We have reported cationic, hydrophobic and aerophilic interfaces with high-density active sites for electrochemical CO2-to-CH3OH conversion in acids. This interface decreases the proton-source concentrations, although improving local CO availability via the strongly polarized electrostatic forces and weakly polarized van der Waals forces; these effects combine to achieve FECH3OH of 61.5% and jCH3OH of 131.6 mA cm-2 in an electrolyte with a pH of ~1, superior to recent reports in neutral or alkaline media8,10,11. Our design is also applicable to other molecular catalysts and can be extended to covalent organic nanosheets bearing different structural configurations (Supplementary Figs. 64-69). This study underscores the modulation of local force fields at the solvent-catalyst interface for enhanced electrocatalysis in strong acids.
All chemicals were purchased and used without further purification. Cobalt(II) 2,9,16,23-tetra(amino)phthalocyanine was obtained from Shanghai Kaiyulin Pharmatech. N,N-dimethylacetamide, acetic acid, N-methyl-2-pyrrolidone and methyl iodide were purchased from J&K Chemical. 2,5-Di-tert-butyl-1,4-benzoquinone, 1-iododecane and 1-iodohexane were purchased from Tokyo Chemical Industry. Ethanol, ethyl acetate, methanol, ethyl ether, sulfuric acid, dimethyl sulfoxide and N,N-dimethylformamide were supplied by AQA. Potassium bicarbonate, potassium hydroxide, potassium sulfate, sodium tetrafluoroborate and potassium chloride were purchased from Aladdin. Deuteroxide and CO were purchased from Cambridge Isotope Laboratories. Anion exchange membrane (FAB-PK-130, FuMA-Tech), 5 wt% of Nafion solution (Sigma-Aldrich), sulfuric acid-d (Sigma-Aldrich), proton exchange membrane (Nafion 117, Fuel Cell Store), carbon black (Cabot, XC 72R), MWCNT (50 nm, XFNANO) and gas diffusion layer (YLS 30T, Suzhou Sinero Technology) were obtained from the corresponding reagent company. Water was purified using a Millipore Milli-Q system (18.2 MΩ cm).
Here 4 mg of CoTAPc, 20 mg of 2,5-di-tert-butyl-1,4-benzoquinone, N,N-dimethylacetamide/ethanol (5 ml/2 ml) and 0.2 ml acetic acid solution (6 M) were added into a 20-ml high-pressure Schlenk tube. The mixture was sonicated for 10 min and then degassed through three freeze-pump-thaw cycles. After sealing, the tube was heated to 120 °C and maintained for 3 days. On cooling, ethyl ether was added to the dispersion, which was then centrifuged at 1,006g for 10 min to remove the bulk precipitates. The nCOP nanosheets were collected by centrifugation at 11,180g for 30 min, followed by sequential washing with N,N-dimethylformamide (three times) and ethanol (three times) to remove the residues. The final product was dried under a vacuum at 60 °C overnight to yield the nCOP powder.
Here 30 mg of nCOP and 10 ml of N,N-dimethylformamide were combined in a 25-ml round-bottom flask. The mixture was sonicated for 10 min, followed by the addition of 200 µl of methyl iodide. It was then heated to 50 °C for 12 h to synthesize iCOP-C1. After the reaction, the product was precipitated by adding diethyl ether into the mixture and washed with ethanol by high-speed centrifugation at 16,107g for 30 min. Finally, the product was freeze dried to give a loose powder.
Here 30 mg of nCOP, 0.02 ml of 1-iodohexane and 10 ml of N-methyl-2-pyrrolidone were added into a 50-ml high-pressure Schlenk tube. The mixture was sonicated for 10 min and then degassed through three freeze-pump-thaw cycles. After sealing, the tube was heated to 120 °C for 3 days. After the reaction, the product was washed with ethyl acetate. Finally, the intermediate product was vacuum dried at 60 °C. The obtained solid was further treated with methyl iodide to prepare fully quaternized catalysts using the same method as iCOP-C1. Supplementary Note 1 provides the details.
The preparation was the same as iCOP-C6, except that 1-iododecane was used instead of 1-iodohexane.
The preparation process was modified from the literature, in which methyl iodide was replaced with 1-iodohexane and CoTAP was replaced with CoTAPc. The obtained alkylated solid was further treated with methyl iodide to produce fully quaternized catalysts.
