Electrocatalytic Nitrogen Reduction to Ammonia by Fe2O3 Nanorod

May 30, 2019 - Electrocatalytic nitrogen fixation provides a green and facile strategy for efficient synthesis of ammonia under ambient conditions, bu...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

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Electrocatalytic Nitrogen Reduction to Ammonia by Fe2O3 Nanorod Array on Carbon Cloth Ziqiang Wang, Kang Zheng, Songliang Liu, Zechuan Dai, You Xu, Xiaonian Li, Hongjing Wang,* and Liang Wang* State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China

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S Supporting Information *

ABSTRACT: Electrocatalytic nitrogen fixation provides a green and facile strategy for efficient synthesis of ammonia under ambient conditions, but it lacks efficient and inexpensive electrocatalysts for the nitrogen reduction reaction. Here, we report the synthesis of porous Fe2O3 nanorods grown on carbon cloth (p-Fe2O3/CC) via a surfactant-free hydrothermal reaction coupled with high-temperature calcination. The obtained p-Fe2O3/CC serves as a superior electrocatalyst for producing ammonia by electrolysis of nitrogen and water in neutral electrolytes (0.1 M Na2SO4). Benefiting from its binder-free characteristic and porous structure, p-Fe2O3/CC achieves superior catalytic performance and excellent long-term stability. This study is significant to design low-cost and binder-free porous Fe-based arrays as effective and stable electrocatalysts for electrochemical ammonia synthesis. KEYWORDS: Porous Fe2O3 nanorod, Self-supported structure, Electrochemical ammonia production, Electrocatalysis, Nitrogen reduction reaction

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INTRODUCTION Ammonia is a significant chemical for production of fertilizers1−5 and is also considered as an attractive hydrogen energy carrier with no CO2 emission and easy storage and transportation.6 Because of extremely strong NN bond energy, it is extremely difficult to directly produce ammonia from nitrogen.7 Industrially, ammonia production has been performed by the Haber−Bosch process, which requires high temperatures and pressures to convert N2 and H2 into NH3 over Fe-based catalysts.8 However, this process consumes 1− 2% of worldwide power source and about 5% of global natural gas annually, leading to great contributors to global CO2 emission.9−11 Hence, the development of sustainable and environmentally benign approaches is highly desired for largescale synthesis of ammonia under ambient conditions. Currently, ever-growing interest has been received in electrochemical synthesis of ammonia as it consumes less energy source and employs water as the hydrogen source.12−19 However, this approach for scalable ammonia production is greatly impeded by the lack of high-performance, cost-effective nanomaterials for catalytic nitrogen reduction reaction (NRR). So far, precious metal based electrocatalysts have been demonstrated to be feasible for electrochemical ammonia synthesis,20−26 but their high cost and low Faradaic efficiency (FE) are far from industrial requirements. Density functional theory calculations have proven the Fe surface as a possible electrocatalyst in ammonia production.27 Inspired by this idea, γ-Fe2O3 nanoparticles and Fe2O3−CNT hybrid were reported © 2019 American Chemical Society

as active NRR electrocatalysts for ammonia synthesis, but they possess a low FE.28,29 Compared with bulk metal oxides, porous metal oxides with well-developed porous structure can offer abundant active sites and facilitate mass/charge transfer, leading to enhanced catalytic performance.30−33 Additionally, constructing self-supported active materials grown on conductive substrates can avoid the usage of conducting binders and ensure the high utilization of active materials, which is a promising strategy to promote electrocatalytic performance.34−38 Moreover, the highly conductive substrate can offer three-dimensional (3D) porous network structure to effectively facilitate charge transfer and mass/charge transport.39,40 Therefore, it is promising to design binder-free porous Fe-based array architecture for electrosynthesis of ammonia. Herein, we demonstrate the preparation of porous Fe2O3 nanorod arrays on carbon cloth (p-Fe2O3/CC) via a simple and efficient hydrothermal route coupled with postannealing treatment. A combination of the binder-free feather and porous structure, p-Fe2O3/CC directly serves as an active NRR electrocatalyst, which exhibits excellent activity, selectivity, and stability for ammonia production. Received: April 9, 2019 Revised: April 30, 2019 Published: May 30, 2019 11754

DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

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ACS Sustainable Chemistry & Engineering

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UV−vis absorption spectra were acquired, and the absorbance was detected at λ = 455 nm. The hydrazine monohydrate solutions with known concentrations were employed to plot a concentrationabsorbance calibration curve.

