The apparent collection efficiency (N, refer to Eq. 13) was determined by using Fe(CN)6 3- /Fe(CN)6 4-redox couple. In 0.1 M KHCO3 with 5 mM K3Fe(CN)6, the disk electrode was scanned from 0.4 to -0.2 VAg/AgCl and the ring electrode potential was set at 0.5 VAg/AgCl, the results are shown in Fig. . N was determined to be 0.35 (±0.01).