Facile Synthesis of Mesoporous α-Fe2O3@g-C3N4-NCs for Efficient Bifunctional Electro-catalytic Activity (OER/ORR).

Mesoporous α-iron oxide@graphitized-carbon nitride nanocomposites (α-Fe2O3@g-C3N4-NCs) were synthesized using urea-formaldehyde (UF) resins at 400 °C/2 h. The mesoporous nature of the prepared nanocomposites was observed from electron microscopy and surface area measurements. The electrochemical measurements show the bifunctional nature of mesoporous α-Fe2O3@g-C3N4-NCs in electrolysis of water for oxygen evolution and oxygen reduction reactions (OER/ORR) using 0.5 M KOH. Higher current density of mesoporous α-Fe2O3@g-C3N4-NCs reveals the enhanced electrochemical performance compared to pure Fe2O3 nanoparticles (NPs). The onset potential, over-potential and Tafel slopes of mesoporous α-Fe2O3@g-C3N4-NCs were found lower than that of pure α-Fe2O3-NPs. Rotating disc electrode experiments followed by the K-L equation were used to investigate 4e- redox system. Therefore, the mesoporous α-Fe2O3@g-C3N4-NCs bifunctional electro-catalysts can be considered as potential future low-cost alternatives for Pt/C catalysts, which are currently used in fuel cells.

Introduction

The cost effective, template free, and environmental friendly synthesis of mesoporous nanostructured materials with controlled size and shape are of great interest till date. The variety of mesoporous nanostructured materials have been studied for various applications including sensing[1], supercapacitors[2,3], electro-catalysis[4], electro-oxidation[5], photo-catalysis[6,7], batteries[8-10], biomedical[11], dehydrogenation[12], adsorption[13-15]. Mesoporous carbon nitride has shown excellent photocatalytic hydrogen generation reactions due to its low band gap and high surface area (1.9 eV)[16-18]. The mesoporous g-C3N4 hetero-structured materials are reported as photo-catalysts in water splitting[6] and dye degradation reactions[19]. The mesoporous hetero-structures of g-C3N4@FeNi3 were also used as an adsorbent in crude oil recovery[20]. Mesoporous nanostructured materials have been synthesized by various methods like solvothermal[10], co-condensation[14], hydrothermal[20], microemulsion[21] and microfluidic synthesis[22]. In this paper, we focus on facile synthesis of mesoporous nanostructured materials and their application in electrolysis of water (OER/ORR).

Electrolysis of water has great interest in renewable energy resources for future development. The hydrogen evolution, oxygen evolution, and oxygen reduction reactions are three main processes in electrolysis of water for renewable energy conversion devices[23,24]. Noble metal based electro-catalysts are considered as the most capable catalysts for OER and ORR[25-27]. These electro-catalysts are very costly. Therefore, they are unwanted for the commercialization. The researchers are devoted to generate the cost-effective and durable electro-catalysts as alternative to replace the expensive noble metal electro-catalysts. Hematite phase of iron oxide (α-Fe2O3) is the most stable phase among other oxides of iron. This material is naturally abundant and inexpensive, which show great attention in several potential applications including supercapacitors[28], batteries[29], sensors[30], adsorbent[31] etc. Hematite suffers from the cycling stability during the electrochemical performance. Graphitized carbon nitride (g-C3N4) exhibits excellent chemical stability for ORR[32]. Therefore, we have designed mesoporous α-Fe2O3@g-C3N4 nanocomposites (NCs) electro-catalysts for electrolysis of water. The α-Fe2O3@g-C3N4-NCs were reported in photo-catalysis[33-35], supercapacitors[36] and photo-electrochemical reactions[37]. Porous core@shell Fe3C@NSC electro-catalyst was reported for efficient ORR in alkaline medium[38]. Suding Yan et al. have reported the g-C3N4/Fe2O3 composite as the photo-catalysts for the oxidation of bisphenol[39]. Other transition metal based nanostructured materials with controlled morphology and composition were also reported for electro-catalysis[40-42] and supercapacitors[40,43]. g-C3N4 decorated FeNi3 and C-decorated iron oxide hybrid nanostructured materials were reported for energy storage[44,45].

Recently, α-Fe2O3@g-C3N4 nanostructured materials were synthesized by co-calcination[46,47], ultrasonic[35], hydrothermal[48] methods and used for waste water treatment and photochemical reactions. Herein, we report the facile synthesis of mesoporous α-Fe2O3@g-C3N4-NCs using urea-formaldehyde (UF) resins at 400 °C/2 h. The structural and morphological characterizations of synthesized nanomaterials were carried out by powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron microscopic techniques. 4-e transferred process in alkaline medium is more desirable for ORR. Therefore, bifunctional electro-catalytic performances of α-Fe2O3@g-C3N4 nanostructured materials were investigated in detail using alkaline medium.

