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Highly active oxygen evolution integrated with efficient CO2 to CO electroreduction
Contributed by Hongjie Dai, October 14, 2019 (sent for review September 4, 2019; reviewed by Dehui Deng and Dunwei Wang)

Significance
Electrochemical reduction of CO2 to useful chemicals or fuels is critical to closing the carbon cycle and preventing further deterioration of the environment/climate. This work addresses the low-energy-efficiency problem of CO2 reduction limited by sluggish oxygen evolution reaction (OER) on the anode side. 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 anode with higher activity and stability than those based on noble-metal catalysts IrO2 and Ir/C. We integrated our anode with a selective CO2 reduction cathode to achieve >97% conversion of CO2 to CO and a record-setting high energy efficiency for CO2 conversion.
Abstract
Electrochemical reduction of CO2 to useful chemicals has been actively pursued for closing the carbon cycle and preventing further deterioration of the environment/climate. Since CO2 reduction reaction (CO2RR) at a cathode is always paired with the oxygen evolution reaction (OER) at an anode, the overall efficiency of electrical energy to chemical fuel conversion must consider the large energy barrier and sluggish kinetics of OER, especially in widely used electrolytes, such as the pH-neutral CO2-saturated 0.5 M KHCO3. OER in such electrolytes mostly relies on noble metal (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover that by anodizing a metallic Ni–Fe composite foam under a harsh condition (in a low-concentration 0.1 M KHCO3 solution at 85 °C under a high-current ∼250 mA/cm2), OER on the NiFe foam is accompanied by anodic etching, and the surface layer evolves into a nickel–iron hydroxide carbonate (NiFe-HC) material composed of porous, poorly crystalline flakes of flower-like NiFe layer-double hydroxide (LDH) intercalated with carbonate anions. The resulting NiFe-HC electrode in CO2-saturated 0.5 M KHCO3 exhibited OER activity superior to IrO2, with an overpotential of 450 and 590 mV to reach 10 and 250 mA/cm2, respectively, and high stability 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 to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O2. The device showed a low cell voltage of 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a current density of 10 mA/cm2.
Large-scale capturing and reduction of CO2 to useful chemicals and fuels could be pivotal to the world’s sustainability and environment. The combined electrochemical CO2 reduction and water oxidation is a promising approach to tackle the world’s growing energy demands without compromising the environment by closing the carbon cycle. In a typical CO2 reduction electrolyzer, OER at the anode is accompanied by CO2 reduction in the cathode into fuels such as CO (1), formic acid (2), ethanol (3), ethane, ethylene (4), and other multicarbon products (5, 6). Intense effort has been taken to develop highly efficient electrocatalysts to increase energy efficiency of CO2 reduction on the cathode side, including materials based on copper (7, 8), gold (9), silver (10), indium (2), tin (11), and metal–nitrogen codoped carbon (5). However, there have been relatively few reports on low-cost, earth-abundant, and efficient oxygen evolution reaction (OER) anodes operating in near-neutral bicarbonate electrolyte to pair with CO2 reduction cathodes for high overall energy efficiency (12). In addition, OER in near-neutral electrolytes is a fundamentally important topic and could be coupled with other oxidative half-reactions such as water splitting and pollutant degradation in biologically relevant systems. The widely used IrO2 (13⇓–15) and Pt (1) anode catalysts were limited by their scarcity, high cost, and insufficient activity. An active NiCoFeP catalyst was reported recently, but required multistep, complex synthesis and noble-metal gold-coated foams (16). An electrodeposited NiOx catalyst on glassy carbon was also reported, with low performance of small current density (<5 mA/cm2) at high overpotentials (467-mV overpotential to reach 1 mA/cm2) (17).
