(6k) Developing Advanced Solid Oxide Fuel Cell (SOFCs) Stacks and Systems

Solid Oxide Fuel Cells (SOFCs) can offer very high electrical efficiencies (55-60 %) when used with commercial Natural Gas as fuels and Air as the oxidant.  They operate in the range of 600 - 1000 deg. C, depending upon the choice of the electrolyte, as per the latter’s ionic conductivity under those temperatures.  SOFC systems have a range of components, from cell materials (Doped Cerias, Stabilized Zirconias as electrolytes, Ni-Ceria/ Ni-Zirconias, along with many novel perovskite compositions as anodes, and La, Sr - doped Ferrites, Manganites, and novel Cuprates/ Nickelates as emerging materials for cathodes), to Interconnects (Ferritic steels with high Cr) to Sealants (glass, mica, Ti-Cu-Ag and other brazes), and followed by the manufacturing and the engineering that goes with the systems build.

However the systems configurations are numerous - beginning with a rudimentary fuel processor (steam reformation or catalytic partial oxidation) to generate syngas upstream of the Fuel Cell stack, and an after-burner downstream of the stack; a Desulfurizer and a humidification apparatus (in case of steam reformer fuel processor) are also necessary.  With the stack and Balance of Plant components in position, it is a process design and optimization problem, involving traditional Chemical Engineering Process Principles - heat integration, partial or total fuel reformation, choice of fuels, necessity of ‘pre-reformation’ in case of C3 or LPG feedstock, stack temperature, recycle configuration (anode gas exhaust) or straight through, recycle to purge ratio, air flow (fuel: air flow ratio) and finally setting ‘fuel utilization’ in the stack. Furthermore systems configurations are a function of the application - combined heat and power, or predominantly electricity generation. Beyond building the cell and integrating it with the stack (usually contingent on materials manufacturing methods), it is mostly chemical engineering, till power conditioning, where the principal domain knowledge is power electronics.

While the cathodic Oxygen Reduction Reaction (ORR) can be rate limiting under certain conditions, its kinetics are determined by either the Oxygen dissociative adsorption or surface diffusion of O-ad-atoms or by the reduction reaction - O + 2e- <--> O^2-which releases the oxide ions that move through the ionic conducting electrolyte towards the anode.  The mechanisms can change with the materials used, the proportion of the electronically conducting phase and the ionic conducting phase, and the microstructure at the electrode-electrolyte interface as well, known as the 3 phase boundary (where the reduction reaction actually occurs).

Similarly the anodic process has its own dynamic, with the key oxidation reactions of H₂- rich syngas taking place at the 3 phase boundary, and in many cases, methane actually undergoing a non-electrochemical but heterogeneous steam reforming reaction internally in the anodic structure, as per the following reactions -

Electrochemical Oxidation (highly exothermic reactions)

H₂ + ½ O^2- <--> H₂O + 2e-

CO + ½ O^2- <--> CO₂ + 2e-

CH₄ + ½ O2- <--> CO + 2 H₂+ 2e-

Steam Reforming of fuels in the anode itself

Endothermic: CH₄ + H₂O <--> CO + 3H₂

Mildly Exothermic (WGS): CO + H₂O <--> CO₂ + H₂

Regardless of the mix of reactions and the rate limiting reaction, certain electrodic properties are essential - porosity, electronic conductivity, 3 phase boundary microstructure; the activation barrier associated with these reactions and transport processes within the electrodes is reflected by a mixture of Ohmic and non-Ohmic resistances that can be measured effectively by Frequency Response Analysis (FRA, commonly known as Electrochemical Impedance Spectroscopy), in addition to measuring cell performance by I-V characteristics.

