Highly Active Oxygen Evolution Integrated with Efficient CO2 to CO

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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 reaction (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 catalyst 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 efficiency for CO2 conversion. by closing the carbon cycle. In a typical CO2 reduction electrolyzer, 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 anodization; 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. 1 H, Inset) and in real space (Fig. 1I) also matched those crystalline NiFe-layer double hydroxide [LDH]) layer. reported for NiFe-HC (18). Note that under the same anodic Spatially resolved TEM imaging and EDX chemical mapping condition, pure Ni foams were also etched, and a layer of α-phase were performed to characterize a thin cross-section of an NiFe Ni hydroxide was produced on the Ni foam (see SI Appendix, Fig. wire in the foam after anodization (wire cross-sectional sample S3 for XRD of α-phase Ni hydroxide). prepared by focused ion beam [FIB]) (Fig. 3). Note that prior to

2of8 | www.pnas.org/cgi/doi/10.1073/pnas.1915319116 Meng et al. Downloaded by guest on October 1, 2021 CHEMISTRY

Fig. 2. Raman, IR, and ToF-SIMS spectroscopic characterization of NiFe-HC. (A) Raman spectrum of the NiFe-HC layer formed by anodization of NiFe foam. − 2− a.u., arbitrary units. (B) IR spectrum of NiFe-HC. Abs., absorption. (C and D) TOF-SIMS mapping showing COx ,COx (x = 2 and 3) detected on the surface and at ∼30-nm depth, respectively, in a freshly made NiFe-HC electrode. (Scale bar: 10 μm.)

anodization, a fresh NiFe foam was composed of ∼100-μm wires NiFe-HC layer exhibiting a ∼586-fold higher ECSA and pseudo with an Fe-rich core surrounded by ∼10-μm-thickness Ni–Fe capacitance than the starting NiFe foam (SI Appendix,Fig.S9), alloy coating (SI Appendix, Figs. S1 and S2). After anodization, which favored high OER electrocatalytic activity due to greatly we observed porous NiFe-HC structures rich in Ni, Fe, C, and O enhanced catalytic sites in the NiFe-HC layer. species over an underlying dense Ni–Fe region (Fig. 3). The In CO2-saturated 0.5 M KHCO3 electrolytes, OER activity of porous structures comprised a layer showing flower-like rough our NiFe-HC electrode exceeded that of well-known precious morphology on top of a less-porous layer with a higher Fe con- metal-based electrocatalysts, including IrO2 and Ir/C (Fig. 4C). tent (see the region between the dashed lines drawn to highlight To reach the benchmark current density of 10 mA/cm2,theNiFe-HC the boundaries in Fig. 3). The existence of Fe in the NiFe-HC electrode required ∼1.68 V vs. reversible hydrogen electrode layer was confirmed by ToF-SIMS data collected from the sur- (RHE) (after iR compensation), while IrO2 required ∼1.79 V vs. face through ∼30-nm depth of the foam (Fig. 2). These imaging RHE (Fig. 4C and SI Appendix,Fig.S10). Note that the peak and elemental mapping data suggested that anodization in the centered at 1.68 V of the CV scan of NiFe-HC was attributed to 0.1 M KHCO3 electrolyte with a bulk pH ∼ 8.6 led to increased the oxidation of Ni(II) to Ni(III, IV) induced by a phase transition + H /reduced pH due to OER, which initiated etching of the NiFe from an Ni(OH)2 type structure to a γ-NiOOH phase (26, 27). A wire to result in a porous surface layer. Ni etching/dissolution Faradaic efficiency (FE) test at 10 mA/cm2 in an H cell with an was much more rapid than Fe, and redeposition of the metal online gas chromatography showed that FE of OER is nearly cations on the etched surface formed the flower-like layer of 100% over a long test period of >100 h (SI Appendix,Fig.S11). To NiFe–hydroxide intercalated by carbonate ions existing in the assess the kinetics of the OER reaction, we fitted the backward electrolyte (25). Underneath the flower-like layer lie an etched, scan of the CV curves to the Tafel equation η = b × log(j/j0), where porous Ni–Fe layer (in the region between the dashed lines η is the overpotential, b is the Tafel slope, j is the current density, drawn to highlight the boundaries) enriched in Fe due to the and j0 is the exchange current density. The NiFe-HC exhibited a more rapid loss of Ni. Strong carbon and oxygen signals were Tafel slope of b = 74 mV/decade in CO2-saturated 0.5 M KHCO3, also detected in this layer, suggesting a layer of NiFe–hydroxide much smaller than the b = 255 and 221 mV/decade for com- with higher Fe content than in the flower-like top layers (see SI mercial IrO2 and Ir/C, respectively (SI Appendix,Fig.S12). Appendix, Fig. S6 for corroborated TOF-SIMS data for a growing The NiFe-HC anode exhibited excellent stability for OER Fe/Ni ratio with increasing depth). operated at a constant voltage of 1.88 V vs. RHE, affording a We performed electrochemical characterization of NiFe-HC stable current of ∼65 mA/cm2 without any decay for over 100 h electrodes at room temperature derived by various anodization (without iR compensation, R ∼ 1.5 Ω; SI Appendix, Fig. S13). To conditions, including temperature, concentration of KHCO3, meet the need of CO2 reduction at higher current densities, we and anodization current density and time (SI Appendix, Figs. S7 tested the stability of NiFe-HC at 250 mA/cm2 (Fig. 4D). Im- and S8). Under the optimized condition, the resulting NiFe-HC pressively, the catalyst retained its stability for >120 h at a voltage exhibited a high electrochemical surface area (ECSA) based on of ∼1.82 V (after iR compensation) without decay. The current cyclic voltammetry (CV) scans in the non-Faradaic region (Fig. was higher than used for most of the OER electrocatalysts repor- 2 4B). Impressively, the anodic etching of NiFe foam led to a porous ted (normally at 10 mA/cm )inCO2-saturated 0.5 M KHCO3,and

