CoFe Nanoalloys Encapsulated in N-Doped Graphene Layers as a Pt

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 12736−12745

pubs.acs.org/journal/ascecg

CoFe Nanoalloys Encapsulated in N‑Doped Graphene Layers as a PtFree Multifunctional Robust Catalyst: Elucidating the Role of CoAlloying and N‑Doping Barun Kumar Barman and Karuna Kar Nanda* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

Downloaded via IOWA STATE UNIV on January 20, 2019 at 20:55:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Pt is known to be a state-of-the-art catalyst for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER), while it can also be used for the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The quest is ongoing to find a suitable catalyst to circumvent the problems associated with the precious metal Pt. Here, we report a facile and green strategy to fabricate CoFe nanoalloys encapsulated in N-doped graphene layers (CoxFe1−x@N-G) by pyrolysis and their catalytic activity toward ORR, HER, and hydrogenation of 4-NP. Intensive studies have been carried out to elucidate the roles of alloying and N-doping. The catalytic activity is found to improve with increasing amounts of Co in the CoFe core and N-doping in the graphene layers. A similar onset potential with better current density as compared to the state-of-the-art Pt/C catalyst in alkaline medium has been achieved for CoxFe1−x@N-G toward ORR activity. These catalysts also show efficient and highly stable HER activity and are very efficient and magnetically separable in the hydrogenation of 4-NP to 4-AP. Overall, the non-precious-metal alloy nanostructures can be exploited as multifunctional catalysts in fuel cells, hydrogen storage systems, and wastewater treatment. KEYWORDS: Nanoalloy, N-doped graphene, Trifunctional robust catalyst, Oxygen reduction reaction, Hydrogen evolution reaction, 4-NP reduction

■

INTRODUCTION Fuel cells,1−8 metal−air batteries,9−13 and water electrolyzers14−22 have attracted considerable interest due to the increasing demand for petroleum resources and concern over global warming. The oxygen reduction reaction (ORR) is a vital cathodic reaction for various electrochemical energy storage devices, such as fuel cells and metal−air batteries, converting chemical energy into electrical energy.23−33 The oxygen electrode kinetic is sluggish, with 15% power loss, which is the major problem in achieving highly efficient energy devices. The hydrogen evolution reaction (HER) is another class of electrochemical reaction for the generation of hydrogen that can be used as fuel in fuel cells. Pt or Ptbased metal alloys have been exploited to overcome the sluggish oxygen kinetics as well as to facilitate the HER.34−36 However, the cost of Pt-based electrocatalysts hinders the commercialization of their use in energy devices. The conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) has fascinated researchers because 4-NP is one of the most common organic pollutants in wastewater generated from industrial sources, while 4-AP is a powerful intermediate in the manufacture of analgesics, antipyretic drugs, anticorrosion lubricants, hair dyes, etc. This is also a model reaction to study the rate kinetics of the catalyst, and mostly noble metals such © 2018 American Chemical Society

as Pt, Pd, Au, etc. and their hybrids are very active toward this conversion of process.37−48 To replace the noble-metal-based catalysts, substantial efforts are being made to develop nonprecious-metal catalysts for ORR, HER, and hydrogenation of 4-NP to 4-AP. Recently, progress on three-dimensional (3-d) transition-metal-based oxides, sulfides, carbides, selenides, nitrides, and phosphides has demonstrated them to be promising catalysts toward the ORR and HER.46−55 Various research groups have also developed CoFe alloys encapsulated in N-doped carbon/graphene layers for various catalytic applications.56−59 The carbon/graphene layers prevent the metals from oxidizing and provide stability, whereas the M-N complex forms active catalytic sites. However, the catalytic activities are still low when compared with those of Pt-based catalysts. It should be noted that the composition of Co-, Fe-, and N-doping is fixed due to the single-source precursor. For example, thermal decomposition of Co-imidazole zeolitic imidazolate frameworks (ZIFs) or Prussian blue (PB) analogues in N2 atmosphere yields Fe-to-Co ratio of ∼1:1 and 3:1.57,58 In order to elucidate the effects of N-doping in Received: April 25, 2018 Revised: July 23, 2018 Published: August 17, 2018 12736

DOI: 10.1021/acssuschemeng.8b01861 ACS Sustainable Chem. Eng. 2018, 6, 12736−12745

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Representation for the Synthesis of CoxFe1−x Nano-alloy Encapsulated in N-Doped Graphene Layers

ance is found to dependent on the nanoalloying as well as the N-doping in the graphene layers. The ORR performance of the X3 hybrid shows a higher limiting current density (JL) and similar onset potential compared with the benchmark precious Pt/C catalyst as well as the reported values. Similarly, catalytic activity toward the conversion of 4-NP to 4-AP in the presence of NaBH4 is demonstrated, and it is shown that the N-doped and Co-alloyed catalysts are magnetically separable, efficient, stable, and overall better as compared with the noble-metalbased catalysts. The HER activity is also found to be improved with Co-alloying and N-doping.

