Switching Electrocatalytic Hydrogen Evolution Pathways through Electronic Tuning of Copper Porphyrins

The electronic structure of metal complexes plays key roles in determining their catalytic features. However, controlling electronic structures to regulate reaction mechanisms is of fundamental interest but has been rarely presented. Herein, we report electronic tuning of Cu porphyrins to switch pathways of the hydrogen evolution reaction (HER). Through controllable and regioselective β-oxidation of Cu porphyrin 1, we synthesized analogues 2-4 with one or two β-lactone groups in either a cis or trans configuration. Complexes 1-4 have the same Cu-N4 core site but different electronic structures. Although β-oxidation led to large anodic shifts of reductions, 1-4 displayed similar HER activities in terms of close overpotentials. With electrochemical, chemical and theoretical results, we show that the catalytically active species switches from a CuI species for 1 to a Cu0 species for 4. This work is thus significant to present mechanism-controllable HER via electronic tuning of catalysts.

FeNi2 S4 -A Potent Bifunctional Efficient Electrocatalyst for the Overall Electrochemical Water Splitting in Alkaline Electrolyte

For a carbon-neutral society, the production of hydrogen as a clean fuel through water electrolysis is currently of great interest. Since water electrolysis is a laborious energetic reaction, it requires high energy to maintain efficient and sustainable production of hydrogen. Catalytic electrodes can reduce the required energy and minimize production costs. In this context, herein, a bifunctional electrocatalyst made from iron nickel sulfide (FeNi2 S4 [FNS]) for the overall electrochemical water splitting is introduced. Compared to Fe2 NiO4 (FNO), FNS shows a significantly improved performance toward both OER and HER in alkaline electrolytes. At the same time, the FNS electrode exhibits high activity toward the overall electrochemical water splitting, achieving a current density of 10 mA cm-2 at 1.63 V, which is favourable compared to previously published nonprecious electrocatalysts for overall water splitting. The long-term chronopotentiometry test reveals an activation followed by a subsequent stable overall cell potential at around 2.12 V for 20 h at 100 mA cm-2 .

A Dynamic Water Channel Affects O2 Stability in [FeFe]-Hydrogenases

[FeFe]-hydrogenases are capable of reducing protons at a high rate. However, molecular oxygen (O2 ) induces the degradation of their catalytic cofactor, the H-cluster, which consists of a cubane [4Fe4S] subcluster (4FeH ) and a unique diiron moiety (2FeH ). Previous attempts to prevent O2 -induced damage have focused on enhancing the protein's sieving effect for O2 by blocking the hydrophobic gas channels that connect the protein surface and the 2FeH . In this study, we aimed to block an O2 diffusion pathway and shield 4FeH instead. Molecular dynamics (MD) simulations identified a novel water channel (WH ) surrounding the H-cluster. As this hydrophilic path may be accessible for O2 molecules we applied site-directed mutagenesis targeting amino acids along WH in proximity to 4FeH to block O2 diffusion. Protein film electrochemistry experiments demonstrate increased O2 stabilities for variants G302S and S357T, and MD simulations based on high-resolution crystal structures confirmed an enhanced local sieving effect for O2 in the environment of the 4FeH in both cases. The results strongly suggest that, in wild type proteins, O2 diffuses from the 4FeH to the 2FeH . These results reveal new strategies for improving the O2 stability of [FeFe]-hydrogenases by focusing on the O2 diffusion network near the active site.

Electroplated electrodes for continuous and mass-efficient electrochemical hydrogenation

Electrocatalytic hydrogenations (ECH) enable the reduction of organic substrates upon usage of electric current and present a sustainable alternative to conventional processes if green electricity is used. Opposed to most current protocols for electrode preparation, this work presents a one-step binder- and additive-free production of silver- and copper-electroplated electrodes. Controlled adjustment of the preparation parameters allows for the tuning of catalyst morphology and its electrochemical properties. Upon optimization of the deposition protocol and carbon support, high faradaic efficiencies of 93 % for the ECH of the Vitamin A- and E-synthon 2-methyl-3-butyn-2-ol (MBY) are achieved that can be maintained at current densities of 240 mA cm-2 and minimal catalyst loadings of 0.2 mg cm-2 , corresponding to an unmatched production rate of 1.47 kgMBE  gcat -1  h-1 . For a continuous hydrogenation process, the protocol can be directly transferred into a single-pass operation mode giving a production rate of 1.38 kgMBE  gcat -1  h-1 . Subsequently, the substrate spectrum was extended to a total of 17 different C-C-, C-O- and N-O-unsaturated compounds revealing the general applicability of the reported process. Our results lay an important groundwork for the development of electrochemical reactors and electrodes able to directly compete with the palladium-based thermocatalytic state of the art.

