# Formate Production from Simulated Quasi-Flue Gas Combining a Molecular Catalyst and a Modified Electrode

Yutzil Segura-Ramirez, Maria Gomez-mingot, Marc Fontecave, Carlos M Sánchez-sánchez

<figure><img src="image_1.png" /></figure>

To cite this version:

Yutzil Segura-Ramirez, Maria Gomez-mingot, Marc Fontecave, Carlos M Sánchez-sánchez. Formate Production from Simulated Quasi-Flue Gas Combining a Molecular Catalyst and a Modified Electrode. ChemSusChem, 2025, 18 (14), pp.e202500392. 10.1002/cssc.202500392. hal-05412417

HAL Id: hal-05412417

https://hal.science/hal-05412417v1

Submitted on 12 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# Formate Production from Simulated Quasi-Flue Gas Combining a Molecular Catalyst and a Modified Electrode

Yutzil Segura-Ramirez, Maria Gomez-Mingot,* Marc Fontecave,* and Carlos M. Sánchez-Sánchez*

Molecular metal complexes form an important class of catalysts for the electroreduction of CO₂ (CO₂RR) to carbon monoxide (CO) or formic acid (HCOOH), key processes in the context of the requested exploration of novel sources of carbon, alternative to fossil fuels. Research studies are most generally carried out with pure gas streams of CO₂, while the available real sources of CO₂ are gases coming out from industrial plants and containing a low share of CO₂, and a great diversity of impurities including nitrogen and sulfur oxides. Herein, it is shown that a molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl (bpy = bipyridine,

Cp* = pentamethylcyclopentadienyl), catalyzes CO₂RR to formic acid using a quasi-flue gas (5–10% CO₂ and 100 ppm NO₂ or 50 ppm SO₂) with substantial selectivity. This is made possible thanks to the modification of the cathode surface with a positively charged imidazolium layer, which greatly favors CO₂RR over competing reactions, proton, NO₂, and SO₂ reductions. These results highlight the potential of combining molecular catalysis and electrode surface modification for the electroreduction of diluted CO₂ without prior carbon capture or purification.

## 1. Introduction

Carbon dioxide (CO₂) emissions contributing to the global greenhouse effect come from a variety of sources, cement and steel production, coal combustion plants, as well as waste incinerators, being some of the most relevant individual contributors.[1] Thus, addressing the conversion and recycling of the annual millions of tons of CO₂ expelled by those specific sources represents an effective strategy to limit CO₂ adverse effects. CO₂ electroreduction[2] (CO₂RR) is an environmentally friendly approach if fueled with low-carbon electricity. It implies complex multielectron and multiproton reactions usually accompanied by competitive hydrogen evolution reaction (HER) and thus necessitates selective and efficient catalysts.[3-9] As shown from reports based on data collected by the U. S. Environmental Protection Agency in 2022,[10] the majority of industrial plants (refineries, power,

metallurgy, and waste incineration plants) release flue gases with low CO₂ concentration (4–25% v/v). Furthermore, depending on the industrial process at work, the flue gas can contain water, oxygen, and pollutants such as particulate matter, amines, aromatic hydrocarbons, halocarbons, heavy metals, as well as nitrogen oxides (NOx), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), or hydrogen cyanide (HCN).[11,12] To date, most research on CO₂RR is conducted in ideal laboratory conditions, using 100% pure CO₂ gas as a reactant into electrolyzers, to avoid any deleterious effect due to both low concentration of CO₂ and the presence of traces of impurities.[12-15] Initially, carbon capture and utilization technologies based on CO₂ purification through CO₂-capturing reactions using amines, which have been already industrially implemented,[16-19] were considered as a suitable strategy to overcome the limitations of working with diluted CO₂ streams. However, this strategy is still energy-intensive and expensive due to the multiple steps involved.[20,21] Thus, the direct utilization of diluted CO₂ from flue gas without capture and purification represents a potential alternative.[22-27]

Molecular catalysts for CO₂RR have concentrated a great deal of attention for direct CO₂ conversion since they reach a very high product selectivity and they can be finely tuned via synthetic modifications of the ligands,[28] as well as variations of the metal ions present within the molecular catalyst.[29-34] This can be achieved for example using molecular catalysts with CO₂ capturing ability, as illustrated by the work of Ishitani and collaborators.[25-27] They have modified the coordination sphere of a Re(I) complex by adding a deprotonated triethanolamine as a ligand and this catalytic complex showed high selectivity for CO production at low concentrations of CO₂ such as 10% and even 1%, with Faradaic efficiencies for CO of 95% and 85%, respectively.[25-27] We have recently proposed an alternative strategy to improve molecular catalysts' performance for CO₂RR by tuning the local electric field at the electrode/electrolyte interface either using ionic liquids as

Y. Segura-Ramirez, M. Gomez-Mingot, M. Fontecave

Laboratoire de Chimie des Processus Biologiques

Collège de France

UMR 8229 CNRS

Sorbonne Université

PSL Research University

11 Place Marcelin Berthelot, 75005 Paris, France

E-mail: maria.gomez@college-de-france.fr

marc fontecave@college-de-france.fr

Y. Segura-Ramirez, C. M. Sánchez-Sánchez

Sorbonne Université

CNRS, Laboratoire Interfaces et Systèmes Electrochimiques (LISE)

4 Place Jussieu, 75005 Paris, France

E-mail: carlos.sanchez@upmc.fr

<figure><img src="image_4.png" /></figure>

Supporting information for this article is available on the WWW under https://doi.org/10.1002/cssc.202500392

<figure><img src="image_5.png" /></figure>

© 2025 The Author(s). ChemSusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_6.png" /></figure>

electrolyte[35,36] or immobilizing charged organic molecules such as imidazolium on the electrode surface.[37] This latter approach based on imidazolium-modified electrodes was successfully scaled up operating at industrially relevant conditions (100 mA cm⁻²) on a gas diffusion electrode in a flow cell type reactor. The effect of the imidazolium layer was specifically assigned to its capacity to interact with the soluble molecular catalyst, namely, the Rh complex ([Rh(bpy)(Cp*)Cl]Cl; bpy = bipyridine, Cp* = pentamethylcyclopentadienyl),[38] via π⁺-π interactions between the imidazolium cation and the reduced bipyridine ligand, which resulted in CO₂RR overpotential decrease and partial HER suppression, as supported by density functional theory (DFT) calculations.[37] This complex is an excellent model molecular catalyst for studying the competition between CO₂RR to formic acid and HER, as it is active for both reactions.[38-40] It was thus tempting to push further the electrode surface modification approach to evaluate its potential for maintaining CO₂RR selectivity even converting diluted CO₂ streams, as this approach was very little explored so far. For example, a recent report exploited an organic hydrophobic polymer (poly(4-vinylpyridine)) for modifying carbon black nanoparticles deposited into a gas diffusion layer, used as the electrode material, and showed enhanced CO generation (Faradaic efficiency 90%) by electrolysis of dilute CO₂ (10% v/v) using a Co complex as a catalyst.[41] Here, we actually show that the imidazolium-modified electrode represents a solution for keeping the Rh model complex substantially selective and exhibiting a high energy efficiency for CO₂ conversion to formic acid versus HER even with gas sources containing low concentrations of CO₂ (10% v/v).

