Accepted Manuscript

Laser Additive Manufacturing of Stainless Steel Micro Fuel Cells

Gianmario Scotti, Ville Matilainen, Petri Kanninen, Heidi Piili, Antti Salminen, TanjaKallio, Sami Franssila

PII: S0378-7753(14)01361-5

DOI: 10.1016/j.jpowsour.2014.08.096

Reference: POWER 19693

To appear in: Journal of Power Sources

Received Date: 21 June 2014

Revised Date: 29 July 2014

Accepted Date: 20 August 2014

Please cite this article as: G. Scotti, V. Matilainen, P. Kanninen, H. Piili, A. Salminen, T. Kallio, S.Franssila, Laser Additive Manufacturing of Stainless Steel Micro Fuel Cells, Journal of Power Sources(2014), doi: 10.1016/j.jpowsour.2014.08.096.

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Laser Additive Manufacturing of Stainless Steel Micro Fuel Cells

Gianmario Scotti*1, Ville Matilainen2, Petri Kanninen3, Heidi Piili2, Antti Salminen2,4, Tanja Kallio3, Sami Franssila1

1 Department of Materials Science and Engineering, Aalto University, P.O. Box 16200, 00076 Aalto, Finland

2 Laser Processing Research Group, Lappeenranta University of Technology, Tuotantokatu 2, 53850 LPR Lappeenranta, Finland

3 Department of Chemistry, Aalto University, P.O. Box 16100, 00076 Aalto, Finland

4 Machine Technology Centre Turku Ltd, Lemminkäisenkatu 28, 20520 Turku, Finland

*Corresponding author. Tel.: +358 503632739 E-mail address: gianmario.scotti@gmail.com

Abstract

This paper introduces laser additive manufacturing as a new method for the fabrication of micro fuel cells: The method opens up the capability of ultrafast prototyping, as the whole device can be produced at once, starting from a digital 3D model. In fact, many different devices can be produced at once, which is useful for the comparison of competing designs. The micro fuel cells are made of stainless steel, so they are very robust, thermally and chemically inert and long-lasting. This enables the researcher to perform a large number of experiments on the same cell without physical or chemical degradation. To demonstrate the validity of our method, we have produced three versions of a micro fuel cell with square pillar flowfield. All three have produced high current and power density, with maximum values of 1.2 A·cm-2 for the current and 238 mW·cm-2 for power.

Keywords: 3D Printing; Proton exchange membrane; Stainless steel; Fuel cells; Prototyping

1. Introduction

Li-ion secondary cell batteries are the current power source of choice for portable electronics, but

micro fuel cells (MFCs) have the potential to further increase energy density [1] [2]. MFCs have

most commonly been microfabricated from silicon [3] [4] [5] [6] [7] [8] [9] but other materials have

been utilized, such as metalized PMMA [10] [11] [12] [13], SU-8 [14], PDMS [15] [16], pyrolized

carbon [17], bulk aluminium [18] and 50 µm thin hydroformed stainless steel sheets [19]. These

materials and techniques do not allow for easy disassembly and re-assembly of the cells, and

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typically, replacing the membrane-electrode assembly (MEA) or the gas diffusion layer (GDL)

requires also the replacement of the bipolar plates. Furthermore, few techniques are suitable for fast

prototyping: a typical microfabrication process flow requires the preparation of one or more

photomasks, the utilization of one or more photolithography steps, and processing with separate

tools for the etching of the microchannels and for the deposition of a metallic current collector.

While moulding, stamping, and hydroforming are faster methods once the mould, or die is ready,

preparation of the mould is, again, a slow process.

