# Efficient Selective Electroreduction of CO2 to CO by a Quaterpyridine Molecular Copper Complex Immobilized onto Carbon

Mahdi Saad, Bhavin Siritanaratkul, Chun-Wei Chang, Yueh-Chun Hsiao, Tung-Han Yang, Corinne Lagrost, Alexander J. Cowan, Noémie Lalaoui, Nicolas Le Poul

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Mahdi Saad, Bhavin Siritanaratkul, Chun-Wei Chang, Yueh-Chun Hsiao, Tung-Han Yang, et al. Efficient Selective Electroreduction of CO2 to CO by a Quaterpyridine Molecular Copper Complex Immobilized onto Carbon. ACS Applied Materials & Interfaces, 2025, 10.1021/acsami.5c17693. hal-05383935

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Efficient, selective electroreduction of CO₂ to CO
by a quaterpyridine molecular copper complex
immobilized onto carbon

Mahdi Saad,† Bhavin Siritanaratkul,¥ Chun-Wei Chang,§ Yueh-Chun Hsiao,§,¥ Tung-Han

Yang,§ Corinne Lagrost,‡ Alexander J. Cowan,*¥ Noémie Lalaoui*‡# and Nicolas Le Poul*†

† UMR CNRS 6521, Univ. Brest, 29200 Brest, France

¥ Stephenson Institute for Renewable Energy and the Department of Chemistry, University of
Liverpool, Liverpool L69 7ZF, United Kingdom

§ Department of Chemical Engineering, National Tsing Hua University, Hsinchu 300044,
Taiwan

‡ Univ. Rennes, CNRS, ISCR-UMR 6226, ScanMAT-UAR 2025, 35000 Rennes, France

# UMR CNRS 5250, Univ. Grenoble-Alpes, 38058 Grenoble, France

KEYWORDS: CO₂ reduction, molecular electrocatalysis, copper complex, quaterpyridine,
heterogeneous catalysis, zero-gap electrolyser
ABSTRACT: We report herein the synthesis, spectroscopic and electrochemical characterization as well as electrocatalytic properties toward heterogeneous electrochemical CO₂ reduction of two mononuclear copper(II) complexes, namely [Cu(L)]²⁺ (1) and [Cu(QTPy)]²⁺ (2) where L = 8-([2,2'-bipyridin]-6-yl)-2-(pyridin-2-yl)quinoline and QTPy = 2,2':6',2":6",2'''-quaterpyridine. The two complexes 1 and 2 have been first successfully immobilized onto multiwalled carbon nanotubes (MWCNTs) grafted on glassy carbon electrodes, as shown by X-ray photoelectron spectroscopy and cyclic voltammetry. Electroreduction of carbon dioxide by the grafted complexes has been investigated in aqueous KHCO₃ medium at neutral pH. Comparative studies highlight the better catalytic performance of the complex bearing L rather than QTPy under the same experimental conditions. Controlled potential electrolysis (CPE) at low overpotential (–0.55 V vs RHE) displays high selectivity for carbon monoxide production with complex 1 (Faradaic Efficiency (FECO = 75%)). Furthermore, in-situ and ex-situ analyses of the electrolysis evidence no modification of the immobilized mononuclear complex upon CPE which is rare for grafted copper systems, as well as no leach of copper in the electrolyte solution. Noteworthy, implementation of the complexes into zero-gap electrolysers allows the strong enhancement of FEco for complex 1, reaching 100% at –0.60 V vs RHE, whereas complex 2 remains poorly selective, hence clearly highlighting the strong impact of the ligand topology on the catalytic properties of the complexes.
# 1. INTRODUCTION

With the increasing amount of carbon dioxide into atmospheric and marine environments due to anthropogenic emissions, electrochemical CO₂ reduction reaction (eCO₂RR) has been identified as one serious green approach to valorize CO₂ into valuable chemicals (CO, HCO₂H, CH₃OH, CH₄, C₂H₄) by using electricity from renewable energy sources.¹,²,³ Nevertheless, the development of eCO₂RR devices at the industrial scale remains still limited by the efficiency and selectivity of the materials which are necessary to catalyze CO₂ conversion. Among the large number of electrocatalysts which have been described for more than 20 years, those based on copper have recently drawn more attention since this metal is one of the rare, if not the only one, to lead to the formation of not only one-carbon (C₁) products, but also two-carbon (C₂) and multi-carbon (C₂+) hydrocarbons and alcohols.¹,⁴ So far, two main approaches have been proposed for eCO₂RR with copper systems. The first is based on the use of elemental copper/copper oxides,¹,⁵ Cu-based metal-organic frameworks (MOFs)⁶ or copper single-site systems⁷,⁸ as heterogeneous catalysts.⁹ Nonetheless, except in rare cases,¹⁰ these systems have mostly demonstrated poor selectivity due to multiple reaction pathways including the competing hydrogen evolution reaction (HER). On the other hand, molecular-based copper electrocatalysts have been considered as both homogeneous and heterogeneous electrocatalysts for CO₂ reduction.¹¹,¹² Compared to copper/copper oxide systems, molecular Cu complexes display the main advantage of being tailored through adequate ligand design, thus offering multitude of possibilities from nuclearity to secondary-sphere effects. Furthermore, they can give access to reaction mechanisms thanks to the characterization of isolated transient species in solution or adsorbed onto electrode surface. Since the first example of eCO₂RR by a molecular copper complex,¹³ major investigations have been focused on copper complexes bearing redox non-innocent ligands such as
phthalocyanine (Pc),<sup>14,15,16,17,18,19,20</sup> phenanthroline (Phen),<sup>21,22,23,24,25,26,27</sup> and porphyrin (Por).<sup>28,29,30,31,32</sup> For these systems and others,<sup>33,34,35</sup> it was generally found that homogeneous electrocatalysis in aqueous or organic media led to the deposition of active copper/copper oxide nanoparticles (NPs) onto electrode surfaces upon application of a reducing potential. Analogously, heterogeneous catalysis performed by Cu complexes grafted onto different electrode materials (carbon cloth or paper, graphene, indium tin oxide) also led to Cu metal or Cu oxide formation. This demetallation and aggregation of copper particles was found to occur in most of the reported studies with Cu heterogeneous catalysts, whatever the nature of the ligand.<sup>14,16,18,22,30,34,36,37,38</sup> Few cases have been described for which the eCO<sub>2</sub>RR process was only driven by grafted molecular copper complexes.<sup>21,24,39,40,41,42</sup>

