CoN Porous Nanowires as Efficient Bifunctional Catalysts for Zn

Feb 14, 2017 - A lower voltage drop is observed at 10 mA cm–2, indicating a good rechargeable property for NiO/CoN PINWs. The battery cycling test w...
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NiO/CoN Porous Nanowires as Efficient Bifunctional Catalysts for Zn−Air Batteries Jie Yin,† Yuxuan Li,† Fan Lv,‡ Qiaohui Fan,⊥ Yong-Qing Zhao,† Qiaolan Zhang,† Wei Wang,† Fangyi Cheng,# Pinxian Xi,*,† and Shaojun Guo*,‡,§,∥ †

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China ‡ Department of Materials Science and Engineering, College of Engineering, §BIC-ESAT, College of Engineering, and ∥Key Laboratory of Theory and Technology of Advanced Batteries Materials, College of Engineering, Peking University, Beijing 100871, China ⊥ Key Laboratory of Petroleum Resources, Gansu Province/Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China # Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: The development of highly efficient bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is crucial for improving the efficiency of the Zn−air battery. Herein, we report porous NiO/CoN interface nanowire arrays (PINWs) with both oxygen vacancies and a strongly interconnected nanointerface between NiO and CoN domains for promoting the electrocatalytic performance and stability for OER and ORR. Extended X-ray absorption fine structure spectroscopy, electron spin resonance, and highresolution transmission electron microscopy investigations demonstrate that the decrease of the coordination number for cobalt, the enhanced oxygen vacancies on the NiO/CoN nanointerface, and strongly coupled nanointerface between NiO and CoN domains are responsible for the good bifunctional electrocatalytic performance of NiO/CoN PINWs. The primary Zn−air batteries, using NiO/CoN PINWs as an air−cathode, display an open-circuit potential of 1.46 V, a high power density of 79.6 mW cm−2, and an energy density of 945 Wh kg−1. The three-series solid batteries fabricated by NiO/CoN PINWs can support a timer to work for more than 12 h. This work demonstrates the importance of interface coupling and oxygen vacancies in the development of highperformance Zn−air batteries. KEYWORDS: oxygen vacancies, nanointerface, NiO/CoN porous nanowires, oxygen evolution, Zn−air battery

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reversible oxygen reaction with high current density at low overpotential and long-term stability is still diffcult.11 To address these challenging issues, ideally the reversible oxygen catalysts are expected to operate the OER/ORR from the equilibrium status to O2/OH− in an opposite direction.12−14 Although recent efforts have focused on nonprecious-metalbased materials such as perovskite, metallic oxide, and their derivatives as bifunctional electrocatalysts,15−24 exploring strategies to develop more efficient and stable catalysts in a

lectrochemical conversion reactions are of great interest due to their important role in renewable energy conversion and storage systems.1−3 Oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the most significant electrochemical reactions at the center of rechargeable Zn−air and Li−air batteries.4−6 High-performance rechargeable Zn−air batteries require highly effective and stable bifunctional catalysts for OER and ORR, because the kinetics of the key charge (OER)/discharge (ORR) reactions are slow at air−cathode: O2 + 2H2O + 4e− ↔ 4OH−.7−9 Traditionally, Ir and Ru are often used for OER, and Pt is efficient for ORR. However, their high cost, scarcity, and poor durability have impeded their commercialization.10 What’s more, the design of an ideal electrocatalyst that can simultaneously drive the © 2017 American Chemical Society

Received: January 19, 2017 Accepted: February 14, 2017 Published: February 14, 2017 2275

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Figure 1. (a) XRD patterns of NiCo2O4 NWs and NiO/CoN PINWs. (b) SEM and (c) TEM images of NiO/CoN PINWs. (d) EDX elemental mapping images of NiO/CoN PINWs. (e) HRTEM image of NiO/CoN PINWs (left) and the related schematic atomic models of the cubic CoN and rhombohedral NiO (right).

