Wenzhong
Huang†
a,
Jiexin
Zhu†
a,
Shanlin
Liu†
a,
Wei
Zhang
a,
Liang
Zhou
*abc and
Liqiang
Mai
*abc
aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, P. R. China. E-mail: mlq518@whut.edu.cn; liangzhou@whut.edu.cn
bHubei Longzhong Laboratory, Wuhan University of Technology (Xiangyang Demonstration Zone), Xiangyang 441000, Hubei, P. R. China
cHainan Institute, Wuhan University of Technology, Sanya 572000, Hainan, P. R. China
First published on 22nd May 2023
The use of electrocatalytic carbon dioxide reduction (ECR) for producing various high-value-added products is critical for achieving carbon neutrality. In the past decades, single-site catalysts (SSCs), such as single-atom catalysts, homogeneous molecular catalysts, metal–organic-framework-supported and covalent-organic-framework-supported SSCs, have shown good selectivity and activity for ECR. In this review, we systematically discuss the design principles and optimization strategies for ECR SSCs, starting with the reaction mechanism and descriptors of ECR. We highlight representative studies conducted in the past decades to elucidate the selectivity and reaction mechanisms of different types of SCCs for ECR. Finally, we describe the remaining challenges and perspectives in the application of these emerging catalysts.
Broader contextExcessive fuel combustion and greenhouse gas emission lead to global environmental deterioration. By converting carbon dioxide into valued chemicals and fuels, electrocatalytic carbon dioxide reduction (ECR) is of great significance to alleviate the environmental issue. However, ECR electrocatalysts suffer from limited activity, low selectivity, and unsatisfactory stability. As a result, developing efficient, stable, and low-cost ECR catalysts is of vital importance. Recently, single-site catalysts (SSCs) have emerged as promising ECR catalysts due to their precisely controllable structure and high atomic utilization efficiency. Herein, to accelerate the development of SSCs for ECR, it is necessary to summarize their recent progress on design principles and optimization strategies. This review shows how SSCs are one of most promising candidates for ECR based on latest studies. The researches on these emerging SSCs for ECR have also been comprehensively summarized and discussed, including their catalytic mechanisms, synthesis, optimization strategies, remaining challenges and perspectives. We hope to provide a comprehensive overview of state-of-art progress to readers and shed light on this important field. |
Since Hori's pioneering research in the 1980s,8 metals and alloys have been extensively investigated for ECR applications. In recent decades, single-site catalysts (SSCs) have emerged as a new type of catalyst because of their precisely controllable structure and high atomic utilization efficiency.9–16 The SSCs with active metal center isolated by non-metal atoms mainly contain single-atom catalysts (SACs),17,18 homogeneous molecular catalysts (HMCs),19,20 as well as metal organic framework-supported (MOF-supported)21,22 and covalent organic framework-supported (COF-supported) SSCs.23,24 The unique coordination environment of SSCs paves a new way for regulating the intrinsic charge, spinning order, and charge transfer of catalysts. Furthermore, the unique coordination structure with a single metal center isolated by non-metal atoms effectively suppresses the competing hydrogen evolution reaction (HER). In addition, the controllable structure of SSCs is also beneficial for the structural design of catalyst, study of reaction mechanism, and evolution of reaction products during the ECR reaction.14,25–27 Numerous theoretical and experimental studies have demonstrated the application potential of SSCs in ECR.28–31
In this review, we comprehensively summarize the researches on these emerging SSCs for ECR, including their catalytic mechanisms, synthesis, optimization strategies, and remaining challenges. Furthermore, we introduce the fundamental ECR reaction mechanism and reaction descriptors to better understand and study the design and reaction mechanisms of different catalysts. We describe recent representative studies on SACs (such as carbon-based and MOF-derived), HMCs (such as phthalocyanine, porphyrin, dipyridine, and their derivatives), MOF-supported SSCs (located in different parts of MOFs) and COF-supported SSCs (such as imine-based and amine-based) to elaborate the advantages and disadvantages of SSCs for ECR (Fig. 1). Finally, we discuss the opportunities and perspectives in the application of these emerging SSCs.
Half-reactions with major reported products are outlined here, without considering the reaction conditions.33 Unless stated otherwise, all potentials in this study are versus the reversible hydrogen electrode (RHE).
CO2 + 2H+ + 2e− → CO + H2O E0 = −0.10 vs. RHE | (1) |
CO2 + 2H+ + 2e− → HCOOH E0 = −0.12 vs. RHE | (2) |
CO2 + 8H+ + 8e− → CH4 + 2H2O E0 = +0.17 vs. RHE | (3) |
2CO2 + 12H+ + 12e− → C2H4 + 4H2O E0 = +0.08 vs. RHE | (4) |
CO2 + 6H+ + 6e− → CH3OH + H2O E0 = +0.03 vs. RHE | (5) |
2CO2 + 12H+ + 12e− → C2H5OH + 3H2O E0 = +0.09 vs. RHE | (6) |
The ECR reaction in aqueous electrolytes is normally divided into four stages: dissolution, activation, hydrogenation, dimerization and polymerization (Fig. 3). The first dissolution process is essential for the ECR in the aqueous electrolyte (Stage 1). In particular, a higher solubility of CO2 usually implies a higher current density, which is vital for achieving industrially viable ECR rates. However, the dissolution and transport of CO2 are limited due to their low solubility in water under ambient conditions (34 mM). This mass transport issue can be resolved by increasing the temperature or pressure to improve the solubility of CO2 in electrolyte; using a gas diffusion flow cell to continually provide CO2 saturated electrolyte, and modifying the electrode surface to optimize the hydrophobicity and aerophilicity of the electrode.
