Synthesis and Regulation of Carbon Supported Non-Noble Metal Based Catalysts for Electrocatalytic Water Splitting

Author:Huang Hua Wei

Supervisor:qiu jie shan yu chang


Degree Year:2019





Electrochemical water splitting is one of effective technologies for clean hydrogen production,which has the advantages of simple technology,production of high purity hydrogen.However,the high overpotentials involving in hydrogen and oxygen evolution reactions limit and affect the energy conversion efficiency of the electrolysis process and the development of this technology.Therefore,reducing the overpotentials involved in the reactions at cathode and anode is important for reducing the operating voltage and improving the energy conversion efficiency.In this study,we use carbon materials with high conductivity and corrosion resistance to acid and alkali as substrates and develop a series of carbon supported non-precious catalysts with high catalytic activity,excellent stability and low cost for electrochemical water splitting,which is mainly based on the earth abundant transition metal elements(such as Fe,Ni and Co).The catalytic activities of these catalysts are improved by optimizing the active sites exposure,conductivity,structure and synthetic process of the catalyst.The main results are as follows:The fluffy graphene prepared by rapid thermal reduction is used as substrate for synthesizing graphene decorated with Fe2P nanodots composite catalyst(Fe2P-ND/FG)through a low-temperature pyrolysis coupling phosphating process.The difference in structure and catalytic properties between the catalyst prepared by using graphene oxide as precursor was investigated.The prepared Fe2P-ND/FG retains the fluffy and open structure of the pre-reduced graphene,which is beneficial to provide more electrocatalytic active surface area.The ultra-small Fe2P nanodots can expose more atoms and edge sites,which may act as catalytic active sites to accelerate the reaction.The Fe2P-ND/FG exhibits excellent electrocatalytic activity for hydrogen evolution reaction,achieving a current density of 10 mA CM-2 at a low overpotential of 91 mV,which is 207 mV lower than that of pure Fe2P and superior to the catalyst synthesized with graphene oxide as precursor.A self-standing electrode(Fe-NiS2/CF)composed of Fe-doped NiS2 nanosheet array anchored on carbon fiber is prepared by simple sulfuration of carbon fiber supported NiFe layered double hydroxide precursor that has a uniform distribution of metal ions.It was found that the introduction of Fe can modulate the electronic structure of NiS2 and enhance its antioxidant capacity in air.Compared with NiS2/CF,the Fe-NiS2/CF features a better oxygen evolution catalytic activity demonstrated by theoretical calculations and experimental results.In addition,Fe-NiS2/CF also has electrocatalytic catalytic activity for hydrogen evolution reaction.It can be used for overall water splitting and achieves a current density of 10 mA cm’2 with an applied voltage of 1.722 V,and can operate stably for more than 30 h.Ni2P with different Fe doping content can be controllably synthesized by low temperature phosphating of layered double hydroxide with different Fe/Ni molar ratios.The effect of Fe doping content on structure and catalytic activities of Ni2O is studied.The experimental results show that the introduction of Fe can significantly improve the catalytic activity of Ni2O for both hydrogen and oxygen evolution reaction.When the Fe/Ni molar ratio is 1:3,the as-prepared Fe0.5Ni1.5P exhibits the best electrocatalytic activity for both oxygen and hydrogen evolution reaction.The self-standing electrode Feo.5Ni1.5P/CF prepared with this Fe/Ni molar ratio exhibits extremely high activity for oxygen evolution reaction and achieves 100 mA cm-2 at an overpotential of 293 mV.The Feo.5Ni1.5P/CF can deliver 10 mA cm-2 at an applied potential 1.589 V together with an excellent stability for overall water splitting.A self-standing Co-NC/CF electrode is fabricated by a rapid microwave-assisted pyrolysis method,in which the carbon fiber paper supported Co-MOF nanosheet arrays(Co-MOF/CF)is used as precursor,and graphene powder is used as wave-absorbing materials.The difference between this method and the traditional temperature-programmed carbonization method is studied.The results show that the microwave-assisted pyrolysis method can be completed in 60 s,and the energy consumption is only 0.37%of the traditional carbonization method.Notably,the yield of the catalyst is increased from 34.0 wt%of the conventional carbonization process to 48.7 wt%.The as-produced Co-NC/CF not only retains the nanosheet array structure of the precursor,but also features the special nanostructure—N-doped graphitized carbon coated on Co nanoparticles.Both theoretical calculations and experimental results confirm that this structure has highly intrinsic catalytic activity for electrocatalytic oxygen and hydrogen evolution reactions.The Co-NC/CF achieves 10 mA cm"2 at low overpotentials of 246 mV and 157 mV for OER and HER,respectively.In the long-term catalytic process,the Co-NC/CF can maintain its catalytic activity and structure for hydrogen evolution.For oxygen evolution,a stable catalytic activity can be maintained,but it is oxidized to cobalt oxides nanoparticles with a low degree of crystallinity that has a high activity for oxygen evolution reaction.