In this context, our aim is to develop new molecular copper complexes that could be easily immobilized onto carbon electrodes and able to sustain as molecular catalyst the electroreduction of CO<sub>2</sub> in water in a selective manner, i.e. ideally one single product (CO) generated with high faradaic efficiency (>70%). For this objective, we have focused on the family of quaterpyridine copper complexes. Indeed, quaterpyridine ligands exhibit several advantages for carbon dioxide reduction: (i) they can stabilize coordination metals with low redox states because of their good π-acceptor properties, (ii) they can be immobilized onto graphene-like substrates or surfaces through π-π interactions; (iii) they can display a macrocyclic-like environment as found with porphyrin and phthalocyanine systems, i.e. good chelating properties for metals.<sup>43</sup> Such properties were previously exploited by Robert and co-workers for the photocatalytic reduction of CO<sub>2</sub>,<sup>44</sup> but not used for heterogeneous eCO<sub>2</sub>RR.

Herein, we present the synthesis and characterization of Cu(II) and Cu(I) complexes based on L ligands ($L = 8-([2,2'-bipyridin]-6-yl)-2-(pyridin-2-yl)quinoline (Scheme 1)$). Compared to QTPy, L features a less flexible and more conjugated backbone, with a smaller binding cavity.<sup>44</sup> These characteristics may improve eCO<sub>2</sub>RR by providing i) stronger binding
between **L** and Cu(II), and ii) higher π-electron delocalization over the entire ligand, thereby increasing interactions with carbon nanotubes. Such properties are thus expected to enhance stability upon bulk electrolysis by comparison with **QTPy**. Furthermore, we have anticipated that the ligand topology may disfavor the formation of dimeric intermolecular Cu(I) species, which are known for tetapyridine copper systems,<sup>45,46,47</sup> thanks to steric constraints.

<figure><img src="image_2.png" /></figure>

**Scheme 1.** Representation of ligands **L** and **QTPy**, as well as their Cu(II) complexes [Cu(**L**)]<sup>2+</sup> (1) and [Cu(**QTPy**)]<sup>2+</sup> (2) studied in this work. Axial ligands for complexes 1 and 2 are not shown.

## 2. EXPERIMENTAL METHODS

### 2.1. Materials

The ligand 2,2':6',2":6",2"'-quaterpyridine (**QTPy**) was synthesized as described in literature.<sup>48</sup> The ligand 8-([2,2'-bipyridin]-6-yl)-2-(pyridin-2-yl)quinoline (**L**) was synthesized by adapting a published method,<sup>49</sup> starting from 8-bromo-2-(pyrid-2'-yl)quinoline (see Scheme S1 and SI for details).<sup>50</sup> All other reagents and solvents are commercially available and were used as received, unless otherwise noted. Multiwalled carbon nanotubes (MWCNTs) NC7000TM were purchased from NANOCYL®. Acetonitrile (MeCN) and tetrahydrofuran (THF) were distilled following standard procedures, degassed and stored over dried molecular sieves. Syntheses were performed using the Schlenk line technique under a
nitrogen atmosphere. For electrochemical measurements, extra dry N,N-dimethylformamide (Acroseal, DMF) 99.8 % over molecular sieve was purchased from Acros. 1-methyl-2-pyrrolidinone (NMP) 99.5% was purchased from Sigma-Aldrich. Milli-Q water was purified through a Millipore system. Indium tin oxide coated glass (ITO) with a resistivity of 12 Ω was purchased from Sigma Aldrich.

## 2.2. Electrochemical methods

The electrochemical studies were conducted with an AUTOLAB PGSTAT 302 potentiostat from Metrohm at room temperature under Argon or CO₂ atmosphere. The glassy carbon disks (Ø 3mm) used as working electrodes (WE) were polished on an alumina slurry (1 µm) and sonicated in water then acetone during 5 minutes each, then dried with N₂ flow. Electrochemistry in organic media was performed in DMF containing 0.1 M n-NBu₄PF₆ in a home-designed 3-electrodes cell displaying separated compartments for the reference (RE) and counter (CE) electrodes (WE: glassy carbon, RE: Ag/AgCl/KCl 3.5 M, CE: Pt wire). Aqueous electrochemistry was performed in KHCO₃ (99.95%, 0.1 M) electrolyte solution using MWCNTs based electrodes (A = 0.07 cm²) as WE. Controlled-potential electrolysis (CPE) was performed with a specifically designed electrochemical air-tight cell (RE: Ag/AgCl/KCl 3.5 M, CE: graphite rod, WE: MWCNTs based electrodes) under stirring. The electrolyte (aqueous 0.1 M KHCO₃) was saturated with CO₂ before the electrolysis (pH = 6.8). Potential values (E / V) obtained experimentally vs Ag/AgCl/KCl 3.5 M at 20°C were referenced against the reversible hydrogen electrode (RHE) by using equation (1):⁵¹

$$E_{V \text{ vs RHE}} = E_{V \text{ vs Ag/AgCl/KCl 3.5 M}} + 0.0591 \text{ pH} + 0.208 \quad (1)$$

