An NADH-Inspired Redox Mediator Strategy to Promote Second-Sphere Electron and Proton Transfer for Cooperative Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin

Design of Cr-Based Molecular Electrocatalyst Systems for the CO2 Reduction Reaction

ConspectusHuman influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO2), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO2 through the use of homogeneous electrocatalysts generally follows a two-electron/two-proton pathway to produce either carbon monoxide (CO) with water (H2O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer-Tropsch process uses CO and H2 to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO2 to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO2 (CO2RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO2RR. However, metals early in the table have sufficient reducing power to mediate the CO2RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO2RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redox-active molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons─often accompanied by protons─to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO2 activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.

Improving co-electrocatalytic carbon dioxide reduction by optimizing the relative potentials of the redox mediator and catalyst

The effects of fixing the redox mediator (RM) reduction potential relative to a series of Cr-centered complexes capable of the reduction of CO2 to CO are disclosed. The greatest co-electrocatalytic activity enhancement is observed when the reduction potentials of the catalyst and RM are identical, implying that controlling the speciation of the Cr complex relative to RM activation is essential for improving catalytic performance. In all cases, the potential where co-catalytic activity is observed matches the reduction potential of the RM, regardless of the relative reduction potential of the Cr complex.

Hydrogen production in microbial electrolysis cells with biocathodes

Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe-electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production.

The meso-substituent electronic effect of Fe porphyrins on the electrocatalytic CO2 reduction reaction

We report Fe porphyrins bearing different meso-substituents for the electrocatalytic CO2 reduction reaction (CO2RR). By replacing two and four meso-phenyl groups of Fe tetraphenylporphyrin (FeTPP) with strong electron-withdrawing pentafluorophenyl groups, we synthesized FeF10TPP and FeF20TPP, respectively. We showed that FeTPP and FeF10TPP are active and selective for CO2-to-CO conversion in dimethylformamide with the former being more active, but FeF20TPP catalyzes hydrogen evolution rather than the CO2RR under the same conditions. Experimental and theoretical studies revealed that with more electron-withdrawing meso-substituents, the Fe center becomes electron-deficient and it becomes difficult for it to bind a CO2 molecule in its formal Fe0 state. This work is significant to illustrate the electronic effects of catalysts on binding and activating CO2 molecules and provide fundamental knowledge for the design of new CO2RR catalysts.

Electrochemically decoupled reduction of CO2 to formate over a dispersed heterogeneous bismuth catalyst enabled via redox mediators

Electrochemical CO2 reduction is a topic of major interest in contemporary research as an approach to use renewably-derived electricity to synthesise useful hydrocarbons from waste CO2. Various strategies have been developed to optimise this challenging reaction at electrode interfaces, but to-date, decoupled electrolysis has not been demonstrated for the reduction of CO2. Decoupled electrolysis aims to use electrochemically-derived charged redox mediators - electrical charge and potential vectors - to separate catalytic product formation from the electrode surface. Utilising an electrochemically generated highly reducing redox mediator; chromium propanediamine tetraacetate, we report the first successful application of decoupled electrolysis to electrochemical CO2 reduction. A study of metals and metal composites found formate to be the most accessible product, with bismuth metal giving the highest selectivity. Copper, tin, gold, nickel and molybdenum carbide heterogeneous catalysts were also investigated, in which cases H2 was found to be the major product, with minor yields of two-electron CO2 reduction products. Subsequent optimisation of the bismuth catalyst achieved a high formate selectivity of 85%. This method represents a radical new approach to CO2 electrolysis, which may be coupled directly with renewable energy storage technology and green electricity.

