Oxygen Vacancy-Rich NiCo<sub>2</sub>O<sub>4</sub> on Carbon Framework with Controlled Pore Architectures as Efficient Bifunctional Electrocatalysts for Zn-Air Batteries

Kim, Min; Hong, Jeong Hoo; Kim, Ki Beom; Koo, Hye Young; Kang, Yun Chan

doi:https://doi.org/10.1155/2024/4459617

International Journal of Energy Research

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Abstract Introduction Experimental Results and Discussion Conclusions Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2024 | Article ID 4459617 | https://doi.org/10.1155/2024/4459617

Oxygen Vacancy-Rich NiCo₂O₄ on Carbon Framework with Controlled Pore Architectures as Efficient Bifunctional Electrocatalysts for Zn-Air Batteries

Min Kim,¹Jeong Hoo Hong,¹Ki Beom Kim,¹Hye Young Koo,²and Yun Chan Kang¹

Academic Editor: Ahmad Azmin Mohamad

Received12 Feb 2024

Revised15 Apr 2024

Accepted02 May 2024

Published16 May 2024

Abstract

Transition metal oxides are considered alternative electrocatalysts for ZAB owing to their multiple oxidation states. However, they have limitations such as low electrical conductivity and the deficiency of reactive sites. In this study, to overcome these shortcomings and improve electrocatalytic activity, oxygen vacancies and porous architectures were introduced through a partial reduction process and a porous carbon framework. Open porous carbon microspheres with uniformly loaded NiCo₂O₄ nanosheets and oxygen vacancies (V-NCO/OPC) displayed enhanced electrocatalytic performance with a low Tafel slope (68 mV dec^-1) in the oxygen reduction reaction (ORR) and a low overpotential (402 mV) at 10 mA cm^–2 in the oxygen evolution reaction (OER). The combined effect of the oxygen vacancies and porous architecture can offer sufficient active sites, modify the electronic structure of the metal oxide surface, and facilitate mass transport, enhancing the electrocatalytic properties of V-NCO/OPC. Furthermore, when applied for ZAB, V-NCO/OPC demonstrated better electrochemical performance including discharge power density (154.9 mW cm^-2) at the current density of 175.9 mA cm^-2, low voltage gap (0.85 V) at the initial cycle, and long-term (250 h) cycle stability at the current density of 10 mA cm⁻² than those of noble-metal electrocatalysts.

1. Introduction

Rechargeable zinc-air batteries (ZABs) are considered one of the most sustainable battery technologies owing to their characteristics such as high theoretical energy density (1086 W h kg⁻¹), low cost, and safe operation [1–4]. Especially, ZABs are inherently ecofriendly due to their utilization of nontoxic zinc as the anode material, their potential recyclability, and the availability of aqueous electrolytes. Owing to the sluggish oxygen catalytic kinetics, the catalytic activity of the cathode material in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays a crucial role in the remarkable performance of ZABs [5–8]. Noble metal-based materials such as Pt and IrO₂/RuO₂ are well-known catalysts with superior ORR and OER catalytic activities, respectively [9–11]. However, the conflict between the increasing energy demand and limited resource supply caused by the low natural abundance of noble metals limits the large-scale applications of these materials [12–18]. In addition, their poor durability, caused by the deformation of the nanoparticles during the electrochemical process, brings about the deactivation of the catalyst [19–21]. Therefore, the development of noble-metal-free bifunctional electrocatalysts possessing high activity and durability at a low cost is important for achieving extensive practical applications [22, 23].

Transition metal oxides (TMOs) have attracted considerable interest as bifunctional electrocatalysts owing to their low cost and multiple oxidation states from their unique crystal structure [22, 24–26]. However, their deficiency in reactive sites and intrinsically inferior electrical conductivity remain challenging [27–30]. Employing oxygen vacancies in the transition metal oxide lattice has been reported as an effective strategy for addressing the issues [31, 32]. Oxygen vacancies in the lattice of the metal oxide modify the electronic structure, which not only forms several active sites for adsorption but also enhances electrical conductivity [33, 34]. Utilizing a carbon framework with a porous architecture is another important strategy for boosting the electrocatalytic activity of transition metal oxides [35–38]. Pore size modulation, including micro-, meso-, and macropores, can facilitate the electrocatalytic activity by regulating the crystal size to expose sufficient active sites, promote mass transport, and facilitate the detachment of the O₂ bubble-blocking surface of the active site [8, 39–41]. Among the various TMOs, NiCo₂O₄ (NCO), which is a spinel oxide with Ni ions occupying octahedral sites in the Co₃O₄ lattice, has attracted significant attention as an electrocatalyst owing to its availability and stability [42–45]. Yuan et al. reported the superior ORR activity of NCO nanosheets on hollow mesoporous carbon with oxygen vacancies through an annealing treatment in a low-oxygen atmosphere [46]. Liu et al. explored ultrathin NCO nanosheets with oxygen vacancies as bifunctional zinc-air cathode materials synthesized under different calcination atmospheres [47].

