Alloys and N-Doped Carbon Nanotubes

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Energy, Environmental, and Catalysis Applications

Coupling Bimetallic Oxides/Alloys and N-doped Carbon Nanotubes as Tri-Functional Catalysts for Overall Water Splitting and Zinc-Air Batteries Qing Qin, Ping Li, LuLu Chen, and Xien Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15612 • Publication Date (Web): 30 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Applied Materials & Interfaces

Coupling Bimetallic Oxides/Alloys and N-doped Carbon Nanotubes as Tri-Functional Catalysts for Overall Water Splitting and Zinc-Air Batteries Qing Qin, Ping Li, Lulu Chen and Xien Liu* State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, P. R. China KEYWORDS: oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, zinc-air battery, water splitting cell

ABSTRACT: An effectively multifunctional electrocatalyst is crucial for catalyzing the reactions occurred at electrodes in zinc-air batteries and water splitting cells, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Herein, two non-noble metal-based multifunctional electrocatalysts of NCNT/CoFeCoFe2O4 and NCNT/MnO-(MnFe)2O3 are prepared by a simple solvothermal procedure, followed by two-step annealing under the argon and ammonia atmosphere. The resultant electrocatalysts exhibit good tri-functional performances for HER, ORR and OER. Notably, the NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 assembled zinc-air batteries exhibit high energy densities of 727 and 776 Wh kgZn-1 at the current density of 20 mA cm-2, respectively. Furthermore, the NCNT/CoFe-CoFe2O4-based rechargeable zinc-air battery remains excellent durability after discharge-charge cycle testing for 22 h, comparable to noble metal-based catalyst (Pt/C + IrO2) assembled zinc-air battery. Furthermore, the NCNT/CoFe-

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CoFe2O4 and NCNT/MnO-(MnFe)2O3 assembled water splitting cells need ~1.65 and 1.70 V, respectively, to deliver a current density of 10 mA cm-2, lower than that of IrO2-Pt/C (1.71 V) and present excellent durability under long-term electrolysis. This work provides a facile strategy to prepare highly active multiple functional electrocatalysts for energy conversion and storage.

1. INTRODUCTION The energy-related technologies such as zinc-air batteries and water splitting cells are urgently needed for environmental friendly society.1 Zinc-air batteries enable convert the chemical energy to electrical energy by chemical reactions ocurred at the electrodes, such as the ORR in the cathode.2 On the contary, water splitting cells can realize electrical energy to chemical energy conversion, and produce hydrogen and oxygen.3 Both the two devices need highly efficient catalysts to reduce energy lost and enhance the reaction kinetics. For rechargable zinc-air batteries, an efficient bifunctional ORR/OER electrocatalyst is considered to be promsing compared with two catalysts, because a bifunctional electrocatalyst can cut the cost and restrain side reactions caused by two different catalysts. On the other hand, hydrogen is considered as a clean energy carrier that is promising for replacing fossil fuel.4-5 In industry, hydrogen is produced using petroleum and coal as raw materials, which leads to waste of natural resource and environmental pollution. At present, electrochemically producing hydrogen is considered as an eco-friendly approach relative to traditional technologies.6-7 Regardless of rechargeable zinc-air batteries and water splitting cells, highly efficient electrocatalysts are needed to speed up hydrogen or oxygen involved pivotal reactions at


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electrodes. As well known, the Ir, Ru and Pt noble metal-based catalysts are the best electrocatalysts for HER, OER or ORR. However, these noble metals are expensive and their reserves are low that restrict scalable industrial application. Thus, exploiting highly active precious metal-free electrocatalysts with cheap price and rich reserves to replace precious metal-based catalysts is an inevitable trend.8-10 Wen et al reported that the N-doped porous carbon nanosheets and mesoporous carbon exhibited excellent ORR acitities in pH-universal electrolyte. In addition, they prepared N-graphene/CNTs presented prominent bifunctional ORR/OER electrocatalytic activities.11-13 At the moment, tri-functional catalysts to catalyze simultaneously the HER, OER and ORR is still undeveloped.14 Transition metal oxides, sulfides and alloys have been considered as the potential substitutes for noble metals for HER,15 ORR16,17 and OER,18,19 due to the low price and abundant in nature.20 Particularly, first family of transition metals, especially for Fe-, Co-, Mn-involved nanocomposites, arouse extensive attention. A composite of bimetal FeCo and N-doped carbon nanotubes prepared by Su’s group, showed excellent performance for rechargeable zinc-air batteries.21 As reported in our previous work, nanocomposite composed by NiCo alloys and their oxides anchored on the N-doped carbon nanotubes also presented excellent ORR and OER bifunctional performance for rechargeable zinc-air batteries.22 The hollow Co3O4 microtube arrays and ultrathin δ-MnO2 nanosheet arrays with defect were used as the HER and OER bifunctional electrocatalyst for overall water splitting,23,24 better than those of noble metals IrO2 and Pt/C. However, these Fe-, Co- and Mn-based nanomaterials rarely own trifunctional catalytic activities. The composite of alloys and metal oxides had been proved to be favorable for improving catalytic activity,22,25 because alloys can enhance electrical



