Double-layer honeycomb (DLHC) materials exhibit tunable electronic properties and enhanced stability over single-layer/bulk honeycomb forms, but existing experimental research has predominantly focused on semiconducting systems, while theoretical studies on superconductivity remain sparse. Through first-principles calculations, we identified three stable metallic DLHC superconductors: ${\mathrm{P}}_2{\mathrm{C}}_2,{\mathrm{Ta}}_2{\mathrm{C}}_2$, and ${\mathrm{W}}_2{\mathrm{C}}_2$, with superconducting transition temperature $T_c$ of 18.3, 6.8, and 4.8 K, respectively. Additionally, anisotropic Migdal-Eliashberg theory predicts that ${\mathrm{P}}_2{\mathrm{C}}_2$ is a single-gap superconductor with a $T_c$ of 27.2 K. The element-dependent electronic characteristics and superconductivity of them are studied. It is revealed that ${\mathrm{P}}_2{\mathrm{C}}_2$ exhibits mixed ionic-covalent bonding, with superconductivity originating from the coupling between $p$-orbital electrons and atomic vibrations of both P and C, highlighting P-C synergy. Conversely, ${\mathrm{Ta}}_2{\mathrm{C}}_2$ and ${\mathrm{W}}_2{\mathrm{C}}_2$, which contain the transition metal elements Ta and W, display ionic character, where superconductivity is governed by the coupling between $d$ orbitals electrons and vibration modes of metal atoms. These comparative results reveal element-specific orbital mechanisms in quantum materials. Additionally, all compounds exhibit nontrivial band topology, providing a tunable platform that may host topological superconductivity.

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• Superconductivity
• 2-dimensional systems
• Topological materials
• Density functional calculations
• Eliashberg theory
• Wannier function methods