Facet Regulation of Fe2O3 via Nanoarray Architecture to Enable High Faradic Efficiency for Electrocatalytic Nitrogen Fixation

XIE Anqi, XIAO Liang＊, QIAO Qiumin, LIU Jinping,2

(1. School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, China; 2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China)

Abstract: We propose a facile facet regulation enabled by nanoarray architecture to achieve a high faradic efficiency of Fe2O3 catalyst for NRR. The α-Fe2O3 nanorod arrays (NAs) were directly grown on carbon cloth (CC) with specific (104) facet exposure. The highly exposed (104) facets provide abundant unsaturated Fe atoms with dangling bonds as nitrogen reduction reaction catalytically active sites. In addition, the NAs architecture enables the enhanced electrochemical surface area (ECSA) to fully manifest the active sites and maintain the mass diffusion. Thus, the selectively exposed (104) facets coupled with the high ECSA of NAs architecture achieve a high FE of 14.89% and a high yield rate of 17.28 μg h-1 cm-2. This work presents an effective strategy to develop highly efficient catalytic electrodes for electrochemical NRR via facet regulation and nanoarray architecture.

Key words: facet regulation; Fe2O3; nanoarray architecture; nitrogen reduction reaction; faradic efficiency

1 Introduction

Ammonia (NH3), synthesized by nitrogen (N2) fixation from the atmosphere, plays an imperative role in the modern society by serving as a precursor to fertilizers and many other nitrogen-containing chemicals[1-3].However, due to the strong N≡N triple bond with a bond energy of 941 kJ mol-1, the chemical reduction of N2to NH3is quite a challenge. Over more than one century, the Haber-Bosch process operated at a high pressure (150-300 bar) and high temperature (350-550℃) is still the dominant method for NH3production,which consumes more than 1% of the global energy consumption[4]. Besides the high energy input, the required hydrogen from steam reformation leads to 3%-5% of the natural gas consumption and large amounts of greenhouse gas emissions[5]. To this end, there is a great need to develop an environmentally benign and energy efficient process for sustainable artificial N2fixation. Alternatively, electrochemically catalytic nitrogen reduction reaction (NRR) under mild conditions using heterogeneous catalysts emerges as a promising approach to produce NH3directly from N2and water,which benefits from clean and renewable energy sources and thus reduce energy consumption and greenhouse gas emissions[6].

However, the electrocatalytic NRR kinetic is still suppressed by the sluggish splitting of strong N≡N triple bond, resulting in a low NH3yield and large overpotential. Furthermore, the competing hydrogen evolution reaction (HER) is always dominant in the electrochemical NRR system because of its faster kinetic than NRR and thermodynamic potential close to NRR. Thus, the Faradaic efficiencies (FEs) of NRR electrocatalysts are extremely low at ambient conditions[7]. Therefore, there are blossoming interests in the rational design of NRR electrocatalysts to achieve high NH3yield and high FE. So far, lots of noble metal and transition metal compounds have been investigated as promising catalysts for electrochemical NRR due to the various electronic structures of these materials. As one of the most abundant transition metals, Fe is not only involved in nitrogenases for biological N2fixation but also extensively utilized in the Haber-Bosch process for industrial-scale NH3synthesis[8]. Hence, iron oxides are also proposed as cost-effective and environment-friendly catalysts for NRR. For instance, Fe2O3-CNT hybrid[9]andγ-Fe2O3nanoparticle[10]are proved to be effective for electrochemical synthesis of ammonia, respectively.However, the reported FEs of iron oxides for NRR are still less than 2% due to the competitive HER. Considerable efforts should be made to further improve the NRR performance of iron oxides towards the FE and NH3production rate.

In previous reports, electronic structure tuning via defect engineering, heteroatom doping, andetc,could improve the intrinsic activities of NRR catalysts[10-20].Another approach to enhance the NRR performance is to improve the apparent activities of catalysts via exposing more active sites[8,11,14,21-23]. For instance, crystal facet regulation has been reported to enhance the apparent activity due to the more active sites in specific facet orientations. Baiet alrevealed that (001) facet of Bi5O7I had a higher activity than (100) facet for photocatalytic N2fixation[24]. Yanget alreported an improved catalytic activity towards NRR with the increase of(110) orientation of Mo nanofilm[25]. A maximum FE of 0.72% was obtained at -0.49 Vvsreversible hydrogen electrode (RHE). For iron oxide catalysts, it might be also possible to create more active sites and improve the exposure of active sites by the facet regulation[26].Besides the facet regulation, electrode architecture engineering is also critical for electrochemical NRR to fully expose active sites and thus enhance the apparent activity. In traditional composite electrodes, the catalyst utilizations are generally low because some active sites might be blocked by the binders. Recently, in order to resolve this issue, novel nanoarray architectures are proposed for catalytic electrodes to expose available catalytic sites for the unrestricted access of reactant molecules, and thus enable the full utilization of the intrinsically catalytic activity[27-29].

