A selective and efficient electrocatalyst for carbon dioxide reduction

Rigidly Axial O Coordination-Induced Spin Polarization on Single Ni-N4-C Site by MXene Coupling for Boosting Electrochemical CO2 Reduction to CO

Regulating the spin states in transition-metal (TM)-based single-atom catalysts (SACs), such as the TM-Nx-C configurations, is crucial for improving the catalytic activity. However, the role of spin in single Ni atoms facilitating the electrochemical CO2 reduction reaction (CO2RR) has been largely overlooked. Using first-principles simulations, we investigated the electrocatalytic performance of Ni-N4-C SACs vertically stacked on the O-terminated MXene nanosheets for the CO2RR. The terminated O atoms on MXene axially interact with the Ni atom due to significant charge transfer between them. Unlike the pure Ni-N4 site, which lacks spin polarization, the newly formed Ni-N4O configuration breaks the spin degeneracy of Ni d orbitals, dramatically lifting the energy level of spin-down d orbitals relative to that of spin-up d orbitals. As a result, the d electrons of Ni in the two spin channels are rearranged, leading to large net spin moments of 1.4 μB. Compared to the Ni-N4 site, the partially filled minority-spin dz2 orbitals of Ni on Ni-N4O weaken the occupied d-π* orbitals between Ni and *COOH, significantly stabilizing the key intermediate. The detailed reaction mechanisms and energetics show that four MXenes, namely, Hf3C2, Zr3C2, Hf2C, and Zr2C, can induce a large spin on the Ni site, thereby improving catalytic activity for CO2 reduction to CO, with a lower onset potential of about -0.75 V vs SHE compared to pure Ni SACs (-1.17 V) according to the potential-constant model with an explicit solvent environment.

The AUREX cell: a versatile operando electrochemical cell for studying catalytic materials using X-ray diffraction, total scattering and X-ray absorption spectroscopy under working conditions

Understanding the structure-property relationship in electrocatalysts under working conditions is crucial for the rational design of novel and improved catalytic materials. This paper presents the Aarhus University reactor for electrochemical studies using X-rays (AUREX) operando electrocatalytic flow cell, designed as an easy-to-use versatile setup with a minimal background contribution and a uniform flow field to limit concentration polarization and handle gas formation. The cell has been employed to measure operando total scattering, diffraction and absorption spectroscopy as well as simultaneous combinations thereof on a commercial silver electrocatalyst for proof of concept. This combination of operando techniques allows for monitoring of the short-, medium- and long-range structure under working conditions, including an applied potential, liquid electrolyte and local reaction environment. The structural transformations of the Ag electrocatalyst are monitored with non-negative matrix factorization, linear combination analysis, the Pearson correlation coefficient matrix, and refinements in both real and reciprocal space. Upon application of an oxidative potential in an Ar-saturated aqueous 0.1 M KHCO3/K2CO3 electrolyte, the face-centered cubic (f.c.c.) Ag gradually transforms first to a trigonal Ag2CO3 phase, followed by the formation of a monoclinic Ag2CO3 phase. A reducing potential immediately reverts the structure to the Ag (f.c.c.) phase. Following the electrochemical-reaction-induced phase transitions is of fundamental interest and necessary for understanding and improving the stability of electrocatalysts, and the operando cell proves a versatile setup for probing this. In addition, it is demonstrated that, when studying electrochemical reactions, a high energy or short exposure time is needed to circumvent beam-induced effects.

Direct Microenvironment Modulation of CO2 Electroreduction: Negatively Charged Ag Sites Going beyond Catalytic Surface Reactions

Electrochemical reduction of CO2 is an important way to achieve carbon neutrality, and much effort has been devoted to the design of active sites. Apart from elevating the intrinsic activity, expanding the functionality of active sites may also boost catalytic performance. Here we designed "negatively charged Ag (nc-Ag)" active sites featuring both the intrinsic activity and the capability of regulating microenvironment, through modifying Ag nanoparticles with atomically dispersed Sn species. Different from conventional active sites (which only mediate the surface processes by bonding with the intermediates), the nc-Ag sites could also manipulate environmental species. Therefore, the sites could not only activate CO2, but also regulate interfacial H2O and CO2, as confirmed by operando spectroscopies. The catalyst delivers a high current density with a CO faradaic efficiency of 97 %. Our work here opens up new opportunities for the design of multifunctional electrocatalytic active sites.

Improving CO2 electroconversion by customizing the hydroxyl microenvironment around a semi-open Co-N2O2 configuration

Constructing local microenvironments is one of the important strategies to improve the electrocatalytic performances, such as in electrochemical CO2 reduction (ECR). However, effectively customizing these microenvironments remains a significant challenge. Herein, utilizing carbon nanotube (CNT) heterostructured semi-open Co-N2O2 catalytic configurations (Co-salophen), we have demonstrated the role of the local microenvironment on promoting ECR through regulating the location of hydroxyl groups. Concretely, compared with the maximum Faradaic efficiency (FE) of 62% for carbon monoxide (CO) presented by Co-salophen/CNT without a hydroxyl microenvironment, the designed Co-salophen-OH3/CNT, featuring hydroxyl groups at the Co-N2O2 structural opening, shows remarkable CO2-to-CO electroreduction activity across a wide potential window, with the FE of CO up to 95%. In particular, through the deuterium kinetic isotope experiments and theoretical calculations, we decoded that the hydroxyl groups act as a proton relay station, promoting the efficient transfer of protons to the Co-N2O2 active sites. The finding demonstrates a promising molecular design strategy for enhancing electrocatalysis.

Synthesis and Structure-Property Relationship of meso-Substituted Porphyrin- and Benzoporphyrin-Thiophene Conjugates toward Electrochemical Reduction of Carbon Dioxide

A novel series of ZnII-trans-A2B2 porphyrins and benzoporphyrins bearing phenyl and thiophene-based meso-substituents was successfully synthesized and characterized by spectroscopic and electrochemical techniques. Systematic comparison among the compounds in this series, together with the corresponding A4 analogs previously studied by our group, led to the understanding of the effects of π-conjugated system extension of a porphyrin core through β-fused rings, replacement of the phenyl with the thiophene-based meso-groups, and introduction of additional thiophene rings on thienyl substituents on photophysical and electrochemical properties. Oxidative electropolymerization through bithiophenyl units of both A4 and trans-A2B2 analogs was achieved, resulting in porphyrin- and benzoporphyrin-oligothiophene conjugated polymers, which were characterized by cyclic voltammetry and absorption spectrophotometry. Preliminary studies on catalytic performance toward electrochemical reduction of carbon dioxide (CO2) was described herein to demonstrate the potential of the selected compounds for serving as homogeneous and heterogeneous electrocatalysts for the conversion of CO2 to carbon monoxide (CO).

