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ACTS-P00358

EXPERIMENTAL STUDY ON REDUCTION OF POROUS IRON OXIDE

FOR SOLID OXIDE IRON-AIR BATTERY

Hiroshi Iwai1 *, Kotaro Itakura1, Motohiro Saito1, Masashi Kishimoto1, Hideo Yoshida1

1Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8540, Japan

*Presenting and Corresponding Author: iwai.hiroshi.4x@kyoto-u.ac.jp

ABSTRACT

Large-scale rechargeable batteries are receiving remarkable attention as energy storage systems. A solid oxide iron-air battery is a new concept rechargeable battery consisting of solid oxide electrochemical cells and porous iron as a redox metal. It is still in its early stage of development and the feasibility of the concept is intensively studied from many different aspects. Understanding and enhancing the slow reduction process of iron oxide is regarded as one of the key issues to realize this system. In this study, we experimentally investigate the reduction process of porous pellets of magnetite (Fe3O4) in a temperature range from 450 to 550 °C to obtain fundamental information on high temperature redox reaction of iron. Starting from iron oxide powders having different particle diameters, we prepared three types of test pellets. Diluted hydrogen was supplied to the test pellets at elevated temperature and the time course of the exhaust gas composition was monitored by using a mass spectrometry. A simple one-dimensional model which considers both the gas diffusion through the pore phase and the reaction was developed and applied to analyze the experimental data. It was found that among the three types of the pellets tested in this study, the pellet made of the largest particle showed the best performance. It is attributed to the combined effects of higher gas diffusivity and reaction rate constant associated with the morphology change of the porous pellet. KEYWORDS: Iron oxide, Reduction, Solid oxide iron-air battery, Experiment

1. INTRODUCTION

A solid oxide fuel cell has an advantage of high power generation efficiency even in systems with a relatively small capacity. Intensive research and development are underway with variety of possible applications. Recently, much attention has also been paid to its reverse reaction to use the cells as electrolyzer. In the electrolysis mode, steam is electrochemically decomposed into hydrogen and oxygen while consuming electrical energy. The high potential of a solid oxide electrochemical cell (SOEC) as a reversible energy conversion device extended its possible applications to energy storage systems [1-4]. As one of the effective utilization methods of reversible SOECs, a rechargeable battery consisting of an SOEC and a redox metal is attracting attention [5]. Two reversible reactions simultaneously proceed at two different locations in the system; a reversible electrochemical reaction occurs at the SOEC and a reversible redox reaction occurs at the redox metal. Hydrogen/steam mixture gas plays a key role as a carrier of oxygen atoms connecting the two simultaneous reactions during both the charge and discharge operations. Iron (Fe) is a typical choice for the redox metal and the system is referred to as a solid oxide iron-air battery (SOIAB). During discharge operations, Fe is oxidized by steam to produce iron oxide and hydrogen. The generated hydrogen is consumed at the SOEC to generate electricity. During charge operations, hydrogen is produced at the SOEC by decomposing steam. The generated hydrogen reduces the iron oxide into Fe, which is to be used in the subsequent discharge operation. Because this battery is a new concept and still in the early stage of development, the feasibility of the concept is being intensively studied from many different aspects [6-8]. To develop a reliable discussion, there are needs for fundamental information particularly on high temperature redox reaction of iron. In this study, we experimentally investigate the reduction process of a porous pellet of iron oxide. An extended unreacted core model is applied to evaluate the combined effects of the gas diffusion and reaction on the reduction process.

Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea

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Fig. 1 Conceptual diagram of solid oxide iron-air battery.

2. CONCEPT OF SOLID OXIDE IRON-AIR BATTERY

Fig. 1 schematically shows the concept of the battery. Iron powder is packed at the bottom of a container. The top of the container is capped with an SOEC with its fuel electrode facing inside. The container is filled with the mixture gas of hydrogen and steam. The air electrode is exposed to the ambient air. In a discharge operation, the SOEC acts as a fuel cell. Hydrogen is consumed at the fuel electrode while steam is generated. The generated steam diffuses to the packed iron and oxidizes the iron into iron oxide. The generated hydrogen diffuses to the SOEC for further power generation. In a charge operation, all the above reactions proceed in the reverse direction. The SOEC acts as an electrolyzer. The iron oxide is reduced by hydrogen. The reactions at the SOEC and iron can be summarized as Eq. (1) and Eq. (2), respectively. The reactions move to right in discharge operations and to left in charge operations.

