Concave surface-enriched reactant and enhanced mass transfer for electrocatalytic ethylene production from low-concentrated acetylene

doi:10.21203/rs.3.rs-4215383/v1

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Concave surface-enriched reactant and enhanced mass transfer for electrocatalytic ethylene production from low-concentrated acetylene

https://doi.org/10.21203/rs.3.rs-4215383/v1

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published 13 Jul, 2024

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Electrocatalytic semihydrogenation of acetylene (C₂H₂) provides a facile and petroleum-independent strategy for ethylene (C₂H₄) production. However, the reliance on the preseparation and concentration of raw coal-derived C₂H₂ hinders its economic potential. Here, density functional theory calculations demonstrate that a concave surface is beneficial for enriching C₂H₂ and optimizing its mass transfer kinetics, thus leading to a high partial pressure of C₂H₂ around active sites, which is suitable for the direct conversion of raw coal-derived C₂H₂. Then, a porous concave carbon-supported Cu nanoparticle (Cu-PCC) electrode was designed to enrich the C₂H₂ gas around the Cu sites. As a result, the as-prepared electrode enables a 91.7% C₂H₄ Faradaic efficiency and a 56.31% C₂H₂ single-pass conversion under a simulated raw coal-derived C₂H₂ atmosphere (~ 15%) at a partial current density of 0.42 A cm^− 2, greatly outperforming its counterpart without concave surface supports. The strengthened intermolecular π conjugation caused by the increased C₂H₂ coverage is revealed to result in the delocalization of π electrons in C₂H₂, consequently promoting C₂H₂ activation, suppressing HER competition, and enhancing C₂H₄ selectivity.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Chemistry/Green chemistry/Sustainability

The production of the essential chemical ethylene (C₂H₄) is highly dependent on high-temperature naphtha cracking, which relies on petroleum resources with excess carbon emissions.^1,2 Hence, developing a petroleum-independent and mild strategy for C₂H₄ production is highly desirable for a low-carbon economy.^3–8 Recently, the electrocatalytic semihydrogenation of coal-derived acetylene (C₂H₂) to ethylene (ESAE) strategy has been developed.^7,8 Inhibiting the competing hydrogen evolution reaction (HER) at an industrial current density (≥ 200 mA cm^− 2) is pivotal for the economic potential of the ESAE strategy. At present, both the C₂H₄ Faradaic efficiency (FE) and the optimal current density are extremely low for low-concentration C₂H₂ hydrogenation (e.g., ˂50% FE at 60 mA cm^− 2 for ~ 1% C₂H₂ impurity hydrogenation in C₂H₄), which is far from the target of practical C₂H₄ production.⁷ Additionally, the cost of separating and concentrating C₂H₂ feed gas accounts for a large proportion of the total C₂H₄ production cost.⁸ Consequently, the cost of C₂H₄ production would further decrease if the raw tail gas (~ 15% C₂H₂) from the arc-plasma process of coal could be directly used as feedstock for the ESAE process.^8–13 However, the HER dominates the whole process as the C₂H₂ concentration decreases (Fig. 1a). Therefore, further development of highly efficient and selective catalysts for converting raw coal-derived C₂H₂ into C₂H₄ with high selectivity and conversion rates is urgently needed.

Generally, for a gas-involved reaction, enriching its local concentration and boosting the mass transfer toward active sites are important for enhancing the reactant’s partial pressure to improve the activity and selectivity.^14–17 Copper (Cu) nanoparticles have been proven to be an appropriate choice for suppressing the competing HER under high C₂H₂ partial pressure.^18–21 Thus, the critical issue for the selective conversion of raw coal-derived C₂H₂ lies in enriching the concentration and guaranteeing the facile mass transfer of C₂H₂ around the surface of the Cu nanoparticles. Structures with high curvatures always lead to a high local electric field, which can gather reactants around the catalyst surface and increase its concentration.^22–26 For example, Liu et al. demonstrated that Cu nanoneedles could increase the adsorption of the *CO intermediate and, in turn, accelerate C–C coupling during the electrocatalytic CO₂ reduction process ²³. In addition to the tips, the concave surface also has a high curvature.^27–29 Moreover, carbon-based supports with porous structures could effectively boost gas capture and transport, benefitting mass transfer.^30–32 In this regard, Cu nanoparticles loaded on porous carbon supports with abundant nanosized concave surfaces (denoted as Cu-PCC) are expected to be efficient at increasing the concentration and increasing the mass transfer of C₂H₂ around Cu sites through the unique concave support, consequently suppressing the HER under low-concentration raw coal-derived C₂H₂ (Fig. 1b). However, the synthesis and exploration of porous concave carbon-supported Cu nanoparticle electrodes for electrocatalytic C₂H₄ production are lacking.