Meng et al. PNAS Latest Articles | 3of8 Downloaded by guest on October 1, 2021 Fig. 3. Cross-sectional imaging and chemical mapping of an anodized NiFe wire in an NiFe foam. (A, Upper) A scanning TEM image of the cross-section of an anodized NiFe wire prepared by removing the wire from an anodized NiFe foam and using FIB to cut a thin section from the wire. The dark NiFe region at the right side of the image was the interior of the NiFe wire not exposed to electrolyte during anodization. The left dashed white line was drawn to highlightthe original surface of NiFe over which NiFe-HC flower-like structures were formed. The right white lines were drawn to show the boundary between etched and unetched NiFe. Note that the thin Pt layer seen in the image was deposited over the sample as a protection layer for the FIB milling step for cross-sectional sample preparation. (A, Lower) EDX line scans showing Ni, Fe, C, and O elemental distributions along the red line in A, Upper. a.u., arbitrary units. (B)EDX mapping of Ni, Fe, C, and O, respectively, for the same sample as in A. (Scale bars: 1 μm.)

the test was over a much longer time period. This was one of the at 0 V. Two characteristic Raman bands of the NiOOH phase – −1 few nonprecious-metal based electrocatalysts capable of catalyz- showed up at ∼482 and 558 cm (I482/558 cm−1 = 1.58) at 1.52 V vs. ing OER in the 0.5-M KHCO3 electrolyte at a high current density RHE. The ratio of I482/558 cm−1 grew to 1.83 at 1.62 V vs. RHE and with high activity and stability. Note that our reported over- stabilized, while further increasing the overpotentials to 2.02 V potential vs. RHE at such high current densities may be over- vs. RHE. The transition of the Raman bands indicated the for- estimated due to the dropped pH on the electrode surface mation of γ-NiOOH at ∼1.52 V that contained high-valence-state compared with the bulk solution. Such local pH effects could 2 Ni(III) and Ni(IV), which contributed to the high OER activity induce a 120- to 180-mV higher overpotential (at 10 mA/cm , (26). The transformation of the valence states of Ni was also planar surface) than the real value (simulation details of the local captured by in situ operando XAS study. The Ni K edge position SI Appendix pH effect can be found in ,Fig.S14). The activity and of NiFe-HC at 0 V was 8,342.8 eV (measured at half-height stability of NiFe-HC also surpassed those of well-known neutral F I = SI Appendix / o 0.5), similar to that of Ni(OH)2 (8,342.4 eV). Under a OER catalyst CoPi ( ,Fig.S15). Remarkably, the cat- positive bias of 1.58 V, the edge position showed an obvious alyst remained intact after a stability test at such high current blue shift to 8,345.2 eV, similar to that of γ-NiOOH (8,345.0 density. Physiochemical analysis of the used catalysts revealed eV), according to literature (27). Further increasing the over- negligible structure, composition, and morphology changes compared to the pristine sample (SI Appendix, Figs. S16–S19). potential slightly blue-shifted the Ni K-edge to 8,346 eV at 1.88 V, The NiFe-HC electrode is chemically and electrochemically stable similar to the edge position of Ni(IV) compounds (27, 29). The in at down to low local pH ∼ 4.3 to 5.3, superior to the common situ XAS results corroborated with the in situ Raman measure- γ NiFe-LDH/Ni foam electrode (28) in our control experiments. ments that NiFe-HC catalyst turned to highly oxidized -NiOOH at about 1.52 to 1.58 V in 2 M KHCO3. Furthermore, the high The