Cooperative promotion of electroreduction of CO to n-propanol by *CO enrichment and proton regulation

Abstract

The CO₂/CO electroreduction reaction (CO₂RR/CORR) to liquid products presents an enticing pathway to store intermittent renewable electricity. However, the selectivity for desirable high-value C₃ products, such as n-propanol, remains unsatisfactory in the CO₂RR/CORR. Here, we report that *CO enrichment and proton regulation cooperatively enhance C₁–C₂ coupling by increasing CO pressure and utilizing proton sponge modification, promoting the production of n-propanol over a Cu⁰/Cu⁺ nanosheet catalyst in the CORR. We obtain an impressive faradaic efficiency (FE) of 44.0% ± 2.3% for n-propanol at a low potential of −0.44 V vs. reversible hydrogen electrode (RHE) under 3 bar CO. Experimental results demonstrated that *H intermediates could be regulated by proton sponge modification. In situ characterization combined with density functional theory (DFT) calculations validate that Cu⁺ species exist stably in proton sponge-modified Cu-based catalysts along with appropriate *CO coverage. This design facilitates the potential-determining C₁–C₁ and C₁–C₂ coupling steps and contributes to the n-propanol production.

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Introduction

Electrochemical CO₂ reduction reaction (CO₂RR) to high value-added chemical feedstocks and fuels is a promising route for the storage of renewable electricity.¹ Presently, much progress has been made in the CO₂RR, however, the major products with satisfying performance reported are limited to C₁ (CO,^2,3 CH₄,^4–6 HCOO⁻,⁷ and CH₃OH^8,9) and C₂ (C₂H₄,^10–12 C₂H₅OH,^13–15 and CH₃COOH¹⁶) products. C₃ products such as n-propanol, a high value-added and high energy-density chemical, have been produced with low FE in previous reports in the CO₂RR.^17–19 Besides, electrolytes with alkaline or neutral pH are typically employed in the CO₂RR, which leads to undesired carbonate formation easily, causing energy/carbon losses and performance degradation.²⁰ Given the conspicuous progress of the highly efficient CO₂RR to CO,²¹ the n-propanol chemical produced by the electrochemical reduction reaction of CO (CORR) following the high-performance CO₂ reduction to CO, could efficiently alleviate these problems.^22,23

Cu-based materials have been reported to be efficient electrocatalysts for producing C₃ products such as n-propanol. Recently, various strategies have been adopted to enhance the FE of n-propanol with Cu-based catalysts, including the engineering of the oxidation state of Cu atoms,²⁴ morphological tuning,^25–27 Cu-based multimetallic materials,^28–31 grain boundary control,³² selective formation of the desired facets,³³etc. However, these catalysts still suffer from several challenges, such as limited selectivity in producing n-propanol (Table S1†). Thus, developing new approaches to enhance n-propanol selectivity in the CORR is still highly challenging and desirable.

As demonstrated in the published reports, high *CO coverage is beneficial for the formation of C₂₊ alcohol products in CO₂/CO electrolysis.^34–37 Interestingly, our previous study of the CO₂RR showed that higher CO₂ pressure could enrich *CO coverage on the catalyst surface, which was beneficial to obtain the ethanol product rather than ethylene.³⁸ Considering that the pathway of the CORR to n-propanol is a multiple proton-coupled electron transfer process, sufficient supplies of *CO and *H intermediates are essential to produce n-propanol during the process. *CO coverage usually could be regulated by pressure variation, while *H intermediates could be modulated via a reductive concerted proton–electron transfer (CPET) mediator.^39,40 Proton sponge (1,8-bis(dimethylamino)naphthalene) shows exceptional proton affinity through bidentate-type coordination by the two dimethylamino groups located at the peri position of the naphthalene skeleton.^41–43 The general feature of all proton sponges is the presence of two basic nitrogen centers in the molecule, which have an orientation that allows the uptake of one proton to yield a stabilized intramolecular hydrogen bond (IMHB). The strength of the IMHB calculated for proton sponges is in the range of 16–21.5 kcal mol⁻¹,^42,44 which is similar to that of the CPET mediator.

