Tailoring the Electrochemical Responses of MOF-74 Via Dual-Defect Engineering for Superior Energy Storage

Rationally designed defects in a crystal can confer unique properties. This study showcases a novel dual-defects engineering strategy to tailor the electrochemical response of metal-organic framework (MOF) materials used for electrochemical energy storage. Salicylic acid (SA) is identified as an effective modulator to control MOF-74 growth and induce structural defects, and cobalt cation doping is adopted for introducing a second type of defect. The resulting dual-defects engineered bimetallic MOF exhibits a discharging capacity of 218.6 mAh g-1, 4.4 times that of the pristine MOF-74, and significantly improved cycling stability. Moreover, the engineered MOF-74(Ni0.675Co0.325)-8//Zn aqueous battery shows top energy/power density performances for Ni-Zn batteries (266.5 Wh kg-1, 17.22 kW kg-1). Comprehensive investigations reveal that engineered defects modify the local coordination environment and promote the in situ electrochemical reconfiguration during operation to significantly boost the electrochemical activity. This work suggests that rational tailoring of the defects within the MOF crystal is an effective strategy to manipulate the coordination environment of the metal centers and the corresponding electrochemical reconfiguration for electrochemical applications.

Metal Organic Framework Cubosomes

We demonstrate a general strategy for the synthesis of ordered bicontinuous-structured metal organic frameworks (MOFs) by using polymer cubosomes (PCs) with a double primitive structure (Im 3 ‾ ${\bar{3}}$ m symmetry) as the template. The filling of MOF precursors in the open channel of PCs, followed by their coordination and removal of the template, generates MOF cubosomes with a single primitive topology (Pm 3 ‾ ${\bar{3}}$ m) and average mesopore diameters of 60-65 nm. Mechanism study reveals that the formation of ZIF-8 cubosomes undergoes a new MOF growth process, which involves the formation of individual MOF seeds in the template, their growth and eventual fusion into the cubosomes. Their growth kinetics follows the Avrami equation with an Avrami exponent of n=3 and a growth rate of k=1.33×10^-4 , indicating their fast 3D heterogeneous growth mode. Serving as a bioreactor, the ZIF-8 cubosomes show high loading of trypsin enzyme, leading to a high catalytic activity in the proteolysis of bovine serum albumin.

Optimizing Acetylene Sorption through Induced-fit Transformations in a Chemically Stable Microporous Framework

Developing practical storage technologies for acetylene (C₂ H₂ ) is important but challenging because C₂ H₂ is useful but explosive. Here, a novel metal-organic framework (MOF) (FJI-H36) with adaptive channels was prepared. It can effectively capture C₂ H₂ (159.9 cm³  cm^-3 ) at 1 atm and 298 K, possessing a record-high storage density (561 g L^-1 ) but a very low adsorption enthalpy (28 kJ mol^-1 ) among all the reported MOFs. Structural analyses show that such excellent adsorption performance comes from the synergism of active sites, flexible framework, and matched pores; where the adsorbed-C₂ H₂ can drive FJI-H36 to undergo induced-fit transformations step by step, including deformation/reconstruction of channels, contraction of pores, and transformation of active sites, finally leading to dense packing of C₂ H₂ . Moreover, FJI-H36 has excellent chemical stability and recyclability, and can be prepared on a large scale, enabling it as a practical adsorbent for C₂ H₂ . This will provide a useful strategy for developing practical and efficient adsorbents for C₂ H₂ storage.

Self-Reconstructed Metal-Organic Framework Heterojunction for Switchable Oxygen Evolution Reaction

Designing metal-organic framework (MOF)-based catalysts with superior oxygen evolution reaction (OER) activity and robust durability simultaneously is highly required yet very challenging due to the limited intrinsic activity and their elusive evolution under harsh OER conditions. Herein, a steady self-reconstructed MOF heterojunction is constructed via redox electrochemistry and topology-guided strategy. Thanks to the inhibiting effect from hydrogen bonds of Ni-BDC-1 (BDC=1,4-benzenedicarboxylic acid), the obatained MOF heterojunction shows greatly improved OER activity with low overpotential of 225 mV at 10 mA cm^-2 , relative to the totally reconstructed Ni-BDC-3 (332 mV). Density function theory calculations reveal that the formed built-in electric field in the MOF heterojunction remarkably optimizes the ad/desorption free energy of active Ni sites. Moreover, such MOF heterojunction shows superior durability attributed to the shielding effect of the surface-evolved NiOOH coating.

