Core–shell heterojunction engineering of Co₃O₄/NiFe LDH nanosheets as bifunctional electrocatalysts for efficient reduction of nitrite to ammonia

Abstract

An electrocatalytic NO₂⁻ reduction reaction (NO₂⁻RR) has cast new light on renewable NH₃ synthesis and treatment of NO₂⁻-contaminated aquatic ecosystems. Numerous studies have been devoted to developing NO₂⁻RR electrocatalysts with superior selectivity. However, few efforts have focused on constructing bifunctional catalysts promoting the NO₂⁻RR and the oxygen evolution reaction (OER) simultaneously, which is significant to reduce the energy consumption and cost of electrocatalytic ammonia production. Herein, we demonstrate the core–shell heterostructures of NiFe LDH nanoarrays attached to Co₃O₄ nanosheets on Ni foam, which exhibit an excellent NO₂⁻RR performance (yield: 4.27 mg h⁻¹ cm⁻², FE: 96.53% at −0.5 V) and a remarkable OER performance (η₁₀₀: 270 mV) as well as decent stability. Furthermore, a two-electrode electrolyzer assembled with Co₃O₄/NiFe LDH heterostructures only requires 1.55 V to reach 10 mA cm⁻², approaching that of the ammonia production system assembled with a noble-metal-based catalyst. The decent NO₂⁻RR and OER properties benefit from the optimized electronic structure due to the heterojunction formation and the increased electrochemically active area owing to the core–shell structure construction. This effort offers new insights into achieving high-efficiency and low-cost electrocatalytic ammonia production.

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1. Introduction

NH₃ is one of the most essential industrial raw materials for synthesizing substantially all nitrogen-based fertilizers, accounting for 5% of the chemical market value.^1–3 Recently, NH₃ has also been acknowledged as an intriguing carbon-free energy carrier due to the merits of competitive delivery cost, large hydrogen capacity, and excellent energy density.⁴ However, with the unceasing employment of the industrial Haber–Bosch route for NH₃ production characterized by high energy consumption and large carbon dioxide emission, the global energy crisis will inevitably be aggravated and the achievement of carbon neutrality will be hindered.⁵ Consequently, it is highly imperative to explore alternative routes for sustainable ammonia production.

Among these alternative routes, electrocatalytic reduction of nitrogenous species under ambient conditions has emerged as an attractive approach on account of its appealing cost and environmental friendliness.¹ In contrast to the extensively studied electrochemical N₂ reduction system with bleak performance, the electrocatalytic NO₂⁻ reduction reaction (NO₂⁻RR) has cast new light on renewable NH₃ synthesis as NO₂⁻ exhibits substantially increased solubility in water and lower cleavage energy than N₂.^2–4 In addition, NO₂⁻ represents one of the severest contaminants of the aquatic ecosystem owing to its poisonous nature and high concentration, potentially jeopardizing public health and disrupting the nitrogen cycle. Apparently, electrocatalytic NO₂⁻ reduction can also contribute to the treatment of NO₂⁻-contaminated aquatic ecosystems, thus restoring the damage caused by the disturbed nitrogen cycle.⁵ However, the reaction kinetics of the NO₂⁻RR remains intractable due to the sophistication of the 6e⁻ process.⁶ Moreover, the NO₂⁻RR faces the challenge of competing with other side reactions under aqueous conditions including the hydrogen evolution reaction (HER). In the aqueous catalytic installations currently employed for ammonia production, the anode reaction coupled with the NO₂⁻RR is the oxygen evolution reaction (OER). The OER also faces challenges regarding sluggish kinetics due to its four-electron transfer.^7–9 Consequently, the exploration of NO₂⁻RR and OER bifunctional catalysts characterized by improved overpotential and superior selectivity contributes to simplifying the requirements for equipment and electrode preparation techniques, leading to electrocatalytic ammonia production with competitive cost and lower energy consumption.^10,11 Nevertheless, few efforts are currently made on exploring highly selective and active NO₂⁻RR and OER bifunctional catalysts.

