Solar Fuels Generation

Overview | Energy Systems | Solar Fuel Generation | Light Directed Growth

Goal

Develop an integrated device that harnesses sunlight to drive water electrolysis and generate hydrogen fuel. Semiconductors are well-suited for capturing light and generating the energetic charge carriers necessary to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, at the electrochemical potential necessary to drive the HER and OER, deleterious side reactions can proceed at the interface of many useful semiconductor materials (e.g. Si and GaAs) that may result in device failure. We aim to develop chemistries and materials that can be applied to semiconductor photoelectrodes to provide selectivity towards the HER or OER reactions and thus enable stable operation.

Photoelectrodes often have various corrosion pathways including dissolution, oxidation, and metal plating. The corrosion can lead to loss in performance and eventual device failure. These pathways can be mitigated or prevented by protecting the surface with a chemically stable and electronically conductive protection layer.

Photoelectrodes often have various corrosion pathways including dissolution, oxidation, and metal plating. The corrosion can lead to loss in performance and eventual device failure. These pathways can be mitigated or prevented by protecting the surface with a chemically stable and electronically conductive protection layer.

Strategy

We use atomic layer deposition and sputtering to deposit protective coatings onto semiconductor interfaces to extend photoelectrode stability from minutes to days and even weeks. Protective coatings are ideally:

  • Chemically stable under the operating conditions of interest;
  • Conductive, to allow efficient transfer of charge from the semiconductor to the interface with water;
  • Catalytic, to favor direction of reactive charges to desired reactions over direction to corrosion reactions;
  • Transparent and anti-reflective, to enable effective capture of sunlight;

In addition, protective coatings should have favorable energy-band alignments with the underlying semiconductor to maximize the power of the device.

Highlights

Nickel Oxide Protective Coatings for Self-Passivating Semiconductors

NiOx

Reactively sputtered nickel oxide coatings are transparent to visible light (top left), and enable stable, high-performance photoanodes to be made using silicon (top right). NiOx-coated Si photoanodes in contact with strongly alkaline water robustly drove the oxidation of water to oxygen gas for over 1200 hours.1

We developed techniques to deposit a multifunctional nickel oxide (NiOx) coating onto n-type Si and n-type InP.  The reactively sputtered NiOx coating is a p-type semiconductor – an electronically ideal partner for an n-type photoanode – that is transparent, anti-reflective, and chemically stable under oxidative conditions in alkaline water.  Furthermore, the NiOx coating is catalytically active for the oxidation of water to oxygen gas. The NiOx coating enabled illuminated Si and InP electrodes immersed in strongly alkaline water – electrodes which would fail immediately without the coating – to be used to robustly drive the OER for over 1200 hours.  Accounting for the ~ 20% capacity factor of sunlight, the 1200 hours of continuous operation is comparable to more than 8 months of outdoor operation.

Electronically “Leaky” Titanium Dioxide Protective Coatings

Thin (~ 2 nm) TiO2 coatings had been found to protect small photoanodes; however, coatings a few nm thick are difficult to deposit uniformly and thus often contain through-film pinholes that limit the ability to scale up.  In addition, it was believed that thicker TiO2 coatings would act as insulators and prevent the flow of electronic charge, and that therefore the coatings needed to be thin enough to allow charges to pass via tunneling.  Specifically,TiO2 is a wide-band-gap (3.05 eV) semiconductor with a valence band positioned at a potential significantly (> 2 V) more positive than the valence bands of Si, GaAs, GaP, or most other semiconductors capable of absorbing significant amounts of sunlight.  For a photoanode, positive charges (holes) generated in the valence band of the light absorber need to pass into the electrolyte, and would not be able to pass through TiO2 on their way to the electrolyte due to the large (> 2V) potential barrier.

(left) Transmission electron micrograph (TEM) of silicon protected by a thick layer of amorphous titanium dioxide bearing a thin layer of nickel. (right) High-resolution TEM of the interface of silicon with the titanium dioxide coating.1

(left) Transmission electron micrograph (TEM) of silicon protected by a thick layer of amorphous titanium dioxide bearing a thin layer of nickel. (right) High-resolution TEM of the interface of silicon with the titanium dioxide coating.5

We used ALD to develop a TiO2 coating that, together with a Ni catalyst coating, protects semiconductors from corrosion while allowing conduction of holes.2  The TiO2 coating can be much thicker (up to 140 nm) than tunneling barriers (few nm), and thus can be prepared pinhole-free over large areas.  The TiO2 coating is transparent to visible light, and the Ni layer can be deposited as islands or thin films that allow transmission of light.  Without the Ni layer, the TiO2 coating does not conduct holes; thus the nickel layer is key to the function of the “leaky” ALD TiO2.  Si photoanodes protected by TiO2/Ni coatings and immersed in strongly alkaline water (1 M KOH) and under simulated solar illumination continuously oxidized water for over 100 hours, while protected GaAs and GaP photoanodes were stable under the same conditions for over 25 hours.  We also demonstrated that the TiO2/Ni coatings could protect CdTe photoanodes for over 100 hours of stable sunlight-driven water oxidation,3 and have shown that the coatings protect Si microwire photoanodes.4   Without the TiO2/Ni coating, all of these photoanodes (Si, GaAs, GaP, and CdTe) would have failed within a few seconds.

