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carbon nanotubes-catalysed anode
Journal of Power Sources 351 (2017) 79e85
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The high utilization of fuel in direct borohydride fuel cells with a PdNix-B/carbon nanotubes-catalysed anode Yaping Zhou a, Sai Li a, b, *, Yuanzhen Chen a, Yongning Liu a, ** a b
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi'an Jiaotong University, Xi'an 710049, PR China School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, PR China
h i g h l i g h t s PdNix-B/CNT catalysts can ease the borohydride hydrolysis and keep high discharge performance. The power density could reach 87 mW cm2 and fuel efficiency could reach 69%. The cost of the battery is greatly reduced by introducing nickel element and using PFM film.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2016 Received in revised form 13 March 2017 Accepted 15 March 2017
Direct borohydride fuel cells (DBFCs) exhibit the potential for a wide range of applications due to their high energy and power density; however, the hydrolysis of BH 4 significantly limits the use of DBFCs. In this paper, PdNix-B/carbon nanotubes (PdNix-B/CNTs) (x ¼ 0, 0.3, 0.6, 0.9) composites have been prepared by a chemical reduction method in which PdNix-B nanoparticles of approximately 3.5 nm are grown on the surface of carbon nanotubes. A cell was assembled with PdNix-B/CNTs as the anode catalyst, a polymer fibre membrane (PFM) as a substitute for the Nafion membrane and LaNiO3 as the cathode catalyst. The results show that the Ni element displays an ability to balance the competition 2 (x ¼ 0.9) was between the hydrolysis and oxidation of BH 4 . A peak power density of 105 mW cm achieved at 25 C. However, the highest fuel efficiency of 69% was achieved at x ¼ 0.3, and the corresponding power density was 87 mW cm2, which represents the best comprehensive performance of these DBFCs. © 2017 Elsevier B.V. All rights reserved.
Keywords: Direct borohydride fuel cell Catalytic activity PdNix-B/CNT catalysts
1. Introduction
BH 4 þ 8OH ¼ BO2 þ 6H2 O þ 8e E0 ¼ 1:24 V
Fuel cells have attracted significant attention as a type of clean and efficient power generation devices. Direct borohydride fuel cells (DBFCs), as liquid fuel cells, are promising for portable applications due to their high theoretical cell voltage and energy density compared with methanol fuel cells. However, the hydrolysis of borohydride, liquid fuel crossover and battery cost still hinder their commercialization [1e4]. In DBFCs, the theoretical anode reaction is shown in the following [5]:
In particular, the anode reaction is usually accompanied by a hydrolysis reaction:
* Corresponding author. State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi'an Jiaotong University, Xi'an 710049, PR China. ** Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.jpowsour.2017.03.056 0378-7753/© 2017 Elsevier B.V. All rights reserved.
BH 4 þ 2H2 O ¼ BO2 þ 4H2
(1)
(2)
Borohydride hydrolysis not only decreases the utilization efficiency of the fuels but also causes fuel shortage due to the hydrogen bubbles generated between the anode and membrane; this subsequently degrades the electrochemical oxidation performance [2]. However, there have been a few studies that focused on solving this problem, and more attention has been paid to the improvement of the power density of these fuels. It is well known that the anode reactions, including hydrolysis and electrochemical oxidation, are strongly affected by the electrode materials. Precious metal catalysts, including Pd, Ag, Au and their compounds, are commonly used to depress the hydrolysis reaction of BH 4 [6e8], but a very low
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power density is obtained at the same time because of their low catalytic kinetic performance. Feng et al. [9] studied Ag and Ag-Ni alloy anodes for DBFCs and found that Ag and AgNi-based anodes could achieve more than 7 electrons transfer, although the discharge current density obtained was only ~ 1e4 mA cm2. More recently, the attention has shifted towards cheaper catalytic materials, such as Ni-B or Co-B alloys, which have shown excellent power performance as the anode catalyst in DBFCs [10] but reduced current efficiency. Therefore, a balance between the power performance and current efficiency of DBFCs is required. Palladium exhibits fast electrode kinetics, good power performance and a relatively low cost compared with platinum. Therefore, alloying Pd with a transition metal, such as Ni, can reduce the cost of the catalyst and maintain or even improve the catalytic activity for BH 4 oxidation and the power output. Liu et al. [11] obtained a power density of 80 mW cm2 in the passive mode at ambient conditions by using an anode containing nickel and carbon-supported Pd catalyst with a Nafion binder. In this paper, composites of PdNix-B/CNTs were synthesized and used to both reduce the hydrolysis of borohydride and maintain high power performance. The results show that this type of precious metal-transition metal boride-carbon material composite with the polymer fiber membrane (PFM) [12e14] displays good performance along with low hydrolysis and high power density. 2. Experimental 2.1. Catalyst preparation Multi-walled carbon nanotubes (CNTs) were purchased from Chengdu Organic Chemicals Co. (China). All reagents were analytical grade. Different metal ratios of PdNix-B/CNTs were prepared by a liquid-phase reaction method. Firstly, CNTs were modified by concentrated HNO3 at room temperature for 24 h, then 0.12 g CNTs were impregnated with a mixed solution of PdCl2 (0.06 M) and NiSO4$6H2O (0.02 M), followed by stirring vigorously for 30 min. Under stirring, the pH of the suspension was adjusted to approximately 8.0 with 1.0 M NaOH solution, and then 0.23 M KBH4 solution were added by drops with a small model micro-injection pump at the speed of 20 mL min1. After this, the solution was stirred for 2 h to guarantee the complete reaction. Lastly, the suspension was filtered, washed and dried overnight at 80 C in a vacuum oven to obtain the PdNi0.3-B/CNT catalyst. Other samples with different ratios of Pd to Ni were prepared using the same method, and they were referred to as PdNix-B/CNTs (x ¼ 0, 0.3, 0.6, 0.9).