Constructing a CdS QDs/silica gel composite with high photosensitivity and prolonged recyclable operability for enhanced visible-light-driven NADH regeneration

Visible-light-driven nicotinamide adenine dinucleotide (NADH) regeneration is one of the most effective measures, and cadmium sulfide (CdS) materials are typically used as low-cost photocatalysts. The CdS photocatalysts, however, still suffer from low regeneration efficiency and poor cycle stability. In this work, the CdS quantum dots (QDs) less than 10 nm embedded onto silica gel (CdS QDs/Silica gel) were constructed for visible-light-driven NADH regeneration by a successive ionic layer adsorption reaction and ball milling method. Results demonstrate that the photosensitivity of the CdS QDs/Silica gel composite was 31 times higher than that of the bulk CdS. Moreover, the conduction band (CB) edge of the CdS QDs/Silica gel composite is -1.34 eV, which is more negative 0.5 eV than that of the bulk CdS. The obtained CdS QDs/Silica gel composites showed the highest NADH regeneration yields of 68.8% under visible-light (LED, 420 nm) illumination and can be reused for over 40 cycles. Finally, the bioactivity of NADH toward enzyme catalysis is further confirmed by the hydrogenation of benzaldehyde to benzyl alcohol catalyzed with an alcohol dehydrogenase as enzyme catalysis.

Co-electrocatalytic CO2 reduction mediated by a dibenzophosphole oxide and a chromium complex

We report a co-electrocatalytic system for the selective reduction of CO₂ to CO, comprised of a previously reported molecular Cr complex and 5-phenylbenzo[b]phosphindole-5-oxide (PhBPO) as a redox mediator. Under protic conditions, the co-electrocatalytic system attains a turnover frequency (TOF) of 15 s^-1 and quantitative selectivity for CO. It is proposed that PhBPO interacts with the Cr-based catalyst, coordinating in an axial position trans to an intermediate hydroxycarbonyl species, M-CO₂H, mediating electron transfer to the catalyst and lowering the barrier for C-OH bond cleavage.

Redox Mediators in Homogeneous Co-electrocatalysis

Homogeneous electrocatalysis has been well studied over the past several decades for the conversion of small molecules to useful products for green energy applications or as chemical feedstocks. However, in order for these catalyst systems to be used in industrial applications, their activity and stability must be improved. In naturally occurring enzymes, redox equivalents (electrons, often in a concerted manner with protons) are delivered to enzyme active sites by small molecules known as redox mediators (RMs). Inspired by this, co-electrocatalytic systems with homogeneous catalysts and RMs have been developed for the conversion of alcohols, nitrogen, unsaturated organic substrates, oxygen, and carbon dioxide. In these systems, the RMs have been shown to both increase the activity of the catalyst and shift selectivity to more desired products by altering catalytic cycles and/or avoiding high-energy intermediates. However, the area is currently underdeveloped and requires additional fundamental advancements in order to become a more general strategy. Here, we summarize the recent examples of homogeneous co-electrocatalysis and discuss possible future directions for the field.

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction

We present a supramolecular approach to catalyzing photochemical CO2 reduction through second-sphere porosity and charge effects. An iron porphyrin box (PB) bearing 24 cationic groups, FePB-2(P), was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO2 reduction reaction (CO2 RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOFmax ) reaching 1400 min-1 . The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin (FeTPP) catalyst, which is far greater than analogs that augment catalysis through porosity (FePB-3(N), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin (Fe-p-TMA), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.

Characterization of the interaction of metal-protoporphyrins photosensitizers with β- lactoglobulin

We investigated the interaction of a series of metal-protoporphyrins (PPIXs) with bovine β- lactoglobulin (BLG) using a combination of optical spectroscopy and computational simulations. Unlike other studies, the simulations were not merely used to rationalize the experimental data but were employed to refine the experimental data itself. The study was carried out at two pH values, 5 and 9, where BLG is known to have different conformation dictated by the so-called Tanford transition which occurs near pH 7.5. The transition is postulated to regulate access to the interior binding cavity of the protein, thus the pH variation was used as a parameter to investigate whether PPIXs access the central cavity of BLG. The results of our study show that indeed binding increases significantly at alkaline pH, however, the increased affinity is not due to the accessibility of the central cavity. Instead, binding appears to be determined by the tendency of PPIXs to form large inhomogeneous aggregates at acidic pH which hinders interactions with proteins. The binding site determined through a combination of experimental and computational methods is located at the interface between two BLG monomers where the long α-helix segment of the protein face each other. This region is rich in positively charged Lys residues that interact with the propionic acid chains of the protoporphyrins. Establishing the modality of binding between protoporphyrins and BLG would have important consequences for the use of BLG:PPIX complexes in applications such as artificial photoreceptors, artificial metallo-enzymes, delivery of photosensitizers for phototherapy and even solar energy conversion.

