Two-Dimensional Lamellar Mo2C for Electrochemical Hydrogen

Subscriber access provided by University of Sunderland

Energy, Environmental, and Catalysis Applications

Two-Dimensional Lamellar Mo2C for Electrochemical Hydrogen Production: Insights into the Origin of HER Activity in Acid and Alkaline Electrolytes Wenjin Yuan, Qing Huang, Xianjin Yang, Zhenduo Cui, Shengli Zhu, Zhaoyang Li, Shiyu Du, Nianxiang Qiu, and Yanqin Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13215 • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

ACS Applied Materials & Interfaces

Two-Dimensional Lamellar Mo2C for Electrochemical Hydrogen Production: Insights into the Origin of HER Activity in Acid and Alkaline Electrolytes

Wenjin Yuana, Qing Huangb, Xianjin Yanga,c, Zhenduo Cuia, Shengli Zhua,c, Zhaoyang Lia,c, Shiyu Dub*, Nianxiang Qiub*, Yanqin Lianga,c*

a School b

of Materials Science and Engineering, Tianjin University, Tianjin 300072, China

Engineering Laboratory of Nuclear Energy Materials, Ningbo Institute of Materials Technology and

Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China c Tianjin

Key Laboratory of Composite and Functional Materials, Tianjin 300072, China

*Corresponding author: [email protected] (Y. Q. Liang) *Corresponding author: [email protected] (S. Y. Du) *Corresponding author: [email protected] (N. X. Qiu)

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

Page 2 of 34

Abstract Developing high surface area Mo2C with certain crystal plane exposed is an efficient strategy but urgent challenge to optimize the HER catalytic performances. In addition, the effects of certain crystal faces on catalytic performance have been limitedly understood. Toward this end, (1 0 0) plane oriented 2D lamellar Mo2C transformed from carbon fiber is synthesized successfully in a molten salt system. Subsequently, the electrocatalytic properties towards HER show that (1 0 0) plane oriented Mo2C functions well in both acidic and basic media. The density functional theory calculations show that the most stable Mo/C termination of (1 0 0) plane contains multiple catalytically active centers. These closeto-zero ΔGH* values verify its better HER performance. Besides, the correlation between hydrogen adsorption behavior and the water dissociation process, as well as their corresponding roles in the overall acid and alkaline HER rate have been discussed in-depth. A simple mechanistic analysis is put forward to explain the favorable HER performance of lamellar structure -Mo2C in alkaline other than acid electrolytes. The molten salt method may provide a new way for developing electrocatalysts with oriented crystal faces.

Keywords: molybdenum carbides, molten salts, crystal faces, electrocatalysts, hydrogen evolution


Page 3 of 34 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


To satisfy the increasing global energy demand, hydrogen has been proved to be one of the advantaged candidates due to its environmental-friendliness and high energy density.1-5 The water-splitting reaction, being composed of two half-reactions (the hydrogen and oxygen evolution reactions, HER&OER), has aroused extensive concern as a sustainable source of hydrogen.6-8 But the practical applications of water splitting are greatly restricted due to high overpotentials of HER and OER.9 Precious metals (e.g., Pt, Pd, and their alloys) have proven to be the competent HER catalysts for making electrochemical hydrogen production feasible.10 However, their high cost and limited reserve on the earth restricted their scale-up application. Therefore, seeking for a more abundant alternative is urgently needed in HER catalytic field.11 Since Hu group reported that commercially available molybdenum carbide microparticles showed good catalytic activity toward HER,12 many efforts have been made to pursue more excellent catalytic properties such as constructing various nanostructures,13-15 doping with nonmetallic elements (e.g., N, S, and P)1, 16-21 or transition metals by altering the electronic and chemical properties (Fe, Co, Ni),22-25 introducing conductive supports26 and so on. Among different Mo carbides, i.e., Mo2C, MoC and Mo3C2, the -Mo2C has been suggested as an active catalytic phase for HER under both acidic and basic conditions, sometimes, the HER activity in basic media is even better than acidic one. This rare property for Mo2C is unusual in HER catalysts since most of the experimental findings reveal that kinetic reaction rate (in terms of j0) in alkaline solution is more sluggish than that in acidic solutions.27-28 However, the intrinsic mechanism of this opposite HER performance of Mo2C in acid and alkaline conditions is still inconclusive to date. Recent research has shown that building specific nanostructures is another way to increase HER activity of molybdenum carbide. Two-dimensional (2D) structures play a crucial role in determining their 3 ACS Paragon Plus Environment


Page 4 of 34

fundamental properties owing to their special structures and rich electronic properties catering for energy storage and conversion application, thus attracting great attention over the past few years. Even though Vojvodic and co-workers reported that 2D MXene materials of M2XTx (M = metal; X = (C, N); and Tx = surface functional groups) can be active and stable catalysts for the HER in acid,29 the controllable synthesis of 2D lamellar β-Mo2C with predominant exposure of a certain crystal plane is still highly challenging. Moreover, there have been no comprehensive studies so far that explore the surface chemical properties with HER catalytic activity on a range of high quality single crystal model catalyst of lamellar -Mo2C at both pH 0 and 14 conditions. Molten salts are widely used in many industrial processes. Due to their thermal stability, low vapor pressure and fast reaction rate, they are well adapted to high-temperature chemistry. Materials for energy storage devices can be successfully prepared by molten salt electrolysis: carbon nanoparticles with very large specific surface for efficient supercapacitors were obtained in molten alkali carbonates.30 It is presumed that the surface energies of polar surfaces can be effectively lowered by the electrostatic interactions in molten salts system and thus hinder the growth along the polar surfaces.31 At present, there are few studies on the preparation of electrocatalysts in molten salt system. Herein, we report a facial, one-step synthetic method which can directly grow preferentially oriented 2D -Mo2C with lamellar structure through unusual template-sacrificed strategy by using carbon fiber paper (CFP). The carbon fibers in CFP are converted directly into lamellar structure -Mo2C by carburization at high temperature in a molten salt system. Cations and anions are inclined to have strong electrostatic interactions with negative or positive charged polar crystal faces, which can lower the surface energy and slow down the growth rate of these polar faces so as to form exposed polar surfaces.32 Then the 4 ACS Paragon Plus Environment