Figure 2. EXAFS spectra in R-space of NiO/CoN PINWs and NiCo2O4 NWs at (a) Ni K-edge and (b) Co K-edge. (c) XPS spectrum of N 1s for the NiO/CoN PINWs. XPS spectra of (d) Ni 2p, (e) Co 2p, and (f) O 1s for the NiO/CoN PINWs and NiCo2O4 NWs.

nitrogenization, thereby resulting in more catalytic sites, which is crucial for electrocatalytic performance. Herein, we report a method for synthesizing the NiO/CoN porous interface NW arrays (NiO/CoN PINWs) as active and stable bifunctional electrocatalysts for boosting OER and ORR through controllable in situ nitrogenization of NiCo2O4 NWs.36−38 The high electrocatalytic activities of NiO/CoN PINWs for OER and ORR are caused by their porous NW structure, the decrease of coordination number for cobalt, the enhanced oxygen vacancies on the NiO/CoN nanointerface, and strongly coupled nanointerface between NiO and CoN domains, proved by diverse techniques like extended X-ray absorption fine structure spectroscopy (EXAFS), electron spin resonance (ESR), and high-resolution transmission electron

cost-effective manner is still a technological task in the development of energy conversion and storage systems. As one of the most famous spinel materials, NiCo2O4 nanowires (NWs) have attracted extensive attention in electrocatalysis and batteries.25−27 Nevertheless, the electrocatalytic performance enhancement of NiCo2O4 NWs is usually limited by the difficulty to find the effective strategies for tuning their structure.28−30 The electrocatalytic performance of lowdimensional materials can be enhanced after heteroatom doping, particularly nitrogen (N) doping.31−35 Nitrogen-rich materials perform the excellent catalytic activity because N doping can optimize the adsorption of oxygen species (e.g., OH− and O2).11 Additionally, a domain will be created after 2276

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Figure 3. (a) LSV of NiO/CoN PINWs, NiCo2O4 NWs, CFP, and Ir/C (20%) for OER. (b) Tafel plots of NiO/CoN PINWs, NiCo2O4 NWs, CFP, and Ir/C (20%). (c) The differences in current density (ΔJ = Ja − Jc) at 1.04 V vs RHE plotted against the scan rate fitted to a linear regression allow for the estimation of Cdl. (d) LSV of NiO/CoN PINWs and Ir/C (20%) before and after CV cycles. The inset in (d) shows the chronoamperometric response at a constant potential of 1.53 and 1.54 V vs RHE for NiO/CoN PINWs and Ir/C (20%). (e) LSV of NiO/CoN PINWs, NiCo2O4 NWs, commercial Ir/C (20%), and Pt/C (20%) catalysts in O2-saturated 0.1 M KOH solution. (f) The K−L plots of NiO/ CoN PINWs, NiCo2O4 NWs, commercial Ir/C, and Pt/C. The inset in (f) shows kinetic current densities and the electron transferred numbers (on the top of the rectangular bars) for oxygen reduction on NiO/CoN PINWs, NiCo2O4 NWs, commercial Ir/C (20%), and Pt/C (20%) at 0.65 V vs RHE.

surface with the mesopores of 20−30 nm than that of the NiCo2O4 NWs (Figure S1). The EDX elemental mapping in Figure 1d illustrates the uniform distribution of Co, Ni, N, and O on porous NWs, verifying the homogeneous distribution of closely interconnected structure of NiO/CoN PINWs. The HRTEM image of NiO/CoN PINWs (Figure 1e) shows two different domains of NiO and CoN, in which the clearly identified lattice fringe spaces of 2.09 and 2.48 Å correspond to the (003) plane of the rhombohedral NiO and the (111) plane of the cubic CoN. Surface Structural Properties of NiO/CoN PINWs. Figure 2a shows the EXAFS of NiO/CoN PINWs and NiCo2O4 NWs at Ni K-edge in R-space.40 Two peaks are shown in the range of 1.0−3.0 Å, in which the first one, ranging from 1.0 to 2.0 Å, corresponds to Ni−O bond, and the second one in the range of 2.0−3.0 Å is attributed to the Ni−Co/Ni bond. The intensity of the first peak is weakened, and its peak position shows no change after nitrogenization. However, the second peak shows a clear shift due to the formation of the NiO/CoN nanointerface during the nitrogenization progress. The local atomic arrangement of the NiO/CoN PINWs is different from that of the NiCo2O4 NWs (Table S1), confirming that they are structurally more disordered.41 Figure 2b shows the EXAFS spectra in R-space at Co K-edge.42,43 The peak in the range of 1.0−2.0 Å, corresponding to Co−O/N bond, shows the shift to long distance after nitrogenization. In addition, the peak in the range of 2.0−3.0 Å, belonging to the Co−Co/Ni bond, displays an enhanced intensity due to the strongly interconnected Co−Co/Ni on the nanointerface. X-ray photoelectron spectra (XPS) measurements were also used to investigate the surface properties of the NiO/CoN PINWs. In Figure 2c, the N 1s spectrum of NiO/CoN PINWs contains three types of nitrogen species, namely pyridinic N, graphitic N, and N−Co.44 The Ni 2p spectrum of NiO/CoN