Furthermore, using a more complex CO₂ stream corresponding to an incinerator simulated quasi-flue gas, which contains diluted CO₂ [1–10% mol] and impurities traces of SO₂ [0–50 ppm] and NO₂ [0–100 ppm],[12,19] we show that the modified electrode also limits the inhibition effect of NO₂ and SO₂ impurities during CO₂RR. Here, we focus exclusively on NO₂ and SO₂ contaminants and do not include O₂, as only a few examples of molecular catalysts based on Fe porphyrins or Co phthalocyanine have been shown, so far, to be capable to reduce CO₂ in the presence of O₂.[42,43] While some studies have analyzed the effect of traces of typical acidic impurities present in industrial incinerator flue gas, such as SO₂ and NO₂, on CO₂RR catalyzed by solid electrocatalysts under 1 atm of CO₂,[44,45] there is no comparable study using molecular catalysts. Such contaminants are likely to deteriorate the CO₂RR performance since they might be prone to bind to and react with molecular catalysts[46] and some of them are easier to reduce, at redox potentials required for CO₂RR.[47] We discuss here possible reasons for the limited effects of NO₂ and SO₂ impurities based on the role of the local electric field induced by imidazolium-modified electrodes.

## 2. Results

Figure 1 shows the electrochemical characterization of the Rh complex by cyclic voltammetry (CV) on bare glassy carbon (GC) electrode (GCE) at a concentration of 2 mM in an acetonitrile

<figure><img src="image_7.png" /><figcaption>Figure 1. Cyclic voltammogram on a bare GC electrode of 2 mM the Rh complex and 0.5 M [TBA][PF₆] either in CH₃CN solution under argon (black plot) or in CH₃CN solution containing 5% v/v H₂O as a proton source under 100% CO₂ (red plot), 10% v/v CO₂ in a N₂ matrix (blue plot), and 5% v/v CO₂ in a N₂ matrix (green plot). Inset: plot of catalytic current enhancement ($J_{cat}/J_p$) versus CO₂ concentration. Scan rate: 0.01 V s⁻¹.</figcaption></figure>

solution containing 0.1 M [TBA][PF₆] as a supporting electrolyte, under inert conditions (argon-saturated electrolyte—black CV), as well as in an acetonitrile solution with 5% v/v of H₂O as a proton source in the presence of either pure (100% CO₂—red CV) or diluted CO₂ in a N₂ matrix (5% and 10% v/v—green and blue CV, respectively).

The Rh complex represents a model molecular catalyst with a well-established mechanism previously described in the literature.[36,38] The Rh complex under Ar (black plot in Figure 1) displays a reversible reduction signal at −1.19 V versus Fc⁺/Fc, corresponding to the two-electron reduction of Rh(III) to Rh(I) coupled with the release of the chloro ligand and a second reduction wave corresponding to the one-electron reduction of the bipyridine ligand,[36,38] which shifts from $E_{1/2} = -2.53$ V versus Fc⁺/Fc in the absence of CO₂ and proton source to $E_p = -2.07$ V versus Fc⁺/Fc under catalytic conditions (in the presence of pure CO₂% and 5% v/v H₂O, red plot in Figure 1). This catalytic reduction wave is assigned to CO₂RR and HER, as confirmed below via bulk electrolysis and in agreement with previous reports.[38-40] In particular, the shoulder at $E_p = -2.20$ V versus Fc⁺/Fc is assigned to HER and the more anodic peak centered at −2.07 V to CO₂RR. Figure 1 shows that upon dilution of CO₂ (blue and green plots), the intensity of the CO₂RR catalytic wave decreases: the $J_{cat}/J_p$ ratio, with $J_{cat}$ being the maximum catalytic current density and $J_p$ the peak current density under Ar, decreases from $J_{cat}/J_p = 13$ for 100% CO₂ to 4 for 10% CO₂ and 2 for 5% CO₂. Interestingly, $J_{cat}/J_p$ is not a linear function of CO₂ concentration and catalysis remains significant even at 5% CO₂.

Figure 2 shows the electrochemical characterization of the Rh complex by CV under diluted CO₂ (10% v/v) catalytic conditions on both a modified GCE (red solid plot) and a bare GCE (black solid plot), as well as under pure CO₂ on the modified GCE (red dashed plot). In particular, the 1-ethyl-3-methylimidazolium cation ([EMIM]⁺) was immobilized on the GCE following the previously described electrochemical procedure.[37] The modified
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 2.** CV of 2 mM Rh complex and 0.5 M [TBA][PF₆] in CH₃CN solution containing 5% v/v H₂O as a proton source under 100% v/v CO₂ (dashed line) or 10% v/v CO₂ in a N₂ matrix (solid line) on bare GC electrode (black plot) and IM⁺EE⁻-modified electrode (red plots). Scan rate: 0.01 V s⁻¹.</figcaption></figure>

GCE is labeled as IM⁺EE⁻-modified electrode throughout this article. It contains a layer of positively charged imidazolium groups at the surface and is designed to enhance CO₂RR and inhibit proton reduction by the Rh complex. The CV in Figure 2 reveals a remarkable effect of the modified electrode on the catalytic performance of the Rh complex under diluted CO₂ (10% v/v), with a more intense and more anodic catalytic wave as compared to the catalytic wave on bare GCE. Indeed, a higher maximum catalytic current density (−3.3 mA cm⁻², red solid plot) is observed on IM⁺E⁻-modified electrode in comparison with bare GCE (−1.9 mA cm⁻², black solid plot), thus with a higher $J_{cat}/J_p$ ratio ($J_{cat}/J_p$ = 7). Remarkably, shifting from 100% to 10% CO₂ results in only a modest decrease of the current density achieved on IM⁺EE⁻-modified electrode, as shown by comparing red dashed and solid plots in Figure 2. Furthermore, using 10% CO₂, the catalytic onset is much more anodic (≈200 mV) on the IM⁺EE⁻-modified electrode than on bare GCE. Finally, the redox process associated with HER at $E_p$ −2.20 V versus Fc⁺/Fc is slightly suppressed on the IM⁺EE⁻-modified electrode (red solid plot in Figure 2) compared to the bare electrode (black solid plot in Figure 2). These features are observed in all conditions using diluted CO₂ as shown in Figure S1, Supporting Information (5% and 1% v/v CO₂), where larger catalytic current densities and lower overpotentials are obtained on the IM⁺EE⁻-modified electrode. As shown in Figure S2, Supporting Information, reduction current associated with CO₂RR in the absence of the Rh complex was only observed at very negative potentials, about 1 V more negative than in the presence of the Rh complex, on both bare and modified GCE, which demonstrates the key role of the Rh complex for CO₂RR catalysis.

## 2.1. Bulk Electrolysis on Bare GC Electrode of Diluted CO₂ in Batch Conditions

The catalytic performance of the Rh complex for CO₂RR on bare GCE under diluted CO₂ (10% v/v CO₂) was evaluated by bulk

electrolysis at constant current density (controlled current electrolysis [CCE] −1.2 mA cm⁻²) during 3.5 h. This current density corresponds to a lower absolute value than the peak current density (−1.9 mA cm⁻²) displayed in Figure 2 and ensures high selectivity toward CO₂RR during electrolysis. The outcome from CCE under 10% CO₂ was compared to the one obtained under 100% CO₂. In both cases, formate (HCOO⁻) was the sole product detected in the liquid phase by ionic chromatography analysis and hydrogen (H₂) the only gas product detected by gas chromatography. First, batch conditions were used with electrolytes only saturated with the corresponding CO₂ stream at the beginning of the electrolysis. **Table 1** summarizes the faradaic efficiencies obtained for HCOO⁻ (FEHCOO⁻) and H₂ (FEH₂) as a function of the concentration of the CO₂ gas stream used in the electrolysis. The mass balance of the electrolysis is efficiently closed and reaches a total FE (FEHCOO⁻ + FEH₂) near 90% in both cases.

