Toward Industrial Electrosynthesis of Ethylene: Energy‐Efficient and Stable Acetylene Semi‐Hydrogenation on a Copper Phosphide/MXene Electrocatalyst

Abstract

Electrocatalytic semi‐hydrogenation of acetylene to ethylene (EHAE) using renewable electricity represents a promising alternative approach for ethylene production. However, its relatively low energy efficiency (EE) and insufficient electrocatalyst stability hinder its industrial applications. The conduct a techno‐economic analysis indicates that the EHAE process becomes profitable when the EE exceeds 22.8% at an industrial current density of 0.2 A cm⁻². Herein, we report a novel electrocatalyst featuring firmly immobilized copper phosphide (Cu₃P) nanoparticles on MXene nanosheets (Ti₃C₂/Cu₃P) for a stable EHAE process at industrial currents using membrane electrode assembly (MEA) system. Specifically, the Ti₃C₂/Cu₃P electrocatalyst achieves an EE of 23.0% at 0.2 A cm⁻², demonstrating its potential for practical application and economic viability. The strong interactions between Cu₃P and Ti₃C₂ MXene prevent the agglomeration and dissolution of Cu₃P nanoparticles during long‐term EHAE process. Notably, in a 4 cm² MEA, Ti₃C₂/Cu₃P catalysts can sustain high performance for 100 h at 1.0 A with an ethylene Faradaic efficiency decay of only 0.051% per hour. Quasi in situ electron paramagnetic resonance spectroscopy and theoretical calculations indicate that Ti₃C₂/Cu₃P facilitates water dissociation and synergistically enhances the adsorption of acetylene and active hydrogen (H^*), thereby accelerating the kinetics of EHAE process.

Cu₃P nanoparticles anchored on MXene via Ti─O─P bonds exhibit enhanced stability and resist agglomeration during the electrochemical semi‐hydrogenation of C₂H₂ to C₂H₄. Ti₃C₂/Cu₃P achieves a record energy efficiency of 23.0% at 0.2 A cm⁻² and stable operation in a 4 cm² MEA for over 100 h at 1.0 A.

Introduction

Ethylene (C₂H₄) is the most important bulk commodity in the modern chemical industry, primarily produced through the steam cracking of oil and shale gas.^{[ 1} , ^{2 ]} Given the global transition toward carbon neutrality, the high energy consumption and massive carbon emissions associated with these processes necessitate the exploration of alternatives to zero‐carbon emission technology for C₂H₄ production.^{[ 3 ]} The electrocatalytic semi‐hydrogenation of acetylene (C₂H₂) to C₂H₄ (EHAE), utilizing water as the proton source and powered by renewable electricity, has emerged as a promising petroleum‐independent route due to its economic viability and environmental friendliness (Figure 1a).^{[ 4} , ⁵ , ⁶ , ^{7 ]} For industrial implementation, it is essential to develop electrocatalysts and reactor systems with significantly enhanced energy efficiency (EE), activity, and selectivity toward C₂H₄.

Figure 1.

Comparison of electrolyzers in the EHAE process and techno‐economic analysis. (a) Schematic illustration for the EHAE route compared with the traditional cracking route, enlarged comparison of the structures of different electrolyzers in electrolysis unit. (b) Comparison of EE with current density in different electrolyzers for commercial copper nanoparticles. (c) The projected cost of the proposed EHAE process as a function of energy efficiency and electricity cost at 0.2 A cm⁻². The area above the dashed‐dotted grey line indicates a profitable region.

