ZnFe₂O₄ nanosheet array: a highly efficient electrocatalyst for ambient ammonia production via nitrite reduction

Abstract

Electrocatalytic reduction of nitrite (NO₂⁻) offers an eco-friendly and sustainable strategy for synthesizing valuable ammonia (NH₃) while simultaneously removing NO₂⁻. Nevertheless, the complexity of the six-electron transfer reaction involved can hamper NH₃ production and faradaic efficiency (FE), necessitating the quest for high-performance electrocatalysts. In this study, we introduce ZnFe₂O₄ nanosheet arrays on nickel foam (ZnFe₂O₄/NF) as an efficient electrocatalyst for the reduction of NO₂⁻ to NH₃. ZnFe₂O₄/NF demonstrates an impressive NH₃ yield of 588.58 μmol h⁻¹ cm⁻² and a remarkable FE of 95.7% at −0.6 V in 0.1 M NaOH containing 0.1 M NO₂⁻. Furthermore, it exhibits robust stability and durability throughout extended electrolysis and continuous cycling tests.

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Ammonia (NH₃), an essential chemical widely recognized by human society, finds extensive applications in cosmetics, fertilizers, disinfectants and dyes, and it also holds the potential as a green energy carrier for its remarkable energy density of 4.3 kW h kg⁻¹.^1–6 To date, the Haber–Bosch process (HBP) remains the dominant method for ammonia (NH₃) synthesis. However, the HBP's demanding production conditions involving high temperatures and pressures have led to significant fossil energy consumption and the release of substantial CO₂ emissions. Consequently, there is a pressing need to explore more environmentally friendly approaches to NH₃ production. Electrochemical reduction of nitrogen (eNRR) has garnered substantial research interest as a sustainable pathway for the electro-synthesis of NH₃.^7–11 Nonetheless, the challenging dissociation enthalpy of the N≡N bond (941 kJ mol⁻¹) and the extremely low solubility of N₂, in conjunction with the competition from the hydrogen evolution reaction (HER), pose significant hurdles, leading to less-than-desirable NH₃ yield and faradaic efficiency (FE).

Fortunately, nitrite (NO₂⁻) offers excellent solubility and significantly lower N=O bond energy (204 kJ mol⁻¹) when compared to N₂, which makes the electrochemical nitrite reduction reaction (eNO₂⁻RR) more advantageous than the HBP and electrochemical nitrogen reduction (eNRR) for NH₃ production.^12–16 Notably, the eNO₂⁻RR not only serves to remove harmful NO₂⁻ from industrial wastewater but also generates value-added NH₃.^17,18 However, the conversion of NO₂⁻ to NH₃ results in the production of additional by-products (N₂H₄, H₂, and N₂), which significantly hinder its selectivity for NH₃. Therefore, it has become essential to search for electrocatalysts for the eNO₂⁻RR with high selectivity.

Noble metal-containing materials show promise for the eNO₂⁻RR, however, their high costs and limited availability hinder their practical large-scale application.^19,20 In contrast, iron (Fe), which is abundant and cost-effective, serves as the active center in biological enzymes responsible for the reduction of NO₂⁻ to NH₃.^21,22 Recent research has pointed to the activity of Fe and its hybrids in the eNO₂⁻RR.^23–27 On the other hand, Fe₃O₄ has garnered favor in the field of electrocatalysis for its ease of synthesis and excellent electrical conductivity,^28–32 making it a promising candidate for eNO₂⁻RR applications. Nevertheless, it's worth noting that Fe₃O₄ faces a challenge in terms of limited adsorption capacity, and thus, further enhancements are required. Zinc (Zn), ranking as the second most abundant non-hazardous metal in the human body after Fe, plays diverse biological roles. These include exerting antioxidant-like effects in specific cells and serving as co-catalytic and regulatory elements in enzymes.^33–35 Recent reports have also shown that Zn can significantly enhance the catalytic performance of electrocatalysts.^36,37 Furthermore, when Zn ions are introduced into Fe₃O₄, they create a “synergistic effect” with Fe ions, efficiently regulating the catalytic activity of Fe₃O₄.³⁸ Hence, it is desirable to enhance the activity of Fe₃O₄ by introducing Zn as an effective means to achieve highly selective conversion of NO₂⁻ to NH₃.