Herein, a cooperative strategy of applying pressure coupled with the use of a proton regulator (proton sponge, PS) for an efficient CORR to n-propanol was developed. A series of proton sponge-modified Cu nanosheet catalysts (denoted as Cu-NS-x% PS, x = 0, 10, 20, 30 and 50, where x is the mass percentage of proton sponge in the catalyst) were constructed. The CORR experiments were carried out under different CO pressures (0.5, 1, 2, 3 and 5 bar) with the as-prepared catalysts. Among these, the Cu-NS-20% PS catalyst for the CORR showed an n-propanol FE of 44.0% ± 2.3% at −0.44 V vs. RHE under 3 bar CO, representing ∼1.4-fold improvement relative to the unmodified Cu-NS under the same CO pressure. Experimental results showed that *H intermediates could be adjusted via proton sponge modification. In situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (in situ ATR-SEIRAS) indicated that sufficient *CO coverage could be provided by elevating CO pressure on the proton sponge modified Cu-NS catalyst for subsequent C–C coupling. In situ Raman spectra combined with DFT calculations further verified that Cu⁺ species in the Cu-NS-20% PS catalyst was persistent during the CORR under 3 bar CO, which was also beneficial for stabilizing *CO, and facilitating both C₁–C₁ and C₁–C₂ coupling to produce the desired n-propanol. This work creatively demonstrated that the cooperative design of a proton regulator with appropriate CO pressure was highly capable of improving the electrosynthesis of n-propanol from CO and H₂O (Scheme 1).

Scheme 1 Schematic illustration of CO electrolysis to n-propanol with proton sponge-modified Cu nanosheets as the catalyst under the assistance of CO pressures.

Results and discussion

Catalyst preparation and characterization

Proton sponge-modified Cu nanosheet catalysts were prepared as displayed schematically in Fig. 1a. Firstly, CuO nanosheets (CuO-NS) were synthesized via a wet chemical method. Proton sponge-modified CuO nanosheets (CuO-NS-x% PS) were then obtained by the addition of 1,8-bis(dimethylamino)naphthalene (proton sponge) dissolved in isopropanol to CuO nanosheets. The representative scanning electron microscopy (SEM) (Fig. 1b) and transmission electron microscopy (TEM) images (Fig. 1c) displayed CuO-NS-20% PS with a nanosheet morphology, which was similar to that of CuO-NS and other CuO-NS-x% PS (Fig. S1 and S2†). The lattice distances of CuO-NS-20% PS and CuO-NS were observed clearly in the high-resolution transmission electron microscopy (HR-TEM) images (Fig. 1d, S3 and S4†), which corresponded to CuO (111) and CuO (1̄10), respectively. Moreover, energy-dispersive X-ray spectroscopy (EDX) elemental mapping of CuO-NS-20% PS showed that the characteristic Cu, O and N elements were uniformly dispersed in CuO-NS-20% PS, indicating that the proton sponge was integrated very well (Fig. 1e). The successful proton sponge modification was further confirmed via Fourier-transform infrared spectroscopy (FT-IR), where representative absorption peaks from proton sponge were also well detected in CuO-NS-20% PS (Fig. 1f). Powder X-ray diffraction patterns (PXRD) of CuO-NS-x% PS (x = 10, 20, 30 and 50) were consistent with those of CuO-NS (Fig. 1g). Raman spectra further demonstrated the characteristic peak of CuO at 276, 320, and 605 cm⁻¹ for both CuO-NS and CuO-NS-20% PS (Fig. S5†).⁴⁵ These results revealed that the mild proton sponge-modification method would not affect the crystallinity of CuO. Additionally, X-ray photoelectron spectroscopy (XPS) was used to monitor the valence state of the catalysts. XPS spectrum of CuO-NS-20% PS revealed characteristic peaks of Cu²⁺ species at 954.3 eV (Cu 2p_1/2) and 934.3 eV (Cu 2p_3/2) (Fig. S6†). The N 1s XPS spectra further evidenced the successful proton sponge-modification (Fig. S7†). Moreover, the mole percentages of proton sponge in CuO-NS-x% PS were cross-checked by thermal gravimetric analysis (TGA), XPS, and SEM-EDX, indicating that the amount of modified PS could be easily regulated (Fig. S8 and Table S2†).

Fig. 1 Synthesis and characterization of CuO-NS-x% PS catalysts (x = 0, 10, 20, 30 and 50). (a) Schematic illustration of the synthetic procedure of Cu-NS-PS catalysts. (b) SEM, (c) TEM and (d) HRTEM images of CuO-NS-20% PS. (e) EDX elemental mappings of Cu, O and N in a single nanosheet of CuO-NS-20% PS. (f) FT-IR spectra of pure CuO-NS, proton sponge and CuO-NS-20% PS. (g) PXRD patterns of CuO-NS and CuO-NS-x% PS with different proton sponge loadings.