Page 5 of 34 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


electrocatalytic properties towards HER of 2D -Mo2C with (1 0 0) oriented crystal face are investigated in both acid and alkaline solutions. The results show that lamellar structure -Mo2C with more orientation of (1 0 0) plane (L-Mo2C) exhibits higher HER electrocatalytic activities in basic condition than that in acid condition. In addition, the -Mo2C with particle-like structure (P-Mo2C) is also synthesized by same strategy except using different carbon source to give a comparison of HER activity based on different morphologies, which further confirms the superiority of 2D lamellar structure with more (1 0 0) exposed planes in HER performance than that of particle-like structure Mo2C. Density functional theory (DFT) calculations are applied to obtain intermediate adsorption energies and kinetics for a certain catalytic process based on the (1 0 0) plane. Then, we further investigate the correlation between catalytic activity and nature of active sites based on the hydrogen adsorption behavior and the water dissociation process, and present an in-depth discussion regarding to the unusual superior HER activity in basic solution than acid solution. Results and discussion Physical characterization of Mo2C on CFP catalysts 2D lamellar Mo2C and particle-like Mo2C are synthesized by a facial one step molten salt method (Figure 1 and Figure S3). It can be seen from Figures 1(a-c) that Mo2C exhibits a lamellar structure which is stacked along the radial direction of carbon fibers, while Mo2C obtained in charcoal activated powder presents a normal grainy morphology in Figure S3. It can be preliminarily inferred that Mo powder may be more likely to react with the CFP in the form of particle dispersions or droplets due to the liquid environment provided by the molten salt.33 With the addition of CNT as the carbon source, the carbon fibers in CFP are converted directly into -Mo2C lamellar structure by carburization at high temperature 5 ACS Paragon Plus Environment


Page 6 of 34

in a molten salt system, which may be due to the fact that the induction of CNT with high reactivity could decrease the surface energy of (1 0 0) plane in Mo2C, thus giving rise to a preferred orientation growth into the lamellar structure. While replacing the carbon source with charcoal activated powder, only a conventional grainy structure could be formed. Therefore, the morphology of Mo2C is predominately determined by addition of different carbon sources. The TEM images also confirm the lamellar morphology of L-Mo2C, as shown in Figure 1(d) and (e). The lattice fringes observed with the interplanar spacing of 0.26 nm are assigned to the (1 0 0) plane of Mo2C. In addition, the SAED pattern of L-Mo2C also clearly shows the lamellar Mo2C is single crystal. Meanwhile, the corresponding elemental mapping (Figure 1(f)) shows that Mo and C atoms are uniformly distributed throughout the entire sheet, besides a small amount of oxygen can also be observed on the surface of the sample. The crystalline structures of L-Mo2C and P-Mo2C are also studied by powder XRD. It can be seen from Figure 1(g) that P-Mo2C is mainly composed of two phases, carbon and -Mo2C. The diffraction peak at 26.5° is assigned to carbon in CFP. The other peaks observed at 34.4°, 38.0°, 39.4°, 52.1° and 61.5° are attributed to -Mo2C crystal faces of (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) , respectively (PDF 35-0787). It is noteworthy that the carbon typical peak in L-Mo2C is negligible but the peak intensity centered at 34.4° is obviously stronger than that of the P-Mo2C. The specific intensity values are shown in Table1. Therefore, it can be inferred that L-Mo2C can be similarly considered as a single phase in which almost all the carbon fibers could be converted into lamellar -Mo2C in the molten salt system, and the addition of CNT is more likely to promote the orientation of (1 0 0) face in Mo2C compared with the addition of charcoal activated powder. In comparison, particle-like Mo2C consists of 6 ACS Paragon Plus Environment

Page 7 of 34 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


two phases: carbon in fiber core and Mo2C coating on the surface of fibers, due to the incomplete transformation from CFP to P-Mo2C. The Raman spectra in Figure 1(h) further confirms the formation of Mo2C in L-Mo2C catalysts, which shows the same Raman typical peaks as commercial Mo2C powders. AFM images of L-Mo2C (Figure 1(i) and (j) show a single layer with thickness of 15 nm and average lateral size of several hundred nanometers, which are consistent with the SEM and TEM results.




Page 8 of 34

Page 9 of 34 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


Figure 1. (a) SEM, (b) high magnificent SEM and (c) cross-section SEM images of L-Mo2C. (d) low resolution and (e) high resolution TEM images of L-Mo2C. The inset in (d) shows the selected area electron diffraction pattern. (f) TEM image and the corresponding elemental mapping images of Mo, C and O of L-Mo2C, (g) XRD patterns of LMo2C and P-Mo2C, (h) Raman spectra of L-Mo2C and commercial Mo2C powders, (i) AFM image and (j) 3D AFM image of L-Mo2C. The inset in (i) shows the thickness of the sample.

Table 1 XRD peak intensity ratio of different crystal faces Material

(1 0 0)

(0 0 2)

(1 0 1)

(1 1 0)

20

25

100

17

L-Mo2C

47.5

19.9

100

32.6

P-Mo2C

21.3

22.2

100

17.2

Mo2C (PDF 35-0787)

Electrocatalytic hydrogen evolution activity The electrocatalytic HER activities are investigated in both acidic media (0.5 M H2SO4 solution) and basic media (1M KOH) by employing a typical three-electrode setup. The HER polarization curves in Figure 2(a) show that L-Mo2C in 0.5M H2SO4 exhibits an overpotential of 170 mV at -10 mA cm-2 (10). While a much smaller 10 of 95.8 mV are required for the L-Mo2C in 1M KOH. The commercial Pt/C shows excellent performance in acid media. But when it comes to the basic media, the current density of L-Mo2C is lower at the same potential in the beginning, but it increases much faster than the Pt/C catalyst. The Tafel plots in Figure 2(b) verify this results. Tafel slopes of L-Mo2C in 0.5 M H2SO4 and 1M KOH are approximately 77 and 99 mV dec-1, respectively. The Tafel slopes indicate that the hydrogen 9 ACS Paragon Plus Environment


