Significance of N bonding configurations in N-doped graphene for enhanced supercapacitive performance: A comparative study in aqueous electrolytes

Herein, Nitrogen doped graphene (NG) samples for supercapacitive applications were synthesized using green approach of microwave assisted hydrothermal synthesis in 300 s. The as prepared NG samples with appreciable N content (2.93-7.48 %) showed all major N bonding configurations fruitful for enhancing the supercapacitance performance. The surface, chemical and morphological features of the synthesized samples were examined, especially the different N configurations available on N doped graphene framework. Detailed analysis revealed that equal distribution of all the N bonding configurations along with high crystalline nature and high surface area was exhibited by the NG3 sample. The supercapacitive performance of NG3 sample was highly appreciable for all electrolytes even when compared with synthesized samples having higher N doping (NG6 and NG8). The present study brings to fore the significance and necessity of balance in N-doped graphene synthesized for supercapacitance electrode applications. It emerged that for good supercapacitive characteristics the synthesized sample should be balanced both in terms of amount of N content vs crystalline integrity of the graphene as well as nearly equal contributions of all major N bonding configurations (pyridinic N, pyrrolic N and quaternary N) rather than dominance of any one configuration. The most balanced sample had enhanced capacitance performance for all aqueous electrolytes even in lowest molarity of 0.5 M. The ion conduction pathways in each individual electrolyte have been discussed on the basis of hydrodynamic radius and ionic mobility of respective anions and cations. The enhanced wettability, high surface area, optimum N content, crystalline integrity and synergistic effect of equal distribution of the relevant N bonding configurations (pyridinic N, pyrrolic N and quaternary N) contributed to high specific capacitance of 113.53 Fg(-1) for NG3-Ni sample. The sample showed high capacitance retention with high coulombic efficiency and low internal resistance, thus, cementing its candidature as an electrode material for supercapacitor industry.