Enhanced Electrosynthetic Hydrogen Evolution by Hydrogenases Embedded in a Redox-Active Hydrogel

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Effect of a single methyl substituent on the electronic structure of cobaltocene studied by computationally assisted MATI spectroscopy

Metallocenes represent archetypical organometallic compounds playing key roles in various fields of fundamental and applied chemistry. Many of their unique properties arise from low ionization energies (IE) which can be tuned by introducing substituents into the rings. Here we report the first mass-analyzed threshold ionization (MATI) spectrum of a methylmetallocene, (Cp')(Cp)Co (Cp' = η5-C5H4Me, Cp = η5-C5H5). The presence of a single Me group allows us to study the "pure" effect of methylation without the mutual influence of substituents. The MATI technique provides an extremely high accuracy in determining the adiabatic IE of (Cp')(Cp)Co which equals 5.2097(6) eV. The effect of a Me group on the IE of cobaltocene appears to be 36% stronger than that in bis(η6-benzene)chromium. The MATI spectrum of (Cp')(Cp)Co shows a rich vibronic structure from which vibrational frequencies of the free ion are determined. This information provides a solid basis for testing the quality of quantum chemical calculations. Various levels of the DFT and coupled cluster computations are used to describe the structural and electronic transformations accompanying the detachment of an elctron from (Cp')(Cp)Co. New aspects of the methyl substituent influence on the potential energy surfaces, as well as on the inhomogeneous changes in charge density and electrostatic potential caused by ionization, are discussed.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Biohydrogen production and purification: Focusing on bioelectrochemical systems

Innovative technologies on green hydrogen production become significant as the hydrogen economy has grown globally. Biohydrogen is one of green hydrogen production methods, and microbial electrochemical cells (MECs) can be key to biohydrogen provision. However, MECs are immature for biohydrogen technology due to several limitations including extracellular electron transfer (EET) engineering. Fundamental understanding of EET also needs more works to accelerate MEC commercialization. Interestingly, studies on biohydrogen gas purification are limited although biohydrogen gas mixture requires complex purification for use. To facilitate an MEC-based biohydrogen technology as the green hydrogen supply this review discussed EET kinetics, engineering of EET and direct interspecies electron transfer associated with hydrogen yield and the application of advanced molecular biology for improving EET kinetics. Finally, this article reviewed biohydrogen purification technologies to better understand purification and use appropriate for biohydrogen, focusing on membrane separation.

Iterative step-growth synthesis and degradation of unimolecular polyviologens under mild conditions

An iterative step-growth addition method was used to expedite the gram-scale synthesis of main-chain polyviologens by several days, while also producing the longest main-chain polyviologen (i.e., 26 viologen subunits) reported to date. Facile degradation using inorganic and organic aqueous bases was also demonstrated for a representative oligoviologen (6V-Me·12Cl), a polyviologen (26V-Me·52Cl), and oligoviologen-crosslinked hydrogels.

Remarkable Stability of a Molecular Ruthenium Complex in PEM Water Electrolysis

The dinuclear Ru diazadiene olefin complex, [Ru₂(OTf)(μ-H)(Me₂dad)(dbcot)₂], is an active catalyst for hydrogen evolution in a Polymer Exchange Membrane (PEM) water electrolyser. When supported on high surface area carbon black and at 80 °C, [Ru₂(OTf)(μ-H)(Me₂dad)(dbcot)₂]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm⁻², 1.9 V). A remarkable turn over frequency (TOF) of 7800 mol_H2 mol_catalyst⁻¹ h⁻¹ is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru₂(OTf)(μ-H)(Me₂dad)(dbcot)₂].

Molecular dinuclear ruthenium complexes deposited on conducting carbon serve as active sites for the evolution of hydrogen from neutral water in a Polymer Exchange Membrane (PEM) water electrolyser.

Bioelectrosynthesis systems

Bioelectrosynthesis (BES) systems exploit extracellular electron transport pathways to augment cellular metabolism. This strategy can be used to improve the economic viability of bio-based syntheses versus conventional methods, most notably petrochemical-based syntheses. It also has the potential to reduce the carbon footprint of biomanufacturing processes. Efficient channeling of cathode-derived electrons towards biosynthesis requires a better understanding of the biological mechanisms of electron transport as well as detailed evaluation of all aspects of process performance. More advanced solutions may deploy cell free systems that use ex situ generated reducing equivalents to improve economic performance.

