An Insight on the Electrocatalytic Mechanistic Study of Pristine Ni MOF (BTC) in Alkaline Medium for Enhanced OER and UOR

Improving the Oxygen Evolution Reaction: Exsolved Cobalt Nanoparticles on Titanate Perovskite Catalyst

Perovskites are an important class of oxygen evolution reaction (OER) catalysts due to highly tunable compositions and adaptable characteristics. However, perovskite-based catalysts can have limited atom utilization efficiency due to large particle size, resulting in low mass activity. Herein, Cobalt nanoparticles are exsolved from La0.2+2x Ca0.7-2x Ti1-x Cox O3 perovskite and applied in OER. Upon reduction in the 5% H2 /N2 atmosphere at 800 °C for 2 h, the Co exsolved perovskite catalyst (R-LCTCo0.11) exhibits optimal OER performance. The mass activity of R-LCTCo0.11 reaches ≈1700 mA mg-1 at an overpotential of 450 mV, which is 17 times and 3 times higher than that of LCTCo0.11 (97 mA mg-1 ) and R-Mix (560 mA mg-1 ) catalysts respectively, surpassing the benchmark catalyst RuO2 (42.7 mA mg-1 of oxide at η = 470 mV). Electrochemical impedance spectroscopy (EIS) data reveals that R-LCTCo0.11 has the lowest charge transfer resistance (Rct  = 58 Ω), demonstrating the highest catalytic and kinetic activity for OER. Furthermore, this catalyst shows high stability during an accelerated durability test of 10 h electrolysis and 1000 cycles cyclic voltammetry (CV). This work demonstrates that nanoparticle exsolution from a doped perovskite is an effective strategy for improving the atom utilization efficiency in OER.

Graphene-Oxide-Modified Metal-Organic Frameworks Embedded in Mixed-Matrix Membranes for Highly Efficient CO2/N2 Separation

MOF-74 (metal-organic framework) is utilized as a filler in mixed-matrix membranes (MMMs) to improve gas selectivity due to its unique one-dimensional hexagonal channels and high-density open metal sites (OMSs), which exhibit a strong affinity for CO2 molecules. Reducing the agglomeration of nanoparticles and improving the compatibility with the matrix can effectively avoid the existence of non-selective voids to improve the gas separation efficiency. We propose a novel, layer-by-layer modification strategy for MOF-74 with graphene oxide. Two-dimensional graphene oxide nanosheets as a supporting skeleton creatively improve the dispersion uniformity of MOFs in MMMs, enhance their interfacial compatibility, and thus optimize the selective gas permeability. Additionally, they extended the gas diffusion paths, thereby augmenting the dissolution selectivity. Compared with doping with a single component, the use of a GO skeleton to disperse MOF-74 into Pebax®1657 (Polyether Block Amide) achieved a significant improvement in terms of the gas separation effect. The CO2/N2 selectivity of Pebax®1657-MOF-74 (Ni)@GO membrane with a filler concentration of 10 wt% was 76.96, 197.2% higher than the pristine commercial membrane Pebax®1657. Our results highlight an effective way to improve the selective gas separation performance of MMMs by functionalizing the MOF supported by layered GO. As an efficient strategy for developing porous MOF-based gas separation membranes, this method holds particular promise for manufacturing advanced carbon dioxide separation membranes and also concentrates on improving CO2 capture with new membrane technologies, a key step in reducing greenhouse gas emissions through carbon capture and storage.

Controllable fabrication of iron-nickel alloy embedded in nitrogen-doped carbon for oxygen evolution

Nickel-based electrocatalysts for the oxygen evolution reaction (OER) show the disadvantages of low activity and poor stability. In this paper, an FeNi alloy is wrapped by an amino-modified MOF-5-derived N-doped carbon layer to address these problems. Additionally, the improvement resulting from Fe doping of NiOOH catalysts is theoretically supported.

