All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications

All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications

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All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications Byeong Gyu Gang a, Sejin Kwon b,* a

Korea Aerospace Research Institute (KARI), Integrated Flight Performance Team, 169-84 Gwahak-ro, Yuseong-gu, Daejeon, 305-806, Republic of Korea b Division of Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea

article info

abstract

Article history:

A portable electric power plant is developed using a NaBH4 (sodium borohydride)-based

Received 4 November 2017

proton exchange membrane fuel cell stack. The power plant consists of a NaBH4-based

Received in revised form

hydrogen generator, a fuel cell stack, a DC-DC converter, a micro-processed controller and

30 January 2018

a data monitoring device. The hydrogen generator can produce 5.9 L/min pure hydrogen

Accepted 1 February 2018

gas using catalytic hydrolysis of 20 wt% NaBH4 to feed a 500-W scale fuel cell stack. Thus,

Available online xxx

the Co/g-Al2O3 and Co-P/Ni foam catalysts in the hydrogen generator play significant roles in promoting hydrogen production rates that are as fast as necessary by enhancing the

Keywords:

slow response that is intrinsic to using only Co-P/Ni foam catalysts. Moreover, different

Portable electric power plant

hydrogen production rates can easily be achieved during the operation by controlling

Proton exchange membrane fuel cell

NaBH4 solution rates using a fuel pump so that the hydrogen storage efficiency can be

Sodium borohydride

improved by supplying required hydrogen gas in accordance with load demands. The

Hydrogen generator

specific energy density of the electric power plant was measured 211 Wh/kg. Therefore, the

Micro-processed controller

power plant described here can be a power source for mobile applications, such as cars and UAVs, as well as a stationary power supplier when electric energy is required. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The increasing demand for clean energy that is occurring in response to pollution by fossil fuels has imposed great challenges on many research groups because different types of emissions from fossil fuel-based ground and aerial vehicles have been contributing to environmental pollution [1]. As green technologies develop, eco-friendly or low-emission fuels, such as a hydrogen-based energy, has been considered

to be a promising alternative to fossil fuels [2] because it has no or low emissions when it is burned as a fuel [3]. In addition, hydrogen can release a high energy density (142 MJ kg-1) compared to liquid hydrocarbons (47 MJ kg-1) [4]. When hydrogen energy is consumed in the fuel cell system, which has very good efficiency, it can provide energy storage up to 33,300 Wh/kg, whereas that of new lithium-ion batteries are limited to approximately 270 Wh/kg [5]. However, with regard to hydrogen and fuel cell implementation, four main obstacles exist: hydrogen production, hydrogen storage, hydrogen

* Corresponding author. E-mail address: [email protected] (S. Kwon). https://doi.org/10.1016/j.ijhydene.2018.02.006 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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distribution, and the high cost of fuel cells [6]. Additionally, pure hydrogen gas is not naturally produced and the finished products are more costly than the costs of petroleum production, which can hinder the integration of hydrogen fuel into high-energy-consumption industries [7]. There are currently two popular types of hydrogensupplying technologies, including compressed hydrogen and chemical hydrides, for mobile applications. Compressed hydrogen is a physical storage method that requires high volume and pressure, and its mass storage efficiency (%H2/kg) at 700 bars is 4.8% [8]. The gravimetric hydrogen storage of compressed hydrogen will increase as the fabrication technologies for the storage tanks develop. However, expensive hydrogen charging stations are only available on-site for filling a hydrogen tank, and high charging pressures remain problematic, making it an unsuitable storage method for small mobile applications [9]. Thus, an economic and safe method are the most significant issues for enhancing mobile hydrogen storage. To meet the storage requirements for the portable power plant discussed herein, we adopted a chemical hydride, NaBH4 (sodium borohydride), as a hydrogen-storage medium. The rationale behind this decision is that NaBH4 has a relatively high volumetric and gravimetric hydrogen density. It is also safe. Furthermore, hydrogen production rates can be controlled in the presence of an appropriate catalyst at room temperature. The hydrolysis reaction of NaBH4 is expressed in Eq. (1). NaBH4 þ 2H2 O/NaBO2 þ 4H2 þ 217 kJ

(1)

