Flexible Waterproof Rechargeable Hybrid Zinc Batteries Initiated by Multifunctional Oxygen Vacancies-Rich Cobalt Oxide Longtao Ma 1, Shengmei Chen 1, Zengxia Pei 1, Hongfei Li 1, Zifeng Wang 1, Zhuoxin Liu 1, Zijie Tang 1, Juan Antonio Zapien 1, Chunyi Zhi 1, 2 * 1 Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China. 2 Chengdu Research Institute, City University of Hong Kong, Chengdu, PR China. *Corresponding Author: Prof. Chunyi Zhi E-mail: cy.zhi@cityu.edu.hk Experiment Materials Cobaltous nitrate (Co(NO 3 ) 2 6H 2 O) is purchased from Alfa. Urea (CH 4 N 2 O), Ammonium fluoride (NH 4 F), Acrylic acid (C 3 H 4 O 2 ), Ammonium persulphate ((NH 4 ) 2 S 2 O 8 ), Sodium hydroxide (NaOH), N, N'-methylenebisacrylamide (C 7 H 10 N 2 O 2 ), 1
and Potassium hydroxide (KOH) are purchased from Aladdin. All chemicals are used without any further purification. The pre-treatment of carbon cloth fiber (CFC) The CFC are cleaned firstly by acetone and DI water under ultrasonic condition for 5 min. Then, the cleaned CFC are immersed into mixture solution of 96% H 2 SO 4 and concentrated 65% HNO 3 with ratio of 2:1(v/v) for 30 min. After that, 6 g KMnO 4 is poured into the mixture solution under vigorously stirring and hold for 1h. Moreover, 100 ml DI is slowly dripped into it. The H 2 O 2 is used to counteract to KMnO 4 and the treated carbon clothes is washed by DI water. Finally, the pretreated CFC is obtained by vacuum dry and calcining at 950 o C for 1h at a rate of 5 o C/min. Synthesis of freestanding Co 3 O 4 electrode materials The Co 3 O 4 nanorods on CFC electrode is fabricated by the protocol. In detail, 4 mmol of Cobaltous nitrate (Co(NO 3 ) 2 6H 2 O), 11.5 mmol urea (CH 4 N 2 O) and 7 mmol ammonium fluoride (NH 4 F) is dissolved in 25 ml distilled water under vigorously stirring and hold for 2h at room temperature. Then, the homogeneous solution is transferred into 25 ml hydrothermal reactor and the pre-treated CFC are immersed into the solution around the reactor wall. After that, the reaction was heated to 120 o C and preserved for 8 h in an electric oven. After the hydrothermal reactor is cooled to room temperature, the CFC coated with pink precursor is picked out and washed with DI water and ethanol for three times, separately. Moreover, the precursor is dried at 60 o C for 12 h under vacuum condition. The freestanding Co 3 O 4 nanorods on CFC are achieved by 2
annealing the precursor at 350 o C for 3h. The loading mass of active materials CFC is determined to be 1.13 mg/cm -2. Preparation of (Polyacrylamide) PAM hydrogel electrolyte. First, 2 g acrylamide was dissolved in 2 ml distilled water under vigorously stirring at 40 o C for 5 min. Then 2 mg N, N'-methylenebisacrylamide and 5 mg potassium persulfate as initiator were added into the solution and reacted for 2 hs. The obtained mixture solution was treated under ultrasonic condition for 30 min. In addition, the free-radical polymerization was conducted at 70 o C for 40 min. Finally, the PAM film hydrogel electrolyte is achieved by soaking in mixture solution of 6 M KOH and 0.2 M Zn(CH 2 COO) 2 overnight. Fabrication of freestanding deposited-zn The freestanding deposited-zn on CFC electrode was prepared by two-electrode electrochemical deposition method. In detail, 100 ml 0.5 M Zn(SO 4 ) 2 serves as electrolyte, Zn Plate as negative electrode and carbon cloth as positive electrode. The electrochemical reaction was conducted by using chronoamperometry procedure (CHI 760E, Chenhua) at 1.0 V for 3000 s. Characterizations The surface microstructure is investigated by using field-emission scanning electron microscopy (FESEM, FEI Quanta 450 FEG) and the crystal structure analysis was conducted by using X-ray diffractometer (Bruker, D2 Phaser) with Cu Kα (λ=1.5418 Å) radiation. In addition, the surface properties and oxygen vacancies were characterized by 3
using X-ray photoelectron spectroscopy (XPS, ESCALB 250) with Al Kα X-ray beam (E = 1486.6 ev). Electrochemical performance Eletrocatalytic activity test The electrocatalytic performance was investigated by rotating disk experiment (RDE) and rotating ring-disk experiment (RRDE). The active materials were prepared by blading Co 3 O 4 and Co 3 O 4-x nanorods from freestanding electrode. All electrochemical measurements were carried out using CHI 760D workstation (Chenhua, China) integrating a rotating disk electrode (RRDE-3A) in a three-electrode system, in which a Ag/AgCl (saturated KCl) was used as reference electrode and a platinum net as counter electrode. The recorded potentials refer to RHE, according to E (RHE) = E (Ag/AgCl) + 0.059 * ph + 0.210. In the experiment, RDE test is conducted with glassy carbon disk electrode (3 mm in diameter) and a RRDE with Pt ring (5 mm in inner diameter and 6.5 mm in outer diameter) and a glassy carbon disk electrode (4 mm in diameter). The catalyst ink was prepared by dispersing 2 mg catalyst in the mixture solution of 800 µl DI water, 200 µl isopropanol and 20 µl Nafion solution (Alfa Acesar, 5 wt%). All oxygen reduction reaction (ORR) measurements were conducted in 0.1 M KOH solution and the loading masses were 140 µg cm -2. The cyclic voltammetry (CV) profiles were conducted in oxygen or nitrogen-saturated 0.1 M KOH solution at scanning rate of 20 mv s -1 and linear scanning voltammetry (LSV) profiles were conducted in oxygen-saturated 0.1 M KOH solution at a scanning rate of 5 mv s -1. Long-term stability evaluation for ORR were carried out by measuring LSV curves using rotating speed of 1600 rpm in oxygensaturated 0.1 M KOH solution. The oxygen evolution reaction (OER) performance was 4
evaluated by LSV using RDE (1600 rpm) in oxygen-saturated 0.1 M KOH solution at scanning rate of 5 mv s -1. To detect peroxide (H 2 O 2 ) species formed in the process of ORR, the RRDE was conducted with Pt ring electrode potential set at 1.3 V. The n can also be evaluated using RRDE measurement profiles according the following equation: = 4 Where the I D is the disk current, I R is the ring current and N is the collection efficiency of Pt ring. The HO 2 - yield was calculated from equation: H = 200 ( + ) Where i d is the disk current, i r is the ring current and N is the current collection efficiency of the Pt ring and in the experiment was determined as 0.44. 5
Figure S1. XRD patterns of Co 3 O 4 and Co 3 O 4-x. The rectangle marked weak peaks is the diffraction peaks of carbon fiber cloth 6
Figure S2. Stability characterization of ORR performance for Co 3 O 4 and Co 3 O 4-x. 7
Figure S3. Stability characterization of OER performance for Co 3 O 4 and Co 3 O 4-x. 8
Figure S4. CV curves in the region of 1.24-1.36 V at scan rates from 2 to 10 mv s -1 and corresponding linear fitting of capacitive current: (a) and (b) for Co 3 O 4 ; (c) and (d) for Co 3 O 4-x. 9
Figure S5. The A.C impedance plots of Co 3 O 4 and Co 3 O 4-x. 10
Figure S6. (a) CV curves of Co 3 O 4 and Co 3 O 4-x cathode in anaerobic condition, (b) Galvanostatic charge/discharge profiles and (c) long-term charge/discharge cycling stability at 2 A g -1. 11
Figure S7. (a) Charge-discharge curves of Zn-Co 3 O 4-x /Zn-air hybrid battery. The points A-I marked the states where are collected for XRD analysis. (b) Ex-situ XRD patterns of Co 3 O 4-x cathode at selected states during the second cycles. Notes: The ex-situ XRD analyses of Co 3 O 4-x nanorods cathode at different chargingdischarging states are utilized to investigate structural evolution of the Co 3 O 4-x cathode in zinc-co 3 O 4-x /zinc-air battery process. The structure of Co 3 O 4-x is fully reversible during charge-discharge process. The new peaks located at 37.3 o is ascribed to the appearance of CoOOH. During charging process (A D), the new phase of CoOOH appears and gradually increase. Notably, the structure of Co 3 O 4-x cathode during process from C to D state does not change, which is attributed to OER process. During discharging process, amounts of CoOOH decreases and completely recover to initial state at H point. The structure of Co 3 O 4-x cathode does not change from H to I point, which is ascribed to ORR process. Hence, a phase transformation between Co 3 O 4-x and CoOOH occurs during zinc- Co 3 O 4-x battery reaction. During zinc-air battery reaction, there is no phase transformation, owing to electrocatalytic reaction in Co 3 O 4-x cathode during discharging process of zincair battery. 12
Figure S8. Representative charge-discharge curves of a hybrid zinc batteries using Co 3 O 4-x cathode at different cut-off discharge voltages from 1.6 to 1.23 V. 13
Figure S9. The A. C impedance plots of zinc-co 3 O 4-x and zinc-air battery. 14
Figure S10. (a) Galvanostatic charge/discharge profiles, (b) rate capacity test and (c) long-term charge/discharge cycling stability at 2 A g -1. 15
Figure S11. Capacity retention of the Zn-Co 3 O 4-x /Zn-air hybrid battery for a long-term soaking test. 16
Figure S12. Comparison of digital photos of immersed Zn-Co 3 O 4-x /Zn-air hybrid battery and Zn-air battery with Pt/C as electrocatalyst in water to power an electronic watch device. Without air, the Zn-air battery with Pt/C as electrocatalyst cannot work anymore, while the hybrid battery can still output power. 17