Environmental monitoring has been one of the key to provide better safety measure in various sectors of life. The most famously featured technology in this area is the gas detecting system. It serves not only in an early-warning application, but also a continuous mean to provide background check of the atmospheric status of a specific area ranges. According to their principals, gas sensors can be separated into two categories; the conductive metal-oxides and the electrochemical-type, with the latter gaining more recognition over the last decade as the future standard component in any gas sensing apparatus. Within this article, the whole concept of an electrochemical gas sensor will be presented, including the basic science and the basic electronical engineering that runs the device. The main concern will be fallen onto a specific amperometric-type sensor as the most common electrochemical sensor in the market. Ultimately, since there are multiple parts that comprise a single electrochemical sensor, a special section will be written within this article to show the array of materials that are responsible to the sensing capabilities of the sensor, particularly in the area of sensitivity and selectivity of gases.

Perovskite-based composite cathodes, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF)–Ce_0.8Sm_0.2O_1.9-carbonate (SDCC), were investigated as cathode materials for low-temperature solid-oxide fuel cells. The LSCF was mixed with the SDC–carbonate (SDCC) composite electrolyte at different weight percentages (i.e., 30, 40, and 50 wt %) to prepare the LSCF–SDCC composite cathode. The effect of SDCC composite electrolyte content on the diffraction pattern, microstructure, specific surface area, and electrochemical performances of the LSCF–SDCC composite cathode were evaluated. The XRD pattern revealed that the SDCC phase diffraction peaks vary according to its increasing addition to the system. The introduction of SDCCs within the composite cathode did not change the LSCF phase structure and its specific surface area. However, the electrical performance of the realized cell drastically changed with the increase of the SDCC content in the LSCF microstructure. This drastic change can be ascribed to the poor in-plane electronic conduction at the surface of the LSCF cathode layer due to the presence of the insulating phase of SDC and molten carbonate. Among the cathodes investigated, LSCF–30SDCC showed the best cell performance, exhibiting a power density value of 60.3–75.4 mW/cm² at 600 °C to 650 °C.

Composite cathodes made of perovskite La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and SDC carbonates (SDC-(Li/Na)2CO3) were investigated in relation to their structure, morphology, thermal expansion coefficient and porosity. As a first step, the LSCF powder was prepared by sol-gel technique. This was followed by the preparation of the LSCF-SDC carbonates composite cathode by mixing the LSCF with SDC-(Li/Na)2CO3 electrolyte via solid state reaction in various compositions, i.e. 30, 40 and 50 wt.%, namely 70LSCF-30SDC7030, 60LSCF-40SDC7030 and 50LSCF-50SDC7030, respectively. The powder mixtures were then calcined at 680oC. The resultant powder was fine with surface area of about 3.39-7.42 m2/g and particle size of 0.56-0.66µm. The powder consists of two distinct phases, i.e. LSCF and SDC-(Li/Na)2CO3 as confirmed with x-ray diffraction. The microstructures were observed under scanning electron microscopy (SEM). Increasing the amount of the SDC-(Li/Na)2CO3 electrolyte in the composite cathode was found to bring the thermal expansion of the cathode closer to that of the electrolyte. The cathode pellets were later compacted at different pressures (27, 32 and 37 MPa) and sintered at 600oC. The optimum porosity (20.99-24.98%) was achieved for samples with SDC-(Li/Na)2CO3 content of 30-50% sintered at 600oC and cold pressed at 37 MPa.