Oxygen Evolution Reaction Kinetics Research Articles

Designing a Phenalenyl-Based Dinuclear Ni(II) Complex: An Electrocatalyst with Two Single Ni Sites for the Oxygen Evolution Reaction (OER).

A new dinuclear Ni(II) complex 1, [Ni2II(dtbh-PLY)2], is synthesized from 9-(2-(3,6-di-tert-butyl-2-hydroxybenzylidene)hydrazineyl)-1H-phenalen-1-one, dtbh-PLYH2 ligand, and structurally characterized by various analytical tools including the single-crystal X-ray diffraction (SCXRD) technique. In the solid state, both Ni(II) metal centers in complex 1 exist in a distorted square planar geometry and display the presence of rare Ni···H-C anagostic interactions to form a one-dimensional (1-D) linear motif in the supramolecular array. Complex 1 is further stabilized in the solid state by π-π-stacking interactions between the highly delocalized phenalenyl rings. The redox features of complex 1 have been analyzed by the cyclic voltammetry (CV) technique in solution as well as in the solid state, revealing the crucial involvement of both the Ni(II) metal centers for undergoing quasi-reversible oxidation reactions on the application of an anodic sweep. A complex 1-modified glassy carbon electrode, GC-1, is employed as an electrocatalyst for oxygen evolution reaction (OER) in 1.0 M KOH, giving an OER onset at 1.45 V, and very low OER overpotential, 300 mV vs the reversible hydrogen electrode (RHE) to reach 10 mA cm-2 current density. Furthermore, GC-1 displayed fast OER kinetics with a Tafel slope of 40 mV dec-1, a significantly lower Tafel slope value than those of previously reported molecular Ni(II) catalysts. In situ electrochemical experiments and postoperational UV-vis, Fourier transform infrared (FT-IR), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS) studies were performed to analyze the stability of the molecular nature of complex 1 and to gain reasonable insights into the true OER catalyst.

Multifunctional carbon armor: Synchronously improving oxygen reaction kinetics, mass transfer dynamics, and robustness of transition metal alloy based hybrid catalyst

Construction of robust protective cover on delicate active sites is a frequently-used scheme to enhance the durability of air-cathode catalyst working in harsh environment, thus the lifespan of rechargeable Zinc-air batteries (ZABs). Paradoxically, this would degrade the activity due to the constricted accessibility of active sites to reactants. Herein, carbon nanotubes with abundant mesoporous defects and Fe-N4 species were elaborately designed to be multifunctional protective armor to encapsulate Ni3Fe nano-alloys (Ni3Fe@CNTs/Fe-N4) for bifunctional oxygen electrocatalyst. In/ex-situ spectroscopy analysis and theoretical calculations reveal that the constructed mesoporous carbon defects effectively facilitate the multiphase mass transfer and the oxygen evolution reaction kinetics via strong electrons coupling with packaged Ni3Fe nano-alloys. Meanwhile, the synchronously introduced Fe-N4 moieties could serve as oxygen reduction active sites. Thus, the obtained Ni3Fe@CNTs/Fe-N4 hybrid electrocatalyst simultaneously exhibits remarkable bifunctional catalytic activity (E1/2=0.86/Ej=10=1.59 V vs RHE) and durability over 450 h in the chronoamperometric test at 1.59 V, endowing the assembled Zn-air batteries (ZABs) with a high power density (150 mW cm−2) and lifespan (307 h), much better than that employing benchmark Pt/C+RuO2 mixed catalyst. This work demonstrates an innovative design route for the multifunctional armor to concurrently enhance the durability, activity and robustness of air-cathode-catalyst for ZABs.

Shock-endurable and reversible evolution between CoOOH and intermediate governed by interfacial strain for fluctuating oxygen evolution

Transition metal oxide usually suffers from reconstruction into metal oxyhydroxide during oxygen evolution reaction (OER), while irreversible evolution between metal oxyhydroxides and intermediate species cannot agilely response to the fluctuating power of renewable energy. Herein, an individual CoMoxO4 phase with a tunable Co/Mo stoichiometry is reconstructed into CoOOH skin as active material for OER. Compressive strain induced by CoMo1.14O4/CoOOH interface, downshifting the d-band center of CoOOH, is an effective strategy to not only accelerate the sluggish kinetics of OER, but also enable inspiring the expeditious evolution and the reversibility between CoOOH and intermediate species. The reversibility is an applicable demand for OER in case of fluctuating potential. Reaction current on CoMo1.14O4 at constant potential is demonstrated by the lower fluctuation of 1.7 %, relative to 13.8 % in CoMoO4 after 12 cycles. Meanwhile, an anion exchange membrane water splitting electrolyzer made of CoMo1.14O4 anode can endure current shock of 50 ∼ 800 mA cm−2 in 0.2 s, which has negligible decay at 800 mA cm−2 after 9 shocks. Designing strategy of interfacial strain modulated by stoichiometry can be universally extended into the other oxide electrocatalysts including NiMoO4 and their derivatives.

