Steering no Dimerization Through Boron‐Enhanced Spin–Orbit Coupling in Ultrathin PdCuB Nanosheets for Durable N 2 ‐Selective Nitrate Reduction

The selectivity bottleneck of electrochemical nitrate‐to‐N 2 conversion is kinetic rather than thermodynamic: dimerization of two adsorbed *NO intermediates is spin‐forbidden, imposing a kinetic barrier invisible to conventional thermodynamic analyses. Spin–orbit coupling (SOC) offers the principal channel for the required spin‐state crossing, yet its deliberate use as a design variable in nitrate reduction remains unexplored. Here we show that covalent Pd─B bonding in ultrathin PdCuB alloy nanosheets breaks the local coordination symmetry at Pd sites, partially lifts the crystal‐field quenching of orbital angular momentum, and enhances the effective SOC without toxic heavy‐metal dopants. Electron paramagnetic resonance reveals a two‐stage evolution of the g‐factor with B content, converging at g = 1.910 for the optimal composition. In situ spectroscopy and density functional theory indicate that, although *NO dimerization is thermodynamically favorable, its spin‐forbidden character dominates selectivity, and the B‐induced electronic and SOC‐related perturbation facilitates intersystem crossing, rendering the N 2 pathway kinetically accessible. The optimized nanosheets deliver 83% N 2 Faradaic efficiency at +0.11 V versus RHE with over 1100 h stability, establishing light p‐block element incorporation as a heavy‐metal‐free strategy to address spin‐forbidden bond‐forming steps in nitrogen electrocatalysis.