Insights into the Molecular Mechanism of Formaldehyde Inhibition of [FeFe]-Hydrogenases

[FeFe]-hydrogenases are efficient H2 converting biocatalysts that are inhibited by formaldehyde (HCHO). The molecular mechanism of this inhibition has so far not been experimentally solved. Here, we obtained high-resolution crystal structures of the HCHO-treated [FeFe]-hydrogenase CpI from Clostridium pasteurianum, showing HCHO reacts with the secondary amine base of the catalytic cofactor and the cysteine C299 of the proton transfer pathway which both are very important for catalytic turnover. Kinetic assays via protein film electrochemistry show the CpI variant C299D is significantly less inhibited by HCHO, corroborating the structural results. By combining our data from protein crystallography, site-directed mutagenesis and protein film electrochemistry, a reaction mechanism involving the cofactor's amine base, the thiol group of C299 and HCHO can be deduced. In addition to the specific case of [FeFe]-hydrogenases, our study provides additional insights into the reactions between HCHO and protein molecules.

Accelerating CO2 electrochemical conversion towards industrial implementation

Despite significant progress in CO2 conversion field, there remains a significant gap between fundamental research and the industrial demands. This Comment discusses key performance parameters for industrial applications and outlines current limitations in the field.

A CuICoII cryptate for the visible light-driven reduction of CO2

Among the rare bimetallic complexes known for the reduction of CO2, CoIICoII and ZnIICoII hexamine cryptates are described as efficient photocatalysts. In close relation to the active sites of natural, CO2-reducing enzymes, we recently reported the asymmetric cryptand {NSNN}m ({NSNN}m = N[(CH2)2SCH2(m-C6H4)CH2NH(CH2)2]3N) comprising distinct sulphur- and nitrogen-rich binding sites and the corresponding CuIMII (MII = CoII, NiII, CuII) complexes. To gain insight into the effect of metals in different oxidation states and sulphur-incorporation on the photocatalytic activity, we herein investigate the CuICoII complex of {NSNN}m as catalyst for the visible light-driven reduction of CO2. After 24 h irradiation with LED light of 450 nm, CuICoII-{NSNN}m shows a high efficiency for the photocatalytic CO2-to-CO conversion with 9.22 μmol corresponding to a turnover number of 2305 and a high selectivity of 98% over the competing H2 production despite working in an acetonitrile/water (4 : 1) mixture. Experiments with mononuclear counterparts and computational studies show that the high activity can be attributed to synergistic catalysis between Cu and Co. Furthermore, it was shown that an increase of the metal distance results in the loss of synergistic effects and rather single-sited Co catalysis is observed.

Adapting Synthetic Models of Heme/Cu Sites to Energy-Efficient Electrocatalytic Oxygen Reduction Reaction

In nature, cytochrome c oxidases catalyze the 4e- oxygen reduction reaction (ORR) at the heme/Cu site, in which CuI is used to assist O2 activation. Because of the thermodynamic barrier to generate CuI , synthetic Fe-porphyrin/Cu complexes usually show moderate electrocatalytic ORR activity. We herein report on a Co-corrole/Co complex 1-Co for energy-efficient electrocatalytic ORR. By hanging a CoII ion over Co corrole, 1-Co realizes electrocatalytic 4e- ORR with a half-wave potential of 0.89 V versus RHE, which is outstanding among corrole-based electrocatalysts. Notably, 1-Co outperforms Co corrole hanged with CuII or ZnII . We revealed that the hanging CoII ion can provide an electron to improve O2 binding thermodynamically and dynamically, a function represented by the biological CuI ion of the heme/Cu site. This work is significant to present a remarkable ORR electrocatalyst and to show the vital role of a second-sphere redox-active metal ion in promoting O2 binding and activation.

Mechanochemical one-pot synthesis of heterostructured pentlandite-carbon composites for the hydrogen evolution reaction

We have utilized carbon sources as milling additives to enable a direct mechanochemical one-pot synthesis of Fe3Co3Ni3S8/carbon (Pn/C) materials using elemental reaction mixtures. The obtained Pn/C materials are thoroughly characterized and their carbon content could be adjusted up to 50 wt%. In addition to carbon black (CB) as an additive, Pn/C materials were produced using graphite, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), which allows the overall physicochemical properties of materials for energy storage applications to be adjusted. By employing the Pn/C materials as electrocatalysts for the HER in a zero-gap proton exchange membrane (PEM) electrolyzer, we were able to reach a current density of 1 A cm-2 at a cell potential as low as 2.12 V using Pn, which was synthesized with 25 wt% CB. Furthermore, electrolysis at an applied current density of 1 A cm-2 for 100 h displays a stable performance, thus providing a sustainable synthesis procedure for potential future energy storage applications. Herein, we show that catalyst supports play an important role in the overall performance.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction

The nucleophilic attack of water or hydroxide on metal-oxo units forms an O-O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1-M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1-M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal-oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1-M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150-mV overpotential to achieve 10 mA/cm2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal-oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal-oxo units bearing trans axial ligands.