Page 10 of 34

evolution might be based on the Volmer-Heyrovsky mechanism,34 which will be discussed in detail in the following section. The exchange current densities (j0) of L-Mo2C are 0.054 mA cm-2 in 0.5 M H2SO4 and 1.089 mA cm-2 in 1M KOH. In catalysis, an effective figure of merit to compare the intrinsic activities of different catalyst materials is by means of their turnover frequency (TOF), that is, the number of H2 molecules evolved per second per active site.35 The number of active sites is inferred from the electrochemically active surface area (ECSA) (see the Supporting Information for details on the calculation of the TOF values). Figure 2(c) shows the TOF values of L-Mo2C in acid and basic media as a function of the applied potential. It can be seen that L-Mo2C in basic media has a relatively higher TOF value, indicating better intrinsic activities than in acid solution. The Electrochemical Impedance Spectroscopy (EIS) curves shown in Figure S4 also reveal the advantagous electrocatalytic kinetics in basic media than acid. In order to illustrate the superior of L-Mo2C in HER activity, the electrochemical performance of P-Mo2C counterpart is also evaluated in both electrolytes. It is found that L-Mo2C shows a smaller Tafel slope, lower 10 and charge transfer resistance, as well as a higher TOF value than PMo2C, suggesting that basal planes in L-Mo2C are also active in HER process (Figure S5-S8 in the supporting information). The stability of L-Mo2C in acid and basic media was also characterized (Figure S9-S11). It is more stable in acid media than that in basic media after operation for 12h. But the performance can be recovered in basic media when having a retest, which indicates that the degradation of current is mainly caused by the mass transfer limitation instead of catalyst dissolving. To analyze the surface state of L-Mo2C during the electrochemical process, XPS spectra are investigated before and after operation in acid and basic media (Figure 2(e) and (f)). The Mo 3d core level XPS spectra suggest that there are four states for Mo species (Mo−C, Mo3+, Mo4+, and Mo6+) on 10 ACS Paragon Plus Environment

Page 11 of 34 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


the surface of the as-prepared L-Mo2C carbides.36-39 After operated in 0.5 M H2SO4 (Oper-H2SO4) or 1M KOH (Oper-KOH), the content of Mo6+ decreases obviously, while the Mo−C and Mo3+ increase compared with the as-prepared L-Mo2C. The C 1s spectra also prove the increase of Mo−C. It is probably caused by the electrochemical reduction process and/or decomposition of Mo oxides during the test. The XPS fitting data are listed in Table S1. The content of Mo3+ in Oper-KOH is higher than Oper-H2SO4, demonstrating more amount of Mo3+OOH can be evolved in the basic media. The CV curve conducted in basic media exhibits a distinct reduction peak, in which the negative current density is much higher than that in acid media in Figure 2(d). The ICP analysis (Figure S12) further reveals that Mo species diminished more in acid solution, indicating that the decrease of Mo species with high oxidation state in basic media is primarily caused by cathodic polarization reduction other than Mo oxides dissolution. While, the decrease of Mo6+ in acid electrolyte after operation is mainly induced by the dissolving of Mo oxides in acid.



Page 12 of 34

Figure 2. (a) Polarization curves and (b) Tafel plots of L-Mo2C and commercial Pt/C tested in 0.5M H2SO4 and 1M KOH respectively. (c) TOF values and (d) CV curves of L-Mo2C. XPS spectra of (e) Mo 3d and (f) C 1s peaks for asprepared L-Mo2C, L-Mo2C operated at -0.2V vs RHE for 30 min in 0.5M H2SO4 and L-Mo2C operated at -0.2V vs RHE for 30 min in 1M KOH.

Thermochemistry of HER by DFT calculations 12 ACS Paragon Plus Environment

Page 13 of 34 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


DFT calculations are performed to evaluate the HER activity of (1 0 0) plane in Mo2C which is mainly exposed in our experiment (except (1 0 1) plane). Generally, a catalyst with ΔGH* ≈ 0 could exhibit excellent performance for the hydrogen evolution reaction. Several possible sites of hydrogen adsorbed on different L-Mo2C terminations have been relaxed to obtain the stable possible hydrogen binding configurations and the corresponding Gibbs free energies ΔGH* (Figure 3). For the Mo termination, the hydrogen atom tends to adsorb on the hollow sites with 3-fold coordination bond (i.e. HM1 and HM2 in Figure 3(a) insert). They have negative ΔGH* values of -0.35 and -0.63 eV, respectively, which indicates a strong Mo-H bonding on Mo termination, hindering the Heyrovsky step of HER. For the C termination, the H atom can bind to a C atom on TC site with a negative value of ΔGH* (-0.58 eV) due to the 3-fold coordinated C atom. And the H atom on HM1 adsorption site coordinates with two Mo atoms in the second layer and one Mo atom in the third layer, while the H atom on HM2 site is bound to one Mo atoms in the second layer and two Mo atoms in the third layer. As shown in Figure 3(c), the calculated ΔGH* values of hydrogen on HM1 and HM2 sites are -0.034 and 0.46 eV, respectively, indicating that the HM1 adsorption site has considerably high catalytic activity for HER. In addition, the energy difference between the HM1 and HM2 sites may be due to the fact that the Mo atom in second layer (0.68 e) possesses more charge and has much stronger binding ability than the one in third layer (0.35 e). With respect to mix Mo/C termination (Figure 3(b)), there are four possible stable adsorption sites (TC, TM, BM and HM). The H atom coordinates with one C atom in the second layer and one Mo atom in the first layer for the TC and TM sites with ΔGH* values of -0.066 and -0.081 eV, respectively. The BM site with ΔGH* of -0.091 eV links one Mo atom in the first layer and one Mo atom in the third layer, and the 2-fold HM site has two Mo atoms in the first layer with ΔGH* of -0.63 eV. This indicates that the TC, TM and BM sites all have 13 ACS Paragon Plus Environment


Page 14 of 34

excellent catalytic activity for HER. Moreover, the C atoms on the surface exhibit better activity than the exposed metal active sites, which avoids the catalytic inactivation of Mo atoms due to the saturation of Mo by the residual carbon atoms during the carbonation process.40 The second stable C-terminal of (1 0 0) plane in Mo2C even exhibits better catalytic activity due to its lower value of ΔGH*. In whole, the mainly exposed (1 0 0) plane of Mo2C exhibits great catalytic performance for HER, which is in excellent agreement with the experimental data.


