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Article

Ag Decorated Co3O4-Nitrogen Doped Porous Carbon as the Bifunctional Cathodic Catalysts for Rechargeable Zinc-Air Batteries

by
Pingshu Leng
1,2,
Hanbin Wang
1,2,*,
Binfeng Wu
2,
Lei Zhao
2,
Yijing Deng
2,
Jinting Cui
2,
Houzhao Wan
1,2 and
Lin Lv
1,2,*
1
Hubei Yangtze Memory Laboratories, Wuhan 430205, China
2
School of Microelectronics, Hubei University, Wuhan 430062, China
*
Authors to whom correspondence should be addressed.
Sustainability 2022, 14(20), 13417; https://doi.org/10.3390/su142013417
Submission received: 10 September 2022 / Revised: 1 October 2022 / Accepted: 13 October 2022 / Published: 18 October 2022
(This article belongs to the Special Issue Energy Storage Materials and Devices)
Browse Figures
Versions Notes

Abstract

:
The use of transition metals as bifunctional catalysts for rechargeable zinc-air batteries has recently attracted much attention. Due to their multiple chemical valence states, the cobalt oxides are considered to be promising catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this work, bifunctional Ag-decorated Co3O4-nitrogen doped porous carbon composite (Co3O4-NC&Ag) catalysts were synthesized by annealing ZIF-67 in N2 and O2, respectively, followed by Ag deposition using chemical bath deposition. Due to the decoration of Ag nanoparticles and high specific surface area (46.9 m2 g−1), the electrochemical activity of Co3O4 increased significantly. The optimized Co3O4-NC&Ag catalysts possessed superior ORR performance with a half-wave potential of 0.84 V (vs. RHE) and OER activity with an overpotential of 349 mV at 10 mA cm−2. The open circuit voltage of the Co3O4-NC&Ag-based zinc-air battery was 1.423 V. Meanwhile, the power density reached 198 mW cm−2 with a specific discharge capacity of 770 mAh g−1 at 10 mA cm−2, which was higher than that of Pt/C-based zinc-air battery (160 mW cm−2 and 705 mAh g−1). At a current density of 10 mA cm−2, the charge-discharge performance was stable for 120 h (360 cycles), exhibiting better long-term stability than the Pt/C&RuO2 counterpart.

