Effect of Cobalt Loading on the Performance and Stability of Oxygen Reduction and Evolution Reactions in Rechargeable Zinc-air Batteries

Abstract

The commercialization of rechargeable metal-air batteries is extremely desirable but designing stable oxygen reduction reaction (ORR) catalysts with non-noble metal still has faced challenges to replace platinum-based catalysts. The nonnoble metal catalysts for ORR were prepared to improve the catalytic performance and stability by the thermal decomposition of ZIF-8 with optimum cobalt loading. The porous carbon was obtained by the calcination of ZIF-8 and different loading amounts of Co nanoparticles were anchored onto porous carbon forming a Co/PC catalyst. Co/PC composite shows a significant increase in the ORR value of current and stability (500 h) due to the good electronic conductive PCN support and optimum cobalt metal loading. The significantly improved catalytic performance is ascribed to the chemical structure, synergistic effects, porous carbon networks, and rich active sites. This method develops a new pathway for a highly active and advantageous catalyst for electrochemical devices.

INTRODUCTION

Rechargeable zinc-air batteries have been widely studied owing to their high stability, low cost, non-explosiveness, and environmentally friendly properties.¹ The commercialization of zinc-air batteries has been limited because of sluggish kinetic reactions and poor stability.²⁻³ The electrochemical performance (performance and stability) has been improved by the appropriate selection of catalysts and their loading amount. Recently, noble metals including platinum (Pt), palladium (Pd), silver (Ag), and gold (Au), and their compounds are most widely used as active catalysts for ORR because of their low over-potential and high performance.⁴⁻⁵ Therefore, it is imperative to overcome these difficulties by developing non-noble metal catalysts for the widespread use of new energy conversion technologies.

Transition metal-derived materials and their oxides, sulfides, phosphides, and carbides have been shown superior catalytic performances for oxygen reactions due to their porosity, structural flexibility, and composition.⁶⁻⁷ Carbon-derived materials with transition metal atoms have attracted substantial attention for the preparation of porous nano-materials.⁸ The most widely used metal catalysts are cobalt-derived carbon materials, cobalt oxide nitrogen, and phosphorous-doped carbon.⁹ Effective ways for the fabrication of bifunctional catalysts include a combination of conductive substrates, abundant active sites and ensuring large surface area.¹⁰

Numerous strategies have been developed, but the inherent degradation of catalysts impedes their catalytic performance.¹¹ There is a need to improve the catalytic performance of zinc air batteries, because of their low cost and need to improve the sluggish kinetics for the ORR/OER. Therefore, a highly active and robust catalyst is desirable for enhanced performance of rechargeable zinc-air batteries. Metal-organic frameworks (MOF) are considered one of the most promising classes of electrocatalysts owing to their porosity, adjustable chemical properties, and structural diversities.¹² The formation of the three-dimensional porous structure of zeolitic frameworks can be featured by bridging the metal centers with imidazolate linkers referred to be the subclass of MOF.¹³

In general, the most desirable features of MOF are their potential use in the preparation of carbon-based materials. Because of the rich heteroatom and easily removable metal species through calcination at a higher temperature such as zinc, MOF can be used as a precursor to prepare porous carbon-based materials with rich active sites and high electrical conductivity.¹⁴⁻¹⁵ The prepared carbon-based material from MOF by removal of zinc species at a higher temperature exhibits higher catalytic activity and stability for ORR in an alkaline medium.¹⁶ The higher catalytic activity is attributed to conductive support, which facilitates electron transport during the catalytic reaction. The synergetic effect and rich active sites of porous carbon materials derived from MOF can enhance their electrochemical properties and stability.

In this work, we prepared a series of cobalt catalysts with various loading amounts on the porous carbon derived from zeolitic-imidazole frameworks (ZIF-8) composites by the calcination of ZIF-8 at a high temperature of 1050℃ under a nitrogenous atmosphere. The porous carbon, which was referred to as PC, was obtained after the calcination of the ZIF-8 composite. By sequential deposition method, Co nanoparticles can be easily anchored onto PC forming a Co/PC catalyst used in air electrodes of rechargeable zinc-air batteries. Co/PC composite shows a significant increase in the ORR value of current due to the electronic conductive PC support. The synthesized Co/PC is in the form of a layered structure, which endows an efficient pathway for the electrolyte and ion transport. Half-cell measurements reveal that Co/PC is stable for 500 h under the current density of 20 mA cm^-2 for both the ORR and OER. The synthesized Co/PC demonstrates a facile approach for the rational design of effective MOF-based electrocatalysts for rechargeable zinc-air batteries with adequate catalytic activity and stability.

