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High-Efficiency Electrosynthesis of Ammonia with High Selectivity under Ambient Conditions Enabled by VN Nanosheet Array Rong Zhang, Ya Zhang, Xiang Ren, Yonglan Luo, Guanwei Cui, Abdullah M. Asiri, Baozhan Zheng, and Xu-Ping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01261 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Sustainable Chemistry & Engineering

High-Efficiency Electrosynthesis of Ammonia with High Selectivity under Ambient Conditions Enabled by VN Nanosheet Array Rong Zhang,†,‡ Ya Zhang,†,‡ Xiang Ren,† Guanwei Cui,§ Abdullah M. Asiri,ǁ Baozhan Zheng,‡,* and Xuping Sun†,‡,* †

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu § 610054, Sichuan, China, ‡College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, China, College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China, ǁChemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia *E-mail: [email protected] (X.S.); [email protected] (B.Z.)

ABSTRACT: The development of efficient earth-abundant electrocatalysts for N2 reduction to ammonia (NH3) under ambient conditions is critical for achieving a low-carbon and sustainable-energy society. Herein, we report the development of VN nanosheet array on Ti mesh as an active and selective electrocatalyst for N2 reduction reaction (NRR) in acid at room temperature and atmospheric pressure in 0.1 M HCl. A rate of NH3 formation of 8.40 × 10–11 mol s–1 cm–2 is obtained at –0.50 V with a Faradaic efficiency of 2.25%. Notably, such catalyst material also exhibits high selectivity (no formation of N2H4) and electrochemical stability. Theoretical and experiment results suggest that VN catalyzes NRR via a Mars-van Krevelen mechanism. This study would offers the potential of utilization of attractive 3D catalyst electrode toward efficient NH3 synthesis for applications. KEYWORDS: VN nanosheet array, N2 reduction reaction, Electrocatalysis, Ambient conditions, Mars-van Krevelen mechanism

INTRODUCTION Ammonia (NH3) is considered as a key precursor of synthetic fertilizer, which also finds applications as hydrogen carrier and combustion fuel.1–3 The Haber-Bosch process, involving the heterogeneous reaction of N2 obtained from air and H2 produced from fossil fuels, still remains the dominant route for industrial production of NH3.4,5 The overall process however operates under harsh conditions of high pressure (200–250 bar) and temperature (400–500 °C). The whole process amounts to 1–3% of global energy supply. Additionally, the necessary H2 feedstock leads to substantial carbon emission and serious safety concerns.6–8 Thus, more sustainable and economical NH3-producing way is urgently demanded.9 Numerous efforts have been devoted toward artificial NH3 synthesis, including biological, photocatalytic, and electrochemical methods.3,10–15 Particularly, electrochemical synthesis of NH3 from N2 and H2O is regarded as an attractive route under ambient conditions but requires efficient electrocatalysts for the N2 reduction reaction (NRR). Noblemetals (Au,16–18 Ru,19,20 and Rh21) show high NRR activity. However, it should be pointed out that those noble metals are scarce and expensive, limiting their wide uses. Some Ru and Rh based electrocatalysts even show low Faradaic efficiencies (FEs) below 1% for the NRR. Thus, active electrocatalysts made of earth-abundant elements are highly desired and several such catalysts have been reported including Mo nanofilm,22 MoS2,23 MoO3,24 γ-Fe2O3,25 Fe2O3-CNT,26 Fe3O4,27 Bi4V2O11/CeO2,28 PEBCD,29 and N-doped porous carbon,30 etc. Generally, the most surface of majority of metals is likely covered with H-adatoms in aqueous solutions, rather than Nadatoms, severely hindering the production of NH3.31

Transition metal nitrides show high electronic conductivity, reliable chemical stability, and rich N-adatoms.32 Recent theoretical investigations suggest that VN is a potential candidate for NRR via a Mars-van Krevelen mechanism under ambient conditions.32,33 Moreover, nanoarray catalysts can expose rich active sites and allow easier diffusion of electrolyte.34,35 Motivated by this prediction, in this study, experiments were conducted to investigate the possible electrochemical NRR over VN nanoarray on Ti mesh (VN/TM) in acidic solution and under ambient conditions. In 0.1 M HCl, such VN/TM achieves a rate of NH3 formation of 8.40 × 10–11 mol s–1 cm–2 with a FE of 2.25% at –0.50 V. Remarkably, this catalyst also shows high electrochemical stability.

