A direct borohydride fuel cell employing a sago gel polymer electrolyte

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A direct borohydride fuel cell employing a sago gel polymer electrolyte A. Jamaludin, Z. Ahmad, Z.A. Ahmad, A.A. Mohamad* School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

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abstract

Article history:

The electrochemistry of a direct borohydride fuel cell based on a gel polymer electrolyte

Received 7 May 2010

was studied. Sago is a type of natural polymer, was employed as the polymer host for the

Received in revised form

electrolyte. An electrolyte with a composition of sago þ 6 M KOH þ 2 M NaBH4 was

6 July 2010

prepared and evaluated as a novel gel polymer electrolyte for a direct borohydride fuel cell

Accepted 6 July 2010

system because it exhibited a high electrical conductivity of 0.270 S cm"1. The rate at which

Available online 11 August 2010

oxygen was consumed at the cathode can be related to the electric current by comparing the calculated number of electrons reacted per molecule of oxygen for different currents

Keywords:

supplied to the fuel cell. From the oxygen consumption data, it was deduced that four

Direct borohydride fuel cell

electrons reacted per molecule of oxygen. The performance of the fuel cell was measured

Gel polymer electrolyte

in terms of its currentevoltage, discharge and open circuit voltage measurements. The

Sago

maximum power density obtained was 8.818 mW cm"2 at a discharge performance of

Oxygen consumption

w230 mA h and nominal voltage of 0.806 V. The open circuit voltage of the cells was about 0.900 V and sustained for 23 h. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Fuel cells are energy conversion device which have become an attractive alternative energy source to oil, natural gas and conventional fossil fuel combustion engine [1,2]. A fuel cell converts chemical energy into electrical energy and generates electricity as long as fuel and oxidant are supplied into the system. Among the assorted types of fuel cells available, H2eO2 fuel cells gained more attention as they produce pure water as a byproduct. Alkaline fuel cell (AFC) is one type of fuel cell classified as H2eO2 fuel cell. In fact, AFC has an advantage because the less corrosive nature of AFC’s components ensures a greater potential longevity [3]. An AFC that directly utilizes a borohydride compound as fuel in an aqueous alkaline medium is termed a direct borohydride fuel cell (DBFC). DBFCs are one of the most exciting energy technologies developed to solve the hydrogen storage and fuel efficiency

issue [4e6]. Their electric performances can be enhanced by using an aqueous solution of potassium borohydride (KBH4) or sodium borohydride (NaBH4) [7,8]. They are grouped as complex hydrides containing large amounts of hydrogen. For example, NaBH4 contains 10.6 wt.% of hydrogen [9]. Furthermore, borohydride is chemically stable and non-combustible hence easy to be store. The reaction product is also recyclable [10]. The chemical reactions at the anode and cathode in a DBFC are given by the following: Anode reaction : 2H2 þ 4OH" /4H2 O þ 4e"

(1)

Cathode : O2 þ 2H2 O þ 4e" /4OH"

(2)

Overall net reaction : 2H2 þ O2 /2H2 O

(3)

A critical safety issue arises, however, from the leakage and evaporation of the conventional alkaline aqueous

* Corresponding author. Tel.: þ604 599 6118; fax: þ604 594 1011. E-mail address: [email protected] (A.A. Mohamad). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.037

