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Research Article
Iodine-Modified Cu Heterointerface Enables Efficient CO2-to-C2H4 Conversion Mengmeng Yang, Sohaib Umer, Ruizhi Wu, Chen Jia, Yu Yang, Zhipeng Ma, Qian Sun, Haochen Lu, Yutong Wu, Zhun Shi, Ruirui Liu, Jun Chen, Fengwang Li, Martina Lessio,* and Chuan Zhao*
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ABSTRACT: Electrochemical CO2 reduction to multicarbon (C2+) products offers a promising route for chemical production. However, uncontrollable catalyst reconstruction and active site loss often limit selectivity and durability. Here, CuI/Cu2(OH)3NO3 heterointerfaces undergo in situ electrochemical reduction to form Cu/ Cu2O(I) structures, in which interfacial engineering generates lowcoordination Cuδ+ sites and stabilizes iodine species under reaction conditions. The catalyst achieves 80% C2+ Faradaic efficiency at –250 mA cm–2, including 61% for ethylene. In situ spectroscopic studies combined with theoretical calculations reveal that iodine stabilization at the Cu/Cu2O interfaces preserves Cuδ+ centers, enhances *CO adsorption, and lowers the energy barrier for C–C bond formation, thereby facilitating C2+ production. This work establishes an interfacial strategy that couples active-site generation with iodine-assisted stabilization, providing mechanistic insights into selective CO2-to-C2+ electroconversion. KEYWORDS: undercoordinated copper sites, iodine stabilizations, interfacial engineering, carbon dioxide reductions, multicarbon products
- INTRODUCTION
underscored that undercoordinated Cu sites (steps, defects, and grain boundaries) and oxidized Cu+ species are pivotal in promoting C2+ formation. These surface motifs generate locally polarized Cu+/Cu0 interfacial environments that lower the kinetic barrier for *CO dimerization and subsequent hydrodeoxygenation, thereby enhancing C2+ productivity.7–10 Unfortunately, the thermodynamic instability of these active surface configurations under high cathodic potentials leads to rapid structural degradation, severely limiting operational durability of the catalysts.11–13 To overcome these limitations, strategic modulation of chemical composition, along with precise engineering of crystallographic and electronic structures, has been recognized as a promising approach for stabilizing metastable Cu+ species.14–16 Experimental and theoretical studies reveal that subsurface oxygen, nitrogen and other heteroatoms can effectively stabilize surface Cuδ+ (0 < δ ≤ 1) sites and facilitate C2+ formation.17
The accelerating accumulation of atmospheric carbon dioxide (CO2), accompanied by the irreversible depletion of fossil resources, has elevated the conversion of CO2 into valueadded chemicals or fuels using renewable energy to one of the defining scientific endeavors.1 Among various approaches, electrochemical CO2 reduction reaction (CO2RR) stands out as a promising strategy for sustainable chemical production.2 Multi-carbon (C2+) products, especially ethylene (C2H4), are particularly attractive due to their superior energy density and extensive applications in chemical manufacturing.3 However, selective C2+ formation involves complex carbon-carbon (C–C) coupling pathways, while the competing hydrogen evolution reaction (HER) often dominates under practical reaction conditions, leading to poor selectivity and energy efficiency.4 Therefore, the rational design of catalyst architectures that stabilize key intermediates and steer reaction pathways toward C2+ products remains a critical frontier in CO2 electrocatalysis.5 As the sole metal capable of producing substantial amounts of C2+ products from CO2RR, copper (Cu) holds exceptional promise owing to its unique ability to catalyze carbon-carbon (C–C) coupling. However, its practical application is hampered by the linear scaling relations governing key intermediate binding energies, which impose a rigorous trade-off between electrocatalytic activity and product selectivity.6 Recent studies have © XXXX American Chemical Society
Received: April 28, 2026 Revised: May 31, 2026 Accepted: June 18, 2026
A
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Among these, iodine (I) modification exhibits a unique capability to induce surface roughness, stabilize Cu+ via specific Cu-I interactions, and modulate the electronic structure to favor the adsorption of CO2 and *CO intermediates.18,19 Despite these merits, the operational CO2RR environment, characterized by reductive potentials, fluctuating local pH, and the presence of reactive species, is inherently corrosive to both Cu and its modifiers.20 Surface-coordinated iodine species are prone to rapid desorption and dissolution into the electrolyte from conventional iodine-modified catalysts. This iodine leaching triggers a cascade of deactivation: the loss of the Cu+ stabilizer leads to uncontrolled site reduction and structural reorganization, while the disappearance of interfacial electronic modulation shifts intermediate binding energies.21 Therefore, the key challenge lies in designing a catalyst that enables sustained interfacial functionality during reconstruction, rather than transient iodine modification, thereby preserving the beneficial role of iodine under CO2RR conditions. Herein, we report an interfacial-confinement strategy that enables sustained catalytic functionality during reconstruction. The iodine-modified and boundary-rich B-Cu-Cu2O(I) catalyst is constructed through in situ electrochemical reduction of a CuI/Cu2(OH)3NO3 precursor. Unlike conventional surfacedoped systems, this reconstruction process generates a robust Cu/Cu2O heterostructure with abundant grain boundaries, which is associated with enhanced iodine retention and suppresses rapid leaching under CO2RR conditions. More importantly, the heterointerface regulates Cu reduction kinetics and induces a high density of undercoordinated Cuδ+ sites with a low Cu–Cu coordination number of approximately 5.9, while the retained iodine further stabilizes these sites against over-reduction. This division of roles enables sustained active-site population during dynamic reconstruction. Benefiting from its unique interfacial configuration, the optimized B-Cu-Cu2O(I) catalyst delivers a C2+ Faradaic efficiency (FE) of 80% with a remarkable 61% FE for ethylene (C2H4) at –250 mA cm–2 and remains stable for 18 h. In contrast, the pristine CuI-derived L-CuCu2O(I) catalyst exhibits only 50% C2+ FE and rapidly deactivates within 6 h because of severe iodine loss. Comprehensive in situ spectroscopic investigations combined with theoretical calculations reveal that the stabilized Cuδ+ sites enhance *CO adsorption and lower the energy barrier for C–C bond formation, while retained iodine further modulates the electronic structure and adsorption environment to promote efficient C2H4 electrosynthesis. This work demonstrates that interfacial engineering can be leveraged to construct dynamically stable catalytic interfaces that couple active-site generation with halogen-assisted stabilization, offering a viable strategy to bridge the selectivitydurability trade-off in CO2RR and related reactions.