EXPERIMENTAL SECTION

Materials and Chemicals. Sodium sulfate anhydrous (Na2SO4, ≥ 99.0%), iron nitrate nonahydrate (Fe(NO3)3·9H2O, ACS grade), sodium citrate dehydrate (≥99.0%), salicylic acid (99.5%), pdimethylaminobenzaldehyde (99.0%), sodium nitroferricyanide dehydrate (99.0%), ammonium chloride (NH4Cl, 99.99%), sodium hypochlorite solution (NaClO, 6−14%), and hydrazine monohydrate (N2H4·H2O, 90%) were received from Aladdin Ltd. Carbon cloth and Nifion 117 membranes were bought from Shanghai Hesen Electric Co., Ltd. Synthesis of p-Fe2O3/CC. For a typical synthesis, 3.5 mmol of Fe(NO3)3·9H2O and 3.5 mmol of Na2SO4 were dissolved in 64 mL of H2O under stirring for 30 min. Then the clear solution and a piece of carbon cloth (2 × 4 cm) were sealed in a Teflon-lined autoclave, which was kept at 120 °C for 6 h. After that, the resultant precursor was collected by centrifugation, fully rinsed with water, and dried under vacuum at 50 °C. Finally, p-Fe2O3/CC was obtained by annealing the precursor under N2 flow at 400 °C for 4 h with a heating rate of 0.5 °C min−1. Characterization. Scanning electron microscopy (SEM) image was obtained from a JEOL-6700F instrument at 5 kV. Transmission electron microscope (TEM) image and elemental mapping images were operated on a JEOL-2100F microscope with an energydispersive X-ray (EDX) analytical system. X-ray diffraction (XRD) measurement was conducted on a Rigaku D/MAX-2200 diffractometer using Cu Kα radiation. X-ray photoelectron spectra (XPS) were performed on ESCALAB 250Xi spectrometer using an Al Kα Xray source. Electrocatalytic Experiments. Electrochemical ammonia synthesis measurements were carried out in a two-compartment cell separated by a Nafion 117 membrane at room temperature. All electrochemical tests were performed using a CHI 660E electrochemical workstation with a three-electrode system. p-Fe2O3/CC, graphite rod, and Ag/AgCl electrode were used as working electrode (WE), counter electrode (CE), and reference electrode (RE), respectively. Before NRR measurements, electrolyte was purged with N2 for 30 min. Chronoamperometric measurements were operated in N2-saturated 0.1 M Na2SO4 solution with continuous N2 bubble. The Na2SO4 solution is widely used as the electrolyte for the NRR, which can suppress the hydrogen evolution reaction. The potentials were converted to the reversible hydrogen electrode. All the current densities were normalized to the geometric area of carbon cloth. Production Quantification. The amounts of ammonia were estimated based on the indophenol blue method.41 In short, 2 mL of electrolyte after electrolysis for 2 h was mixed with 2 mL of a mixture solution consisting of NaOH (1 M), sodium citrate (5 wt %), and salicylic acid (5 wt %), followed by adding 1 mL of a NaClO solution (0.05 M) and 0.2 mL of a sodium nitroferricyanide solution (1 wt %). The mixture solution was kept for 2 h. We employed an ultraviolet− visible (UV−vis) spectrophotometer to measure the absorption spectrum of a mixture solution. The absorbance of the solution was detected at λ = 680 nm. In order to calculate the amount of produced NH3, the calibration curve was fitted using standard ammonia chloride solutions. The ammonia yield (rNH3) and FE can be calculated as follows:

rNH3 = (c NH3 × V )/(t × m)

