Fuel cells are highly promising energy conversion devices because they are renewable and have low emissions and high efficiencies. The oxygen reduction reaction (ORR) significantly affects the cathode performance in fuel cells, and efficient ORR electrocatalysts are necessary for practical applications of fuel cells [1-5]. The main products of the ORR are OH− and HO−2, and are determined by the major component of the electrode material, electrode potential, and electrolyte [6-8]. Pt and its alloys are currently regarded as the best catalysts for the ORR [9, 10]. However, the limited reserves of Pt, high cost, activity deterioration with time, and poor durability severely hinder large-scale use of Pt in ORRs [11-13]. The development of nonprecious-metal and Pt-free catalysts for fuel cells is therefore important [14-18].
Carbon materials have many advantages such as excellent catalytic activity, low costs, long-term stability, and good tolerance of the methanol crossover effect [19-21]. Carbon materials for use in the ORR as alternatives to Pt-based catalysts have therefore attracted enormous interest. It was recently reported that nitrogen-doped carbon materials show excellent ORR electrocatalytic activity because of their unique electronic properties derived from conjugation between nitrogen lone-pair electrons and the graphene system, i.e., nitrogen doping can improve the electron-donor properties, improving interactions between carbon and oxygen molecules to create metal-like conductivity [22-25]. Dai’s group [26] used chemical vapor deposition to produce a nitrogen-doped graphene (NG) that showed better ORR catalytic activity than Pt in alkaline fuel cells. They also developed nitrogen-doped carbon nanotube arrays that showed better electrocatalytic activity, long-term operational stability, and tolerance to the methanol effect than did Pt in oxygen reduction in alkaline fuel cells [27]. Shen’s group [28] prepared an ORR catalyst consisting of nitrogen-doped porous carbon particles with a high proportion of pyridinic nitrogen atoms using a combination of the Maillard reaction and thermal annealing in an NH3 atmosphere. Xian’s group [29] synthesized a non-precious-metal oxygen electrode electrocatalyst consisting of MoS2/NG by physically mixing MoS2 sheets with NG; the catalyst showed improved electrocatalytic activity in the ORR based on a four-electron pathway in alkaline solutions.
Different preparation conditions can change the structural properties of material such as the specific surface area, electronic conductivity, doping content, and doping state, and these significantly affect the ORR electrocatalytic performance of nitrogen-doped carbon materials. Various direct synthetic methods such as spray pyrolysis, chemical vapor deposition, and solvothermal reactions are effective for preparing nitrogen-doped carbon materials, but high costs, low yields, and complicated processes make these methods unsatisfactory [30-32]. Despite much effort using various synthetic methods, the development of cheap, efficient, and simple methods for the production of nitrogen-doped nanocarbons with excellent catalytic performance in ORRs is still a challenge [33-36].
In this study, we developed an improved approach based on direct one-step pyrolysis for preparing nitrogen-doped carbon nanoblocks (NCNBs) with high nitrogen contents (4.11%). We used tris(hydroxymethyl)aminomethane (Tris) as the carbon and nitrogen sources because it is a cheap and readily available small molecule containing hydroxyl and amino groups. The NCNBs obtained by pyrolysis below 800 °C had diameters of approximately 60 nm and gave a good catalytic performance in the ORR. This can be ascribed to the electron-accepting ability of the nitrogen atoms resulting in creation of net positive charges on adjacent carbon atoms, and these readily attract electrons. The morphology of the prepared NCNBs was studied using scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to identify the surface nitrogen groups. The electrochemical performance of the NCNBs in the ORR was evaluated in detail.
Tris was purchased from Sigma. A saturated solution of Tris was prepared using double-distilled water. A 20% Pt/C catalyst was purchased from Aladdin. All other chemicals (analytical grade) were purchased from the Beijing Chemical Reagent Company (Beijing, China) and used without further purification. Ultra-pure water was obtained using a Milli-Q plus water purification system (18 MW, Milli-pore Co., Ltd., USA).