In this study, we developed a highly selective electrocatalyst for the eNO₂⁻RR by manufacturing ZnFe₂O₄ nanosheet arrays grown on nickel foam (ZnFe₂O₄/NF). The ZnFe₂O₄/NF electrocatalyst demonstrates exceptional eNO₂⁻RR catalytic activity, yielding a large NH₃ yield of 588.58 μmol h⁻¹ cm⁻², with a corresponding FE of 95.7% at −0.6 V. Notably, it maintains this high performance over a 16 h testing period, demonstrating excellent stability.

The synthesis process for ZnFe₂O₄/NF is presented in Fig. 1a (see the ESI† for details). Fig. 1 shows the X-ray diffraction (XRD) pattern for ZnFe₂O₄/NF. Three distinct diffraction peaks at 44.5°, 51.9°, and 76.4° correspond to the NF substrate (JCPDS no. 71-3740), and the four peaks at 18.2°, 36.9°, 42.9°, and 62.50° can be attributed to the ZnFe₂O₄ phase (JCPDS no. 03-7430). The scanning electron microscopy (SEM) images (Fig. 1c and d and S1†) illustrate the full coverage of ZnFe₂O₄ nanosheet arrays on NF. We also prepared Fe₃O₄/NF (Fig. S2†) using the same procedure but without the inclusion of a Zn source. The nanosheet structure of ZnFe₂O₄ is unveiled as shown by the transmission electron microscopy (TEM) image (Fig. 1e). The high-resolution TEM (HRTEM) image (Fig. 1f) reveals a d-spacing of 0.150 nm indexed to the (440) plane of ZnFe₂O₄. In Fig. 1g, the energy dispersive X-ray (EDX) elemental mapping images confirm a uniform distribution of the Zn, Fe, and O elements throughout the entirety of ZnFe₂O₄.

Fig. 1 (a) Schematic illustration for the synthesis of ZnFe₂O₄/NF. (b) XRD pattern, and (c) and (d) SEM images for ZnFe₂O₄/NF. (e) TEM and (f) HRTEM images for ZnFe₂O₄. (g) EDX elemental mapping images for ZnFe₂O₄.

To gain insight into the surface composition and bonding state of ZnFe₂O₄, X-ray photoelectron spectroscopy (XPS) analysis was performed. The survey XPS spectrum presented in Fig. 2a reveals the presence of the Zn, Fe, O, and Ni elements in ZnFe₂O₄, with the Ni component originating from NF. As described in Fig. 2b, the double peaks at 1020.9 and 1044.2 eV in the Zn 2p core-level spectrum are deconvoluted into 2p_3/2 and 2p_1/2 of Zn²⁺.^39,40 In the Fe 2p region of Fig. 2c, the existence of Fe³⁺ and Fe²⁺ in ZnFe₂O₄ is evidenced by the characteristic peaks at 715.3, 727.9, 711.7, and 724.4 eV, respectively.^40,41 Additional peaks at 719.5 and 733.2 eV are classified as shake-up satellites (Sat.). The spectrum for O 1s (Fig. 2d) is divided into two main peaks at 529.9 and 531.6 eV, which match lattice O in the catalyst and adsorbed O on its surface, respectively.^39,42

Fig. 2 XPS spectra of (a) survey, (b) Zn 2p, (c) Fe 2p, and (d) O 1s regions for ZnFe₂O₄.