heating the resulting film at 340 C for 1 h. The catalyst layer consists of 30 wt % LaNiO3, 45 wt % CNTs and 25 wt % PTFE, which was also coated on the Ni foam. The three-layer electrode was formed by pressing the coated Ni-foam and the gas diffusion layer at a pressure of 3 MPa into a sheet with a thickness of 0.6 mm. The mass loading of LaNiO3 in the cathode was 7.5 mg cm2. The details of the cell assembly process are described in Fig. 1. The anode, membrane and cathode were pressed together. The gas diffusion layer of the cathode was exposed to O2, while the anode was connected to the fuel container. The SEM and EDS elemental mapping images of the gas diffusion electrode cross-sectional are shown in Supplementary Fig. S1. 2.3. Electrochemical measurements The cell performance, including the cell polarization, power density, stability and fuel efficiency, was measured using a battery testing system (Neware Technology Limited, Shenzhen, China). Cyclic voltammetry (CV) was conducted using an electrochemical workstation (CHI750D, ChenHua, Shanghai, China) at a sweep rate of 50 mV s1. A standard three-electrode system was used in these electrochemical measurements. In the CV tests, a glassy carbon (GC) electrode with an area of 0.196 cm2 was modified with anode catalyst, Pt wire and a Hg/HgO electrode were used as the working electrode, counter electrode and reference electrode, respectively. The modified GC electrode was fabricated as follows: a catalyst ink was prepared by ultrasonically mixing 3 mg of the catalyst, 0.5 mL ethanol and 15 mL Nafion (5 wt%) into a slurry and then spreading 10 mL of the slurry onto the surface of the glass carbon electrode. 3. Results and discussion 3.1. Structural characterization The XRD patterns of the prepared catalyst are illustrated in Fig. 2. All samples display a peak at approximately 2q ¼ 26.4 , which belongs to the (002) characteristic peak of CNTs [16]. The PdNix-B/CNT samples exhibit obvious Pd-peaks at 40.1, 46.7 and 68.2 [17]. As the Ni content increases, the Pd peaks in the samples become weaker and broader. In addition, additional weak peaks gradually appear at 2q ¼ 33.5, 45 and 60 , which should belong to Ni-B [18]. The SEM and TEM images of PdNix-B/CNTs are shown in Fig. 3.
2.2. Electrode preparation LaNiO3 was selected as the cathode catalyst for the DBFCs-PFM due to its excellent ORR activity in alkaline solution and excellent BH 4 tolerance; the method outlined in our previous work [15]. The PFM was purchased from the Nippon Kodoshi Corporation, and its physical properties were also described in our previous work [14]. The anode was prepared by mixing 85 wt % PdNix-B/CNTs (x ¼ 0, 0.3, 0.6, 0.9) and 15 wt% of the binder (Nafion or PTFE solution) in alcohol solution. Then, the mixture was coated on Ni foam, and dried at 80 C for 2 h under vacuum. Finally, the anode electrode was pressed under a pressure of 1.5 M Pa. The mass loading in the anode was 20 mg cm2. The cathode was a sandwich structure consisting of a gas diffusion layer, a catalyst layer and a current collector layer. The gas diffusion layer was prepared by mixing 60 wt % acetylene black and 40 wt % PTFE (30 wt % PTFE solution) with ethanol into a slurry, then rolling the slurry into a film with a thickness of 0.3 mm and
Fig. 1. Structure of DBFC-PFM.
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Ni element was added, the particle morphology evolved. The TEM images for different ratios of Pd and Ni are shown in Fig. 3eeg. The average particle sizes become smaller as the Ni content increases by statistical analysis (see Fig. 3h), which is attributed to the introduction of Ni element.
3.2. Comparative cyclic voltammetry of KBH4 on PdNix-B/CNTs
Fig. 2. XRD patterns of PdNix-B/MCNTs (x ¼ 0, 0.3, 0.6, 0.9), CNTs and Ni-B.
Fig. 3a presents the SEM image of the CNTs, and Fig. 3b is the SEM image of PdNi0.3-B/CNTs. It is observed that the PdNi0.3-B nanoparticles of PdNi0.3-B/CNTs are uniformly dispersed on the surface of the CNTs. Fig. 3c shows the TEM image of PdNix-B/CNTs. The mean nanometre particle diameter, of which the morphology is generally spherical, is approximately 3.5 nm (shown in Fig. 3h) with a narrow particle size distribution. Pd nanoparticles can also be observed from the HR-TEM image (Fig. 3d) with a lattice spacing of 0.23 nm, which corresponds to the (111) lattice of metal Pd. When
The cyclic voltammograms (CVs) recorded for the PdNix-B/CNT (x ¼ 0, 0.3, 0.6, 0.9) electrodes with 0.01 M KBH4 þ 1.0 M KOH solution at a sweep rate of 50 mV s1 in the potential range of 1.2 V to 0 V vs. Hg/HgO are shown in Fig. 4. For comparison, the cyclic voltammogram in the 1.0 M KOH solution (curve P0) with the PdNi0.3-B/CNT electrode was also obtained; no obvious reaction peak was observed for this electrode. On the other hand, in the mixed solution of 0.01 M KBH4 þ 1.0 M KOH, every catalyst exhibited similar catalytic behaviour, and a clear peak (a3) potential at 0.37 V to 0.5 V vs. Hg/HgO can be observed in all curves, which results from the direct oxidation of BH 4 . It has been reported that different catalysts have a unique catalytic reaction mechanism; in addition, for the same type of catalyst, the CV curves still rely on the fuel concentration, scanning speed and the solution pH value [19]. Gyenge et al. [20] obtained a direct oxidation peak potential at 0.15 V to 0.05 V vs. Ag/AgCl on Pt in 2 M NaOH. Similar anodic peak patterns were also reported by Duan [17] at 0.1 V vs. SHE on Pd/C in 1 M NaOH. Additionally, it was also discovered that the higher Ni content in the catalyst, the larger the peak current
Fig. 3. SEM images of (a) CNTs and (b) PdNi0.3-B/MCNTs, (c) is the TEM image of PdNix-B/CNTs, (d) is the HR-TEM image of PdNix-B/CNTs, (e) is the TEM image of PdNi0.3-B/CNTs, (f) is the TEM image of PdNi0.6-B/CNTs, (g) is the TEM image of PdNi0.9-B/CNTs, and (h) is the particle size distribution.
