Preparation Of MnO₂ Nanobundles Composite Electrodes And Study On Electrocatalytic Oxidation Of Methanol

Direct methanol fuel cell（DMFC）is a device that takes methanol as fuel and converts chemical energy into electrical energy through redox reaction.Methanol is considered one of the most popular fuels for fuel cells due to its low molecular weight,light weight and high energy density.Although,DMFC is considered as a promising power source due to its superior specific energy,operational reliability and environmental protection,there are still some technical barriers to its large-scale commercial application.It is generally believed that the main problems are catalyst poisoning caused by COads and the high cost of electrode materials.COads is the intermediate product of methanol electrocatalytic oxidation process,Therefore,the development of new electrode materials with high electrocatalytic activity and low price has become the key to the electrocatalytic oxidation of methanol.MnO₂ nanobundles was synthesized from potassium permanganate and sodium thiosulfate at 140℃,2.85 MPa by hydrothermal method.And modified by organic N and rare earth metal oxide Ce O₂.The aim is to reduce the cost of electrocatalytic methanol oxidation by replacing Pt catalyst.MnO₂-PVA/GP,MnO₂-N-PVA/GP and MnO₂-Ce O₂-PVA/GP electrodes were prepared by coating method.The catalysts and composite electrode materials were physically characterized by FT-IR,XRD,SEM,BET and XPS,and the electrochemical properties of the electrodes were characterized by cyclic volt-ampere curve,Tafel curve and polarization curve.The results of FT-IR showed that MnO₂ nanobundles contained Mn-OH structure,which was conducive to the rapid generation of MnO_X+1.XRD indicated that the nano MnO₂ was tetragonal polycrystallineα-MnO₂.Nano Ce O₂ was a single-phase cubic crystal system and it belongs to fluorspar structure.PVA was a semi-crystalline structure.BET characterization results showed that the specific surface area of MnO₂ nanobundles,nano MnO₂-N and nano MnO₂-Ce O₂ were 227.77,133.45 and 46.18 m²·g^-1,respectively.The SEM characterization showed that the diameter of a single nanowire in the MnO₂ nanobundles was about 30 nm and the length was over 2.0μm.XPS further confirmed the oxidation states of Mn⁴⁺and Ce⁴⁺,and the synthesized substances were determined to be MnO₂ and Ce O₂.Electrochemical characterization showed that the methanol oxidation peak potential of MnO₂-Ce O₂-PVA/GP electrode was the lowest（1.08 V）and the oxygen evolution overpotential was the highest（1.47V）.MnO₂-N-PVA/GP electrode is the least prone to corrosion,and MnO₂-PVA/GP electrode has the slowest corrosion rate.The electrocatalytic performance of nano MnO₂ composite electrode was investigated by changing the catalyst content,methanol concentration,current density,H₂SO₄ concentration,temperature and other factors.Under the optimal conditions,the experimental results showed the conversion rate of methanol oxidized by electrocatalytic oxidation of MnO₂-PVA/GP electrode,MnO₂-N-PVA/GP electrode and MnO₂-Ce O₂-PVA/GP electrode for 3.0 h was 67.77wt%,77.13 wt%and 93.47 wt%,respectively.Under the optimal experimental conditions,the coulomb efficiency of the electrocatalytic oxidation of methanol on MnO₂-PVA/GP electrode,MnO₂-N-PVA/GP electrode and MnO₂-Ce O₂-PVA/GP electrode was 23.28%,33.94%and 44.12%,respectively.The electrocatalytic oxidation of methanol on three electrodes conforms to the second-order kinetic model,and the reaction rate constants k are 0.30,0.44 and 0.5 L（mg·min）^-1,respectively.