Flexible fuel cell using stiffness-controlled endplate

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Flexible fuel cell using stiffness-controlled endplate Ikwhang Chang a,b, Taehyun Park c, Jinhwan Lee c, Ha Beom Lee c, Seung Hwan Ko c,**, Suk Won Cha a,c,* a

Graduate School of Convergence Science and Technology(GSCST), Seoul National University, Gwanakro 1 Gwanakgu, Seoul, 151744, Republic of Korea b Material Science and Engineering, Georgia Institute of Technology, 771 Ferst Dr NW, Atlanta, GA 30332, USA c Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanakro 1 Gwanakgu, Seoul, 151744, Republic of Korea

article info

abstract

Article history:

We investigate the use of stiffness-controlled polydimethylsiloxane (PDMS) endplates with

Received 17 December 2015

Young's modulus of 7.50
105 Pa and 8.68
105 Pa for improving the performance of

Received in revised form

flexible fuel cells. The maximum power densities of stacks with PDMS endplates with

14 February 2016

Young's modulus of 7.50
105 Pa and 8.68
105 Pa are 82 mWcm2 and 117 mWcm2,

Accepted 14 February 2016

respectively. The flexible fuel cells produce a maximum absolute power of 1.053 W (i.e., the power density is 117 mWcm2) under a bending radius of 15 cm. Interestingly, their impedance spectra reveal that the ohmic and faradaic resistances decrease under the bent

Keywords:

condition. Furthermore, the decreased resistance and corresponding performance

Bendable

enhancement are due to the increased compressive force normal to the membrane elec-

Flexible

trode assembly, which is investigated using a finite element method of stress distribution

Stiffness

within the flexible fuel cells. As our experiments show, the faradaic impedance decreases

Fuel cell

significantly because the bending radius decreases from 36 cm to 15 cm.

Endplate

Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Polymer electrolyte fuel cell

Introduction Demand for portable power sources with high energy density continues to increase as the development, commercialization, and diffusion of portable electronics such as smart phones and laptops increase [1e3]. Current energy sources for portable electronics largely depend upon secondary rechargeable batteries such as lithium-ion batteries; however, secondary rechargeable batteries have limited storage capacities of

reserved.

volumetric and gravimetric energy densities due to the intrinsic properties of materials such as lithium and carbon [4,5]. Since a fuel cell's feature of continuous operation can also eliminate the charging time, which is essential for normal secondary batteries, many researchers are working to improve a fuel cell's energy densities for secondary batteries [1,6e9]. Among various fuel cells, polymer electrolyte fuel cells (PEFCs) have high energy conversion efficiency, are environment friendly, and can even continuously produce DC electric power as long as an equivalent amount of fuel is provided [4,10]. Also,

* Corresponding author. Graduate School of Convergence Science and Technology(GSCST), Seoul National University, Gwanakro 1 Gwanakgu, Seoul, 151744, Republic of Korea. Tel.: þ82 2 880 8050; fax: þ82 2 880 1696. ** Corresponding author. Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanakro 1 Gwanakgu, Seoul, 151744, Republic of Korea. Tel.: þ82 2 880 7114; fax: þ82 2 880 880 8302. E-mail addresses: [email protected] (S.H. Ko), [email protected] (S.W. Cha). http://dx.doi.org/10.1016/j.ijhydene.2016.02.087 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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recent studies focus mainly on wearable electronic devices, as well as other general portable electronics, all of which require portable bendable and stretchable power sources to be fully flexible electronics [11,12]. Flexible/bendable fuel cells and batteries can be utilized in one of the power sources of biorelated power solutions such a blood glucose sensor and a blood pressure sensor. Recently, the batteries and fuel cells of flexible power sources have been improved. Kwon et al. created a cable-type lithium-ion battery that is fully bendable, practical, and aesthetically pleasing [2]. Tominaka et al. reported that using bendable fuel cells is feasible but the total power (1.9 mW) of the stack was too low to operate real electrical applications [13]. Wheldon et al. did not show the in situ operational characteristics of fuel cells [14]. Hsu et al. reported that bendable fuel cells had relatively low bendability due to a carbon lump that was used as a current collecting layer [15]. The above mentioned studies are still in their infancy. In a fuel cell operation, except for a performance loss due to mass transport, the fuel cell output voltage associated with drawing current can be described in the following way [4]: Vcell ¼ VOCV  helectrode  hohmic Here, Vocv represents the ideal voltage calculated using the Nerst equation, and helectrode and hohmic are mainly the overpotentials of the electrode and the electrolyte, respectively. Also, an ohmic loss (i.e., IR overpotential) can be expressed in the following way: hohmic ¼ iðRelec  Rionic Þ Here, hohmic consists of both electronic (Relec) and ionic (Rionic) resistances within an electrolyte. In particular, Relec and Rionic can be expressed as follows: R¼

Fig. 1 e Internal stress schematics of bendable fuel cells; (A) original stacks and (B) bent stacks.

