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 m(2)/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/cm(2) at 260 mV).

sp(2)-Bonded boron nitride aerogels are synthesized from graphene aerogels via carbothermal reduction of boron oxide and simultaneous nitridation. The color and chemical composition of the original gel change dramatically, while structural features down to the nanometer scale are maintained, suggesting a direct conversion of the carbon lattice to boron nitride. Scanning and transmission electron microscopies reveal a foliated architecture of wrinkled sheets, a unique morphology among low-density, porous BN materials. The converted gels display a high degree of chemical purity (>95%) and crystalline order and exhibit unique cross-linking structures.

An impedancemetric method for NO x sensing using an yttria-stabilized zirconia ͑YSZ͒-based electrochemical cell is described. The sensor cell consists of a planar YSZ electrolyte and two identical YSZ/Cr 2 O 3 composite electrodes exposed to the test gas. The sensor response to a sinusoidal ac signal applied between the two electrodes is measured via two parameters calculated from the complex impedance, the modulus ͉Z͉ and phase angle ⌰. While either of these parameters can be correlated to the NO x concentration in the test gas, ⌰ was found to provide a more robust metric than ͉Z͉. At frequencies below approximately 100 Hz, ⌰ is sensitive to both the NO x and O 2 concentrations. At higher frequencies, ⌰ is predominantly affected by the O 2 concentration. A dual frequency measurement is demonstrated to compensate for changes in the O 2 background between 2 and 18.9%. Excellent sensor performance is obtained for NO x concentrations in the range of 8-50 ppm in background. An equivalent-circuit model was used to extract fitting parameters from the impedance spectra for a preliminary analysis of NO x -sensing mechanisms.

An electrochemical cell ͓Au/yttria-stabilized zirconia ͑YSZ͒/Au͔ serves as a model system to investigate the effect of O 2 and NO x . 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 NO x response. Impedance data were obtained over a range of frequencies ͑0.1 Hz to 1 MHz͒, temperatures ͑600-700°C͒, and oxygen ͑2-18.9%͒ and NO x ͑10-100 ppm͒ concentrations. Spectra were fit with an equivalent circuit, and values of the circuit elements were evaluated. In the absence of NO x , the effect of O 2 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 O 2 concentrations. When both O 2 and NO x 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 NO x sensing are also discussed.The development of NO x sensors has been motivated primarily by environmental concerns and the automotive industry's desire to monitor gases in the exhaust stream. 1 Fast, reliable sensors are needed in order to meet increasingly stringent governmental regulations for emission limits. Ceramic metal oxides are candidate materials for operation in harsh, high-temperature environments, especially the oxygen-ion conductor yttria-stabilized zirconia ͑YSZ͒. YSZ is currently used for automotive oxygen sensors and has shown good stability and operation at temperatures 700°C and higher. 1-3 NO x sensor development poses significant challenges due to a number of issues including cost, sensitivity, stability, and response time. In the past decade, development of YSZ-based NO x sensors has focused on amperometric and potentiometric operation. 1-3 Amperometric operation typically measures a diffusion-limited current and has been shown to be effective as an NO-selective or a total-NO x sensor. Typically, research focuses on various metal-oxide electrodes to optimize the response. 4-7 In order to isolate the NO x from the O 2 response, a separate pumping cell may be necessary to maintain a constant O 2 concentration at the sensing electrode, leading to complicated device structures. 1-3 Potentiometric sensors correlate the measured open circuit potential ͑OCP͒ to the gas composition. The OCP can be measured between an electrode in the test atmosphere and another electrode in reference gas, or between dissimilar electrodes in the same atmosphere. In potentiometric operation, the response to NO 2 is generally opposite in sign to that of NO, and generally larger, making total-NO x sensing difficult. 8 More recently, a YSZ-based impedancemetric technique has been reported for...