Glycerine-based synthesis of a highly efficient Fe₂O₃ electrocatalyst for N₂ fixation

Abstract

The electrochemical nitrogen reduction reaction (NRR) is a promising approach to convert N₂ into high value-added NH₃. However, it is still a challenge to achieve an efficient electrocatalyst for the NRR. Herein, it is demonstrated that the Fe₂O₃ nanoparticles (NPs) generated from a glycerine-based synthesis can be applied as highly efficient catalysts for the NRR. The Fe₂O₃ NPs show good performance with a high NH₃ yield (22 μg mg_cat⁻¹ h⁻¹) and a favorable Faradaic efficiency (FE) (3.5%) at −0.5 V vs. reversible hydrogen electrode (RHE). The facile synthesis strategy and satisfactory electrochemical properties demonstrate the potential application of the as-synthesized Fe₂O₃ NPs for NRR.

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1. Introduction

Ammonia (NH₃) is a highly important chemical in many aspects of industrial production and daily life.^1,2 Fixation of N₂ to NH₃ is an important step for the natural N₂ cycle.^3–5 The current synthesis of NH₃ mainly depends on the industrial Haber–Bosch process, which involves serious energy consumption and leads to large amounts of greenhouse gas emissions.^6,7 To realize a green and sustainable strategy for N₂ fixation, electrochemical reduction of N₂ has recently attracted much attention, being an environmentally friendly route involving mild conditions.^8–10

To date, a number of catalysts have been developed for the NRR, including noble metals,^11–13 transition metals,^14,15 metal-free materials,^16–18 metal–C composite materials^19–21 and Au–Fe₃O₄.²² These catalysts have demonstrated potential applications in the NRR with improved FE and NH₃ yield. Most of the catalysts were synthesized with the assistance of surfactants (structure-directing agents) through solution methods.²³ However, the surfactants could passivate the catalyst surface, which decreases the activity of the catalyst since the reactions take place on the catalyst surface. Therefore, the catalyst with a clean surface could be very important for achieving the high activity.

Herein, we successfully synthesized the clean-surface Fe₂O₃ electrocatalyst for NRR by using glycerine as solvent with a subsequent calcination process. There was no need of surfactants in the synthesis process and the particles were further calcinated, guaranteeing the clean nature of the Fe₂O₃ surface. The as-prepared Fe₂O₃ NPs demonstrated a good electrocatalytic performance for NRR, with a high NH₃ yield (22 μg mg_cat⁻¹ h⁻¹) and a favorable FE (3.5%) at −0.5 V vs. reversible hydrogen electrode (RHE).

2. Materials and reagents

Glycerine (C₃H₈O₃) (purity, 99.5%), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) (purity, 98.5%) and ethanol (CH₃CH₂OH) (purity ≥ 98.5%) were purchased from Sinopharm Chemical Reagent Co. Ltd.

2.1. Synthesis of Fe₂O₃ nanoparticles (NPs)

121.2 mg of iron(III) nitrate nonahydrate was added into 5 mL glycerin. Then the solution was ultrasonicated for 10 min. The uniform solution was transferred into the Teflon-lined stainless-steel autoclave and heated at 180 °C for 20 h. The obtained product washed with ethanol and water for three times and dried at room temperature for 12 h. Then the product was put into a tubular furnace, heated to 450 °C for 2 h at the heating rate of 10 °C min⁻¹ under air. Finally, Fe₂O₃ NPs was obtained.

2.2. Characterization

A Rigaku Dmax-rc X-ray diffractometer was used to perform X-ray diffraction (XRD) characterization. The transmission electron microscopy (TEM) images were obtained on a JEM 1400 TEM instrument. The X-ray photoelectron spectroscopy (XPS) characterization was conducted with ESCALAB 250.

2.3. Electrode preparation

5 mg as-obtained Fe₂O₃, 40 μL Nafion (5% wt) and 960 μL ethanol were mixed by ultrasound for forming suspension. 50 μL suspension was modified on 1 × 1 cm carbon cloth (CC). The Fe₂O₃–CC was used as working electrode.

2.4. Electrochemical reduction of N₂

Electrochemical reduction of N₂ was carried out in a typical three-electrode gastight two-compartment reaction vessel separated by an anion exchange membrane (Nafion 211) on a CHI760 electrochemical workstation (Chenhua, Shanghai). The experiment was performed in 0.1 M Na₂SO₄ solution (50 mL each compartment). A Pt wire was used as the counter electrode and an Ag/AgCl (4.0 M KCl) was used as reference. All potentials were converted to RHE. The electrolyte was then purged with N₂ for at least 30 min. N₂ was delivered into the cathodic compartment at a constant rate of 20 mL min⁻¹. The potentiostatic tests were performed in 0.1 M Na₂SO₄ aqueous solution at different potentials such as −0.3, −0.4, −0.5, −0.6, and −0.7 V vs. RHE. The NH₃ yields and FEs of products were calculated as follows:

NH₃ yields (μg mg_cat⁻¹ h⁻¹) = m/(t × m_cat) (1)

FE (%) = αmF/MQ (2)

where m is the mass of NH₃, m_cat is the mass of catalyst, α is the quantity of transferred electrons for producing NH₃, M is the relative molecular mass, and Q is the total quantity of the electric charge.

