The research on anion-exchange membrane fuel cells (AEMFCs) is motivated by the ability of alkaline systems to improve their efficiencies without the use of precious metal catalysts, which is the major limitation for the widespread implementation of proton-exchange membrane fuel cells [1, 2]. Furthermore, in alkaline medium, AEMFCs have the additional advantages of faster electrode reaction kinetics and flexible fuel choices [3-5]. The development of low-cost oxygen reduction reaction (ORR) catalysts is one of the major challenges for electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries. In recent studies on transition metal oxides, conducting-polymer-derived materials have been intensively investigated as ORR catalysts because of their greater abundance and lower cost compared with Pt-based catalysts [6-10]. However, most non-precious metal (NPM) catalysts exhibit insufficient ORR activity and stability, which limits their wide application in fuel cell technologies. A better understanding of the nature of the ORR on the catalytic sites and the design of new catalysts with optimal activity and durability are needed.
NPM catalysts include non-noble-metal-based oxides and chalcogenides. Jasinski [11] was the first to report that Pt-free metal phthalocyanines (also called metal-N4 macrocycles, such as Fe-and Co-macrocycles) could catalyze the ORR as effectively as Pt-based materials. Ever since this seminal work, various other N-coordinated transition metals and metal chalcogenides, oxides, oxynitrides, carbonitrides, and transition-metal-doped conductive polymers have been explored, and their potential to catalyze the ORR and other reactions has been evaluated. Among these materials, state-of-the-art nitrogen-doped graphene oxides (N/GOs) with or without transition metals (Fe or Co) are generally accepted as potential substitutes for Pt for both cost reduction and enhancement of the stability of ORR electrocatalysts [7, 12-15]. Deliberate nitrogen doping can enhance the electron-donor property of the graphene or carbon matrix, resulting in an improvement of the interaction between carbon and guest molecules. However, the development of a convenient synthesis method for N/GO catalysts remains a challenge [16-20]. Novel Co or Fe composites containing N/GO with high electrical conductivity and a stable structure are also highly desirable for further improvement of the efficiency, stability, and electrochemical activity, as reported in previous investigations [12, 21-29].
To date, various strategies to introduce N atoms into GO have been intensively employed. Specifically, NH3, conducting polymers, and methane are effective nitrogen-doping agents [30-33]. Polypyrrole (Ppy) is a well-known electrically conducting polymer and has been used for surface modification of carbon particles, enabling the immobilization of Co ions at the surface via coordination processes [16, 17, 34]. In this work, Ppy was selected as a multifunctional agent, serving as a crosslinker of Co2+ ions with GO and especially as the precursor of nitrogen for the doping of graphene. In our previous work [35], Fe2O3/Ppy/GO composites were synthesized using a hydrothermal method, and their effect on the ORR performance was investigated. The decisive factors for improving the electrocatalytic and durable performance of Fe2O3/Ppy/GO were the intimate, large interfaces between the Fe2O3 nanocrystals and Ppy/GO and the high electron withdrawing/storing ability and high conductivity of GO doped with nitrogen from Ppy. In this work, novel Co and N co-doped graphene networks (Co3O4/Ppy/GO) were prepared by the simple pyrolysis of a mixture of pyrroles, cobalt(Ⅱ) nitrate, and GO. The as-prepared Co3O4/Ppy/GO catalysts exhibited comparable catalytic activity and high selectivity for the four-electron ORR in alkaline solutions.
GO was prepared using the modified Hummers' method [36]. First, 3 g of expandable graphite and 18 g of KMnO4 were added to a 1-L beaker containing 360 mL of concentrated sulfuric acid and 40 mL of H3PO4. The beaker was placed in an ice-water bath (0 ℃) and continuously stirred until the graphite and KMnO4 were completely dissolved. After cooling the system to ambient temperature, 400 mL of ice water was added to the beaker to keep the solution at 0 ℃. Then, 30 mL of 30% H2O2 solution was added to the solution with continuous stirring for 4 h. Finally, the suspension was centrifuged and washed several times with deionized water, 30% HCl, and ethanol until a pH of approximately 7 was attained. The black homogeneous supernatant was dried at 30 ℃ for 48 h to obtain GO.