Water-Stable Fluorous Metal-Organic Frameworks with Open Metal Sites and Amine Groups for Efficient Urea Electrocatalytic Oxidation

Urea oxidation reaction (UOR) is one of the promising alternative anodic reactions to water oxidation that has attracted extensive attention in green hydrogen production. The application of specifically designed electrocatalysts capable of declining energy consumption and environmental consequences is one of the major challenges in this field. Therefore, the goal is to achieve a resistant, low-cost, and environmentally friendly electrocatalyst. Herein, a water-stable fluorinated Cu(II) metalorganic framework (MOF) {[Cu2 (L)(H2 O)2 ]·(5DMF)(4H2 O)}n (Cu-FMOF-NH2 ; H4 L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) is developed utilizing an angular tetracarboxylic acid ligand that incorporates both trifluoromethyl (-CF3 ) and amine (-NH2 ) groups. The tailored structure of Cu-FMOF-NH2 where linkers are connected by fluoride bridges and surrounded by dicopper nodes reveals a 4,24T1 topology. When employed as electrocatalyst, Cu-FMOF-NH2 requires only 1.31 V versus reversible hydrogen electrode (RHE) to deliver 10 mA cm-2 current density in 1.0 m KOH with 0.33 m urea electrolyte and delivered an even higher current density (50 mA cm-2 ) at 1.47 V versus RHE. This performance is superior to several reported catalysts including commercial RuO2 catalyst with overpotential of 1.52 V versus RHE. This investigation opens new opportunities to develop and utilize pristine MOFs as a potential electrocatalyst for various catalytic reactions.

In Situ Formation of CoP/Co3 O4 Heterojunction for Efficient Overall Water Splitting

Electrochemical water splitting is an environmentally friendly and effective energy storage method. However, it is still a huge challenge to prepare non-noble metal based electrocatalysts that possess high activity and long-term durability to realize efficient water splitting. Here, we present a novel method of low-temperature phosphating for preparing CoP/Co3 O4 heterojunction nanowires catalyst on titanium mesh (TM) substrate that can be used for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting. CoP/Co3 O4 @TM heterojunction showed an excellent catalytic performance and long-term durability in 1.0 M KOH electrolyte. The overpotential of CoP/Co3 O4 @TM heterojunction was only 257 mV at 20 mA cm-2 during the OER process, and it could work stably more than 40 h at 1.52 V versus reversible hydrogen electrode (vs. RHE). During the HER process, the overpotential of CoP/Co3 O4 @TM heterojunction was only 98 mV at -10 mA cm-2 . More importantly, when used as anodic and cathodic electrocatalyst, they achieved 10 mA cm-2 at 1.59 V. The Faradaic efficiencies of OER and HER were 98.4 % and 99.4 %, respectively, outperforming Ru/Ir-based noble metal electrocatalysts and other non-noble metal electrocatalysts for overall water splitting.

CeO2/Ni-MOF with Synergistic Function of Enrichment and Activation: Efficient Reduction of 4-Nitrophenol Pollutant to 4-Aminophenol

The conversion of organic pollutants to value-added chemicals has been considered as a sustainable approach to solve environmental problems. However, it is still a challenge to construct a suitable heterogeneous catalyst that can synchronously achieve the enrichment and activation of organic pollutants (such as 4-nitrophenol, 4-NP). Here, an organic-inorganic hybrid catalyst (CeO₂/Ni-MOF) was successfully fabricated for efficiently reducing 4-NP to 4-aminophenol (4-AP) with water as the hydrogen source. Based on the synergistic effect of Ni-MOF (adsorption action) and CeO₂ (active sites), CeO₂/Ni-MOF could achieve a reaction rate of 1.102 μmol min^-1 mg^-1 with an ultrahigh Faraday efficiency (FE) (99.9%) and conversion (97.6%). In addition, the catalytic mechanism of 4-NP reduction over CeO₂/Ni-MOF was elaborated in depth. This work presents a new avenue for the effective reduction of pollutants and provides a new strategy for designing high-performance catalysts for rare-earth metals.

Bimetallic Cu/Fe MOF-Based Nanosheet Film via Binder-Free Drop-Casting Route: A Highly Efficient Urea-Electrolysis Catalyst

