Synthesis Of Metal Carbides And Carbonitride Based Catalyst For Oxygen Reduction In Alkaline Solution

Cathode materials play an important role during oxygen reduction reaction?ORR?which is relevant for electrochemical energy conversion in the fuel cells.The efficiency of this ORR process depends on the catalyst used;highly dispersed platinum?Pt?nanoparticle on carbon is currently being used for this reaction.However,Pt is prohibitively expensive and easily corroded in alkaline/acidic solution.Transition metal carbides as well as carbonitrides possess desirable properties including high electrical conductivity(104?^-11 m^-1),easy structural tunability and high chemical stability.Their synthesis is environmental friendliness.In fact the precursors often used for their synthesis are earthly abundant making them cheap and available in various forms.As such,the materials have received plentiful intentions to replace noble metal based catalysts?Pt?in order to achieve electrochemical energy conversion in a low-cost and sustainable manner.In particular,metal carbides have seen a lot of advances for fuel cell application wherein power density in the range of?14.5-120 mW/cm²?is often observed.With this,this thesis investigates use of novel synthetic methods to tune the structural property of metal carbides and carbonitrides for enhancing ORR activity in alkaline solution.The challenges which are currently facing by researchers in tuning these properties are also highlighted along with possible solutions and perspectives.Firstly,it is observed that titanium carbonitrides have promising catalytic properties for oxygen reduction reaction?ORR?.However its synthesis normally requires high temperatures?1200-1800 ^oC?,thus limiting researches carried out on them.Also,catalytic property of carbonitrides has so far been improved through post-synthesis partial surface oxidation.This makes the entire process of making the desired catalyst cumbersome and complex.In this thesis,single walled titanium carbonitride nanotube supported cobalt nanoparticle(Co@TiC_0.25N_0.75)catalyst is reported via solvothermal method followed by solid-solid separation process.The problem of high temperature required for TiCN synthesis is successfully overcome,and Co doping strategy is used to improve the catalytic performance.Specifically,different mass ratios of Co nanoparticles are loaded onto TiC_0.25N_0.75;it is discovered that only a fraction of Co is dissolved at TiC_0.25N_0.75.75 lattice due to the solubility limit being⁰.98%.The remaining fraction of Co is found on the wall of TiC_0.25N_0.75.In alkaline solution,Co@TiC_0.25N_0.75.75 shows appreciable ORR activity with onset and half-wave potential located at 0.85 and 0.73 V vs RHE respectively;the performance is higher than that of TiN?i.e.0.69 and 0.55 V?and single phase TiC_0.25N_0.75?i.e.0.74 and 0.62 V vs RHE?.The Co nanoparticles hence act as an active site modifiers by increasing the proportion of Ti 2p3/2 in the low valence state.This correlates with catalytic activity observed.The results indicate charge transfer effect between Co and TiC_0.25N_0.75 due to strong electronic interaction between the two.This gives a catalyst with appreciable ORR activity and stability.Secondly,it is also demonstrated that transition metal carbides are good candidates for ORR activity,due to their good conductivity.However,a lot of challenges have been previously faced during their synthesis,owing to formation of metallic cobalt and cobalt oxides.In fact,this has limited research work carried out on them.The cobalt carbide nanoparticles?CoC_x?are also known to form agglomeration;a problem causes low current density and poor active site exposures.As such,solid-solid separation method is used to prepare carbon nanotube supported CoC_x?C@CoC_x?for ORR activity.In fact,low current density which prevents cobalt carbide based electrocatalysts from being practically used is overcome.Here,the synthetic approach used ensures good distribution and protection of CoC_x on in-situ generated carbon nanotubes.This results in robust activities with onset potential⁰.92 V,half-wave potential⁰.8 V and durability of¹00%for²2 h of continuous operation.In alkaline medium,C@CoC_x compares favorably with commercial platinum dispersed on carbon?Pt/C 20wt%?,and shows higher current density of³00?A cm² at electrode potential 0.6 V.The results indicate that there is synergistic charge transfer between CoC_x and in-situ generated Carbon-nanotube.This is due to intimate contact between the two phases;it is in fact responsible for high methanol tolerance making C@CoC_x a good electrocatalyst for ORR activity in direct methanol fuel cells?DMFCs?.At the third phase,another synthetic strategy is also used to solve the problem of coarsening and grain growth,a problem which often leads to poor active sites exposure in the Fe₃C,CoC_x and Ni₃C materials.In fact the synthetic approach leads to discovery of another family of ternary metal carbides,wherein Zn is incorporated into the lattice of Fe₃C,CoC_x and Ni₃C to give anti-perovskite Fe₃ZnC,Co₃ZnC and Ni₃ZnC electrocatalysts.In particular,it is discovered that the ORR activity of binary metal carbides?i.e.Fe₃C,CoC_x and Ni₃C?can be significantly enhanced through incorporation of Zn into the interstitial sites of the binary carbides;this results in the formation of ternary metal carbides.Here Co₃ZnC,Fe₃ZnC and Ni₃ZnC electrocatalysts with anti-perovskite structures are successfully synthesized via solvothermal method followed by solid state reaction.Reitveld refinement along with XPS is used to indicate that Zn enhances the charge density around the central atom.The as-prepared samples show good catalytic performances in O₂ saturated 0.1 M KOH.Among the samples prepared,Fe₃ZnC and Co₃ZnC electrocatalysts show the most positive onset potential?0.92 V vs RHE?and half-wave potential?0.8 V vs RHE?.These values are close to that of the prohibitively expensive commercial Pt/C,which has onset and half-wave potential of?0.98 V vs RHE?and?0.82 V vs RHE?respectively.The Fe₃ZnC electrocatalyst exhibits the highest limiting current density(i.e.5.3 mA cm^-2)at low electrode potential?0.2-0.6 V?;it is noted that in terms of limiting current density,it is even better than commercial Pt/C.Both Co₃ZnC and Fe₃ZnC electrocatalysts show promising and robust activity.With their use,they show activity retention of 99.9%for¹5 h;good methanol tolerance is also observable which is superior to the tolerance exhibited by commercial Pt/C.This work shows another interesting and rather simple chemical strategy to enhance the ORR activity of binary metal carbides for fuel applications.All the investigations carried out in this thesis have resulted in a number of key results and conclusions related to ORR activity of metal carbides and carbonitrides in alkaline solution.