Research Article
a Cu(NO3)2 solution. Separately, 3 mmol KI was dissolved in 80 mL deionized water and added to the Cu(NO3)2 solution under vigorous stirring for 60 min. The resulting products were collected by centrifugation, washed three times with deionized water, and dried at 60 °C for 12 h to yield the CuI precursor. L-Cu-Cu2O(I) was generated by electrochemical reduction at –50 mA cm–2 for 5 min in 0.5 M KOH under flowing CO2. 2.2.2. Synthesis of B-Cu-Cu2O(I). B-Cu-Cu2O(I) was synthesized following the same procedure as L-Cu-Cu2O(I), except that 2 mmol KOH was additionally introduced together with KI during precursor synthesis to obtain the CuI/Cu2(OH)3NO3 precursor, which was subsequently electrochemically reduced under identical conditions.
2.3. Electrochemical Performance Tests 2.3.1. Cathode Preparation. Initially, 10 mg of catalyst powder was dispersed in 1 mL of ethanol solution (ethanol: deionized water = 1:1) with 100 μL of Nafion solution and sonicated for 30 min to obtain a homogeneous catalyst ink. Subsequently, the ink was loaded onto carbon paper (1 cm × 0.5 cm) and dried under an infrared lamp for 5 min. The loading mass was 1 mg cm–2. For flow-cell electrodes, the CO2RR cathode employed hydrophobic polytetrafluoroethylene-coated carbon paper as the gas diffusion layer (GDL). The catalyst ink was prepared by dispersing 20 mg of catalyst in 2 mL ethanol solution (ethanol: deionized water = 1:1) and 200 μL Nafion solution (5 wt %), followed by sonication for 30 min. The ink was uniformly sprayed onto a 3 cm × 6 cm GDL using a spray gun. Subsequently, the GDL was dried overnight before the test. The catalyst loading on the GDL was approximately 1.0 mg cm–2. 2.3.2. CO2RR tests in H-Cell. CO2 electrochemical reduction (CO2RR) was performed in an H-cell with a 1 × 0.5 cm2 window for CO2 electrolysis. A three-electrode configuration was employed using the prepared catalyst-coated carbon paper as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a Pt wire as the counter electrode. A 0.1 M KHCO3 aqueous solution was used as both the catholyte and anolyte. The CO2 flow rate was set at 20 sccm. The potential was controlled using an electrochemical workstation (Autolab PGSTAT 302N). The gaseous and liquid products generated during CO2RR were analyzed using gas chromatography (GC, Shimadzu 2010) and 400 MHz nuclear magnetic resonance spectroscopy (NMR, Bruker Avance III HD), respectively. The solution resistance (R) in the H-cell was evaluated using potentiostatic electrochemical impedance spectroscopy (PEIS). PEIS measurements were conducted before CO2RR over a frequency range from 0.1 MHz to 100 Hz. All potentials vs SCE were converted to the reversible hydrogen electrode (RHE) scale and corrected for the iR drop using the following eq 1: EðRHEÞ = EðSCEÞ + 0:2412 V + 0:0591∗ pH–50%iR
(1)
2.3.3. CO2RR Tests in Flow Cell. CO2 electroreduction experiments were performed in a flow-cell configuration using the prepared gas diffusion electrode (GDE) as the working electrode, an Ag/AgCl electrode as the reference electrode, and Ni foam (1 × 1 cm2) as the counter electrode. A Cu strip was used as the current collector. A proton exchange membrane was used to separate the cathode and anode chambers. A 0.5 M KOH aqueous solution was used as both the catholyte and anolyte (20 mL each), which were circulated at 17 mL min–1. The CO2 flow rate was maintained at 20 sccm. 2.3.4. Product Analysis. The Faradaic efficiencies of gaseous products were calculated as the following eq 2:
- MATERIALS AND METHODS 2.1. Chemicals Copper nitrate trihydrate (Cu(NO3)2·3H2O, 99%), potassium hydroxide (KOH, 96%), potassium iodide (KI, 99%), and potassium bicarbonate (KHCO3, 99.5%) were purchased from Sigma-Aldrich. Nafion 117 ionomer (∼5 wt %) and carbon paper were purchased from Fuel Cell Store.