L As

where L is the path lengths of electrons and ions, A is the cross-sectional area of the path, and s is the conductivity. Therefore, the resistances commonly depend on the path lengths for moving ions between two electrodes and for minimizing the contact resistance between compartments (i.e., catalyst layer and gas diffusion layer). In particular, previous literature reported deformation (or stiffness) of endplates in fuel cells directly influences on their performances in which the optimization of stiffness and clamping pressure are important parameters of a fuel cell stack [16,17]. Our previous study reported that the decreased resistance and corresponding performance enhancement were due to the increased compressive force normal to the membrane electrode assembly (MEA) [16,18e20]. As shown in Fig. 1A (flat) and Fig. 1B (bent condition), we designed stiffness-controlled polydimethylsiloxane (PDMS) endplates (Young's modulus: 7.50
105 Pa / 8.68
105 Pa) for increasing the clamping force between two PDMS endplates, and measured 117 mWcm2 at the same bending.

Experimental First, the PDMS layer was mixed with a curing agent in a stainless steel mold where the anode and the cathode flow

channels were machined to feature the channels on the PDMS endplates. The mixing ratios of the PDMS and the curing agent were 10:1 (Young's modulus: 7.50
105 Pa) and 5:1 (Young's modulus: 8.68
105 Pa) [21]. Then, the mold was heated at 70  C for 4 h to solidify the PDMS. As shown in Fig. 2, the dimensions of the cross-sectional areas of the flow channels in the anode and cathode PDMS endplates were 1 (W)
1 (H) mm2 and 2 (W)
1 (H) mm2, respectively. The thicknesses of the asymmetric PDMS pads are 6 mm (anode) and 4 mm (cathode). Fig. 3A and B describe the process of fabricating Ag nanowires (NWs) as current collectors on the PDMS endplate. The Ag NWs were fabricated using conventional polyol synthesis. As shown in Fig. 3C, a successive multi-step growth (SMG) method was used to increase the length of the Ag NWs so they can be used as stable current collectors under various bent conditions [22]. Since our study focuses mainly on the scheme of the bendable fuel cell, we excluded the details of fabricating Ag NWs from our explanation. These details were discussed in our previous studies [18,20,22]. Fig. 3D shows the bent stack whose bending radius was 15 cm. An MEA (0.45 mgcm2 Pt/C loading, Fuel Cell Power Inc., Korea) with an active area of 3
3 cm2 was attached to the center of the anode PDMS endplate using polypropylene tape (3M, USA). The thicknesses of the Nafion™ 212 membrane and carbon paper electrodes were 50 mm and 350 mm, respectively. This

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Fig. 2 e PDMS endplates coated with Ag NWs; anode side (A) and cathode side (B). Two inset images show that the dimensions of flow channels at the anode and the cathode were 1 (W) £ 1 (H) mm2 and 2 (W) £ 1 (H) mm.2.

Fig. 3 e (A) Synthesis of Ag NWs using AgNO3, PVP, EG solution, salt, and seed. (B) Schematic of successive multistep growth of long Ag NWs. (C) SEM image of Ag NWs on PDMS endplate. (D) Image of bending stack experimental setup.

assembled anode PDMS endplate was then attached to the PDMS endplate of the cathode using a silicone sealant (SS900, Okong, Korea). Last, the two PDMS pads in contact with the MEA were clamped together with two steel clips. To supply fuel gases, flexible polyurethane tubes were inserted into each of the inlets of the endplate (Fig. 2). Fully humidified hydrogen

and air were provided to the anode and the cathode at volumetric flow rates of 0.5 Lmin1 and 1 Lmin1, respectively, at room temperature and pressure. The currentevoltage (IeV) curves and electrochemical impedance spectra (EIS) were measured using Solartron 1260/1287. The IeV curves were measured using the galvanodynamic mode at a scanning rate

