Considering the Influence of Polymer-Catalyst Interactions on the Chemical Microenvironment of Electrocatalysts for the CO2 Reduction Reaction

Electroreduction of CO2 to Methanol Using a Coordination-Moiety-Anchored Carbon-Based Electrode

Electrochemical reduction of carbon dioxide (CO2ER) has gained wide attention lately because of its potential to create a closed carbon loop, offering a sustainable solution toward environmental as well as energy crisis. However, the key challenge lies in the selective conversion of CO2 into electrofuels, such as methanol, which necessitates six proton-coupled electron transfers. In this work, we report the first instance of an electrochemically prepared Cu-coordinated 2,5-dimercapto-1,3,4-thiadiazole-modified carbon fiber paper electrode (CDM@CFP). The hence-engineered novel electrode was applied for the CO2ER reaction to produce methanol exclusively with an F.E. of 59.6% at a low potential of -0.73 V versus RHE. Unlike most of the copper-based electrocatalysts, which result in multiple hydrocarbons, here, we have optimized a potential-dependent selectivity for maximum efficiency, which is a significant milestone in the field.

Tuning the microenvironment of immobilized molecular catalysts for selective electrochemical CO2 reduction

The electrochemical CO2 reduction reaction (CO2RR), as a novel technology, holds great promise for carbon neutrality. Immobilized molecular catalysts are considered efficient CO2RR catalysts due to their high selectivity and fast electron transfer rates. However, at high current densities, changes in the microenvironment of molecular catalysts result in a decrease in the local CO2 concentration, leading to suboptimal catalytic performance. This work describes an effective strategy to control the local CO2 concentration by manipulating the hydrophobicity. The obtained catalyst exhibits high CO selectivity with a Faradaic efficiency (FE) of 96% in a membrane electrode assembly. Moreover, a consistent FE exceeding 85% could be achieved with a total current of 0.8 A. Diffusion impedance testing and interface characterization confirm that the enhanced hydrophobicity of the catalyst layer leads to an increase in the thickness of the Nernst diffusion layer and an expansion of the three-phase interface, thereby accelerating CO2 adsorption to enhance the performance.

Polymer Binder Blends Stabilize Alkaline Hydrogen Evolution by Heterogenized Molecular Phen-Based Cobalt Electrocatalysts through Coordination and Environmental Control

With the increase in greenhouse gas emissions and their detrimental effect on the environment, there is a push to develop a renewable way to produce H2, a fuel source that has nonharmful byproducts, unlike traditional methods of energy production. Alkaline water electrolysis has seen increasing focus as a viable way to produce H2, but efficient and stable electrocatalysts are required to facilitate this process. Here, a heterogenized Co(II) phenanthroline-based complex for the production of H2 from alkaline water is disclosed. Activity was improved by considering the role of axial Co ligation and the reaction environment created by the polymer binder in the catalyst inks by using variable ratios of Nafion and poly-4-vinylpyridine (P4VP). A ratio of 1:1 Nafion:P4VP was found to have the highest stability, and Co nanoparticle formation was not observed when P4VP was included as part of the binder mixture. The activity and stability enhancement could not be replicated by the addition of molecular pyridine or the use of poly-2-vinylpyridine, which is sterically prevented from coordinating to Co. The increased electrochemical performance caused by the inclusion of Nafion as part of the polymer binder is attributed to a role in mass transfer to and from the Co active site during catalysis, complementing the stabilizing effect of P4VP on the molecular active site.

Electrocatalytic CO2 reduction by a cobalt porphyrin mini-enzyme

Cobalt-mimochrome VI*a (CoMC6*a), a synthetic mini-enzyme with a cobalt porphyrin active site, is developed as a biomolecular catalyst for electrocatalytic CO2 reduction in water. The catalytic turnover number reaches ∼14 000 for CO production with a selectivity of 86 : 5 over H2 production under the same conditions. Varying the applied potential and the pK a of the proton donor was used to gain insight into the basis for selectivity. The protected active site of CoMC6*a is proposed to enhance selectivity for CO2 reduction under conditions that typically favor H2 production by related catalysts. CoMC6*a activity and selectivity change only marginally under air, indicating excellent oxygen tolerance.

