Ultra compact direct hydrogen fuel cell prototype using a metal hydride hydrogen storage tank for a mobile phone

Applied Energy 134 (2014) 382–391

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Ultra compact direct hydrogen fuel cell prototype using a metal hydride hydrogen storage tank for a mobile phone Sung Han Kim a, Craig M. Miesse b, Hee Bum Lee a, Ik Whang Chang c, Yong Sheen Hwang d, Jae Hyuk Jang a, Suk Won Cha e,⇑ a

Corporate R&D Institute, Samsung Electro Mechanics, 150 Maeyoung-Ro, Youngtong-Gu, Suwon-Si, Gyeonggi-Do 443-743, South Korea Energy Solutions, Samsung SDI America, 601 McCarthy Blvd., Milpitas, CA 95035, USA Department of Intelligent Convergence Systems, Seoul National University, 599 Gwanak-Ro, Gwanak-Gu, Seoul 151-744, South Korea d Dept. of Green Technology, Korea Construction Equipment Technology Institute, 36, Sandan-Ro, Gunsan-Si, Jeollabuk-Do 573-540, South Korea e Department of Mechanical and Aerospace Engineering, Seoul National University, 599 Gwanak-Ro, Gwanak-Gu, Seoul 151-744, South Korea b c

h i g h l i g h t s  A compact direct hydrogen fuel cell of 25 ml operates mobile phone without a battery.  An 8 ml AB5 metal hydride tank has reversible hydrogen storage capacity of 4 L at 25 °C.  The 3 mm thick, 8 ml air-breathing planar stack can supply maximum 2.68 W of power.  A miniature DC–DC buck converting circuit has 90% overall efficiency.  Estimated system energy density of 204 W h L

a r t i c l e

i n f o

Article history: Received 18 April 2014 Received in revised form 29 July 2014 Accepted 4 August 2014 Available online 1 September 2014 Keywords: Direct hydrogen fuel cell (DHFC) AB5 metal hydride alloy Hydrogen storage tank Air-breathing planar stack High efficiency DC–DC buck converting circuit

1

is sufficient to make a 6 h voice call.

a b s t r a c t The small fuel cell is being researched as an alternative power source to the Li-ion battery in mobile phone. In this paper, a direct hydrogen fuel cell system which powers a mobile phone without a supplementary battery is compactly integrated below 25 ml volume at the backside of the phone. The system consists of a small (8 ml) metal hydride hydrogen storage tank with 4 L hydrogen storage or an energy density of 640 W h/L, a thin air-breathing planar polymer electrolyte membrane fuel cell (PEMFC) stack (13.44 cm2  3 mm for a volumetric power density of 335 W/L), miniature pressure regulator, and a high efficiency DC–DC voltage converting circuitry. The hydrogen storage tank is packed with the AB5 type metal hydride alloy. The eight-cell air-breathing planar stack (8 ml) is very thin (3 mm) due to a thin flexible printed circuit board current collectors as well as a unique riveting assembly and is capable of a robust performance of 2.68 W (200 mW/cm2). A miniature pressure regulator is compact with fluidic and electrical connections within 4 ml. A miniature DC–DC voltage converter operates at an overall efficiency of 90%. Consequently, the estimated energy density of a fully integrated fuel cell system is 205 W h/L (70.5 W h/kg). Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Since the alkaline fuel cell system was selected for power generation in the Apollo spacecraft, fuel cells have been demonstrated in various applications such as automobile, power plant, marine, and portable electronics equipment [1,2]. Various types of fuel

⇑ Corresponding author. Address: Department of Mechanical and Aerospace Engineering, Seoul National University, Building 301, Room 1505, 599 Gwanak-Ro, Gwanak-Gu, Seoul 151-744, South Korea. Tel.: +82 2 880 1700; fax: +82 2 880 1513. E-mail address: [email protected] (S.W. Cha). http://dx.doi.org/10.1016/j.apenergy.2014.08.019 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

cells, such as the polymer electrolyte membrane fuel cell (PEMFC), the solid oxide fuel cell (SOFC), the molten carbide fuel cell (MCFC), and the phosphoric acid fuel cell (PAFC) have been applied to applications where their individual strengths are best suited [3]. For portable electronics applications such as mobile phones, there is customer discontent in the operating time limitations imposed by the current capacity of Li-ion batteries. Fuel cell technologies are thus being developed as an alternative power sources to the Li-ion battery. Especially, polymer electrolyte membrane fuel cell (PEMFC) technologies – including direct methanol fuel cell (DMFC) as well as a direct hydrogen fuel cell (DHFC) – are accepted