Improved electrochemical reduction of CO2 to syngas with a highly exfoliated Ti3C2Tx MXene-gold composite

Transforming carbon dioxide (CO2) into valuable chemicals via electroreduction presents a sustainable and viable approach to mitigating excess CO2 in the atmosphere. This report provides fresh insights into the design of a new titanium-based MXene composite as a catalyst for the efficient conversion of CO2 in a safe aqueous medium. Despite its excellent electrocatalytic activity towards CO2 reduction and high selectivity for CO production, the high cost of Au and the decline in catalytic activity on a larger scale hinder its large-scale CO2 conversion applications. In this research, we have successfully prepared an Au/Ti3C2Tx composite and tested its catalytic activity in the electrochemical CO2 reduction reaction (ECRR). The as-prepared composite features strong interactions between gold atoms and the MXene support, achieved through the formation of metal-oxygen/carbon bonds. The Au/Ti3C2Tx electrode demonstrated a significant current density of 17.3 mA cm-2 at a potential of -0.42 V vs. RHE, in a CO2 saturated atmosphere (faradaic efficiency: CO = 48.3% and H2 = 25.6%). Nyquist plots further indicated a reduction in the charge-transfer resistance of the Au/Ti3C2Tx layer, signifying rapid charge transfer between the Au and Ti3C2Tx. Furthermore, it is known that liquid crossover through the Gas Diffusion Electrode (GDE) significantly improves CO2 diffusion to catalyst active sites, thereby enhancing CO2 conversion efficiency. The goal of this work is to design an interface between metal and MXene so that CO2 can be electroreduced to fuels and other useful chemical compounds with great selectivity.

Unveiling the role of silicon in boosting electrochemical carbon dioxide reduction via carbon nanotubes@bismuth silicates

Electrochemical CO2 reduction reaction (CO2RR) is one of the most attractive measures to achieve the carbon neutral goal by converting CO2 into high-value chemicals such as formate. Si in Bi silicates is promising to enhance CO2 adsorption and activation due to its strong oxygenophilicity. Whereas, its role in boosting CO2RR via the cheap Bi-based catalysts is still not clear. Herein, we design CNT@Bi silicates catalyst, demonstrating the highest FEHCOOH of 96.3 % at -0.9 V vs. reversible hydrogen electrode with good stability. Through X-ray photoelectron spectroscopy (XPS), in-situ Attenuated Total Reflectance-Fourier Transform Infrared (In-situ ATR-SEIRAS) experiments, and Density Functional Theory (DFT) calculations, the role of Si in Bi silicates was unveiled: tuning the electronic structure of Bi, weakening the Bi-O bond, and strengthening electron transfer from Bi to CO2, thereby promoting the generation of CO2*- and *OCHO intermediates. Additionally, carbon nanotubes (CNTs) promote not only the conductivity but also the generation of abundant oxygen vacancies in CNT@Bi silicates evidenced by the electron transfer from CNT to Bi silicates from XPS results. Further, the CNT@Bi silicates endows it with the highest electrochemical activation area. These findings suggest the effectiveness of Si in Bi silicates and structure tuning to design highly selective CO2RR catalyst for HCOOH production.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Turning copper into an efficient and stable CO evolution catalyst beyond noble metals

Using renewable electricity to convert CO2 into CO offers a sustainable route to produce a versatile intermediate to synthesize various chemicals and fuels. For economic CO2-to-CO conversion at scale, however, there exists a trade-off between selectivity and activity, necessitating the delicate design of efficient catalysts to hit the sweet spot. We demonstrate here that copper co-alloyed with isolated antimony and palladium atoms can efficiently activate and convert CO2 molecules into CO. This trimetallic single-atom alloy catalyst (Cu92Sb5Pd3) achieves an outstanding CO selectivity of 100% (±1.5%) at -402 mA cm-2 and a high activity up to -1 A cm-2 in a neutral electrolyte, surpassing numerous state-of-the-art noble metal catalysts. Moreover, it exhibits long-term stability over 528 h at -100 mA cm-2 with an FECO above 95%. Operando spectroscopy and theoretical simulation provide explicit evidence for the charge redistribution between Sb/Pd additions and Cu base, demonstrating that Sb and Pd single atoms synergistically shift the electronic structure of Cu for CO production and suppress hydrogen evolution. Additionally, the collaborative interactions enhance the overall stability of the catalyst. These results showcase that Sb/Pd-doped Cu can steadily carry out efficient CO2 electrolysis under mild conditions, challenging the monopoly of noble metals in large-scale CO2-to-CO conversion.

The Selectivity Origins in Ag-Catalyzed CO2 Electroreduction

Ag exhibits high selectivity of electrochemical CO2 reduction (CO2R) toward C1 products, while the hydrogenation involving the concerted proton-electron transfer (CPET) or sequential electron-proton transfer (SEPT) mechanism is still in debate. Toward a better understanding of the Ag-catalyzed electrochemical CO2R, we employed a microkinetic model based on the Marcus electron transfer theory to thoroughly investigate the selectivity of C1 products of electrochemical CO2R over the Ag(111) surface. We found that at an acidic condition of pH = 1.94, formate is the main product when U < -0.94 V via the CPET mechanism, whereas CO becomes the primary product when U > -0.94 V via the SEPT mechanism. Conversely, at an alkaline condition of pH = 13.95, formate is the main product following the SEPT mechanism. Our findings provide novel insights into the influence of external factors (applied potential and pH) on the product selectivity and hydrogenation mechanism of electrochemical CO2R.

Rate-Determining Step for Electrochemical Reduction of Carbon Dioxide into Carbon Monoxide at Silver Electrodes

Silver is one of the most studied electrode materials for the electrochemical reduction of carbon dioxide into carbon monoxide, a product with many industrial applications. There is a growing number of reports in which silver is implemented in gas diffusion electrodes as part of a large-scale device to develop commercially relevant technology. Electrochemical models are expected to guide the design and operation toward cost-efficient devices. Despite decades of investigations, there are still uncertainties in the way this reaction should be modeled due to the absence of scientific consensus regarding the reaction mechanism and the nature of the rate-determining step. We review previously reported studies to draw converging conclusions on the value of the Tafel slope and existing species at the electrode surface. We also list conflicting experimental observations and provide leads to tackling these remaining questions.

Easily constructed porous silver films for efficient catalytic CO2 reduction and Zn-CO2 batteries

For the electroreduction of carbon dioxide into high value-added chemicals, highly active and selective catalysts are crucial, and metallic silver is one of the most intriguing candidate materials available at a reasonable cost. Herein, through a novel two-step operation of Ag paste/SBA-15 coating and HF etching, porous silver films on a commercial carbon paper with a waterproofer (p-Ag/CP) could be easily fabricated on a large scale as highly efficient carbon dioxide reduction reaction (CO2RR) electrocatalysts with a CO Faraday efficiency (FECO) as high as 96.7% at -1.0 V vs. the reversible hydrogen electrode (RHE), and it still reaches up to 90% FECO over applied potentials ranging from -0.8 to -1.1 V vs. the RHE. Meanwhile, the membrane electrode assembly (MEA) utilizing the p-Ag/CP catalyst has achieved a current density, FECO, and stability of ∼60 mA cm-2, >91%, and 11 h, respectively. Furthermore, the assembled aqueous Zn-CO2 battery using p-Ag/CP cathode yielded a peak power density of 0.34 mW cm-2, 75 charge-discharge cycles for 25 h, and 64% FECO at 2.5 mA cm-2. Compared with flat Ag/CP, the significant enhancement in the CO2RR activity of p-Ag/CP was mainly attributed to the distinctive porous structure and an improved three-phase boundary, which is capable of inducing the stabilization of *COOH intermediates, increased active specific surface areas, fast electron transfer kinetic and mass transportation. Further, theoretical calculations revealed that p-Ag/CP possessed an optimized energy barrier for *COOH intermediates.

