LAUSR

Ammonia (NH3 ) is a promising hydrogen (H2 ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO2 ) polyhedrons with exposed high-index facets are prepared by a one-step NH3 -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO2 facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO2 and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH3 and exhibit a potential improvement of 260 mV for electrocatalytic AOR at 10 mA cm-2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH3 to *NH2 ) is presented on high-index CuFeO2 facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.