Effects of cobalt precursor on pyrolyzed carbon-supported cobalt-polypyrrole as electrocatalyst toward oxygen reduction reaction
© Yuan et al.; licensee Springer. 2013
Received: 11 October 2013
Accepted: 5 November 2013
Published: 14 November 2013
A series of non-precious metal electrocatalysts, namely pyrolyzed carbon-supported cobalt-polypyrrole, Co-PPy-TsOH/C, are synthesized with various cobalt precursors, including cobalt acetate, cobalt nitrate, cobalt oxalate, and cobalt chloride. The catalytic performance towards oxygen reduction reaction (ORR) is comparatively investigated with electrochemical techniques of cyclic voltammogram, rotating disk electrode and rotating ring-disk electrode. The results are analyzed and discussed employing physiochemical techniques of X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma, elemental analysis, and extended X-ray absorption fine structure. It shows that the cobalt precursor plays an essential role on the synthesis process as well as microstructure and performance of the Co-PPy-TsOH/C catalysts towards ORR. Among the studied Co-PPy-TsOH/C catalysts, that prepared with cobalt acetate exhibits the best ORR performance. The crystallite/particle size of cobalt and its distribution as well as the graphitization degree of carbon in the catalyst greatly affects the catalytic performance of Co-PPy-TsOH/C towards ORR. Metallic cobalt is the main component in the active site in Co-PPy-TsOH/C for catalyzing ORR, but some other elements such as nitrogen are probably involved, too.
With advantages of high power density, low-operating temperature, and low emissions, proton exchange membrane fuel cells (PEMFCs) have become new electrical sources for transportable and stationary applications in recent years. There is no doubt that oxygen reduction reaction (ORR) in the cathode of PEMFCs is a key factor determining the cell performance. The ORR generally proceeds by two pathways : the direct four-electron-transfer reduction of oxygen that produces H2O and the two-electron-transfer reduction of oxygen yielding H2O2 which may be further reduced to H2O. Between them, the former route is an ideal path. Therefore, it is imperative to find an efficient catalyst that can enhance the direct four-electron-transfer reduction of oxygen to give H2O, in order to improve the efficiency of PEMFCs. To date, carbon supported Pt and/or its alloys have been widely accepted to be the most active catalyst for ORR, but the high cost and limited resource of Pt greatly hinder the large-scale commercialization. Hence, the development of low cost, efficient and stable non-precious metal catalysts for ORR has become the goal of worldwide fuel cell people.
In the last few decades, several types of non-precious metal ORR catalysts, including transition metal macrocyclic compounds [2, 3] and chalcogenides [4, 5], enzymatic catalysts , inorganic oxide composites , and conducting polymers or nitrogen containing catalysts [8–10], have been explored and the heat-treated transition metal-based nitrogen-containing complexes [11–17], such as porphyrins, phthalocyanines, dibenzotetraazaannulenes, phenanthrolines, polypyrrole (PPy), triethylenetetramine chelate, tripyridyl triazine, have been considered to be the most promising alternate. Among them, PPy has been paid much more attention because of the porous structure, high surface area, high conductivity, easy synthesis and excellent environmental adaptability [18, 19]. It can be used as a carrier of transition metal in the nitrogen-containing complex catalysts, where the metal particles can be fixed on its surface and physically dispersed, the interaction between PPy and metal particles can work as efficient active site for ORR [20, 21]. Recent researches on transition metal-based PPy-containing catalyst Co-PPy/C [1, 10, 21, 22] have demonstrated promising ORR activity and durability with both electrochemical experiments and single-cell performance measurements. More work is needed, however, to identify the ORR mechanism, the actual ORR active site and the effects of preparation techniques/parameters on the catalytic performance of this kind of catalyst.
In our previous work , a p-Toluenesulfonic acid (TsOH)-doped Co-PPy/C catalyst, namely Co-PPy-TsOH/C, has been successfully developed. The H2-O2 PEMFC with it as the cathode catalyst exhibited a peak power density of 203 mW · cm−2 with no back pressure used on either side of the cell. In the present research, a series of Co-PPy-TsOH/C catalysts have been synthesized with various cobalt precursors, and the catalytic performance towards ORR has been comparatively investigated in order to explore the effect of cobalt precursor. Then, diverse physiochemical techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), elemental analysis (EA), and extended X-ray absorption fine structure (EXAFS) analysis, have been employed to understand the results.