SEM images were obtained using a Hitachi S-2600N instrument. Elemental analysis was performed using a Flash EA 1112 instrument. XPS was performed using a VG Micro-tech ESCA 2000 instrument with a monochromatic 15 Al X-ray source. Fourier-transform infrared (FTIR) spectroscopy was performed using an EQUINOX 55 instrument (Bruker, Germany). Electrochemical measurements were performed using a computer-controlled electrochemical analyzer (CHI600E, Chenhua, China) in a two-compartment electrochemical cell with a bare or modified glassy carbon electrode (GCE, diameter 3 mm) and a rotating ring disk electrode (RRDE; PINE) of diameter 4 mm as the working electrode. The geometric area of the RRDE was 0.1256 cm2. Pt wire was used as the counter electrode, and the reference electrode was Ag/AgCl (3 mol/L-KCl).
A saturated solution of Tris was placed in a quartz-tube reactor and dried overnight at 70 °C. The quartz-tube reactor was transferred to a tube furnace and pyrolyzed in a vacuum at 700, 800, or 900 °C for 2 h, to prepare different NCNBs, denoted by NCNBs-700, NCNBs-800, and NCNBs-900, respectively.
A GCE of diameter 3 mm and geometric area 0.07065 cm2 was used as the working electrode. The GCE was polished with 1.0, 0.3, and 0.05 μm alumina slurries, and then washed with ultraclean water and ethanol, respectively, to form a mirror-like surface. NCNBs (6 mg) were ultrasonically dispersed in dimethylformamide (DMF, 1 mL). The prepared ink solution (5 μL) was dropped onto the GCE. The working electrode was prepared as follows. The NCNB suspension (5 μL, 6 mg/ mL in DMF) was dropped onto the GCE surface, and exposed to air to evaporate the solvent. The electrode surface was covered with Nafion solution (5 μL, 0.5%, 5 diluted 10 times with deionized water) and dried to form a modified working electrode. The electrode was rinsed thoroughly with deionized water to remove physically adsorbed materials to give an NCNBs/GCE. The modified working electrodes were used to study ORR activity.
All electrochemical experiments were performed at room temperature. The NCNBs/GCE was pretreated by electrochemical oxidation in a phosphate-buffered solution (pH = 6.8) at a potential of 1.7 V (vs Ag/AgCl) for 300 s at room temperature, followed by potential sweeping from 0.0 to 1.4 V in H2SO4 (0.5 mol/L) until a stable voltammogram was obtained. The surface ORR activity of the catalyst in O2-saturated KOH (0.1 mol/L) was investigated by linear sweep voltammetry (LSV) of the NCNBs/GCE from 0.2 to −1.0 V at a rate of 10 mV/s. For quantitative measurements of the ORR activity, LSV of the catalyst-coated RRDE at a scan rate of 5 mV/s in O2-saturated KOH solution was conducted at various rotation rates from 400 to 2025 r/min [37, 38].
Fig. 1 shows the X-ray diffraction (XRD) patterns of the samples. Pyrolysis significantly changes the Tris diffraction peak. The NCNBs formed at different temperatures all give two broad peaks characteristic of turbostratic carbon. The diffraction peaks of NCNBs-800 are stronger than those of NCNBs-700 and NCNBs-900, indicating a high graphitization degree at this temperature. The NCNBs-800 peak is much narrower than those of the other samples, showing that 800 °C can promote graphitization of the carbon formed during carbonization.
The ORR activities of the NCNBs were investigated by performing LSV in N2- and O2-saturated KOH solution (0.1 mol/L). Fig. 2 shows that the ORR reduction peak potentials for the NCNBs-700 and NCNBs-900 are −0.26 and −0.24 V (vs Ag/AgCl), respectively. However, the ORR reduction peak potential for the NCNBs-800 is −0.20 V (vs Ag/AgCl), i.e., positive shifts of −0.06 and −0.04 V compared with those for the NCNBs-700 and NCNBs-900, respectively, occurred; a concomitant increase in the peak current density was observed. These results clearly show significant enhancement of the ORR electrocatalytic activity for NCNBs produced at 800 °C. The high electrocatalytic activity of the NCNBs can be attributed to the electron-accepting ability of the nitrogen atoms, which creates a net positive charge on adjacent carbon atoms in the carbon plane of the NCNBs; electrons are readily attracted from the anode, facilitating the ORR.