Spray pyrolysis, which is a continuous reaction process possessing high efficiency, can be considered an effective synthetic process for porous carbon frameworks [48]. Moreover, the use of an aqueous solution containing various components and short-time synthesis for spray pyrolysis are very powerful factors for ecofriendliness and scalability. Various removable organic/inorganic materials contained in the droplets can be used to easily fabricate porous structures suitable for supporting transition metal compounds after spray pyrolysis. In this study, NCO with oxygen vacancies on three-dimensional (3D) open-pore-structured porous carbon microspheres (V-NCO/OPC) was prepared as a bifunctional electrocatalyst. The unique porous structure of the carbon framework provides a high surface area for uniform decoration of NCO nanosheets, as well as desired diffusion channels for electrolyte and oxygen. Moreover, with the accompanying morphological advantages, the oxygen vacancies created in NCO by the cost-effectiveness of the NaBH₄ reducing agent with water enhanced the electrocatalytic activity by changing the valence state and electronic structure within a short time. Consequently, V-NCO/OPC exhibited outstanding performance in the ORR and OER processes in alkaline media, as well as low polarization and long-term cycle stability in ZABs.

2. Experimental

MgO/open-pore-structured carbon composite microspheres (Mg-OPC) were synthesized by a one-pot spray pyrolysis process using a carbon source and a pore-forming agent. A spray solution was prepared by dissolving 0.1 M magnesium nitrate and 4 g of carbon sources composed of dextrin and polyvinylpyrrolidone (PVP; ) in a weight ratio of 1 : 1, followed by dispersing 2 g of 100 nm polystyrene (PS) nanobeads in distilled water (200 mL). The colloidal solution was nebulized into droplets by an ultrasonic spray generator, and the droplets passed through a reactor maintained at 800°C with an N₂ gas flow rate of 10 L min^-1. Based on the synthesis through spray pyrolysis on a lab scale, approximately 1.3 g of powder can be obtained from a 1 L precursor solution. After the etching process using HCl solution to remove MgO particles and the subsequent carbonization process at 800°C under Ar gas flow for 3 h, 3D open-pore structured carbon microspheres (denoted as OPC) were obtained. To decorate NCO on the microspheres, 0.01 g of nickel acetate, 0.02 g of cobalt acetate, and 0.025 g of hexamethylenetetramine (HMTA) were dissolved in 50 mL of ethanol under continuous stirring and 0.02 g of OPC was added to the homogenous solution. The reaction mixture was transferred into a Teflon-lined autoclave and was treated at 110°C for over 16 h. The resultant precipitate was washed with DI water several times and calcined at 280°C for 2 h in the air to obtain the NCO nanosheets/OPC (NCO/OPC). Subsequently, the NCO/OPC was dispersed in a 0.025 M NaBH₄ aqueous solution for 45 min, resulting in V-NCO/OPC. For comparison, porous carbon microspheres without open macropores decorated with NCO nanosheets (denoted as NCO/PC) were synthesized using an identical procedure without PS nanobeads. MgO/PC composite microspheres prepared from the spray solution without PS nanobeads transformed into porous carbon microspheres (denoted as PC) without open macropores. Detailed information including characterization methods, ORR and OER measurements, and manufacture and measurement of the ZABs is described in supporting information.

