(682a) Nanomaterials As Interfacial Modifiers in Energy Applications

Introduction

Energy and nanomaterials are intimately related.  Most energy processes occur at the interface.  Nanomaterials, when used as interfacial modifiers have the potential to significantly alter the energy landscape.  Although carbon nanomaterials have been explored for a suite of energy applications, to-date the approach has been based on replacing bulk materials, not as interfacial modifiers.  The work presented here will highlight this latter approach by showing results for a suite of energy applications from the author’s own work.

Experimental

Experimental details will be presented at the presentation with regards to the synthesis and utilization of the nanomaterials.  Our applications include energy conversion, generation, storage, efficiency, conservation and control.

Results and Discussion

Nanoscale materials are redefining the relation between material composition, size and properties.  Chemical properties (e.g. reactivity) and physical properties (e.g. surface area) become a strong function of size at the nanoscale.  Applications illustrated include the following selected examples from the author’s work: catalysts, composite materials, supercapacitors, batteries, sensors, nanofluids, and lubricants.  The key concept is that the nanomaterials serve as interfacial modifiers.  Since most energy related processes are dominated by interfacial reactions, nanomaterials have the potential to dramatically affect energy conversion rates and magnitudes.  Examples follow.

With regards to overall system integration and control, sensors will play a prominent role [1].  With ultrahigh surface exposure relative to bulk material, nanoscale materials are exceedingly sensitive to gas adsorption.  Exploitation of nanoscale properties will lead to new NO_x sensors and in-cylinder oxygen sensors for combustion control in power generation facilities.  Examples include catalyst coated metal oxide semiconductors capable of low temperature operation amidst complex gas environments.

Lightweight materials, a prime route to energy conservation particularly in transportation will reduce weight significantly, yielding substantial benefits in fuel efficiency with reduced emissions.  Composites with substantial gains in Young’s modulus, tensile strength, and EM shielding may be realized in polymeric composites using carbon nanotubes as an interfacial modifier rather than bulk filler [2]. The purpose of the foil is to serve as a gas impermeable barrier layer within the polymer composite.

Catalysis is central to fuel cell reactions and CO cleanup from the feeding, fuel stream. Preferential CO oxidation, so-called “PROX” is necessary to prevent catalyst poisoning by CO chemisorption, deactivating the catalyst.  Catalytic conversion at low temperature is advantageous for continuity of thermal management and can be accomplished by Au, supported on reducible oxides such as CeO₂, TiO₂ and Fe₂O₃ [3]. Other applications for these catalysts systems include volatile organic compounds (VOCs) and potentially exhaust hydrocarbons in present automotive transportation systems. 

Energy generation is often synonymous with energy harvesting of sunlight, a renewable and “free” source of energy, though quite arguably this is yet an energy conversion process – photons to electrons.  Whether by dye-sensitized solar cell or all semiconductor-based architecture, anodic capture and conversion of incoming photons is drastically increased by nanomaterials with the anode [4].  Crystalline nanowires offer vectorial electron transport with mobilities ~ 100-fold greater than polycrystalline counterpart.  Moreover their high surface area facilitates charge carrier injection from anchored dye or from an exterior semiconductor in the form of quantum dots or cladding as in a core-shell construction.

Advances in energy storage include batteries, ultra-capacitors.  These can support lighting, appliances, a starter, cooling fans, transmission and hydraulic systems, fuel and air handling systems and ultimately enable hybrid systems.  Towards these goals, substantial gains in Li ion battery cathode and anode materials have been realized using carbon nanofibers, coating processes and including elements such as tin and silicon.  Modifications of the CNT surfaces increases the Li ion capacity beyond the theoretical limit of normal graphite.  As an illustration, CNTs were directly fabricated for this end-application use upon ultra-fine SS mesh.  Significantly no harvesting, purification or processing (using binders) was required.  The CNTs could be used directly as synthesized [5].

Thermal energy transfer will benefit from nanofluids.  Nanofluids can increase thermal conductivity and reduce radiator and heat exchanger size in power generation and cooling operations.  Carbon-based nanofluids using nano-onions and carbon nanotubes have increased water conductivity by ~ 20% [6].  Being nanoscaled, they will not sediment, and with graphitic carbon content, can provide lubricity rather than abrasion to seals, valves and gears within fluid systems.

Lubrication is critical to efficient energy transfer many engine components and powertrain systems.  Nanolubricants can bridge the gap between fluid and solid materials [7].  As additives within liquids or greases, synergistic properties may be realized, particularly in boundary-phase lubrication by preventing rheological thinning of the lubricant.  Improved coating formulations and properties can reduce or eliminate fretting and pitting.  Results with single-walled carbon nanotubes show superior performance relative to graphite, diamond like carbon (DLC) and even Teflon.

Conclusions

Nanoscale materials are redefining the relation between material composition, size and properties.  Their applications to energy processes include selected examples from the author’s work.  Selected technologies include catalysis, composite materials, energy storage, sensors, thermal management, and tribology.  Interfacial modification serves to affect changes in energy conversion rates, and transfer magnitudes.  This appears to be a rather universal concept and suggests R&D directions for nano-engineering of interfaces.

Acknowledgements

Funding through The Penn State Institutes for Energy and the Environment (PSIEE) and the Pennsylvania Keystone Innovation Starter Kit (KISK) is gratefully acknowledged.

References

1. Hunter, G. W., Vander Wal, R. L., Liu, C. C., Xu, J. C., and Berger, G. M., Sensor and Actuators B 2009, 138, 113-119.

2. Vander Wal, R. L., and Hall, L. J., Advanced Engineering Materials 2004, 6, 48-52.

3. R. L. Vander Wal, Nanomaterials and Energy Applications: Interfacial Engineering – A Survey. (in preparation).

4. ibid.

5. Luo, Y., Vander Wal, R. L., and Scherson, D. A., Electrochemical and Solid State Letters 2003, 6, A56-A58.Vander Wal, R. L., Mozes, S. D., and Pushkarev, Nanotechnology 2009, 20, 105702-10.

6. Vander Wal, R. L., Mozes, S. D., and Pushkarev, Nanotechnology 2009, 20, 105702-10.

7. Street, K. W., Miyoshi, K., and Vander Wal, R. L., “Application of Carbon Based Nano-particles to Aeronautics and Space Lubrication”, in Superlubricity, Chapter 19, pp 311-340.  Edited by Drs. Jean-Michel Martin and Ali Erdemir, Elsevier, Amsterdam (2007).  (also NASA/TM-2007-214473).