Understanding the Electrochemical Window of Solid-State Electrolyte in Full Battery Application

In recent years, solid-state Li batteries (SSLBs) have emerged as a promising solution to address the safety concerns associated. However, the limited electrochemical window (ECW) of solid-state electrolytes (SEs) remains a critical constraint full battery application. Understanding the factors that influence the ECW is an essential step toward designing more robust and high-performance electrochemical systems. This review provides a detailed classification of the various "windows" of SEs and a comprehensive understanding of the associated interfacial stability of SEs in full battery application. The paper begins with a historical overview of SE development, followed by a detailed discussion of their structural characteristics. Next, examination of various methodologies used to calculate and measure the ECW is presented, culminating in the proposal of standardized testing procedures. Furthermore, a comprehensive analysis of the numerous parameters that influence the thermodynamic ECW of SEs is provided, along with a synthesis of strategies to address these challenges. At last, this review concludes with an in-depth exploration of the interfacial issues associated with SEs exhibiting narrow ECWs in full SSLBs.

Closed-Loop Navigation of a Kinetic Zone Diagram for Redox-Mediated Electrocatalysis Using Bayesian Optimization, a Digital Twin, and Automated Electrochemistry

Molecular electrocatalysis campaigns often require tuning multiple experimental parameters to obtain kinetically insightful electrochemical measurements, a prohibitively time-consuming task when performing comprehensive studies across multiple catalysts and substrates. In this work, we present an autonomous workflow that combines Bayesian optimization and automated electrochemistry to perform fully unsupervised cyclic voltammetry (CV) studies of molecular electrocatalysis. We developed CV descriptors that leveraged the conceptual framework of the EC' (where EC' denotes an electrochemical step followed by a catalytic chemical step) kinetic zone diagram to enable efficient Bayesian optimization. The CV descriptor's effect on optimization performance was evaluated using a digital twin of our autonomous experimental platform, quantifying the accuracy of obtained kinetic values against the known ground truth. We demonstrated our platform experimentally by performing autonomous studies of TEMPO-catalyzed ethanol and isopropanol electro-oxidation, demonstrating rapid identification of kinetically insightful conditions in 10 or less iterations through the closed-loop workflow. Overall, this work highlights the application of autonomous electrochemical platforms to accelerate mechanistic studies in molecular electrocatalysis and beyond.

A Dinuclear Nickel Complex for Electrocatalytic Nitrite Reduction

While many mononuclear complexes for electrocatalytic NOx- reduction have been reported, there are few examples of dinuclear electrocatalysts. Here, we report on the electroreduction of NO2- by the dinuclear complex [Ni2(tpmc)(NO3)2]2+ (tpmc = 1,4,8,11-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and a mononuclear analogue [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane). Notably, the presence of the second metal ion in [Ni2(tpmc)(NO3)2]2+ shifts the onset of catalysis anodically by ca. 0.3 V compared to [Ni(cyclam)]2+, while maintaining a similar electrocatalytic rate and Faradaic efficiency. Both complexes produce NH4+ as the primary reduction product. Interestingly, in some respects, Ni2+(aq) is a better electrocatalyst for NO2- than [Ni(cyclam)]2+, with faster electrocatalysis occurring at a more anodic onset potential, although the Faradaic efficiency for NH4+ is lower.

Deciphering Reaction Mechanisms of Molecular Proton Reduction Catalysts with Cyclic Voltammetry: Kinetic vs Thermodynamic Control

ConspectusThe kinetics and thermodynamics of elementary reaction steps involved in the catalytic reduction of protons to hydrogen define the reaction landscape for catalysis. The mechanisms can differ in the order of the elementary proton transfer, electron transfer, and bond-forming steps and can be further differentiated by the sites at which protons and electrons localize. Access to fully elucidated mechanistic, kinetic, and thermochemical details of molecular catalysts is crucial to facilitate the development of new catalysts that operate with optimal efficiency, selectivity, and durability. The mechanism by which a catalyst operates, as well as the kinetics and thermodynamics associated with the individual steps, can often be accessed through electroanalytical studies.This Account details the application of cyclic voltammetry to interrogate reaction mechanisms and quantify the kinetics and thermodynamics of elementary reaction steps for a series of molecular catalysts that mediate electrochemical proton reduction. I distinguish the limiting scenarios wherein a catalyst operates under kinetic control vs thermodynamic control, with a focus on detecting how cyclic voltammetry features shift with proton source strength and concentration, as well as scan rate. For systems that operate under kinetic control, catalytic currents are observed at, or slightly positive toward, the formal potential for the redox process that triggers catalysis. Under thermodynamic control, catalytic responses shift as a function of the proton source pKa and effective pH of the solution. After drawing this distinction, we introduce the appropriate voltammetry experiments and accompanying analytical expressions for extracting key metrics from the data.To illustrate analytical strategies to quantify elementary reaction steps of catalysts operating under kinetic control, I describe our studies of proton reduction catalysts Co(dmgBF2)2(CH3CN)2 (dmgBF2 = difluoroboryl-dimethylglyoxime) and [Ni(P2PhN2Ph)2]2+ (P2PhN2Ph = 1,5-phenyl-3,7-phenyl-1,5-diaza-3,7-diphosphacyclooctane). Here, peak shift analysis, foot-of-the-wave analysis, and plateau current analysis are applied to data sets wherein voltammetric response are recorded as a function of catalyst concentration, proton source concentration, proton source strength, and scan rate to quantify rate constants for elementary proton transfer and bond-forming steps in a catalytic cycle. Further, the case study of [Ni(P2PhN2Ph)2]2+ illustrates how complementary spectroscopic methods can bolster the mechanistic assignment. Collectively, these two studies showcase how detailed mechanistic studies inform on rate-limiting elementary steps in catalysis and other key processes underpinning catalysis.Second, I present analytical strategies to interrogate catalysts operating under thermodynamic control, centered on the case study of [NiII(P2PhN2Bn)2]2+ (P2PhN2Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane). Here, the application of nonaqueous Pourbaix theory to extract thermodynamic information is introduced, and the construction of a coupled Pourbaix diagram is detailed. This study identifies ligand-based protonation as the key process that places catalysis under thermodynamic control and influences the reaction mechanism.Together, the work detailed in this Account showcases the utility of electroanalytical methods to disentangle complex reaction mechanisms and extract key thermochemical and kinetic parameters for elementary steps of catalysis. Through detailed presentation of the key analytical expressions that underpin these analyses, this Account seeks to facilitate the adoption of cyclic voltammetry by the community to fully extract kinetic, thermochemical, and mechanistic information on electrochemical small-molecule activation.

