mo2C in lowering the overpotentials of OER in 1.0M KOH. As discussed above, B and N species remain in the catalyst (confirmed by XPS) even after in-situ oxidation of Mo2C during OER to exert the promotional effects. As displayed in Figure 5c, the Tafel slope of B,N:Mo2C@BCN (61 mV/dec) is much lower than those for IrO2 electrode (82 mV/dec), Mo2C@C (84 mV/dec) and N:Mo2C@NC (64 mV/dec). The exchange currents J0 and η100 values are also compared in Figure S13. Interestingly, the J0 values for B,N:Mo2C@BCN catalyst is almost the same or higher than those for IrO2, Mo2C@C and N:Mo2C@NC, indicating faster intrinsic OER kinetics. The B,N:Mo2C@BCN-2

requires

the

lowest

overpotential

(η100~360

mV)

relative

to

B,N:Mo2C@BCN-1 (372 mV), N:Mo2C@NC (380 mV), B,N:Mo2C@BCN-3 (379 mV), B,N:Mo2C@BCN-4 (383 mV) and commercial IrO2@NF (435 mV). The EIS measurements are also conducted for B,N:Mo2C@BCN catalysts at a constant overpotential of 350 mV in Figure S14. The lower charge-transfer resistance Rct of B,N:Mo2C@BCN-2 (~1.5 ohm) than those of 13 Environment ACS Paragon Plus

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N:Mo2C@NC (1.9 ohm), B,N:Mo2C@BCN-1 (1.81 ohm), B,N:Mo2C@BCN-3 (3.7 ohm) and B,N:Mo2C@BCN-4 (3.9 ohm) is related to the faster ions/electrons transfer on the B,N:Mo2C@BCN-2 electrode for water oxidation, confirming the promotional effect of dual B, N-doping for improved OER performance. The summary of electrochemical OER data is tabulated in Table S7 of SI. The stability test for 20-h OER was also performed by CA at a constant overpotential of ~350 mV for IrO2, N:Mo2C@NC and B,N:Mo2C@BCN electrodes in Figure 5d. The initial decrease in current density for Mo2C-based catalysts is related to in-situ formation of Mo-oxides. The current gets stabilized from 30 min for B,N:Mo2C@BCN, but IrO2 and N:Mo2C@NC electrodes show significant continuous decreases in the current. The overall rate of current loss of IrO2 electrode (0.585 mAcm-2h-1) is much higher than that of B,N:Mo2C@BCN (0.15 mA. cm-2h-1). The polarization curves were also compared (Figure 5e) before and after CA for 20 h. Only a minor change in η100 for B,N:Mo2C@BCN catalyst (Figure 5e, inset) displays an excellent stability for OER due to B, N co-doping. The electrochemical OER data in alkaline media of all catalysts are compared with other reported Mo-based electrocatalysts in Table S6 of SI. All above results of HER and OER activities of B,N:Mo2C@BCN catalyst have demonstrated that the B and N dual-doping has remarkably improved activity along with stability of both reactions that exceed performances of precious metal catalysts of Pt/C and IrO2 and most of Mo-based electrocatalysts reported in the literature as summarized in Table S4-S7. Next, we tested a whole electrolyzer cell in 1.0 M KOH by using B,N:Mo2C@BCN as both the anode and the cathode. For comparison, a cell with cathode-anode combination of Pt/C@NF IrO2@NF was also tested, as displayed in Figure 5f. The B,N:Mo2C@BCN electrolyzer displays an outstanding and stable performance for overall water splitting, exhibiting a much lower cell


