2D metal–organic frameworks and their derivatives for the oxygen evolution reaction

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

On-surface fabrication of two-dimensional (2D) metal organic frameworks (MOFs) has been continuously attracting attentions for years. However, the synthesis of 2D MOFs with large-amplitude flexibility was rarely carried out since the bonding configurations in the coordination nodes are typically highly directional. Here we demonstrate that single alkali ions, which are fully isotropic in ionic bonding, can act as pivot joints for constructing tunable 2D MOFs by bonding to dihalogen groups in organic molecules. We take 2,3,6,7,10,11-hexabromotriphenylene, a 3-fold polycyclic molecule with three ortho-dibromo groups, and sodium (Na) atoms as a model system and successfully construct Na-based MOFs on Au(111) surface. The deflection angle of the Na coordination nodes is variable in an unprecedentedly large range between ±36° that allows the construction of multiple 2D MOF architectures. Such a flexible alkali-halogen bonding may provide a unique toolbox for designing and constructing more tunable MOFs by choosing various alkali atoms and halogen moieties.

Metal–organic frameworks (MOFs) have been widely studied due to their versatile applications and easily tunable structures. However, heteroatom‐metal coordination dominates the MOFs community, and the rational synthesis of carbon–metal coordination‐based MOFs remains a significant challenge. Herein, two‐dimensional (2D) MOFs based on silver–carbon linkages are synthesized through the coordination between silver(I) salt and isocyanide‐based monomers at ambient condition. The as‐synthesized 2D MOFs possess well‐defined crystalline structures and a staggered AB stacking mode. Most interestingly, these 2D MOFs, without π–π stacking between layers, exhibit narrow band gaps down to 1.42 eV. As electrochemical catalysts for converting CO2 to CO, such 2D MOFs demonstrate Faradaic efficiency over 92 %. Surprisingly, the CO2 reduction catalyzed by these MOFs indicates favorable adsorption of CO2 and *COOH on the active carbon sites of the isocyanide groups rather than on silver sites. This is attributed to the critical σ donor role of isocyanides and the corresponding ligand‐to‐metal charge–transfer effect. This work not only paves the way toward a new family of MOFs based on metal–isocyanide coordination but also offers a rare platform for understanding the electrocatalysis processes on strongly polarized carbon species.

Abstract2D metal organic frameworks (MOF) have received tremendous attention due to their organic–inorganic hybrid nature, large surface area, highly exposed active sites, and ultrathin thickness. However, the application of 2D MOF in light‐to‐hydrogen (H2) conversion is rarely reported. Here, a novel 2D MOF [Ni(phen)(oba)]n·0.5nH2O (phen = 1,10‐phenanthroline, oba = 4,4′‐oxybis(benzoate)) is for the first time employed as a general, high‐performance, and earth‐abundant platform to support CdS or Zn0.8Cd0.2S for achieving tremendously improved visible‐light‐induced H2‐production activity. Particularly, the CdS‐loaded 2D MOF exhibits an excellent H2‐production activity of 45 201 µmol h−1 g−1, even exceeding that of Pt‐loaded CdS by 185%. Advanced characterizations, e.g., synchrotron‐based X‐ray absorption near edge structure, and theoretical calculations disclose that the interactive nature between 2D MOF and CdS, combined with the high surface area, abundant reactive centers, and favorable band structure of 2D MOFs, synergistically contribute to this distinguished photocatalytic performance. The work not only demonstrates that the earth‐abundant 2D MOF can serve as a versatile and effective platform supporting metal sulfides to boost their photocatalytic H2‐production performance without noble‐metal co‐catalysts, but also paves avenues to the design and synthesis of 2D‐MOF‐based heterostructures for catalysis and electronics applications.

Two-dimensional (2D) metal-organic framework (MOF) membranes are considered potential gas separation membranes of the next generation due to their structural diversity and geometrical functionality. However, achieving a rational structure design for a 2D MOF membrane and understanding the impact of MOF nanosheet stacking modes on membrane separation performance remain challenging tasks. Here, we report a novel kind of 2D MOF membrane based on [Cu2 Br(IN)2 ]n (IN=isonicotinato) nanosheets and propose that synergetic stacking modes of nanosheets have a significant influence on gas separation performance. The stacking of the 2D MOF nanosheets is controlled by solvent droplet dynamic behaviors at different temperatures of drop coating. Our 2D MOF nanosheet membranes exhibit high gas separation performances for H2 /CH4 (selectivity >290 with H2 permeance >520 GPU) and H2 /CO2 (selectivity >190 with H2 permeance >590 GPU) surpassing the Robeson upper bounds, paving a potential way for eco-friendly H2 separation.

