A Durable Free-standing Hierarchical Porous Electrode for

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A Durable Free-standing Hierarchical Porous Electrode for Rechargeable Zn-air Batteries Xiaoyi Cai, Linfei Lai, Lijun Zhou, and Ze Xiang Shen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02101 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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ACS Applied Energy Materials

A Durable Free-standing Hierarchical Porous Electrode for Rechargeable Zn-air Batteries Xiaoyi Cai,[a] Linfei Lai,*[b] Lijun Zhou,[b] and Zexiang Shen*[c] X.Cai, [a] Energy Research Institute @NTU (ERI@N), Nanyang Technological University 50 Nanyang Drive, X-Frontiers Block, Level 5, Singapore 637553 Prof. L. Lai, L. Zhou [b] Key Laboratory of Flexible Electronics & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) Nanjing Tech University, Nanjing 210009, China *E-mail: [email protected] Prof. Z. Shen [c] School of Physical and Mathematical Sciences, Nanyang Technological University 21 Nanyang Link, Singapore 637371 *E-mail: [email protected] Keywords: vertically aligned CNTs • Zn air battery • free-standing • oxygen reduction reaction • oxygen evolution reaction

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Abstract:

The development of free-standing bifunctional air-cathodes for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for next generation of flexible rechargeable metal-air batteries. It remains challenging to achieve efficient OER and ORR bifunctionality on a single lightweight and inexpensive electrode. In this article, metal free, freestanding air-cathode based on vertically aligned carbon nanotubes (VACNTs) functionalized with N, P heteroatoms doped carbon is first reported. In addition to the high catalytic activity caused by N, P heteroatoms doping, the importance of the efficient gas diffusion and electron transfer provided by the VACNT-GF hierarchical structure is highlighted. The carbonization temperature has been identified to have pronounced effect on catalytic activity, the samples with P-N bonds has smaller ORR and OER overpotentials, while the quantitative atomic ratio of either P or N has little effect on catalytic activity. The resulting air electrode achieved a high peak power density of 56mW cm-2 at a current density of 120 mA cm-2, outperforming Pt/C and IrO2 based rechargeable Zn-air batteries. The zinc-air battery assembled with the air electrode also show good cyclability which exceeds cells with the Pt/C//IrO2 catalyst. The increase of voltage difference between the charge and discharge platform was 0.2V for the cell assembled with N, P doped VACNT-based free-standing air cathode after 75 hours of operation at 10 mA cm-2, which was less than half of that of cells with Pt/C//IrO2 catalyst. Impedance analysis further reveals the good performance results from the favorable mass transfer of the electrode.


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1. Introduction Zinc-air batteries (ZABs) have attracted increasing attention because of its high theoretical volumetric and gravimetric energy densities, low cost, environmental friendliness, and inherent safety, which are desirable for applications ranging from stationary energy storage to electric vehicles and portable electronic devices1. Primary Zn-air batteries are already available in the market mainly as button cells to power hearing aid devices, while the rechargeable Zn-air battery is not commercially available yet. The oxygen electrochemical reactions are at the center of this technology and are considered as the bottleneck in practical applications of rechargeable Zn-air batteries. The sluggish ORR and OER during discharge and charging indicate the high standard requirement of catalyst in order to improve the kinetics of these reactions. The best catalyst available is currently Pt-based, however, its application offsets one of ZABs’ major advantage, which is their low cost per kWh stored. Non-noble metal-based catalyst and metal-free catalysts are both intensively studied to replace Pt. Heteroatoms doped CNTs,2, 3 especially N-doped CNTs4, 5 have long been considered a candidate for effective metal-free ORR catalysts, co-doping with other heteroatoms is reported to increase both OER and ORR catalytic activity. CNTs have also been used to support other catalysts in composite materials while being catalytically active themselves, improving the overall performance of the material6-8. Recently ternary doped carbon materials have also gained much attention due to good performance reported9, 10. Researchers are also attempting to gain more understanding of active catalytic sites and effects of doping8. New carbon materials such as doped graphdiyne are also been investigated as ORR catalysts.11 Many CNT based catalysts for OER have also been reported, including pristine12 and oxidized CNTs13 and boron doped CNTs14, carbon nitride-CNTs composite,15 nitrogen-doped16 and sulfur-doped 17 graphene-CNT composite.


