Atomically Thin Two-Dimensional Solids: An Emerging Platform for

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Atomically-Thin Two-Dimensional Solids: An Emerging Platform for CO2 Electroreduction Wentuan Bi, Changzheng Wu, and Yi Xie ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01343 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Energy Letters

Atomically-Thin Two-Dimensional Solids: An Emerging Platform for CO2 Electroreduction Wentuan Bi, Changzheng Wu*, and Yi Xie* Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, China.

ABSTRACT

Benefiting from the high specific surface area and the ensuing novel electronic structures, atomically-thin two-dimensional (2D) solids have drawn intense interest in the field of carbon dioxide (CO2) catalysis. Here we review the major advantages of atomically-thin 2D solids for electrochemical CO2 reduction and special emphasis will be paid to the recent advances in fine characterization and controllable tailoring of the local atomic and electronic structure. Since surface atoms in atomically-thin 2D solids are comparable to the overall atoms, we highlight surface modifications, such as molecular functionalization, heteroatom incorporation, and defect engineering as effective ways to manipulate the reactivity. Finally, the major challenges and opportunities for future development of this field are discussed. As a well-defined model system,

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atomically-thin 2D solids offer practical possibilities to study the structure–catalytic activity relationship at the atomic level.

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Global warming caused by excessive emissions of greenhouse gases, especially CO2, has become a widespread concern in recent years. Despite numerous efforts devoted to developing renewable, carbon-neutral energy sources, fossil fuels are predicted to remain the dominant energy source in the near future. Therefore, removing excess CO2 from the atmosphere, particularly converting it to high value-added chemicals is crucial to achieving energy and environmental sustainability.[1] To this end, various CO2 conversion methods have been proposed and intensively investigated in the past decades.[2-4] Among these methods, electrochemical CO2 reduction reaction (CO2RR) is particularly promising in terms of its operation under ambient temperature and pressure conditions. Such a process also facilitates the coupling of intermittent electricity generated from renewable energy.[5] However, due to the chemical inertness of CO2 molecules, this technique is still subject to the high reaction barriers and the competing proton reduction reaction.[6,7] Therefore, efficient electrocatalysts with high selectivity toward a single product are urgently demanded to promote the practicality of CO2RR. The development of electrocatalysts for selective CO2RR has gone from the bulk metals to their nanostructures and then to the emerging atomically-thin 2D solids. Early studies on electrochemical CO2RR mainly focus on polycrystalline bulk metals and have established a primary classification based on the selectivity of reduction products.[8] For example, noblemetal catalysts, especially Au and Ag, show high selectivity for CO evolution; p-block metals such as Sn, Bi, and In prefer to convert CO2 into formic acid; while Cu is capable of reducing CO2 to hydrocarbons. In recent years, nanostructured metals, alloys and their oxides, chalcogenides have further improved the CO2RR performance because they possess several attractive features over bulk catalysts. The most immediate advantage is that nanostructured


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materials usually provide much more surface active sites because of their high specific surface area.[9,10] Nevertheless, it should be noted that the concomitant variety of active sites presents a substantial challenge to improve the selectivity towards CO2RR.[11] In contrast, molecular catalysts with uniform active sites can be systematically optimized to achieve both high activity and high selectivity, yet generally, suffer from performance degradation in aqueous solution. Intriguingly, as the thickness is reduced to the molecular or even atomic scale, surface atoms become comparable to the overall atoms. Accordingly, atomically-thin materials exhibit significant molecular adjustability.[12,13] In particular, as shown in Figure 1, atomically-thin 2D solids with size scalability and molecular adjustability are considered as an ideal model system combining the advantageous features of both molecular and heterogeneous catalysts, thus drawing intense attention in the field of catalysis.[14-16]

Figure 1. Atomically-thin 2D solids fill the gap between theoretical models and real catalysts in terms of their well-defined structure and molecule-like adjustability. The image of atomically-


