Fuel cells that operate between approximately 150 ^oC and 250 ^oC accrue certain advantages, most notably facilitated operation with low grade / low cost hydrogen sourced from carbon feedstocks, robust cathode performance in the presence of air contaminants, absence of balance of plant components for stack water management, high quality heat, and readily sourced materials of construction. Fuel cells running at these intermediate temperatures with phosphoric acid as the electrolyte currently hold the record for a commercial lifetime warrantee (1). Yet, lower temperature polymer electrolyte fuel cells (PEFC) operating at under 100 ^oC and comprised of a conductive membrane, electrodes, and gas diffusion layers offer the potential for low cost assembly at high volumes, especially if these components are made in roll-to-roll processes. The better of these two technologies would be a hybrid high temperature Membrane Electrode Assembly (MEA) based on phosphoric acid electrolyte.

The first hybrid high temperature (HT) MEA made use of polybenzimidazole (PBI) as a membrane matrix for phosphoric acid and originated from the Case Western Reserve University team of Litt, Savinell and Wainright in the early 1990s as part of a research proposal to the Defense Advanced Research Projects Agency (DARPA). Their initial motivation was to develop a system that could overcome the limitations of direct methanol oxidation at under 100 ^oC (2). Although ultimately not viable for direct methanol oxidation, hydrogen oxidation with 1-2 % CO proved good enough to license the technology to PEMEAS, the company that developed the first commercial PBI HT MEA based on pre-formed membranes of PBI.

However, using a preformed PBI membrane posed some limitations on acid content, and through the efforts of Calundann and Benicewicz, a second generation PBI membrane was invented (3). This new process identified a technique to polymerize PBI in the presence of polyphosphoric acid (PPA), and then hydrolyze the cast film to phosphoric acid (PA). The change of solvent from PPA to PA also radically changed the morphology of the PBI from a viscous solution into a tough second generation PBI plastic that held over 90 % by weight PA. This development also radically simplified the construction of HT MEAs by eliminating the acid soaking step. Due to the nature of these materials, new gas diffusion electrode architectures were developed and optimized to accommodate a membrane-electrode interface substantially different than that found in LT MEA assemblies. This electrode effort on roll coaters was awarded a Department of Energy Manufacturing R&D award (4). Ultimately, BASF purchased PEMEAS, as well as the electrode team at PEMEAS.

In parallel to the development of the polybenzimidazole (PBI) system, Advent Technologies identified and developed a pyridine polymer based TPS^® system also using PA imbibed into the membrane matrix. Advent’s TPS membranes operate over a wide temperature range from ~120 °C to greater than 200 °C. In 2014, Advent became the licensee of BASF’s PBI MEA and gas diffusion electrode technology. Advent develops, manufactures, and markets both the PBI and TPS MEAs.

This presentation will present an overview of the commercial and technical history of developing HT MEAs, including some of the latest membrane advances like the third generation PBI MEAs, and cross-linked TPS MEAs operating at 210 ^oC, and conclude with future directions such as the application of HT MEAs in facilitating direct oxidation of dimethyl ether (DME).

1. http://www.doosanfuelcell.com/en/main.do
2. Robert F. Savinell and Jesse Wainright, "Overture: The Early History of PBI/Phosphoric acid membranes," in High Temperature Polymer Electrolyte Membrane Fuel Cells: Approaches, Status and Perspectives, edited by Qingfeng Li, David Aili, Hans Aage Hjuler and Jens Oluf Jensen, Spring International Publishing, 2016.
3. For example, G. Calundann et al. US Patent 7,820,314.
4. US Department of Energy Annual Merit Review, 2013.