Bamboo fiber–derived bifunctional electrocatalyst for rechargeable Zn-air batteries

Low-cost and high-performance bifunctional electrocatalysts for efficient oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) are vital for the applications of rechargeable Zn-air batteries (ZABs). Herein, a porous carbon material is fabricated by traditional pulp and papermaking and carbonization process from bamboo. The resultant carbon paper catalyst possesses a high surface area, porous structure, and high content of nitrogen. Benefiting from these characteristics, this material exhibits remarkable ORR and OER catalytic performances. The aqueous rechargeable Zn-air batteries assembled with this catalyst exhibit a high power density of 279.5 mW cm−2. This work paves an encouraging way for the industrial production of cost-effective catalysts in a sustainable manner. Bamboo-derived N-doped porous carbon material with enhanced electrocatalytic for promoting electrochemical energy storage Bamboo-derived N-doped porous carbon material with enhanced electrocatalytic for promoting electrochemical energy storage


Introduction
With the continuous development of society, the development of energy storage equipment is very important, so people have carried out a lot of research in the field of energy storage [1][2][3]. ZABs have been considered as a new generation of energy conversion and storage devices, owing to their high theoretical energy density, long life, low cost, and environmental friendliness [4,5]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play critical role in ZABs. However, the sluggish kinetics of cathodic ORR and OER in the ZABs leads to several problems such as large overpotential, low-energy efficiency, and short cycle life [5][6][7]. Thus, the development of high-performance bifunctional electrocatalysts is crucial, and great efforts have been made on this field in the past decade. Although noble metal-based catalysts such as Pt and Ru show high activity for ORR and OER, their shortage, high price, and poor durability limit their practical application [8,9]. To this end, efforts have been devoted to finding low-cost, environmentally friendly, and highly efficient metal-free carbon-based catalysts [10][11][12][13][14][15][16][17][18][19]. Heteroatom dopants, such as N [6,20], S [21,22], P [23,24], and B [25,26], have been widely used to enhance electrocatalytic activities of carbon materials. The inherent differences in the electronegativity and atomic size between C and these heteroatoms can modify the electronic structures of carbon materials, thereby creating new catalytic sites and facilitating the mass transfer process of reactants to enhance the catalytic activities [6,20,24,26,27]. Furthermore, numerous studies demonstrate that the porous structure of carbon material also plays an important role in the electrocatalytic oxygen reaction [28][29][30]. Therefore, the adjustment of the porous structure and doping heteroatoms is beneficial to construct superior carbon-based electrocatalysts.
Bamboo, which is a natural, green, and environmentally friendly raw material, grows and renews rapidly and can replace wood, cotton, or other plant resources for sustainable use. Bamboo-derived active carbon has high specific surface area and strong adsorption capacity due to the unique ultra-fine micropore structure. Therefore, bamboo fiber can be used to prepare carbon materials with a high specific surface area. Meanwhile, the abundant hydroxyl group on the surface of fibers makes it more reactive, beneficial to the doping process [31,32]. After being treated by chemical and mechanical processes, bamboo cellulose exposes more hydroxyl groups, rendering these properties even more prominent. Considering the above characteristics, bamboo is an ideal precursor for the preparation of carbonbased catalysts.
Herein, a high-performance N-doped carbon material was successfully prepared from bamboo fiber via a simple papermaking and carbonization process. With etching by NH 3 (produced from the decomposition of NH 4 Cl at high temperature), nitrogen is successfully doped in carbon paper to produce a lot of active sites. The specific surface area of carbon paper is high, and many micro/mesopores are formed. Benefiting from high N content and micro-mesoporous structure, the obtained electrocatalyst has favorable ORR/ OER bifunctional catalytic activity with a ΔE of only 0.79 V and exhibits excellent performance as a cathode catalyst for conventional liquid ZABs, with a peak power density of 279.5 mW cm −2 . Such an approach is promising for the cost-effective and large-scale preparation of carbon-based metal-free catalysts.

Fabrication of bamboo pulp paper
Bamboo pulp paper was gotten by pulp and papermaking of bamboo. Fifty grams of absolutely dry bamboo slices was added into a cooking pot containing 1 L 5% NaOH and 95% water and heated to 120 °C for 2 h. The pretreated bamboo was ground in two stages with a high concentration continuous grinding machine, with a grind clearance of 0.5 mm in the first stage and 0.2 mm in the second stage. Finally, we obtained bamboo pulp paper (90 g cm −2 ) with paper-making equipment.