**Figure 3a** shows the cathode potential response as a function of electrolysis time, which is compared to the thermodynamic potential of CO₂ to HCOOH under the studied experimental conditions. The cathode potential evolution under 100% CO₂ saturation (red plot) is perfectly stable all along CCE. In contrast, the cathode potential evolution under 10% v/v CO₂ saturation (blue plot) exhibits a more negative value, as well as a sudden decrease after 90 min of electrolysis followed by a second plateau at a more cathodic potential. In contrast, no cathode potential drop is observed when the 10% v/v CO₂ gas stream is continuously flowing in the electrolyte all along the electrolysis (black plot). Thus, the potential drop under batch conditions using diluted CO₂ is likely due to a shortage in CO₂ after 90 min electrolysis, not occurring with 100% CO₂, leading to the shifting in the main reaction catalyzed by the Rh complex from CO₂RR to HER in order to cope with the imposed current density. Figure 3b,C supports this interpretation. They report the time course of formate and H₂ production as a function of electrolysis time using 100% CO₂ or 10% v/v CO₂ under batch conditions, respectively. The production of formate was approximately linear with time all along the electrolysis in both cases (filled squares in Figure 3b,c, R² = 0.9931 and 0.9959, respectively) with a rate slightly lower at 10% CO₂ as compared to 100% CO₂. In contrast, the evolution of H₂ production (empty squares in Figure 3b,c) was different depending whether CO₂ was pure (100%) or diluted (10%). Figure 3b (100% CO₂) shows a quasi-linear time dependence of H₂ production (R² = 0.9849), whereas Figure 3c (10% CO₂) shows a greater acceleration of H₂ production after a low production during the first 100 min of electrolysis. Interestingly, the initial H₂ production rate

<table><caption>**Table 1.** CCE on a bare GCE at −1.2 mA cm⁻² of 2 mM the Rh complex and 0.5 M [TBA][PF₆] in CH₃CN solution containing 5% v/v H₂O and different CO₂ streams in a two compartments H-type cell under batch conditions. Total circulated charge 15 C. Solution stirring rate: 300 rpm.</caption><thead><tr><th>CO₂ v/v [%]</th><th>E<sub>cat</sub> average [V vs Fc<sup>+</sup>/Fc]</th><th>FE<sub>HCOO⁻</sub><sup>a)</sup> [%]</th><th>FE<sub>H₂</sub><sup>a)</sup> [%]</th></tr></thead><tbody><tr><td>100</td><td>−1.65</td><td>58 ± 4</td><td>29 ± 4</td></tr><tr><td>10</td><td>−2.19</td><td>48 ± 3</td><td>38 ± 4</td></tr></tbody></table>

<footnote>a) Faradaic efficiencies after a passed total charge of 15 C.</footnote>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** a) Bare GC cathode potential evolution during CCE in H-type cell at -1.2 mA cm⁻² of 2 mM of the Rh complex and 0.5 M [TBA][PF₆] in CH₃CN solution containing 5% v/v H₂O under different gas saturation conditions: saturated atmosphere of 100% CO₂ (red plot), 10% v/v CO₂ in a N₂ matrix (blue plot), or under continuous gas flow (14 mL min⁻¹) of 10% v/v CO₂ in a N₂ matrix (black plot). Solution stirring rate 300 rpm.
b) Production of H₂ (empty squares) and HCOO⁻ (filled squares) for saturated atmosphere of 100% CO₂ and c) saturated atmosphere of 10% CO₂ in a N₂ matrix. Linear regression lines are also included. H₂ and HCOO⁻ quantification samples are indicated by gray squares in graph (a).</figcaption></figure>

is lower at 10% CO₂ as compared to that at 100% CO₂, so that selectivity for CO₂RR, as probed by the formate/H₂ ratio, is larger using diluted CO₂ (formate/H₂ = 4) as compared to pure CO₂ (formate/H₂ = 2) during the first 100 min of electrolysis.

## 2.2. Bulk Electrolysis of Diluted CO₂ in Flow Conditions

The catalytic performance of the Rh complex for CO₂RR using a bare GC and an IM⁺EE⁻-modified electrode under 100% CO₂ and diluted CO₂ (10%, 5% and 1% v/v CO₂) was evaluated by bulk CCE during 76 min at -3.3 mA cm⁻² (Table 2). In all cases, the CO₂ stream was continuously flowed in the electrolyte all along the electrolysis, as schematically described in Figure 4a, to avoid any shortage in CO₂ substrate and subsequent drop in the cathode potential. Note that under flow configuration, the H₂ produced was not quantified and thus total FE ≤ 72% was reached in all cases since only formate production was reported. Nevertheless, it can be assumed that the missing circulated charge corresponded to H₂ production based on the previous results obtained under batch conditions and reported in Table 1, where formate and H₂ were the only detected products. Figure 4b nicely shows a much faster (3.5 fold) formate production rate using the IM⁺EE⁻-modified cathode (red plot) as compared to the bare GCE (black plot) during electrolysis of diluted 10% v/v CO₂, reflecting again the positive effect in CO₂RR of the modified electrode. Figure 4c compares the Faradaic efficiency values for formate (FEHCOO⁻) obtained at the end of the electrolysis using the IM⁺EE⁻-modified electrode (red bars) to those obtained with the bare GCE (black bars) when flowing different types of CO₂ streams (100%, 10%, 5%, and 1% v/v CO₂). As expected, a dramatic decrease of FEHCOO⁻ in both electrodes is observed as the result of decreased concentration of available CO₂, from a pure CO₂ to 1% CO₂ stream. Interestingly, this decrease is much more limited for the imidazolium-modified electrode. This is particularly visible using 10% CO₂ (red bars), where the FEHCOO⁻ remains constant at 66% as for 100% CO₂, as compared to 19% with GCE (black bars), and to a lesser extent at 5% CO₂. Thus, the imidazolium-modified electrode allows to keep an energy efficiency for formate production (EEHCOO⁻) close to 50% using a diluted 10% v/v CO₂ stream. Table 2 summarizes the results from all bulk CCE at -3.3 mA cm⁻² for both diluted CO₂ and simulated quasi-flue gas streams under flowing conditions, including the final formate concentration [HCOO⁻] and cathode potential (E), as well as cathodic overpotential (η), FEHCOO⁻ and EEHCOO⁻.

For the case of diluted 10% v/v CO₂ stream, all parameters reported in Table 2 reflect a higher performance with the IM⁺EE⁻-modified electrode than GCE. Besides larger formate selectivity, probed by FEHCOO⁻ increasing from 19% to 66%, already discussed above, we observe a large decrease of the cathode potential by more than 1 V (translated into a 1.05 V decrease of the cathodic overpotential η—see Figure S3b, Supporting Information) and thus a fivefold increase of the EEHCOO⁻. However, while the modified electrode system is still significantly superior to the bare electrode one using 5% CO₂ gas stream, this is not the case anymore for a too diluted CO₂ stream (1% CO₂). Figure S3, Supporting Information, shows not only a more anodic cathode potential during electrolysis with the IM⁺EE⁻-modified cathode (red plots) as compared to the bare GC cathode (black plots) but also a more stable potential behavior for all three diluted CO₂ streams. Moreover, additional blank electrolysis at -3.3 mA cm⁻² were performed in order to exclude the potential contribution in CO₂RR activity from Rh⁰ nanoparticles deposited on the GCE due to the decomposition of the Rh molecular catalyst
<figure><img src="image_12.png" /></figure>

