|Power density curves of the PVPA-doped MEA measured at 25°C (orange line), 40°C (pink line), 60°C (blue line), 80°C (green line), 100°C (purple line) and 120°C (red line). Berber et al. Click to enlarge.|
Researchers in Japan have developed a novel polymer electrolyte membrane fuel cell (PEFC) that shows high durability (>400,000 cycles) together with high power density (252 mW/cm2) at high temperatures of 120°C under a non-humidified condition.
In a paper published in Scientific Reports, the open access journal of the Nature Publishing Group, they suggest that the study “opens the door” for the next-generation high temperature and non-humidified PEFC for use in the “real world”.
Higher-temperature PEM fuel cells are of interest due to a number of performance and cost advantages, including higher power efficiency; elimination of the cooling device and water management system; reduction of CO poisoning of the platinum (Pt) and/or enabling the use of a non-precious metal catalyst. Volkswagen, as an example, in 2006 reported developing a PEM (proton exchange membrane) fuel cell that operates at temperatures of about 120 °C. (Earlier post.)
In the current PEFC systems, water-assisted proton conduction in the conventional polyelectrolytes limit the operation temperature below 100 °C. The limitation of operating temperature originated from the poor proton conductivity of the conventional polyelectrolytes such as Nafion at temperatures above 100 °C, thus the development of a new polyelectrolyte with a high proton conductivity at high temperature has been strongly desired. To overcome this temperature limitation problem, the use of polymer membrane doped with a non-volatile liquid acid as a polymer electrolyte membrane (PEM) has been proposed, in which the mobile acid such as phosphoric acid (PA) is responsible for the proton conduction through the vehicle mechanism....However, recent studies have revealed that leaching of liquid PA...causes inhomogeneous PA distribution that results in deterioration of PEFC performance during long-term operation.
In this study, in order to prevent acid leaching from the high temperature PEFC system, we used poly(vinylphosphonic acid) (PVPA) in place of PA because PVPA is a polymeric acid and is stably bound to the PBIs via multipoint acid-base reactions.—Berber et al.
The researchers used a membrane electrode assembly (MEA) with Pt on poly(vinylphosphonic acid)-doped polybenzimidazole wrapped on carbon nanotube and poly(vinylphosphonic acid)-doped polybenzimidazole for the electrocatalyst and electrolyte membrane, respectively.
Among their findings was that the power density of the MEA increased when increasing the measurement temperature up to 120°C—a result in good agreement with the increase of the proton conductivity at higher temperatures. As a result, they said, they expect a higher power density above 120°C. This study is underway now by using a new apparatus capable of 120–200 °C measurements.
Mohamed R. Berber, Tsuyohiko Fujigaya, Kazunari Sasaki & Naotoshi Nakashima (2013) Remarkably Durable High Temperature Polymer Electrolyte Fuel Cell Based on Poly(vinylphosphonic acid)-doped Polybenzimidazole. Sci. Rep. doi: 10.1038/srep01764