Enabling Effective Electrocatalytic N2 Conversion to NH3 by the TiO2

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Enabling Effective Electrocatalytic N2 Conversion to NH3 by TiO2 Nanosheets Array under Ambient Conditions Rong Zhang, Xiang Ren, Xifeng Shi, Fengyu Xie, Baozhan Zheng, Guo Xiaodong, and Xuping Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06647 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Applied Materials & Interfaces

Enabling Effective Electrocatalytic N2 Conversion to NH3 by TiO2 Nanosheets Array under Ambient Conditions Rong Zhang,†,‡ Xiang Ren,† Xifeng Shi,║ Fengyu Xie,∫ Baozhan Zheng,*,‡ Xiaodong Guo,*,§ 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 273165, Shandong, China, ∫ College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China, §College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China ABSTRACT: NH3 serves as an attractive hydrogen storage medium and a renewable energy sector for a sustainable future. Electrochemical reduction is a feasible ambient reaction to convert N2 to NH3 while needs efficient electrocatalysts for the N2 reduction reaction (NRR) to meet the challenge associated with N2 activation. In this letter, we report on our recent experimental finding that TiO2 nanosheets array on Ti plate (TiO2/Ti) is effective for electrochemical N2 conversion to NH3 at ambient conditions. When tested in 0.1 M Na2SO4, such TiO2/Ti attains a high NH3 yield of 9.16 × 10–11 mol s–1·cm–2 with corresponding Faradaic efficiency of 2.50% at –0.7 V vs. reversible hydrogen electrode, outperforming most reported aqueous-based NRR electrocatalysts. It also shows excellent selectivity for NH3 formation with high electrochemical stability. The superior NRR activity is due to the enhanced adsorption and activation of N2 by oxygen vacancies in-situ generated during electrochemical test. Keywords: TiO2 nanosheets, N2 reduction reaction, ambient conditions, electrolysis, oxygen vacancies Apart from being an activated nitrogen building block for the manufacture of fertilizers, medicaments, dyes, explosives, and resins,1–3 NH3 is also recognized as a carbon-neutral energy vector with high energy density for the hydrogen economy.4 N2 is the richest gas in earth’s atmosphere, but the strong triple bond makes it inert and and thus difficult to take part in a chemical reaction.5 In the industrial Haber-Bosch process for NH3 production from N2 and H2, the limitation in kinetics can be overcome by using an Fe- or Ru-based catalyst, repeated cycle, high pressure (200-300 atmosphere) and elevated temperature (300-500 oC).6,7 This process is not only energy-intensive, but emits large amounts of CO2. With the deplition of fossil fuels,8-11 it is urgent for us to turn to new N2fixing approaches which are environmentally-benign and sustainable. In nature, N2 fixation is realized by nitrogenases through multiple proton and electron transfer steps with a significant energy input delivered by ATP at ambient conditions.12–14 Inspired by the natural process, with the use of N2 and water as the raw materials, adding protons and electrons through an electrochemcial method leads to artificial N2 fixation at room temperature and 1 atmosphere.15–17 Although electrochemical reduction is a feasible ambient reaction to convert N2 to NH3,18-20 it needs efficient electrocatalysts for the N2 reduction reaction (NRR) to meet the challenge associated with N2 activation. Noble metals (Au,21 Ru,22 and Rh23) based catalysts show attractive activity for the NRR, but the high cost of these catalysts limit their wide uses. have been recent Transition metal oxides (TMOs) are earth-abundant and can be easily prepared on a large scale. Nevertheless, only limited such NRR electrocatalysts have been documented so far, including γ-Fe2O3,24 Fe2O3/CNT,25 and Bi4V2O11/CeO2,26 etc. Moreover, nanoarray catalysts can expose rich active sites and allow

easier diffusion of electrolyte.27,28 Therefore, idenfication of new TMOs nanostructures for effective N2 reduction electrocatalysis is still of critical importance for fundamental and application research. In this letter, we describe our recent experimental finding that TiO2 nanosheets array on Ti plate (TiO2/Ti) is effective for electrochemical N2 conversion to NH3 at ambient conditions. In 0.1 M Na2SO4, such TiO2/Ti achieves a high NH3 yield of 9.16 × 10–11 mol s–1·cm–2 with a high Faradaic efficiency (FE) of 2.50% at –0.7 V vs. reversible hydrogen electrode (vs. RHE), outperforming most reported aqueous-based NRR electrocatalysts. Notably, it also shows excellent selectivity for NH3 formation with high electrochemical stability. It suggests that the oxygen vacancies (VO) in-situ formed during electrochemical test enhance the adsorption and activation of N2, enabling superior N2 reduction electrocatalysis. As shown in Figure 1a, the X-ray diffraction (XRD) pattern for TiO2/Ti displays the peaks characteristics of TiO2 (JCPDS No. 21–1272), which also shows several peaks corresponding to Ti plate (JCPDS No. 44–1294), without any impurity. The energy-dispersive X-ray (EDX) spectrum (Figure S1) confirms the presence of Ti and O elements in the sample. The scanning electron microscopy (SEM) images of TiO2/Ti are depicted in Figure 1b, suggesting Ti substrate is fully covered by TiO2 nanosheets array. The SEM image and corresponding EDX elemental mapping images (Figure 1c) further confirm the uniform distribution of Ti and O elements. The nanosheet nature is further evidenced by the transmission electron microscopy (TEM) image (Figure 1d). Furthermore, the highresolution TEM (HRTEM) images presents uniform and welldefined lattice fringes. The interval is then determined as 0.35 nm corresponding to the (101) plane of anatase TiO2 (Figure 1d inset).

