Non-Noble Metal-Based Nano-Structured Electrocatalysts

Energy issues have been one of the earth's most major problems with gradual economic development.Hydrogen energy has indeed been ubiquitously implicated as important new energy.As a zero-carbon renewable energy source,hydrogen not only freely can burn,but could also transform clean power into hydrogen,without having to worry regarding air pollution of carbon dioxide.Many methods are available for hydrogen preparation.Even though the hydrogen preparation from electrolytic water electrolysis is the greenest and cheapest way,but the energy barrier for direct electrochemical water electrolysis is comparatively high.To minimize the energy barrier for water electrolysis and speed up hydrogen generation,catalysts for the water electrolysis are needed.We all know that materials based on the noble metals are excellent catalysts for electrolysis of water.Conversely,owing to the scarce resources and high prices,the massive scale production of catalysts based on noble metal is restricted.Thus,it is essential to discoverinexpensive and highly active non-noble metal catalysts for the water electrolysis.Cobalt and nickel as a transitional metal material with copiousquantity and high conductivity have been widely studied in the area of electrocatalytic water electrolysis for the hydrogen or oxygen generation.This thesis is primarily for the synthesis of Nickel-nanocrystal assemblednanoparticles,amorphous poly elemental hollow nanoparticles and nanosheets and their application as electrolyzer in alkaline and neutral solution.This thesiscomprises the following five chapters:The first chapter attributed to the introduction.Hereinthe in-depth history and importance of hydrogen preparation from water electrolysis water,along with the nature and core principle of the reaction on the oxygen electrode and the hydrogen electrode were presented.Then we present the different types of catalystsbased on non-noble metals for oxygen evolution reaction(OER)and the hydrogen evolution reaction(HER)with different nano-structure.Lastly,we presented the electrolytic cell constructed with two dissimilar catalysts or just one bifunctional catalyst,and some research groupsattach the waster electrolysis cell with photovoltaic cell in sequence,whichattained the conversion of solar light energy into hydrogen.In the second chapter,a nickel nanocrystal assembled nanoparticles(Ni-SP)synthesis through a simple solution-phase ultrasonic-assisted chemical reduction method was reported.The addition of tungsten(W)metal as a non-3d high valency element and non-metals(B and P)as a low valency element could modulate the hydrogen adsorption energy for efficient hydrogen evolution reaction(HER).For HER in a neutral-pH buffer solution,the Ni-SP needs 38 and 214 mV overpotentials to drive 10 and 100 mA·cm-2 current densities,respectively.Moreover,the catalytic activity of Ni-SP remains unaffected after 10 h hydrogen production at 100 mA·cm-2current density,presenting superior stability and activity as compared to Pt/C.In chapter third,a simple synthesis of Ni-doped Co hollow nanoparticles(CoNi-HNP)through an easy ultrasonic-assisted chemical reduction method in the presence of non-metal(B and P)and metal(W)as an additive was described.The addition of low valency non-metals(B and P)and non-3d high valency metals like tungsten(W)improve the near-optimal adsorption energy for hydrogen evolution reaction(HER).The CoNi-HNP requires a 41 mV overpotential to drive a 10 mA·cm-2 current density for HER in 1.0 M potassium phosphate buffer solution(pH=7).Remarkably,the CoNi-HNP achieves a 400 mA·cm-2 current density at a low overpotential of 417 mV in neutral pH aqueous electrolyte.The CoNi-HNP achieved high current densities as compared to Ni-SP at lower overpotential.Moreover,the CoNi-HNP shows better performance during long-term electrolysis as compared to Pt/C.To the best of our knowledge,the CoNi-HNP is the most stable,active and efficient non-noble metal electrocatalyst for hydrogen generation from a neutral solution.In the fourth chapter,the ultrasonic-assisted template-free synthesis of amorphous polyelemental(Co,Ni,Fe,W,B,P)hollow nanoparticles(PE-HNP)as an efficient and stable electrocatalyst for both OER and HER was reported.Highly active and stable electrocatalysts could be achieved by amorphous materials with tunable polyelemental compositions,and further enhanced with proper nanostructures.For HER,as-prepared PE-HNP attained 1200 mA·cm-2 current density at a low overpotential of 195 mV.For OER,PE-HNP was in situ oxidized to form PEO-HNP,which attained 1200 mA·cm-current density at a low overpotential of 309 mV.The activities of PE-HNP and PEO-HNP remained unaffected after 20 h HER or OER test at high current densities,showing higher activity and stability than the benchmark electrocatalysts Pt/C and IrO2,respectively.Moreover,thePEO-HNP and PE-HNP achieved high current densities as compared to CoFeNi-NSA and CoFeNiO-NSA at lower overpotential.The PEO-HNP ?PE-HNP electrolyzer needs a 1.85 V cell voltage to drive a high current density of 400 mA·cm-2,which is much better than the IrO2?Pt/C electrolyzer(2.15 V for 400 mA·cm-2).Our strategy provides a cost-effective and convenient method to synthesize amorphous polyelemental materials with designed hollow nanostructures for the development of high-performance electrocatalysts.In the fifth chapter,the synthesis of crystalline multi-metal nanosheets array amalgamated with oxygen vacancies by a facile electrosynthesis method for efficient and robust water splitting in alkaline solutions was described.We have also demonstrated a simple electrochemical oxidation and reduction methods to generate the oxygen vacancies in CoFeNi-NSA and CoFeNiO-NSA without using harsh reaction condition.Owing to the multi-metal composition,the oxygen vacancies and the porous nanosheet structure,The CoFeNiO-NSA and CoFeNi-NSA catalysts have good stability along with very good OER and HER performance in KOH solution.The CoFeNiO-NSA and CoFeNi-NSA catalysts with high mass loading can deliver high current densities for OER and HER at a low overpotential for 150 h.The CoFeNiO-NSA and CoFeNi-NSA electrodes are assembled into an electrolytic cell,whose water splitting performance and stability are better as compared to noble metal electrolytic cell IrO21?Pt/C.The synthesis of nanosheets array via electrosynthesis is a suitable technique for large scale fabrication of active electrocatalysts.