The non-Nernstian behavior of zirconia-based electrochemical cells with various metal oxide electrodes for 0–500 ppm C1-C4 hydrocarbons, hydrogen, and carbon monoxide was studied in the presence of 10% oxygen at 600°C. The cell using an In 2 O 3 electrode exhibited a largely negative electromotive force (emf) for propene but gave moderately negative emfs for hydrogen and carbon monoxide. The addition of 0.1 wt % MnO 2 to the In 2 O 3 electrode significantly improved the selectivity of the parent electrode for propene over hydrogen and carbon monoxide. The negative emf of the cell using the MnO 2 -containing In 2 O 3 electrode for the hydrocarbons became enhanced as the carbon number of the hydrocarbon increased, the C-C linkage was unsaturated, and the chain structure was branched. This enhanced effect, as well as the addition effect of MnO 2 , was discussed on the basis of the measurements of the polarization curves of the MnO 2 -containing In 2 O 3 electrode for the hydrocarbons and oxygen and the nonelectrochemical oxidation of the hydrocarbons by oxygen. In addition, the MnO 2 -containing In 2 O 3 electrode for the hydrocarbons was compared with an Au electrode with respect to sensitivity to the hydrocarbons and reproducibility of the sensing properties. © 2000 The Electrochemical Society. All rights reserved.

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A single chamber cell constructed from an yttria-stabilized zirconia solid electrolyte with a nickel anode and a strontium-doped lanthanum manganese oxide (La 0.8 Sr 0.2 MnO 3 ) cathode can generate an electromotive force (emf) of 795 mV and a power density of 121 mW cm -2 in a mixture of methane and air at a flow rate of 300 mL min -1 and 950°C. The simultaneous additions of 25 wt % gadolinium-doped cerium oxide (Ce 0.8 Gd 0.2 O 1.9 ) and 15 wt % manganese oxide (MnO 2 ) to the anode and cathode, respectively, further increased the emf to 833 mV and the power density to 162 mW cm -2 .Conventional fuel cells consist of two gas chambers partitioned by gastight electrolytes. The working principle of such cells is based on the separate supply of fuel and air to the anode and cathode, respectively. Another type of fuel cell based on a different principle has been suggested by many researchers. 1-6 This type of fuel cell consists of only one gas chamber, where both the anode and the cathode are exposed to the same mixture of fuel and air. According to the working principle most commonly mentioned, 1,2,5,6 one electrode has a higher electrocatalytic activity for the anodic oxidation of the fuel, whereas the other electrode has a higher electrocatalytic activity for the cathodic reduction of oxygen, thus resulting in an electromotive force (emf) between the two electrodes even in a uniform atmosphere. The construction of this type of cell is simpler than for conventional fuel cells, because there is no need for separating the supply of fuel and air. However, all one-chamber fuel cells proposed so far employ impractical electrode and/or electrolyte materials, which are expensive or unstable under the proposed operating conditions. For example, the Pt|BaCe 0.8 Y 0.2 O 3-α |Au cell reported by our research group showed a short lifetime of ca. 5 h in a flowing mixture of methane and air at 950°C, because BaCe 0.8 Y 0.2 O 3-α reacted with carbon dioxide to form barium carbonate and/or the Au electrode was gradually sintered. 7 Subsequently, although yttria-stabilized zirconia (YSZ) and YSZ-doped by some metal cations, were examined as the solid electrolytes, these cells showed very low power densities, because of the large overpotentials at both the Pt and Au electrodes. 8 A cell consisting of Ni|YSZ|La 0.8 Sr 0.2 MnO 3 (LSM) is widely used as a conventional solid oxide fuel cell (SOFC). All of the component parts of this cell are inexpensive and very stable at high temperatures. Therefore, we applied this cell as a single chamber SOFC working in a flowing mixture of methane and air. This paper demonstrates that the Ni and LSM electrodes can be used in place of the Pt and Au electrodes, respectively, for a single chamber SOFC. In addition, the simultaneous additions of Ce 0.8 Gd 0.2 O 1.9 (GDC) and MnO 2 to the Ni and LSM electrodes, respectively, make it possible for this cell to exhibit high performance.Experimental Figure 1a shows a two-chamber cell constructed from a YSZ solid electrolyte with three electrodes for measur...

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...

The sensing properties of zirconia-based electrochemical cells with various metal oxide electrodes for 0-500 ppm hydrocarbons (primarily propene), hydrogen, and carbon monoxide were studied in the presence of 10% oxygen at 600°C. The addition of 0.1 wt % manganese oxide (MnO 2 ) to an indium oxide (In 2 O 3 ) electrode significantly improved the selectivity of the parent electrode to propene over hydrogen and carbon monoxide. This addition effect was based on the promotion of nonelectrochemical oxidations of hydrogen and carbon monoxide in preference to that of propene. The sensor using the 0.1 wt % MnO 2 -added In 2 O 3 electrode was more reproducible and sensitive to the hydrocarbons than that using a gold electrode.The oxygen sensors constructed from zirconia-based solid electrolytes rarely generate different electromotive forces (emfs) from the values calculated by the Nernst equation, when the reducing gases such as carbon monoxide, 1-4 hydrogen sulfide, 5 nitrogen monoxide, 6,7 and hydrogen 8,9 coexist with oxygen. This behavior has been explained by the appearance of a mixed potential at the working electrode, where the cathodic reaction of oxygen gas and the anodic reaction of the reducing gas occur simultaneously. 10 Recently, some research groups have reported that a gold (Au) electrode shows large mixed potentials for hydrocarbons at high temperatures up to 600°C. 11-13 For example, we found that the mixed potential at the Au electrode was enhanced by increasing the carbon number of the hydrocarbons, unsaturating the C-C linkage and branching the chain structure. 12 This property meets the hydrocarbon sensing in the exhaust gases from diesel engines, because unsaturated and higher hydrocarbons significantly cause photochemical smog in the atmosphere. However, Mukundan et al. pointed out that the mixed potential at the Au electrode, especially deposited on an yttria-stabilized zirconia (YSZ), was easily affected by its surface morphology, thus resulting in a poor reproducibility of the sensing properties. 13 This is mainly because of its relatively low melting point of 1063°C. Hence, we have investigated the use of metal oxide electrodes in place of the Au electrode for the purpose of developing a more reliable hydrocarbon sensor. In this paper, we report that a 0.1 wt % manganese oxide (MnO 2 )-added indium oxide (In 2 O 3 ) electrode showed larger mixed potentials for the hydrocarbons but smaller mixed potentials for hydrogen and carbon monoxide than the Au electrode. In addition, this electrode yields much more reproducible mixed potentials for the hydrocarbons than does the Au electrode.Experimental A two-chamber cell constructed from a YSZ solid electrolyte with working, counter, and reference electrodes is used as the hydrocarbon sensor. A detailed illustration of this cell is reported in a previous paper. 12 The sintered YSZ disk (8 mol % yttria, 14 mm diam, 1.0 mm thick) was purchased from Nikkato Co., Ltd. Various metal oxide working electrodes were deposited on the YSZ surface as follows: In t...