Advancing the Preparation Strategy of High-Performance Integrated Electrodes for eCO2RR via Sublimation

The uniform dispersion and loading of phthalocyanine molecular catalysts on conductive carbon substrates are crucial for exposing their active sites. The significant amount of solvent needed to achieve appropriate dispersion of phthalocyanine leads to the risk of reaggregation during solvent evaporation. Hence, a solventless strategy is adopted by many to bypass the use of a solvent. In this study, we showcase the deposition of transition metal phthalocyanine (TMPc) molecules onto a self-supporting conductive carbon cloth electrode using an environmentally friendly sublimation technique for efficient electrocatalytic CO2 reduction. We meticulously investigated the preparation conditions, including the heating temperature and TMPc type, to assess their impact on the CO2 reduction activity. The as-prepared CC-CoPc-450 electrode demonstrated an outstanding comprehensive performance, showcasing a remarkable maximum CO Faradaic efficiency (FECO) of 97.1% at -0.86 V with a current density of 8.3 mA cm-2. The electrode exhibited excellent stability during the 16 h long-term eCO2RR process. Density functional theory (DFT) calculations demonstrated the role of d-orbitals in TM-N4 and the synergy with π-conjugation electrons in facilitating the efficient electron transfer process in eCO2RR. This study offers a fresh perspective on the eco-friendly dispersion of TMPcs on conductive substrates and provides insights into the design of π-species macrocyclic electrocatalyst electrodes.

Amine groups alter product selectivity and rate of catalytic hydride transfer reactions

Primary amines are common functional groups in the reaction environment surrounding an (electro)catalyst, and this includes catalysts ranging from metalloenzymes surrounded by amino acids, to electrocatalysts operating in amine industrial sorbents for CO2 capture and conversion. This report explores the behavior of amine functional groups at the surface of an electrocatalyst. The possible effects of those amine secondary coordination sphere (SCS) groups on a CO2 electro-reduction mechanism include stabilization of intermediates and positioning substrate near the active site. Two different clusters were synthesized: [PPN][Fe4N(CO)11(Ph2PCH2CH2NH2)] (PPN-1) has one amine, and [PPN][Fe4N(CO)10(Ph2PCH2CH2NH2)2] (PPN-2) has two covalently appended amine functional groups (PPN: bis(triphenylphosphine)iminium). Infra-red spectroscopic studies show a direct reaction of each cluster with CO2 to afford an SCS carbamate functional group, and cyclic voltammetry investigations reveal a variety of roles for the amine SCS groups in the mechanism of catalyst hydride formation and hydride transfer (HT) to CO2. The most prominent effect of the amine functional group is stabilization of the intermediate hydride to lower formate yield. With PPN-1, these combined effects serve to shut down HT to CO2. With PPN-2, the combined effects result in some loss of selectivity, so that formate and H2 mixtures (6 : 1) are obtained.

Engineered metallobiocatalysts for energy-relevant reactions

Engineering metallobiocatalysts is a promising approach to addressing challenges in energy-relevant electrocatalysis and photocatalysis. The design freedom provided by semisynthetic and fully synthetic approaches to catalyst design allows researchers to demonstrate how structural modifications can improve selectivity and activity of biocatalysts. Furthermore, the provision of a superstructure in many metallobiocatalysts facilitates active-site microenvironment engineering. Recurring themes include the role of the biomolecular scaffold in enhancing reactivity in water and catalyst robustness, the impact of the outer sphere on reactivity, and the importance of tuning system components in full system optimization. In this perspective, recent strategies to design and modify novel biocatalysts, understand proton and electron transfer mechanisms, and tune system activity by modifying catalysts and system conditions are highlighted within the field of energy-related catalysis. Opportunities in this field include developing robust structure-function relationships to support approaches to engineering second-sphere interactions and identifying ways to enhance biocatalyst activity over time.

