Interfacial catalysis in a single-phase magnesium–cerium intermetallic anode enables high-power magnesium–air and seawater batteries

Magnesium (Mg)–air batteries are promising for military backup power, marine monitoring, and remote microgrids due to their high energy density, inherent safety, and long shelf life. However, their high-power output is limited by effectively low reactivity under high-current conditions, a severe negative differential effect characterized by a sharp voltage drop at high current, and the blockage of active surface sites by adherent Mg(OH) 2 discharge products. To address these issues, we overcome the conventional solubility limits of Ce (< 0.74 wt.%) in Mg‐based solid solutions and develop a novel single‐phase Mg 3 Ce intermetallic anode. This anode features a homogeneous distribution of Ce and Mg atoms within the lattice, forming atom-level active sites that promote rapid H 2 O adsorption and dissociation and thereby enhancing surface reactivity. Meanwhile, strengthened Mg–OH surface interactions suppress parasitic proton reduction to H 2 . During Mg discharge, in situ segregation of Ce clusters provides catalytic “hot spots” and further lowers activation barriers, while the formation of a loose, detachable Mg(OH) 2 layer preserves continuous electrolyte contact, enabling stable high-current operation. These combined effects balance high intrinsic reactivity with enhanced corrosion resistance, yielding significantly improved high-current discharge performance and stability compared to conventional Mg and commercial AZ31 anodes. Consequently, at an ultra-high current density of 100 mA cm −2 , a regime rarely explored in previous studies, the Mg 3 Ce anode delivers stable discharge with an energy density of 375 Wh kg −1 (⁓80% higher than AZ31 and ⁓56% higher than pure Mg), a peak power density of 104 mW cm −2 (⁓22% and ⁓30% higher, respectively), and an average voltage of 0.81 V over 5 h (⁓50% and ⁓35% higher, respectively). Seawater-activated batteries deliver > 1.1 V at 100 mA cm −2 with > 140 mW cm −2 peak power. Remarkably, the Mg 3 Ce anode maintains high activity across a wide temperature range (0–80 °C) and even under oxygen-depleted deep-sea conditions. These results demonstrate a novel materials strategy for overcoming the power-density bottleneck in Mg–air/seawater batteries and provide new guidance for next-generation anode alloy design.