Functionalised biphenylene and graphenylene: excellent choices for supercapacitor electrodes

Quantum capacitance (C_Q) is a crucial parameter that reflects the energy storage capacity of supercapacitors. In this work, we extensively investigate the effect of vacancy induced defects on quantum capacitance of well studied biphenylene (BPN) and graphenylene (GPN) monolayers. Based on density functional theory (DFT), we have systematically studied the consequence of vacancies on structural stability, charge distribution, electronic band structure of pristine systems, and correlated this with the variation of quantum capacitance with applied voltage. The results demonstrate that insertion of a vacancy significantly improves the density of states (DOS) profile around the Fermi level for both structures, attributed to the localisation of charge carriers. This leads to higher C_Q values for relatively lower potential, with the highest value of C_Q being 221 μF cm^-2 for defective BPN. Furthermore, we have calculated the density of surface charge for different voltages to evaluate the adaptability of particular materials as a cathode or anode. It is found that both pristine and defective BPN are more preferable as anode materials. It is noteworthy that GPN and its vacancy induced structure, stand-out as superior candidates for a symmetric supercapacitor in aqueous systems. This work will provide valuable insights to design BPN and GPN-based high performance electrode materials for electric double layer (EDL) supercapacitors.