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as a most promising fuel cell technologies for small portable applications such as laptop PCs, personal multimedia players (PMPs), personal digital assistants (PDAs), and mobile phones because of their ease of miniaturization and moderate operating condition requirements [4]. DMFC prototypes for cellular phones, laptop PCs, and PDAs have been released by many companies including Toshiba [5], Fujitsu [6], Hitachi [7], Samsung Advanced Institute of Technology [8] and MTI micro fuel cells [9]. However, DMFC technology has yet to resolve the fuel crossover issues caused by high concentration methanol fuel and low system volumetric energy densities [10]. Distinctively, the Ultracell [11] has released a reformed methanol fuel cell (RMFC) prototype that uses hydrogen produced from liquid methanol fuel through a catalytic reforming process. Such a system is also often referred to as a reformed hydrogen fuel cell (RHFC). Although the RMFC systems guarantee fuel storage convenience and high power performance, additional parts for reforming significantly increase total system volume and reduces the system’s volumetric energy density [12– 14]. Thus, RMFC technology is thought to be more suitable for relatively larger portable applications than for handheld consumer electronics [15]. On the other hand, DHFC systems using a metal hydride based hydrogen storage tank have recently drawn attention for handheld electronics applications due to their high power and volumetric energy densities. Angstrom Power [16], Canon [17], and Horizon fuel cell technologies [18] recently released DHFC prototypes for mobile phones, digital cameras, and battery chargers. Although metal hydride hydrogen storage technology trials for transportation applications have failed in the past due to low gravimetric energy densities, the high volumetric energy density of metal hydride hydrogen storage tanks make the technology attractive to mobile fuel cell system designers whose focus is on making compact systems [19,20]. In spite of research efforts geared towards DMFC, RMFC, and DHFC systems for mobile electronics application, their current size requirements are still too big to surpass the volumetric energy density of state of the art Li-ion secondary batteries. Thus, the miniaturization of fuel cells is a major technical issues for making marketable fuel cell powered handheld electronics. In this paper, a direct hydrogen fuel cell prototype with an AB5 type metal hydride hydrogen storage tank is compactly integrated with a mobile phone as is shown in Fig. 1(a). Among the various hydrogen storage alloys, AB5 type metal hydride alloys have fairly moderate hydrogen adsorption and desorption properties around 10 MPa in room temperature and can be conveniently packaged in a miniature hydrogen storage tank [21,22]. The AB5 metal hydride miniature hydrogen storage tank is 8 ml and is inserted into 12 ml void originally intended for a mobile phone’s Li-ion battery. In the remaining 4 ml of space, there is a miniature pressure regulator for reducing the outlet hydrogen pressure from hydrogen storage tank as well as components connecting hydrogen flow from the tank to an air-breathing planar stack through the miniature pressure regulator as is shown in Fig. 1(b). The fuel cell stack is a thin air-breathing planar stack which operates silently and without the need for additional balance of plant (BOP). To minimize the stack’s thickness, thin flexible printed circuit boards of 100 lm thickness are used as current collectors and a riveting assembly method is employed. The eight cell air-breathing planar stack is just 8 ml. A tiny DC–DC voltage converting circuit designed inhouse uses a step-down buck type converter for high efficiency. All of metal hydride hydrogen storage tank, miniature pressure regulator, air-breathing planar stack, and converting circuit are assembled compactly for a total system volume of 25 ml. The ultra compact direct hydrogen fuel cell is designed to supply a mobile phone sufficient power to carry out all functions such as calling, playing music, and playing video without a supplementary secondary battery.

55.3 mm Air-breathing planar stack

14.5 mm

1.8 mm 115.2 mm

Mobile phone (Samsung SCH-W4200)

(a) Pressure regulator Circuit

AB5 metal hydride hydrogen storage tank

Air-breathing planar stack

(b) Fig. 1. (a) Concept design of ultra compact direct hydrogen fuel cell prototype for mobile phone and (b) detailed component arrangement in the concept design (DC– DC converting circuit as well as an air-breathing planar stack is built into the backside cover of the mobile phone. The metal hydride hydrogen storage tank and regulator parts are placed in the space for the original secondary battery).