Results and Discussion

XRD patterns of mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs were shown in Fig. 1a,b. The resulting XRD reflections of the prepared materials were found to be <012>, <104>, <110>, <113>, <024>,<116> and <018> which resemble to the hexagonal crystal system of α-Fe2O3. The entire xrd patterns match with the JCPDS number of 86-0550. A small peak was also detected in XRD at ~26.50, which reveals the presence of graphitized (g) carbon nitride (C3N4) in the nanocomposites at 400 °C (Fig. 1a). No peak of g-C3N4 was detected in XRD of pure α-Fe2O3 at 500 °C/24 h (Fig. 1b). Note that the shifting of XRD peaks in small extent to the higher angle side is clearly visible in the nanocomposites, which could be due to the presence of g-C3N4.

Figure 1

XRD patterns of (a) mesoporous α-Fe2O3@g-C3N4-NCs and (b) α-Fe2O3-NPs.

Figure 2 shows the XPS spectra of mesoporous α-Fe2O3@g-C3N4-NCs. Full length XPS spectrum confirms the presence of C, N, Fe and O elements in mesoporous α-Fe2O3@g-C3N4-NCs. High resolution XPS spectrum of Fe element shows two peaks at ~711 and ~725 eV, which represent to the Fe 2p3/2 and Fe 2p1/2 respectively[24]. We have observed no characteristic peaks of Fe (II) or Fe (0) in XPS. The peaks between 397–400 eV correspond to the graphitized nitrogen (i.e., pyridinic-N, pyrrolic-N and graphitic-N) as also reported[24]. Moreover, FTIR studies clearly show the difference between mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs in the spectra as shown in Fig. 3. A strong peak at 1200–1600 cm−1 represent to the vibrations of C=C, C-N and C-H in the mesoporous α-Fe2O3@g-C3N4-NCs (Fig. 3a) while no FTIR bands were detected in pure α-Fe2O3 as expected (Fig. 3b). A small peak at ~810 cm−1 belongs to triazine ring of g-C3N4 (Fig. 3a). The bands at lower wavenumbers (465 and 545 cm−1) correspond to Fe-O[49]. Figure 4 shows Raman spectra of g-C3N4, mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs. The characteristic Raman bands of α-Fe2O3-NPs were identified at ~221, ~294, ~410, ~498, and ~607 cm−1, which resemble to the A1g, E1g, E1g, A1g, and E1g Raman modes, respectively. Raman peaks were also observed at ~220, ~480, ~708, ~760, and ~980 cm−1 for g-C3N4. The most intense peak of g-C3N4 at ~708 cm−1 represented the s-triazine ring[50] and also consistent with FTIR results.

Figure 2

Full length XPS spectrum of mesoporous α-Fe2O3@g-C3N4-NCs and high resolution XPS spectra of N and Fe.

Figure 3

FTIR spectra of (a) mesoporous α-Fe2O3@g-C3N4-NCs and (b) α-Fe2O3-NPs.

Figure 4

Raman spectra of g-C3N4, mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs.

Figure 5 shows the electron microscopic studies of mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs. TEM and high magnification TEM studies of α-Fe2O3@g-C3N4-NCs clearly show the porous nature of the materials and also reveal that the α-Fe2O3-NPs are embedded to the g-C3N4 matrix at 400 °C/2 h (Fig. 5a,b). The average size of embedded nanoparticle is found to be ~10 nm. High resolution TEM (HRTEM) image show the d-spacing (~2.90 A) of <110> plane (Fig. 5c). Inset of Fig. 5c shows selected area electron diffraction (SAED) patterns of mesoporous α-Fe2O3@g-C3N4-NCs. SAED patterns correspond to the hematite phase of Fe2O3 nanoparticles. TEM micrograph of pure α-Fe2O3-NPs is shown in Fig. 5d. We observed that pure α-Fe2O3-NPs are more agglomerated with larger particle size as compared to α-Fe2O3-NPs of mesoporous α-Fe2O3@g-C3N4-NCs. From our results, we can conclude that the presence of the graphitic carbon nitride in the mesoporous nanocomposites could be helpful to control the agglomeration of nanoparticles.

