Ionic Liquid–Mediated Selective Conversion of CO 2 to CO at Low Overpotentials

Due to the increasing global energy demands, scarce fossil fuel supplies, and environmental issues, the pursued goals of energy technologies are being sustainable, more efficient, accessible, and produce near zero greenhouse gas emissions. Electrochemical water splitting is considered as a highly viable and eco-friendly energy technology. Further, electrochemical carbon dioxide (CO2 ) reduction reaction (CO2 RR) is a cleaner strategy for CO2 utilization and conversion to stable energy (fuels). One of the critical issues in these cleaner technologies is the development of efficient and economical electrocatalyst. Among various materials, metal-organic frameworks (MOFs) are becoming increasingly popular because of their structural tunability, such as pre- and post- synthetic modifications, flexibility in ligand design and its functional groups, and incorporation of different metal nodes, that allows for the design of suitable MOFs with desired quality required for each process. In this review, the design of MOF was discussed for specific process together with different synthetic methods and their effects on the MOF properties. The MOFs as electrocatalysts were highlighted with their performances from the aspects of hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and electrochemical CO2 RR. Finally, the challenges and opportunities in this field are discussed.

Pd–In catalysts were electrochemically prepared for application in unitized regenerative fuel cells based on carbon dioxide and formic acid. The morphology and composition of the Pd–In catalysts were controlled by varying the concentration of the In precursor in a deposition bath. The catalytic activity was then investigated for both formic acid oxidation and carbon dioxide reduction reactions. In order to enhance their activity and stability, the Pd–In catalysts were annealed under Ar atmosphere, resulting in better activities than those of the as‐deposited Pd–In catalysts. Although the activity was not excellent, this effort aimed at the development of bimetallic catalysts paves the way toward inter‐conversion electrodes active in both the electrochemical carbon dioxide reduction and formic acid oxidation reactions.

Rising levels of greenhouse gases has become a global issue, due to their negative environmental impact. A significant rise in atmospheric carbon dioxide (CO2) levels has correlated with the rise in the use of fossil fuels, resulting in it being a major contributor to global warming. Current research implements various methods to electrochemically reduce CO2, however, the challenge has been to develop a method that is efficient and sustainable. Prior studies have supported the use of electrocatalysts to assist in the reduction of environmental CO2. Carbon dioxide reduction reactions (CO2RR) are able to turn a damaging gas into a useful C1-feedstock, CO, achieving carbon neutrality. In recent years, studies have suggested the addition of ionic liquids into electrochemical CO2RR systems may have a positive impact on CO2RR. Ionic liquids have emerged as a major focus of research because of their ability to act as a cocatalyst, supporting electrolyte, and solvent depending on the system. Intermediates and products created during CO2RR experiments can vary depending on the solvent used, nature of the supporting electrolyte and as well as the material of the electrode. Utilizing an established catalyst and electrolyte/solvent system for CO2 reduction reactions, investigations can be done on the impact of additional supporting co-reactants into the system, such as ionic liquids. For this study, we took an established catalyst/electrolyte/solvent system and investigated the mechanistic effects the addition of ionic liquids may potentially exhibit on the system. Specifically, we performed electrochemical and spectroelectrochemical studies to determine catalytic activity and mechanistic differences of the highly studied CO2RR catalyst, Re(bipy-tBu)(CO)3Cl, in the neat solvent acetonitrile (CH3CN) with supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6), in neat room temperature ionic liquids (1-butyl-1methylpyrrolidinium bis(trifluoromethyl sulfonyl)imide and -butyl-3methylimidazolium) and in mixtures of CH3CN with each of them. The reaction mechanisms were determined by combining multistep-amperometry with FTIR using an optically transparent thin-layer electrochemical cell (OTTLE cell). We determined that in the ionic liquid + CH3CN systems resulted in an increase in catalytic activity, showing that the ionic liquids are “promoters” for the CO2RR. We also found remarkably different reaction mechanisms for each of the systems studied.

