Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Co3O4 Nanoparticles Anchored on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as an Enhanced Oxygen Electrocatalyst for the Rechargeable and Flexible Solid-State Zn−Air Battery Zongxiong Huang,† Xueping Qin,‡ Guanzhou Li,† Weicong Yao,† Jun Liu,† Naiguang Wang,† Kemakorn Ithisuphalap,§ Gang Wu,*,§ Minhua Shao,*,‡ and Zhicong Shi*,† †
Downloaded by 79.110.17.172 at 07:06:50:621 on May 30, 2019 from https://pubs.acs.org/doi/10.1021/acsaem.9b00675.
Guangdong Engineering Technology Research Center for New Energy Materials and Devices, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510000, China ‡ Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China § Department of Chemical and Biological Engineering, University at Buffalo, State University of New York, Buffalo, New York 14260, United States S Supporting Information *
ABSTRACT: This work presents a desirable bifunctional catalystCo3O4 nanoparticles anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Co3O4/N-p-MCNTs)for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the rechargeable and flexible solid-state Zn−air battery. The Co3O4/N-p-MCNTs demonstrates good catalytic performance with the ORR half-wave potential of 0.760 V (vs RHE). Additionally, the Co3O4/N-p-MCNTs exhibits superior limiting current density with higher stability than Pt/C in alkaline solutions. The catalyst obtains a low operating potential (Ej10) of 1.62 V (vs RHE) to achieve a 10 mA cm−2 current density for OER. The potential difference (ΔE) between Ej10 of OER and ORR half-wave potential is 0.86 V, which is smaller than that of many highly active bifunctional catalysts reported recently. Moreover, a Zn−air battery utilizing Co3O4/N-p-MCNTs as the catalyst in cathode could successfully generate a specific capacity of 768 mAh g−1 at 10 mA cm−2, and there is no voltage loss after a continuous discharge of 135 h. The fabricated solid-state rechargeable Zn−air battery displays a high power density and superior long-term cycling stability. Furthermore, first-principles density functional theory simulations were conducted to explore the interfacial properties of the hybrid catalyst, hinting that the N-p-MCNTs could significantly enhance the electrical conductivity of Co3O4 nanoparticles. The free energy diagrams generated from our simulations suggest that the N-p-MCNTs influence the superior ORR performance, while cobalt oxide affects the favored performance of OER. The obtained results confirm that the Co3O4/N-p-MCNTs catalyst would have a broad impact and could be used for renewable energy conversion devices. KEYWORDS: Co3O4 nanoparticles, oxygen electrocatalysts, partially exfoliated multiwall carbon nanotubes, density functional theory simulations, rechargeable Zn−air battery
1. INTRODUCTION The rechargeable and flexible solid-state Zn−air battery is a promising energy storage candidate for emerging wearable devices. Among the transition-metal oxides, Co3O4 is a promising oxygen electrocatalyst due to its relatively high catalytic activity, low economic cost, and excellent stability.1−3 However, pure Co3O4 exhibits an inferior activity compared to commercial electrocatalysts because of the intrinsic low electrical conductivity, relatively large particle size, and poor dispersion.4 There are various methods attempted to improve the catalytic activity of Co3O4 including developments on the dispersion of Co3O4 nanoparticles with controlled sizes2,5,6 and © XXXX American Chemical Society
control loading of the Co3O4 nanoparticles on conductive support materials (such as carbon) with enhanced electrical conductivity.5,7−9 Because of the desirable properties such as high electrical conductivity, low cost, and strong stability, carbon-based catalysts have been chosen as bifunctional electrocatalysts for metal−air batteries.10,11 Generally, it is one useful method to promote the electronic transfer of Co3O4 nanoparticles by Received: April 1, 2019 Accepted: May 17, 2019
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DOI: 10.1021/acsaem.9b00675 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials
Nafion solution were uniformly dispersed in a mixed solvent consisting of 4 mL of ethanol and water. The catalyst thin film on RRDE (area: 0.1256 cm2) was controlled at a loading of 0.1 mg cm−2 for 20% Pt/C and at a loading of 0.2 mg cm−2 for other catalysts. The OER polarization curves were recorded with the scan rate of 10 mV s−1. To evaluate the Zn−air battery performance, the air cathode was prepared by depositing the catalysts onto the pressed stainless-steel net (325 mesh) current collector uniformly. The Zn−air battery was constructed with one air diffusion cathode, a Zn plate as an anode, and a solution of 0.2 M zinc acetate and 6.0 M KOH as an electrolyte. A mixture of RuO2 and Pt/C catalyst (with a mass ratio of 1:1) was also used for a comparison in the rechargeable battery testing. The solid-state Zn−air battery was constructed including the zinc foil as anode, PVA gel polymer as a solid electrolyte, and stainlesssteel net (325 mesh) as the cathode collector. PVA gel polymer was prepared from mixing 5.0 g of poly(vinyl alcohol) (PVA) powder with 50 mL of H2O at continuous stirring rate and temperature