Effective Enhancement of the Ferroelectric Performance of Polar Co-Gallate MOF by Doping M2+ Ions (M = Mg, Mn, Ni) into Framework Nodes

MOF ferroelectrics have been demonstrated to be a promising candidate owing to various structures and controllable properties. However, weak ferroelectricity hampers their boom. Herein, a convenient strategy, doping metal ions into the framework nodes of parent MOF, is adopted to enhance ferroelectric performance. A series of M-doped Co-Gallate (M = Mg, Mn, Ni) were synthesized to improve ferroelectric properties. The electrical hysteresis loop demonstrated its ferroelectric behaviors, exhibiting obviously improved ferroelectric properties compared with the parent Co-Gallate. The remanent polarization was enhanced by two times for Mg-doped Co-Gallate, six times for Mn-doped Co-Gallate, and four times for Ni-doped Co-Gallate. The promoted ferroelectric performances are ascribed to the enhanced polarity of the overall structure triggered by framework distortion. Intriguingly, ferroelectric behaviors increase in the order Mg < Ni < Mn, displaying the same tendency as the difference value in the ionic radius between Co²⁺ ions and M²⁺ metal ions (M = Mg, Mn, Ni). These results demonstrate doping of metal ions is a valid strategy to enhance ferroelectric performances, which may serve as a guide in modulating ferroelectric behaviors.

Hydrogel Polymer-PBA Nanocomposite Thin Film-Based Bifunctional Catalytic Electrode for Water Splitting: The Unique Role of the Polymer Matrix in Enhancing the Electrocatalytic Efficiency

A novel approach to efficient bifunctional catalytic electrodes for water splitting is developed, based on a counterintuitive choice of an insulating hydrogel polymer (chitosan, CS)-Prussian blue analogue (PBA, KCoFe) nanocomposite thin film on nickel foam. The polymer matrix in KCoFe-CS enables the formation of framelike structures of the non-noble metal-based catalyst nanocrystals, in addition to improving their stability. An optimized cycling protocol leads to a substantial enhancement of the electrocatalytic efficiency for oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER), achieving relatively low overpotentials of 272 and 320 mV (@ 10 and 20 mA cm^-2) and 146 mV (@ 10 mA cm^-2), respectively, reduced Tafel slopes, and increased Faradaic efficiencies of 98 and 96%; the overpotentials estimated based on the electrochemically active surface area show similar trends. The polymer encapsulation and the cycling protocol are key to the realization of the desirable combination of enhanced efficiency and stability demonstrated up to 50 h for both OER and HER. Detailed characterizations of the postcycling catalytic electrode show that favorable morphological changes of the polymer matrix with concomitant reduction in the PBA nanocrystal size lead to the enhanced activity. The bifunctional activity of the catalytic electrode is demonstrated by the stable water splitting achieved with a 20 mA cm^-2 current density at 1.55 V. The present study unravels the utility of hydrogel polymer matrices (without the use of binders like Nafion) in realizing sustainable water splitting electrocatalysts with high stability and efficiency, through the combined effect of confining the electrolyte within and favorably modifying the catalyst nanoparticles and the nanocomposite morphology.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Nanocrystals of CuCoO2 derived from MOFs and their catalytic performance for the oxygen evolution reaction

In this work, two different solvothermal synthesis routes were employed to prepare MOF-derived CuCoO₂ (CCO) nanocrystals for electrocatalytic oxygen evolution reaction (OER) application. The effects of the reductants (ethylene glycol, methanol, ethanol, and isopropanol), NaOH addition, the reactants, and the reaction temperature on the structure and morphology of the reaction product were investigated. In the first route, Cu-BTC derived CCO (CCO1) nanocrystals with a size of ∼214 nm and a specific surface area of 4.93 m² g^-1 were prepared by using Cu-BTC and Co(NO₃)₂·6H₂O as the Cu and Co source, respectively. In the second route, ZIF-67 derived CCO (CCO2) nanocrystals with a size of ∼146 nm and a specific surface area of 11.69 m² g^-1 were prepared by using ZIF-67 and Cu(NO₃)₂·3H₂O as the Co and Cu source, respectively. Moreover, the OER performances of Ni foam supported CCO1 (Ni@CCO1) and CCO2 (Ni@CCO2) electrodes were evaluated in 1.0 M KOH solution. Ni@CCO2 demonstrates a better OER catalytic performance with a lower overpotential of 394.5 mV at 10 mA cm^-2, a smaller Tafel slope of 82.6 mV dec^-1, and long-term durability, which are superior to those of some previously reported delafossite oxide or perovskite oxide catalysts. This work reveals the preparation method and application potential of CCO electrocatalysts by using Cu-BTC/ZIF-67 as the precursor, providing a new approach for the preparation of delafossite oxide CCO and the enhancement of their OER performances.

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Metal–organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

Nitrogen-doped carbon encapsulating RuCo heterostructure for enhanced electrocatalytic overall water splitting

The kinetically sluggish electrochemical water splitting reaction still faces great challenges, and the rational design of excellent electrocatalysts is the key to solving the problem. Herein, an etching and pyrolysis...

Fe-Induced electronic optimization of mesoporous Co–Ni oxide nanosheets as an efficient binder-free electrode for the oxygen evolution reaction

An efficient binder-free OER electrode CoNiFeO_x/NF with mesoporous structure was synthesized by a facile strategy of hydrothermal method and post-annealing.