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CNTs have also been selected as support for transitional metal based OER catalysts such as cobalt oxides18, 19, carbides20,hydroxides21 and their composites22. P has the same number of valence electrons as N and often shows similar chemical properties, but opposite electronegativities compared to that of carbon.23 N, P dual doped carbon nanomaterials have been demonstrated to have excellent catalytic activity.24-30 Recent research result has suggested that N, P co-doped carbon with P-N bonds have high OER and ORR bi-functional activity31, while the ORR activity is also closely related to the N dopant structure. Past studies have reported dual-doped carbon materials with improved activity than carbon materials doped with a single element. The moststudied type of dual-doped carbon material is B and N dual doped carbon32 materials and BCN nanotubes33, 34, while S, N dual-doped carbon materials also see many reports35-37. On the other hand, N and P dual-doped carbon materials have been shown to have good bi-functional catalytic activity towards OER and ORR, but there are only a few reports utilizing this type of materials as air electrode for ZABs. The design of the electrode structure for efficient mass/charge transport is crucial to the performance of the ZAB. The porosity, pore size, pore volume and surface area of electrode have been found to have a significant effect on the performance of metal-air batteries. The presence of interconnected macropores is important for O2 gas diffusion while the smaller pores provide interfaces for the reaction to proceed, therefore ZABs can greatly benefit from a hierarchical porous structure. VACNTs have good alignment and uniform length, as well as good thermal and electrical conductivity, which make them attractive support for Pt nanoparticles,38, 39 transition metal oxides,40 B, N34 N, P30 heteroatoms co-doping, and modification of Fe–N–C,41 FeN4 structures42 for ORR applications. Although VACNTs has been identified as an excellent substrate over entangled CNTs powder as an electrocatalyst, their applications in Zn-air batteries are rarely


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reported. To prepare an air cathode, multistep processes are usually needed to transfer the conventional grown VACNTs from the 2D flat substrate onto a 3D porous substrate to be breathable, which is complicated and extremely challenging. Due to the good conductivity, chemical and mechanical stability,43 3D graphene have been used as substrates for numerous energy storage and conversion applications such as Li-ion batteries44, supercapacitors45 and hydrogen production.46 Graphene-CNT composites, such as N-doped graphene CNT composite, 16, 47-49

N-S co-doped graphene-CNT,50 partial unzipped CNTs/graphene,51 Se-doped CNT-

graphene, etc., have been studied as ORR catalysts. All these composites are in powder form and still requires conventional air-electrode fabrication process utilizing a large amount of inactive carbon black and insulate binder additive, which not only increases the thickness and weight of the battery but also reduce the conductivity and mass transportation of the electrode, limiting its power density. The aforementioned drawbacks highlight the importance of construction of freestanding air-cathode, earlier reports of metal foam/mesh-based air-electrodes, such as Co3O4 nanowire on stainless steel mesh,52 Ag-Cu nanoalloy on Ni foam,53 carbon particles@2~8 μm nickel fibers54, or the modification of commercially available carbon fiber based gas diffusion layer with MnO2 43 or phosphorous-doped g-C3N455 etc. Flexible Zn-air batteries have also been in high demand.56 It is noteworthy that many types of scaffolds for free-standing or flexible electrodes do not possess catalytic active sites or high surface area properties, resulting in inert mass in electrode and lowered mass activity. In this paper, we report the construction of free-standing and hierarchical porous N and P codoped VACNT-GF foam (NP-VACNT-GF) for ZABs, and X-ray and ultraviolet photoelectron spectroscopy were adopted to systematically reveal the relationship of chemical structure and the catalytic activity. The NP-VACNT-GF annealed at 800oC (NP8-VACNT-GF) in inert gas


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environmental has the best catalytic activity, with N and P atomic ratio of 0.76% and 0.41% respectively. NP8-VACNT-GF also has the lowest work function than other heteroatoms doped VACNT-GF as synthesized. The ZABs assembled using NP8-VACNT-GF can outperform the ones made with conventional catalysts (Pt/C and IrO2) in terms of both power density and cyclability, showcasing high catalytic activity and good stability. The excellent performance of NP8-VACNT-GF mainly results from high catalytically active surface area and favorable mass transport properties, which highlights the importance of the rational design of electrode structure in ZABs. The 3D vertically aligned structure of hierarchical CNTs on graphene foam possesses a well-engineered structure which promotes the diffusion of oxygen to reach the catalytic active sites effectively, then the reaction can be catalyzed efficiently on the surface of VACNTs. The unique characteristic of NP8-VACNT-GF, combined with high conductivity, made them able to achieve high performance as ultralight ZAB air-cathodes.

2. Experimental procedures 2.1 Growth of Graphene Foam The free-standing graphene foam (GF) was grown using a method similar to previously reported in the literature57. Nickel foam with a pore density of 100PPI and thickness of 1mm was cut to pieces with dimensions of 10cm × 5cm, washed with ethanol and used as the template for growth of 3D graphene foam. The nickel foam was placed inside a horizontal quartz tube in a First Nano ET3000 CVD system and the tube was pumped down to 100 mTorr to remove astrosphere oxygen and water. The tube was then filled with a 10:1 mixture of Ar and H2 and heated to 1000°C under an Ar flow of 500 sccm and H2 flow of 100 sccm. The temperature and air flow were then maintained at 1000°C for 10 min to clean the surface of nickel and remove the oxide layer. After


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that, the Ar flow was increased to 800 sccm while keeping the same H2 flow rate, and 100 sccm of methane was supplied into the chamber. After a growth time of 10 min, the gases were purged, and the chamber was cooled down to room temperature.