of 90 °C until a transparent gel was observed, followed by the addition of 5 mL of 6 M KOH with 0.2 M zinc acetate. After 30 min of stirring, the gel was transferred into Petri dishes, cooled in a commercial freezer at −20 °C over 24 h, and then thawed at room temperature. Lastly, a hot sealing machine (MSK-140) was used to assemble the packaging film. Computational Methodology. DFT Calculations. First-principles density functional theory (DFT) simulations were conducted to investigate the electronic structure and catalytic performances for the composite catalyst (Co3O4/N-p-MCNTs). The Vienna Ab-initio Simulations Package (VASP)19,20 was used by adopting the projected augmented wave (PAW)21,22 method while considering the periodic boundary conditions. During the theoretical simulations, the exchange-correlation energies were evaluated by the Perdew− Burke−Ernzerhof generalized gradient approximation (PBE-GGA)23 functionals, which are provided in the VASP database. For the planewave basis, the cutoff energies were set to be 450 eV. The conjugategradient scheme was selected to relax all the bulk and slab models, and the force convergence criterion was 0.03 eV Å−1. The Fermi level of the surfaces was smeared by using the Gaussian smearing with a width of 0.05 eV. Spin polarization was considered. The Hubbard scheme (PBE+U) was applied in the calculations to deal with the strong correlation between 3d electrons of Co. The effective U value was set to be 2.0 eV, which has been proven to provide reliable simulation results such as lattice constant and band gap from previous work.24−26 Simulation Models. To verify the origin of desirable catalytic performances of the Co3O4/N-p-MCNTs, we carefully designed the interfacial model containing the Co3O4 slab as the substrate with one N-doped graphene layer attached above the cobalt oxide surface. The Co3O4 (311) surface was selected as the slab model in response to the experimental characterization results obtained from the highresolution TEM. Two models with different surface terminations ((311)-A and (311)-B) were created for this Co3O4 slab, and detailed calculations regarding their surface energies are provided in the Supporting Information (Figure S1). The convergence test was conducted for the selected surface slab (311-B) with the lower surface energy by increasing the slab thickness from 6 Å (containing 132 atoms) to 11 Å (containing 244 atoms), with the result indicating that the surface energy converged within 0.04 J m−2 (Figure S2). With the relatively similar calculation accuracy, the slab with a thickness of 6 Å was used for the following simulations to curtail computation expense. The simulation cell had the lattice parameters of a = 19.84 Å, b = 11.45 Å, and c = 20.91 Å with the vacuum layer of 15 Å in the z direction to avoid the interfacial interactions between the periodic structures. The graphene layer was used to represent the multiwall carbon nanotube.27 The pyrrolic N and pyridinic N were doped into the graphene layer based on the XPS results, which indicated these two types of N as the essential part of the material sample. The van der Waals correction was introduced to appropriately define the nonbonding interaction between the Co3O4 slab and the graphene layer via the DFT-D2 method (Grimme’s scheme).28 Nørskov’s method was employed to evaluate the catalytic ORR and OER
growing them on carbon-based materials, including graphene and carbon nanotubes.12−16 In addition, N-doped carbon matrix can enhance the catalytic performance of the Co3O4 because of the strong synergistic interaction between Co3O4 and N-doped carbon.17 Because graphene and graphene ribbons are easy to reunite, here one simple approach was shown to prepare the 3D nitrogen-doped multiwall carbon nanotubes with partial exfoliation (p-MCNTs) loaded with Co3O4 nanoparticles as a high-performance ORR/OER bifunctional electrocatalyst. Within the system, the partially exfoliated carbon nanotubes provided the reactive sites for the formation of nanoparticles, and the reduction of p-MCNTs and oxidation of Co2+ happened via hydrothermal reactions. The aggregations of Co3O4 were prevented by agitating the reacting solution, ensuring uniform dispersion of the material on p-MCNTs. The 3D structure of nitrogen-doped p-MCNTs network with uniformly coated Co3O4 nanoparticles (Co3O4/ N-p-MCNTs) manifested superior kinetic current density with comparable ORR activity as Pt/C. Moreover, the catalyst exhibited a low operating potential of 1.62 V (vs RHE) upon achieving a 10 mA cm−2 current density for OER. The potential difference (ΔE) between Ej10 of OER and ORR halfwave potential is 0.86 V, comparable to the highly active bifunctional catalysts previously reported. Moreover, a Zn−air battery utilizing Co3O4/N-p-MCNTs as the cathode catalyst generated a specific capacity of 768 mAh g−1 at 10 mA cm−2 successfully, and there is no voltage loss after continuous discharge for 135 h. First-principles density functional theory (DFT) calculations were also performed to investigate the interfacial properties of Co3O4/N-p-MCNTs. The overpotentials for both ORR and OER were obtained through the theoretical Gibbs free energy diagrams, reporting low overpotentials for this hybrid catalyst.