Enzymatic Hydrogen Electrosynthesis at Enhanced Current Density Using a Redox Polymer

High-temperature tolerant enzymes offer multiple advantages over enzymes from mesophilic organisms for the industrial production of sustainable chemicals due to high specific activities and stabilities towards fluctuations in pH, heat, and organic solvents. The production of molecular hydrogen (H2) is of particular interest because of the multiple uses of hydrogen in energy and chemicals applications, and the ability of hydrogenase enzymes to reduce protons to H2 at a cathode. We examined the activity of Hydrogen-Dependent CO2 Reductase (HDCR) from the thermophilic bacterium Thermoanaerobacter kivui when immobilized in a redox polymer, cobaltocene-functionalized polyallylamine (Cc-PAA), on a cathode for enzyme-mediated H2 formation from electricity. The presence of Cc-PAA increased reductive current density 340-fold when used on an electrode with HDCR at 40 °C, reaching unprecedented current densities of up to 3 mA·cm−2 with minimal overpotential and high faradaic efficiency. In contrast to other hydrogenases, T. kivui HDCR showed substantial reversibility of CO-dependent inactivation, revealing an opportunity for usage in gas mixtures containing CO, such as syngas. This study highlights the important potential of combining redox polymers with novel enzymes from thermophiles for enhanced electrosynthesis.

Reversible catalysis

We describe as 'reversible' a bidirectional catalyst that allows a reaction to proceed at a significant rate in response to even a small departure from equilibrium, resulting in fast and energy-efficient chemical transformation. Examining the relation between reaction rate and thermodynamic driving force is the basis of electrochemical investigations of redox reactions, which can be catalysed by metallic surfaces and biological or synthetic molecular catalysts. This relation has also been discussed in the context of biological energy transduction, regarding the function of biological molecular machines that harness chemical reactions to do mechanical work. This Perspective describes mean-field kinetic modelling of these three types of systems - surface catalysts, molecular catalysts of redox reactions and molecular machines - with the goal of unifying concepts in these different fields. We emphasize that reversibility should be distinguished from other figures of merit, such as rate or directionality, before its design principles can be identified and used to engineer synthetic catalysts.

Following Electroenzymatic Hydrogen Production by Rotating Ring-Disk Electrochemistry and Mass Spectrometry*

Gas-processing metalloenzymes are of interest to future bio- and bioinspired technologies. Of particular importance are hydrogenases and nitrogenases, which both produce molecular hydrogen (H₂ ) from proton (H⁺ ) reduction. Herein, we report on the use of rotating ring-disk electrochemistry (RRDE) and mass spectrometry (MS) to follow the production of H₂ and isotopes produced from deuteron (D⁺ ) reduction (HD and D₂ ) using the [FeFe]-hydrogenase from Clostridium pasteurianum, a model hydrogen-evolving metalloenzyme. This facilitates enzymology studies independent of non-innocent chemical reductants. We anticipate that these approaches will be of value in resolving the catalytic mechanisms of H₂ -producing metalloenzymes and the design of bioinspired catalysts for H₂ production and N₂ fixation.

From Enzyme Stability to Enzymatic Bioelectrode Stabilization Processes

Bioelectrocatalysis using redox enzymes appears as a sustainable way for biosensing, electricity production, or biosynthesis of fine products. Despite advances in the knowledge of parameters that drive the efficiency of enzymatic electrocatalysis, the weak stability of bioelectrodes prevents large scale development of bioelectrocatalysis. In this review, starting from the understanding of the parameters that drive protein instability, we will discuss the main strategies available to improve all enzyme stability, including use of chemicals, protein engineering and immobilization. Considering in a second step the additional requirements for use of redox enzymes, we will evaluate how far these general strategies can be applied to bioelectrocatalysis.

Putative Extracellular Electron Transfer in Methanogenic Archaea

It has been suggested that a few methanogens are capable of extracellular electron transfers. For instance, Methanosarcina barkeri can directly capture electrons from the coexisting microbial cells of other species. Methanothrix harundinacea and Methanosarcina horonobensis retrieve electrons from Geobacter metallireducens via direct interspecies electron transfer (DIET). Recently, Methanobacterium, designated strain YSL, has been found to grow via DIET in the co-culture with Geobacter metallireducens. Methanosarcina acetivorans can perform anaerobic methane oxidation and respiratory growth relying on Fe(III) reduction through the extracellular electron transfer. Methanosarcina mazei is capable of electromethanogenesis under the conditions where electron-transfer mediators like H₂ or formate are limited. The membrane-bound multiheme c-type cytochromes (MHC) and electrically-conductive cellular appendages have been assumed to mediate the extracellular electron transfer in bacteria like Geobacter and Shewanella species. These molecules or structures are rare but have been recently identified in a few methanogens. Here, we review the current state of knowledge for the putative extracellular electron transfers in methanogens and highlight the opportunities and challenges for future research.