Among the established techniques only laser ablation [10] [13] [20] and CNC machining [11] [12]

are suitable for rapid prototyping, in the sense that a testable MFC can be produced directly from a

design made on a computer, with few processing steps. In our previous work we have optimized the

laser ablation parameters for speed [20], disregarding the small irregularities in flow-field channels,

as they proved to be inconsequential to the functioning of the MFCs. However, even with the high

processing speeds achieved by methods such as laser ablation or CNC machining, the drawbacks of

the serial subtractive approach come to the fore when large cavities must be produced. Many MFC

designs require the presence of large volume cavities, such as fuel reservoirs [3] [4] [11], or basins

to accommodate thick gas diffusion layers [7] [18]. Speeding up CNC machining by using larger

milling bits with CNC, or increasing laser ablation speed with larger beam waist and sustained

power, is not usually feasible, as these actions would then make it impossible to create sufficiently

small microchannels or inlet holes. Finally, it should be mentioned that CNC-milled [11] [12] or

laser-ablated [10] [13] PMMA flow-field plates require a separate metallization step, so that they

may collect the produced current.

In this work we introduce laser additive manufacturing (LAM) for the rapid prototyping of stainless

steel micro-fuel cells: LAM is a layer-wise material addition technique where complex 3D parts are

manufactured by selective melting and solidification of consecutive layers of powder material on

top of each other (Fig. 1) [21] [22]. LAM can be used both for rapid prototyping as well as for

manufacturing of complex metallic objects. The material for our MFCs is 316L stainless steel, a

corrosion and acid resistant steel alloy also used for medical implants. Its excellent corrosion

resistance combined with its high hardness and toughness permits the experimenter to re-use the

MFCs fabricated from it, many times. The electrical resistivity of 316L stainless steel is 74 µΩ·cm,

which compares favorably to the 10 mΩ·cm of highly-doped silicon used in [7] and [20]. Because

of the low electrical resistivity, the steel flowfield plates are good current collectors.

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To assess the viability of LAM for MFC fast prototyping, we have fabricated and tested three

variants of a cell with square pillar flowfield and inserted carbon cloth GLD. The difference

between the three designs was the size and aspect ratio of the flowfield: 1x1 cm2, 2x1 cm2, and 4x1

cm2. Scaling the flowfield in one dimension is intended to determine if elongated MFCs, suitable

for form factors of devices such as mobile phones, would still generate sufficient power and current

density compared with 1x1 aspect ratio flowfields.

2. Experimental

2.1 MFC structure and construction

The design of the MFCs in this work is similar to the one in [7] and [18]: we have a square pillar

flowfield and a basin for accommodating a commercial carbon cloth GDL. An exploded schematic

view of the MFC construction is presented on Fig. 2a. All the main feature sizes are summarized on

Fig. 2b: the pillars have a 1 mm x 1 mm cross section, while the channel (inter-pillar distance) is

500 µm wide. There is a 200 µm gap from the top of the pillars to the edge of the flowfield plate.

This is done in order to accommodate a commercial carbon cloth GDL. The GDL used was GDL-

CT® (Fuel Cell Etc), with the microporous side turned towards the MEA. The GDL was 410 µm

thick when not compressed. The MEA was a Gore® Primea membrane (a proton-conductive

ionomer similar to Nafion®) with a platinum loading of 0.1 mg cm-2 on the anode and 0.3 mg cm-2

on the cathode side. Thread seal Teflon® tape (also known as plumber’s tape) was stretched on the

edge of the flowfield plates, to act as a simple gasket. The MFC was mounted in a matching jig

made of ABS polymer by 3D printing.

Fig. 3 shows the 3D models of the three MFC flowfield plates fabricated by LAM. These plates

differ only by the flowfield size in one dimension. The flowfield sizes are (a) 1 cm x 1 cm, (b) 2 cm

x 1 cm, and (c) 4 cm x 1 cm. The MFCs made with these plates will from now on be called “1x1”,

“2x1”, and “4x1”, for convenience.

2.2 LAM of flowfield plates

Fig. 1 presents the process cycle of LAM. In this process, we first create a digital 3D model of the

object to be manufactured. This 3D model is then sliced in the Z direction into 20 µm thick layers.

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Using these layers, a cross-section of desired geometry is melted locally by a laser beam to form a

solid layer. Once the cross-section is melted a new layer of powder material is spread. This cycle is

repeated until the desired object is finished. The platform on which the pieces rest is preheated to

80ºC, to decreases temperature gradient and internal stresses during the manufacturing. The process

usually takes place in an inert atmosphere, such as nitrogen or argon, to avoid oxidation [21] [22].

