Impedance Characterization of a Model Au/Yttria-Stabilized Zirconia/Au Electrochemical Cell in Varying Oxygen and NO\u3csub\u3e\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3e Concentrations

An electrochemical cell [Au/yttria-stabilized zirconia (YSZ)/Au] serves as a model system to investigate the effect of O2 and NOx. Possible mechanisms responsible for the response are presented. Two dense Au electrodes are co-located on the same side of a dense YSZ electrolyte and are separated from the electrolyte by a porous YSZ layer, present only under the electrodes. While not completely understood, the porous layer appears to result in enhanced NOx response. Impedance data were obtained over a range of frequencies 0.1 Hz to 1 MHz, temperatures 600–700°C, and oxygen 2–18.9% and NOx 10–100 ppm concentrations. Spectra were fit with an equivalent circuit, and values of the circuit elements were evaluated. In the absence of NOx, the effect of O2 on the low-frequency arc resistance could be described by a power law, and the temperature dependence by a single apparent activation energy at all O2 concentrations. When both O2 and NOx were present, however, the power-law exponent varied as a function of both temperature and concentration, and the apparent activation energy also showed dual dependence. Adsorption mechanisms are discussed as possibilities for the rate-limiting steps. Implications for impedancemetric NOx sensing are also discussed

Effect of Electrode Composition and Microstructure on Impendancemetric nitric oxide sensors based on YSZ Electrolyte

The role of metal (Au, Pt, and Ag) electrodes in yttria-stabilized zirconia (YSZ) electrolyte-based impedancemetric nitric oxide (NO) sensors is investigated using impedance spectroscopy and equivalent circuit analysis. Focus on the metal/porous YSZ interface is based on previous studies using a symmetric cell (metal/YSZporous/YSZdense/YSZporous/metal) and attempts to further elucidate the important processes responsible for sensing. The current test cell consists of a rectangular slab of porous YSZ with two metal-wire loop electrodes (metal/YSZporous/metal), both exposed to the same atmosphere. Of the electrode materials, only Au was sensitive to changes in NO concentration. The impedance behavior of porous Au electrodes in a slightly different configuration was compared with dense Au electrodes and was also insensitive to NO. Although the exact mechanism is not determined, the composition and microstructure of the metal electrode seem to alter the rate-limiting step of the interfering O2 reaction. Impedance behavior of the O2 reaction that is limited by processes occurring away from the triple-phase boundary may be crucial for impedancemetric NO sensing

Ultralow Density, Monolithic WS₂, MoS₂, and MoS₂/Graphene Aerogels

We describe the synthesis and characterization of monolithic, ultralow density WS₂ and MoS₂ aerogels, as well as a high surface area MoS₂/graphene hybrid aerogel. The monolithic WS₂ and MoS₂ aerogels are prepared via thermal decomposition of freeze-dried ammonium thio-molybdate (ATM) and ammonium thio-tungstate (ATT) solutions, respectively. The densities of the pure dichalcogenide aerogels represent 0.4% and 0.5% of full density MoS₂ and WS₂, respectively, and can be tailored by simply changing the initial ATM or ATT concentrations. Similar processing in the presence of the graphene aerogel results in a hybrid structure with MoS₂ sheets conformally coating the graphene scaffold. This layered motif produces a ∼50 wt % MoS₂ aerogel with BET surface area of ∼700 m²/g and an electrical conductivity of 112 S/m. The MoS₂/graphene aerogel shows promising results as a hydrogen evolution reaction catalyst with low onset potential (∼100 mV) and high current density (100 mA/cm² at 260 mV)