A nickel (II) PY5 complex as an electrocatalyst for water oxidation

Polyoxometalate-incorporated NiFe-based oxyhydroxides for enhanced oxygen evolution reaction in alkaline media

NiFe-based oxyhydroxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline media, but further enhancing their OER performance remains a significant challenge. Herein, we in situ incorporated polyoxometalates into NiFe oxyhydroxides to form a homogeneous/heterogeneous hybrid material, which induces the electronic interaction between Ni, Fe and Mo sites, as revealed by a variety of characterization experiments and theoretical calculations. The resulting hybrid electrocatalyst delivers a low overpotential of 203 mV at 10 mA cm-2 and a TOF of 2.34 s-1 at 1.53 V in alkaline media. This work presents a critical step towards developing high-performance OER catalysts by constructing metal-POM hybrids.

Structural modification of nickel tetra(thiocyano)corroles during electrochemical water oxidation

In this study, we present two fully characterized nickel tetrathiocyanocorroles, representing a novel class of 3d-metallocorroles. These nickel(II) ions form square planar complexes, exhibiting a d8-electronic configuration. These anionic complexes are stabilized by the electron-withdrawing SCN groups on the bipyrrole unit of the corrole. The reduced aromaticity in these anionic nickel(II) corrole complexes is evidenced by single crystal X-ray diffraction (XRD) data and a markedly altered absorption profile, with stronger Q bands compared to Soret bands. Notably, the UV-Vis and electrochemical data exhibit significant differences from previously reported nickel(II) corrole radical cation and nickel(II) porphyrin complexes. While these electrochemical data bear a resemblance to those of the anionic nickel(II) corrole by Gross et al., the UV-Vis data show substantial distinctions. Additionally, we explore the utilization of nickel(II)-corrole@CC (where CC denotes carbon cloth) as an electrocatalyst for the oxygen evolution reaction (OER) in an alkaline medium. During electrochemical water oxidation, the molecular catalyst is partially converted to nickel (oxy)hydroxide, Ni(O)OH. The structure reveals the coexistence of the molecular complex and Ni(O)OH in the active catalyst, achieving a turnover frequency (TOF) of 3.32 × 10-2 s-1. The synergy between the homogeneous and heterogeneous phases improves the OER activity, providing more active sites and edge sites and enhancing interfacial charge transfer.

Efficiency Conceptualization Model: A Theoretical Method for Predicting the Turnover of Catalysts

In recent times, the theoretical prediction of catalytic efficiency is of utmost urgency. With the advent of density functional theory (DFT), reliable computations can delineate a quantitative aspect of the study. To this state-of-the-art approach, valuable incorporation would be a tool that can acknowledge the efficiency of a catalyst. In the current work, we developed the efficiency conceptualization model (ECM) that utilizes the quantum mechanical tool to achieve efficiency in terms of turnover frequency (TOF). Twenty-six experimentally designed transition metal (TM) water oxidation catalysts were chosen under similar experimental conditions of temperature, pressure, and pH to execute the same. The computations conclude that the Fe-based [Fe(OTf)2(Me2Pytacn)] (MWOC-17) is a highly active catalyst and, therefore, can endure for more time in the catalytic cycle. Our results conclude that the Ir-based catalysts [Cp*Ir(κ2-N,O)X] with MWOC-23: X=Cl; and MWOC-24: X=NO3 report the highest computed turnover numbers (TONs),τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ of 406 and 490 against the highest experimental TONs,τ e x p e r i m e n t a l T O N ${\tau _{experimental\;TON} }$ of 1200 and 2000 respectively, whereas the Co-based [Co(12-TMC)]2+ (MWOC-19) has the lowest TONs (τ c o m p u t e d T O N 0 ${\tau _{computed\;TON}^0 }$ =19, τexperimental TON=16) among the chosen catalysts and thereby successful in corroborating the previous experimental results.

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.