2.2 Growth of VACNTs The VACNTs was grown on GF using an approach similar to the method reported by Tian et al.39 A piece of as-grown GF with Ni substrate was treated with oxygen plasma with a power of 80W for 90s and coated with 3nm of Alumina using atomic layer deposition. The GF with Ni substrate was then fixed on a hotplate set at 90oC, 10 mL of 7mM iron acetate and 7mM cobalt acetate ethanol solution were evenly sprayed on each side of the GF. Heat treatment was carried out at 500 °C for 10 min in air to synthesize Fe-Co bimetallic catalyst. The GF was then put in the chamber of plasma-enhanced chemical vapor deposition (PECVD) system. The sample chamber was first evacuated to a pressure of 10-5 Pa, then 20 sccm of H2 gas was supplied into the chamber. The pressure was then maintained at 133Pa. The temperature inside the chamber was ramped from room temperature to 500 °C at a rate of 100 °C min-1. Once the temperature reached 500 °C, then a radio frequency alternating magnetic field with a power of 30 W was switch on to treat the FeCo bimetallic catalyst using hydrogen plasma for 2 min. Then the 20 sccm flow rate of H2 flow was maintained and 40 sccm of C2H2 was introduced into the chamber. The air flow was kept for 20 min to grow VACNTs. Finally, the chamber was cooled down to room temperature. The resulting material was then cut to 1cm × 5cm stripes and submerged in the 1M/1M FeCl3/HCl solution overnight to completely remove the Ni template.


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2.3 N and P doping of VACNTs The VACNT-GF material was submerged in 16 mol L-1 HNO3 and kept at 80oC for 24 hours to mildly oxidize the surface of the VACNT-GF and prepare it for the next step of experiment. The sample is designated COOH-VACNT-GF. It was then washed until the pH reached neutral and placed in a 50ml beaker supported by a glass slide. 30mL of DI water was added to the beaker. 365μL (4 mmol) of aniline was solved in 10ml of 16 wt% phytic acid and slowly added to the beaker. The mixture was stirred for 20 min to let the VACNTs absorb the solution. 1.36g (6 mmol) of ammonium persulfate was then solved in 10mL of DI water and added drop by drop to the mixture while stirring. The beaker was then placed in an ice bath overnight. The polyaniline (PANI) coated VACNTs on GF was taken out, washed thoroughly with DI water and dried under 70 oC. It was then annealed in N2 atmosphere at temperatures of 700 oC, 800 oC, 900 oC, and 1000 oC respectively, each for 2 hours. The same doping process was also applied to CNT powders to synthesis NP8-CNT. 2.4 Synthesis of 40% Pt/C sample The Pt/C catalyst was prepared by adding 0.77 mmol of chloroplatinic acid and 90mg of XC-72 activated carbon into 15ml of DI water with stirring. The pH value of the solution was adjusted to 8 using 0.1mol L-1 NaOH solution. 1.54 ml of 1 mol L-1 NaBH4 solution and 2ml of ethylene glycol was then added to the solution. The solution was then transferred into a Teflon lined hydrothermal reactor. The reactor was put under 180°C for 4 hours. The resulting sample was washed and dried for further use. 2.5 Characterization The samples were examined using a JEOL JSM-6700F scanning electron microscope (SEM) and high-resolution JEOL 2100 transmission electron microscope (TEM). The crystal structure of


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the samples was examined by a Bruker D8 Advanced X-ray diffractometer (XRD). X-ray photoelectron spectroscopy (XPS) was used with Al Kα X-ray source (1486 eV) to characterize the surface chemical compositions of the synthesized samples. 2.6 Electrochemical measurements The 0.1 M KOH electrolyte and Ag/AgCl reference electrode was used for ORR test on a rotating-disk electrode (RDE) system with a Pt plate counter electrode. The rotating speed was varied from 100rpm to 2500rpm and the scan rate was 2mV s-1. For Pt/C catalyst, 10mg of 40% Pt/C, was ultrasonically dispersed into 1mL of ethanol containing 10μL of 5 wt% Nafion solution. 5 μL of the catalyst ink was then coated onto the glassy carbon electrode and dried in air. The NPVACNT-GF samples were cut to 2 mm diameter pieces and directly adhered to the surface of glassy carbon by applying 1 μL of Nafion solution (0.5 wt%). The weight of NP-VACNT-GF samples were weighted to be 50 μg each. For rotating ring disk electrode (RRDE) tests, the experiment was performed with a RRDE with a collection efficiency of 37% set at 0.5V vs. Ag/AgCl. 2.7 Zn-air battery assembly and testing Home-made zinc-air batteries were used for the electrochemical measurements. The anode was made of a polished zinc plate, and the cathode was prepared using ~1cm2 of NP8-VACNT-GF sample. 100 μL of 0.5% PTFE solution was drop cast onto the sample as a waterproof additive and dried at 120°C. A microporous membrane (glass microfiber filter, thickness: 0.68mm, pore size: 2μm) was used as the separator and 6M KOH was used as the electrolyte. For comparison, 40 wt% Pt/C at a concentration of 10 g L-1 was drop-coated on GF and dried. All tests of Zn-air batteries were conducted in air and room temperature (~25°C) with no force-feeding of gases. The polarization curves, constant current discharge curves, and impendence were measured using a