2D metal-organic frameworks (MOFs) have emerged as potential candidates for electrocatalytic oxygen evolution reactions (OER) due to their inherent properties like abundant coordination unsaturated active sites and efficient charge transfer. Herein, a versatile and massively synthesizable self-etching assembly strategy wherein nickel-iron foam (NFF) acts as a substrate and a metal ion source. Specifically, by etching the nickel-iron foam (NFF) surface using ligands and solvents, Ni/Fe metal ions are activated and subsequently reacted under hydrothermal conditions, resulting in the formation of self-supporting nanosheet arrays, eliminating the need for external metal salts. The obtained 33 % NiFeMOF/NFF exhibits remarkable OER performance with ultra-low overpotentials of 188/231 mV at 10/100 mA cm-2 , respectively, outperforming most recently reported catalysts. Besides, the built 33 % NiFeMOF/NFF(+) ||Pt/C(-) electrolyzer presents low cell voltages of 1.55/1.83 V at 10/100 mA cm-2 , superior to the benchmark RuO2 (+) ||Pt/C(-) , implying good industrialization prospects. The excellent catalytic activity stems from the modulation of the electronic spin state of the Ni active site by the introduction of Fe, which facilitates the adsorption process of oxygen-containing intermediates and thus enhances the OER activity. This innovative approach offers a promising pathway for commercial-scale sustainable energy solutions.

Co‐based bimetallic metal–organic frameworks (MOFs) have emerged as a kind of promising electrocatalyst for oxygen evolution reaction (OER). However, most of present works for Co‐based bimetallic MOFs are still in try‐and‐wrong stage, while the OER performance trend and the underlying structure‐function relationship remain unclear. To address this challenge, Co‐based MOFs on carbon cloth (CC) (CoM MOFs/CC, M = Zn, Ni, and Cu) are prepared through a room‐temperature method, and their structure and OER performance are compared systematically. Based on the results of overpotential and Tafel slope, the order of OER activity is ordered in the decreasing sequence: CoZn MOF > CoNi MOF > CoCu MOF > Co MOF. Spectroscopic studies clearly show that the better OER performance of CoM MOFs results from the higher oxidation state of Co, which is related to the choice of second metal. Theoretical calculations indicate that CoZn MOFs possess strengthened adsorption for O‐containing intermediate, and lower energy barrier towards OER. This study figures out the effect of second metal on the OER performance of Co‐based bimetallic MOFs and suggests that tuning the electronic structure of the metal site can be an effective strategy for other MOFs‐based OER catalysts.

The design of cost-effective and highly active, durable electrocatalysts for the sluggish oxygen evolution reaction (OER) is critical for promoting various energy conversion process such as water splitting and rechargeable metal-air batteries. Due to a wider selection of earth-abundance and low cost catalyst candidates (e.g., mixed 3d-transition metal oxides), OER in alkaline environment has attracted considerable interests during past years. Besides the recently developed Ni-Fe layered double hydroxides (LDH) catalysts, spinel phase transition metal oxides have also drawn particular interests owing to their well-defined rigid structure and thus potentially high stability during electrocatalysis. While improved OER activities have been demonstrated, currently most spinel transition metal oxides (mainly Co-based spinel oxides) still underperformed the state-of-the-art benchmark OER catalyst IrO2and the Ni-Fe LDH catalysts. Moreover, atomic insights into the surface structure that governs their OER reactivities and stabilities still remain very limited. We report here the exploration of iron-based spinel oxide nanoparticles (MxFe3-xO4, M= Mn, Fe, Co, Ni, Cu) as a new class of alkaline OER electrocatalysts with potentially both high electrocatalytic activity and high stability. Emphasis will be placed on atomistic understanding on the surface cation chemistry and surface atomic structures of the spinel oxide nanoparticles and their correlations with OER activities and stabilities. This will be achieved by comparative microscopic and spectroscopic studies of the nanoparticle surfaces before and after OER electrocatalysis using high-resolution (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray photon spectroscopy, which will be further complemented by density functional theory calculations. The results will provide important insights into the structure-activity-stability relations of spinel phase transition metal oxides as well as other mixed metal oxides for OER electrocatalysts.

