(410f) Nanostructured Silicon Photocathodes for Solar Water Splitting

Photoelectrochemical (PEC) water splitting is a promising route for synthesizing hydrogen using solar energy without direct CO₂ emissions.^[1] PEC water splitting devices require highly active hydrogen evolution reaction (HER) catalysts to efficiently produce hydrogen at the photocathode.^[2] Platinum and its alloys are the most active HER catalysts but the low availability of platinum may create problems for large scale implementation of solar water splitting.^[3] There is a need to replace platinum with a non-precious metal earth-abundant alternative to enable cost effective, scalable devices.^[4]

Recently, transition metal phosphides and sulfides have been shown to be among the most active nonprecious HER catalysts on a total electrode activity and turnover frequency (TOF) basis.^[5] Though extremely active, the high surface area morphologies of these previously synthesized electrodes present additional challenges for integration onto PEC water splitting electrodes.^[6] New methods are required to synthesize HER catalysts with a high density of active sites that also transmit a large portion of incident light.

We discuss our progress in synthesizing highly active transition metal phosphide and sulfide catalysts in nanostructured, low light-absorbing geometries. First, we compare nanostructured silicon semiconductors covered with optically thin catalysts to analogous flat silicon electrodes. These results provide insight into the general design considerations necessary to maximize the device efficiency for any given catalyst-absorber system. Next, we present a device structure which decouples the catalyst nanostructure from that of the silicon absorber. In so doing, we achieve low levels of recombination within the silicon and maximize the current-voltage characteristics of the device. Based on our findings, we discuss the application of our catalyst structure to other semiconductor systems and suggest general strategies for further improving the performance of solar water splitting electrodes.

[1] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori, N. S. Lewis, Chemical Reviews 2010, 110, 6446.

[2] L. C. Seitz, Z. Chen, A. J. Forman, B. A. Pinaud, J. D. Benck, T. F. Jaramillo, ChemSusChem 2014, 7, 1372.

[3] J. Greeley, I. Stephens, A. Bondarenko, T. P. Johansson, H. A. Hansen, T. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, Nature Chemistry 2009, 1, 552.

[4] P. C. Vesborg, T. F. Jaramillo, RSC Advances 2012, 2, 7933.

[5] J. Kibsgaard, T. F. Jaramillo, Angew. Chem.-Int. Edit. 2014, 53, 14433.

[6] J. D. Benck, T. R. Hellstern, J. Kibsgaard, P. Chakthranont, T. F. Jaramillo, ACS Catalysis 2014, 4, 3957.