2. System configuration of ultra-compact direct hydrogen fuel cell prototype 2.1. Air-breathing miniature planar stack For use as a power source for mobile electronics, it is desired that a fuel cell stack be designed as an air-breathing planar stack. The air-breathing planar stacks are being studied broadly lately [23–29]. Since oxygen is supplied to the cell via an open cathode by free convection, the planar stack does not have internal flow channels or any kind of air pump to facilitate forced air convection. The elimination of the air pump gives three advantages to the fuel cell system. The first is quiet operation, the second is removal of the parasitic power draw, and the third is increasing the system’s energy density by reducing the total system volume. Though the maximum power of air-breathing planar stack can be lower than performance of an active stack with flow channels and air pump, air-breathing planar stacks do not need the parasitic power consumption for making forced air flows. Fig. 2 shows the cathode side of the stack, its’ air-breathing cathode openings, rivet fasteners, and the thinness afforded by the FPCB current collectors. The planar stack thickness of total 3 mm is achievable due to the advantages of rivet fastening. Fastening using bolts imposes a minimum thickness limitation because bolts require a relatively longer tap lead length in the plates and additional space for the bolt heads. The air-breathing planar stack has 8 unit cells serially connected and a total active area of 13.44 cm2. Unit cells are 1.2 cm  1.4 cm each. The hydrogen fuel inlet is placed on an edge of the anode end plate and a PTFE tube is connected to the hydrogen fuel inlet. The stack anode outlet is normally closed by a screw

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Cathode end plate Purge Hole (Normally closed) 66 mm

FPCB Current collector ≈ 3 mm Anode end plate

H2 inlet 40 mm Riveting

Fig. 2. Eight-cell air-breathing planar stack using flexible printed circuit boards as current collectors (rivet fastening is used to compress two gaskets, two current collectors, and an MEA and finally to assemble the anode endplate and the cathode endplate. The end of anode flow channel is blanked off by inserting a bolt with an Oring).

and O-ring, unless the purging of any accumulated impurity gases in the anode feed channel chamber is desired. More detail of the stack configuration using an FPCB current collector can be found by referencing the author’s previous paper entitled ‘‘Air-breathing miniature planar stack using the flexible printed circuit board as a current collector’’ [30]. The cathode opening ratio is 65% and cathode open geometry is a rectangular hole. The dimensions of the airbreathing planar stack are 40 mm  66 mm  2.8 mm and the stack volume is about 8 ml. 2.2. The hydrogen storage tank packed with AB5 type metal hydride alloy There are several hydrogen storage methods such as cryogenic liquid hydrogen storage, high pressure compressed hydrogen storage, and hydrogen storage via hydrogen storage materials (e.g., high surface area carbon-based materials and metal hydride alloys) [31–34]. In principle, liquid hydrogen storage and compressed hydrogen storage have relatively higher gravimetric storage capacity as well as volumetric storage capacity than that of the hydrogen storage by metal hydride alloys [35]. However, the system of cryogenic liquid hydrogen storage and compressed hydrogen storage inevitably require complicated and large components and extreme operating condition requirement which decrease the system-level volumetric and gravimetric capacity and make portability impossible. And cryogenic liquid hydrogen storage and compressed hydrogen storage are more suitable for large transportation application than small portable application [36–39]. On the other hand, a hydrogen storage tank packed with a group of metal hydrides alloys can be easily miniaturized all the while retaining good system-level capacity because intermetallic hydrides alloys such as AB2 and AB5 type metal hydrides have moderate operating temperatures (around 25 °C) and relatively low operation pressures (0.1–1 MPa) [40–42]. Nanostructured carbon-based materials of superior gravimetric hydrogen storage capacity, however, still demand high pressure or very low temperature for hydrogen adsorption [43]. Thus, at this point in time, AB2 and AB5 type metal hydride alloys are best suited for compact fuel cell systems for mobile portable consumer electronics applications. In this paper, AB5 type metal hydride alloys are used as a hydrogen storage material. AB5 type metal hydride alloys are mainly metal alloys of Mischmetal (Mm) and nickel, with reversible hydrogen storage capacities of about 1.5 wt.%. While, AB2 type metal hydride alloys of Ti–Zr–Mn–V or Ti–Zr–Cr–Fe have 1.5– 1.9 wt.% of hydrogen storage capacity, and while BCC type metal hydride alloys of Ti, Cr, and V have relatively higher hydrogen storage capacity of 2.3–2.7 wt.%, relatively higher gravimetric storage