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electrolyte employed in an AFC system. Instead of utilizing a solid electrolyte that gives poor conductivity and poor surface contact, the introduction of the gel polymer electrolyte (GPE) into a DBFC system was found to be the best alternative. To be applied in fuel cells or any other electrochemical devices, GPEs need to exhibit high ionic conductivity and an ability to sustain high electrochemical activities. Therefore, a suitable polymer host must be chosen to ensure that the GPE has good characteristics. Recently, electrolytes based on natural polymers, such as starch [11,12], cellulose [13] and chitosan [14e16], have been proposed. Sago, or metroxylon sago, is another possible candidate starch to be the polymer host to produce the GPE. Sago is a white, tasteless and odorless natural polymer possessing biodegradability and low toxicity. Furthermore, sago exhibits gel-like characteristics when dissolved in water. In the present work, the possibility of using sago as a gel was investigated. Sago-based GPEs with an optimum composition of potassium hydroxide (KOH) and NaBH4 were prepared to achieve high ionic conductivities. The influence of different concentrations of KOH and NaBH4 in the GPE were investigated by impedance spectroscopy and viscosity studies. The electrochemical properties of the DBFCs were then studied through oxygen consumption, discharge characteristic, open circuit voltage (Voc), currentevoltage (IeV) and current densityepower-density (JeP) measurements.

2.

Experimental

Sago powder (Nee Seng Ngeng & Sons Sago Industries Sdn. Bhd., Sarawak), KOH (Merck) and NaBH4 (Sigma Aldrich) were used as the starting materials in the preparation of the GPEs. Firstly, 2.0 g of sago powder was added to 20 ml of deionized water, and the mixture was stirred and heated until the sago powder was fully dissolved and formed a transparent gel solution. The gel solution was then cooled to room temperature. After that, 20 ml of KOH solution was added to the gel solution in different concentrations. Homogenization of the GPE was achieved by continuously stirring the mixed solutions. Finally, the GPE preparation was followed by the addition of different concentrations of 20 ml NaBH4 solution. The KOH pellets and NaBH4 granules were not directly added to the sago gel solution to prevent agglomeration. The compositions of the samples were sago þ x M KOH þ y M NaBH4, where x ¼ 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9, and y ¼ 0, 1, 2 and 3. Conductivity measurements of the GPE samples were then carried out using an Autolab PGSTAT 30 system with a frequency response analyzer (FRA) module at room temperature. The samples were connected to the system using a Teflon casing with stainless steel (SS) electrodes and a spacer filled with the GPE. The conductivities of the GPE samples without fuel addition were measured first, followed by the GPE samples with fuel addition. The bulk resistances (Rb) of the GPE samples obtained from the impedance spectrum were collected over a frequency range between 0.1 Hz and 1.0 MHz with amplitude of 0.01 V.

Viscosity (h) measurements were then carried out for three batches of GPE samples. The first batch was pure sago GPEs with different weights of sago powder in deionized water. The second batch was sago GPEs with different KOH concentrations, and the final batch was sago þ KOH GPEs with different NaBH4 concentrations. The preparation methods of the samples were same as for the conductivity measurements. The viscosities of the samples were measured using a Haake viscotester VT550 at room temperature. The commercial anode, C/Ni (ECT, UK), was placed at the bottom of a plastic beaker, while the commercial cathode, MnOx/C fiber on Ni mesh (ECT, UK), was attached to one of the open ends of a hollow plastic cylinder. Both the anode and cathode had an active area of 16.0 cm2. The prepared 60 ml of sago þ KOH þ NaBH4 GPE solution was then poured into the beaker to fill the space between the anode and cathode. The distance between anode and cathode is 1.2 cm. One side of the cathode was in contact with the GPE solution, while the other side was exposed to the air. In order to measure the current and voltage of the DBFC system, two wires were connected to the anode and cathode terminals. The performance of the DBFC system was evaluated by employing the optimum GPE composition that gave the best conductivity value. Firstly, oxygen (O2) consumption at the cathode in the DBFC system was measured by the gasdisplacement method. From the O2 reduction reaction occurring at the cathode as shown in Eq. (2), the rate at which the O2 was consumed can be related to the electric current. The numbers of electrons reacted per molecule of oxygen were calculated by dividing the number of electrons that flowed round the circuit by the number of oxygen molecules involved in the reaction. The ideal gas law formula, as shown in Eq. (4), was used in the calculation. PV ¼ nRT

