Density Functional Study of Hydrogen Evolution on Cobalt-Embedded

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Density Functional Study of Hydrogen Evolution on Cobalt-Embedded Carbon Nanotubes: Effects of Doping and Surface Curvature Lianming Zhao, Sheng Guo, Haijun Liu, Houyu Zhu, Saifei Yuan, and Wenyue Guo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01466 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Nano Materials

Density Functional Study of Hydrogen Evolution on Cobalt-Embedded Carbon Nanotubes: Effects of Doping and Surface Curvature Lianming Zhao, Sheng Guo, Haijun Liu, Houyu Zhu, Saifei Yuan, Wenyue Guo* School of Materials Science and Engineering, Institute of Advanced Materials, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China. KEYWORDS: hydrogen evolution reaction; density functional theory; carbon nanotubes; surface curvatures; Co doping and Co and N co-doping; catalytic activity.

ABSTRACT: Exploring low-cost, efficient, and stable non-precious alternatives for Pt-based catalysts is of significance in the hydrogen evolution reaction (HER) in acidic environments. Previous experiments have found that 3d transition metals Fe, Co, and Ni incorporated with inert carbon templates or carbon-nitrogen materials exhibit long-term durability and high HER activity in acidic electrolytes. To clarify the underlying mechanism determining the HER activity, here, we report a theoretical investigation of the HER on a series of defective carbon nanotubes (CNTs), doped with atomic Co (CoCNT(n,n), n = 3, 5, 7, and 9) and co-doped with Co and double N (CoN2CNT(5,5)), based on the first-principle density functional calculations. Our calculations indicate that the HER on these Co- and Co, N-(co)doped CNTs occurs via the Volmer-Heyrovsky mechanism, and the primary active sites are the C atoms adjacent to the

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metal center. The enhancement of the HER activity is due to uplifting of the p-band center (p) of the active C atoms induced by using a CNT with appropriate curvature, Co doping, and Co and N co-doping. The HER activity of CoCNT(n,n)’s follows a volcano dependence with surface curvature, showing nearly six orders of magnitude difference in exchange currents, peaked at CoCNT(5,5), with the activity comparable with Pt-catalysts. Doped with double N atoms in CoCNT(5,5), the exchange current could be further substantially enhanced (by 30 times), even one order of magnitude higher than that of Pt(111). The fact that CoN2CNT(5,5) has an p (4.16 eV) very close to the optimum value for the maximum exchange current (4.14 eV) justifies the advance in improving the HER activity of CNTs.

1. INTRODUCTION As a clean prospective alternative to fossil fuels to meet the improving energy requirements, molecular hydrogen (H2) has caused growing concerns owing to its high energy density, clean burning, and potentially renewable properties.1 In contrast to the traditional steam reforming of natural gas, electrochemical water splitting provides one of the most promising routes to sustainable hydrogen production. Hydrogen evolution reaction (HER) as an essential reaction in the electrolysis of water is of importance for hydrogen economy. At present, Pt and Pt-based materials are regarded as the most effective electro-catalysts for HER. However, their general scarcity and thus high price limit the scalable commercial applications. Therefore, exploring lowcost and efficient non-precious alternatives for Pt-based HER catalysts has been of significance in the recent developing hydrogen landscape. In the past decades, considerable efforts have been devoted to discovering non-precious-metal based HER catalysts, mainly including metal phosphide (such as Ni2P, MoP, CoP, and FeP),2-5


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metal sulphides (MoS2, WS2, and CoS2 ),6-8 metal carbides (Mo2C and WC),9,10 metal nitrides (Co0.6Mo1.4N2),11 metal selenides (MoSe2, CoSe2),12,13 and so on. Especially, some electrocatalysts composed of earth-abundant 3d transition metals (such as Fe, Co, and Ni) have been found to show exciting catalytic activity for HER in the alkaline electrolytes.14,15 However, 3d transition metals inevitably suffer from inherent corrosion and oxidation in an acidic reaction environment, which is considered to be more efficient and exquisite for HER compared to the alkaline condition. To address these problems, 3d transition metals are ingeniously incorporated with the inert carbon templates, which are cooperated to catalyze HER efficiently. Wu et al. reported that a carbon-nitrogen material incorporating with iron and cobalt shows superior stability for 700 h at a fuel cell voltage of 0.4 V.16 Deng et al. reported that ultrathin grapheneencapsulated CoNi nanoalloy displays high stability and activity for HER in strongly acidic solution, which is comparable to the performance of commercial 40% Pt/C catalysts.17 In addition, the FeCo alloy encapsulated in N-doped carbon nanotubes (CNTs) was also found to exhibit long-term durability and high HER activity in acidic electrolytes.18 Qiu et al. demonstrated that single-atom nickel dopants anchored to nanoporous graphene have superior HER catalysis with a low overpotential and excellent cycling stability in 0.5 M H2SO4 solution.19 Wang et al. further verified that the equilibrium C and N atoms (1: 1) around the metal center for Co and N co-doped graphene are more favorable to the adsorption and desorption of hydrogen.20 In a word, 3d transition metals cooperating with carbon materials could significantly keep the transition metals stable and exhibit high HER activity. Recently, Zou et al.21 have successfully synthesized the cobalt-embedded nitrogen-rich carbon nanotubes (Co-NRCNTs) through simple treatment of Co2+-functionalized graphitic carbon nitride (Co2+-g-C3N4) followed by acid treatment of the resulting material, and found the


