Cathodes with MnO₂ catalysts for metal fuel battery

Abstract

A series of cathodes with MnO₂ catalysts of metal fuel battery were prepared. The catalyst slurry was treated by ultrasonic dispersion under the ultrasonic time of 20 min, 40 min and 60 min. The cathodes were also dried with the temperature of 90°C, 120°C and 150°C. Besides, the microstructures of the cathodes and discharging performance were investigated. The results indicated that the ultrasonic time and drying temperature had a remarkable influence on the electric current densities, but had little effect on the open-circuit voltage. The effects of oxygen on the current density and voltage of cathode were also studied, and it was found that the method of blowing oxygen to cathode could increase the current density of the metal fuel battery.

Keywords

metal fuel battery / cathode / current density / ultrasonic dispersion / oxygen supply

Introduction

The traditional fossil energy is gradually depleted, and the negative impact of burning fossil fuels on the environment is intensifying day by day. The current society is in a transition phase from fossil fuel to clean energy. Several clean energy sources like solar, wind, hydropower, natural gas, and hydrogen energy have been getting rapid development. It is important to develop safe, flexible, efficient, and even suitable ways for large scale energy storage technologies [¹]. Battery is one of the most popular methods to store energy due to its flexibility and discharge efficiency [²,³]. Lithium-ion batteries, which have an energy density of 100‒200 Wh/kg as well as good energy efficiency, are especially widely used in electric vehicles (EVs) [⁴,⁵]. However, the lithium-ion batteries still cannot fulfill the future demand of EVs for high energy density and high charging rate. Hydrogen fuel cell has also become one of the research hotspots due to its many advantages including the pollution-free combustion products, fast fuel injection rate, and so on [⁶,⁷]. However, several disadvantages, such as the high cost of hydrogen, the high price of platinum catalyst, and the unsolved safety, limit the development and popularization of hydrogen fuel cell [8‒10].

Among these energy storage technologies, in recent years metal fuel batteries represented by metal air battery have gained great interest due to its high energy density and capacity, low cost, high safety, and excellent discharging properties [¹¹‒¹³]. Magnesium (Mg), aluminum (Al), zinc (Zn), lithium (Li) and other metals can be used as metal fuels, which can release electric energy with the participation of oxygen and water solutions. Metal fuels are abundant on the earth and could be reusable. The battery capacity and energy density are high, the discharge voltage is stable, and the storage life of the metal fuel is quite stable and safe. They are suitable for large scale energy storage, and can also be used as small battery.

It was reported that an Al battery of 100 kg was fabricated and could drive an EV to over 3000 km, and many research groups have been conducting researches to increase the capacity and lifetime of metal fuel battery systems [¹⁴,¹⁵]. However, there are still many science and engineering problems which should be solved.

The cathode is an essential part for metal fuel battery, which is generally composed of a current collector and a catalytic active layer. Sometimes a gas diffusion layer is also needed. Current collectors are made by the metal mesh or metal foam, which can collect the electrons and connect to the external circuit [¹⁶]. The catalytic active layer is generally composed of catalysts, carbon material and a binder. The oxygen reduction reaction (ORR) takes place in the catalytic active layer [¹⁷]. The gas diffusion layer is generally composed of the carbon material and the binder such as PTFE, which oxygen can pass through but water cannot. With regards to the cathode, the low efficiency of the ORR and short service lifespan are the main barriers for the application of mental fuel batteries.

The most common catalysts are noble metal catalysts including platinum (Pt), palladium (Pd), gold (Au), and so on [¹⁸]. However, scarce resources and the high cost of noble metal catalysts limit their large-scale application. Transition metal oxides catalysts like MnO₂ have prominent advantages such as high catalytic activity and low cost because the complex clusters of transition metal oxides could be the catalytically active centers of oxygen evolution [¹⁹,²⁰]. Additionally, the variable valences and abundant structures of transition metal oxides can provide many opportunities to develop non-precious metal catalysts. Therefore, up to the present, transition metal oxides have been promising catalysts for large-scale application [²¹].

In this paper, MnO₂ was used as catalyst to prepare the cathode for the metal fuel battery. A series of cathodes were fabricated by using different preparation technical parameters. Ultrasonic dispersion is an important procedure to make all particles fully mixed and play the better effect of the catalyst. While overlong ultrasonic treatment does not always play a positive role according to other research results [22‒24]. Drying procedure for the catalytic and diffusion layer is also necessary to evaporate the liquid and to leave micropores, which makes oxygen molecules penetrate to attend ORR. The catalyst slurry was treated by ultrasonic dispersion at different ultrasonic times. The cathodes were also dried with different temperatures. The microstructures of the cathodes and discharging properties were investigated. To study the effect of oxygen on the current density and voltage, additional oxygen was added to by using several test methods.