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CHI760e electrochemical workstation. The cycle performance of the rechargeable Zn-air battery was measured in a home-made tri-electrode cell with 6M KOH and 0.2 M zinc acetate solution as electrolyte using a Neware battery testing system. 3. Results and Discussion

Scheme 1 Synthesis process of NP-VACNTs-GF samples. Scheme 1 shows the synthesis method of the NP-VANCT-GF samples. The VACNT-GF was fabricated with a method developed by our group that allows the growth of VACNTs on 3D porous substrates58. GF was prepared by CVD synthesis on a Ni foam as a template. The as-grown GF on Ni foam was treated by oxygen plasma to prepare them for ALD deposition, then a uniform thin layer(~3nm) of Al2O3 was deposited on the GF to act as a buffer layer to assist the synthesis of the


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catalyst. The catalyst was prepared by using an airbrush to coat the sample evenly with Fe and Co acetate, then quickly calcinating the sample. PECVD was then used to grow VACNTs. It has been reported that phytic acid can dope into polyaniline by a high content to form a phytic acid doped polyaniline complex59, 60, which can be then calcinated to yield carbon materials with N and P codoping. After removal of the nickel template and treatment by the HNO3 solution to mildly oxidize the surface of the CNTs, the COOH-VACNT-GF was submerged in 16 wt% phytic acid solution. Aniline and ammonium persulfate were then slowly added to the solution under stirring to coat the CNTs with phytic acid doped PANI, followed by calcination at temperatures varying from 700oC to 1000oC. Figure S1 shows the SEM images after each step of synthesis. The samples were then tested using RDE and assembled into zinc-air battery prototypes. The microstructure of the sample was examined using the scanning electron microscope (SEM) and transmission electron microscope (TEM) shown in Figure 1. The SEM and TEM images of pristine CNTs (Figure 1 a~c) show that they are vertically aligned on the 3D GF surface with lengths of 5~6 μm and diameters of 5-10 nm. The pristine CNTs have smooth surfaces composited of 10~15 walls. The SEM image of the NP8-VACNT-GF sample (Figure 1 d) shows that the vertically aligned structure is retained after mild oxidation, coating, and heat treatment. TEM images (Figure 1 e,f ) show that compared with the CNTs before the treatments, the N, P co-doped CNTs’ sidewalls have visibly thickened with a sleeve-like outer layer with rough surface and a lot of curvatures and carbon flakes, while the inner layers of the CNTs retains the graphitic structure of the pristine CNTs. During the HNO3 treatment aimed to improve the hydrophilicity of VACNTs a few walls of the CNTs were removed, then the CNTs were coated with PANI doped with phytic acid, therefore after annealing the outer layer derived from the N and P rich PANI and phytic acid have large amounts of dopants and defects which can act as catalytic active sites, while the inner


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core of the CNTs retains its highly graphitic structure and can act as electron highways. Figure S2 shows the macropores with sizes of around 200µm, which serves to facilitate oxygen diffusion to the reaction sites. EDX spectra suggests that N and P were successfully doped onto the VACNTs after annealing (Figure S3 and Table S1) with less P content than N. Oxygen are also present due to the acid oxidization the samples went through prior to doping. Fe and Co were introduced during the synthesis of CNTs and Cu is present in the sample holder. Oxygen-containing functional groups can also act as active sites for OER61, 62 these functional groups catalyze OER by altering the electronic structures of the adjacent carbon atoms and facilitating the adsorption of OER intermediates13. Both oxygen-containing functional groups and the defects left by removal of these functional groups by heat treatment also showed catalytic activity towards oxygen reduction, however, past studies show two-electron reduction of O263, 64 on these active sites, suggesting inferior activity than N doped ones, which mainly catalyze ORR through the 4-electron route.


