Postface: Nanomaterials for Energy, A Look Forward - ACS Publications

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Chapter 11

Postface: Nanomaterials for Energy, A Look Forward Stephen Creager* Hunter Laboratory, Clemson University, Clemson, South Carolina 29634, United States *E-mail: [email protected]

Structural control of materials on the nanoscale is essential to achieve the properties needed for the efficient collection, transduction, storage and consumption of energy. Many recent advances in energy science and technology have been enabled by advances in synthesis and characterization of nanomaterials. This chapter will consider various scenarios for how such advances might continue in the future.

Introduction From the burning of wood or coal for heat to the refining of petroleum to power automobile engines to the generation of electricity from renewable sources such as wind or solar, the ability to collect, transduce, store and consume energy effectively has been fundamental to modern life since the dawn of civilization. Just as the light bulb transformed society by enabling us to work/play at night, green energy is the next revolution in decentralized, smart-grid, clean energy for the society of today and tomorrow. Energy-related issues continue to be important in the public arena; one need only think about how global geopolitics is affected by factors such as global climate change, the changing cost and availability of fossil fuels such as coal, petroleum, and natural gas, and the emergence of renewable energy sources such as wind and solar, to understand the importance of energy to modern society. Against this backdrop of significance, the importance of nanomaterials for energy technology has only recently come to be appreciated. Nanomaterials harness time and space dimensions that are not normally accessed in bulk © 2015 American Chemical Society In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


materials and because of this fact they often exchibit chemical, electrical, optical properties that are unmatched in their bulk counterparts. Of course, nanomaterials are not new; many materials that have been used for centuries in energy harvesting, storage and consumption are quite complex at the nanoscale. Rather, the recent focus on nanomaterials is due to a lack of appreciation until relatively recently that the properties of many materials including materials used in energy-related technologies are dominated by structure and dynamics on the nanoscale. This realization regarding the new properties of nanomaterials, combined with the relatively recent availability of tools for imaging and studying structure and dynamics on the nanoscale, has greatly deepened out understanding of nanomaterials for energy and in many other areas of science and technology. This book on Nanomaterials for Sustainability has focused on select recent developments for topics related in some way to nanomaterials for energy sustainability. The book is divided into thematic chapters, where each chapter describes a recent development in a specific area. There are chapters on fuel cells including proton exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC) that are the engines driving new generations of electrochemical power sources. There are chapters on the electrocatalysts that operate within fuel cells, catalyzing critical electrode reactions such as the electrochemical oxygen reduction reaction (ORR). There are also chapters on electrode modeling, electrode characterization techniques, and lastly but equally important energy policy. In this final “postface” chapter, I will attempt to look forward to where I see continued research and development efforts in five areas having the most impact. In doing so, I am venturing my opinion and as such will keep in mind the timeless proverb, attributed to many wise men including Yogi Berra, Mark Twain, Niels Bohr and others, which says something to the effect that “It’s tough to make predictions, especially about the future.” These are indeed wise words.

Nanomaterials for Electrochemical Energy Conversion: Fuel Cells and Electrolysis Cells Research activity on new materials for fuel cells and electrolysis cells, including catalysts, electrolytes and supporting structures, has steady increased over the past several decades and seems likely to at least remain steady. Two technology platforms receiving much attention are proton exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC). These platforms differ principally in the temperature range in which they operate; PEMFC devices typically operate between 80-100 °C whereas SOFC devices require much higher temperatures, typically between 500 and 1000 °C. Because of these widely differing operating temperature regimes, nanomaterials research follows very different paths for these two platforms. In the PEMFC realm, there is a pressing need for oxygen reduction reaction (ORR) catalysts having higher activity, lower cost, and greater durability. Research has focused mostly on catalysts containing nanostructured platinum-group metals having high specific surface area that resist sintering. Recent reports on platinum-nickel nanoframe structures for ORR catalysis are promising and further developments seem likely. Catalysts that 270 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