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thin 2D solids was reproduced with permission from ref 9, Copyright 2011, Nature Publishing Group. The image of the bulk was reproduced with permission from ref 10, Copyright 2017, American Chemical Society. The image of nanomaterials was reproduced with permission from ref 11, Copyright 2013, American Chemical Society. The image of cluster/molecule was reproduced with permission from ref 13, Copyright 2014, Nature Publishing Group. With great efforts devoted to atomically-thin 2D solids, continuous advances have been made in synthetic methodologies and fine characterization technologies over the past decade.[17-20] The advances in these controllable synthetic strategies enable the desired structural features of 2D solids, providing sufficient scope for the design of high-efficient catalyst. Meanwhile, the development of fine characterization techniques provides the solid foundation for obtaining a clear structure-activity relationship.[14] Considering the lack of long-range order of atomicallythin 2D solids in the third dimension, conventional techniques such as X-ray diffraction (XRD) are unable to identify the spatial atomic distribution. Of note, X-ray absorption fine structure (XAFS) spectroscopy, a highly sensitive technique with elemental specificity, is widely applied to acquire the local atomic structures and chemical states. In addition, owing to the nonnegligible role of defects in catalysis, fine characterization techniques probing various defects in atomically-thin 2D solids, such as aberration-corrected electron microscopes,[20] scanning tunneling microscopy (STM),[21] positron annihilation spectroscopy,[22] and electron spin resonance spectroscopy[23] have been successfully developed. The combination of these technologies has provided the premise for revealing the structure-activity relationship of atomically-thin 2D solids in CO2RR. Here we discuss the major advantages and opportunities of atomically-thin 2D solids for electrochemical CO2RR. Special emphasis will be paid to recent advances in the controllable


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tailoring of their local atomic and electronic structure via molecular functionalization, heteroatom incorporation, defect engineering and so on. In the last section of this perspective, we also present the major challenges and opportunities for future development of this field in terms of both fundamental understanding and industrial applications. We anticipate that atomically-thin 2D solids as a well-defined model system offer practical possibilities to study the structure– catalytic activity relationship at the atomic level.

Figure 2. (a) CO2 adsorption isotherms for the single-unit-cell Bi2WO6 layers and bulk Bi2WO6. Reproduced with permission from ref 27, Copyright 2015, Wiley-VCH. (b) Temperatureprogrammed desorption spectra of CO2 over ultrathin SnNb2O6 nanosheets and bulk SnNb2O6. Reproduced with permission from ref 28, Copyright 2016, Royal Society of Chemistry. The adsorption of CO2 on active sites is the prerequisite for the subsequent catalytic reduction reaction. However, electrocatalytic CO2RR generally takes place at the gas-solid-liquid threephase interface where the mass transport is significantly subject to the low solubility of CO2 in the working electrolyte. Thus increasing the local concentration of CO2 is considered as a straight-forward way to realize high efficiency.[24,25] For example, Sargent and co-workers recently reported that nanoneedle-like metallic electrodes produced high local electric fields that concentrate electrolyte cations, which in turn concentrate CO2 locally and enhance the reaction


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ACS Energy Letters

rate.[26] Intriguingly, owing to the high specific surface area and unsaturated surface atoms, atomically-thin 2D solids generally possess high CO2 adsorption capacity. As shown in Figure 2a, CO2 adsorption isotherm on single-unit-cell Bi2WO6 layers depicts that CO2 adsorption capacity at 298 K and 1 atm could reach 22.7 mg g−1, over 3-times higher than that of bulk Bi2WO6 (7 mg g−1).[27] Temperature-programmed desorption (TPD) of CO2 was also used to evaluate the interaction between CO2 and the catalysts. In the case of ultrathin SnNb2O6 nanosheets (Figure 2b), the substantially enhanced intensity of desorption peaks at about 100 °C and 400 °C relative to that of bulk SnNb2O6 indicates a higher CO2 adsorption capacity as well as an increased number of basic sites.[28] Benefiting from this advantage, atomicallythin 2D solids enable CO2 enrichment within a local environment and therefore improve mass transport, eventually promoting CO2 reduction.

Figure 3. (a, b) The charge-density wave of the bulk and single-layered Fe7S8, respectively. (c) Density of state (DOS) for the bulk and single-layered Fe7S8. (d) Comparison of temperaturedependent electronic resistivity of Fe7S8 nanosheets and bulk sample. Reproduced with permission from ref 33, Copyright 2017, American Chemical Society.


