Steering CO2 electroreduction toward ethanol 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 electrode 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. overpotentials. In 0.5 M KHCO3/CO2 aqueous solutions, Faradaic N-doped carbon electrodes 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. catalysis. 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 overpotential, 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 adsorption–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.