Sung-Dae Yim

Self-Supported Mesostructured Pt-Based Bimetallic Nanospheres Containing an Intermetallic Phase as Ultrastable Oxygen Reduction Electrocatalysts

Developing highly active and stable cathode catalysts is of pivotal importance for proton exchange membrane fuel cells (PEMFCs). While carbon-supported nanostructured Pt-based catalysts have so far been the most active cathode catalysts, their durability and single-cell performance are yet to be improved. Herein, self-supported mesostructured Pt-based bimetallic (Meso-PtM; M = Ni, Fe, Co, Cu) nanospheres containing an intermetallic phase are reported, which can combine the beneficial effects of transition metals (M), an intermetallic phase, a 3D interconnected framework, and a mesoporous structure. Meso-PtM nanospheres show enhanced oxygen reduction reaction (ORR) activity, compared to Pt black and Pt/C catalysts. Notably, Meso-PtNi containing an intermetallic phase exhibits ultrahigh stability, showing enhanced ORR activity even after 50 000 potential cycles, whereas Pt black and Pt/C undergo dramatic degradation. Importantly, Meso-PtNi with an intermetallic phase also demonstrated superior activity and durability when used in a PEMFC single-cell, with record-high initial mass and specific activities.

Enhancement of oxygen reduction reaction activities by Pt nanoclusters decorated on ordered mesoporous porphyrinic carbons

The high cost of Pt-based membrane electrode assemblies (MEAs) is a critical hurdle for the commercialization of polymer electrolyte fuel cells (PEFCs). Recently, non-precious metal-based catalysts (NPMCs) have demonstrated much enhanced activity but their oxygen reduction reaction (ORR) activity is still inferior to that of Pt-based catalysts resulting in a much thicker electrode in the MEA. For the reduction of mass transport and ohmic overpotential we adopted a new concept of catalyst that combines an ultra-low amount of Pt nanoclusters with metal–nitrogen (M–Nx) doped ordered mesoporous porphyrinic carbon (FeCo–OMPC(L)). The 5 wt% Pt/FeCo–OMPC(L) showed a 2-fold enhancement in activities compared to a higher loading of Pt. Our experimental results supported by first-principles calculations indicate that a trace amount of Pt nanoclusters on FeCo–OMPC(L) significantly enhances the ORR activity due to their electronic effect as well as geometric effect from the reduced active sites. In terms of fuel cell commercialization, this class of catalysts is a promising candidate due to the limited use of Pt in the MEA.

The Analysis on the Activation Procedure of Polymer Electrolyte Fuel Cells

It is, in general, believed that during the activation process, the proton conductivity increases due to wetting effect and the electrochemical resistance reduction, resulting in an increase in the fuel cell performance with time. However, until now, very scant information is available on the understanding of activation processes. In this study, dominant variables that effect on the performance increase of membrane electrode assemblies (MEAs) during the activation process were investigated. Wetting, pore restructuring and active metal utilization were analyzed systematically. Unexpectedly, the changes for both ohmic and reaction resistance characterized by the electrochemical impedance spectroscopy (EIS) after initial wetting process were much smaller when considering the degree of cell performance increases. However, the EIS spectra represents that the pore opening of electrode turns into gas transportable structure more easily. The increase in the performance with activation cycles was also investigated in a view of active metals. Though the particle size was grown, the number of effective active sites might be exposed more. The impurity removal and catalytic activity enhancement measured by cyclic voltammetry (CV) could be a strong evident. The results and analysis revealed that, not merely wetting of membrane but also restructuring of electrodeand catalytic activity increase are important factors for the fast and efficient activation of the polymer electrolyte fuel cells.