EXPERIMENTAL

The ZIF-8 composites were synthesized using a similar procedure as reported previously.¹⁷ The mixture of zinc nitrate hexahydrate (Zn (NO₃)₂. 6H₂O, Sigma-Aldrich, 4.39 g, 2.31 × 10^-2 mol) and 2-methyl imidazole (C₄H₆N₂, Sigma-Aldrich, 9.74 g, 11.5 × 10^-2 mol) in 300 mL of methanol (Dae-jung Chemicals and Metals) were prepared separately. These two prepared solutions were mixed under stirring at room temperature. The white color crystals were obtained after stirring via centrifugation and washed with methanol several times. The ZIF-8 crystal was collected after drying in a vacuum oven. The obtained ZIF-8 crystals were calcined under the nitrogenous atmosphere at a higher calcination temperature of 1050℃ to obtain porous carbon (PC).

First, PC (0.8 g) was dissolved in 300 mL of distilled water. Then, cobalt chloride hexahydrate (CoCl₂. 6H₂O, Sigma-Aldrich) was dissolved in distilled water (680 mL) separately. These two solutions were mixed under magnetic stirring. On the other hand, sodium borohydride (4 times higher molar ratio than Co precursor) was dissolved in distilled water (680 mL) with the addition of sodium hydroxide. Then, the sodium borohydride (Alfa Aesar) solution was added dropwise to the mixed solution of PC and cobalt chloride hexahydrate under magnetic stirring. After dropping, the solution was stirred for 6 h and then the resulting solution was filtered and dried in an oven. The obtained crystals were referred to as Co(X)/PC (Scheme 1). The number X in the parentheses indicates the amounts of cobalt (wt.%) that were loaded on the PC to observe the effect of loading metal on the performance and stability.

Scheme 1. Synthetic procedure of Co/PC catalysts for oxygen reduction and evolution reactions.

The crystallinity of PC and Co/PC were analyzed using X-ray diffractometer (XRD) with Cu Kα radiation source with wavelength of 0.154 nm in the 2θ range of 10-80º. The morphological alteration in PC after cobalt deposition was observed by using a scanning electron microscope (SEM, Hitachi, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL, Japan), operating at an accelerating voltage of 200 kV. The presence of cobalt was determined by energy-dispersive X-ray spectroscopy (EDX) analysis. The porosity and surface area of these catalysts were measured via Brunauer– Emmett–Teller (BET, BELSORP-max), and the adsorption/desorption profiles were determined by the BJH (Barrett Joyner Halenda) method. Thermogravimetric analysis (TGA, SDT Q600) was performed under atmospheric conditions with a heating rate of 10℃/min from 100 to 1000℃ to determine the carbon contents. The nature of carbon material was analyzed by the Raman spectroscopy (NT-MDT spectrometer) with an excitation wavelength of 532 nm.

All electrochemical measurements of PC and Co/PC were performed using potentiostat/galvanostat (WBCS 3000, WonATech, Seocho, Seoul) at a scan rate of 1 mV/s. The catalyst’s performance was measured using a three-electrode system in 8 M KOH at room temperature. The reference and counter electrodes used during the three-electrode system were zinc wire (Alfa Aesar, Lancashire, UK) and Pt mesh, respectively. The working electrode in the three-electrode system was prepared a similar method reported previously.¹⁸ The mixture of active material (catalyst), Ni powder (INCO 255; Vale, Toronto Canada), and carbon black (CB, Vulcan, XC-72; Massachusetts, USA) were dispersed in a solution containing deionized (DI) water and ethanol. The subsequent slurry was stirred using sonication, followed by the addition of polytetrafluoroethylene (PTFE, 60 wt.%, Aldrich, St. Louis, MO, USA) as a binder material. The resultant slurry was dried overnight to evaporate the solvent and then isopropyl alcohol was added to the dried material. The material was rolled to form a sheet having a thickness of 500 µm, then attached to the carbon sheet (GDS 1120, Ballard; B.C. Canada). Thus, the electrode was formed by placing the squeezed sheets in the hot press at 350℃. ¹⁹

The linear sweep voltammetry for ORR and OER was performed using half-cell measurement. The three-electrode cell system used 8 M KOH solution as the electrolyte, while the reference and counter electrodes were Zn wire (0.385 V vs. RHE) and Pt mesh, respectively. The charging/discharging of the Co/PC was measured using a durability test. The voltage was measured at a rate of 20 mA cm^-2 against Pt wire (reference electrode) for long-term operation.

