Preparation Of Transition Metal Oxide Composites With Layered Double Hydroxides Precursors Toward Oxygen Evolution

In this thesis,to enhance the electrocatalytic performance of electrocatalysts for oxygen evolution reaction(OER),the multicomponent and highly dispersed composites with nano-size were fabricated by a simple hydrothcrmal rcduction or oxidation method based on layered double hydroxides(LDHs)as precursor.The reason for choosing LDHs as precursor is that all mental atoms have an atom level dispersion in the layer of LDHs,which could ensure the uniformly distribution of all components in the as-produced samples,thus affording abundant active sites and fast mass transport ability.Moreover,LDHs can act as a support preventing sintering of the nanoparticles,which have built a strong foundation for develop OER electrocatalysts with high activity.Our main researches are summarized as follow:(1)A nanocomposite of Ni/Fe3O4 has been fabricated through a facial hydrothermal method based on NiFe layered double hydroxides(NiFe-LDH)as precursor.The homogeneous dispersion of Ni and Fe3O4 in the nanocomposite is beneficial to the transportation of electrons in electrochemical reaction process.Moreover,the growth mechanism was also investigated for obtaining more detailed explanation for the synthesize process.Owing to the synergetic electrical coupling effect of Fe3O4 and Ni,the Ni/Fe3O4 nanocomposite displays excellent oxygen evolution reaction(OER)activity in 1 M KOH,affording a small overpotential of a mere 275 mV at a current density of 20 mA cm-2 with a small Tafel slope of?70 mV dec-1.The overpotential is among the best reported values for non-noble metal catalysts.Furthermore,the obtained catalyst also presents outstanding stability(there was no obviously OER current density degradation in 11 hour).In addition,the nanocomposite materials were synthesized through reduction of NiFe-LDH precursors in different reduction temperature.It is found that the Ni/Fe-doped ?-Ni(OH)2/Fe3O4 obtained through reduction of NiFe-LDH precursors in 140? displays excellent OER activity,affording a low onset potential of 1.46 V versus RHE,a current density of 20 mA cm-2 at a small overpotential of a mere 250 mV and a small Tafel slope of?59 mV dec-1.These results indicated that the possibility for the fabrication of a promising electrocatalyst with effective and low-price by reducing NiFe-LDH into composite material.(2)The Fe-doped Co3O4@C nanoparticles were devised and fabricated via a simple and novel method.First,CoFe layered double hydroxide(CoFe-LDH)nanosheets was synthesized as precursor through a simple and general of co-precipitation method.Using an soft oxidation procedure,we successfully obtained Fe-doped C03O4 nanoparticles.After the glucose was strictly covering Fe-doped Co3O4 nanoparticles via hydrothermal processing,Fe-doped Co3O4@C nanoparticles are prepared through high-temperature calcinations.we studied the structure and morphology of the Fe-doped C03O4@ C nanoparticles via SEM,TEM,HRTEM,XPS and BET.The result shows that the Fe-doped Co3O4@C nanoparticles were highly dipersed,smaller sizes(20-40 nm)and the nanoparticles form a mesoporous,which could afford abundant active sites and fast mass transport ability.In electrochemical measurement,Fe-doped Co3O4@C nanoparticles exhibited excellent electrocatalytic property in OER process,which revealed a low overpotential of 260 mV at a current density of 20 mA cm-2 with a small Tafel slope of 70 mV dec-1 and good durability(there was no obviously OER current density degradation in 100 hours)in alkaline solution.