Design,Synthesis And Electrochemical Performance Of Cobalt-Based Semiconductor Junctions

Using hydrogen as an energy carrier,through electrochemical conversion and energy storage systems,sustainable clean energy with shortcomings such as intermittent and unpredictability can be safely incorporated into the grid.Therefore,synthesis and design of electrocatalysts play a vital role in the future energy transformation.In order to prepare electrocatalysts with low cost,high efficiency and excellent stability,in recent years,non-noble metal-based electrocatalysts have attracted great attention.Co-based materials have been applied in various energy fields due to their rich and changeable chemical states,which are conducive to catalyzing rapid oxidation-reduction reactions.Based on the understanding of the OER mechanism and the cause of the overpotential,in this dissertation,starting from the aspects of constructing heterostructures,and controlling composition and phase transition,the intrinsic structures of the prepared Co-based composite catalysts can be successfully regulated through the electron distribution,and its affects on the electrochemical activity have been investigated.Finally,the efficient electrocatalysts are obtained.The main findings and results are listed as follow(1)CoP two-dimensional(2D)nanosheet arrays and amorphous FeNi-LDH nanosheets are sequentially constructed on a flexible carbon cloth(CC)to form an open and hierarchical p-n junction(FeNi-LDH/CoP/CC)as a self-supporting electrode According to semiconductor physics,electrons transfer and redistribute at the p-n junction interface,and FeNi-LDH as n-type semiconductor become the positively charged active center,which is confirmed by spectroscopic and electrochemical characterization.Density functional theory(DFT)calculations show the adsorption energy of OH-on the surface of FeNi-LDH in the FeNi-LDH/CoP junction is-2.76 eV,which is lower than-1.45 eV on the surface of the unstructured FeNi-LDH,that is,OH-has a stronger trend to adsorb on the surface of FeNi-LDH side of FeNi-LDH/CoP p-n junction.In an alkaline medium,at a low overpotential(?)of 254mV,the current density(j)of the FeNi-LDH/CoP/CC electrode is as high as 350 mA cm-2,and a rather small Tafel slope can be achieved as 33.5 mV dec-1,with a large turnover frequency(TOF)of 0.131 s-1.At 1.485 V(vs RHE),the j of the p-n junction for electrocatalytic OER is about 10 and 100 times that of the n-type FeNi-LDH/CC and p-type CoP/CC,respectively.In addition,FeNi-LDH/CoP/CC also has good stability for OER,and also shows excellent activity of electrochemical water splitting in the alkaline system(2)By optimizing the composition to adjust the electronic structure of the heterojunction,the electrocatalytic activity can be improved.Using CC as the substrate,various p-type Co-based phosphides containing different molar ratios of Ni atoms are obtained by electrochemical deposition and low-temperature phosphating,followed by in situ electrodeposition of n-type FeNi-LDH for constructing a Ni-containing p-n heterojunction catalyst.The different addition of Ni can effectively adjust the energy level position of the heterojunction.As the amount of Ni increases,the Fermi level(Ef)gradually shifts positively,thereby regulating the energy level position of the FeNi-LDH in p-n junction.In turn,it affects the OER activity of the corresponding catalyst In the alkaline environment,when the molar ratio of Co to Ni is 2:1,the energy level of the obtained FeNi-LDH/Co2/3Ni1/3P/CC p-n junction catalyst is the most suitable,that is,the optimal balanced point between OH-adsorption and effect of built-in electric field is achieved.Therefore,FeNi-LDH/Co2/3Ni1/3P/CC is endowed with the excellent activity for OER.The FeNi-LDH/Co2/3Ni1/3P/CC can achieve excellent j at a small ?.Furthermore,the Tafel slope of 33.6mV dec-1 shows that it has highly efficient kinetic characteristics.It also shows good stability for catalyzing OER and electrocatalysis for water splitting.By adding Fe instead of Ni,an amorphous Fe-doped Co-based phosphate is obtained.FeNi-LDH/Co2/3Fe1/3Pi/CC shows the most excellent OER electrocatalytic activity.It is worth mentioning that FeNi-LDH/Co2/3Fe1/3Pi/CC has an efficient kinetic process and exhibits a very small Tafel slope of 30.3 mV dec-1(3)Phase control is utilized to improve the electrocatalytic activity of materials thorough adjusting the electronic system.Various cobalt-based phases with doped Ni atom and carbon-coated layers are in situ grown on the nickel foam(NF)by hydrothermal synthesis and following calcination under different atmospheres,and show a nano-flower shape.The modified CoO(C-Ni-CoO/NF)shows the best OER catalytic activity with corresponding Tafel slope of 75.3 mV dce-1 in alkaline media.Moreover,FeNi-LDH nanosheets are electrodeposited on the C-Ni-CoO/NF surface to form a FeNi-LDH/C-Ni-CoO/NF heterostructure.It exhibits superior OER catalytic activity because Ni-CoO can accept electrons from FeNi-LDH quickly transported through carbon materials.The small Tafel slope of 31.9 mV dce-1 indicates that this heterostructure possesses excellent dynamic characteristics.(4)An environmentally-friendly one-step pyrolysis method without any post-processes is developed with combination of various concepts to adjust the electronic structure of the catalyst,such as forming solid solution,heteroatoms doping,phase controlling and heterostructures.Triphenylphosphine sulfide(Ph3PS),melamine,and metal nitrates are used as precursors to synthesize a heterostructure with 'new-diamond'-containing nanoporous carbon co-doped with N,P and S as 'shell',and the crystalline CoFe alloys and Co2P nanoparticles as 'core'.When the catalyst with the Co:Fe at 2:1 mol ratio(NPSC-Co2Fe1)acts as a bifunctional oxygen electrode,the potential gap(?E)between Ej=10(the potential to obtain a 10.0 mA cm-2 at 1.6 V vs RHE for OER)and E1/2(the half-wave potential at 0.85 V vs RHE for ORR)of 0.75 V can be achieved in alkaline media.In addition,NPSC-Co2Fe1 exhibits excellent electrochemical activities when used as a catalyst on air electrodes in rechargeable zinc-air batteries(ZABs).Furthermore,the flexible all-solid-state ZABs using NPSC-Co2Fe1 as the air electrode also show good charge-discharge performances and cycling stabilities,indicating that the catalyst has great application prospects.