conductivity of composite, and the disparateness of lattice strain can cause change of potentials and structural phase.26 Except alloys and metal oxides, the N-doped carbon nanotubes are generally used as the support of active components because they own excellent electrical conductivity, thermal and chemical durability, once modified with carboxylic group, which can restrict the dissolution and aggregation of metallic nanoparticles.27,28 To combine alloys, metal oxides and N-doped carbon nanotubes into a composite as tri-functional catalysts are promising for energy storage and conversion. Herein, we prepared two tri-functional catalysts composed of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3, which exhibit good tri-functional performances for HER, ORR and OER. Especially for NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 assembled zinc-air batteries and water splitting cells, their performances are comparable to noble metal Pt/C and IrO2 assembled devices. This work delivers a facile strategy for fabricating highly active multiple functional electrocatalysts for the application in zinc-air batteries and water splitting cells. 2. EXPERIMENTAL SECTION 2.1 Synthesis of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 In a typical synthetic procedure, 50 mg of multiwall carbon nanotubes (MWCNTs) were homogeneously dispersed into the deionized water (20 mL) by consecutive ultrasound for 0.5 h. Then, 145 mg of Co(NO3)2·6H2O, 87.5 mg of Fe(OAc)2, 300 mg of urea, and 92.5 mg of NH4F were added into the homogeneously dispersed solution under the vigorously stirring. About 10 min later, the above solution was sealed in a 50 mL of stainless-steel autoclave and heated to 180 oC. After maintained for 12 h, the black products were fully washed with


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deionized water and absolute ethyl alcohol. Next, the above products were redispersed in deionized water and freeze-dried. After that, the collected products were annealed at 500 oC for 2 h under the argon atmosphere. And then the cooled products were reannealed at 500 oC for 1 h under argon and ammonia gas with both gas flowing of 500 mL min-1 and cooled under argon gas naturally. Finally, the resultant products were collected and used for the electrochemical measurements. The NCNT/MnO-(MnFe)2O3 products were prepared via the same procedure, except for the replacement of Co(NO3)2·6H2O with Mn(OAc)2·4H2O (122 mg). 2.2

Electrochemical measurements.

Hydrogen evolution reaction (HER). HER activity tests of the two catalysts were operated in a typical three-electrode setup, including a graphite rod counter electrode, reversible hydrogen reference electrode and Ni foam working electrode that was prepared by method of spincoating the catalyst ink on a 1 cm  1 cm Ni foam. 1 M KOH solution was used as electrolyte. The HER activity of commercial Pt/C was also test for comparison. All the LSV curves were obtained at a scan speed of 5 mV s-1 without iR compensation. The electrochemical stability test was by chronoamperometry, which was conducted at -0.20 V (vs RHE) and continuously processed for 10 h. The Ni foam working electrode was prepared following the steps below: 3 mg of catalysts were uniformly dispersed in an ethanol and ultrapure water mixture (vethanol : vultrapure water = 2 : 1) under a continuous ultrasonic for 20 min. Then, 24 µL of the prepared catalytic ink was loaded onto the Ni foam (1 cm2) by spin-coating and dried naturally.



Oxygen evolution reaction (OER). The OER activity measurements of the as-prepared two catalysts and commercial IrO2 were conducted in a three-electrode setup with a glassy carbon (3 mm in diameter) working electrode, a Pt wire counter electrode and an Hg/HgO reference electrode. The electrolyte was 1 M KOH solution. The catalyst loading was 0.42 mg cm-2. Durability tests adopted chronoamperometry at a potential of 1.54 V. Oxygen reduction reaction (ORR). ORR properties of the as-prepared catalysts were tested through rotating ring-disk electrode measurements with Hg/HgO reference electrode and Pt wire counter electrode in 0.1 M KOH solution. Before all the tests, the electrolyte was purging by bubbling high-purity O2 gas for 20 min. Moreover, the high-purity O2 was bubbled continuously into KOH electrolyte throughout the whole testing process. CV curves were recorded within 0.3 – 1.05 V vs RHE with the scan rate of 10 mV s-1. Polarization curves were obtained at scan speed of 10 mV s-1 with varying rotation rate from 400 to 2000 rpm. Overall water splitting. The two-electrode alkaline electrolyzer was fabricated with the asprepared catalysts powder both as the cathode and anode. LSV curves were obtained at a scan rate of 5 mV s-1. The durability over extended periods was conducted by controlled potential electrolysis. Zinc-air batteries. Activated charcoal and PTFE with a weight ratio of 7 : 3 were coated on a Ni foam, which was used on air cathodes. Catalysts (10 mg) and Nafion (5 wt %, 200 μL) were dispersed in ethanol (0.25 mL) by ultrasonic blending. Then, 200 μL of catalytic ink was coated onto the above Ni foam electrode and dried in vacuum for 0.5 h. A mildly pressing process was needed. The anode was Zn plate and a nylon polymer membrane used as separator. The electrolyte was 6 M KOH.