Access to Heterobimetallic MII/CuI Complexes with a Multichelate Platform and Their Reactivity Studies in CO2RR

We describe the selective formation of heterobimetallic complexes, exploiting the coordination trends of the developed bis-terpyridyl trans-1,2-cyclohexadiamine platform (L). Following a stepwise addition, we first reacted ligand L toward tetrakisacetonitrile transition metal precursors, [M(MeCN)4][BF4]2 (where M = Fe or Ni), to generate the monometallic complexes 1 ([FeL][BF4]2) and 2 ([NiL][BF4]2). These species were later combined with the tetrakisacetonitrile precursor [Cu(MeCN)4][BF4], generating the corresponding heterobimetallic complexes 3 ([FeCuL(MeCN)2][BF4]3) and 4 ([NiCuL(MeCN)2][BF4]3). The four species obtained, in high yields, have been structurally characterized. Their cyclic voltammetry analysis revealed the impact of the CuI-atom presence on the heterobimetallic complexes under argon and carbon dioxide (CO2) atmospheres. Controlled potential electrolysis studies revealed the instability of complexes 1-4 toward CO2RR, generating the heterogeneous material in solution and on the electrode surface. In contrast, CO2 photoreduction studies revealed higher stability and photocatalytic activity for the FeII-based complexes (1 and 3), generating CO with 88% selectivity.

Testing mixed metal bimetallic, and monometallic, cryptates for electrocatalytic hydrogen evolution

Appropriately designed catalysts help to minimise the energy required to convert the energy-poor feedstock H2O into energy-rich molecular H2. Herein, two families of pyridazine-based cryptates, mononuclear [MIILi](BF4)2 and mixed metal dinuclear [MIICuILi](BF4)3 (M = Fe, Co, Cu or Zn; Li is the Schiff base cryptand made by 2 : 3 condensation of tris(2-aminoethyl)amine and 3,6-diformylpyridazine), are investigated as potential electrocatalysts for the hydrogen evolution reaction (HER) in MeCN with acetic acid as the proton source. The synthesis and structures of a new mixed metal cryptate, [ZnIICuILi](BF4)3, and the tetrafluoroborate analogue of the previously reported perchlorate salt of the mono-zinc cryptate, [ZnIILi](BF4)2·0.5H2O, are reported. Electrocatalytic HER testing showed that a deposit forms on the glassy carbon working electrode during electrolysis and it is the active species responsible for the very modest activity observed. The deposits formed by the heterobinuclear cryptates had higher activities (2.0 < TON2h < 3.5) than the deposits formed by the mononuclear cryptates (TON2h < 0.75). But unfortunately the control, using CuI(MeCN)4BF4, had a similar TON2h (2.3) to those seen for the heterobinculear cryptates, which indicates that it is the deposit formed by the CuI cation present in the heterobinuclear cryptates that is likely responsible for the observed, very modest, HER activity.

Overcoming the Tradeoff Between Reaction Rate and Overpotential in Dinuclear Cobalt Complex Catalyzed Electrochemical Water Oxidation

This study focuses on enhancing the water oxidation reaction (WOR) efficacy of dinuclear cobalt complex catalysts from both kinetic (turnover frequency, TOF) and thermodynamic (overpotential, η) perspectives. For this purpose, we synthesized six dinuclear cobalt complexes 1-6 comprising non-innocent ligands with different electronically active substituents (-OMe (1), -Me (2), -H (3), -F (4), -Cl (5), and -CN (6)). The electronic effects on the electrochemical WOR under neutral, acidic, and alkaline conditions were investigated experimentally and computationally. X-ray crystallography revealed that the valence state of the cobalt complexes was affected by electronic effects, affording different catalytic potentials, as evidenced by electrochemical measurements. Kinetic and spectroscopic studies confirmed that the synergistic effect of catalyst identity and reaction media changes the reaction mechanism of each catalyst. The WOR performance of 1-6 was tunable through ligand electronics and solvent effects, with the log(TOF)-η analysis highlighting an operational η as low as 40 mV. This study provides valuable insights into optimizing WOR catalysis through molecular design and environmental tuning.

Pyridyl Aroyl Hydrazone-Based Metal Complexes Showing a Ligand-Centered Metal-Assisted Pathway for Electrocatalytic Hydrogen Evolution

In the past few decades, the concern over the excessive use of fossil fuels and consequent environmental damage has triggered the search for cleaner and renewable energy resources. This has led to the consideration of hydrogen as our future fuel. It can be produced from various sources, and if used as fuel, it generates only water as a byproduct. Electrocatalytic reduction of protons (2H+ + 2e- → H2) is one of the available methods for producing hydrogen gas at large scales. Currently, the most efficient electrocatalysts are Pt-based complexes. In order to make the entire process more cost-effective, it has become necessary to find electrocatalysts that are derived from earth-abundant metals such as Ni, Zn, Fe, etc. Herein, we have demonstrated the electrocatalytic hydrogen evolution reaction (HER) catalyzed by pyridyl aroyl hydrazone ligand (HL)-based metal complexes (M = Fe, Co, Ni, Cu, and Zn). Rate calculations using controlled potential electrolysis revealed the optimal overall catalytic performance of NiL2 among the investigated complexes. For NiL2, a maximum turnover frequency of 7040 s-1 with a 0.42 V overpotential was obtained when triethylamine hydrochloride was used as a proton source. Both experimental and density functional theory (DFT) calculations suggested a ligand-centered metal-assisted catalysis pathway for NiL2 in the presence of triethylammonium chloride.