FEgas =
2.2. Catalyst Synthesis 2.2.1. Synthesis of L-Cu-Cu2O(I). 3 mmol of Cu(NO3)2·3H2O was dissolved in 40 mL of ethanol under stirring for 30 min to form
z × VCO2 × F × x × 100% jtotal
(2)
where z is the number of electrons transferred for producing a target product; F is the Faraday constant; x is the molar fraction of a target B
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product determined by GC; V is the molar flow rate of gas; and jtotal is the total current density. The Faradaic efficiencies of liquid products were calculated as the following eq 3: FEliquid =
α×F×n × 100% Q
the bottom two layers in each system were fixed to their bulk structures and only the top two layers and the adsorbates were allowed to relax. The electronic energy was considered self-consistent when the difference between two electronic steps reached 10–5 eV, while the geometries were relaxed until the forces on each atom were less than 0.03 eV Å–1. The surface Brillouin-zone integrations were sampled according to the Γ-centered Methfessel-Paxton grid with the size of 3 × 3 × 1. Finally, the adsorption energies (ΔEads) of the key intermediates including CO2 and CO were calculated by the eq 7 below:
(3)
where α is the number of electrons transferred for the formation of the liquid product; n is the number of moles of the specific liquid product; F is the Faraday constant (96485 C/mol); Q is the total charge (C), which was calculated by Q = It (I: tested current (mA), t: CO electrolysis time (s)) According to previous reports, the electrochemically active surface area (ECSA) and roughness factor (RF) were calculated using the following equations: ECSA = Cdl=Cs
RF = ECSA=GSA
ΔEads = Esurf + ads –Esurf –Eads
ðj × 60sÞ=ðn × FÞ ðV × 1 minÞ × Vm
(7)
where Esurf+ads is the total energy of the catalyst surface with the adsorbate, Esurf is the energy of the clean surface, and Eads is the energy of the isolated adsorbate in the gas phase. The reaction energy for the coupling of two CO molecules adsorbed on the surface to form the *OCCO dimer was calculated as follows:
(4)
ΔE∗OCCO = E∗OCCO –E∗2CO
(8)
where E*OCCO refers to the energy of the OCCO dimer adsorbed on the surface, while E2CO denotes the energy of two *CO molecules adsorbed independently at different sites on the surface. Figure S23 reports the optimized adsorption geometries of the intermediates on the different surface models. The electron density difference plots were generated using the following eq 9:
(5)
In eq 4, Cdl is the measured double-layer capacitance of the samples in 0.1 M KHCO3 (mF) and Cs is the specific capacitance of the catalyst (Cs = 0.04 mF cm–2 in 0.1 M KHCO3). In eq 5, RF represents the roughness factor, and GSA is the geometric surface area of the samples (1.00 cm–2). The single-pass carbon dioxide conversion efficiency (SPCE) was evaluated in an electrochemical flow cell setup by modulating the CO2 flow rate. To optimize the SPCE, a CO2 flow rate of 0.5 mL·min–1 was employed in the flow cell system with an electrode area of 1 cm2. Data acquisition for SPCE commenced at a current density of –200 mA·cm–2 in CO2-saturated 0.5 M KOH electrolyte. The SPCE value was determined using the following eq 6 under the conditions of 298.15 K and 101.3 kPa: SPCE =
Research Article
Δρ = ρsurface + adsorbate –ρsurface –ρadsorbate
(9)
To further examine the bonding interactions and electronic structure of the surfaces, projected density of states (PDOS) were computed using a denser k-point mesh (7 × 7 × 1) with the tetrahedron smearing method. The d-band centre (εd) was defined as:26 E
εd =
(6)
∫ –F εnðεÞdε
(10)
E
∫ –F nðεÞdε
where ε represents the Kohn–Sham eigenvalues, n(ε) is the calculated total d-PDOS, and EF denotes the Fermi energy.
where j is the partial current density of a specific group of products from CO2RR and N is the number of electrons transferred for every product molecule. F is the Faraday constant.
- RESULTS AND DISCUSSION 3.1. Catalyst Preparation and Characterization
2.4. Computational Methods
To engineer the interfacial confinement environment, we synthesized a composite precursor comprising CuI nanoparticles (NPs) embedded within layered Cu2(OH)3NO3 (CHN), denoted as CuI/CHN. This precursor was prepared via a KOH coprecipitation method and subsequently subjected to in situ electrochemical reduction to generate the boundaryrich B-Cu-Cu2O(I) catalyst with abundant Cu/Cu2O interfacial sites (Figure 1a). For comparison, a pure CuI precursor was prepared under identical conditions but without KOH addition, yielding the L-Cu-Cu2O(I) catalyst after electroreduction. The as-synthesized CuI/CHN precursor exhibits a more uniform dispersion and notably smaller particle size, as evidenced by scanning electron microscopy (SEM, Figure S1) and transmission electron microscopy (TEM, Figure S2). TEM images further show that CuI NPs are closely surrounded by CHN, creating a dense distribution of pre-forming heterointerfaces (Figure S2c). Detailed phase identification was conducted using energy-dispersive spectrometry (EDS) mapping and powder X-ray diffraction (XRD), which confirms the intimate mixing of crystalline CuI
Spin-polarized density functional theory (DFT) calculations were carried out using the Vienna ab initio simulation package (VASP)22 to investigate the interaction of key CO2RR intermediates with the catalytic surfaces. Electron-ion interactions were addressed utilizing the projector-augmented wave (PAW) approach,23 while the valence electrons were modelled using plane waves with a cutoff energy of 500 eV. The electronic exchange correlation interactions were described using the generalized gradient approximation (GGA) framework with the Perdew-Burke-Ernzerhof (PBE)24 functional. Dispersion corrections were included through the DFT-D325 method to account for the long-range van der Waals interactions. The Cu(111) surface was built using a 4 × 4 supercell with four Cu layers, while Cu2O(111) was modelled using a 2 × 2 four trilayered (O–Cu–O) supercell terminated by oxygen atoms. The Cu2O(111)||Cu(111) interface was modelled by placing the two optimized surfaces next to each other 1.90 Å apart to make a lateral heterostructure. The surface models are presented in Figure S22. A vacuum of 15 Å was added along the z-axis to avoid the interaction between the periodic images. In addition, a dipole correction was included along the z-direction given the asymmetry of the slab and the adsorbates being placed only on one surface of the slab. During optimization, C
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Figure 1. Catalyst synthesis and structural characterization. (a) Schematic illustration of the synthesis process of B-Cu-Cu2O(I). (b) XRD patterns of B-Cu-Cu2O(I) and L-Cu-Cu2O(I). (c) TEM of B-Cu-Cu2O(I). (d) EDS mapping of B-Cu-Cu2O(I). (e) HRTEM of L-Cu-Cu2O(I). (f) HRTEM of B-Cu-Cu2O(I). (g) Enlarged image of green square area in f.