Cu-NS-x% PS catalysts (x = 0, 10, 20, 30 and 50) were obtained via in situ electroreduction of CuO-NS-x% PS upon four runs of linear sweep voltammetry (LSV) with a scan speed of 20 mV s⁻¹ over a potential range from open-circuit potential (OCP) to −0.64 V vs. RHE. As observed in SEM and TEM images (Fig. 2a and b), Cu-NS-20% PS displayed a rougher surface with distinguishable nanoparticles on the nanosheet when compared with the precursor, CuO-NS-20% PS (Fig. 1b and c).⁴⁶ Due to the unavoidable oxidation during sample transfer in ex situ XRD, Cu₂O species were detected in both Cu-NS and Cu-NS-20% PS following electroreduction. The PXRD pattern of the Cu-NS-20% PS proved the formation of Cu⁰ and Cu⁺ species, and the major exposed planes were Cu₂O (111), Cu (111) and Cu (200) of face-centered cubic Cu (fcc-Cu), while Cu (111) was the most remarkable characteristic peak in the PXRD pattern of the pure Cu-NS sample (Fig. 2c). Cu LMM Auger spectra in Cu-NS-20% PS further confirmed the existence of Cu⁺ species (Fig. S9†). To acquire more accurate surface features of the Cu-NS and proton sponge modified Cu-NS, we investigated the electrochemical adsorption of OH⁻ on the Cu-NS and Cu-NS-20% PS catalysts considering their facet-dependent OH⁻ adsorption behaviors. The cyclic voltammetry curves in Fig. S10† showed OH⁻ adsorption peaks at around 0.40, 0.44 and 0.50 V vs. RHE for Cu-NS and Cu-NS-20% PS, corresponding to the OH⁻ adsorption on the Cu (100), Cu (110) and Cu (111) surfaces, respectively.^47–49 These results indicate that both Cu-NS and Cu-NS-20% PS are dominated by Cu {100}, {110} and {111} facets.

Fig. 2 Synthesis and characterization of Cu-NS-20% PS catalysts. (a) SEM and (b) TEM images of Cu-NS-20% PS. (c) PXRD patterns of Cu-NS and Cu-NS-20% PS. (d) Schematic illustration of the custom-made high-pressure in situ Raman cell setup for the CORR. (e) In situ Raman spectra of Cu-NS under 3 bar CO at −0.44 V vs. RHE for 50 min. (f) In situ Raman spectra of Cu-NS-20% PS under 3 bar CO at −0.44 V vs. RHE for 90 min. The electrode was pretreated via in situ electroreduction before recording the amperometric curve and in situ Raman spectra testing.

In situ Raman spectra were collected in a high-pressure in situ Raman cell setup (Fig. 2d), and Raman bands that appeared around 510 cm⁻¹ belonging to Cu₂O^50–52 remained in the proton sponge-modified Cu-based catalysts (Cu-NS-20% PS and Cu-NS-50% PS) at −0.44 V vs. RHE during the in situ observation (Fig. 2f and S11†), while no Cu₂O was detected for the pure Cu-NS sample during the CORR process (Fig. 2e). These results validated that Cu⁺ species existed stably in the proton sponge-modified Cu-based catalysts during the CORR process, which was obviously different from Cu-NS without proton sponge modification. In order to observe the detailed changes in the valence state of Cu species during the in situ electroreduction process, in situ PXRD and in situ Raman characterization were conducted. Results showed that the Cu-NS catalyst possessed both Cu⁺ and Cu⁰ before converting to Cu⁰ completely (Fig. S12 and S13†).

CORR performance on proton sponge modified Cu-NS under different pressures

The electrocatalytic activity and selectivity for the CORR under different pressures were systematically evaluated in a custom-made pressurized cell with a two-compartment polytetrafluoroethylene (PTFE) lining separated by a proton exchange membrane (Fig. 3a and S14†). The CORR product distributions from Cu-NS and Cu-NS-20% PS under 1 bar and 3 bar CO are presented in Fig. 3b. As the CO pressure elevated from 1 bar to 3 bar, both Cu-NS and Cu-NS-20% PS exhibited an increase in FE_n-propanol and FE_acetate, while the FE_C2H4 and FE_H2 decreased. Notably, compared with pure Cu-NS, the FE_H2 of proton sponge modified Cu-NS increased under ambient pressure, which was attributed to low *CO coverage and high *H intermediates caused by the proton sponge. While the FE_H2 of Cu-NS-20% PS under 3 bar CO was similar to that of Cu-NS under the same CO pressure, Cu-NS-20% PS exhibited higher FE_n-propanol and lower FE_acetate. This was because the higher *CO coverage under 3 bar CO could match the higher *H intermediates to produce n-propanol rather than acetate. Overall, the FE_n-propanol reached 44.0 ± 2.3% over Cu-NS-20% PS under 3 bar CO at −0.44 V vs. RHE, which was ∼1.4-fold higher than that over the pure Cu-NS catalyst under the same CO pressure. Moreover, the total current density (j_total) and partial current density for n-propanol (j_n-propanol) of Cu-NS or Cu-NS-20% PS could be enhanced during the CORR when CO pressure elevated from 1 bar to 3 bar (Fig. 3c). The positive feedback further indicated that higher activity for producing n-propanol in the CORR could be achieved with the assistance of proton sponge under elevating CO pressure.