Electrochemical studies with zero-gap cells were conducted with a Vertex S5A potentiostat (Ivium) at room temperature under either argon or CO₂ atmosphere. The membrane electrode assembly (MEA) was constructed by sandwiching the anion exchange membrane (Fumasep FAB-PK-130) between the anode and cathode layers. The anode was IrOₓ on carbon paper.
The MEA was assembled into a 5 cm² electrolyser from Dioxide Materials. The electrolyser consists of a titanium anode plate with an active area of 9 cm² and a stainless-steel cathode plate with an active area of 5 cm², separated by Teflon spacers. The MEA is sandwiched between these plates to form a zero-gap assembly, with CO₂ gas supplied to the cathode and liquid electrolyte to the anode. The cell was assembled with a torque of 3 Nm. CO₂ was flowed at 20 sccm directly to the cathodic chamber. Anolyte (0.1 M KHCO₃) was circulated across the anode at 15 ml/min. In order to better control the applied voltage, the anion exchange membrane was cut to a size that extended out of the cell, and an Ag/AgCl reference electrode was placed in contact with the cathode side of the membrane in a chamber with 0.1 M KHCO₃, following previously reported procedure.⁵² The uncompensated resistance was measured before each experiment, using electrochemical impedance spectroscopy at 5000 Hz, and iR compensation was applied at 85%.

### 3. RESULTS AND DISCUSSION

#### 3.1 Complexes synthesis and characterization

The complexes have been prepared by stoichiometric reaction between **L** or **QTPy** ligands and copper(II) or copper(I) salts (see ESI for synthetic details). Characterization of the resulting complexes has been carried out by X-ray diffraction (XRD), UV-Visible and ¹H Nuclear Magnetic Resonance (NMR) spectroscopies, and high-resolution mass spectrometry (HR-MS) (Figures S1-S7). Both triflate-based **L** and **QTPy** copper(II) complexes are mononuclear according to solid-state and solution analyses, yielding [Cu²⁺(**L**)]²⁺ (**1**) and [Cu²⁺(**QTPy**)]²⁺ (**2**) respectively, in agreement with reported studies on quaterpyridine copper(II) analogues.⁴⁴,⁵³ On the other hand, the **L**-based copper(I) complex (**3**) is dinuclear, as previously found with the **QTPy** complexes.⁴⁵,⁴⁶ This monomer-dimer molecular switch was already reported for quaterpyridine-based complexes and ascribed to the possible torsion
of the central C-C bond separating the two bipyridine units.⁴⁷ At Cu(II) redox state, the metal ion lies in a pseudo-octahedral geometry featuring a tetraaza plane and one or two counteranions (or solvent (H₂O)) axial ligands. This is well exemplified by the structures obtained from XRD analysis for both Cu(II) complexes (Figure 1 for **1** and Figure S8 for **2** with crystallographic data in Table S1). As expected, the geometric features of **2** are close to those previously obtained with other counter-anions (ClO₄⁻, NO₃⁻)⁴⁴,⁵³ as it exhibits Cu-N bond lengths ranging from 1.950 to 2.029 Å as well as a N1-Cu-N4 angle of 119° (Table S2). In contrast, the complex **1** displays a narrower range of copper- nitrogen bond distances (1.974 to 1.995 Å) and a significantly lower N1-Cu-N4 angle (103°) (Table S2). It also shows a significant torsion of the N4 frame by 22° (Figure 1B) that is not present for **2**. Such distortion was previously observed for methyl-substituted quaterpyridine Cu(II) complexes.⁴⁷ Here, the structural difference with complex **2** is likely due to the smaller binding cavity of **L** (Scheme 1) which might constrain the ligand to lie in a more closed configuration, as a pseudo-macrocycle, hence inducing a slightly distorted geometry.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Left (A) and right (B) ORTEP views of the mononuclear complex **1** with ellipsoids drawn at the 50 % probability level. Hydrogen atoms and one triflate ion are omitted for clarity. For bond distances and angles details, see Table S2.</figcaption></figure>
As evidenced by XRD, the structure is different at the copper(I) redox state (see Figure S9).
Indeed, the metal is stabilized in a pseudo-tetrahedral $N_4$ environment leading to helical
rearrangement thanks to $\pi-\pi$ favored interactions between pyridine cycles (approx. 3.5 Å, see
Figure S9). The Cu-Cu bond distance is found to be equal to 4.635 Å which is significantly
longer than that reported for its **QTPy** analogue (3.173 Å)$^{45}$ (Figure S10 and Table S3). This
confirms that **L** is less flexible than **QTPy** to accommodate for an optimized tetrahedral
geometry around each copper(I) ion.

