Chapter 2. Activating MoS 2 Basal Plane with Ni 2 P Nanoparticles for Pt-Like Hydrogen Evolution
2.4 Results
2.4.2 Electrochemical HER Performance
38
Figure 2.5. High-resolution transmission electron (HR-TEM) images of Ni2P/MoS2/N:CNT heterostructure. (a) TEM image showing the Ni2P NPs distributed on MoS2 and CNT, and magnified image (inset) displaying MoS2, Ni2P and CNTs together. (b) HR-TEM image, magnified image of (c) Ni2P NPs, (d) a few layered MoS2 with enhanced interlayer distance and corresponding Fast Fourier transforms (FFTs).
Figure 2.6. (a) High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) image and (b) Energy-dispersive X-ray spectroscopy (EDS) TEM elemental mappings for Ni2P/MoS2/N:CNT hybrid catalyst.
39
possible because they are synthesized from the same precursor NiMoS4 containing both metal atoms.
The HER activity of Ni2P/MoS2 was further enhanced by anchoring it on highly conductive and porous N:RGO or N:CNT substrates. Evidently, the carbon supports improve the electronic conductivity, hinder the aggregation of Ni2P nanoparticles, and prevent stacking of MoS2 layers as confirmed in HR- TEM in Figure 2.5. Therefore, the hybrid electrocatalysts Ni2P/MoS2/N:RGO and Ni2P/MoS2/N:CNT require much lower η1 (6.79, 27.9 mV) and η10 (39.5, 93.9 mV), respectively. Especially, the η10 value for Ni2P/MoS2/N:RGO (39.5 mV) is close to commercial 20 wt% Pt/C (13.4 mV). In particular, both Ni2P/MoS2/N:RGO and Ni2P/MoS2/N:CNT electrocatalysts demonstrate even superior performance to that of Pt/C at practically more important high current densities (>200 mA cm−2). The Tafel slopes and exchange currents at zero voltage (j0) are important parameters to determine reaction mechanism and intrinsic kinetics for HER. Thus, Tafel analysis was performed by using the Tafel equation (η = b × log (j) + a, where j = current density and b = Tafel slope) as displayed in Figure 2.7c. The Tafel slopes and exchange current densities of Ni2P/MoS2/N:RGO (39.52 mV dec−1 and 0.93 mA cm−2) are comparable to Pt/C (31 mV dec−1 and 3.46 mA cm−2), and the Tafel slopes suggest that the catalysts follow the Volmer–Heyrovsky mechanism for HER in acids. The η10 and j0 values for all catalysts are also compared in Figure 2.7d. The lowest η10 value of Ni2P/MoS2/N:RGO relative to previously reported state-of-the-art catalysts in Figure 2.8 derived from Table 2.1 demonstrates that our heterostructured hybrid catalysts are unique in showing Pt-like HER activity in acidic media.
40
Figure 2.7. HER performance and stability of Ni2P/MoS2 heterostructures in 0.5 M H2SO4 electrolyte.
(a) iR-corrected polarization curves for HER. (b) Comparison of HER performance of samples synthesized at different reduction temperatures (c) Tafel slopes. (d) η10 values (left) and exchange currents (right).
Figure 2.8. Comparison of overpotential required for 10 mA/cm2 (η10) with previously reported catalysts enlisted in Table 2.1.
41
Table 2.1. Comparison of HER activity of Ni2P/MoS2/N:RGO, Ni2P/MoS2/N:CNT and Ni2P/MoS2
hybrid electrocatalysts with other reported electrocatalysts in 0.5 M H2SO4 electrolyte.