microscopy (HRTEM). The Zn−air batteries with NiO/CoN PINWs as air−cathode show a maximum power density of 79.6 mW cm−2, an energy density of 945 Wh kg−1, a high open circuit of 1.46 V, and excellent stability without obvious decay more than 500 min. Furthermore, the solid rechargeable Zn− air batteries with the NiO/CoN PINWs as air−cathode display an excellent performance with the high open-circuit voltage (single solid battery of 1.335 V and three-series solid batteries of 3.375 V), long-time cycling stability, and long life cycle (more than 500 min at different current densities). Three-series solid batteries were used to power a commercial timer, which could continually work for more than 12 h, indicating the feasibility for practical applications, such as power sources in daily life. This methodology for the interface NWs with oxygen vacancies should open opportunities to low-cost, large-scale production of high-performance transition-metal-based bifunctional oxygen electrocatalysts for renewable energy technologies and other applications.

RESULTS AND DISCUSSION Synthesis and Characterization of NiO/CoN PINWs. A strategy was developed to synthesize the NiO/CoN PINWs by annealing NiCo2O4 NWs on carbon fiber paper (CFP) in NH3 atmosphere for 3 h at 380 °C.39 Figure 1a shows the X-ray diffraction (XRD) pattern of the NiO/CoN PINWs. The diffraction peaks of the NiO/CoN PINWs are attributed to NiO and CoN, belonging to the space groups of R3m ̅ (JCPDS card no. 22−1189; a = b = 2.954 Å; c = 7.236 Å) with rhombohedral structure and F4̅3m (JCPDS card no. 16−116; a = b = c = 4.28 Å) with cubic structure, respectively. Figure 1b shows the scanning electron microscopy (SEM) image of NiO/ CoN PINWs, illustrating the spindled and crooked NWs structure. The transmission electron microscopy (TEM) results reveal that the NiO/CoN PINWs (Figure 1c) have rougher 2277

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Figure 4. EXAFS spectra in the R-space of NiO/CoN PINWs before and after OER at the (a) Ni K-edge and (b) Co K-edge. XPS spectra of (c) Co 2p and (d) O 1s of NiO/CoN PINWs before and after OER. (e) ESR spectra of NiO/CoN PINWs before and after OER. (f) HRTEM image of NiO/CoN PINWs after OER.

PINWs is similar to that of NiCo2O4 NWs (Figure 2d). This result is in good agreement with the fitting results at Ni K-edge (Table S1). The binding energies of Co are increased after nitridation (Figure 2e),45,46 indicating enhanced oxidability for NiO/CoN PINWs, which is also similar to the fitting results at Co K-edge (Table S1). The enhanced coupling between NiO and CoN is further confirmed by the increased binding energy of O 1s (Figure 2f).47 Furthermore, the NiO/CoN PINWs show stronger ESR signal at g = 2.004 (Figure S2), illustrating their higher oxygen vacancy density. Electrochemical Catalytic Performance. Linear scan voltammogram (LSV) of the NiO/CoN PINWs were used to evaluate their OER activity in 1.0 M KOH at the scan rate of 2 mV s−1.48 The NiO/CoN PINWs exhibit the overpotential of 300 mV (vs reversible hydrogen electrode (RHE)) at a current density of 10 mA cm−2 (Figure 3a), a metric related to solar fuel synthesis,49 which is lower than that of Ir/C (310 mV). Noticeably, the NiO/CoN PINWs show the higher OER current densities than Ir/C above the potential of 1.53 V vs RHE. They also show a lower Tafel slope of 35 mV dec−1 than that of Ir/C (50 mV dec−1) (Figure 3b), suggesting that NiO/ CoN PINWs are a better OER catalyst (Table S2). The electrochemical double layer capacitance (Cdl) was used to evaluate the active surface area of the catalysts.50 The Cdl of NiO/CoN PINWs is 42.7 mF cm−2, much higher than that of NiCo2O4 NWs (17.6 mF cm−2) (Figure 3c and Figure S3), indicating their larger active surface area. The rotating ring− disk electrode (RRDE) results (Figure S4) reveal that the reversible OER/ORR process, which occurred on NiO/CoN PINWs, is dominated by a desirable four-electron pathway with the negligible formation of peroxide intermediates, that is, 4OH− → O2 + 2H2O + 4e−. Moreover, the observed oxidation current can be fully attributed to OER with a high Faradaic efficiency of 98.7% (Figure S5).48 The mass activity and turnover frequency (TOF) of the catalysts at the overpotential of 400 mV were further used to evaluate their OER catalytic ability. The calculated mass activity of NiO/CoN PINWs is 853.0 A g−1, superior to that of Ir/C (238.5 A g−1).51