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obtained Co-NRCNTs can efficiently eletrocatalyze HER under acidic media. Despite the exciting experimental advances, the reaction mechanisms of the HER process on transition metals embedded in carbon nanotubes remain unknown, especially the effect of CNT curvatures as well as metal (such as Co) and nonmetal (N) dopants. It is well known that curvatures can effectively adjust the energy band structure of CNTs,22,23 and thus may have an important influence on the catalytic activity of CNTs.24,25 However, the research of the curvature effect on HER is rather scarce. Inspired by those, in this work, we designed a series of single-atom Co embedded carbon nanotubes (CoCNTs) to systematically examine the HER activities by the firstprinciple density functional theory (DFT) computations. Firstly, the geometrical and electronic structure of CoCNT(n,n) (n = 3, 5, 7, and 9) was carefully investigated. The stability of the CoCNTs was confirmed by the cohesive energy, formation energy, and phonon dispersion spectra. Then, the adsorption of single H and double H atoms was studied systematically, and the HER mechanisms on CoCNT(n,n) (see Scheme 1) were studied in term of thermodynamic and kinetic properties. Especially, the influence of CNT curvatures on the HER activities was analyzed based on the H adsorption free energy and free energy barriers involved. Thirdly, further nitrogen doping was used to improve the HER activity. Finally, the relationships among exchange current, H adsorption free energy, free energy barrier, as well as active C p-band center were discussed to further reveal the dependence of the HER activity on the CNT curvature, Co doping, and Co and N co-doping. We believe that this work could provide a bright perspective for understanding the detailed HER mechanism and searching effective non-precious-metal HER catalysts to substitute the Pt and its group. 2. THEORETICAL APPROACH According to reference,26 an energetically favorable route was applied to embed a Co atom into


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CNTs, that is: (i) removing two bonding C atoms from a periodic (1×1×5) supercell of CNTs to create a vacancy, and (ii) incorporating a metal atom into the center of the under-coordinated carbon atoms (see Figure 1). The built models of Co-embedded CNT(3,3), CNT(5,5), CNT(7,7), and CNT(9,9) are named as CoCNT(3,3), CoCNT(5,5), CoCNT(7,7), and CoCNT(9,9), respectively. To explore the effect of nitrogen doping on HER, the CoN2CNT(5,5) model was built via replacing two ortho-C atoms with two N atoms in CoCNT(5,5) (see Figure 1c and Figure S1). For all the models, a vacuum box in the a and b directions (see Figure 1a) was set to ~15 Å to avoid interactions between periodic CNTs. Since the effects of water can’t be negligible,27 one hydrated proton and eight free water molecules were adopted to approximately simulate a water monolayer over the CoCNT surfaces (Figure S2). All the DFT computations were performed with spin-polarization treatment in the DMol3 code.28,29 The generalized gradient approximation (GGA) treated by the Perdew-Burke-Ernzerh functional was used to describe the electronic exchange and correlation effects.30 Density functional semicore pseudopotential (DSPP) was used to calculate the metal ion cores, and the double numerical plus polarization (DNP) basis set was adopted to represent the valence electron functions.31 Particularly, a Grimme Van der Waals (vdW) corrected PBE+D2 method was employed,32 considering the long-range electrostatic interactions between adsorbates and substrates. In addition, the PBE0 hybrid XC functional including an ab-initio vdW correction was further employed to evaluate the method we used. The convergence criterion of energy change, max force, and displacement were 1×10-5 Ha, 2×10-3 Ha Å-1, and 5×10-3 Å, respectively. A 1×1×4 k points set Brillouin zone was sampled, and all structural atoms were fully relaxed during all calculations. Transition state (TS) searches were performed with the complete Linear Synchronous Transit/Quadratic Synchronous Transit (LST/QST) method at the same theoretical