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3. RESULTS AND DISCUSSION 3.1 Morphological and component characterization of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3

Figure 1. Characterizations of the as-synthesized NCNT/CoFe-CoFe2O4 composites: a) Panorama SEM image, b) TEM image, c) HRTEM image, d-h) HAADF-STEM image and the corresponding elemental mapping, i) XRD pattern, j) XPS survey spectrum. The bimetallic CoFe-CoFe2O4 coupled with NCNT catalyst was obtained by a hydrothermal reaction step and subsequent phase transformation process via annealing the resultant precursor in Ar/NH3 atmosphere. Figure S1a is the typical XRD pattern of the precursor of NCNT/CoFeCoFe2O4. It is evidently that all the diffraction peaks match well with the peak position of tetragonal structural FeOOH with lattice constants of a = 10.535 Å and c = 3.03 Å (JCPDS No.



34-1266) and Co(OH)2 (JCPDS No. 02-0925). After annealed in Ar/NH3 flowing, the precursors thoroughly transformed into the composite of alloy and their oxide supported on the N-doped CNTs, which was systematically characterized by SEM, TEM, HRTEM, HAADFSTEM, XRD, and XPS. As shown in Figure 1a, it is the typical panorama SEM image of the as-prepared catalyst, indicating numerous small nanoparticles supported on the CNT. TEM image in Figure 1b is more evidence the modification of CNT with several nanoparticles. The clear lattice fringes can be observed in Figure 1c and the interplane distances are 0.205 nm and 0.256 nm, corresponding to the (110) crystal planes of cubic metallic Fe and (311) crystal planes of cubic CoFe2O4, respectively.29 The HAADF-STEM image and the corresponding elementals mapping in Figure 1d-h demonstrate the components of hybrids and the distribution of elementals C, Co, Fe, and O. No peak of elemental N is observed due to its low content. The corresponding XRD pattern of the catalyst is shown in Figure 1i. Except for the peak at 26.6° belongs to CNT, all the other diffraction peaks are perfectly assigned to the cubic structural CoFe2O4 (JCPDS No. 22-1086), metallic Fe (JCPDS No. 01-1252), and metallic Co (JCPDS No. 15-0806), respectively. The XPS survey spectrum shows the existing of elementals C, Co, Fe, O, and N (Figure 1j). The above results clearly demonstrate the as-prepared catalyst is NCNT/CoFe-CoFe2O4.


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Figure 2. Characterizations of the as-synthesized NCNT/MnO-(MnFe)2O3 composites: a) Panorama SEM image, b) TEM image, c) HRTEM image, d-h) HAADF-STEM image and the corresponding elementals mapping, i) XRD pattern, j) XPS survey spectrum. The NCNT/MnO-(MnFe)2O3 catalyst was obtained via the same procedure except different metal salts. As evidenced by XRD pattern in Figure S1b, the precursor prepared by the hydrothermal reaction is a composite phase of tetragonal structural FeOOH (JCPDS No. 341266), rhombohedral structural MnCO3 (JCPDS No. 44-1472) and carbon nanotube. After the high-temperature annealing process in Ar/NH3 atmosphere, the precursor released the CO2 and H2O, and was completely transformed into more stable phases of NCNT/MnO-(MnFe)2O3. Figure 2a is the typical SEM image of the NCNT/MnO-(MnFe)2O3 catalyst, showing numerous small nanoparticles supported on the CNTs. Figure 2b and c are the corresponding TEM and



