Feasibility study of porous copper fiber sintered felt

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Feasibility study of porous copper fiber sintered felt: A novelporous flow field in proton exchange membrane fuel cells

Yong Tang a, Wei Yuan a,b,*, Minqiang Pan a, Zhenping Wan a

aKey Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes,

South China University of Technology, Guangzhou 510640, People’s Republic of Chinab School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

a r t i c l e i n f o

Article history:

Received 29 January 2010

Received in revised form

27 June 2010

Accepted 28 June 2010

Available online 31 July 2010

Keywords:

Proton exchange membrane fuel cell

Porous copper fiber sintered felt

Porous flow field

Contact angle

Corrosion resistance

* Corresponding author. Key Laboratory ofSouth China University of Technology, Guan

E-mail address: [email protected]/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.06.101

a b s t r a c t

A novel porous flow field made of the porous copper fiber sintered felt (PCFSF) is developed

for proton exchange membrane (PEM) fuel cells. The feasibility of this material is

systematically investigated involving fabrication, characterization and application. The

experimental results reveal that a lower cutting speed helps prevent the fibers from

formation failure and morphological defects. A lower feed rate and a smaller back-cutting

depth both reduce the equivalent diameter of the copper fibers. The sintering temperature,

time and pressure combine to affect the formation quality of the PCFSF which has three-

dimensional network structure with open pores distributed stochastically. The wettability

tests show that the PCFSF is hydrophobic and the contact angle increases with the increase

of porosity. The corrosion behaviors of the PCFSF in simulated PEM fuel cell environment

prove that the PCFSF without any coating and alloying treatment is not quite competent,

although the Cu corrosion does not critically happen in the tested fuel cell. The resistance

tests demonstrate that the combined total electrical resistance of the composite bipolar

plate is smaller than the graphite plate. The single-cell tests show that the performance of

the fuel cell with the PCFSF flow field is acceptable. The effects of the operating parameters

such as the anode operating pressure and cathode air flow rate are also explored.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction weight and 45% of total cost [4]. It serves for current conduc-

The proton exchange membrane (PEM) fuel cell technology

has drawn considerable attention during past two decades in

the field of hydrogen energy. It has the ability to generate high

power densities under low-temperature conditions. This

makes it potentially attractive for automotive, portable and

distributed power generation [1e3]. A typical PEM fuel cell

consists of the bipolar/end plate and the membrane electrode

assembly (MEA). The bipolar plate (BP) is a key component in

the PEM fuel cell architecture, which accounts for 80% of total

Surface Functional Strucgzhou 510640, People’s Rcn (W. Yuan).ssor T. Nejat Veziroglu. P

tion, heat dissipation, gas flow distribution and residual water

removal, thereby directly influencing the fuel cell perfor-

mance. A large number of R&D activities have concentrated

on the flow field design and fabrication in PEM fuel cells. A

common method is to make grooved channels on the surface

of the bipolar plates through mechanical or electrochemical

methods so as to transport reactant gases uniformly to the

whole electrodes. Fig. 1 illustrates some traditional flow field

patterns usually employed in PEM fuel cells. However, there is

no certain conclusion concerning which type is the best.

ture Manufacturing of Guangdong Higher Education Institutes,epublic of China. Tel./fax: þ86 2085516947.

ublished by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.elsevier.com/locate/he

http://dx.doi.org/10.1016/j.ijhydene.2010.06.101



Fig. 1 e Schematic of various flow field patterns for PEM fuel cells: (a) serpentine; (b) parallel; (c) parallel serpentine; (d)

interdigitated; (e) spiral; (f) porous mesh.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 6 1e9 6 7 79662

Additionally, the fabrication of these complex flow fields on

graphite and metal plates costs too much. This severely

hinders quantity manufacturing and inevitably delays the

market commercialization of PEM fuel cells. Therefore, it is

necessary to develop newmaterials, structures and processes

for bipolar plates and flow fields so as to satisfy various

application requirements of PEM fuel cells.

Compared with graphite, the metallic materials seem to be

more promising candidates when used for BPs due to higher

heat conductivity andmechanical strength, better durability to

shocks and vibration, and superior machinability and cost

effectiveness [5]. For example, aluminum [6,7], titanium [8,9],

copper [10e14] and stainless steel [9,15,16] are all popular

materials used tomakemetallic bipolar plates. In spite of their

advantages mentioned above, most of them have to face

a crucial issuewith regard to thematerial corrosion in the acid

and humid environment of PEM fuel cells. They are likely to

suffer dissolution when operating at a temperature of around

80 �C and a pH of 2e4 [17]. When they are corroded, the

producedmetal ions tend to poison the catalysts and even the

polymer membrane. Besides, the metal-oxide layer may grow

on the material surface, which increases the interfacial elec-

trical resistancebetween the current collector andneighboring

components. These two factors both possibly deteriorate the

performance of PEM fuel cells. Therefore, protective pretreat-

ments suchas surfacecoatingandalloyingareusually adopted

to avoid or at least alleviate the material corrosion.

In recent years, a new trend is worth noting that the porous

metals (e.g. stainless steel foam and nickel foam) have

exhibited great feasibility to make current collectors and gas

diffusion layers in PEM fuel cells. According to different fabri-

cation methods, the porous metals can be mainly classified

into following types: the powder/fiber sintered, mold casted

and electro/vapor deposited (i.e. metal foams), which are

widely used for filtration and separation, heat exchange,

catalyst support, fluid flow control and so on [18]. Nowadays,

they are regarded as potential alternative materials for

construction of PEM fuel cells because of their excellent

advantages in weight, current conductivity, self-support

ability and specific surface area, thereby attracting increasing

attentions of the fuel cell community [19e28]. There have been

published reports studying on the porous flow field models in

PEM fuel cells, although the related knowledge is still limited.

Hontanon et al. [19] focused on optimization of the gas flow

distribution in PEM fuel cells by using the FLUENT software

package. They compared the porous medium with traditional

groovedflowfield, proving that thepermeability of the gasflow

distributor is a key parameter affecting consumption of the

reactant gases in the electrodes. The simulation results

demonstrated that the porous medium produces better

performance than the grooved flow channels because the

permeability reduction benefits utilization of the reactant

gases, especially on the porous anode side. Kumar and Reddy

[20] developed a three-dimensional steady-state model to

investigate the PEM fuel cell, which provided an efficient

solution forporousmaterialswithdifferentpermeability. They

usedmetal foamas the gas distributionmedium instead of the

widely used rectangular channels and meanwhile prepared



i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 6 6 1e9 6 7 7 9663

amulti-parallel flowfield for comparison. They also confirmed

that the permeability has prominent effect on the cell perfor-

mance. This result is consistent with Ref. [19]. Based on above

review, a general conclusion can be drawn that the fuel cell

performance increases with a decrease in the permeability of

the flow field. This phenomenon may be explained by the

increase of the pressure drop which will enhance the mass

transfer from the flow field to the electrode-membrane reac-

tion interface through forced convection. In this regard, the

traditional machined flow field seems less competent because

it cannot reduce the permeability value below 10�8 m2 due to

machining limitation of the micro channels [20].

