VELCRO-TYPE ATTACHMENT OF BLACK SILICON AND CARBON CLOTH
FOR IMPROVED GALVANIC CONTACT IN MICRO FUEL CELLS
1
Gianmario Scotti1,2*, M. Mäkinen1, P. Kanninen3 T. Kallio3 S. Franssila1
Department of Materials Science and Engineering, Aalto University, Espoo, FINLAND
2
Micronova, Centre for Nanofabrication, Aalto University, Espoo, FINLAND
3
Department of Chemistry, Aalto University, Espoo, FINLAND
*Presenting Author: gianmario.scotti@gmail.com
Abstract: Black silicon is a surface morphology of silicon etched in reactive plasma under passivating conditions
of high oxygen to SF6 ratio. It’s a quasi-regular array of narrow pyramidal pillars. We have studied the effects of
coupling black silicon with carbon cloth, to increase galvanic contact for micro fuel cells. We found that the micro
fuel cells with the current collector covered with black silicon performed better than the otherwise identical
counterparts without the black silicon.
Keywords: black silicon, carbon cloth, micro fuel cell, current collector, conductivity
INTRODUCTION
Micro fuel cells (MFC) are microfabricated
electrochemical sources of electrical energy, which
could enable longer active times of portable electronic
devices. Silicon MFCs [1-6] in particular have the
advantage of using well-established microfabrication
techniques, as well as easier integration with CMOS as
well as silicon MEMS components. In a previous work
[Scotti 2010] we presented a silicon MFC where the
gas diffusion layer (GDL) is created from the bulk of
the silicon by reactive ion etching (RIE) to form black
silicon. The black silicon produced by using our recipe
is about 1.5 µm to 2 µm tall with a base of about 300
nm, and compared to micro-columns created with
lithography or other techniques, has the advantage of
having slightly slanted sidewalls, allowing for good
coverage of evaporated or sputtered metals.
A GDL implemented as black silicon is easy to
add to the existing silicon MFC designs, since it
requires only one RIE step. However, with a thickness
of only about 2 µm, it is shallow and increasing this
thickness should increase the performance of the MFC
because the gas flow is less restricted. We decided to
look for more traditional GDL materials for our silicon
MFCs, such as carbon cloth. In our case, this is a
woven web, 330 µm thick uncompressed, with catalyst
nanoparticle loading of 0.25 mg cm-1 applied to one
side. Creating a good galvanic contact between the
carbon cloth and the silicon current collector now
becomes an important issue, compared to the chips
presented in [1], where the GDL is created from the
same bulk as the current collector and there is no
transition between materials. One way to improve this
contact is to increase the compressive force on the
MFC stack. Another is to increase the total contacting
area. Given our existing experience with black silicon,
we decided to try to increase the contacting area
between silicon and carbon cloth, by creating black
silicon on the surface facing the carbon cloth. The
black silicon nanograss would penetrate the carbon
cloth web and better electron conduction between the
two materials should be obtained. Our experiments
show that the performance of silicon MFCs with black
silicon is significantly improved compared to the same
design but without the black silicon treatment.
EXPERIMENTAL
Fabrication
Similarly to the device described in [Scotti 2010],
the MFC presented here is composed of two silicon
chips, with a membrane-electrode assembly (MEA)
sandwiched in between. The GDL function is,
however, now performed by a separate element; the
carbon cloth. For this reason, the chips have a 100 µm
deep depression (basin) intended to receive the layer of
carbon cloth, as illustrated on figure 1. The area of the
flowfield is 1x1 cm2 consisting of a simple array of
pillars, 200 x 200 µm2 wide and 50 µm tall.
Fig. 1: Construction of the silicon micro fuel cell.
A scanning electron microscope (SEM) image of
black silicon as created on the surface of the silicon
chips, is visible on figure 2.
Fig. 2: SEM image of black silicon on top of a pillar.
Inset: SEM of pillar flowfield.
The carbon cloth is an ELAT® gas diffusion
electrode (LT120EWALTSI) made by E-Tek Inc. It
has a platinum catalyst in a microporous layer on one
side. The cloth has been treated with a standard
ionomer application. Figure 3 shows a SEM
micrograph of such carbon cloth but without the
catalyst and the ionomer. The MEA is made by hotpressing a 1 x 1 cm2 square of carbon cloth on both
sides of a Narion®-115 membrane (130°C, 2 tons, 2
minutes) with the catalyst sides towards the Nafion®
membrane. The MFC is formed by placing such MEA
between two silicon chips, so that the carbon cloth is
inserted into the depression in the chips.
flowfield (Oxford Instruments Plasmalab System
100®, ICP 2 KW, CCP 3 W, SF6 100 SCCM, O2 15
SCCM, T=-110°C, t=7 min.) (c). The resist is removed
and a longer DRIE step with the same parameters as
above (but t=14 min.), sinks the whole flowfield by
about 100 µm (d). A thin layer (200 nm) of aluminum
is sputtered on the back of the wafer, patterned and the
wafer is DRIE-etched through to form the gas inlets
(e). Finally, a black silicon etch is performed with the
same DRIE equipment(ICP 1 KW, CCP 2 W, SF6 40
SCCM, O2 18 SCCM, T=-110°C, t=10 min.), and a
thin metal layer of about 40 nm gold on 10 nm TI-W
adhesion layer is sputtered over it (MRC 903®, RF for
Ti-W, DC for Au) (f). The fabrication of the
counterparts without black silicon, is analogous to the
one above, except that the DRIE etch step to form the
black silicon over the flowfield (f) is omitted. In every
other aspect the chips are identical and
interchangeable.