Fig. 3 Electrochemical CORR performances of Cu-NS and Cu-NS-20% PS under ambient and high CO pressures. (a) Schematic CORR setup under both ambient and high CO pressures. (b) CORR product distributions for Cu-NS and Cu-NS-20% PS under 1 bar and 3 bar CO, respectively. (c) The total current density (j_total) and partial current density for n-propanol (j_n-propanol) on Cu-NS-20% PS and Cu-NS catalysts under 1 bar, 3 bar CO vs. the cathodic potentials, respectively. (d) FE_n-propanol and FE_n-propanol/FE_C2+ ratios on Cu-NS-20% PS under different CO pressures (0.5, 1, 2, 3 and 5 bar CO). (e) FE_oxygenates/FE_ethylene ratios on Cu-NS-20% PS under different CO pressures (1 bar vs. 3 bar CO). Error bars represent the standard deviation of three independent measurements.

To explore the effects of CO pressure on CORR performance, CORR experiments with Cu-NS-20% PS at 0.5, 1, 2, 3 and 5 bar CO were further conducted (Fig. S19†). According to the experimental data, we built the correlations between FE_n-propanol and CO pressures in the CORR with the Cu-NS-20% PS catalyst (Fig. 3d). FE_n-propanol presented a volcanic trend with the CO pressures increased from 0.5 to 5 bar CO, in which FE_n-propanol rose steeply with CO pressure increasing from 0.5 to 3 bar. FE_n-propanol exhibited a drop when the CO pressure was further elevated to 5 bar, which could be attributed to excessive *CO coverage and insufficient *H intermediates required for the n-propanol production pathway. These results indicated that an appropriate *CO coverage was beneficial for C₁–C₁ coupling and C₁–C₂ coupling to produce n-propanol during the CORR process. We observed that the FE_oxygenates/FE_ethylene ratio on Cu-NS-20% PS increased with the CO pressures elevated from 1 bar to 3 bar CO (Fig. 3e), and a similar trend appeared on Cu-NS (Fig. S20†). These results proved that oxygenated products could be enhanced accompanied by decreased ethylene by facilely increasing the CO pressure.^53,54 The CO pressure regulation could also remarkably affect the C₂ and C₃ selectivities in the CORR. In detail, with the CO pressure elevated, the FE of C₂H₄ and CH₃CH₂OH decreased, while the FE of CH₃COOH and n-propanol increased on the majority of nano Cu-based catalysts (Table S3†).

The role of proton sponge was then investigated. We conducted a series of CORR experiments on Cu-NS without proton sponge and Cu-NS-x% PS with different loadings of proton sponge (x = 10, 20, 30 and 50) under 3 bar CO pressure (Fig. 4a and S21†). Initially, with the increased amount of proton sponge in Cu-NS, the FE_n-propanol was promoted efficiently, and reached the peak until Cu-NS combined with 20% of proton sponge, and then showed a downward trend. We also constructed the relationship between FE_n-propanol/FE_C2+ ratio and the amount of proton sponge (Fig. 4a). The FE_n-propanol/FE_C2+ ratio over Cu-NS-20% PS reached almost 0.5 under 3 bar CO, while the ratio was only 0.38 for Cu-NS under 3 bar CO, which evidenced that the n-propanol production was synergistically promoted by the proton sponge and elevating CO pressure. Interestingly, both FE_acetate and the FE_acetate/FE_oxygenates ratio dropped after being modified with 20% proton sponge on Cu-NS compared with the pure Cu-NS under the same CO pressure (Fig. 3b and 4b). Meanwhile, the proton sponge modified Cu-NS increased HER selectivity especially under ambient CO pressure (Fig. S22†). According to the above results, it can be concluded that acetate formation with the least proton requirement per carbon among the produced oxygenates could be favored because the proton donor was in short supply on Cu-NS without modification of the proton sponge. However, the n-propanol production pathway, in demand for more proton introduction, is adaptively regulated and enhanced accordingly by the Cu-NS with modification of proton sponge.

