Electrochemical reduction of CO2 at metal-porphyrin supported gas diffusion electrodes under high pressure CO2

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO2

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO2 and H2O reached as high as ∼10 to 50 mA cm-2, and remained stable for >2 days. Selectivity for the reduction of CO2 to CO vs. H2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO2 reduction at sizable current densities.

Bioinspired Hydrophobicity for Enhancing Electrochemical CO2 Reduction

Electrochemical carbon dioxide reduction (CO2 R) is a promising pathway for converting greenhouse gasses into valuable fuels and chemicals using intermittent renewable energy. Enormous efforts have been invested in developing and designing CO2 R electrocatalysts suitable for industrial applications at accelerated reaction rates. The microenvironment, specifically the local CO2 concentration (local [CO2 ]) as well as the water and ion transport at the CO2 -electrolyte-catalyst interface, also significantly impacts the current density, Faradaic efficiency (FE), and operation stability. In nature, hydrophobic surfaces of aquatic arachnids trap appreciable amounts of gases due to the "plastron effect", which could inspire the reliable design of CO2 R catalysts and devices to enrich gaseous CO2 . In this review, starting from the wettability modulation, we summarize CO2 enrichment strategies to enhance CO2 R. To begin, superwettability systems in nature and their inspiration for concentrating CO2 in CO2 R are described and discussed. Moreover, other CO2 enrichment strategies, compatible with the hydrophobicity modulation, are explored from the perspectives of catalysts, electrolytes, and electrolyzers, respectively. Finally, a perspective on the future development of CO2 enrichment strategies is provided. We envision that this review could provide new guidance for further developments of CO2 R toward practical applications.

Copper Cobalt Selenide as a Bifunctional Electrocatalyst for the Selective Reduction of CO2 to Carbon-Rich Products and Alcohol Oxidation

Copper cobalt selenide, CuCo2Se4, has been identified as an efficient catalyst for electrocatalytic CO2 reduction, exhibiting high selectivity for carbon-rich and value-added products. Achieving product selectivity is one of the primary challenges for CO2 reduction reactions, and the catalyst surface plays a pivotal role in determining the reaction pathway and, more importantly, the intermediate adsorption kinetics leading to C1- or C2+-based products. In this research, the catalyst surface was designed to optimize the adsorption of the intermediate CO (carbonyl) group on the catalytic site such that its dwell time on the surface was long enough for further reduction to carbon-rich products but not strong enough for surface passivation and poisoning. CuCo2Se4 was synthesized through hydrothermal method, and the assembled electrode showed the electrocatalytic reduction of CO2 at various applied potentials ranging from -0.1 to -0.9 V vs RHE. More importantly, it was observed that the CuCo2Se4-modified electrode could produce exclusive C2 products such as acetic acid and ethanol with 100% faradaic efficiency at a lower applied potential (-0.1 to -0.3 V), while C1 products such as formic acid and methanol were obtained at higher applied potentials (-0.9 V). Such high selectivity and preference for acetic acid and ethanol formation highlight the novelty of this catalyst. The catalyst surface was also probed through density functional theory (DFT) calculations, and the high selectivity for C2 product formation could be attributed to the optimal CO adsorption energy on the catalytic site. It was further estimated that the Cu site showed a better catalytic activity than Co; however, the presence of neighboring Co atoms with the residual magnetic moment on the surface and subsurface layers influenced the charge density redistribution on the catalytic site after intermediate CO adsorption. In addition to CO2 reduction, this catalytic site was also active for alcohol oxidation producing formic or acetic acid from methanol or ethanol, respectively, in the anodic chamber. This report not only illustrates the highly efficient catalytic activity of CuCo2Se4 for CO2 reduction with high product selectivity but also offers a proper insight of the catalyst surface design and how to obtain such high selectivity, thereby providing knowledge that can be transformative for the field.

Local environment-mediated efficient electrocatalysis of CO2 to CO on Zn nanosheets

Polytetrafluoroethylene-modified Zn nanosheets inhibit the hydrogen evolution reaction and then enhance the selectivity for electrochemical CO2-to-CO conversion.

