The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C₂ products in tandem electrocatalytic CO₂ reduction

Abstract

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO₂. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cu_cub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cu_cub/C, wherein the CoPc and Cu_cub share an interface, and CoPc-C/Cu_cub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cu_cub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C₂ products almost doubles for the CoPc-Cu_cub/C, whereas it decreases by half for the CoPc-C/Cu_cub, compared to the Cu_cub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C–C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

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Introduction

The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive route for CO₂ recycling, wherein value-added chemicals and fuels are produced from CO₂ while also storing electricity generated from renewable energy sources.^1,2 Among all catalysts, Cu-based materials provide suitable intermediate binding energies to drive CO₂RR towards highly reduced products (e.g. methane, ethylene, ethanol), which are of interest due to their high energy densities.^3,4 To date, tremendous effort has been dedicated to increasing the selectivity towards targeted products of the reaction, which produces over sixteen different products in various yields when performed over a polycrystalline Cu foil.⁵

Cu-based tandem schemes have emerged as a valid strategy to enhance the selectivity of CO₂RR towards multicarbon products (C₂₊) by decoupling the CO₂ to CO and the CO to C₂₊ reduction steps.^6–9 Previous work has demonstrated that increasing the local concentration of CO and/or the surface coverage of adsorbed CO (*CO) decreases the energetic barrier for C–C coupling, which is the rate-determining step towards C₂₊ products on Cu surfaces.^10–17 In addition to their intrinsic catalytic properties (i.e. turnover frequency, selectivity, overpotential), the relative spatial arrangement of the CO-producing component and of the Cu catalyst plays an important role in defining the efficiency of such tandem systems.^9,18–20 For example, researchers have synthesized reverse core–shell structures, with the CO-producing catalyst (Ag) in the core and the Cu as a shell, which was found to maximize CO utilization and enhance the C₂₊ activity compared to core–shell structures with Cu in the core.¹⁹ In an alternative approach, similar benefit was obtained by rationally segmenting the Ag and Cu components in gas-diffusion electrodes.²⁰ In both cases, the spatial distribution of the two catalysts impacts the modality of CO transport to the active sites, which can occur either via surface diffusion or via sequential adsorption following transport through the gas phase or the electrolyte. Thus, optimization of the tandem catalyst configuration emerges as an important parameter to modulate the CO utilization efficiency.

Most studies on the design of tandem systems have so far focused on the coupling of Cu with a second metallic domain (Zn,^21,22 Au²³ or Ag^24–27) as the CO-generating catalyst. Molecular catalysts, such as cobalt phthalocyanine²⁸ and iron porphyrin^29,30 have also been successfully employed as the CO-generating catalyst in the context of tandem schemes. Compared to metals, molecular catalysts offer additional versatility for varying the local CO concentration and production rate thanks to the superior chemical tunability via synthetic modifications of molecules versus materials.³⁰ Despite their promise, no experimental work has investigated yet the influence of the spatial arrangement of the components on the catalytic performance of molecular-Cu based tandem systems.

In this work, we investigate the impact of a direct interface between the molecular catalyst and the Cu electrode on the C–C coupling efficiency. We use colloidally dispersible Cu nanocubes (Cu_cub) enclosed by (100) facets as model catalysts. Their high surface to volume ratio, which is ideal for exploring surface phenomena, as well as their intrinsic selectivity towards ethylene production, justify this choice.^31–33 To work alongside Cu_cub, we select cobalt phthalocyanine (CoPc) as the CO-producing molecular catalyst. In addition to being highly active and stable when immobilized on conductive substrates for CO₂RR, the reduction potential window matches the applied potentials of Cu_cub in aqueous solutions.^34–37 Additionally, CoPc readily adsorbs on different surfaces, including carbon and metallic Cu, via electrostatic interactions, thanks to the Pc planar structure.^38–40 Finally, both catalysts (i.e. the CoPc and the Cu_cub) can be produced in the form of inks, which facilitates the manipulation of their surface interactions. We exploit these intrinsic characteristics of the Cu_Cub and of the CoPc to create tandem catalysts comprising CoPc-Cu_cub/C, where the CoPc shares a direct interface with the Cu_cub, and CoPc-C/Cu_cub, where the CoPc is first adsorbed on carbon black and then mixed with the Cu_cub. We find that the CoPc-Cu_cub/C catalyst exhibits a faradaic efficiency (FE) and partial current density for C₂ products which are double the values measured for pristine Cu_cub/C without the molecular component present. In contrast, the FE for C₂ products decrease by half for the CoPc-C/Cu_cub compared to the Cu_cub/C. These results demonstrate that CO utilization is much more efficient in the CoPc-Cu_cub/C, where a close contact between the two components of the tandem catalyst exists. Thus, we propose that surface transport, which requires a direct interface, dominates over electrolyte-mediated diffusion-readsorption and is the favorable pathway to maximize the CO utilization efficiency and C₂ product formation in these molecular-based tandem catalysts.