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Figure 1 The SEM images of VACNT-GF (a,b), TEM images of VACNT-GF (c), SEM image of NP8-VACNT-GF (d) and TEM image of NP8-VACNT-GF (e,f). The heat-treatment temperature was reported to play a crucial role in the catalytic performance of heteroatom-doped carbon materials. X-ray photoelectron spectroscopy (XPS) measurements were conducted for samples annealed at different temperatures. The XPS narrow scans for N1s and P2p are shown in Figure 2. The N1s scan shows peaks corresponding to pyridinic N (398 eV), pyrrolic N (399 eV), graphitic N (401 eV) and oxidized N (402 eV). The pyridinic N atom has long been regarded as a key factor to the ORR activity for N-doped nanocarbon catalysts since they change oxygen chemisorption mode change from end-on adsorption to side-on adsorption, which largely weakened the O-O bond and increased the reduction rate of oxygen. Graphitic N is responsible for the increased electric conductivity in N-doped carbons and our early study found that graphitic N can improve the limiting current density of ORR catalysts. The P2p spectrum samples contained a major peak at about 132~133 eV due to C-P bonds65, a peak at 133.2eV from N-P bond and a peak at 134.2 eV caused by P-O bond66. No detectable P was found in NP7VACNT-GF samples since phytic acid does not fully decompose until 800 degrees.67 The peaks corresponding to N-P bond can be observed in temperatures higher than 700oC and reach the highest ratio of total N at 800 oC, however, the ratio decreases when carbonization temperature was increased. Chai et al reported that N-P bonds near the edge of carbon materials have excellent ORR and OER bi-functional activity, which is the best among all P-containing structures investigated by them by computational studies. It is also shown in experiment that samples with a combination of N-P bonds and pyridinic N shows the best bi-functional activity among all samples tested31. Zhou et.al also suggested that N-P bonds are effective in improving the ORR activity68.


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The NP8-VACNT-GF has both the highest amount of N-P bond and a large amount of pyridinic N, which can be a cause of its good OER and ORR activity.

P-O

Figure 2 XPS N1s (a) and P2p (b) narrow scan, UPS cut-off region (c) and valence band region (d) spectra of NP-VANCT-GF samples.


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Ultraviolet photoelectron spectroscopy (UPS) measurement results are shown in Figure 2c and d. The valence band region (Figure 2d) reveal that compared to VACNT-GF, both COOHVACNT-GF and NP-VACNT-GF have distorted π-bonds, as a result of defects and doping. The cut-off region (Figure 2c) shows that he as-grown VACNT-GF have a work function of 4.56 eV, which is typical of multiwall carbon nanotubes, and further oxidation and doping with heteroatoms are shown to increase the work function.69.The mildly oxidized VACNT-GF sample (COOHVACNT-GF) have a work function of 4.96 eV, while NP7-VACNT-GF and NP8-VACNT-GF have a lower work function of 4.68eV and 4.72eV, respectively. The increase of the work function reflects the lowering of Fermi level by 0.4 eV, 0.12 eV, and 0.16eV, respectively. Work function have been used as an indicator for ORR activity in doped nanocarbons, since a smaller work function causes lower energetic barrier for electron donation from the catalyst surface to the adsorbed molecular oxygen, thus facilitating the formation of the OOH species, which is widely accepted to be the rate-determining step in the ORR process.70-72 However, the work function is not the independent factor of ORR in VACNT samples. For example, the VACNT-GF have the lowest work function of all samples, its lack of catalytic active sites makes it a poor catalyst. To evaluate the ORR catalytic activity, cyclic voltammetry (CV) tests were first conducted on NP-VACNTs-GF, COOH-VACNTs-GF, and 40 wt% Pt/C catalyst as a reference in nitrogensaturated and oxygen-saturated electrolyte. The CV curves are shown in Figure 3(a). In N2saturated electrolyte, CV curves of both NP-VACNT-GF samples show typical rectangular shape, indicating that no redox reaction happened. In O2-saturated electrolyte, the CV curves for all samples show ORR peak. NP8-VACNT-GF exhibits onset potential of -0.13 V vs. Ag/AgCl, much higher than that of COOH-VACNT-GF (-0.33V vs. Ag/AgCl), suggesting the superior ORR catalytic activity of NP8-VACNTs-GF compared to COOH-VACNT-GF. The ORR activities of


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these catalysts were further evaluated using the RDE technique. Heat treatment has a significant impact on the electrocatalytic performance of heteroatoms doped carbon, namely increasing the temperature raises the level of graphitization thus increases the conductivity, but also result in loss of dopants, thus a loss of active sites. On the other hand, the lower annealing temperature can result in poor electrical conductivity due to the incomplete carbonization of precursors. Figure 3(b) shows the linear scanning voltammetry (LSV) curves of NP-VACNT-GF catalysts annealed at different temperatures with COOH-VACNT-GF and Pt/C as reference samples measured on a rotating disk electrode at a rotation speed of 1600 rpm normalized to geometrical area. NP8VACNTs-GF sample shows a significantly higher limiting current density than other samples, and an onset potential 80mV higher than that of COOH-VACNT-GF, indicating the superior ORR activity of N and P doped carbon. It can be seen by comparing the LSV curves of NP-VACNTGF samples annealed under different temperatures that 800oC is the optimal annealing temperature. The LSV measurement results are consistent with the work function of the doped CNT samples, indicating the good performance of NP8-VACNTs-GF comes from its better catalytic activity than other NP-VACNTs-GF samples and COOH-VACNT-GF and the better ability to transfer electron to adsorbed oxygen. To provide more in-depth information about the ORR activity of NP8-VACNT-GF samples, a powder sample was produced using the same procedure and labelled as NP8-CNT. Figure 3c shows the LSV scan of NP8-CNT powder samples under different scan rates. Figure 3d shows the Koutecky–Levich (K-L) plots at a variety of potentials. The relation between rotating speed and current can be plotted in a linear relation according to the K-L equation 1 1 1 1 ―1/2 1 = + = 𝜔 + 𝑗 𝑗𝑘 𝑗𝑙 𝐵 𝑗𝑘


