The Preparation of Transition Metal Nano Functional Materials for Electrocatalytic Water Splitting

Author:Li Jia Chen

Supervisor:ma hai xia


Degree Year:2019





In order to meet the needs of social and economic development,the rapid exhaustion of traditional fossil energy will result in some serious environment problems,including atmospheric pollution,greenhouse effect,moreover,the scant natural resources will bring great energy crisis etc.Therefore,the clean energy sources for sustainable energy applications have received much research interest.Electrochemical water splitting,that is,hydrogen evolution reaction(HER)has been considered to be an ideal and simple yet efficient method alternative to conventional fossil energy.The oxygen evolution reaction(OER)is a important half-reaction in water splitting.However,the mechanism of OER is a kinetically sluggish four-electron transfer process because of its multistep elementary reactions.Hence,the research on the OER has become a significant component of water splitting and flourishing research field over the past few years.The commercial benchmark electrocatalysts are noble metal based catalysts including Pt,IrO2,and RuO2.However,in view of the high cost,instability,and low natural abundance significantly limit their industrial-scale applications.Thus,the study of cheaper and high efficient electrocatalysts have become a current research hotspot.In this paper,we aimed at Fe/Co/Ni based transition metal nanomaterials for water splitting and several electrocatalysts were prepared including graphene supported iron cobalt phosphate nanorings(rGO@CF:Pi NRs)and graphene supported iron doped nickel phosphide nanoparticles(rGO@Fe-Ni2P NPs)for OER.In addition,the three-dimensional(3D)carbon fiber supported cobalt carbonate hydroxide for ultrafine ruthenium deposition(CF@Ru-CoCH)for HER in alkaline solution.The research works in this paper are summarized as follows:(1)The amorphous rGO@CF:Pi nanoring was first prepared by two-step method of ydrothermal reaction and followed by phosphorylation process,and use SEM、TEM、XPS、FTIR、Raman and EXAFS to characterize the mataerials.It was reported that the morphous materials with special disordered structures possess more unsaturated metal toms as active sites on the surface of catalyst,thus can facilitate OER performance.In ddition,the hollow structure in rGO@CF:Pi can enlarge electrochemical surface area(ECSA),promote more active sites,and accelerate mass transfer.Besides,the phosphate pecies act as proton acceptors to participate in proton-coupled electron transfer(PCET) nd facilitate the oxidation of the Co/Fe-OH2 centers during OER.In the mechanism tudy,it is discovered that the Co/Fe center connected by di-μ-oxo bridges arranged in M4O4”cubane motifs are the active sites in rGO@CF:Pi.In the alkaline media,eaturing by the overpotential of 300 mV to achieve the current density of 10 mA cm–2 nd the Tafel value is 36 mV dec–1.Such performance is much superior to commercial rO2.(2)The rGO@Fe-Ni2P electrocatalyst can be prepared from the facile hydrothermal reaction nd followed by the phosphorization step.The iron was successfully doped into the attice of Ni2P.By adjusting the amount of iron,the OER performance of rGO@Fe-Ni2P as successfully optimized.The presence of P in Ni2P has a large impact on the OER,ignificantly improved the turnover frequency(TOF)value of Ni2P,and the ncorporation of iron can modulate the electron structure of center atoms(Ni),which can mprove the intrinsic activity of Ni2P and thus promote the OER performance.In the lkaline solution,the obtained rGO@Fe-Ni2P demonstrated a lower OER overpotential f 260 mV at a current density of 10 mA cm–2.The Tafel value is 35 mV dec–1.Such uperior catalytic performance compared favorable to commercial IrO2.(3)Using the 3D carbon fiber as the conductive substrate,the CF@Ru-CoCH was uccessfully prepared by facile hydrothermal-electrodeposition process.The obtained ybrid structure possesses a high density of interfaces between Ru and CoCH,which ooperate synergistically towards alkaline HER.To be specific,the H2O molecular ould adsorb electron to dissociated into hydrogen adsorption species(H*)and hydroxyl on(OH–),then the H*would be adsorbed on a nearby empty Ru site to form H2 by the eyrovsky step and showed superior HER activity.The as-prepared CF@Ru-CoCH xhibited the overpotential of 66 mV and 121 mV to achieve the current density of–10 A cm–2 and–100 mA cm–2,respectively.The Tafel solpe is 65 mV dec–1.Such erformance is next only to commercial Pt.