Electroreduction of CO2to ethanol by electrochemically deposited Cu-lignin complexes on Ni foam electrodes

A low cost, non-toxic and highly selective catalyst based on a Cu-lignin molecular complex is developed for CO₂electroreduction to ethanol. Ni foam (NF), Cu-Ni foam (Cu-NF) and Cu-lignin-Ni foam (Cu-lignin-NF) were prepared by a facile and reproducible electrochemical deposition method. The electrochemical CO₂reduction activity of Cu-lignin-NF was found to be higher than Cu-NF. A maximum faradaic efficiency of 23.2% with current density of 22.5 mA cm^-2was obtained for Cu-lignin-NF at -0.80 V (versus RHE) in 0.1 M Na₂SO₄towards ethanol production. The enhancement of catalytic performance is attributed to the growth of the number of active sites and the change of oxidation states of Cu and NF due to the presence of lignin.

Effect of Cobalt Speciation and the Graphitization of the Carbon Matrix on the CO2 Electroreduction Activity of Co/N-Doped Carbon Materials

The electroreduction of carbon dioxide is considered a key reaction for the valorization of CO₂ emitted in industrial processes or even present in the environment. Cobalt-nitrogen co-doped carbon materials featuring atomically dispersed Co-N sites have been shown to display superior activities and selectivities for the reduction of carbon dioxide to CO, which, in combination with H₂ (i.e., as syngas), is regarded as an added-value CO₂-reduction product. Such catalysts can be synthesized using heat treatment steps that imply the carbonization of Co-N-containing precursors, but the detailed effects of the synthesis conditions and corresponding materials' composition on their catalytic activities have not been rigorously studied. To this end, in the present work, we synthesized cobalt-nitrogen co-doped carbon materials with different heat treatment temperatures and studied the relation among their surface- and Co-speciation and their CO₂-to-CO electroreduction activity. Our results reveal that atomically dispersed cobalt-nitrogen sites are responsible for CO generation while suggesting that this CO-selectivity improves when these atomic Co-N centers are hosted in the carbon layers that cover the Co nanoparticles featured in the catalysts synthesized at higher heat treatment temperatures.

First principles studies of mononuclear and dinuclear Pacman complexes for electrocatalytic reduction of CO2

An iron-containing Pacman complex exhibited high activity and selectivity for the reduction of CO₂ to CH₄.

Boosting Membrane Hydration for High Current Densities in Membrane Electrode Assembly CO2 Electrolysis

Despite the advantages of CO₂ electrolyzers, efficiency losses due to mass and ionic transport across the membrane electrode assembly (MEA) are critical bottlenecks for commercial-scale implementation. In this study, more efficient electrolysis of CO₂ was achieved by increasing cation exchange membrane (CEM) hydration via the humidification of the CO₂ reactant inlet stream. A high current density of 755 mA/cm² was reached by humidifying the reactant CO₂ in a MEA electrolyzer cell featuring a CEM. The power density was reduced by up to 30% when the fully humidified reactant CO₂ was introduced while operating at a current density of 575 mA/cm². We reduced the ohmic losses of the electrolyzer by fourfold at 575 mA/cm² by fully humidifying the reactant CO₂. A semiempirical CEM water uptake model was developed and used to attribute the improved performance to 11% increases in membrane water uptake and ionic conductivity. Our CEM water uptake model showed that the increase in ohmic losses and the limitation of ionic transport were the result of significant dehydration at the central region of the CEM and the anode gas diffusion electrode-CEM interface region, which exhibited a 2.5% drop in water uptake.

Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

Iron complexes with a N₂ O₂ -type N,N'-bis(salicylaldehyde)-1,2-phenylenediamine salophen ligand catalyze the electrochemical reduction of CO₂ to CO in acetonitrile with phenol as the proton donor, giving rise to 90-99 % selectivity, faradaic efficiency up to 58 %, and turnover frequency up to 10³  s^-1 at an overpotential of 0.65 V. This novel class of molecular catalyst for CO₂ reduction operate through a mononuclear Fe^I intermediate, with phenol being involved in the process with first-order kinetics. The molecular nature of the catalyst and the low cost, easy synthesis and functionalization of the salophen ligand paves the way for catalyst engineering and optimization. Competitive electrodeposition of the coordination complex at the electrode surface results in the formation of iron-based nanoparticles, which are active towards heterogeneous electrocatalytic processes mainly leading to proton reduction to hydrogen (faradaic efficiency up to 80 %) but also to the direct reduction of CO₂ to methane with a faradaic efficiency of 1-2 %.

