Structural Design Of Molybdenum-based Core-shell Electrocatalysts For Water Splitting

Electrocatalytic water splitting plays a key role in the sustainable production of hydrogen.Combining renewable energy power generation technologies to produce hydrogen,e.g.,solar energy,thermal energy,tidal energy and wind energy,is expected to become a low-carbon emission method for storing excess electricity or producing large amounts of clean fuel.Water electrolysis can be divided into two semi-redox reactions,the hydrogen evolution reaction（HER）at cathodes and the oxygen evolution reaction（OER）at anodes.For HER,the state-of-the-art electrocatalysts are Pt-based materialss.However,the high cost and low earth-abundance have severely hindered the large-scale application.For OER,only noble metals Ir and Ru can be used in acidic environments,which also suffer the limitation by high cost and low abundance.Therefore,it’s highly desired to exploit cost-efficient electrocatalysts free-from or with few noble metals for water electrolysis.As previously evidenced,core-shell nanostructures can afford particular merits,e.g.,a highly-exposed active surface,modulated electronic configurations,strain effects,interfacial synergy,or reinforced stability,to promote the kinetics and electrocatalytic performance of the HER,OER and overall water splitting.This thesis is devoted to designing molybdenum-based core-shell electrocatalysts toward the efficient HER and OER.1.N,P co-doped carbon-coated molybdenum carbide core-shell electrocatalyst（Mo₂C@NPC）was prepared by pyrolyzing ammonium molybdate tetrahydrate and the carbon matrix derived from kapok.As evidenced,the presence of the carbon shell prevents the aggregation of Mo₂C nanoparticles during the high-temperature synthesis and also protects Mo₂C nanoparticles from the corrosion of the electrolyte during the electrochemical process.Thereby,the carbon shell coated fine Mo₂C nanoparticles uniformly dispersed on the carbon substrate.More importantly,due to the co-doping of N and P,the electronic structure of Mo₂C is optimized towards the enhanced HER performance.Compared with commercial Mo₂C and Mo₂C@NC,Mo₂C@NPC exhibits excellent HER activity that featuresη₁₀ of 127 and 155 m V and the Tafel slope of 63 and64 m V dec^-1 in 1.0 M HCl O₄ and 1.0 M KOH,respectively.2.The Ir Mo O_xcore-shell electrocatalyst which exhibits excellent OER activity in acidic electrolytes is prepared by high-temperature pyrolysis and in-situ electrochemical oxidation dealloying.Ir-Mo-PAN nanofibers are used as precursors to synthesize Ir_1.02Mo_0.98/C by pyrolysis,which is then electrochemically oxidized and dealloyed in 1.0 M KOH to achieve core-shell Ir Mo O_x.PMMA was added during the preparation of the precursor,resulting in porous nitrogen-doped carbon skeleton after the volatilization of PMMA.After pyrolysis,Ir_1.02Mo_0.98/C with a large specific surface area was obtained.In the electrochemical oxidation dealloying process,Mo species is leached into the solution.Optimizing the electrochemical oxidation to control the leaching content of Mo species can lead to an Ir Mo O_x core-shell electrocatalyst with the designed Ir and Mo contents.In addition,the porous structure of nitrogen-doped nanofibers can ensure the rapid transfer of electrons and reduce the limitation by mass transport.As expected,the optimal core-shell electrocatalyst obtained by CV scanning 300 cycles exhibits the best OER activity in0.1 M HCl O₄ with aη₁₀ of 252 m V.In summary,Mo₂C@NPC and Ir Mo O_x core-shell electrocatalysts have been designed for the HER and OER in an acidic electrolyte,which achieve the high performance associated with the elaborately designed core-shell nanostructures.Illustrating the adjusting of surface/interfacial activity of core-shell electroctalyts,this work will provide a reference method for exploring various cost-effective electrocatalysts in practical applications.