1. Introduction

A rechargeable zinc-air battery is recognized as a promising electrochemical energy storage technology owing to its advantages of high theoretical energy, low price, environmental friendliness, and strong durability. However, restricted by the slow electrocatalytic reaction kinetics of cathode, the overall efficiency of zinc-air battery needs to further improve to meet practical applications [1]. To build efficient cathodes for zinc-air batteries, commercial Pt/C and RuO2/IrO2 are always chosen as bifunctional catalysts due to their excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance in alkaline electrolytes [2]. Considering the scarcity and high cost of noble metals, it is urgent to continuously explore non-precious metal based bifunctional catalysts with good performance, low cost, and sustainability.
A variety of studies have been conducted on various electrocatalysts that can replace noble metals, such as transition metals and their oxides [3,4], nitrides [5,6], sulfides [7], phosphates [8], alloys [9], metal-free carbon materials [10,11,12] and biomass-derived heteroatom doped carbon materials [13]. Among these materials, cobalt oxides have exhibited superior OER performance and considerable ORR activity in alkaline media [1]. However, the low electron conductivity as well as the insufficient active site of transition-metal oxides, limit their performance. To seek a solution to such problems, several different approaches have been attempted. Firstly, controlling the microstructure of the catalysts or selecting appropriate supports helps to expose more active sites. For example, Li et al. studied two-dimensional Co3O4−x nanosheets as the bifunctional catalysts, in which the highly exposed active sites and the O2/OH loose binding endowed the cathodes with the superior catalytic activity of ORR and OER [14]. Liu et al. [15] and Wang et al. [16] prepared bifunctional Co3O4 particles using nitrogen-doped porous carbon spheres and ZIF-67/ZIF-8 as the supports, respectively, which reduced agglomeration and enhanced the activity and stability of the catalysts. Secondly, the cooperation of Co3O4 with other transition metal oxides, such as MnO2 and Mn3O4, has been reported, which led to enhanced performance in zinc-air batteries. For example, the Co3O4 nanocrystals were anchored on MnO2 nanorods by a simple two-step hydrothermal method. It was found that the Co3O4 active sites homogenously distributed and reduced agglomeration, while the presence of MnO2 further promoted the ORR performance [17]. Huang et al. carried out nanoscale hybridization of Co3O4 and Mn3O4 and used graphene as the carrier [18]. Density functional theory calculation showed that the Co3O4/Mn3O4 improved the transport kinetics and thus promoted the catalytic activities with higher ORR half-wave potential (E1/2) and lower OER overpotential.
Along with the above strategies, cooperation metallic atoms/particles with cobalt oxides also promotes their bifunctional catalytic activity and enhances electrical conductivity. For instance, Jose et al. deposited Fe atoms on Co3O4 with hollow nitrogen-doped carbon spheres as the supports. It was found the introduction of Fe atoms effectively promoted the ORR and OER catalytic activities [19,20]. Huang et al. synthesized Ru-incorporated Co3O4 nanoparticles by calcinating the mixture of Ru(OH)3 and ZIF-67. The zinc-air battery using Ru@Co3O4 as a cathode presented a high specific capacity (788.1 mAh g−1) and power density (101.2 mW cm−2) [21]. Bifunctional electrocatalytic activities were also significantly improved by combining metallic Ni with Co3O4 [22]. Especially, Ag particles have been reported to show good ORR activity in alkaline electrolytes. The decoration of cobalt oxide by metallic silver NPs would improve the conductivity of the catalysts and contribute more active centers in ORR. The combination of Co oxides with Ag has drawn attention in the field of fuel cells due to their synergistic effect [23,24]. However, the composites are rarely reported as the bifunctional cathodic catalysts in the development of rechargeable zinc-air batteries. In addition, from the perspective of catalyst structure. ZIF-67 is a typical cobalt-coordinated MOF material [25]. Therefore, the carbonized Co-MOF materials would inherit intrinsic advantages such as high porosity, rich nitrogen active sites, large specific surface area, adjustable pore diameter, diversity and tailoring of topological structure, and so on. Cooperated with the outstanding electrochemical activity of Co3O4, it is expected that the ZIF-67 derived Co3O4 nanoparticles-nitrogen-doped porous carbon would exhibit novel performance towards the rechargeable zinc-air battery.
In these studies, Ag decorated Co3O4 nanoparticles-nitrogen doped porous carbon (Co3O4-NC&Ag) were prepared by chemical bath deposition of Ag nanoparticles (Ag NPs) on MOF-derived Co3O4. The synergistic effects between Ag NPs and Co3O4-NC significantly promoted the ORR performance (E1/2 = 0.84 V (vs. RHE)) and also showed a superior OER performance (overpotential of 349 mV at 10 mA cm−2). By employing the Co3O4-NC&Ag as cathode catalysts, the zinc-air battery yielded a power density of 198 mW cm−2, and the specific discharge capacity was as high as 770 mAh g−1. The charging and discharging can be stable for 120 h at the current density of 10 mA cm−2. This study suggests a prospective strategy to develop a bifunctional cathodic catalyst in zinc-air batteries by exploring the optimum ratio between Ag and Co-based catalysts.

2. Materials and Methods

2.1. The Process of Material Preparation

2.1.1. Synthesis of Co3O4-Nitrogen Doped Porous Carbon

Preparation of cobalt salt and 2-methylimidazole aqueous solutions: 582.4 mg of Co(NO3)2·6H2O and 1.3136 g of 2-methylimidazole were added to 40 mL of deionized water, respectively, and stirred quickly for 40 min. The two solutions were mixed and stirred quickly for 5 min and then kept at room temperature for 24 h. After standing, the mixture was centrifuged, and the precipitate was collected, cleaned with deionized water by three times, and dried in a drying oven at 60 °C for 24 h. Then the powder was annealed in N2 at 800 °C for 2 h with a heating rate of 5 °C/min. Finally, the product was further annealed in air at 300 °C for 2 h at a heating rate of 5 °C/min.

2.1.2. Preparation of Co3O4-NC&Ag

The decoration of Ag NPs on Co3O4-NC was conducted by chemical bath deposition. In detail, 4.2 mg (0.025 M) of CH3COOAg and 25.73 mg (0.0875 M) of trisodium citrate dihydrate were added to 50 mL of deionized water and stirred until completely dissolved. To tune the concentration of Ag ions in solution, the mass of CH3COOAg was changed to 2.1 mg, 8.35 mg, and 16.7 mg in 50 mL of deionized water, respectively. Accordingly, the mass of trisodium citrate dihydrate was changed to 12.86 mg, 51.4 mg, and 102.93 mg. The trisodium citrate solution was slowly dropped into the CH3COOAg solution under intense stirring and then heated to 100 °C for 30 min. After 30 min, 20 mg of prepared Co3O4-NC powder was poured into the above solution and stirred at 100 °C for 5 min. After the deposition, the precipitation was collected by centrifugation, washed three times with deionized water, and dried at 60 °C in a drying oven for 24 h. The resulting products were denoted as sample C0–C4 with different Ag ions concentrations. After fixing the optimal concentration of Ag ions (0.025 M) in the chemical bath deposition, the catalytic properties of samples were further optimized by changing the deposition time, and the yielded products were donated as T1–T4. Table 1 shows the details of sample naming.

