INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 3, pp. 501-504 MARCH 2013 / 501

© KSPE and Springer 2013

Air-breathing Flexible Polydimethylsiloxane (PDMS)-

based Fuel Cell

Ikwhang Chang1, Min Hwan Lee2, Ji-Hyun Lee3, Youn-Sang Kim3,4, and Suk Won Cha5,#

1 Department of Intelligent Convergence Systems, Seoul National University, Gwanakro 1, Gwanak-gu, Seoul, Republic of Korea, 151-7442 School of Engineering, University of California, Merced, 5200 North Lake Road, Merced, California 95343, USA

3 Department of Nano Convergence, Seoul National University, Gwanakro 1, Gwanak-gu, Seoul, Republic of Korea, 151-7444 Advanced Institute of Convergence Technology, 64-1 Iui-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do, Republic of Korea, 443-270

5 Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanakro 1, Gwanak-gu, Seoul, Republic of Korea, 151-744# Corresponding Author / E-mail: swcha@snu.ac.kr, TEL: +82-2-880-1700, FAX: +82-2-880-1696

KEYWORDS: Polydimethylsiloxane (PDMS), Flexible fuel cell, Air-breathing, Polymer Electrolyte Fuel Cell (PEFC)

The paper examines a fabrication method of flexible fuel cells (FCs), and its feasibility through a set of electrical measurements both

in the as-prepared and the bended condition. The flexible FC consists of three parts: membrane electrode assembly (MEA), anode

and cathode endplates with current collectors. The endplate material for anode and cathode used in this study is Polydimethylsiloxane

(PDMS), and metallic films are sputtered on a patterned PDMS to use the resulting structure as current collector. The power density

of bended cell with the curvature of ~1.8 m-1

decreased by ~30% compared to the as-prepared (non-bended) cell.

Manuscript received: July 2, 2012 / Accepted: December 11, 2012

1. Introduction

Among various renewable energy devices, fuel cells (FCs) are

considered one of the most promising direct energy conversion devices

generating electrical energy because of its low carbon emission and

high efficiency.1-3 In particular, polymer electrolyte fuel cells (PEFCs)

are known to have the highest output power density and cell

durability.4-6 Even more they are considered to be the most plausible

type for mobile applications due to its low temperature operation. To be

usable as a mobile/portable devices, the system needs to be simple,

easy to exchange fuel, and stable in performance regardless of

surrounding conditions.7,8 Recently, the need of flexible devices surges

for a variety of applications including energy devices, and flexible

substrates such as polymers and metal foils have gained more attention

gradually for flexible displays and electronic sensors.9-11 The meaning

of the term “flexibility” is categorized roughly into three categories:

how much a system of interest is bendable, permanently shaped, or

elastically stretchable. Among these meanings, studies related to

flexible electronics involve generally either bendable or stretchable

levels.12 Among flexible substrates such as glass, plastic film and metal

foil, flexible electronics based on Polydimethylsiloxane (PDMS) have

been investigated widely by many research groups.13-17 Rodgers et al.

highlighted many researches related to bio-integrated electronics and

optical electronics based on flexible substrate.18,19 Wheldon et al.

originally reported that H2-O2 flexible FCs with 10-100 mm2 active

area, and its peak power density was 57 mW/cm2.20 This study

suggested a simple stack configuration with a single cell which uses

organic material and Au plated Cu mesh. In this paper, PDMS was

employed as the material for endplates in polymer based fuel cells.

PDMS, comprised of silicone elastomer with a low Young’s modulus,

can be patterned via 3-dimensional nano-patterning and lithography.16,18,19

Moreover, we investigated the possible use of PDMS as flexible

endplates for fuel cells to take advantage of the flexibility of PDMS.

Although several studies demonstrated micro fuel cells based on

PDMS, our report presents a fabrication method of a flexible PDMS-

based fuel cell and demonstrates the performance feasibility under

bendable/non-bendable conditions in air-breathing flexible electrolyte

fuel cell for the first time.21,22 We confirmed a critical design issues

relative to the performance losses in our studies. Also, the experimental

methods and preparation processes were proposed.

2. Fabrication and experimental procedure

PDMS endplate fabrication - The fabrication process of a PDMS-

based endplate is depicted in Fig. 1. PDMS is considerably flexible

DOI: 10.1007/s12541-013-0067-1

502 / MARCH 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 3

(360-870 KPa) compared to other candidates for endplate material such

as polycarbonate (2.4 GPa), graphite (10 GPa) and stainless steel

(190 GPa).23,24 The dimension of PDMS endplate is 45 mm × 45 mm.