In our case the chamber was filled with 99.8% pure nitrogen gas.

Once the building process is finished the part is surrounded by loose powder, which is removed and

it can be reused after sieving. The part itself must be detached from the platform, and usually needs

some post-processing. In our case the flowfield plates were placed with the longer edge on top of

the platform for easier detachment, and to make the best use of the available real estate.

The powder used for LAM of the MFC flowfield plates was EOS StainlessSteel 316L, which is a

316L stainless steel with median particle size of 31 µm. The main components of the alloy, by

weight%, are Fe 62%, Cr 17-19%, Ni 13-15%, Mo 2.25-3%, Mn 2%, Si 0.75%, and Cu 0.5%.

Because of the relatively large particle size of the powder, the edge of the plate on which the

Teflon® tape was stretched required some polishing, which was done with a wet grinder. This was

necessary to ensure good gas sealing.

The LAM equipment used in the experiment can be seen on Fig. 4: the bespoke setup was

assembled by EOS GmbH, and it uses a 200 W continuous wave yttrium fiber laser source

operating at 1070 nm wavelength, and a Scanlab hurrySCAN® 20 galvanometer scanner (visible

on the top of the process chamber) for fast beam movement.

The processing time for 10 pieces of 1x1 flowfield plates (2 mm x 14 mm x 14 mm total volume for

one 1x1 plate, Fig. 2) is 5 hours, and if the whole building platform were utilized with 154 pieces of

1x1 plates the building time would be ~ 30 hours. These building times are simulated by the LAM

machine’s process control software PSW 3.2. In our case, we fabricated four of each type of

flowfield plate (Fig. 3) which took about 6 hours 30 min.

2.3 MFC characterization

The MFCs were fueled with humidified hydrogen, while the oxidant was non-humidified oxygen.

The cells were not heated, so their temperature was near room temperature of 20ºC. The flow rate of

both gases was kept at 25 mL min-1 per cm2 of flowfield area. This means that the gases were

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flowing at a rate of 25 mL min-1 in the 1x1 cells, 50 mL min-1 in the 2x1 cells and 100 mL min-1 in

the 4x1 cells. The over-pressure was estimated at ~ 0.1 bar. Polarization and chronoamperometric

measurements were both performed using a Metrohm Autolab PGSTAT100 potentiostat. For the

polarization curve measurements a BSTR10A booster was added to the setup, because the total

current produced by the micro fuel cells exceeded the capacity of the potentiostat alone.

The experimental protocol was as thus:

1) First a set of polarization curves were measured. The measurement was repeated until the

curves became stable and did not differ between iterations. The polarization curves were

obtained by sweeping the potential across the cells at a rate of 3 mV s-1, from open-circuit

potential of about 0.97 V down to 0.05 V.

2) Once the fuel cell performance was deemed stable, a chronoamperometric test was

performed, for 15 h. During the chronoamperometric measurement the potential on the cells

was kept at 0.6 V.

3) After the chronoamperometric experiment was over, a new set of polarization curves was

obtained, much the same way as in step 1).

An additional experiment was made, with the 1x1 MFC: polarization curves were obtained with the

gas flow rates increased from 25 mL min-1 to 50 mL min-1, which are the same conditions as in [7]

and [18]. This allowed us to make a better comparison with published works.

3. Results and discussion

For the 1x1 cell, a maximum current density of 1172 mA·cm-2 and a power density of 233 mW·cm-2

were obtained. When the gas flow rates of the 1x1 cell were increased from 25 mL·min-1 to 50

mL·min-1 to better compare the steel MFCs with our previous results [18], the maximum power

density increased further to 238 mW cm-2 and the current density to 1190 mA·cm-2. This power

density is nearly double compared to the silicon MFCs in [7] (127 mW cm-2) and an improvement

compared to the aluminium MFCs in [18] (228 mW cm-2). These values are comparable even with

macro fuel cells made of 316 stainless steel (similar to the 316L used in this work) [23]. Note that

in [23] the cells were kept at 50ºC and there was an overpressure of 2 bar for both gases, whereas

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our set-up was run under conditions normal for micro fuel cells: very small over-pressure and close

to room temperature (~20ºC).