Enlightened by the previous reports, the combination of facet regulation and nanoarray architecture design could be a synergistic strategy to improve the NRR catalytic performances of iron oxides. Herein, we propose a facile facet regulation strategy only enabled by the construction of nanoarray architecture to achieve high FE and NH3yield for electrocatalytic NRR. In detail, α-Fe2O3nanorod arrays (NAs) on carbon cloth (CC)with highly exposed (104) facets were prepared as electrochemical NRR catalysts. On one hand, the exposed(104) facet provides abundant unsaturated Fe atoms with dangling bonds as NRR catalytically active sites.On the other hand, the nanoarray architecture achieves enhanced electrochemical surface area (ECSA) to fully manifest the active sites and maintain the sufficient mass diffusion. Thus, the selectively exposed (104) facets coupled with the high ECSA of nanoarray architecture achieve the superior electrocatalytic performance of Fe2O3NAs. This work presents an effective strategy to develop highly efficient catalytic electrodes for electrochemical NRR via facet regulation and nanoarray architecture.

2 Experimental

2.1 Materials and electrodes preparation

α-Fe2O3nanorod arrays (Fe2O3NAs) on carbon cloth (CC) were prepared by a hydrothermal route followed by calcination in air. Firstly, a piece of CC(2 cm×2 cm, Cetech Crop.) were treated in 70 mL aqueous solution containing 1.75 mmol FeCl3and 1.75 mmol Na2SO4at 160 ℃ for 6 hours by a 100 mL Teflon-lined stainless-steel autoclave. The obtained CC was washed with distilled water several times and then dried at 80 ℃. Finally, the dried precursor on CC was calcinated at 550 ℃ for 2 hours in Ar to prepare Fe2O3NAs. The catalyst loading of Fe2O3NAs is around 0.2 mg·cm-2. As the reference material, Fe2O3nanoparticles(Fe2O3NPs) were prepared by the same route without CC as the substrate. In order to prepare a reference working electrode, 20 mg Fe2O3NPs and 20 μL Nafion solution (5%) were dispersed in 980 μL isopropanol by 2-hours sonication to form a homogeneous ink. Then 15 μL catalyst ink was loaded onto carbon paper to enable a mass loading of about 0.2 mg cm-2.

2.2 Materials characterization

The X-ray diffraction (XRD) patterns of prepared samples were recorded by a Rigaku SmartLab X-ray diffractometer operating at 45 kV and 200 mA with Cu Kα radiation (λ=1.540 6 Å). The morphologies were characterized using a Hitachi S-4800 scanning electron microscopy (SEM). Transmission electron microscopy(TEM) observations, selected area electron diffraction(SAED) analyses, and energy dispersive X-ray (EDX)analyses were performed on a FEI Tecnai F30 transmission electron microscopy (TEM) with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy(XPS) was measured by a Kratos Axis Ultra Imaging photoelectron spectrometer equipped with a monochromatic Al-Kα X-ray source (hν=1 468.7 eV).

2.3 Electrocatalytic performance studies

The electrocatalytic performances were tested via a two-compartment H-typed electrolysis cell with a piece of Nafion-117 separator between the cathodic and anodic compartments. 0.1 mol·L-1KOH solutions were added into the cathodic and anodic compartments,respectively. Fe2O3NAs or Fe2O3NPs were used as the working electrode in the cathodic compartment with a saturated AgCl electrode as the reference electrode. A Pt foil was used as the counter electrode in the anodic compartment. Dry ultra-pure nitrogen gas was bubbled into the catholyte with an inlet pressure of 1 bar,and the outlet gas tube was connected to an ammonia trap filled with 60 mL of 0.05 mol·L-1H2SO4aqueous solution. All the electrochemical tests were performed on a CHI-660E electrochemical workstation, and all the electrode potentials refer to reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV)and cyclic voltammetry (CV) tests were performed to investigate the current responses and electrochemical surface areas (ECSA). In the NH3yield and FE studies,the working electrodes were kept at fixed potentials under constant N2gas flow (50 mL·min-1) for 2 hours, and then the catholyte and the adsorbent of outlet gas were collected for product quantification, respectively.

