Article

Investigating the Nature of the Active Sites for the CO 2 Reduction Reaction on Carbon-Based Electrocatalysts

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Abstract

To achieve a better understanding of the CO2 reduction reaction on carbon-based electrocatalysts, we synthesized a library of nitrogen-doped carbonaceous materials with atomically dispersed 3d transition metals and corresponding metal-free electrocatalysts. The sacrificial support method was used yielding catalyst materials of high dispersity and high graphitic content. The resulting electrocatalysts were impurity free, hence allowing a better understanding of the mechanism of CO2 reduction. By combining the electrochemical results with density functional theory, we were able to separate the electrocatalysts into several categories, based on their CO2 → COOHads free energy and their COads binding strength. The ‘strong-CO binder’ electrocatalysts (e.g. Cr, Mn and Fe-N-C) achieved a Faradaic efficiency up to 50% at – 0.35 V vs. RHE (at pH = 7.5, in 0.1 M phosphate buffer). Such Faradaic efficiency was also achieved for a metal-free electrocatalyst, therefore showing the high activity of the metal-free, N-containing, moieties toward the CO2 re-duction reaction. This was confirmed by near ambient pressure X-ray photoelectron spectrosco-py that confirmed pyridinic and hydrogenated (pyrrolic) nitrogen moieties act as preferential ad-sorption sites for the CO2 on the Fe-N-C catalyst surface.

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... Atomically dispersed active sites enable control over reaction paths by varying the coordination environment and transition metal species, among other considerations [42]. Stable and active M-N-C reaction sites have been identified through experimental efforts and DFT, including M-N-C 2 single atom sites, [43] bimetallic dimer N-C 2 sites, [44] and M-N 4 -C 10 type structures [45]. Fe-N 4 -C 10 has been established to be a durable active site at the high positive potentials necessary for ORR, [33] and Cu-N-C catalysts have been reported for CO 2 RR and undergo no active site reconstruction up to -0.6 V applied potential versus the reversible hydrogen electrode [46]. ...
... Density functional theory (DFT) computations have been proven to be an effective tool for explaining and predicting the experimental performance of M-N-C catalysts towards the oxygen reduction and evolution reactions [33,41,47,51] as well as for CO 2 RR [45,52,53]. Binding energy of intermediates correlates well with experimental overpotential for these catalysts, [53] thus DFT computations that explore a large number of sites are a valuable approach for identifying target active sites for synthesis before time-consuming experimental work is performed. ...
... While there are many modeling studies on M-N-C catalysts, [32,36,37,42,43,[54][55][56], and data-driven approaches have been applied to other classes of single atom catalysts, [57,58] our present study aims to examine more closely M-N 4 C 10 active sites for CO 2 RR. These have been established to be active and durable not only for fuel cell applications [33] but also for CO 2 RR [45]. By providing free energy pathways for 26 different M-N 4 C 10 sites (with varied M-speciation) calculated in a systematic, reproducible manner, we not only provide a feature-based analysis in the present work, but also enable future data-driven efforts by providing our full set of free energies as well as optimized structures for each surface adsorbed intermediate. ...
... Research on electrochemical CO 2 R covers a huge range of development aspects: catalyst development, [2][3][4][5] electrode structure optimization, [6,7] and cell and process design. [8,9] Electrochemical analysis methods are widely used for characterization and performance evaluation at all three aspects. ...
... Besides the electrochemical reaction and mass transport processes, the pH-dependent carbonate equilibrium in the electrolyte plays an important role [Eq. (3)- (5)]. ...
... The pH value is a measure of hydrogen and CO 2 production, as OH À is produced in both reactions. In addition, OH À is consumed by carbonation in the electrolyte according to Equation (3)- (5). Therefore, most processes strongly change the pH near and at the electrode. ...
Article
Electrochemical CO 2 reduction is crucial for mitigating emissions by converting them into valuable chemicals. Stationary methods suffer from drawbacks like gas bubble distortion and long measurement times. However, dynamic cyclovoltammetry in rotating disc electrode setups is employed to infer performance. This study uncovers limitations when applying this approach to CO 2 reduction in aqueous electrolyte. Here, we present a model‐based analysis considering electrochemical reactions, species and charge transport, and chemical carbonation. Experimental and simulated potential cycles demonstrate scan rate dependence, significantly deviating from stationary curves at low rotation rates (50 rpm). Such low rotation rates mimic real diffusion layer thicknesses in practical cell systems, thus a transport impact can be expected also on cell level. This behavior arises from slow transport and carbonation, causing time‐dependent CO 2 depletion and electrolyte buffering. Dynamic investigation reveals strong species transport effects. Furthermore, dynamic operation enhances Faradaic efficiency due to a shift in the carbonate reaction system, favoring electrochemical CO 2 consumption over chemical CO 2 consumption. By clarifying dynamic vs. stationary operation, this research contributes to understanding electrochemical CO 2 reduction processes, how to determine transport limitations via dynamic measurements, and provides guidelines for more accurate performance assessment.
... Among non-precious metal catalysts explored, atomically dispersed, transition metal-nitrogen-carbon (MÀ NÀ C) catalysts have proven to be highly selective for CO 2 reduction. [10,11] Metalfree NÀ C catalysts also show substantial selectivity for CO 2 reduction with lower activity compared to their metal counterparts. [12] While Fe-containing MÀ NÀ C catalysts are known to catalyze the reduction of CO 2 to CO at low overpotential, Mo-containing MÀ NÀ C catalysts have active MoÀ N x centers which may catalyze the competing HER reaction. ...
... [12] While Fe-containing MÀ NÀ C catalysts are known to catalyze the reduction of CO 2 to CO at low overpotential, Mo-containing MÀ NÀ C catalysts have active MoÀ N x centers which may catalyze the competing HER reaction. [10][11][12][13][14] The active sites of MÀ NÀ C catalysts are diverse and complex, containing both nitrogen coordinated metal sites (M-N x ), and a variety of metal-free (NÀ C) sites whose participation in the CO 2 RR vs. HER is highly dependent on the specific N-moiety and the surrounding chemical environment. [10,11,13] FeÀ NÀ C catalysts are highly selective for CO formation at low overpotential (E > À 0.50 V RHE ), while the NiÀ NÀ C catalyst boasts the lowest Tafel slope for CO 2 reduction to CO and generally the highest overall CO production rates at higher overpotential (E < À 0.75 V RHE ). ...
... [10][11][12][13][14] The active sites of MÀ NÀ C catalysts are diverse and complex, containing both nitrogen coordinated metal sites (M-N x ), and a variety of metal-free (NÀ C) sites whose participation in the CO 2 RR vs. HER is highly dependent on the specific N-moiety and the surrounding chemical environment. [10,11,13] FeÀ NÀ C catalysts are highly selective for CO formation at low overpotential (E > À 0.50 V RHE ), while the NiÀ NÀ C catalyst boasts the lowest Tafel slope for CO 2 reduction to CO and generally the highest overall CO production rates at higher overpotential (E < À 0.75 V RHE ). [15] Generally, mono-metallic MÀ NÀ C catalysts which have substantial selectivity to CO (e. g., MÀ NÀ C catalysts where M=Fe, Ni, Mn, Cr, and to a lesser extent Co) have a potential-dependent faradaic efficiency for CO formation. ...
Article
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The production of syngas by traditional processes such as steam methane reforming is energetically intensive and produces a large amount of CO2 emissions. In contrast, the electrochemical CO2 reduction reaction (CO2RR) enables the carbon neutral production of syngas at ambient conditions. Among non‐precious metal catalysts, metal‐nitrogen‐carbon electrocatalysts are inexpensive and highly selective towards syngas production. This study examined the selectivity of mono‐ and bi‐metallic (M−N−C, M=Fe, Mo or FeMo) electrocatalysts towards syngas production. The ratio of the CO : H2 in the syngas was tuned by modifying the ratio of the metallic precursors in the bi‐metallic FeMo−N−C catalysts, tailoring the catalysts’ selectivity towards the CO2RR or the hydrogen evolution reaction (HER). The catalyst synthesis temperature(s) were considered as they influence the catalyst morphology and activity. Further, the dependence of the ratio of CO : H2 in the syngas as a function of the potential was explored for the different bi‐metallic catalysts. This work showed that by tailoring both the ratio of Fe : Mo in the bi‐metallic catalyst and optimizing the reductive potential, a CO : H2 ratio between 0.25 to 5 was achievable. This study demonstrated a novel approach in which the ratio of the product syngas composition could be tailored in a single reaction, without the need for further downstream processing to reach a desired composition. Fine tuning: Different syngas compositions (CO : H2) between 0.25 and 5.0 are achievable by varying the ratio of atomically dispersed Fe−Nx to Mo−Nx sites. The effect is more pronounced at more negative cathodic potential primarily through changes in the CO partial current density (jco).
