WO₃ nanosheets rich in oxygen vacancies for enhanced electrocatalytic N₂ reduction to NH₃

Abstract

The Haber–Bosch process for industrial-scale NH₃ production suffers from harsh conditions and serious CO₂ release. Electrochemical N₂ reduction is an alternative approach to synthesize NH₃ under ambient conditions, but it requires highly-efficient electrocatalysts for the N₂ reduction reaction (NRR). In this Communication, we demonstrate that WO₃ nanosheets rich in oxygen vacancies (R-WO₃ NSs) exhibit greatly enhanced NRR performances. In 0.1 M HCl, such R-WO₃ NSs achieve a large NH₃ yield of 17.28 μg h⁻¹ mg_cat.⁻¹ and a high faradaic efficiency of 7.0% at −0.3 V vs. a reversible hydrogen electrode, much superior to the WO₃ nanosheets deficient in oxygen vacancies (6.47 μg h⁻¹ mg_cat.⁻¹ and 1.02%). Remarkably, R-WO₃ NSs also show high electrochemical stability.

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As an indispensable chemical in today's chemical industry, NH₃ is an activated nitrogen feedstock for fertilizers, plastics, pharmaceuticals, dyes etc.^1–3 It is also regarded as a green energy carrier.⁴ The synthesis of NH₃ from the hydrogenation of atmospheric N₂, however, is extremely difficult due to the inherent nature of the strong N≡N bond.⁵ To date, industrial NH₃ production is still dominated by the Haber–Bosch process under harsh conditions.^6,7 This process however consumes large energy and releases enormous CO₂. Therefore, it is needed to develop sustainable and eco-friendly routes for efficient N₂ fixation under milder conditions.

Electrochemical reduction, which can be operated under ambient conditions using renewable electric energy, is regarded as a promising choice for artificial N₂ fixation.^8,9 However, the N₂ reduction reaction (NRR) must be driven by efficient electrocatalysts.^8–11 Precious metal-based catalysts (Au, Ag, Pd, Ru, etc.) show favourable NRR activity,^12–16 but their high price and low abundance impede their large-scale applications. Considerable attention has thus been focused on Earth-abundant alternatives.^17–41 Recent density functional theory (DFT) calculations suggest that a single tungsten atom anchored on N-doped graphyne can activate the inert N≡N triple bond.⁴² As an important transition metal oxide, tungsten oxide (WO₃) has been widely applied in electrochemical fields due to its Earth abundance, highly tunable composition, and good electrochemical stability,⁴³ but its use as a NRR electrocatalyst still remains unreported.

Introducing oxygen vacancies (OVs) into electrocatalysts stands out as an effective strategy to manipulate the electronic structure, lower the activation energy barrier and further improve the electrocatalytic activities.^44,45 Recent studies emphasize the significant role of VOs in electrocatalytic N₂ adsorption and activation.^46–49 Furthermore, two-dimensional (2D) nanosheets with a large number of active sites are a promising structure for realizing high electrocatalytic NRR performance. Herein, we report the first experimental demonstration of using 2D WO₃ nanosheets rich in OVs (R-WO₃ NSs) as an active and selective electrocatalyst for artificial NH₃ synthesis under ambient conditions. In 0.1 M HCl, such R-WO₃ NSs achieve a large NH₃ yield (V_NH3) of 17.28 μg h⁻¹ mg_cat.⁻¹ and a high faradaic efficiency (FE) of 7.0% at −0.30 V vs. the reversible hydrogen electrode (RHE), much superior to the WO₃ nanosheets deficient in OVs (D-WO₃ NSs: 6.47 μg h⁻¹ mg_cat.⁻¹ and 1.02%). Remarkably, R-WO₃ NSs also show excellent electrochemical and structure stability.