the retention of iodine species within the reconstructed architecture after electroreduction. Compared with L-Cu-Cu2O(I), the B-Cu-Cu2O(I) exhibits a higher iodine retention, as evidenced by SEM-EDS mapping (Figure S4). HRTEM imaging (Figure 1e,f) of the activated catalysts demonstrates abundant Cu/Cu2O heterointerfaces in B-Cu-Cu2O(I), whereas only limited contact regions are observed in L-Cu-Cu2O(I) control sample. The enlarged HRTEM image in Figure 1g resolves interplanar spacings of 0.20 and 0.25 nm, corresponding to Cu (111) and Cu2O (111) planes, respectively, which confirms the formation of dense Cu/Cu2O heterointerfaces induced by the initial precursor geometry. X-ray photoelectron spectroscopy (XPS) was performed to clarify the surface electronic structure differences between BCu-Cu2O(I) and L-Cu-Cu2O(I) (Figure 2a,b and Figure S6) with the XPS spectrum of the precursor provided in Figure S5. Compared with L-Cu-Cu2O(I), B-Cu-Cu2O(I) shows no pronounced shift in the Cu 2p binding energy, which can be attributed to the competing effects between the increased Cu+ character and iodine-induced interfacial charge redistribution at Cu sites (Figure 2a).28 In Figure 2b, the I 3d spectrum of B-Cu-Cu2O(I) displays a significant shift toward higher binding energy compared to that of L-Cu-Cu2O(I), suggesting strengthened Cu-I interactions and more stabilized interfacial iodine species.29 Furthermore, the retention of iodine under operational CO2RR conditions was confirmed to be several times higher than that of CuI-derived L-Cu-Cu2O(I), as evidenced by inductively coupled plasma mass spectrometry (ICP-MS)
and CHN phases (Figure S2b–e). High-resolution TEM (HRTEM) images and the corresponding inverse Fast Fourier transform (FFT) patterns provide further evidence of the tightly interfaced bicontinuous structure of CuI/CHN,27 displaying distinct lattice spacings of 0.35 and 0.25 nm, which correspond to CuI (111) and CHN (120) planes, respectively (Figure S2f,g). Notably, this pre-organized Cu+/Cu2+ interfacial architecture is expected to facilitate the formation of well-defined Cu+/Cu0 (Cu2O/Cu) heterointerfaces during subsequent electrochemical activation. After electrochemical activation, the crystalline compositions of the catalysts were identified by XRD analysis. As shown in Figure 1b, the diffraction patterns for both catalysts show peaks at 43.3°, 50.4°, and 74.1°, corresponding to the (111), (200), and (220) planes of metallic Cu (PDF No. 089–2838), as well as peaks at 36.4°, 42.3°, and 61.4° assigned to the (111), (200), and (220) planes of Cu2O (PDF No. 05–0667), respectively, confirming that the as-synthesized precursors successfully evolve into Cu2O/Cu heterostructures. Notably, the CuI/CHN-derived B-Cu-Cu2O(I) catalyst shows sharper diffraction peaks, suggesting improved crystallinity compared to pure CuI-derived L-Cu-Cu2O(I), which also ensures better structural stability during electrolysis. In contrast, the pure CuI precursor undergoes drastic structural collapse, as illustrated by structural evolution analysis in Figure S3. TEM image and corresponding EDS mapping of B-Cu-Cu2O(I) (Figure 1c,d) reveal irregular nanoparticle aggregation together with the uniform distribution of Cu, O, and residual iodine species, further verifying D
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Figure 2. Spectroscopic analysis of catalysts. (a) and (b) XPS spectra of Cu 2p and I 3d. (c) I retention ratio across time measured by ICP-MS (after 60 min). (d) Cu LMM Auger spectra. (e) Cu K-edge XANES spectra. (f) FT EXAFS k3-weighted χ(k) function spectra of L-Cu-Cu2O(I) and B-CuCu2O(I). (g) EXAFS fitting curve of B-Cu-Cu2O(I). (h) and (i) WT spectra for L-Cu-Cu2O(I) and B-Cu-Cu2O(I), respectively.
coordination environment and electronic structure of Cu in the materials (Figure 2 and Figure S7).33 For the precatalyst, Cu K-edge X-ray absorption near edge structure (XANES) spectra indicate the coexistence of Cu+/Cu2+ species, while extended X-ray absorption fine structure (EXAFS) and wavelet transform analyses reveal the coexistence of Cu-I, Cu-O, and Cu-Cu coordination shells, evidencing the formation of an Imodified Cu-oxide interfacial structure in the precursor (Figure S7). After electrochemical activation, the Cu K-edge XANES spectra of both B-Cu-Cu2O(I) and L-CuCu2O(I) shift to positions between Cu2O and metallic Cu, confirming the coexistence of Cu+ and Cu0 species (Figure 2e). Notably, B-Cu-Cu2O(I) exhibits a noticeable shift toward higher edge energy relative to L-Cu-Cu2O(I), indicative of a slightly more oxidized Cuδ+ electronic environment. This observation is consistent with the higher iodine retention in B-Cu-Cu2O(I), where interfacial iodine exerts an electronic modulation effect and partially stabilizes oxidized Cu species. Furthermore, the abundant Cu/Cu2O interfaces in B-Cu-
analysis (Figure 2c). These results suggest that enhanced iodine retention facilitates charge redistribution between Cu and I atoms, thereby optimizing and stabilizing the polarized heterointerfaces in B-Cu-Cu2O(I). Cu LMM Auger spectroscopy further supports this electronic modulation. As shown in Figure 2d, the Cu+-related Auger feature of B-CuCu2O(I) shifts toward lower kinetic energy (KE) relative to that of L-Cu-Cu2O(I), indicating a more electron-deficient Cu environment (Figure 2d).30 For the O 1s XPS spectra, B-CuCu2O(I) presents a significantly more intense signal compared to the CuI-derived control sample, which can be attributed to the abundant surface-adsorbed oxygenated species derived from the nitrate hydroxide precursor (Figure S6a).31 Electron paramagnetic resonance (EPR) measurements reveal negligible differences between the two catalysts, indicating that oxygen vacancies are not the dominant origin of their divergent catalytic behaviors (Figure S6b).32 Synchrotron-based X-ray absorption fine structure (XAFS) measurements were conducted to elucidate the bulk E