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level, confirmed by vibrational frequency calculations. Vibrational frequencies were calculated from the Hessian matrix with the harmonic approximation, to estimate zero-point energies (ZPE) that were included in all the reported energies and entropies for computing the free energies as well as to characterize the stationary points as local minima or TSs involved in the HER. The atomic charge populations were analyzed by the Hirshfeld method.33 Deformation electron densities (DED) were computed by the total electron density with the density of the isolated atoms subtracted.29 This computational strategy has been successfully employed to study the mechanism of oxygen reduction reaction (ORR) on CoN4 embedded graphene34,35 and Feembedded hexagonal boron nitride (h-BN) sheet.36 In order to determine the stability of the Co-doped CNTs, the phonon dispersion spectra of the prototype CoCNT(5,5) were calculated by CASTEP code with the norm-conserving pseudopotential. The cutoff energy was set as 680 eV, while other details were consistent with the above DMol3 computations. The cohesive energy representing the energy required to decompose a CoCNT structure into isolated atoms was derived from:37 Ecoh = (∑niEi – ECoCNT)/∑ni where ni is the number of atom i in the structure, ECoCNT and Ei represent the total energies of CoCNT and isolated atom i, respectively. The formation energy (∆Ef) of CoCNT was defined as:38 ∆Ef = ECoCNT + xμC − (ECNT + ECo)


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where ECNT and ECo represent the total energy of the pristine CNT and Co atom, respectively; x refers to the number of the removed C atoms for forming the defective CNT; and μC refers to the chemical potential of a single carbon atom, which was defined as the total energy per C atom in the pristine CNT. The binding energy (Eb) of Co with defective CNT in CoCNT was calculated by Eb = ECoCNT – (ECo + ECNT) where ECNT is the total energy of the defective CNT with a vacancy by removing two bonding C atoms. According to this definition, a negative binding energy means stable doping of Co in CNT. The adsorption energy (Eads) of H on substrates was calculated by Eads = Esub+H – (Esub + 1/2𝐸H2) where Esub+H and Esub are the total energies of the substrate with and without an adsorbed H atom; 𝐸H2is the energy of a free H2 molecule in the gas phase. Free energy (G) calculations were performed for H+ + e- and intermediates (minima and TSs) involved in the elementary steps of HER, where G(H+ + e-) was set as 1/2G(H2) under the standard conditions corresponding to the equilibrium potential U = 0 V, 𝑃H2= 1 bar, pH = 0, and T = 298 K.39,40 Then, the free energy of intermediates was determined by the following expression: G = E + ZPE - TS


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where E and S are the total energy and entropy of species, ZPE is the zero point energy, and T is the reaction temperature at 298 K in this work.40 The exchange current (i0) reflects the intrinsic rate of electrons in a HER at the equilibrium potential (U = 0).40 If the electron transfer is going on in the exothermic direction (free energy change for H adsorption ∆GH* < 0), i0 at pH = 0 is expressed as i0 = ek0/[1 + exp(∆GH*/kBT)] If the electron transfer process is endothermic (∆GH* > 0), the value of i0 can be calculated by i0 = ek0·exp(∆GH*/kBT)/[1 + exp(∆GH*/kBT)] where rate constant k0 (k0 = 200 s-1 site-1 40) includes all effects relating to the recoganization of solvent during protons transfer to surface, and we thus assumed it to be independent of catalysts at the equilibrium potential. In this work, i0 were evaluated at the general experiment conditions with T = 298 K.40 3. RESULTS AND DISCUSSION 3.1. Structures of clean and H adsorbed CoCNTs 3.1.1. Structures of CNT(n,n) and CoCNT(n,n) (n = 3, 5, 7, and 9) Considering the similarity in structures, Figure 1a,b shows only the optimized configurations of the representative CNT(5,5) and CoCNT(5,5) together with the definitions of some important structural parameters, on behave of various CNTs and CoCNTs, respectively. The corresponding structural parameters of all the CNTs and CoCNTs are summarized in Tables S1 and S2 in the