graphene layers and the amount Co in CoFe toward their catalytic activity, it is desirable to fabricate CoFe nanoalloys with different Co:Fe ratios encapsulated in N-doped graphene and carry out systematic studies. It is well known that the performance of N-doped carbon ORR catalysts improves due to the doping-induced charge relocation around the N dopants that weakens the O−O bonding, helping the ORR by lowering the potential. It has been also predicted that N-doping lowers the H* adsorption energy and favors HER. Furthermore, the CoFe core transfers charge to the graphene support and enhances the HER activity as well.56 Overall, CoFe encapsulated in N-doped graphene is expected to be an efficient catalyst that can be exploited for ORR as well as 4-NP reduction apart from HER. There are several approaches for the fabrication of the encapsulated CoFe nanostructures. However, it is desirable to develop a simple synthesis technique that can produce encapsulated nanoalloys with different Co:Fe ratios along with different levels of N-doping. In this work, we demonstrate a one-step simple pyrolysis for the synthesis of CoxFe1−x nanoalloy encapsulated in N-doped graphene layers (CoxFe1−x@N-G) with tunable N-doping in the graphene layers and different percentages of Co-alloying in the core region. Thereafter, their catalytic activity toward ORR, HER, and hydrogenation of 4-NP to 4-AP is investigated in order to elucidate the roles of alloying and N-doping. Interestingly, the activity is found to be dependent on the amounts of Co in the CoFe core and of N in graphene layers. Co-alloying and high N-doping in CoxFe1−x@NG show outstanding ORR activity, offering the potential to replace precious Pt/C catalysts. CoxFe1−x@NG also shows efficient HER activity and hydrogenation of 4-NP. The encapsulated nanostructures are fabricated by pyrolyzing a mixture of Prussian blue (PB, Fe4[Fe(CN)6]3) and cobaltocene (Co(C5H5)2) or cobalt nitrate hexahydrate (Co(NO3)2·6H2O) as presented in Scheme 1. Choosing different precursors helps to control the N-doping in graphene and Co-alloying in Fe. The pyrolysis of PB alone forms Fe-Fe3C@N-G (X0). Pyrolysis of Co(C5H5)2 and PB forms CoxFe1−x/NG in 1:10 and 1:5 ratios, named as X1 and X2, respectively. With Co(C5H5)2, the Co:Fe ratio increases along with the reduction of N-doping. Therefore, PB and (Co(NO3)2·6H2O) are pyrolyzed to obtain CoxFe1−x@NG (X3), with a higher percentage of N-doping. All the encapsulated nanostructures were subjected to various catalytic applications. Interestingly, the electrocatalytic ORR perform-

■

EXPERIMENTAL SECTION

Materials. Prussian blue, cobaltocene, and sodium borohydride were purchased from Sigma-Aldrich. Cobalt nitrate hexahydrate and 4-nitrophenol were purchased from S D Fine-Chem Limited. Commercial Pt/C (20 wt%) was purchased from Johnson Matthey. Methods. Various encapsulated nanostructures in N-doped graphene layers (CoxFe1−x@N-G) were synthesized by simply pyrolyzing mixtures with different ratios of PB and cobaltocene/ cobalt nitrate hexahydrate. The precursor mixtures were taken in 1 cm diameter quartz tubes in closed condition and pyrolyzed at 750 °C for 2 h. The synthesis parameters for various CoxFe1−x@N-G nanoalloys are presented in Table 1.

Table 1. Synthesis Parameters and Characterization Results precursors (mg) sample

Prussian blue

Co(C5H5)2

Co(NO3)2·6H2O

%N

% Co

X0 X1 X2 X3

500 500 500 500

0 50 100 0

0 0 0 40

8.3 4.02 1.01 8.3

0 8 12 10

Characterization. The crystal structures of the as-synthesized materials were characterized by X-ray diffraction (XRD) with a PANalytical instrument using a Cu Kα (λ= 1.54 Å) radiation source. The Raman measurements were performed with a WITec Raman system at room temperature using a Nd:YAG laser (532 nm) as an excitation source. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) data were taken on a FEI-INSPECTF50 instrument. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were acquired using a JEOL JEM-2100F instrument. All the TEM samples were prepared by dispersing the sample in ethanol solution using an ultrasonic bath for 15 min; it was then drop-casted on a carbon-coated copper grid and dried at 70 °C. X-ray photoelectron spectroscopy (XPS) was carried 12737


Research Article


Figure 1. (a,b) Low- and high-magnification SEM images of CoxFe1−x/N-G (X3). (c,d) TEM and HRTEM images of CoxFe1−x/N-G (X3). (e,f) TEM and HRTEM images of Fe-Fe3C/N-G (X0). out for the elemental investigation on an ESCALAB 250 instrument (Thermo Electron) with a monochromatic Al Kα (1486.6 eV) radiation source. Electrochemical Measurements. Cyclic voltammetry (CV) studies were performed using an electrochemical workstation with a rotating disk electrode (RDE) from CH Instruments. Each time, the catalyst ink was prepared by ultrasonically dispersing 5 mg of catalyst in a solution of 0.1 mL of 5% Nafion and 0.9 mL of ethanol solution. The loading on the RDE was 0.64 mg/cm2 for CoxFe1−x@N-G and 0.360 mg/cm2 of 20% (72 μg (Pt)/cm2) of the Pt/C catalyst. A conventional three-electrode cell was used where Ag/AgCl is the reference electrode, a Pt wire is the counter electrode, and the catalyst film-coated RDE is the working electrode. The RDE measurements were carried out at different rotation speeds between 600 and 2400 rpm in the O2-saturated 0.1 M KOH aqueous solution to evaluate the kinetics of ORR performance. The stability was monitored by linear