Self-Supported WO3@RuO2 Nanowires for Electrocatalytic Acidic Water Oxidation.

Developing catalysts with high catalytic activity and stability in acidic media is crucial for advancing hydrogen production in proton exchange membrane water electrolyzers (PEMWEs). To this end, a self-supported WO3@RuO2 nanowire structure was grown in situ on a titanium mesh using hydrothermal and ion-exchange methods. Despite a Ru loading of only 0.098 wt %, it achieves an overpotential of 246 mV for the oxygen evolution reaction (OER) at a current density of 10 mA·cm-2 in acidic 0.5 M H2SO4 while maintaining excellent stability over 50 h, much better than that of the commercial RuO2. After the establishment of the WO3@RuO2 heterostructure, a reduced overpotential of the rate-determining step from M-O* to M-OOH* is confirmed by the DFT calculation. Meanwhile, its enhanced OER kinetics are also greatly improved by this self-supported system in the absence of the organic binder, leading to a reduced interface resistance between active sites and electrolytes. This work presents a promising approach to minimize the use of noble metals for large-scale PEMWE applications.

γ-MnO2 as an Electron Reservoir for RuO2 Oxygen Evolution Catalyst in Acidic Media.

Developing highly active and durable catalysts in acid conditions remains an urgent issue due to the sluggish kinetics of oxygen evolution reaction (OER). Although RuO2 has been a state-of-the-art commercial catalyst for OER, it encounters poor stability and high cost. In this study, the electronic reservoir regulation strategy is proposed to promote the performance of acidic water oxidation via constructing a RuO2/MnO2 heterostructure supported on carbon cloth (CC) (abbreviated as RuO2/MnO2/CC). Theoretical and experimental results reveal that MnO2 acts as an electron reservoir for RuO2. It facilitates electron transfer from RuO2, enhancing its activity prior to OER, and donates electrons to RuO2, improving its stability after OER. Consequently, RuO2/MnO2/CC exhibits better performance compared to commercial RuO2, with an ultrasmall overpotential of 189mV at 10mA cm-2 and no signs of deactivation even after 800h of electrolysis in 0.5m H2SO4 at 10mA cm-2. When applied as the anode in a proton exchange membrane water electrolyzer, the cost-efficient RuO2/MnO2/CC catalyst only requires a cell voltage of 1.661V to achieve the water-splitting current of 1 A cm-2, and the noble metal cost is as low as US$ 0.00962 cm-2, indicating potential for practical applications.

Specific roles of Fe in NiFe-MOF nanosheet arrays on oxygen evolution reaction kinetics in alkaline media

Although great progress has been made in the study of the performance of NiFe-based electrocatalysts, the specific role of Fe is still under debate, largely due to the lack of desirable platforms. Metal-organic frameworks (MOFs) with uniform pore size, large specific surface area and uniform distribution of active sites are ideal platforms. Therefore, we synthesized a series of NiM-MOF (M = Fe, Co, Mn, Cu, or Zn) nanosheet arrays by solvothermal method with MOF as a platform and systematically investigated their OER properties. XRD and XPS characterizations show that the introduction of Fe into Ni-MOF not only leads to lower crystallinity, but also increases the oxidation state of Fe and modifies the electronic structure of Ni sites. This is further confirmed by DFT calculations, where the d-band center is closer to the Fermi level, enhancing the ability to adsorb/desorb oxygen-containing intermediates and thereby improving the intrinsic activity of NiFe-MOF.

Amorphous/Crystalline Phases Mixed Nanosheets Array Rich in Oxygen Vacancies Boost Oxygen Evolution Reaction of Spinel Oxides in Alkaline Media.

