(457a) Effect of Particle Size On Hydrogen Production by Reaction of Liquid Water and Aluminum Powders

Abstract

Reaction of aluminum with water is of interest for hydrogen
generation for a wide range of applications, from in-situ operated fuel cells
to propulsion of underwater vehicles. 
This reaction is extensively studied and various aluminum-based
composite materials and alloys are under development with the aims to
accelerate the rate of hydrogen production and enable controlled operation of
the hydrogen-generating systems.  It
would be particularly interesting to achieve the desired rate of hydrogen
production at low operating temperatures, using liquid water as opposed to
water vapor.  However, the understanding
of the rate-limiting processes is lacking for the reaction of pure aluminum and
liquid water.  Recent research by
different authors showed, somewhat unexpectedly, that reaction dynamics are
different for micron sized spherical aluminum powders with different size
distributions.  This result can be
interpreted considering evolution of aluminum hydroxide layers and possible
changes in their integrity and diffusion resistance when the layer thickness
becomes comparable to its radius of curvature.  
In addition, porosity in such layers can develop when they are
sufficiently thick; however, such thick layers may not necessarily grow on
finer Al particles.  This work attempts
to understand the effects of particle size on its reaction dynamics in liquid
water using experimental data from time-resolved micro-calorimetry.  The experimental data tracking energy release
caused by the Al/H₂O reaction are processed using the actual
measured particle size distributions for three different Al powders.   The reaction rate is assumed to be
proportional to the particle surface; thus the entire heat flow measured by the
calorimeter is partitioned among Al particles based on their size distribution.  Evolution of particle sizes and thickness of
the grown Al(OH)₃ layers are tracked during
the micro-calorimetry experiment.  The results are processed using a simplified
reaction model and diffusion coefficients are found for particles of different
sizes and at different reaction stages. 
Ideally, such coefficients should only depend on temperature, so that
their observed dependencies on particle size and reaction progress serve to
indicate where the modifications of the simplified reaction model are
necessary.  Furthermore, reaction rates
are compared for individual particles of the same size which are present in
aluminum powders with different size distributions.  Once the correct reaction model is
implemented, the reaction rates for such identical particles must
coincide.  Additional comparisons and
measurements will be presented and discussed, aimed at establishing a corrected
reaction model for aluminum and liquid water. 
Based on the developed reaction mechanism, a composition and morphology of
an optimized, Al-based material for hydrogen generation will be proposed.