Tandem electrocatalytic CO₂ reduction with Fe-porphyrins and Cu nanocubes enhances ethylene production

Abstract

Copper-based tandem schemes have emerged as promising strategies to promote the formation of multi-carbon products in the electrocatalytic CO₂ reduction reaction. In such approaches, the CO-generating component of the tandem catalyst increases the local concentration of CO and thereby enhances the intrinsic carbon–carbon (C–C) coupling on copper. However, the optimal characteristics of the CO-generating catalyst for maximizing the C₂ production are currently unknown. In this work, we developed tunable tandem catalysts comprising iron porphyrin (Fe-Por), as the CO-generating component, and Cu nanocubes (Cucub) to understand how the turnover frequency for CO (TOF_CO) of the molecular catalysts impacts the C–C coupling on the Cu surface. First, we tuned the TOF_CO of the Fe-Por by varying the number of orbitals involved in the π-system. Then, we coupled these molecular catalysts with the Cu_cub and assessed the current densities and faradaic efficiencies. We discovered that all of the designed Fe-Por boost ethylene production. The most efficient Cu_cub/Fe-Por tandem catalyst was the one including the Fe-Por with the highest TOF_CO and exhibited a nearly 22-fold increase in the ethylene selectivity and 100 mV positive shift of the onset potential with respect to the pristine Cu_cub. These results reveal that coupling the TOF_CO tunability of molecular catalysts with copper nanocatalysts opens up new possibilities towards the development of Cu-based catalysts with enhanced selectivity for multi-carbon product generation at low overpotential.

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Introduction

The electrochemical CO₂ reduction reaction (CO₂RR) offers the opportunity to convert CO₂ into fuels and chemical feedstocks, while mitigating anthropogenic CO₂ emissions and storing renewable energy.^1,2 Multi-carbon (C₂₊) compounds (e.g., ethylene, ethanol, propanol) are among the most attractive CO₂RR products owing to their commercial value and high energy densities.^1,2 However, optimizing the activity of CO₂RR electrocatalysts towards one particular product remains a crucial challenge that is not trivial to address because multiple intermediates are involved in the CO₂ to C₂₊ conversion pathway.^3–5

Cu-based materials have been widely employed as CO₂RR catalysts due to their unique ability to facilitate the electroreduction of CO₂ beyond two electron reduction products.^5,6 Numerous studies on catalyst development and mechanistic investigations have demonstrated that the surface coverage of the adsorbed CO (indicated as *CO) regulates the activation barrier of the C–C coupling step and, thus, the formation of C₂₊ products.^7–9 An attractive approach to realize this high *CO surface coverage is via tandem catalysis, wherein Cu is coupled with a CO-producing component to realize the sequential CO₂-to-CO and CO-to-C₂₊ product conversion on complementary active sites that are coupled at the nanometer scale.^10–21 However, the correlation between the intrinsic catalytic properties of the CO-generating component (e.g. turnover frequency, TOF, and overpotential) and its impact on the outcome of tandem catalysis (e.g. C–C coupling efficiency) is still critically missing. Thus, the fundamental design principles required to rationally promote C₂₊ production while decreasing the overpotential in tandem catalysis remain to be elucidated.

Bimetallic systems comprising Cu and another metal (Au, Ag, or Zn) as the CO-producing component have been extensively investigated as electrocatalysts in tandem CO₂ reduction.^15–21 One drawback of these catalysts is that composition (e.g. alloying) and morphology can change during operation.^15–18 These changes affect both the physical and electronic structure, thus complicating the interpretation of the catalytic behavior and delaying the establishment of design principles required to further improve the catalytic activity of these systems.^18–22 Additionally, tuning the intrinsic activity and efficiency of metals is difficult and requires their coupling with organic modifiers.^23,24 Consequently, these systems are not ideal for guiding the understanding of the performance-determining factors in tandem catalysis.