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absorption spectra of these electrolytes show negligible change.

Figure 1. (a) XRD pattern and (b) SEM images for TiO2/Ti. (c) SEM image and corresponding EDX elemental mapping images of Ti and O elements for TiO2/Ti. (d) TEM and HRTEM (inset) images of TiO2 nanosheet.

The electrocatalytic NRR activity of TiO2/Ti was tested using an aqueous-based electrochemical setup at ambient conditions. All potentials were reported on a RHE scale. The NRR performance of catalyst was then evaluated by chronopotentiometry in N2-saturated 0.1 M Na2SO4 for 3 h. N2 was continually supplied to the cathode during the electrocatalytic process. H+ can be transported through electrolyte and react with N2 on the surface of catalyst to generate NH3 with potentiostatic reduction. The generated NH3 was then transformed into the indophenol after reaction with phenol in alkaline condition.29–31 Figure S2 shows the polarization curves of TiO2/Ti in Ar- or N2-saturated 0.1 M Na2SO4. TiO2/Ti shows higher current density in N2, implying the occurrence of electrocatalytic N2 reduction. Figure 2a shows the ultraviolet-visible (UV-Vis) absorption spectra of these electrolytes with absorption at 655 nm attributed to indophenol. The concentrations of generated NH3 at different applied potentials are calculated according to the calibration curve of NH3 (Figure S3). As shown in Figure 2b, the NH3 yields are determined as 1.59 × 10–11, 3.95 × 10–11, 9.16 × 10–11, 7.64 × 10–11, 6.11 × 10–11, and 5.60 × 10–11 mol s–1 cm–2 at –0.5, –0.6, –0.7, –0.8, –0.9, and –1.0 V, respectively. The highest NH3 yield is obtained at –0.7 V for TiO2/Ti, which is much higher than the values of the reported electrocatalysts under comparable reaction conditions (Table S1). The FEs of TiO2/Ti for NH3 formation are shown in Figure 2c. The highest FE of 3.34% is obtained at –0.6 V. However, when the applied potential is below –0.7 V, the competing H2 evolution reaction greatly reduces the FEs of NRR. Considering the NH3 yield and FE, –0.7 V is thus chosen as the optimum potential. Also note that the FE (2.50%) at –0.7 V is higher than that of most reported aqueous-based NRR electrocatalysts under ambient conditions (Table S1). Also note that Ti plate has very poor NRR performance compared with TiO2/Ti at –0.7 V (Figure 2d). Note that no complexing agent is added in our experiments.25 For comparison, C6H5Na3O7·2H2O was also added during the determination of NH3 to conceal the potential metal ions in electrolytes. As shown in Figure S4, the UV-Vis

Figure 2. (a) UV-Vis absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis at a series of potentials. (b) NH3 yields and (c) FEs of TiO2/Ti for the NRR at a series of potentials. (d) UV-Vis absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis using TiO2/Ti and Ti plate at –0.7 V.

Figure 3. (a) UV-Vis absorption spectra of the electrolytes stained with indophenol indicator after NRR electrolysis under different conditions using TiO2/Ti catalyst. (b) UV-Vis absorption spectra of the electrolytes stained with p-C9H11NO indicator after NRR electrolysis at a series of potentials. (c) Cycling stability tests of TiO2/Ti at –0.7 V. (d) Time-dependent current density curves of TiO2/Ti at a series of potentials for 24 h.

In control experiments (Figure 3a), when N2 gas is bubbled into the cathode at an open-circuit and replacing N2 with Ar at –0.7 V, no NH3 is detected in either case. We further performed 15N isotopic labeling experiment to verify the N source of the NH3 produced, using a doublet coupling for 15 NH4+ standard sample as reference. Figure S5 shows the 1H nuclear magnetic resonance spectra. As observed, 15NH4+ signals can be detected when 15N2 is bubbled into the cathode. These observations also confirm that the NH3 in the electrolyte is indeed generated via electrocatalytic N2 reduction by TiO2/Ti. During electrochemical N2 conversion to NH3, N2H4 could generate as the by-product.32 In our present study, N2H4


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is also estimated by the method of Watt and Chrisp.33 Figure S6 shows the corresponding calibration curve. However, no N2H4 is detected at all potentials (Figure 3b), indicating the excellent selectivity of TiO2/Ti catalyst for NH3 synthesis. During stability test, the current densities are almost stable for 10 consecutive cycles at –0.7 V (Figure S7). As shown in Figure 3c, 90.2% and 88.8% of the initial NH3 yield and FE are retained after cycling tests, respectively, which may be ascribed to the decay effect of the electrode after long time electrolysis. Figure 3d shows the time-dependent current density curves of TiO2/Ti at different potentials, revealing the high stability of TiO2 with current densities being remained for 24 h. XRD analyses confirms the TiO2 nature of catalyst (Figure S8) and SEM images (Figure S9) further suggests that this TiO2/Ti still keeps its initial morphology after stability test.

