(58c) Updated Assessment of Microreactors for Improved Thermal Management of H2O2 Decomposition as a Fuel Cell Oxidant Aboard Unmanned Undersea Vehicles

Abstract

            The catalytic
decomposition of hydrogen peroxide (H₂O₂) into water (H₂O)
and oxygen (O₂) [H₂O_2(l) → H₂O_(l,g)
+ ½ O_2(g)] is notoriously susceptible to thermal runaway (Heat of
Reaction: ΔH_rxn = -98 kJ/mol). However, liquid H₂O₂
has high oxygen density making it a particularly attractive oxidant for
applications where air-independence and space limitations elicit additional
power supply design challenges. Microchemical technologies have intrinsically
higher surface-to-volume ratios (S/V) to potentially mitigate this problem by facilitating
heat and mass transfer. This study further developed previous work on the
capabilities of a micro-scale packed bed (MPB) reactor (radius 0.5 mm) to
enhance thermal management and oxygen production during the multiphase
decomposition of H₂O₂ as an oxidant for an unmanned
undersea vehicle (UUV) solid oxide fuel cell (SOFC). Despite an order of
magnitude increase in S/V over conventional reactors (S/V:  ~100 m²/m³), the micro-scale reactor channel (SV: 2254 m²/m³) generated significant heat rise (T_rise ~
100 K) during simulated decomposition under convective cooling using the
program COMSOL. The extension of surface area around the reactor channel (SV: 188,439
m²/m³) improved passive cooling, resulting in simulated MPB
temperature rises less than 7 K and experimental MPB temperature rises less 4 K.
S/Vs at least three orders of magnitude larger than conventional S/Vs were
required to achieve thermal control over the reaction zone during H₂O₂
decomposition. Although thermal management was successfully accomplished with
the addition of the high area heat exchanger, experimental oxygen production rates
were lower than simulated oxygen production rates. The simulated MPB reactor
assumed a homogenously dispersed multiphase product stream, whereas the
experimentally observed product stream more closely resembled two-phase plug
flow (the formation of oxygen bubbles was directly observed within the experimental
MPB). In the experimental two-phase plug flow, oxygen bubbles situated on active
catalyst sites likely prevented reactant from reaching the active sites, in
turn contributing to lower oxygen production during experimentation. The
presence of phosphorous based stabilizers in the concentrated H₂O₂
stocks probably caused deactivation of manganese dioxide (MnO₂) active
sites over time, also lowering oxygen flow rates observed at the outlet. Recommendations
are made to overcome the mass transfer limitations associated with oxygen
bubbles occupying active sites. For example, using a thin layer of inert
adhesive to hold catalyst particles in place down the length of the reactor
channel could encourage uninterrupted flow and more readily dislodge oxygen
bubbles. Overall these findings show improved thermal control during H₂O₂
decomposition using a micro-scale reactor relative to conventionally-sized
or even miniature (mm-scale) reactors, oxygen production capabilities of
micro-scale reactors during H₂O₂ decomposition, and
suggests mechanisms for overcoming mass transfer limitations naturally present
in multiphase reactions like H₂O₂ decomposition.