2. EXPERIMENTAL SECTION Materials Synthesis. Preparation of Partially Exfoliated MCNTs (p-MCNTs), N-p-MCNTs, Co3O4/p-MCNTs, and Co3O4/N-p-MCNTs. For the detailed synthesis process of p-MCNTs, please refer to the previous work,18 and only the procedures for the latter three material preparations were shown here. To prepare the N-p-MCNTs, Co3O4/ p-MCNTs, and Co3O4/N-p-MCNTs, 40 mL of 2 mg mL−1 pMCNTs and 2.0 mL of cobalt acetate solution (0.02 mol L−1) were added into a 200 mL of PTFE autoclave and sonicated for 10 min. Then, 120 mL of 28−30% concentrated ammonia was added, and the solution was sonicated for another 10 min. The reaction was stirred and kept at 180 °C for 12 h. The precipitate was vacuum filtered after cooling to room temperature and washed by distilled water. Then it was dried via freeze-drying. The dried product was denoted as Co3O4/N-p-MCNTs 2. The Co3O4/N-p-MCNTs 1 was prepared similarly to the Co3O4/N-p-MCNTs 2, except for the stirring process during the hydrothermal reaction step. The preparations of N-pMCNTs and Co3O4/p-MCNTs were similar to the Co3O4/N-pMCNTs 1, except for the additions of cobalt acetate solution and concentrated ammonia during the hydrothermal reaction, respectively. Materials Characterization. Scanning electron microscopy (SEM) S-3400N and field emission transmission electron microscopy (TEM) Talos F200S were conducted to characterize the material morphologies. X-ray photoelectron spectroscopy (XPS) ESCALAB 250Xi was used to identify the oxidation state of elements. Electrochemical Measurement. The electrocatalytic ORR and OER performances were evaluated by rotating disk ring electrode (RRDE) with a gold ring. An Ag/AgCl electrode and Pt wire were used as a reference and counter electrode, respectively. Typically, 8.0 mg of the electrocatalyst sample (Co3O4/N-p-MCNTs, Co3O4/pMCNTs, N-p-MCNTs, or (20 wt %) Pt/C) and 80 μL of DuPont 5% B
DOI: 10.1021/acsaem.9b00675 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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ACS Applied Energy Materials Scheme 1. Schematic Synthesis of Co3O4/N-p-MCNTs Composite Catalysts
Figure 1. TEM images of (a, c) Co3O4/N-p-MCNTs 1 and (b, d) Co3O4/N-p-MCNTs 2. performances of the hybrid material from free energy diagrams and overpotential calculations29,30 (details can be found in the Supporting Information). Bader charge analysis31,32 was conducted to estimate the interfacial charge transfer. The binding energy was calculated at the Co3O4/graphene interface according to the equation
ΔE binding = Etotal − (ECo3O4 + Egraphene)
The morphology of the MCNTs-based materials was characterized by SEM and TEM. The SEM images present the long MCNTs (Figure S3a) that were cut off (Figure S3b) and partially exfoliated by strong oxidants (Figure S4c,d). The morphology of the N-p-MCNTs (Figure S3c) was akin to that of the p-MCNTs (Figure S3b). The introduction of Co2+ during the hydrothermal synthesis process produced ellipsoidal nanoparticles, which can be vividly observed in the Co3O4/pMCNTs (Figure S3d) and the Co3O4/N-p-MCNTs 1 (Figure S3e). Interestingly, the ellipsoidal nanoparticles were not observed with enforced stirring during the reaction process for the Co3O4/N-p-MCNTs 2 sample as evident from Figure S3f. Distinct structural morphologies of different MCNTs-based materials can be observed with TEM images (Figure S4a,b). Figure S4c,d provides insights into the exfoliated graphene nanosheets obtained from MCNTs. The morphologies of Co3O4/N-p-MCNTs 1 and Co3O4/N-p-MCNTs 2 are displayed in Figures 1a and 1b, respectively. The TEM images in Figures 1c and 1d show well-resolved lattice fringes with a dspacing of 0.244 and 0.202 nm, corresponding to the (311) and (400) crystalline planes of Co3O4, which suggested the
(1)
where Etotal, ECo3O4, and Egraphene correspond to the energies of total interfacial structure, Co3O4 slab, and N-doped graphene layer, respectively.
