Energy-efficient CO2/CO interconversion by homogeneous copper-based molecular catalysts

Integrated Capture and Electrocatalytic Conversion of CO2: A Molecular Electrocatalysts Perspective

The ever-increasing concentration of atmospheric CO2, primarily driven by anthropogenic activities, has raised urgent environmental concerns, spurring the development of carbon capture and utilization (CCU) technologies. This review focuses on the integrated capture and electrochemical conversion of CO2 (ICECC), a promising approach that combines carbon capture with its direct electroreduction into value-added products. By eliminating energy-intensive steps such as CO2 release, compression, and transportation, ICECC offers a more energy-efficient and cost-effective alternative to conventional CCU methods. In this review, particular attention is given to molecular electrocatalysts, which offer high tunability and selectivity in electrochemical CO2 reduction reaction (eCO2RR). The role of capturing agents, including both external and dual-functional molecular systems, is critically examined to understand their influence on CO2 binding and catalytic efficiency. Whereas ICECC has significant potential, research in this area remains underexplored compared to conventional CO2 reduction methods. The review discusses the mechanistic insights into ICECC processes, highlighting key challenges and potential future research directions for improving catalyst design, enhancing capture efficiency, and scaling up ICECC technologies. These developments can make ICECC a critical component in achieving carbon neutrality and addressing climate change.

Electronic Structure Tuning in Cu-Co Dual Single Atom Catalysts for Enhanced COOH* Spillover and Electrocalytic CO2 Reduction Activity

The development of efficient electrocatalysts for CO2 reduction to CO is challenging due to competing hydrogen evolution and intermediate over-stabilization. In this study, a Cu-Co dual single-atom catalyst (CuCo-DSAC) anchored on carbon black was synthesized via scalable pyrolysis. The catalyst achieves 98.5% CO Faradaic efficiency at 500 mA cm-2, maintaining > 95% selectivity across a 400 mV window with < 6% decay over 48 h, which is superior to the corresponding single-atom control samples. In situ spectroscopy and DFT calculations reveal a synergistic mechanism: Co sites activate CO2 and stabilize *COOH intermediates, while adjacent Cu sites facilitate CO desorption by lowering the energy barrier through charge redistribution. This dynamic buffer system mitigates active-site blocking and suppresses HER by weakening H adsorption. The electronic interplay between Cu and Co optimizes intermediate energetics, enabling industrial-level performance. This work demonstrates the potential of tailored dual-site architectures for complex electrocatalytic processes, offering a promising approach to overcoming traditional limitations.

Efficacy of Redox-Active Cu(II) Dipyrrin Complexes toward Electrochemical Reduction of CO2

New D-A-type catalysts based on Cu (II) complexes (C1 and C2) including dipyrrin ligands with phenothiazine/carbazole as the meso-substituent have been described. The complexes have been thoroughly characterized by various methods (1H, 13C, ESI-MS, EPR, and UV-vis studies), and structures of both C1 and C2 unequivocally determined by X-ray single crystal analyses. The catalysts C1 and C2 are stable at room temperature and exhibit Faradaic efficiency values of ∼56% (C1) and ∼46% (C2) toward homogeneous reduction of CO2 to CO. The release of CO has been validated by gas chromatographic (GC) studies. Electron-rich phenothiazine and carbazole included in the catalysts facilitate proton transfer, enabling rapid and selective formation of CO over H2 with FEH2 values of ∼22% for C1 and ∼7% for C2 and turnover numbers (TON) of ∼46 for C1 and ∼21 for C2. Furthermore, the formation of formate ions has been affirmed by ion chromatography (C1, ∼16%; C2, ∼18%). Detailed electrochemical studies and product analyses suggested that C1 displays superior catalytic activity relative to C2 which has been further supported by theoretical studies.

Monitoring chalcogenide ions-guided in situ transform active sites of tailored bismuth electrocatalysts for CO2 reduction to formate

Although bismuth catalysts enable accelerated electrochemical CO2-to-formate conversion, the intrinsic active sites and forming mechanisms under operating conditions remain elusive. Herein, we prepared Bi2O2NCN, Bi2O3, and Bi2O2S as precatalysts. Among them, Bi2O2NCN-derived catalyst possesses optimum performance of electrochemical CO2-to-formate, exhibiting an upsurge of Faradaic efficiency to 98.3% at -0.6 V vs. reversible hydrogen electrodes. In-situ infrared and electrochemical impedance spectra trace and interpret the superior performance. Multimodal structural analyses utilizing quasi-in-situ X-ray diffraction, in-situ X-ray absorption near edge structure and in-situ Raman spectra provide powerful support to monitoring the catalysts' in-situ transforms to metallic Bi, identifying the formation of the active sites influenced by the chalcogenide ions-guided: Carbodiimide promotes to form of the dominant Bi(003) facet exposure, which distinguishes from sulfide- and oxide-preferred dominant Bi(012) facets exposure. Concurrently, theoretical insights garnered from multiscale/multilevel computational analyses harmoniously corroborate the experimental findings. These findings show the pivotal role of chalcogenide in tailoring bismuth electrocatalysts for selective CO2 reduction to formate, illuminating the significance of controlling structural chemistry in designing catalysts toward high-efficiency renewable energy conversion.

