Preparation Of Transition Metal Compounds Loaded On Carbon Nanomaterials As Bifunctional Catalyst And Its Application In Zinc-air Batteries

Nowadays,due to the rising demand of social industrial development,the burning of limited energy such as fossil energy,and the greenhouse effect caused by the emission of carbon dioxide and other gases is becoming more and more serious.Exploring the"green"new energy technology and its related storage/conversion system has become the key to solve the energy and environmental problems.Among them,zinc-air batteries（ZABs）have become one of the hot spots in the field of new energy batteries because of their high specific energy,good safety,environmental friendliness and low preparation cost.However,the multi-phase interface is the place where the discharge and charging of the battery occur,and the kinetics of oxygen reduction/precipitation reaction（ORR/OER）is relatively slow,which makes the energy efficiency and cycle life of ZABs decrease.In view of this,Pt/Ir/Ru based noble metal catalysts are undoubtedly an effective way to improve the performance of ORR/OER.However,precious metals are usually scarce,and their high price and instability in operation to some extent hinder the development and application of such catalysts.At the same time,these noble metal catalysts often show unidirectional activity when they catalyze ORR or OER.Therefore,the two catalysts need to be loaded on the working electrode to complete the charge and discharge work,which further increases the cost.Therefore,it is of great significance to explore efficient and economical non noble metal bifunctional oxygen electrocatalysts.In this paper,carbon nanotubes（CNTs）and carbon nanofibers（CNFs）were used as carbon material substrates.Firstly,S-CoNi-CNTs with high catalytic performance were prepared by hydrothermal/calcination method combined with vacuum vulcanization method.Secondly,metal sulfide catalysts with excellent ORR/OER dual functional properties were prepared by doping Mg ions with heteroatoms,which is named（Co,Mg）S₂@CNTs.Thirdly,CNFs were synthesized by electrospinning technology,and FeCo-CNFs catalysts were prepared by N-doping of melamine and pore forming agent.In order to explore the catalytic mechanism of the catalyst,various physical characterization methods,were used the performance of each catalyst was analyzed in depth,and the related power,charge and discharge performance of the prepared catalyst assembled ZABs were tested and analyzed to explore the feasibility of its practical application.The main research contents and achievements of this paper are summarized as follows:（1）A series of graded catalysts for transition metal oxide composite CNTs were prepared by a simple and green hydrothermal/calcination method.The ORR/OER properties of Fe,Co,Ni and Mn were investigated.The more excellent Co and Ni were selected as bimetallic dopants（the doping amount was less than 5%）to prepare CoNi-CNTs catalysts.The intercalation between metal oxide particles and carbon nanotubes was observed by physical characterization.As an electron transfer channel,CNTs can improve the conductivity and reduce the resistance of oxygen catalytic reaction.At the same time,metal oxide particles can better mass transfer（O₂ adsorption and desorption）.It was found by XRD that the metal oxide was spinel NiCo₂O₄.Thiophene sulfur after sulfur doping further promoted ORR/OER performance,which also explained the role of sulfur doping from a deeper chemical mechanism.Finally,through electrochemical test and single cell assembly test,(E_1/2-E_j=10)=1.72 V–0.72 V=1.00 V,which is better than that of commercial catalyst（1.02 V,Pt/C-RuO₂）;the power density of single cell is 198 m W cm^-2,which is equivalent to that of commercial catalyst Pt/C-RuO₂,and its discharge/charge and discharge duration are significantly better than that of commercial catalyst.（2）A kind of hierarchical bifunctional catalyst supported on CNTs with controllable Mg substitution for bimetallic sulfides was prepared by Mg ion doping to enhance lattice basicity to form（Co,Mg）S₂@CNTs.The effects of hydrothermal temperature/time and calcination temperature/time on preparation conditions were investigated.By comparing with Mg S@CNTs,Co S₂@CNTs,it is found that Mg ions play an important role in the formation of metal sulfides.In the hydrothermal process,the concentration of magnesium ions can reduce the effective lattice basicity and form stable（Co,Mg）S₂ nanoparticles better.The jellyfish like shape and CNTs as electron channels form a good synergistic effect.Therefore,it can promote the 4e-transfer process and improve the mass transfer and accelerate the reaction kinetics of oxygen catalysis.It was found that（Co,Mg）S₂@CNTs its half wave potential(E_1/2)is 0.80 V（vs.RHE）,and the limiting diffusion current is equivalent to Pt/C-RuO₂(6 m A cm^-2),which is close to the theoretical maximum current density.Its oer performance E_j=10 is 1.59 V,which is far better than other samples and Pt/C-RuO₂（1.83 V）.In addition,（Co,Mg）S₂@CNTs the value of△E is only 0.79 V,which is better than that in many reported literatures.For primary zinc air batteries,the peak power density of 268 m W cm^-2 was higher than that of commercial Pt/C-RuO₂(190 m W cm^-2)and other reported catalysts.For rechargeable ZABs,the long-term stable charge discharge cycle(current density:5 m A cm^-2)was realized in 52 hours,and the voltage drop was only 0.67 V.at the same time,it also had good performance in large current charge and discharge,indicating that（Co,Mg）S₂@CNTs The catalyst has the potential to be used in high power and high current devices.（3）A kind of carbon nanofiber supported Fe/Co embedded bimetallic oxide bifunctional catalyst,FeCo-CNFs,was prepared by electrospinning/calcination method with high efficiency and low economic cost.The feasibility of melamine as pore forming agent and N-dopant in the calcination process was investigated.The catalytic activity mechanism was studied by physical characterization methods.It was found that the CNFs prepared by electrospinning had hierarchical structure.The embedded metal oxides could improve the oer performance of the catalyst,while pyridine nitrogen and graphite nitrogen formed by N-doping could help to improve the OER performance of the catalyst.The results show that the ORR performance is improved,and the polyacrylonitrile precursor has better carbonization degree,which can carry out the 4e-transfer process well,improve the oxygen catalytic performance and reduce the over potential.Through CV,LSV and other electrochemical tests,FeCo-CNFs have a peak initiation potential(E_onset)of0.90 V,half wave potential(E_1/2)of 0.80 V,oer performance(E_j=10)of 1.72 V,and its△E=0.92V is better than that of Pt/C-RuO₂.The electron transfer number measured by RRDE technology is close to 4 and the conversion rate of hydrogen peroxide is less than 5%.Furthermore,FeCo-CNFs was applied to the liquid/flexible ZABs to achieve a power density of 232 m W cm^-2.In the liquid zinc air battery,FeCo-CNFs could maintain a charge discharge voltage drop of 0.76 V at a current density of 5 m A cm^-2 for 129 h（774 cycles）;in the flexible zinc air battery,FeCo-CNFs could stably charge and discharge at a current density of 2 m A cm^-2 for 12 hours.The results show that FeCo-CNFs is a kind of dual functional catalyst with high efficiency,high activity and low cost,which fully shows the potential of application in liquid/flexible zinc air battery devices.