Fig. 4 Electrochemical CORR performance of Cu-NS-x% PS (x = 0, 10, 20, 30 and 50) under ambient and high CO pressures. (a) FE_n-propanol and FE_n-propanol/FE_C2+ ratios on Cu-NS-x% PS (x = 0, 10, 20, 30 and 50) under 3 bar CO. (b) The comparison of FE_acetate/FE_oxygenates ratios on Cu-NS and Cu-NS-20% PS under 3 bar CO pressure. (c) FE_n-propanolvs. potentials for various Cu-based catalysts in the CORR. (d) The chronopotentiometric potential time curve for Cu-NS-20% PS under 3 bar CO at −12 mA cm⁻² along with the corresponding FE_n-propanol. (e) The charging current density differences against scan rates with Cu-NS-x% PS (x = 0, 10, 20, 30 and 50) as catalysts. (f) Nyquist plots for Cu-NS and Cu-NS-20% PS electrodes in CO-staurated 1 M KOH electrolyte under 1 bar CO.

The FE_n-propanol of the CORR on a catalyst with higher amount of proton sponge and CO pressure (Cu-NS-30% PS under 5 bar CO pressure) was further investigated. Cu-NS-30% PS exhibited FE_n-propanol of 30.6% at −0.49 V vs. RHE under 5 bar CO, which was lower than that of Cu-NS-20% PS under 3 bar CO pressure and higher than that of Cu-NS under 1 bar CO pressure (Fig. 3b and S23†). These results revealed that an excess amount of proton sponge could cover the active sites of the Cu-based catalyst and further suppress the n-propanol production. Besides, the CORR performance of Cu-NS-20% PS using different concentrations of KOH (0.5 M and 2 M) as the electrolyte was also scrutinized under 3 bar CO (Fig. S24†). The FE_n-propanol and FE_C2+ of Cu-NS-20% PS using 1 M KOH as the electrolyte were higher than those of the same catalyst using 0.5 M or 2 M KOH as the electrolyte under 3 bar CO pressure (Fig. S25†). This observation suggests that in higher concentrated alkaline media (e.g., 2 M KOH), the limited proton availability may suppress CO reduction, thereby diminishing C₂₊ product selectivity. Compared with the two catalysts from commercial CuO and proton sponge modified commercial CuO, the as-prepared Cu-NS and Cu-NS-20% PS exhibited higher activity and selectivity towards n-propanol during the CORR, which could be attributed to the more abundant active sites in the nanosheet CuO precursor (Fig. S1 and S26†). In order to investigate the CORR performance of CuO-based catalysts from different proton sponge modification methods, CuO-NS was first in situ reduced at −0.44 V vs. RHE for 30 min, then 20% proton sponge was added to the surface of the Cu-NS to form the Cu-NS-after 20% PS catalyst. As displayed in the results in Fig. 3b and S27,† the FE_n-propanol from Cu-NS-after 20% PS was lower than that from Cu-NS-20% PS at different tested potentials.

Moreover, we directly compared the catalytic performance of Cu-NS-20% PS for n-propanol production with that of the reported catalysts (Fig. 4c and Table S1†). It was clearly demonstrated that Cu-NS-20% PS possessed one of the highest FE_n-propanol values to date, and revealed one of the lowest potentials amongst results reported thus far. And the partial current density for n-propanol was also greatly improved compared with the bare Cu-NS catalyst under ambient CO pressure, exhibiting pretty good results in H-type cells (Fig. 3c and S28†).

The stability of Cu-NS-20% PS was also evaluated via a long-term chronopotentiometry testing. It was found that there was no apparent decay of activity within 10 h operation time (Fig. 4d). Furthermore, the PXRD, XPS, SEM and TEM data of the electrode after the CORR under 3 bar CO test were collected. The characteristic peaks of Cu₂O and Cu observed obviously from the PXRD patterns were found to be similar to the original Cu-NS-20% PS (Fig. 2c and S29†). XPS spectra of the Cu-NS-20% PS after the CORR also exhibited the presence of Cu⁺ peaks (Fig. S30†). Besides, the SEM and TEM images showed that the sample still retained the nanosheet morphology (Fig. S31†).

Based on the results and analysis above, the selectivity and activity of n-propanol were raised distinctly by rationally introducing a certain amount of proton sponge and appropriate CO pressure. This prompted us to investigate the intrinsic reasons for the enhanced CO conversion to n-propanol. To verify the importance of the Cu-NS-20% PS structure, we determined the double-layer capacitance (C_dl), which was obtained from the charging current density at −0.026 V vs. RHE against the slope of scan rate (Fig. 4e and S32†). The C_dl values of Cu-NS-x% PS (x = 0, 10, 20, 30 and 50) presented a volcanic trend, and Cu-NS-20% PS showed the highest electrochemically active surface area (ECSA) (Fig. S33 and Table S4†). The trend was similar to the relationship between FE_n-propanol and proton sponge amount on Cu-NS, which further indicated that the active sites of the Cu-based catalyst and the activity of the CORR to n-propanol could be enhanced with an optimized proton sponge/Cu ratio, while an excess amount of proton sponge would shield the active sites of the Cu-based catalyst indeed. The Nyquist plots were also measured (Fig. 4f and S34†), and the charge-transfer resistance (R_ct) of Cu-NS-20% PS was found to be smaller than that of Cu-NS, suggesting that the proton sponge-modified Cu-based catalyst could efficiently promote the charge transport during the electrocatalytical process in contrast to the control Cu-NS.⁵⁵