... 27,28 N−C materials are promising, cost-effective alternatives to traditional catalysts in several electrochemical processes, such as in the oxygen reduction, 29 hydrogen evolution, 30 and CO 2 R reactions. 31,32 In CO 2 R using metalfree N−C catalysts, mainly CO 28,33,34 and formic acid were produced, but in some cases the formation of alcohols and hydrocarbons has also been reported. 35,36 In TC CO 2 conversion reactions, however, N-doped carbons alone are rarely used as catalysts. ...
... Identifying the active centers of (M)−N−C catalysts and getting insights into the reaction mechanism are challenging, as the chemical structure of these materials is not well-defined and multiple active centers might be present in one material. 31,41,42 Moreover, several factors other than the chemical identity of the active sites (e.g., morphology and local environment of active center N-defects as preferential adsorption sites) can play a role in defining the catalytic performance. 31,32,43,44 Because of these factors, there is still no consensus about the nature of active sites and the role of surface functional groups of the N−C materials in the CO 2 R reaction. ...
... 31,41,42 Moreover, several factors other than the chemical identity of the active sites (e.g., morphology and local environment of active center N-defects as preferential adsorption sites) can play a role in defining the catalytic performance. 31,32,43,44 Because of these factors, there is still no consensus about the nature of active sites and the role of surface functional groups of the N−C materials in the CO 2 R reaction. Many works have suggested the importance of pyridinic N, 28,45−47 but pyrrolic N, 48,49 the partially positive C atoms next to pyridinic N, 50 and intrinsic carbon defects 51 (achieved by heteroatom removal) were also suspected. ...
Article
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N-doped carbon (N-C) materials are increasingly popular in different electrochemical and catalytic applications. Due to the structural and stoichiometric diversity of these materials, however, the role of different functional moieties is still controversial. We have synthesized a set of N-C catalysts, with identical morphologies (∼27 nm pore size). By systematically changing the precursors, we have varied the amount and chemical nature of N-functions on the catalyst surface. The CO2 reduction (CO2R) properties of these catalysts were tested in both electrochemical (EC) and thermal catalytic (TC) experiments (i.e., CO2 + H2 reaction). CO was the major CO2R product in all cases, while CH4 appeared as a minor product. Importantly, the CO2R activity changed with the chemical composition, and the activity trend was similar in the EC and TC scenarios. The activity was correlated with the amount of different N-functions, and a correlation was found for the -NO x species. Interestingly, the amount of this species decreased radically during EC CO2R, which was coupled with the performance decrease. The observations were rationalized by the adsorption/desorption properties of the samples, while theoretical insights indicated a similarity between the EC and TC paths.
... (i) CO; (ii) formic acid (if the CO2 adsorb through one of its oxygen atoms instead of the carbon atoms [149]) and, on Cu-based electrocatalysts, (iii) C2+ (including C2H4, ethanol, etc.), although with a low selectivity for a given product. Metal-N-Cs mainly belong to the first category, due to (i) the initial adsorption of CO2 through its carbon atom and (ii) the fact that the adsorption of two adsorbed CO (COads-COads) on a single Metal-N4 moiety is highly unfavourable (∆GCOads → COads-COads = ca 0.7 eV [150]), thus greatly limiting the Metal-N-Cs capability to form a C-C bond and thus generate C2+ species. Despite the simple nature of its main product, i.e. ...
... First, the role of Metal-free N-moieties to the electrocatalytic activity of Metal-N-C electrocatalysts is non-negligible: (i) an entire sub-field of the CO2RR is exclusively dedicated to the study of nitrogen-doped carbon [154][155][156], which often exhibit great faradaic efficiency at low overpotentials (e.g. FECO = 80 % on nitrogen-doped carbon nanotubes or graphene); (ii) it was also observed that, on Fe-N-Cs, the activity was increasing with the N-pyridinic abundancy and that those moieties acted as preferential CO2 chemisorption sites [150]. On the other hand, the presence of metallic NP/nanocluster leads to a decrease of the faradaic efficiency toward CO, and catalyse the HER, as recently shown by Huan et al. [100]. ...
Chapter
While supported metal nanoparticles cannot achieve full electrochemical utilization of metal atoms, catalysts featuring single‐metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting‐edge characterization techniques that can target single‐metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure–catalytic activity–selectivity–stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal–nitrogen–carbon single‐metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems.
... (i) CO; (ii) formic acid (if the CO2 adsorb through one of its oxygen atoms instead of the carbon atoms [149]) and, on Cu-based electrocatalysts, (iii) C2+ (including C2H4, ethanol, etc.), although with a low selectivity for a given product. Metal-N-Cs mainly belong to the first category, due to (i) the initial adsorption of CO2 through its carbon atom and (ii) the fact that the adsorption of two adsorbed CO (COads-COads) on a single Metal-N4 moiety is highly unfavourable (∆GCOads → COads-COads = ca 0.7 eV [150]), thus greatly limiting the Metal-N-Cs capability to form a C-C bond and thus generate C2+ species. Despite the simple nature of its main product, i.e. ...
... First, the role of Metal-free N-moieties to the electrocatalytic activity of Metal-N-C electrocatalysts is non-negligible: (i) an entire sub-field of the CO2RR is exclusively dedicated to the study of nitrogen-doped carbon [154][155][156], which often exhibit great faradaic efficiency at low overpotentials (e.g. FECO = 80 % on nitrogen-doped carbon nanotubes or graphene); (ii) it was also observed that, on Fe-N-Cs, the activity was increasing with the N-pyridinic abundancy and that those moieties acted as preferential CO2 chemisorption sites [150]. On the other hand, the presence of metallic NP/nanocluster leads to a decrease of the faradaic efficiency toward CO, and catalyse the HER, as recently shown by Huan et al. [100]. ...
... There are excellent reviews on the mechanistic aspects and effectiveness of CO2R catalysts [2][3][4][5][6]. Artificial CO2R can be achieved by processes, such as electrochemical [7][8][9], photochemical [10][11][12], and photoelectrochemical (PEC), by which electrons are transferred to CO 2 molecules to increase their energy content [13,14]. Electrochemical CO2R is an electron transfer reaction between the cathode and the adsorbed CO 2 . ...
... High-CO-affinity electrocatalysts (i.e., Cr, Mn, and Fe-N-C) exhibited high carbon monoxide (CO) Faradaic efficiency. The pyridinic and hydrogenated (pyrrolic) nitrogen moieties of the carbonaceous support are active sites for CO 2 adsorption [9]. Accordingly, a relatively basic surface (such as the presence of γ − Al 2 O 3 overlayer and pyridinic modification) would have a positive effect on CO2R efficiency enhancement, likely through increasing the affinity of the catalyst surface toward CO 2 adsorption. ...
Article
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Managing the concentration of atmospheric CO2 requires a multifaceted engineering strategy, which remains a highly challenging task. Reducing atmospheric CO2 (CO2R) by converting it to value-added chemicals in a carbon neutral footprint manner must be the ultimate goal. The latest progress in CO2R through either abiotic (artificial catalysts) or biotic (natural enzymes) processes is reviewed herein. Abiotic CO2R can be conducted in the aqueous phase that usually leads to the formation of a mixture of CO, formic acid, and hydrogen. By contrast, a wide spectrum of hydrocarbon species is often observed by abiotic CO2R in the gaseous phase. On the other hand, biotic CO2R is often conducted in the aqueous phase and a wide spectrum of value-added chemicals are obtained. Key to the success of the abiotic process is understanding the surface chemistry of catalysts, which significantly governs the reactivity and selectivity of CO2R. However, in biotic CO2R, operation conditions and reactor design are crucial to reaching a neutral carbon footprint. Future research needs to look toward neutral or even negative carbon footprint CO2R processes. Having a deep insight into the scientific and technological aspect of both abiotic and biotic CO2R would advance in designing efficient catalysts and microalgae farming systems. Integrating the abiotic and biotic CO2R such as microbial fuel cells further diversifies the spectrum of CO2R.
