Construction And Electrocatalytic Performance Of Composite Niobium-based Bifunctional Catalysts

As a green and efficient energy conversion device,fuel cells have great potential for development.However,the anode and cathode reaction kinetics of fuel cells are very slow,so it is necessary to design and develop highly active catalysts.At present,platinum and platinum-based catalysts have shown excellent activity in catalytic reactions.However,the high price of platinum and the instability in the catalytic process greatly limit its large-scale application.Metal niobium is abundant,cheap and easy to obtain,and niobium-based nitrides and oxides have good physical and chemical properties,which can effectively improve the catalytic performance of catalysts.Therefore,the design and development of efficient niobium-based catalysts to replace traditional noble metal platinum-based catalysts has important application value.In this paper,a niobium nitride/niobium dioxide-coated carbon film catalyst was designed and synthesized using niobium chloride and urea as reactants,and then the electrocatalytic activity was improved by loading Pd and composite carbon nanotubes,which was used as a direct ethylene glycol fuel cell anodic alcohol oxidation and cathodic oxygen reduction electrocatalysts.The details are as follows:（1）The carbon film-coated niobium nitride,niobium dioxide and niobium nitride/niobium dioxide two-phase composite（NbN@C,NbO₂@C,NbN-NbO₂@C）catalysts were synthesized by the solid-phase one-step calcination method and by adjusting the dosage ratio of niobium chloride and urea.The oxygen reduction electrocatalytic activity of the catalyst was investigated under alkaline conditions.The results show that the electrocatalytic activity of NbN-NbO₂@C catalyst is superior to that of NbN@C and NbO₂@C,and its oxygen reduction half-wave potential is comparable to that of commercial 10%Pt/C.In addition,molecular simulation calculations were performed using density functional theory（DFT）,and the results showed that there is a good synergy between NbN and NbO₂ in the NbN-NbO₂@C catalyst,which is beneficial to improve its electrocatalytic activity.On the other hand,the encapsulated graphitized carbon film not only increases the electrical conductivity of the samples but also enhances their stability.（2）The NbN-NbO₂@C catalyst with high electrocatalytic activity was selected,and it became a bifunctional catalyst（Pd/NbN-NbO₂@C）by loading Pd.Compared with the NbN-NbO₂@C catalyst,the Pd/NbN-NbO₂@C not only improved the oxygen reduction activity,but also exhibited certain catalytic activity for the oxidation of ethylene glycol.The half-wave potential of the Pd/NbN-NbO₂@C catalyst for the oxygen reduction reaction can reach 0.90 V in an alkaline environment（compared to the reversible hydrogen electrode（RHE））,outperformed the NbN-NbO₂@C(E_1/2=0.83V)and commercial 20%Pt/C(E_1/2=0.86 V)catalysts.On the other hand,the catalytic activity of the Pd/NbN-NbO₂@C catalyst for ethylene glycol oxidation is about twice that of commercial 10%Pd/C,with excellent anti-poisoning ability.However,its durability for electrocatalytic oxygen reduction reaction needs to be improved.（3）Carbon nanotubes were introduced into NbN-NbO₂@C catalyst by solvothermal method,and a matrix material with hierarchical structure of carbon nanotubes+nanosheets was constructed,and then Pd nanoparticles were supported by reflux reduction method to obtain Pd/NbN-NbO₂@C-CNTs catalyst.Compared with Pd/NbN-NbO₂@C,the catalytic activity and durability of the catalyst are greatly improved.In an alkaline environment,the Pd/NbN-NbO₂@C-CNTs catalyst can achieve a half-wave potential of 0.91 V and a limiting current density of 6.1 m A cm^-2,which is superior to commercial 20%Pt/C.In addition,the catalyst can achieve a peak current density of 703.02 m A mg^-1 for ethylene glycol oxidation in an alkaline environment,which is superior to that of the Pd/NbN-NbO₂@C catalyst(421.54 m A mg^-1).In particular,the hierarchical structure of nanotubes+nanosheets in the Pd/NbN-NbO₂@C-CNTs catalyst can expose more active sites,which is favorable for anchoring Pd nanoparticles and greatly improves the catalytic durability of the catalyst,which is better than commercial 10%Pd/C.