Comparison of OER activities of Co9S8-MoS2 heterostructure synthesized at (a) different hydrothermal treatment time, (b) reduction temperatures in 1 M KOH electrolyte. Comparison of HER activity of Co9S8-MoS2/NF heterostructure with other reported electrocatalysts in 1M KOH electrolyte.
Introduction
To mitigate energy depletion and environmental problems, hydrogen (H2) as an energy carrier would be one of the leading options for future carbon-neutral energy systems in the sense that it has the highest gravimetric energy density (figure 1.4).6 In the existing industries, hydrogen is used for refining. In the hydrogen economy, the various clean energy technologies, for example fuel cells, CO2 conversions, use hydrogen as the primary energy carrier (Figure 1.5).7-9 Currently, around 44.5 million tons of hydrogen are produced per year worldwide.
Water Electrolysis …
Because water has a poor proton conductivity, water electrolysis is usually carried out in aqueous acidic, alkaline or some buffer solutions. 32 On the other hand, the electrolysis of alkaline water is carried out in an alkaline medium, which extends the choice of electrocatalysts to earth-rich materials.
Fundamentals of Electrochemical Water Splitting
Mechanism of Hydrogen Evolution Reaction (HER)
Mechanism of hydrogen evolution reaction on the surface of an electrode in (a) acidic and (b) alkaline electrolytes. As discussed above, the rate of total water splitting strongly depends on the first hydrogen adsorption step (Volmer reaction).
Mechanism of Oxygen Evolution Reaction (OER)
The volcano relationship is plotted against experimentally measured exchange current densities against ΔGH from density functional theory (DFT). The volcano relationship is plotted with experimentally measured overpotentials at 1 mA cm-2 against ΔGO−ΔGOH from density functional theory (DFT).
Evaluation Approaches of HER and OER
- Electrocatalytic Activity
- Tafel Analysis
- Electrocatalytic Stability
- Electrochemical Impedance Spectroscopy (EIS)
- Electrochemical Active Surface Area (ECSA)
- Turnover Frequency
- Faradaic Efficiency
- Theoretical Descriptor
TOF is determined by the total number of molecules transformed from the reactant molecules per catalytically active site per unit time. The total charging of the external circuits can be calculated from the galvanostatic or potentiostatic measurement.
Transition Metal Dichalcogenides (TMDs)
Structure of TMDs
These stable 2H and 3R phases share most of the properties such as Raman spectra, electronic band structure except for minor differences in its band structure and the location of the UV-Vis absorption peak because the different order of aggregation changes the ionic contribution. The dashed lines show how the top and side views match each other.
Properties of TMD for Electrochemical Application
The states at the Γ point and the minimum point of the conduction band have strong characteristics of the d orbitals of Mo and the antibonding of the pz orbitals of S which means that they have a strong dependence on interlayer coupling.67. Reprinted with permission from ref 67. 1.19b, the HER activity of MoS2 depends linearly on the edge length of MoS2.71 Thus, the edge sites of MoS2 are the catalytically active sites while the basal plane is inert as confirmed theoretically and experimentally.
Strategies for Enhancing Electrocatalytic Activity of TMDs …
On the other hand, TMD can be synthesized via chemical reactions called bottom-up techniques. There are primary bottom-up techniques such as chemical vapor deposition (CVD)75-77 and wet chemical synthesis (e.g. solvothermal or hydrothermal reactions)78-80 as shown in Figure 1.20c, d.81 During the synthesis, defect engineering, heteroatom doping, heterostructure construction, or tuning phase can be performed.
Research Scope
Furthermore, a phase transition from the 2H to the 1T phase can also be induced during exfoliation with Li intercalation (Figure 1.20b). Schematic strategies for (a, b) top-down and (c, d) bottom-up production of 2D nanomaterials. a) Mechanical splitting using adhesive tape.
Wang, J.; Xu, F.; Jin, H.; Chen, Y.; Wang, Y., Non-noble metal-based carbon composites in the hydrogen evolution reaction: Fundamentals for applications. Shi, Y.; Zhang, B., Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Y.; Zhang, Y.; Liu, Z., Controllable growth and transfer of MoS2 monolayer on Au foils and its potential application in hydrogen evolution reaction.
Y.; Wang, X., Ultrathin MoS2 nanoplates with rich active sites as highly efficient catalyst for hydrogen evolution.
Activating MoS 2 Basal Plane with Ni 2 P Nanoparticles for Pt-Like Hydrogen Evolution
Introduction
11 Molybdenum disulfide (MoS2) has received much attention as a pH-universal electrocatalyst13, but its HER activity is modest because active catalytic sites are limited to the small number of edge sites, while broader basal planes of MoS2 remain inert.10, 14 Numerous efforts are have been devoted to improving the HER activity of MoS2 through maximizing the number of exposed active sites, 13, 15 generation of sulfur vacancies and defects, 13, 16 doping with heteroatoms, 17-20 assembly with conductive carbon materials, 21-24 and control the phase.25-26 Despite all these efforts, it is still a great challenge to increase the catalytic activity of MoS2-based electrocatalysts at the level of commercial Pt catalysts.27-. Herein, we propose a strategy to increase the activity of MoS2 by forming a Ni2P/MoS2 heterostructure to activate the basal planes of MoS2 with Ni2P nanoparticles (NPs). The Ni2P/MoS2 heterostructure shows a HER activity much higher than that of individual MoS2 and Ni2P components.
