I. Materials Genome for Accelerated Discovery of New Solar Materials and Catalysts
The discovery and development of advanced energy materials and their large-scale, low-cost manufacturing and deployment is and will be a critical research need for the foreseeable future for economic growth of any region. Unfortunately, the current strategy used for discovery to development uses a linear methodology of discovery-manufacturing-verification, which takes over twenty years or more for translation to market. The White House and several federal agencies have outlined a materials genome initiative specifically to accelerate the discovery and development of new advanced materials in conjunction with advanced manufacturing to reduce the time required for their deployment to market (Figure 1, Holdren, J. P. Materials Genome Initiative Strategic Plan. Initiative, 2014). It is predicted that if one combines materials genome type algorithms to materials/manufacturing development continuum methodology, the time scales for discovery to deployment of both materials and advanced manufacturing will reduce by half to less than 10 years.
Figure 1. Acceleration of manufacturing to practice with a new materials continuum strategy (adapted from Holdren et al., 2014).
Materials Genome Initiative
The future materials/manufacturing discovery to deployment continuum methodology will involve a strategy outlined in the Figure 2 and will rely on combinatorial experimental tools, rapid device screening facilities, computational intelligence methods to analyze experimental data, and advanced low cost and scalable manufacturing. Specifically, they include: (a) combinatorial experimental tools for preparing and characterizing materials for structure-property-process-functionality data sets; (b) infrastructure necessary for an experimental data repository with data flowing directly from lab note books and the web-based analysis tools; and (c) discovery and manufacturing roadmaps for specific challenges. This new infrastructure design will facilitate workforce development for advanced manufacturing and enable the state of Kentucky to establish an economic ecosystem for the next generation manufacturing.
Figure 2. Proposed materials genome based discovery to manufacturing continuum strategy for rapid deployment of new materials into manufacturing.
II. Advanced Materials Processing for Renewable Energy
A. Large Single Crystal Growth of Diamond
B. Bulk growth of Gallium Nitride
Gallium nitride is a direct bandgap (3.45eV) semiconductor that has the potential to revolutionize the fields of solid state lighting, electronics and power conversion. GaN’s inherent high breakdown strength, high saturation electron velocity and good thermal conductivity make this material very promising for power switches applications; helping to reduce the consumption of energy in grid, automotive and industrial applications.1
Despite the huge technological interest, the progress in high power electronics and long-term applications has been slow due to the high cost of low defect density bulk GaN wafers. GaN crystals cannot be grown using typical Bridgman or Czochralski type techniques because decomposition occurs prior to melting. Vapor phase heteroepitaxy on single crystal substrates such as SiC, and sapphire has been the most common method to synthesize GaN. However, the large threading dislocation density (~ 1010–1014 cm-2) obtained in heteroepitaxial growth due to lattice mismatch between the GaN and the underlying substrate2 is significantly higher than the required for high power devices (<104 cm-2). Bulk synthesis methods have been used to obtain GaN crystals with low defect density. Among these, the ammono-thermal method has been the one that provides the best quality.3 In general, bulk synthesis methods are performed under extreme temperatures and pressures for long periods of time (in the order of days) making them cost prohibitive. The high costs of producing GaN based power electronics makes this material noncompetitive and limits its widespread use.
Our group at Conn Center has been working to produce at low cost large area (~ 1-2 inch) single crystal GaN wafers with low defect density (<107/cm2). In contrast to other bulk growth techniques, our proposed processes are performed at low pressures (sub-atmospheric), relatively low temperatures (~900 oC) and are considerably faster (in the order of hours) making them easy to scale and more importantly, cheaper than current GaN bulk techniques. If successful, these techniques will enable wide spread applications of GaN in power electronics and eventually could result in enormous energy savings.
Homoepitaxial growth of GaN via plasma-assisted liquid phase epitaxy (LPE). GaN is grown homoeptaxially by a controlled nitridation of Ga using nitrogen plasma. This technique offers a contaminant-free and safe environment as it only uses Ga and N2 as the precursors instead of TMG and NH3 in the case of MOCVD, or HCL, Ga, and NH3 in the case of HVPE.
C. Halide Vapor Phase Epitaxy (HVPE) of Novel III-V Materials
Conn Center researchers have identified a number III-V alloys whose band gaps and band edge energetics can be engineered for various solar cell and solar fuel applications. However, their deposition/synthesis methods are either complicated or expensive. Here, Conn Center researchers are developing a halide/hydride vapor phase approaches for developing high growth rate methods for growing near single crystal quality films. The long term objectives include the development of roll to roll processing of these materials systems for large area growth and flexible substrates.
Left, Tauc plots of diffuse reflectance measurements of a GaSbP alloy containing 4 % of Antimony, compared to pure GaP. Right, proposed band edge alignments of the ternary alloy versus pure GaP.4
Scanning Electron Micrograph of the GaSbP alloy cross section.4
D. Metal Oxide Nanowires and Complex Metal Oxide Nanoparticles
Metal oxide nanowires and complex metal oxides play an important role in various energy conversion and storage applications. Examples include heterogeneous catalysts and electrocatalysts. Advanced Energy Materials, LLC, a company started with original efforts from Conn Center, has been pioneering the scalable manufacturing of both metal oxide nanowires and complex metal oxide particles with precise compositional control.
Nanowires provide uniformity of active surfaces, great stability and improved diffusion processes among other advantages. They have attracted a lot of attention into material design in recent years. In our group, the development of solid adsorbents in nanowire morphology have been studied successfully with major focus in carbon capture technologies where the nanowire morphology enhanced the adsorption kinetics and provide stability for the recycling of adsorbent ceramics.5 Materials synthesized through solvo-plasma or thermal oxidation techniques have described ultrafast CO2 uptake reaching surface saturation in 3 minutes, when typically this region is reached after 80 minutes.6
Figure 3. A schematic illustration of various steps used in our solvo-plasma synthesis method.