3. RESULTS AND DISCUSSION The synthesis schematic of Co3O4/N-p-MCNTs is shown in Scheme 1. The outer walls of MCNTs were induced by KMnO4 and resulted in partial exfoliation formed graphene oxide, attaching to the intact inner wall of nanotubes. The partially exfoliated carbon nanotubes provided the reactive sites for the formation of nanoparticles.33 The oxidation of Co2+ to Co oxides, nitrogen doping, and Co3O4 nanoparticle growth on N-p-MCNTs were completed in a hydrothermal reaction. C
DOI: 10.1021/acsaem.9b00675 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 2. (a) XRD patterns of Co3O4/N-p-MCNTs 1 and Co3O4/N-p-MCNTs 2 with the reference Co3O4 diffraction pattern (JCPDS-42-1467). (b) Raman spectra of MCNTs, p-MCNTs, Co3O4/N-p-MCNTs, and Co3O4. The high-resolution (c) C 1s, (d) N 1s, (e) Co 2p, and (f) O 1s XPS spectra of Co3O4/N-p-MCNTs 2.
anchored of Co3O4 nanoparticles onto the MCNTs-based materials, forming Co3O4/N-p-MCNTs composites. XPS measurements were performed to measure the element constitution of Co3O4/N-p-MCNTs 2. The obtained XPS spectrum of Co3O4/N-p-MCNTs 2 is shown in Figure S5. The characteristic peaks of Co 3s, Co 2p, C 1s, O 1s, and N 1s indicate the existence of Co, C, O, and N elements in the samples. The XPS characteristic peaks of Co 2p3/2 and Co 2p1/2 (Figure 2e) were located at 780 and 795 eV with the energy difference ca. of 15 eV. The positions of the peaks are consistent with the results reported previously.1,35,36 The Co3+/Co2+ atomic ratio of the Co3O4/N-p-MCNTs 2 can be obtained through the fitted curve with a reported valued of 2.03, resembling the ratio of the Co2+(Co3+)2O4 formula. Four Co 2p satellite peaks that belonged to Co3+ and Co2+ can be observed in Figure 2e. Figure 2f reveals two oxygen peaks contributions of O 1s region of the Co3O4/N-p-MCNTs 2. The O1 peak at 529.8 eV was assigned to the cobalt−oxygen bonds, whereas the O2 peak at 532.5 eV represented the surface oxygen defect species, C−O or CO of p-MCNTs.37 Four different kinds of N atoms in the Co3O4/N-p-MCNTs were determined by the N 1s spectrum with high resolution. The peaks with binding energies of 398.4, 399.6, 401.1, and 403.9 eV corresponded to pyridinic N, pyrrolic N, graphite N, and oxidized N, respectively.38−40 The peak intensity confirms that the pyrrolic N and pyridinic N were the major
formation of Co3O4 nanoparticles deposited on the p-MCNTs skeletons. As mentioned that no apparent nanoparticles can be seen in the SEM image (Figure S3f) with enforced stirring during the synthesis process, the corresponding TEM image of the material shows small Co3O4 nanoparticles with a diameter 0.8, which indicates that mesopores and macropores were the primary pores in MCNTs. The main pore sizes of MCNTs were 38.7, 50.6, and 68.5 nm (Figure S6b). After oxidation of the materials by KMnO4, the MCNTs were partially cut off, and the length of p-MCNTs became shorter. At the same time, the formerly long cylindrical pores were divided into different types of smaller cylindrical pores. New pores at 27.4, 34.7, and 50.1 nm were observed in p-MCNTs. After the hydrothermal reaction, the original pores were decorated with Co3O4 nanoparticles, resulting in the disappearance of pores. From Figure S6d,f, it was evident that the pore at 2.9 nm of pMCNTs disappeared, leaving some relatively large pores. The specific surface area of MCNTs was 249.1 m2 g−1. After oxidation, as shown in Figures S3b and S4c, the p-MCNTs became shorter with some of them were partially exfoliated to E