2.5. Determination of NH₃ and N₂H₂

The detections of NH₃ and N₂H₂ were made by indophenol blue and Watt–Chrisp methods, respectively, according to the reported literature.²⁴

3. Results and discussion

The Fe₂O₃ electrocatalysts were synthesized by using glycerine as the solvent with a subsequent calcination process. In the synthesis system, there was no surfactant and the particles were further calcinated. Therefore, the Fe₂O₃ particles with clean surface could be obtained by our present synthetic route. Fig. 1(a) shows the X-ray diffraction (XRD) pattern of the as-synthesized Fe₂O₃, which agrees well with the JCPDS No. 33-0664. This demonstrates the successful formation of Fe₂O₃. Low peak intensity may be caused by small size of Fe₂O₃ particles. Transmission electron microscopy (TEM) images of the Fe₂O₃ suggests that the diameters of as-synthesized Fe₂O₃ nanoparticles are in the range of 4–6 nm (Fig. 1(b) and (c)).

Fig. 1 (a) XRD pattern and (b and c) TEM images of the Fe₂O₃ after calcination at 450 °C.

To investigate the surface elemental state of Fe₂O₃ NPs, we analyzed the chemical states of the Fe₂O₃ NPs by XPS analysis. There are two Fe³⁺ peaks located at 711.8 eV and 725.3 eV, corresponding to Fe 2p_1/2 and Fe 2p_3/2,²⁵ agreeing well with Fe₂O₃ NPs (Fig. 2). The XPS spectrum result further demonstrated the successful synthesis of Fe₂O₃ NPs.

Fig. 2 High resolution XPS spectrum of Fe 2p.

The synthesized Fe₂O₃ NPs were then used as the electrocatalysts for the electrochemical nitrogen reduction reaction (NRR). The produced ammonia was analyzed and quantified based on the indophenol blue method. Before NRR experiment, the corresponding calibration curve for ammonia by indophenol blue method was first determined and shown in Fig. 3.

Fig. 3 (a) UV-vis absorption spectra of different concentration ammonia in 0.1 M Na₂SO₄ solution, (b) standard curve of ammonia in 0.1 M Na₂SO₄ solution.

The N₂-fed electrolytes in 0.1 M Na₂SO₄ electrolytes at different potentials for 2 h were mixed with the indophenol reagent, and their absorbance changed at 660 nm were depicted in Fig. 4(a). It indicated the substantial electroreduction of inert N₂ into valuable NH₃ was achieved using the as-synthesized Fe₂O₃ electrocatalyst. Fig. 4(b) presented the chronoamperometric curves as a function of reaction time at varying applied potentials in 0.1 M Na₂SO₄, demonstrating the Fe₂O₃ nanoparticles have good stability for NRR from −0.3 V to −0.6 V. The average NH₃ yield and the corresponding Faradaic efficiency (FE) was given in Fig. 4(c), in which the favorable NH₃ yield was 22.0 μg mg_cat⁻¹ h⁻¹ with FE value of 3.5% at −0.5 V. The NRR performance of the as-synthesized Fe₂O₃ is comparable to lots of the NRR electrocatalysts (Table S1†). Therefore, the present synthesized Fe₂O₃ with clean surface might be as a potential catalyst for electrochemical NRR in consideration that there are abundant Fe element in the earth. TEM image of catalyzed Fe₂O₃ was tested after nitrogen fixation at −0.5 V. It was found that the morphology did not changed significantly, which indicated the good stability of the as-synthesized Fe₂O₃ (Fig. S1†).

Fig. 4 UV-vis absorption spectra in (a) 0.1 M Na₂SO₄ electrolytes stained with the indophenol indicator after NRR electrolysis at a series of potentials for 2 h, (b) time-dependent current density curves for Fe₂O₃ at different potentials in 0.1 M Na₂SO₄ solution, (c) NH₃ yields and FEs for Fe₂O₃ at a series of potentials in 0.1 M Na₂SO₄.

The NNR performance is highly related to the electrochemically active surface area (ECSA). Therefore, the ECSA of the as-prepared Fe₂O₃ was further studied. Here, the ECSA was reflected by double layer capacitance (C_dl) since there was a linear proportional relationship between ECSA and C_dl, which could be obtained by cyclic voltammetry curves in the range of 0.1–0.2 V (Fig. 5(a)). Based on Fig. 5(b), the high C_dl of 1.6 mF cm⁻² (Fig. 5(b)) further demonstrated that Fe₂O₃ had high ECSA for NRR.

Fig. 5 (a) Cyclic voltammetry curves of Fe₂O₃ at different scanning rate of potential in 0.1 M Na₂SO₄ solution, (b) the double-layer capacitance of Fe₂O₃.

The hydrazine byproduct was further determined through a Watt–Chrisp method. Fig. 6(a) showed the corresponding calibration curve based on UV-vis absorption at 455 nm. As shown in Fig. 6(b), the by-product of hydrazine was not detected, implying the high selectivity of Fe₂O₃ for NH₃ production (Fig. 6). Hence, the as-prepared Fe₂O₃ catalyst could be used as a high-selective catalyst for producing NH₃ by electrochemical NRR.

Fig. 6 (a) Standard curve of hydrazine in 0.1 M Na₂SO₄ solution, (b) UV-vis absorption spectra of hydrazine in 0.1 M Na₂SO₄ electrolytes after NRR electrolysis at a series of potentials for 2 h.

We also explored the NRR performance of Fe₂O₃ in 0.1 M KOH. The selected potential was −0.5 V. Unfortunately, the current density declined rapidly within 2 h (Fig. S2†).

4. Conclusions

In summary, Fe₂O₃ synthesized by glycerine-based route with subsequent calcination process can be adopted as highly efficient NRR catalysts. Thanks to the clean surface of the catalyst, the Fe₂O₃ NPs exhibited good performance with a 22 μg mg_cat⁻¹ h⁻¹ NH₃ yield and a 3.5% Faraday efficiency at −0.5 V for NH₃ production, which outperformed lots of the previous catalysts. The flexible strategy and the good electrochemical performance endow Fe₂O₃ with potential application in NRR.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research was supported by Science and technology development foundation, Nanjing Medical University, China (No. NMUB2019273).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05831a

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