Ulf-Peter Apfel

• Dr.
• Group Leader at Ruhr-university Bochum & Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT

The design of non-noble metal containing heterogenous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and wor...

The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe4.5Ni4.5S8) as a direct ‘rock’ electrode material for hydrogen evolution under acidic conditions with an ove...

Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe : Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0 - 1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry...

[FeFe]-hydrogenases are the most efficient enzymes for catalytic hydrogen turnover. Their H2 production efficiency is hitherto unrivalled. However, functional details of the catalytic machinery and possible modes of application are discussed controversially. The incorporation of synthetically modified cofactors and utilization of semi-artificial en...

In order to design more powerful electrocatalysts, developping understanding of the role of surface atomic-structural composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts that can replace platinum. We report scanning electrochemical cell microsc...

Delivering CO2 molecules to catalyst sites is a vital step in the CO2 reduction reaction (CO2RR). Achievements have been made to develop efficient catalysts, but few efforts have been dedicated to improving CO2 delivering in solutions. Herein, we report on electrocatalytic CO2‐to‐CO conversion using Fe tetraphenylporphyrin (FeTPP) as a catalyst and...

In recent years, formic acid (FA) has garnered attention as a compelling molecule for various chemical and everyday applications. Additionally, with recent studies demonstrating direct FA generation through CO2 electrolysis, it can serve as a stable liquid hydrogen carrier. Nevertheless, FA‐permeability via semi‐permeable ion–exchange membranes (FA...

Tailoring the properties of the catalytic layer (CL) and its architecture is crucial for enhancing both the efficiency and selectivity of CO2 electrolysers. Traditionally, CLs for CO2 reduction comprise of a single binder material or a combination that handles both ion conductance and the maintenance of a hydrophobic environment. In this work, we d...

Delivering CO2 molecules to catalyst sites is a vital step in the CO2 reduction reaction (CO2RR). Achievements have been made to develop efficient catalysts, but few efforts have been dedicated to improving CO2 delivering in solutions. Herein, we report on electrocatalytic CO2‐to‐CO conversion using Fe tetraphenylporphyrin (FeTPP) as a catalyst and...

Electrosynthesis has the potential to revolutionize industrial organic synthesis sustainably and efficiently. However, high cell voltages and low stability often arise due to solubility issues with organic solvents, while protic electrolytes restrict substrate options. We present a three-layered electrode design that enables the use of concentrated...

Hydrogenases offer a sustainable alternative to noble metals for catalyzing H₂ oxidation and H₂ production. The heterodimeric [FeFe]-hydrogenase from Desulfovibrio desulfuricans ATCC 7757 (DdHydAB) is most promising due to its exceptional catalytic activity and high-yield heterologous expression of its apo-form. Scalable production of the holo-form...

The commercialisation of PEM water electrolysis is still hindered by the necessity of using noble metals that are rare, expensive and therefore unsustainable. To replace the benchmark HER catalyst Pt with more abundant materials, promising non‐noble catalysts need to be identified and optimal electrode preparation and electrolysis conditions need t...

Ag, Cu and Sn based electrocatalysts promise high CO2 reduction kinetics and efficiencies on gas diffusion electrodes. Ag, Cu, Sn sulfide catalysts in particular may offer altered electronic properties and product selectivity, while still being easy to manufacture in scaleable synthesis routes. Comparing the CO2 reduction (CO2RR) performance of Cu3...

A key challenge in modern society is developing the sustainable processes for producing vital chemicals, such as hydrocarbons, from renewable raw materials. The OX‐ZEO process, which uses bifunctional catalysts to convert syngas into hydrocarbons, offers a potential alternative to the nonsustainable cracking process. Combining oxide materials with...

Electrosynthetic processes powered by renewable energy present a viable solution to decarbonize the chemical industry, while producing essential chemical products for modern society. However, replacing well‐established thermocatalytic methods with renewable‐powered electrosynthesis requires cost‐efficient and highly optimized systems. Current optim...

In biological systems, nitrite reductase enzymes (NIRs) are responsible for reduction of nitrite (NO2-) to nitric oxide (NO). These NIRs have mostly Cu- or Fe-containing active sites, surrounded by amine-containing ligands. Therefore, mononuclear Cu complexes with N-donor ligands are highly relevant in the development of NIR model systems and in th...

