The Study On Ion Doping Of Cobalt-Based Compounds And Design Of New Electrocatalysts

During past few decades,the excessive exploitation of traditional fossil fuel has aroused severe energy crisis and environmental problems.Recently,chemical electrolysis,which can convert commercial electricity into sustainable and non-polluted novel energy resources(such as hydrogen),is regarded as a promising energy-supply device and has triggered much attention from fields of both science and industry.However,the practical usage of electrolysis is still hindered by several essential issues.The most important one is how to replace currently commercial noble-metal catalysts(such as Pt/C and Ir/C)with cheap,active and stable alternative,which can tremendously minimize cost.Focusing on this hotspot,in this dissertation,we first reviewed recent advances of non-noble-metal based transition metal compounds(including design,synthesis and application in electrocatalysis),highlighting the modification of their electrocatalytic performance(such as activity and stability).Note that the catalytic property of the certain material is still far from industrial application,and their true active sites during catalysis is also unclear.Based on such assessments,we rationally selected transition metal phosphides and selenides,with the advantage of low cost,high yield and large room to enhance their activity and stability,as host objects.By cation and anion doping strategies,their electronic structures can be successfully modulated,leading to enhanced electrocatalytic property.We aimed at realizing their true active sites atomically and bringing them much closer to practical usage.Main achievements are summarized and listed as follows:1.Using a facile two-step method,namely electrochemical deposition followed by solid phosphication,we have fabricated CoP nanosheets onto commercial carbon fiber paper(CoP-CFP).By in-situ introducing of Ni ion,CoP nanosheets with different Ni dopant were successfully achieved.The influence of deposition time and Ni doping amount on the electrocatalytic performance(including hydrogen evolution reaction,HER,and oxygen evolution reaction,OER)of final products is systematically discussed.With the products with best performance(10%Ni doped CoP,donated as Ni0.1Co0.9P)in hand,we gave some insights into their enhanced activity.Experimental and theoretical data reveal that compared with pure CoP,Ni0.1Co0.9P has an optimized electronic state due to the incorporation of Ni.As a result,the Gibbs free energy of catalysts and active species during HER and OER is lowered,so that their HER and OER activity is improved.Moreover,a home-made electrolyzer,directly using Ni0.1Co0.9P-CFP as both cathode and anode,is constructed.They can catalyze full water splitting even better than commercial noble-metal catalysts.We anticipate that such Ni0.1Co0.9P ternary catalysts will advance practical hydrogen generation from electrolyzer,as well as the rational design of novel highly efficient materials in field of energy.2.Once again,we chose CoP as target material,and developed a F-anion-doping-method to improve its HER activity.By using NH4F as starting materials,F doped CoP(CoP|F)have been successfully synthesized.Related characterizations suggested that the introduction of F ion into CoP could modulate the electronic structure,and resulted in the enhanced HER performance.By comparison of the XPS of CoP|F before and after HER stability test,as well as the operado XAS analysis of CoP|F and CoP,we concluded that Co and P in transition metal phosphide would change into elementary substances during HER.With the assist of F ion,the change from Co and P into their elementary substances could occur under lower overpotential in case of CoP|F.Also,F ion would diffuse to the surface of CoP|F during HER,forming a thin shell consisted of ternary CoPF and acting as true active HER composite.Such results reveal the atomic active sites of transition metal phosphides during HER,and pave a new path for preparing novel efficient catalysts.3.Inspired by the doping concept in CoP system,we selected CoSe2 nanobelt as major material and developed a "one-pot" solve-thermal method to achieve general metal ion doping.Ten kinds of metal ions could be single-atomically introduced into CoSe2 nanobelt,resulting in single atomic doping strategy.Analysis of XAS data and theoretical calculations revealed that these metal ions could make the Co coordination environment different from each other.As a result,the electronic structure of different products differs from each other,thus modulating their HER activity.These results not only enrich the synthetic methods of transition metal compounds,but also explore the rule under which doping could tune the catalytic performance,which might facilitate the rational design of novel effective electrocatalysts.