Murphyetal. [21]proposeda low-costapproach toassemble

light-weigh PEM fuel cell stacks. Their core idea was to use

expanded metals or metal foams to make the flow field,

forming a combined bipolar platewith the polymer cell frames

integrated. They claimed that this new solution was able to

remarkably reduce both cost and weight as a result of the

simple fabrication process. They experimentally demon-

strated that the flow field made of porous metal could yield

even flow, and that the three-dimensional feature of the

porous materials could prevent formation of a laminar flow

regime, where the gas flow itself would develop a concentra-

tion gradient and deprive the electrodes of sufficient reactive

gas. Kumar and Reddy [22] also performed single-cell experi-

ments to validate their rationale developed in Ref. [20]. The

results showed that the best fuel cell performance was

obtainedwhenNieCrmetal foamwas used, followed by SS316

metal foam, multi-parallel flow field and carbon cloth. This

trend effectively strengthened the modeling result they

previously published [20]. Hottinen et al. [23] evaluated the

performance of a PEM fuel cell which used the titanium sinter

material as the gas diffusion backing (GDB). Their study aimed

at reducing the high contact resistance between the MEA and

the titanium sinter with platinum and carbon coatings on its

surface.Zhanget al. [24] reportedanovel gasdiffusionmedium

with controllable porosity. They prepared a 12.5 mmcopper foil

as theGDLbymeansofMEMStechnologyandcompared itwith

the conventional carbon GDL. The results showed that the cell

performance was improved due to increased in-plane mass

transport. Liu et al. [25] adopted a stainless steel sintered fiber

felt as the GDB in an air-breathing direct methanol fuel cell

(DMFC). Compared with the conventional carbon paper, this

GDB pattern helps increase the power density of the fuel cell

because of its higher electronical conductivity and hydrophi-

licity. Chen and Zhao [26] tried to fabricate and test current

collectors utilizing the porous metal foam made from NieCr

alloy for a passive DMFC. The results suggested that the fuel

cell with porous current collectors yielded higher performance

in a stable manner than with perforated-plate current collec-

tors. This interesting behavior could be attributed to a larger

specific transport area, a lower effective thermal conductivity

and faster water removal as a result of the capillary action.

Arisetty et al. [27] conducted experimental studies on DMFCs

incorporating a metal foam flow field. The authors chiefly

focused on the effects of the physical parameters (i.e. pore size

and density) of aluminum metal foam and the operating

parameters (i.e. concentration and flow rate) of methanol

feeding on the polarization behavior and power output. Shudo

and Suzuki [28] employed a corrosion-resisting porous

stainless steel as the flow field used for a DMFC. They ascribed

the improved cell performance to its high porosity around 90%

that benefits both the reactant supply and product removal. As

enumerated above, these literatures greatly illuminate the

feasibility of using porous metals as the flow field in PEM fuel

cells, and fully affirm their advantages in cell performance

optimization.

As aforementioned, the metallic bipolar plate must effec-

tively handle the two important issues such as corrosion

resistance and electrical conductivity. It is known that the

copper has poor corrosion resistance but good electrical and

thermal conductivity. This fact seems to be an intractable

conflict when the copper is used as BPs in PEM fuel cells. This

may explain why the copper is seldom considered as a BP

material. In general, surface coating or alloying treatment is

necessary for copper and other corrodible metals to meet the

functional demands of BPmaterials. Nikam and Reddy [10e12]

solved this issue by using a coppereberyllium alloy (C-17200)

and successfully fabricated channeled and corrugated BPs for

PEM fuel cell application. They reported that this alloy was

a competent candidate material for BPs, which was proved to

have good corrosion resistance in simulated PEM fuel cell

environment. Lee et al. [13] tested the chemical behavior and

electrical conductivity of the Cue5.3 (wt.%) Cr alloy for

possible application in PEM fuel cells. They found that the

formation of Crenitride improved the corrosion resistance

especially in an anode environment and the electrical

conductivity was acceptable without any recession after

polarization. Hsieh et al. [14] made the micro-fabricated

copper BPs through the electroforming method and per-

formed the polarization tests. They did not give out more

details about the corrosion property of the copper material,

although they claimed that it was suitable to produce BPs in

PEM fuel cells.

As summarized above, the previous work has made

constructive contribution to the potential application of

porous metals in PEM fuel cells and facilitated further studies

on this promising solution for flow field design. However, few

efforts were made with respect to the porous copper used in

PEM fuel cells, although it hasmuch favorable properties such

as higher electrical and thermal conductivity, better ductility

andmanufacturability, andmassive production. In this study,

a novel porous flow field of the PEM fuel cell is presented,

which is made of the porous copper fiber sintered felt (PCFSF).

It can be regarded as a multi-functional material with opti-

mized structures serving for flow distribution and mass

transfer enhancement. A systematical investigation is carried

out to explore a series of concerned issues including fabrica-

tion process, morphological description, functional charac-

terization and application in PEM fuel cells. Several important

physical indexes such as wettability, electrical conductivity

and corrosion resistance, are emphatically analyzed.

2. Experimental

2.1. Fabrication of the PCFSF

The metal fibers can be used to produce porous metal mate-

rials partially or entirely instead of metal powders, which is



Fig. 2 e Copper fiber fabrication usingmulti-tooth cutter: (a) photograph of the copper fiber cutting on the lathe; (b) schematic

of the multi-tooth cutter.


generally called metal fiber metallurgy. A common process of

this technique includes fiber drawing, felting and sintering. In

this study, the copper fibers are made by the cutting method

developed in our laboratory [29,30], which is considered to be

simple, cost effective and controllable. A special multi-tooth

cutter was prepared through wire-cutting and employed for

fiber fabrication, as shown in Fig. 2.