(4)

where P is the atmospheric pressure, V is the volume of water rose in the pipette, n is the amount of oxygen gas in moles, R is the gas constant, and T is the room temperature. The fuel cell was connected to a variable resistor in series with an ammeter. The oxygen consumption measurement kit was also set up with a clamped 10 ml pipette, which was connected to the fuel cell with a flexible tube. The pipette was half immersed in a beaker filled with water. The variable resistor was adjusted to control the current flow within a range of between 20 and 80 mA. Once the circuit was completed, it was observed that the water level rose up the pipette very slowly. A timer was started when the water level inside the pipette was just above the outside level. The time for the water level to change by 1.0 cm3 was taken. While the water was rising, the load resistor was adjusted to keep the current constant. After that, the load was disconnected, and the pipette was removed from the water to empty it and then replaced. These steps were repeated with different currents. The discharge profile at a constant current of 10.0 mA and the Voc of the DBFC system were also evaluated using a Neware BTS. In order to plot the IeV and JeP curves, the DBFC was discharged with various current ranges using an Autolab PGSTAT 30 with a general purpose electrochemical system (GPES) module. All the measurements and evaluations were carried out at room temperature.

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Fig. 1 e Impedance plot for GPEs with different concentrations of KOH: (a) pure sago, (b) 1e3 M KOH, (c) 4e6 M KOH and (d) 7e9 M KOH.

3.

Results and discussion

3.1. Sago gel polymer electrolyte characterization: conductivity Fig. 1 shows the Nyquist plot for the GPE based on sago and KOH. The results show that with the addition of KOH to the sago gel, the plots were identified by non-vertical spikes intersecting on the Zr axis. These behaviors were presumably due to an inhomogeneous electrode/electrolyte surface contact. However, the spike angles were just slightly (about 80$ ) lower than the theoretical perpendicular angle. The performance of these gels were therefore better than the reported impedance spectrum performance of solid polymer electrolytes, which had spike angles of less than 70$ [17]. Fig. 1 (a) gives the average Rb value for the pure sago GPE at (1027 % 80) U. The addition of KOH into the sago GPE significantly decreased the bulk resistance in the electrolyte system. The average Rb value for the sago GPE with 3 M KOH added was (1.154 % 0.020) U while the lowest Rb value was (0.675 % 0.003) U with 6 M KOH added, as presented in Fig. 1(b) and (c) respectively. However, the Rb value was slightly increased to (0.688 % 0.016) U when a high concentration of 9 M KOH was added, as shown in Fig. 1(d). The Rb values were then converted into ionic conductivities (s) using the following equation: s¼

t Rb A

(5)

where t is the thickness of the Teflon spacer where the sago GPE fills, and A is the electrode/electrolyte surface contact area. Fig. 2 shows the dependence of the conductivity of the sago GPE with different concentrations of KOH. The pure sago GPE had a very low conductivity of (2.01 % 15.7) & 10"4 S cm"1. The ionic conductivity increased steadily as 1 M KOH was introduced into the sago GPE. As the concentration of KOH was increased, the ionic conductivity increased sharply from the order of 10"4 S cm"1 to 10"1 S cm"1. The ionic conductivity continued to increase as greater concentrations of KOH was added until 6 M KOH is reached. The highest ionic conductivity value at this

Fig. 2 e Conductivity dependence of the sago gel polymer electrolyte with different concentrations of KOH at 25 $ C.

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Fig. 3 e Conductivity of the sago D 3, 6 and 9 M KOH GPEs with different NaBH4 concentrations at 25 $ C.

ionic conductivity dropped slightly and remained constant once a maximum was reached at 6 M KOH added. This can be explained by the limitations of the free charge carrier. As the KOH concentration increased, the free ions would tend to come closer to each other, resulting in the formation of ion pairs. Ion pairs are held together by Coulomb attraction, which is a direct consequent of the mass law effect on ionization equilibria in electrolytic solutions. Therefore, the number of free ions will decrease at high concentrations and result in the observed decrease in conductivity. NaBH4 was added into the sago GPE as the fuel for the DBFC system. Because the dissolved NaBH4 was directly added into the system, the hydrogen gas (H2) would be readily available for the hydrogen oxidation reaction at the anode without any external supply. The H2 was produced through hydrolysis of NaBH4 as shown in Eq. (6). The presence of the borate product would not interfere the system because it is relatively inert [1]: NaBH4 þ 2H2 O/NaBO2 þ 4H2