Fig. 1a presents the X-ray diffraction (XRD) patterns for R-WO₃ NSs and D-WO₃ NSs. The XRD peaks for both R-WO₃ NSs and D-WO₃ NSs can be well distributed to monoclinic WO₃ (JCPDS No. 83-0951). The transmission electron microscopy (TEM) images show that R-WO₃ NSs (Fig. 1b) and D-WO₃ NSs (Fig. S1a†) exhibit a sheet-like morphology. A slight lattice disorder and dislocations are locally observed in the R-WO₃ NSs (Fig. 1c), highlighting the existence of numerous OVs. D-WO₃ NSs have a relatively flat and smooth surface without OVs (Fig. S1b†). As shown in the high-resolution TEM (HRTEM) image (inset in Fig. 1c), the nanosheet displays clear lattice fringes with the lattice spacings of 0.38 and 0.36 nm, which correspond to the (002) and (200) planes of WO₃, respectively. The polymorphic nature of R-WO₃ NSs is supported by the selected area electron diffraction pattern (Fig. S2†). The energy-dispersive X-ray (EDX) spectrum (Fig. S3†) reveals the existence of W and O. TEM and the corresponding EDX elemental mapping images (Fig. 1d) further indicate the uniform distribution of W and O.

Fig. 1 (a) XRD patterns of R-WO₃ NSs and D-WO₃ NSs. (b, c) TEM and HRTEM images of R-WO₃ NSs. (d) TEM and the corresponding EDX mapping images of W and O elements for R-WO₃ NSs.

The chemical states of R-WO₃ NSs are characterized by X-ray photoelectron spectroscopy (XPS). The XPS survey spectrum further confirms the co-existence of W and O elements (Fig. 2a). As shown in the W 4f XPS spectra (Fig. 2b), two peaks at 35.5 and 37.6 eV correspond to the characteristic W 4f_7/2 and W 4f_5/2 peaks of W⁶⁺, respectively. The other two peaks at 36.4 and 34.3 eV are assigned to the typical binding energies of W⁵⁺.⁵⁰ W⁵⁺ usually has a great relationship with OVs.^50,51 In the O 1s region (Fig. 2c), two peaks at 529.9 and 531.7 eV are attributed to the lattice oxygen and the oxygen species chemisorbed at OVs, respectively.⁵² The OVs were further resolved by electron spin resonance (ESR) spectroscopy. Room-temperature ESR spectra (Fig. 2d) indicate that R-WO₃ NSs rather than D-WO₃ NSs exhibit a significant ESR signal at g = 2.002, suggesting the electron trapping at OVs.⁵³ All of these results support the successful preparation of WO₃ rich in OVs.

Fig. 2 (a) XPS survey spectrum of R-WO₃ NSs. XPS spectra of R-WO₃ NSs in the (b) W 4f and (c) O 1s regions. (d) Room-temperature ESR spectra of R-WO₃ NSs and D-WO₃ NSs.

A carbon paper (CP) electrode was loaded with R-WO₃ NSs (R-WO₃/CP, R-WO₃ loading: 0.1 mg cm⁻²) for NRR tests in N₂-saturated 0.1 M HCl electrolytes. All potentials are reported on a RHE scale. We applied the indophenol blue method and the Watt and Chrisp's method to measure the produced NH₃ and the possible by-product (N₂H₄), respectively.⁵⁴ Fig. S4 and S5† present the corresponding calibration curves. NRR electrolysis experiments were conducted by applying controlled potentials from −0.25 to −0.45 V, as shown in Fig. S6.† The UV-Vis absorption spectra for electrolytes stained with the indophenol indicator at different potentials are shown in Fig. 3a. Fig. 3b presents the calculated results of V_NH3 and FEs. Obviously, V_NH3 and FE are enhanced as the negative potential increases until −0.30 V, where the values reach 17.28 μg h⁻¹ mg_cat.⁻¹ and 7.0%, respectively. This outstanding performance is superior to most reported NRR electrocatalysts. Table S1† lists more detailed comparison. When applying a more negative potential, both the V_NH3 and FEs decrease sharply because of the competition of the hydrogen evolution reaction.⁵⁵ N₂H₄ was not detected at a series of potentials (Fig. S7†), indicating that this catalyst possesses good selectivity for NH₃ formation. The electrocatalytic NRR performance of D-WO₃ NSs on the CP (D-WO₃/CP) was investigated for comparison. V_NH3 and FE of D-WO₃/CP can only achieve 6.47 μg h⁻¹ mg_cat.⁻¹ and 1.02% at −0.30 V, respectively, which are much lower than those of R-WO₃/CP (Fig. 3c and d).

