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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, 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 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 carbene has been successfully anchored to the surface. 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 con- tent was evaluated by the expression Q = ΓnFA, where Q is the charge obtained from redox peak integration, Γ is the amount of Fig. 2. TEM image (A) and N2 adsorption–desorption isotherm (B) of RuPC/NPC. Fig. 1. 2 of 6 Schematic illustration illustrating preparation of the RuPC/NPC electrode. | www.pnas.org/cgi/doi/10.1073/pnas.1907740116 electroactive Ru(II) polypyridyl carbene (mol·cm−2), n is the num- ber 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 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 was lower than on the RuPC/NPC under the same conditions, Liu et al. Downloaded by guest on May 6, 2021PDF Image | Steering CO2 electroreduction toward ethanol production
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