Metal-air batteries have attracted considerable interest because of their many advantages,such as environmental friendliness,high conversion efficiency,quick start-up and high energy density. However,improvements in the cathode catalysts are still the challenge for their application [1]. Bifunctional catalysts play key roles in metal-air batteries for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Platinum and its alloys are often used as mono-functional catalysts for ORR [2-6]. Ruthenium or iridium-group metals remain the best known catalysts for OER but show poor performance for ORR [7, 8]. Therefore,a cheap and bifunctional catalyst should be developed to meet the application.
Heteroatom-doped carbon has been much studied for ORR. The introduction of a heteroatom changes the local charge density as well as asymmetry spin density of the carbon lattice,which creates catalytic sites for ORR [9-20]. Doped carbon has been explored as a bifunctional oxygen catalyst. Zhang et al. [21],Li et al. [22] and Ma et al. [23],respectively,explored nitrogen and phosphorus co-doped carbon as a bifunctional oxygen catalyst. With N,P co-doped carbon,OER is facilitated by N,P co-doped graphene,and the active site for ORR is the N dopant and it is enhanced by the P dopant [21, 22].
Efforts to obtain bifunctional catalysts have resulted in cobalt-basedcatalysts supported on heteroatom-doped carbon. The cobalt-based catalysts have variable valence states for ORR and OER,but are hindered by a low electrical conductivity,which can be improved by its being hybridized with heteroatom-doped carbon. Liang et al. [24] designed Co3O4 hybridized with N-doped reduced graphene oxide (rGO) as a bifunctional oxygen catalyst,in which the N species enhanced ORR activity and Co3O4 catalyzed OER. Cobalt and N-doped carbon nanotubes showed high performance in a wide pH range for ORR and OER because of the combination of the embedded structure and doped nitrogen [25]. Co3O4 on N-doped carbon nanoweb has a large specific surface area and abundant active sites for catalyzing ORR and OER [26]. MnCo2O4 hybridized with N-rGO exhibited enhanced ORR reactivity [27, 28]. Liang et al. [27] demonstrated that the substitution of Co3+ sites by Mn3+ increased the ORR reactivity compared with the pure Co3O4,but decreased the OER activity,which was attributed to Co2+ as the active sites for ORR [29] and Co3+ for OER [27]. NiCo2S4 supported on N- and S-doped rGO has high ORR activity because of a synergistic effect. The doped rGO served as a synergist while NiCo2S4 was critical for OER reactivity [30]. CoFe2O4 supported on N- and S-doped rGO has enhanced activity for ORR and OER because of a similar synergistic effect [31]. Jin et al. [32] developed a one-pot method to prepare cobalt-cobalt oxide/N-doped carbon hybrids for OER,which showed excellent activity because of the variable valence stat es of cobalt,carbon-encapsulated structure and nitrogen dopant.
There are still few studies on cobalt-based catalyst supported on N,P co-doped carbon. Here,Co3O4 supported on N,P co-doped carbon (Co3O4/NPC) was fabricated as a bifunctional catalyst for ORR and OER. Co3O4/NPC was prepared by two steps. First,NPC was pyrolyzed from melamine and phytic acid supported on carbon particles. Then Co3O4/NPC was synthesized by the solvothermal method followed by oxidization in air. The activity of Co3O4/NPC for OER was enhanced due to a synergistic effect between Co3O4 and NPC,and the stability was improved mainly by Co3O4. For ORR,even though the activity was enhanced by the P dopant in NPC,the N,P co-dopants have little effect on the activity of Co3O4/NPC and Co3O4 played the key role. Because of its overall oxygen electrode activity and stability,Co3O4/NPC will be useful as a bifunctional electrocatalyst for OER and ORR.
To prepare NPC,0.55 g melamine,0.2 g Vulcan XC-72,32 mL deionized water and 4 mL formaldehyde solution (37%) were heated and stirred at 70 °C for 5 min to form a homogeneous ink. Then phytic acid amine solution,which contained 2 mL phytic acid solution (50%),8 mL deionized water and 2.5 mL ammonia solution (25%) was added and the solution was stirred for another 10 min. The mixture was transferred into a 100 mL autoclave and heated at 160 °C for 4 h with a heating rate of 10 °C/min. After cooling,the powder was collected by filtration,washed,and dried. The dried powder was heated at 750 °C for 1.5 h under Ar. N-doped carbon (NC) was prepared without phytic acid amine solution and the other conditions were the same.
