Tuning Microstructures Of Iron Group Catalysts For Enhanced Electrochemical Processes

Hydrogen,an efficient and clean energy,has become one of best candidates for replacing traditional fossil fuels.Electrolysis of H₂O is an important route in supplying clean hydrogen,which however is heavily limited by sluggish,multistep and four-electron oxygen evolution reaction（OER）kinetics in anode.Electrocatalysts have been applied to this reaction in order to lower activation energy and increase kinetics.Nowadays,precious Ru and Ir based oxides are still the most effective OER electrocatalysts,but limited reserves and high cost restrict their wide applications.Thus it is desired to get low-cost but efficient OER catalysts.A great deal of transitional metal and non-metal catalysts have been developed,among which FeNi alloys demonstrate attractive activities.When it comes to FeNi alloys,a large quantity of researches on FeNi₃ could be found.However,there is no systematic study of compositions,microstructures and catalytic activities of FeNi alloy catalysts.On the one hand,diverse chemical compositions of alloys make intrinsic activities of these catalysts different;On the other hand,apparent catalytic activities of catalysts are also closely related to their microscopic structures.Dispersion density of active sites,degree of exposure and mass transfer efficiency in the catalytic layer all have significant impacts on the catalytic performance of catalysts.In view of this,this paper has carried out the following two aspects of research work:Firstly,we prepared N-doped carbon（NC）coated iron/nickel alloy catalysts loaded on the carbon fiber papers（CFP）by firstly hydrothermal depositing iron–nickel based hydroxide/carbonates,then coating dopamine on their surface,and finally annealing the materials.Particularly,the active alloy species can be tuned to be Fe_0.64Ni_0.36 and FeNi₃,and the microscopic structures of the catalysts show to be nanotubes,nanoflowers and nanosheets upon different atomic ratio of Fe²⁺to Ni²⁺.In an alkaline media,the alloy catalysts exhibit superior OER catalytic activities,which only require 277³06 mV overpotentials to reach a current density of 20 mA/cm²,and331⁴05 mV overpotentials to reach 100 mA/cm².At the same time,FeNi₃@NC（1:3）displays superior OER catalytic activities and no obvious activity loss after continuous running for 30 h at 20 mA/cm²,indicating a good durability.The excellent performances of the alloy catalysts in alkaline solution can be ascribed to their microstructural features:（1）The synergistic interaction between NC and alloy nanoparticles creates copious active sites;（2）The NC coating layer prevents the erosion of alloy nanoparticles which thus improves the durability;（3）The three-dimensional structures well promote mass transfer and effectively suppress the gas peeling effect.Secondly,FeNi₃@NC（1:3）powders were mixed with transition metal@NC catalyst to prepare bifunctional catalysts composed of two phases and applied to oxygen electrodes of rechargeable Zn-oxygen batteries.We prepared NC coated cobalt nanoparticles catalyst with a good catalytic activity by firstly hydrothermal synthesis of cobalt-based precursor powders,then coating dopamine on their surface,finally annealing and acid etching the material.A new method of synthesizing transition metal@NC powders in enormous quantities is developed.FeNi₃@NC（1:3）powders prepared by the above mentioned method were mixed with Co@NC catalyst to prepare bifunctional catalysts composed of two phases.Through exploration,it is found that when the mass ratio of FeNi₃@NC（1:3）powders to Co@NC catalyst is 2:3,the prepared two-phase catalyst of FeNi₃@NC（1:3）2-3 Co@NC shows excellent ORR activity（the half-wave potential reaches 0.791 V in 0.1 M KOH solution）and OER activity（the catalyst only requires 324 mV overpotentials to reach a current density of20 mA/cm² in 1 M KOH solution）.When the two-phase catalyst was applied to rechargeable Zn-oxygen batteries,charge-discharge polarization and constant-current cycling tests show that the total overpotential of charge and discharge of FeNi₃@NC 2-3Co@NC is lower than Pt/C-RuO₂,indicating a better performance of charge and discharge.At the same time,the bifunctional catalyst shows higher energy conversion efficiency than Pt/C-RuO₂.The above results show a potential prospect of the bifunctional catalyst for commercial application.