LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

Electrochimica Acta 56 (2011) 7523–7529

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

LaNi0.9 Ru0.1 O3 as a cathode catalyst for a direct borohydride fuel cell Xiaozhu Wei, Xiaodong Yang, Sai Li, Yuanzhen Chen, Yongning Liu ∗ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China

a r t i c l e

i n f o

Article history: Received 29 March 2011 Received in revised form 28 June 2011 Accepted 28 June 2011 Available online 6 July 2011 Keywords: Direct borohydride fuel cell Oxygen reduction reaction Membraneless fuel cell Perovskite-type oxide Cathode catalyst

a b s t r a c t LaNi0.9 Ru0.1 O3 as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi0.9 Ru0.1 O3 exhibited higher electrochemical performance compared with LaNiO3 , which suggested incorporation of element Ru into LaNiO3 could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi0.9 Ru0.1 O3 catalyst was found to have good tolerance of BH4 − . Meanwhile the maximum power density of 171 mW cm−2 was obtained at 65 ◦ C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi0.9 Ru0.1 O3 , which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Fuel cells, which directly convert the chemical energy into electricity by electrochemical reactions, are viewed as promising power sources, owing to their high energy efficiency, environmental compatibility, and quiet operation [1–3]. Of several different types of fuel cells under development today, the direct borohydride fuel cell (DBFC) has been studied extensively as a potential candidate for portable applications and automobiles, due to its high fuel utilization efficiency [4–6], high theoretical open circuit voltage (OCV), and easy handling of liquid fuel [7–10]. As one of the key materials in the fuel-cell system, cathode catalyst is crucially important to the development of DBFC. At present, platinum and platinum-based catalysts are most frequently used cathode catalysts for ORR, and the scarcity and high price of them is one of the factors that limited the commercialization of DBFCs [11,12]. To become commercially viable, finding alternative cathode catalysts which have high catalytic activity and low cost is of prime concern and has been the focus of numerous investigations in recent years. In view of its reasonable ORR activity, Ag has been used as cathode catalyst in some literatures [7,11,13]. However, further researches have shown that Ag is not a suitable cathode catalyst, owing to its performance deterioration related to its poor property and stability in its water management and high sensitivity towards borohydride crossover [12,14]. Moreover, some kinds of oxides, such as MnO2 [15–17], Eu2 O3 [18], demonstrate considerable electrochemical activity for ORR as well as good borohydride

∗ Corresponding author. Tel.: +86 29 8266 4602; fax: +86 29 8266 3453. E-mail address: [email protected] (Y. Liu). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.06.100

tolerance, however, the discharge performances are not satisfactory. The OCV of MnO2 is only about 0.87 V, and the maximum power density of the cell using Eu2 O3 is only 66.4 mW cm−2 . Apart from these, several transition-metal macrocycle compounds have been developed and successfully applied as cathode catalysts in DBFCs, including iron/cobalt phthalocyanine (Fe/CoPc) [19,20], iron tetramethoxyphenyl porphyrin (FeTMPP) [13] and cobalt polypyrrole carbon composite (Co-PPY-C) [21]. Another problem existing in the current DBFCs is “BH4 − crossover”, which results in the deactivation of the cathode catalyst and low utilization of BH4 − . In order to overcome this problem, an ion exchange membrane is applied to prevent BH4 − from contacting the cathode [9,22] and Nafion® membrane is the most common choice to be employed currently [12,21,23–25]. But some other accompanied disadvantages emerge, such as carbonate fouling, accumulation of alkali in the cathode, and high cost [9,22]. To avoid these membrane-related issues, more attention is paid to construct DBFCs without membrane, and air-breathing laminar flow-based fuel cell (LFFC) has been developed. On the micro-scale, fuel crossover can only occur via diffusion and may be minimized by adjusting stream flow rates and varying channel dimensions [26,27]. From the angle of materials, cathode catalysts with good tolerance of BH4 − may be beneficial. With this kind of cathode catalyst, a simple DBFC can be built, which avoids not only the degradation of the cathode performances arising from BH4 − but also the use of precious membrane [15,18–20,28,29]. Perovskite-type oxides, with the general formula of ABO3 where A is rare earth metal and B is transition metal, can be employed as catalysts for the ORR because of their relatively available specific surface areas, easy synthesis, high conductivity, low price, and good oxygen reduction in alkaline solution [30–32]. Furthermore,