sweep voltammetry (LSV) up to 1000 cycles with a scan rate of 50 mV/s and a rotation speed of 600 rpm. The RDE data were analyzed using Koutecky−Levich (K-L) plots (J−1 vs ω−1/2) at different applied potentials. The slopes of the linear fit were used to calculate the electron transfer (ET) number (n) based on the K-L equation: 1/J = 1/JL + 1/Jk = 1/Bω1/2 + 1/Jk, with B = 0.62nFC0(D0)2/3ν−1/6, where JL is the measured current density, Jk is the kinetic current, ω is the electrode rotation rate, F is the Faraday constant (96485 C/mol), C0 is the saturation concentration of O2 in 0.1 M KOH solution at room temperature (1.2 × 10−6 mol/cm3), D0 is the diffusion coefficient of O2 in KOH (1.9 × 10−5 cm2/s), and ν is the kinetic viscosity of the electrolyte (0.01 cm2/s). According to the K-L plot, the slope (1/B) can be used to calculate n: n = B/0.62FC0(D0)2/3ν−1/6. The HER activity was recorded in N2-saturated 0.1 and 1 M KOH solutions with scan rates of 10 and 100 mV/s and a rotation speed of 1400 rpm. The stability test was performed by using LSV measurements at a scan 12738


Research Article


Figure 2. (a) XRD patterns of X0, X1, X2, and X3. (b) Typical Raman spectrum of X3. (c) XPS survey spectra of X0, X1, X2, and X3. (d−f) Deconvoluted XPS spectra of Fe-2p, Co-2p, and N-1s of the X3 hybrid, respectively. rate of 100 mV/s in 1 M KOH solution. The HER performance was also studied using carbon paper as both working and counter electrodes. Catalytic Reduction of 4-NP to 4-AP. The conversion of 4-NP to 4-AP is an important reaction that also requires a catalyst. The catalytic conversion of 4-NP was monitored using a Hitachi UV−vis spectrophotometer in the range of 200−500 nm at room temperature. An aqueous solution of 10 mL of 1 mM 4-NP was added to 10 mL of freshly prepared 0.125 M NaBH4 solution. Next, 2 mg of solid catalyst was added and mixed thoroughly. The catalytic performance with different NaBH4 concentrations was also evaluated. Each catalytic experiment was performed at least three times (from each batch) to ensure reproducibility.

nanoarchitectures. Figure 1e,f displays the TEM and HRTEM images of the Fe-Fe3C-encapsulated N-doped graphitic nanostructure (X0), clearly revealing that the metallic nanostructures are encapsulated by graphitic layers. The dspacing of 0.202 nm, corresponding to the (110) crystal plane of Fe, is apparent from Figure 1, and 32−40 layers of graphene are seen, with an interlayer distance of 0.336 nm. SEM, TEM, and HRTEM images of other hybrids (X0, X1, and X2) also confirm core−shell-like structures (Figures S3 and S4). HRTEM images reveal that Co-alloying of the X3 hybrid reduces the number of graphene layers from 32−40 to 8−15 (Figure S5), which can also influence the catalytic activity. Figure 2a shows the XRD patterns of all the samples. When PB alone is pyrolyzed, core−shell structures of Fe-Fe3C/Ndoped graphene (Fe3C is assigned by * of X0) are obtained.26,60 However, inclusion of Co seems to resist the formation of Fe3C. Three foremost peaks centered around 44.74, 65.2, and 82.50° for (110), (200), and (211) planes that correspond to the γ phase of the Fe (JCPDS no. 870722) are observed. No appreciable change in the XRD patterns is observed after the incorporation of Co, which is believed to be due to the low amount of Co alloying with the Fe. There are no XRD peaks corresponding to metallic Co, which indicates that Co substitutes Fe in the lattice. Figure 2b represents the Raman spectrum of X3, with two most prominent peaks at 1344 and 1579 cm−1 corresponding to D and G bands, respectively. The presence of a prominent D peak with an ID/ IG ratio of 1.1 signifies the defective nature of the graphene layer and is due to the N-doping.61,62 Figure 2c displays the XPS survey spectra of all the hybrids. The survey spectra clearly show a strong peak of C-1s at 284.6 eV due to graphene layers, a small peak around 400 eV due to N-doping, and a peak around 533 eV due to the adsorbed atmospheric oxygen. The full survey spectrum of X3 displays the presence of C, N, Co, and Fe (Figure S6). Figure 2d,e displays the deconvoluted

■

RESULTS AND DISCUSSION Figure 1a,b shows the typical SEM images of X3, which clearly reveal the metallic part encapsulated by graphene layers. Figure 1c,d shows TEM and HRTEM images of X3. The TEM image reveals the black portion corresponding to the metallic nanostructures covered with the crumpled graphene layers. The size of the overall nanostructures is between 30 and 50 nm. The HRTEM image reveals the nanoalloy of CoxFe1−x with a d-spacing of 0.20 nm, corresponding to the (110) crystal plane of Fe, wrapped with a few layers (6−12 layers) of graphene layers. The interlayer distance is 0.336 nm, which corresponds to the van der Waals bonding distance of two graphene layers. A high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure S1) reveals bright spots corresponding to the alloy (Fe and Co) core of the nanostructures. Elemental distributions ensure the incorporation of Co into the Fe nanoparticles inside the core region as well as N-doping in the graphene. Fe (red) and Co (green) mappings indicate that CoxFe1−x nanoparticles are in the core, while C (yellow) and N (pink) are distributed all over (Figure S2). Overall, it is confirmed that CoxFe1−x nanoalloy is encapsulated in N-doped graphene to form core−shell 12739