Highly Durable Supportless Pt Hollow Spheres Designed for Enhanced Oxygen Transport in Cathode Catalyst Layers of Proton Exchange Membrane Fuel Cells

Supportless Pt catalysts have several advantages over conventional carbon-supported Pt catalysts in that they are not susceptible to carbon corrosion. However, the need for high Pt loadings in membrane electrode assemblies (MEAs) to achieve state-of-the-art fuel cell performance has limited their application in proton exchange membrane fuel cells. Herein, we report a new approach to the design of a supportless Pt catalyst in terms of catalyst layer architecture, which is crucial for fuel cell performance as it affects water management and oxygen transport in the catalyst layers. Large Pt hollow spheres (PtHSs) 100 nm in size were designed and prepared using a carbon template method. Despite their large size, the unique structure of the PtHSs, which are composed of a thin-layered shell of Pt nanoparticles (ca. 7 nm thick), exhibited a high surface area comparable to that of commercial Pt black (PtB). The PtHS structure also exhibited twice the durability of PtB after 2000 potential cycles (0-1.3 V, 50 mV/s). A MEA fabricated with PtHSs showed significant improvement in fuel cell performance compared to PtB-based MEAs at high current densities (>800 mA/cm2). This was mainly due to the 2.7 times lower mass transport resistance in the PtHS-based catalyst layers compared to that in PtB, owing to the formation of macropores between the PtHSs and high porosity (90%) in the PtHS catalyst layers. The present study demonstrates a successful example of catalyst design in terms of catalyst layer architecture, which may be applied to a real fuel cell system.

Novel catalyst layer synthesized by an in situ sol–gel process with tetraethoxysilane in a Nafion ionomer solution with Pt/C for PEFCs: the effect of self-assembled Nafion–SiO2 on Pt ORR activity and an increased water content in the polymer membranes

A highly optimized catalyst layer, under low humidity conditions, was developed through an in situ sol–gel process with tetraethoxysilane (TEOS) in a Nafion ionomer solution with Pt/C in order to prepare a polymer electrolyte fuel cell (PEFC). This method is quick and does not require the use of additional solvents to accomplish the sol–gel reaction of TEOS at high temperatures compared to other ex situ sol–gel methods. Amorphous SiO2 nanoparticles of <5 nm were distributed uniformly in the vicinity of the graphitic carbons and the Pt electrocatalysts without significant particle aggregation. In the in situ sol–gel reaction, due to their very high acidity, –SO3H groups attached to the Nafion polymer conveniently served as catalyst to facilitate the hydrolysis reaction. Hydrolyzed alkoxysilanes migrated to the negatively charged sulfonate groups (–SO3−) due to a rapid increase in the ζ potential of the SiO2 nanoparticles at pH < 2, which resulted in a core–shell structure in the catalyst ink during the drying of the membrane–electrode assemblies (MEAs). A core–shell structure that consisted of positively charged SiOH2+ groups and negatively charged –SO3− in the Nafion ionomer was evaluated for its enhancement of the water content in the polymer membranes, as well as its effect on the specific activity of Pt. When an MEA has SiO2 nanoparticles only within the cathode catalyst layer under low humidity, the chemically-adsorbed water molecules on the silanol groups are released into the cathode catalyst layer, increasing the water content at the cathode catalyst layer, which increases the water transport from the cathode to the anode (i.e., back diffusion). The enhanced back diffusion mitigated the dehydration of the membrane under low humidity conditions. Furthermore, it is interesting to note that the core–shell structure between positively-charged SiOH2+ groups and negatively-charged –SO3− groups in the Nafion ionomer may reduce the specific adsorption of –SO3− groups on the surface of Pt, which enhances the kinetics of the oxygen reduction reaction (ORR) in the catalyst layer of the cathode.

메탄올 수증기 개질반응에서의 상용촉매 비교연구

− The comparison work was conducted for the methanol steam reforming among commercial Cu-based catalysts, viz. ICI-M45, which is for the methanol synthesis, MDC-3 and MDC-7, which are for the water-gas shift reaction. The catalytic activity for the water-gas shift reaction was also compared over three catalysts. Among them, MDC-7 showed the highest methanol conversion and formation rate of hydrogen and carbon dioxide at 473 K for the methanol steam reforming. To find out any promotional effect between ICI-M45 and MDC-7, three different packing methods with these two catalysts were examined. However, no synergistic effect was observed. The catalytic activity for watergas shift reaction decreased in the following order: MDC-7 > MDC-3 > ICI-M45. The highest activity of MDC-7 for the methanol steam reforming as well as the water-gas shift reaction can be due to its high surface area, copper dispersion, and an adequate Cu/Zn ratio.