The [4Fe-4S]-Cluster of HydF is not Required for the Binding and Transfer of the diiron site of [FeFe]-Hydrogenases

The active site of [FeFe]-hydrogenases contains a cubane [4Fe-4S]-cluster and a unique diiron cluster with biologically unusual CO and CN- ligands. The biogenesis of this diiron site, termed [2FeH], requires the maturation proteins HydE, HydF and HydG. During the maturation process HydF serves as a scaffold protein for the final assembly steps and the subsequent transfer of the [2FeH] precursor, termed [2FeP], to the [FeFe]-hydrogenase. The binding site of [2FeP] in HydF has not been elucidated, however, the [4Fe-4S]-cluster of HydF was considered as a possible binding partner of [2FeP]. By targeting individual amino acids in HydF from Thermosipho melanesiensis using site directed mutagenesis, we examined the postulated binding mechanism as well as the importance and putative involvement of the [4Fe-4S]-cluster for binding and transferring [2FeP]. Surprisingly, our results suggest that binding or transfer of [2FeP] does not involve the proposed binding mechanism or the presence of a [4Fe-4S]-cluster at all.

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine

In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ⁴ mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations─bis(triflato), bis(acetonitrile), and mixed coligand species─could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]²⁺ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment─all of the {N₆}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

Evidence of Sulfur Non-Innocence in [CoII (dithiacyclam)]2+ -Mediated Catalytic Oxygen Reduction Reactions

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)]²⁺ (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S₂ N₂ -cyclam)]²⁺ (2; S₂ N₂ -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the Co^II centers in 1 and 2 point to the involvement of sulfur in the O₂ reduction process as the key for the improved catalytic ORR capabilities of 2.

Substrate-Gated Transformation of a Pre-Catalyst into an Iron-Hydride Intermediate [(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2]- for Catalytic Dehydrogenation of Dimethylamine Borane

Continued efforts are made on the development of earth-abundant metal catalysts for dehydrogenation/hydrolysis of amine boranes. In this study, complex [K-18-crown-6-ether][(NO)₂Fe(μ-^MePyr)(μ-CO)Fe(NO)₂] (3-K-crown, ^MePyr = 3-methylpyrazolate) was explored as a pre-catalyst for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution of H_2(g) from DMAB triggered by 3-K-crown, parallel conversion of 3-K-crown into [(NO)₂Fe(N,N'-^MePyrBH₂NMe₂)]^- (5) and an iron-hydride intermediate [(NO)₂(CO)Fe(μ-H)Fe(CO)(NO)₂]^- (A) was evidenced by X-ray diffraction/nuclear magnetic resonance/infrared/nuclear resonance vibrational spectroscopy experiments and supported by density functional theory calculations. Subsequent transformation of A into complex [(NO)₂Fe(μ-CO)₂Fe(NO)₂]^- (6) is synchronized with the deactivated generation of H_2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)₂Fe(η²-BH₄)] (4-Na-crown) with CO_(g) as an alternative synthetic route, isolated intermediate [Na-18-crown-6-ether][(NO)₂(CO)Fe(μ-H)Fe(CO)(NO)₂] (A-Na-crown) featuring catalytic reactivity toward dehydrogenation of DMAB supports a substrate-gated transformation of a pre-catalyst [(NO)₂Fe(μ-^MePyr)(μ-CO)Fe(NO)₂]^- (3) into the iron-hydride species A as an intermediate during the generation of H_2(g).

Increasing the O2 Resistance of the [FeFe]-Hydrogenase CbA5H through Enhanced Protein Flexibility

The high turnover rates of [FeFe]-hydrogenases under mild conditions and at low overpotentials provide a natural blueprint for the design of hydrogen catalysts. However, the unique active site (H-cluster) degrades upon contact with oxygen. The [FeFe]-hydrogenase fromClostridium beijerinckii (CbA5H) is characterized by the flexibility of its protein structure, which allows a conserved cysteine to coordinate to the active site under oxidative conditions. Thereby, intrinsic cofactor degradation induced by dioxygen is minimized. However, the protection from O₂ is only partial, and the activity of the enzyme decreases upon each exposure to O₂. By using site-directed mutagenesis in combination with electrochemistry, ATR-FTIR spectroscopy, and molecular dynamics simulations, we show that the kinetics of the conversion between the oxygen-protected inactive state (cysteine-bound) and the oxygen-sensitive active state can be accelerated by replacing a surface residue that is very distant from the active site. This sole exchange of methionine for a glutamate residue leads to an increased resistance of the hydrogenase to dioxygen. With our study, we aim to understand how local modifications of the protein structure can have a crucial impact on protein dynamics and how they can control the reactivity of inorganic active sites through outer sphere effects.