3. Results and Discussion

A schematic illustration for synthesizing V-NCO/OPC is shown in Scheme 1. The spray pyrolysis process involves atomizing a spray solution into droplets using ultrasonic force, followed by their evaporation and decomposition in a heated reactor. When a droplet containing dextrin, PVP, Mg nitrate, and PS nanobeads passes through the reactor, which is exposed to high temperatures, solvent evaporation and decomposition for each component occur. This rapid process ensures the formation of aggregate-free spherical microspheres from each droplet, with particles spending only a short time in the reactor furnace. Then, the Mg nitrate and carbon sources (dextrin and PVP) were converted into MgO and carbon frameworks, respectively, and the PS nanobeads were decomposed into gas phases including CO₂ and H₂O to create macropores [49]. In this process, MgO nanocrystals act as inorganic catalysts that convert the carbon sources into a carbon framework [50]. The existence of MgO in Mg-OPC was demonstrated by the XRD pattern in Figure S1. The resultant microspheres were etched using a dilute HCl solution to eliminate MgO and complete the carbonization of the carbon framework with further heat treatment to form the OPC microspheres. NCO nanosheets were decorated on OPC via a hydrothermal method and subsequent oxidation. During the hydrothermal process, the Ni²⁺ and Co²⁺ ions, initially absorbed onto the surface of OPC, underwent a chemical transformation facilitated by the presence of OH^- ions generated through the hydrolysis of HMTA. This led to the formation of NiCo₂(OH)₆ nanosheets, and the nanosheets were transformed into NCO nanosheets during an oxidation process. The overall reactions are conducted by following the chemical equations [51, 52].

Through reaction with NaBH₄ solution, oxygen vacancies are introduced into the NCO lattice by the abundant hydrogen gas generated by NaBH₄, which produces V-NCO/OPC.

The structural difference between OPC and porous carbon without open pores (PC) was compared using scanning electron microscopy (SEM) images in Figure 1. Numerous macropores generated by the decomposition of PS nanobeads in Mg-OPC and no visible morphological changes after etching to remove the MgO crystals in OPC were confirmed in Figures 1(a) and 1(b). In contrast, the MgO/PC composite microspheres exhibited no macropores owing to the absence of PS nanobeads in the spray solution, and PC possessed only pores formed by etching MgO (Figures 1(c) and 1(d)). To further compare the differences between OPC and PC, their surface areas and pore structures were analyzed. The isotherms shown in Figure S2(a) exhibit a typical type IV hysteresis, indicating that the samples had an abundance of micro/mesopores [53]. The BET surface areas of OPC and PC were 757.9 and 519.8 m² g^-1, respectively. The open-macroporous structure enhances the surface area of the OPC. The Barrett-Joyner-Halenda (BJH) pore size distributions of OPC and PC in Figure S2(b) revealed mesopores with a maximum peak of around 4.5 and 4.9 nm, respectively. The increased specific surface area of OPC with micro/mesopores can be considered as a more suitable carbon framework for not only decorating the NCO component but also encouraging the mass transfer of oxygen molecules and the electrolyte.

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After the hydrothermal method and oxidation treatment, NCO nanosheets were successfully loaded onto OPC and PC, as confirmed by the X-ray diffraction (XRD) patterns shown in Figure S3. The morphologies of NCO/OPC and NCO/PC were investigated by SEM and transmission electron microscopy (TEM) in Figure 2 and Figure S4, respectively. Ultrafine NCO nanosheets were uniformly decorated on the surface of the OPC, and the open pore structure was maintained after NCO loading in Figures 2(a) and 2(b). However, in the case of NCO/PC, large NCO nanosheets were deposited on the surface of the PC (Figures S4(a) and S4(b)). The presence of macropores prevents the large growth of NCO nanosheets on the surface and allows NCO to be uniformly distributed in small sizes. From the high-resolution TEM (HR-TEM) image of NCO/OPC in Figure 2(c), the two lattice fringe spacings of 0.21 nm matching the (400) and (040) planes of the cubic structure were observed, which is in line with the results from the XRD pattern of NCO nanosheets (JCPDS 20-07810). The selected-area electron diffraction (SAED) pattern in Figure 2(d) also reveals well-defined diffraction rings in the (511), (400), (311), and (111) crystal planes of NCO. The uniform distributions of Co, Ni, O, and C on the OPC surface are demonstrated in the elemental mapping images in Figure 2(c). The HR-TEM and SAED patterns presented in Figures S4(c) and S4(d), respectively, demonstrated the successful decoration of PC with the NCO nanosheets.

