Expediting in-Situ Electrochemical Activation of Two-Dimensional Metal–Organic Frameworks for Enhanced OER Intrinsic Activity by Iron Incorporation

Stabilizing Oxygen Vacancy in Entropy-Engineered CoFe2O4-Type Catalysts for Co-prosperity of Efficiency and Stability in an Oxygen Evolution Reaction

We used entropy engineering to design a series of CoFe₂O₄-type spinels. Through microstructural characterization, electrochemical measurements, and X-ray photoelectron spectroscopy, we demonstrated that the entropy-stabilized oxide (Co_0.2Mn_0.2Ni_0.2Fe_0.2Zn_0.2)Fe₂O₄ has a single-phase spinel structure and exhibits both efficient and stable catalytic oxygen evolution. This is attributable to disordered occupation of multivalent cations, which induces severe lattice distortion and increases configurational entropy, thereby facilitating formation of structurally stable, high-density oxygen vacancies on the exposed surface of the spinel. Thus, more catalytic sites on the surface are activated and retained over the course of long-duration testing for oxygen evolution. Entropy engineering expands researchers' access to catalysts that link entropy-stabilized structures to useful properties.

In situ semi-transformation from heterometallic MOFs to Fe-Ni LDH/MOF hierarchical architectures for boosted oxygen evolution reaction

Metal-organic frameworks (MOFs) with large surface area, abundant coordination metal centers and tunable structures are regarded as promising electrocatalysts for the water splitting reaction. However, the less accessible active sites and poor stability of MOFs hinder their potential practical applications. Hierarchical double-layer hydroxide (LDH)/MOF electrocatalysts that combine the advantages of two materials are expected to overcome these drawbacks. Herein, we develop a simple and universal strategy, in situ pseudomorphic transformation, to construct hierarchical LDH/MOF electrocatalysts. Accordingly, ultra-thin Fe-Ni LDH nanosheets are in situ produced in the heterometallic MOF during the transformation process. Profiting from the abundant metal sites and the extended electron transport channel from the inserted ultra-thin LDH arrays, the hierarchical Fe-Ni LDH/MOFs exhibit striking electrochemical activities for the oxygen evolution reaction (OER). In particular, the as-synthesized Fe-Ni LDH/MOF-b2 delivers the best OER performance, exhibiting an ultralow overpotential (255 mV at 10 mA cm^-2), minimum Tafel slope (24 mV dec^-1) and outstanding cycling durability. Meanwhile, the evolution process of the hierarchical Fe-Ni LDH/MOF has been monitored with the controllable in situ semi-transformation strategy. This also provides an opportunity to decipher the original active species for the OER process. Mechanism analysis indicates that the bimetallic MOF and bimetallic LDH are both active species, and the excellent OER performance of hierarchical Fe-Ni LDH/MOF could be attributed to the effect of "a whole greater than the sum of the parts".

Modulation engineering of in situ cathodic activation of FePx based on W-incorporation for the hydrogen evolution reaction

In situ electrochemical activation as a new pretreating method to adjust electrocatalytic performance attracts extensive attention. However, the activation mechanisms of electrocatalysts are still ambiguous. Herein, we propose a facile modulation strategy of in situ cathodic activation of FeP_x based on W-incorporation (W-FeP_x/IF) for the hydrogen evolution reaction (HER). The activated W-FeO_x with obvious surface reconstruction demonstrates the role of W-incorporation for driving the cathodic activation of FeP_x, which suggests the larger surface area and more active sites. In fact, W incorporation can not only accelerate the cathodic activation process but also act as the adsorption sites for H_ad to form the synergistic effect with FeO_x for water dissociation. The obtained W-FeO_x/IF exhibits greatly enhanced HER activity featuring decreased overpotential from 237.7 to 154.0 mV at 100 mA cm^-2, which may be ascribed to W-FeO_x with double catalytic active sites after cathodic activation. Additionally, the modulation effects of cathodic activation can be exactly achieved by changing electrochemical parameters such as CV cycles. W-FeO_x/IF also shows excellent long-term stability for at least 100 h at 100 mA cm^-2. This modulation engineering based on metal doping is expected to provide inspiration for the understanding of the cathodic activation process for efficient electrocatalysts.

Optimized Enhancement Effect of Sulfur in Fe-N-S Codoped Carbon Nanosheets for Efficient Oxygen Reduction Reaction