Redox-Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

Microbial electrochemical systems in which metabolic electrons in living microbes have been extracted to or injected from an extracellular electrical circuit have attracted considerable attention as environmentally-friendly energy conversion systems. Since general microbes cannot exchange electrons with extracellular solids, electron mediators are needed to connect living cells to an extracellular electrode. Although hydrophobic small molecules that can penetrate cell membranes are commonly used as electron mediators, they cannot be dissolved at high concentrations in aqueous media. The use of hydrophobic mediators in combination with small hydrophilic redox molecules can substantially increase the efficiency of the extracellular electron transfer process, but this method has side effects, in some cases, such as cytotoxicity and environmental pollution. In this Review, recently-developed redox-active polymers are highlighted as a new type of electron mediator that has less cytotoxicity than many conventional electron mediators. Owing to the design flexibility of polymer structures, important parameters that affect electron transport properties, such as redox potential, the balance of hydrophobicity and hydrophilicity, and electron conductivity, can be systematically regulated.

Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO2 reduction remains a highly creative approach.

Selective single-atom electrocatalysts: a review with a focus on metal-doped covalent triazine frameworks

Single-atom electrocatalysts (SACs), which comprise singly isolated metal sites supported on heterogeneous substrates, have attracted considerable recent attention as next-generation electrocatalysts for various key reactions from the viewpoint of the environment and energy. Not only electrocatalytic activity but also selectivity can be precisely tuned via the construction of SACs with a defined coordination structure, such as homogeneous organometallics. Covalent organic frameworks (COFs) are promising supports for single-atom sites with designed coordination environments due to their unique physicochemical properties, which include porous structures, robustness, a wide range of possible designs, and abundant heteroatoms to coordinate single-metal sites. The rigid frameworks of COFs can hold unstable single-metal atoms, such as coordinatively unsaturated sites or easily aggregated Pt-group metals, which exhibit unique electrocatalytic selectivity. This minireview summarizes recent advances in the selective reactions catalysed by SACs, mainly those supported on triazine-based COFs.

Single-atom electrocatalysts (SACs) have attracted considerable attention as selective electrocatalysts. Metal-doped covalent triazine frameworks will be a novel platform for selective SACs to solve energy and environmental issues.

Electrode Materials Engineering in Electrocatalytic CO2 Reduction: Energy Input and Conversion Efficiency

Electrocatalytic CO₂ reduction (ECR) is a promising technology to simultaneously alleviate CO₂ -caused climate hazards and ever-increasing energy demands, as it can utilize CO₂ in the atmosphere to provide the required feedstocks for industrial production and daily life. In recent years, substantial progress in ECR systems has been achieved by the exploitation of various novel electrode materials. The anodic materials and cathodic catalysts that have, respectively, led to high-efficiency energy input and effective heterogenous catalytic conversion in ECR systems are comprehensively reviewed. Based on the differences in the nature of energy sources and the role of materials used at the anode, the fundamentals of ECR systems, including photo-anode-assisted ECR systems and bio-anode-assisted ECR systems, are explained in detail. Additionally, the cathodic reaction mechanisms and pathways of ECR are described along with a discussion of different design strategies for cathode catalysts to enhance conversion efficiency and selectivity. The emerging challenges and some perspective on both anode materials and cathodic catalysts are also outlined for better development of ECR systems.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. To achieve CO₂ conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

