Belabbes Merzougui

Rotating Disk Electrode Investigations of Fuel Cell Catalyst Degradation Due to Potential Cycling in Acid Electrolyte

Fuel cells operated under automotive cyclic conditions are more vulnerable to membrane and electrode degradation. Degradation of Pt catalyst due to potential cycling was characterized by loss of hydrogen adsorption (HAD) area and shift in the half-wave potential for oxygen reduction at a thin film catalyst rotating disk electrode. It is shown from an analysis of the assumptions involved in calculating the HAD area that uncertainty in the potential dependence of hydrogen coverage and the inability to separate a priori the double layer charging current at Pt can lead to a 34% underestimation of the HAD area of Pt. Potential cycling between 0 and 1.2 V (RHE) for 500 cycles caused about a 20-30% decrease in HAD area for two carbon-supported platinum catalysts. This decrease followed second order kinetics, indicating that the loss of surface area is probably caused by agglomeration of Pt particles due to carbon corrosion. Analysis of oxygen reduction kinetic losses shows an increase in Tafel slope probably due to a change in the morphology of the carbon support caused by corrosion reactions. Three mechanisms are discussed for the loss of surface area and activity of Pt catalyst due to cycling.

Oxidation Resistance of Bare and Pt-Coated Electrically Conducting Diamond Powder as Assessed by Thermogravimetric Analysis

A corrosion-resistant electrocatalyst support was prepared by overcoating high surface-area diamond powder (3-6 nm diameter, 250 m 2 /g) with a thin layer of boron-doped ultrananocrystalline diamond (B-UNCD) by microwave plasma-assisted chemical vapor deposition. This core-shell approach produces electrically conducting (0.4-0.5 S/cm) and high surface-area (150―170 m 2 /g) diamond powder (B-UNCD-D). Accelerated degradation testing was performed by thermogravimetric analysis (TGA) to assess the oxidation resistance (i.e., corrosion resistance) of powder in the absence and presence of nanoscale Pt. The oxidation onset temperature for B-UNCD-D powder decreased with the Pt loading from 0 to 30 wt % (Pt/C). However, compared with the bare powder, the rate of carbon consumption was significantly greater for Pt-(XC-72) as compared to the platinized diamond powder. For example, the temperature of the maximum carbon consumption rate, T d , occurred at 426°C for Pt-(XC-72) (20% Pt/C), which was 295°C lower than the T d for bare XC-72. In contrast, T d for Pt-(B-UNCD-D, 20% Pt/C) was 558°C; a temperature that was only 62°C lower than that for bare diamond. Isothermal oxidation at 300°C for 5 h produced negligible weight loss for Pt-UNCD-D (20% Pt/C) while a 75% weight loss was observed for Pt-(XC-72) (20% Pt/C). The results clearly demonstrate that platinized diamond is more resistant to gas phase oxidation than is platinized Vulcan at elevated temperatures.

Single-Pot Synthesis of ⟨001⟩-Faceted N-Doped Nb2O5/Reduced Graphene Oxide Nanocomposite for Efficient Photoelectrochemical Water Splitting.

Due to exciting catalytic activity and selectivity, tailoring of nanocatalysts consisting of preferred crystal facets and desired structural properties remains at the forefront of materials engineering. A facile one-step nonhydrolytic solvothermal synthesis of a nanocomposite of reduced graphene oxide and one-dimensional nitrogen-doped Nb2O5 (N-NbOx) with exposed ⟨001⟩ facet is described. Triethylamine performed the dual role as nitrogen source and capping agent to control the size and unidirectional growth of Nb2O5 nanocrystallites. The nanocomposite showed efficient visible-light-mediated (λ > 420 nm) water splitting in a photoelectrochemical cell. A plausible mechanism for the formation of N-NbOx nanorods and improved photoelectrochemical efficacy in terms of their oriented growth is proposed.

Rapid microwave synthesis of high aspect-ratio ZnO nanotetrapods for swift bisphenol A detection.

Highly crystalline and high aspect-ratio ZnO nanotetrapods were grown by a novel and swift microwave synthesis. FESEM analysis revealed that each tetrapod has four thin arms and are derived from the midst of the crystal. The diameter of each arm is larger at the base and smaller at the tip. Structural analysis revealed that these nanotetrapods are single crystalline and have a wurtzite hexagonal crystal structure. These ZnO nanotetrapods were used for the detection of BPA. The electrochemical sensor based on the ZnO nanotetrapods modified electrode showed electrocatalytic activity in terms of significant improvement of the anodic current of BPA and lowering of the detection limit. Under optimized conditions, the squarewave oxidation peak current of BPA was linear over the concentration range of 12.4 nM to 1.2 μM with the detection limit of 1.7 nM and sensitivity of 5.0 μA nM(-1) cm(-2). This sensor showed high sensitivity and response compared with other electrochemical sensors reported for the detection of BPA.

Anode Materials for Mitigating Hydrogen Starvation Effects in PEM Fuel Cells

Localized hydrogen starvation at a polymer electrolyte membrane (PEM) fuel cell anode can lead to the formation of local cells in the membrane electrode assembly, which cause performance degradation at the fuel cell cathode due to carbon corrosion. We propose using hydrogen spillover materials as a hydrogen reservoir in the fuel cell anode in order to compensate for any temporary proton deficit caused by local flooding of the anode channels. We tested composite electrodes containing TiO 2 , WSi 2 , and WO 3 , and compared their behavior to that of commercial Pt/Vulcan XC-72 carbon (Pt/Vu) benchmark catalysts, using gas-diffusion electrodes in a 0.1 M HClO 4 solution and pellet electrodes in a 0.5 M H 2 SO 4 solution. While TiO 2 yields no benefits, both WSi 2 and WO 3 can significantly delay the voltage excursion of the gas-diffusion electrode into the oxygen evolution region upon the cessation of hydrogen flow. X-ray data indicate that the beneficial effect of WSi 2 may be caused by WO 3 , because we observed conversion of WSi 2 to W0 3 during voltage cycling, without a significant loss in the apparent hydrogen adsorption―desorption area. Electrodes with WO 3 yielded the best results, with a hydrogen storage charge higher by a factor of 6 than for the Pt/Vu benchmark.

Metal Carbides in Fuel Cell Cathode

Moderate-temperature fuel cells are clean power generators for both stationary and mobile applications. In particular, polymer electrolyte membrane fuel cells (PEMCs) have attracted much attention due to their high gravimetric and volumetric power densities. However, due to their acidic environment, platinum-based nanocatalysts are the only feasible electrocatalyts for such systems. High cost and limited resources of this precious metal hinder the commercialization of PEMFCs. As a result, tremendous efforts are being exerted to either reduce Pt loading or substitute Pt metal with other non-noble metals. In this context, metal carbides have been extensively investigated due to their bifunctional mechanism as a catalyst as well as a catalyst support. Hence, the aim of using metal carbides is to replace carbon support since carbon suffers from corrosion problem and at the same time to reduce a substantial amount of Pt in fuel cell cathode. In this chapter, we have given an overview on metal carbides and their benefits as catalyst support for fuel cell cathode reactions.