DOI: 10.1021/acsaem.9b00675 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Figure 4. (a) Schematic representation of the rechargeable Zn−air battery. (b) Discharge and power density curves of the Zn−air batteries with Pt/C and Co3O4/N-p-MCNTs 2 as catalysts. (c) Mechanically recharging of the Zn−air battery with Co3O4/N-p-MCNTs 2 catalyst using new Zn anodes. (d) Charge and discharge curves of the Zn−air batteries with the RuO2 + Pt/C and Co3O4/N-p-MCNTs 2. (e) Galvanostatic discharge− charge cycling curves at 10 mA cm−2 of rechargeable Zn−air batteries with the RuO2 + Pt/C and Co3O4/N-p-MCNTs 2 as catalysts.
KOH. The RuO2 afforded a low operating potential of 1.59 V (vs RHE) to achieve a 10 mA cm−2 current density (Ej10) for OER and with a Tafel slope of 72 mV dec−1, while the Pt/C (20%) catalyst exhibited a low OER activity with a Tafel slope of 192 mV dec−1 (Figure 3e). Co3O4/N-p-MCNTs 2 had an Ej10 potential at 1.62 V with a Tafel slope of 78 mV dec−1. The electrochemical impedance spectra of different catalysts conducted on glassy carbon electrode at 1.55 V vs RHE for OER in 1.0 M KOH aqueous solution are shown in Figure 3f. Co3O4/N-p-MCNTs 2 displayed a smaller semicircle than Co3O4/N-p-MCNTs 1, Co3O4/p-MCNTs, and Pt/C at the potential of 1.55 V vs RHE, suggesting that the activation energy of OER was the lowest on Co3O4/N-p-MCNTs 2 among these catalysts. However, Co3O4/N-p-MCNTs 2 is still inferior as compared to RuO2, being consistent with the results obtained from LSV measurements. Besides, the catalytic performances of Co3O4/N-p-MCNTs 2 also similar to those of bifunctional electrocatalysts reported previously (Table S1). Because of the novel 3D porous structure and good ORR/ OER activity, Co3O4/N-p-MCNTs 2 was introduced as the catalyst in cathode for the Zn−air battery as shown in Figure 4a. In Figure S11a, the assembled battery administered a high OCV (open-circuit voltage) of 1.47 V for an extended period. Corresponding polarization curve and power density plots of
four-electron pathway. LSV was further used to measure the kinetics process of ORR (Figure S8). According to the requirements of the US Department of Energy, the durability of the Co3O4/N-p-MCNTs 2 and Pt/C was tested by CV from 0.6 to 1.0 V (vs RHE) in 0.1 M KOH at 50 mV s−1 under the O2 atmosphere. Figure S9 displays the half-wave potential E1/2 of the Pt/C catalyst with ∼32 mV as the negative shift, which was higher than the Co3O4/N-p-MCNTs catalyst (12 mV). The tolerance against the methanol crossover effect of catalyst was also investigated. The current density and time curves are shown in Figure S10a. The currents of Pt/C and Co3O4/N-pMCNTs were stabilized within a small range without the addition of methanol. After 3.0 M methanol was added into the electrolytic cell, methanol was oxidized by Pt/C catalyst. However, the current of Co3O4/N-p-MCNTs 2 stabilized with over 90% even after adding 3.0 M methanol. The amperometric response of these two catalysts on RDE for 10 h is shown in Figure S10b. After 10 h, Co3O4/N-p-MCNTs and Pt/C suffered from 18% and 28% activity loss, respectively. Again, the above results confirmed the superior tolerance of Co3O4/N-p-MCNTs over Pt/C. The electrocatalytic OER performance was also investigated. Figure 3d shows the LSV of Co3O4/N-p-MCNTs, Pt/C (20%), and RuO2 catalysts loaded on glassy carbon in 1.0 M F
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Figure 5. (a) Schematic representation of solid-state rechargeable Zn−air battery. Photographs of (b) the PVA electrolyte and (c) a solid-state Zn−air microbattery with an open-circuit voltage of 1.33 V. (d) Photograph of solid Zn−air battery lights up a LED light. (e) Galvanostatic discharge−charge cycling curves at 2 mA cm−2 of the rechargeable solid Zn−air battery using the Co3O4/N-p-MCNTs 2 as catalysts.