A copper rod was firstly cleaned and then fitted on the

C6132A lathe. After enough fibers were manufactured, they

were categorized and mixed uniformly according to certain

length, diameter and aspect ratio. Then the fibers were spread

in the groove of a stainless steel mold plate with Al2O3

powders filled simultaneously. Later, the cover plate was

installed and fastened using M8 screw bolts. The detailed

setup configuration is shown in Fig. 3. Subsequently, the

whole assemblywas sealed and sintered in a sintering furnace

with a programmable temperature controller for 30e60 min at

850e950 �C. The sintering process depended on the tempera-

ture level by stages. When the temperature was below 800 �C,the heating rate was kept at 300 �C/h; and when the temper-

ature was above 800 �C, the heating rate was reduced to

200 �C/h. The furnace chamber was purged by nitrogen to get

rid of the air and then filled with hydrogen to maintain

a reductive atmosphere. The pressure of the hydrogen in the

furnace chamber was kept at 0.3 MPa.When the sintering was

finished, the sample was taken out and cooled to room

temperature. Then the Al2O3 powderswerewashed away, and

the PCFSF with a similar size to the groove was finally

obtained.

Fig. 3 e Schematic of the assembly for PCFSF sintering.

2.2. Characterization of the PCFSF

2.2.1. Morphological analysisThemorphological features of the PCFSFwere inspected using

the scanning electron microscope (SEM) technique. The

complex intertexture of the solid phase and holes were

magnified to characterize the detailed microstructures that

closely influence the physical characteristics of the PCFSF.

The relationship between the processing parameters and

surface formation of the PCFSF was also investigated.

2.2.2. Porosity, pore diameter and permeabilityThe porous metal materials are treated as a kind of complex

consisting of solid phases and holes. Its porosity is defined as

a key parameter related to various physical quantities such as

electrical/thermal conductivity, optical/acoustical perfor-

mance, tensile intensity and compressibility. Here, it can be

expressed by the percentage of the interspace volume in the

whole volume:

3 ¼ Vk

V(1)

where Vk and V are the volumes of the holes and the whole

PCFSF, respectively.

In the present study, the weight-volume method was used

to determine the porosity of the PCFSF. Three samples were

cautiously prepared without deforming the original porous

structure. Then they were weighted using an analytical

balance. The volume value could be acquired averagely from



Fig. 4 e The relationship between wettability and contact

angle (q).


the results of repeated measurements. Thus, the porosity of

the PCFSF can be deducted as:

3 ¼�1� M

Vars

�� 100% (2)

where M is the mass of the PCFSF; rs is the density of the

substrate material of solid copper; Va is the average value of

the measured volume.

Besides the porosity, the average pore diameter is also an

important structural parameter for the PCFSF. The relation-

ship between these two physical quantities can be calculated

by the bundle-of-capillary model. It is assumed that the pores

in the PCFSF are all parallel cylindrical capillary tubes. Thus,

the average pore diameter of the PCFSF can be computed by

the following equation:

da ¼ 3df=ð1� 3Þ (3)

where df is the fiber diameter. In this study, it is approximately

100 mm.

However, the above method only yields a theoretical value

which is far from the real condition. Thus, the average pore

diameter is usually obtained using the statistical method. In

this study, the average pore diameter was calculated using Eq.

(4) presented by Wang et al. [31]:

da ¼ 6:15df33:35 (4)

Since the PCFSF was applied as the flow field in PEM fuel

cells, its permeability has to be evaluated and optimized for

fluid distribution and penetration. Here, the Darcy’s law can

be used to describe the fluidic flow through the PCFSF:

DPh

¼ m

Ku (5)

where DP is the hydrostatic pressure difference across the

PCFSF; h is the thickness of the PCFSF; m is the fluid dynamic

viscosity; u is the average flow rate in unit cross-sectional

area; and K is the permeability coefficient which is closely

related with the porosity and average pore size. The value of K

is a very important indicator to distinguish the permeability

difference between the PCFSF and grooved gas flow field.

According to the KozenyeCarman equation, in the porous

material, K can be defined as following form:

K ¼ C33=S2v (6)

where C¼ 0.2 for the irregular metal fiber sintered felt. And Svis the specific surface area of the pore, which can be expressed

as:

Sv ¼ 43=da (7)

2.3. Wettability test

Since the PCFSF serves as the flow field and GDL in PEM fuel

cells, its wettability becomes an important index that is

closely related to the water management mechanisms

[32e34]. In this research, the wettability tests were carried out

to verify the characteristics of hydrophobicity or hydrophi-

licity. A group of samples obtained from the PCFSFs with

different porosities were used to explore the influence of the

porosity on wettability degree. For the purpose of microscopic

insight, the quantitative measurement of the surface contact

angle (i.e. wetting angle, q) was performed on the video optical

platform, OCA20 (Dataphysics, Inc.) by using the sessile

dropletmethod [35,36]. The contact angle is the angle at which

a liquid/vapor interface meets the solid surface. It is a specific

parameter for any given system and is determined by the

interactions across the three interfaces. The relationship

betweenwettability and the value of q is shown in Fig. 4.When

q< 90�, the material is hydrophilic; contrarily when q> 90�,the material is hydrophobic.

2.4. Corrosion resistance test

The porous metal materials used under harsh operating

conditions have to be corrosion-resistant especially in acid

environment. They are usually damaged internally by corro-

sion behavior due to existence of a lot of pores. In this study,

corrosion experiment for the PCFSF was conducted in simu-

lated PEM fuel cell environment. The polarization tests were

carried out in a self-designed electrochemical cell, which was

filled with 200 ml acid solution of 0.05 M SO42� and 2 ppm F� in

deionized water. The three-electrode system was set up with

a platinum foil acting as the counter electrode (CE) and

a saturated calomel electrode (SCE) acting as the reference

electrode (RE). A cleaned sample of a copper foil

(10 mm� 10 mm� 1 mm) was prepared as the working elec-

trode (WE), which had experienced the same high-tempera-

ture process in the furnace as the PCFSF. It was connected

with a standard copper wire through the welding method. A



Fig. 5 e Comparison between the PCFSF porous flow field

and the single-channel serpentine flow field.

Table 1 e The detailed dimensional parameters of theporous flow field plate and serpentine flow field plate.

Dimensionalparameters

Porous flowfield plate

Serpentine flowfield plate

Materials PCFSF & 316 L SS Graphite

Size (mm) 70� 27� 4 70� 27� 4

Channel

depth (mm)

1 1

Channel

width (mm)

1.7

Rib width (mm) 0.5


Luggin capillary was filled with saturated KCl solution and

encapsulated with agar, working as a salt bridge to maintain

a constant distance of 2 mm between the RE and WE. In order

to simulate the anodic and cathodic environments in PEM fuel

cells, the solution was bubbled with pure hydrogen and air,

respectively. The gas flowwas regulated by a flowmeter before

entering the gas feeding inlet of the electrochemical cell. The

operating temperature was automatically controlled using

water bath. Before starting each test, pre-bubbling was per-

formed for about 30 min. The AUTOLAB (PGSTAT302) elec-

trochemical workstation was utilized to conduct the

potentiodynamic tests. All the samples were stabilized in the

simulated solution at the open circuit potential (OCP) for

about 5 min every time before measurement. In the poten-

tiodynamic tests, the potential increases with a scan rate of

1 mV/s from �1 V to þ1.5 V.