Co₃O₄ has been broadly studied for electrocatalytic conversion of nitrogen-oxyanions (NO_x⁻) into NH₃ due to the decent intrinsic electrocatalytic activity.¹² However, the actual ammonia production performance of Co₃O₄ still falls short of expectations owing to the inferior electrical conductivity caused by the large intrinsic band gap. Various strategies have been employed to improve Co₃O₄ performance, such as introducing vacancies,¹³ heteroatom doping¹⁴ and constructing heterojunctions.¹² The obtained Co₃O₄-based materials exhibit favourable NH₃ yields and superior Faraday efficiencies; nevertheless, the availability of Co₃O₄-based materials as NO₂⁻RR and OER bifunctional catalysts is restricted by the poor inherent OER activity of Co₃O₄.¹⁵ Recently, tremendous interest has been dedicated to interfacial engineering of excellent core–shell structures by coupling two components,^16–18 which is conducive to achieving enhanced electrocatalytic activity and better integration of the advantages of each component, by accelerating electron transfer, decreasing the ion diffusion distance and creating strong coupling effects between different components. For example, a core–shell heterostructured composite is formed by coupling MoS₂, possessing remarkable HER activity, and NiFe LDH, characterized by promising OER activity, requiring only 1.61 V for achieving 10 mA cm⁻² in overall water splitting.¹⁹ The decent HER and OER bifunctional properties of the material are mainly derived from two aspects: (1) the optimized electron configuration and regulated adsorption/desorption energy on account of heterogeneous structures; (2) the enlarged contact surface between the electrode and electrolyte caused by the formation of core–shell structures. Considering the promising feasibility of core–shell interfacial engineering²⁰ and the prominent OER performance of NiFe LDH,^21,22 obtaining remarkable NO₂⁻RR and OER bifunctional electrocatalysts is extremely promising through the construction of core–shell Co₃O₄/NiFe LDH heterostructures.

Herein, we report core–shell Co₃O₄/NiFe LDH heterostructured nanosheets with NiFe LDH nanoarrays attached to Co₃O₄ nanosheets, which serve as efficient and robust NO₂⁻RR and OER bifunctional catalysts. To further enhance the electrical conductivity and structural toughness of the electrocatalyst, Co₃O₄/NiFe LDH nanoarrays are grown directly on Ni foam. The combination with NiFe LDH not only introduces OER active sites but also optimizes the charge distribution of NO₂⁻RR active sites on Co₃O₄. Furthermore, the construction of the optimized core–shell structure is achieved after regulating the time of growing NiFe LDH, increasing the contact area between the electrolyte and catalyst and creating more abundant electrolyte permeation channels. The optimized Co₃O₄/NiFe LDH nanosheets demonstrate an excellent NO₂⁻RR performance (yield: 4.27 mg h⁻¹ cm⁻², FE: 96.53% at −0.5 V) and a remarkable OER performance (η₁₀₀: 270 mV) as well as decent stability. And a two-electrode cathode–anode coupled ammonia production system assembled with Co₃O₄/NiFe LDH heterostructures only requires 1.55 V to reach 10 mA cm⁻², approaching that of the system assembled with precious metal-based materials.

2. Experimental section

2.1. Synthesis of electrocatalysts

The heterostructured Co₃O₄/NiFe LDH nanoarrays were obtained by an electrodeposition–calcination–electrodeposition process on nickel foam. The first electrodeposition was conducted in a three-electrode system by employing clean nickel foam (2.5 × 2.5 cm), a Pt electrode, and a mercury oxide electrode as the working, counter, and reference electrode, respectively. The first electrolyte is a reddish solution (0.05 M Co(NO₃)₂·6H₂O) used for the electrochemical preparation of Co(OH)₂/NF. The relevant conditions were adjusted to potential reaching −1.0 V vs. Hg/HgO for 10 min. Then, Co₃O₄/NF was obtained by calcining Co(OH)₂/NF after fully drying at 350 °C under a N₂ atmosphere for 2 h. In order to obtain Co₃O₄/NiFe LDH, the second electrodeposition was performed on Co₃O₄/NF. The conditions in the second electrodeposition of NiFe LDH are similar to those of the first, apart from the electrolyte (0.15 M Ni(NO₃)₂·6H₂O + 0.15 M FeSO₄·7H₂O) and deposition duration. By adjusting the deposition duration (20, 40, and 60 s), a series of catalysts (Co₃O₄/NiFe LDH-x, x represents the number of seconds used for the growth of NiFe LDH) were obtained. In the series of catalysts, Co₃O₄/NiFe LDH-40 shows the best NO₂⁻RR and OER bifunctional performance. For comparison, pure NiFe LDH/NF can also be grown on NF via the same electrodeposition approach.