Fabrication of carbon-based metal-free catalyst
A certain mass of bamboo pulp paper and NH 4 Cl was loaded into a porcelain boat at a mass ratio of 1:10 and placed in a tubular furnace. Under the atmosphere of inert gas N 2 , the paper was heated from room temperature to 250 °C with a heating rate of 3 °C min −1 , and then heated from 250 to 900 °C with a heating rate of 5 °C min −1 , and finally carbonized at 900 °C for 2 h to obtain a paper-based catalyst (named as CP-N-C@900). For comparison, carbon without N (CP@900) and carbon prepared by replacing NH 4 Cl with urea (CP-UN-C@900) or dicyandiamide (CP-DN-C@900) under the same carbonization process were prepared.

SEM and TEM analysis
Paper samples were cut into 2 mm × 2 mm, then the microstructures of samples were characterized with transmission electron microscopy (TEM, JEM-2100F, 200 kV) and scanning electron microscope (SEM, SU5000, 10 kV).

XRD analysis
The paper samples were first ground into powder through a mortar. The crystalline structure of the samples was analyzed using an XRD (XRD, D8 Discover, Bruker, Germany) operating with Cu Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA. The samples were scanned at 12°/min between 10 and 90° (reflection mode).

Other analysis
The chemical composition of the samples was determined by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific K-Alpha) with an exciting source of Al Kα (1486.8 eV). Paper samples were cut into 1 cm × 1 cm, and the Raman spectroscopy (LabRAM Aramis-Horiba Jobin Yvon) was used to characterize the graphitization and defect degree of carbon in the material. Take 5-mg sample powder into the Elemental analyzer (VarioEL III, Elementar) to analyze the C, N, and O element content of the material. The N 2 adsorption-desorption isotherm (Micromeritics, ASAP 2460) was measured at − 196 °C to analyze the Brunauer-Emmett-Teller (BET)-specific surface area (SSA) and pore structure. Before analysis, the sample was degassed in a vacuum of 180 °C for at least 12 h.

Electrochemical measurements
All electrochemical measurements were carried out on CHI 760E (CHI Instruments) with a rotating ring-disk electrode (RRDE, Pine Instrument Company, USA) in a standard three-electrode configuration at room temperature. Platinum plate and Ag/AgCl electrodes were used as counter and reference electrodes, respectively. The recorded potentials were given with respect to the reversible hydrogen electrode (RHE). The catalyst ink was prepared by dispersing 5 mg of catalyst in the mixture of 490 μL ethanol, 490 μL ultrapure water, and 20 μL Nafion solution (Alfa Acesar, 5 wt %) with 30-min sonication. Then, 10 μL catalyst ink was loaded onto a glassy carbon electrode (5 mm in diameter). The mass loading of catalyst was 255 μg cm −2 . For comparison, the commercial Pt/C catalyst (20 wt %) was also onto a glassy carbon electrode with a loading mass of ~ 102 μg cm −2 .
The cyclic voltammogram (CV) curves were carried out in N 2 or O 2 -saturated potassium hydroxide electrolyte (0. where j (mA cm −2 ) is the measured current, jk (mA cm −2 ) is the kinetic-limiting current, and ω (rad s −1 ) is the electrode rotation rate. The theoretical value of the Levich slop (B) is evaluated from the following equation (2): where n is the overall number of transferred electrons in the ORR process, F is the Faradaic constant (96,485 C mol −1 ), C 2 is the oxygen concentration (solution) in 0.1 M KOH (1.2 × 10 −6 mol cm −3 ), D 2 is the oxygen diffusion coefficient in 0.1 M KOH (1.9×10 −5 cm 2 s −1 ), and v is the kinematic viscosity of the 0.1 M KOH (0.01 cm 2 s −1 ).
Rotating ring-disk electrode (RRDE) measurements were recorded to determine the selectivity of the four-electrode reactions. The hydrogen peroxide yield (H 2 O 2 %) and the electron transfer number (n) were calculated according to the following Eqs. (3) and (4): where I D (mA cm −2 ) and I R (mA cm −2 ) are disk current and ring current respectively, and N (0.37) is the collection efficiency of platinum ring. (1)

