Light elements, such as hydrogen and boron, provide a possible route to high-temperature superconductivity. However, the pursuit of light-element superconductors with critical temperature ($T_{\mathrm{c}}$) exceeding the McMillan limit at ambient pressure still remains challenging. Here, we design a two-dimensional (2D) superconductor, ${\mathrm{ScB}}_2{\mathrm{H}}_2$, via hydrogenation. This material features a unique sandwich structure where the scandium layer is asymmetrically functionalized with boron and hydrogen on opposite surfaces. The boron layer plays the key role in high-temperature superconductivity of ${\mathrm{ScB}}_2{\mathrm{H}}_2$ with the metallization of σ bands that induce a Van Hove singularity near the Fermi level. This enables strong coupling between the σ band and the in-plane vibration of B atoms, yielding a large electron-phonon coupling of 1.69. Remarkably, the monolayer ${\mathrm{ScB}}_2{\mathrm{H}}_2$ exhibits three-gap superconductivity with $T_{\mathrm{c}}$ of $51-61\mathrm{K}$ (${\mu }^{*}=0.1-0.2$), surpassing the McMillan limit. Although hydrogen is lighter than boron, the electron-phonon coupling of hydrogen layer is weaker than that of the boron layer. Nevertheless, hydrogen not only stabilizes ${\mathrm{ScB}}_2{\mathrm{H}}_2$ via hydrogenating naked activity Sc sites of monolayer ${\mathrm{ScB}}_2$, but also controls the metallization of the σ band in the honeycomb B sublattice. Our finding provides a theoretical guidance for regulating the 2D superconductivity through functionalizing two faces of 2D materials with light elements.

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• Chemical bonding
• Chemisorption
• Electronic structure
• First-principles calculations
• Multiband superconductivity
• Phonons
• Van Hove singularity
• High-temperature superconductors
• Superconductivity
• Density functional theory