An Intermediate-Temperature Solid Oxide Iron–Air Redox Battery

An Intermediate-Temperature Solid Oxide Iron−Air Redox Battery Operated on O2−-Chemistry and Loaded with Pd-Catalyzed Iron-Based Energy Storage Material Cuijuan Zhang and Kevin Huang* Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: The solid oxide iron−air redox battery (SOIARB) operated on high-temperature O2−-chemistry is an emerging all-solid-state battery suitable for large-scale energy storage with strong advantages in rate capacity and safety. However, it faces a serious challenge, particularly at lower temperatures, in rechargeability controlled by sluggish reduction kinetics of iron oxide. This work demonstrates that the slow iron oxide reduction kinetics can be significantly enhanced by loading Pd nanoparticles into the Fe-based energy storage material, achieving high cycle efficiency at high energy and power density. A representative result shows that at 500 °C and C/5.3 (10 mA cm−2, or 239.6 mA g−1-Fe) rate, the battery delivers a discharge specific energy of 960.3 Wh kg−1-Fe at 80% iron utilization (UFe) and ∼600 Wh kg−1-Fe at UFe = 50% with an average cycle efficiency of 62.9% over 25 cycles.

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solid oxide fuel cell (RSOFC) and energy storage unit (ESU). The basic configuration and reactions are shown schematically in Figure 1. Operated with a dense O2−-electrolyte and porous air- and fuel-electrodes, the RSOFC serves as an electrical charger−discharger, whereas located inside the fuel-electrode chamber, the ESU functions as an oxygen (energy) store via a metal/metal oxide redox couple mediated by the H2/H2O oxygen shuttle. A full discharge and charge cycle can be described as follows. During the discharge, Me is oxidized by H2O to form MeOx and H2; the latter is then electrochemically oxidized into H2O at the anode of the RSOFC operating as a fuel cell, and the produced H2O diffuses back to ESU and reacts with Me, producing more H2 to sustain the electrochemical oxidation (and discharge cycle). When all Me (or a controlled portion) is depleted, the discharge cycle is stopped and the battery needs to be recharged. During the recharge, RSOFC operates as an electrolyzer with electricity as the energy input to split H2O into H2 at the anode; the produced H2 then diffuses back to ESU and reduces MeOx into Me, producing more H2O to sustain electrochemical reduction of H2O to yield H2 (and charge cycle). When all MeOx (or a controlled portion)

he increasing environmental concern over carbon pollution and more strict environmental regulations have sparked broad global research and development activities in renewable solar and wind energy in recent decades. However, because of the inherent intermittency the harnessed renewable energy cannot be efficiently transmitted across the utility grid that has already been stressed by the daily peak-hour imbalance between generation and load.1−3 Energy storage is deemed the best viable solution to balance the generation and load.3−6 An energy storage system suitable for large-scale grid and renewable energy applications is preferred to be fast in response time, low in life-cycle cost, high in cycle efficiency and rate capability, safe, and scalable.6 Among all the energy storage technologies available, electrochemical batteries such as Na−S, redox flow batteries, and alkaline metal−air batteries stand out as potential candidates to meet these requirements.3,6−10 However, the current performance of these batteries can only partially satisfy the requirements. To enable these batteries for commercial applications, significant breakthroughs in materials and engineering designs are necessary.2,6,9−12 Alternatively, developing advanced batteries with novel chemistry could be another way to break the performance barrier. Recently, a solid oxide metal−air redox battery (SOMARB) has been demonstrated as a new type of all-solid-state battery suitable for grid and renewable energy storage.13−20 This new battery is operated on O2−-chemistry, consisting of a reversible © XXXX American Chemical Society

Received: October 13, 2016 Accepted: November 14, 2016

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DOI: 10.1021/acsenergylett.6b00529 ACS Energy Lett. 2016, 1, 1206−1211

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ACS Energy Letters

Figure 1. Schematic of components and working principle of SOMARB consisting of reversible solid oxide fuel cell (RSOFC) and energy storage unit (ESU). The RSOFC is composed of porous air electrode, fuel electrode, and dense electrolyte. The ESU is loaded with an energystoring Me/MeOx redox couple. The solid and dashed arrows indicate the discharge and charge cycles, respectively.