Assembly and measurement of aqueous Zn-air batteries
The rechargeable aqueous Zn-air batteries were assembled in a two-electrode configuration. A polished Zn foil with a thickness of 0.3 mm was used as the active anode. A mixed solution of 6 M KOH and 0.2 M Zn(Ac) 2 was used as electrolyte to ensure reversible zinc electrochemical reactions at the Zn anode. For powder samples, 5 mg of CP-N-C@900 was dispersed in 490 μL ethanol, 490 μL ultrapure water, and 20 μL Nafion solution, and the resulting mixture was sonicated for 30 min to form a homogenous ink. The catalyst ink was then sprayed onto a cleaned carbon cloth with a gas diffusion layer (GDL) to prepare cathode. The mass loading of catalyst is 2.0 mg cm −2 and the gas diffusion area of the air cathode (to the electrolyte and air) is 1 cm 2 . The polarization curve measurements were performed by LSV at 5 mV s −1 . Galvanostatic charge-discharge cycling test was performed with a cycling interval of 10 min (5 min for charging and 5 min for discharging) at various current densities by using Land-CT2001A.
The specific capacity (C) was calculated according the Eq. (5): where the I (mA) represents the current density and t (h) is the discharge time and m (g) is the mass loading of the consumed Zn anode.
The power density (P) of the Zn-air battery was calculated by Eq. (6): where I (mA) is the discharge current density and V (V) is the corresponding voltage. Figure 1a illustrates the preparation process of CP-N-C@900, mainly including the process of pulp and papermaking and high-temperature carbonization. Firstly, pulp fibers were prepared by chemical mechanical pulping of bamboo, and then papers were obtained by papermaking process. The pulping process included chemical treatment and mechanical disc grinding. During chemical treatment, bamboo was put into a cooking pot with a certain concentration of NaOH solution, which was warmed up to 120 °C for 2 h. In this process, bamboo fibers were fully softened,

Catalyst preparation and characterization
and a certain degree of delignification occurred under such alkaline conditions. After chemical treatment, mechanical treatment was carried out to further shear bamboo fibers by shearing force of mechanical disc mill, so as to expose more fibers. In the papermaking process, bamboo fibers were interconnected with each other by hydrogen bonding to form a two-dimensional network structure, which may be conducive to the formation of more conductive pathways. Finally, bamboo paper was carbonized with NH 4 Cl at high temperature to acquire porous N-doping carbonbased catalyst, named as CP-N-C@900. The morphology and microstructure of the samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). CP-N-C@900 shows a 2D network structure with interwoven fibers, which can provide many channels for electron transport (Fig. 1b-d).
In the TEM and high-resolution TEM (HR-TEM) image (Fig. 1e, f), the carbon substrate shows a lattice fringe spacing of 0.34 nm, which corresponds to the (002) plane of carbon, indicating the formation of partially graphitized carbon. The energy-dispersive X-ray (EDX) elemental mapping analysis confirms the distribution of N element in the carbon substrate (Fig. 1g). The N content is determined to be 8.6 wt% by element analyzer, indicating a relatively high N active species (Table S1). CP-N-C@900 can fold simply and is very light (Fig. 1h, i). The contact angle is about 35°, indicating highly hydrophilic property, which is conducive to the electrolyte infiltration and interfacial reaction (Fig. 1j).
The corresponding X-ray diffraction patterns (XRD) of the samples are shown in Fig. 2a. There are two obvious broad peaks at 24.4° and 43.2°, corresponding to the (002) and (100) planes of graphite, respectively. The Raman spectra of CP@900, CP-N-C@900, CP-UN-C@900, and CP-DN-C@900 show two distinct peaks, corresponding to disordered carbon (D) and graphitic carbon (G), respectively (Fig. 2b). Obviously, compared with CP@900, CP-UN-C@900, and CP-DN-C@900, CP-N-C@900 displays a small ratio of I D to I G , indicating a higher graphitization degree, which can facilitate electron transport [33]. In order to analyze the specific surface area and size distribution of pore, nitrogen isotherm adsorption-desorption analysis was carried out. As shown in Table S2, the CP-N-C@900 possesses the highest specific surface area of 1109 m 2 g −1 . It can be seen that CP-N-C@900 has a high adsorption capacity in a low relative pressure range (P/P 0 < 0.1), demonstrating the existence of micropores (Fig. 2c) [34]. The pore size distribution diagram indicates that CP-N-C@900 shows a micro-mesoporous structure with the pore size mainly concentrating at 1-3 nm (Fig. 2d). This micro-mesoporous structure and high specific surface area facilitate the exposure of active sites and the rapid transport of species during the electrocatalysis process [35]. The X-ray photoelectron spectroscopy (XPS) was carried out to analyze the elemental composition and chemical state of the material. The XPS spectrum (Fig. S1) and quantitative analysis (Table S2) of CP-N-C@900 show the coexistence of C (88.57 at%), N (3.48 at%), and O (7.3 at%), further confirming the successful N doping in the carbon substrate. For the C 1s spectrum (Fig. 2e), there are four peaks, centering at 284.8, 285.6, 288.8, and 292.0 eV, corresponding to C-C, C-O, C-N, and C=O, respectively. The high-resolution N 1s spectrum (Fig. 2f) reveals four different peaks, attributed to pyridine N (397.9 eV), pyrrolic N (399.1 eV), graphitic N (402.3 eV), and oxide N (405.3 eV) [34,36]. In addition, CP-N-C@900 has a high content of pyridine N, which has been widely reported as the active site for ORR and OER. [37]. The above results indicate that N element is successfully doped in the carbon substrate and forms the active sites that are conducive to the catalytic reaction.

