Iron-Salen Complex and Co2+ Ion-Derived Cobalt-Iron Hydroxide/Carbon Nanohybrid as an Efficient Oxygen Evolution Electrocatalyst

The Role of Metal-Organic Framework Induced Confinement Effects on Molecular Electrocatalysts Relevant to the Energy Transition

Metal organic frameworks (MOFs) are promising materials for (electro)catalysis as they can improve stability, reusability, and catalytic current densities of molecular catalysts, thereby combining the advantages of homogeneous- and heterogeneous catalysts. However, much is unknown about the effects of confinement of a catalyst within an MOF on the overall catalytic behavior. The performance of a series of electrocatalysts confined in MOFs is compared to that of the corresponding homogeneous catalysts to evaluate to what extend the catalytic site is affected by confinement in terms of stability, activity, and selectivity. Together the examples discuss depict what happens to a catalyst when it is incorporated into an MOF, and recommendations are made on how to evaluate the electrochemical activity of an MOF in a way that allows for description of such confinement effects on the catalyst performance. It is noted that the limiting factor for the catalytic reaction in MOFs is found in 1) slow electron transport, 2) slow mass transport of reactants and products, or 3) a low activity of the catalytic site itself. Understanding the changes in mass- and electron transport and the resulting effects on catalytic mechanism is essential to be able to bring MOF systems to practical applications.

A facile and efficient method for preparing La-doped Co3O4 by electrodeposition as an efficient air cathode in rechargeable zinc-air batteries: Role of oxygen vacancies

A rechargeable zinc-air battery (ZAB) is a promising candidate for simple and low-cost energy storage systems. However, preparing the air cathode material using a binder-free method and a bifunctional catalyst is still the major challenge in the field. Herein, we demonstrate the effect of different La contents doped into the Co3O4 spinel structure in the presence of oxygen vacancies prepared by a facile and efficient electrodeposition technique on the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and ZAB performance. Incorporating the La dopant into the Co3O4 improves the OER and ORR performances and thus enhances the specific capacity and energy density of ZAB. The optimal La-doping amount in the CoLa-1 catalyst demonstrates high feasibility for practical application with a capacity of 780 ± 24 mAh/g and an energy density of 901 ± 39 mW g-1, significantly outperforming the pristine Co3O4. The stability and cycling tests reveal good durability performance after 300 cycles and 100 h of testing without degradation, which is much more stable than the benchmark Pt/C + RuO2 electrode. The performance enhancement is attributed to the synergetic effect of high active surface area, low charge transfer resistance, and optimal oxygen vacancies. A kinetic micromechanism is proposed to illustrate the importance of the oxygen vacancy amount in trapping oxygen gas and maximizing the number of ORR and OER reactions.

Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 μmol g-1  h-1 ) and H2 (434.7 μmol g-1  h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

Intensive adsorption of tetracycline by cobalt oxide quantum dots-loaded mineral carbon

Spent bleaching earth (SBE), a waste by-product produced from the bleaching step of edible oil by montmorillonite clays (bleaching earth), causes serious public health and environmental problems. Accordingly, in this study, SBE was pyrolyzed to yield mineral carbon materials (SBE@C) and cobalt oxide (Co3O4) was loaded to improve the active site of those materials. Due to the carrier function of SBE@C, ultra-fine Co3O4 quantum dots (QDs) (2-6 nm) were homogeneously and robustly immobilized onto SBE@C. The obtained adsorbent exhibited high regeneration performance and an outstanding adsorption capacity (253.36 mg/g). It can be attributed to the surface complexation of cobalt with TC being the dominant process contributing to adsorption behavior. Further, Co3O4 QDs-SBE@C still maintained adequate sorption capacity at a broad range of pH values and in the presence of co-occurring ions. These results suggested the significant application potential of SBE and demonstrated the efficiency of using Co3O4 QDs-SBE@C for wastewater remediation.

