Enhanced Interface with Strong Charge Delocalization toward Ultralow Overpotential CO2 Electroreduction

The construction of an interface has been demonstrated as one of the most insightful strategies for designing efficient catalysts toward electrochemical CO2 reduction (CO2RR). However, the weak interfacial interaction and inherent instability inevitably hinder a further performance enhancement in CO2RR attributable to the interface effect. Herein, 2 nm Ag nanoclusters (Ag NCs) are embedded onto CeO2 nanospheres (CeO2 NSs) with highly interconnected porosity (Ag NCs@CeO2 NSs) to exclusively study the pure interface effect toward CO2RR. The enhanced Ag–CeO2 pure interface endows Ag NCs@CeO2 NSs with a remarkably larger current density, significantly higher Faraday efficiency (FE), and energy efficiency as compared to Ag NCs, CeO2 NSs, and the one with Ag NCs dispersed on CeO2 nanoparticles. More importantly, an impressively high CO FE of over 70.0% is achieved at an ultralow overpotential (η) of 146 mV. The free energy and differential charge calculations, coupled with X‐ray photoelectron spectroscopy results jointly imply that the effective initiation of CO2RR to CO at a lower η over Ag NCs@CeO2 NSs derives from the enhanced interface‐induced charge delocalization, which enhances the electron transfer ability toward *COOH intermediate, thus overcoming the energy barrier demanded for the rate‐determining step.


Introduction
Global climate change and the associated environmental issues are exacerbated by fossil fuel burning, which sharply increases CO 2 emissions, the cause of global warming. [1]To battle against the greenhouse effect, various technologies (e.g., thermochemistry, [2] photochemistry, [3] biochemistry, [4] and electrochemistry [5] ) have been employed with varying degrees of success to reduce CO 2 emissions and simultaneously utilize CO 2 as a carbon source to produce valuable carbon-neutral chemicals/fuels.Among them, electrochemical CO 2 reduction (CO 2 RR) into high-energydensity chemicals/fuels (e.g., CO, CH 4 , HCOOH, and C 2 H 4 ), powered by renewable but intermittent energy sources (e.g., tide, solar, and wind), provides an attractive solution to both CO 2 emissions control and energy-demanding challenges toward a sustainable future for humankind. [6]espite the huge prospect, CO 2 RR usually suffers from thermodynamically high energy barriers, kinetically sluggish reaction rate, and undesirable selectivity toward target products, due to the high energy demand for transforming the extremely stable linear CO 2 molecule to the bent radical anion, the complicated reaction pathways involving proton-coupled electron transfer steps, and the competitive hydrogen evolution reaction (HER). [7]Thus, it has never been more imperative to design and construct highly electroactive and stable nanomaterials capable of driving efficient CO 2 RR to high-value target products.
To this end, substantial amounts of effort, such as size [8] and morphology control, [9] composition tuning, [10] interface engineering, [11] defect incorporation, [12] and ligand modification, [13] have been dedicated to devising high-performance nanomaterials toward CO 2 RR.Among them, the effective construction of an interface, particularly between nanostructured metals and metal oxides, has been demonstrated to be one of the most intellectual strategies, since it not only preserves and stabilizes the active sites of nanostructured metals with excellent dispersions, but also introduces an interface effect.An extensive body of prior experiments (e.g., Au/CeO 2 , [14] Bi/CeO 2 , [15] In/In 2 O 3 , [16] and Cu/In 2 O 3 [17] ), coupled with persuasive theoretical computations, has revealed that atoms situated at interfacial sites are characterized by a coordination deficiency in comparison to their The construction of an interface has been demonstrated as one of the most insightful strategies for designing efficient catalysts toward electrochemical CO 2 reduction (CO 2 RR).However, the weak interfacial interaction and inherent instability inevitably hinder a further performance enhancement in CO 2 RR attributable to the interface effect.Herein, 2 nm Ag nanoclusters (Ag NCs) are embedded onto CeO 2 nanospheres (CeO 2 NSs) with highly interconnected porosity (Ag NCs@CeO 2 NSs) to exclusively study the pure interface effect toward CO 2 RR.The enhanced Ag-CeO 2 pure interface endows Ag NCs@CeO 2 NSs with a remarkably larger current density, significantly higher Faraday efficiency (FE), and energy efficiency as compared to Ag NCs, CeO 2 NSs, and the one with Ag NCs dispersed on CeO 2 nanoparticles.More importantly, an impressively high CO FE of over 70.0% is achieved at an