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Electrochemical active surface areas (ECSAs), determined from the double-layer capacitance (Cdl), show that B-CuCu2O(I) possesses a larger Cdl (1.08 mF cm–2) than L-CuCu2O(I) (0.86 mF cm–2), indicating a higher electrochemically accessible surface area and more exposed active sites (Figure S14a). Electrochemical impedance spectra (EIS) further reveals that B-Cu-Cu2O(I) exhibits a lower charge-transfer resistance, suggesting its faster CO2RR kinetics (Figure S14b). This is further reflected in the Tafel analysis (Figure S14c), where B-Cu-Cu2O(I) shows a lower slope (276 mV dec–1) than LCu-Cu2O(I) (378 mV dec–1), suggesting optimized kinetics for electrochemical reactions. To further clarify the role of iodine stabilization in promoting C2+ formation, an iodine-free CHN-derived Cu-Cu2O catalyst was also evaluated under identical H-cell conditions (Figure S15). The catalyst achieved a maximum C2H4 Faradaic efficiency of 31.5% at –1.25 V vs RHE, with a total C2+ Faradaic efficiency of 48%. However, the iodine-free CuCu2O catalyst exhibited higher selectivity toward formate production relative to B-Cu-Cu2O(I). These results suggest that although Cu-Cu2O interfacial structures intrinsically contribute to C2+ formation, retained iodine species play a critical role in stabilizing Cuδ+ interfacial sites and promoting efficient C–C coupling. Notably, varying the ratio of the precursor components does not lead to a pronounced change in catalytic activity, suggesting that the intrinsic active sites remain essentially unaffected by the CHN content (Figure S16a). To further evaluate catalytic performance and validate the robustness of these active sites under industrial-relevant current densities, CO2RR was conducted in a flow-cell electrolyzer (0.5 M KOH) equipped with a gas diffusion electrode (GDE), a Ni counter electrode, and an Ag/AgCl reference electrode (Figure 3d). Over a broad current density range from –100 to –350 mA cm–2, B-Cu-Cu2O(I) predominantly produces C2H4 and consistently outperforms L-Cu-Cu2O(I), delivering a peak C2H4 FE of 60.6% with jC2H4 of –150 mA cm–2, along with a maximum C2+ FE of 79.5% at a total current density of –250 mA cm–2 (Figure 3e–g). In contrast, L-Cu-Cu2O(I) mainly yields H2, exhibiting a much lower C2+ FE of 27.6% at –250 mA cm–2. Crucially, stability tests conducted in an alkaline membrane electrode assembly (MEA) electrolyzer at a constant current density of –250 mA cm–2 demonstrate a pronounced difference in durability. While L-Cu-Cu2O(I) rapidly deactivates within 6.5 h due to extensive iodine depletion and Cu+/Cu0 interface destruction, the B-Cu-Cu2O(I) catalyst maintains a stable C2H4 FE of ∼50% for 18.5 h with only minimal potential fluctuations (Figure 3h). This sustained performance corroborates our structural hypothesis that iodine confinement within oxide-derived grain boundaries preserves metastable Cu+ sites against overreduction and leaching under CO2RR conditions. Compared with recently reported catalysts for selective C2H4 formation, B-CuCu2O(I) displays superior performance (Figure 3i).32,35,36 To evaluate the influence of CO2 feed rate, flow-cell tests were conducted under different CO2 flow rates (Figure S16b). The catalytic activity and selectivity remain nearly unchanged, whereas the single-pass carbon efficiency (SPCE) changes with the CO2 flow rate. Importantly, B-Cu-Cu2O(I) achieves an SPCE of 50.2% for C2H4 at a CO2 flow rate of 0.5 sccm, highlighting its excellent carbon utilization capability. Post-catalytic structural analysis for both catalysts after CO2RR was carried out to elucidate their reconstruction behaviors (Figure S17). XRD patterns (Figure S17a) show that both
Cu2O(I) effectively stabilize a larger proportion of oxidized Cu species (Cu+) compared with the more metallic L-CuCu2O(I) control sample. Quantitative XANES analysis further supports this trend, yielding higher average Cu oxidation states for B-Cu-Cu2O(I) (δ = 0.89) than for L-Cu-Cu2O(I) (δ = 0.71) (Figure S8). The Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra (Figure 2f) together with corresponding curve fitting results (Figure 2g and Figure S9) were analyzed to uncover distinct coordination features and quantitatively assess the Cu-centered coordination. The k3-weighted Cu K-edge EXAFS spectra of both catalysts are well reproduced by their respective structural models. Compared to the L-Cu-Cu2O(I) control (1.45 Å), the Cu-O peak for B-Cu-Cu2O(I) shifts to a higher R-value of 1.47 Å. This lattice expansion is associated with abundant interfacial sites derived from the hybrid precursor, where retained iodine species induce local tensile strain rather than lattice incorporation. Wavelet transform (WT) analysis further reveals distinct Cu-Cu and Cu-O contributions, supporting the presence of residual Cu+ species (Figure 2h,i). Moreover, B-CuCu2O(I) exhibits an exceptionally low Cu-Cu coordination number (CN) of 5.9, which is substantially lower than that of L-Cu-Cu2O(I) (CN = 7.7) and bulk Cu2O (CN = 10.5) or metallic Cu (CN = 12) reference samples (Figure S9 and Table S1). This reduced CN indicates a high density of undercoordinated Cu sites and abundant grain boundaries, which are known to be catalytically active motifs for CO2-to-C2+ conversion.34 Consequently, these results collectively demonstrate that the interfacial structure effectively generates lowcoordination Cuδ+ sites, while the retained iodine contributes to stabilizing these sites during reconstruction, thereby creating a favorable environment for C–C coupling. 3.2. Electrochemical Performance
