Electroreduction of CO2 From Flue Gas: Impurity Tolerance and Mechanistic Insights in Molecular Catalysis

TL;DR

This paper explores how a rhenium-based catalyst efficiently converts CO2 from industrial flue gas into CO, even in the presence of impurities like SO2 and NO2.

Contribution

The study reveals the impurity tolerance of a molecular Re catalyst during CO2 electroreduction, offering new insights into catalyst design for real-world CO2 sources.

Findings

The Re catalyst maintains high CO selectivity even with 10% CO2 and impurities like O2, NO2, and SO2.

DFT calculations show low catalyst affinity for impurities due to their reduced forms during CO2RR.

O2 reduction accounts for most of the charge consumed during CO2 electrolysis.

Abstract

Direct utilization of diluted CO2 from industrial flue gas containing SO2, NO2 and O2 impurities is economically appealing and circumvents capture and purification prior to conversion. We present experimental data and density functional theory (DFT) calculations for understanding the effect of those impurities during CO2RR catalyzed by a model Re molecular catalyst, [Re(bpy)(CO)3Cl] (bpy = bipyridine). Under both 10% and 1% v/v CO2 gas streams, high selectivity toward CO production was maintained, with faradaic efficiency (FECO) above 90% and 70%, respectively. O2 reduction reaction (ORR) on the electrode surface represents a substantial competitive reaction that accounts for ≈80% of the charge consumed when a realistic CO2 source mimicking an industrial waste‐incinerator stream (10% v/v CO2, 10% v/v O2, 100 ppm NO2, and 50 ppm SO2 in N2 matrix) is used. Neither NO2 nor SO2 incorporated in the diluted CO2 stream provoke a significant decrease in the CO production (FECO ≥ 82%). DFT calculations indicate low affinity of the catalyst for impurities, because they are in their reduced form at the CO2RR potential. This is experimentally proven by cyclic voltammetry and ion chromatography. These findings call for new strategies that enhance the moderate O2‐tolerance exhibited by Re complex on CO2RR from flue gas.

Impurity‐tolerance exhibited by Re molecular complex for CO2 conversion from flue gas containing 10% CO2, 10% O2, 100 ppm NO2 and 50 ppm SO2 in N2 matrix. This tolerance is based on electrochemical reduction of gas impurities on the electrode surface under CO2 electrolysis conditions.© 2026 WILEY‐VCH GmbH

Figures

Cyclic voltammograms on a bare GCE disc of 1 mM of Re complex and 0.5 M [TBA][PF6] either in CH3CN solution under argon (black plot) or in CH3CN solution containing 1.5 M TFE as a proton source under 100% CO2 (red plot), 10% CO2 (blue plot), 1% CO2 (purple plot), and 10% CO2 + 10% O2 (green plot), the latter three in a N2 matrix. Scan rate 0.1 V s−1.

Cyclic voltammogram of 1 mM of Re complex in a solution of 0.5 M [TBA][PF6] in CH3CN containing 1.5 M TFE as a proton source, recorded under 10% v/v CO2 in a N2 matrix with varying impurity levels: a) NO2: 0 ppm (blue plot), 100 ppm (green), and 500 ppm (red); b) SO2: 0 ppm (blue), 50 ppm (green) and 500 ppm (red); c) 10% v/v CO2, 10% v/v O2 and different concentration of NO2, following the same color code than in (a); and d) 10% v/v CO2, 10% v/v O2 and different concentration of SO2, the same color code than in (b). Scan rate 0.1 V s−1.

Selection of FE for H2 (black bars) and CO (color bars) production obtained after controlled potential electrolysis (CPE) by applying the corresponding electrolysis potentials indicated in Table 3 and using 1 mM Re complex in CH3CN + 1.5 M TFE as proton source under various gas streams as indicated in the top X‐axis. Error bars for FECO are indicated in black, and for FEH2 in blue. The presence of NO2 and/or SO2 is indicated in the bottom X‐axis. The corresponding EECO (%, red squares) is indicated on the secondary right Y‐axis. Stirring rate 300 rpm.

Tables

TABLE 1. Comparison of the catalytic parameters from CV of 1 mM Re complex in CH3CN solution containing 1.5 M TFE as a proton source under different CO2 atmospheres and flue gas compositions. Potentials are referred versus Fc+/Fc. Catalytic potential (E cat/2) corresponds to the potential value at half of the catalytic current. The overpotential (η) was determined from the difference between E cat/2 and E 0CO2/CO (CH3CN, TFE) = −1.36 V versus Fc+/Fc [19].

Entry	CO2 (%) in N2 matrix	O2 (%) in N2 matrix	Impurities (ppm)	E cat/ 2 (V)	η (V)	j cat/j p
a	100	—	—	−1.96	0.60	30.7
b	10	—	—	−1.87	0.51	13.7
c	1	—	—	−1.76	0.40	2.2
d	10	10	—	−1.90	0.54	12.5
e	10	—	NO2, 100	−1.87	0.51	13.7
f	10	—	SO2, 50	−1.93	0.57	12.2
g	10	10	NO2, 100	−1.94	0.58	12.7
h	10	10	SO2, 50	−1.92	0.56	11.4

TABLE 2. Calculated DFT thermodynamic data for the interaction of selected flue gas components and their corresponding reduction products with the active form of the Re complex.

Species in solution	Calculated binding energy to the active form of Re complex (2e−) ΔG (kcal mol−1)
CO2	+3.0
O2	−20.5
SO2	−25.6
NO2	−46.2
NO2 −	+14.9
O2 •−	+9.3
SO2 •−	+6.3
H2O2	+3.5

TABLE 3. CPE at different potentials versus Fc+/Fc of 1 mM Re complex and [TBA][PF6] in CH3CN solution containing 1.5 M TFE in a two compartments H‐type cell under different CO2 concentration in a N2 matrix, with the presence of O2 (10% v/v) and/or NO2 and SO2 gas impurities when indicated. Total circulated charge 14 C. Solution stirring rate 300 rpm.