Research Article


Figure 3. CV of (a) X3 and (b) 20% Pt/C in N2- and O2-saturated and 1 MeOH solution in 0.1 M KOH aqueous solution at a scan rate of 50 mV/ s with rotation speed 600 rpm. (b) CV of X3 in the presence of 1 M MeOH solution. (c) Comparison of the polarization curves of X0, X1, X2, and X3 with Pt/C in O2-saturated solution at scan rate of 10 mV/s with 1400 rpm. (d,e) Polarization curves with different rotation rates at 10 mV/s and K-L plots (J−1 versus ω−1/2) at different potentials for X3. (f,g) Polarization curves with different rotation rates at 10 mV/s and K-L plots (J−1 versus ω−1/2) at different potentials for the Pt/C. (h) Number of electrons transferred for X3 and Pt/C. (i) Comparisons of ORR stability plot of X3 with that of Pt/C up to the 1000th cycle.

from 8.3% to 4.02% and 1.01% for X0, X1, and X2, respectively, due to the absence of N in the cyclopentadiene ring in cobaltocence. When the PB-to-cobaltocene ratio is 10:1, 8% Co is incorporated (Figure S7a) in Fe (X1), while 11% Co is incorporated (Figure S7b) when the ratio is 5:1 (X2). When PB is pyrolyzed with Co(NO3)2·6H2O in a 12:1 ratio (X3), 10% Co in the CoFe core region (Figure S7c) and ∼8.3% of N-doping in graphene as in the case of X0 is realized. The overall elemental composition of all the encapsulated nanostructures is presented in Table 1. Figure 3a,b displays the CV curves of X3 and commercial Pt/C in N2- and O2-saturated 0.1 M KOH solution. Both X3 and Pt/C clearly show a sharp O2 reduction peak at −0.3 V vs Ag/AgCl in O2-saturated solution which is absent in N2saturated solution. X3 shows a limiting current density (JL) of 4.8 mA/cm2, which is superior compared to those of Pt/C (4.2 mA/cm2) and other catalysts reported in the literature for N-

XPS spectra of Fe-2p and Co-2p spectra of X3. Figure 2d shows a peak centered on 706.6 eV corresponding Fe(0). Two broad peaks at 710.1 and 713.3 eV are attributed to the 2p3/2 orbitals of the Fe2+ and Fe3+ oxidation states, respectively. Interestingly, the peak at higher binding energy (713.3 eV) indicates the presence of Fe2+−N bonding.63 Similarly, the deconvoluted XPS spectrum of Co-2p in Figure 2e displays two major peaks centered at 778.4 and 780.5 eV, corresponding to the Co(0) and Co(III) oxidation states of the 2p3/2 valence state. The deconvoluted N-1s spectrum as shown in Figure 2f clearly reveals different types of N environments: pyridine-type (398.4 eV), pyrrolic-type (399.4 eV), and graphite-type (401.1 eV), with 26.08%, 44.67%, and 29.92%, respectively. The XPS results of Fe-Co and N-Fe further confirm the presence of chemical metallic and metalnitron-type bonding in the hybrid nanostructure, respectively. In this context, it may be noted that the N-doping decreases 12740


Research Article


Figure 4. (a) HER performance of all the encapsulated nanostructures in 0.1 M KOH solution. (b,c) HER performance in 1 M KOH solution and the Tafel slope of X3 and Pt/C. (d) Stability of X3 toward HER activity in 1 M KOH solution.

X2 is inferior to that of X1, which is attributed to the drastic decrease in N-doping (4.02% to 1.01%) despite increasing Co content from 8% to 11%. However, the ORR activity of X3 is far superior as compared to other hybrids (X0, X1, and X2), as both the amount of Co in the CoFe core and N-doping in graphene are optimum for all the CoxFe1−x@N-G investigated. Overall, incorporation of Co and N-doping in graphene are ideal for ORR activity. It is well established that N-doping in a hexagonal carbon matrix enhances the ORR performance. N is more electronegative than C, which induces a positive charge on C, favoring the adsorption of O2 and improving the ORR performance.65,66 Similarly, Co is more electronegative as compared to Fe, and the further enhancement in the ORR activity is attributed to partial electron transfer from the Fe to Co, creating charge imbalance. Overall, the charge imbalance in both the CoFe core and N-doped graphene layers leads to a drastic enhancement in the catalytic site toward ORR performances. In an attempt to increase the percentage of Co in CoFe, the ratio of Co(NO3)2·6H2O to PB was doubled as compared to that in X3. Bigger and irregular-shaped particles without any graphene layer on the surface were obtained due to the insufficient availability of carbon. As expected, the ORR performance (Figure S10) was very low compared to other hybrids. The electrocatalytic HER performance of different catalysts is investigated in N2-purged 0.1 and 1 M KOH. Figure 4a displays the HER polarization curves for different catalysts in 0.1 M KOH solution, where the X3 shows the best HER performance compared to the others. It requires only 85 mV of overpotential (η) to overcome the HER activity in alkaline medium. Figure 4b compares the HER activity of X3 and commercial benchmark Pt/C catalyst. A current density of 10 mA/cm2 is achieved at 272 mV. Figure 4c shows the Tafel plot