Table 1 contains a summary of the MFC characterization results, while. Fig. 5, 6, and 7 show all the

obtained results, using the experimental protocol outlined in subsection 2.3. The maximum current

density for the 2x1 MFC was 1089 mA·cm-2 and the power density was 217 mW·cm-2, while for the

4x1 cell the values were 848 mA·cm-2 for current and 164 mW·cm-2 for power density. Both of

these results are still comparable or better than the best MFCs in literature. Comparing the MFCs

with each other, we see that the longer the flowfield, the lower the maximum current and power

densities (Fig. 5 and 6). Also the sustained current density decreases with the longer flowfields (Fig.

7). This is likely due to a lower and more uneven contact pressure between the components caused

by clamping the MFC by bolts [24]. However, the loss in performance is relatively small — less

than 10% going from 1x1 to 2x1 and 15% from 2x1 to 4x1 for the sustained current density (Table

1) — and if these cells were to be used in a portable electronic device, it would be rational to adjust

the form factor of the MFC to that of the device it powers; we will obtain higher power and overall

higher energy density by using a 4x1 cell instead of three 1x1 cells.

It was observed that the performance of all the cells presented in this work was in general excellent.

This can be attributed to the presence of the thick GDL integrated with the MFC flowfield. The

same approach was used in [7] and [18], where it was very beneficial. Fig. 8 is a photograph of the

LAM fabricated flowfield plates with a 1 EUR coin for size comparison. Fig. 9 shows an optical

micrograph of the LAM flowfield plate. The rough surface of the flowfield, due to the relatively

large median size of the steel powder particles (31 µm) that composes it, increases the electrical

contact surface between the flowfield plates and the GDL. This, also, was observed in [7] and [18],

although the nature of the asperity in those previous studies was entirely different from the one

present with LAM fabricated flowfield plates: in [7] we used silicon nanograss for contact

enhancement, in [18] the asperity was caused by non-etched components in the aluminium alloy,

and in this work, the asperity is the result of the particle size of the sintered steel dust. From Fig. 9

we can draw the conclusion that the dimensions of the pillars are not precisely 1 mm x 1 mm. This

could be an issue with more complex flowfield topologies, especially with channels narrower than

500 µm. According to our measurements, the 200 µm gap from the top of the pillars to the upper

edge of the flowfield plates was reproduced very accurately (Fig. 2b), guaranteeing an even

compression force across the GDL.

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All three MFC designs showed improved performance after the chronoamperometric measurement

due to catalyst activation [25] seen as an earlier increase of current between 1.0 V and 0.8 V in Fig.

5b compared to Fig. 5a.

As mentioned previously, fabrication of four of each type of flowfield plate took approximately 6

hours 30 minutes. In [26], flowfield plates from 316L stainless steel were prepared by CNC

micromilling. The authors report a milling time of 5 minutes for a 47.5 mm long, 300 µm deep and

1 mm wide channel, which results in a milling speed of 2.85 mm3 per minute. Since the total

volume of the channels, the inlet holes, and the basin in our 1x1 type cell is ~27.5 mm3, the

fabrication time using CNC milling would have been close to 10 minutes. The total time for 4 of

each type of flowfield plate would have been 4 hours 30 minutes, not counting the time to

dice/separate the CNC milled plates from the stainless steel substrate. This compares favourably to

LAM. However, with deeper channels and larger cavities LAM has a speed advantage over CNC

milling, where processing time increases with removed volume.

4. Conclusions

In this work we have validated the method of using LAM for rapid prototyping of MFCs: we found

that the whole process from the CAD design of three different versions of a MFC, to finished,

testable prototypes, is straightforward and quick. We also found that the performance of the MFCs

was excellent for all three designs. Finally, the study yielded a useful finding on the aspect ratio of

MFC flowfields: we came to the conclusion that the performance of the MFC can scale almost

linearly with the lengthening of the flowfield. This is a valuable result since the intended purpose of

MFCs is to power mobile devices, which can have widely varying form factors.

