Nano PdO Activated Iron Molten Air Battery - ACS Publications

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A Nano PdO Activated Iron Molten Air Battery Baochen Cui, Yumeng Shao, Wei Xiang, Shuzhi Liu, Xianjun Liu, Wei Han, Jianhua Zhang, and Stuart Licht J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01145 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

A Nano PdO Activated Iron Molten Air Battery Baochen Cui,†,‡ Yumeng Shao,† Wei Xiang,† Shuzhi Liu,*,†,‡ Xianjun Liu,† Wei Han,† Jianhua Zhang,† and Stuart Licht*,§ †

Province Key Laboratory of Oil and Natural Gas Chemical Industry, College of Chemistry and

Chemical Engineering, Northeast Petroleum University, Daqing 163318, P.R. China. ‡

School of Chemistry and Environmental Engineering, Harbin University of Science and

Technology, Harbin 150080, P.R. China. §

Department of Chemistry, George Washington University, Washington DC 20052, United

States.

ABSTRACT: Iron molten air is a new battery class that combines multi-electron cathodic storage, high capacity air electrodes with the high redox activity of a molten electrolyte. The need for a bifunctional air electrode to improve the kinetics of oxygen reduction and evolution reactions (ORR/OER) is a major challenge of this battery technology. Here, we demonstrate for the first time that a highest-activity air electrode catalyzed by lithiated PdO nanoislands decorating lithiated NiO nanoparticles enhance the iron molten air battery. It is found that the lithiated NiO nanoparticles uniquely decorated with lithiated PdO nanoislands are in situ formed during the battery cycling. The air electrode exhibits more efficient activity at catalyzing OER/ORR and a superior rate performance compared with that with the bare NiO catalyst. Cycling tests for the iron molten air battery show stable performance through 100 cycles with the

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highest coulombic efficiency of 98.7% and average discharge potential of ~1.10 V. This study is expected to open up exciting opportunities for developing an efficient air electrode for iron molten air batteries.


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1. INTRODUCTION Iron molten air battery has attracted strong interests as a revolutionary new technology for electronic, transportation and greenhouse gas reduction power generation devices,1 due to its large capacity, in addition to iron being low cost, sustainable, and not toxicologically threatening to the environment.2 The iron molten air batteries use an iron anode, a nickel cathode, and molten salts electrolytes such as Li2CO3 with adding Li2O,1 Li0.87Na0.63K0.50CO3 with adding Li2O,3 KCl– LiCl–LiOH with adding NaOH,2 and so on. To date, iron molten air battery chemistries have been demonstrated using 3e- iron anode charging/discharging. The reactions in the iron molten air battery are represented by eqn (1), including eqn (2a) or (2b) showing the dissolution of Fe2O3 by Li2O or Na2O to form LiFeO2 or NaFeO2, followed by quasi-reversible charge/discharge of LiFeO2 or NaFeO2 to iron, shown in eqn (3a) or (3b). 1-3 1/2Fe2O3 ⇌ Fe + 3/4O2

(1)

Molten electrolyte dissolution: 1/2Fe2O3 + 1/2Li2O⇌LiFeO2

(2a)

or 1/2Fe2O3 + 1/2Na2O⇌ NaFeO2

(2b)

LiFeO2⇌Fe + 1/2Li2O + 3/4O2

(3a)

3e- cycling:

or NaFeO2⇌Fe + 1/2Na2O + 3/4O2

(3b)

Compared to the iron molten air batteries with the molten Li2CO3 and Li0.87Na0.63K0.50CO3 eutectic electrolytes, the battery operating temperatures were lowered from 730 and 600 °C to 500 °C using eutectic electrolytes including KCl–LiCl–LiOH with adding NaOH and 56.5mol% Li0.87Na0.63K0.50CO3 eutectic–43.5 mol% LiOH with adding NaOH.2,4 Recently, we improved the