Recent studies have suggested that CO-producing molecular catalysts are alternative candidates to metals in tandem schemes.^25–27 Molecular components offer additional versatility compared to metals because their electronic and structural features can be tuned via synthetic modifications.^28–31 While the examples are still limited, studies in the literature on tandem catalysts including CO-producing molecular catalysts and Cu are encouraging.^25–27 For example, the addition of either an iron porphyrin or a cobalt phthalocyanine to sputtered Cu resulted in enhanced C₂₊ product yields.^25,26 Specifically, the coupling of iron porphyrin with sputtered Cu resulted in increased faradaic efficiency for ethanol from 29% to 41% and in a shift the detection potential of this product from −0.84 to −0.82 V vs. RHE.²⁵ The cobalt phthalocyanine/Cu catalyst enhanced the faradaic efficiency for C₂₊ products from 45% to 82% in a highly basic electrolyte.²⁶ Instead, the combination of a cobalt phthalocyanine with a Zn–N–C catalyst promoted formation of methane with a total faradaic efficiency of 18%, a product not observed on the pristine Zn–N–C, again in a highly basic electrolyte.²⁷ However, these studies have so far focused only on the integration of commercial molecular catalysts in tandem CO₂RR, thus not profiting from the chemical tunability of these systems. Furthermore, they remain a few isolated examples each utilizing a different electrocatalyst and with testing performed under different conditions, which make any comparison among them difficult to make.

Herein, we exploit the intrinsic tunability of molecular catalysts (in terms of TOF_CO and overpotential) to understand the impact of the CO-generating component on subsequent C–C coupling on Cu surfaces during tandem catalysis. Iron porphyrin (F-Por) molecular catalysts are selected as the CO-producing entity because of their high efficiency for CO₂ to CO conversion at neutral pH when immobilized on conductive substrates.^32–34 Furthermore, their versatile chemistry enables facile tuning of their redox behaviors via modification of the π-system.^35–37 To this end, we synthesized three Fe-Por with different numbers of orbitals involved in the π-system. Colloidally dispersible Cu nanocubes (Cu_cub) were chosen as Cu catalysts. Their intrinsic selectivity towards C–C coupling, and ethylene production in particular, as well as the demonstrated possibility to integrate them in different electrochemical cell designs, justifies this choice.^38–41 Furthermore, the fact that both Fe-Por and Cu_cub catalysts can be employed as inks is beneficial for catalyst processability, scalability, and device integration. By studying the designed series of Fe-Por molecules in tandem CO₂RR, we investigate how their TOF_CO impacts the C–C coupling on Cu_cub. Importantly, we discover that the most active Fe-Por generates an optimal local environment that boosts the intrinsic activity of Cu_cub towards ethylene while also increasing energy efficiency by anodically shifting the potential at which this product is detected.

Results and discussion

A series of Fe-Por complexes with a varying number of orbitals involved in the π-system of the porphyrin ligand were synthesized and tested as molecular catalysts for CO₂RR (Fig. 1). These complexes are iron(III)-chlorido-tetrabicyclo[2.2.2]octadiene-tetraphenyl porphyrin (Fe-TbcTPP), iron(III)-chlorido-tetraphenyl porphyrin (Fe-TPP), and an iron(III)-chlorido-tetrabenzo-tetraphenyl porphyrin (Fe-TBTPP). The synthesis of Fe-TPP was carried out following a procedure reported in the literature,⁴² whereas Fe-TbcTPP and Fe-TBTPP were prepared according to the synthetic approach described in detail in the ESI and illustrated in Scheme S1†. In brief, the initial step corresponds to the synthesis of the free base porphyrin H₂TbcTPPvia a “Lindsey condensation reaction”.⁴³ Refluxing H₂TbcTPP in a tetrahydrofuran (THF) solution containing FeBr₂ and 2,6-lutidine generates the desired Fe-TbcTPP. The last step is a retro-Diels–Alder reaction achieved by heating the Fe-TbcTPP complex at 200 °C under vacuum to produce Fe-TBTPP. The ¹H and ¹³C-NMR spectra of H₂TbcTPP are presented in Fig. S1 and S2,† while the MALDI-TOF spectra of the iron-derivatives (Fe-TbcTPP and Fe-TBTPP) are provided in Fig. S3 and S4.† The aromatic delocalization effect in Fe-Por is revealed by optical absorption spectroscopy (Fig. 1 and Table S1†). According to expectation, as the number of π-orbitals contributing to the system increases, the respective Soret band red-shifts, following the peak wavelength (λ_max) order λ_max (Fe-TBTPP) > λ_max (Fe-TPP) > λ_max (Fe-TbcTPP).