ultralow overpotential (η) of 146 mV.The free energy and differential charge calculations, coupled with X-ray photoelectron spectroscopy results jointly imply that the effective initiation of CO 2 RR to CO at a lower η over Ag NCs@CeO 2 NSs derives from the enhanced interface-induced charge delocalization, which enhances the electron transfer ability toward *COOH intermediate, thus overcoming the energy barrier demanded for the rate-determining step.counterparts in the bulk phase, which are usually the active sites for the activation of CO 2 .Furthermore, in a diverse array of systems (e.g., Ag/SnO x , [18] Ag/ZnO, [19] Cu/ZrO 2 , [20] and Cu/Ce(OH) x [21] ), there has been a conspicuous observation of synergistic behavior at interfacial sites.This synergism, which is manifested through the regulation of charge distribution, facilitation of electron transfer and alteration in the binding energy to reaction intermediates consequently deliver an equivalent or even higher CO 2 RR performance with a much lower metal loading.Although it has been previously demonstrated with the aid of in situ characterization and theoretical calculations that the construction of metal-metal oxide interfaces can enhance CO 2 adsorption and intermediates transformation on catalysts. [14,20]owever, few reports have further explored the underlying mechanism of the role of interfacial sites for such an enhanced adsorption ability.What is even more critical is the difficulty in distinguishing pure interface effect from other influencing factors, which hinders an in-depth mechanism understanding underpinning performance enhancement in CO 2 RR attributable to pure interface effect.To surmount the traditional bottlenecks of interface engineering, the prerequisites for creating pure metal-metal oxide interfacial sites capable of exclusively delving into the underlying mechanism of interface effect have normally depended on the compatibility and harmony between two components that constitute the interfacial sites, the plenitudinous exposure of active interfacial sites possessing favorable electronic structures, the elimination of interferences from other interfacial sites but the target ones, the strengthened stability of interfacial sites in a high-energy states, and the sufficient porosity for mass transfer to expedite the kinetics.
Here, 2 nm Ag nanoclusters (Ag NCs, Figure S1, Supporting Information) were embedded onto CeO 2 nanospheres (CeO 2 NSs, Figure S2, Supporting Information) with highly interconnected porosity, denoted as Ag NCs@CeO 2 NSs, to exclusively study the pure interface effect toward CO 2 RR.The results show that, due to the being-tuned charge delocalization induced by the enhanced Ag-CeO 2 pure interface, a significantly accelerated proton-coupled electron transfer to form reaction intermediates and a reduced energy barrier to initiate rate-determining step (RDS) was achieved, which enables Ag NCs@CeO 2 NSs to be particularly active for CO formation, especially at an ultralow overpotential (η).

Results and Discussion
The morphological structure of Ag NCs@CeO 2 NSs was analyzed using bright-field scanning transmission electron microscopy (STEM).Clearly, the CeO 2 NSs with an average size of %50 nm are comprised of substantial ultrasmall CeO 2 nanoparticles (NPs) with a crystallite size of %4 nm, which are inextricably interwoven with each other to form highly porous NSs shape (Figure 1a,b).The specific NSs nanostructure undoubtedly could well preserve and stabilize the 2 nm Ag NCs with superb dispersions on the interconnected CeO 2 NPs, thereby creating Ag-CeO 2 pure interface and enhanced interfacial sites.A close inspection of the crystal lattice in the selected region (Figure 1b) illustrates that the two adjacent areas correspond to Ag NCs and CeO 2 NSs with interplanar spacings of 0.204 and 0.312 nm (Figure 1c), respectively, which closely align with the values determined by X-ray diffraction patterns (Figure S3, Supporting Information) for Ag(200) (PDF #04-0783) and CeO 2 (111) (PDF #34-0394).This persuasively demonstrates the successful construction of pure interface between Ag NCs and CeO 2 NSs, as further confirmed by the corresponding selected area electron diffraction (SAED) pattern in Figure 1d.Concurrently, high-angle annular dark field (HAADF)-STEM, together with elemental mappings of energy-dispersive X-ray (EDX) spectroscopy, was also carried out, and the results validated that the uniformly distributed %2 nm species on CeO 2 NSs were indeed Ag NCs (Figure 1e), as appeared in blue in Figure 1f.Apparently, the homogeneous dispersion guarantees an optimal utilization of Ag-CeO 2 interfacial sites and presages an enhanced interface-induced CO 2 RR.