Dual In Situ Laser Techniques Underpin the Role of Cations in Impacting Electrocatalysts

Understanding the electrode/electrolyte interface is crucial for optimizing electrocatalytic performances. Here, we demonstrate that the nature of alkali metal cations can profoundly impact the oxygen evolution activity of surface-mounted metal-organic framework (SURMOF) derived electrocatalysts, which are based on NiFe(OOH). In situ Raman spectroscopy results show that Raman shifts of the Ni-O bending vibration are inversely proportional to the mass activities from Cs+ to Li+ . Particularly, a laser-induced current transient technique was introduced to study the cation-dependent electric double layer properties and their effects on the activity. The catalytic trend appeared to be closely related to the potential of maximum entropy of the system, suggesting a strong cation impact on the interfacial water layer structure. Our results highlight how the electrolyte composition can be used to maximize the performance of SURMOF derivatives toward electrochemical water splitting.

Electric-Field-Assisted Alkaline Hydrolysis of Metal-Organic Framework Bulk into Highly Porous Hydroxide for Energy Storage and Electrocatalysis

Metal-organic frameworks (MOFs) have attracted tremendous attention in the field of supercapacitors and electrocatalysis due to their open metal sites and high surface area. However, their inherent instability and poor electrical conductivity lead to limited electrochemical performance. Herein, we have employed a new and simple strategy for converting MOF bulk into porous Zn-Co hydroxide composites with the assistance of electric fields with different cycles. This method can alter the migration behavior of charged molecules/ions and improve the nucleation rate of hydroxide, thus adjusting the morphology of derivatives. As a supercapacitor electrode, the optimal material of Zn_0.3Co_0.7(OH)₂ with an electric-field application time of 1200 cycles shows excellent electrochemical performance with a high specific capacity of 981.2 C g^-1 at 1 A g^-1. Additionally, the fabricated asymmetric supercapacitor exhibits an energy density of 42.5 Wh kg^-1 at a power density of 750.0 W kg^-1 and a remarkable cycling stability (99% after 11,000 cycles). Simultaneously, the as-prepared Zn_0.3Co_0.7(OH)₂ with an electric-field application time of 1200 cycles delivers prominent OER performances, which can exhibit low overpotentials of 300 and 326 mV at 50 and 100 mA cm^-2, respectively, and shows a small Tafel slope of 31.5 mV dec^-1. This study represents a new strategy for the synthesis of economical and efficient electrode materials for supercapacitors and OER electrocatalysts and offers a novel way for the mild preparation of nanoderivatives from MOFs.

Synthesis and Electronic Modulation of Nanostructured Layered Double Hydroxides for Efficient Electrochemical Oxygen Evolution

Water electrolysis is considered to be one of the most promising technologies to produce clean fuels. However, its extensive realization critically depends on the progress in cost-effective and high-powered oxygen evolution reaction (OER) electrocatalysts. As a member of the big family of two-dimensional (2D) materials, nanostructured layered double hydroxides (nLDHs) have made significant processes and continuous breakthroughs for OER electrocatalysis. In this Review, the advancements in designing nLDHs for OER in recent years were discussed with a unique focus on their electronic modulations and in situ analysis on catalytic processes. After a brief discussion on different synthetic methodologies of nLDHs, including "bottom-up" and "top-down" approaches, the general strategies to enhance the catalytic performances of nLDHs reported so far were summarized, including compositional substitution, heteroatom doping, vacancy engineering, and amorphous/crystalline engineering. Furthermore, the in situ OER processes and mechanism analysis on engineering efficient nLDHs electrocatalysts were discussed. Finally, the research trends, perspectives, and challenges on designing nLDHs were also carefully outlined. This progress Review may offer enlightening experimental/theoretical guidance for designing highly catalytic active nLDHs and provide new directions to promote their future prosperity for practical utilization in water splitting.