2.2. Characterization

The chemical compositions of the materials were analyzed by X-ray powder diffraction (XRD, Rigaku Smart Lab, with Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS, PHI 5000). The morphologies of materials were analyzed by scanning electron microscopy (FE-SEM, Jeol JSM-7800) and transmission electron microscopy (TEM, Jeol JEM-2800).

2.3. Electrochemical measurement

The NO₂⁻RR performances were measured in a classic H-type cell with Nafion 117 membrane separation, employing the synthesized catalysts (1 × 1 cm²) as the working electrode with a WaveDrive 20 Bipotentiostat/Galvanostat (Pine Research Instrumentation, USA). A Pt wire and Ag/AgCl electrode were employed as the counter and reference electrodes. 35–40 mL Ar-saturated electrolyte solutions (0.1 M NaOH with and without 0.1 M NO₂⁻) were added to two cells of the H-type electrolyzer. The OER performance was evaluated using a traditional three-electrode system with an electrolyte (1 M KOH), a reference electrode (Ag/AgCl electrode) and a counter electrode (graphitic rod). The electrocatalytic ammonia production was carried out in a two-electrode system. All voltages used in our effort were transformed into the potential of the reversible hydrogen electrode (RHE) using the equation:

E_RHE = E_Ag/AgCl + 0.197 V + 0.059 V × pH.

3. Results and discussion

3.1 Materials synthesis and characterization

The fabrication process of Co₃O₄/NiFe LDH supported on Ni foam is depicted in Fig. 1a. First, Co₃O₄/NF was obtained by an electrodeposition–calcination process. Co₃O₄ nanosheets were grown vertically on nickel foam (Fig. 1b). Then Co₃O₄/NiFe LDH was obtained by subsequent electrodeposition on Co₃O₄/NF. NiFe LDH/NF can also be obtained by a similar electrodeposition on Ni foam. The SEM image of NiFe LDH/NF (Fig. 1c) shows staggered NiFe LDH nanosheets with smaller dimensions uniformly distributed on the Ni foam. Obviously, the deposition duration of NiFe LDH on Co₃O₄/NF is critical for constructing core–shell Co₃O₄/NiFe LDH heterostructures and obtaining a prominent active surface area. After the optimum electrodeposition of NiFe LDH on Co₃O₄/NF, Co₃O₄/NiFe LDH-40 exhibited a core–shell structure, clearly depicted in Fig. 1d, which is based on Co₃O₄ nanosheets with larger dimensions as cores and interlaced NiFe LDH nanoarrays as shells. Such a core–shell structure is extremely conducive to increasing the electrochemically active reaction area and constructing more abundant electrolyte permeation channels, thus exposing more active sites and accelerating mass transfer.^23–25 Excessively long and short electrodeposition times were also probed for their effects on the construction of core–shell Co₃O₄/NiFe LDH heterostructures. A short time led to insufficient exposure of active sites (Fig. S1a†), while an overly long time induced agglomeration of NiFe LDH on the surface, resulting in the blockage of active sites and slower mass transport (Fig. S1b†).

Fig. 1 (a) Illustration of the Co₃O₄/NiFe LDH synthesis. SEM images of (b) Co₃O₄/NF, (c) NiFe LDH/NF and (d) Co₃O₄/NiFe LDH-40. (e) XRD pattern of Co₃O₄/NiFe LDH-40. (f) TEM and (g) HRTEM images of Co₃O₄/NiFe LDH-40. (h) TEM and the corresponding elemental mapping images of Co₃O₄/NiFe LDH-40.

