Three-dimensional electrodes based on an array of carbon microfibers were prepared by a process called direct-charging electrostatic flocking. The cathodes comprised carbon fibers 11
m in diameter and 500
m long that protruded from a titanium foil support like blades of grass. The fiber density was 125,000 fibers per cm2 of geometric area. The fibers were coated with an alloy of Pd and Ir to catalyze hydrogen peroxide reduction. The electrochemical performance of catalyst-coated carbon microfiber arrays
CMAs was investigated in a flowing electrolyte Mg–H2O2 semi-fuel cell. In these studies, CMA-based cathodes showed higher voltages at high current densities, better power densities, and equivalent H2O2 utilization compared to planar cathodes with the same loading of catalyst. © 2008 The Electrochemical Society. DOI: 10.1149/1.2901043 Jo ecs 155(6)b558-b562(2008)

1. Fabrication and Rate Performance of a Microfiber Cathode in
a Mg–H2O2 Flowing Electrolyte Semi-Fuel Cell
Charles J. Patrissi,a,
*,z
Russell R. Bessette,a,b,
* Yong K. Kim,c
and
Christian R. Schumachera
a
Naval Undersea Warfare Center, Newport, Rhode Island 02841, USA
b
Department of Chemistry and Biochemistry and c
Department of Textile Sciences,
University of Massachusetts, Dartmouth, Massachusetts 02747, USA
Three-dimensional electrodes based on an array of carbon microfibers were prepared by a process called direct-charging electro-
static flocking. The cathodes comprised carbon fibers 11 ␮m in diameter and 500 ␮m long that protruded from a titanium foil
support like blades of grass. The fiber density was 125,000 fibers per cm2
of geometric area. The fibers were coated with an alloy
of Pd and Ir to catalyze hydrogen peroxide reduction. The electrochemical performance of catalyst-coated carbon microfiber arrays
͑CMAs͒ was investigated in a flowing electrolyte Mg–H2O2 semi-fuel cell. In these studies, CMA-based cathodes showed higher
voltages at high current densities, better power densities, and equivalent H2O2 utilization compared to planar cathodes with the
same loading of catalyst.
© 2008 The Electrochemical Society. ͓DOI: 10.1149/1.2901043͔ All rights reserved.
Manuscript submitted October 23, 2007; revised manuscript received January 23, 2008. Available electronically April 10, 2008.
Efforts to reduce the effects of slow mass transport in flowing
electrolytes have led to the development of electrodes with in-
creased surface area and surface roughness. Surface roughness is a
function of the size, shape, orientation, and packing density of par-
ticles used in the electrodes.1
Metal meshes,2,3
electrochemically
roughened metals,4
porous carbons,5
and carbon fiber electrodes6-9
are examples of three-dimensional ͑3D͒ electrodes specifically de-
signed to improve mass transport in flowing electrolyte batteries10
and electrochemical reactors.11,12
Such electrodes impede the flow
of electrolyte, causing eddies and increased electrolyte mixing be-
tween the bulk and the electrode surface. In addition to electrolyte
mixing, rough, 3D electrodes have a higher surface area than their
planar counterparts. Increased surface area reduces current density
and the concentration gradient of electroactive species at the elec-
trode surface. In flowing electrolytes, the benefit of a rough elec-
trode surface and high surface area is increased concentration of
electroactive material at the electrode surface, higher voltage, and
reduced effects from concentration polarization.