Here, we developed a simple, 1-step approach to a nonprecious Ni–Fe hydroxide carbonate (NiFe-HC) electrode for superior OER anode in neutral (pH ∼ 7.4) 0.5 M KHCO3 electrolyte widely used for CO2 electroreduction. A piece of commercial Ni–Fe foam was anodized against a platinum mesh in a 0.1 M KHCO3 solution maintained at 85 °C at a constant current of 250 mA/cm2 for 16 h (see Methods for details and voltage vs. time curve in Fig. 1B), after which the original metallic NiFe foam turned into a dark foam. Obvious etching of the foam was seen from the debris and color change in the electrolyte. The original smooth NiFe wires (∼100-µm wires; Fig. 1D and SI Appendix, Figs. S1 and S2) in the foam evolved into highly porous and rough structures fully covered with 2- to 3-µm-sized flower-shaped plates (Fig. 1 E and F). Energy-dispersive X-ray spectroscopy (EDX) mapping (Fig. 1E) revealed an atomic ratio of Ni:Fe:C ∼ 15:1:3.4. X-ray diffraction (XRD) of the material showed broad and weak peaks, suggesting poorly crystalline structures in a discernable NiFe-HC phase (similar to α-phase Ni hydroxide [Joint Committee on Powder Diffraction Standards {JCPDS} 38-0715] and NiFe-double hydroxide phase) (18⇓–20) with a large d spacing of ∼0.7 nm observed for (003) planes (Fig. 1G and synchrotron XRD data in SI Appendix, Fig. S3). Transmission electron microscopy (TEM) showed the ultrathin nature of the plates (Fig. 1 H and I). The (012), (015), and (110) planes observed in electron diffraction (Fig. 1 H, Inset) and in real space (Fig. 1I) also matched those reported for NiFe-HC (18). Note that under the same anodic condition, pure Ni foams were also etched, and a layer of α-phase Ni hydroxide was produced on the Ni foam (see SI Appendix, Fig. S3 for XRD of α-phase Ni hydroxide).
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 anodization 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.
The anodized NiFe foam probed by Raman and infrared (IR) spectroscopy showed a strong Raman vibration band at 1,071 cm−1 (21) (Fig. 2A), corresponding to symmetric stretching of CO32− and the characteristic bending mode of CO32− in IR spectroscopy at 1,360 cm−1 (22⇓–24) (Fig. 2B). X-ray photoelectron spectroscopy (XPS) also suggested a significant amount of carbonate in the materials with a binding energy of 288.6 eV for C1s (SI Appendix, Figs. S4 and S5). These results, together with time-of-flight secondary ion mass spectroscopy (TOF-SIMS) imaging (Fig. 2 C and D and SI Appendix, Fig. S6), unambiguously showed abundant CO32− anions in the anodized NiFe foam, forming a NiFe–hydroxide carbonate (NiFe-HC, or poorly crystalline NiFe-layer double hydroxide [LDH]) layer.
Raman, IR, and ToF-SIMS spectroscopic characterization of NiFe-HC. (A) Raman spectrum of the NiFe-HC layer formed by anodization of NiFe foam. a.u., arbitrary units. (B) IR spectrum of NiFe-HC. Abs., absorption. (C and D) TOF-SIMS mapping showing COx−, COx2− (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.)
