Supporting Information

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1 Supporting Information Electrospun Thin-Walled CuCo 2 O Nanotubes as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries Xiaojun Wang, Yang Li, Ting Jin, Jing Meng, Lifang Jiao *,,,Min Zhu # and Jun Chen, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin , China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin , China #School of Materials Science and Engineering and Guangdong Provincial Key Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou , China Corresponding author: Lifang Jiao, jiaolf@nankai.edu.cn

2 Experimental Section Preparation of thin-walled mesoporous CuCo 2 O nanotubes: The N-doped mesoporous CuCo 2 O nanotubes were prepared through a simple coaxial electrospinning technique followed by heat treatment. After optimization of synthetic conditions, 1.5 g low-molecular-weight Polyacrylonitrile (PAN, average Mw = 85000, J&K) dissolved in 18 ml N,N-dimethylformamide (DMF, anhydrous, 99.8%) was used as inner fluid, while 2.0 mmol cupric nitrate (Cu(NO 3 ) 2 3H 2 O, 99%), 4.0 mmol cobalt acetate (Co(CH 3 COO) 2 4H 2 O, 99%), and 1.3 g high-molecular-weight PAN (average Mw = , J&K) were added in 18 ml DMF as outer fluid. Both of the precursors were under continuous magnetic stirring at 80 for 12 h to obtain a homogeneous viscous solution. These mixtures serve as working liquid for electrospinning. The precursor solution was transferred into 5 ml plastic syringe equipped with coaxial stainless-steel tubes. A high voltage of 16 KV was applied at needle tip with feeding rate of solution set to be 0.9 ml h -1. A copper foil used as the receiving plate to collected polymer fibers. The obtained fiber paper was first drying at vacuum box, and then the paper stabilized at 250 for 2 h, followed by a carbonized at 800 for 1 h with a heating rate of 2 min -1 under high-purity nitrogen (N 2 ) environment. In the heating process, PAN carbonized and nitrate decomposed to form nitrogen doped carbon nanofibers, and gases like NO 2, CO 2 released to generate pores. Finally, the CuCo 2 O nanotubes with pure crystal structure were obtained by heating treated at 350 for 2 h in flowing air. For comparison, CCO@C composite with

3 higher/lower carbon content were prepared by just altering the oxidation temperature of 300 and 400 (denoted as and H-CCO), respectively, with other synthetic conditions unchanged. It is worth mentioning that we use different molecular weight of PAN precursor to control the escape velocity of gases during heat treatment, thus creating the open-ended hollow nanotubes. Similarly, in order to altering the carbon content in the CuCo 2 O composites, we use the different oxidation temperature to adjust the lost quantity of carbon. Characterizations: The phase purity and crystallinity of the prepared samples were examined by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu Kα radiation (λ = Å)). The microstructure was observed by field-emission scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscopy (TEM, Tecnai G2 F20). Raman spectra were studied on a confocal Raman microscope with a laser excitation at nm. TG-DSC analyzer (NETZSCH, STA 449 F3) was used to determine the percentage of CuCo 2 O 4 in the composites from 50 to 700 with a heating rate of 5 min -1 in air. X-ray photoelectron spectrometry (XPS, Perkin Elmer PHI 1600 ESCA) was performed to evaluate the elemental compositions and analyzed the N 2 -doping configurations in carbon nanofibers. The specific surface area of the materials was measured by nitrogen adsorption-desorption isotherms and pore size distribution was estimated from the adsorption isotherms using the Barrette-Joynere-Halenda (BJH) method.

4 Electrochemical measurements The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) measurement were performed on an electrochemical workstation (CHI 760E, CH Instrument) equipped with a rotating ring-disk electrode apparatus (RRDE-3A, ALS). Conventional three-electrode system was used, including Glassy carbon electrode (GCE, 4 mm in diameter), platinum net (Pt), and saturated calomel electrode (SCE) as working electrode, counter electrode and reference electrode, respectively. All potentials are reference to the reversible hydrogen electrode (RHE, (E RHE =E SCE pH )). To prepare the catalyst slurry, 4.0 mg of electrocatalyst was sonicated in the mixture of 500 μl ethanol, 500 μl isopropanol and 50 μl Nafion solution (5 wt%, Sigma Aldrich) for 60 min to form homogeneous catalyst ink. For ORR, 3.3 μl of the ink was dropped onto the rotating ring-disk electrode (RRDE) to give a catalyst loading of 0.1 mg cm -2. The ORR activities of catalysts were measured via linear sweep voltammetry (LSV) from 0.1 to -0.7 V (vs. SCE) at a scan rate of 5 mv s -1 with 0.1 M oxygen saturated KOH electrolyte at a series of rotating electrode speeds (400, 625, 900, 1225, 1600, 2025 and 2500 rpm). The number of transferred electrons (n) per O 2 molecule in the ORR was calculated from the Koutecky-Levich equations: 1, 2 1 J 1 J L 1 J K 1 Bω 1/2 1 J K B 0.2nFC 2 / 3 0 ( D0) v 1 / 6