The U.S. Navy continues to pursue electrochemical power
sources for undersea applications because they are quiet, recharge-
able, environmentally friendly, and promise high energy density. En-
ergy content is a key driver because it impacts mission duration and
capability. The Naval Undersea Warfare Center ͑NUWC͒ has inves-
tigated a Mg–H2O2 semi-fuel cell ͑SFC͒ for the U.S. Navy’s un-
manned undersea vehicle program.13
The SFC is composed of a
consumable metal anode and a replenishable catholyte. Electrolytes
are pumped through the anode and cathode chambers, which are
separated by an ion exchange membrane. The catholyte in this work
was an aqueous solution of hydrogen peroxide, acid, and sodium
chloride. The power and energy densities of fuel cells and SFCs for
undersea platforms is typically not as high as for similar terrestrial
systems. This is largely because undersea power sources are re-
quired to be air-independent. Therefore, undersea fuel cells must
carry an oxidizer that stores oxygen efficiently, such as hydrogen
peroxide. Liquid or compressed oxygen is not an option at this time
due to safety concerns on board submarines as well as packaging
efficiency. The use of H2O2 results in high energy density for an
air-independent fuel cell, but the consequence is concentration po-
larization when it is diluted in the flowing catholyte. The microfi-
brous cathode was developed to reduce the effects of H2O2 concen-
tration polarization. The novelty of the work presented here is the
morphology of the cathode and the process used to prepare it.
Electrostatic flocking is a simple and efficient textile technique
which is used to coat a surface with short fibers for functional and
aesthetic reasons.14-16
The flocked fibers are oriented approximately
perpendicular to the supporting surface. Using electrostatic flocking,
it is straightforward to control fiber length and density ͑i.e., surface
roughness and surface area͒. This paper shows a method which we
call direct charging electrostatic flocking ͑DCEF͒ for the preparation
of 3D, carbon microfiber array ͑CMA͒-based cathodes. These cath-
odes were comprised of conductive carbon microfibers ͑11 ␮m
diam, 500 ␮m long͒ that protruded from a current collector approxi-
mately normal to the surface. In addition, it shows the polarization
and power performance of CMA-based cathodes compared to planar
control cathodes in a flowing H2O2-based catholyte.
Experimental
DCEF.— Figure 1 shows a schematic diagram of the electro-
static flocking method used to prepare the CMAs for this study.
Pitch-based carbon fibers ͑Mitsubishi Chemical, K63712, 11 ␮m
diam͒ were chopped to a length of 0.5 mm ͑Engineered Fibers Tech-
nology, LLC, Shelton, CT͒ and dried at 100°C in air to remove
moisture. To begin the DCEF process, a known mass of carbon
fibers was evenly distributed onto a conductive plate that was the
same dimensions as the current collector to be flocked. An electri-
cally conductive, carbon-based adhesive ͑112-48, Creative Materi-
als, Tewksbury, MA͒ was screen-printed onto a titanium foil current
collector/support ͑Hamilton Precision Metals, Grade 1, 100 ␮m
thick͒. First, a thin coating of room temperature adhesive was spread
onto the Ti. A macroporous filter ͑Spectra/Mesh, 100 ␮m opening,
78 ␮m thickness͒, which functioned as the printing screen, was then
pressed down into the adhesive, the excess was removed with a
squeegee, and then the screen was removed. The low-viscosity ad-
hesive flowed to a uniform thickness. The filter has an open area of
47% and chemical stability toward the organic solvents in the adhe-
sive. The adhesive was screen-printed using the mesh filter so its
thickness would be uniform across the surface of each electrode and
from electrode to electrode. Prior to application of the adhesive, the
Ti current collector was abraded to remove the oxide film. The
adhesive-coated Ti was suspended horizontally over the fiber sup-
port plate and connected to the ground. The support holding the
loose fibers was then connected to a high-voltage power supply
͑EK100, Maag Flockmaschinen, Germany͒. The field strength to
prepare these CMAs was 5 kV/cm. High voltage was typically ap-
plied for 10–30 s until no fibers remained on the high-voltage plate.
The flocked adhesive/titanium support was weighed before and after
flocking to determine fiber density. The resulting CMA was heated
for 1 h at 100°C in air to cure the adhesive.
