Graduation Year

2008

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Chemical Engineering

Major Professor

John T. Wolan, Ph.D.

Keywords

Solid Acid, Permeability, CsHSO4, Ionic Conductivity, RuO2/CoS2

Abstract

This study describes the development of a novel thin membrane exchange assembly (MEA) from a solid acid material, cesium hydrogen sulfate (CsHSO4), and from a composite anode electrocatalyst for electrolytic splitting of (100 %) H2S feed content gas operating at 135 kPa and 150 °C. A new class of anode electrocatalyst with the general composition, RuO2/CoS2, and an improved proton conductor, CsHSO4, have shown great stability and desired properties at typical operating conditions. This configuration demonstrated stable electrochemical operation for 24 h with a (100 %) H2S fuel stream at 423 K. This same system showed a maximum current density of (19 mA/cm²) at 900 mV. The performance of this new anode electrocatalyst when compared to that of Pt black investigated in a previous study showed an overall superiority in application.

We have achieved a 30 % reduction in the overall system performance by fabricating a thin (200 µm) CsHSO4 electrolyte, which reduced the whole MEA thickness from 2.3 mm to 500 µm. The result of permeability measurements proved that this thin solid electrolyte is impermeable to H2S gas and physical integrity was preserved throughout the experimental period. Further resistance losses were compensated by using a high energy planetary milling system to enhance the ionic conductivity of CsHSO4. The difference in stability and electrochemical performance of these cells compared to that of Pt anode based systems is directly attributable to the anode materials developed in this project. Factorial experiments were used to characterize the effect of controllable process variables (electrolyte thickness, time, age of the electrolyte) on the cell current density and interfacial polarization resistances.

As expected, cell current density and interfacial polarization resistances were a function of electrolyte thickness and age. Nevertheless, the effect of electrolyte thickness has a more prominent effect on the measured parameters. In addition, these experiments were used to identify regions of optimum system performance. Tafel plots were constructed to investigate the kinetic behavior of various anode based electrocatalysts. Exchange current densities, which are directly a measure of the electrochemical reaction, increased with RuO2/CoS2-based anodes. These experiments also suggested that high levels of feed utilization were possible using these materials. This was an impressive result considering the drastic improvement in electrochemical performance, current density, and sulfur tolerance compared to the other anode configurations.

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