Electrochemical reduction of CO2 on graphene supported transition metals - towards single atom catalysts

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.

NP monolayer supported transition-metal single atoms for electrochemical water splitting: a theoretical study

The development of cost-effective and highly efficient electrocatalysts for water splitting is highly desirable but remains an ongoing challenge. Numerous single-atom catalysts (SACs) have achieved satisfactory performances in this area; however, non-carbon metal-free substrates have been rarely explored. Herein, we report a series of single-metal atoms supported on a novel two-dimensional NP monolayer as promising electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) by theoretical calculations. Our results disclose that Ti@NP, V@NP and Ir@NP exhibit desirable catalytic activity for the HER with extremely low of -0.004, -0.051, and 0.017 eV, respectively. More importantly, the calculated activation barriers for the Tafel reactions of these SACs are much lower than those for the benchmark Pt catalysts. In addition, Pt@NP shows the lowest η^OER of 0.495 V, followed by Rh@NP (η^OER = 0.548 V), which are even superior to that of state-of-the-art IrO₂. This work highlights the potential application of metal-free supports in SACs, which also further enriches the application of a NP monolayer in other related electrochemical processes.

Catalytic potential of post-transition metal doped graphene-based single-atom catalysts for the CO2 electroreduction reaction

Catalysts are required to ensure electrochemical reduction of CO₂ to fuels proceeds at industrially acceptable rates and yields. As such, highly active and selective catalysts must be developed. Herein, a density functional theory study of p‐block element and noble metal doped graphene‐based single‐atom catalysts in two defect sites for the electrochemical reduction of CO₂ to CO and HCOOH is systematically undertaken. It is found that on all of the systems considered, the thermodynamic product is HCOOH. Pb/C₃, Pb/N₄ and Sn/C₃ are identified as having the lowest overpotential for HCOOH production while Al/C₃, Al/N₄, Au/C₃ and Ga/C₃ are identified as having the potential to form higher order products due to the strength of binding of adsorbed HCOOH.

Electrochemical reduction of NO catalyzed by boron-doped C60 fullerene: a first-principles study

The electrochemical reduction of nitrogen monoxide (NO) is one of the most promising approaches for converting this harmful gas into useful chemicals. Using density functional theory calculations, the work examines the potential of a single B atom doped C₆₀ fullerene (C₅₉B) for catalytic reduction of NO molecules. The results demonstrate that the NO may be strongly activated over the B atom of C₅₉B, and that the subsequent reduction process can result in the formation of NH₃ and N₂O molecules at low and high coverages, respectively. Based on the Gibbs free energy diagram, it is inferred that the C₅₉B has excellent catalytic activity for NO reduction at ambient conditions with no potential-limiting. At normal temperature, the efficient interaction between the *NOH and NO species might lead to the spontaneous formation of the N₂O molecule. Thus, the findings of this study provide new insights into NO electrochemical reduction on heteroatom doped fullerenes, as well as a unique strategy for fabricating low-cost NO reduction electrocatalysts with high efficiency.

Using DFT calculations, the potential of B-doped C₆₀ fullerene is evaluated for electrochemical reduction of nitrogen monoxide. The B-doped C₆₀ exhibits exceptional catalytic activity and high selectivity for reduction of nitrogen monoxide.

Theoretical insights into CO2 reduction reaction on a CuPc/graphene single-atomic catalyst

A theoretical insight into the CO2 reduction reaction on a CuPc/graphene single atomic catalyst.

Electrochemical Reduction of CO2: A Review of Cobalt Based Catalysts for Carbon Dioxide Conversion to Fuels

Electrochemical CO2 reduction reaction (CO2RR) provides a promising approach to curbing harmful emissions contributing to global warming. However, several challenges hinder the commercialization of this technology, including high overpotentials, electrode instability, and low Faradic efficiencies of desirable products. Several materials have been developed to overcome these challenges. This mini-review discusses the recent performance of various cobalt (Co) electrocatalysts, including Co-single atom, Co-multi metals, Co-complexes, Co-based metal-organic frameworks (MOFs), Co-based covalent organic frameworks (COFs), Co-nitrides, and Co-oxides. These materials are reviewed with respect to their stability of facilitating CO2 conversion to valuable products, and a summary of the current literature is highlighted, along with future perspectives for the development of efficient CO2RR.