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The changes in the surface area and pore structure after NCO loading were analyzed using N₂ gas adsorption/desorption isotherms, and the corresponding results are shown in Figure S5. The hysteresis types of NCO/OPC and NCO/PC (Figure S5(a)) were identical to those of OPC and PC. However, the surface area of NCO/OPC decreased slightly to 718.5 m² g^-1, while that of NCO/PC decreased significantly to 353.2 m² g^-1. The macropores of NCO/OPC were not blocked even after the loading of NCO nanosheets, which also led to the inhibition of crystal growth to form ultrafine NCO nanosheets. However, the micro/mesopores of NCO/PC were blocked by large-sized NCO nanosheets owing to the absence of macropores; therefore, NCO/OPC exhibited a smaller decrease in surface area than NCO/PC. The Barrett-Joyner-Halenda (BJH) pore-size distribution in Figure S5(b) showed a decrease in the volume fraction of pores less than 5 nm in NCO/OPC and NCO/PC compared to that in OPC and PC, which suggests the successful loading of NCO in the carbon framework. To determine the actual NCO content in the carbon framework, the thermogravimetric analysis (TGA) of NCO/OPC and NCO/PC was performed under an airflow from 25 to 800°C at a heating rate of 10°C min⁻¹, as shown in Figure S6. The initial weight loss below 150°C corresponded to the evaporation of water molecules that had been adsorbed on each sample. The subsequent dramatic drop occurring from 300 to 500°C was related to the combustion of the carbon framework. The TGA results confirmed that NCO/OPC and NCO/PC contained the same amount of NiCo₂O₄ (approximately 23 wt.%).

In general, the presence of oxygen vacancies in the lattice of metal oxide materials enhances their electrocatalytic performance in the ORR and OER. To introduce oxygen vacancies into the NCO/OPC, a partial reduction process using a NaBH₄ solution was conducted. The XRD pattern of V-NCO/OPC, as shown in Figure S7, exhibited no change in the crystal structure but showed lower peak intensities, indicating the creation of oxygen vacancies in the NCO lattice. The SEM image in Figure 3(a) demonstrated that, even after the chemical reduction step, the morphology of the sample was approximately intact without structural destruction, and the TEM image in Figure 3(b) confirmed that the shape of the NCO nanosheets was maintained after the formation of V-NCO. The HR-TEM image in Figure 3(c) also showed two clear lattice fringes at an angle of 90°, which suggests that the cubic crystal structure was well-maintained after the introduction of oxygen vacancies. The diffraction rings in the SAED pattern were also in agreement with the crystal planes of NCO in Figure 3(d). However, unlike the SAED pattern of NCO/OPC, some diffraction rings of NCO were not observed, which could be attributed to the decrease in crystallinity caused by the formation of oxygen vacancies.

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To verify the formation of oxygen vacancies in the lattice of NCO, X-ray photoelectron spectroscopy (XPS) was performed, and all the spectra of NCO/OPC and V-NCO/OPC were compared. The survey scans of both samples, shown in Figure 4(a), indicate the presence of elements corresponding to NCO and carbon. The Co 2p spectrum (Figure 4(b)) was divided into four major peaks and two satellite peaks. Peaks related to Co³⁺/Co²⁺ in Co 2p_3/2 and Co 2p_1/2 were located at 779.7/781.1 eV and 795.1/796.6 eV, respectively, and two satellite peaks were assigned to 785.8 and 802.3 eV [54]. In the Ni 2p spectrum, peaks corresponding to Ni³⁺/Ni²⁺ were observed at 854.1/855.5 and 871.9/873.1 eV in the Ni 2p_3/2 and Ni 2p_1/2 orbitals, respectively, and satellite peaks were displayed at 861.1 and 879.2 eV (Figure 4(c) [55–57]. Three deconvoluted peaks located at 529.3, 531.6, and 533.4 eV in the O 1s spectrum corresponded to the lattice oxygen atoms, defect sites with oxygen vacancy, and absorbed water molecules, respectively (Figure 4(d)) [58]. When comparing all the spectra of each element between V-NCO/OPC and NCO/OPC, the ratios of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ increased in V-NCO/OPC compared to those in NCO/OPC, indicating that M³⁺ ions were reduced to M²⁺ ions originating from the introduction of oxygen vacancies in the NCO lattice [59]. Furthermore, the O 1s spectrum of V-NCO/OPC exhibited decreased M-O bonding and an increased number of defect sites with oxygen vacancies compared to those of NCO/OPC, which also demonstrated the successful introduction of oxygen vacancies in the NCO lattice [60]. UV-vis analysis was conducted to investigate oxygen vacancies, comparing absorbance between bare NCO and V-NCO sheets without carbon. The lower absorbance in V-NCO sheets suggests the presence of oxygen vacancies affecting its structure and reducing bandgap energy compared to NCO sheets in Figure S8(a). Bandgap energies extrapolated from the linear part following the Tauc plot in Figure S8(b) were determined as 3.2/1.6 eV for NCO sheets and 2.9/1.4 eV for V-NCO sheets, which is evidence for the existence of oxygen vacancy in V-NCO [61]. To further demonstrate the existence of oxygen vacancies in the NCO lattice, an electron paramagnetic resonance (EPR) analysis was conducted. The EPR spectra shown in Figure S9 demonstrated that the signals in both samples with could be assigned to paramagnetic oxygen vacancies. V-NCO/OPC exhibits a higher signal intensity at than NCO/OPC, which verifies the existence of oxygen vacancies in V-NCO/OPC.