Advances in Biomimetic Photoelectrocatalytic Reduction of Carbon Dioxide

Emerging photoelectrocatalysis (PEC) systems synergize the advantages of electrocatalysis (EC) and photocatalysis (PC) and are considered a green and efficient approach to CO₂ conversion. However, improving the selectivity and conversion rate remains a major challenge. Strategies mimicking natural photosynthesis provide a prospective way to convert CO₂ with high efficiency. Herein, several typical strategies are described for constructing biomimetic photoelectric functional interfaces; such interfaces include metal cocatalysts/semiconductors, small molecules/semiconductors, molecular catalysts/semiconductors, MOFs/semiconductors, and microorganisms/semiconductors. The biomimetic PEC interface must have enhanced CO₂ adsorption capacity, preferentially activate CO₂ , and have an efficient conversion ability; with these properties, it can activate CO bonds effectively and promote electron transfer and CC coupling to convert CO₂ to single-carbon or multicarbon products. Interfacial electron transfer and proton coupling on the biomimetic PEC interface are also discussed to clarify the mechanism of CO₂ reduction. Finally, the existing challenges and perspectives for biomimetic photoelectrocatalytic CO₂ reduction are presented.

Electrocatalytic metal hydride generation using CPET mediators

Transition metal hydrides (M-H) are ubiquitous intermediates in a wide range of enzymatic processes and catalytic reactions, playing a central role in H+/H2 interconversion1, the reduction of CO2 to formic acid (HCOOH)2 and in hydrogenation reactions. The facile formation of M-H is a critical challenge to address to further improve the energy efficiency of these reactions. Specifically, the easy electrochemical generation of M-H using mild proton sources is key to enable high selectivity versus competitive CO and H2 formation in the CO2 electroreduction to HCOOH, the highest value-added CO2 reduction product3. Here we introduce a strategy for electrocatalytic M-H generation using concerted proton-electron transfer (CPET) mediators. As a proof of principle, the combination of a series of CPET mediators with the CO2 electroreduction catalyst [MnI(bpy)(CO)3Br] (bpy = 2,2'-bipyridine) was investigated, probing the reversal of the product selectivity from CO to HCOOH to evaluate the efficiency of the manganese hydride (Mn-H) generation step. We demonstrate the formation of the Mn-H species by in situ spectroscopic techniques and determine the thermodynamic boundary conditions for this mechanism to occur. A synthetic iron-sulfur cluster is identified as the best CPET mediator for the system, enabling the preparation of a benchmark catalytic system for HCOOH generation.

Deciphering Distinct Overpotential-Dependent Pathways for Electrochemical CO2 Reduction Catalyzed by an Iron-Terpyridine Complex