2.1.3. Preparation of Electrode Liquid for Test

Afterward, 2 mg of Co3O4-NC&Ag sample and 2 mg of conductive carbon black were put into a small centrifuge tube, then 490 μL of isopropanol and 10 μL of Nafion (5 wt.%, Sigma-Aldrich, St. Louis, MO, USA) were added into the centrifuge tube using a pipetting gun. The mixture was ultrasonic treated for 40 min before use. For comparison, Pt/C (20 wt.%, Sigma-Aldrich) or RuO2 (99.9%, Aladdin, Bay City, MI, USA) catalyst electrode solutions were prepared with the same concentration, and no conductive carbon black was added to the Pt/C electrode solution.

2.2. Material Characterization

X-ray diffraction (XRD) (Bruker D8 Advance, 40 kV, 40 mA Cu-ka radiation, λ = 1.5406 A) and field emission scanning electron microscopy (SEM) (JEOL, JSM-7100F, Tokyo, Japan) were employed to characterize the structure and morphology of the material. The content of Ag on the surface of Co3O4-NC was detected by inductively coupled plasma emission spectroscopy (ICP-OES). The chemical compositions were analyzed using an X-ray photoelectron spectrometer (XPS) (Therma Fisher Scientific Esca lab 250Xi), with reference to the C 1S peak (284.60 eV) for all binding energies [26]. Nitrogen desorption isotherms were collected on an accelerated specific surface area and porosity measurement system (ASAP 2015). The Braeuer–Emmett–Teller (BET) method was used to calculate the specific surface area.

2.3. Electrochemical Test Methods

All electrochemical tests were performed on an electrochemical workstation (CHI660E) connected with a rotating disc electrode (RDE). In the oxygen-saturated KOH solution (the concentration of the test ORR was 0.1 mol/L, and the concentration of the test OER was 1 mol/L), the three-electrode system was adopted at room temperature. A glassy carbon electrode coated with Co3O4-NC&Ag (Pt/C or RuO2) was used as the working electrode (407 μg/cm2 catalyst load), a platinum electrode as the counter electrode, and an Ag/AgCl electrode as the reference electrode to evaluate the catalytic activity of Co3O4-NC&Ag. Firstly, the CV method was used for activation in a 0.1 M oxygen-saturated KOH solution to recede the adhesive hydrophobic film on the surface and fully expose the active site. The ORR polarization curve was obtained at a speed of 1600 rpm and a scanning rate of 5 mV/s, and the OER polarization curve was obtained at a scanning rate of 5 mV/s [27]. The potential scale was converted from a reference AgCl to reversible hydrogen electrode (RHE) standard using the Nyquist formula: E(RHE) = E(Ag⁄AgCl) + 0.059 × pH + 0.1976. According to the LSV curve, the Tafel curve of different samples in the appropriate potential interval was made [28]. The Tafel equation is shown in Equation (1), where η is the overpotential, j is the measured current density, b is the Tafel slope, and a is a constant.
η = a + bLog j
The reaction kinetics discussed contains the reaction path of oxygen reduction reaction in an alkaline environment. It is known that there are two reaction pathways for ORR reaction in an alkaline medium. The first is the four-electron way, in which the oxygen molecule obtains four electrons and becomes H2O or OH−; The other is the two-electron pathway, where the oxygen molecule gets two electrons and becomes H2O2 or HO2− and eventually converts to H2O or OH−. The Koutechky–Levich (K-L) equation is used to analyze the ORR performance of the catalyst in the diffusion and kinetic restricted areas. Through analyzing the limiting current density obtained from the LSV curves at different speeds from 400 to 2400 pm, the relationship diagram of w−1/2 and j−l was drawn. Then the electron transfer number “n” was deduced according to Equations (2) and (3) [29]. In Equations (2) and (3), j is the measured current density, jk is the kinetic current density, jL is the diffusion limit current density, w is the angular velocity of the rotating disk electrode, F is the Faraday constant (F = 96,485 C/mol), C0 is the concentration of O2 in the electrolyte (C0 is 1.2 × 10−6 mol/cm−3), D is the diffusion coefficient of O2 in the electrolyte (D is 1.9 × 10−5 cm2/s), and v is the kinetic viscosity (v is 0.01 cm2/s) [30].
1 j = 1 j k + 1 j L = 1 j k + 1 Bw 1 2
B = 0.62 nFC 0 ( D ) 2 3 v 1 6

2.4. Assembly and Test of Zinc-Air Battery

A 3 × 4 cm zinc sheet was taken, and the impurities and oxide layer on the surface were polished and cleaned to serve as the anode of the battery. To prepare the air cathode of the battery, the Co3O4-NC&Ag electrode liquids were dropped onto the electrode composite substrate, which was composed of carbon paper, waterproof film, and foam nickel (the manufacturer is Changsha spring) of the same size, followed by drying at 60 °C in air. The area occupied by the catalysts in the cathode was 1 cm2. The zinc-air battery was finally assembled using a 6 mol/L KOH and 0.2 mol/L zinc acetate solution as the electrolyte. The specific discharge capacity and charge/discharge cycle of the battery were carried out on the battery testing system BTS 7.6. X (battery testing system 7.6. X).