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To demonstrate the effect of hybridization with an open porous structure of the carbon framework and oxygen vacancies in the NCO lattice on electrocatalytic activity, linear sweep voltammetry (LSV) measurements were carried out in an O₂-saturated alkaline electrolyte (0.1 M KOH). In the ORR LSV curves shown in Figure 5(a), V-NCO/OPC exhibited a higher onset potential ( V) and half-wave potential ( V) than NCO/OPC (0.94 V (), 0.81 V ()) and NCO/PC (0.93 V (), 0.77 V ()). Although the onset and half-wave potential of V-NCO/OPC were still lower than those of the benchmark catalyst (Pt/C, 1.0 V (), 0.92 V ()), the limited current density of V-NCO/OPC (4.72 mA cm^-2) was higher than that of Pt/C (4.57 mA cm^-2). Moreover, Tafel plots, which are the criteria for reaction kinetics, showed that V-NCO/OPC possessed the lowest Tafel slope (68 mV dec^-1) compared to NCO/OPC (81 mV dec^-1), NCO/PC (113 mV dec^-1), and Pt/C (99 mV dec^-1) (Figure 5(b)). The EIS experiments were carried out to explore catalytic kinetics (Figure S10). The EIS spectrum under ORR conditions revealed that the smaller charge transfer resistance () observed in V-NCO/OPC and NCO/OPC, compared to NCO/PC, is attributed to the presence of macropores facilitating electrolyte transport. These results indicate the superior ORR activity and kinetics of V-NCO/OPC compared with those of the other samples. To further investigate this synergistic effect, the electrochemical surface area (ECSA) of each sample was compared using the double-layer capacitance () values calculated by cyclic voltammetry (CV) measured in the non-Faraday region at various sweep rates (Figure S11). Based on the CV data of the prepared samples, the obtained value of V-NCO/OPC (21.8 mF cm^-2) was higher than those of NCO/OPC (18.1 mF cm^-2) and NCO/PC (1.7 mF cm^-2), which indicates that V-NCO/OPC have numerous adsorption sites compared to other samples. To calculate the electron transfer number () for the ORR, LSV curves were measured at different rotation speeds from 400 to 2025 rpm, and the Koutecky-Levich (K-L) equation was applied using the LSV curves. As shown in Figure 5(c), all values based on the K-L plots at each potential point are approximately 4, which is consistent with the four-electron transfer reactions during the ORR process. In ORR, since the durability of electrocatalysts is as important as activity and kinetics, the chronoamperometric responses of V-NCO/OPC and Pt/C were measured at the half-wave potential of each sample for 10 h. As shown in Figure 5(d), V-NCO/OPC maintained 73% of its initial current density, whereas Pt/C maintained only 58% of its initial current density, demonstrating the superior ORR durability of V-NCO/OPC compared to Pt/C. Furthermore, chronoamperometry was used to analyze the tolerance of methanol on the catalysts by adding 3 M methanol solution at 300 s in a 0.1 M oxygen-saturated KOH solution. While the commercial Pt/C catalyst exhibited a decline in performance due to oxidation with byproducts upon methanol addition, the V-NCO/OPC catalyst demonstrated remarkable durability under the same conditions, maintaining its performance (Figure S12). This result suggests the superior methanol tolerance property of V-NCO/OPC compared to Pt/C.