Fig. 3 The relationship between catalyst-intermediate binding energy and selectivity of products (* represents active site on catalysts surface). |
CO2 is adsorbed and stabilized on the electrode surface after dissolution, resulting in two main products: *HCOO and *COOH (Stage 2 to Stage 3). The hydrogenation of *HCOO on the surface of the Group I catalysts, which have a weak binding force to *HCOO, results in HCOOH (Stage 3). *COOH is reduced to *CO (Stage 4), on the catalysts (Group II and Group III) with strong *COOH binding ability. Finally, Group II catalysts with weak binding to *CO produce CO. In contrast, Group III catalysts with strong binding with *CO prefer to undergo C–C coupling and generates C2/C2+ products. In particular, the formation of C2 and C2+ is called “dimerization and polymerization”, respectively. Furthermore, the selectivity and adsorption capacity of intermediates is the most critical research field during the entire process. The Sabatier principle should be followed in the design of catalysts, which states that the interaction between the catalysts and intermediates should not be too strong or too weak.
The target product's current density is not only an important indicator of reaction rate, but also an important factor in terms of electricity cost. This governs the size of the electrolyzer required for a given production rate. Generally, a large electrolyzer requires high current density to cover the capital cost. Sargent et al. found that the impact of the current density on the electricity cost gradually weakened once it exceeded 300 mA cm−2.32 Therefore, a high current density (>300 mA cm−2) should be regarded as the basic standard to reduce the overall cost.
FE is another key indicator for describing the efficiency of charge (electron) transfer in systems that promotes targeted electrochemical reactions. A high FE toward a specific product could minimize the cost of product separation, which can help reduce current density and electricity cost. Thus, achieving a high FE, close to 100%, is always a worthwhile goal.
EE is the ratio of the output energy from the products of “what we gain” to the input energy from “what it cost” to produce them. Generally, the calculation of this parameter requires a comprehensive consideration of the FE and voltage efficiency of the product. Voltage efficiency reflects the part of the total input voltage that is actually used to drive the thermodynamic process, which is also related to the electricity cost. Therefore, a high EE is generally desired at low cost.
Durability is an extremely important parameter of ECR catalysts. After reaching the target current density, FE, and EE, robust stability effectively reduces the cost of maintenance and replacement. However, most current reports do not reach an ideal test time (more than 8000 h), which is important in the study of stability.
Generally, a suitable high current density (>300 mA cm−2), high FE (∼100%) and EE, and good stability (over 8000 h) are of great significance to promote the widespread application of ECR.
The bottom-up strategy is one of the most common methods for obtaining carbon-based SACs. In this method, metal ions are usually adsorbed on the surface of the carbon matrix; this is followed by drying and high/low-temperature thermal treatment to achieve firm anchoring.46 Zhang et al. synthesized some SACs with different metal atoms (M-SACs, M = Ru, Fe, Ni, Cr, Cu, Zn, Pt, Mn, Co, etc.) (Fig. 4a–i) by employing a “ligand-mediated” synthetic strategy.47 The strong covalent bond between the metal atoms and abundant defect sites on the surface of carbon black prevented the aggregation of metal species. Moreover, the use of carbon-supporting substrates circumvented the instability caused by high-temperature pyrolysis. Only graphite peaks were observed in the X-ray diffraction (XRD) patterns of these M-SACs, indicating that the formation of metal nanoclusters could be prevented (Fig. 4j). Different M-SACs were measured at −1.2 V to compare the effect of different central metals on the ECR performance. A “volcano curve” is displayed in Fig. 4k, showing that the appropriate interaction between intermediates and Ni atoms makes Ni-SACs the most suitable candidate for ECR compared with other as-prepared M-SACs.
Fig. 4 (a–i) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for M-SACs. Scale bar: 5 nm. (j) XRD patterns for the M-SACs. (k) FECO for M-SACs at −1.2 V. Reproduced with permission.47 Copyright (2019) Springer Nature. |
Efforts have been devoted to heteroatom doping (such as N, O, and S) in the carbon lattice to improve the catalytic performance.48 Furthermore, the N dopant could be incorporated into carbon via pyrolysis of N-containing compounds (such as dicyandiamide,47 melamine49 and urea50) with carbon matrix (such as graphene,51 carbon black,50 and graphdiyne52), and products with different architectures (such as nanotubes,53 nanosphere,54 and nanosheets53) can be obtained. The introduction of N can effectively stabilize single-atomic centres for N-doped carbon-based SACs.55,56 For example, Xie et al. enabled the activation and protonation of ECR through anchoring Snδ+ atoms on N-doped graphene.57 Furthermore, they performed in situ Fourier transform infrared (FT-IR) spectroscopy coupled with computational analysis and found that the N dopant could enhance the rate-determining HCOOH desorption step by decreasing the Gibbs free energy and elongating the bond length of Sn-HCOO− (Fig. 5a and b). Their experimental results agreed with the theoretical predictions. The as-prepared catalysts exhibited an onset overpotential below 60 mV and robust stability of more than 200 h (Fig. 5c), whereas their turnover frequency (TOF) was up to 11930 h−1 (Fig. 5d).