Synthesis and Characterization of Phosphorus-Containing Isocyclam Macrocycles and Their Nickel Complexes

The tetradentate azamacrocycle cyclam (=1,4,8,11-tetraazacyclotetradecane) was studied profoundly for the coordination of transition metal ions, and the resulting complexes were investigated extensively for their catalytic performance in, e.g., O₂ activation and electrocatalytic CO₂ reduction. Although the successful synthesis of analogous P₄ macrocycles was described earlier, no tetradentate N,P mixed 14-membered macrocycles have been prepared to date and their chemistry remains elusive. Thus, in this work, we showcase the synthesis of phospha-aza mixed cyclam-based macrocycles by selectively "exchanging" one or two secondary amines in the macrocycle isocyclam (=1,4,7,11-tetraazacyclotetradecane) with tertiary phosphines. In addition, we herein present the preparation of the corresponding nickel complexes along with their complex chemical and structural characterization to provide first coordination studies.

Influence of the ZnCrAl Oxide Composition on the Formation of Hydrocarbons from Syngas

The conversion of syngas into value-added hydrocarbons gains increasing attention due to its potential to produce sustainable platform chemicals from simple starting materials. Along this line, the "OX-ZEO" process that combines a methanol synthesis catalyst with a zeolite, capable of catalyzing the methanol-to-hydrocarbon reaction, was found to be a suitable alternative to the classical Fischer-Tropsch synthesis. Hitherto, understanding the mechanism of the OX-ZEO process and simultaneously optimizing the CO conversion and the selectivity toward a specific hydrocarbon remains challenging. Herein, we present a comparison of a variety of ZnCrAl oxides with different metal ratios combined with a H-ZSM-5 zeolite for the conversion of syngas to hydrocarbons. The effect of aluminum on the catalytic activity was investigated for ZnCrAl oxides with a Zn/Cr ratio of 4:1, 1:1, and 1:2. The product distribution and CO conversion were found to be strongly influenced by the Zn/Cr/Al ratio. Although a ratio of Zn/Cr of 1:2 was best to produce lower olefins and aromatics, with aromatic selectivities of up to 37%, catalysts with a 4:1 ratio revealed high paraffin selectivity up to 52%. Notably, a distinct effect of aluminum in the oxide lattice on the catalytic activity and product selectivity was observed, as a higher Al content leads to a lower CO conversion and a changed product spectrum. We provide additional understanding of the influence of different compositions of ZnCrAl oxides on their surface properties and the catalytic activity in the OX-ZEO process. Furthermore, the variation of the zeolite component supports the important role of the channel topology of the porous support material for the hydrocarbon production. In addition, variation of the gas hourly space velocity showed a correlation of contact time, CO conversion, and hydrocarbon selectivity. At a gas hourly space velocity of 4200 mL/g_cat h, CO conversion as high as 44% along with a CO₂ selectivity of 42% and a lower paraffin (C₂ ⁰-C₄ ⁰) selectivity of 41% was observed.

Opening the pathway towards a scalable electrochemical semi-hydrogenation of alkynols via earth-abundant metal chalcogenides

Electrosynthetic methods are crucial for a future sustainable transformation of the chemical industry. Being an integral part of many synthetic pathways, the electrification of hydrogenation reactions gained increasing interest in recent years. However, for the large-scale industrial application of electrochemical hydrogenations, low-resistance zero-gap electrolysers operating at high current densities and high substrate concentrations, ideally applying noble-metal-free catalyst systems, are required. Because of their conductivity, stability, and stoichiometric flexibility, transition metal sulfides of the pentlandite group have been thoroughly investigated as promising electrocatalysts for electrochemical applications but were not investigated for electrochemical hydrogenations of organic materials. An initial screening of a series of first row transition metal pentlandites revealed promising activity for the electrochemical hydrogenation of alkynols in water. The most active catalyst within the series was then incorporated into a zero-gap electrolyser enabling the hydrogenation of alkynols at current densities of up to 240 mA cm-2, Faraday efficiencies of up to 75%, and an alkene selectivity of up to 90%. In this scalable setup we demonstrate high stability of catalyst and electrode for at least 100 h. Altogether, we illustrate the successful integration of a sustainable catalyst into a scalable zero-gap electrolyser establishing electrosynthetic methods in an application-oriented manner.

Boosting the overall electrochemical water splitting performance of pentlandites through non-metallic heteroatom incorporation

We report on synthesis of the heterotrimetallic pentlandite-type material Fe₃Co₃Ni₃S₈ (FCNS) in presence of suitable phosphorus-(FCNSP) and nitrogen-(FCNSN) donors for the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N into the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP facilitate the oxygen evolution reaction (OER) at 100 mAcm⁻² in 1.0M KOH with overpotentials of 479, 440, and 427 mV, respectively, outperforming the benchmark IrO₂ (564 mV) and commercial Ni metal powder (>600 mV). Likewise, FCNSN and FCNSNP reveal an improved performance toward the hydrogen evolution reaction (HER) in 0.5M H₂SO_4, outperforming the pristine FCNS. All materials revealed high stability and morphological robustness during OER and HER. Notably, DFT calculation suggests that N and P doping boost the OER activity of the pristine FCNS, whereas N doping enhances the HER activity.