To date, most EHAE studies are conducted in three‐electrode flow cell systems using gas‐diffusion electrodes (GDEs).^{[ 1} , ⁸ , ⁹ , ^{10 ]} Satisfactory C₂H₄ Faradaic efficiency (FE) values (>90%) are readily achieved at relatively large current densities (>0.2 A cm⁻²) by optimizing copper‐based electrocatalysts.^{[ 11} , ¹² , ^{13 ]} In our previous work, we achieved a C₂H₄ FE exceeding 90% at current densities above 0.5 A cm⁻² by introducing oxygen/nitrogen vacancies into nano‐copper electrocatalysts.^{[ 14} , ^{15 ]} However, the electrolyte in the gap between the cathode and anode in flow cells causes elevated resistance at large current densities, which require higher overpotentials and result in low full‐cell EE values. Notably, the FE value and cell voltage, a key trade‐off pair, determine the commercial viability of the EHAE process.^{[ 16 ]} In contrast, zero‐gap membrane electrode assemblies (MEAs) featuring near zero‐gap between electrodes can significantly reduce the internal resistance and enable higher current density and lower cell voltage (Figure 1a). Using commercial copper nanoparticles as the control electrocatalyst, we compare the variation of EE with current density within two reaction systems. The EE value in the MEA system reached 21.8% at 0.2 A cm⁻², approximately 1.9 times higher than that of the flow cell system, and the difference becomes even more pronounced at higher current density (4.6 times at 1.0 A cm⁻²) (Figure 1b). Furthermore, the techno‐economic analysis (TEA) demonstrates that the EHAE process is profitable when the EE exceeding 22.8% at a current density of 0.2 A cm⁻² with electricity price of US$ 0.033 per kWh (Figure 1c and Supplementary Note 1). Additionally, the profitability range further expand as the EE increases and the price of renewable electricity decreases.

In previous EHAE studies, the electrocatalyst stability is commonly overlooked compared to the reaction activity and selectivity.^{[ 10} , ¹⁷ , ^{18 ]} Most reported long‐term stability was below 55 h at industry‐scale current densities (0.2 A cm⁻²), which remains insufficient for potential commercialization.^{[ 1} , ⁸ , ^{12 ]} Generally, anchoring catalysts on appropriate matrices can stabilize and disperse the active species via strong catalyst‐support interactions, thereby improving both catalytic activity and stability.^{[ 19} , ²⁰ , ^{21 ]} 2D transition metal carbides, known as MXenes, are considered ideal matrix owing to their large surface area, superior conductivity, and tunable surface chemistry.^{[ 22} , ²³ , ²⁴ , ^{25 ]} The surface terminal groups of MXenes are negatively‐charged, which reinforce electrostatic interactions with positively‐charged metal cations, therefore promoting high dispersion and binding of metal nanoparticle catalysts on MXene.^{[ 21} , ²⁵ , ^{26 ]} Zang et.al. demonstrated that loading metal nanoclusters onto MXene effectively suppresses their aggregation during the electroreduction process, thereby improving the stability of the electrocatalyst.^{[ 27 ]} Nevertheless, the weak interactions mediated solely by the surface functional groups of MXenes are unlikely to ensure structural robustness under industrially relevant current densities. A more resilient interface can be obtained by introducing metal compounds capable of forming covalent bonds with the support.^{[ 28} , ^{29 ]} Copper phosphides represent a rational candidate due to their demonstrated structural stability and favorable electrochemical performance under hydrogenation conditions.^{[ 30 ]} Therefore, anchoring copper phosphides onto MXene to construct a covalently bonded interface is expected to offer significant promise for enabling an efficient and stable EHAE process—yet this strategy remains unexplored.

Herein, we present a stable electrocatalyst featuring firmly anchored copper phosphide (Cu₃P) nanoparticles on MXene support (Ti₃C₂/Cu₃P) for efficient C₂H₄ electrosynthesis through EHAE process. The strong interactions between Cu₃P and Ti₃C₂ MXene via covalent Ti─O─P bonds facilitate the uniform dispersion of Cu₃P and prevent its aggregation and dissolution during long‐term operation. Moreover, Quasi in situ electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations reveal that Ti₃C₂/Cu₃P facilitates water dissociation to produce active H^*, and the enhanced adsorption of C₂H₂ and H^* accelerates semi‐hydrogenation kinetics. As a result, the Ti₃C₂/Cu₃P electrocatalyst delivers a high EE value of 23.0% at a current density of 0.2 A cm⁻² with a C₂H₄ FE of 94.3%. By inputting these operation parameters measured at different current densities (0.2‐1.0 A cm⁻²), we predict that the EHAE process is economically feasible. Furthermore, in a 4 cm² MEA system, Ti₃C₂/Cu₃P exhibits remarkable stability exceeding 100 h at 1.0 A with a C₂H₄ FE decay of only 0.051% per hour. This work provides insights into the industrial implementation and economic feasibility of electrosynthesis of C₂H₄ from C₂H₂.