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Where j, jk, and jl are the measured, kinetic-limited and mass transfer-limited current densities, 𝜔 is the angular velocity of the RDE and B = 0.20D2/3𝜐 ―1/6𝑛𝐹𝐶 ∗ , where D is the diffusion coefficient of the reactant (O2), 𝜐 is the kinetic viscosity of the electrolyte, F is the Faraday constant and 𝐶 ∗ is the concentration of the reactant in the bulk electrolyte. The electron transfer number n can be derived from the slope of the K-L plot and the abovementioned constants. Rotating ring disk electrode (RRDE) was also applied to show the electron transfer number as shown in Figure 3e. The electron transfer number n can also be derived from: n=4

𝐼𝐷 𝐼𝐷 +

𝐼𝑅 𝑁

Where ID is the disk current, IR is the ring current and N is the collection efficiency of the RRDE. Figure 3f show the electron transfer numbers derived from both RRDE and K-L plot. The electron transfer number shows a trend of decreasing then increasing again, showing that in order to optimize the performance the voltage of the cell should be better controlled in the region that has higher electron transfer numbers. It is worth noting that the operating condition of ZABs correspond to >-0.2V vs. Ag/AgCl. Figure S4 shows the LSV curves of NP-VACNT-GF with different concentration of phytic acid during the polymerization process. The samples were all annealed at 800 °C. The N-16%P-VACNT-GF sample has more positive onset potential than that of N-8%P-VACNT-GF (-0.13V vs. Ag/AgCl) and N-32%P-VACNT-GF (-0.16V) samples. The specific current for N-16%P-VACNT-GF electrode was also much larger than that of other samples, implying that the NP-VACNT-GF synthesized using 16wt% phytic acid solution has the best catalytic performance among the NP-VACNT-GF samples synthesized with different phytic acid concentrations. The low acidity of 8% phytic acid and the high viscosity of the 32% phytic acid can lead to inhomogeneous coating layer on VACNTs, which may hinder electron


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transportation and gas diffusion. The Tafel slope of NP8-VACNT-GF (Figure 3g) shows two linear regions with slops of 73 mV dec-1 and 139 mV dec-1, both larger than Pt/C, indicating that NP8CNT is intrinsically less active than Pt/C and highlighting the contribution of the structure of the material to the good performance. Figure S5 shows LSV of samples annealed at different temperatures and synthesized with different phytic acid concentrations normalized to mass. Figure S6 shows scan of non-faradic region used to calculate the electrochemical surface area (ECSA), which is estimated to be 165.9 m2 g-1, and Figure S7 shows the LSVs of the samples normalized to ECSA. The OER performance of the as-prepared samples was also investigated and compared. Figure 3h shows the RDE measuring results of the OER activities of NP8-VACNT-GF, COOHVACNT-GF, and Pt/C samples. It can be seen from the graph that NP8-VACNT-GF has the largest current at potentials over 0.7V vs. Ag/AgCl, as well as the smallest Tafel slop. This result indicates that NP8-VACNT-GF possesses better OER catalytic activity than that of COOH-VACNT-GF and Pt/C.


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Figure 3 (a)CV curves of NP8-VACNT-GF, Pt/C and COOH-VACNT-GF samples; (b)LSV curves of samples annealed at different temperatures; (c) LSV curves of NP8-CNT at different rotating rates; (d)K-L plot of NP8-CNT at different potentials; (e) RRDE measuring result of NP8CNT; (f) Electron transfer number calculated from K-L equation and RRDE measurement (g) Tafel plot of NP8-CNT and Pt/C (h) OER performance and corresponding Tafel plot of NP8VACNT-GF, Pt/C and COOH-VACNT-GF samples. In the commercial primary Zn-air batteries, the air electrode was commonly fabricated using a hot press method. The catalyst powder is mixed with carbon black and PTFE binder to form the slurry for the catalyst layer, while another slurry for the gas diffusion layer is prepared using a similar method without the catalyst powder but with an increased amount of PTFE and poreforming agent. The slurries are then cast onto a mesh current collector with the catalyst layer at one side and gas diffusion layer at the other side. The preparation procedure is relatively complicated, and greatly increases the weight of the electrode. The pores that host the reactions and allows air diffusion was formed by the combination of PTFE, carbon black and pore forming agent. However, the PTFE additive is also highly insulating, which will increase the overall electrical resistance of the electrode and severely limit the ZAB’s ability to operate at high current density, which is needed for the much more important high-power ZABs. Therefore, binder-free electrodes with built-in catalysts and porous structure that allows ion/gas diffusion is highly desired. Recently, there have been several reports of high-performance Zn-air batteries based on free-standing air electrodes either with CNT-based films or metal mesh as the substrate. As the free-standing air-electrode for a primary Zn-air battery, the NP8-VACNTs-GF exhibited a high peak power density of 56mW cm-2 at a current density of 120mA cm-2, which was much higher than the reference samples with Pt/C catalyst on GF and COOH-VACNT-GF (Figure 4b). All the