The X-ray diffraction (XRD) pattern (Fig. 1e) was employed for probing the crystal structure of the optimal Co₃O₄/NiFe LDH-40 sample. In Fig. 1e, the diffraction peaks positioned at around 2θ = 36.9°, 94.1° and 99.3° can correspond to the (311), (731) and (800) crystallographic planes of Co₃O₄ (PDF # 42-1467),²⁶ respectively. Other diffraction peaks at the vicinity of 2θ = 31.3° and 65.1° can be well indexed to the (101) and (116) characteristic crystalline planes belonging to NiFe LDH (PDF # 40-0215).²⁷ Sufficient evidence is provided in Fig. 1e to verify that Co₃O₄/NiFe LDH-40 is successfully synthesized. The detailed structure of the optimized Co₃O₄/NiFe LDH-40 sample was further investigated employing a transmission electron microscope (TEM). The shell made of interlaced nanosheets of Co₃O₄/NiFe LDH-40 is reconfirmed in Fig. 1f. Two lattice fringe spacings of 0.243 and 0.143 nm are displayed on the HRTEM image (Fig. 1g), corresponding to the (311) plane of Co₃O₄ and the (116) plane of NiFe LDH, separately, revealing the successful construction of a heterojunction interface constituted by NiFe LDH and Co₃O₄. Elemental mapping images (Fig. 1h) were employed to investigate the distribution of NiFe LDH on the Co₃O₄/NF surface. The distributions of Ni, Fe, Co and O elements are uniform in Fig. 1h, which further confirms the success of Co₃O₄/NiFe LDH-40 synthesis. Furthermore, the selected area electron diffraction (SAED) pattern (Fig. S2†) also provides definite proof of construction of the heterojunction structure. These results suggest that Co₃O₄/NiFe LDH-40 is endowed with a complex interface structure.

X-ray photoelectron spectroscopy (XPS) was utilized to analyze the atomic valence states and surface compositions of catalysts.²⁸ The survey spectrum (Fig. 2a) was employed to reaffirm the co-existence of the Ni, Fe, Co and O elements in Co₃O₄/NiFe LDH-40. As illustrated in Fig. 2b, the peaks located at 779.3, 780.65, 794.4 and 796.1 eV are present in the high-resolution Co 2p XPS spectrum of Co₃O₄/NiFe LDH-40,¹³ which correspond to Co³⁺ 2p^3/2, Co²⁺ 2p^3/2, Co³⁺ 2p^1/2 and Co²⁺ 2p^1/2, respectively. Positive shifts in Co₃O₄/NiFe LDH-40 are observed compared to Co₃O₄/NF, which indicate intense electron interactions between Co₃O₄ and NiFe LDH as well as electron depletion of Co sites. The dominant valence state of Ni in Co₃O₄/NiFe LDH-40 is Ni²⁺, which is distinctly evidenced by the peaks at 855.6 (Ni²⁺ 2p^3/2) and 873.4 (Ni²⁺ 2p^1/2) eV in Fig. 2c.²⁹ In addition, the peak at 853.8 eV should be attributed to Ni⁰ of the conductive substrate (Ni foam).³⁰ Notably, Co₃O₄/NiFe LDH-40 in Fig. 2c exhibits distinct negative shifts compared with NiFe LDH/NF, demonstrating the intense electron interactions within Co₃O₄ and NiFe LDH accompanied by the electron-rich structure of Ni sites. Two fitting peaks at 711.8 and 724.9 eV are displayed in the Fe 2p XPS spectrum (Fig. 2d), corresponding to the Fe³⁺ oxidation state.³¹ The peaks show significant positive shifts compared with those of NiFe LDH/NF, which manifests electron depletion of Fe sites when NiFe LDH is coupled with Co₃O₄. The existence of surface adsorbed –OH (M–OH) and lattice oxygen (M–O) in Co₃O₄/NiFe LDH-40 is confirmed by the O 1s spectrum (Fig. S3†), which contains peaks located at 531.4 and 529.6 eV.³²

Fig. 2 (a) XPS survey spectrum of Co₃O₄/NiFe LDH-40. (b) Co 2p, (c) Ni 2p and (d) Fe 2p XPS spectra of related materials.

Overall, the XPS spectra mentioned above unveil interfacial electronic interactions between Co₃O₄ and NiFe LDH in the heterostructure for optimizing the charge-transfer behavior of metal active sites. After electrodepositing NiFe LDH, the electrons stream from the Fe and Co sites to the Ni site. The electron-deficient Co sites have stronger binding effects with NO₂⁻, facilitating the subsequent hydrogenation processes during the NO₂⁻RR.³³ Fe ions with a higher valence state can enrich the hydroxyl on the surface of the electrocatalysts to accelerate the OER reaction.³⁴

