Collaborators

• W. Higgins, RMD, MA
• I. D. Kim, KAIST, Korea
• M. Martin & R. DeSouza, Aachen University, Germany
• D. Nocera, MIT
• R. Moos, Univ. Bayreuth Germany
• B. Yildiz, MIT
• K. Van Vliet, MIT
• J. Varela, UNESP, Brazil

Graduate Students

• G. Whitfield, Res. Asst., MSE
• D. Chen, Res. Asst., MSE
• J. Engel, Res. Asst., MSE
• J. J. Kim, Res. Asst., MSE
• N. Thompson, Res. Asst., MSE
• A. Geupel, visiting student, MSE
• A. Felix, visiting student, MSE
• A. Mani, visiting student, MSE

Postdoctoral Associates

• S. Bishop, MSE
• D.J. Yang, MSE
• P.S. Cho, MSE
• Y. Kuru, MSE, NSE
• M. Kuhn, MSE

Visiting Scientist

• J. Rupp, MSE, NSE
• M. Orlandi, MSE

Support Staff

• A. Rothwell, Administrative Assistant II

Publications

S. R. Bishop, W. Higgins, G. Ciampi, A. Churilov, K. S. Shah and H. L. Tuller, The Defect and Transport Properties of Donor Doped Single Crystal TlBr, J. Electrochem. Soc., 158, J47-J51 (2011)

Y. Chen, W.C. Jung, Y. Kuru, H. Tuller, and B. Yildiz, Chemical, Electronic and Nanostructure Dynamics on Sr(Ti_1-xFe_x)O₃ Thin-Film Surfaces at High Temperatures, ECS Transactions, 35 (1) 2409-2416 (2011), Solid Oxide Fuel Cells 12 (SOFC-XII). Eds. S. Singhal, K. Eguchi.

M. Schulz, J. Sauerwald, H. Seh, H. Fritze, H. L. Tuller, Defect Chemistry Based Design of Monolithic Langasite Structures for High Temperature Sensors, Solid State Ionics 184, 78–82 (2011)

S. R. Bishop, T. S. Stefanik and H. L. Tuller, Electrical Conductivity and Defect Equilibria of Pr_0.1Ce_0.9O_2-δ, Physical Chemistry Chemical Physics, 13, 10165-10173 (2011).

Y. Kuru , S. R. Bishop, J. J. Kim, B. Yildiz, and H. L. Tuller, Chemomechanical Properties and Microstructural Stability of Nanocrystalline Pr-doped Ceria: an in situ X-ray Diffraction Investigation, Solid State Ionics, 193, 1-4 (2011).

H. L. Tuller and S. R. Bishop, Point Defects in Oxides: Tailoring Materials Through Defect Engineering, Annu. Rev. Mater. Res. 41, 13.1–13.30  (2011).  Invited Review.

M. Burbano, D. Marrocchelli, B. Yildiz, H. L. Tuller, S. T. Norberg, S.Hull, P. A. Madden, and G. W. Watson, A Dipole Polarizable Potential for Reduced and Doped CeO₂ from First-Principles. Journal of Physics-Condensed Matter, 23 (25), 255402 (9 pg) (2011).

S. Bishop; H. L Tuller; Y. Kuru; B.Yildiz, Chemical Expansion of Nonstoichiometric Pr_0.1Ce_0.9O_2-δ: Correlation with Defect Equilibrium Model, J. Euro. Ceram. Soc. 31, 2351-2356 (2011).

Il-Doo Kim, Tae-Seon Hyun, H. Tuller, Duck-Kyun Choi, and Doo-Young Youn, Facile Synthesis of Highly Conductive RuO₂-Mn₃O₄ Composite Nanofibers via Electrospinning and Their Electrochemical Properties, J. Electrochem. Soc., 158 (8), A970-A975 (2011).

N. G. Cho, D. J. Yang, M.-J. Jin, H.-G. Kim, H. L. Tuller, and I.-D. Kim, Highly Sensitive SnO₂ Hollow Nanofiber-based NO₂ Gas Sensors, Sensors and Actuators B, 160, 1468-1472 (2011).

Y. Kuru, H. Jalili, Z. Cai, B. Yildiz, and H. L. Tuller, Direct Probing of Nano-dimensioned Oxide Multilayers with Aid of Focused Ion Beam Milling, Advanced Materials. 23, 4543-4548 (2011).

S. R. Bishop, J-J. Kim, N. Thompson, and H. L. Tuller, Defect Chemistry and Electrical Properties of a Pr-CeO₂ Solid Solution: From Nano- to Micro-scale, Symposium K, Frontiers of Solid State Ionics, Mater. Res. Soc. Symp. Proc. Vol. 1331, mrss11-1331-k02-02 (6 pages) (2011).

M. Søgaard, A. Bieberle- Hütter, P. Vang Hendriksen, M. Mogensen, H. L. Tuller, Oxygen Incorporation in Porous Thin Film of Strontium Doped Lanthanum Ferrite, J. Electroceramics, 27, 134-142 (2011).

