(Keynote) Nanomaterials and Interfaces for Efficient Solar Energy Conversion

Nanostructured materials and interfaces are critical to the functioning of solar energy conversion devices. For photovoltaics, hybrid organic/inorganic halide perovskite solar cells have emerged as among the most competitive photovoltaic technologies of the future thanks to their superb and rapidly improving power conversion efficiencies (PCEs). For realistic deployment of the perovskite photovoltaic technology in large scale, however, device stability has become more and more important issue of the day. I will update our recent efforts in the material and interface innovations to develop high-efficiency and high-stability perovskite solar cells: (1) the relationship between device stability and ion movement of the perovskite layer; 2) the use of NiO and carbon nanostructures for efficient hole extraction and enhanced device stability; and 3) the development of perovskite and interface engineering techniques for improving both efficiency and stability. Implications of these results on the future development of perovskite solar cells will be discussed. For electrocatalysts needed for generating solar fuels, I will focused on two-dimensional (2D) materials. In particular, molybdenum sulfide (MoS 2 ) is an attractive noble-metal-free electrocatalyst for the hydrogen evolution reaction (HER). We have recently demonstrated significantly enhanced HER kinetics by controllably fabricating a stepped MoS 2 surface structure. Vertical arrays of MoS 2 sheets terminated with such a stepped surface structure have proved to be an outstanding HER electrocatalyst with overpotential of 104 mV at 10 mA/cm 2 , exchange current density of 0.2 mA/cm 2 and high stability. Experimental and theoretical results indicate that the enhanced electrocatalytic activity of the vertical MoS 2 arrays is associated with the unique vertically terminated, highly exposed, stepped surface structure with a nearly thermoneutral H-adsorption energy. The HER electrocatalyst we engineered above, albeit based on the earth-abundant 2D MoS 2 , is only active in acidic media, and the HER reaction kinetically retarded in alkaline media. Thus, improving the sluggish kinetics for HER in alkaline media is crucial for advancing the performance of water-alkali electrolyzers. Toward this end, we have demonstrated a dramatic enhancement of HER kinetics in base by judiciously hybridizing vertical MoS 2 sheets with another earth-abundant material, layered double hydroxide (LDH). The resultant MoS 2 /NiCo-LDH hybrid exhibits an extremely low HER overpotential of 78 mV at 10 mA/cm 2 and a low Tafel slope of 76.6 mV/dec in 1 M KOH solution. At the current density of 20 mA/cm 2 or even higher, the MoS 2 /NiCo-LDH composite can operate without degradation for 48 hr. This work not only brought forth a cost-effective and robust electrocatalyst, but more generally opened up new vistas for developing high-performance electrocatalysts in unfavorable media recalcitrant to conventional catalyst design.