Steering CO₂ electroreduction toward ethanol production by a surface-bound Ru polypyridyl carbene catalyst on N-doped porous carbon

Significance

Electrochemical reduction of CO₂ can convert CO₂ emission back to value-added fuels and chemicals and store renewable electricity. Reducing CO₂ to multicarbon products has attracted great interest because of their higher energy densities and associated economic values. We report here a promising strategy for steering CO₂ electroreduction toward ethanol production by exploiting a surface-bound Ru polypyridyl carbene catalyst on an N-doped porous carbon electrode. We show the synergistic effects of Ru polypyridyl carbene for CO intermediate production with a porous carbon for C–C coupling that could boost ethanol production at relatively low overpotentials. The strategy provides insights on how to improve selectivity and efficiency for CO₂ reduction toward multicarbon products.

Abstract

Electrochemical reduction of CO₂ to multicarbon products is a significant challenge, especially for molecular complexes. We report here CO₂ reduction to multicarbon products based on a Ru(II) polypyridyl carbene complex that is immobilized on an N-doped porous carbon (RuPC/NPC) electrode. The catalyst utilizes the synergistic effects of the Ru(II) polypyridyl carbene complex and the NPC interface to steer CO₂ reduction toward C2 production at low overpotentials. In 0.5 M KHCO₃/CO₂ aqueous solutions, Faradaic efficiencies of 31.0 to 38.4% have been obtained for C2 production at −0.87 to −1.07 V (vs. normal hydrogen electrode) with 21.0 to 27.5% for ethanol and 7.1 to 12.5% for acetate. Syngas is also produced with adjustable H₂/CO mole ratios of 2.0 to 2.9. The RuPC/NPC electrocatalyst maintains its activity during 3-h CO₂-reduction periods.

Electrocatalytic reduction of CO₂ to useful fuels and chemical feedstocks is a promising strategy for carbon utilization and greenhouse gas mitigation. Among the CO₂-reduction products including CO, formate, methanol, methane, acetate, ethanol, etc., liquid multicarbon products such as ethanol and acetate are desirable because of their high energy densities and economic values (1, 2). A variety of electrocatalysts have been explored for CO₂ reduction, including metals (3, 4), metal oxides (5), heteroatom-doped carbon nanomaterials (6), molecular complexes (7–9), immobilized molecular complexes (10), and hybrid catalysts (11). Manipulation of morphology (12, 13), oxidation state (14), and introduction of dopants (15), alloys (16), and single-metal atoms (17, 18) have been employed to control overpotential, steer product distributions, enhance activity, and control selectivity toward specific products. Significant progress has been made, but, to date, the most common products for CO₂ electroreduction are CO and formate.

Immobilized molecular complexes such as porphyrins (19, 20), phthalocyanines (21), polypyridyl carbenes (22), and their hybrid catalysts (23, 24) have been investigated for electrochemical reduction of CO₂. They offer the merits of tailorable catalytic sites and molecular structures for enabling electrocatalytic performance optimization. By immobilizing molecular complexes on the electrode surface, the catalysts are easy to reuse and show improved CO₂-reduction performance (25, 26). In previous works, we have demonstrated that the Ru(II) polypyridyl carbene complex [Ru^II(tpy)(Mebim-py)(H₂O)]²⁺ (tpy, 2,2′:6′,2″-terpyridine; Mebim-py, 3-methyl-1-pyridyl-benzimidazol-2-ylidene) is an effective catalyst for electrochemical reduction of CO₂ to CO with high selectivity in solution and on surfaces (22, 27, 28). CO₂ reduction occurs by proton-coupled electron transfer through a carbene complex stabilized intermediate [Ru^II(tpy)(Mebim-py)(CO)]⁺ to give CO as the product, which could reduce overpotential for CO₂ reduction. A significant challenge that remains is development of electrocatalysts that steer CO₂ reduction toward multicarbon products with high selectivity at low overpotentials. Formation of C–C bonds necessitates coupling reactions between a CO intermediate and/or intermediates from CO protonation (29–31). Assembling the Ru(II) polypyridyl carbene on an electrode surface that is capable of C–C dimerization offers an attractive strategy for reducing CO₂ to multicarbon products.

