Dual-descriptors tailoring: The hydroxyl adsorption energies

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Dual-descriptors tailoring: The hydroxyl adsorption energies-dependent hydrogen evolution kinetics of high-valance state doped Ni3N in alkaline media Bao Zhang, Jinsong Wang, Jia Liu, Lishang Zhang, Houzhao Wan, Ling Miao, and Jianjun Jiang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01637 • Publication Date (Web): 05 Sep 2019 Downloaded from pubs.acs.org on September 5, 2019

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Dual-descriptors tailoring: The hydroxyl adsorption energies-dependent hydrogen evolution kinetics of high-valance state doped Ni3N in alkaline media Bao Zhang†, Jinsong Wang†, Jia Liu†, Lishang Zhang†, Houzhao Wan‡, Ling Miao†,

Jianjun Jiang†* † School

of Optical and Electronic Information, Huazhong University of Science and Technology,

Wuhan 430074, China. ‡ Hubei

Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics and

Electronic Science, Hubei University, Wuhan 430062, China *Corresponding

author: Jianjun Jiang E-mail: [email protected]

ABSTRACT: The requirement of both water discharge and hydrogen adsorption free energy restricts the activity of most electrocatalysts for hydrogen evolution reaction (HER) in alkaline medium. Herein, the dual-descriptors guided design without time-consuming transition state calculations is proposed. Theory-driven precise surface reactivity tailoring of Ni3N is demonstrated toward the balance of hydrogen and hydroxyl species adsorption energetics. The results reveal that the rate-determining step of Ni3N catalyst mainly originates from the strong hydrogen adsorption. By higher valance-state Mo, W, V doping, the electronic structure of Ni3N is modulated, leading to lower surface reactivity and

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favorable hydrogen adsorption/desorption thermodynamics. Notably, Mo doped Ni3N exhibits optimal hydroxyl adsorption energy and fast water discharge kinetics, while W doping lead to hydroxyl poisoning and sluggish kinetics of water discharge. The experimental investigations confirm the theoretical prediction and the Mo-Ni3N realizes about 12-fold, 9-fold and 3-fold enhancement in alkaline HER activity compared to pure Ni3N, W-Ni3N and V-Ni3N, respectively. This dual-descriptors guided design opens up opportunities for developing superior alkaline HER electrocatalysts.

Keywords: dual-descriptors, DFT calculations, hydroxyl poisoning, microkinetics analysis, alkaline hydrogen evolution


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The Hydrogen is widely considered as one of the most important chemicals and energy carrier. Alkaline hydrogen evolution reaction (HER) is critical to reducing the energy consumption in water-alkali and chlor-alkali electrolyzers1–3 and water-alkali has been regarded as the most feasible way for the industrial production of hydrogen4–7. The requirement of both optimal hydrogen and hydroxyl species adsorption energies restricts the electrocatalyst performances of most electrocatalysts in alkaline medium8. Take Pt for example, the alkaline HER activity of Pt is limited by the sluggish kinetics of water dissociation step, leading to about two to three orders of magnitude lower than in acid solutions9. Thus, the carefully surface reactivity design for favorable hydrogen and hydroxyl species adsorption energies is necessary for accelerating the alkaline hydrogen evolution kinetics. Recently, tremendous efforts have been undertaken to improve the electrocatalytic activity3,10–12. One common strategy is to tune their surface electronic structure and assessed on the basis of hydrogen adsorption free energy (ΔGH)13–16, which has been widely accepted as hydrogen evolution activity descriptor. Various cations doping and anions doping are employed to modulate the electronic structure toward more optimal