2.5. Preparation of single PEM fuel cell

2.5.1. MEA fabricationThe MEA was fabricated with an active area of 11 cm2

(5.5 cm� 2 cm) and catalyst loadings of 0.8 mgPt/cm2 on both

the anode and the cathode sides. Nafion� membranes of NRE-

212 (Dupont, Inc.) and Toray� Carbon papers were used as the

proton exchange membrane and the backing layer (BL), respec-

tively. Firstly, the membrane was boiled in 3% H2O2 for 1 h and

then boiled in 0.5 M H2SO4 solution to activate the membrane.

Afterward, themembranewaspurifiedinboileddeionizedwater

for 1 h to get rid of the residual sulfuric acid. A mixed PTFE/C

(Vulcan XC72, E-Tec, Inc.) layer was bonded upon the wet-

proofed BL surface to form a complete GDL. The catalyst ink for

the electrodes was prepared by mixing the catalyst powders

(40 wt.% Pt/C, E-Tec, Inc.), Nafion� solution, and isopropyl

alcohol together. Then the prepared catalyst inkwas sprayedon

the PTFE/C layer surface by using a sprayingmachine and dried

in a vacuum oven, forming a so-called gas diffusion electrode

(GDE). Subsequently, the Nafion� solution was again used to

brush the CL in order to realize effective contact between the

catalyst and the polymer electrolyte. Finally, a complete MEA

can be obtained by hot-pressing the pretreated membrane and

the electrodes together for 3 min at 120 �C.

2.5.2. Bipolar plate and flow field designThe PCFSF was used as the porous flow field in a PEM fuel cell

(FC1). To validate the feasibility and effectivity of such

a special structure, a traditional graphite fuel cell (FC2) with

a single-channel serpentine flow field was prepared for

comparison analysis, as illustrated in Fig. 5. The detailed

geometric parameters of these two flow field plates are listed

in Table 1. In this study, the flow field was machined on one

side of the bipolar plate for single-cell tests.

2.5.3. Single-cell configuration and experimentsFig. 6 describes the detailed configuration of the tested PEM

fuel cell. The PCFSF was embedded in the groove of the SS316

support plate (see Fig. 6(b)). A copper plate with an exerted

wiring terminal served as the current collector and an insu-

lated nylon plate served as the end plate. The connections

between relevant parts were sealed with Teflon gaskets.

During the whole testing, the fuel cell worked in a counter-

flowmode with the gas inlet and outlet both on the same side.

It was operated at the normal temperature without any

external humidification. A schematic diagram of the experi-

mental system for fuel cell test is shown in Fig. 7.

2.6. Electrical resistance measurement

Except for corrosion resistance, the electrical conductivity of

the metallic components included in PEM fuel cells also

deserves much attention. Generally speaking, the level of

electrical resistance (ER) has great effect on the operating

performance and power output of PEM fuel cells. In particular,

the interfacial contact resistance (ICR) between two materials

in PEM fuel cells is regarded as an important criterion for

feasibility evaluation. Wang et al. once presented a useful

method to measure the ICR which was commonly used later

by other groups [16]. However, the ICR only reflects the



Fig. 6 e Schematic of the configuration of a PEM fuel cell with the PCFSF flow field.


contacting quality of two components. A combined value of

both the body resistance (BR) and ICR is more significant in

real application because the BR is always inevitable. Moreover,

a variety of PEM fuel cell components with different materials

and configurations are involved in this study, namely the

PCFSF, SS316 support plate, copper plate, graphite plate, and

carbon paper. Therefore, different from Ref. [16], a new

concept of a combined resistance Rt0 (see Fig. 8) containing one

BR and two ICRs, is proposed to express the related variables

when a new material is chosen to make the bipolar plate and

flow field. This symbolic resistance was used as an objective

function to evaluate the componential electrical property. In

the real testing process, Rt0 can be replaced by a total ER (Rt)

through deductive calculation. The overall experiment was

divided into five groups according to the assembly patterns, as

illustrated in Fig. 9. A series of tests were carried out to directly

measure the value of Rt under a forced compaction and the

results were characterized as functions of the applied pres-

sure. A pressure transducer connected to a digital screen was

used to capture the compaction values. And two polished

copper sheets with four standard wires embedded were used

as clamping plates on both the upper and lower sides. The

expressions of the total resistance (Rt) in each case are

respectively described as follows:



Fig. 7 e Schematic of the experimental system for single-cell tests.


(a) PCFSF/SS316 vs. Carbon paper

The PCFSF flow filed is incompressible because it is placed

in the groove of the support plate. Thus, the SS316 support

platewith the PCFSF together is assumed to be an entire object

representing a composite bipolar plate (CBP) used in this

study. The total body resistance of CBP can be defined as

R1 ¼ RPCFSF þ RSS þ RPCFSF=SS (8)

And the ICR between CBP and carbon paper is

R2 ¼ 1=�1=RPCFSF=C þ 1=RSS=C

�(9)

The total resistance is

Rt�a ¼ 2RCP þ RC þ R1 þ R2 þ RSS=CP þ RC=CP (10)

where the subscripts C and CP denote the carbon paper and

clamping plate, respectively.

(b) Graphite plate vs. Carbon paper

Rt�b ¼ 2RCP þ RC þ RG þ RG=C þ RG=CP þ RC=CP (11)

where the subscript G denotes the graphite plate.

Fig. 8 e Combined electrical resistance with one body

resistance and two interfacial contact resistances.

(c) PCFSF/SS316 vs. Current collector (copper)

R3 ¼ 1=�1=RPCFSF=CP þ 1=R0

SS=CP

�(12)

Rt�c ¼ 2RCP þ RCC þ R1 þ R3 þ RSS=CC þ RCC=CP (13)

where the subscript CC denotes the copper current collector.

(d) Graphite vs. Current collector (copper)

Rt�d ¼ 2RCP þ RCC þ RG þ RG=CP þ RG=CC þ RCC=CP (14)

(e) SS316 plate vs. Carbon paper

Rt�e ¼ 2RCP þ RC þ R0SS þ RSS=CP þ R0

SS=C þ RC=CP (15)

3. Results and discussion

3.1. Processing mechanisms of the PCFSF

Fig. 10(a) shows the SEM image of the single fiber made by the

multi-tooth cutter. The technique used in this study can easily

achieve an appropriate control of the fiber-processing param-

eters such as cutting speed, back-cutting depth and feed rate.

