Design,Synthesis And Electrocatalytic Properties Of Cobalt Based Electrocatalysts

The excessive consumption of fossil fuels has caused the rapid decrease of traditional energy resources and serious deterioration of environment.It is urgently required to explore the abundant,clean and renewable energy sources.Hydrogen fuel cells,rechargeable metal-air batteries and hydrogen from water electrolysis are the clean and environmentally friendly energy conversion systems,which have attracted considerable attention in this field.Notably,these systems highly depend on a series of electrochemical reactions,such as oxygen reduction reaction?ORR?,oxygen evolution reaction?OER?and hydrogen evolution reaction?HER?.It is well known that highly efficient electrocatalyts can increase the ORR/OER/HER reaction rate,efficiency of total system and selectivity of the chemical transformations.Currently,noble metals,such as Pt and Ir,Ru,have been recognized as the good electrocatalysts for the ORR,HER and OER,respectively.Unfortunately,the high cost and scarcity of the metals resources limit their widespread application.Therefore,substantial efforts have been devoted to the developments of non-precious metal based electrocatalysts with highly active,low-cost and superior durability for these electrochemical reactions.This work mainly focused on the improvement of the electrocatalytic performances by designing and controlling the microstructure of nanomaterials.Herein,a series of cobalt-based electrocatalysts were synthesized with outstanding properties for ORR,OER and HER.Meanwhile,these catalysts were used in metal-air batteries and electrolysis of water.The main contents are as following.?1?A novel of cobalt based metal-organic framework?Co-MOFs?was obtained and then used as a starting material for the synthesis of Co nanoparticles embedded in N,S-codoped carbon layer and supported on multi-walled carbon nanotubes?Co@NSC/MWCNTs?through the high temperature calcination under nitrogen atmosphere.As used as the electrocatalyst,the Co@NSC/MWCNTs displayed excellent oxygen reduction reaction?ORR?electrocatalytic activity in alkaline medium,which behaved the onset potential of 0.834 V vs.RHE,the half-wave potential of 0.782 V vs.RHE and the limiting current density of 5.49 mA cm^-2?0.5 V vs.RHE,1600 rpm?.In comparison with the commercial Pt/C electrocatalyst,the Co@NSC/MWCNTs showed superior stability and better tolerance to methanol crossover and CO poisoning.The synthergistic effects between N,S doping carbon layer sites and CoN_x active sites significantly improved the catalytic performance of Co@NSC/MWCNTs.Moreover,Co nanoparticles were covered by carbon shells,which displayed an outstanding durability in the electrocatalytic process,but controlled the thickness of carbon layer is still needed to be improved.?2?A bifunctional electrocatalyst(Fe_1.2Co@NC/NCNTs),consisting of ultrathin N-doped carbon?1³ graphitic carbon layers?coated Fe_1.2Co nanoparticles and supported N-doped carbon nanotubes,was synthesized through high temperature calcinations of precursor under nitrogen atmosphere.The precursor was prepared from the metal source of cobalt phthalocyanine?CoPc?,iron phthalocyanine?FePc?and nitrogen source of melamine supported on CNTs.Thus,metal nanoparticles were encapsulated into ultrathin carbon layer to prevent the corrosion of metal nanoparticles from a harsh environment and simultaneously promote the catalytic reaction on the carbon surface due to the electron transfer from encapsulated metal nanoparticles.As a result,the electrocatalyst had an extremely low Fe/Co content?6.7 wt.%?,but with highly efficient and durable bifunctionality for ORR and OER.Specifically,the Fe_1.2Co@NC/NCNTs exhibited onset potential(E_onset=0.842 V vs.RHE)and half-wave potential(E_1/2=0.82 V vs.RHE)for ORR and overpotential of 355 mV at 10 mA cm^-2 for OER.Most notably,when used in the Zn-air battery,the Fe_1.2Co@NC/NCNTs displayed superior charge-discharge performance and cycling stability.Furthermore,it has potential application in wearable electronic devices field.?3?A novel electrocatalyst of CoNi/CoFe₂O₄/NF with three-dimensional?3D?porous nanostructure was fabricated through a facile hydrothermal synthesis method and followed by electrodeposition.The electrocatalyst showed highly efficient electrocatalytic activity in alkaline medium toward OER with an overpotential of 230?±2?and 360?±5?mV as to achieve a current density of 10 and 1000 mA cm^-2,respectively.Using as a cathode catalyst,the electrocatalyst exhibited excellent performance toward HER with overpotentials as low as 82?±3?mV to deliver current densities of 10 mA cm^-2.Furthermore,when it was applied as a bifunctional electrocatalyst for water electrolysis,the current densities of 10 mA cm^-2 and 100 mA cm^-2 were obtained at applied potentials of 1.57 and 1.75 V,respectively,together with excellent durability.During the water electrolysis,the CoNi/CoFe₂O₄/NF electrodes displayed high O₂ and H₂ generation efficiencies with nearly99%faradaic yield.The outstanding performances stemed from the 3D interconnected porous networks exposed more catalytic active sites and benefited for the effective contact between electrode material and electrolyte,thus facilitating the rapid transfer electron and ions,and boosting the OER/HER electrocatalytic.?4?A core-shell nanostructured electrocatalyst of CoS₂@MoS₂/CP was fabricated by a two-step hydrothermal strategy.The optimum CoS₂@MoS₂/CP electrocatalyst exhibited outstanding HER activity over a wide range pH values.The electrocatalyst showed that the overpotentials was 69 mV in acidic solution,145 mV in neutral solution and 82 mV in alkaline solution,respectively,to afford a current density of10 mA cm^-2.Furthermore,it also behaved superior stability under different pH conditions for at least 48 h.The generation of H₂ from the water electrolysis in a wide pH range media exhibited over 97%faradaic efficiencies.Benefiting from the unique core-shell structure,the active sites can not noly be provided by the formation of abundant interface defects,but also the synergistic interactions can be realized between CoS₂and MoS₂ as well as the fast electron transport from CoS₂ core to MoS₂shell.Both of them facilitate the adsorption of hydrogen intermediates and enhance the electrochemical HER activity in a broad pH range.