3. Results and Discussion

Figure 1 illustrates the synthesis process of Co3O4-NC&Ag. Firstly, 2-methylimidazole and Co(NO3)2 react in water to form a sheet-like ZIF-67. Usually, the shape of ZIF-67 is polyhedral when the solvent is methanol. However, it suffers from a low product yield. In our actual operation, the yield of ZIF-67 in methanol is far less than that made in water. From the perspective of the catalyst cost, the sheet-like ZIF-67 was chosen as the precursor catalyst in the studies. The ZIF-67 was pyrolyzed at high temperature in N2 to obtain nitrogen-doped porous carbon embedded with metallic Co, and then the Co was oxidized in air and formed Co3O4-NC. It was proven that the optimal conditions for the synthesis of ZIF-67-based Co3O4-NC by calcination method were calcination at 800 °C in an N2 atmosphere and at 300 °C in air [30,31,32]. Subsequently, the Co3O4-NC powder was immersed in a mixture of silver acetate and sodium citrate. After the chemical bath deposition reaction, Ag NPs were uniformly deposited to the surface of Co3O4-NC particles, forming the bifunctional catalyst Co3O4-NC&Ag.

3.1. Material Characterization

The morphology of the samples was characterized by SEM and shown in Figure 2. Figure 2a,b shows the images of precursor ZIF-67 before annealing. Figure 2c,d shows the Co3O4-NC, and Figure 2e,f shows the images of Co3O4-NC after Ag deposition. It was observed that after high-temperature pyrolysis and oxidation treatment, the ZIF-67 nanosheet changed into polyhedral Co3O4-NC particles, which was caused by the collapse of the zeolite imidazole framework structure at high temperatures. Through chemical bath deposition, the Ag NPs uniformly attached to the Co3O4-NC surface. Figure S1 is the SEM images of Co3O4-NC &Ag prepared at different concentrations of Ag ions in the chemical bath deposition T1–T4. Due to the high electron conductivity of Ag, the Ag NPs exhibited high contrast in the SEM images. With the increase of Ag concentrations, more Ag NPs were found to deposit on the surface of Co3O4-NC, while the average size of the Ag NPs gradually increased. Among them, the sample in Figure S1b showed the best dispersion and lowest agglomeration.
The phase characterization and valence state analysis of the samples were performed using an X-ray diffractometer and X-ray photoelectron spectroscopy. Figure 3a shows the XRD patterns of different Co3O4-NC&Ag samples (C0–C4). The three diffraction peaks at 31.2°, 36.85°, and 65.2° can be indexed to the (220), (311), and (440) crystal planes of Co3O4 (PDF#42-1467), which belong to the face-centered cubic phase. The diffraction peaks corresponding to the fcc structured Ag (PDF#04-0783) are observed at 38.1° and 44.2°. Increasing the concentration of Ag ions in the chemical bath deposition resulted in larger Ag NPs on the Co3O4-NC with higher crystallinity. The locally amplified XRD spectrum in Figure 3b further showed that as the concentration of Ag ions reached 0.1 M, the diffraction peak intensity of Ag exceeded that of Co3O4-NC, suggesting that Ag agglomerated seriously on the surface of Co3O4-NC. Further optimization of the Co3O4-NC&Ag catalysts was performed by tuning the time of Ag deposition while fixing the concentration of Ag ions at 0.025 M. The XRD patterns of the Co3O4-NC&Ag samples (T1–T4) are shown in Figure S2, in which the XRD peaks corresponding to Co3O4 and Ag are evidenced. Table 2 summarizes the details of Co/Ag content in samples C1–C4, with their chemical compositions determined by ICP.
The N2 adsorption–desorption isothermal curve and pore size distribution curve of a typical Co3O4-NC&Ag sample C2 were exhibited in Figure 3c. The adsorption–desorption curve does not overlap in the high-pressure region, forming an adsorption hysteresis loop. The pore size ranges from 2 to 50 nm with an average size of 26.1 nm, indicating the mesoporous structure of the Co3O4-NC [33]. The BET-specific surface area is 46.9 m2 g−1 which is beneficial to the transfer of electrons and reactive species in the catalysis of the cathode. The valence state of Co and Ag in the catalysts were analyzed by XPS. Figure 3d is the full spectra of sample C2. The presence of five elements, Co, Ag, N, C, and O, are all observed in the sample. Figure 3e shows the fine spectrum of Co 2p. The spectra include two dominating peaks located at 780.44 eV (2p3⁄2) and 796.1eV (2p1/2) and a pair of satellite peaks. The Co 2p3/2 and Co 2p1/2 peaks were divided into two pairs of sub-peaks, corresponding to the peaks of Co3+ (779.81 eV, 794.49 eV) and Co2+ (781.44 eV, 796.45 eV) in the Co3O4 nanoparticles. The fine XPS spectrum of Ag in Figure 3f consists of two main peaks corresponding to the binding energies of Ag 3d5/2 (368.2 eV) and Ag 3d3/2 (374.2 eV) in the metallic state. In addition, the XPS spectrum of N1s in Figure 3g was fitted with two sub-peaks, which represented two N species, namely pyridine N (398.4 eV) and graphite N (401.6 eV) [34]. The two kinds of N species led to different chemical and electronic environments around neighboring carbon atoms, leading to different electrocatalytic activities. In general, pyridine N and graphite N are the major active sites for OER and ORR reactions [27]. Therefore, the existence of N dopants in the porous carbon would improve the bifunctional electrocatalytic properties of Co3O4-porous carbon composites and, eventually, the performance of the rechargeable zinc-air batteries.

