Durability test with fuel starvation using a Pt/CNF catalyst in PEMFC
© Jung et al; licensee Springer. 2012
Received: 9 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
In this study, a catalyst was synthesized on carbon nanofibers [CNFs] with a herringbone-type morphology. The Pt/CNF catalyst exhibited low hydrophilicity, low surface area, high dispersion, and high graphitic behavior on physical analysis. Electrodes (5 cm2) were prepared by a spray method, and the durability of the Pt/CNF was evaluated by fuel starvation. The performance was compared with a commercial catalyst before and after accelerated tests. The fuel starvation caused carbon corrosion with a reverse voltage drop. The polarization curve, EIS, and cyclic voltammetry were analyzed in order to characterize the electrochemical properties of the Pt/CNF. The performance of a membrane electrode assembly fabricated from the Pt/CNF was maintained, and the electrochemical surface area and cell resistance showed the same trend. Therefore, CNFs are expected to be a good support in polymer electrolyte membrane fuel cells.
Keywordspolymer electrolyte membrane fuel cell catalyst carbon nanofiber durability
Each type of carbon has a different performance and durability, but carbon characteristics can also affect the corrosion rate [2, 18, 19]. Cell performance will decrease due to the increase in cell resistance with reduced thickness of the catalyst layer and electric contact of the current collector caused by carbon corrosion . Recently, graphitized carbon types, such as carbon nanofibers [CNFs], carbon nanotubes, and graphene, have been studied. Graphitic carbons are known to have high corrosion resistance as they have good thermal and electrochemical stability [19, 21–25]. CNFs have higher electric conductivity and durability than commercial CBs as catalyst support materials [19, 26]. However, it is difficult to synthesize platinum nanoparticles for loading and dispersion. CNFs with different structure and morphology have been used for electrode materials fabricated via various synthesis methods in order to achieve different surface chemistries [27–31]. CNFs are potentially suitable materials for high platinum loading and dispersion due to their many functional groups. The number of functional groups increases with increasing surface oxidation treatment time, producing a hydrophobic carbon surface which accelerates carbon corrosion .
In this study, we synthesized a catalyst with Pt particles of high loading and distribution on the CNFs. Membrane electrode assemblies [MEAs] were prepared using the Pt/CNF catalyst, and the performance changes caused by fuel starvation were evaluated via electrochemical analysis.
Synthesis of the Pt/CNF catalyst
The surface treatment and functionalization were carried out as follows. The CNFs (herringbone type) were placed in a flask, and H2SO4/HNO3 (v/v = 4:1) was added; the solution was ultrasonicated and stirred for 4 h. The CNFs were separated from the acids and washed with deionized water. A Pt salt precursor, H2PtCl6·6H2O (Sigma-Aldrich Corporation, St. Louis, MO, USA) was dissolved in ethylene glycol, and the CNF support was dispersed in the solution. The suspension was filtered and dried at 60°C for 4 h in a vacuum oven. The heat treatment was performed in argon atmosphere at 350°C for 2 h.
Manufacturing of the membrane electrode assembly
Catalysts used for the preparation of MEAs
A thermogravimetric analyzer [TGA] (Q50, TA Instruments, New Castle, DE, USA) was used to measure the amount of Pt loaded onto the carbon support. The crystal structure and particle size of the Pt were confirmed using an X-ray diffractometer [XRD] (RAD-3C, Rigaku Corporation, Tokyo, Japan with Cu-Kα (λ = 1.541 Å) at a scan rate of 1.5° min-1. The shape and dispersion of the Pt particles supported on the CNFs were verified by transmission electron microscopy [TEM] (JEM-2010, JEOL Ltd., Akishima, Tokyo, Japan) Brunauer-Emmett-Teller [BET] (ASAP2020, Micromeritics Instrument Co., Norcross, GA, USA) analysis was performed in order to measure the specific surface areas of the Pt/CNF and Pt/C catalysts.
The polarization curves of the unit cell were used to gauge the cell temperature at 70°C under atmospheric pressure using H2 and air at the anode and cathode, respectively. After obtaining the polarization curves, cyclic voltammetry [CV] was performed in the range of 0.05 to 1.2 V at a sweep rate of 50 mV/s with 20 and 100 cm3/min flow rates of H2 and N2 to the anode and cathode, respectively.
The durability of the assembled MEA was determined by acceleration tests using reverse potential operation under fuel starvation conditions. The acceleration experiment was operated at a current density of 400 mA/cm2. The hydrogen stoichiometry of the anode was maintained at 0.5. If the cell potential reached at -0.5 V, then the recovery system would be driven for 30 s under an open circuit voltage [OCV] state. In the OCV condition, the stoichiometric ratios of hydrogen and air were maintained at 1.5 and 2.0. This process was considered as 1 cycle, and experiments were repeated 200 times. After the acceleration tests, the performance curve and CV were obtained using the same method.
Results and discussion
Physical characteristics of the catalyst
Properties of Pt/CNF and Pt/C catalysts
Weight percentage by TGA (wt.%)
Surface area by BET (m2/g)
Dispersion of pore size (nm)
Pt particle size (nm)
Electrochemical measurement with fuel starvation
Summary of changes before and after the fuel starvation test
Performance at 0.6 V (mA/cm2)
Electrochemical surface area (m2/g)
Polarization resistance (Ω)
The 47.5 wt.% Pt/CNF catalyst was synthesized with a highly dispersed platinum. The Pt/CNF was used on the anode and on both electrodes. The MEAs were evaluated for durability against fuel starvation. After 200 cycles of reverse voltage drops, the performance of MEA-1 with Pt/C using both electrodes decreased by 59%, whereas the performance of the MEA-2 and MEA-3 was maintained. In the CV and EIS analyses, the ESA and cell resistance of the MEAs with Pt/CNF were nearly unchanged. As a result, a catalyst on a CNF support which has higher graphitization, lower specific surface area, and lower hydrophilicity has higher carbon corrosion resistance than a commercial Pt/C catalyst.
electrochemical surface area
membrane electrode assembly
open circuit viltage
polymer electrolyte membrane fuel cell
transmission electron microscopy
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