Furthermore, the TOF of NiO/CoN PINWs for OER reaches 0.132 s−1, higher than that of Ir/C (0.118 s−1).52,53 Figure 3d shows the LSV curves of NiO/CoN PINWs before and after 30,000 CV cycles. NiO/CoN PINWs also display a negligible LSV curve shift after long-term stability testing and a more stable anodic current density than that of the commercial of Ir/C (the inset of Figure 3d). Furthermore, after 48 h of operation, there is no structure change on the XRD pattern and no morphology changes for NiO/CoN PINWs (Figure S6). All the data indicate their excellent OER durability. The ORR performances of NiO/CoN PINWs, NiCo2O4 NWs, Ir/C (20%), and Pt/C (20%) were further explored in 0.1 M KOH (Figure 3e).54 Both the onset potential (0.89 V) and half-wave potential (0.68 V) of NiO/CoN PINWs are close to those of benchmarked Pt/C (onset potential: 0.95 V, halfwave potential: 0.78 V) and superior to those of commercial Ir/ C and NiCo2O4 NWs (Table S4). The reversible oxygen electrode activity can be evaluated by the difference of OER and ORR metrics, that is, ΔE = Ej=10 − E1/2, where Ej=10 is the OER potential obtained at a current density of 10 mA cm−2, and E1/2 is the ORR potential taken at the half wave.48 The NiO/CoN PINWs exhibit a ΔE value of 0.85 V, less than those of Pt/C (1.13 V) and Ir/C (0.91 V) (Table S5). The Koutechy−Levich (K−L) plots (Figure 3f) of the NiO/CoN PINWs obtained from the LSV curves at different rotating rates (Figure S7) according to the K−L equation (experimental part in the SI) yield an electron transferred number (n) of approximately 3.97 and kinetic current density (jk) of 16.9 mA cm−2 (inset of Figure 3f). These results are comparable to those of Pt/C (4.08 and 17.8 mA cm−2) and better than those of Ir/C (3.34 and 11.7 mA cm−2), indicating their superior ORR catalytic activity through a four-electron pathway. The NiO/CoN PINWs are also considerably stable for ORR by displaying no obvious shift of LSV curves after 10,000 CV cycles (Figure S8). All these results make NiO/CoN PINWs among the most effective bifunctional catalysts for OER and ORR (Table S6). 2278

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Figure 5. (a) The schematic diagram of a homemade Zn−air battery. (b) The open-circuit plot, (c) discharge voltage curve, and the corresponding power density plot of NiO/CoN PINWs tested in the Zn−air battery. (d) Battery cycling test at charging current density of 50 mA cm−2 and discharging current density of 1 mA cm−2 in a short interval (10 min per cycle) for NiO/CoN PINWs and Pt/C. (e) Photograph of a blue LED (∼3.0 V) powered by two-series liquid Zn−air batteries. (f) Open-circuit plots of single and three-series solid batteries built by NiO/CoN PINWs. (g) A single solid battery and (h) three-series solid batteries cycling both tested at charging and discharging current densities of 1 and 3 mA cm−2 in a short interval (10 min per cycle). (i) Photograph of three-series solid batteries drives a timer to work continuously.

