The Effects of Adding Heteropolyacids to Phosphoric Acid Doped Polybenzimidazole for Enhanced Conductivity in High Temperature Fuel Cell Applications

Abstract

With the current expansion of the global population, the demand to reduce our dependence on fossil fuels and reduce our emissions of pollutants has led to the development of fuel cells as alternatives for clean power generation. There are several types of fuel cells, but polymer electrolyte membrane fuel cells (PEMFCs) have stood out because they are well suited for transportation and small scale stationary power generation. PEMFCs have outstanding power density, rapid start-up, and high efficiency.1 In addition, PEMFC’s operation is simple and free of pollution. Despite several advantages, current PEMFCs are not widely commercialized because they are still too expensive and need higher durability. At the moment, PEMFCs generally operate below 100ºC due to the need to fully humidify commonly used perfluorosulfonic acid electrolytes such as Nafion®. As a result, current PEMFCs require high purity hydrogen that can only be cost effectively produced from natural gas at this time. At low operating temperatures, the catalysts are poisoned by CO that must be completely removed. The catalysis problem can be solved by increasing the PEMFCs operating temperatures above 120ºC.2 High temperature operation is a promising way to improve PEMFC performance; it has been shown that higher operating temperatures would increase chemical kinetics at the anode electrode, which introduces the use of lower cost hydrogen. In addition, operating at elevated temperatures would also provide easier water and thermal management.2 One approach in designing a high temperature electrolyte membrane for PEMFCs is acid doping into aromatic polymer materials that have high temperature stability. The polymer is typically chosen to contain basic sites that serve as proton acceptors, forming an ion-pair after doping with an inorganic acid.3 One of the most interesting acid-base membranes that have been discovered is based on polybenzimidazole (PBI) doped with phosphoric acid, shown in Figure 2. PBI has long been recognized as an excellent candidate for use in the high temperature fuel cell environment.4 Although the proton conductivity of pure PBI is low, doping with phosphoric acid causes a remarkable increase in its proton conductivity. PBI contains imide groups that can be easily doped with phosphoric acid, and it forms a membrane with excellent thermal and mechanical stability upon casting. More importantly, the resulting electrolyte membrane has very high proton conductivity at temperatures above 120ºC, even in the absence of water. The proton transport in acid-base membranes occurs mainly through a Grotthuss Mechanism, where the proton transfer occurs through the hopping of protons between two molecules via rearrangement of their hydrogen bonds.1 This mechanism allows for proton conductivity even in the absence of water at temperatures above 120ºC. Although the use of phosphoric acid in PBI introduces high proton conductivity, the phosphoric acid still dissolve in the presence of liquid water and does not enhance the oxygen reduction reaction at the cathode4. Long-term operation of phosphoric acid doped PBI membranes often loses proton conductivity due to acid leaching. The free phosphoric acid also degrades and embrittles the polymer. As a result, the membrane is degraded at elevated temperatures, has poor mechanical properties, and has a narrow operational temperature range. Our preliminary results indicate that doping phosphoric acid in PBI in the presence of super-protonic conducting oxides such as heteropolyacids (HPA) successfully generated high temperature electrolyte membranes for PEMFCs (Figure 2). More importantly, HPA not only increases the barrier to prevent water from escaping the membrane at elevated temperatures, but also adds more acid sites, as a consequence, giving additional paths for proton transport.3These membranes at different doping levels and with various percentages of HPA conduct well at temperatures above 100ºC and can potentially be used as PEMs in high temperature applications.