Lab-on-Fiber Operando Deciphering of a MOF Electrocatalyst

Despite the great success in deploying metal-organic frameworks (MOFs) as efficient electrocatalysts, the low adoption of operando methods hinders the understanding of underlying mechanism. By leveraging the subtle refractive index evolution, including both the real and the imaginary parts, an entirely new concept of a lab-on-fiber operando method and its feasibility for "pristine-immersion-operando-post analysis" of electrocatalyts are demonstrated. Concurrent collection of absorption spectra and surface plasmon resonance is achieved by engineering fiber-optic waveguides to simultaneously induce guided light attenuation and plasmonic coupling. In situ-formed Co hydroxide and oxide reactive intermediates in zeolitic imidazolate framework-67 (ZIF-67) electrocatalyst are optically identified, which shows its underlying self-reconstruction conversions at different stages during electrocatalytic oxygen evolution reactions, and address the gap in knowledge concerning whether ZIF-67 is a precatalyst for real catalyst production. This illuminating operando method offers intriguing opportunities to collect all the observables in a single fiber, provides an exciting potential of a new class of device with long-sought integration and miniaturization capability, and is expected to enable the electrocatalysis community to conquer challenges with conducting multimodal operando experiments outside the laboratory.

Mechanistic insights into multimetal synergistic and electronic effects in a hexanuclear iron catalyst with a [Fe3(μ3-O)(μ2-OH)]2 core for enhanced water oxidation

Multinuclear molecular catalysts mimicking natural photosynthesis have been shown to facilitate water oxidation; however, such catalysts typically operate in organic solutions, require high overpotentials and have unclear catalytic mechanisms. Herein, a bio-inspired hexanuclear iron(III) complex I, Fe6(μ3-O)2(μ2-OH)2(bipyalk)2(OAc)8 (H2bipyalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol); OAc = acetate) with desirable water solubility and stability was designed and used for water oxidation. Our results showed that I has high efficiency for water oxidation via the water nucleophilic attack (WNA) pathway with an overpotential of only ca. 290 mV in a phosphate buffer of pH 2. Importantly, key high-oxidation-state metal-oxo intermediates formed during water oxidation were identified by in situ spectroelectrochemistry and oxygen atom transfer reactions. Theoretical calculations further supported the above identification. Reversible proton transfer and charge redistribution during water oxidation enhanced the electron and proton transfer ability and improved the reactivity of I. Here, we have shown the multimetal synergistic and electronic effects of catalysts in water oxidation reactions, which may contribute to the understanding and design of more advanced molecular catalysts.

Complexes of a Noncyclic Carbazole-based N5-donor Schiff base: Structures, Redox, EPR and Poor Activity as Hydrogen Evolution Electrocatalysts

A new noncyclic pentadentate N5-donor Schiff-base ligand, HL2Etpyr (1,1'-(3,6-ditert-butyl-9H-carbazole-1,8-diyl)bis(N-(2-(pyridin-2-yl)ethyl)methanimine)), prepared from 1,8-diformyl-3,6-ditertbutyl-carbazole (HUtBu) and two equivalents of 2-(2-pyridyl)ethylamine, along with four tetrafluoroborate complexes, [MIIL2Etpyr](BF4), where M = Co, Ni, Cu, and Zn, and two [CoIIL2EtPyr]·1/2[CoIIX4] complexes where X = NCS or Cl, isolated as solvates, are reported. All six complexes were structurally characterized, revealing the cations to be isostructural, with M(II) in a trigonal bipyramidal N5-donor environment. Only the Zn(II) complex is fluorescent. Cyclic voltammograms of [MIIL2Etpyr](BF4) in MeCN reveal reversible redox processes at positive potentials: 0.61 (Zn), 0.62 (Cu), 0.57 (Ni), and 0.25 V (Co), and for the cobalt complex a second quasi-reversible process occurs at 0.92 V vs Fc+/Fc. EPR data for the first oxidation product clearly demonstrate that the Zn complex undergoes a ligand centered oxidation, and support this being the case for the Ni and Cu complexes, although this is not definitively shown. After both oxidations the EPR data shows that the Co complex is best described as a low spin Co(III)-ligand radical. In the presence of 80 mM acetic acid, controlled potential electrolysis carried out on [MIIL2Etpyr](BF4) at -1.68 V in MeCN shows some electrocatalytic hydrogen evolution reaction (HER) performance in the order Ni(II) > Cu(II) > Co(II) - but the control, Ni(II) tetrafluoroborate, is more active than all three of the complexes.

