Synergistic Effect of Oxygen and Nitrogen Co-doping in Metal-Organic Framework-Derived Ultramicroporous Carbon for an Exceptionally Stable Solid-State Supercapacitor via a Proton Trap Mechanism

Because carbon-based materials with heteroatom doping are at the forefront of research on energy storage devices as a result of their excellent physical properties, such as electrical conductivity and wettability, we report the synthesis of nitrogen and oxygen co-doped ultramicroporous/microporous carbon nanomaterials (MNOUCs), derived from a composite of Zn-MOF-74 and melamine, wherein the metal-organic framework (MOF) acted as a self-sacrificial template and melamine acted as the nitrogen source. In three-electrode system, MNOUC synthesized at 800 degrees C (MNOUC-2) was tested as a supercapacitor and achieved a high capacitance of 368 F g(-1) at 1 mV s(-1) with a superior rate capability of 80%. Furthermore, an outstanding cyclic stability (113%) after 25 000 cycles has been observed. A symmetric solid-state supercapacitor (SSSC) device with MNOUC-2 as the electrode and PVA/H2SO4 as a gel electrolyte/separator exhibited an excellent specific capacitance of 116 F g(-1) at 0.5 A g(-1) with a high energy density of 31.6 Wh kg(-1) and an exceptional cyclic stability (103%) after 25 000 cycles. Capacitances obtained from electrochemical impedance spectroscopy and galvanostatic charge/discharge provided a supercapacitor performance efficiency of 42%, which is more superior than carbon-based materials in aqueous electrolytes. The outstanding performance of MNOUC as a supercapacitor is attributed to (a) presence of ultramicropores/micropores, aiding in better ion accumulation, (b) oxygen functionalities that introduced pseudocapacitance as a result of quinone groups, (c) incorporation of pyridinic units in consequence of N-doping, leading to a proton-trap mechanism, which assisted in stabilizing the quinone functionalities of MNOUC-2 during redox transformation and also improved wettability, thus rendering outstanding cyclic stability, and (d) graphitic nitrogen, which improved conductivity of the material, thereby helping in improvement of ion percolation and minimization of charge transfer resistance at the electrode/electrolyte interface. Hence, this work endeavored to provide a better understanding of the synergistic effects of heteroatom doping along with the presence of ultramicropores/micropores for further advancement of supercapacitors.

Automated summary

A nitrogen and oxygen co-doped ultramicroporous/microporous carbon nanomaterials (MNOUCs) was synthesized and applied as a supercapacitor, demonstrating excellent capacitance and cyclic stability. By optimizing heteroatom doping and microporous structure, the material's ion accumulation and conductivity were improved, resulting in superior supercapacitor performance.