Fig. 1 The Fe-Por molecules synthesized and investigated as molecular catalysts in this study (top) along with the UV-vis spectra of the same in THF (bottom).

To understand the impact of the different porphyrin ligands on the electrocatalytic properties, we performed cyclic voltammetry (CV) experiments in homogeneous conditions (Fig. 2 and ESI† for details). A very similar behavior was observed for all Fe-Por, with each characterized by three redox processes that are assigned to Fe^III/Fe^II, Fe^II/Fe^I, and Fe^I/Fe⁰, as depicted in Fig. 2 and Table S1.† The second and third reduction waves are associated with two consecutive one electron reductions that are mainly ligand-based.^44,45 As expected, the different degree of π-delocalization of the Fe-Por results in a significant shift of the respective redox potentials. More specifically, the Fe-TbcTPP containing the more σ-donating and less π-accepting bicyclo[2.2.2]octadiene groups has the lowest potentials. On the other hand, the Fe-TBTPP containing benzo groups and phenyl substitution at the meso positions has more anodic values. The Fe-TPP with no additional functionalization onto the pyrrole moieties has redox potentials between those of Fe-TbcTPP and Fe-TBTPP.

Fig. 2 Cyclic voltammetry of 0.25 mM Fe-Por dissolved in DMF containing 0.1 M TBAPF₆ as the supporting electrolyte, carried out at a scan rate of 100 mV s⁻¹. Fe-TbcTPP (red), Fe-TPP (gray), and Fe-TBTPP (blue).

We then investigated the CO₂RR catalytic properties of the Fe-Por in an H-cell using CO₂ saturated 0.1 M KHCO₃ as the electrolyte (see ESI† for details). Results of these experiments are presented in Fig. 3. Linear sweep voltammetry (LSV) (Fig. 3a) reveals an anodic shift of the onset potential (i.e. the potential at which the current starts to be detectable from the background) for CO₂ reduction from −0.57 V (Fe-TbcTPP) to −0.50 V (Fe-TPP) and to −0.40 V (Fe-TBTPP) vs. RHE (Reversible Hydrogen Electrode). This anodic shift follows the π-delocalization trend of the Fe-Por reported above.^46–48 Furthermore, the faradaic efficiency for CO (FE_CO) is higher for Fe-TBTPP than for either Fe-TPP or Fe-TbcTPP across the entire potential range (Fig. 3b and S5†), with a maximum of 91% at −0.55 V vs. RHE. The TOF_CO (see ESI† for details) follows the same trend, with TOF_CO (Fe-TBTPP) > TOF_CO (Fe-TPP) > TOF_CO (Fe-TbcTPP) (Fig. 3c). Specifically, Fe-TBTPP exhibits the highest TOF_CO (0.26 s⁻¹) compared to Fe-TPP (0.11 s⁻¹) and Fe-TbcTPP (0.02 s⁻¹) at −0.95 V vs. RHE.

Fig. 3 (a) LSV curves of the Fe-Por catalysts measured at a scan rate of 10 mV s⁻¹, with the vertical dashed lines representing the onset potentials for each catalyst; (b) FE_CO and (c) TOF_CO as a function of applied potential. These measurements were performed at Fe-Por loadings of 16 nmol cm⁻² on carbon nanotubes deposited onto glassy carbon electrodes (S = 1.33 cm²) using an H-cell with CO₂ saturated 0.1 M KHCO₃ electrolyte. The carbon nanotubes were chosen as commonly used as a conductive support to homogeneously disperse the molecular catalysts via non-covalent interactions.^32–34 The reported FE and TOF values are the average of three independent experiments with error bars indicating the standard deviations.

