chemical, [5] gas, humidity, [6] and strain sensing applications. [7,8] Various types of paper-based sensors have been reported, and these can be broadly categorized as optical sensors and electrochemical sensors. Optical sensors include colorimetric, surface-enhanced Raman spectroscopy, and fluorescence sensors, whereas electrochemical sensors include voltammetrybased, potentiometric, and chemiresistive sensors. [9] Chemiresistive sensors are preferred for gas sensing applications due to their low-cost fabrication, portability, and easy-to-read output. [10] In contrast to paper-based chemical sensors and biosensors that are typically disposable, [11] gas sensors, especially chemiresistive sensors, can be used to reversibly detect gases over extended periods of time. Hence, paper-based chemiresistive sensors are promising as ultra-low-cost chemiresistive sensors for gases such as NO 2 , [12] NH 3 , [13] H 2 S, [14] and H 2 . [15] Such paper-based chemiresistive gas sensors often outperform sensors using other (nonporous) substrates such as silicon or polymers, due to synergistic effects of the flexible porous paper coupled with simplicity of fabrication and use. [16] Hydrogen is a highly explosive gas with a lower explosion limit of 4% in air. It also has high diffusivity through many materials due to its small size, creating challenges in safely storing and transporting it. [17] These limitations are a key factor influencing future development of a H 2 -based energy economy. Hence, even with its obvious advantages such as high energy density, compatibility with efficient fuel cells, and carbon-free combustion products, the role of H 2 as a next generation fuel remains uncertain. [18] The cost of H 2 sensors is a significant factor in widespread adoption of H 2 as a fuel, as such sensors will be required at H 2 production units, distribution units, and in H 2 fuel cell-powered vehicles. The US Department of Energy (DOE) has set cost targets for such H 2 sensors, with an ambitious goal of <$15 per sensor for fuel cellpowered vehicles. [19] This goal for low-cost H 2 sensors could be achieved using paper-based Pd chemiresistive H 2 sensors. Pd can selectively absorb a large amount of H 2 at room temperature and form palladium hydride, PdH x , changing both the volume and resistivity of the Pd. [18] Although the material cost of Pd is higher than most conventional metals, low operating cost and selective room-temperature H 2 detection make 2D palladium nanostructures enable sensitive room-temperature detection of H 2 . However, they can be limited by stability and fabrication costs. Stability may be improved by alloying Pd with other metals, while cost could be reduced by using paper as a substrate. An ultra-low-cost sensor using Pd alloy (PdMoY) nanosheets (NS) on paper is reported. The 2D Pd alloy nanosheets are prepared by a solution-phase route, drop cast onto paper (≈1 × 1 cm) with silver contacts drawn on it, and dried. The same material is deposited on an interdigitated electrode (IDE). Both sensors are teste...

We introduce a flame-based aerosol process for producing supported non-noble metal nanocatalysts from inexpensive aqueous metal salt solutions, using catalysts for the dry reforming of methane (DRM) as a prototype. A flame-synthesized nickel-doped magnesia (MgO) nanocatalyst (NiMgO-F) was fully physicochemically characterized and tested in a flow reactor system, where it showed stable DRM activity from 500 to 800 °C. A kinetic study was conducted, and apparent activation energies were extracted for the temperature range of 500−650 °C. It was then compared with a Ni-decorated MgO nanopowder prepared by wet impregnation of (1) flame-synthesized MgO (NiMgO-FI) and (2) a commercial MgO nanopowder (NiMgO-CI) and with (3) a NiMgO catalyst prepared by co-precipitation (NiMgO-CP). NiMgO-F showed the highest catalytic activity per mass and per metallic surface area and was stable for continuous H 2 production at 700 °C for 50 h. Incorporation of potential promoters and co-catalysts was also demonstrated, but none showed significant performance improvement. More broadly, nanomaterials produced by this approach could be used as binary or multicomponent catalysts for numerous catalytic processes.