To evaluate CO 2 RR electrocatalytic activity for exclusively unraveling the enhanced Ag-CeO 2 pure interface effect, Ag NCs@CeO 2 NSs, coupled with Ag NCs, CeO 2 NSs, and the one with Ag NCs dispersed on CeO 2 NPs (Ag NCs@CeO 2 NPs, Figure S4, Supporting Information), were all fabricated onto a glassy carbon electrode with a geometric area of 0.785 cm 2 by an equivalent Ag loading.All the materials were examined in 0.1 M KHCO 3 within a custom-built cell separated by a Nafion-117 membrane; all potentials are with reference to the reversible hydrogen electrode.Linear scanning voltammetry (LSV) measurements were first performed for all samples under Ar and CO 2 atmospheres, respectively, to roughly investigate their electrocatalytic activity.Remarkably, the trend in current density ( j) growth follows Ag NCs > Ag NCs@CeO 2 NPs > Ag NCs@CeO 2 NSs under Ar atmosphere as the applied potential increases (Figure S5, Supporting Information), whereas it exhibits a completely opposite trend under CO 2 atmosphere (Figure 2a), suggesting that the Ag-CeO 2 interface is specifically active for CO 2 RR rather than HER.However, since the LSV results include current contributions from both CO 2 RR and the competitive HER, it only qualitatively concluded that CO 2 RR was favored on Ag NCs@CeO 2 NSs.Thus, coupling CO 2 RR potentiostatic measurements at different potentials with product analyses for extracting partial js toward target products is of prime importance to exclusively demonstrate the preferential CO 2 RR rather than HER over Ag NCs@CeO 2 NSs.Gas chromatography and ion chromatography were employed to quantitatively determine the gaseous fuels and liquid products, respectively, and the results confirm that CO and H 2 are the only two gaseous fuels (Figure 2b).A close observation of Faraday efficiencies (FEs) in Figure 2c finds that standalone CeO 2 NSs possess an almost negligible ability to convert CO 2 to CO with a maximum FE CO of only 16.6% and a low j of %2.1 mA cm -2 at -1.156 V (Figure 2a).However, the introduction of a modest quantity of Ag NCs on CeO 2 NPs to engender Ag-CeO 2 interface culminates in a considerably enhanced j of 6.4 mA cm À2 , even surpassing that of 5.6 mA cm À2 on Ag NCs.Meanwhile, the maximum FE CO significantly increases to 80.7%, preliminarily indicating that Ag-CeO 2 interface could indeed bolster CO 2 RR electrocatalytic activity.Nevertheless, the highly porous Ag NCs@CeO 2 NSs with interconnected channels result in additional increments in both js and CO FEs as compared to Ag NCs@CeO 2 NPs due to an enhanced Ag-CeO 2 pure interface effect.Specifically, the Ag NCs@CeO 2 NSs show higher FEs toward CO formation than all the counterparts at all potentials, and reach its maximum FE of 95.6% at -0.956 V, comparable with most of the state-of-the-art Ag-based and nonnoble-metal-based nanomaterials. [22]More importantly, Ag NCs@CeO 2 NSs maintain CO FEs over 70% in an ultrawide potential range from -0.296 to -1.056 V, where, particularly, it reaches a FE CO of %70.0% at an ultralow η of 146 mV, over 7-fold higher than that of 9.2% on Ag NCs, superior to most of the previously reported catalysts (Figure S6, Supporting Information).The significantly enhanced Ag-CeO 2 pure interface effect is further verified by CO partial js [js (CO) ], which was determined by integrating js obtained at various potentials (Figure S7, Supporting Information) and CO FEs (Figure 2c).Clearly, a j (CO) of over 3.90 mA cm À2 on Ag NCs@CeO 2 NSs was obtained at -1.056 V, almost 1.3-and 1.6-fold larger than that of 3.07 mA cm À2 on Ag NCs@CeO 2 NPs and that of 2.36 mA cm À2 on Ag NCs (Figure 2d), respectively.In addition, a remarkably ultralow onset η of 46 mV and a η of 0.60 V to deliver a j (CO) of 1.0 mA cm À2 (Figure 3a) was achieved over Ag NCs@CeO 2 NSs, comparably lower than those over Ag NCs, CeO 2 NSs, and Ag NCs@CeO 2 NPs.Furthermore, the low η together with the high FE of Ag NCs@CeO 2 NSs contributes to maximum energy efficiency (EE) of 63.1% at -0.756 V (Figure 3b), and particularly achieves a comparably higher EE of 57.0% at an ultralow potential of -0.456 V, much higher than those over Ag NCs (21.7%) and Ag NCs@CeO 2 NPs (16.8%), which is also comparable with most of the benchmarking nanomaterials for CO formation, [23] further emphasizing the positively enhanced pure interface effect toward CO 2 RR.