Herein, a preliminary density functional theory (DFT) calculation was first conducted to show that a concave surface is beneficial for the enrichment and facile mass transfer of C₂H₂, increasing its partial pressure around the active sites. Then, we designed a facile self-template method to synthesize Cu-PCC, which was found to be an outstanding electrocatalyst for the ESAE process, using simulated raw coal-derived C₂H₂ as feedstocks. Cu-PCC delivered a C₂H₄ FE of 91.70% and a single-pass C₂H₂ conversion of 56.31% at a potential of − 1.2 V versus a reversible hydrogen electrode (vs. RHE) at a partial current density of 0.42 A cm^− 2, greatly outperforming the Cu nanoparticles supported on carbon without a concave surface counterpart. Moreover, C₂H₂ temperature-programmed desorption (C₂H₂-TPD) and in situ spectroscopic characterization experiments revealed that the polarization field induced by the concave surface over Cu-PCC increased C₂H₂ coverage and strengthened the intermolecular π-conjugation of C₂H₂, thus leading to the delocalization of the π electrons of C₂H₂ to promote the activation of C₂H₂ and enhance the C₂H₄ selectivity of the ESAE with raw coal-derived C₂H₂.

The design and synthesis of electrocatalyst

We first conducted DFT calculations to evaluate the local field induced by the concave surface. As shown in Fig. 2a, the electrons are enriched at the concave carbon surfaces to build a polarization field, which could enhance the conjugation between C₂H₂ and the negative center (Fig. 2b and Supplementary Fig. 1), thus leading to the downshifting of the bonding orbital and benefiting the enrichment of C₂H₂ (Fig. 2c).^21,29,33 Once the low-concentration C₂H₂ accumulated on the concave carbon surfaces, facile migration to the Cu sites was still a prerequisite for the following reaction. In that case, simulations of the migration pathway of the C₂H₂ molecule in solution over the Cu-C and Cu-PCC interfaces were conducted (Supplementary Figs. 2–3). For gas-involved reactions, there will be a few layers of water clusters (WC) due to the hydrogen bonding network around the gas‒solid-liquid three-phase interface and the gap between the WC and solid surface (labeled d in Fig. 2d) provides a diffusion channel for gaseous reactants. As shown in Fig. 2d, the diffusion channel for Cu-PCC is larger than that for Cu-C, thus leading to a straight-line migration pathway rather than a distorted pattern over the counterpart. The associated migration energy barriers were calculated, as shown in Fig. 2e. The maximum migration energy for C₂H₂ diffusion over Cu-PCC is 0.41 eV, which is much lower than that of Cu−C (1.23 eV), indicating that the mass transfer of C₂H₂ is significantly greater over the concave surface. These theoretical results indicate that a carbon support with concave surfaces could efficiently gather low-concentration raw coal-derived C₂H₂ feedstocks and increase the mass transfer kinetics for subsequent hydrogenation over Cu sites.

