The continuous rise in the concentration of CO2 in the atmosphere has caused serious worries about global climate change [1], and there is an urgent need for advanced technologies to neutralize or even reduce CO2 emissions [2]. Due to the rapid development of renewable power grid technology, the cost of electricity could be continuously reduced [3]. The continuous decline in electricity costs enhanced the market competitiveness of producing chemicals through electrochemical CO2 reduction reaction (eCO2RR) [4-7]. Compared with traditional synthesis methods, eCO2RR has several advantages for producing chemicals and fuels, including operating under mild reaction conditions such as room temperature and ambient pressure, and high energy conversion efficiency [8, 9]. Depending on the electrocatalyst used, a variety of chemicals and fuels could be obtained, ranging from C1 (e.g., CO, formic acid, methane, and methanol) [10, 11] to higher economic value and energy density C2 (e.g., ethylene, ethanol and acetic acid) products [12, 13]. Compared with gas-phase products, liquid fuels have drawn particular interest due to their high energy density, ease of transportation and storage, and spontaneous separation from the input CO2 stream [14, 15]. However, the economic feasibility and large-scale application of producing liquid fuels through eCO2RR still faced significant challenges [16].
Currently, the catalytic selectivity and activity of high-value C2 liquids, especially the acetate, were still not within the scope of industrial requirements, and many other practical challenges required technological developments beyond the scope of catalytic material design [17, 18]. Among these challenges, the energy consumption and issues of the purity and concentration of the produced liquid fuel were the prominent challenges that need to be addressed. After all, the separation of diverse reduction products involved considerable separation costs [19].
To obtain 100% pure and high-concentration liquid fuel from eCO2RR with low energy consumption, the development of highly active and selective catalysts is a prerequisite. Presently, copper-based catalysts are considered as the main ones that can efficiently electrosynthesis the production of hydrocarbon products and oxides from eCO2RR [20, 21]. A lot of efforts have been invested in developing new copper-based catalysts for eCO2RR [22, 23]. However, the performance of most catalysts in eCO2RR to acetate was still limited, the relative purity of the obtained acetate product is usually ≤ 95% [24]. Generally speaking, the performance of nanometer catalysts depends to some extent on their morphology and particle size [25]. Nanometer catalysts with uniform size and sub-nanometer particle size usually exhibited high catalytic activity [26, 27]. Besides, the chemical microenvironment surrounding the catalytic center also exerted a crucial role in enhancing the catalytic performance [28-30].
Metal-organic frameworks composed of various organic ligands and inorganic nodes were highly designable and tunable. Due to the highly regular dispersion of metal ions in metal-organic framework (MOF), they were considered to be ideal precursors for the synthesis of various carbon-based single-atom catalysts [31] or metal oxide catalysts [32, 33]. Currently, it has been reported that some copper-based MOFs with poor stability were destroyed in the process of eCO2RR to generate copper oxide or copper-based nanomaterials such as copper oxide [34] and copper monoxide [22]. In the field of eCO2RR to produce ethylene, the catalytic performance of these copper-based nanomaterials was superior to that of commercial copper-based nanomaterials [35]. However, there was no relevant research on the eCO2RR to produce pure acetate. As a two-dimensional conductive Cu-based MOF, Cu–THQ (H4THQ = tetrahydroxy-1, 4-quinone) was composed of square planar CuO4 nodes of high density and THQ4- ligands [36, 37]. Previous work showed that the CuO4 node in Cu–THQ was unstable and it could be reduced into Cu(111) nanoparticle under electric field conditions in neutral electrolyte [38]. Compared to copper(0), cuprous ions had a unique stabilizing effect on the C2 product intermediate [39]. Moreover, cuprous oxide might can be generated from CuO4 node under electric field conditions in alkaline electrolyte. Therefore, we used Cu–THQ as a precursor to prepare ultrasmall cuprous oxide nanoparticles in an alkaline electrolyte under electric field conditions, and studied its performance and mechanism for eCO2RR to acetate.