To prepare Co3O4/NPC,0.09 mmol Co(OAc)2·4H2O and 0.18 mmol urea were added into 10 mL ethylene glycol solution under continuous magnetic stirring. After the metal salt was dissolved,0.075 g NPC was added and stirring continued for 30 min to form a homogeneous suspension. The mixture was transferred to a 25 mL autoclave and heated at 180 °C for 4 h. After cooling,the product was collected by centrifugation,washed with deionized water and dried overnight. The product was calcined in air at 350 °C for 2 h. Co3O4/C was prepared using Vulcan XC-72 as the support instead of NPC.
Pt/C (20 wt% Pt on Vulcan XC72) was purchased from Sigma-Aldrich.
The sample was pressed into a powder for X-ray diffraction (XRD) detection on a Rigaku UItimaIV X diffractometer using nickel filtered Cu Kα radiation (λ = 0.15406 nm). Transmission electron microscope (TEM) images were taken by a Tecnai G2 F20 microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250Xi with a monochromic Al X-ray source.
All electrochemical tests were carried out on a Princeton Parstat 2273 electrochemical work station with a three- electrode cell. A platinum wire was employed for the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. A glassy carbon electrode (5 mm in diameter) coated with catalyst was used as the working electrode and as the rotating disk electrode. The working electrode was modified with the catalyst by dropping catalyst ink on the glassy carbon surface. The catalyst ink consisted of 1.31 mg of catalyst,0.9 mL deionized water and 0.1 mL Nafion solution (5 wt%) mixed by sonication. The catalyst ink (0.015 mL) was added dropwise onto the glassy carbon surface. The catalyst loading was about 0.1 mg/cm2.
Cyclic voltammetry (CV) experiments were done with a scan rate of 50 mV/s in N2 or O2-saturated KOH solution (0.1 mol/L). Linear sweep voltammetry (LSV) experiments with a rotating disk electrode were carried out with a scan rate of 10 mV/s at various rotating rates in O2-saturated KOH (0.1 mol/L). The stability of the samples was evaluated by chronoamperometry (current versus time) in O2-saturated KOH (0.1 mol/L). Electrochemical impedance spectra (EIS) were recorded in the frequency range from 100 kHZ to 10 mHZ with an AC signal amplitude of 5 mV. To ensure O2 saturation,the gas was bubbled into the electrolyte prior to the start of each experiment and maintained over the electrolyte during each measurement.
The preparation of Co3O4 supported on NPC is shown in Fig. 1. Initially,melamine (M) was reacted with formaldehyde (F) to form M-F,followed by adding phytic acid (PA) solution. Then the mixture was heated to 160 °C for 4 h for the cross-linking of M-F and the esterification of M-F and PA in the presence of the carbon nanoparticles (C) to form (M-F-PA)n-C,in which C was covered by (M-F-PA)n. To obtain NPC,(M-F-PA)n-C was pyrolyzed at 750 °C for 1.5 h under Ar. Co3O4 was synthesized by the solvothermal method followed by oxidization in air at 350 °C.
XRD patterns shown in Fig. 2 revealed the crystalline structure of Co3O4/C,NPC and Co3O4/NPC. The diffraction peak at 2θ = 24.6° originated from the (002) plane of carbon. The peaks at 2θ = 18.8°,31.1°,36.6°,44.6°,59.0° and 65.0° corresponded to the (111),(220),(311),(400),(511) and (440) planes of crystalline Co3O4 in Co3O4/C,respectively. They are less well defined in Co3O4/NPC. The TEM images of these samples revealed that some nanoparticles and nanorods grew on the carbon powder (Fig. 3).
