Abstract
High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are being studied due to a number of benefits offered versus their low temperature counterparts, including co-generation of heat and power, high tolerance to fuel impurities, and simpler system design. Approximately 90% of the literature on HT-PEM is related to the electrolyte and, for the most part, these electrolytes all use free phosphoric acid, or similar free acid, as the ion conductor. A major issue with using phosphoric acid based electrolytes is the free acid in the electrodes. The presence of the acid on the catalyst sites leads to poor oxygen activity, low solubility/diffusion, and can block electrochemical sites through phosphate adsorption. This review will focus on these issues and the steps that have been taken to alleviate these obstacles. The intention is this review may then serve as a tool for finding a solution path in the community.
Highlights
Despite the advantages of HT-PEMFCProton exchange membrane fuel cells (PEMFC) versus LT-PEMFCs, there are major roadblocks which prevent them from being a viable alternative
The widespread use of phosphoric acid doping leads to the inevitable presence of free electrolyte in the fuel cell electrodes. This free electrolyte becomes crucial to cell performance as it acts as the ionic phase conductor in the electrode. This leads to issues that have been plaguing the phosphoric acid fuel cell (PAFC) community for decades, including poor oxygen activity in phosphoric acid, low oxygen permeability in phosphoric acid, phosphate adsorption onto the catalyst surface, and electrolyte movement within the cell [17,18,19,20]
This leads to issues that have been plaguing the phosphoric acid fuel cell (PAFC) community for decades, issues including poor oxygen activity in phosphoric acid, low oxygen permeability in phosphoric acid, phosphate adsorption onto the catalyst surface, and electrolyte movement within the cell
Summary
Proton exchange membrane fuel cells (PEMFC) are well-established, having demonstrated excellent performance and respectable durability. The state-of-the-art for this technology is based on a NafionTM membrane and platinum (Pt) on carbon catalyst, assembled with carbon/carbon Teflon gas diffusion layers, and carbon bipolar plates. The key cost sensitive elements of this technology are identified as the high cost of the NafionTM membrane, platinum catalysts, and carbon machining for bipolar plates. These costs added to the lack of pure hydrogen infrastructure and the high cost of hydrogen itself, have presented an obstacle to the introduction of commercial fuel cells for several decades. In an effort to alleviate some of the operational issues with the state of the art (SOA) low temperature PEMFCs, a new classification was developed, high temperature PEMFCs (HT-PEMFCs). The HT-PEMFCs can be broken into two ranges for further analysis
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