Structure of NiO/CoN PINWs after OER. The EXAFS, XPS, and ESR measurements were performed to verify the catalytic mechanism of NiO/CoN PINWs for OER. As shown in Figure 4a, the intensity and k3χ(k) function for the spectra of NiO/CoN PINWs in R-space at Ni K-edge do not show an obvious change during OER (Table S3), but the spectra of NiO/CoN PINWs in R-space at Co K-edge show a slight shift to the long distance of the k3χ(k) function in the range of 1.0− 2.0 Å, corresponding to a Co−O/N bond (Figure 4b).50 Moreover, the Co coordination number for NiO/CoN PINWs becomes high after OER (Table S3), being in good agreement with that of the XPS result (the binding energy of Co 2p decreases by 0.4 eV after OER, Figure 4c), which indicates the slight oxidation of CoN in alkaline condition. The O 1s spectra of NiO/CoN PINWs show the enhanced area at 531.2 eV after OER, which implies many oxygen vacancies (Figure 4d).55 Furthermore, the ESR signal at g = 2.004 was used to evidence the electrons trapped on oxygen vacancies.56 As shown in Figure 4e, the enhanced signal intensity illustrates that the NiO/CoN PINWs possess more oxygen vacancies after OER. In addition, the nanointerface of NiO−CoN domains still exists after OER in NiO/CoN PINWs (Figure 4f) and ensures the efficient and stable performance for oxygen reaction. Battery Performance. Considering the excellent bifunctional catalytic activity of NiO/CoN PINWs for OER and ORR, a homemade Zn−air battery was assembled to further

identify its performance under practical battery operation conditions. This battery uses the zinc plate as the anode, NiO/ CoN PINWs as the air−cathode, and 6.0 M KOH as the electrolyte (Figure 5a).4−9,57 The battery can work stably with a high open-circuit (OCV) of 1.46 V for more than 12 h (Figure 5b). The maximum power density of NiO/CoN PINWs is 79.6 mW cm−2 at 200 mA cm−2 (Figure 5c). In addition, the battery with NiO/CoN PINWs shows the voltage plateaus of ∼1.37 at 5 mA cm−2 and ∼1.29 V at 10 mA cm−2 with specific capacities of 690 and 648 mAh g−1 when normalized to the weight of the consumed zinc electrode (Figure S9).5,11 Furthermore, the corresponding energy densities are 945 and 836 Wh kg−1. A lower voltage drop is observed at 10 mA cm−2, indicating a good rechargeable property for NiO/CoN PINWs. The battery cycling test was conducted to further confirm the excellent recharge ability of NiO/CoN PINWs. The ideal reversible oxygen catalysts should possess low charging voltage (Echarging), high discharging voltage (Edischarging), and minimal fluctuation of Echarging and Edischarging. As expected, the battery fabricated with NiO/CoN PINWs shows a considerably stable charging voltage of 2.19 V and a discharging voltage of 1.35 V with the virtually negligible voltage fading for 500 min (Figure 5d). Figure 5e shows two-series Zn−air batteries based on the NiO/CoN PINWs air−cathode, which are used to power a blue lightemitting diode (LED, 3.0 V) with an excellent operation stability (e.g., without obvious change in brightness for more 2279