3.2. Electrochemical Performance Test

The electrochemical activity of the different Co3O4-NC&Ag samples was tested using a rotating disk electrode equipped with an electrochemical workstation. The CV curves of the Co3O4-NC&Ag samples prepared at different concentrations (C1–C4) were plotted in Figure 4a and Figure S3a, in which obvious ORR peaks of all samples were observed. The LSV curves of the above samples were measured and shown in Figure 4b. The bare Co3O4-NC exhibited the E1/2 with an initial potential of 0.82 V (vs. RHE). The decoration of Ag NPs significantly improved the ORR catalytic activity of Co3O4-NC. The best catalytic properties were achieved on sample C2 with E1/2 of 0.84 V (vs. RHE) and initial potential of 0.96 V (vs. RHE), respectively, where the values were higher than most reported Ag/cobalt-based bifunctional catalysts for zinc-air batteries [20,35]. As shown in Table 1, E1/2 of Co3O4-NC&Ag does not increase with the increase in Ag concentration in the chemical bath deposition. The initial potential and limiting current density also showed the same trend, reflecting that prepared Co3O4-NC&Ag had the strongest synergistic effects as the Ag concentration was 0.025 M. The Tafel slope of the prepared samples was calculated according to the LSV curves. As shown in Figure 4c, the Tafel slope of Pt/C was calculated to be 76.8 mV/dec, which was similar to the reported values [32,36,37]. The lowest Tafel slope of 91 mV/dec was achieved for sample C2, which was close to that of Pt/C and suggested that it had the fastest kinetics in oxygen reduction.
After fixing the optimal concentration of Ag ions (0.025 M) in the chemical bath deposition, the catalytic properties of samples were further optimized by changing the reaction time. Figure S3b shows the LSV curves of the Co3O4-NC&Ag samples prepared at different deposition times. The best ORR performance was achieved when the deposition time was 5 min. As the deposition time increased from 5 min to 10 min, the E1/2 and initial potential of the samples showed a decreasing trend. The possible reason was that the excessive Ag loading covered the surface-active sites of Co3O4-NC and caused agglomeration. In addition, the calculated Tafel slopes are shown in Figure S3c, which are consistent with the trend of LSV curves. The ORR polarization curves of sample C2 at different rotation speeds are shown in Figure 4d. According to the Koutecky–Levich curves in Figure 4e, the number of electron transfers of our catalyst was calculated to be 3.85. It indicates that the main ORR reaction process is a four-electron pathway, which is desired in the discharging process of the rechargeable zinc-air batteries.
The ORR stability of sample C2 was evaluated by cyclic voltammetry in 0.1 M KOH solution between 0.7~1.0 V (vs. RHE). For comparison, the stability of Pt/C catalysts was also measured under the same conditions. The CV curves before and after cycling are shown in Figure S3d. It is seen from Figure 4f that after 8000 electrochemical cycles, the LSV curve of sample C2 shows almost no decay on the initial potential and only 10 mV of decay on E1/2. In contrast, the E1/2 of Pt/C decreased significantly to 82 mV while the initial potential decreased to 0.99 V (vs. RHE). It is then concluded that the Co3O4-NC&Ag catalysts possess excellent ORR durability over Pt/C.
In order to obtain the bifunctional catalysts with the best comprehensive performance, the OER performance of the Co3O4-NC&Ag catalyst samples was also tested. Figure 5a shows the LSV curves of sample C0–C4 and RuO2, with the corresponding overpotentials at 10 mA cm−2 given in Table 1. It was seen that the deposition of proper silver NPs improved the OER activity of the Co3O4-NC, while the excessive deposition of Ag reduced the activity. The best OER performance was obtained for sample C2, which showed an overpotential of 349 mV at 10 mA cm−2, being slightly higher than that of RuO2 (300 mV). The decreased OER activities of C3 and C4 were probably due to the influence of sodium citrate in the deposition. It is known that the chelation between citrate and oxides surface will lead to lattice disorder on the crystal surface. Therefore, the presence of high concentrations of sodium citrate might adversely affect the active sites of Co3O4-NC in the OER reaction. Based on the LSV curves, the Tafel curves of C0–C4 were plotted in Figure 5b. In accordance with the LSV curves, sample C2 has the lowest Tafel slope (100 mV/dec), which is very close to that of RuO2 (98.4 mV/dec) [38,39].
The OER stability of sample C2 was also evaluated by cyclic voltammetry and compared with the commercial RuO2 catalyst. The LSV curves of samples after 3000 electrochemical cycles are shown in Figure 5c. The overpotential of sample C2 attenuated by 30 mV was less than that of RuO2 (50 mV). This indicates that the long-term stability of sample C2 is better than that of RuO2 in OER. Combined with all the above analyses on ORR and OER, it can be concluded that sample C2 has the best bifunctional catalytic activity. The electrochemical properties of the optimized Co3O4-NC&Ag catalysts were compared with recently reported similar catalysts, and the results are labeled in Table S1. Therefore, the Co3O4-NC&Ag composites in our studies possess some advantages and have the potential for application in rechargeable zinc-air batteries.

