Efficient and durable N₂ reduction electrocatalysis under ambient conditions: β-FeOOH nanorods as a non-noble-metal catalyst†

Xiaojuan Zhu‡ ^ab, Zaichun Liu‡ ^c, Qin Liu ^ab, Yonglan Luo ^b, Xifeng Shi ^d, Abdullah M. Asiri ^e, Yuping Wu *^c and Xuping Sun *^ab
aChemical Synthesis and Pollution Control, Key Laboratory of Sichuan Province, School of Chemistry and Chemical engineering, China West Normal University, Nanchong 637002, Sichuan, China. E-mail: xpsun@uestc.edu.cn
^bInstitute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China
^cState Key Laboratory of Materials-oriented Chemical Engineering, and School of Energy Science and Engineering, and Institute for Advanced Materials, Nanjing Tech University, Nanjing 211816, jiangsu, China. E-mail: wuyp@fudan.edu.cn
^dCollege of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China
^eChemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

First published on 17th September 2018

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NH₃, as an easily transportable energy carrier with a high energy density and no CO₂ emission, has played a key role in the agriculture, pharmaceutical and textile industries.^1–5 To date, more than 90% of NH₃ is produced via the Haber–Bosch process under harsh reaction conditions (200–250 bar, 400–500 °C).^6,7 Moreover, the necessary H₂ feedstock is produced by the steam reformation of natural gas (CH₄ + 2H₂O → 4H₂ + CO₂), leading to large energy consumption and carbon emission.^8,9 Compared with the traditional Haber–Bosch process, the electrochemical N₂ reduction reaction (NRR) can occur under ambient conditions, and can be regarded as a sustainable and less energy-intensive approach.^10–13

Most studies on the electrochemical NRR are based on aqueous electrolytes at elevated or ambient pressure.^10–18 However, aqueous electrolyte approaches suffer from competition with the hydrogen evolution reaction (HER), which limits overall efficiency, causing a low overall reaction rate. It is reported that this question can be solved by utilizing an electrochemical lithium cycling strategy.¹⁹ Chen et al. reported that the incorporation of Li⁺ into poly(N-ethyl-benzene-1,2,4,5-tetracarboxylic diimide) (PEBCD) retards the HER process and affords a reaction site for the NRR with a high faradaic efficiency (FE) of 2.91%.²⁰ Song et al. also reported that Li⁺ can enhance the electric field at the sharp spikes and increase the N₂ concentration within the Stern layer, resulting in carbon nanospikes that achieve a higher FE of 11.56%.²¹ Although progress has been made in this respect, there is great room to design and develop efficient electrocatalysts to enhance the NRR performance through the Li⁺ strategy. As a low cost and abundant material, β-FeOOH has been widely used as a positive electrode material for lithium batteries.^22,23 Additionally, this material has a large specific surface area and significant surface effect.²⁴ To the best of our knowledge, however, the use of β-FeOOH for electrocatalytic N₂ reduction has not been reported before.

In this communication, we report our recent finding that β-FeOOH nanorods act as an efficient electrocatalyst for the conversion of N₂ to NH₃ under ambient conditions. When tested in 0.5 M LiClO₄, such β-FeOOH nanorods are capable of achieving a high NH₃ yield of 23.32 μg h⁻¹ mg_cat.⁻¹ and a faradaic efficiency (FE) of 6.7%, outperforming most of the current aqueous-based NRR electrocatalysts. Remarkably, they also demonstrate good electrochemical stability and excellent selectivity. The catalytic mechanism of the NRR on the FeOOH(110) surface is further discussed using density functional theory (DFT) calculations.

Fig. 1a shows the X-ray diffraction (XRD) pattern for β-FeOOH, which gives peaks characteristic of β-FeOOH (JCPDS No. 78-1594). The scanning electron microscopy (SEM) images show a well-defined nanorod morphology (Fig. 1b). Fig. 1c shows a transmission electron microscopy (TEM) image of β-FeOOH, further confirming its nanorod nature. High-resolution TEM (HRTEM) images taken from a nanorod show well-resolved lattice fringes with an interplane spacing of 0.524 nm corresponding to the (200) plane of the β-FeOOH phase (Fig. 1d). Fig. S1 (ESI†) exhibits the X-ray photoelectron spectroscopy (XPS) survey spectrum of β-FeOOH, indicating the presence of Fe and O elements. In the Fe 2p region (Fig. 1e), two distinct peaks at binding energies (BEs) of 711.5 and 725.2 eV can be assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively. Besides, there are two satellites peaks at BEs of 719.1 and 733.9 eV, which indicates the coexistence of Fe²⁺ and Fe³⁺.²⁵ The high-resolution O 1s spectrum (Fig. 1f) exhibits two peaks at 530.2 and 531.6 eV assigned to the BEs of Fe–O–Fe and Fe–O–H bonds.²⁶ All these results strongly support the successful formation of β-FeOOH nanorods.