CO2 (%) in N2 matrix	O2 (%) in N2 matrix	Presence of impurities, (ppm)	E (V)	FECO (%)a	FEH2 (%)b	EECO (%)c
100	—	—	−1.95	91 ± 5	2 ± 1	62 ± 5
10	—	—	−1.95	92 ± 4	1 ± 1	63 ± 5
10	—	—	−2.05	92 ± 2	1 ± 1	61 ± 1
1	—	—	−1.80	72 ± 5	2 ± 1	54 ± 4
10	—	NO2, 100	−2.05	90 ± 2	2 ± 1	60 ± 1
10	—	NO2, 500	−2.05	84 ± 5	1 ± 1	56 ± 4
10	—	NO2, 1000	−2.05	88 ± 3	1 ± 1	58 ± 2
10	—	SO2, 50	−2.05	91 ± 3	2 ± 1	60 ± 2
10	—	SO2, 500	−2.05	85 ± 4	1 ± 1	56 ± 3
10	—	SO2, 1000	−2.05	82 ± 1	1 ± 1	54 ± 1
10	—	NO2, 100 SO2, 50	−2.05	94 ± 2	1 ± 1	62 ± 1
10	10	—	−2.05	22 ± 3	1 ± 1	14 ± 2
10	10	NO2, 100 SO2, 50	−2.05	20 ± 3	2 ± 1	13 ± 2
30	10	—	−2.05	21 ± 3	1 ± 1	14 ± 2
50	10	—	−2.05	20 ± 2	1 ± 1	14 ± 1

Full text

Introduction

1

The electrochemical reduction reaction of carbon dioxide (CO_2_RR) has emerged as a promising route to transform CO_2_ into value‐added e‐fuels [1, 2, 3, 4]. The 2e^−^/2H^+^ reduction of CO_2_ can generate CO, a product that holds industrial relevance as a component of synthetic gas (syngas), which serves as a feedstock in processes such as Fischer‐Tropsch and water‐gas‐shift (WGS) and as a building block for different industrial commodity chemicals such as methanol and acetic acid [5, 6]. Nowadays, the flue gases emitted by industrial exhausts (such as power plants, cement plants, and incinerators) represent point sources of CO_2_ [7]. However, unlike the pure CO_2_ sources commonly used for laboratory studies, these exhausts contain low share of CO_2_ (4%–25% v/v) alongside other compontents such as water, N_2_, O_2_, and gaseous impurities such as NO_ x _ and SO_ x _ [8]. In this case, catalysts are required to selectively convert CO_2_ over other competitive reactions such as hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), as well as NO_ x /SO x _ reduction [9, 10, 11, 12, 13, 14]. Molecular electrocatalysts offer a high product selectivity and the possibility of tuning them via synthetic ligand modification. For this reason, they have been deeply investigated as catalysts for CO_2_RR [15, 16]. Among them, [Re(bpy)(CO)3_Cl] (bpy = bipyridine), herein refered as Re complex, was introduced in the 80's by J. M. Lehn as a model molecular catalyst for the photoreduction and electrochemical reduction of CO_2 to CO [17]. Re(I) complex represents a model molecular catalyst for CO_2_RR studies thanks to its well‐established mechanism [18, 19], high selectivity for CO production and almost negligible H_2_ production from HER. Thus, the use of the model Re(I)‐molecular catalyst for CO_2_RR under realistic industrial conditions by direct utilization of low‐concentrated CO_2_ from a flue gas source is of great interest, since it avoids the energy intense and costly steps of CO_2_ capture and concentration prior to conversion [7].

Ishitani and collaborators [20, 21, 22] have explored different variations in the functional groups within the Re complex, making the catalyst suitable for the reduction of low‐concentrated CO_2_ gas streams (down to 1% CO_2_), but the effects of O_2_, NO_2_, and SO_2_ impurities, have not been addressed yet. Recently, the multiple scientific challenges present in the direct CO_2_RR from industrial flue gases, such as those related to low concentration of CO_2_ and the presence of impurities, have been reviewed [23]. However, the effects of impurities (NO_2_ and SO_2_) have been mainly studied in the context of heterogeneous systems with deleterious results towards CO_2_RR [9, 24]; meanwhile, the use of homogeneous molecular complexes for the CO_2_RR under conditions mimicking those of industrial gas effluents remains limited [25, 26, 27]. Particularly scarce are those dealing with the presence of O_2_ in the gas stream during CO_2_RR catalyzed by a molecular complex. A notable example stands out: using a Fe‐phorphyrin catalyst with four ferrocene groups, kinetic studies revealed that CO_2_ reacts with the catalyst 500 times faster than O_2_, leading to selective conversion of CO_2_ to CO while any O_2_ was reduced to benign H_2_O [27]. Some heterogenized molecular catalysts for CO_2_RR have been explored under flue gas conditions. As examples, a cobalt phthalocyanine heterogenized together with vinylpyridine polymer onto carbon black nanoparticles allowed a FE for CO of 90% under a diluted CO_2_‐stream [28], as well as a cobalt phthalocyanine immobilized on carbon nanotubes with a microporous polymer on top [29]. Thus, it is of interest to extend such studies to a model molecular catalyst such as the Re complex, owing to its well defined CO_2_RR mechanism. So far, only the catalytic reduction of pure nitrous oxide (N_2_O) gas to N_2_ has been deeply studied using the Re complex [30, 31].

In this work, we aim to evaluate the electrocatalytic performance of the Re complex for CO_2_ to CO conversion under an environment which mimics the flue gas composition of an industrial waste‐incinerator [32, 33] containing diluted CO_2_ (1%–10% v/v), O_2_ (1%–10% v/v), and impurity traces of NO_2_ (0–1000 ppm) and SO_2_ (0–1000 ppm). To the best of our knowledge, this is the first electrochemical investigation of a Re‐based molecular catalyst operating under these conditions with controlled impurity levels, thereby addressing a gap in the use of homogeneous molecular systems for CO_2_RR under realistic flue gas environments. Thus, we explore herein by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) the behavior of the Re complex as a molecular platform during CO_2_RR under a sequence of different gas mixtures by introducing each component, individually and in combination, and quantifying its effect on the faradaic efficiency and energy efficiency (EE) of CO production. Finally, density functional theory (DFT) calculations provide additional mechanistic insights regarding the exceptional tolerance of Re complex during CO_2_RR in the presence of NO_2_ and SO_2_ impurities, as well as the role of O_2_ and the contribution of ORR.

