USC team finds Li-Al nanoparticles produce hydrogen from water with high rate and yield; potential for industrial scaling
Aluminum and water react exothermically to form aluminum hydroxide and hydrogen; this basic property has lured numerous researchers interested in generating hydrogen from the aluminum-water reaction for modern transportation systems for at least 35 years. (Earlier post.) However, among the barriers to the practical application of this reaction are the low reaction rate and poor yield.
Now, results of large quantum molecular dynamics (QMD) simulations by a team at the University of Southern California suggest that alloying aluminum particles with lithium to produce hydrogen from water can produce orders-of-magnitude faster reactions with higher yields. Their paper is published in the ACS journal Nano Letters.
Metals such as Al can be used in renewable energy cycles. In a two-step thermochemical cycle, an exothermic reaction between metal and water produces hydrogen gas, followed by endothermic reduction of the metal-oxide product assisted by solar energy to regenerate metal fuel. One potential application of this technology is on-board hydrogen production for hydrogen-powered vehicles, but conventional metal−water reaction kinetics is not fast enough to make such on-demand hydrogen production commercially viable. Previous experimental and theoretical works suggested that remarkable reactivity of “superatoms” (i.e., clusters consisting of a magic number of Al atoms) with water may solve this problem.
… While this superatomic design achieves high reaction rates in nanometer-size clusters, unfortunately, it does not scale up to larger particle sizes of industrial relevance. For larger particles, surface atoms begin to lose acid−base distinction that originates from local geometrical differences on nanocluster surfaces. More seriously, formation of an inert oxide or hydroxide layer associated with the hydrogen-production reaction protects the metal core and thereby stops the reaction incomplete. This leaves a large fraction of Al atoms unreacted, leading to low yields. Radically new design principles are thus needed for scalable high-yield H2 production.—Shimamura et al.
In the study, the USC team performed large QMD simulations on a parallel supercomputer consisting of 786,432 processors to provide spatially and temporally resolved reaction dynamics of LinAln particles at the atomic resolution (n = 30, 135, and 441). The total numbers of atoms involved in the simulation were 606, 4,836 and 16,611, respectively, for the Li30Al30, Li135Al135, and Li441Al441 systems.
A total of 1, 4, and 19 H2 molecules were produced from water using Li30Al30 within 10 ps at temperatures 300, 600, and 1500 K (26.85 ˚C, 326.85 ˚C and 1,226.85 ˚C), respectively. The reaction rate was drastically higher than Aln (n is between 12 and 55), for which no H2 production was observed at 300 and 600 K within a similar time frame.
The LinAln particles also appear to have overcome the problem of formation of a passive oxide or hydroxide coating layer on the particle surface, which prohibits reaction of the inner Al core with water—i.e., the aluminum core remains unreacted. This is one of the key barriers in aluminum-water technology.
The USC team found a high yield of H2 production reactions from water using LinAln particles, with no Al atom remaining unreacted at the end of the simulation. By contrast, most of the Aln particles remained unreacted.
The team determined that the abundance of neighboring Lewis acid−base pairs is a key nanostructural design element through which water-dissociation and hydrogen-production require very small activation energies. These reactions are facilitated by charge pathways across Al atoms that collectively act as a “superanion” and a “surprising” autocatalytic behavior of bridging Li−O−Al products.
Furthermore, they found, the dissolution of Li atoms into water produces a corrosive basic solution that inhibits the formation of a the reaction-stopping oxide layer on the particle surface, thereby increasing the yield.
These atomistic mechanisms not only explain recent experimental findings but also predict the scalability of this hydrogen-on-demand technology at industrial scales.—Shimamura et al.
Kohei Shimamura, Fuyuki Shimojo, Rajiv K. Kalia, Aiichiro Nakano, Ken-ichi Nomura, and Priya Vashishta (2014) “Hydrogen-on-Demand Using Metallic Alloy Nanoparticles in Water,” Nano Letters doi: 10.1021/nl501612v