The base mold made of stainless steel was covered with a mixture of

PDMS and curing agent. Since it is difficult to lift off a comb-like

structure with high depth-to-width aspect ratio due to a significant

adhesion at the side walls of flow channels, the dimensions of PDMS-

endplate were chosen to ease the lift-off process. The width, depth (or

height) and length of H2 flow channel at the anode side were 1, 1 and

30 mm, respectively, and those of cathode(rectangle hole type) was 2.5,

6 and 28 mm. Since the cathode side is open to air without forced air

injection/compression system (i.e., air-breathing type) and oxygen

reduction reaction at the cathode is usually known to cause one of the

most significant losses, the open area was designed to be wider.25

However, the requirement of structural stability limited the maximum

open area and thus, we compromised some of fluent mass transport by

having the portion of open area to be less than 50% (for our

experiment, it was ~38%) whereby the clamping force can be conveyed

well through to the MEA.25 The PDMS and a curing agent were mixed

with the ratio of 10:1 in an experimental dish, and heated at 70oC for

4 hours (Fig. 1(b)). After 5 min sonication in ethanol solution, thin-film

metals layers that act as current collector were deposited on PDMS via

DC sputtering method. The target-substrate distance was 6 cm and

sputter deposition power was 200 W under 5 mtorr Ar pressure. First,

as shown in Figure 2(a), an 880 nm thick Ni was deposited on PDMS

in 5 minutes. Another 3.8 μm thick Au film was deposited in

20 minutes on top of the Ni layer with the same deposition conditions.

Here, Ni serves as adhesion layer for Au film that may have peeling

issue from PDMS. We selected Ni (electric conductivity = 0.143 × 106

S/cm) over more common Ti (electric conductivity = 0.023 × 106 S/cm)

or Cr (electric conductivity = 0.077 × 106 S/cm) as adhesion layer due

to higher electric conductivity.26

MEA preparation - Commercial MEAs (CNL, Korea) with a

polymer membrane (Nafion 212, DuPont, USA) and Pt catalyst with

loading of 0.4 mg/cm2 were employed in the test. The gas diffusion

layers (GDLs) of both sides are 420 μm thick SGL 10BC carbon paper

(SGL, Germany). The second MEA samples comprised of same

material but have no gas diffusion layer. The active area of each MEA

was 3 cm × 3 cm. The three layer parts (Ni/Au coated anode/cathode

endplates and MEA) were simply assembled by two paper clips

without additional clamps or adhesions.

Fuel cell test - The current-voltage (I-V) and electrochemical

impedance spectroscopy (EIS) were measured with Solartron 1287/

1260 combinations. The I-V was obtained in galvanodynamic mode at

3 mAsec-1 and EIS measurements were performed with 30 mV of ac

perturbation under a constant bias of 0.3 V and in the frequency range

of 105-10 Hz. Humidified H2 in 20oC was supplied to the anode at a

rate of 50 sccm and the cathode was open to the atmospheric

environment (i.e. air-breathing). Experimental sequences were 1) H2

supply, 2) OCV measurement for 10 minutes, 3) galvanostatic

measurement at 0.1, 0.3 and 0.5A for 10 minutes each for membrane

and catalyst layer humidification, 4) I-V measurement and 5) EIS

measurement. PDMS endplate cross section was obtained using a

focused ion beam (Quanta 3D FEG; FEI Inc., Netherland) for a

scanning electron imaging.

