Chapter 4. Heterostructuring Cobalt Sulfide with MoS 2 to Induce Bifunctionality of Overall
4.4 Results and Discussion
4.4.4 Application to practical PV-EC system
Ultimately, the electrochemical water splitting should be connected with sustainable energy sources such as photovoltaic cells. To demonstrate the bifunctionality of Co9S8-MoS2/NF, we constructed a photovoltaic cell-electrochemical cell (PV-EC) system with a c-Si PV module, and Co9S8-MoS2/NF for both cathode and anode for overall water splitting (Figure 4.9a). The LSV curve of the c-Si PV module is displayed in Figure 4.9b under AM 1.5G solar irradiation (100 mW cm-2). The power conversion efficiency (PCE) of the PV module was 13.81%. The predicted operating current density point (Jop) and voltage point (Vop) of the integrated PV-EC system are determined by the intersection of the LSV curves of PV and EC in a two-electrode system; Jop = 7.57 mA/cm2, and Vop = 1.68 V. Consequently, the solar to hydrogen efficiency (STH) was estimated as 9.70% without considering resistive losses. The conversion efficiency (ηcon) of the system was 70.24%, which is a comparable value to demonstrate the
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practical PV-EC system. Therefore, all these results make Co9S8-MoS2/NF a promising non-noble metal-based bifunctional catalyst for practical water electrolysis.
Figure 4.9. (a) Schematic diagram of the PV-EC combining c-Si PV module with Co9S8-MoS2/NF (both anode and cathode) in 1.0 M KOH. (b) Current density-voltage (j-V) and power-voltage curves of the c-Si PV module with its specification. (c) LSV curves of the Si solar cell (9 cm2) under simulated 1 ± 0.05 sun (AM 1.5 G filtered, 100 mW cm−2) illumination, and EC in a two-electrode configuration.
4.5 Conclusion
The electrocatalytic application of MoS2 was confined as HER in acidic electrolyte due to the low water dissociation property. The electrocatalytic properties of MoS2 nanosheets are modified via heterostructuring with Co9S8 nanoparticles to utilize in an alkaline electrolyte. Reflecting the strategy that incorporating multiple active components into a single catalyst leads to bifunctionality, the Co9S8- MoS2 heterostructure was also applied to OER as well as HER. The Co9S8-MoS2 is used as a pre-catalyst for OER oxidized to form active Co oxide, soluble Mo oxide, and sulfate phases making porosity. Thus, the OER active Co3O4 can be effectively exposed at the surface and used for highly active OER electrocatalyst. In addition, the Co9S8-MoS2/NF exhibit remarkably enhanced OER activity after reductive annealing treatment compared to Co3O4-(am-MoSx) which includes OER active species. As a result, Co9S8-MoS2/NF has superior OER performance to Ir/C catalyst. Eventually, the bifunctional Co9S8-MoS2/NF electrocatalyst is utilized in overall water splitting by a two-electrode system composed of Co9S8-MoS2/NF for both cathode and anode. The Co9S8-MoS2/NF in alkaline water electrolyzer shows high performance and stability. Finally, the Co9S8-MoS2/NF was applied to construct a PV-EC system. The PCE of c-Si PV module was 13.81% and the STH of the PV-EC system was 9.70% which demonstrates the conversion efficiency of 70.24% for our EC system. Considering the high performance and stability of Co9S8-MoS2/NF, we believe the Co9S8-MoS2/NF can be a promising non-noble metal-based bifunctional catalyst for practical water electrolysis.
103 4.6 References
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21. Zhu, H.; Zhang, J.; Yanzhang, R.; Du, M.; Wang, Q.; Gao, G.; Wu, J.; Wu, G.; Zhang, M.; Liu, B.; Yao, J.; Zhang, X., When cubic cobalt sulfide meets layered molybdenum disulfide: a core-shell system toward synergetic electrocatalytic water splitting. Adv Mater 2015, 27 (32), 4752-4759.
22. Ramos, M.; Berhault, G.; Ferrer, D. A.; Torres, B.; Chianelli, R. R., HRTEM and molecular modeling of the MoS2–Co9S8interface: understanding the promotion effect in bulk HDS catalysts.
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23. Zhang, X.; Liu, S.; Zang, Y.; Liu, R.; Liu, G.; Wang, G.; Zhang, Y.; Zhang, H.; Zhao, H., Co/Co9S8@S,N-doped porous graphene sheets derived from S, N dual organic ligands assembled Co- MOFs as superior electrocatalysts for full water splitting in alkaline media. Nano Energy 2016, 30, 93-
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25. Ray, C.; Lee, S. C.; Sankar, K. V.; Jin, B.; Lee, J.; Park, J. H.; Jun, S. C., Amorphous Phosphorus-Incorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values. ACS Appl Mater Interfaces 2017, 9 (43), 37739-37749.