HRTEM images, further confirming that the CNTs are interspersed with small nanoparticles with 13-20 nm in the diameter. The clear lattice spacing of 0.252 nm corresponds to the (321) crystal planes of cubic (MnFe)2O3. HAADF-STEM image and the corresponding elementals mapping in Figure 2d-h further confirm the components of the catalyst, as well as the distribution of elementals C, Mn, Fe, and O. Because of low content, no signal of elemental N can be observed. The dominated X-ray diffraction peaks in Figure 2i correspond to (220), (222), (321), (411), (420), (521), (433), (600), (620), and (543) planes of cubic (MnFe)2O3 (JCPDS No. 08-0010), whereas the diffraction peaks marked with blue triangle and red dot can be assigned to cubic MnO with lattice constants a = 4.445 Å (JCPDS No. 07-0230) and CNT, respectively. As shown in Figure 2j, XPS survey spectrum further demonstrates the catalyst components of elemental C, Mn, Fe, O, and N. Accordingly, NCNT/MnO-(MnFe)2O3 catalyst was successfully synthesized. Further insight into the structural and electronic states of two prepared catalysts were gained by XPS, XANES, and EXAFS measurements. Evidenced by Figure 3a, high resolution XPS of Co 2p for NCNT/CoFe-CoFe2O4 possesses two dominant peaks at 796.2 and 780.8 eV with two shoulders at 803.3 and 785.3 eV, respectively, assigning to Co2+.30,31 While a small peak at 778.8 eV in Co 2p spectrum indicates the containing of metallic Co in the surface layer of NCNT/CoFe-CoFe2O4.32 The subpeaks at 724.9, 713.6 and 710.9 eV in Fe 2p (Figure 3b) belong to Fe3+,33-34 while the other two peaks located at 719.8 and 707.6 eV are associated with metallic Fe.35 The O 1s spectrum is shown in Figure 3c, showing three types of oxygen species, where the peaks at 530.2, 531.7, 533.3 eV are attributed to metal-O, C-O-C and C-OH, respectively.36 There are three subpeaks can be observed in the N 1s with the binding energy


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of 398.5, 399.6 and 401.7 eV, corresponding to pyridinic-N, pyrrolic-N and graphitic-N, respectively.37 The high-resolution spectrum of C 1s for NCNT/CoFe-CoFe2O4 (Figure S2a) shows two peaks at approximately 284.6 and 285.2 eV, belong to C=C and C-N, respectively.38,39 The presence of C-N clearly demonstrates that the successful incorporation of N element into carbon matrix. The XANES of Co K-Edge for NCNT/CoFe-CoFe2O4 and precursor are shown in Figure 3e. The principal absorption peaks for both two samples are approximately located at 7726 eV, attributed to the transition of Co 1s to the mixing state of Co 4p and O 2p.40 Notably, a shoulder peak appeared at 7712 eV in the spectroscopy of NCNT/CoFe-CoFe2O4 demonstrates the existence of metallic Co species.30 As shown in Figure 3f, the EXAFS of NCNT/CoFe-CoFe2O4 is very different to that of corresponding precursor. The subpeak at 2.2 Å for NCNT/CoFe-CoFe2O4 belongs to the Co-Co interaction from metallic Co species, whereas the peak at 2.6 Å can be attributed to the Co-O-Co/Fe edge-shared bonding.41 Figure 3g shows the XANES spectroscopies of Fe K edge for NCNT/CoFe-CoFe2O4 and precursor. A significant shift to lower energy above the absorption edge is observed for NCNT/CoFe-CoFe2O4 followed with a decrease of peak intensity. The peak of approximately 7132 eV for both NCNT/CoFe-CoFe2O4 and precursor is consistent with reported Fe3+,42 while the shoulder peak at 7113 eV effectively attests that metallic Fe species indeed exist in the NCNT/CoFe-CoFe2O4.41 Moreover, the EXAFS of Fe K edge for NCNT/CoFe-CoFe2O4 is very different from that of precursor. The strongest peak at 1.5 Å for precursor is assigned to Fe-O interactions, while the relatively weak peak at 2.7 Å is typically assigned to Fe-Fe interactions. For NCNT/CoFe-CoFe2O4, the first shell of Fe absorber is located at 1.5 Å, representing the Fe-O interactions, and the Fe−O−Fe/Co edge-shared bonding is located at



about 2.5 Å. It is noteworthy that the peak located at 2.0 Å in EXAFS spectrum for NCNT/CoFe-CoFe2O4 is perfectly consistent with the Fe-Fe interaction from metallic Fe species, again proving the presence of metallic Fe.41 The composite would yield high activity and stability for multifunctional electrocatalysis due to the synergetic effect from CoFe alloys and the corresponding bimetallic oxides coupled with N-doped CNT.


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Figure 3. High-resolution XPS spectra of a) Co 2p, b) Fe 2p, c) O 1s, d) N 1s for NCNT/CoFeCoFe2O4, e) XANES spectra, and f) EXAFS spectra for Co K edge of precursor and NCNT/CoFe-CoFe2O4, g) XANES spectra, and h) EXAFS spectra for Fe K edge of precursor and NCNT/CoFe-CoFe2O4.