Focus of the current Poster Presentation - overview of several aspects of electrode development, cell, stack and systems build

In this presentation, I would like to focus on the manufacturing, design and process development for a ‘metal supported SOFC (m-SOFC)’ stack and system (which have the potential for reducing costs immensely due to its use of ferritic steel anode supports). Due to the usage of SS430 or SS441 as porous anode supports, m-SOFCs can operate at 600 deg. C, with the usage of doped ceria based electrolytes that have high ionic conductivity at those temperatures. The following aspects from this project will be discussed -

• Developing colloidal inks for deposition - Inkjet printing, Screen Printing
• Plasma Spray Deposition of electrolytes and electrodes on substrates
• Fabrication of Porous Ferritic steel substrates by pelletization and tape casting - tailoring porosity and pore structure for optimal performance
• Stack Design, and fabrication
• Cell and Stack Testing
• Developing steam reforming catalyst testing station for integration with SOFC stack
• Overview of the following -
  • Technology challenges in manufacturing
  • Major R&D advances in the areas of electrodics and minimization of cell degradation
  • Capital cost estimations of m-SOFC systems

 Research Interests in critical technologies for electrochemical systems

• Investigation of the interplay of electrochemical oxidation, and internal steam reforming reactions (Natural Gas) in Solid Oxide Fuel Cell (SOFC) anodes - The anode compartment in a stack with internal reformation, will have competing exothermic and endothermic reactions, and there is a need to balance the two in order to prevent very high exotherms (to bring temperatures down, flow rates of air have to be increased in the cathode, thereby increasing the power on the blower, and adding to parasitic power loss). 

A functionally graded electrocatalyst system is necessary wherein the electrochemically active layer will also have enough reformation sites to ensure internal steam reformation and Water gas shift reaction to enhance local Hydrogen concentrations. 

Also, to promote Steam Reformation (SR) at lower temperatures, investigation of quick ‘light-off’ catalysts, i.e., high activity catalyst, with advanced bi-metallic compositions, like Ni-Ru, or Ni-Rh with aluminate supports are a major topic of study, which can activate steam reforming (internal) under SOFC stack operating conditions.  

The research focus is in two areas -

1. Catalyst Development to kick off SR of Natural Gas at low temperatures
2. Engineering a balance between the exotherm and the endotherm to regulate temperature rise - to achieve this, a ‘graded’ catalyst composition in the anode ink formulation needs to be investigated

Keywords:Internal Reformation, Colloidal inks, Functionally graded anodes, Bi-metallic catalyst compositions

• Solid State Electrochemistry, Characterization of electrodes - interpretation and modeling of Electrochemical Impedance Spectroscopy data - The usage of advanced colloidal preparations, to make electrodes is a major challenge.  A strong effort in the physical chemistry of colloids, that takes into account the nanoparticle preparation of electrodic materials like Cerias, Zirconias, Manganites, and Ferrites, typically used in the SOFC industry today, has to be commenced. This requires exploration of various techniques for synthesis from Sol Gel preparation, combustion synthesis, co-precipitation, followed by communition methods (dispersion milling) under a carefully adjusted mix of dispersants and binders to finally yield electrode inks with a high three phase boundary, ideal for high activity and performance. 

 Particle size analysis with estimates of Zeta Potential, are the regular tools used to determine the state of the colloid, and its stability.

Electrochemical Impedance Spectroscopy is a powerful tool to de-convolute Ohmic and non-Ohmic processes in the operating fuel cell/ membrane system.  Development of advanced models and curve-fits and correlation of kinetic phenomena as functions of electrocatalyst microstructure is a major area of my interest.

 Keywords:Electrochemical Impedance Spectroscopy, Modeling, Three Phase boundary, Electrode inks

• CO₂ conversion by electrolysis - development of highly efficient electrocatalysts for CO₂ activation - I will be interested in exploring CO₂ utilization towards chemicals, e.g., primarily by the electrolysis pathways. The research focuses on electrolysis of CO₂, with electrocatalyst studies, in a bid to activate the cathodic process.  Characterization tools such as - CV, EIS, V-I curves will be used extensively, followed by cell/stack designs, viz., flow field designs and heat/water management. 

CO₂ emission reduction by conversion (as opposed to sequestration) as mentioned above is one of the ways, which if successfully scaled up can make a big impact globally.  Initially I would like to work on catalyst testing using simple electrolysis cells with reference electrodes and testing catalytic activity by methods such as Cyclic Voltammetry.  Over time this area of research will get bigger, and hopefully elements of process design, scale up, and development can be incorporated as well.