Dae-Soo Yang

Phosphorus-Doped Ordered Mesoporous Carbons with Different Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media

Phosphorus-doped ordered mesoporous carbons (POMCs) with different lengths were synthesized using a metal-free nanocasting method of SBA-15 mesoporous silica with different sizes as template and triphenylphosphine and phenol as phosphorus and carbon sources, respectively. The resultant POMC with a small amount of P doping is demonstrated as a metal-free electrode with excellent electrocatalytic activity for oxygen reduction reaction (ORR), coupled with much enhanced stability and alcohol tolerance compared to those of platinum via four-electron pathway in alkaline medium. Interestingly, the POMC with short channel length is found to have superior electrochemical performances compared to those with longer sizes.

Ultra-high Li storage capacity achieved by hollow carbon capsules with hierarchical nanoarchitecture

Hollow core–mesoporous shell carbon (HCMSC) with hierarchical nanoarchitecture was prepared and explored as an anode with ultra-high Li storage capacity in Li ion batteries. Compared with commercial graphite and ordered mesoporous carbon (CMK-3), the HCMSC not only demonstrates higher Li storage capacity, but also better cycling performance and rate capability. HCMSC possesses unique structural characteristics such as large surface area and mesopore volume. Particularly the hierarchical macro-scaled hollow core/mesoporous shell nanostructure along with 3D large interconnected interstitial volume guarantees fast mass transport in HCMSC, resulting in ultra-high Li storage capacity and excellent cycling performance and rate capability. Furthermore, the hollow macro-scaled core encapsulated in a well-developed 3D interconnected mesoporous shell serves as an efficient Li storage and buffer reservoir to reduce volume change during the charge–discharge cycling especially at high rates, which contributes greatly to the enhanced cycling performance and rate capability.

Morphology-dependent Li storage performance of ordered mesoporous carbon as anode material.

Rod-shaped ordered mesoporous carbons (OMCs) with different lengths, prepared by replication method using the corresponding size-tunable SBA-15 silicas with the same rodlike morphology as templates, are explored as anode material for Li-ion battery. All of the as-synthesized OMCs exhibit much higher Li storage capacity and better cyclability along with comparable rate capability as compared with commercial graphite. Particularly, the OMC-3 with the shortest length demonstrates the highest reversible discharge capacity of 1012 mAh g(-1) at 100 mA g(-1) and better cyclability with 86.6% retention of initial capacity after 100 cycles. Although the Coulombic efficiencies of all the OMCs are relatively low at the beginning, they improve promptly and after 10 cycles reach the level comparable to commercial graphite. Based on their specific capacity, cycle efficiency, and rate capability, the OMC-3 outperforms considerably its carbon peers, OMC-1 and OMC-2 with longer length. This behavior is mainly attributed to higher specific surface area, which provides more active sites for Li adsorption and storage along with the larger mesopore volume and shorter mesopore channels, which facilitate fast Li ion diffusion and electrolyte transport. The enhancement in reversible Li storage performance with decrease in the channel length is also supported by low solid electrolyte interphase resistance, contact resistance, and Warburg impedance in electrochemical impedance spectroscopy.

Preparation of Nitrogen-Doped Porous Carbon Nanofibers and the Effect of Porosity, Electrical Conductivity, and Nitrogen Content on Their Oxygen Reduction Performance

Nitrogen‐doped carbon nanostructures are considered as a possible alternative to platinum‐based catalysts for fuel cells. The surface density of catalytic sites, electrical conductivity, and nitrogen content play important roles in designing electrode materials for fuel cells. Herein, N‐doped carbon fibers are prepared by electrospinning the poly(acrylonitrile) (PAN) solution followed by carbonization. Some of the key issues of the oxygen reduction reaction (ORR) are addressed in terms of nitrogen content, porosity, and electrical conductivity in the N‐containing carbon nanofibrous system. Nitrogen content and the amount of the graphitic phase are varied by changing the carbonization temperature. In addition, N‐doped carbon fibers with high porosity are prepared by electrospinning the solution mixture of poly(ethylene oxide) (PEO)/PAN followed by carbonization, and the porosity is tuned by varying the ratio of PEO to PAN. The effect of porosity or the surface density of catalytic sites on the ORR is studied. A medium porous sample prepared from the PEO/PAN mixture in a 1:1 ratio by carbonization at 1000 °C is found to be favorable for improved ORR performance for such a system. The observations made herein are explained in terms of trade‐offs between electrical conductivity, nitrogen content, and surface density of active sites.