3.2. Electrocatalytic NO₂⁻RR and OER performances

The electrocatalytic NO₂⁻ reduction performance of Co₃O₄/NiFe LDH-40 was investigated in a classical H-type cell containing 35 mL catholyte with 0.1 M NaOH and 0.1 M NO₂⁻ under an Ar-saturated atmosphere. As illustrated in Fig. 3a and Fig. S6,† the most distinct enhancement of current density belonging to Co₃O₄/NiFe LDH-40 is demonstrated on the linear scanning voltammetry curve in the presence of NO₂⁻ compared to that without NO₂⁻, preliminarily confirming the possibility of high-efficiency reduction of NO₂⁻via Co₃O₄/NiFe LDH-40. Chronoamperometric (CA) measurements were conducted on Co₃O₄/NiFe LDH-40 at working voltages ranging from −0.3 to −0.7 V (Fig. S7a†), in order to evaluate in detail the catalytic NH₃ production performance of the electrocatalyst. The indophenol blue photometric method was adopted to analyze the NH₃ produced at various potentials, and the corresponding UV–vis absorption spectrum is shown in Fig. 3b. NH₃ yields and the associated Faraday efficiencies (FEs) of the Co₃O₄/NiFe LDH-40 composite at various potentials are displayed in Fig. 3c. At −0.5 V, Co₃O₄/NiFe LDH-40 simultaneously achieves a maximum NH₃ yield reaching 4.27 mg h⁻¹ cm⁻² and an optimal FE reaching 96.53%. The outstanding NH₃ yield and FE of Co₃O₄/NiFe LDH-40 could also be competitive with those of the various advanced electrocatalysts that have been reported for electrocatalytic NO₂⁻ reduction (Table S1†). Moreover, the Watt and Chrisp method was employed for the detection of the by-product (N₂H₄) at diverse potentials, and the corresponding results revealing the virtually undetectable N₂H₄ confirm the excellent selectivity of Co₃O₄/NiFe LDH-40 (Fig. S8†).

Fig. 3 (a) LSV curves of Co₃O₄/NiFe LDH-40 in 0.1 M NaOH with/without 0.1 M NO₂⁻. (b) UV-vis spectra, (c) NH₃ yields and FEs of Co₃O₄/NiFe LDH-40 from −0.3 to −0.7 V. (d) NH₃ yields and FEs belonging to 1. NiFe LDH/NF, 2. Co₃O₄/NF, 3. Co₃O₄/NiFe LDH-40, 4. Co₃O₄/NiFe LDH-20, 5. Co₃O₄/NiFe LDH-60, and 6. NF. (e) Long-term experiment of electrochemical nitrate reduction at −0.5 V and the associated NH₃ yields and FEs.

With a view of demonstrating the significance of the heterostructure formation and the optimized core–shell structure in Co₃O₄/NiFe LDH, in Fig. 3d the contrast in the properties between Co₃O₄/NiFe LDH-40 and related single component materials at the same potential is illustrated. Co₃O₄/NiFe LDH-40 demonstrates superior electrocatalytic activity compared to Co₃O₄/NF (70%, 2 mg h⁻¹ cm⁻²), NiFe LDH/NF (20.3%, 0.8 mg h⁻¹ cm⁻²), and NF (16.8%, 0.5 mg h⁻¹ cm⁻²). In detail, the NH₃ yield in Co₃O₄/NiFe LDH-40 is 2.14, 5.31, and 8.62 times higher than that of Co₃O₄/NF, NiFe LDH/NF, and NF, respectively. The distinct difference in the NO₂⁻RR performance among Co₃O₄/NiFe LDH-40, Co₃O₄/NF and NiFe LDH/NF reaffirms the favorable contribution of heterostructure construction and core–shell nanostructure design for the NO₂⁻RR process.

Co₃O₄/NiFe LDH-40 also exhibits noticeable enhancement in performance compared to Co₃O₄/NiFe LDH-20 and Co₃O₄/NiFe LDH-60, which further confirms the importance of the optimized LDH deposition process for the exposed active area.^35,36 Long-term durability is indisputably a criterion indispensable for assessing the performance of a catalyst,^37–39 and successive electrolysis in Co₃O₄/NiFe LDH-40 was conducted for 15 h at −0.5 V. As depicted in Fig. 3e, the Co₃O₄/NiFe LDH-40 heterostructure maintains relatively stable current density for 15 h at −0.5 V with slight fluctuations in both NH₃ yield and FE, indicating the superior robustness of Co₃O₄/NiFe LDH-40. More intriguingly, scarcely any changes could be noticed in the electronic structure (Fig. S10 and 11†) and morphology (Fig. S12†) of Co₃O₄/NiFe LDH-40 undergoing the 15 h robustness test, further verifying the decent robustness of Co₃O₄/NiFe LDH-40.