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performance of the iron molten air battery with a fin air electrode, and found that Li-doped nickel oxide catalytic nanoparticles microstructure was formed in situ on the air electrode, facilitating the ORR kinetics of air electrode.5 However, one of the critical challenges that limit the practical use is the development of a bifunctional air electrode to improve the kinetics of ORR/OER. A key to the development of bifunctional air electrode for a metal-air battery is how to design cathode electrocatalysts. Recently, it has been demonstrated to be an effective way for promoting the ORR/OER using Pd nanoparticles supported on carbonaceous materials as the catalysts, such as Pd/C,6 Pd/Vulcan,7 Pd/Ketjen black,8 Pd/mesoporous carbon,9 Pd/carbon nanofibers,10 Pd/carbon nanotubes (CNTs)11,12. However, during the charging process, the carbon materials on the air electrode can undergo corrosion challenges at high operation potentials, affecting the stability of the catalysts.7,10 To replace carbon materials, much attention has been paid to using metal oxides such as MnO213-17, TinO2n-118 and Co3O4

19,20

as alternatives for supporting Pd

nanoparticles. Another way to improve the catalytic activity is to combine Pd with other transition metals to form bimetallic or trimetallic catalysts. For example, some studies have demonstrated that Pd-based binary alloy nanocatalysts such as PdAu/C,21 PdAu/β–MnO2,22 Pd3Pb/C,23 Ni3Pd7/rGO(reduced graphene oxide),24 and

Pd17Se15/rGO25 showed excellent

catalytic performances due to the change of the electronic structure of Pd.

26,27

Reportedly, the

ternary alloy nanocatalysts such as PdSnCo/NG (nitrogen-doped reduced graphene)28 and Pd2FeCo/C29 also present a high electrocatalytic activity, being attributed to the synergetic effect of the three nanostructured metals. Although the understanding of oxygen electrocatalysts in metal-air batteries such as Li–air or Zn–air batteries has been developed over the last few decades, this process is just beginning for


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the iron molten air battery. What is learned from Li–air or Zn–air batteries can promote the development of the iron molten air battery. Especially, it has been shown that for the Pd and Pdcontaining electrodes, PdO species are actively involved in processes of oxygen reduction, and PdO showed higher stability than Pd under oxygen reduction conditions.30-32 It is also a useful guide to developing an air cathodes for the iron molten air battery. Here, we report the successful fabrication of an air cathode with lithiated PdO nanoislands decorating lithiated NiO nanoparticles, through the combination of galvanic displacement method with in situ oxidation and lithiation. To the best of our knowledge, this represents the first clear verification of nanoislands-on-nanoparticles growth on the nickel air electrode in the molten salt electrolyte. The results of electrochemical investigations of the air cathode revealed a good catalytic activity toward OER/ORR. The battery exhibited better cycling stability with a higher average discharge voltage of 1.10 V and the highest coulombic efficiency of 98.7%, and a higher discharge rate capacity compared to the electrode without lithiated PdO nanoislands decorating. The present study highlights a novel strategy for preparing high-performance air electrode with hybrid nanocatalysts and provides a new direction for catalyst performance optimization for iron molten air batteries. 2. EXPERIMENTAL SECTION 2.1.Chemicals and materials. Lithium carbonate (Li2CO3, 97%), sodium carbonate (Na2CO3,anhydrous,99.8%),

potassium

carbonate

(K2CO3,anhydrous,99%),

lithium

hydroxide (LiOH·H2O,95%), sodium hydroxide (NaOH,96%) and ferric oxide (Fe2O3, 99.5%) are combined to form the molten eutectic electrolyte consisting of 56.5 mol % Li0.87Na0.63K0.50CO3 eutectic–43.5 mol% LiOH with 0.5 m (m=molal, moles per kilogram of electrolyte) Fe2O3 and 3 m NaOH. Palladium chloride (PdCl2,59% Pd basis), aqua


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ammonia (NH3·H2O, 25–28%NH3), ethylenediamine (EDTA) (C10H16N2O8, 99.5%), ammonium chloride (NH4Cl, 99.5%), and acetone (CH3COCH3, 99.5%) were used to deposit Pd on a Ni air electrode with the galvanic method. 2 mm diameter pure Ni wire (99.5%), pure Ni foil (99.95%), 1.2 mm Fe wire (annealed), steel foil (316 steel), and pure alumina (99.7%) crucible are used to make electrodes and are combined to form various cell configurations. 2.2. Electrodes Preparation. The air electrode with deposited Pd was prepared using a Ni electrode by the galvanic method, modified from the literature.33 A nickel fin air electrode described in our previous report