Catalysts Loading (mg/cm2)
η1
(mV) η10
(mV)
Tafel slope
(mV/dec) Reference
Ni2P/MoS2/N:RGO 0.526 6.79 39.49 39.52 This work Ni2P/MoS2/N:CNT 0.526 27.9 93.9 57.8 This work Ni2P/MoS2 0.526 23.8 92 87.69 This work
Ni2P/CC47 10.8 - 69 55 J. Mater. Chem. A, 6, 4088-4094 (2018) Ni2P@graphene48 1.0 - 98 56 Mater. Chem. Front. 1,
973-978 (2017)
MoP44 0.86 50 140 54 Energy Environ. Sci. 7,
2624-2629 (2014) NiMo@NCNT49 2.0 - 65 67 ACS Nano, 10, 10397-10403
(2016) NiPS3/defective
graphene29 0.2 - 73 39 J. Mater. Chem. A, 5, 23536-23542 (2017) metallic MoS226 0.043 - 175 41 Nat. Commun. 7, 10672 (2016) S vacancy MoS250 - - 170 60 Nat. Mater. 15, 48-53 (2016)
MoSx/NCNT21 0.102 75 110 40 Nano Lett. 14, 1228-1233 (2014) MoP|S/Ti foil31 3.0 - 64 50 Angew. Chem. Int. Ed. 53,
14433-14437 (2014) NiCoPS/CC51 0.53 26 57 44.9 Nano Res. 10, 814-825 (2017)
MoP/SN-G32 0.5 30 99 54.41 ACS Catal. 7, 3030-3038 (2017) N-doped MoS220 1.0 - 160 36 Nanoscale 10, 14726–14735
(2018) CoP/Ni5P4/CoP
/Ni foam52 - - 33 43 Energy Environ. Sci. 11, 2246-2252 (2018)
The number of active sites and turnover frequencies (TOFs) were also estimated according to a previously reported method.13 The number of exposed metal ion sites (m) for each electrocatalyst was titrated from cyclic voltammetry (CV) curve measured in 0.1 M phosphate buffer solution.
m = 𝑄𝐴
2𝐹
Here, the anodic charge (QA, Coulombs) was integrated from the anodic part of CV curves measured in
42
0.1 M phosphate buffer solution (PBS, pH 7) as displayed in Figure 2.9a, F is Faraday constant (96485.4 C/mol). The TOF is defined as follows:
TOF = J × A
2𝐹𝑚
where J is the current density (A cm-2 = C s-1 cm-2) at a specific overpotential, A is the surface area of the electrode (cm2), the factor 1/2 is related to the number of electrons required to generate one molecule of H2, and m is the number of surface active sites (moles) those take part in electrochemical hydrogen evolution reaction. Then TOF was calculated by normalizing the HER current with the titrated active sites according to the procedure. As in Figure 2.9b, the number of active sites and the activity per site (TOF) at 150 mV of Ni2P/MoS2/N:RGO (1.14 × 10−8 mol, 12.27 s−1) and Ni2P/MoS2/N:CNT (1.22 × 10−8 mol, 5.37 s−1) are much higher than those of Ni2P (5.45 × 10−9 mol, 0.13 s−1), MoS2 (1.27 × 10−9 mol, 0.20 s−1), and Ni2P/MoS2 (7.83 × 10−9 mol, 3.53 s−1). The results reveal that Ni2P/MoS2
heterostructure formation and introduction of the porous/conductive carbon supports increase not only the number of active sites, but also its intrinsic catalytic activity of TOFs.
The double layer capacitance (Cdl), which is proportional to electrochemically active surface area (ECSA), is another parameter that can affect the electrocatalytic performance. Cdl values are obtained from the linear slope of the plot between scan rate and Δj (=ja – jc) as displayed in Figure 2.9c. The Cdl
and ECSA values for Ni2P/MoS2/N:CNT (61.7 mF cm−2 and 1542.5 cm2) and Ni2P/MoS2/N:RGO (13.8 mF cm−2 and 345 cm2) are much higher than Ni2P (0.34 mF cm−2 and 8.5 cm2), MoS2 (0.34 mF cm−2 and 8.5 cm2), Ni2P+MoS2 (0.4 mF cm−2 and 10.0 cm2), and Ni2P/MoS2 (5.9 mF cm−2 and 147.5 cm2), revealing the favorable effect of the carbon supports. The Cdl and ECSA values of Ni2P/MoS2/N:CNT are larger than Ni2P/MoS2/N:RGO, but the latter displays higher HER activity than the former, indicating that all the surfaces represented by ECSA do not become active sites of HER.46
Electrochemical impedance spectroscopy (EIS) was applied to all electrocatalysts in 0.5 M H2SO4 at an overpotential of 100 mV in order to probe charge transfer characteristics, and the Nyquist plot was fitted to an equivalent circuit model in Figure 2.9d. The similar solution (series) resistance (Rs ≈ 5.1 Ω) for each sample was determined, which was then used for iRs correction in polarization curves. The charge transfer resistance Rct for Ni2P/MoS2/N:RGO (13.95 Ω) was comparable to Ni2P/MoS2/N:CNT (14.48 Ω), but both were much lower than those of Ni2P/MoS2 (109.3 Ω), Ni2P + MoS2 (1198 Ω), MoS2
(1202 Ω), and Ni2P (679.2 Ω). The heterostructure Ni2P/MoS2 shows a highly decreased Rct value. In other words, less conductive basal plane of MoS2 changed to have an improved charge transfer kinetics through coupling with conductive Ni2P NPs.20 A little bit smaller Rct of Ni2P/MoS2/N:RGO than Ni2P/MoS2/N:CNT may be related to better contact between both 2D MoS2 and N:RGO materials,
43 which results in the higher electronic conductivity.