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For electrocatalysis applications, reducing the resistance of electron transport not only at the reactive interface but also between the electrocatalysts and the current collectors is critical to improving the catalytic performance. Right now three-dimensional current collectors have been widely used to lower the contact resistance. Nevertheless, the resistive loss caused by the long electron transport distance from the active site to the current collectors remains to be eliminated. To address this issue, enhancing the intrinsic electronic conductivity of electrocatalysts becomes an inevitable trend in the design of highly efficient and selective electrocatalytic systems. As we all know, metallic structures are characterized by delocalized valence electrons which are responsible for the high electronic conductivity of metals. Intriguingly, when confined to one dimension, electron couples strongly to the lattice, giving rise to plenty of novel electronic transport behaviors.[29,30] With the aid of theoretical simulations, we demonstrated that singlelayered metallic solids such as Ni3N,[31] NiSe2,[32] Fe7S8[33] show increased density of states near the Fermi level. As shown in Figure 3, the enhanced orbital hybridization of single-layered Fe7S8 relative to the bulk materials indicates that electrons are more delocalized in these atomic layers, which means a higher conductivity. As expected, the temperature dependence of resistivity measurements showed an approximately linear increase with the temperature, confirming the intrinsic metallic conductivity. Additionally, the values of resistivity for Ni3N and Fe7S8 nanosheets at room temperature are 9.9 ×10−5 Ω·m, 10.9 ×10−5 Ω·m, about 4 and 9 times lower than that of the corresponding bulk counterparts, respectively. Apart from these intrinsic metallic atomic layers, semiconducting nanosheets such as Bi2WO6,[27] Co3O4,[34] and layered hydroxides[35] also exhibit enhanced electronic conductivity due to the dimensional confinement.


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Figure 4. Facet matters. (a) Catalytic site distribution of Au nanoparticle vs nanosheet. (b) Free energy diagrams for electrochemical reduction of CO2 to CO, and (c) protons to hydrogen on Au (111) (yellow symbols), Au (211) (orange symbols), or a 13-atom Au cluster (red symbols). Reproduced with permission from ref 11, Copyright 2013, American Chemical Society. As is well-known, heterogeneous catalysis occurs on the surface and is thus closely dependent on their exposed facets which possess diverse atomic and electronic structures.[11,36,37] Especially, for electrocatalysts in CO2RR, facet effect not only affects the activity but also has a great impact on the selectivity. As shown in Figure 4, low-index surfaces, edge sites, and corner sites coexist in nanoparticles, yet their catalytic activity and selectivity are different. According to the theoretical calculation, Au (211) facet is expected to yield high CO selectivity due to the weak binding of H* relative to COOH*. On the contrary, Au13 clusters mimicking the corner sites exhibit increased affinity for H*, thus promoting H2 evolution.[11] Given these multiple active sites in nanoparticles, optimizing the selectivity remains a substantial challenge.


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Fortunately, with the thickness down to a few nanometers, atomically-thin 2D solids are considered to expose only a specific facet, thus providing an ideal platform to optimize the catalytic selectivity.[38] As a proof of concept, Liu et al. demonstrated that triangular Ag nanoplates require lower energy to initiate the formation of COOH*, which is the rate determining step, and showed the best CO selectivity of 96.8% with a much lower overpotential of 0.746 V, as compared to similarly sized Ag nanoparticles (65.4%, 0.846 V) and bulk Ag (57.2%, 0.946 V).[39] Likewise, Bi nanosheets formed via in situ electro-reduction of BiOCl exhibited a surprisingly high selectivity towards CO2 reduction, with the Faradic efficiency of formate up to 92% at −1.5 V (vs. SCE), which is much higher than that of commercial bismuth powder (55%).[40]

Figure 5. (a) 2D TMDs with covalent nature and electrical conductance break transition-metal scaling relations for CO2 electrochemical reduction. Reproduced with permission from ref 43, Copyright 2016, American Chemical Society. (b) CO formation turnover frequency (TOF) of WSe2 NFs, bulk MoS2, and Ag NPs in ion liquid electrolyte. (c) Overview of different catalysts’ performance at different overpotentials (η). Reproduced with permission from ref 5, Copyright 2016, AAAS.