Results

2

Molecular Catalyst Characterization Under Diluted CO2 and O2 by Cyclic Voltammetry

2.1

Figure 1 shows the cyclic voltammograms of a 0.5 M solution of [TBA][PF_6_] in CH_3_CN containing 1 mM of [Re(bpy)(CO)3_Cl] (Re complex) on bare glassy carbon electrode (GCE) under inert conditions (argon‐saturated, black plot). Previous reports describing the mechanism of Re complex under inert conditions attribute the first reversible wave centered at −1.72 V versus Fc^+^/Fc to the one‐electron reduction on the bpy ligand (bpy/bpy^•−^) and the second wave located at −2.04 V versus Fc^+^/Fc to the quasireversible metal center reduction (Re^I^/Re^0^) [5, 18, 19]. Since proton‐coupled electron transfer is required for CO_2 reduction [19], 1.5 M of trifluoroethanol (TFE), a weak Brønsted acid (pKa = 35.4) [19], was added to the solution as a proton source. Upon TFE addition, the catalytic response of Re complex under different concentrations of CO_2_ with or without the simultaneous presence of O_2_ was studied. Under 100% CO_2_ (red plot), a catalytic wave for CO_2_ reduction with an onset potential at −1.76 V versus Fc^+^/Fc appeared [5]. Upon dilution of CO_2_ to 10% (blue plot) and 1% (purple plot), the intensity of the catalytic current densities (j cat) decreases, as expected because of the limited reactant availability. In addition to this, Figure 1 also includes the effect of diluted CO_2_ in the presence of O_2_, both gases at 10% v/v, (green plot). This CV exhibits two well separated reduction processes: (i) a new irreversible reduction peak at −1.35 V versus Fc^+^/Fc associated with ORR and (ii) the catalytic peak previously displayed under 10% CO_2_ with an onset potential at −1.76 V versus Fc^+^/Fc and associated to the CO_2_ reduction to CO. The different catalytic current density reached by those two peaks (0.75 mA cm^−2^ and 3.5 mA cm^−2^ for ORR and CO_2_RR, respectively) is mainly related to the different type of process taking place: direct conversion on the GCE surface for ORR and mediated conversion in solution for the CO_2_RR. Moreover, 35 times lower solubility displayed by O_2_ than CO_2_ in acetonitrile might also play a role (Table S1) [34, 35].

Cyclic voltammograms on a bare GCE disc of 1 mM of Re complex and 0.5 M [TBA][PF6] either in CH3CN solution under argon (black plot) or in CH3CN solution containing 1.5 M TFE as a proton source under 100% CO2 (red plot), 10% CO2 (blue plot), 1% CO2 (purple plot), and 10% CO2 + 10% O2 (green plot), the latter three in a N2 matrix. Scan rate 0.1 V s−1.

Table 1 (entries a–c) reports the j cat/j _ p _ ratio values derived from the CVs shown in Figure 1, with j cat being the maximum catalytic current density and j _ p _ the current density under Ar gas. As it can be observed from the different ratios, 30.7, 13.7, and 2.2 for 100, 10, and 1% CO_2_ gas stream, respectively, the decrease of j cat/j _ p _ is not a linear function of the CO_2_ concentration, showing that even at 1% CO_2_, the catalyst still produces a current only 15 times lower than that obtained with 100% CO_2_. In addition to this, the j cat/j _ p _ ratio using 10% v/v CO_2_ in the presence of 10% v/v O_2_ (green plot) is only slightly decreased as compared to the ratio obtained in the absence of O_2_ (entries d with 12.5 vs. b with 13.7).

TABLE 1: Comparison of the catalytic parameters from CV of 1 mM Re complex in CH3CN solution containing 1.5 M TFE as a proton source under different CO2 atmospheres and flue gas compositions. Potentials are referred versus Fc+/Fc. Catalytic potential (E cat/2) corresponds to the potential value at half of the catalytic current. The overpotential (η) was determined from the difference between E cat/2 and E 0CO2/CO (CH3CN, TFE) = −1.36 V versus Fc+/Fc [19].

Figure S1 presents different voltammograms for studying the ORR associated peak in the presence and the absence of the Re complex under 10% (v/v) O_2_ in a N_2_ matrix. Figure S1 shows an almost identical reduction wave in potential and current density with an onset of −1.1 V versus Fc^+^/Fc, in the presence and the absence of the Re complex. Therefore, these results showed that this reduction peak originates from the ORR on the GCE surface and is not mediated by the Re complex. In addition, the effect of varying CO_2_ concentration in the presence of 1 mM Re complex and 10% v/v O_2_ in a N_2_ matrix was studied via CV on a bare GCE. The CVs in Figure S2 show that neither the onset potential for the CO_2_‐related catalytic peak (onset at −1.76 V vs. Fc^+^/Fc) nor the one corresponding to ORR (onset at −1.1 V vs. Fc^+^/Fc) are shifted with higher CO_2_ concentration in the gas stream. The only evident change in CV corresponds to the increase of current density of the CO_2_RR catalytic peak, consistent with greater reactant availability. However, the O_2_ reduction signal remains unchanged, indicating that ORR proceeds under these conditions, independently of the concentration of CO_2_ available.

Molecular Catalyst Characterization Under Simulated Flue Gas by Cyclic Voltammetry

2.2

Figure 2a,b compare the CVs under a diluted CO_2_ atmosphere (10% v/v in a N_2_ matrix), in the absence and the presence of either NO_2_ introduced at 100 and 500 ppm (Figure 2a) or SO_2_ at 50 and 500 ppm (Figure 2b), two common flue gas impurities. As previously stated, this specific CO_2_ concentration, together with a low presence of NO_2_ and SO_2_ (100 ppm and 50 ppm, respectively), reflect realistic conditions typically found in gas effluents of industrial waste‐incinerators. Higher NO_2_ and SO_2_ impurity levels were introduced to test the tolerance limit of the catalyst. The results from these CVs show that in all tested conditions, the catalytic current is about the same and thus CO_2_RR operates without interference from NO_2_ and SO_2_ impurities under the tested conditions. Figure 2c,d present the CVs under a more representative flue gas composition (10% v/v CO_2_ + 10% v/v O_2_ in a N_2_ matrix with the addition of either NO_2_ or SO_2_, respectively). The reduction of diluted CO_2_ remains unchanged, and ORR process appears mainly insensitive to the presence of NO_2_ (Figure 2c) and SO_2_ (Figure 2d). Table 1 (entries e–h) reports the catalytic parameters (E cat/2, η and j cat/j _ p ) calculated from CVs displayed in Figure 2. It shows that in both low‐concentrated CO_2 streams (10% v/v CO_2_ and 10% v/v CO_2_ + 10% v/v O_2_), the presence of either SO_2_ or NO_2_ impurities at low concentrations (50–100 ppm range) has a minimal impact in all three catalytic parameters.