They together directly affect the fiber formation. The cutting

speed is closely related to the internal stresses and morpho-

logical defects because of the higher plasticity of the copper. A

lower cutting speedbenefits continuous collectionof relatively

long copperfiberswithmanycoarsemicrostructures (inmicro/

nano scale) on the surface. But a higher cutting speed easily

brings forth a higher cutting temperature and curly deforma-

tion so that the internal stresses and defects become more

serious. Furthermore, a higher cutting speed tends to induce

vibration during fiber fabrication. Hence, a relatively low

cutting speed (n¼ 180 r/min) was employed to reduce the



Fig. 9 e Experiments of electrical resistance between the involved components in PEM fuel cells.


deformation coefficient andmaintain a relatively steady state.

It is also discovered that the increase of the feed rate ( f ) and

back-cutting depth (ap) results in an increase of the equivalent

fiber diameter. According to the basic principle of cutting

process, the teethdepth (h)must be bigger than the real cutting

depth to promote efficient chip bifurcation.When the feed rate

increases, the cutting depth gets bigger so that the tool teeth

have to be made deeper accordingly. This tends to cause cut

edge crack. Meanwhile, an increase of the feed rate helps

achieve a larger fiber diameter. Therefore, an optimum feed

rate is necessary to create fibers with a required equivalent

diameter and improve the production efficiency. In addition,

the back-cutting depth also affects the fiber diameter because

it is related to the contact length between the multi-tooth

cutter and work piece. For this reason, the feed rate and back-

cutting depth were kept at appropriate values ( f¼ 0.10 mm/r,

ap¼ 1.25 mm)during thewholemachiningprocess.With these

parameters prescribed, the produced fibers have an average

diameter of less than 100 mm. According to Eqs. (5) and (6), the

fiber diameter interacts on theporosity andpermeability of the

PCFSF.When the porosity is certain, the average pore diameter

increases with a growth of the fiber diameter so that the

permeability also increases. However, the fibers with an

excessively big diameter are not suitable for fiber arrangement

andsintering. Conversely,when thefiberdiameter is too small,

the permeability of the PCFSF is quite limited so that it cannot

beused as theflowfield in PEM fuel cells. Therefore, an average

fiber diameter of 100 mm obtained in this study is found to be

suitable for PCFSF fabrication.

As for the sintering process, there are also a series of tech-

nical factors,namely thesintering temperature, sintering time,

sintering pressure and so forth. Fig. 10 compares the shape of

the single fiber before and after sintering. It is obvious that the

fiberedgebecomesa little smootherafter the sinteringprocess.

This trend can be explained by the material migration

behavior. Before sintering, the fibers have plenty of salient

burrs enlarging the surface area of the fiber bundles. So, the

fibers have higher surface free energy under this unsteady

condition. The sintering process forces the surfacematerial of

the fibers tomigrate so as to reduce the surface area. The fibers

finally reach a relatively steady state with lower surface

energy. Generally, a higher sintering temperature benefits

formationof the sintering joints (see Fig. 11) betweenfibersdue

to thematerialmigrationmechanisms. On the other hand, the

copper fiber felts cannot be sintered at a lower temperature

because the fibers contact each other loosely without proper

melting condition. The sintering time is determined by the

sintering temperature.When the sintering temperature is kept

at a relatively high level, the sintering time should not be long.

But when the sintering temperature is lower, the processing

time should be prolonged to satisfy the sintering requirement.

Additionally, the sintering pressure helps to accelerate the

forming velocity of the PCFSF. Therefore, the sintering

parameters used in this study are optimized to facilitate better

formation of the fiber-to-fibermetallurgy junction. To sumup,

the processing parameters have great influence on the micro-

structures of the PCFSF, thereby affecting the functional

characteristics (e.g. flow distribution, fluidic permeation,

electrical conduction) in practical applications.

3.2. Morphological description of the PCFSF

Fig. 11 shows the SEM images of the PCFSF (90% porosity)

sintered for 1 h at 900 �C. It is shown that the copper fibers



Fig. 10 e SEM photos of the single fiber: (a) before sintering;

(b) after sintering.


intervene stochastically and bond with each other, forming

many contact points among the fiber clusters. This layout can

be regarded as a type of three-dimensional network micro-

structure with countless open pores, which is quite different

from the metal foams. All of the pores are connected so that

the fluidic phase can pass through. Thus, there are only

Fig. 11 e SEM photo and structu

through holes inside the PCFSF, without blind holes formed.

The solid phase constitutes a skeleton of the network struc-

ture and the non-solid phase makes up the porous space.

However, the formed holes are not regularly and uniformly

distributed because the copper fibers cross and overlap each

other in a chaotic mode. It is usually acknowledged that the

structural uniformity depends on the regulation of fiber

layers. More fiber layers help achieve more uniform porous

structures. As shown in Fig. 11, there exist three kinds of

“path” in the PCFSF architecture, which can be defined as

a connected string of fiber segments [37]:

(a) “branchebranch” (BeB) path that connects to joints at

each end;

(b) “brancheleaf” (BeL) path that connects to a joint at only

one end leaving the other end free;

(c) “leafeleaf” (LeL) path that stands for an isolated fiber.

These paths combine to strengthen the mechanical and

physical properties of the PCFSF with different fiber length

and orientation. Compared with the sintered powders, the

sintered fiber felts are able to produce a large range of pore

diameters and porosities. Such an irregular fiber layout can be

treated as a fractal profile which benefits a deep under-

standing of the internal heat and mass transfer mechanisms

in porous medium [38e41]. It is also noticed that the edges of

copper fibers havemany coarse antler-like burrs revealing the

cutting trace during themulti-toothmachining process. These

microstructures increase both the specific surface area of the

internal pores and the surface roughness, which have great

influence on the surface free energy, capillary intrusion,

wetting behavior and so on. In addition, the number of joints

determines the effective contact between copper fibers. It is

also an important parameter that improves the electrical

conductivity by constructing a conductive skeleton. In a word,

the morphology of the PCFSF is closely related to the func-

tional characteristics and practical applications.

3.3. Wettability characterization of the PCFSF

Thewettabilityof thePCFSF is considered tobeakeyparameter

when it isusedas theflowfieldandGDLmaterial,whichgreatly

ral schematic of the PCFSF.