Electrochemical Reactors for CO2 Conversion

Increasing risks from global warming impose an urgent need to develop technologically and economically feasible means to reduce CO2 content in the atmosphere. Carbon capture and utilization technologies and carbon markets have been established for this purpose. Electrocatalytic CO2 reduction reaction (CO2RR) presents a promising solution, fulfilling carbon-neutral goals and sustainable materials production. This review aims to elaborate on various components in CO2RR reactors and relevant industrial processing. First, major performance metrics are discussed, with requirements obtained from a techno-economic analysis. Detailed discussions then emphasize on (i) technical benefits and challenges regarding different reactor types, (ii) critical features in flow cell systems that enhance CO2 diffusion compared to conventional H-cells, (iii) electrolyte and its effect on liquid phase electrolyzers, (iv) catalysts for feasible products (carbon monoxide, formic acid and multi-carbons) and (v) strategies on flow channel and anode design as next steps. Finally, specific perspectives on CO2 feeds for the reactor and downstream purification techniques are annotated as part of the CO2RR industrial processing. Overall, we focus on the component and system aspects for the design of a CO2RR reactor, while pointing out challenges and opportunities to realize the ultimate goal of viable carbon capture and utilization technology.

Recent advances in metalloporphyrin-based catalyst design towards carbon dioxide reduction: from bio-inspired second coordination sphere modifications to hierarchical architectures

Research in the development of new molecular catalysts for the selective transformation of CO2 to reduced forms of carbon is attracting enormous interest from chemists. Molecular catalyst design hinges on the elaboration of ligand scaffolds to manipulate the electronic and structural properties for the fine tuning of the reactivity pattern. A cornucopia of ligand sets have been designed along this line and more and more are being reported. In this quest, the porphyrin molecular platform has been under intensive focus due to the unmatched catalytic properties of metalloporphyrins. There have been rapid advances in this particular field during the last few years wherein both electronic and structural aspects in the second coordination spheres have been addressed to shift the overpotential and improve the catalytic rates and product selectivity. Metalloporphyrins have also attracted much attention in terms of the elaboration of hybrid materials for heterogeneous catalysis. Here too, some promising activities have made metalloporphyrin derivatives serious candidates for technological implementation. This review collects the recent advances centred around the chemistry of metalloporphyrins for the reduction of CO2.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using hetero- and homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.

CO2 electrochemical catalytic reduction with a highly active cobalt phthalocyanine

Molecular catalysts that combine high product selectivity and high current density for CO₂ electrochemical reduction to CO or other chemical feedstocks are urgently needed. While earth-abundant metal-based molecular electrocatalysts with high selectivity for CO₂ to CO conversion are known, they are characterized by current densities that are significantly lower than those obtained with solid-state metal materials. Here, we report that a cobalt phthalocyanine bearing a trimethyl ammonium group appended to the phthalocyanine macrocycle is capable of reducing CO₂ to CO in water with high activity over a broad pH range from 4 to 14. In a flow cell configuration operating in basic conditions, CO production occurs with excellent selectivity (ca. 95%), and good stability with a maximum partial current density of 165 mA cm⁻² (at −0.92 V vs. RHE), matching the most active noble metal-based nanocatalysts. These results represent state-of-the-art performance for electrolytic carbon dioxide reduction by a molecular catalyst.

Molecular electrocatalysts reducing CO₂ to CO with high selectivity and high rate are urgently needed. A cobalt phthalocyanine complex is capable of reducing CO₂ to CO in water with a maximum partial current density up to 165 mA cm⁻², matching the most active noble metal-based nanocatalysts.

Controlling the nucleophilic properties of cobalt salen complexes for carbon dioxide capture

The nucleophilic properties of cobalt salen complexes are examined using density functional theory to investigate its carbon fixing capacity. In particular, carbon dioxide attack on neutral and anionic cobalt salen molecules is considered. Carbon fixation occurs for the anionic cobalt salen complex and is due to the nucleophilic interaction between the cobalt center and carbon dioxide molecule in a Co d z 2 -CO2 π* interaction. A minimum energy path search by a nudged elastic band calculation reveals a lower forward activation energy for the anionic complex than the neutral complex, indicating that the formation of the anionic complex is thermodynamically and kinetically favored. In this case, the CO2 molecule is chemisorbed as partial charge transfer from the cobalt center to carbon dioxide is observed. Proposed reaction mechanisms explain how the Co-C bond energy of the CO2-cobalt salen complex can be tuned by appropriate substitutions of electron donating or withdrawing groups on the phenyl ring.

