β–phase hydroxide-steered inner-hosted metal sites for exceptional hydrogen production

High-density metal single-atom catalysts (M–SACs) tend to aggregate during synthesis and electrocatalytic processes. To prevent this aggregation, it is essential to develop ultra-low-density M–SACs that exhibit high catalytic activity and stability, which is highly challenging. Additionally, M–SACs maximize the utilization of the active sites and thus increase the atomic efficiency for electrocatalysis. Here, we present the β–phase and α–phase hydroxide-functionalized metals [β–Ni(OH)₂ and α–Co(OH)₂] as sacrificial templates to produce various M–SACs (M = Pt, Ir, Pd, and Ru) embedded in porous nitrogen-bonded carbon sheets, where the metal hydroxides interact strongly with dicyandiamide–metal complexes, effectively preventing the aggregation of isolated metal atoms. The β–Ni(OH)₂-driven platinum variant catalyst (Pt−0.38 wt%:β–Pt_SAs/S₈₀₀; Pt−0.54 wt%:β–Pt_SAs/S₈₅₀) demonstrates zero-onset potential, ultra-low overpotential (15 mV at 10 mA cm⁻²), exceptional stability over 10 days of operation, and unprecedented turnover frequencies of 3.68/3.38 H₂ s⁻¹/Pt-site, which are 78/72 times higher than that of 20 wt%Pt/C (0.047 H₂ s⁻¹/Pt-site) for the hydrogen evolution reaction (HER). Notably, β–Pt_SAs/S₈₅₀-based proton-exchange-membrane water electrolysis (PEMWE) achieves a current density of 3.0 A cm⁻²at a low voltage of 1.75 V_cell@80 ℃ [exceeding the Department of Energy 2026 target], along with stable operation for over 200 h at a current density of 1.0 A cm⁻². Experimental observation and theoretical calculations indicate that the inner-hosted PtN₂ moieties remain intact within the graphitic sheets due to their lower formation energy under acidic conditions, effectively reducing the overall HER energy barriers and showcasing the true active sites responsible for the remarkable catalytic activity.