Fig. 12 e Contact angles of the different PCFSF samples

with porosities changing from 70% to 90%.


influences the efficiency of water management in PEM fuel

cells [42,43]. It is related to the interfacial tensionwhich can be

identified by the contact angle. The relationship between

interfacial tension and contact angle is given by Young’s law

ss;g ¼ ss;l þ sl;gcos q (16)

where s is the interfacial tension.

Fig. 13 e Amplified photos of the contact angles o

According to the surface thermodynamics, the free energy

drop per unit area of the gaseliquidesolid system is

�DG ¼ ss;g þ sl;g � ss;l (17)

where sl,g is a constant for water.

The above two equations combine to interpret the effects

of contact angles on the wetting conditions. Besides, the

capillary degree in porous media deserves considerable

attention, which is defined as the difference between the

wetting and non-wetting phase pressures:

Pc ¼ Pwp � Pnwp (18)

It relates the solideliquid interface to surface conditions

and pore geometries via the modified YoungeLaplace equa-

tion [43]:

Pc ¼ 4s cos qdp

(19)

Fig. 12 illustrates the measured contact angles obtained

fromfive sampleswith various porosities from 70% to 90%. For

each sample, inhomogeneity of the surface microstructure is

taken into account. So three sub-samples picked from

different parts are used for each contact angle measurement.

The results show that the contact angles are all far beyond 90�

between 112� and 136.4�, showing evident hydrophobicity of

the PCFSF. According to Eqs. (21) and (22), when q> 90�, DG> 0.

This indicates that the wetting behavior does not occur, and

the liquid tends to reduce the solideliquid interfacial area. In

the obtuse angle region, a bigger contact angle leads to worse

wettability. Meanwhile, according to Eq. (24), the capillary

pressure acts negatively when q> 90�. This means that it is

more difficult for the liquid to wet the objective material. It is

also obvious that the contact angle increaseswith the increase

of porosity. This trend can be explained by the fact that an

enlarged pore size due to the higher porosity worsens its

capillary ability, thereby making it less possible for the liquid

to infiltrate through the porous material.

Fig. 13 displays the contact angles of the PCFSF with 90%

porosity. There is just a slight difference among the contact

angles of the three sub-samples. This is not absolutely caused

by the inhomogeneous surface structure, because the violent

adsorption of immersing water may also lead to measurement

errors occasionally. The detailed values are listed in Table 2.

The maximum value is q¼ 136.4� while the minimum value is

f the three PCFSF samples with 90% porosity.



Table 2 e The measurement results of the contact anglesof the three PCFSF samples with 90% porosity.

Code Contact angle (�)

Sample a 131.8

Sample b 136.4

Sample c 134.1


q¼ 131.8�. The water droplet shows a spherical profile indi-

cating that the PCFSF is hydrophobicwith lower surface energy.

The merits of this special property lie in following two points.

On the one hand, lower surface energy of the PCFSF helps to

prevent the electrodes from dehydration through effective

control of the water transfer and removal rates, especially

when such a small-scale PEM fuel cell operates without

external humidification. For most of the time, the fuel cell runs

at a medium current density so that it does not produce too

much byproduct water. Thus, the hydrophobic PCFSF contrib-

utes to water conservation in the electrodes and membranes.

On the other hand, even if the fuel cell operates under a high

current load, the PCFSF with hydrophobic characteristics is

helpful to remove accumulated water [44,45]. Thus, it helps

protect the porous flow field from being blocked by residual

water so that the gas flow is able to successfully reach the

catalyst layer. These two advantages together improve the

water and gas management in PEM fuel cells. Additionally, in

view of the practical design and cost control, the auxiliary

system could be simplified without the complex humidifying

equipments. Most importantly, the fabrication method used in

this study can easily realize an effective control of the material

wettability by regulating the porosity instead of the traditional

process of water-proof treatment. For example, the GDLs are

conventionally made hydrophobic using PTFE that increases

the weight by 5e30% and also reduces the electronic and

thermal conductivities. Therefore, the PCFSF shows potential

feasibility to be used in PEM fuel cells due to its advantages in

both functional performance and control flexibility.

3.4. Application of the PCFSF as porous flow field in PEMfuel cells

3.4.1. Corrosion resistance of the PCFSF in simulated PEMfuel cell environmentSince the present work focuses on the feasibility of the PCFSF

to be used as the flow field in PEM fuel cells, the corrosion

problemsmust be investigated in order to explore an effective

way to protect the material being corroded. During the

corrosion test, the dissolution of the copper sample is

observed. The overall reaction can be formulated by following

equations:

CuþO2þ 2Hþ¼Cu2þþH2O (20)

Since Cu is nobler than Hþ in the electromotive series,

a cathodic reaction other than displacement of Hþ accounts

for metal dissolution in terms of O2 reduction [46]:

O2þ 4Hþþ 4e�¼ 2H2O (21)

In fact, the detailed reaction process can be divided into

following three steps (Eqs. (27)e(29)). They together affect the

current variation by changing the concentrations of Hþ and

O2. At the beginning of the dissolution on the anode side, the

reaction is mainly controlled by:

Cu¼Cu2þþ 2e� (22)

Subsequently, the dissolution is controlled by the following

equation [46]:

4CuþþO2þ 4Hþ¼ 4Cu2þþ 2H2O (23)

Thus, the concentration of Cu2þ increases and the current

density climbs up due to the auto-catalytic reaction given

below [46]:

CuþCu2þ¼ 2Cuþ (24)

Fig. 14 shows the polarization curves of the copper samples

in simulated anodic environment of PEM fuel cells. It is

observed that there is no activationepassivation turning

point, which means the material gets corroded continuously

without any passive film formed on the sample surface.