(0.68 V), suggesting that the Co3O4/N-p-MCNTs 2 exhibited a long cycle life. In addition, a sandwich configuration type for solid Zn−air batteries was adopted, where the gas diffusion cathode, PVA electrolyte, and Zn foil anode were in a planar configuration with layer-by-layer assembly (Figure 5a). Figure 5b shows the PVA gel electrolyte, and Figure 5c depicts the photograph of a solid-state Zn−air microbattery with an open-circuit voltage of 1.33 V. Three Co3O4/N-p-MCNTs 2-based solid Zn−air batteries could power red LED lamp beads as shown in Figure 5d. Also, the solid Zn−air battery maintained a stable discharge and charge cycle at a current density of 2 mA cm−2 (Figure 5e), which are comparable to the results for previously reported Zn−air batteries.44−46 As mentioned, the Co3O4 has been considered as a promising electrocatalyst among transition-metal oxides but exhibits inferior activity because of the low conductivity and poor dispersion. The existence of N-doped graphene layer is expected to enhance the electrical conductivity of cobalt oxides and prevent the nanoparticles from aggregation on the interfacial structure of Co3O4/N-p-MCNTs. In addition, this hybrid material could benefit from high stability due to the interfacial binding of Co3O4 and graphene layer. Therefore, the interfacial properties of this hybrid catalyst were explored by theoretical DFT calculations.
batteries are shown in Figure 4b. The optimum power density of the battery using Co3O4/N-p-MCNTs 2 as catalysts reached 112 mW cm−2, which is equivalent to a battery that utilizes Pt/ C (133 mW cm−2). The discharge curves of the Zn−air battery are shown in Figure S11b, indicating the voltage of the constructed battery could be stabilized. Also, the specific capacity was evaluated to be 768 mA h g−1 at 10 mA cm−2 (Figure S11b). Additionally, when operated as a rechargeable battery, the Co3O4/N-p-MCNTs 2 air electrode could work for 135 h continuously (Figure 4c), confirming remarkable stability of Co3O4/N-p-MCNTs 2 catalyst for Zn−air batteries. In addition, the cycling test was applied to the Zn−air batteries. The mixture of RuO2 and Pt/C (1:1, mass ratio) catalyst also tested for comparison, and the charge and discharge curves are shown in Figure 4d. As shown in Figure 5e and Figure S12, the initial cycling test of the mixed RuO2 + Pt/C electrode displayed a higher discharge voltage and lower charge voltage than Co3O4/N-p-MCNTs 2. However, during the continuous cycling test, the performance of the RuO2 + Pt/ C electrode decreased faster than that of Co3O4/N-p-MCNTs 2. After operating for 55 h, highly stable discharge and charge voltage plateaus were observed for both the RuO2 + Pt/C electrode and Co3O4/N-p-MCNTs 2 electrode. Even at the 600th cycle, the voltage gap of Co3O4/N-p-MCNTs 2 electrode (0.79 V) was slightly larger than the initial cycle G
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ACS Applied Energy Materials OH* + e− → OH− + ∗
Figure 6 shows the models of Co3O4 (311) slab (Figure 6a), N-doped graphene layer (Figure 6b), and the combined
For the N-doped graphene layer in Co3O4-N-G, Bader charge analysis was conducted, and results are shown in Figure S13. The possible active sites for ORR were screened based on the charge analysis results, and O2 adsorption energies on possible active sites (with positive Bader charge) are summarized in Table S3. The C near the pyrrolic N was the active site, and other intermediate adsorption structures (OOH*, O*, and OH*) were optimized (Figure S14). The Gibbs free energy diagram (Figure 7a) was constructed by the
Figure 6. (a) Simulation models of Co3O4 (311) slab, (b) N-doped graphene, (c) interfacial structure Co3O4-N-G, and (d) charge density difference distribution at the interface of Co3O4-N-G. Color code: Co, dark blue; O, red; C, brown; N, green; H, white. The yellow area is the electron-rich area, cyan is the electron-depletion area, and the dark blue area at the interface denotes the cross-section display.
