We report a facile and low-cost bottom-up synthesis of ultrathin Zn(bim)(OAc) MOF nanosheets (with thicknesses of ∼5 nm and a high yield of ∼65%) and their derived N-doped porous ultrathin (2.5 ± 0.8 nm) carbon nanosheets (UT-CNSs) for energy storage.

Although the planar square Fe-N 4 is considered to be the basic unit of the active Fe-N 4 -based moieties, the exact local structure of such moieties has not yet been determined due to that the axial ligands and the second coordination sphere (i.e., the surrounding carbon matrix) of Fe-N 4 are unclear. [8] Based on the computational hydrogen electrode (CHE) model, the theoretical ORR onset potentials of Fe-N 4 in different models were as low as 0.25-0.43 V versus RHE (V RHE ), [9,10] much lower than the experimentally measured ones on Fe-N-C (0.82-0.95 V RHE ). Besides, some new active moieties in Fe-N-C (e.g., FeON 4 , Fe(OH)N 4 , and FeN 4 Cl) were proposed by using in situ characterizations or CHE modeling, [11][12][13][14] revealing that the axialligand coordination played a critical role in high-activity Fe-N 4 -based moieties. However, these studies were carried out on Fe-N-C that contained not only the Fe-N 4 sites but also a large number of other ORR active sites (such as the N-doped carbon and intrinsic defects of carbon), in which the implemented modifications would affect the ORR activities of different active sites and sometimes even alter the structure of the carbon matrix. [15] That is, the observed ORR activity improvement of the Fe-N-C after such modifications could not be solely attributed to the enhancement of the intrinsic ORR activity of Fe-N 4 and thus the mechanism associated with the improvement is still ambiguous. To this end, developing a model catalyst as a studying platform is highly demanded to reveal the regulation mechanism of axial-ligand coordination on the catalytic activity of Fe-N 4 sites.Iron porphyrin (FePr) and iron phthalocyanine (FePc) are widely used molecular model catalysts with Fe-N 4 as the only kind of ORR active sites. [16] Although some modified FePc and FePr with axially coordinated Fe-N 4 moieties have been reported in recent papers, [17][18][19] only a few strong ligands (e.g., CN − and pyridine) could perform such axial coordination due to that the FePc and FePr are more inclined to exist as the D4h symmetry structure with highly concentrated local electrons. [17] In order to systematically study the correlation between the axial ligands and the catalytic activity of Fe-N 4 sites, it is necessary to explore a Fe-N 4 -based molecular model catalyst with a stronger axial coordination ability. Poly(phthalocyanine iron) (PFePc) is an alkali-soluble Identifying the actual structure and tuning the catalytic activity of Fe-N 4based moieties, well-recognized high-activity sites in the oxygen reduction reaction (ORR) are challenging problems. Herein, by using poly(iron phthalocyanine) (PFePc) as an Fe-N 4 -based model electrocatalyst, a mechanistic insight into the effect of axial ligands on the ORR catalytic activity of Fe-N 4 is provided and it is revealed that the ORR activity of Fe-N 4 sites with OH desorption as a rate-determining step is related to the energy level gap between the OH p x p y and Fe 3d z 2 , which can be tuned by regulating the field strength of the...

The recent progress on the fabrication of two-dimensional metal–organic frameworks and their derivatives as well as their applications in electrochemical energy storage and electrocatalysis are reviewed.

Atomically dispersed iron doped-MOF-derived carbon with high iron loading and nitrogen content for the oxygen reduction reaction via a cage-confinement strategy shows excellent catalytic performance.

Rational design and facile synthesis of highly active and stable electrocatalysts for oxygen reduction reaction (ORR) are crucial in the field of metal-air batteries. Here, we present a facile two-stage thermal synthesis of Fe-N codoped porous carbon (Fe-N/C) with abundant Fe-N x active sites and mesopores from Fe-doped ZIF-8 precursors. The first-stage preheating treatment of the Fe-doped ZIF-8 precursors before the second-stage carbonization is the key to boost the coordination between the doped Fe and N-containing ligands, which contributes to a higher N content and more Fe-N x sites in the final carbonized product. Besides, the preheating and Fedoping both affect the morphology, porous structure, and catalytic performance of the fabricated Fe-N/C. The optimized Fe-N/C catalyst exhibits an outstanding ORR catalytic performance with a half-wave potential of 0.88 V and limiting current density of 6.0 mA cm −2 in 0.1 M KOH. A Mg-air battery assembled with a neutral electrolyte using the optimized Fe-N/C catalyst as the cathode exhibits an excellent power density of 72 mW cm −2 at 0.72 V. This developed two-stage synthesis strategy is facile, and the preheating stage could be integrated into any carbonization process as an intermediate step for the fabrication of various metal, N codoped carbon materials with enhanced electrocatalytic performance.

Fundamental kinetics of the V2+/V3+ of vanadium redox flow battery (VRFB) are still not well understood despite tremendous efforts in improving the sluggish kinetics of V2+/V3+. This article first reveals the rate‐determining step in the electrochemical oxidation of V2+ to V3+ by exploring the reaction kinetics. Thereafter, TiB2 with abundant electron‐deficient sites, which possesses a strong electron‐accepting ability, is demonstrated to improve the rate‐determining step of V2+/V3+ by enhancing the electron transfer from V2+ to the electrode. The mechanism of TiB2 for boosting V2+/V3+ kinetics is also unraveled by analyzing the reaction order. VRFB with the electron‐deficient TiB2 demonstrates a remarkable enhancement in the electrochemical performance, exhibiting an excellent rate performance from 70 to 300 mA cm−2. The energy efficiency is improved by 14.06% compared to the cell with the pristine electrode at 150 mA cm−2 for 300 cycles. This study is critical for not only proposing the promising electron‐deficient catalyst in VRFB application but also promoting fundamental understanding and offering a design strategy for achieving superior performance electrocatalysts in VRFB.

No abstract

NixFe3−xO4/Ni hybrid nanosheet arrays have been directly fabricated on different substrates via template-free and annealing-free one-step electrodeposition, which show excellent catalytic performance for the oxygen evolution reaction.