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

Yanming Liua,b, Xinfei Fanc, Animesh Nayakb, Ying Wangb, Bing Shanb, Xie Quana, and Thomas J. Meyerb,1

aKey Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China; bDepartment of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599; and cCollege of Environmental Science and Engineering, Dalian Maritime University, Dalian 116024, China

Contributed by Thomas J. Meyer, November 10, 2019 (sent for review May 6, 2019; reviewed by Andrew B. Bocarsly and Clifford P. Kubiak)

Electrochemical reduction of CO2 to multicarbon products is a sig- necessitates coupling reactions between a CO intermediate and/or nificant challenge, especially for molecular complexes. We report intermediates from CO protonation (29–31). Assembling the Ru(II) here CO2 reduction to multicarbon products based on a Ru(II) poly- polypyridyl carbene on an surface that is capable of pyridyl carbene complex that is immobilized on an N-doped porous C–C dimerization offers an attractive strategy for reducing CO2 carbon (RuPC/NPC) electrode. The catalyst utilizes the synergistic to multicarbon products. effects of the Ru(II) polypyridyl carbene complex and the NPC N-doped carbon nanomaterials have been widely used for interface to steer CO2 reduction toward C2 production at low electroreduction due to their electrocatalytic activity and low cost. . In 0.5 M KHCO3/CO2 aqueous solutions, Faradaic N-doped carbon have been shown to be capable of C–C efficiencies of 31.0 to 38.4% have been obtained for C2 production dimerization (6, 32). Combining the Ru(II) polypyridyl carbene at −0.87 to −1.07 V (vs. normal hydrogen electrode) with 21.0 to catalyst with an N-doped porous carbon electrode (RuPC/NPC) 27.5% for ethanol and 7.1 to 12.5% for acetate. Syngas is also pro- provides an appealing approach to facilitate CO2 electroreduction duced with adjustable H2/COmoleratiosof2.0to2.9.TheRuPC/ toward multicarbon products. The immobilized Ru(II) polypyridyl NPC electrocatalyst maintains its activity during 3-h CO2-reduction carbene can provide atomically distributed active sites for electro- periods.

. The large surface area and porous structure of NPC CHEMISTRY favors Ru(II) polypyridyl carbene attachment at catalytic inter- CO2 reduction | electrocatalysis | Ru(II) polypyridyl complex | porous carbon faces and exposes reactive sites. In the experiments described here, the pyrene-derivatized Ru(II) π–π lectrocatalytic reduction of CO2 to useful fuels and chemical polypyridyl carbene complex was attached to NPC by stacking Efeedstocks is a promising strategy for carbon utilization and on the surface. As noted below, the catalytic results were notable in greenhouse gas mitigation. Among the CO2-reduction products identifying a greatly enhanced reactivity toward the formation of including CO, formate, methanol, methane, acetate, ethanol, ethanol as a major product at relatively low overpotentials. etc., liquid multicarbon products such as ethanol and acetate are desirable because of their high energy densities and economic Results and Discussion values (1, 2). A variety of electrocatalysts have been explored for Synthesis and Characterization of the RuPC/NPC Electrode. The RuPC/ CO2 reduction, including metals (3, 4), metal oxides (5), NPC hybrid catalyst was prepared by attaching the pyrene-derivatized heteroatom-doped carbon nanomaterials (6), molecular com- Ru(II) polypyridyl carbene (22, 27, 28, 33) on NPC bonded by π–π plexes (7–9), immobilized molecular complexes (10), and hybrid catalysts (11). Manipulation of morphology (12, 13), oxidation Significance state (14), and introduction of dopants (15), alloys (16), and single-metal atoms (17, 18) have been employed to control Electrochemical reduction of CO2 can convert CO2 emission back , steer product distributions, enhance activity, and to value-added fuels and chemicals and store renewable elec- control selectivity toward specific products. Significant progress tricity. Reducing CO2 to multicarbon products has attracted great has been made, but, to date, the most common products for CO2 interest because of their higher energy densities and associated electroreduction are CO and formate. economic values. We report here a promising strategy for Immobilized molecular complexes such as porphyrins (19, 20), steering CO2 electroreduction toward ethanol production by phthalocyanines (21), polypyridyl carbenes (22), and their hybrid exploiting a surface-bound Ru polypyridyl carbene catalyst on an catalysts (23, 24) have been investigated for electrochemical N-doped porous carbon electrode. We show the synergistic ef- reduction of CO2. They offer the merits of tailorable catalytic fects of Ru polypyridyl carbene for CO intermediate production sites and molecular structures for enabling electrocatalytic per- with a porous carbon for C–C coupling that could boost ethanol formance optimization. By immobilizing molecular complexes on production at relatively low overpotentials. The strategy pro- the electrode surface, the catalysts are easy to reuse and show vides insights on how to improve selectivity and efficiency for