[Fe(tpyPY2Me)]²⁺ ([Fe]²⁺) is a homogeneous electrocatalyst for converting CO₂ into CO featuring low overpotentials of <100 mV, near-unity selectivity, and high activity with turnover frequencies faster than 100 000 s^-1. To identify the origins of its exceptional performance and inform future catalyst design, we report a combined computational and experimental study that establishes two distinct mechanistic pathways for electrochemical CO₂ reduction catalyzed by [Fe]²⁺ as a function of applied overpotential. Electrochemical data shows the formation of two catalytic regimes at low (η_TOF/2 of 160 mV) and high (η_TOF/2 of 590 mV) overpotential plateaus. We propose that at low overpotentials [Fe]²⁺ undergoes a two-electron reduction, two-proton-transfer mechanism (electrochemical-electrochemical-chemical-chemical, EECC), where turnover occurs through the dicationic iron complex, [Fe]²⁺. Computational analysis supports the importance of the singlet ground-state electronic structure for CO₂ binding and that the rate-limiting step is the second protonation in this low-overpotential regime. When more negative potentials are applied, an additional electron-transfer event occurs through either a stepwise or proton-coupled electron-transfer (PCET) pathway, enabling catalytic turnover from the monocationic iron complex ([Fe]⁺) via an electrochemical-chemical-electrochemical-chemical (ECEC) mechanism. Comparison of experimental kinetic data obtained from variable controlled potential electrolysis (CPE) experiments with direct product detection with calculated rates obtained from the energetic span model supports the PCET pathway as the most likely mechanism. Moreover, we build upon this mechanistic understanding to propose the design of an improved ligand framework that is predicted to stabilize the key transition states identified in our study and explore their electronic structures using an energy decomposition analysis. Taken together, this work highlights the value of synergistic computational/experimental approaches to decipher mechanisms of new electrocatalysts and direct the rational design of improved platforms.

Porous silver microrods by plasma vulcanization activation for enhanced electrocatalytic carbon dioxide reduction

Metal electrode is considered as an ideal candidate for electrocatalytic carbon dioxide (CO₂) reduction considering its excellent chemical stability, application potential and eco-friendly properties. Optimization process such as morphological control, non-metallic doping, alloying is widely studied to improve the efficiency of metal electrodes. In this work, we successfully improved the CO₂ reduction performance of silver using a facile plasma vulcanization treatment. The obtained sulfide derived silver (Ag) porous microrods (SD-AgPMRs) are optimized from both morphology and composition aspects, and demonstrates high Faradaic efficiency and partial current density for carbon monoxide (CO) production at low potentials. The larger specific surface area of porous microrod structure and the improved adsorption energy of important intermediates in comparison with Ag foil are realized by introduction of sulfur (S) atoms after plasma vulcanization activation, as suggested by density functional theory (DFT) calculations. This work presents a novel strategy to optimize metal electrocatalysts for CO₂ reduction as well as to improve catalysis in other fields.

Mediated Inner-Sphere Electron Transfer Induces Homogeneous Reduction of CO2 via Through-Space Electronic Conjugation

The electrocatalytic reduction of CO₂ is an appealing method for converting renewable energy sources into value-added chemical feedstocks. We report a co-electrocatalytic system for the reduction of CO₂ to CO comprised of a molecular Cr complex and dibenzothiophene-5,5-dioxide (DBTD) as a redox mediator, which achieves high activity (TOF=1.51-2.84×10⁵  s^-1 ) and quantitative selectivity. Under aprotic or protic conditions, DBTD produces a co-electrocatalytic response with 1 by coordinating trans to the site of CO₂ binding and mediating electron transfer from the electrode with quantitative efficiency for CO. This assembly is reliant on through-space electronic conjugation between the π frameworks of DBTD and the bpy fragment of the catalyst ligand, with contributions from dispersive interactions and weak sulfone coordination.

Inverse Potential Scaling in Co-Electrocatalytic Activity for CO2 Reduction Through Redox Mediator Tuning and Catalyst Design

Electrocatalytic CO2 reduction is an attractive strategy to mitigate the continuous rise in atmospheric CO2 concentrations and generate value-added chemical products. A possible strategy to increase the activity of molecular...

