Activated carbon prepared from silk fibroin, which is free of metal elements, showed a high catalytic activity for the oxygen-reduction reaction (ORR). The activated carbon had a very high onset potential of E onset = 0.83 V (vs. RHE) in oxygen saturated 0.5 M H 2 SO 4 at 60 °C. The ORR on the activated carbon proceeded by a four-electron process in the high-electrode-potential region; this gradualy decreased to a 3.5-electron reaction below about 0.6 V (vs. RHE). Only about 1 % of nitrogen atoms (mostly quaternary) remained in the activated carbon by heat treatment at up to 1200 °C are responsible for the high catalytic activity. The open circuit voltage of a polymer electrolyte fuel cell using the activated carbon as the cathode and a platinum/carbon black anode under pure oxygen and hydrogen gases, respectively, both at 1 atmosphere, was 0.96 V at 27 °C.
Carbonized silk fibroin (CS), which is free of metallic elements, showed high catalytic activity for oxygen-reduction reaction (ORR). The catalytic activity of CS for ORR was greatly enhanced by steam activation forming silk-derived activated carbon (CS-AC). The surface morphology, surface area, pore structure and remaining nitrogen species of the CSs were compared with those of the CS-ACs. The open-circuit potential and the power density of a polymer electrolyte fuel cell using a CS900-AC, which was heat-treated at 900°C prior to the steam activation, and a platinum/C (C: carbon black) anode under pure oxygen and hydrogen gases, respectively, both at 0.2 MPa, were 0.92 V and 142 mW cm -2 at 80°C. The ORR on the 1 activated carbon, CS900-AC, proceeded with a 3.5-electron reaction at 0.6 V (vs. RHE);however, this was improved to a 3.9-electron reaction with the addition of zirconium oxide at 20 wt% to CS900-AC. Durability test of the electrode was also performed in O 2 -saturated HClO 4 solution.
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