Mitigating Cobalt Phthalocyanine Aggregation in Electrocatalyst Films through Codeposition with an Axially Coordinating Polymer

Cobalt phthalocyanine (CoPc) is a promising molecular catalyst for aqueous electroreduction of CO2, but its catalytic activity is limited by aggregation at high loadings. Codeposition of CoPc onto electrode surfaces with the coordinating polymer poly(4-vinylpyridine) (P4VP) mitigates aggregation in addition to providing other catalytic enhancements. Transmission and diffuse reflectance UV-vis measurements demonstrate that a combination of axial coordination and π-stacking effects from pyridyl moieties in P4VP serve to disperse cobalt phthalocyanine in deposition solutions and help prevent reaggregation in deposited films. Polymers lacking axial coordination, such as Nafion, are significantly less effective at cobalt phthalocyanine dispersion in both the deposition solution and in the deposited films. SEM images corroborate these findings through particle counts and morphological analysis. Electrochemical measurements show that CoPc codeposited with P4VPonto carbon electrode surfaces reduces CO2 with higher activity and selectivity compared to the catalyst codeposited with Nafion.

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

Modified Working Electrodes for Organic Electrosynthesis

Organic electrosynthesis has gained much attention over the last few decades as a promising alternative to traditional synthesis methods. Electrochemical approaches offer numerous advantages over traditional organic synthesis procedures. One of the most interesting aspects of electroorganic synthesis is the ability to tune many parameters to affect the outcome of the reaction of interest. One such parameter is the composition of the working electrode. By changing the electrode material, one can influence the selectivity, product distribution, and rate of organic reactions. In this Review, we describe several electrode materials and modifications with applications in organic electrosynthetic transformations. Included in this discussion are modifications of electrodes with nanoparticles, composite materials, polymers, organic frameworks, and surface-bound mediators. We first discuss the important physicochemical and electrochemical properties of each material. Then, we briefly summarize several relevant examples of each class of electrodes, with the goal of providing readers with a catalog of electrode materials for a wide variety of organic syntheses.

Microenvironment Engineering of Heterogeneous Catalysts for Liquid-Phase Environmental Catalysis

Environmental catalysis has emerged as a scientific frontier in mitigating water pollution and advancing circular chemistry and reaction microenvironment significantly influences the catalytic performance and efficiency. This review delves into microenvironment engineering within liquid-phase environmental catalysis, categorizing microenvironments into four scales: atom/molecule-level modulation, nano/microscale-confined structures, interface and surface regulation, and external field effects. Each category is analyzed for its unique characteristics and merits, emphasizing its potential to significantly enhance catalytic efficiency and selectivity. Following this overview, we introduced recent advancements in advanced material and system design to promote liquid-phase environmental catalysis (e.g., water purification, transformation to value-added products, and green synthesis), leveraging state-of-the-art microenvironment engineering technologies. These discussions showcase microenvironment engineering was applied in different reactions to fine-tune catalytic regimes and improve the efficiency from both thermodynamics and kinetics perspectives. Lastly, we discussed the challenges and future directions in microenvironment engineering. This review underscores the potential of microenvironment engineering in intelligent materials and system design to drive the development of more effective and sustainable catalytic solutions to environmental decontamination.