The Operating Characteristics in an Air-Breathing PEMFC

Air-breathing polymer electrolyte membrane fuel cells (PEMFC) are highly promising particularly for small-power applications up to tens watts class. A distinctive feature of the air-breathing PEMFC is its simple system configuration in which axial fans operate for dual purposes, supplying both oxidant and coolant in a single manner. In the present study, a nominal 80W air-breathing PEMFC system is developed and investigated to determine the optimal operating strategy through parametric studies (i.e., reactant humidity, and fan-blowing flow rate). The cell voltage distributions are examined as a function of time to evaluate the system performance under various operating conditions.Copyright

Influence of Water Behavior in the Gas Diffusion Layer on the Performance of PEMFC

The affect of water behavior on the performance of the polymer electrolyte membrane fuel cell (PEMFC) was investigated experimentally. To understand the water transportation phenomena systematically, the gas diffusion layers were divided into two parts: the gas diffusion medium (GDM) and the micro-layer (ML). In this work, different Teflon (PTFE) contents in the GDM were intensively investigated under various single cell operation conditions. Current-Voltage (I-V) performance curves of single cells were compared and analyzed with respect to water transportation in the GDM. Through the results of this work, the dominant driving forces of the water transportation in the gas diffusion layer were determined which aids in designing the gas diffusion layers.Copyright

Principal design parameters of electro-catalysts for PEMFCs

ABSTRACT Design parameters of the anode catalyst for the polymer electrolyte membrane fuel cells(PEMFCs) were investigated in the aspect of active metal size and inter-metal distance. Pt-Ru catalysts which have various morphologies could be prepared by using systematically pretreated Ketjenblacks. The electro-catalysts were characterized to get their physicochemical properties by XRD, and Ar adsorption/desorption. And the performances of electro-catalysts were evaluated and compared as an electrode of PEMFCs by single cell test. Based on the relationship between the I-V performance and the morphology of catalysts, the basic parameters for the preparation of catalysts are suggested to be in the ranges of about 2.0 to 2.8nm and 5.0 to 14.2nm for the active metal size and inter-metal distance, respectively. In addition, as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than intermetal distances

An Improved Design of Microchannel Fuel Processors

This paper focuses on a new systematic configuration of micro-channel fuel processors, particularly designed for portable applications. An alternative integration method of the micro-channel fuel processors is attempted to overcome the serious thermal unbalance and to minimize the system volume by introducing the direct contact method of the sub-components. An integrated micro-channel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bounding micro-channel patterned stainless steel plates, including fuel vaporizer, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al2 O3 catalyst (ICI Synetix 33-5) was coated inside micro-channel of the unit reactor for steam reforming. The steam reforming reaction was conducted in the temperature range of 200°C to 260°C in the basis of reformer side end-plate and the temperature was controlled by varying methanol feeding into the combustor. More than 99% of methanol was converted at 240°C of reformer side temperature. A mechanism-based numerical model aimed at enhancing physical understanding and optimizing designs has been developed for improved micro-channel fuel processors. A two-dimensional numerical model in the reformer section created to model the phenomena of species transport and reaction occurring at the catalyst surface. The mass, momentum, and species equations were employed with kinetic equations that describe the chemical reaction characteristics to solve flow-field, methanol conversion rate, and species concentration variations along the micro-channel. This mechanism-based model was validated against the experimental data from the literature and then applied to various layouts of the micro-channel fuel processors targeted for the optimal catalyst loading and fuel reforming purpose. The computer-aided models developed in this study can be greatly utilized for the design of advanced fast-paced micro-channel fuel processors research.Copyright