Monday, January 21, 2019

Understanding Combustion of H/O Gases inside Nanobubbles Generated by Water Electrolysis Using Reactive Molecular Dynamic Simulations.

S. Jain and L. Qiao, The Journal of Physical Chemistry A,122, 5261 2018
Highlighted by Tina Mihm, Colleen Lasar, Matthew Emerson

Abstract Image

It was found (by accident) that nanobubbles containing both H2 and O2 gas would form during the electrolysis of water and spontaneously combust. This is surprising because at smaller scales, the surface-to-volume ratio is large enough that heat loss becomes a real factor when trying to create and sustain a combustion reaction. Jain et al. believe this nanobubble combustion reaction is due to the low temperature and high pressure zone that takes place in the bubble. The initial thought with this discovery was that the combustion reaction inside said nanobubbles could be used to produce energy. However, most of the temperature from the reaction was found to be lost to the walls of the bubbles indicating low energy yields.

The reaction mechanism is as follows: 2 H2(g) + O2(g)  →  2 H2O(g). Both experimental and older computational methods have looked into the temperature changes and kinetics of this reaction, however, the mechanism has not been looked into in detail. Jain et al. uses molecular dynamic simulations to explore the mechanism of this phenomenon. They explored the characteristics of H2/O2 reactions at high pressure and low temperature as a function of Hydrogen radical concentration and found that H2O2 was the dominant species produced instead of the expected H2O.

In the simulations, they used a force field designed specifically to investigate the reaction kinetics of H2/O2 system at high pressures and low temperatures. Specifically, the first-principles derived reactive force field ReaxFF was employed, as implemented in the open-source molecular dynamics simulation code LAMMPS.  After thermalizing the system to 300 K with a Nose-Hoover thermostat, production runs  of 100 fs were carried out, using a 0.1 fs time step. The model was then validated using existing more generalized force fields that were not designed for the H2/O2 system. They also found that increasing the concentration of H radical or the system pressure increased reactivity. While this result was initially thought to be able to increase energy output, it was found that most of the energy from the reaction was found to be lost to the walls of the combustion chamber. If this happened in an automobile, the engine would become so hot that the hood would melt off.

In conclusion, it was found that hydrogen and oxygen gas in nano bubbles formed during electrolysis of water and would spontaneously combust. The mechanism for this reaction was investigated using reactions at high pressure and low temperature as a function of Hydrogen radical concentration and found that H2O2 was the dominant species produced instead of the expected H2O. The increase in reactivity due to increased pressure and H radical concentration during simulation was thought to increase energy output, and, therefore, create a source of clean energy. However, further computational simulations found that most of the heat was lost to the walls of the bubbles, greatly decreasing energy output, making a lousy nano-engine.

1 comment:

  1. Uniper Energy Storage GmbH pools decades of experience and all the Uniper’s competence for European gas storage.

    ReplyDelete