Erkan, S.

Eroglu, I.

Ozkar, S.

Yurdakul, A.

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Design of a Hydrogen Gas Generator Using Aqueous SodiumBorohydride Solution for Portable Fuel Cell ApplicationsA. Yurdakul, S. Erkan, S. Ozkar, I. ErogluThis document appeared inDetlef Stolten, Thomas Grube (Eds.):18th World Hydrogen Energy Conference 2010 - WHEC 2010Parallel Sessions Book 4: Storage Systems / Policy Perspectives, Initiatives and Co-operationsProceedings of the WHEC, May 16.-21. 2010, EssenSchriften des Forschungszentrums Jülich / Energy & Environment, Vol. 78-4Institute of Energy Research - Fuel Cells (IEF-3)Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, 2010ISBN: 978-3-89336-654-5Design of a Hydrogen Gas Generator Using Aqueous Sodium Borohydride Solution for Portable Fuel Cell Applications Asli Yurdakul, Serdar Erkan, Saim Ozkar, Inci Eroglu, Middle East Technical University (METU), Turkey 1 Introduction Due to increasing demand for power in portable electronic devices fuel cells attained considerable attention on replacement of lithium batteries. The last two century heavily depends on fossil fuels. Due to drawbacks of fossil fuels, researches have been concentrated on alternative energy sources.  Hydrogen can be considered as ideal fuel for fuel cell.  Since hydrogen can be stored, transported, and converted easily to other energy forms, there has been intense scientific, industrial, and governmental interest in the development of hydrogen based energy production technology [1,2].  Also it is considered as clean fuel but it has still a problem, storage that is crucial for the supply of hydrogen for portable fuel cells. Hydrogen has been stored in tanks in compressed or liquefied form, in hydrogen-storing alloys, and on activated carbon or nanoscale materials such as carbon nanotubes but, none of these methods are suitable for portable applications due to the low volumetric and gravimetric efficiency of hydrogen storage as well as the associated safety issues. Instead of such hydrogen storage methods, liquid fuels (methanol, ethanol, gasoline, etc.) and chemical hydrides (NaBH4, KBH4, LiH, NaH, etc.) could be employed as hydrogen sources for portable PEMFC [2]. Among them, hydrogen generation from the hydrolysis reaction of an alkaline sodium borohydride solution (NaBH4) has widely investigated due to its theoretically high hydrogen storage capacity (10.8 wt %). Also, because of the high purity of produced hydrogen, it can be used as hydrogen supplier for proton exchange membrane (PEM) fuel cells [3].  Hydrogen is produced from NaBH4 according to the following irreversible, heterogeneous and highly exothermic, with the heat of reaction of 210kJ/mol:  Catalytic generation of hydrogen from NaBH4 solutions has several advantages listed below [4]:  NaBH4 solutions are non-flammable and not toxic.  NaBH4 solutions are stable in air for months.   H2 generation only occurs in the presence of selected catalysts. Proceedings WHEC2010 281But this reaction can occur to some extent even without a catalyst if the solution pH < 9. However, to increase the shelf life of NaBH4 solutions and to suppress the self hydrolysis of it, NaBH4 solutions are typically maintained as a strongly alkaline solution by adding NaOH. According to the reactor type and amount of reactants, NaOH can be added in various amounts but generally in the range of 5-15 wt% of reactants. It must be noted that the excess amount of NaOH decreases the hydrogen yield. Mostly NaOH in the amount of 3-5% of reactant is thought to be sufficient to control hydrogen release. [4]   The only other product in the gas stream is water vapour.  The presence of water vapour is beneficial for use in PEM fuel cells where the water vapour can be used to humidify the PEM membrane. The H2 gas generated is sufficiently pure and it can be used directly in PEM fuel cells without further cleanup. As a reactant, water is important since approximately 95% of the reactant mass is occupied by it.  Furthermore fuel cell applications needs low weight hydrogen devices so, water content must be decreased as possible. [4]  Reaction products are environmentally safe.  Since reaction (1) is totally inorganic and does not contain sulphur, it produces virtually no fuel cell poisons such as sulphur compounds, CO, soot, or aromatics.  H2 generation rates are easily controlled.  