An electrode catalyst that maintains catalytic activity under conditions
of an electrode potential as high as 0.4 V or above and exhibits improved
stability. The metal oxynitride electrode catalyst is composed of an
oxynitride containing at least one transition metal element selected from
the group consisting of La, Ta, Nb, Ti, and Zr, the metal oxynitride
electrode catalyst being used at a potential of 0.4 V or higher relative
to the reversible hydrogen electrode potential in an acidic electrolyte.
The metal oxynitride electrode catalyst is useful as an electrode
catalyst for electrochemical systems used in acidic electrolytes in the
fields of water electrolysis, organic electrolysis, fuel cells, etc.