Fig. 4:Process flow for the microfabrication of a
single silicon chip
Fig. 3: SEM image of carbon cloth. The image is taken
from the side of a cut sheet of carbon cloth. Inset:
magnified detail showing carbon nanoparticle cluster.
The fabrication steps for the silicon chips are
outlined on figure 4: a thermally oxidized (700 nm
thick oxide) silicon wafer is patterned with
photolithography followed by buffered HF etching, to
reveal the area that will later be covered by black
silicon (a). Next, normal photoresist is patterned (b)
and then used for the deep RIE (DRIE) etching of the
Measurement
The MFCs were clamped in an aluminum jig that
provides electrical contacts to and reactant gas flow
into the cells. Vertical pressure on the MFCs was
controlled with the use of a torque wrench for the
tightening of the jig.
The fuel gas was hydrogen and the oxidant was
oxygen, the flow of which was regulated with a mass
flow controller to be 50 mL min-1 for both. The
electrical load on the cells was provided by a
computer-controlled
potentiostat
Autolab
PGSTAT100®. Polarization curves were obtained by
sweeping the voltage on the cells from open-circuit
voltage (OCV), which is about 1V for hydrogen fuel,
down to 0.1 V. The OCV was attained by a
stabilization period of 20-30 minutes. Measurements
were performed at a room temperature of 20°C. All
environmental, electrical and chemical conditions were
kept the same for both cells with and without black
silicon. The polarization curves thus obtained can be
seen on figure 5 (without black silicon) and 6 (with
black silicon).
Figure 5. Polarization curves for MFC without black
silicon.
Figure 6. Polarization curves for MFC with black
silicon.
The maximum values obtained compare as thus:
current density: 75 mA cm-2 vs. 202 mA cm-2, power
density: 30 mW cm-2 vs. 64 mW cm-2, for the MFCs
without and with black silicon, respectively.
Preliminary long-term (chronoamperometric)
measurements exhibited excellent stability of the
MFCs with black silicon, yielding a current density of
20 mA cm-2 at 600 mV load.
DISCUSSION
The polarization curves in figure 4 and 5 suggest
that the MFCs in which the silicon current
collectors/flowfields have been treated with a black
silicon etch step, have significantly improved
performance. As mentioned, the mechanical pressure
on the cells was controlled and kept identical for all
measurements. Since the carbon cloth, even under
compression, is two orders of magnitude thicker than
the black silicon layer (~100 µm vs. ~2 µm), increase
of GDL thickness should not be a significant factor.
The most likely reason for the phenomenon of drastic
current and power density increases should be the
increased electron conductivity in the current collector
to GDL interface zone. From the SEM picture of the
commercial carbon cloth used in our experiments
(figure 3), we conclude that the black silicon needlelike pillars form the strongest contact with the
nanostructured clusters attached to the carbon fiber
mesh rather than the mesh fibers, since the mesh is
sparsely woven compared to the black silicon pillars
frequency, and it functions mainly as a support for the
carbon nanoparticle clusters. It is, however, difficult to
say whether the only substantial effect taking place is
increased contact area, or whether the small radius of
curvature at the tip of the black silicon needles has an
influence as well. Small radius of curvature creates a
strong electric field at the tips of the silicon needles,
which could lead to field electron emission [7].
However, field electron emission requires an electric
field of the order of 5 V nm-1, which is unlikely to be
reached with the low voltages generated by the MFC
and the radius of curvature of our average goldmetalized black silicon needle, which is close to 100
nm [7, 8]. Whether the black silicon needle array
produced with our recipe is capable of generating a
low-macroscopic-field electron emission [9], is still
unclear but unlikely, again due to the relatively large
(~100 nm) size of the structures. At any rate, given the
low potential difference between the silicon current
collector (and hence, the metalized black silicon) and
the carbon cloth, the increase of galvanic contact
between the two bodies due to increased contact area,
seems the most plausible explanation for the improved
performance of the MFCs.
The instability seen in the polarization curves is
probably caused by water management problems. The
water condensation seems to cause more issues for the
MFCs without black silicon. This may be due to the
weaker galvanic contact between the plain goldmetalized silicon surface and the carbon cloth, where a
lower level of condensation can disrupt the electrical
flow more readily than in the case of metalized black
silicon needles lodged into the bulk of the carbon
cloth.
CONCLUSION
We have demonstrated the benefit of using black
silicon etching on current collector surfaces of silicon
MFCs, for contacting a carbon cloth gas diffusion
layer. Using carbon cloth for this purpose allows for a
relatively thick GDL (~100 µm), which is usually a
problem with common microfabrication techniques.
With this thick GDL the performance of the MFCs is
comparable to the best in the category of silicon MFCs
[2, 3] while keeping the fabrication process very
simple.
[5]
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