Improvement of electrocatalytic water oxidation activity of novel copper complex by modulating the axial coordination of phosphate on metal center

The structure of organic ligand scaffolds of copper complexes critically affects their electrocatalytic properties toward water oxidation, which is widely regarded as the bottleneck of overall water splitting. Herein, two novel mononuclear Cu complexes, [Cu(dmabpy)](ClO4)2 (1, dmabpy = 6,6'-bis(dimethylaminomethyl)-2,2'-bipyridine) and [Cu(mabpy)](ClO4)2 (2, mabpy = 6,6'-bis(methylaminomethyl)-2,2'-bipyridine), with four-coordinated distorted planar quadrilateral geometry were synthesized and explored as efficient catalysts for electrochemical oxygen evolution in phosphate buffer solution. Interestingly, complex 1 with a tertiary amine group catalyzes water oxidation with lower onset overpotential and better catalytic performance, while complex 2 containing a secondary amine fragment displays much lower catalytic activity under identical conditions. The water oxidation catalytic mechanism of the two complexes is proposed based on the electrochemical test results. Experimental methods indicate that phosphate coordinated on the Cu center of the two complexes inhibits their reaction with substrate water molecules, resulting in lower activity toward water oxidation. Electrochemical tests reveal that the structure of the coordinated nitrogen atom improves the catalytic performance of the Cu complexes by modulating the coordination of phosphate on the Cu center, indicating that a minor alteration of the coordinating nitrogen atom of the ligand has a detrimental effect on the catalytic performance of electrochemical WOCs based on transition metal complexes.

Pivotal Role of Geometry Regulation on O-O Bond Formation Mechanism of Bimetallic Water Oxidation Catalysts

In this study, we highlight the impact of catalyst geometry on the formation of O-O bonds in Cu2 and Fe2 catalysts. A series of Cu2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O-O bond formation in Cu2 and Fe2 catalysts. In Cu2 systems, the coupling takes place between a terminal CuIII -OH and a bridging μ-O⋅ radical. Whereas in Fe2 systems, it involves the coupling of two terminal Fe-oxo entities. Furthermore, an in-depth structure-activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ-O⋅ radical, ultimately leading to the O-O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O-O bonding pathway.

O-O bond formation via radical coupling in a dinuclear iron water oxidation catalyst with high catalytic activity

The use of iron-based catalysts for the water oxidation reaction is highly attractive due to the high abundance of iron. While many molecular catalysts have been made, most show limited activity and short lifetimes. An exception with higher activity was presented by Thummel and co-workers in 2015. Herein we present a study on the feasibility of the coupling of two O centered radicals originating from the two subunits of the dinuclear catalyst. The reaction pathway includes the oxidation to the active species FeIV-O-FeIV but avoids further high potential oxidations which previous mechanistic proposals have relied on.

Electrochemical Water Oxidation and CO2 Reduction with a Nickel Molecular Catalyst

Mimicking the photosynthesis of green plants to combine water oxidation with CO2 reduction is of great significance for solving energy and environmental crises. In this context, a trinuclear nickel complex, [NiII3(paoH)6(PhPO3)2]·2ClO4 (1), with a novel structure has been constructed with PhPO32- (phenylphosphonate) and paoH (2-pyridine formaldehyde oxime) ligands and possesses a reflection symmetry with a mirror plane revealed by single-crystal X-ray diffraction. Bulk electrocatalysis demonstrates that complex 1 can homogeneously catalyze water oxidation and CO2 reduction simultaneously. It can catalyze water oxidation at a near-neutral condition of pH = 7.45 with a high TOF of 12.2 s-1, and the Faraday efficiency is as high as 95%. Meanwhile, it also exhibits high electrocatalytic activity for CO2 reduction towards CO with a TOF of 7.84 s-1 in DMF solution. The excellent electrocatalytic performance of the water oxidation and CO2 reduction of complex 1 could be attributed to the two unique µ3-PhPO32- bridges as the crucial factor for stabilizing the trinuclear molecule as well as the proton transformation during the catalytic process, while the oxime groups modulate the electronic structure of the metal centers via π back-bonding. Therefore, apart from the cooperation effect of the three Ni centers for catalysis, simultaneously, the two kinds of ligands in complex 1 can also synergistically coordinate the central metal, thereby significantly promoting its catalytic performance. Complex 1 represents the first nickel molecular electrocatalyst for both water oxidation and CO2 reduction. The findings in this work open an avenue for designing efficient molecular electrocatalysts with peculiar ligands.

Copper Complex Catalyzed Two-Electron and Proton Shuttle Mechanism of O-O Bond Formation from DFT-Based Metadynamics Simulations

We performed first-principles metadynamics simulations to explore the mechanistic pathway of oxygen-oxygen bond formation catalyzed by cis-bis(hydroxo) and cis-(hydroxo)oxo copper complexes. The ligands of considered complexes involve modified bipyridine ligands with oxo and hydroxo groups on 6, 6' positions. The study focuses on the kinetics and thermodynamics of the oxygen-oxygen bond formation. The individual migration of the proton to the hydroxyl group and hydroxide to the oxo and hydroxo moieties of the complexes was examined. The proton transfer requires more kinetic barrier than the hydroxide migration. The nature of the electronic density was analyzed with the help of spin population analysis. The molecular orbitals and natural orbital analysis were carried out to examine the nature of the orbitals involved in the oxygen-oxygen bond formation. The σ*(d_x²_-y²-p_x) molecular orbital of the Cu-O or Cu-OH bond overlaps with the p_z orbital of the hydroxide ion in forming the oxygen-oxygen bond. The two-electron two-centered (2e^--2C) bond is observed in the oxygen-oxygen bond formation. In the oxidation process, these ligands stabilize the electron density from the water or hydroxide ion. These redox-active ligands also help stabilize the formed hydrogen peroxide or peroxide complexes.