TEM images were taken on a Philips Technai 12 at an accelerating voltage of 120 kV and a JEM-2100F instrument equipped with an energy-dispersive X-ray spectroscopy detector at an accelerating voltage of 200 kV. AFM images were obtained on a Bruker Icon device in the tapping mode. The samples for AFM measurements were prepared by depositing a drop of the nanosheet dispersion onto a fresh silicon wafer (Innotronix Technologies) and dried in air at room temperature. The silicon wafer was sequentially cleaned with acetone, isopropanol, deionized water, isopropanol and then dried by N purging. No additional hydrophilic treatment (for example, plasma cleaning) was applied. Fourier transform infrared spectroscopy was measured using KBr pellets on a PerkinElmer Spectrum 100 spectrometer in the range of 500-4,000 cm. Solid-state C nuclear magnetic resonance (NMR) spectra were measured on Bruker Avance III 500. The elemental composition of the catalysts was analysed by X-ray photoelectron spectroscopy on a Thermo Scientific K-Alpha equipped with an Al X-ray excitation source (1,486.6 eV). All the binding energies were referenced to the C1s peak at 284.8 eV. Ultraviolet-visible spectra were measured on Shimadzu-UV1700. Zeta potentials were recorded on a dynamic light scattering particle size analyser (Malvern Zetasizer Nano-ZS). CO/CO adsorption isotherms were recorded on Micromeritics ASAP2020 V4.0. All samples were degassed at 120 °C for 12 h in a vacuum before analysis.
To prepare the samples for contact angle measurement, 80 mg of the sample powder was evenly spread onto the surface of a 1 × 1 cm tape. The surface of the powder was then covered with weighing paper, and the tape was transferred into a mould. A force of 3 t was applied to obtain a well-compacted powder layer on the tape surface. The prepared sample was carefully removed from the mould once the pressure was released.
Electrochemical CORR measurements were performed in a glassy H-type cell (Gaoss Union, C007-10) with 0.5 M of KHCO as the electrolyte. The volumes of catholyte and anolyte were 10 ml each. The working electrode was prepared through these steps. First, 10 mg of iCOP-C6 and 10 mg of MWCNTs were dispersed in 10 ml of ethanol and labelled as dispersions A and B, respectively. Subsequently, 2 µl of 5 wt% of Nafion solution was added to a mixture of 0.2 ml of dispersion A and 1 ml of dispersion B. Then, the mixture was sonicated for 30 min. Finally, 100 µl of this ink was drop cast onto a glassy carbon electrode with an area of 0.5 cm at room temperature. The counter and reference electrodes used were a platinum foil and an Ag/AgCl electrode, respectively. Before measurement, CO gas (99.99% purity, Linde) was bubbled into the catholyte for 20 min to ensure a CO-saturated electrolyte solution. CO was continuously bubbled into the cathodic compartment at a flow rate of 5 s.c.c.m. LSV was recorded at a scan rate of 50 mV s. Electrolysis was performed for 20 min at each applied potential. The gaseous products were analysed via online gas chromatography (Ruimin GC 2060), with each analysis lasting 8 min. After electrolysis at each given potential, the electrolytes were collected, and liquid products were characterized by H NMR (Bruker AVANCE AV III 300) with dimethyl sulfoxide as the internal standard. For the H NMR analysis, 450 µl of the electrolyte was mixed with 50 µl of a 10-mM dimethyl sulfoxide solution in DO.
Electrochemical CORR measurements were carried out in a custom-made H-cell, with catholyte and anolyte volumes of 1.75 ml each. Then, 10 mg of carbon black together with 50 µl of Nafion solution were dispersed in 10 ml of ethanol. Then, 2 ml of this dispersion was mixed with 0.2 ml of dispersion A and sonicated for 15 min. Finally, 400 µl of the catalyst ink was drop cast onto the gas diffusion layer electrode with an area of 2 × 2 cm.
The catalyst electrode was fabricated as follows: 2.5 mg of the catalyst, 2.5 mg of carbon black, and 25 µl of 5 wt% of Nafion solution were mixed in 1 ml of ethanol and sonicated for 30 min to form a catalyst ink. We used carbon black in the flow cell as the metal impurity is lower. Previous studies show that different dispersed states on carbon supports can affect the activity of molecules. However, we observed a minor change using carbon black or MWCNT as supports, which could be attributed to the spatially confined molecules in layered form. The TEM images of iCOP-C6 and the carbon black composite are shown in Supplementary Fig. 10, demonstrating that iCOP-C6 is well dispersed on carbon black. The acidic CORR performance of iCOP-C6 using different conducting additives is compared in Supplementary Fig. 20, showing similar performance. This ink was then dip coated onto a 1 × 2.5 cm gas diffusion layer to serve as the working electrode, with an active area of 0.5 × 2 cm. The cathode and anode chambers were separated by a proton exchange membrane. The electrolytes were 3 M of KCl + 0.05 M of HSO. Then, 50 ml of catholyte was circulated at a flow rate of 5 ml min, whereas the anolyte's flow rate was maintained at 10 ml min by a peristaltic pump. Gas feeds of CO and CO were introduced from the backside of the gas diffusion layer to the liquid electrolyte with flow rates of 3 s.c.c.m. and 5 s.c.c.m., respectively. The gaseous and liquid products in the flow cell were analysed using the same methods as in the H-type cell. The electrolysis in an alkaline environment was conducted using a mixed solution of 0.2 M of KOH + 1.5 M of KCl, with an anion exchange membrane used.