... By employing electrical energy, these catalytic technologies can efficiently facilitate the conversion under mild conditions, promoting a circular carbon economy [20,21]. Nonetheless, traditional catalytic materials, like metals [22,23], metal oxides [24][25][26][27][28][29], semiconductors [30], and carbonbased materials [31][32][33][34][35][36], used for the electrocatalytic CO 2 reduction reaction (CO 2 RR), show several limitations, including lower surface area, limited tunability, and restricted overall catalytic performance [37,38]. Consequently, it is crucial to develop novel catalytic materials for the electrocatalytic CO 2 RR to enhance catalytic efficiency and performance [39][40][41]. ...
Article
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The conversion of carbon dioxide (CO 2 ) to the reduced chemical compounds offers substantial environmental benefits through minimizing the emission of greenhouse gas and fostering sustainable practices. Recently, the unique properties of metal-organic frameworks (MOFs) make them attractive candidates for electrocatalytic CO 2 reduction reaction (CO 2 RR), providing many opportunities to develop efficient, selective, and environmentally sustainable processes for mitigating CO 2 emissions and utilizing CO 2 as a valuable raw material for the synthesis of fuels and chemicals. Here, the recent advances in MOFs as efficient catalysts for electrocatalytic CO 2 RR are summarized. The detailed characteristics, electrocatalytic mechanisms, and practical approaches for improving the electrocatalytic efficiency, selectivity, and durability of MOFs under realistic reaction conditions are also clarified. Furthermore, the outlooks on the prospects of MOF-based electrocatalysts in CO 2 RR are provided.
... Electrocatalytic conversions of small molecules play essential roles in the storage and utilization of intermittent renewable energy. 1,2 The transition metal−nitrogen−carbon (M−N−C) type single-atom catalyst is an important family of non-noble metal-based electrocatalysts widely used in the CO 2 reduction reaction (CO 2 RR), 3,4 the O 2 reduction reaction (ORR), 5 and other electrocatalytic processes. 6 The most widely used methods to prepare M−N−C catalysts are the pyrolysis of metal organic precursors. ...
Article
Metal−nitrogen−carbon single-atom catalysts with metal sites at the edge of the carbon layers are widely used in electrocatalytic processes such as CO 2 reduction. For the single-atom catalysts prepared with pyrolysis methods, the local structure of the metal sites, including the type of the ligands and the coordination number, cannot be precisely controlled, making it difficult to investigate the relationship between the electrocatalytic properties and the local structure. In this work, Co−N x complexes with 2−4 pyridinic ligands are prepared and anchored at the edge of carbon black with conjugated pyrazine linkers as the model edge-site Co single-atom catalysts. Only the Co−N 4 catalyst shows high selectivity toward CO in CO 2 electroreduction, with the turnover frequency 1 order of magnitude higher than that of Co−N 2 and Co−N 3 catalysts. Compared with Co−N 2 and Co−N 3 sites, the Co−N 4 site shows stronger adsorption of COOH and CO species and weaker adsorption of H 2 O and the H atom due to the low-spin electron configuration, which rationalizes the high CO 2 reduction activity and low hydrogen evolution activity of the Co−N 4 catalyst.
... [34][35][36] The SSM has been extensively applied for atomically dispersed Fe-N-C catalysts for the ORR and CO2RR. 27,37 Here, extending beyond Fe-N-C to a variety of 3d-, 4d-, 5d-and f-block metals, a set of atomically dispersed M-N-C catalysts (M = Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, La, Ce, and W) were synthesized, as shown in Figure 1a. The metallic centers ideally have a first coordination shell of 4-nitrogen atoms and are coordinated in either an in-plane or out-of-plane configuration as shown in Figure 1b. ...
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Electrocatalytic reduction of waste nitrates (NO3-) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts with varying metal centers uniquely favor mono-nitrogen products (e.g., NH3), as well as provide synergistic supports for nanoparticle catalysts. But the reaction fundamentals remain largely underexplored. Herein, we report a set of 3d-, 4d-, 5d- and f-block atomically dispersed M-N-C catalysts with a well-defined M-Nx coordination. The selectivity and activity of NO3- reduction to NH3 in neutral media were thoroughly studied, with a specific focus on deciphering the role of the NO2- intermediate in the reaction cascade, wherein strong correlations (R=0.9) were found between the NO2- reduction activity and NO3- reduction selectivity for NH3. Moreover, theoretical computations identified the associative/dissociative adsorption pathways for NO2- evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for certain oxyphilic metals. The free energies for the reductive adsorption of nitrate [∗ + NO3− → ∗NO2 ], correlated strongly with experimental NH3 selectivity. This work provides a platform for designing multielement NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.
... In comparison with Ni-N-C SSCs, Fe-N-C SSCs present a low overpotential (about − 0.45 V vs. RHE) and still maintain a high FE CO (~ 90%), although they usually deliver relatively low response currents (< 10 mA cm −2 ) [40,[74][75][76]. Hence, the Fe-N-C SSCs are also promising catalysts for the CO 2 RR to CO [211,212], and the Fe-N 4 moieties are usually regarded as the main active sites [213]. As shown in Fig. 10, the operando XANES and EXAFS demonstrated the evolution of the Fe valence of Fe-N-C SSCs during the CO 2 RR. ...
Article
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The carbon dioxide electroreduction reaction (CO2RR) to fuels and/or chemicals is an efficient prospective strategy to realize global carbon management using intermittent electric energy harvested from renewable sources. Highly efficient inexpensive electrocatalysts are required to achieve high energy and faradaic efficiencies as well as fast conversion. Metal-nitrogen-carbon (M-N-C) single-site catalysts (SSCs) are highly competitive over precious metal catalysts in the CO2RR to CO due to their high performance, easy regulation and low cost. In the past six years, intensive studies of M-N-C SSCs for CO2RR to CO have been performed, and great progress has been achieved. This review focuses on the important topic of CO2RR to CO with M-N-C SSCs. We first introduce the reaction mechanism of the CO2RR to CO and the regulation of the electronic structure from a theoretical viewpoint. Then, the construction of M-N-C SSCs and the regulation of the electronic structure are demonstrated experimentally. The up-to-date electrocatalytic performance of M-N-C SSCs with different metal centers (Ni, Fe, Co and others) are summarized and compared systematically to highlight structure-performance correlations that were considered from both theoretical and experimental perspectives. Finally, the opportunities, challenges and future outlooks are summarized to deepen and widen research and applications in this promising field.
... It is worth noticing that, when it comes to liquid electrolyte systems, the selectivity towards either CO or H 2 can also be achieved by changing the type of electrolyte. As can be seen from most studies, the most adopted electrolyte for obtaining a high CO selectivity is KHCO 3 , since CO 2 replenishment at the electrode is favored by it [45]. On the other hand, the main limitation of an H-type configuration is that the mass transportation of gas species is rather low [46,47]. ...
Article
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Nowadays, transition towards green chemistry is becoming imperative. In this scenario, an attractive perspective consists in the generation of CO through the electrochemical reduction of CO2 under ambient conditions. This approach allows storage of the electrical energy from intermittent renewable sources in the form of chemical bonds, and simultaneously reduces greenhouse gas emissions, giving carbon a second chance of life. However, most catalysts adopted for this process, i.e., noble metal-based nanoparticles, still have several issues (high costs, low current densities, high overpotentials), and in the view of generating syngas through co-electrolysis of H2O and CO2, do not enable a widely tunable CO/H2 ratio. Single-atom catalysts with N-doped carbon supports have been recently introduced to face these challenges. The following review aims to answer the demand for an extended and exhaustive analysis of the metal single-atom catalysts thus far explored for the electro-reduction of CO2 in aqueous electrolyte solution. Moreover, focus will be placed on the objective of generating a syngas with a tunable CO/H2 ratio. Eventually, the advantages of single-atom catalysts over their noble metal-based nano-sized counterparts will be identified along with future perspectives, also in the view of a rapid and feasible scaling-up.
