(Invited) Developing Electrodes for Solid Oxide Fuel Cells What I Learned from Anil Virkar about Designing High Performance Electrodes

Abstract

The performance of state-of-the-art solid oxide fuel cells (SOFC) and electrolyzers (SOEC) are largely limited by the overpotentials at the electrodes, both anode and cathode. The electrode overpotentials can be categorized by i) concentration of gas species, ii) charge transfer, and, iii) ionic transport near the interface, with the magnitude and contribution of each dependent on the cell geometry and operating conditions. One of the strategies of reducing overpotentials and improving electrode performance is the use of functionally grade electrodes wherein individual layers are optimized for porosity, conductivity, and three phase boundaries. Outer current collecting layers are designed with substantial porous microstructures and from highly electronically conductive materials resulting in low sheet resistance. Whereas the electrode layer near the electrolyte interface can be comprised of a two phase electrolyte-electrocatalyst composite in order to extend the effective three phase boundary layer beyond the electrolyte surface.The effects of various microstructural and material properties on the various electrode overpotentials were investigated. The influence of porosity and pore structure on the effective N2-O2 binary gas diffusion in the air electrode and resulting concentration polarization was studied. Likewise, highly porous anode supports were developed and characterized with the intent to decrease gas transport concentration overpotentials. Given electrodes that are adequately thin or have sufficient porosity in order to facilitate gas diffusion and reduce concentration polarizing, a substantial portion of the electrode overpotential can be attributed to the charge transfer and transport of the oxide ion from the electrode into the electrolyte, especially at lower temperatures. The critical transition region between the electrode and the electrolyte may span only a few microns but is essentially the equivalent of the ‘last mile problem’ for a fuel cell.Composite electrode layers nearest to the electrolyte were studied with regard to microstructure and materials selection. The activation overpotential was measured and related to the electrode properties including, three phase boundary density, the ionic transport within this layer, and the charge transfer of the electrocatalyst. In particular, the influence of the ionic conductivity of the electrolyte phase in the composite electrode on the electrode polarization was investigated. In addition, the microstructure of this electrolyte phase within the composite phase was studied and the sensitivity of necks between sintered electrolyte particles on ionic transport was estimated. A unique fabrication method was developed that produced an electrolyte skeleton with improved particle-to-particle necking and yet extremely porous, that could subsequently be infiltrated with an electrocatalyst material. Applying and combining the various strategies outlined above, highly efficient electrodes were developed and demonstrated for solid oxide fuel cells.