The large size of the steel powder used in LAM of the flowfield plates creates a rough surface,

which is not necessarily bad; such surface increases the contact between the current collecting

flowfield and the GDL. We also discovered that the feature sizes are not controlled very accurately,

at the sub-millimeter scale of the MFCs in this work. Considering the high performance obtained in

this work, this size variation does not seem to matter for a square pillar flowfield with a thick GDL

on top of it.

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However, other types of MFCs and flowfields may require better dimensional control, which should

be possible to achieve by using powders with smaller particle sizes, and by optimizing the geometry

of the manufactured cells. For instance, Sandvik Osprey Ltd. produces 316L stainless steel powder

with 5 µm particle size. The geometry optimization can be done so that the MFCs better follow the

design rules of additive manufacturing. For example, there should not be structures just laying on

top of powder since the powder does not serve as support and the minimum build angle for stainless

steel material is 45°. Also the build orientation has an effect on the quality and accuracy of the

features to be manufactured. For instance, by optimizing the build orientation, the overhanging

features can be minimized and with this optimization the pillars could be more accurate.

Acknowledgments

The MFC flowfield plates were fabricated at Lappeenranta University of Technology. The MFC

characterization was done at the Fuel Cell Laboratory, Department of Chemistry, Aalto University.

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MFC type Max. current density

[mA cm-2 ]

Max. power density

[mW cm-2]

Average sustained current

density (2h-15h) [mA cm-2 ]

1x1 1172 233 214

2x1 1089 217 205

4x1 848 164 174

Table 1: Summary of MFC characterization results. The results in the 2nd and 3rd column relate to

the polarization curves made after the chronoamperometric measurements. The 4th column contains

an averaged value of the current density obtained during the time interval between 2 h and 15 h of

the chronoamperometric measurement.

Figure 1. Diagram of the laser additive manufacturing (LAM) process. On the left: file manipulation

from 3D solid model to 3D STL model and to 2D slices. Center: basic principle of laser additive

manufacturing process. First the powder is spread then the laser beam melts the geometry of one

layer. Finally, then building platform is lowered. The powder is then spread again, and the cycles

repeat until the part is finished.

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Figure 2. Construction of a MFC with LAM-fabricated flowfield plates. (a) Exploded view. (b)

Cross section of a flowfield plate with annotated feature sizes. The top of the pillars is 200 µm

below the edge of the flowfield, to accommodate the GDL.

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Figure 3. Three versions of LAM-fabricated flowfield plates. The flowfield sizes are (a) 1 cm x 1

cm, (b) 1 cm x 2 cm, and (c) 1 cm x 4 cm. The inset shows an enlarged view of the model near one

of the inlet holes.

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Figure 4. Photograph of the LAM setup at Lappeenranta University of Technology. On the right

side are the control computer and the laser source. In front is the process chamber, with the

galvanometric scanner on top.

(a) (b)

Figure 5. Polarization curves showing the current densities obtained (a) before, and (b) after the 15

h chronoamperometric measurement. The current density, for all three cell designs, has increased

after the polarization measurement.

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(a) (b)

Figure 6. Polarization curves showing the power density (a) before, and (b) after the 15 h

chronoamperometric measurement. Power density has increased after the 15 h chronoamperometric

measurement, for all three MFC designs.

Figure 7. Summary of the 15 h chronoamperometric measurement results. The voltage across the

cells was kept at 0.6 V. After a brief fall, the current density kept increasing in all three MFC

designs.

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Figure 8. Photograph of the LAM fabricated flowfield plates next to a 1 EUR coin, for size

comparison. The flowfield edge is somewhat irregular, but that does not present a problem for the

intended use.

(a) (b)

Figure 9. Low magnification micrographs of LAM fabricated flowfield plate. (a) optical micrograph

of slanted view. (b) SEM top view of an inlet hole.

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Highlights

• First ever stainless steel micro fuel cells made by laser additive manufacturing. • Maximum current density: 1.19 A cm-2, maximum power density: 238 mW cm-2.

• The results are comparable to those of macro fuel cells.

• We demonstrated that the method is suitable for fast prototyping. • The method was used to test flowfield aspect ratio modifications.