doped graphene-based nanostructures/hybrids (Table S1). Another advantage associated with our CoxFe1−x@N-G is the methanol tolerance. The catalytic activity does not change in the presence of 1 M methanol, while in the case of Pt/C the ORR activity is completely suppressed and a sharp methanol oxidation peak at −0.177 V with high current density (12 mA/ cm2) is observed. Overall, the higher ORR current density, similar onset potential compared to Pt/C, and methanol tolerance suggest that nanoalloy-encapsulated nanostructures could serve as replacements for the precious Pt/C catalyst. Figure 3c compares the LSV curves of X0, X1, X2, X3, and 20% Pt/C. JL is found to be 3.2, 3.4, 2.9, and 4.1 mA/cm2 for X0, X1, X2, and X3, while it is 3.6 mA/cm2 for Pt/C. It is evident from Figure 3c that X3 shows better JL and similar onset potential as compared to Pt/C. The kinetic study of the ORR is performed by RDE measurements. Figure 3d,e represents the polarization curves with different rotation speed from 600 to 2400 rpm at 10 mV/s and K-L plot that displays the inverse current density (J−1) versus inverse of the square root of the rotation speed (ω−1/2) at different potentials of X3. Similarly, Figure 3f,g represents the polarization curves with different rotation speed from 600 to 2400 rpm at 10 mV/s and K-L plot of the benchmark 20% Pt/C catalyst. The linearity with parallelism in K-L plots of X3 and Pt/C (Figure S8) suggests first-order rate kinetics of the O2 reduction.64 Using the K-L equation, the number of electrons transferred (n) is evaluated to be 3.98 and 3.94 for X3 and Pt/C, respectively, as shown in Figure 3h, which clearly suggests a 4e− transfer process. Figure 3e compares the ORR stability of X3 and Pt/C up to the 1000th cycle (Figure S9). When the activities of X0 and X1 are compared, it is realized that X1 is superior to X0, which can be attributed to the 8% Co-alloying even though N-doping (8.3% vs 4.02%) is less. The activity of 12741


Research Article


Figure 5. CV curves of (a) X0, (b) X1, (c) X2, and (d) X3. (e) Difference in current density variation (ΔJ = Ja − Jc) at 0.55 V plotted against scan rate. Half of these values from the linear regression enable the estimation of Cdl.

Figure 6. Successive reduction of 4-NP catalyzed by (a) X0, (b) X1, and (c) X3 in the presence of NaBH4 (10 mL of 1 mM 4-NP and 10 mL of 0.125 M NaBH4 solution with 2 mg of catalyst). (d) Pseudo-first-order rate plot: ln(At/A0) at 400 nm vs time for the reduction of 4-NP. (e) Optical photographs of 4-NP (left) to 4-AP (right) conversion followed by magnetic separation of the X3 catalyst. (f) Recyclability performance of X3 during five successive cycles.

stability. Recent studies have shown that the use of Pt-based counter electrodes for HER studies without using an ionexchange membrane to separate the working electrode from the counter electrode can influence the HER performance substantially due to the dissolution−deposition of Pt on the working electrode. Therefore, HER performance has been

of X3 and Pt/C. The Tafel slope of X3 is 96 mV/dec, and that of Pt/C is 92 mV/dec. Figure 4d displays the HER stability of X3 up to the 1000th cycle, which proves the stability of the catalyst up over 1000 cycles with almost no change in the current density (97.6% retention). Overall, the HER performance of X3 is comparable to that of Pt/C with very high 12742


Research Article


Interestingly, the presence of Co-alloying in the core region along with high % of N-doping in the graphene improves the catalytic activity toward ORR, HER, and the conversion of organic pollutant 4-NP to 4-AP. In addition, the ORR activity is found to be superior as compared to that of traditional electrocatalysts in terms of current density. Overall, encapsulating the nanoalloy in N-doped graphene enhances the stability of the catalysts. We strongly believe that this methodology can be applied to synthesize a variety of nanoalloys (with the other 3-d transition metals) encapsulated in N-doped graphene layers, which can be tuned further for a variety of other applications.

examined using the carbon paper as both the working electrode (catalyst loaded) and the counter electrode (Figure S11). This system also shows good HER performance, with an onset potential of ∼82 mV and long-term stability.67 In order to rationalize the Co-alloying with Fe in the core region and N-doping in graphene toward the electrocatalytic surface area, electrochemical double-layers capacitance (Cdl), which is directly proportional to the electrochemically active surface area (ECSA), is evaluated for all CoxFe1−x@N-G structures. The ECSA has been determined by CV studies in the non-redox regime in 1 M KOH solution with different scan rates (Figure 5), followed by a plot of difference in current density (difference between cathodic and anodic currents) with the scan rate. Figure 5a−d displays the CVs of X0, X1, X2, and X3. Figure 5e displays the Cdl of all the hybrid nanostructures. It can be seen that Cdl of X3 is 19 mF/cm2, whereas Cdl = 8.5, 14.5, and 12 mF/cm2 for X0, X1, and X2, respectively. This result indicates that X3 has the best electrochemically active sites toward HER due to high Ndoping as well as Co-alloying with the Fe that possesses high ECSA as compared to other hybrids, leading to better ORR and HER performance. Generally, the catalytic conversion of 4-NP to 4-AP is carried out in the presence of NaBH4. During the addition of NaBH4 to 4-NP, the color of the solution changes from pale yellow to dark greenish yellow due to the formation of 4nitrophenolate ions, and when the catalyst is added the solution changes from greenish yellow to colorless. The conversion of 4-NP ions to 4-AP is monitored by UV−visible spectroscopy. Figure 6a−c displays the successive reduction of 4-NP to 4-AP by X0, X1, and X3, respectively. The catalytic performance of 4-NP reduction of X2 is provided in Figure S12. The absorbance peak at ∼400 nm corresponding to 4-NP decreases, while the peak at ∼296 nm corresponding to 4-AP increases. It can be noted that X3 exhibits better activity as compared to the others, and it takes only 3 min to complete the conversion. This is believed to be due to the high atom-% N-doping in graphene as well as the presence of Co, as is the case with ORR activity. The apparent rate constant (kapp) of the reaction is estimated by the pseudo-first-order rate kinetic relation: −ln(At/A0) = kappt, where At and A0 are the absorbance values at times t and 0 (initial value), respectively, as shown in Figure 6d. The kapp values of the catalysts are 0.406, 0.52, 0.46, and 0.71 min−1 for X0, X1, X2, and X3, respectively. Interestingly, the rate constant is high for X3, even as compared with the noble metal (Pt, Ag, Au, and its hybrids)-assisted reduction of 4-NP.35−45 One of the advantages of the present catalysts is that they can be recovered by magnetic separation (Figure 6e). Furthermore, the recycling test of X3 indicates that the time of complete conversion of 4-NP to 4-AP only increased from 3 to 3.5 min from the first to the fifth cycle, suggesting the robustness of the catalyst (Figure 6f). Overall, the developed catalyst exhibits far superior activity as compared to Pt-based materials (typical reduction time is 10−15 min) toward the conversion of 4-NP into 4-AP.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b01861. SEM images, EDX and XPS spectra, cyclic voltammograms, and LSV, including Figures S1−S12 and Table S1 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-080-2293 2996. Fax: +91-80-2360 7316. E-mail: [email protected]. ORCID