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electrolyte solution. The higher the ratio of OH/BH 4 is, the lower the amount of hydrogen evolved [25]. An improvement of the fuel utilization, to as large an extent as possible, based on changing the solution concentration was expected. Therefore, a fuel concentration of 6 M KOH and 0.8 M KBH4 was used, which, in our previous study, proved to be an appropriate proportion of both solutions to improve the power density. The following Equation (5) described the utilization of the fuel:
Rutilization ¼
Fig. 4. CV curves: P(1~4) stand for PdNix-B/CNTs (x ¼ 0.9, 0.6, 0.3, 0) tested in a mixed solution of 0.01 M KBH4 and 1.0 M KOH, and P0 is the CV curve of PdNi0.3-B/CNTs tested in the blank solution of 1.0 M KOH.
density. Meanwhile, during the discharge process, it is difficult to achieve the overall eight-electron reaction because of the hydrolysis of BH 4 . The a1 peak that appears at the most negative potential could be attributed to the oxidation of the hydrogen gas, which evolved during the partial hydrolysis of BH 4 , and the a2 peak presumably originates from the oxidation of the primary BH3OH by-products [21e23]. Moreover, as shown in Fig. 4, as the molar ratio of Ni to Pd decreased, the peak current density around 0.8 V became weaker and even disappeared. This phenomenon indicates that the introduction of Ni improves the reaction dynamics at relatively positive electrode voltages. The improved reaction dynamics not only promotes the direct oxidation of BH 4 but also increases the probability of the hydrolysis reaction. When the Ni content was further increased to Pd:Ni ¼ 1:0.9, the peak current density that corresponds to the hydrolysis reaction significantly increased at more negative electrode potentials, which would lead to a reduction of the fuel utilization. For the BH 4 oxidation reaction mechanism, as shown in Formulas (3) and (4), the electro-oxidation process is mainly determined by the hydrolysis reaction and the direct oxidation of BH 4. The relative speed of the reaction depends on the chemical properties of the anode catalyst surface and the electrode potential. According to the CV curves, the direct oxidation reaction of BH 4 is more likely to occur when the electrode reaction potential is positive, while the hydrogenation reaction will be more severe when the electrode reaction potential is negative [24]. BHn ðOHÞ 4n þ 2OH ¼ BHn1 ðOHÞ5n þ H2 O þ 2e
n
¼ 4; 3; 2; 1 BHn ðOHÞ 4n þ H2 O ¼ BHn1 ðOHÞ5n þ 2H2
(3) n ¼ 4; 3; 2; 1
(4)
3.3. Influence of the anode catalyst on cell performance In DBFCs-PFM systems, a cheaper fibre membrane, which still maintained a high cell performance, was used as an alternative to the proton exchange membrane. In a full cell, the competition between the direct oxidation and hydrolysis of BH 4 is governed not only by the anode catalyst but also by the composition of the
Eautual 100% Etheory
(5)
where Eactual can be determined by the current and time of the constant current discharge, and Etheory is the theoretical discharge capacity of 4 Ah g1 for the KBH4 fuel. A similar test method has been reported by Kim et al. [26], where Pt catalyst with NaBH4 solution was used, and Feng et al., where AgNi was used with the KBH4 solution [9]. Fig. 5a shows the polarization and power density curves obtained with PdNix-B/CNTs as the anode catalyst at ambient temperature and atmosphere. It can be observed that the performance of Pd/CNTs as the anode catalyst is poor, and its peak power density is only 69 mW cm2. However, for the PdNix-B/CNT catalysts, with increased Ni content, the peak power density (P.D.) increases from 69 to 110.7 mW cm2. Table 1 compared the power density of DBFCs using Pd [20] or Pt [27e31] alloys as anode catalysts reported in the recent years and in this work. It can be seen that the power performance of PdNix-B/CNT is higher than that of most platinum or palladium alloys which reported in the literature. When the value of x is close to 0.9, a stable peak power density is likely observed. The results once again confirm that the addition of Ni can improve the catalytic activity of the anode. Moreover, the fuel utilization is also affected by the Ni content. As shown in Fig. 5b, even though the cells have a similar power density value at the current density of 100 mA cm2, their fuel utilization is different. The PdNi0.9-B/CNT catalyst exhibits a much better electro-catalytic activity and a high operation voltage at the same current density but the lowest fuel utilization efficiency (less than 50%). Gradually, as the Ni content decreases, the operation voltage decreases but the fuel utilization is greatly enhanced. It is observed that the discharge efficiencies of PdNi0.3-B/CNTs and Pd/CNTs as the anode catalyst are as high as 69.1% and 70.1%, respectively. Therefore, the fuel utilization should be evaluated using the energy density because the addition of Ni decreases the discharge capacity but increases the operation voltage. From the inset of Fig. 5b, it is observed that the cell with PdNi0.3-B/CNTs as the anode catalyst exhibits the highest energy density. The difference in the energy density is attributed to the competition between the hydrolysis and oxidation of BH 4. According to the analysis above, the addition of the Ni element can increase the operation voltage, but it also accelerates the hydrolysis reaction of BH 4 , while the Pd element mainly catalyses the oxidation of BH 4 . As shown in Fig. 5c, V1 and V2 represent the generation speed of H2 and the diffusion speed of BH 4 , respectively. For the catalysts (x ¼ 0, 0.3), there is a slow H2 generation on the surface of the catalysts because the rate of BH 4 hydrolysis (corresponding to V1) is slower than the rate of BH 4 diffusion (corresponding to V2). Therefore, both of these catalysts (x ¼ 0, 0.3) exhibit a relatively low power density (as observed in Fig. 5a). Meanwhile, PdNi0.3-B/CNTs shows a relatively high operation voltage and a slow hydrolysis reaction, and it exhibits the largest energy density. 3.4. Influence of the binder on cell performance At present, there are mainly two types of binders used in
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Fig. 5. (a) Comparison of the power density for different current densities for the PdNix-B/CNT (x ¼ 0, 0.3, 0.6, 0.9) anode catalysts, (b) Comparison between the fuel efficiency and energy density (inset) for the discharge of DBFCs at a current density of 100 mA cm2 and a temperature of 25 C. (c) An illustration of the hydrolysis and oxidation reactions of BH 4.
Table 1 The power density of DBFCs using different Pt or Pd alloys as anode catalysts reported in recent years and this work. Anode
Cathode
T( C)
Power density (mW cm2)
Ref.