3.3. Application of Co3O4-NC&Ag(C2) in Zinc-Air Batteries

The sample C2 catalysts were used as the cathode to form a zinc-air battery. At the same time, the air cathode was made with Pt/C&RuO2 catalysts as a comparison under similar conditions. As shown in Figure 6a, the open circuit voltage of the as-made zinc-air battery reached 1.423 V, being close to 1.453 V for the Pt/C& RuO2 zinc-air battery (Figure S5). As shown in Figure 6b, the open-circuit voltage of the Co3O4-NC&Ag zinc-air battery is stable and does not change within 400 s. The electrochemical workstation was used to test the charge and discharge polarization of the above two zinc-air batteries. It is obvious in Figure 6c that the Co3O4-NC&Ag zinc-air battery has good charge-discharge characteristics. Especially as the current density is less than 250 mA cm−2, the discharge voltage is even higher than that of Pt/C. The charging voltage stability of the Co3O4-NC&Ag battery also exceeded that of RuO2. According to the discharge polarization curve, the power density of the battery is plotted in Figure 6d. The power density of the Co3O4-NC&Ag zinc-air battery reached 198 mW cm−2, which was significantly higher than that of the Pt/C&RuO2 zinc-air battery (160 mW cm−2).
The specific discharge capacity is an important parameter in evaluating battery performance. The specific discharge capacity of Co3O4-NC&Ag and Pt/C were tested at the current density of 10 mA cm−2. In the process of constant current discharge, the zinc anode will be constantly consumed and eventually broken off. According to the quality of the zinc consumed, the following Equation (4) is used to calculate the specific discharge capacity (CSP) of the battery. Where I is the magnitude of the current applied in the test, t is the time when the voltage drops to 0, and m is the quality of the zinc sheet consumed after the test [5].
CSP = It m
As shown in Figure 6e,f, the Co3O4-NC&Ag zinc-air battery can operate for 60 h at a stable current density of 10 mA cm−2. The discharge voltage is 1.26 V–1.19 V, slightly higher than that of a Pt/C-based zinc-air battery, which corresponds to 1.24 V–1.16 V. Further, the specific discharge capacity is as high as 770 mAh g−1, which is higher than 705 mAh g−1 of Pt/C-based zinc-air battery. The cycle stability of the Co3O4-NC&Ag zinc-air battery was also tested and compared with the Pt/C&RuO2-based zinc-air battery. The cycle period was set to 20 min with a charging/discharging current density of 10 mA cm−2. The battery cycle stability test comparison is shown in Figure 6g. Although the Pt/C&RuO2 zinc-air battery exhibited better charge-discharge properties than that of the Co3O4-NC&Ag in the first hour, its voltage gap between charge and discharge (ΔE) decays rapidly with the extension of test time. On the contrary, the ΔE of the Co3O4-NC&Ag zinc-air battery showed little change. Figure 6h is a local magnification of the cyclic stability. After 120 h of cycling, the ΔE of the Co3O4-NC&Ag zinc-air battery increased from 0.88 V to 1.1 V (Figure 6g), while the ΔE of the Pt/C& RuO2 zinc-air battery increased from 0.75 V to 1.25 V after only 40 h of cycling. Our Co3O4-NC&Ag zinc-air battery showed certain advantages with respect to open circuit voltage, power density, and specific discharge capacity when compared with the reported Co or Co/Ag cathode catalysts (Table S2, references [11,20,40,41,42,43,44,45,46,47,48] are cited in the supplementary file).
We have also done an EDS survey on the cathode. For the cathode made of sample C2, the average atomic ratio of Ag:Co was calculated to be ~2%. After cycling, the average atomic ratio of Ag:Co was less than 1%. Within the error range of the instrument, it is inferred that the Ag NPs suffered from corrosion in the long-term charging-recharging process, especially the OER process. In addition, the cycle life of the battery is affected by zinc dendrites. The generation of dendrites is mainly due to irregular dissolution during discharge and uneven zinc deposition during charging during repeated cycles [49,50,51]. In the assembled batteries, the addition of zinc acetate supplied zinc ions during the charging and discharging process, which helped to reduce the difference in electron transfer rate and retarded the generation of zinc dendrites.
According to the structural and electrochemical analysis of Co3O4-NC&Ag catalysts, the superior performance of Co3O4-NC&Ag zinc-air batteries was attributed to two reasons. First, the MOF-derived porous carbon support enables the Co3O4 NPs uniformly disperse and avoid agglomeration. The large specific surface areas of supports and the decoration of Ag NPs provide sufficient active sites for ORR. Second, both the pyridine N and graphitic N binding energies undergo a positive shift compared to the reported binding energies. This indicates the existence of electronic interactions between the carbon support and Co3O4, which will greatly affect the adsorption energies of intermediates in the ORR process, thereby optimizing the ORR activity.