Spatially resolved TEM imaging and EDX chemical mapping were performed to characterize a thin cross-section of an NiFe wire in the foam after anodization (wire cross-sectional sample prepared by focused ion beam [FIB]) (Fig. 3). Note that prior to anodization, a fresh NiFe foam was composed of ∼100-μm wires with an Fe-rich core surrounded by ∼10-μm-thickness Ni–Fe alloy coating (SI Appendix, Figs. S1 and S2). After anodization, we observed porous NiFe-HC structures rich in Ni, Fe, C, and O species over an underlying dense Ni–Fe region (Fig. 3). The porous structures comprised a layer showing flower-like rough morphology on top of a less-porous layer with a higher Fe content (see the region between the dashed lines drawn to highlight the boundaries in Fig. 3). The existence of Fe in the NiFe-HC layer was confirmed by ToF-SIMS data collected from the surface through ∼30-nm depth of the foam (Fig. 2). These imaging and elemental mapping data suggested that anodization in the 0.1 M KHCO3 electrolyte with a bulk pH ∼ 8.6 led to increased H+/reduced pH due to OER, which initiated etching of the NiFe wire to result in a porous surface layer. Ni etching/dissolution was much more rapid than Fe, and redeposition of the metal cations on the etched surface formed the flower-like layer of NiFe–hydroxide intercalated by carbonate ions existing in the electrolyte (25). Underneath the flower-like layer lie an etched, porous Ni–Fe layer (in the region between the dashed lines drawn to highlight the boundaries) enriched in Fe due to the more rapid loss of Ni. Strong carbon and oxygen signals were also detected in this layer, suggesting a layer of NiFe–hydroxide with higher Fe content than in the flower-like top layers (see SI Appendix, Fig. S6 for corroborated TOF-SIMS data for a growing Fe/Ni ratio with increasing depth).
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 highlight the 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.)
We performed electrochemical characterization of NiFe-HC electrodes at room temperature derived by various anodization conditions, including temperature, concentration of KHCO3, and anodization current density and time (SI Appendix, Figs. S7 and S8). Under the optimized condition, the resulting NiFe-HC exhibited a high electrochemical surface area (ECSA) based on cyclic voltammetry (CV) scans in the non-Faradaic region (Fig. 4B). Impressively, the anodic etching of NiFe foam led to a porous NiFe-HC layer exhibiting a ∼586-fold higher ECSA and pseudo capacitance than the starting NiFe foam (SI Appendix, Fig. S9), which favored high OER electrocatalytic activity due to greatly enhanced catalytic sites in the NiFe-HC layer.
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-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, and 20% Ir/C in CO2-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, with iR compensation).
In CO2-saturated 0.5 M KHCO3 electrolytes, OER activity of our NiFe-HC electrode exceeded that of well-known precious metal-based electrocatalysts, including IrO2 and Ir/C (Fig. 4C). To reach the benchmark current density of 10 mA/cm2, the NiFe-HC electrode required ∼1.68 V vs. reversible hydrogen electrode (RHE) (after iR compensation), while IrO2 required ∼1.79 V vs. RHE (Fig. 4C and SI Appendix, Fig. S10). Note that the peak centered at 1.68 V of the CV scan of NiFe-HC was attributed to the oxidation of Ni(II) to Ni(III, IV) induced by a phase transition from an Ni(OH)2 type structure to a γ-NiOOH phase (26, 27). A Faradaic efficiency (FE) test at 10 mA/cm2 in an H cell with an online gas chromatography showed that FE of OER is nearly 100% over a long test period of >100 h (SI Appendix, Fig. S11). To assess the kinetics of the OER reaction, we fitted the backward scan of the CV curves to the Tafel equation η = b × log(j/j0), where η is the overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density. The NiFe-HC exhibited a Tafel slope of b = 74 mV/decade in CO2-saturated 0.5 M KHCO3, much smaller than the b = 255 and 221 mV/decade for commercial IrO2 and Ir/C, respectively (SI Appendix, Fig. S12).
