High temperature tolerance polybenzimidazoles

Phosphoric acid is also used as the electrolyte in so called High Temperature PEFC. In this case, the phosphoric acid is imbibed in a polybenzimidazole polymer matrix. While management of the liquid electrolyte in PAFC requires careful differential pressure control, High Temperature PEFCs are more tolerant Furthermore, the basic nature of the polybenzimidazole matrix prevents electrolyte migration. High Temperature PEFCs therefore are an attractive alternative to PAFC. Nevertheless, phosphoric acid is washed out from PAFC and high temperature PEFC once liquid water can form inside the fuel cell. Therefore, a continuous mode of operation is preferred in both cases. 

BASF Fuel Cells (formerly PEMEAS or Celanese Ventures) produces polybenzimidazole (PBI)-based high-temperature membrane and electrode assemblies sold under the brand name Celtec . These MEAs operate at temperatures between 120 °C and 180 °C. One of the distinct advantages of high-temperature PEMFCs is exhibited in their high tolerance toward fuel gas impurities, such as CO (up to 3%), H2S (up to 10 ppm), NH3, or methanol (up to several percent), compared to low-temperature PEMFCs. Additionally, waste heat can be effectively used and, therefore, the overall system efficiency is increased. 

The proton conduction based on the phosphoric acid is the basis of HT-PEMFC Celanese technology [37], mostly referred to as phosphoric acid-doped PBI (polybenzimidazole) This membrane enables operation at temperatures as high as 180°C, without the need for external humidification. Heat dissipation at this temperature is much easier than at the 70-80°C operating temperature of fuel cell systems using standard PFSA membranes. The CO tolerance at 180°C is such that even 1 % CO leads to a minor loss of power density compared to that using the same membrane on pure hydrogen. The downside of this membrane is its low conductivity below 1(X)°C, making a cold start impossible, as well as the lower power density at its optimal temperature. 

High working temperatures benefit PEMFC performances because of faster electrode kinetics, higher CO tolerance and the possibility to use the residual heat for energy cogeneration [5, 6]. Among the polybenzimidazole derivatives, the most widely studied are the commercially available poly[2,2 -(m-phenylene)-5,5 -bibenzimidazole] with the acronyms w-PBI and poly(2,5-benzimidazole), AB-PBI (see Fig. 8.1). The polymers contain a basic imidazole functionality that allows the uptake of the acid protic electrolyte, which is responsible, and required, for the proton... 

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