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of 0.1 mA1. To identify the area specific resistance (ASR), the EIS were measured using a perturbation amplitude of 30 mV and a DC bias of 0.8 V with respect to the open circuit voltage (OCV). The frequency range was 105 to 1 Hz. The Ag NWcoated PDMS pads were observed using a field emission secondary electron microscope (SEM: SUPRA 55VP, Germany). The percolation networks in bendable fuel cells are crucial due to their reliable electrical conductivity under bent conditions [18,22]. As the bending radius of the fuel cell stack decreases, the electrical conductance of the current collector with short Ag NWs (<15 mm) decreases and thus significantly degrades the performance of the fuel cell [22]. However, long Ag NWs (j50 mm) improve the electrical and mechanical stability under various bent conditions [20,22]. The electrical conductance of high density Ag NWs in bendable fuel cells increases under mechanical stress and bent conditions due to the reliable Ag NWs percolation networks which have high mechanical stretchability. The reliable electrical conductance under bent conditions can be improved using very long Ag NWs. Interwoven networks of longer Ag NWs can better maintain the electrical connection than those of shorter Ag NWs, even under severe bent conditions.

Results & discussion FEM model Bending as a key parameter that affects the performance of the bendable fuel cell generates the internal stress of the PDMS pad and the MEA. To investigate the internal stress variation in the bendable fuel cell, the bending of the PDMS pad was simulated using COMSOL Multiphysics which uses the MooneyeRivlin model; the MooneyeRivlin model is suitable for an element with a small strain (<0.45) [23]. Two previous reports assume bending moments at two edge points [18,20]. Our FEM model uses only two forces in the x-directions to describe a substantial experimental condition indicated in Fig. 4A because the boundary conditions we suggest in this study are closer to experimental conditions. Fig. 3D shows that the fuel cell bent due to the compressive force applied at each end, and not the bending moment. The roller that was used for simplifying a model was the constraint for the analysis; only the x-directions constraint is shown in Fig. 4B. This constraint depicts real movements by outer forces in the x-directions at the edge. Unlike in previous studies, the stiffness of the PDMS endplates was controlled in the bending experiments, resulting in the variation of Poisson's ratio, Young's modulus, and density. Armani et al. reported that the approximate Poisson ratio of the PDMS can be calculated as follows [21]: m¼

E 1 2G

Here, G ¼ 250 kPa, E ¼ 750 kPa (10:1 mixing ratio), and E ¼ 868 kPa (5:1 mixing ratio). Its Poisson ratios are 0.499 (10:1) and 0.736 (5:1). Other parameters for FEM analysis are shown in Table 1. As shown in Fig. 4C, an upper PDMS pad and a lower PDMS pad were tensile (red arrow) and compressive (blue

Fig. 4 e (A) Original FEM model, (B) simplified FEM model; forces in x-directions and roller constraint in y-direction, and (C) visualization of principal stresses; red and blue arrows indicate tensile and compressive stresses, respectively. COMSOL finite element analysis of internal stress distribution inside the PDMS endplates in the flexible fuel cell. Distribution of von Mises stress at radii of (D) 25 cm and (E) 15 cm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 e Parameters for calculation using COMSOL Multiphysics: the parameters of Poisson's ratio and Young's modulus for calculating two cases are different. Parameters Bulk modulus(kPa) Poisson's ratio Young's modulus(kPa) Coefficient of thermal expansion Heat capacity at constant pressure(Jkg1K1) Relative permittivity Thermal conductivity(WmK1) C01(kPa) C10(kPa)

5:1/10:1 600 0.736/0.499 868/750 kPa 9
104 1460 2.75 0.16 254 kPa 146 kPa

arrow) stresses, respectively; the black dotted line indicates the original assembled position of the MEA. V$s ¼


vWs $ðI þ VuÞ ve

1 Ws ¼ C10 ðI1  3Þ þ C01 ðI2  3Þ þ kðJ  1Þ2 2 i 1h ε ¼ ðVuÞT þ Vu þ ðVuÞT Vu 2 where s, ε, k, and u are Cauchy stress, strain, bulk modulus, and deformation, respectively, and Ws, J, C, and I represent strain energy density, determinant value of deformation gradient of left Cauchy-Green deformation tensor, MooneyRivlin constant, and uni-modular component variant, respectively. The calculated stress distributions are indicated in Fig. 4D and E. To help compare the bent shapes in Fig. 4D and E, the original stacks are indicated by a black line. The bending radii and the performance of the fuel cell were measured. As suggested in previous research, our modified FEM model also shows the generation of compressive stress due to bending [13]. As Fig. 4D shows, the stress caused by bending is distributed at the central position of the lower PDMS pad. As mentioned above, the stress resulted from the difference of boundary conditions, the compressive force, and the bending moment. We believe that our boundary conditions are more suitable because the upper side of the PDMS pad was not largely stretched. Clearly, the bending generated the compressive stress inside the fuel cell, especially the normal force to the center where the MEA was located.

