Preparation And Properties Of Nickel-cobalt Binary Oxide As Cathode Catalyst For Direct Methanol Fuel Cell

In recent years,the environmental problem is becoming seriously,and the sustainable and green energy technologies have attracted widespread attention.Direct methanol fuel cells(DMFCs)as a new type of energy conversion technology can directly convert chemical energy into electrical energy.They have received widespread attention in the field of portable equipments due to the advantages of higher energy density and pollution-free.Compared with the usually used H2-O2 fuel cells,the methanol fuel used in DMFCs is easy to storage and transportation.However,the current bottlenecks for the commercialization of DMFCs are as follows:Firstly,the high cost of DMFCs resulted from the expensive noble metal Pt or Pt-based alloys as catalysts,as well as the Nafion membrane;Secondly,methanol could easily penetrate through the Nafion membrane poisoning Pt or Pt-based catalysts and generating amixed potential" at the cathode,which would lower the output performance of DMFCs.Moreover,the poor power density and stability of DMFCs still have a far way to meet the commercial requirements.This thesis aims to reduce the cost and improve the performance of DMFCs.The nickel and cobalt binary oxide materials and their composite were applied as cathode catalysts,which have excellent methanol poisoning resistance.In order to resolve the issues including the poor conductivity of metal oxides and weak stability of DMFCs,this thesis mainly contains the following contents:(1)A composite of nitrogen-doped carbon-supported Co1.29Ni1.71O4(NC/Co1.29Ni1.71O4)was synthesiszed by a simple thermal bath method.Firstly,Co1.29Ni1.71O4 nanosheets were growed on a nano-flower-like urea-formaldehyde resin-based carbon material.And then ec600 was physically adsorbed on the surface of the Co1.29Ni1.7O4 nanosheet,which further accelerated the electron transport on the Co1.29Ni 1.71 O4 nanosheet during the reaction.A multistage conductive network was conducted in NC/Co1.29Ni1.71O4 composite.The single DMFCs were consisted with NC/Co1.29Ni1.71O4 or Co1.29Ni1.71O4 samples as the cathode catalyst,PtRu/C as the anode catalyst and the polymer fiber membrane as an electrolyte membrane.When Co1.29Ni1.71O4 was used as the cathode catalyst,the peak power density of the DMFC was only 1.90 mW·cm-2,while the peak power density of NC/Co1.29Ni1.71O4 could reached 7.40 mW·cm-2.Therefore,the three-dimensional conductive network structure can greatly improve the conductivity of transition metal oxides and oxygen reduction catalytic performance.(2)In order to solve the weak stability of DMFCs poor,NiCo2O4 with three-dimensional network structure was synthesized by salt precipitation method using PMMA colloid crystal as template.It displays hierarchical pore structure including ordered macropores and mesopores.A passive DMFC was assembled using NiCo2O4 as the cathode catalyst,PtRu/C as the anode catalyst and PFM as the electrolyte membrane.The peak power densities were 14.25 and 26.00 mW·cm-2 at 15 and 40?,respectively.Notably,NiCo2O4-500-based DMFC could run continually for 1406 hours,which is approximately 6 times higher than the highest values reported in literature.The outstanding stability of this DMFC can be ascribed to the special hierarchical pore structure of NiCo2O4 at cathode.The mesopores provided abundant catalytic active sites to improve its catalytic activity,while the interconnected and ordered macropores could acted as structural buffer to mitigate the structural changes and also provided an effective "water management system" to solve the problem of water flooding at cathode during the long-term operation.Overall,this work provided a promising strategy to fabricate high-stable and cost-effective DMFCs.(3)In the aboved study,the preparation process of the cathode is complicated and the negative effect of PTFE binder used in the catalyst layer would reduce the performance of DMFCs.This work provided a simple two-step method to in-situ growing NiCo2O4 and nitrogen-doped carbon on Ni-foam(Ni-foam@NC/NiCo2O4)to form an integrated electrode.A passive DMFC was assembled using the integrated Ni-foam@NC/NiCo2O4 electrode as cathode,PtRu/C as anode catalyst,and PFM as the electrolyte membrane.The peak power densities of this DMFC were 11.76 mW·cm-2,21.12 mW·cm-2 and 28.35 mW·cm-2 at 20?,40? and 60?,respectively.The integrated electrode simplifies the process steps reduce the contact resistance between catalyst and Ni-foam and shortens the transmission channels for electrons and ions.Moreover,the integrated electrode also shows a wider pore size distribution and numerous macropores,which could used as transport channel for electrolyte,oxygen and cathodic product to obtain a high-stability DMFC.But due to the current catalyst loading is still unsatisfactory,its output performance needs to be further improved in the following study.