Beomgyun Jeong

Unravelling the electrochemical double layer by direct probing of the solid/liquid interface

The electrochemical double layer plays a critical role in electrochemical processes. Whilst there have been many theoretical models predicting structural and electrical organization of the electrochemical double layer, the experimental verification of these models has been challenging due to the limitations of available experimental techniques. The induced potential drop in the electrolyte has never been directly observed and verified experimentally, to the best of our knowledge. In this study, we report the direct probing of the potential drop as well as the potential of zero charge by means of ambient pressure X-ray photoelectron spectroscopy performed under polarization conditions. By analyzing the spectra of the solvent (water) and a spectator neutral molecule with numerical simulations of the electric field, we discern the shape of the electrochemical double layer profile. In addition, we determine how the electrochemical double layer changes as a function of both the electrolyte concentration and applied potential.

Excavated Fe-N-C sites for enhanced electrocatalytic activity in the oxygen reduction reaction.

Platinum (Pt) is the best electrocatalyst for the oxygen reduction reaction (ORR) in hydrogen fuel cells, but it is an extremely expensive resource. The successful development of a cost-effective non-Pt ORR electrocatalyst will be a breakthrough for the commercialization of hydrogen-air fuel cells. Ball milling has been used to incorporate metal and nitrogen precursors into micropores of carbon more effectively and in the direct nitrogen-doping of carbon under highly pressurized nitrogen gas in the process of the preparation of non-noble ORR catalysts. In this study, we first utilize ball milling to excavate the ORR active sites embedded in Fe-modified N-doped carbon nanofibers (Fe-N-CNFs) by pulverization. The facile ball-milling process resulted in a significant enhancement in the ORR activity and the selectivity of the Fe-N-CNFs owing to the higher exposure of the metal-based catalytically active sites. The degree of excavation of the Fe-based active sites in the Fe-N-CNFs for the ORR was investigated with cyclic voltammetry, X-ray photoelectron spectroscopy, and pore-size distribution analysis. We believe that this simple approach is useful to improve alternative ORR electrocatalysts up to the level necessary for practical applications.

Diagnosis of the measurement inconsistencies of carbon-based electrocatalysts for the oxygen reduction reaction in alkaline media

Finding inexpensive alternative catalysts for the oxygen reduction reaction (ORR) is considered as one of the most overriding challenges in the development of electrochemical technologies. Although significant progress has been made in developing carbon-based ORR catalysts, there is difficulty in judging improvements in the catalysts due to the inconsistent results arising from differences in experimental conditions. In this review, we provide a diagnosis of the influence of key factors in the measured ORR activity of catalysts. Knowing the exact conditions when measuring ORR activity is of paramount importance in establishing a reference for relevant comparison of ORR performance in developed catalysts.

Electrocatalytic Oxidation of Formic Acid: Closing the Gap Between Fundamental Study and Technical Applications

Electro-catalytic oxidation of formic acid has a significant importance in fundamental research of small organic molecule oxidation, as well as practical application to fuel cells. Notwithstanding intensive research efforts for last couple of decades, there are still fundamental questions in debate on the mechanistic origin. This perspective presents underlying issues in the electro-catalytic oxidation of formic acid. Until now, the oxidation mechanism of formic acid is not fully understood in spite of its importance of this work, since the role of adsorbed formate is not clearly identified. In addition, we will discuss on the role of Bi on Pt that unambiguously enhances the activity of Pt in fuel cell systems but is in debate with single crystal Pt surface. We finally accentuate the causes of the deactivation of Pd catalysts, because the utilization of non-Pt electrocatalysts could be one of the key researches for cost reduction of fuel cell systems. Thus, we intend to introduce different views toward formic acid oxidation proposed by the other researchers and provide perspectives on the further research to close the gap between fundamental study and technical applications.

Iron-Cobalt Modified Electrospun Carbon Nanofibers as Oxygen Reduction Catalysts in Alkaline Fuel Cells

Iron, cobalt, and carbon nanofiber (FeCo-CNF) composite electrocatalysts for the oxygen reduction reaction (ORR) in alkaline fuel cells (AFCs) have been fabricated via a simple and cost-effective process―electrospinning and the subsequent pyrolysis of a mixture of a nitrogen-containing polymer and organo-metallic compounds. From subsequent electrochemical characterizations, the resultant FeCo-CNFs catalysts demonstrated comparable electrocatalytic activity and stability to commercial carbon-supported platinum (Pt/C) for ORR, a direct 4-electron reduction pathway, and better ethanol tolerance than Pt/C in an alkaline electrolyte. According to material characterizations, metal inclusion seems to result in oxygen-related active site formations and enhancement of electrical conductivity. Therefore, with this synthesis approach, we expect that it is possible to make advanced cathode materials with high conductivity, surface area, ORR activity, and stability for AFCs without requiring noble metal catalysts.