Results and Discussions

Synthesis and Characterizations of Catalysts

The synthesis procedures for Ti₃C₂/Cu₃P involve etching the Ti₃AlC₂ phase through redox reactions between Al³⁺ and CuCl₂ as a Lewis acid molten salt (Figure 2a).^{[ 23 ]} Subsequently, Cu₃P nanoparticles were anchored onto Ti₃C₂ MXene through a facile phosphidation process. Each step in the preparation process of Ti₃C₂/Cu₃P was confirmed by powder X‐ray diffraction (PXRD) patterns (Figure S1 and Supplementary Note 2). The morphology at each stage was also characterized using scanning electron microscopy (SEM) (Figures S2 and S3). EDS analysis further confirms that Al is effectively exchanged by Cu (Figure S4 and Supplementary Note 3). After complete etching, the initial dense structure evolved into a typical accordion‐like MXene structure, which remained stable throughout the phosphidation process (Figure 2b).^{[ 23} , ^{31 ]} Transmission electron microscopy (TEM) images revealed the lamellar microstructures of both Ti₃C₂/Cu and Ti₃C₂/Cu₃P, which can be attributed to the fact that the generated AlCl₃ vapors facilitated the delamination of MXene (Figures S5 and S6).^{[ 23 ]} Furthermore, high‐resolution TEM (HRTEM) images showed the (002) facet of Ti₃C₂ with an interlayer spacing of 1.2 nm and the (111) facet of Cu with an interlayer spacing of 2.08 Å (Figure S7).^{[ 32 ]} Upon ultrasonic exfoliation, Cu₃P nanoparticles were evenly dispersed on the surfaces of ultrathin MXene nanosheets (Figure 2c). Elemental mapping analysis confirmed the uniform distributions of C, O, Ti, Cu, and P elements throughout Ti₃C₂/Cu₃P (Figure 2d). The lattice fringes with an interlayer spacing of 2.01 Å belonged to the (300) facet of Cu₃P (Figure 2e).^{[ 33} , ^{34 ]} The specific surface area slightly decreased following phosphidation (Figure S8). The inductively coupled plasma‐mass spectrometry (ICP‐MS) results demonstrated the Cu content of about 17.2 wt% on Ti₃C₂/Cu₃P.

Figure 2.

The synthesis and structural characterization of Ti₃C₂/Cu₃P. (a) Schematic of the synthesis process of Ti₃C₂/Cu₃P. (b) SEM image (scale bars: 2 µm), (c) TEM image (scale bars: 200 nm), (d) HAADF‐STEM image and corresponding elemental mappings (scale bars: 50 nm), and (e) HRTEM image (scale bars: 5 nm) of Ti₃C₂/Cu₃P. Normalized Cu K‐edge XANES (f) and Fourier transform of EXAFS (g) spectra of Ti₃C₂/Cu, Ti₃C₂/Cu₃P, and references. High‐resolution XPS spectra of P 2p (h), Ti 2p (i), and O 1s (j).

In the Cu K‐edge X‐ray absorption near edge structure (XANES) spectra, the absorption edge of Ti₃C₂/Cu resembled that of the CuO reference, indicating a high oxidation state of Cu.^{[ 35 ]} After phosphidation, the absorption edge of Ti₃C₂/Cu₃P in the Cu K‐edge XANES spectra shifted to lower energy and approached that of the Cu₂O reference, suggesting a partial reduction of Cu²⁺ to Cu⁺ (Figure 2f).^{[ 36 ]} Notably, a peak near 8986 eV was preserved, similar to CuO reference, which indicates the coexistence of Cu─P and Cu─O coordinations in Ti₃C₂/Cu₃P. The Fourier transform from extended X‐ray absorption fine structure (EXAFS) spectra at the Cu K‐edge of Ti₃C₂/Cu showed two peaks at 1.56 and 2.23 Å, corresponding to Cu─O and Cu─Cu bonds, respectively (Figure 2g).^{[ 35 ]} Furthermore, the spectra of Ti₃C₂/Cu₃P exhibited two peaks at 1.65 and 2.02 Å, which were assigned to the Cu─P/O and Cu─Cu bonds, respectively.^{[ 37 ]}

