For the broad application of polymer electrolyte fuel cells (PEFCs), the development of nonprecious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalysts have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors; however, they suffer from low density and uncertain chemical structure of their active sites. This study reports a novel 14-membered macrocyclic Fe complex, which was inspired by FeN4 centers in pyrolyzed catalysts, unlike typical macrocyclic MN4 complexes that have 16-membered rings. This 14-membered macrocycle possesses strong Fe–N bonds with an average bond distance of 1.90 Å, as evidenced by single-crystal X-ray diffraction (XRD), which are markedly shorter than those in porphyrin (2.0 Å). This complex demonstrates high electrocatalytic activity for oxygen reduction in both acidic and basic media.

Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this study, the durability of the Fe-14MR complex was compared to that of Fe phthalocyanine (FePc), which has a 16-membered ring ligand, using in situ X-ray absorption spectroscopy; demetalation of the Fe complexes was directly observed during electrochemical experiments performed under acidic ORR conditions. It was found that Fe-14MR is significantly more resistant to demetalation than FePc during the ORR.

Naturally occurring polysaccharides, such as cellulose, hemicellulose, and chitin, have roles in plant skeletons and/or related properties in living organisms. Their hierarchically regulated production systems show potential for designing nanocomposite fabrication using engineered microorganisms. This study has demonstrated that genetically engineered Gluconacetobacter hansenii (G. hansenii) individual cells can fabricate naturally composited nanofibrils by simultaneous production of hyaluronan (HA) and bacterial cellulose (BC). The cells were manipulated to contain hyaluronan synthase and UDP-glucose dehydrogenase genes, which are essential for HA biosynthesis. Fluorescence microscopic observations indicated the production of composited nanofibrils and suggested that HA secretion was associated with the cellulose secretory pathway in G. hansenii. The gel-like nanocomposite materials produced by the engineered G. hansenii exhibited superior properties compared with conventional in situ nanocomposites. This genetic engineering approach facilitates the use of G. hansenii for designing integrated cellulose-based nanomaterials.

Liquid flow dynamics through the armchair (6,6)-(160,160) carbon nanotubes (CNTs) is elucidated by molecular dynamics simulations. The liquid is modeled by nonpolar argon atoms to understand the fundamental flow physics. The velocity profiles and slip lengths are discussed considering the radial distributions of the fluid density by the presently proposed finite difference-based velocity fitting method. It is found that as the CNT diameter D increases, the slip length and the flow rate enhancement show three-step transitional profiles in the region of D≤2.3 nm. The slip length and the flow rate stepwise increase at the first transition while they drop at the second and third transitions. The first transition corresponds to the structural change from the single-file chain to single-ring structures of the molecule cluster. The second and third transitions take place when the ring structure starts to develop another inner layer.

For the broad application of polymer electrolyte fuel cells (PEFCs), the development of nonprecious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalysts have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors; however, they suffer from low density and uncertain chemical structure of their active sites. To date, pyrolyzed Fe/N/C catalysts have been regarded as more active than typical macrocyclic MN4 complexes such as Fe porphyrin. The exact reason for this difference in catalytic activity has remained obscure because of the indistinct chemical structure of the active sites on the pyrolyzed catalyst. However, the recent significant progress in the direct microscopic observation of atomically dispersed Fe sites has prompted researchers to embed FeN4 sites in graphene sheets. Interestingly, these Fe centers are considered to be surrounded by a 14-membered ring consisting of C and N atoms, unlike the case of typical macrocyclic MN4 complexes that have 16-membered rings (e.g., porphyrin and phthalocyanine). This has inspired our research group to focus on the ORR catalytic activity of 14-membered macrocyclic Fe complexes. This study reports a novel 14-membered macrocyclic Fe complex. This 14-membered macrocycle possesses strong Fe–N bonds with an average bond distance of 1.90 Å, as evidenced by single-crystal X-ray diffraction (XRD), which are markedly shorter than those in porphyrin (2.0 Å). This complex demonstrates high electrocatalytic activity for oxygen reduction in both acidic and basic media. Figure 1