Investigation Of Catalysts For Water Splitting And Energy Driven Water Splitting Hybrid System

With the depletion of fossil fuels,such as coal,oil and natural gas,finding a renewable and clean energy has become an essential research issue.Hydrogen has been received more attention because of its many advantages.Electrochemical water splitting for hydrogen evolution reaction(HER)is of great significance,because it is a clean,pollution-free and efficient H2 production method,and can be combined with distributed energy such as solar and wind energy,converting theses excess energy into storable hydrogen energy.The key problem of electrochemical water splitting is the high energy consumption.At present,it still needs a high overpotential for both the anodic oxygen evolution reaction and the cathodic hydrogen evolution reaction,which greatly limits the efficiency of the electrochemical water splitting.In order to reduce the overpotential of electrochemical water splitting and reduce the power consumption,it can be solved mainly from the following two aspects:on one hand,exploring high-active electrocatalysts is an effective way to improve the efficiency of water splitting and reduce the overpotential for hydrogen and oxygen generation;On the other hand,the combination of solar energy and other distributed energy such as wind energy and thermal energy,and converting them into electrical energy can also effectively utilize these natural energy and reduce the cost of hydrogen production from electrochemical water splitting.Thus,this work aims at reducing the overpotential and power consumption of electrochemical water splitting.From the two aspects mentioned above,the energy driven water splitting hybrid system and the corresponding catalysts were investigated.On one hand,we explored the OER electrocatalyst with high activity and extreme stability in acidic electrolytes,which can solve the problem that the general transition-metal oxide based OER electrocatalysts are easy to dissolve in acidic electrolytes and unstable.Meanwhile,the phosphorization of transition-metal oxides applied in the hydrogen evolution reaction was also investigated.The studied acidic stable OER electrocatalysts and.the corresponding transition-metal phosphide as HER electrocatalysts have far-reaching significance for the construction of overall water splitting with a two-electrode configuration.On the other hand,this study also investagates the photocatalytic H2 generation and thermoelectric-driven water splitting systems based on photoelectric conversion,photothermal and thermoelectric conversion effects,which can also effectively utilize solar energy and reduce the cost of hydrogen production from electrochemical water splitting.The main contents of this study are as following:1.Electrochemical water splitting:Iron oxide embedded titania nanowires-an active and stable electrocatalyst for oxygen evolution in acidic media and the phosphide as HER electrocatalysts(1)Under alkaline conditions,the widely reported earth-abundant catalysts based on transition-metal(Ni,Fe,and Co)oxides can efficiently and stably catalyze the OER.However,they are unstable and quickly corrode in acidic condition.In order to solve the dissolution,this study proposes that K2Ti4O9 nanowires with layered structure were first grown on the surface of titanium foam by chemical bath method,and then the iron oxide embedded in the titanium dioxide nanowires(Fe-TiOx LNWs/Ti)electrocatalyst was successfully grown on the surface of Ti foam by ion exchange followed by calcination process.The structiure characterization proved that the Fe3+ was successfully substituted K+ of K2Ti4O9 and embedded in the titanate layered structure during the ion exchange process,then the iron oxide embedded in titania nanowire was obtained after being calcined in the air atmosphere.In this structure,TiO2 nanowires can not only expose the active sites of FeOx,but also fix FeOx,which can inhibit its dissolution in acidic electrolyte and improve its stability.(2)The electrochemical characterization implies that the prepared Fe-TiOx LNWs/Ti sample exhibits high OER activity in acidic electrolytes with an onset potential of 1.49 V vs.RHE(1 mA/cm2)and a Tafel slope of 126.2 mV/dec.This excellent OER performance can be attributed to two factors.First,the one-dimensional nano wire structure with a high specific surface area is conducive to exposing more active sites.Second,the macroporous structure caused by the crosslinked nanowire contributes to the charge transfer and diffusion between electrocatalysts and electrolyte.