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Supporting Information. As shown in Table 1, the CNT curvature of CNT(3,3), CNT(5,5), CNT(7,7), and CNT(9,9) is calculated to be 0.49, 0.29, 0.21, and 0.16, respectively. When a CNT(n,n) (n = 5, 7, or 9) is formed from a graphene nanosheet (dCC = 1.420 Å), CC bond length in different directions changes differently, i.e., stretching for bonds perpendicular to the CNT (c) axis (d1) and shrinking for bonds in other directions, e.g., d2 and d3 (see Tables S1). Curvature of the formed CNT is another factor affecting the variations gently, showing a 0.006  0.009 Å stretching for d1 and a minor shrinking of 0.003 to 0.002 Å for d2/d3 when turning n from 9 to 5. The C-C-C bond angles show a little deviations ( CoCNT(7,7) > CoCNT(9,9) > CoCNT(3,3). Note that the free energy barrier of all the CoCNTs except CoCNT(3,3) is comparable to or even lower than that of Pt(111) (Ea = 0.85 eV),52 indicating the good performance of Co doped CNTs as a promising material for HER. Moreover, the free energy of hydrogen adsorption (∆GH*) has been adopted as a key descriptor to examine the HER activity of catalytic materials18,53 and a moderate H adsorption free energy (∆GH* ≈ 0) would be a good candidate for HER.40 The values of ∆GH* are calculated to be 0.50 eV for CoCNT(3,3), 0.11 eV for CoCNT(5,5), 0.13 eV for CoCNT(7,7), and 0.21 eV for CoCNT(9,9) by the PBE+D2 functional, following the same order of the CoCNTs’ HER activity determined according to the free energy barrier Ea of rate-determining step. The ∆GH* values also indicate that the HER activity of CoCNT(5,5) is comparable to Pt (∆GH* = 0.09 eV),40,52 suggesting further the good performance of the Co doped CNT for the promising application in HER. Note that benchmark calculations using PBE0 hybrid XC functional evaluate the accuration of the PBE+D2 method for the calculations of ∆GH* (see Table 1).


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3.3 Effect of further nitrogen doping on HER Nitrogen doping has been widely used to tune the physical and chemical properties of catalysts for HER and ORR.18,35,54,55 In this work, we introduced nitrogen atoms to substitute orthocarbons to investigate the role of the doped nitrogen in HER. For CoNx embedded graphene, the CoN2 embedded graphene has been found to have better ORR catalytic activity.35 Therefore, the CoN2 embedded CNT (CoN2CNT(5,5)) was selected in this work. CoN2CNT(5,5) has three possible configurations because of the relative positions of the two substituted N atoms (Figure S1), and in the most stable configuration the two N atoms are located along the c axis (see Figure 1c). This structure accounts for an formation energy of 3.69 eV, much higher than CoCNT(5,5) (1.74 eV). Structurally, Co in CoN2CNT(5,5) interacts equivalently with the two adjacent C or N atoms, as compared to the four equivalent CoC bonds in CoCNT(5,5); the central Co atom in CoN2CNT(5,5) moves down into the CNT surface. The binding energy of Co with N2CNT(5,5) (8.75 eV, see Table S2) is 0.8 eV lower than with CNT(5,5) (9.55 eV). For the covalent interactions among Co, N, and C atoms, the lowest-energy N 2p states are expected to interact with the C 2p states more strongly than with the highest Co 3d, 4s, and 4p states with a larger energy difference. Thus, the CoC bonds are relatively strong, while the CoN bonds are relatively weak. This structural feature is mirrored by the relevant bond lengths in Table S2 as well as the PDOSs in Figure S5b, in which the ortho-C p states interact with Co d, s, p states more strongly than N p states, especially at high energies. Note that the disappearance of the * peak across the Fermi level in CoN2CNT(5,5) indicates the ortho-CC  interaction is weakened. Hirshfeld charge analyses suggest that the N, ortho-C, meta-C, and Co atoms in CoN2CNT(5,5)


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are charged by 0.075, 0.063, 0.012, and 0.081 e, respectively, indicating both N and ortho-C would have high reactivity. However, the PDOS analyses suggest that the population of the ortho-C p states is much closer to the Fermi level than the N p states (see Figure S4), indicating that ortho-C would interact more strongly with a proton than N, and thus is the optimum site for proton adsorption on CoN2CNT(5,5). Furthermore, after N doping, the p (ortho-C) of CoN2CNT(5,5) shifts up from 4.39 eV of CoCNT(5,5) to 4.16 eV (see Figure 2), indicating N doping could further enhance the reactivity of CoCNT(5,5). As shown in Figure S5b, the uplifting of p is mainly due to the bonding and anti-bonding interactions of ortho-C p states with Co d, s, p states at the higher energies. The possible H adsorption sites of CoN2CNT(5,5) are shown in Figure 3b and Figure S8i,j. As expected, the most stable H adsorption sites of CoN2CNT(5,5) are at ortho-carbon for SHA (Eads = 0.34 eV) and at adjacent ortho- and meta-carbons for DHA (Eads = 0.35 eV). Compared to CoCNT(5,5) (Eads = 0.30 eV), the SHA energy is slightly increased on CoN2CNT(5,5), whereas the adsorption energy of DHA on CoN2CNT(5,5) (Eads = 0.35 eV) is 0.32 eV lower, favoring the desorption of the newly-formed H2. The free energetic diagram of the HER process on CoN2CNT(5,5) at U = 0, pH = 0, and T = 298 K is displayed in Figure 5b. We can find that in this case the energetic diagram is relatively smooth compared to that on CoCNT(5,5). The solvated proton is physisorbed with an endothermic free energy of 0.14 eV, which is slightly under that of CoCNT(5,5). Then, the Volmer reaction is exothermic by 0.14 eV with a free energy barrier of 0.33 eV to form a chemisorbed hydrogen. Subsequently, the Tafel reaction is endothermic by 0.58 eV and overcomes a free energy barrier of 0.65 eV. Alternatively, the Heyrovsky reaction is exothermic


