• Ultrathin 2D MoCoNi medium-entropy alloy (MEA) synthesized via ionic layer epitaxy exhibits exceptional OER performance with an overpotential of only 167 mV at 10 mA/cm² and a Tafel slope of 33.2 mV/dec.
• The catalyst achieves an ultrahigh mass activity of 3359.6 A/g, three orders of magnitude higher than commercial RuO₂, significantly reducing noble metal dependence.
• The synergy of multiple active metals and ultrathin thickness enhances charge transfer and exposes abundant active sites, driving superior electrocatalytic activity.
• The entropy-stabilizing effect ensures outstanding durability, retaining 90% of initial current after 134 hours of continuous operation, promising for sustainable hydrogen production.