Figure 5

(a,b) TEM, and (c) HRTEM of mesoporous α-Fe2O3@g-C3N4-NCs. (d) TEM of α-Fe2O3-NPs. Inset of Fig. 4c shows electron diffraction of mesoporous α-Fe2O3@g-C3N4-NCs.

The BET surface area of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs were calculated using the range of relative pressure (P/P) of 0.05–0.35. Figure 6a,b show the BET plots of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs. The resulting surface area of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs were found to be ~26 and ~115 m2g−1 respectively. N2 adsorption-desorption analysis of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs exhibit the type IV isotherms followed by hysteresis-3 type as shown in Fig. 6a. The particle size from BET studies matches closely with the particle size obtained from electron microscopy. The pore size distributions of the nanostructured materials were examined by using the Barrett–Joyner–Halenda (BJH) and Dubinin-Astakhov (DA) model. This is noteworthy that BJH method is good for mesoporous compounds while DA model is suitable for microporous materials to find the pore size distributions from the isotherms. The BJH pore size distribution of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs were found to be 15 and 65 Å respectively (Fig. 6c). These results confirmed that the α-Fe2O3@g-C3N4-NCs exhibit mesoporous nature while Fe2O3-NPs show microporous nature. The DA pore size of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs were found to be 18 and 48 Å respectively (Fig. 6d). The resulting DA pore sizes validate the micro and mesoporous nature of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs and corroborate well with BJH studies. Mesoporous nature and high surface area advocate the more active sites available at the α-Fe2O3@g-C3N4-NCs.

Figure 6

(a,b) BET surface area, (c) BJH pore size distribution, and (d) DA pore size distribution plots of α-Fe2O3-NPs and α-Fe2O3@g-C3N4-NCs.

Electrochemical measurements of α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs were examined using 0.5 M KOH vs Ag/AgCl for OER/ORR. The amount of electro-catalysts (~0.21 mg/cm2) has been loaded on the working electrode (0.07 cm2), which is used to calculate the current density. The cathodic and anodic sweeps in cyclic voltammetry (CV) show the bifunctional redox behavior (OER/ORR) within the potential window of −1 to +1 V at 50 mV/s (Fig. 7a). Figure 7b shows the linear sweep voltammetry (LSV) curves of mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs at 50 mV/s for OER (anodic region) to confirm the water oxidation reaction. Figure 7c shows the choronoamperometric (CA) studies for stability check during OER measurements. The CA studies confirm the excellent stability and higher electro-catalytic behavior of α-Fe2O3@g-C3N4-NCs than that of pure α-Fe2O3-NPs at 0.5 V for 600 seconds. Figure 7d shows the LSV curves of mesoporous α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs for ORR (cathodic sweep) at 50 mV/s vs Ag/AgCl. Low onset potential and high current density of α-Fe2O3@g-C3N4-NCs were observed for OER and ORR as compared to Fe2O3-NPs. Inset of Fig. 7b shows the Tafel plots of α-Fe2O3@g-C3N4-NCs (~280 mV/dec) and α-Fe2O3-NPs (~320 mV/dec) for OER while the Tafel values of α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs were found to be ~90 and ~215 mV/dec for ORR. The over-potentials of the α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs for OER were found to be ~425 and ~550 mV, respectively, for OER at the current density (η10) of 10 mA/cm2. However, while the over-potentials of the α-Fe2O3@g-C3N4-NCs and α-Fe2O3-NPs for ORR were found to be ~350 and ~530 mV, respectively, at 10 mA/cm2. The Nernst equation[42] has been used to convert the potential from Ag/AgCl to reversible hydrogen electrode in order to understand the over-potential of water oxidation reactions. The following reactions could be summarized on the basis of OER and ORR from the electrolysis of water in alkaline medium (0.1MKOH) i.e. 4OH− → O2(g) + 2H2O(l) + 4e− (for OER) and O2 + 2H2O(l) + 4e− → 4OH− (for ORR). The energy conversion tools include electro-catalytic ORR and OER at cathode and anode of an electrolytic cell consisting of two half-cells reactions. The resulting onset potentials, over-potentials, and Tafel slopes are inversely proportional to the electro-catalytic activity, while the resulting current density is directly proportional to the electro-catalytic activity of the materials.

Figure 7

(a) CV for electrolysis of water (OER/ORR) using α-Fe2O3-NPs and mesoporous α-Fe2O3@g-C3N4-NCs electrocatalysts in 0.5 M KOH vs Ag/AgCl. (b) LSV curves for OER in 0.5 M KOH vs Ag/AgCl and (c) Stability test for OER in 0.5 M KOH at 0.5 V for 600 seconds. (d) LSV curves ORR in 0.5 M KOH vs Ag/AgCl. Inset of Fig. 5b shows Tafel plots of α-Fe2O3-NPs and mesoporous α-Fe2O3@g-C3N4-NCs.

