Fig. 1: One-atom-thick co-catalyst covered OER electrodes.

a–c Fabrication of monolayer hexagonal boron nitride films. a Schematic of the chemical vapor deposition (CVD) method. b Optical image of a centimeter-sized film on a Si/SiO₂ substrate. c High Angle Angular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) image of a monolayer hBN crystal. Inset shows the Electron diffraction patterns of our hBN film, indicating its polycrystal nature. d–e Fabrication of the NiFeO_xH_y catalytic layer on Au contacts. d Schematic of the electrochemical deposition method. e Raman spectrum of the resulting layer. Two characteristic peaks of Ni²⁺ at 450 cm⁻¹ and 540 cm⁻¹ are observed⁵⁵. The broad Raman peak at 680 cm^–1 can be attributed to Fe–O bonds in the NiFeO_xH_y⁵⁶. f–h OER electrodes fabricated by covering the NiFeO_xH_y catalytic layer using one-atom-thick hBN. f Schematic of our heterostructure electrodes and measurement set up. g Height profile of the hBN layer on NiFeO_xH_y. h Optical image of a final device.