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The OER characteristics of V-NCO/OPC, NCO/OPC, NCO/PC, and RuO₂ are shown in Figure 6. The LSV curves shown in Figure 6(a) revealed that V-NCO/OPC possessed a lower overpotential (402 mV) at 10 mA cm^-2 than NCO/OPC (486 mV) and NCO/PC (516 mV), and higher than that of RuO₂ (360 mV). However, in terms of the OER kinetics, the Tafel slope of V-NCO/OPC is 195 mV dec^-1, which is lower than those of NCO/OPC (219 mV dec^-1), NCO/PC (222 mV dec^-1), and RuO₂ (204 mV dec^-1), as shown in Figure 6(b). Moreover, when the charge-transfer resistance () was evaluated using electrochemical impedance spectroscopy (EIS), V-NCO/OPC exhibited the smallest among the samples in Figure 6(c). These results indicate that V-NCO/OPC has better OER activity than the other samples. The excellent bifunctional electrocatalytic activity of the V-NCO/OPC was triggered by the synergistic effect between the suitable pore structure of the carbon framework and the oxygen vacancies of NCO. The porous structure (micro/meso/macropores) of OPC can regulate the crystal growth to expose sufficient active sites, promote mass transport, and facilitate the detachment of the O₂ bubble-blocking surface of the active site. Moreover, oxygen vacancies in the NCO lattice modify the electronic structure of the NCO surface, which not only forms several active sites for adsorption but also enhances electrical conductivity.

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Based on the superior bifunctional electrocatalytic properties of V-NCO/OPC, a homemade ZAB cell was assembled using V-NCO/OPC as an air cathode to demonstrate its practical application; its electrochemical properties are shown in Figure 7. A Pt/C+RuO₂ mixed electrocatalyst was used as the benchmark for comparison. The charge-discharge polarization curves in Figure 7(a) revealed that the V-NCO/OPC electrode possessed a smaller voltage gap with a higher discharge voltage and lower charge voltage than the Pt/C+RuO₂ electrode, in agreement with the previous ORR and OER results. Moreover, as shown in Figure 7(b), V-NCO/OPC showed a peak power density of 154.9 mW cm^-2 at a current density of 175.9 mA cm^-2, which was 1.3 times higher than that of Pt/C+RuO₂ (118.3 mW cm^-2 at a current density of 123.0 mA cm^-2). The discharge-specific capacity, which is calculated based on the weight of the consumed zinc, is shown in Figure 7(c). At a current density of 10 mA cm^-2, V-NCO/OPC achieved a specific capacity of 758.8 mA h g_Zn^-1 and a corresponding energy density of 940.9 W h kg_Zn^-1, which is better than Pt/C+RuO₂ (699.1 mA h g_Zn⁻¹ and 894.8 W h kg_Zn^-1). In the rate capability shown in Figure 7(d), the discharge voltages of V-NCO/OPC were 1.31, 1.30, 1.28, 1.27, 1.25, 1,24, 1.23, and 1.22 V at 0.5, 1, 2, 4, 8, 10, and 20 mA cm⁻², respectively, which demonstrated superior reaction kinetics of V-NCO/OPC compared to Pt/C+RuO₂ even at high current density without sudden voltage drop for the ZAB system. The durability of the ZAB cathode was tested using galvanostatic charge and discharge measurements at 10 mA cm⁻² with a 20 min period for one cycle (Figure 7(e)). The initial voltage gap between discharge (1.17 V) and charge (2.02 V) of V-NCO/OPC was 0.85 V, which altered to 0.95 V after 100 h of continuous cycling. In contrast, the voltage gap of Pt/C+RuO₂ was increased from 0.89 to 1.09 V at the initial cycle and after 100 h, respectively. Moreover, V-NCO/OPC exhibited long-term cycle stability for 250 h with a restricted increased voltage gap of 1.04 V, with 81% efficiency, as illustrated in Figure 7(f). To investigate the structural and chemical properties of V-NCO/OPC during the electrocatalytic process, ex situ SEM and XPS analyses were employed. SEM images of V-NCO/OPC shown in Figure S13 exhibited that the spherical shape was well-maintained after the discharge and recharge process, which suggests excellent structural stability. In the case of the XPS spectra shown in Figure S14, overall peak shifts from high to low binding energy were observed in the Ni 2p and Co 2p spectra. In the O 1s spectrum, several broad peaks corresponding to oxygen vacancies () and absorbed water were observed in a pristine state. However, the peak related to bonding was increased after the recharge process, and the shape of the overall peaks became gradually sharper due to the dominance of . These phenomena are attributed to the formation of carbon oxidation because of the high charging potential, which is consistent with results in previous literatures [62–64]. When comparing the ZAB performances of various composite materials comprised of NCO and carbonaceous materials reported in previous studies, V-NCO/OPC exhibited superior electrocatalytic properties compared to those of composite materials reported in the literatures (Table S1). Based on these results, V-NCO/OPC can be considered a superior ZAB cathode material compared to noble metal benchmarks in practical applications, owing to its pore structures, accelerating mass transfer, and large active sites from oxygen vacancies.