Developing efficient electrocatalysts for urea oxidation reaction (UOR) can be a promising alternative strategy to substitute the sluggish oxygen evolution reaction (OER), thereby producing hydrogen at a lower cell-voltage. Herein, we synthesized a binder-free thin film of ultrathin sheets of bimetallic Cu-Fe-based metal–organic frameworks (Cu/Fe-MOFs) on a nickel foam via a drop-casting route. In addition to the scalable route, the drop-casted film-electrode demonstrates the lower UOR potentials of 1.59, 1.58, 1.54, 1.51, 1.43 and 1.37 V vs. RHE to achieve the current densities of 2500, 2000, 1000, 500, 100 and 10 mA cm⁻², respectively. These UOR potentials are relatively lower than that acquired by the pristine Fe-MOF-based film-electrode synthesized via a similar route. For example, at 1.59 V vs. RHE, the Cu/Fe-MOF electrode exhibits a remarkably ultra-high anodic current density of 2500 mA cm⁻², while the pristine Fe-MOF electrode exhibits only 949.10 mA cm⁻². It is worth noting that the Cu/Fe-MOF electrode at this potential exhibits an OER current density of only 725 mA cm⁻², which is far inconsequential as compared to the UOR current densities, implying the profound impact of the bimetallic cores of the MOFs on catalyzing UOR. In addition, the Cu/Fe-MOF electrode also exhibits a long-term electrochemical robustness during UOR.

Electrocatalytic reduction of 4-nitrophenol over Ni-MOF/NF: understanding the self-enrichment effect of H-bonds

The chemical adsorption and active sites play a key role in electrocatalysis, so Ni-MOF/nickel foam was fabricated for efficiently reducing 4-nitrophenol (4-NP) without any sacrificial agents. The coordinated water molecules induced the formation of hydrogen bonds (H-bonds) with the nitro group, contributing to the self-enrichment of 4-NP. The reaction rate reached 0.351 μmol min^-1 mg^-1. Therefore, this work provides a new insight into the H-bond effect in the field of electrocatalysis.

Nickel-Based Metal-Organic Frameworks as Electrocatalysts for the Oxygen Evolution Reaction (OER)

The exploration of earth-abundant electrocatalysts with high performance for the oxygen evolution reaction (OER) is eminently desirable and remains a significant challenge. The composite of the metal-organic framework (MOF) Ni₁₀Co-BTC (BTC = 1,3,5-benzenetricarboxylate) and the highly conductive carbon material ketjenblack (KB) could be easily obtained from the MOF synthesis in the presence of KB in a one-step solvothermal reaction. The composite and the pristine MOF perform better than commercially available Ni/NiO nanoparticles under the same conditions for the OER. Activation of the nickel-cobalt clusters from the MOF can be seen under the applied anodic potential, which steadily boosts the OER performance. Ni₁₀Co-BTC and Ni₁₀Co-BTC/KB are used as sacrificial agents and undergo structural changes during electrochemical measurements, the stabilized materials show good OER performances.

Enhancing the Overall Electrocatalytic Water-Splitting Efficiency of Mo2C Nanoparticles by Forming Hybrids with UiO-66 MOF

For efficient electrocatalytic water-splitting, developing a nonprecious-metal-based stable and highly active material is the most challenging task. In this paper, we have devised a synthesis strategy for a hybrid catalyst composed of molybdenum carbide (Mo₂C) and a Zr-based metal-organic framework (MOF) (UiO-66) via the solvothermal process. Synergistic effects between Mo₂C and UiO-66 lead to a decrease in the hydrogen adsorption energy on the catalysts, and Mo₂C/UiO-66 hybrids offer excellent catalytic activity in an alkaline environment for water-splitting. Particularly, the optimized Mo₂C/UiO-66 hybrid, termed MCU-2 with 50:50 wt % of both components, displayed the best catalytic performance for both hydrogen and oxygen evolution reactions (HER/OER). It offered a small overpotential of 174.1 mV to attain a current density of 10 mA/cm² and a Tafel plot value of 147 mV/dec for HER. It also offered a low overpotential of around 180 mV to attain a current density of 20 mA/cm² and a Tafel plot value of 134 mV/dec for OER. Additionally, the catalyst was stable for over 24 h and ∼1000 cycles with a very minute shift in performance, and the electrolyzer indicates that a potential of ∼1.3 V is required to reach 10 mA/cm² current density. It can be inferred from the results that the Mo₂C/UiO-66 hybrid is a promising candidate as a nonexpensive and active catalyst for overall electrocatalytic water-splitting as the devised catalyst exhibits enhanced kinetics for both OER and HER, a more exposed surface area, faster electron transport, and enhanced diffusion of the electrolyte.

