The development of thin-film solid oxide fuel cells (TFSOFCs) and a method of fabricating them have progressed to the prototype stage. A TFSOFC consists of the following:
- A fuel electrode (anode) of nickel or other suitable metal, about 10 micrometers thick, made porous in a required pattern as described below;
- A solid electrolyte deposited on the anode of 0.5- to 2-micrometer thickness;
- An oxidizer electrode (cathode) in the form of a layer of a mixed ionic-electronic conductive oxide deposited to a typical thickness between 1 and 10 micrometers on the solid-electrolyte face opposite that of the anode; and
- An electrically insulating structure that encloses the aforementioned components and includes manifolds for the delivery of fuel to the anode, delivery of air or other oxidizing gas mixture to the cathode, and removal of combustion products.
The porosity of the anode in a TFSOFC is necessary to enable delivery of the fuel to the anode side of the solid electrolyte. The cathode is required to be porous or at least permeable to the oxidizer to enable delivery of oxygen to the cathode side of the solid electrolyte. The solid electrolyte layer is required to be dense and free of defects so that neither the fuel nor the oxidizer leaks through it. The relatively small thickness of the electrolyte also makes it possible to operate the TFSOFC at temperature lower than is necessary for a thickerel-ectrolyte fuel cell of older design. In turn, operation at lower temperature increases the reliability and enables a wider choice of materials for constructing the TFSOFC.
In the fabrication of a TFSOFC, nickel foil to be used as the anode material can be rolled or otherwise processed to produce an ordered crystal structure so that subsequent epitaxial deposition of the solid electrolyte material on the anode will cause the solid electrolyte to be also crystallographically ordered and, therefore, to be dense and relatively free of defects, as required. The epitaxial deposition of the solid electrolyte and the deposition of the electronically and ionically conductive cathode layer on the electrolyte can be effected by any of several established processes for deposition of thin oxide films. After deposition of the solid electrolyte, the required porosity is introduced into the nickel by photolithographic patterning and etching.
The cathode layer can be deposited either before or after patterning of the anode. Optionally, to enhance the activity of the porous anode structure, a mixed-ionic-and-electronic-conductor film can be deposited on the anode patterning and etching.
Typically, the total thickness of the anode/solid electrolyte/cathode sandwich of a TFSOFC is only about 15–25 micrometers. Operating at a temperature between 450 and 500 °C, a TFSOFC can utilize hydrogen or methane as a fuel. The power density of a TFSOFC can exceed 10 W/cm3 (10 kW/liter), while the power per unit mass is ≈3 W/g (or ≈3 kW/kg). Relative to older thicker-electrolyte fuel-cell designs, TFSOFC designs can reduce costs of materials and reduce the volumes and masses of fuel cells capable of a generating a given amount of electric power.
This work was done by Xin Chen, NaiJuan Wu, and Alex Ignatiev of the University of Houston for Marshall Space Flight Center.