N-doped carbon nanomaterials have been widely used for electroreduction due to their electrocatalytic activity and low cost. N-doped carbon electrodes have been shown to be capable of C–C dimerization (6, 32). Combining the Ru(II) polypyridyl carbene catalyst with an N-doped porous carbon electrode (RuPC/NPC) provides an appealing approach to facilitate CO₂ electroreduction toward multicarbon products. The immobilized Ru(II) polypyridyl carbene can provide atomically distributed active sites for electrocatalysis. The large surface area and porous structure of NPC favors Ru(II) polypyridyl carbene attachment at catalytic interfaces and exposes reactive sites.

In the experiments described here, the pyrene-derivatized Ru(II) polypyridyl carbene complex was attached to NPC by π–π stacking on the surface. As noted below, the catalytic results were notable in identifying a greatly enhanced reactivity toward the formation of ethanol as a major product at relatively low overpotentials.

Results and Discussion

Synthesis and Characterization of the RuPC/NPC Electrode.

The RuPC/NPC hybrid catalyst was prepared by attaching the pyrene-derivatized Ru(II) polypyridyl carbene (22, 27, 28, 33) on NPC bonded by π–π interactions between pyrene and the hexatomic carbon rings of NPC (Fig. 1). The pyrene unit enables stable immobilization of Ru polypyridyl carbene on carbon surface (25). Briefly, the resol was synthesized from phenol and formaldehyde polymerization under basic conditions. The thermosetting resin was prepared by solvent evaporation-induced self-assembly between resol, Pluronic F127 (soft template), and dicyandiamide (nitrogen source) (34). The synthesized resin was pyrolyzed at 750 °C to obtain NPC. An RuPC/NPC electrode was prepared by loading the pyrene-derivatized Ru(II) polypyridyl carbene complex on NPC by π–π interactions. In brief, the NPC suspension (120 g/L in isopropanol) and RuPC solution (100 μM in dimethyl sulfoxide [DMSO]-D6) were mixed (vol:vol = 10:1) by sonication for 10 min and stirring for 12 h, followed by drop-coating on a carbon fiber paper substrate. As a comparison, the NPC electrode was prepared with the same method without the added Ru(II) polypyridyl carbene.

Fig. 1.

Schematic illustration illustrating preparation of the RuPC/NPC electrode.

A transmission electron microscopic (TEM) image shows that the NPC has a mesoporous structure (Fig. 2A). Its surface area, measured from N₂ adsorption–desorption isotherms, is 302.4 m²⋅g⁻¹ (Fig. 2B). The pore-size distribution curve (SI Appendix, Fig. S1) confirms that NPC has mesopores with sizes around 19.1 nm. The total pore volume of NPC is 0.32 cm³⋅g⁻¹. The scanning electron microscopic (SEM) image shows abundant pores on the NPC surface (Fig. 3A). After attaching the Ru(II) polypyridyl carbene on NPC, the surface of the RuPC/NPC hybrid catalyst maintains the porous structure (Fig. 3B). Energy-dispersive X-ray spectroscopic maps (Fig. 3 C and D) show the uniform distribution of Ru and N on an RuPC/NPC hybrid catalyst, revealing that the Ru(II) polypyridyl carbene is homogeneously distributed on NPC.

Fig. 2.

TEM image (A) and N₂ adsorption–desorption isotherm (B) of RuPC/NPC.

Fig. 3.

SEM images of NPC (A) and RuPC/NPC (B) and energy-dispersive X-ray spectroscopic maps of N (C) and Ru (D) on RuPC/NPC.

The N-containing species and surface content of NPC was investigated by X-ray photoelectron spectroscopy (XPS). As shown in the N 1s spectrum (Fig. 4A), pyridinic N (398.5 eV), pyrrolic N (400.1 eV), and graphitic N (401.2 eV) are observed on NPC (32). Among the 3 N species, pyridinic N is the main N species for NPC, with a percentage of 49.9%, which has been proved to be the most active N species for electrocatalysis (32). The total N content is 5.0 atomic% for NPC. In the XPS spectrum of the RuPC/NPC catalyst (Fig. 4B), the small peak centered at 281.1 eV arises from Ru (35), showing that Ru(II) polypyridyl carbene has been successfully anchored to the surface.

Fig. 4.

N 1s XPS spectrum of NPC (A), Ru 3d and C 1s spectrum of RuPC/NPC (B), and cyclic voltammograms of NPC (C) and RuPC/NPC (D) electrodes in 0.1 M TBAPF₆/MeCN at a scan rate of 10 mV⋅s⁻¹. a.u., arbitrary units.