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

The global warming and the dangerous climate change arising from the massive emission of CO2 from the burning of fossil fuels have motivated the search for alternative clean and sustainable energy sources. However, the industrial development and population necessities make the decoupling of economic growth from fossil fuels unimaginable and, consequently, the capture and conversion of CO2 to fuels seems to be, nowadays, one of the most promising and attractive solutions in a world with high energy demand. In this respect, the electrochemical CO2 conversion using renewable electricity provides a promising solution. However, faradaic efficiency of common electro-catalysts is low, and therefore, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is critical. Carbon-based materials present some advantages such as relatively low cost and renewability, excellent electrical conductivity, and tunable textural and chemical surface, which show them as competitive materials for the electro-reduction of CO2. In this review, an overview of the recent progress of carbon-based electro-catalysts in the conversion of CO2 to valuable products is presented, focusing on the role of the different carbon properties, which provides a useful understanding for the materials design progress in this field. Development opportunities and challenges in the field are also summarized.

Electrochemical Reduction of Carbon Dioxide on Graphene-Based Catalysts

The current environmental situation requires taking actions regarding processes for energy production, thus promoting renewable energies, which must be complemented with the development of routes to reduce pollution, such as the capture and storage of CO₂. Graphene materials have been chosen for their unique properties to be used either as electrocatalyst or as catalyst support (mainly for non-noble metals) that develop adequate efficiencies for this reaction. This review focuses on comparing experimental and theoretical results of the electrochemical reduction reaction of carbon dioxide (ECO₂RR) described in the scientific literature to establish a correlation between them. This work aims to establish the state of the art on the electrochemical reduction of carbon dioxide on graphene-based catalysts.

Regulating the electronic properties of MoSe2 to improve its CO2 electrocatalytic reduction performance via atomic doping

The atomic environment should heavily influence the performance of CO₂ reduction, and the regulated electronic property of reaction intermediates and metals (Cu) is responsible for the high catalytic performance of CH₄ production.

Identifying the Types and Characterization of the Active Sites on M-X-C Single-Atom Catalysts

Single atom catalysts (SACs) have attracted much attention in recent years. As an essential group in SACs, M-X-C (X=nonmetallic element) materials have been demonstrated to be efficient in many reactions. However, identifying the active sites on M-X-C, especially under working conditions, is still challenging, which is crucial for chemists to further understand the mechanism underlying the reaction and better design proper SACs for specific reactions. Herein, the types and characterization of M-X-C are comprehensively summarized and discussed in this review. In addition to the basic information above, the challenges and opportunities remaining in this field will be also proposed to present a perspective to the research on the next step.

Electronic Metal-Support Interaction of Single-Atom Catalysts and Applications in Electrocatalysis

The electronic metal-support interaction (EMSI), which acts as a bridge between theoretical electronic study and the design of heterogenous catalysts, has attracted much attention. Utilizing the interaction between the metal and the support is one of the most essential strategies to enhance electrocatalytic efficiency due to structural and synergetic promotion. To date, as the ideal model for realizing EMSI, many types of single-atom catalysts (SACs) have been developed. The understanding of the electronic interaction on SACs has also been pushed to a higher level. However, systematic theories and operando experiments are seldom reported, and will be necessary to put forward and be carried out, respectively. Herein, the types, characterization, mechanism, and electrocatalytic applications of EMSI are comprehensively summarized and discussed. In addition to the basic information above, the challenges, opportunities, and future development of the EMSI on SACs are also proposed to present an overall view and reference to the later research.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Three-Dimensional Cathodes for Electrochemical Reduction of CO2: From Macro- to Nano-Engineering

Rising anthropogenic CO2 emissions and their climate warming effects have triggered a global response in research and development to reduce the emissions of this harmful greenhouse gas. The use of CO2 as a feedstock for the production of value-added fuels and chemicals is a promising pathway for development of renewable energy storage and reduction of carbon emissions. Electrochemical CO2 conversion offers a promising route for value-added products. Considerable challenges still remain, limiting this technology for industrial deployment. This work reviews the latest developments in experimental and modeling studies of three-dimensional cathodes towards high-performance electrochemical reduction of CO2. The fabrication-microstructure-performance relationships of electrodes are examined from the macro- to nanoscale. Furthermore, future challenges, perspectives and recommendations for high-performance cathodes are also presented.