7524

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529

ABO3 allows the possibility of not using any membrane [28,29,33]. Among the candidate perovskite-type oxides, LaNiO3 has attracted considerable attention for its good electrochemical performance. Suntivich et al. [32] have compared the ORR activities of LaNiO3 , LaCu0.5 Mn0.5 O3 , and La0.75 Ca0.25 FeO3 , finding that the activity of LaNiO3 is much higher, which is shown to be competitive with current platinum-based cathodes. Ma et al. [29] have presented a DBFC using LaNiO3 as cathode catalyst, with the maximum power density of 127 mW cm−2 at 65 ◦ C and a high stability at room temperature. On the other hand, it has been found that the B-site cation in ABO3 directly contributes to the observed catalytic performances [34]. As a result, many transition-metal ions have been explored for partial replacements at the B-site to further improve the ORR. As different classes of Ru-based catalysts are among the most promising alternative catalysts for ORR in fuel cells [35–37], in the present work, we attempt to replace partial Ni for Ru to form perovskite-type oxides LaNi0.9 Ru0.1 O3 in order to further improve the electrochemical performance of LaNiO3 . Although, much work has been done on the ORR of LaNiO3 and Ru-based catalysts, respectively, few investigations have been carried out on LaNiO3 , in which small stoichiometric quantities Ru is substituted for Ni. Hence, it will be interesting to use LaNi0.9 Ru0.1 O3 as an alternative cathode catalyst in DBFC. The structures and morphologies of the prepared electrocatalysts were characterized by various techniques, and their performances were tested in half and single fuel cells by electrochemical methods. 2. Experimental 2.1. Preparation of catalyst The citrate-based sol–gel method was adopted to prepare LaNi0.9 Ru0.1 O3 , in which analytical grade lanthanum nitrate (La(NO3 )3 ·6H2 O), nickel nitrate (Ni(NO3 )2 ·6H2 O), ruthenium chloride (RuCl3 ·nH2 O), citric acid (C6 H8 O7 ·H2 O), and ammonia water (NH3 ·H2 O) were used as raw materials. According to the stoichiometric composition of the reactants, specified amounts of La(NO3 )3 ·6H2 O, Ni(NO3 )2 ·6H2 O, and RuCl3 ·nH2 O were dissolved in excess deionized water with citric acid. The molar rate for the citric acid to the metal ions was 1:1. Under vigorous stirring, NH3 ·H2 O (25–28 wt% NH3 ) was slowly added to adjust the pH value of the solution in the range of 7–8 at room temperature. After this, the sol was vaporized on water bath at a temperature of 70 ◦ C to obtain deep-gray viscous gel, followed by drying in a vacuum at 80 ◦ C for 8 h. Subsequently the dry gel was ignited in air, and a loose precursor powder was gained. Finally, the sample was subjected to heat-treatment in air at 800 ◦ C for 2 h. 2.2. Preparation of electrodes and cells The cathode used in this system was a three-layer electrode consisting of a gas diffusion layer, an active layer and a current accumulating matrix. The gas diffusion layer of 0.2 mm thick was prepared by mixing 60 wt% acetylene black and 40 wt% polytetrafluoroethylene (PTFE) with ethanol, and heat treatment at 340 ◦ C for 1 h. The active layer was prepared by mechanically mixing 30 wt% LaNi0.9 Ru0.1 O3 , 45 wt% carbon nano-tube (CNT), and 25 wt% PTFE emulsion. Then, the slurry was coated onto a 1 cm × 1 cm Ni foam sheet (thickness = 1.7 mm, porosity > 95%). The electrode sheet was dried at 80 ◦ C in air for 2 h. Finally, the three-layer gas electrode was finished by pressing the coated Ni-foam and the gas diffusion layer into a 0.7 mm thick sheet. To prepare the anode, cobalt (II) oxide (CoO) powder (97 wt% CoO) was mixed together with 30% PTFE solution (3 wt% PTFE) and then the mixture was smeared onto a 1 cm × 1 cm