Cyclic voltammogram of 1 mM of Re complex in a solution of 0.5 M [TBA][PF6] in CH3CN containing 1.5 M TFE as a proton source, recorded under 10% v/v CO2 in a N2 matrix with varying impurity levels: a) NO2: 0 ppm (blue plot), 100 ppm (green), and 500 ppm (red); b) SO2: 0 ppm (blue), 50 ppm (green) and 500 ppm (red); c) 10% v/v CO2, 10% v/v O2 and different concentration of NO2, following the same color code than in (a); and d) 10% v/v CO2, 10% v/v O2 and different concentration of SO2, the same color code than in (b). Scan rate 0.1 V s−1.

The electroreductions of NO_2_ (Figure S3a) and SO_2_ (Figure S3b) in the presence of the Re complex, but in the absence of CO_2_ and O_2_, are also studied. These catalytic processes are analogous to the CO_2_ to CO conversion and could be of interest in flue gas streams. Moreover, the effect of increasing the NO_2_ and SO_2_ concentration beyond the usual concentration limit found in typical waste‐incinerator flue gas is also reported. For the electroreduction of 100 ppm NO_2_ (Figure S3a) and 50 ppm SO_2_ (Figure S3b), a poor catalytic current from Re complex is observed (onset potential −1.75 V vs. Fc^+^/Fc). Increasing the SO_2_ concentration beyond 500 ppm (Figure S3b) provokes the appearance of an additional reduction peak with an onset of −1.0 V versus Fc^+^/Fc, which might be associated to direct reduction of SO_2_ on GCE. To support that, additional CVs studying the reduction of NO_2_ and SO_2_ directly on the GCE are also included (Figure S4a, b). In particular, Figure S4b exhibits a clear reduction peak at an onset of −1.0 V versus Fc^+^/Fc that fits well the second reduction peak found in Figure S3b.

Density Functional Theory Calculations on the Interaction of Different Flue Gas Components and [Re(bpy)(CO)3Cl]

2.3

It is well established in the literature that NO_ x _ and SO_ x _ gas species might compete with CO_2_ in reduction processes catalyzed by molecular complexes [9], which in some cases might reduce catalytically gas impurities such as NO_2_ and SO_2_. However, the cyclic voltammetry results reported in the previous section suggest that Re complex catalytic intermediate does not significantly react neither with NO_2_ nor SO_2_, and no competition is established with CO_2_. Therefore, density functional theory (DFT) calculations were set up for reaching a deeper mechanistic insight. Table 2 shows the calculated binding energies of all flue gas components (CO_2_, O_2_, SO_2_, and NO_2_), as well as the binding energies of some of their corresponding reduction products (NO_2_ ^−^, SO_2_ ^•−^, O_2_ ^•−^ and H_2_O_2_), with the catalytic active form of the Re complex. Those calculations show that the Re^0^ center in the active form of the catalyst exhibits lower affinity (less favorable binding energy) for CO_2_ compared to NO_2_ and SO_2_, which should come associated with a significant decrease in CO production from CO_2_RR in the presence of either NO_2_ or SO_2_ impurities. However, the CV data shown in Figure S4 and the calculated reduction potentials for these molecules on the electrode surface (0.0 and −1.0 V vs. Fc^+^/Fc, respectively) indicate that both NO_2_ and SO_2_ molecules are electrochemically reduced on the electrode surface at the required potential for CO_2_RR mediated by the Re complex (ca. −1.8 to −1.9 V vs. Fc^+^/Fc). The one electron reduction product of NO_2_ and SO_2_ produces new ionic species NO_2_ ^−^ and SO_2_ ^•−^, respectively. Given their anionic nature, these species exhibit less favorable binding energy (Table 2) than CO_2_ and are less prone to bind to the Re^0^ center in the anionic active form of the catalyst, which should explain why NO_2_ and SO_2_ molecules do not compete with CO_2_ binding in flue gas under electrocatalytic conditions. Data from Table 2 are summarized graphically in Figure S5.

The binding energy of O_2_ with the active form of the Re complex is also included in Table 2. Similarly, DFT calculations show that the metal center in the catalyst active form exhibits higher affinity for O_2_ than to CO_2_. However, as happened with NO_2_ and SO_2_, the O_2_ does not occupy the Re^0^ active site, because ORR is favored directly at the GCE surface and depletes all available O_2_ prior to the reduction of the Re complex, as experimentally shown by CV (Figure 2 and Figure S1). In addition, the O_2_ reduction products formed from one or two electron transfers, O_2_ ^•−^ and H_2_O_2_, respectively, exhibit less favorable binding energy to the active form of the Re complex than CO_2_ (Table 2).

Effect of Diluted CO2 on CO2RR by Controlled Potential Electrolysis

2.4

The catalytic performance of the Re complex was further evaluated under flue gas conditions by bulk electrolysis at constant potential (controlled potential electrolysis (CPE)) in batch conditions (solution saturated with the corresponding CO_2_ stream at the beginning of the electrolysis). A total charge of 14 C was circulated in all CPEs, leading to a different electrolysis duration depending on the current displayed at the applied electrolysis potential in each case. Different gas streams, with different concentrations of CO_2_ and with or without the addition of O_2_, NO_2_, and/or SO_2_, were evaluated on GCE, as summarized in Figure 3 and Table 3. CO and H_2_ were the only reduction products detected in the gas phase after CO_2_ electrolysis, with the absence of any additional product from CO_2_RR in solution. Under both 100% and 10% v/v CO_2_ gas streams, high selectivity toward CO production was obtained after CPE at −1.95 and −2.05 V versus Fc^+^/Fc, with FE_CO_ above 90% (Figure 3, red and blue bars, respectively) and minimal HER. When the CO_2_ concentration was reduced to 1% (Figure 3, purple bar, CPE at −1.8 V vs. Fc^+^/Fc), high CO selectivity was still obtained (FE_CO_ = 72 ± 5%). However, a poor total FE ≤ 75% was achieved in this case, which seems to indicate that a portion of the total charge is being consumed by processes not accounted by our product analysis.

Selection of FE for H2 (black bars) and CO (color bars) production obtained after controlled potential electrolysis (CPE) by applying the corresponding electrolysis potentials indicated in Table 3 and using 1 mM Re complex in CH3CN + 1.5 M TFE as proton source under various gas streams as indicated in the top X‐axis. Error bars for FECO are indicated in black, and for FEH2 in blue. The presence of NO2 and/or SO2 is indicated in the bottom X‐axis. The corresponding EECO (%, red squares) is indicated on the secondary right Y‐axis. Stirring rate 300 rpm.