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hydrogen adsorption free energy17–20. X. P. Sun et al. demonstrated Zn doped CoP with more thermal-neutral hydrogen adsorption free energy and thus exhibits enhanced HER activity21. G. M. Wang et al. reported that transition‐metal doping can intrinsically boost the HER kinetics of Co4N by tuning the potential of the d‐band center leading to more thermoneutral ΔGH value22. S. Z. Qiao et al. developed a multifaceted heteroatom doping method to optimize the HER activity of non-noble metal catalysts23,24. The theoretical investigation based only on hydrogen adsorption energetics has been a great success in HER design. However, this method can hardly meet with the experimental observation in alkaline medium, which involves water dissociation step9,25,26. In this case, the most common strategy is bi-functional mode by inducing water dissociation promoter4,27,28, like transition metal oxides29,30 and hydroxides32. And the water dissociation energy barriers are proposed to be crucial, while the involved transition state calculations are more timeconsuming than adsorption energy calculations. Besides, the hydroxyl adsorption energy has also been proposed8,34. The reported hydroxyl adsorption energy (ΔGOH) ranges from about -3.4 eV to -4.6 eV26,34,35, and there is hardly report about the optimal ΔGOH value. Overall, the simple and effective design method is still a big challenge.


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In this study, the optimal free energy of ΔGH and ΔGOH are proposed according to thermodynamic and kinetic calculation and analysis. This novel dual-descriptors can serve as efficient quantitative descriptors for high-efficiency alkaline HER electrocatalysts design. Theory-driven precise surface reactivity tailoring of Ni3N is demonstrated toward the balance of hydrogen and hydroxyl species adsorption energetics. The experimental investigations and related microkinetics analysis confirm the dual-descriptors prediction. The resulting Mo-Ni3N exhibits highly enhanced alkaline HER performance with an overpotential of 12 mV at 10 mA/cm2 and a low Tafel slope of 64 mV/dec, and realizes about 12-fold, 9-fold and 3-fold enhancement in alkaline HER activity compared to pure Ni3N, W-Ni3N and V-Ni3N, respectively. This study provides effective theoretical guidance for understanding the alkaline HER mechanisms and electrocatalysts design.

RESULTS AND DISCUSSION As shown in Fig. 1a, the hydrogen and hydroxyl species are the two key reaction intermediates during water dissociation, hydroxyl desorption, and subsequent hydrogen production. The energetics of hydrogen and hydroxyl species is critical for the alkaline


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HER, and the balance between the dual-descriptors is necessary for optimizing efficiency for hydrogen production.8 Herein, the energy of hydroxyl ion is employed as the reference of hydroxyl adsorption energy. According to thermodynamic and kinetic calculation and analysis (for details see Supplemental Information), the optimal free energy of hydrogen adsorption (ΔGH) and hydroxyl adsorption energy (ΔGOH) are both assumed about thermoneutral 0 eV.

Figure 1 a) Schematic illustration of water activation, H* intermediate formation and hydrogen generation processes with different hydrogen and hydroxyl adsorption. b) The density of states of d band of surface atoms. c) The local charge density difference isosurfaces of H adsorption and OH


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adsorption surface (0.0035 e per Å3 isosurfaces, cyan:-, yellow:+). d) Calculated free energies of hydrogen adsorption and hydroxyl adsorption energies. e) Predicted polarization curves with a reference prefactor of 1000 mA/cm2.

Nickel-based materials have been regarded as promising non-noble metal electrocatalysts toward HER15,36,37. Herein, we take the metallic Ni3N as example and demonstrate dual-descriptors driven design. Ni3N has been considered to be appropriate electrocatalysts for OH–H bond cleavage during the Volmer step. As shown in Fig. 1d, our density functional theory (DFT) calculations reveal that facile hydroxyl adsorption free energy of 0.03 eV can be achieved on the Ni3N(111) surface, which is favorable for water dissociation energetics and subsequent hydroxyl desorption

26,28.

This is consistent with

previous reports that Ni3N can act as water dissociation promoter 38. As shown in Fig. S3, the NEB calculation also confirms the favorable water dissociation step on Ni3N (111) surface. However, the DFT calculations show that the typical ΔGH value on Ni3N (111) surface is about -0.31 eV, which is significantly stronger than optimal value (ΔGH=0 eV). The overactive surface reactivity leads to unfavorable for hydrogen desorption and subsequent H2 production.


