Sunghyun Uhm

A Stable and Cost-Effective Anode Catalyst Structure for Formic Acid Fuel Cells†

The high energy density, fast oxidation kinetics, and convenience of use of liquid formic acid (HCOOH), as well as the simplicity of power-system integration make direct formic acid fuel cells (DFAFCs) a promising power source for portable device applications. Considerable progress in aspects of DFAFC research and engineering has been achieved in recent years, which has enabled the fuel-cell technology to be implemented in practical devices. However, large-scale practical commercialization has been limited by several challenging issues such as high anode overpotential, excessive fuel and water permeability of the polymer electrolyte membrane, and questionable long-term durability of the fuel cells. The issue of high overpotential for anode catalysts is associated with the formation of poisons on the catalyst surface and also with the large amount of catalyst loading. To date, despite the problem of being strongly poisoned by intermediate species, Pt is the best-known anode catalyst for the oxidation of small organic molecules. Besides Pt, Pd catalysts have recently shown superior performances compared to platinum-based catalysts in the oxidation ultrapure HCOOH in DFAFCs, because of the great initial activity of Pd, even at low temperature. However, Pd catalysts have a significant drawback; their high performance is not sustained for extended time periods, largely because of the vulnerability of these catalysts towards uncharacterized intermediate species and the potential for the dissolution of Pd in acidic solutions. 12] To enhance the power performance as well as the durability of the catalyst, we have recently demonstrated that irreversible modification of the Pt metal surface with foreign metal adatoms such as Bi, 13, 14] Pb , and Sb 16] is a powerful method to drive a practical DFAFC system. In addition, an effective anode structure that alleviates mass transport of HCOOH in the membrane electrode assembly (MEA) enabled us to reduce the amount of Pt loading and extend the range of the HCOOH concentration windows. Herein, we report a novel approach for the fabrication of more stable and cost-effective anode catalysts for DFAFC by using a three-step electrochemical process. The newly developed catalyst, which contains Pt modified with Bi, is directly applied to the fuel cell to evaluate its catalytic activity and performance. The PtBi catalyst was fabricated in three consecutive electrochemical steps, namely: 1) electrochemical oxidation of carbon paper to form the adequate catalyst support, followed by 2) Pt electrodeposition, and 3) underpotential deposition (UPD) of Bi onto the Pt. The conceptual design of the final electrode is illustrated schematically in Figure 1 and

Electrodeposition of PbO2 onto Au and Ti substrates

The electrodeposition of lead dioxide (PbO2) onto Au and Ti substrates was anodically executed at 65°C and the electrochemical characteristics and the mechanism in the PbO2 films deposition were investigated by the using an in-situ electrochemical quartz crystal microbalance, cyclic voltammogram, and chronoamperometry experiments. An X-ray diffractometer and scanning electron microscope were also used for investigation of the formed lead dioxide films onto both substrates. Considering the experimental and theoretical mass/charge ratios, PbO2 deposition on Au by applying constant potential was in excellent agreement with the theoretical value, while relatively higher values of the mass/charge ratio due to film hydration were found when Ti was used as substrate. Analysis of X-ray diffractometer and scanning electron microscope show different film structures on each substrate, especially the additional hydration of the lead dioxide film deposited on Ti leads to some structural effects, identified as Sky-Lotus PbO2.

Sustainable production of formic acid by electrolytic reduction of gaseous carbon dioxide

A tin (Sn) nanostructure has been applied to a gas diffusion electrode for the direct electro-reduction of carbon dioxide (CO2) in a zero-gap electrolytic cell. A Sn catalyst layer was evenly applied to a carbon substrate by a controlled spraying technique and the efficient catalytic conversion of gas-phase CO2 to formic acid (HCOOH) demonstrated. We observed that the overall mean faradaic efficiency towards HCOOH remained above 5.0% over the entire reduction time. In addition, due to its compact configuration and surroundings at near ambient conditions the approach described is promising in both modularity and scalability. Sustainable energy sources such as solar, wind, or geothermal electricity could be used as a power source to minimize the large-scale operating cost.

An etched nanoporous Ge anode in a novel metal-air energy conversion cell.

We first report the successful synthesis of porous germanium with ordered hierarchical structures, via controlled etching, and show its performance as an anode in a new metal-air battery. Our experimental results demonstrate the potential use of porous germanium in a high power density Ge-air energy conversion cell, showing a stable long-term discharge profile at various current drains.

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.

Enhancement of methanol tolerance in DMFC cathode: addition of chloride ions.

In the operation of a direct methanol fuel cell, the modification by chloride ions on the surface of a Pt cathode can facilitate the extraordinary increase of power performance and long-term stability. Analyzing the results of cyclic voltammograms and electrochemical impedance spectroscopy, the positive shift of Pt oxidation onset potential and the depression of oxidation current are observed, which results from the role of chloride as surface inhibitor. In addition, O(2) temperature-programmed desorption and X-ray photoelectron spectroscopy also reveal that the suppression of Pt surface oxide can be best understood in terms of lower binding of oxygen species by the alteration of electronic state of Pt atoms. Such a reduced surface oxide formation not only provides more efficient proton adsorption sites with high selectivity but also decreases the mixed potential by crossover methanol, resulting in higher performance and stability even under high voltage long-term operation.

Electrocatalytic Oxidation of HCOOH on an Electrodeposited AuPt Electrode: its Possible Application in Fuel Cells

ABSTRACT: Controlled electrodeposition of dendritic nano-structured gold-platinum (AuPt) alloy onto an elec-trochemically pretreated carbon paper substrate was conducted in an attempt to improve catalystutilization and to secure an electronic percolation network toward formic acid (FA) fuel cell appli-cation. The AuPt catalysts were obtained by potentiostatic deposition. AuPt catalysts synthesizedas bimetallic alloys with 60% Au content exhibited the highest catalytic activity towards formicacid electro-oxidation. The origin of this high activity and the role of Au were evaluated, in par-ticular, by XPS analysis. Polarization and stability measurements with 1 mg cm −2 AuPt catalyst(only 0.4 mg cm −2 Pt) showed 52 mW cm −2 and sustainable performance using 3 M formic acid anddry air at 40 o C.Keywords : electrodeposition, AuPt alloy, dendritic nanostructure, formic acid fuel cells. Received August 23, 2010 : Accepted September 7, 2010 1. Introduction Small organic molecules (SOM) such as methanol,formic acid, formate, and ethanol have great potentialas possible fuels in direct small organic fuel cells.

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.