Compared with the coppereberyllium alloy C-17200 reported

in Ref. [12], the polarization curves of the PCFSF have the

similar shape to C-17200. However, the current density of the

PCFSF in simulated PEM fuel cell solution with 0.05 M H2SO4 is

much higher than the copper alloy, because the PCFSF is

mainly made of pure copper without any coating or alloying

treatment. The copper is directly exposed to the sulfuric acid

and easily dissolved during the anodic reaction. Fig. 15 shows

the cathodic behaviors of the copper samples in simulated

oxidizing environment of PEM fuel cells. It is also noticed that

the material does not experience an obvious activeepassive

transition. However, compared with the anodic behavior, the

cathodic polarization curves have two distinct features. On

the one hand, there exists serious vibration of the current

density in the negative low-potential regions. This can be

explained by the violent reaction on the surface of pure copper

especially when the solution is bubbled with air/oxygen. The

oxide film forms and dissolves repeatedly, showing an

unstable feature of the current density. Another possible

reason may be the presence of porosity in the corrosion

products as claimed in Ref. [12]. On the other hand, the current

density experiences a sudden jump in the positive high-

potential regions. Different from the anodic manner, the

current density here takes two steps to reach the level of

around 10�1 A cm�2. This pronounced phenomenon is due to

the release of slow passivation after a gradual linear rise of

current density. During the reaction process in an oxidizing

environment, more and more oxygen-rich regions near the

electrode emerge so that the kinetic behaviors are quite

different from the non-oxidizing activity. In single-cell tests

(see Section 3.4.3), the cell performance is not inhibited seri-

ously by the corrosion effects because the PCFSF does not

directly touch the membrane and catalyst layer, and the

testing period does not last very long. Another possible reason

may be the room-temperature operation that keeps the

corrosion rate at a lower level, alleviating the influence of



Fig. 16 e Curves of total electrical resistances vs.

compaction force for the situations (a), (b) and (e)

mentioned in Section 2.6. The inset shows the detailed

resistance when the compaction force is between 1.2 and

1.8 MPa.

Fig. 14 e Anodic behavior of the PCFSF in simulated PEM

fuel cell environment at 65 �C purged with hydrogen. The

anode operating potential is marked.


corrosion behavior. However, in view of long-term operation,

the copper corrosion is inevitable without stable passivation

regions formed in whatever the anodic and cathodic envi-

ronments. Therefore, it is necessary to take measures to

develop corrosion-resistant methods when the PCFSF is used

in real environment of PEM fuel cells.

3.4.2. Electrical resistance of bipolar plate materials in PEMfuel cellsThe curves of the total electrical resistance (Rt) vs. compaction

force (Fc) under different assembly conditions are illustrated in

Figs. 16 and 17. It is evident that during the initial stage, the

total resistance drops sharply with a slow increase of

compaction force. When Fc is around 0.5 MPa, Rt decreases

more slowly than before. When Fc rises to 1.5 MPa, Rt gradually

approaches a horizontal line at a relatively steady value. This

dynamic trend can be explained by the variation of interfacial

contact area among the tested components. The change of

Fig. 15 e Cathodic behavior of the PCFSF in simulated PEM

fuel cell environment at 65 �C purged with air. The cathode

operating potential is marked.

body resistance due to slight deformation is not considered. At

the beginning, the compaction force is unavailable so that the

components touch each other naturally with very small

contact area. Subsequently, the compaction force continuously

increases, the contact area gets enhanced, forming more and

more conductive paths. When the force is big enough to make

the components contact each other completely, the contact

area arrives at a maximum value so that the total resistance

will not increase further. Thus, the value of Rt actually reveals

the involved contact resistances in the experiments.

As mentioned above, the values of Rt-a and Rt-b can

approximately represent the values of Rt-a0 and Rt-b

0, which are

more significant in this study. It can be judged fromFig. 16 that

the order of the three quantities is Rt-a< Rt-e< Rt-b. Meanwhile,

the inset plot in Fig. 16 magnifies the details of total electrical

Fig. 17 e Curves of total electrical resistance vs. compaction

force for the situations (c) and (d) mentioned in Section 2.6.

The inset shows the detailed resistance when the

compaction force is between 1.2 and 1.8 MPa.




resistances when the compaction force is around 1.5 MPa. It is

obvious that even during the steady stage, Rt-a keeps about

10 mU lower than Rt-b. This result illuminates the fact that the

CBP with the PCFSF does not bring forth more resistance than

traditional graphite bipolar plate. On the contrary, it is able to

reduce both the body resistance and interfacial contact resis-

tance efficiently so as to improve the performance of PEM fuel

cells. Furthermore, by comparison between Rt-a and Rt-e, it can

be seen that the PCFSF flow field is not a barrier for current

collection because the effective contacts among fibers can

Fig. 18 e Comparison of the polarization curves between

the PCFSF fuel cell and the traditional graphite fuel cell.

Fig. 19 e Effects of the anode operating pressure and

cathode flow rate on the performance of PEM fuel cell with

the PCFSF flow field.

guarantee a higher electrical conductivity. In addition, Fig. 17

suggests that the CBP also exhibits better electrical perfor-

mance than graphite when it is connected with the current

collector. Furthermore, it can be inferred that the electrical

resistance is supposed to decrease to amuch lower level when

we use the SS316L support plate itself for current collection

instead of the copper current collector.

3.4.3. Polarization behavior of the PEM fuel cell with thePCFSF flow fieldThe performance of PEM fuel cells can be evaluated by using

polarization curves including the current vs. voltage (IeV) and

current vs. power (IeP) curves. Fig. 18 illustrates the polariza-

tion curves of the laboratory-scale single cell under both inac-

tivated and activated conditions, respectively. The range of

current density is typically chosen from 100 to 700 mAcm�2.

This region accounts for the ohmic losses, showing a linear

drop of potentials. The effects of the bipolar plate with the

PCFSF flow filed on cell performance are compared with

a traditional graphite fuel cell. It is evident that the cell

performance after activation is much higher than the inacti-

vated one. Before activation (see Fig. 18(a)), the maximum

power densities of FC1 and FC2 are 102.2 mWcm�2 and

208.6 mWcm�2, respectively. After activation, the maximum




power densities of FC1 and FC2 increase to 434 mWcm�2 and

429.8 mWcm�2, respectively (see Fig. 18(c)). This prominent

rejuvenation of the cell performance can be explained by the

complex evolving process of water and gas transfer as well as

the electron and proton transport. At the beginning of the

electrochemical reaction, the polymer electrolyte membrane is

not well hydrated for lack of enough water, especially at lower

current densities. It is known that in a PEM fuel cell, the water

molecules are driven in following three ways: the electro-

osmotic drag from the anode to the cathode due to potential

difference, the back diffusion from the anode to the cathode

due to concentration gradient and the pressure differences

between the cathode and anode. The activation process

accelerates these activities and finally reaches a water balance

in the PEM fuel cell. Meanwhile, at the three-phase interface of

the electrodes, the activation process improves mass transfer

by generating more channels for gas diffusion. In addition, the

activation process particularly benefits formation of the

transport networks for electrons and protons in the MEA so

that the ohmic losses can be reduced. The cell voltages in Fig. 18

(b) and (c) decrease more slowly than Fig. 18(a) with the

increase of the current density. This indicates that the activa-

tion process indeed promotes reduction of the ohmic losses.