The NiFe-HC anode exhibited excellent stability for OER operated at a constant voltage of 1.88 V vs. RHE, affording a stable current of ∼65 mA/cm2 without any decay for over 100 h (without iR compensation, R ∼ 1.5 Ω; SI Appendix, Fig. S13). To meet the need of CO2 reduction at higher current densities, we tested the stability of NiFe-HC at 250 mA/cm2 (Fig. 4D). Impressively, the catalyst retained its stability for >120 h at a voltage of ∼1.82 V (after iR compensation) without decay. The current was higher than used for most of the OER electrocatalysts reported (normally at 10 mA/cm2) in CO2-saturated 0.5 M KHCO3, and the test was over a much longer time period. This was one of the few nonprecious-metal–based electrocatalysts capable of catalyzing OER in the 0.5-M KHCO3 electrolyte at a high current density with high activity and stability. Note that our reported overpotential vs. RHE at such high current densities may be overestimated due to the dropped pH on the electrode surface compared with the bulk solution. Such local pH effects could induce a 120- to 180-mV higher overpotential (at 10 mA/cm2, planar surface) than the real value (simulation details of the local pH effect can be found in SI Appendix, Fig. S14). The activity and stability of NiFe-HC also surpassed those of well-known neutral OER catalyst CoPi (SI Appendix, Fig. S15). Remarkably, the catalyst remained intact after a stability test at such high current density. Physiochemical analysis of the used catalysts revealed negligible structure, composition, and morphology changes compared to the pristine sample (SI Appendix, Figs. S16–S19). The NiFe-HC electrode is chemically and electrochemically stable at down to low local pH ∼ 4.3 to 5.3, superior to the common NiFe-LDH/Ni foam electrode (28) in our control experiments. The NiFe-HC electrode also holds promise for pairing with CO2 reduction reaction (CO2RR) in near-neutral electrolytes at high-pressure systems.
The working mechanism of NiFe-HC for catalyzing neutral OER was investigated by conducting in situ Raman and X-ray absorption spectroscopy (XAS) in CO2-saturated 2 M KHCO3 (pH 8.1) (Fig. 5). The initial broad Raman bands at 490 and 550 cm−1 featured a highly disordered Ni–Fe hydroxide structure at 0 V. Two characteristic Raman bands of the NiOOH phase showed up at ∼482 and 558 cm−1 (I482/558 cm−1 = 1.58) at 1.52 V vs. RHE. The ratio of I482/558 cm−1 grew to 1.83 at 1.62 V vs. RHE and stabilized, while further increasing the overpotentials to 2.02 V vs. RHE. The transition of the Raman bands indicated the formation of γ-NiOOH at ∼1.52 V that contained high-valence-state Ni(III) and Ni(IV), which contributed to the high OER activity (26). The transformation of the valence states of Ni was also captured by in situ operando XAS study. The Ni K edge position of NiFe-HC at 0 V was 8,342.8 eV (measured at half-height F/Io = 0.5), similar to that of Ni(OH)2 (8,342.4 eV). Under a positive bias of 1.58 V, the edge position showed an obvious blue shift to 8,345.2 eV, similar to that of γ-NiOOH (8,345.0 eV), according to literature (27). Further increasing the overpotential slightly blue-shifted the Ni K-edge to 8,346 eV at 1.88 V, similar to the edge position of Ni(IV) compounds (27, 29). The in situ XAS results corroborated with the in situ Raman measurements that NiFe-HC catalyst turned to highly oxidized γ-NiOOH at about 1.52 to 1.58 V in 2 M KHCO3. Furthermore, the high OER activity was enhanced by the intercalated CO32−, which strongly bounded to the active sites act as stronger proton acceptors and electron donors, as compared with monovalent anions. The strongly bounded proton acceptor was attributed to lower the activation barrier of water oxidation and promote the reaction thermodynamically (22). Photocatalytic activity test of NiFe-HC revealed very small photocurrents; thus, the possible enhancement of OER activity due to the direct photooxidation of bicarbonate is at the minimum level (SI Appendix, Fig. S20).
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.