Experimental

Preparation of cathode

The cathode with MnO₂ catalyst was prepared under different preparation conditions. The catalytic layer contained MnO₂, carbon black, activated carbon, and PTFE. The waterproof diffusion layer contained activated carbon, carbon black, and PTFE. Ethanol was used as the dispersant and pore-forming agent in the catalytic layer and diffusion layer. The porous nickel was used as collector electrode. The diffusion layer was first coated on the porous nickel, and then the catalytic layer was sequentially coated on the diffusion layer. Therefore, the cathode contained three layers including the porous nickel layer, the diffusion layer, and the catalytic layer.

During the preparation process of catalytic materials, the mixture of MnO₂, carbon black, activated carbon, and PTFE was treated by using the ultrasonic dispersion method. Three kinds of catalytic mixtures were prepared with an ultrasonic dispersion of 20 min (U-20), 40 min (U-40), and 60 min (U-60), respectively. All the electrodes were dried at 120°C for 4 h and then cooled at room temperature.

The other three cathodes were prepared according to the above preparation process. Mixtures of catalytic materials were prepared with an ultrasonic dispersion of 20 min. Three cathodes were dried at 90°C (T-90), 120°C (T-120), and 150°C (T-150) for 4 h and then cooled at room temperature, respectively.

Scanning electron microscope

Scanning electron microscope (SEM, FEI Quanta 200) was used to observe micro-morphologies of all cathode samples, and the energy-dispersive X-ray spectrometer (EDS) was used to characterize the chemical compositions of catalytic layer.

Discharging performance tests

The cathodes prepared in this work and magnesium plate comprised the positive pole and negative pole of the homemade cell test system, respectively. The distance between the positive pole and negative pole was 10 mm and 5% of KCl water solution was used as electrolyte. The circuit current and open-circuit voltage were tested by using the cell test system with the corresponding test mode. The effective working area for both electrodes was 25 cm², and the current densities were calculated and compared in this work.

The effect of oxygen on the current density and voltage of different cathodes was studied by using three testing methods. The first testing method was in the air without adding extra oxygen. The second testing method was that the oxygen gas was pumped into the 5% KCl electrolyte, and the third testing method was to blow the oxygen to the cathode at a flow rate of 50 mL/min. The current densities and open-circuit voltage were tested.

Results and discussion

Figure 1 shows the SEM morphology and EDS composition analysis of the catalytic layer of cathode U-20. It can be seen from Fig. 1 that the white particles disperse sporadically in the catalytic layer, and the EDS composition analysis reveals that these particles are MnO₂. The original size of MnO₂ powder was about 2 mm, while in the catalytic layer the size of partial MnO₂ particles had grown to 50‒80 mm. This phenomenon indicated that MnO₂ particles agglomerated to some extent even treated with ultrasonic dispersion. The black particles with sizes of 20‒50 mm were also discovered in the catalytic layer, and the EDS composition analysis revealed that these black particles were activated carbon.

The carbon blacks were added to the catalytic layer during the preparation process, but there was no carbon black obviously observed in the SEM morphology. The particle size of carbon black was about dozens of nanometers, coupled with the ultrasonic treatment. Therefore, it was difficult to find carbon black particles in the mixture with PTFE by using SEM. The substrate composition in the white rectangular as shown in Fig. 1 was analyzed by using EDS. The elements includes fluorine, carbon, and oxygen, which were the mixtures of PTFE binder and carbon blacks. The results indicated that the PTFE emulsion and carbon blacks were fully mixed during the process of ultrasonic dispersion.

Figure 2 illustrates the SEM morphologies of the catalytic layers of cathodes U-40 and U-60. There were more agglomerate MnO₂ and activated carbon particles in catalytic layers. The sample U-60 had more obvious MnO₂ agglomerations, and the size of partial MnO₂ agglomerations reached about 100 mm, which meant that overlong time of ultrasonic treatment could intensify the MnO₂ particles agglomerating.

Yu et al. discovered the agglomeration phenomenon of catalyst particles in PTFE after long time ultrasonic treatment [²²]. Yang et al. also reported that the size of TiO₂ particles in water would grow larger after 15 min of ultrasonic treatment [²³]. Ultrasonic treatment of long time could cause temperature increasing of the mixtures due to the fact that the heat generated by ultrasound concussion could not diffuse rapidly to external environment [²⁴]. Besides, the fasten motion of catalyst particles would increase the collision chance among the particles. Therefore, some dispersed particles would re-agglomerate to become larger particles.

Figure 3 displays the variation of the current density and voltage as a function of time for three cathodes U-20, U-40, and U-60. It can be observed from Fig. 3(a) the current densities of three cathodes ranges from 3 to 7 mA/cm². The current density of cathode U-20 was about 6.7 mA/cm². The current density of cathode U-40 was about 4.3 mA/cm², which was less than the current density of U-20. The current density of U-60 cathode was only about 3.1 mA/cm², which was obvious lower than that of the other two cathodes.