The electrocatalytic performances of L-Cu-Cu2O(I) and B-CuCu2O(I) were systematically evaluated in both CO2-saturated KHCO3 H-cell and KOH flow-cell configurations (Figure 3). Gas-phase products (hydrogen, carbon monoxide, methane, and ethylene) and liquid-phase products (formate, ethanol, acetate, and propanol) were quantified by gas chromatograph (GC) and 1 H nuclear magnetic resonance (1H-NMR) spectroscopy, respectively (Figures S10 and S11). The corresponding calibration curves are provided in Figure S12. Linear sweep voltammetry (LSV) curves in H-cell tests show significantly enhanced current densities under CO2 atmosphere compared to Ar for both catalysts, confirming their CO2RR activity. Notably, B-CuCu2O(I) exhibits a higher current density and a more positive onset potential than L-Cu-Cu2O(I) (Figure 3a). The CO2RR product distribution over B-Cu-Cu2O(I) in the potential range from –0.95 to –1.40 V vs RHE is displayed in Figure 3b, where its C2+ Faradaic efficiency (FE) follows a volcano-shaped trend. As shown in Figure S13, L-Cu-Cu2O(I) shows a similar product distribution of C2H4, C2H5OH and n-C3H7OH, whereas B-CuCu2O(I) achieves a markedly higher C2H4 FE of 50.4% compared with L-Cu-Cu2O(I) (21.7%) at –1.3 V vs RHE. The C2+ FE and corresponding partial current densities for both catalysts are summarized in Figure 3c. At –1.3 V vs RHE, the B-CuCu2O(I) catalyst reaches a C2+ FE of 72% and a maximum C2+ partial current density of 62.1 mA·cm–2, nearly double those of L-Cu-Cu2O(I) (37% and 27.6 mA cm–2, respectively). These results clearly demonstrate that B-Cu-Cu2O(I) promotes both higher selectivity and faster kinetics toward C2+ production. F
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Figure 3. Electrochemical characterizations. (a) LSV curves of the catalysts under different gas feeding conditions in an H-cell. (b) Product distribution over B-Cu-Cu2O(I) under different working potentials. (c) FEC2+ and partial current densities of C2+ products of different catalysts in an Hcell. (d) Illustration for the flow-cell setup in this work. (e) Product distribution over B-Cu-Cu2O(I) in a flow cell. (f) FEC2H4 and jC2H4 and (g) FEC2+ over L-Cu-Cu2O(I) and B-Cu-Cu2O(I) under different current densities. (h) Stability tests of B-Cu-Cu2O(I) and L-Cu-Cu2O(I) at –250 mA cm–2. (i) Comparison of FE of C1, C2+, C2H4, jC2H4 and single-cell efficiency to C2H4 of B-Cu-Cu2O(I) with previous reports.
substantial reconstruction occurring under reaction conditions (Figure S18a). Post-electrolysis STEM-EDS elemental mapping further shows that iodine remains detectable throughout the BCu-Cu2O(I) catalyst after prolonged CO2RR operation. In comparison, a noticeably weaker iodine signal is observed in L-Cu-Cu2O(I), suggesting that the boundary-rich Cu/Cu2O architecture facilitates iodine retention during electrolysis (Figure S18b). In addition, ex situ XPS measurements reveal a detectable I 3d signal after the stability test, although with reduced intensity relative to the fresh catalyst, indicating partial iodine loss during prolonged operation (Figure S19). These results collectively suggest that the boundary-rich Cu/ Cu2O(I) catalyst retains part of its interfacial structure and iodine species after extended CO2RR operation.
samples evolve into Cu-rich phases after electrolysis, while BCu-Cu2O(I) uniquely preserves more discernible Cu2O reflections, indicating its stabilized Cu+/Cu0 domains. The critical role of iodine is further revealed by ex situ surface spectroscopic analysis: B-Cu-Cu2O(I) displays a clear Cu 2p shift toward lower binding energy and a Cu LMM Auger shift toward higher kinetic energy, which directly correlates with the high retention of surface iodine (Figure S17b–d). In contrast, L-CuCu2O(I) shows almost no metallic Cu feature in the Cu LMM spectrum because its surface is structurally unstable and undergoes rapid oxidation upon air exposure. This implies that the retained iodine acts as a protective ligand, preserving the metastable metallic/oxide interfacial structure required for C–C coupling against both electrochemical degradation and environmental oxidation. Post-catalytic microscopic and spectroscopic characterizations were further conducted to investigate the structural evolution of B-Cu-Cu2O(I) after long-term CO2RR operation. HRTEM analysis reveals that Cu and Cu2O domains remain in close contact after electrolysis, indicating that portions of the Cu/Cu2O interfacial architecture are retained despite the
3.3. Electrocatalytic Mechanism Studies
To probe the dynamic evolution of Cu species under reaction conditions, in situ XAS measurements were performed. The XANES spectra show that both B-Cu-Cu2O(I) and L-CuCu2O(I) undergo pronounced shifts of the absorption edge toward lower energy with increasing cathodic potential, G
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more thorough reduction to metallic Cu (Figure 4c and Figure S20). This difference is consistent with the presence of stabilized interfacial iodine species in B-Cu-Cu2O(I), which help suppress excessive Cu reduction and maintain partially oxidized Cuδ+ sites during reconstruction. To elucidate the catalytic mechanism and reaction pathway, in situ infrared absorption spectroscopy was conducted. As shown in Figure 4d,e, the appearance of a band at ∼1430 cm–1 (*COOH) is observed, signaling initial activation and reduction of CO2.37 Additional bands located at around
indicating substantial reduction of Cu species (Figure 4a). Quantitative analysis of the edge position (E0) reveals that BCu-Cu2O(I) maintains a relatively higher average Cu valence state compared with L-Cu-Cu2O(I) under identical conditions (Figure 4b). EXAFS analysis further shows that, despite dominant Cu-Cu coordination, B-Cu-Cu2O(I) retains a discernible Cu-O contribution, whereas L-Cu-Cu2O(I) is almost entirely characterized by Cu-Cu coordination. These results indicate that B-Cu-Cu2O(I) preserves a greater quantity of Cu+ species under reduction conditions, while L-Cu-Cu2O(I) undergoes
Figure 4. Mechanism investigations. (a) In situ Cu K-edge XANES spectra. (b) Derived normalized χμ(E) spectra. (c) In situ Fourier-transform k3weighted EXAFS spectra. In situ Fourier transform infrared spectra obtained during chronopotentiometry in a potential window from –0.9 to –1.5 V vs RHE for (d) L-Cu-Cu2O(I) and (e) B-Cu-Cu2O(I) under CO2RR. (f−h) In-situ Raman spectra of L-Cu-Cu2O(I) and B-Cu-Cu2O(I). (i) Comparison of the HFB/(HFB + LFB) ratio as a function of applied potential for different catalysts. (j) Adsorption energy comparison of *CO2 and *CO, (k) CO2RR energy diagram showing the pathways alongside the key intermediates toward the C2H4 products on different models. H