3. Results and discussion

Figure 3(a) shows I-V behaviors and their resulting power density

of cells with and without a GDL. As the curves indicate, the cell with

GDL shows superior I-V behavior to the one without GDL. The OCV

is also close to 1 V in the case of the cell with GDL while the other

barely reaches 0.9 V. This is the OCV comparisons with GDL/without

GDL flexible fuel cell using PDMS endplate. The controlled parameter

is only the GDL. The lower OCV was measured in the fuel cell without

GDL. These are significantly attributed to poor gas tightness between

the rugged thin film current collectors. Also, we suspect that the poor

contact resistance between the catalyst layer and the PDMS endplate

can be the other possible reason. As shown in Figure 3(b), the cell’s

ohmic resistance with GDL is 4 times larger than that without GDL. It

shows that the poor contact resistance between the catalyst layer and

PDMS endplate exists.It shows that the role of GDL can be the

appropriate gap filler in flexible fuel cells. In a more elaborate effort to

improve Au adhesion on PDMS, the surface of PDMS was roughened

by sandpapers beforehand. The GDL must have acted as a gap-filler

and also a buffer that facilitates even distribution of mechanical

pressure. As shown in Figure 3(b), the faradaic impedance of the cell

with a GDL clearly smaller than that of the cell without GDL. It is

Fig. 1 Fabrication procedure of PDMS endplate: (a) mold fabrication

using stainless steel, (b) PDMS curing at 70oC, (c) PDMS lift-off from

stainless steel mold, (d) Ni thin film sputtering on PDMS, (e) Au

sputtering on Ni film, and (f) assembly of anode and cathode endplates

with MEA

Fig. 2 (a) A cross-sectional image of thin-film current collector on

PDMS: 880 nm thick Ni and 3.8 μm thick Au (b) A picture showing

the assembly of flexible fuel cell

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 3 MARCH 2013 / 503

believed that a GDL in the cell can retain water both at the anode and

the cathode, which could facilitate electrode reactions.27 Although

humidified H2 is supplied to the electrodes, the GDL has a beneficial

role in terms of stable water management at catalyst layers.25,27 The

possible reason is partly due to high mass diffusion loss in the

vicinity of ribs. The dependency of EIS results coincides that the

GDL role directly influences on activation overpotentials. Figure 3(c)

reveals I-V performances of cells in an as-prepared condition and in

a bended condition. The peak power densities of these two cells were

29.1 and 20.5 mW/cm2, respectively. A close similarity of the two

cells in the OCV (~1.0 V) indicates that the gas tightness of both cells

were reliable based on other studies.20,22,25 Impedance spectra shown

in Figure 3(d) also shows that activation overpotentials are similar

each other. From the I-V and EIS results, the discrepancy in the

power density can be ascribed to the difference in the ohmic loss. The

impedance spectra also indicate a clear gap in the ohmic resistances

between the two cells. The high ohmic loss in the bended cell was

attributed to two reasons: the rigidity of GDL and possible

delamination of Ni/Au film. In the bended condition, the rigid GDL

made of carbon paper must resist the deformations, which could

consequently result in an uneven pressure in the cell and possible

poor electrical contact especially at the far end from the center.

Unlike carbon cloth GDLs, the flexibility of carbon paper is not

enough, and thus the carbon paper resists the bending condition of

flexible fuel cell, resulting in the possible increase of ohmic resistance.

This effect may be alleviated by employing a softer material such as

carbon cloth in a flexible FC assembly. Second, possible delamination

of Ni/Au film during the bending process could have detrimental

impact on the ohmic resistance. As shown in Figure 3(e) and (f), the

cracks of Ni/Au film propagate as the number of bending cycles

increase. We confirmed that the electrical resistance variation (in-

plane direction) of thin film current collectors occur under bended

conditions. Under the non-bended condition (Fig. 4(a)) / the bended

condition (Fig. 4(b)) by a table vise, the dimension of original endplate

was 45 mm and it shrank to 40 mm (neutral axis) when compressed.

The strain (ε), defined as the ratio of length decrement to the original

length along the center line, was 11% in the case of bended cell. Since

the MEA was located almost in the middle of two endplates, we can

postulate it was under neither tensile nor compressive stress.

4. Conclusion

We fabricated flexible air-breathing fuel cells successfully utilizing

PDMS as a mechanical buffer layer and characterized their

performances under as-prepared and bended conditions. Metal layers

were deposited on the PDMS to be used as current collectors. The

PDMS-based current collectors were assembled with conventional

polymer-based MEA and carbon paper-based GDL. The peak power

density of the cell bended by κ = 1.8 m-1 of curvature was 70% of the

original cell. The decrease in the performance was found mostly caused

by increased ohmic resistance in the cell via electrical characterization.

Even though a novel methodology suggested in this paper, the power

density of flexible fuel cell is still lower than that of conventional air-

breathing fuel cell. The use of more flexible mechanical buffer and

GDL material and development of a scheme to paste current collecting

metal to the mechanical structure more adhesively should improve the

performance significantly as future works.

ACKNOWLEDGEMENT

This research was supported by National Research Foundation of

Korea (Grant No. 2010-0024889) contracted through the Institute of

Advanced Machinery and Design at Seoul National University. Also, this

research was supported by Basic Science Research Program through the

National Research Foundation of Korea (NRF) funded by the Ministry

of Education, Science and Technology (Grant No. 2012-0000921).

Fig. 3 (a) A set of galvanodynamic curves and resulting power density

curves measured on cells with and without GDL (b) A set of EIS

spectra on the same cells (c) A set of galvanodynamic curves and

resulting power density curves measured on a non-bended (curvature =

0) and a bended (curvature = 1.8 m-1) cell (d) A set of EIS spectra on

the same cells. Ni/Au film at (e) non-bending(initial) and (f) bending

condition(final)

Fig. 4 (a) (b) Experimental setup of non-bended condition (curvature =

0) and bended condition (curvature = 1.8 m-1), respectively

504 / MARCH 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 3

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