This improved performance is related to the activation of the MoS2 basal plane by forming a broadly cross-doped and chemically linked heterostructure with Ni2P nanoparticles.
Experimental Methods
- Synthesis of Single Phase Ni 2 P and MoS 2
- Physical Characterization
- Electrochemical Measurements
There are several reports of non-noble metal-based electrocatalysts showing Pt-like HER activity in alkaline media, but they are rare in “acidic” media. Therefore, the high HER activity and durability of our Ni2P/MoS2/N:RGO or Ni2P/MoS2/N:CNT catalysts in acidic media suggest their high potential to replace expensive Pt-based electrocatalysts in practical water electrolyzers. The electrochemical stability tests were performed up to 3000 CV cycles in the range from 0.03 to -0.17 VRHE.
EIS was performed in the frequency range 100 kHz to 1 mHz with a modulation amplitude of 10 mV.
Results
- Structure and Properties of Ni 2 P/MoS 2 Heterostructures
- Electrochemical HER Performance
High-resolution transmission electron microscopy (HRTEM) image of Ni2P/MoS2/N:CNT heterostructure in Figure 2.5 shows that Ni2P NPs deposited on MoS2 sheets are successfully grown on N:CNT. The particle size of Ni2P is in the range of 8-15 nm as confirmed by TEM images of Ni2P/MoS2/N:CNT hybrid. HER performance and stability of Ni2P/MoS2 heterostructures in 0.5 M H2SO4 electrolyte. a) iR-corrected polarization curves for HER.
HER performance of Ni2P/MoS2 heterostructures in 1.0 M KOH electrolyte. a) iR-compensated polarization curves for HER and (b) the corresponding Tafel slopes at.
Discussion
As a result, Ni2P NPs appear to activate HER-inert basal levels of MoS2 nanosheets to synergistically enhance HER activity. It is also worth noting that the heterostructuring is more effective than the formation of solid solutions, which has recently been proposed as an effective method to improve the HER activity of MoS2.31, 56 The hybridization with N-doped carbon supports increases the number of active sites by improves dispersion of the Ni2P/MoS2 heterostructure, and increases the intrinsic kinetics (TOF) of HER. Although there have been several non-noble metal electrocatalysts reported to exhibit Pt-like HER activity and stability in alkaline media, there is no such report in acids to the best of our knowledge.
Therefore, our Ni2P/MoS2/N:CNT and Ni2P/MoS2/N:RGO could be promising candidates for HER in acidic media.
Conclusion
It is of practical importance that the promotion effect works more effectively in acidic electrolytes than in alkaline media.
Mr.; Zhang, P.; Cao, X.; Song, B.; Jin, S., Contributions of phase, sulfur vacancies, and edges to the catalytic activity of the hydrogen evolution reaction of porous molybdenum disulfide nanosheets. Cao, P.; Peng, J.; Lee, J.; Zhai, M., Amorphous molybdenum disulfide supported on highly conductive carbon black for efficient hydrogen evolution reaction. Lin, H.; Lee, H.; Li, Y.; Liu, J.; Wang, X.; Wang, L., Hierarchical CoS/MoS2 and Co3S4/MoS2/Ni2P Nanotubes for Efficient Electrocatalytic Hydrogen Evolution in Alkaline Media.
Wang, T.; Guo, Y.; Zhou, Z.; Chang, X.; Zheng, J.; Li, X., Ni-Mo nanocatalysts on N-doped graphite nanotubes for highly efficient electrochemical hydrogen evolution in acid.
Introduction
16-17 Despite all these efforts, the HER activity of MoS2 is still limited due to the poor electrical conductivity and dominant basal planes. Fabrication of the heterostructures with other transition metal compounds is considered as another strategy to improve the electrocatalytic performance of MoS2.18-23. In particular, covalent coupling of MoS2 with metallic or metal-like compounds modifies its electronic characteristics and enhances the HER activity more effectively than simple van der Waals coupling.24 Recently, our group reported a covalent Ni2P/MoS2/N:C heterostructure that exhibits superior HER activity in acidic media due to activation of the inert basal plane by growing the highly conductive Ni2P nanoparticles on MoS2. Theoretical and experimental studies reveal that doping Co atoms in the structure of MoS2 reduces the hydrogen bond energy, a prerequisite for higher HER activity.
Both experimental and theoretical results confirm that Co species increase the electron density of Mo atoms in the MoS2 basal plane in the synthesized Co9S8-MoS2 heterostructure.