values of AB2 alloys are offset by AB5 alloy’s density and therefore AB5 alloys have better volumetric storage values [44]. Moreover, AB5 alloys are much cheaper than BCC and maybe even AB2 alloys and also have better cyclic durability and superior anti-contaminant durability [45]. Thus, AB5 metal hydride alloys are more practical and suitable as reversible hydrogen storage materials than any other metal hydride at this point in time. Fig. 3 shows a hydrogen storage tank packed with AB5 type metal hydride alloys of Mm–Ni–Mn–Co, which dimension is 39 mm  34 mm  6 mm of little bit smaller size than the original lithium-ion battery’s size of the mobile phone. 2.3. The miniature gas pressure regulator and connecting parts The remaining void of the mobile phone’s original battery compartment is used for a miniature gas regulator and connecting parts as shown in Fig. 4. The miniature gas pressure regulator is needed for reducing the gas exit pressure from the metal hydride hydrogen storage tank from approximate 1 MPa to atmospheric pressure (around 0.1 MPa) to prevent damage and tearing of the MEA. On the left side of the miniature gas pressure regulator, there is a male connector which is matched to a female connector in the hydrogen storage tank. The female connector of the hydrogen storage tank is normally closed, but opens when coupled with male connector. On the right side of miniature gas pressure regulator, there is a nipple connected with 0.8 mm diameter PTFE tube, which becomes a hydrogen flow channel from the gas pressure regulator to air-breathing planar stack. In the upper side of the gas pressure regulator, there is electrical connector which is connected with stack’s power FPCB output terminals. 2.4. A tiny DC–DC buck converting circuit There are two different types of DC–DC voltage converting circuits. One is buck converter (alternatively referred to as a stepdown converter) and another is boost converter (alternatively referred to as a step-up converter) [46]. A buck converter reduces the output voltage from the input voltage and increases the system output current to a value higher than the input current from stack. Contrarily, the output from a boost converter is of a higher voltage and lower current than the input.

Fig. 3. Small hydrogen storage tank with AB5 type metal hydride alloy (the dimension is 39  34  6 mm).

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Connecting part with a hydrogen storage tank Miniature pressure regulator Hydrogen outlet to an air-breathing planar stack Fig. 4. Miniature gas pressure regulator, an electrical connecting part, and fluid connectors.

If a buck converter is employed, a stack voltage higher than target converter output voltage is required. Contrarily, if a boost converter is employed, stack voltage lower than target converter output voltage is possible. Because the boost converter requires higher current output (lower efficiency) from a stack than a buck converter, and because air-breathing stacks might not respond sufficiently fast enough to transient power demands when operating at higher current output levels, a system configuration with a higher voltage output to a buck converter is better suited to provide robust performance. In this study, a buck voltage converting circuit reduces a stack output voltage of P4.2–4.0 V for output to the mobile phone as shown in Fig. 5. The output voltage is at 4.0 V to emulate the voltage of the original lithium-ion battery. The system’s output power is delivered to mobile phone’s power-in terminals via the thin flexible PCB. The serially connected eight cell air-breathing planar stack is capable of voltage range 4.2–7.2 V and a currents up to 600 mA. 3. Experiment

o

Charging in cooled water of 0 C o

Charging in just water of 21 C o

Charging in ambient air of 25 C

Hydrogen absorption flow rate (ml/min)

Electrical connected part with a FPCB of circuit

metal hydride alloys is an exothermic reaction. Thus, the hydrogen charging and discharging process of the AB5 metal hydride hydrogen storage tank is strongly related with heat flux to its’ surroundings [49–51]. As it is shown in Fig. 6(a), hydrogen charging rate of AB5 metal hydride hydrogen storage tank was measured in three the different environments of cooled water slightly above 0 °C, water maintained at 21 °C, and ambient air at 25 °C. In all cases, charging rates started high and gradually leveled out. Especially, in the first two cases, where heat transfer was to a liquid medium, the tank was charged to 95% of capacity (about 3.8 L) within 12–13 min of charging. Meanwhile, in case of surrounding circumstance of ambient air at just room temperature (25 °C), almost 40 min are required for a hydrogen full charging. The surrounding water functioning as a much more effective heat sink than ambient air facilitated this fast charging. The relatively slow hydrogen adsorption

2000 1750

12~13min

1500 1250 1000 750 500 250 0 0

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10

15

25

30

35

40

45

50

Time (min)

3.1. Hydrogen charging characteristics of AB5 metal hydride hydrogen storage tank The metal hydride increases a little during the tank’s temperature discharging [47,48]. This

20

(a)

hydrogen storage tank’s temperature hydrogen charging, on the other hand, decreases just a bit during hydrogen means that hydrogen adsorption on AB5

o

Charging in cooled water of 0 C o

Charging in just water of 21 C o

Charging in ambient air of 25 C Chip inductor

DC-DC Buck converter

Hydrogen absorption amount (ml)

5000 4500

4.0L ~ 4.2L

4000 3500 3000 2500 2000 1500 1000 500 0 0

FPCB electrical output to mobile phone throughout electrical connectors

Fig. 5. A tiny DC–DC buck type converting circuit with thin FPCB.