Moreover, LSV measurements of mesoporous α-Fe2O3@g-C3N4-NCs were carried out for ORR by rotating disk electrode using 0.5 M KOH vs Ag/AgCl at the scan rate of 25 mV/s (Fig. 8). The rotating disk electrode works as working electrode in the cathodic sweep for ORR. The rotation of the electrode varies from 400 to 2000 rpm. This is noticeable that the current densities of mesoporous α-Fe2O3@g-C3N4-NCs were increased significantly with rotation due to the diffusion distance of O2. Inset of Fig. 7 shows linear fitted curves obtained from Koutecky–Levich (K–L) equation[51]. On the basis of K-L equation, the linear fitted curves have been used to estimate the average number of electrons involved (n) in ORR. The following K-L equation has been used for the calculation of transferred electrons during electrolysis of water:

Figure 8

LSV studies of mesoporous α-Fe2O3@g-C3N4-NCs for ORR with the rotation from 400–2000 rpm at 25 mV/s. Inset shows the K-L plots of mesoporous α-Fe2O3@g-C3N4-NCs for ORR at the potential from −0.30 to −0.50 V vs Ag/AgCl.

J: Current density (A.cm−2); JK: Kinetic current density (A.cm−2); JL: Diffusion-limiting current densities (A.cm−2); F: Faraday’s constant (C.mol−1); Do: Diffusion coefficient of O2 in 0.5 M KOH; ν: Kinematic viscosity of the electrolyte (cm2s−1), Co: Saturation concentration of O2 in 0.5 M KOH at 1 atm O2 pressure (mol.cm−3); ω: rotation rate (rad.s−1).

Linear fitting curves of 1/current density (mA−1cm2) and ω−1/2 (rpm−1/2) reveals that the K-L plots follow the 1st order reaction kinetics with around four electrons ORR process in 0.5 M KOH. Our results are in good agreement with the reported values of transferred electrons during the water redox reactions[52]. Based on the present studies, mesoporous α-Fe2O3@g-C3N4-NCs show excellent electro-catalytic behavior for OER/ORR compared to pure α-Fe2O3-NPs in alkaline medium and other reported works. The synergic effect arises with the interaction of iron oxide and g-C3N4, which is also important for better ORR performance of the nanocomposite. The mesoporous nature of the prepared materials provides high surface area as well as more active sites for electro-chemical reactions to enhance the ORR performance. Table 1 shows the comparison of present work with the highly active noble electro-catalysts. Therefore, mesoporous α-Fe2O3@g-C3N4-NCs could be used as potential low-cost alternatives Pt/C electrode materials in fuel cells.

Table 1

Comparison table of present work with the reported work of highly active noble electro-catalysts.

Electro-catalysts	Loaded amount(mg/cm2)	Electrolyte	Scan rate(mV/s)	Over-potential(mV)	Rotation speed of RDE (rpm)	Tafel slopes (mV/dec)	Reference
α-Fe2O3@g-C3N4	~2.1	0.5 M KOH	50	~425 (OER) ~350 (ORR)	400–2000 (ORR)	~280 (OER) ~90 (ORR)	Present Work
α-Fe2O3NPs	~2.1	0.5 M KOH	50	~550 (OER) ~530 (ORR)	400–2000 (ORR)	~320 (OER) ~215 (ORR)	Present Work
IrO2	0.35	1.0 M HClO4	10–500	450 (OER)	—	~120 (OER)	[55]
IrO2	0.30	1.0 N H2SO4	1	—	—	~100 (OER)	[56]
Pt, Ir, Ru	—	0.1 M HClO4	50–500	—	1600	~210 (OER)	[27]
Pt/C	0.30	2.0 M KOH	5	—	400–1600	~65 (ORR)	[57]
Strontium Iron oxyhalides	0.56	1.0 M KOH	50	350–970 (OER)	500–2000	~100 (OER) ~98 (ORR)	[58]
Co3O4	1.0	1.0 M NaOH	1	350–970 (OER)	—	~70 (OER)	[59]
Pt	1.0	1.0 M NaOH	1	450 (OER)	—	~220 (OER)	[59]