Fig. 3(a) shows SEM images of the NCNBs-800. The images obtained at various magnifications show that the NCNBs-800 are distributed uniformly in blocks of diameter about 60 nm. The molecular structure of the NCNBs-800 was investigated using FTIR; the spectrum is shown in Fig. 4. The peak at 3424 cm−1 can be assigned to N-H stretching vibrations. The peaks at 2550, 1661, 1634, and 700 cm−1 are assigned to the stretching vibrations of C≡N, C=N, C=C, and cis-olefin, respectively, suggesting elimination of -OH and -NH2 groups. The FTIR results suggest that nitrogen and carbon atoms with different degrees of saturation were formed during high-temperature Tris pyrolysis. The improved ORR performance is attributed to the electron-accepting ability of nitrogen atoms, which creates a net positive charge on adjacent carbon atoms in the NCNBs-800, enabling attraction of electrons and ORR facilitation [39, 40].
XPS was performed to determine the surface elements in the NCNB structure; the spectra are shown in Fig. 5. The XPS survey spectrum of the NCNBs-800 (Fig. 5(a)) shows narrow graphitic C 1s (284.86 eV), N 1s (399.82 eV), and O 1s (532.67 eV) peaks, without any impurities, indicating the presence of a few oxygen-containing groups on the NCNBs-800, which will assist NCNBs-800 dispersion. The spectrum clearly indicates the presence of nitrogen in the NCNBs-800, indicating efficient nitrogen doping. The C 1s peak for the NCNBs-800 is centered at approximately 284.6 eV, with a tail at higher binding energy, which is a common effect for nitrogen-doped carbon materials. The carbon species in the NCNBs-800 were further investigated based on the high-resolution C 1s spectrum (Fig. 5(b)). The spectrum has peaks at 285.5, 289.3, 284.5, and 286.7 eV, which correspond to C=C, C-N, C-C, and C-O, respectively. The π-π conjugated C=C functional groups can accelerate electron transfer in the ORR. The N 1s XPS spectrum of the NCNBs-800 can be deconvoluted into three peaks, centered at 398.3, 400.0, and 401.1 eV. The three individual peaks indicate the presence of three types of nitrogen bonding. The peaks at 398.3 and 400.0 eV are assigned to pyridinic and pyrrolic nitrogen, respectively, contributing one and two π electrons, respectively, to the π-conjugated network. The peak at 401.1 eV is assigned to quaternary nitrogen, indicating that nitrogen atoms replaced carbon atoms in the NCNBs-800. All these nitrogen atoms play important roles in the ORR catalytic activities of the carbon materials, especially the pyridinic and pyrrolic nitrogen. The reported positions of different nitrogen types differ widely for different studies, possibly because of different nitrogen environments. The ORR performance depends on the bonding configurations of the nitrogen atoms in carbon materials [4]. When the nitrogen lone-pair electrons are hybridized with sp2 carbon atoms, they provide electrocatalytically active sites in the NCNBs-800. The peak areas change significantly depending on the pyrolysis temperature, which suggests that different numbers of nitrogen atoms with various bonding configurations were formed during pyrolysis at various temperatures. Fig. 5(f) shows the percentages of different types of nitrogen in the samples produced at different pyrolysis temperatures. For pyrolysis at 800 °C, pyrrolic nitrogen is dominant, but the pyrrolic nitrogen contents at 700 and 900 °C are lower. The amount of quaternary nitrogen formed at 800 °C is lower than those formed at 700 and 900 °C. These results imply that the pyrolysis temperature influences the ratios of pyridinic, pyrrolic, and quaternary nitrogen. It can promote the formation of pyrrolic nitrogen and suppress the generation of quaternary nitrogen; this is beneficial because nitrogen provides highly active sites for the ORR.