Results and discussion

The CoPc-Cu_cub/C catalyst was obtained by first mixing the CoPc molecules with the Cu_cub in dimethylformamide (DMF), followed by the addition of carbon black into CoPc-Cu_cub. Prior to mixing, the native ligands of the Cu_cub were removed via solvent washing, which allowed them to become dispersible in DMF. The molecular catalyst was added in excess to the washed Cu_cub in order to maximize the Cu surface coverage (see Experimental section in ESI† for details). The amount of the adsorbed molecular catalyst (∼1 nmol per cm² per working electrode) was quantified by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Table S1†). The CoPc-C/Cu_cub catalyst was prepared by adsorbing the same total amount of CoPc on the carbon black prior to mixing with the Cu_cub. For reference, carbon black was also added to the individual Cu_cub and CoPc systems (Cu_cub/C and CoPc/C) to enable a direct comparison among each of the different systems.

Fig. 1 reports the transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) characterization of the as-obtained catalysts. TEM reveals the intact cubic shape of the Cu_cub in both CoPc-Cu_cub/C (Fig. 1A) and CoPc-C/Cu_cub (Fig. 1B), which proves that the synthesis procedure does not impact the Cu_cub morphology. The low contrast material in both images is the carbon black. The FTIR data presented in Fig. 1C show that, first of all, the washed Cu_cub (Cu_cub–ligand stripped) has ligand-free surfaces. Indeed, the intensity of the –CH– stretching at 2915 cm⁻¹ is negligible compared to the as-synthesized sample and the P=O vibration is completely absent. Both vibrational modes are representative of the native trioctylphosphine oxide ligands of the Cu_cub (area shaded in blue), confirming their removal from the Cu nanoparticles. The characteristic peaks of CoPc (area shaded in yellow: with Pc ring at 753 cm⁻¹, pyrrole C–N asymetric stretch at approximately 1086 cm⁻¹, and the isoindole and pyrrole stretch at 1200–1500 cm⁻¹) are present in the CoPc-Cu_cub sample even after thoroughly washing, proving that the CoPc is strongly adsorbed on the Cu_cub surface, likely due to the expected electrostatic interactions. Similarly, the characteristic IR peaks of CoPc are present for the CoPc-C/Cu_cub catalyst. The background signal in the region from 700 cm⁻¹ to 2000 cm⁻¹, is attributed to the carbon black (Fig. S1†).

Fig. 1 (A and B) Bright field TEM images of CoPc-Cu_cub/C (A) and CoPc-C/Cu_cub (B), confirming that the structure of Cu_cub is retained in both cases following catalyst preparation. The synthesized tandem catalyst structures are schematically indicated on the left. (C) FTIR spectra of TOPO ligand, the as-synthesized Cu_cub, ligand-stripped Cu_cub, CoPc-Cu_cub, CoPc-C/Cu_cub and CoPc. These results demonstrate the successful removal of TOPO ligand from Cu_cub and the retention of CoPc within both catalyst assemblies.