... The metal single-atom sites exhibit high active site exposure and 100% metal atom utilization. More importantly, the well-defined active site configuration makes it a perfect catalytic site model for studying reaction mechanism (Asset et al. 2019;Zhang and Guan 2020;Zhang et al. 2020b). However, the limited loading of single-atom sites on the support greatly impedes the further improvement of the catalytic performance. ...
Article
Carbon neutral becomes one of the most important environmental goals due to the excessive emission of CO2. The reduction of CO2 into valuable chemicals or fuels is one of the important strategies to solve the carbon cycle and achieve carbon neutral. Atomic site catalysts show high activity and selectivity in CO2 reduction reactions. However, due to the complexity of the multistep CO2 reduction reaction, it is difficult for an isolated atomic site to achieve multi-functional requirements. Bimetallic atomic site catalysts can take advantage of the high activity and selectivity of atomic sites, and the synergies between bimetallic atomic sites can fully optimize the CO2 reduction reactions. In this review, firstly, we summarize the design considerations of catalysts for CO2 reduction reactions. Secondly, the preparation and characterization of bimetallic atomic site catalysts are reviewed. Thirdly, the role of bimetallic atomic sites in CO2 reduction reactions was analyzed in detail.
... 26 Moreover, Atanassov et al. confirmed with near-ambient pressure XPS that pyridinic and hydrogenated (pyrrolic) nitrogen moieties, rather than Fe−N 4 moieties, act as preferential adsorption sites for CO 2 over pyrolyzed Fe−N−C catalysts. 73 Thus, it is likely that the MO x species and the neighboring N-defects cocatalyze the eCO 2 RR. Moreover, further work will be needed to elucidate whether Zn plays a role (direct or indirect) in stabilizing such hydroxide particles or not. ...
Article
The local microenvironment of single-atom electrocatalysts (SACs) governs their activity and selectivity. While previous studies have focused on the first coordination shell (FCS) of metal centers, functional species beyond FCS...
Article
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Electrocatalytic reduction of waste nitrates (NO3⁻) enables the synthesis of ammonia (NH3) in a carbon neutral and decentralized manner. Atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts demonstrate a high catalytic activity and uniquely favor mono-nitrogen products. However, the reaction fundamentals remain largely underexplored. Herein, we report a set of 14; 3d-, 4d-, 5d- and f-block M-N-C catalysts. The selectivity and activity of NO3⁻ reduction to NH3 in neutral media, with a specific focus on deciphering the role of the NO2⁻ intermediate in the reaction cascade, reveals strong correlations (R=0.9) between the NO2⁻ reduction activity and NO3⁻ reduction selectivity for NH3. Moreover, theoretical computations reveal the associative/dissociative adsorption pathways for NO2⁻ evolution, over the normal M-N4 sites and their oxo-form (O-M-N4) for oxyphilic metals. This work provides a platform for designing multi-element NO3RR cascades with single-atom sites or their hybridization with extended catalytic surfaces.
Article
Metal-nitrogen-carbon materials, especially Fe–N–C, are currently at the forefront of interests in electrocatalysis owing to their low price and excellent performance in electrochemical CO2 reduction reaction (CO2RR). It was reported recently that Fe4N and Fe2N modified FeN4 structures, namely penta-coordinated FeN4–L with an axial ligand, would show better performance than the pristine FeN4 structure for CO2 reduction reaction. In this work, microkinetic models were developed and the production rate of CO was calculated over FeN4-based structures. The results suggested that the weaker adsorption of CO over FeN4-L structures, which is originated from the weakened donation and backdonation between CO and the central Fe upon ligand coordination, significantly enhances the CO production rate. Inspired by these results, we further constructed 27 penta-coordinated single iron models with axial ligands and addressed the CO production rate explicitly, and several candidates that would show high activity towards the CO production are identified from the activity map.
Article
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.
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To date, the electrochemical reduction of CO2 (CO2RR) into valuable chemicals is a promising method for combating global warming. However, finding a suitable electrocatalyst for CO2RR is critical to reducing CO2 emissions. Herein, the CO2RR mechanism on a pristine monoclinic Bi2O3 (120) surface was thoroughly investigated using density-functional theory (DFT) calculations. The CO2 adsorption modes of side-on, end-on, and chemisorbed were compared, with the chemisorbed species outperforming for CO2 activation. Furthermore, analyses of the Bader charge, the density of states, and electron density difference strongly suggest that the Bi2O3 (120) surface has excellent activation for CO2 molecules. More importantly, the DFT calculations revealed that the electrochemical CO2RR over the Bi2O3 surface favors both formic acid and methanol in the limiting-potential step of *CO2 → *COOH with the free energy barriers of 0.86 eV and 0.87 eV with and without solvent, respectively. The microkinetic simulation also supports the DFT result, showing the possibility of more selective production by predicting desorption temperatures. This research suggests a promising way for CO2RR mechanisms on pristine monoclinic Bi2O3 surfaces to produce formic acid (HCOOH) and methanol (CH3OH).
Chapter
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Oxygen‐regulated Ni‐based single‐atom catalysts (SACs) show great potential in accelerating the kinetics of electrocatalytic CO2 reduction reaction (CO2RR). However, it remains a challenge to precisely control the coordination environment of NiO moieties and achieve high activity at high overpotentials. Herein, a facile carbonization coupled oxidation strategy is developed to mass produce NiO clusters‐decorated NiNC SACs that exhibit a high Faradaic efficiency of CO (maximum of 96.5%) over a wide potential range (−0.9 to −1.3 V versus reversible hydrogen electrode) and a high turnover frequency for CO production of 10 120 h−1 even at the high overpotential of 1.19 V. Density functional theory calculations reveal that the highly dispersed NiO clusters induce electron delocalization of active sites and reduce the energy barriers for *COOH intermediates formation from CO2, leading to an enhanced reaction kinetics for CO production. This study opens a new universal pathway for the construction of oxygen‐regulated metal‐based SACs for various catalytic applications. NiO clusters‐decorated NiNC single‐atom catalysts show remarkable CO2 reduction reaction (CO2RR) performance over a wide potential range. Even at a high overpotential of 1.19 V, NiO/NiNC‐800 exhibits a high CO production turnover frequency of 10120 h−1, which exceeds the value recently reported for MNC catalysts recently. The highly dispersed NiO clusters induce electron delocalization of NiNx sites and lower the reaction energy barrier for CO2RR.
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Engineering the electronic structure of metal, N‐doped carbon catalysts is a potential strategy for increasing the activity and selectivity of CO2 electroreduction reaction (CO2RR). However, establishing a definitive link between structure and performance is extremely difficult due to constrained synthesis approaches that lack the ability to precisely control the specific local environment of MNC catalysts. Herein, a soft‐template aided technique is developed for the first time to synthesize pyrrolic N4Ni sites coupled with varying N‐type defects to synergistically enhance the CO2RR performance. The optimal catalyst helps attain a CO Faradaic efficiency of 94% at a low potential of −0.6 V and CO partial current density of 59.6 mA cm⁻² at −1 V. Results of controlled experimental investigations indicate that the synergy between NiN4 and metal free defect sites can effectively promote the CO2RR activity. Theoretical calculations revealed that the pyrrolic N coordinated NiN4 sites and C atoms next to pyrrolic N (pyrrolic NC) have a lower energy barrier for the formation of COOH* intermediate and optimum CO* binding energy. The pyrrolic N regulate the electronic structure of the catalyst, resulting in lower CO2 adsorption energy and higher intrinsic catalytic activity.
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Atomic catalysts supported on optimal supports exhibit an ideal strategy to maximize the utilization of active atoms for improving the catalytic efficiency and reducing the cost of catalysts. Among these atomic catalysts, double atom catalysts (DAC), and triple atom catalysts (TACs) are emerging materials, which represent the most basic active sites of the bridge and hollow sites and show high activity and high selectivity. In this chapter, the synthesis routes, characterization techniques, and important applications of DACs and TACs are summarized. Moreover, this chapter outlines the opportunities and challenges in developing DACs and TACs, which provide a comprehensive and distinct understanding of DACs and TACs and inspire further research in the field of catalysis.