Barun Kumar Barman: 0000-0002-9894-1890 Karuna Kar Nanda: 0000-0001-9496-1408 Author Contributions

B.K.B. and K.K.N. designed the experiments, and B.K.B. performed the experiments. Both B.K.B. and K.K.N. analyzed the data and wrote the paper. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the Council of Scientific and Industrial Research (CSIR), India, for financial support. The authors also acknowledge the Chemical Science Division of IISc for providing access to the FETEM facility. Dr. Sanjoy Mukherjee is acknowledged for helping with the preparation of the schematics.

■

REFERENCES

(1) Steele, B. C. H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, 345−352. (2) Shao, Y.; Cheng, Y.; Duan, W.; Wang, W.; Lin, Y.; Wang, Y.; Liu, J. Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow Batteries: A Selected Review. ACS Catal. 2015, 5, 7288−7298. (3) Goswami, G.; Nandan, R.; Barman, B. K.; Nanda, K. K. Excellent performance of Pt-free cathode in alkaline direct methanol fuel cell at room temperature. J. Mater. Chem. A 2013, 1, 3133−3139. (4) Raj, C. R.; Samanta, A.; Noh, S. H.; Mondal, S.; Okajima, T.; Ohsaka, T. Emerging new generation electrocatalysts for the oxygen reduction reaction. J. Mater. Chem. A 2016, 4, 11156−11178. (5) Liu, X.; Dai, L. Carbon-based metal-free catalysts. Nature Reviews Materials 2016, 1, 16064. (6) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652−657. (7) Li, Y.; Dai, H. Recent advances in zinc-air batteries. Chem. Soc. Rev. 2014, 43, 5257−5275.

■

CONCLUSION We have demonstrated a facile approach to synthesize nonprecious-metal nanoalloys encapsulated in N-doped graphene (CoxFe1−x/N-G) by pyrolyzing a mixture of Prussian blue and cobaltocene/cobalt nitrate. For the first time, the roles of both Co in the CoFe core and N-doping in graphene are elucidated. 12743


Research Article

ACS Sustainable Chemistry & Engineering (8) Li, X.; Faghri, A. Review and Advances of Direct Methanol Fuel Cells (DMFCs) Part I: Design, Fabrication, and Testing with High Concentration Methanol Solutions. J. Power Sources 2013, 226, 223− 240. (9) Blurton, K.; Sammells, A. F. Metal/Air Batteries: Their Status and Potential - A Review. J. Power Sources 1979, 4, 263−269. (10) Nandan, R.; Nanda, K. K. J. Mater. Chem. A 2017, 5, 16843− 16853. (11) Cheng, F.; Chen, J. Metal-air batteries: from oxygen reduction electrochemistry to cathode catalysts. Chem. Soc. Rev. 2012, 41, 2172−2192. (12) Soloveichik, G. L. Flow Batteries: Current Status and Trends. Chem. Rev. 2015, 115, 11533−11558. (13) Wu, S.; Zhu, K.; Tang, J.; Liao, K.; Bai, S.; Yi, J.; Yamauchi, Y.; Ishida, M.; Zhou, H. A long-life lithium ion oxygen battery based on commercial silicon particles as the anode. Energy Environ. Sci. 2016, 9, 3262−3271. (14) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A comprehensive review on PEM water electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901−4934. (15) Hisatomi, T.; Kubota, J.; Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43, 7520−7535. (16) Barman, B. K.; Nanda, K. K. Ultrafast-Versatile-DomesticMicrowave-Oven Based Graphene Oxide Reactor for the Synthesis of Highly Efficient Graphene Based Hybrid Electrocatalysts. ACS Sustainable Chem. Eng. 2018, 6, 4037−4045. (17) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 9351−9355. (18) Hu, C.; Dai, L. Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the ORR. Angew. Chem., Int. Ed. 2016, 55, 11736−11758. (19) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal-Organic Framework. ACS Nano 2015, 9, 6288−6296. (20) Deng, J.; Ren, P.; Deng, D.; Bao, X. Enhanced electron penetration through an ultrathin graphene layer for highly efficient catalysis of the hydrogen evolution reaction. Angew. Chem., Int. Ed. 2015, 54, 2100−2104. (21) Cui, X.; Ren, P.; Deng, D.; Deng, J.; Bao, X. Single layer graphene encapsulating non-precious metals as high-performance electrocatalysts for water oxidation. Energy Environ. Sci. 2016, 9, 123− 129. (22) Barman, B. K.; Nanda, K. K. A noble and single source precursor for the synthesis of metal-rich sulphides embedded in an Ndoped carbon framework for highly active OER electrocatalysts. Dalton Trans. 2016, 45, 6352−6356. (23) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat. Nanotechnol. 2015, 10, 444−452. (24) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X. Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 371−375. (25) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc. 2010, 132, 13612−13614. (26) Barman, B. K.; Nanda, K. K. Prussian blue as a single precursor for synthesis of Fe/Fe3C encapsulated N-doped graphitic nanostructures as bi-functional catalysts. Green Chem. 2016, 18, 427−432. (27) Deng, J.; Yu, L.; Deng, D.; Chen, X.; Yang, F.; Bao, X. Highly active reduction of oxygen on a FeCo alloy catalyst encapsulated in pod-like carbon nanotubes with fewer walls. J. Mater. Chem. A 2013, 1, 14868−14873. (28) Muthukrishnan, A.; Nabae, Y.; Okajima, T.; Ohsaka, T. Kinetic Approach to Investigate the Mechanistic Pathways of Oxygen