Thickness-dependent β/γ-NiOOH transformation of Ni-MOFs in oxygen evolution reaction

CoX4 (X = NH, S, and O) motifs have demonstrated their high catalytic activity in the platforms of metal organic frameworks (MOFs), however, the underlying reason is still unrevealed. Herein, we propose monolayers constructed by linking TMNxO4-x motifs (TM = Fe, Co, Ni, Cu) with trioxotriangulenes (TOTs) as suitable models to clarify the structure-property-performance relationship of 2D MOFs for the oxygen evolution/reduction reaction (OER/ORR). The highly robust catalytic activity of CoNxO4-x for both the OER and the ORR has been confirmed, even surpassing that of most previously reported 2D MOFs and SACs. This activity is attributed to the moderate interaction between Co and the key intermediate species, which can be modulated by the coordinating atoms. We reveal spin momentum as a reliable activity descriptor in rationalizing the OER/ORR activity, which can be extended to many other 2D MOFs. The elucidated structure-activity relationship is significant for the development of effective bifunctional OER/ORR electrocatalysts.

Large scale energy storage solutions are an imperative feature of our future, greener energy grid. Recently, many megawatts-worth of solar photovoltaic arrays and wind turbines have been added to the energy network. While this sets a largely positive trajectory, the nature of their unpredictable, uncontrollable and intermittent output poses a threat to grid stability. For their continued addition to the energy network, storage solutions are needed to control their energy output. To date, very low volumes of commercial electrochemical storage system exist, relative to pumped hydro storage systems. A wider breadth of distributable electrochemical devices is needed to harness the crucial, seismic change the energy network has ahead. Among the potentially significant emerging technologies include the metal-air and hybrid redox flow batteries. [1]Expanding the range of catalysts used in electrochemical devices is of vital importance for the exploration of new types of energy storage system. Reducing the use of expensive platinum group metal catalysts is a common goal among researchers too. This work develops and optimizes the synthesis of an iridium (Ir)-supported ZIF 67-derived nitrogen doped carbon (Ir/NC) bifunctional oxygen catalyst using a previously developed synthesis protocol [2] and attempts to fully characterize the best performing material using a series of physical characterization techniques including BET, EDX, ICP, SEM, TEM, XAS, XPS and XRD.On inspection of the surface of the material via TEM, a better description of the catalyst active site can be deciphered. Ir particles are shown to be co-located with the cobalt metal agglomerates and clearly demonstrated to positively affect the catalyst by providing the majority of the oxygen evolution reaction (OER) performance. XPS and XRD analyses shed additional light on the surface chemistry of the catalyst and the former serves as a crucial indicator of both the nitrogen content and the form of Ir at the catalyst surface. ICP measurements, often used to calculate catalyst ‘mass activity’, are systematically carried out too, and we rigorously demonstrate the importance of thorough sample preparation and the dangers associated metal-content misrepresentation.Several aspects of the physical characterization findings carried out in this work are rationalized and confirmed during electrochemical tests using a rotating ring disk electrode. As expected, inclusion of Ir in the catalyst augments the OER performance [3] and has very little effect on the oxygen reduction reaction performance – assertions are therefore made about the active site for the oxygen reactions. Investigation of catalyst performance in different pH media shows extremely high OER performance in acid, however, durability tests show that the catalyst performs best overall in basic media and may find utility in the previously mentioned devices.In attempting to make a clear comparison between this catalyst and previously reported materials, a pervasive issue associated with defining performance of these catalysts is found. The ΔE metric, commonly used to compare bifunctional oxygen catalysts, is calculated using arbitrary potentials (E10 and E1/2, which, when used to compare single function catalysts is perfectly valid). We suggest an alternative ‘Figure of Merit’ which provides a potentially alternative performance metric and better way of comparing bifunctional catalysts.In summary, we synthesize and characterize a new material and carry out fundamental electrochemical tests that reveal a material with compelling performance and promising qualities for next generation energy storage/conversion devices.[1]: Electrochemical Energy Storage for Renewable Sources and Grid Balancing, Moseley, P.T. and Garche, J., Elsevier (2015).[2]: A. Mehmood, J. Pampel, G. Ali, H. Y. Ha, F. Ruiz‐Zepeda and T.‐P. Fellinger, Facile Metal Coordination of Active Site Imprinted Nitrogen Doped Carbons for the Conservative Preparation of Non‐Noble Metal Oxygen Reduction Electrocatalysts. Adv. Energy Mater., 8, 1701771 (2008).[3]: T. Reier, M. Oezaslan and P. Strasser, Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal., 2, 8, 1765-1772 (2012).