Reversible H2 Oxidation and Evolution by Hydrogenase Embedded in a Redox Polymer Film

Efficient electrocatalytic energy conversion requires the devices to function reversibly, i.e. deliver a significant current at minimal overpotential. Redox-active films can effectively embed and stabilise molecular electrocatalysts, but mediated electron transfer through the film typically makes the catalytic response irreversible. Here, we describe a redox-active film for bidirectional (oxidation or reduction) and reversible hydrogen conversion, consisting of [FeFe] hydrogenase embedded in a low-potential, 2,2’-viologen modified hydrogel. When this catalytic film served as the anode material in a H₂/O₂ biofuel cell, an open circuit voltage of 1.16 V was obtained - a benchmark value near the thermodynamic limit. The same film also acted as a highly energy efficient cathode material for H₂ evolution. We explained the catalytic properties using a kinetic model, which shows that reversibility can be achieved despite intermolecular electron transfer being slower than catalysis. This understanding of reversibility simplifies the design principles of highly efficient and stable bioelectrocatalytic films, advancing their implementation in energy conversion.

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.

Closing the Gap for Electronic Short-Circuiting: Photosystem I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices

Well-defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi-artificial energy conversion devices, capable of providing unidirectional electron flow when light-harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short-circuiting processes that typically limit the performance of PSI-based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light-induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]-hydrogenase confirms the possibility to realize light-induced H2 evolution.

Proton Transfer Mechanisms in Bimetallic Hydrogenases

Hydrogenases are metalloenzymes that catalyze proton reduction and H₂ oxidation with outstanding efficiency. They are model systems for bioinorganic chemistry, including low-valent transition metals, hydride chemistry, and proton-coupled electron transfer. In this Account, we describe how photochemistry and infrared difference spectroscopy can be used to identify the dynamic hydrogen-bonding changes that facilitate proton transfer in [NiFe]- and [FeFe]-hydrogenase.[NiFe]-hydrogenase binds a heterobimetallic nickel/iron site embedded in the protein by four cysteine ligands. [FeFe]-hydrogenase carries a homobimetallic iron/iron site attached to the protein by only a single cysteine. Carbon monoxide and cyanide ligands in the active site facilitate detailed investigations of hydrogenase catalysis by infrared spectroscopy because of their strong signals and redox-dependent frequency shifts. We found that specific redox-state transitions in [NiFe]- and [FeFe]-hydrogenase can be triggered by visible light to record extremely sensitive "light-minus-dark" infrared difference spectra monitoring key amino acid residues. As these transitions are coupled to protonation changes, our data allowed investigation of dynamic hydrogen-bonding changes that go well beyond the resolution of protein crystallography.In [NiFe]-hydrogenase, photolysis of the bridging hydride ligand in the Ni-C state was followed by infrared difference spectroscopy. Our data clearly indicate the formation of a protonated cysteine residue as well as hydrogen-bonding changes involving a glutamic acid residue and a "dangling water" molecule. These findings are in excellent agreement with crystallographic analyses of [NiFe]-hydrogenase. In [FeFe]-hydrogenase, an external redox dye was used to accumulate the Hred state. Infrared difference spectra indicate hydrogen-bonding changes involving two glutamic acid residues and a conserved arginine residue. While crystallographic analyses of [FeFe]-hydrogenase in the oxidized state failed to explain the rapid proton transfer because of a breach in the succession of residues, our findings facilitated a precise molecular model of discontinued proton transfer.Comparing both systems, our data emphasize the role of the outer coordination sphere in bimetallic hydrogenases: we suggest that protonation of a nickel-ligating cysteine in [NiFe]-hydrogenase causes the notable preference toward H₂ oxidation. On the contrary, proton transfer in [FeFe]-hydrogenase involves an adjacent cysteine as a relay group, promoting both H₂ oxidation and proton reduction. These observations may guide the design of organometallic compounds that mimic the catalytic properties of hydrogenases.