タイトル平成21年度成果報告書 エコイノベーション推進事業「レアメタル代替材料となる燃料電池用の炭素触媒の探索研究」
報告書年度2009 - 2009
 シルク活性炭をカソード触媒に用いた燃料電池を作製し、電極構造の違いによる性能の評価を行った。一般的な燃料電池のカソード触媒層と異なり、シルク活性炭触媒は活性が低いが安価な触媒である事を考慮し、白金の場合と異なる設計を行う必要がある。触媒とイオン伝導体の混合比、触媒塗布量などをパラメータとして、電極構造の最適化を行ったところ、測定温度80℃、常圧において約70 mWcm-2の出力密度が得られた。これは本研究開始前の同条件における出力より2割程度向上している。本研究で作製したシルク活性炭の中で最も高い性能を示したのは、昇温速度が1℃、二次炭化温度が1000℃で作製した触媒であった。これは通常の活性炭よりも炭化温度の高い条件であり、シルクを原料とした活性炭特有の反応サイトが酸素還元活性に影響を与えている可能性が示唆された。今後、触媒作製条件、電極構造の最適化を進めていくことでレアメタル代替材料となる燃料電池用の炭素触媒として期待できることが明らかとなった。
英文要約Climate change is one of the most important issues that human being is facing. Development of highly efficient energy system is necessary, and polymer electrolyte fuel cell (PEFC) is one of the key technologies. However, in most PEFCs, platinum (Pt), which is a rare metal, is used for the electrode catalyst. Since Pt is a metal of low abundance and high cost, alternative catalyst is required to put PEFCs to practical use.
Many non-Pt catalysts have been investigated. However, much of them exhibit low oxygen reduction reaction (ORR) activity, or low durability. Furthermore, metal containing catalysts dissolve in a low pH environment. In recent years, metal oxides and nitrogen containing carbon are reported as novel catalysts which exhibit high activity and durability, but they are not yet ready for practical use. Therefore, in this study, silk-derived activated carbon (SAC) was investigated as a metal free activated carbon catalyst for cathode of PEFC.
SAC was prepared using silk fibroin as a starting material after removing sericin from the silk fiber. Temperature rate during the primary carbonization, and maximum temperature during the secondary carbonization were varied and the obtained SAC was evaluated. Comparing thermogravimetric (TG) analysis, SAC prepared at higher temperature rate showed faster weight change and smaller total weight loss. From the scanning electron microscope (SEM) images, the particle size of the sample prepared by high temperature rate was smaller.
From the nitrogen adsorption isotherm analysis, it was revealed that SAC exhibit large volume of micropores. Decrease in micropores and increase in mesopores were observed for SAC prepared at high carbonization temperature. The average pore size of SAC prepared at low carbonization temperature was small, which indicates formation of strong molecular potential field. By changing the carbonization temperature, characteristics of nano-space changed, such as average pore size and mesopore fraction. Development of highly active carbon catalysts is expected by further investigations.
Catalytic activity toward ORR was measured by linear-sweep voltammograms. The onset potentials for ORR showed little dependence to the steam activation time, and its value was slightly lower than those before this study. Further investigation on impurities, relative surface area, and properties of microstructure are necessary. SCA as a support material for Pt was also investigated. The onset potential for ORR was higher than carbon black indicating the validity of SCA as support material.
Membrane electrode assembly was prepared, and the relationship between the electrode microstructure and the performance was evaluated. Since SCA is a catalyst of lower activity but is less expensive than Pt, different design from a general electrode should be optimum. By varying parameters such as amount of catalyst and mixing ratio of catalyst and ionomer, the microstructure of the electrode was optimized. The maximum power density was 70 mWcm-2 at 80~C under ambient pressure, which was 20% higher than the value obtained before the investigation. The highest performance was obtained when the maximum temperature during carbonization was 1000 ~C. This temperature is higher than the carbonization of usual active carbon, which indicates an existence of a unique active site. Further investigation would make SCA a promising material for an alternative of rare metal catalyst for PEFCs.