solid oxide iron-air rechargeable battery - a new energy storage mechanism

Solid Oxide Iron-Air Rechargeable Battery - A New Energy Storage Mechanism

X. Zhao, N. Xu, X. Li, Y. Gong, K. Huang*

Department of Mechanical Engineering, University of South Carolina Columbia, SC 29201

Abstract

Cost effective large-scale energy storage mechanisms are of critical importance to our future energy infrastructure consisting of a large portion of renewables and smart-grid. Rechargeable batteries have a great potential to provide a cost-effective energy storage mechanism for this grand demand. We here report on a novel solid oxide iron-air rechargeable battery derived from solid oxide fuel cell (SOFC) and chemical looping hydrogen technologies, featuring a separate energy storage unit and electrode configuration. A systematic study shows that the battery’s capacity and round trip efficiency have strong and competing reliance on how much iron is actually utilized. The energy capacity increases with increasing iron utilization, but at the expense of lowered round-trip efficiency. A practical strategy to operate the new battery is to maintain a low iron utilization of inexpensive iron-based energy storage materials as a mean of achieving required energy/power rating with high efficiency.

Introduction

Constant variations in utility demand require a close matchup of electricity supply from the electrical grid. In order to do so, electrical energy storage (EES) system coupled to premium power plants is essential [1-3]. Current EES technologies rely primarily on pumped-hydro energy storage [4], which is complemented by compressed air energy storage [5], supercapacitors [6] and rechargeable batteries [7-10]. Rechargeable battery technology is an attractive option because of its fast response time, high energy/power density and potential cost effectiveness. However, in current energy storage market, rechargeable batteries only accounts for a tiny fraction, largely due to the limitations in the areas of performance, reliability and safety.

Recently, rechargeable metal-air batteries have attracted much attention due to their extremely high energy density and inexhaustible air cathode. However, traditional

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metal-air batteries such as Li-air batteries and Zn-air batteries suffer from oxygen diffusion limitation caused by air-pathway clogging, mass loss, carbonation and decomposition of the electrolyte, all of which have resulted in poor rechargeability and low energy output [11-13]. These issues need to be properly solved before they can become a commercially viable product.

Recently, our group demonstrated a novel solid oxide metal-air rechargeable battery based on solid oxide fuel cell (SOFC) and chemical looping hydrogen technologies with high energy density and round trip efficiency [14]. We report more characteristics of this newly emerged metal-air battery in this paper. Working Principle and Advantages of the New Battery Fig. 1 shows the working principle of the new battery. The free-standing energy storage unit (ESU) is loaded into the inner space next to the fuel electrode of an anode-supported tubular cell, where a mixture of H2O/H2 is circulated in a closed loop. The ESU converts electrical energy and chemical energy via a H2O/H2 mediated Metal/Metal oxide (Me/MeOx) redox reaction. During the discharge, SOFC works in the fuel cell mode, in which H2 is oxidized to H2O and the produced electrons supply electricity to the external circuit. The produced H2O proceeds towards ESU to oxidize the Me into MeOx, and meanwhile H2 is produced, which can be further oxidized into H2O. During the charge, SOFC operates in the electrolyzer mode and all the discharge reactions are reversed accordingly. Overall, a redox cycle at the fuel electrode compartment is completed during one discharge-charge cycle as shown in Fig.1, while O2 in the air undergoes reduction and evolution reaction at the air-electrode during the discharge and charge cycles, respectively.

Fig. 1 Working principle of the new solid oxide metal-air rechargeable battery: energy storage unit is decoupled from the electrode of the SOFC supported on a tubular anode substrate

By combining fuel electrode and air electrode reactions in Fig.1, the global reaction of the battery becomes

arg

2 arg2

disch e

ch e

xMe O MeOx→+ ←

(1)

ECS Transactions, 50 (45) 115-123 (2013)



which essentially suggests the nature of a metal-air battery. Some key features of the new battery are observed as follows: • Different from most traditional rechargeable batteries that rely on single-electron

charge transfer limited by single-charge electrolytes employed (e.g. Li+, H+ or Na+), the new battery present a double-electron transfer process enabled by solid O2- electrolytes, promising a higher storage-capacity at a higher rate.

• The new battery features a design of separated fuel electrode and ESU, which allows faster charge-discharge cycles without the concerns of structural damages as commonly encountered in conventional storage batteries.

• The new battery can be designed independently with energy and power to meet different applications.

• The new battery can be thermally cycled without the concerns of structural damages resulted from liquid-solid transition encountered in conventional high-temperature Na-S and liquid metal batteries.

• The new battery possesses higher rate capacity enabled by its higher operating temperature (typically from 500oC-800oC), which is a valuable asset to rapidly harvest energy from renewable sources when the natural high energy flux is available.

• The new battery features sustainability, environmental-friendliness, scalability and safety due to the use of earth-abundant and inexpensive iron as the energy storage materials and the mature design developed by SOFC technology.

Thermodynamics of the New Battery

The energy storage capacity of the new battery relies upon thermodynamics of the Fe-FeOx redox reaction. The phase diagram of Fe-O in Fig.2 suggests two potential redox couples that can be utilized for the redox reaction: Fe-FeO operating at above 600oC and Fe-Fe3O4 operating at below 600oC. The phase stability diagram of Fe-O-H system in Fig. 3 further confirms the temperature-dependent phase relationship shown in Fig.2. On the other hand, fixing a temperature leads to a fixed pH2/pH2O ratio, which for a concentration cell with air as the oxidant, it implies a fixed Nernst potential. Fig. 4 shows the calculated theoretical EMF based on the two redox couples. For instance, the EMF corresponding to Fe-Fe3O4 equilibrium prevalent at 550oC is 1.067 V, while it is 0.970 V at 800oC, corresponding to the phase equilibrium of Fe-FeO. Conversely, if the EMF is measurable as a function of temperature, it can also be used to confirm the phase relationship predicted by Figs. 2 and 3 [14-15].