Preparation of catalyzed SFC cathode.— Palladium and iridium
were electrodeposited onto CMAs to catalyze H2O2 reduction. The
* Electrochemical Society Active Member.
z
E-mail: patrissicj@npt.nuwc.navy.mil
Journal of The Electrochemical Society, 155 ͑6͒ B558-B562 ͑2008͒
0013-4651/2008/155͑6͒/B558/5/$23.00 © The Electrochemical Society
B558

2. deposition method, developed previously,17-19
was performed using
a cyclic potential sweep between −0.15 and −0.30 V ͑vs Ag/AgCl͒
at 1 mV/s for 25 cycles in a three-electrode cell with a carbon rod
counter electrode. A paddle stirrer was used for convection. During
deposition, any exposed Ti was masked with Teflon tape ͑3M Com-
pany͒. The deposition bath was 2 mM PdCl2, 2 mM Na2IrCl6·6H2O,
0.2 M KC1, and 0.1 M HCl in deionized water heated to 70°C. All
chemicals were used as obtained from Aldrich Chemical. Approxi-
mately 700 mL of solution was used for a CMA geometric area of
77 cm2
. The CMA was weighed before and after deposition to de-
termine catalyst loading. Catalyzed CMAs are called high electro-
lyte penetration ͑HElP͒ electrodes because the catalyst-coated fibers
protrude into the flowing catholyte.
Preparation of planar control electrode.— Planar electrodes were
prepared similarly to the HElP electrodes. Titanium foil was abraded
to a bright finish, wiped with ethyl alcohol, screen printed with
conductive adhesive, and heated at 100°C for 1 h. Pd:Ir deposition
was performed using the cyclic potential sweep method as described
above.
SFC electrochemistry.— The electrochemical performance of
CMA and planar control cathodes was investigated in NUWC’s
Mg–H2O2 SFC. The half-cell and full-cell reactions and standard
potentials for the SFC couple are shown in Eq. 1-3
E0
Mg ——→ Mg2+
+ 2e−
2.37
͓1͔
H2O2 + 2H+
+ 2e−
——→ 2H2O 1.78 ͓2͔
Mg + H2O2 + 2H+
——→ Mg2+
+ 2H2O 4.15 ͓3͔
Because it strongly affects cathode voltage, catholyte pH was con-
tinuously monitored and manually maintained between 0.7 and 1.0
by addition of concentrated H2SO4.
Figure 2 shows a schematic diagram of the SFC constructed at
NUWC for these experiments. The anode was a Mg wafer ͑2.5
ϫ 3.5 ϫ 0.2 cm͒ press fitted onto a stainless steel ͑SS͒ bus rod.
Epoxy ͑Epoxy 907, Miller-Stephenson͒ was applied around the SS
bus rod on both faces to secure it to the Mg anode. Aqueous anolyte
and catholyte were recirculated through each compartment from
150 mL three neck flasks using Tygon tubing. The cathode bus bar
was a SS wafer with a press-fit SS bus rod. Silver epoxy ͑118-06,
Creative Materials͒ was used to bond the HElP cathode to the SS
bus plate. One drop was applied to the SS, the cathode and bus plate
were pressed together, and then the entire assembly was heated at
100°C for 4 h. A bead of Epoxy 907 was then placed around the
perimeter of the Ti and onto the SS bus bar to anchor the cathode
and prevent contact between the Ag epoxy and the catholyte. During
testing the electrode inserts were recessed into the polysulfone elec-
trode housings. The bus rods protruded from the back of the half-
cell bodies as electrical leads. Rubber O-rings were placed around
the bus rods and an outer polycarbonate plate ͑not shown͒ was
clamped to the back of each electrode housing to compress the
O-rings and prevent leakage. Rubber gaskets defined a channel of
approximately 0.7 mm ϫ 2.5 cm ϫ 3.5 cm between the electrodes
and membrane. The SFC was assembled with a dry cation exchange
membrane ͑Nafion 115͒ prepared as explained below. A polymeric
mesh spacer ͑not shown͒ was used in the anode compartment for
even flow over the anode. No spacer was used in the cathode com-
partment.