From this equation, the NaBH4 concentration is an important factor that if its concentration is increased to hold more hydrogen gas, the amount of water to dissolve the NaBH4 is decreased. In consequence, the formation of sodium metaborate (NaBO2$xH2O) occurs readily at lower water solubility with higher NaBH4 concentrations. If the system weight is not a critical factor, it is possible to adopt the lower concentrations of sodium borohydride as a hydrogen storage, which results in the minimized byproduct precipitation. However, the system weight is an important factor in designing mobile systems, especially UAVs. Moreover, one of the difficult challenges in using NaBH4 as a hydrogen storage medium is the decrease in gravimetric efficiency that is associated with the hydration of reaction byproducts, sodium metaborate which is the highly viscous solution so that it can lower reaction yield by clogging the reactor [10] and may damage the fuel cell stack if it is supplied with hydrogen gas. Thus, the byproduct must be removed during the hydrolysis of NaBH4. To eliminate NaBO2, we equipped the liquid-gas separator with a purging pump that periodically pumped NaBO2 out of the fuel cell system by the control logic during the operation. In this way, the negative effects of NaBO2-hydration during the catalytic reaction process were eradicated, which resulted in system weight reduction as operational time elapsed. Therefore, after separating NaBO2, pure hydrogen gas could be fed into the proton exchange membrane fuel cell stack (PEMFCS) to generate stable power. Moreover, the separated NaBO2 after

the operation of the PEMFCS was collected in the chemical container and sent to the special chemical factory for the wasting process. To produce hydrogen gas at required rates, the addition of catalysts is necessary. Therefore, numerous catalysts based on metal-alloys have been investigated. Among them, noble metals such as ruthenium and platinum show high activity in the sodium borohydride hydrolysis [11] however, their high cost and availability may cause the system price to increase. Thus, Co and Ni borides are considered as good candidates for catalyzed hydrolysis reaction of NaBH4 owing to their good catalytic activity and low cost [12,13]. In our previous research we also proved that Co-P/Ni foam catalysts was capable of increasing the catalytic activities of sodium borohydride [8]. Thus, we adopted the synthesis of mixed Co/g-Al2O3 and Co-P/ Ni foam catalysts to enhance the catalytic activity. Then, the NaBH4-based hydrogen system was integrated into the 500-W scale proton exchange membrane fuel cell stack because it has high power density and low operating temperature. With this integration, the fuel cell system could constitute a small and portable electric power plant with other devices such as a controller and DC-DC converter. Thus, the PEM fuel cells with its associated hydrogen production system could be developed for transportation applications as well as for stationary and portable applications [14]. To effectively control components in our portable electric power plant (such as a fuel pump, purging pump, and cooling fan in the hydrogen generator as well as a purge valve and oxygen-supplier fan in the stack), the micro-processed controller was built based on the systematic control logic. The controller allowed the NaBH4 solution rate to be adjusted using a fuel pump in accordance with the load variations, thereby extending the operational time because of higher fuel efficiency. Therefore, our portable electric power plant could generate a maximum of 500-W power with the consumption of 5.9 L/min hydrogen gas that is supplied by the NaBH4hydrogen generator, and this electric power source could be utilized for mobile applications such as cars and small UAVs as well as a mobile power stations where electric energy is required.

Development of the portable electric power plant Description of the hydrogen generator and the fuel cell system The fuel cell system includes a fuel tank, fuel pump, purge pump, hydrogen generator, liquid-gas separator and two consecutive containers and a 500-W scale fuel cell stack. The hydrogen generator can supply 5.9 L/min hydrogen gas to the PEM stack from the catalytic hydrolysis of 20 wt% NaBH4 using two different types of catalysts, including the Co/g-Al2O3 and Co-P/Ni foam catalysts. These catalysts were embedded in the catalyst bed of hydrogen generator to promote the hydrolysis of NaBH4. As such, the catalysts played important roles in increasing hydrogen production rates to be as fast as required for the operation of the fuel cell stack. However, when the NaBH4 solution started to flow into the hydrogen generator with the fuel pump, not only was a required volume of

Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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hydrogen generated but viscous NaBO2 solutions were also produced during the hydrolysis of NaBH4. Thus, NaBO2 solutions must be separated and pumped out of the system before flowing into the stack with a given volume of hydrogen gas. The liquid-gas separator was designed to pump the byproduct, NaBO2, out of the hydrogen generation system periodically using a purge pump so that only pure hydrogen was allowed to flow. However, due to exothermic chemical reactions in the hydrogen generator, heated hydrogen gas may cause dry conditions in the fuel cell stack, resulting in degradation of the performance of the stack if it is continuously supplied for a long period of time. To reduce the high temperature in the hydrogen gas, a finned container that contained water was fabricated so that the heat radiated outward over a maximum surface area. Furthermore, a cooling fan was installed near the finned container to promote rapid cooling of the heated hydrogen gas. Next, pure hydrogen gas at operating room temperature flowed into the 500-W scale PEM stack to produce power through a container that contained silica grains to capture moisture. The controller had the authority to manipulate five components, including the fuel pump, purge pump and cooling fan in the hydrogen generation system, as well as a purge valve and air-supply fan in the stack. The stack fan was regulated to supply ambient oxygen and to cool the fuel cell stack. The water purge valve was opened every 10 s for 0.2 s to remove water and extra hydrogen gas out of the stack. The hydrogen gas temperature, stack temperature and stack power were monitored and transmitted to the ground station through the controller so that the electric power system could be efficiently operated at remote locations. Fig. 1 is a

configurational diagram of the fuel cell system and Fig. 2 displays the portable electric power plant that uses a fuel cell system.

Preparation of Co-P/Ni foam catalysts for NaBH4 hydrolysis The Co-P/Ni foam catalysts were built according to the electroless deposition method reported in previous research [8,15], in which cobalt coated the surface of Ni foam, subsequently applying reducing agents in the coating bath, so that it could be utilized to accelerate the hydrolysis of NaBH4. Moreover, compared to other noble metal catalysts, such as Pt and Ru catalysts, the performance of a metallic Ni or Co catalyst that coats a Ni foam substrate and is used in a batch reactor was tested and verified for the hydrolysis of NaBH4 [13]. The procedures included the following: the nickel foam samples (110 pores per inch) were cut in the dimensions of 2.5 cm (width)  2.5 cm (thickness)  5.5 cm (height) based on a volume of hydrogen generator. Then, the nickel foam samples were cleaned with ethanol and were immersed in solutions of hydrochloric acid (HCl, 0.3 mL) and distilled water (300 mL) for etching. After the pretreatment, the coating bath was filled with cobalt chloride (CoCl2$6H2O), glycine (NH2CH2COOH), sodium phosphinate (NaH2PO2$H2O) and sodium hydroxide (NaOH). Finally, the weight of the Co-P/Ni foam catalysts was measured to calculate the deposited catalysts. The measured weight percent (wt%) of deposited catalyst is expressed in Eq. (2). MassCoP=Ni Foam  MassNi Foam  100 MassNi Foam

(2)

Fig. 1 e Diagram showing the configuration of the fuel cell system that uses a NaBH4-hydrogen generator. Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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Fig. 2 e The portable electric power plant that uses the fuel cell system.

The weight percent of catalyst deposition on the Ni foam surfaces was approximately 17 wt%.

Preparation of Co/g-Al2O3 catalysts Co/g-Al2O3 catalysts were made using a simple wet impregnation method to enhance the slow response time from the catalytic hydrolysis of NaBH4 when using only Co-P/Ni foam catalysts. The procedures include the following: the g-Al2O3 was crushed to 16e20 mesh size. Then, the catalysts were water-cleaned and desiccated in a convection oven at 120  C. Afterwards, the catalysts were impregnated with a 25 wt% CoCl2 aqueous solution followed by heat treatment at 150  C in a convection oven. In the last procedure, Co/g-Al2O3 catalysts were calcinated in a furnace at 300  C to remove impurities on the surface.