Plastron effect enhanced electrochemical CO2 reduction activity over hydrophobic three-dimensional nanoporous silver

The electrochemical reduction of CO2 on catalyst surfaces is hindered by the inefficient mass transfer of CO2 in aqueous solutions. In this study, we employed an electrochemical reduction approach to fabricate a hydrophobic three-dimensional nanoporous silver catalyst with a plastron effect, aiming to enhance the CO2 diffusion. The resulting catalyst exhibited an exceptional performance with the FECO peaking at 95% at -0.65 V (vs. RHE) and demonstrated remarkable stability during continuous electrolysis for 48 hours. Control experiments, together with Tafel analysis, EIS measurements, and contact angle results, confirmed that the notable enhancement of performance was attributed to the hydrophobic porous structure that facilitated efficient storage and rapid mass transfer of low-solubility CO2 gas reactants.

Strategies to Modulate the Copper Oxidation State Toward Selective C2+ Production in the Electrochemical CO2 Reduction Reaction

The electrochemical reduction of CO2 to form value-added chemicals receives considerable attention in recent years. Copper (Cu) is recognized as the only element capable of electro-reducing CO2 into hydrocarbons with two or more carbon atoms (C2+), but the low product selectivity of the Cu-based catalyst remains a major technological challenge to overcome. Therefore, identification of the structural features of Cu-based catalysts is of great importance for the highly selective production of C2+ products (ethylene, ethanol, n-propanol, etc.), and the oxidation state of Cu species in the catalysts is found critical to the catalyst performance. This review introduces recent efforts to fine-tune the oxidation state of Cu to increase carbon capture and produce specific C2+ compounds, with the intention of greatly expediting the advance in the catalyst designs. It also points to the remaining challenges and fruitful research directions for the development of Cu-based catalysts that can shape the practical CO2 reduction technology.

High performance electrochemical CO2 reduction over Pd decorated cobalt containing nitrogen doped carbon

Efficient electrocatalytic CO2 reduction reaction (eCO2RR) to various products, such as carbon monoxide (CO), is crucial for mitigating greenhouse gas emissions and enabling renewable energy storage. In this article, we introduce Pd nanoparticles which are deposited over in-house synthesized nitrogen doped tubular carbon (NC) whose ends are blocked with cobalt oxide (CoOx). This composite material is denoted as Pd@CoOx/NC. Among the series of synthesized electrocatalysts, the optimum ratio (Pd@CoOx/NC1) within this category exhibits exceptional performance, manifesting an 81% faradaic efficiency (FE) for CO generation which was quantitatively measured using a gas chromatograph. This remarkable efficiency can be attributed to several scientific factors. Firstly, the presence of Pd nanoparticles provides active sites for CO2 reduction. Secondly, the NC offer enhanced electrical conductivity and facilitate charge transfer during the reaction. Thirdly, the CoOx capping at the ends of the NC serves to stabilize the catalyst, favoring the formation of CO. The remarkable selectivity of the catalyst is further confirmed by the qualitative CO detection method using PdCl2 strips. Pd@CoOx/NC1 exhibits a high current density of 55 mA cm-2 and a low overpotential of 251 mV, outperforming Pd decorated multiwalled carbon nanotubes (Pd@MWCNTs) which shows a higher overpotential of 481 mV. Pd@CoOx/NC1 shows long-term stability at different potentials and rapid reaction kinetics. These findings highlight Pd@CoOx/NC1 as promising CO2 reduction catalysts, with implications for sustainable energy conversion techniques.

Electronic Structure Design of Transition Metal-Based Catalysts for Electrochemical Carbon Dioxide Reduction

With the increasingly serious greenhouse effect, the electrochemical carbon dioxide reduction reaction (CO2RR) has garnered widespread attention as it is capable of leveraging renewable energy to convert CO2 into value-added chemicals and fuels. However, the performance of CO2RR can hardly meet expectations because of the diverse intermediates and complicated reaction processes, necessitating the exploitation of highly efficient catalysts. In recent years, with advanced characterization technologies and theoretical simulations, the exploration of catalytic mechanisms has gradually deepened into the electronic structure of catalysts and their interactions with intermediates, which serve as a bridge to facilitate the deeper comprehension of structure-performance relationships. Transition metal-based catalysts (TMCs), extensively applied in electrochemical CO2RR, demonstrate substantial potential for further electronic structure modulation, given their abundance of d electrons. Herein, we discuss the representative feasible strategies to modulate the electronic structure of catalysts, including doping, vacancy, alloying, heterostructure, strain, and phase engineering. These approaches profoundly alter the inherent properties of TMCs and their interaction with intermediates, thereby greatly affecting the reaction rate and pathway of CO2RR. It is believed that the rational electronic structure design and modulation can fundamentally provide viable directions and strategies for the development of advanced catalysts toward efficient electrochemical conversion of CO2 and many other small molecules.

Ir-Doped CuPd Single-Crystalline Mesoporous Nanotetrahedrons for Ethylene Glycol Oxidation Electrocatalysis: Enhanced Selective Cleavage of C-C Bond

The development of highly efficient electrocatalysts for complete oxidation of ethylene glycol (EG) in direct EG fuel cells is of decisive importance to hold higher energy efficiency. Despite some achievements, their progress, especially electrocatalytic selectivity to complete oxidated C1 products, is remarkably slower than expected. In this work, we developed a facile aqueous synthesis of Ir-doped CuPd single-crystalline mesoporous nanotetrahedrons (Ir-CuPd SMTs) as high-performance electrocatalyst for promoting oxidation cleavage of C-C bond in alkaline EG oxidation reaction (EGOR) electrocatalysis. The synthesis relied on precise reduction/co-nucleation and epitaxial growth of Ir, Cu and Pd precursors with cetyltrimethylammonium chloride as the mesopore-forming surfactant and extra Br- as the facet-selective agent under ambient conditions. The products featured concave nanotetrahedron morphology enclosed by well-defined (111) facets, single-crystalline and mesoporous structure radiated from the center, and uniform elemental composition without any phase separation. Ir-CuPd SMTs disclosed remarkably enhanced electrocatalytic activity and excellent stability as well as superior selectivity of C1 products for alkaline EGOR electrocatalysis. Detailed mechanism studies demonstrated that performance improvement came from structural and compositional synergies, which kinetically accelerated transports of electrons/reactants within active sites of penetrated mesopores and facilitated oxidation cleavage of high-energy-barrier C-C bond of EG for desired C1 products. More interestingly, Ir-CuPd SMTs performed well in coupled electrocatalysis of anode EGOR and cathode nitrate reduction, highlighting its high potential as bifunctional electrocatalyst in various applications.

Conjugated Microporous Polymers for Catalytic CO2 Conversion

Rising carbon dioxide (CO2) levels in the atmosphere are recognized as a threat to atmospheric stability and life. Although this greenhouse gas is being produced on a large scale, there are solutions to reduction and indeed utilization of the gas. Many of these solutions involve costly or unstable technologies, such as air-sensitive metal-organic frameworks (MOFs) for CO2 capture or "non-green" systems such as amine scrubbing. Conjugated microporous polymers (CMPs) represent a simpler, cheaper, and greener solution to CO2 capture and utilization. They are often easy to synthesize at scale (a one pot reaction in many cases), chemically and thermally stable (especially in comparison with their MOF and covalent organic framework (COF) counterparts, owing to their amorphous nature), and, as a result, cheap to manufacture. Furthermore, their large surface areas, tunable porous frameworks and chemical structures mean they are reported as highly efficient CO2 capture motifs. In addition, they provide a dual pathway to utilize captured CO2 via chemical conversion or electrochemical reduction into industrially valuable products. Recent studies show that all these attractive properties can be realized in metal-free CMPs, presenting a truly green option. The promising results in these two fields of CMP applications are reviewed and explored here.