As promising oxygen evolution reaction (OER) catalysts, spinel-type oxides face the bottleneck of weak adsorption for oxygen-containing intermediates, so it is challenging to make a further breakthrough in remarkably lowering the OER overpotential. In this study, a novel strategy is proposed to substantially enhance the OER activity of spinel oxides based on amorphous/crystalline phases mixed spinel FeNi2O4 nanosheets array, enriched with oxygen vacancies, in situ grown on a nickel foam (NF). This unique architecture is achieved through a one-step millisecond laser direct writing method. The presence of amorphous phases with abundant oxygen vacancies significantly enhances the adsorption of oxygen-containing intermediates and changes the rate-determining step from OH*→O* to O*→OOH*, which greatly reduces the thermodynamic energy barrier. Moreover, the crystalline phase interweaving with amorphous domains serves as a conductive shortcut to facilitate rapid electron transfer from active sites in the amorphous domain to NF, guaranteeing fast OER kinetics. Such an anodic electrode exhibits a nearly ten fold enhancement in OER intrinsic activity compared to the pristine counterpart. Remarkably, it demonstrates record-low overpotentials of 246 and 315 mV at 50 and 500 mA cm-2 in 1 m KOH with superior long-term stability, outperforming other NiFe-based spinel oxides catalysts.

Enhancing the oxygen evolution activity and stability of Pb anode in Mn2+-containing acidic solution by embedding MnCo2O4 particles

Due to the excellent activity and stability of MnCo2O4 toward the oxygen evolution reaction (OER) in acidic solution, a Pb–MnCo2O4 composite anode for zinc electrowinning was prepared by embedding dispersed MnCo2O4 particles into a Pb matrix in a powder metallurgy process. In this work, the phase structure, chemical composition, and morphology of the oxide layers formed on Pure-Pb and Pb–MnCo2O4 were analyzed by XRD, SEM, and EDS. Galvanostatic polorization, Tafel tests, and EIS measurements were performed to investigate the anodic potential variation and OER kinetics of the Pure-Pb and Pb–MnCo2O4 composite anodes. Compared with that on the Pure-Pb anode, the oxide layer on Pb–MnCo2O4 is thinner, more compact, and more stable in a 160 g L−1 H2SO4 solution containing 4 g L−1 Mn2+. Consequently, the Pb–MnCo2O4 composite anode exhibited a much lower anode slime production (8.7 mg) during 72 h of the simulated zinc electrowinning process. Despite the smaller surface area and lower PbO2 content of the oxide layer, the Pb–MnCo2O4 composite anode presented preferable OER kinetics with a lower OER charge transfer resistance (0.729 Ω cm2) and a smaller Tafel slope (90.74 mV dec−1), which contributed to a 70 mV reduction in the anodic potential compared with that of the Pure-Pb anode.

Direct solar energy conversion on zinc–air battery

A concept of solar energy convertible zinc-air battery (SZAB) is demonstrated through rational design of an electrode coupled with multifunction. The multifunctional electrode is fabricated using nitrogen-substituted graphdiyne (N-GDY) with large π-conjugated carbonous network, which can work as photoresponsive bifunctional electrocatalyst, enabling a sunlight-promoted process through efficient injection of photoelectrons into the conduction band of N-GDY. SZAB enables direct conversion and storage of solar energy during the charging process. Such a battery exhibits a lowered charge voltage under illumination, corresponding to a high energy efficiency of 90.4% and electric energy saving of 30.3%. The battery can display a power conversion efficiency as high as 1.02%. Density functional theory calculations reveal that the photopromoted oxygen evolution reaction kinetics originates from the transition from the alkyne bonds to double bonds caused by the transfer of excited electrons, which changes the position of highest occupied molecular orbital and lowest unoccupied molecular orbital, thus greatly promoting the formation of intermediates to the conversion process. Our findings provide conceptual and experimental confirmation that batteries are charged directly from solar energy without the external solar cells, providing a way to manufacture future energy devices.