Reduction Reaction on Fe-Containing N-Doped Carbon Catalysts. ACS Catal. 2015, 5, 5194−5202. (29) Tan, H.; Li, Y.; Jiang, X.; Tang, J.; Wang, Z.; Qian, H.; Mei, P.; Malgras, V.; Bando, Y.; Yamauchi, Y. Perfectly Ordered Mesoporous Iron-Nitrogen Doped Carbon as Highly Efficient Catalyst for Oxygen Reduction Reaction in Both Alkaline and Acidic Electrolytes. Nano Energy 2017, 36, 286−292. (30) Tang, J.; Liu, J.; Li, C.; Li, Y.; Tade, M. O.; Dai, S.; Yamauchi, Y. Synthesis of Nitrogen-Doped Mesoporous Carbon Spheres with Extra-Large Pores through Assembly of Diblock Copolymer Micelles. Angew. Chem., Int. Ed. 2015, 54, 588−593. (31) Wen, Z.; Ci, S.; Zhang, F.; Feng, X.; Cui, S.; Mao, S.; Luo, S.; He, Z.; Chen, J. Nitrogen-Enriched Core-Shell Structured Fe/Fe3C-C Nanorods as Advanced Electrocatalysts for Oxygen Reduction Reaction. Adv. Mater. 2012, 24, 1399−1404. (32) Wang, X.; Orikasa, Y.; Uchimoto, Y. Platinum-Based Electrocatalysts for the Oxygen-Reduction Reaction: Determining the Role of Pure Electronic Charge Transfer in Electrocatalysis. ACS Catal. 2016, 6, 4195−4198. (33) Li, C.; Sato, T.; Yamauchi, Y. Electrochemical Synthesis of One-Dimensional Mesoporous Pt Nanorods Using the Assembly of Surfactant Micelles in Confined Space. Angew. Chem., Int. Ed. 2013, 52, 8050−8053. (34) Yang, J.; Sun, H.; Liang, H.; Ji, H.; Song, L.; Gao, C.; Xu, H. A Highly Efficient Metal-Free Oxygen Reduction Electrocatalyst Assembled from Carbon Nanotubes and Graphene. Adv. Mater. 2016, 28, 4606−4613. (35) Jiang, B.; Li, C.; Malgras, V.; Imura, M.; Tominaka, S.; Yamauchi, Y. Mesoporous Pt nanospheres with designed pore surface as highly active electrocatalyst. Chem. Sci. 2016, 7, 1575−1581. (36) Hoque, M. A.; Hassan, F. M.; Jauhar, A. M.; Jiang, G.; Pritzker, M.; Choi, J.-Y.; Knights, S.; Ye, S.; Chen, Z. Web-like 3D Architecture of Pt Nanowires and Sulfur-Doped Carbon Nanotube with Superior Electrocatalytic Performance. ACS Sustainable Chem. Eng. 2018, 6, 93−98. (37) Pradhan, N.; Pal, A.; Pal, T. Silver nanoparticle catalyzed reduction of aromatic nitro compounds. Colloids Surf., A 2002, 196, 247−257. (38) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Catalytic Activity of Palladium Nanoparticles Encapsulated in Spherical Polyelectrolyte Brushes and Core-Shell Microgels. Chem. Mater. 2007, 19, 1062−1069. (39) Barman, B. K.; Nanda, K. K. Uninterrupted galvanic reaction for scalable and rapid synthesis of metallic and bimetallic sponges/ dendrites as efficient catalysts for 4-nitrophenol reduction. Dalton Trans. 2015, 44, 4215−4222. (40) Barman, B. K.; Nanda, K. K. The dual role of Zn−acid medium for one-step rapid synthesis of M@rGO (M = Au, Pt, Pd and Ag) hybrid nanostructures at room temperature. Chem. Commun. 2013, 49, 8949−8951. (41) Mogudi, B. M.; Ncube, P.; Bingwa, N.; Mawila, N.; Mathebula, S.; Meijboom, R. Appl. Catal., B 2017, 218, 240−248. (42) Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes. J. Phys. Chem. C 2010, 114, 8814−8820. (43) Wang, H.; Dong, Z.; Na, C. Hierarchical Carbon Nanotube Membrane-Supported Gold Nanoparticles for Rapid Catalytic Reduction of p-Nitrophenol. ACS Sustainable Chem. Eng. 2013, 1, 746−752. (44) Li, Y.; Zhang, Z.; Shen, J.; Ye, M. Hierarchical nanospheres based on Pd nanoparticles dispersed on carbon coated magnetite cores with a mesoporous ceria shell: a highly integrated multifunctional catalyst. Dalton Trans. 2015, 44, 16592−16601. (45) El-Sheikh, S. M.; Ismail, A. A.; Al-Sharab, J. F. Catalytic reduction of p-nitrophenol over precious metals/highly ordered mesoporous silica. New J. Chem. 2013, 37, 2399−2407. 12744


