Microenvironment Evolution at Triple-phase Interface on the CO2RR Process of Hydrophobic Oxide-derived Copper

doi:10.21203/rs.3.rs-3812973/v1

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Microenvironment Evolution at Triple-phase Interface on the CO2RR Process of Hydrophobic Oxide-derived Copper

https://doi.org/10.21203/rs.3.rs-3812973/v1

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The local microenvironment of electricity-powered CO₂ electroreduction reaction (CO₂RR) surrounding the catalyst-electrolyte-gas triple-phase interface plays a crucial role in catalytic activity and selectivity as it affects reaction pathways and species transport. However, it still needs to be explored and understood regarding the impact of microenvironment evolution on the CO₂RR performance. We report here a hydrophobic oxide-derived copper foam with villous nanowires on the surface that demonstrates significant suppressed HER and enhanced C₂₊ selectivity in H-type cell. In-situ 3D Raman mapping and in-situ Raman spectra investigation on micro-environmental species reveal that high local pH and fast CO₂ mass transfer were simultaneously allowed in the microenvironment of the triple-phase interface because of the special hydrophobic structure. On this mechanism, the material reaches a minimum H₂ Faradaic efficiency (FE) of 6.6% and maximum C₂₊ FE of 74.4% at the current density of 300 mA cm^-2in a flow cell under acidic conditions (pH=4) without an additional gas-diffusion layer (GDL). This study not only highlighted the importance of the microenvironment but also provided an effective method for tuning the triple-phase interface of CO₂RR and demonstrated a promising application of the pure metal foam-based GDEs.

Physical sciences/Engineering/Chemical engineering

Physical sciences/Chemistry/Catalysis/Electrocatalysis

CO2RR

Copper

hydrophobicity

triple-phase interface

microenvironment

The renewable electricity-powered CO₂ electro-reduction (CO₂RR) offers a promising route to simultaneously achieve renewable energy storage and effectively address the challenge of climate change.^1–4 In this context, the value-added multi-carbon (C₂₊) products from CO₂RR like ethylene, ethanol, and propanol, are particularly desired due to their substantial energy density and economic returns.^4–8 Accordingly, extensive efforts have been dedicated to developing potent electrocatalysts, such as Cu-based catalysts, for efficient conversion of CO₂ to C₂₊ products.^9–11 Besides seeking for novel catalysts, it is also critical to understand the reaction microenvironment,^{11, 12} which can influence reaction pathways through the affecting mass transfer and local concentration of reaction species. ^{11, 13} Significantly, the reaction microenvironment involves the local environment surrounding the reaction interface between the catalyst,¹⁴ reactants,^{15, 16}, and electrolyte¹⁷, which decides both the kinetics and thermodynamics during reactions.^{11, 18, 19} For example, the electrolyte's local pH, species, and buffer capacity, etc., have significant effects on the local reaction conditions and the product distribution of the electrochemical reactions, and the hydrophilicity and hydrophobicity of electrode/catalysts can affect the mass transfer of interfacial water thereby modulating the process of the reactions.^17,20

As for the electrochemical CO₂RR process, the structure and properties of the catalyst-electrolyte-gas triple-phase interface, which interacts with the reaction microenvironment, play a crucial role in the catalytic activity and selectivity.²¹ As a typical work, Warkerley et al. introduced a hydrophobic Cu dendrite catalyst with a triple-phase interface, which can trap gas under hydrophobic hairs in water like a spider to modulate the reaction process.²² In this catalyst-electrolyte-gas system, the CO₂RR process only evolved near the triple-phase boundary with CO₂ diffusion, and the microenvironment like the local ion and gas transfer was significantly changed, which led to the suppressed hydrogen evolution reaction (HER) with a Faradaic efficiency (FE) below 10% and induced high C₂₊ product selectivity.²² Recently, the cells with gas-diffusion electrodes (GDE) were developed and upgraded to enhance the mass transfer of reactants, which can achieve high current density at an industrial scale because the limitation of CO₂ mass transfer was alleviated.^{11, 23, 24} For example, Dinh et al. achieved enhanced operational stability of constant ethylene selectivity for an initial 150 operating hours though the Polytetrafluoroethylene (PTFE)-based gas diffusion layer, which sandwiches the reaction interface between separate hydrophobic and conductive carbon layer.²⁴ It was emphasized that the stability of the CO₂RR in gas-type reactors was significantly affected by the durability of the triple-phase interface, which was successfully kept with the additional carbon materials on the electrode.^{21, 24} These studies have demonstrated the substantial influence of the catalyst-electrolyte-gas triple-phase interface and its microenvironment in gas-involved electrochemical reactions.^{11, 25} However, most of the reported electrode is either carbon-based or PTFE derived GDE, metal foam based GDE without GDL is rarely reported. And there is still need to be explored and understood regarding that the impact of catalyst microenvironment and reaction interfaces on the mass transport and kinetics of metal foam-based GDE, which was conducive to further design of the effective CO₂RR system with an optimal microenvironment at the catalyst-electrolyte-gas interfaces.