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ACS Catalysis

voltage (E100=1.84 V) for current densities 100 mA/cm2, which is 110 mV lower than that for the Pt/C-RuO2 (E100=1.95 V) after the 20-h CA. Interestingly, there is a no loss in current density in the case of the B,N:Mo2C@BCN electrolyzer even after 20 h of continuous operation in contrast to the ≈60 % loss for the Pt/C-RuO2 electrolyzer at a cell voltage of 1.8 V, as shown in Figure 5f (inset). The overall rate of current loss of B,N:Mo2C@BCN electrolyzer (0.7 mA.cm-2h-1) is much lower than that of Pt/C@NF-IrO2@NF electrolyzer (2.3 mA.cm-2h-1), as detremined by choronoamperometry (CA) analysis in Figure 5g. The reversibility of B,N:Mo2C@BCN electrodes from OER (anode) to HER (cathode) catalysts was also tested by pulse chronopotentiometry. As displayed in Figure 5h, the B,N:Mo2C@BCN shows high reversibility up to 50 cycles over 30,000 seconds when the electrodes were reversed every 600 sec. The electrolyzer cell voltage still remains at a constant voltage of ≈−1.72 V for cathode and ≈1.71 V for anode in each reversed cycle to produce a constant density of 10 mA cm−2 for both HER and OER. The high reversibility between OER and HER of B,N:Mo2C@BCN can be attributed to the synergistic effect of BCN network and flexible valence changes of Mo (e.g., from oxidized Mo to reduced/metallic Mo and vice versa) on the surface of B,N:Mo2C@BCN NPs in the widepotential window.42 Therefore, a short delay near the zero voltage (HER/HOR region) was observed before going to next OER/HER potential region. This test further ensures that the B,N:Mo2C@BCN electrocatalyst indeed displays good bifunctional catalytic performance for both HER and OER, probably because of the BCN network that stabilizes Mo2C NPs.


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Figure 5: (a) iR-corrected polarization curves of OER in 1.0 M KOH for Mo2C@C, N:Mo2C@NC and B,N:Mo2C@BCN loaded on NF electrode. (b) Comparison of η10 values with reported Mo-based electrocatalysts enlisted in Table S5. (c) Tafel plots. (d) Electrode durability determined by CA. (e) LSV curves and comparison of η100 values (inset) before and after 20-h durability test. (f) Polarization curves of two-electrode electrolyzer using with cathode-anode combinations of B,N:Mo2C@BCN - B,N:Mo2C@BCN, and Pt/C - RuO2@NF at a sweeping rate of 2 mV/s and comparison of current densities at cell voltage of 1.8 V before and after CA (inset). (g) Electrolyzers stablity determined by choronoamperometry (CA). (h) Pulse chronopotentiometric curve for H2/O2 evolution in a B,N:Mo2C@BCN electrolyzer with the current density of −10/10 mA cm−2. The anode and cathode were reversed after a time interval of 600 s in 1 M KOH solution.

Mechanism of HER and OER over B,N:Mo2C@BCN catalyst. As confirmed by XPS, B and N elements are directly/indirectly attached to the C atom, which are then bonded to Mo atoms in B,N:Mo2C@BCN catalyst. Nitrogen is more electronegative than C and B, and


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ACS Catalysis

therefore withdraws electron density from C–B bond through C–N bond and then donates its lone pair electrons through sp3 hybrid orbital to vacant 3d orbital of Mo atom attached to adjacent C, and hence reduces the Mo oxidation state. As shown in Figure 6 (upper HER part), the electron deficient B adsorbs water by coordinating with lone pair electrons, which then weakens O–H bond (Eq.2). The aqueous proton ( ) and adsorbed on a Mo-C-N site then

combine to form a H2 molecule (Eq.3). The hydroxyl anion at B (OH–B) reacts with the nearby hydroxyl group (OH–Mo) of water molecule adsorbed on adjacent Mo atom to form O2 by releasing two protons as described in Eq. 4 and in the bottom OER part of Figure 6. These protons then recombine electrochemically at cathode to make H2 molecule as in Eq. 5.

! " # " !