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

Diiron Complexes with Rigid and Conjugated S-to-S Bridges for Electrocatalytic Reduction of CO2

Electroreduction of CO2 to value-added low-carbon chemicals is a promising way for carbon neutrality and CO2 utilization. It was found that the diiron complex [(μ-bdt)Fe2(CO)6] (bdt = benzene-1,2-dithiolate) has high catalytic activity for electrocatalytic CO2 reduction. To further study the effect of the S-to-S bridge on the catalytic performances of diiron complexes for electrochemical CO2 reduction, four diiron complexes 1-4 with different rigid and conjugated S-to-S bridges were either selected or designed. The electrocatalytic studies showed that under optimal conditions, 2 with a 2,3-naphthalenedithiolato bridge exhibited the lowest catalytic onset potential (Eonset = -1.75 V vs Fc+/0), while 4 with a diphenyl-1,2-vinylidene bridge displayed the highest catalytic activity (TOFmax = 295 s-1), which is 1.5 times that of [(μ-bdt)Fe2(CO)6]. The controlled potential electrolysis experiments of 4 in 0.1 M MeOH/MeCN at -2.35 V vs Fc+/0 gave a total faradaic yield close to 100%, with selectivities of 77%, 9%, and 14% for HCOOH, CO, and H2, respectively. The mechanism for CO2 reduction was studied using density functional theory, IR spectroelectrochemistry, and electrochemical methods. The results indicate that modifying the structure of the S-to-S bridge is an effective strategy to improve the catalytic performance of diiron complexes for electrocatalytic CO2 reduction.

Dramatic Improvement of Homogeneous Carbon Dioxide and Bicarbonate Electroreduction Using a Tetracationic Water-Soluble Cobalt Phthalocyanine

Electrochemical conversion of carbon dioxide (CO2) offers the opportunity to transform a greenhouse gas into valuable starting materials, chemicals, or fuels. Since many CO2 capture strategies employ aqueous alkaline solutions, there is interest in catalyst systems that can act directly on such capture solutions. Herein, we demonstrate new catalyst designs where the electroactive molecules readily mediate the CO2-to-CO conversion in aqueous solutions between pH 4.5 and 10.5. Likewise, the production of CO directly from 2 M KHCO3 solutions (pH 8.2) is possible. The improved molecular architectures are based on cobalt(II) phthalocyanine and contain four cationic trimethylammonium groups that confer water solubility and contribute to the stabilization of activated intermediates via a concentrated positive charge density around the active core. Turnover frequencies larger than 103 s-1 are possible at catalyst concentrations of down to 250 nM in CO2-saturated solutions. The observed rates are substantially larger than the related cobalt phthalocyanine-containing catalysts. Density functional theory calculations support the idea that the excellent catalytic properties are attributed to the ability of the cationic groups to stabilize CO2-bound reduced intermediates in the catalytic cycle. The homogeneous, aqueous CO2 reduction that these molecules perform opens new frontiers for further development of the CoPc platform and sets a greatly improved baseline for CoPc-mediated CO2 upconversion. Ultimately, this discovery uncovers a strategy for the generation of platforms for practical CO2 reduction catalysts in alkaline solutions.

Tuning Electrocatalytic Water Oxidation Activity: Insights from the Active-Site Distance in LnCu6 Clusters

Atomically precise metal clusters serve as a unique model for unraveling the intricate mechanism of the catalytic reaction and exploring the complex relationship between structure and activity. Herein, three series of water-soluble heterometallic clusters LnCu6, abbreviated as LnCu6-AC (Ln = La, Nd, Gd, Er, Yb; HAC = acetic acid), LnCu6-IM (Ln = La and Nd; IM = Imidazole), and LnCu6-IDA (Ln = Nd; H2IDA = Iminodiacetic acid) are presented, each featuring a uniform metallic core stabilized by distinct protected ligands. Crystal structure analysis reveals a triangular prism topology formed by six Cu2+ ions around one Ln3+ ion in LnCu6, with variations in Cu···Cu distances attributed to different ligands. Electrocatalytic oxygen evolution reaction (OER) shows that these different LnCu6 clusters exhibit different OER activities with remarkable turnover frequency of 135 s-1 for NdCu6-AC, 79 s-1 for NdCu6-IM and 32 s-1 for NdCu6-IDA. Structural analysis and Density Functional Theory (DFT) calculations underscore the correlation between shorter Cu···Cu distances and improves OER catalytic activity, emphasizing the pivotal role of active-site distance in regulating electrocatalytic OER activities. These results provide valuable insights into the OER mechanism and contribute to the design of efficient homogeneous OER electrocatalysts.

Electrocatalytic oxidation of pyrrole on a quasi-reversible silver nanodumbbell particle surface for supramolecular porphyrin production

Photoactive supramolecular porphyrin assemblies are attractive molecules for light-harvesting applications. This is due to their relatively non-toxicity, biological activities and charge and energy exchange characteristics. However, the extreme cost associated with their synthesis and requirements for toxic organic solvents during purification pose a challenge to the sustainability characteristics of their applications. This work presents the first report on the sustainable synthesis, spectroscopic and photophysical characterizations of a near-infrared (NIR) absorbing Ca(II)-meso-tetrakis (4-hydroxyphenyl)porphyrin using an electrolyzed pyrrole solution. The latter was obtained by cycling the pyrrole solution across the silver nanodumbbell particle surface at room temperature. The electrolyzed solution condensed readily with acidified p-hydroxybenzaldehyde, producing the targeted purple porphyrin. The non-electrolyzed pyrrole solution formed a green substance with significantly different optical properties. Remarkable differences were observed in the voltammograms of the silver nanodumbbell particles and those of the conventional gold electrode during the pyrrole cycling, suggesting different routes of porphyrin formation. The rationale behind these formations and the associated mechanisms were extensively discussed. Metalation with aqueous Ca2+ ion caused a Stokes shift of 38.75 eV. The current study shows the advantage of the electrochemical method towards obtaining sustainable light-harvesting porphyrin at room temperature without the need for high-energy-dependent conventional processes.

Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO2 Reduction

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy and sustainable organic synthesis. Metalloporphyrins are well-known to aggregate due to π-π stacking, but surprisingly, the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO2 to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of the kinetics on the catalyst concentration. The inhibition extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and redox state. Periodic boundary calculations provide additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. This study demonstrates that homogeneous metalloporphyrins can aggregate severely in well-solubilizing organic electrolytes, that aggregation can be easily modulated through experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.