Electrocatalytic performance of catalyst
To evaluate the catalytic activity of the obtained catalysts, electrochemical tests were performed using a standard threeelectrode system in 0.1 M KOH solution under O 2 saturation. The CV curves show that CP-N-C@900 and Pt/C have similar oxygen reduction peaks at 0.9-1.0V (Fig. S2), indicating a good ORR performance. The ORR performance of the catalyst was further characterized by linear sweep voltammetry (LSV) at 1600 rpm. As shown in Fig. 3a, CP-N-C@900 exhibits the best ORR performance with an onset potential (E onset ) of 1.00 V and a half-wave potential (E 1/2 ) of 0.86 V, which are superior to those of commercial Pt/C catalyst (E onset = 0.98V and E 1/2 = 0.837 V), and many recently reported metal-free carbon-based catalysts or even metalbased catalysts (Table. S3) [21,[38][39][40][41][42][43][44][45][46]. Moreover, CP-N-C@900 has the lowest Tafel slope (85.4 mV dec −1 ), which is slightly smaller than that of Pt/C (87.2 mV dec −1 ), indicating the better ORR activity (Fig. 3b). The kinetic electron transfer numbers (n) of CP-N-C@900 at different potentials (0.2-0.6 V) were calculated according to the LSV curves at different rotational speeds and the Koutecky-Levich (K-L) equation. As shown in Figs. 3c and S3, K-L curves at different potentials show a good linear relationship, indicating typical first-order reaction kinetics. At the same time, the electron transfer numbers at different potentials are all greater than 3.76, manifesting that the ORR undergoes a close four-electron (4e − ) process. To further illustrate the ORR pathway, a rotating ring-disk electrode (RRDE) was employed to test the electron transfer number and H 2 O 2 yield. As shown in Fig. S4, the lower H 2 O 2 yield and electron transfer number of CP-N-C@900 confirm that the ORR has an ideal 4e − process and is similar to that of Pt/C. Apart   Fig. 2 a XRD patterns, b Raman spectra, c nitrogen adsorption-desorption isotherms, and d pore size distribution curves calculated from DFT method of CP-N-C@900 and reference samples. e The deconvoluted C 1s spectrum, and f deconvoluted N 1s spectrum of CP-N-C@900 from catalytic activity, stability and the ability to resist methanol poisoning are other important indexes to evaluate the performance of electrocatalysts. Therefore, this work compares the timing current curve of CP-N-C@900 with that of commercial Pt/C to evaluate the stability and methanol toxicity resistance of CP-N-C@900. As shown in Fig. 3d, the timing current of commercial Pt/C catalyst decreases rapidly, and only 67.7% of the initial value remains after 40,000 s. In contrast, CP-N-C@900 maintains 76.8% of its initial current density after 40,000 s, indicating better stability. Furthermore, in order to compare the change of chemical composition of the samples before and after the cyclic test for ORR, XPS test was conducted. It can be clearly seen that the content of pyridine N decreases after the cyclic test (Fig. S5), which proves that pyridine N plays an important role in ORR. Subsequently, methanol toxicity resistance tests were conducted on CP-N-C@900 and commercial Pt/C, and the results are presented in Figure S6. When 1 M methanol is injected in electrolyte, the oxygen reduction current of Pt/C catalyst suddenly attenuated, while the oxygen reduction current of CP-N-C@900 catalyst does not attenuate significantly, demonstrating that CP-N-C@900 has a stronger ability to resist methanol toxicity.
Similarly, the OER performance of the catalysts was tested in 0.1 M KOH solution at 900 rpm. CP-N-C@900 reveals a promising OER activity with an overpotential of only 420 mV at current density of 10 mA cm −2 , which is significantly higher than those of comparative examples and comparable to that of RuO 2 (360 mV) (Fig. 3e). The corresponding Tafel slope values for CP @900, CP-N-C@900, CP-UN-C@900, CP-DN-C@900, and RuO 2 are 137, 112, 194, 116, and 106 mV dec −1 , respectively. The Tafel slope of CP-N-C@900 is comparable to that of RuO 2 , indicating that CP-N-C@900 has a good OER kinetic process (Fig. S7). Similarly, the OER stability was tested, and the result is shown in Fig. S8. After chronoamperometry tests for 20,000 s, CP-N-C@900 displays a current retention of 80.0%, which is higher than that of benchmark RuO 2 catalyst (67.8% current retention). In order to explore the active sites of OER, XPS test was performed on CP-N-C@900 before and after OER cyclic test. The result shows that after a long time of OER test, the content of pyridine N shows a significant decrease, which suggests that pyridine N is the active site for OER (Fig. S9). Finally, the overall performance of ORR/OER was evaluated. As depicted in Fig. 3f, the difference ΔE between the E 1/2 of the ORR and the OER potential of CP-N-C@900 at current density of 10 mA cm −2 is only 0.79 V, showing good reaction reversibility. Compared with CP-UN-C@900 and CP-DN-C@900, CP-N-C@900 has higher nitrogen content (Table S1) and larger surface area (Table S2), thus resulting in a better electrocatalytic performance. Concretely, it has been reported that the introduction of nitrogen hetero atoms with stronger electron affinity than carbon atoms can reduce the electron density on neighboring carbon atoms, which is helpful for the adsorption and deionization of oxygen molecules on carbon. Therefore, high nitrogen content, especially pyridine N, are proposed to be ORR and OER active sites, providing active sites with high density to ensure effective ORR. Moreover, abundant micropores and mesopores are expected to promote mass transfer of ORR/OER-related species and ultimately increase ORR/OER activity. The high surface area maximizes the exposure of ORR/OER active sites, resulting in high utilization of active sites.

