(560e) Direct Ammonia Fuel Cell Utilizing an OH- Ion Conducting Membrane Electrolyte

Authors: 
Yan, Y., University of Delaware

We describe the
techno-economic background and the R&D work scheduled for the  ARPA-E
project :

“Direct
Ammonia Fuel Cells ( DAFCs) for Transportation  Applications”

 which is about to start under the REFUEL program. The project
is led by Shimshon Gottesfeld &Yushan Yan, University of Delaware, Jia Wang
& Radoslav Adzic, Brookhaven National Laboratory, Chulsung Bae, Rensselaer
Polytechnic Instituteand Bamdad Bahar, Xergy Inc. The multidisciplinary R&D
work scheduled will cover  the fields of advanced membrane and electrocatalyst
development, MEA development and fabrication and stack engineering. The latter
two activities will be supported by work at POCellTech, with Miles Page as
lead.  

The Project Vision is creation of a high power density,
direct ammonia fuel cell suitable for transportation applications , using a
hydroxide exchange membrane electrolyte and  operating the cell near 100 degC. 
A practical ammonia fuel cell should  enable use of  the lowest cost,
carbon-neutral liquid fuel for clean, long-range transportation.

A detailed techno-economic  evaluation of carbon free and
carbon neutral fuels , made in the preparatory  phase of this Project, revealed
that the combined cost  of fuel storage and fuel transport is the lowest for
liquid ammonia.  This is a direct result of the high energy density and the liquid
form of the fuel under conditions very close to ambient .  

 The choice of a polymer electrolyte fuel cell for operation in
direct oxidation mode with ammonia as fuel, has been made in light of the inherent
advantages of this type of low temperature fuel cells in powering passenger
vehicles which typically require a number of stop-restart cycles per  day.
Making such choice, requires, however, to answer successfully the challenge of the
low rate of anodic oxidation of ammonia, reported to date for DAFCs operating
at low cell temperatures. It was concluded by our team, that operation near or
somewhat above 100 degC, could allow, by use of advanced anode
electrocatalysts, to achieve the power density levels required for transport applications.
This strategy requires, however, OH- ion conducting ionomers which are stable
near , or somewhat above 100 degC , whereas, to date, stability of this type membrane
above 70 decC  has not been established.  Hence, the targeting of high power
density DAFCs operating around 100 degC,  requires successful combined development
of advanced electrocatalysts and advanced hydroxide-conducting membranes, with
the catalyst enabling operation at cell temperature widely considered to date
as too low for direct anodic oxidation of ammonia and, the advanced membrane
exhibiting good stability near 100 degC.

Early tests performed  by us with some well chosen  binary-metal
 anode catalysts  and  using two types of OH- ion conducting membranes,
resulted in DAFC power levels around 90 degC that  were more than an order of
magnitude higher than reported to date.   Results of voltage vs. current density
and, power density vs. current density, are shown below for a DAFC operating at
95 degC with vendor-supplied hydroxide conducting membrane , a bimetallic anode
catalyst and a silver catalyzed cathode.

The performance
shown here  for a polymer electrolyte DAFC operating at such temperature, is
the result of not only the quality of the cayst and the membrane, but, to significant
 degree, the quality of the membrane/electrode assemblies prepared and, last
but not least, optimized gas flow rates and inlet RH levels.