Figure 4. Mechanistics of the morphology enhanced CO2 capture and full isotherms of CO2 adsorption (60%CO2/N2) on Li silicates: (a) agglomerates between 10 and 200 μm, (b) 2–20 μm particles, and (c) nanowire aggregates, with diameters less than 50 nm and lengths around 5 μm. Schematic.
E. Novel Processing Using Plasma Catalysis
Conn Center researchers are working to develop fundamental understanding of catalytic phenomena and exploit plasma catalysis to advance catalyst synthesis, design and their synergistic interactions, enabling transformative advances in chemical transformation of nitrogen, methane and carbon dioxide.
Grand Challenges in Chemical Transformations: The chemical transformation of nitrogen (N2), methane (CH4) and carbon dioxide (CO2) into technologically relevant chemicals is a grand challenge in catalysis science, with a potentially enormous impact on our energy infrastructure. Specifically, these three species are relatively inert, and the key rate-limiting step in their chemical transformation is the dissociation of these molecules. The energetic barrier of dissociation can be decreased effectively using synergistic action between gas phase activation combined with catalysis.
The most important transformation routes for the above three molecules are the following: (a) Conversion of nitrogen and hydrogen to ammonia for fertilizers and fuel; (b) conversion of methane to higher fuels and plastics; and (c) CO2 conversion, dry reforming and fuel production. The overall reactions are shown below.
Ammonia Synthesis: N2 + 3H2 2NH3 (Haber-Bosch)
Methane Processing: 2CH4 + O2 C2H4 + 2H2O (Oxidative coupling of methane)
CO2 Reduction: CO2 + CH4 2CO + 2H2 (Dry reforming of methane)
CO2 + 2H2O CH3OH + 3/2 O2 (Artificial photosynthesis)
Current catalysts and processes typically have low conversion, high temperature and pressure process conditions, short life span, and more often than not, severe environmental impacts.
Proposed Concept of Plasma Catalysis: The concept of plasma catalysis has grown in significance in the last ten years or so.7 Plasma can be created via exciting gas phase molecules with an applied electric field through ionization and electron-molecule reactions. The energized neutral or charged species can directly impact the catalytic process by significantly decreasing the energetic barrier, or by changing the reaction pathway, eventually enhancing the rate of reaction. In conventional catalysis, the gas phase molecules (reactants) are exposed to catalyst beds at high temperatures and pressures to overcome the energetic barrier, resulting in dissociation of gas phase molecules. The dissociated species further react to form desired products. Second, the activity and selectivity depend on the nature of catalyst materials in terms of composition, surfaces, and defects, as well as on processing conditions. Third, plasmas present as non-equilibrium techniques for synthesizing complex materials and at time scales necessary for both discovery and development of new catalyst materials in terms of scalability and throughput.
III. DURABILITY OF SURFACES AND INTERFACES
Conn Center is utilizing a number of in-situ and in-operando techniques to obtain the necessary fundamental understanding of the durability of various materials and interfaces used in our conversion and storage devices. Most importantly, the Conn Center researchers are developing standardized approaches for understanding long term durability for electrocatalysts; solar cells and li-ion materials and also various fundamental descriptors that can be used for predicting long term durability.
1. Tamura, S.; Anda, Y.; Ishida, M.; Uemoto, Y.; Ueda, T.; Tanaka, T.; Ueda, D. In Recent Advances in GaN Power Switching Devices, Compound Semiconductor Integrated Circuit Symposium (CSICS), 2010 IEEE, IEEE: 2010; pp 1-4.
2. Davis, R.; Einfeldt, S.; Preble, E.; Roskowski, A.; Reitmeier, Z.; Miraglia, P., Gallium nitride and related materials: challenges in materials processing. Acta materialia 2003, 51 (19), 5961-5979.
3. Dwiliński, R.; Doradziński, R.; Garczyński, J.; Sierzputowski, L.; Kucharski, R.; Zając, M.; Rudziński, M.; Kudrawiec, R.; Strupiński, W.; Misiewicz, J., Ammonothermal GaN substrates: Growth accomplishments and applications. physica status solidi (a) 2011, 208 (7), 1489-1493.
4. Martinez‐Garcia, A.; Russell, H. B.; Paxton, W.; Ravipati, S.; Calero‐Barney, S.; Menon, M.; Richter, E.; Young, J.; Deutsch, T.; Sunkara, M. K., Unassisted Water Splitting Using a GaSbxP (1− x) Photoanode. Advanced Energy Materials 2018, 1703247.
5. Akram, M. Z.; Atla, V.; Nambo, A.; Ajayi, B. P.; Jasinski, J. B.; He, J.; Gong, J. R.; Sunkara, M., Low-Temperature and Fast Kinetics for CO2 Sorption Using Li6WO6 Nanowires. Nano Letters 2018.
6. Nambo, A.; He, J.; Nguyen, T. Q.; Atla, V.; Druffel, T.; Sunkara, M., Ultrafast Carbon Dioxide Sorption Kinetics Using Lithium Silicate Nanowires. Nano Letters 2017, 17 (6), 3327-3333.
7. Neyts, E. C.; Ostrikov, K.; Sunkara, M. K.; Bogaerts, A., Plasma catalysis: synergistic effects at the nanoscale. Chemical reviews 2015, 115 (24), 13408-13446.