Selective Electrochemical Oxygen Reduction to Hydrogen Peroxide by Confinement of Cobalt Porphyrins in a Metal-Organic Framework

Sustainable alternatives for the energy intensive synthesis of H2O2 are necessary. Molecular cobalt catalysts show potential but are typically restricted by undesired bimolecular pathways leading to the breakdown of both H2O2 and the catalyst. The confinement of cobalt porphyrins in the PCN-224 metal-organic framework leads to an enhanced selectivity towards H2O2 and stability of the catalyst. Consequently, oxygen can now be selectively reduced to hydrogen peroxide with a stable conversion for at least 5 h, illustrating the potential of catalysts confined in MOFs to increase the selectivity and stability of electrocatalytic conversions.

Manipulating the Rate and Overpotential for Electrochemical Water Oxidation: Mechanistic Insights for Cobalt Catalysts Bearing Noninnocent Bis(benzimidazole)pyrazolide Ligands

Electrochemical water oxidation is known as the anodic reaction of water splitting. Efficient design and earth-abundant electrocatalysts are crucial to this process. Herein, we report a family of catalysts (1-3) bearing bis(benzimidazole)pyrazolide ligands (H 2 L1-H 2 L3). H 2 L3 contains electron-donating substituents and noninnocent components, resulting in catalyst 3 exhibiting unique performance. Kinetic studies show first-order kinetic dependence on [3] and [H2O] under neutral and alkaline conditions. In contrast to previously reported catalyst 1, catalyst 3 exhibits an insignificant kinetic isotope effect of 1.25 and zero-order dependence on [NaOH]. Based on various spectroscopic methods and computational findings, the L3Co2 III(μ-OH) species is proposed to be the catalyst resting state and the nucleophilic attack of water on this species is identified as the turnover-limiting step of the catalytic reaction. Computational studies provided insights into how the interplay between the electronic effect and ligand noninnocence results in catalyst 3 acting via a different reaction mechanism. The variation in the turnover-limiting step and catalytic potentials of species 1-3 leads to their catalytic rates being independent of the overpotential, as evidenced by Eyring analysis. Overall, we demonstrate how ligand design may be utilized to retain good water oxidation activity at low overpotentials.

Intricacies of Mass Transport during Electrocatalysis: A Journey through Iron Porphyrin-Catalyzed Oxygen Reduction

Electrochemical steps are increasingly attractive for green chemistry. Understanding reactions at the electrode-solution interface, governed by kinetics and mass transport, is crucial. Traditional insights into these mechanisms are limited, but our study bridges this gap through an integrated approach combining voltammetry, electrochemical impedance spectroscopy, and electrospray ionization mass spectrometry. This technique offers real-time monitoring of the chemical processes at the electrode-solution interface, tracking changes in intermediates and products during reactions. Applied to the electrochemical reduction of oxygen catalyzed by the iron(II) tetraphenyl porphyrin complex, it successfully reveals various reaction intermediates and degradation pathways under different kinetic regimes. Our findings illuminate complex electrocatalytic processes and propose new ways for studying reactions in alternating current and voltage-pulse electrosynthesis. This advancement enhances our capacity to optimize electrochemical reactions for more sustainable chemical processes.

Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt-porphyrin complexes

A CoII-porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (i.e., Li+, Na+, K+, Ca2+, Sr2+, and Ba2+) bind to the aza-crown ether group of 1. The binding constant for Li+ is comparable to that observed for a free aza-crown ether. LA binding causes an anodic shift in the CoII/CoI couple of between 10 and 40 mV and also impacts the CoIII/CoII couple. The magnitude of the anodic shift of the CoII/CoI couple varies linearly with the strength of the LA as determined by the pKa of the corresponding metal-aqua complex, with dications giving larger shifts than monocations. The extent of the anodic shift of the CoII/CoI couple also increases as the ionic strength of the solution decreases. This is consistent with electric field effects being responsible for the changes in the redox properties of 1 upon LA binding and provides a novel method to tune the reduction potential. Density functional theory calculations indicate that the bound LA is 5.6 to 6.8 Å away from the CoII ion, demonstrating that long-range electrostatic effects, which do not involve changes to the primary coordination sphere, are responsible for the variations in redox chemistry. Compound 1 was investigated as a CO2 reduction electrocatalyst and shows high activity but rapid decomposition.

Mechanistic Insights into Electrocatalytic Hydrogen Evolution by an Exceptionally Stable Cobalt Complex

Co(aPPy) is one of the most stable and active molecular first-row transition-metal catalysts for proton reduction reported to date. Understanding the origin of its high performance via mechanistic studies could aid in developing even better catalysts. In this work, the catalytic mechanism of Co(aPPy) was electrochemically probed, in both organic solvents and water. We found that different mechanisms can occur depending on the solvent and the acidity of the medium. In organic solvent with a strong acid as the proton source, catalysis initiates directly after a single-electron reduction of CoII to CoI, whereas in the presence of a weaker acid, the cobalt center needs to be reduced twice before catalysis occurs. In the aqueous phase, we found drastically different electrochemical behavior, where the Co(aPPy) complex was found to be a precatalyst to a different electrocatalytic species. We propose that in this active catalyst, the pyridine ring has dissociated and acts as a proton relay at pH ≤ 5, which opens up a fast protonation pathway of the CoI intermediate and results in a high catalytic activity. Furthermore, we determined with constant potential bulk electrolysis that the catalyst is most stable at pH 3. The catalyst thus functions optimally at low pH in an aqueous environment, where the pyridine acts as a proton shuttle and where the high acidity also prevents catalyst deactivation.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Breaking a Molecular Scaling Relationship Using an Iron-Iron Fused Porphyrin Electrocatalyst for Oxygen Reduction