improved CO2-reduction performance (25, 26). In previous works, CO2 reduction toward multicarbon products. we have demonstrated that the Ru(II) polypyridyl carbene complex II 2+ [Ru (tpy)(Mebim-py)(H2O)] (tpy, 2,2′:6′,2″-terpyridine; Mebim-py, Author contributions: Y.L. and T.J.M. designed research; Y.L., X.F., A.N., and Y.W. per- 3-methyl-1-pyridyl-benzimidazol-2-ylidene) is an effective catalyst formed research; Y.L., X.F., A.N., Y.W., B.S., X.Q., and T.J.M. analyzed data; and Y.L., X.F., A.N., Y.W., B.S., X.Q., and T.J.M. wrote the paper. for electrochemical reduction of CO2 to CO with high selectivity in Reviewers: A.B.B., Princeton University; and C.P.K., University of California San Diego. solutionandonsurfaces(22,27,28).CO2 reduction occurs by proton-coupled electron transfer through a carbene complex sta- The authors declare no competing interest. II + bilized intermediate [Ru (tpy)(Mebim-py)(CO)] to give CO as Published under the PNAS license. 1 the product, which could reduce overpotential for CO2 reduction. To whom correspondence may be addressed. Email: [email protected]. A significant challenge that remains is development of electro- This article contains supporting information online at https://www.pnas.org/lookup/suppl/ catalysts that steer CO2 reduction toward multicarbon products doi:10.1073/pnas.1907740116/-/DCSupplemental. with high selectivity at low overpotentials. Formation of C–C bonds

www.pnas.org/cgi/doi/10.1073/pnas.1907740116 PNAS Latest Articles | 1of6 Downloaded by guest on September 26, 2021 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. A transmission electron microscopic (TEM) image shows that the NPC has a mesoporous structure (Fig. 2A). Its surface area, 2 −1 measured from N2 –desorption isotherms, is 302.4 m ·g (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 dis- tribution of Ru and N on an RuPC/NPC hybrid catalyst, re- vealing that the Ru(II) polypyridyl carbene is homogeneously distributed on 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 Fig. 2. TEM image (A)andN adsorption–desorption isotherm (B) of RuPC/NPC. carbene has been successfully anchored to the surface. 2 The amount of electrochemically active Ru(II) polypyridyl − carbene attached on NPC was estimated from cyclic voltammetry electroactive Ru(II) polypyridyl carbene (mol·cm 2), n is the num- − measurements on an RuPC/NPC hybrid electrode. Surface con- ber of electrons transferred, F is the Faraday constant (C·mol 1), Q = ΓnFA Q tent was evaluated by the expression ,where is the and A is the electrode area (cm2). Fig. 4C shows that there are no Γ charge obtained from redox peak integration, is the amount of redox peaks in the cyclic voltammogram (CV) for the NPC elec- trode from 0.6 to 1.3 V (vs. normal hydrogen electrode [NHE]), while reversible redox waves do appear at E1/2 = 1.05 V (vs. NHE) in the CV curve for the RuPC/NPC electrode (Fig. 4D). The waves in the CV arise from the RuIII/RuII redox couple at a potential similar to results found previously (27). The amount of Ru poly- pyridyl 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 to- ward 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 Ap- pendix, Fig. S3). The net current density for CO2 reduction on NPC Fig. 1. Schematic illustration illustrating preparation of the RuPC/NPC electrode. was lower than on the RuPC/NPC under the same conditions,