Pore size effect of graphyne supports on CO2 electrocatalytic activity of Cu single atoms

Steric effects of graphyne supports on the intermediates and coordination number of metal atoms determine the CO₂ electrocatalytic activity of SACs.

Theoretical insights into single-atom catalysts

Schematic diagram of theoretical models and applications of single atom catalysts. A review on the theoretical models, intrinsic properties, and the related application of SACs.

Electrochemical CO2 Reduction to C1 Products on Single Nickel/Cobalt/Iron-Doped Graphitic Carbon Nitride: A DFT Study

Electrocatalytic CO₂ reduction reaction (CRR) is one of the most promising strategies to convert greenhouse gases to energy sources. Herein, the CRR was applied towards making C₁ products (CO, HCOOH, CH₃ OH, and CH₄ ) on g-C₃ N₄ frameworks with single Ni, Co, and Fe introduction; this process was investigated by density functional theory. The structures of the electrocatalysts, CO₂ adsorption configurations, and CO₂ reduction mechanisms were systematically studied. Results showed that the single Ni, Co, and Fe located from the corner of the g-C₃ N₄ cavity to the center. Analyses of the adsorption configurations and electronic structures suggested that CO₂ could be chemically adsorbed on Co-C₃ N₄ and Fe-C₃ N₄ , but physically adsorbed on Ni-C₃ N₄ . The H₂ evolution reaction (HER), as a suppression of CRR, was investigated, and results showed that Ni-C₃ N₄ , Co-C₃ N₄ , and Fe-C₃ N₄ exhibited more CRR selectivity than HER. CRR proceeded via COOH and OCHO as initial protonation intermediates on Ni-C₃ N₄ and Co/Fe-C₃ N₄ , respectively, which resulted in different C₁ products along quite different reaction pathways. Compared with Ni-C₃ N₄ and Fe-C₃ N₄ , Co-C₃ N₄ had more favorable CRR activity and selectivity for CH₃ OH production with unique rate-limiting steps and lower limiting potential.

Interface Properties of the Partially Oxidized Pt(111) Surface Using Hybrid DFT-Solvation Models

This article reports a theoretical-computational effort to model the interface between an oxidized platinum surface and aqueous electrolyte. It strives to account for the impact of the electrode potential, formation of surface-bound oxygen species, orientational ordering of near-surface solvent molecules, and metal surface charging on the potential profile along the normal direction. The computational scheme is based on the DFT/ESM-RISM method to simulate the charged Pt(111) surface with varying number of oxygen adatoms in acidic solution. This hybrid solvation method is known to qualitatively reproduce bulk metal properties like the work function. However, the presented calculations reveal that vital interface properties such as the electrostatic potential at the outer Helmholtz plane are highly sensitive to the position of the metal surface slab relative to the DFT-RISM boundary region. Shifting the relative position of the slab also affects the free energy of the system. It follows that there is an optimal distance for the first solvent layer within the ESM-RISM framework, which could be found by optimizing the position of the frozen Pt(111) slab. As it stands, manual sampling of the position of the slab is impractical and betrays the self-consistency of the method. Based on this understanding, we propose the implementation of a free energy optimization scheme of the relative position of the slab in the DFT-RISM boundary region. This optimization scheme could considerably increase the applicability of the hybrid method.

Size, Composition, and Support-Doping Effects on Oxygen Reduction Activity of Platinum-Alloy and on Non-platinum Metal-Decorated-Graphene Nanocatalysts

Recent investigations reported in the open literature concerning the functionalization of graphene as a support material for transition metal nanoparticle catalysts have examined isolated systems for their potential Oxygen Reduction Reaction (ORR) activity. In this work we present results which characterize the ability to use functionalized graphene (via dopants B, N) to upshift and downshift the adsorption energy of mono-atomic oxygen, O* (the ORR activity descriptor on ORR Volcano Plots), for various compositions of 4-atom, 7-atom, and 19-atom sub-nanometer binary alloy/intermetallic transition metal nanoparticle catalysts on graphene (TMNP-MDG). Our results show several important and interesting features: (1) that the combination of geometric and electronic effects makes development of simple linear mixing rules for size/composition difficult; (2) that the transition from 4- to 7- to 19-atom TMNP on MDG has pronounced effects on ORR activity for all compositions; (3) that the use of B and N as dopants to modulate the graphene-TMNP electronic structure interaction can cause shifts in the oxygen adsorption energy of 0.5 eV or more; (4) that it might be possible to make specific doped-graphene-Ni_xCu_y TMNP systems which fall close to the Volcano Peak for ORR. Our results point to systems which should be investigated experimentally and may improve the viability of future fuel cell or other ORR applications, and provide new paths for future investigations of more detail for TMNP-MDG screening.