The NiFe-HC OER anode was integrated into a CO2 reduction electrolyzer as a proof-of-concept demonstration (Fig. 6A) by coupling with a CO2 electro-reduction cathode made of a molecular catalyst, cobalt phthalocyanine/carbon nanotube (CoPc/CNT) hybrid recently developed by one of our groups (SI Appendix, Fig. S21) (30). Near-neutral CO2-saturated 2 M KHCO3 was used as the electrolyte to minimize the solution resistance of the electrolyzer and avoid operation with corrosive chemicals in practice. NiFe-HC showed higher OER activity while retaining its high stability in 2 M KHCO3 (SI Appendix, Fig. S22). Moreover, the CoPc/CNT hybrids exhibited stable activity and highly selective production of CO in CO2-saturated 2 M KHCO3 (SI Appendix, Figs. S23 and S24). The generated CO could be used as feedstock in the Fischer–Tropsch process to produce valuable chemicals (31⇓–33). Linear sweep voltammetry (LSV) of the full CO2 reduction electrolyzer cell with the NiFe-HC anode and CoPc/CNT cathode showed a low-onset cell voltage of ∼1.9 V (Fig. 6B). Electroreduction of CO2 was conducted at current densities from 5 to 30 mA/cm2 with Faradaic efficiencies for carbon monoxide FE(CO) of 97.2% achieved at 20 mA/cm2 (Fig. 6C), without appreciable decay of performance in long-term electrolysis (Fig. 6D and SI Appendix, Fig. S24). We calculated the electricity to chemical energy efficiencies of the electrolyzer, a key metric to evaluate the performance of such devices and which was defined as the chemical energy stored in produced CO over the total electric energy input. Our device achieved a high efficiency of 59% and 57% with and without iR compensation, respectively, at 10 mA/cm2. The performance was on par with a reported work using a noble-metal gold CO2 electro-reduction cathode and NiFeCoP catalysts loaded on gold-coated Ni foam as anode (16). Our electrolyzer was also operated at 20 mA/cm2 with excellent stability and energy efficiencies of 58% and 54% with and without iR compensation, respectively. Inductively coupled plasma atomic emission spectrometry tests of the anolyte detected a low leaching of Fe and Ni (0.9 ppm Fe and 3.8 ppm Ni) from the anode, and no impact on the CO2RR cathode was observed with the use of an anion-exchange membrane in the electrolyzer to block the diffusion of Fe and Ni into the catholyte (Fig. 6C and SI Appendix, Fig. S23). The high activity, selectivity, and stability of the CO2 electrolyzer confirmed the superior performance of both the NiFe-HC anode and CoPc/CNT cathode catalysts.
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 M KHCO3 electrolyte. (D) Long-term stability of the CO2 electrolyzer operating at 10 and 20 mA/cm2 in CO2-saturated 2 M KHCO3 electrolyte. Data with and without iR compensation (comp.) (resistance ∼8.0 Ω) are shown.
An interesting finding by this work was that by anodizing a metallic Ni–Fe composite foam under a harsh condition in a low-concentration 0.1 M KHCO3 solution at ∼85 °C, OER on the NiFe foam was accompanied by anodic etching, and the surface layer evolved into a NiFe-HC material composed of a porous, near-amorphous carbonate anion intercalated NiFe LDH. This led to a facile single-step approach to making a Ni–Fe-based electrocatalyst in CO2-saturated 0.5 M KHCO3 electrolyte widely used for CO2 reduction. Anodization of metal or metal alloy in the presence of anions tends to form double hydroxides intercalated with anions, a phenomenon we are currently investigating in terms of its generality and implications. The single-step electrochemically derived NiFe-HC anode was highly active and stable, surpassing precious-metal-based OER catalysts in neutral solutions. The anode was ideal for integration with CO2 reduction electrolyzes to afford a high overall energy conversion efficiency of ∼58% with >97% conversion of CO2 to CO. The simple approach to low-cost and high-performance oxygen anode could facilitate scalable/sustainable CO2 reduction potentially at the industrial scale in the future.
Methods
In Situ Synthesis of NiFe-HC on Metal Foam Substrates.