Generally, the collapse and reconstruction of a surface increases the roughness and results in many nanosized concave surfaces.^34,35 Thus, a sequential self-template transformation method based on the Kirkendall effect was proposed for the synthesis of Cu-PCC (Fig. 3a).^36,37 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed the successful preparation of Cu-based metal-organic framework precursors (Cu-MOF) with planar surfaces and octahedral-like morphologies (Supplementary Fig. 4). After the reaction of Cu-MOF precursors with tannic acid (TA), the octahedron-like shapes can be maintained, and the surface collapses inward (denoted as Cu-TA, Supplementary Fig. 5). After the annealing of Cu-TA under an H₂ atmosphere, the wall of the Cu-TA complex was converted into porous carbon with abundant nanosized concave surfaces, as confirmed by scanning transmission electron microscopy (STEM), SEM, and TEM images (Figs. 3b, c and Supplementary Fig. 6).³⁸ However, Cu-C directly calcinated from Cu-MOF precursors without the collapse process exhibited a planar carbon surface (Figs. 3d, e and Supplementary Fig. 7). Moreover, the atomic force microscopy (AFM) images also demonstrated the rougher surface of Cu-PCC caused by these concave surfaces compared to that of Cu-C (Fig. 3f). In addition, X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectra, and Raman spectra were obtained to monitor the sequential conversion process from Cu-MOF precursors to Cu-TA and eventually to Cu-PCC (Supplementary Fig. 8). The Raman, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) results show that there are no other differences between Cu-PCC and Cu-C, other than the nanosized concave surfaces over the PCC supports (Supplementary Figs. 9–11). Note that the negative shift in the binding energies of C-O and C = O over Cu-PCC compared to that over Cu-C verifies the existence of a polarization field (Fig. 3g).^39–41 Furthermore, C₂H₂-TPD was employed to evaluate the C₂H₂ gas enrichment ability of Cu-PCC.⁴² The greater C₂H₂ adsorption on Cu-PCC than on its Cu-C counterpart, along with the ever-increasing desorption temperature under similar specific surface areas (Fig. 3h and Supplementary Fig. 12), indicated that the enriched C₂H₂ and optimized mass transfer were the main reasons for the enhanced interactions between the C₂H₂ feedstocks and Cu-PCC.

ESAE reaction analysis and performance evaluation

The ESAE process was evaluated in a three-electrode flow cell with a gas diffusion layer under potentiostatic conditions using simulated raw coal-derived C₂H₂ (~ 15%) as the feeding gas. First, online differential electrochemical mass spectrometry (DEMS) was conducted under linear sweep voltammetry (LSV) mode to explore and analyze the ESAE process (Fig. 4a). In addition to Cu-PCC possessing a more positive onset potential for C₂H₂ hydrogenation (− 0.06 V vs. RHE) than Cu-C (− 0.1 V vs. RHE), Cu-PCC has a much more negative HER onset potential, endowing Cu-PCC with a better ability to activate C₂H₂ and suppress the HER (Figs. 4b, c and Supplementary Figs. 13–14). Moreover, the signal intensity of C₂H₄ over Cu-C displays a nearly volcanic shape, and the response of H₂ acutely increases under potentials more negative than − 0.6 V vs. RHE. However, the C₂H₄ signal always dominated the whole product until the end of the LSV over Cu-PCC. In other words, the production gap between C₂H₄ and the H₂ byproduct becomes larger with decreasing potential over Cu-PCC, while it shows an inverse trend over Cu-C, further verifying the superiority of Cu-PCC in the ESAE process. In addition, the same electron transfer number of C₂H₂ to C₂H₄ and H₂O to H₂ is likely the reason for the similar LSV curves of the two catalysts. Then, we performed the DEMS test under square wave potentials. For Cu-PCC, H₂ emerges under a much more negative potential than does its Cu-C counterpart. In addition, unlike the almost unchanged or even decreased C₂H₄ signal observed for Cu-C, the C₂H₄ signal increases with decreasing potential, indicating better C₂H₄ selectivity in Cu-PCC under a raw coal-derived C₂H₂ atmosphere (Figs. 4d, e).