Clam-shaped cyclam-functionalized porphyrin for electrochemical reduction of carbon dioxide

A clam-shaped molecule comprising a Zn(II)-porphyrin and a Zn(II)-cyclam is synthesized and characterized. Its electrochemical behavior and catalytic activity for homogeneous electrochemical reduction of carbon dioxide (CO[Formula: see text] are investigated by cyclic voltammetry and compared with those of Zn(II)-meso-tetraphenylporphyrin and Zn(II)-cyclam. Under N2-saturated conditions, cyclic voltammetry of the featured complex has characteristics of its two constituents, but under CO2-saturated conditions, the target compound exhibits significant current enhancement. Iterative reduction under electrochemical conditions indicated the target compound has improved stability relative to Zn(II)-cyclam. Controlled potential electrolysis demonstrates that, without addition of water, methane (CH[Formula: see text] is the only detectable product with 1% Faradaic efficiency (FE). The formation of CH4 is not observed under the catalysis of the Zn(II)-porphyrin benchmark compound, indicating that the CO2-capturing function of the Zn(II)-cyclam unit contributes to the catalysis. Upon addition of 3% v/v water, the electrochemical reduction of CO2 in the presence of the target compound gives carbon monoxide (CO) with 28% FE. Dominance of CO formation under these conditions suggests enhancement of proton-coupled reduction. Integrated action of these Zn(II)-porphyrin and Zn(II)-cyclam units offers a notable example of a molecular catalytic system where the cyclam ring captures and brings CO2 into the proximity of the porphyrin catalysis center.

Synthesis and investigation of tetraphenyltetrabenzoporphyrins for electrocatalytic reduction of carbon dioxide

We report the synthesis and electrochemical properties of freebase tetraphenyltetrabenzoporphyrin and its complexes of Zn(ii), Co(ii), Ni(ii), Cu(ii) and Sn(iv) towards electrochemical reduction of carbon dioxide (CO2). Based on cyclic voltammetry, it is shown that central metals significantly affect the electrocatalytic performance in the reduction of CO2 in terms of reduction potential and catalytic current enhancement. At an applied potential of -1.90 V vs. an Ag/AgCl quasi reference electrode for 20 h, the electrocatalytic reduction of CO2 realized by Zn(ii)- and Cu(ii)-tetraphenyltetrabenzoporphyrins to carbon monoxide resulted in faradaic efficiencies of around 48% and 33%, respectively.

An Integrated Design with new Metal-Functionalized Covalent Organic Frameworks for the Effective Electroreduction of CO2

One of the long-standing issues that prohibits large-scale CO₂ reutilization is the low aqueous solubility of CO₂ and the incurring inefficient mass transport of CO₂ . Herein, we suggest a feasible way to promote the CO₂ reutilization by integrating the storage and reduction, with a new covalent organic framework (COF) series constituted by cobalt-phthalocyanine and boronic acid linkers. We find that the porous structure of the cobalt COF is competitive in the CO₂ storage and can sustain a high CO₂ concentration around the reduction center, whereas the mass transport of CO₂ as well as the efficiency of the CO₂ reduction is significantly improved. The predicted cobalt COF exhibits an overpotential of 0.27 V and a CO production rate, which is 97.7 times higher than in aqueous solution, for the CO₂ reduction. Our work provides a promising candidate for the CO₂ reutilization, with valuable insights and an important prototype for future practical design.

Oxygenates from the Electrochemical Reduction of Carbon Dioxide

Electrochemical reduction of carbon dioxide (CO2 ) driven by renewable electricity to give chemicals and fuels is considered an ideal approach that can alleviate both carbon emission and energy tension stress. High-value chemicals such as oxygenates can be effectively produced from the electroreduction of CO2 , and this is highly attractive to promote the economy and applicability of CO2 utilization. This review focuses on recent advancements in the electrochemical reduction of CO2 to formic acid, methanol, ethanol, acetic acid, and other oxygenates. The principles of the process, influencing factors, and typical catalysts are summarized. On the basis of the aforementioned discussions, we present future prospects for further development of the electroreduction of CO2 to oxygenates.