Tetrafunctional electrocatalyst for oxygen reduction, oxygen evolution, hydrogen evolution, and carbon dioxide reduction reactions

Controlling the chemical environment of the atomically dispersed central atoms doped in the graphene lattice is critical to achieve desirable catalytic performances in carbon dioxide reduction reaction (CO2RR), however, how the local structures of non-transition metal-based single atom (SAs) affect the catalytic performances of CO2RR remains less understood. This study reports the immobilization of bismuth single atom catalysts (SACs) on pristine and nitrogenated graphene nanosheets with switchable catalytic selectivity in the carbon dioxide reduction reaction (CO2RR). Based on systematic physical characterizations and electrochemical analysis, it has been demonstrated that the Bi atom coordinated with four adjacent nitrogen atoms (Bi-N-C) selectively produces carbon monoxide (CO) with high selectivity at low overpotential, whereas Bi SACs bounded with carbon atoms (Bi-C) almost exclusively generate formate (FA). Theoretical investigations reveal that the Bi-N-C catalyst displays the lowest activation barrier for the first hydrogenation step of CO2 to produce *COOH, while Bi-C shows the most preferable pathway towards the formation of *OCHO, which is considered as the important intermediate species to generate FA. The controllable product distributions are dictated by the different local structures of Bi centers in Bi-N-C and Bi-C, and such differences could induce distinct electronic properties of Bi centers and subsequently switch the CO2RR products from CO to FA. This work has substantiated the importance of the fine-regulation of the coordination environment of one of the representative p-blocking SAs to steer the selectivity of CO2RR.

In this comprehensive research endeavor, we introduce two groundbreaking methodologies poised to reshape the landscape of nanocomposite material synthesis, unlocking unprecedented potential across a spectrum of electrochemical applications. The Joule heating method, also known as Carbothermal Shock (CTS), emerges as a remarkably simple yet highly effective technique for generating multi-metallic nanoparticles (NPs), including high-entropy alloys (HEA). The CTS process involves loading a metal precursor onto a carbon substrate and swiftly elevating the temperature through the passage of an electric current, inducing rapid thermal shock and resulting in the formation of small and uniform NPs. While the CTS method has showcased high performance in various applications, such as rechargeable energy storage systems and catalytic conversion, challenges persist in achieving optimal surface coverage of NPs, particularly for electrochemical applications. For example, metal NPs formed through the CTS method have not been used for electrocatalytic carbon dioxide (CO2) or nitrogen (N2) reduction reactions because the hydrogen evolution reaction (HER) and unwanted reactions occur on the exposed carbon substrate as a competing reactionTo overcome this challenge, we present a pioneering approach involving the utilization of partially carbonized cellulose as a novel carbon substrate. The cellulose matrix, distinguished by its unique structural characteristics, including interconnected aromatic rings and numerous edge sites, facilitates an unprecedented high surface coverage of various single and copper (Cu)-based polyelemental alloy NPs. The controlled manipulation of defect sites, such as vacancies and dangling bonds, within the carbonized cellulose plays a pivotal role in the nucleation and stabilization of metal NPs during the CTS process. The study underscores the correlation between defect sites on the carbon substrate and the resulting surface morphology of metal NPs, providing valuable insights for future advancements in nanocomposite material synthesis through the CTS method.Moreover, the cellulose-enabled high surface coverage Cu NPs exhibit remarkable potential in electrocatalytic carbon dioxide (CO2) reduction reactions. The Cu NPs on cellulose/carbon paper (Cu/cellulose/CP) demonstrated a high ethylene selectivity of 48.92% at a potential of −0.529 V versus the reverse hydrogen electrode (RHE), maintaining stability over 30 hours of reaction in a 10 M KOH electrolyte. This study presents an exciting prospect for expanding the applications of polyelemental alloy NPs synthesized via the CTS method in diverse electrochemical applications.Transitioning to the second part of the research, we explore the application of MXenes, a recently discovered family of two-dimensional materials composed of transition metal carbides and carbonitrides. These atomically thin layers exhibit a unique combination of metallic conductivity and high surface area due to their layered structure. To enhance the properties of MXenes and broaden their applications, various components, including organic small molecules, polymers, metals, and semiconducting materials, have been incorporated into their intrinsic nanostructures.However, existing methods for fabricating MXene composite hybrid materials predominantly rely on solution processing techniques, introducing challenges such as severe MXene oxidation and nanoparticle aggregation. Addressing these limitations, we introduce a revolutionary rapid Joule heating approach, a solution-free method designed to synthesize a diverse range of MXene hybrid nanocomposites. This approach minimizes MXene oxidation on the surface and ensures a uniform distribution of nanoparticles on MXene surfaces without the drawbacks of severe aggregations. It became possible to synthesize metal NPs from unary to senary combinations (including Pt, Co, Ni, Fe, Cu components) with minimal MXene oxidation through rapid heating. The ability to easily modify the precursor ratio and synthesize unlimited combinations of MXene composite hybrid nanostructures enables the modulation of material properties to achieve high performance in diverse electrochemical applications. The resulting Pt-MXene hybrid nanostructure, synthesized through this novel approach, exhibits promising electrocatalytic performance for the hydrogen evolution reaction (HER) by minimizing MXene oxidation during the synthesis process. This breakthrough method holds immense potential for a wide range of catalytic applications, where the synergistic effects of MXene composites can significantly contribute.