The bench-top Mg–H2O2 SFC ͑Fig. 3͒ consisted of separate
electrolyte loops for the anode and cathode. Electrolyte ͑100 mL͒
was continuously pumped from 150 mL reservoirs to the SFC by
peristaltic pumps. The anode electrolyte for these experiments was
40 g/L NaCl in DI water and the catholyte was H2O2,
0.2 M H2SO4, and 40 g/L NaCl in DI water. The anolyte flow rate
was 200 mL/min for all experiments, while the catholyte flow rate
Figure 1. Schematic diagram of the flocking station. Arrow shows fiber
direction during application of the electric field, E. Figure 2. Exploded view of the 9.6 cm2
Mg–H2O2 SFC.
Figure 3. Schematic diagram of bench-top Mg–H2O2 SFC. Anode and cath-
ode potentials ͑A and B, respectively͒ were measured vs Ag/AgCl reference
electrodes ͑D͒ immersed in the electrolyte reservoirs. Current control and
full cell measurement were performed at location “C.”
B559Journal of The Electrochemical Society, 155 ͑6͒ B558-B562 ͑2008͒ B559

3. was varied between 50 and 200 mL/min. Hydrogen peroxide ͑Ato-
fina, 50%͒ was added to the catholyte reservoir with a micropump
͑Masterflex, Cole-Parmer͒ at a constant flow rate to maintain H2O2
concentration at 0.060 M. A colorimetric redox titration with
Ce4+
/Ce3+
and iron ͑II͒ phenanthroline was used to determine H2O2
concentration.20
The ion exchange membranes were pretreated by
boiling for 2 h each in 1 M H2SO4 and DI water. Commercial
Ag/AgCl reference electrodes ͑Accumet, Fisher Scientific͒ were im-
mersed in the electrolyte reservoirs and used to determine anode and
cathode half-cell potentials. SFC current was controlled using a
Princeton Applied Research 362 potentiostat/galvanostat. All SFC
data was acquired using Lab View and a high-impedance National
Instruments data acquisition package ͑SCXI 1120 signal input mod-
ule and PCI 6023E data acquisition card͒. A JEOL 6300 and Prince-
ton Gamma Tech IMIX system were used for scanning electron
microscopy ͑SEM͒ and energy dispersive spectroscopy, respectively.
Results and Discussion
Flocking process.— Electrostatic flocking is the application of
chopped fiber to an adhesive coated surface such that the fiber pro-
trudes approximately normal from the supporting surface.14-16,21
In
our DCEF process ͑Fig. 1͒, charged fibers accelerate toward the
grounded Ti foil and embed in the wet conductive adhesive. The
perpendicular orientation occurs because fibers align with the field
for dipole moment and aerodynamic reasons. Factors that influence
fiber density and distribution are anode–cathode separation distance,
electric field strength, substrate conductivity, and amount of loose
flock. The DCEF method requires little hardware and no moving
parts compared to our previous method17
and commercial flocking
methods in general. In both of the latter two methods, the flock is
suspended in a hopper directly over the substrate on a close mesh
screen. In this configuration, the fibers are charged using a corona
discharge22
from an electrode inside the hopper. CMAs prepared
using the direct charging method were more homogeneous in fiber
distribution and fiber density was more reproducible compared to
the hopper method. This is likely because in our previous method
the hopper was shaken slightly to aid fiber transit through the sup-
porting screen. Shaking imparted lateral motion to the fibers, which
caused some of them to miss the target. This does not happen with
DCEF, where fibers transit against the direction of gravity, are uni-
formly charged by contact with the conductive support, and are
pulled to the target by the electric field, which ensures high flocking
efficiency. Corona discharge is an inefficient and potentially damag-
ing method of charging carbon fibers. This is because ions and elec-
trons created in this process react with the atmosphere, creating
species, such as ozone and nitrous oxides, that can alter the surface
properties of the carbon microfibers. Further, corona charging is a
line-of-sight process, so there is likely a wide range of charge den-
sity on fibers transiting to the substrate. This would affect fiber
velocity and, ultimately, fiber distribution. Therefore, the new DCEF
method is easier to use in the laboratory and for larger applications.