## 3.2 Electrochemical and surface spectroscopic studies

Cyclic voltammetry (CV) studies of complexes **1** and **3** in dimethylformamide (DMF) under argon have confirmed the monomer-dimer process with the redox state of copper, as previously reported for various quaterpyridine-substituted copper complexes.<sup>46,47</sup> The one-electron reduction of the monocopper(II) complex **1** occurs irreversibly at –0.14 V vs Ag/AgCl at a glassy carbon electrode at $v = 0.1 \text{ V.s}^{-1}$ (Figure S11). The backward scan displays an irreversible anodic peak at +0.67 V vs Ag/AgCl, which is ascribed to the two-electron oxidation of the dicopper(I) generated from the forward reduction, according to studies performed with analogous complexes.<sup>46,47</sup> Increase of the scan rate up to 1 V.s<sup>-1</sup> shows the appearance of an oxidation peak at –0.03 V likely related to the oxidation of the transient mono-Cu(I) species (Figure S11). This redox behavior is slightly different from that obtained with the analogous complex **2**. Indeed, although both complexes **1** and **2** behave similarly at low scan rate (irreversible cathodic and anodic peaks), the **QTPy** complex **2** does not exhibit any reversibility at higher scan rate for the reduction of mono-Cu(II) species at ca. –0.1 V vs Ag/AgCl (Figure S12). This result clearly demonstrates that the Cu(I) monomer-to-dimer reaction is relatively slow for **1** (approximatively 1 s. under those conditions) whereas it is fast for **2**. CVs of the bis-Cu(I) complex **3** are also consistent with voltammetric data obtained with **1**, i.e. an oxidation of the dimer at +0.68 V vs Ag/AgCl followed by dissociation into
monomeric species which can be reduced at ca. -0.14 V (Figure S13). Comparative analysis of the redox potential values found in DMF at v = 0.1 V·s⁻¹ (Table S4) indicates no effect of the ligand **L** or **QTPy** on the reduction potential of the copper(II) species (-0.14 V and -0.13 V vs Ag/AgCl for **1** and **2**, respectively). However, the oxidation potential of the dimer is significantly more positive with **L** compared to **QTPy** (+0.67 V and +0.38 V for **1** and **2**, respectively), suggesting a lower stability of the dicopper(I) **L**-based complex compared to the **QTPy** one.

In order to investigate the heterogeneous electroreduction of CO₂ in water, complexes **1** and **2** have been immobilized onto multi-walled carbon nanotubes (MWCNTs) by taking advantage of strong π-π interactions between the ligand backbones and the CNTs sidewalls. Furthermore, MWCNTs offer the possibility to increase the active surface area without interfering with the catalyst for the CO₂ reduction at low overpotential.¹⁵ The MWCNTs-based electrodes coined as GC-CNT-**1** and GC-CNT-**2** have been prepared according to a classical drop-casting procedure (see experimental part for details, SI), involving two main deposition steps: firstly, MWCNTs onto glassy carbon (GC) surfaces, followed by the deposition of complex **1** or **2** respectively. For full comparison, glassy carbon electrodes with only MWCNTs deposited (named as GC-CNT) have also been prepared. The same strategy of deposition has been applied to graft onto flat ITO/glass surfaces for chemical surface analysis, yielding ITO-CNT and ITO-CNT-**1** electrodes. X-ray photoelectron spectroscopy (XPS) measurements provide the chemical composition of ITO-CNT-**1** surfaces before and after electrolysis (Figures S14-15). The signal of C1s, N1s and Cu2p photoelectron peaks are observed in both survey and high-resolution core level spectra. C1s signal is the dominant contribution because of the presence of MWCNTs as demonstrated by the asymmetric lineshape of the corresponding signal. In both cases, ITO-CNT-**1** clearly shows the presence of copper through a Cu2p doublet component at 933.1 eV and 953 eV due to spin-orbit
splitting with the expected energy separation of 19.9 eV between Cu2p<sub>3/2</sub> and Cu2p<sub>1/2</sub> as well as nitrogen N1s at 400.1 eV (Figures S14-S15).<sup>54,55</sup> In addition, the elemental ratio of nitrogen to copper is equal to 3.2, in good agreement with the expected theoretical ratio in complex **1**. All these observations evidence the deposition of the complex in MWCNTs. For comparison purposes, XPS experiments with ITO-CNT modified electrodes do not exhibit the presence of any copper or nitrogen (Figure S16). Noteworthy, the peak-fitted high-resolution core level spectra in the Cu2p region of ITO-CNT-**1** before and after CPE (Figures S17) displays a dominant contribution of Cu(I) over Cu(II) which likely results from reduction of Cu(II) under the X-ray beam during the acquisition process, as evidenced by control experiments with complex **1** alone (Figure S18) or deposited on ITO without MWCNTs (Figures S19-S20), and in agreement with reported studies.<sup>56,57</sup>

We also investigated the chemical environment of complex **1** deposited onto a carbon paper substrate by synchrotron X-ray absorption spectroscopy (XAS). The ex-situ XANES (X-ray near-edge structure) Cu K-edge spectrum of complex **1** displays a main white-line peak at 8896 eV (Figure S21), consistent with the presence of a tetracoordinated Cu(II) species.<sup>58</sup> The +II redox state of copper is corroborated by the comparison with spectra obtained with Cu foil and Cu<sub>2</sub>O which both clearly show different features, while CuO exhibits more similarities with **1**. The ex-situ Fourier-transformed X-ray absorption fine structure (FT-EXAFS) of complex **1** (Figure S22) shows a major peak at ~1.5 Å which can be assigned to Cu-N/O bonds, as observed with analogous tetranuclear Cu complexes.<sup>14,30,31</sup>