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three electrodes have similar discharge voltages at a current density of 5 mA cm-2, while NP8VACNTs-GF has higher discharge voltage of 200 mV higher than that of the cell with Pt/C air cathode at a high discharge current of 20 mA cm-2 (Figure 4d). The good performance of the VACNT-GF based samples can be attributed to the hierarchical porous structure of the electrodes, which provides plenty of surface area for the formation of tri-phase interface and allows efficient gas diffusion, together with the good electrical conductivity. The Nyquist plots (Figure 4c) were fitted to an equivalent circuit according to the electrode circuit model proposed by Ma et al73. The results (Table S2) shows that the ZAB with NP8-VACNT-GF as the electrode has a significantly increased tri-phase interface area represented by the higher double layer capacitance, and greatly reduced mass transfer resistance attributed to the favorable structure of the NP8-VACNT-GF sample.


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56mW cm-2

Figure 4 The setup of primary ZAB (a), the polarization curve of primary ZABs (b), the EIS measurement results (c), the constant current discharge results (d)


22

(a)

O2

ORR electrode

−

−

Zn + 4OH ⇄

−

2

4

)

e e

−

2

e

O

2

−

2.0

−

e

O-2

-

2

Pt/C IrO2

1.5

NP800-VACNT-GF 1.0 0.5

−

0.00

0.05

0.10

0.15

Current Density/ A cm-2

H OH Initial 0.70 V OH

(c) Voltage / V

2.5

−

Charging

−

4OH → O + 4e + 2H O 2

+ 2e

Discharging

OER electrode −

2−

Zn(OH)

2

Electrolyte (6M KOH + 0.2M Zn(Ac)

(b)

Zn plate

O + 4e + 2H O → 4OH

1

After 125 hours 0.85 V

0 0

5

10

105

110

115

120

125

Time / h

Voltage / V

(d)

NP8-VANCT-GF 0.73 V

NP8-VANCT-GF 0.70 V 2

1

Pt/C//IrO2 0.85 V

Pt/C//IrO2

Pt/C//IrO2 1.83 V

NP800-VACNT-GF

0 0

3

6

9

45

48

51

54

Time / h

Voltage (V)

(e)

Initial 0.90 V 2

1

After 145 hours 1.20 V 0 0

1

2

3

140

141

142

143

144

145

Time (h)

(f) Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Voltage / V

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NP8-VANCT-GF 0.90 V

NP8-VANCT-GF 0.90 V

2

1

Pt/C//IrO2 0.85 V

Pt/C//IrO2

Pt/C//IrO2 1.50 V

NP800-VACNT-GF

0 0

1

2

3

4 Paragon 70 Plus71Environment 72 73 ACS

74

75

76

Time / h

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Figure 5 The schematic representation of setup of rechargeable ZAB (a), the polarization curve of rechargeable ZABs (b), the cycling performance of the recharge ZAB at 2 mA cm-2 with 2 hours per cycle (c), The comparison of NP8-VACNT-GF based cells and Pt/C//IrO2 cells, at 2 mA cm2,

2 hours per cycle (d) 10 mA cm-2 with 20 min per charge/discharge cycle (e), 10 mA cm-2, and

20 min per cycle (f). The NP-VACNT-GF samples are also catalytically active towards OER, which makes it suitable for rechargeable ZABs. Tri-electrode rechargeable Zn-air battery was constructed as shown in Figure 5a. The third electrode, especially for OER process, is submerged in the same electrolyte together with the ORR electrode and Zn plate. This configuration allows us to separately study the effect of the long-term operation on OER and ORR catalytic activity of the sample and prevent the effects of carbon corrosion at high voltages from affecting the surface functionalities of ORR electrode. The polarization curves of tri-electrode rechargeable Zn-air batteries are shown in Figure 5b. The polarization of a ZAB with NP8-VACNTs-GF air-electrode for both ORR and OER process was compared with the polarization curves of Pt/C and IrO2 for ORR and OER process, respectively. NP8-VACNTs-GF sample has similar current densities as IrO2 on carbon paper for OER process and better ORR performance than Pt/C on GF. NP8VACNTs-GF has a current density of 21mA cm-2 at a discharge voltage of 1.0 V, much larger than that of Pt/C (13 mAcm-2), indicating the superior bifunctionality of NP8-VACNTs-GF. The cycling performance of NP8-VACNTs-GF based rechargeable ZABs are shown in Figure 5c~f. The ZABs are able to achieve a stable operation of over 125 hours after pulse chargedischarged at a current density of 2 mA cm-2 (2h per cycle) and high rate operation at a current density of 10 mA cm-2 (20 min per cycle). Under the current density of 2mA cm-2 The chargedischarge voltage gap (ΔE) for NP8-VACNTs-GF based rechargeable ZABs increased from 0.70