Electrocatalytic Conversion of CO2 to Formic Acid: A Journey from 3d-Transition Metal-Based Molecular Catalyst Design to Electrolyzer Assembly

ConspectusElectrochemical CO2 reduction to obtain formate or formic acid is receiving significant attention as a method to combat the global warming crisis. Significant efforts have been devoted to the advancement of CO2 reduction techniques over the past few decades. This Account provides a unified discussion on various electrochemical methodologies for CO2 to formate conversion, with a particular focus on recent advancements in utilizing 3d-transition-metal-based molecular catalysts. This Account primarily focuses on understanding molecular functions and mechanisms under homogeneous conditions, which is essential for assessing the optimized reaction conditions for molecular catalysts. The unique architectural features of the formate dehydrogenase (FDH) enzyme provide insight into the key role of the surrounding protein scaffold in modulating the active site dynamics for stabilizing the key metal-bound CO2 intermediate. Additionally, the protein moiety also triggers a facile proton relay around the active site to drive electrocatalytic CO2 reduction forward. The fine-tuning of FDH machinery also ensures that the electrocatalytic CO2 reduction leads to the production of formic acid as the major yield without any other carbonaceous products, while limiting the competitive hydrogen evolution reaction. These lessons from the enzymes are key in designing biomimetic molecular catalysts, primarily based on multidentate ligand scaffolds containing peripheral proton relays. The subtle modifications of the ligand framework ensure the favored production of formic acid following electrocatalytic CO2 reduction in the solution phase. Next, the molecular catalysts are required to be mounted on robust electroactive surfaces to develop their corresponding heterogeneous versions. The surface-immobilization provides an edge to the molecular electrocatalysts as their reactivity can be scaled up with improved durability for long-term electrocatalysis. Despite challenges in developing high-performance, selective catalysts for the CO2 to formic acid transformation, significant progress is being made with the tactical use of graphene and carbon nanotube-based materials. To date, the majority of the research activity stops here, as the development of an operational CO2 to formic acid converting electrolyzer prototype still remains in its infancy. To elaborate on the potential future steps, this Account covers the design, scaling parameters, and existing challenges of assembling large-scale electrolyzers. A short glimpse at the utilization of electrolyzers for industrial-scale CO2 reduction is also provided here. The proper evaluation of the surface-immobilized electrocatalysts assembled in an electrolyzer is a key step for gauging their potential for practical viability. Here, the key electrochemical parameters and their expected values for industrial-scale electrolyzers have been discussed. Finally, the techno-economic aspects of the electrolyzer setup are summarized, completing the journey from tactical design of molecular catalysts to their appropriate application in a commercially viable electrolyzer setup for CO2 to formate electroreduction. Thus, this Account portrays the complete story of the evolution of a molecular catalyst to its sustainable application in CO2 utilization.

Rational Designing Microenvironment of Gas-Diffusion Electrodes via Microgel-Augmented CO2 Availability for High-Rate and Selective CO2 Electroreduction to Ethylene

Efficient electrochemical CO2 reduction reaction (CO2RR) requires advanced gas-diffusion electrodes (GDEs) with tunned microenvironment to overcome low CO2 availability in the vicinity of catalyst layer. Herein, for the first time, pyridine-containing microgels-augmented CO2 availability is presented in Cu2O-based GDE for high-rate CO2 reduction to ethylene, owing to the presence of CO2-phil microgels with amine moieties. Microgels as three-dimensional polymer networks act as CO2 micro-reservoirs to engineer the GDE microenvironment and boost local CO2 availability. The superior ethylene production performance of the GDE modified by 4-vinyl pyridine microgels, as compared with the GDE with diethylaminoethyl methacrylate microgels, indicates the bifunctional effect of pyridine-based microgels to enhance CO2 availability, and electrocatalytic CO2 reduction. While the Faradaic efficiency (FE) of ethylene without microgels was capped at 43% at 300 mA cm-2, GDE with the pyridine microgels showed 56% FE of ethylene at 700 mA cm-2. A similar trend was observed in zero-gap design, and GDEs showed 58% FE of ethylene at -4.0 cell voltage (>350 mA cm-2 current density), resulting in over 2-fold improvement in ethylene production. This study showcases the use of CO2-phil microgels for a higher rate of CO2RR-to-C2+, opening an avenue for several other microgels for more selective and efficient CO2 electrolysis.