Fig. 5 (a) In situ FT-IR spectra of Sn-SACs. (b) Free energy diagrams. (c) Chronoamperometry tests at −1.6 V vs. SCE. (d) FEHCOOH and TOF of the Sn-SACs. Reproduced with permission.57 Copyright (2019) Wiley-VCH. |
Li et al. found that the metal-N site could serve as the active center.54 They computationally showed that the as-prepared Co–N5 is the rate-determining center for the transformation from *CO2 to *COOH (Fig. 6a and b), which is a crucial process for CO desorption. The obtained CoN5 coordinated on hollow N-doped porous carbon spheres (Co–N5/HNPCSs) achieved a satisfactory FECO of approximately 90% (Fig. 6c and d).
Fig. 6 (a) Schematic diagram of ECR reaction process on M–N5/HNPCSs. (b) Calculated free energy. (c) FE of M–N5/HNPCSs. (d) FECO of Co–N5/HNPCSs-T (T = 400, 600, 800, and 1000 °C). Reproduced with permission.54 Copyright (2018) American Chemical Society. |
Although N can contribute to ECR through pyrrolic N, pyridinic N or graphitic N. In general, the dominant contribution is derived from metals sites.58,59 Strasser et al. found that once with the participation of metals, CO is further reduced to methanol and/or methane, indicating that the metal centers are dominant for the further reduction of intermediates during ECR by conducting controlled tests with or without metal (Fe or Mn) centers.60
Unsaturated coordination is also an important ECR performance improvement strategy for carbon-supported SSCs.61,62 The bonding of common transition metal-N-doped carbon SACs to the reaction intermediates during the ECR process is relatively weak owing to the saturated coordination environment. Therefore, properly adjusting the coordination number to form an unsaturated coordination state helps improve the adsorption capacity of intermediates. Feng et al. proposed novel Cu–N2 SACs on an ultrathin graphene matrix with unoccupied 3d orbitals of the central metal.63 A reduction in the N coordination number shortens the Cu–N bond length. Meanwhile, electron transfer from Cu–N2 to *CO2 is accelerated, thus promoting the hydrogenation of *CO2 as well as the Zn–CO2 battery performance.
Defect engineering also functions as an effective route to avoid the migration of isolated metal atoms and optimize the ECR process of carbon-based SACs.53,61 Particularly, defects not only change the coordination environment but also modify the surrounding electronic structure, thereby forming unsaturated coordination sites or vacancies. These as-obtained coordination sites or vacancies are effective for anchoring metal atoms. In addition, carbon defects can be engineered to anchor metal atoms through electron transfer between carbon and metal atoms for carbon-based materials. These carbon defects usually result in a negative charge on the surface of the carbon material, which is helpful for the uniform adsorption of metal cations.64,65 Furthermore, Wang et al. designed graphene oxide with high-density defects, which made the surface full of negative charges and helped to uniformly anchor a monolayer of Ni cations (Fig. 7a–c).66 The density of single atom active sites could be increased to achieve high catalytic activity: a nearly 95% CO selectivity at 550 mV over-potential with the assistance of carbon defects (Fig. 7d and e). Furthermore, they found that by anchoring other transition metal (TM) single atoms (Co, Mn, etc.) into layered graphene vacancies (M-NG), the HER kinetics could be effectively suppressed, and suitable CO binding could be realized.
Fig. 7 (a–c) Structure characterization of Ni–NG; (d) FEH2 and FECO and (e) the linear sweep voltammetry (LSV) curves of Ni–NG. Reproduced with permission.66 Copyright (2018) Royal Society of Chemistry. |
Through the design of carbon defects, the coordination number of metals can be regulated, thereby achieving the goal of anchoring more metals species with different characteristics.63,67 Shui et al. first anchored the rare-earth metal atoms Y and Sc in large carbon defects (M/NC) by adjusting the coordination anion.67 Furthermore, six M–N/M–C coordination bonds are required for anchoring each metal atom because of the large radii of Y and Sc (Fig. 8). Therefore, the catalyst exhibited a different coordination structure from that of MN4, and it exhibited excellent performance in the nitrogen reduction reaction (NRR) and ECR.