Tungsten Disulfide-Interfacing Nickel-Porphyrin For Photo-Enhanced Electrocatalytic Water Oxidation

Covalent functionalization of tungsten disulfide (WS₂ ) with photo- and electro-active nickel-porphyrin (NiP) is reported. Exfoliated WS₂ interfacing NiP moieties with 1,2-dithiolane linkages is assayed in the oxygen evolution reaction under both dark and illuminated conditions. The hybrid material presented, WS₂ -NiP, is fully characterized with complementary spectroscopic, microscopic, and thermal techniques. Standard yet advanced electrochemical techniques, such as linear sweep voltammetry, electrochemical impedance spectroscopy, and calculation of the electrochemically active surface area, are used to delineate the catalytic profile of WS₂ -NiP. In-depth study of thin films with transient photocurrent and photovoltage response assays uncovers photo-enhanced electrocatalytic behavior. The observed photo-enhanced electrocatalytic activity of WS₂ -NiP is attributed to the presence of Ni centers coordinated and stabilized by the N₄ motifs of tetrapyrrole rings at the tethered porphyrin derivative chains, which work as photoreceptors. This pioneering work opens wide routes for water oxidation, further contributing to the development of non-noble metal electrocatalysts.

Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO4 2- anion

The water oxidation reaction is the bottleneck problem of the artificial photosynthetic system. In this work, the mechanism of water oxidation catalyzed by a mononuclear copper complex in alkaline conditions was studied by density functional calculations. Firstly, a water molecule coordinating with the copper center of the complex (Cuii, 1) generates Cuii-H2O (2). 2 undergoes two proton-coupled electron transfer processes to produce intermediate (4). The oxidation process occurs mainly on the ligand moiety, and 4 (˙L-Cuii-O˙) can be described as a Cuii center interacting with a ligand radical antiferromagnetically and an oxyl radical ferromagnetically. 4 is the active species that can trigger O-O bond formation via the water nucleophilic attack mechanism. This process occurs in a step-wise manner. The attacking water transfers one of the protons to the HPO4 2- coupled with an electron transfer to the ligand radical, which generates a transient OH˙ interacting with the oxyl radical and H2PO4 -. Then the O-O bond is formed through the direct coupling of the oxo radical and the OH radical. The triplet di-oxygen could be released after two oxidation processes. According to the Gibbs free energy diagram, the O-O bond formation was suggested to be the rate-limiting step with a calculated total barrier of 19.5 kcal mol-1. More importantly, the copper complex catalyzing water oxidation with the help of a redox non-innocent ligand and HPO4 2- was emphasized.

Three-Electron Two-Centered Bond and Single-Electron Transfer Mechanism of Water Splitting via a Copper-Bipyridine Complex

We report the atomistic and electronic details of the mechanistic pathway of the oxygen-oxygen bond formation catalyzed by a copper-2,2'-bipyridine complex. Density functional theory-based molecular dynamics simulations and enhanced sampling methods were employed for this study. The thermodynamics and electronic structure of the oxygen-oxygen bond formation are presented in this study by considering the cis-bishydroxo, [Cu^III(bpy)(OH)₂]⁺, and cis-(hydroxo)oxo, [Cu^IV(bpy)(OH)(═O)]⁺, complexes as active catalysts. In the cis-bishydroxo complex, the hydroxide transfer requires a higher kinetic barrier than the proton transfer process. In the case of [Cu^IV(bpy)(OH)(═O)]⁺, the proton transfer requires a higher free energy than the hydroxide one. The peroxide bond formation is thermodynamically favorable for the [Cu^IV(bpy)(OH)(═O)]⁺ complex compared with the other. The hydroxide ion is transferred to one of the Cu-OH moieties, and the proton is transferred to the solvent. The free energy barrier for this migration is higher than that for the former transfer. From the analysis of molecular orbitals, it is found that the electron density is primarily present on the water molecules near the active sites in the highest occupied molecular orbital (HOMO) state and lowest unoccupied molecular orbital (LUMO) of the ligands. Natural bond orbital (NBO) analysis reveals the electron transfer process during the oxygen-oxygen bond formation. The σ*Cu(d_xz)-O(p) orbitals are involved in the oxygen-oxygen bond formation. During the bond formation, three-electron two-centered (3e^--2C) bonds are observed in [Cu^III(bpy)(OH)₂]⁺ during the transfer of the hydroxide before the formation of the oxygen-oxygen bond.

