Potentiometric CO sensors using anion-conducting polymer electrolyte: Effects of the kinds of noble metal-loaded metal oxides as sensing-electrode materials on CO-sensing properties

Abstract Potentiometric CO-sensing properties of electrochemical gas sensors using an anion-conducting polymer as an electrolyte and a metal oxide (MO; Bi 2 O 3 , CeO 2 , In 2 O 3 , SnO 2 , or V 2 O 5 ) loaded with or without 2.0 wt% noble metal (N; Ag, Au, Ir, Ru, Rh, Pd, or Pt) as an electrode material (EC(N/MO) or (EC(MO) sensor; N/MO: N-loaded MO) were investigated in wet synthetic air (57%RH) at 30 °C. Among all the EC(MO) sensors, the EC(CeO 2 ) sensor showed the largest CO response and excellent CO selectivity against H 2 and the EC(Bi 2 O 3 ) sensor showed relatively large CO and H 2 responses (no CO selectivity against H 2 ). Other EC(MO) sensors hardly showed both CO and H 2 responses. However, the Au loading just onto In 2 O 3 and SnO 2 was effective in improving the magnitude of CO and H 2 responses of the EC(In 2 O 3 ) and EC(SnO 2 ) sensors, respectively, which resulted in relatively poor CO selectivity against H 2 . On the other hand, the Pt loading only onto SnO 2 extremely enhanced only the magnitude of CO response of the EC(SnO 2 ) sensor, and thus the EC(Pt/SnO 2 ) sensor showed the most excellent CO selectivity against H 2 , among all the sensors in this study. The heat treatment of N/MO powders in H 2 at 250 °C reduced only the H 2 response of the EC(Au/SnO 2 ) sensor, leading to an improvement of the CO selectivity of only the EC(Au/SnO 2 ) sensor against H 2 . On the other hand, the heat treatment drastically enhanced both CO and H 2 responses of the EC(Pt/SnO 2 ) sensor, resulting in a decrease in its CO selectivity against H 2 . The CO and H 2 responses of other EC(N/SnO 2 ) sensors before and after the heat treatment in H 2 at 250 °C were also examined, and the effectiveness of the noble metal loading and the heat treatment on the CO-sensing properties was discussed in this study. In addition, gas-sensing mechanism was also proposed on the basis of chemical surface states of representative N/MO.