Fabrication of the hydrogen generator and liquid-gas separator A flow-through-type of hydrogen generator was made to evaluate the performance of the catalysts and to supply a constant volume of hydrogen to the PEM fuel cell stack. The generator was made of stainless steel and had a catalyst bed volume of 2.5 cm (width)  2.5 cm (length)  6.5 cm (height) to hold the Co/g-Al2O3 catalysts at the top and Co-P/Ni foam catalysts in the remaining volume. Thus, Co/g-Al2O3 catalysts could react with 20 wt% NaBH4 solution first. The inlet diameter for NaBH4 solution was 0.32 cm, and the outlet diameter was 0.64 cm so that a volume of hydrogen gas containing byproduct (NaBO2) could be easily discharged into the liquidgas separator. The two gaskets, which were made of Teflon, were installed to prevent hydrogen leakage. It was designed to produce 5.9 L/min hydrogen gas to operate a 500-W scale PEM fuel cell stack. A schematic of the hydrogen generator is illustrated in Fig. 3. The liquid-gas separator was also made of stainless steel and had a volume of 10 cm (diameter)  7 cm (height). A 0.5cm hole for the NaBO2 outlet was located at the lower side of

the separator and was connected to the suction port of a purge pump with a silicon tube. Thus, accumulated NaBO2 solutions were purged every 3 min for a period of 20 s from the separator during the catalytic hydrolysis of sodium borohydride. The design of the separator is illustrated in Fig. 2.

Fabrication of finned container as a heat exchanger Not only was aqueous byproduct (NaBO2) problematic, but heated hydrogen gas associated with the catalytic hydrolysis reaction may have caused dry conditions in the stack, which could lower the performance of the stack performance if it was continuously fed without cooling. Thus, maintaining hydrogen gas at operational room temperature was essential, especially when it was necessary to have a long period of operation of the fuel cell system. To reduce the high temperature of the hydrogen gas, fins that were 1 cm long and 0.2 cm thick were attached lengthwise to a cylindrical container which had a volume of 10 cm (diameter)  10 cm (height), as shown in Fig. 2. Thus, the heat could easily radiate out through the enlarged surface area of the container with the aid of a cooling fan. Additionally, water and silica grains in the consecutive containers were used to decrease high hydrogen temperature further and to absorb the moisture in the hydrogen gas. Thus, pure hydrogen was allowed to flow into the fuel cell stack.

Design of the controller for the fuel cell system The controller was designed to efficiently control five components including the fuel pump, purge pump, cooling fan, purge valve and oxygen supply fan in the fuel cell system. The controller was composed of relays, a DC-DC regulator, programmable potentiometer, RS232 communication ports and micro control unit. The DC-DC regulators were allowed to adjust voltages for each component by changing resistance values from a programmable potentiometer so that each component was controlled during the operation.

Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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Fig. 3 e A schematic of a hydrogen generator: (a) Exterior structure (b) Catalyst bed volume.

The fuel pump was run at 7e12 V to provide different rates of NaBH4 solution. The purge pump and cooling fan were operated at 24 V to remove NaBO2 solutions and increase cooling effects on heated hydrogen gas, respectively. The purge valve was opened at 12 V every 10 s for a period of 0.2 s to bring generated water out of the fuel cell stack, which was a critical factor for the long-term operation of the fuel cell stack. The stack fan was run at 12e14 V, depending on the stack power requirements to supply oxygen and for cooling for the stack. RS232 communication ports were used to transmit and/or receive data through other devices, such as a data monitoring device. The monitoring device could monitor the power of the fuel cell stack and hydrogen gas temperature in real-time during the operation. Fig. 4 shows the design of the controller, and Fig. 5 demonstrates the communication between the controller and data monitoring device through an RS 232 port.

Installation of a DC-DC converter for the active power management The fuel cell system was equipped with a DC-DC converter. The system had input voltage ranges of 9 Ve60 V and output

ranges of 0 Ve60 V at a maximum current of 40 A with 95% efficiency; thus, the system could regulate the voltage and control fuel cell power in accordance with the load demand. This power converter was suitable for applications that require low and medium DC voltage, but when the required DC voltage was higher, multiple converters placed in cascade are strongly recommended [16]. Thus, the active control with a DC-DC converter was feasible to manage the power output from the fuel cell system.