4,5

was washed with acetone under sonication,

subsequently flushed with deionized water. PdCl2 (0.0667g) and 2.4 ml of aqua ammonia were mixed and stirred with a magnetic stirrer at room temperature for 12 hours to form a homogeneous solution. 0.25 g EDTA and 4 g NH4Cl were added to the solution, then added deionized water with a continuous magnetic stirring until solution volume of 50 ml. The obtained solution was heated to 40 °C. The obtained air electrode was placed into the obtained solution for galvanic displacement for 2 min. Pd cations in the solution replace the surface atoms of Ni via a simple redox reaction: Pdliquid2++ 2Nisurface →2Pdsurface+ Niliquid2+ because Pd has a more positive equilibrium potential than Ni.33 Finally, the obtained air electrode was washed with deionized water and dried in a vacuum oven at 70°C for 2 h. The rotating disk electrode (RDE) consisted of the Ni disc of 4 mm diameter which was insulated by a pure alumina tube of 4 mm internal diameter and 7.5 mm outer diameter. The Ni disc was polished using a succession of metallographic abrasive papers and a 0.3 µm-alumina


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slurry. The solid was then rinsed with deionized water until it was clean. After drying, the RDE was modified with Pd by the same procedure as the preparation of air electrode described above. 2.3.Characterizations. X-ray diffraction (XRD) measurements were operated on a powder X–ray diffraction with a Cu Kα source (Rigaku D/MAX–2200). Scanning electron microscopy (SEM) was performed on a field-emission scanning electron microscopy (FE-SEM, Zeiss Sigma) coupled with energy dispersive spectrometry (EDS, Oxford Instruments X–Max). The electrode samples were prepared by gold sputtering treatment. 2.4.Battery Assembly and Testing. The cells were configured using a steel foil (2 cm × 2.5 cm) as the current collector of the anode, a nickel air electrode with twelve vertical fins as the air electrode, and 56.5 mol % Li0.87Na0.63K0.50CO3 eutectic–43.5 mol % LiOH with 0.5 m Fe2O3 and 3 m NaOH as the electrolyte. Details of cell configurations were described in our previous studies.4,5 Cyclic voltammetry (CV) measurements were recorded using an electrochemical workstation (CS350, Wuhan CorrTest instruments Co. Ltd., China). A nickel air electrode was used as the working electrode. A steel foil (2 cm × 2.5 cm) was used as the counter and the reference electrode. The CV was scanned between 1.7 V and 0.2 V (vs. steel foil electrode) at sweep rates of 50 mV sec-1. Linear sweep voltammetry (LSV) was determined using a rotating disk electrode (RDE) as the working electrode. A steel foil with a diameter of 2.0 cm was used as the counter and the reference electrode. LSV was scanned between 0.2 V and -1.2 V (vs. steel foil electrode) at sweep rates of 10 mV sec-1 using an electrochemical workstation (CS350, Wuhan CorrTest instruments Co. Ltd., China).The charge-discharge behaviors of the cell were


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performed on a battery testing system (CT2001A, Wuhan LAND electronics Co. Ltd, China) under air at 500°C. 3. RESULTS AND DISCUSSION 3.1. .Characterization of air electrode. The nickel fin air electrode deposited Pd was used as the cathode for the iron molten air battery. For investigating the cycle performances of the iron molten air battery, the nickel fin air electrode was tested for 100 cycles in air, cycle charges at 0.05 A and discharges over a 100 Ω load to 0.7 V cutoff. Subsequent to the test, the nickel fin air electrode was extracted from the cell and washed with water to remove the electrolyte. The crystal structures of the nickel fin air cathode before and after use were characterized by powder X–ray diffraction (XRD). As shown in Figure 1, compared with the nickel fin air electrode before the galvanic displacement with Pd, the XRD pattern for the nickel fin air electrode before used (after the galvanic displacement with Pd) reveals not only three 2-theta peaks of pure nickel at 44.5, 51.8, and 76.4°, but also three additional peaks at 40.2, 46.8, and 68.3°, matching the library XRD of palladium (Pd) (MDI Jade 5.0, PDF: 87–0638) (Figure 1). Figure S1 in the Supporting Information presents SEM images and EDS mapping of the nickel fin air electrode after the galvanic displacement with Pd. The EDS spectrum analysis indicates the existence of Pd and Ni elements on the surface of the nickel fin air electrode (Figure S2 in the Supporting Information).The atomic ratio of Ni to Pd is approximately 0.63:1. SEM image also shows that a Pd layer emerged and covered homogenously on the nickel shim. From the XRD pattern of the nickel fin air electrode after used (Figure 1), it can be seen that the XRD exhibits not only three 2-theta peaks of nickel at 44.5, 51.8, and 76.4°, but also four 2theta peaks of nickel oxide at 37.2, 43.3, 62.8, and 75.4°. Moreover, the XRD pattern also exhibits new peaks at 2θ = 18.5, 37.3, 44.7, and 76.6°, which are assigned to lithium nickel oxide,