RESULTS AND DISCUSSION

The ZIF-8, a sub-class of three-dimensional metal-organic framework (MOF), consists of imidazolate (organic linker) and transition metal (Zn²⁺) ions forming tetrahedral frameworks similar to zeolite topologies. The fascinating chemical and mechanical stability as well as the excellent electrical conductivity of ZIFs-derived material make their extensive application in the electrocatalytic field. The synthesis of a single ZIF material by pyrolysis cannot retain many merits. Therefore, Co/PC was synthesized by cobalt deposition on PC derived from ZIF-8 crystals. The carbon layer wrapped by the cobalt particles of Co/PC creates a pathway for the gas to reach the active sites for the oxygen reactions. Furthermore, this porous nanomaterial (Co/PC) exhibits a high specific surface area and large pore volume.

The crystallinity of synthesized ZIF-8 crystals was measured by the XRD pattern. The PC derived from the heat treatment of ZIF-8 crystals reveals two peaks at ~25º and 44º, which are assigned to graphitic carbon, as shown in Fig. 1(a). The XRD pattern of Co/PC is shown in Fig. 1(b). Co (2.5)/PC had sharp peaks which are assigned to ZnO (JCPDS No. 36-1451). After calcination, the residue of zinc remained in PC. The peaks for ZnO disappeared or their intensities dramatically decreased because most of the Zn residues were dissolved by the addition of sodium hydroxide during the deposition process of Co particles. The cubic Co₃O₄ (JCPDS # 43-1003) was not detected in Co/PC.²⁰ We assumed Co particles were formed on PC as an amorphous state. The characteristic peak at ~25.5º (002) can be assigned to the graphitic carbon.²¹

Figure 1. XRD of (a) ZIF-8 and PC and (b) Co/PC catalysts.

As proven by the XRD pattern, SEM images of Co/PC in Fig. 2(a-e) show an irregular 3D structure of carbon, which is beneficial for rapid mass transfer, the convenient pathway for electron transmission, and exposure of the active layer.¹⁶ For evaluating the metal effect on PC, the agglomeration increases as we increase the number of cobalt particles in the PC, as shown in SEM images. Furthermore, we have not found any noticeable difference in the morphology by altering the cobalt loading. Also, TEM images of Fig. 2 (f,g) show Co/PC which are composed of plate-shaped particles with amorphous metal or metal oxide. To verify the presence of various metals and their dispersion in the catalysts, EDX mapping (Fig. 2(i-l)) was measured which reveals the homogenous distribution of cobalt and carbon.

Figure 2. SEM images of (a) PC, (b) Co(2.5)/PC, (c) Co(5.0)/PC, (d) Co(7.5)/PC, and (e) Co(10)/PC catalysts, (f-g) TEM images of Co(5.0)/PC, and (h) STEM image of Co(5.0)/PC, with their corresponding EDX mapping (i) C, (j) Co, (k) O, and (l) Zn.

The amounts of Co loading in Co/PC were measured by EDX analysis, as shown in Fig. S1 (supporting information). As expected, the amount of cobalt content increases with the increase in loading amount. EDX analysis suggests the presence of Co, C, and O species in the Co-based PC catalysts. The EDX line making of Co/PC reveals the uniform distribution of Co and C, which may be beneficial for the catalytic active sites for the ORR. The pore size distribution and specific surface area were measured by nitrogen adsorption-desorption isotherms, as shown in Fig. 3(a). The hysteresis loop observed by the adsorption-desorption isotherm belongs to type II curves for Co/PC and indicates mesoporous characteristics of the material.

Figure 3. (a) Nitrogen adsorption/desorption isotherm of Co (5.0)/PC (V_a indicates the pore volume, and P/Po indicates the relative pressure), and (b) TGA plots of Co/PC catalysts.

The specific surface and total pore volume of PC are 806.43 m²/g and 0.3433 cm³/g, while those of Co(5)/PC are 329.4 m²/g and 0.273 cm³/g, respectively. The decrease in the surface area and pore volume was caused by the loading of nonporous and high-density Co NPs. The mesoporous structure and higher surface area enable gas and electrolytes to diffuse and penetrate toward active sites. The high surface area might relieve the enormous volume changes during the charging-discharging processes.²² The nitrogen adsorption-desorption isotherms of all the Co-derived catalysts were presented in Fig. S2 (supporting information). Thermogravimetric analysis (TGA) was performed under atmospheric air, as shown in Fig. 3(b). The weight loss of around 80℃ was attributed to the evaporation of water molecules. The further weight loss between 380 and 580℃ was ascribed to the decomposition of the ligand and collapse of the structure. The cobalt contents in Co/PC were calculated from TGA as shown in Table S1.

