New PBI/H3PO4 Fuel Cell Starts Up at Room Temperature
A Japanese research group announced that it developed a non-humidification polymer electrolyte fuel cell (PEFC) that can generate power in temperatures ranging from room temperature to intermediate temperatures (100-200°C).
As a solid electrolyte membrane, the research group used a PBI/H3PO4 composite membrane made of polybenzimidazole (PBI) doped with phosphoric acid. The group was led by Masahiro Rikukawa, professor of the Faculty of Science and Technology at Sophia University.
PEFCs that are nearly at a practical level have an acid group such as sulfonic acid at the end and protons (H+) are carried through water. In contrast, the PEFC studied by Rikukawa uses a composite membrane of PBI, a basic polymer with an N or NH group, doped with phosphoric acid. The base in the PBI chain and the doped acid interact with each other to immobilize phosphoric acid through which protons migrate.
A fuel cell using a PBI/H3PO4 composite membrane can eliminate auxiliaries to control water because it does not require humidification, and thus reduces the cost and the size. Moreover, because it shows ionic conductivity in a temperature range of 100-200°C, which is higher than a range (70-90°C) applicable to the existing PEFCs, it has a number of advantages such as a high power generation efficiency and a capability to prevent catalyst poisoning.
Thus, the development of fuel cells with a PBI/H3PO4 composite membrane is promoted by manufacturers like BASF and Volkswagen of Germany.
The existing fuel cells with the PBI/H3PO4 composite membrane need to be heated to 100°C or higher by means of auxiliaries because they cannot be started up at room temperature. In contrast, the latest fuel cell, which can be started up at room temperature, is gradually heated by the reaction. This makes it possible for the cell to generate power by itself until the temperature reaches an intermediate range.
At present, the power density of the new PEFC at room temperature (23°C) is only 8.23mW/cm2. The research group confirmed that the power density is increased to 270-280mw/cm2 when the temperature rises to 160°C.
The optimization of the adsorption amount of phosphoric acid greatly contributed to enabling room temperature startup, the company said. When the amount of phosphoric acid in a solid electrolyte is increased, the ionic conductivity improves accordingly and hence the power generation efficiency. But phosphoric acid in the catalyst layer may adhere to platinum, deteriorating the catalytic performance to lower the power generation efficiency. Phosphoric acid in the electrolyte migrates to the catalyst layer with protons.
In addition, the catalyst layer itself contains phosphoric acid to carry protons to platinum. Therefore, the research group reportedly improved the PBI molecular and catalyst structures to increase the amount of phosphoric acid in the electrolyte while reducing its amount in the catalyst layer as much as possible.
Rikukawa's group is considering using various kinds of basic polymers besides PBI. The group plans to improve the durability of membrane, the power generation characteristics, etc by further optimizing the adsorption amount of phosphoric acid.
The membrane is expected to be used for alkaline fuel cells and direct methanol fuel cells (DMFC), in addition to PEFCs. The group aims to put the fuel cell to practical use as the next-generation stationary power source, the automotive power supply system and the power source of mobile devices in the next 10 years.
The research was conducted as part of the project called "Strategic Development of PEFC Technologies for Practical Application," which is sponsored by Japan's New Energy and Industrial Technology Development Organization (NEDO). The achievement will be presented at the 57th Polymer Symposium, which runs from Sept 24-26, 2008, at Osaka City University.