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The excessive cost and scarcity of the noble-metal catalysts needed for catalyzing the bifunctional oxygen evolution and reduction reactions (OER and ORR) for metal-air batteries limit the commercialization of clean-energy technologies. In this work, gadolinium-2-methylimidazole (Gd-2-mim) metal–organic framework-functionalized 2D-graphitic carbon nitride (2D-g-C3N4) heterostructure (Gd-2-mim/2D-g-C3N4) as a bifunctional electrocatalyst is demonstrated for OER and ORR. The electrocatalyst is synthesized through mixing and coprecipitation methods. The structural, interfacial, thermal, and microstructural properties of the synthesized heterostructure are revealed through several characterizations. A stable nanoporous morphology with improved electrocatalytic active sites and enhanced specific surface area of Gd-2-mim/2D-g-C3N4exhibits the highest onset potential (0.85 V) and lowest charge transfer resistance (Rct= 96.2 Ω) for ORR and lowest overpotential (59 mV @10 mA cm–2) for OER with the highest turnover frequency (TOF = 0.42 s–1). A plausible bifunctional mechanism is portrayed based on the electrocatalytic reaction kinetics. The catalyst is capable of substituting the state-of-the-art electrocatalysts Pt/C and RuO2for ORR and OER respectively.

Earth-abundant transition-metal-based metal–organic frameworks (MOFs) are of immense interest for the development of efficient and durable heterogeneous water splitting electrocatalysts. This repot explores the design of two-dimensional (2D) MOFs with redox-active metal centers (Ni(II), Co(II), and Cu(II)) containing two types of electron-rich linkers such as bis(5-azabenzimidazole), linear L1and angular L2, and aromatic dicarboxylates. The electron-rich linkers are considered to stabilize the higher oxidation state of the redox-active metal centers in the course of the electrocatalytic oxygen evolution reaction (OER) process. The 2D MOFs of L1and L2with Co(II) (1and 3) and Ni(II) (2and 4) have been produced viathe conventional hydrothermal synthesis, while the MOFs of Cu(II) (Cu@1and Cu@3) are obtained by the postsynthetic transmetallation reaction of MOFs 1and 3. The electrocatalytic OER activities of the six MOFs have been studied to explore the influence of the redox potential of the transition-metal quasi-reversible couples and the coordination environment around the redox-active metal centers in the electrocatalytic activity. The lowest overpotential of 370 mV exhibited by MOF 2with the highest current density and TOF value indicates the importance of the presence of coordinated water molecules and the lowest redox potential value of the most favorable quasi-reversible couple Ni+2/Ni+3. These catalysts exhibit a remarkable stability up to 1000 OER cycles. These studies pave the way for the design of MOF materials toward the development of a promising heterogeneous OER electrocatalyst.

With the rapid advancement of electronic technology, the necessity for efficient electrode materials has been tremendously increasing. With this endeavor, the design of cost-saving, environment-friendly, and stable electrocatalysts from earth-abundant transition elements for water splitting is immensely desired. To this contribution, herein, we have aimed to design two stable Co(II)- and Ni(II)-containing redox-active metal–organic frameworks (Co-MOFand Ni-MOF) with a nitrogen-rich linear bis(azabenzimidazole) ligand and electron-rich dicarboxylic acids and explored the redox-active metal–organic frameworks (MOFs) as efficient heterogeneous catalysts for oxygen evolution reaction (OER) and CO2sorption studies. The structural analysis of Co-MOFshows that it exhibits a double-walled square grid-type two-dimensional structure, whereas Ni-MOFexhibits a rectangular grid-type two-dimensional layered structure. Further, three-dimensional packing of both MOFs is formed by two-dimensional (2D) metal–ligand coordination layers via N–H···O hydrogen bonding interactions with the imidazole moiety and solvent molecules, resulting in one-dimensional cavities. These cavities have void spaces of 27.6 and 33.2% for Co-MOFand Ni-MOF, respectively, which are occupied by water molecules in Co-MOFand dimethylformamide (DMF) molecules in Ni-MOF. Electrochemical studies of these pristine MOFs infer that both MOFs display efficient OER activity in alkaline solutions. These MOFs demand overpotentials of 470 and 450 mV to generate a current density of 10 mA/cm2, which is nearly equivalent to 10% solar energy conversion efficiency. The 2D layers with nitrogen-rich ligands and oxygen-rich linkers are expected to promote better diffusion of electrons and protons in the course of the electrocatalytic process. The presence of hydrophilic channels further promoted to adsorb considerable amounts of CO2gas (46 and 15 cm3/g, respectively) at 273 K and 1 bar pressure.

The design of highly efficient, cost-effective non-noble metal-based electrocatalysts with superior stability for overall water splitting (OWS) reactions is of great importance as well as of immense challenge for the upcoming sustainable and green energy conversion technologies. Herein, a convenient and simple in situsolvothermal method has been adopted to fabricate a self-supported, binder-free 3D electrode (Mn-MOF/NF) by the direct growth of a newly synthesized carboxylate-based pristine Mn(II)-metal–organic framework (Mn-MOF) upon the conducting substrate nickel foam (NF). The binder-free Mn-MOF/NFelectrode exhibits excellent performances toward OWS with ultralow overpotentials of 280 mV@20 mA cm–2for the oxygen evolution reaction (OER) and 125 mV@10 mA cm–2for the hydrogen evolution reaction (HER) with remarkable durability. Mn-MOF/NFcan also attain a current density of 10 mA cm–2with a low cell voltage of 1.68 V in a 0.1 M KOH solution in a two-electrode system for OWS. The direct growth of nonconducting electroactive Mn-MOFmaterials upon conducting substrate NF provides an excellent mass transport of the electrolyte with a relatively low contact resistance due to the strong catalyst–substrate contact and enhances the efficient electron transport for OWS. The redox chemical etching of the self-sacrificial substrate NF during solvothermal synthesis introduces redox-active Ni2+in Mn-MOF/NF. Thus, the excellent OWS electrocatalytic activity can mainly be attributed to the bimetallic synergistic collaboration of the two redox active metal centers (Mn2+and Ni2+) along with the excellent support surface of NF, which provides a high specific surface area and maximum utilization of the electroactive metal ion sites by preventing the self-aggregation of the active sites. The Mn-MOF/NFelectrode also exhibits superb stability and durability for a prolonged time throughout the multiple cycles of full water splitting reactions. Therefore, this work elucidates a convenient and smart approach for constructing MOF-based bifunctional electrocatalysts for OWS.