Electrode work function and absolute potential scale in solid state electrochemistry

摘要

A two Kelvin probe arrangement was recently used to measure for the first time in situ the work function, Φ, of the gas exposed surfaces of porous Pt, Au and Ag working and reference electrodes, exposed to O_2-He, H_2-He and O_2-H_2 mixtures, and deposited on 8%Y_2O_3-stabilized-ZrO_2 (YSZ) in a three-electrode solid electrolyte cell. It was found that at temperatures above 600 K the potential difference, U_(WR), between the working (W) and reference (R) electrode reflects the difference in the actual, spillover and adsorption-modified, work functions, Φ_w and Φ_R of the two electrodes: eU_(WR) = Φ_W-Φ_R [1] This equation, typically valid over 0.8-1 V wide U_(WR) ranges, was found to hold for any combination of the Pt, Au and Ag electrodes. It is consistent with the previously reported equation: e△U_(WR) = △Φ [2] also confirmed here and allows the definition of a natural absolute electrode potential U_(O_2)(abs) in solid state electrochemistry from: U_(O_2)(abs) =Φ/e [3] where Φ is the work function of the gas exposed electrode surface of the metal (any metal) electrode in contact with the YSZ solid electrolyte. It expresses the energy of "solvation" of an electron from vacuum to the Fermi level of the solid electrolyte. The value U_(O_2)~o (abs)-5.14(±0.05)V was determined as the standard U_(O_2)(abs) value for YSZ at P_(O_2)=lbar and T-673 K.