A DFT study of CO2 electroreduction catalyzed by hexagonal boron-nitride nanosheets with vacancy defects

In addition to providing a sustainable route to green alternative energy and chemical supplies from a cheap and abundant carbon source, recycling CO2 offers an excellent way to reduce net anthropogenic global CO2 emissions. This can be achieved via catalysis on 2D materials. These materials are atomically thin and have unique electrical and catalytic properties compared to bigger nanoparticles and conventional bulk catalysts, opening a new arena in catalysis. This paper examines the efficacy of hexagonal boron nitride (h-BN) lattices with vacancy defects for CO2 electroreduction (CO2RR). We conducted in-depth investigations on different CO2RR electrocatalytic reaction pathways on various h-BN vacancy sites using a computational hydrogen model (CHE). It was shown that CO binds to h-BN vacancies sufficiently to ensure additional electron transfer processes, leading to higher-order reduction products. For mono-atomic defects VN (removed nitrogen), the electrochemical path of (H+ + e-) pair transfers that would lead to the formation of methanol is most favorable with a limiting potential of 1.21 V. In contrast, the reaction pathways via VB (removed boron) imposes much higher thermodynamic barriers for the formation of all relevant species. With a divacancy VBN, the hydrogen evolution reaction (HER) would be the most probable process due to the low rate-determining barrier of 0.69 eV. On the tetravacancy defects VB3N the pathways toward the formation of both CH4 and CH3OH impose a limiting potential of 0.85 V. At the same time, the HER is suppressed by requiring much higher energy (2.15 eV). Modeling the edges of h-BN reveals that N-terminated zigzag conformation would impose the same limiting potential for the formation of methanol and methane (1.73 V), simultaneously suppressing the HER (3.47 V). At variance, the armchair conformation favors the HER, with a rate-determining barrier of 1.70 eV. Hence, according to our calculations, VB3N and VN are the most appropriate vacancy defects for catalyzing CO2 electroreduction reactions.

Silver frameworks based on a tetraphenylethylene-imidazole ligand for electrocatalytic reduction of CO2 to CO

Metal-organic frameworks (MOFs) can be used as electrocatalysts for the CO2 reduction reaction (CO2RR) because of their well-dispersed metal centers. Silver is a common electrocatalyst for reduction of CO2 to CO. In this study, two Ag-MOFs with different structures of [Ag8O2(TIPE)6](NO3)4 (Ag-MOF1) and [Ag(TIPE)0.5CF3SO3] (Ag-MOF2) [TIPE = 1,1,2,2-tetrakis(4-(imidazol-1-yl)phenyl)ethene] were synthesized and used for CO2 electroreduction. The results show that Ag-MOF2 is superior to Ag-MOF1 and exhibits high CO faradaic efficiency (FE) of 92.21% with partial current density of 29.51 mA cm-2 at -0.98 V versus reversible hydrogen electrode (RHE). The FECO is higher than 80% in the potential range of -0.78 to -1.18 V. The difference may be caused by different framework structures leading to different electrochemical active surface areas and charge transfer kinetics. This study provides a new strategy for designing and constructing CO2 electroreduction catalysts and provides potential ways for solving environmental and energy problems caused by excessive CO2 emission.

The Influence of Mesoscopic Surface Structure on the Electrocatalytic Selectivity of CO2 Reduction with UHV-Prepared Cu(111) Single Crystals

The key role of morphological defects (e.g., irregular steps and dislocations) on the selectivity of model Cu catalysts for the electrocatalytic reduction of CO2 (CO2RR) is illustrated here. Cu(111) single-crystal surfaces prepared under ultrahigh vacuum (UHV) conditions and presenting similar chemical and local microscopic surface features were found to display different product selectivity during the CO2RR. In particular, changes in selectivity from hydrogen-dominant to hydrocarbon-dominant product distributions were observed based on the number of CO2RR electrolysis pretreatment cycles performed prior to a subsequent UHV surface regeneration treatment, which lead to surfaces with seemingly identical chemical composition and local crystallographic structure. However, significant mesostructural changes were observed through a micron-scale microscopic analysis, including a higher density of irregular steps on the samples producing hydrocarbons. Thus, our findings highlight that step edges are key for C-C coupling in the CO2RR and that not only atomistic but also mesoscale characterization of electrocatalytic materials is needed in order to comprehend complex selectivity trends.

Highly selective photoelectrochemical CO2 reduction by crystal phase-modulated nanocrystals without parasitic absorption

Photoelectrochemical (PEC) carbon dioxide (CO2) reduction (CO2R) holds the potential to reduce the costs of solar fuel production by integrating CO2 utilization and light harvesting within one integrated device. However, the CO2R selectivity on the photocathode is limited by the lack of catalytic active sites and competition with the hydrogen evolution reaction. On the other hand, serious parasitic light absorption occurs on the front-side-illuminated photocathode due to the poor light transmittance of CO2R cocatalyst films, resulting in extremely low photocurrent density at the CO2R equilibrium potential. This paper describes the design and fabrication of a photocathode consisting of crystal phase-modulated Ag nanocrystal cocatalysts integrated on illumination-reaction decoupled heterojunction silicon (Si) substrate for the selective and efficient conversion of CO2. Ag nanocrystals containing unconventional hexagonal close-packed phases accelerate the charge transfer process in CO2R reaction, exhibiting excellent catalytic performance. Heterojunction Si substrate decouples light absorption from the CO2R catalyst layer, preventing the parasitic light absorption. The obtained photocathode exhibits a carbon monoxide (CO) Faradaic efficiency (FE) higher than 90% in a wide potential range, with the maximum FE reaching up to 97.4% at -0.2 V vs. reversible hydrogen electrode. At the CO2/CO equilibrium potential, a CO partial photocurrent density of -2.7 mA cm-2 with a CO FE of 96.5% is achieved in 0.1 M KHCO3 electrolyte on this photocathode, surpassing the expensive benchmark Au-based PEC CO2R system.

From Biomimicking to Bioinspired Design of Electrocatalysts for CO2 Reduction to C1 Products

The electrochemical reduction of CO2 (CO2 RR) is a promising approach to maintain a carbon cycle balance and produce value-added chemicals. However, CO2 RR technology is far from mature, since the conventional CO2 RR electrocatalysts suffer from low activity (leading to currents <10 mA cm-2 in an H-cell), stability (<120 h), and selectivity. Hence, they cannot meet the requirements for commercial applications (>200 mA cm-2 , >8000 h, >90 % selectivity). Significant improvements are possible by taking inspiration from nature, considering biological organisms that efficiently catalyze the CO2 to various products. In this minireview, we present recent examples of enzyme-inspired and enzyme-mimicking CO2 RR electrocatalysts enabling the production of C1 products with high faradaic efficiency (FE). At present, these designs do not typically follow a methodical approach, but rather focus on isolated features of biological systems. To achieve disruptive change, we advocate a systematic design methodology that leverages fundamental mechanisms associated with desired properties in nature and adapts them to the context of engineering applications.