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Catalysis, in principle, is governed by the interaction of the reactants and intermediates with the active sites. Previous theoretical studies on transition metals have revealed a linear scaling relationship between the adsorption energies of intermediates and products. In the case of CO2 reduction to CO, which requires a sequential two-electron reduction, the ideal catalyst should bind the *COOH intermediate strongly to promote the first proton-coupled electron transfer to the chemically stable CO2; while bind *CO weakly to facilitate the desorption process. However, as shown in Figure 5a, transition metals generally show a significant positive correlation between the binding energy of *COOH and the binding energy of *CO, posing a fundamental limitation to design high-efficient catalysts for CO2 electroreduction.[41] Recently, Nørskov et al. demonstrated that the active edges of TMDs such as MoS2 do not follow this linear scaling relation.[42] Furthermore, based on first-principles-based high-throughput screening, Kim and co-workers found that 2D metallic TMDs can entirely break the strong correlation between the adsorption energies of *COOH and *CO, opening up a new avenue for the design of CO2-to-CO conversion catalysts.[43] Salehi-Khojin et al. first experimentally demonstrated that MoS2 with Mo-terminated edges showed selective electrochemical reduction of CO2 to CO in an ionic liquid solution, with a maximum current density of 65 mA cm−2 at −0.764 V vs. RHE.[44] Furthermore, as the thickness is reduced to atomic scale, 2D TMDs exhibit remarkably improved catalytic performance (Figure 5b and 5c). In particular, WSe2 nanoflakes (NFs) reached a record current density of 330 mA cm−2 at the same overpotential.[5] Recently our group put forward an alloying strategy to further optimize the electrocatalytic performances of 2D TMDs.[45] The partially delocalized charge in MoSeS alloy monolayers not only help stabilize *COOH intermediate, but also facilitates the rate-limiting CO desorption step. As a result, MoSeS alloy monolayers exhibit a current density of 43 mA cm-2 at


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-1.15 V vs. RHE, approximately 2.7 and 1.3 times higher than that of MoS2 monolayers and MoSe2 monolayers, respectively. More importantly, as the thickness of 2D solids is reduced to the molecular or even atomic scale, surface atoms become comparable to the overall atoms. As such, engineering the impressionable surface provides a rational way to tune the intrinsic physical properties and the adsorption energies of reactant and/or intermediates, hence modulating the reaction kinetics.[12] Based on this principle, a series of surface modification strategies have been developed, including surface molecular functionalization, surface heteroatom incorporation, defect engineering and so on. These surface modification strategies successfully tailor the local atomic and electronic structure of atomically-thin 2D solids at the atomic level, shedding new light on the design of high-efficiency electrocatalysts for CO2 reduction. Surface molecular functionalization, which has been widely applied in Au cluster catalysis,[46] also represents a feasible and effective way to tune the reactivity of 2D electrocatalysts for CO2 reduction. Among these molecules, amines are intensively investigated in surface functionalization because of the lone pair of electrons on the nitrogen atom, which makes these compounds not only bases that facilitate the CO2 capture, but also good electron donors tailoring the electronic properties of 2D solids.[47-50] For example, polyaniline (PANI) decorated graphene shows a considerable increase in CO2 absorption due to the chemical interaction between CO2 and amine groups.[48] Meanwhile, the electron-donating effect of amine groups activates 2D electrocatalysts by increasing the electron density. Zhang et al. found that the introduction of polyethylenimine (PEI) can help suppress HER and stabilize the CO2 · —


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ACS Energy Letters

intermediate.[50] As a result, PEI modified reduced graphene oxide-MoSx system turns out to be a highly efficient and selective electrocatalyst for the reduction of CO2 to CO.

Figure 6. (a) Schematic illustration of the topo-chemical transformation strategy to construct exclusive Ni−N4 sites in graphene. Reproduced with permission from ref 54, Copyright 2017, American Chemical Society. (b) Selectivity of N-doped carbon materials for CO2 reduction over proton reduction is governed by the ratio of isolated FeN4 sites vs Fe-based nanoparticles. Reproduced with permission from ref 55, Copyright 2017, American Chemical Society. In addition to surface molecular functionalization, surface heteroatom incorporation which immobilizes exotic ions on the surface stands out as a more facile and effective way to tailor the reactivity of atomically-thin 2D solids.[51] Recently, through this strategy, our group has made considerable progress in optimizing the CO2 electroreduction performance of N-doped graphene.