The composition of Co3O4/NPC was investigated by XPS to reveal the valence state of the elements in the near surface region,as shown in Fig. 4(a). XPS revealed 84.8 at% C,10.9 at% O,2.1 at% Co,1.2 at% N and 0.9 at% P in Co3O4/NPC. In the Co 2p spectra in Fig. 4(b),the Co 2p3/2 and Co 2p1/2 doublets were located at 781 and 797 eV,respectively. The peaks at 781.7 and 796.4 eV were assigned to Co3+,and the peaks at 782.9 and 797.9 eV were assigned to Co2+. The satellite peaks at 786 and 803 eV were two shake-up peaks of Co at the high binding energy side of the Co 2p3/2 and Co 2p1/2 edge,which indicated the presence of Co2+ or Co3+ in the sample [31, 33]. The peak of N 1s (Fig. 4(c)) with a binding energy at 400 eV was deconvoluted into three peaks,revealing the presence of three types of nitrogen,pyridinic N (398.8 eV),pyrrolic N (399.9 eV) and graphitic N (400.6 eV). The high resolution P 2p spectrum (Fig. 4(d)) was also deconvoluted into three peaks,which were assigned to P-O (134.4 eV),P-N (133.7 eV) and P-C (133.1 eV) [19, 23].
To investigate the catalytic activity for OER,linear sweep voltammetry was carried out at a rotating rate of 1600 r/min in O2-saturated KOH solution (0.1 mol/L) for Co3O4/C,NPC and Co3O4/NPC (Fig. 5(a)). The OER reactivity over the samples was evaluated by the onset potential and current density. Other samples,including single N-doped carbon (NC) and commercial 20 wt% Pt/C,were also examined for comparison. NC only showed a plateau in the tested potential range. By contrast,the onset potential of NPC occurred at 0.66 V (vs SCE) and the current density reached 8.17 mA/cm2 at 0.80 V. These results demonstrated that the N-dopant has little activity and the N,P co-dopants were critical for OER,which was in agreement with other reported work [21, 22]. Co3O4/C was more active than NPC,which was evident from the lower onset potential (0.57 V) and higher current density (11.04 mA/cm2 at 0.80 V). Co3O4/NPC showed the highest activity of all the samples with
an onset potential of 0.54 V and current density of 21.95 mA/cm2 at 0.80 V. The improved OER reactivity of Co3O4/NPC was further confirmed by the Tafel slope of 93 mV/dec at a low overpotential (Fig. 5(b)),while that of NPC was 104 mV/dec and Co3O4/C was 132 mV/dec. Because Co3O4 supported on N-doped carbon has little improving effect on OER [24],the high activity of Co3O4/NPC was attributed to the synergistic effect of Co3O4 and N,P co-dopants.
The charge transfer of the samples was compared by the electrochemical impedance spectra (EIS). As shown in Fig. 5(c),the smallest diameter of the semicircle in the Nyquist plot of Co3O4/NPC suggested that Co3O4/NPC had the lowest charge transfer resistance during OER. The stability of these three samples was compared by chronoamperometry (current versus time) at a rotating rate of 1600 r/min in O2-saturated KOH for OER. As shown in Fig. 5(d),the percentage of the remaining current dropped to 21% for NPC after 6 h,to 79% for Co3O4/C and to 46% for Co3O4/NPC. Hence,the surface N,P co-dopants were unstable for OER but were improved by being hybridized with Co3O4. In addition,the active site for OER may be related to Co3O4 in Co3O4/NPC,which was shown by the similar remaining current of Co3O4/NPC (1.10 mA/cm2) and Co3O4/C (1.04 mA/cm2) after this test.
To investigate the electrocatalytic properties for ORR,cyclic voltammetry (CV) was first carried out in N2 and O2-saturated KOH solution (0.1 mol/L) (Fig. 6(a)). The CV showed well- defined ORR peaks at ~0.3 for all samples. The catalytic activity of the samples was further examined by LSV at a rotating rate of 1600 r/min in O2-saturated KOH solution (0.1 mol/L) (Fig. 6(b)). NC showed a plateau of limited diffusion current density when E was more negative than -0.45 V. For NPC,the current density began to increase at -0.48 V and reached -4.43 mA/cm2 at -0.80 V. Hence,doped N was critical for ORR and the catalytic activity was enhanced by doped P at negative potential. To further improve the activity,Co3O4 was synthesized directly on NPC. For comparison,Co3O4/NPC showed a close ORR catalytic activity (onset potential of -0.18 V,current density of -4.49 mA/cm2 at -0.80 V) to commercial 20 wt% Pt/C (-0.10 V,-4.76 mA/cm2) for ORR. As for Co3O4/NPC,unexpectedly,there was a negative shift for the current plateau in contrast with Co3O4/C,which suggested the N,P co-dopants gave a small disadvantage for ORR. The catalytic stability was tested by chronoamperometry (current vs time). As shown in Fig. 6(c),the current of Co3O4/NPC remained at 95% after 6 h,which was higher than NPC (75%) and similar to Co3O4/C (92%). Methanol was added into the electrolyte at the beginning of the chronoamperometry to compare the resistance. As shown in Fig. 6(g),after adding methanol,the remaining current of Pt/C dropped to 20 % immediately,but with Co3O4/NPC,it remained at 100%. These results suggested that Co3O4 /NPC outperformed Pt/C in term of resistance to methanol. In summary,it was found that even though ORR was catalyzed by the N,P co-dopants in NPC,the N,P co-dopants gave little advantageous effect for the activity and Co3O4 played the key role for ORR in Co3O4/NPC.