Ni-foam sheet. After drying at 80 ◦ C under vacuum for 2 h, the electrode was pressed with a pressure of 3 MPa [38]. Before cell testing, the anode was dipped in the 6 M KOH–0.8 M KBH4 aqueous solution for 2 h to activate. The mass loadings of LaNi0.9 Ru0.1 O3 in the cathode and CoO in the anode were 7.5 mg cm−2 and 150 mg cm−2 , respectively. 2.3. Characterization of catalyst The catalyst structure of the synthesized powders was investigated with an X-ray diffractometer (XRD) (D/MAX-3A, Japan) using a Cu K˛ ( = 15.444 nm) source. The elemental composition of LaNi0.9 Ru0.1 O3 was analyzed with energy-dispersive X-ray spectroscopy (EDS) (IE350, OXFORD Instrument plc, UK). The particle size and the morphology of the samples were observed by scanning electron microscopy (SEM) (FE SEM, JSM-6700F, JEOL, Japan). The BET surface area and pore width of catalysts were acquired on a specific surface area and porosity analyzer (ASAP2020, Mack Instrument Company, USA). The roughness of electrodes was evaluated by violet laser color 3d laser scanning microscope (VK-9710, KEYENCE, Japan). 2.4. Electrochemical measurements Polarization curves were carried out to characterize the electrochemical performance of the cathode by Electrochemistry Workstation (CHI650C, ChenHua, Shanghai, China) employing Line sweep voltammetry (LSV). A standard three-electrode system was used in these electrochemical measurements. The cathode made in the above mentioned procedure served as the working electrodes, a Hg/HgO/6 M KOH electrode as a reference electrode, and a Pt wire as a counter electrode. The reference electrode was placed in a Luggin capillary with its tip positioned close to the working electrode. The LSV was plotted at ambient atmosphere at room temperature (22 ◦ C) within a potential region of 0 to −0.6 V at a scan rate of 5 mV s−1 . Cyclic voltammetry (CV) was employed to reveal the reactivity for BH4 − of LaNi0.9 Ru0.1 O3 catalyst by Electrochemistry Workstation using the same method as our previous work [39]. Graphite electrode and Hg/HgO/6 M KOH electrode was used as counter electrode and reference electrode, respectively. The cyclic voltammogram was drawn at 25 ◦ C and a scan rate of 100 mV s−1 . The discharging performances of this DBFC were performed by a battery testing system (from Neware Technology Limited, Shenzhen, China). In our experiment, two different kinds of cell systems were used to measure the performances of DBFCs at different gas atmosphere. As described in Fig. 1(a), the cathode was sandwiched between a container with a square window of 1 cm2 and a plate with the same square window, and the gas diffusion layer of the cathode was exposed to air. In Fig. 1(b), the cathode was placed between the container and a plate with an oxygen channel and a sealed square window of 1 cm2 , and the gas diffusion layer of the cathode was exposed to pure oxygen. The oxygen rates were 5 sccm at 22 ◦ C and 1 atm. In both Fig. 1(a) and (b), the active layer was in contact with the electrolyte fuel, and the anode was placed inside the container, facing the cathode and 2 cm away from it. The electrolyte fuel was 0.8 M KBH4 –6 M KOH. All the experiments about the discharging performances were performed at these conditions. 3. Results and discussion 3.1. Structural characterization Fig. 2 presents the XRD results of synthesized LaNi0.9 Ru0.1 O3 (a) and LaNiO3 (b) along with the standard diffraction pattern for LaNiO3 as a reference. Also we find that weak nickel oxide (NiO)

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529

7525

Fig. 1. Schematic diagram of DBFCs using two different kinds of catholytes: (a) air under nature convection; (b) pure oxygen; oxygen flow rate: 5 sccm; temperature: 22 ◦ C. pressure: 1 atm.