Pd50Cu50/C PtRu/C Pt0.4Dy0.6 Pt67Co33/C Pt-Sn/C Pt/graphene PdNix-B/CNTs
Pt/C Pt/C Pt Pt/C Pt/C Au/C LaNiO3
60 65 25 25 25 25 25
98 110 298 70.7 91.5 42 110
[20] [27] [28] [29] [30] [31] This
catalysts, polytetrafluoroethylene (PTFE) and Nafion binder solution. PTFE is hydrophobic, while Nafion is hydrophilic [5]. Kim et al. [26] reported that the addition of Nafion in the anode of a DBFC can significantly improve the cell performance at room temperature due to the reduction of the interfacial resistance. In addition, Li et al. [32] found that the addition of Nafion in the anode could reduce the hydrogen evolution not only from the hydrolysis reaction but also from the electrochemical reaction. Here, we studied the effect of Nafion binder on the DBFCs-PFM cell system. Fig. 6 compared the effects of Nafion addition with that of PTFE addition on cell performance and hydrogen evolution. Peak power densities of 125 mW cm2 and 159.8 mW cm2 are achieved at 25 C and 60 C, respectively, when Nafion is used as the binder. However, peak power densities of 87 and 125 mW cm2 are achieved when PTFE is used as the binder under the same condition. This is mainly due to the hydrophilic Nafion binder solution, which can not only reduce the interfacial resistance but also ease the
transport of BH 4 to the reaction interface thereby resulting in excellent electrochemical performance. With regards to the fuel efficiency, the entire picture is presented in Fig. 6b, the direct oxidation of BH 4 is significantly favoured compared to the hydrolysis reaction with the Nafion solution binder. In particular, it is worth mentioning that when the discharge current density is reduced to 20 mA cm2, the fuel efficiency rapidly decreases to only 32% compared with 69% obtained for a discharge current density of 100 mA cm2 with PTFE as the anode catalyst binder. On the contrary, when Nafion solution is used as the anode catalyst binder, there is no obvious change in the fuel efficiency. It can also be observed from Fig. 6c that the energy density obtained with the Nafion binder decreased slightly, while it decreased significantly with the PTFE binder. This mainly occurs because the Nafion binder improves the hydrogen adsorption intensity on the electrode surface, and therefore, the formation and release of hydrogen can be suppressed under the condition of a low current discharge. If the discharge current density is high, which simultaneously requires more BH 4 participation in the reaction, a large amount of BH4 will be oxidized accompanied by the generation of a large amount of hydrogen; subsequently, the fuel efficiency will decline. Therefore, the properties of the electrode surface are altered by different binders. On the other hand, the hydrophobic properties of PTFE enable the formation of a gas channel. Therefore, when the discharge current is small, it requires more time to deplete the same amount of fuel, and the reaction dynamic will be slow; subsequently, the hydrogen generated can be easily released from the channel, and the fuel utilization would reduce [33]. Whereas, along with the increase in the discharge current density, although the
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Fig. 6. (a) Comparison of the power density tested at different temperatures using different binders with PdNi0.3-B/CNTs as the anode catalyst, (b) the fuel efficiency of DBFCs discharged at 20, 60 and 100 mA cm2 using the Nafion and PTFE solution binder, respectively, (c) energy density at different current densities using the Nafion and PTFE solution binder, respectively, and (d) Stability tests (E-t) of the DBFCs-PFM at a current density 20 mA cm2.
reaction power increases, the reaction time is shorter, more BH 4 tend to be oxidized at the positive electrode reaction potential, and a high fuel efficiency is obtained. The stability is another determinant of the cell performance. Fig. 6d shows the stability tests of the DBFCs-PFM at a constant current density of 20 mA cm2 for approximately 100 h at room temperature. The fuel solution was refreshed every 12 h. The fluctuating points are due to the addition and consumption of the fuel solution. In the 100 h test, the cell maintained a good operating state and voltage without attenuation. 4. Conclusions In this work, PdNix-B/CNTs as DBFCs-PFM anode catalysts were synthesized by a chemical reduction method in aqueous solution. PdNi0.9-B/CNTs shows the highest power density (110 mW cm2) and the lowest fuel efficiency (41%) compared to catalysts with other ratios of Pd and Ni due to severe hydrogenation. It is worth mentioning that the PdNi0.3-B/CNT catalysts not only exhibit a relative higher electrochemical catalytic activity for the oxidation of 2 BH 4 (87 mW cm ) but also have a higher fuel efficiency (69%) and the highest energy density observed. In addition, the best comprehensive performance of the assembled DBFCs-PFM using PdNi0.3-B/CNTs as the anode catalyst with the Nafion binder reached 127 mW cm2, while the fuel utilization was 55%. In particular, when the discharge current density of the cell dropped from 100 to 20 mA cm2, the energy density with Nafion as the binder reduced by 4.9%, compared with the reduction of the energy density by 29.8% with the PTFE binder; this shows the advantage of the hydrophilic Nafion binder in small current discharge
conditions. Furthermore, stability tests of the cell showed a constant current density of 20 mA cm2 that continued for approximately 100 h without decay. Therefore, the new DBFCs-PFM cell structure using the PdNi0.3-B/CNT anode catalyst shows potential for widespread use in different applications along with its high performance and low cost. Acknowledgments This work was financially supported by State Key Laboratory for Mechanical Behavior of Materials (Grant No. 20161812), and the National Natural Science Foundation of China (Grant No. 21543004). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.056. References [1] C.P. de Leon, F.C. Walsh, D. Pletcher, D.J. Browning, J.B. Lakeman, J. Power Sources 155 (2006) 172e181. [2] B.H. Liu, Z.P. Li, J. Power Sources 187 (2009) 291e297. nez, C. Ponce de Leo n, A.A. Shah, F.C. Walsh, J. Power Sources [3] I. Merino-Jime 219 (2012) 339e357. nez, C. Ponce de Leo n, F.C. Walsh, in: Q. Xu, T. Kobayashi (Eds.), [4] I. Merino-Jime Advanced Materials for Clean Energy, CRC Press, 2015, pp. 589e607. [5] J. Ma, N.A. Choudhury, Y. Sahai, Renew. Sustain. Energy Rev. 14 (2010) 183e199. k, Electrochimica Acta 51 [6] M. Chatenet, F. Micoud, I. Roche, E. Chainet, J. Vondra (2006) 5452e5458. [7] M.H. Atwan, C.L.B. Macdonald, D.O. Northwood, E.L. Gyenge, J. Power Sources
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The high utilization of fuel in direct borohydride fuel cells with a PdNix-B/carbon nanotubes-catalysed anode Yaping Zhou a, Sai Li a, b, *, Yuanzhen Chen a, Yongning Liu a, ** a b
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi'an Jiaotong University, Xi'an 710049, PR China School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, PR China
h i g h l i g h t s PdNix-B/CNT catalysts can ease the borohydride hydrolysis and keep high discharge performance. The power density could reach 87 mW cm2 and fuel efficiency could reach 69%. The cost of the battery is greatly reduced by introducing nickel element and using PFM film.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2016 Received in revised form 13 March 2017 Accepted 15 March 2017
Direct borohydride fuel cells (DBFCs) exhibit the potential for a wide range of applications due to their high energy and power density; however, the hydrolysis of BH 4 significantly limits the use of DBFCs. In this paper, PdNix-B/carbon nanotubes (PdNix-B/CNTs) (x ¼ 0, 0.3, 0.6, 0.9) composites have been prepared by a chemical reduction method in which PdNix-B nanoparticles of approximately 3.5 nm are grown on the surface of carbon nanotubes. A cell was assembled with PdNix-B/CNTs as the anode catalyst, a polymer fibre membrane (PFM) as a substitute for the Nafion membrane and LaNiO3 as the cathode catalyst. The results show that the Ni element displays an ability to balance the competition 2 (x ¼ 0.9) was between the hydrolysis and oxidation of BH 4 . A peak power density of 105 mW cm achieved at 25 C. However, the highest fuel efficiency of 69% was achieved at x ¼ 0.3, and the corresponding power density was 87 mW cm2, which represents the best comprehensive performance of these DBFCs. © 2017 Elsevier B.V. All rights reserved.