Highly porous Co-doped NiO nanorods: facile hydrothermal synthesis and electrocatalytic oxygen evolution properties

Highly porous 3d transition metal oxide nanostructures are opening up the exciting area of oxygen evolution reaction (OER) catalysts in alkaline medium thanks to their good thermal and chemical stability, excellent physiochemical properties, high specific surface area and abundant nanopores. In this paper, highly porous Co-doped NiO nanorods were successfully synthesized by a simple hydrothermal method. The porous rod-like nanostructures were preserved with the added cobalt dopant ranging from 1 to 5 at% but were broken into aggregated nanoparticles at higher concentrations of additional cobalt. The catalytic activity of Co-doped NiO nanostructures for OER in an alkaline medium was assayed. The 5%Co-NiO sample showed a drastically enhanced activity. This result could originate from the combination of advantageous characteristics of highly porous NiO nanorods such as large surface area and high porosity as well as the important role of Co dopant that could provide more catalytic active sites, leading to an enhanced catalytic activity of the nanocatalyst.

Engineering single-atomic ruthenium catalytic sites on defective nickel-iron layered double hydroxide for overall water splitting

Rational design of single atom catalyst is critical for efficient sustainable energy conversion. However, the atomic-level control of active sites is essential for electrocatalytic materials in alkaline electrolyte. Moreover, well-defined surface structures lead to in-depth understanding of catalytic mechanisms. Herein, we report a single-atomic-site ruthenium stabilized on defective nickel-iron layered double hydroxide nanosheets (Ru₁/D-NiFe LDH). Under precise regulation of local coordination environments of catalytically active sites and the existence of the defects, Ru₁/D-NiFe LDH delivers an ultralow overpotential of 18 mV at 10 mA cm⁻² for hydrogen evolution reaction, surpassing the commercial Pt/C catalyst. Density functional theory calculations reveal that Ru₁/D-NiFe LDH optimizes the adsorption energies of intermediates for hydrogen evolution reaction and promotes the O–O coupling at a Ru–O active site for oxygen evolution reaction. The Ru₁/D-NiFe LDH as an ideal model reveals superior water splitting performance with potential for the development of promising water-alkali electrocatalysts.

Rational design of single atom catalyst is critical for efficient sustainable energy conversion. Single-atomic-site ruthenium stabilized on defective nickel-iron layered double hydroxide nanosheets achieve superior HER and OER performance in alkaline media.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Heptanuclear brucite disk with cyanide bridges in a cocrystal and tracking its pyrolysis to an efficient oxygen evolution electrode

The development of efficient oxygen evolution reaction (OER) catalysts is still lacking in exploration of the mechanism of controlled pyrolysis of precursors among new material platforms. Here, a novel Co-based coordination molecular cluster has been first introduced as precursor to obtain metallic cobalt core shelled by N-doped carbon (Co@NC) structure which operates as an oxygen evolution electrode. Specifically, a new cocrystal compound, [Co^II₇(μ₃-CN)₆(mmimp)₆] [Co^IICl₃N(CN)₂]·3CH₃OH (Co₇₊₁, mmimp = 2-methoxy-6-((methylimino)-methyl)phenol), was isolated consisting of Brucite disks of cobalt where the usual bridging μ₃-OH is replaced by μ₃-CN produced by the in-situ decomposition of dicyanamide (N≡C-N-C≡N^-). The cobalt atoms are bonded through the nitrogen atom of the cyanide. Remarkably, time dependent thermogravimetric-mass spectrometry (TG-MS) analysis was utilized to track its pyrolysis process. It allowed us to propose a possible formation process of the Co@NC structure from Co₇₊₁. Interestingly, an extremely superior OER electrode is optimized for Co@NC-600 having the lowest overpotential of 257 mV at 10 mA/cm² in 1 mol/L KOH solution. The present study pins down the importance of clusters of transition metals on realizing distinct nanostructures operating as highly efficient OER electrocatalyst.