To date we have flocked electrodes as large as 11 in. ϫ 15 in. with
90% efficiency ͑see below͒.
Morphology of the carbon microfiber array.— Overhead and
cross-sectional images of a carbon microfiber array are shown in
Fig. 4. SEM investigations show the thickness of the cured adhesive
was approximately 25 ␮m. SEM images of CMAs ͑not shown͒ in-
dicate that the fibers penetrate through the entire thickness of the
adhesive. The fiber density for these investigations was approxi-
mately 125,000 per cm2
of geometric area ͑ca. 12% fibers by vol-
ume͒. This is in good agreement with the maximum fiber densities
that can be obtained for flocked textiles.14,16
Fiber density was de-
termined from the mass of the fibers flocked onto the Ti substrate,
fiber specific gravity ͑2.12 g/cm3
͒, and fiber dimensions ͑11 ␮m
diam, 0.5 mm long͒. High-magnification SEM images show that the
fibers are smooth and nonporous. The calculated surface area, then,
of smooth, nonporous fibers was 0.17 m g−1
, and the calculated sur-
face area of the flocked CMA was approximately 22 cm2
of fiber
area per cm2
of electrode geometric area. Fiber density was repro-
ducibly controlled by weighing a specific amount of fibers onto the
conductive support plate. In these investigations, flocking efficiency
͑the mass of fibers adhered to the substrate compared to the mass of
loose fibers͒ was observed to scale with electrode dimensions. Inef-
ficiency occurred because some fibers followed the electric field to
the back of the substrate. Flocking efficiency was greater with larger
geometric area, presumably because of the increase in ratio of sub-
strate surface area to perimeter length. For example, during scale up
of the HElP cathodes, the typical flocking efficiency for 1.5
ϫ 8 in. CMAs was approximately 60%, while for 11 ϫ 15 in.
CMAs it was over 90%. Fibers not embedded in the adhesive were
recovered and reused.
Preparation of HElP and planar cathodes.— A wide variety of
materials have been explored to catalyze the reduction19,23-27
and
decomposition28-30
of H2O2. Efficient utilization and rapid reduction
kinetics of H2O2 are critical for high SFC specific energy and power
density. In these experiments, an electrodeposited alloy of Pd and Ir
Figure 4. SEM images of top view ͑A͒ and cross section ͑B͒ of a carbon
microfiber array prepared using DCEF.
Figure 5. ͑A͒ Top-view SEM images of
Pd:Ir-coated carbon microfiber array. ͑B͒
The fractal nature of the Pd:Ir deposit that
forms at the tips of some carbon fibers.
B560 Journal of The Electrochemical Society, 155 ͑6͒ B558-B562 ͑2008͒B560

4. ͑denoted here as Pd:Ir͒17,18
was used to catalyze the reduction of
H2O2. Cathodes with Pd:Ir show higher voltage and power density
than uncatalyzed carbon cathodes. Figure 5 shows the morphology
of the Pd:Ir-coated CMA ͑i.e., the HElP cathode͒. Note that un-
coated fibers shown in Fig. 4B appear darker than the gray
Pd:Ir-coated fibers in Fig. 5. This shows that the length of each fiber
is well covered by the catalyst. It was not possible, however, to
image the fibers all the way to the adhesive surface. Figure 5B
shows the fractal-like nodules of Pd:Ir that form at the tips of some
of the microfibers. These nodules are likely the result of a higher
diffusion rate to the tips during electrodeposition. Previous X-ray
diffraction studies at NUWC indicate the deposit is a 1:1 ͑atomic
ratio͒ alloy. The Pd:Ir loading for these experiments was 10 mg/cm2
for both CMA and planar cathodes. The SEM images in Fig. 6 show
the morphology of the planar cathode. The Pd:Ir nodules are visible
across the planar surface in the top view ͑Fig. 6A͒ and at an 80°
stage tilt angle ͑Fig. 6B͒. The film is continuous, and the nodules are
5–10 ␮m in height.