The Ppy/GO suspension was prepared according to previously reported procedures [35]. The solution was prepared by mixing 0.2 g GO and 20 mL of H2O. Another solution of 4 mL of ethanol and 2 mL of pyrrole was then slowly added into the GO solution under continuous stirring. Then, 2 mL of 30% H2O2 was added and stirred for 30 min. Finally, the mixture was transferred to an autoclave and stored at 180 ℃ for 12 h. The resulting product was obtained by washing with water and ethanol five times. The clear product was dried at 60 ℃ for 24 h in a vacuum drying oven.
The Co3O4/Ppy/GO catalysts were prepared according to previously reported procedures [35]. Briefly, 0.2 g GO was dispersed in 20 mL of water with ultrasonic processing for 1 h to obtain a GO suspension. Then, 4 mL of ethanol and 2 mL of pyrrole were added into the GO solution to form a composite dispersion under vigorous stirring for 30 min. Next, 4 mL of 0.069 mol/L Co(NO3)2 was dissolved into the composite solution. The composite solution was then transferred into an autoclave and stored at 180 ℃ for 12 h. A powder was obtained by washing with water and ethanol three times. The clear product was dried at 60 ℃ for 24 h in a vacuum drying oven to obtain Co3O4/Ppy/GO hybrids. After that, the resulting hybrids were placed into a tube furnace and heated to 400, 600, or 800 ℃ for 3 h under N2 flow. The resulting catalysts were labeled Co3O4/Ppy/GO-400, Co3O4/Ppy/GO-600, and Co3O4/Ppy/ GO-800, respectively.
The morphology and structure of the as-prepared Co3O4/Ppy/GO composites were investigated using scanning electron microscopy (SEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Morphological characterization was performed using SEM (QUANTA-450), and all the samples were measured after gold sputter coating. The FT-IR spectroscopy analysis was performed using a Bruker-Tenson 27 FT-IR spectrometer in the 4000-400 cm−1range to determine the characteristic functional groups of the samples. XPS data were obtained using a Thermo ESCALAB 250XI multifunctional imaging electron spectrometer with a monochromic Al X-ray source. TGA was performed using a TA Instruments TGA-Q50 in a nitrogen flow (40 mL/min) with a linear heating rate of 10 ℃/minto evaluate the thermal stabilities of the samples.
A glassy carbon electrode (GCE, 5 mm in diameter, geometric area of 0.197 cm2) was used as the working electrode fixed on a rotating apparatus. The GCE was polished with 0.05-mm alumina slurries and sonicated in deionized water three times before the electrochemical measurements. Then, 10 μL of catalyst ink, prepared by dispersing 2 mg of the catalyst into 1 mL of ethanol containing 10 μL 5% Nafion, was deposited on the working electrode with a catalyst loading of 0.1 mg/cm2.
Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed on a ZAHNER ZENNIUM electrochemical workstation with a three-electrode system. A glassy carbon rotating disk electrode (RDE) with a 5-mm diameter loaded with as-prepared GO, Ppy/GO, and Co3O4/Ppy/GO was used as the working electrode. A Pt gauze electrode was employed as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. All the electrode potentials here are reported versus the SCE. During the electrocatalytic measurements, the working electrode loaded with the as-prepared catalysts was continuously electrochemically looped. O2 was bubbled into the electrolyte for 10 min before each experiment, and a flow of O2 was maintained over the electrolyte during the measurements to ensure O2 saturation. The CV and LSV measurements were performed at a scan rate of 10 mV/s.
The slopes of Koutecky-Levich (K-L) plots were used to calculate the number of transfer electrons according to Eqs. (1) and (2), where J, JK, and JL are the measured current density, kinetic current density, and diffusion-limiting current density, respectively, n is the number of transfer electrons in the ORR, F is the Faraday constant (96487 C/mol), C0 is the concentration of molecular oxygen in the 0.1 mol/L KOH electrolyte (1.117 × 10−6 mol/mL), D0 is the diffusion coefficient of O2 (1.9 × 10−5 cm2/s), n is the kinetic viscosity of the electrolyte (0.01073 cm2/s for alkaline solution), ω is the angular velocity in rad/s [37].
