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Enhanced catalytic activity for the oxygen reduction reaction with co-doping of phosphorus and iron in carbon
Journal of Power Sources 277 (2015) 161e168
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Enhanced catalytic activity for the oxygen reduction reaction with codoping of phosphorus and iron in carbon Zhenrong Yang, Jiao Wu, Xiangjun Zheng, Zhangjun Wang, Ruizhi Yang* College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215006, China
h i g h l i g h t s The The The The
P and Fe co-doped carbon (PeFeeC) is facilely synthesized. as-prepared PeFeeC shows high catalytic activity for the ORR. catalytic site of PeFeeC for the ORR is proposed as FeePx. pyrolysis temperature plays an important role in the structure and activity of PeFeeC.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2014 Received in revised form 10 November 2014 Accepted 4 December 2014 Available online 5 December 2014
The phosphorus (P) and iron (Fe) co-doped carbon (PeFeeC) has been facilely synthesized via an in situ solegel polymerization method followed by a pyrolysis process using resorcinol and formaldehyde as the carbon source, tetraphenylphosphonium bromide as the P source and iron nitrate as the Fe source. Benefiting from the high doping content of P and Fe and large specific surface area, the as-prepared PeFe eC electrocatalysts demonstrate considerable catalytic activity for the oxygen reduction reaction (ORR) as evidenced by rotating ring-disk electrode studies and show long-term stability superior to the commercial Pt/C (20 wt.%). The catalytic site of PeFeeC for the ORR is proposed as Fe-Px (x is the coordination number of P atoms to Fe) embedded in carbon. The pyrolysis temperature is found to play an important role in the microstructure, texture and the doping content of P and Fe in carbon, which further affects the catalytic activity of PeFeeC for ORR. © 2014 Elsevier B.V. All rights reserved.
Keywords: Oxygen reduction reaction Electrocatalysts Phosphorus Iron Doping Carbon
1. Introduction Fuel cells and metal-air batteries, as the environmental friendly energy storage and conversion devices, have attracted much attention over the past few years due to their high energy density and the potential to reduce the negative impact of climate change [1e3]. However, the sluggish oxygen reduction reaction (ORR) at the cathode, which requires highly efficient and low cost electrocatalysts, is one of the bottlenecks that restrict the wide application of these devices [4e7]. Exploring inexpensive and sustainable electrocatalysts with high efficiency to alternate the platinumbased noble electrocatalysts for the ORR is crucial for the development of these energy devices [8e11].
* Corresponding author. E-mail address: [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jpowsour.2014.12.018 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Carbon-based materials have emerged as a class of promising alternative electrocatalyts for the ORR due to its low price, high earth abundance and environmental friendliness. Especially the catalytic activity of carbon materials can be further improved by doping of heteroatom into carbon framework. The non-metal heteroatom doped in carbon is usually N, S, B, F and P et al., which can modify the electronic structure (like charge and/or spin density redistribution) of carbon and create the active sites favorable for the adsorption of O2 molecule and facilitate the ORR process [12e17]. In addition, transition metal (TM) heteroatom dopants (such as Fe, Co, Ni, Cu, Mn et al.), especially being co-doped with the non-metal heteroatom mentioned above, have been reported to show great potential in improving the catalytic activity of carbon although the nature of catalytic sites in these carbon is still on debate [18e20]. Considerable efforts have been devoted to the study of transition metal (mainly Fe and Co) and N co-doped carbon (TMeNeC) catalysts for the ORR [19e23]. The catalytic sites with
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FeeN4/C or FeN2þ2/C configuration bridging two adjacent graphitic sheets hosted in micropores were proposed by Dodelet and coworkers to exist in these catalysts [19,24]. Recently, P-doped carbon is of thrive interest since P doping can modify the structure and activity of carbon more effectively due to the lower electronegativity and larger covalent radius of P as compared with N [13,25,26]. P-doped carbon with different structure, such as graphite layers [27], graphene nanosheets [28], microporous carbon [29,30] and mesoporous carbon [31,32], have been reported to show improved electrocatalytic activity for ORR. Nevertheless, there have been few reports about the ORR activity of P and transition metal co-doped carbon. In our previous work, we prepared P and Co co-doped mesoporous carbon (PeCoeMC) and studied the ORR activity of PeCoeMC in alkaline media [32]. Our results showed that codoping of Co with P can significantly improve the catalytic activity of carbon as compared with only P-doping or Co-doping. To better understand the effect of co-doping of transition metal with P on the structure and ORR activity of carbon, other transition metal dopants need to be explored. It's reported that for the transition metal and N co-doped carbon, Fe-based catalysts show higher activity and especially higher selectivity than the Co-based catalysts [33e36]. This motivates us to explore the activity of Fe and P codoped carbon for the ORR. To the best of our knowledge, the preparation and electrocatalytic activity of Fe and P co-doped carbon for ORR are rarely reported. Herein, we developed a facile method to synthesize the P and Fe co-doped carbon xerogel (PeFeeC) via an in-situ solegel polymerization method followed by a pyrolysis process for the first time. The resorcinol and formaldehyde were used as the carbon source. Iron nitrate and tetraphenylphosphonium bromide were used as sources of Fe and P, respectively. The P and Fe were introduced into carbon precursors during the polymerization process.
The influence of Fe and P co-doping on the electrocatalytic activity of the resulting PeFeeC for the ORR in alkaline media was investigated. The pyrolysis temperature has a strong impact on the structure and texture of the PeFeeC, which further affects the ORR activity and durability of PeFeeC. 2. Experimental 2.1. Sample preparation The P and Fe co-doped carbon xerogel was synthesized via an insitu solegel polymerization method followed by a pyrolysis process. In a typical experiment, 0.05 mol of resorcinol and 0.10 mol of formaldehyde were mixed and dissolved in 25 ml of deionized water to form a homogeneous solution by stirring. Meanwhile 0.0167 mol of tetraphenylphosphonium bromide (C24H20BrP, 99.0%, Sigma Aldrich) was dissolved in 50 ml of ethanol and added to the above solution. Then 0.0025 mol Fe(NO3)3$9H2O (99.0%, Guoyao Chemical Reagent Co. Ltd.) was added to the solution. Afterwards, aqueous ammonia solution was added and kept stirring until a wet organic xerogel was obtained. Next, the wet organic gel was dried and cured in a vacuum oven for 7 days at a temperature of 85 C. Eventually, the organic gel was ground into uniform powder and subjected to carbonization under N2 atmosphere at a temperature of 800 C with a heating rate of 5 C min1 and holding time of 2 h. The pyrolyzed samples were stirred in 500 mL of 1.0 M HCl solution for 12 h to dissolve the residue Fe in the carbon. Finally, the samples were washed with ultrapure water until a neutral pH was reached and then dried in an oven at 90 C. The sample obtained was denoted as PeFeeC-800, where the number reflects the pyrolysis temperature. To study the influence of pyrolysis temperature on the structure and catalytic activity of
Scheme 1. Schematic illustration of the preparation of P and Fe co-doped carbon xerogels.