contain no platinum group metals (non-PGM catalysts) are also quite promising and are becoming much better understood, and it seems likely that further developments will be realized on these materials. The successful development of non-PGM ORR catalysts having activity and durability approaching that of the best PGM catalysts would be a breakthrough development, driving down cost and increasing availability of PEMFC devices. SOFC platforms also benefit from improved ORR catalysis but not to the same degree as PEMFCs. Because of the higher operating temperature for SOFC devices, electrode reaction proceed relatively rapidly even without benefit of catalysis. Even so, higher activity and improved selectivity and durability are always desired in catalysts and will continue to be sought. Most PEMFCs, and to a lesser extent SOFCs, utilize hydrogen gas as fuel. Hydrogen is convenient because it is lightweight and its electrochemical oxidation reaction (HOR) is catalyzed relatively easily with high activity and selectivity by small amounts of a platinum catalyst. Hydrogen is also inconvenient because there is no hydrogen-mine or natural tap to draw it like coal or petroleum, thus hydrogen would need to be generated, for example from water. Here, improved HOR catalysts are critical and are always being sought but the more difficult catalysis problem is electrooxidation of other fuels, usually fuels containing carbon. Another disadvantage of hydrogen is that it remains a gas under all except the most extreme conditions (low temperatures and high pressures). For many applications, particularly involving automotive transportation, it is desirable for fuels to be liquids under ambient conditions. Unfortunately, electro-oxidation of most liquid organic fuels is mechanistically complex with many microscopic steps and reaction intermediates. Even the best catalysts have low activity with the possibility of multiple reaction pathways. A pressing need exists for electrocatalysts having higher activity and greater selectivity for electrooxidation of liquid organic fuels. Many paths are possible for developing such catalysts, and it is not yet clear how much improvement may be realistically expected. Even so, it seems very likely that whatever catalysts are developed, their properties on the nanoscale will be intimately involved in determining catalyst activity. Catalyst development for electrochemical energy conversion is certain to involve a significant amount of nanomaterials research. Electrocatalysis is important to accomplish fuel oxidation and oxygen reduction at electrodes, but equally important is the electrolyte that transports ions to and from the electrocatalyst active sites and between the anode and cathode in the electrochemical cell. PEMFC devices typically use a polyelectrolyte material, often a fluoropolymer, as a proton-conducting electrolyte, whereas SOFC devices typically use ceramic materials that conduct via oxide ions as electrolyte. Materials design considerations are very different for these two device classes, yet some things remain constant. For example, nanoscale integration of electrolyte with electrode / electrocatalyst is always important to provide a means for electrons and ions to move to and from the catalyst active sites, as they must for oxidation / reduction to occur. This tight integration of catalyst and electrolyte on the nanoscale is essential to achieving high electrocatalyst activity. For PEMFC devices the principal drivers for improved electrolytes are increased ionic conductivity at temperatures below (possibly down to -40 °C) 271 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


and above (possibly up to 120 °C) the usual operating range of 80-100 °C, and also increased conductivity under relatively dry conditions (e.g. less than 50% relative humidity). Most current PEM materials are poor ion conductors at these extremes of temperature and humidity, and research efforts will continue to provide materials having improved performance at these limits. There is also much interest in creating polymer electrolytes in which hydroxide anions are the charge carriers. Oxygen reduction under strongly alkaline conditions may be catalyzed using different and less-expensive and more earth-abundant materials from those commonly used under acid conditions. Many organic materials have poor long-term stability when exposed to hot and concentrated alkali solutions, and further research is needed to create new alkaline exchange materials suitable for use in fuel-cell devices. For SOFC devices, there is much interest in creating new ceramic / solid-state electrolytes that could enable fuel-cell operation at temperatures in the 300 – 500 °C range. Mixed conductors, e.g. materials that conduct both ions and electrons, are of interest for electrodes, as are catalyst layers that allow for use of chemically complex fuels, e.g. diesel fuel, and/or dirty fuel streams, e.g. fuels containing carbon monoxide, carbon dioxide, or other impurities / poisons. Mixed conductors are also potentially useful in applications involving gas purification. Another area receiving high attention is that of electrolysis cells, which may be thought of simply as fuel cells run in reverse. Electrolysis technology is already well developed however the emergence of intermittent electrical power generation technologies from sources such as wind or solar has created a need for storing the electrical energy for later use. One way of storing electrical energy is via water electrolysis, which converts water into separate streams of hydrogen and oxygen. Much of the research on improved PEMFC and SOFC devices is also applicable to electrolyzers and has driven strong improvements in electrolyzer performance and cost over the past decade. Further improvements in performance, cost and durability are likely in response to the emerging high need for ways to accomplish large-scale energy storage using electrical energy produced by renewable but intermittent power sources that are already being planned for introduction on a 5-10 year timetable.

Nanomaterials for Electrochemical Energy Storage The past twenty years have seen remarkable improvements in the basic understanding and engineering performance of batteries for storing and providing electrical energy. Lithium batteries are of great interest due to their high cell voltage and low mass, which provide for high energy and power density. Most battery materials are inherently nanomaterials, and battery charge/discharge reactions nearly always involve electrochemical conversions on nanometric length scales. Much of the recent improvement in battery performance has come about due to improved understanding of nanoscale chemistry in battery materials and electrodes. Further incremental improvements in lithium battery performance are likely from ongoing research for which a comprehensive review is not possible in 272 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


this short chapter. Significant advances are also possible from several emerging battery chemistries and technologies that may be loosely grouped together as “Beyond Lithium Ion”, which is also the name of an annual battery meeting sponsored by the US Department of Energy. Emerging chemistries in this category include lithium-sulfur and lithium-air, sodium and magnesium ion intercalation compounds, and others. Flow batteries are also of high interest particularly for applications in stationary energy storage, e.g. on the electric grid. Flow batteries are attractive in part because they are inherently scalable and tend to have very low capacity fade and be highly energy efficient, because all the reactions occur in a concentrated liquid phase rather than a solid phase. Work on traditional vanadium flow battery chemistry is still of interest and is being supplemented by emerging electrode chemistries including bromine/bromide, chlorine/chloride, and sulfur/sulfide. A particularly interesting approach involves fluids containing suspended micro/nanoparticles of redox-active solids, for example intercalation materials from traditional lithium-ion batteries. This approach to flow battery development combines the high energy density of a solid-state electrode material with the desirability of having an electrode that is a fluid. Nanoparticulate-based redox fluids are very interesting nanomaterials and I think that further developments are likely in this novel but promising approach to flow-battery development.

