Oxyhydroxide is widely reported as a low-cost and intrinsically active electrocatalyst to expedite oxygen evolution reaction (OER) in water electrolysis. For applications, it is important to further improve its specific activity (activity per atomic site) and maximize its number of active sites. An effective approach to realize the above strategy is using core-shell structures, where the core can introduce profound nucleation sites and modify the electronic structure of surface oxyhydroxide. In this talk, I will present our recent studies of CoPx@FeOOH electrocatalysts for OER. The structures were synthesized on highly-conductive nickel foams by hydrothermal and subsequent phosphidation at elevated temperatures. X-ray photoelectron spectroscopy indicates that the Fe peaks for CoPx@FeOOH are positively shifted to a higher oxidation state compared with that of pristine FeOOH, which is more favorable to catalyze OER. In addition, the electrochemically active surface area is also improved by approximately 2.5 times. As a result, it can drive an OER current density of 800 mV/cm2 at 303 mV overpotential in 1 M KOH solution, which is superior to pristine FeOOH and IrO2. Besides the performance in fresh water based electrolyte, I will also introduce its activity and stability in saline water as well as designing core-shell structures in other materials systems.