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from the Fuel Cell Store. All of the other materials for electrochemical measurements were of analytical grade and used without further purification. Fabrication of NiCo2O4 NWs. The NiCo2O4 NWs were obtained according to literature.36−38 Fabrication of NiO/CoN PINWs. In a typical procedure, the NiCo2O4 NWs were placed in a tube and heated to 380 °C with a rate of 10 °C min−1 under a flowing NH3 atmosphere. After reacting for 3 h at 380 °C, the system was cooled under a flowing NH3 atmosphere to room temperature naturally.32 Structure Characterization. XRD experiments were conducted by the X’Pert Pro X-ray diffractometer with Cu Kα radiation (λ= 0.1542 nm) under a voltage of 40 kV from 20° to 90°. The fieldemission scanning electron microscopy (FESEM) was used to investigate the morphologies of the samples at an accelerating voltage of 5 kV. TEM and HRTEM observations were performed under an acceleration voltage of 200 kV with a JEOL JEM 2100 TEM. The absorption spectra at Co and Ni K-edge were collected in transmission mode using a Si (111) double-crystal monochromator at the BL14W1 station of the Shanghai Synchrotron Radiation Facility (SSRF). XPS analyses were made with a VG ESCALAB 220I−XL device and corrected with a C 1s line at 284.6 eV. Sample compositions were determined by ICP−AES (HITACHI P−4010, Japan). OER Test. All the electrochemical measurements were carried out in a typical three-electrode system consisting of NiO/CoN PINWs on CFP (geometric area = 1 cm2) directly used as a working electrode, a Pt auxiliary electrode, and a Ag/AgCl reference electrode (3 M KCl) connected to a CHI 760 E Electrochemical Workstation (CHI Instruments, Shanghai Chenhua Instrument Corp., China) at a scan rate of 2 mV s−1 in the electrolyte solution. The potentials were referenced to the RHE through RHE calibration (E(RHE) = E(Ag/ AgCl) + 0.0951pH + 0.197). Also, a resistance test was made, and the iR compensation was applied using the CHI software. In this work, all electrochemical experiments were conducted at 20 ± 0.2 °C. The Ir/C (20%) and Pt/C (20%) catalysts were prepared according the reported method.37 The loading amount of the catalysts is 0.2 mg cm−2. The mass activity (A g−1) values of catalysts for OER were calculated at η = 400 mV:51−53

than 24 h). The multiple Zn−air batteries with the NiO/CoN PINWs as air−cathode can also be used for various applications. Relative to those of control catalysts (Table S7), the low charging and high discharging voltages of NiO/CoN PINWs are indicative of their excellent electrocatalytic performance for Zn−air batteries. To demonstrate potential applications in a daily life device, solid Zn−air batteries composed of NiO/CoN PINWs air− cathode, zinc anode, and hydrogel polymer electrolyte (Figure S10) were further developed.58−60 Figure 5f displays the OCV of the single solid battery (1.335 V and more than 15 h) and the three-series solid batteries (3.375 V and more than 10 h), indicating good performance for NiO/CoN PINW-based solid Zn−air batteries. The single solid battery shows a voltage plateau of ∼1.15 V at a current density of 0.5 mA cm−2 and a standing time of more than 7 h and ∼1.02 V at a current density of 1.0 mA cm−2 for about 6 h. Additionally, the threeseries solid batteries show higher voltage plateau of ∼2.51 V at a current density of 0.5 mA cm−2 and ∼2.64 V at a current density of 1 mA cm−2 (Figure S11). Both the single solid and three-series solid batteries with NiO/CoN PINWs as an air− cathode exhibit excellent rechargeable performance. As shown in Figure 5g, the discharge/charge cycles for single solid battery can be maintained for 500 min at the current densities of 1 and 3 mA cm−2 (10 min per cycle). Furthermore, the three-series solid batteries show a long cycle time under the same condition (Figure 5h). The three-series solid batteries can drive a timer to work continuously for more than 12 h (Figures 5i and S12).

CONCLUSIONS In summary, we report porous NiO/CoN interface NWs with both oxygen vacancies and the strongly interconnected nanointerface between NiO and CoN domains as efficient bifunctional electrocatalysts for developing high-performance Zn−air batteries. The NiO/CoN PINWs exhibit superior performance for OER with a low overpotential of 300 mV at 10 mA cm−2 and good stability (more than 48 h). They are also active and stable for ORR. The EXAFS and ESR investigations reveal that the high electrocatalytic performance of NiO/CoN PINWs for OER and ORR is attributed to their porous NW structure, the synergistic effect of the high oxidation capability for cobalt and oxygen vacancies, and the strongly interconnected nanointerface between NiO and CoN domains. The NiO/CoN PINWs as an air−cathode for Zn−air batteries show an open-circuit potential of 1.46 V, high energy density of 945 Wh kg−1, a high power density of 79.6 mW cm−2, and a high stability for 500 min. Moreover, the solid rechargeable Zn−air batteries with the NiO/CoN PINWs air−cathode also exhibit a higher open-circuit potential of 1.335 V and an excellent cycling stability. The three-series solid batteries can drive a timer to work continuously for more than 12 h. The design of the interface NWs for boosting catalysis should also be extended to other interface materials, thereby providing a promising approach for fabricating multifunctional materials for efficient renewable energy applications.

