Preparation And Performance Study Of Electrocatalysts For Oxygen Electrode Reaction

Fossil fuels such as oil,natural gas,and coal are the main energy source of human production and life.They are unsustainable,while they are rapidly consumed in recent two hundred years.More importantly,the combustion of them also brings serious global climate change,toxic gas emission,and environmental degradation.Therefore,development of high-efficiency energy conversion equipment in line with the protection of the natural environment has been a major challenge.Recently,the development of economy from fossil fuels to clean energy has attracted more and more attention.Hydrogen production by electrolytic water,fuel cells,metal-air batteries are representative as new electrochemical energy sources.Fuel cells use hydrogen or other fuels and oxygen as the reactants,which can convert the chemical energy into electrical energy through electrochemical reactions to produce water.The water electrolysis technology can split water into hydrogen and oxygen,which can further be used as the fuels of fuel cells,hence forming a benign green cycle.In the two processes mentioned above,the oxygen evolution reaction?OER?is the anodic reaction of water electrolysis and the oxygen reduction reaction?ORR?is the cathodic reaction of fuel cells.Both of them strongly restrict the overall energy conversion efficiency due to their sluggish kinetics.Thus far,the typical electrocatalysts for OER/ORR are still the noble metal-based catalysts such as RuO₂,IrO₂ and Pt-based materials.These catalysts have high activity for OER and ORR,whereas their high cost and relatively low stability limit their commercial applications.Therefore,the development of cheap,efficient and stable electroccatalysts that can replace precious metal catalysts is of great significance.This work aimed to design and synthesize non-noble metal electrocatalysts with high electrocatalytic activity and stability for OER and ORR,and disclose the relationship between the nanostructure and catalytic performances.Specific work includes the following aspects:1.Co-MOF@CNTs composite materials were prepared through a hydrothermal method by combining the Co-MOF with the conductive carbon nanotubes?CNTs?via self-assemble technique.The copposite materials were used as the electrocatalysts for ORR in 0.1 M KOH electrolyte.The effects of the loading amount of CNTs on the electrocatalytic performance for ORR are investigated.Metal organic framework?MOF?is a kind of promising functional material,which is porous and composed of metal ions and organic ligands.However,the MOF materials always have a poor conductivity,which limits their applications as an electrode material.Therefore,we combined the highly conductive CNTs with Co-MOF to synthesize the composite material Co-MOF@CNTs.This material can not only retain the basic structure of the MOF,but also improve the conductivity of the composite catalyst.Results show that the addtiton of CNTs can strongly improve the electrocatalytic performance for ORR,and the performance is closely related to the loading amount of CNTs.Co-MOF@CNTs with the loading amount of CNTs being 15 wt%exhibits the highest ORR activity.The onset potential of ORR is found at 0.85 mV and the half-wave potential is 0.79 mV.The number of ORR electron transfer is 3.7,which is close to the theoretical value of 4,suggesting a four-electron transfer process of ORR on Co-MOF@CNTs.2.Cu-doped RuO₂ composites?Cu-RuO₂?were synthesized via one-step calcination of amorphous RuCu samples in air atmosphere.The amorphous RuCu samples were prepared by ion exchange method with Cu-MOF as the precursor.The electrocatalytic performances of Cu-RuO₂ toward OER were tested in 0.5 M H₂SO₄ electrolyte and the effects of calcination temperature on the electrocatalytic performance were investigated.Results show that Cu-RuO₂prepared at a calcination temperature of 300°C exhibits the highest electrocatalytic activity for OER.At a current density of 10 mA cm^-2,the overpotential of OER on Cu-RuO₂-300 is only197 mV,which is about 40 mV lower than that on commercial RuO₂ catalyst?237 mV?.The Tafel slope is as low as 41.6 mV dec^-1,which is also lower than that on RuO₂ catalyst(49.7 mV dec^-1).In addition,the potential shows almost no attenuation during the 24 h of durability test.The above results indicate that the doping of copper can significantly improve the catalytic activity and stability of ruthenium-based metal catalysts.3.A two-dimensional?2D?ultrathin hybrid Co-NiFe layered double hydroxide?LDH?was synthesized via a facile hydrothermal method.In 1.0 M KOH electrolyte,Co-NiFe LDH exhibits remarkable activities for OER.At the current density of 10 mA cm^-2,it only needs an overpotential of 278 mV,which is ca.50 mV and 20 mV lower than those for NiFe LDH?328mV?and RuO₂ catalysts?298 mV?,respectively.In addition,Co-NiFe LDH also shows impressive long-term stability for OER.Besides the stable morphology and crystal structure,the potential is always kept at 1.50 V and shows almost no attenuation during the 20 h of durability test.Changes in the electronic structure of LDH due to introduction of Co ions,as well as the large specific surface area facilitate the mass/electron transfer and the oxygen bubbles release,and thus lead to the enhanced catalytic properties for OER.This work can be informative not only for understanding the role of physical and electronic structures on OER but also for designing high-performance non-precious metal OER electrocatalysts.