Fabrication Of Multiple Metal Oxide/Hydroxide Based Electrodes And Their Electrocatalytic Oxygen Evolution Reaction Performance

Hydrogen derived from electrocatalytic water splitting is regarded as promising renewable energy with ever-increasing energy and environmental problems.However,oxygen evolution reaction(OER),the anodic half-reaction,is a sluggish four-electron transfer process,which limits the improvement of overall energy efficiency.The key to accelerating OER process is to develop highly-efficient OER electrocatalysts.In acidic or alkaline media,the emphasis is different.In alkaline media,researches focus on the coordinated interaction among components to further accelerate OER reaction rate.In acidic media,the focus is on the improvement of mass activity and long-term stability of noble metal based catalysts.Currently,transition metal oxides/hydroxides attracted much attention due to their superior activities,adjustable structures and other advantages.The activity of catalyst depends on the amount of exposed active sites on the surface and intrinsic activity of each site.Therefore,on the one hand,we designed surface morphology of catalysts to modify its contact situation with electrolyte.On the other hand,by regulating the electronic structures of internal components,the adsorption energy of OER intermediates was optimized,thereby improving the OER performance of electrocatalysts.Simultaneously,developing simple preparation methods of catalysts is significant to the large-scale application of electrocatalytic water splitting.In this thesis,we prepared highly efficient multiple metal oxide/hydroxide based electrodes for water oxidation in acidic or alkaline electrolyte respectively.The specific works are as follows:(1)In order to improve the OER activities of transition metal based hydroxides,NiCoFe-OH nanocage with hollow hierarchical structure was synthesized from ZIF-67 precursor through a self-sacrifice template method.The nanosheets grown on the surface of nanocage increased its electrochemically active area and exposed more active sites on the surface.The reaction time and Fe amount deeply impacted the surficial morphology and electrocatalytic performance of NiCoFe-OH nanocage.The reaction time will affect the growth of surface nanosheets,which was related to the internal charge transfer resistance of nanocage.Appropriate Fe amount can effectively enhance the OER activity of NiCoFe-OH.But excessive Fe ions caused serious agglomeration and damaged surface morphology of nanocage.The regulated NiCoFe-OH nanocage exhibited excellent OER performance in alkaline electrolyte,which only need an overpotential of 232 mV to reach 10 mA cm-2,314 mV to reach 300 mA cm-2,with a Tafel slope of 54.4 mV dec-1.(2)Enhancing the bonding ability between catalysts and conductive substrate is regarded as an effective way to avoid extra energy loss caused by adhesives.Two transition-metal-based hydroxide arrays were synthesized in situ on the foam substrates:Fe and Nb co-doped ?-Ni(OH)2 nanosheet arrays on Ni foam,Ni doped goethite FeOOH arrays on Fe foam.In Fe,Nb co-doped ?-Ni(OH)2 electrodes,The representative NiFe3Nb2-OH possessed outstanding OER performance with an overpotential of 294 mV to reach 100 mA cm-2,317 mV to reach 300 mA cm-2,and a Tafel slope of 47 mV dec-1.Simultaneously,the electrode exhibited excellent long-term stability without activity decline for 90 h.After doping Fe and Nb atoms into?-Ni(OH)2 nanosheets,its electrochemical active area and electronic transfer capability was enhanced obviously.The synergistic interaction among Fe,Nb and Ni modulated the electronic structure of cations and apparently boosted OER performance of the electrode.Density functional theory(DFT)calculations revealed that doping Fe and Nb into ?-Ni(OH)2 could reduce its Gibbs free energy of the rate determining step for OER process and accelerate the reaction rate.NiFe3Nb2-OH and MoS2/NF were combined to perform electrocatalytic overall water splitting,which exceeded many other overall water splitting catalysts.The potential of NiFe3Nb2-OH I MoS2/NF to reach 300 mA cm-2 was only 1.91 V.The results proved that Fe,Nb co-doped(3-Ni(OH)2 was a promising electrocatalyst for practical application.In Ni doped goethite FeOOH array,the amount of Ni dopant had a critical influence on the morphology of Ni-FeOOH electrode.When Ni doping amount was moderate,the Ni-FeOOH-2 electrode with nanorod array morphology exhibited better OER performance,which only need an overpotential of 233 mV to reach 10 mA cm-2 with a Tafel slope of 60 mV dec-1.(3)Compared with ordinary alkaline electrolytic cells,proton exchange membrane electrolysis cells(PEM)are more promising because of their safety and faster reaction rate.The acidic environment on the membrane demands the noble-metal-based catalysts on the anode.Due to its high cost and scarce reserves,it is particularly critical to improve the mass activity of noble-metal-based catalysts in acidic media.In the thesis,carbon supported Y doped RuO2 nanoparticle was prepared for OER in acidic media.In 0.5 M H2SO4,Y-RuO2 showed good OER performance,which need an overpotential of 244 mV to reach 10 mA cm-2.Compared with other noble metal based catalysts in acidic media,Y-RuO2 exhibited superior mass activity.DFT calculations indicated that doped Y could decrease the Gibbs free energy of rate determining step on Ru sites,which significantly accelerated reaction kinetics of Y-RuO2.More importantly,Y-RuO2 exhibited outstanding stability in acidic media,whose overpotential only increased 63 mV after stability test for 50 h.Furthermore,Y-RuO2 was combined with Pt/C to fabricate Membrane electrode assembly(MEA)for overall water splitting in PEM electrolytic cell under acidic condition.At 50 mA cm-2,Y-RuO2 | Pt/C maintained stable more than 48 h,whose potential only increased 30 mV.This part provides new ideas for the design and application of OER catalysts in acidic media.