Shengwen Yuan

Microporous polyphenylenes with tunable pore size for hydrogen storage

A series of highly porous polymers with similar BET surface areas of higher than 1000 m(2) g(-1) but tunable pore ranging from 0.7 nm to 0.9 nm were synthesized through facile ethynyl trimerization reaction to demonstrate the surface property-hydrogen adsorption relationship.

A Highly Active and Support‐Free Oxygen Reduction Catalyst Prepared from Ultrahigh‐Surface‐Area Porous Polyporphyrin

A new approach for preparing non-precious-metal electrocatalysts using a porous organic polymer (POP) as precursor is presented. Polyporphyrin, containing a high density of nitrogen-coordinated iron macrocyclic centers, was prepared by oxidative coupling to form a porous network with a very high specific surface area and narrow pore-size distribution. Upon pyrolysis, the POP was converted into a highly active electrocatalysts for the oxygen reduction reaction in an acidic electrolyte. Proton-exchange membrane fuel cells, prepared with such catalyst at the cathode, achieved very high measured volumetric and gravimetric current densities of 20.2 Acm 3 and 39.4 Ag 1 at 0.8 V, respectively, and a peak power density of 730 mWcm 2 at 0.4 V. The proton-exchange membrane fuel cell (PEMFC) is among the most efficient energy conversion devices for future transportation applications. The PEMFC is operated through the electrochemical hydrogen oxidation reaction (HOR) at the anode and oxygen reduction reaction (ORR) at the cathode. The ORR generally faces higher kinetic barrier than HOR and therefore requires more catalyst. At present, the electrocatalysts of choice are precious metals, such as platinum supported on a carbon substrate. High costs and limited reserves of the precious metals pose a major challenge for large scale commercialization of PEMFCs. Non-precious-metal catalysts (NPMCs) made of Fe and Co in carbon composites have attracted a great deal of attentions since they were discovered with promising activities towards ORR in acidic media. Their activities in alkaline or neutral media were also extensively studied, although the subject is beyond the scope of the current discussion. Extensive characterizations have been carried out in attempts to understand the roles of transition metals, nitrogen, and surface properties in the catalytic activity of these NPMCs. The durability of these NPMCs in the protonic medium has been a major concern, although recent work by Wu et al. demonstrated a catalyst with improved stability in the PEMFC operating environment. At present, the catalytic activities of NPMCs are still significantly less than that of precious metals. To make NPMCs truly competitive, substantial improvements in two critical properties have to be accomplished: 1) a higher turnover frequency (TOF) per active site; and 2) a greater catalytic site density per unit volume. To improve TOF requires an in-depth understanding of the influences by transition metals, organic ligands, and the support on the active site. The interdependences between these factors are still under intensive investigation. To improve active site density, a NPMC precursor with densely populated metal– ligand sites and high surface exposure, and preferably free of inactive support, such as carbon, would be a rational starting point. For example, the volumetric current density of NPMCs prepared by impregnating transition metal salt over porous carbon appeared to have reached an upper limit, although performances were recently elevated through pore filler and pore former approaches. 7] More recently, NPMCs prepared using the metal–organic frameworks (MOFs) as precursors have generated excellent catalytic performances. In MOFs, the frameworks are built through the metal–ligand interaction with well-defined coordination chemistry and the highest possible precursor site density. One key issue with MOFbased NPMC preparation is the removal of the high level of metal, which is currently accomplished by either high-temperature vaporization during the thermolysis or an acid wash after heat treatment. In either approach, limitations on the experimental conditions affected the versatility of the method. Herein we describe a new approach of preparing highly active NPMCs using porous organic polymer (POP) precursors containing densely populated transition-metal–nitrogen coordination sites uniformly decorated over the micropore surface. POPs have recently emerged as a new class of gasstorage and separation materials. A broad selection of monomers and cross-linking reactions provide great flexibility for producing very-high-surface-area POPs containing different functional groups. When nitrogen-containing macrocyclic functional groups, such as porphyrin or phthalocyanine, are employed as the oligomers for the synthesis, the new [*] Dr. S. Yuan, Dr. J. Shui, C. Chen, S. Commet, B. Reprogle, Dr. D.-J. Liu Chemical Sciences & Engineering Division Argonne National Laboratory, Argonne, IL 60439 (USA) E-mail: djliu@anl.gov

Conjugated block copolymers and co-oligomers: from supramolecular assembly to molecular electronics

This article reviews the progress in conjugated block copolymers and co-oligomers. The discussion focuses on recent advances in the synthesis and supramolecular assemblies of conjugated diblock copolymers. New knowledge in supramolecular chemistry learned from studies of rod–coil conjugated diblock copolymers is discussed. This article also points out the limited success in obtaining unique electronic properties brought about by the conjugated nanostructures and discusses progress in another important class of block copolymers, rod–rod diblock copolymers. Progress in synthesis and characterization of the rod–rod block co-oligomers is summarized. Unique rectification effects in conjugated diblock co-oligomers are illustrated with several molecular systems.

Charge transport mediated by d-orbitals in transition metal complexes

This communication reports an asymmetric charge transport with a large rectification ratio and finely featured NDR (negative differential resistance) by d-orbitals of a neutral ruthenium(ii) complex with a C(2) axis of symmetry.

Improving Hydrogen Adsorption Enthalpy Through Coordinatively Unsaturated Cobalt in Porous Polymers

The design and synthesis of a new porous organic polymer (POP) incorporated with cobalt carbonyl complexes through built-in bipyridinic coordination sites for hydrogen storage are described. A thermal activation process was developed to remove the ligated carbonyl and carbon dioxide in order to expose the cobalt atomically inside of porous structure. Various spectroscopic and physical characterization techniques were used to study the coordinated Co sites and the POPs surface property. Upon thermal activation, this new cobalt-containing POP showed improved hydrogen uptake capacity and isosteric heat of adsorption.