The Figure 2A (red curve) displays CV of GC-CNT-1 in KHCO<sub>3</sub> 0.1 M under CO<sub>2</sub> (pH = 6.8) at v = 0.1 V·s<sup>-1</sup>. A well-defined reversible system is detected at E<sub>1/2</sub> = +0.20 V vs RHE (E<sub>pc</sub> = 0.01 V, ΔE<sub>p</sub> = 380 mV, no ohmic drop compensation) together with a catalytic wave starting at an onset potential of -0.45 V vs RHE. Control experiments with a GC-CNT electrode show no catalytic wave in this potential range (Figure 2A, black curve), thus evidencing that the
observed electrocatalytic process is driven by the immobilized copper complexes. Moreover, CV studies of GC-CNT-1 under argon in KHCO₃ 0.1 M only exhibit the reversible system (Figure S25). Accordingly, this redox process can be ascribed to the reversible 1-electron reduction of the grafted mono-Cu(II) complex. Plots of anodic and cathodic peak current densities with v show a linear trend as expected for a redox process with immobilized species (Figure S26). Integration of the voltammetric signal gives a surface concentration of Γ = 253 ± 11 nmol.cm⁻² (Table S5). Successive scanning (50 cycles, Figure S27) demonstrates the very good stability of the anchored species.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** CVs (v = 0.1 V.s⁻¹) in 0.1 M KHCO₃ (pH = 6.8) under CO₂ of A) GC-CNT (black) and GC-CNT-1 (red) electrodes; B) GC-CNT-1 electrodes before (red) and after (blue) 1 hour CPE at – 0.55 V vs RHE. The arrow indicates the scanning direction.</figcaption></figure>

Comparative CV studies with the GC-CNT-2 electrode reveal a Cu(II)/Cu(I) reversible system at E₁/₂ = +0.30 V vs RHE (Epc = 0.09 V, ΔEₚ = 420 mV), a 100 mV more positive value than that found for GC-CNT-1 (Figure S28B). Scan-rate variation shows a linear increase of the cathodic peak current with v, as expected for a surface-confined redox process (Figure S29). Calculations of the surface concentration for this system give Γ = 311 ± 10 nmol cm⁻² (Table S5). A catalytic wave is also observed upon reduction under CO₂ at an onset
potential of -0.45 V vs RHE, with similar current densities to those found for GC-CNT-1 (Figure S28). An additional irreversible oxidation process is detected on the backward scan under Ar or CO₂. Based on solution studies in DMF, it may be assigned to the oxidation of the dicopper(I) species generated through the fast dimerization of the immobilized monocopper complex on the electrode surface.

When performing voltammetric measurements with bis-Cu(I)-based GC-CNT-3, two redox systems are detected. The main one is the same as that found with GC-CNT-1, i.e. a reversible process at E₁/₂ = +0.20 V corresponding to the Cu(II)/Cu(I) electron exchange for the monomer (Figure S30A). A second anodic peak appears at Eₚₐ = +0.80 V vs RHE. This peak is irreversible and remains present upon second cycling. As for GC-CNT-2 and by analogy with solution studies, it can be ascribed to the oxidation of the immobilized dicopper(I) species. Interestingly, this observation suggests that upon deposition onto carbon nanotubes, most of the dimeric Cu(I) species 1 dissociates into monomeric entities.

### 3.3 Controlled-potential electrolysis (CPE) with GC electrodes

CPE of the CO₂-saturated solution was conducted for 1 hour, applying different potential values ranging from -0.55 to -0.70 V vs RHE allowed the determination of the different products electrogenerated by GC-CNT-1, GC-CNT-2 and GC-CNT-3 electrodes (Figure 3 and Table S6). So far, only CO and H₂ have been detected by gas chromatography (no generation of methane) from the headspace of the cell. For GC-CNT-1, a maximum of Faradaic Efficiency (FE) for CO is obtained when the electrolysis is carried out at -0.55 V vs RHE (FE<sub>CO</sub> = 75%) with a small amount of H₂ is electrochemically produced (FE<sub>H₂</sub> = 3%). No formic acid has been detected by ion chromatography in the liquid electrolytic solution after CPE. Chronoamperometric monitoring of the electrolysis at -0.55 V indicates a stable current density *j* value of 0.75 mA·cm⁻² (Figure S31A). This value remains constant when
CO₂ is gently bubbled in the solution (1 atm.) during electrolysis (Figure S32A), showing that current density limitation is not due to CO₂ mass transport effect or depletion in the solution with time. The turnover number (TON) and frequency (TOF) for CO production for 1 hour at -0.55 V vs RHE are found to be equal to 135 and 0.04 s⁻¹, respectively. When the applied potential is more negative than -0.55 V, a decrease of FEₐᵣ is observed whereas FE₂H₂ increases, as a consequence of a more competitive HER process catalyzed by the complex itself and/or carbon nanotubes (Figure 3). When a more positive (-0.45 V vs RHE) potential is applied, the FEₐᵣ decreases drastically while FE₂H₂ increases substantially. Importantly, the GC-CNT-1 electrode gives an identical CV signal after 1-hour electrolysis at -0.55 V vs RHE, indicating that the immobilized complex is highly stable at this potential value (Figure 2B). Interestingly, this is corroborated by XPS experiments that do not show any modification of the spectra for the electrode material after CPE (Figure S15). Especially, the elemental ratio N/Cu is found equal to 3.8 after CPE, close to the value of 3.2 found before the reaction, highlighting the very good stability of the grafted complex 1 during electrocatalysis. Moreover, dynamic light scattering (DLS) analysis of the solution after CPE with the supported catalyst did not show any presence of nanoparticles (< 70 nm) in solution hence conforming the robustness of the GC-CNT-1 electrode upon electrolysis at this potential (Figure S33). Control CPE experiments performed with GC-CNT under CO₂ at -0.55 V yield dihydrogen as main product (FE(H₂) = 38 %) while CO is generated with low amounts (FEₐᵣ = 3%). This result emphasizes the high efficiency and selectivity of the immobilized complex 1 for CO₂-to-CO conversion. At last, electrolysis at -0.55 V carried out with GC-CNT-3 yields same results as for GC-CNT-1, i.e. high selectivity for CO vs H₂ (FEₐᵣ = 80 %, FE₂H₂ = 3 %) and similar current density (j = -0.60 mA.cm⁻²) (Figure S32B and Table S6)
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Faradaic efficiency for CO (green) and $H_2$ (blue) calculated from gas chromatography analysis after CPE for 1 hour in 0.1 M $KHCO_3$ (pH = 6.8) at -0.70 V, -0.65 V, -0.60 V and -0.55 V vs RHE with A) GC-CNT-1 and B) GC-CNT-2 electrodes.</figcaption></figure>