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The adsorption strength of hydrogen decreases as the d-band center is downshifted from the Fermi level (Ef) on metal and metal-like surface.39,40 As demonstrated in Fig. 1b, the d-band center (d) of Ni3N(111) surface is about -1.71 eV below the Fermi level. To weaken the hydrogen adsorption, the d should be more far away from the Fermi level. The higher valance-state doping has been proven to shift the d away from the Fermi level in nitrides41. Inspired by this, the higher valance-state Mo, W, V doped Ni3N are investigated. The results show the d of Mo, W, V doped Ni3N is clearly downshift to -1.84 eV, -2.13 eV, -1.99 eV. And the isosurfaces of local charge density difference visualized in Fig. 1c, show quite similar local charge distributions around the hydrogen in these four structures, yielding an optimal ΔGH of -0.17 eV, -0.16 eV, -0.16 eV, respectively. On the other hand, the hydroxyl species adsorption, in principle, can also be weaken as the d downshift. Indeed, theoretical results show that only the hydroxyl adsorption on Mo doped Ni3N surface is slightly weakened. This is consistent with previous report that Mo-N species can facilitate hydroxyl species adsorption during water dissociation42. The isosurfaces of local charge density difference show quite different local charge distributions around the hydroxyl in these four structures, and the resulting hydroxyl


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adsorption free energies of W and V doped Ni3N are enhanced to -0.81 eV and -0.14 eV, respectively. Apparently, it is rather tricky to modulate the bonding strength of hydroxyl. Thus, more precise surface reactivity tailoring is necessary for accelerate the alkaline HER rate. This strong hydroxyl adsorption strength indicates the hydroxyl poisoning would occur to some extent, especially on the W doped surface. A simplified dualpathway kinetics model is employed to predict the HER performance43. As shown in Fig. 1e, the Mo doped Ni3N with favorable hydrogen and hydroxyl adsorption energetics can exhibits much higher alkaline HER activity.


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Figure 2 a) Schematic illustration of the synthesis process and related SEM images of NiMo LDH and Mo-Ni3N. b) Energy-dispersive X-ray (EDX) elemental mapping images of Ni, Mo, and N for Mo-Ni3N. c-e) TEM and HRTEM images of Mo-Ni3N. f) High-resolution XPS of Ni 2p, N 1s, Mo 3p, and Mo 3d and particle sizes distribution.

Encouraged by these theoretical predications, Ni3N and M-Ni3N (M=Mo, W, V) are synthesized via employing gas-phase nitridation onto nickel hydroxide/NiM layered


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double hydroxides (LDH). Fig. 2a shows schematic illustration of the synthesis process and typical scanning electron microscopy (SEM) images of hydroxide precursors and nitrides with lamellar morphology. As reported before44, the lamellar Ni3N nanosheets are composed of stacked nanoparticles with more active sites exposed. The M-Ni3N has a similar nanoparticles stacked morphology with a mean particle size about 10.5 nm. The high active surface area and high intrinsic conductivity of these nickel nitrides are beneficial to exhibits high alkaline HER performance. The X-ray powder diffraction (Figure S7) patterns of the nickel hydroxide precursor and nitrided product match well with Ni(OH)2 (JCPDS No. 01-073-1520) and Ni3N (JCPDS No. 01-089-5144), while the NiM LDH and M-Ni3N present less characteristic diffraction peaks. The Raman spectra of Ni3N and Mo-Ni3N (Fig. S9) show prominent bands around 700, 550, 370 and 210 cm−1, which can be assigned to Ni3N45–47. Besides, the high-resolution TEM images of Mo-Ni3N nanoparticles shows well-resolved lattice fringes with interplanar distances of 0.21 nm, consistent with the (111) plane of Ni3N and proves the synthesis of Ni3N. The corresponding energy-dispersive X-ray (EDX) images also confirm the existence of Ni, Mo, and N.