In this context, our work aimed at constructing an intuitive and conveniently researchable catalyst-electrolyte-gas triple-phase interface, deepening the understanding of the triple-phase interface microenvironment and guiding the design of the efficient CO₂RR system. We prepared the oxide-derived copper with villous nanowire structure (OBC)²⁶ base on the porous copper foam framework, and then on its basis synthesized the corresponding hydrophobic materials (OBC-OT) with the treatment of 1-octadecanethiol. CO₂RR performance in H-type cell showed that the HER of OBC-OT was effectively suppressed to 12.6%, and the C₂₊/C₁ selectivity of OBC-OT was improved to 3.3 compared to the 1.4 of OBC. The weak CO adsorption in TPD results showed that the surface active site variation caused by the 1-octadecanethiol adsorption was not the deep reason for elevated C₂₊ selectivity. The in-situ 3D Raman mapping was used to visually observe the catalyst-electrolyte-gas triple-phase interface and demonstrated the significant increase of reactants and local current density at the triple-phase interface. The in-situ Raman spectra further verified the high local current density under different applied potentials and revealed the changes from C₁ products favored to C₂₊ products along with the applied potential negative shifted or current density increase. Additionally, combining the measurements with simulations of micro-environmental species, it is found that high local pH and fast CO₂ mass transfer can be simultaneously allowed in the microenvironment of the triple-phase interface as a result of the special hydrophobic structure of OBC-OT, which led to the high C₂₊ selectivity and suppressed HER. Leveraging this mechanism and the porous hydrophobic structure, the OBC-OT was further applied in the flow cell under acidic conditions (pH = 4) and displayed a low H₂ FE of 6.6% and high C₂₊ FE of 74.4% at the current density of 300 mA cm^− 2 without additional gas-diffusion layer (GDL).

Catalyst Synthesis and Characterization. Catalyst-electrolyte-gas triple-phase interface and its microenvironment play a crucial role in the CO₂RR performance. Therefore, we prepared the hydrophobic and porous OBC-OT based on Cu foam as the catalysts, which can intuitively observe the interface and conveniently research the microenvironment. Firstly, we synthesized a layer of Cu(OH)₂ nanowires on the surface of Cu foam with the alkaline solution according to the previous report^26–28, and the blue Cu(OH)₂ layer was turned to CuO layer after the annealing process. After that, the OBC materials were then obtained as the CuO layer was electro-reduced to oxide-derived Cu with a villous nanowire structure^{8, 26}. Based on OBC, the OBC-OT materials were prepared with the hydrophobic treatment of 1-octadecanethiol (Fig. 1a and Figure S1).

The scanning electron microscopy (SEM) images of the OBC-OT showed that the villous and twisted nanowires were grown on the surface of Cu foam and a large amount of dense and interlaced channels can be formed based on this structure (Fig. 1b). The typical transmission electron microscopy (TEM) images in Fig. 1c further demonstrated certain rough surfaces belong to the oxide-derived Cu. Additionally, the corresponding energy dispersive spectroscopy (EDS) mapping in Fig. 1c also showed that the oxide-derived Cu was different from the pure metallic and there are still some O remained after the electro-reduction, which was in accord with the previous report.²⁶ The 1-octadecanethiol was verified to be successfully absorbed with the thickness of ~ 1.5 nm (Fig. 1d) on the nanowires based on the EDS images of C and S element (Fig. 1c) and the S 2p spectra in X-ray photoelectron spectroscopy (XPS) result (Figure S7). The BET results showed that there was no obvious pore blinding after the processing of 1-octadecanethiol as the surface area even increased (Figure S9). The CuO, OBC, and OBC-OT were also characterized by the Raman and infrared absorption (IR) spectra in Figure S8a-b. Although there was no obvious Raman peak of 1-octadecanethiol detected in OBC-OT because of the low content and Raman scattering area, the weak IR peak of 1-octadecanethiol was detected in OBC-OT.

High-resolution transmission electron microscopy (HRTEM) indicated that the lattice space of oxide-derived Cu (1.90, 2.17, 2.21, and 3.55 Å) differs from the pure Cu, CuO, or Cu₂O, which can be designed as the CuO_x (Fig. 1d and Figure S2). The X-ray diffraction (XRD) results of OBC with different electrochemical reduction time (Figure S4a-b) further confirmed that this phenomenon was not resulted from the incomplete reduction of CuO. The CuO_x on OBC was formed in 10 min and can be stable with the potential of -1.0 V vs. RHE applied for 10 h. The in-situ Raman spectra (Figure S5) monitored the process of this conversion process from CuO to OBC at -1.0 V vs. RHE and the peak of 616 cm^− 1 (Cu-O) was gradually weakened. When the 1-octadecanethiol directly adsorbed on CuO nanowires (CuO-OT), a thick layer (~ 15 nm) of 1-octadecanethiol was averse to the exposure of reaction sites, which confirmed the necessity of the electrochemical reduction process (Figure S6).

The XPS was conducted to reveal the valence and chemical bonding information of the catalysts (Fig. 1e and Figure S7). The Cu LMM spectra showed that the proportion of Cu(0) increased as the 1-octadecanethiol adsorbed on OBC, which was in accord with the previous report.²² While, the proportion of Cu(Ⅰ) decreased concomitantly. Combining the S-Cu(Ⅰ) existed on OBC-OT in Figure S7a,^{22, 29, 30} it can be speculated that the decrease in the signal strength of Cu (I) can be attributed to the coverage of long-chain 1-octadecanethiol on Cu(Ⅰ).^{31, 32} The weakened peak intensity of OBC-OT compared to OBC in the O 1s spectra further verified the increased content of Cu(0) on the surface. The comparison of XPS data between OBC-OT and the OBC-OT-tested (after testing at -1.57 V vs. RHE for 1 h) demonstrated the considerable adsorption stability of the 1-octadecanethiol on OBC-OT as there was a small variation between them.

