Theoretical Study On Quantum Capacitance Of Transition Metal Oxide-modified Graphene Electrode Materials

The problems of energy shortage and environmental pollution have promoted the development of new energy sources.However,the development of new energy sources requires advanced energy storage devices.As a green energy storage device,supercapacitors have attracted widespread attention due to their advantages of small size,light and thin materials,and high power density.Electrode materials can affect key parameters such as specific capacitance and energy density of supercapacitors,and are currently the focus of research on supercapacitors.Graphene has become a widely used electrode material for supercapacitors due to its good conductivity and large specific surface area,but its low quantum capacitance limits its development in supercapacitors.The study found that the effective adjustment of the electronic structure of graphene through heteroatom doping and transition metal oxide adsorption can increase the localized state of the system near the Fermi level,and the peak value becomes larger,thereby improving the quantum capacitance value.At the same time,the introduction of transition metal oxides can improve the easy stacking problem of graphene in the process of synthesis,and can also improve the capacitance characteristics of the material.In addition,the redox reaction of binary metal oxides is more abundant,and the synergistic effect between multiple metals makes it have higher conductivity,which is a good electrode material for supercapacitors.In this paper,the effects of different transition metal oxides（TMO=Mn O,Fe O,Co O,Ni O,Cu O,Zn O）on graphene and nitrogen doping on the electrochemical performance of graphene were systematically investigated through first-principles.The results show that transition metal oxides can be stably adsorbed on the graphene surface,and the introduction of transition metal oxides changes the electronic properties of the system and increases the total density of states（TDOS）of the system.The Ni O-G system achieves the maximum quantum capacitance value under negative bias,and can be used as a positive electrode material for asymmetric supercapacitors.The modification effects of different numbers and positions of N atoms on graphene were calculated,and the results showed that the modification effect of the NG system was the best.N atoms are introduced into the TMO-G system.Compared with the TMO-G system,N doping further improves the quantum capacitance value of the system.Among them,Zn O-NG obtains the maximum quantum capacitance value under negative bias,which can be used as Cathode material for asymmetric supercapacitors.The analysis of Local Density of States（LDOS）shows that N doping increases the density of states（DOS）value of TM and O atoms in the TMO-NG system,and the TDOS value of the system increases.Thus,the quantum capacitance value is increased.In addition,we investigated the effect of N doping amount on TMO-G,and replaced the N-adjacent C atoms in TMO-NG with N atoms to obtain the TMO-2N_oG system.The research results show that with the increase of the amount of N doping,the effects on different TMO-G systems are different.Among them,the quantum capacitance value of the Ni O-G system increases gradually with the increase of the amount of N doping,and the amount of N doping increases.The difference changes the electrode selectivity of the material.In addition,the introduction of other transition metals（TM=Mn,Fe,Ni）into Co₃O₄ to modify graphene was also investigated in this paper.The optimal substitution position of TM is determined by calculation,and the obtained system TMCo₂O₄-G can exist stably.The study found that the introduction of metal elements and Co atoms synergistically increased the DOS value of the system near the Fermi level,and the graphene was transformed from semi-metallic to metallic,which significantly improved the electrochemical performance of the system.And compared with the original Co₃O₄-G system,the binary metal oxide further improves the quantum capacitance value of graphene,and the Fe Co₂O₄-G system has the best effect,which can achieve the maximum quantum capacitance and surface charge value under negative bias,applicable to cathode materials for asymmetric supercapacitors.