(JCPDS, card number: 044-1159) peaks appear both in Fig. 2(a) and (b), and weak nickel lanthanum oxide (La2 NiO4 ) (JCPDS, card number: 034-0314) peaks exist in Fig. 2(b). The spectrum of LaNi0.9 Ru0.1 O3 shows that La2 NiO4 phase disappears and a special phase of which structure is similar to lanthanum palladium complex oxide (La2 O3 ·xPdO) (JCPDS, card number: 047-0574) is

LaNiO3 NiO

La2NiO4 La2O3 xPdO

20

30

40

50 2θ/degree

60

70

(310)

(300)

(220)

(211)

(210)

(200)

JCPDS, card number : 033-0710 (111)

(110)

(b)

(100)

Intensity/a.u.

(a)

80

Fig. 2. XRD patterns for (a) LaNi0.9 Ru0.1 O3 and (b) LaNiO3 . Also shown is the standard pattern of LaNiO3 from JCPDS, card number: 033-0710.

identified. However, we did not use any element Pd in the synthesis process and the result of EDS in Fig. 3 also confirms the absence of element Pd. We speculate that there may be a composite oxide of La2 O3 and RuO with the same structure of La2 O3 ·xPdO in the synthesized LaNi0.9 Ru0.1 O3 , suggesting that not all Ru3+ enters the crystalline lattice, and some ions form impure phases, which may lead to the formation of the lattice defects. Additionally, (110) peak of LaNiO3 in Fig. 1(b) slightly shift from 32.778◦ to 32.388◦ , because Ru3+ (8.2 nm) is relatively larger in size than Ni2+ (6.9 nm), and the introduction of Ru3+ to the perovskite lattice engenders a moderate expansion in lattice parameters. Fig. 4 provides the SEM images of the LaNi0.9 Ru0.1 O3 and LaNiO3 powders. It is found that globular particles are displayed uniformly with average diameter around 60–100 nm. Furthermore, the Rusubstitution somewhat promotes the process of agglomeration due to high activity of doped Ru and sintering treatments at high temperature. Table 1 shows roughness of the electrodes. It can be found that two electrodes with different catalysts almost have the same roughness, so it is reasonable for us to compare the electrochemical performance of catalysts without considering the influence of roughness. In Table 2, the BET surface area and pore width of the catalysts are revealed. It is obvious that the LaNiO3 catalyst possesses higher surface area and smaller pore width than LaNi0.9 Ru0.1 O3 , which is in accordance with the result of SEM that more agglomerates exist in LaNi0.9 Ru0.1 O3 powder.

7526

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529

Fig. 3. EDS pattern for the synthesized LaNi0.9 Ru0.1 O3 catalyst. Table 1 Measured roughness of electrodes with different catalysts.

Table 2 Measured BET surface area and pore width of two catalysts.

Electrodes with different catalysts

Roughness (Ra ) (␮m)

Catalysts

SBET (m2 g−1 )

Pore width (nm)a

LaNi0.9 Ru0.1 O3 LaNiO3

1.542 1.508

LaNi0.9 Ru0.1 O3 LaNiO3

6.6451 23.6189

51.11 7.74

a

3.2. Polarization properties of cathodes Polarization curves for the LaNi0.9 Ru0.1 O3 and LaNiO3 catalyzed cathodes are shown in Fig. 5. The polarization trend of LaNi0.9 Ru0.1 O3 is smaller than that of LaNiO3 . A current density of 305 mA cm−2 (at −0.6 vs (Hg/HgO)/V) is obtained when using LaNi0.9 Ru0.1 O3 as cathode catalyst. It suggests that the former reveals superior catalytic performance for the ORR in alkaline solution. The results may be attributed to lattice defects of the LaNi0.9 Ru0.1 O3 perovskite-type phase which consequently increases the concentration of oxygen vacancies. Hence, both ionic conductivity and the presence of main active sites are enhanced. For comparison, the polarization curve of commercially available

Width of pores is calculated by the BJH equation.