For the Tafel (E - log (JK)) plot, the kinetic current was calculated from the mass-transport correction of the RDE using the following equation [12, 38, 39]:
The SEM images of the GO and Ppy/GO nanocomposites in Fig. 1(a) and (b) reveal typical layered structures, similar to the layered structure of pure graphene in terms of regularity. The corresponding energy-dispersive X-ray spectroscopy (EDS) spectrum of Ppy/GO in Fig. 1(c) indicates that the systems contained 3.03 at% N. After treatment of Ppy/GO in N2 for 2 h at 600 ℃, the resulting Ppy/GO-600 still exhibited a layered structure, as observed in Fig. 1(d). The images of Co3O4/Ppy/GO in Fig. 1(e) and (f) indicate that Co3O4 and Ppy were inserted into adjacent GO layers, resulting in large interspaces between GO layers with a thickness range of 10-200 nm. The corresponding XPS spectrum of Co3O4/Ppy/GO in Fig. 2 indicates the presence of 1.3 at% Co and 8.5 at% N in the systems.
The XPS analysis of Co3O4/Ppy/GO in Fig. 2(a) reveals mainly C, O, N, and Co, as indicated by the presence of the C 1s, O 1s, N 1s, and Co 2p core levels, with no evidence of impurities. A clear N 1s peak is observed, confirming the presence of N (Fig. 2(b)). The N 1s peak can be divided into three components located at 398.4, 399.0, and 400.4 eV, which correspond to pyridinic, pyrrolic, and graphitic nitrogen, respectively [14, 16, 40]. The estimated N doping level was 8.5 at%, of which 59.48% was pyrrolic nitrogen, indicating that the main form of nitrogen was pyrrolic nitrogen. Importantly, the presence of graphitic N is an indicator of the in situ formation of N-doped graphitic carbon.
As shown in Fig. 2(c), two sharp peaks at 780.1 and 794.9 eV were detected for the Co3O4 samples, which correspond to the Co 2p1/2 and Co 2p3/2 spin-orbit peaks of Co3O4 spinel, respectively. The XPS spectra of the O 1s electronic levels of the Co3O4 samples are presented in Fig. 2(d). Two component peaks are observed, which indicates the presence of two types of oxygen species on the surface of the Co3O4 samples. The peak with low binding energy (529.8 eV) can be assigned to the lattice oxygen O2− [38]. The signal at 531.3 eV indicates the presence of surface hydroxyl (OH−) species, which most likely led to the generation of Co-OH. The higher concentration of surface hydroxyl ions (OH−) than lattice oxygen in the Co3O4 sample indicates that the addition of Co3O4 improves the ORR performance [41, 42]. The detailed analysis results of each surface species are presented in Table 1.
As observed in Fig. 3, the FT-IR spectrum of GO contains strong bands centered at approximately 3405, 1550, and 1734 cm−1, which are attributed to O-H, carbon double bonds (C=C), and strong carbonyl (C=O) stretching vibration, respectively. The peaks at approximately 1056, 1125, and 1250 cm−1 can be assigned to the epoxy C-O stretching vibration, C-OH stretching vibration, and carboxy C-O stretching vibration, respectively [43].
With the polymerization process of Ppy within or on the GO film, the peaks related to C=C (at approximately 1550 cm−1) and C=O (1734 cm−1) stretching vibration were relatively weakened. The epoxy C-O, C-OH, and carboxy C-O stretching vibration almost disappeared, suggesting the introduction of GO. Several new peaks at approximately 1452 cm−1 can be observed on Ppy/GO, which can be assigned to symmetric stretching modes of the pyrrole ring, further demonstrating the existence of Ppy in the samples [43-46]. The strong and sharp absorption peaks at ~750 and ~800 cm−1 are attributed to the vibrational modes of Co-O and Co=O bonds in Co3O4 [42].
TGA curves of GO, Ppy/GO, and Co3O4/Ppy/GO under a N2 atmosphere are presented in Fig. 4. GO showed considerable ( > 40%) mass loss before 250 ℃ because of the low molecular mass and volatile feature of the adsorbed H2O molecules. The mass loss of Ppy/GO upon heating to 800 ℃ was 40%, which was slightly higher than that of Co3O4/Ppy/GO (25%). Nevertheless, the slow mass loss indicates the high stability of GO/Ppy and Co3O4/Ppy/GO resulting from the Ppy bonding effect between GO and Co3O4. This high stability is beneficial for the use of these composites as durable ORR catalysts. Co3O4/Ppy/GO was subsequently pyrolyzed at various temperatures (400, 600, and 800 ℃) in an inert atmosphere for 2 h to produce N-doped porous Co3O4 anchored on GO network materials.