interfacial structure (Figure 6c, denoted as Co3O4-N-G). Bader charge analysis was conducted to explore the charge transfer in the interfacial structures. Figure 6d shows the charge density difference distribution diagram, where the yellow color near the cobalt oxide surface is the electron-rich area, and the cyan color means the electron-depletion distribution with the dark blue areas at the interface denotes the cross-section display. From the charge analysis results, it was found that 1.23 e− were transferred to Co3O4 substrate from graphene layer, which demonstrated that the catalyst conductivity of cobalt oxide was significantly improved due to the existence of N-doped graphene layer. In this hybrid interfacial structure, the binding energy between the Co3O4 (311) slab and N-doped graphene layer was calculated according to eq 1. The considerable binding energy of −3.65 eV at the interface hinted that the composite catalyst obtained excellence stability, agreeing with the stability measurement results from the experiments (Figure 4c,e). In general, based on the charge analysis results and binding energy calculations, the anchored N-doped graphene could contribute to the conductivity enhancement of Co3O4 nanoparticles with high stability. The experimental results suggested that the Co3O4/N-pMCNTs hybrid material exhibited high ORR and OER performances in alkaline media. Generally, Co 3 O 4 is considered as a promising catalyst for OER47−49 while ORR activity of the graphene layer could be enhanced by the doping of N.50 Therefore, for the Co3O4-N-G interfacial model, ORR and OER performances were evaluated by DFT calculations in N-doped graphene layer and Co3O4 (311) slab, respectively. The catalyst performances were evaluated from the free energy diagrams and the calculated overpotentials of ORR and OER according to Nørskov’s method.22,23 In alkaline media, the ORR mechanism is suspected to follow the four-electron reaction pathway,51,52 which was also demonstrated by the measured polarization curves from our experiments (Figure S7). The elementary steps are as follows:
Figure 7. Schematic energy profiles for the (a) ORR pathway and (b) OER pathway.
energy differences of these adsorbed intermediates, which include the zero point energy and entropy correction (Table S4) according to the ΔG = ΔE + ΔZPE − TΔS. In Figure 7a, the OOH formation was the rate-determining step (RDS) for ORR. Under the voltage of 0.402 V in alkaline solution, the OOH formation was endothermic with a small energy barrier observed at zero external potential, which disappeared at −0.19 eV. According to the free energy diagrams under different external potentials, the overpotential for ORR of the N-doped graphene in the Co3O4-N-G hybrid catalyst was 0.59 V. For comparison, the same C site in single N-doped graphene layer was used as the active site for ORR, and the corresponding free energy diagram is shown in Figure S15. Upon comparison of Figure 7a and Figure S15, the most significant difference observed for ORR in single N-doped graphene was the barrier for OOH* formation that only disappeared after the external potential of −0.68 V was applied, leading to the considerably large overpotential of 1.08 V. Based on the above calculation results, the Co3O4 nanoparticles in the hybrid catalyst could modulate the electronic structure of N-doped graphene layer, contributing to the remarkably reduced overpotential for ORR through adjusting the intermediate adsorptions.
O2 + ∗ + H 2O + e− → OOH* + OH−
OOH* + e− → O* + OH− O* + H 2O + e− → OH* + OH− H
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MCNTs mainly originated from their novel structural arrangements. First of all, p-MCNTs can prevent the restacking of carbon nanoribbons and construct a 3D carbon nanotube network with connected carbon nanoribbon structure. The 3D skeletons favored the adequate electrolyte penetration and the mass transport in porous network structure was promoted. Second, the efficient N doping provides additional active sites for the ORR/OER. Third, as a non-noble-metal catalyst, the Co3O4/N-p-MCNTs catalysts demonstrate a high ORR and OER catalytic activity. Owing to the synergistic interaction between Co3O4 and N-p-MCNTs, Co3O4/N-p-MCNTs show excellent bifunctional oxygen electrocatalyst activity and could have broad applications for renewable energy conversion devices.