Research Progress of Oxygen Evolution Reaction Catalysts for Electrochemical Water Splitting

The development of a low-cost and high-efficiency oxygen evolution reaction (OER) catalyst is essential to meet the future industrial demand for hydrogen production by electrochemical water splitting. Given the limited reserves of noble metals and many competitive applications in environmental protection, new energy, and chemical industries, many studies have focused on exploring new and efficient non-noble metal catalytic systems, improving the understanding of the OER mechanism of non-noble metal surfaces, and designing electrocatalysts with higher activity than traditional noble metals. This Review summarizes the research progress of anode OER catalysts for hydrogen production by electrochemical water splitting in recent years, for noble metal and non-noble metal catalysts, where non-noble metal catalysts are highlighted. The categories are as follows: (1) Transition metal-based compounds, including transition metal-based oxides, transition metal-based layered hydroxides, and transition metal-based sulfides, phosphides, selenides, borides, carbides, and nitrides. Transition metal-based oxides can also be divided into perovskite, spinel, amorphous, rock-salt-type, and lithium oxides according to their different structures. (2) Carbonaceous materials and their composite materials with transition metals. (3) Transition metal-based metal-organic frameworks and their derivatives. Finally, the challenges and future development of the OER process of water splitting are discussed.

Advances in Understanding the Electrocatalytic Reconstruction Chemistry of Coordination Compounds

Coordination compounds including mainstream metal-organic frameworks and Prussian blue analogues receive extensive researches when they directly serve as electrocatalysts. Their reconstruction phenomena, that are closely associated with actual contributions and intrinsic catalytic mechanisms, are expected to be well summarized. Here, the recent advances in understanding reconstruction chemistry of coordination compounds are reviewed, including their main classifications and structural properties, reconstruction phenomena in electrocatalysis (e.g., oxygen/hydrogen evolution reaction, CO₂ reduction), influence factors of reconstruction parameters (e.g., reconstruction rate and reconstruction degree), and reconstruction-performance correlation. It is outlined that the reconstruction processes are influenced by electronic structure of coordination compounds, pH and temperature of testing solution, and applied potentials. The characterization techniques reflecting the evolution information before and after catalysis are also introduced for reconstruction-related mechanistic study. Finally, some challenges and outlooks on reconstruction investigations of coordination compounds are proposed, and the necessity of studying and understanding of these themes under actual working conditions of devices is highlighted.

Electrocatalytic performance of NiNH2BDC MOF based composites with rGO for methanol oxidation reaction

Present work comprehensively investigated the electrochemical response of Nickel-2 Aminoterephthalic acid Metal-Organic Framework (NiNH2BDC) and its reduced graphitic carbon (rGO) based hybrids for methanol (CH3OH) oxidation reaction (MOR) in an alkaline environment. In a thorough analysis of a solvothermally synthesized Metal-Organic Frameworks (MOFs) and its reduced graphitic carbon-based hybrids, functional groups detection was performed by FTIR, the morphological study by SEM, crystal structure analysis via XRD, and elemental analysis through XPS while electrochemical testing was accomplished by Chronoamperometry (CA), Cyclic Voltametric method (CV), Electrochemically Active Surface Area (EASA), Tafel slope (b), Electron Impedance Spectroscopy (EIS), Mass Activity, and roughness factor. Among all the fabricated composites, NiNH2BDC MOF/5 wt% rGO hybrid by possessing an auspicious current density (j) of 267.7 mA/cm2 at 0.699 V (vs Hg/HgO), a Tafel slope value of 60.8 mV dec-1, EASA value of 15.7 cm2, and by exhibiting resistance of 13.26 Ω in a 3 M CH3OH/1 M NaOH solution displays grander electrocatalytic activity as compared to state-of-the-art platinum-based electrocatalysts.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Ni2P nanoflakes for the high-performing urea oxidation reaction: linking active sites to a UOR mechanism

Urea electrolysis is regarded as an effective method for addressing both energy and environment issues. Herein, we successfully synthesized Ni2P nanoflakes for catalyzing the urea oxidation reaction (UOR). Due to the higher electrical conductivity as well as the prevailing tendency in triggering the UOR via a direct electro-oxidation mechanism, Ni2P nanoflakes exhibit comparable UOR activity (1.33 V vs. RHE for onset-potential, and 95.47 mA·cm-2 at 1.6 V vs. RHE) to the most active state-of-the-art catalysts, rendering them an effective alternative to precious metals such as Pt and Rh. The accelerated proton-coupled electron transfer (PCET) process caused by PO43- facilitates the in situ generation of NiOOH; thus, the UOR process is initiated at a lower onset-potential on Ni2P nanoflakes than on β-Ni(OH)2 nanoflakes. The in situ generated NiOOH instead of the Ni2P phase in Ni2P nanoflakes functions as an active site during the UOR process, while both NiOOH and the Ni2P phase serve as active sites in the OER process. This work provides insights into the understanding of the UOR mechanism and opens a new avenue to design low-cost Ni-based phosphide UOR catalysts.