With the rapid growth of the world population and economy, the greenhouse effect caused by CO2 emissions is becoming more and more serious. To achieve the "two-carbon" goal as soon as possible, the carbon dioxide reduction reaction is one of the most promising strategies due to its economic and environmental friendliness. As an analog of graphene, monolayer h-BN is considered to be a potential catalyst. To systematically and theoretically study the effect of O doping on the CO2 reduction catalytic properties of monolayer h-BN, we have perform a series of first-principle calculations in this paper. The structural analysis demonstrates that O preferentially replaces N, leading to decreasing VBM of monolayer h-BN, which is conducive to improving its capability for CO2 reduction. The preferential CO2 adsorption sites on monolayer h-BN before and after O doping are the N-t site and B-t site, respectively. O doping increases the adsorption strength of CO2, which is favorable in the further hydrogenation of CO2. During the conversion of CO2 into CO and HCOOH via a two-electron pathway and CH3OH and CH4 via a six-electron pathway, O doping can reduce the energy barrier of the rate determining step (RDS) and change the key steps from uphill reactions to downhill reactions, thus increasing the probability of CO2 reduction. In conclusion, O(N)-doped h-BN exhibits the excellent CO2 reduction performance and has the potential to be a promising catalyst.

Theoretical insights into the selective and activity of CuAu catalyst for O2 and CO2 electroreduction

Carbon dioxide reduction reaction (CO₂RR) offers significant potential for closing the carbon loop by converting CO₂ into value-added feedstocks for industries or renewable fuels. Among the diverse categories of CO₂RR electrocatalysts, single-atom catalysts (SACs), in particular, metal-nitrogen-doped carbon (M-N-C) catalysts are emerging as promising candidates, owing to their optimal atom utilization efficiency, well-defined active sites, relatively uniform active centers. Up to date, M-N-C catalysts have demonstrated excellent performance in electrochemical CO2-to-CO conversion, but only within very narrow potential windows. Such limited working potentials will negatively affect the real-world industrial applications with complex operation variations, which may introduce unwanted byproducts (i.g. H2).Herein, we successfully synthesized a high-efficient Fe-N-C electrocatalysts for CO2RR, achieving near 100% CO conversion efficiency across a broaden potential range. In this work, zeolitic imidazolate frameworks (ZIF-8) were utilized as a template to create a porous structure, providing a high surface area and well-distributed pores, while the mechanical collisions from ball milling process can enhance the interaction of Fe source with the support materials. As a result, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the synthesized catalyst possesses atomically dispersed Fe single atom sites, which are uniformly distributed and anchored within a nitrogen-doped carbon matrix. The X-ray photoelectron spectroscopy analysis further corroborates this, showing characteristic peaks corresponding to Fe-N x coordination bonds, indicative of strong interactions between the Fe atoms and the surrounding nitrogen-doped carbon framework.The as-prepared Fe-N-C catalysts were implemented in CO2RR, achieving an exceptionally high faradaic efficiency (FE) for CO production, exceeding 99% across a wide potential range from -0.3 V to -1.2 V vs. RHE. In addition, the partial current density of CO at -1.2 V vs. RHE in a flow cell reaches up to 27.25 mA/cm2. Stability studies reveal that our catalysts exhibit consistent CO2RR performance in 5 hours, maintaining the CO selectivity with near 100%. XANES and theoretical studies will reveal the inherent electronic structure for great CO2RR performance with near-unity CO conversion. The details will be shown in the presentation. This work highlights the potential of Fe single-atom catalysts derived from ZIF-8 precursors as a scalable and robust solution for CO₂ electroreduction.