<table><caption>**Table 2.** CCE at –3.3 mA cm⁻² of 2 mM the Rh complex and 0.5 M [TBA][PF₆] in CH₃CN solution containing 5% v/v H₂O in a two compartments H-type cell under continuous purging flow (14 mL min⁻¹) of different CO₂ streams without or with impurities NO₂ [100–400 ppm] or SO₂ [50–200 ppm] in bare GC electrode or IM⁺EE-modified electrode. Two control experiments with no Rh complex are also included. Total circulated charge 15 C. Solution stirring rate: 300 rpm.</caption><thead><tr><th>CO<sub>2</sub><br>v/v [%]</th><th>Gas impurity<br>[ppm]</th><th>IM<sup>+</sup><sub>EE</sub>-modified<br>electrode</th><th>[HCOO<sup>-</sup>]<br>[µmol/L]<sup>a)</sup></th><th>η [V]<sup>b)</sup></th><th>E [V] versus<br>Fc<sup>+</sup>/Fc<sup>c)</sup></th><th>FE<sub>HCOO<sup>-</sup></sub><br>[%]<sup>d)</sup></th><th>EE<sub>HCOO<sup>-</sup></sub><br>[%]<sup>e)</sup></th></tr></thead><tbody><tr><td>100<sup>f)</sup></td><td>–</td><td>No</td><td>587 ± 163</td><td>2.00</td><td>–3.32</td><td>14 ± 4</td><td>6 ± 1</td></tr><tr><td>100<sup>f)</sup></td><td>–</td><td>Yes</td><td>446 ± 54</td><td>1.98</td><td>–3.30</td><td>11 ± 1</td><td>4 ± 1</td></tr><tr><td>100</td><td>–</td><td>No</td><td>2844 ± 356</td><td>0.44</td><td>–1.76</td><td>72 ± 9</td><td>54 ± 7</td></tr><tr><td>100</td><td>–</td><td>Yes</td><td>2608 ± 11</td><td>0.44</td><td>–1.76</td><td>66 ± 1</td><td>50 ± 1</td></tr><tr><td>10</td><td>–</td><td>No</td><td>756 ± 17</td><td>1.75</td><td>–3.07</td><td>19 ± 1</td><td>8 ± 1</td></tr><tr><td>10</td><td>–</td><td>Yes</td><td>2599 ± 252</td><td>0.70</td><td>–2.02</td><td>66 ± 6</td><td>43 ± 4</td></tr><tr><td>5</td><td>–</td><td>No</td><td>401 ± 14</td><td>1.50</td><td>–2.82</td><td>10 ± 1</td><td>4 ± 1</td></tr><tr><td>5</td><td>–</td><td>Yes</td><td>796 ± 45</td><td>0.79</td><td>–2.11</td><td>20 ± 1</td><td>13 ± 1</td></tr><tr><td>1</td><td>–</td><td>No</td><td>214 ± 16</td><td>1.10</td><td>–2.42</td><td>5 ± 1</td><td>3 ± 1</td></tr><tr><td>1</td><td>–</td><td>Yes</td><td>185 ± 10</td><td>0.79</td><td>–2.11</td><td>5 ± 1</td><td>3 ± 1</td></tr><tr><td>10</td><td>NO<sub>2</sub>, 100</td><td>No</td><td>460 ± 51</td><td>1.86</td><td>–3.18</td><td>12 ± 1</td><td>5 ± 1</td></tr><tr><td>10</td><td>NO<sub>2</sub>, 100</td><td>Yes</td><td>1125 ± 127</td><td>0.90</td><td>–2.22</td><td>29 ± 4</td><td>17 ± 2</td></tr><tr><td>10</td><td>NO<sub>2</sub>, 200</td><td>Yes</td><td>1432 ± 105</td><td>0.80</td><td>–2.11</td><td>35 ± 5</td><td>22 ± 3</td></tr><tr><td>10</td><td>NO<sub>2</sub>, 400</td><td>Yes</td><td>821 ± 171</td><td>0.82</td><td>–2.14</td><td>21 ± 5</td><td>13 ± 3</td></tr><tr><td>10</td><td>SO<sub>2</sub>, 50</td><td>No</td><td>560 ± 177</td><td>1.95</td><td>–3.27</td><td>14 ± 4</td><td>6 ± 2</td></tr><tr><td>10</td><td>SO<sub>2</sub>, 50</td><td>Yes</td><td>1317 ± 252</td><td>0.89</td><td>–2.21</td><td>33 ± 7</td><td>18 ± 2</td></tr><tr><td>10</td><td>SO<sub>2</sub>, 100</td><td>Yes</td><td>1692 ± 152</td><td>1.03</td><td>–2.32</td><td>42 ± 5</td><td>20 ± 3</td></tr><tr><td>10</td><td>SO<sub>2</sub>, 200</td><td>Yes</td><td>1208 ± 88</td><td>0.80</td><td>–2.11</td><td>31 ± 2</td><td>13 ± 3</td></tr></tbody></table>

a) Total formate concentration at the end of the CCE (total volume of electrolyte 12.5 mL). b) Cathodic overpotential calculated at final electrolysis time. c) Cathodic potential at the end of the CCE. d) Faradaic efficiency for formate (FE<sub>HCOO<sup>-</sup></sub>) after a total circulated charge of 15 C. e) Energy efficiency for formate (EE<sub>HCOO<sup>-</sup></sub>) after a total circulated charge of 15 C. f) Control CCEs performed in the absence of the Rh catalyst.

during electrolysis. Tables S1–S2, Supporting Information, show the results of two consecutive electrolysis performed on either the same bare GCE or the IM⁺EE-modified electrode, respectively. On the first electrolysis under catalytic conditions, formate is the main product on both electrodes. In contrast, on the second electrolysis, using the same electrode after a rinsing step, but in the absence of the Rh complex in solution, a minor production of formate and FE<sub>H2</sub> larger than 80% are detected. This excludes deposited Rh nanoparticles as the catalytic species for CO₂ conversion to formate.

## 2.3. Bulk Electrolysis of Simulated Quasi-Flue Gas in Flow Conditions

Next, we explore the effect on CO₂RR activity and selectivity of the presence of gas impurities, specifically NO₂ and SO₂ in a diluted CO₂ stream (10% v/v CO₂ in N₂ matrix), and the possible advantage of using the IM⁺EE-modified cathode. Thus, these impurities were introduced separately in the diluted CO₂ gas stream at trace concentrations simulating those found in industrial incinerator flue gas[12,19] (SO₂ at 50 ppm and NO₂ at 100 ppm), as schematically described in Figure 4a. Figure S4, Supporting Information, reports the cathode potential versus time evolution during CCE at –3.3 mA cm⁻² comparing the bare GCE (black plots) and the IM⁺EE-modified electrode (red plots) in a diluted 10% CO₂ stream without (solid line) and with NO₂ (dashed line) or SO₂ (dotted line) during 76 min of electrolysis. It shows almost no effect of impurities

on cathode potential, which allows keeping a significant overpotential decrease on IM⁺EE-modified electrode versus GCE of 0.96 V and 1.06 V (see Table 2) in the presence of NO₂ and SO₂, respectively. Table 2 demonstrates the deleterious effect of both NO₂ and SO₂, even at such low concentrations, on formate production on both electrodes. Nevertheless, Table 2 also shows the positive effect of the modified electrode regarding the concentration of formate at the end of CCE, FE<sub>HCOO<sup>-</sup></sub> and EE<sub>HCOO<sup>-</sup></sub> values, which are more than twice greater working with the IM⁺EE-modified cathode as compared to the bare GCE: compare FE<sub>HCOO<sup>-</sup></sub> = 29% (NO₂) and 33% (SO₂) on the IM⁺EE-modified cathode to FE<sub>HCOO<sup>-</sup></sub> = 12% (NO₂) and 14% (SO₂) on the bare GCE. Thus, CO₂RR occurs at more anodic potentials and is more selective to formate production on the IM⁺EE-modified electrode, even in the presence of either SO₂ or NO₂ impurities. Figure 5 shows the effect of increasing the concentration of those impurities, either NO₂ [100–400 ppm] or SO₂ [50–200 ppm], in the diluted CO₂ stream (10% v/v CO₂ in N₂ matrix). A very limited impact on FE<sub>HCOO<sup>-</sup></sub> is observed in both cases, exhibiting a minor decrease from 29% to 20% in the presence of 400 ppm NO₂ and from 33% to 30% in the presence of 200 ppm SO₂. Figure S5, Supporting Information, reports the cathode potential versus time evolution during those electrolysis on the IM⁺EE-modified electrode. Figure S6, Supporting Information, displays the linear sweep voltammetry profiles corresponding to the Rh complex in solution together with H₂O as proton source, 5% v/v NO₂, 5% v/v SO₂, or 5% v/v CO₂ on bare and modified GCE to evaluate its affinity to react with each of those compounds.
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 4.** a) Schematic illustration of the bulk electrolysis setup in a two compartments H-type cell under flow conditions for diluted CO₂ and simulated quasi-flue gas experiments, highlighting the placement of a gas humidifier at the gas inlet and a gas trap at the gas outlet, which ensures optimal gas hydration and capture of exhaust gases. The outlet of the gas trap is open to the atmosphere to prevent overpressure due to continuous gas flow during CCE experiments. Mass flow controllers regulate the flow of pure gases and custom-made bottles containing CO₂, SO₂, and NO₂ (each diluted in N₂), allowing precise gas mixture preparation for studies. b) Formate production during CCE at −3.3 mA cm⁻² of 2 mM the Rh complex and 0.5 M [TBA][PF₆] in CH₃CN solution containing 5% v/v H₂O and under a continuous gas flow of 14 mL min⁻¹ of 10% v/v CO₂ in a N₂ matrix on the bare GC electrode (black plot) and the IM⁺EE⁻-modified electrode (red plot). c) FE₃HCOO⁻ (bars) and EE₃HCOO⁻ (symbols) after 15 C of circulated charge from different CCE at −3.3 mA cm⁻² on bare GC electrode (black) and IM⁺EE⁻-modified electrode (red) as a function of the type of dilution of CO₂ in a N₂ matrix (1% v/v CO₂, 5% v/v CO₂, 10% v/v CO₂, or 100% v/v CO₂).</figcaption></figure>