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4. Conclusions

In summary, we developed open porous-structured carbon microspheres with a scaffold design containing micro/mesopores using an aerosol-based spray pyrolysis method. By using MgO and PS nanobeads as pore agents, OPC possessed micro/mesopores to regulate the crystal size when NCO nanosheets were loaded, as well as open macropores to facilitate mass transport and detachment of the O₂ bubble-blocking surface of the active site. Using the characteristics of OPC, ultrafine NCO nanosheets were uniformly decorated via a hydrothermal method and a subsequent oxidation process. The abundant micro/mesopores of OPC not only enable the uniform decoration of NCO nanosheets during the synthesis process but also inhibit their crystal growth. Moreover, to increase the number of active sites in the NCO nanosheets, oxygen vacancies were introduced into the NCO lattice via a partial reduction process using a NaBH₄ solution. Owing to the hybridization effect of each characteristic, V-NCO/OPC exhibited enhanced ORR and OER performances compared with noble-metal-based electrocatalysts. This study provides significant ideas for the fabrication of an efficient bifunctional oxygen electrocatalyst for ZAB based on pore design and defect introduction.

Data Availability

Data is available on request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Min Kim and Jeong Hoo Hong are the co-first authors of this paper.

Acknowledgments

This research was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. CAP22082-100).

Supplementary Materials

Additional supporting information may be found online in the Supporting Information at the end of the article. Experimental Section (continued). Supplementary Figure S1: XRD pattern of Mg-OPC. Supplementary Figure S2: (a) N2 gas adsorption and desorption isotherm and (b) pore-size distribution of OPC and PC. Supplementary Figure S3: XRD patterns of NCO/OPC and NCO/PC. Supplementary Figure S4: morphologies and SAED pattern of NCO/PC: (a) SEM image, (b) TEM image, (c) HR-TEM image (insert: fast Fourier transform electron diffraction pattern), and (d) SAED pattern. Supplementary Figure S5: (a) N2 gas adsorption and desorption isotherm and (b) pore-size distribution of NCO/OPC and NCO/PC. Supplementary Figure S6: TG curves of NCO/OPC and NCO/PC. Supplementary Figure S7: XRD pattern of V-NCO/OPC. Supplementary Figure S8: (a) UV-vis spectra and (b) Tauc plot of V-NCO and NCO. Supplementary Figure S9: EPR spectra of V-NCO/OPC and NCO/OPC. Supplementary Figure S10: the Nyquist plots of NCO/PC, NCO/OPC, V-NCO/OPC, and Pt/C under ORR condition. Supplementary Figure S11: cyclic voltammogram curves of (a) V-NCO/OPC, (b) NCO/OPC, and (c) NCO/PC, and (d) corresponding linear fitting of capacitive current. Supplementary Figure S12: methanol tolerance tests of V-NCO/OPC and Pt/C. Supplementary Figure S13: ex situ SEM images of V-NCO/OPC: (a) discharge and (b) recharge in ZABs. Supplementary Figure S14: ex situ XPS spectra of V-NCO/OPC: (a) Co 2p, (b) Ni 2p, and (c) O 1s. Supplementary Table S1: Zn-air battery performances of various composite electrocatalysts comprised of transition metal oxide and carbonaceous materials for air cathode reported in the previous studies. (Supplementary Materials)

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International Journal of Energy Research

Oxygen Vacancy-Rich NiCo2O4 on Carbon Framework with Controlled Pore Architectures as Efficient Bifunctional Electrocatalysts for Zn-Air Batteries

Abstract

1. Introduction

2. Experimental

3. Results and Discussion

4. Conclusions

Data Availability

Conflicts of Interest

Authors’ Contributions

Acknowledgments

Supplementary Materials

References

Copyright

Oxygen Vacancy-Rich NiCo₂O₄ on Carbon Framework with Controlled Pore Architectures as Efficient Bifunctional Electrocatalysts for Zn-Air Batteries