First-Principles Study of Transition-Metal Doping in Monolayer Black and Blue Phosphorene: A Promising Pathway for High-Performance Electrode Design for Electric Double-Layer Supercapacitors

Abstract This study provides an extensive theoretical investigation of group IV-(Ti/Zr/Hf), VI-(Cr/Mo/W), X-(Ni/Pd/Pt), and XI-(Cu/Ag/Au) transition-metal (TM)-doped black phosphorene (BP) and blue phosphorene (BLP) for electric double-layer (EDL) supercapacitor (SC) electrode design, using density functional theory (DFT)-based first-principles calculations. The feasibility of TM doping is assessed by the doping formation energy and structural integrity, as indicated by bond-length and buckling-height modulations. The effects of TM doping on E-k dispersion and density-of-states profiles are thoroughly correlated with atomic orbital hybridization and valence charge redistribution. The electrode performance of TM-doped BP/BLP is quantified by excess charge density, quantum capacitance (CQ), surface charge density, and total interfacial capacitance profiles. This study reveals that doping-mediated formation of in-gap defect states and band-edge shifts near the Fermi level are primary factors contributing to improvement of electrode performance in TM-doped BP and BLP. The increase in doping concentration leads to more in-gap defect states, suggesting improved electrode performance. Specifically, Cu/Ag-doped BP (Au/Hf-doped BLP) exhibits a high CQ value at lower and moderate doping concentrations. Irrespective of doping concentration, Cu- and Ag-doped BP act as best cathode materials, whereas Au (Hf)-doped BLP is suitable for cathode (anode) materials. Based on both the charge-storage mechanism and the energetic feasibility of TM-doped systems, Cu-BP and Ag-BP (W-BLP and Cr-BLP) show a greater tendency toward anodic operation. Finally, nudged elastic band calculations indicate low dopant diffusion in Cu-doped BP and Au-doped BLP, ensuring stable electrode performance during long-term EDL SC cycling.