Mechanism analysis

To gain in-depth insights into the mechanism of proton sponge-modification of Cu-NS and the pressure-driven electrochemical CORR to n-propanol, in situ ATR-SEIRAS data were further collected on a custom-made high-pressure spectroelectrochemical setup to probe the intermediates associated with CORR reactivity on Cu-NS-20% PS and Cu-NS under CO pressures of 1 bar and 3 bar, respectively (Fig. S35–S37†). A prominent absorption band in the range of 2072–2025 cm⁻¹, attributed to the C=O stretching mode of atop-adsorbed CO (CO_atop), was observed on the Cu-NS-20% PS and Cu-NS at potentials from 0.1 to −0.6 V vs. RHE under both 1 bar and 3 bar CO (Fig. 5a, b, d and e). The CO_atop band red-shifted as the potential decreased from 0.1 to −0.6 V vs. RHE, which was ascribed to the Stark shift.^56,57

Fig. 5 Mechanism investigation. Potential-dependent SEIRAS investigation of CO_atop and CO_b bands on Cu-NS-20% PS under (a) 1 bar CO, (b) 3 bar CO in 0.1 M KOH electrolyte. (c) Potential-dependent SEIRAS investigation of CO_atop peak positions on Cu-NS-20% PS and Cu-NS under CO pressures of 1 bar and 3 bar, respectively. Potential-dependent SEIRAS investigation of CO_atop and CO_b bands on Cu-NS under (d) 1 bar CO, (e) 3 bar CO in 0.1 M KOH electrolyte. (f) Potential-dependent SEIRAS investigation of CO_atop band areas on Cu-NS-20% PS and Cu-NS under pressures of 1 bar and 3 bar CO, respectively. The maximum peak area at each CO pressure was scaled to 1.0. Error bars represent the standard deviation of three independent measurements. (g) The linear sweep voltammetry curve per ECSA in 1 M KOH under an Ar atmosphere for the Cu-NS-20% PS and Cu-NS catalysts, respectively. (h) KIE of H/D in the CORR to C₂H₄ performance on Cu-NS and Cu-NS-20% PS at −0.44 V vs. RHE under 1 bar and 3 bar CO, respectively. (i) Ratios of FE_n-propanol/FE_C2+ on Cu-NS and Cu-NS-20% PS at −0.44 V vs. RHE in 1 M KOH solution with the addition of 0 M, 0.1 M and 0.5 M t-BuOH under 3 bar CO pressure.

Meanwhile, the peak position of the CO_atop band could serve as a proxy to compare absolute CO coverages.⁵⁸ The wavenumber of the CO_atop band was higher under 3 bar than those under 1 bar for both Cu-NS-20% PS and Cu-NS (Fig. 5c), indicating that the absolute CO_atop coverage at 3 bar of CO was higher than that under ambient CO pressure at a controlled potential.⁵⁸ Besides, the CO_atop band areas of Cu-NS-20% PS and Cu-NS were normalized to the maximum value under both 1 bar and 3 bar CO pressures, respectively (Fig. 5f). We speculated that the elevated CO pressure and modification with proton sponge on Cu-NS could enrich the *CO intermediates, and cause moderate impact of rapid CO_atop consumption at large overpotentials. On the other hand, the bands in the range of 1839–1763 cm⁻¹ can be assigned to the C=O stretching mode of the bridge-adsorbed CO (CO_b). The Cu-NS-20% PS catalyst presented the strongest CO_b peaks during the CORR from 0.1 to −0.6 V vs. RHE under 3 bar CO compared with those of control experiments (Fig. 5a, b, d and e), and it was reported that the cooperation of CO_atop and CO_b could facilitate C–C coupling.⁵⁹