Synthesis of Co-PPy-TsOH/C catalysts
The Co-PPy-TsOH/C catalysts were synthesized from various cobalt precursors with a procedure previously reported . Specifically, 0.6 g BP2000 carbon powder (Cabot company, Boston, MA, USA), previously treated with 6 M HNO3 for 8 h at 100°C, was ultrasonically dispersed in 100 ml isopropyl alcohol for 30 min, followed by an addition of 3 mmol of freshly distilled pyrrole and 100 ml double-distilled water and stirring for another 30 min. Subsequently, 100 ml ammonium peroxydisulfate solution with a concentration of 0.06 M and 0.1902 g TsOH were added and then stirred at room temperature for 4 h. Finally, the mixture was filtered, washed at least 3 times with double distilled water and alcohol alternately, and then dried at 45°C under vacuum for 12 h to obtain PPy-modified carbon which is named as PPy-TsOH/C. Then, 0.5 g PPy-TsOH/C and appropriate amount of cobalt salt (cobalt chloride, cobalt nitrate, cobalt oxalate, or cobalt acetate) were blended with 200 ml double-distilled water. After ultrasonic mixing for 1 h and vigorous stirring for 2 h, the solvent was evaporated under reduced pressure. The obtained powders were then heat-treated at 800°C for 2 h under an argon atmosphere to obtain the Co-PPy-TsOH/C catalysts.
Electrochemical characterization of Co-PPy-TsOH/C catalysts
Electrochemical performance evaluation of the Co-PPy-TsOH/C catalysts was performed at room temperature of about 25°C with a standard three-electrode system. A Pt wire was used as the counter electrode, while a saturated calomel electrode (SCE) was used as the reference electrode and a catalyst-covered glassy carbon disk with a diameter of 4 mm as the working electrode. A 0.5 M H2SO4 aqueous solution was used as the supporting electrolyte. The ink-type working electrode was fabricated with the following steps: a catalyst ink was prepared using 6 mg catalyst and 50 μL Nafion® solution (5 wt%, DuPont, Wilmington, DE, USA) in 1 ml double-distilled water. After sonicating for 30 min, 10 μl of the ink was deposited onto the glassy carbon disk to completely cover the surface with a thin film and then air-dried. The catalyst was electrochemically activated by repeatedly scanning the potential in a range from 0.8 to −0.2 V (vs. SCE) at a rate of 50 mV · s−1 in an oxygen-saturated H2SO4 solution until stable voltammograms were achieved. Then, the cyclic voltammogram (CV) curve was recorded, in oxygen-saturated 0.5 M H2SO4 solution, in the same potential range at a scan rate of 5 mV · s−1 controlled by an electrochemical station (CHI instrument, Austin, TX, USA). The rotating disk electrode (RDE) measurement of the catalysts after activation was conducted by scanning the electrode potential from 0.8 to −0.2 V (vs. SCE) at a rate of 5 mV · s−1 and with an electrode rotating rate of 900 rpm in argon and oxygen-saturated 0.5 M H2SO4 solution, respectively. The rotating ring-disk electrode (RRDE) measurement was conducted with the same three-electrode system controlled by a CHI 750 bipotentiostat (CHI instrument, USA) along with a model 636 RRDE system (Pine Instrument, Grove City, PA, USA). A RRDE was employed as the working electrode, while the counter electrode, the reference electrode, and the electrolyte were the same as described above. During the working electrode fabrication, 20 μl of the catalyst ink was spread onto the surface of the disk only. The polarization curves were measured in argon and oxygen-saturated 0.5 M H2SO4 solution, respectively, at a potential scanning rate of 5 mV · s−1 from 0.8 to −0.2 V (vs. SCE), electrode rotating rate of 900 rpm and ring potential of 1.0 V (vs. SCE).
In the following contents, all the potentials reported are quoted to normal hydrogen electrode (NHE) except specially stated.
Physicochemical characterization of Co-PPy-TsOH/C catalysts
Crystal/phase structure of the Co-PPy-TsOH/C catalysts were identified by a Rigaku D/MAX-2200/PC XRD instrument (Shibuya-ku, Japan) using Cu K α radiation (λ = 1.546 Å) at a tube current of 30 mA and a tube potential of 40 kV. The scanned two-theta range was from 20° to 80° at a rate of 6° · min−1 with a step size of 0.02°.
Microstructure of the Co-PPy-TsOH/C catalysts was recorded on a JEOL JEM-2100 TEM instrument (Akishima-shi, Japan) operated at 200 kV. After ultrasonic dispersion in ethanol, a drop of the sample was dispersed on a Cu grid for analysis under different magnifications.