Unlocking the Potential for Methanol Synthesis via Electrochemical CO2 Reduction Using CoPc-Based Molecular Catalysts

The electrochemical CO2 reduction reaction (CO2RR) to produce methanol (CH3OH) is an attractive yet challenging approach due to a lack of selective electrocatalysts. An immobilized cobalt phthalocyanine (CoPc) molecular catalyst has emerged as a promising electrocatalyst for CH3OH synthesis, demonstrating decent activity and selectivity through a CO2-CO-CH3OH cascade reaction. However, CoPc's performance is limited by its weak binding strength toward the CO intermediate. Recent advancements in molecular modification aimed at enhancing CO intermediate binding have shown great promise in improving CO2-to-CH3OH performance. In this Perspective, we discuss the competitive binding mechanism between CO2 and CO that hinders CH3OH formation and summarize effective molecular modification strategies that can enhance both the binding of the CO intermediate and the conversion of the CO2-to-CH3OH activity. Finally, we offer future perspectives on optimization strategies to inspire further research efforts to fully unlock the potential for methanol synthesis via the CO2RR using molecular catalysts.

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

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

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.

Confinement of ionomer for electrocatalytic CO2 reduction reaction via efficient mass transfer pathways

Gas diffusion electrodes (GDEs) mediate the transport of reactants, products and electrons for the electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assemblies. The random distribution of ionomer, added by the traditional physical mixing method, in the catalyst layer of GDEs affects the transport of ions and CO2. Such a phenomenon results in elevated cell voltage and decaying selectivity at high current densities. This paper describes a pre-confinement method to construct GDEs with homogeneously distributed ionomer, which enhances mass transfer locally at the active centers. The optimized GDE exhibited comparatively low cell voltages and high CO Faradaic efficiencies (FE > 90%) at a wide range of current densities. It can also operate stably for over 220 h with the cell voltage staying almost unchanged. This good performance can be preserved even with diluted CO2 feeds, which is essential for pursuing a high single-pass conversion rate. This study provides a new approach to building efficient mass transfer pathways for ions and reactants in GDEs to promote the electrocatalytic CO2RR for practical applications.

Multi-Functional Polymer Membranes Enable Integrated CO2 Capture and Conversion in a Single, Continuous-Flow Membrane Reactor under Mild Conditions

Herein, we present a membrane-based system designed to capture CO2 from dilute mixtures and convert the captured CO2 into value-added products in a single integrated process operated continuously under mild conditions. Specifically, we demonstrate that quaternized poly(4-vinylpyridine) (P4VP) membranes are selective CO2 separation membranes that are also catalytically active for cyclic carbonate synthesis from the cycloaddition of CO2 to epichlorohydrin. We further demonstrate that quaternized P4VP membranes can integrate CO2 capture, including from dilute mixtures down to 0.1 kPa of CO2, with CO2 conversion to cyclic carbonates at 57 °C and atmospheric pressure. The catalytic membrane acts as both the CO2 capture and conversion medium, providing an energy-efficient alternative to sorbent-based capture, compression, transport, and storage. The membrane is also potentially tunable for the conversion of CO2 to a variety of products, including chemicals and fuels not limited to cyclic carbonates, which would be a transformative shift in carbon capture and utilization technology.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