The concentrations of produced ammonia and hydrazine were determined by spectrophotometric indophenol blue method and Watt-Chrisp method, respectively[18]. The concentrations of produced ammonia in the catholytes were tested by spectrophotometric indophenol blue method. Typically, 5 mL of catholyte or absorbent was firstly mixed with 1 000 μL 0.5 mol L-1NaOH aqueous solution containing 5 wt% salicylic acid and 5 wt% sodium citrate. Then, 500 μL NaClO (ρCl=4-4.9) aqueous solution and 100 μL 1 wt% C5FeN6Na2O(sodium nitroferricyanide) aqueous solution were added into the above mixed solution. After the aging at room temperature for an hour, the UV/Vis absorbance curves were determined by an Agilent Cary5000 UV-Vis-NIR spectrophotometer using deionized water as the reference. Standard solutions were prepared by dissolving vacuum dried NH4Cl into either 1.0 mol·L-1KOH or 0.05 mol·L-1H2SO4. The calibration UV-vis adsorption curves of NH4+were plotted by the absorbance of prepared standard solutions. In experiment studies, the absorbance of catholyte was transfered to a concentration of NH3by checking the calibration curves.

The concentrations of hydrazine in the catholytes were determined by a spectrophotometric method reported by Watt and Chrisp. A mixture of 3.99 g para-(dimethylamino) benzaldehyde, 30 mL concentrated HCl, and 300 mL ethanol was used as a color reagent.In a typical test, 5 mL catholyte or absorbent was mixed with 5 mL color reagent and then stirred for 30 min. The UV-vis absorption of mixed solution at 460 nm was collected for hydrazine determination. Standard solutions were prepared by diluting 85% hydrazine hydrate with 1.0 M KOH into about 0.85 μg·mL-1,followed by serial dilutions to get standard solutions.

The Faradic efficiency was calculated using the following equation:FE= 3n(NH4+)F/Q, wheren(NH4+) is the detected total amount of ammonia (mol),Fis the Faraday constant, andQis the total charge consumed (C). The ammonia yield rate was calculated using the following equationr(NH3) = 17.043×n(NH4+)/(t×s), wherer(NH3) is the average ammonia yield rate(μg·h-1·cm-2), 17.034 is the molecular weight of ammonia,tis the reaction time in hour, andsis the surface area in cm2. In the present work, the ammonia yield rate was normalized to the geometric area for comparison considering that the areal mass loading of catalysts in literature reports were varied.

3 Results and discussion

3.1 Materials characterization

The crystalline structures, morphologies, and chemical components of prepared Fe2O3NAs were firstly investigated by XRD, SEM, and XPS, respectively. As shown in Fig.1(a), besides the wide and strong diffraction peak at 25.3° that is ascribed to the(002) plane diffraction of CC, all the other well-defined peaks in the XRD pattern of prepared NAs are well indexed to α-Fe2O3(JCPDS No. 33-0664)[30]. The SEM images in Fig.1(b) and 1(c) clearly show the nanorod arrays morphology of Fe2O3NAs. The nanorods grown on the CC fibers are approximately 200 nm long and tens nm wide. The survey XPS spectrum of Fe2O3NAs in Fig.1(d) shows the signals of iron, oxygen, and carbon without other impurities. Further, the fine Fe 2pand O 1sspectra are presented in Fig.1(e) and Fig.1(f),respectively. In Fig.1(e), the peaks at 724.7 and 711.1 eV are attributed to Fe 2p3/2and Fe 2p1/2, respectively.The binding energy gap between Fe 2p3/2and Fe 2p1/2is 13.6 eV confirming the presence of Fe3+species[31].As shown in Fig.1(f), O 1sXPS spectrum of Fe2O3NAs can be resolved to two peaks located atE1= 529.8 eV andE2= 531.5 eV respectively.E1is attributed to the Fe-O bonds[26], andE2is the chemisorbed water on the surface[30]. All the characterizations indicate that the Fe2O3NAs on CC were successfully prepared. As the reference material, Fe2O3NPs with a pure α-Fe2O3phase were also successfully prepared by the same method without the CC substrate.