5 J K nfkc 0 Where J is measured current density, J K and J L are the kinetic and diffusion-limiting current densities, respectively. ω is the angular velocity of the disk, n is the number of electrons transferred in ORR, F is the Faraday constant (96485 C mol -1 ), C 0 is the bulk concentration of O 2 ( mol cm -3 ), D 0 is the diffusion coefficient of O 2 in 0.1 M KOH ( cm 2 s -1 ), v is the kinematic viscosity of the electrolyte (0.01 cm 2 s -1 ). The constant 0.2 is adopted when the rotation speed is expressed in rpm. The generated peroxide ion content during ORR was determined by a rotating ring-disk electrode (RRDE). The disk electrode was scanned at a rate of 5 mv s 1, and the ring potential was constant at 1.5 V vs SCE. The electron transfer number (n) and percentage of HO 2 intermediate production (HO 2 %) were determined by the followed equations: 200I r HO NI I % 2 d r n 4NId NI I d r Where I d is the disk current, I r is the ring current, and N is the current collection efficiency of the Pt ring, which was determined to be For OER, 9.2 μl of the ink was pipetted on the glassy carbon electrode to give a mass loading of 0.28 mg cm -2. LSV was measured at a scanning rate of 5 mv s -1 from 0 to 0.8 V in 1 M O 2 -saturated KOH solution. The electrochemical double layer capacitances (C dl ) of catalysts are measured by using a simple CV method with a mass loading of 0.1 mg cm -2. It is known that C dl value is expected to be linearly proportional to the electrochemically active surface area of the electrode.

6 Additionally, the catalytic activity of H-CCO samples, commercial Pt/C and IrO 2 were also tested at the same condition with CCO@C nanotubes. Fabrication of Zn-air battery Rechargeable Zn-air batteries were tested in homemade full cells, where catalysts carried on carbon paper (1 mg cm -2 ), CCO@C and Pt/C-IrO 2 electrode as the air cathodes and Zn plate as anode in 6.0 M KOH with 0.2 M zinc oxide to ensure reversible Zn electrochemical reaction. The polarization curve measurements were performed by LSV at 5 mv s -1 with Autolab electrochemical working. A Land CT2001A system was used to carry out galvanostatic discharge-charge cycling curves measurements with each discharge and charge period set to be 30 min at a current density of 2 ma cm -2, and 10 min at a current density of 10 ma cm -2.

7 Figure S1. Raman spectrum of the Figure S2. N 2 adsorption desorption isotherm and pore size distribution curve (inset) Figure S2 depicts the nitrogen adsorption-desorption isotherm of CCO@C nanotubes, exhibiting a type-iv behavior with a hysteresis loop at relative pressure (P/P 0 ) range of This means the CCO@C nanotubes possess a large number of mesoporous. 3 Significantly, the abundant mesopores give rise to a large specific surface area with up to 514 m 2 g 1, which not only increases the exposure active sites of bifunctional catalyst but also facilitates the diffusion kinetics of oxygen and hydroxyl.

8 Figure S3. Particle size analysis of Figure S4. N-doped character of nanotubes: (a) survey XPS spectrum, (b) N1s and (c) C1s high resolution XPS spectra. X-ray photoelectron spectroscopy (XPS) was measured to reveal the close to-surface chemical composition of nanotubes. No peaks of elements other than Cu, Co, O, C and N are discovered in survey spectrum (Figure S4a). The high-resolution N1s in Figure S4b reveals the co-existence of pyridinic N (398.2 ev), pyrrolic N (399.5 ev), and graphitic N (400.8 ev) as doped nitrogen in CCO@C, which is consistent with the results of EA (Figure S9a). Figure S4c further demonstrates the formation of C-N and C=N bond. As we all known, pyridinic N and graphitic N in carbon networks plays an important role in improving ORR and OER

9 properties owing to the enhanced π bonding (i.e., C N bonding as active sites). This atom would favor the adsorptions of OH and oxygen (transfer of feasible intermediates, such as O 2 and O 2 2 ). 4 Synergistic effect of doping-n and porosity on CCO@C nanotubes making this material anticipated to deliver high ORR and OER activity. Figure S5. SEM images of as-spun Cu(NO 3 ) 2 3H 2 O/Co(CHCOO) 2 4H 2 O@PAN (denoted as CuCo@PAN) nanofibers. Figure S6. SEM images of as-spun CuCo@PAN nanofibers at different carbonization temperatures: (a) 700, (b) 800, (c) 900.