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Fine Fe₃Co₃Ni₃S₈ (FCNS) nanoparticles were prepared through ball milling methods

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Subsequent regulated thermal treatment in inert ambientes produces N- and P-doped FCNS

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The obtained materials showed high activity toward electrochemical water splitting

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DFT calculation confirms that N and P doping boosts OER, whereas N enhances HER

Trimetallic Pentlandites (Fe,Co,Ni)9S8 for the Electrocatalytical HER in Acidic Media

Recently, pentlandite materials have been shown to exhibit promising properties with respect to the hydrogen evolution reaction (HER). A whole series of trimetallic FeCoNi-pentlandite materials and composites have been synthesized from the elements using high-temperature synthesis and categorized in terms of purity. Furthermore, the electrocatalytic properties regarding the HER were determined and correlated to hydrogen adsorption energies, which were determined by means of density functional theory (DFT) calculations. The relationships between activity and its origin generated in this way help to better understand the pentlandite system and provide meaningful approaches for catalyst synthesis.

Trapping an Oxidized and Protonated Intermediate of the [FeFe]-Hydrogenase Cofactor under Mildly Reducing Conditions

The H-cluster is the catalytic cofactor of [FeFe]-hydrogenase, a metalloenzyme that catalyzes the formation of dihydrogen (H₂). The catalytic diiron site of the H-cluster carries two cyanide and three carbon monoxide ligands, making it an excellent target for IR spectroscopy. In previous work, we identified an oxidized and protonated H-cluster species, whose IR signature differs from that of the oxidized resting state (Hox) by a small but distinct shift to higher frequencies. This "blue shift" was explained by a protonation at the [4Fe-4S] subcomplex of the H-cluster. The novel species, denoted HoxH, was preferentially accumulated at low pH and in the presence of the exogenous reductant sodium dithionite (NaDT). When HoxH was reacted with H₂, the hydride state (Hhyd) was formed, a key intermediate of [FeFe]-hydrogenase turnover. A recent publication revisited our protocol for the accumulation of HoxH in wild-type [FeFe]-hydrogenase, concluding that inhibition by NaDT decay products rather than cofactor protonation causes the spectroscopic "blue shift". Here, we demonstrate that HoxH formation does not require the presence of NaDT (or its decay products), but accumulates also with the milder reductants tris(2-carboxyethyl)phosphine, dithiothreitol, or ascorbic acid, in particular at low pH. Our data consistently suggest that HoxH is accumulated when deprotonation of the H-cluster is impaired, thereby preventing the regain of the oxidized resting state Hox in the catalytic cycle.

Metal-Corrole-Based Porous Organic Polymers for Electrocatalytic Oxygen Reduction and Evolution Reactions

Integrating molecular catalysts into designed frameworks often enables improved catalysis. Compared with porphyrin-based frameworks, metal-corrole-based frameworks have been rarely developed, although monomeric metal corroles are usually more efficient than porphyrin counterparts for the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). We herein report on metal-corrole-based porous organic polymers (POPs) as ORR and OER electrocatalysts. M-POPs (M=Mn, Fe, Co, Cu) were synthesized by coupling metal 10-phenyl-5,15-(4-iodophenyl)corrole with tetrakis(4-ethynylphenyl)methane. Compared with metal corrole monomers, M-POPs displayed significantly enhanced catalytic activity and stability. Co-POP outperformed other M-POPs by achieving four-electron ORR with a half-wave potential of 0.87 V vs. RHE and reaching 10 mA cm^-2 OER current density at 340 mV overpotential. This work is unparalleled to develop and explore metal-corrole-based POPs as electrocatalysts.

Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects

For the efficient electrochemical conversion of CO₂ into valuable chemical feedstocks, a well-coordinated interaction of all electrolyzer compartments is required. In addition to the catalyst, whose role is described in detail in the part “Electrochemical CO₂ Reduction toward Multicarbon Alcohols - The Microscopic World of Catalysts & Process Conditions” of this divided review, the general cell setups, design and manufacture of the electrodes, membranes used, and process parameters must be optimally matched. The authors' goal is to provide a comprehensive review of the current literature on how these aspects affect the overall performance of CO₂ electrolysis. To be economically competitive as an overall process, the framework conditions, i.e., CO₂ supply and reaction product treatment must also be considered. If the key indicators for current density, selectivity, cell voltage, and lifetime of a CO₂ electrolyzer mentioned in the techno-economic consideration of this review are met, electrochemical CO₂ reduction can make a valuable contribution to the creation of closed carbon cycles and to a sustainable energy economy.

Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions

Tackling climate change is one of the undoubtedly most important challenges at the present time. This review deals mainly with the chemical aspects of the current status for converting the greenhouse gas CO2 via electrochemical CO2 reduction reaction (CO2RR) to multicarbon alcohols as valuable products. Feasible reaction routes are presented, as well as catalyst synthesis methods such as electrodeposition, precipitation, or sputtering. In addition, a comprehensive overview of the currently achievable selectivities for multicarbon alcohols in CO2RR is given. It is also outlined to what extent, for example, modifications of the catalyst surfaces or the use of bifunctional compounds the product distribution is shifted. In addition, the influence of varying electrolyte, temperature, and pressure is described and discussed.

A bioinspired redox-modulating copper(II)-macrocyclic complex bearing non-steroidal anti-inflammatory drugs with anti-cancer stem cell activity

Copper(II) coordination compounds have been investigated for their anticancer properties for decades, however, none have reached advanced human clinical trials. The poor translation of copper(II) complexes from in vitro studies...

Introducing Water-Network-Assisted Proton Transfer for Boosted Electrocatalytic Hydrogen Evolution with Cobalt Corrole

Proton transfer is vital for many biological and chemical reactions. Hydrogen-bonded water-containing networks are often found in enzymes to assist proton transfer, but similar strategy has been rarely presented by synthetic catalysts. We herein report the Co corrole 1 with an appended crown ether unit and its boosted activity for the hydrogen evolution reaction (HER). Crystallographic and 1H NMR studies proved that the crown ether of 1 can grab water via hydrogen bonds. By using protic acids as proton sources, the HER activity of 1 was largely boosted with added water, while the activity of crown-ether-free analogues showed very small enhancement. Inhibition studies by adding (1) external 18-crown-6-ether to extract water molecules and (2) potassium ion or N-benzyl-n-butylamine to block the crown ether of 1 further confirmed its critical role in assisting proton transfer via grabbed water molecules. This work presents a synthetic example to boost HER through water-containing networks.

Magnetic NiFe2 O4 Nanoparticles Prepared via Non-Aqueous Microwave-Assisted Synthesis for Application in Electrocatalytic Water Oxidation

Phase‐pure spinel‐type magnetic nickel ferrite (NiFe₂O₄) nanocrystals in the size range of 4 to 11 nm were successfully synthesized by a fast and energy‐saving microwave‐assisted approach. Size and accessible surface areas can be tuned precisely by the reaction parameters. Our results highlight the correlation between size, degree of inversion, and magnetic characteristics of NiFe₂O₄ nanoparticles, which enables fine‐tuning of these parameters for a particular application without changing the elemental composition. Moreover, the application potential of the synthesized powders for the electrocatalytic oxygen evolution reaction in alkaline media was demonstrated, showing that a low degree of inversion is beneficial for the overall performance. The most active sample reaches an overpotential of 380 mV for water oxidation at 10 mA cm⁻² and 38.8 mA cm⁻² at 1.7 V vs. RHE, combined with a low Tafel slope of 63 mV dec⁻¹.

An asymmetric cryptand for the site-specific coordination of 3d metals in multiple oxidation states

Bis-tren (tren = tris(2-aminoethyl)amine) azacryptands were previously studied profoundly for the coordination of two +II metals and subsequent binding of substrates within their cavity. Likewise, cryptates including metals in different oxidations states were reported with the rather unstable hexa-imine analogues but also revealed only little stability. In this work, we report the synthesis of an asymmetric hexa-amine cryptand analogue by selectively exchanging three of the secondary amines of one binding site with sulphur atoms. We show that the presence of two distinct binding sites allows for the selective formation of stable dinuclear complexes of metals with different oxidation numbers and present the formation of distinct Cu^ICo^II, Cu^INi^II and Cu^ICuI^II cryptates.

Synergistic Electrocatalytic Hydrogen Evolution in Ni/NiS Nanoparticles Wrapped in Multi-Heteroatom-Doped Reduced Graphene Oxide Nanosheets

Hydrogen production is a key driver for sustainable and clean fuels used to generate electricity, which can be achieved through electrochemical splitting of water in alkaline solutions. However, the hydrogen evolution reaction (HER) is kinetically sluggish in alkaline media. Therefore, it has become imperative to develop inexpensive and highly efficient electrocatalysts that can replace the existing expensive and scarce noble-metal-based catalysts. Herein, we report on the rational design of nonprecious heterostructured electrocatalysts comprising a highly conductive face-centered cubic nickel metal, a nickel sulfide (NiS) phase, and a reduced graphene oxide (rGO) doped with phosphorous (P), sulfur (S), and nitrogen (N) in one ordered heteromaterial named Ni/NiS/P,N,S-rGO. The Ni/NiS/P,N,S-rGO electrode shows the best performance toward HER in 1.0 M KOH media among all materials tested with an overpotential of 155 mV at 10.0 mA cm^-2 and a Tafel slope of 135 mV dec^-1. The performance is comparable to the herein used Pt/C-20% benchmark catalyst examined under the same experimental conditions. The chronoamperometry and chronopotentiometry measurements have reflected the high durability of the Ni/NiS/P,N,S-rGO electrode for technological applications. At the same time, the current catalyst showed a high robustness and structure retention after long-term HER performance, which is reflected by SEM, XRD, and XPS measurements.