The GO sample was selected as a tentative catalyst to examine the ORR activity. The electrocatalytic activity of GO for the ORR was evaluated by CV scanning in KOH aqueous solution (0.1 mol/L) saturated with either N2 or O2 gas at room temperature. As observed in Fig. 5(a), the CV scan of GO was featureless in N2-saturated KOH (line (1)). The sample exhibited a dramatic increase in current density from the potential −0.18 V vs. SCE in an O2-saturated electrolyte compared with that in a N2-saturated electrolyte (line (2) in Fig. 5(a)). The cathodic reduction current increase near −0.18 V vs. SCE in the O2 saturated electrolyte corresponds to the electrochemical reduction of oxygen molecules [47].
The impressive electrocatalytic activity of GO was confirmed by the LSV curves on a RDE (Fig. 5(b)). The potential at a current density of 0.1 mA/cm2, also known as the onset potential [7], was −0.31 V vs. SCE, and the limiting current density at −1.0 V vs. SCE for 1600 r/min reached approximately 0.36 mA/cm2, which is higher than that of GO-based materials [48]. However, GO alone exhibited very distinct ORR activity.
The Ppy/GO could impart electrocatalytic activities toward the ORR and was compared with the metal oxides supported on it. A well-defined O2 reduction peak centered at −0.38 V vs. SCE emerged as the electrolyte solution became saturated with O2 (the curve (2) in Fig. 6(a)), which is close to the value of other N-doped GO catalysts in alkaline solution [15, 49, 50]. As observed in Fig. 6(b), the onset potential of the ORR for the Ppy/GO electrode was −0.20 V vs. SCE with the cathodic reduction peak at approximately −0.38 V vs. SCE. As observed in Fig. 7, the cathodic reduction peak started at approximately 0.10 V vs. SCE (Fig. 7(a)), and the onset potential was at −0.10 V vs. SCE (Fig. 7(b)) for the ORR for the Ppy/GO-600 electrode. These findings indicate that nitrogen doping and high-temperature pyrolysis are the primary contributors to the ORR performance, forming active sites and facilitating electron or charge transfer on the electrodes [20].
After the Co3O4 nanoparticles were assembled, the Co3O4/Ppy/GO catalysts were heated to 400, 600, or 800 ℃. To efficiently analyze the effect of temperature on the ORR performance, a comparison between the RDE voltammograms for Co3O4/Ppy/GO at a rotation rate of 1600 r/min was made. As observed in Fig. 8(a), the onset potentials of the catalysts at 0.1 mA/cm2 were −0.20, −0.18, −0.10, and −0.10 V vs. SCE for Co3O4/Ppy/GO, Co3O4/Ppy/GO-400, Co3O4/Ppy/GO-600, and Co3O4/Ppy/GO-800, respectively. In addition, the diffusion limiting current density for Co3O4/Ppy/GO-800 was higher than those for Co3O4/Ppy/GO, Co3O4/Ppy/GO-400, and Co3O4/Ppy/ GO-600. These results indicate that pyrolytic carbonization of Co3O4/Ppy/GO (a necessary step in generating active N-doped or Co, N-co-doped carbon species) requires high temperatures.
In Fig. 8(b), the CVs of the Co3O4/Ppy/GO-800 catalyst show a reductive peak at −0.18 V vs. SCE in O2-saturated KOH (0.1 mol/L), which is close to the value of −0.18 V for commercial carbon-supported Pt and superior to those of other reduced GO-based catalysts [12]. Therefore, the enhanced ORR on Co3O4/Ppy/GO is due to the excellent dispersion of Co3O4 nanoparticles with smaller size and the nitrogen doping effect on GO sheets.
To obtain further insight into the ORR involving the Co3O4/Ppy/GO-800, LSV curves on a RDE at various rotation rates were constructed. Fig. 9(a) demonstrates that the onset potential for Co3O4/Ppy/GO-800 at various rotation rates was almost constant, and more importantly, at a rotation rate of 1600 r/min, Co3O4/Ppy/GO-800 exhibited a comparable limiting current density to that of a Pt/C catalyst. These results suggest that the ORR catalytic activity of Co3O4/Ppy/GO is closer to that of commercial Pt/C catalysts.