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Performance under various bent conditions Fig. 5 shows the IeVeP results of the bendable fuel cell. The maximal power densities of flexible fuel cells that have PDMS pads with mixing ratios of 5:1 and 10:1 were 82 mWcm2 and 117 mWcm2, respectively, at the bending radius of 15 cm. Compared with the performance of the original stack and previous our reports, the maximal performance of the flexible fuel cells that have PDMS endplates with mixing ratios of 10:1 and 5:1 increased up to 95% and 173%, respectively [18,20]. To best of my knowledge, this is the best power density under bent conditions. These experimental results prove that increasing the stiffness of PDMS pads from 750 kPa to 868 kPa increases power densities. Fig. 6 also shows the IeVeP results of decreasing the bending radii from infinite (no bending) to 15 cm under various bent conditions. Interestingly, due to the decreased bending radii and the enhanced clamping forces between the two PDMS pads at the anode and cathode sides, the performance of the bendable fuel cell improved [18,20]. As stated in the introduction, our previous studies explained that the performance enhancement was due to the decreased faradaic and ohmic resistances, both of which were caused by the increased compressive normal force to the MEA. Fig. 7A

Fig. 6 e IeVeP measurements of stack assembly under bent conditions of R ¼ ∞, 25 cm, 20 cm, 17 cm, and 15 cm.

Fig. 5 e IeVeP measurements of (A) symmetric cell stack and (B) cell stack using stiffness-controlled PDMS endplates under bent condition of R ¼ 15 cm.

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Fig. 7 e (A) Calculations of average normal forces to MEA based on finite element analysis and measurements of ohmic and faradaic resistances consistent with the curvatures of PDMS-based flexible fuel cells, (B) Faradaic impedance measurements and fitting data of measurement data under bent conditions (radii ¼ 15 cm and 36 cm).

shows the variation of ohmic resistances due to stack bending. As the bending radii of a stack decreased from 25 cm to 15 cm, the clamping forces increased from 10 N to 41 N and thus the ohmic resistances also decreased from 1.31 Ucm2 to 1.08 Ucm2. The ohmic resistances of a flexible fuel cell stack that uses 5:1 PDMS pads are lower than those of bending radii of less than 20 cm. This is mostly due to the enhanced normal force to the MEA. The compressive stress inside the fuel cell was further increased by increasing the stiffness of the PDMS pads. As another parameter, we discuss the decreased faradaic impedance under bent conditions. Fig. 7B shows the impedance spectra and fitting data of stacks at bending radii of 15 cm and 36 cm. In general, the equivalent circuit of a polymer electrolyte fuel cell (PEFC) is one resistance for a membrane and two parallel resistances with constant phase elements for electrodes. It is believed that R1 at the anode is 0.02 Ucm2 due to the hydrogen oxidation reaction resistance, despite the bent condition, whereas R2 at the cathode decreases from 4.93 Ucm2 to 2.03 Ucm2 due to the bending effects of the flexible fuel cell stacks. Decreasing R2 helps improve oxygen delivery from a channel to a catalyst layer through a gas diffusion layer at the cathode side. It is assumed that the decreased width and depth of the flow channel under the bent conditions may facilitate this process under bent conditions, and thus enhance the performance. Cha et al. reported that flow channels with smaller dimensions can enhance oxygen diffusion at the cathode side, and thus the decreased flow channels can improve performance (or decrease oxygen reduction reaction) at the cathode sides under bent conditions because of a reduced dead zone and increased gas velocity [24].

Conclusion In this study, we demonstrated the high performance of the bendable PEFC using Ag NW percolation networks on PDMS film as the current collecting endplate. To improve the performance, we suggested a simple method of employing a stiffness-controlled PDMS endplate. While the previous methods only modified the structure of the cell, our method

controlled the stiffness of endplates. A simplified FEM simulation showed that the increased pressure in the cell thickness direction lowered the ohmic and faradaic resistances. From our conclusion, the simple modified endplates without adding other fasteners for assembly may be beneficial to various electronic devices. From a series bendable fuel cell studies, we believe that bendable fuel cells using enzyme catalysts will be improved in a new type fuel cell.

Acknowledgment This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2011-0031569). Also, this research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0029576, and 2011-0028662) contracted through the Institute of Advanced Machinery and Design at Seoul National University.

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