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Fig. 1. XRD patterns of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples.
the sample, the organic xerogel powder was also heated at 600 C, 700 C, 900 C and 1000 C. The samples obtained were denoted as PeFeeC-600, PeFeeC-700, PeFeeC-900 and PeFeeC-1000, respectively. Pure carbon without adding tetraphenylphosphonium bromide and Fe(NO3)3$9H2O was also prepared for comparison.
glassy carbon (GC) disk (5 mm diameter, 0.196 cm2 of geometric surface area) surrounded by a Pt ring (0.125 cm2 of geometric surface area). The electrochemical measurements were conducted in a standard three-electrode electrochemical cell at room temperature. The working electrode was the catalyst film-coated GC disks. A Pt-foil was used as the counter electrode, and an Ag/AgCl (3 M Cl, Cypress) electrode in a double-junction chamber was used as reference electrode. The electrolyte was 0.1 M KOH solution prepared from ultrapure water (Millipore, 18.2 MU cm). The catalyst inks were prepared by mixing 10 mg of catalyst powder, 10 mL of Nafion solution (5% wt from Aldrich) and 700 mL of ethanol. The catalyst loading on the GC disk is 503 mg cm2. For the ORR test, the electrolyte was purged with high-purity O2 gas for at least 30 min to ensure O2 saturation. Linear sweep voltammetry (LSV) measurements during the ORR were performed in O2-saturated 0.1 M KOH by sweeping the potential from 0.9 V anodically to 0 V at 10 mV s1 with the electrode rotated at 400, 900, 1600 and 2500 rpm. For all the RRDE measurements, the ring potential was held at 0.5 V vs. Ag/AgCl in order to oxidize any HO 2 produced in alkaline solution [37]. The % HO 2 produced and the electron number (n) transferred during the ORR were calculated using the following equations [29,38].
%HO 2 ¼ 100
n¼4 2.2. Physical characterizations The crystal structure of the samples was examined with Xeray diffraction (XRD) using a Xeray diffractometer (UK, Bede Scientific Ltd.; Cu Ka radiation; operated at 40 kV, 45 mA; l ¼ 0.15418 nm). Raman spectroscopy of the samples was performed on a Jobin Yvon LabRAM HR 800 instrument with a 514 nm excitation laser at a power of around 1 mW. The binding environment of the elements in samples was analyzed with a X-ray photoelectron spectroscopy spectrometer (XPS, VG ESCALAB MKII) using a monochromatized Al Ka (1486 eV) source. The spectra were corrected for the background using the Shirley approach and the surface composition of the samples was determined by measuring the ratio of C1s to P2p and Fe 2p intensities (integrated peak area) normalized by their respective sensitivity factors. The specific surface area and the pore structure of the samples were analyzed by adsorption/desorption measurements of nitrogen at 77 K (Quantachrome, QuadraSorb SI). Prior to measurements, the samples were degassed at 250 C overnight under vacuum. Surface area was calculated by BrunauereEmmetteTeller (BET) method, micropore volume from N2 sorption was calculated using the t-plot method. Pore size distributions were calculated using BarretteJoynereHalenda (BJH) method for mesopores and HorvatheKawazoe (HK) method for micropores. The morphology of the sample was examined with scanning electron microscopy (SEM, FEI Quanta 200). The bulk content of P and residual metal in the carbons were measured with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis (Vista MPX).
163
2IR =N ID þ ðIR =NÞ
ID ID þ ðIR =NÞ
(1)
(2)
where ID is the Faradaic current at the disk, IR is the Faradaic current at the ring, and N ¼ 0.22 is the disk electrode collection efficiency.
3. Results and discussion 3.1. Structure of PeFeeC The preparation of P and Fe co-doped carbon xerogels is shown in Scheme 1 based on the mechanisms of the base-catalyzed phenoleformaldehyde reaction proposed by Knop and Pilato et al. [39] and other researchers [40,41]. Under alkaline condition, the
2.3. Electrochemical measurements The electrocatalytic activity for the ORR of the samples was studied with the rotating ring-disk electrode (RRDE) technique using a Pine electrochemical system (AFMSRX rotator, and AFCBP1 bipotentiostat). The RRDE electrode consisted of a catalyst-coated
Fig. 2. Raman spectroscopy for PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples. The ratio of D-band to G-band (ID/IG) is indicated for each sample.
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Fig. 3. (a) The N2 sorption isotherms of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples; (b) and (c) SEM images of PeFeeC-900.
positively charged group of tetraphenylphosphonium bromide (i.e. C24H20Pþ) and Fe3þ interact with the alkylated phenoxide ion and then further get polymerized. After the pyrolysis (i.e. carbonization) of the organic xerogels at high temperature under N2 atmosphere, the P and Fe co-doped carbon xerogels (i.e. PeFeeC) are formed. The XRD patterns of the as-synthesized PeFeeC samples are shown in Fig. 1. Two broad peaks at about 2q ¼ 23.4 and 43.6 can be indexed to (002) and (100) reflections of carbon phase,
Current Density / mA cm
-2
0.50 0.25 0.00
respectively, suggesting the formation of amorphous carbon. Note that Fe2P2O7, FeP and Fe2P are formed in the PeFeeC-1000 sample. The detailed structure evolution of the PeFeeC samples with the pyrolysis temperature as well as the structural defects induced by P and Fe doping was studied by Raman spectroscopy. As shown in Fig. 2, the samples exhibit two typical peaks centered at approximately 1340 cm1 and 1586 cm1, which are known as Dband and G-band, respectively. The D band is commonly attributed to the disorder and defects of the carbon crystallites. The G band is due to the first-order scattering of the E2g mode of twodimensional hexagonal graphitic layer [42,43]. The ID/IG ratio, as a gauge for the structure defects in carbon, is given for each sample (Fig. 2). Remarkably, the ID/IG increases with the increasing of pyrolysis temperature and reaches a maximum value of 1.03 at a temperature of 900 C despite that common fact that the structure of carbon becomes more ordered with the increasing of pyrolysis temperature. The increased ID/IG indicates the increasing defects in carbon, which results from the incorporation of heteroatoms into carbon xerogels. This also suggests that introduction of defects by
-0.25
P-Fe-C-600 P-Fe-C-700 P-Fe-C-800 P-Fe-C-900 P-Fe-C-1000
-0.50 -0.75 -1.00 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
Table 1 BET specific surface areas, total pore volumes and average pore size of PeFeeC samples. Samples
0.2
Potential / V vs.Ag/AgCl Fig. 4. Cyclic voltammograms of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC900 and PeFeeC-1000 samples. Experiments were conducted in Ar-saturated 0.1 M KOH at 298 K with a sweep rate of 10 mV s1.