OHO21H 222 (1)

2432 4HOFeO4H3Fe (2)

Among these reactions, the reduction of Fe3O4 is reported to be the slowest reaction. Understanding its detailed process is important and we perform a fundamental experiment on this reaction. Note that when iron is oxidized, the iron oxide can be wustite (FeO), magnetite (Fe3O4), or hematite (Fe2O3) depending on the temperature and oxygen partial pressure. According to the potential diagram of the Fe-O system, the dominant redox reaction is between iron and magnetite under operation conditions assumed in this study.

3. EXPERIMENTAL METHOD

3.1 SAMPLE PREPARATION A wide variety of Fe2O3 fine power is commertially available. We employed Bayferrox 110M, 160M and 180M (LANXESS) as the starting materials. The technical data sheets show that the mean diameters of the Fe2O3 particles in the three starting materials are 90, 400 and 700 nm, respectively. The Fe2O3 powers were first reduced into Fe by supplying a mixture gas of H2 : Ar = 40 : 60 ml/min at 500 oC for 15 hours. The obtained Fe powders were then oxidized into Fe3O4 by supplying a mixture gas of H2O : Ar = 40 : 60 ml/min at 500 °C for 15 hours. Test pellets of Fe3O4 were made by pressing. Table 1 summarizes the porosity and median pore diameter of the pellets evaluated by mercury intrusion porosimetry. Hereafter, we use the particle diameter of the starting material as the names of the test pellets: 90 nm, 400 nm and 700 nm.

Table 1 Porosity and pore diameter of the test pellets. Particle diameter

of starting material Porosity Median pore diameter

090 nm 0.59 087 nm 400 nm 0.56 170 nm 700 nm 0.55 270 nm

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Fig. 2 Experimental apparatus and test section.

Fig. 3 Reaction pocess model.

3.2 EXPERIMENTAL APPARATUS AND CONDITIONS Fig. 2 shows the experimental appratus. The test section is a cylindrical metal block. One end of it is half hollowed with an inner diameter of 10 mm. The Fe3O4 powder was put in the bottom of the hollow part and pressed at 7 MPa for 1 minute to form a cylindrical pellet. The amount of Fe3O4 powder was controlled by measuring its weight so that the thickness of the pellet became 3 mm after the pressing. Note that only one side of the pellet is exposed to the gas phase. The test section is connected to the gas flow pipes and placed in an elecric furnace. A mixture gas of H2 and Ar was supplied to the test section. The composition and flow rate of the supply gas was precisely controlled by using mass flow controllers. After removing steam content at the cold trap, the exhaust gas composition was measured by a mass spectrometry with a time interval of 10 s. The reaction temperature was kept constant during each experiment at 450, 500 or 550 °C by controlling the electric furnace. The H2 molar fraction of the suply gas was varied in a range of 20 - 60 %, while keeping the total flow rate constant at 100 sccm. When the exhaust gas comosition reached to that of the supply gas, we judged the ruduction process was completed.

3.3 DATA REDUCTION Fig. 3 schematically shows the model we employed to evaluated the reduction process. It is an extened version of the unreacted core model. L and r are the thickness and radius of the pellet, respectively. In this model, the reduction process is assumed to be one-dimenasional. When we start to supply H2, the reaction starts at the surface of the pellet. The reaction front gradually proceeds from the surface to the bottom of the pellet until the reaction stops when it reaches to the bottom. H2 is supplied to the reaction front by diffusion through the pores. For simplisity, we assume the bulk shape of the pellet does not change. We also assume a quasi-steady state of the gas diffusion. Its characteristic time scale, L2/Deff, is approximately 5 s, (L = 3mm, Deff m2/s) and is much shorter than the total reaction time in this study ( > 5000 s). Because the Ar flow rate is constant in each experiment, we can calculate the H2 consumption rate at the test section from the difference of the gas composition between the supply gas and the exhaust gas. By integrating the H2 consumption rate, we can obtain the reduction rate of the pellet, , that is defined as the ratio of the amount of consumed H2 to the amount of H2 required to completely reduce the pellet. The reduction depth, X, in Fig. 3 is obtained as L. Using the molar density of Fe3O4 in the pellet, M, the amount of remaining Fe3O4 in the pellet, NM, is expressed as follows.

