Catalyst layer

A catalyst layer comprising an electrocatalyst and an oxygen evolution catalyst, wherein the oxygen evolution catalyst comprises a crystalline metal oxide comprising:(i) one of more first metals selected from the group consisting of yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, magnesium, calcium, strontium, barium, sodium, potassium, indium, thallium, tin, lead, antimony and bismuth;(ii) one or more second metals selected from the group consisting of Ru, Ir, Os and Rh; and(iii) oxygencharacterised in that:(a) the atomic ratio of first metal(s) : second metal(s) is from 1:1.5 to 1.5:1(b) the atomic ratio of (first metal(s) + second metal(s)) : oxygen is from 1:1 to 1:2 is disclosed.