The hydrophobic property of the OBC and OBC-OT was investigated via the contact angle result and photography water immersion test. After the hydrophobic treatment, the contact angle of OBC-OT was turned to 129° compared to the 17° of OBC (Fig. 1f), and it remained 127° after testing at -1.57 V vs. RHE for 1 h (Figure S11), which illustrated the stability of the hydrophobic layer as well. As the OBC-OT was immersed in the water (Figure S3), the white surface liquid film appeared because of the high contact and refractive index difference between liquid and gas. Additionally, the plucked OBC-OT can keep dry from visual observation although it had been completely submerged in water.

The electrochemical double-layer capacitance (C_dl) was calculated to evaluate the electrochemical active surface area (ECSA) in Fig. 1g. The OBC-OT showed a C_dl of 0.021 mF cm^− 2, which was well below the 4.60 mF cm^− 2 of OBC, which indicated the great reduction of the catalyst-electrolyte contact area due to the hydrophobic property. Furthermore, the C_dl of polished plate Cu was 0.067 mF cm^− 2 according to a previous study²⁸, higher than that of OBC-OT, which further verified the hollow space as the OBC-OT was immersed in the electrolyte.

CO₂ RR Performances in H-type cell. The CO₂RR properties of the OBC and OBC-OT catalysts were evaluated in the H-type cell setup with the CO₂-saturated 0.1 M KHCO₃ (pH ∼ 6.8) solution as the electrolyte, the prepared catalysts as the working electrode, the Pt mesh as the counter electrode, and the saturated Ag/AgCl as the reference electrode. The FE of CO₂-reduction products was calculated by nuclear magnetic resonance (NMR) spectroscopy (Figure S13-14) and gas chromatography (GC, Figure S15). The voltage drop from solution resistance was compensated. The solution resistance was measured by the electrochemical impedance spectroscopy (EIS) in Figure S12.

The linear sweep voltammetry (LSV) was conducted in the CO₂-saturated and N₂-saturated 0.1 M KHCO₃ solution to estimate the CO₂RR and competitive HER activity (Fig. 2a) of OBC and OBC-OT. The general current density of OBC-OT was lower than that of OBC owing to the hydrophobic property of OBC-OT. The OBC in CO₂-saturated electrolyte showed an approximate current density with that in N₂-saturated electrolyte, indicating a high competitive HER on the OBC. However, the OBC-OT showed higher current density in the presence of CO₂ than that of N₂, which reflected its preferable CO₂RR activity.

The CO₂RR selectivity results (Fig. 2b-c and Figure S16a-c) also showed that the HER was effectively suppressed on OBC-OT. The OBC exhibited a high FE_H2 of about 40%, while the FE_H2 of OBC-OT can reach a small value of 12.6% at the testing potential (Figure S16a). Besides, the OBC-OT showed a higher C₂H₄ FE (~ 36.5%) and C₂₊ FE (~ 67.9%) than the OBC that exhibited the C₂H₄ FE and C₂₊ FE of ~ 17.0% and 33.8%, respectively (Figure S16b-c). The C₂₊, C₁, and H₂ selectivity of OBC and OBC-OT were arranged in Fig. 2d and it intuitively indicated the improvement of CO₂RR performance via the hydrophobic processing of OBC. Additionally, to ensure that the improved C₂₊ FE was not simply resulted from the decreased HER, the FE ratio of C₂₊/C₁ of OBC and OBC-OT was presented in Fig. 2d. The C₂₊/C₁ selectivity of OBC-OT was significantly elevated to 3.3 compared to the 1.4 of OBC, which meant that the C₂₊ selectivity was indeed improved even disregarding the HER factor.

However, it should be noted that the CO FE was significantly increased (reached ~ 21.3%) with the hydrophobic processing when the applied potential was not too negative (-1.2 V to -1.4 V). While, as the applied potential became more negative, the CO FE rapidly declined to ~ 4.5%. Similarly, the C₁ product of HCOOH showed the same variation trend as the applied potential changed. Therefore, combining the analysis of the opposite trend of C₂H₄ or C₂H₅OH in Fig. 2c, it can be concluded that the C₂₊ selectivity of OBC-OT under the low applied potential was not ideal enough (similar to the OBC) and the C₂₊ selectivity can be elevated with more negative potential and higher current density applied.

The long-term CO₂RR stability of OBC and OBC-OT regarding the C₂H₄ FE and C₂₊ FE retention over a 6-hour operation period in H-type cell were investigated (Figure S17). After the 6 h operation period, the C₂H₄ FE and C₂₊ FE of OBC-OT were stable and can be held at 34.2% and 61.2%, respectively. Furthermore, the SEM and EDS of OBC-OT with 10 h of CO₂RR test (Figure S10) also demonstrated that the absorbed hydrophobic layer was durable, which was consistent with the high CO₂RR stability of OBC-OT.

In addition, the influence of the 1-octadecanethiol adsorption quantity on OBC was investigated. We prepared the OBC-sOT which contained a slight content of 1-octadecanethiol molecules on the surface (Figure S18). Unlike the OBC-OT that possessed strong HER suppression capacity, OBC-sOT exhibited a higher H₂ evolution. However, the HER was still lower than the untreated OBC, which indicated the certain effect of 1-octadecanethiol. The LSV curves of OBC-sOT in CO₂-saturated and N₂-saturated 0.1 M KHCO₃ solution indicated the not strong enough HER suppression as well (Figure S18c). Besides, the CO FE on OBC-sOT was higher than that on OBC-OT, which demonstrated the preferable C₁ selectivity when the adsorption quantity of 1-octadecanethiol was not enough. The ECSA results of OBC-sOT showed that certain hydrophobic property was achieved and the C_dl was also significantly reduced with slight 1-octadecanethiol absorbed (Figure S18b).