The linearity of the K-L plots (Fig. 9(b)) and near parallelism of the fitting lines suggest first-order reaction kinetics for the concentration of dissolved oxygen and similar electron transfer numbers for the ORR at different potentials [37, 51]. The electron-transfer number was calculated to be approximately 3.4 between −0.4 V and approximately −0.6 V vs. SCE based on the slopes, indicating that the ORR at the Co3O4/Ppy/GO-800 electrode was selected by an approximately four-electron reduction pathway. The Tafel slope calculation was used to analyze the ORR catalyzed by Co3O4/Ppy/GO-800 at 1600 r/min, and the kinetic parameters of the reaction were obtained (Fig. 9(c) and (d)). The Tafel slopes (∂η/∂ log JK) for Co3O4/Ppy/GO-800 in the overpotential ranges of −0.2 to −0.3 V vs. SCE and −0.3 to −0.6 V vs. SCE were calculated to be 100 and 160 mV/dec, respectively. These values indicate that the first step involving oxygen adsorption is fast, whereas the reduction of the hydrogen peroxide intermediate is relatively slower. Both these values differed from the Tafel slopes for the ORR on Pt (60 and 120 mV/dec at potentials higher and lower than 0.8 V vs RHE, respectively) [52]. The Tafel slopes for the Pt/C catalyst suggest that the smaller Tafel slope in the low-overpotential region corresponds to the ORR on a Pt surface covered with an oxide and the large slope in the high-overpotential region corresponds to that on a clean Pt surface [53]. For Co3O4-Ppy/GO-800, the Tafel slope of 100-160 mV/dec is indicative of a more complicated ORR mechanism, with the rate-determining step likely involving both high activation energy for O-O bond splitting and extremely weak adsorption of the H2O2 intermediate molecule on the Co-based active sites. The difference in the Tafel slopes for Co3O4-Ppy/GO and Pt/C also implies the different nature of the active ORR site in these two cases. The detailed electrocatalytic activities of the catalysts are listed in Table 2.
To further analyze the experimental results obtained in this study, we used simplified reaction pathways (Scheme 1) [54, 55]. Path A shows how O2 is reduced directly into H2O through a four-electron transfer. Path B represents the sequential reaction path wherein O2 is first reduced to H2O2 through a two-electron transfer, followed by a two-electron reduction to H2O (Path C) or the release of the formed H2O2 into the bulk solution (Path C'). The latter is less desirable because it is inefficient for fuel conversion; in addition, the H2O2 it produces can lead to corrosion of many types of metal-based catalysts. For Co3O4/Ppy/GO, the ORR reaction can proceed with the high selectivity of path A; that is, the catalyst yields the more desirable reduced product, H2O, during the ORR.
Based on these observations, it is postulated that the presence of cobalt oxide assembled nitrogen-GO could be the main contributor to the remarkable capability of Co3O4/Ppy/GO to catalyze the ORR with high activity. In our study, the cobalt oxide assembled nitrogen-GO hybrid material was Ppy-stabilized Co3O4 clusters embedded in N-doped graphitic carbon materials (Scheme 2). The Ppy chains not only stabilized the Co3O4 centers but also aided electron transfer, facilitating the ORR process. The N-doped graphitic carbon formed in situ not only directly served as the active site for improved intrinsic activity but also provided robust support to anchor the metal active sites for enhanced corrosion resistance to oxidative attack during the ORR.
In this study, Co3O4/Ppy/GO nanocomposites were prepared, and their performances as ORR electrocatalysts were evaluated and compared with those of GO and Ppy/GO. Unexpectedly, Co3O4/Ppy/GO exhibited excellent electrocatalytic performance in terms of higher catalytic activity and selectivity for the ORR in alkaline media. This finding can be attributed to the bridging of Co3O4 nanoparticles and GOs with Ppy as well as the effective N doping of GO. The fine distribution of Co species and their excellent activity in the ORR will enable the wide application of Co3O4 in fuel cells and other energy conversion devices.
2017,
Vol. 38 Issue (7): 1281-1290
PDF
2017,
Vol. 38 Issue (7): 1281-1290 PDF