The amount of electrochemically active Ru(II) polypyridyl carbene attached on NPC was estimated from cyclic voltammetry measurements on an RuPC/NPC hybrid electrode. Surface content was evaluated by the expression Q = ΓnFA, where Q is the charge obtained from redox peak integration, Γ is the amount of electroactive Ru(II) polypyridyl carbene (mol⋅cm⁻²), n is the number of electrons transferred, F is the Faraday constant (C⋅mol⁻¹), and A is the electrode area (cm²). Fig. 4C shows that there are no redox peaks in the cyclic voltammogram (CV) for the NPC electrode from 0.6 to 1.3 V (vs. normal hydrogen electrode [NHE]), while reversible redox waves do appear at E_1/2 = 1.05 V (vs. NHE) in the CV curve for the RuPC/NPC electrode (Fig. 4D). The waves in the CV arise from the Ru^III/Ru^II redox couple at a potential similar to results found previously (27). The amount of Ru polypyridyl carbene loaded on the RuPC/NPC electrode is estimated to be 1.5 ± 0.2 nmol⋅cm⁻² based on peak area measurements of 3 electrodes.

Electrocatalytic Reduction of CO₂.

The activity of RuPC/NPC toward electrochemical reduction of CO₂ was examined by linear-sweep voltammetry in 0.5 M KHCO₃ aqueous solutions (Fig. 5A). Its current density in a CO₂-saturated solution at potentials more negative than −0.62 V (vs. NHE) was notably enhanced compared to an Ar-saturated solution, implying the reduction of CO₂ at a low onset potential by the RuPC/NPC electrode. The catalytic current was stable during 3 scans (SI Appendix, Fig. S2). CO₂ reduction by the NPC electrode was also evaluated (SI Appendix, Fig. S3). The net current density for CO₂ reduction on NPC was lower than on the RuPC/NPC under the same conditions, consistent with enhanced CO₂ reduction by the surface-bound Ru(II) polypyridyl carbene.

Fig. 5.

(A) Linear-sweep voltammograms for RuPC/NPC in Ar- or CO₂-saturated 0.5 M KHCO₃ aqueous solutions. (B) Faradaic efficiencies for ethanol, acetate, methanol, formate, and CO production on RuPC/NPC over a 1-h period. (C) Mole ratios of H₂/CO produced on RuPC/NPC. (D) Faradaic efficiencies for C2 products at NPC or RuPC/NPC electrodes over a 1-h period in 0.5 M KHCO₃ aqueous solutions.

To probe for CO₂-reduction products, bulk electrolysis was performed on RuPC/NPC electrodes in CO₂-saturated 0.5 M KHCO₃ aqueous solutions with potentials from −0.87 to −1.17 V (vs. NHE). The liquid and gas products were detected by gas chromatography and ¹H NMR. Ethanol, acetate, methanol, CO, and formate have been identified as CO₂-reduction products for RuPC/NPC (SI Appendix, Fig. S4). To confirm that these products originated from catalyzed CO₂ reduction, bulk electrolysis on the RuPC/NPC electrode was performed in Ar-saturated 0.5 M KHCO₃ aqueous solutions at −0.97 and −1.07 V (vs. NHE). As expected, there was no evidence for carbon-containing compounds after 1 h of electrolysis. The ¹³CO₂-reduction experiment was performed at −0.97 V (vs. NHE), and the liquid products were analyzed by ¹H NMR. In the ¹H NMR spectrum (SI Appendix, Fig. S5), H–¹³C signals for ethanol, acetate, methanol, and formate with peak splitting were clearly observed, while H–¹²C signals were negligible, confirming that the products were derived from CO₂ reduction.