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and also new peaks at 2θ = 19.1, 37.5, 43.3, and 63.6°, being characteristic ones for the formation of lithium palladium oxide phase. Compared to the XRD pattern of the nickel fin air electrode before used, no palladium peaks show at 40.2, 46.8, and 68.3° in the XRD pattern of the sample used. These XRD results clearly indicate the presence of lithiated PdO-NiO hybrid materials on the surface of the nickel fin air electrode. We suggest that Ni and Pd were in situ oxidized and lithiated to lithium nickel oxide and lithium palladium oxide inside the cell during charge-discharge cycling, respectively.

.. . ..

pure palladium pure nickel nickel oxide lithium nickel oxide lithium palladium oxide

Intensity (Counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


..

. . . . . . .. .. .

. . . .. .. (1)

(2) .

(3)

pure Pd PDF#87-0638 pure Ni PDF#70-1849 NiO PDF#78-0423 (Li0.05Ni0.05)(NiO2) PDF#85-1987 Li2PdO2 PDF#38-1325

10

20

30

40

50

60

70

80

o

2-Theta ( )

Figure 1. XRD analysis of the nickel fin air electrode. (1) XRD pattern of the nickel fin air electrode before the galvanic displacement with Pd. (2) XRD pattern of the nickel fin air electrode after the galvanic displacement with Pd. (3) XRD pattern of the nickel fin air electrode after charge-discharge cycle.

Manufacturing nano hierarchical surface structure is one of the major research efforts to achieve and optimize air electrode performance. The morphology of the air electrode was examined by SEM images with various magnifications, given in Figure 2. Well-dispersed nanoparticles were formed on the nickel foils surface from the low magnification of Figure 2a


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and b. The particle size of the nanoparticles varies from 100 to 200 nm. From the high magnification of Figure 2c and d, it is seen that the lithiated PdO phases are brighter, owing to their heavier atomic weight, whereas lithiated NiO phases are darker. Almost all the highly dispersed lithiated PdO phases in form of nanoislands are uniformly distributed over lithiated NiO nanoparticles surfaces to form a unique nanoislands-on-nanoparticle structure, which is expected to enable full Pd utilization. Such nanostructured electrodes should allow the creation of large surface areas to offer enough active sites for both the ORR/OER so that the battery can work more efficiently. For convenience, we defined the lithiated NiO nanoparticle as the PdOdecorated NiO nanoparticle.

(a)

(b)

(c)

(d)

Figure 2. SEM images of the nickel fin air electrode after 100 cycles at various magnification: (a) 10000×, (b) 50000×, (c)100000×, and (d) 200000×. The composition of the PdO-decorated NiO nanoparticles was also confirmed by EDS analysis. According to the EDS mapping shown in Figure 3 and the elemental analysis details given in Figure S3 in the Supporting Information, the existence of C, O, Pd, Ni, Fe and Au elements on the surface of the nickel fin air electrode is evident. The appearance of the C element is (only) a byproduct of the use of conductive adhesive when the sample was prepared


10

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for SEM and EDS analysis, and the Fe element presence is due to residual Fe2O3. The Au element in the EDS spectrum is attributed to the gold sputtering treatment prior to SEM and EDS analysis. Hence, we could confirm that the elements on the surface of the nickel fin air electrode are mainly composed of O, Pd, and Ni. Notably, Pd particles were homogenously dispersed on the nickel fin air electrode. Further, the atomic ratio of Ni to Pd increased from 0.63:1 before use to 4.76:1 after use, due to the existence of a large amount of lithiated NiO nanoparticles instead of the smooth substrate on the surface of the air electrode. (a)

(b)

Pd La1 (d)

(c)