As illustrated in Fig. 1a, Cu–THQ, was synthesized by copper nitrate hydrate and H4THQ, and its purity was verified by the powder X-ray diffraction (PXRD) patterns (Fig. S1a in Supporting information) [36]. The microcrystalline powder of Cu–THQ was coated on a gas diffusion layers with Nafion binder to prepare the working electrode for in situ electrosynthesis of Cu2O@Cu–THQ. After electrochemical reduction of Cu–THQ in a 2 mol/L KOH electrolyte at –1.0 V vs. RHE for 30 minutes, the PXRD patterns of the collected sample showed the diffraction peak of Cu–THQ was still present, but its intensity became weakened and a diffraction peak of Cu2O appeared. Notably, a new diffraction peak at 36.4° was observed, consistent with the (111) plane of Cu2O (Fig. S1b in Supporting information) [40], indicating that a new catalyst, denoted as Cu2O@Cu–THQ, was obtained (Fig. 1a). In the meanwhile, the Cu 2p XPS spectra and auger electron spectroscopy (AES) of Cu LMM (Fig. S2 in Supporting information) revealed that Cu2O@Cu–THQ mostly consisted of Cu(Ⅰ), the transformation of Cu(Ⅱ) to Cu(Ⅰ) was observed [41]. As a result, it also could be confirmed that the partial Cu–THQ transferred to Cu2O via a facile electroreduction process, and the remaining part Cu–THQ was left during the process of eCO2RR. The reason why the final product was cuprous oxide instead of elemental copper might be that under strong alkaline conditions, cuprous ions were more stable. The morphologies of Cu–THQ and Cu2O@Cu–THQ were characterized by scanning electron microscopy (SEM) and spherical aberration-corrected transmission electron microscope (AC-TEM). As demonstrated by SEM, Cu2O@Cu–THQ and Cu–THQ both exhibited a lamellar structure (Fig. S3a and b in Supporting information). The AC-TEM image of Cu–THQ also displayed a lamellar structure without any visible nanoparticles observed (Fig. S3c and d in Supporting information). Simultaneously, the AC-TEM image of Cu2O@Cu–THQ established an extensive number of nanoparticles with an average size of ca. ~2.5 nm (Fig. S4 in Supporting information), and the lattice fringe was 0.252 nm, which corresponded to the (111) plane of Cu2O nanoparticles. Meanwhile, Cu, C, and O species were consistently distributed in Cu2O@Cu–THQ, as shown by the elemental mapping images in Fig. 1b.
To further investigate their local coordination environments and distinguish the copper species in Cu–THQ and Cu2O@Cu–THQ, X-ray absorption spectroscopy (XAS) was performed. As shown in the X-ray absorption near edge structure (XANES) spectra of Cu K-edge in Fig. 1c, a significant change in Cu K-edge and visible Cu(Ⅰ) signal could be noticed, corresponding to Cu+ species were observed for Cu2O@Cu–THQ with respect to that of Cu–THQ, indicating that partial Cu2+ centers were reduced to Cu2O species after electrochemical treatment [42, 43]. Furthermore, the primary curve of Cu2O@Cu–THQ was similar to that of Cu–THQ, suggesting that the structure of Cu–THQ was partly maintained. Moreover, as shown in the extended X-ray absorption fine structure (EXAFS) spectra (Fig. 1d) for Cu–THQ and Cu2O@Cu–THQ, the peak at 1.5 Å attributed to Cu–O scattering path was observed for both of Cu–THQ and Cu2O@Cu–THQ. In addition, the absence of Cu–Cu peak at 2.21 Å in Cu2O@Cu–THQ supported the generation of Cu2O instead of Cu nanoparticles.
The Cu2O@Cu–THQ sample coated on carbon paper obtained by in situ electrosynthesis was directly used as the working electrode for eCO2RR in a CO2-saturated 2 mol/L KOH electrolyte using flow cell. Compared with the linear sweep voltammetry (LSV) of CO2 and Ar (Fig. 2a), showing the notable enhancement of current density in the CO2-saturated electrolyte, implying that the Cu2O@Cu–THQ had the potential for eCO2RR. The gaseous products were monitored by online gas chromatography (GC) equipment (Fig. S7 in Supporting information), while the liquid products were analyzed by 1H nuclear magnetic resonance (1H NMR) spectroscopy (Fig. S8 in Supporting information), all the potentials indicated in this work were with reference to the reversible hydrogen electrode (RHE). Cu2O@Cu–THQ was discovered to exhibit a high Faradaic efficiency (FE) of acetate to 65(3)% with the current density of 10.5 mA/cm2 at –0.3 V vs. RHE (Fig. 2b and Fig. S5 in Supporting information). The half cell (cathodic) energy conversion efficiency (ECE%) was 47.6% and energy conversion current density (ECCD = jacetate × (1.23-Eacetate0/1.23-Eapplied) was 5 mA/cm2. Importantly, in the liquid electrolyte, no other eCO2RR products were detected except for acetate, meaning that the relative purity of acetate in liquid products was 100% (Fig. S8f), which the isotope labeling experiment also proved (Fig. S9 in Supporting information). At –0.4 V vs. RHE, the relative purity of acetate and FE(acetate) were slightly reduced to 96.8% and 54.6%, respectively, but the ECCD was increased from 5 mA/cm2 to 11.2 mA/cm2. Considering the relative purity and ECCD, the performance of Cu2O@Cu–THQ was higher than those of all the reported catalysts (Fig. 2c). Commercial Cu2O NPs was applied for the eCO2RR under the same conditions, the FE of acetate was only 40% (Fig. S6 in Supporting information). To evaluate the durability, Cu2O@Cu–THQ was operated at –0.4 V vs. RHE for 40 h and we found that the selectivity of acetate was preserved while the current density decreased slightly (Fig. 2d). The electrochemical double-layer capacitance (Cdl) value of Cu2O@Cu–THQ was 0.16 mF/cm2, suggesting an approximate electrochemical surface area that of the Cu2O (Fig. S10 in Supporting information). The sample of Cu2O@Cu–THQ after electrocatalysis was also collected to further characterization, the SEM, AC-TEM, HRTEM images and XPS spectra all showed that Cu2O@Cu–THQ maintained the structure (Figs. S11–S14 in Supporting information).
