Synergistic Spin‐Polarization and Single‐Atom Engineering in Magnetic Heterojunctions for Efficient Solar Water Splitting

The advancement of photocatalytic water splitting requires going beyond charge migration control to the greater challenge of utilizing the spin degree of freedom as a handle to precisely steer reaction pathways. Here, it is identified 67 magnetic heterojunctions composed of magnetic 2D transition metal halide and non‐magnetic transition metal chalcogenide monolayers via high‐throughput screening. These selected structures exhibit a lattice mismatch below 5%, with each constituent monolayer possessing a band gap in the optimal range of 0.5 – 2.5 eV. First‐principles calculations confirm a type‐II staggered band alignment and a built‐in electric field that promotes efficient charge separation. More importantly, the unique spin‐polarized electronic configuration of Cr 3+ in CrI 3 preferentially facilitates the generation of triplet oxygen, significantly boosting the oxygen evolution reaction. Meanwhile, the anchoring of Pt single atoms on the MoTe 2 and WTe 2 layers addresses their weak hydrogen adsorption, enhancing the HER kinetics to balance the overall water splitting process. This synergistic engineering of spin‐polarization for OER and single‐atom sites for HER works cooperatively within the heterojunction. Together with strong visible‐light absorption and a predicted solar‐to‐hydrogen efficiency that surpasses industrial benchmarks, this study highlights a high‐throughput‐guided strategy for designing high‐performance magnetic photocatalysts through multi‐component active site optimization.