<p>Ampere-level hydrogen peroxide (H₂O₂) electrosynthesis in brine and seawater via two-electron oxygen reduction reaction (2e⁻ ORR) is promising, but limited by the slow water dissociation and insufficient protons in neutral media. Hence, we design a multifunctional Ni(OH)₂ nanoplates anchored on carbon nanotubes (CNTs) as 2e⁻ ORR catalyst towards H₂O₂ electrosynthesis, where Ni(OH)₂ nanoplates accelerate water dissociation and proton transfer, resolving the critical proton shortage for H₂O₂ formation. Combined with exceptional chloride tolerance and suppressed hydrogen evolution, the catalyst achieves a high H₂O₂ yield of 141 mol g⁻¹ h⁻¹ (14.1 mmol cm⁻² h⁻¹) at 1 A cm⁻² and a long operation time over 150 h at 200 mA cm⁻² in 1 M NaCl solution with &gt;80% H₂O₂ selectivity. In natural seawater, it achieves a Faraday efficiency over 70% at 100 mA cm⁻². This work enables water purification/disinfection via simultaneous H₂O₂/active chlorine production, bridging electrosynthesis with environmental remediation.</p>