Modulating the electrocatalytic activity of mononuclear nickel complexes toward water oxidation by tertiary amine group

Water oxidation is the bottleneck of water splitting, which is a promising strategy for hydrogen production. Therefore, it is significant to develop efficient water oxidation catalysts. Herein, electrochemical water oxidation catalyzed by three nickel complexes, namely [Ni(bptn)(H₂O)](ClO₄)₂ (1), [Ni(mbptn)(CH₃CN)](ClO₄)₂ (2), and [Ni(tmbptn)(H₂O)](ClO₄)₂ (3) (bptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane, mbptn = 5-methyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane, and tmbptn = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), is studied under near-neutral condition (pH 9.0). Meanwhile, the homogeneous catalytic behaviors of the three mononuclear nickel complexes were investigated and confirmed by scanning electron microscopy, energy dispersive spectrometry, X-ray photoelectron spectroscopy and electrochemical method. Complex 1 stabilized by a pentadentate ligand with three N-H fragments homogeneously catalyzes water oxidation to oxygen with the lowest onset overpotential. Complex 2 stabilized by a similar ligand with two N-H groups and one N-CH₃ group exhibits relatively higher onset overpotential but higher catalytic current and turnover frequency. However, complex 3 with three N-CH₃ coordination environment shows the highest onset overpotential and the highest catalytic current at higher potential. Comparison of catalytic behaviors and ligand structure of the three complexes reveals that the methyl group on the polypyridine amine ligand affects the water oxidation activity of the complexes obviously. The electronic effect of N-CH₃ coordination environment leads to higher redox potential of the metal center and potential demand for water oxidation, while it leads to higher reaction activity of high-valent intermediates, which account for higher catalytic current and efficiency of water oxidation. This work reveals that electrocatalytic water oxidation performance of nickel complexes can be finely modulated by constructing suitable N-CH₃ coordination.

Homoleptic Ni(II) dithiocarbamate complexes as pre-catalysts for the electrocatalytic oxygen evolution reaction

Four new functionalized Ni(II) dithiocarbamate complexes of the formula [Ni(L_x)₂] (1-4) (L₁ = N-methylthiophene-N-3-pyridylmethyl dithiocarbamate, L₂ = N-methylthiophene-N-4-pyridylmethyl dithiocarbamate, L₃ = N-benzyl-N-3-pyridylmethyl dithiocarbamate, and L₄ = N-benzyl-N-4-pyridylmethyl dithiocarbamate) have been synthesized and characterized by IR, UV-vis, and ¹H and ¹³C{¹H} NMR spectroscopic techniques. The solid-state structure of complex 1 has also been determined by single crystal X-ray crystallography. Single crystal X-ray analysis revealed a monomeric centrosymmetric structure for complex 1 in which two dithiocarbamate ligands are bonded to the Ni(II) metal ion in a S^S chelating mode resulting in a square planar geometry around the nickel center. These complexes are immobilized on activated carbon cloth (CC) and their electrocatalytic performances for the oxygen evolution reaction (OER) have been investigated in aqueous alkaline solution. All the complexes act as pre-catalysts for the OER and undergo electrochemical anodic activation to form Ni(O)OH active catalysts. Spectroscopic and electrochemical characterization revealed the existence of the interface of molecular complex/Ni(O)OH, which acts as the real catalyst for the OER. The active catalyst obtained from complex 2 showed the best OER activity achieving 10 mA cm^-2 current density at an overpotential of 330 mV in 1.0 M aqueous KOH solution.

Buffer Assists Electrocatalytic Nitrite Reduction by a Cobalt Macrocycle Complex

This work reports a combined experimental and computational study of the activation of an otherwise catalytically inactive cobalt complex, [Co(TIM)Br₂]⁺, for aqueous nitrite reduction. The presence of phosphate buffer leads to efficient electrocatalysis, with rapid reduction to ammonium occurring close to the thermodynamic potential and with high Faradaic efficiency. At neutral pH, increasing buffer concentrations increase catalytic current while simultaneously decreasing overpotential, although high concentrations have an inhibitory effect. Controlled potential electrolysis and rotating ring-disk electrode experiments indicate that ammonium is directly produced from nitrite by [Co(TIM)Br₂]⁺, along with hydroxylamine. Mechanistic investigations implicate a vital role for the phosphate buffer, specifically as a proton shuttle, although high buffer concentrations inhibit catalysis. These results indicate a role for buffer in the design of electrocatalysts for nitrogen oxide conversion.