Applications of the Zn-air battery.
In view of the bifunctional catalytic activity, CP-N-C@900 was utilized as an air cathode catalyst for liquid rechargeable ZAB to evaluate its practical application (Fig. 4a). The open circuit voltage of the CP-N-C@900-based battery is 1.48 V, which is higher than that of Pt/C+RuO 2 -based battery (1.4 V) (Fig. S10). As shown in Fig. 4b, in the galvanostatic discharge test at current densities of 2-40 mA cm −2 , CP-N-C@900-based ZAB has a small voltage difference. When the current density returns to 2 mA cm −2 , the discharge plateau recovers to the original level, indicating that the catalyst has a high rate capability. The polarization curves show that Fig. 4 a Schematic of aqueous rechargeable ZAB using CP-N-C@900 as air cathode catalyst. b Discharge curves of the CP-N-C@900-based ZAB at various current densities. c Charge and discharge polarization curves of Pt/C+RuO 2 and CP-N-C@900 air electrodes for the rechargeable ZABs and the corresponding power density curves of ZABs. d Galvanostatic discharge curves at different current densities. The specific capacity is normalized by the mass of the consumed Zn anode. e Cyclic stability at a current density of 1 mA cm −2 and 5 mA cm −2 . f Image of the LED powered by two liquid ZABs with CP-N-C@900 powder. g Open circuit voltage of liquid flow battery assembled by CP-N-C@900. h, i Cyclic stability at a current density of 1 mA cm −2 and 5 mA cm −2 of liquid flow battery CP-N-C@900-based battery has a remarkable peak power density of 279.5 mW cm −2 (Fig. 4c), which is better than that of commercial Pt/C+RuO 2 -based battery (223.5 mW cm −2 ). Meanwhile, CP-N-C@900-based battery has a specific capacity of 726.3 mAh g −1 at current density of 10 mA cm −2 (Fig. 4d), corresponding to an energy density of 958.6 Wh kg −1 , which is close to that of Pt/C+RuO 2 -based battery (741.9 mAh g −1 at 5 mA cm −2 ). The capacities of 766.9 mAh g −1 and 703.4 mAh g −1 can be maintained at 5 mA cm −2 and 20 mA cm −2 , manifesting that CP-N-C@900 has a good rate performance. Cyclic charge-discharge tests were carried out at current density of 1 mA cm −2 and 5 mA cm −2 (Fig. 4e). The battery based on CP-N-C@900 has a small initial charge-discharge voltage gap and exhibits a relatively small voltage change even after cycling for 150 h at 1 mA cm −2 . Moreover, it can maintain voltage stability for 70 h at 5 mA cm −2 . However, the battery based on Pt/ C+RuO 2 shows a large voltage gap in the initial state, and a serious polarization after only 20-h cycling. The charge-discharge voltage gap and round-trip voltaic efficiency of the CP-N-C@900-based liquid ZAB are 0.82 V and 58.7% at the first cycle, 1.09 V and 50.4% at the 384th cycle, respectively (Fig. S11). In contrast, the Pt/C+RuO 2 -based liquid ZAB exhibits a growing charge-discharge voltage gap from 1.03 to 1.31 V and a decaying round-trip voltaic efficiency from 47.3 to 39.1%. Furthermore, as shown in Fig. 4f, two CP-N-C@900-based ZABs connected in series can power lightemitting diodes (LED ~ 3 V), demonstrating its practical application. In addition, the catalyst was used to assemble liquid flow battery. The liquid flow battery assembled with the catalyst can also reach an open circuit voltage of 1.46 V (Fig. 4g). At the same time, at current density of 1 mA cm −2 and 5 mA cm −2 , the stable cycle time is more than 120 h, and the voltage gap is only about 0.8 V (Fig. 4h, i).