The evaluation of system performance Hydrogen generation test with two catalysts To evaluate the performance of the Co-P/Ni foam catalysts, a hydrogen generation test was done to produce 5.9 L/min hydrogen gas, which corresponds to a hydrogen consumption rate of the 500-W scale PEM fuel cell stack. To produce such a volume of hydrogen gas, 12 mL/min of 20 wt% NaBH4 solution was inserted into the hydrogen generator using a fuel pump. Then, the volume of produced hydrogen gas was measured

Fig. 4 e The design of the system controller: (a) Front view (b) Rear view. Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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solved by placing Co/g-Al2O3 catalysts on top of Co-P/Ni foam catalysts in the hydrogen generator, in which Co/g-Al2O3 catalysts react with NaBH4 solution first, resulting in increasing temperature. Thus, this catalyst played an important role as an initiator to produce heat for the faster chemical reaction between aqueous NaBH4 solutions and Co-P/Ni foam catalysts. This was due to the fact that Co-P/Ni foam catalysts had smaller reacting surface areas (0.1 m2/g) than the reacting surface area of Co/g-Al2O3 (184 m2/g) [8]. This result is in agreement with investigations that show that metal nanoparticles with large surface areas provide a potential way to increase catalytic activity [19]. The enhanced hydrogen generation rate with two catalysts is illustrated in Fig. 7. Fig. 5 e The communication test between controller and monitoring device through an RS 232 Port.

The evaluation of performance of the fuel cell system

during the catalytic hydrolysis of NaBH4. The results in Fig. 6 show that the Co-P/Ni foams have a slow initial response when they come into contact with the NaBH4 solution so that the fuel cell system could not produce essential power for the initial 10 min to meet the load demand. This slow response can be problematic, and causes a waste of fuel when the fuel cell system is required to be restarted. To amend this shortcoming, a lower solution flow rate was needed initially because the production rate at the lower solution flow rate is higher with slower initial filling of the reactor volume, due to the increase of temperature inside the hydrogen generator and the enhanced kinetics of NaBH4 hydrolysis [17]. This result was supported by the investigation of the startup times at various operations, and showed that lower volume flow rates of the solution resulted in better startup characteristics due to the decreased cooling solution effects in the generator that are associated with the rapidly increased reaction temperature [18]. Moreover, the slow response issue could be

The commercial PEM fuel cell stack (Horizon, UL-500 W) was selected and integrated with the NaBH4-based hydrogen generator to evaluate the performance of the system. 20 wt% NaBH4 solution was used as a hydrogen supplier which could produce 5.9 L/min hydrogen gas. However, the intrinsic problems of using a NaBH4 solution as a hydrogen supplier include the production of a byproduct (NaBO2) and heated hydrogen gas during the catalytic exothermic reaction, which could both degrade the performance of the PEM fuel cell stack. Thus, these two problems must be solved to provide pure hydrogen at operational room temperature. To overcome those problems, a liquid-gas separator with a purge pump was designed to periodically pump the NaBO2 out of the system every 3 min for 20 s, while the heated hydrogen gas was cooled down using the finned container (containing 100 g of water) with an aid of a cooling fan. Then, the moisture in the hydrogen gas was captured by silica grains in another container. Therefore, pure hydrogen at operational room temperature could be supplied by the NaBH4-based hydrogen generator. With these techniques, the 500-W scale PEM fuel cell stack was operated in accordance with the load demand at about 15 V with 33 A for 2 h, as shown in Fig. 8. It shows that the hydrogen gas temperature (75  C), increased by the

Fig. 6 e The results of hydrogen generation rates during an hour when using Co-P/Ni foam catalysts.

Fig. 7 e The result of enhanced hydrogen generation rates using Co/g-Al2O3 and Co-P/Ni foam catalysts.

Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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Fig. 8 e The hydrogen gas temperature and stack voltage measured in the fuel cell system.