Non-heme oxoiron complexes as active intermediates in the water oxidation process with hydrogen/oxygen atom transfer reactions

In this study, we explore the water oxidation process with the help of density functional theory. The formation of an oxygen-oxygen bond is crucial in the water oxidation process. Here, we report the formation of the oxygen-oxygen bond by the N5-coordinate oxoiron species with a higher oxidation state of Fe^IV and Fe^V. This bond formation is studied through the nucleophilic addition of water molecules and the transfer of the oxygen atom from meta-chloroperbenzoic acid (mCPBA). Our study reveals that the oxygen-oxygen bond formation by reacting mCPBA with Fe^VO requires less activation barrier (13.7 kcal mol^-1) than the other three pathways. This bond formation by the oxygen atom transfer (OAT) pathway is more favorable than that achieved by the hydrogen atom transfer (HAT) pathway. In both cases, the oxygen-oxygen bond formation occurs by interacting the σ*d_z²-2p_z molecular orbital of the iron-oxo intermediate with the 2p_x orbital of the oxygen atom. From this study, we understand that the oxygen-oxygen bond formation by Fe^IVO with the OAT process is also feasible (16 kcal mol^-1), suggesting that Fe^VO may not always be required for the water oxidation process by non-heme N5-oxoiron. After the oxygen-oxygen bond formation, the release of the dioxygen molecule occurs with the addition of the water molecule. The release of dioxygen requires a barrier of 7.0 kcal mol^-1. The oxygen-oxygen bond formation is found to be the rate-determining step.

A theoretical study of the role of the non-innocent phenolate ligand of a nickel complex in water oxidation

Water oxidation is the bottleneck of artificial photosynthesis. A novel nickel phenolate complex with a redox-active ligand has been designed to manage multiple electron transfers during water oxidation (D. Wang and C. O. Bruner, Inorg. Chem., 2017, 56, 13638). However, the mechanism of the reaction is not well understood and verified from a theoretical aspect. Density functional theory calculations were conducted to investigate the mechanism of water oxidation catalyzed by the nickel(II)-phenolate complex. Because only two cyclic voltammogram (CV) peaks were observed and the phenolate ligand is redox-active, the active species was proposed to be Ni^III-OH by the experiment. Based on the calculated results, the first CV peak is phenolate ligand-centered and the second peak is a single two-proton-coupled-two-electron process. In addition, the activation barrier of O-O bond formation of Ni^III-OH is higher than that of Ni^IV-2OH by 15.3 kcal mol^-1. Thus, the redox-active phenolate ligand does not lower the oxidation state of Ni in the active species to Ni^III. The oxidation state of the active species is still Ni^IV, the same as other Ni complexes for WOCs. As the phenolate ligand and the hydroxyl ligand can act as an internal base, three pathways are compared for O-O bond formation: normal WNA, phenolate-involving single electron transfer (SET)-WNA, and OH-involving SET-WNA. The OH-involving SET-WNA pathway is the most favorable because the hydroxyl ligand is more nucleophilic than the oxygen radical of the phenolate ligand. Based on the experimental observation and theoretical results, the phenolate ligand is not stable and easily oxidized because of the hydrogen at the benzyl position. Thus, WOC candidates should not have the presence of hydrogen at the benzyl position near the active center.

Covalent Metalloporphyrin Polymer Coated on Carbon Nanotubes as Bifunctional Electrocatalysts for Water Splitting

Metalloporphyrins have exhibited excellent electrocatalytic activities for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In order to improve the efficiency and conductivity, these molecular catalysts need to be immobilized on conductive electrode materials. Herein, a facile "one-pot" strategy was developed to coat a covalent metalloporphyrin polymer on a carbon nanotube (CNT) as bifunctional catalysts [denoted as MTIPP@CNTs, H₂TIPP = 5,10,15,20-tetra(4-(imidazole-1-yl)phenyl)porphyrin)] for water splitting in alkaline solution. MTIPP@CNTs have shown excellent electrocatalytic activities for both the HER and OER when metalloporphyrin's central metal is optimized as well as the amount of catalysts that is loaded on the CNT. The overpotential (η₁₀) of NiTIPP@CNT-2 for the OER is only 320 mV at a current density of 10 mA cm^-2 in 1.0 M KOH, and CoTIPP@CNT-1 exhibited an excellent electrocatalytic activity for the HER (η₁₀ = 450 mV for 10 mA cm^-2). Furthermore, the remarkable bifunctional electrocatalytic performance (a cell voltage of 2.04 V with a current density of 10 mA cm^-2) was also explored in the overall water splitting test.