The study on the design and preparation of oxygen reduction reaction (ORR) electrocatalysts with high efficiency is currently attracting great concern. Among different types of catalysts, heteroatom-doped carbon-based catalysts have exhibited promising potential, and the exploration of optimized matching of the doping elements is crucial to the design and fabrication of this category of catalysts. Herein, by annealing commercially available and cost-effective precursors, Fe-N-S codoped graphene-like carbon nanosheet catalysts were prepared. The atomically dispersed Fe atoms coordinated with the N atoms to form FeN₄ sites as proved by X-ray absorption spectroscopy. By facile modulation of the relative amount of the precursors, the contents of thiophene-S (Th-S) and Fe-N₄ sites could be tuned and a series of catalysts with different Th-S/Fe ratios were prepared. The doped sulfur exhibited an enhancement effect on ORR performance, and strikingly, the enhancement efficiency could be optimized by fine modulation of the Th-S/Fe ratio in the catalysts. Furthermore, it was found that when the Th-S/Fe ratio reached an optimal value of 1.8, the ORR performance was significantly boosted, especially in acidic media. The experimental data were supported by density functional theory calculation results, which indicated that the ORR overpotential of the S₂(FeN₄) configuration model (corresponding to the Th-S/Fe ratio of 2) was lower than that of S₃(FeN₄) and S₁(FeN₄). The optimized catalyst (denoted as FeN/SNC-900-3) displayed highly efficient ORR activity in both alkaline and acidic media. In alkaline media, the half-wave potential was 49 mV more positive than that of the commercial Pt/C catalyst, and in acidic media, the half-wave potential was close to that of Pt/C. Moreover, the stability of FeN/SNC-900-3 was outstanding, and the relative current density showed only a slight decay in both alkaline and acidic media after 40,000 s. A primary Zn-air battery with FeN/SNC-900-3 as the cathode catalyst exhibited a high peak power density of up to 153 mW cm^-2 and superior cycling stability over 200 cycles.

Rational Design of a N,S Co-Doped Supermicroporous CoFe-Organic Framework Platform for Water Oxidation

It remains a challenge to rational design of a new metal-organic framework (MOF) as highly efficient direct electrocatalysts for the oxygen evolution reaction (OER). Herein, we developed a simple and effective method to explore a new pillared-layered MOF with syringic acid as a promising OER electrocatalyst. The isostructural mono-, heterobimetallic MOF and N,S co-doped MOF by mixing thiourea were quickly synthesized in a high yield under solvothermal condition. Moreover, the optimized N,S co-doped MOF exhibits the lowest overpotential of 254 mV at 10 mA cm^-2 on a glass carbon electrode and a small Tafel slope of 50 mV dec^-1 , especially, this catalyst also possesses long-term electrochemical durability for at least 16 h. According to the characterization, the incorporation of N and S atoms into this heterobimetallic CoFe-based MOF could modify its pore structure, tune the electronic structure, accordingly, improve the mass and electron transportation, and facilitate the formation of active species, as a consequence, the improved activity of this new N,S co-doped MOF for OER should be mainly be ascribed to higher electrochemical activation toward the active species via in situ surface modification during the OER process.

Optimized trimetallic benzotriazole-5-carboxylate MOFs with coordinately unsaturated active sites as an efficient electrocatalyst for the oxygen evolution reaction

Enhanced OER performance of bimetallic and trimetallic MOFs were gained through synergistic effect in Fe/Co/Ni unsaturated coordination sites.

Direct observation of chemical origins in crystalline (NixCo1−x)2B oxygen evolution electrocatalysts

A crystalline bimetal (Ni_0.75Co_0.25)₂B catalyst synthesized by a simple ball-milling method exhibits excellent OER activity, and the special roles of Co and Ni were accurately certified by CV cycling on hydrophobic and hydrophilic electrodes.

Coordination polymers as heterogeneous catalysts in hydrogen evolution and oxygen evolution reactions

This article highlights various strategies of designing coordination polymers for catalysing water splitting reactions.

Single-atom implanted two-dimensional MOFs as efficient electrocatalysts for the oxygen evolution reaction

A facile and controllable synthesis strategy for bimetallic electrocatalysts for the OER from a two-dimensional iron-based metal–organic framework precursor has been reported, in which the Fe/Ni type exhibits the best efficiency and conversion.

Metal-Layer Assisted Growth of Ultralong Quasi-2D MOF Nanoarrays on Arbitrary Substrates for Accelerated Oxygen Evolution

Controlled growth of metal-organic frameworks (MOFs) nanocrystals on requisite surfaces is highly desired for myriad applications related to catalysis, energy, and electronics. Here, this challenge is addressed by overlaying arbitrary surfaces with a thermally evaporated metal layer to enable the well-aligned growth of ultralong quasi-2D MOF nanoarrays comprising cobalt ions and thiophenedicarboxylate acids. This interfacial engineering approach allows preferred chelation of carboxyl groups in the ligands with the metal interlayers, thereby making possible the fabrication and patterning of MOF nanoarrays on substrates of any materials or morphologies. The MOF nanoarrays grown on porous metal scaffolds demonstrate high electrocatalytic capability for water oxidation, exhibiting a small overpotential of 270 mV at 10 mA cm^-2 , or 317 mV at 50 mA cm^-2 as well as negligible decay of performance within 30 h. The enhanced performance stems from the improved electron and ion transport in the hierarchical porous nanoarrays consisting of in situ formed oxyhydroxide nanosheets in the electrochemical processes. This approach for mediating the growth of MOF nanoarrays can serve as a promising platform for diverse applications.