Following the analysis of the Fe-Por, we prepared the Cu_cub/Fe-Por by mixing the Fe-Por and the Cu_cub (Fig. S6†) (see ESI† for details). The inks were then drop cast onto glassy carbon electrodes and the activities of the tandem catalysts were tested from −0.45 V to −1.05 V vs. RHE. Fig. 4 and S7† provide an overview of the obtained data. At low potentials (−0.45 V and −0.55 V vs. RHE), the addition of the Fe-Por to Cu_cub suppresses the hydrogen evolution reaction (HER) and enhances the production of CO and formate. However, no evidence of C–C coupling reactions is observed at these potentials (Fig. S7†). The HER suppression and CO₂RR promotion induced by the Fe-Por become more pronounced at more negative potentials (−0.65 V to −1.05 V vs. RHE, Fig. 4a). Indeed, the enhancement of C₂H₄ faradaic efficiency from Cu_cub/Fe-Por relative to Cu_cub is clearly observed for all tandem-catalysts (Fig. 4b). Interestingly, C₂H₄ is produced at a potential as low as −0.65 V vs. RHE for the Cu_cub/Fe-TBTPP, which maintains the highest FE_C2H4 across the entire potential range (Fig. 4b), reaching a FE_C2H4 of 36% at −1.05 V vs. RHE. By comparison, the Cu_cub in the absence of Fe-Por only achieved a FE_C2H4 of 19% at the same potential. At more negative potential, CH₄ is favored over C₂H₄ (Fig. S8†). Thus, while Fe-TBTPP still promotes C–C coupling, the FE_C2H4 decreases (∼25% at −1.15 V). The FE_C2H4 was much lower also for Cu_sphere/Fe-TBTPP (∼15% at −1.05 V), which highlights the importance of the Cu morphology to maximize ethylene production (Fig. S9†).

Fig. 4 (a) Total FEs and (b) FE_C2H4 for Cu_cub/Fe-TBTPP, Cu_cub/Fe-TPP, Cu_cub/Fe-TbcTPP and Cu_cub as a function of potential; (c) C₂H₄ product enhancement factor of the Cu_cub/Fe-Por compared to the pristine Cu_cub (enhancement factor = (FE_C2H4 of Cu_cub/Fe-Por)/(FE_C2H4 of Cu_cub) as a function of potential; (d) j_C2H4/ECSA for Cu_cub/Fe-TBTPP, Cu_cub/Fe-TPP, Cu_cub/Fe-TbcTPP and Cu_cub as a function of potential. The electrodes for these experiments were prepared by loading 3.12 × 10² nmol cm⁻² of Cu_cub and 8 nmol cm⁻² of the Fe-Por onto glassy carbon electrodes. The reported values are an average of three independent experiments with error bars indicating the standard deviations.

To quantify the impact of the Fe-Por on C–C coupling, we define an enhancement factor for C₂H₄ production as the ratio of FE_C2H4 of Cu_cub/Fe-Por to the FE_C2H4 of Cu_cub at a given potential. The corresponding enhancement factors for Cu/Fe-TBTPP, Cu/Fe-TPP and Cu/Fe-TbcPP are 21.5, 9.5 and 3.2 at −0.75 V vs. RHE, respectively (Fig. 4c). Analysis of the electrochemically active surface area (ECSA) of Cu_cub/Fe-Por reveals a decreased solid/liquid contact area compared to the Cu_cub, which is likely a consequence of the blocking of Cu active sites by the molecules (Fig. S10†). However, the ECSA-normalized current density (j_C2H4/ECSA) indicates an increase in the intrinsic activity of the unpassivated sites in the presence of the Fe-Por, with a trend that is consistent with the FE and the enhancement factor (Fig. 4d).

To exclude the contribution of C₁ product suppression on the observed trends and to provide further evidence for the tandem mechanism, we analyzed the C₁ products in greater detail (Fig. 5). The FE_C1 and the j_C1,ECSA of the Cu_cub/Fe-Por catalysts are generally higher than those for the pristine Cu_cub across the entire potential range (Fig. 5a, b and S11†). Furthermore, the trend of FE_C1 and j_C1/ECSA among the Cu_cub/Fe-Por follows that of the FE_C2H4, which are higher values measured for higher molecular TOF_CO. The improved capacity of Cu_cub to catalyze C–C coupling at more negative potentials results in an increase of FE_C2+/FE_C1 for all catalysts at higher overpotentials (Fig. 5c). The decrease of FE_CO (Fig. 5d) and the saturation of j_CO (Fig. 5e) with increasing overpotential is consistent with the increased consumption of CO as more ethylene is generated via the tandem catalytic mechanism. In contrast, FE_CO and j_CO remain approximately constant for pure Cu_cub across all potentials, reflecting the unmodified native activity of the nanocubes in the absence of tandem CO generation.

