Abstract: An electrode catalyst for a fuel cell comprising at least one boride of molybdenum is disclosed. The electrode catalyst is far more excellent than conventional catalysts such as platinum black in that the resistance to acids is very high, that the polarization is little, that the price is low, etc. It is particularly suitable as an electrode catalyst for a fuel cell employing an acidic electrolyte.

Abstract: Nitric acid is added to an aqueous solution of organic acid salts of metals to be included in a resulting complex oxide powder, into which an aqueous solution of an inorganic salt of other metal component has been incorporated according to need, and the resulting solution is heated until the organic acid salts are decomposed and the decomposition product is heated, whereby a intended complex powder which is very fine and highly bulky and has a large specific surface area is obtained.

Abstract: A gas-sensor element for detecting oxidizable gases and vapors such as alcohols for carbon monoxide, which is characterized by comprising a complex metal oxide having the K.sub.2 MgF.sub.4 -type crystal structure and represented by the general formula A.sub.2-x A'.sub.x BO.sub.4-.sub..delta., wherein A is at least one element selected from the group consisting of rare earth elements of the atomic numbers from 57 to 71, yttrium, and hafnium, A' is at least one element selected from the group consisting of alkaline earth metals and lithium, B is at least one element selected from the group consisting of transition elements of the atomic numbers from 21 to 30, 0 is oxygen, x is in the range of 0.ltoreq.x.ltoreq.2, and .delta. is a nonstoichiometric parameter.