Water Oxidation by Pentapyridyl Base Metal Complexes? A Case Study

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O–O bond formation, and O₂ release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]⁺ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)₃]²⁺/S₂O₈^2– system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV–vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the M^III/M^II oxidation, but none was able to yield a detectable amount of a M^IV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl^– or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond M^III. On that basis, design features for next-generation molecular WOCs are suggested.

We scrutinize the water oxidation activity for pentapyridyl metal complexes [M^II(Py5R)Cl]⁺ (M = Mn, Fe, Co, Ni; R = OH, OMe). Analysis of their stability, redox, and water-binding properties shows that the complexes are not able to reach high-valent intermediate states and do not catalyze water oxidation in their molecular form.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Molecular Mechanism of the Mononuclear Copper Complex-Catalyzed Water Oxidation from Cluster-Continuum Model Calculations

Cluster-continuum model calculations were conducted to decipher the mechanism of water oxidation catalyzed by a mononuclear copper complex. Among various O-O bond formation mechanisms investigated in this study, the most favorable pathway involved the nucleophilic attack of OH^- onto the ^.+ L-Cu^II -OH^- intermediate. During such process, the initial binding of OH^- to the proximity of ^.+ L-Cu^II -OH^- would result in the spontaneous oxidation of OH^- , leading to OH⋅ radical and Cu^II -OH^- species. The further O-O coupling between OH⋅ radical and Cu^II -OH^- was associated with a barrier of 14.8 kcal mol^-1 , leading to the formation of H₂ O₂ intermediate. Notably, the formation of "Cu^III -O^.- " species, a widely proposed active species for O-O bond formation, was found to be thermodynamically unfavorable and could be bypassed during the catalytic reactions. On the basis the present calculations, a catalytic cycle of the mononuclear copper complex-catalyzed water oxidation was proposed.

Homogeneous Water Oxidation Catalyzed by First-Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

The utilization of earth-abundant low-toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first-row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

Finding the True Catalyst for Water Oxidation at Low Overpotential in the Presence of a Metal Complex

The design of molecular-based catalysts for oxygen-evolution reaction (OER) requires more investigations for the true catalyst to be found. First-row transition metal complexes are extensively investigated for OER, but the role of these metal complexes as a true catalyst is doubtful. Some doubts have been expressed about the role of first-row transition metal complexes for OER at high overpotentials (η > 450). Generally, the detection of the true catalyst has so far been focused on high overpotentials (η > 450) because at low overpotentials (η < 450), many methods are not sensitive enough to detect small amounts of heterogeneous catalysts on the electrode surface during the first seconds of the reaction. Ni(II) phthalocyanine-tetra sulfonate tetrasodium (1) is in moderate conditions (at 20-50 °C and pH 5-13) in the absence of electrochemical driving forces, which could make it noteworthy for OER. Herein, the results of OER in the presence of 1 at low overpotentials under alkaline conditions are presented. In addition, in the presence of Ni complexes, using an Fe ion is introduced as a new method for detecting Ni (hydr)oxide under OER. Our experiments indicate that in the presence of a homogeneous OER (pre)catalyst, a deep investigation is necessary to rule out the heterogeneous catalysts formed. Our approach is a roadmap in the field of catalysis to understand the OER mechanism in the presence of a molecular Ni-based catalyst design. Our results shown in this study are likely to open up new perspectives and discussion on many molecular catalysts in a considerable part of the chemistry community.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Electrochemical water oxidation catalyzed by a mononuclear cobalt complex of a pentadentate ligand: the critical effect of the borate anion

A cobalt complex is found as a homogeneous water oxidation electrocatalyst. Electrochemical examinations indicate that the implementation of proton-couple electron transfer process and formation of O–O bond are assisted by borate anion.

Electrocatalytic Hydrogen Production by CN‑ substituted Cobalt Triaryl Corroles

Four cobalt corrole complexes bearing 0–3 cyano groups on the para-position of the three meso-phenyl rings of the macrocycle were synthesized, characterized and applied for electrocatalytic H2 production under both...