mass activity = j /m

(1)

The TOF values were calculated by the following equation:

TOF = jS /4nF

(2)

−2

where j (mA cm ) is the current density at η = 400 mV, S is the surface area of the working electrode, the number 4 means 4 electrons mol−1 of O2, F is Faraday’s constant (96485.3 C mol−1), and n is the moles of coated metal atom on the electrode. The FE is calculated as follows:

FE = n/(Q /4F )

(3)

where n is the moles of theoretical oxygen production, and Q is the total charge passing through the three-electrode system.37 For the RRDE measurements, the ring potential was constant at 1.5 V vs RHE. The % HO2− and transfer number (n) were determined by the followed equations:48

HO2− = 200(Ir /N )/(Id + Ir /N )

(4)

n = 4Id /(Id + Ir /N )

(5)

where Id is disk current, Ir is ring current, and N was determined to be 0.40. ORR Test. The electrochemical measurements for ORR were conducted on a RRDE at 1600 rpm in oxygen-saturated 0.1 M KOH solution. RDE was measured at rotating rates varying from 400 to 2400 rpm at the scan rate of 2 mV s−1. The as-synthesized NiCo2O4 NWs and NiO/CoN PINWs were carefully scraped off from the CFP. Then, 3 mg of the catalyst was ultrasonically dispersed in 1470 μL DMF solvent with 30 μL Nafion as the binder. Finally, 12.6 μL of the ink was coated onto the RRDE and dried in the air (the loading

MATERIALS AND METHODS Materials. Ni(NO3)2·6H2O (98.0%), Co(NO3)2·6H2O (99.0%), CO(NH2)2 (99.0%), and KOH (90.0%) were purchased from Aladdin. The deionized (DI) water for solution preparation was obtained from a Millipore Autopure system (18.2 MΩ, Millipore Ltd., USA). In the electrochemical experiment, 0.1 and 1.0 M KOH were used as the electrolyte. Commercial carbon fiber papers (CFP) were purchased 2280

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ACS Nano amount is 0.2 mg cm−2). The relationship between the measured currents (j) with various rotating speeds (ω) under fixed potentials can be expressed on the basis of the K−L equation:4

1/j = 1/jk + 1/(Bω1/2)

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(6)

where jk is the kinetic current and ω is the electrode rotating rate. B is determined from the slope of the K−L plots based on the Levich equation below: B = 0.2nF(DO2)2/3 v−1/6CO2

(7)

where n represents the transferred electron number per oxygen molecules, DO2 is the diffusion coefficient of O2 in 0.1 M KOH (DO2 = 1.9 × 10−5 cm2 s−1), v is the kinetic viscosity (v = 0.01 cm2 s−1), and CO2 is the bulk concentration of O2 (CO2 = 1.2 × 10−6 mol cm−3). The constant 0.2 is adopted when the rotation speed is expressed in rpm. Battery Test. The Zn−air battery was assembled by the zinc plate anode. 25−30 mL of 6.0 M KOH was used as the electrolyte without a separator. All the Zn−air batteries were tested under ambient atmosphere. The polarization curve measurements were performed by LSV (2 mV s−1) at 25 °C with CHI 760 E electrochemical working station (CH Instrument). Both the current and power densities were normalized to the effective surface area of air−cathode electrode.5,11 The specific capacity was calculated according to the equation:

current × service hours weight of consumed zinc

(8)

The energy density was calculated according to the equation: current × service hours × average discharge voltage weight of consumed zinc

(9)

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00417. Supporting Figures S1−S12 and Tables S1−S7 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pinxian Xi: 0000-0001-5064-5622 Shaojun Guo: 0000-0003-4427-6837 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (nos. 21571089, 51671003, 51571125, 41573128, 21503102, and 21505062), the National Key Research and Development Program of China (no. 2016YFB0100201), the start-up funding from Peking University, Young Thousand Talented Program, and the Fundamental Research Funds for the Central Universities (lzujbky-2016-k02, lzujbky-2016-k09, lzujbky-2016-38, and Lzujbky-2014-177). We also thank the staff at the BL14W1 station of the Shanghai Synchrotron Radiation Facility (SSRF) for EXAFS measurements and data analysis. 2281

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