To exclude the influences caused by different morphologies and sizes, and consequently confirm the intrinsic origins credited for the remarkably enhanced electrocatalytic activity of Ag NCs@CeO 2 NSs for CO 2 RR, electrochemical surface areas (ECSAs) of Ag NCs@CeO 2 NSs, Ag NCs, CeO 2 NSs, and Ag NCs@CeO 2 NPs were evaluated, and the results in Figure S8 and S9, Supporting Information show that Ag NCs@CeO 2 NSs obtained a double layer capacitance (C dl ) of 2.88 mF cm À2 , approximately 0.86 that on Ag NCs (3.34 mF cm À2 ).However, the obtained j (CO) at -1.056 V over Ag NCs@CeO 2 NSs was 2.36 times higher than Ag NCs, delivering an intrinsically 2.74-fold higher electrocatalytic activity.This, beyond any doubt, points out that the intrinsic activity of Ag NCs@CeO 2 NSs originates from the massive highly active Ag-CeO 2 interfacial sites.To further clarify the reaction kinetics, an ECSA-normalized Tafel plot was thus illustrated in Figure 3c.As compared to the Tafel slopes of 159 mV dec À1 on Ag NCs, 237 mV dec À1 on CeO 2 NSs, and 176 mV dec À1 on Ag NCs@CeO 2 NPs, a much lower Tafel slope of 155 mV dec À1 was achieved over Ag NCs@CeO 2 NSs, much closer to the theoretical value of 120 mV dec À1 , indicative of higher intrinsic electrocatalytic activity of Ag NCs@CeO 2 NSs toward the key CO •2 intermediate formation over the special architecture. [24]This was further verified by electrochemical impedance spectroscopy results in Figure S10, Supporting Information, where Ag NCs@CeO 2 NSs exhibited a smaller interface charge transfer resistance, indicating that Ag-CeO 2 interface facilitates charge transfer to reaction intermediates more effectively during CO 2 RR.The long-term stability, as a crucial parameter to evaluate practicability, was further assessed by recording the j as a function of time at a constant potential of -0.856 V. Remarkably, Ag NCs@CeO 2 NSs displayed a slight CO FE decay of below 4.4% during the stability test (Figure 3d), significantly lower than Ag NCs@CeO 2 NPs (16.1%) and Ag NCs (10.3%) (Figure S11, Supporting Information).The superior stability of Ag NCs@CeO 2 NSs in terms of j and CO FE can be attributed to the anchoring effect of CeO 2 NSs with abundant mesoporous channels on Ag NCs, which, to some extent, enhances the stability of high-energy Ag-CeO 2 interfacial sites.
To theoretically deepen the enhanced pure interface effect over Ag NCs@CeO 2 NSs for CO 2 RR, density functional theory (DFT) calculations were conducted using a structure of Ce 3 O 7 H 7 -Ag(100) (Figure S12, Supporting Information) to simulate Ag-CeO 2 pure interface, [14] and the computational hydrogen electrode model was employed to calculate the free energy diagrams.As depicted in Figure 4a, the pathways through which CO 2 is transformed to CO via a carboxyl intermediate (*COOH) coupled with two protons and electrons were analyzed on Ce 3 O 7 H 7 -Ag(100) and Ag(100).Evidently, the formation of the key intermediate of *COOH, involving the first proton-coupled electron transfer (i.e., CO 2 þ H þ þ e -! *COOH), encounters a comparably high-energy barrier of 1.41 eV on Ag(100), which inevitably accumulates in the initial η for CO formation as the RDS.In contrast, the ΔG required to form the key *COOH intermediate on Ce 3 O 7 H 7 -Ag(100) is only 0.59 eV, approximately 41.8% of that on Ag(100), indicating that the enhanced Ag-CeO 2 pure interface can initiate CO 2 RR at lower applied potentials, which is consistent with previously observed high CO FEs achieved at ultralow η (Figure 4c).The interactions and electronic structure between Ce 3 O 7 H 7 cluster and Ag(100) facet were also investigated by calculating differential charge density maps.A conspicuous decrease in charge density was observed on both Ce and Ag atoms, accompanied by a corresponding accumulation of charge throughout the entire interface (Figure 4b), suggesting that the construction of Ce 3 O 7 H 7 -Ag(100) pure interface induces a strong charge delocalization from Ag and Ce atoms toward the interface region, which is believed to be the origin credited for the high electrocatalytic activity at the interfacial