Fig. 18 also compares the cell performance between FC1 and

FC2. Before activation, the traditional FC2 shows a higher

performance than FC1 with a porous flow field made of the

PCFSF. Comparedwith themachined serpentine channels, the

PCFSF seems less able to deliver the reactants to GDLs and CLs

at the beginning of the testing operation, because it takesmore

time to feed enough gases to the three-phase reaction area due

to a lower diffusion rate. However, this is only a temporary

phenomenon. With the reaction going on, the diffusion

networks in GDLs and CLs gradually build up so that the fuel

cell output can keeps at a relatively higher level. Thus, the

polarization curve of FC1 draws close to FC2, as shown in Fig. 18

(b). Subsequently, the power output of FC1 continues to

improve and eventually reaches a maximum value of

434 mWcm�2 while FC2 still keeps at a relatively stable level as

before (see Fig. 18(c)). This can be interpreted by the enhanced

mass transfer in the porous flow field. The PCFSF with a lower

permeability tends to cause forced convection by which the

reactant gas can easily arrive at the electrode-membrane

reaction interface. This mechanism facilitates efficient utili-

zation of the reactants and improves the performance of PEM

fuel cells. This result agrees well with that presented by other

research groups [19,20,22]. From Eqs. (6) and (7), it can be seen

that the permeability of the porous material is determined by

porosity and pore geometry (i.e. average pore diameter). In this

study, the permeability of 90% PCFSF is calculated to be

2.1� 10�9 m2 which is quite smaller than themagnitude order

of 10�8. Therefore, it canbepredicted that FC1 is able toproduce

anevenhigherperformanceby furtheroptimizationof theflow

fieldpermeability, although thefinal output of FC1 is just a little

higher than FC2. However, as for the traditionalmachined flow

field, a decrease of permeability cannot be easily achieved by

minimizing the feature size on account of processing limita-

tion. This test proves that the PCFSF flow field yields better

performances in mass transfer than the traditional grooved

channels. This is consistent with the conclusion made by

Kumar and Reddy [20].

Fig. 19 shows the effects of anode operating pressure and

cathode flow rate on the performance of PEM fuel cells with

the PCFSF flow field. It is shown in Fig. 19(a) that the PCFSF fuel

cell is insensitive to the change of anode operating pressure,

because there is almost no difference between the two cases

when P1¼ 0.04 MPa and P2¼ 0.12 MPa. Theoretically speaking,

an increase of operating pressure results in the improvement

of cell performance because themass transport resistance can

be reduced. However, the PCFSF flow field with a lower

permeability here contributes to an increase of pressure drop

that forces the gas to diffuse from the flow field to the reaction

area. Thus, it can be inferred that the porous structure itself

may play a more important role in gas distribution and

diffusion than the high-pressure operation. Moreover, the

tested fuel cell is not fed with humidified gas. Under this

condition, the high-pressure operation at the anode may dry

out the membrane due to a pressure gradient. This may

depress the cell performance to a great extent. Therefore, the

negative water transfer from the anode to the cathode side

may offset the contribution of high-pressure operation to the

electrode kinetics. The result shows that it is possible to

operate the fuel cell at a lower pressure when the PCFSF is

used. Therefore, the PCFSF has great advantages in system

efficiency and running cost for the engineering practice.

Fig. 19(b) reveals the effects of air flow rate on the perfor-

mance of PEM fuel cells with the PCFSF flow field. On the one

hand, a higher air flow rate leads to a higher oxygen concen-

tration so as to elevate the reaction rate. On the other hand,

a higher air flow rate helps to remove excessive water which

may cause flooding at the cathode. These two factors combine

to improve the cell performance efficiently. However, in this

study, it isworthemphasizing that theair humidification isnot

applied so that the latter mechanism may result in a higher

water removal rate and gradually dry out the membrane. As

shown in Fig. 19(b), a lower air flowrate (V1¼ 0.3 L/min) ismore

beneficial to the PEM fuel cell performance especially at high

current densities. This result may be attributed to the hydro-

phobicity of the PCFSF. Asmentioned in Section 3.3, the PCFSF

is hydrophobic so that the water cannot freely accumulate at

the cathode. Therefore a lower air flow rate facilitates not only

the utilization of oxygen but also themaintenance of adequate

water. Based on this test, it can be concluded that the PCFSF is

competent to be used as a porous flow field in PEM fuel cells

with the air fed at a lower flow rate.

4. Conclusions

A comprehensive study on a novel porous flow field made of

the PCFSF is conducted in order to investigate the feasibility

for this material to be used in the special environment in PEM

fuel cells. A series of experiments were systematically carried

out, which covered the PCFSF fabrication, the structural and

functional characterization, and the application in PEM fuel

cells. The copper fibers can be obtained directly by using

a self-designedmulti-tooth cutter. A lower cutting speed helps

prevent the fibers from formation failure and morphological

defects. A lower feed rate and a smaller back-cutting depth

both reduce the equivalent diameter of the fibers. The




sintering temperature, time and pressure have great effects

on the formation of fiber-to-fiber joints in the PCFSF.

It is found that the PCFSF is hydrophobic and the contact

angle increases with the increase of porosity. This property

benefitswaterandgasmanagement inPEMfuel cells especially

when the reactant gases are not humidified. Meanwhile, it is

proved that the PCFSF without any coating or alloying treat-

ment is not quite competent in corrosion resistance. In view of

long-term operation, the work regarding this critical issue is

now under progress and will be reported deeply in future

papers. It is also demonstrated that the electrical resistance of

the CBP decreases, when compared with the graphite plate.

The polarization tests demonstrate that the performance of

the fuel cellwith thePCFSFflowfiled is comparablewith that of

the graphite-plate fuel cell. The output of the PCFSF fuel cell is

expected to get further improved if the permeability of the

material is better optimized and the corrosion problem is

successfully resolved. In addition, the PCFSF flow field can be

used at lower anodic pressures and cathodic flow rates. This

facilitates both efficiency enhancement and cost reduction of

the fuel cell system. To sum up, the PCFSF is proved to be

a promising material used as the flow field in PEM fuel cells,

which further ignites the feasibility of using the porous flow

field instead of the traditional machined channels.

Acknowledgment

The research was supported by the National Natural Science

Foundation of China (NSFC), projects No. 50930005 and

50805052, the Natural Science Foundation of Guangdong

Province, project No. 07118064. The authors also would like to

acknowledge the Joint-training Program (No. 2009615064)

sponsored by China Scholarship Council and the Doctorate

Dissertation Innovation Funds supported by South China

University of Technology. In addition, the constructive

suggestions by professor Jun Wang in The University of New

South Wales are also gratefully appreciated.

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Feasibility study of porous copper fiber sintered felt

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