" $

# " ! %& " !

(Eq. 2) (Eq. 3)

2 ! (Eq.4)

2$

(Eq. 5)

Figure 6: A proposed mechanism of overall water splitting on a B,N:Mo2C@BCN nanoparticle showing the synergestics effect of B and N attached to C atom of Mo2C for HER and OER.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

As noted, Mo and other elements (B, N and C) are partially oxidized during OER reaction as confirmed by XPS, TEM and elemental analysis. The heteroatoms (B, N, C and O) are still interconnected directly/indirectly with Mo atoms through their oxides (–COx, –NOx and –B–O) even after in-situ oxidation as confirmed by XPS spectra of post-OER B,N:Mo2C@BCN catalyst. It has been revealed that these species play critical roles in enhancing the OER performance of in-situ partially oxidized B,N:Mo2C@BCN catalyst. In order to confirm the importance of in-situ converted Mo-oxide, the OER activity was also compared with as-synthesized MoO2, a physical mixture of Mo2C+MoO2, and Mo2C@C NPs. As shown in Figure S15, the prepared MoO2 catalyst displays an inferior activity due to its lower electronic conductivity and already oxygen rich sites. When 50 wt% Mo2C NPs are physically mixed with MoO2, the overpotential is slightly reduced, which can be ascribed to increased conductivity due to presence of carbon and in-situ conversion of Mo2C to oxygen deficient MoO2. The OER activity is further increased when pure Mo2C@C NPs are used but still lower than in-situ oxidized B,N:Mo2C@BCN catalyst. The defects and polycrystalline nature of in-situ oxidized catalyst is confirmed by HRTEM image and corresponding FFT pattern in Figure S16. These results confirm the synergistic effect of B and N doping in Mo2C of B,N:Mo2C@BCN catalyst remains effective in in-situ produced oxides. CONCLUSIONS In summary, the electronic/chemical properties of Mo2C are regulated by doping N and B into its structure through a novel Mo-imidazole complex route, in which the monomer, imidazole, works as a source of C and N, and controls the particle size of Mo2C, while H3BO4 is the source of B. The as-synthesized bifunctional (HER and OER) B,N:Mo2C@BCN displays excellent electrocatalytic performance and remarkable stability during both HER and OER of water splitting in alkaline media. The performances exceed those of conventional noble metals like


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ACS Catalysis

Pt/C and IrO2, and other Mo2C-based electrocatalysts reported in the literature. A mechanistic idea on the role of B and N species as co-dopant in Mo2C structure for water electrolysis to generate H2 and O2 are also proposed, whereby the excellent performance of B,N:Mo2C@BCN is related to the improved charge transfer and wetting properties due to the modifications of their electronic structure by B and N co-doping, in addition to formation of tiny Mo2C NPs imbedded in the BCN network. The present unique synthesis method could also become a general approach to tune the electrochemical characteristics of other transition metal carbides for use as electrocatalysts for various applications.

EXPERIMENTAL SECTION Synthesis of B and N co-doped Mo2C nanoparticles. All chemicals were used as purchased without further purification. A Mo-imidazole complex was synthesized by slightly modifying previously reported methods.17,

36

Typically, 15 ml of aqueous MoCl5 solution (3.0 mmol,

Sigma-Aldrich) was gradually poured in an ethanoic solution (35 mL) of imidazole (10.0 mmol, Sigma-Aldrich) to form a brownish Mo-imidazole complex (Scheme 1) that precipitates in a three-neck round bottom flask (250 mL) equipped with a magnetic stirrer and nitrogen purging line. To control the grain size and reaction kinetics, 5 mL of glycerol (Sigma) was added to the above solution. The reaction mixture was heated to 80-90 oC until the removal of whole water and ethanol. After cooling the reactor, the brownish precipitates of Mo(ImH)1-xClx were collected and washed with plenty of ethanol to remove glycerol and unreacted species. Then, this freshly prepared suspension of Mo(ImH)1-xClx was dried at 60 oC to obtain the precursor powder. The precursor Mo(ImH)1-xClx powder was annealed under Ar gas with ramping temperatures (5 ⁰C/min) to a pre-optimized temperature (900 ⁰C), kept there for 4 h and then cooled down naturally to obtain N:Mo2C@NC NPs. For B and N co-doped molybdenum carbides 19 Environment ACS Paragon Plus