We further characterized the catalysts by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy to learn more about the spatial distributions of the different components (Fig. 2). For the case of the CoPc-Cu_cub catalyst, the Cu and Co signals are co-localized on the Cu_cub and no free CoPc molecules are present on the grid (Fig. 2A and B). By contrast, for the case of CoPc-C/Cu_cub catalyst, the CoPc is present only on the carbon black and not on the Cu_cub (Fig. 2C and D). These results confirm the successful assembly of two different catalyst motifs according to the designed structures depicted by the schematic illustrations in Fig. 1A.

Fig. 2 (A and C) HAADF-STEM images and corresponding EDX elemental maps, along with (B and D) EDX spectra for CoPc-Cu_cub/C (A and B) and CoPc-C/Cu_cub (C and D). The EDX maps show the spatial distributions of Cu (red) and Co (blue). The carbon is the low contrast material in (C) and is not colored for the sake of clarity. The EDX spectra report the intensity of the Co signals in two different areas of the map, which correspond to the Cu_cub (area 1) and to the carbon (area 2), as indicated on the colored maps.

Having completed the characterization of the two catalysts, we evaluated the behavior of CoPc-Cu_cub/C and CoPc-C/Cu_cub towards CO₂RR in a H-cell system, using CO₂-saturated 0.1 M aqueous KHCO₃ as the supporting electrolyte (Fig. 3). We chose an electrochemical potential of −1.0 V vs. RHE (reversible hydrogen electrode) for initial comparison based on the previous knowledge that Cu_cub exhibits a significant selectivity for C₂H₄ and that CoPc possesses maximum activity for CO near this working potential. For reference, the product distributions for the tandem catalyst assemblies are also compared to those possessing only the individual components, Cu_cub/C and CoPc/C. All reported current densities are normalized by the electrochemically active surface area (ECSA) (Fig. S2†).

Fig. 3 (A) FEs for all gaseous products (i.e., H₂, CO, CH₄, C₂H₄) and the main liquid products (i.e., formate, C₂H₅OH), (B) FE_C2 and corresponding partial current density j_C2 normalized by the ECSA for the Cu_cub/C, CoPc-Cu_cub/C, CoPc-C/Cu_cub and CoPc/C. The loading of Cu_cub was 20 μg cm⁻² and the loading of CoPc anchored on the Cu_cub and on the carbon black was 1.1 nmol cm⁻² and 0.9 nmol cm⁻², respectively. These data were collected in an H-cell configuration using 0.1 M KHCO₃ as the electrolyte with the working electrode poised at −1.0 V vs. RHE for 1 h. The reported values are an average of three independent experiments with error bars indicating the standard deviations. Compared to the reference Cu_cub/C, a significant increase of FE_C2 is observed for CoPc-Cu_cub/C, while a decrease is observed for CoPc-C/Cu_cub.

The product distribution analysis (Fig. 3A), which is represented by the FE values, reveals that the major products observed for Cu_cub/C are still present for the CoPc-Cu_cub/C, but with a clear increase of the FE for C₂ products (i.e. ethylene + ethanol). However, for the CoPc-C/Cu_cub, CO production notably increases while the other products are suppressed. Importantly, FE_H2 is suppressed for both CoPc-Cu_cub/C and CoPc-C/Cu_cub compared to the Cu_cub/C. As expected, the major product for CoPc/C is CO, with minor production of H₂, indicating that the activity of the heterogenized molecular component is preserved.

To facilitate the comparison of C₂ products, Fig. 3B reports the FE_C2 and the corresponding partial current densities (j_C2) of all Cu_cub-based systems. The CoPc-Cu_cub/C clearly shows an enhancement of C₂ products with the highest FE of 39.4% and j_C2 of 1.07 mA cm⁻². In contrast, the FE_C2 for CoPc-C/Cu_cub is only 10%, much lower than that of the Cu_cub/C, which is 18.3%. The j_C2 are instead comparable and equal to 0.2 mA cm⁻² for CoPc-C/Cu_cub and 0.16 mA cm⁻² for the Cu_cub.