This work presents the functional design of an ionic liquid (IL) for reactive capture and conversion of CO2 (RCC) where enhanced Faradic efficiencies, stable electrolysis, and lowered overpotentials were obtained toward CO and C2+ products on Ag and Cu electrodes, respectively. Critical experimental factors that impact the evaluation of such electrolytes for RCC including the cosolvent selection, working electrode construction, reference electrode choice, and the cell design are discussed. As the IL chemisorbs CO2, the changes in the bulk viscosity and conductivity as well as the speciation (e.g, ion-CO2 adducts and newly formed hydrogen bonded species) near the electrode surface alter RCC activation overpotential, thus impacting conversion reaction outcomes as well as the interpretation of results. We showcase the utilization of in-situ surface enhanced Raman spectroscopy (SERS) in the characterization of the electrode-electrolyte interface in corroboration of the mechanisms proposed for the RCC products obtained from constant potential electrolysis. This study serves as a basis for the understanding of RCC mechanism in the presence of reactive ILs, which could lead to the development of more efficient and selective CO2 reduction electrolytes.

: The reduction of carbon dioxide by alkali-metal amalgams was studied as a potential means for reclamation of carbon dioxide waste gas in space systems. The carbon dioxide reduction reactions were investigated at moderate temperatures and pressures - typically 25 to 200 C and 1 to 1.1 atmospheres. The investigations demonstrated that carbon dioxide can be reduced effectively by an appropriate alkali-metal amalgam system. Moreover, the alkali-amalgam expended in the carbon dioxide reduction process can be reclaimed by electrolysis from an aqueous or nonaqueous salt solution containing the fixed carbon dioxide products. The carbon dioxide fixation products can be drained- off or, in some cases, recycled for further reduction. In the carbon dioxide reduction reactions by alkali-amalgam systems, a wide variety of products ranging from simple organic acids, such as formic and oxalic acid, to totally reduced carbon dioxide in the form of carbon black have been identified. The types of products formed are dependent upon such factors as the type of alkali- metal amalgam used, temperature, pressure, and electrolytic reducing conditions. In electrolytic recovery of the alkali-metal amalgam from aqueous media, oxygen also is produced as a useful by-product of the carbon dioxide reduction system.

Extensive research efforts have been concentrated into the conversion of CO2 into value-added chemicals as it provides a route to a circular carbon economy. Electroreduction of CO2 on Au surfaces allows for the selective transformation of CO2 into CO via carbon dioxide reduction reaction (CO2RR), and the catalytic activity depends on the concentration and identity of cations present at the electrode-electrolyte interface. Experimental reports performed under typical CO2RR-operating conditions have widely shown that the CO2RR is enabled by the presence of metal or organic cations in the cathodic interfacial microenvironment. A remaining question is to address if CO2RR can occur in the absence of metal or organic cations and, if so, what the mechanism may be. Here, we show that CO2 can be electrochemically reduced to CO on Au in acidic electrolytes rigorously controlled to avoid the presence of metal and organic cations and systematically suggest the important contributions allowing this reaction to proceed. The formation of CO is confirmed by both qualitative and quantitative methods using potentiodynamic CO-stripping scans and chromatography-assisted constant potential electrolysis. Calculations indicate that H3O+ is able to stabilize the formation of *CO2 -, albeit at more negative potentials than when an alkali metal cation is present. Spectroelectrochemical experiments show that the electric field at the interface is reduced when metal cations are not added, indicating that the decreased field stabilization of intermediates could play an important role in increased overpotential required for the CO2RR to occur.