The design of efficient electrocatalysts is limited by scaling relationships governing trade-offs between thermodynamic and kinetic performance metrics. This ″iron law″ of electrocatalysis arises from synthetic design strategies, where structural alterations to a catalyst must balance nucleophilic versus electrophilic character. Efforts to circumvent this fundamental impasse have focused on bioinspired applications of extended coordination spheres and charged sites proximal to a catalytic center. Herein, we report evidence for breaking a molecular scaling relationship involving electrocatalysis of the oxygen reduction reaction (ORR) by leveraging ligand design. We achieve this using a binuclear catalyst (a diiron porphyrin), featuring a macrocyclic ligand with extended electronic conjugation. This ligand motif delocalizes electrons across the molecular scaffold, improving the catalyst's nucleophilic and electrophilic character. As a result, our binuclear catalyst exhibits low overpotential and high catalytic turnover frequency, breaking the traditional trade-off between these two metrics.

Copper-based electrocatalyst for hydrogen evolution in water

In aqueous pH 7 phosphate buffer, during controlled potential electrolysis (CPE) at -1.10 V vs. Ag|AgCl the literature square planar copper complex, [CuIILEt]BF4 (1), forms a heterogeneous deposit on the glassy carbon working electrode (GCWE) that is a stable and effective hydrogen evolution reaction (HER) electrocatalyst. Specifically, CPE for 20 hours using a small GCWE (A = 0.071 cm2) gave a turnover number (TON) of 364, with ongoing activity. During CPE the brownish-yellow colour of the working solution fades, and a deposit is observed on the small GCWE. Repeating this CPE experiment in a larger cell with a larger GCWE (A = 2.7 cm2), connected to a gas chromatograph, resulted in a TON of 2628 after 2.6 days, with FE = 93%, and with activity ongoing. After this CPE, the working solution had faded to nearly colourless, and visual inspection of the large GCWE showed a material had deposited on the surface. In a 'rinse and repeat test', this heterogeneous deposit was used for further CPE, in a freshly prepared working solution minus fresh catalyst, which resulted in similar ongoing HER activity to before, consistent with the surface deposited material being the active HER catalyst. EDS, PXRD and SEM analysis of this deposit shows that copper and oxygen are the main components present, most likely comprising copper and copper(I) oxide ((Cu2O)n) formed from 1. The use of 1 leads to a deposit that is more catalytically active than that formed when starting with a simple copper salt (control), likely due to it forming a more robustly attached deposit, which also enables the observed long-lived catalytic activity.

Synthesis and Optimization of Ni-Based Nano Metal-Organic Frameworks as a Superior Electrode Material for Supercapacitor

Metal-organic frameworks (MOFs) are hybrid materials that are being explored as active electrode materials in energy storage devices, such as rechargeable batteries and supercapacitors (SCs), due to their high surface area, controllable chemical composition, and periodic ordering. However, the facile and controlled synthesis of a pure MOF phase without impurities or without going through a complicated purification process (that also reduces the yield) are challenges that must be resolved for their potential industrial applications. Moreover, various oxide formations of the Ni during Ni-MOF synthesis also represent an issue that affects the purity and performance. To resolve these issues, we report the controlled synthesis of nickel-based metal-organic frameworks (NiMOFs) by optimizing different growth parameters during hydrothermal synthesis and by utilizing nickel chloride as metal salt and H2bdt as the organic ligand, in a ratio of 1:1 at 150 °C. Furthermore, the synthesis was optimized by introducing a magnetic stirring stage, and the reaction temperature varied across 100, 150, and 200 °C to achieve the optimized growth of the NiMOFs crystal. The rarely used H2bdt ligand for Ni-MOF synthesis and the introduction of the ultrasonication stage before putting it in the furnace led to the formation of a pure phase without impurities and oxide formation. The synthesized materials were further characterized by powder X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), and UV-vis spectroscopy. The SEM images exhibited the formation of nano NiMOFs having a rectangular prism shape. The average size was 126.25 nm, 176.0 nm, and 268.4 nm for the samples (1:1)s synthesized at 100 °C, 150 °C, and 200 °C, respectively. The electrochemical performances were examined in a three-electrode configuration, in a wide potential window from -0.4 V to 0.55 V, and an electrolyte concentration of 2M KOH was maintained for each measurement. The charge-discharge galvanostatic measurement results in specific capacitances of 606.62 F/g, 307.33 F/g, and 287.42 F/g at a current density of 1 A/g for the synthesized materials at 100 °C, 150 °C, and 200 °C, respectively.

Soluble Gd6Cu24 clusters: effective molecular electrocatalysts for water oxidation

The water oxidation half reaction in water splitting for hydrogen production is extremely rate-limiting. This study reports the synthesis of two heterometallic clusters (Gd6Cu24-IM and Gd6Cu24-AC) for application as efficient water oxidation catalysts. Interestingly, the maximum turnover frequency of Gd6Cu24-IM in an NaAc solution of a weak acid (pH 6) was 319 s-1. The trimetallic catalytic site, H2O-GdIIICuII2-H2O, underwent two consecutive two-electron two-proton coupled transfer processes to form high-valent GdIII-O-O-CuIII2 intermediates. Furthermore, the O-O bond was formed via intramolecular interactions between the CuIII and GdIII centers. The results of this study revealed that synergistic catalytic water oxidation between polymetallic sites can be an effective strategy for regulating O-O bond formation.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel-Bipyridine Complexes

Hydrogenation reactions of carbon-carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C-C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.

