Demonstration of a 20 W class high-temperature polymer electrolyte fuel cell stack with novel fabrication of a membrane electrode assembly
Introduction
Fuel cells offer several advantages for solving problems such as environmental contamination and fossil fuel diminishment. Among various kinds of fuel cells, polymer electrolyte fuel cells (PEFCs) have been in the spotlight because of their fast start-up, low operation temperature and high efficiency. However, low-temperature PEFCs (LTPEFCs) should be operated below 100 °C and water management has to be carefully controlled to achieve high performance of this type of cell. In addition, LTPEFCs demand CO (carbon monoxide)-free hydrogen as a fuel. For these reasons, high-temperature PEFCs (HTPEFCs) (operating at >120 °C) have been studied to overcome the drawbacks of LTPEFCs [1], [2].
Most HTPEFCs employ acid-doped polybenzimidazole (PBI) derivatives in the fabrication of a fuel cell membrane [2], [3], [4]. Although PBI itself does not have proton conductivity, it develops proton conductivity after being doped with a strong inorganic acid, such as phosphoric acid. Acid-doped PBI can be prepared by two different methods: post acid-doping with PBI membrane and in situ acid-doping by direct membrane casting from a polymerization mixture in polyphosphoric acid (PPA) [5]. The former method produces a membrane which has strong mechanical strength, but a limited doping level. The latter results in a high doping level, but low mechanical strength. In order to obtain an acid-doped PBI membrane for effective HTPEMFC operation, the two methods are used by carefully controlling the acid-doping level; however, there has been no solid strategy for fabricating a PBI membrane which has a high doping level, uses an easy membrane-casting method and exhibits strong mechanical strength.
Previously we reported several results which addressed the synthesis, membrane fabrication, and fabrication of the membrane electrode assembly (MEA) for PBI membranes. For high-temperature fuel cell operation, in situ fuel cell membrane was fabricated from a polymerization mixture in PPA. In this case, high proton conductivity could be obtained, but low mechanical strength remained a problem for MEA fabrication and long-term stability. In this report, we use a novel MEA fabrication method that produces a very durable HTPEFC. The MEA has an 8-layered structure consisting of 1 phosphoric acid-doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-diffusion media, and 2 gas-sealing gaskets (Fig. 1). Most components of this MEA are similar to those of other types for low-and high-temperature PEFCs. The unique component of this MEA is a sub-gasket. The sub-gasket ensures that the membrane structure maintains proton conductivity and long-term durability. Commercialized MEAs of low-temperature PEFCs are equipped with a sub-gasket which has at least the same thickness as that of an electrolyte membrane [6], [7]. In our case, however, the thickness of the sub-gasket is thinner than that of the phosphoric acid-doped PBI membrane. During single cell fabrication, the phosphoric acid-doped PBI membrane is squeezed only as far as the thickness of the sub-gasket. As a result, the structure does not degrade for single cell and stack operation, resulting in excellent durability.
In order to achieve good performance of the MEA of our HTPEFC, the electrode was also optimized. Recently, binders such as polytetrafluoroethylene (PTFE) and PBI for the Pt/C catalyst have been studied to fabricate the electrodes [8], [9]. In this work, we used a mixture of PTFE and PBI as the catalyst binder to provide good proton conduction and maintain a pathway for air. Using this type of MEA, a 20 W class HTPEFC stack was fabricated. In the sections which follow, we present new findings associated with our novel MEA and 20 W class HTPEFC stack which demonstrate their capacity for long-term durability.
Section snippets
Materials
3,3-Diaminobenzidine (99%, Aldrich), terephthalic acid (>99%, TCI), isophthalic acid (>99%, TCI), phosphoric acid (85%, Aldrich) and polyphosphoric acid (PPA) (115% phosphoric acid equivalent, Aldrich) were used as received. A 46.1 wt% Pt/C catalyst (Tanaka), gas-diffusion cloth (with microporous layer, HT1410-W from E-Tek), and 60 wt% PTFE dispersion in water (Aldrich) were used for the MEA fabrication.
Synthesis of p-PBI (poly[2,2′-(p-phenylene)-5,5′-bibenzimidazole]) and membrane fabrication
3,3-Diaminobenzidine (3.9 g, 0.018 mol) and terephthalic acid (3.0 g, 0.018 mol) were
MEA fabrication and cell performance
Highly acid-doped p-PBI membrane has a relatively low mechanical strength. The membrane breaks easily during single cell preparation or fuel cell operation. The boundary of the active area is particularly susceptible to damage, probably due to the pressure difference between the electrolyte membrane under gas-diffusion media and the gas-sealing gaskets. Fig. 2 shows the MEA with and without a sub-gasket after single cell fabrication. Without the sub-gasket, the membrane was destroyed by the
Conclusions
In order to improve HTPEFC performance, PTFE and m-PBI were combined to make catalyst binder in the electrode. In addition, a hot-pressing method was shown to be very effective for the mitigation of Ohmic resistance, resulting in high cell performance. A novel MEA fabrication method was developed for the acid-doped PBI membrane. It involved the insertion of a sub-gasket to avoid deterioration of the PBI membrane during cell fabrication and operation. We fabricated a 20 W class HTPEFC stack
Acknowledgements
This work was supported by the New and Renewable Energy R&D Program (2008-N-FC12-J-01-2-100), and a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (MKE), Republic of Korea.
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Distribution characteristics of phosphoric acid and PTFE binder on Pt/C surfaces in high-temperature polymer electrolyte membrane fuel cells: Molecular dynamics simulation approach
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Equally contributed to as a first author.