X‐ray photoelectron spectroscopy (XPS) analysis elucidated the surface composition and chemical states of Ti₃C₂/Cu and Ti₃C₂/Cu₃P. The Cu 2p peaks at 935.3/955.1 eV were assigned to the Cu─O bond, formed by the reaction of Cu²⁺ and oxygen‐containing surface groups (Figure S9 and Supplementary Note 4). Compared to Ti₃C₂/Cu, the peak intensity of Ti₃C₂/Cu₃P at 932.5/952.3 eV markedly increased, indicating the formation of Cu─P bonds during the phosphidation process.^{[ 34 ]} The P 2p spectrum displayed Cu─P and P─O─Ti bonds at 129.7 and 133.1 eV, respectively (Figure 2h).^{[ 38 ]} For Ti₃C₂/Cu, the Ti 2p signal exhibited multiple Ti valence states and Ti─O bonds (Figure 2i).^{[ 35 ]} After phosphidation, the characteristic peaks for Ti (III) disappeared, and a pair of peaks emerged at 458.3/464.2 eV, corresponding to the Ti─O─P bond.^{[ 38} , ^{39 ]} Furthermore, the Ti─O─P bond was also confirmed by the O 1s signal at 531.0 eV in Ti₃C₂/Cu₃P (Figure 2j).^{[ 38 ]} These results indicated that the Cu₃P nanoparticles were bonded to the MXene support through Ti─O─P and Cu─O bonds.^{[ 38} , ^{39 ]}

Electrocatalytic Semi‐Hydrogenation of C₂H₂ to C₂H₄

The electrocatalytic performance of EHAE was first evaluated in a standard three‐electrode flow cell. Linear sweep voltammetry (LSV) curves demonstrated that the addition of C₂H₂ led to a higher responsive current density than that of Ar (Figure 3a).^{[ 17 ]} Moreover, Ti₃C₂/Cu₃P delivered a cathodic current density of 350.4 mA cm⁻² at −1.1 V (vs. the reversible hydrogen electrode, RHE), surpassing the value of 271.3 mA cm⁻² for Cu₃P nanoparticles (Cu₃P NPs) and 218.0 mA cm⁻² for Ti₃C₂/Cu. These results showed the superior activity of Ti₃C₂/Cu₃P to drive C₂H₂ electrochemical hydrogenation. The superior EHAE activity of Ti₃C₂/Cu₃P remained even after normalization by the electrochemical surface area (ECSA) (Figures S10 and S11). Moreover, steady‐state chronoamperometric measurements of C₂H₂ semi‐hydrogenation were conducted at various cathodic current densities ranging from 0.1 to 0.8 A cm⁻². Notably, the Ti₃C₂ matrix without Cu nanoparticles predominantly produced hydrogen (H₂), suggesting that Cu served as the primary active site for the EHAE process (Figure S12). In contrast, C₂H₄ was the main hydrogenation product on Ti₃C₂/Cu₃P, along with minor amounts of C₄ (mainly 1,3‐butadiene) and H₂ (Figure 3b). Across a wide range of current density from 0.2 to 0.7 A cm⁻², the FE for C₂H₄ consistently exceeded 94.2%, outperforming both Ti₃C₂/Cu and Cu₃P NPs (Figure 3c and Figure S13). Additionally, the C₂H₄ yield on the Ti₃C₂/Cu₃P catalyst reached 12.59 mmol h⁻¹ cm⁻² at a current density of 0.8 A cm⁻², surpassing that of Ti₃C₂/Cu (6.44 mmol h⁻¹ cm⁻²) and Cu₃P NPs (11.20 mmol h⁻¹ cm⁻²), as well as most previously reported electrocatalysts (Figures S14 and S15 and Table S1).^{[ 1} , ⁴ , ¹³ , ^{40 ]} Meanwhile, the Ti₃C₂/Cu₃P catalyst demonstrated an outstanding capability to inhibit HER and over‐hydrogenation (Figures S16 and S17 and Supplementary Notes 5–6).

Figure 3.