"1

"1

point was (3.05 % 0.01) & 10 S cm . Beyond 6 M of KOH, the ionic conductivity decreased slightly and then remained constant. This conductivity of the sago GPE was higher than the alkaline solid polymer electrolyte which had conductivity on the order of 10"4 S cm"1 [18,19]. The observed trend of ionic conductivity results can be explained by the following. The pure sago GPE had a very low conductivity, which may be attributed to possible impurities present. The effect of KOH addition can be seen clearly from the steady increase in the ionic conductivity of the sago GPE. KOH supplied free OH" ions which are the charge carriers in the electrolyte system. At lower concentrations of KOH, few charge carriers existed so that ionic conductivity correspondingly kept low in value [20]. The addition of higher concentrations of KOH increased the ionic conductivity due to an increase in the number of OH" ions available in the electrolyte. However, the

(6)

It was expected that adding more NaBH4 into the sago GPE would improve the conductivity of the sago GPE. However, there was fluctuation in conductivity as seen in Fig. 3 due to the effervescence of H2 (g) from the system since the H2 affect the conductivity of GPE system. The presence of H2 may provide extra energy to the GPE system, thereby enhancing or/ and interfering the mobility of the charge carriers. The GPE with the optimum composition of sago þ 6 M KOH þ 2 M NaBH4 gave the best conductivity of (0.270 % 0.005) S cm"1 and was chosen to be employed in the DBFC fabrication.

3.2. Sago gel polymer electrolyte characterization: viscosity Normally, the structure of sago consists of linear amylose (15e20%) and branched amylopectin (80e85%) with molecular

a

b

Fig. 4 e Molecular structure of (a) amylose and (b) amylopectin in sago.

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Fig. 5 e Hydrogen bond in amylopectin structure.

structures as shown in Fig. 4(a) and (b) respectively. When the sago was stirred and heated in water, the kinetic energy of the water and sago molecules increased to allow solvation. As shown in Fig. 5, the formation of hydrogen bonds between the water molecules and the hydroxyl group of the sago molecules facilitated solvation and created a gelling effect. The sagosolvent interactions were stronger than the sagoesago attraction forces. Chain segments started to absorb solvent molecules, thereby increasing the volume of the sago matrix and turning into gel. The viscosity of the pure sago GPE was (2.993 % 0.234) Pa s when 2.0 g sago powder was dissolved in 20 ml water (Fig. 6). Further additional of sago powder induce higher gel viscosity. This composition was then used for the next test. The viscosity of the sago GPE decreased dramatically to (0.262 % 0.018) Pa s when 1 M KOH was added, as shown in Fig. 7. This decrease is attributed to the breakdown of intermolecular hydrogen bonds of amylopectin and amylose by the abundant number of OH" ions from KOH, as illustrated in Fig. 8. Rupturing of H-bond between H2O molecule of starch chain is more severe. Meanwhile, adding higher concentrations of KOH did not visibly affect the sago GPE viscosity; the viscosity remained constant up to 9 M of KOH added. Apparently, the viscosity did not limit the conductivity of the sago GPE significantly. Based on the poly(methylmethacrylate) gel electrolytes [21,22], this

Fig. 6 e Viscosity of the pure sago GPE at 25 $ C.