The partial current density of C₂₊ products on OBC and OBC-OT was normalized³³ (Figure S16d) with the value of 0.067 mF cm^− 2 was used as the C_dl of polished Cu from the previous report⁸. The C_dl of OBC-OT was small and the electrocatalytic reaction was conducted on a small solid-liquid contact area. It emphasized the high local current density at the solid-liquid-gas interface, which may lead the microenvironment variation with such mass transfer occurred at the interface.

Surface properties and catalyst-electrolyte-gas triple-phase interfaces. To gain insight into the mechanism of CO₂RR performance improvement, we studied the surface properties and the difference between OBC and OBC-OT. The kelvin probe force microscope (KPFM) was applied to further explore the origin of CO2RR performance difference with the analysis of surface potential. Figure 3a-b displayed the KPFM images and the corresponding height and potential difference of OBC and OBC-OT. The OBC-OT showed a higher potential difference of − 34.2 mV (vs. silicon substrate) compared with − 6.2 mV (vs. silicon substrate) of OBC, which implied the lower local work function of OBC-OT. The lower local work function indicated the faster charge separation and transfer, thereby contributing to spontaneous polarization and enhanced energetics for CO₂RR,^{34, 35} which was consistent with the weak HER and strong CO₂RR of OBC-OT.

As displayed in Fig. 3c, the temperature-programmed desorption (TPD) of CO and H₂ was carried out to evaluate the CO and H₂ desorption on catalysts^{36, 37}. The similar H₂-TPD curves of OBC and OBC-OT implied the similar H₂ adsorption on the catalysts. However, comparing to the OBC, the OBC-OT presented a CO desorption peak with obviously lower temperature in the CO-TPD spectra, indicating that the CO was not easy to adsorbed on the OBC-OT surface³⁶. The weak adsorption of CO on OBC-OT was not beneficial to the CO coupling and high C₂₊ selectivity^38–40, which was inconsistent with the elevated C₂₊ selectivity of OBC-OT. The surface-active site variation caused by the 1-octadecanethiol adsorption was not the root cause of elevated C₂₊ selectivity. However, it was noteworthy that the TPD was not tested under operating conditions for CO₂RR as no additional potential was applied in the TPD measurement. Besides, the TPD result of OBC-OT was in accord with the high C₁ selectivity at less negative potential owing to the similar microenvironment when no potential or low potential was applied. The microenvironment could be changed with the different potential applied to the catalysts, which also affect the product's selectivity.

The catalyst-electrolyte-gas triple-phase interface of the OBC-OT, where the CO₂RR mainly occurred, was primarily investigated via the in-situ 3D Raman mapping technique. Firstly, the 3D Raman mapping (based on the peak intensity of surface CuO_x in 615 cm^− 1)⁴¹ of OBC-OT at air atmosphere (Fig. 3d and Figure S20a) showed the clear structure of the foam framework, indicating the feasibility of the measurement method for the materials. In addition, the corresponding 2D Raman mapping represented high consistency with the white light imaging as for the OBC-OT (Fig. 3g).

Figure 3e demonstrated the in-situ 3D Raman mappings (based on the peak intensity of CuO_x in 615 cm^− 1 and HCO₃⁻ in 1070 cm^− 1)⁴¹ of OBC-OT at -0.2 V vs. RHE. Combining with the analysis of the corresponding white light imaging and the sliced mapping along depth (Figure S20b), it can be easily concluded that the gas chamber and solid-liquid-gas triple-phase reaction interface caused by hydrophobicity were presented in the immersed OBC-OT. Additionally, the white liquid film formed on the surface of immersed OBC-OT owing to the high contact angle and the refractivity difference of the electrolyte and gas (Fig. 3h). Below the white liquid film, the obvious CuO_x peak was presented and there was no HCO₃⁻ peak being detected even though the frame of OBC-OT can be seen in the white light imaging, which further verified the existence of gas chamber and the catalyst-electrolyte-gas triple-phase interface.

Different from the OBC-OT, the peak of CuO_x vanished and only the HCO₃⁻ peak was shown in the in-situ 3D Raman mapping because of the O atoms detachment from OBC under the negative potential (Fig. 3f and Figure S20c). The corresponding outer surface white light imaging and 2D Raman mapping showed that there was no gas chamber in the immersed OBC (Fig. 3i). On the other hand, noteworthy that the peak intensity of HCO₃⁻ on the outer side was higher than that on the inner side of the foam, which can be attributed to the higher solution IR drop on the inner side.

Comparing the peak intensity of HCO₃⁻ absorbed on the surface of catalysts (Fig. 3h-i), it can be revealed that the peak intensity of HCO₃⁻ on OBC-OT was nearly ten times that on OBC, which meant that the large amount of HCO₃⁻ was gathered around the triple-phase interface of OBC-OT. The high local current density resulted in high local mass transfer, including the transfer of HCO₃⁻ and the CO₂RR reactants. Based on the data above, it can be speculated that the reaction microenvironment changed by the accumulation of local reactants and intermediates at the solid-liquid gas triple-phase interface with high negative potentials applied, led to the variation in the catalytic reaction pathway and the significant increase in C₂₊ selectivity.