Figure 4a presents the spectrum of Mn 2p for NCNT/MnO-(MnFe)2O3, Mn 2p1/2 and Mn 2p3/2 at BEs of 653.8 and 641.4 eV are assigned to Mn3+, and the peak at 642.5 eV corresponds to Mn2+.43 As shown in Figure 4b, two peaks relative to Fe 2p1/2 and Fe 2p3/2 appears at 724.7 and 710.5 eV, similar to the reported value of Fe3+ (Figure 4b).33 The high-resolution spectrum of O 1s for NCNT/MnO-(MnFe)2O3 is deconvoluted into three subpeaks at position of 530.3, 531.8, and 533.7 eV, refers to metal-O, C-O-C and C-OH, respectively (Figure 4c). Three different types of N coordination environments for NCNT/MnO-(MnFe)2O3 are verified by N 1s spectrum in Figure 4d, i.e. pyridinic-N (398.2 eV), pyrrolic-N (399.9 eV), graphitic-N (401.4 eV).44,45 The C 1s spectrum in Figure S2b for NCNT/MnO-(MnFe)2O3 is deconvoluted into two peaks: 284.6 eV for C=C, 285.4 eV for C-N, suggesting the successful doping of nitrogen into carbon framework.35 The XANES for Mn K-edge of NCNT/MnO-(MnFe)2O3 and their precursor are shown in Figure 4e. The sharp peaks at approximately 6551.2 eV and 6562.2 eV in the spectrum of precursor are the typical adsorption features of MnCO3.46,47 For the spectrum of NCNT/MnO-(MnFe)2O3, a dominate peak at 6555.3 eV corresponds to Mn3+, whereas the shoulder peak at 6551.5 eV belongs to Mn2+.48 As shown in Figure 4f, the strongest peak at 1.7 Å for precursor belong to Mn-O interactions, while the relatively weak peak at 3.5 Å is typically assigned to Mn-O-Mn/Fe corner-shared bonding. For NCNT/MnO-(MnFe)2O3, the first peak is located at 1.7 Å, representing the Mn-O interactions, and the Mn-O-Mn/Fe edge-shared bonding is located at approximately 2.7 Å. Figure 4g shows the XANES of Fe K edge for NCNT/MnO-(MnFe)2O3 and precursor. The NCNT/MnO-(MnFe)2O3 shows a slightly low energy-shifted relative to that of the precursor, in which the maximum absorption feature peak at 7132.4 eV corresponds to Fe3+.49 As shown in Figure 4h, for Fe K edge of NCNT/MnO-


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(MnFe)2O3, the peak at 1.46 Å corresponds to Fe-O interactions, while the relatively weak peak at 2.87 Å is typically assigned to Fe-O-Fe/Mn edge-shared bonding, which are high energyshifted relative to that of precursor.

Figure 4. High-resolution XPS spectra of a) Mn 2p, b) Fe 2p, c) O 1s, d) N 1s for NCNT/MnO-



(MnFe)2O3, e) XANES spectra, and f) EXAFS spectra for Mn K edge of precursor and NCNT/MnO-(MnFe)2O3, g) XANES spectra, and h) EXAFS spectra for Fe K edge of precursor and NCNT/MnO-(MnFe)2O3. 3.2 Electrocatalytic Performance of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 catalysts The electrocatalytic HER activities of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 supported on Ni foam were firstly evaluated by polarization linear sweep voltammograms (LSVs) with a scan rate of 5 mV s-1 in 1 M KOH solution. For comparison, the performance of commercial Pt/C (20 wt %) as well as the bare Ni foam were also recorded. Evidenced by Figure 5a, the NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 exhibit optimal HER activities with small onset potentials of -110 mV and -148 mV at the current density of 2 mA cm-2,50 respectively, beyond which the current densities of two catalysts increase rapidly at higher applied biases, demonstrating the excellent electrocatalytic activities. The overpotentials at j = 10 mA cm-2 for NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 are 204 and 212 mV, respectively, which are smaller than that of bare Ni foam (278 mV), and even smaller than that of many reported non-noble metal electrocatalysts (Table S1). Tafel plots presented in Figure 5b is used to investigate the HER reaction kinetics of the two catalysts. Small Tafel slopes of 91 mV dec-1 for NCNT/CoFe-CoFe2O4 and 102 mV dec-1 for NCNT/MnO-(MnFe)2O3 are obtained, suggesting favorable HER reaction kinetics. For comparison, the histogram of Tafel slopes and the overpotentials at the current density of 10 mA cm-2 for NCNT/CoFe-CoFe2O4, NCNT/MnO-(MnFe)2O3, Pt/C and Ni foam are compared in Figure 5c, intuitively demonstrating good HER activities of as-synthesized catalysts. Furthermore, the durability of