Page 15 of 34 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


Figure 3. The Gibbs free energy change ΔGH* of H* adsorbed on (a) Mo termination, (b) mixed Mo/C termination and (c) C termination of (1 0 0) plane in Mo2C. Inserts show the possible stable adsorption sites.



Page 16 of 34

Figure 4. Schematic of distinct mechanisms in acid and alkaline media of L-Mo2C (H1, H2, Mo1, Mo2 represent the hydrogen atom in H2O, the hydrogen ion in acid, the molybdenum atom in Mo2C and the molybdenum atom in MoOx, respectively)

Correlation of HER activity and oriented planes In order to give an in-depth investigation of the kinetics of cathodic HER, it is necessary to unveil the mechanism of the reaction by correlating theoretical with experimental results. The intrinsic activity for HER is usually determined by several parameters, e.g. overpotentials, Tafel slopes, exchange current densities, and TOF values. These characterization techniques reveal that HER kinetics is strongly influenced by its reaction pathway, which reduces protons into hydrogen in acid medium (reduces water if in alkaline medium), as shown in the three following steps:41 i) electroreduction of hydroxonium ion or water molecules with hydrogen adsorption; Volmer reaction ii) electrochemical hydrogen desorption; Heyrovsky reaction iii) chemical desorption; Tafel reaction (a) Volmer reaction [Equation (1), (2)] 16 ACS Paragon Plus Environment

Page 17 of 34 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


H+ + M + e− ⇌ M−H* (acid solution)

(1)

H2O + M + e− ⇌ M−H* + OH− (alkaline solution)

(2)

followed by (b) Heyrovsky reaction [Equation (3), (4)] M−H* + H+ + e−⇌ M + H2 (acid solution)

(3)

M−H* + H2O + e−⇌ M + OH− + H2 (alkaline solution)

(4)

or (c) Tafel reaction [Equation (5)] 2 M−H* ⇌ 2M + H2 (both acid and alkaline solutions)

(5)

Where H* designates a hydrogen atom chemically adsorbed on an active site of the electrode surface (M). Practically, a small Tafel slope corresponds to a steep rise of the electrocatalytic current density by applying certain overpotential. Tafel slope value from the HER polarization curve is commonly taken as an indication of determining the possible HER rate-controlling steps and may provide some valuable insight into possible reaction pathways. For platinum in present work, the reaction proceeds via a Volmer-Tafel or Volmer-Heyrovsky mechanism (supported by Tafel slope of 32 and 157 mV dec-1) in acid and alkaline media. While Tafel slopes of L-Mo2C obtained in acid and basic solutions (77 and 99 mV dec-1) indicate that Volmer-Heyrovsky mechanism is dominated in both electrolytes. It is noteworthy that higher Tafel slopes have been observed on both Pt/C and L-Mo2C in alkaline media compared to those in acidic media. It is generally considered that only proton reduction into an adsorbed hydrogen atom is weighed in Volmer step to evaluate the HER kinetic properties, thus suggesting the catalytic 17 ACS Paragon Plus Environment


Page 18 of 34

activity is mainly determined by the adsorption energy of the hydrogen atom. In contrast, the catalyst need to break the H–O–H bonding firstly in alkaline electrolytes before adsorbing free H+ ion, thereby the Volmer step not only correlates to the adsorption energy of the hydrogen atom, but also to the adsorption energy of water, water dissociation, as well as the desorption energy of the hydroxide anion in alkaline condition. Therefore, low water adsorption energy can lead to insufficient reactants. Energy barriers introduced by water dissociation process are very likely to govern the overall reaction rate, and high hydroxide anion adsorption energy also causes a poisoning effect owing to a loss of catalytically active site. Even though the Pt/C catalyst starts to catalytically function earlier than L-Mo2C does and requests smaller overpotential to reach current density of 10 mA cm-2 in alkaline solution, the later one presents even more steeper rising of the cathodic current density in the same range of potential than that of the former one, which suggests that L-Mo2C is supposed to be a promising substitution of noble metal catalysts in basic condition. The j0 is another key parameter in determining electrocatalytic activity, which can give a description of reaction rate at the equilibrium potential. These parameters e.g. j0 and Tafel slope are not entirely independent of each other. It is more often to see that HER electrocatalysts with lower Tafel slopes appear a smaller j0, and vice versa. The performance of L-Mo2C in acid and alkaline condition happens to be in accordance with this fundamental rule. Therefore, the potential to achieve a current density of 10 mA cm-2 is commonly used to discern and quantify HER activity. Following this principle, the HER process of L-Mo2C in alkaline condition is more favorable in term of overpotential of 95.8 mV at 10 mA cm-2 than that of 170 mV in acid condition. ΔGH* is considered as one of the key descriptors in theoretical prediction of the optimal catalytic activity of catalysts, which means that the targeted surface should have well balance between hydrogen bonding and releasing 18 ACS Paragon Plus Environment

Page 19 of 34 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


properties since either too strong or too weak hydrogen adsorption and desorption on the electrode surface go against the HER process. Compared with the P-Mo2C, L-Mo2C with a larger proportion of (1 0 0) basal planes exposed shows a significant improved HER activity in both acid and alkaline media. As expected, L-Mo2C exposes higher basal plane surface area than P-Mo2C, supported by larger electrochemically active surface area in Figure S5 and S7. The improved HER activity compared to that of P-Mo2C, in terms of low overpotential, high TOF, as well as gentle Tafel slope suggests that, unlike MoS2 catalysts,42 the basal planes of L-Mo2C are indeed catalytically active, and this is also borne out by DFT calculation results that H atoms sited in mixed Mo/C termination exhibiting close-to-zero ΔGH*. From detailed discussion based on experimental and theoretical investigation above, we can safely infer that the water dissociation barrier, or ΔGH*, would play a crucial part, possibly in concert with factors affecting the overall rate in the alkaline electrolytes. What’s more, the intrinsic per-site activity of a catalyst described by TOF value is also a significant descriptor in evaluating the HER catalysis activity among different catalyst materials, which basically involves the quantity of catalytically active sites on the surface. Herein, TOF value is normalized by total amount of surface sites in the whole electrochemically active surface area obtained by CV measurements, which means that the higher the ECSA of the catalyst, the lower the TOF (assuming at the same current density). Practically, for L-Mo2C catalysts, the number of hydrogen molecules evolved per second per active site in alkaline solution is much higher than that in acid solution when the values are taken at the same potential range. There are mainly two probable reasons contributing to this phenomenon. i) It is noticed from Mo 3d XPS spectra that Mo2C before operation is suffered from surface oxidation to form molybdenum oxides, such as MoO3 and MoO2, while after conditioning in both acid and alkaline solutions, the amounts of MoO3 and 19 ACS Paragon Plus Environment