Crossing the Valley of Death: From Fundamental to Applied Research in Electrolysis

The growing societal and political focus on the use of environmentally friendly technologies has led to an ever-increasing interest in electrolysis technologies in the scientific communities. This development is reflected by the plethora of candidate catalysts for the hydrogen and oxygen evolution reactions, as well as the CO₂ reduction reaction, reported in the literature. However, almost none of them entered the stage of application yet. Likewise, the reports on process engineering inadequately address the utilization of these catalysts, as well as electrode and cell concepts, that might be suitable for the market. Evidently, a closer collaboration between chemists and engineers from industry and academia is desirable to speed up the development of these disruptive technologies. Herein, we elucidate the critical parameters and highlight the necessary aspects to accelerate the development of industrially relevant catalysts capable of fulfilling the forthcoming challenges related to energy conversion and storage. The aim of this Perspective, composed by industrial and academic partners, is to critically question current undertakings and to encourage researchers to strike interdisciplinary research pathways.

A dithiacyclam-coordinated silver(i) polymer with anti-cancer stem cell activity

A cancer stem cell (CSC) active, solution stable, silver(i) polymeric complex bearing a dithiacyclam ligand is reported. The complex displays similar potency towards CSCs to salinomycin in monolayer and three-dimensional cultures. Mechanistic studies suggest CSC death results from cytosol entry, an increase in intracellular reactive oxygen species, and caspase-dependent apoptosis.

Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

Achieving a selective 2 e^- or 4 e^- oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso-phenyls each bearing a bulky ortho-amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers αβαβ and αααα catalyze ORR with n=2.10 and 3.75 (n is the electron number transferred per O₂ ), respectively, but ααββ and αααβ show poor selectivity with n=2.89-3.10. Isomer αβαβ catalyzes 2 e^- ORR by preventing a bimolecular O₂ activation path, while αααα improves 4 e^- ORR selectivity by improving O₂ binding at its pocket, a feature confirmed by spectroscopy methods, including O K-edge near-edge X-ray absorption fine structure. This work represents an unparalleled example to improve 2 e^- and 4 e^- ORR by tuning only steric effects without changing molecular and electronic structures.

A bioinspired oxoiron(IV) motif supported on a N2S2 macrocyclic ligand

A mononuclear oxoiron(iv) complex 1-trans bearing two equatorial sulfur ligations is synthesized and characterized as an active-site model of the elusive sulfur-ligated FeIV[double bond, length as m-dash]O intermediates in non-heme iron oxygenases. The introduction of sulfur ligands weakens the Fe[double bond, length as m-dash]O bond and enhances the oxidative reactivity of the FeIV[double bond, length as m-dash]O unit with a diminished deuterium kinetic isotope effect, thereby providing a compelling rationale for nature's use of the cis-thiolate ligated oxoiron(iv) motif in key metabolic transformations.

Site-selective protonation of the one-electron reduced cofactor in [FeFe]-hydrogenase

Hydrogenases are bidirectional redox enzymes that catalyze hydrogen turnover in archaea, bacteria, and algae. While all types of hydrogenase show H2 oxidation activity, [FeFe]-hydrogenases are excellent H2 evolution catalysts as well. Their active site cofactor comprises a [4Fe-4S] cluster covalently linked to a diiron site equipped with carbon monoxide and cyanide ligands. The active site niche is connected with the solvent by two distinct proton transfer pathways. To analyze the catalytic mechanism of [FeFe]-hydrogenase, we employ operando infrared spectroscopy and infrared spectro-electrochemistry. Titrating the pH under H2 oxidation or H2 evolution conditions reveals the influence of site-selective protonation on the equilibrium of reduced cofactor states. Governed by pKa differences across the active site niche and proton transfer pathways, we find that individual electrons are stabilized either at the [4Fe-4S] cluster (alkaline pH values) or at the diiron site (acidic pH values). This observation is discussed in the context of the complex interdependence of hydrogen turnover and bulk pH.