Fig. 5 (a) FE_C1 and (b) partial current density for the C₁ products (j_C1) normalized by ECSA, (c) FE ratio of C₂₊ to C₁ products, (d) FEs and (e) partial current density for CO (j_CO) normalized by ECSA of Cu_cub/Fe-TBTPP, Cu_cub/Fe-TPP, Cu_cub/Fe-TbcTPP and Cu_cub at different potentials. The reported values are an average of three independent experiments with error bars indicating the standard deviations.

To investigate possible changes of electronic and chemical properties of Cu_cub due to interactions with the molecular constituents, which could influence the overall catalytic properties of the Cu_cub/Fe-Por, we performed X-ray photoelectron spectroscopy (XPS) (Fig. S12†). The analysis of the Cu 2p peaks and Cu LMM spectra indicated that Cu is present in its metallic form in all systems. Furthermore, the absence of any peak shifts in spectra from Cu_cub/Fe-Por compared to Cu_cub indicated that there are no changes in surface composition and/or charge transfer effects. This lack of a specific chemical or electronic change of the Cu surface, together with the systematic correlations between C–C coupling efficiency of Cu_cub/Fe-Por with the TOF_CO trends among Fe-Por, indicates that overall selectivity and activity is governed by the tandem catalytic process.

Having established that a tandem mechanism is dominant in defining activity and selectivity of Cu_cub/Fe-Por, optimization of ethylene production requires understanding of the relationship between the loading of the CO-generating component and the CO conversion rate of the Cu catalyst. With this aim, we tested the impact of Fe-Por loading on enhancing C–C coupling in Cu_cub/Fe-Por tandem catalysts (Fig. 6). We observe that the FE_C2H4 increases with the Fe-Por loading until reaching its maximum value at 8 nmol cm⁻² (Fig. 6a). This trend suggests that addition of more Fe-Por results in the local accumulation of CO in the vicinity of Cu_cub, which promotes C–C coupling. However, a further increase of the Fe-Por coverage leads to decreased C₂H₄ selectivity. Concomitantly, the FE_CO increases (Fig. S13†), which indicates that the additionally generated CO escapes the system.

Fig. 6 (a) FE_C2H4 and (b) j_C2H4/ECSA of Cu_cub/Fe-TBTPP, Cu_cub/Fe-TPP, Cu_cub/Fe-TbcTPP and Cu_cub at different loadings of the Fe-Por (2, 4, 8 and 16 nmol cm⁻²) at −0.95 V vs. RHE in CO₂ saturated 0.1 M KHCO₃ H-cell. The reported values are an average of three independent experiments with error bars indicating the standard deviations.

As discussed above, the Fe-Por molecules also block Cu active sites. Consistent with this conclusion, we observe a continuous decrease of the ECSA with increased loading (Fig. S14†). The enhancement in intrinsic activity of the unpassivated sites on the Cu_cub compensates the decreasing concentration of sites, but only up to a certain loading. Above that point, the C₂H₄ selectivity is suppressed, which is also reflected in the j_C2H4,ECSA (Fig. 6b). In particular, the j_C2H4,ECSA in Cu_cub/Fe-Por increases up to 8 nmol cm⁻² and thereafter saturates.

The comparison of different Fe-Por at the same loadings reveals more efficient C₂H₄ production for the Cu_cub/Fe-TBTPP. Furthermore, we note that lower loadings of the Fe-TBTPP generate higher FE_C2H4 and j_C2H4/ECSA compared to higher loadings of the Fe-TbcTPP (e.g. 4 nmol cm⁻² of Fe-TBTPPvs. 8 nmol cm⁻² of Fe-TbcTPP). As the Fe-TBTPP possesses the highest TOF_CO, these data prove that the CO production rate plays a crucial role in achieving optimal C–C coupling in the tandem catalysts.