sites. [19]As a result, the significantly increased charge density at the interfacial sites dramatically boosts the electronic supply ability to the reaction intermediates.Moreover, the Bader charge analyses provide quantitative evidence that Ce 3 O 7 H 7 -Ag(100) transfers 0.68 electron to the *COOH intermediate, more than twice the 0.32 electron delivered by Ag(100) (Figure 4a), which is further supported by the projected density of states (PDOS) calculations.In comparison to the Ag(100), although the Ag in Ce 3 O 7 H 7 -Ag(100) remains in the metallic state, its d-band center shifts positively toward the Fermi level (Figure 4d), resulting in a stronger binding ability to the *COOH intermediate, and hence benefiting the CO production.Likewise, X-ray photoelectron spectroscopy (XPS) results also confirm the existence of the metallic state of Ag in Ag NCs@CeO 2 NSs (Figure S13, Supporting Information), indicating that the construction of Ag-CeO 2 interface did not change the metallic nature of Ag.More importantly, the spectra of Ce 3d XPS show that the presence of Ce in Ag NCs@CeO 2 NSs, Ag NCs@CeO 2 NPs, CeO 2 NSs, and CeO 2 NPs is in the form of a mixture of Ce 4þ and Ce 3þ , where the atomic fraction of Ce 4þ increases from %81.5% in CeO 2 NSs to %84.4% in Ag NCs@CeO 2 NSs (Figure 4e).Apparently, the increase in Ce 4þ concentration is not caused by the oxidizing agents during synthesis but induced by the enhanced strong interfacial interaction between Ag and CeO 2 , which facilitates the delocalization of electrons initially bound within Ce atoms to the interfacial sites, leading to an elevation of the valence.Therefore, it is concluded that the impressive CO 2 RR to CO at ultralow η over Ag NCs@CeO 2 NSs stems from the enhanced interface-induced charge delocalization, which significantly accelerates electron transfer to the key *COOH intermediate, and consequently mitigates the energy barrier required for the RDS.

Conclusion
In summary, Ag NCs@CeO 2 NSs with enhanced Ag-CeO 2 pure interface were successfully synthesized and found that Ag NCs@CeO 2 NSs could effectively promote CO 2 RR to CO with remarkably higher electrocatalytic activity, near-unity selectivity, and superior long-term stability relative to Ag NCs, CeO 2 NSs, and Ag NCs@CeO 2 NPs.More importantly, Ag NCs@CeO 2 NSs could initiate CO 2 RR at an impressively low η, together with a considerably high FE CO in 0.1 M KHCO 3 .DFT calculations, coupled with differential charge calculation on the interface model, Bader analyses on key intermediate adsorption models, and XPS results, collectively suggest that the effective activation of CO 2 RR to CO at a lower η over Ag NCs@CeO 2 NSs originates from the enhanced interface-induced charge delocalization.This phenomenon significantly increases the electron density at interfacial sites and considerably enhances the electron transfer ability toward the key *COOH intermediate, thus strongly reducing the energy barrier demanded for the RDS.This study, thus, paves an avenue to construct enhanced pure interface through intellectually customizing specific metal oxide nanostructures capable of well preserving, dispersing, and stabilizing metal nanostructures.
Y.-F.Tang, T. Zhang, H.-C. Mi, M. Yu, S. Liu School of Minerals Processing and Bioengineering Central South University Changsha, Hunan 410083, China E-mail: subiao@csu.edu.cnP.-F.Sui, J.-L.Luo Department of Chemical and Materials Engineering University of Alberta Edmonton, Alberta T6G 1H9, Canada X.-Z.Fu, J.-L.Luo College of Materials Science and Engineering Shenzhen University Shenzhen, Guangdong 518000, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smsc.202300169.© 2023 The Authors.Small Science published by Wiley-VCH GmbH.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.DOI: 10.1002/smsc.202300169

Figure 1 .
Figure 1.a) TEM and b) HRTEM images of Ag NCs@CeO 2 NSs; c) enlarged square area in (b) showing the crystal lattices and d) the corresponding SAED patterns of individual Ag NC and ultrasmall CeO 2 NP; e) HAADF-STEM image and f ) the associated EDX mapping profiles of Ag, Ce, and O on Ag NCs@CeO 2 NSs.