Magnetic field-assisted water splitting at ternary NiCoFe magnetic Nanocatalysts: Optimization study

This study addresses the enhanced splitting of water into H2 and O2 assisted with applied magnetic field at ternary NiCoFe catalyst. The catalyst's activity towards water electrolysis is enhanced in the presence of AC-induced magnetic field (B = 720 μT) and approaches the performance of Pt/C∥IrO2. That is the overpotential at a current density of 10 mA cm−2 (η10) is reduced by 90 mV and 210 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Besides, a significant reduction in the cell voltage of 0.3 V and a power saving of 215.6 kWh during the production of one kg of H2 are obtained when AC magnetic field of 720 μT is applied. A markedly high stability of the catalyst is obtained under magnetic field during a prolonged electrolysis. The magnetohydrodynamic effect is believed to drive water electrolysis at NiCoFe film by reducing the ohmic potential drop across the electrode/solution interface, and increases the rate of charge transfer at the catalyst surface. The results are explained by capacitance measurements revealing maximum capacitance of NiFeCo catalyst in the presence of AC magnetic flux of 720 μT. Thus, favourable adsorption conditions lead to accelerated kinetics of OER and HER.

Atomic Strain Wave-Featured LaRuIr Nanocrystals: Achieving Simultaneous Enhancement of Catalytic Activity and Stability toward Acidic Water Splitting.

Rare earth microalloying nanocrystals have gotten widespread attention due to their unprecedented performances with customization-defected nanostructures, divided energy bands, and ensembled surface chemistry, regarded as a class of ideal electrocatalysts for oxygen evolution reaction (OER). Herein, a lanthanide microalloying strategy is proposed to fabricate strain wave-featured LaRuIr nanocrystals with oxide skin through a rapid crystal nucleation, using thermally assisted sodium borohydride reduction in aqueous solution at 60°C. The atomic strain waves with alternating compressive and tensile strains, resulting from La-stabilized edge dislocations in form of Cottrell atmospheres. In 0.5m H2SO4, the LaRuIr displays an overpotential of 184mV at 10mA cm-2, running at a steadily cell voltage for 60h at 50mA cm-2, eightfold enhancement of IrO2||Pt/C assemble in PEMWE. The coupled compressive and tensile profiles boost the OER kinetics via faster AEM and LOM pathways. Moreover, the tensile facilitates surface structure stabilization through dynamic refilling of lattice oxygen vacancies by the adsorbed oxyanions on La, Ru, and Ir sites, eventually achieving a long-term stability. This work contributes to developing advanced catalysts with unique strain to realize simultaneous improvement of activity and durability by breaking the so-called seesaw relationship between them during OER for water splitting.

Role of Electrolyte pH on Water Oxidation for Iridium Oxides.

Understanding the effect of noncovalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy (XAS), and surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the effect of noncovalent interactions on the oxygen evolution reaction (OER) activity of IrOx in acidic and alkaline electrolytes. Our results suggest that the active species for the OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species are higher in alkaline electrolytes. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilize oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolytes, the larger repulsive interaction between them results in a significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of the OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.

Eco-friendly high entropy oxide rock-salt type structure for oxygen evolution reaction obtained by green synthesis

Global energy consumption increases year after year, causing the depletion of non-renewable sources. According to the International Energy Agency (IEA), global demand for electrical energy is expected to increase by 3.3 % in 2024. Therefore, developing new renewable sources is urgent, including new devices for energy storage and conversion, particularly those based on electrochemical reactions. Water splitting is a clean and sustainable technology capable of facing this issue by producing oxygen and hydrogen from water and electricity. However, an issue related to this technology is the slow kinetics of oxygen evolution reaction, making it necessary to develop new electrocatalysts with high electrochemical performance. To meet this requirement, this work deals, for the first time, with a high entropy oxide with a rock-salt structure synthesized by a green sol–gel synthesis using red seaweed (Rhodophyta) as a polymerizing agent. Sol-gel synthesis allows the large-scale production of nanomaterials with high uniformity and dispersion of the involved chemical elements. The literature, which discussed the synthesis of these oxides, reveals that agents harmful to the environment are employed, including sodium hydroxide, acetic acid, hexadecyltrimethylammonium bromide, urea, and ammonium hydroxide. The composition of the high entropy oxide is (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O. As electrocatalyst for oxygen evolution reaction, it exhibits a low overpotential (336 mV vs. RHE at 10 mA cm−2), a Tafel slope of 68 mV dec-1, and excellent durability. The electrochemical performance of the high entropy oxide prepared in this work is superior to other electrocatalysts of the same class that were produced using transition metal-based precursors.

Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates.