Experimental Methods
- Synthesis of Co-pyrazole Complex
- Synthesis of Co 3 O 4 -(am-MoS x )
- Synthesis of Co 9 S 8 -MoS 2 Heterostructure
- Synthesis of Single Phases of Co 9 S 8 and MoS 2
- Materials Characterizations
- Electrochemical Measurements
- Theoretical Section
In addition, the in situ generated S basal plane defects near the Co9S8 nanoparticles (NPs) tune the d-band center of the adjacent Mo site, leading to a near-zero Gibbs free energy of H adsorption. The surface chemical states were identified by X-ray photoelectron spectroscopy (XPS, Thermo-Fisher , K-alpha). Transmission electron microscopy (TEM, JEOL, JEM-2100) and high-resolution transmission electron microscopy (HR-TEM, JEOL, JEM-2100F) were used to investigate the structural information, elemental mapping, and chemical composition of the heterostructures.
There are two configurations for the adsorption site of the Co9S8 nanoparticle on the MoS2 monolayer, S-top and Mo-top.
Results and Discussion
- Structure and Properties of Co 9 S 8 -MoS 2 as a Covalent 0D-2D Heterostructure
- Electrochemical HER Performances
- Theoretical Insight for Improved HER Performance
Similarly, Co3O4 NPs are converted to Co9S8 NPs at the basal plane of the MoS2 sheets in the Co9S8-MoS2 heterostructure as shown in Figure 3.8b. The elemental composition of the Co9S8-MoS2 heterostructure is further investigated using line scanning HAADF-STEM in Figure 3.8f. Stronger H adsorption on the Mo2 site of double-defect Co9S8-MoS2 benefits by concentrating H*, but.
Gibbs free energies of hydrogen adsorption at each Mo site for (c) one- and (d) two-defect Co9S8-MoS2.
Conclusion
Rodriguez, P.; Xiang, B.; Wang, Z.; Liang, Y.; Gu, M., Design of active nickel single-atom decorated MoS2 as a pH-universal catalyst for hydrogen evolution reaction. Huang, J.; Hou, D.; Zhou, Y.; Zhou, W.; Li, G.; Tang, Z.; Li, L.; Chen, S., MoS2 nanowire-coated CoS2 nanowire arrays on carbon cloth as three-dimensional electrodes for efficient electrocatalytic hydrogen evolution. Li, H.; Qian, X.; Xu, C.; Huang, S.; Zhu, C.; Jiang, X.; Shao, L.; Hou, L., Hierarchical Porous Co9S8/Nitrogen-Doped Carbon@MoS2 Polyhedrons as pH Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction.
Shao, L.; Qian, X.; Wang, X.; Li, H.; Yan, R.; Hou, L., Low-cost and highly efficient CoMoS4/NiMoS4-based electrocatalysts for hydrogen evolution reactions over a wide pH range.
Heterostructuring Cobalt Sulfide with MoS 2 to Induce Bifunctionality of Overall
- Introduction
- Experimental Methods
- Synthesis of Co-pyrazole Complex
- Synthesis of Co 3 O 4 -(am-MoS x )
- Synthesis of Co 9 S 8 -MoS 2 Heterostructure
- Synthesis of Single Phases of Co 9 S 8 and MoS 2
- Materials Characterizations
- Electrochemical Measurements
- Results and Discussion
- Electrocatalytic OER Performance
- Bifunctional Electrocatalytic Performance
- The Origin of Bifunctionality
- Application to practical PV-EC system
- Conclusion
- References
Overall water splitting was performed by a two-electrode system consisting of Co9S8-MoS2/NF for both cathode and anode. The synergistic effect of Co9S8-MoS2/NF is definitely confirmed for both high HER and OER activities. The Co9S8-MoS2/NF electrode exhibits good stability with a small overpotential increase (64 mV for HER, 2 mV for OER after 50 h).
Comparison of overall water splitting performance of Co9S8-MoS2/NF heterostructure with other reported electrocatalysts in 1M KOH electrolyte.
Summary and Suggestion for future works
Suggestion for future works
Thus, the modified MoS2 dispersion was pre-sprayed onto graphite sheets as depicted in Figure 5.3. The modified MoS2 was successfully deposited on the substrate without clogging the nozzle of the sputtering system. XPS of the Mo 3d spectrum for modified MoS2. a) SEM images of modified MoS2 sputtered on graphite sheet and (b) magnified image of the electrode.
In this regard, the modified MoS2 prepared by the simple one-step hydrothermal method can be applied to N2RR under ambient conditions by controlling the phase ratio and heteroatom doping leading to the enhancement of the conductivity and the reconstruction of the electron density.
먼저, 언제나 변함없는 열정으로 학위과정을 이끌어주신 이재성 교수님께 감사의 말씀을 전하고 싶습니다. 또한 가속기 분석에 큰 도움을 주신 최선희 박사님께도 감사드립니다. 또한 방문을 허락해주신 Kevin Sivula 교수님께도 감사의 말씀을 전하고 싶습니다.
나도 원해요. 염준호님 감사합니다.