The quantification of the ESAE process showed that the FE of C₂H₄ over Cu-PCC exceeded ~ 90%, and the C₂H₆ byproduct was almost undetectable throughout the whole range (Fig. 5a). However, the FE of C₂H₄ decreased rapidly, with more H₂ produced at more negative potentials than − 0.8 V vs. RHE over Cu-C (Fig. 5b). Furthermore, the obtained C₂H₄ production rate of Cu-PCC at a potential of − 1.2 V vs. RHE was 3.42 mol g_cat⁻¹ h^− 1 with a partial current density and C₂H₂ conversion of 0.42 A cm^− 2 and 56.31%, respectively, greatly surpassing those of its Cu-C counterpart (2.00 mol g_cat⁻¹ h^− 1, 0.26 A cm^− 2 and 33.21% C₂H₂ conversion) (Figs. 5a, b). The superiority of the C₂H₄ production rates of Cu-PCC became more obvious after normalization by the electrochemical surface area (ECSA) or Cu loading capacity (Fig. 5c, and Supplementary Figs. 15–17). In addition, considering the demand for industrial production, stability evaluation experiments at various concentrations and step potentials for the hydrogenation of C₂H₂ were performed. Accordingly, the 95% confidence intervals of FEs were calculated to evaluate the selectivity stability, as shown in Fig. 5d. The narrower confidence intervals of the C₂H₄ and H₂ FEs over Cu-PCC than Cu-C demonstrate that the fitted lines of FEs under different concentrations of Cu-PCC are more precise,⁴³ which means that the influence of concentration on FEs is less significant over Cu-PCC,⁴⁴ indicating its better stability. In addition, the FE and selectivity of C₂H₄ remain unchanged at different potentials over Cu-PCC, which is superior to its counterpart (Fig. 5e). These results indicate that the proposed Cu-PCC exhibits potential- and concentration-independent ESAE activity, which is suitable for practical application. Note that both the FE of C₂H₄ and the C₂H₂ conversion over Cu-PCC remained unchanged within the error range during the 12 h continuous test, suggesting its robust durability (Fig. 5f and Supplementary Fig. 18).

Mechanistic exploration of the high selectivity for ethylene

To elucidate the reason for the increase in C₂H₄ FE and selectivity over Cu-PCC under a raw coal-derived C₂H₂ atmosphere, in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) and Raman spectroscopies were used to evaluate the status and coverage of C₂H₂ with the catalytic surface (Supplementary Figs. 19–20). Generally, the peak frequency of the IR or Raman characteristic peak is determined by the strength of the corresponding bond.^45–48 For C₂H₂, the enhanced π conjugation led to the redistribution of bonding electrons, and the corresponding π bond was weakened due to the delocalization of electrons, consequently leading to a negative shift in the peak frequency (redshift).⁴⁹ As shown in Figs. 6a, b, both ν(C-H) (~ 3200 cm^− 1) and ν(C ≡ C) (~ 1620 cm^− 1) of C₂H₂ over Cu-PCC shift to lower frequencies than do their Cu-C counterparts at each potential (Supplementary Figs. 21, 22),^7,8,50,51 indicating that the triple bond of C₂H₂ becomes unstable over Cu-PCC due to the delocalization of π electrons; thus, the C₂H₂ molecule is easier to activate. Moreover, the redshift of the peak attributed to adsorbed C₂H₂ in the Raman spectrum from 1700 cm^− 1 over Cu-C to 1685 cm^− 1 over Cu-PCC also confirmed the attenuation of the π bonds of C₂H₂, further demonstrating that support with concave surfaces is beneficial for C₂H₂ activation (Figs. 6c, d).^20,21 In addition, considering that the integrals of the IR bands are related to the coverage of the respective adsorbate on the surface, the area ratio between ν(C ≡ C) and δ(H-O-H) was viewed as the descriptor of the relative coverage of C₂H₂ over the catalyst surface. The plot of ν(C ≡ C)/δ(H-O-H) over Cu-C exhibited a volcano-like profile, which began to decrease at potentials more negative than − 0.2 V vs. RHE, indicating that H₂O adsorption improved with the negative shift potential and accounted for the strong HER competition. Conversely, the plot of ν(C ≡ C)/δ(H-O-H) over Cu-PCC presented a nearly monotonically increasing trend with negatively shifted potentials (Fig. 6e), demonstrating the higher C₂H₂ coverage of Cu-PCC under the applied potential range and accounting for the enhanced intermolecular π conjugation and the delocalization of π electrons from C₂H₂.⁵² Finally, we also conducted DFT calculations to evaluate the C₂H₂ hydrogenation energy barrier under high and low coverage to verify our experimental results. As shown in Fig. 6f, the hydrogenation barriers under high C₂H₂ coverage are lower than those under low coverage (Supplementary Figs. 23–27), indicating easier activation of C₂H₂ and better hydrogenation kinetics over Cu-PCC. Accordingly, the in-depth origins for the enhanced C₂H₄ selectivity obtained using low-concentration raw coal-derived C₂H₂ over Cu-PCC are summarized in Fig. 6g. The C₂H₂ feedstocks could be enriched by the nanosized concave carbon surfaces and then effectively transferred to the Cu sites, consequently resulting in high C₂H₂ partial pressure and coverage. Then, the electron delocalization effect due to the increased C₂H₂ coverage promoted C₂H₂ activation, thus leading to satisfactory C₂H₄ selectivity and FE over Cu-PCC.