cathode with Pt (1 mg Pt cm−2 ) was also demonstrated in Fig. 5 [21]. It can be found that the polarization of Pt/C catalyzed cathode is much higher, and the current densities are 160 mA cm−2 and 226 mA cm−2 at a potential of −0.45 V, respectively, reconfirming that LaNi0.9 Ru0.1 O3 has a good activity for ORR in alkaline solution. 3.3. Borohydride tolerance of cathode Fig. 6 gives the polarization curves of the LaNi0.9 Ru0.1 O3 catalyzed cathode in 6 M KOH solutions with and without the addition of 0.8 M KBH4 . As shown in Fig. 6, although a small deviation is observed, overall, the polarization curve of the LaNi0.9 Ru0.1 O3

Fig. 4. SEM images of the prepared catalysts (a) LaNi0.9 Ru0.1 O3 ; (b) LaNiO3 .

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529

10

0.0 LaNi0.9Ru0.1O3

8

LaNiO3

6

-2

-0.1

Current density/10 A cm

Pt/C [ref.21 ]

-0.2

-4

Potential vs (Hg/HgO)/V

7527

-0.3 -0.4 -0.5 -0.6

4 2 0 -2 -4 -6

-3

In 1 mol dm KOH solution -3 -3 In 1 mol dm KOH+0.1 mol dm KBH4 solution

-8 0

50

100

150

200

250

300

350

-10

-2

Current density/mA cm

-1.2

-0.6 -0.4 -0.2 Potential vs SHE/V

0.0

3.4. Performance of DBFCs Fig. 8 displays the performances of the cells at different discharge current densities using LaNi0.9 Ru0.1 O3 and LaNiO3 as cathode catalysts in 0.8 M KBH4 –6 M KOH. As shown in Fig. 8(a), the OCV of the DBFC using LaNi0.9 Ru0.1 O3 as cathode catalyst is about 1.13 V, which is higher than that of LaNiO3 (1.11 V). Com-

1.2

(a)

100

1.0

80

0.8

60

0.6

40

0.4

LaNiO3

20

-2

LaNi0.9Ru0.1O3

0.2

0

Pt/C [ref. 23] 0.0 0

50

100 150 200 250 -2 Current density/mA cm

300

0.0

120 1.2

(b)

without KBH4

-0.1

100

with KBH4

Cell potential /V

1.0

-0.2 -0.3

60

0.6

40

0.4

LaNiO3

20

LaNi0.9Ru0.1O3

-0.5 0.2

-0.6

-2

-0.4

80 0.8

Power density/mW cm

Potential vs (Hg/HgO)/V

0.2

Power density/mW cm

catalyzed cathode in the presence of BH4 − ions is approximately identical to that in the absence of BH4 − ions, even the largest difference of current obtained at the potential of −0.413 V is only 4.5%, indicating that the existence of BH4 − ions has almost no negative influences on the discharge performance of the cathode. Therefore, LaNi0.9 Ru0.1 O3 can be used in the DBFC system without using any ion exchange membranes, owing to its acceptable tolerance for the BH4 − ions in alkaline solution. In order to further investigate the reactivity for BH4 − of LaNi0.9 Ru0.1 O3 catalyst, we characterized the cyclic voltammogram of the GC electrode modified with LaNi0.9 Ru0.1 O3 in fuel solution (0.1 M KBH4 + 1 M KOH) and electrolyte solution (1 M KOH), respectively. To eliminate the influence of the oxygen reduction reaction, the solution was saturated with nitrogen gas to exclude oxygen. As shown in Fig. 7, the cyclic voltammograms of LaNi0.9 Ru0.1 O3 cathodes are almost identical regardless of the presence of BH4 − with exception for a very small deviation. Besides, there is no apparent oxidation peak, which indicates that the LaNi0.9 Ru0.1 O3 catalyst does not react with BH4 − and has a strong tolerance to BH4 − . As for the slight decrease in the current density of the LaNi0.9 Ru0.1 O3 catalyst in the presence of BH4 − , further work is in progress to understand the mechanism.