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Coordination engineering has recently emerged as a promising strategy to boost the activity of single atom catalysts (SACs) in electrocatalytic CO2 reduction reactions (CO2RR). Understanding the correlation between activity/selectivity and the coordination environment would enable the rational design of more advanced SACs for CO2 reduction. Herein, via density functional theory (DFT) computations, we systematically studied the effects of coordination environment regulation on the CO2RR activity of Ni SACs on C, N, or B co-doped graphene. The results reveal that the coordination environments can strongly affect the adsorption and reaction characteristics. In the C and/or N coordinated Ni-BXCYNZ (B-free, X = 0), only Ni acts as the active site. While in the B, C and/or N coordinated Ni-BXCYNZ (X ≠ 0), the B has transition-metal-like properties, where B and Ni function as dual-site active centers and concertedly tune the adsorption of CO2RR intermediates. The tunability in the adsorption modes and strengths also results in a weakened linear scaling relationship between *COOH and *CO and causes a significant activity difference. The CO2RR activity and the adsorption energy of *COOH/*CO are correlated to construct a volcano-type activity plot. Most of the B, C, and/or N-coordinated Ni-BXCYNZ (X ≠ 0) are located in the left region where *CO desorption is the most difficult step, while the C and/or N coordinated Ni-BXCYNZ (X = 0) are located in the right region where *COOH formation is the potential-determining step. Among all the possible Ni-BXCYNZ candidates, Ni-B0C3N1 and Ni-B1C1N2-N-oppo are predicted to be the most active and selective catalysts for the CO2RR. Our findings provide insightful guidance for developing highly effective CO2RR catalysts based on a codoped coordination environment.
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Electrochemical CO2 reduction to value‐added chemicals/fuels provides a promising way to mitigate CO2 emission and alleviate energy shortage. CO2‐to‐CO conversion involves only two‐electron/proton transfer and thus is kinetically fast. Among the various developed CO2‐to‐CO reduction electrocatalysts, transition metal/N‐doped carbon (M‐N‐C) catalysts are attractive due to their low cost and high activity. In this work, recent progress on the development of M‐N‐C catalysts for electrochemical CO2‐to‐CO conversion is reviewed in detail. The regulation of the active sites in M‐N‐C catalysts and their related adjustable electrocatalytic CO2 reduction performance is discussed. A visual performance comparison of M‐N‐C catalysts for CO2 reduction reaction (CO2RR) reported over the recent years is given, which suggests that Ni and Fe‐N‐C catalysts are the most promising candidates for large‐scale reduction of CO2 to produce CO. Finally, outlooks and challenges are proposed for future research of CO2‐to‐CO conversion. The activity of M‐N‐C catalysts on CO2 reduction reaction (CO2RR) can be adjusted through regulation of active center and local atomic environments, including tuning the central metal atom and its neighbored coordinated atoms structure, or doping other heteroatoms and forming dual‐atoms sites. The role of nonmetal moieties and metal nanoparticles in M‐N‐C is also included.
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To identify high‐efficiency metal—nitrogen‐doped (M—N—C) electrocatalysts for the electrochemical CO2‐to‐CO reduction reaction (CO2RR), a method that uses density functional theory calculation is presented to evaluate their selectivity, activity, and structural stability. Twenty‐three M—N4—C catalysts are evaluated, and three of them (M = Fe, Co, or Ni) are identified as promising candidates. They are synthesized and tested as proof‐of‐concept catalysts for CO2‐to‐CO conversion. Different key descriptors, including the maximum reaction energy, differences of the *H and *CO binding energy (ΔG*H−ΔG*CO), and *CO desorption energy (ΔG*CO→CO(g)), are used to clarify the reaction mechanism. These computational descriptors effectively predict the experimental observations in the entire range of electrochemical potential. The findings provide a guideline for rational design of heterogeneous CO2RR electrocatalysts.
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Electrochemical conversion of CO2 to CO or syngas (CO/H2 mixture) is considered one of the most promising approaches to valorise waste-CO2. To develop the process on industrial scale, it would be necessary to use selective and inexpensive electrodes and to obtain high productivities with low energy consumption. In this frame, Ni−NC catalysts are considered among the most interesting ones because of their relatively low cost, high faradaic efficiency in CO (FECO), and high stability. However, up to now, quite low productivities were obtained as a result of low current densities achieved in aqueous electrolytes. In this work, we have evaluated the performances of a Ni−NC electrocatalyst at relatively high carbon dioxide pressures (5−30 bar) in a wide range of cell potentials and current densities. It is found that proper selection of CO2 pressure and catalyst loading improves drastically the performance of the process, obtaining high FECO(close to 100 %), high current densities (>100 mA cm⁻²), and high productivities. Furthermore, it is shown that it is possible to obtain syngas with a target ratio of two between H2 and CO under various operating conditions. As an example, syngas was obtained with FE close to 100% and a productivity of ∼18 mol h⁻¹ m⁻² working at 30 bar with a current density close to 195 mA cm⁻².
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The increasing release of carbon dioxide into atmosphere has caused serious environmental consequences and closing the carbon loop is therefore essential for promoting the transition towards a sustainable development. Electrochemical reduction of carbon dioxide (E−CO2RR) represents a powerful strategy for reducing CO2 levels in atmosphere and obtaining value-added chemicals and fuels using renewable energy sources. Despite the important achievements obtained so far, major issues associated with activity and selectivity of electrocatalysts toward the production of multi-carbon (C2+) products hinder large-scale applications. Hence, a thorough understanding of catalytic mechanisms is needed for advancing the design of efficient electrocatalysts to drive the reaction pathway to the desired products. This review summarizes the latest advances in the design of nanostructured metal-based catalysts for E−CO2RR, with a special emphasis on the synthesis procedures and electrochemical performance of metal-nitrogen-carbon catalysts. An overview on the catalytic mechanisms is included along with a discussion of the experimental and computational techniques for mechanistic studies and catalyst development. Finally, we outline a perspective on the relationship between structure, morphology and electrochemical activity highlighting challenges and outlook on developing metal-nitrogen-carbon electrocatalysts for E−CO2RR to multi-carbon products.
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Benefits of utilizing cascade reactions for chemical synthesis include minimizing waste and decreasing experimentation times. However, these complex systems lack an efficient screening platform for evaluating their progression. A paper‐based microfluidic platform was developed to monitor cascade systems without the need for off‐line product verification. Two types of paper‐based platforms were fabricated to facilitate this study: electrochemical and spectro‐electrochemical. Electrochemical platforms were integrated with stencil‐printed electrodes and used to perform electrochemical alcohol oxidation reactions with two derivatives of the organocatalyst TEMPO (TEMPO=2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl). The catalyst 4‐amino‐TEMPO (TEMPO‐NH2) was used as a model catalyst that was studied not immobilized and immobilized (pyrene‐amido‐TEMPO) on electrochemical platforms. These platforms were designed to provide quasi‐stationary flow allowing constant electrochemical data collection as catalytic reactions proceeded. TEMPO‐NH2 and pyrene‐amido‐TEMPO were evaluated for the partial oxidation of glycerol and its intermediates. In comparison to TEMPO‐NH2, the pyrene‐amido‐TEMPO catalyst produced higher current outputs. Spectro‐electrochemical platforms were integrated with stencil‐printed electrodes, and a surface enhanced Raman spectroscopy (SERS) detection zone. The spectro‐electrochemical platforms allowed for catalytic conversions at the electrodes and subsequent delivery of catalytically transformed analytes to a SERS detection zone for product analysis. This platform was demonstrated for pyrene‐amido‐TEMPO and was shown to convert glycerol to mesoxalic acid. The experimental procedures for making components of the spectro‐electrochemical device were described and include: preparation of the paper‐based platform, construction of stencil‐printed electrodes, and fabrication of SERS detection zones. These platforms provide an approach to analyzing multi‐step cascade chemical reactions.