Synthesis and Evaluation of Copper-Supported Titanium Oxide Nanotubes as Electrocatalyst for the Electrochemical Reduction of Carbon Oxide to Organics

Carbon dioxide (CO2) is considered as the prime reason for the global warming effect and one of the useful ways to transform it into an array of valuable products is through electrochemical reduction of CO2 (ERC). This process requires an efficient electrocatalyst with high faradaic efficiency at low overpotential and enhanced reaction rate. Herein, we report an innovative way of reducing CO2 using copper-metal supported on titanium oxide nanotubes (TNT) electrocatalysts. The TNT support material was synthesized using alkaline hydrothermal process with Degussa (P-25) as a starting material. Copper nanoparticles were anchored on the TNT by homogeneous deposition-precipitation method (HDP) with urea as precipitating agent. The prepared catalysts were tested in a home-made H-cell with 0.5 M NaHCO3 aqueous solution in order to examine their activity for ERC and the optimum copper loading. Continuous gas-phase ERC was carried out in a solid polymer electrolyte (SPE) reactor. The 10% Cu/TNT catalysts were employed in the gas diffusion layer (GDL) on the cathode side with Pt-Ru/C on the anode side. Faradaic efficiencies for the three major products namely methanol, methane, and CO were found to be 4%, 3%, and 10%, respectively at −2.5 V with an overall current density of 120 mA/cm2. The addition of TNT significantly increased the catalytic activity of electrocatalyst for ERC. It is mainly attributed to their better stability towards oxidation, increased CO2 adsorption capacity and stabilization of the reaction intermediate, layered titanates, and larger surface area (400 m2/g) as compared with other support materials. Considering the low cost of TNT, it is anticipated that TNT support electrocatalyst for ECR will gain popularity.

Recent Progress in Graphene-Based Noble-Metal Nanocomposites for Electrocatalytic Applications

The fast industrialization process has led to global challenges in the energy crisis and environmental pollution, which might be solved with clean and renewable energy. Highly efficient electrochemical systems for clean-energy collection require high-performance electrocatalysts, including Au, Pt, Pd, Ru, etc. Graphene, a single-layer 2D carbon nanosheet, possesses many intriguing properties, and has attracted tremendous research attention. Specifically, graphene and graphene derivatives have been utilized as templates for the synthesis of various noble-metal nanocomposites, showing excellent performance in electrocatalytic-energy-conversion applications, such as the hydrogen evolution reaction and CO₂ reduction. Herein, the recent progress in graphene-based noble-metal nanocomposites is summarized, focusing on their synthetic methods and electrocatalytic applications. Furthermore, some personal insights on the challenges and possible future work in this research field are proposed.

Emerging Carbon-Based Heterogeneous Catalysts for Electrochemical Reduction of Carbon Dioxide into Value-Added Chemicals

The electrocatalytic reduction of CO₂ provides a sustainable way to mitigate CO₂ emissions, as well as store intermittent electrical energy into chemicals. However, its slow kinetics and the lack of ability to control the products of the reaction inhibit its industrial applications. In addition, the immature mechanistic understanding of the reduction process makes it difficult to develop a selective, scalable, and stable electrocatalyst. Carbon-based materials are widely considered as a stable and abundant alternative to metals for catalyzing some of the key electrochemical reactions, including the CO₂ reduction reaction. In this context, recent research advances in the development of heterogeneous nanostructured carbon-based catalysts for electrochemical reduction of CO₂ are summarized. The leading factors for consideration in carbon-based catalyst research are discussed by analyzing the main challenges faced by electrochemical reduction of CO₂ . Then the emerging metal-free doped carbon and aromatic N-heterocycle catalysts for electrochemical reduction of CO₂ with an emphasis on the formation of multicarbon hydrocarbons and oxygenates are discussed. Following that, the recent progress in metal-nitrogen-carbon structures as an extension of carbon-based catalysts is scrutinized. Finally, an outlook for the future development of catalysts as well as the whole electrochemical system for CO₂ reduction is provided.