Keywords: Direct borohydride fuel cell Catalytic activity PdNix-B/CNT catalysts
1. Introduction
BH 4 þ 8OH ¼ BO2 þ 6H2 O þ 8e E0 ¼ 1:24 V
Fuel cells have attracted significant attention as a type of clean and efficient power generation devices. Direct borohydride fuel cells (DBFCs), as liquid fuel cells, are promising for portable applications due to their high theoretical cell voltage and energy density compared with methanol fuel cells. However, the hydrolysis of borohydride, liquid fuel crossover and battery cost still hinder their commercialization [1e4]. In DBFCs, the theoretical anode reaction is shown in the following [5]:
In particular, the anode reaction is usually accompanied by a hydrolysis reaction:
* Corresponding author. State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi'an Jiaotong University, Xi'an 710049, PR China. ** Corresponding author. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.jpowsour.2017.03.056 0378-7753/© 2017 Elsevier B.V. All rights reserved.
BH 4 þ 2H2 O ¼ BO2 þ 4H2
(1)
(2)
Borohydride hydrolysis not only decreases the utilization efficiency of the fuels but also causes fuel shortage due to the hydrogen bubbles generated between the anode and membrane; this subsequently degrades the electrochemical oxidation performance [2]. However, there have been a few studies that focused on solving this problem, and more attention has been paid to the improvement of the power density of these fuels. It is well known that the anode reactions, including hydrolysis and electrochemical oxidation, are strongly affected by the electrode materials. Precious metal catalysts, including Pd, Ag, Au and their compounds, are commonly used to depress the hydrolysis reaction of BH 4 [6e8], but a very low
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power density is obtained at the same time because of their low catalytic kinetic performance. Feng et al. [9] studied Ag and Ag-Ni alloy anodes for DBFCs and found that Ag and AgNi-based anodes could achieve more than 7 electrons transfer, although the discharge current density obtained was only ~ 1e4 mA cm2. More recently, the attention has shifted towards cheaper catalytic materials, such as Ni-B or Co-B alloys, which have shown excellent power performance as the anode catalyst in DBFCs [10] but reduced current efficiency. Therefore, a balance between the power performance and current efficiency of DBFCs is required. Palladium exhibits fast electrode kinetics, good power performance and a relatively low cost compared with platinum. Therefore, alloying Pd with a transition metal, such as Ni, can reduce the cost of the catalyst and maintain or even improve the catalytic activity for BH 4 oxidation and the power output. Liu et al. [11] obtained a power density of 80 mW cm2 in the passive mode at ambient conditions by using an anode containing nickel and carbon-supported Pd catalyst with a Nafion binder. In this paper, composites of PdNix-B/CNTs were synthesized and used to both reduce the hydrolysis of borohydride and maintain high power performance. The results show that this type of precious metal-transition metal boride-carbon material composite with the polymer fiber membrane (PFM) [12e14] displays good performance along with low hydrolysis and high power density. 2. Experimental 2.1. Catalyst preparation Multi-walled carbon nanotubes (CNTs) were purchased from Chengdu Organic Chemicals Co. (China). All reagents were analytical grade. Different metal ratios of PdNix-B/CNTs were prepared by a liquid-phase reaction method. Firstly, CNTs were modified by concentrated HNO3 at room temperature for 24 h, then 0.12 g CNTs were impregnated with a mixed solution of PdCl2 (0.06 M) and NiSO4$6H2O (0.02 M), followed by stirring vigorously for 30 min. Under stirring, the pH of the suspension was adjusted to approximately 8.0 with 1.0 M NaOH solution, and then 0.23 M KBH4 solution were added by drops with a small model micro-injection pump at the speed of 20 mL min1. After this, the solution was stirred for 2 h to guarantee the complete reaction. Lastly, the suspension was filtered, washed and dried overnight at 80 C in a vacuum oven to obtain the PdNi0.3-B/CNT catalyst. Other samples with different ratios of Pd to Ni were prepared using the same method, and they were referred to as PdNix-B/CNTs (x ¼ 0, 0.3, 0.6, 0.9).