Electrocatalytic performance and cost analysis of the process. In a three‐electrode flow cell. (a) LSV curves for Ti₃C₂/Cu₃P, Cu₃P NPs, and Ti₃C₂/Cu. (b) The FE of EHAE products at different current densities of Ti₃C₂/Cu₃P. (c) The FE of C₂H₄ at different current densities for Ti₃C₂/Cu₃P, Cu₃P NPs, and Ti₃C₂/Cu. (d) The full‐cell voltage of Ti₃C₂/Cu₃P in the different electrolyzers and anode. (e) The energy efficiency of Ti₃C₂/Cu₃P and Ti₃C₂/Cu at different current densities in MEA. (f) A comparison of the reported full‐cell energy efficiency. (g) The subdivided cost of the EHAE process under a current density of 0.6 A cm⁻², full‐cell voltage of 2.24 V, and C₂H₄ FE of 90.9% at the given electricity price of US$ 0.033/kWh. (h) Single‐variable sensitivity analysis for the production cost of C₂H₄.

We further investigated the electrocatalytic performance of EHAE in a two‐electrode zero‐gap MEA system (Figure S18). As a result, the C₂H₄ FE reached 94.3% at 0.2 A cm⁻² and remained as high as 90.9% at 0.6 A cm⁻², which is comparable to that in the flow cell (Figure S19 and Table S2). Notably, although the use of a more efficient oxygen evolution reaction (OER) catalyst can partially decrease the full‐cell voltage, the intrinsic structural advantages of the electrolyzer play a more decisive role in determining its operational performance. At a current density of 0.2 A cm⁻² without iR compensation, the full‐cell voltage was significantly reduced from 3.81 V to 2.02 V, and this drop became more significant at higher current densities (Figure 3d). The reduction in full‐cell voltage substantially enhanced the EE for C₂H₄ production, increasing from 12.4% to 23.0%, which is beneficial for reducing the overall energy consumption of the EHAE process (Figure S20). Additionally, the Ti₃C₂/Cu₃P catalyst in the MEA system consistently surpassed Cu₃P NPs and Ti₃C₂/Cu in both electrochemical activity and C₂H₄ selectivity, which is consistent with the results in the flow cell (Figures S21–S23 and Supplementary Notes 7–8). Notably, even at an ultrahigh current density of 1.0 A cm⁻², Ti₃C₂/Cu₃P maintained an EE value of 16.1%, outperforming Cu₃P NPs (11.1%), Ti₃C₂/Cu (5.8%), and other reported electrocatalysts (Figure 3e,f and Table S3).^{[ 1} , ⁴ , ¹⁴ , ¹⁵ , ⁴⁰ , ^{41 ]} Note that the calculation method for the EE value has been standardized based on reported data. Additionally, the C₂H₄ EE of Ti₃C₂/Cu₃P at different current densities consistently exceeded the minimum threshold required for profitability, thereby highlighting the considerable economic potential of Ti₃C₂/Cu₃P in the EHAE process (Figure S24 and Supplementary Note 9).

The detailed cost analysis of the EHAE process has been performed (Tables S4 and S5). As shown in Figure 3g, electricity cost represents the largest component, accounting for 49.5%. Thanks to the ongoing decrease in renewable electricity prices and improvements in system energy efficiency, the overall C₂H₄ production cost for the EHAE process is expected to be further reduced. Moreover, the single‐variable sensitivity analysis also revealed that the production cost was most sensitive to fluctuations in the electricity price (Figure 3h). Moreover, the price of C₂H₂ feedstock is also a critical factor, which can potentially be reduced by advancements in arc electric arc pyrolysis technology for coal and natural gas in the near future. From the perspective of electrochemical process, the design of efficient electrocatalysts should not only aim to achieve high C₂H₄ FEs at high current densities, but also focus on reducing the full‐cell voltage and improving long‐term stability.