The electrochemical reduction reaction of carbon dioxide (CO2RR) has garnered significant attention due to its potential for the formation of carbon monoxide, which has industrial relevance. Herein, an oxide‐derived Cu–Zn electrocatalyst with an optimized Cu x O layer that shows high selectivity toward CO with a faradic efficiency of 75% at a low overpotential (−0.8 V vs reversible hydrogen electrode) is reported. Various structural characterizations and activity tests are conducted to understand the origin of this improvement depending on the Cu x O amount. Electrochemical surface area and electrochemical impedance spectroscopy measurements suggest that the addition of Cu x O increases double‐layer capacitance and decreases charge transfer resistance. Scanning electron microscopy images indicate that the electrodes undergo a severe reconstruction process, which is further confirmed by X‐ray diffraction that shows the formation of CuZn4 alloy during the reduction reaction. Furthermore, X‐ray photoelectron spectroscopy depth profile analysis shows that after CO2RR at −0.8 V, the Cu/Zn ratio is higher than that after −1.2 V, which suggests that applied potential plays a significant role in the reconstruction process and hence the difference in selectivity. The presence of copper in the surface layer has a significant impact on the improvement of selectivity toward CO.

Since photocatalytic and electrocatalytic technologies are crucial for tackling the energy and environmental challenges, significant efforts have been put into exploring advanced catalysts. Among them, perovskite type ABO3 oxides show great promising catalytic activities because of their flexible physical and chemical properties. In this review, the fundamentals and recent progress in the synthesis of perovskite type ABO3 oxides are considered. We describe the mechanisms for electrocatalytic oxygen evolution reactions (OER), oxygen reduction reactions (ORR), hydrogen evolution reactions (HER), nitrogen reduction reactions (NRR), carbon dioxide reduction reactions (CO2RR), and metal-air batteries in details. Furthermore, the photocatalytic water splitting, CO2 conversion, pollutant degradation, and nitrogen fixation are reviewed as well. We also stress the applications of perovskite type ABO3 oxides in solid oxide fuel cells (SOFs). Finally, the optimization of perovskite type ABO3 oxides for applications in various fields and an outlook on the current and future challenges are depicted. The aim of this review is to present a broad overview of the recent advancements in the development of perovskite type ABO3 oxides-based catalysts and their applications in energy conversion and environmental remediation, as well as to present a roadmap for future development in these hot research areas.

Optimizing the electrochemical carbon dioxide reduction reaction (CRR) to fuels is one of the most significant challenges in materials science and chemistry. Recently, single metal atom catalysts based on 2D materials have shown promise to improve the electroreduction performance of pristine 2D materials in the CRR. The physical origins of such performance enhancements are still poorly understood. Herein, we report the potential of a single Cu atom doped phosphorene catalyst for CO2 electroreduction based on density functional theory (DFT) calculations. The doping sites (hollow, bridge, and on-top) of Cu on phosphorene are investigated first. Phosphorene with a Cu atom anchored on the hollow site is chosen for further study. The pathways for different CRR products, including HCOOH, CO, CH3OH, and CH4, are examined via constructing free energy diagrams and via comparing the limiting potentials. CH4 is the most likely product after analysis of the adsorption energies and free energy pathways. Cu-Doped phosphorene in general shows improved CRR performance with lower limiting potential values. Cu doping leads to a decrease in the band gap value (about 0.2 eV), which is likely to be the physical origin of the CRR performance enhancement. Our study provides a novel promising CRR candidate catalyst based on phosphorene.