2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1907740116 Liu et al. Downloaded by guest on September 26, 2021 0.5 M KHCO3 aqueous solutions at −0.97 and −1.07 V (vs. NHE). As expected, there was no evidence for carbon-containing 13 compounds after 1 h of . The CO2-reduction ex- periment 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 for- mation 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 pro- duction 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 Fig. 3. SEM images of NPC (A) and RuPC/NPC (B) and energy-dispersive X- current densities (SI Appendix, Fig. S6) and efficiencies for ray spectroscopic maps of N (C) and Ru (D) on RuPC/NPC. 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 in- consistent with enhanced CO reduction by the surface-bound 2 creased gradually. The electrolysis results show that ethanol Ru(II) polypyridyl carbene. production is favored on the RuPC/NPC electrode at less neg-

To probe for CO -reduction products, bulk electrolysis was CHEMISTRY 2 ative potentials. Increasing the potential to or beyond −1.17 V performed on RuPC/NPC electrodes in CO2-saturated 0.5 M (vs. NHE) caused CO to become the major CO2-reduction KHCO3 aqueous solutions with potentials from −0.87 to −1.17 V product to give syngas, CO, and H2 (SI Appendix, Fig. S7). Syngas (vs. NHE). The liquid and gas products were detected by gas 1 is an important chemical feedstock for synthesizing bulk chemicals chromatography and H NMR. Ethanol, acetate, methanol, CO, and fuels such as methanol, acetic acid, and others. The molar and formate have been identified as CO2-reduction products for ratio of H2/CO syngas was 2.9:1 at −0.87 V (vs. NHE), which RuPC/NPC (SI Appendix, Fig. S4). To confirm that these prod- decreased gradually to 2:1 as potential shifted negatively from ucts originated from catalyzed CO2 reduction, bulk electrolysis −0.87 to −1.17 V (vs. NHE) (Fig. 5C). However, it increased to on the RuPC/NPC electrode was performed in Ar-saturated 2.4:1 at more negative potential (−1.17 V vs. NHR). The potential

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 −1 TBAPF6/MeCN at a scan rate of 10 mV·s . a.u., arbitrary units.

Liu et al. PNAS Latest Articles | 3of6 Downloaded by guest on September 26, 2021 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.