When performing the same experiments with GC-CNT-2, different results are obtained since $H_2$ is the main product resulting from electrolysis (Figure 3B). At -0.55 V vs RHE, the functionalized electrode generates $H_2$ and CO with Faradaic efficiencies of 44% and 30%, respectively (Table S6). These values clearly indicate a much lower selectivity of GC-CNT-2 than that of GC-CNT-1 ($FE_{CO}$ = 75%, $FE_{H_2}$ = 3%). The current density during CPE linearly decreases from 2 to 1.2 mA.cm$^{-2}$ over 1 hour CPE at -0.55 V, indicating the loss of catalyst under operation (Figure S31B). This is corroborated by CV of GC-CNT-2 after CPE which shows a large decrease of the Cu(II)/Cu(I) Faradaic signal at 0.30 V (by 30%) together with the capacitive current (Figure S30B). This is likely due to leaching of both MWCNTs and complex 2 upon electrolysis, probably because of production of gas bubbles that peel some parts of the deposited layer off.
Obviously, GC-CNT-1 and GC-CNT-2 electrodes display highly different efficiencies for eCO₂RR. On one hand, CV studies in organic media with copper(II) complexes demonstrate that [Cu¹L]⁺ can be transiently generated at the electrode surface at v = 0.5 V·s⁻¹ whereas it is not the case for its **QTPy** analogue (Figures S11 and S12). On the other hand, when grafted onto MWCNTs, complex 1 remains mononuclear at the Cu(I) redox state in the CV timescale, while complex 2 displays both mono and dimeric forms (Figure S28B). Moreover, XRD structural data suggest that the dicopper(I) is less stabilized with **L** than with **QTPy**. These results may indicate that the mono-reduced form is statistically more present on GC-CNT-1 than on GC-CNT-2. From CPE studies, it would thus suggest that the better efficiency and stability of GC-CNT-1 originates from its monomeric form at reduced state. Another possible reason might be that the supplementary phenyl group on **L** vs **QTPy** may strengthen the interactions with multiwalled-carbon nanotubes, and consequently increases electron transfer kinetics between the surface and the catalyst.

### 3.4 Controlled-potential electrolysis (CPE) with a zero-gap electrolyser

Further studies were conducted using zero-gap electrolyser cells to improve the performances (Faradic efficiency, current density) of the catalysts for CO production and examine the properties of both molecular complexes 1 and 2 on gas diffusion electrodes in an operating electrolyser. In such electrolysers, the cathode is directly fed with a continuously humidified CO₂ stream, with CO₂ reduction occurring at the interface between the membrane and the cathode electrode. One main advantage of zero-gap cells over flow ones is that it simplifies the catalytic process by significantly increasing the local CO₂ concentration through improved gas transport, which in principle leads to higher current densities and faster reaction rates.⁵⁹,⁶⁰,⁶¹ Additionally, these cells offer a key benefit compared to liquid-fed flow cells as they exhibit lower ohmic losses and a reduced risk of catalyst poisoning from impurities in the cathode electrolyte. Here the electrolyser was modified to include a reference electrode in the
manner described previously elsewhere.⁵² The reference electrode allowed for accurate control of the cathode potential and direct comparison to H-cell behaviour. Immobilization of the complexes is performed by drop-casting a mixed solution of MWCNTs and catalyst (1 mM) onto a 1 cm² carbon cloth electrode (see experimental details in SI).