In-situ Raman spectroscopy characterization. To elucidate the origin of the promoted C₂₊ selectivity on OBC-OT, we conducted an extensive set of investigations via the in-situ Raman spectra and mapping. The intermediates and adsorbates on the OBC-OT with an elevated potential applied were displayed in the in-situ Raman spectra for the triple-phase interface of the OBC-OT catalysts (Fig. 4a-b). When the weak negative potential was applied, the strong peak of HCO₃⁻ (1070 cm^− 1) appeared and the peak of CuO_x (1581 cm^− 1) was weakened due to the HCO₃⁻ adsorption and O detachment in the lattice under the negative potential⁴¹. Additionally, the COOH* peak of 1581 cm^− 1 was detected as the applied potential was not too negative, which was consistent with the result of CO-TPD and the preferable C₁ selectivity of OBC-OT at weak negative potential.⁴² Furthermore, the *CO peak of 2000–2100 cm^− 1 was also studied with the applied potential shifted negatively. The *CO peak is an important index to judge the C₂₊ selectivity of catalysts, as the CO₂RR to multi-carbon products undergo a critical CO dimerization step.⁴³ However, there was no obvious *CO peak until the applied potential was negatively shifted to -1.1 V vs. RHE, which was in accord with the C₂₊ FE variation trend with the potential change.

Besides the *CO peak, the peaks around 1296 cm^− 1 and 704 cm^− 1, which can be assigned to the adsorbed C-O bonds of CO₂ and the in-plane δCO₂⁻,^{41, 44} respectively, were also obviously raised with the potential applied from − 1.1 V vs. RHE to -1.8 V vs. RHE, indicating the improved activation of CO₂. In addition, owing to the high local current density and reaction, the peaks of C-C stretching (1103 cm^− 1), symmetrical CH₃ deformation (1132 cm^− 1), C-H vibration of hydrogenated intermediates after C-C coupling (1332 cm^− 1), and COO stretching vibration of CH₃COOH (1437 cm^− 1) from the CO₂RR intermediates and reactants were clearly presented, demonstrated the hydrocarbons were generated during the CO₂RR process.^{42, 45, 46} It was noteworthy that the HCO₃⁻ peak declined with the peak of C-C stretching and symmetrical CH₃ deformation raised, which implied the competition between HCO₃⁻ and multi-carbons on the surface of catalysts. The HCO₃⁻ was gradually replaced by the multi-carbon intermediates, reflecting the strong C₂₊ production accordingly. The bands related to symmetric -CH₂ (ν_sCH₂) and -CH₃ (ν_sCH₃) stretching gradually appeared around 2854 and 2930 cm^− 1 in Fig. 4b, which further indicated the hydrocarbons generation and consistency with the elevated C₂₊ selectivity in highly negative potential.⁴⁷

However, Fig. 4c showed the weak peaks of intermediates and adsorbates on the OBC and the *CO peak nearly vanished as the applied potential was negative than − 0.9 V vs. RHE, which was agreed to its low C₂₊ selectivity. The weak *CO and HCO₃⁻ signal can be detected with weak negative potential applied, but they disappeared because of the active site occupation from strong HER.

The in-situ 2D Raman mapping of OBC-OT and OBC (Fig. 4d-e) further verified that the reaction intermediates accumulated at the solid-liquid-gas triple-phase interface, and the Raman signal on OBC-OT was stronger than that on OBC. Under the weak negative potential, the intermediate of OBC-OT was mainly the *COOH with strong peak, while intermediates of OBC mainly consisted of CH₃COOH, *COOH and *CO with weak peaks. The intermediates are disorderly distributed on the immersed OBC through the 2D Raman mapping (based on the peak intensity of CH₃COOH, *COOH and *CO) in Fig. 4e. However, the 2D Raman mapping (based on the peak intensity of *COOH) in Fig. 4d showed the relatively clear shape of the immersed OBC-OT as the *COOH can not be detected in the area below the liquid film.

Combining the analysis of the in-situ Raman measurement and ECSA normalized partial current density of C₂₊ products, it can be confirmed that the high local current density and high mass transfer prevail at the triple-phase interface on OBC-OT. The in-situ Raman further verified that the changes from C₁ products favored to C₂₊ products favored with the applied potential negative shift, which was in accord with the CO₂RR performance in H-type cell. From another perspective, it meant that properly increasing the applied current density can further improve the C₂₊ selectivity for OBC-OT.

Microenvironment evolution. According to the previous report,⁴ the local pH, which impacted the C₂₊ selectivity on catalysts, was significantly affected by the current density. Therefore, it can be reasonably hypothesized that the improved C-C coupling and generation of C₂₊ products at the triple-phase interface originated from the large consumption of local protons and local pH increase. To further verify this supposition, we simulated local pH changes along the catalyst surface (plate electrode) at various applied current densities (Fig. 5a). The local pH was gradually elevated and reached pH13 with the applied current density was improved to 10 mA cm^− 2. Besides the local pH, the local reactant CO₂ concentration was also an important factor in the HER suppression and C₂₊ FE. The modeled local CO₂ concentration profiles in Fig. 5b showed that the local CO₂ concentration was nearly zero at the current density of 15 mA cm^− 2. Due to the limitation of CO₂ mass transfer in typical H-type cells, the excessive current density was not beneficial for CO₂RR performance, although the high local pH was produced. However, according to the previous report^{22, 48}, this kind of hydrophobic microscale and nanoscale surface can trap gas under hydrophobic hairs in water like a spider, which means that the CO₂ mass transfer of OBC-OT can be effectively improved based on its special hydrophobic structure. In addition, the injected CO₂ can be stored in the internal pore of the foam structure and diffuse to the triple-phase interface for the CO₂RR reaction. During the CO₂RR test, the CO₂ will be constantly supplied from both the electrolyte and gas chamber of OBC-OT, which is different from the individual CO₂ supply from electrolytes like OBC. Therefore, the special hydrophobic structure of OBC-OT can allow high local CO₂ concentration and high current density for high local pH.