NiFe-HC electrode also holds promise for pairing with CO2 − OER activity was enhanced by the intercalated CO 2 ,which reduction reaction (CO2RR) in near-neutral electrolytes at high- 3 pressure systems. strongly bounded to the active sites act as stronger proton ac- The working mechanism of NiFe-HC for catalyzing neutral ceptors and electron donors, as compared with monovalent OER was investigated by conducting in situ Raman and X-ray anions. The strongly bounded proton acceptor was attributed to absorption spectroscopy (XAS) in CO2-saturated 2 M KHCO3 lower the activation barrier of water oxidation and promote the (pH 8.1) (Fig. 5). The initial broad Raman bands at 490 and reaction thermodynamically (22). Photocatalytic activity test of − 550 cm 1 featured a highly disordered Ni–Fe hydroxide structure NiFe-HC revealed very small photocurrents; thus, the possible

4of8 | www.pnas.org/cgi/doi/10.1073/pnas.1915319116 Meng et al. Downloaded by guest on October 1, 2021 Fig. 4. ECSA, OER activity, and stability of NiFe-HC in CO2-saturated 0.5 M KHCO3 electrolytes. (A) CV scans of the starting NiFe foam measured in CO2- CHEMISTRY saturated 0.5 M KHCO3 at various scan rates between 5 and 50 mV/s. (B) CV scans of anodized NiFe foam with a layer of NiFe-HC on the surface measured in CO2- saturated 0.5 M KHCO3 at various scan rates between 1 and 10 mV/s. Much larger current loops were observed compared to the starting NiFe foam before anodization in A.(C) Forward branch of CV scans of NiFe-HC, commercial IrO2,and20%Ir/CinCO2-saturated 0.5 M KHCO3 electrolytes. The CV curves were taken between 1.3 and 2 V vs. RHE at a scan rate of 1 mV/s. Resistance was ∼1.4 Ω and was not compensated. (D) Chronopotentiometry of NiFe-HC electrode under OER

operation at a constant current of ∼250 mA in CO2-saturated 0.5 M KHCO3 electrolyte for 120 h (resistance ∼1.4 Ω,withiR compensation).

enhancement of OER activity due to the direct photooxidation production of CO in CO2-saturated 2 M KHCO3 (SI Appendix, of bicarbonate is at the minimum level (SI Appendix, Fig. S20). Figs. S23 and S24). The generated CO could be used as feedstock The NiFe-HC OER anode was integrated into a CO2 re- in the Fischer–Tropsch process to produce valuable chemicals A duction electrolyzer as a proof-of-concept demonstration (Fig. 6 ) (31–33). Linear sweep voltammetry (LSV) of the full CO2 re- by coupling with a CO2 electro-reduction cathode made of a duction electrolyzer cell with the NiFe-HC anode and CoPc/ molecular catalyst, cobalt phthalocyanine/carbon nanotube CNT cathode showed a low-onset cell voltage of ∼1.9 V (Fig. SI (CoPc/CNT) hybrid recently developed by one of our groups ( 6B). Electroreduction of CO2 was conducted at current densities 2 Appendix,Fig.S21) (30). Near-neutral CO2-saturated 2 M KHCO3 from 5 to 30 mA/cm with Faradaic efficiencies for carbon mon- was used as the electrolyte to minimize the solution resistance of oxide FE(CO) of 97.2% achieved at 20 mA/cm2 (Fig. 6C), without the electrolyzer and avoid operation with corrosive chemicals in appreciable decay of performance in long-term electrolysis (Fig. practice. NiFe-HC showed higher OER activity while retaining its 6D and SI Appendix,Fig.S24). We calculated the electricity to high stability in 2 M KHCO3 (SI Appendix,Fig.S22). Moreover, chemical energy efficiencies of the electrolyzer, a key metric to the CoPc/CNT hybrids exhibited stable activity and highly selective evaluate the performance of such devices and which was defined