To further understand how the *H supply affected the C₂ and C₃ products, well-controlled sets of experiments including linear sweep voltammetry tests, kinetic isotopic effect (KIE) of H/D measurements and *H trapping experiments were conducted. Firstly, the intrinsic H₂O dissociation activity of Cu-NS, with and without proton sponge modification, was investigated by comparing the normalized polarization curves of Cu-NS and Cu-NS-20% PS per ECSA in 1 M KOH under an Ar atmosphere (Fig. 5g and S38†). Both the ECSA-normalized current density and geometric current density of Cu-NS-20% PS were higher than those of Cu-NS, demonstrating that modifying Cu-NS with a proton sponge enhanced the dissociation of H₂O,⁶⁰ in good agreement with experimental results (Fig. S22†). Besides, we measured the KIE of H/D on the Cu-NS and Cu-NS-20% PS catalysts. The KIE of H/D is defined as the ratio of FE_C2H4 in H₂O and D₂O, a KIE value larger than 1.5 indicates that water activation affects the reaction rate.⁶¹ The Cu-NS catalyst showed large KIE values of 1.53 and 1.85 at 1 bar and 3 bar CO, respectively, indicating that the sluggish kinetics of H₂O dissociation limited the electrocatalytic CORR process.⁶² The Cu-NS-20% PS catalyst exhibited lower KIE values of 1.02 and 1.35 at 1 bar and 3 bar CO, respectively, indicating that the Cu-NS catalyst modified with proton sponge could effectively accelerate H₂O dissociation and promote proton transfer at both 1 bar CO and 3 bar CO pressures (Fig. 5h).^63,64 To further investigate whether *H supply of Cu-NS-20% PS tuned the n-propanol selectivity, we conducted the *H trapping experiments using tert-butanol (t-BuOH) as the *H trapping agent.^64,65 The decrease in the ratio of FE_n-propanol/FE_C2+ on Cu-NS-20% PS was much larger than that on Cu-NS when t-BuOH was added to the reaction system (Fig. 5i). This result indicates that the generation of n-propanol is closely related to the *H supply on the catalyst surface, and a faster *H supply (proton-sufficient supply) could directionally promote the CORR to produce n-propanol.⁶⁴

Density functional theory (DFT) calculations

To further rationalize the impact of the proton sponge on the catalyst performance towards the selective and efficient propanol synthesis, DFT simulations related to the impact of the proton sponge on the oxidation state of the Cu-NS and on the key C-chain growth stage were performed. The proton sponge could adsorb on the Cu₂O surface in different configurations, including the one depicted in Fig. 6a. The proton sponge was mainly attracted to the Cu₂O surface via carbon copper interactions, as well as weak interactions of hydrogen atoms with the surface. The presence of the proton sponge substantially increased the energy of formation of oxygen vacancy on the Cu₂O surface (Fig. 6b and S39–S41†), as compared with the pristine Cu₂O (111) surface,⁶⁶ preventing the removal of oxygen from the reactive surface and thus preserving the oxidation state of the Cu⁺, which coexisted with the metallic Cu⁰. The hydrophobic nature of the proton sponge heightened the chemical potential of water ⁶⁶ in the vicinity of the catalyst surface, therefore, making its formation unfavorable. That also contributed to the coexistence of Cu⁺ and Cu⁰ oxidation states. Moreover, its non-polar fragments could serve to attract the non-polar hydrogen molecules. Therefore, Cu⁺/Cu⁰ became stabilized in the presence of the proton sponge.

Fig. 6 The impact of the proton sponge on the copper oxidation state on the surface and the adsorption energetics of the key C-chain growth intermediates. (a) Adsorption of the proton sponge molecule on the Cu₂O (111) surface. (b) The formation energies of oxygen vacancies on the pristine Cu₂O (111) surface (blue, from ref. 66) and Cu₂O (111) surface covered with proton sponge (pink). (c) Adsorption intermediates and energies on the Cu⁰/Cu⁺ (111) Cu₂O surface and Cu (111) surface. (d) Schematic illustration of the cooperative promotion of electroreduction of CO to n-propanol on Cu-based catalysts by the proton regulator and elevated pressure.

Given that only *CO was observed and hydrogen-containing species were not detected in the in situ ATR-SEIRAS, we are more inclined to rationally follow the *CO + *CO and *CO + *OCCO coupling mechanisms to generate n-propanol.^28,67 Considering the C-chain growth on the Cu⁺/Cu⁰ and Cu⁰ surfaces, it turned out that the adsorption of *CO, *OCCO and *OCCOCO intermediates on Cu⁺/Cu⁰ was more favorable than that on the fully reduced (111) surface of fcc-Cu (Fig. 6c and S42–S44†). Indeed, for *OCCOCO, the adsorption energy on Cu⁺/Cu⁰ was −1.76 eV; while for the metallic Cu system, it was −0.66 eV. In both Cu⁺/Cu⁰ and Cu⁰ systems, *OCCOCO attached to the surface via the terminal carbon atoms on both sides interacting with surface copper atoms, while neither the middle carbon atom nor oxygen atoms participated in the adsorption. However, in particular, for the mixed valence Cu⁺/Cu⁰ system, the distance between copper sites perfectly matched with the length of the C₃ chain of the *OCCOCO intermediates (Fig. S45 and S46†), which was another factor responsible for its enhanced selectivity and activity towards the C₃ product.

