After 18 hours the output has dropped to 20mV - no surprise considering corrosion and the absence of any ambient hydrogen.
But Argon is a non-corrosive atmosphere? Further, this data from R. K. Rout about anomalous emissions from table 1 Gas, Fogging Density : Air,0.23 ; Oxygen, 0.13 ; hydrogen, .03 ; Argon, 0. It seem corrosion is more like necessary.