Page 20 of 34

MoO2 noticeably diminish, while Mo2C in Mo containing species shows a remarkable increase in the both electrolytes (evidenced by Mo 3d and C1s spectra). It is proposed that the reductive removal of surface oxides could related to two different routes. For L-Mo2C catalyst which is immersed in acid solution, the negative current density is much smaller than that of the one in alkaline condition during the CV scanning (supported by Figure 2(d)). This means that the L-Mo2C catalyst is reduced more significantly in alkaline than in acid condition. The comparable amount of Mo2C after operation in both electrolytes indicates that the diminishment of the oxidized species in acid solution is predominantly caused by dissolution of molybdenum oxides in acid, while the decrease of those species in alkaline is primarily resulted from electrochemical reduction reaction under cathodic HER potentials. As shown in schematic of distinct mechanisms aforementioned in Figure 4, the (1 0 0) plane after operation in acid solution may expose more active sites due to the random resolving of oxides species than those in alkaline condition. While in alkaline condition, molybdenum oxides on the surface of (1 0 0) are mainly reduced to Mo species with low oxidation states by applying cathodic potential but without extensive damage in the targeted plane, thus the ECSA would be lower than that after conditioning in acid solution (supported by double-layer capacitance of 53.3 mF cm-2 in acid and 48.7 mF cm-2 in alkaline). ii) Besides, the presence of Mo3+OOH in Mo 3d XPS spectra is supposed to provide a significant contribution in facilitating the water dissociation and the generation of hydrogen intermediates in alkaline solution, thus more easily to initiate the HER process than in acid electrolyte. The promotion of HER activity in wateralkali electrolyte was also observed in Ni(OH)2-Pt catalyst because of metal-hydride formation during the HER.43 Conclusion 20 ACS Paragon Plus Environment

Page 21 of 34 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


In summary, we have developed a facial one step molten salt method to produce a 2D lamellar Mo2C HER electrocatalyst through unusual template-sacrificed strategy in which the carbon fibers are directly transformed to lamellar structure Mo2C. The addition of carbon source is critical to realize preferential orientation of Mo2C plane. The electrocatalytic properties towards HER show that L-Mo2C possesses a lower overpotential and superior HER kinetics in both acid and basic media compared with the P-Mo2C catalysts. Moreover, the DFT calculations show that the most stable Mo/C termination of (1 0 0) plane contains multiple catalytically active centers (TC, TM and BM sites with ΔGH* values of -0.066, -0.081 and -0.091 eV, respectively), and the second stable C-terminal of (1 0 0) plane presents a more catalytically active center (HM1 site with ΔGH* of -0.034 eV). Moreover, for the most stable Mo/C termination, the C atoms on the surface exhibit better activity than the exposed metal active sites, which avoids the catalytic inactivation of Mo atoms due to the saturation of Mo by the residual carbon atoms during the carbonation process. In whole, the close-to-zero ΔGH* values suggest a good balance between Volmer and Heyrovsky steps in HER kinetics. Experimental and theoretical investigation suggests that the basal plane of L-Mo2C is also active for HER reaction. It is shown that the hydrogen evolution reaction for L-Mo2C proceeds via Volmer-Heyrovsky mechanism in both acid and basic solutions (Tafel slopes of 77 and 99 mV dec-1, respectively). Furthermore, the HER process of L-Mo2C in alkaline condition requests lower 10 and presents higher TOF value than that does in acid condition. It is suggested L-Mo2C exhibits favorable performance in basic media allowing it to be coupled with the oxygen-evolving catalysts which also play crucial roles in the overall water-splitting reaction. Together, we offer new insight into the distinct catalytic behavior of L-Mo2C in acid and basic solutions by associating the Mo oxides changing processes in different solutions, marking an important step forward 21 ACS Paragon Plus Environment


Page 22 of 34

in our understanding of the HER catalysis of this molybdenum carbide system. Experimental Section Synthesis of different oriented Mo2C on CFP. CFP was firstly dipped in acetone for 5 h and then dried at room temperature. NaCl, KCl, Mo powder and CNT were mixed thoroughly, wherein 4.5 g of NaCl and KCl were mixed with a molar ratio of 1:1 and 1.44 g of Mo powder and 0.01 g of CNT were added. The mixture was settled in an alumina crucible. Took a piece of CFP (1.5 cm × 1.5 cm) and covered it up with the powder. Then the sample was put into a tube furnace and sintered at 200°C for 20min, 600°C for 10 min with a heating rate of 5°C min-1 and then 1000°C for 90 min with a heating rate of 3°C min-1 in an Argon gas flow. Figure 5 schematically illustrates the preparation process. After reaction in the molten salts, the product was washed several times in distilled water. Then the CFP was taken out and dried at room temperature to obtain L-Mo2C on CFP. The sample of P-Mo2C on CFP was prepared by a similar procedure, except that the CNT was replaced by charcoal activated powder and increased to 0.06 g.

Figure 5. Schematic illustration for the preparation process (the precursor in (a) consists of NaCl, KCl, Mo powder


Page 23 of 34 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


and CNT, while the precursor in (b) consists of NaCl, KCl, Mo powder and charcoal activated powder)

Sample Characterization. The phase composition of the as-prepared samples was analyzed by X-ray diffractometer, XRD, (D8 Advance, Bruker AXS, Germany) with Cu Kα radiation. Raman spectra were obtained on a Raman-RENISHAW (Renishaw inVia Reflex). The microstructures were observed by Field Emission Scanning Electron Microscope, (SEM, S-4800, Hitachi). Transmission electron microscopy (TEM) including selected-area electron diffraction (SAED) was carried out on a Tecnai F20 (FEI, USA) electron microscope at an acceleration voltage of 200 kV. The thickness of L-Mo2C was tested by Atomicforce microscopy (AFM, CPSM5500A). X-ray photoelectron spectroscopy (XPS, PHI5000 Versa Probe) of the samples was performed by using Al Kαradiation (1486.7 eV). All the binding energies were calibrated using the C 1s peak (BE = 284.8 eV) as standard. Electrochemical

measurements.