3.2 Electrocatalytic performances

Fig.1 (a) XRD pattern; (b) and (c) SEM images; (d) XPS survey spectrum; (e) Fe 2p XPS spectrum; and (f) O 1s XPS spectrum of Fe2O3 NAs

Fig.2 Linear sweep voltammetry (LSV) curves of (a) Fe2O3 NAs and (b) Fe2O3 NPs under N2 and Ar; current responses of (c) Fe2O3 NAs and (d) Fe2O3 NPs at -0.2 V under Ar and N2; NH3 yield rate and Faradic efficiency of (e) Fe2O3 NAs and (f) Fe2O3 NPs at different potentials

The NRR catalytic selectivity of Fe2O3NAs and Fe2O3NPs were preliminarily studied by LSV tests.Fig.2(a) and 2(b) compare the current responses at a scan rate of 1.0 mV·s-1from 0.0 to -0.80 VvsRHE under nitrogen (N2) and argon (Ar) gas bubbling. In Fig.2(a), the current density in N2-saturated electrolyte(jN2) for Fe2O3NAs was higher than that in Ar-saturated electrolyte (jAr), especially in the potential range from-0.2 to -0.8 V. In the aqueous electrochemistry system,jAris attributed to the hydrogen evolution reaction(HER). Under N2bubbling, HER and NRR are two competitive processes. Therefore, the higher difference betweenjN2andjArmeans higher reaction rate of NRR.At -0.2 V,jN2is 2.37 times as much asjArfor Fe2O3NAs. By contrast, there are only slight differences betweenjN2andjArfor Fe2O3NPs in Fig.2(b). In conclusion, Fe2O3NAs give a higher catalytic selectivity for NRR than Fe2O3NPs, although they have identical chemical components.

The NRR electrocatalytic activities of Fe2O3NAs and Fe2O3NPs were studied via potentiostatic polarization. Fig.2(c) and 2(d) show the current responses of Fe2O3NAs and Fe2O3NPs at -0.2 V for 2 hours,respectively. The current responses difference under N2and Ar for Fe2O3NAs are obviously higher than that for Fe2O3NPs, confirming the higher electrocatalytic selectivity of Fe2O3NAs for NRR. After the potentiostatic polarization, the generated NH3and byproduct N2H4were determined by indophenol blue method.The NH3yield rates and the corresponding Faradic efficiencies (FEs) under a series of applied potentials were determined and plotted in Fig.2(e) and 2(f). It can be seen that NH3yields gradually increase from -0.1 V to -0.4 V. At -0.4 V, Fe2O3NAs achieve the highest NH3yield rate of 17.28 μg·h-1·cm-2. Beyond this potential, NH3yield start to decrease, which is ascribed to the rapidly increased hydrogen evolution. However,FEs gradually decrease from -0.1 V to -0.5 V. Fe2O3NAs show the highest faradic efficiency of 14.89% at-0.1 V. The FE of Fe2O3NAs is higher than most of reported NRR electrocatalysts under ambient conditions.A more-detailed comparison is presented in Fig.3. As the reference materials, Fe2O3NPs shows a lower NH3yield rate of 10.42 μg h-1cm-2at -0.5 V and a lower FE of 7.81% at -0.1 V than that of Fe2O3NAs.

Fig.3 Summarization of NRR electrocatalytic performance of previously reported catalysts and Fe2O3 NAs in our work

3.3 Electrocatalytic mechanism

The advanced performance of Fe2O3NAs might be attributed to the three dimensional nanorod arrays architecture that fully exposes catalytically active sites for NRR. Herein, the electrochemically active surface areas (ECSAs) were determined by cyclic voltammetry tests in double layer regions. Fe2O3NAs shows a higher double layer capacitance (Cdl) than Fe2O3NPs,suggesting the higher ECSA of the Fe2O3NAs. Therefore, Fe2O3NAs shows higher NH3yield rate in the electrochemical reduction of N2. The high ECSA contributes to the enhanced yield rate of Fe2O3NAs, but might not result in the improved selectivity of Fe2O3NAs in the competition of NRR and HER. Thus, more detailed structural factors should be investigated to illuminate the catalytic activity of Fe2O3NAs. To this end, TEM investigation was carried out to study the detailed structure. Fig.4(a) and 4(d) compare the brightfiled TEM images of Fe2O3NAs and Fe2O3NPs. Fe2O3NAs shows a single crystallized nanorod in Fig.4(a),however Fe2O3NPs shows a morphology composed of packing nanograins in Fig.4(d). The high-resolution TEM (HR-TEM) image of Fe2O3NAs in Fig.4(b)shows well-resolved lattice fringes with an interplanar distance of 0.268 nm which could be indexed to the (104) plane of Fe2O3. The selected area electron diffraction (SAED) pattern in Fig.4(c) indicates the single crystalline rhombohedral structure of α-Fe2O3. It is demonstrated that the nanorods of Fe2O3NAs were induced via CC to grow along the (104) lattice plane.By contrast, the high-resolution TEM (HRTEM) image of Fe2O3NPs in Fig.4(e) shows multiple lattice planes suggesting the random nanograins packing. The corresponding selected area electron diffraction (SAED)pattern of Fe2O3NPs in Fig.4(f) exhibited concentrated rings for the lattice planes of (012), (110), (214), and others which are consistent with XRD results.