The OER properties of related materials were evaluated in 1.0 M KOH. As illustrated in Fig. 4a, Co₃O₄/NiFe LDH-40 simultaneously possesses the most remarkable NO₂⁻RR and OER activity. The OER polarization curves (Fig. 4b) demonstrate in detail the optimal OER activity of Co₃O₄/NiFe LDH-40, achieving 100 and 200 mA cm_geometric⁻² with ultralow overpotentials reaching 270 and 300 mV, respectively. Co₃O₄/NiFe LDH-40 simultaneously has the smallest Tafel slope (47.46 mV dec⁻¹) among various electrocatalysts, which also confirms the excellent OER kinetics and the extraordinary charge transfer coefficient of Co₃O₄/NiFe LDH-40. Such an OER performance is intermediate among the Co-based and LDH-based catalysts developed currently (Table S2†), but the activity of Co₃O₄/NiFe LDH-40 is moderately increased compared to that of NiFe LDH and Co₃O₄ synthesized in this work. Moreover, the electrochemical surface area (ECSA) and electrochemical impedance spectra (EIS) were employed to study the remarkable OER performance of Co₃O₄/NiFe LDH-40. Typically, the ECSA can be derived from double-layer capacitance (C_dl, Fig. 4d), which is correlated with the active surface area of electrocatalysts.¹² Co₃O₄/NiFe LDH-40 has a larger C_dl value (5.675 mF cm⁻²) than NiFe LDH/NF (1.947 mF cm⁻²) and Co₃O₄/NF (2.095 mF cm⁻²), confirming that the core–shell structure is conducive to exposing more active sites and enhancing the intrinsic activity of the material. In addition, in order to eliminate the effect of surface area,⁴⁰ ECSA normalized LSV curves were also explored for investigating the intrinsic catalytic activity of the catalysts. As illustrated in Fig. S14,† Co₃O₄/NiFe LDH-40 still exhibits better OER performance than other contrast electrodes.

Fig. 4 (a) The comparison of NH₃ yield at −0.5 V and overpotential at 100 mA cm⁻² of different materials. (b) OER polarization curves based on geometric area, (c) OER Tafel curves, (d) C_dl values and (e) Nyquist plots of different electrocatalysts. (f) Chronoamperometric measurements of Co₃O₄/NiFe LDH-40.

Meanwhile, the C_dl value of Co₃O₄/NiFe LDH-40 is larger than that of Co₃O₄/NiFe LDH-20 (2.213 mF cm⁻²) and Co₃O₄/NiFe LDH-60 (4.925 mF cm⁻²), reaffirming the effect of the NiFe LDH electrodeposition time on the exposure of active sites, which is favorable for maximizing the contact between the catalyst and electrolyte and avoiding the blockage of active sites due to agglomeration. EIS (Fig. 4e) was employed to investigate the charge transfer and mass diffusion processes of the catalysts at the electrode/electrolyte interface. Co₃O₄/NiFe LDH-40 exhibits the smallest charge transfer resistance compared to NiFe LDH/NF and Co₃O₄/NF, which confirms that the formation of heterogeneous interfaces enhances the electron transfer efficiency and boosts the reaction kinetics for the OER. As for the stability of the electrocatalyst, Co₃O₄/NiFe LDH-40 demonstrates outstanding robustness with no significant decay in current density after 30 hours of stability testing (Fig. 4f). The morphology (Fig. S16†) and electronic structure (Fig. S17†) of the material did not display any significant changes during the long period of the robustness test, reconfirming the promising robustness.

To further confirm the significance of the core–shell structure of Co₃O₄/NiFe LDH-40, we prepared non-core–shell Co₃O₄/NiFe LDH by changing the time and voltage of NiFe LDH electrodeposition. As depicted in Fig. S18a,† the NiFe LDH nanosheets no longer grow uniformly on the Co₃O₄ nanosheets, but grow interleaved with the Co₃O₄ nanosheets. The non-core–shell Co₃O₄/NiFe LDH exhibits worse NO₂⁻RR and OER properties (Fig. S18g and h†), and the unsatisfactory stability is verified by the morphology with rapid collapse and agglomeration after the NO₂⁻RR and OER (Fig. S18b and c†). The remarkable difference between Co₃O₄/NiFe LDH-40 and non-core–shell Co₃O₄/NiFe LDH may be derived from the structure. The core–shell structure with Co₃O₄ as the core may prevent the contact between part of Co₃O₄ nanosheets and the electrolyte, resulting in a limited improvement of NO₂⁻RR performance. However, the NiFe LDH shell also effectively protects the Co₃O₄ nanosheets thereby achieving more sustained NH₃ production. In addition, the tight coupling between NiFe LDH and Co₃O₄ in the core–shell structure is more beneficial for regulating the electronic structure of Co₃O₄ and accelerating electron transfer than the loose coupling in the non-core–shell Co₃O₄/NiFe LDH.

