Electrode work function and absolute potential scale in solid-state electrochemistry

A two-Kelvin-probe arrangement was used to measure, for the first time in situ, the work functions, Phi, 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% Y2O3-stabilized ZrO2 (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, Phi (W) and Phi (R) of the two electrodes eU(WR) = Phi (W) - Phi (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 DeltaU(WR) = Delta Phi (W) [2] which is also confirmed here, and allows for the definition of a natural absolute electrode potential U-O2 (abs) in solid-state electrochemistry from U-O2(abs) = Phi /e [3] where Phi 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-O2(0)(abs) = 5.14 +/- 0.05 V was determined as the standard U-O2(abs) value at p(O2) = 1 bar and T = 673 K. (C) 2001 The Electrochemical Society. [DOI: 10.1149/1.1362547] All rights reserved.