Polarization performance.— Figure 7 shows the raw polariza-
tion data from SFCs with HElP and planar cathodes. These data
show anode and cathode half-cell potentials at current densities be-
tween 10 and 75 mA per cm2
of cathode geometric area. Flow rates
for the anolyte and catholyte were 200 mL/min. Each polarization
current was held for 30 s, followed by 30 s at open circuit. Half-cell
potentials were measured between Ag/AgCl reference electrodes
immersed in the electrolyte reservoirs and their respective electrodes
and recorded at 1 s intervals. Figure 6 shows much higher voltage
loss at the HElP and planar cathodes, especially at higher current
densities, compared to the Mg anodes. This shows that SFC voltage
is dominated by cathode polarization at high current densities.
The raw data in Figure 7 provide a direct comparison of the
polarization performance of the HElP and planar cathodes. The
open-circuit potentials for both cathodes were approximately 0.6 V
vs the Ag/AgCl reference electrode. This is a mixed potential be-
tween H2O2 and O2 in the acidic electrolyte.31
During discharge,
cathode potential was also likely affected by the formation of a
hydride surface layer during H2O2 reduction.27,32
This layer has
been shown to passivate the electrode surface and impede the pro-
cess of H2O2 reduction. Hydride passivation appears to be especially
likely for the planar cathodes in this study because of their more
cathodic potentials at high current densities. The data in Fig. 7 show
that from 25 to 100 mA/cm2
there is greater polarization at the
planar cathode than at the HElP cathode. The data indicate that the
HElP cathode has improved rate performance and higher voltage at
high currents compared to planar cathodes. For these investigations,
small-scale ͑1 in. square͒ electrodes were used repeatedly ͑between
5 and 10 times͒ in 4 h experiments. After each experiment, the cell
was opened and the microfiber cathode was rinsed with DI water
and left to dry in air at room temperature. During the first use of a
cathode, some fibers were observed floating in the cathode reservoir.
This detachment was not observed in subsequent experiments and
was not observed to affect performance from one experiment to the
next. To date, 11 ϫ 15 in. HElP cathodes have been discharged con-
tinuously for as long as 40 h and over multiple experiments with no
decrease in performance and obvious loss of fibers after the first
experiment.
The effect of cathode architecture and catholyte flow rate on SFC
power performance is shown in Fig. 8. These data were determined
using average voltages from raw polarization data like those shown
in Fig. 7. The anolyte flow rate for all these experiments was held
Figure 8. Mg–H2O2 SFC power density with HElP and planar cathodes.
SFC catholyte flow rate ͑mL/min͒ is given in parentheses. Anode flow rate
was 200 mL/min for all experiments.
Figure 6. Top ͑A͒ and cross section ͑B͒ SEM images of the planar ͑control͒
Pd:Ir cathode used in these flowing electrolyte investigations.
Figure 7. Mg–H2O2 SFC cathode and anode potentials at various current
densities: ͑—͒ SFC with HElP cathode and ͑- - -͒ SFC with planar cathode.
B561Journal of The Electrochemical Society, 155 ͑6͒ B558-B562 ͑2008͒ B561

5. constant at 200 mL/min. The data show that at 200 mL/min
catholyte flow rate, the HElP cathode shows an increasing power
density up to 75 mA/cm2
with a high power density of 84 mW/cm2
.
In contrast, the power density of the SFC with the planar cathode
peaks at approximately 65 mA/cm2
at 53 mW/cm2
. At all flow rates
the SFC with the HElP cathode shows a greater power density and a
wider power range than the SFC with planar cathode at the highest
flow rate of 200 mL/min. This is likely due to increased mass trans-
port at the HElP cathode due to its microfibrous architecture ͑see
below͒.
Utilization efficiencies for H2O2 were measured for HElP and
planar cathodes using 3 h galvanostatic experiments at 25 mA/cm2
.