26. Yang, Y.; Zhang, K.; Lin, H.; Li, X.; Chan, H. C.; Yang, L.; Gao, Q., MoS2–Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catalysis 2017, 7 (4), 2357-2366.
27. Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X., Interface Engineering of MoS2 /Ni3 S2 Heterostructures for Highly Enhanced Electrochemical Overall- Water-Splitting Activity. Angew Chem Int Ed Engl 2016, 55 (23), 6702-6707.
28. Jia, N.; Liu, J.; Gao, Y.; Chen, P.; Chen, X.; An, Z.; Li, X.; Chen, Y., Graphene-Encapsulated Co9S8 Nanoparticles on N,S-Codoped Carbon Nanotubes: An Efficient Bifunctional Oxygen Electrocatalyst. ChemSusChem 2019, 12 (14), 3390-3400.
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41. Ji, D.; Peng, S.; Fan, L.; Li, L.; Qin, X.; Ramakrishna, S., Thin MoS2 nanosheets grafted MOFs-derived porous Co–N–C flakes grown on electrospun carbon nanofibers as self-supported bifunctional catalysts for overall water splitting. Journal of Materials Chemistry A 2017, 5 (45), 23898- 23908.
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Chapter 5.
Summary and Suggestion for future works
5.1 Summary
Heavy reliance on fossil fuels in industrial growth gives rise to severe environmental problems by emitting greenhouse gases. To prevent further climate change, the industry is moving toward a sustainable energy system called hydrogen economy. The greenest way to produce hydrogen is water electrolysis which requires efficient electrocatalysts such as noble metal-based catalysts. The replacement of these scarce material-based electrocatalysts is essential to accomplish commercial water electrolysis. Therefore, MoS2 based heterostructure was developed and applied to hydrogen evolution reaction and oxygen evolution reaction, and overall water splitting in this dissertation. Constructing heterostructure with other components, MoS2 overcome the poor electrical conductivity and limitation in active sites. As a result, Ni2P/MoS2/N:C shows Pt-like HER performance in an acidic electrolyte, and Co9S8-MoS2/NF exhibit remarkable HER activities in all-pH electrolytes. Due to the multicomponent in Co9S8-MoS2/NF, the catalyst has bifunctionality which is working for OER as well as HER.
Eventually, the overall water splitting is achieved using Co9S8-MoS2/NF for both cathode and anode in an alkaline electrolyte with great activity and durability.
5.2 Suggestion for future works
The 2H MoS2 has hydrophobicity, so it is difficult to apply to scale-up equipment such as printing or spraying due to poor dispersibility. The improvement of the intrinsic property for MoS2 is essential for various practical energy applications. Tuning the phase of the MoS2, the conductivity and reaction selectivity can be tuned because 1T phase of MoS2 is hydrophilic and 107 times more conductive than 2H MoS2.1-2 However, 1T MoS2 or 1T incorporated 2H MoS2 is typically synthesized in two main methods: (1) Top-down synthesis such as exfoliation,3-7 mechanical ball-milling,8-9 and (2) bottom-up synthesis such as hydrothermal preparation by introducing S vacancies and high-temperature annealing.10-12 Among them, hydrothermal methods are the most common approaches for fabricating MoS2 and their heterostructures because the exfoliation using alkali metal must be carried out in an inert atmosphere, usually in an argon-filled glove box. Thus, hydrothermal synthesis for 1T MoS2 is easily adoptable but typically needs two steps: making 2H MoS2 in the first step, and phase transformation to 1T phase in the second step via S vacancy control using another solvothermal process10 or high-
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temperature annealing in NH3 atmosphere.13 Besides, the yield of 1T MoS2 in the final product is low as 15~25%.14
Recently, we are developing 1T/2H mixed phase of MoS2 prepared using one-step hydrothermal method to modify dispersibility. The mixed phase of modified MoS2 is identified via Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. In Raman spectra, Raman peaks at 149.0, 191.8, and 337.8 cm-1 exhibit J1, J2, and J3 mode of 1T phase while weak peaks at 375.1 and 402.9 cm-1 imply typical E12g and A1g modes of 2H MoS2 (Figure 5.1).10, 15-17 In addition, the 1T MoS2
ratio in the modified MoS2 was also investigated using XPS analysis. As shown in Figure 5.2, the modified MoS2 has a mixed phase of ~80% 1T phase and ~20% 2H phase. The high content of 1T MoS2
leads to the hydrophilicity of the material which can be applied to spraying or printing method for scale- up in commercialization. Thus, the dispersion of the modified MoS2 was preliminary sprayed on graphite foil as depicted in Figure 5.3. The modified MoS2 was successfully deposited on the substrate without clogging the nozzle of the spray system. Consequently, the 1T/2H MoS2 prepared by a simple one-step hydrothermal method can be a good candidate for commercial energy conversion systems.