Fig. 8 (a) Structural diagram of type A and type B substrates with small-size and large-size carbon defects, respectively. (b) Calculated adsorption energies of different active sites of Y/NC. (c) FECO and FEH2 of Y/NC and Sc/NC from −0.58 V to −0.88 V. Reproduced with permission.67 Copyright (2020) American Chemical Society. |
MOF-derived SACs are another type of carbon-based SACs. Recently, MOFs have already received extensive attention due to their adjustable structure, size, and tunable active sites.68–70 Furthermore, they inherit advantages from MOFs included rich three-dimensional (3D) channels and large surface areas, which are beneficial for electrolyte wettability and acceleration of mass diffusion. In addition, a favourable electronic environment of the active metal sites can be obtained in MOF-derived SACs owing to the coordination of various heteroatoms from ligands, which is beneficial for ECR performance. The most common synthetic strategy for this type of material is using a zinc-based zeolitic imidazolate framework (ZIF-8) with rich N atoms as the supporting material. After one-step pyrolysis, Zn2+ is reduced to Zn and evaporated owing to its low boiling point, leaving abundant free N atoms to stabilize other high-boiling-point metals, such as Fe,71 Co,72 Ni,73 Mn,71 and Cu.74
There are two common strategies for the synthesis of SACs from ZIF-8. The first method involves directly mixing zinc salt with other metal sources and ligands in a system followed by calcination at a high temperature to obtain SACs.75,76 The second method is to use the as-obtained ZIF-8 as a sacrificial template to mix with other metal species and form a uniform composite by ball milling,77 electrospinning,74 or other treatment,78 and removing the Zn species through high-temperature pyrolysis. Furthermore, Jaouen et al. designed an atomically dispersed metal bonded to N atoms (metal-Nx) with a similar coordination environment by ball milling and pyrolysis of ZIF-8 and M2+ acetate (Fig. 9a).77 As shown in Fig. 9b, a volcano trend was observed between the different metal centers and their activities toward CO formation. Operando X-ray absorption near-edge structure spectroscopy (XANES) and density functional theory (DFT) were performed to monitor the evolution of the active sites during ECR. The valence states of Co and Mn remained unchanged during ECR. However, Fe, Ni, and Cu were partially or completely reduced (Fig. 9c). The DFT results indicated that M2+N4–H2O played the most important role in Fe- and Co-based catalysts, whereas Ni1+N4 was considered to be the intrinsic active sites in Ni samples at 0.5–0.6 V (Fig. 9d).
Fig. 9 (a) K-edge EXAFS spectra of MNC. (b) Partial current densities (Jp) over the MNC catalysts for different products. (c) K-edge XANES spectra of MNC before (solid curve) and after (dashed curve) the chronoamperometry test at different potentials. (d) CO jp at −0.6 V for other MNC and computational trendency (U = −0.6 V). Reproduced with permission.77 Copyright (2019) American Chemical Society. |
Fig. 10 (a–c) Structural diagrams of Cu doped CeO2, with 1–3 Vo. (d and e) CO2 adsorption and activation structural diagrams of these structure. Reproduced with permission.87 Copyright (2018) American Chemical Society. |
Fig. 11 Free-energy profiles for ECR on (a) TiC and (b) TiN. Density of states of (c) TiC, (d) Ir, and (e) Ir@d-TiC interacting with *CO. (f) The electron density isosurfaces at noted panels of (c–e). Reproduced with permission.88 Copyright (2017) American Chemical Society. |
MoS2 is a two-dimensional (2D) layered material with many application scenarios.91,92 Various single atoms anchored on MoS2 also have good application prospects in ECR.93,94 It should be noted that MoS2 could also be employed directly for ECR, and its edges are more active than its inert surface.93,95 Activating the surface of MoS2 by anchoring various single-atomic metals could also be a good choice for enhancing the ECR performance because the stability of edges is usually limited by the edge structure and reaction conditions.96,97 By changing the mass loading of Pt, Zeng et al. prepared Pt monomers on MoS2 with different percentages of isolated Pt and neighboring Pt (Fig. 12a–c).96 These two kinds of monomer Pt with different coordination environments showed completely different selectivities towards ECR. Pt single atoms prefer the conversion of CO2 into CH3OH instead of HCOOH. However, CO2 is easily hydrogenated into HCOOH and CH3OH for neighboring Pt atoms (Fig. 12d and e).
Fig. 12 (a) Pt K-edge XANES spectra and corresponding R space (b) for different Pt/MoS2. (c) Bar graph showing the different Pt monomer contents of Pt/MoS2 samples based on HAADF-STEM results. (d and e) Steps for the hydrogenation process of Pt1/MoS2 and Pt2iii/MoS2. Reproduced with permission.96 Copyright (2018) Springe Nature. |
Although SACs have a number of advantages, they also face some challenges: (1) materials that can be applied as precursors are still limited. As for MOF-derived SACs, the common MOFs used are limited to ZIF-8, UiO-66-NH2, MIL-101-NH2, and ZIF-67, and the supported single metal atoms are restricted to Fe, Co, Ni, Cu, Bi, etc. Thus, efforts should be dedicated to exploring additional supporting substrates, precursors, and metal species. (2) High-temperature treatment is typically required to obtain SACs. The metal loading is limited to avoid the agglomeration of metal atoms in a high-temperature environment. Therefore, the development of novel large-scale tactics to prepare SACs with high single-metal loadings is very important. (3) Morphology regulation is an important method to improve ECR activity. However, the morphologies of SACs synthesized are usually irregularly shaped particles after high temperatures treament. The design of better-defined morphologies remains a challenge. (4) Their structural characterization techniques are often limited to particular methods such as X-ray absorption spectroscopy and spherical aberration-electron microscopy because of the low loading of single atoms and their special structural properties. Therefore, it is important to develop new characterization methods for SACs.