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We report novel structure-activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO2 reduction reaction (CO2RR) catalyzed by a series of porous iron-nitrogen-carbon (FeNC) catalysts. The FeNC catalysts were synthesized from different nitrogen precursors and, as a result of this, exhibited quite distinct physical properties, such as BET surface areas and distinct chemical N-functionalities in varying ratios. The chemical diversity of the FeNC catalysts was harnessed to set up correlations between the catalytic CO2RR activity and their chemical nitrogen-functionalities, which provided a deeper understanding between catalyst chemistry and function. XPS measurements revealed a dominant role of porphyrin-like Fe-N x motifs and pyridinic nitrogen species in catalyzing the overall reaction process. Operando EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe2+ to Fe1+. This redox change coincides with the onset of catalytic CH4 production around -0.9 VRHE. The ability of the solid state coordinative Fe1+-N x moiety to form hydrocarbons from CO2 is remarkable, as it represents the solid-state analogue to molecular Fe1+ coordination compounds with the same catalytic capability under homogeneous catalytic environments. This finding highlights a conceptual bridge between heterogeneous and homogenous catalysis and contributes significantly to our fundamental understanding of the FeNC catalyst function in the CO2RR.
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Nitrogen‐doped carbon materials are proposed as promising electrocatalysts for the carbon dioxide reduction reaction (CRR), which is essential for renewable energy conversion and environmental remediation. Unfortunately, the unclear cognition on the CRR active site (or sites) hinders further development of high‐performance electrocatalysts. Herein, a series of 3D nitrogen‐doped graphene nanoribbon networks (N‐GRW) with tunable nitrogen dopants are designed to unravel the site‐dependent CRR activity/selectivity. The N‐GRW catalyst exhibits superior CO2 electrochemical reduction activity, reaching a specific current of 15.4 A gcatalyst⁻¹ with CO Faradaic efficiency of 87.6% at a mild overpotential of 0.49 V. Based on X‐ray photoelectron spectroscopy measurements, it is experimentally demonstrated that the pyridinic N site in N‐GRW serves as the active site for CRR. In addition, the Gibbs free energy calculated by density functional theory further illustrates the pyridinic N as a more favorable site for the CO2 adsorption, *COOH formation, and *CO removal in CO2 reduction.
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Electrochemical reduction of CO2 provides an opportunity to reach a carbon‐neutral energy recycling regime, in which CO2 emissions from fuel use are collected and converted back to fuels. The reduction of CO2 to CO is the first step toward the synthesis of more complex carbon‐based fuels and chemicals. Therefore, understanding this step is crucial for the development of high‐performance electrocatalyst for CO2 conversion to higher order products such as hydrocarbons. Here, atomic iron dispersed on nitrogen‐doped graphene (Fe/NG) is synthesized as an efficient electrocatalyst for CO2 reduction to CO. Fe/NG has a low reduction overpotential with high Faradic efficiency up to 80%. The existence of nitrogen‐confined atomic Fe moieties on the nitrogen‐doped graphene layer is confirmed by aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine structure analysis. The Fe/NG catalysts provide an ideal platform for comparative studies of the effect of the catalytic center on the electrocatalytic performance. The CO2 reduction reaction mechanism on atomic Fe surrounded by four N atoms (Fe–N4) embedded in nitrogen‐doped graphene is further investigated through density functional theory calculations, revealing a possible promotional effect of nitrogen doping on graphene.
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The electrochemical reduction of carbon dioxide (CO2) has attracted considerable attention as a means of maintaining the carbon cycle. This process still suffers from poor performance,including low faradaic efficiencies and high overpotential. Herein, we attempted to use coordination number as a control parameter to improve the electrocatalytic performance of metal species that have previously been thought to have no CO2 reduction activity. 3d metal single atoms (Co, Ni or Cu)-modified covalent triazine frameworks (CTF) were developed for efficient electroreduction of CO2. Co-CTF and Ni-CTF materials effectively reduced CO2 to CO from -0.5 V versus RHE. The faradaic efficiency of the Ni-CTF during CO formation reached 90% at -0.8 V versus RHE. These performances are much higher than that of the corresponding metal-porphyrins (using tetraphenylporphyrin; TPP), in which only Co-TPP has CO2 reduction activity in neutral solutions. First principles calculations demonstrated that the intermediate species (adsorbed COOH) was stabilized on the metal atoms in the CTF due to the low-coordination structure of this support. Thus, the free energy barriers for the formation of adsorbed COOH on the metal atoms in the CTF supports were lower than those on the TPP supports.
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Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–Nx moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–Nx moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomicscale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–Nx moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.
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Bimetallic catalysts are promising for the most difficult thermal and electrochemical reactions but modeling the many diverse active sites on polycrystalline samples is an open challenge. We present a general framework for addressing this complexity in a systematic and predictive fashion. Active sites for every stable low-index facet of a bimetallic crystal are enumerated and cataloged yielding hundreds of possible active sites. The activity of these sites is explored in parallel using a neural-network based surrogate model to share information between the many Density Functional Theory (DFT) relaxations, resulting in activity estimates with an order of magnitude fewer explicit DFT calculations. Sites with interesting activity were found and provide targets for follow-up calculations. This process was applied to the electrochemical reduction of CO2 on nickel gallium bimetallics and indicated that most facets had similar activity to Ni surfaces, but a few exposed Ni sites with a very favorable on-top CO configuration. This motif emerged naturally from the predictive modeling and represents a class of intermetallic CO2 reduction catalysts. These sites rationalize recent experimental reports of nickel gallium activity and why previous materials screens missed this exciting material. Most importantly these methods suggest that bimetallic catalysts will be discovered by studying facet reactivity and diversity of active sites more systematically.
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A series of copper-based electrocatalysts were prepared by the Sacrificial Support Method (SSM) with variation of synthesis parameters. Thin films of the materials were evaluated for their electrocatalytic activities towards CO2 electroreduction to short-chain (C1-C2) hydrocarbons by standard electrochemical methods. Gas-phase reaction products were quantified using an online gas chromatography system. At −0.98 V vs reversible hydrogen electrode (RHE), copper oxide-derived catalysts were found to have selectivity toward C2H4 approximately one order of magnitude higher than to CH4. The highest selectivity towards C2H4 production at −0.98 V was demonstrated by the catalyst with cube morphology, synthesized with 20 wt% Cu: 80 wt% SiO2 ratio in the precursor. Possible causes for this shift in selectivity are discussed in terms of the morphology and surface/core composition as determined by scanning electron microscopy (SEM), X-Ray diffraction (XRD), and X-Ray photoelectron spectroscopy (XPS).
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Electrocatalytic conversion of carbon dioxide has gained much interest for the synthesis of value-added chemicals and solar fuels. Important issues such as high overpotentials and competition of hydrogen evolution still need to be overcome for deeper insight into the reaction mechanism in order to steer the selectivity towards specific products. Herein we report on several metalloprotoporphyrins immobilized on a pyrolytic graphite electrode for the selective reduction of carbon dioxide to formic acid. No formic acid is detected on Cr-, Mn-, Co- and Fe-protoporphyrins in perchloric acid of pH 3, while Ni-, Pd-, Cu- and Ga-protoporphyrins show only a little formic acid. Rh, In and Sn metal centers produce significant amounts of formic acid. However, the faradaic efficiency varies from 1% to 70% depending on the metal center, the pH of the electrolyte and the applied potential. The differentiation of the faradaic efficiency for formic acid on these metalloprotoporphyrins is strongly related to the activity of the porphyrin for the hydrogen evolution reaction. CO2 reduction on Rh-protoporphyrin is shown to be coupled strongly to the hydrogen evolution reaction, whilst on Sn- and In-protoporphyrin such strong coupling between the two reactions is absent. The activity for the hydrogen evolution increases in the order In < Sn < Rh metal centers, leading to faradaic efficiency for formic acid increasing in the order Rh < Sn < In metal centers. In-protoporphyrin is the most stable and shows a high faradaic efficiency of ca. 70%, at a pH of 9.6 and a potential of −1.9 V vs RHE. Experiments in bicarbonate electrolyte were performed in an attempt to qualitatively study the role of bicarbonate in formic acid formation.