4. Conclusions

In summary, a novel Co3O4-NC&Ag nanocomposite was prepared by depositing Ag NPs on the surface of Co3O4-NC particles by chemical bath deposition. The ORR and OER performance of Co3O4-NC was significantly improved after depositing Ag NPs, in which the best Co3O4-NC&Ag catalysts possessed superior ORR performance with E1/2 of 0.84 V (vs. RHE) and OER activity with an overpotential of 349 mV at 10 mA cm−2. The assembled Co3O4-NC&Ag zinc-air battery has a power density of 198 mA cm−2 and better long-term stability, and the specific discharge capacity is up to 770 mAh g−1. By rationally designing and utilizing ZIF-67 materials, this work provides a simple but effective approach for fabricating durable, cost-effective, and active bifunctional electrocatalysts and demonstrates their practical application prospects in rechargeable zinc-air batteries.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su142013417/s1, Figure S1: (a–d) SEM patterns of samples C1–C4; Figure S2: (a–b) XRD patterns of T0–T4; Figure S3: (a) CV curves of Co3O4-NC&Ag samples T0–T4; (b) LSV curves compared with Pt/C; (c) Corresponding Tafel slope plot; (d) CV curves of Pt/C and sample C2 before and after cyclic voltammetry test. Figure S4: (a) LSV curves of samples T0–T4 prepared with RuO2; (b) Corresponding Tafel slope plot; Figure S5: Physical diagram of open-circuit voltages of Zinc-air battery assembled by sample C2 and Pt/C& RuO2 respectively; Table S1: Comparison table of electrochemical performance of Co or its oxid-based catalyst reference system; Table S2: Performance comparison table of Co or its oxide-based catalyst for zinc-air battery system.

Author Contributions

Conceptualization, H.W. (Hanbin Wang); methodology, H.W. (Hanbin Wang) and P.L.; formal analysis, P.L. and B.W.; investigation, P.L. and L.Z.; writing-original draft preparation, P.L.; writing—review and editing, H.W. (Hanbin Wang), P.L. and J.C.; visualization, Y.D. and J.C.; supervision, L.L. and H.W. (Houzhao Wan); project administration, L.L., H.W. (Houzhao Wan) and H.W. (Hanbin Wang). All authors have read and agreed to the published version of the manuscript.

Funding

The Natural Science Foundation of Hubei Province, China (2020CFB446).

Institutional Review Board Statement

Not applicable.

Informed Consent statement

Not applicable.