Fig. 3 (a) UV-Vis absorption spectra of the 0.1 M HCl electrolytes stained with the indophenol indicator after NRR electrolysis. (b) V_NH3 and FEs at each given potential. (c) V_NH3 and (d) FEs at different potentials on R-WO₃/CP and D-WO₃/CP.

The enhancement of V_NH3 and FE as an NRR electrocatalyst could be attributed to the following reasons: (1) the existing OVs in R-WO₃ NSs could act as the active sites for boosting the adsorption and activation of N₂; (2) the introduction of OVs could advantageously affect the electronic structure of WO₃ NSs, enabling enhanced electron transfer. Fig. S8† shows that R-WO₃/CP possesses a much lower impedance⁵⁶ and thus obviously faster NRR kinetics than D-WO₃/CP; (3) the creation of OVs significantly increases the surface area (Fig. S9†) and provides more active sites for catalytic reactions (Fig. S10†).

Stability is another crucial indicator to evaluate NRR performance. As shown in Fig. 4a, this catalyst reveals a slight fluctuation in V_NH3 and FEs during consecutive recycling experiments for 6 times. As observed in Fig. 4b, the current density shows almost no fluctuation during 24 h NRR electrolysis. We also tested the NRR performance of R-WO₃/CP after 24 h electrolysis. Fig. S11† shows that there is only a slight decrease of V_NH3 after 24 h electrolysis. XRD (Fig. S12a†) confirms that this catalyst is still WO₃ in nature. The TEM image shows that the R-WO₃ NSs still kept their vacancy structure post-NRR (Fig. S12b†). These results indicate that R-WO₃ NSs show stable NRR performance under ambient conditions. Moreover, we executed a series of control experiments to verify the N source of the detected NH₃. The amount of NH₃ (m_NH3) produced on blank CP was examined at −0.30 V for 2 h (Fig. S13a and b†). Note that blank CP shows no obvious activity to the NRR. We also carried out NRR tests in a N₂-saturated electrolyte at the open circuit potential (OCP) for 2 h and an Ar-saturated electrolyte at −0.30 V for 2 h. As expected, no apparent NH₃ was detected in either case (Fig. S13c and d†). We conducted a total of 12 h cycles with the intervals of 2 h in N₂- and Ar-saturated electrolytes at −0.30 V and NH₃ was formed only in the N₂-saturated electrolyte (Fig. 4c). Fig. 4d shows that the m_NH3 nearly linearly increases with increased electrolysis time. All the above results prove that the N source of the detected NH₃ originates from N₂. We also explored the influence of electrolytes with different concentrations on NRR performance. Fig. S14† shows the V_NH3 and FEs of R-WO₃/CP at different concentrations of the electrolyte, indicating that the optimum NRR performance is obtained when the HCl electrolyte concentration is 0.1 M.

Fig. 4 (a) Recycling experiments for R-WO₃/CP during NRR at −0.30 V. (b) Time-dependent current density curve of R-WO₃/CP at −0.30 V. (c) V_NH3 and FEs of R-WO₃/CP with alternating 2 h cycles between N₂- and Ar-saturated electrolytes for a total of 12 h at −0.30 V. (d) The amount of produced NH₃vs. time recorded at −0.30 V.

In summary, R-WO₃ NSs are successfully proven as a highly-active 2D electrocatalyst for NRR under ambient conditions. R-WO₃ NSs exhibit a high activity and selectivity with a NH₃ yield of 17.28 μg h⁻¹ mg_cat.⁻¹ and a FE of 7.0% at −0.30 V vs. RHE, outperforming D-WO₃ NSs (NH₃ yield: 6.47 μg h⁻¹ mg_cat.⁻¹; FE: 1.02%). Notably, R-WO₃ NSs show excellent electrochemical stability as well. This study not only provides us with an attractive non-noble-metal catalyst for electrocatalytic NH₃ synthesis, but also guides the rational design of W-based catalysts with high performance for applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21575137). The Science & Technology Fund Planning Project of Shandong Colleges and Universities (J16LA13 & J18KA112), the Qingdao Basic & Applied Research Project (15-9-1-100-jch), and the Qingdao Science & Technology Planning Project (17-6-3-15-gx).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c9nr03678d

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