PeFeeC-600 PeFeeC-700 PeFeeC-800 PeFeeC-900 PeFeeC-1000
BET specific surface area/m2 g1
Total pore volume/micropore volume/m3 g1
Average micropore/mesopore size/nm
572.32 594.38 667.92 1243.29 972.68
0.23/0.21 0.25/0.22 0.28/0.25 0.55/0.32 0.56/0.25
0.58/e 0.59/e 0.60/e 0.70/3.68 0.67/3.77
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heteroatom doping takes precedence over the ordering/graphitization of carbon during the synthesis of PeFeeC. After reaching a maximum, the ID/IG decreases to 0.99 for PeFeeC-1000, which mainly owes to the formation of graphitic structure at the high temperature of 1000 C [44,45]. The textural properties of the P and Fe co-doped carbon xerogels were investigated by N2 sorption measurement. Fig. 3a shows the N2 adsorptionedesorption isotherms of PeFeeC samples. The PeFeeC-600, PeFeeC-700 and PeFeeC-800 samples show type I isotherm (Fig. 4a), indicating the samples possess micropores. In contrast, PeFeeC-900 and PeFeeC-1000 samples exhibit a combined type I and Ⅳ isotherm, which suggests that besides micropores, mesopores are developed in the samples pyrolyzed above 800 C. The determined BET surface area, total pore volume and average pore size of all the samples are summarized in Table 1. It can be clearly seen that the BET specific surface area of the samples increases with the increasing of pyrolysis temperature and reaches a maximum of 1243.29 m2 g1 for PeFeeC-900. However, the BET specific surface area decreases to 972.68 m2 g1 for PeFeeC-1000. This mainly results from the decreasing of micropores and increasing of mesopores in PeFeeC-1000 as compared to PeFeeC900 sample, which is evidenced from the changes in the total pore/ micropore volume and the average pore size of the samples (Table 1). Moreover, the macropores with about several mm in diameter are found on the surface of PeFeeC-900 as shown in the SEM image of the sample (Fig. 3b). A high degree of porosity can also be observed from the high magnification SEM image of this sample (Fig. 3c). The macropores along with the mesopores and micropores in the sample provide efficient channels for the mass transfer of O2 and electrolyte inside the material during the ORR process. 3.2. Catalytic activity of PeFeeC for the oxygen reduction
Fig. 5. (a) Ring and disk current density obtained with Linear sweeping voltammograms (LSVs) on rotating ring-disk electrode for PeFeeC-600, PeFeeC-700, PeFeeC800, PeFeeC-900 and PeFeeC-1000 samples in O2-saturated 0.1 M KOH at a rotating speed of 1600 rpm. The disk potential was scanned at 10 mV s1 and the ring potential was fixed at 0.5 V; (b) Calculated electron transfer number (n) and determined peroxide percentage (HO 2 %) at various potentials based on the corresponding RRDE data in (a); (c) LSVs of oxygen reduction on PeFeeC-900 in O2-saturated 0.1 M KOH
To investigate the catalytic activity of the PeFeeC samples for the ORR, the CVs of the samples in N2-saturated 0.1 M KOH solution were first compared. As shown in Fig. 4, the area under CV (i.e. capacitance) first increases as the pyrolysis temperature increases to 900 C and then decreases as the pyrolysis temperature increases further to 1000 C. This is in good agreement with the trend observed from the BET specific surface area of the samples. Fig. 5a shows the LSVs of the PeFeeC samples in O2-saturated 0.1 M KOH solution measured with RRDE. The onset potential of pure carbon xerogel is 0.307 V and the current density of it at 0.8 V is 2.30 mA cm2. As compared with pure carbon xerogel, the onset potential of PeFeeC samples increases positively. The PeFeeC-600 shows a much higher onset potential of 0.247 V, it further increases to 0.238, 0.202 and 0.139 V for PeFeeC-700, PeFeeC-800 and PeFeeC-900 samples, respectively. But it decreases to 0.170 V for PeFeeC-1000. The diffusion limiting current density of the samples follows the same trend. The results clearly show that the improved catalytic activity of carbon xerogels for ORR can be achieved by P and Fe doping in carbon and the activity of PeFeeC is closely related to the pyrolysis temperature. The sample pyrolyzed at 900 C exhibits the highest catalytic activity for the ORR. Furthermore, as can be seen in Fig. 5a, although the obtained PeFeeC in this work shows high catalytic activity for ORR, its half-wave potential is 110 mV lower than that of the commercial Pt/C (20 wt.%) and is also lower than that of the Co and P co-doped carbon reported in our previous work (66 mV lower than that of the commercial Pt/C (20 wt.%)) [32]. The reason for the activity
with a scan rate of 10 mV s1 at different electrode rotating speeds. Inset: the corresponding KouteckyeLevich plots for ORR on PeFeeC-900.
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Fig. 7. The bulk P and Fe content in PeFeeC samples pyrolyzed at different temperatures.
calculated based on the RRDE data (Fig. 5a) using Equations (1) and (2), respectively, as shown in Fig. 5b. It can be seen that the corresponding HO 2 yield on pure carbon over the measured potential range (0.8 to 0.3 V) is about 55.0e91.3% and it decreases after P and Fe doping, following C > PeFeeC-600 > PeFeeC700 > PeFeeC-800 > PeFeeC-1000 > PeFeeC-900 > Pt/C. The HO 2 on PeFeeC-900 is only about 7.8e15.0% and slightly higher than the amount of HO2 produced on Pt/C (below 7.0%). The corresponding electron number transferred during the ORR process decreases in the reverse order. The electron number on PeFeeC900 is 3.74e3.89 over the potential range of 0.8 to0.3 V. This indicates that P and Fe doping modifies the ORR catalytic pathway of carbon xerogel, which is significantly influenced by pyrolysis temperature. Fig. 5c shows the KouteckyeLevich (KeL) plots at 0.4, 0.5 and 0.6 V for the PeFeeC-900 sample. The KeL plots are constructed using the following KeL equation [46]:
1 1 1 1 1 ¼ þ ¼ þ i iK iD nFkCO2 0:62nFC D2=3 n1=6 u1=2 O2 O2
Fig. 6. (a) XPS survey spectra of PeFeeC-900. (b) XPS spectra for the P 2p of PeFeeC900. (c) XPS spectra for the Fe 2p of PeFeeC-900.
difference between “co-doping of Fe and P” and “co-doping of Co and P” requires further considerable studies. The corresponding peroxide species (HO 2 ) produced on these catalysts and the electron numbers (n) transferred during the ORR process were
(3)
Where i is the measured current density on the polarization curve; iK and iD are the kinetic and diffusion limiting current densities, respectively; n is the apparent number of electrons transferred during the ORR; F is the Faraday constant (96,485C mol1), DO2 is the diffusion coefficient of O2 ðDO2 ¼ 1:86 105 cm2 s1 Þ, y is the kinetic viscosity of the solution (y ¼ 0.01 cm2 s1), CO2 is the concentration of O2 dissolved in electrolyte ð CO2 ¼ 1:21 106 mol cm3 Þ [47,48], and u is the electrode rotation speed. The average electron number calculated from the K-L plots at different potentials for PeFeeC-900 is 3.81. This is in good agreement with the result (n z 3.74e3.89) obtained from the RRDE measurements, suggesting a dominant four-electron pathway of ORR on PeFeeC-900. Note that the diffusion-limiting current densities (ID) of the samples vary significantly due to the different intrinsic catalytic activities of the samples, which are related to the electron transfer number n during the ORR based on Equation (3). To gain insight into the catalytic activity enhancement of P and Fe co-doped carbon xerogels, X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical species incorporated into carbon as well as their electronic states. The full XPS survey spectrum of a typical PeFeeC sample (PeFeeC-900) in Fig. 6a confirms the presence of C, O, P and Fe species in the sample. The
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are proposed as catalytic active sites of PeFeeC for ORR as analyzed from the XPS results. The high content of P and Fe along with the high specific surface area are believed to account for the high electrocatalytic activity of PeFeeC-900 catalyst. The stabilities of the PeFeeC samples for the ORR were tested with the chronoamperometric method as shown in Fig. 8. The stability of the commercial Pt/C (20 wt.%) catalyst is also included for comparison. The ORR current of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 is decreased by 33.8%, 27.3%, 17.2%, 12.4% and 13.8%, respectively, over 43000 s of continuous operation. The PeFeeC-900 catalyst exhibits the slowest performance attenuation. However, the commercial Pt/C suffers a significant current loss of 52.3%. The high durability of PeFeeC samples, especially PeFeeC-900, can be attributed to the covalent bonding of Fe and P to carbon as well as the absence of metal agglomeration and migration (like Pt) during the long-term operation. Fig. 8. Current-time (iet) chronoamperometric responses for the ORR on PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900, PeFeeC-1000 and commercial Pt/C in O2saturated 0.1 M KOH at 0.35 V (vs. Ag/AgCl) for ORR at a rotating speed of 1600 rpm.