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Furthermore, the chemical states of the Ni3N and Mo-Ni3N are characterized by Xray photoelectron spectroscopy shown in Fig. 2f. For the Ni3N, two peaks at 853.0 eV and 870.3 eV are observed in the spectra, which correspond to the Ni+ 2p1/2 and Ni+ 2p3/2 of Ni3N48. The two peaks at 855.5 eV and 873.3 eV can be ascribed to Ni2+ 2p1/2 and Ni2+ 2p3/2 of surface partial oxidation. The N 1s spectrum shows two peaks at 398.0 eV and 399.8 eV, corresponding to N-Ni bond and N-H bond49. As for the Mo-Ni3N spectra, the characteristic peaks of Ni+ 2p1/2 (852.9 eV) and Ni+ 2p3/2 (870.1 eV) shows slightly negative shift. The Mo 3d spectrum shows three valence states: Mo3+, Mo4+, Mo6+. The two peaks at 229.0 eV and 232.5 eV can be attributed to Mo3+ 3d5/2 and Mo3+ 3d3/2, respectively50. The peaks at 229.6 eV and 233.3 eV, 232.5 eV and 235.4 eV can be attributed to Mo4+ and Mo6+ due to surface oxidation, respectively51. The three peaks at 394.9 eV, 397.3 eV and 399.4 eV can be ascribed to Mo 3p3/2, N-Ni/Mo bond and N-H bond, indicating the valance-state of N become more negative with the high valance-state of Mo doping. These XPS spectra further prove the formation of the Ni3N and Mo-Ni3N. Next, the alkaline HER electrocatalytic activity of Mo-Ni3N is tested in 1M KOH. For comparison, the HER activities of Ni3N, W-Ni3N, V-Ni3N and commercial 20% Pt/C are


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also investigated in a typical three electrode cell. The IR-corrected polarization curves and Tafel plots are shown in Fig. 3a-b. It can be seen that Pt/C (0.5 mg/cm2) shows excellent alkaline HER activity with a low overpotential of 10 mV to drive 10 mA/cm2 and a Tafel slope of 70 mV/dec. The Ni3N exhibits unsatisfied alkaline HER activity with a high overpotential of 133 mV at 10 mA/cm2 (10) and a high Tafel slope of 103 mV/dec. The Mo, W, V doped Ni3N shows significantly lower overpotential of 12 mV, 40 mV, 10 mV to drive 10 mA/cm2, respectively. These is consistent to the DFT calculated ΔGH values. The transition‐metal doped Ni3N exhibits more optimal hydrogen adsorption strength than the pristine, leading to lower 10.


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Figure 3 a) Polarization curves and b)Tafel plots of 20% Pt/C, Ni3N, Mo-Ni3N, W-Ni3N and V-Ni3N with a scan rate of 2 mV/s. c) Nyquist plots obtained by EIS at 150 mV overpotential for HER. d) The TOF values for Ni3N, Mo-Ni3N, W-Ni3N and V-Ni3N. e) Polarization curves of the Mo-Ni3N electrocatalyst before and after long-term stability tests at 20 mA/cm2. The overpotential as a function of time is shown in the inset. f) Comparison of overpotential at a current density of 10 mA/cm2 and Tafel slope on various non-noble catalysts.

However, the Tafel slopes of W and V doping are getting worse and reach to 110 mV/dec and 118 mV/dec, indicating sluggish Volmer kinetics in alkaline condition. In contrast, the Mo-Ni3N shows effectively facilitated kinetics with a Tafel slope of 64 mV/dec. As a result, the Mo-Ni3N needs of overpotential of only 88 mV to drive 100 mA/cm2, which is 150 mV, 67 mV and 50 mV less than that for Ni3N, W-N3N and V-Ni3N, respectively. The electrochemical impedance spectroscopy (EIS) also shows that the charge-transfer resistance of the Mo-Ni3N electrocatalyst is lower than that of the pristine Ni3N, W-N3N and V-Ni3N electrocatalysts. These trends are consistent with DFT predications, and the Mo doped Ni3N exhibits excellent alkaline HER performance among


