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Highly Active Oxygen Evolution Integrated with Efficient CO2 to CO Highly active oxygen evolution integrated with efficient CO2 to CO electroreduction Yongtao Menga,b,1, Xiao Zhangb,c,1, Wei-Hsuan Hungb,d, Junkai Hee, Yi-Sheng Tsaid, Yun Kuangb, Michael J. Kenneyb, Jing-Jong Shyuef, Yijin Liug, Kevin H. Stoneg, Xueli Zhengh, Steven L. Suibe, Meng-Chang Lina, Yongye Liangc, and Hongjie Daib,2 aCollege of Electrical Engineering and Automation, Shandong University of Science and Technology, 266590 Qingdao, China; bDepartment of Chemistry, Stanford University, Stanford, CA 94305; cDepartment of Materials Science and Engineering, South University of Science and Technology of China, 518055 Shenzhen, China; dInstitute of Materials Science and Engineering, National Central University, 32001 Taoyuan, Taiwan; eInstitute of Materials Science, University of Connecticut, Storrs, CT 06269; fResearch Center of Applied Science, Academia Sinica, 115 Taipei, Taiwan; gStanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025; and hDepartment of Material Science and Engineering, Stanford University, Stanford, CA 94305 Contributed by Hongjie Dai, October 14, 2019 (sent for review September 4, 2019; reviewed by Dehui Deng and Dunwei Wang) – Electrochemical reduction of CO2 to useful chemicals has been ac- in biologically relevant systems. The widely used IrO2 (13 15) and tively pursued for closing the carbon cycle and preventing further Pt (1) anode catalysts were limited by their scarcity, high cost, and deterioration of the environment/climate. Since CO2 reduction re- insufficient activity. An active NiCoFeP catalyst was reported re- action (CO2RR) at a cathode is always paired with the oxygen cently, but required multistep, complex synthesis and noble-metal evolution reaction (OER) at an anode, the overall efficiency of gold-coated foams (16). An electrodeposited NiOx catalyst on electrical energy to chemical fuel conversion must consider the glassy carbon was also reported, with low performance of small large energy barrier and sluggish kinetics of OER, especially in current density (<5mA/cm2) at high overpotentials (467-mV 2 widely used electrolytes, such as the pH-neutral CO2-saturated overpotential to reach 1 mA/cm ) (17). 0.5 M KHCO3. OER in such electrolytes mostly relies on noble metal Here, we developed a simple, 1-step approach to a nonprecious (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover Ni–Fe hydroxide carbonate (NiFe-HC) electrode for superior that by anodizing a metallic Ni–Fe composite foam under a harsh OER anode in neutral (pH ∼ 7.4) 0.5 M KHCO3 electrolyte condition (in a low-concentration 0.1 M KHCO3 solution at 85 °C CHEMISTRY widely used for CO2 electroreduction. A piece of commercial Ni–Fe under a high-current ∼250 mA/cm2), OER on the NiFe foam is ac- foam was anodized against a platinum mesh in a 0.1 M KHCO3 companied by anodic etching, and the surface layer evolves into a solution maintained at 85 °C at a constant current of 250 mA/cm2 nickel–iron hydroxide carbonate (NiFe-HC) material composed of for 16 h (see Methods for details and voltage vs. time curve in porous, poorly crystalline flakes of flower-like NiFe layer-double Fig. 1B), after which the original metallic NiFe foam turned into a hydroxide (LDH) intercalated with carbonate anions. The resulting dark foam. Obvious etching of the foam was seen from the debris NiFe-HC electrode in CO -saturated 0.5 M KHCO exhibited OER 2 3 and color change in the electrolyte. The original smooth NiFe activity superior to IrO2, with an overpotential of 450 and 590 ∼ μ D SI Appendix mV to reach 10 and 250 mA/cm2, respectively, and high stability wires ( 100- mwires;Fig.1 and , Figs. S1 and S2)in the foam evolved into highly porous and rough structures fully for >120 h without decay. We paired NiFe-HC with a CO2RR cata- lyst of cobalt phthalocyanine/carbon nanotube (CoPc/CNT) in a CO2 electrolyzer, achieving selective cathodic conversion of CO2 Significance to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O2. The device showed a low cell voltage of Electrochemical reduction of CO2 to useful chemicals or fuels is 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a critical to closing the carbon cycle and preventing further de- current density of 10 mA/cm2. terioration of the environment/climate. This work addresses the low-energy-efficiency problem of CO2 reduction limited by oxygen evolution | CO2 reduction | CO2 electrolyzer | electrocatalysis | sluggish oxygen evolution reaction (OER) on the anode side. pH-neutral electrolyte The only active OER catalysts for coupling CO2 reduction in neutral conditions are based on noble metals such as Ir, Ru, and gold. Herein, we developed a nonprecious-metal-based OER