high-resolution P 2p spectrum of PeFeeC-900 (Fig. 6b) shows two deconvolved contributions at 133.2 eV and 134.6 eV, which reveals the presence of PeC and PeO bonding [49,50], respectively. The results indicate that the P atoms have been successfully incorporated into the carbon lattice. The deconvolution of Fe 2p spectrum shows three peaks (Fig. 6c). The peaks at 712.5 and 726.8 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively [51e53]. The peak at 718.1 eV is a satellite peak. These suggest that Fe3þ cations exist in the sample. Since no Fe-based carbide and phosphide are found in PeFeeC-900 as confirmed from the XRD results, these Fe3þ cations are most likely bound to P to form Fe-Px moieties embedded in carbon (like Fe-Nx, x is the coordination number of P atoms to Fe in this work), which can act as active sites for the ORR. Further investigation will be done to understand the nature of these active sites in future studies. The surface and bulk content of P and Fe in the PeFeeC samples were determined from XPS and ICP-AES measurements, respectively. They are very close to each other for the content of either P or Fe higher than 1.0 at%. Some samples contain P and/or Fe content below 1.0 at%, only the bulk contents of P and Fe in PeFeeC are shown in Fig. 7. It can be seen that both P and Fe content increase as the pyrolysis temperature increases and reach a maximum at 900 C, but decrease as the temperature increases further to 1000 C. The doping content of Fe and P are 1.06% and 1.97%, respectively, for the sample pyrolyzed at 900 C (i.e. PeFeeC-900). The results suggest that 900 C is the optimal pyrolysis/carbonization temperature for doping both Fe and P into carbon lattice. The increasing P and Fe content in carbon with the increasing pyrolysis temperature up to 900 C is due to the extraction of H,O and C in the form of H2O, CO and CO2 et al. during the carbonization, which increases as the pyrolysis temperature increases [54]. Meanwhile, the releasing of H2O, CO and CO2 et al. also creates large amount of pores in the bulk of samples. This is evidenced by the increasing BET specific surface areas of the samples pyrolyzed up to 900 C (Fig. 3 and Table 1). In contrast, both the P and Fe content decrease as the pyrolysis temperature further increases to 1000 C, which most likely results from the loss of FeePx groups at this high temperature because of the additional formation of Fe2P2O7, FeP and Fe2P in the sample as confirmed from the XRD results (Fig. 1). This also leads to the decreased BET specific surface area of the samples pyrolyzed at 1000 C (Fig. 3 and Table 1). Therefore, the catalytic activity of PeFeeC-1000 decreases. The FeePx moieties
4. Conclusions In conclusion, we have reported the successful fabrication of PeFeeC catalyst via an in situ solegel polymerization method followed by a pyrolysis process. The pyrolysis temperature has a significant effect on the structure, texture and the doping content of P and Fe, which further affects greatly the ORR catalytic activity of the PeFeeC electrocatalysts. The catalytic site of PeFeeC for the ORR is proposed as FeePx (x is the coordination number of P atoms to Fe), the nature of which will be further studied. Due to the high doping content of P and Fe and large specific surface area, the PeFeeC pyrolyzed at 900 C shows the highest catalytic activity for ORR among the samples pyrolyzed at different temperatures. Meanwhile, the PeFeeC-900 sample exhibits long-term durability, superior to commercial Pt/C. This in-situ polymerization process coupled with carbonization approach provides a tunable method for fabricating heteroatom-doped carbon for use in alkaline fuel cells and metal-air batteries. The species and content of heteroatom can be further tailored with this method for the catalytic activity optimization of the heteroatom-doped carbon toward ORR. Acknowledgments This work is supported by National Natural Science Foundation of China (Nos. 51272167 and 21206101). References [1] H.A. Gasteiger, S.S. Kocha, B. Somapalli, F.T. Wagner, Appl. Catal. B Environ. 56 (2005) 9. [2] P.G. Bruce, L.J. Hardwick, K.M. Abraham, Mater. Res. Soc. Bull. 36 (2011) 506. [3] M. Armand, J.M. Tarascon, Nature 451 (2008) 652. [4] S.Y. Wang, E. Iyyamperumal, A. Roy, Y.H. Xue, D.S. Yu, L.M. Dai, Angew. Chem. 123 (2011) 11960. [5] J. Liang, Y. Jiao, M. Jaroniec, S.Z. Qiao, Angew. Chem. Int. Ed. 51 (2012) 11496. [6] R.Z. Yang, W.Y. Bian, P. Strasser, M.F. Toney, J. Power Sources 222 (2013) 169. [7] Z.T. Cui, S.G. Wang, Y.H. Zhang, M.H. Cao, J. Power Sources 259 (2014) 138. [8] Y.Y. Shao, J.H. Sui, G.P. Yin, Y.Z. Gao, Appl. Catal. B Environ. 79 (2008) 89. [9] Z.S. Wu, S.B. Yang, Y. Sun, K. Parvez, X.L. Feng, K. Müllen, J. Am. Chem. Soc. 134 (2012) 9082. [10] F.Y. Cheng, J. Shen, B. Peng, Y.D. Pan, Z.L. Tao, J. Chen, Nature Chem. 3 (2011) 79. [11] Z.Y. Lin, G.H. Waller, Y. Liu, M.L. Liu, C.P. Wong, Carbon 53 (2013) 130. [12] L.J. Yang, S.J. Jiang, Y. Zhao, L. Zhu, S. Chen, X.Z. Wang, Q. Wu, J. Ma, Y.W. Ma, Z. Hu, Angew. Chem. Int. Ed. 123 (2011) 7270. [13] D.S. Yu, Y.H. Xue, L.M. Dai, J. Phys. Chem. Lett. 3 (2012) 2863. [14] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 6 (2012) 205. [15] M. Zhang, L.M. Dai, Nano Energy 1 (2012) 514. [16] K.P. Gong, F. Du, Z.H. Xia, M. Durstock, L.M. Dai, Science 323 (2009) 760. [17] L.P. Zhang, Z.H. Xia, J. Phys. Chem. C. 115 (2011) 11170. [18] Z.W. Chen, D. Higgins, A.P. Yu, L. Zhang, J.J. Zhang, Energy Environ. Sci. 4
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Enhanced catalytic activity for the oxygen reduction reaction with codoping of phosphorus and iron in carbon Zhenrong Yang, Jiao Wu, Xiangjun Zheng, Zhangjun Wang, Ruizhi Yang* College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215006, China
h i g h l i g h t s The The The The
P and Fe co-doped carbon (PeFeeC) is facilely synthesized. as-prepared PeFeeC shows high catalytic activity for the ORR. catalytic site of PeFeeC for the ORR is proposed as FeePx. pyrolysis temperature plays an important role in the structure and activity of PeFeeC.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2014 Received in revised form 10 November 2014 Accepted 4 December 2014 Available online 5 December 2014
The phosphorus (P) and iron (Fe) co-doped carbon (PeFeeC) has been facilely synthesized via an in situ solegel polymerization method followed by a pyrolysis process using resorcinol and formaldehyde as the carbon source, tetraphenylphosphonium bromide as the P source and iron nitrate as the Fe source. Benefiting from the high doping content of P and Fe and large specific surface area, the as-prepared PeFe eC electrocatalysts demonstrate considerable catalytic activity for the oxygen reduction reaction (ORR) as evidenced by rotating ring-disk electrode studies and show long-term stability superior to the commercial Pt/C (20 wt.%). The catalytic site of PeFeeC for the ORR is proposed as Fe-Px (x is the coordination number of P atoms to Fe) embedded in carbon. The pyrolysis temperature is found to play an important role in the microstructure, texture and the doping content of P and Fe in carbon, which further affects the catalytic activity of PeFeeC for ORR. © 2014 Elsevier B.V. All rights reserved.
