Cobalt-nanocrystral-assembled Materials As High-performance Electrocatalysts For Overall Water Splitting

With the rapid economic and social development,energy issues become one of the world's most important problems.Hydrogen energy as an important new energy has been widely concerned.Hydrogen as a zero carbon source of clean energy not only can burn directly without worrying about emissions of carbon dioxide,but also can convet other clean energy into hydrogen.There are many methods for the preparation of hydrogen.The preparation of hydrogen from electrolytic water splitting is the simplest and cleanest way,but the energy barrier of direct electrolysis of water is relatively high.So that,the catalyst for the electrolysis of water is required to reduce the energy barrier of water splitting and accelerate the generation of hydrogen.As we all know,noble metal based material is a very good catalysts for the electrolysis of water.However,the large scale used of noble metal based catalysts are limited due to the scarce reserves and expensive prices.Therefore,it is necessary to find cheap and high-performance non-precious metal catalysts for the electrolysis of water.Cobalt as a transitional metal material with abundant content and high conductivity has been widely studied in the field of electrocatalytic of water for the generation of hydrogen or oxygen.This paper is mainly for the synthesis of cobalt-nanocrystal assembled hollow nanoparticles or nanosheets and their application in the electrolytic of alkaline solution and neutral solution.This paper includes the following chapters:The first chapter is the introduction.We introduced in detail the history and significance of the preparation of hydrogen in electrolytic water,as well as the type and basic principle of the reaction on the hydrogen electrode and the oxygen electrode.Then we introduce the cobalt-based catalyst for the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)with different nano-structure.Finally,we introduced the electrolytic cell built with two different catalysts or just one double-function catalyst,and there are some group connect the electrolytic cell and the photovoltaic cell in series,which achieved the transformation of solar energy into hydrogen.In the second chapter,we synthesized the high specific surface area of cobalt-nanocrystals assembled hollow nanoparticles(Co-HNP)by chemical reduction method as a high performance HER catalysts.So that the active sites of cobalt-nanocrystals can be more exposed,an excellent stability is due to the amorphous part which will protect cobalt-nanocrystals from being oxidated,and also play a supporting role.For this Co-HNP,a highly activation HER performance close to the Pt/C and an excellent stability was obtained in the neutral solution.Our results show that the synthesis of a hollow structure catalyst composed of non-noble metal nanocrystals is a promising direction for the research and development of high performance electrocatalysts.In the third chapter,two kinds of different synthesis methods were used to obtain the precursors of the amorphous cobalt-based materials.Then,in situ electrochemical activation of the precursors was carried out on the nickel foam electrode.The hydrogen electrode or oxygen electrode of the cobalt-based nanosheets was obtained after this activation.For the preparation of hydrogen electrode,in-situ electrodeposition was used to systhesis amorphous cobalt alloy on the nickel foam electrode.The electrochemical activation was carried out in alkaline solution.Benefiting from the activation for about 30 min,the obtained cobalt nanosheet array(Co NS)exhibited excellent HER activity,even better than the performance of the commercial Pt/C under the same condition.For the preparation of oxygen electrode,the precursor of cobalt-iron-boron alloy was obtained by a chemical reduction method,and also electrochemical activation was used to obtain the cobalt-iron-boron oxide nanosheets array(CoFeBO NS)on the Ni foam,which exhibited an excellent OER performance,much better than the noble metal IrO2 in the alkaline solution.Then an electrolytic cell was built by this two different electrode,which have a much better performance than the noble metal Pt/C ? IrO2 under same condition.This study shows that catalytic performance and stability of this non-noble metal based nanosheets array structure in alkaline solution can already exceed that of noble metals based catalysts,due to the high specific surface area and the high conductivity between the catalyst and the substrate.For this kind of cobalt-based nanosheets,a very simple preparation methods and an excellent catalytic performance for the water splitting show a wide range of applications in the future.In the fourth chapter,a precursor of Cobalt-Iron-Tungsten-Boron alloy obtained by the chemical reduction method was loaded on the nickel foam electrode by a drop-and-dry method.The low-oxidation Co-Fe-W-B quaternary metal nanosheets(CoFeWB NS)array catalysts was obtained in situ on the electrode by the electrochemically reductive in the alkaline solution.In the neutral solution,a highly HER effeicent was tested by this CoFeWB NS electrode which was better than that of Co-HNP in chapter 2,also very close to the catalytic performance of Pt/C under the same conditions.If the precursor on the nickel foam electrode was electrooxidized in alkaline solution,a high-oxidation CoFeWB oxides nanosheets array(CoFeWBO NS)structure will obtained,in the neutral solution,the OER performance of this CoFeWBO NS is much better than that of the noble metal IrO2.An electrolytic cell assembled by this two electro-activited ternary metal based nanosheets arrays showed an amazing performance for overall water splitting in the neutral solution,which was even better than that of the noble metal based electrolytic cell(Pt/C ? IrO2).Our results provide a very simple and efficient direction for the design of high performance electrocatalysts in the neutral solutions,which can further reduce the cost of hydrogen production from electrolyzed of water.