In summary, Cu nanoparticles loaded on carbon supports with abundant nanosized concave surfaces were designed to enhance C₂H₂ adsorption for direct utilization of raw coal-derived C₂H₂. Cu-PCC delivered a C₂H₄ FE of 91.7% and a single-pass C₂H₂ conversion of 56.31% under a potential of − 1.2 V vs. RHE at a partial current density of 0.42 A cm^− 2, greatly outperforming its counterpart without the concave surface. Notably, the nanosized concave surfaces were significantly enriched in C₂H₂ gas and had lower mass transfer kinetics, resulting in higher C₂H₂ coverage. Moreover, the delocalization of π electrons in C₂H₂ due to the strengthened intermolecular π conjugation caused by the increased C₂H₂ coverage promoted the activation of C₂H₂, thus endowing Cu-PCC with robust HER suppression ability and better C₂H₄ selectivity. Our work may not only demonstrate an efficient and selective catalyst for nonpetroleum C₂H₄ electrosynthesis but also open a facile way to access low-concentration gaseous reactants for various catalytic applications.

Synthesis of Cu-MOF precursors. According to previous literature³⁷, the Cu-MOF precursors were prepared by a PVP-assisted strategy as follows. First, 1.46 g of Cu(NO₃)₂·3H₂O and 0.7 g of H₃BTC were dissolved in 20 mL of DMF to form solution A and solution B, respectively. Subsequently, 0.5 g PVP was added to solution A and stirred for 5 min to obtain a homogenous solution. Then, solution B was mixed with solution A and stirred for an additional 10 min. Afterward, the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 80°C for 24 h. Finally, the blue precipitates were harvested by centrifugation, washed with DIW and ethanol several times, and dried in a vacuum oven overnight to produce the Cu-MOF precursors.

Synthesis of Cu-TA. The as-prepared Cu-MOF precursors (100 mg) and tannic acid (TA) (50 mg) were first dispersed into 50 mL of DIW to form two solutions. The two solutions were subsequently mixed at room temperature and stirred for 30 min. Afterward, the mixture was put into an oil bath at 50°C and refluxed under continuous magnetic stirring (stirring speed: 700 rpm) for 7 h. The precipitate was then washed with DIW and absolute ethanol at least three times to remove the residual TA and dried at 70°C in a vacuum oven overnight.

Synthesis of Cu-PCC and Cu-C. To obtain Cu-PCC and Cu-C, the as-prepared Cu-TA and Cu-MOF precursors were annealed at 400°C for 2 h at a heating rate of 5°C min^− 1 under a 3% H₂/Ar atmosphere. The mixture was then naturally cooled to room temperature.