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

Collaborative role of co-doping and surface modifying for adjusting surface state of hematite to release the photoelectrochemical activity

Inhibition of bulk and interfacial charge carrier recombination is an urgent solution to release furthest the photoelectrochemical potential abilities of hematite photoelectrode. Herein, the Zr-Y dual metal co-doping engineering and Co/Fe dual metal-organic frameworks cocatalyst layer were employed for modifying hematite photoanode. The novel CoFe-MOF/Zr-Y:Fe2O3 nanoarray composite photoanode exhibits higher PEC activity, the photocurrent density at 1.23 V vs.RHE is increased by 4.7 times than that of the pristine α-Fe2O3. The gain in bulk charge separation efficiency was mainly attributed to the synergistic effect between the two doping ions to improve the electrical conductivity. Wherein, Zr4+ doping endows more free electrons in the bulk α-Fe2O3 and Y3+ introduction causes polaron mobility enhancement. For another, co-doping could reduce the recombination surface state, but form the intermediate surface state to promote hole transfer, thus effectively inhibiting the severe recombination of photogenerated electrons and holes. Furthermore, the CoFe-MOF cocatalyst coating with abundant active sites not only expedites the oxygen evolution reaction kinetics process, but also passivates the surface hole trapping state more thoroughly, resulting in efficient interfacial hole transfer and utilization. This work affords a new collaborative role of co-doping and surface modifying effective avenue for adjusting surface state of hematite to release the photoelectrochemical activity.

Lattice contraction-driven design of highly efficient and stable O–NiFe layered double hydroxide electrocatalysts for water oxidation

The slow progress of the oxygen evolution reaction (OER) is a key challenge in advancing water splitting as a viable technology for sustainable hydrogen production. Utilizing a self-standing NiFe layered double hydroxide (LDH) is a promising approach to address the slow kinetics of OER. Herein, the oxidized Fe2+-containing NiFe-LDH supported on Ni foam (O–NiFe-LDH@NF) is developed through corrosion engineering and aging. The growth mechanism is studied by adjusting urea content, hydrothermal temperature and time. It is revealed that the transformation of Fe2+ to Fe3+ in Fe2+-containing NiFe-LDH and the increase of Fe content leads to lattice contraction. Furthermore, detailed experiments and theoretical simulations illustrate that the substantial Fe content and lattice shrinkage promote the generation of Ni in high oxidation state and enhance the adsorption affinity toward the reaction species. Consequently, O–NiFe-LDH@NF exhibits enhanced OER activity and stability, with an exceptionally low overpotential of 197 mV and 354 mV observed at the current densities of 10 and 500 mA cm−2, respectively, and remarkable stability over 300 h. This approach can be generalized for designing advanced electrocatalysts.

Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.

Improved Oxygen Evolution Reaction Kinetics with Titanium Incorporated Nickel Ferrite for Efficient Anion Exchange Membrane Electrolysis

Improved Oxygen Evolution Reaction Kinetics with Titanium Incorporated Nickel Ferrite for Efficient Anion Exchange Membrane Electrolysis

Electronic structure-modulated NiFe-metal–organic framework nanosheets for enhance electrochemical oxygen Evolution

Oxygen Evolution Reaction (OER) is a critical half-reaction that hinders water decomposition due to its sluggish kinetics. Developing efficient, stable electrocatalysts for OER is crucial for addressing the challenges in energy crisis and green hydrogen production. Compared to single-metal Metal-Organic Frameworks (MOFs), synergistic interactions between metal ions in bimetallic MOFs provide sufficient space for tuning the electronic structure and oxygen-containing intermediate adsorption energies, and therefore bimetallic MOFs shows great potential in enhancing the OER kinetics. In this work, NiFe-MIL-88A nanosheets were synthesized on nickel foam via a facile one-step hydrothermal method. The catalyst exhibited exceptional OER performance in 1.0 M KOH solution. Notably, it achieved a current density of 50 mA cm−2 at a low overpotential of 220 mV and displayed a Tafel slope of 43.2 mV/dec. Furthermore, it had impressive electrocatalytic durability suitable for commercial applications. This work elucidates the performance enhancement of NiFe bimetallic MOF from the perspective of electron migration. In-situ Raman spectroscopy was adopted to investigate the structural reconfiguration of the catalyst to reveal the true catalytic active sites. First-principle calculation demonstrate that introduction of Ni atoms with low electronegativity is conducive to optimizing the catalytic d-band center and ƒe value. This work provides insights for the designing bimetallic MOFs with superior OER performance.

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation.

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.