To further elaborate the foundation for practical applications, stability tests were conducted in flow cell and MEA with a larger electrode area (4 cm²). Under a current of 1.0 A for 10 h in the flow cell, ICP‐MS analysis of the catholyte confirmed the dissolution of Cu from catalyst. Notably, the Cu dissolution from Ti₃C₂/Cu₃P was markedly lower than that from Ti₃C₂/Cu, indicating that Ti₃C₂/Cu₃P exhibits superior anti‐dissolution performance under industrial‐level current densities (Figure 4a and Figure S25). Furthermore, severe flooding of the cathodic GDE was consistently observed during the stability tests (Figure S26). In flow cells, the applied electrocatalysts are in direct contact with the electrolyte, rendering a delicate balance at the tri‐interface that is highly susceptible to pressure fluctuations on either side.^{[ 42 ]} When the interface is at equilibrium, C₂H₂ can permeate through the gas diffusion layer (GDL) and rapidly access the reaction interface. However, over time, flooding becomes inevitable, causing electrolyte intrusion into the electrocatalyst and impeding C₂H₂ diffusion (Figure 4b).^{[ 43 ]} In contrast, the cathode in MEA requires a minimal volume of electrolyte, and water molecules are supplied through the anion exchange membrane (AEM), thus minimizing the risk of flooding. Therefore, we conducted further evaluation using a 4 cm² MEA system. Under a pure C₂H₂ flow, the full cell current reached 1.73 A at a voltage of 2.14 V (Figure S27). Furthermore, the Ti₃C₂/Cu₃P catalyst demonstrated exceptional stability over 100 h at 1.0 A, consistently maintaining high C₂H₄ FEs above 88.5% and outperforming most reported electrocatalysts (Figure 4c).^{[ 1} , ⁴ , ¹⁷ , ⁴¹ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ⁴⁸ , ^{49 ]} In contrast, Cu₃P NPs and Ti₃C₂/Cu rapidly deactivated within the first 10 h. Specifically, Ti₃C₂/Cu showed an average C₂H₄ FE decay rate of 0.39% per hour, which is significantly higher than that of Ti₃C₂/Cu₃P (0.051% per hour) (Figure 4d). In addition, during the stability test, Ti₃C₂/Cu₃P exhibited a C₂H₂ conversion decay rate of 0.048% per hour while maintaining C₂H₄ selectivity consistently above 93.5% (Figure S28). Notably, after 100 h of continuous operation, the MEA cell was disassembled, and no signs of flooding were detected behind the cathode (Figure S29).

Figure 4.

Investigation of catalyst stability. (a) the dissolution of Cu in Ti₃C₂/Cu and Ti₃C₂/Cu₃P electrode at 1.0 A for 10 h. (b) Schematic illustration of flow cell at a three‐phase interface. (c) Comparison of the C₂H₂ semi‐hydrogenation stability over Ti₃C₂/Cu₃P with previously reported works. (d) Stability test of the 4 cm² MEA measured at 1 A, with a flow rate of 20 mL min⁻¹. (e) TEM images, particle size distribution, and schematic illustration of Ti₃C₂/Cu and Ti₃C₂/Cu₃P after long‐term stability tests, scale bars: 200 nm.

To further elucidate the cause of electrocatalyst deactivation, TEM characterizations were recorded for both Ti₃C₂/Cu and Ti₃C₂/Cu₃P after the long‐term stability test. The average size of Cu nanoparticles in Ti₃C₂/Cu significantly increased from 4.6 nm to 82.7 nm (Figure 4e left). In sharp contrast, for Ti₃C₂/Cu₃P, the average size of Cu₃P nanoparticles only slightly increased from 6.6 nm to 7.8 nm (Figure 4e right and Figures S30 and S31). Therefore, compared with the pronounced aggregation observed in Ti₃C₂/Cu, the aggregation propensity on Ti₃C₂/Cu₃P has been effectively suppressed, thereby preventing rapid deactivation and enhancing long‐term stability.