Metal-Organic-Framework-Supported Molecular Electrocatalysis for the Oxygen Reduction Reaction

Synthesizing molecule@support hybrids is appealing to improve molecular electrocatalysis. We report herein metal-organic framework (MOF)-supported Co porphyrins for the oxygen reduction reaction (ORR) with improved activity and selectivity. Co porphyrins can be grafted on MOF surfaces through ligand exchange. A variety of porphyrin@MOF hybrids were made using this method. Grafted Co porphyrins showed boosted ORR activity with large (>70 mV) anodic shift of the half-wave potential compared to ungrafted porphyrins. By using active MOFs for peroxide reduction, the number of electrons transferred per O₂ increased from 2.65 to 3.70, showing significantly improved selectivity for the 4e ORR. It is demonstrated that H₂ O₂ generated from O₂ reduction at Co porphyrins is further reduced at MOF surfaces, leading to improved 4e ORR. As a practical demonstration, these hybrids were used as air electrode catalysts in Zn-air batteries, which exhibited equal performance to that with Pt-based materials.

Enzyme-Inspired Iron Porphyrins for Improved Electrocatalytic Oxygen Reduction and Evolution Reactions

Nature uses Fe porphyrin sites for the oxygen reduction reaction (ORR). Synthetic Fe porphyrins have been extensively studied as ORR catalysts, but activity improvement is required. On the other hand, Fe porphyrins have been rarely shown to be efficient for the oxygen evolution reaction (OER). We herein report an enzyme-inspired Fe porphyrin 1 as an efficient catalyst for both ORR and OER. Complex 1, which bears a tethered imidazole for Fe binding, beats imidazole-free analogue 2, with an anodic shift of ORR half-wave potential by 160 mV and a decrease of OER overpotential by 150 mV to get the benchmark current density at 10 mA cm^-2 . Theoretical studies suggested that hydroxide attack to a formal Fe^V =O form the O-O bond. The axial imidazole can prevent the formation of trans HO-Fe^V =O, which is less effective to form O-O bond with hydroxide. As a practical demonstration, we assembled rechargeable Zn-air battery with 1, which shows equal performance to that with Pt/Ir-based materials.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction

We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst.

Sustainable and rapid preparation of nanosized Fe/Ni-pentlandite particles by mechanochemistry

In recent years, metal-rich sulfides of the pentlandite type (M9S8) have attracted considerable attention for energy storage applications. However, common synthetic routes towards pentlandites either involve energy intensive high temperature procedures or solvothermal methods with specialized precursors and non-sustainable organic solvents. Herein, we demonstrate that ball milling is a simple and efficient method to synthesize nanosized bimetallic pentlandite particles (Fe4.5Ni4.5S8, Pn) with an average size of ca. 250 nm in a single synthetic step from elemental- or sulfidic mixtures. We herein highlight the effects of the milling ball quantity, precursor types and milling time on the product quality. Along this line, Raman spectroscopy as well as temperature/pressure monitoring during the milling processes provide valuable insights into mechanistic differences between the mechanochemical Pn-formation. By employing the obtained Pn-nanosized particles as cathodic electrocatalysts for water splitting in a zero-gap PEM electrolyzer we provide a comprehensive path for a potential sustainable future process involving non-noble metal catalysts.

Interplay of Spin Crossover and Coordination-Induced Spin State Switch for Iron Bis(pyrazolyl)methanes in Solution

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N₅O} to {N₆} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.

Dual-Heteroatom-Doped Reduced Graphene Oxide Sheets Conjoined CoNi-Based Carbide and Sulfide Nanoparticles for Efficient Oxygen Evolution Reaction

Intensive research is being conducted into highly efficient and cheap nanoscale materials for the electrocatalytic oxidation of water. In this context, we built heterostructures of multilayered CoNi-cyanide bridged coordination (CoNi-CP) nanosheets and graphene oxide (GO) sheets (CoNi-CP/GO) as a source for heterostructured functional electrodes. The layered CoNi-CP/GO hybrid components heated in nitrogen gas (N₂) at 450 °C yield CoNi-based carbide (CoNi-C) through thermal decomposition of CoNi-CP, while GO is converted into reduced GO (rGO) to finally form a CoNi-C/rGO-450 composite. The CoNi-C/rGO-450 composite shows a reasonable efficiency for oxygen evolution reaction (OER) through water oxidations in alkaline solution. Meanwhile, regulated annealing of CoNi-CP/GO in N₂ with thiourea at 450 and 550 °C produces CoNi-based sulfide (CoNi-S) rather than CoNi-C between rGO sheets co-doped by nitrogen (N) and sulfur (S) heteroatoms (NS-rGO) to form CoNi-S/NS-rGO-450 and CoNi-S/NS-rGO-550 composites, respectively. The CoNi-S/NS-rGO-550 shows the best efficiency for electrocatalytic OER among all electrodes with an overpotential of 290 mV at 10 mA cm^-2 and a Tafel slope of 79.5 mV dec^-1. By applying the iR compensation to remove resistance of the solution (2.1 Ω), the performance is further improved to achieve a current density of 10 mA cm^-2 at an overpotential of 274 mV with a Tafel slope of 70.5 mV dec^-1. This result is expected to be a promising electrocatalyst compared to the currently used electrocatalysts and a step for fuel cell applications in the future.