Non‐Nernstian Behavior at Modified Au Electrodes for Hydrocarbon Gas Sensing

Abstract

The electrochemical cells using yttria-stabilized zirconia (YSZ) electrolytes generate electromotive forces (emfs) according to Nernst's equation E(V) ϭ Ϫ(RT/4F) ln[p O2(anode) /p O2(cathode) ][1]However, the measured emf value often deviates from the value calculated by Eq. 1, especially when a reducing gas coexists with oxygen. Fleming first interpreted this deviation as the non-Nernstian behavior which results from the simultaneous presence of two electrochemical reactions occurring at the working electrode: one reaction involves oxygen, and the other involves the coexisting carbon monoxide. 1 Subsequently, many research groups have found similar behavior for other reducing gases such as hydrogen, 2,3 nitrogen oxide, 4,5 hydrogen sulfide, 6 and propene. 7,8 As a result of such efforts, not only has it become apparent that the mechanism of the non-Nernstian behavior is based on the mixed potential of the working electrode, but also it has been possible to apply the non-Nernstian behavior to sensors for monitoring combustible gas components.According to the mechanism of the mixed potential most commonly mentioned, 2-13 the cathodic reaction of oxygen and the anodic reaction of the reducing gas simultaneously proceed at the working electrodeanodic reaction for example using COThese reaction rates are equal to each other due to the formation of a local cell at the working electrode, resulting in the appearance of a mixed potential. Therefore, the mixed potential becomes more negative as the working electrode favors the anodic reaction of the reducing gas rather than the cathodic reaction of oxygen. We previously reported that a gold (Au) working electrode shows large mixed potentials for hydrocarbons, which have a carbon number up to 8 and are not only aliphatic but also aromatic, at an operating temperature of 600ЊC. 14 The mixed potential is enhanced by increasing the carbon number of the hydrocarbons, unsaturating the C-C linkage, and branching the chain structure. This property matches the need for hydrocarbon gas sensing in the exhaust gases from diesel engines, because unsaturated and higher hydrocarbons easily cause photochemical smog in the atmosphere. However, the sensitivity of the mixed potential for hydrocarbons is degraded at higher temperatures. Therefore, we have modified the Au working electrode in an attempt to maintain a large mixed potential for the hydrocarbons at high temperatures. Based on the described mechanism of the mixed potential, this can be accomplished by either promoting the anodic reaction of the hydrocarbons or depressing the cathodic reaction of oxygen. This paper deals with three kinds of modifications of the Au working electrode. We demonstrate that the addition of the oxides of group V a metals such as niobium oxide (Nb 2 O 5 ) and tantalum oxide (Ta 2 O 5 ) to the Au electrode is the best process for enhancing the mixed potential. In addition, we discuss the enhancing effect due to the addition of the metal oxide to the Au working electrode on the basis of the measurements of...

Solid‐State Electrochemical Sensors for Automotive Applications

“…4,5 Mixed potential sensors have been used in the past for the detection of CO 6,7 , NOx 8 , H 2 9 and HCs. 10 These sensors rely on the fact that two dissimilar electrodes on an oxygen-ion conducting electrolyte exhibit different non-equilibrium potentials in the presence of a reducing gas and oxygen. The potential at any one electrode is fixed by the rate of electrochemical oxidation of the reducing gas (CO, HC's or NO 2 ) and the rate of electrochemical reduction of oxygen (Eqns.…”

Hierarchical Nanoceramics for Industrial Process Sensors

“…Past sensor studies have shown the importance of gas-phase catalytic activity on sensor EMF [Fleming 1977, Hibino 1999, Elumalai 2005, Elumalai 2009, Striker 2011 Au sensors]. Namely, gas-phase conversion to equilibrium concentrations reduces the EMF caused by mixed-potential.…”

“…The mixed-potential results in a deviation from the Nernst predicted EMF. Gas phase reactions over catalytic electrodes have also been considered to influence sensor EMF [Fleming 1977, Hibino 1999, Elumalai 2005, Elumalai 2009]. In addition, the semiconducting nature associated with the adsorption and desorption of gas species on select metal-oxide electrodes can alter the EMF [Wachsman 2001, Di Bartolomeo JAmCeramSoc 2004, Di Bartolomeo JElectroChemSoc 2004.…”

“…Altering CO concentration causes a change in overpotential at the Au electrode, corresponding to a change in voltage between the electrodes [Fergus 2007]. Although work has studied the sensing behavior effect of varying Au particle size, post-calcination electrode structures maintained larger than 1 m particles despite using fine starting powders [Hibino 1999]. Results showed an increased sensitivity to propene when reducing particle size.…”

“…Since gas-phase catalytic conversion of test gas occurs competitively with mixed potential reactions in non-nernstian sensors [Fleming 1977, Hibino 1999, it is believed that nanoparticle, highly catalytic, gold should decrease sensitivity. Therefore it is of interest to develop nanostructured gold electrodes for use in potentiometric sensors.…”