Reversible Electrocatalytic NAD+/NADH Interconversion Mediated by a Pyrazine-Amidate Iridium Complex

Herein, we report reversible electrocatalytic NAD+/NADH interconversion mediated by [Cp*Ir(pyza)Cl] (1, pyza = pyrazine amidate). 1 was designed through a rational approach aimed at lowering the overpotential of NAD+ to NADH reduction with respect to that observed for electrocatalyst [Cp*Ir(pica)Cl] (2, pica = picolinamidate). The peculiar properties of pyza, which is substantially less σ electron-donator and more π electron-acceptor than pica, resulted in an easier bielectronic reduction process occurring at -0.29 V (instead of ca. -0.65 V for 2), very close to the equilibrium potential of NAD+/NADH redox couple (E°eq = -0.32 V vs NHE, 298 K, pH 7). 1 catalyzes both NAD+ reduction and NADH oxidation in response to even a small departure from equilibrium potential, with a catalytic bias for the former (|ipred/ipox| = 6.2, 333 K). The reversibility of NAD+/NADH interconversion was ascertained by 1H EXSY NMR spectroscopy that clearly demonstrated the rapid establishment of 1_H + NAD+ ⇌ 1 + NADH equilibrium (Keq = 3, ΔG = -0.6 kcal/mol, 298 K) and a similar hydridicity of NADH (28.9 kcal/mol, 298 K) and 1_H (28.3 kcal/mol, 298 K).

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

Noble Metal Plasmon-Molecular Catalyst Hybrids for Renewable Energy Relevant Small Molecule Activation

Significant endeavors have been dedicated to the advancement of materials for artificial photosynthesis, aimed at efficiently harvesting light and catalyzing reactions such as hydrogen production and CO2 conversion. The application of plasmonic nanomaterials emerges as a promising option for this purpose, owing to their excellent light absorption properties and ability to confine solar energy at the nanoscale. In this regard, coupling plasmonic particles with molecular catalysts offers a pathway to create high-performance hybrid catalysts. In this review, we discuss the plasmonic-molecular complex hybrid catalysts where the plasmonic nanoparticles serve as the light-harvesting unit and promote interfacial charge transfer in tandem with the molecular catalyst which drives chemical transformation. In the initial section, we provide a concise overview of plasmonic nanomaterials and their photophysical properties. We then explore recent breakthroughs, highlighting examples from literature reports involving plasmonic-molecular complex hybrids in various catalytic processes. The utilization of plasmonic materials in conjunction with molecular catalysts represents a relatively unexplored area with substantial potential yet to be realized. This review sets a strong basis and motivation to explore the plasmon-induced hot-electron mediated photelectrochemical small molecule activation reactions. Utilizing in situ spectroscopic investigations and ultrafast transient absorption spectroscopy, it presents a comprehensive template for scalable and sustainable antenna-reactor systems.

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.

Connecting Biological and Synthetic Approaches for Electrocatalytic CO2 Reduction

Electrocatalytic CO2 reduction has developed into a broad field, spanning fundamental studies of enzymatic 'model' catalysts to synthetic molecular catalysts and heterogeneous gas diffusion electrodes producing commercially relevant quantities of product. This diversification has resulted in apparent differences and a disconnect between seemingly related approaches when using different types of catalysts. Enzymes possess discrete and well understood active sites that can perform reactions with high selectivity and activities at their thermodynamic limit. Synthetic small molecule catalysts can be designed with desired active site composition but do not yet display enzyme-like performance. These properties of the biological and small molecule catalysts contrast with heterogeneous materials, which can contain multiple, often poorly understood active sites with distinct reactivity and therefore introducing significant complexity in understanding their activities. As these systems are being better understood and the continuously improving performance of their heterogeneous active sites closes the gap with enzymatic activity, this performance difference between heterogeneous and enzymatic systems begins to close. This convergence removes the barriers between using different types of catalysts and future challenges can be addressed without multiple efforts as a unified picture for the biological-synthetic catalyst spectrum emerges.