(Invited) Engineering Strategies for the Industrial-Scale Production of Readily Deployable Renewable Energy Devices
Authors: Vaddiraju Sreeram 
Published: February 2020

Over the last two decades, nanowires have been employed not only for efficiently converting solar energy into electricity, but also efficiently converting thermal energy into electricity. They have also been demonstrated to be useful as photocatalysts for producing hydrogen fuel from water. Enhanced charge transfer resulting from the single-crystalline nature of nanowires, coupled with the control of their electrical, thermal and electronic properties through size variations, allowed for accomplishing this task.
Translating these successes to industrial-scale production and deployment of nanowire-based devices requires extending enhanced energy conversion performances demonstrated in single nanowires and small-scale nanowire arrays and mats to devices composed of large-scale nanowire assemblies. The need for such a task is demonstrated here using two illustrative examples. Considering a hydrogen production rate of 10 mM/hour per gram of nanowire photocatalyst, 1.65 x 108 Kg of nanowires are required to generate energy from hydrogen at a rate equal to 1% of the world’s energy consumption rate (estimated to be 13 terawatts as of 2007). To put this in perspective, the scale of production of nanowires should be on a level similar to that accomplished by the steel industry (≈2x1012 kg of steel was produced in 2005). Therefore, photoelectrochemical production of hydrogen from water requires the production of the photocatalysts on million metric ton scales. Similarly, a simple estimate based on the recently reported enhancement in the thermoelectric performance of silicon nanowires shows that the amount of energy generated from a single nanowire is in the range of 10-16-10-13 Watts. Therefore, 1013-1016 individual nanowires have to be assembled for producing 1 Watt of energy. Again, this translates to a requirement of 107-109 kg of nanowires for producing electricity at a rate equal to 1% of the rate of energy consumption worldwide, assuming that bulk nanowire devices also exhibit the enhanced thermoelectric performances observed in individual nanowires. Hence, deployment of nanowire-based devices requires strategies not only for mass producing them on a ton scale, but also integrating them into macroscale devices in a manner that offers the ability to engineer the interfaces between the assembled nanowires. Such assemblies should at a minimum ensure the translation of any favorable properties exhibited by individual nanowires to large-scale nanowire assemblies. In addition, ensuring reliable device performance necessitates that the comprising nanowires retain their chemical composition and structure over extended periods of time. So, nanowires need to be inert and remain unreactive with air and moisture. Such inertness also minimizes the environmental impact of nanowires.
Mass producing such corrosion-resistant nanowires is ideally possible by developing strategies that rely only on direct reactions of the component elements/molecules at low/moderate temperatures, without reliance on expensive catalysts/templates. Ensuring that the properties exhibited by individual nanowires are extendable to large-scale nanowire assemblies is ideally possible by welding nanowires during assembly, and ensuring that the bridges formed between the assembled nanowires are of the same chemical composition as the nanowires themselves. Ideal assembly strategies should also offer the possibility of consolidating nanowires either in a randomly oriented fashion or aligned fashion. Assemblies of aligned nanowires aid in utilizing their anisotropic properties. Imparting corrosion resistance to nanowires is ideally possible by decorating their surfaces with organic-inorganic molecules.
In this talk, strategies developed for the mass production of nanowires that involve the direct reaction of component elements will be presented (1). Strategies for mass producing nanowires in a by-product free manner will be presented and discussed. The latter strategy eliminates the need to purify nanowires following their production. In-situ methods useful for decorating nanowires with inorganic/organic molecules during/immediately after their synthesis for imparting them corrosion resistance will also be discussed (1,2). Implementation of pressure-assisted and shear-assisted strategies for the respective assembly of randomly oriented nanowires and aligned nanowires via welding will be discussed (3-6). Finally, the electrical and thermal transport behavior of these nanowire assemblies will be discussed (3-4). Illustrations will be presented using Si, Zn3P2, ZnO and Mg2Si nanowire systems as examples.
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