Keywords: Oxygen reduction reaction Electrocatalysts Phosphorus Iron Doping Carbon
1. Introduction Fuel cells and metal-air batteries, as the environmental friendly energy storage and conversion devices, have attracted much attention over the past few years due to their high energy density and the potential to reduce the negative impact of climate change [1e3]. However, the sluggish oxygen reduction reaction (ORR) at the cathode, which requires highly efficient and low cost electrocatalysts, is one of the bottlenecks that restrict the wide application of these devices [4e7]. Exploring inexpensive and sustainable electrocatalysts with high efficiency to alternate the platinumbased noble electrocatalysts for the ORR is crucial for the development of these energy devices [8e11].
* Corresponding author. E-mail address: [email protected] (R. Yang). http://dx.doi.org/10.1016/j.jpowsour.2014.12.018 0378-7753/© 2014 Elsevier B.V. All rights reserved.
Carbon-based materials have emerged as a class of promising alternative electrocatalyts for the ORR due to its low price, high earth abundance and environmental friendliness. Especially the catalytic activity of carbon materials can be further improved by doping of heteroatom into carbon framework. The non-metal heteroatom doped in carbon is usually N, S, B, F and P et al., which can modify the electronic structure (like charge and/or spin density redistribution) of carbon and create the active sites favorable for the adsorption of O2 molecule and facilitate the ORR process [12e17]. In addition, transition metal (TM) heteroatom dopants (such as Fe, Co, Ni, Cu, Mn et al.), especially being co-doped with the non-metal heteroatom mentioned above, have been reported to show great potential in improving the catalytic activity of carbon although the nature of catalytic sites in these carbon is still on debate [18e20]. Considerable efforts have been devoted to the study of transition metal (mainly Fe and Co) and N co-doped carbon (TMeNeC) catalysts for the ORR [19e23]. The catalytic sites with
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FeeN4/C or FeN2þ2/C configuration bridging two adjacent graphitic sheets hosted in micropores were proposed by Dodelet and coworkers to exist in these catalysts [19,24]. Recently, P-doped carbon is of thrive interest since P doping can modify the structure and activity of carbon more effectively due to the lower electronegativity and larger covalent radius of P as compared with N [13,25,26]. P-doped carbon with different structure, such as graphite layers [27], graphene nanosheets [28], microporous carbon [29,30] and mesoporous carbon [31,32], have been reported to show improved electrocatalytic activity for ORR. Nevertheless, there have been few reports about the ORR activity of P and transition metal co-doped carbon. In our previous work, we prepared P and Co co-doped mesoporous carbon (PeCoeMC) and studied the ORR activity of PeCoeMC in alkaline media [32]. Our results showed that codoping of Co with P can significantly improve the catalytic activity of carbon as compared with only P-doping or Co-doping. To better understand the effect of co-doping of transition metal with P on the structure and ORR activity of carbon, other transition metal dopants need to be explored. It's reported that for the transition metal and N co-doped carbon, Fe-based catalysts show higher activity and especially higher selectivity than the Co-based catalysts [33e36]. This motivates us to explore the activity of Fe and P codoped carbon for the ORR. To the best of our knowledge, the preparation and electrocatalytic activity of Fe and P co-doped carbon for ORR are rarely reported. Herein, we developed a facile method to synthesize the P and Fe co-doped carbon xerogel (PeFeeC) via an in-situ solegel polymerization method followed by a pyrolysis process for the first time. The resorcinol and formaldehyde were used as the carbon source. Iron nitrate and tetraphenylphosphonium bromide were used as sources of Fe and P, respectively. The P and Fe were introduced into carbon precursors during the polymerization process.
The influence of Fe and P co-doping on the electrocatalytic activity of the resulting PeFeeC for the ORR in alkaline media was investigated. The pyrolysis temperature has a strong impact on the structure and texture of the PeFeeC, which further affects the ORR activity and durability of PeFeeC. 2. Experimental 2.1. Sample preparation The P and Fe co-doped carbon xerogel was synthesized via an insitu solegel polymerization method followed by a pyrolysis process. In a typical experiment, 0.05 mol of resorcinol and 0.10 mol of formaldehyde were mixed and dissolved in 25 ml of deionized water to form a homogeneous solution by stirring. Meanwhile 0.0167 mol of tetraphenylphosphonium bromide (C24H20BrP, 99.0%, Sigma Aldrich) was dissolved in 50 ml of ethanol and added to the above solution. Then 0.0025 mol Fe(NO3)3$9H2O (99.0%, Guoyao Chemical Reagent Co. Ltd.) was added to the solution. Afterwards, aqueous ammonia solution was added and kept stirring until a wet organic xerogel was obtained. Next, the wet organic gel was dried and cured in a vacuum oven for 7 days at a temperature of 85 C. Eventually, the organic gel was ground into uniform powder and subjected to carbonization under N2 atmosphere at a temperature of 800 C with a heating rate of 5 C min1 and holding time of 2 h. The pyrolyzed samples were stirred in 500 mL of 1.0 M HCl solution for 12 h to dissolve the residue Fe in the carbon. Finally, the samples were washed with ultrapure water until a neutral pH was reached and then dried in an oven at 90 C. The sample obtained was denoted as PeFeeC-800, where the number reflects the pyrolysis temperature. To study the influence of pyrolysis temperature on the structure and catalytic activity of
Scheme 1. Schematic illustration of the preparation of P and Fe co-doped carbon xerogels.
