Anion-exchange membranes (AEMs) with high conductivity are crucial for realizing next-generation energy storage and conversion systems in an alkaline environment, promising a huge advantage in cost reduction without using precious platinum group metal catalysts. Layered double hydroxide (LDH) nanosheets, exhibiting a remarkably high hydroxide ion (OH–) conductivity approaching 10–1 S cm–1 along the in-plane direction, may be regarded as an ideal candidate material for the fabrication of inorganic solid AEMs. However, two-dimensional anisotropy results in a substantially low conductivity of 10–6 S cm–1 along the cross-plane direction, which poses a hurdle to achieve fast ion conduction across the membrane comprising restacked nanosheets. In the present work, a composite membrane was prepared based on mixing/assembling micron-sized LDH nanosheets with nanosized LDH platelets (nanoparticles) via a facile vacuum filtration process. The hybridization with nanoparticles could alter the orientation of LDH nanosheets and reduce the restacking order, forming diversified fast ion-conducting pathways and networks in the composite membrane. As a result, the transmembrane conductivity significantly improved up to 1000-fold higher than that composed of restacked nanosheets only, achieving a high conductivity of 10–2 to 10–1 S cm–1 in both in-plane and cross-plane directions.
Ultrathin nanosheets of Ni-, Co-, and Fe-based (oxy)hydroxides exhibit promising catalytic activity for the oxygen evolution reaction (OER) in water electrolysis under alkaline conditions. It has been revealed that Fe may play a crucial role in the catalytic process of Ni- or Co-based catalysts. However, it lacks effective methods to prepare and study pure Fe hydroxide nanosheets. In the present work, we report a topochemical synthesis of mixed-valent Fe2+–Fe3+ layered double hydroxides (LDHs), i.e., Green Rust (GR), featured with much higher quality and crystallinity compared to the traditionally synthesized ones. Monolayer Fe2+–Fe3+ LDH nanosheets with a thickness of ∼0.8 nm were derived from the GR product. In addition, a series of Ni-bearing Fe-rich LDH nanosheets were successfully prepared by the same method. The OER catalytic performance of the obtained Fe-rich LDH nanosheets achieved a small overpotential with η = 290 mV@10 mA cm–2 in 1 M KOH, which either outperforms or is comparable to the most active Ni–Fe LDH catalysts. The monolayer Fe2+–Fe3+ LDH nanosheets may be ideal for exploring the fundamental physicochemical properties of iron hydroxides from a molecular-scale perspective as well as serving as a building block for the assembly with other functional materials to obtain hybrid nanocatalysts.
The design and development of efficient catalysts for electrochemical nitrogen reduction reaction (ENRR) under ambient conditions are critical for the alternative ammonia (NH3) synthesis from N2 and H2O, wherein iron‐based electrocatalysts exhibit outstanding NH3 formation rate and Faradaic efficiency (FE). Here, the synthesis of porous and positively charged iron oxyhydroxide nanosheets by using layered ferrous hydroxide as a starting precursor, which undergoes topochemical oxidation, partial dehydrogenated reaction, and final delamination, is reported. As the electrocatalyst of ENRR, the obtained nanosheets with a monolayer thickness and 10‐nm mesopores display exceptional NH3 yield rate (28.5 µg h−1 mgcat.−1) and FE (13.2%) at a potential of −0.4 V versus RHE in a phosphate buffered saline (PBS) electrolyte. The values are much higher than those of the undelaminated bulk iron oxyhydroxide. The larger specific surface area and positive charge of the nanosheets are beneficial for providing more exposed reactive sites as well as retarding hydrogen evolution reaction. This study highlights the rational control on the electronic structure and morphology of porous iron oxyhydroxide nanosheets, expanding the scope of developing non‐precious iron‐based highly efficient ENRR electrocatalysts.
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