Nanomaterials Characterization Techniques Nanomaterials are traditionally characterized using electron and proximal probe microscopy techniques, often coupled with X-ray, electron and neutron diffraction techniques and techniques such as X-ray photoelectron spectroscopy (XPS), dynamic light scattering, zeta-potential measurements, nuclear magnetic resonance (liquid and solid state), gas porosimetry, and others. All of these techniques have undergone significant advancement over the past twenty years which has helped to drive research on a wide range of nanoscale phenomena and materials across many technology areas including energy. Incremental advancements in these (and nearly all) materials characterization techniques will continue to drive nanomaterials research over the coming decade. An emerging trend in the characterization area is an increased emphasis on in-situ characterization in high-pressure gas or liquid environments, possibly to include studies within operating devices such as batteries, fuel-cells or electrolysis cells. In-situ operation can involve some significant compromises, e.g. electron microscopy and spectroscopy techniques are traditionally thought to require vacuum conditions however ways around these limitations continue to be found, such that electron microscopy and photoelectron spectroscopy are now possible on samples at close to atmospheric conditions, and/or on samples covered by a thin liquid layer. Another emerging trend is towards combining information-rich spectroscopic techniques with sample imaging in both two and three dimensions. Raman Imaging, scanned probe and tomographic techniques provide images with unprecedented resolution containing highly informative localized information, for example on local elemental composition, water content, and substrate-metal 273 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

homogeneity. Instrument developers continue to “push the envelope” on what is possible for in-situ sample characterization and imaging, and it seems likely that this trend will continue.


Electrode Modeling Computer modeling capabilities in all areas of science and engineering have undergone spectacular development over the past several decades, to the point where modeling is now an expected and often required part of any new research project. Recent federal initiatives aimed at “Materials by Design” and a “Materials Genome” speak to the power and prevalence of computational modeling in materials science and engineering. In the area of energy, system-level models that include multiple types of physics all at once are commercially available and may be easily used to model a wide range of energy storage and conversion devices. Ab-initio modeling at atomic/molecular scales has also advanced on many fronts and is available via both commercial and proprietary avenues. Application of atomic / molecular models to nanomaterials can be problematic due to the large size and large number of atoms that must be included in the simulation to achieve realistic results, however rapid advancements in supercomputer technology are making calculations routine that were once thought to be nearly impossible. This trend is almost certain to continue and will lead to advancements in understanding the relationship between structure and properties, and hopefully, to the design and ultimately the synthesis of new nanomaterials for energy.

Energy Policy and Nanomaterials Public policy is not normally thought of as a science and engineering area, but in the present national and global climate, with so many new paradigms being considered for energy harvesting, conversion, storage and consumption, it is only reasonable to think that informed policy decisions will be needed to allow for new technologies to safely enter the marketplace. The relationship between nanomaterials and energy policy is perhaps a circuitous one, but as nanomaterial-enabled technologies such as plug-in hybrid electric vehicles, fuel-cell-powered hydrogen vehicles, and grid-scale electrical energy storage systems become more well developed and widespread, it will become necessary to develop public policies that are forward-looking enough to enable new technologies to develop, while still allowing for the fair market competition that provides the selective pressure to determine which technologies will ultimately win the day. In fact, energy needs in society are so great and so diverse that there will likely be many winners for different markets. Predictions in the public policy area are particularly difficult to make because of the high uncertainty associated with global geopolitics, and also the price volatility that accompanies select fossil fuels such as petroleum and natural gas. Even so, it seems likely that over the next five to ten years, public policy will continue to evolve in areas relating to renewable energy and the electric grid, electrical energy distribution 274 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


(charging) for electric cars, and hydrogen, bioethanol, biodiesel and natural gas as automotive transportation fuels. In concluding, and keeping in mind the wise proverb about making predictions, we may speculate that the demand for energy will certainly increase as the number of people who have access to energy-consuming devices increases. The number of electric and green-energy automobiles and portable devices in the world is likely to increase, as is the incorporation of nanomaterials into offices and homes. Windows that generate electricity and energy-efficient appliances will become more widespread. Green-energy tax credits will mean that the children of the authors of this volume may well be compelled to write a companion piece outlying their challenges and their solutions with which they are faced. I imagine their approach will be innovative, far reaching and hypothesis-driven as are the solutions offered herein. The demand for food, energy and new materials for ancient and modern communities is as old as fire or the wheel and drives human desire to control their environment forward towards the next millennium.

275 In Nanomaterials for Sustainable Energy; Liu, Jingbo Louise, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Postface: Nanomaterials for Energy, A Look Forward - ACS Publications

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