Figure 5.1. Raman Spectroscopy of modified MoS2 and commercial 2H MoS2.
110 Figure 5.2. XPS of Mo 3d spectra for the modified MoS2.
Figure 5.3. (a) SEM images of modified MoS2 sprayed on graphite foil and (b) magnified image of the electrode.
Since the commercialization of the energy conversion and storage system has been accelerated, making large-scale electrodes and scale-up of the system are essential. Add to this, hydrogen storage technologies have been studied actively to reduce transportation cost. Since ammonia contains large gravimetric hydrogen storage density as shown in Figure 5.4 and is readily liquefied at room temperature, atmospheric pressure being convenient to transport, it has great attention as a strong candidate of hydrogen carrier.18
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Figure 5.4. Volumetric (kg/m3, in blue) and gravimetric (wt% multiplied by ten, in orange) hydrogen storage densities of considered technologies. Reprinted with permission from ref 18. Copyright © 2019 Elsevier B.V.
Since Haber and Bosch developed the industrial ammonia production process, ammonia has been generated via the Haber-Bosch process which requires a high temperature over 400 oC and pressures above 200 bar to react N2 and H2 from steam reforming of hydrocarbon. However, the environmental issues from greenhouse gas emissions make the industry pursue sustainable production methods. As demonstrated in Figure 5.5, we can expect a series of overlapping technology. The 1st generation involves carbon capture, utilization, and storage (CCUS) technologies which can be referred as a blue ammonia. In the 2nd generation, green or blue hydrogen is utilized instead of H2 from steam reforming of hydrocarbon. In the 3rd generation, ammonia is produced by electrochemical N2 reduction reaction which can be called green ammonia. All generation technologies will be in progress overlapping each other but, ultimately, we should go toward green ammonia production using an electrochemical system.
Thus, highly efficient and durable electrocatalysts are required to reduce the overpotential. Recently, MoS2 based catalysts have been studied for N2 reduction reaction (N2RR). To break the nonpolar triple bond of N2, some strategies have been developed such as heteroatom doping, making vacancy, and heterostructuring. In this regard, the modified MoS2 prepared by simple one-step hydrothermal method can be applied to N2RR at ambient condition by controlling phase ratio and doping heteroatom which leads to enhancement of conductivity and electron density reconstruction. As a result, we expect the MoS2 based materials can be utilized to accomplish the hydrogen economy from green hydrogen production to green ammonia production as a hydrogen carrier with high abundancy, efficiency, durability, and scalability in a commercial system.
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Figure 5.5. Current and 1st generation (red), 2nd generation (blue), and 3rd generation (green) of ammonia production technologies.
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13. Wang, T.; Sun, C.; Yang, M.; Zhao, G.; Wang, S.; Ma, F.; Zhang, L.; Shao, Y.; Wu, Y.; Huang, B.; Hao, X., Phase-transformation engineering in MoS 2 on carbon cloth as flexible binder-free anode for enhancing lithium storage. Journal of Alloys and Compounds 2017, 716, 112-118.
14. Shi, S.; Sun, Z.; Hu, Y. H., Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2. Journal of Materials Chemistry A 2018, 6 (47), 23932-23977.
15. Park, S.; Kim, C.; Park, S. O.; Oh, N. K.; Kim, U.; Lee, J.; Seo, J.; Yang, Y.; Lim, H. Y.; Kwak, S. K.; Kim, G.; Park, H., Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability, and Scalability via Molten-Metal-Assisted Intercalation. Adv Mater 2020, 32 (33), e2001889.
16. Saber, M. R.; Khabiri, G.; Maarouf, A. A.; Ulbricht, M.; Khalil, A. S. G., A comparative study on the photocatalytic degradation of organic dyes using hybridized 1T/2H, 1T/3R and 2H MoS2 nano- sheets. RSC Advances 2018, 8 (46), 26364-26370.