Fig. 5B shows the Faradaic efficiency for the products that appeared after CO₂ reduction on the RuPC/NPC electrode for 1 h. In the data, it is notable that the Faradaic efficiency for ethanol (21.0 to 27.5%) is much higher than efficiencies for other products at potentials from −0.87 to −1.07 V (vs. NHE). For example, at −0.97 V (vs. NHE), the efficiency for ethanol formation was 2.5 to 5.2 times higher than efficiencies for acetate, methanol, CO, and formate. The experimental observations show that RuPC/NPC has a high selectivity for ethanol production at relatively low overpotentials. The total efficiencies for the multicarbon products, ethanol and acetate, were 31.0 to 38.4% at −0.87 to −1.07 V (vs. NHE). When the potential was shifted negatively from −0.87 to −1.17 V (vs. NHE), both the current densities (SI Appendix, Fig. S6) and efficiencies for ethanol and acetate increased initially and then decreased at potentials more negative than −1.07 V (vs. NHE). Over the same potential range, the current density and efficiencies for CO increased gradually. The electrolysis results show that ethanol production is favored on the RuPC/NPC electrode at less negative potentials. Increasing the potential to or beyond −1.17 V (vs. NHE) caused CO to become the major CO₂-reduction product to give syngas, CO, and H₂ (SI Appendix, Fig. S7). Syngas is an important chemical feedstock for synthesizing bulk chemicals and fuels such as methanol, acetic acid, and others. The molar ratio of H₂/CO syngas was 2.9:1 at −0.87 V (vs. NHE), which decreased gradually to 2:1 as potential shifted negatively from −0.87 to −1.17 V (vs. NHE) (Fig. 5C). However, it increased to 2.4:1 at more negative potential (−1.17 V vs. NHR). The potential dependent H₂/CO ratios can meet the requirements for downstream chemical and fuel production (36).

The NPC electrode also reduced CO₂ to ethanol, acetate, methanol, formate, and CO, but the product distribution was different from the RuPC/NPC electrode. Fig. 5D compares C2 product efficiencies between NPC and RuPC/NPC. Ethanol efficiency on RuPC/NPC was enhanced by 1.9 to 2.2 times relative to that on NPC, from −0.87 to −1.07 V (vs. NHE), but there was no obvious difference in acetate efficiency with or without the Ru(II) polypyridyl carbene catalyst. Since CO is the only product of CO₂ reduction by the Ru(II) polypyridyl carbene (22, 27, 28), the high ethanol efficiency for RuPC/NPC must arise from the synergistic effect of Ru(II) polypyridyl carbene and NPC at the interface.

The potential for CO₂ reduction on RuPC/NPC is more positive than for the Ru(II) polypyridyl carbene in solution (−1.20 to −1.50 V vs. NHE) (27, 28) or as heterogenous catalyst (−0.96 to −1.16 V vs. NHE) (22). Dimerization or protonation of adsorbed CO have been reported as the main pathways for C2 production (SI Appendix, Fig. S8) (30, 31, 37). CO₂ reduction on Ru(II) polypyridyl carbene occurs through initial 2e⁻ transfer to the ligands, followed by reaction with CO₂ to give [Ru^II(tpy)(Mebim-py)(COO²⁻)]⁰, and 1e⁻/1H⁺ reduction to give [Ru^II(tpy⁻)(Mebim-py)(COOH)]⁰. In the subsequent steps, it undergoes further reduction to give [Ru^II(tpy⁻)(Mebim-py)(CO)]⁺ as the intermediate (27, 28). The enhanced ethanol efficiency for RuPC/NPC may arise from that CO intermediates adsorbed at the Ru polypyridyl carbene, [Ru^II(tpy⁻)(Mebim-py)(CO)]⁺, are transformed to C2 products at the RuPC/NPC interface by C–C coupling between CO intermediates or intermediates from CO protonation. The porous structure of RuPC/NPC probably can facilitate C–C coupling reaction via nanoconfinement of CO₂ or CO₂ reduction intermediates (38, 39). The results highlighted here point to the strategy, assembling molecular complexes that are active toward producing C1 intermediates on carbon materials where C–C coupling can occur, being promising for steering CO₂ electroreduction toward multicarbon products.

Achieving high stability is a significant challenge for heterogenous molecular catalysis. For CO₂ reduction by the RuPC/NPC electrode, electrochemical reduction was investigated over a 3-h electrolysis period at −0.97 V (vs. NHE) in 0.5 M KHCO₃ aqueous solution. During the electrolysis period, the current density for RuPC/NPC was nearly stable except an initial decrease (break-in period) (Fig. 6A). The Faradaic efficiency for multicarbon products, ethanol, and acetate was 33.1 to 37.3% for a 3-h experiment (Fig. 6B), and the efficiencies for other CO₂-reduction products presented no obvious change, suggesting that the RuPC/NPC electrode was stable during 3 h of CO₂ reduction. A Leach test with inductively coupled plasma atomic emission spectroscopy analysis confirmed its stability (details are in SI Appendix). The RuPC/NPC hybrid catalyst is more stable than the reported Ru(II) polypyridyl carbene heterogeneous catalyst, which has a lifetime of ∼15 min under similar conditions (22).