Fig. 10 e The stack power variations achieved by changing a fuel pump voltage.

catalytic hydrolysis reaction, in the separator was cooled down to approximately 30  C before entering the PEM fuel cell stack, so the finned container with a cooling fan proved to be a good heat exchanger. Moreover, the stack voltage (15 V) was constantly maintained to follow the load demand. According to Fig. 7, an initial hydrogen production rate reached approximately 10 L/min, which was much higher than required for hydrogen gas (5.9 L/min) for operating a 500W PEM stack. Subsequently, this rate may cause the system to burst with high build-up pressures in a short time period. To reduce pressure build-up at the beginning, the purge valve at the outlet of the stack, which is normally closed, was opened for 1 min and was then open every 10 s for a period of 0.2 s to allow exhaust of the fuel cell, such as water and unreacted hydrogen gas, to escape. In this way, the pressure was stabilized as time elapsed. Fig. 9 illustrates the hydrogen pressure variation in the fuel cell system during the catalytic hydrolysis of NaBH4. As shown in Fig. 9, the maximum hydrogen

pressure (1.44 bar) is reached at the beginning due to an initial high hydrogen production rate, and then, the pressure was reduced to above 1 bar for 1 h. From this pressure measurement, it was observed that hydrogen gas generated by the hydrolysis reaction was pushed forward to flow into the PEM fuel cell stack by small pressure build-up in the fuel cell system. To be utilized as an electric power engine in mobile applications such as UAVs and cars, the power level must be adjusted in accordance with load conditions so that a constant hydrogen gas supply is not necessary. To achieve power variations from the fuel cell system, the NaBH4 solution rates were controlled by changing the fuel pump voltage with the control logic, as shown in Fig. 10. It shows that a different voltage from a fuel pump produces a different stack voltage in accordance with extraction of different currents. In other words, more hydrogen gas could be supplied to the fuel cell stack with higher feeding voltages, which permit more NaBH4 solution to flow into the hydrogen generator so that the fuel cell stack could produce a direct current from 17 A to 29 A. The result is a voltage drop from 18 V to 15 V, in accordance with the load demand. By using this technique, hydrogen consumption rates could be matched for the power requirements from the stack, thereby extending the operational period by maximizing fuel consumption rates. Therefore, this fuel cell system could handle different load variations in operational conditions. In turn, it could be the portable electric power plant to supply energy for mobile applications such as cars and UAVs [20].

Specific energy density of the fuel cell system

Fig. 9 e The pressure variation measured during catalytic hydrolysis of NaBH4.

The specific energy density was calculated based on the weight breakdown of the fuel cell system (4720 g), as shown in Fig. 11. The NaBH4 solution accounted for the most weight at 1440 g (31%), which can be used as a hydrogen supplier for 2 h of operation of the fuel cell system. However, this weight could be reduced as the hydrogen gas was consumed by the

Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006

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Fig. 11 e The weight breakdown of the portable electric power plant using the fuel cell system.

fuel cell stack. Subsequently, the specific energy density of the system was enhanced as time elapsed. Then, the fuel cell stack accounted for 700 g (15%) of the total weight, and the weight of the finned water container and silica container accounted for 400 g (8%) and 320 g (7%), respectively. Thus, the hydrogen generation system, excluding the stack and DC-DC converter, accounts for 3720 g (79%) of the total weight. This NaBH4-based hydrogen generation system could operate at a maximum power of a 500-W scale PEM fuel cell stack for 2 h, which resulted in 1000 Wh energy storage capacity. As a result, the specific energy density of our portable fuel cell system was 211 Wh/kg, which is slightly higher or close to the specific energy density of a typical lithium polymer battery that holds 200 Wh/kg. Nevertheless, at present, the portable electric fuel cell system is not optimized in terms of weight. Thus, the specific energy density will improve if the weight is reduced from the hydrogen generation system.

Conclusions The portable electric power plant that uses a NaBH4-based fuel cell system was developed to supply stable electric power for mobile applications. The power plant includes a 20 wt% NaBH4-based hydrogen generator, micro-processed controller and PEM fuel cell stack. This portable power plant can provide stable electric power as long as hydrogen gas is supplied from the NaBH4-based hydrogen generator. Moreover, the output power was adjusted not only by regulating the voltage through a DC-DC converter but also by controlling NaBH4 solution flow rates in accordance with the load demand. In this method, the fuel cell power could cope with load variations in operating

conditions. However, the compact system design for hydrogen generation process was required to reduce weight of the fuel cell system. Thus, the current specific ergy density (211 Wh/kg) will be improved by reducing total system weight.

the the enthe

Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2014M1A3A3A02034777).

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Please cite this article in press as: Gang BG, Kwon S, All-in-one portable electric power plant using proton exchange membrane fuel cells for mobile applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.006