Fig. 5. In situ characterization of NiFe-HC at neutral electrolyte. In situ Raman spectra of NiFe-HC (A) and in situ Ni K-edge XAS of NiFe-HC (B) as a function of

applied potential vs. RHE in CO2-saturated 2 M KHCO3 are shown. B, Inset shows an enlarged region that clearly indicates the blue shifts of the spectra while increasing the potentials. a.u., arbitrary units.

Meng et al. PNAS Latest Articles | 5of8 Downloaded by guest on October 1, 2021 Fig. 6. CO2 electrolyzer with NiFe-HC anode and CoPc/CNT cathode. (A) Experiment setup of the CO2 electrolyzer. (B) LSV of the CO2 electrolyzer with NiFe- HC anode and CoPc/CNT cathode in CO2-saturated 2 M KHCO3 electrolyte. (C) Cell voltage and FE for CO of the CO2 reduction electrolyzer in CO2-saturated 2 2 M KHCO3 electrolyte. (D) Long-term stability of the CO2 electrolyzer operating at 10 and 20 mA/cm in CO2-saturated 2 M KHCO3 electrolyte. Data with and without iR compensation (comp.) (resistance ∼8.0 Ω) are shown.

as the chemical energy stored in produced CO over the total could facilitate scalable/sustainable CO2 reduction potentially at electric energy input. Our device achieved a high efficiency of 59% the industrial scale in the future. and 57% with and without iR compensation, respectively, at 10 mA/cm2. The performance was on par with a reported work using Methods In Situ Synthesis of NiFe-HC on Metal Foam Substrates. The Ni–Fe foam and anoble-metalgoldCO2 electro-reduction cathode and NiFeCoP catalysts loaded on gold-coated Ni foam as anode (16). Our pure Ni foam were purchased from Suzhou Jiashide Metal Foam Co. A piece – × electrolyzer was also operated at 20 mA/cm2 with excellent sta- of Ni Fe foam (4 1 cm; thickness: 1 mm; number of pores per inch [ppi]: 110 iR ppi; atomic ratio of Ni/Fe = 1:3) was cleaned by sonicating the foam in ac- bility and energy efficiencies of 58% and 54% with and without etone and ethanol for 15 min at each solvent and dry, followed by annealing compensation, respectively. Inductively coupled plasma atomic in 9% H2 (diluted by Ar, flow rate of Ar:H2 = 200 standard cubic centimeters emission spectrometry tests of the anolyte detected a low leaching per minute [sccm]:20 sccm) at 500 °C to remove the native oxides on the of Fe and Ni (0.9 ppm Fe and 3.8 ppm Ni) from the anode, and no metal surface. The foam was glued in the middle by epoxy (Loctite EA 1C); × × impact on the CO2RR cathode was observed with the use of an this left only an active area of 1 1 cm on one end and an area of 0.5 to 1 anion-exchange membrane in the electrolyzer to block the diffu- 1 cm on the other end that was clamped by the electrode holder. The foam = sion of Fe and Ni into the catholyte (Fig. 6C and SI Appendix, Fig. was used as anodes and a platinum mesh (d 2 cm, 52 mesh) was used as counterelectrode, and the 2 electrodes were placed in a distance at ∼5 mm. S23). The high activity, selectivity, and stability of the CO2 elec- A concentration of 0.1 M KHCO3 solution was used as the electrolyte, and trolyzer confirmed the superior performance of both the NiFe-HC the electrodes were assembled in a 2-electrode Teflon electrochemical cell; anode and CoPc/CNT cathode catalysts. the whole cell was placed into an 85 °C oil bath. The electrodes were con- An interesting finding by this work was that by anodizing a nected to a LANHE battery tester and ran at constant current of 250 mA for