arge-scale capturing and reduction of CO2 to useful chem- Licals and fuels could be pivotal to the world’s sustainability anode with higher activity and stability than those based on and environment. The combined electrochemical CO reduction noble-metal catalysts IrO2 and Ir/C. We integrated our anode 2 > and water oxidation is a promising approach to tackle the world’s with a selective CO2 reduction cathode to achieve 97% con- growing energy demands without compromising the environment version of CO2 to CO and a record-setting high energy effi- ciency for CO2 conversion. by closing the carbon cycle. In a typical CO2 reduction electro- lyzer, OER at the anode is accompanied by CO2 reduction in the Author contributions: Y.M. and H.D. designed research; Y.M., X. Zhang, W.-H.H., J.H., cathode into fuels such as CO (1), formic acid (2), ethanol (3), Y.-S.T., M.J.K., J.-J.S., K.H.S., and X. Zheng performed research; Y.M., X. Zhang, W.-H.H., ethane, ethylene (4), and other multicarbon products (5, 6). In- J.H., Y.-S.T., Y.K., J.-J.S., Y. Liu, K.H.S., X. Zheng, S.L.S., M.-C.L., Y. Liang, and H.D. contrib- tense effort has been taken to develop highly efficient electro- uted new reagents/analytic tools; Y.M., X. Zhang, W.-H.H., J.H., Y.-S.T., and H.D. analyzed data; and Y.M., X. Zhang, and H.D. wrote the paper. catalysts to increase energy efficiency of CO2 reductiononthe cathode side, including materials based on copper (7, 8), gold (9), Reviewers: D.D., Dalian Institute of Chemical Physics, Chinese Academy of Sciences; and silver (10), indium (2), tin (11), and metal–nitrogen codoped car- D.W., Boston College. bon (5). However, there have been relatively few reports on low- The authors declare no competing interest. cost, earth-abundant, and efficient oxygen evolution reaction Published under the PNAS license. (OER) anodes operating in near-neutral bicarbonate electrolyte to 1Y.M. and X. Zhang contributed equally to this work. 2 pair with CO2 reduction cathodes for high overall energy efficiency To whom correspondence may be addressed. Email: [email protected]. (12). In addition, OER in near-neutral electrolytes is a funda- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. mentally important topic and could be coupled with other oxida- 1073/pnas.1915319116/-/DCSupplemental. tive half-reactions such as water splitting and pollutant degradation www.pnas.org/cgi/doi/10.1073/pnas.1915319116 PNAS Latest Articles | 1of8 Downloaded by guest on October 1, 2021 Fig. 1. Anodization-derived NiFe-HC on NiFe foam, morphology, and structure. (A) Drawing of the starting commercial NiFe foam. (B) Voltage vs. time curve at constant current density 250 mA/cm2 during anodization of NiFe foam at 85 °C. B, Inset shows the setup. (C) Drawing of the resulting NiFe-HC after anod- ization; the metallic surface turns to a dark color with rough surfaces. (D) SEM image of the starting NiFe foam. (E and F) SEM images of the foam after an- odization at low and high magnifications. (G) Powder XRD of the anodized foam NiFe-HC. The lines correspond to a standard XRD pattern of α-Ni(OH)2 (JCPDS card 38-0715). a.u., arbitrary units. (H and I) TEM images of NiFe-HC flakes at low and high resolution. H, Inset shows the selected area diffraction (SAD) pattern. covered with 2- to 3-μm-sized flower-shaped plates (Fig. 1 E and The anodized NiFe foam probed by Raman and infrared (IR) − F). Energy-dispersive X-ray spectroscopy (EDX) mapping (Fig. spectroscopy showed a strong Raman vibration band at 1,071 cm 1 1E) revealed an atomic ratio of Ni:Fe:C ∼ 15:1:3.4. X-ray dif- (21) (Fig. 2A), corresponding to symmetric stretching of 2− 2− fraction (XRD) of the material showed broad and weak peaks, CO3 and the characteristic bending mode of CO3 in IR − suggesting poorly crystalline structures in a discernable NiFe-HC spectroscopy at 1,360 cm 1 (22–24) (Fig. 2B). X-ray photoelec- phase (similar to α-phase Ni hydroxide [Joint Committee on tron spectroscopy (XPS) also suggested a significant amount of Powder Diffraction Standards {JCPDS} 38-0715] and NiFe-double carbonate in the materials with a binding energy of 288.6 eV for hydroxide phase) (18–20) with a large d spacing of ∼0.7 nm ob- C1s (SI Appendix, Figs. S4 and S5). These results, together with served for (003) planes (Fig. 1G and synchrotron XRD data in SI time-of-flight secondary ion mass spectroscopy (TOF-SIMS) Appendix,Fig.S3). Transmission electron microscopy (TEM) imaging (Fig. 2 C and D and SI Appendix, Fig. S6), unambigu- 2− showed the ultrathin nature of the plates (Fig. 1 H and I). The ously showed abundant CO3 anions in the anodized NiFe (012), (015), and (110) planes observed in electron diffraction foam, forming a NiFe–hydroxide carbonate (NiFe-HC, or poorly (Fig.
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