The electrocatalytic performance of ZnFe₂O₄/NF for the eNO₂⁻RR was evaluated using an H-type cell, incorporating a three-electrode system, in a 0.1 M NaOH solution containing 0.1 M NO₂⁻ under argon saturation. Taking into account the high volatility of NH₃, the cathodic compartment was sealed during testing. All electrochemical measurements were conducted at a stirring rate of 600 rpm. The primary product, NH₃, and the associated by-products, N₂H₄, H₂, and N₂, resulting from the eNO₂⁻RR, were quantitatively determined using UV-vis spectrophotometry and gas chromatography. Calibration curves for NH₃ and N₂H₄ are given in Fig. S3 and S4.† We collected the linear sweep voltammetry (LSV) curves for ZnFe₂O₄/NF with a scan rate of 5 mV s⁻¹. The LSV curves (Fig. 3a) suggest a substantial improvement in the current density (j) of ZnFe₂O₄/NF in the presence of NO₂⁻, reaching 201.40 mA cm⁻² at −1.0 V, much higher than the value of 102.95 mA cm⁻² in pure NaOH solution. This increase underscores the superior performance of ZnFe₂O₄/NF for NO₂⁻ reduction. Additionally, under the same test conditions, NF and Fe₃O₄/NF exhibits significantly lower current densities of 108.19 and 136.28 mA cm⁻², respectively, at −1.0 V. We further conducted chronoamperometry (CA) tests at various potentials, ranging from −0.3 to −0.8 V. The NH₃ concentrations were determined using UV-vis spectrophotometry (refer to Fig. 3b and c and S5†). As depicted in Fig. 3d, the NH₃ yield of ZnFe₂O₄/NF steadily increases with the rise in cathode potential, reaching a maximum value of 771.93 μmol h⁻¹ cm⁻² at −0.8 V. Meanwhile, the NH₃ FE achieves its peak at 95.7% at −0.6 V. Notably, in order to further investigate NH₃ yield and FE of ZnFe₂O₄/NF, chronoamperometry (CA) tests were enforced at various potentials (from −0.3 to −0.8 V) and NH₃ concentrations were determined by UV-vis spectrophotometry (Fig. 3b and c and S5†). Remarkably, the catalytic activity of ZnFe₂O₄/NF for the eNO₂⁻RR surpasses that of the majority of electrocatalysts reported in the recent literature (Fig. 3e and Table S1†). Similarly, control samples underwent electrolysis for 1 h at −0.6 V (Fig. S6a†), with the corresponding NH₃ product absorbances provided in Fig. S6b.† Ultimately, as depicted in Fig. 3f, it becomes evident that the catalytic activity of ZnFe₂O₄/NF significantly outperforms that of NF (14.95 μmol h⁻¹ cm⁻², 20.42% FE) and Fe₃O₄/NF (229.54 μmol h⁻¹ cm⁻², 50.05% FE). We determined the electrochemical double-layer capacitance (C_dl) to assess the electrochemically active surface area (ECSA) of the prepared samples, as illustrated in Fig. S7a and b.† Cyclic voltammograms (CV) were collected within the voltage range of 0.8 to 0.9 V. Fig. S7c† reveals that the C_dl values for Fe₃O₄/NF and ZnFe₂O₄/NF are 1.10 and 1.61 mF cm⁻², respectively. This indicates that ZnFe₂O₄/NF exhibits higher surface roughness, thus exposing a greater number of catalytically active sites. Additionally, the electrochemical impedance spectra (Fig. S7d†) indicate that the charge transfer resistance of ZnFe₂O₄/NF is lower compared to that of Fe₃O₄/NF, which facilitates faster charge transfer. These results collectively demonstrate that the introduction of the Zn elements significantly enhances the catalytic performances.

Fig. 3 (a) LSV curves of ZnFe₂O₄/NF in 0.1 M NaOH with and without 0.1 M NO₂⁻. (b) CA curves and (c) corresponding UV-vis spectra of ZnFe₂O₄/NF at different potentials. (d) NH₃ yields and FEs of ZnFe₂O₄/NF at different potentials. (e) Comparing FEs and NH₃ yields of ZnFe₂O₄/NF with recently reported eNO₂⁻RR electrocatalysts. (f) NH₃ yields and FEs of ZnFe₂O₄/NF, Fe₃O₄/NF, and NF.