### 3. Discussion

Achieving CO₂RR using a gas stream released from industrial plants directly, without CO₂ purification, is a great challenge. This is largely related to the competing reactions occurring specifically when the CO₂ substrate accounts for a small part of the gas, making CO₂RR more kinetically unfavorable. Among these competing reactions, HER is the most prominent one, but catalytic reduction of gas impurities such as NO₂ and SO₂ by molecular complexes can also deteriorate, even at the low concentration found in some industrial flue gases, the CO₂RR performance during electrolysis because of the similar redox potentials required for CO₂, NO₂, and SO₂ reductions.[44,47] Our results provide an original illustration that not only a molecular catalyst, such as the model Rh complex used, allows significant CO₂RR activity when

CO₂ accounts only for 5% and 10% of the gas mixture, in line with rare previous examples of other molecular catalysts converting diluted CO₂ streams,[25-27] but, more importantly, that this is possible thanks to a molecular modification on the GCE. Here, the electrode is modified with a positively charged imidazolium layer providing a suitable local environment, which limits HER contribution and enhances CO₂RR performance when the gas stream contains little CO₂ (5% and 10%), as well as redox active impurities (NO₂ or SO₂), an observation with no precedent in molecular catalysis literature.

As previously established experimentally and computationally by DFT calculations in homogeneous systems,[36] we propose that the effect of the imidazolium-based modified electrode is due to the ability of the Rh complex to develop π-π interactions with the imidazolium group on the electrode surface, even
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 5.** FE<sub>HCOO</sub><sup>-</sup> after 15 C of circulated charge from different CCE at -3.3 mA cm<sup>-2</sup> of 2 mM Rh complex and 0.5 M [TBA][PF<sub>6</sub>] in CH<sub>3</sub>CN solution containing 5% v/v H<sub>2</sub>O on IM<sup>+</sup><sub>EE</sub>-modified electrode as a function of the amount of either NO<sub>2</sub> (brown squares) or SO<sub>2</sub> (green squares) present in diluted 10% v/v CO<sub>2</sub> in N<sub>2</sub> matrix (continuous purging flow of 14 mL min<sup>-1</sup>).</figcaption></figure>

though such interactions might be weaker in the case of immo-
bilized imidazolium as compared to imidazolium in solution. First,
this would indeed make redox potentials of the catalyst more
positives and facilitate its reduction to generate the catalytically
active intermediate, which explains the observed decrease in
cathodic overpotential, as well as in full cell potential. Second,
it contributes to inhibit HER via electrostatic repulsion between
the positively charged imidazolium and free protons and via
increasing the free-energy barrier of the HER pathway, which
explains the enhanced selectivity observed for formate produc-
tion. Third, it would help accumulating the active form of the
molecular catalyst at the surface of the electrode, explaining
the increased current density achieved. Such a concentration
effect would partly compensate for the reduction of CO₂ concen-
tration available under diluted gas streams. Finally, in agreement
with previous reports describing strong interactions between CO₂
and imidazolium[⁴⁸,⁴⁹] the surface modification might also contrib-
ute to concentrate CO₂ at the electrode interface. All these effects
are strong enough to offset the low CO₂RR kinetics associated
with the low concentration of CO₂ in the gas stream.

In conclusion, there is a clear negative effect of adding traces
of NO₂ (100–400 ppm) and SO₂ (50–200 ppm) on CO₂RR catalyzed
by the Rh complex. Our working hypothesis for this behavior is
that the same Rh-hydride active catalytic intermediate might
react with NO₂ and SO₂, in competition with CO₂. Figure S6,
Supporting Information, shows a comparative study under iden-
tical concentration (5% v/v in N₂) of individually added NO₂, SO₂,
and CO₂ in the presence of the Rh complex and either on bare
GCE or IM⁺EE⁻-modified electrode. Figure S6a, Supporting
Information, on bare GCE exhibits a nonzero reduction current
associated with the presence of both NO₂ and SO₂ within the
same potential window where the Rh complex catalyzes
CO₂RR (E < -2.0 V vs Fc⁺/Fc), being CO₂ reduction the main con-
tribution up to -2.25 V, from where the reaction associated to

NO₂ becomes the most relevant current contributor. In contrast,
Figure S6b, Supporting Information, on IM⁺EE⁻-modified electrode
exhibits CO₂ reduction as the main current contribution up to
-2.40 V, despite nonzero reduction current associated with the
presence of both NO₂ and SO₂ is also displayed within the same
potential window. These voltammograms indicate that both NO₂
and SO₂ present some affinity for the Rh complex and compete
for its active form. Notably, the IM⁺EE⁻-modified electrode exhibits,
in addition to the selective enhancement of CO₂RR in comparison
with bare GCE, a moderate inhibition effect for the reaction asso-
ciated to the NO₂ presence, because the catalytic current in the
presence of NO₂ is half of that exhibited on bare GCE at -2.40 V.
However, further studies are required for reaching a deep mech-
anistic understanding of all those associated processes, which are
out of the scope of the present work. Interestingly, the modified
electrode surface provides a significant decrease in cathodic
overpotential and helps maintaining a reasonable selectivity
for formate production, allowing a FE<sub>HCOO</sub><sup>-</sup> value more than two-
fold larger as compared to a bare electrode (Table 2). Thus, for
example with challenging quasi-flue gas streams containing
10% CO₂ and 50 ppm SO₂ in a N₂ matrix, a FE<sub>HCOO</sub><sup>-</sup> value of
33% and EE<sub>HCOO</sub><sup>-</sup> of 20% were obtained. As the matter of fact,
the negative effect of adding traces of NO₂ and SO₂ in the diluted
CO₂ stream seems to be concentration independent below
400 ppm. Furthermore, the enhanced CO₂RR performance exhib-
ited on the modified electrode in the presence of NO₂ and SO₂
impurities in comparison with bare GCE might be linked to the
different dipolar moments displayed by all three competing reac-
tants and its orientation within the electrode/electrolyte interface
due to the local electric field effect: CO₂ (linear molecule, zero
dipolar moment, insensitive to electric field effect) and NO₂
and SO₂ impurities (bent molecules, nonzero dipolar moment,
sensitive to electric field effect). Thus, the modified electrode,
thanks to the local electrostatic effect induced by the positively
charged imidazolium, is likely to impact on the molecular orien-
tation of NO₂ and SO₂, which might reduce the probability to
reach a strong interaction with the Rh complex and thus, difficult
the competitive NO₂ and SO₂ reduction reactions.

