Highly active oxygen evolution integrated with efficient CO₂ to CO electroreduction

• oxygen evolution
• CO₂ reduction
• CO₂ electrolyzer
• electrocatalysis
• pH-neutral electrolyte

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 NiO_x 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).

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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 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.

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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. a.u., arbitrary units. (B) IR spectrum of NiFe-HC. Abs., absorption. (C and D) TOF-SIMS mapping showing CO_x⁻, CO_x²⁻ (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).

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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 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.

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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-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 (I_{482/558 cm⁻¹} = 1.58) at 1.52 V vs. RHE. The ratio of I_{482/558 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/I_o = 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).

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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.

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.

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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 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.