MaterialsAll chemicals used in the experiments were analytically pure and used without further purification. Copper nitrate (Cu(NO3)2·3H2O, 99.0%), N,N-dimethylformamide (DMF), polyvinyl pyrrolidone (PVP, Mw = 15,000–19,000), tannic acid (TA, 99.0%), anhydrous ethanol (CH3CH2OH, 99.9%), and 1,3,5-benzene tricarboxylic acid (H3BTC) were purchased from Aladdin Ltd. (Shanghai, China). Simulated raw coal-derived C2H2 with concentrations of ~15% and 3% H2/Ar were purchased from Lian Bo (Tianjin, China) Co., Ltd. A Nafion 117 exchange membrane with a thickness of 183 μm was purchased from DuPont and used after sequent heat treatments in 5% H2O2 and H2SO4 solutions under 80 °C. The Hg/HgO reference electrode (diameter 1.8 mm) and the gas diffusion electrode (GDE) (Carbon paper 29BC) was purchased from Shanghai Chuxi Industrial Co., Ltd. Deionized water (DIW) was used in all the experimental processes.Synthesis of Cu-MOF precursorsAccording 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-TAThe 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-CTo 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.Fabrication of Cu-PCC and Cu-C electrodesThe electrodes used in this work were fabricated by the traditional spin-casting method. The commercial GDE was cut into a square shape with a size of 1.2 × 1.2 cm2 as the electrode substrate. Specifically, Cu-PCC and Cu-C were dissolved in a mixed solvent of water and ethanol with a volume ratio of 1/3 to form a solution at a concentration of 2 mg mL−1, respectively. Then, the as-prepared solutions were spin-coated on the GDE substrates with 1 mL on each substrate under a constant spin speed of 500 rpm to obtain the electrodes with a loading of 1 mg Cu-PCC and/or Cu-C.General characterizationsQuasi-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 1 s 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 microscopy (AFM) was carried out on a Bruker Dimension Icon.Electrochemical measurements in the flow cellElectrochemical measurements were carried out in a typical flow cell consisting of a GDE as the working electrode, Pt foil as the counter electrode, and Hg/HgO (Note that the Hg/HgO electrode was calibrated with respect to a reversible hydrogen electrode in a high-purity hydrogen-saturated electrolyte with a Pt foil as the working electrode.) as the reference electrode using a CS150H electrochemical workstation. The volume of each compartment is around 1.5 mL. In addition, the potentials were scaling to RHE using Eq. (1). 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 freshly prepared KOH solution, of which the pH value is around 13.6 ± 0.3, 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. Before LSV, the resistance (R) was measured firstly using CS150H electrochemical workstation and the R values were 1.5 ± 0.1 Ω. Only the LSV curves provided in this work 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. After LSV process, the electrolysis was conducted under a potentiostatic mode in the range from −0.6 to −1.2 V vs. RHE.$$E\,{{{{{\rm{vs}}}}}}.\,{{{{{\rm{RHE}}}}}}=E\,{{{{{\rm{vs}}}}}}.\,{{{{{\rm{Hg}}}}}}/{{{{{\rm{HgO}}}}}}+{E}^{{{{{{\rm{\theta }}}}}}}({{{{{\rm{Hg}}}}}}/{{{{{\rm{HgO}}}}}})+0.0591\times {{{{{\rm{pH}}}}}}$$
(1)
Quantitative analysis of the C2H2 conversion, evolution rate, and FE of the obtained productsThe 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 Eqs. (2)−(5), and the FEs of the different products were calculated using Eq. (6). All the experiments were repeated three times.$$C({{\mbox{X}}})=k({{\mbox{X}}})\times {{\mbox{peak area}}}$$
(2)
$$n({{\mbox{X}}})=C({{\mbox{X}}}) \times \it {{\mbox{V}}}$$
(3)
$${{{{{\rm{Conversion}}}}}}\,(\%)=\frac{{{\it{{n}}}({{{\rm{C}}}}_{2}{{{\rm{H}}}}_{2})}_{{{\rm{in}}}}-{{\it{{n}}}({{{\rm{C}}}}_{2}{{{\rm{H}}}}_{2})}_{{{\rm{out}}}}}{{{\it{{n}}} ({{{\rm{C}}}}_{2}{{{\rm{H}}}}_{2})}_{{{\rm{in}}}}} \times 100\%$$
(4)
$${{{{{\rm{Evolution}}}}\; {{{\rm{Rate}}}}}} \, ({{{{{\rm{mmol}}}}}}/{{{{{\rm{mg}}}}}}/{{{{{\rm{h}}}}}})=\frac{{{n}} ({{{\rm{X}}}})}{m} \times S$$
(5)
$${{{\mbox{FE}}}}_{{{\mbox{X}}}}(\%)=\frac{{{{{a}}}} \times {{{{n ({{{\rm{X}}}})}}}}\times {{{{F}}}}}{Q}$$
(6)
X: The feedstock and products, including C2H2, H2, C2H4, and C2H6.C: The concentration of feedstock and products.m: The mass of catalysts over the electrode.n: The moles of feedstock and products.k: The slope of the calibration curves for feedstock and products.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 analysisOperando 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 DEMS was conducted in the same flow cell electrolyzer with our performance evaluation 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 and the schematic of DEMS testing has been provided in Supplementary Fig. 18. In addition, the membrane employed was only for the separation of cathode and anode using Nafion 117 proton exchange membrane. Note that all the ion currents plotted in this work are provided without any correction or subtraction. The LSV test and rectangular wave potentials were applied from 0.3 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 measurementsIn 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 measurementsIn 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 detailsAll the DFT calculations were performed using the Vienna ab initio simulation package (VASP) (Supplementary Data 1 for the optimized DFT computational models)53. The projector augmented wave (PAW) pseudopotential with the PBE generalized gradient approximation (GGA) exchange-correlation function was utilized in the computations54,55. The cut-off 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 VASPsol56. The relative permittivity of the media was chosen to be ϵr = 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.In the ab initio molecular dynamics (AIMD) simulations, canonical ensemble (NVT) conditions were imposed by a Nose‒Hoover thermostat with a target temperature of 300 K. The MD time step was 1 fs, and all the systems were run for 10 ps to reach equilibrium. In the plane and angle models, 114 and 120 water molecules were added to ensure that the density of water in the model was approximately 1 g/cm3. The last 1 ps of data in the AIMD process are selected for analysis. In the process of hydrogen bond analysis, we set the maximum distance of the hydrogen bond to 3.5 Å and the angle cut-off to 40°.The adsorption energy of the reaction intermediates can be computed using Eqs. (7)−(8):$$\Delta E=\it {E}_{* {{{\rm{ads}}}}}-(\it {E}_{ * }+\it {E}_{{{\rm{ads}}}})$$
(7)
$$\Delta G=\Delta E+{\Delta E}_{{ZPE}}-T\Delta S$$
(8)
where ∆EZPE is the zero-point energy change and ∆S is the entropy change. In this work, the values of ∆EZPE and ∆S were obtained via vibration frequency calculations.
Ethylene electrosynthesis from low-concentrated acetylene via concave-surface enriched reactant and improved mass transfer