-0.8

Fig. 7. Cyclic voltammograms of LaNi0.9 Ru0.1 O3 -modified glassy carbon in 1 M KOH solution and 1 M KOH + 0.1 M KBH4 solution saturated with N2 . Operation temperature: 25 ◦ C. Scan rate: 100 mV s−1 . Init E: 0.2 vs (Hg/HgO)/V; High E: 0.2 vs (Hg/HgO)/V; Low E: −1.2 vs (Hg/HgO)/V.

Cell potential/V

Fig. 5. Polarization curves of the LaNi0.9 Ru0.1 O3 and LaNiO3 -catalyzed cathodes at ambient atmosphere in 6 M KOH. Operation temperature: 22 ◦ C. Scan rate: 5 mV s−1 . Referenced polarization curves of Pt/C [21].

-1.0

0 0

50

100

150

200

250

300

-2

0

50

100

150

200

250

300

350

-2

Current density/mA cm

Fig. 6. Polarization curves of a LaNi0.9 Ru0.1 O3 -catalyzed cathode in 6 M KOH solution with or without the addition of 0.8 M KBH4 . Operation temperature: 22 ◦ C. Scan rate: 5 mV s−1 .

Current density/mA cm

Fig. 8. Electrochemical performance of DBFCs using LaNi0.9 Ru0.1 O3 and LaNiO3 as cathode and CoO as anode catalyst in (a) air at nature convection; (b) pure oxygen. Operation temperature: 22 ◦ C. Fuel solution: 6 M KOH–0.8 M KBH4 . Referenced electrochemical performance using Pt/C as cathode catalyst, Ni + Pd/C as anode catalyst, and N112 [23].

7528

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529

1.2

1.2

180

(a)

LaNi0.9Ru0.1O3

160 120 0.8

100 80

0.6

60

25 35 45 55 65

0.4

0.0 -50

20

-2

0.2

40

0.8 0.6 0.4

0

0.2

-20 0

Pt/C [ref. 40]

1.0

Cell potential/V

Cell potential/V

140

Power density/mW cm

1.0

50 100 150 200 250 300 350 400 450 500 -2

Current density/mA cm

0.0 0

1.2

Cell potential/V

120 0.8

100 80

0.6

60 0.4

40 20

LaNiO365

0

-2

0.0 -50

LaNi0.9Ru0.1O365

Power density/mW cm

140

-20 0

20

30

40

50

60

70

Time/hour

160 1.0

0.2

10

180

(b)

50 100 150 200 250 300 350 400 450 500 -2

Current density/mA cm

Fig. 9. (a) Electrochemical performance of DBFCs using LaNi0.9 Ru0.1 O3 as cathode and CoO as anode catalyst at different temperatures; (b) LaNi0.9 Ru0.1 O3 and LaNiO3 as cathode catalyst at 65 ◦ C. Fuel solution: 6 M KOH–0.8 M KBH4 .

pared with the cell using LaNiO3 as cathode catalyst, the cell using LaNi0.9 Ru0.1 O3 demonstrates a higher performance. A maximum power density of 96.8 mW cm−2 can be achieved at 0.537 V even under air at room temperature (22 ◦ C), while the maximum power density is only about 91 mW cm−2 when using LaNiO3 as cathode catalyst under the same conditions. Performance of the DBFC employing Pt/C (1 mg Pt cm−2 ) as cathode catalyst was also given [23]. This figure shows that a relatively high power density can be achieved when using LaNi0.9 Ru0.1 O3 . Further improvement is observed while using pure oxygen instead of air, as displayed in Fig. 8(b), maximum power density of 106 mW cm−2 and 96 mW cm−2 are obtained for LaNi0.9 Ru0.1 O3 and LaNiO3 , respectively. 3.5. Performance of DBFC at different operating temperatures Fig. 9(a) shows the performance of the DBFC employing LaNi0.9 Ru0.1 O3 as cathode catalyst at different temperatures. It can be found that the maximum power density obviously increases with increasing temperature. The maximum power densities obtained are 115, 128, 138, 151, and 171 mW cm−2 at 25, 35, 45, 55, and 65 ◦ C, respectively. The increase in performance is primarily attributed to a combined effect of enhancing kinetics of the ORR, reducing the electrode overpotential, increasing the electrode reaction rates and reducing the cell resistance. Fig. 9(b) compares power densities of the DBFC using LaNi0.9 Ru0.1 O3 as cathode catalyst with that using LaNiO3 at 65 ◦ C. This figure shows that a relatively high maximum power density can be achieved when using LaNi0.9 Ru0.1 O3 as cathode catalyst. This result further confirms that LaNi0.9 Ru0.1 O3 owns a better catalytic ability for ORR in alkaline solution.

