Synthesis,modification And Properties Towards Oxygen Reduction Reaction Of Transition Metal Selenides

Proton exchange membrane fuel cells?PEMFC?are electrochemical devices that convert chemical energy directly into electrical energy.They are regarded as one of the most promising power sources for transportation,generation of power,and portable electronic devices,etc.,due to their high energy-conversion efficiency,low temperature of operation,and zero or low emission.However,compared with the oxidation reaction of fuels at the anode,the oxygen reduction reaction?ORR?at the cathode is the main bottleneck due to its sluggish kinetics.Although platinum and its alloys have demonstrated so far the most active ORR catalysts,the prohibitive cost,scarcity hindered the large-scale application of PEMFC.Recently,the transition metal selenides have been attracted widespread attentions due to the high oxygen reduction catalytic activity and stability.The main contents and results are summarized as follows:?1?The controllable synthesis of Co-based selenides and their application in oxygen reduction reactionCo-based selenides with different selenium contents were successfully synthesized by a one-pot solvothermal method.The physical properties of the prepared catalysts were characterized by X-ray diffraction,scanning electron microscopy,X-ray photoelectron spectroscopy and thermogravimetric analysis.While their electrocatalytic performances were evaluated by cyclic voltammetry,linear sweep voltammetry,electrochemical impedance spectroscopy and chronoamperometry.The results showed that Co-based selenides with 75mol%selenium content?Co-Se-75?displayed the best electrocatalytic activity towards oxygen reduction reaction with higher onset potential?0.708 V?,reduction current(2.81 mA cm^-2)and a four-electron oxygen reduction process.Furthermore,the catalyst exhibited better stability,stronger resistance to methanol,ethanol and ethlene glycol than that of the Pt/C?20wt%Pt?catalyst.?2?The controllable synthesis of iron,nickel and molybdenum doped CoSe₂ and their applications in oxygen reduction reactionIron,nickel and molybdenum doped the above Co-Se-75 were synthesized to form a series of Co_xFe_1-xSe₂,Co_x Ni_1-xSe₂ and Co_1-xMo_xSe₂ ternary selenides,respectively.The effects of iron,nickel and molybdenum contents on the oxygen reduction reaction were studied by electrochemical measurements,and the highest activities were obtained on Co_0.7Fe_0.3Se₂,Co0.7Ni0.3Se2 and Co0.85Mo0.15Se2,respectively.Co0.7Fe0.3Se2,Co0.7Ni0.3Se2 and Co_0.85Mo_0.15Se₂ all presented four-electron pathway.Furthermore,Co_0.7Fe_0.3Se₂ exhibited more positive cathodic peak potential?0.564 V?and onset potential?0.759 V?than those of Co_0.7Ni_0.3Se₂?0.558 V and 0.741 V?.And Co_0.7Fe_0.3Se₂ and Co_0.85Mo_0.15Se₂ displayed even superior stability and better tolerance to methanol,ethanol and ethylene glycol crossover effects than those of the commercial Pt/C?20 wt%Pt?.?3?The controllable synthesis of M/CoSe₂?M=MoS₂,Fe₃O₄?hybrids and their applications in oxygen reduction reactiona:A series of MoS₂/CoSe₂ hybrids were prepared by two different hydrothermal methods.The synthesized MoS₂/CoSe₂ hybrids with different MoS₂ contents were characterized by X-ray diffraction,X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy.And the electrochemical activity and durability of MoS₂/CoSe₂ hybrids towards oxygen reduction reaction were studied in O₂-saturated 0.5 M H₂SO₄ using rotating disk electrode.The results showed all of them demonstrated catalytic activity towards oxygen reduction reaction.Furthermore,loading 30%MoS₂ on CoSe₂ by the first method?CoSe₂-30a?displayed the best electrocatalytic activity.The onset potential?0.741 V?and half-wave potential?0.570 V?were higher than those of loading 20%MoS₂ on CoSe₂ by the second method?CoSe₂-20b,0.737 V and 0.565 V?,CoSe₂?0.708 V and 0.560 V?and MoS₂?0.698 V and 0.429 V?.Most important,CoSe₂-30a showed superior stability and better methanol tolerance than CoSe₂-20b,CoSe₂ and MoS₂.b:A series of Fe₃O₄/CoSe₂ hybrids were used as the oxygen reduction reaction catalysts.The effects of different contents of Fe₃O₄ on the structure and properties of CoSe₂ were investigated.A series of electrochemical tests showed that the obtained catalysts with loading30%Fe₃O₄ on CoSe₂ by the second method?C30b?exhibited the highest oxygen reduction reaction activity with an onset potential of 0.775 V,a half-wave potential of 0.600 V,and a small Tafel slope of 65 mV decade^-1 in O₂-saturated 0.5 M H₂SO₄ solution,was better than those of loading 20%Fe₃O₄ on CoSe₂ by the first method(C20a,0.753 V,0.567 V and 67 mV decade^-1),CoSe₂(0.708 V,0.560 V and 66 mV decade^-1)and Fe₃O₄(0.700 V,0.556 V and 79mV decade^-1).The high oxygen reduction reaction activity of C30b was likely due to the electrocatalytic synergistic effects between CoSe₂ and Fe₃O₄ materials and the more increased catalytic sites.Most importantly,C30b showed much higher stability and better methanol,ethanol and ethylene glycol tolerance.