Here 2.5 mg of CoTAPc, 2.5 mg of carbon black and 150 µl of 5 wt% of Nafion solution were mixed in 1 ml of ethanol and sonicated for 30 min to form a catalyst ink. This ink was drop coated onto a gas diffusion layer to serve as the working electrodes with working area of 0.5 × 2 cm. The electrocatalytic CORR was conducted under the same conditions as previously described.
For the HER experiments, the RDE electrode was prepared as follows: 2 mg of catalyst and 2 mg of carbon black were combined with 1 ml of ethanol and 20 µl of 5 wt% of Nafion solution. This mixture was sonicated for 20 min to form a homogeneous ink. Subsequently, 10 µl of the ink was drop cast onto the RDE (Pine Research) with a diameter of 5 mm and dried at room temperature. The electrolyte was saturated with argon, and the LSV curves were recorded at a scan rate of 10 mV s. The electrolytes used in acidic, neutral and alkaline environments are 0.5 M of KSO + 0.05 M of HSO, 0.5 M of KSO, and 0.1 M of KOH + 0.45 M of KSO, respectively.
For CO oxidation, 5 mm of Pt RDE was used as the working electrode, with 5 mM of pyridinium solution as the electrolyte. For pyridine, we used 5 mM of pyridine + 5 mM of NaBF. N-Methylpyridinium tetrafluoroborate, N-hexylpyridinium tetrafluoroborate and N-dodecylpyridinium tetrafluoroborate were purchased from Beijing Hwrk Chemicals.
In situ ATR-SEIRAS spectroscopy measurements were conducted using a Nicolet iS50 Fourier transform infrared spectrometer, which is equipped with a liquid-nitrogen-cooled HgCdTe detector. Catalyst ink was prepared by mixing 5 mg of electrocatalyst, 5 mg of carbon black, 50 μl of 5 wt% of Nafion solution and 10 ml of ethanol. Then, 1 ml of this ink dispersion was then carefully dropped onto a Au-film-coated Si prism (60°, 20 × 0.95 mm) and dried in air. The ATR-SEIRAS measurements were performed by 32 scans at a spectral resolution of 4 cm. The spectrum collected under the open-circuit voltage was used as the background.
In situ spectroscopy measurements were conducted using a Shimadzu 1700 spectrometer. Catalyst ink was prepared by mixing 5 mg of electrocatalyst, 50 μl of 5 wt% of Nafion solution and 2.5 ml of ethanol. Then, 40 μl of this ink dispersion was then drop cast onto 1.2 × 0.4 cm transparent conductive fluorine-doped tin oxide substrates. In situ measurements were conducted in a quartz cell with Ag/AgCl and Pt serving as the reference and counter electrodes, respectively. The cell was filled with CO-saturated 0.5-M KHCO, with CO continuously bubbled into the cell at a flow rate of 1 s.c.c.m.
For the CO stripping experiments, CO was adsorbed by continuously flowing CO into the H-cell for 20 min at a potential of -0.6 V in 0.5 M of KHCO. After CO adsorption, the electrolyte was purged with Ar for 20 min to remove the dissolved CO. Cyclic voltammetry was then carried out by sweeping the potential from 0 to 1.83 V at a scan rate of 10 mV s.
The flow cell was used during the DEMS measurements. LSV was performed from -0.53 V to -1.74 V in 3 M of KCl + 0.05 M of HSO at a scan rate of 5 mV s, until a stable baseline was achieved. Mass signals were then recorded. After each electrochemical test, the cell was allowed to return to baseline before starting the next cycle under identical conditions to minimize measurement errors.
All the MD calculations were performed using the LAMMPS software. All the covalent terms (bonds, angles and dihedrals) were described using the universal force field potential. The van der Waals interactions were also described using the universal force field. Atomic charges were computed using the charge equilibrium scheme. Modified charge equilibrium parameters were used to describe the charged N species and the H and O atoms of charged HO species. Specifically, the charge equilibrium electronegativities of the N and HO atoms were modified to allow a charge accumulation of +0.6 on the ions, in line with the charges predicted from Mulliken analyses. All the catalysts were surrounded by 500 water molecules with a density matching that of liquid water. Five HO ions were then solvated, followed by nine Cl ions (to restrict the total system charge to 0). All the species were randomly spawned using Packmol to avoid bias. Systems were first minimized, followed by heating from 0.1 to 298.15 K using the canonical (NVT) ensemble. Systems were then maintained at 298.15 K for 2 ns. Volumes were chosen such that the system densities were equal to that of liquid water. The S and A values of all CO molecules were quantified for the final 100 ps (in intervals of 5 and 20 ps) using the 2PT method.