Modifying the surface of HMCs is an important strategy to alleviate the aggregation issue.108,109 HMCs are more prone to agglomeration in aqueous solutions than in organic solutions because of their special synthesis environment. This reunion further blocks or severely hinders the mass transfer process around the active sites, thereby reducing the stability decay and current density. This problem can be addressed by introducing hydrophilic groups. Furthermore, Officer et al. effectively improved the solubility of the catalyst by introducing alkoxy groups to CoPc (CoPc-A) (Fig. 14a), which prevented its agglomeration. However, the inherent activity of the catalyst was greatly enhanced (TOF of ∼5 s−1 at an overpotential of 0.480 V). In addition, the as-prepared catalysts showed stable CO conversion for more than 30 h (Fig. 14b–d).108
Fig. 14 (a) Structural diagram of CoPc-A. (b) Total current density (Jt) of chemically converted graphene/CoPc-A (CCG/CoPc-A) and CCG/CoPc hybrids. Stability test of (c) CCG/CoPc and (d) CCG/CoPc-A at same potential (−0.69 V). Reproduced with permission.108 Copyright (2019) American Chemical Society. |
Using a water-soluble HMC-modified electrode also alleviates the aggregation of the catalysts. Wang et al. effectively suppressed HER in the aqueous solution by depositing water-soluble 1,10-phenanthroline–Cu (phen–Cu) molecular complexes on a mesoporous graphene electrode.109In situ attenuated total reflection infrared (ATR-IR) study revealed that Cu molecules would heterogenize near the electrode during the test, resulting in an increase in electron density under catalytic conditions (Fig. 15a and b). Furthermore, they found that the phen–Cu surface structure of the graphene electrode affected charge distribution during the ECR process based on the infrared signal. The change in the applied potential confirms that the external electric field is crucial for the reversible heterogenization of Cu molecules (Fig. 15c). In addition, more electrons would aggregate on the ligands, owning to the reduction of the potential, thus leading to a blue shift in the IR spectra (Fig. 15d). Furthermore, the mesostructure of the graphene matrix limits mass transfer from the bulk solution to the electrode, thus suppressing the HER and greatly improving the selectivity of ECR. The modified catalysts exhibited enhanced TOF (45 s−1 at −1.0 V) and high FECO around 90% at −0.6 V. These results emphasize that the modification of the electrode surface with a suitable molecular catalyst can also achieve high ECR selectivity.
Fig. 15 (a and b) In situ Raman spectrum tests of Ni–TAPc on gold electrode under (a) Ar and (b) CO2. Electrochemical performance: (c) FECO and Jt at different potentials acquired on a carbon cloth electrode in CO2-saturated 0.5 M KHCO3 solution. (d) TOF of Ni–CNT–CC (Ni–CNT represents Ni–TAPc anchored on carbon nanotubes) compared with other CO2-to-CO catalysts. Reproduced with permission.109 Copyright (2018) Wiley-VCH. |
The reduction and demetallation of center metals are common side reactions of HMCs during the ECR process.110–112 Actually, proper metal reduction could be helpful to the activation of ECR. Based on operational spectroscopy and electrochemical kinetics studies, Liu et al. found that Ni+ produced by in situ reduction of Ni2+ in Ni(II) 2,9,16,23-tetra(amino) phthalocyanine (Ni–TAPc) is very active for ECR activation and can act as the intrinsic catalytic sites for ECR.113 When the cathode potential was increased above 0.57 V under Ar atmosphere, a red-shift occurred for the Ni–N vibration in Ni–TAPc. Electrons were transferred to the Ni 3d orbital during the test, which led to the reduction of Ni2+ to Ni+ (Fig. 16a). Once CO2 was pumped into the system, the vibration of Ni–N will return to its pristine state (Fig. 16b), and no longer change with the changing cathode potential (from open circuit potential to 0.37 V). The quick oxidation of Ni+ is believed to be more helpful than that of Ni2+ for the activation of CO2 because it overlaps better with the C 2p* orbital in CO2. Furthermore, the developed Ni SSCs showed high ECR activity with a FECO of 99% and TOF of 100179 h−1 (Fig. 16c and d).