(3)Meanwhile,this study also focuses on the stability of the OER electrocatalyst in acidic electrolyte.It was found that after the amperometric i-t measurement for 20 hours,the current density only decreased 18.7%.The OER stability in acidic electrolyte is very prominent,which indicates that in the embedded structure of Fe-TiOx LNWs/Ti sample,titanium dioxide nanowires can immobilize iron oxide and improve its anti-corrosion ability,so as to improve the acidic OER stability.(4)In addition,this study discovers that the FeP/TiO2 NWs/Ti sample obtained by phosphorizing Fe2O3/TiO2 NWs/Ti has excellent HER activity,and the overpotential of hydrogen evolution reaction in 0.5 M H2SO4 is only 133 mV.The characterization results suggest that the existence of TiO2 nanowires leads to more FeP nanoparticles loading on its surface,which can expose more active sites for HER.At the same time,the nanowire structure is also conducive to the diffusion of electrolytes,which leads to the high efficiency of electrochemical water splitting.2.Photocatalytic water.splitting:CdS Nanoparticles/MoS2 nanosheets heterostructure on porous molybdenum sheet for enhanced photocatalytic H2 evolution(1)This study demonstrates that a CdS/MoS2/Mo sheets system simultaneously containing photocatalysts,co-catalysts,and conductive supports,was prepared via the one-step hydrothermal process by Mo sheets as template and Mo sources.The obtained CdS/MoS2/Mo sheets possess the superior photocatalytic H2 production via water splitting under visible light irradiation,which achieved an extraordinary H2 production of 4.54 mmol/h/g,up to 28.6 and 3.6 times greater than that of CdS alone and Pt/CdS.(2)The study shows that the mechanism of enhanced hydrogen production in CdS/MoS2/Mo porous sheets photocatalytic system is as follows:the synergetic effect of MoS2 as co-catalysts and Mo sheets as conductive supports contribute to the dramatically improved photocatalytic H2 evolution activity of CdS nanoparticles,by means of facilitating charge carriers separation and providing active sites for proton reduction.(3)This study concludes the exploration of non-noble metal electrocatalysts(MoS2)as the cocatalyst instead of the expensive noble metal(Pt)for photocatalytic hydrogen production.The synthesis method in this stdudy is simple and easy to fabricate,which provides a straightforward and practical route to produce cheap and efficient co-catalysts for large-scale water splitting.3.Thermoelectric-driven water splitting:an earth-abundant Ni nanosheets array as electrocatalysts and heat absorption layer integrated thermoelectric device for overall water splitting(1)Ni(OH)2 nanosheets were firstly synthesized on the hot-end ceramic chip of thermoelectric device via the hydrothermal method,and then the Ni nanosheet arrays were obtained by in-situ topological reduction reaction.The structural,photothermal transformation and electrocatalytic characterizations confirm that the earth-abundant Ni nanosheets array grown on the hot end of TE is not only an efficient photothermal conversion layers for providing temperature difference(?T)of thermoelectric device,but also an active electrocatalysts for hydrogen evolution reaction(HER).(2)Ni nanosheet array integrated thermoelectric device was fabricated by using the ceramic chip with Ni nanosheet array grown on the surface as the hot-end of thermoelectric device.Under the irradiation of simulated sunlight,a high temperature difference(?T)between the hot-end and the cold-end is obtained,and thus outputs a certain voltage.Instead of the external power source,the power output from thermoelectric device can be directly used for water splitting.(3)In this study,' two kinds of electrolyzer-thermoelectric hybrid devices are designed for overall water splitting in a two-electrode configuration.The Ni nanosheet array on the hot-end of the thermoelectric device is connected to the negative electrode,and the synthesized NiFe LDH/CFC is connected to the positive electrode.No matter illumination in the electrolyte or in the air,it possesses a long-term extraordinary durability and a high efficiency of water splitting.(4)The proposed earth-abundant Ni nanosheets array as electrocatalysts for HER and light absorption layer integrated thermoelectric device herein offered great advantages in terms of designing the overall water splitting system with integrated structure and earth-abundant metal materials,which are helpful for its practical applications.In addition,this assumption is also universally applicable to other metal nanomaterials.