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NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 was examined at a constant overpotential of - 0.20 V (vs RHE) (Figure S3). Compared with starting current densities, the NCNT/CoFeCoFe2O4 and NCNT/MnO-(MnFe)2O3 show loss of approximately 10 % of current density under the work condition for 10 h, indicating a normal stability. The above results clearly indicate the high electrocatalytic activity, favorable reaction kinetics and good electrochemical stabilities for the two as-prepared catalysts during the long-term HER processes. The OER performance of the two catalysts were investigated by RDE technique in a standard three-electrode setup with O2-saturated 1 M KOH solution as electrolyte. The catalytic activity of commercial IrO2 was also measured as the reference. As shown in Figure 5d, the polarization curve of NCNT/CoFe-CoFe2O4 increases rapidly beyond the onset potential of 1.48 V. The overpotential at the current density of 10 mA cm-2 for NCNT/CoFe-CoFe2O4 is 310 mV, superior to that of benchmark IrO2 (360 mV) and NCNT/MnO-(MnFe)2O3 (410 mV). As shown in Figure 5e, the NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 exhibit small Tafel slopes of 63 and 137 mV dec-1, respectively, indicating the fast reaction kinetics for water oxidation. The histograms of overpotential at the current density of 10 mA cm-2 and Tafel slopes are gathered in Figure 5f for intuitively reflecting the OER activity of NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3. The durability of OER was evaluated at a constant overpotential of 310 mV (Figure S4). It is remarkable that an increasing current density for NCNT/CoFeCoFe2O4 was observed after running 12 h, indicating a long-term excellent activity and stability. While NCNT/MnO-(MnFe)2O3 catalyst also delivers a good durability with a slight loss of the current density during 12 h continuous electrolysis. Accordingly, the as-synthesized NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 hybrid nanomaterials possess outstanding



electrochemical activities and stabilities for long-term water oxidation, which are comparable to many reported excellent electrocatalysts (Table S2).

Figure 5. a) Polarization curves and b) Tafel plots of NCNT/CoFe-CoFe2O4, NCNT/MnO(MnFe)2O3, Pt/C, and Ni foam for HER in N2-saturated 1.0 M KOH solution with scan rate of 5 mV s-1. c) Comparison of overpotentials at the current density of 10 mA cm-2 and Tafel slopes of Pt/C, NCNT/CoFe-CoFe2O4, NCNT/MnO-(MnFe)2O3, and Ni foam for HER. d) Polarization curves and e) Tafel plots of NCNT/CoFe-CoFe2O4, NCNT/MnO-(MnFe)2O3, and IrO2 for OER in O2-saturated 1.0 M KOH solution with scan rate of 5 mV s-1. f) Comparison of overpotentials at the current density of 10 mA cm-2 and Tafel slopes of NCNT/CoFeCoFe2O4, NCNT/MnO-(MnFe)2O3 and IrO2 for OER. Inspired by the outstanding performance towards both OER and HER, it is anticipated that the as-prepared hybrid nanomaterials could serve as highly effective bifunctional electrocatalysts for overall water splitting. We constructed water electrolyzers with the NCNT/CoFe-CoFe2O4/Ni foam as the anode and cathode, and the configuration of water


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electrolyzer was labelled as NCNT/CoFe-CoFe2O4‖NCNT/CoFe-CoFe2O4. For NCNT/MnO(MnFe)2O3 and noble metal Pt/C-IrO2, water electrolyzers were labelled as NCNT/MnO(MnFe)2O3‖NCNT/MnO-(MnFe)2O3 and Pt/C‖IrO2. As shown as Figure 6a, the observed potentials of overall water splitting for NCNT/CoFe-CoFe2O4‖NCNT/CoFe-CoFe2O4 is 1.65 V, and for NCNT/MnO-(MnFe)2O3‖NCNT/MnO-(MnFe)2O3 is 1.70 V at the current density of 10 mA cm-2, overmatching that of Pt/C‖IrO2 (1.71 V). Aside from the competitive activity, a better long-term electrocatalytic durability was achieved for both NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3. As demonstrated by Figure 6b, a high current density of 15.4 mA cm2

for NCNT/CoFe-CoFe2O4 is well maintained for 80000 s while bubbles are incessantly

evolved and released from the surface of electrodes, especially for NCNT/MnO-(MnFe)2O3, the current density gradually increase at the first 20000 s and then keep relatively stable. Therefore, both NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 are promising for the application of alkaline electrolyzer due to their competitive OER and HER activities and longterm electrocatalytic stabilities. The performance of NCNT/CoFe-CoFe2O4 and NCNT/MnO(MnFe)2O3 for overall water splitting are comparable, even better than many previously reported catalysts, as shown in Table S3.