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by 0.28 eV with a free energy barrier of 0.15 eV, suggesting HER on CoN2CNT(5,5) also follows preferentially the Volmer-Heyrovsky mechanism. However, the rate-determining step is changed to the Volmer process on CoN2CNT(5,5) rather than the Heyrovsky process on CoCNTs. The relevant free energy barrier on CoN2CNT(5,5) (Ea = 0.33 eV) is 0.34 eV lower than that on CoCNT(5,5) (Ea = 0.67 eV), and 0.52 eV lower than that on Pt(111) (Ea = 0.85 eV).52 Also, the ∆GH* descriptor of HER on CoN2CNT(5,5) is calculated to be as low as 0.01 eV by the PBE+D2 method and 0.005 eV by the PBE0 hybrid XC functional, much close to the standard of ∆GH* ≈ 0 for a good HER candidate.40 These facts indicate double N substitution could substantially improve the HER activity of CoCNTs. In this study, we find that the HER activity depends strongly on the CNT curvature, Co doping, and Co and N co-doping. In order to reveal the underlying mechanism, Figure 7 shows the HER activity descriptors (Ea and ∆GH*) as a function of the p-band center (p) of ortho-C in all the modified CNTs studied. Similar to the relations between the d-band centers and the ethylene and ethyl dehydrogenation activation barriers on Pd based alloys,56 here, both Ea and ∆GH* also show clearly linear dependence with p, no matter whether the modification is through changing the surface curvature or N substitution on CoCNTs. These facts clearly indicate that the improvement of HER activity is a result of tuning the electronic structure rather than geometric structure. Together with the linear relationship between Ea and |∆GH*| (Ea = 2.56|∆GH*| + 0.39, R2= 0.9837, see Figure 8b), we can conclude that in addition to the standard ∆GH*, both Ea and p are also good descriptors of the HER activity of CoCNTs. 3.4 Exchange current analysis To clearly evaluate the HER catalytic ability of the above-mentioned candidates, the exchange


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current (i0) was computed based on ∆GH* at U = 0, pH = 0, and T = 298 K. All calculated results together with the experimental i0 of Pt, Pd, Rh, Au, Ag, Mo, Ni, Co, etc. under the same conditions40 are gathered into a volcano curve (Figure 8a), where the i0 of HER is as a function of ∆GH*. As the figure displays, the value of ∆GH* = 0 divides the volcano curve into two branches. In the ascending branch (left side of ∆GH* = 0), the strong bonding between hydrogen atoms and catalysts results in a sluggish hydrogen desorption with a low HER activity. While in the descending branch (right side of ∆GH* = 0), a weak interaction between hydrogen atoms and catalysts also impedes the HER. Thus, only these catalysts with ∆GH* approaching zero are favorable for HER. In our calculations, CoCNT(3,3) is located far away from ∆GH* = 0 at the left side due to the strong binding with hydrogen atoms, resulting in a much low i0 (1.68 × 10-10 A cm-2); whereas CoCNT(9,9), CoCNT(7,7), and CoCNT(5,5) on the right side of ∆GH* = 0 show an increasing trend of i0 with the decrease of ∆GH* (1.35 × 10-5, 3.02 × 10-4, and 6.54 × 10-4 A cm-2 for CoCNT(9,9), CoCNT(7,7), and CoCNT(5,5), respectively). Note that CoCNT(5,5) and CoCNT(7,7) show comparable exchange currents with Pt-catalysts, which is six orders larger than that of CoCNT(3,3). Importantly, the double N doped CoCNT(5,5) is almost located at the apex of the volcano, and its exchange current (i0 = 1.94 × 10-2 A cm-2), even one order higher than Pt (i0 = 8.06 × 10-4 40 and 2.34 × 10-3 A cm-2 57), shows nearly a 30-times increase from the parent CoCNT. These facts indicate co-doping with double N could improve the HER activity of CoCNT(5,5) efficiently. Interestingly, the exchange current i0 is found to decrease exponentially with the increase of free energy barrier Ea for HER on CoCNTs and CoN2CNT (see Figure 8c). The relationship between i0 and Ea can be described by an expression of i0 = 2.22×105exp(8.28Ea) (R2 = 0.9998). The relationship between i0 and p for CoCNTs and CoN2CNTs is also shown in Figure 8d. The curve