General characterizations. Quasi-in situ powder X-ray diffraction (XRD) was performed on a Bruker D8 Focus Diffraction System (Germany) using a Cu Kα radiation source (λ = 0.154178 nm). Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were conducted with an FEI Apreo S LoVac microscope (10 kV). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL-2100F system equipped with an EDAX Genesis XM2. X-ray photoelectron spectroscopy (XPS) was conducted with a PHI-1600 X-ray photoelectron spectrometer equipped with Al Kα radiation. All the peaks were calibrated with the Ti 2p spectrum since C 1s is a key parameter in our research. The Raman spectra were obtained with a Renishaw inVia reflex Raman microscope under excitation with a 514 nm laser at a power of 20 mW. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet IS50 instrument. The Brunauer–Emmett–Teller (BET) surface area was measured by N₂ adsorption using a Micromeritics ASAP 2460. Inductively coupled plasma‒optical emission spectrometry (ICP‒OES) was conducted with an Agilent 5110 instrument (OES). Atomic Force Microscope (AFM) was carried out on a Bruker Dimension Icon.

Electrochemical measurements in the flow cell. Electrochemical measurements were carried out in a typical flow cell consisting of a GDL as the working electrode, Pt foil as the counter electrode, and Hg/HgO as the reference electrode using a CS150H electrochemical workstation. The cathode cell and anode cell were separated by a Nafion 117 proton exchange membrane. The cathode and anode electrolytes were both composed of 1.0 M KOH solution, and a peristaltic pump was used to circulate the liquid phase. The gas flow rate was controlled by a mass flowmeter. Before the performance tests, the working electrode was fixed at the interface between the gas flow block and the cathodic electrolyte block by conductive copper tape. First, the electrochemical semihydrogenation of acetylene was conducted at different applied potentials for 10–20 min to achieve relatively stable and reliable performance parameters before quantitative analysis. The gas at the flow cell outlet was directly introduced into the gas chromatography system for analysis of the products. All the LSV curves were iR compensated with a compensation level of 70%. For the Tafel slopes, the LSV curves were replotted by using the logarithms of the current density as the x-axis and the potential as the y-axis. The obtained slopes of the linear part of the replotted figure were the Tafel slopes.

Quantitative analysis of the C₂H₂ conversion, evolution rate, and FE of the obtained products. The products were subjected to a GC-2010 gas chromatograph equipped with an activated carbon-packed column (with He as the carrier gas) and a barrier discharge ionization detector. The C₂H₂ conversion and evolution rate of the different products were calculated using equations (1) and (2), and the FEs of the different products were calculated using Eq. (3). All the experiments were repeated three times.

$$\text{Conversion }\left(\text{%}\right)\text{ = }\frac{\text{the peak area of B-peak area of A}}{\text{the peak area of B}}\text{ ×100% (1) }$$

$$\text{Evolution Rate (}\text{mmol}\text{/mg/h)=}\frac{\text{the peak area of X ×}\text{C}}{\text{the peak area of standard }\text{gas×}\text{m}\text{ }}\text{ × }\text{S }\text{ (2)}$$

$${\text{FE}}_{\text{X}}\left(\text{%}\right)\text{=}\frac{\text{a × }{\text{n}}_{\text{X}}\text{ ×}\text{F}}{\text{Q}} \text{(3)}$$

X: The different products, including H₂, C₂H₄, and C₂H₆.

C: The concentration of X in standard gas.

m: The mass of catalysts over the electrode.

n: The moles of different products, including H₂, C₂H₄, and C₂H₆.

A: Area of the C₂H₂ outlet; B: area of the C₂H₂ inlet.

S: The gas flow rate.

a: The electron transfer number.

F: Faraday constant.

Q: The total Coulomb number of the ESAE process.

Electrochemical operando online DEMS analysis. Operando online DEMS analysis was conducted with a QAS 100 instrument provided by Linglu Instruments (Shanghai) Co., Ltd. Because the products in the proposed ESAE process were all in the gas phase, operando experiments were conducted to monitor the distribution of the products during the on-stream reaction, clarifying the selectivity issues more directly and clearly. The flow cell used in the performance evaluation and the DEMS were coupled to ensure that the gas at the flow cell outlet was directly injected into the negatively pressured gas circuit system of the DEMS through a quartz capillary that was inserted into the outlet of the flow cell. The LSV test and rectangular wave potentials were applied from − 0.6 to − 1.2 V vs. RHE with a constant interval of 400 s using a CS150H electrochemical workstation. During the experiment, the flow rates of C₂H₂ gas and the electrolyte were set the same as those used for the performance evaluation.

