Preparation And Performance Of Manganese Oxides As Oxygen Reduction Catalyst

In recent ten years, the new power technology has been accelerated to meet therequirement of portable electrical appliance and electric vehicles. Zinc-air battery hasbeen a major cause of concern, due to its advantages compared with traditionalbatteries, such as high power, flat discharge voltage, safety, non-polluting, and lowcost. But some key technologies like oxygen reduction catalyst, alkali leakage andcarbonation of electrolyte should also be concerned. In this dissertation, thefabrication of oxygen reduction electrodes and their polarization test conditions wereoptimized, and then manganese- based oxygen reduction catalysts were synthesizedand characterized. Manganese oxides (MnO_x/AC and Mn₃O₄) were synthesizedthrough the pyrolysis of manganese nitrate or the reduction of potassiumpermanganate. The phase composition of as-prepared materials was characterizedusing XRD, TEM and SEM, their electrocatalytic activity towards oxygen reductionwas tested by linear polarization. The detailed results are described as follow:It is shown that many factors can affect the performance of oxygen reductionelectrode, including the layer structure of electrode, thickness, content of PTFE binderin catalyst film and gas-diffusion film, hydrophilic surfactant containing in PTFEsuspension, and so on. Hydrophilic surfactant containing in PTFE suspension is notgood for the electrode performance, so the catalyst layer and gas-diffusion layershould be pretreated by marinating in acetone to remove the hydrophilic surfactantwhich could lead to leakage of oxygen electrode. The content of PTFE affects theelectrode performance, and its preferable content (weight ratio) in catalyst film andgas-diffusion film are as follows: AC:PTFE (60wt.% suspension)=1:0.4, AB:PTFE(60wt.% suspension)=1:2. Electrode with three-layer structure presents betterperformance than that of with two- layer, and the preferable electrode thickness is0.6mm. The best condition for linear polarization test is to execute testing after theelectrolyte being added into the 3-electrode electrolytic cell for 6h at a scan rate of 1 mV/s. The button zinc-air batteries assembled meet the designing parameter, theaverage discharge voltage of the prototype battery is 1.2121V at 30mA.Mn₃O₄ was synthesized by combusting the mixture of manganese nitrate andactive carbon, and its catalytic activity towards oxygen reduction was tested. TheSEM pictures show that combusting with 47% Mn (NO₃)₂ led to little grainy Mn₃O₄with size of about 40nm, while initial addition by 27% or 63% Mn (NO₃)₂ led to flakeparticle. The as-prepared Mn₃O₄ sample presented a lower catalytic activity towardsoxygen reduction in comparison with electrolytic manganese dioxide (EMD) sample.The polarization potential of 15% EMD was 10-42mV higher than that of 15% Mn₃O₄under 40-120mA·cm^-2. However, the catalytic activity of EMD and Mn₃O₄ mixturewas higher than that of EMD or Mn₃O₄. The polarization potential of 10%(EMD+Mn₃O₄) was 11-41mV higher than that of 15% EMD under 120-160mA·cm^-2.Mn₃O₄/AC was synthesized through a thermal decomposition process ofmanganese nitrate, and samples doped with La, Ce, Co, or Ni were also synthesizedby adding La, Ce, Co, or Ni nitrate into manganese nitrate precursors. The XRDpatterns show that the pyrolytic product of Mn (NO₃)₂ supported on active carbon wasmainly Mn₃O₄. The SEM pictures indicate that Mn₃O₄ was coated on AC by differentmorphologies, i.e. needles like with diameter of about 30nm, rods like with diameterof about 50rim or particle clusters. 9% Mn₃O₄/AC heated for 1.Sh under 300℃presented a higher catalytic activity towards oxygen reduction, and current density ofelectrode under -0.2V, -0.35V, and -0.45V approached 60, 133.8, and 179.6 mA·cm^-2,respectively. Mn₃O₄/AC doped with La, Ce, Co, Ni could improve the open-circuitvoltage. As an instance, the open circuit potential (OCP) of electrode withMn₃O₄-Ce/AC (Mn:Ce=1:1) approached 97mV, which was 30mV higher than that ofnon-doping Mn₃O₄ /AC. The OCPs of Mn₃O₄ /AC doped with La and Co wererespectively increased by 18-22mV and 18-33mV at a discharge current of 40-120mA·cm^-2。MnO_x/AC was also prepared by the reduction of KMnO₄, which had a lowerpolarization performance than that by the pyrolysis method. It is indicated that MnO_x prepared by this method was amorphous MnO₂, and could be transformed intocircular-plate like Mn₃O₄ with diameter of about 80nm after being treated under200-500℃. The catalytic activity of MnO_x/AC would also be enhanced after heattreatment. The polarization potentials of 7.5%, 10%, 12.5% and 15% MnO_x/ACtreated at 300℃for 1.5h were increased by 48, 46, 130 and 149 mV at a dischargecurrent of 100 mA·cm^-2.