Iron–polypyrrole electrocatalyst with remarkable activity and stability for ORR in both alkaline and acidic conditions: a comprehensive assessment of catalyst preparation sequence

A new facile template-free method is presented to synthesize Fe-treated N-doped carbon (Fe/N–C) catalysts for oxygen reduction reaction (ORR) by employing a synthesis protocol of pyrolysis–leaching–stabilization (PLS) sequence of polypyrrole in the presence of ferric source, which serves dual purposes of an oxidant for pyrrole polymerization and an iron source. Each step in the PLS sequence is assessed in detail in terms of the related structural properties of the resulting carbon catalysts, and their effects on ORR activities are elaborated to confirm the validity of the current synthesis protocol. It is found that the as-prepared carbon catalyst exhibits outstanding high catalytic activity in both alkaline and acidic conditions. The carbon catalyst prepared at a pyrolysis temperature of 900 °C (FePPyC-900) shows remarkably high ORR activity with onset potential of 0.96 V (vs. RHE), which is similar to that of Pt/C, whereas the half-wave potential (E1/2) of FePPyC-900 is 0.877 V, more positive than that of Pt/C at the same catalyst loading amount under alkaline conditions. Furthermore, the FePPyC-900 catalyst also illustrates exceptionally high activity under acidic conditions with onset and half-wave potentials of 0.814 and 0.740 V, respectively, which are almost comparable to those (0.817 and 0.709 V) of the state-of-the-art Pt/C catalyst, which is rarely observed for non-Pt-based carbon catalysts. In addition, the FePPyC-900 catalyst displays much better stability and methanol tolerance than the Pt/C and exhibits a four electron transfer pathway under both alkaline and acidic conditions. Such extraordinary high ORR activity and stability of the FePPyC-samples can be attributed to the implementation of extra stabilization step in addition to conventional sample preparation steps of pyrolysis and subsequent leaching in current PLS synthesis protocol as well as to the use of highly conducting PPy as a single precursor of carbon and nitrogen in the presence of Fe.

Nitrogen‐Doped Ordered Mesoporous Carbon with Different Morphologies for the Oxygen Reduction Reaction: Effect of Iron Species and Synergy of Textural Properties

Nitrogen‐doped ordered mesoporous carbons (N‐OMCs) with different morphologies are prepared as oxygen reduction reaction (ORR) catalysts through pyrolysis of iron phthalocyanine‐infiltrated SBA‐15 silica with different mesochannel lengths. Excellent ORR activity with a nearly four‐electron transfer process is observed in both alkaline and acidic media. In particular, the difference in half‐wave potential for ORR relative to commercial Pt/C catalyst is only 50 mV negative in acidic medium, whereas it is 50 mV more positive in alkaline medium. Interestingly, it is found that although the use of iron is necessary for the preparation of highly active nitrogen‐doped ORR carbon catalysts, its presence is not necessary for N‐OMC to be active in the ORR in either alkaline or acidic media. In addition, the ORR activity increases gradually with decreasing mesopore channel length, with maximum activity in N‐OMC with short channels, demonstrating the high synergistic influence of structural morphology on ORR in heteroatom‐doped carbon.

Simple approach to advanced binder-free nitrogen-doped graphene electrode for lithium batteries

A simple binder-free synthesis approach of just rubbing nitrogen-doped reduced graphene oxide (N-RGO) powder on a mechanically grinded Cu-foil substrate with a rough surface is proposed for a lithium ion battery (LIB). The nitrogen content of N-RGO is found to be 2.1 wt%. The binder-free N-RGO electrode shows excellent reversible capacity of 551 mA h g−1 as compared to 433 mA h g−1 of the binder-added N-RGO electrode at a current density of 50 mA g−1 after 100 cycles. The process is not only highly reproducible and successful, but also results in high LIB performance, proposing easy scaling-up of such an electrode for commercial application.

Poly(p-phenylene)-based membrane materials with excellent cell efficiencies and durability for use in vanadium redox flow batteries

Poly(p-phenylene)-based ionomers with remarkable durability and rate capability for use in vanadium redox flow batteries (VRFBs) are reported. The family of synthesized ionomers, sPBPSP-z, exhibited not only well-developed phase separation between hydrophilic domains and hydrophobic domains but also well-connected hydrophilic channels, resulting in enhanced proton conductivities and excellent dimensional stabilities. sPBPSP-8, which has an ion exchange capacity of 1.83 meq g−1, showed high discharge capacity retention and superior efficiencies over 100 cycles at a current density of 50 mA cm−2. In addition, the sPBPSP-8 ionomer exhibited stable performance at various current densities (50–180 mA cm−2) and retained high efficiencies at high current densities. Furthermore, this material exhibited superior chemical stability under oxidizing conditions, excellent capacity retention, and high efficiencies during long-term VRFB operation (1000 cycles). These results indicate that the sPBPSP-8 membrane is a superb material for VRFB applications.