To further understand the ORR kinetics on Co3O4/NPC,LSV was carried out at a different rotating rate in O2-saturated KOH solution (0.1 mol/L) (Fig. 6(d)). Koutecky-Levich plots (Fig. 6(e)) were generated from Fig. 6(d),which were used to investigate the ORR mechanism on Co3O4/NPC. The least squares- fitted slopes were used to calculate the number of electrons transferred (n) on the basis of the Koutecky Levich equation [34]:
where j is the measured current density,jL and jK are the diffusion and kinetic-limiting current density,ω is the angular velocity (r/min),n is the transfer electron number,C0 is the bulk concentration of O2 (1.2 × 10-6 mol/cm3),D0 is the diffusion coefficient of O2 (1.9 × 10-5 cm2/s),ν is the kinematic viscosity of the electrolyte (0.0113 cm2/s),F is the Faraday constant (96485000 mA s/mol),and k is the electron-transfer rate constant. The n of Co3O4/NPC was calculated to be from 3.66 to 3.76 at the indicated potentials (Fig. 6(e)). The n of Co3O4/C and NPC was obtained by the same method and the results are shown in Fig. 6(f). All samples favored a four-electron oxygen reduction pathway.
The overall oxygen electrode activity is usually evaluated by the difference of the OER and ORR metrics,△E (△E = EOER - EORR ) [35]. EOER is the operating potential to deliver 10 mA/cm2 current density for OER,which is a critical metric for solar fuel synthesis [36]. EORR is the operating potential to deliver -1 mA/cm2 current density for ORR. A smaller △E indicates a catalyst closer to an ideal reversible oxygen electrode. But △E is affected by the catalyst loading. Our experience is that Ej=10 can be shifted negatively and Ej=-1 can be shifted positively by increasing the catalyst loading on the working electrode. The overall activity for the samples was shown in Fig. 7. △E of Co3O4/C,NPC and Co3O4/NPC was 1.01,1.12 and 0.89 V,respectively,with a catalyst loading of 0.1 mg/cm2. Co3O4/NPC outperformed some noble-metals,such as RuO2,IrO2 and Pt/C (1.10,1.32 and 0.94 V,respectively,with 0.21 mg/cm2) and was close to (or even outperformed) similar materials such as CoxOy/NC and MnxOy/NC (0.86 and 0.87 V,respectively,with 0.21 mg/cm2) [35]. This overall activity of Co3O4/NPC suggested that Co3O4/NPC will be useful as a bifunctional electrocatalyst for OER and ORR.
Co3O4 supported on N,P co-doped carbon (Co3O4/NPC) was prepared and examined as a bifunctional electrocatalyst for OER and ORR. For OER,N-doped carbon has little activity and the N,P co-dopants were critical for the catalysis by NPC. Co3O4/NPC showed higher activity than NPC or Co3O4/C for OER,which was due to a synergistic effect of the N,P co-dopants and Co3O4. The surface N,P co-dopants were unstable for OER but were improved by being hybridized with Co3O4,which is the active site for OER. For ORR,even though the N,P co-dopants exhibited catalytic activity,there was little improvement of Co3O4/NPC compared to Co3O4/C and Co3O4 played the key role for ORR. Co3O4/NPC exhibited an activity close to commercial Pt/C for ORR. Combining the advantages of Co3O4 and NPC,Co3O4/NPC showed improved stability and high overall oxygen electrode activity,and is anticipated would be a good bifunctional oxygen electrode for metal-air batteries.
2016,
Vol. 37 Issue (8): 1249-1256 
PDF (1400 KB)
2016,
Vol. 37 Issue (8): 1249-1256 PDF (1400 KB)