Strategies for photoelectrode protection: A case study of an extremely stable TiO2 protected c-Si device
Dowon Bae a, Brian Seger a, Thomas Pedersen b, Ole Hansen b, Peter Vesborg a, Ib Chorknedorff a
a Technical University of Denmark, Department of Physics, Fysikvej, 312, Kongens Lyngby, Denmark
b Technical University of Denmark, Department of Micro- and Nanotechnology, 345B, Kgs. Lyngby, 2800, Denmark
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2017 (NFM17)
SF1: Material and Device Innovations for the Practical Implementation of Solar Fuels (SolarFuel17)
Barcelona, Spain, 2017 September 4th - 9th
Organizers: Wilson Smith and Ki Tae Nam
Oral, Dowon Bae, presentation 092
Publication date: 20th June 2016

Photoelectrochemical (PEC) water splitting is a promising approach to provide clean and storable fuel (i.e. hydrogen) directly from sunlight and water. However, major challenges still have to be overcome before commercialization can be achieved. One of the largest barriers to overcome is to obtain a stable PEC reaction in either strongly basic or acidic electrolytes without degradation of the semiconductor photoelectrodes.1 In this work, we discuss fundamental aspects of protection strategies for achieving stable solid/liquid interfaces. Besides, we also cover protection layer approaches and their stabilities for a wide variety of experimental photoelectrodes for water splitting.

Then as a case study, transparent TiO2 protected c-Si based photoelectrodes for both hydrogen and oxygen evolution (HER and OER, respectively) are discussed in-depth. The detailed working principles based on the band alignment are outlined to understand how the carriers can be injected and transferred to solid/liquid interfaces in PEC systems. Particularly, we demonstrate an extremely stable HER at pH 0 for more than 80 days under the red-light (λ ≥ 635 nm) using a TiO2 protected MOS (metal-oxide-semiconductor) based c-Si photocathode.2 So far, this is the longest stability reported in photocatalytic water reduction. Importantly, the long-term stability experiment contains day/night cyclic test that can evaluate the intrinsic stability of the protection layer.

Detailed analysis using SEM (scanning electronic microscopy), XPS (X-ray photoelectron spectroscopy) and ICP-MS (inductively coupled plasma mass spectrometry) revealed that the degradation in photocurrent mainly results from carbon contamination, which decreases reactivity and blocks the light absorption. Finally, we discuss key aspects which should be addressed in continued work on realizing a reliable and practical PEC solar water splitting device.

1. D. Bae, B. Seger, P. C. K. Vesborg, O. Hansen and I. Chorkendorff, Chem. Soc. Rev., 2017.

2. D. Bae, T. Pedersen, B. Seger, B. Iandolo, O. Hansen, P. C. K. Vesborg and I. Chorkendorff, Catal. Today, 2016.

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