The platinum electrode/yttria-stabilized zirconia metal/solid electrolyte interface is used in a wide variety of sensing applications including lambda oxygen sensors, wide range oxygen sensors, HC and NOx sensors.^1–3 In many cases the electrode is protected by a porous ceramic layer to shield the device from exhaust particle impingement,¹ or in the case of some types of mixed potential sensors, the electrolyte itself is porous.⁴The electrochemically active surface area and porosity are important factors that may control the performance of all types of zirconia-based sensors. While measuring AC impedance behavior in the gas phase is possible, the high diffusivity of gases through the porous layer makes extracting diffusion parameters difficult.

Mixed potential electrochemical sensors being electro-kinetic devices, are particularly sensitive to these parameters, but few techniques have been developed to measure active surface areas and characterize transport through porous layers. We have developed sensors consisting of a yttria-stabilized zirconia (YSZ) substrate coated with an insulating ceramic onto which dense electrodes of Pt, La_0.8Sr_0.2CrO₃, or Au_0.5Pd_0.5alloy are screen printed. Finally, a porous YSZ electrolyte is deposited on top of the electrodes. Measuring the effective diffusivity through the porous layer and the electrochemically active surface area of the underlying electrodes is useful in optimizing the performance of the devices.

The electrochemically active surface area of the Pt electrodes beneath the porous electrolyte layer was determined by immersing the sensor in aqueous H₂SO₄ solution and measuring the double layer capacitance or adsorption of a monolayer of H. The oxidation and reduction of [Fe(CN)₆]^-4/-3 observed by cyclic voltammetry (CV) acts as a probe for the effective diffusion constant through the porous layer as the kinetics of this reaction are fast and the reaction is easily reversible. The effective diffusion coefficient is obtained by fitting to the Randles-Secvik equation for the peak CV current as a function of scan rate. Figure 1 shows the CVs obtained on (a) a Pt disk and (b) the Pt sensor electrode beneath a porous YSZ electrolyte. The effective diffusion coefficient is on the order of 10^-6 cm²/s for the Pt disk and is consistent with values reported in the literature.⁴ The diffusion coefficient is 10^-10 cm²/s for the transport through the porous layer on sensor indicating significant hindrance by the YSZ.

Measuring these characteristics for both sensors produced by screen printing (ESL ElectroScience) and sensors produced by additive manufacturing in our lab will guide the optimization of sensor performance characteristics. The measurements are correlated to the device sensing and AC impedance response.

References

1. J. Riegel, H. Neumann, and H.-M. Wiedenmann, Solid State Ionics, 152–153, 783–800 (2002).

2. F. H. Garzon, R. Mukundan, and E. L. Brosha, Solid State Ionics, 136–137, 633–638 (2000).

3. E. Ivers-Tiffée, K. H. Härdtl, W. Menesklou, and J. Riegel, Electrochim. Acta, 47, 807–814 (2001).

4. P. K. Sekhar, E. L. Brosha, R. Mukundan, W. Li, M. A. Nelson, P. Palanisamy, F. H. Garzon, Sensors Actuators, B Chem., 144, 112–119 (2010).

5. S. J. Konopka and B. McDuffie, Anal. Chem., 42, 1741–1746 (1970).

Figure 1. Cyclic voltammetry of [Fe(CN)₆]^-4/-3 oxidation and reduction on (a) Pt disk electrode and (b) Pt sensor electrode beneath a porous YSZ electrolyte. Current densities are normalized to electrochemically active surface area.