GPE can also be thought of as a conductive liquid encaged in a amylopectin structure with continuous free OH" conduction paths available through it. As discussed earlier in Section 3.1, the direct addition of NaBH4 into the electrolyte system also had a small effect on the viscosity of the sago þ KOH GPE. The viscosity behaviors of the sago þ 3 M KOH, sago þ 6 M KOH and sago þ 9 M KOH GPEs is shown in Fig. 9. For different KOH concentrations, the viscosity of the GPE showed a decreasing pattern as higher concentrations of NaBH4 was added. The optimum viscosity of (0.074 % 0.003) Pa s was obtained for the GPE with a composition of sago þ 6 M KOH þ 2 M NaBH4. The addition of NaBH4 reduced the viscosity of the GPE system due to the presence of H2. H2 is a gas with very low viscosity that will definitely reduce the shear stress in the GPE system and, consequently, decrease the viscosity of the GPE.

3.3. Direct borohydride fuel cell characterizations: oxygen consumption Table 1 shows the calculated values for the number of electrons involved in the cathode reaction at different applied currents of 20, 40, 60 and 80 mA. The calculated numbers were the same as the theoretically estimated values at different currents applied to the DBFC system. Verma et al. [23] reported

Fig. 7 e Viscosity of sago GPEs with different KOH concentrations at 25 $ C.

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Fig. 8 e Breakdown of the intermolecular hydrogen bond by hydroxyl ions in the sago gel solution.

that at an average current of 43.25 mA, the calculated electrons reacted per oxygen molecule is 4.54. In this work, the value of 4.45 electrons reacted per oxygen molecule was obtained at 40 mA current flow. At the other currents investigated, i.e., 20, 60 and 80 mA, the calculated numbers of electrons reacted per oxygen molecule were still equal to the theoretical values. These results confirmed that the GPE based DBFC performances were comparable to the liquid electrolytebased DBFC.

3.4. Direct borohydride fuel cell characterizations: discharge and open circuit voltage profiles Fig. 10 shows the discharge profile of the 2 M NaBH4 DBFC system employing the sago þ 3 M KOH, sago þ 6 M KOH and sago þ 9 M KOH GPEs as a function of time. The DBFCs were discharged at a constant current of 10 mA. It can be observed that the DBFC system employing the sago þ 6 M KOH þ 2 M NaBH4 GPE had the best discharge characteristic with a nominal voltage of 0.806 V sustained for 23 h. Even though the DBFC system employing the sago þ 9 M KOH þ 2 M NaBH4 GPE also had a 23 h lifetime, the nominal voltage was lower at 0.7741 V. This lower nominal voltage may be the result of excess product accumulation within the fuel cell at high

Table 1 e Oxygen consumption measurement results for the sago GPE with 6 M KOH D 2 M NaBH4 with different currents at 25 $ C. Current, Time for 1 ml No. of electrons No. of electrons I (mA) water to rise flow through reacted per up (min) the circuit oxygen molecule

Fig. 9 e Viscosity of sago D 3, 6 and 9 M KOH GPEs with different NaBH4 concentrations at 25 $ C.

20 40 60 80

13.24 7.30 4.68 3.73

9.93 & 1.10 & 1.05 & 1.12 &

1019 1020 1020 1020

4.04 4.45 4.28 4.55

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the VOC were recorded every 60 min for 24 h at room temperature. It can be seen that all the DBFCs demonstrated good voltage stability. The DBFC employing the sago þ 6 M KOH GPE gave highest average VOC value of (0.926 % 0.009) V. DBFCs employing the sago þ 3 M KOH and sago þ 9 M KOH GPEs exhibited slightly lower VOC values of (0.882 % 0.006) V and (0.904 % 0.004) V respectively. The stable voltage achieved by the DBFCs may be explained by the good reactant tolerance within the system that was sufficient for 24 h of storage. Consequently, these results show that the DBFCs had good durabilities at room temperature and were able to satisfy the requirements as power sources.