Single-molecule fluorescence microscopy enables the direct observation of individual reaction events at the surface of a catalyst. It has become a powerful tool to image in real time both intra- and interparticle heterogeneity among different nanoscale catalyst particles. Single-molecule fluorescence microscopy of heterogeneous catalysts relies on the detection of chemically activated fluorogenic probes that are converted from a nonfluorescent state into a highly fluorescent state through a reaction mediated at the catalyst surface. This review article describes challenges and opportunities in using such fluorogenic probes as proxies to develop structure-activity relationships in nanoscale electrocatalysts and photocatalysts. We compare single-molecule fluorescence microscopy to other microscopies for imaging catalysis in situ to highlight the distinct advantages and limitations of this technique. We describe correlative imaging between super-resolution activity maps obtained from multiple fluorogenic probes to understand the chemical origins behind spatial variations in activity that are frequently observed for nanoscale catalysts. Fluorogenic probes, originally developed for biological imaging, are introduced that can detect products such as carbon monoxide, nitrite, and ammonia, which are generated by electro- and photocatalysts for fuel production and environmental remediation. We conclude by describing how single-molecule imaging can provide mechanistic insights for a broader scope of catalytic systems, such as single-atom catalysts.

Amphiphilic Cobalt Phthalocyanine Boosts Carbon Dioxide Reduction

Due to the easy accessibility, chemical stability, and structural tunability of the macrocyclic skeleton, cobalt phthalocyanines immobilized on carbon supports offer an ideal research model for advanced electrochemical carbon dioxide reduction reaction (eCO2 RR). In this work, an amphiphilic cobalt phthalocyanine (TC-CoPc) is loaded on multiwalled carbon nanotubes to reveal the roles of hydrophilic/hydrophobic properties on catalytic efficiency. Surprisingly, the resultant electrode exhibits a CO Faradaic efficiency (FECO ) of 95% for CO2 RR with turnover frequency (TOF) of 29.4 s-1 at an overpotential of 0.585 V over long-term electrolysis in a H-type cell. In the membrane electrode assembly (MEA) device, the boosted transport of water vapor to the catalyst layer slows down carbonate crystallization and enhances the stability of the electrode, with FECO value of >99% over 27 h at -0.25 A, representing the best selectivity and stability among reported molecular catalysts in MEA devices. The amphiphilic cobalt phthalocyanine, which decreases interfacial charge and mass transfer resistance and maintains effective contact between active sites and the electrolyte, highlights the exceptional CO2 conversion from a molecular perspective.

Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing

Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining─the use of electrochemical processes to tune and recover specific products from wastewaters─as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncovers a mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

Contemporary Strategies for Immobilizing Metallophthalocyanines for Electrochemical Transformations of Carbon Dioxide

Metallophthalocyanine (PcM) coordination complexes are well-known mediators of the electrochemical reduction of carbon dioxide (CO2). They have many properties that show promise for practical applications in the energy sector. Such properties include synthetic flexibility, a high stability, and good efficiencies for the reduction of CO2 to useful feedstocks, such as carbon monoxide (CO). One of the ongoing challenges that needs to be met is the incorporation of PcM into the heterogeneous materials that are used in a great many CO2-reduction devices. Much progress has been made in the last decade and there are now several promising approaches to incorporate PcM into a range of materials, from simple carbon-adsorbed preparations to extended polymer networks. These approaches all have important advantages and drawbacks. In addition, investigations have led to new proposals regarding CO2 reduction catalytic cycles and other operational features that are crucial to function. Here, we describe developments in the immobilization of PcM CO2 reduction catalysts in the last decade (2013 to 2023) and propose promising avenues and strategies for future research.

Zn-induced electron-rich Sn catalysts enable highly efficient CO2 electroreduction to formate

Renewable-energy-driven CO2 electroreduction provides a promising way to address the growing greenhouse effect issue and produce value-added chemicals. As one of the bulk chemicals, formic acid/formate has the highest revenue per mole of electrons among various products. However, the scaling up of CO2-to-formate for practical applications with high faradaic efficiency (FE) and current density is constrained by the difficulty of precisely reconciling the competing intermediates (*COOH and HCOO*). Herein, a Zn-induced electron-rich Sn electrocatalyst was reported for CO2-to-formate with high efficiency. The faradaic efficiency of formate (FEformate) could reach 96.6%, and FEformate > 90% was maintained at formate partial current density up to 625.4 mA cm-1. Detailed study indicated that catalyst reconstruction occurred during electrolysis. With appropriate electron accumulation, the electron-rich Sn catalyst could facilitate the adsorption and activation of CO2 molecules to form a intermediate and then promoted the carbon protonation of to yield a HCOO* intermediate. Afterwards, the HCOO* → HCOOH* proceeded via another proton-coupled electron transfer process, leading to high activity and selectivity for formate production.