Fig. 6.

Current density (A) and Faradaic efficiencies (B) for CO₂ reduction on RuPC/NPC over a 3-h period in 0.5 M KHCO₃ aqueous solutions (−0.97 V vs. NHE).

Conclusions

A method is described here for constructing a heterogenous molecular catalyst that steers electroreduction of CO₂ toward C–C-bonded products, notably ethanol. It is based on anchoring a Ru(II) polypyridyl carbene complex on NPC. With the synergistic effects of the Ru(II) polypyridyl carbene catalyst for CO intermediate production and NPC for C–C coupling, electrochemical reduction of CO₂ to ethanol occurs with a Faradaic efficiency of 27.5% at relatively low overpotentials. Appearance of ethanol is in competition with a syngas mixture of H₂/CO at a mole ratio of 2.0 to 2.9. The RuPC/NPC electrocatalyst is stable toward CO₂ reduction for a period of 3 h and adds a promising lead for the reduction of CO₂ to multicarbon products.

Materials and Methods

Preparation of RuPC/NPC Electrode.

A total of 60.0 mg of NPC was added into 10.0 μL of Nafion (5 wt%) and 490.0 μL of isopropanol. RuPC dissolved at DMSO-D6 (100 μM, 50 μL) was added into the suspension. After sonication for 10 min and stirring for 12 h, the suspension was drop-coated on carbon-fiber substrate with a catalyst loading of 0.03 g⋅cm⁻². The prepared RuPC/NPC electrode was washed by isopropanol to remove Ru polypyridyl carbene, which was not anchored on NPC, and vacuum dried at 80 °C.

Electrochemical Experiments.

All of the electrochemical tests were conducted in a 3-electrode system at room temperature. RuPC/NPC or NPC was used as the working electrode, and Pt foil was used as the counterelectrode. The amount of Ru polypyridyl carbene anchored on the RuPC/NPC electrode was measured by cyclic voltammograms in Ar saturated in 0.1 M TBAPF₆/MeCN with Ag as reference electrode. The potential was converted to vs. NHE by using ferrocene for calibration.

All CO₂-reduction experiments were tested with Ag/AgCl as reference electrode, and the potentials were converted to vs. NHE by using the equation of E_{vs. NHE} = E_{vs. Ag/AgCl} + 0.2 (V). The activity of RuPC/NPC for electrocatalytic CO₂ reduction was examined by linear-sweep voltammograms in CO₂- or Ar-saturated 0.5 M KHCO₃ aqueous electrolyte (scan rate of 10 mV⋅s⁻¹). CO₂ bulk electrolysis was performed at −0.87 to approximately −1.17 V (vs. NHE) in a gas-tight H-type 2-chamber cell separated by Nafion 117 membrane, which was filled with CO₂-saturated 0.5 M KHCO₃ aqueous solution.

Product Analysis.

After bulk electrolysis, gas samples were drawn from the headspace of the gas-tight cell and injected into gas chromatography (Varian 450) equipped with a thermal conductivity detector. The liquid products were measured by a ¹H NMR spectrum (Bruker B600) and gas chromatography (Shimadzu, catalog no. GC-2010) equipped with a flame ionization detector (FID). For ¹H NMR spectra, solvent suppression was applied for CO₂-reduction product analysis to reduce the intensity of water peak. The samples were collected into 10% D₂O with DMSO as internal standard for quantification. The amount of ethanol produced was confirmed by gas chromatography (Shimadzu, catalog no. GC-2010) equipped with an FID detector and DB-Wax column (30 m × 0.25 mm × 0.50 μm).

The Faradaic efficiency (FE) for CO₂ reduction was calculated via the equation of FE = nNF/Q, where n is the number of electron transferred for CO₂ reduction to products, N is the molar quantity of CO₂-reduction products (mol), F is the Faraday constant (C⋅mol⁻¹), and Q is the amount of charge passed through the cell (C).

Data Availability.

All data are included in the main text and SI Appendix.