5

10

15

20

25

30

35

40

45

50

Time (min)

(b) Fig. 6. (a) Hydrogen adsorption flow rate during hydrogen charging and (b) hydrogen accumulation for three different environments (in water maintained just above 0 °C, water maintained at 21 °C, and air at 25 °C).

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Table 1 The specifications of the hydrogen storage tank and the metal hydride alloy. Components

Specification

Values

Metal hydride

Metal hydride type Material of metal hydride alloys Weight of metal hydride alloys Approximate density of alloys Gravimetric hydrogen storage capacity

AB5 type Mm–Ni–Mn–Co 24.8 g 3.31 g cm 3 1.5 wt.%

Hydrogen storage tank

Size of the tank Volume of the tank Weight of the tank Material of the tank Storing temperature Charging pressure Charging time Charging temperature Discharging pressure (after a small pressure regulator) Hydrogen storage content Volumetric storage capacity of the tank Gravimetric storage capacity of the tank

39 mm  34 mm  6 mm 8 ml 43.4 g (including MH alloys) Steel 0–40 °C Max. 1 MPa @ 20 °C <13 min (in 21 °C water) 0–30 °C <0.1 MPa 4.1 NL (0.37 g) 513 NL L 1 94.5 NL kg 1

Table 2 The specifications of eight-cell air-breathing planar stack and its components.

Voltage Power density 0.25

8

0.2 W/cm

0.20

Voltage (V)

6 5

0.15

4.2 V

4

2

0.37 mA/cm

0.10

3 2

0.05

1

Power density (W/cm2)

2

7

0.00

0 0.0

0.1

0.2

0.3

0.4

Components

Specification

Values

MEA

Membrane Membrane thickness GDL material GDL thickness Active area Peak power density

Nafion 212 50.8 lm Carbon cloth 0.3 mm 13.44 cm2 (1.2 cm  1.4 cm  8 ea) 0.35 W cm 2

Gasket

Gasket material Gasket thickness

Teflon PFA sheet 0.24 mm

Current collector

Current collector material Au coating thickness

Au coated FPCB

End Plate

End plate material Fastening method

Teflon coated aluminum alloy Riveting

Stack

Size Weight Volume Peak power Operating pressure Operating temperature Humidity of cathode air

40 mm  66 mm  3 mm 20.6 g About 8 ml 2.68 W 1 atm (atmospheric pressure) 25 °C 25% RH (no artificial humidification) Dry hydrogen 0.2 W cm 2 335 W L 1

0.5

Current density (A/cm2) Fig. 7. Polarization curve for the eight-cell air-breathing planar stack with a wellhydrated membrane.

reaction in ambient air shows that in order to facilitate faster charging, an effective heat sink is required. In spite of the difference in charging times, the total accumulated hydrogen amount in the 8 ml tank was almost similar at about 4 L in all three charging environments, as shown in Fig. 6(b). The total accumulated hydrogen amount value was determined by comparing the tank weights before and after hydrogen charging. This method was found to be superior to integrating the flow meter data, due to the relative accuracy tolerances of the balance and the flow meter. With a 4 L hydrogen storage capacity in an 8 ml space, the tank has an energy density of over 640 W h/L. The detail specifications of the hydrogen storage tank and the metal hydride alloy are given in Table 1. A hydrogen fuel cell can supply 1.7 W of power for 3 h using 4 L hydrogen. Because the state-art-of the energy density of lithiumion battery is about 400 W h/L, the AB5 metal hydride hydrogen storage tank has the ability to supply power for longer periods of time or than a similarly sized lithium-ion battery. Also, relatively faster hydrogen charging time, particularly when a heat sink is employed, is another competitive advantage of this tank to a lithium-ion battery. 3.2. The performance characteristics of the air-breathing miniature planar stack The air-breathing miniature planar stack is operated by free air convection at a room temperature and an atmospheric pressure

Humidity of anode fuel Peak power density Volumetric power density Gravimetric power density