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As is well-known, N-doped carbon materials are promising electrocatalysts for CO2 reduction because of their abundance, high electrical conductivity, and good durability, yet they still face the challenges of poor activity and selectivity.[52] It is worth mentioning that, with the high specific surface area and abundant Lewis basic nitrogen sites, N-doped graphene can readily coordinate with metal ions. Especially, some nitrogen sites constitute the active moieties of macrocyclic molecules such as porphyrins, phthalocyanines. These macrocyclic complexes are excellent molecular catalysts with high selectivity towards CO2 reduction.[53] As such, when Ndoped graphene meets transition metal ions, an ideal platform bridging the heterogeneous catalysis and molecular catalysis emerges, and consequently provides a rational way to tune the catalytic performance of N-doped graphene. Based on the above considerations, we reported the construction of exclusive Ni−N4 sites in graphene to improve the CO2 reduction performance.[54] The topo-chemical transformation strategy we developed successfully inhibits the aggregation of Ni species, thus ensuring the maximum preservation of Ni–N4 sites (Figure 6a). Theoretical simulations indicate that Ni–N4 sites reduce the activation energy required to generate *COOH, which is the rate determining step; and the difference between the limiting potentials for CO2 reduction and H2 evolution (i.e., UL(CO2)–UL(H2)) is also more positive compared with that of N-doped graphene, corresponding to higher selectivity toward CO2 reduction. Electrochemical measurements verified that the formation of Ni−N4 sites in graphene leads to particularly selective electrochemical reduction of CO2, achieving near unity faradaic efficiency for CO production at −0.81 V with a current density of 28.6 mA cm–2. Likewise, Huan et al also demonstrated that isolated Fe-N4 sites are able to selectively reduce CO2 to CO with the faradaic efficiency of over 90%, while metallic Fe-based nanoparticles are mainly active for proton reduction under similar conditions (Figure 6b).[55] More recently, Strasser's research suggests


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that, different from the high selectivity of Ni−N4 and Fe-N4 sites toward CO2 reduction, Co-Nx sites prefer proton reduction; whereas Cu-Nx sites fail to catalyze the reduction of CO2 due to spontaneous reduction to metallic Cu nanoparticles under the experimental condition.[56,57] Apart from N-doped graphene, it is envisioned that the wide variety of 2D solids will create different chemical environments for heteroatoms on the surface, hence providing ample opportunities for designing high-efficient catalysts for CO2 reduction.

Figure 7. (a) Scheme of VO-rich and VO-poor Co3O4 single-unit-cell layer, respectively. (b) O 1s XPS spectra. (c) Fourier transforms of the Co K-edge EXAFS oscillations. (d) LSV curves in a CO2-saturated (solid line) and N2-saturated (dashed line) 0.1 M KHCO3 aqueous solution. (e) Faradaic efficiencies of formate at different applied potentials. (f) Electrochemical impedance spectra. Reproduced with permission from ref 59, Copyright 2017, Nature Publishing Group.


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Defects are common in catalysts. However, to date, insights into the role of defects at the atomic level are still lacking due to the complexity of the practical catalysts. In this regard, it is necessary to simplify the model through designing well-defined catalysts. In contrast to the bulk materials which contain both internal and surface defects, atomically-thin 2D solids are considered to contain only surface defects in view of the ultrahigh fraction of surface atoms that are comparable to the overall atoms.[58] Inspired by this unique structural feature, we systemically studied the influence of surface defects on the reactivity of 2D electrocatalysts for CO2 conversion. Taking oxygen-deficient Co3O4 single-unit-cell layers as an example, we elucidated the role of surface oxygen vacancies in CO2 reduction catalysis via tuning their concentration (Figure 7).[59] Theoretical simulations suggested that surface oxygen vacancies confined in Co3O4 single-unit-cell layers reduced the activation barrier of rate-limiting proton transfer step from 0.51 to 0.40 eV via stabilizing the HCOO−* intermediate. Moreover, the presence of surface oxygen vacancies also facilitated CO2 adsorption. Benefiting from these advantages, oxygen-deficient Co3O4 layers showed an onset potential of −0.78 V and achieved 85% faradaic efficiency towards HCOOH at −0.87 V vs SCE with a current density of 2.7 mA cm–2. Overall, atomically-thin 2D solids provide an ideal platform to study the role of surface defects not only in CO2 reduction but also in related catalytic applications.


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Figure 8. (a) Schematic formation process of the partially oxidized and pure-Co 4-atomic-layer. (b) High-resolution TEM image. (c) Linear sweep voltammetric curves in a CO2-saturated (solid line) and N2-saturated (dashed line) 0.1 M Na2SO4 aqueous solution. (d) Faradaic efficiencies of formate at each given potential for 4 h. (e) Charging current density differences plotted against scan rates. (f) CO2 adsorption isotherms. (g) ECSA corrected Tafel plots for formate production. Data are shown for the partially oxidized Co 4-atom-thick layers (red), Co 4-atomthick layers (blue), partially oxidized bulk Co (violet) and bulk Co (black). Reproduced with permission from ref 65, Copyright 2016, Nature Publishing Group.


