(348f) Stabilizing Electrodeposited Nanoparticle Electrocatalysts on Si MIS Photocathodes for Solar Water Splitting

As the cost of solar energy continues to drop, the major hurdle limiting the widespread use of intermittent renewable solar energy is the lack of efficient and cost-effective energy storage. Photoelectrochemical cells (PECs) offer one solution through their ability to convert solar energy directly into storable solar fuels such as hydrogen. Hydrogen is a versatile energy carrier which can be used for a variety of applications (e.g., chemical precursor for ammonia production, fuel for transportation, or converted back to electricity). The essential component of a PEC is the semiconducting photoelectrode that absorbs sunlight to drive non-spontaneous reactions such as water electrolysis. Commercialization of PEC technology is inhibited by challenges to identify and develop photoelectrodes that have long term stability, high efficiency, and low-cost materials.

One promising approach to achieving stable and efficient PEC water splitting is the composite photoelectrode architecture known as a metal-insulating-semiconductor (MIS) photoelectrode. [1,2,3,4] Within the MIS design, the semiconductor efficiently absorbs sunlight and a thin insulating layer (< 2 nm thick) protects the underlying semiconductor from corrosion and facilitates carrier transport between the semiconductor and metal through quantum mechanical tunneling. A metal catalyst is deposited on top of the insulator, to collect photo-generated carriers and catalyze the water splitting reaction. Through this composite design, the MIS architecture is able to decouple efficiency and stability tradeoff that typically limits conventional photoelectrodes, and has been applied to c-Si and InP photoelectrodes with great success in recent years. [1,2,3,4] In prior work on MIS photoelectrodes, the metal layer is typically deposited by physical vapor deposition in vacuum, which is an expensive and non-scalable method for deposition. If this technology is to become commercially viable, a scalable fabrication process is needed in addition to further improvements in MIS photoelectrode performance. In particular, improved photovoltage and maximized photo-current density generated by MIS photoelectrodes is essential for achieving DOE solar-to-hydrogen conversion efficiency targets.

In this work, electrodeposition has been explored as a potentially low-cost and scalable means of depositing ultra-low loadings (1 to 20 μg cm^-2) of discontinuous Pt nanoparticles onto SiO₂-covered p-Si photoelectrodes. Well-defined, nano-scale catalytic metal islands,[3,5] such as those achieve by electrodeposition, can facilitate high optical transmission of incident light into the semiconductor to improve photo-current density. However, as-deposited MIS photoelectrodes are found to exhibit very poor stability and performance. We report how this issue can be overcome through application of a thin transparent SiO_x overlayer. The SiO_x/Pt nanoparticle structure is similar in nature to core-shell nanoparticles that have previously been used for high temperature heterogeneous catalysis to enhance thermal stability [5], as well as for reducing undesired back-reactions in photoelectrochemical applications [6]. Surprisingly, it is found that the overlayer also results in a substantial improvement in performance in comparison to as-deposited MIS photoelectrodes of identical Pt loading. The combination of electrodeposition with the transparent overlayer thus offers an exciting opportunity for improving both the efficiency and durability of photoelectrochemical energy conversion. After describing a side-by-side comparison of MIS photoelectrodes with and without overlayer, we highlight reasons for the differences in their performance and stability.

References:

[1] Y.W. Chen, P. McIntyre, et al., Nat. Mater., 10 (2011)

[2] M.H. Lee, et al., Angew. Chem. Int. Ed., 51 (2012).

[3] J.C. Lee, A. Bard, J. Ekerdt, et al., Nature Nanotech., 10, 84-90 (2015)

[4] D.V. Esposito, I. Levin, T.P. Moffat, and A.A. Talin. Nature Materials, 12, 562-568 (2013).

[5] T. D. Gould, A. Izar, A. W. Weimer, J. L. Falconer, J. W. Medlin, ACS Catal. 2014, 4, 2714â??2717.

[6] K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue, K. Domen, Angew. Chemie - Int. Ed. 2006, 45, 7806â??7809.