Overall, Co₃O₄/NiFe LDH-40 is an NO₂⁻RR and OER bifunctional catalyst with low overpotential and superior selectivity. The decent NO₂⁻RR and OER bifunctional properties of Co₃O₄/NiFe LDH-40 are primarily attributed to two factors: (1) the formation of the heterogeneous structure optimizes the local electronic structure of the active sites, and the electron-deficient Co sites promote the enhanced adsorption of NO₂⁻ leading to weakening and activation of the N=O bond during the NO₂⁻RR,³³ while Fe ions with a higher valence state accelerate the OER and facilitate the release of O₂ from the catalyst surface.³⁴ (2) The construction of the core–shell structure establishes abundant electrolyte permeation channels and broadens the contact area between the electrocatalyst and electrolyte, which is favorable for obtaining an increased electrochemically active surface area and expedited mass transport.³⁰

3.3. Electrocatalytic performance of the NO₂⁻RR||OER system

In view of the prominent NO₂⁻RR and OER bifunctional activity of Co₃O₄/NiFe LDH-40, it was directly employed as both the anode and cathode of a two-electrode device for ammonia production (Fig. 5a). For comparison, an electrolyzer was also assembled by replacing the anode with advanced RuO₂ having the same load (2.0 mg cm⁻²) as Co₃O₄/NiFe LDH-40. As shown in Fig. 5b, the Co₃O₄/NiFe LDH-40||Co₃O₄/NiFe LDH-40-assembled electrolytic cell only requires 1.55 V to achieve 10 mA cm⁻², which is approaching that of the advanced electrolyzer RuO₂||Co₃O₄/NiFe LDH-40 (1.53 V), confirming the superior OER activity of Co₃O₄/NiFe LDH-40. Intriguingly, the electrolyzer with the cathode reaction supplanted by the HER requires 1.79 V to achieve 10 mA cm⁻², significantly exceeding the 1.55 V of the OER||NO₂⁻RR system, demonstrating the inhibitory effect of Co₃O₄/NiFe LDH-40 on the HER. In the Co₃O₄/NiFe LDH-40||Co₃O₄/NiFe LDH-40-assembled electrolytic equipment for ammonia production, the current densities and cathodic NH₃ yields exhibit an increasing trend with rising voltage (Fig. 5c and d). Notably, the faradaic efficiencies of NH₃ remain above 85% under the broad imposed voltage range (1.5–1.9 V), suggesting that the competition of the HER could be effectively inhibited by the approach. The excellent robustness of the electrolyzer assembled with the Co₃O₄/NiFe LDH-40 catalyst is demonstrated by long-term electrolysis (Fig. S19†).

Fig. 5 (a) Illustration of the two-electrode OER||NO₂⁻RR electrolyzer. (b) LSV curves of relevant two-electrode systems. (c) Chronoamperometry curves, (d) NH₃ yield rate and FE of Co₃O₄/NiFe LDH-40 in the OER||NO₂⁻RR system at various applied cell voltages.

4. Conclusions

In conclusion, the core–shell Co₃O₄/NiFe LDH heterostructures have been developed to serve as remarkable NO₂⁻RR and OER bifunctional electrocatalysts. Benefiting from the optimized electronic structure due to the heterojunction formation and the increased electrochemically active area owing to the core–shell structure construction, Co₃O₄/NiFe LDH exhibits a highly selective and active NO₂⁻RR and OER bifunctional performance. The Co₃O₄/NiFe LDH||Co₃O₄/NiFe LDH-assembled electrolytic cell only requires 1.55 V to deliver 10 mA cm⁻², which is comparable to that of the electrolyzer assembled with an advanced noble-metal-based catalyst. This effort not only opens up new approaches to explore efficient catalysts via regulating interfacial electronic interactions but also offers new insights into achieving high-efficiency and low-cost electrocatalytic ammonia production.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22179065).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00795b

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