O Ka1

Ni Ka1

Figure 3. SEM image (a) and EDS elemental maps of Pd (b), O (c), and Ni (d) for the nickel fin air electrode after 100 cycles. 3.2. .Electrochemical Properties of the Nickel Fin Air Electrode. The PdO-decorated NiO nanoparticles were investigated by cyclic voltammetry to obtain information regarding their activity toward the OER and ORR. The cyclic voltammograms during the initial five cycles in the voltage range of 0.2–1.7 V are shown in Figure 4. As shown in Figure 4, the PdO-decorated NiO electrode exhibited a distinctly higher peak current at around 0.5 V than that of the NiO electrode without PdO. This implied that the PdO-decorated NiO electrode has a superior performance for the ORR. During anodic scans, the PdO-decorated NiO electrode showed a


11


more negative onset potential and much greater current values for the OER than the NiO electrode without PdO, clearly indicating that the PdO-decorated NiO nanoparticles were more efficient at catalyzing OER than the bare NiO catalyst. 0.4 0.3 Current (A)

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0.2

with PdO without PdO

0.1 0.0 -0.1 -0.2 -0.3 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Voltage (V vs. steel foil electrode)

Figure 4. Cyclic voltammograms for the nickel fin air electrodes in 56.5mol% Li0.87Na0.63K0.50CO3 eutectic–43.5mol% LiOH electrolyte containing 0.5 m Fe2O3 and 3 m NaOH at a sweep rate of 50 mVsec-1 and in the range of 0.2 to 1.7 V at 500 °C. To probe the intrinsic catalytic activity of the catalysts, linear sweep voltammograms recorded using a RDE modified with Pd by the galvanic method at various revolution rates are determined (Figure S4 in the Supporting Information) at 500°C. Prior to determination, the RDE was immersed in the electrolyte for twenty hours so that Ni and Pd on RDE could be in situ oxidized and lithiated to lithium nickel oxide and lithium palladium oxide. As shown in Figure S4, the RDE exhibited a current peak of oxygen reduction at around -0.93 V at various revolution rates. The peak current values are influenced by rotation rate, indicating that the oxygen reduction is under a limiting-diffusion control. In order to verify further the activity of the PdO-decorated NiO nanoparticles toward the OER/ORR, charge-discharge curves using the air electrodes with and without PdO were compared in Figure 5. As shown in Figure 5, it was found that the discharge potential plateau curve shifted toward the high average potential of 1.10 V using the PdO-decorated NiO air electrode, whereas the charge potential plateau curve shifted toward the low average potential of


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1.50 V. The potential difference between oxygen reduction and evolution (∆V1) was 0.40 V, lower than ∆V2 (0.52 V) on the air electrode without PdO, demonstrating that the PdO-decorated NiO air electrode should be highly effective for lowering the charging potential and increasing the discharging potential close to the theoretical value. That is, the air electrode also showed a good bifunctional activity of OER/ORR. 1.8 1.6 Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4 ∆V1= 0.40 V

1.2

∆V2= 0.52 V

1.0 with PdO without PdO

0.8 0.6

0

5

10

15

20

25

Capacity (C)

Figure 5. Charge-discharge curves for the iron molten air battery using the nickel fin air electrodes with and without PdO in the atmosphere. Cycles charge at 0.05 A for 8 minutes and discharge over a 100 Ω load to 0.7 V cutoff at 500 °C.

Figure 6 compares the discharge voltage profiles for the PdO-decorated NiO air electrode and the air electrode without PdO in 56.5 mol % Li0.87Na0.63K0.50CO3 eutectic–43.5 mol % LiOH electrolyte containing 0.5 m Fe2O3 and 3 m NaOH at 500 °C. As seen in Figure6, the battery with the PdO-decorated NiO air electrode exhibits lower polarization losses, that is higher discharge voltages over similar ohmic loads, than the configuration with the air electrode without PdO. Down to a 50 Ω load, the battery discharge is supported without significant potential drop when compared to the 1.28 V of open circuit potential. At constant resistive loads of 20 Ω or less (at higher current densities), a significant potential drop occurs.