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Coordinatively unsaturated surface atoms are generally considered as the active sites in catalysis, yet their quantity and reactivity are still limited. In this sense, constructing new active sites via microstructure tailoring at the atomic level becomes an inevitable trend in the development of high-efficient catalytic systems. Previous works have reported that oxide-derived metal nanostructures showed much enhanced CO2 reduction performance compared with the pristine metal nanostructures.[60-64] For example, Kanan et al. demonstrated that oxide-derived Au exhibit highly selective CO2 reduction to CO at overpotential as low as 140 mV, whereas polycrystalline Au electrodes require an additional overpotential of at least 200 mV to attain comparable CO2 reduction activity.[62] By means of in situ attenuated total reflectance infrared (ATR-IR) spectroscopy, Bocarsly and co-workers observed a metastable oxide layer formed on Sn electrodes under the electrochemical conditions, and they proposed that surface-confined tin carbonate is the key intermediate in CO2 reduction.[64] However, unraveling the atomic-level correlation between the electrocatalytic activity of these metals and their native oxides is still challenging in view of variable microstructural features. Notably, benefiting from the atomic thickness, partially oxidized atomic metal layers possess almost complete surface metal atoms and metal ions, hence providing an ideal platform to reveal the crucial role of these metals and their native oxides in CO2 reduction. To this end, we fabricated freestanding 4-atom-thick Co sheets with and without surface oxide layer via a ligand-confined growth strategy (Figure 8).[65] Despite the comparable electrochemical surface area, partially oxidized Co 4-atom-thick layers generated a current density of 10.59 mA cm−2 at −0.85 V vs SCE, about 10 times larger than that of the pure-Co 4-atom-thick layer. Moreover, the as-obtained partially oxidized Co 4-atom-thick layers achieved a stable reduction current density up to 10 mA cm−2 and approximately 90% formate selectivity at an overpotential of only 0.24 V. In addition to the oxide system, Zhang et


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al. revealed that the reduced metallic Sn forms from SnS2 under the working condition and serve as the catalytic sites for CO2 reduction, while the residual SnS2 helps stabilize the intermediate CO2 · — .[66] These findings point to new opportunities to improve the CO2 electroreduction properties of metal systems via manipulating their oxidation states. Moreover, metal-support interaction, especially strain and electronic effect are prevalent yet crucial to tailor the catalytic performance. However, previous studies have shown that these effects mainly affect the interfacial region and are essentially negligible at metal catalysts thicker than three monolayers.[67, 68] In this regard, the surface properties of atomically-thin metal overlayers can be effectively tailored through metal-support interaction. Recently, Yang and coworkers demonstrated that the order−disorder transformation of Au-Cu bimetallic support, applied to three-atoms-thick Au layer, significantly changes the selectivity of CO2 electroreduction.[69] In contrast to the disordered Au-Cu support, which is catalytically active for HER, underlying Au-Cu with ordered intermetallic structure compressively strains the Au overlayers, bringing about selective CO2 conversion to CO with faradaic efficiency up to 80%. On the other hand, despite highly active, atomically-thin metal layers are easily oxidized. To cope with this challenge, we developed a confined strategy to improve the electrocatalytic activity and stability of tin (Sn) quantum sheets.[70] Compared with Sn nanoparticles and their mixture with graphene, both of which show obvious oxidation at above ca. 200 °C, Sn quantum sheets confined in graphene exhibit excellent stability in air up to 570 °C. In addition, grapheneinduced confined effect allows Sn quantum sheets to possess reduced coordination number, as compared with Sn nanoparticles and Sn bulk, hence helping stabilize the CO2·— intermediate and lowering the overall activation energy. As a result, graphene-confined Sn quantum sheets achieve a maximum faradaic efficiency of 89% at −1.8 V (vs. SCE), which was roughly 1.45, 1.5