(1)

FE = (3F × c NH3 × V )/(17 × Q )

(2)

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RESULTS AND DISCUSSION As illustrated in Scheme 1, p-Fe2O3/CC has been fabricated by a surfactant-free hydrothermal method with the assistance of Scheme 1. Schematic Illustration for Preparing p-Fe2O3/CC

high-temperature calcination in inert gas. During the hydrothermal process, one-dimensional nanorod precursors (FeOOH) are uniformly grown on the carbon cloth using Na2SO4 as a structure-directing agent,39,43 which are converted to porous Fe2O3 after annealing. The as-obtained product possesses a free-standing nanorod structure with good flexibility, which can be directly employed for electrochemical synthesis of ammonia. The mass loading of porous Fe2O3 nanorods on carbon cloth was measured to approximately 0.5 mg cm−2. The generalization of this method and commercialization of conductive carbon cloth would allow large-scale production of p-Fe2O3/CC. The morphology of products was investigated by SEM images. After hydrothermal reaction, it can be clearly observed that highly dense FeOOH nanorods with the diameter of ca. 45 nm are vertically grown on the surface of the carbon cloth (Figure S1). This is due to the fact that Na2SO4 plays a structure-directing role in one-dimensional growth.43 After annealing treatment, p-Fe2O3/CC still maintains the initial morphology with smaller diameter due to the structural shrinkage during the pyrolysis process (Figure 1a,b). For comparison, the samples prepared from the different hydrothermal times, which are denoted as Fe2O3−4/CC and Fe2O3−8/CC, exhibit similar morphology to p-Fe2O3/CC (Figure S2). The diameter of the nanorods has a slight increase raising the hydrothermal time. Moreover, some pores in the Fe2O3 nanorods are obviously illustrated by a TEM image (highlighted areas in Figure 1c), which are generated from dehydration and lattice contraction during the annealing process.39 Further information about the microstructure and phase of Fe2O3 was studied. The HRTEM image displays the clear lattice fringes in the entire nanorod, in which lattice plane spacings of 0.368 nm are observed in the fast Fourier transformation (FFT) pattern, corresponding with the (012) lattice facet of hematite α-Fe2O3 (Figure 1d,e). The result indicates the well-developed crystalline of the product. Additionally, elemental mapping was performed to reveal the product composition. In accordance with the high-angle annular dark-field scanning TEM (HAADF-STEM) image and corresponding EDX elemental mapping images of one

where cNH3 is the NH3 concentration, V is the electrolyte volume, t is the electrolysis time, m is the catalyst weight, F (96485 C mol−1) is the Faraday constant, and Q is the total charge during electrolysis. The hydrazine concentration was estimated by the method of Watt and Chrisp.42 In brief, the color reagent was first obtained by dissolving 5.99 g of p-dimethylaminobenzaldehyde in a mixture solution containing 30 mL of concentrated hydrochloric acid and 300 mL of ethanol. Afterward, 5 mL of electrolyte was taken out and mixed with color reagent (5 mL). After 10 min, the corresponding 11755

DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

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ACS Sustainable Chemistry & Engineering

33−0664). To explore the chemical components and states of the products, XPS analysis was performed. In the XPS survey spectrum, C, Fe, and O elements can be observed in the product (Figure 2b). The C signal is derived from the carbon cloth, and the Fe/O ratio is estimated to about 2/3, consistent with the EDX analysis. In the Fe 2p spectrum (Figure 2c), the peaks at the binding energies (BEs) of 711.1 and 724.3 eV reflect Fe 2p3/2 and Fe 2p1/2 orbits, respectively, and their satellite peaks are located at 718.9 and 732.5 eV. These peaks can be assigned to Fe3+ species.44,45 The O 1s XPS spectrum can be resolved into two peaks at the BEs of 529.8 and 531.7 eV (Figure 2d). The former peak is attributed to the Fe−O group in Fe2O3, and the latter peak is ascribed to the O−H group derived from the chemisorbed water on the surface of Fe2O3.44,46 Above results demonstrate the successful synthesis of p-Fe2O3/CC. Due to its free-standing and porous structures, p-Fe2O3/CC is considered as a promising binder-free electrocatalyst for electrochemical synthesis of ammonia. During the NRR process, p-Fe2O3/CC would adsorb and activate the bubbled N2 to nitrogen species, which are combined with electrons and water to produce NH3, as described in Figure 3a. The NRR