The heat generated by reaction Eq. (1), 75 kJ/mole H2 formed, is considerably less than the typical > 125 kJ/mole H2, produced by reacting other chemical hydrides with water. This promises a safer, more controllable reaction.  Moreover, to generate H2, NaBH4 solution is allowed to contact catalyst either by dipping catalyst into a NaBH4 solution or injecting NaBH4 solution on catalyst. This ensures fast response to H2 demand, i.e., H2 is generated only when NaBH4 solution contacts with catalyst. When H2 is no longer needed, NaBH4 solution is removed from catalyst and H2 production ceases.   Volumetric and gravimetric H2 storage efficiencies are high.  The reaction products can be recycled.  H2 can be generated even at 0C.  Also hydrogen could be generated at temperatures below 0C if water-methanol or water-ethylene glycol mixtures were used. [5]  2 Experimental In this work, we construct a batch system for hydrogen production where hydrogen generating reactor is approximately 30mL three neck tube reactor that is stayed in a water bath. From one neck, alkaline NaBH4 solution is introduced, from the other neck the reaction temperature is measured and recorded online. The generated gas that exits from reactor comes to the water displacement column that is filled with water. The scale is placed under the column for determining the change in the water amount and the scale is also connected to the computer for online recording the data. From that displacement of water the generated amount of hydrogen is determined. Figure 1 shows the experimental setup. 282 Proceedings WHEC2010Figure 1: Experimental setup for hydrolysis of sodium borohydride. In experiments first, water displacement column is filled with water by the help of vacuum pump. In order to prevent the water leakage to the pump, there is a gas washing column for collecting coming water. Then the solutions are prepared. Since NaBH4 spontaneously reacts with water, while the spontaneous hydrolysis can be depressed in alkaline solutions, we prepared solutions on sodium hydroxide (NaOH) medium. The commercial catalyst that is 20% HP Pt on Vulcan XC-72 (ETEK®) is used in powder form to initiate the reaction. The desired amount of catalyst is weighted and placed into the reactor. Some amount of deionized water (1-2mL) is added on the catalyst to wet and introduce a homogenous medium. Then necessary amount of NaOH is weighted and dissolved in 14mL of deionized water. When alkaline medium is established the NaBH4 is added to finish the solution preparation. During the set of experiments amount of water used is fixed. For the high temperature experiments this prepared solution is heated until it reaches to the reaction temperature before adding to the reactor. After that the solution is introduced to the reactor. When it comes to contact with catalyst the hydrogen is started to be generated. The generated gas comes to the water displacement column from the bottom. After it enters the column the water level reduces and this pressurizes the scale under it. The change in the scale is measured at every 0.2s. Also the temperature of the reaction and the ambient temperature are recorded at every 0.2s. Proceedings WHEC2010 2833 Results There are mainly four parameters that are affecting the hydrogen generation such that NaBH4 concentration, catalyst amount, NaOH concentration and temperature. To investigate the order of magnitude of their effects the controlled experiments were carried out. The effect of NaBO2 is not considered in this work. To analyze the effect of NaBH4 concentration on H2 generation rate three sets of experiment were prepared. In these sets, the only parameter that was changing is NaBH4, others were kept as constant. These three experiments were carried out at constant temperature of 20C.  The pressure of system was 0.94atm. It was found that as NaBH4 concentration increases the amount of hydrogen increases. The initial hydrogen generation rates are nearly the same for all these three experiments but, through the completion of reaction the rates were changing. This shows that the reaction rate is dependent of concentration of sodium borohydride solution catalyzed with Pt/C. As mentioned before, the solutions of NaBH4 were prepared in NaOH in order to prevent self hydrolysis of sodium borohydride. To investigate the effect of NaOH concentration again three sets of experiments were conducted that had 1ww% NaOH, 5ww%NaOH and 10%NaOH. It was found  that the amount of hydrogen produced is