β-FeOOH was deposited on carbon paper (β-FeOOH/CP with a β-FeOOH loading of 0.2 mg) as the working electrode. Then, NRR tests were carried out in a two-compartment electrocatalytic cell separated by a Nafion 211 membrane in 0.5 M LiClO₄ solution at room temperature under atmospheric conditions. All potentials were reported on an RHE scale. The concentrations of produced NH₃ were spectrophotometrically determined using the indophenol blue method²⁷ and possible by-products were detected by using the method of Watt and Chrisp.²⁸ Their detecting calibration curves are shown in Fig. S2 and S3 (ESI†).

The time-dependent current density curves of β-FeOOH/CP at various potentials in N₂-saturated 0.5 M LiClO₄ are displayed in Fig. 2a, revealing that the current densities remain almost constant for 2 h. Fig. 2b shows the UV-vis absorption spectra of the electrolytes stained with indophenol indicator after charging at different potentials for 2 h. The electrolyte shows the highest absorbance intensity when electrolyzed at −0.75 V. The corresponding FEs and NH₃ yields at given potentials are plotted in Fig. 2c. The highest FE (6.7%) is achieved at a potential of −0.70 V and a maximum NH₃ yield (23.32 μg h⁻¹ mg_cat.⁻¹) can be obtained at −0.75 V, outperforming most reported NRR catalysts, including Fe₂O₃/CNT (0.22 μg h⁻¹ mg_cat.⁻¹, 0.15%),²⁹ Mo nanofilm (1.89 μg h⁻¹ cm⁻², 0.72%)³⁰ and γ-Fe₂O₃ (0.212 μg h⁻¹ cm⁻², 1.9%).³¹ More detailed performance comparisions under ambient conditions are listed in Table S1 (ESI†). Although in this potential range (−0.70 V to −0.75 V), the NH₃ yields increase as the negatively applied potential increases, the FEs decrease gradually, which is attributed to the compromise between increasing current density and competitive selectivity toward the HER rather than the NRR (Fig. S4, ESI†). When the applied potential is below −0.75 V, both the NH₃ yields and FEs decrease gradually, which is attributed to a rapid increase of the FE_HER and H₂ occupying the active sites on β-FeOOH, which prevents N₂ adsorption and reduction.³² Note that bare CP has a poor electrocatalytic activity for the NRR (Fig. 2d), revealing that the β-FeOOH nanorods are highly active for N₂ elelctroreduction electrocatalysis. Significantly, no N₂H₄ is detected in the electrolytes after 2 h of electrolysis (Fig. S5, ESI†), indicating that β-FeOOH exhibits an excellent selectivity for N₂ reduction to NH₃.

To verify that the detected NH₃ is generated via N₂ reduction electrocatalyzed by β-FeOOH, we performed electrolysis in Ar-saturated solution at −0.70 V and N₂-saturated solution at open circuit potential (Fig. S6, ESI†). Clearly, almost no NH₃ is detected in either case. Furthermore, we have also performed electrolysis at −0.70 V with alternating 2 h cycles between N₂-saturated and Ar-saturated electrolytes, for a total of 12 h. As shown in Fig. S7 (ESI†), the positive NRR results are only obtained in the durations of N₂-saturated electrolyte while the measurement in Ar-saturated electrolytes only provided blank results. Additionally, we further performed ¹⁵N isotopic labeling experiments, using a doublet coupling for the ¹⁵NH₄⁺ standard sample as a reference. Fig. S8 (ESI†) shows the ¹H nuclear magnetic resonance (NMR) spectra. As observed, ¹⁵NH₄⁺ signals can be detected when ¹⁵N₂ is bubbled into the cathode. All these results strongly confirm the N source of the NH₃ produced.

