Cobalt Single Atoms in Hexagonal Boron Nitride Host for Efficient Electrochemical Nitrate Reduction

Stabilizing transition-metal single atoms in chemically inert hexagonal boron nitride (hBN) remains a fundamental challenge due to the absence of intrinsic anchoring sites. Here, we report an in situ co-pyrolysis strategy that simultaneously constructs hBN nanosheets and anchors atomically dispersed Co single atoms. Our first experimental demonstration shows that nitrate-to-ammonia electroreduction (NO₃RR) performance of Co single atoms in hBN (Co₁/hBN) is far superior to Co single atoms in N-doped graphene (Co₁/NG), with over sixfold enhancement in per-site turnover frequency compared with carbon-supported Co single atoms. The Co₁/hBN catalyst delivers high Faradaic efficiency, yield rate, low Tafel slope, and definitive ¹⁵N isotopic validation. Importantly, the structural adaptability of the polar hBN lattice significantly enhances nitrate adsorption, lowers the adsorption free energy of NO₃ ^--derived intermediates, and strongly accelerates NO₃RR kinetics relative to Co₁/NG. Combined EXAFS and DFT analyses identify a Co-N₆ coordination environment, while DFT reveals that NO₃ ^- adsorption induces a support-driven Co-N₆ → distorted Co-N₄ transformation, lowering the rate-determining energy barrier to 0.43 eV. In contrast, Co₁/NG retains a rigid Co-N₄ configuration with slower kinetics (0.62 eV). These findings highlight host-induced coordination flexibility as a powerful design principle for high-performance single-atom electrocatalysts in nitrogen-conversion reactions.