Developing non-precious metal catalysts for oxygen reduction reaction(ORR)has been receiving more attention in recent years due to the high cost of existing precious metal catalysts and the critical roles of ORR in next generation energy conversion/storage devices.Significant progress has been made on developing catalysts using transition metal oxides,perovskites,doped carbons,etc.Among them,manganese dioxides(MnO2)have been found promising because of the abundance of manganese in the earth,low cost,low toxicity,and the relative high activity.MnO2 is a material that has been intensively studied for supercapacitors in the past years due to its high specific capacitance.Motivated by supercapacitor application,the major effort in developing MnO2 nanomaterials has been focused on methods to synthesize MnO2 with high specific surface area.To date,various methods have been reported to synthesize nanosized MnO2.Those methods are able to produce MnO2 nanomaterials in plate,needle,sphere,flower,wire,rod,belt,tube,and many other morphologies on carbon substrates.However,it should be noted that these methods provided good control on the shape of MnO2 nanomaterials,while their sizes are usually big,ranging from few hundred nanometers to micrometers.Challenges remain in controlling the size uniformity of MnO2 nanomaterials,especially for small and ultrathin free-standing MnO2 nanomaterials.A small,uniform,and thin MnO2 should be able to lower the mass loading of MnO2 in ORR catalyst as well as to facilitate the mass transportation of gas reactants within the composite electrode.Most importantly,the free-standing MnO2 will offer the flexibility to choose various conductive supports with variable mass loading ratios for catalyst electrode fabrication.That may not be readily achieved by conventional MnO2 nanocomposites designed for supercapacitors.This presentation will introduce a facile synthesis of free-standing,ultrathin MnO2 nanoflakes with uniform size of~47 nm.These are the smallest MnO2 flakes to be reported to date.The thickness of nanoflakes is~1.5nm.So-produced small MnO2 nanoflakes showed a very high mass activity(21±1.2mA/mg at 0.75V vs RHE)toward ORR in alkaline conditions as compared to previously reported MnO2,some Mn-based perovskite and doped carbon catalysts.Such an enhancement was achieved by nanostructuring MnO2 into small and ultrathin flakes.In addition,these nanoflakes were also used as a standard active material for a rational comparison study of three carbon supports,graphene,CNTs,and carbon black,in terms of supportive behaviors in MnO2/C supercapacitors.