Figure 2.9. (a) Cyclic voltammetry of all synthesized samples in a phosphate buffer solution (pH 7).
The integrated charge of the anodic curve for the whole potential range was used to estimate the number of active sites for TOF. (b) The number of active sites (left) and TOF at 150 mV (right). (c) Double layer capacitance calculated from the linear slope between ∆j (= ja - jc) and scan rates. (d) EIS Nyquist plots (solid line: fitted, dots: experimental) measured at an overpotential of 100 mVand corresponding equivalent circuit model (inset).
The electrochemical stability is a great concern for any non-Pt electrocatalysts, especially in acidic solutions. Thus, the stability of the hybrid electrocatalysts was determined by an accelerated degradation test of 3000 continuous CV cycles in the range of 0.03 to −0.17 VRHE in 0.5 M H2SO4 in Figure 2.10a.
The Ni2P/MoS2/N:CNT catalyst displays remarkable stability showing even an improved HER performance after the 3000 CV cycles. This could be related to the in situ electrochemical reduction of surface oxides. The η10 value for Ni2P/MoS2/N:CNT reduces from 93.9 to 79.3 mV after 3000 CV cycles.
The 60 h long chronoamperometry (CA) test was conducted at a constant overpotential of 150 mV as shown in Figure 2.10b. Interestingly, the Ni2P/MoS2/N:CNT catalyst displays better stability with
44
negligible current loss (3.44%) after a continuous 60 h long CA test. Therefore, Ni2P/MoS2/N:RGO gives better initial activity, but a slight degradation is observed after 3000 CV cycles, which could be attributed to the removal of the catalyst from the surface of speedy rotating disc electrode (RDE). The XPS analysis and HR-TEM analysis after 60 h of continuous HER are shown in Figures 2.11 and 2.12.
In XPS analysis, the peaks for surface oxides are evidently decreased. Moreover, HR-TEM images indicate that there is no sign of morphological degradation during the reaction in Ni2P/MoS2/N:CNT.
Figure 2.10. (a) LSV curves before (solid) and after (dotted) 3000 CV cycles for Ni2P/MoS2/N:CNT and Ni2P/MoS2/N:RGO. (b) Long-term durability of Ni2P/MoS2/N:CNT determined by chronoamperometry (CA) at overpotential of 150 mV for 60 h in 0.5 M H2SO4 aqueous solution.
45
Figure 2.11. High resolution XPS scans of (a, e) Mo 3d, (b, f) Ni 2p, (c, g) S 2p, (d, h) P 2p for Ni2P/MoS2/N:CNT hybrid catalyst on carbon cloth. (a), (b), (c), (d) are for before 60 h CA and (e) , (f), (g), (h) are for after 60 h CA.
46
Figure 2.12. (a) HR-TEM analysis of Ni2P/MoS2/N:CNT hybrid catalyst after long-term HER durability test.