Electrochemical

measurements

were

performed

with

an

electrochemical workstation (CHI 660E, Chenhua Instruments Inc.) in 0.5 M H2SO4 (pH=0) and 1 M KOH (pH=14), respectively. An Hg/Hg2Cl2 electrode (SCE, saturated KCl) and carbon rod were used as the reference and counter electrode, separately. The Mo2C grown on CFP was employed as the working electrodes. Polarization curves were achieved by sweeping the potential from 0 to -0.4 V vs. RHE at a sweep rate of 5 mV s-1. Unless specifically mentioned, all the electrochemical measurements were iRuncorrected. Cyclic voltammetry (CV) curves with sweep rates of 5, 10, 20, 30, 50 and 100 mV s-1 were applied to measure the electrochemical double layer capacitance at nonfaradaic potentials. AC impedance was detected with frequency from 0.01 Hz to 100 kHz and an amplitude of 5 mV. DFT calculation. The density functional theory was performed with the revised Perdew-BurkeErnzerhof functional (RPBE)44-45 by using ultrasoft pseudopotentials46 along with a 370 eV cutoff energy 23 ACS Paragon Plus Environment


Page 24 of 34

and the plane-wave basis as implemented in the Cambridge sequential total energy package (CASTEP).47 This functional could provide reliable descriptions for the crystal structures and chemisorption behaviors of Mo2C based catalyst.44, 48-52 The bulk Mo2C has a structure with hexagonal-close-packed Mo atoms and C atoms randomly occupying half of octahedral interstitial sites, called hexagonal Mo2C. Shi et al.,50 Han et al.52 and Wang et al.53 have proposed the most stable eclipsed configuration with a Mo−C−Mo−C stacking pattern. For the geometric optimization, the convergence threshold was set as 5 × 10-6 eV in energy and 0.001 eV Å-1 in force. The calculated lattice parameters of this bulk structure in current study were a=6.075 Å, b=6.075 Å and c=4.714 Å, using 4 × 4 × 4 Monkhorst-Pack grid k-points, which were in good agreement with the experimental data (a=b=2 × 3.002 Å and c=4.724 Å, PDF 35-0787).54 The mainly exposed (1 0 0) plane of hexagonal Mo2C observed in experimental XRD spectrum was built by six layers of atoms with the surface area of 6.075 × 4.714 Å2 and a 16 Å vacuum between the adjacent slabs in z direction. The adsorbate and top three layers were relaxed during the geometry optimization, and the bottom three layers were fixed at the bulk-terminated geometry. A 8 × 12 × 1 Monkhorst-Pack grid k-points was applied for the surface calculations. For the (1 0 0) plane of Mo2C, there were three possible terminals (Figure S13), corresponding to the reported (0 1 0) plane by Shi et al..50 The Moterminal exposed four Mo atoms, and the mixed Mo/C termination consisted of four Mo atoms and two C atoms on the 4-fold hollow sites, while the C termination contained two more C atoms with 3-fold coordination compared to the Mo-terminal. It should be noted that the mixed Mo/C termination had a surface free energy of 3.084 J m-2, and was thermodynamically most stable among the three terminations compared with 3.413 and 3.280 J m-2 for the Mo termination and C termination, which was in good agreement with the results reported by Shi et al.50 and Wang et al..53 24 ACS Paragon Plus Environment

Page 25 of 34 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


The hydrogen adsorption energy was defined by Equation (6):

EH *  Eslab  H  Eslab  1/ 2 EH 2

(6)

where Eslab+H, Eslab and EH2 are the total energies of L-Mo2C slab with a hydrogen atom adsorbed on the surface, clean slab and a gaseous hydrogen molecule, respectively. The corresponding Gibbs free energy of hydrogen adsorption was expressed as Equation (7):55 (7)

GH *  EH *  ZPE  T S

where ΔZPE and ∆S are the zero-point-energy difference and entropy difference between the adsorbed and the gas phase.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Chemicals and materials; Raman spectra; calculated electrochemical active surface area; turnover frequency calculations; SEM images; Nyquist plots; electrochemical test curves; stability test curves; ICP results; molecular structure models; DFT calculations of (1 1 0) plane; XPS fitting parameters; performance of Mo2C in articles. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant numbers 51771131].

References



1.

Page 26 of 34

Shi, Z.; Nie, K.; Shao, Z. J.; Gao, B.; Lin, H.; Zhang, H.; Liu, B.; Wang, Y.; Zhang, Y.; Sun, X.,

Phosphorus-Mo2C@carbon Nanowires toward Efficient Electrochemical Hydrogen Evolution: Composition, Structural and Electronic Regulation. Energy Environ. Sci. 2017, 10, 1819-1826. 2.

Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H., MoS2 Nanoparticles Grown on Graphene:

an Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. 3.

Trancik, J. E., Renewable energy: Back the Renewables Boom. Nature 2014, 507, 300-302.

4.

Ji, X.; Liu, B.; Ren, X.; Shi, X.; Asiri, A. M.; Sun, X., P-Doped Ag Nanoparticles Embedded in N-

Doped Carbon Nanoflake: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction. ACS Sustainable Chem. Eng. 2018, 6, 4499-4503. 5.

Wang, Z.; Ren, X.; Luo, Y.; Wang, L.; Cui, G.; Xie, F.; Wang, H.; Xie, Y.; Sun, X., An Ultrafine

Platinum-cobalt Alloy Decorated Cobalt Nanowire Array with Superb Activity toward Alkaline Hydrogen Evolution. Nanoscale 2018, 10, 12302-12307. 6.

Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., ChemInform Abstract: Design of Electrocatalysts for

Oxygen‐ and Hydrogen‐Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 46, 2060-2086. 7.

