3.4 Results and Discussion
3.4.2 Electrochemical HER Performances
The electrochemical performances of Co9S8-MoS2, Co3O4-(am-MoSx), Co9S8 and MoS2 were evaluated in aqueous 1.0 M KOH (pH=14), 0.5 M H2SO4 (pH=0) and 0.1 M phosphate buffer solution (PBS, pH=7) electrolytes by using a standard three-electrode cell and compared with commercial 20 wt% Pt/C (Alfa Aesar). Through initial optimization experiments in 1.0 M KOH (Figure 3.10), the hydrothermal reaction time of 12 h and the reduction temperature of 400 oC were selected to prepare Co9S8-MoS2 of the best HER performance. Figure 3.11a compares HER performances of synthesized electrocatalysts by the polarization curves in 1.0 M KOH. The Co9S8-MoS2 catalyst displays a higher performance in comparison to Co3O4-(am-MoSx) and its individual components of Co9S8 and MoS2. Thus, it requires an overpotential (𝜂10) of 167 mV to generate a current density of 10 mA/cm2, which is much lower than those of bare MoS2 (349 mV), Co9S8 (380 mV), and Co3O4-(am-MoSx) (190 mV).
This improved performance may be attributed to the activation of the inert basal plane of MoS2 by Co9S8
and defects that improve electronic clouds near Mo atoms as suggested by XPS and XAS analyses. This synergistic effect of heterostructuring becomes more prominent when Co9S8-MoS2 is grown on highly conductive and porous nickel foam (NF) support. The digital photographs of bare NF, Co3O4-(am- MoSx)/NF, and Co9S8-MoS2/NF are displayed in Figure 3.12. Due to the promotional effect of NF, the 𝜂10 value of Co9S8-MoS2/NF is further lowered to 110 mV. Notably, the activity of Co9S8-MoS2/NF is much better than already reported core-shell Co9S8@MoS2 (143 mV)44, Co-MoS2/CC (203 mV)15, CoSx/MoSx chalcogels (230 mV)59, and also superior to Pt/C in a high current density region (>170 mA/cm2), which is more meaningful in practical applications.
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Figure 3.10. Comparison of HER activities of samples synthesized at (a) different hydrothermal treatment times and (b) reduction temperatures in 1 M KOH electrolyte.
Two important kinetic parameters, Tafel slopes and exchange current densities (j0) at zero voltage, were determined from the Tafel equation (𝜂 = 𝑏 × log(𝑗) + 𝑎, where j = current density, b = Tafel slope) as displayed in Figure 3.11b. The Tafel slopes for Co9S8-MoS2/NF, Co9S8-MoS2, Co3O4-(am- MoSx), MoS2 and Co9S8 are 81.7, 85.5, 86.1, 86.1, and 91.8 mV/dec, respectively. These Tafel slopes suggest that all catalysts follow the Volmer-Heyrovsky mechanism in alkaline media. The exchange current densities (j0) were obtained by extrapolating Tafel plots to zero overpotential, giving 0.438, 0.116, 0.057, 0.001 and 0.001 mA/cm2 for Co9S8-MoS2/NF, Co9S8-MoS2, Co3O4-(am-MoSx), MoS2 and Co9S8, respectively. The higher j0 values of Co9S8-MoS2 and Co9S8-MoS2/NF reveal much faster HER kinetics of these electrodes than those of MoS2 and Co9S8 electrocatalysts.
The number of active sites and turnover frequency (TOF, s-1) were also calculated to find the intrinsic catalytic activity according to a reported method13. The number of exposed metal ion sites (m) for each electrocatalyst was obtained by using the equation below.
m = 𝑄
2𝐹
Here, Q is the charge (C) that was calculated by half-integration of cyclic voltammetry (CV) curve for the whole potential range in Figure 3.11c measured in 0.1 M phosphate buffer solution (pH 7). Thus the number of exposed metal ion sites (m) for each electrocatalyst was estimated from integrated charge.
Then TOF was calculated by normalizing the HER current with the titrated active sites according to the equation below.
TOF = 𝐽×𝐴
2𝐹𝑚
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where J is the current density (C∙s-1∙cm-2), A is the surface area of the electrode (cm2), F is Faraday constant (96485 C/mol), m is the number of surface active sites (moles) and the factor 1/2 is related to the number of electrons required to produce one molecule of H2. The number of active sites (left) and TOF (right) values for as-synthesized catalysts are compared in Figure 3.11d. The TOF values at –0.2 VRHE are Co9S8-MoS2/NF (3.10 s-1) > Co9S8-MoS2 (0.54 s-1) > Co3O4-(am-MoSx) (0.42 s-1) > Co9S8
(0.19 s-1) > MoS2 (0.07 s-1). The higher TOF and larger number of active sites for Co9S8-MoS2/NF and Co9S8-MoS2 disclose that heterostructuring increases not only the number of active sites, but also its intrinsic catalytic activity of each site. The lowest η10 value of Co9S8-MoS2/NF compared to previously reported state-of-the-art catalysts in Figure 3.11e derived from Table 3.2 displays that the covalent heterostructuring with Co9S8 turns MoS2 a Pt-like HER catalyst in alkaline media.