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Electroreduction of carbon dioxide into higher-energy liquid fuels and chemicals is a promising but challenging renewable energy conversion technology. Among the electrocatalysts screened so far for carbon dioxide reduction, which includes metals, alloys, organometallics, layered materials and carbon nanostructures, only copper exhibits selectivity towards formation of hydrocarbons and multi-carbon oxygenates at fairly high efficiencies, whereas most others favour production of carbon monoxide or formate. Here we report that nanometre-size N-doped graphene quantum dots (NGQDs) catalyse the electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates at high Faradaic efficiencies, high current densities and low overpotentials. The NGQDs show a high total Faradaic efficiency of carbon dioxide reduction of up to 90%, with selectivity for ethylene and ethanol conversions reaching 45%. The C2 and C3 product distribution and production rate for NGQD-catalysed carbon dioxide reduction is comparable to those obtained with copper nanoparticle-based electrocatalysts.
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Improving cobalt catalysts Tethering molecular catalysts together is a tried and trusted method for making them easier to purify and reuse. Lin et al. now show that the assembly of a covalent organic framework (COF) structure can also improve fundamental catalytic performance. They used cobalt porphyrin complexes as building blocks for a COF. The resulting material showed greatly enhanced activity for the aqueous electrochemical reduction of CO 2 to CO. Science , this issue p. 1208
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This study explores the kinetics, mechanism, and active sites of the CO2 electroreduction reaction (CO2RR) to syngas and hydrocarbons on a class of functionalized solid carbon-based catalysts. Commercial carbon blacks were functionalized with nitrogen and Fe and/or Mn ions using pyrolysis and acid leaching. The resulting solid powder catalysts were found to be active and highly CO selective electrocatalysts in the electroreduction of CO2 to CO/H2 mixtures outperforming a low-area polycrystalline gold benchmark. Unspecific with respect to the nature of the metal, CO production is believed to occur on nitrogen functionalities in competition with hydrogen evolution. Evidence is provided that sufficiently strong interaction between CO and the metal enables the protonation of CO and the formation of hydrocarbons. Our results highlight a promising new class of low-cost, abundant electrocatalysts for synthetic fuel production from CO2 . © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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We report an experimental-computational study of mechanistic reaction pathways during the electrochemical reduction of CO2 to CH4, catalyzed by solid-state, single-site Fe-N-C catalysts. Fe-N-C catalysts feature molecularly dispersed catalytically active Fe-N motifs and represent a type of non-Cu-based catalysts that yield “beyond CO” hydrocarbon products. The various multi-step mechanistic pathways toward hydrocarbons with these catalysts has never been studied before and is the focus of this study. A number of different reactant molecules with varying formal carbon redox states, more specifically CO2, CO, CH2O, CH3OH and formate were electrochemically converted at the Fe-N sites, yet only CO2, CO and CH2O could be protonated into methane. Also, we observed a distinctly different pH dependence of the catalytic CH4 evolution from CO and CH2O, suggesting differences in the proton participation of rate determining steps. In comparing the experimental observations with Density Functional Theory (DFT) -derived Free Energy Diagrams of reactive intermediates along the reaction coordinates, we unraveled the distinctly different dominant mechanistic pathways and roles of CO and CH2O along the catalytic CO2-to-CH4 cascade and their rate-determine-steps (RDS). We close with the first comprehensive reaction network of the CO2 electroreduction on a M-N-C catalyst. Our findings offer valuable insights in the catalysis of the CO2RR on single site Fe-N-C catalysts that may prove useful in developing efficient, non-Cu-based catalysts for direct electrochemical hydrocarbons production.
Article
The electrochemical CO2 reduction reaction (CO2RR) to pure CO streams in electrolyzer devices is poised to be the most likely process for near-term commercialization and deployment in the polymers industry....
Article
The CO2 electrochemical reduction reaction (CO2RR) is a promising technology for converting CO2 into chemicals and fuels, using surplus electricity from renewable sources. The technological viability of this process, however, is contingent on finding affordable and efficient catalysts. A range of materials containing abundant elements, such as N, C, and non‐noble metals, ranging from well‐defined immobilized complexes to doped carbon materials have emerged as a promising alternative. One of the main products of the CO2RR is CO, which is produced on these catalysts with selectivities comparable to those of noble metal catalysts. Furthermore, other valuable products, such as formic acid, hydrocarbons, and alcohols, have also been reported. The factors that control the catalytic performance of these materials, however, are not yet fully understood. A review of recent work is presented on heterogeneous nitrogen‐containing carbon catalysts for the CO2RR. The synthesis and characterization of these materials as well as their electrocatalytic performance are discussed. Combined experimental and theoretical studies are included to bring insight on the active sites and the reaction mechanism. This knowledge is key for developing optimal catalyst materials that meet the requirement in terms of activity, selectivity, and stability needed for commercial applications. Materials containing abundant elements such as N, C, and non‐noble metals have recently emerged as a promising alternative to metal‐based catalysts for the CO2 reduction reaction. Such catalysts include immobilized complexes, metal organic frameworks, and doped carbon materials.
Article
Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.
Article
We discuss perspectives and challenges in applying density functional theory for the calculation of spectroscopic properties of platinum group metal (PGM)-free electrocatalysts for oxygen reduction. More specifically, we discuss recent advances in the density functional theory calculations of core-level shifts in binding energies of N 1s electrons as measured by X-ray photoelectron spectroscopy. The link between the density functional theory calculations, the electrocatalytic performance of the catalysts, and structural analysis using modern spectroscopic techniques is expected to significantly increase our understanding of PGM-free catalysts at the molecular level.
Article
We prepare a series of atomically dispersed Co catalysts with different nitrogen coordination numbers and explore their CO2 electroreduction catalytic performance. Our best catalyst, atomically dispersed Co with two coordinate nitrogen atoms, achieves both high selectivity and superior activity with 94% CO formation Faradaic efficiency and a current density of 18.1 mA cm−2 at an overpotential of 520 mV. The CO formation turnover frequency reaches a record value of 18,200 hour−1, surpassing most of reported metal-based catalysts under comparable conditions. Our experimental and theoretical calculating results demonstrate that lower coordination number facilitates activation of CO2 into the CO2*− intermediate and hence enhances CO2 electroreduction activity, which may provide an effective strategy for catalysts design in important reactions.
Article
Electrochemical reduction of carbon dioxide (CO2RR) to formate provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Here, we hypothesized that the presence of sulfur atoms in the catalyst surface could promote undercoordinated sites, and thereby improve the electrochemical reduction of CO2 to formate. We explored, using density functional theory, how the incorporation of sulfur into tin may favor formate generation. We used atomic layer deposition of SnSx followed by a reduction process to synthesize sulfur-modulated tin (Sn(S)) catalysts. X-ray absorption near-edge structure (XANES) studies reveal higher oxidation states in Sn(S) compared with that of tin in Sn nanoparticles. Sn(S)/Au accelerates CO2RR at geometric current densities of 55 mA cm⁻² at −0.75 V versus reversible hydrogen electrode with a Faradaic efficiency of 93%. Furthermore, Sn(S) catalysts show excellent stability without deactivation (<2% productivity change) following more than 40 hours of operation.
Article
Multiple approaches will be needed to reduce the atmospheric CO2 levels which has been linked to the undesirable effects of global climate change. The electroreduction of CO2 driven by renewable energy is one approach to reduce CO2 emissions while producing chemical building blocks, but current electrocatalysts exhibit low activity and selectivity. Here we report the structural and electrochemical characterization of a promising catalyst for the electroreduction of CO2 to CO: Au nanoparticles supported on polymer-wrapped multiwall carbon nanotubes. This catalyst exhibits high selectivity for CO over H2: 80-92% CO, as well as high activity: partial current density for CO as high as 160 mA/cm2. The observed high activity, originating from a high electrochemically active surface area (23 m2/g Au), in combination with the low loading (0.17 mg/cm2) of the highly-dispersed Au nanoparticles underscores the promise of this catalyst for efficient electroreduction of CO2.
