Synthesis,Characterization And Structure Evolution Of Iron,Nickel And Nitrogen Co-Doped Carbon-Based Oxygen Electrocatalysts

Metal-air batteries and electrolysers for water spilitting have received wide attention in energy storage and conversion field.While limited by the sluggish kinetic properties,the oxygen reduction reaction(ORR)and the oxygen evolution reaction(OER)have to be derived by a high overpotential,which greatly limited the energy utilization and conversion.Therefore,we need to find a valid substitute.Among those known catalysts,Pt has become a commercial oxygen reduction catalyst,while Ir and has been widely used in OER.These precious metal catalysts have low deposits in the crust and are expensive.On the other hand,it is difficult to meet both oxygen reduction and oxygen evolution,simutaneously,which hinders their application as oxygen electrocatalysts(especially in secondary metal-air batteries).Therefore,it's of significance to develop a cost effective bifunctional catalyst for oxygen electrocatalysis.Otherwise,in rotation disk electrode(RDE)test,the wetability of the catalysts are always ignored,which leads to inaccurate electrochemical performance.In respect of the above issues,we first report a robust and facile approach to eliminate these bubbles and thereby improve the wettability of thin-filmed catalyst electrodes.With this test method,we develop a high performance bifunctional FeNi-N/C catalyst.And role of different doped element,compositions and their corresponding structure evolutions during the catalytic process are further investigated.We first studied the relationship between the wetability of the catalyst and the accuracy of the electro test,and then reported a robust and facile approach to eliminate these bubbles and thereby improve the wettability of thin-filmed catalyst electrodes.We found that,insufficient wettability of catalyst electrodes due to visible or even invisible bubbles can lead to under-estimated activity for oxygen evolution reaction as well as inaccurate oxygen reduction reaction measurement.And reductive constant potential treatment before test can improve the wetability,which was achieved by triggering oxygen reduction at the electrode-electrolyte-bubble triple phase boundaries at a constant reductive potential for a short time,leading to oxygen depletion in the bubbles and thereby vanishing the bubbles.Then we studied the synthesis of Me-N/C catalyst,including the radio of catalyst precursor,drying method,grinding,synthetic route,etc.It was found that although Fe-N/C had high ORR activity,the OER activity was poor,while the Ni-N/C catalyst had high OER activity.Combining them,we found that FeNi-N/C could be a high-performance bifunctional catalyst.Finally,In virtue of XPS,TEM-EELS and electrochemical treatment we studied the role of different elements and compositions of FeNi-N/C catalyst,and we found that: The doped nitrogen provide a poor oxygen electrochemical activity.While the metal elements,with higher OER activity have to be fixed by the doping nitrogen;the metal particles themselves do not have ORR activity,and can be converted to metal oxides in OER process to produce a part of OER activity;The metal-nitrogen coordination structures in the carbon area play the key role in OER process.While during the OER process,the nitrogen cannot suffer from the hostile high applied potential,and thus dissolved.Although the damaged metal nitrogen coordination structure may not affect the OER performance since the metal oxide is also OER active,the ORR activity decline dramatically.