heating the resulting film at 340 C for 1 h. The catalyst layer consists of 30 wt % LaNiO3, 45 wt % CNTs and 25 wt % PTFE, which was also coated on the Ni foam. The three-layer electrode was formed by pressing the coated Ni-foam and the gas diffusion layer at a pressure of 3 MPa into a sheet with a thickness of 0.6 mm. The mass loading of LaNiO3 in the cathode was 7.5 mg cm2. The details of the cell assembly process are described in Fig. 1. The anode, membrane and cathode were pressed together. The gas diffusion layer of the cathode was exposed to O2, while the anode was connected to the fuel container. The SEM and EDS elemental mapping images of the gas diffusion electrode cross-sectional are shown in Supplementary Fig. S1. 2.3. Electrochemical measurements The cell performance, including the cell polarization, power density, stability and fuel efficiency, was measured using a battery testing system (Neware Technology Limited, Shenzhen, China). Cyclic voltammetry (CV) was conducted using an electrochemical workstation (CHI750D, ChenHua, Shanghai, China) at a sweep rate of 50 mV s1. A standard three-electrode system was used in these electrochemical measurements. In the CV tests, a glassy carbon (GC) electrode with an area of 0.196 cm2 was modified with anode catalyst, Pt wire and a Hg/HgO electrode were used as the working electrode, counter electrode and reference electrode, respectively. The modified GC electrode was fabricated as follows: a catalyst ink was prepared by ultrasonically mixing 3 mg of the catalyst, 0.5 mL ethanol and 15 mL Nafion (5 wt%) into a slurry and then spreading 10 mL of the slurry onto the surface of the glass carbon electrode. 3. Results and discussion 3.1. Structural characterization The XRD patterns of the prepared catalyst are illustrated in Fig. 2. All samples display a peak at approximately 2q ¼ 26.4 , which belongs to the (002) characteristic peak of CNTs [16]. The PdNix-B/CNT samples exhibit obvious Pd-peaks at 40.1, 46.7 and 68.2 [17]. As the Ni content increases, the Pd peaks in the samples become weaker and broader. In addition, additional weak peaks gradually appear at 2q ¼ 33.5, 45 and 60 , which should belong to Ni-B [18]. The SEM and TEM images of PdNix-B/CNTs are shown in Fig. 3.
2.2. Electrode preparation LaNiO3 was selected as the cathode catalyst for the DBFCs-PFM due to its excellent ORR activity in alkaline solution and excellent BH 4 tolerance; the method outlined in our previous work [15]. The PFM was purchased from the Nippon Kodoshi Corporation, and its physical properties were also described in our previous work [14]. The anode was prepared by mixing 85 wt % PdNix-B/CNTs (x ¼ 0, 0.3, 0.6, 0.9) and 15 wt% of the binder (Nafion or PTFE solution) in alcohol solution. Then, the mixture was coated on Ni foam, and dried at 80 C for 2 h under vacuum. Finally, the anode electrode was pressed under a pressure of 1.5 M Pa. The mass loading in the anode was 20 mg cm2. The cathode was a sandwich structure consisting of a gas diffusion layer, a catalyst layer and a current collector layer. The gas diffusion layer was prepared by mixing 60 wt % acetylene black and 40 wt % PTFE (30 wt % PTFE solution) with ethanol into a slurry, then rolling the slurry into a film with a thickness of 0.3 mm and
Fig. 1. Structure of DBFC-PFM.
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Ni element was added, the particle morphology evolved. The TEM images for different ratios of Pd and Ni are shown in Fig. 3eeg. The average particle sizes become smaller as the Ni content increases by statistical analysis (see Fig. 3h), which is attributed to the introduction of Ni element.
3.2. Comparative cyclic voltammetry of KBH4 on PdNix-B/CNTs
Fig. 2. XRD patterns of PdNix-B/MCNTs (x ¼ 0, 0.3, 0.6, 0.9), CNTs and Ni-B.
Fig. 3a presents the SEM image of the CNTs, and Fig. 3b is the SEM image of PdNi0.3-B/CNTs. It is observed that the PdNi0.3-B nanoparticles of PdNi0.3-B/CNTs are uniformly dispersed on the surface of the CNTs. Fig. 3c shows the TEM image of PdNix-B/CNTs. The mean nanometre particle diameter, of which the morphology is generally spherical, is approximately 3.5 nm (shown in Fig. 3h) with a narrow particle size distribution. Pd nanoparticles can also be observed from the HR-TEM image (Fig. 3d) with a lattice spacing of 0.23 nm, which corresponds to the (111) lattice of metal Pd. When
The cyclic voltammograms (CVs) recorded for the PdNix-B/CNT (x ¼ 0, 0.3, 0.6, 0.9) electrodes with 0.01 M KBH4 þ 1.0 M KOH solution at a sweep rate of 50 mV s1 in the potential range of 1.2 V to 0 V vs. Hg/HgO are shown in Fig. 4. For comparison, the cyclic voltammogram in the 1.0 M KOH solution (curve P0) with the PdNi0.3-B/CNT electrode was also obtained; no obvious reaction peak was observed for this electrode. On the other hand, in the mixed solution of 0.01 M KBH4 þ 1.0 M KOH, every catalyst exhibited similar catalytic behaviour, and a clear peak (a3) potential at 0.37 V to 0.5 V vs. Hg/HgO can be observed in all curves, which results from the direct oxidation of BH 4 . It has been reported that different catalysts have a unique catalytic reaction mechanism; in addition, for the same type of catalyst, the CV curves still rely on the fuel concentration, scanning speed and the solution pH value [19]. Gyenge et al. [20] obtained a direct oxidation peak potential at 0.15 V to 0.05 V vs. Ag/AgCl on Pt in 2 M NaOH. Similar anodic peak patterns were also reported by Duan [17] at 0.1 V vs. SHE on Pd/C in 1 M NaOH. Additionally, it was also discovered that the higher Ni content in the catalyst, the larger the peak current
Fig. 3. SEM images of (a) CNTs and (b) PdNi0.3-B/MCNTs, (c) is the TEM image of PdNix-B/CNTs, (d) is the HR-TEM image of PdNix-B/CNTs, (e) is the TEM image of PdNi0.3-B/CNTs, (f) is the TEM image of PdNi0.6-B/CNTs, (g) is the TEM image of PdNi0.9-B/CNTs, and (h) is the particle size distribution.