This study presents the correlation between electrolyte pH, surface morphology, chemical speciation and electro‐catalytic oxygen evolution activity of additive‐free electrodeposited NiFe catalysts for application in anion exchange membrane water electrolysis. Spherical morphologies were identified at pH 0, shifting towards honey‐combed structures a...

The metalloenzyme [FeFe]‐hydrogenase is of interest to future biotechnologies targeting the production of “green” hydrogen (H2). We recently developed a simple two‐step functionalized procedure to immobilize the [FeFe]‐hydrogenase from Clostridium pasteurianum (“CpI”) on mesoporous indium tin oxide (ITO) electrodes to achieve elevated H2 production...

Metalloporphyrins are widely studied in the field of electrochemical CO2 reduction (CO2R), whereas they are mainly investigated in homogenous catalysis. Herein six metalloporphyrins (M = Fe, Co, Ni, Cu, Zn,...

The hydrogenation of organic substances is a central process type in various industries [1] . The usage of gaseous hydrogen, which is primarily sourced from fossil fuels, entails the release of CO 2 during its production. In addition, the usage of gaseous hydrogen also carries a safety risk, which should be reduced as much as possible. An alternati...

Powered by renewable sources the electrochemical conversion of CO 2 can be a sustainable opportunity to generate fuels. In the past various molecular complexes have been studied in homogeneous, lab-scale systems for the electrochemical CO 2 reduction. Lately the field of incorporating molecular metal complexes in heterogenous electrocatalysis emerg...

As the world transitions to renewable energy sources, efficient energy storage is crucial for managing power fluctuations and decarbonize crucial high temperature processes. Achieving grid balance and optimizing renewable energy capacity requires innovative solutions. One promising option is the conversion of electrical energy into hydrogen through...

The metalloenzyme [FeFe]‐hydrogenase is of interest to future biotechnologies targeting the production of “green” hydrogen (H2). We recently developed a simple two‐step functionalized procedure to immobilize the [FeFe]‐hydrogenase from Clostridium pasteurianum (“CpI”) on mesoporous indium tin oxide (ITO) electrodes to achieve elevated H2 production...

In recent years, CO2 electrolysis, particularly the electrochemical reduction of CO2 to CO in zero‐gap systems, has gained significant attention. While Ag‐coated gas diffusion electrodes are commonly used in state‐of‐the‐art systems, heterogenized molecular catalysts like bis‐coordinated homoleptic silver(I) N,N‐bis(arylimino)‐acenaphthene (Ag‐BIAN...

Improving proton transfer is vital for electrocatalysis with porous materials. Although several strategies are reported to assist proton transfer in channels, few studies are dedicated to improving proton transfer at the local environments of active sites in porous materials. Herein, we report on new Co‐corrole‐based porous organic polymers (POPs)...

Enzymes and heterogenous catalysts for CO2 reduction reactions (CO2RR) use secondary interactions between metal sites and protein-derived coordination spheres to control the precise transfer of protons and electrons to minimize overpotential and maximize selectivity over the competitive hydrogen evolution reaction. We now report a molecular cobalt...

Nitrite (NO2⁻) serves as a pool of nitric oxide (NO) in biological systems under hypoxic conditions, and it is transformed to NO by nitrite reductase (NiR) enzyme in the presence of acid (H⁺ ions). However, NO synthases (NOSs) generate NO via L‐arginine oxidation in normoxic conditions. Previously, acid‐induced NO2⁻ reduction chemistry was modeled...

Despite considerable efforts to develop electrolyzers for energy conversion, progress has been hindered during the implementation stage by different catalyst development requirements in academic and industrial research. Herein, a coherent workflow for the efficient transition of electrocatalysts from basic research to application readiness for the...

The direct conversion of syngas into hydrocarbons, catalyzed by a combination of metal oxide and acidic zeolite, has garnered increasing attention in the scientific community. The remarkable hydrocarbon selectivity achieved in the OX‐ZEO process is a key factor in this growing interest. This process involves two distinct steps: CO activation over t...

Improving proton transfer is vital for electrocatalysis with porous materials. Although several strategies are reported to assist proton transfer in channels, few studies are dedicated to improving proton transfer at the local environments of active sites in porous materials. Herein, we report on new Co‐corrole‐based porous organic polymers (POPs)...

Tailoring the properties of the catalytic layer (CL) alongside its architecture is a key development towards ensuring both improve efficiency and selectivity for CO2 electrolyzers. Traditionally, CLs for the CO 2 R consist of a single binder-material or a combination of them overtaking both the ion-conductance and maintenance of a hydrophobic envir...