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V to 0.73V after 55 hours of operation, significantly lower compared to the gap increase of from 0.85 V to1.83 V for Pt/C in the similar measurement conditions. At a current density of 10mA cm-2, the ΔE increased by 0.3V from 0.9 V to 1.2 V after 145 hours of operation, significantly lower than that of Pt/C//IrO2 cells, in which the charge-discharge gap increased by 0.65V after 75 hours. As shown in Table S3, NP8-VACNTs-GF based ZABs has a better or at least comparable cycle life than the other recently reported rechargeable ZABs based on carbon materials, indicating the cycle stability of free-standing electrodes can be achieved. The excellent VACNTs based air-cathode in ZAB devices inspired us to explore the underlying mechanism. In practice, powder form electrocatalyst needs to be coated on a conductive substrate (e.g. metallic mesh, carbon paper/cloth) and then an air diffusion layer is further applied (Figure 6a). The conductive substrate/scaffolds do not possess active sites or provide a high surface area for electrocatalysis. What’s more, the severe aggregation of nanosized catalysts not only has reduced tri-phase interfaces for oxygen electrocatalysis but also hinder electron transfer and mass diffusion. The hierarchical CNTs arrays provide a 3D carbon scaffold that possesses excellent mechanical strength, high conductivity and is lightweight. The well-aligned structure allows the diffusion of oxygen through the interstitials of CNTs and reaches the catalytic active sites effectively, then the reaction can be catalyzed efficiently on the surface of VACNTs (Figure 6b). The electron transfer was facilitated due to the excellent conductivity of CNT arrays. The unique characteristic of free-standing VACNTs–based electrocatalysts to serve as ultralight air-cathodes with efficient mass transfer and excellent performance in metal-air battery devices.


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Figure 6 Schematic illustration of the electrode structure and the gas and electron diffusion path for conventional powder catalysts (a), and the free-standing VACNTs (b) based air cathode. 4. Conclusions In summary, N and P co-doped VACNT-GF composite has been developed as free-standing aircathode for ZABs. N, P co-doped VACNTs-GF prepared by coating of phytic acid doped PANI on mildly oxidized CNTs with subsequent heat treatment has a dopant-rich outer layer and a highly conductive CNT core. Electrochemical characterizations reveal that the NP-VACNTs-GF sample annealed at 800°C shows the best catalytic activity among the VACNTs-GF samples, and XPS reveals the presence of the highest amount N-P bonds indicating highly bi-functionally active sites and optimum work function with a lower energetic barrier for electron transfer. NP8-VACNTsGF shows significantly better performance and higher peak power density than the similarly prepared Pt-C based electrode due to its high conductivity, hierarchical structure, and abundant highly active catalytic active sites. NP8-VACNTs-GF achieved stable operation over 145 hours in rechargeable Zn-air battery prototype, much longer than that of the Pt/C//IrO2 coupled Zn-air battery. In conclusion, the 3D doped VACNT-GF structure holds great promise as a free-standing


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electrode for rechargeable metal-air batteries with high power and energy densities and long cycle life.

ASSOCIATED CONTENT Supporting Information. The elemental composition, EIS fitting results and a comparison between the performance reported here and in other publications are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors: Prof. L. Lai, Key Laboratory of Flexible Electronics & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM) Nanjing Tech University, Nanjing 210009, China E-mail: [email protected] Prof. Z. Shen School of Physical and Mathematical Sciences, Nanyang Technological University 21 Nanyang Link, Singapore 637371 E-mail: [email protected] Funding Sources The authors gratefully acknowledge National Natural Science Foundation of China for supporting this research through Grant No. 51502135, and Singapore Ministry of Education for supporting this research through Grant AcRF Tier 1 (Reference No: RG103/16).


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ABBREVIATIONS Oxygen evolution reaction (OER), Oxygen reduction reaction (ORR), Vertically aligned carbon nanotubes (VACNTs), Zinc-air batteries (ZABs), Graphene foam (GF), Plasma-enhanced chemical vapor deposition (PECVD), Scanning electron microscope (SEM), Transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), Rotating-disk electrode (RDE).


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Graphical Abstract 338x200mm (96 x 96 DPI)


A Durable Free-standing Hierarchical Porous Electrode for

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