Important safety applications include detection of carbon monoxide and leaks of domestic mains and bottled gas. A more unusual application is as “electronic noses” for perfume, fruit and wine. An even more amazing application is an array of three thick film ceramic gas sensors that is said to be able to detect heart failure on a person’s breath.

Electronic nose: schematic and collection
of filters - sources web via scielo.org.ar
and abc science

Collection of capacitive humidity sensors
- source Wikipedia

Humidity sensors are a category of gas detector, aimed at water molecules. A version used in microwave ovens to detect the onset of cooking uses magnesium chromium titanate with about 35% porosity, but there are so many contaminants in the oven during cooking that a small heater is incorporated to periodically burn them off. Other applications of humidity detectors are in industrial air conditioning (particularly when fine powders are being prepared), food production and clothes dryers.

16.23 Fuel Cells and Batteries

Fuel cells and batteries directly convert chemical energy into electrical energy. In the case of fuel cells, continuous streams of fuel and oxidants are supplied to produce electricity without mixing or burning, with the production of considerably less greenhouse gas than fossil fuel generation. As there is no combustion and no moving parts fuel cells are clean and quiet. Fundamentally the cell is made up of an anode and a cathode separated by an electrolyte that is a conductor only for very specific charged particles provided by the fuel or oxidant.

Schematic of fuel cell

Theoretically they can have very high efficiencies, above 90%, although achieving 60 - 70% is more practical, which is still twice as efficient as conventional electricity generators. In 1845 William Grove demonstrated the principle using hydrogen and oxygen gases. However, it was not until the space race in the 1960s that innovation got underway, and fuel cells were first used on the Apollo spacecraft for night time electricity generation. This was followed in the 1980s by significant development for military applications. However, their enormous future potential is for clean energy generation, particularly in vehicles.

There are two types of fuel cell involving ceramics that operate at high temperatures, so they can process hydrocarbons such as natural gas or methane. The alternative lower temperature fuel cells are limited to pure hydrogen, requiring a precious metal catalyst. The first high temperature type is the “Molten Carbonate Fuel Cell” - MCFC. The molten salts, typically potassium and lithium carbonates at 650 ºC, form the electrolyte that is immobilised in a porous ceramic. This ceramic matrix is made from tape cast Lithium Aluminate with specially prepared edges. The edges form a “bubble barrier” to stop leakage of the molten salt to the anode or cathode, which are made of porous nickel. The molten salts are very corrosive, making the selection of materials for the electrodes and packaging difficult. In operation the carbonate ion formed at the cathode passes through the electrolyte and reacts with the hydrogen in the fuel to form water and carbon dioxide at the anode, freeing electrons to flow as electricity through an external circuit.