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reveals that the value of i0 increases exponentially with the increase of p, described by i0 = 1.05×1019exp(11.56p) (R2 = 0.9992). The green dashed line in Figure 8d shows the optimum p value (4.14 eV) for the maximum i0 at ∆GH* = 0, and the dots shown in the figure display clearly how to approach the optimum p of CoCNTs by changing the CNT curvature and N substitution. 4. Conclusions DFT calculations have been carried out to study the performance of HER on the Co-embedded CNT(n,n)’s (n = 3, 5, 7, and 9) (CoCNT(n,n)) and Co and double N co-embedded CNT(5,5) (CoN2CNT(5,5)). The HER proceeds favorably through the Volmer-Heyrovsky mechanism on the C sites adjacent to the metal center of CoCNTs and CoN2CNT(5,5). The HER activity of CNTs can be enhanced by uplifting the position of p-band center (p) of the active C atoms via changing the CNT curvature, Co doping, and Co and N co-doping. The HER activity of CoCNT(n,n)’s shows a volcano-shape dependence with the curvature, peaked at n = 5 with an exchange current comparable to Pt-catalysts, or six orders of magnitude larger than that of n = 3. Double N doping in CoCNT(5,5) could further result in a nearly 30-times increase in the HER activity, giving an exchange current even one order larger than that of Pt(111). The advance in improving the HER activity of CoCNTs by the double N doping is justified by its p value (4.16 eV) which is very close to the optimum value for the maximum exchange current (4.14 eV). These findings are beneficial for exploring novel HER catalysts for the hydrogen production application based on non-precious 3d transition metals and carbon materials. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: . Structural parameters of the pristine CNTs, structural parameters, formation energy, cohesive energy, and Co binding energy for CoCNTs and CoN2CNT(5,5), structural parameters and Hirshfeld charges for single/double H adsorbed CoCNTs and CoN2CNT(5,5), the possible configurations of CoN2CNTs, top and side views of the stable CoCNT(5,5) configuration with a water layer, the deformation electron density of CoCNT(5,5), the PDOS for CoCNTs and CoN2CNT(5,5), the detailed analysis of the upshift of p of the active C in CoCNT(5,5) and CoN2CNT(5,5), the phonon dispersion spectra of CoCNT(5,5), the possible configurations of SHA and DHA on CoCNTs and CoN2CNT(5,5), potential energy profile for the HER process on CoN2CNT(5,5). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Wenyue Guo: 0000-0002-7537-984X Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21776315 and 21805307), Natural Science Foundation of Shandong Province (ZR2017MB053 and ZR2016BL12), PetroChina Innovation Foundation (2018D-5007-0504 and 2017D-5007-0402),


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the Fundamental Research Funds for the Central Universities (17CX02031A and 15CX08010A) and Qingdao independent innovation program (16-5-1-88-jch). REFERENCES (1) Schlapbach, L.; Züttel, A. Hydrogen-Storage Materials for Mobile Applications. Nature 2001, 414, 353358. (2) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 92679270. (3) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J.Y.; Lim, K. H.; Wang, X. Molybdenum Phosphide as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction. Energ. Environ. Sci. 2014, 7, 26242629. (4) Zeng, Y.; Wang, Y.; Huang, G.; Chen, C.; Huang, L.; Chen, R.; Wang, S. Porous CoP Nanosheets Converted from Layered Double Hydroxides with Superior Electrochemical Activity for Hydrogen Evolution Reaction at All pH Values. Chem. Commun. 2018, 54, 14651468. (5) Xu, Y.; Wu, R.; Zhang, J.; Shi, Y.; Zhang, B. Anion-Exchange Synthesis of Nanoporous FeP Nanosheets as Electrocatalysts for Hydrogen Evolution Reaction. Chem. Commun. 2013, 49, 66566658. (6) Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction. Nano Lett. 2013, 13, 62226227.