As a proof of concept, first CPE measurements have been carried out with the gas-diffusion layer modified with the quaterpyridine complex (GDE-2). To evaluate the stability of the gas diffusion electrode, electrolysis was performed at –0.50 V vs RHE (–1.10 V vs Ag/AgCl) for four hours, with a break every hour to allow analyses by CV and gas chromatography (Figure S34). Chronoamperometry monitoring shows that the GDE-2 system can display a continuous and stable current density of 1.5 mA.cm⁻² over 4 hours (Figure S34A). Analysis of the reaction product by gas chromatography (Figure S34B, Table S7) reveals that H₂ is the major gaseous species whereas CO is produced in low amounts, in agreement with results obtained with GC-CNT-2 at –0.55 V vs RHE (Figure 3B). CVs after 2 and 4 hours (Figure S34C) display a redox Cu(II)/Cu(I) with no significant variation of the current density on the back scan. The same analysis was performed with the BPQP complex 1 immobilized onto a gas diffusion layer and assembled in the zero-gap electrolyser, namely GDE-1. For full comparison with GDE-2, CPE has been carried out at –0.50 V vs RHE. Gas chromatography monitoring over a 3-hours CPE shows that GDE-1 is able to sustain high Faradic efficiency for CO (>90 %, Figure 4A), demonstrating much higher selectivity for carbon monoxide than GDE-2 in the same conditions (Figure S34A), but also better FE<sub>CO</sub> than GC-CNT-1 (Figure 3A). In addition, the effect of the applied potential has then been investigated for short-time electrolysis using the same approach (gas chromatography and voltammetric analyses). Application of a potential of –0.50 V vs RHE yields selectively carbon monoxide with high efficiency (FE<sub>CO</sub> = 97%, Table S7 and Figure 4B) at a current density around 0.75 mA.cm<sup>-2</sup> (Figure S35A). The CV after CPE displays a well-defined redox system ascribed to the
Cu(II)/Cu(I) process (Figure S35B). At –0.6 V vs RHE, the high selectivity is maintained (FE<sub>CO</sub> = 100%) and the current density is significantly increased as it reaches 4 mA.cm<sup>-2</sup> (Figure S36A). Interestingly, when a potential of –0.70 V vs RHE is applied (j = 5 mA.cm<sup>-2</sup>, Figure S37A), a mixture of products (CO, H<sub>2</sub>, CH<sub>4</sub>, and C<sub>2</sub>H<sub>4</sub> with 5.7%, 40.2%, 1%, and 1% FEs respectively) is generated according to gas chromatography analysis (Figure 4B), although the CV remains similar to what was obtained at –0.6 V vs RHE (Figures S36B and S36B). This loss of CO selectivity as well as large increase of dihydrogen production are consistent with results obtained with GC-CNT-1 at –0.70 V vs RHE (Figure 3A), thus excluding effects arising from the electrolyser configuration (static/flow mode).

In-situ XAS measurements were finally conducted with GDE-1 in CO₂-purged 0.1 M KHCO₃
in order to probe the redox state and coordination environment of the grafted complex during
electrocatalysis. The measurements were started at open-circuit potential (OCP), then stepped
to –0.1 V vs RHE, –0.5 V and –0.7 V, then finally driven back to OCP. Accordingly, all
spectra obtained at OCP with GDE-1 (Figures S23-S24) are quasi-identical to those obtained
ex-situ in air (Figure S21-S22), evidencing the high stability of the complex under electrolytic
conditions. As shown by XANES (Figure S23) and FT-EXAFS (Figure S24) spectra, no
significant change could be observed upon application of a reductive potential under CO₂
purging. In particular, the absence of edge shift in the XANES spectra and the lack of increase
in the FT-EXAFS intensity in the ~2.2 Å region even at the lowest applied potential (–0.7 V
vs RHE) clearly indicates that complex 1 maintains its coordination as a molecular complex
and does not decompose into Cu(0) nanoparticles within the potential ranges used in this
paper, in full agreement with CV data before and after electrolysis with GC-CNT-1 (Figure
2B).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** A) Monitoring of the Faradaic efficiency for CO (green) and $H_2$ (blue) calculated from gas chromatography analyses at different time intervals during CPE for 3 hours at -0.50 V vs RHE with GDE-1. B) FEs for CO (green), $H_2$ (blue), $CH_4$ (red) and $C_2H_4$ (cyan) determined from gas chromatography analysis after 3 hours CPE with GDE-1 for different applied potential values (-0.50 V, -0.60 V and -0.70 V vs RHE).</figcaption></figure>

<figure><img src="image_8.png" /></figure>

Finally, the results obtained with GC-CNT-1 and GDE-1 for $CO_2$ reduction can be compared with those obtained from other reported heterogeneous molecular copper complexes (Table S8). So far, very few $eCO_2RR$ molecular catalysts immobilized onto electrode surface have shown stability over CPE in aqueous media as most of them lead to the formation of copper nanoclusters or copper oxide upon application of a reducing potential.<sup>14,15,16,17,18,21,24,28,30,34,36,37,38,62</sup> Among the few molecular copper catalysts able to recover their initial molecular state after electrolysis in aqueous media, the dinuclear bis(µ-OH) bis-phenanthroline catalyst developed by Francke and co-workers yielded various mixtures of $C_1$ and $C_2$ products upon CPE between -1.1 and -1.35 V vs RHE in KOH.<sup>21</sup> In a second example, Liao *et al.* performed electrocatalytic $CO_2$ reduction at a gas diffusion
electrode modified with a bis-(Cu(phenanthrolinone)₂) complex. In KHCO₃, the complex was shown to produce three main products (H₂, C₂H₄ and CH₄) at high current density at an applied potential varying between -1.0 V to -1.6 V vs RHE.²⁴ More recently, Liu and co-workers used crystalline indazole-based tricopper systems deposited on a gas diffusion layer electrode to perform electroreduction of CO₂ in acidic and basic aqueous media. At low overpotential (-0.60 V vs RHE), a FEₐouth of approximately 50 % was obtained together with dihydrogen (FEH₂ ~ 10%) and other C₁ and C₂ products.³⁹ Finally, a NHC-based cluster bearing 17 copper atoms deposited on carbon paper was shown to be highly stable in KOH while producing essentially CO (FEₐouth ~ 90%) at -0.4 V vs RHE, and interestingly ethylene (37%) and CO (33%) at -1.1 V.⁴⁰ To recall and for comparison, the GDE-1 electrode herein reported displays very high selectivity (FEₐouth = 100%) at low potential values (-0.60 V vs RHE), making it as one of the best molecular Cu electrocatalysts for eCO₂RR so far.