Figure S6 shows the evolution of current density over time during CPE under different CO_2_ concentrations: 100%, 10%, and 1% v/v CO_2_ in N_2_ matrix. Moreover, two different applied potentials are reported for the 10% v/v CO_2_ gas stream. These current profiles show a continued diminution of current density all along the electrolysis, which is expected for electrolysis in batch conditions (solution saturated with the corresponding CO_2_ stream at the beginning of the electrolysis) due to the progressive consumption and decrease in reactant available over time. As anticipated from the CV data in Figure 1, the current density signal displayed in Figure S6 shows an intensity that depends on the initial CO_2_ concentration, with the highest values observed for 100% v/v CO_2_ and decreasing progressively for 10% and 1% v/v CO_2_. This behavior confirms the concentration‐dependent catalytic response relaying on the CO_2_ availability. Using 10% v/v CO_2_ gas stream a lower current density was obtained at −1.95 V compared to that at −2.05 V versus Fc^+^/Fc (blue and brown plot in Figure S6, respectively). Thus, all additional CPE experiments shown in Figure 3 and Table 3 were performed at −2.05 V versus Fc^+^/Fc in order to evaluate the effects of flue gas components without any additional effect associated with the applied potential.

Effect of NO2 and SO2 Impurities on CO2RR by Controlled Potential Electrolysis

2.5

Under flue gas composition, the presence of 100 ppm NO_2_ or 50 ppm SO_2_ impurities in a 10% v/v CO_2_ gas stream did not significantly affect either the FE_CO_ value, which remained at 90%, or the EE_CO_ value, which was kept at 60% (Figure 3 and Table 3). Notably, the combination of 10% v/v CO_2_, 100 ppm NO_2_ and 50 ppm SO_2_ (Figure 3, navy blue bar and Table 3), which approaches the composition of an industrial flue gas effluent, preserved high CO selectivity (FE_CO_ = 94 ± 2% and EE_CO_ = 62 ± 1%), confirming the great tolerance of the Re catalyst under these impurity conditions. Figure S7a (NO_2_),b (SO_2_) show the current density profiles over time recorded during CPE under different gas compositions in 10% v/v CO_2_. The presence of increasing concentrations of NO_2_ (100, 500, and 1000 ppm) in the gas mixture did not significantly affect or alter the intensity of the current density compared to the impurity‐free conditions (0 ppm), indicating no observable impact of NO_2_ impurity on the catalytic response over the electrolysis time. The presence of SO_2_ in the gas mixture below 1000 ppm did not significantly impact the current density evolution over time during electrolysis. However, once 1000 ppm of SO_2_ were present in the gas stream, a minimal deleterious effect on the current density was observed (orange plot in Figure S7b). These observations are consistent with both the CV results (Figure 2a,b) and the FE_CO_ data from Table 3, which also show minimal variation in the catalytic performance of the Re complex in the presence of these impurities.

Ion chromatography was also used to analyze any potential byproduct derived from either NO_2_ or SO_2_ electrochemical reduction on the GCE. In particular, nitrite anion (NO_2_ ^−^) from NO_2_ reduction during electrolysis was detected by ion chromatography. However, the one electron reduction product from SO_2_ considered in the DFT calculations (SO_2_ ^•−^) is not a stable anion in solution and was not detected. Figure S8a shows the ion chromatography data collected from a blank experiment where 5% v/v NO_2_ in N_2_ matrix was bubbled in CH_3_CN solution to demonstrate that neither NO_2_ ^−^ (retention time 10.34 min) nor NO_3_ ^−^ (retention time 15.75 min) were initially present in the solution, contrary to what has been reported in aqueous solution [13]. Figures S8b,c show the ion chromatography data from two independent electrolysis under a 10% v/v CO_2_ gas stream containing NO_2_ at 100 ppm (black plot) and 1000 ppm (red plot) from the anolyte and the catholyte, respectively. The presence of NO_2_ ^−^ formed during the CPE is confirmed in both compartments, being more evident in the anolyte. Moreover, the total amount of NO_2_ ^−^ detected is, as expected, larger in the electrolysis using 1000 ppm of NO_2_ (red plot) than 100 ppm of NO_2_ (black plot).

Effect of Simulated Flue Gas on CO2RR by Controlled Potential Electrolysis

2.6

The introduction of 10% v/v O_2_ in the gas matrix containing 10% v/v CO_2_ provokes an increase in the current density during the electrolysis (Figure S9, green plot), but led to a sharp decline in CO selectivity during CO_2_RR electrolysis (FE_CO_ = 22%), as observed in Table 3 and Figure 3 (green bar). This is attributed to the competitive ORR taking place at more anodic potential, as shown by CV in Figure 1 (green plot) and Figure S1 (green plot). However, the Re complex remains active for CO_2_RR to CO, since ORR does not consume all the available electrons. This is probably because the applied potential during electrolysis in the presence of Re complex is negative enough to ensure the constant production of the Re active intermediate all along the electrolysis. Then, Re complex represents a suitable system to evaluate the effect of the complete flue gas composition by introducing both 50 ppm SO_2_ and 100 ppm NO_2_, together with 10% v/v O_2_ and 10% v/v CO_2_ (Figure S9, black plot). The use of this simulated flue gas composition presents no additional deleterious effect to the one observed upon 10% v/v O_2_ incorporation in the gas stream. Therefore, the FE_CO_ and EE_CO_ remain unchanged at around 20% and 13%, respectively (Table 3 and Figure 3, olive bar). Comparing the current density evolution over time in the presence and absence of NO_2_ and SO_2_ impurities under the low‐concentrated CO_2_ conditions containing O_2_ shows no difference (Figure S9, green and black plots), in accordance with the CVs shown in Figure 2c,d. Finally, a strategy based on increasing CO_2_:O_2_ ratio was tested to improve the CO_2_RR performance in the presence of O_2_ at 10% (v/v). Additional electrolysis in Table 3 show the results of increasing the CO_2_ concentration from 10% (v/v) to 30% and 50%, while keeping the concentration of O_2_ at 10% (v/v) in the N_2_ matrix, likewise fail to increase the selectivity towards CO (FE_CO_ = 20%, Table 3). Therefore, neither the increase of CO_2_ availability nor the presence of NO_2_ or SO_2_ at the chosen concentrations affect the thermodynamically favored ORR.