Biohybrid Systems for Improved Bioinspired, Energy-Relevant Catalysis

Biomimetic catalysts, ranging from small-molecule metal complexes to supramolecular assembles, possess many exciting properties that could address salient challenges in industrial-scale manufacturing. Inspired by natural enzymes, these biohybrid catalytic systems demonstrate superior characteristics, including high activity, enantioselectivity, and enhanced aqueous solubility, over their fully synthetic counterparts. However, instability and limitations in the prediction of structure-function relationships are major drawbacks that often prevent the application of biomimetic catalysts outside of the laboratory. Despite these obstacles, recent advances in synthetic enzyme models have improved our understanding of complicated biological enzymatic processes and enabled the production of catalysts with increased efficiency. This review outlines important developments and future prospects for the design and application of bioinspired and biohybrid systems at multiple length scales for important, biologically relevant, clean energy transformations.

Ligand Conjugation Directs the Formation of a 1,3-Dihydropyridinate Regioisomer

The selective formation of the 1,4-dihydropyridine isomer of NAD(P)H is mirrored by the selective formation of 1,4-dihydropyridinate ligand-metal complexes in synthetic systems. Here we demonstrate that ligand conjugation can be used to promote selective 1,3-dihydropyridinate formation. This represents an advance toward controlling and tuning the selectivity in dihydropyridinate formation chemistry. The reaction of (I₂P^2-)Al(THF)Cl [1; I₂P = bis(imino)pyridine; THF = tetrahydrofuran] with the one-electron oxidant (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) afforded (I₂P^-)Al(TEMPO)Cl (2), which can be reduced with sodium to the twice-reduced ligand complex (I₂P^2-)Al(TEMPO) (3). Compounds 2 and 3 serve as precursors for high-yielding and selective routes to an aluminum-supported 1,3-dihydropyridinate complex via the reaction of 2 with 3 equiv of potassium metal or the reaction of 3 with KH.

Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH₂) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H₂) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH₂) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photoredox catalyst, and a cobalt(III) complex, Co^III(dmgH)₂pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H₂ and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H₂ evolution from tetrachlorohydroquinone (Cl₄QH₂) with Acr⁺-Mes and Co^III(dmgH)₂pyCl and H₂O in deaerated MeCN was determined to be 10%. Photocatalytic H₂ evolution is started by electron transfer (ET) from Cl₄QH₂ to the triplet ET state of Acr⁺-Mes to produce Cl₄QH₂^•+ and Acr^•-Mes with a rate constant of 7.2 × 10⁷ M^-1 s^-1, followed by ET from Acr^•-Mes to Co^III(dmgH)₂pyCl to produce [Co^II(dmgH)₂pyCl]^-, accompanied by the regeneration of Acr⁺-Mes. On the other hand, Cl₄QH₂^•+ is deprotonated to produce Cl₄QH^•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co^II(dmgH)₂pyCl]^- to produce a cobalt(III) hydride complex, [Co^III(H)(dmgH)₂pyCl]^-, which reacts with H⁺ to evolve H₂, accompanied by the regeneration of Co^III(dmgH)₂pyCl. The formation of [Co^II(dmgH)₂pyCl]^- was detected by electron paramagnetic resonance measurements.

Multifaceted aspects of charge transfer

Charge transfer and charge transport are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible. Involving multiple electron-transfer steps, photosynthesis and cellular respiration have been principally responsible for managing the energy flow in the biosphere of our planet since the Great Oxygen Event. It is impossible to imagine living organisms without charge transport mediated by ion channels, or electron and proton transfer mediated by redox enzymes. Concurrently, transfer and transport of electrons and holes drive the functionalities of electronic and photonic devices that are intricate for our lives. While fueling advances in engineering, charge-transfer science has established itself as an important independent field, originating from physical chemistry and chemical physics, focusing on paradigms from biology, and gaining momentum from solar-energy research. Here, we review the fundamental concepts of charge transfer, and outline its core role in a broad range of unrelated fields, such as medicine, environmental science, catalysis, electronics and photonics. The ubiquitous nature of dipoles, for example, sets demands on deepening the understanding of how localized electric fields affect charge transfer. Charge-transfer electrets, thus, prove important for advancing the field and for interfacing fundamental science with engineering. Synergy between the vastly different aspects of charge-transfer science sets the stage for the broad global impacts that the advances in this field have.