Diamond‐Based Supercapacitors with Ultrahigh Cyclic Stability Through Dual‐Phase MnO₂‐Graphitic Transformation Induced by High‐Dose Mn‐Ion Implantation

Abstract While occasionally being able to charge and discharge more quickly than batteries, carbon‐based electrochemical supercapacitors (SCs) are nevertheless limited by their simplicity of processing, adjustable porosity, and lack of electrocatalytic active sites for a range of redox reactions. Even SCs based on the most stable form of carbon (sp³ carbon/diamond) have a poor energy density and inadequate capacitance retention during long charge/discharge cycles, limiting their practical applications. To construct a SC with improved cycling stability/energy density Mn‐ion implanted (high‐dose; 10¹⁵–10¹⁷ ions cm⁻²) boron doped diamond (Mn‐BDD) films have been prepared. Mn ion implantation and post‐annealing process results in an in situ graphitization (sp² phase) and growth of MnO₂ phase with roundish granular grains on the BDD film, which is favorable for ion transport. The dual advantage of both sp² (graphitic phase) and sp³ (diamond phase) carbons with an additional pseudocapacitor (MnO₂) component provides a unique and critical function in achieving high‐energy SC performance. The capacitance of Mn‐BDD electrode in a redox active aqueous electrolyte (0.05 M Fe(CN)₆^3‐/4− + 1 M Na₂SO₄) is as high as 51 mF cm⁻² at 10 mV s⁻¹ with exceptional cyclic stability (≈100% capacitance even after 10 000 charge/discharge cycles) placing it among the best‐performing SCs. Furthermore, the ultrahigh capacitance retention (≈80% retention after 88 000 charge/discharge cycles) in a gel electrolyte containing a two‐electrode configuration shows a promising prospect for high‐rate electrochemical capacitive energy storage applications.