The electrochemical HER performances were also determined in 1.0 M KOH solution. Similar activity trends are observed, but all hybrid catalysts displayed almost similar performances with η1
(81.2–92.8 mV), η10 (149–182.7 mV), and Tafel slopes in the range of 60.22–69.46 mV dec−1 as displayed in Figure 2.13a, b. The promotional effect of hybridization between Ni2P and MoS2 is also observed in alkaline media, but the effect is less pronounced than in the acidic electrolyte. The η10 values of Ni2P/MoS2 hybrid catalysts (149–159 mV) are much lower than pure Ni2P (277 mV), MoS2 (282 mV), and most of the other reported catalysts in Table 2.2. The η10 and exchange current densities (j0) of all electrocatalysts are compared in Figure 2.13c. The double layer capacitances (Cdl) of all hybrid catalysts are in the range of 14.8–33.3 mF cm−2 and are larger than Ni2P (0.3 mF cm−2), MoS2 (3.95 mF cm−2), and a physical mixture of Ni2P+MoS2 (4.01 mF cm−2) as exhibited in Figure 2.14a. Similarly, the charge transfer resistances (Rct) of all Ni2P/MoS2 catalysts are much smaller than individual Ni2P, MoS2, and a Ni2P+MoS2 mixture as determined by the Nyquist plots at a constant voltage of −0.1 VRHE
in 1.0 M KOH as in Figure 2.14b. The η10 values are also compared with previously reported catalysts in 1.0 M KOH electrolyte in Figure 2.15 derived from Table 2.2.
Figure 2.13. HER performance of Ni2P/MoS2 heterostructures in 1.0 M KOH electrolyte. (a) iR- compensated polarization curves for HER and (b) corresponding Tafel slopes in. (c) Comparison of η10
values (left) and exchange currents (right) for all electrocatalysts.
47
Figure 2.14. (a) Double layer capacitance (Cdl) calculated from the linear slope between ∆j (= ja - jc) and scan rates of all synthesized samples. (b) Electrochemical impedance spectroscopy (EIS) Nyquist plots (solid line: fitted, symbol: experimental) measured at overpotential – 0.1 VRHE for all synthesized samples and equivalent circuit used for fitting of EIS data (inset).
Figure 2.15. Comparison of overpotential required to generate current density of 10 mA cm-2 with catalysts in 1.0 M KOH and enlisted in Table 2.2.
48
Table 2.2. Comparison of HER activity of Ni2P/MoS2/N:RGO, Ni2P/MoS2/N:CNT and Ni2P/MoS2
hybrid electrocatalysts with other reported electrocatalysts in 1.0 M KOH electrolyte.
Catalysts Loading (mg/cm2)
η1
(mV) η10
(mV)
Tafel slope
(mV/dec) Reference
Ni2P/MoS2/N:RGO 0.526 81.2 149 67.29 This work Ni2P/MoS2/N:CNT 0.526 82.7 152.1 65.54 This work Ni2P/MoS2 0.526 89.6 159 69.46 This work Ni2P/MoO2/MoS2
/Ti foil36 - 73 159 77 Nanoscale 9,
17349-17356 (2017) Co3S4/MoS2
/Ni2P nanotubes/C34 0.182 60 178 98 J. Mater. Chem. A 5, 25410-25419 (2017) CoS/MoS2/C34 0.182 120 214 106 J. Mater. Chem. A 5, 25410-25419 (2017) MoS2/Ni3S2
/Ni foam53 13.1 50 110 83.1 Angew.Chem.Int. Ed. 55, 6702-6707 (2016) NiFeSP/Ni foam33 4.2 - 91 82.6 ACS Nano. 11, 10303-10312
(2017)
Ni2P/CC47 10.8 - 73 73 J. Mater. Chem. A 6,
4088-4094 (2018) NiPS3/defective
graphene29 0.2 - 99 36 J. Mater. Chem. A 5, 23536-23542 (2017) Ni5P4|S
/Ni foam30 - - 103 - ChemElectroChem. 4,
1108-1116 (2016) Ni-MoS2/CC18 0.89 - 98 60 Energy Environ. Sci. 9,
2789-2793 (2016) MoP/SN-G32 0.5 7 49 31.4 ACS Catal. 7, 3030-3038
(2017) Porous-MoS2/Ni3S2/
Ni Foam54 58.74 - 99 71 ACS Appl. Energy Mater. 1, 3929−3936 (2018) Ni−MoxC/NC55 0.86 58 161 104.8 ACS Appl. Mater. Interfaces
10, 35025−35038 (2018)