Sensor element

A sensor element for a limiting-current type gas sensor measuring concentration of NOx in a measurement gas, said sensor element comprising:

• a base part made of an oxygen-ion conductive solid electrolyte;
• a gas inlet through which said measurement gas is introduced from an external space;
• a first internal space communicating with said gas inlet under predetermined diffusion resistance;
• a main pump cell as an electrochemical pump cell including an inner pump electrode located to face said first internal space, an out-of-space pump electrode located to face a space other than said first internal space, and said solid electrolyte located between said inner pump electrode and said out-of-space pump electrode;
• a measurement electrode located inside said sensor element, at least one diffusion control part being located between said measurement electrode and said first internal space;
• a reference electrode located inside said sensor element and capable of being in contact with a reference gas;
• a measurement pump cell as an electrochemical pump cell including said measurement electrode, said out-of-space pump electrode, and said solid electrolyte located between said measurement electrode and said out-of-space pump electrode;
• and
• a heater part buried in said sensor element and heating said sensor element, wherein
• said inner pump electrode is at least made of a cermet of a Pt—Au alloy and zirconia, and includes:
• a first portion located on a surface farther from said heater part from among surfaces opposing each other in said first internal space;
• and
• a second portion located on a surface closer to said heater part from among said surfaces opposing each other in said first internal space,
• an Au content with respect to said Pt—Au alloy as a whole of said second portion is 0.3 wt % or more smaller than an Au content with respect to said Pt—Au alloy as a whole of said first portion, and
• a total area of said first portion and said second portion is 10 mm2 to 25 mm2.