Ma, M.; Zhu, G.; Xie, F.; Qu, F.; Liu, Z.; Du, G.; Asiri, A. M.; Yao, Y.; Sun, X., Homologous

Catalysts Based on Fe-Doped CoP Nanoarrays for High-Performance Full Water Splitting under Benign Conditions. ChemSusChem 2017, 10, 3188-3192. 8.

Ji, Y.; Yang, L.; Ren, X.; Cui, G.; Xiong, X.; Sun, X., Full Water Splitting Electrocatalyzed by

NiWO4 Nanowire Array. ACS Sustainable Chem. Eng. 2018, 6, 9555-9559. 9.

Karunadasa, H. I.; Chang, C. J.; Long, J. R., A Molecular Molybdenum-oxo Catalyst for Generating

Hydrogen from Water. Nature 2010, 464, 1329-1333. 26 ACS Paragon Plus Environment

Page 27 of 34 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


10. Wang, J.; Xu, F.; Jin, H.; Chen, Y.; Wang, Y., Non-Noble Metal-based Carbon Composites in Hydrogen Evolution Reaction: Fundamentals to Applications. Adv. Mater. 2017, 29, 1605838. 11. Deng, J.; Deng, D.; Bao, X., Robust Catalysis on 2D Materials Encapsulating Metals: Concept, Application, and Perspective. Adv. Mater. 2017, 29, 1606967. 12. Vrubel, H.; Hu, X., Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in both Acidic and Basic Solutions. Angew. Chem. 2012, 124, 12875-12878. 13. Liao, L.; Wang, S.; Xiao, J.; Bian, X.; Zhang, Y.; Scanlon, M. D.; Hu, X.; Tang, Y.; Liu, B.; Girault, H. H., A Nanoporous Molybdenum Carbide Nanowire as an Electrocatalyst for Hydrogen Evolution Reaction. Energy Environ. Sci. 2013, 7, 387-392. 14. Fan, X.; Liu, Y.; Peng, Z.; Zhang, Z.; Zhou, H.; Zhang, X.; Yakobson, B. I.; Guo, X.; Hauge, R. H., Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst. ACS Nano 2017, 11, 384-394. 15. Yang, H.; Gong, Q.; Song, X.; Feng, K.; Nie, K.; Zhao, F.; Wang, Y.; Min, Z.; Zhong, J.; Li, Y., Mo2C Nanoparticles Dispersed on Hierarchical Carbon Microflowers for Efficient Electrocatalytic Hydrogen Evolution. ACS Nano 2016, 10, 11337-11343. 16. Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., Frontispiece: Coupling Mo2C with Nitrogen‐Rich Nanocarbon Leads to Efficient Hydrogen‐Evolution Electrocatalytic Sites. Angew. Chem. 2015, 54, 10752-10757. 17. Ma, R.; Zhou, Y.; Chen, Y.; Li, P.; Liu, Q.; Wang, J., Ultrafine Molybdenum Carbide Nanoparticles Composited with Carbon as a Highly Active Hydrogen‐Evolution Electrocatalyst. Angew. Chem. 2016, 54, 14723-14727. 27 ACS Paragon Plus Environment


Page 28 of 34

18. Tang, C.; Wang, W.; Sun, A.; Qi, C.; Zhang, D.; Wu, Z.; Wang, D., Sulfur Decorated Molybdenum Carbide Catalysts for Enhanced Hydrogen Evolution. ACS Catal. 2015, 5, 6956-6963. 19. Jeon, J.; Park, Y.; Choi, S.; Lee, J.; Lim, S. S.; Lee, B. H.; Song, Y. J.; Cho, J. H.; Jang, Y. H.; Lee, S., Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C/MoS2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide. ACS Nano 2018, 12, 338-346. 20. Jin Jia; Tanli Xiong; Lili Zhao; Fulei Wang; Hong Liu; Renzong Hu; Jian Zhou; Weijia Zhou; Chen, S., Ultrathin N-Doped Mo2C Nanosheets with Exposed Active Sites as Efficient Electrocatalyst for Hydrogen Evolution Reactions. ACS Nano 2017, 11, 12509-12518. 21. Yu-Yun Chen; Yun Zhang; Wen-Jie Jiang; Xing Zhang; Zhihui Dai; Li-Jun Wan; Hu, J.-S., Pomegranate-like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution. ACS Nano 2016, 10, 8851-8860. 22. Wan, C.; Leonard, B. M., Iron-Doped Molybdenum Carbide Catalyst with High Activity and Stability for the Hydrogen Evolution Reaction. Chem. Mater. 2015, 27, 4281-4288. 23. Xiong, K.; Li, L.; Zhang, L.; Ding, W.; Peng, L.; Wang, Y.; Chen, S.; Tan, S.; Wei, Z., Ni-doped MoC Nanowires Supported on Ni Foam as a Binder-free Electrode for Enhancing the Hydrogen Evolution Performance. J. Mater. Chem. A 2015, 3, 1863-1867. 24. Yu, Z. Y.; Duan, Y.; Gao, M. R.; Lang, C. C.; Zheng, Y. R.; Yu, S. H., A One-Dimensional Porous Carbon-supported Ni/Mo2C Dual Catalyst for Efficient Water Splitting. Chem. Sci. 2017, 8, 968-973. 25. Xu, X.; Nosheen, F.; Wang, X., Ni-Decorated Molybdenum Carbide Hollow Structure Derived from Carbon-Coated Metal-Organic Framework for Electrocatalytic Hydrogen Evolution Reaction. Chem. Mater. 2016, 28, 6313-6320. 28 ACS Paragon Plus Environment