Fig.4 TEM images, HRTEM images, and SAED patterns: (a), (b), (c) Fe2O3 NAs and (d), (e), (f) Fe2O3 NPs; (g) the catalytic mechanism of(104) facet of Fe2O3 NAs

Fig.5 (a) the current responses at -0.2 V under alternative Ar and N2 flows; (b) NH3 yield rate and FE of Fe2O3 NAs at -0.2 V under Ar and N2; (c) the current responses at -0.2 V under N2 during repeated cycling; (d) NH3 yield rate and FE of Fe2O3 NAs during repeated cycling

The superior activity and selectivity of Fe2O3NAs could be ascribed to the selectively exposed (104)facets coupled with the high ECSA of nanoarray architecture. As illustrated in Fig.4(g), the exposed (104)facet is terminated with Fe atoms, and the Fe atoms lying on the (104) surface are in an unsaturated coordination form. The abundant dangling bonds on (104)plane make the surface highly active for the adsorption of nitrogen species, which would likely contribute to the enhanced nitrogen reduction reactions on the surface[26,32]. In the present work, as demonstrated in the TEM studies, carbon nanofiber surface of CC provides nucleation site for the hydrothermal preparation of Fe2O3NAs and induces the growth of Fe2O3nanorods along (104) facets. Thus, the unsaturated Fe atoms with dangling bonds on the (104) surface could be fully exposed via the nanoarray architecture as the active sites for NRR. In summary, the nanoarray architecture enables Fe2O3NAs achieve enhanced NRR electrocatalytic performance via both enhanced ECSA and induced(104) facets exposure.

Fig.6 (a) the current response of Fe2O3 NAs at -0.2 V for 20 hours; (b) the UV-Vis absorption spectra of catholytes with indophenol indicator after 2-hours and 20-hours electrolysis; (c) NH3 yield rate and FE of Fe2O3 NAs at -0.2 V with different electrolysis time

In order to verify the source of detected NH3,control experiments were carried out under Ar and air flow at -0.2 V. Electrolysis were carried out at -0.20 V under alternative N2and Ar flows. Each gas flows were kept for 2 hours, and the NH3yield rates were quantified for each 2 hours. Fig.5(a) clearly shows the difference of current densities under N2and Ar. As expected,Fig.5(b) suggest that no NH3was detected under Ar flow, which further confirms that the produced ammonia comes from N2gas. Also, the by-product of N2H4was not detected, indicating that the Fe2O3nanorods has excellent selectivity for NH3formation.

3.4 Stability of catalysts

Besides the high catalytic activity, the stability of NRR catalysts is another critical parameter of catalytic performance. Fig.5(c) shows the current responses at-0.20 V during repeated 2-hours chronoamperometric cycles. The corresponding NH3yield rate and FE are plotted in Fig.5(d) with only small fluctuation proving the favorable cycling stability. Fe2O3NAs can also give a constant current response during 20-hours chronopotentiometric test in Fig.6(a). Fig.6(b) compares the UV-Vis adsorption of the electrolytes after 2-hours and 20-hours electrolysis, the NH3yield rate and FEs of Fe2O3NAs in Fig.6(c) show negligible loss when the electrolysis time increases from 2 hours to 20-hours.

4 Conclusions

In summary, we prepared Fe2O3NAs on CC as a self-standing catalyst electrode towards artificial N2fixation to NH3with high selectivity under ambient conditions. Due to the facet regulationof Fe2O3via nanoarray architecture(Fe2O3NAs), a high FE of 14.89% and a high NH3yield rate of 17.28 μg h-1cm-2are achieved. The highly exposed (104) facet provides abundant unsaturated Fe atoms with dangling bonds as NRR catalytically active sites, and the nanoarray architecture achieves enhanced electrochemical surface area(ECSA) to fully manifest the active sites. Thus, the selectively exposed (104) facets coupled with the high ECSA of nanoarray architecture achieve an enhanced catalytic performance. This work presents an effective strategy to develop highly efficient catalytic electrodes for electrochemical NRR via facet regulation enable by nanoarray architecture.