Investigation of the Pt/YSZ interface at low oxygen partial pressure by solid electrochemical mass spectroscopy under high vacuum conditions

Abstract

The Pt/YSZ interface was investigated at low oxygen partial pressure under high vacuum (HV) conditions at 400°C. Two different electrochemical techniques were coupled to mass spectrometric gas analysis using a new solid electrochemical mass spectrometric monitoring device. Under cathodic polarization, the lack of oxygen in the gas phase induces the reduction of the YSZ solid electrolyte which acts as oxygen source for the formation of O 2-ions migrating to the anode. Under anodic polarization, both platinum oxidation and oxygen evolution reaction are identified. PtO x is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms. At the Pt/YSZ interface, PtO x formation is an electrochemical process following a parabolic growth law, while the presence of PtO x at the Pt/gas interface is related to the diffusion of PtO x formed at the triple phase boundary towards the Pt/gas interface. It is proposed that the side oxygen evolution reaction stabilizes thermodynamically the PtO x diffusion toward the gas exposed interface during the anodic polarization.

Electrochemical promotion of CO combustion over Pt/YSZ under high vacuum conditions

“…This unexpected plateau also occurred in both MS-cyclic voltammetry (MS-CV) and MS-double step chronopotentiometry (MS-DSCP) measurements at the same conditions [11]. During anodic polarization, a fraction of PtO x forms at the TPB according to reaction (R3) and diffuses to the Pt/gas interface.…”

“…The polarization only caused a Faradaic increase of the catalytic rate with = 0.1 [11]. The lack of non-Faradaic processes is attributed to the thermodynamic instability of PtO x under operation conditions according to [13].…”

“…During the galvanostatic step, zirconia is electrochemically reduced at the cathode to release O 2− which migrates to the anode across YSZ according to reaction (R1) [16]. When the potential of Pt electrode (vs. gold reference) is higher than 300 mV [11], oxygen evolution takes place at the triple phase boundary (TPB) of the anode according to reaction (R2):…”

“…Under anodic polarization, oxygen evolution reactions are identified. We assume that PtO x is formed at both the Pt/YSZ and the Pt/gas interface according to two different mechanisms [11]. The PtO x species on the Pt/gas interface acts as oxygen promoter and induces the EP effect.…”

“…At the Pt/YSZ interface, PtO x formation is an electrochemical process following a parabolic growth law, while the presence of PtO x at the Pt/gas interface is related to the diffusion of PtO x formed at the triple phase boundary (TPB) towards the Pt/gas interface [12]. However, the stability of PtO x is thermodynamically not favored at 400 • C under HV conditions restraining the appearance of EP [11]. In order to investigate the influence of PtO x on EP, decreasing the operation temperature to stabilize PtO x seems an alternate way according to thermodynamic data [13].…”