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those non-noble metal electrocatalysts (Fig 3f and Table S1), even higher than the stateof-the-art Pt/C. To further reveal the intrinsic HER activities, the hydrogen turnover frequency (TOF) is analysed. Firstly, the electrochemical double layer capacitances (Cedl) are assessed. As shown in Fig. S15, the Cedl of Ni3N is about 60 mF/cm2. And the Mo doping only make a slight difference, indicating that the enhanced HER activities originate from the improved intrinsic HER activities, rather than the active surface area. As shown in Fig. 3d, the Mo-Ni3N displays maximum TOF values. A TOF value of 0.083 H2/s, 0.009 H2/s, 0.023 H2/s and 0.007 H2/s is reached under an overpotential of 100 mV for Mo-Ni3N, WNi3N, V-Ni3N and Ni3N, respectively. This confirms that the Mo doped Ni3N with both favorable hydrogen and hydroxyl adsorption energetics and high intrinsic HER activity. A 24 h cycling test is conducted at 20 mA/cm2 in 1 M KOH media to assess the electrochemical HER stability of Mo-Ni3N electrocatalyst. As shown in Fig. 3e, there is no noticeable increase in potential and the LSV polarization curves before and after cycling test only show a tiny difference in current density. These indicate the strong long-term electrochemical stability of Mo-Ni3N electrocatalyst in HER process. Furthermore, the


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structural stability of the Mo-Ni3N are checked by using SEM shown in Fig. S16. The nanoparticles stacked lamellar morphology can still remain after 24 h cycling test, suggesting the high robustness of Mo-Ni3N. Overall, these results clearly confirm the necessity of hydroxyl adsorption energetics for alkaline HER electrocatalytic activities and demonstrate a theory-driven precise surface reactivity tailoring of Ni3N with highly enhanced HER performance.

Figure 4 a) Schematic illustration of four main stages in alkaline HER. b) ECSA normalized polarization curves (symbols) of pure Ni3N and Mo-Ni3N with the fits using the dual-pathway microkinetic model. c) The energy barriers of Volmer step, Heyrovsky step, and Tafel step. d) Free energy diagram of dominant pathway.


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Furthermore, the microkinetics analysis is performed to evaluate the kinetic factors of the three elementary reaction steps (Volmer step, Tafel step and Heyrovsky step) of HER.43,52By fitting the polarization current density curves using the dual-pathway kinetic model, the free energy diagram with four standard activation free energy parameters (GH for hydrogen adsorption free energy, G*0 V for Volmer step, G*0 T for Tafel step, and G*0 H for Heyrovsky step) is constructed. As shown in Figure 4a, the Volmer-Heyrovsky pathway dominates HER for pure Ni3N while the Volmer-Tafel pathway dominates HER for doped Ni3N at low polarization region. The calculated energy barriers are shown in Fig 4c, the rate-determining step (RDS) for Ni3N is determined to be the Heyrovsky step with a high activation energy (G*0 H -GH) of 0.29 eV, which mainly originates from the low GH value. Consistent with the DFT calculations, the doped Ni3Ns show similar mixed reaction pathway with more optimal GH value. Notably, the Ni3N and Mo-Ni3N show low the activation energies for Volmer step, while the W- and V-Ni3N exhibit much higher water discharge energy barrier. This may well come from hydroxyl poisoning. This microkinetics analysis is in good agreement with the dual-descriptors proposed.


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CONCLUSION In summary, we have demonstrated that the energetics of both hydrogen and hydroxyl species is critical for the alkaline HER. By theory-driven precise surface reactivity tailoring of Ni3N, various transition-metal doped Ni3N has been theoretically proven as effective promoter to modulate the HER performance. The Mo, W, V doping with favorable hydrogen adsorption energetics can exhibit low onset overpotential, while only Mo doping of favorable hydroxyl adsorption energetics can effectively facilitate the alkaline HER kinetics with low Tafel slope. Notably, our experimental results are quite consistent with the theoretical predictions. The Mo-Ni3N exhibits highly enhanced HER performance with an overpotential of 12 mV at 10mA/cm2 and a low Tafel slope of 64 mV/dec. The combination of theoretical calculation, experimental data, and microkinetics analysis demonstrates that both hydrogen and hydroxyl species are critical to optimize the alkaline HER electrocatalytic performance. This dual-descriptors is helpful for understanding the HER mechanisms and open up avenues to screen high-activity alkaline HER electrocatalysts in future high-throughput calculations and experiments.


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Supporting Information Computational method, experimental details, and additional figures, table. Acknowledgements This research work is supported by National Natural Science Foundation of China (Grant No. 51302097 and No. 51571096). The authors thank the Analytical and Testing Center of Huazhong University of Science and Technology for their support.

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Dual-descriptors tailoring: The hydroxyl adsorption energies

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