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(B,N:Mo2C@BCN-X) NPs, the different molar ratios of boric acid to Mo(ImH)1-xClx precursor (X= 1:4, 2:4, 3:4 and 4:4) was ground and annealed at the above conditions. For comparison, undoped Mo2C@C NPs were also prepared by using simply annealing in H2 atmosphere. Physical characterization. Crystallographic structure of the electrocatalysts was investigated by powder X-ray diffraction (XRD on PANalytical pw 3040/60 X'pert) with Cu Kα radiation. The morphologies were probed with field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800, 15 kV) and transmission electron microscope (TEM, JEOL, JEM-2100). The chemical compositions, elemental mapping and in-depth crystal information of samples were collected by high resolution transmission electron microscope (HR-TEM, JEOL, JEM-2100F). X-ray photoelectron spectroscopy (XPS, ThermoFisher, K-alpha) was used to identify the chemical states of the surface atoms. Electrochemical

measurements.

The

catalyst

ink

was

prepared

by

dispersing

B,N:Mo2C@BCN, N:Mo2C@NC and undoped Mo2C@C electrocatalysts (5.0 mg/mL) in an equal volume mixture of deionized water and ethanol, and 10 µL of 5 % Nafion solution. The working electrode for HER and ORR was prepared by drop casting of 10 µL sonicated ink onto a glassy carbon electrode (0.4-0.5 mg/cm2 loading) and drying at room temperature. The asprepared ink of nanoparticles (1.0 mg cm-2) was loaded on a nickel foam (NF) to make the electrodes for OER. Electrochemical HER, OER/ORR activity and stability tests were carried out in a three electrode cell configuration using a rotating disc electrode (RDE, PAR Model 636 RDE) attached to a potentiostat (Ivium technologies). An Ag/AgCl (3.0M NaCl) electrode and a graphite rod were used as reference and counter electrodes, respectively. All potentials were iRcorrected and referenced to the reversible hydrogen electrode (RHE) by the equation ERHE=E(Ag/AgCl) + 0.059pH + 0.20.


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ACS Catalysis

The HER performance was measured in aqueous 1.0 M KOH (pH 13.7) at a scan rate of 2 mV/s after 20 cyclic voltammetry (CV) cycles in the range of 0.4 to -0.3 VRHE. The electrochemical stability tests were conducted by performing chronoamperometry (CA) for 20 h. Electrochemical impedance spectroscopy (EIS) was conducted in the same setup in the frequency range of 100 kHz to 1 mHz with a modulation amplitude of 10 mV. To evaluate the electrochemical active surface area (ECSA), CV was conducted from -0.8 to -0.6 V in 1.0 M KOH vs. Ag/AgCl with different sweep rates between 20 to 100 mV s-1. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. HER performance comparisons with literature, summary of electrochemical data, elemental composition, XRD patterns, SEM images, EDS-STEM images, XPS spectra, ECSA calculations, polarization LSV curve, EIS data, and Tafel analysis (PDF). AUTHOR INFORMATION Corresponding Author *Jae Sung Lee, Professor: [email protected] Present Addresses Ulsan National Institute of Science & Technology (UNIST) 50 UNIST-gil, Ulsan, 44919 Korea. Notes The authors declare no competing financial interest. Author Contributions


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J.S.L conceived the project and oversaw all the research phases. M.A.R.A. conceived the idea for the study, designed the experiments, synthesized the materials, performed the electrochemical measurements and the XRD, XPS, SEM, and TEM analyses, and analyzed the data. M.H.L assisted with HR-TEM imaging. J.S.L. and M.A.R.A. wrote the manuscript and discussed the results. All the authors contributed and commented on the manuscript. ACKNOWLEDGMENT This work was supported by the Climate Change Response project (2015M1A2A2074663, 2015M1A2A2056824), the Basic Science Grant (NRF-2015R1A2A1A10054346), Korea Center for Artificial Photosynthesis (KCAP, No. 2009-0093880), Next Generation Carbon Upcycling Project (2017M1A2A2042517) funded MSIT, and Project No. 10050509 and KIAT N0001754 funded by MOTIE of Republic of Korea. This work was also supported by the 2017 Research Fund (1.170053) of UNIST.

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