metallic Ni–Fe composite foam under a harsh condition in a low- 16 h as the prime condition for NiFe-HC. The concentration of KHCO3,cur- concentration 0.1 M KHCO3 solution at ∼85 °C, OER on the rent, and synthesis time were varied for control samples. For synthesis of the NiFe foam was accompanied by anodic etching, and the surface Ni-HC sample, Ni foam (4 × 1 cm; 420 m2/g; thickness: 1 mm) was used; all of layer evolved into a NiFe-HC material composed of a porous, the pretreatment and synthesis procedures followed the same methods as near-amorphous carbonate anion intercalated NiFe LDH. This NiFe-HC, except that 0.1 M KHCO3,50mA,and8hwereusedasthe optimized condition. led to a facile single-step approach to making a Ni–Fe-based electrocatalyst in CO2-saturated 0.5 M KHCO3 electrolyte Characterization. The powder XRD was carried out at room temperature by widely used for CO2 reduction. Anodization of metal or metal using a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 Å) at the alloy in the presence of anions tends to form double hydroxides beam voltage of 40 kV and current of 44 mA and a scan rate of 1°/min. intercalated with anions, a phenomenon we are currently inves- Synchrotron XRD was conducted by using quasimonochromatic X-rays (λ = tigating in terms of its generality and implications. The single- 0.9744 Å) at fixed energy of 12.7 keV at beamline 11-3 of the Stanford step electrochemically derived NiFe-HC anode was highly active Synchrotron Radiation Light Source (SSRL) at SLAC National Accelerator Δ ∼ × −4 and stable, surpassing precious-metal-based OER catalysts in Laboratory. The energy resolution at this beamline was E/E 5 10 .The diffraction patterns of the samples were collected by using an area detector neutral solutions. The anode was ideal for integration with CO2 (Raxyonics 225, pixel size at 73.242 × 73.242 μm) placed at ∼200 mm reduction electrolyzes to afford a high overall energy conversion downstream of the sample. The direct beam stop also served as a moni- efficiency of ∼58% with >97% conversion of CO2 to CO. The tor for recording the intensity of the direct beam for normalization of the simple approach to low-cost and high-performance oxygen anode data. The initial data reduction was carried out by using wide angle X-ray

6of8 | www.pnas.org/cgi/doi/10.1073/pnas.1915319116 Meng et al. Downloaded by guest on October 1, 2021 scattering tools, an in-house-developed software package. Raman spec- electrochemistry was performed by using catalysts on carbon paper as a troscopy was carried out by using a Horiba Raman spectrometer equipped working electrode, an Ag/AgCl (saturated KCl) reference electrode, a platinum-

with an Olympus BX41 microscope and a Spectra-Physics 532-nm Ar laser. IR wire counterelectrode, and a flowing CO2-saturated 2 M HKCO3 solution as the spectra were taken on a Nicolet iS50 Fourier transform/IR spectrometer with electrolyte. Ni K-edge XAS measurements were carried out at beamline 2-2 at