dependent H2/CO ratios can meet the requirements for down- where C–C coupling can occur, being promising for steering CO2 stream chemical and fuel production (36). electroreduction toward multicarbon products. The NPC electrode also reduced CO2 to ethanol, acetate, Achieving high stability is a significant challenge for heter- methanol, formate, and CO, but the product distribution was ogenous molecular catalysis. For CO2 reduction by the RuPC/ different from the RuPC/NPC electrode. Fig. 5D compares C2 NPC electrode, electrochemical reduction was investigated over product efficiencies between NPC and RuPC/NPC. Ethanol ef- a 3-h electrolysis period at −0.97 V (vs. NHE) in 0.5 M KHCO3 ficiency on RuPC/NPC was enhanced by 1.9 to 2.2 times relative aqueous solution. During the electrolysis period, the current to that on NPC, from −0.87 to −1.07 V (vs. NHE), but there was density for RuPC/NPC was nearly stable except an initial de- no obvious difference in acetate efficiency with or without the crease (break-in period) (Fig. 6A). The Faradaic efficiency for Ru(II) polypyridyl carbene catalyst. Since CO is the only product multicarbon products, ethanol, and acetate was 33.1 to 37.3% of CO2 reduction by the Ru(II) polypyridyl carbene (22, 27, 28), for a 3-h experiment (Fig. 6B), and the efficiencies for other the high ethanol efficiency for RuPC/NPC must arise from the CO2-reduction products presented no obvious change, sug- synergistic effect of Ru(II) polypyridyl carbene and NPC at the gesting that the RuPC/NPC electrode was stable during 3 h of interface. CO reduction. A Leach test with inductively coupled plasma The potential for CO reduction on RuPC/NPC is more pos- 2 2 atomic emission spectroscopy analysis confirmed its stability itive than for the Ru(II) polypyridyl carbene in solution (details are in SI Appendix). The RuPC/NPC hybrid catalyst is (−1.20 to −1.50 V vs. NHE) (27, 28) or as heterogenous − − more stable than the reported Ru(II) polypyridyl carbene het- catalyst ( 0.96 to 1.16 V vs. NHE) (22). Dimerization or ∼ protonation of adsorbed CO have been reported as the main erogeneous catalyst, which has a lifetime of 15 min under similar pathways for C2 production (SI Appendix, Fig. S8) (30, 31, 37). conditions (22). CO2 reduction on Ru(II) polypyridyl carbene occurs through − Conclusions initial 2e transfer to the ligands, followed by reaction with CO2 − − + to give [RuII(tpy)(Mebim-py)(COO2 )]0, and 1e /1H reduction A method is described here for constructing a heterogenous − to give [RuII(tpy )(Mebim-py)(COOH)]0. In the subsequent steps, molecular catalyst that steers electroreduction of CO2 toward − + – it undergoes further reduction to give [RuII(tpy )(Mebim-py)(CO)] C C-bonded products, notably ethanol. It is based on anchor- as the intermediate (27, 28). The enhanced ethanol efficiency for ing a Ru(II) polypyridyl carbene complex on NPC. With the RuPC/NPC may arise from that CO intermediates adsorbed at the synergistic effects of the Ru(II) polypyridyl carbene catalyst for II − + – Ru polypyridyl carbene, [Ru (tpy )(Mebim-py)(CO)] ,are CO intermediate production and NPC for C C coupling, transformed to C2 products at the RuPC/NPC interface by C–C electrochemical reduction of CO2 to ethanol occurs with a coupling between CO intermediates or intermediates from CO Faradaic efficiency of 27.5% at relatively low overpotentials. protonation. The porous structure of RuPC/NPC probably can Appearance of ethanol is in competition with a syngas mixture facilitate C–C coupling reaction via nanoconfinement of CO2 or of H2/COatamoleratioof2.0to2.9.TheRuPC/NPCelec- CO2 reduction intermediates (38, 39). The results highlighted trocatalyst is stable toward CO2 reduction for a period of 3 h and here point to the strategy, assembling molecular complexes that adds a promising lead for the reduction of CO2 to multicarbon are active toward producing C1 intermediates on carbon materials products.

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1907740116 Liu et al. Downloaded by guest on September 26, 2021 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.Thepre- pared 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 Evs. NHE = Evs. Ag/AgCl + 0.2 (V). The activity of RuPC/NPC for

electrocatalytic CO2 reduction was examined by linear-sweep voltammo-

grams in CO2- or Ar-saturated 0.5 M KHCO3 aqueous electrolyte (scan rate of −1 10 mV·s ). 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 1 ionization detector (FID). For H NMR spectra, solvent suppression was CHEMISTRY

applied for CO2-reduction product analysis to reduce the intensity of

water peak. The samples were collected into 10% D2OwithDMSOasin- ternal 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 prod- ucts (mol), F is the Faraday constant (C·mol−1), and Q is the amount of charge passed through the cell (C).

Fig. 6. Current density (A) and Faradaic efficiencies (B) for CO reduction on 2 Data Availability. All data are included in the main text and SI Appendix. RuPC/NPC over a 3-h period in 0.5 M KHCO3 aqueous solutions (−0.97 V vs. NHE). ACKNOWLEDGMENTS. This work was supported by the US Department Materials and Methods of Energy, Office of Basic Energy Sciences Award DE-SC0015739; and National Natural Science Foundation of China Grants 21707016 and 51708085. Preparation of RuPC/NPC Electrode. A total of 60.0 mg of NPC was added into Y.L. was supported by the State Scholarship Fund from the China 10.0 μL of Nafion (5 wt%) and 490.0 μL of isopropanol. RuPC dissolved at Scholarship Council.

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