Conclusion
In conclusion, based on the traditional pulp and paper making and carbonization process of bamboo, we prepare a carbon-based metal-free catalyst. High specific surface area, abundant porous structure, and high N content enable CP-N-C@900 to exhibit remarkable ORR/OER bifunctional catalytic activity. More importantly, this material can serve as a bifunctional electrocatalyst for liquid ZABs and exhibit excellent battery performances. This work provides a new and sustainable way to prepare low-cost and high-performance carbon-based metal-free catalysts for rechargeable ZABs via a simple method.
Author contribution All the authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Jian Yu. Zehong Chen, Tingzhen Li, and Yongfa Huang were responsible for directing the experiment process. The first draft of the manuscript was written by Jian Yu. Zehong Chen was responsible for the first revision of the manuscript. Linxin Zhong, Wu Yang and Xinwen Peng were responsible for the final revision of the manuscript. All the authors commented on previous versions of the manuscript. All the authors read and approved the final manuscript.
Funding This work was supported by the National Natural Science Foundation of China, State Key Laboratory of Pulp and Paper Engineering. We acknowledge the financial support from National Program for Support of Topnotch Young Professionals, Guangdong Basic and Applied Basic Research Foundation, State Key Laboratory of Pulp and Paper Engineering.
Data availability All data generated or analyzed during this study are included in this article. The database used and/or analyses during the current study are available from the corresponding author on reasonable request.

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Ethics approval This research work did not involve any human or animal participants.
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Competing interests
The authors declare no competing interests.