Figure 1. (a, b) SEM images of p-Fe2O3/CC. (c) TEM and (d) HRTEM images of p-Fe2O3. (e) Corresponding FFT pattern in square area. (f) Elemental mapping images of one p-Fe2O3 nanorod.

nanorod (Figure 1f), it can be obviously found that Fe and O elements in the Fe/O ratio of approximately 2/3 are uniformly distributed in the entire nanorod. p-Fe2O3/CC with a 3D architecture provides abundant highly active sites and facilitates the mass/charge transfer for electrosynthesis of ammonia. XRD was employed to investigate the crystalline structure of the as-prepared sample. The broad peak with the 2θ value of 26.3° is indexed to the C(002) lattice facet resulting from carbon cloth in the sample (Figure 2a). The other XRD peaks are ascribed to the lattices of hematite α-Fe2O3 (JCPDS card:

Figure 3. (a) Schematic of typical NRR process. (b) Chronoamperometry curves for p-Fe2O3/CC in N2-saturated 0.1 M Na2SO4 at different potentials. (c) UV−vis absorption spectra of electrolytes after charging for 2 h. (d) Comparison of NRR performance for pFe2O3/CC at corresponding potentials.

performance of p-Fe2O3/CC is estimated by electrocatalysis in N2-saturated 0.1 M Na2SO4 under various potentials for 2 h (Figure 3b). Corresponding UV−vis absorption spectra are shown in Figure 3c, which have a higher absorbance than that of the standard electrolyte (0.1 M Na2SO4). This phenomenon demonstrates that p-Fe2O3/CC can effectively catalyze nitrogen reduction to produce ammonia between −0.3 and −0.5 V. In accordance with the calibration curve of NH3 (Figure S3), we calculate the NH3 yields and FEs of p-Fe2O3/CC. As shown in Figure 3d, the NH3 yields and FEs gradually increase from −0.3 to −0.4 V. The highest NH3 yield and FE of 6.78 μg h−1 cm−2 (or ∼13.56 μg h−1 mg−1cat.) and 7.69% are obtained at −0.4 V, respectively. Unfortunately, the NH3 yields and FEs have a significant decrease below −0.4 V, which can be explained by the overwhelming hydrogen adsorption on the catalyst surface. The optimum NRR performance of p-Fe2O3/ CC is superior to that of most reported electrocatalysts (Table

Figure 2. (a) XRD pattern, (b) XPS survey spectrum, and (c) XPS Fe 2p and (d) O 1s spectra of p-Fe2O3/CC. 11756

DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

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ACS Sustainable Chemistry & Engineering S1), such as Fe3O4/Ti (3.42 μg h−1 cm−2, 2.6%),47 Mn3O4 nanocube (11.6 μg h−1 mg−1cat., 3.0%),48 and MoS2 (4.94 μg h−1 cm−2, 1.17%).10 Moreover, the selectivity of NH 3 production for p-Fe2O3/CC can be confirmed by the detection of the byproduction (N2H4). As expected, the N2H4 cannot be detected in all electrolytes (Figure S4 and S5), suggesting that p-Fe2O3/CC may proceed via the distal associative pathway with complete conversion of N2 to NH3.24 In order to demonstrate superior performance of p-Fe2O3/ CC for the NRR, we compare the NRR performance of precursor and p-Fe2O3/CC after electrolysis at −0.4 V. As shown in Figure S6, p-Fe2O3/CC achieves much higher NH3 yield and FE compared with the precursor, due to its porous structure. Furthermore, we investigate the effect of hydrothermal time on catalytic performance of samples. As shown in Figure 4, p-Fe2O3/CC prepared with hydrothermal time for 6

