Celtec® membranes

Another approach for producing PBI membranes for fuel cell applications is through a sol-gel process (Xiao et al. 2005a, b). This synthesis route was developed by researchers from BASF Fuel Cell and from Rensselaer Polytechnic Institute, respectively, in a collaborative effort. Figure 2 sketches the production pathway of the now-called Celtec membranes. The polymerization of the two monomers, viz.. 

Fig. 3 (a) Phosphoric acid evaporation rates in Celtec membrane electrode assemblies (MEAs) at the beginning of life as a function of temperature, (b) Phosphoric acrid evaporation rate in a Celtec MEA operated at 160°C over a period of 1 year (left axis) and the t ell resistanc es measured by 1-kHz AC impedance specriroscopy (right axis)... 

PEMEAS, a 2004 spin-off from Celanese AG, has developed a membrane made from the heat-resistant polymer PBI. The PBI membrane marketed by PEMEAS under the brand name Celtec enables a fuel cell to operate at temperatures of up to 200°C (392°F), while more conventional technologies allow PEMFC-operating temperatures of up to 100°C (212°F). Due... 

A commercial phosphoric acid doped PBI membrane, Celtec V by BASF Fuel Cells, tailored for DMFC is based in a blend of PBI and poly(vinyl phosphonic acid) (PVPA). The PVPA poly acid is immobilized in the PBI matrix by interpenetration, crosslinking and covalent bonding [201]. 

H3PO4 doped PBI membranes (A = 2.5) 100 pm thick exhibit methanol crossover current densities less than 10 mA.cm [403, 410], while the PBI/PVPA composite commercial membrane Celtec-V shows crossover current densities higher than 100 mA.cm at 90 °C in 1 M methanol [201]. 

An isomer of ABPBI which contains head-to head and tail-to-tail benzimidazole sequences, was recently synthesized [449] and membranes were prepared by using the PPA sol- gel process (see Fig. 6.11). The membranes, like those prepared with PBI using this method PBI [200], have a much higher doping degree and their conductivities are above 200 mS.cm at 180 °C, even without humidihcatimi. Proton conductivities above 200 mS.cm were also reported for commercial crosslinked ABPBI membranes by Fumatech [425, 447], at 120 and 140 °C and partial humidification. ABPBI/MMA membranes exhibit only modest conductivities [419, 445], while an ABPBI/PVPA composite [450], which is the equivalent to the commercial PBI-based Celtec V by BASF Fuel Cells shows a poor conductivity. 

Gubler L, Kramer D, Belack J, Unsal O, Schmidt TJ, Scherer GG (2007) Celtec-V. A polybenzimidazole-based membrane for the direct methanol fuel cell. J Electrochem Soc... 

Mocoteguy P, Ludwig B, Scholia J, Nedellec Y, Jones DJ, Roziere J (2010) Long-term testing in dynamic mode of HT-PEMFC H3PO4/PBI Celtec-P based membrane electrode assemblies for micro-CHP applications. Fuel Cells 10(2) 299-311... 

BASF Fuel Cell GmbH (2010) Celtec MEAs membrane electrode assemblies for high temperature PEM fuel cells, http //www.basf-fuelcell.com/en/projects/celtec-mea.html... 

For all experiments reported in this chapter, BASF Celtec -based MEAs were used. These MEAs exhibit a thickness of approximately 820 pm including a membrane thickness of around 50-75 pm. They furthermoie cmisist of a highly H3P04-doped FBI membrane, electrodes with a symmetrical platinum loading of 1 mgp,cm with an active area of 45.15 cm and a carbon paper (Sects. 14.2 and 14.3) or carbon cloth (Sect. 14.4) gas diffusion layer. Furthermore, if it was not specifically noted otherwise, only dry gases were used for start/stop simulation and investigation. 

The ohmic resistance (/ ohm) function of contact pressure cycling is shown in Fig. 17.19 for Dapozol -G55 and Celtec PllOOW MEAs. The values have been extracted from the Nyquist plots at 0.3 A/cm shown in [21]. As previously defined, ohmic or high frequency resistances are used to differentiate the resistance of conduction of protons and electrons and resistances of wires and MEA contact interfaces as BBP/GDL and GDL/CL. If one assumes a constant or nearly constant contribution from proton conductivity resistance through the membrane as the membrane thickness is nearly constant in comparison with GDL thickness changes with increasing the contact pressure [17] and resistance related to fuel cell wires and connections is constant [40, 41], the observed changes in the ohmic resistance will be mainly due to variation of contact between BPP/GDL and GDL/CL interfaces. / ohm decreases in the first cycle of contact pressure as shown in Fig. 17.19 for both the MEAs so contact between BPP/GDL and GDL/CL interfaces has been enhanced. This... 

Jd — On Dt20 [134] Celtec -P 1000. Fictitious water diffusivity of 1 X 10 m s used. Effective water diffusivity in catalyst layer accounts for transport in the gas and membrane phase... 

Table 24.1 Input and output of gases that were purified through hydrogen pumping using a Celtec-P membrane at 160 °C...

Further testing was performed on a gas mixmre containing 1.12 % CO, 43.8 % CO2, with a H2 balance using a Celtec-P membrane at 160 °C at 0.2 A cm. The carbon monoxide was found to be 1 ppm at the outlet. These findings show... 

Fig. 24.6 Purification ratios for carbon dioxide and carbon monoxide at various current densities using a Celtec-P membrane at 160 C...

FIGURE 11.3 Cell potentials at 0.2 A as a function of lifetime of a constant load test with and without start/ stop cycling for a PEM fuel cell with phosphoric acid-doped PBI membrane. Hj-air (stoichiometry 1.2/2), T = 160 C, p = 1 bar. (Reprinted from Journal of Power Sources, 176, Schmidt, T.J. and J. Baurmeister, Properties of high-temperature PEFC Celtec -P 1000 MEAs in start/stop operation mode, 428-434, Copyright (2008), with permission from Elsevier.)... 

Fig. 2 (a) Polybenzimidazole (PBI) polymerization reaction in polyphosphoric acid (PPA). (b) State diagram for the Celtec PBI membrane... 

From the PBI/polyphosphoric acid sol the membrane is cast and further hydrolyzed. During the hydrolysis, the membrane undergoes a sol-gel transition which results in the final membrane used in Celtec MEAs (Fig. 2b). This production pathway not only produces high molecular weight PBIs with inherent viscosities of up to 7, but the final membrane also has a phosphoric acid content of more than 95 wt%, or up to 70 phosphoric acid molecules per PBI repeat unit. 

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. 

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