Self‐Terminating Surface Reconstruction Induced by High‐Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen

Abstract

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.

Vacancy-Driven Ammonia Electrooxidation Reaction on the Nanosized CeOx Electrode in Nonaqueous Electrolyte

“…This suggests that the introduction of oxygen vacancies enhances NH 3 adsorption, thereby promoting its activation. Previous studies have shown that the dehydrogenation of *NH 3 to *NH 2 is usually the rate-determining step (RDS) for AER on metal oxides. , As shown in Figure c, the energy barrier of *NH 3 to *NH 2 on CeO 2 is 2.13 eV, while the energy barrier on CeO 2 –Vo is only −0.26 eV, indicating that NH 3 is readily deprotanated. Obviously, the NH 3 oxidation process is energetically unfavorable on the CeO 2 surface, and the presence of oxygen vacancies in CeO 2 significantly reduces the energy barrier of *NH 3 to *NH 2 , which is consistent with the enhanced AER activity of CeO 2 –Vo.…”

“…Eventually, the N–N bond forms between these intermediates, and they subsequently dehydrogenate to produce N 2 . The overall process in the G–M mechanism can be represented as NH 3 → *NH 2 → *H x NNH y → ··· → N 2 . , The AER process on CeO 2 –Vo was investigated by following the two mechanisms mentioned above. As shown in Figures c and S25, the energy change from *NH 2 to *NH on the CeO 2 –Vo surface is higher than the formation of *H 2 NNH 2 .…”

“…Hydrogen is currently regarded as the optimal energy solution to meet the demand for carbon neutrality. However, the widespread adoption of hydrogen energy faces significant challenges, primarily related to safety and efficiency in hydrogen supply and storage. − Ammonia is increasingly recognized as a promising hydrogen carrier due to its advantages, such as high hydrogen storage density, ease of liquefaction at ambient temperatures, and the presence of established storage and transport networks. − Moreover, the well-developed technology for industrial ammonia synthesis enhances its economic competitiveness compared to other energy sources. , Therefore, NH 3 -to-H 2 conversion technologies have emerged as a promising and effective approach for high-performance H 2 energy applications.…”