In this paper, mixed potential electrochemical sensors based on YSZ and La₂NiO₄sensing electrode were fabricated for the sub-ppm H₂S detection. The sensing materials La₂NiO₄was prepared by a sol-gel method. The different moles citric acid (CA) were used as complex agent to change the micro-structure of the La₂NiO₄. In comparison with other mole proportions, the sensor based on La₂NiO₄-SE with n(CA):n(La³⁺+ Ni²⁺)= 1:1 performed the highest response of -55 mV to 500 ppb H2S at 500℃. Moreover, a short temperature pulse of 700 ℃ was used to decrease the recovery time and the result shows a significance decrease for the sensor to 500 ppb H₂S, which is changed from 20 min to about 150s. In addition, the sensor displayed good stability for H₂S during the testing period and excellent selectivity toward H₂S in comparison to the other interference gases, such as SO₂, NO₂, NO, CO, CH₄and NH₃. The sensing mechanism related to the mixed potential ws proposed for the sensor based on YSZ and oxide electrodes.