The high FE_CO and the suppressed FE_C2 for CoPc-C/Cu_cub suggests that CO released from the molecule diffuses away rather than being utilized by the Cu_cub. Longer reaction times up to 10 hours do eventually result in a slight increase of FE_C2 and j_C2 for CoPc-C/Cu_cub (Fig. S3†), which might result from the overall increasing concentration of CO in the electrolyte. However, the performance of CoPc-C/Cu_cub remain always lower than CoPc-Cu_cub/C and Cu_cub/C. Decreasing the loading of the CoPc does decrease the FE_CO but does not improve the FE_C2 compared to CoPc-Cu_cub/C with same CoPc loading and to Cu_cub/C (Fig. S4†), which indicates that the CO transfer from CoPc-C to Cu_cub does not efficiently occur in these catalysts.

To further investigate the mechanism behind the observed catalytic activity differences, we investigated the CoPc-Cu_cub/C and CoPc-C/Cu_cub CO₂RR performance over a wide range of applied potentials and compared them to those of Cu_cub/C and CoPc/C (Fig. 4). We note that the CoPc stability decreases and the selectivity for methane increases at more negative voltages (Fig. S5†). Consistent with the data in Fig. 3, CoPc-Cu_cub/C exhibits a similar potential-dependent product distribution compared to Cu_cub/C, but with an increased C₂ product yield and suppressed hydrogen generation across the entire potential range (Fig. 4A). In particular, the FE_C2 and the corresponding partial current density of the CoPc-Cu_cub/C are consistently higher than the values measured for the Cu_cub/C and increase at more cathodic potentials (Fig. 4B). The best-performing CoPc-Cu_cub/C possess a FE_C2 of 48% with a partial current density of 1.5 mA cm⁻² at −1.05 V vs. RHE, which is 1.7 times the FE_C2 of the Cu_cub/C. In contrast, for the CoPc-C/Cu_cub assembly, CO is the major product and the FE_C2 is reduced over the whole potential range compared to Cu_cub/C (Fig. 4D). Furthermore, the partial current density of the C₂ products in CoPc-C/Cu_cub and Cu_cub/C is always similar, which indicates that the Cu_cub intrinsic selectivity is unaffected by the CoPc molecules (Fig. 4D). We also note that the total current densities of the tandem catalysts are higher compared to those of either the Cu_cub or the CoPc alone, which indicates that both components are active (Fig. S8†). Furthermore, the overall C₂ production rate is higher in the tandem catalysts compared to the Cu_cub alone (Fig. S8†), which is similar to what has been reported for some of the metallic/Cu tandem catalysts.^41,42 We highlight that morphology and composition of all catalysts were maintained after CO₂RR at −1.05 V vs. RHE for 1 hour (Fig. S6†).

Fig. 4 (A and D) FEs for all gaseous products (i.e., H₂, CO, CH₄, C₂H₄) and the main liquid products (i.e., formate, C₂H₅OH), (B and E) FEs and partial current densities of C₂ products (i.e. C₂H₄ + C₂H₅OH), j_C2, normalized by the ECSA and (C and F) FEs for CO as a function of potential for Cu_cub/C, CoPc-Cu_cub/C, CoPc-C/Cu_cub and CoPc/C. The loading of Cu_cub was 20 μg cm⁻² and the loading of CoPc anchored on the Cu_cub and on the carbon black was 1.1 nmol cm⁻² and 0.9 nmol cm⁻², respectively. These data were collected in a H-cell system using 0.1 M KHCO₃ as the electrolyte for 1 h. The reported values are an average of three independent experiments with error bars indicating the standard deviations.

To understand whether the C–C coupling enhancement in the CoPc-Cu_cub/C catalyst is consistent with a tandem mechanism, we plotted the FE_CO of the different systems at various electrochemical potentials (Fig. 4C and F). The FE_CO for the CoPc-Cu_cub/C decreases as the potential becomes more cathodic (Fig. 4C), concomitant with the C₂ production increase. This correlation is consistent with a tandem mechanism in which the CO formed on CoPc is consumed by Cu_cub. By contrast, the FE_CO of the CoPc-C/Cu_cub catalyst remains approximately constant and is similar to that of CoPc/C across the entire potential range (Fig. 4F), proving that the CO formed on CoPc is not consumed by Cu_cub and the sequential tandem reaction does not occur.