Efficient homogeneous electrochemical water oxidation by a copper(ii) complex with a hexaaza macrotricyclic ligand

A copper complex [CuII(L)](ClO4)2 with a hexaaza macrotricyclic ligand is found to be an efficient homogeneous electrocatalyst for water oxidation with onset overpotential of 480 mV and a turnover frequency of 3.65 s−1.

Homogeneous or heterogeneous electrocatalysis: reinvestigation of a cobalt coordination compound for water oxidation

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans., 2017, 46, 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s^-1) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds.

Bioinspired Trinuclear Copper Catalyst for Water Oxidation with a Turnover Frequency up to 20000 s-1

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O-O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O₂ to water. This synthetic mimic exhibits a turnover frequency of 20000 s^-1 in sodium bicarbonate solution, which is about 150 and 15 times higher than that of the mononuclear Cu catalyst (F-N₂O₂Cu, 131.6 s^-1) and binuclear Cu₂ complex (HappCu₂, 1375 s^-1), respectively. This work shows that the cooperation between multiple metals is an effective strategy to regulate the formation of O-O bond in water oxidation catalysis.

Bio-Inspired Molecular Catalysts for Water Oxidation

The catalytic tetranuclear manganese-calcium-oxo cluster in the photosynthetic reaction center, photosystem II, provides an excellent blueprint for light-driven water oxidation in nature. The water oxidation reaction has attracted intense interest due to its potential as a renewable, clean, and environmentally benign source of energy production. Inspired by the oxygen-evolving complex of photosystem II, a large of number of highly innovative synthetic bio-inspired molecular catalysts are being developed that incorporate relatively cheap and abundant metals such as Mn, Fe, Co, Ni, and Cu, as well as Ru and Ir, in their design. In this review, we briefly discuss the historic milestones that have been achieved in the development of transition metal catalysts and focus on a detailed description of recent progress in the field.

Mechanistic Insight into the O2 Evolution Catalyzed by Copper Complexes with Tetra- and Pentadentate Ligands

The mononuclear complexes ([(bztpen)Cu] (BF₄)₂ (bztpen = N-benzyl-N,N',N'-tris (pyridin-2-yl methyl ethylenediamine))) and ([(dbzbpen)Cu(OH₂)] (BF₄)₂ (dbzbpen = N,N'-dibenzyl-N,N'-bis(pyridin-2-ylmethyl) ethylenediamine)) have been reported as water oxidation catalysts in basic medium (pH = 11.5). We explore the O₂ evolution process catalyzed by these copper catalysts with various ligands (L) by applying the first-principles molecular dynamics simulations. First, the oxidation of catalysts to the metal-oxo intermediates [LCu(O)]²⁺ occurs through the proton-coupled electron transfer (PCET) process. These intermediates are involved in the oxygen-oxygen bond formation through the water-nucleophilic addition process. Here, we have considered two types of oxygen-oxygen bond formation. The first one is the transfer of the hydroxide of the water molecule to the Cu═O moiety; the proton transfer to the solvent leads to the formation of the peroxide complex ([LCu(OOH)]⁺). The other is the formation of the hydrogen peroxide complex ([LCu(HOOH)]²⁺) by the transfer of proton and hydroxide of the water molecule to the metal-oxo intermediate. The formation of the peroxide complex requires less activation free energy than hydrogen peroxide formation for both catalysts. We found two transition states in the well-tempered metadynamics simulations: one for proton transfer and another for hydroxide transfer. In both cases, the proton transfer requires higher free energy. Following the formation of the oxygen-oxygen bond, we study the release of the dioxygen molecule. The formed peroxide and hydrogen peroxide complexes are converted into the superoxide complex ([LCu(OO)]²⁺) through the transfer of proton, electron, and PCET processes. The superoxide complex releases an oxygen molecule upon the addition of a water molecule. The free energy of activation for the release of the dioxygen molecule is lesser than that of the oxygen-oxygen bond formation. When we observe the entire water oxidation process, the oxygen-oxygen bond formation is the rate-determining step. We calculated the rates of reaction by using the Eyring equation and found them to be close to the experimental values.