On the other side, it should be emphasized that the practical local current density was influenced by the structure of catalysts, which differed from the current density modeled by the plate electrode or valued from the area of the macroscopic catalyst. Based on the above ECSA and in-situ Raman result, the obviously higher practical local current density was conducted on OBC-OT than that on OBC. Accordingly, the local pH was directly measured by surface-enhanced Raman spectroscopy (SERS) with pH-sensitive molecules (4-MBA) and further verified the local pH difference with the same applied current density of 5 mA cm^− 2 (Fig. 5c). The pH at 10 µm of distance to the catalyst surface for OBC-OT reached pH ~ 11, higher than the pH ~ 10 for OBC, which indicated the higher local pH induced by the higher practical local current density.

As shown in Fig. 5d, during the process of CO₂RR on OBC-OT, the reaction only occurred in the area that contacted the electrolyte (catalyst-electrolyte-gas triple-phase interface), and the inside pores of foam were filled with CO₂. The reaction microenvironment of CO₂RR evolved to the favored C₂₊ products and suppressed HER (Fig. 5e). The high local pH was generated with high practical local current density, and simultaneously the CO₂ mass transfer can be efficiently improved by CO₂ supply from both the electrolyte and gas chamber because of its special hydrophobic structure.

Application in flow cells. According to previous reports^49–53, the massive CO₂ assumption caused by the carbonate formation, and the liquid product crossover owing to the anion exchange membrane, reflects a major obstacle of the traditional alkaline or neutral pH electrolytes system for the economical CO₂RR catalysis. The acid electrolyte and accordingly used Nafion membrane can effectively address the challenges, however, are not beneficial for the multi-carbon production and HER suppression.^{4, 54, 55} Therefore, we artfully applied the OBC-OT in flow cells with the acid electrolyte for high C₂₊ selectivity and strong HER suppression, as high local pH can be achieved at the thee-phase interface of OBC-OT based on our investigation above (Figure S23a).

Based on the villous hydrophobic structure of porous foam, the OBC-OT was directly applied in the flow cells as the cathode without an additional gas-diffusion layer (GDL) as the brief flow-cell configuration showed in Fig. 6a. The acid electrolyte of 0.5 M K₂SO₄ (pH = 4) was employed according to the previous report.⁴ The gas diffusion scheme of the CO₂RR process on OBC-OT with the local pH variation was demonstrated in Fig. 6b. The CO₂ diffused through the porous foam frame and then further diffused across the villous nanowires on OBC-OT, participated in the reaction and led to local pH variation at the triple-phase interface.

The OBC-OT exhibited a high C₂₊ FE of 74.4% and low H₂ FE of 6.6% with the current density of 300 mA cm^− 2 (Fig. 6d and Figure S22), which further verified the proposed mechanism that the local pH can be elevated for improved C₂₊ selectivity and suppress HER even in the acid electrolyte. For comparison, the Cu foam adsorbed with 1-octadecanethiol (Cu-OT) was also investigated in the flow cell (Figure S23b-c). The strong and obvious cathode flooding suggested the necessity of the villous hydrophobic structure on OBC-OT. In addition, the H₂ FE and C₂₊ FE on OBC-OT with the current density of 300 mA cm^− 2 applied were relatively stable and can be held at 13.5% and 59.5% after the 10 h operation, respectively, which further demonstrated the application prospect of OBC-OT.

In summary, a hydrophobic oxide-derived copper foam with villous nanowires on the surface has been constructed, representing significant improvement on HER suppression and C₂₊ selectivity for the CO₂RR performance in H-type cell. The in-situ 3D Raman mapping was adopted to probe the catalyst-electrolyte-gas triple-phase interface and indicated the significant increase of reactants and local current density at the triple-phase interface. Besides, the in-situ Raman spectra further verified the variations from C₁ preferred to C₂₊ preferred with the applied potential negative shifted or current density increased, which was consistent with the CO₂RR performance and TPD results. Combining the measurements and simulations of micro-environmental species, high local pH and CO₂ mass transfer can be simultaneously allowed in the microenvironment of the triple-phase interface based on the special hydrophobic structure, being the internal mechanism of the high C₂₊ selectivity and HER suppression. Therefore, on this mechanism, both the low HER of 6.6% and high C₂₊ selectivity of 74.4% were achieved in the flow cell under acidic conditions at the current density of 300 mA cm^− 2 without an additional gas-diffusion layer (GDL). This work provides valuable theoretical insights for the design of catalysts and catalytic devices and opens up promising opportunities for the application of metal foam-based GDEs without fragility.

