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Enhancing aluminum corrosion for hydrogen generation Skrovan, John

Abstract

Aluminum powder when ball milled with secondary particles will corrode in water releasing hydrogen. With corrosion rates approaching 90% of available aluminum within 20 minutes, this process is of interest as a portable hydrogen source. Electrochemical polarization and hydrogen capture tests are used to study changes in solution pH and temperature during the reaction as well as calculate the activation energy for corrosion of the ball milled powder. Evidence that the ball milled aluminum is increasing the solution pH is presented along with tests indicating the pH shift is not sufficient to account for the increase in corrosion rate. The effect of solution temperature on reaction products and corrosion rate for aluminum powders is measured, and the hypothesis that the exothermic nature of the reaction combined with a deformed surface is creating high temperature micro-environments is discounted. Activation energy for the rate limiting step in the corrosion of ball milled aluminum is calculated between 72 – 74 kJ/mol, similar to that seen for aluminum disks at a similar pH. Finally BET measurements show an increase in surface area of the aluminum particles after ball milling between 10x-20x. The amount of hydrogen evolved in the first hour is seen to correlate almost exactly with the aluminum surface area. The addition of alumina powder without ball milling is shown to increase the corrosion rate of aluminum powders by an order of magnitude or greater and to delay or prevent passivation of the aluminum. Two models are proposed to explain this observation and tests run to support them. The high surface area (10m²/g) of alumina is thought to provide an alternative deposition site for hydroxide formed during aluminum corrosion and the positive surface charge alumina acquires at pH 7 to provide a source of protons for hydrogen evolution. The hydrogen exchange current densities on aluminum and platinum surfaces are shown to increase by over an order of magnitude in the presence of alumina particles. The acceleration of aluminum corrosion is only seen with electrical contact between the aluminum and alumina, but contact is not required to delay passivation.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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