Engineering an Oxygen-Binding Protein for Photocatalytic CO2 Reductions in Water

While native CO2 -reducing enzymes display remarkable catalytic efficiency and product selectivity, few artificial biocatalysts have been engineered to allow understanding how the native enzymes work. To address this issue, we report cobalt porphyrin substituted myoglobin (CoMb) as a homogeneous catalyst for photo-driven CO2 to CO conversion in water. The activity and product selectivity were optimized by varying pH and concentrations of the enzyme and the photosensitizer. Up to 2000 TON(CO) was attained at low enzyme concentrations with low product selectivity (15 %), while a product selectivity of 74 % was reached by increasing the enzyme loading but with a compromised TON(CO). The efficiency of CO generation and overall TON(CO) were further improved by introducing positively charged residues (Lys or Arg) near the active stie of CoMb, which demonstrates the value of tuning the enzyme secondary coordination sphere to enhance the CO2 -reducing performance of a protein-based photocatalytic system.

Heterogeneous Structure of Sn/SnO2 Constructed via Phase Engineering for Efficient and Stable CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂RR) catalyzed by Sn-based materials shows great potential for CO₂-to-formate conversion. The presence of tin species with different oxidation states can promote the catalytic performance, most likely due to the interfaces of metallic and oxide phases that induce a synergistic effect. Therefore, it is desirable yet challenging to synthesize a hybrid catalyst with abundant active heterogeneous interfaces. Herein, we synthesize a hybrid catalyst constructed by decorating nanosized SnS₂ in the SnO₂ matrix. The uniformly distributed SnS₂ nanoparticles are first reduced to metallic tin, which assists in the generation of abundant Sn/SnO₂ heterogeneous interfaces under the in situ reduction process. Because of the electronic modulation at the heterogeneous interfaces, the resulting catalyst delivers a high current density of 200 mA·cm^-2 at -0.86 V vs RHE, and the performance is stable for over 20 h. This work suggests a potentially powerful interface engineering strategy for the development of high-performance electrocatalysts for the CO₂RR.

Two-Electron-Induced Reorganization of Cobalt Coordination and Metal-Ligand Cooperative Redox Shifting Co(I) Reactivity toward CO2 Reduction

Electrochemical reorganization of complex structures is directly related to catalytic reactivity; thus, the geometric changes of catalysts induced by electron transfer should be considered to scrutinize the reaction mechanism. Herein, we studied electron-induced reorganization patterns of six-coordinate Co complexes with neutral N-donor ligands. Upon two-electron transfer into a Co center enclosed within a bulky π-acceptor ligand, the catalytic site exhibited different reorganization patterns depending on the ligand characteristics. While a bipyridyl ligand released Co-bound solvent (CH₃CN) to open a reaction site, a phenanthroline ligand caused Co-N_arm (side "arm" of NNN-ligand) bond dissociation. The first electron transfer occurred in the Co(II/I) reduction step and the second electron entered the bulky π-acceptor, of which redox steps were assigned from cyclic voltammograms, magnetic moment measurements, and DFT calculations. In comparison, the Co complex of [NNN^NCH₃-Co(CH₃CN)₃](PF₆)₂ ([1-(CH₃CN)₃](PF₆)₂) showed a high H₂ evolution reactivity (HER), whereas a series of Co complexes with bulky π-acceptors such as [NNN^NCH₃-Co(L)(CH₃CN)](PF₆)₂ (L = phen ([2-CH₃CN](PF₆)₂), bpy ([3-CH₃CN](PF₆)₂), [NNN^NCH₃-Co(tpy)](PF₆)₂ ([4](PF₆)₂), and [NNN^CH₂-Co(phen)(CH₃CN)](PF₆)₂ ([5-CH₃CN](PF₆)₂)) suppressed the HER but rather enhanced the CO₂ reduction reaction. The metal-ligand cooperative redox steps enabled the shift of Co(I) reactivity toward CO₂ reduction. Additionally, the amine pendant attached to the NNN^NCH₃-ligand could stabilize the CO₂ reduction intermediate through the hydrogen-bonding interaction with the Co-CO₂H adduct.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

