Synthesis And Properties Of Nitrogen/Phosphorus Doped Carbon-Supported Cobalt Phosphide Electrocatalysts

Nowadays,it has become a global goal to develope renewable energy such as solar energy,wind energy and tide energy for replacing fossil fuels.Because of the intermittent and imbalance of these energy sources in spatial and temporal distribution,the development of energy conversion and storage technology has become one of the keys to support the application of these new energy sources.Hydrogen energy,as a kind of energy conversion and storage carrier with high energy density,environmental friendliness and wide sources,has developed rapidly in recent years.Among them,the technologies of electrolytic overall water slitting and hydrogen fuel cel,which are highly related to the development and utilization of hydrogen energy,have gradually attracted intensive attention.These two technologies mainly involve four reactions:hydrogen evolution reaction?HER?,oxygen evolution reaction?OER?,hydrogen oxidation reaction?HOR?,and oxygen reduction reaction?ORR?.However,these reactions require higher certain reaction energy barriers than thermal values,resulting in high overpotential and low energy efficiency.Up to now,noble metal catalysts have shown excelent HER,OER and ORR activities,but their practical applications are greatly limited by their high cost,scarcity,and unsatisfied stability.To solve these issues,this thesis mainly focuses on non-noble metal materials,i.e.transition metal based compounds.Cobalt phosphides confined in porous P-doped carbon materials and N,P co-doped carbon-based cobalt phosphides hybrids were studied as catalysts for HER,OER and ORR.Details are as followings:1.The development of low-cost and highly efficient electrocatalysts via an eco-friendly synthetic method is of great significance for future renewable energy storage and conversion systems.Herein,cobalt phosphides confined in porous P-doped carbon materials?Co-P@PC?were fabricated by calcinating the cobalt-phosphonate complex formed between 1-hydroxyethylidenediphosphonic acid and Co?NO₃?₂ in alkaline solution.The P-containing ligand in the complex acted as the in situ phosphorizing agent as well as carbon source for the formation of cobalt phosphides and doping P element into carbon material upon calcination.The Co-P@PC exhibited high activity for all-pH hydrogen evolution reaction(overpotentials of 72,85,and 76 mV in acidic,neutral,and alkaline solutions at the current density of 10 mA cm^-2)and oxygen evolution reaction in alkaline solution(an overpotential of 280 mV at the current density of 10 mA cm^-2).The alkaline electrolyzer assembled from the Co-P@PC electrodes delivered the current density of 10 mA cm^-2 at the voltage of 1.60 V with a durability of 60 h.The excellent activity and long-term stability of the Co-P@PC derive from the synergistic effect between the active cobalt phosphides and the porous P-doped carbon matrix.2.N and P co-doped carbon-based cobalt phosphides hybrids?Co₂P@NPC?were synthesized by calcining,pickling and re-calcining the above-prepared cobalt phosphate for used as bifunctional OER/ORR electrocatalysts.Electrochemical tests confirm that the Co₂P@NPC hybrids exhibited comparable ORR and OER activity and better stability compared with commercial Pt/C and RuO₂ catalyst in 0.1 M KOH solution,indicating the key roles of the synergistic effect between the active cobalt phosphides with OER activity and the porous N,P-doped carbon matrix with ORR activity.The hybrids fabricated here are expected to promote the development of related energy technologies and devices.