Fig. 2 Phase diagram of Fe-O system (The red dash square indicates the operating temperature of the new battery.)

Fig. 3 Phase stability diagram of Fe-O-H system as a function of temperature




Fig.4 Theoretical EMFs of Fe-FeOx redox couples as a function of temperature

Experimental Procedure The experimental setup consisted of two major components: a planar battery cell and circulating pump. The battery cell was assembled with Fe-containing ESU and a reversible SOFC (RSOFC). The details on the assembly as well as the experimental setup can be found in ref. [15]. The initial ESU material Fe2O3-ZrO2 (5-10wt%) and RSOFC were first activated with 5%H2-N2 at 800oC. At the beginning of electrical cycles, pure H2 was first introduced into the cell, and a constant current was drawn to oxidize 35% H2 into H2O until a chemical equilibrium between Fe and FeO can be established. The establishment of the Fe-FeO equilibrium was constantly monitored with EMF. As predicted by the thermodynamics in Figs.3 and 4, an EMF=0.970 volt at 800oC would indicate the reach of the equilibrium. The electrical charge-discharge cycles were then allowed to proceed. During electrical cycles, the reaction gas was circulated in a closed-loop to homogenize the concentration. To prevent condensation, all pipelines were heat-wrapped and kept at 110oC. A Solartron 1260/1287 Electrochemical System was employed to measure the cyclic as well as impedance performance of the battery with software modules such as OCV (open circuit voltage)-t, impedance spectroscopy, potential-dynamic, and galvanic cycles. Results and Discussion

The total charge stored in Fe, full discharge time, and Fe utilization follow respectively:




Total charge *FeQ (Coulomb) stored in Fe: F2

MW

QFe

Fe*Fe ××= (2)

Full discharge time *Fet (sec):

d

*Fe*

Fe I

Qt = (3)

Fe utilization UFe: *Fe

Fe*Fe

FeFe t

t

Q

QU == (4)

where WFe (g) and MFe (g/mol) are the actual Fe loading and molecular weight of Fe, respectively; Id is the galvanic discharge current in amperes; QFe and tFe are the measured charge capacity (Coulomb) and actual cycle time (seconds), respectively. The experimental results in Fig.5 show that the storage capacity is strongly affected by current density and cycle time, combination of which determines the Fe utilization (UFe) as suggested by eq. (4). Despite having similar shapes and trends, Figs.5 (a)-(c) reflect the ability of the battery to store electrical energy on the basis of mass, volume of Fe and active area of the electrode, respectively. It is generally observed that higher UFe leads to a higher storage capacity since more active solid “fuel” is involved in the redox reaction, but accompanied by a more pronounced voltage degradation. Such a trend is probably caused by a slower kinetics in the ESU and resistance increase resulting from RSOFC. When UFe approaches 100%, see Fig.5 (a), the energy capacity reaches 957mAh/g Fe, which is in agreement with the theoretical value. However, the volume specific energy capacity cannot reach the theoretical value since the ESU volume includes those of redox materials and pores, see Fig.5 (b). The abrupt drop-off in voltage at UFe=100% indicates a complete consumption of metallic Fe, and the Fe-FeO equilibrium was shifted to the next FeO-Fe3O4 equilibrium of lower energy density.

(a)




Fig. 5 Energy storage characteristics measured at different Iron utilization and 800oC: (a) Mass specific energy capacity (b) Volume specific energy capacity (c) rate capacity

Among the parameters listed in eq. (2)-(4), UFe is a key parameter reflecting the combined effect of operating current and cycle duration. The relationships between energy capacity, round-trip efficiency and UFe are shown in Fig. 6. Overall, higher UFe leads to higher energy capacity. Energy capacity follows the theoretical line at low UFe, but quickly deviates to lower values at higher UFe. On the other hand, the observed decrease in round-trip efficiency favors lower UFe. When the iron utilization reaches 100%, the energy capacity of the iron-air battery at 800oC is 957mAh/g Fe, equal to the theoretical value. However, the round trip efficiency is only about 55%. In contrast, when the iron utilization is 1.0%, the round trip efficiency is around 90%; however, the energy capacity is only 9.31mAh/g Fe. The competing trend exhibited between capacity-UFe and efficiency-UFe suggests that the capacity and efficiency of the new solid oxide iron-air storage battery can be balanced with a proper choice of iron utilization. One strategy is to load and sacrifice the inexpensive ESU materials in the system to maintain a low UFe yet guarantee the total amount of active “fuel” to meet the energy storage requirement. In this way, high energy capacity and high round-trip efficiency can be achieved simultaneously.

(b)

(c)




Fig.6 Energy storage capacity and round-trip efficiency as a function of iron utilization [15]

Summary

The new battery has demonstrated the unique ability to meet key technical requirements for grid scale energy storage application. Energy storage capacity and round trip efficiency show strong but competing dependence on iron utilization, which can be balanced with a proper choice of iron utilization. One strategy is to load and sacrifice the inexpensive ESU materials in the system to maintain a low UFe yet guarantee enough amounts of active energy storage materials to meet the energy storage requirement. Recent modeling work in our laboratory has been dedicated to optimize UFe for achieving high capacity and round trip efficiency simultaneously.

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solid oxide iron-air rechargeable battery - a new energy storage mechanism

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