Peroxide efficiency was determined by comparing moles of H2O2
consumed ͑mc͒ to coulombs passed through the external circuit. The
number of moles of peroxide consumed ͑mc͒ was determined using
mc = ma + ͑ms − me͒
where ma is the number of moles added via the micropump, ms is the
number of moles present in the tubing and reservoir at the start, and
me is the number of moles present after 3 h. Peroxide efficiency and
average voltage for HElP cathodes were 88͑4͒% and 1.75͑3͒ V and
for planar cathodes were 86͑3͒% and 1.55͑3͒ V. These data show
that greater SFC specific energy was obtained with HElP cathodes
compared to planar cathodes for these small-scale experiments.
As shown above, the morphology of the CMA delayed electrode
polarization to higher current while having a neutral effect on reac-
tant utilization efficiency. The surface area of CMA-based cathodes
is approximately 22 times higher than for the planar electrodes. This
likely results in lower current density and a higher H2O2 concentra-
tion at the CMA-based cathodes resulting in higher electrode poten-
tial at high current densities.
In addition to our work, modeling studies were performed on
heat and mass transport at CMAs.33,34
This work showed four times
greater heat transport for gases at CMAs compared to smooth sur-
faces in circular and planar ducts under laminar flow conditions.
This is important for flowing electrolytes, because mass- and heat
transport are linked by a common set of equations and
assumptions.35
The model was subsequently extended to flowing
aqueous systems, where it predicted increased mass transport for
CMAs compared to smooth surfaces under the same conditions.
Reduced polarization at HElP electrodes, then, is likely the result of
increased electrode roughness ͑electrolyte mixing͒ and surface area
͑reduced current density͒ compared to the planar control electrode.
They combine to increase voltage at high currents and reduce the
effects of concentration polarization. Our future work will explore
the effect of fiber density and fiber length ͑surface roughness͒ on
CMA mass-transport coefficient over a wide range of electrolyte
flow rates. The limiting current technique is being used for these
investigations.36,37
These studies will also compare the relative ef-
fects of CMA surface roughness and surface area on electrode rate
performance. In these studies the effect of CMA morphology on
other system parameters, such as pressure drop across the cathode
and current distribution, will also be considered, because they con-
tribute to overall fuel cell system efficiency and performance.
Conclusions
An approach for preparing 3D, microfibrous electrodes has been
demonstrated using a textile technique called DCEF. This method
was used to attach 0.5 mm long carbon fibers to a flat current col-
lector support such that they protruded like blades of grass. The
carbon fibers were coated with an alloy of Pd and Ir to complete the
preparation of microfibrous cathodes for the flowing-electrolyte
Mg–H2O2 SFC. The catalyzed CMAs have higher surface roughness
and surface area compared to planar Pd:Ir electrodes. Polarization
and galvanostatic experiments performed on the Mg–H2O2 SFC
showed higher electrode potentials, at high current densities, for the
CMA cathodes compared to planar cathodes. This is likely due to
increased electrolyte mixing in the flowing electrolyte and decreased
current density at the CMA-based cathode because of its unique
architecture. The combination of higher cathode potential and high
H2O2 utilization result in higher specific energy and power density
for SFCs with CMA-based cathodes compared to planar cathodes.
DCEF is simple, low cost, efficient, and scalable. It can be used
to prepare a 3D, microfibrous architecture on surfaces with widely
varying geometric shapes and dimensions. We have prepared CMAs
as small as 1.5 cm in diameter and as large as 30 ϫ 40 cm. Fiber
density and fiber length ͑i.e., surface roughness and surface area͒
can be controlled over a wide range. This presents an opportunity to
optimize mass transport in flowing electrolytes and reduce current
density. Also, a variety of fibrous materials can be flocked, such as
metals, carbons, and semiconductors, thus expanding the applica-
tions of this technology in energy conversion and electrochemical
synthesis.
Acknowledgments
This work was supported by the Office of Naval Research ͑Dr.
Richard Carlin and Dr. Michele Anderson͒ as well as NUWC’s in-
house laboratory independent research program ͑Mr. Richard Phil-
ips͒.
Naval Undersea Warfare Center assisted in meeting the publication costs
of this article.
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