**4. Conclusion**

In conclusion, our results demonstrate the potential of appropri-
ate surface modifications on noncatalytic GC electrodes for
achieving CO₂RR catalyzed by molecular catalysts using simu-
lated gas streams from industrial plants, containing low share
of CO₂ and deleterious gaseous impurities. This is illustrated here
with a model molecular Rh complex allowing high selectivity for
formate production from gas mixtures containing 5–10% CO₂ and
traces of NO₂ and SO₂, thanks to the presence of a positively
charged imidazolium layer on the cathode of the electrolyzer.
These results point out surface modification of the electrode
as an interesting approach combined with molecular electroca-
talysts to treat direct industrial CO₂ streams and overcome the
expenses associated with CO₂ capture, separation, and subse-
quent purification.
<figure><img src="image_17.png" /></figure>

Finally, the choice of a water soluble molecular catalyst as the
Rh complex, together with applying a high constant current dur-
ing electrolysis, properly places the results of the present work as
the initial step toward large-scale applications. Further work
devoted to evaluate the feasibility of formate production from
quasi-flue gas in acidic aqueous solution in a flow cell-type reac-
tor is required for that purpose.

**5. Experimental Section**

Reactants

Anhydrous acetonitrile of 99.99% purity (CH₃CN), tetrabutyl
ammonium hexafluorophosphate ([TBA][PF₆], >99%) 2,2'-Bipyridyl
of ≥99% purity, and ferrocene (98%) were purchased from Sigma-
Aldrich. The Rhodium complex precursor dichloro(pentamethylcyclo-
pentadienyl) rhodium(III) dimer [Rh(Cp*)Cl₂]₂ of 99% purity was
purchased from Strem Chemicals or from Merck (Chiralyst P618).
The ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate
([EMIM][PF₆]) (99%) was acquired from Iol-Li-Tec (Germany). All reac-
tants were utilized without any additional purification. Ultrapure water
(18.2 MΩ cm, Millipore) was used to conduct experiments.

Synthesis of Rhodium Complex

The described synthesis of [Rh(bpy)(Cp\*)Cl]Cl was previously adapted within the research team, from protocols outlined in the existing literature.[50] An ethanol solution (30 mL) of 1 equivalent [Rh(Cp\*)Cl\_2]\_2 (200 mg, 0.32 mmol) and 2 equivalents 2,2'-bipyridine (120 mg, 0.76 mmol) was stirred at room temperature for 2 h in the dark. Evaporation to dryness of the resultant solution, clear and orange-yellow in appearance, was performed next. The yellow solid was dissolved in a minimal quantity of acetonitrile (CH₃CN) and subsequently precipitated by adding ethyl acetate (AcOEt). The precipitate was then collected on a Buchner funnel and dried under vacuum. The dried powder purity was verified by ¹H NMR spectroscopy according to the literature. The ¹H NMR spectrum of [Rh(bpy)(Cp\*)Cl] is shown in Figure S7, Supporting Information (300 MHz, CD₃CN): δ/ppm, 1.66 (s, 15H), 7.88 (dt, *J* = 6.5 Hz, 2H), 8.34 (dt, *J* = 7.9 Hz, 2H), 8.70 (d, *J* = 7.9 Hz, 2H), 8.99 (d, *J* = 5.4 Hz, 2H).

Electrochemical Studies

CV and chronopotentiometry experiments were performed on Biologic Science Instruments SAS potentiostats (SP-300 or VSP-300). All electrochemical trials were conducted at room temperature (20 ± 2 °C) in acetonitrile (CH3CN) with 5% vol. H2O content. [TBA][PF6] was used as a supporting electrolyte in solution (0.5 M). Pure gases Ar (>99.99%), N2 (>99.99%), and CO2 (>99.99%) used to saturate solutions were purchased from Air Liquide. Bottles containing the diluted gases: 10% v/v CO2 in a N2 matrix, 5% v/v NO2 in a N2 matrix, and 5% v/v SO2 in a N2 matrix were custom made by Air Liquide. The electrochemical studies under 10% v/v CO2 in N2 matrix atmosphere were conducted using the custom-made gas bottle from Air Liquide to saturate the solution. For lower CO2 concentrations (5% v/v and 1% v/v CO2 in a N2 matrix), the gas mixtures used to purge the solutions were prepared by properly mixing pure CO2 and N2 gases at different ratios using mass flow controllers (Bronkhorst, EL-FLOW Prestige, model FG-201CV). Gas flow rates were controlled online using FlowSuite 2 software. The preparation of the simulated quasi-flue gas was done by simultaneously mixing pure CO2 and N2 gases, as well as SO2 and NO2 (Bronkhorst's mass flow controllers, EL-FLOW Prestige, model FG-200CV) from the custom-made bottles containing 5% v/v SO2% and 5% v/v NO2 in a N2 matrix, respectively.

A three-electrode setup was used to carry out CV experiments, using a GC disc (diameter = 3 mm, area = 0.07 cm², BioLogic) as working electrode (WE), a platinum wire (diameter = 0.5 mm, Alfa Aesar, 99.5% purity) as a counter electrode (CE), and an Ag/AgCl/KClsat reference electrode (RE). The WE was polished on a polishing cloth (DP-Nap 200 mm, Struers) on a 1 µm diamond suspension (Struers), followed by a 10 s sonication in water and subsequent drying before initiating experiments. The CE was previously flame annealed and the RE was separated from the solution in all cases using a salt bridge. All potentials were calibrated at the end of each experiment by adding the ferrocenium/ferrocene (Fc⁺/Fc) redox couple as an internal standard, since they were carried out in CH₃CN. CVs were run at 0.01 V s⁻¹ scan rate and only the third steady-state cycle of all CVs is shown. E₁/₂ potential (V) corresponds to the half wave potential from CV. Eₚ potential (V) corresponds to the potential at the peak current density. Catalytic response from CV was determined by calculating the current density ratio (Jₕₐt/Jₚ), where (Jₕₐt) corresponds to the catalytic current density in the presence of CO₂ and H₂O; meanwhile, (Jₚ) corresponds to the reduction peak current density under inert conditions (Ar).

Modified Electrode Synthesis by Imidazolium Immobilization
on GCEs (IM⁺EE-Modified Electrode)

The grafting of imidazolium (IM⁺) was done by an electrochemical–
electrochemical (EE) two-step covalent method, optimized in our
prior study,[37] and gives a result an IM⁺EE-modified electrode. The
IM⁺ immobilization was carried using a three-electrode cell: a GC disc
or plate (Alfa Aesar) as a WE, where the grafting took place, a GC rod
as a CE, and an all-solid Ag/AgCl wire directly immersed in solution
was used as the pseudoreference electrode. The IM⁺ immobilization
occurs in a 0.5 M [EMIM][PF₆] in CH₃CN solution purged with argon by
using CV (scan rate 0.1 V s⁻¹) as the electrochemical deposition tech-
nique. It involves two reaction steps: (1) the electrochemical reduc-
tion of [EMIM]⁺ on the GC electrode by performing a forward sweep
from −1.45 to −3.95 V versus Fc⁺/Fc and (2) a subsequent electro-
chemical oxidation step in the reverse scan of CV at open-circuit
potential to form the positively charged imidazolium layer on the
GC electrode. After this, the modified electrode (IM⁺EE) was removed
from the cell, washed and sonicated 10 s in acetonitrile, and placed in
a new solution for further analysis and evaluation of its catalytic
properties.