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Fig. 1. XRD patterns of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples.
the sample, the organic xerogel powder was also heated at 600 C, 700 C, 900 C and 1000 C. The samples obtained were denoted as PeFeeC-600, PeFeeC-700, PeFeeC-900 and PeFeeC-1000, respectively. Pure carbon without adding tetraphenylphosphonium bromide and Fe(NO3)3$9H2O was also prepared for comparison.
glassy carbon (GC) disk (5 mm diameter, 0.196 cm2 of geometric surface area) surrounded by a Pt ring (0.125 cm2 of geometric surface area). The electrochemical measurements were conducted in a standard three-electrode electrochemical cell at room temperature. The working electrode was the catalyst film-coated GC disks. A Pt-foil was used as the counter electrode, and an Ag/AgCl (3 M Cl, Cypress) electrode in a double-junction chamber was used as reference electrode. The electrolyte was 0.1 M KOH solution prepared from ultrapure water (Millipore, 18.2 MU cm). The catalyst inks were prepared by mixing 10 mg of catalyst powder, 10 mL of Nafion solution (5% wt from Aldrich) and 700 mL of ethanol. The catalyst loading on the GC disk is 503 mg cm2. For the ORR test, the electrolyte was purged with high-purity O2 gas for at least 30 min to ensure O2 saturation. Linear sweep voltammetry (LSV) measurements during the ORR were performed in O2-saturated 0.1 M KOH by sweeping the potential from 0.9 V anodically to 0 V at 10 mV s1 with the electrode rotated at 400, 900, 1600 and 2500 rpm. For all the RRDE measurements, the ring potential was held at 0.5 V vs. Ag/AgCl in order to oxidize any HO 2 produced in alkaline solution [37]. The % HO 2 produced and the electron number (n) transferred during the ORR were calculated using the following equations [29,38].
%HO 2 ¼ 100
n¼4 2.2. Physical characterizations The crystal structure of the samples was examined with Xeray diffraction (XRD) using a Xeray diffractometer (UK, Bede Scientific Ltd.; Cu Ka radiation; operated at 40 kV, 45 mA; l ¼ 0.15418 nm). Raman spectroscopy of the samples was performed on a Jobin Yvon LabRAM HR 800 instrument with a 514 nm excitation laser at a power of around 1 mW. The binding environment of the elements in samples was analyzed with a X-ray photoelectron spectroscopy spectrometer (XPS, VG ESCALAB MKII) using a monochromatized Al Ka (1486 eV) source. The spectra were corrected for the background using the Shirley approach and the surface composition of the samples was determined by measuring the ratio of C1s to P2p and Fe 2p intensities (integrated peak area) normalized by their respective sensitivity factors. The specific surface area and the pore structure of the samples were analyzed by adsorption/desorption measurements of nitrogen at 77 K (Quantachrome, QuadraSorb SI). Prior to measurements, the samples were degassed at 250 C overnight under vacuum. Surface area was calculated by BrunauereEmmetteTeller (BET) method, micropore volume from N2 sorption was calculated using the t-plot method. Pore size distributions were calculated using BarretteJoynereHalenda (BJH) method for mesopores and HorvatheKawazoe (HK) method for micropores. The morphology of the sample was examined with scanning electron microscopy (SEM, FEI Quanta 200). The bulk content of P and residual metal in the carbons were measured with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis (Vista MPX).
163
2IR =N ID þ ðIR =NÞ
ID ID þ ðIR =NÞ
(1)
(2)
where ID is the Faradaic current at the disk, IR is the Faradaic current at the ring, and N ¼ 0.22 is the disk electrode collection efficiency.
3. Results and discussion 3.1. Structure of PeFeeC The preparation of P and Fe co-doped carbon xerogels is shown in Scheme 1 based on the mechanisms of the base-catalyzed phenoleformaldehyde reaction proposed by Knop and Pilato et al. [39] and other researchers [40,41]. Under alkaline condition, the
2.3. Electrochemical measurements The electrocatalytic activity for the ORR of the samples was studied with the rotating ring-disk electrode (RRDE) technique using a Pine electrochemical system (AFMSRX rotator, and AFCBP1 bipotentiostat). The RRDE electrode consisted of a catalyst-coated
Fig. 2. Raman spectroscopy for PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples. The ratio of D-band to G-band (ID/IG) is indicated for each sample.
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Fig. 3. (a) The N2 sorption isotherms of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 samples; (b) and (c) SEM images of PeFeeC-900.
positively charged group of tetraphenylphosphonium bromide (i.e. C24H20Pþ) and Fe3þ interact with the alkylated phenoxide ion and then further get polymerized. After the pyrolysis (i.e. carbonization) of the organic xerogels at high temperature under N2 atmosphere, the P and Fe co-doped carbon xerogels (i.e. PeFeeC) are formed. The XRD patterns of the as-synthesized PeFeeC samples are shown in Fig. 1. Two broad peaks at about 2q ¼ 23.4 and 43.6 can be indexed to (002) and (100) reflections of carbon phase,
Current Density / mA cm
-2
0.50 0.25 0.00
respectively, suggesting the formation of amorphous carbon. Note that Fe2P2O7, FeP and Fe2P are formed in the PeFeeC-1000 sample. The detailed structure evolution of the PeFeeC samples with the pyrolysis temperature as well as the structural defects induced by P and Fe doping was studied by Raman spectroscopy. As shown in Fig. 2, the samples exhibit two typical peaks centered at approximately 1340 cm1 and 1586 cm1, which are known as Dband and G-band, respectively. The D band is commonly attributed to the disorder and defects of the carbon crystallites. The G band is due to the first-order scattering of the E2g mode of twodimensional hexagonal graphitic layer [42,43]. The ID/IG ratio, as a gauge for the structure defects in carbon, is given for each sample (Fig. 2). Remarkably, the ID/IG increases with the increasing of pyrolysis temperature and reaches a maximum value of 1.03 at a temperature of 900 C despite that common fact that the structure of carbon becomes more ordered with the increasing of pyrolysis temperature. The increased ID/IG indicates the increasing defects in carbon, which results from the incorporation of heteroatoms into carbon xerogels. This also suggests that introduction of defects by
-0.25
P-Fe-C-600 P-Fe-C-700 P-Fe-C-800 P-Fe-C-900 P-Fe-C-1000
-0.50 -0.75 -1.00 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
Table 1 BET specific surface areas, total pore volumes and average pore size of PeFeeC samples. Samples
0.2
Potential / V vs.Ag/AgCl Fig. 4. Cyclic voltammograms of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC900 and PeFeeC-1000 samples. Experiments were conducted in Ar-saturated 0.1 M KOH at 298 K with a sweep rate of 10 mV s1.