results in Figure S1), morphology (transmission electron microscopy (TEM) images in Figure S2) and phase composition (X-ray diffraction (XRD) results in Figure S3) of the prepared Pd-loaded ESU can be found in the Supporting Information. The RSOFC employed is an anode-supported button cell with (ZrO 2 ) 0.89 (Sc 2 O 3 ) 0.1 (CeO 2 ) 0.01 as the electrolyte, Ni-(ZrO2)0.89(Sc2O3)0.1(CeO2)0.01 as anode, and La0.8Sr0.2MnO3(Bi0.75Y0.25)0.93Ce0.07O1.5 as cathode; a typical microstructure and electrochemical performance of the RSOFC at 500 °C are given in Figures S4 and S5, respectively. It is to be noted that neither the conventional CeO2-based electrolytes24 nor Bi2O3-based bilayer electrolytes25 can be used as an electrolyte for SOIARB because the cell open-circuit potential with these electrolytes is lower than the Nernst potential of Fe/Fe3O4 redox couple at 500 °C (1.088 V) due to the electronic conduction. The leaked oxygen flux (current) would gradually consume all the Fe in the ESU without producing useful work during the energy conversion process. The initial electrochemical performance of 500 °C SOIARB cycled at 10 mA cm−2 (239.6 mA g−1-Fe) and 10 min/10 min charge/discharge time with and without Pd catalyst in the Fe-based ESU is compared in Figure 2. The testing condition is equivalent to C/5.3 rate and UFe = 3.1% according to the Fe−O2 chemistry. The incorporation of Pd into ESU is clearly seen to improve rechargeability of the battery. Without Pd, Figure 2a shows that the battery is virtually nonrechargeable. With Pd catalyst, the battery can cycle >50 times without obvious degradation. Compared to the charge cycle, the discharge cycle exhibits much flatter voltage with lower polarization, inferring that there is still room to improve the rechargeability by a further promotion of Fe3O4 reduction kinetics. The corresponding discharge and charge specific energy and round-trip efficiency (RTE) of SOIARB with Pd-impregnated ESU are shown in Figure 2b. The average discharge specific energy is ∼38 Wh kg−1-Fe over 50 cycles at UFe = 3.1%. The slight decline in RTE mainly arises from the slight increase in charge specific energy as the discharge specific energy remains virtually constant during cycling. An average RTE of 77% was achieved for the Pd-catalyzed ESU tested within its cycle life. The battery can also cycle 500 times with rather stable voltage under the same conditions, as shown in Figure S6.

is reduced, the battery is ready for the next-round discharge cycle. Compared to conventional liquid-based metal−air batteries, the SOMARB is advantageous in terms of solid structure, decoupled RSOFC and ESU components, two-electron transfer, high energy density and rate capability, and strong safety features. A large body of early work has demonstrated that Fe/FeOx is the most promising ESU material for SOMARB.13−18,21 The thermodynamic analysis of Fe/FeOx equilibrium for a solid oxide iron−air redox battery (SOIARB) indicates two distinct redox couples: Fe/Fe3O4 at 80%, however, the discharging voltage experiences a sharp decrease toward the end of the cycle. Similarly, the charging voltage also suffers a sharp increase toward the end of the cycle. The observed “fast voltage drop/ rise” toward the end of the cycle results from mass transport limitation caused by a thick buildup of a Fe3O4/Fe layer at high UFe, seriously impeding the transport of H2 and H2O and

Figure 3. Performance of 500 °C SOIARB with Pd-catalyzed Fe-based ESU at a fixed cycle current density of 10 mA cm−2 (239.6 mA g−1-Fe), but different UFe (5−100%). (a) Battery voltage profiles vs time; (b) discharge and charge specific energy and round-trip efficiency vs UFe. 1208

DOI: 10.1021/acsenergylett.6b00529 ACS Energy Lett. 2016, 1, 1206−1211

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revealed that the RSOFC performance has declined (current− voltage curves and impedance spectra in Figure S7) in conjunction with coarsening of the ESU (Table S1) for both C/4.8 and C/2.9 cases. However, the degree of reduction in RSOFC performance and surface area is smaller for the C/2.9 case, which suggests that the slow kinetic rate of Fe3O4 reduction that is unable to match up with the electrolysis (H2O splitting) current, rather than the microstructural changes in RSOFC electrode and ESU, is the root cause for the observed faster performance decay at higher rate. On the other hand, the discharge of Fe oxidation is rather stable, yielding 238.6 Wh kg−1-Fe for 50 cycles and 228.9 Wh kg−1-Fe for 10 cycles at C/4.8 and C/2.9, respectively. The corresponding average RTE values are 67.8 and 59.9%, respectively. To compare with other energy storage technologies, we also plot power versus energy (Ragone plot) for the SOIARB tested, which is shown in Figure 6. The specific power of SOIARB

Figure 4. Cycling performance of a 500 °C battery with Pd-catalyzed Fe-ESU at high UFe= 50% and J = 10 mA cm−2 (239.6 mA g−1-Fe) with a cycle duration of 2.65 h for 25 cycles. (a) Battery voltage profiles vs time; (b) discharge and charge specific energy and roundtrip efficiency vs cycle number.