On the other hand, the proton sponge can locally stabilize the dissociated water molecules, collecting the H⁺ inside. The [proton sponge–H]⁺ complex attracts OH⁻ along with hydration water molecules due to the electrostatic interaction. The system with H⁺ in the proton sponge and hydrated OH⁻ is less energetically favorable by 0.43 eV than the proton sponge––water system (Fig. S47†). However, the electrochemical overpotential applied to the system during the reaction (−0.44 V vs. RHE) is sufficient to overcome this energy barrier, that is, the negatively charged catalyst surface by electrostatic interaction provokes water dissociation; the proton sponge collects the resulting protons and promotes proton supply (Fig. S48†).

Based on a series of control experiments and DFT calculations above, it could be observed that both the elevated CO pressure and proton sponge-modification on Cu-based catalysts played indispensable roles in enhancing *CO coverage and *H supply for subsequent *CO + *CO and *CO + *OCCO coupling. Besides, the mixed-valence Cu⁺/Cu⁰ system exhibits an optimal geometric configuration, where the interatomic spacing between adjacent copper sites aligns precisely with the molecular dimensions of the C₃ backbone in the *OCCOCO intermediate. The synergistic optimization of multiple reaction parameters in proton sponge-functionalized Cu catalysts leads to significantly enhanced n-propanol generation efficiency in the CORR (Fig. 6d).

Conclusions

In summary, we present a strategy to significantly improve the selectivity of n-propanol in CO electrolysis by cooperation of the binding proton sponge on Cu-NS and the CO pressure regulation. Through conveniently regulating the amount of proton sponge and CO pressure, we demonstrate a CO to n-propanol conversion with an FE of 44.0% ± 2.3% for n-propanol on the Cu-NS-20% PS catalyst at −0.44 V vs. RHE under 3 bar CO pressure, which is ∼1.4-fold higher than that on the pure Cu-NS catalyst under the same CO pressure. The oxygenated products are enhanced accompanied by a decrease of ethylene selectivity by increasing the CO pressure for both pure Cu-NS and Cu-NS-20% PS. Simultaneously, the selectivity of the multiple proton coupled electron transfer product (n-propanol) was enhanced effectively on Cu-NS-20% PS compared with the pure Cu-NS. This is due to the appropriate *CO coverage, matched with the suitable *H intermediates, and the perfect distance between copper sites matching the length of the C₃ chain of the *OCCOCO intermediates after modification with proton sponge under elevated CO pressure. Experimental results and in situ ATR-SEIRAS demonstrated that sufficient *CO coverage and *H intermediate could be provided by appropriate CO pressure and proton sponge-modification, respectively, for subsequent C₁–C₁ and C₁–C₂ coupling. In situ Raman spectra and DFT calculations further revealed that Cu⁺ species existed stably in the proton sponge-modified Cu-based catalyst, leading to a low energy barrier in the potential-determining C₁–C₁ and C₁–C₂ coupling steps.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

L. P., S. Y., J. L. and B. H. supervised the project. R. Q. designed the high-pressure in situ Raman cell setup and in situ ATR-SEIRAS cell setup. R. Q. and B. Z. synthesized the catalysts. R. Q., L. C., J. L., C. L. and J. Z. carried out electrochemical experiments. R. Q., L. C., L. D. and Y. D. characterized the materials. O. A. S. and M. A. S. carried out DFT calculations. R. Q., N. F. and J. Y. conducted the in situ ATR-SEIRAS experiment. R. Q., Y. J., J.-C. D. and J.-F. L. conducted the in situ Raman spectroscopy experiment. T. X., I. A. and J. Z. contributed to data analysis. R. Q., J. L., L. P., J. L., S. Y. and B. H. co-wrote the manuscript. All authors discussed the results and contributed to the preparation of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

L. P. acknowledges funding support from the National Natural Science Foundation of China (grant no. 22373080) and Fujian Provincial Natural Science Foundation of China (grant no. 2024J08008). J. L. acknowledges funding support from the National Natural Science Foundation of China (grant no. 22078274). S. Y. acknowledges funding support from the Natural Science Foundation of Fujian Province of China (no. 2022J05009), the Fundamental Research Funds for the Central Universities (grant no. 20720240054), the Nan-qiang Youth Scholar Program of Xiamen University and Xiaomi Young Talents Program/Xiaomi Foundation.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc00274e

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