PeFeeC-600 PeFeeC-700 PeFeeC-800 PeFeeC-900 PeFeeC-1000
BET specific surface area/m2 g1
Total pore volume/micropore volume/m3 g1
Average micropore/mesopore size/nm
572.32 594.38 667.92 1243.29 972.68
0.23/0.21 0.25/0.22 0.28/0.25 0.55/0.32 0.56/0.25
0.58/e 0.59/e 0.60/e 0.70/3.68 0.67/3.77
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heteroatom doping takes precedence over the ordering/graphitization of carbon during the synthesis of PeFeeC. After reaching a maximum, the ID/IG decreases to 0.99 for PeFeeC-1000, which mainly owes to the formation of graphitic structure at the high temperature of 1000 C [44,45]. The textural properties of the P and Fe co-doped carbon xerogels were investigated by N2 sorption measurement. Fig. 3a shows the N2 adsorptionedesorption isotherms of PeFeeC samples. The PeFeeC-600, PeFeeC-700 and PeFeeC-800 samples show type I isotherm (Fig. 4a), indicating the samples possess micropores. In contrast, PeFeeC-900 and PeFeeC-1000 samples exhibit a combined type I and Ⅳ isotherm, which suggests that besides micropores, mesopores are developed in the samples pyrolyzed above 800 C. The determined BET surface area, total pore volume and average pore size of all the samples are summarized in Table 1. It can be clearly seen that the BET specific surface area of the samples increases with the increasing of pyrolysis temperature and reaches a maximum of 1243.29 m2 g1 for PeFeeC-900. However, the BET specific surface area decreases to 972.68 m2 g1 for PeFeeC-1000. This mainly results from the decreasing of micropores and increasing of mesopores in PeFeeC-1000 as compared to PeFeeC900 sample, which is evidenced from the changes in the total pore/ micropore volume and the average pore size of the samples (Table 1). Moreover, the macropores with about several mm in diameter are found on the surface of PeFeeC-900 as shown in the SEM image of the sample (Fig. 3b). A high degree of porosity can also be observed from the high magnification SEM image of this sample (Fig. 3c). The macropores along with the mesopores and micropores in the sample provide efficient channels for the mass transfer of O2 and electrolyte inside the material during the ORR process. 3.2. Catalytic activity of PeFeeC for the oxygen reduction
Fig. 5. (a) Ring and disk current density obtained with Linear sweeping voltammograms (LSVs) on rotating ring-disk electrode for PeFeeC-600, PeFeeC-700, PeFeeC800, PeFeeC-900 and PeFeeC-1000 samples in O2-saturated 0.1 M KOH at a rotating speed of 1600 rpm. The disk potential was scanned at 10 mV s1 and the ring potential was fixed at 0.5 V; (b) Calculated electron transfer number (n) and determined peroxide percentage (HO 2 %) at various potentials based on the corresponding RRDE data in (a); (c) LSVs of oxygen reduction on PeFeeC-900 in O2-saturated 0.1 M KOH
To investigate the catalytic activity of the PeFeeC samples for the ORR, the CVs of the samples in N2-saturated 0.1 M KOH solution were first compared. As shown in Fig. 4, the area under CV (i.e. capacitance) first increases as the pyrolysis temperature increases to 900 C and then decreases as the pyrolysis temperature increases further to 1000 C. This is in good agreement with the trend observed from the BET specific surface area of the samples. Fig. 5a shows the LSVs of the PeFeeC samples in O2-saturated 0.1 M KOH solution measured with RRDE. The onset potential of pure carbon xerogel is 0.307 V and the current density of it at 0.8 V is 2.30 mA cm2. As compared with pure carbon xerogel, the onset potential of PeFeeC samples increases positively. The PeFeeC-600 shows a much higher onset potential of 0.247 V, it further increases to 0.238, 0.202 and 0.139 V for PeFeeC-700, PeFeeC-800 and PeFeeC-900 samples, respectively. But it decreases to 0.170 V for PeFeeC-1000. The diffusion limiting current density of the samples follows the same trend. The results clearly show that the improved catalytic activity of carbon xerogels for ORR can be achieved by P and Fe doping in carbon and the activity of PeFeeC is closely related to the pyrolysis temperature. The sample pyrolyzed at 900 C exhibits the highest catalytic activity for the ORR. Furthermore, as can be seen in Fig. 5a, although the obtained PeFeeC in this work shows high catalytic activity for ORR, its half-wave potential is 110 mV lower than that of the commercial Pt/C (20 wt.%) and is also lower than that of the Co and P co-doped carbon reported in our previous work (66 mV lower than that of the commercial Pt/C (20 wt.%)) [32]. The reason for the activity
with a scan rate of 10 mV s1 at different electrode rotating speeds. Inset: the corresponding KouteckyeLevich plots for ORR on PeFeeC-900.
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Fig. 7. The bulk P and Fe content in PeFeeC samples pyrolyzed at different temperatures.
calculated based on the RRDE data (Fig. 5a) using Equations (1) and (2), respectively, as shown in Fig. 5b. It can be seen that the corresponding HO 2 yield on pure carbon over the measured potential range (0.8 to 0.3 V) is about 55.0e91.3% and it decreases after P and Fe doping, following C > PeFeeC-600 > PeFeeC700 > PeFeeC-800 > PeFeeC-1000 > PeFeeC-900 > Pt/C. The HO 2 on PeFeeC-900 is only about 7.8e15.0% and slightly higher than the amount of HO2 produced on Pt/C (below 7.0%). The corresponding electron number transferred during the ORR process decreases in the reverse order. The electron number on PeFeeC900 is 3.74e3.89 over the potential range of 0.8 to0.3 V. This indicates that P and Fe doping modifies the ORR catalytic pathway of carbon xerogel, which is significantly influenced by pyrolysis temperature. Fig. 5c shows the KouteckyeLevich (KeL) plots at 0.4, 0.5 and 0.6 V for the PeFeeC-900 sample. The KeL plots are constructed using the following KeL equation [46]:
1 1 1 1 1 ¼ þ ¼ þ i iK iD nFkCO2 0:62nFC D2=3 n1=6 u1=2 O2 O2
Fig. 6. (a) XPS survey spectra of PeFeeC-900. (b) XPS spectra for the P 2p of PeFeeC900. (c) XPS spectra for the Fe 2p of PeFeeC-900.
difference between “co-doping of Fe and P” and “co-doping of Co and P” requires further considerable studies. The corresponding peroxide species (HO 2 ) produced on these catalysts and the electron numbers (n) transferred during the ORR process were
(3)
Where i is the measured current density on the polarization curve; iK and iD are the kinetic and diffusion limiting current densities, respectively; n is the apparent number of electrons transferred during the ORR; F is the Faraday constant (96,485C mol1), DO2 is the diffusion coefficient of O2 ðDO2 ¼ 1:86 105 cm2 s1 Þ, y is the kinetic viscosity of the solution (y ¼ 0.01 cm2 s1), CO2 is the concentration of O2 dissolved in electrolyte ð CO2 ¼ 1:21 106 mol cm3 Þ [47,48], and u is the electrode rotation speed. The average electron number calculated from the K-L plots at different potentials for PeFeeC-900 is 3.81. This is in good agreement with the result (n z 3.74e3.89) obtained from the RRDE measurements, suggesting a dominant four-electron pathway of ORR on PeFeeC-900. Note that the diffusion-limiting current densities (ID) of the samples vary significantly due to the different intrinsic catalytic activities of the samples, which are related to the electron transfer number n during the ORR based on Equation (3). To gain insight into the catalytic activity enhancement of P and Fe co-doped carbon xerogels, X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical species incorporated into carbon as well as their electronic states. The full XPS survey spectrum of a typical PeFeeC sample (PeFeeC-900) in Fig. 6a confirms the presence of C, O, P and Fe species in the sample. The
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are proposed as catalytic active sites of PeFeeC for ORR as analyzed from the XPS results. The high content of P and Fe along with the high specific surface area are believed to account for the high electrocatalytic activity of PeFeeC-900 catalyst. The stabilities of the PeFeeC samples for the ORR were tested with the chronoamperometric method as shown in Fig. 8. The stability of the commercial Pt/C (20 wt.%) catalyst is also included for comparison. The ORR current of PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900 and PeFeeC-1000 is decreased by 33.8%, 27.3%, 17.2%, 12.4% and 13.8%, respectively, over 43000 s of continuous operation. The PeFeeC-900 catalyst exhibits the slowest performance attenuation. However, the commercial Pt/C suffers a significant current loss of 52.3%. The high durability of PeFeeC samples, especially PeFeeC-900, can be attributed to the covalent bonding of Fe and P to carbon as well as the absence of metal agglomeration and migration (like Pt) during the long-term operation. Fig. 8. Current-time (iet) chronoamperometric responses for the ORR on PeFeeC-600, PeFeeC-700, PeFeeC-800, PeFeeC-900, PeFeeC-1000 and commercial Pt/C in O2saturated 0.1 M KOH at 0.35 V (vs. Ag/AgCl) for ORR at a rotating speed of 1600 rpm.