Figure 6. a) Polarization curves of the two-electrode alkaline electrolyzers using NCNT/CoFe-



CoFe2O4 ‖ NCNT/CoFe-CoFe2O4, NCNT/MnO-(MnFe)2O3 ‖ NCNT/MnO-(MnFe)2O3, IrO2‖Pt/C, and Ni foam‖Ni foam as both the cathode and anode at a scan rate of 5 mV s-1 in 1.0 M KOH. b) Corresponding chronoamperometric responses of NCNT/CoFe-CoFe2O4 ‖ NCNT/CoFe-CoFe2O4, NCNT/MnO-(MnFe)2O3‖NCNT/MnO-(MnFe)2O3, IrO2‖Pt/C at a constant potential of 1.70 V for 80000 s. The as-prepared NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 nanocomposites also displayed appreciable activities for ORR. Figure 7a shows the LSV curves for ORR obtained by RDE technique. The NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 exhibit the halfwave potentials of 0.74 and 0.72 V (vs RHE), which are negatively shifted 70~90 mV compared to that of Pt/C. Although the NCNT/MnO-(MnFe)2O3 achieves a current density of 6.93 mA cm-2 at the potential of 0.3 V that is higher than that of Pt/C, but it does not exhibit a limiting current platform like that of Pt/C, indicating a poor catalytic activity relative to Pt/C. The Koutecky-Levich (K-L) plots for NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 derived from the RDE test at various rotating speeds are linearly fitted (Figure S5), indicating a firstorder reaction kinetics relative to the dissolved oxygen concentration.51,52 The average electron transfer numbers (n) obtained from the slopes of K-L plots at different potentials are 3.75 and 3.73 for NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3, respectively, demonstrating an efficient 4e- pathway for ORR processes.53 The tolerance to methanol is also a very significant criterion for assessing the catalysts for practical application. In this case, it was investigated by decanting 8.5 mL of methanol into O2-saturated KOH solution (70 mL) during the ORR process. As demonstrated by Figure S6, a sharp current density decrease (~ 27 %) occurs for Pt/C in the presence of methanol, confirming the deactivation of Pt/C due to the methanol crossover effect.


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On the contrary, both the NCNT/MnO-(MnFe)2O3 and NCNT/CoFe-CoFe2O4 exhibit strong tolerance to methanol confirmed by a high current retention ratio of ~ 90 % after the injection of methanol into the KOH electrolyte. The above results demonstrate the high ORR activities and selectivities of NCNT/MnO-(MnFe)2O3 and NCNT/CoFe-CoFe2O4 in alkaline media. As a proof-concept application toward good ORR and OER, the primary zinc-air batteries are assembled with NCNT/MnO-(MnFe)2O3 and NCNT/CoFe-CoFe2O4 as the air-cathode, aqueous KOH (6 M) as electrolyte and the polished zinc plate as anode. Figure 7b shows the power densities and discharge curves of the two-electrode primary zinc-air batteries, which are assembled with NCNT/MnO-(MnFe)2O3, NCNT/CoFe-CoFe2O4 and Pt/C. Evidently, the NCNT/MnO-(MnFe)2O3 and NCNT/CoFe-CoFe2O4 catalysts deliver the open circuit voltages of 1.45 and 1.56 V, respectively. At the potential of 0.6 V, the batteries assembled with the two catalysts show similar current densities (~ 160 mA cm-2) and power densities (98 mW cm−2) to those of Pt/C. The galvanostatic discharge curves in Figure 7c uncover the discharge potential platforms of the NCNT/MnO-(MnFe)2O3, NCNT/CoFe-CoFe2O4 are slightly lower than that of Pt/C (1.22 V) at current density of 20 mA cm-2.



Figure 7. a) ORR polarization curves of NCNT/MnO-(MnFe)2O3, NCNT/CoFe-CoFe2O4, and Pt/C in O2-saturated 0.1 M KOH with a scan rate of 10 mV s-1 at 1600 rpm. b) The power densities and polarization plots of primary zinc-air batteries with NCNT/MnO-(MnFe)2O3, NCNT/CoFe-CoFe2O4 and Pt/C as air-cathodes. c) The galvanostatic discharge curves of NCNT/MnO-(MnFe)2O3, NCNT/CoFe-CoFe2O4, and Pt/C assembled primary zinc-air batteries at the current density of 20 mA cm-2. d) The specific capacities of NCNT/MnO(MnFe)2O3, NCNT/CoFe-CoFe2O4, and Pt/C assembled primary zinc–air batteries normalized to the mass of the consumed Zn. As evidenced by Figure 7d, the NCNT/MnO-(MnFe)2O3-based zinc-air battery exhibits a higher specific capacity (647 mAh g-1) than that of Pt/C (619 mAh g-1), which corresponds to the energy densities of 776 and 755 Wh kg-1 at the current density of 20 mA cm-2, respectively. Even NCNT/CoFe-CoFe2O4, also shows a specific capacity of 606 mAh g-1 and energy density