10 Figure S7. (a) XRD patterns of the as-prepared H-CCO and the standard card of CuCo 2 O 4 (JCPDS No ), (b) Raman spectra of the L-CCO@C and H-CCO. For comparison, CuCo 2 O nanofibers with lower and higher carbon content were also prepared by altering the oxidation temperature (higher and lower oxidation temperature obtained production named H-CCO and L-CCO@C, respectively). The XRD patterns of L-CCO@C and H-CCO are displayed in Figure S7 (SI), which are in accordance with spinel-cuco 2 O 4. In combination with Raman spectra of these samples, it is found that L-CCO@C possesses distinctly carbon peaks (1345 cm -1, 1590 cm -1 ), while H-CCO does not contain carbon element. 5 This result is also confirmed by element analysis (EA), as summarized in Figure S9a (SI). The carbon contents of L-CCO@C and H-CCO are and 1.12 wt%, separately, which is in good agreement with the thermogravimetric analysis (TGA, Figure S9b, SI).

11 Figure S8. SEM and TEM images of as-spun nanofibers at different oxidation temperatures at 800 carbonization temperature: (a) (c) 300, (b) (d) 400. Figure S9. (a) Element analysis (EA) test of and H-CCO samples, (b) Thermogravimetric (TG) curves of the three samples at a heating rate of 5 min -1 in a flowing air environment.

12 Figure S10. N 2 adsorption desorption isotherm and pore size distribution curve of (a) L-CCO@C and (b) H-CCO. The L-CCO@C and H-CCO samples possess specific surface area of 156 and 133 m 2 g 1, respectively (Figure S10). The relative low active areas would restrain the diffusion kinetics of hydroxide and the transportation of oxygen. 6 Figure S11. (a) Rotating ring-disk electrode voltammograms recorded with CCO@C and Pt/C electrodes, (b) peroxied yield and electron transfer number of CCO@C and Pt@C electrodes.

13 Figure S12. Methanol resistance test of and Pt/C.

14 Figure S13. CVs of (a) (b) and (c) H-CCO at scan rates from 10 to 100 mv s 1. (d) Scan rate dependence of the current densities of the three catalysts. To explore the reasons of excellent performance of CCO@C better than L-CCO@C and H-CCO, we have measured the electrochemically active surface area (ECSA) from the electrochemical double-layer capacitance (C dl ) by using CV. As shown in Figure S13, a linear fit determined the specific capacitance to be mf cm 2 for CCO@C, 3.20 mf cm 2 for L-CCO@C and 2.45 mf cm 2 for H-CCO. The specific capacitance can be converted into an electrochemical active surface area (ECSA) using the specific capacitance value for a flat standard with 1 cm 2 of real surface area. CV scans of the CuCo 2 O 4 in a non-faradaic region at different scan rates (10 to 100 mv s 1 ) are shown in Figure S13. The current measured from CV scans

15 were linearly fitted with CV scan rates using the following equation: i C dl where ν is the scan rate and i is the current. C dl value can be obtained from the slope of the above equation. ECSA was then calculated by dividing the C dl using the specific surface capacitance (C s ) of electrode surface: ECSA C C dl s A commonly used Cs value for metal surfaces (40 μf cm 2 ) was adopted here. 7, 8 Thus, the ECSA of CCO@C, L-CCO@C, H-CCO was estimated to be cm 2, 80.0 cm 2, 61.3 cm 2, respectively, which means that CCO@C has a more electrochemically active surface than the other catalysts. The increased ECSA for CCO@C can render a large specific surface area for it catalytically active sites, excellent gas bubble diffusion ability, and thus leading to the superior catalytic 9, 10 performance. Calculated electrochemical active surface area: CCO@C: 12.55mF cm 40μF cm per cm 2 A CCO@C ECSA 2 2 ECSA cm 2 ECSA L-CCO@C: 3.20 mf cm 40μF cm per cm 2 A L - CCO@C ECSA 2 2 ECSA cm 2 ECSA H-CCO 2.45 mf cm 40μF cm per cm 2 A H - CCO ECSA 2 2 ECSA cm 2 ECSA

16 Figure S14. Nyquist plots of H-CCO, and IrO 2 at 1.6 V. Figure S15. Galvanostatic discharge-charge cycling curves at 2 ma cm -2 L-CCO@C and H-CCO as air electrodes. using