Mechanism of the EHAE Conversion

To elucidate the underlying mechanism for the superior performance of the EHAE conversion on Ti₃C₂/Cu₃P, we conducted a series of experimental characterizations and theoretical calculations. First, the H^* source in the EHAE process was identified. The kinetic isotope effect (KIE) values (H/D) for Ti₃C₂/Cu₃P consistently exceeded 2, indicating that water dissociation is kinetically involved as the rate‐determining step in the EHAE (Figure S32). Moreover, increasing the electrolyte pH resulted in a corresponding rise in C₂H₄ FE and suppression of HER, indicating that C₂H₂ preferentially undergoes hydrogenation via H^* dissociated from water, rather than through the conventional proton‐coupled electron transfer (PCET) processes at high pH values (Figure S33).^{[ 5} , ^{12 ]} Furthermore, the hydrogen radicals (denoted as •H) were detected by Quasi in situ electron paramagnetic resonance (EPR) measurements using 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) as the spin‐trapping agent (Figure 5a and Figure S34). Under Ar flow, the stronger DMPO‐H signal intensity observed for Ti₃C₂/Cu₃P indicated its superior water dissociation capacity compared to Ti₃C₂/Cu.^{[ 1} , ^{50 ]} Notably, for Ti₃C₂/Cu₃P, water dissociation preferentially occurs on the Cu₃P nanoparticles rather than on the Ti₃C₂ support (Figure S35 and Supplementary Note 10). Meanwhile, the enhanced water dissociation does not lead to H₂ evolution during the EHAE process. The Bode plots obtained from EIS date reflect both electron‐transfer process and their kinetics during electrocatalysis. As shown in Figure 5b, the phase angle peak intensities decreased when the atmosphere was switched from Ar to C₂H₂ over Ti₃C₂/Cu₃P, indicating that C₂H₂ hydrogenation kinetically outcompetes HER. Notably, under Ar flow, Ti₃C₂/Cu₃P exhibited a higher phase angle intensity and a lower peak frequency compared to Ti₃C₂/Cu, suggesting suppressed HER kinetics on Ti₃C₂/Cu₃P (Figure S36).^{[ 51 ]}

Figure 5.

Mechanism of C₂H₄ production. (a) Quasi in situ EPR measurements of Ti₃C₂/Cu and Ti₃C₂/Cu₃P at −0.6 V vs. RHE. (b) Bode phase plots with Ar and C₂H₂ at various potentials for Ti₃C₂/Cu₃P. (c) The projected density of states (PDOS) and corresponding structure models for Ti₃C₂/Cu and Ti₃C₂/Cu₃P. (d, e) Free energy diagram for (d) H₂O dissociation and (e) HER pathways. (f) In situ Raman spectra at different potentials on Ti₃C₂/Cu₃P electrodes. (g) Free energy profiles for the hydrogenation of C₂H₂ on Ti₃C₂/Cu and Ti₃C₂/Cu₃P. (h) Schematic illustration of the EHAE pathway on Ti₃C₂/Cu₃P.

Density functional theory (DFT) calculations revealed that the incorporation of P atoms shifted the d‐band center of Cu closer to the Fermi level on Ti₃C₂/Cu₃P, providing more empty antibonding d‐orbitals for active H^* and thus enhancing H^* adsorption (Figure 5c).^{[ 20} , ^{52 ]} We further analyzed the free energy profiles for water dissociation and HER on Ti₃C₂/Cu₃P and Ti₃C₂/Cu (Table S6). As shown in Figure 5d, Ti₃C₂/Cu₃P exhibited stronger water adsorption (−0.61 eV) and a lower activation energy for water dissociation (0.50 eV) compared to Ti₃C₂/Cu (−0.52 eV and 0.79 eV, respectively). These results indicated that Ti₃C₂/Cu₃P efficiently supplied H^* and promoted the protonation process during the EHAE process. Moreover, Ti₃C₂/Cu₃P exhibited a higher energy barrier for HER (0.1 eV) compared to Ti₃C₂/Cu (−0.04 eV), which highlights its ability to suppress HER (Figure 5e). Raman spectroscopy was performed on Ti₃C₂/Cu₃P before and after EHAE process. The bands at 151 cm⁻¹ and 622 cm⁻¹ were attributed to the in‐plane and out‐of‐plane Ti─C vibrations of Ti₃C₂ with E _g and A _1g symmetry, respectively, and these remained unchanged after the reaction (Figure S37).^{[ 53} , ^{54 ]} Additionally, EDS analysis confirmed the uniform distribution of Ti, Cu, P, and C elements after the EHAE reaction (Figure S38).