100 lm

130 W kg

1

without any artificial humidification [52–55]. Fig. 7 shows the maximum performance characteristics of the air-breathing planar stack. A maximum power density of 0.2 W/cm2 occurs at 4.2 V and 0.37 A/cm2. Based on the stack’s total active area of 13.44 cm2, the maximum power output is 2.68 W. The peak power demand of the mobile phone is 2.0 W at 4 V, and the conversion efficiency of the buck voltage converter is approximately 90%. Thus, according to the data in Fig. 7, the system is capable to meet the maximum power requirements of the phone. The detail specifications of eight-cell air-breathing planar stack and its components are given in Table 2. However, the maximum power can be achieved only when the Nafion membrane is fully self-hydrated and the planar stack is properly warmed up after a considerable constant current operation. Fig. 8(a) shows the current output of the stack during a

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Load current from mobile phone during turn-on 0.6

Vibration

0.5

Signaling Sounding

Current (A)

0.4

Display On

0.3 0.2 0.1

Pressing a power-on button of mobile phone 0.0 0

10

20

30

Time (s)

(a) Load current from mobile phone 0.6 Pressing a power-on button

Current (A)

0.5

Abrupt turn-off of mobile phone

0.4 0.3 0.2 0.1 0.0 0

20

40

60

80

100

120

Time (s)

(b) Fig. 8. (a) Normal power performance of an air-breathing planar stack during boot-up of the mobile phone and (b) the transient power performance of an air-breathing planar stack that is incapable of completing on the boot-up of the mobile phone despite several turn-on trials.

successful boot-up procedure of the mobile phone after the stack has been sufficiently hydrated and warmed up. Fig. 8(b), however, shows the planar stack’s current output when the boot-up is performed with a cold stack. As can be seen, the stack failed to deliver sufficient power to the stack despite the phone’s power demand being less than its maximum. The several boot-up attempts were made for 120 s by pressing the power button without success. It is believed that the polymer membrane was not significantly hydrated and/or too cold, leading to less efficient stack operation than that shown in Fig. 7. During the subsequent boot-up attempts, self-hydration and self-heating gradually occurs in the cell and membrane until the efficiency is high enough that the stack and system can meet the demands of the phone. Similarly, the performance of an air-breathing planar stack depends on the operating environment variables such as temperature and relative humidity (RH). At fairly low temperatures, the product water on the cathode GDL layer either condenses easily or freezes and disturbs fresh air supply. On the other hand, the ion conductivity of the membrane decreases significantly at the relatively high temperatures because the Nafion membrane dries out. Thus, if the stack operating temperature goes beyond an ordinary temperature (25 °C ± 15 °C), the stack performance becomes lower. Also, high humidity at low current densities makes the stack performance increase with increasing RH from 20% to 100% due to slower water evaporation and higher ionic conductivity of the Nafion membrane. In contrast, higher RH at high current densities can

lead to a steeper mass transport loss since the effluent product water could not be easily removed from the cathode. As a result, the stack performance was lowered somewhat. Therefore, the stack designer should have a few strategies to guarantee a more stable and robust power output from an airbreathing planar stack. The well-hydrated MEA which can contain comparably more water in the membrane should be chosen. The shape and opening ratio of cathode air holes should be optimized to be able to remove the effluent product water more easily. Also, the end plate material should radiate heat efficiently from the stack. Most of all, the power demand should be designed to be quite lower than the maximum power of the air-breathing planar stack, in order for the integrated fuel cell to operate robustly regardless of initial condition, temperature, humidity, and so on. Fig. 9 shows the stack power and system temperature during long time operations at constant current loads (500 mA). The system temperature is maintained at the state of heat equilibrium because metal hydride alloys in the hydrogen storage tank plays the role of a heat sink and absorbs the heat that is generated from the air-breathing planar stack [56]. Any fluctuation of either power or temperature is caused by the product water, which blocks the cathode air holes and disturbs the air mass transfer. Thus, the product water on the cathode should be removed properly and the water management is pretty important for a long-term stable system operation [57].