Article
The binding of nitric oxide (NO) to heme-proteins is an important biochemical process involved in a variety of physiological functions. Here, using hybrid density-functional calculations, we systematically investigate the adsorption of NO to first-transition-row metal centers in metal–ligand complexes. Through the comparative study for different transition metal (TM) centers, we provide a unified understanding of the microscopic interactions of NO with the TM centers and related chemical trends. We found that as the atomic number of the TM center increases, the binding strength of NO is largely reduced from 207 kJ/mol to near zero due to the low d-orbital energies for late TM centers. The intermolecular spin coupling between the localized spins at the TM center and the NO molecule is generally antiferromagnetic, except for the case of Sc. The spin-spin coupling is determined in such a way to avoid the energy penalty associated with the electron occupation in the antibonding states of the NO-bound complex. The adsorption strength of NO is generally larger than of CO because the unpaired electron of NO occupies the associated bonding state.
Article
A key challenge of the carbon dioxide electroreduction (CO2RR) on Cu-based nanoparticles is its low faradic selectivity towards higher-value products such as ethylene. Here, we demonstrate a facile method for tuning the hydrocarbon selectivities on CuOx nanoparticle ensembles by varying the nanoparticle areal density. The sensitive dependence of the experimental ethylene selectivity on catalyst particle areal density is attributed to a diffusional interparticle coupling which controls the de- and re-absorption of CO and thus the effective coverage of COad intermediates. Thus, higher areal density constitutes dynamically favoured conditions for CO re-adsorption and *CO dimerization leading to ethylene formation independent of pH and applied overpotential.
Article
For targeted development of platinum group metal-free (PGM-free) catalysts for proton exchange membrane fuel cell applications, it is critically important to elucidate the catalytic moieties of Fe-N-C materials as they relate to the structure/morphology of the graphitic layers of carbon – the catalyst basic building blocks. In this report, X-ray diffraction analysis with a carbon-specific structure refinement algorithm was performed on 12 Fe-N-C catalysts. Samples with fewer graphitic layers exhibit increased kinetic performance in fuel cell testing. This trend is consistent with the dominant active species residing within the graphitic plane as opposed to at the edges. We also we report performance of an optimized catalyst based on structure-to-property predictions derived in the recently published report. This catalyst produces 0.44 mA cm-2 at 0.85 V and has a maximum power density of 490 mW cm-2 in 1 bar O2 (not iR corrected).
Article
Glycerol is a common fuel considered for bioenergy applications. Computational docking studies were performed on formate dehydrogenase from Candida boidinii cbFDH that showed that mesoxalate can bind to the buried active site of the holo form predicting that mesoxalate, a byproduct of glycerol oxidation, may act as its substrate. Spectroscopic assays and characterization by HPLC and GC/TCD have shown for the first time that cbFDH can act as decarboxylase with mesoxalate. From this assessment, cbFDH was combined with NH2-TEMPO to form a novel hybrid anode to oxidize glycerol to carbon dioxide at near-neutral pH.
Article
Cu films electrodeposited from plating baths containing 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor exhibit high surface area and high CO2 reduction activities. By changing pH and deposition current density, the morphologies of the Cu films are varied to exhibit wire, dot, or amorphous structures. Among these Cu films, the CuDAT-wire samples exhibit the best CO2 reduction activities activity with a Faradaic Efficiency (FE) for C2H4 product formation reaching 41% at -0.47 V vs. RHE, a FE for C2H5OH formation reaching 22% at -0.55 V vs. RHE, and a mass activity for CO2 reduction at -0.65V vs. RHE of ~ 720 A/g.
Article
Development of platinum group metal free catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) requires understanding of the interactions between surface chemistry and performance, both of which are strongly dependent on synthesis conditions. To elucidate these complex relationships, a set of Fe-N-C catalysts derived from the same set of precursor materials is fabricated by varying several key synthetic parameters under controlled conditions. The results of physicochemical characterization are presented and compared with the results of rotating disk electrode (RDE) analysis and fuel cell testing. We find that electrochemical performance is strongly correlated with three key properties related to catalyst composition: concentrations of 1) atomically dispersed Fe species, 2) species in which N is bound to Fe, and 3) surface oxides. Not only are these factors related to performance, these types of chemical species are shown to correlate with each other. This study provides evidence supporting the role of iron coordinated with nitrogen as an active species for the ORR, and offers synthetic pathways to increase the density of atomically dispersed iron species and surface oxides for optimum performance.
Article
Currently, no catalysts are completely selective for the electrochemical CO2 Reduction Reaction (CO2RR). Based on trends in density functional theory calculations of reaction intermediates we find that the single metal site in a porphyrine-like structure has a simple advantage of limiting the competing Hydrogen Evolution Reaction (HER). The single metal site in a porphyrine-like structure requires an ontop site binding of hydrogen, compared to the hollow site binding of hydrogen on a metal catalyst surface. The difference in binding site structure gives a fundamental energy-shift in the scaling relation of ∼0.3 eV between the COOH* vs. H* intermediate (CO2RR vs. HER). As a result, porphyrine-like catalysts have the advantage over metal catalyst of suppressing HER and enhancing CO2RR selectivity.
Article
We report the first in-situ Ambient Pressure X-Ray Photoelectron Spectroscopy(APXPS) study of the binding of oxygenated species to the active sites of iron-nitrogen-carbon Oxygen Reduction Reaction (ORR) electrocatalysts. To better interpret the results DFT calculations were used to calculate absorption energies of reactants and intermediates on potential active sites and calculate the core level shifts for those. The observed oxygen binding to nitrogen coordinated to iron centers correlates with the enhanced measured ORR fuel cell activity of these materials with respect to metal-free analogs and sheds light on the ORR mechanism on PGM-free electrocatalysts.
Article
A combination of N 1s X-ray photoelectron spectroscopy (XPS) and first principle calculations of nitrogen-containing model electrocatalysts was used to elucidate the nature of the nitrogen defects that contribute to the binding energy (BE) range of the N 1s XPS spectra of these materials above ~400 eV. Experimental core level shifts were obtained for a set of model materials, namely N-doped carbon nanospheres, Fe-N-carbon nanospheres, polypyrrole, polypyridine, and pyridinium chloride, and were compared to the shifts calculated using density functional theory. The results confirm that the broad peak positioned at ~400.7 eV in the N 1s XPS spectra of N-containing catalysts, which is typically assigned to pyrrolic nitrogen, contains contributions from other hydrogenated nitrogen species such as hydrogenated pyridinic functionalities. Namely, N 1s BEs of hydrogenated pyridinic-N and pyrrolic-N were calculated as 400.6 eV and 400.7 eV, respectively, using the Perdew-Burke-Ernzerhof exchange-correlation functional. A special emphasis was placed on the study of the differences in the XPS imprint of N-containing defects that are situated in the plane and on the edges of the graphene sheet. Density functional theory calculations for BEs of the N 1s of in-plane and edge defects show that hydrogenated N defects are more sensitive to the change in the chemical environment in the carbon matrix than the non-hydrogenated N defects. Calculations also show that edge hydrogenated pyridinic-N and pyrrolic-N defects only contribute to the N 1s XPS peak located at ~400.7 eV if the graphene edges are oxygenated or terminated with bare carbon atoms.
Article
In the last few years, there has been increased interest in electrochemical CO2 reduction (CO2R). Many experimental studies employ a membrane separated, electrochemical cell with a mini H-cell geometry to characterize CO2R catalysts in aqueous solution. This type of electrochemical cell is a mini-chemical reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are constant throughout the study. We show that operating cells with high catalyst surface area to electrolyte volume ratios (S/V) at high current densities can have subtle consequences due to the complexity of the physical phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temperature and bulk electrolyte CO2 concentration. Both effects were evaluated quantitatively in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temperature is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high current density, 20 mA cm⁻², the temperature increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concentration is predicted. In contrast, limits on the CO2 gas-liquid mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concentration, significant undersaturation of CO2 is observed in the bulk electrolyte, even at more modest current densities of 10 mA cm⁻². Undersaturation of CO2 produces large changes in the faradaic efficiency observed on Cu electrodes, with H2 production becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is critical for maintaining the equilibrium CO2 concentration in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equilibrium CO2 concentration at current densities up to 15 mA cm⁻².