Fig. 10 e Discharge profiles of the DBFC employing the sago D 2 M NaBH4 GPE with different KOH concentrations at 25 $ C.

reactant concentrations, which affected the performance. A nominal voltage of 0.743 V was obtained for the DBFC with the sago þ 3 M KOH þ 2 M NaBH4 GPE, which lasted for 22 h. At low KOH concentrations, reactant depletion may cause the DBFC system to have shorter lifetimes. From the results, different KOH concentrations slightly affected the performance of the DBFC systems. The calculated discharge performance for the DBFCs employing the sago þ 3 M KOH þ 2 M NaBH4, sago þ 6 M KOH þ 2 M NaBH4 and sago þ 9 M KOH þ 2 M NaBH4 GPEs were 220, 230 and 230 mA h respectively. If the DBFCs were continuously supplied with fresh reactant and fuel, we believe that the discharge efficiencies could be improved as fuel crossover issues could be reduced. Since this work was carried out in order to study the performance of DBFCs with a fixed amount of sago þ KOH þ NaBH4, the set up did not involve tanks for reactant and fuel stocks. The voltage stability of the DBFC was investigated by measuring the VOC. Fig. 11 shows the VOC of the 2 M NaBH4 DBFCs employing the sago þ 3 M KOH, sago þ 6 M KOH and sago þ 9 M KOH GPEs. Without any load applied to the DBFCs,

Fig. 11 e Voc profiles of the DBFC employing the sago D 2 M NaBH4 GPE with different KOH concentrations at 25 $ C.

3.5. Direct borohydride fuel cell characterizations: IeV and JeP curves Fig. 12 presents the IeV and JeP curves for the DBFC system employing the same GPE compositions as the discharge profile measurements. From the gradient of the IeV curve, it is clearly observed that the DBFC with the sago þ 6 M KOH þ 2 M NaBH4 GPE gave the lowest internal resistance (r) of 1.341 U. This result supports the finding discussed in Section 3.1 that the optimum conductivity for the GPE was obtained at a composition of sago þ 6 M KOH þ 2 M NaBH4. The sago þ 9 M KOH þ 2 M NaBH4 and sago þ 3 M KOH þ 2 M NaBH4 GPEs had higher r values of 1.345 U and 2.158 U respectively. Lower KOH concentrations cause reactant exhaustion and affect the kinetic reaction rate within the fuel cell [24]. However, higher KOH concentrations, such as 9 M, are also unfavorable due to a limitation of charge carrier movement. As a result, there is an optimized KOH concentration to maximize the cell power density. The DBFC with the sago þ 6 M KOH þ 2 M NaBH4 GPE achieved the best performance by providing the highest power density of 8.188 mW cm"2 from the JeV curve. The power density obtained for the DBFC with the sago þ 9 M KOH þ 2 M NaBH4 GPE was 7.084 mW cm"2. The lowest power density of 4.893 mW cm"2 was obtained with the sago þ 3 M KOH þ 2 M NaBH4 GPE.

Fig. 12 e Plot of IeV and JeP for the DBFC employing the sago D 2 M NaBH4 GPE with different KOH concentrations at 25 $ C.

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Conclusion

New GPEs based on sago þ KOH þ NaBH4 were prepared. The best GPE composition of sago þ 6 M KOH þ 2 M NaBH4 yielded a high ionic conductivity of (0.270 % 0.005) S cm"1. The viscosity of the GPE did not show major changes and there were no major effects on the system. A DBFC utilizing this GPE system demonstrated good electrochemical capability. The effects of reactant and fuel concentration on the DBFC system were investigated through cell characterizations. The theoretical number of electrons reacted per oxygen molecule was also verified through oxygen consumption measurements. These results suggest that a sago-based GPE is suitable to be applied in a DBFC for electrochemical device applications. Its performance is also comparable with a liquid-based DBFC.

Acknowledgements A.J. would like to thank MOSTI for the NSF scholarship and USM-RU-PRGS for their financial support via Grant (8031001). Z.A.A and A.A.M. wish to thank USM-RU for their financial support via Grant (811103).

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