RESULTS AND DISCUSSION VN/TM was synthesized by nitridation of VO2 nanosheet array hydrothermally grown on TM (VO2/TM) in NH3 atmosphere at 700 oC for 3 h (see SI for preparative detail). After hydrothermal treatment, the color of Ti mesh changes from silver to dark green. The corresponding X-ray diffraction (XRD) pattern (Figure S1) presents diffraction peaks characteristic of monoclinic VO2 (JCPDS No. 65-7960). After nitridation, the product shows the peaks characteristic of cubic VN (JCPDS No. 35-0768) (Figure 1a) without any impurities, implying the complete conversion of VO2 into VN. Figure 1b and 1c display the scanning electron microscopy (SEM) images. It can be clearly observed the formation of VO2 nanosheet array on TM after hydrothermal reaction. Interestingly, the resulting VN is still perfectly integrated on the substrate with the maintenance of nanoarray feature (Figure 1d and 1e). Energy-dispersive X-ray (EDX) spectrum elemental mapping analysis suggests the uniform distribution

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of V, N, and Ti elements in VN/TM, as shown in Figure 1f. High-resolution transmission electron microscopy (HRTEM) image (Figure 1h) of VN (Figure 1g) displays lattice fringes and the interval is determined as 0.207 nm indexed to the (200) plane. Selected area electron diffraction (SAED) pattern of VN (Figure 1i) shows four bright bright rings made up of discrete spots corresponding to the (111), (200), (220), and (311) planes.

Figure 1. (a) XRD pattern for VN scraped down from TM. SEM images for (b,c) VO2/TM and (d,e) VN/TM. (f) SEM and corresponding EDX elemental mapping images of V, N, and Ti for VN/TM. (g) TEM image of VN nanosheets. (h) HRTEM image and (i) SED pattern taken from VN.

Figure 2. (a) XPS survey spectrum for VN. XPS spectra in (b) V 2p and (c) N 1s for NV.

Figure 2a shows the X-ray photoelectron spectroscopy (XPS) survey spectrum of VN, revealing the existence of V and N. Three peaks based on values in literature are presented in V 2p3/2 region with three peaks shown in V 2p1/2 region.36 The peak with binding energy (BE) of 513.7 eV can be ascribed to V in VN.37 Two peaks located at 514.7 and 516.6 eV are assigned to the BEs for V3+ and V5+ states in surface oxides owing to the passivation of VN in air, respectively.38 The N 1s spectrum in Figure 2c shows a strong peak at 397.2

eV, which is consistent with the N environment in metal nitride.39,40 The peak at 398.3 eV is assigned to the N bonding to surface oxide layer, which can be dissolved and eliminated in acid.

Figure 3. (a) LSV curves of VN/TM in Ar- and N2-saturated 0.1 M HCl with a scan rate of 5 mV s-1. (b) UV-Vis absorption spectra of indophenol blue formed from generated NH3 in the electrolytes after charging at a series of potentials for 3 h. (c) Average NH3 yields and FEs for VN/TM at a series of potentials. (d) Proposed pathway for the NH3 electrosynthesis using VN (blue, purple, and grey balls represent H, N, and V atoms, respectively).

The N2 electroreduction experiments were performed in a two-compartment cell separated by Nafion 211 membrane at ambient temperature (~25 oC) and atmospheric pressure. The NRR performance of VN/TM (VN loading: 1.43 mg cm-2) was initially evaluated by linear sweep voltammetry (LSV) in N2and Ar-saturated 0.1 M HCl. As shown in Figure 3a, a slight current density increasement in N2 (99.999% purity) indicates that VN/TM is active for NRR. The generated NH3 is was measured by using the indophenol blue method.41 The possible hydrazine (N2H4) by-product can be determined through method of Watt and Chrisp.42 The calibration curves of NH3 and N2H4 are shown in Figure S2 and S3, respectively. After feeding N2 gas into the cathodic compartment at open-circuit potential for 3 h, there was almost no NH3 generated (Figure S4). Figure 3b shows the UV-Vis absorption spectra of indophenol blue formed from generated NH3 in the electrolytes after charging at a series of potentials for 3 h. It is clear that the electrochemical NRR can be realized at applied potentials ranging from –0.38 to –1.10 V. The electrolyte shows the highest absorbance intensity when electrolyzed at – 0.50 V. Also note that TM has negligible NRR activity at – 0.50 V compared with that for VN/TM (Figure S5). Moreover, no N2H4 as reaction by-product is detected (Figure S6), suggesting the high selectivity of VN/TM for the NRR. The