Electrochemical in situ ATR-FTIR measurements. In situ ATR-FTIR was performed on a Nicolet 6700 FTIR spectrometer equipped with an MCTA detector with silicon as the prismatic window and an ECIR-II cell by Linglu Instruments. First, Cu-PCC was carefully dropped on the surface of the gold film, which was chemically deposited on the surface of the silicon prismatic material before each experiment. Then, the deposited silicon prismatic material served as the working electrode. Pt foil and Hg/HgO with an internal reference electrolyte of 1.0 M KOH were used as the counter and reference electrodes, respectively. A 1 M KOH solution was used as the electrolyte. The electrolyte was presaturated with pure C₂H₂ gas, and the gas was continuously bubbled through during the whole measurement. The spectrum was recorded every 30 s under an applied potential ranging from 0.2 to − 1.0 V vs. RHE.

Electrochemical in situ Raman measurements. In situ electrochemical Raman spectra were recorded via an electrochemical workstation on a Renishaw inVia reflex Raman microscope under 532 nm laser excitation under controlled potentials. We used a homemade Teflon electrolytic cell equipped with a piece of round quartz glass for the incidence of lasers and protection of the tested samples1. Before the experiments, the electrolyte was pretreated with pure C₂H₂ gas to obtain C₂H₂-saturated KOH. The working electrode was parallel to the quartz glass to maintain the plane of the sample perpendicular to the incident laser. The Pt wire was rolled to a circle around the working electrode to serve as the counter electrode. The reference electrode was Hg/HgO with an internal reference electrolyte of 1.0 M KOH. The spectrum was recorded under applied potentials ranging from 0.2 to − 1.0 V vs. RHE.

Computational details. All the DFT calculations were performed using the Vienna ab initio simulation package (VASP). ⁵³ The projector augmented wave (PAW) pseudopotential with the PBE generalized gradient approximation (GGA) exchange-correlation function was utilized in computations .^54,55 The cutoff energy of the plane wave basis set was 500 eV, and a Monkhorst-Pack mesh of 3×3×1 was used in K-sampling for the adsorption energy calculations and other nonself-consistent calculations. The long-range dispersion interaction was described by the DFT-D3 method. The electrolyte was incorporated implicitly with the Poisson-Boltzmann model implemented in VASPsol ⁵⁶. The relative permittivity of the media was chosen to be ϵ_r = 78.4, corresponding to that of water. All atoms were fully relaxed with an energy convergence tolerance of 10^− 5 eV per atom, and the final force on each atom was < 0.05 eV Å⁻¹.

The transition state (TS) searches were performed using the Dimer method in the VTST package. The final force on each atom was < 0.1 eV Å⁻¹. The TS search is conducted by using the climbing-image nudged elastic band (CI-NEB) method to generate initial guess geometries, followed by the dimer method to converge to the saddle points.

The adsorption energy of the reaction intermediates can be computed using Eqs. (4)-(5):

∆E = E_*ads - (E_*+ E_ads) (4)

∆G= ∆E + ∆E_ZPE - T∆S (5)

where ∆E_ZPE is the zero-point energy change and ∆S is the entropy change. In this work, the values of ∆E_ZPE and ∆S were obtained via vibration frequency calculations.

Data availability

The source data underlying Figs. 1–6 are provided as a Source Data file. The data that support other plots within this paper are available from the corresponding author upon reasonable request.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (Nos. 22271213 and 22209120).

Author Contributions

B. Zhang conceived the idea and directed the project. L. Li, B.-H. Zhao, and B. Zhang designed the experiments. L. Li. and F. P. Chen carried out the experiments and characterization. Y. F. Yu assisted in some experiments. C. Q. Cheng performed the DFT calculations. L. Li and B.-H. Zhao wrote the paper. B. Zhang revised the paper. All the authors discussed the results and commented on the paper.

Competing Financial Interests

The authors declare no competing interests.

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Concave surface-enriched reactant and enhanced mass transfer for electrocatalytic ethylene production from low-concentrated acetylene

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The design and synthesis of electrocatalyst

ESAE reaction analysis and performance evaluation

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