Design of molecular water oxidation catalysts with earth-abundant metal ions

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

Unveiling the High Catalytic Activity of a Dinuclear Iron Complex for the Oxygen Evolution Reaction

The dinuclear iron complex [(H₂O)-Fe^III-(ppq)-O-(ppq)-Fe^III-Cl]³⁺ (Fe^III(ppq), ppq = 2-(pyrid-2'-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline) demonstrates a catalytic activity about one order of magnitude higher than the mononuclear iron complex [Cl-Fe^III(dpa)-Cl]⁺ (Fe^III(dpa), dpa = N,N-di(1,10-phenanthrolin-2-yl)-N-isopentylamine) for the oxygen evolution reaction (OER). However, the mechanism behind such an unusually high activity has remained largely unclear. To solve this puzzle, a decomposition-and-reaction mechanism is proposed for the OER with the dinuclear Fe^III(ppq) complex as the initial state of the catalytic agent. In this mechanism, the high-valent dinuclear iron complex first dissociates into two mononuclear moieties, and the oxidized mononuclear iron complexes directly catalyze the formation of an O-O bond through a nitrate attack pathway with nitrate functioning as a cocatalyst. Density functional theory calculations reveal that it is the electron-deficient microenvironment around the iron center that gives rise to the remarkable catalytic activity observed experimentally. Therefore, the outstanding performance of the Fe^III(ppq) catalyst can be ascribed to the high reactivity of its mononuclear moieties in a high oxidation state, which is concomitant with the structural stability of the low-valent dinuclear complex. The theoretical insights provided by this study could be useful for the optimization and design of novel iron-based water oxidation catalysts.

Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework

Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl]pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (M^II) and the one-electron oxidized (M^III) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Electrochemical water oxidation by a copper complex with an N4-donor ligand under neutral conditions

A mononuclear copper(ii) complex [Cu(H2L)](NO3)2 with an N4-donor redox-active ligand is found to be an efficient homogeneous catalyst for electrochemical water oxidation with the assistance of ligand oxidation.

Nickel is a Different Pickle: Trends in Water Oxidation Catalysis for Molecular Nickel Complexes

The development of novel water oxidation catalysts is important in the context of renewable fuels production. Ligand design is one of the key tools to improve the activity and stability of molecular catalysts. The establishment of ligand design rules can facilitate the development of improved molecular catalysts. In this paper it is shown that chemical oxidants can be used to probe oxygen evolution activity for nickel-based systems, and trends are reported that can improve future ligand design. Interestingly, different ligand effects were observed in comparison to other first-row transition metal complexes. For example, nickel complexes with secondary amine donors were more active than with tertiary amine donors, which is the opposite for iron complexes. The incorporation of imine donor groups in a cyclam ligand resulted in the fastest and most durable nickel catalyst of our series, achieving oxygen evolution turnover numbers up to 380 and turnover frequencies up to 68 min-1 in a pH 5.0 acetate buffer using Oxone as oxidant. Initial kinetic experiments with this catalyst revealed a first order in chemical oxidant and a half order in catalyst. This implies a rate-determining oxidation step from a dimeric species that needs to break up to generate the active catalyst. These findings lay the foundation for the rational design of molecular nickel catalysts for water oxidation and highlight that catalyst design rules are not generally applicable for different metals.

Potential- and Buffer-Dependent Catalyst Decomposition during Nickel-Based Water Oxidation Catalysis

The production of hydrogen by water electrolysis benefits from the development of water oxidation catalysts. This development process can be aided by the postulation of design rules for catalytic systems. The analysis of the reactivity of molecular complexes can be complicated by their decomposition under catalytic conditions into nanoparticles that may also be active. Such a misinterpretation can lead to incorrect design rules. In this study, the nickel-based water oxidation catalyst [NiII (meso-L)](ClO4 )2 , which was previously thought to operate as a molecular catalyst, is found to decompose to form a NiOx layer in a pH 7.0 phosphate buffer under prolonged catalytic conditions, as indicated by controlled potential electrolysis, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy measurements. Interestingly, the formed NiOx layer desorbs from the surface of the electrode under less anodic potentials. Therefore, no nickel species can be detected on the electrode after electrolysis. Catalyst decomposition is strongly dependent on the pH and buffer, as there is no indication of NiOx layer formation at pH 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer. Under these conditions, the activity stems from a molecular species, but currents are much lower. This study demonstrates the importance of in situ characterization methods for catalyst decomposition and metal oxide layer formation, and previously proposed design elements for nickel-based catalysts need to be revised.

Macrocyclic cyanocobalamin (vitamin B12) as a homogeneous electrocatalyst for water oxidation under neutral conditions

Homogeneous electrochemical water oxidation under neutral conditions using impressively stable vitamin B₁₂.

Is nickel phosphide an efficient catalyst for the oxygen-evolution reaction at low overpotentials?

At low overpotentials, the oxygen-evolution reaction by Ni₂P in the presence of Fe ions was investigated.