CO₂ Conversion by Bulk Electrolysis

A gastight two-compartment electrochemical H-type glass cell with a glass frit separating anolyte (3 mL) and catholyte (9.5 mL) solutions was used in all electrolysis reported here. CCE was performed at different current density values in acetonitrile solution containing 5% v/v H₂O and 0.5 M of supporting electrolyte ([TBA][PF₆]) previously saturated by gas bubbling (100%~1% CO₂ or the different quasi-flue gas mixtures) in both anolyte and catholyte, during 5 and 30 min, respectively. The gas was humidified in a solution containing acetonitrile +5% v/v H₂O before entering the H-cell compartments. For experiment under static conditions, no continuous CO₂ gas was purged during the electrolysis; meanwhile, experiments under continuous flow conditions were purged in the catholyte with CO₂ gas, diluted gas, or quasi-flue gas mixture at a flow rate of 14 mL min⁻¹. 2 mM of the Rh complex was only added in the catholyte. The working electrode was a 1 cm² GC plate (1 mm thick, type 2, from Alfa Aesar), the counter electrode was a 5 cm² GC rod (Alfa Aesar), and a conventional Ag/AgCl/KClsat electrode separated from the solution by a salt bridge, which was calibrated with ferrocene as an internal redox reference, was used as a reference electrode. Ohmic losses in the cell were minimized by achieving the minimal distance between electrodes and keeping magnetic stirring during the electrolysis. In all
<figure><img src="image_18.png" /></figure>

cases, the current density was calculated using the electrode geometrical area. All CCE experiments were performed with two or three replicates to check results reproducibility.

## Diluted CO₂ Gas and Simulated Incinerator Quasi-Flue Gas

CCE experiments under diluted (10%–1% CO₂) or the different quasi-flue gas mixtures were performed by purging both anolyte and catholyte compartments. A flow rate of 14 mL min⁻¹ was used to purge the electrochemical cell under continuous flow conditions. Studied impurities concentration range was either (50–200) ppm of SO₂ or (100–400) ppm of NO₂ in 10% v/v CO₂ diluted in a N₂ matrix. An additional gas trap open to the atmosphere containing 0.1 M NaOH was added at the outlet of the H-cell compartments when studying the effects of impurities to avoid the release of these gases in the open atmosphere.

## Analytical Quantification of Products

Formate (HCOO⁻) was the sole product detected in the liquid phase by ionic chromatography analysis (Figure S8, Supporting Information) and hydrogen (H₂) was the only gas product detected by gas chromatography from CO₂RR bulk CCE. H₂ was quantified by gas chromatography (Model 8610C, SRI Instruments) equipped with a TCD detector and any other potential gas product was evaluated by the FID detector from 100 µL gas aliquots of the headspace of both compartments in the H type cell. The GC was periodically calibrated using a custom standard gas mixture in CO₂ (0–85 nmol H₂) (Figure S9a, Supporting Information). Formate was evaluated using an ionic exchange chromatograph (IC) (Metrohm 883 Basic IC), automatized by a 863 Compact IC autosampler and equipped with a Metrosep A Supp 5 column and a conductivity detector. A solution of 1 mM NaHCO₃ and 3.4 mM Na₂CO₃ was used as eluent with a flow of 0.7 mL min⁻¹. Calibration curve for HCOOH quantification (0–15 µM) was periodically checked before analysis (Figure S9b, Supporting Information). Formate samples for IC quantification were prepared by diluting 200 times in ultrapure water a sample of 50 µL from both the catholyte and anolyte solutions. 20 µL of the diluted sample were injected into the IC chromatograph.

Faraday efficiency (FE, %) of each reaction product is calculated from the ratio between the charge consumed to form each product and the total circulated charge.[51] A constant total circulated charge (15 C) was used in all electrolysis in order to reach a fair comparison among all CCE results. However, this total circulated charge is corrected to account for the initial three electrons consumed by the Rh complex (2 mM in solution) to generate its active form. Catalyst activation charge = [number of electrons × Faraday constant × mol of catalyst] = [3 × 96485 × 1.9 × 10⁻⁵] = 5.47 C and thus effective circulated charge is 9.5 C in all bulk electrolysis. The cathodic overpotential (η) was calculated from the difference between the cathode potential at final electrolysis time and E°CO₂/HCOOH (CH₃CN, H₂O) = -1.32 V versus Fc⁺/Fc in acetonitrile.[52] The cathodic half reaction energy efficiency (EE, %) was calculated for CO₂ conversion to formate reaction according to the following Equation (1).[53]

$$ \text{Cathodic energy efficiency, EE}(\%) = (E_T/E) \times \text{FE}_{\text{HCOO}^-} \quad (1) $$

where E_T is the thermodynamic potential in volts required for the electrocatalytic reduction of CO₂ to formate (E°CO₂/HCOOH (CH₃CN, H₂O) = -1.32 V vs Fc⁺/Fc), whereas E and FE_HCOO⁻ represent the experimental obtained cathode potential in volts (vs Fc⁺/Fc) and the formate Faradaic efficiency (%), respectively.

## Acknowledgements

This work was supported by the "Génie des Procédés" Ph.D. Program of Sorbonne Université. The authors acknowledge the support of the Centre National de la Recherche Scientifique (CNRS).

## Conflict of Interest

The authors declare no conflict of interest.

## Data Availability Statement

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

Keywords: CO₂ electroreduction · immobilized imidazolium · impurity tolerance · molecular electrochemistry · simulated flue gas

[1] T. Al-Attas, S. K. Nabil, A. S. Zeraati, H. S. Shiran, T. Alkayyali, M. Zargartalebi, T. Tran, N. N. Marei, Md. A. Al Bari, H. Lin, S. Roy, P. M. Ajayan, D. Sinton, G. Shimizu, M. G. Kibria, ACS Energy Lett. 2023, 8, 107.

[2] I. E. L. Stephens, K. Chan, A. Bagger, S. W. Boettcher, J. Bonin, E. Boutin, A. K. Buckley, R. Buonsanti, E. R. Cave, X. Chang, S. W. Chee, A. H. M. da Silva, P. de Luna, O. Einsle, B. Endrödi, M. Escudero-Escribano, J. V. F. de Araujo, M. C. Figueiredo, C. Hahn, K. U. Hansen, S. Haussener, S. Hunegnaw, Z. Huo, Y. J. Hwang, C. Janáky, B. S. Jayathilake, F. Jiao, Z. P. Jovanov, P. Karimi, M. T. M. Koper, et al., J. Phys. Energy 2022, 4, 042003.

[3] J. E. Huang, F. Li, A. Ozden, A. Sedighian Rasouli, F. P. García de Arquer, S. Liu, S. Zhang, M. Luo, X. Wang, Y. Lum, Y. Xu, K. Bertens, R. K. Miao, C.-T. Dinh, D. Sinton, E. H. Sargent, Science 2021, 372, 1074.

[4] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo, I. Chorkendorff, Chem. Rev. 2019, 119, 7610.

[5] Y. Li, M. Gomez-Mingot, T. Fogeron, M. Fontecave, Acc. Chem. Res. 2021, 54, 4250.

[6] A. Leal-Duaso, Y. Adjez, C. M. Sánchez-Sánchez, ChemElectroChem 2024, 11, e202300771.

[7] M. C. O. Monteiro, F. Dattila, N. López, M. T. M. Koper, J. Am. Chem. Soc. 2022, 144, 1589.

[8] C. M. Sánchez-Sánchez, J. Souza-Garcia, E. Herrero, A. Aldaz, J. Electroanal. Chem. 2012, 668, 51.

[9] E. Vichou, A. Perazio, Y. Adjez, M. Gomez-Mingot, M. W. Schreiber, C. M. Sánchez-Sánchez, M. Fontecave, Chem. Mater. 2023, 35, 7060.

[10] 2023 Greenhouse Gas Emissions from Large Facilities, EPA Facility Level GHG Emissions Data, 2023, https://ghgdata.epa.gov/ghgp (accessed: February 2025).