Fig. 10. Life-time test of the DBFC employing LaNi0.9 Ru0.1 O3 as cathode and CoO as anode catalyst at a current density of 20 mA cm−2 at ambient atmosphere. Operation temperature: 22 ◦ C. Fuel solution: 6 M KOH–0.8 M KBH4 . Referenced performance stabilities using Pt/C as cathode, Au/C as anode catalyst, and N117 at the same current density [40].

3.6. Stability of DBFC The stability of the DBFC is investigated by monitoring changes in cell potential during galvanostatic operation. Fig. 10 shows the changes of the cell potential at a constant current discharge of 20 mA cm−2 , which lasted for approximately 70 h at ambient atmosphere. New fuel solution was added every 12 h, and data points were randomly picked. As can be seen from Fig. 10, the cell potential reaches a stable value around 0.97 V, which is much higher than the cell using LaNiO3 as cathode catalyst (0.9 V) in Ref. [29]. Despite some slight fluctuations, no obvious decay occurs during the operation time, suggesting that the cell using LaNi0.9 Ru0.1 O3 as catalyst has good performance stability. The high stability of the cell performance may be attributed to the good tolerance for BH4 − of LaNi0.9 Ru0.1 O3 catalyst. Compared with that using Pt/C (2 mg Pt cm−2 ) as the cathode catalyst [40], the DBFC employing LaNi0.9 Ru0.1 O3 has better performance stability and higher cell potential. 4. Conclusion Here we have demonstrated a perovskite oxide LaNi0.9 Ru0.1 O3 catalyst synthesized via a sol–gel method, and studied its electrochemical properties for possible use as the cathode catalyst in a DBFC. Our experiments indicate that LaNi0.9 Ru0.1 O3 exhibits a higher electrochemical performance than LaNiO3 . As for LaNi0.9 Ru0.1 O3 , a current density of 305 mA cm−2 (at −0.6 vs (Hg/HgO)/V) is obtained. The acceptable borohydride tolerance enables this DBFC work without using expensive ion exchange membranes, and the maximum power densities of 115 mW cm−2 and 171 mW cm−2 are achieved at 25 ◦ C and 65 ◦ C, respectively. Furthermore, such catalyst promises to work without noticeable loss in performance over a relative long-time operation. It is concluded that, without using expensive ion exchange membranes, LaNi0.9 Ru0.1 O3 provides a simple way to construct DBFCs, and it may be promising cathode catalyst for DBFCs with high performance and low cost. References [1] C.Y. Wang, Chem. Rev. 104 (2004) 4727. [2] K.V. Kordesch, G.R. Simader, Chem. Rev. 95 (1995) 191. [3] A. Kirubakaran, S. Jain, R.K. Nema, Renew. Sust. Energ. Rev. 13 (2009) 2430.