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(37) Sang, P.; Zhao, L.; Xu, J.; Shi, Z.; Guo, S.; Yu, Y.; Zhu, H.; Yan, Z.; Guo, W. Excellent Membranes for Hydrogen Purification: Dumbbell-Shaped Porous γ-Graphynes. Int. J. Hydrogen Energ. 2017, 42, 51685176. (38) Yang, Y.; Li, K.; Meng, Y.; Wang, Y.; Wu, Z. A Density Functional Study on the Oxygen Reduction Reaction Mechanism on FeN2-Doped Graphene. New J. Chem. 2018, 42, 68736879. (39) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 1788617892. (40) Nørskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the Exchange Current for Hydrogen Evolution. J. Electrochem. Soc. 2005, 152, J23J26. (41) Sun, G.; Kürti, J.; Kertesz, M.; Baughman, R. H. Variations of the Geometries and Band Gaps of Single-Walled Carbon Nanotubes and the Effect of Charge Injection. J. Phys. Chem. B 2003, 107, 69246931. (42) Gan, L. H.; Zhao, J. Q. Theoretical Investigation of [5,5], [9,0] and [10,10] Closed SWCNTs. Physica E. 2009, 41, 12491252. (43) Vojvodic, A.; Nørskov, J. K.; Abild-Pedersen, F. Electronic Structure Effects in Transition Metal Surface Chemistry. Top. Catal. 2014, 57, 2532. (44) Sinthika, S.; Waghmare, U. V.; Thapa, R. Structural and Electronic Descriptors of Catalytic


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Scheme 1. Mechanisms of hydrogen evolution reaction on CoCNTs or CoN2CNT.


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Table 1. Curvatures, Hirshfeld charges q (in e), ortho-C p-band centers εp (in eV), and H adsorption free energies ∆GH* (in eV) relevant to CoCNTs and CoN2CNT(5,5).

Species

CoCNT(3,3)

CoCNT(5,5) CoCNT(7,7) CoCNT(9,9) CoN2CNT(5,5)

Curvaturea

0.49

0.29

0.21

0.16

0.29

qCo

0.117

0.125

0.124

0.122

0.081

qortho-C

0.042

0.041

0.042

0.043

0.063

qmeta-C

0.006

0.002

0.002

0.002

0.012

pb

4.90

4.39

4.56

4.67

4.16

∆GH* c

0.50(0.481)

0.11(0.127)

0.13(0.155)

0.21(0.218)

0.01(0.005)

a

Values are those of the corresponding pristine CNTs;

b

Values are relative to the Fermi energy level;

c Values

in the parentheses are the results calculated by the PBE0 hybrid XC functional including an ab-initio Van der Waals correction.


33


Page 34 of 42

Figure 1. Representative configurations and definitions of important structural parameters and H adsorption sites for the studied CNT(n,n)’s, CoCNT(n,n)’s, and Co, N codoped CNT(5,5). (a) CNT(5,5), (b) CoCNT(5,5), and (c) CoN2CNT(5,5).

34


Page 35 of 42

5

5

n=3

0

Energy (eV)

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


5

n=5

-4.90

-4.39 -5.38

-5.82 -10

0.4

0.8

PDOS (States eV-1)

-15 0.0

0.4

0.8

PDOS (States eV-1)

-4.67

-5

0.4

0.8

PDOS (States eV-1)

-4.16 -5

-4.39

-6.43 -10

-15 0.0

N doping

0

-6.28 -10

-10

-15 0.0

-4.56

-5

-5

5

n=9

0

0

0

-5

5

n=7

-10

-15 0.0

0.4

0.8

PDOS (States eV-1)

-15 0.0

0.4

0.8

PDOS (States eV-1)

Figure 2. Projected density of states (PDOSs) for the p band of ortho-C in CoCNT(n,n)’s (n = 3, 5, 7, and 9) and CoN2CNT(5,5) (the green boundary filled with light gray shadow). The Fermi level marked by blue dashed lines is set as the energy zero. The band centers (in eV) are shown by green dashed lines. For comparison, the PDOSs of the C p band of the corresponding CNT(n,n)’s and CoCNT(5,5) are also shown (black dot lines), and the band centers (in eV) are marked by black dashed lines.


35


Page 36 of 42

Figure 3. Top and side views of the most stable configurations of single and double H adsorption on (a) CoCNT(5,5) and (b) CoNCNT(5,5). The adsorption energies Eads and bond lengths d are given in eV and Å, respectively.


36

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Figure 4. Potential energy profiles together with structures involved in the elementary steps of HER on CoCNT(5,5). Relative energies of the intermediates and transition states are given in eV. The white, red, gray, and royal blue balls represent hydrogen, oxygen, carbon, and cobalt atoms, respectively, while the hydrogen atoms taking directly part in the surface reaction are highlighted by green balls.