To further validate the conclusion that the obtained results are due to a tandem effect, we performed control experiments to investigate whether other factors could be involved. XPS revealed that the electronic properties of the catalysts are not impacted by interactions between the different components and are, therefore, not responsible for the observed catalytic differences (Fig. S8 and Table S2†). In addition, we verified that neither the phthalocyanine ligand nor the Co alone on the Cu surface can provide the enhanced C–C coupling observed for CoPc-Cu_cub/C (Fig. S9–S11†). Finally, no major changes in hydrophobicity, which could justify hydrogen suppression, were observed upon adsorption of the CoPc on the Cu_cub (Fig. S12†). Overall, the tandem effect emerges as the main reason underlying the C–C coupling enhancement observed in the CoPc-Cu_cub/C catalysts. Finally, encouraged by the results measured in a H-cell configuration, we included the spatial configurated systems into a gas-fed flow cell, comprising the CoPc-Cu_cub/C and the CoPc-C/Cu_cub supported on a gas diffusion electrode as the cathode, in order to realize CO₂ to C₂ conversion at high current density (Fig. S13†). In agreement with the data discussed above, the CoPc-Cu_cub/C catalyst demonstrated enhanced C₂ production compared to the Cu_cub/C reaching a FE for C₂ of 62% with a partial current density of 125 mA cm⁻² in 1 M KHCO₃.

Fig. 5 summarizes the results of this study and the proposed mechanism responsible for the observed electrocatalytic activity of molecule-copper tandem catalysts. We find that the close vicinity and direct interface between the CO-generating molecules and the Cu surface is essential to promote C–C coupling via the tandem effect. When the molecular component is spatially separated from the copper, the sequential tandem process does not occur and the overall multicarbon production is suppressed. These results suggest that the tandem mechanism in these catalysts is enabled by surface diffusion of the CO rather than CO diffusion in the electrolyte and sequential reabsorption.

Fig. 5 Schematic illustration summarizing the effect of the spatial distribution of CO-producing molecular catalysts and Cu catalysts in tandem systems. The close spatial coupling and direct interface between the two components is crucial for enhancing C–C coupling via enhanced CO generation in the vicinity of Cu.

Conclusions

In summary, we have demonstrated that tandem catalysts must be carefully engineered to promote efficient C–C coupling. In particular, our results reveal that a direct interface between the CO-generating molecular catalyst and the Cu is a key factor for efficient CO utilization by the copper. Indeed, we have reported a CO₂-to-C₂ conversion with FE of 48% at −1.05 V vs. RHE for CoPc-Cu_cub/C catalysts, which is 1.7 times higher than that of the Cu_cub/C when tested under the same conditions. In contrast, the CoPc-C/Cu_cub assembly, in which the Cu_cub and the CoPc were physically separated, exhibited a dramatically reduced FE_C2 compared to the Cu_cub. Overall, these findings advance our fundamental understanding of the performance-driving parameters necessary for constructing more efficient hybrid molecule/nanocrystal tandem catalysts in the future. For example, the discovery that a close spatial coupling is crucial for efficient tandem catalysts suggests that their integration into gas diffusion electrodes operating at high current density may benefit from functional groups to anchor the molecular catalysts on the surface of the nanocatalysts.

Data availability

Data for this paper, including TEM images, FT-IR, HAADF-STEM images and corresponding EDX and electrochemical characterization, are available at Zenodo at: https://doi.org/10.5281/zenodo.7327628.

Author contributions

M. W. carried out the synthesis of the tandem catalysts and electrocatalytic testing for CO₂RR. A. L. performed X-ray photoelectron spectroscopy, HAADF-STEM images and corresponding EDX and contributed to daily discussions. V. O. and I. D. S. contributed to discussions. R. B. supervised this work and coordinated the project. All the authors contributed to the writing of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the support from EU-funded LICROX FET project (Grant agreement 951843).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2sc06359j

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