Environment-responsive phase transformation of Ni-modified cobalt phosphate aerogels for durable hydrogen evolution in freshwater and seawater

Abstract

Non-precious 3d transition metal-based compounds are attractive electrocatalysts for the hydrogen evolution reaction (HER), but their structural instability under practical environments, particularly in seawater, remains a major challenge. Here, we report nickel modified cobalt phosphate (NCP) aerogels synthesized via a metal coordination-initiated sol-gel process, in which Ni incorporation governs environment-dependent reconstruction pathways. In freshwater, pristine cobalt phosphate undergoes surface oxidation toward crystalline oxyhydroxide species, whereas in seawater, Co(OH)₂ phases dominate due to Cl⁻-induced corrosion, resulting in compromised durability. Remarkably, Ni incorporation fundamentally alters these behaviors: in freshwater, it weakens the crystallinity of the CoOOH phase, and in seawater, it drives the formation of an amorphous CoOOH-rich matrix containing only minor Co(OH)₂ domains. This Ni-induced disorder suppresses long-range crystalline hydroxide growth, stabilizes the surface against chloride attack, and facilitates charge transfer. The optimized NCP2.5 catalyst achieves low overpotentials in freshwater and seawater, respectively, along with Tafel slopes of 67.6 and 103.4 mV dec⁻¹. It retains outstanding stability with only ~1% performance loss in freshwater after 100 h and ~ 8% in natural seawater after 100 h. These findings unveil a generalized design principle where dopant-induced amorphization guides environment-specific reconstruction pathways, providing design principles for robust and seawater-compatible hydrogen production.