electrolyte after electrolysis. Considering the presence of possible impurity gases in the supplying gas and atmosphere, the electrolysis of the 0.1 M Na2SO4 solution with continual N2 bubble is conducted at an open circuit potential. The UV− vis absorption spectrum reveals that very little NH3 is detected, indicative of negligible disturbance on NRR performance from supplying gas and atmosphere. These results demonstrate that porous Fe2O3 nanorods are active sites for catalytic nitrogen reduction. On the other hand, durability of catalysts is another crucial parameter to assess their NRR performance. The longterm stability of p-Fe2O3/CC is estimated by a chronoamperometry measurement at −0.4 V (Figure 5b). The current density of p-Fe2O3/CC maintains stable without obvious fluctuation after long-term electrolysis for 20 h. It is noted that the UV−vis absorbance is only a little smaller than the initial one, indicating slight loss of corresponding NH3 yield and Faradaic efficiency (Figure S8). The above results confirm the excellent stability of p-Fe2O3/CC for the NRR due to its selfsupported array structure.

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CONCLUSIONS In summary, p-Fe2O3/CC has been successfully synthesized by a surfactant-free hydrothermal method with the assistance of high-temperature calcination using Na2SO4 as a structuredirecting agent, which acts as a high-performance catalyst for electrochemical ammonia synthesis under ambient condition. This well-developed porosity and binder-free structure of pFe2O3/CC can provide rich active sites and accessible channels for N2 activation, which possesses a high NH3 yield (6.78 μg h−1 cm−2) and FE (7.69%) at −0.4 V in 0.1 M Na2SO4 and excellent stability for electrochemical NH3 synthesis. This work highlights the design of earth-abundant and self-supported porous Fe-based arrays as active and robust electrocatalysts for electrochemical ammonia synthesis.

Figure 4. (a) UV−vis absorption spectra of electrolytes after electrolysis for the different samples and (b) corresponding NH3 yields and FEs.

h possess the highest NH3 yield and FE at −0.4 V. The high intrinsic NRR activity of p-Fe2O3/CC is highly related to the electrochemically active surface area (ECSA), which is estimated from the double layer capacitances (Cdl).34,37 The Cdl was determined by the cyclic voltammetry (CV) method (Figure S7). The Cdl value of p-Fe2O3/CC is estimated to be 46 mF cm−2, larger than that of Fe2O3−4/CC (28 mF cm−2) and Fe2O3−8/CC (21 mF cm−2). The higher Cdl represents the larger ECSA, which indicates the increased number of active sites, thus leading to the improvement of NRR catalytic performance. To verify that the ammonia is produced from nitrogen reduction electrocatalyzed by p-Fe2O3/CC, some comparative experiments were carried out (Figure 5a). For comparison, the bare carbon paper exhibits negligible absorbance increase relative to the standard electrolyte, indicating the inertness of carbon cloth for electrochemical ammonia synthesis. Moreover, there is no detection of NH3 in the Ar-saturated

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01991. SEM images, UV−vis absorption spectra, CV curves, and comparison of NRR performance (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongjing Wang: 0000-0003-0641-3909 Liang Wang: 0000-0001-7375-8478 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21601154, 21776255, and 21701141).

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Figure 5. (a) UV−vis absorption spectra of the electrolytes measured under different conditions. (b) Long-term stability test for p-Fe2O3/ CC at −0.4 V for 20 h and comparison of UV−vis absorption spectra before and after the test.

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DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

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DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759

Research Article

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DOI: 10.1021/acssuschemeng.9b01991 ACS Sustainable Chem. Eng. 2019, 7, 11754−11759