Chemicals

Copper foam (0.3 mm, > 99.99%), copper foil (0.3 mm, > 99.99%), sodium hydroxide (NaOH, Aladdin, ≥ 99%), ammonium persulfate ((NH₄)₂S₂O₈, Aladdin, ≥ 99%), 1-octadecanethiol (C₁₈H₃₈S, Macklin, ≥ 99%), potassium bicarbonate (KHCO₃, Macklin, ≥ 99.5%), ethanol (C₂H₅OH, Sinoreagent, AR), carbon dioxide (CO₂, 99.99%), argon (Ar, 99.999%), nitrogen (N₂, 99.999%), 4-mercaptobenzoic acid (4-MBA, Aladdin, ≥ 99%), silver nitrate (AgNO₃, Aladdin, ≥ 99.9%). chloroauric acid (HAuCl₄, Aladdin, ≥ 99.9%), sodium citrate (Aladdin, ≥ 99.9%), ascorbic acid (Aladdin, ≥ 99.9%).

Catalyst Preparation

Synthesis of OBC materials

The CuO nanowires were synthesized by annealing method from Cu(OH)₂ nanowires according to a previously reported procedure.⁵⁶ In a typical synthesis, the Cu foam (1 mm×0.5 mm) was cleaned by hydrochloric acid solution (1 M), acetone, ethanol and distilled water in sequence, then the cleaned Cu foam was immediately soaked into the mixture solution with 4.8g NaOH, 2.28g (NH₄)₂S₂O₈ and distilled water (100 mL) at 3℃ for 30 min. After that, the Cu foam turned to blue and a layer of Cu(OH)₂ nanowires was obtained on the surface after it was thoroughly rinsed with distilled water and dried in 60℃. The black CuO MPNWs were prepared after the Cu foam with a layer of Cu(OH)₂ nanowires was annealed in air at a preset temperature of 180°C for 1 h. The CuO MPNWs were used as the pre-catalysts and they were reduced to dark-red OBC materials in the 0.1M KHCO₃ aqueous solution with the potential of -1.0 V (vs. RHE) for 10 min before the CO₂ reduction test.

Synthesis of OBC-OT/OBC-sOT materials

The OBC-OT materials were synthesized by the functionalization of OBC materials with 1-octadecanethiol. The OBC materials were soaked in the Ar-saturated ethyl acetate solution with 10 wt% (0.01wt%) 1-octadecanethiol at room temperature for 30 min (2 min). The processed OBC materials were removed to ethyl acetate to clean the residual 1-octadecanethiol molecules which were not strongly absorbed on the OBC materials, and dried with nitrogen to obtain the OBC-OT (OBC-sOT) materials.

Materials Characterization

SEM and EDS measurements were performed on a scanning electron microscope (G500). The HRTEM data were obtained on a Field Emission Transmission Electron Microscope (Tecnai F30). The Raman spectroscopy was tested by a Laser confocal Raman Spectrometer (Renishaw). The X-ray photoelectron spectroscopy (XPS) was measured by the instrument ESCALAB and the X-ray diffraction (XRD) measurement was conducted by an X-ray diffractometer (SmartLab). A Kelvin probe force microscopy (KPFM, Bruker) was used to study the morphology and surface potential properties variation of the catalyst. The quantification of solution-phase and gas-phase products was carried out by nuclear magnetic resonance (NMR) spectroscopy (Figure S14) and gas chromatography (GC) (Figure S15), respectively.

In-situ Raman spectra tests

The in-situ Raman spectra were measured via a homemade electrolytic cell with CO₂-saturated 0.1 M KHCO3 aqueous solution flowing inside. This electrolytic cell was connected to the electrochemical station with a triple-electrode system, and the prepared electrode with catalysts, Ag/AgCl electrode, and Pt wire were used as work electrode, reference electrode, and counter electrode, respectively. The reduction process of the CuO MPNWs to OBC materials was monitored at the excitation laser source of 532 nm. The reaction intermediates of the CO₂RR process were monitored at the excitation laser source of 785 nm.⁵⁷ The measured potentials were converted to V versus RHE (E_RHE = E_Ag/AgCl + 0.197 V + 0.0591 V × pH) without the iR_s compensation.

Electrochemical measurements

Electrochemical properties and CO₂RR performance in H-type electrochemical cell were measured with three electrode system. The prepared electrode, Ag/AgCl electrode, and platinum nets electrode were used as the working, reference, and counter electrodes, respectively. The CO₂-saturated 0.1 M KHCO₃ aqueous solution (pH∼6.8) was used as the electrolyte and the cathode and anode chamber of the H-type electrochemical cell were separated by an anion exchange membrane (FAA-3-50). The measured potentials were converted to V versus RHE (E_RHE = E_Ag/AgCl + 0.197 V + 0.0591 V × pH ‒ iR_s).⁵⁸ The R_s was the resistance of the electrolyte solution between the working and reference electrode, which was measured by the impedance method.

Flow cell application test

The application of materials in the flow cell (Figure S23) was conducted with the acid electrolyte of 0.5 M K₂SO₄ (pH = 4) solution. Three pieces of OBC-OT were stacked to avoid the cathode flooding and used as the working electrode without any other gas diffusion attachment because of its hydrophobic and porous properties. The Ag/AgCl electrode and platinum nets electrode were used as the reference and counter electrodes, respectively. The cathode and anode chambers were separated with the Nafion membrane (N211) in between. The liquid flow rate in the cathode and anode chamber was 3 mL min^− 1 and 40 mL min^− 1, respectively. The CO₂ flow rate was set as 30 mL min^− 1.

ECSA measurements

The Electrochemical Active Surface area (ECSA) was evaluated by the double-layer capacitance (C_dl). The CV curves were taken over a range of scan rates (20, 40, 60, 80, and 100 mV/s), and the potential range was 0.15 V to 0.25 V vs. RHE.