Conclusions

Successfully, we have prepared the low cost mesoporous α-Fe2O3@g-C3N4 nanostructured materials using the UF resins at 400 °C/2 h for water electrolysis application. XRD, FTIR, XPS, Raman and TEM studies were used to characterize the synthesized nanocomposite materials. Mesoporous α-Fe2O3@g-C3N4-NCs show excellent bifunctional electro-catalytic behavior (OER/ORR) as compared to pure α-Fe2O3-NPs using 0.5 M KOH electrolyte. Our results show low energy loss with α-Fe2O3@g-C3N4-NCs (~90 mV/dec) as compared to pure α-Fe2O3-NPs (~215 mV/dec) during electrolysis of water for ORR. Low onset potential, low over-potential, low Tafel slope, high current density and excellent stability of mesoporous α-Fe2O3@g-C3N4-NCs have been observed, which makes it an alternative electro-catalyst over expensive noble metal based electro-catalysts. Therefore, mesoporous low cost α-Fe2O3@g-C3N4-NCs can be considered as potential candidate in electrochemical water splitting reactions for renewable energy conversion devices in near future.

Materials and Methods

Mesoporous α-Fe2O3@g-C3N4 nanocomposites have been synthesized using urea-formaldehyde UF resins at 400 °C/2 h. A typical UF resin was obtained from aqueous urea (0.1 mole, 10 mL) and formaldehyde (0.2 mole) at pH of 10. Thereafter, 0.01 mole of FeCl3 solution was made with 100 mL of DI water. These two systems were mixed together on magnetic stir at 80 °C for 30 minutes. The resulting brown colored precipitates were appeared. The precipitates were filtered and then dried at 80 °C/24 h. These precipitates were used as the single source precursor for the synthesis of mesoporous α-Fe2O3@g-C3N4-NCs at 400 °C/2 h. The Fe2O3 nanoparticles (NPs) were also synthesized using the same precursor at 500 °C/24 h. Figure 9 shows the synthetic scheme of the prepared nanostructured materials.

Figure 9

Reaction scheme for the synthesis of mesoporous α-Fe2O3@g-C3N4-NCs.

Powder X-ray diffraction (XRD) data was recorded on Rigaku MiniFlex using Ni-filtered-CuKα radiation (λ = 1.54056 Å). The data was collected in two theta range from 10 to 60° with step size and step time of 0.05° and 1 s respectively. X-ray photoelectron spectroscopy (XPS) was used to identify the oxidation states of the elements. XPS data was recorded in an ultra-high vacuum chamber (Kratos Axis Ultra-DLD electron-spectrometer) with the pressure of 5 × 10−10 Torr. FTIR data was measured on Bruker TENSOR-27 spectrometer. The powder samples were pelletized with KBR and run the FTIR measurements under transmittance mode in the range from 400 to 4000 cm−1. Raman measurements were carried out on Bruker Sentera Raman microscope. High resolution transmission electron microscopic (HRTEM) images were captured on JEOL (JSM-2100F) using C-coated Cu TEM grid to identify the morphology and size of the particles. Nitrogen adsorption–desorption isotherm data were collected on the Micromeritics ASAP-2020 physisorption at 77 K. The nanoparticles were degassed at 150 °C for 10 h to remove the moisture, contaminants and adsorbed gases on the surface of the materials. The BET surface area was calculated from the adsorption data obtained at the relative pressure ranging from 0 to 1. Pore size distribution plots were figured using the desorption isotherms followed by the Barrett, Joyner, and Halenda (BJH) method.

The electrolysis of water was investigated on potentiostat/galvanostat electrochemical work station (CHI 660E). The reference, counter and working electrodes were Ag/AgCl, Pt-wire, and glassy carbon respectively. The area of working electrode (0.07 cm2) was used to calculate the current density of the electrodes. 0.5 M KOH solution was used as an electrolyte in the electrochemical studies of the electrode materials. The electro-catalysts (5.0 mg of) and isopropanol (1.0 ml) were mixed with nafion (0.2 ml) and sonicated for 10 minutes. One drop of the suspension was put on to the glassy carbon (GC) surface and then dried. The loaded amount of electro-catalysts was of ~0.21 mg/cm2 on the working electrode. Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and Tafel studies were carried out for electrolysis of water to OER and ORR in 0.5 M KOH vs Ag/AgCl at at 50 mV/s. In order to investigate the number of involved electrons in electrolysis of water, we have conducted rotating disc electrode (RDE) experiments with the rotation of 400–2000 rpm in ORR followed by Koutecky–Levich (K–L) equation[53,54]. Note that all the electrochemical measurements were conducted at room temperature and repeated three times to check the reproducibility of the results.