Non-Noble Metal Incorporated Transition Metal Dichalcogenide Monolayers for Electrochemical CO2 Reduction: A First-Principles Study

Using non-noble metal atoms as catalysts is attractive for decreasing the cost of the CO2 reduction reaction (CO2RR). By screening first-row transition metals and noble metals through extensive first-principles calculations, non-noble Sc and Ti single atoms binding on vacancy-defected transition metal dichalcogenide (TMD) monolayers exhibit better catalytic performance and selectivity for electrochemical CO2RR than noble metal single atoms. The overpotentials of Sc and Ti atoms for the CO2RR can be reduced lower than 0.09 V after applying suitable biaxial tensile strains on vacancy-defected TMDs, which are approximately 1 order of magnitude lower than that of most reported metal atom catalysts. The vacancy defects of TMDs and charge transfer to metal atoms induced by tensile strain play a key role in improving the catalytic activity of non-noble metal single atoms. These results highlight a possible way to design new single atom catalysts for electrochemical CO2RR by utilizing the combination of non-noble metal atoms, defected TMDs, and strain engineering.

Nanocluster Surface Microenvironment Modulates Electrocatalytic CO2 Reduction

The catalytic activity and product selectivity of the electrochemical CO2 reduction reaction (eCO2 RR) depend strongly on the local microenvironment of mass diffusion at the nanostructured catalyst and electrolyte interface. Achieving a molecular-level understanding of the electrocatalytic reaction requires the development of tunable metal-ligand interfacial structures with atomic precision, which is highly challenging. Here, the synthesis and molecular structure of a 25-atom silver nanocluster interfaced with an organic shell comprising 18 thiolate ligands are presented. The locally induced hydrophobicity by bulky alkyl functionality near the surface of the Ag25 cluster dramatically enhances the eCO2 RR activity (CO Faradaic efficiency, FECO : 90.3%) with higher CO partial current density (jCO ) in an H-cell compared to Ag25 cluster (FECO : 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO2 . Remarkably, the hydrophobic Ag25 cluster exhibits jCO as high as -240 mA cm-2 with FECO >90% at -3.4 V cell potential in a gas-fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO2 RR over 120 h. Operando surface-enhanced infrared absorption spectroscopy and theoretical simulations reveal how the ligands alter the neighboring water structure and *CO intermediates, impacting the intrinsic eCO2 RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.

Dual Localized Surface Plasmon Resonance effect enhances Nb2AlC/Nb2C MXene thermally coupled photocatalytic reduction of CO2 hydrogenation activity

Nb2AlC/Nb2C MXene (NAC/NC) heterojunction photocatalysts with Schottky junctions were obtained by selective etching of the Al layer, resulting in 146.25 μmol·g-1 electrons and 15.28 μmol·g-1 holes stored in the heterojunction. The average conversion of NAC/NC thermally coupled photocatalytic reduction of CO2 under the simulated solar irradiation reached 110.15 μmol⋅g-1⋅h-1, and the CO selectivity reached over 92%, which was 1.49 and 1.74 times higher than that of pure Nb2AlC and Nb2C MXene, respectively. After light excitation, the localized surface plasmon resonance (LSPR) effect of holes distributed on the surface of Nb2C MXene crystals in the heterojunction will form high-energy thermal holes to dissociate H2 to H+ and reduce CO2 to form H2O at the same time. The high-energy electrons formed by the LSPR effect of Nb2C MXene and the conduction band electrons generated by the photoexcitation of Nb2C MXene can be migrated to Nb2AlC under the action of the interfacial Schottky junction to supplement the electrons needed for the LSPR effect of Nb2AlC, which continuously forms high-energy hot electrons to convert the adsorbed CO2 into *CO2-, b-HCO3, and HCOO. Subsequently, HCOO releases ⋅OH in a cyclic reaction to continuously reduce to form CO. The dual LSPR effect of Nb2AlC and Nb2C MXene is used to enhance the hydrogenation activity of thermally coupled photocatalytic reduction of CO2, which provides a new research idea for the application of MXene in thermally coupled photoreduction of CO2.

In operando NMR investigations of the aqueous electrolyte chemistry during electrolytic CO2 reduction

The electrolytic reduction of CO2 in aqueous media promises a pathway for the utilization of the green house gas by converting it to base chemicals or building blocks thereof. However, the technology is currently not economically feasible, where one reason lies in insufficient reaction rates and selectivities. Current research of CO2 electrolysis is becoming aware of the importance of the local environment and reactions at the electrodes and their proximity, which can be only assessed under true catalytic conditions, i.e. by in operando techniques. In this work, multinuclear in operando NMR techniques were applied in order to investigate the evolution of the electrolyte chemistry during CO2 electrolysis. The CO2 electroreduction was performed in aqueous NaHCO3 or KHCO3 electrolytes at silver electrodes. Based on 13C and 23Na NMR studies at different magnetic fields, it was found that the dynamic equilibrium of the electrolyte salt in solution, existing as ion pairs and free ions, decelerates with increasingly negative potential. In turn, this equilibrium affects the resupply rate of CO2 to the electrolysis reaction from the electrolyte. Substantiated by relaxation measurements, a mechanism was proposed where stable ion pairs in solution catalyze the bicarbonate dehydration reaction, which may provide a new pathway for improving educt resupply during CO2 electrolysis.

Toward high-efficiency photovoltaics-assisted electrochemical and photoelectrochemical CO2 reduction: Strategy and challenge

The realization of a complete techno-economy through a significant carbon dioxide (CO2) reduction in the atmosphere has been explored to promote a low-carbon economy in various ways. CO2 reduction reactions (CO2RRs) can be induced using sustainable energy, including electric and solar energy, using systems such as electrochemical (EC) CO2RR and photoelectrochemical (PEC) systems. This study summarizes various fabrication strategies for non-noble metal, copper-based, and metal-organic framework-based catalysts with excellent Faradaic efficiency (FE) for target carbon compounds, and for noble metals with low overvoltage. Although EC and PEC systems achieve high energy conversion efficiency with excellent catalysts, they still require external power and lack complete bias-free operation. Therefore, photovoltaics, which can overcome the limitations of these systems, have been introduced. The utilization of silicon and perovskite-based solar cells for photovoltaics-assisted EC (PV-EC) and photovoltaics-assisted PEC (PV-PEC) CO2RR systems are cost-efficient, and the III-V semiconductor photoabsorbers achieved high solar-to-carbon efficiency. This work focuses on PV-EC and PV-PEC CO2RR systems and their components and then summarizes the special cell configurations, including the tandem and stacked structures. Additionally, the study discusses current issues, such as low energy conversion, expensive PV, theoretical limits, and industrial scale-up, along with proposed solutions.

Influence of Water Molecules on CO2 Reduction at the Pt Electrocatalyst in the Membrane Electrode Assembly System

CO2 electroreduction using a Pt catalyst in an aqueous solution system is known to produce only H2. Recently, a remarkable result has been reported that CH4 can be obtained by reducing CO2 using a membrane electrode assembly (MEA) containing a Pt catalyst. A big difference that exists between the two systems is the number of water molecules. Therefore, this study investigated the influence of water molecules on the CO2-reduction process at the Pt electrocatalyst in the MEA system. As a result, cyclic voltammetry indicated that adsorbed CO (COads) was formed by CO2 reduction in the MEA system more preferably than the aqueous solution system. In detail, the ratio of COads at the atop sites (linear CO, COL) on Pt, which participates in the CH4 generation reaction, to the total COads formed by the CO2 reduction became higher as the lower relative humidity (RH) at 50 °C in the MEA system. Cyclic voltammetry combined with in-line mass spectrometry revealed that the amount of COL and CH4 generated by the CO2 reduction reached their maximums at 63.1% RH. CH4 production by the extremely low-overpotential CO2 reduction was significantly achieved under all the RH conditions. Consequently, the Faradaic efficiency of the CH4 production at 63.1% RH was improved by 1.35 times compared to that at 100% RH. These results would be mainly obtained based on the H2O-involved chemical equilibrium of the reactions for the COads and CH4 formation. Overall, the present study experimentally clarified that the formation of COads (particularly COL) and the following CH4 from the CO2 reduction at the Pt electrocatalyst in the MEA system was facilitated by appropriately controlling the water-molecule content.