Figure 4. (a) HRTEM images taken from TiO2 nanosheet before and after electrolysis. (b) Raman spectra, (c) Room-temperature

ESR spectra, and (d) valence band XPS spectra of pristine TiO2 and TiO2 after electrolysis. Figure 4a shows the HRTEM images of TiO2 before and after electrolysis. The lattice image of TiO2 becomes blurred after electrolysis, indicating the distortion of atomic lattice structure after electrochemical test. Figure 4b displays the Raman spectra of TiO2 before and after electrolysis. Both samples show several Raman vibrational modes of anatase TiO2.34 However, a slight blueshift of the peak at 144.4 cm–1 is observed in TiO2 after electrolysis. Figure 4c shows the valence band X-ray photoelectron spectroscopy (XPS) of TiO2 before and after electrolysis. TiO2 after electrolysis displays a decreased band edge at 0.18 eV compared to pristine TiO2. Room-temperature electron spin resonance (ESR) spectrum of TiO2 after electrolysis (Figure 4d) shows a pronounced VO signal (g = 2.002) along with the presence of Ti3+ centers.35 All results indicate the increased surface disorder of TiO2 after electrolysis due to the introduction of VO. Figure S10 shows the XPS spectra of TiO2 in Ti 2p region before and after electrolysis at –0.7 V for different time. Before catalysis, the high-resolution XPS spectrum of Ti 2p is comprised of two distinct peaks at binding energies (BEs) of 464.1 and 458.3 eV assigned to Ti 2p1/2 and Ti 2p3/2, respectively, which are consistent with the characteristic of Ti4+ in anatase TiO2.36 After electrolysis, the new peaks at 457.7 eV for Ti 2p3/2 and

463.5 eV for Ti 2p1/2 clearly suggests the existence of Ti3+.37 The O 1s spectra (Figure S11) can be resolved into two peaks at BEs of 529.9 and 531.2 eV. The peak at 529.9 eV is ascribed to lattice oxygen in TiO2, while a broader peak at high energy position is assigned to surface Ti-OH species.38 The area of the peak at 531.1 eV in O 1s XPS spectrum increases obviously compared to that of pristine TiO2, indicating more surface defects caused by VO.39 Moreover, we have calculated the concentrations of VO in TiO2 after electrolysis for different time at –0.7 V (see Experimental section for calculation details). As shown in Figure S12, the concentrations of VO increase initially and remain nearly unchanged after 2 h. Figure S13 shows the NRR performance of TiO2/Ti after electrolysis for different time at –0.7 V, which has small changes in NH3 yields and FEs. TiO2/Ti after 2-h electrolysis shows higher current density in 0.1 M N2-saturated Na2SO4 (Figure S14). For comparison, we also prepared TiO2/Ti with VO treated in Ar/H2 (Figure S15).35 Such catalyst also shows enhanced current density in 0.1 M N2-saturated Na2SO4 (Figure S16), imply the enhanced NRR performance of TiO2/Ti with VO. All these results suggest that the resulting VO favor more effective N2 adsorption and activation of the N≡N triple bond, leading to superior electrocatalytic NRR activity.36,40 We further calculated the concentrations of VO in TiO2 after electrolysis at different potentials for 2 h. As shown in Figure S17, the concentrations of VO increase with the potential being more negative. In summary, TiO2 nanosheets array has been proven as an efficient NRR electrocatalyst under ambient conditions. Such catalyst achieves a high NH3 yield of 9.16 × 10–11 mol·s–1·cm–2 with a high FE of 2.50% at –0.7 V in 0.1 M Na2SO4, rivaling the performances of most reported aqueous-based NRR electrocatalysts. It also exhibits excellent selectivity for NH3 synthesis with high electrochemical stability. The superior NRR activity is rationally attributed to the enhanced adsorption and activation of N2 by VO in-situ generated during electrochemical test. This study not only offers us an attractive earth-abundant electrocatalyst for N2 reduction under ambient conditions, but would open up an exciting new avenue to design and develop Ti-based nanocatalysts for NH3 electrosynthesis.

ASSOCIATED CONTENT Supporting Information Experimental Section; EDX spectrum; SEM image; UV-Vis absorption spectra; calibration curve; time-dependent current density curves; XRD pattern. 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.); [email protected] (X.G.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21575137). We appreciate Hui Wang



from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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Enabling Effective Electrocatalytic N2 Conversion to NH3 by the TiO2

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