Photo-Activated Low Temperature, Micro Fuel Cell Power Source Page: 4 of 18
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I. EXECUTIVE SUMMARY
The electrochemical performance of electrodes is presently the key limiting factor in achieving
satisfactory performance in micro-SOFC and conventional SOFC designed to operate at
intermediate or reduced temperatures. Determining the rate limiting step/s remains a challenge
from both scientific and technological standpoints, and despite extensive research efforts, no
consensus exists about the rate limiting step/s in SOFC electrodes.
This program has several inter-related objectives. Most broadly, to identify electrodes that will
make it possible to significantly reduce the operating temperature of micro-SOFC and thin film-
based SOFCs. Towards that objective, key goals of this program include: 1) identify the key rate
limiting steps limiting presently utilized electrodes from performing at reduced temperatures and
2) investigate the use of optical, as opposed to thermal energy, as a means for photocatalyzing
electrode reactions and enabling reduced operating temperatures. This requires a multifaceted
approach which addresses the need to a) work with well defined and reproducible electrode
structures and model electrode compositions thereby enabling conclusions to be made about
the rate limiting mechanisms controlling electrode performance that can then be broadly
applied, b) utilize measurement techniques, including illumination, Kelvin Probe and Impedance
Spectroscopy, which provide the ability to isolate the contributions of e.g. gas adsorption,
charge transfer and diffusive kinetics towards the overall electrode impedance and c) offer
prototype structures demonstrating reduced temperature operation and the potential
advantages of illumination. This report summarizes progress achieved during the Phase I
In Phase I (one year's funding), we have focused on a) assembling and testing our unique
Microprobe Thin Film Characterization System, b) demonstrating that the model materials
system can be fabricated in thin film form, c) that it exhibits expected physical properties and d)
that it can be configured and operated as model cathode material. A key objective was also to
test the potential of illumination in enhancing electrode performance. These were all achieved
as summarized below.
SrTi1XFex03 (STF) was chosen as a model system given the ability to manipulate the band
structure as well as the level of mixed ionic-electronic conduction by controlling the fraction of
Fe substitution for Ti. We demonstrated that they could be prepared systematically in thin film
form as a function of composition and that these films largely replicated the bulk materials
properties whose defect, MIEC and optical properties were characterized and modeled in detail
in earlier studies. Specifically a subset of STF films (x=0.05, 0.35 and 0.50) were prepared by
PLD and ink jet printing and their properties were confirmed to exhibit expected characteristics.
Microelectrodes with varying diameters were fabricated and scaling with area was confirmed
consistent with their mixed ionic-electronic conducting properties. Further they where shown to
exhibit typical electrode characteristics when applied to solid electrolytes. A number of
STF/YSZ/Pt and STF/YSZ/STF cells were prepared and studied by complex impedance
spectroscopy over wide temperature limits (400-9000) and as a function of PO2 (p02 = 1.5x102
to 1atm). The electrode impedance typically exhibited 3 impedance signatures, i.e. two
somewhat overlapping semicircles at higher frequency and a 450 Warburg like contribution at
reduced frequency and temperature. Of particular note, has been the demonstration of as much
as a factor of 73% reduction in electrode impedance of SrTi1zFex03 (x = 0.35) model electrode
under low intensity illumination confirming the ability to modulate electrode impedance by
illumination of a model electrode-electrolyte interface. In other cases, the effect of illumination
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Tuller, Harry L. Photo-Activated Low Temperature, Micro Fuel Cell Power Source, report, March 30, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc898588/m1/4/: accessed November 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.