In recent years, there has been a growing need for new renewable energy sources due to the rapid depletion of conventional energy sources and an escalating energy demand. Since environmental protection concerns are increasing, electrochemical processes are becoming increasingly essential [1]. Specifically, the focus has shifted towards storing ener...

Electrocatalytic hydrogenation reactions (ECH) are increasingly recognized as promising approaches for the reductive production of both commodity and fine chemicals. Particularly when powered by renewable energies, ECH offers a sustainable alternative, avoiding the need for gaseous hydrogen, high-pressure conditions, and stoichiometric reducing age...

Electrochemical hydrogenations are becoming increasingly important in today’s world, representing a gateway reaction for the sustainable production of base and fine chemicals. However, finding optimal reaction conditions for such systems can be challenging due to a vast variety of adjusting screws that influence the overall performance of the react...

Due to the challenge of cleaving O−O bonds at single Co sites, mononuclear Co complexes typically show poor selectivity for the four‐electron (4e⁻) oxygen reduction reaction (ORR). Herein, we report on selective 4e⁻ ORR catalyzed by a Co porphyrin with a hanged ZnII ion. Inspired by Cu/Zn‐superoxide dismutase, we designed and synthesized 1‐CoZn wit...

Reducing the cost of PEM water electrolyzers is crucial for making green hydrogen economically viable achievable by cheaper catalysts as well as bipolar plates. This study demonstrates for the first time the use of uncoated carbon bipolar plates (C‐BPs) as an alternative to titanium bipolar plates (Ti‐BPs). The corrosion phenomena of C‐BPs versus T...

Metal sulfides are promising alternatives to noble metal electrocatalysts for water‐based hydrogen evolution. Pentlandites, notably, exhibit high activity in acidic environments. To explore their potential in alkaline conditions, pentlandite electrodes are tested in both conventional three‐electrode setups and scaled up to a 12.6 cm membrane electr...

[FeFe]-hydrogenase is nature’s most efficient proton reducing and H2-oxidizing enzyme. However, biotechnological applications are hampered by the O2 sensitivity of this metalloenzyme and the mechanism of aerobic deactivation is not...

Due to the challenge of cleaving O−O bonds at single Co sites, mononuclear Co complexes typically show poor selectivity for the four‐electron (4e−) oxygen reduction reaction (ORR). Herein, we report on selective 4e− ORR catalyzed by a Co porphyrin with a hanged ZnII ion. Inspired by Cu/Zn‐superoxide dismutase, we designed and synthesized 1‐CoZn wit...

Cobalt(III) compounds with tetradentate ligands have been widely employed to deliver cytotoxic and imaging agents into cells. A large body of work has focused on using cobalt(III)–cyclam scaffolds for this purpose. Here, we investigate the cytotoxic properties of cobalt(III) complexes containing 14-membered macrocycles related to cyclam. A breast c...

This special collection is dedicated to the field of application‐oriented electrochemical processes and contains a diverse selection of original research contributions in this field. It reaches from a fundamental electrochemical understanding, over catalyst material development, to engineering technology. With this Special Collection key factors in...

Aprotic organic electrolytes for electrocatalytic CO2 reduction (CO2R) offer a promising yet little-explored alternative to avoid the selectivity issues of alkaline media. We present a model-assisted study of the local reaction environment at a Cu cathode in aprotic organic electrolytes to determine the kinetics of the CO2R and the kinetic limitati...

Shape optimization of flow fields for electrochemical cells is performed. Simulation models of the flow field are introduced with and without the porous transport layer. The latter model is less detailed and used for shape optimization whereas the former one is used to validate the obtained results. Three objective functions are proposed based on t...

Electrochemical hydrogenations are considered a sustainable alternative to classical thermocatalytic processes prevalent in industrial conversions. Using a base metal sulfide of the pentlandite class, the hydrogenation of glutaraldehyde and propionic acid was investigated. While propionic acid could not be converted, glutaraldehyde was conveniently...

Electrochemical hydrogenations (EChH) constitute a sustainable alternative to conventional thermocatalytic hydrogenation. Here, we explored the EChH of the vitamin E and A synthon 2‐methyl‐3‐butyn‐2‐ol in a scalable zero‐gap electrolyzer and revealed crucial effects of electrolyte composition and convection parameters to consider for optimization....

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 product...

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). Th...

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 c...

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). Th...