Complexation of copper(II) 2,3,9,10,16,17,23,24-octahydroxy-29H,31H-phthalocyanine (CuPcOH) with copper(II) ions gives a two-dimensional (2D) metal-organic framework (MOF). This is the first report of a phthalocyanine-based MOF. This 2D MOF was obtained as a black powder and showed an electrical conductivity of 1.6×10-6 S cm-1 at 80 °C. When this MOF is used as a cathode of lithium ion battery (LIB), large charge/discharge capacities of 151/128 mAh g-1 were obtained. In addition, it showed a good stability during 200 charge/discharge cycles. The obtained LIB performance mainly originates from the electrically conductive and redox-active framework of the phthalocyanine-based 2D MOF and its hierarchical microporous/mesoporous structure.

The structure and properties of materials are determined by a diverse range of chemical bond formation and breaking mechanisms, which greatly motivates the development of selectively controlling the chemical bonds in order to achieve materials with specific characteristics. Here, an orientational intervening bond‐breaking strategy is demonstrated for synthesizing ultrathin metal–organic framework (MOF) nanosheets through balancing the process of thermal decomposition and liquid nitrogen exfoliation. In such approach, proper thermal treatment can weaken the interlayer bond while maintaining the stability of the intralayer bond in the layered MOFs. And the following liquid nitrogen treatment results in significant deformation and stress in the layered MOFs’ structure due to the instant temperature drop and drastic expansion of liquid N2, leading to the curling, detachment, and separation of the MOF layers. The produced MOF nanosheets with five cycles of treatment are primarily composed of nanosheets that are less than 10 nm in thickness. The MOF nanosheets exhibit enhanced catalytic performance in oxygen evolution reactions owing to the ultrathin thickness without capping agents which provide improved charge transfer efficiency and dense exposed active sites. This strategy underscores the significance of orientational intervention in chemical bonds to engineer innovative materials.

The most important topics in the world today are environmental and resource issues. The development of green and clean energy is still one of the great challenges of social sustainable development. Two-dimensional(2D) metal-organic frameworks(MOFs) and derivatives have exceptional potential as high-efficiency electrocatalysts for clean energy technologies. This review summarizes various synthesis strategies and applications of 2D MOFs and derivatives in electrocatalysis. Firstly, we will outline the advantages and uniqueness of 2D MOFs and derivatives, as well as their applicable areas. Secondly, the synthetic strategies of 2D MOFs and derivatives are briefly classified. Each category is summarized and we list classic representative fabrication methods, including specific fabrication methods and mechanisms, corresponding structural characteristics, and insights into the advantages and limitations of the synthesis method. Thirdly, we separately classify and summarize the application of 2D MOFs and derivatives in electrocatalysis, including electrocatalytic water splitting, oxygen reduction reaction(ORR), CO2 reduction reaction(CO2RR), and other electrocatalytic applications. Finally, the development prospects and existing challenges to 2D MOFs and derivatives are discussed.

Constructing nanomaterials mimicking the coordination environments of natural enzymes may achieve biomimetic catalysis. Here we construct a two-dimensional (2D) metal-organic framework (MOF) nanosheet catalyst as an artificial antioxidase for nanocatalytic rheumatoid arthritis treatment. The 2D MOF periodically assembles numbers of manganese porphyrin molecules, which has a metal coordination geometry analogous to those of two typical antioxidases, human mitochondrial manganese superoxide dismutase (Mn-SOD) and human erythrocyte catalase. The zinc atoms of the 2D MOF regulate the metal-centered redox potential of coordinated manganese porphyrin ligand, endowing the nanosheet with both SOD- and catalase-like activities. Cellular experiments show unique anti-inflammatory and pro-biomineralization performances of the 2D MOF, while in vivo animal model further demonstrates its desirable antiarthritic efficacy. It is expected that such a nanocatalytic antioxidation concept may provide feasible approaches to future anti-inflammatory treatments.