Engineering Molybdenum Disulfide Protected Silicon Photocathodes for Corrosion Resistance in Acid

Abstract

Photoelectrochemical (PEC) water splitting devices offer a promising path to sustainable production of hydrogen.1 Silicon shows great promise as a small band gap photocathode in tandem photoelectrochemical (PEC) water splitting due to its appropriate band structure, energetic alignment for driving hydrogen evolution, and its band gap of 1.1 eV.2 However, some challenges remain to ensure that silicon photoelectrodes possess the required catalytic activity and stability for use in highly effect PEC water splitting devices. Molybdenum disulfide (MoS2) is a low permeability material known for its excellent stability and HER activity in acid. We have previously demonstrated MoS2 protected silicon photocathodes which operate for 100 hrs without significant loss of activity.3 Here, we further this study to design strategies, such as the utilization of a molybdenum silicide interface layer, for improved activity and over 600 hour stability with unchanged photoelectrochemical performance (onset potential, fill factor and light limited photocurrent density), which is among the best performance of any precious metal free photocathode. Critical to developing future electrodes with enhanced stability, we probe the failure mechanism of the MoS2 protected photocathodes. By XPS we observe that the MoS2 is fully absent post stability testing. Detailed failure mechanisms of the photocathodes and electrolyte are uncovered by electrochemical methods, SEM, Raman microscopy and ICP. Furthermore, we implement increased light intensity (10 suns), as well as light and dark cycling to probe accelerated durability testing protocols utilizing MoS2 protected photocathodes. From such analyses we uncover a feedback loop that enables insightful design of future corrosion resistant protection schemes. (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (2) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7 (5), 1372–1385. (3) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Adv. Energy Mater. 2014, 4 (18), 1400739.