Covalent triazine framework modified with coordinatively-unsaturated Co or Ni atoms for CO2 electrochemical reduction

The electrochemical reduction of carbon dioxide (CO2) has attracted considerable attention as a means of maintaining the carbon cycle. This process still suffers from poor performance, including low faradaic efficiencies and high overpotential. Herein, we attempted to use coordination number as a control parameter to improve the electrocatalytic performance of metal species that have previously been thought to have no CO2 reduction activity. Covalent triazine frameworks (CTF) modified with coordinatively-unsaturated 3d metal atoms (Co, Ni or Cu) were developed for efficient electroreduction of CO2. Co-CTF and Ni-CTF materials effectively reduced CO2 to CO from -0.5 V versus RHE. The faradaic efficiency of the Ni-CTF during CO formation reached 90% at -0.8 V versus RHE. The performance of Ni-CTF is much higher than that of the corresponding metal-porphyrin (using tetraphenylporphyrin; TPP). First principles calculations demonstrated that the intermediate species (adsorbed COOH) was stabilized on the metal atoms in the CTF due to the low-coordination structure of this support. Thus, the free energy barriers for the formation of adsorbed COOH on the metal atoms in the CTF supports were lower than those on the TPP supports.

Cobalt-porphine catalyzed CO2 electro-reduction: a novel protonation mechanism

The urgent need for artificially fixing CO₂ calls for catalysts of high efficiency. The transition metal functionalized porphyrin (TMP) is one of the most important types of organic catalysts for CO₂ reduction. However, the catalytic mechanisms of TMP in CO₂ reduction still remain controversial. Starting from the previously neglected catalyst self-protonation model, we uncover a new CO₂ reduction mechanism on cobalt-porphine, which involves an indirect proton transfer step occurring at the beginning of the reduction cycle. Based on this protonation mechanism, we demonstrate the different correlations between producing rate and pH for the formation of CO and methane, in good agreement with available experimental observations. Our results reveal how pH and potential affect the CO₂ reduction process, providing important clues and insights for further optimization of TMP catalysts.

Electroreduction of CO2 Catalyzed by a Heterogenized Zn-Porphyrin Complex with a Redox-Innocent Metal Center

Transition-metal-based molecular complexes are a class of catalyst materials for electrochemical CO₂ reduction to CO that can be rationally designed to deliver high catalytic performance. One common mechanistic feature of these electrocatalysts developed thus far is an electrogenerated reduced metal center associated with catalytic CO₂ reduction. Here we report a heterogenized zinc-porphyrin complex (zinc(II) 5,10,15,20-tetramesitylporphyrin) as an electrocatalyst that delivers a turnover frequency as high as 14.4 site^-1 s^-1 and a Faradaic efficiency as high as 95% for CO₂ electroreduction to CO at -1.7 V vs the standard hydrogen electrode in an organic/water mixed electrolyte. While the Zn center is critical to the observed catalysis, in situ and operando X-ray absorption spectroscopic studies reveal that it is redox-innocent throughout the potential range. Cyclic voltammetry indicates that the porphyrin ligand may act as a redox mediator. Chemical reduction of the zinc-porphyrin complex further confirms that the reduction is ligand-based and the reduced species can react with CO₂. This represents the first example of a transition-metal complex for CO₂ electroreduction catalysis with its metal center being redox-innocent under working conditions.

Noncovalent Immobilization of a Molecular Iron-Based Electrocatalyst on Carbon Electrodes for Selective, Efficient CO2-to-CO Conversion in Water

Catalysis of fuel-producing reactions can be transferred from homogeneous solution to surface via attachment of the molecular catalyst. A pyrene-appended iron triphenyl porphyrin bearing six pendant OH groups on the phenyl rings in all ortho and ortho' positions was immobilized on carbon nanotubes via noncovalent interactions and further deposited on glassy carbon. X-ray photoelectron spectroscopy and electrochemistry confirm catalyst immobilization. Using the carbon material, highly selective and rapid catalysis of the reduction of CO2 into CO occurs in water (pH 7.3) with 480 mV overpotential. Catalysis could be sustained for hours without loss of activity and selectivity, and high turnover number was obtained.