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1240 and 1540 cm–1, corresponding to the *OCCO intermediate, reflect C–C coupling via symmetric *CO/*CO interactions.38 Meanwhile, the atop-adsorbed CO (*COatop, ∼2140 cm–1) and bridge-adsorbed CO (*CObridge, ∼1960 cm–1) bands evolve with increasing cathodic potential, indicating that the CO2RR pathway is modulated by the surface structure.39 The *OCCO intermediate, formed via CO dimerization and serving as a key precursor for C2H4 production, becomes increasingly discernible at more negative potentials. Notably, the *OCCO band is more prominent on B-Cu-Cu2O(I) than on L-Cu-Cu2O(I), evidencing that the iodine-confined interface promotes C–C coupling more effectively and thus favors C2+ product formation, consistent with its superior CO2RR performance. In-situ Raman spectroscopy further clarified the surface adsorbate dynamics (Figure 4f–i and Figure S22). Three characteristic *CO-related bands at ∼265, 351, and 2101 cm–1, assigned to γ(Cu-CO), ν(CuCO), and ν(CO) vibrations of atop-bound *CO,27 were observed on both catalysts. In the low-frequency region, bands associated with frustrated rotation (P1, 260–310 cm–1) and stretching (P2, 350–390 cm–1) modes of Cu-*CO were detected. The P2/P1 ratio, indicative of *CO availability and mobility on Cu surfaces,40 is consistently higher for B-CuCu2O(I), highlighting the role of abundant undercoordinated Cu sites in stabilizing reactive *CO configurations (Figure 4f). Deconvolution of the 1950–2100 cm–1 region reveals contributions from *CObridge and two *COatop components (lowfrequency *COLFB and high-frequency *COHFB bands).41 The *COHFB modes, typically associated with step or undercoordinated sites, are known to favor CO dimerization. On BCu-Cu2O(I), these *CO features persist under all potentials, reflecting effective stabilization of *CO intermediates by undercoordinated Cu sites (Figure 4h). In contrast, L-Cu-Cu2O(I) is dominated by *COLFB with negligible *COHFB, and its overall *CO intensity drops rapidly with increasing negative potentials (Figure 4g), indicating less efficient *CO retention. Analysis of the *COHFB/(*COHFB+*COLFB) ratio indicates that B-CuCu2O(I) possesses a higher density of undercoordinated sites across applied potentials (Figure 4i), consistent with enhanced *CO-*CO coupling. For comparison, in situ Raman spectra of the iodine-free CHN-derived Cu-Cu2O catalyst were also collected (Figure S21). Unlike B-Cu-Cu2O(I), the CHNderived Cu-Cu2O catalyst does not exhibit the distinct Cu2Orelated Raman feature at ∼533 cm–1 under CO2RR conditions, indicating that persistent Cuδ+ interfacial domains cannot be effectively maintained in the absence of iodine species.42 Collectively, these results demonstrate that the interfacial structure generates abundant undercoordinated Cuδ+ sites that stabilize *CO intermediates, while iodine helps maintain these active sites during reconstruction, thereby promoting efficient C–C coupling and enhanced C2+ selectivity. Density functional theory (DFT) calculations were performed to elucidate the origin of the spectroscopic and catalytic trends. Cu2O(111) and Cu(111) were selected as model facets, constructed in accordance with the experimental observations (Figures S22 and S23). Incorporation of iodine at the Cuoxide interface markedly enhances *CO2 adsorption, enabling more efficient capture and activation of CO2 for subsequent reduction steps.43 The calculations further reveal that *CO is preferentially stabilized at undercoordinated interfacial Cu sites, where metallic and oxide characteristics coexist in the presence of iodine, whereas purely metallic or oxide-terminated surfaces
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bind *CO weakly.44 This strengthened *CO retention at the iodine-stabilized interface increases the steady-state *CO coverage, thereby providing a sustained pool of reactive *CO species for C–C coupling. Such a trend is fully consistent with the persistent atop-*CO signatures in the in-situ Raman spectra, and the experimentally enhanced C2+ formation (Figure 4j). The results establish that iodine incorporation simultaneously promotes CO2 uptake and strengthens *CO retention at undercoordinated interfacial Cu sites, underpinning the superior C2+ selectivity observed experimentally. Specifically, the undercoordinated interfacial Cu sites in B-Cu-Cu2O(I) induce an upward shift of the Cu d-band center (–2.14 eV), positioning it closer to the Fermi level than those of Cu2O-Cu (–2.27 eV), Cu2O-I (–2.26 eV), and metallic Cu (–2.29 eV). This d-band upshift strengthens the interaction between Cu-d states and the antibonding orbitals of adsorbates, facilitating electron back-donation to *CO2/*CO intermediates (Figure S24).45 Differential electron density analyses further reveal pronounced electron accumulation on the O-C-O unit accompanied by electron depletion on neighboring Cu atoms at the interfacial sites, indicating enhanced stabilization of activated *CO2 intermediates (Figure S25).43 These computational insights are fully supported by temperature programmed desorption (TPD) measurements (Figure S26), which show that B-Cu-Cu2O(I) exhibits a higher CO2 adsorption capacity than the other sample, confirming that the multi-interfaces and iodine incorporation synergistically promote CO2 binding and improve CO2 activation and *CO retention. To further quantify the impact of iodine-modified interfaces on C–C coupling, energy diagrams for the reaction pathway were constructed (Figure 4k). The adsorption of *CO on the interfacial Cu-Cu2O(I) surface (ΔE = –1.87 eV) is substantially stronger than on Cu (–1.05 eV), Cu2O(I) (–1.65 eV), and CuCu2O (–1.56 eV), evidencing strong CO stabilization induced by the iodine-modified interfaces. Moreover, the subsequent 2CO adsorption step is also favorable on Cu-Cu2O(I), exhibiting a moderate adsorption energy of –0.19 eV. Importantly, the *OCCO formation energy on Cu-Cu2O(I) (–0.17 eV) is downhill and markedly lower than those on Cu (0.93 eV), Cu2O(I) (–0.02 eV), and Cu-Cu2O (0.95 eV). These results collectively demonstrate that the Cu/Cu2O heterointerface provides active interfacial Cu sites for *CO stabilization, while iodine incorporation further enhances *CO retention and lowers the energy barrier for its dimerization to form *OCCO, thereby promoting the C2 pathway on B-CuCu2O(I).