Discussion

3

The CV and bulk electrolysis data presented here demonstrate that the catalytic performance of the Re complex for CO_2_ conversion to CO (selectivity and efficiency) is not affected either by decreasing the CO_2_ concentration in the gas stream to 10% v/v (FE_CO_ ≥ 90 and EE_CO_ ≥ 60%) or by adding traces of NO_2_ (100 ppm) and SO_2_ (50 ppm) in such a low‐concentrated CO_2_ stream (10% v/v) (Figure 2 and Table 3). With such a complex gas mixture, we obtained performances comparable to that with pure CO_2_ (FE_CO_ = 94% and EE_CO_ = 62%, Table 3). A very minor effect is only observed for SO_2_ far beyond its typical flue gas concentration (1000 ppm) in the diluted CO_2_ gas stream (Figure 2 and Table 3). This could be due to the competitive reduction of SO_2_ directly on the GCE as experimentally observed (Figures S3 and S4) and theoretically modeled. In contrast, the Re complex catalytic performance is significantly impacted by the presence of O_2_ in the gas stream (10% v/v) (Figure 1), and a substantial portion of the circulated charge during CPE is consumed for ORR taking place on the GCE (Table 3, FE_CO_ = 22%). ORR presents a reduction wave in CV at an onset of −1.1 V versus Fc^+^/Fc, in the presence and the absence (Figure S1) of the Re complex in solution, which means that ORR takes place on the GCE surface, and is not mediated by the Re complex. This peak in CV associated to ORR remains mainly insensitive to the simultaneous presence of NO_2_ and SO_2_ under flue gas composition (Figure 2). Thus, the presence of 10% v/v O_2_ provokes a severe deleterious effect in FE_CO_ during flue gas electrolysis (FE_CO_ = 20% and EE_CO_ = 13%). Nevertheless, the Re complex exhibits a moderate O_2_‐tolerance thanks to its ability to remain active for the CO_2_RR keeping its CO selectivity in the presence of O_2_ [26, 27]. Control electrolyses increasing the ratio of (CO_2_:O_2_) in the gas stream from (1:1) up to (5:1) do not overcome this limitation.

The DFT results show that O_2_, NO_2_, and SO_2_ present high affinity for the Re complex and could favorably compete with CO_2_ for binding to the catalytically active form of the complex (Table 2 and Figure S5). However, at the required potential for CO_2_RR mediated by the Re complex (ca. −1.8 to −1.9 V vs. Fc^+^/Fc), O_2_, NO_2_, and SO_2_ molecules are electrochemically reduced on the electrode surface, which produces new ionic species O_2_ ^•−^, NO_2_ ^−^ and SO_2_ ^•−^ presenting much less affinity for the Re complex than CO_2_ as reported in Table 2. The electroreduction of NO_2_ on GCE and the formation of NO_2_ ^−^ has been confirmed by ion chromatography as shown in Figure S8.

Conclusions

4

Achieving CO_2_RR directly using a gas stream released from industrial plants, without CO_2_ purification, is a great challenge. The competing reduction reactions occurring during CO_2_RR to CO electrolysis of simulated flue gas, where the CO_2_ accounts for a small part of the gas stream and additional reducible gases such as O_2_, NO_2_, and SO_2_ are also present in solution, might impact on CO_2_RR selectivity and efficiency. We were able to demonstrate the ability of the model Re molecular catalyst to sustain CO_2_RR to CO without degradation under a realistic CO_2_ source by mimicking an industrial waste‐incinerator stream (10% v/v CO_2_, 10% v/v O_2_, 100 ppm NO_2_, and 50 ppm SO_2_ in N_2_ matrix). The high NO_2_ and SO_2_ impurities tolerance demonstrated by Re complex (FE_CO_ = 94%) is contrasted by a moderated O_2_‐tolerant CO_2_RR catalytic performance (FE_CO_ = 20%). DFT calculations demonstrated the high affinity of impurities for the Re complex (NO_2_ > SO_2_ > O_2_ > CO_2_), as well as their instability under electrolysis conditions due to their reduction on the electrode surface before activating the Re complex.

In conclusion, the deep mechanistic understanding of the flue gas impurities role during CO_2_RR provided here points molecular catalysts, such as the Re complex, as an attractive alternative to achieve efficient and selective CO_2_RR and mitigate some of the deleterious impacts of direct usage of flue gas streams. Nevertheless, several challenges remain unsolved before allowing high conversion efficiency of gas stream input from practical/industrial CO_2_ sources.

Experimental Section

5

Reactants and Characterization

5.1

All chemicals were purchased from commercial suppliers and used without further purification. Anhydrous acetonitrile of 99.8% purity (CH_3_CN), tetrabutyl ammonium hexafluorophosphate ([TBA][PF_6_]) of 99% purity, 2,2,2‐Trifuoroethanol (TFE, CF_3_CH_2_OH) of ≥99% purity, and ferrocene (98%) were purchased from Sigma‐Aldrich. The Re‐molecular catalyst Chlorotricarbonyl(2,2′‐bipyridine)rhenium(I), ([Re(bpy)(CO)3_Cl]), of 99% chemical purity was purchased from Strem Chemicals. Pure gases Ar (>99.99%), N_2 (>99.99%), CO_2_ (>99.99%), and diluted gases 10% v/v CO_2_ in a N_2_ matrix, 10% v/v CO_2_ + 10% v/v O_2_ in N_2_, 5% v/v NO_2_ in N_2_, and 5% v/v SO_2_ in N_2_ were custom‐prepared by Air Liquide.

Proton Nuclear Magnetic Resonance (NMR) was used to characterized the Re complex by using a Bruker AVANCE III 300 MHz spectrometer. The NMR spectrum of the complex is shown in Figure S10.

Simulated Incinerator Flue Gas Mixture

5.2

Prior to entering the electrochemical cell, CO_2_ gas streams (100% CO_2_ and 10% v/v CO_2_) were passed through a gas purifier from Agilent (model CP17973) to remove residual oxygen and moisture. The complex gas mixtures used to purge the solutions were prepared by properly mixing gases at the appropriate ratio using mass flow controllers (Bronkhorst, EL‐FLOW Prestige, models FG‐201CV, 50 mL min^−1^ or FG‐200CV, 1 mL min^−1^). The gas flow rates were regulated online using FlowSuite 2 software. For all experiments involving the use of SO_2_, a high‐pressure stainless‐steel regulator (DIM 200‐3‐5 T PURGE, AirLiquide) was used for the customized SO_2_ cylinder to ensure safe gas delivery. The manometer includes a purge inlet, which helps to safely purge the system and avoid contamination and corrosion. The concentration range of the studied impurities was either 50, 500 or 1000 ppm of SO_2_ or 100, 500 and 1000 ppm of NO_2_, in 10% v/v CO_2_ diluted in a N_2_ matrix or 10% v/v CO_2_ + 10% v/v O_2_ in N_2_.