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supply and an electric loader before and after converting through a DC–DC converting circuit. The converting efficiency of the DC– DC buck converting circuit employed in this study is measured at various input voltage and is shown in Fig. 10. Input power is supplied from an AGILENT 661C power supply (0–8 V, 0–5 A) and an output power fixed at 4.0 V is loaded at various currents below 0.7 A by using a KISHSHUI PLZ-70UA electric loader (0–150 V, 0–15 A). The overall conversion efficiency is over 85% for all converter input voltages tested except for 4.5 V, which is too close to the target output voltage. For the 4.5 V converter input case, only currents less than 0.1–0.2 A resulted in efficiencies less than 85%. However, considering the output characteristics of the stack, operation in this inefficient conversion region is not likely, and would only be expected if the stack is significantly degraded. The conversion efficiency of the converting circuit is expected to be maintained around 90% in real operation because

Temperature of the system 40

3.5

35

3.0

30

2.5

25

2.0

20

1.5

15

1.0

10

0.5

5

0.0

0

60

120

180

240

300

360

Temperature ( oC)

Power (W)

Power of the planar stack 4.0

0

Time (min) Fig. 9. The stack power and system temperature during a long time operation at constant current loads (500 mA) (the double size metal hydride hydrogen storage tank (16 ml) was used to observe the phenomena of a long time operation).

100 90

Efficiency (%)

80

Vout = 4.0 V

70 60 50 40

V in =4.5 Vi n =5.0 V in =6.0 V in =7.0

30 20 10 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Load current (A) Fig. 10. The conversion efficiency of the DC–DC buck type converting circuit.

Fig. 11. The components of an ultra compact direct hydrogen fuel cell system for a mobile phone.

3.3. The electrical converting efficiency of a tiny DC–DC buck converting circuit The conversion efficiency is calculated as the output power divided by the input power, which is measured from a DC power

Fig. 12. The front back view of an ultra compact direct hydrogen fuel cell system connected a mobile phone (Samsung SCH-W4200).

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Load current from mobile phone Output power of DHFC to mobile phone 0.6

3.0

Turn on

2.5

Max. 0.52 A Max. 2.0 W

0.4

2.0

0.3

1.5

0.2

1.0

0.1

0.5

0.0

Power (W)

Current (A)

0.5

Turn off

0.0 0

10

20

30

40

50

Time (s) Fig. 13. The output power of the direct hydrogen fuel cell system with mobile phone’s load current during mobile phone’s boot-up and shut-down cycle.

Table 3 The specification comparisons of compact direct hydrogen fuel cell system with NTT’s prototype for mobile phone. Specification

Values

NTT’s prototype [19]

Peak power density of the MEA Output voltage of the stack Peak power of the stack

200 mW cm 2 <8.0 V (eight cell) 2.68 W (8 ml stack) 335 W L 1 4.1 NL (in 8 ml Tank) 513 NL L 1

298 mW cm 2 <1.0 V (unit cell) 4.42 W (7.8 ml stack) 553 W L 1 (calculated) 6.7 NL (in 13 ml tank) 508 NL L 1

>90% Buck (step-down) Not necessary 72.3 g 25 ml 5.1 W h (=1.7 W  3 h) 70.5 W h kg 1

<75% Boost (step-up) Included 144 g 57 ml 8.25 W h (=1 W  8.25 h) 58 W h kg 1

Volumetric power density of the stack Hydrogen storage content Hydrogen storage capacity of the tank Efficiency of the circuit Type of DC–DC converter Auxiliary secondary battery Weight of the system Volume of the system Electric energy Gravimetric energy density of the system Volumetric energy density of the system

205 W h L

1

140 W h L

1

a high stack voltage (Vin) corresponds with a low current, while a low stack voltage (Vin) corresponds with high current operation. Additionally, if the system volume is not critically limited, the overall conversion efficiency of circuit can be further improved simply by using a bigger inductor.

4. Results and discussions The total direct hydrogen fuel cell system is compactly composed of an AB5 metal hydride tank for hydrogen storage, a miniature pressure regulator for reducing the output pressure from hydrogen storage tank, the connecting parts for hydrogen flow link as well as electrical connection, an air-breathing planar stack for generating the power, and a tiny DC–DC converting circuit for dropping the entrance voltage into mobile phone as is shown in Fig. 11. A PTFE tube links the pressure reducer to the inlet of air-breathing planar stack. Also, a thin FPCB from the DC–DC converting circuit is linked electrically to the positive and negative slide-terminals when the back-side cover of mobile phone and integrated stack