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electrolyte solution. The higher the ratio of OH/BH 4 is, the lower the amount of hydrogen evolved [25]. An improvement of the fuel utilization, to as large an extent as possible, based on changing the solution concentration was expected. Therefore, a fuel concentration of 6 M KOH and 0.8 M KBH4 was used, which, in our previous study, proved to be an appropriate proportion of both solutions to improve the power density. The following Equation (5) described the utilization of the fuel:
Rutilization ¼
Fig. 4. CV curves: P(1~4) stand for PdNix-B/CNTs (x ¼ 0.9, 0.6, 0.3, 0) tested in a mixed solution of 0.01 M KBH4 and 1.0 M KOH, and P0 is the CV curve of PdNi0.3-B/CNTs tested in the blank solution of 1.0 M KOH.
density. Meanwhile, during the discharge process, it is difficult to achieve the overall eight-electron reaction because of the hydrolysis of BH 4 . The a1 peak that appears at the most negative potential could be attributed to the oxidation of the hydrogen gas, which evolved during the partial hydrolysis of BH 4 , and the a2 peak presumably originates from the oxidation of the primary BH3OH by-products [21e23]. Moreover, as shown in Fig. 4, as the molar ratio of Ni to Pd decreased, the peak current density around 0.8 V became weaker and even disappeared. This phenomenon indicates that the introduction of Ni improves the reaction dynamics at relatively positive electrode voltages. The improved reaction dynamics not only promotes the direct oxidation of BH 4 but also increases the probability of the hydrolysis reaction. When the Ni content was further increased to Pd:Ni ¼ 1:0.9, the peak current density that corresponds to the hydrolysis reaction significantly increased at more negative electrode potentials, which would lead to a reduction of the fuel utilization. For the BH 4 oxidation reaction mechanism, as shown in Formulas (3) and (4), the electro-oxidation process is mainly determined by the hydrolysis reaction and the direct oxidation of BH 4. The relative speed of the reaction depends on the chemical properties of the anode catalyst surface and the electrode potential. According to the CV curves, the direct oxidation reaction of BH 4 is more likely to occur when the electrode reaction potential is positive, while the hydrogenation reaction will be more severe when the electrode reaction potential is negative [24]. BHn ðOHÞ 4n þ 2OH ¼ BHn1 ðOHÞ5n þ H2 O þ 2e
n
¼ 4; 3; 2; 1 BHn ðOHÞ 4n þ H2 O ¼ BHn1 ðOHÞ5n þ 2H2
(3) n ¼ 4; 3; 2; 1
(4)
3.3. Influence of the anode catalyst on cell performance In DBFCs-PFM systems, a cheaper fibre membrane, which still maintained a high cell performance, was used as an alternative to the proton exchange membrane. In a full cell, the competition between the direct oxidation and hydrolysis of BH 4 is governed not only by the anode catalyst but also by the composition of the
Eautual 100% Etheory
(5)
where Eactual can be determined by the current and time of the constant current discharge, and Etheory is the theoretical discharge capacity of 4 Ah g1 for the KBH4 fuel. A similar test method has been reported by Kim et al. [26], where Pt catalyst with NaBH4 solution was used, and Feng et al., where AgNi was used with the KBH4 solution [9]. Fig. 5a shows the polarization and power density curves obtained with PdNix-B/CNTs as the anode catalyst at ambient temperature and atmosphere. It can be observed that the performance of Pd/CNTs as the anode catalyst is poor, and its peak power density is only 69 mW cm2. However, for the PdNix-B/CNT catalysts, with increased Ni content, the peak power density (P.D.) increases from 69 to 110.7 mW cm2. Table 1 compared the power density of DBFCs using Pd [20] or Pt [27e31] alloys as anode catalysts reported in the recent years and in this work. It can be seen that the power performance of PdNix-B/CNT is higher than that of most platinum or palladium alloys which reported in the literature. When the value of x is close to 0.9, a stable peak power density is likely observed. The results once again confirm that the addition of Ni can improve the catalytic activity of the anode. Moreover, the fuel utilization is also affected by the Ni content. As shown in Fig. 5b, even though the cells have a similar power density value at the current density of 100 mA cm2, their fuel utilization is different. The PdNi0.9-B/CNT catalyst exhibits a much better electro-catalytic activity and a high operation voltage at the same current density but the lowest fuel utilization efficiency (less than 50%). Gradually, as the Ni content decreases, the operation voltage decreases but the fuel utilization is greatly enhanced. It is observed that the discharge efficiencies of PdNi0.3-B/CNTs and Pd/CNTs as the anode catalyst are as high as 69.1% and 70.1%, respectively. Therefore, the fuel utilization should be evaluated using the energy density because the addition of Ni decreases the discharge capacity but increases the operation voltage. From the inset of Fig. 5b, it is observed that the cell with PdNi0.3-B/CNTs as the anode catalyst exhibits the highest energy density. The difference in the energy density is attributed to the competition between the hydrolysis and oxidation of BH 4. According to the analysis above, the addition of the Ni element can increase the operation voltage, but it also accelerates the hydrolysis reaction of BH 4 , while the Pd element mainly catalyses the oxidation of BH 4 . As shown in Fig. 5c, V1 and V2 represent the generation speed of H2 and the diffusion speed of BH 4 , respectively. For the catalysts (x ¼ 0, 0.3), there is a slow H2 generation on the surface of the catalysts because the rate of BH 4 hydrolysis (corresponding to V1) is slower than the rate of BH 4 diffusion (corresponding to V2). Therefore, both of these catalysts (x ¼ 0, 0.3) exhibit a relatively low power density (as observed in Fig. 5a). Meanwhile, PdNi0.3-B/CNTs shows a relatively high operation voltage and a slow hydrolysis reaction, and it exhibits the largest energy density. 3.4. Influence of the binder on cell performance At present, there are mainly two types of binders used in
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Fig. 5. (a) Comparison of the power density for different current densities for the PdNix-B/CNT (x ¼ 0, 0.3, 0.6, 0.9) anode catalysts, (b) Comparison between the fuel efficiency and energy density (inset) for the discharge of DBFCs at a current density of 100 mA cm2 and a temperature of 25 C. (c) An illustration of the hydrolysis and oxidation reactions of BH 4.
Table 1 The power density of DBFCs using different Pt or Pd alloys as anode catalysts reported in recent years and this work. Anode
Cathode
T( C)
Power density (mW cm2)
Ref.