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

Electrocatalytic CO2 reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti3 C2 Tx MXene-regulated Ag-ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag-ZnO/Ti3 C2 Tx catalyst achieves an outstanding CO2 conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm-2 at -0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag-ZnO interfaces contributes the high selectivity of CO. The CO2 conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO2 reduction.

X-ray Tomography Applied to Electrochemical Devices and Electrocatalysis

X-ray computed tomography (CT) is a nondestructive three-dimensional (3D) imaging technique used for studying morphological properties of porous and nonporous materials. In the field of electrocatalysis, X-ray CT is mainly used to quantify the morphology of electrodes and extract information such as porosity, tortuosity, pore-size distribution, and other relevant properties. For electrochemical systems such as fuel cells, electrolyzers, and redox flow batteries, X-ray CT gives the ability to study evolution of critical features of interest in ex situ, in situ, and operando environments. These include catalyst degradation, interface evolution under real conditions, formation of new phases (water and oxygen), and dynamics of transport processes. These studies enable more efficient device and electrode designs that will ultimately contribute to widespread decarbonization efforts.

Achieving Efficient CO2 Electrolysis to CO by Local Coordination Manipulation of Nickel Single-Atom Catalysts

Selective electroreduction of CO2 to C1 feed gas provides an attractive avenue to store intermittent renewable energy. However, most of the CO2-to-CO catalysts are designed from the perspective of structural reconstruction, and it is challenging to precisely design a meaningful confining microenvironment for active sites on the support. Herein, we report a local sulfur doping method to precisely tune the electronic structure of an isolated asymmetric nickel-nitrogen-sulfur motif (Ni1-NSC). Our Ni1-NSC catalyst presents >99% faradaic efficiency for CO2-to-CO under a high current density of -320 mA cm-2. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and differential electrochemical mass spectrometry indicated that the asymmetric sites show a significantly weaker binding strength of *CO and a lower kinetic overpotential for CO2-to-CO. Further theoretical analysis revealed that the enhanced CO2 reduction reaction performance of Ni1-NSC was mainly due to the effectively decreased intermediate activation energy.

Highly Efficient CO2 Reduction at Steady 2 A cm-2 by Surface Reconstruction of Silver Penetration Electrode

Electroreduction of CO2 to CO is a promising route for greenhouse gas resource utilization, but it still suffers from impractical current density and poor durability. Here, a nanosheet shell (NS) vertically standing on the Ag hollow fiber (NS@Ag HF) surface formed by electrochemical surface reconstruction is reported. As-prepared NS@Ag HF as a gas penetration electrode exhibited a high CO faradaic efficiency of 97% at an ultra-high current density of 2.0 A cm-2 with a sustained performance for continuous >200 h operation. The experimental and theoretical studies reveal that promoted surface electronic structures of NS@Ag HF by the nanosheets not only suppress the competitive hydrogen evolution reaction but also facilitate the CO2 reduction kinetics. This work provides a feasible strategy for fabricating robust catalysts for highly efficient and stable CO2 reduction.

CO electroreduction on single-atom copper

Electroreduction of carbon dioxide (CO₂) or carbon monoxide (CO) toward C₂₊ hydrocarbons such as ethylene, ethanol, acetate and propanol represents a promising approach toward carbon-negative electrosynthesis of chemicals. Fundamental understanding of the carbon─carbon (C-C) coupling mechanisms in these electrocatalytic processes is the key to the design and development of electrochemical systems at high energy and carbon conversion efficiencies. Here, we report the investigation of CO electreduction on single-atom copper (Cu) electrocatalysts. Atomically dispersed Cu is coordinated on a carbon nitride substrate to form high-density copper─nitrogen moieties. Chemisorption, electrocatalytic, and computational studies are combined to probe the catalytic mechanisms. Unlike the Langmuir-Hinshelwood mechanism known for copper metal surfaces, the confinement of CO adsorption on the single-copper-atom sites enables an Eley-Rideal type of C-C coupling between adsorbed (*CO) and gaseous [CO(g)] carbon moxide molecules. The isolated Cu sites also selectively stabilize the key reaction intermediates determining the bifurcation of reaction pathways toward different C₂₊ products.

Smart manufacturing inspired approach to research, development, and scale-up of electrified chemical manufacturing systems

As renewable electricity becomes cost competitive with fossil fuel energy sources and environmental concerns increase, the transition to electrified chemical and fuel synthesis pathways becomes increasingly desirable. However, electrochemical systems have traditionally taken many decades to reach commercial scales. Difficulty in scaling up electrochemical synthesis processes comes primarily from difficulty in decoupling and controlling simultaneously the effects of intrinsic kinetics and charge, heat, and mass transport within electrochemical reactors. Tackling this issue efficiently requires a shift in research from an approach based on small datasets, to one where digitalization enables rapid collection and interpretation of large, well-parameterized datasets, using artificial intelligence (AI) and multi-scale modeling. In this perspective, we present an emerging research approach that is inspired by smart manufacturing (SM), to accelerate research, development, and scale-up of electrified chemical manufacturing processes. The value of this approach is demonstrated by its application toward the development of CO₂ electrolyzers.

Potential-Dependent Temporal Dynamics of CO Surface Concentration in Electrocatalytic CO2 Reduction

Beyond the identity and structure of an intermediate, changes in its concentration on and near the electrode surface with time are a critical component to understand and improve selectivity and reactivity in electrochemical transformations. We apply pulsed-potential electrochemical Raman scattering microscopy to measure the potential-dependent temporal evolution of CO formed during electrocatalytic CO₂ reduction in acetonitrile on Ag electrodes. At driving potentials positive of the onset potential as determined by cyclic voltammetry, CO accumulates on the electrode surface at time scales longer than 1 s. Near the ensemble onset potential, CO resides on the electrode surface for approximately 100 ms. At potentials known to evolve CO from the electrode surface, CO remains adsorbed on the electrode for less than 10 ms. The time scales accessible in our strategy are nearly 3 orders of magnitude faster than transient Raman or infrared measurements, allowing direct measurement of the temporal evolution of intermediates.