The Ni–Fe foam and pure Ni foam were purchased from Suzhou Jiashide Metal Foam Co. A piece of Ni–Fe foam (4 × 1 cm; thickness: 1 mm; number of pores per inch [ppi]: 110 ppi; atomic ratio of Ni/Fe = 1:3) was cleaned by sonicating the foam in acetone and ethanol for 15 min at each solvent and dry, followed by annealing in 9% H2 (diluted by Ar, flow rate of Ar:H2 = 200 standard cubic centimeters per minute [sccm]:20 sccm) at 500 °C to remove the native oxides on the metal surface. The foam was glued in the middle by epoxy (Loctite EA 1C); this left only an active area of 1 × 1 cm on one end and an area of 0.5 to 1 × 1 cm on the other end that was clamped by the electrode holder. The foam 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. A concentration of 0.1 M KHCO3 solution was used as the electrolyte, and the electrodes were assembled in a 2-electrode Teflon electrochemical cell; the whole cell was placed into an 85 °C oil bath. The electrodes were connected to a LANHE battery tester and ran at constant current of 250 mA for 16 h as the prime condition for NiFe-HC. The concentration of KHCO3, current, and synthesis time were varied for control samples. For synthesis of the Ni-HC sample, Ni foam (4 × 1 cm; 420 m2/g; thickness: 1 mm) was used; all of the pretreatment and synthesis procedures followed the same methods as NiFe-HC, except that 0.1 M KHCO3, 50 mA, and 8 h were used as the optimized condition.
Characterization.
The powder XRD was carried out at room temperature by using a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 Å) at the beam voltage of 40 kV and current of 44 mA and a scan rate of 1°/min. Synchrotron XRD was conducted by using quasimonochromatic X-rays (λ = 0.9744 Å) at fixed energy of 12.7 keV at beamline 11-3 of the Stanford Synchrotron Radiation Light Source (SSRL) at SLAC National Accelerator Laboratory. The energy resolution at this beamline was ΔE/E∼5 × 10−4. The diffraction patterns of the samples were collected by using an area detector (Raxyonics 225, pixel size at 73.242 × 73.242 μm) placed at ∼200 mm downstream of the sample. The direct beam stop also served as a monitor for recording the intensity of the direct beam for normalization of the data. The initial data reduction was carried out by using wide angle X-ray scattering tools, an in-house-developed software package. Raman spectroscopy was carried out by using a Horiba Raman spectrometer equipped with an Olympus BX41 microscope and a Spectra-Physics 532-nm Ar laser. IR spectra were taken on a Nicolet iS50 Fourier transform/IR spectrometer with attenuated total reflectance sampling. Scanning electron microscopy (SEM) was performed by using a thermal field emission electron microscope operating at 5 kV and equipped with an energy-spectrum analyzer (JEOL model JSM-7100F). High-resolution TEM (HR-TEM) images were collected on a Talos F200X microscope operating at 200 kV equipped with an energy-dispersive spectrometer. XPS was performed by using the PHI 5000 VersaProbe system, using microfocus (25 W, 100 μm) and Al Kα (1,486.6 eV) as the radiation source.
FIB Sample Preparation.
The cross-section TEM samples were prepared by using a FEI Versa 3D DualBeam with field emission gun and FIB in back-scattered electron mode. A thin layer of Pt was predeposited for protection 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 milling process, the desired thin film was lifted out and directly deposited on thin copper TEM grids under optical microscopes.
TOF-SIMS Mapping.
TOF-SIMS was performed by using the PHI TRIFT V nanoToF (Chigassaki) ToF-SIMS system. The primary ion source is a pulsed bismuth liquid metal ion gun with an incident angle of 50° to show the 2D molecular distribution of the NiFe-HC sample.
Electrochemical Measurement.