Pd50Cu50/C PtRu/C Pt0.4Dy0.6 Pt67Co33/C Pt-Sn/C Pt/graphene PdNix-B/CNTs
Pt/C Pt/C Pt Pt/C Pt/C Au/C LaNiO3
60 65 25 25 25 25 25
98 110 298 70.7 91.5 42 110
[20] [27] [28] [29] [30] [31] This
catalysts, polytetrafluoroethylene (PTFE) and Nafion binder solution. PTFE is hydrophobic, while Nafion is hydrophilic [5]. Kim et al. [26] reported that the addition of Nafion in the anode of a DBFC can significantly improve the cell performance at room temperature due to the reduction of the interfacial resistance. In addition, Li et al. [32] found that the addition of Nafion in the anode could reduce the hydrogen evolution not only from the hydrolysis reaction but also from the electrochemical reaction. Here, we studied the effect of Nafion binder on the DBFCs-PFM cell system. Fig. 6 compared the effects of Nafion addition with that of PTFE addition on cell performance and hydrogen evolution. Peak power densities of 125 mW cm2 and 159.8 mW cm2 are achieved at 25 C and 60 C, respectively, when Nafion is used as the binder. However, peak power densities of 87 and 125 mW cm2 are achieved when PTFE is used as the binder under the same condition. This is mainly due to the hydrophilic Nafion binder solution, which can not only reduce the interfacial resistance but also ease the
transport of BH 4 to the reaction interface thereby resulting in excellent electrochemical performance. With regards to the fuel efficiency, the entire picture is presented in Fig. 6b, the direct oxidation of BH 4 is significantly favoured compared to the hydrolysis reaction with the Nafion solution binder. In particular, it is worth mentioning that when the discharge current density is reduced to 20 mA cm2, the fuel efficiency rapidly decreases to only 32% compared with 69% obtained for a discharge current density of 100 mA cm2 with PTFE as the anode catalyst binder. On the contrary, when Nafion solution is used as the anode catalyst binder, there is no obvious change in the fuel efficiency. It can also be observed from Fig. 6c that the energy density obtained with the Nafion binder decreased slightly, while it decreased significantly with the PTFE binder. This mainly occurs because the Nafion binder improves the hydrogen adsorption intensity on the electrode surface, and therefore, the formation and release of hydrogen can be suppressed under the condition of a low current discharge. If the discharge current density is high, which simultaneously requires more BH 4 participation in the reaction, a large amount of BH4 will be oxidized accompanied by the generation of a large amount of hydrogen; subsequently, the fuel efficiency will decline. Therefore, the properties of the electrode surface are altered by different binders. On the other hand, the hydrophobic properties of PTFE enable the formation of a gas channel. Therefore, when the discharge current is small, it requires more time to deplete the same amount of fuel, and the reaction dynamic will be slow; subsequently, the hydrogen generated can be easily released from the channel, and the fuel utilization would reduce [33]. Whereas, along with the increase in the discharge current density, although the
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Fig. 6. (a) Comparison of the power density tested at different temperatures using different binders with PdNi0.3-B/CNTs as the anode catalyst, (b) the fuel efficiency of DBFCs discharged at 20, 60 and 100 mA cm2 using the Nafion and PTFE solution binder, respectively, (c) energy density at different current densities using the Nafion and PTFE solution binder, respectively, and (d) Stability tests (E-t) of the DBFCs-PFM at a current density 20 mA cm2.
reaction power increases, the reaction time is shorter, more BH 4 tend to be oxidized at the positive electrode reaction potential, and a high fuel efficiency is obtained. The stability is another determinant of the cell performance. Fig. 6d shows the stability tests of the DBFCs-PFM at a constant current density of 20 mA cm2 for approximately 100 h at room temperature. The fuel solution was refreshed every 12 h. The fluctuating points are due to the addition and consumption of the fuel solution. In the 100 h test, the cell maintained a good operating state and voltage without attenuation. 4. Conclusions In this work, PdNix-B/CNTs as DBFCs-PFM anode catalysts were synthesized by a chemical reduction method in aqueous solution. PdNi0.9-B/CNTs shows the highest power density (110 mW cm2) and the lowest fuel efficiency (41%) compared to catalysts with other ratios of Pd and Ni due to severe hydrogenation. It is worth mentioning that the PdNi0.3-B/CNT catalysts not only exhibit a relative higher electrochemical catalytic activity for the oxidation of 2 BH 4 (87 mW cm ) but also have a higher fuel efficiency (69%) and the highest energy density observed. In addition, the best comprehensive performance of the assembled DBFCs-PFM using PdNi0.3-B/CNTs as the anode catalyst with the Nafion binder reached 127 mW cm2, while the fuel utilization was 55%. In particular, when the discharge current density of the cell dropped from 100 to 20 mA cm2, the energy density with Nafion as the binder reduced by 4.9%, compared with the reduction of the energy density by 29.8% with the PTFE binder; this shows the advantage of the hydrophilic Nafion binder in small current discharge
conditions. Furthermore, stability tests of the cell showed a constant current density of 20 mA cm2 that continued for approximately 100 h without decay. Therefore, the new DBFCs-PFM cell structure using the PdNi0.3-B/CNT anode catalyst shows potential for widespread use in different applications along with its high performance and low cost. Acknowledgments This work was financially supported by State Key Laboratory for Mechanical Behavior of Materials (Grant No. 20161812), and the National Natural Science Foundation of China (Grant No. 21543004). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.03.056. References [1] C.P. de Leon, F.C. Walsh, D. Pletcher, D.J. Browning, J.B. Lakeman, J. Power Sources 155 (2006) 172e181. [2] B.H. Liu, Z.P. Li, J. Power Sources 187 (2009) 291e297. nez, C. Ponce de Leo n, A.A. Shah, F.C. Walsh, J. Power Sources [3] I. Merino-Jime 219 (2012) 339e357. nez, C. Ponce de Leo n, F.C. Walsh, in: Q. Xu, T. Kobayashi (Eds.), [4] I. Merino-Jime Advanced Materials for Clean Energy, CRC Press, 2015, pp. 589e607. [5] J. Ma, N.A. Choudhury, Y. Sahai, Renew. Sustain. Energy Rev. 14 (2010) 183e199. k, Electrochimica Acta 51 [6] M. Chatenet, F. Micoud, I. Roche, E. Chainet, J. Vondra (2006) 5452e5458. [7] M.H. Atwan, C.L.B. Macdonald, D.O. Northwood, E.L. Gyenge, J. Power Sources
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