[11] G. V. Last, M. T. Schmick, Identification And Selection Of Major Carbon Dioxide Stream Compositions, Pacific Northwest National Laboratory, Richland, Washington 2011.

[12] I. Aouini, A. Ledouxha, L. Estel, S. Mary, P. Evrard, B. Valognes, Procedia Eng. 2012, 42, 704.

[13] G. V. Last, M. T. Schmick, Environ. Earth Sci. 2015, 74, 1189.

[14] U. Legrand, U.-P. Apfel, D. C. Boffito, J. R. Tavares, J. CO₂ Util. 2020, 42, 101315.

[15] Y. Cheng, P. Hou, X. Wang, P. Kang, Acc. Chem. Res. 2022, 55, 231.

[16] S. E. Jerng, B. M. Gallant, iScience 2022, 25, 104558.

[17] O. Gutiérrez-Sánchez, B. Bohlen, N. Daems, M. Bulut, D. Pant, T. Breugelmans, ChemElectroChem 2022, 9, e202101540.

[18] W. Gao, S. Liang, R. Wang, Q. Jiang, Y. Zhang, Q. Zheng, B. Xie, C. Y. Toe, X. Zhu, J. Wang, L. Huang, Y. Gao, Z. Wang, C. Jo, Q. Wang, L. Wang, Y. Liu, B. Louis, J. Scott, A.-C. Roger, R. Amal, H. He, S.-E. Park, Chem. Soc. Rev. 2020, 49, 8584.

[19] Q. Thabit, A. Nassour, M. Nelles, Appl. Sci. 2022, 12, 5236.

[20] S. Kar, M. Rahaman, V. Andrei, S. Bhattacharjee, S. Roy, E. Reisner, Joule 2023, 7, 1496.
<figure><img src="image_19.png" /></figure>

[21] J. Safipour, A. Z. Weber, A. T. Bell, ACS Energy Lett. **2023**, *8*, 5012.

[22] M. Bhattacharya, S. Sebghati, R. T. VanderLinden, C. T. Saouma, *J. Am. Chem. Soc.* **2020**, *142*, 17589.

[23] Q. Sun, Y. Zhao, W. Ren, C. Zhao, *Appl. Catal. B Environ.* **2022**, *304*, 120963.

[24] R. E. Siegel, M. Aceves, L. A. Berben, ACS Energy Lett. **2024**, *9*, 2896.

[25] Y. Yamazaki, M. Miyaji, O. Ishitani, *J. Am. Chem. Soc.* **2022**, *144*, 6640.

[26] H. Kumagai, T. Nishikawa, H. Koizumi, T. Yatsu, G. Sahara, Y. Yamazaki, Y. Tamaki, O. Ishitani, *Chem. Sci.* **2019**, *10*, 1597.

[27] M. Miyaji, Y. Tamaki, K. Kamogawa, Y. Abiru, M. Abe, O. Ishitani, ACS Catal. **2024**, *14*, 10403.

[28] L. Rotundo, S. Ahmad, C. Cappuccino, A. J. Pearce, H. Nedzbala, S. R. Bottum, J. M. Mayer, J. F. Cahoon, D. C. Grills, M. Z. Ertem, G. F. Manbeck, *J. Am. Chem. Soc.* **2024**, *146*, 24742.

[29] R. Francke, B. Schille, M. Roemelt, *Chem. Rev.* **2018**, *118*, 4631.

[30] E. E. Benson, C. P. Kubiak, A. J. Sathrum, J. M. Smieja, *Chem. Soc. Rev.* **2008**, *38*, 89.

[31] A. Taheri, L. A. Berben, *Chem. Commun.* **2016**, *52*, 1768.

[32] J. Qiao, Y. Liu, F. Hong, J. Zhang, *Chem. Soc. Rev.* **2013**, *43*, 631.

[33] N. Elgrishi, M. B. Chambers, X. Wang, M. Fontecave, *Chem. Soc. Rev.* **2017**, *46*, 761.

[34] K. Elouarzaki, V. Kannan, V. Jose, H. S. Sabharwal, J.-M. Lee, *Adv. Energy Mater.* **2019**, *9*, 1900090.

[35] E. Vichou, Y. Li, M. Gomez-Mingot, M. Fontecave, C. M. Sánchez-Sánchez, *J. Phys. Chem. C* **2020**, *124*, 23764.

[36] E. Vichou, A. Solé-Daura, C. Mellot-Draznieks, Y. Li, M. Gomez-Mingot, M. Fontecave, C. M. Sánchez-Sánchez, *ChemSusChem* **2022**, *15*, e202201566.

[37] E. Vichou, Y. Adjez, Y. Li, M. Gómez-Mingot, M. Fontecave, C. M. Sánchez-Sánchez, *J. Am. Chem. Soc.* **2024**, *146*, 2824.

[38] C. Caix, S. Chardon-Noblat, A. Deronzier, *J. Electroanal. Chem.* **1997**, *434*, 163.

[39] C. Caix, S. Chardon-Noblat, A. Deronzier, J.-C. Moutet, S. Tingry, *J. Organomet. Chem.* **1997**, *540*, 105.

[40] T. K. Todorova, T. N. Huan, X. Wang, H. Agarwala, M. Fontecave, *Inorg. Chem.* **2019**, *58*, 6893.

[41] B. Chen, Y. Rong, X. Li, J. Sang, P. Wei, Q. An, D. Gao, G. Wang, ACS Energy Lett. **2024**, *9*, 911.

[42] B. Mondal, P. Sen, A. Rana, D. Saha, P. Das, A. Dey, ACS Catal. **2019**, *9*, 3895.

[43] X. Lu, Z. Jiang, X. Yuan, Y. Wu, R. Malpass-Evans, Y. Zhong, Y. Liang, N. B. McKeown, H. Wang, *Sci. Bull.* **2019**, *64*, 1890.

[44] D. J. D. Pimlott, A. Jewlal, B. A. W. Mowbray, C. P. Berlinguette, ACS Energy Lett. **2023**, *8*, 1779.

[45] J. Leverett, J. A. Yuwono, P. Kumar, T. Tran-Phu, J. Qu, J. Cairney, X. Wang, A. N. Simonov, R. K. Hocking, B. Johannessen, L. Dai, R. Daiyan, R. Amal, ACS Energy Lett. **2022**, *7*, 920.

[46] A. Bhattacharya, A. Kumar Nath, A. Ghatak, A. Nayek, S. Dinda, R. Saha, S. Ghosh Dey, A. Dey, *Angew. Chem. Int. Ed.* **2023**, *62*, e202215235.

[47] B. H. Ko, B. Hasa, H. Shin, E. Jeng, S. Overa, W. Chen, F. Jiao, *Nat. Commun.* **2020**, *11*, 5856.

[48] F. A. Hanc-Scherer, M. A. Montiel, V. Montiel, E. Herrero, C. M. Sánchez-Sánchez, *Phys. Chem. Chem. Phys.* **2015**, *17*, 23909.

[49] C. Kim, J. C. Bui, X. Luo, J. K. Cooper, A. Kusoglu, A. Z. Weber, A. T. Bell, *Nat. Energy* **2021**, *6*, 1026.

[50] U. Kölle, M. Grützel, *Angew. Chem. Int. Ed.* **1987**, *26*, 567.

[51] H.-R. "Molly" Jhong, S. Ma, P. J. Kenis, *Curr. Opin. Chem. Eng.* **2013**, *2*, 191.

[52] T. N. Huan, P. Simon, G. Rousse, I. Génois, V. Artero, M. Fontecave, *Chem. Sci.* **2016**, *8*, 742.

[53] J. Medina-Ramos, J. L. DiMeglio, J. Rosenthal, *J. Am. Chem. Soc.* **2014**, *136*, 8361.