same within 3 cases as expected since the NaBH4 amount did not changed. Also, as NaOH concentration increases the H2 generation rate decreases accordingly. So we conclude that for high production rates we have to optimize the amount of NaOH that also stabilize the solution. When catalyst amount and temperature was considered as expected it is seen that the hydrogen production rate increases with increase of catalyst and temperature whereas the maximum amount of hydrogen produced does not changes. To determine the kinetic model of hydrolysis reaction on commercial Pt/C catalyst, method of excess is used. After the experiments we concluded that the reaction rate depends on concentration of NaBH4 and NaOH, catalyst amount and temperature. By using power law we propose a model:  2 42 4/( , , , )aH NaOH NaBH catE RTH NaBH NaOHr f T C C Cr ke C Cα β−== After calculations the rate model proposed is of the form that: 420.2353 0 20.405' 5.2 10         for 20 C and 0.048gPt/C with R 0.994NaBHHNaOHCrC−= × × = For the temperature analysis Arrhenius plot is drawn shown in Figure 2. 284 Proceedings WHEC2010 Figure 2: Arrhenius plot. From Figure 2, the pre-exponential factor and activation energy are found as 62 10.minmolL×  and 50.65 kJ/mol respectively. So the rate expression takes the form:  420.23550.656 20.405' 2 10         for 0.048g Pt/C with R 0.986NaBHRTHNaOHCr eC−= × × × = The catalyst effect must be introduced to the model in the activation energy but this is not investigated through this work.  4 Conclusions Hydrogen generation from the hydrolysis of stabilized sodium borohydride solution offers a convenient, practical and effective way for portable fuel cell applications. Using NaBH4 solutions reduces inherent safety concerns associated with long-term gaseous H2 storage. H2 production occurs on demand and reaction products are not toxic. Even the presence of water vapour is beneficial for use in PEM fuel cells where the water vapour can be used to humidify the PEM membrane. The H2 gas generated is sufficiently pure and it can be used directly in PEM fuel cells without further cleanup.  In the present work, hydrogen generation system for portable applications has been studied and developed. The 20% Pt/C catalyst was utilized in the form of powder to initiate the hydrolysis reaction. The four parameters that are affecting the H2 generation rate such as amount of catalyst, NaBH4concentration, NaOH concentration and temperature has been widely investigated. It was seen that under Pt catalyst reaction behaves nearly zero order with respect to the NaBH4. Also, the increase in NaOH concentration which is used to prevent self hydrolysis of NaBH4 results decrease in the rate. The catalyst amount and Proceedings WHEC2010 285temperature significantly affect the H2 generation rate in a positive manner as expected.  At the end of controlled experiments the rate equation for hydrolysis reaction has been derived. Moreover, at ambient conditions the generation rate was estimated as 2.14L/min.g Pt catalyst for 0.23M NaBH4-0.27M NaOH solution.  This indicates that our generator system is suitable for portable applications of PEM fuel cells.  References [1]  Erce Şengül, “Preparation and performance of membrane electrode assemblies with Nafion and alternative polymer electrolyte membranes” MS thesis, Chemical Engineering Department, METU, Ankara, September, 2007 [2]  Kim et al.  “Hydrogen generation system using sodium borohydride for operation of a 400W-scale polymer electrolyte fuel cell stack” Journal of Power Sources 170 (2007) 412-418  [3]  Hung, A., Tsai, S., Hsu, Y., Ku, J., Chen, Y., Yu, C. “Kinetics of sodium borohydride hydrolysis reaction for hydrogen generation” International Journal of Hydrogen Energy 33 (2008) 6205–6215 [4]  Çakanyıldırım, Ç., Gürü, M. “Hydrogen cycle with sodium borohydride” International Journal of Hydrogen Energy 33 (2008) 4634–4639 [5]  Schlesinger, H.I., Brown, H.C., Finholt, A.E., Gilbreath, J.R., Hockstra, H.R., Hyde, E.K. “Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen” J Am Chem Soc 75 (1953) 215–219. 286 Proceedings WHEC2010

Design of a Hydrogen Gas Generator Using Aqueous Sodium Borohydride Solution for Portable Fuel Cell Applications

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Design of a Hydrogen Gas Generator Using Aqueous Sodium Borohydride Solution for Portable Fuel Cell Applications

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