Fig. 16 (a and b) Raman spectra of Ni–TAPc collected on an Au electrode at various potentials under (a) Ar (1.0 atm) and (b) CO2 (1.0 atm). Electrochemical performance: (c) FECO and Jt at different potentials acquired on a carbon cloth electrode in CO2-saturated 0.5 M KHCO3 solution. (d) The TOF of Ni–CNT–CC (Ni–CNT represents Ni–TAPc anchored on carbon nanotubes) compared with those of other CO2-to-CO reduction catalysts. Reproduced with permission.113 Copyright (2020) Wiley-VCH. |
However, the irreversible metal reduction or demetallation reaction can easily lead to the formation of large-size metal particles, which would lead to the decay of the catalytic activity. Wang et al. designed a controlled ECR experiment with three Cu complexes: CuPc, Cu-based HKUST-1, and [Cu(cyclam)]Cl2 (Fig. 17a–c).110 They found that the CuPc exhibited the highest activity and selectivity among all catalysts; its partial current density for methane and FECH4 reached 13 mA cm−2 and 66% at −1.06 V, respectively (Fig. 17d and e). Additionally, the CuPc exhibited the highest activity and selectivity among all the catalysts. It showed high partial current density for methane (13 mA cm−2) and FECH4 (∼70%), respectively (Fig. 17d and e). Operando XANES and extended X-ray absorption fine structures (EXAFS) were applied to probe the reconstruction of the local coordination environment during the ECR. However, during the ECR process, the CuPc molecules underwent reconstruction and produce nearly 2 nm metallic Cu clusters. Additionally, the Cu nanocluster quickly reverted to the pristine structure after applying the negative electrode potential (at 0.64 V) (Fig. 17f–h). On the contrary, for [Cu(cyclam)]Cl2 and HKUST-1, the complexes decomposed and agglomerated into larger Cu particles, which also explains their poor ECR activities. They applied DFT to examine the thermal kinetics of the reductive demetalization and recovery of the CuPc structure to better study the reconstruction process. The calculations show that the demetallization process has lower reduction potential than those of the other two catalysts. This indicates that CuPc has the most stable structure among all Cu-based catalysts. Thermodynamic calculations also revealed that the binding affinity of the metal ions of the copper complex with the ligands affects the threshold potential and reversibility of the reductive demetallization process, influencing the ECR performance further.
Fig. 17 Structural diagram of (a–c) three Cu complexes. (d) Potential-dependent FE and (e) jp of products for ECR catalyzed by CuPc. Fitted (f) R-space and (g) k-space EXAFS spectra of CuPc. (h) First-shell Cu–Cu coordination numbers of CuPc from −1.5 to −0.65 V. Reproduced with permission.110 Copyright (2018) Springer Nature. |
For HMCs, their ligands can be easily fine-tuned during the synthesis process, giving them greater controllability and advantages in performance and model establishment. Their good solubility in organic solvents and water also expands their application scenarios, though they still face many challenges: (1) the low loading capacity of HMCs makes them insensitive to traditional characterization methods. However, HMC must prioritize selectivity and stability. (2) The HMCs currently used in the ECR direction are limited to bipyridine, porphyrin, and phthalocyanine derivatives. Thus, the development of HMCs with novel ligands and product selectivity is critical. (3) The methods for HMC immobilization are currently limited (such as π–π interactions, physical confinement, and covalent bonding). Stability is another big challenge; the choice of immobilization needs to consider the problems of catalyst leaching, demetallation, aggregation, and steric hindrance of ligands. Furthermore, it will be necessary to further develop assembly technology and introduce more kinds of ligands into the skeleton to help improve the diversity and mechanism research of molecular catalysts in the future.
Fig. 18 (a) Schematic diagram of TCPP(Co)/Zr-BTB. (b) FECO and (c) CO Jp for all catalysts at different applied voltages. (d) FEH2 and (e) FECO for PABA-, PSBA-, and PSABA-modified TCPP(Co)/Zr-BTB at different potentials. Reproduced with permission.18 Copyright (2019) Wiley-VCH. |
Using an external electric field to promote the coordination of molecules with single metal sites and MOFs is also an effective method to prepare SSCs located on MOF nodes. Furthermore, Joseph et al. realized the heterogeneous electrochemical conversion of carbon dioxide (∼100% FE(CO+H2)) to fuel under high flux conditions, by electrophoretic deposition of a large number of Fe-porphyrin (Fe-TPP) based MOF-525 (Fe_MOF-525) on fluorine-doped tin oxide (FTO), (Fig. 19a).22 Additionally, they achieved a high concentration of Fe-TPP adsorption, equivalent to approximately 900 layers of Fe-TPP adsorbed on the FTO surface by selecting appropriate MOFs (Fig. 19b). In addition to the introduction of Fe single-site molecular catalysts, the ECR selectivity of the original electrodes could be optimized. Moreover, the abundant nanoscale porosity of MOFs is also conducive for solvents, reactants, and electrolytes to access catalytic sites. Fe-TPP anchored on the metal nodes of MOF-525 serves as both an electrocatalyst and a redox-hopping conduit for transporting reduction equivalents to catalytic sites beneath the surface of the electrode.