Assessing the overall effectiveness of the catalyst also entails the detection of potential by-products, namely N₂H₄, H₂, and N₂. The data presented in Fig. S8† indicates the absence of N₂H₄ formation in the electrolyte within the test voltage range spanning from −0.3 to −0.8 V. Additionally, the NH₃ yields and FEs of H₂ and N₂ in the electrolyte after electrolysis at various potentials are significantly lower than those of NH₃ (Fig. 4a). This observation elucidates that NH₃ is the primary product in the NO₂⁻ reduction process and that the HER is predominantly suppressed. Furthermore, in Fig. 4b, it is evident that the partial current density (j_partial) of NH₃ (94.65 mA cm⁻²) significantly surpasses that of H₂ (0.01 mA cm⁻²) and N₂ (0.07 mA cm⁻²) at −0.6 V, confirming the exceptional selectivity of ZnFe₂O₄/NF for NH₃ production from NO₂⁻.

Fig. 4 (a) NH₃ yields and FEs of H₂ and N₂. (b) j_partial values of NH₃, H₂, and N₂. (c) Comparison of the amount of NH₃ production at different conditions. (d) Long-time stability test of consecutive production of NH₃. (e) Recycling tests of ZnFe₂O₄/NF at −0.6 V.

To ascertain that the generated NH₃ indeed arises from the eNO₂⁻RR on ZnFe₂O₄/NF, a series of control experiments were carried out at −0.6 V. The corresponding UV-vis absorption spectra are presented in Fig. S9.† The results obtained after testing under various conditions are illustrated in Fig. 4c, where substantial amounts of NH₃ were detected in the electrolyte. Simultaneously, the NH₃ yields in the cathode chamber were only 0.070, 0.072, and 0.071 μmol after 1 h electrolysis in blank electrolyte, an open circuit potential (OCP), and pure 0.1 M NaOH. This unequivocally rules out nitrogen contamination from the electrolyzer and reagents, thus indicating that the generated NH₃ originates exclusively from NO₂⁻ in the electrolyte. Stability is a crucial parameter for assessing catalyst activity. To evaluate the stability of ZnFe₂O₄/NF, it underwent an electrolysis test for 16 h in a solution of 0.1 M NaOH and 0.1 M NO₂⁻ (Fig. 4d). Over the extended test duration, ZnFe₂O₄/NF exhibited only a minor decrease in j, indicating its exceptional electrochemical stability. Simultaneously, we assessed the NH₃ yields and FEs both before and after the 16 h electrolysis to further confirm the stability of ZnFe₂O₄/NF. Even after 16 h electrolysis, the NH₃ yield attains 577.13 μmol h⁻¹ cm⁻², with a slight decrease in FE to approximately 93.7% (Fig. S10†). The LSV curves remain identical before and after long-term electrolysis (Fig. S11†), demonstrating the high stability of the electrocatalyst. In addition, we conducted continuous cycling tests at −0.6 V, replacing the electrolyte after each cycle. Remarkably, we observed nearly overlapping CA curves and UV-vis absorption spectra for each cycle (Fig. S12†). Furthermore, negligible fluctuations in NH₃ yields and FEs were observed over the course of 11 cycles (Fig. 4e), providing strong evidence of the exceptional long-term durability of ZnFe₂O₄/NF. Of note, the XRD pattern and SEM images (Fig. S13†) and XPS spectra (Fig. S14†) indicate minimal changes in the crystal phase, microstructure, and valence state of ZnFe₂O₄ after 16 h electrolysis. All above observations evidence the remarkable electrochemical and structural stability of our catalyst.

In conclusion, the ZnFe₂O₄ nanoarray is proven as a highly active electrocatalyst for the eNO₂⁻RR in alkaline environments. Incorporating Zn into the structure significantly enhances both the ECSA and electrical conductivity. It achieves an outstanding FE of 95.7% with a large NH₃ yield of 588.58 μmol h⁻¹ cm⁻² at −0.6 V. This work not only provides us with an efficient electrocatalyst for the production of value-added NH₃ but would be an inspiration for future design and exploration of Fe-based bimetallic oxides for applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding support through large group Research Project under (grant no. RGP2/257/44).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01176c

‡ Both authors contributed equally to this work.

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