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average NH3 yields and corresponding FEs at different potentials are calculated (see SI for details) and plotted in Figure 3c. The NH3 yields have been subtracted by the value that obtained in N2 at open-circuit potential. Both rate of NH3 formation and FE increase with potential being more negative until –0.50 V, where a rate of NH3 formation of 8.40 × 10–11 mol s–1 cm–2 (or 3.57 µg h–1 mg–1cat.) and a FE of 2.25% are achieved. When the applied potential is below –0.50 V, both values reduce rapidly. It can be ascribed to the competitive adsorption of N2 and H species on electrode surface.17 Although NRR performance of VN/TM is inferior to Au-based catalysts16–18 and N-doped carbon nanospikes,43 VN/TM still outperforms most reported aqueous-based electrocatalysts under ambient conditions (Table S1). The performances are even comparable with those under hard conditions (Table S2). Based on the fact that no N2H4 was detected in the final product, a possible NRR mechanism over VN can be proposed according to the previous study.33 NH3 can be formed over VN via a Mars-van Krevelen mechanism. A N atom in VN is reacted with H to form a NH3. The catalyst then regenerated under N2 gas. As shown in Figure 3d, firstly, a N atom on the surface is reduced to NH3 resulting in the generation of a N vacancy. Then, this vacancy is replenished with N2. Finally, hydrogenation of VN occurs by adding H atoms one-by-one. Experimentally, Ar (99.999% purity) was bubbled at –0.5 V for 3 h. The UV-Vis absorption spectrum (Figure S4) reveal that NH3 also can be produced under Ar atmosphere. EDX spectra of VN/TM after long-term electrolysis in Ar-saturated 0.1 M HCl (Figure S5) suggests the absence of N in VN after electrolysis. We further performed 15N isotopic labeling experiment to verify the N source of the NH3 produced, using doublet coupling for 15NH4+ and triplet coupling for 14NH4+ standard sample as reference. Figure S6 shows the 1H nuclear magnetic resonance (1H NMR) spectra. As observed, only 14 NH4+ is produced in Ar. However, both 14NH4+ and 15NH4+ are detected when 15N2 is bubbled into the cathode. All above results confirm that VN catalyzes NRR via the Mars-van Krevelen mechanism. Moreover, after feeding N2 gas into the cathodic compartment at open-circuit potential for 3 h, there was almost no NH3 generated (Figure S7), indicating that there is no source of NH3 present as contaminants in our standard experiments and the detected NH3 is only electrochemically produced from N2 reduction catalyzed by VN/TM. Also note that TM has negligible NRR activity at –0.50 V compared with that for VN/TM (Figure S8). Moreover, no N2H4 as reaction by-product is detected (Figure S9), suggesting the high selectivity of VN/TM for the NRR. Stability is also an critical criterion to estimate the performance of the catalyst. Figure 4a shows the amperometric i-t curves of VN/TM at a series of potentials, indicating our catalyst material maintains its NRR performance for 8 h. Both XRD (Figure 4b) and XPS (Figure S10) analyses demonstrate that this catalyst is still VN in nature after NRR electrolysis. SEM images (Figure 4c) further suggest that this VN/TM still keeps its initial morphology. The recycling experiment was also carried out. NRR experiment was repeatedly conducted at –0.5 V for 3 h for 10 times using VN/TM catalyst. Fresh electrolyte were used in each cycle. As shown in Figure 4d, VN/TM has negligible changes in NH3

yields and FEs through the 10-time recycling stability tests, suggesting its excellent stability for NRR.

Figure 4. (a) Amperometric i-t curves of VN/TM for NRR at a series of potentials in 0.1 M HCl. (b) XRD pattern for VN after stability test. (c) SEM images for VN/TM after stability test. (d) Recycling test of VN/TM at a potential of –0.50 V.

CONCLUSION In summary, VN nanosheet array was proven experimentally as a high-efficient and stable noble metal-free NRR catalyst for electrochemical NH3 synthesis in acid under ambient conditions. In 0.1 M HCl, this catalyst achieves a FE and a NH3 formation rate of 2.25% and 8.40 × 10–11 mol s–1 cm–2 at – 0.50 V, respectively, outperforming most reported aqueousbased NRR electrocatalysts and even comparable with those under high temperatures. This VN/TM also shows high selectivity (no formation of N2H4) and electrochemical stability for 8 h. Theoretical and experiment results suggest that VN catalyzes NRR via a Mars-van Krevelen mechanism. This work opens an attractive new avenue to utilize the transition metal nitride nanoarray as an attractive 3D catalyst electrode toward efficient and selective electrosynthesis of NH3 from ubiquitous N2 and H2O for applications.44,45

ASSOCIATED CONTENT Supporting Information Experimental section; XRD pattern; UV-vis absorption, EDX, 1H NMR and XPS spectra; calibration curves; Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.S.); [email protected] (B.Z.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT



This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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to NH3 on MoN nanosheets array under ambient conditions. Unpublished results.



For Table of Contents Use Only

VN nanosheet array on Ti mesh (VN/TM) shows high performance for N2 reduction reaction under ambient conditions. In 0.1 M HCl, such VN/TM achieves a rate of NH3 formation of 8.40 × 10–11 mol s–1 cm–2 and a FE of 2.25% at –0.50 V at room temperature and atmospheric pressure.


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