We have explored the mechanism of conduction – why are TiO2/Ni coatings electronically “leaky”? – using several experimental techniques.  We have shown using operando X-ray photoelectron spectroscopy to examine TiO2/Ni/electrolyte interfaces and found that the metallic Ni coating serves as an electrical contact to the electrolyte.6  We have also used solid-state measurements and X-ray photoelectron spectroscopy to  characterize the behavior and electronic structure of interfaces between Si and TiO2/Ni coatings.7

Kinetic Stabilization of III-V Photocathodes

Our work in kinetic stabilization of III-V photocathodes focuses on enhancing the long-term stability and efficiency of p-GaInP and p-InP photocathodes used in the solar-driven hydrogen-evolution reaction (HER) in both acidic and alkaline environments.8,9 We have observed that etched p-GaInP and p-InP photocathodes tend to corrode cathodically under illumination, leading to the formation of metallic In0 on the electrode surface, which negatively impacts their performance. However, the electrodeposition of Pt on these photocathodes has shown to significantly improve their stability and HER kinetics. Specifically, p-GaInP/Pt and p-InP/Pt electrodes demonstrate stable current densities under simulated 1 sun illumination for extended periods, with the stability being influenced by changes in surface chemistry and the dissolution processes of the photocathodes. In acidic conditions, p-InP/Pt electrodes exhibit nearly constant current density-potential (J–E) behavior over time, despite the slow leaching of In ions. In contrast, in alkaline conditions, the formation of an InOx layer on p-InP/Pt electrodes results in negligible dissolution but leads to degradation in J–E characteristics.

References

  1. Sun, K.; McDowell, M. T.; Nielander, A. C.; Hu, S.; Shaner, M. R.; Yang, F.; Brunschwig, B. S.; Lewis, N. S., Stable solar-driven water oxidation to O2(g) by Ni-oxide coated silicon photoanodes. J. Phys. Chem. Lett. 2015, 6, 592-598.
  2. Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S., Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science 2014, 344, 1005-1009.
  3. Lichterman, M. F.; Carim, A. I.; McDowell, M. T.; Hu, S.; Gray, H. B.; Brunschwig, B. S.; Lewis, N. S., Stabilization of n-cadmium telluride photoanodes for water oxidation to O2(g) in aqueous alkaline electrolytes using amorphous TiO2 films formed by atomic-layer deposition. Energy Environ. Sci. 2014, 7, 3334-3337.
  4. Shaner, M. R.; Hu, S.; Sun, K.; Lewis, N. S., Stabilization of Si microwire arrays for solar-driven H2O oxidation to O2(g) in 1.0 M KOH(aq) using conformal coatings of amorphous TiO2. Energy Environ. Sci. 2015, 8, 203-207.
  5. McDowell, M. T.; Lichterman, M.; Carim, A. I.; Liu, R.; Hu, S.; Brunschwig, B. S.; Lewis, N. S., The influence of structure and processing on the behavior of TiO2 protective layers for stabilization of n-Si/TiO2/Ni photoanodes for water oxidation. ACS Appl. Mater. Interfaces 2015, 7, 15189-15199.
  6. Lichterman, M. F.; Hu, S.; Richter, M. H.; Crumlin, E. J.; Axnanda, S.; Favaro, M.; Drisdell, W.; Hussain, Z.; Mayer, T.; Brunschwig, B. S.; Lewis, N. S.; Liu, Z.; Lewerenz, H.-J., Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy. Energy Environ. Sci. 2015, 8, 2409-2416.
  7. Hu, S.; Richter, M. H.; Lichterman, M. F.; Beardslee, J.; Mayer, T.; Brunschwig, B. S.; Lewis, N. S., Electrical, photoelectrochemical and photoelectron spectroscopic investigation of the interfacial transport and energetics of amorphous TiO2/Si heterojunctions. J. Phys. Chem. C 2016, 120, 3117-3129.
  8. Yu, W.; Richter, M. H.; Buabthong, P.; Moreno-Hernandez, I. A.; Read, C. G.; Simonoff, E.; Brunschwig, B. S.; Lewis, N. S. Investigations of the Stability of Etched or Platinized P-InP(100) Photocathodes for Solar-Driven Hydrogen Evolution in Acidic or Alkaline Aqueous Electrolytes. Energy Environ. Sci. 2021, 14, 6007–6020.
  9. Yu, W.; Young, J. L.; Deutsch, T. G.; Lewis, N. S. Understanding the Stability of Etched or Platinized P-GaInP Photocathodes for Solar-Driven H2 Evolution. ACS Appl. Mater. Interfaces 2021, 13, 57350–57361.