Electrochemical impedance spectroscopy (EIS) measured at –0.14 VRHE gives the charge transfer properties between electrode and electrolyte. The solution (series) resistance (Rs) was used to compensate iR-correction in all polarization curves. The charge transfer resistances (Rct) for Co9S8- MoS2/NF (10.72 Ω) and Co9S8-MoS2 (59.23 Ω) are much smaller than those of Co3O4-(am-MoSx) (15402 Ω), MoS2 (43014 Ω) and Co9S8 (3858 Ω) as shown in Figure 3.11f. The lower charge transfer resistance of Co9S8-MoS2 is attributed to the formation of the heterostructure with metallic Co9S8, leading to improved charge transfer through the covalently connected Co-S-Mo bonds at the Co9S8/MoS2 interface, which eventually accelerates HER reaction.
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Figure 3.11. The electrochemical measurements of all synthesized catalysts in 1 M KOH. (a) HER polarization curves. (b) Tafel plots. (c)Cyclic voltammetry of all synthesized samples in a 0.1 M PBS (pH 7). The half of integrated charge of CV for the whole potential range was used to calculate the number of active sites for TOF. (d) The number of active sites (left) and TOF at –200 mV (right). (e) Comparison of overpotential (η10) required to generate current density of 10 mA/cm2 for reported catalysts. (f) EIS Nyquist plots measured at an overpotential of 140 mV and magnified plots (inset).
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Table 3.2. Comparison of HER activity of Co9S8-MoS2/NF heterostructure with other reported electrocatalysts in 1M KOH electrolyte.
Catalyst η10 (mV) Reference
Co9S8-MoS2/NF 110 This Work
Co9S8-MoS2 167 This Work
Co3O4-(am-MoSx) 189 This Work
NiCo2S4/Ni3S2/NF60 119 ACS Appl. Mater. Interfaces, 10, 10890−10897 (2018) CoNC@MoS2/CNF61 143 J. Mater. Chem. A, 5, 23898-23908
(2017)
CoS-Co(OH)2@aMoS2+x/NF23 143 Adv. Funct. Mater. 26, 7386–7393 (2016)
NiS-Ni(OH)2@aMoS2+x/NF23 226 Adv. Funct. Mater. 26, 7386–7393 (2016)
CoMoS4-H (MoS2+CoS2 phase)62 153 Electrochimica Acta, 213, 236–243 (2016)
Co-MoS2/CC15 203 Energy Environ. Sci. 9, 2789-2793 (2016)
CoSx/MoSx chalcogels59
(0.1M KOH) 230 Nature Materials, 15, 197–203
(2016)
Co9S8@MoS2 core-shell44 143 ACS Appl. Mater. Interfaces, 10, 1678-1689 (2018)
CoS/MoS2 + commercial C63 214 J. Mater. Chem. A, 5, 25410–25419 (2017)
Ni2.3%–CoS2/CC64 231
@ η100
Electrochemistry Communications, 63, 60–64 (2016)
2D-MoS2/Co(OH)265 125 Adv.Mater. 30, 1801171 (2018) 2D-MoS2/Ni(OH)265 185 Adv.Mater. 30, 1801171 (2018) TiO2@Co9S866 139 Adv. Sci. 5, 1700772 (2018) Co9S8@NOSC50 320 Adv. Funct. Mater. 27, 1606585
(2017)
Figure 3.12. Digital photos of (a) bare Ni foam, (b) Co3O4-(am-MoSx)/NF, and (c) Co9S8-MoS2/NF.
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The electrochemically active surface area (ECSA) is another important parameter to evaluate the HER performance of an electrocatalyst, which is calculated from electrochemical double-layer capacitance (Cdl). This Cdl is obtained from the linear slope of the plot of ∆J (=Ja-Jc) from CV in a non- Faradaic window at different scan rates as shown in Figure 3.13. The ECSA is then calculated by dividing Cdl by Cs, where Cs is the specific capacitance for a flat electrode per unit surface area (cm2) and was assumed to be 40 μF/cm2 in this study. The Cdl and ECSA values of Co9S8-MoS2/NF (63.05 mF/cm2, 1576.25 cm2) and Co9S8-MoS2 (10.29 mF/cm2, 257.25 cm2) are much higher than those of Co3O4-(am-MoSx) (1.73 mF/cm2, 43.25 cm2), Co9S8 (5.85 mF/cm2, 146.25 cm2) and MoS2 (10.02 mF/cm2, 250.5 cm2). The results indicate that the promotional effect comes from multiple sources; the supramolecular Co9S8-MoS2 heterostructure, in-situ formed defects, and porous NF support.