Figure 6. Ragone plot of SOIARB compared with that of several other rechargeable batteries.3

depends on the cycling rate, ranging from 230 W kg−1-Fe at C/5.3 to 386.2 W kg−1-Fe at C/2.9. The specific energy depends on UFe, varying from 38.7 Wh kg−1-Fe at UFe = 5% to 960.3 Wh kg−1-Fe at UFe = 80%. This level of performance not only represents a significant improvement over reported early results obtained at higher temperatures (Table S2) but also is superior over other rechargeable batteries in achieving high specific energy at high specific power (Figure 6). The specific energy is higher than that of the state-of-the-art Na−S battery (150−240 Wh kg−1) and significantly higher than that of the redox flow battery (10−25 Wh kg−1).5,8 Although it appears lower than Li−air (1700 Wh kg−1) and Na−air batteries (∼1500 Wh kg−1) (calculated based on 100% utilization of metal),5,8 the higher cycling current density (C-rate),5 e.g., 18.5 versus 0.1 mA cm−2, and better reversibility are the clear advantages of SOIARB over Li/Na−air batteries. Given the high operating current density and strong safety features, SOIARB holds promise for grid and renewable energy storage where high cycling rate and safety are critically important. There are two sources for energy losses in SOIARB: RSOFC and ESU. From the impedance analysis results shown in Figure S7b, a large portion of polarization losses of RSOFC stems from the anode substrate where significant Ni coarsening and poor electrolyte−anode bonding are noted (Figure S8d-f). Further improvement in low-temperature RSOFC will further

Figure 5. Performance of 500 °C battery with Pd-catalyzed ESU at different cycle current densities but the same UFe = 21%. (a) Voltage profiles vs time at a cycle rate of C/4.8 and C/2.9; (b) comparison of discharge and charge specific energy and round-trip efficiency at different C-rates.

50 cycles can be sustained with the highest charging voltage below 1.40 V, whereas at a rate of C/2.9 only 10 cycles can be completed before the charging voltage rises to 2.0 V. This observation indicates that the current density is more impactful than the cycle time on the rechargeability. Post-test analysis 1209

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ACKNOWLEDGMENTS This work was funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award DE-AR0000492.

boost the performance of SOIARB. Fe coarsening is also found by the reduced surface area and pore volume determined after testing (Table S1), which contributes to the gradual performance degradation. However, the faster decay at higher rate (e.g., C/2.9) during the charging cycle is fundamentally limited by the slow intrinsic kinetic rate of reduction that is unable to match the rate of water splitting by RSOFC. With the addition of Pd into Fe-ESU, the battery has been enabled to cycle at C/4.8 with a reasonable UFe. To push the battery to cycle at even higher C-rate, further improvement in Fe3O4-reduction kinetics is needed. The fundamental reasons for Pd-promoted Fe3O4 reduction kinetics have also been discussed based on the temperature-programed reduction results in Figure S9a, and it was concluded that the spillover mechanism (Figure S9b) is possible. In summary, the rechargeability of the SOIARB operated at 500 °C is critically limited by the sluggish kinetics of Fe3O4 reduction to Fe. Dispersing Pd nanoparticles into the Fe-based ESU material is an effective way to promote the reduction kinetics. The battery loaded with Pd-catalyzed Fe-based ESU exhibits significantly improved cyclability. At a lower C/5.3 rate, the battery shows stable performance at UFe as high as 80%, delivering a discharge specific energy of 960.3 Wh kg−1-Fe. At 50% iron utilization, SOIARB can cycle more than 25 cycles with a discharge specific energy of 595−608 Wh kg−1-Fe and average RTE of 62.9%. The SOIARB can also cycle at a high rate of C/2.9, but at the expense of shortened cycle life and lowered RTE. Compared with other rechargeable batteries, the SOIARB is capable of achieving high specific energy and high specific power without safety concerns. Higher operating C-rate, inherent safety features, and low maintenance indicate SOIARB is an excellent candidate for stationary grid and renewable energy storage. Future power enhancement in RSOFC, redox activity, and sintering resistance improvement in ESU will further extend the battery’s cycle life at high C-rate and Fe utilization. With concurrent advances in SOFC stack and system design, SOIARB is expected to play an important role in future grid and renewable energy storage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00529. Experimental details; XPS and XRD data of ESU materials with and without Pd-impregnation; TEM image of Pd-impregnated ESU material; SEM images of fresh and tested cells; current−voltage, current−power curves, and AC impedance spectra of cells fresh and regenerated cells; battery performance for 500 cycles; tables of surface area and pore volume of fresh and tested ESU materials; comparison of battery performance of the present SOIARB with Pd-catalyzed ESU materials with reported results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kevin Huang: 0000-0002-1232-4593 Notes

The authors declare no competing financial interest. 1210

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ACS Energy Letters (23) Jin, X.; Zhao, X.; Zhang, C.; White, R. E.; Huang, K. Computational analysis of performance limiting factors for the new solid oxide iron-air redox battery operated at 550 °C. Electrochim. Acta 2015, 178, 190−198. (24) Shao, Z.; Haile, S. M. A high-performance cathode for the next generation of solid-oxide fuel cells. Nature 2004, 431, 170−173. (25) Wachsman, E. D.; Lee, K. T. Lowering the temperature of solid oxide fuel cells. Science 2011, 334, 935−939.

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