- CONCLUSIONS In summary, this work demonstrates a robust interfacial engineering strategy that resolves the critical trade-off between activity and stability in CO2RR to C2H4. By leveraging the unique topotactic transformation of a CuI/Cu2(OH)3NO3 precursor, we construct a B-Cu-Cu2O(I) catalyst featuring abundant Cu+/Cu0 heterointerfaces that enable higher iodine retention within the catalyst structure. This interfacial architecture effectively suppresses iodine leaching and mitigates excessive surface reconstruction and active site loss under CO2RR conditions. Comprehensive analyses reveal that the heterointerface generates undercoordinated Cuδ+ sites essential for *CO adsorption, while the increased iodine retention I
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Transformation of Carbon Dioxide, The University of Sydney, Sydney, New South Wales 2006, Australia Zhipeng Ma – School of Chemical Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia; orcid.org/0000-0002-0719-7710 Qian Sun – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Haochen Lu – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Yutong Wu – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Zhun Shi – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Ruirui Liu – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Jun Chen – Intelligent Polymer Research Institute, Australian Institute for Innovative Materials, Innovation Campus, North Wollongong, New South Wales 2500, Australia Fengwang Li – School of Chemical and Biomolecular Engineering and ARC Centre of Excellence for Green Electrochemical Transformation of Carbon Dioxide, The University of Sydney, Sydney, New South Wales 2006, Australia; orcid.org/0000-0003-1531-2966 Complete contact information is available at: https://pubs.acs.org/doi/10.1021/acscatal.6c03153
plays a critical role in stabilizing these sites and tuning their electronic structure. This synergistic effect strengthens *CO retention and lowers the thermodynamic barrier for C–C coupling. As a result, the optimized catalyst achieves an impressive C2+ FE of ∼80% (with 61% selectivity for C2H4) and sustained stability over 18 h at an industrial-relevant current density of –250 mA cm–2. Beyond this specific system, our work underscores the broad potential of I-mediated interfacial regulation as a general strategy for catalyst design. Stabilizing metastable oxidation states through interfacial confinement offers a clear conceptual framework for the development of nextgeneration electrocatalysts that combine high activity with long-term durability. Future research could extend this strategy to dynamic regeneration systems and other multivalent metal surfaces, thereby promoting the development of efficient and durable systems for the electrocatalytic industry. ■ ASSOCIATED CONTENT Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon request. Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.6c03153. Details of materials characterization. Supplementary characterization of the catalysts: SEM, TEM, EDS, XRD, XPS, XAS, EPR, and TPD. Electrochemical measurements: LSV, CV, ECSA, EIS, and potential-dependent product distributions. Product analysis: GC profiles of gaseous products and 1H NMR spectra of liquid products. DFT calculations: computational models and PDOS of different electrocatalysts (DOCX)
Author Contributions
The manuscript was written through contributions of all authors.
■ AUTHOR INFORMATION
Funding
Corresponding Authors
This work was supported by the Australian Research Council (CE230100017, FL250100099, IC200100023, DP250101509)
Martina Lessio – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia; orcid.org/0000-0002-5143-9924; Email: martina.lessio@unsw.edu.au Chuan Zhao – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia; orcid.org/0000-0001-7007-5946; Email: chuan.zhao@unsw.edu.au
Notes
The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors acknowledge financial support from the Australian Research Council (CE230100017, FL250100099, IC200100023, DP250101509). The authors also thank the UNSW Mark Wainwright Analytical Centre (MWAC) for access to SEM, TEM, XRD, NMR, FTIR, and XPS facilities. XAS measurements were carried out at the X-ray Absorption Spectroscopy Beamline of the Australian Synchrotron (ANSTO, Melbourne). This work includes computations carried out using the computational cluster Katana supported by Research Technology Services at UNSW Sydney and computational resources from the National Computational Infrastructure (NCI) provided by the Australian government.
Authors
Mengmeng Yang – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia; orcid.org/0009-0009-8229-0323 Sohaib Umer – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Ruizhi Wu – School of Chemistry, Faculty of Science, University of New South Wales, Sydney, New South Wales 2052, Australia Chen Jia – School of Food Science and Pharmaceutical Engineering, Nanjing Normal University, Nanjing, Jiangsu 210023, China; orcid.org/0000-0001-7086-1832 Yu Yang – School of Chemical and Biomolecular Engineering and ARC Centre of Excellence for Green Electrochemical
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