GC analysis

The gas products were quantified by a gas chromatography (GC9790plus, FULI INSTRUMENTS) connecting to the cathode chamber of the electrochemical cell, which can automatically sample at the set time. The gas chromatography used Argon as the carrier gas, and it was calibrated by the standard curves obtained from a series of concentrations of the standard gas mixture. The CO₂ was sparged into the cathode chamber by a mass flow controller (Sevenstar) and the electrolyte was stirred (600 rpm). With the online measurement system, the gas products were typically sampled at 15, 35 and 55 min.

¹H NMR spectroscopy

The liquid components were analyzed by ¹H NMR spectroscopy undertaken with a Bruker Avance III 400 MHz spectrometer. During the NMR test, the mixture solution containing 500 µL electrolyte after the CO₂RR test, 100 µL D₂O, and 100 µL DMSO aqueous solution (10 ppm, volume/volume) was added to the NMR tube for quantification. A Pre-SAT180 water suppression method was used to diminish the effect of the excessively strong water peak of each spectrum. The standard curve of each liquid product was fitted to quantify the concentration of liquid production before the CO₂RR tests.

Faradaic Efficiency calculation: The Faradaic Efficiency of different gas components was calculated by the following method: FE_j = (nFV_gνp₀)/(RT₀I_total), where n is the electron transfer number from the CO₂ to its reduced gas product; F means the Faraday Constant (96485 C/mol); V_g is the flow rate of CO₂; ν is the gas component concentration quantified by the gas chromatography; R is the universal gas constant (8.314 J/mol·K); the p₀ and T₀ mean the pressure and temperature during the electrochemical catalytic reduction process; I_total is the averaged current over the whole test period. The Faradaic Efficiency of different liquid products was calculated by the following method: FE_j=nFVC/Q, where n means electron transfer number from the CO₂ to its reduced liquid product; F is the Faraday constant (96485 C/mol); V means the volume of the electrolyte for cathode; C is the liquid component concentration calculated from the standard curve; Q means the total passed charge (C) during the whole electrochemical catalytic test period.

In-situ local pH measurement

The in-situ local pH was tested via Raman signal of the pH-sensitive molecules (4-MBA) on the prepared SERS substrates (Figure S21a). The electrode was separated with the SERS substrate through a very thin insulating layer. As the potential was applied to the electrode, the local pH was varied and the peaks of 1372 cm^− 1 (νCOO⁻) and 1633 cm^− 1 (νC = O) were sensed (Figure S21b). Before the pH detection, we made the standard curve based on the peak area ratio of 1372 cm^− 1 and 1633 cm^− 1 (Figure S21c). The laser was 532 nm and the line image mode was used for local pH detection.

The pH-sensitive SERS substrates were prepared according to previous reports.^{59, 60} The 1.28 mL sodium citrate solution (1 wt%) was rapidly added to the boiling mixture of 31.6 mL distilled water and 390.4 µL HAuCl₄ (1 wt%), and then the Au seeds were obtained after the 30 min heating reflux. The Au@Ag soliquid was synthesized via the dropwise addition of the 2.5 mL AgNO₃ solution to the mixture of 32 mL distilled water, 2 mL sodium citrate (1 wt%), 630 µL ascorbic acid (0.1 M), and 930 µL Au seeds solution for 30 min. A piece of gold-plated silicon wafer was immerged in the 4-MBA solution (ethanol, 10 mM) for 24 h and then washed with massive ethanol to get the 4-MBA adsorbed gold-plated silicon wafer. Then the Au@Ag soliquid (concentrated 20 times) was dropped on the 4-MBA adsorbed gold-plated silicon wafer and dried for 12 h naturally. The obtained sample was immerged in the 4-MBA solution (ethanol, 10 mM) for 24 h, and then cleaned the dissociative 4-MBA with ethanol to gain the pH-sensitive SERS substrate.

COMSOL simulation

The local pH and CO₂ concentration profiles were simulated with COMSOL (COMSOL Multiphysics v5.6, Stockholm, Se) based on previous publications^{4, 24}. Two-dimensional (2D) domain (20 µm*100 µm) including the electrode surface in the left boundary (x = 0 µm) and the bulk solution in the right boundary (100 µm) was used for simulation. The zero flux of aqueous species was set in the top and bottom boundaries. The model considered the acid-base equilibria (equations 1–5) of CO₂ hydrolysis reactions:

where j meant the current density applied; F was the Faraday’s constant; \({F}_{{CO}_{2}RR}\) and \({F}_{HER}\) meant the Faradaic efficiencies of the CO₂RR and HER, respectively, which were set as 70% and 30%, respectively; \({n}_{{CO}_{2}RR}\) and \({n}_{HER}\) was the number of electrons required for the reduction reaction. The “eps” was added to the code to avoid zero or negative concentration. The model parameters are listed in Table S1.

Data Availability Statement

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

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (22172196, 21902188), the Guangxi Science and Technology Program (AD21220067), Natural Science Foundation of Guangxi Province (2022GXNSFAA035467).

Author contributions

Y. T. and K.W. supervised this this project. Z. X., J. Y and H. Yperformed most of the experiments and analyzed the experimental data. Q. W. and J. C helped with the physical characterization and data analysis. S. S., C. M., and K.W. helped with visualization. Z. X., J. Y and K.W. wrote and revised the manuscript. All authors discussed the results and assisted during manuscript preparation.

Competing Interest

The authors declare no competing financial interest.

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Microenvironment Evolution at Triple-phase Interface on the CO2RR Process of Hydrophobic Oxide-derived Copper

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