*3.2. Catalysts' Synthesis*

The citrate complexation method was used to prepare the K-doping catalysts [43]. Firstly, a solution with La, K, Co, Ga nitrates and citric acid, glycol are mixed by using deionized water, in which the molar ratio of lanthanum:potassium:cobalt:gallium:citric acid:glycol is 1−x:x:0.65:0.35:2.4:0.48. Secondly, the above prepared mixed solution was continuously stirred to gel at 80 ◦C, and then dried overnight at 120 ◦C. Finally, the powder catalysts were calcined at 350 ◦C for 2 h and 700 ◦C for 5 h (with a heating speed of 2 ◦C min−1), respectively. The obtained La1−xKxCo0.65Ga0.35O3 were labeled as LKCG-x (x = 0.1 or 0.2, which is the content of K in perovskite).

For comparison, LaCo0.65Ga0.35O3 without K doping was prepared according to the above method, and the sample was labeled as LCG.