Fig. 19 (a) FE over approximately 4 h of bare FTO, Fe_MOF-525 with/without 2,2,2-trifluoroethanol (TFE). (b) Proposed ECR mechanism on the Fe porphyrin-based MOF. Reproduced with permission.22 Copyright (2015) American Chemical Society. |
Fig. 20 (a) Schematic diagram of TPY-MOL-CoPP and the synergetic role of two different moieties (CoPP and TPY) for ECR. (b) FECO and FEH2 as well as (c) HER current density for TPY- and BTB-MOL-CoPP. Reproduced with permission.122 Copyright (2019) American Chemical Society. |
Lan et al. synthesized polyoxometalate–metalloporphyrin organic frameworks (PMOFs) coordinated by M-TCPP linkers (tetrakis[4-carboxyphenyl]-porphyrin-M) and reductive Zn-ε-Keggin clusters via a hydrothermal method.123 The obtained PMOFs had the general formula [PMoV8MoVI4O35(OH)5Zn4]2[M-TCPP][2H2O][1.5TBAOH] (M = Fe, Co, Ni, and Zn, TBAOH = tetrabutylammonium hydroxide) (Fig. 21a). In this structure, the four Zn-ε-Keggin chains were assembled by M-TCPP via the coordination connection between the carboxyl group and Zn clusters. M-TCPP and Zn clusters both served as electron donors and ECR active sites (Fig. 21b). The PMOFs demonstrated excellent performance in ECR, especially for the Co-PMOF, achieving a high FECO of 99%, a high TOF (nearly 1700 h−1), and robust stability over 35 h (Fig. 21c).
Fig. 21 (a) Structural diagram of M-PMOFs. (b) Proposed ECR mechanism on Co-PMOF. (c) FECO of M-PMOFs. Reproduced with permission.123 Copyright (2018) Springer Nature. |
Fig. 22 (a) The comparison of MCp2@MOF and MOF in ECR. (b) LSV and (c) FECO for MCp2@MOF-545-Co. Reproduced with permission.21 Copyright (2020) Elsevier Ltd. |
SSCs anchored on MOFs have various adjustable properties because of their adjustable structure and porosity. Furthermore, they have become an important research topic for ECR. However, they also face many challenges: (1) the introduction of single-site active centers in MOFs, either in the pores or anchored on the metal nodes, may hinder the mass transfer process. (2) It is difficult to achieve precise control of catalysts and high single-site metal loading because of the mutual restriction of MOFs and single-site active centers. (3) It is difficult to achieve precise positioning and structural analysis of single-site catalysts because of the limited characterization technology. Therefore, efforts should be dedicated to developing more in situ and ex situ spectroscopic characterizations as well as theoretical simulations and calculations. Only in this way can we realize the precise control of SSC-anchored MOFs and unlock their great potential for ECR.
By incorporating metal macrocyclic units (such as porphyrin,127 phthalocyanine128etc.), the advantages of both molecular (high ECR selectivity) and heterogeneous catalysts (stable in water) can be achieved simultaneously in the obtained COFs. For instance, by using imine bonds to link cobalt porphyrin catalysts and organic struts, Yaghi and his co-workers reported a modular optimization strategy of COFs for ECR (Fig. 23a).129 The covalent coordination of cobalt porphyrin within a COF was shown to affect the ECR mechanism in electrochemical experiments. The as-obtained COF-367-Co catalyst showed a high FECO (90%), a high turnover number of nearly 300000, and an impressive TOF of approximately 9400 h−1 under neutral conditions (Fig. 23b and c).
Fig. 23 (a) Design of metalloporphyrin-based COFs. (b) Long-term electrolysis at −0.67 V and gas production of different Co COFs catalysts. (c) Stability test and TON of Co COFs catalysts at −0.67 V. Reproduced with permission.129 Copyright (2015) American Association for the Advancement of Science. |
COFs constructed by amine bonds have also received increasing attention for ECR applications because of their robust stability in aqueous electrolytes. Deng et al. synthesized COF-300-AR and COF-366-M-AR with amine linkages with both 3D and 2D structures by reducing the COFs with imine linkers.24 Spectroscopic results confirmed that the synergetic effect of the COFs and the Ag electrode at their interface was responsible for the enhanced ECR performance (Fig. 24a). In addition, the amine linkage showed a robust structure in 6 M HCl and 6 M NaOH (Fig. 24b). In particular, the COF-300-AR deposited on Ag electrode promoted the ECR with a FECO of 80% at −0.85 V and 53% at −0.7 V (Fig. 24c).
Fig. 24 (a) Schematic diagram of ECR at the interface of electrode. (b) XRD of COFs samples in HCl or NaOH solution. (c) FECO of the different COFs electrodes at −0.70 V and −0.85 V. Reproduced with permission.24 Copyright (2018) Elsevier Ltd. |
In general, COF-supported SSCs have unique advantages for studying the structure–performance relationship because of their regular pores, adjustable pore sizes, high surface areas, and high adjustability. The activity and selectivity of ECR can also be enhanced by introducing different building blocks into the COFs. However, COF-supported SSCs also face many challenges: (1) the synthesis and collection of COFs are complicated and costly, which limits their large-scale synthesis. In addition, the good dynamic covalent nature of CN and B–O would also result in stability issues, limiting their application under harsh conditions. (2) The conductivity of the COFs is in the range of insulators and semiconductors. Further improvement in its electrical conductivity is of great significance for electrocatalysis. (3) The current ECR performance of COF-supported SSCs remains limited. Therefore, it is desirable to improve intrinsic ECR performance and selectivity. More research is needed to determine the exact structure-ECR performance relationship of COF-supported SSCs, which is beneficial for further performance optimization. As technology grows by leaps and bounds, COF-supported SSCs with high ECR performance may play a more significant role in the future.
