Synthesis Of Sub-nanometric Iridium Based Materials And Their Electrocatalytic Performance Towards Oxygen Evolution Reaction

Hydrogen which is clean and possesses high energy density,is an ideal energy carrier.As a promising technology for hydrogen production via water electrolysis powered by renewable energy sources,proton exchange membrane water electrolysis technology has attracted great attention due to easy operation and high energy conversion efficiency.To date,Ir based electrocatalysts are almost the only choice to promote the kinetically sluggish oxygen evolution reaction in acid media due to its outstanding stability and activity.In the field of oxygen reduction reactions,the nanostructure engineering of platinum-based catalysts that can adjust the electronic structure and enhance the activity and stability has been extensively studied.However,synthesizing Ir nanocrystals with well-defined morphologies is still difficult,owing to its tendency to self-nucleate and grow into small particles via island growth mode,Meanwhile,conventional carbon is not suitable anymore for Ir based electrocatalysts as support material due to excessively high operating voltages.Therefore,developing highly efficient and robust Ir based OER electrocatalysts via optimize Ir atomic utilization and interface regulation in acid medium still is highly desirable.The main contents and conclusions are as follows:（1）Ti N supported ultrafine Ru Ir nanocrystals:Using ethylene glycol as reducing agent,high conductivity and oxidation-resistant nano Ti N as the supports,Ru Ir/Ti N nanocrystals loaded with different components were successfully synthesized.The mean particle size of Ti N and the Ru Ir nanocrystals is 20nm and 1.82 nm respectively.The effects of the ratio of Ru,Ir and the Ti N supports on their surface chemical state and electrocatalytic properties have been investigated.Electrochemical tests indicate that Ru₁Ir₁/Ti N possess the best OER activity.in which the overpotential at 10 m A cm^-2 with the Ru Ir loading of 35 ug cm^-2 is 270 m V and the corresponding mass activity of 1.17 A mg^-1 is 7.8 times higher than commercial Ir O₂(0.15A.mg^-1)at 350 m V of overpotential.The chronopotential stability test shows that the overpotential of Ru₁Ir₁/Ti N only increases by 8 m V in period of 9.8hours.The improvement of OER activity and stability is mainly attributed to the optimization of Ru Ir alloy composition and the interaction between metal supports.（2）Ultrathin-shell IrCo hollow nanospheres:Co@Ir nano core-shell spheres are synthesized using sodium borohydride as reducing agent and amorphous Co₂B as an intermediate template.After further acid leaching,Ultrathin-shell IrCo hollow nanospheres are obtained.Furthermore,formation mechanism and structural characteristics of IrCo hollow sphere are investigated.IrCo hollow nanospheres prepared are characterized by the spheres size of around 80.0 nm,the shell thickness of 2.2 nm and strong lattice strain effect induced by the specific hollow structure and alloying Co into Ir crystal lattice.These unique properties endow the hollow nanospheres excellent performance for oxygen evolution reaction（OER）in the aspect of activity and stability,in which the overpotential at 10 m A cm^-2 with the Ir loading of 32.5ug cm^-2 is 284 m V and the corresponding mass activity of 1.91A mg^-1 is 12.7 times higher than commercial Ir O₂(0.15 A.mg^-1)at 350 m V of overpotential.Meanwhile,the feasibility of hollow nanospheres for practical application is validated by a small overpotential of 318 m V to achieve a current density of 10 m A cm^-1 in a prototype of overall water splitting device fabricated by using Pt/C as cathode and IrCo hollow spheres as cathode.Furthermore,the performance of the hollow nanospheres is also better than that of other morphologies of previously reported Ir based catalysts（IrNi Fe nanoparticles,IrNi Cu nanoframe,IrNi Co nanowire,Ir nanoneedle,etc）.（3）Ultrathin-shell IrNi,IrCo Ni hollow nanospheres:Using sodium borohydride as reducing agent and amorphous Ni₂B and Co₂B-Ni₂B composites as intermediate templates to synthesize different transition metal alloyed IrNi,IrCo Ni hollow nanospheres.We explore the influence of different transition metal alloying in aspect of the phase structure of iridium-based hollow nanospheres,lattice stress,and oxygen evolution performance.The IrCo hollow nanospheres prepared are characterized by the spheres size of around 20 nm and 40 nm.Among them,IrNi hollow nanospheres possess the largest lattice compression effect.The enhanced stress of Ni effect endows IrNi hollow nanospheres the best OER activity,in which the overpotential at10 m A cm^-2 with the Ir loading of 32.5ug cm^-2 is 270 m V and the corresponding mass activity of 2.92 A mg^-1 is 1.53 and 19.5 times higher than IrCo hollow nanospheres(1.91 A mg^-1)and commercial Ir O₂(0.15 A.mg^-1)at350 m V of overpotential respectively.The above results show that the Ir-based hollow nanospheres and Ti N-supported Ru Ir alloys through sequential reduction and polyol co-reduction have significantly improved OER activity and stability compared to commercial Ir O₂.Therefore,the synthetic methods of Ir-based hollow nanospheres and Ti N-supported Ru Ir alloys proposed in this thesis provide ideas for developing high-efficiency acidic OER catalysts.