Steric and Lewis Basicity Influence of the Second Coordination Sphere on Electrocatalytic CO2 Reduction by Manganese Bipyridyl Complexes

This study aims to provide a greater insight into the balance between steric (bpy vs (Ph)₂bpy vs mes₂bpy ligands) and Lewis basic ((Ph)₂bpy vs (MeOPh)₂bpy vs (MeSPh)₂bpy ligands) influence on the efficiencies of the protonation-first vs reduction-first CO₂ reduction mechanisms with [Mn^I(R₂bpy)(CO)₃(CH₃CN)]⁺ precatalysts, and on their respective transition-state geometries/energies for rate-determining C-OH bond cleavage toward CO evolution. The presence of only modest steric bulk at the 6,6'-diphenyl-2,2'-bipyridyl ((Ph)₂bpy) ligand has here allowed unique insight into the mechanism of catalyst activation and CO₂ binding by navigating a perfect medium between the nonsterically encumbered bpy-based and the highly sterically encumbered mes₂bpy-based precatalysts. Cyclic voltammetry conducted in CO₂-saturated electrolyte for the (Ph)₂bpy-based precatalyst [2-CH₃CN]⁺ confirms that CO₂ binding occurs at the two-electron-reduced activated catalyst [2]^- in the absence of an excess proton source, in contrast to prior assumptions that all manganese catalysts require a strong acid for CO₂ binding. This observation is supported by computed free energies of the parent-child reaction for [Mn-Mn]⁰ dimer formation, where increased steric hindrance relative to the bpy-based precatalyst correlates with favorable CO₂ binding. A critical balance must be adhered to, however, as the absence of steric bulk in the bpy-based precatalyst [1-CH₃CN]⁺ maintains a lower overpotential than [2-CH₃CN]⁺ at the protonation-first pathway with comparable kinetic performance, whereas an ∼2-fold greater TOF_max is observed at its reduction-first pathway with an almost identical overpotential as [2-CH₃CN]⁺. Notably, excessive steric bulk in the mes₂bpy-based precatalyst [3-CH₃CN]⁺ results in increased activation free energies of the C-OH bond cleavage transition states for both the protonation-first and the reduction-first pathways relative to both [1-CH₃CN]⁺ and [2-CH₃CN]⁺. In fact, [3-CH₃CN]⁺ requires a 1 V window beyond its onset potential to reach its peak catalytic current, which is in contrast to the narrower (<0.30 V) potential response window of the remaining catalysts here studied. Voltammetry recorded under 1 atm of CO₂ with 2.8 M (5%) H₂O establishes [2-CH₃CN]⁺ to have the lowest overpotential (η = 0.75 V) in the series here studied, attributed to its ability to lie "on the fence" when providing sufficient steric bulk to hinder (but not prevent) [Mn-Mn]⁰ dimerization, while simultaneously having a limited steric impact on the free energy of activation for the rate-determining C-OH bond cleavage transition state. While the methoxyphenyl bpy-based precatalyst [4-CH₃CN]⁺ possesses an increased steric presence relative to [2-CH₃CN]⁺, this is offset by its capacity to stabilize the C-OH bond cleavage transition states of both the protonation-first and the reduction-first pathways by facilitating second coordination sphere H-bonding stabilization.