4. CONCLUSIONS

In summary, copper complexes bearing either quaterpyridine-based L or QTPy ligands have been synthesized, characterized and their eCO₂RR catalytic properties have been investigated. Solid-state and solution studies demonstrate that copper(II) complexes are mononuclear whereas they form dimeric species at Cu(I) redox state. Heterogeneous CO₂ electroreduction in KHCO₃ has been carried out thanks to the non-covalent immobilization of the complexes onto MWCNTs-based glassy carbon electrodes. Strikingly, electrocatalysis at -0.55 V vs RHE with complex 1 is more efficient than with complex 2, since CO is produced with high selectivity (75 % FE) without interference of HER (3 % FE). This highlights the significant impact of the ligand topology on the catalytic properties. Noteworthy, the immobilized mononuclear complex 1 remains unchanged with electrolysis. Such property is rarely reported
for copper-based heterogeneous molecular systems, since they lead for most of the time to copper nanoparticles or copper oxide deposition upon CPE. Incorporation of complex **1** into a gas diffusion layer for a zero-gap electrolyser showed a strong enhancement of current density, reaching 4 mA.cm⁻² at -0.60 V vs RHE, as well as product selectivity, since CO was almost the unique product generated with FE > 90 % over 4 hours. For comparison, complex **2** was shown to produces essentially dihydrogen with such an electrolyser configuration, thus emphasizing the crucial role of the ligand on the electrocatalytic performances. Possibly, the enhanced properties of complex **1** compared to complex **2** arise from the geometric constraints imposed by ligand **L** as well as its better interactions with MWCNTs sidewalls. In conclusion, this work lays the foundation for future research aimed at enhancing both the performance and stability of copper-based molecular catalysts for electrochemical CO₂ reduction with the aim of achieving relevant operational conditions for real applications.

Future works will be devoted to the synthesis of new complexes in this family of tetpyridines with the main objective to improve current densities and better decipher by theoretical calculations the effect of the ligand topology on the electrocatalytic performances.

## ASSOCIATED CONTENT

**Supporting Information.** The Supporting Information is available free of charge on the ACS Publications website.

Materials and methods, detailed synthesis, spectroscopic characterization (NMR, UV-Vis), mass spectrometry analysis, XRD diffraction data of complexes **1-3**, CV analysis, controlled-potential electrolysis curves of grafted complexes, XPS data, XAS data.

## AUTHOR INFORMATION

### Corresponding Authors
## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Funding Sources

The authors thank the University of Brest for PhD grant (M. S.) and the French National Research Agency (ANR) for funding (ANR-21-CE50-0034 and ANR-17-EURE-0003), the Labex ARCANE and the CDP-DefiCO2. The doctoral School EDS3M is thanked for providing a mobility grant for M. Saad. The XPS measurements have been performed on ASPHERYX platform (ScanMAT, UAR 2025, University of Rennes-CNRS, CPER MAT &Trans 2021-2027). B.S. and A.J.C. thank UKRI-EPSRC for support (EP/W038021/1 and EP/V011863/1).

## ACKNOWLEDGMENT

The authors would like to thank Cyril Colas from the "Fédération de Recherche" ICOA/CBM (FR2708) and the ICMG analytic platform (FR 2607) for XRD and HRMS analyses. François Michaud at UBO is also thanked for XRD analysis. N.L.P. acknowledges Paul-Alain Jaffrès and Mathieu Berchel at UBO for access to DLS measurements.

## ABBREVIATIONS

eCO₂RR: electrochemical carbon dioxide reduction reaction; CPE: controlled potential electrolysis; FE: Faradaic efficiency; TON: turnover number; TOF: turnover frequency; HER:

<footnote>*Email: nicolas.lepoul@univ-brest.fr</footnote>

<footnote>*Email: noemie.lalaoui@univ-grenoble-alpes.fr</footnote>

<footnote>*Email: A.J.Cowan@liverpool.ac.uk</footnote>
hydrogen evolution reaction; XAS: X-ray absorption spectroscopy; XRD: X-ray diffraction; XPS: X-ray photoelectron spectroscopy; CV: cyclic voltammetry; QTPy: quaterpyridine; MWCNTs: multi-walled carbon nanotubes.

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For Table of Contents only

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

✓ $FE_{CO} = 100\% @ -0.6 V$ vs RHE

<figure><img src="image_11.png" /></figure>

✓ High stability in $KHCO_3$

<figure><img src="image_12.png" /></figure>

✓ Molecular electrocatalysis