There is a considerable interest in the design of functional materials and devices for solar energy conversion, in anticipation of the renewable energy needs associated with limited resource of fossil fuels [1]. Of a range of materials that have been investigated in pursuit of greater utilization of solar energy, titanium dioxide is one of the most effective semiconductor photocatalysts, which has an electronic band gap in the ultraviolet wavelength regime. Narrowing the band gap is therefore vital to achieve efficient absorption of sunlight. While doping is a well-established method of tuning the band gap, its application to titanium dioxide has only limited success. We demonstrated [2] a conceptually different approach to enhancing solar absorption by tailoring disorders in nanophase titanium dioxide through hydrogenation. We showed that disorder-engineered titanium dioxide nanocrystals exhibited substantial solar-driven photocatalytic activity and are capable of producing hydrogen from water at room temperature.

[1] X. Chen, S. Shen, L. Guo, and S. S. Mao, “Semiconductor-based photocatalytic hydrogen generation.” Chemical Reviews, V.110, 6503 (2010).

[2] X. Chen, L. Liu, P. Y. Yu, and S. S. Mao, “Increasing Solar Absorption for Photocatalysis with Black, Hydrogenated Titanium Dioxide Nanocrystals.” Science, V.331, 746 (2011).

As clean energy conversion devices, polymer electrolyte membrane fuel cells (PEMFCs) have many advantages, including high energy conversion efficiency, which make them attractive for both mobile and stationary applications. While platinum (Pt) has been regarded as the most active catalyst for oxygen reduction reaction, the ultimate applications of PEMFCs are hindered, at least partially, due to the high cost of the noble material. We recently demonstrated [1] that highly crystalline CrN nanoparticles of fcc structure exhibit attractive catalytic activity and stability for the oxygen reduction reaction in PEMFCs. The result offers a possible solution to the need of cost-effective electrocatalyst alternatives for fuel cells. Additionally, we are also developing solid-oxide fuel cells (SOFCs) with perovskite-phase BaSrCoFeO (BSCF) thin film cathodes prepared on NiO-YSZ/YSZ composite substrates using pulsed laser deposition (PLD) and traditional screen-printing method [2]. The electrochemical performance of the SOFC cathode fabricated by PLD outperformed that based on the screen-printing method, which can be attributed to the smaller internal resistance within the cathode and the interfacial resistance between the cathode and the electrolyte. This work  demonstrated the potential of PLD as a promising cathode deposition technique for achieving high performance SOFC.

[1] H. Zhong, X. Chen, H. Zhang, M. Wang, and S. S. Mao, “Proton exchange membrane fuel cells with chromium nitride nanocrystal catalysts.” Applied Physics Letters, V.91, 163103 (2007).

[2] B. Liu, X. Chen, Y. Dong, S. S. Mao, and M. Cheng, “A high performance nanostructured BaSrCoFeO cathode for solid-oxide fuel cells.” Advanced Energy Materials, V.1, 343 (2011).

There is a need of thermal management to control battery temperature as the trend to high power densities for vehicles would result in more heat generated within a smaller volume. A temperature increase of 15 ^oC could reduce the life of a Li-ion cell by 50%. Typically Li-ion cells have two electrodes separated by a thin membrane separator. When charging, electrons are conducted through the external electrical connection, and Li+ ions diffuse through the separator towards the negative electrode. When discharging, electrons and Li+ ions flow in the opposite direction. We studied through numerical modeling the effects of material and design modifications on the temperature distribution of Li-ion cells, such as the incorporation of an internal structure for better heat transfer, and the use of a carbon nanotube negative electrode and of separators made of Separion and different ceramics mixed with functioning particles.

[1] M. Sievers, U. Sievers, and S. S. Mao, “Thermal modeling of new Li-ion cell design modifications.” Engineering Research / Forschung im Ingenieurwesen, V.74, 215 (2010).

Since the discovery of extreme surface wetting phenomenon induced by ultraviolet (UV) photocatalysis, titanium dioxide has become the material of choice for environmentally friendly applications such as self-cleaning and anti-contamination coatings. Nevertheless, it remains a significant challenge to realize surfaces exhibiting persistent super-hydrophilicity but without the need of external stimuli, especially UV irradiation that also degrades the wetting surface over a period of time. We demonstrated [1] a porous titanium dioxide nanostructure that shows super-hydrophilicity without the need of light activation, and with stability against successive wetting-dewetting cycles. The wetting surface exhibits high optical transmittance over a wavelength range from near UV to the infrared, enabling practical anti-fogging technologies where transparency is critical.

[1] V. Zorba, X. Chen, and S. S. Mao, “Superhydrophilic TiO₂ surface without photocatalytic activation.” Applied Physics Letters, V.96, 093702 (2010).