Catalyst | Main product | Maximum FE | Reaction condition | Ref. |
---|---|---|---|---|
Note. a, b, c and d indicate that catalysts are SACs, HMCs, MOF-supported SSCs and COF-supported SSCs, respectively. jp means partial current density, and jtol means total current density. | ||||
aBi SACs | CO | 97% (−0.5 V, jp = 3.9 mA cm−2) | H-cell, 0.1 M NaHCO3 | 130 |
aCo SACs | CO | 97% (−0.6 V, jp ≈ 22.5 mA cm−2) | H-cell, 0.1 M KHCO3 | 131 |
aNi SACs | CO | ∼99% (−0.681 V, jp = 100 mA cm−2) | H-cell, 0.5 M KHCO3 | 50 |
aPd–NC | CO | 55% (−0.5 V, jp ≈ 0.57 mA cm−2) | H-cell, 0.5 M NaHCO3 | 132 |
aCu SACs | CH3OH, CO | 44% (methanol, −0.9 V, jtol ≈ 90 mA cm−2); 56% (CO, −0.9 V, jtol ≈ 90 mA cm−2) | H-cell, 0.1 M KHCO3 | 47 |
aNi-SAC-2.5 | CO | 98.9% (−1.2 V, jtol ≈ 7.5 mA cm−2) | H-cell, 0.1 M KHCO3 | 133 |
aFe–NC | CO | 93% (∼−0.6 V, jp = 2.8 mA cm−2) | H-cell, 0.1 M KHCO3 | 72 |
aCu–CeO2-4% | CH4 | ∼58% (−1.8 V, jtol = 70 mA cm−2) | H-cell, 0.1 M KHCO3 | 54 |
bCuPc | CH4 | ∼66% (−1.06 V, jp = 13 mA cm−2) | H-cell, 0.5 M KHCO3 | 134 |
bNiTAPc | CO | 99% (∼−0.74 V, jtol = 32.3 mA cm−2) | H-cell, 0.5 M KHCO3 | 135 |
bCoPc-Pyr | CO | 95% (−0.7 V, jtol = 2.5 mA cm−2) | H-cell, 0.05 M K2CO3 | 136 |
bCr-bipyridine based HMCs | CO | ∼96% (−2.1 V vs. Fc+/Fc) | H-cell, 0.1 M TBAPF6/DMF, 0.62 M PhOH | 137 |
bMn-bipyridine based HMCs | HCOOH | 90% (−1.77 V vs. Fc+/Fc) | H-cell, 0.2 M Bu4NBF4/MeCN, 2.0 M TFE | 138 |
bNi-complexes | CO | 87% (−2.44 V vs. Fc+/0) | 3-Neck pear-shaped glass cell, 0.1 M Bu4NBF4/CH3CN | 139 |
bRe-bipyridine based HMCs | CO | 81% (−2.8 V vs. Fc0/+) | H-cell, MeCN (0.1 M [nBu4N][PF6]) | 140 |
bCu-porphyrin based HMCs | CH4+ C2H2 | 44% (−0.976 V, jp = 13.2 + 8.4 mA cm−2) | H-cell, 0.5 M KHCO3 | 141 |
cCu2(CuTCPP) | HCOOH | 68.4% (−1.55 V vs Ag/Ag+, jp ≈ 3 mA cm−2) | H-cell, 1 M H2O and 0.5 M EMIMBF4 | 142 |
cCo-MOF | CO | 98.7% (−0.8 V, jp = 18.8 mA cm−2) | H-cell, 0.5 M KHCO3 | 123 |
cMOL-CoPP | CO | 92.2% (−0.86 V, jp = 1314 mA mg−1) | H-cell, 0.1 M NaHCO3 | 122 |
cCoCp2@MOF-545-Co | CO | 97% (−0.7 V jp = 15 mA cm−2) | H-cell, 0.5 M KHCO3 | 21 |
cTCPP(Co)/Zr-BTB-PSABA | CO | 85.1% (−0.769 V, jtol = 6 mA cm−2) | H-cell, 0.5 M KHCO3 | 143 |
cPcCu–O8–Zn | CO | 88% (−0.7 V, jp = 2.5 mA cm−2) | H-cell, 0.1 M KHCO3 | 143 |
dCOF-366-Co | CO | 90% (–0.67 V, jp ≈ 80 mA mg−1) | 0.2 M K2HPO4 buffer, 0.5 M KHCO3 | 23 |
dCo-porphyrin based COF | CO | 95% (−0.7 V, jp ≈ 4.5 mA cm−2) | H-cell, 0.5 M KHCO3 | 127 |
dCOF-300-AR | CO | 80% (−0.85 V) | H-cell, 0.1 M KHCO3 | 24 |
dCo-porphyrin based COF | CO | 95% (−0.6 V) | 3-Compartment cell, 0.5 KHCO3 | 144 |
dCOF-366-Co | CO | 87% (−0.67 V, 65 mA mg−1) | H-cell, 0.5 KHCO3 | 145 |
dFe-porphyrin based COF | CO | ∼80% (−2.2 V vs. Ag/AgCl) | One-compartment cell, 0.5 M TFE/MeCN | 146 |
Footnote |
† W. H., J. Z. and S. L. contributed equally to this work. |
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