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XLrN 2MM (4)

From Eq. (2), NM is also related to the amount of the H2 consumption, NH2, as follows.

dt

dNdt

dN 2HM

41 (5)

The negative sign on the left-hand side appears because NM shows the remaining Fe3O4. H2 consumption should balance with its supply. H2 is supplied to the reaction front by quasi-steady state 1-D diffusion. Employing Fick’s law for simplicity and assuming a uniform total pressure, the H2 consumption rate can be expressed as follows.

X

CCDrdt

dN X,HHeff2

H 222 (6)

CH2 and CH2, X are the H2 concentrations at the surface and the reaction front, respectively. Deff is an effective diffusion coefficient. Assuming the reaction rate is proportional to the local concentration and introducing reaction rate constant, kr, the H2 consumption rate can also be expressed as follows.

XrCkrdt

dN,H

2H2

2 (7)

Combining Eqs. (4) – (7) and integrating from t = 0 to an arbitrary time, we obtain Eq. (8). It can be rearranged as Eq. (9). By plotting the left-hand side of Eq. (9) against X, the experimental data show a liner distribution. From its slope and intercept, we can separately obtain the effective diffusion coefficient and the reaction rate constant.

⎟⎟⎠

⎞⎜⎜⎝

⎛

rkDX

CXt 22

effH

M

2

(8)

rkD

XXtC 1

24 effM

H2 (9)

4. RESULTS AND DISCUSSION

Fig. 5 shows the time course of the conversion ratio from Fe3O4 to Fe for pellets of 700 nm. The H2 concentration of the supply gas was varied while the reaction temperature was kept constant at 550 °C. Reduction of Fe3O4 proceeds faster when the the supply gas contains more H2. The reaction rate is generally high at the beginning and it gradually decreases because the diffusion length through the porous Fe increases with time. It took 5.49*103 s to complete the reaction for H2 concentration of 60%, 9.08*103 s for 40% and 1.84*104 s for 20%. Eq. (8) shows that the reaction time required to complete the reduction, t*, is inversely proportional to the H2 concentration of the supply gas. This relationship can clearly be confirmed in Fig. 6, showing the validity of the experimental data. Fig. 7 shows the

Fig. 5 Time course of conversion ratio to Fe. Fig. 6 Effect of hydrogen concentration on reaction time.

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(a) (b) (c)

Fig. 7 Effects of temperature on reduction characteristics of three test pellets: (a) Reaction time, (b) Effective gas diffusion coefficient, (c) Reaction rate constant.

effects of temperature on the reduction characteristics of the pellets. The H2 concentration of the supply gas was set at 40 %. The reaction time tends to be shorter while both Deff and kr become higher for the pellet of larger particles and/or operation at higher temperature. The high Deff should be related to the large pore diameter. To clarify the dependency of kr on the particle diameter, we performed FIB-SEM analysis [9] on the pellets after the experiment. In the pellets of 400 nm and 700 nm, fine nano-scale pores are observed inside the particles. They dramatically increase the surface area density and enhance the reaction. It may, however, cause a durability problem when the iron is repeatedly used in the redox cycles.

5. CONCLUSION

To obtain fundamental information on high temperature redox reaction of iron, we experimentally investigated the reduction process of porous pellets of magnetite (Fe3O4) in a temperature range from 450 to 550 °C. Applying a simple 1-D model extended from the conventional unreacted core model, the effective gas diffusion coefficient and the reaction rate constants were evaluated. Among the three types of the pellets tested in this study, the pellet made of the largest particle shows the best performance. It is attributed to the combined effects of higher gas diffusivity and reaction constant associated with the morphology change of the pellet.

ACKNOWLEDGMENT

The authors would like to thank Dr. Hiroko Ohmori for useful discussions on the topic of solid oxide iron-air battery.

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