37


a

2.01 TS2'

Volmer Tafel Heyrovsky

0.69 Free Energy (eV)

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

+

0.00 2H++2e-

2

b 0.00 2H++2e-

2

0.26 H +TS1+e + H +H'+e

4

6

0.47 0.14 H++TS1+eH++H'+e-

4

Page 38 of 42

6

0.11 H++H*+e-

H*+H' -0.03 -0.36 H*+H*

8

-0.01 H++H*+e-

8

10

0.28 H*+H'

0.64 TS2 0.00 H2

12

TS2 0.43 0.07 TS2'

-0.58 H*+H*

Reaction Coordinate

10

12

14

0.00 H2

14

Figure 5. Free energy diagrams for HER on (a) CoCNT(5,5) and (b) CoN2CNT(5,5) at a potential U = 0, pH = 0, and T = 298 K. The reference energy is set as the total free energy of a gas hydrogen molecule, 9 H2O molecules, and CoCNT(5,5) or CoN2CNT(5,5). H+, H, and H* denotes free proton, physisorbed hydrated proton, and chemisorbed H, respectively.


38

Page 39 of 42

1.14

n=3 n=5 n=7 n=9

1.0

Free Energy (eV)

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


0.97

0.5

0.0

0.76 0.64

0.71

0.69

0.69 0.56

0.36 0.25 0.00

0.25

0.21 0.13 0.11

0.20

0.02

0.00

-0.03

-0.11 -0.50

-0.5

-0.70 +

-

2H +2e

+

-

H +H'+e

+

-

+

-

H +TS1+e H +H*+e

H*+H'

TS2

H2

Reaction Coordinate

Figure 6. Free energy diagrams of the Volmer-Heyrovsky reactions involved in the HER on CoCNT(n,n)’s (n = 3, 5, 7, and 9) at U = 0 V, pH = 0, and T = 298 K. Notations are the same as in Figure 5.


39


b 0.6

a 1.6

CoCNT(3,3)

0.5

CoCNT(9,9) CoCNT(7,7)

CoCNT(5,5)

-4.8

-4.6 -4.4 p (eV)

-4.2

0.3 0.2

CoN2CNT(5,5)

0.4

0.0 -5.0

GH*| (eV)

0.8

CoCNT(3,3)

0.4

1.2 Ea (eV)

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 40 of 42

-4.0

0.1 0.0 -5.0


CoCNT(5,5) CoN2CNT(5,5)

-4.8

-4.6 -4.4 p (eV)

-4.2

-4.0

Figure 7. Relationship of the (a) free energy barrier Ea and (b) H adsorption free energy ∆GH* with the active C p-band center εp involved in the HER on Co doped and Co and N codoped CNTs at U = 0 V, pH = 0, and T = 298 K.


40

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Ni

Cu

W

-6 Nb

1.2

CoCNT(9,9)

Co

Au

Mo

0.8

CoCNT(5,5)

Ag

-8

0.4 0.5

0.0

d

24 20

i0 (10-3 A cm-2)

16 12 8

0 0.2

0.4

0.6

0.8

1.0 1.2 Ea (eV)

CoCN T(3,3 )

4

0.1

0.2 0.3 GH*| (eV)

1.4

1.6

1.8

0.4

0.5

24 20

CoN2CNT(5,5)

CoCN T CoCN (5,5) T(7,7 ) CoCN T(9,9 )

c

0.0 GH* (eV)

CoN2CNT(5,5)

CoN2CNT(5,5)

16 12 8 4 0 -5.0

CoCN T(3,3 )

-10

CoCNT(3,3) -0.5

CoCNT(9,9)

CoCNT(7,7)

-4.8

CoCN T(7,7 )

-4


p = -4.14 eV

Rh Ir

CoCN T(5,5 )

Re Pt

CoCNT(3,3) 1.6

CoCN T(9,9 )

Pd

Log(i0(A cm-2))

b

CoN2CNT(5,5)

-2

Ea (eV)

a

i0 (10-3 A cm-2)

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


-4.6 -4.4 p (eV)

-4.2

-4.0

Figure 8. (a) Volcano curve for the HER activity of various materials. The black circles are experimental data taken from ref 40, while the red triangles represent the present DFT results; (b) relationship of HER free energy barrier Ea with H adsorption free energy ∆GH* for Co doped and Co and N codoped CNTs; (c) relationship of exchange current i0 with Ea for Co doped and Co and N codoped CNTs; and (d) relationship of exchange current i0 with active C p-band center εp for Co doped and Co and N codoped CNTs. The data are given at the conditions of U = 0 V, pH = 0, and T = 298 K.


41


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Table of Contents (TOC) Graphic:


42

Density Functional Study of Hydrogen Evolution on Cobalt-Embedded

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