H. L. Tuller, J. Engel, S. J. Litzelman, and S. R. Bishop, Nano-Structured Materials for Next Generation Fuel Cells and Photoelectrochemical Devices, Symposium on Renewable Fuels and Nanotechnology, Mater. Res. Soc. Symp. Proc. Vol. 1326, mrss11-1326-f08-01 (12 pages) (2011),

W. Jung and H. L. Tuller, A New Model Describing Solid Oxide Fuel Cell Cathode Kinetics: Model Thin Film SrTi_1-xFexO_3-δ Mixed Conducting Oxides – a Case Study, Advanced Energy Materials, 1, 1184-1191 (2011). 

Combustion of fossil fuels, essential for electricity generation and vehicular propulsion, is generally incomplete, leading to harmful NOx, CO, and unburned hydrocarbons emissions. Great progress in minimizing such emissions has relied on the operation of “three-way catalysts” (TWCs), which utilize a combination of precious metals and metal oxides with the ability to take up or release oxygen for reduction/oxidation of pollutants (NOx to N₂ and CO and HC to CO₂ and H₂O, respectively) ^[1] . In this project, we are investigating the rate at which oxygen storage materials (OSM, typically Ce_xZr_1-xO_2-δ) exchange oxygen with the atmosphere and the magnitude of oxygen they store with the aid of geometrically well-defined thin film structures. Impedance spectroscopy, Kelvin probe, thermogravimetry, coulometric titration, and electrical conductivity measurement methods are used to determine electrochemical performance and oxygen storage capabilities. These properties, when correlated to actual TWC performance, using a differential flow reactor, will allow for a more detailed understanding of performance criteria. Previous studies on Pr_xCe_1-xO₂in our group have demonstrated the feasibility of these methods ^{[2] [3] [4]} .

1. P. Forzatti, L. Castoldi, I. Nova, L. Lietti, and E. Tronconi, “NOx removal catalysis under lean conditions,” Catalysis Today, vol. 117, pp. 316-320, June 2006.
2. D. Chen, S. Bishop, and H. L. Tuller, “Praseodymium-cerium oxide thin film cathodes: Study of oxygen reduction reaction kinetics,” Journal of Electroceramics, vol. 28, pp. 62-69, Jan. 2012.
3. D. Chen, S. Bishop, and H. L. Tuller, “The chemical capacitance of praseodymium-cerium oxide thin films and relationship to nonstoichiometry,” under review.
4. S. R. Bishop, T. S. Stefanik, and H. L. Tuller, “Electrical conductivity and defect equilibria of Pr_0.1Ce_0.9O_2-δ,” Physical Chemistry Chemical Physics, vol. 13, pp. 10165-73, Apr. 2011.

With continuously growing energy demands, new alternative energy solutions become essential. In order to achieve sustainability, efficient conversion and storage of solar energy are imperative ^{[1] [2]} .  Photoelectrolysis utilizes solar energy to evolve hydrogen and oxygen from water, thereby enabling energy storage via chemical means. This work investigates photoelectrodes, which offer high conversion efficiency, long-term, stability and low cost. The focus is initially on semiconducting metal oxides in which the energy band-, defect-, and micro-structure are tuned to optimize optical absorption, charge transport, and reduced overpotentials. For high efficiency, a cobalt-based oxidation catalyst ^[3] is implemented at the photoelectrode. The electro-deposition kinetics of this catalyst are studied as part of this project to allow further insights into the catalytic mechanism.

1. N. S. Lewis and D. G. Nocera, “Powering the planet: Chemical challenges in solar energy utilization,” Proc. Natl. Acad. Sci. U.S.A. vol. 103, pp. 15729-15735, 2006.
2. R. van de Krol and Y. Liang, J. Schoonman, “Solar hydrogen production with nanostructured metal oxides,” J. Mater. Chem. vol. 18, pp. 2311-2320, 2008.
3. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science, vol. 321, pp .1072-1075, 2008.

Gas sensors are essential in the monitoring, control, and reduction of harmful emissions in the environment ^[1] .  Conductometric gas sensors based on semiconducting metal oxides are advantageous in many applications due to high sensitivity, manufacturability, and small size.  However, there are a number of drawbacks, including difficulty in control over the semiconductor/substrate interface, high power consumption, and reduced selectivity at high temperatures (300-400˚C) required for operation ^{[2] [3]} . The development of gas sensors with innovative designs and advanced functional materials has attracted considerable scientific interest due to their great technological potential ^[4] . This work presents new insight towards the development of high-performance p-type semiconductor gas sensors.  Gas sensor test devices, based on copper (II) oxide (CuO) with innovative and unique urchin-like structures, were prepared by a microwave-assisted synthesis method. An assembly of urchin-like structures was found to be most effective for hydrogen detection in the range of parts-per-billion (300 ppb) at low temperatures (200˚C). These results show that morphology plays an important role in the gas sensing performance of p-type semiconducting CuO gas sensors.