Improved Specific Capacitance of Amorphous Vanadium Pentoxide in a Nanoporous Alumina Template

This article investigates the physicochemical properties of vanadium pentoxide (V 2 O 5 ) prepared by anodic electrodeposition onto two nanoporous alumina templates having pore sizes of 50 and 200 nm. V 2 O 5 formed inside the templates was an amorphous structure with flakelike (50 nm) and round (200 nm) shapes. The capacitance of V 2 0 5 was measured using cyclic voltammetry in 1 M KCl electrolyte, and ca. 900 F g -1 of maximum specific capacitance was obtained from V 2 O 5 in 50 nm alumina oxide. Improved specific capacitance of V 2 O 5 is ascribed to the high surface area of the porous structure by the use of the template.

Electrode Architecture in Galvanic and Electrolytic Energy Cells

Electrodes in galvanic and electrolytic energy cells are complicated structures comprising redox-active materials, ionic/electronic conductors, and porous pathways for mass transfer of reactants. In contrast to breakthroughs in component development, methods of optimizing whole-system architectural design to draw maximum output have not been well explored. In this Minireview, we introduce generalized types of electrode architecture, discuss fabrication strategies, and characterize already built structures. Systematic efforts to discover optimal electrode configurations will resolve long-standing discrepancies that arise between whole systems and the sums of their parts for a number of electrochemical reactions and technologies.

In situ analysis of post-annealing effect on Sn-doped indium oxide films

Oxygen post-annealing effects on tin (Sn) doped indium oxide (ITO) film are investigated with various analytical tools as a function of temperature, including in situ XRD, ambient pressure XPS (AP-XPS), and Hall measurement. As the annealing temperature increases up to 200 °C under the oxygen pressure of 100 mTorr, the in situ XRD shows the evidence of crystallization of the film while the AP-XPS reveals the formation of oxygen vacancy and Sn4+ states on surface. In addition, the mobility of ITO thin film is increased as the post-annealing temperature increases, supporting the results of both in situ XRD and AP-XPS. The results of angle-resolved XPS reveal that the degree of Sn segregation changes little after post-annealing procedure.

Chemical states of surface oxygen during CO oxidation on Pt(1 1 0) surface revealed by ambient pressure XPS

The study of CO oxidation on Pt(110) surface is revisited using ambient pressure x-ray photoemission spectroscopy. When the surface temperature reaches the activation temperature for CO oxidation under elevated pressure condition, both the -phase of PtO2 oxide and chemisorbed oxygen are formed simultaneously on the surface. Due to exothermic nature of CO oxidation, the temperature of Pt surface increases as CO oxidation takes places. As the CO/O2 ratio increases, the production of CO2 increases continuously and the surface temperature also increases. Interestingly, during the mass transfer limiting regions, the amount of surface oxide changes little while the chemisorbed oxygen is being reduced. .The study of CO oxidation on Pt(1 1 0) surface is revisited using ambient pressure x-ray photoemission spectroscopy. When the surface temperature reaches the activation temperature for CO oxidation under elevated pressure conditions, both the α-phase of PtO2 oxide and chemisorbed oxygen are formed simultaneously on the surface. Due to the exothermic nature of CO oxidation, the temperature of the Pt surface increases as CO oxidation takes place. As the CO/O2 ratio increases, the production of CO2 increases continuously and the surface temperature also increases. Interestingly, within the diffusion limited regions, the amount of surface oxide changes little while the chemisorbed oxygen is reduced.

On the Origin of Reactive Pd Catalyst for an Electrooxidation of Formic Acid

We investigated the reactivation of Pd catalysts during the electrocatalytic oxidation of formic acid. Pd catalyst was electrochemically deposited onto carbon paper substrate that was characterized by XRD, SEM, and XPS. Pd catalysts showed fast deactivation, though their activity could be simply recovered by applying a reduction potential at which hydrogen generation can occur. XPS results revealed that the surface of Pd catalysts is significantly affected by interaction with formic acid. High coverage of formic acid and/or formate is found at active state of Pd catalyst and diminished as deactivation proceeds. We concluded that cathodic polarization may remove poisoning species on the Pd electrode and form an active site for formic acid oxidation. This reactivation method can sustain a durability of Pd catalysts.