attenuated total reflectance sampling. Scanning electron microscopy (SEM) SSRL. The intensity of the incident X-ray radiation, I0, was monitored with a was performed by using a thermal field emission electron microscope op- nitrogen-filled ionization chamber. Data were calibrated and analyzed by erating at 5 kV and equipped with an energy-spectrum analyzer (JEOL using Athena software. model JSM-7100F). High-resolution TEM (HR-TEM) images were collected on a Talos F200X microscope operating at 200 kV equipped with an In Situ Raman Spectroscopy. Raman spectra was taken on the sample under energy-dispersive spectrometer. XPS was performed by using the PHI 5000 working conditions by using homemade Teflon cells with a quartz window. VersaProbe system, using microfocus (25 W, 100 μm) and Al Kα (1,486.6 eV) The working electrode was carbon paper-loaded NiFe-HC (loading was ∼1to as the radiation source. 2 mg), the reference electrode was a Ag/AgCl (saturated KCl), and a platinum wire was used as counterelectrode. The spectra were taken after a FIB Sample Preparation. The cross-section TEM samples were prepared by controlled constant voltage was applied on the working electrodes for 50 s, using a FEI Versa 3D DualBeam with field emission gun and FIB in back- a typical exposure time was set as 30 s, and 20 to 30 times of accumulation scattered electron mode. A thin layer of Pt was predeposited for protection was used for each potential. A 532-nm Ar laser was used for the excitation. of the preserved examine area. The milling acceleration voltage was used in a range of 30 kV to remove the materials from the unwanted area. After the FIB Preparation of CoPc/CNT Electrode. The CoPc/CNT hybrid catalyst was syn- milling process, the desired thin film was lifted out and directly deposited on thesized by a reported method with a Co loading of ∼0.76% (30). Catalyst ink thin copper TEM grids under optical microscopes. was prepared by dispersing 4 mg of hybrid material in a mixture of 16 μLof 5 wt% Nafion solution and 984 μL of ethanol with the assistance of soni- TOF-SIMS Mapping. TOF-SIMS was performed by using the PHI TRIFT V cation. The electrodes were prepared by drop-coating 250 μL of catalyst ink nanoToF (Chigassaki) ToF-SIMS system. The primary ion source is a pulsed on carbon fiber paper (Toray catalog no. TGP-H-060) to cover an active area bismuth liquid metal ion gun with an incident angle of 50° to show the 2D of 1 × 1cm2 (loading: 1 mg/cm2). molecular distribution of the NiFe-HC sample.

CO2 Electrolyzer. A Teflon H-cell with a gas-tight cathode chamber was used to Electrochemical Measurement. All of the electrochemical measurements were assemble the CO2 electrolyzer. NiFe-HC and CoPc/CNT electrodes with active 2 performed at ambient conditions in a standard 3-electrode configuration, areas of 1 cm were used as anode and cathode in the CO2 electrolyzer, using a CHI 760 electrochemical working station. The as-prepared electrodes respectively, and were separated by a Selemion AMV anion-exchange 2 were clamped by a Teflon-wrapped platinum electrode holder and used as membrane (∼5 × 3cm). CO2-saturated 2 M aqueous KHCO3 solution (pH working electrode, Pt mesh (round shape, inner diameter = 2 cm) was used 8.1) was used as electrolyte. The solution resistances between anode and CHEMISTRY as the counterelectrode, and saturated calomel electrodes (SCEs) was used as cathode were measured to be ∼8.0 Ω.. Twenty sccm CO2 was introduced to reference electrode and calibrated before each use. The electrolyte was CO2- the cathode chamber and then directed to an online gas chromatography saturated in 0.5 M KHCO3 (pH 7.4) prepared by continuously flowing CO2 at (SRI MG#5) to analyze the product distribution. 30 sccm at least 40 min before and during the test. The CV was taken at a scan rate of 1 mV/s. No iR compensation was performed unless otherwise Data availability. Additional materials and methods used in this study are Ω noted. A typical resistance in our system was between 1.0 and 1.9 . The described in detail in SI Appendix. Information includes optimization of the second forward scans were used as the LSV curves and presented. The scan synthesis and growth mechanism of NiFe-HC, sample preparation for elec- range was from the open circuit potential to 1.98 V (vs. RHE). The potential trochemical tests, electrochemical characterization (ECSA measurement, vs. RHE was converted by using formula ERHE = ESCE + 0.244 + 0.059 × pH (pH measurement of FE of OER, and quantification of FE of CO2RR), and esti- 7.4 in CO2-saturated 0.5 M KHCO3). Stability tests of the catalyst were per- mation of surface pH under OER conditions. formed in both constant voltage and the chronopotentiometry modes of the instruments. ACKNOWLEDGMENTS. W.-H.H. was supported by Ministry of Science and Technology, Taiwan, Grant MOST-106-2918-I-035-002. Y.M. thanks Dr. Kai In Situ XAS Study. A homemade in situ electrochemical cell was used for XAS Zhou from University of Connecticut for the assistance of theoretical measurements under operating conditions. In the homemade flow cell, simulation.

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