Nitrogen–carbon (N–C) species is a potential electrocatalyst for oxygen reduction reaction (ORR) in electrochemical energy conversion cells, but its mechanistic origin of ORR on the N–C surface is still unclear. We show our facile approach to the synthesis of highly active Co-modified N–C catalyst and investigated the origin of ORR activity of electrospun N–C species by removing the metal with hydroxide carbon etching and acid metal leaching. Through the detailed investigation on the origin of ORR electrocatalysis for electrospun N–C nanofibers, we revealed that pyrrolic-N and highly graphitized carbon structure are mainly responsible for the enhanced ORR activity of metal-free N–C nanofiber and embedded Co metal got involved in the creation of the pyrrolic N site.

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We performed in situ magnetotransport measurements on ultrathin Bi(111) films [4-30 bilayers (BLs), 16-120 Å thick] to elucidate the role of bulk or surface states in the transport phenomena. We found that the temperature dependence of the film conductivity shows no thickness dependence for the 6-16 BL films and is affected by the electron-electron scattering, suggesting surface-state dominant contribution. In contrast, the weak antilocalization effect observed by applying a magnetic field shows clear thickness dependence, indicating bulk transport. This apparent inconsistency is explained by a coherent bulk-surface coupling that produces a single channel transport. For the films thicker than 20 BLs, the behavior changes drastically which can likely be interpreted as a bulk dominant conduction.

An optimal catalyst testing methodology that could allow precise benchmarking to obtain standardized ORR activity is put forward.