For OER, the elementary steps in alkaline media are deemed to contain the adsorbed OH, O, and OOH species (denoted as OH*, O*, and OOH*), which are represented by the following scheme: OH− + ∗ → OH* + e− OH* + OH− → O* + H 2O + e−
O* + OH− → OOH* + e− OOH* + OH− → O2 + ∗ + H 2O + e−
The possible active sites for OER on Co3O4 (311) slab (Figure 6a) were screened by OH adsorptions according to the first elementary step. Three configurations of OH* were optimized (Figure S16) with the surface Co site (Figure S16a) indicated as the active site with the most stable adsorption structure. Figure S17 shows the optimized adsorption structures for the intermediates of O* and OOH*. Figure 7b presents the Gibbs free energy diagram for OER of Co3O4 (311), which was generated from the energy difference calculations of the adsorbed intermediates with the zero point energy and entropy correction (Table S5). When the applied electrode potential was 0 V (black line shown in Figure 7b), the rate of OER was uphill except for the first OH adsorption step. Inversely, the OER elementary reaction steps were downhill when the external potential of 0.912 V (0.51 V in overpotential) was applied. Based on the above discussions, as the bifunctional material (Co3O4-N-G), the calculation results confirmed that the good ORR performance originated from the N-doped graphene, and the excellent OER performance was attained from the cobalt oxide. Additionally, the feasibility of single component in the hybrid material as a bifunctional ORR and OER catalyst was examined by further DFT simulations. From the calculation results in Figure S18, the graphene layer with N doping was not a suitable OER catalyst with a large overpotential of 1.33 V (larger than 0.51 V), and the Co3O4 nanoparticle was not the ideal ORR catalyst with the corresponding overpotential of 0.76 V (larger than 0.59 V), which eliminated the possibility of single component as the bifunctional ORR and OER catalyst. This hybrid material had both relatively small overpotentials for ORR (0.59 V) and OER (0.51 V), although they are not as low as those of theoretically identified best catalysts (∼0.45 V on Pt53 and ∼0.42 V on RuO2 for ORR and OER, respectively).23 As a bifunctional catalyst, the Co3O4-N-G demonstrated performance improvements in both ORR and OER activities, emerging from the enhanced electrical conductivity of Co3O4 nanoparticles and N-doped graphene in the unique interfacial structures.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00675. Experimental details, material characterizations, evaluation of catalyst performance, calculation methods, simulation models, and supplementary results (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: gangwu@buffalo.edu (G.W.). *E-mail:
[email protected] (M.S.). *E-mail:
[email protected] (Z.S.). ORCID
Gang Wu: 0000-0003-0885-6172 Minhua Shao: 0000-0003-4496-0057 Zhicong Shi: 0000-0003-2360-7668 Author Contributions
Z.H. and X.Q. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Z.S. acknowledges the financial support from the National Natural Science Foundation of China (21673051), the Guangdong Science and Technology Department (2017B010119003), the “One-hundred Talents plan” (220418056), and the analysis and test center of the Guangdong University of Technology. G.W. acknowledges the financial support from the National Science Foundation (CBET-1604392) along with the Sustainable Manufacturing and Advanced Robotics Technology (SMART) Community of Excellence program at the University at Buffalo, SUNY. M.S. thanks the financial support from the National Key R&D Program of China (No. 2017YFB0102900), the Research Grant Council (N_HKUST610/17) of the Hong Kong Special Administrative Region, Guangdong Special Fund for Science and Technology Development (Hong Kong Technology Cooperation Funding Scheme) (201604030012, 201704030019, and 201704030065), and the Shenzhen Science and Technology Innovation Commission (JCYJ20180507183818040).
4. CONCLUSION In summary, a facile hydrothermal method was developed to synthesize hybrid material where the Co3O4 nanoparticles anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes. The hybrid material Co3O4/N-p-MCNTs exhibited good ORR and OER performance. Additionally, a Zn−air battery utilizing Co3O4/N-p-MCNTs as the cathode catalyst generated a specific capacity of 768 mA h g−1 at 10 mA cm−2 successfully. The fabricated aqueous rechargeable Zn−air batteries exhibited a high-power density and superior longterm cycling stability. The constructed flexible solid-state rechargeable Zn−air batteries with excellent cycling stability were presented. The ORR and OER kinetics of Co3O4/N-pI
DOI: 10.1021/acsaem.9b00675 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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