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of 727 Wh kg-1. Although the ORR performance of our catalysts is poorer than that of Pt/C, primary zinc air batteries are comparable to, even better than that of Pt/C, which caused by many factors such as the concentration of electrolyte, the thickness of electrodes and membrane, and so on. The rechargeable zinc-air batteries were also assembled with 0.2 M zinc acetate adding into 6 M KOH as electrolyte (Figure 8a). Figure 8b displays the charge and discharge polarization curves of rechargeable zinc-air batteries with NCNT/CoFe-CoFe2O4 as bifunctional ORR/OER catalysts assembled into air electrodes. Notably, the sum of charge and discharge potentials for NCNT/CoFe-CoFe2O4 is 0.54 V at the current density of 20 mA cm-2, comparable to Pt/C+IrO2 cathode. The charge-discharge cycle stability tests were performed at the current density of 20 mA cm-2 over extended more than 20 h.

Figure 8. The performance of NCNT/CoFe-CoFe2O4 and Pt/C assembled rechargeable zincair batteries with Ni foam as the supports in 6 M KOH. a) Schematic diagram of the rechargeable zinc-air battery. b) The charge and discharge polarization curves of NCNT/CoFe-



CoFe2O4 and Pt/C assembled rechargeable zinc-air batteries. c) The cycling tests of NCNT/CoFe-CoFe2O4 and Pt/C assembled rechargeable zinc-air batteries at the current density of 20 mA cm-2. As demonstrated by Figure 8c, the rechargeable zinc air batteries were tested by switching the current densities between 20 and -20 mA cm-2 each 5 min (300 s) for one cycle. The battery assembled with NCNT/CoFe-CoFe2O4 air-cathode exhibited obvious potential platform that can be obviously observed by enlarging the X-axis (Figure S7), small voltage gap and high trip efficiency, which is comparable to that of the Pt/C+IrO2 cathode, and better than those of many reported noble metal-free oxide bifunctional catalysts (Table S4). Therefore, the rechargeable zinc air battery assembled with bifunctional NCNT/CoFe-CoFe2O4 catalysts is highly feasible in practical metal-air batteries with low charge/discharge overpotential and enhanced cyclic stability. However, the NCNT/MnO-(MnFe)2O3-based battery displays capacitive behaviors without obvious potential platform, which is unbeneficial for the assembly of zinc air battery (Figure S7). Of course, the above mentioned batteries performance is caused by the synergetic effect of both catalysts and Ni foam support because Ni foam itself is active for OER. In addition, as evidenced by Figure S8, after zinc battery testing, Zn dendrites is easily observed, which is also major obstacle to push zinc air battery to the industry. 4. CONCLUSIONS In summary, we have successfully achieved in situ coupling of bimetallic CoFe-CoFe2O4 and MnO-(MnFe)2O3 with N-doped carbon nanotubes via a facile and efficient two-step growth-annealing approach. The resultant NCNT/CoFe-CoFe2O4 and NCNT/MnO-(MnFe)2O3 were used as two trifunctional electrocatalysts for overall water splitting and zinc-air batteries.


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Evidently,

the

NCNT/CoFe-CoFe2O4-

and

NCNT/MnO-(MnFe)2O3-based

alkaline

electrolysers exhibited low cell voltages of 1.65 V and 1.70 V (vs RHE), respectively, and excellent long-term electrochemical durability, significantly superior to the benchmark IrO2. Meanwhile, the zinc-air batteries assembled by NCNT/CoFe-CoFe2O4 on air-cathode delivered small charge/discharge overpotentials and extraordinary cyclic stability, comparable to the performance of (Pt/C + IrO2). The outstanding electrocatalytic performance of hybrids could be ascribed to the synergistic effect between bimetallic oxides and N-doped carbon nanotubes. It is anticipated this fundamental approach and insight could be developed to design advanced bimetallic oxides/CNT hybrid trifunctional catalysts as potential alternatives to noble metals for various renewable energy storage and conversion technologies. ASSOCIATED CONTENT Supporting Information. Details on characterization method, and sample supplementary characterization AUTHOR INFORMATION *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Taishan Scholar Program of Shandong Province, China (ts201712045). The Key Research and Development Program of Shandong Province (2018GGX104001). Natural Science Foundation of Shandong Province of China



(ZR2017MB054, ZR2018BB008). Doctoral Found of QUST (0100229001). Qingdao Postdoctoral Application Program (04000637). The XAFS beam time was granted by the 1W1B beamline of Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Science. The authors would like to thank the staff of 1W1B for their technical support in XAFS measurement and guide for data analysis. REFERENCES (1)

Zheng, X. R.; Han, X. P.; Liu, H.; Chen, J. J.; Fu, D. J.; Wang, J. H.; Zhong, C.; Deng, Y. D.; Hu, W. B. Controllable Synthesis of NixSe (0.5

Alloys and N-Doped Carbon Nanotubes

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