Electrochemical impedance spectra can show changes in the surface properties of a modified electrode. The diameter of the semicircle at higher frequencies corresponds to the electron transfer resistance, and diffusion corresponds to the linear portion at low frequencies. Fig. 6 shows that the resistance, including the series resistance and charge-transfer resistance, of the NCNBs-800 is comparable to that of the Pt/C electrocatalyst, but significantly lower than that of the bare GCE, indicating that the fabricated NCNBs-800 have good conductivity and superior electrocatalytic activity.
We performed LSV measurements using an RRDE in an O2-saturated electrolyte solution consisting of KOH (0.1 mol/L) KOH to gain further insights into the ORR activity of the NCNBs-800. Fig. 7(a) shows the LSV curves of the NCNB/GCE at various rotational speeds from 400 to 2025 r/min. RRDE voltammograms were obtained at a disk electrode scanning rate of 5 mV/ s and a constant ring potential of 0.5 V, to oxidize the HO−2 intermediate. The electron transfer number n and HO−2 intermediate production percentage (HO−2 (%)) were determined as
where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be 0.4. The calculated HO−2 (%) value for the NCNBs-800 is about 52% to 35% at potentials ranging from −0.3 to −0.8 V. The calculated n value for the NCNBs-800 is about 3.0-3.4 from −0.3 to −0.8 V. These results show that electrocatalysis of the ORR using the NCNBs-800 involves a simultaneous four-electron pathway and a two-electron transfer pathway. This excellent electrocatalytic activity shows that most of the doped nitrogen atoms are on the NCNBs-800 surface, especially the quaternary and pyridinic nitrogen, and oxygen molecules can be activated on both these types of doped nitrogen. Adjacent carbons transfer electrons to quaternary nitrogen atoms and the nitrogens donate electrons back to the adjacent C pz orbitals. The donation and back-donation processes play an important role in the ORR by facilitating O2 dissociation on the adjacent carbon atoms, assisting formation of strong O-C chemical bonds. Pyridinic nitrogen located on the NCNBs-800 surface can activate oxygen via direct bonding with the nitrogen lone-pair electrons; this promotes the ORR.
Long-term stability is a vitally important parameter for the ORR performance of a fuel cell. The durabilities of the NCNBs-800 and Pt/C catalysts in the ORR were tested using chronoamperometric measurements at −0.2 V; the stability curves of a commercial (20%)Pt/C catalyst were also obtained under the same conditions for comparison. Fig. 8 shows that the current densities for both the NCNBs-800 and Pt/C electrodes initially decreased with time. However, the decrease for the NCNBs-800 electrode was much slower than that for the Pt/C electrode. After 2000 s, the NCNBs-800 catalyst lost 17.56% of its initial catalytic activity; this is better than the performance of the commercial Pt/C catalyst, which lost 30.71% of its initial catalytic activity. These results indicate that the NCNBs-800 prepared in this study are much more stable than a Pt/C catalyst in alkaline solution.
We have developed metal-free NCNBs with high nitrogen contents (4.11%). They gave good electrocatalytic performance in ORRs. The NCNBs were fabricated using a scalable, one-step process involving Tris pyrolysis at 800 °C. This synthetic method can be used to produce low-cost and scalable carbon-based and nitrogen-doped carbon materials. The ORR onset potential of the catalyst was −0.05 V (vs Ag/AgCl) and the ORR reduction peak potential was −0.20 V (vs Ag/AgCl). The NCNBs showed higher electrocatalytic activity, better long-term stability, and better tolerance of the methanol crossover effect compared with a commercial Pt/C ORR electrocatalyst. These results show that NCNBs prepared by pyrolysis of Tris as nitrogen and carbon sources at 800 °C provide a promising ORR catalyst for fuel cells.
2016,
Vol. 37 Issue (8): 1275-1282 
PDF (738 KB)
2016,
Vol. 37 Issue (8): 1275-1282 PDF (738 KB)