Figure 3.13. (a-e) CV curves at different scan rates for all synthesized catalysts. (f) Double layer capacitance estimated from the linear slope between ∆j (= ja - jc) and scan rates.
The electrochemical stability of the catalyst is a prime concern in the commercialization of a water electrolyzer. Therefore, a 60-h long, reversible step-wise chronoamperometry (CA) stability analysis was also carried out to see the electrode stability at lower and higher current densities as displayed in Figure 3.14a because the supplied potential in a real system is not constant. The applied overpotential oscillated between –0.14 and –0.2 VRHE for 12 h for each step. Interestingly, the current density remains constant in every interval at the lower overpotential, while HER performance continuously increases at the higher overpotential. This might be due to the rapid reduction of surface oxides at the higher current density. In addition, a 60-h long, chronopotentiometry (CP) stability test was conducted at a constant
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current density of –30 mA/cm2 as shown in Figure 3.14b. The Co9S8-MoS2/NF electrode displays good durability with a minor increase in overpotential (64 mV) after 60 h. The η10 value for Co9S8-MoS2/NF in Figure 3.14c slightly increases from 120.7 to 130.5 mV after the stability test. The reductive annealing synthesis can make the heterostructure have strong chemical bonds of Co-S-Mo type between the phases with the conversion of inactive oxides to active sulfides, amplifying both activity and stability. All experimental results described so far reveal that Co9S8-MoS2/NF can be a potential non- precious metal electrocatalyst for the practical alkaline water electrolysis system in terms of the high HER activity and durability.
Figure 3.14. (a) Step-wise chronoamperometry test of Co9S8-MoS2/NF. The potentials of –0.14 and – 0.2 VRHE are applied by turns for 60 h. (b) A long-term durability for Co9S8-MoS2/NF determined by CP at –30 mA/cm2 for 60 h. (c) LSV curves before and after 60-h CP test.
By extension, the electrochemical HER performance of all synthesized electrocatalysts was also tested in acidic (0.5 M H2SO4) and neutral (0.1 M PBS) electrolytes. As in Figure 3.15, the hydrothermal time of 12 h and the reduction temperature of 400 oC were found to be the optimum synthetic conditions in both electrolytes just like in alkaline media. Linear sweep voltammetry (LSV) curves and corresponding Tafel plots are compared in acidic (Figure 3.16a, b) and neutral (Figure 3.16c, d) electrolytes. The 𝜂10 values in 0.1 M PBS for Co9S8-MoS2/NF, Co9S8-MoS2, Co3O4-(am- MoSx), MoS2 and Co9S8 are 152.1, 161.6, 174.3, 398.4 and 370.7 mV, respectively. The performance of Co9S8-MoS2/NF is impressive because it outperforms even incumbent Pt/C in the high current region (>60 mA/cm2). The Tafel slopes in both electrolytes of Co9S8-MoS2/NF (99.4 mV/dec in 0.1 M PBS), Co9S8-MoS2 (79.58, 106 mV/dec) are much lower than those of its individual components MoS2 (87.84, 173.8 mV/dec) and Co9S8 (193.9, 168.3 mV/dec). The LSV curves, 𝜂10 and 𝜂50 values in all pH values are also compared in Figure 3.16e, f. In general, all the non-precious metal catalysts and Pt/C give better performance in an alkaline aqueous solution than in a neutral electrolyte. These results indicate that the intrinsic catalytic activity of MoS2 could be enhanced dramatically by heterostructuring with Co9S8 NPs, and it could be a promising alternative as non-precious material-based catalysts in alkaline
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and neutral electrolytes. The activity of Co9S8-MoS2/NF was not determined in an acidic electrolyte because of the instability of NF. However, the high activity approaching that of Pt/C is expected if an acid-stable, porous substrate like carbon cloth is employed.26
Figure 3.15. Comparison of HER activities of samples synthesized at different hydrothermal treatment times and reduction temperatures (a, b) in 0.5 M H2SO4 electrolyte, and (c, d) in 0.1 M PBS electrolyte, respectively.
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Figure 3.16. HER performances and Tafel plots of all synthesized catalysts in (a, b) 0.5 M H2SO4 and (c, d) 0.1 M PBS electrolytes. (e) Comparison of HER performances of Co9S8-MoS2 in different pH of electrolytes. (f) The 𝜂10 and 𝜂50 values in each electrolyte are compared.