Synthesis and characterization of lead-based metal-organic framework nano-needles for effective water splitting application

Metal organic frameworks (MOFs) are a class of porous materials characterized by robust linkages between organic ligands and metal ions. Metal-organic frameworks (MOFs) exhibit significant characteristics such as high porosity, extensive surface area, and exceptional chemical stability, provided the constituent components are meticulously selected. A metal-organic framework (MOF) containing lead and ligands derived from 4-aminobenzoic acid and 2-carboxybenzaldehyde has been synthesized using the sonochemical methodology. The crystals produced were subjected to various analytical techniques such as Fourier-transform infrared spectroscopy (FT-IR), Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Brunauer-Emmett-Teller (BET), and thermal analysis. The BET analysis yielded results indicating a surface area was found to be 1304.27 m2 g-1. The total pore volume was estimated as 2.13 cm3 g-1 with an average pore size of 4.61 nm., rendering them highly advantageous for a diverse range of practical applications. The activity of the modified Pb-MOF electrode was employed toward water-splitting applications. The electrode reached the current density of 50 mA cm-2 at an overpotential of - 0.6 V (vs. RHE) for hydrogen evolution, and 50 mA cm-2 at an overpotential of 1.7 V (vs. RHE) for oxygen evolution.

Bioinspired Active Site with a Coordination-Adaptive Organosulfonate Ligand for Catalytic Water Oxidation at Neutral pH

Many enzymes use adaptive frameworks to preorganize substrates, accommodate various structural and electronic demands of intermediates, and accelerate related catalysis. Inspired by biological systems, a Ru-based molecular water oxidation catalyst containing a configurationally labile ligand [2,2':6',2″-terpyridine]-6,6″-disulfonate was designed to mimic enzymatic framework, in which the sulfonate coordination is highly flexible and functions as both an electron donor to stabilize high-valent Ru and a proton acceptor to accelerate water dissociation, thus boosting the catalytic water oxidation performance thermodynamically and kinetically. The combination of single-crystal X-ray analysis, various temperature NMR, electrochemical techniques, and DFT calculations was utilized to investigate the fundamental role of the self-adaptive ligand, demonstrating that the on-demand configurational changes give rise to fast catalytic kinetics with a turnover frequency (TOF) over 2000 s^-1, which is compared to oxygen-evolving complex (OEC) in natural photosynthesis.

Mechanistic Investigations into the Selective Reduction of Oxygen by a Multicopper Oxidase T3 Site-Inspired Dicopper Complex

Understanding how multicopper oxidases (MCOs) reduce oxygen in the trinuclear copper cluster (TNC) is of great importance for development of catalysts for the oxygen reduction reaction (ORR). Herein, we report a mechanistic investigation into the ORR activity of the dinuclear copper complex [Cu₂L(μ-OH)]³⁺ (L = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine). This complex is inspired by the dinuclear T3 site found in the MCO active site and confines the Cu centers in a rigid scaffold. We show that the electrochemical reduction of [Cu₂L(μ-OH)]³⁺ follows a proton-coupled electron transfer pathway and requires a larger overpotential due to the presence of the Cu-OH-Cu motif. In addition, we provide evidence that metal-metal cooperativity takes place during catalysis that is facilitated by the constraints of the rigid ligand framework, by identification of key intermediates along the catalytic cycle of [Cu₂L(μ-OH)]³⁺ . Electrochemical studies show that the mechanisms of the ORR and hydrogen peroxide reduction reaction found for [Cu₂L(μ-OH)]³⁺ differ from the ones found for analogous mononuclear copper catalysts. In addition, the metal-metal cooperativity results in an improved selectivity for the four-electron ORR of more than 70% because reaction intermediates can be stabilized better between both copper centers. Overall, the mechanism of the [Cu₂L(μ-OH)]³⁺ -catalyzed ORR in this work contributes to the understanding of how the cooperative function of multiple metals in close proximity can affect ORR activity and selectivity.

Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal-oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of CuII and CuIII. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical-oxo coupling pathways, which were previously proposed for Ru-based catalysts. [CuII(HL)(OTf)2] [HL = Hbbpya = N,N-bis(2,2'-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [CuII(L-)(H2O)]+ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH⧧ value of 4.49 kcal·mol-1, a ΔS⧧ value of -42.6 cal·mol-1·K-1, and a ΔG⧧ value of 17.2 kcal·mol-1 were found. A computational study supported the formation of a Cu-oxyl intermediate, [CuII(L•)(O•)(H2O)]+. From this intermediate onward, formation of the O-O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the CuII center is proposed to be redox-inactive.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO₂ ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO₂ and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO₂ . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO₂ .

Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β'-β' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 107  s-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.

Potential- and Buffer-Dependent Selectivity for the Conversion of CO2 to CO by a Cobalt Porphyrin-Peptide Electrocatalyst in Water

A semisynthetic electrocatalyst for carbon dioxide reduction to carbon monoxide in water is reported. Cobalt microperoxidase-11 (CoMP11-Ac) is shown to reduce CO₂ to CO with a turnover number of up to 32,000 and a selectivity of up to 88:5 CO:H₂. Higher selectivity for CO production is favored by a less cathodic applied potential and use of a higher pK _a buffer. A mechanistic hypothesis is presented in which avoiding the formation and protonation of a formal Co(I) species favors CO production. These results demonstrate how tuning reaction conditions impact reactivity toward CO₂ reduction for a biocatalyst previously developed for H₂ production.