In situ Raman spectroscopy was employed to identify reaction intermediates during the EHAE process (Figure 5f and Figures S39 and S40). The peaks at 1332 and 1578 cm⁻¹ are assigned to the D and G bands of carbon, respectively. Upon applying a cathodic potential from 0 to −0.2 V versus RHE, two new peaks emerged at 1119 and 1500 cm⁻¹, which correspond to the C─C and C═C bonds in polyacetylene, respectively. As the potential was further reduced, the intensities of these polyacetylene characteristic peaks increased on both Ti₃C₂/Cu₃P and Ti₃C₂/Cu, indicating progressive intensification of the EHAE process.^{[ 40} , ⁵⁵ , ^{56 ]} In addition, a broad band between 3100 and 3600 cm⁻¹ was monitored and attributed to the O─H stretching of interfacial water. Gaussian deconvolution resolved three hydrogen‐bonding states in this region: 4HB‐H₂O (3220 cm⁻¹), 2HB‐H₂O (3405 cm⁻¹), and cation‐bound water (K⁺‐H₂O, 3550 cm⁻¹), respectively. As the potential became more negative, the intensity of the K⁺‐H₂O signal increased, indicating accelerated dissociation of interfacial water that correlates with an increased hydrogenation rate.^{[ 57} , ^{58 ]} Notably, the relative fraction of K⁺‐H₂O was consistently higher on Ti₃C₂/Cu₃P than on Ti₃C₂/Cu, suggesting that Ti₃C₂/Cu₃P promotes interfacial water dissociation more effectively—consistent with the EPR results (Figure S41 and Figure 5a).

Regarding the kinetics of the EHAE process, Figure 5g illustrated that the free energy for C₂H₂ adsorption on Ti₃C₂/Cu and Ti₃C₂/Cu₃P was −0.97 eV and −1.54 eV, respectively, indicating a stronger C₂H₂ affinity on Ti₃C₂/Cu₃P. Notably, the adsorption of H^*on the Ti₃C₂/Cu₃P surface was exothermic (−0.07 eV), whereas it was endothermic (0.068 eV) on Ti₃C₂/Cu, suggesting that H^* adsorption was energetically more favorable on Ti₃C₂/Cu₃P.^{[ 9 ]} For the following two hydrogenation steps, the adsorbed C₂H₂ was hydrogenated to form C₂H₄ ^* with favorable downhill energy profiles. The transformation from C₂H₄ ^* + H^* to C₂H₅ ^* is commonly recognized as the rate‐determining step for the over‐hydrogenation of C₂H₄.^{[ 11} , ^{59 ]} For Ti₃C₂/Cu₃P, this step was endothermic (0.42 eV), whereas it was exothermic (−0.64 eV) for Ti₃C₂/Cu, indicating that Ti₃C₂/Cu tended to over‐hydrogenate C₂H₄ (Table S7). These results support a plausible EHAE mechanism on the Ti₃C₂/Cu₃P surface: initially, water and C₂H₂ molecules are adsorbed onto the surface, where rapid water dissociation generates abundant active H^* that react with the adsorbed C₂H₂ to form C₂H₄. The resulting C₂H₄ is then rapidly desorbed as C₂H₄ (g). Owing to its excellent hydrogenation activity and relatively high HER energy barrier, H^* is more inclined to participate in the hydrogenation of C₂H₂ rather than couple to form H₂. This behavior leads to superior EHAE performances at industrial current densities (Figure 5h).

Conclusions

In summary, we report a highly efficient and stable electrocatalyst, Ti₃C₂/Cu₃P, for EHAE conversion. By anchoring Cu₃P nanoparticles onto the MXene matrix through Ti─O─P bonds, the strong interactions not only facilitated water dissociation but also synergistically enhanced the adsorption of C₂H₂ and H^*, while effectively preventing catalyst dissolution and agglomeration. This enabled both efficient and stable electrochemical C₂H₄ synthesis. When assembled as the cathode, Ti₃C₂/Cu₃P achieved a record energy efficiency of 23.0% at 0.2 A cm⁻². Moreover, it demonstrated continuous operation for over 100 h at 1.0 A in a 4 cm² MEA, outperforming most previously reported electrocatalysts. The breakthrough in full‐cell energy efficiency and durability represents a significant advancement toward achieving environmental sustainability and economic viability in C₂H₄ productions.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.