Electrochemical Degradation, Kinetics & Performance Studies Of Solid Oxide Fuel Cells

Open Access
Author:
Das, Debanjan
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 24, 2015
Committee Members:
  • Serguei Lvov, Dissertation Advisor
  • Serguei Lvov, Committee Chair
  • Derek Elsworth, Committee Member
  • Sarma V Pisupati, Committee Member
  • Michael John Janik, Committee Member
Keywords:
  • Solid Oxide Fuel Cell
  • Impedance Spectroscopy
  • Electrohcemical Degradation
  • Electrochemical Frequency Modulation
  • Kinetics
Abstract:
The Pennsylvania State University The Graduate School Department of Energy & Mineral Engineering ELECTROCHEMICAL DEGRADATION, KINETICS & PERFORMANCE STUDIES OF SOLID OXIDE FUEL CELLS BY LINEAR AND NON-LINEAR FREQUENCY TECHNIQUES A Dissertation in Energy & Mineral Engineering by Debanjan Das © 2016 Debanjan Das Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2016 The Dissertation of Debanjan Das was reviewed and approved* by the following: Serguei N. Lvov Professor of Energy and Mineral Engineering & Materials Science and Engineering, Director of Electrochemical Technologies Program Dissertation Adviser Chair of Committee Derek Elsworth Professor of Energy and Mineral Engineering Sarma Pisupati Professor of Energy & Mineral Engineering Michael Janik Associate Professor of Chemical Engineering Luis F. Ayala Associate Professor of Petroleum and Natural Gas Engineering Associate Department Head for Graduate Education * Signatures are on file in the Graduate School Abstract Linear and Non-linear electrochemical characterization techniques and equivalent circuit modelling were carried out on miniature and sub-commercial Solid Oxide Fuel Cell (SOFC) stacks as an in-situ diagnostic approach to evaluate and analyze their performance under the presence of simulated alternative fuel conditions. The main focus of the study was to track the change in cell behavior and response live, as the cell was generating power. Electrochemical Impedance Spectroscopy (EIS) was the most important linear AC technique used for the study. The distinct effects of inorganic components usually present in hydrocarbon fuel reformates on SOFC behavior have been determined, allowing identification of possible "fingerprint" impedance behavior corresponding to specific fuel conditions and reaction mechanisms. Critical electrochemical processes and degradation mechanisms which might affect cell performance were identified and quantified. Sulfur and siloxane cause the most prominent degradation and the associated electrochemical cell parameters such as Gerisher and Warburg elements are applied respectively for better understanding of the degradation processes. Electrochemical Frequency Modulation (EFM) was applied for kinetic studies in SOFCs for the very first time for estimating the exchange current density and transfer coefficients. EFM is a non-linear in-situ electrochemical technique conceptually different from EIS and is used extensively in corrosion work, but rarely used on fuel cells till now. EFM is based on exploring information obtained from non-linear higher harmonic contributions from potential perturbations of electrochemical systems, otherwise not obtained by EIS. The baseline fuel used was 3 % humidified hydrogen with a 5-cell SOFC sub-commercial planar stack to perform the analysis. Traditional methods such as EIS and Tafel analysis were carried out at similar operating conditions to verify and correlate with the EFM data and ensure the validity of the obtained information. The obtained values closely range from around 11 mA cm-2 - 16 mA cm-2 with reasonable repeatability and excellent accuracy. The potential advantages of EFM compared to traditional methods were realized and our primary aim at demonstrating this technique on a SOFC system are presented which can act as a starting point for future research efforts in this area. Finally, an approach based on in-situ State of Health tests by EIS was formulated and investigated to understand the most efficient fuel conditions for suitable long term operation of a solid oxide fuel cell stack under power generation conditions. The procedure helped to reflect the individual effects of three most important fuel characteristics CO/H2 volumetric ratio, S/C ratio and fuel utilization under the presence of a simulated alternative fuel at 0.4 A cm-2. Variation tests helped to identify corresponding electrochemical/chemical processes, narrow down the most optimum operating regimes considering practical behavior of simulated reformer-SOFC system arrangements. At the end, 8 different combinations of the optimized parameters were tested long term with the stack, and the most efficient blend was determined.