Hydrogen Evolution Electrocatalysis with a Molecular Cobalt Bis(alkylimidazole)methane Complex in DMF: a Critical Activity Analysis

[Co(HBMIM^Ph2 )₂ ](BF₄ )₂ (1) [HBMIM^Ph2 =bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane] was investigated for its electrocatalytic hydrogen evolution performance in DMF using voltammetry and during controlled potential/current electrolysis (CPE/CCE) in a novel in-line product detection setup. Performances were benchmarked against three reported molecular cobalt hydrogen evolution reaction (HER) electrocatalysts, [Co(dmgBF₂ )₂ (solv)₂ ] (2) (dmgBF₂ =difluoroboryldimethylglyoximato), [Co(TPP)] (3) (TPP=5,10,15,20-tetraphenylporphyrinato), and [Co(bapbpy)Cl](Cl) (4) [bapbpy=6,6'-bis-(2-aminopyridyl)-2,2'-bipyridine], showing distinct performances differences with 1 being the runner up in H₂ evolution during CPE and the best catalyst in terms of overpotential and Faradaic efficiency during CCE. After bulk electrolysis, for all of the complexes, a deposit on the glassy carbon electrode was observed, and post-electrolysis X-ray photoelectron spectroscopy (XPS) analysis of the deposit formed from 1 demonstrated only a minor cobalt contribution (0.23 %), mainly consisting of Co²⁺ . Rinse tests on the deposits derived from 1 and 2 showed that the initially observed distinct activity was (partly) preserved for the deposits. These observations indicate that the molecular design of the complexes dictates the features of the formed deposit and therewith the observed activity.

Two Birds with One Stone: Concurrent Ligand Removal and Carbon Encapsulation Decipher Thickness-Dependent Catalytic Activity

A carbon shell encapsulating a transition metal-based core has emerged as an intriguing type of catalyst structure, but the effect of the shell thickness on the catalytic properties of the buried components is not well known. Here, we present a proof-of-concept study to reveal the thickness effect by carbonizing the isotropic and homogeneous oleylamine (OAm) ligands that cover colloidal MoS₂. A thermal treatment turns OAm into a uniform carbon shell, while the size of MoS₂ monolayers remains identical. When evaluated toward an acidic hydrogen evolution reaction, the calcined MoS₂ catalysts deliver a volcano-type activity trend that depends on the calcination temperature. Rutherford backscattering spectrometry and depth-profiling X-ray photoelectron spectroscopy consistently provide an accurate quantification of the carbon shell thickness. The same variation pattern of catalytic activity and carbon shell thickness, aided by kinetic studies, is then persuasively justified by the respective limitations of electron and proton conductivities on the two branches of the volcano curve.

Dinuclear Cobalt Complexes for Homogeneous Water Oxidation: Tuning Rate and Overpotential through the Non-Innocent Ligand

In this study, dinuclear cobalt complexes (1 and 2) featuring bis(benzimidazole)pyrazolide-type ligands (H₂ L and Me₂ L) were prepared and evaluated as molecular electrocatalysts for water oxidation. Notably, 1 bearing a non-innocent ligand (H₂ L) displayed faster catalytic turnover than 2 under alkaline conditions, and the base dependence of water oxidation and kinetic isotope effect analysis indicated that the reaction mediated by 1 proceeded by a different mechanism relative to 2. Spectroelectrochemical, cold-spray ionization mass spectrometric and computational studies found that double deprotonation of 1 under alkaline conditions cathodically shifted the catalysis-initiating potential and further altered the turnover-limiting step from nucleophilic water attack on (H₂ L)Co^III ₂ (superoxo) to deprotonation of (L)Co^III ₂ (OH)₂ . The rate-overpotential analysis and catalytic Tafel plots showed that 1 exhibited a significantly higher rate than previously reported Ru-based dinuclear electrocatalysts at similar overpotentials. These observations suggest that using non-innocent ligands is a valuable strategy for designing effective metal-based molecular water oxidation catalysts.

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

Two Ni complexes are reported with κ⁴-P₂N₂ β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF₄)₂, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF₄)₂ and X = Ph for 2(BF₄)₂. Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 1²⁺ and 2²⁺ to the negatively charged β-diketiminato (BDK) analogues (1-H)⁺ and (2-H)⁺ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1⁺ and 2⁺ can also rapidly evolve to the BDK (1-H)⁺ and (2-H)⁺, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at E_app = -1.8 V vs Fc^+/0, being the active species the one-electron-reduced derivative 1-H and 2-H.

Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins

Bicarbonate-based electrolytes are ubiquitous in aqueous electrochemical CO₂ reduction, particularly in heterogenous catalysis, where they demonstrate improved catalytic performance relative to other buffers. In contrast, the presence of bicarbonate in organic electrolytes and its roles in homogeneous electrocatalysis remain underexplored. Here, we investigate the influence of bicarbonate on iron porphyrin-catalyzed electrochemical CO₂ reduction. We show that bicarbonate is a viable proton donor in organic electrolyte (pK_a = 20.8 in dimethyl sulfoxide) and that urea pendants in the second coordination sphere can be used to template bicarbonate in the vicinity of a molecular iron porphyrin catalyst. The templated binding of bicarbonate increases its acidity, resulting in a 1500-fold enhancement in catalytic rates relative to unmodified parent iron porphyrin. This work emphasizes the importance of bicarbonate speciation in wet organic electrolytes and establishes second-sphere bicarbonate templating as a design strategy to harness this adventitious acid and enhance CO₂ reduction catalysis.

Electrocatalytic Semihydrogenation of Alkynes with [Ni(bpy)3]2

Electrifying the production of base and fine chemicals calls for the development of electrocatalytic methodologies for these transformations. We show here that the semihydrogenation of alkynes, an important transformation in organic synthesis, is electrocatalyzed at room temperature by a simple complex of earth-abundant nickel, [Ni(bpy)₃]²⁺. The approach operates under mild conditions and is selective toward the semihydrogenated olefins with good to very good Z isomer stereoselectivity. (Spectro)electrochemistry supports that the electrocatalytic cycle is initiated in an atypical manner with a nickelacyclopropene complex, which upon further protonation is converted into a putative cationic Ni(II)-vinyl intermediate that produces the olefin after electron-proton uptake. This work establishes a proof of concept for homogeneous electrocatalysis applied to alkyne semihydrogenation, with opportunities to improve the yields and stereoselectivity.

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.

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

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.