The presence of graphitic carbon was further confirmed by the Raman spectrum (Fig. S3). The presence of D (sp³) and G (sp²) bands at ~1365 and ~1588 cm⁻¹ were attributed to A_1g and E_2g symmetries for graphitic carbon. Furthermore, the presence of (002) in the XRD pattern ascribes graphitization.

The effect of cobalt loading on the electrochemical properties was analyzed by linear sweep voltammetry (LSV), using a three-electrode system and presented in Fig. 4 after 20^th cycles. Furthermore, the comparison of the current density of Co/PC catalysts with Co powder and Co₃O₄ was presented in Fig. S4. The crucial factor for the satisfactory performance of the battery is highly dependent on the choice of electrolyte material, which plays a dominant role in the determination of rechargeability and its electrochemistry.²³ KOH has been considered a primary electrolyte owing to its high ionic conductivity, low viscosity, and excellent performance. Furthermore, 8 M KOH solutions are used as electrolytes due to low corrosion and high conductivity.²⁴

Figure 4. Linear sweep voltammetry of Co/PC catalysts for ORR and OER at the 20^th cycle.

Fig. 4 shows the comparison of various Co catalysts which reveals that Co(5)/PC has maximum current density for both ORR and OER. The current density for the ORR at 0.5 V for Co(5)/PC and Co(10)/PC was 350 mA cm^-2 and 168 mA cm^-2 respectively. Similarly, the current density for OER at 2.4 V for Co(5)/PC and Co(10)/PC were 280 mA cm^-2 and 151 mA cm^-2 respectively. The Co(5)/PC shows the highest current density which might be due to an increase in the active sites caused by the synergistic effects. Electrical conductivity is another factor that improved the current density and is mainly dependent on graphitic carbon or porous carbon network. The catalytic performance was verified several times and the average results of six different batches were presented in Fig. 4. When comparing the Co/PC with Co powder and Co₃O₄, the increase in current density was almost 5~6 times higher, revealing their significance in the rechargeable zinc-air batteries.

To understand the effect of Co loading on a PC, the stability test was performed for almost 520 h under a constant current density of 20 mA cm^-2 for 2 h charging/discharging. The stability profiles of Co/PCN were shown in Fig. 5. The voltage gap between ORR/OER for Co(2.5)/PC was increased to 1.37 mV per cycle while the Co(7.5)/PC reveals a significant decrease in the voltage gap of 2.72 mV per cycle. On the other hand, the increase in voltage gap for Co(5.0)/PC and Co(10)/PC were 0.82 and 1.67 mV per cycle. This comparison reveals the clear difference for the Co loading on the stability. The Co(5.0)/PC shows much better performance as compared to others and negligible voltage fading during the operation of 520 h. The comparison with other materials reported previously in the literature has been listed in Table S2.²⁵

Figure 5. Stability profiles of (a) Co(2.5)/PC, (b) Co(5.0)/PC, (c) Co(7.5)/PC, and (d) Co(10)/PC respectively.

The electrode was tested before and after the stability test of 520 h to verify the crystal structure variation in the XRD patterns as shown in Fig. S5. The XRD pattern reveals the presence of some nickel hydroxide peaks after the long-term stability test, which might be because of the conductive material in the electrode formation. Furthermore, no peak related to cobalt hydroxides has been detected. The Co/PC catalysts show a significant approach to the rational design of ZIF-derived catalysts with excellent performance and stability.

CONCLUSION

The Co/PC catalysts were synthesized by the sequential reduction method using various metal loadings. The physical structure of Co/PC endows an efficient pathway for the electrolyte and ion transport, demonstrating good performance for ORR and OER in alkaline solution as air electrodes in rechargeable zinc-air batteries. The Co/PC reveals excellent performance having current density of 350 mA cm^-2 for the ORR at 0.5 V after 20^th cycles as a cathode material in rechargeable zinc-air batteries. The durability of Co/PC was measured for 500 h under the current density of 20 mA cm^-2 for 2 h charging/discharging and practicality of the catalyst is highlighted by the admirable performance of Co/PC. The optimum loading amount of cobalt on porous carbon enhances the performance and stability for both the OER/ORR. This reveals that metal loading is significant for better performance and stability. In short, the Co(5.0)/PC shows significant performance for OER/ORR as well as catalytic stability.