All of the electrochemical measurements were performed at ambient conditions in a standard 3-electrode configuration, using a CHI 760 electrochemical working station. The as-prepared electrodes were clamped by a Teflon-wrapped platinum electrode holder and used as working electrode, Pt mesh (round shape, inner diameter = 2 cm) was used as the counterelectrode, and saturated calomel electrodes (SCEs) was used as reference electrode and calibrated before each use. The electrolyte was CO2-saturated in 0.5 M KHCO3 (pH 7.4) prepared by continuously flowing CO2 at 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 noted. A typical resistance in our system was between 1.0 and 1.9 Ω. The second forward scans were used as the LSV curves and presented. The scan range was from the open circuit potential to 1.98 V (vs. RHE). The potential vs. RHE was converted by using formula ERHE = ESCE + 0.244 + 0.059 × pH (pH 7.4 in CO2-saturated 0.5 M KHCO3). Stability tests of the catalyst were performed in both constant voltage and the chronopotentiometry modes of the instruments.
In Situ XAS Study.
A homemade in situ electrochemical cell was used for XAS measurements under operating conditions. In the homemade flow cell, electrochemistry was performed by using catalysts on carbon paper as a working electrode, an Ag/AgCl (saturated KCl) reference electrode, a platinum-wire counterelectrode, and a flowing CO2-saturated 2 M HKCO3 solution as the electrolyte. Ni K-edge XAS measurements were carried out at beamline 2-2 at SSRL. The intensity of the incident X-ray radiation, I0, was monitored with a nitrogen-filled ionization chamber. Data were calibrated and analyzed by using Athena software.
In Situ Raman Spectroscopy.
Raman spectra was taken on the sample under working conditions by using homemade Teflon cells with a quartz window. The working electrode was carbon paper-loaded NiFe-HC (loading was ∼1 to 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 controlled constant voltage was applied on the working electrodes for 50 s, a typical exposure time was set as 30 s, and 20 to 30 times of accumulation was used for each potential. A 532-nm Ar laser was used for the excitation.
Preparation of CoPc/CNT Electrode.
The CoPc/CNT hybrid catalyst was synthesized by a reported method with a Co loading of ∼0.76% (30). Catalyst ink was prepared by dispersing 4 mg of hybrid material in a mixture of 16 μL of 5 wt% Nafion solution and 984 μL of ethanol with the assistance of sonication. The electrodes were prepared by drop-coating 250 μL of catalyst ink on carbon fiber paper (Toray catalog no. TGP-H-060) to cover an active area of 1 × 1 cm2 (loading: 1 mg/cm2).
CO2 Electrolyzer.
A Teflon H-cell with a gas-tight cathode chamber was used to assemble the CO2 electrolyzer. NiFe-HC and CoPc/CNT electrodes with active areas of 1 cm2 were used as anode and cathode in the CO2 electrolyzer, respectively, and were separated by a Selemion AMV anion-exchange membrane (∼5 × 3 cm2). CO2-saturated 2 M aqueous KHCO3 solution (pH 8.1) was used as electrolyte. The solution resistances between anode and cathode were measured to be ∼8.0 Ω.. Twenty sccm CO2 was introduced to the cathode chamber and then directed to an online gas chromatography (SRI MG#5) to analyze the product distribution.
Data availability.
Additional materials and methods used in this study are described in detail in SI Appendix. Information includes optimization of the synthesis and growth mechanism of NiFe-HC, sample preparation for electrochemical tests, electrochemical characterization (ECSA measurement, measurement of FE of OER, and quantification of FE of CO2RR), and estimation of surface pH under OER conditions.
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 Zhou from University of Connecticut for the assistance of theoretical simulation.
Footnotes
↵1Y.M. and X. Zhang contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: hdai1{at}stanford.edu.
Author contributions: Y.M. and H.D. designed research; Y.M., X. Zhang, W.-H.H., J.H., Y.-S.T., M.J.K., J.-J.S., K.H.S., and X. Zheng performed research; Y.M., X. Zhang, W.-H.H., 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. contributed 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.
Reviewers: D.D., Dalian Institute of Chemical Physics, Chinese Academy of Sciences; and D.W., Boston College.
The authors declare no competing interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1915319116/-/DCSupplemental.
Published under the PNAS license.
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