Engineering iron carbide catalyst with aerophilic and electron-rich surface for improved electrochemical CO2 reduction

Highly efficient electrocatalyst for carbon dioxide reduction (CO₂RR) is desirable for converting CO₂ into carbon-based chemicals and reducing anthropogenic carbon emission. Regulating catalyst surface to improve the affinity for CO₂ and the capability of CO₂ activation is the key to high-efficiency CO₂RR. In this work, we develop an iron carbide catalyst encapsulated in nitrogenated carbon (SeN-Fe₃C) with an aerophilic and electron-rich surface by inducing preferential formation of pyridinic-N species and engineering more negatively charged Fe sites. The SeN-Fe₃C exhibits an excellent CO selectivity with a CO Faradaic efficiency (FE) of 92 % at -0.5 V (vs. RHE) and remarkably enhanced CO partial current density as compared to the N-Fe₃C catalyst. Our results demonstrate that Se doping reduces the Fe₃C particle size and improves the dispersion of Fe₃C on nitrogenated carbon. More importantly, the preferential formation of pyridinic-N species induced by Se doping endows the SeN-Fe₃C with an aerophilic surface and improves the affinity of the SeN-Fe₃C for CO₂. Density functional theory (DFT) calculations reveal that the electron-rich surface, which is caused by pyridinic N species and much more negatively charged Fe sites, leads to a high degree of polarization and activation of CO₂ molecule, thus conferring a remarkably improved CO₂RR activity on the SeN-Fe₃C catalyst.

Sub-100 mA/cm2 CO2-to-CO Reduction Current Densities in Hierarchical Porous Gold Electrocatalysts Made by Direct Ink Writing and Dealloying

While most research efforts on CO₂-to-CO reduction electrocatalysts focus on boosting their selectivity, the reduction rate, directly proportional to the reduction current density, is another critical parameter to be considered in practical applications. This is because mass transport associated with the diffusion of reactant/product species becomes a major concern at a high reduction rate. Nanostructured Au is a promising CO₂-to-CO reduction electrocatalyst for its very high selectivity. However, the CO₂-to-CO reduction current density commonly achieved in conventional nanostructured Au electrocatalysts is relatively low (in the range of 1-10 mA/cm²) for practical applications. In this work, we combine direct ink writing-based additive manufacturing and dealloying to design a robust hierarchical porous Au electrocatalyst to improve the mass transport and achieve high CO₂-to-CO reduction current densities on the order of 64.9 mA/cm² with CO partial current density of 33.8 mA/cm² at 0.55 V overpotential using an H-cell configuration. Although the current density achieved in our robust hierarchical porous Au electrocatalyst is one order of magnitude higher than the one achieved in conventional nanostructured electrocatalysts, we found that the selectivity of our system is relatively low, namely 52%, which suggests that mass transport remains a critical issue despite the hierarchical porous architecture. We further show that the bulk dimension of our electrocatalyst is a critical parameter governing the interplay between selectivity and reduction rate. The insights gained in this work shed new light on the design of electrocatalysts toward scale-up CO₂ reduction and beyond.

Turning Carbon Dioxide into Sustainable Food and Chemicals: How Electrosynthesized Acetate Is Paving the Way for Fermentation Innovation

ConspectusThe agricultural and chemical industries are major contributors to climate change. To address this issue, hybrid electrocatalytic-biocatalytic systems have emerged as a promising solution for reducing the environmental impact of these key sectors while providing economic onboarding for carbon capture technology. Recent advancements in the production of acetate via CO₂/CO electrolysis as well as advances in precision fermentation technology have prompted electrochemical acetate to be explored as an alternative carbon source for synthetic biology. Tandem CO₂ electrolysis coupled with improved reactor design has accelerated the commercial viability of electrosynthesized acetate in recent years. Simultaneously, innovations in metabolic engineering have helped leverage pathways that facilitate acetate upgrading to higher carbons for sustainable food and chemical production via precision fermentation. Current precision fermentation technology has received much criticism for reliance upon food crop-derived sugars and starches as feedstock which compete with the human food chain. A shift toward electrosynthesized acetate feedstocks could help preserve arable land for a rapidly growing population.Technoeconomic analysis shows that using electrochemical acetate instead of glucose as a fermentation feedstock reduces the production costs of food and chemicals by 16% and offers improved market price stability. Moreover, given the rapid decline in utility-scale renewable electricity prices, electro-synthesized acetate may become more affordable than conventional production methods at scale. This work provides an outlook on strategies to further advance and scale-up electrochemical acetate production. Additional perspective is offered to help ensure the successful integration of electrosynthesized acetate and precision fermentation technologies. In the electrocatalytic step, it is critical that relatively high purity acetate can be produced in low-concentration electrolyte to help ensure that minimal treatment of the electrosynthesized acetate stream is needed prior to fermentation. In the biocatalytic step, it is critical that microbes with increased tolerances to elevated acetate concentrations are engineered to help promote acetate uptake and accelerate product formation. Additionally, tighter regulation of acetate metabolism via strain engineering is essential to improving cellular efficiency. The implementation of these strategies would allow the coupling of electrosynthesized acetate with precision fermentation to offer a promising approach to sustainably produce chemicals and food. Reducing the environmental impact of the chemical and agricultural sectors is necessary to avoid climate catastrophe and preserve the habitability of the planet for future generations.

Guanosine-derived atomically dispersed Cu-N3-C sites for efficient electroreduction of carbon dioxide

Single-atom copper (Cu) embedded within carbon catalysts have demonstrated significant potential in the electrochemical reduction of carbon dioxide (CO₂) into valuable chemicals and fuels. Herein, we develop a straightforward and template-free strategy for synthesizing atomically dispersed CuNC catalysts (CuG) by annealing the self-assembled guanosine. The CuG catalysts display two-dimensional morphology, tunable pore size and large surface areas that can be adjusted by changing carbonization temperature. Spherical aberration-corrected transmission electron microscopy reveals that single-atom Cu are homogeneously dispersed on the surface of carbon nanosheets. The optimum CuG-1000 catalysts achieve a high CO Faradaic efficiency (FE_co) up to 99% and a high CO current density of 6.53 mA cm^-2 (-0.65 V vs. RHE). Besides, the flow cell test of CuG-1000 shows a high current density up to 25.2 mA cm^-2 and the FE_co still exceeded 91% after more than 20 h of testing. Specifically, the existence of Cu-N₃-C active sites was proved by extended X-ray absorption fine structure (EXAFS). Density functional theory evidences that tricoordinated copper with N can largely regulate the adsorption and desorption of key intermediates by transferring electrons to *COOH through Cu atoms, thereby improving selectivity toward CO. This work demonstrates the active origin of CuNC catalysts in CO₂ electroreduction and offers a blueprint to construct atomically dispersed transition site catalysts by supramolecular self-assembly strategy.

Multiscale model to resolve the chemical environment in a pressurized CO2-captured solution electrolyzer

The community of electrochemical CO₂ reduction is almost exclusively focused on gaseous CO₂-fed electrolyzers. Here, we proposed a pressurized CO₂-Captured solution electrolyzer to produce solar Fuel of CO (abbreviated "CCF") without the need to regenerate gaseous CO₂. Specifically, we developed an experimentally validated multiscale model to quantitatively investigate the effect of pressure-induced chemical environment and to resolve the complex relationship between this effect and the activity and selectivity of CO production. Our results show that the pressure-induced variation of the cathode pH has a negative effect on the hydrogen evolution reaction, whereas the species coverage variation positively affects CO₂ reduction. These effects are more pronounced at pressures below 15 bar (1 bar = 101 kPa). Consequently, a mild increase in the pressure of the CO₂-captured solution from 1 to 10 bar leads to a dramatic enhancement in selectivity. Using a commercial Ag nanoparticle catalyst, our pressurized CCF prototype achieved CO selectivity higher than 95% at a low cathode potential of -0.6 V versus reversible hydrogen electrode (RHE), comparable to that under the gaseous CO₂-fed condition. This enables the demonstration of a solar-to-CO efficiency of 16.8%, superior to any known devices with an aqueous feed.