Page 29 of 34 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


26. Youn, D. H.; Han, S.; Kim, J. Y.; Park, H.; Choi, S. H.; Lee, J. S., Highly Active and Stable Hydrogen Evolution Electrocatalysts Based on Molybdenum Compounds on Carbon Nanotube-graphene Hybrid Support. ACS Nano 2014, 8, 5164-5173. 27. Durst, J.; Siebel, A.; Simon, C.; Hasché, F.; Herranz, J.; Gasteiger, H. A., New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7, 2255-2260. 28. Rheinländer, P. J.; Herranz, J.; Durst, J.; Gasteiger, H. A., Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure. J. Electrochem. Soc. 2014, 161, F1448-F1457. 29. Zhi, W. S.; Fredrickson, K. D.; Anasori, B.; Kibsgaard, J.; Strickler, A. L.; Lukatskaya, M. R.; Gogotsi, Y.; Jaramillo, T. F.; Vojvodic, A., Two-Dimensional Molybdenum Carbide (MXene) as an Efficient Electrocatalyst for Hydrogen Evolution. ACS Energy Lett. 2016, 1, 589-594. 30. Frederic Lantelme, H. G., Molten Salts Chemistry: From Lab to Applications; Elsevier: Amsterdam, 2013; Preface, pp xvii- xvii. 31. Jiang, Z. Y.; Xu, T.; Xie, Z. X.; Lin, Z. W.; Zhou, X.; Xu, X.; Rongbin Huang, A.; Zheng, L. S., Molten Salt Route toward the Growth of ZnO Nanowires in Unusual Growth Directions. J. Phys. Chem. B 2005, 109, 23269-23273. 32. Xu, T.; Zhou, X.; Jiang, Z.; Kuang, Q.; Xie, Z.; Zheng, L., Syntheses of Nano/Submicrostructured Metal Oxides with All Polar Surfaces Exposed via a Molten Salt Route. Cryst. Growth Des. 2009, 9, 192-196. 33. Zhang, H.; Dasbiswas, K.; Ludwig, N. B.; Han, G.; Lee, B.; Vaikuntanathan, S.; Talapin, D. V., 29 ACS Paragon Plus Environment


Page 30 of 34

Stable Colloids in Molten Inorganic Salts. Nature 2017, 542, 328-331. 34. Bockris, J. O. M.; Potter, E. C., The Mechanism of the Cathodic Hydrogen Evolution Reaction. J. Electrochem. Soc. 1952, 99, 169-186. 35. Boudarl, M., Turnover Rates in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 661-666. 36. Ledoux, M. J.; Huu, C. P.; Guille, J.; Dunlop, H., Compared Activities of Platinum and High Specific Surface Area Mo2C and WC Catalysts for Reforming Reactions : I. Catalyst Activation and Stabilization: Reaction of n-Hexane. J. Catal. 1992, 134, 383-398. 37. Oshikawa, K.; Masatoshi Nagai, A.; Omi, S., Characterization of Molybdenum Carbides for Methane Reforming by TPR, XRD, and XPS. J. Phys. Chem. B 2001, 105, 9124-9131. 38. Wan, C.; Regmi, Y. N.; Leonard, B. M., Multiple Phases of Molybdenum Carbide as Electrocatalysts for the Hydrogen Evolution Reaction. Angew. Chem. 2014, 126, 6525-6528. 39. Gao, Q.; Zhao, X.; Xiao, Y.; Zhao, D.; Cao, M., A Mild Route to Mesoporous Mo2C-C Hybrid Nanospheres for High Performance Lithium-ion Batteries. Nanoscale 2014, 6, 6151-6157. 40. Jia, J.; Zhou, W.; Wei, Z.; Xiong, T.; Li, G.; Zhao, L.; Zhang, X.; Liu, H.; Zhou, J.; Chen, S., Molybdenum Carbide on Hierarchical Porous Carbon Synthesized from Cu-MoO2 as Efficient Electrocatalysts for Electrochemical Hydrogen Generation. Nano Energy 2017, 41, 749-757. 41. Lasia, A., In Handbook of Fuel Cells-Fundamentals, Technology and Applications; Vielstich Wolf, Lamm Arnold, Gasteiger Hubert A., John Wiley & Sons, Ltd, 2003; Chapter 29, pp 416-440.

42. Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I., Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102. 30 ACS Paragon Plus Environment

Page 31 of 34 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


43. Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M., Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. 44. Hammer, B., Improved Adsorption Energetics within Density-Functional Theory Using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413-7421. 45. Perdew, J. P.; Burke, K.; Ernzerhof, M., ERRATA: Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1998, 77, 3865-3868. 46. Vanderbilt, D., Soft Self-consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892-7895. 47. Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H., Electronic Structure, Properties, and Phase Stability of Inorganic Crystals: A Pseudopotential Plane‐wave Study. Int. J. Quantum Chem. 2000, 77, 895-910. 48. Liu, P.; Rodriguez, J. A.; Asakura, T.; Gomes, J.; Nakamura, K., Desulfurization Reactions on Ni2P(001) and Alpha-Mo2C(001) Surfaces: Complex Role of P and C Sites. J. Phys. Chem. B 2005, 109, 4575-4583. 49. Kim, S. K.; Zhang, Y. J.; Bergstrom, H.; Michalsky, R.; Peterson, A., Understanding the LowOverpotential Production of CH4 from CO2 on Mo2C Catalysts. ACS Catal. 2016, 6, 2003-2013. 50. Shi, X. R.; Wang, S. G.; Wang, H.; Deng, C. M.; Qin, Z.; Wang, J., Structure and Stability of βMo2C Bulk and Surfaces: A Density Functional Theory Study. Surf. Sci. 2009, 603, 852-859. 51. Shi, X. R.; Wang, J.; Hermann, K., CO and NO Adsorption and Dissociation at the Β-Mo2C(0001) Surface: A Density Functional Theory Study. J. Phys. Chem. C 2010, 114, 13630-13641. 31 ACS Paragon Plus Environment


Page 32 of 34

52. Han, J. W.; Li, L.; Sholl, D. S., Density Functional Theory Study of H and CO Adsorption on AlkaliPromoted Mo2C Surfaces. J. Phys. Chem. C 2011, 115, 6870-6876. 53. Wang, T.; Liu, X.; Wang, S.; Huo, C.; Li, Y. W.; Wang, J.; Jiao, H., Stability of β-Mo2C Facets from ab Initio Atomistic Thermodynamics. J. Phys. Chem. C 2011, 115, 22360-22368. 54. Fries, R. J.; Kempter, C. P., 195. Dimolybdenum Carbide. Anal. Chem. 1960, 32, 1898-1898. 55. Noerskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U., Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23-J26.

For Table of Contents Only


Page 33 of 34 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




Table of Contents Graphic 307x286mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 34 of 34

Two-Dimensional Lamellar Mo2C for Electrochemical Hydrogen

Recommend Documents