To test whether the bottleneck for further improving C₂H₄ generation with the highest Fe–Por loading is the CO consumption, we tested increased Cu-cube loadings for the best performing Cu_cub/Fe-TBTPP (Fig. 7 and S15†). The Fe-TBTPP loading was kept at 16 nmol cm⁻² for comparison with Fig. 6. An increase in Cu_cub loading from 3.12 × 10² to 6.24 × 10² nmol cm⁻² does improve the C₂H₄ production, with both the FE_C2H4 and j_C2H4,_ECSA increasing (Fig. 7a). A FE_C2H4 of 33% is thus measured at −0.95 V vs. RHE. The concomitant decrease FE_CO and j_CO,_ECSA (Fig. 7b) indicates that the CO produced by Fe-TBTPP is indeed further consumed by the increased Cu_cub loading.

Fig. 7 (a) FE and partial current density for the C₂H₄ product normalized by ECSA (j_C2H4,ECSA) and (b) FE and partial current density for the CO product normalized by ECSA (j_CO,ECSA) for Cu_cub/Fe-TBTPP at different loading of Cu_cub (3.12 × 10², 6.24 × 10² and 9.36 × 10² nmol cm⁻²) with the same Fe-TBTPP loading (16 nmol cm⁻²) at −0.95 V vs. RHE in the CO₂ saturated 0.1 M KHCO₃ H-cell. The reported values are an average of three independent experiments with error bars indicating the standard deviations.

For higher Cu_cub loading (9.36 × 10² nmol cm⁻²), the FE_C2H4 and j_C2H4,_ECSA decrease and the FE_CO and j_CO,ECSA don't change further. In a similar manner to the Fe-Por loading experiments, the ratio between the CO-generating component and the CO conversion rate of the Cu catalyst must be balanced for optimal performance.

Finally, we characterized the Fe-Por by Fourier-transform infrared spectroscopy (FT-IR) and ultraviolet-visible spectroscopy (UV-vis) (Fig. S16 and S17†), and the Cu_cub morphology by TEM (Fig. S18†), which proved the intact structure of Fe-Por and cubic shape of Cu_cub after 1.5 hours of electrolysis. Furthermore, the best performing tandem catalyst in terms of C₂H₄ production, which is Cu_cub/Fe-TBTPP, displayed an excellent stability and selectivity over 10 hours of electrolysis (Fig. S19†). Altogether, these data indicate that the addition of the Fe-Por has a stabilizing effect on the morphology of the Cu cubes. While a dedicated study is needed, we speculatively attribute the origin of this observation to the formation of the CO intermediate on a different surface than Cu.

Conclusions

In summary, we built tandem catalysts including Fe-Por with tunable CO₂ to CO selectivity (Fe-TBTPP > Fe-TPP > Fe-TbcTPP) and Cu_cub. We discovered that the C–C coupling enhancement in these tandem catalysts correlates directly with the Fe-Por selectivity and conversion rates for CO, with the Cu_cub/Fe-TBTPP being the best performing one.

We found a 100 mV anodic potential shift for C₂H₄ detection, a 22 times enhancement of C₂H₄ production at −0.75 V vs. RHE and a doubled FE_C2H4 of 36% at −1.05 V vs. RHE in the Cu_cub/Fe-TBTPP compared to the Cu_cub. This performance is comparable to the state-of-the-art in this class of catalysts.^25–27 Going beyond it, these results demonstrate that the rate and potential at which CO is generated plays a defining role for the selectivity and overpotential of multiple-carbon products in tandem schemes (Fig. 8).

Fig. 8 Schematic representation of the proposed mechanism for C₂H₄ promotion in Cu_cub/Fe-Por tandem catalysts. The C–C coupling enhancement and C₂H₄ promotion correlate with the Fe-Por selectivity and conversion rate for CO.

Overall, this study encourages further investigations on the design of the homogenous catalyst as a promising strategy to further optimize molecular-based tandem systems.

Data availability

Data for this paper, including UV-vis and electrochemical characterization, are available at Zenodo at https://doi.org/10.5281/zenodo.7229402.

Author contributions

M. W. carried out the synthesis of the tandem catalysts and all electrocatalytic testing for CO₂RR. V. N. synthesized and characterized the molecular catalysts. A. L. performed X-ray photoelectron spectroscopy and contributed to daily discussions. A. L. supervised the work on the molecular catalysts. I. S. contributed to discussions. R. B. supervised the work on the tandem catalysts and CO₂RR 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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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2sc04794b

‡ These authors have equally contributed.

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