Engineering the interlayer sodium density in layered sodium cobalt oxide for boosted chlorine evolution reaction

Layered sodium cobaltate (Na_xCoO₂), characterized by CoO₂ slabs and intralayer edge-shared CoO₆ octahedra, holds promising potential as an electrocatalyst for chlorine evolution reaction (CER). However, the suboptimal adsorption of the intermediate on Na_xCoO₂ resulted in unsatisfactory activity. Herein, Na_xCoO₂ flakes with varying sodium densities (x = 0.6, 0.7, 0.9) were engineered for efficient CER. Excitingly, the optimal Na_0.7CoO₂ achieves an ultralow overpotential (55.47 mV) outperforming commercial RuO₂ at 10 mA/cm², while remaining inactive toward the competing OER. Experimental and theoretical calculations demonstrate that appropriate interlayer sodium density has optimized the d-band center level of Co atoms in Na_xCoO₂, thereby weakening the strength of Co-Cl bonds. This modulation facilitates the adsorption-desorption equilibrium of Cl species (∆G_Cl* = -0.109 eV) on the surface and kinetically accelerating Cl₂ release. This work is anticipated to elucidate the mechanism by which interlayer sodium density modifies the catalytic performance of Na_xCoO₂, and present new insights for the rational design of advanced CER electrocatalysts.