Data Availability Statement

All data used during the study are available from the corresponding author by request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Synthetic diagram of Co3O4-NC&Ag.
Figure 1. Synthetic diagram of Co3O4-NC&Ag.
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Figure 2. SEM images of (a,b) ZIF-67; (c,d) Co3O4-NC; and (e,f) sample C2 with different magnifications.
Figure 2. SEM images of (a,b) ZIF-67; (c,d) Co3O4-NC; and (e,f) sample C2 with different magnifications.
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Figure 3. XRD patterns of (a) sample C1–C4 and (b) their local magnifications; (c) N2 adsorption-desorption isothermal curve and pore size distribution of sample C2; XPS spectra of (d) full spectrum, (e) Co 2p, (f) Ag 3d and (g) N 1s of sample C2.
Figure 3. XRD patterns of (a) sample C1–C4 and (b) their local magnifications; (c) N2 adsorption-desorption isothermal curve and pore size distribution of sample C2; XPS spectra of (d) full spectrum, (e) Co 2p, (f) Ag 3d and (g) N 1s of sample C2.
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Figure 4. (a) CV curves, (b) LSV curves, and (c) corresponding Tafel slope of Pt/C and sample C0–C4; (d) ORR polarization curves of sample C2 at different rotation speeds; (e) The calculated Koutecky–Levich curves at 0.4, 0.5, 0.6, and 0.7 V for sample C2; (f) LSV curves of Pt/C and sample C2 before and after stability test.
Figure 4. (a) CV curves, (b) LSV curves, and (c) corresponding Tafel slope of Pt/C and sample C0–C4; (d) ORR polarization curves of sample C2 at different rotation speeds; (e) The calculated Koutecky–Levich curves at 0.4, 0.5, 0.6, and 0.7 V for sample C2; (f) LSV curves of Pt/C and sample C2 before and after stability test.
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Figure 5. (a) LSV curves of samples prepared with RuO2 and C1–C4; (b) corresponding Tafel slope plot; (c) LSV curves of RuO2 and sample C2 before and after stability test.
Figure 5. (a) LSV curves of samples prepared with RuO2 and C1–C4; (b) corresponding Tafel slope plot; (c) LSV curves of RuO2 and sample C2 before and after stability test.
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Figure 6. (a) Physical diagram of the Co3O4-NC&Ag zinc-air battery; (b) open circuit voltage comparison diagram of a battery composed of Pt/C and (c) charge-discharge polarization curves; (d) The power density curve calculated from the discharge polarization curve; (e) volt-time diagram of constant current flux at a 10 mA cm−2 current density; (f) comparison diagram of specific discharge capacity; (g,h) battery cycle stability test comparison chart and local comparison chart.
Figure 6. (a) Physical diagram of the Co3O4-NC&Ag zinc-air battery; (b) open circuit voltage comparison diagram of a battery composed of Pt/C and (c) charge-discharge polarization curves; (d) The power density curve calculated from the discharge polarization curve; (e) volt-time diagram of constant current flux at a 10 mA cm−2 current density; (f) comparison diagram of specific discharge capacity; (g,h) battery cycle stability test comparison chart and local comparison chart.
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Table
Table 1. Experimental parameters of different Co3O4-NC&Ag samples and their corresponding electrochemical properties.
Table 1. Experimental parameters of different Co3O4-NC&Ag samples and their corresponding electrochemical properties.
LabelsAg Ions Concentration
(M)		Ag Ions Deposition Time (min)ORR E1/2
(V vs. HRE)		The Initial Potential
(V vs. HRE)		Limiting Current Density (mA cm−2)OER Overpotential
(mV)
C0050.820.89−2.86370
C10.012550.80.91−4.2364
C20.02550.840.966−4.6349
C30.0550.830.94−3.86400
C40.150.8220.95−5.37438
T10.0252.50.8230.93−2.2350
T2(C2)0.02550.840.966−4.6349
T30.0257.50.820.94−3.56370
T40.025100.7970.88−2.89380
Pt/C--0.891.02−4.9-
RuO2-----300
Table
Table 2. ICP data of samples C1–C4.
Table 2. ICP data of samples C1–C4.
Sample NameContent of Co (mg/L)Content of Ag (mg/L)Elemental Molar Ratio (Co/Ag)Content of Ag in the Sample (mg/kg)
C118.90.3106:19
C254.81.757:117
C339.92.135:128
C445.017.95:1166
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Leng, P.; Wang, H.; Wu, B.; Zhao, L.; Deng, Y.; Cui, J.; Wan, H.; Lv, L. Ag Decorated Co3O4-Nitrogen Doped Porous Carbon as the Bifunctional Cathodic Catalysts for Rechargeable Zinc-Air Batteries. Sustainability 2022, 14, 13417. https://doi.org/10.3390/su142013417

AMA Style

Leng P, Wang H, Wu B, Zhao L, Deng Y, Cui J, Wan H, Lv L. Ag Decorated Co3O4-Nitrogen Doped Porous Carbon as the Bifunctional Cathodic Catalysts for Rechargeable Zinc-Air Batteries. Sustainability. 2022; 14(20):13417. https://doi.org/10.3390/su142013417

Chicago/Turabian Style

Leng, Pingshu, Hanbin Wang, Binfeng Wu, Lei Zhao, Yijing Deng, Jinting Cui, Houzhao Wan, and Lin Lv. 2022. "Ag Decorated Co3O4-Nitrogen Doped Porous Carbon as the Bifunctional Cathodic Catalysts for Rechargeable Zinc-Air Batteries" Sustainability 14, no. 20: 13417. https://doi.org/10.3390/su142013417

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