Cutting-Edge Electrocatalysts for CO2RR

A world-wide growing concern relates to the rising levels of CO₂ in the atmosphere that leads to devastating consequences for our environment. In addition to reducing emissions, one alternative strategy is the conversion of CO₂ (via the CO₂ Reduction Reaction, or CO₂RR) into added-value chemicals, such as CO, HCOOH, C₂H₅OH, CH₄, and more. Although this strategy is currently not economically feasible due to the high stability of the CO₂ molecule, significant progress has been made to optimize this electrochemical conversion, especially in terms of finding a performing catalyst. In fact, many noble and non-noble metal-based systems have been investigated but achieving CO₂ conversion with high faradaic efficiency (FE), high selectivity towards specific products (e.g., hydrocarbons), and maintaining long-term stability is still challenging. The situation is also aggravated by a concomitant hydrogen production reaction (HER), together with the cost and/or scarcity of some catalysts. This review aims to present, among the most recent studies, some of the best-performing catalysts for CO₂RR. By discussing the reasons behind their performances, and relating them to their composition and structural features, some key qualities for an "optimal catalyst" can be defined, which, in turn, will help render the conversion of CO₂ a practical, as well as economically feasible process.

Metal functionalization of two-dimensional nanomaterials for electrochemical carbon dioxide reduction

With the mechanical exfoliation of graphene in 2004, researchers around the world have devoted significant efforts to the study of two-dimensional (2D) nanomaterials. Nowadays, 2D nanomaterials are being developed into a large family with varieties of structures and derivatives. Due to their fascinating electronic, chemical, and physical properties, 2D nanomaterials are becoming an important type of catalyst for the electrochemical carbon dioxide reduction reaction (CO₂RR). Here, we review the recent progress in electrochemical CO₂RR using 2D nanomaterial-based catalysts. First, we briefly describe the reaction mechanism of electrochemical CO₂ reduction to single-carbon (C₁) and multi-carbon (C₂₊) products. Then, we discuss the strategies and principles for applying metal materials to functionalize 2D nanomaterials, such as graphene-based materials, metal-organic frameworks (MOFs), and transition metal dichalcogenides (TMDs), as well as applications of resultant materials in the electrocatalytic CO₂RR. Finally, we summarize the present research advances and highlight the current challenges and future opportunities of using metal-functionalized 2D nanomaterials in the electrochemical CO₂RR.

Achieving Tunable Selectivity and Activity of CO2 Electroreduction to CO via Bimetallic Silver-Copper Electronic Engineering

Limited comprehension of the reaction mechanism has hindered the development of catalysts for CO₂ reduction reactions (CO₂ RR). Here, the bimetallic AgCu nanocatalyst platform is employed to understand the effect of the electronic structure of catalysts on the selectivity and activity for CO₂ electroreduction to CO. The atomic arrangement and electronic state structure vary with the atomic ratio of Ag and Cu, enabling tunable d-band centers to optimize the binding strength of key intermediates. Density functional theory calculations confirm that the variation of Cu content greatly affects the free energy of *COOH, *CO (intermediate of CO), and *H (intermediates of H₂ ), which leads to the change of the rate-determining step. Specifically, Ag₉₆ Cu₄ reduces the free energy of the formation of *COOH while maintaining a relatively high theoretical overpotential for hydrogen evolution reaction(HER), thus achieving the best CO selectivity. While Ag₇₀ Cu₃₀ shows relatively low formation energy of both *COOH and *H, the compromised thermodynamic barrier and product selectivity allows Ag₇₀ Cu₃₀ the best CO partial current density. This study realizes the regulation of the selectivity and activity of electrocatalytic CO₂ to CO, which provides a promising way to improve the intrinsic performance of CO₂ RR on bimetallic AgCu.

Electrochemical C-N coupling of CO2 and nitrogenous small molecules for the electrosynthesis of organonitrogen compounds

Electrochemical C-N coupling reactions based on abundant small molecules (such as CO₂ and N₂) have attracted increasing attention as a new "green synthetic strategy" for the synthesis of organonitrogen compounds, which have been widely used in organic synthesis, materials chemistry, and biochemistry. The traditional technology employed for the synthesis of organonitrogen compounds containing C-N bonds often requires the addition of metal reagents or oxidants under harsh conditions with high energy consumption and environmental concerns. By contrast, electrosynthesis avoids the use of other reducing agents or oxidants by utilizing "electrons", which are the cleanest "reagent" and can reduce the generation of by-products, consistent with the atomic economy and green chemistry. In this study, we present a comprehensive review on the electrosynthesis of high value-added organonitrogens from the abundant CO₂ and nitrogenous small molecules (N₂, NO, NO₂^-, NO₃^-, NH₃, etc.) via the C-N coupling reaction. The associated fundamental concepts, theoretical models, emerging electrocatalysts, and value-added target products, together with the current challenges and future opportunities are discussed. This critical review will greatly increase the understanding of electrochemical C-N coupling reactions, and thus attract research interest in the fixation of carbon and nitrogen.

Strategies to Enhance CO2 Electrochemical Reduction from Reactive Carbon Solutions

CO₂ electrochemical reduction (CO₂ ER) from (bi)carbonate feed presents an opportunity to efficiently couple this process to alkaline-based carbon capture systems. Likewise, while this method of reducing CO₂ currently lags behind CO₂ gas-fed electrolysers in certain performance metrics, it offers a significant improvement in CO₂ utilization which makes the method worth exploring. This paper presents two simple modifications to a bicarbonate-fed CO₂ ER system that enhance the selectivity towards CO. Specifically, a modified hydrophilic cathode with Ag catalyst loaded through electrodeposition and the addition of dodecyltrimethylammonium bromide (DTAB), a low-cost surfactant, to the catholyte enabled the system to achieve a FE_CO of 85% and 73% at 100 and 200 mA·cm^-2, respectively. The modifications were tested in 4 h long experiments where DTAB helped maintain FE_CO stable even when the pH of the catholyte became more alkaline, and it improved the CO² utilization compared to a system without DTAB.

Recent Advances of the Confinement Effects Boosting Electrochemical CO2 Reduction

Powered by clean and renewable energy, electrocatalytic CO₂ reduction reaction (CO₂ RR) to chemical feedstocks is an effective way to mitigate the greenhouse effect and artificially close the carbon cycle. However, the performance of electrocatalytic CO₂ RR was impeded by the strong thermodynamic stability of CO₂ molecules and the high susceptibility to hydrogen evolution reaction (HER) in aqueous phase systems. Moreover, the numerous reaction intermediates formed at very near potentials lead to poor selectivity of reaction products, further preventing the industrialization of CO₂ RR. Catalysis in confined space can enrich the reaction intermediates to improve their coverage at the active site, increase local pH to inhibit HER, and accelerate the mass transfer rate of reactants/products and subsequently facilitate CO₂ RR performance. Therefore, we summarize the research progress on the application of the confinement effects in the direction of CO₂ RR in theoretical and experimental directions. We first analyzed the mechanism of the confinement effect. Subsequently, the confinement effect was discussed in various forms, which can be characterized as an abnormal catalytic phenomenon due to the relative limitation of the reaction region. In specific, based on the physical structure of the catalyst, the confinement effect was divided in four categories: pore structure confinement, cavity structure confinement, active center confinement, and other confinement methods. Based on these discussions, we also have summarized the prospects and challenges in this field. This review aims to stimulate greater interests for the development of more efficient confined strategy for CO₂ RR in the future.