Jianglan Shui

Highly Efficient Non-Precious Metal Electrocatalysts Prepared from One-Pot Synthesized Zeolitic Imidazolate Frameworks

A facile synthesis of non-PGM ORR electrocatalysts through thermolysis of one-pot synthesized ZIF is demonstrated. The electrocatalysts exhibit excellent activity, with a maximum volumetric current density of 88.1 A cm(-3) measured at 0.8 V in PEFC tests. This approach not only makes ZIFs-based electrocatalysts easy to scale up, but also paves the way for the tailored synthesis of electrocatalysts.

Highly efficient nonprecious metal catalyst prepared with metal–organic framework in a continuous carbon nanofibrous network

Significance The performance of conventional carbon-supported catalysts is strongly influenced by the support morphology, which contains micropores, mesopores, and macropores. Whereas micropores host the majority of the active sites and macropores promote effective reagent/product mass transfer, mesopores contribute a limited role in both but occupy a significant fraction of the total pore volume. For catalytic applications where maximizing active site number and mass/charge transports with the highest possible catalyst density is essential, conventional carbon supports are no longer suitable. In this paper, we introduce a previously unidentified catalyst’s morphology with a high catalytic active surface concentrated nearly exclusively in micropores while transferring reactant/product via a macroporous nanofiber framework. The nonprecious metal catalyst with such architecture demonstrated unprecedented activity in fuel cell tests. Fuel cell vehicles, the only all-electric technology with a demonstrated >300 miles per fill travel range, use Pt as the electrode catalyst. The high price of Pt creates a major cost barrier for large-scale implementation of polymer electrolyte membrane fuel cells. Nonprecious metal catalysts (NPMCs) represent attractive low-cost alternatives. However, a significantly lower turnover frequency at the individual catalytic site renders the traditional carbon-supported NPMCs inadequate in reaching the desired performance afforded by Pt. Unconventional catalyst design aiming at maximizing the active site density at much improved mass and charge transports is essential for the next-generation NPMC. We report here a method of preparing highly efficient, nanofibrous NPMC for cathodic oxygen reduction reaction by electrospinning a polymer solution containing ferrous organometallics and zeolitic imidazolate framework followed by thermal activation. The catalyst offers a carbon nanonetwork architecture made of microporous nanofibers decorated by uniformly distributed high-density active sites. In a single-cell test, the membrane electrode containing such a catalyst delivered unprecedented volumetric activities of 3.3 A⋅cm−3 at 0.9 V or 450 A⋅cm−3 extrapolated at 0.8 V, representing the highest reported value in the literature. Improved fuel cell durability was also observed.

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

In Operando Spatiotemporal Study of Li2O2 Grain Growth and its Distribution Inside Operating Li–O2 Batteries

Nanocrystalline lithium peroxide (Li2 O2 ) is considered to play a critical role in the redox chemistry during the discharge-charge cycling of the Li-O2 batteries. In this report, a spatially resolved, real-time synchrotron X-ray diffraction technique was applied to study the cyclic formation/decomposition of Li2 O2 crystallites in an operating Li-O2 cell. The evaluation of Li2 O2 grain size, concentration, and spatial distribution inside the cathode is demonstrated under the actual cycling conditions. The study not only unambiguously proved the reversibility of the Li2 O2 redox reaction during reduction and evolution of O2 , but also allowed for the concentration and dimension growths of the peroxide nanocrystallites to be accurately measured at different regions within the cathode. The results provide important insights for future investigation on mass and charge transport properties in Li2 O2 and improvement in cathode structure and material design.