Raman spectra of the Co-PPy-TsOH/C catalysts were captured on a UV–vis Raman System 1000 equipped with a charge-coupled device (CCD) detector (Renishaw, Wotton-under-Edge, UK). A CCD camera system with monitor was used to select the location on the sample from which the Raman spectra were taken. Each Raman spectrum was calibrated by an external pen-ray Ne-lamp.
Chemical state of nitrogen in the Co-PPy-TsOH/C catalysts was acquired on a PHI Quantum 2000 XPS instrument (Chanhassen, MN, USA) using Al K α radiation with a power of 250 W and pass energy of 14 eV. The data analysis was conducted by AugerScan3.21 software and the peak fitting was carried out with XPS Peak 4.1 software.
Cobalt content in the Co-PPy-TsOH/C catalysts was detected by a Thermal iCAP 6000 Radial ICP spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). By soaking the catalyst samples in aqua regia, cobalt ions can be dissolved in the solution. The content of cobalt in the catalysts can then be determined by measuring the concentration of Co ions in the solution. Contents of non-metallic elements, including N, C, S, and H, in the Co-PPy-TsOH/C catalysts were determined by EA with a PerkinElmer PE 2400 II CHNS/O analyzer (Waltham, MA, USA). To ensure the reliability of the results, each sample was measured twice and the average was recorded as the elemental content. The residual other than Co, N, C, S, and H was calculated to be the oxygen content.
EXAFS analysis of the Co-PPy-TsOH/C catalysts was performed at beamline BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF). Si (111) double-crystal monochromator was used to select the energy. X-ray absorption data were collected at room temperature in the transmission mode. Gas ion chamber detectors were used. The specimens were prepared by brushing the powders over an adhesive tape and folding it several times for uniformity. Some samples were also made as pellets. Data processing and analysis were done with IFEFFIT software.
Results and discussion
Hereto, it could be summarized with the electrochemical study of CV, RDE, and RRDE experiments that the cobalt precursor for the Co-PPy-TsOH/C catalysts significantly affects the ORR activity as well as the mechanism. The electrochemical performance, including both the ORR activity and four-electron-transfer reaction selectivity, of the Co-PPy-TsOH/C catalysts decrease in the order that cobalt acetate > cobalt nitrate > cobalt chloride > cobalt oxalate.
Crystallite size of metallic cobalt in Co-PPy-TsOH/C catalysts prepared from various cobalt precursors
Crystallite size of metallic co/nm
D -band and G -band intensities of carbon in Co-PPy-TsOH/C catalysts prepared from various cobalt precursors and calculated graphitization degree
D-band intensity (I D /a.u.)
G-band intensity (I G /a.u.)
Graphitization degree (I G /I D )
Surface atomic concentration of different types of nitrogen in Co-PPy-TsOH/C catalysts prepared from various cobalt precursors
Effects of cobalt precursors on electrochemical performance of Co-PPy-TsOH/C as catalyst towards ORR have been comparatively studied, and the results have been analyzed with diverse physiochemical techniques. The following conclusions could be drawn from this research: (1) cobalt precursors affect both the catalytic activity of the Co-PPy-TsOH/C catalysts and the corresponding ORR mechanism; (2) the electrochemical performance, including both the ORR catalytic activity and the selectivity to four-electron-transfer reaction, of the Co-PPy-TsOH/C catalysts follows the order with respect to the used cobalt precursor that cobalt acetate > cobalt nitrate > cobalt chloride > cobalt oxalate; (3) the synthesis process, especially the high-temperature pyrolysis, of the catalyst could be interfered by the used cobalt precursors, resulting in different microstructure, morphology, elemental state as well as the ORR performance; (4) lower graphitization degree of carbon and smaller crystallite/particle size of metallic cobalt and the uniform distribution in Co-PPy-TsOH/C catalysts lead to better ORR performance; (5) metallic cobalt is a main component forming the ORR active site in the Co-PPy-TsOH/C catalysts, but some other elements such as nitrogen is probably also involved; and (6) Co-N bond/structure is not necessary to forming a catalytic active site toward ORR in Co-PPy-TsOH/C catalysts, and a small-amount coexistence of CoO in the catalysts does not have an adverse effect on the electrochemical performance.
The authors are grateful for the financial support of this work by the National Science Foundation of China (21176155), the STCSM of China (10JC1406900) and the Open Foundation of State Key Laboratory of Physical Chemical of Solid Surfaces in Xiamen University (No.200708). The authors also thank beamlines BL14W1 and BL08UA1(STXM) of SSRF (Shanghai Synchrotron Radiation Facility) for providing the beam time.
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