high-resolution P 2p spectrum of PeFeeC-900 (Fig. 6b) shows two deconvolved contributions at 133.2 eV and 134.6 eV, which reveals the presence of PeC and PeO bonding [49,50], respectively. The results indicate that the P atoms have been successfully incorporated into the carbon lattice. The deconvolution of Fe 2p spectrum shows three peaks (Fig. 6c). The peaks at 712.5 and 726.8 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively [51e53]. The peak at 718.1 eV is a satellite peak. These suggest that Fe3þ cations exist in the sample. Since no Fe-based carbide and phosphide are found in PeFeeC-900 as confirmed from the XRD results, these Fe3þ cations are most likely bound to P to form Fe-Px moieties embedded in carbon (like Fe-Nx, x is the coordination number of P atoms to Fe in this work), which can act as active sites for the ORR. Further investigation will be done to understand the nature of these active sites in future studies. The surface and bulk content of P and Fe in the PeFeeC samples were determined from XPS and ICP-AES measurements, respectively. They are very close to each other for the content of either P or Fe higher than 1.0 at%. Some samples contain P and/or Fe content below 1.0 at%, only the bulk contents of P and Fe in PeFeeC are shown in Fig. 7. It can be seen that both P and Fe content increase as the pyrolysis temperature increases and reach a maximum at 900 C, but decrease as the temperature increases further to 1000 C. The doping content of Fe and P are 1.06% and 1.97%, respectively, for the sample pyrolyzed at 900 C (i.e. PeFeeC-900). The results suggest that 900 C is the optimal pyrolysis/carbonization temperature for doping both Fe and P into carbon lattice. The increasing P and Fe content in carbon with the increasing pyrolysis temperature up to 900 C is due to the extraction of H,O and C in the form of H2O, CO and CO2 et al. during the carbonization, which increases as the pyrolysis temperature increases [54]. Meanwhile, the releasing of H2O, CO and CO2 et al. also creates large amount of pores in the bulk of samples. This is evidenced by the increasing BET specific surface areas of the samples pyrolyzed up to 900 C (Fig. 3 and Table 1). In contrast, both the P and Fe content decrease as the pyrolysis temperature further increases to 1000 C, which most likely results from the loss of FeePx groups at this high temperature because of the additional formation of Fe2P2O7, FeP and Fe2P in the sample as confirmed from the XRD results (Fig. 1). This also leads to the decreased BET specific surface area of the samples pyrolyzed at 1000 C (Fig. 3 and Table 1). Therefore, the catalytic activity of PeFeeC-1000 decreases. The FeePx moieties
4. Conclusions In conclusion, we have reported the successful fabrication of PeFeeC catalyst via an in situ solegel polymerization method followed by a pyrolysis process. The pyrolysis temperature has a significant effect on the structure, texture and the doping content of P and Fe, which further affects greatly the ORR catalytic activity of the PeFeeC electrocatalysts. The catalytic site of PeFeeC for the ORR is proposed as FeePx (x is the coordination number of P atoms to Fe), the nature of which will be further studied. Due to the high doping content of P and Fe and large specific surface area, the PeFeeC pyrolyzed at 900 C shows the highest catalytic activity for ORR among the samples pyrolyzed at different temperatures. Meanwhile, the PeFeeC-900 sample exhibits long-term durability, superior to commercial Pt/C. This in-situ polymerization process coupled with carbonization approach provides a tunable method for fabricating heteroatom-doped carbon for use in alkaline fuel cells and metal-air batteries. The species and content of heteroatom can be further tailored with this method for the catalytic activity optimization of the heteroatom-doped carbon toward ORR. Acknowledgments This work is supported by National Natural Science Foundation of China (Nos. 51272167 and 21206101). References [1] H.A. Gasteiger, S.S. Kocha, B. Somapalli, F.T. Wagner, Appl. Catal. B Environ. 56 (2005) 9. [2] P.G. Bruce, L.J. Hardwick, K.M. Abraham, Mater. Res. Soc. Bull. 36 (2011) 506. [3] M. Armand, J.M. Tarascon, Nature 451 (2008) 652. [4] S.Y. Wang, E. Iyyamperumal, A. Roy, Y.H. Xue, D.S. Yu, L.M. Dai, Angew. Chem. 123 (2011) 11960. [5] J. Liang, Y. Jiao, M. Jaroniec, S.Z. Qiao, Angew. Chem. Int. Ed. 51 (2012) 11496. [6] R.Z. Yang, W.Y. Bian, P. Strasser, M.F. Toney, J. Power Sources 222 (2013) 169. [7] Z.T. Cui, S.G. Wang, Y.H. Zhang, M.H. Cao, J. Power Sources 259 (2014) 138. [8] Y.Y. Shao, J.H. Sui, G.P. Yin, Y.Z. Gao, Appl. Catal. B Environ. 79 (2008) 89. [9] Z.S. Wu, S.B. Yang, Y. Sun, K. Parvez, X.L. Feng, K. Müllen, J. Am. Chem. Soc. 134 (2012) 9082. [10] F.Y. Cheng, J. Shen, B. Peng, Y.D. Pan, Z.L. Tao, J. Chen, Nature Chem. 3 (2011) 79. [11] Z.Y. Lin, G.H. Waller, Y. Liu, M.L. Liu, C.P. Wong, Carbon 53 (2013) 130. [12] L.J. Yang, S.J. Jiang, Y. Zhao, L. Zhu, S. Chen, X.Z. Wang, Q. Wu, J. Ma, Y.W. Ma, Z. Hu, Angew. Chem. Int. Ed. 123 (2011) 7270. [13] D.S. Yu, Y.H. Xue, L.M. Dai, J. Phys. Chem. Lett. 3 (2012) 2863. [14] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 6 (2012) 205. [15] M. Zhang, L.M. Dai, Nano Energy 1 (2012) 514. [16] K.P. Gong, F. Du, Z.H. Xia, M. Durstock, L.M. Dai, Science 323 (2009) 760. [17] L.P. Zhang, Z.H. Xia, J. Phys. Chem. C. 115 (2011) 11170. [18] Z.W. Chen, D. Higgins, A.P. Yu, L. Zhang, J.J. Zhang, Energy Environ. Sci. 4
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