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and converting circuit is placed at the back side of mobile phone. This PTFE tube and FPCB connection was carefully designed to properly connect the hydrogen and electrical flows because the stack and circuit together at the back-side cover should be frequently separated from the mobile phone to facilitate hydrogen refilling of the hydrogen storage tank. Finally, a direct hydrogen fuel cell system completely integrated with the mobile phone (Samsung SCHW4200) is shown in Fig. 12. Only air-breathing planar stack can be seen from the outside of the case because oxygen from the air must be supplied at the stack cathode. Here, a tiny DC–DC converting circuit also can be seen through a transparent back side cover. Because the hydrogen storage tank, pressure regulator, and connectors are placed in the void originally intended for a Li-ion battery, the full power system investigated in the paper adds only 1.8 mm of thickness, and a total system volume increase of just below 25 ml. In spite of this incredibly compact size, the system can supply maximum 2.3 W of power at 4.0 V to a mobile phone and can make the mobile phone operate normally without any additional secondary battery. Fig. 13 shows the output power of the direct hydrogen fuel cell system (after sufficient membrane hydration and warmup) with the current supplied to the mobile phone during bootup and shut-down cycle of the mobile phone. A peak current of 0.52 A is momentarily demanded from the system which supplies maximum 2.0 W to mobile phone without any unintended abrupt turn-off. This suggests that the direct hydrogen fuel cell, when properly conditioned, is possible as an alternative power source for a mobile phone because the peak power demand of the mobile phone’s boot-up is higher than the demands for playing mp3s, playing videos, voice calling, and almost all services except a video calling.

5. Conclusion An ultra compact direct hydrogen fuel cell prototype using a metal hydride hydrogen storage tank was successfully made for a mobile phone application. A hydrogen storage tank packed with AB5 metal hydride alloy has a hydrogen storage capacity of 4 L in an 8 ml tank volume, and charges and releases hydrogen at a moderate pressure (0.1–1 MPa) at room temperate. The hydrogen storage tank’s volumetric energy density is estimated at 640 W h/L, which is 1.6 times higher than the volumetric energy density of state-of-the-art lithium-ion battery (about 400 W h/L) of the same size. The 3 mm planar stack has a volume of 8 ml and can supply a maximum 2.68 W (0.2 W/cm2), 0.37 A/cm2 at 4.2 V. The volumetric power density of the air-breathing planar stack reaches at 335 W/L by using FPCB current collectors and by using rivets for fastening for stack assembly. A tiny DC–DC buck converting circuit has an overall efficiency of 90%. The specifications of compact direct hydrogen fuel cell system in this paper were compared with those of NTT’s prototype for mobile phone, as shown in Table 3. The total direct hydrogen fuel cell system is compactly assembled below 25 ml volume and can supply a maximum 2.3 W after step-down conversion to a mobile phone (Samsung SCH-W4200). The total system volumetric energy density is estimated at 205 W h/L, which is enough to enable continuous voice calling for approximately 6 h. Though the energy density of the hydrogen storage tank is higher than that of a lithium-ion battery, the total volumetric energy density of the direct hydrogen fuel cell system is lower than that of a lithium-ion battery because the volume of air-breathing planar stack, regulator, DC–DC converting circuit, collectors must be factored in. In a miniature fuel cell system, the easiest way to elevate the energy density is to reduce either the volume or the weight of components in the system. The volume can be reduced mechanically by

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getting rid of either abundant thicknesses or unnecessary volumes and by modifying the assembling methods like using rivets for fastening stack assemblies in this paper. The weight can be reduced by substituting a little bit of the heavy material used commonly for slightly light material like the way of using FPCB current collectors in this paper. But the boosting of the energy density can be achieved only by synthesis of new material applicable to fuel cells. The membrane that has high ion conductivity in dry conditions, a catalyst that has higher catalytic activity and can keep it for a longer time without contamination, and the catalyst support that can guarantee more TPB (triple phase boundary)’s active sites in the same area with better electric conductivity should be synthesized anew [58]. Also, the new hydrogen storage material should have either high volumetric or high gravimetric hydrogen storage capacities and it should be reversible near ambient temperature and normal pressure with cyclic durability of superior anti-contamination [59,60]. Therefore, miniaturization of stack, regulator, connectors, and the circuit, as well as performance improvements for hydrogen storage alloys and MEA’s must be accomplished in both mechanical aspects and material synthetic aspects to make such a system a viable alternative energy source to Li-ion secondary batteries [61–65].

Acknowledgments This research was supported by the Global Frontier R&D Program on Center for Multiscale Energy Systems funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2011-0031569). Also, this research was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (2013R1A1A2A10065234). BK21 Plus is also acknowledged for their partial support. The authors are grateful to the editors and the anonymous reviewers for their insightful comments and suggestions.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2014. 08.019.

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