To investigate the reaction intermediates during the eCO2RR process, the operando electrochemical attenuated total reflection Fourier transform infrared spectroscopy (ATR–FTIR) measurements were performed in an electrolytic cell (Fig. 3a). The reaction intermediates were observed, the peak at 1434 cm–1 was attributed to the *COOH, which was widely regarded as the key intermediate of CO2 electroreduction to CO [44]. The peak at 1554 cm–1 could be attributed to *CH2COOH intermediate [45] and the peak that appeared at 1000 cm–1 was assigned to the O-H in-plane bending vibration of *HOCCOH [46], which could be converted to the acetate product by further hydrogenation. Based on the FTIR results, the density functional theory (DFT) calculations were performed for further studying the catalytic mechanism (Figs. 3b and c). Specifically, when hydrogen bonding interaction exists, for potential limiting step (PLS) *OCCOH → *HOCCOH, the free energy change (∆G) (0.46 eV) was much lower than that (0.79 eV) without hydrogen bonding interaction, indicating that the hydrogen bonding interaction between the C2 key intermediate and the hydroxyl group on the organic ligand THQ of Cu–THQ was conducive to the generation of acetate as main product.
In summary, through electrochemical treatment of the conducting Cu–THQ containing periodically arranged Cu–O4 nodes, uniformly deposited Cu2O(111) ultrasmall nanoparticles on Cu–THQ had been successfully fabricated. Owing to the crystal structure of Cu–THQ, some Cu(Ⅱ) centers within Cu–THQ transformed into active Cu2O ultrasmall nanoparticles with an average size of 2.5 nm and a highly exposed single (111) crystal plane, while the uncoordinated THQ ligands provided hydroxyl groups. Under very-low potential conditions, the prepared Cu2O@Cu–THQ exhibited outstanding activity in the process of eCO2RR to produce acetate, and the relative purity of the produced liquid phase acetate reached 100%, with a performance surpassing most of the reported catalysts. The uniform Cu2O ultrasmall nanoparticles with extremely small size and single (111) crystal plane, as well as the abundant hydroxyl groups (-OH) in the released THQ ligands of Cu–THQ, contributed to the high selectivity for acetate and prevented the formation of ethanol by-products. The high conductivity of the THQ supported facilitates electron transfer between the active sites and the substrate, thereby generating a large current density. The operando ATR–FTIR of eCO2RR measurements and Gibbs free energy calculations clarified the significance of the hydrogen bonding interaction between the intermediate products of the acetate reaction pathway and the released THQ ligands. This work provided an efficient strategy for regulating the selectivity of high value-added C2 products in eCO2RR by designing active site, stabilizing key intermediates using hydrogen bonds, and increasing the current density with conductive framework materials.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.
Zhi-Xin Li: Writing – original draft, Investigation, Formal analysis, Data curation. Xiao-Feng Qiu: Writing – review & editing, Data curation. Pei-Qin Liao: Writing – review & editing, Software, Resources, Conceptualization.
This work was supported by the National Key Research and Development Program of China (No. 2021YFA1500401), National Natural Science Foundation of China (NSFC, Nos. 21821003, 22371304, and 223B2123), Fundamental Research Funds for the Central Universities, Sun Yat-Sen University (No. 24lgzy006), Science and Technology Innovation Special Support Project of Guangdong Province, China (No. STKJ2023078), and the Guangzhou Science and Technology Program (No. SL2023A04J01767).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 (a) Illustration of the synthesis of Cu–THQ and Cu2O@Cu–THQ. Color code: Cu, pink; C, grey; O, red. Note: the hydrogen atoms are omitted for clarity. (b) AC-TEM image of Cu–THQ and AC-TEM image, HRTEM image and EDX elemental mapping images of Cu2O@Cu–THQ. (c) Normalized Cu K-edge XANES spectra of Cu foil, Cu–THQ and Cu2O@Cu–THQ. (d) Fourier transform EXAFS spectra of Cu foil, Cu–THQ and Cu2O@Cu–THQ.
Figure 2 (a) LSV curves of Cu2O@Cu–THQ in 2 mol/L KOH electrolyte under Ar and CO2. (b) Faradaic efficiencies of different products for Cu2O@Cu–THQ. (c) Comparison of energy conversion current density for the purity of acetate between Cu2O@Cu–THQ and the reported electrocatalysts. (d) The durability of Cu2O@Cu–THQ in the electrocatalysis at the potential of –0.4 V vs. RHE.
Figure 3 (a) The operando ATR–FTIR spectra on Cu2O@Cu–THQ collected at –0.4 V vs. RHE. (b) Proposed mechanism of Cu2O@Cu–THQ for the formation of acetate. (c) Reaction free energies to form acetate on Cu2O@Cu–THQ with and without hydrogen bonding interaction, respectively. Dotted lines are displayed to guide eyes.
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