13


1.4

1000Ω

Voltage (V)

1.2 500Ω

1.0

100Ω

without PdO with PdO

50Ω

0.8 0.6

20Ω

0.4

10Ω 5Ω 3Ω

0.2 0.0

0

300

1Ω

600 900 1200 1500 1800 Time (seconds)

Figure 6. Comparison of discharge potentials for the iron molten air battery using the nickel fin air electrodes with and without PdO at 500 °C. 3.3. Rate Performance. For the assessment of the rate performance of the air electrodes under cycling conditions, the battery was systematically cycled over various loads (or at different current densities) for 20 cycles each. The charge-discharge curves are shown in Figure 7. As shown in Figure 7, it is clearly shown that the discharge voltages using the PdO-decorated NiO air electrode are higher at various loads than that using the air electrode without PdO. By applying a load of 100 Ω, the PdO-decorated air electrode showed an average discharge voltage of 1.10 V. The discharge voltage decreased as the current density increased; however, event at 9.3 mA cm-2 the PdO-decorated air electrode still retained the average discharge voltage of 0.83 V (the average discharge voltage of 0.83 V/the load of 20 Ω, then normalized by the 5 cm2 electrode surface). This indicates that the rate performance of the current iron molten air battery is reasonably high, which can be attributed to the high activity of the present air electrode.

Average discharge voltage (V)

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1.2 1.0

100 Ω 50 Ω

0.8

40 Ω 20 Ω

0.6

without PdO with PdO

0.4 0.2 0.0

0

20

40 Cycle number

60

80


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Figure 7. The rate capacities for the iron molten air battery using the nickel fin air electrodes with and without PdO at 500 °C. Cycle charges at 0.05 A for 8 minutes and discharges over loads of 100,50, 40 and 20 Ω to 0.7 V cutoff. 3.4. Cycle Performances. Building on this study’s results, longer-term charge-discharge tests were performed to determine the term stability of the PdO-decorated NiO air electrode enhancement effect. All cycles were conducted with charging at 0.05 A for 8 minutes and subsequent discharging over a 100 Ω load to 0.7 V cutoff. The results of repeated chargedischarge cycling can be seen in Figure S5 in the Supporting Information. As evident, the air electrode cycled stably for 100 cycles in molten 56.5 mol % Li0.87Na0.63K0.50CO3 eutectic–43.5 mol % LiOH with 0.5 m Fe2O3 and 3 m NaOH in the air at 500 oC. As seen in Figure 8 a and b, the battery exhibits a discharge voltage plateau at approximately 0.98 V and an initial coulombic efficiency of only 12.1% in the first cycle because the formation of catalysts is only just beginning on the air electrode. Thereafter, the discharge plateau voltage and coulombic efficiency increase quickly and reach to 1.09 V and 80.7% in the 9th cycle, respectively, indicating that the catalysts on the air electrode have formed in the first nine cycles. The charging curves and discharging curves for the 20th, 40th, 60th, 80th, and 100th cycle were highly constant and consistent (Figure 8a). Moreover, no evident coulombic efficiency fading was observed for 100 cycles, and the highest coulombic efficiency reached 98.7% (Figure 8b). As seen in Figure 8c and d, for the majority of cycles, the average charge potential, the average open circuit voltage, and the average discharge potential were around ~1.49, ~1.29 V, and ~1.10 V, respectively. Compared to the average discharge potential of ~1.08 V in our previous study 4, the average discharge potential for the PdO-decorated NiO air electrode increased by 18.5%.The voltage efficiencies of the majority of cycles were above 73%, the highest reached to 74.8%. No


15


significant voltage efficiency degradation was also observed for 100 cycles. This showed that the

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 (c) 0.0 0 20

100

charge

Coulombic efficiency (%)

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 (a) 0.0 0 4

discharge 1st 20th 40th

60th 80th 100th

8 12 16 Capacity (C)

20

80 60 40 20 (b) 0

24

0

20

40 60 Cycle number

80

100

20

40 60 80 Cycle number

100

100 Voltage efficiency (%)

Voltage (V)

battery exhibits excellent long-term stability and repetitiveness of the battery cycling.

