Abstract
Elevation of operational temperatures of polymer electrolyte membrane fuel cells (PEMFCs) has been demonstrated with phosphoric acid-doped polybenzimidazole (PA/PBI) membranes. The technical perspective of the technology is simplified construction and operation with possible integration with, e.g., methanol reformers. Toward this target, significant efforts have been made to develop acid-base polymer membranes, inorganic proton conductors, and organic-inorganic composite materials. This report is devoted to updating the recent progress of the development particularly of acid-doped PBI, phosphate-based solid inorganic proton conductors, and their composite electrolytes. Long-term stability of PBI membranes has been well documented, however, at typical temperatures of 160°C. Inorganic proton-conducting materials, e.g., alkali metal dihydrogen phosphates, heteropolyacids, tetravalent metal pyrophosphates, and phosphosilicates, exhibit significant proton conductivity at temperatures of up to 300°C but have so far found limited applications in the form of thin films. Composite membranes of PBI and phosphates, particularly in situ formed phosphosilicates in the polymer matrix, showed exceptionally stable conductivity at temperatures well above 200°C. Fuel cell tests at up to 260°C are reported operational with good tolerance of up to 16% CO in hydrogen, fast kinetics for direct methanol oxidation, and feasibility of nonprecious metal catalysts. The prospect and future exploration of new proton conductors based on phosphate immobilization and fuel cell technologies at temperatures above 200°C are discussed.
Highlights
Energy and environment are of a global concern, which has stimulated worldwide research on clean, efficient, and sustainable energy technologies
For polymer electrolyte membrane fuel cells (PEMFCs) operating at below 80°C, the catalyst tolerance to CO is very poor, below 10-20 ppm [8] and careful removal of CO after reforming is demanded, which increases the complexity of the fuel processing systems
Solid inorganic proton conductors have been synthesized for fuel cell applications at elevated temperatures of 100-300°C
Summary
Energy and environment are of a global concern, which has stimulated worldwide research on clean, efficient, and sustainable energy technologies. Hydrogen and fuel cells are recognized as possible solutions in this context. Despite the high mutuality of the PEMFC technology, establishment of a hydrogen infrastructure, i.e., the storage, transportation, and distribution of pure H2 is the main barrier to large-scale commercialization [4, 5]. Compared to H2 (1.9 MJ L-1 at 20 MPa), liquid alcohol fuels, e.g., methanol (CH3OH, 17.28 MJ L-1), have a high energy density and flexible resources and are easy in storage and distribution [6]. Wide applications of PEMFCs based on direct oxidation of alcohol fuels are retarded by the low catalyst activity at low temperatures [3, 7]. An alternative solution is to generate hydrogen by an on-board reformer. The reformate hydrogen is, containing CO2 and CO, the latter being a strong poison to the anode catalyst
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