Ultrathin 4 nm FeCo-PBA nanosheets were fabricated for the alkaline oxygen evolution reaction (OER), exhibiting an overpotential of 266 mV and a Tafel slope of 43.5 mV dec-1 at 10 mA cm-2. Density functional theory (DFT) calculations confirm the reduced energy barrier at the Co active site for the rate-determining step.