Average voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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charging open circuit discharging

40 60 80 Cycle number

100

80 60 40 20 (d) 0

0

Figure 8. Cycle performance of the iron molten air battery with the PdO-decorated NiO air electrode in a 56.5mol% Li0.87Na0.63K0.50CO3 eutectic–43.5 mol% LiOH electrolyte containing 0.5 m Fe2O3 and 3 m NaOH at 500 °C. (a) Voltage profiles for 1st, 20th, 40th, 60th, 80th and 100th cycles. (b) Coulombic efficiency. (c) The average voltage of charging, open circuit and discharging. (d) Voltage efficiency. 4. CONCLUSIONS In summary, an air electrode with lithiated PdO nanoislands decorating lithiated NiO nanoparticles for the iron molten air battery was successfully prepared by deposition of Pd on a nickel air electrode via a galvanic exchange methodology, and then was lithiated during the battery cycling. It was found that the lithiated NiO nanoparticles have been successfully decorated with the lithiated PdO nanoislands. The decoration of lithiated PdO nanoislands effectively improved OER/ORR catalytic activity of the air electrode. The battery with the PdOdecorated NiO nanoparticle air electrode had not only a higher discharging voltage, but also a lower charging voltage and a superior rate performance than that without PdO decorating.


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Moreover, the battery facilitated the achievement of a long cycle life up to 100 cycles without the coulombic efficiency and voltage efficiency fading. The highest coulombic efficiency and average discharge potential reached respectively 98.7 % and ~1.10 V at 500 °C. These findings are potentially important to design practical application iron molten air batteries with high performance. ■ASSOCIATED CONTENT Supporting Information EDS data for the air electrode after the galvanic displacement with Pd, EDS data for the air electrode after 100 cycles, cycled charge-discharge behavior of the iron molten air battery with the PdO-decorated NiO air electrode in the 56.5 mol% Li0.87Na0.63K0.50CO3 eutectic–43.5 mol% LiOH electrolyte containing 0.5 m Fe2O3 and 3 m NaOH at 500 °C, polarization curves for ORR on the PdO-decorated NiO nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. ■AUTHOR INFORMATION Corresponding Author *E-mail:[email protected] (S.-Z. L.); [email protected] (S.L.). ■ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21473028), the Natural Science Foundation of Heilongjiang Province, China (Grant No. B2015011), and the Postdoctoral Scientific Research Development Fund of Heilongjiang Province, China (Grant No. LBH–Q14029). S. Liu is grateful to the Foundation of Northeast Petroleum University, China for partial support of this study. We appreciate the assistance of


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Analysis Test Center of College of Chemistry and Chemical Engineering, Northeast Petroleum University in collecting XRD, FE-SEM and EDS data. Licht is grateful to the US NSF (Award 1505830) for partial support of this study. ■REFERENCES (1) Licht, S.; Cui, B. C.; Stuart,J.; Wang, B. H.; Lau, J. Molten Air–a New, Highest Energy Class of Rechargeable Batteries. Energy Environ.Sci.2013, 6, 3646–3657. (2) Liu,S.Z.; Li,X.; Cui,B. C.; Liu,X. J.; Hao,Y. L.; Guo,Q.; Xu, P. Q.; Licht, S. Critical Advances for the Iron Molten Air Battery: A New Lowest Temperature, Rechargeable, Ternary Electrolyte Domain. J. Mater. Chem. A 2015, 3, 21039–21043. (3) Cui,B.C.;Licht,S.A Low Temperature Iron Molten Air Battery. J. Mater. Chem. A 2014,2,10577–10580. (4) Cui,B.C.;Xiang,W.;Liu,S.Z.;Xin, H.Y.;Liu,X.J.;Licht,S.A Long Cycle Life, High Coulombic Efficiency Iron Molten Air Battery. Sust. Energ. Fuels 2017,1, 474–481. (5) Cui, B.C.;Xin,H.Y.;Liu,S.Z.;Liu,X.;Hao,Y.L.;Guo,Q.;Licht,S.Improved Cycle Iron Molten Air Battery Performance Using a Robust Fin Air Electrode. J. Electrochem. Soc.2017,164, A88– A92. (6)McKerracher,R.D.; Alegre,C.; Baglio,V.; Aricò,A.S.; Ponce de León,C.; Mornaghini,F.; Rodlert,M.; Walsh,F.C.A Nanostructured Bifunctional Pd/C Gas–Diffusion Electrode for Metal– Air Batteries. Electrochim. Acta 2015,174,508–515. (7) Alegre,C.; Stassi, A.; Modica, E.; Vecchio, C. L.; Aricò,A. S.; Baglio, V. Investigation of the Activity and Stability of Pd Based Catalysts Towards the Oxygen Reduction (ORR) and Evolution Reactions (OER) in Iron–Air Batteries. RSC Adv.2015,5,25424–25427.


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Nano PdO Activated Iron Molten Air Battery - ACS Publications

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