J. Yang et al. / Electrochimica Acta 54 (2009) 6300–6305
6301
◦
at 29 C for 7 days. The pH of the medium was adjusted to 6.0–6.2
◦
by 2.5 M NaOH. BC pellicles were purified by soaking in DI at 70 C
for 3 h and then 1 M NaOH in DI at 70 C for 90 min. Samples were
rinsed with DI to pH 7 and stored in refrigerator at 4 C prior to use.
◦
◦
2.2. In situ preparation of Pt/BC nanocomposite membranes
For in situ preparation of platinum nanoparticles in 3D net-
work structure of BC membrane was conducted through liquid
phase chemical deoxidization method. Firstly, the BC pellicles were
cut into small pieces, and comminuted by high speed homog-
enizer. Secondly, the BC homogenates were soaked in a 5 mM
solution of hexachloroplatinic acid (H PtC1 ·6H O) dissolved in
2
6
2
◦
5
0 mM sodium citrate, pH 5.0 and incubated at 40 C for 12 h, the
hexachloroplatinate is not spontaneously reduced inside the cellu-
lose. Thirdly to induce platinum nanoparticles precipitation, the
soaked BC homogenates were then rinsed with DI and reduced
by 1.5 M solution of NaBH4 or HCHO into the cellulose matrix at
◦
4
5 C for 24 h, under vigorous stirring, when the pellicles were
completely black in appearance. This material was termed Pt/BC
nanocomposite. The corresponding samples are denoted as BH-
Pt/BC or HC-Pt/BC. Finally, the obtained platinum/BC composites
were rinsed with DI and freeze dried.
2.3. Chemical modification of BC membranes
In the present study, doping with proton acid or inorganic acid
on the BC pellicles was performed in an attempt to enhance the
capability of proton exchange [13–14]. Modification of matrix was
prepared by equilibrating dehydrated BC composites in 5% solu-
tion of H PW12O40·29H O (PWA), for 12 h. The modification of BC
Fig. 1. The construction and testing of the fuel cell (a) the construction of single
fuel cell and (b) fuel cell testing system; 1-load resistance, 2-voltage and current
measurement, 3-single fuel cell, 4-air fan.
3
2
◦
◦
membranes was frozen at −40 C and dried in a vacuum at −52 C.
2.4. Characterization
nativeorchemicalmodificationofBC. TheBH-Pt/BC(20 wt%)orPt/C
(E-TEK 20 wt% Pt/C) and the acetylene black (3 wt%) were ultrason-
The morphology and composition of the obtained compos-
icated and stirred in the distilled water–isopropanol (1:3 volume
ratio) solution for 12 h to obtain a homogeneous black suspen-
sion solution. The catalyst ink was brushed onto both sides of the
ite were analyzed by using scanning electron microscopy (SEM,
JEOLJSM-6380LV) transmission electron microscopy (JEM-2100),
and energy dispersive spectroscopy (EDS, ISIS30). Thermo-
gravimetric analysis (TG) and differential thermal analysis (DTA)
were carried out by using a TGA/SDTA85 instrument. The samples
were kept in a platinum crucible and heated in a furnace, flushed
−
2
membrane with the Pt loading of 0.5 mg cm , and then this assem-
bly was rapidly dried applying a vacuum dryer. The drying step
caused the assembly to become dehydrated to MEA. The MEA may
be assembled together by H-bonding between the fibrils without
adhesives or glues, so the catalyst layers can prevent the bonding
or the catalysts can be destroyed due to the bonding process [15].
Current generated by application of H2 to the anode of the MEA,
was measured by an ammeter connected to the current collectors.
−
1
◦
◦
with air at the rate of 200 ml min , from 30 C to 700 C, at a
◦
−1
heating rate of 10 C min . Platinum/BC nanocomposite were crys-
tallographically characterized by X-ray diffractometry (Bruker D8
ADVANCE) with an area detector using a Cu K␣ source (ꢀ = 1.54056
A) operating at 40 kV and 40 mA.
The total thickness of the MEA was approximately 100 m and the
2
active area of MEA was 6.25 cm . The flow rate of H and O2 was
2
3
2
.5. Electrochemical measurements
regulated by a flow meter at 10 and 20 cm /min. The single fuel cell
testing system is shown in Fig. 1b.
The electrochemical characterization was carried out by the
cyclic voltammetry (CV) using a potentiostat (CHI630B) connected
to a three-electrode test cell. The working electrode was a thin layer
of BH-Pt/BC or CH-Pt/BC cast on a piece of PTFE hydrophobized
carbon paper (0.5 cm × 1.0 cm). The loading of Pt on the carbon
3. Results and discussion
3.1. SEM observation and EDS analysis
−2
paper was 0.5 mg cm . Platinum plate and a saturated calomel
electrode (SCE) were used as the counter and the reference elec-
trode. The Pt surface areas of the catalysts were estimated from
hydrogen adsorption charges in cathodic voltammograms, which
were obtained between −0.2 V and +1.2 V versus SCE at a scan rate of
The SEM images of bare BC nanofibers and the TEM images of
Pt/BC hybrid nanofibers are presented in Fig. 2. The SEM image of
Fig. 2ashowsasideviewoftheBCnanofibers, withanaveragediam-
eter of about 30 nm and a length ranging from micrometers up to
dozens of micrometers. The well-organized three-dimensional net-
workstructureissynthesizedbyA. xylinumduringculture. Ascanbe
seen from Fig. 2a, the BC is porous with interconnecting pores. The
pore size varies in a 5–10 m range. With this structure, BC own the
ability to incorporate fine divided metals [16]. The TEM images of
Pt/BC nanocomposite membranes show that nanoparticles are dis-
crete in BC (Fig. 2b and c). The migration of platinum nanoparticles
50 mV/s in N-purged electrolytes. For cyclic voltammetry of hydro-
gen adsorption, the electrolyte solution which is 0.5 M H SO4 was
2
de-aerated with high-purity nitrogen for 2 h prior to measurement.
A single fuel cell was assembled from a MEA, two copper net
plates on the supply sides for gas, and two Teflon gaskets. The con-
struction of single fuel cell is shown in Fig. 1a. The PEM is hydrated