1. F. Rock, N. Barsan, and U. Weimar, “Electronic nose: Current status and future trends,” Chemical Reviews, vol. 108, no. 2, pp. 705-725, Jan. 2008
2. K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, and D. R. Walt, “Cross-reactive chemical sensor arrays,” Chemical Reviews, vol. 100, no. 7, pp. 2595-2626, June, 2000.
3. K. Wiesner, H. Knozinger, M. Fleischer, and H. Meixner, “Working mechanism of an ethanol filter for selective high-temperature methane gas sensors,” IEEE Sensors Journal, vol. 2, no. 4, pp. 354-359, Aug., 2002.
4. D.-J. Yang, I. Kamienchick, D. Y. Youn, A. Rothschild, and I.-D. Kim, “Ultrasensitive and highly selective gas sensors based on electrospun SnO(2) nanofibers modified by Pd loading,” Advanced Functional Materials, vol. 20, no. 24, pp. 4258-4264, Dec., 2010.

Solid oxide fuel cells (SOFCs) directly convert chemical to electrical energy with high efficiency and can operate using a wide variety of fuels ranging from hydrogen and kerosene to gasified coal ^{[1] [2]} . Many of the more advanced oxides utilized in SOFCs experience significant changes in oxygen content, or oxygen stoichiometry, during operation, resulting in changes in volume and elastic properties termed chemomechanics ^{[3] [4]} . This lattice dilation known as chemical expansion, analogous to temperature induced thermal expansion, is oxygen nonstoichiometry-induced and can cause large stress gradients across a SOFC stack, with potential for negative impact on device performance ^[5] . Therefore, a fundamental understanding regarding the coupling between solid-state electrochemistry and mechanical deformation is required for successful development of functionally superior and long-lived fuel cell systems. In this project, we are studying the chemical expansion coefficient, elastic properties, and oxygen stoichiometry of thin film and bulk SOFC oxide materials. Thin films are of particular interest since they allow control of strain and increase the surface-to-volume ratio, particularly important for electrode performance. Furthermore, there is a trend towards the use of thinner structures such as m-SOFCs ^[6] . The chemomechanical properties are being investigated using high temperature and atmosphere-controlled nanoindentation, high-resolution x-ray diffraction, dilatometry, impedance spectroscopy, and thermo-gravimetry techniques.

1. B. C. H. Steele and A. Heinzel, “Materials for fuel-cell technologies,” Nature, vol. 414, pp. 345-352, Nov 2001.
2. M. Mogensen and K. Kammer, “Conversion of hydrocarbons in solid oxide fuel cells,” Ann. Rev. Mater. Res., vol. 33, pp. 321-331, Aug 2003.
3. H. L. Tuller and S. R. Bishop, “Point defects in oxides: tailoring materials through defect engineering,” Ann. Rev. Mater. Res., vol. 41, pp. 369-398, Aug 2011.
4. Y. Kuru, S. R. Bishop, J. J. Kim, B. Yildiz, and H. L. Tuller, “Chemomechanical properties and microstructural stability of nanocrystalline Pr-doped ceria: An in situ X-ray diffraction investigation,” Solid State Ionics, vol. 193, pp.1-4, June 2011.
5. K. Sato, H. Omura, T. Hashida, K. Yashiro, H. Yugami, T. Kawada, and J. Mizusaki, “Tracking the onset of damagemechanism in ceria-based solid oxide fuel cells under simulated operating conditions,” Journal of Testing and Evaluation, vol. 34, pp. 246-250, May 2006.
6. A. Evans, A. Bieberle-Hütter, J. L. M. Rupp, and L. J. Gauckler, “Review on microfabricated micro-solid oxide fuel cell membranes,” Journal of Power Sources, vol. 194, pp. 119-129, Oct 2009.

Detection of high-energy radiation (e.g., γ-rays) is key in nuclear non-proliferation strategies.  When a wide-band gap semiconductor detector intercepts a γ-ray, electron-hole pairs are formed, resulting in an increase in electrical conductivity. This change in conductivity is maximized if the conductivity in the non-illuminated (dark) state is very low. Current semiconductor technologies require cooling to very low temperatures, which adds to cost and reduces portability.  TlBr is an attractive detector material given its low room-temperature dark conductivity, as well as its high mass density, leading to higher radiation absorption.

In this project, we have characterized the dominant ionic conduction properties in TlBr using impedance spectroscopy.  Through doping techniques, we have determined that TlBr is primarily a Schottky-type ionic conductor, meaning that Tl and Br move through the material by vacancy motion.  These measurements have led us to predict a doping strategy to minimize the dark conductivity, and we are collaborating with a local company (RMD) to implement this technology as well as developing it further by studying new TlBr based material systems.  In addition, our newfound understanding of TlBr has led us to investigate novel device designs never before used in ionic conducting systems.