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and 2 times higher than that of Sn nanoparticles mixed with graphene, Sn nanoparticles, and bulk Sn, respectively. Overall, atomically-thin 2D solids fill the gap between theoretical models and real catalysts in terms of their well-defined structure and molecule-like adjustability. In this perspective, we discuss the advantages of atomically-thin 2D solids for electrochemical CO2RR and different routes available to tune their electronic states and active sites. Despite numerous theoretical and experimental investigations in this field, it still seems quite challenging to efficiently reduce CO2 to desirable products. Here we propose several major challenges and opportunities for future development of this field. From the point of view of material preparation, despite the advances in synthetic methodologies, it is still challenging to construct catalytic sites construct active sites with atomic accuracy. Although ultrathin metal-organic frameworks (MOFs) with well-defined active sites become a potential choice, their stability and electronic conductivity need to be further optimized.[71] On the other hand, it should be noted that the applications in catalysis require mass production of 2D solids with controllable structural features, such as size, thickness, crystal phase, defects, and so on. Therefore, the development of high-yield, efficient, and controllable synthesis methods is of vitally necessary to advance these materials from the laboratory to industry. From the perspective of CO2RR performance, many atomically-thin 2D solids electrocatalysts reported have shown high activity and selectivity for simple C1 products but rarely for multicarbon hydrocarbons. As is well-known, the interaction between various reaction intermediates and electrocatalysts is the key factor governing the catalytic activity and final distribution of


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products. Multi-carbon products are typically generated through a series of intermediates. Atomically-thin 2D solids can reduce the reaction barriers for CO2RR via stabilizing a specific reaction intermediate; however, it is hard to simultaneously optimize the interactions between different intermediates and the same active site. By means of the synergistic effect of 2D heterostructures, tandem catalysis is expected to mitigate this problem.[2] Nørskov's recent theoretical studies have demonstrated that dual binding sites result in an overall deviation from the linear scaling on pure transition-metal surfaces.[41] The design of vertical/in-plane heterostructures of atomically-thin 2D solids is expected to improve the production of multicarbon products. More importantly, given the high overpotential required to drive CO2RR, electrocatalysts typically undergo dynamic changes under the working conditions. For example, spectroscopic changes attributed to Co(II)/Co(I) transition are observed in cobalt porphyrins based covalent organic frameworks, as evidenced by UV-Vis spectra.[72] While palladium (Pd) has transformed into β-phase hydride (β-PdH) during CO2RR.[73] In this respect, monitoring these dynamic changes under the reaction conditions such as oxidation state, composition and structure is an indispensable prerequisite for understanding the mechanism. In situ/operando spectroscopic techniques can provide information about the nature of active sites and reaction intermediates. For instance, as revealed by in situ ATR-IR, metastable surface oxides under the electrochemical conditions appear to play a crucial role in the CO2RR.[64] Moreover, the local structure and oxidation state of atomically-thin 2D solids can be probed by synchrotron-based XAFS spectroscopy. In particular, this technique does not require ultra-high vacuum conditions, which is easy to perform in-situ experiments.[74] At present, these in situ/operando studies on CO2RR


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are in the early stages. We anticipate that the insights gained from these in situ/operando spectroscopic techniques will guide the rational design of CO2RR electrocatalysts.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. Biographies Wentuan Bi received his BS degree from China University of Geosciences (2010) and a Ph.D. degree from University of Science and Technology of China (USTC, 2015). After that, he worked as a postdoctoral fellow in the Department of Chemistry. He is currently a Research Associate in Department of Chemistry, USTC. His research focuses on the design of lowdimensional solids for photocatalysis/electrocatalysis. Changzheng Wu obtained his BS (2002) and Ph.D. (2007) degrees in the Department of Chemistry, University of Science and Technology of China (USTC). Since then, he worked as a postdoctoral fellow in the Hefei National Laboratory for Physical Sciences at Microscale. He is now a full professor of Department of Chemistry, USTC. His current research interests focus on the synthesis and characterization of inorganic two-dimensional nanomaterials and regulation of their intrinsic physical properties for a wide range of applications in energy storage or energy conversion.


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Yi Xie received her BS degree from Xiamen University (1988) and a Ph.D. from the University of Science and Technology of China (USTC, 1996). She is now a Principal Investigator of Hefei National Laboratory for Physical Sciences at the Microscale and a full professor of the Department of Chemistry, USTC. She was appointed as the Cheung Kong Scholar Professor of inorganic chemistry in 2000 and elected as a member of the Chinese Academy of Sciences in 2013. Her research interests focus on the design and synthesis of inorganic functional solids with efforts to modulate their electron and phonon structures.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2015CB932302),

National

Key

R&D

Program

of

China

(2017YFA0303500,

2017YFA0207301), National Natural Science Foundation of China (91745113, 21701164, U1532265, 11621063), and the Fundamental Research Funds for the Central Universities (WK2060190084). We also appreciate the support from the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology and USTC Center for Micro and Nanoscale Research and Fabrication.


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