Electrochemical Studies

5.3

Cyclic Voltammetry

5.3.1

All electrochemical experiments were performed on Biologic Science Instruments SAS potentiostats (SP‐300 or VSP‐300) at room temperature (20 ± 2°C) in acetonitrile (CH_3_CN). [TBA][PF_6_] was used as a supporting electrolyte in solution (0.5 M) and TFE (1.5 M) was used as the proton donor. These catalytic conditions were used as the optimal conditions [19]. A three‐electrode setup was used with a glassy carbon (GC) disc electrode (diameter = 3 mm, area = 0.07 cm^2^, BioLogic) as the working electrode (WE). Prior to use, the GC electrode was polished on a polishing cloth (DP‐Nap 200 mm, Struers) on a 1 μm diamond suspension (Struers), sonicated in water for 10 s and then dried before starting the experiments. The counter electrode (CE) was a platinum wire (diameter = 0.5 mm, Alfa Aesar, 99.5% purity), which was previously flame‐annealed. The reference electrode (RE) was a conventional Ag/AgCl/KCl_sat_, separated from the solution via a salt bridge. All potentials were calibrated by adding the internal standard ferrocenium/ferrocene (Fc^+^/Fc) redox couple at the end of each experiment. CVs were run at 0.1 V s^−1^ scan rate and only the third steady state cycle of all CVs is shown. E cat/2 (V) corresponds to the potential at half of the catalytic peak current. The cathodic overpotential (η) was calculated from the difference between E cat/2 and E ^0^ CO2/CO (CH_3_CN, TFE) = −1.36 V vs. Fc^+^/Fc in acetonitrile [19]. Catalytic response from CV was determined by calculating the current density ratio (j cat/j _ p ), where (j cat) corresponds to the catalytic current density in the presence of CO_2 (at all different concentrations and conditions) and TFE, and (j _ p _) corresponds to the reduction peak current density under inert conditions (Ar).

CO2 Conversion by Bulk Electrolysis

5.3.2

Chronoamperometric studies (controlled potential electrolysis) were carried out in a gastight two‐compartment electrochemical H‐type glass cell with a glass frit to separate anolyte and catholyte chambers. 9.5 mL of 0.5 M [TBA][PF_6_] + 1.5 M TFE in CH_3_CN was used in the catholyte, while 3 mL of 0.5 M [TBA][PF_6_] was used in the anolyte. The indicated volumes were constant in all electrolysis reported here. The working electrode was a 1 cm^2^ GC plate (1 mm thick, type 2, from Alfa Aesar), the counter electrode was a 5 cm^2^ GC rod (Alfa Aesar), and the reference electrode was a Ag/AgCl/KCl_sat_ electrode within a salt bridge. To minimize ohmic losses in the cell, a minimal distance between electrodes was kept, and both H‐cell chambers were stirred during the electrolysis. In addition, 85% of the electrolyte resistance was compensated using the potentiostat's Ohmic drop compensation module. Controlled potential electrolysis (CPE) were performed by gas saturating both anolyte and catholyte chambers (with 100%–1% CO_2_ or the different gas mixtures) for 5 and 30 min, respectively. The gas was humidified in a solution containing the electrolyte before entering the H‐cell compartments. No continuous CO_2_ gas was purged during the electrolysis. One millimolar of the Re complex was added only in the catholyte. An additional gas trap open to the atmosphere to avoid overpressure containing 0.1 M NaOH was added at the outlet of the H‐cell compartments to avoid the release of these gases in the open atmosphere. A schematic illustration describing the electrochemical setup is included in Figure S11. All CPE experiments were performed with two or three replicates to check the reproducibility of results.

Analytical Quantification of Products

5.4

Gas products were quantified by gas chromatography (Model 8610C, SRI Instruments) equipped with a thermal conductivity detector (TCD) to detect H_2_ and a flame ionization detector (FIC) to detect CO. Gas aliquots of volumes varying from 25 to 100 µL were taken from the headspace of both compartments in the H type cell and injected into the GC. The GC was periodically calibrated using a custom standard gas mixture in CO_2_ (0–75 nmol H_2_). Calibration curves are shown in Figure S12. An ion exchange chromatograph (IC) (Metrohm 883 Basic IC), automatized by a 863 Compact IC autosampler and equipped with a Metrosep A Supp 5 (250 × 4 mm) column and a conductivity detector, was used to evaluate liquid products, with results indicating no appreciable quantities of liquid products. Faraday efficiency (FE, %) for a giving product is calculated from the ratio between the charge consumed to form each product and the total charge passed [36]. A constant total circulated charge (14 C) was used in all electrolysis in order to reach a fair comparison among all CPE results. A correction in the total circulated charge is made to account for the initial two electron reduction of the Re complex (1 mM in solution) to generate its active form. Catalyst activation charge = [number of electrons × Faraday constant × mol of catalyst] = [2 × 96 485 × 9.5 × 10^−6^] = 1.83 C. With this correction, an effective circulated charge of 12.2 C for all bulk electrolysis is calculated. The cathodic half reaction energy efficiency (EE, %) was calculated for the production of CO from CO_2_ using the following Equation (1) [37].

$$[eqn]$$

where E ^0^CO_2_/CO is the thermodynamic potential in volts required for the electrocatalytic reduction of CO_2_ to CO (E ^0^CO_2_/CO (CH_3_CN, TFE) = −1.36 V vs. Fc^+^/Fc) [19], whereas E and FE_CO_ represent the actual cathodic potential applied in volts (vs. Fc^+^/Fc) and the CO Faradaic Efficiency (%), respectively.