17 Figure S16. Galvanostatic discharge-charge cycling curves at 10 ma cm -2 nanotubes as air electrode. using Table S1: The electrocatalytic performance of CuCo 2 O 4 samples and the commercial noble metal catalyst. ORR OER Samples Onset potential (V) Half-wave potential (V) Limiting current density (ma cm -2 ) Overpotential at 10 ma cm -2 (V) Tafel slope (mv dec -1 ) CCO@C L-CCO@C H-CCO Pt/C ~ ~ IrO 2 ~ ~ ~

18 Table S2. The electrocatalytic activities of the recently reported bifunctional catalysts for ORR/OER. ORR OER Overall oxygen Materials Mass Loading Onset potential Half-wave potential 10 ma cm -2 Tafel slope electrode activity Reference (mg cm -2 ) (V vs. RHE) E 1/2 (E j=10) (mv ΔE (V vs. RHE) (mv vs. RHE) dec -1 ) (E j=10-e 1/2) (V) NiCo/PFC aerogels PS-CNS Nano Lett. 2016, ACS Nano, 2017, 11, Angew. Chem. Int. Ed. PCN-CFP , 54, ACS Appl. Mater. NiFeO@MnO x (1:0.8) ~ ~ Interfaces, 2017, 9, NPMC ~ ~ ~ Nat. Nanotechnol. 2015, 10, Single-crystal CoO nanorods ~ 0.71 Nat. Commun. 2016, 7, Mn xo y/n-c Angew. Chem. Int. Ed. 2014, 53, J. Am. Chem. Soc. Ni 0.75Co 0.25O x 0.07 ~ ~ ~ 2012, 134, ACS Appl. Mater. HP-Fe-N/CNFs ~ ~ ~ Interfaces, 2017, 9, CoO/N-G Cu-MOF/GO ~ ~ Energy Environ. Sci. 2014, 7, Adv. Funct. Mater. 2013, 23, NiO/CoN PINWs (400 mv) 0.85 ACS Nano 2017, 11, NCNF ~ 1.02 Adv. Mater. 2016, 28, N-doped graphene/cnt ~ 1.00 Small 2015, 10, Fe@N-C ~ 0.88 Nano Energy 2015, 13,

19 Co 0.50Mo 0.50O yn z/c ~ ~ ~ ~ NCNT/Co 0.51Mn 0.49O Co 3O 4/Co 2MnO ~ 1.09 CuFe 2O 4/NS-rGO 0.57 ~ ~ ~ Zn xco 3-xO ~ ~ ~ MnCo 2O 4/N-rmGO ~ 0.70 Angew. Chem. Int. Ed. 2013, 52, Nano Energy 2016, 20, Nanoscale 2013, 5, Adv. Sci. 2017, Chem. Mater. 2014, 26, J. Am. Chem. Soc. 2012, 134, CCO@C 0.1(ORR) 0.28(OER) This work

20 Table S3. The performance of rechargeable Zn-air batteries with various electrocatalysts. Rechargeable Zn-air batteries Materials Mass Loading Open circuit Reference (mg cm -2 ) potential Recharge ability (V) NiO/CoN PINWs Self-supported min per cycle for 50 cycles NCNF min per cycle for 500 cycles NiO/Ni(OH) 2 2 ~ 70 min per cycle for 70 cycles PS-CNS min per cycle for 500 cycles Fe@NC 2.2 ~ min per cycle for 100 cycles NiCo/PFC aerogels 0.13 ~ 2 h per cycle for 300 cycles c-comn 2/C oxide ~ 400 s per cycle for 155 cycles LaNiO 3/N-CNT min per cycle for 75 cycles ACS Nano 2017, 11, Adv. Mater. 2016, 28, Nano Lett. 2016, 16, ACS Nano 2017, 11, Nano Energy 2015, 13, Nano Lett. 2016, Nat. Commun. 2015, 6, Nano Lett. 2012, 12, ACS Appl. Mater. NiFeO@MnO x (1:0.8) 0.25 ~ 100 cycles Interfaces, 2017, 9, NCNT/CoO-NiO-NiCo 0.53 ~ 10 min per cycle for 100 cycles LBSCFO ox ~ 10 min per cycle for 100 cycles BNC 3 ~ min per cycle for 66 cycles ZnCo 2O 4/N-CNT min per cycle for 32 cycles Angew. Chem. Int. Ed. 2015, 54, Energy Environ. Sci., 2016, 9, Adv. Mater. 2015, 27, Adv. Mater. 2016, 28, N/P co-doped carbon foam min per cycle for 180 cycles Nat. Nanotech. 2015, 10, ACS Appl. Mater. HP-Fe-N/CNFs 1.0 ~1.42 ~ Interfaces, 2017, 9, NCNT/Co xmn 1 xo 0.53 ~ 10 min per cycle for 72 cycles Nano Energy 2016, 20, CCO@C min per cycle for 160 cycles This work

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