We further studied the NRR activity of β-FeOOH/CP in other different electrolytes (0.5 M NaClO₄ and 0.5 M KClO₄). NH₃ was spectrophotometrically determined by using indophenol blue and the detected calibration curves are shown in Fig. S9 and S10 (ESI†). As shown in Fig. 3a, the FEs are the highest at all given potentials in LiClO₄. Similar results in NH₃ yields are observed (Fig. 3b), meaning a favorable role for Li⁺ in enhancing the electrochemical NRR performance of β-FeOOH. Moreover, the FEs and NH₃ yields dropped with increasing cation size (the order of the cation size is Li⁺ < Na⁺ < K⁺), which may plausibly be ascribed to the steric effect of the counterions and the relatively strong interaction between the counterions and N₂. The desolvated Li⁺ counterion layer may serve to restrict the approach of water molecules to the electrode surface to reduce the competitive HER, thereby raising the FE for the NRR.²¹ We also performed electrolysis at −0.70 V using Hg/Hg₂SO₄ as a reference electrode. The NH₃ yield and FEs show no significant variation (Fig. S11, ESI†), verifying that Ag⁺ cations do not influence the NRR performance.

Stability is a critical factor to evaluate the performance of catalysts for the NRR. As observed in Fig. 3c, this catalyst can maintain its current density for at least 20 h at −0.70 V. In addition, β-FeOOH/CP has no obvious variation in FE and NH₃ yield during consecutive recycling tests at −0.70 V, indicating the high stability of β-FeOOH/CP for the electrochemical NRR (Fig. 3d). SEM images (Fig. S12, ESI†) suggest that the β-FeOOH nanorods still retain their initial morphology after long-term electrocatalysis. The XRD pattern and XPS spectra also show almost no changes in the crystalline phase (Fig. S13, ESI†) and the chemical state, respectively (Fig. S14, ESI†), after the stability tests. All these results indicate that this catalyst is extremely stable under ambient conditions for the NRR.

To understand the unique activity of NRR on FeOOH, we carried out DFT calculations for the energetics of the NRR steps on the (110) surface of FeOOH. Both end-on and side-on configurations are energetically favorable for N₂ adsorption on the FeOOH(110) surface (Fig. 4a). Charge density difference results revealed that the Fe-edge of the (110) surface plays a key role in polarizing and activating the N₂ molecules for both the charge exchange and transfer, which mainly took place between the Fe atoms of the (110) surface and N₂, resulting in the formation of the N–Fe bond and the intensive weakening of the NN triple bond. After adsorption, the N–N bond length is elongated from 1.128 Å in free N₂ to 1.190 and 1.151 Å in the adsorbed ones with the N₂ adsorption energies of about −1.01 and −0.84 eV, respectively. As N₂ activation plays a key role in the NRR process, FeOOH should show an extraordinary catalytic performance. To evaluate the potential of FeOOH as the electrocatalyst for the conversion of activated N₂ to NH₃, we canvassed the subsequent two typical NRR possible associative pathways, including distal and enzymatic mechanisms (Fig. 4b), while Fig. 4c and d summarize the corresponding free energy profiles. The N₂ fixation on the FeOOH(110) surface in the distal mechanism is due to the protonation of NNH* to form NNH₂* species, and that in the enzymatic mechanism is due to the formation of the NHNH₂* group. At zero electrode potential (U = 0 V), along the distal pathway, there is a key energy barrier of 0.52 eV required for the potential-limiting protonation step of the NNH* species (NNH* + H⁺ + e⁻ → NNH₂*), while it is much lower than that of enzymatic (0.68 eV) pathway for the protonation step of the NHNH* species and suggests that the distal pathway is more favorable. Favorably, it is much lower than the limiting barriers for flat surfaces of common metals, ranging from 1 to 1.5 eV.³³ With an applied potential of U = −0.52 V, all electrochemical steps will be downhill in free energy. Therefore, our theoretical calculation results and experimental observations indicate that FeOOH is a very promising catalyst for the NRR.

In summary, β-FeOOH nanorods are proven to be an efficient electrocatalyst for N₂ fixation to NH₃ with excellent selectivity under ambient conditions. In 0.5 M LiClO₄, this electrocatalyst achieves a high FE of 6.7% and an NH₃ yield of 23.32 μg h⁻¹ mg_cat.⁻¹, with a good electrochemical stability. The choice of counterions in the aqueous electrolyte is also critically important and Li⁺ plays a favorable role in enhancing the electrochemical NRR performance. This study not only provides us with an attractive earth-abundant catalyst material for electrochemical N₂ reduction, but could open up an exciting new avenue to explore transition metal hydroxides in artificial N₂ fixation.

This work was supported by the National Natural Science Foundation of China (No. 21575137, 51773092 and 51772147), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grants No. Kycx18_1122). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization and appreciate the High Performance Computing Center of Nanjing Tech University for the computational resources generously provided.

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c8cc06366d

‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018