X. Wei et al. / Electrochimica Acta 56 (2011) 7523–7529 [4] J.H. Kim, H.S. Kim, Y.M. Kang, M.S. Song, S. Rajendran, S.C. Han, D.H. Jung, J.Y. Lee, J. Electrochem. Soc. 151 (2004) A1039. [5] R. Jamard, A. Latour, J. Salomon, P. Capron, A. Martinent-Beaumont, J. Power Sources 176 (2008) 287. [6] G.J. Wang, Y.Z. Gao, Z.B. Wang, C.Y. Du, G.P. Yin, Electrochem. Commun. 12 (2011) 1070. [7] B.H. Liu, Z.P. Li, K. Arai, S. Suda, Electrochim. Acta 50 (2005) 3719. [8] V. Kiran, S. Srinivasan, J. Indian Inst. Sci. 89 (2009) 447. [9] J. Ma, N.A. Choudhury, Y. Sahai, Renew. Sust. Energy Rev. 14 (2010) 183. [10] Z.P. Li, B.H. Liu, K. Arai, S. Suda, J. Alloy Compd. 404 (2005) 648. [11] H. Cheng, K. Scott, K. Lovell, Fuel Cells 6 (2006) 367. [12] B.H. Liu, S. Suda, J. Power Sources 164 (2007) 100. [13] H. Cheng, K. Scott, J. Electroanal. Chem. 596 (2006) 117. [14] M. Chatenet, F. Micoud, I. Roche, E. Chainet, Electrochim. Acta 51 (2006) 5452. [15] R.X. Feng, H. Dong, Y.D. Wang, X.P. Ai, Y.L. Cao, H.X. Yang, Electrochem. Commun. 7 (2005) 449. [16] Y.G. Wang, Y.Y. Xia, Electrochem. Commun. 8 (2006) 1775. [17] A. Verma, A.K. Jha, S. Basu, J. Power Sources 141 (2005) 30. [18] X.M. Ni, Y. Wang, Y.L. Cao, X.P. Ai, H.X. Yang, M. Pan, Electrochem. Commun. 12 (2010) 710. [19] J.F. Ma, J. Wang, Y.N. Liu, J. Power Sources 172 (2007) 220. [20] J.F. Ma, Y.N. Liu, P. Zhang, J. Wang, Electrochem. Commun. 10 (2008) 100. [21] H.Y. Qin, Z.X. Liu, W.X. Yin, J.K. Zhu, Z.P. Li, J. Power Sources 185 (2008) 909. [22] N.A. Choudhury, S.K. Prashant, S. Pitchumani, P. Sridhar, A.K. Shukla, J. Chem. Sci. 121 (2009) 647.

[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

7529

B.H. Liu, Z.P. Li, S. Suda, J. Power Sources 175 (2008) 226. H. Cheng, K. Scott, J. Appl. Electrochem. 36 (2006) 1361. C. Kim, K.J. Kim, M.Y. Ha, J. Power Sources 180 (2008) 154. P.J. Kenis, R. Jayashree, F. Brushett, A. Hollinger, D. Natatajan, L. Markoski, ECS Meeting Abstracts 902 (2009) 1136. F.R. Brushett, R.S. Jayashree, W.-P. Zhou, P.J.A. Kenis, Electrochim. Acta 54 (2009) 7099. Y. Liu, J.F. Ma, J.H. Lai, Y.N. Liu, J. Alloy Compd. 488 (2009) 204. J. Ma, Y. Liu, Y. Yan, P. Zhang, Fuel Cells 8 (2008) 394. A. Martinez-Juarez, L. Sanchez, E. Chinarro, P. Recio, C. Pascual, J.R. Jurado, Solid State Ion. 135 (2000) 525. G.L. Wang, Y.Y. Bao, Y.M. Tian, J. Xia, D.X. Cao, J. Power Sources 195 (2010) 6463. J. Suntivich, H.A. Gasteiger, N. Yabuuchi, S.-H. Yang, J. Electrochem. Soc. 157 (2010) 1263. Y. Liu, Y.N. Liu, J.F. Ma, J.H. Lai, J. Power Sources 195 (2009) 1854. S.K. Tiwari, S.P. Singh, R.N. Singh, J. Electrochem. Soc. 143 (1996) 1505. J. Prakash, H. Joachin, Electrochim. Acta 45 (2000) 2289. J.W. Lee, B.N. Popov, J. Solid State Electrochem. 11 (2007) 1355. C.C. Chang, T.C. Wen, J. Appl. Electrochem. 27 (1997) 355. S. Li, Y.N. Liu, Y. Liu, Y.Z. Chen, J. Power Sources 195 (2010) 7202. X.D. Yang, S. Li, Y. Liu, X.Z. Wei, Y.N. Liu, J. Power Sources 196 (2011) 4992. H. Cheng, K. Scott, J. Power Sources 160 (2006) 407.