Figure 3(b) depicts the voltage variation as a function of time for three cathodes U-20, U-40, and U-60. The voltage values and variations along with time were similar for all cathodes. The voltage for all cathodes was approximately 1.6 V, which meant that the time of ultrasonic treatment had no evident effect on the voltage of the cathode.

According to the analysis from Figs. 1 and 2, some dispersed MnO₂ particles would re-agglomerate to become larger particles with the increase of the time of ultrasonic treatment. Therefore, the large agglomerate MnO₂ particles as electrocatalyst would not work adequately due to the lower specific surface area compared to small particles. Additionally, the ethanol as the dispersant would volatilize easily with the rising temperature caused by ultrasonic treatment. The content of ethanol in the catalyst mixtures would decrease along with increasing ultrasonic dispersion time. Therefore, the porosity formed by ethanol volatilization during the catalytic layer drying process would correspondingly decrease, which resulted in the lower current without enough oxygen supplement.

Drying is an important procedure to prepare high performance cathodes [²⁵]. Drying at a proper temperature could evaporate liquids such as water and ethanol, and micropores would be left in the catalytic and diffusion layer. On the other hand, the microstructures of the cathode would be restructured during the drying process. The micropore density, size, and the microstructure play important roles in determining the property of the cathode.

Figure 4 exhibits the SEM morphologies of three cathodes T-90, T-120 and T-150. It can be seen from Fig. 4(a) that there are many silk-like PTFE binders. With the increase of dry temperature to 150°C, the silk-like PTFE binders disappeared. However, it was difficult to observe the specific difference between T-120 and T-150 samples from SEM morphologies.

Figure 5 demonstrates the variation of current density and voltage as a function of time for three cathodes T-90, T-120 and T-150. It can be seen from Fig. 5(a) that the current densities of three cathodes range from 4 to 7 mA/cm². The current densities of T-90 and T-150 cathodes were about 4.6 mA/cm², which was lower than that of T-120 cathode. It can be concluded that the current density is related, to some degree, to the temperature of drying. The voltage variation as a function of time for three cathodes T-90, T-120 and T-150 is shown in Fig. 5(b). It can be seen that the voltage variations along with time for three cathodes are similar and the average voltage is about 1.6 V.

For the drying temperature of 90°C, water and ethanol in the catalytic and diffusion layer could not evaporate sufficiently because of the lower drying temperature, and there was not enough micropores formed. So the permeability of the cathode was not good enough to let more oxygen attend electrochemical reaction. With the increase of the dry temperature, water and ethanol could evaporate more easily and form more micropores and larger specific surface area in the catalytic and diffusion layer. Therefore, more oxygen could permeate the diffusion layer to catalytic layer to attend electrochemical reaction, and exhibited higher current density like the drying temperature of 120°C. Further increasing the drying temperature, higher temperature would not be in favor of forming pore structures and increasing the specific surface area [²⁵], which resulted in the current density decreasing.

The experimental results of the effect of oxygen on the current density and voltage were plotted in Fig. 6. It can be found that oxygen has different influences on the current as shown in Fig. 6(a). The current density for pumping oxygen into the KCl electrolyte was about 6.7 mA/cm², which was similar to that tested in the air without adding extra oxygen. For the method of blowing oxygen to cathode, the current density increased to 7.9 mA/cm², which was about 18% higher than those of the other two testing methods.

The voltage variation as a function of time for three testing styles is shown in Fig. 6(b). It can be seen that the voltage values and variations along with time for three testing methods were similar and the average voltage was about 1.6 V, which meant the oxygen had no evident effect on the voltage.

The catalytic active layer consists of MnO₂ catalyst, carbon material, and PTFE binder, and is the place where the oxygen reduction reaction takes place [²⁶]. The three phase interface of solid/liquid/gas is important for ORR process, and more oxygen supply would promote ORR and generate high electric current [²⁷]. This test indicated that blowing oxygen to cathode is effective to supply more oxygen to the three phase interface of solid/liquid/gas, which increased ORR. While for the method of pumping oxygen into electrolyte, the oxygen has quite low solubility in aqueous solution, and there was not more oxygen in the three phase interface for ORR.

Conclusions

A series of cathodes with MnO₂ catalysts of metal fuel cell were prepared with different synthesis parameters. The agglomerate MnO₂ particles were found in the catalytic layer. Increasing the ultrasonic treatment time from 20 min to 60 min for catalysts slurry decreased the current densities caused by the MnO₂ agglomeration and ethanol volatilization. The cathode with a drying temperature of 120°C exhibited the relatively high current density about 6.7 mA/cm², and excessive low or high temperature was not in favor of improving the current density. Blowing oxygen to cathode could increase the current density compared to the method of pumping oxygen into the KCl electrolyte or no extra oxygen supply. The open-circuit voltage for all tests was about 1.6 V, and the preparing methods of cathodes in this work had little effect on the voltage.

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