Computational Methods and Modeling (Density Functional Theory Calculations)

5.5

DFT calculations of the binding energy were used to evaluate the affinity between incinerator flue‐gas components (CO_2_, O_2_, NO_2_, and SO_2_) and the catalytically active form of the Re complex. Moreover, the standard reduction potential of those molecules was also calculated by DFT. Calculations were carried out using the Gaussian 16 (rev. A.03) quantum chemistry software [38] at the B3LYP‐D3BJ [39, 40, 41, 42] level of theory. The LANL2DZ(f) basis set and associated pseudopotentials [43, 44] were used for the Re center, while remaining atoms were described using the all‐electron cc‐pVDZ basis set [45, 46, 47]. Implicit solvent effects (acetonitrile) were included both in geometry optimizations and energy calculations using the IEF‐PCM model [48] as implemented in Gaussian 16. Electronic energies were corrected by performing single‐point calculations on optimized geometries with a more extended basis set: LANL2TZ(f) [43, 44, 49] for Re and aug‐cc‐pVTZ [45, 46, 47, 50] for the other elements. Gibbs free energies were further corrected from the 1 atm reference state used in the Gaussian code to the 1.0 M solution standard state at 25°C using a +1.89 kcal mol^−1^ correction [51]. Quasiharmonic entropy and enthalpy corrections (Grimme's [52] and Head‐Gordon's [53] methods, respectively) were applied as implemented in the GoodVibes code [54]. Final Gibbs free energy values are reported in kcal mol^−1^, and standard redox potentials are reported in volts (V) versus Fc^+^/Fc. A dataset collection of the optimized structures is available in the ioChem‐BD repository [55] and can be accessed via https://doi.org/10.19061/iochem‐bd‐1‐404.

Supporting Information

Additional supporting information can be found online in the Supporting Information section. Supporting Figure S1: Cyclic voltammograms recorded for a solution of 0.5 M [TBA][PF_6_] in CH_3_CN and 1.5 M TFE as a proton source under 10% (v/v) O_2_ in an N_2_ matrix (green plot) and under the same conditions, but adding 1 mM Re complex (red plot). CV containing 1 mM of Re complex and 0.5 M [TBA][PF_6_] in CH_3_CN solution under Argon plotted for reference (black plot). Scan rate 0.1 V s^−1^. Supporting Figure S2: Cyclic voltammogram on a 3 mm diameter bare GC electrode disc of 1 mM of Re complex in a solution of 0.5 M [TBA][PF_6_] in CH_3_CN containing 1.5 M TFE as a proton source under different CO_2_/O_2_/N_2_ (% vol.) atmospheres: 10/10/80 (green plot), 30/10/60 (blue plot) and 50/10/40 (orange plot). Scan rate 0.1 V s^−1^. Supporting Figure S3: Cyclic voltammograms recorded on GCE in a solution of 1 mM of Re complex, 0.5 M [TBA][PF_6_] and 1.5 M TFE in CH_3_CN under different concentrations of gas impurities in N_2_ matrix of (a) NO_2_ and (b) SO_2_. Scan rate 0.1 V s^−1^. Supporting Figure S4: Cyclic voltammograms recorded on GCE for a solution of 0.5 M [TBA][PF_6_] and 1.5 M TFE in CH_3_CN under different concentrations of gas impurities in N_2_ matrix of (a) NO_2_ and (b) SO_2_. Inset in (b) shows a magnified view of reduction region. Scan rate V s^−1^. Supporting Figure S5: Schematic representation of the interaction between the active form of the Re complex and selected flue gas components. Computed binding energies (ΔG, kcal mol^−1^) indicated next to each structure. Supporting Figure S6: Current density evolution during constant potential electrolysis (CPE) of 1 mM Re complex in a solution of 0.5 M [TBA][PF_6_] in CH_3_CN containing 1.5 M TFE on bare GCE under different gas atmospheres: 100% (v/v) CO_2_ (red plot) at −1.95 V vs Fc^+^/Fc, 10% CO_2_ at −1.95 V vs Fc^+^/Fc (blue plot) and at −2.05 V (brown plot), and 1% CO_2_ (purple plot) at −1.8 V vs Fc^+^/Fc. Stirring rate for all CPE = 300 rpm. Supporting Figure S7: Current density evolution during constant potential electrolysis (CPE) at −2.05 V vs Fc^+^/Fc of 1 mM Re complex and 0.5 M [TBA][PF_6_] in CH_3_CN solution containing 1.5 M TFE on bare GCE under different gas atmospheres. (a) NO2 (0, 100, 500 and 1000 ppm) and (b) SO_2_ impurities (0, 50, 500 and 1000 ppm), both in 10% v/v CO_2_ in a N_2_ matrix. In both cases, the same color code is used to indicate increasing impurity concentration: blue (0 ppm), brown (100 ppm NO_2_ or 50 ppm SO_2_), gray (500 ppm) and orange (1000 ppm). Stirring rate 300 rpm. Supporting Figure S8: Representative IC chromatograms recorded for blank and post‐electrolysis electrolyte samples. (a) Blank solution: anhydrous CH_3_CN bubbled with 5% v/v NO_2_ in N_2_ matrix. (b) Anolyte samples taken after CPE (1 mM Re complex, 0.5 M [TBA][PF_6_] in CH_3_CN containing 1.5 M TFE; total circulated charge = 14 C) under a 10% v/v CO_2_ atmosphere containing NO_2_ at 100 ppm (black plot) and 1000 ppm (red plot). (c) Catholyte samples for the same experiments, with same color‐coding as in (b). The corresponding NO_2_ ^−^ peak is labeled in panels (b) and (c) next to its retention time (10.34 min). Supporting Figure S9: Current density evolution during constant potential electrolysis (CPE) at −2.05 V vs Fc^+^/Fc of 1 mM Re complex in a solution of 0.5 M [TBA][PF_6_] in CH_3_CN containing 1.5 M TFE on bare GCE under different gas streams: 10% v/v CO_2_ in a N_2_ matrix (blue plot), 10% v/v CO_2_ + 10% v/v O_2_ in a N_2_ matrix (green plot) and with the addition of 50 ppm SO_2_ and 100 ppm NO_2_ (flue gas composition, black plot). Stirring rate 300 rpm. Supporting Figure S10: ^1^H NMR spectrum of [Re(CO)(bpy)Cl] (300 MHz, CD_3_CN) δ/ppm, 1. 7.66 (dt, J = 6.5 Hz, 2H), 8.23 (dt, J = 8 Hz, 2H), 8.45 (d, J = 8.5 Hz, 2H), 9.05 (d, J = 5.7 Hz, 2H). Supporting Figure S11: Schematic illustration of the mass flow controller setup employed to create the different gas mixtures used to purge the H‐type cell, where constant potential electrolysis (CPE) were carried out. Supporting Figure S12: Calibration curves for (a) H_2_ and (b) CO quantification using gas chromatography. The linear fit for both products follows the equation y = a + b*x, intercepting at 0 (a = 0), with b = 1.32 for H_2_ and 277.97 for CO and R^2^ = 0.999 for both cases. Supporting Table S1: Solubility of CO_2_ and O_2_ in different solvents at 298 K.

Conflict of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary Material