Convenient synthesis of polymetallic metal–organic gels for efficient methanol electro-oxidation

Novel Ni-based AlNiCu-MOG and AB&AlNiCu-MOG composite materials were successfully fabricated, which exhibited superior MOR activities with a current density of 17.1 and 33.24 mA cm⁻², respectively.

Recent advances in pristine tri-metallic metal-organic frameworks toward the oxygen evolution reaction

Pristine metal-organic frameworks (MOFs) have received much attention in recent years due to their high specific surface areas, large porosity, excellent pore size distributions, flexible structure, and remarkable catalytic properties. The design of functional MOFs that can function as efficient HER and OER catalysts is significant in solving the energy crisis but remains a big challenge. Tri-metallic metal-organic frameworks show a good application prospect in water oxidation. In this review, we are going to focus on the latest progress and future trends in the development of pristine trimetallic MOFs with respect to the OER. The synergistic effect between multi-metal active sites is effective at improving the intrinsic activity of MOFs toward the OER. By summarizing the synthesis method of tri-metallic MOFs and observing their performance toward the oxygen evolution reaction, we hope that this review will trigger new developments in coordination chemistry, electrochemistry, nanomaterials and energy materials.

Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction

Urea oxidation reaction (UOR) is the underlying reaction that determines the performance of modern urea-based energy conversion technologies. These technologies include electrocatalytic and photoelectrochemical urea splitting for hydrogen production and direct urea fuel cells as power engines. They have demonstrated great potentials as alternatives to current water splitting and hydrogen fuel cell systems with more favorable operating conditions and cost effectiveness. At the moment, UOR performance is mainly limited by the 6-electron transfer process. In this case, various material design and synthesis strategies have recently been reported to produce highly efficient UOR catalysts. The performance of these advanced catalysts is optimized by the modification of their structural and chemical properties, including porosity development, heterostructure construction, defect engineering, surface functionalization, and electronic structure modulation. Considering the rich progress in this field, the recent advances in the design and synthesis of UOR catalysts for urea electrolysis, photoelectrochemical urea splitting, and direct urea fuel cells are reviewed here. Particular attention is paid to those design concepts, which specifically target the characteristics of urea molecules. Moreover, challenges and prospects for the future development of urea-based energy conversion technologies and corresponding catalysts are also discussed.

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.

Controllable synthesis of a mesoporous NiO/Ni nanorod as an excellent catalyst for urea electro-oxidation

Mesoporous rod-like structured composites of NiO/Ni have been successfully prepared via a low temperature heat treatment of the precursor NiC₂O₄·2H₂O in N₂.

Coral-like hierarchical architecture self-assembled by cobalt hexacyanoferrate nanocrystals and N-doped carbon nanoplatelets as efficient electrocatalyst for oxygen evolution reaction

It is challenging to develop novel oxygen evolution reaction (OER) electrocatalysts with high performance and low cost to replace the noble metal-based catalysts for large-scale electrochemical water splitting. To settle such issue, herein, self-assembled porous coral-like architecture constructed by cobalt hexacyanoferrate (CoHCF) nanocrystals and nitrogen-doped carbon (NC) nanoplatelets network is fabricated for the first time by a facile electroless deposition approach. The porous coral-like CoHCF/NC hybrid exhibits an excellent OER electrocatalytic activity in alkaline medium with an ultra-low onset overpotential of 165 mV (vs. RHE) and a small Tafel slope of 73.97 mV dec^-1, which are much lower than that of bare CoHCF (onset overpotential of 296 mV and Tafel slope of 113.25 mV dec^-1); it also exhibits a lower overpotential of 357 mV (vs. RHE) at current density of 10 mA cm^-1 and superior durability even after 16 h. The excellent electrocatalytic performance of CoHCF/NC hybrid can be assigned to its unique coral-like architecture self-assembled by CoHCF nanocrystals and NC nanoplatelets network, which significantly increases the electrochemical active surface area and remarkably facilitates the electron and ion transfer. This work offers rational design and facile synthesis strategy for transition metal hexacyanoferrate-based nonprecious electrocatalysts with unique nano-architecture and excellent electrocatalytic efficiency towards OER.