Research Article

ACS Sustainable Chemistry & Engineering (46) Pandey, S.; Mishra, S. B. Catalytic reduction of p-nitrophenol by using platinum nanoparticles stabilised by guar gum. Carbohydr. Polym. 2014, 113, 525−531. (47) Lin, X.; Wu, M.; Wu, D.; Kuga, S.; Endo, T.; Huang, Y. Platinum nanoparticles using wood nanomaterials: eco-friendly synthesis, shape control and catalytic activity for p-nitrophenol reduction. Green Chem. 2011, 13, 283−287. (48) Cheng, Z.; Liao, J.; He, B.; Zhang, F.; Zhang, F.; Huang, X.; Zhou, L. One-Step Fabrication of Graphene Oxide Enhanced Magnetic Composite Gel for Highly Efficient Dye Adsorption and Catalysis. ACS Sustainable Chem. Eng. 2015, 3, 1677−1685. (49) Hu, C.; Dai, L. Multifunctional Carbon-Based Metal-Free Electrocatalysts for Simultaneous Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution. Adv. Mater. 2017, 29, 1604942. (50) Lukowski, M. A.; Daniel, A. S.; English, C. R.; Meng, F.; Forticaux, A.; Hamers, R. J.; Jin, S. Highly active hydrogen evolution catalysis from metallic WS2 nanosheets. Energy Environ. Sci. 2014, 7, 2608−2613. (51) Cheng, L.; Huang, W. J.; Gong, Q. F.; Liu, C. H.; Liu, Z.; Li, Y. G.; Dai, H. J. Ultrathin WS2 Nanoflakes as a High-Performance Electrocatalyst for the Hydrogen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53, 7860−7863. (52) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (53) Tian, J. Q.; Liu, Q.; Asiri, A. M.; Sun, X. P. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0−14. J. Am. Chem. Soc. 2014, 136, 7587−7590. (54) Das, D.; Nanda, K. K. One-step, integrated fabrication of Co2P nanoparticles encapsulated N, P dual-doped CNTs for highly advanced total water splitting. Nano Energy 2016, 30, 303−311. (55) Barman, B. K.; Das, D.; Nanda, K. K. Facile and one-step synthesis of a free-standing 3D MoS2-rGO/Mo binder-free electrode for efficient hydrogen evolution reaction. J. Mater. Chem. A 2017, 5, 18081−18087. (56) Yang, Y.; Lun, Z.; Xia, G.; Zheng, F.; He, M.; Chen, Q. Nonprecious alloy encapsulated in nitrogen-doped graphene layers derived from MOFs as an active and durable hydrogen evolution reaction catalyst. Energy Environ. Sci. 2015, 8, 3563−3571. (57) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 2009, 324, 71−74. (58) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. HighPerformance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443−447. (59) Hu, M.; Belik, A. A.; Imura, M.; Yamauchi, Y. Tailored Design of Multiple Nanoarchitectures in Metal-Cyanide Hybrid Coordination Polymers. J. Am. Chem. Soc. 2013, 135, 384−391. (60) Hou, Y.; Huang, T.; Wen, Z.; Mao, S.; Cui, S.; Chen, J. Metal− Organic Framework-Derived Nitrogen-Doped Core-Shell-Structured Porous Fe/Fe3C@C Nanoboxes Supported on Graphene Sheets for Efficient Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4, 1400337. (61) Zafar, Z.; Ni, Z. H.; Wu, X.; Shi, Z. X.; Nan, H. Y.; Bai, J.; Sun, L. T. Evolution of Raman spectra in nitrogen doped graphene. Carbon 2013, 61, 57−62. (62) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (63) An, L.; Jiang, N.; Li, B.; Hua, X.; Fu, F.; Liu, J.; Hao, W.; Xia, D.; Sun, Z. A highly active and durable iron/cobalt alloy catalyst encapsulated in N-doped graphitic carbon nanotubes for oxygen reduction reaction by a nano-fibrous Dicyandiamide template. J. Mater. Chem. A 2018, 6, 5962−5970.

(64) Zhou, R.; Zheng, Y.; Jaroniec, M.; Qiao, S.-Z. Determination of the Electron Transfer Number for the Oxygen Reduction Reaction: From Theory to Experiment. ACS Catal. 2016, 6, 4720−4728. (65) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. NitrogenDoped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760−764. (66) Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170−11176. (67) Tian, M.; Cousins, C.; Beauchemin, D.; Furuya, Y.; Ohma, A.; Jerkiewicz, G. Influence of the working and counter electrode surface area ratios on the dissolution of platinum under electrochemical conditions. ACS Catal. 2016, 6, 5108−5116.

12745


CoFe Nanoalloys Encapsulated in N-Doped Graphene Layers as a Pt

Recommend Documents