Electrochemical hydrogenation reactions offer a green and sustainable production pathway for both bulk and fine chemicals employed in the modern chemical industry. However, optimizing such systems can be tremendously challenging due to the number of variables that potentially influence the overall performance. The tailored and scalable electrode fa...

The electrochemical conversion of carbon dioxide by means of renewable electricity holds great promise. However, despite significant progress in current literature, there remains a significant gap between fundamental research and the industrial demands to establish new disruptive technologies in real world applications. This gap primarily arises fr...

[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 seco...

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 e...

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 energ...

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 energ...

Akademische Forschung und industrielle Anwendung – wie die Zusammenarbeit zwischen Industrie und Akademia zu optimieren ist, zeigt ein Blick aus der technischen Elektrochemie auf deren Anwendung in der Industrie. Wichtig ist dabei eine klare Kommunikation auf beiden Seiten.

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 e...

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 sulphu...

[FeFe]-hydrogenase is nature’s most efficient proton reducing and H2 oxidizing enzyme. However, biotechnological applications are hampered by the pronounced O2 sensitivity of this metalloenzyme, and the mechanism of aerobic deactivation is poorly understood. Here, we explore the oxygen reactivi-ty of four mimics of the organometallic active site co...

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 ma...

This work reports on ligand exchange reactions between a [FeFe] hydrogenase model containing the higher homologue (PhosDT) and phosphines selected to cover a variety of electronic properties and possible coordination modes. Additionally, the amount of the phosphines and the reaction temperature were varied to study the formation of complexes with m...

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...

Cobalt complexes are extensively studied as bioinspired models for non‐heme oxygenases as they facilitate both the stabilization and characterization of metal‐oxygen intermediates. As an analog to the well‐known Co(cyclam) complex Co{N4} (cyclam=1,4,8,11‐tetraazacyclotetradecane), the CoII complex Co{i‐N4} with the isomeric isocyclam ligand (isocyc...

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 sy...

Electrosynthesis from carbon dioxide using hybrid systems, which combine electrochemical hydrogen production with microorganisms as selective biocatalysts, is a promising approach to convert renewable electricity into transportable fuels and commodities. Herein, Rad et al. develop a zero-gap electrolyzer operating in line with a methanogenic reacto...

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...

This study investigates the modification of materials by doping with foreign elements to enhance electrocatalytic activity and focuses on the engineering of an inorganic material composed of transition heterometal-rich pentlandite (Fe3Co3Ni3S8, FCNS) doped with silicon (FCNSSi) as a bifunctional catalyst for the overall electrochemical water splitt...

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...

Abstract Compared to other battery technologies, metal–air batteries offer high specific capacities because the active material at the cathode side is supplied by ambient atmosphere. To secure and further extend this advantage, the development of highly active and stable bifunctional air electrodes is currently the main challenge that needs to be r...

Eine der größten Herausforderungen der elektrochemischen Umwandlung von CO 2 zu CO in Zero‐Gap‐Elektrolyseuren ist die Skalierung dieser Systeme in den industriellen Maßstab. Trotz der bemerkenswerten Ergebnisse der letzten Jahre, sind bisher nur wenige Fortschritte bei der Skalierung dieser Technologie erzielt worden. In diesem Beitrag wird gezeig...

Electrifying the chemical industry is a key step towards the generation of green and sustainable chemical products. Electrochemical hydrogenations have gained a central role in this effort, generating important synthons in compact and modular zero-gap electrolyzers. Despite the significant achievements in recent years, clear guidelines to improve t...

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 mole...

Numerous catalysts have been reported with enhanced performance, e.g. longer lifetime and improved selectivity, for the electrochemical CO2 reduction reaction (CO2RR). Respectively little is, however, known about the influence of the electrode structuring and pre-treatment on this reaction for catalytic layers. Thus, we herein report on the modific...

Enzymes can activate otherwise unreactive substrates by using residues precisely located in the active sites. We herein report on a Ga porphyrin which bears a second-sphere sulfonic group (named as 1) and its high efficiency for the electrocatalytic hydrogen evolution reaction (HER). Complex 1 can achieve a large TOF value of 1.3 × 10⁵ s⁻¹ at low 2...

With electrocatalysis being the very foundation of multiple energy conversion technologies, the search for more effective, and affordable catalysts is becoming increasingly important for their further development.

The multi-component approach to materials design, together with the adaptation of the processing conditions, is gaining increasing popularity in energy-conversion-oriented applications allowing highly effective catalysts to be obtained.