Surface strategies for catalytic CO2 reduction: from two-dimensional materials to nanoclusters to single atoms

This work constructively reviewed and predicted the surface strategies for catalytic CO₂ reduction with 2D material, nanocluster and single-atom catalysts

C2N-graphene supported single-atom catalysts for CO2 electrochemical reduction reaction: mechanistic insight and catalyst screening

Single-atom catalysts (SACs) have emerged as an excellent platform for enhancing catalytic performance. Inspired by the recent experimental synthesis of nitrogenated holey 2D graphene (C2N-h2D) (Mahmood et al., Nat. Commun., 2015, 6, 6486-6493), we report density functional theory calculations combined with computational hydrogen electrode model to show that C2N-h2D supported metal single atoms (M@C2N) are promising electrocatalysts for CO2 reduction reaction (CO2 RR). M confined at pyridinic N6 cavity promotes activation of inert O[double bond, length as m-dash]C[double bond, length as m-dash]O bonds and subsequent protonation steps, with *COOH → *CO → CHO predicted to be the primary pathway for producing methanol and methane. It is found that *CO + H+ + e- → *CHO is most likely to be the potential determining step; breaking the scaling relation of *CO and *CHO binding on M@C2N SACs may simply be a rare event that is sensitively controlled by the detailed geometry of the adsorbate. Among twelve metals screened, M@C2N SACs where M = Ti, Mn, Fe, Co, Ni, Ru were identified to be effective in catalyzing CO2 RR with lowered overpotentials (0.58 V-0.80 V).

Single-Atom Catalysts of Precious Metals for Electrochemical Reactions

Single-atom catalysts (SACs), in which metal atoms are dispersed on the support without forming nanoparticles, have been used for various heterogeneous reactions and most recently for electrochemical reactions. In this Minireview, recent examples of single-atom electrocatalysts used for the oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), formic acid oxidation reaction (FAOR), and methanol oxidation reaction (MOR) are introduced. Many density functional theory (DFT) simulations have predicted that SACs may be effective for CO₂ reduction to methane or methanol production while suppressing H₂ evolution, and those cases are introduced here as well. Single atoms, mainly Pt single atoms, have been deposited on TiN or TiC nanoparticles, defective graphene nanosheets, N-doped covalent triazine frameworks, graphitic carbon nitride, S-doped zeolite-templated carbon, and Sb-doped SnO₂ surfaces. Scanning transmission electron microscopy, extended X-ray absorption fine structure measurement, and in situ infrared spectroscopy have been used to detect the single-atom structure and confirm the absence of nanoparticles. SACs have shown high mass activity, minimizing the use of precious metal, and unique selectivity distinct from nanoparticle catalysts owing to the absence of ensemble sites. Additional features that SACs should possess for effective electrochemical applications were also suggested.

Preparation of a Cu(BTC)-rGO catalyst loaded on a Pt deposited Cu foam cathode to reduce CO2 in a photoelectrochemical cell

To increase the reaction productivity and selectivity of the CO₂ photoelectrochemical reduction reaction, a Cu (benzene 1,3,5-tricarboxylic acid [BTC])-reduced graphite oxide (rGO) catalyst was prepared by using a facile hydrothermal method and used in a CO₂ photoelectrochemical cell (PEC) as a cathode catalyst. Characterization of the catalyst proved that successfully bonding of rGO to Cu(BTC) not only facilitated faster transfer of electrons on the surface of the catalyst but also created more active sites. CO₂ photoelectrochemical reduction experimental results showed that the total carbon atom conversion rate was up to 3256 nmol h⁻¹ cm⁻² which was much higher than when pure Cu(BTC) was used as a cathode catalyst. The liquid product's selectivity to alcohols was up to 95% when −2 V voltage was applied to the system with Cu(BTC)-rGO used as the cathode catalyst.

Schematic of a photoelectrochemical cell for CO₂ reduction: the H⁺ generation process and the CO₂ process run in two separated chambers respectively.

CO2electroreduction performance of a single transition metal atom supported on porphyrin-like graphene: a computational study

Single transition metal atoms supported by porpyrin-like graphene exhibit high catalytic activity for the electroreduction of CO₂.