Protective Metal Oxides that Electronically Couple Catalysts to Efficient Light Absorbers

Transcription

1 Protective Metal Oxides that Electronically Couple Catalysts to Efficient Light Absorbers Co-PI: Christopher Chidsey Personnel: Andrew Scheuermann, Olivia Hendricks, and Kyle Kemp Support: GCEP Leverage: Stanford Graduate Fellowship, NSF Graduate Fellowship, NSF CBET (US-Ireland Program)

2 How can technology address the greenhouse gas problem?

3 How can technology address the greenhouse gas problem? I. Stop putting GHGs in the environment II. Remove GHGs before they re emitted Renewable sources Efficiency, Demand Reduction Capture Store and Renewable Fuels and Chemicals III. Artificial Photosynthesis Recycle GHGs

4 Hydrogen Synthesis: Water Splitting Photoelectrochemical Cell Schematic of solar hydrogen synthesis by photolysis of water using a semiconducting photo-anode with E g = 3.0 ev (such as TiO 2,* SrTiO 3 ). Basic Research Needs for Solar Energy Utilization, US DOE (2005) * A. Fujishima and K. Honda, Nature 238, (1972). 4

5 Photoanode Selection Bandgap energy (ev) Irradiance (Wm -2 nm -1 ) TiO 2 0 Si Fe 2 O 3 WO Wavelength (nm) Fraction of irradiance (%) 5

6 ALD protection of high quality semiconductor absorbers 6

7 Atomic-Layer-Deposition (ALD) Protection of Silicon Electrode Combine chemical stability of TiO 2 with efficient photoabsorption by Si substrate TDMAT (g) Saturated adsorption Coat thin TiO 2 by ALD as corrosion resistant tunnel oxide TiO 2(s) HNMe 2 (g) H 2 O (g) Deposit thin surface layer of a known water oxidation catalyst (e.g. Ir) Y.W. Chen et al., Nature Mater. 10, 539 (2011). 7

8 Water Oxidation in Simulated Solar Light Electrode: Ir/TiO 2 /n-si Use lightly-doped n-si as substrate Holes must be photo-generated for efficient oxidation Without illumination No observable peaks With AM 1.5 illumination Overpotentials at 1 ma/cm 2 1M NaOH: -171 mv ph 7: -219 mv 1M H 2 SO 4 : -200 mv Large water oxidation current density below equilibrium (dark) potential Inferred photovoltage 550 mv Y.W. Chen et al., Nature Mater. 10, 539 (2011). 8

9 OER Stability: 2nm Ir/x nmtio 2 /SiO 2 /n-si Longer-term solar water splitting (AM 1.5 illumination) No clear relationship between stability and ALD-TiO 2 thickness Stability > 72 hours demonstrated for 2-12 nm thick TiO 2 9

10 Electrical resistivity of the protection layer 10

11 TiO 2 Thickness Effect: Overpotential Ir/TiO 2 /p + Si Overpotential (mv) Base ph7 Acid FFC TiO 2 thickness (nm) Increasing TiO 2 thickness requires increasing overpotential for the same water oxidation rate; ~ 20 mv/nm TiO 2 At very small thicknesses, the overpotential for water splitting is approximately constant A.G. Scheuermann et al., Energy Environ. Sci. 6, (2013). 11

12 Methods to Thin the SiO 2 Interlayer 1: HF etch ~1.3 nm SiO 2 Uncontrolled Regrown SiO 2 Si Si ALD-TiO 2 HF-etch H H H H H Si Islanded TiO 2 Poor control Incomplete protection at lower thicknesses 2: HF etch, ALD-SiO 2 growth H H H H H Si Ultrathin, controlled SiO 2 x nm TiO 2 Si ALD-SiO 2 with TDMS Uniform TiO 2 Si ALD-TiO 2 with TDMAT 3: O-scavenging postgrowth 20 nm W 50 nm Ti x nm TiO 2 ~1.3 nm SiO 2 Si 300⁰ C FGA Etch, deposit contacts Catalyst x nm TiO 2 ~0.6 nm SiO 2 Si Back contact P.F. Satterthwaite, A.G. Scheuermann et al, Manuscript in preparation (2015). 12

13 Maximizing photovoltage of Si water splitting anodes 13

14 p + nsi Anodes Remove Photovoltage Loss Water Oxidation Overpotential (mv) p+si ALD-TiO 2 Thickness (nm) acid p+si ph7 p+si base p+si acid nsi ph7 nsi base nsi acid p+nsi ph7 p+nsi base p+nsi A.G. Scheuermann et al., Nature Materials. (2015). 14

15 Water Oxidation Overpotential (mv) mv nsi p + nsi Anodes Remove p+si nsi Photovoltage Loss negative nsi ALD-TiO 2 Thickness (nm) acid p+si ph7 p+si base p+si acid nsi ph7 nsi base nsi acid p+nsi ph7 p+nsi base p+nsi A.G. Scheuermann et al., Nature Materials. (2015). 15

16 Water Oxidation Overpotential (mv) mv nsi p + nsi Anodes Remove p+si nsi p+nsi Photovoltage Loss negative nsi p+nsi 600+ mv constant ALD-TiO 2 Thickness (nm) Observed with various insulator stacks: 1) SiO 2 2) TiO 2 / SiO 2 3) TiO 2 /SiO 2 4) Al 2 O 3 /SiO 2 acid p+si ph7 p+si base p+si acid nsi ph7 nsi base nsi A.G. Scheuermann et al., Nature Materials. (2015). acid p+nsi ph7 p+nsi base p+nsi Record photovoltage for a Si photoelectrode: 630 mv 16

17 Oxygen evolution reaction (OER) catalyst engineering 17

18 Elemental Abundance in the Earth s Crust Unfortunately, Ir is the rarest metal on Earth. Ru is about 3 orders of magnitude more abundant (similar to Pt, a widely used industrial catalyst) but still rare. 18

19 OER Catalysts in TiO 2 -Protected MIS A.G. Scheuermann et al., Energy Environ. Sci. 6, (2013). 19

20 Ruthenium ALD for OER Catalyst Layers 1 nm vs 2 nm ALD-Ru comparison (on 2 nm TiO 2 /SiO 2 /p+si) 4 Current Density (ma/cm2) OH c Ru OH c Ru FGA OH c Ru Potential (mv vs Ag AgCl) Ferri/ferrocyanide redox FGA = 400 C, 30 min, 5% H 2 /95% N 2 anneal Smaller E p1/2 and lower OER overpotential for 100 cycle compared to 50 cycle Ru samples E pa (mv) E pc (mv) E p (mv) E p1/2 Overpotential Pre FGA Post FGA

21 Solar-to-hydrogen (STH) efficiency 21

22 Matching PV with an Electrochemical Load DOE target 15% 22

23 Tandem Water Splitting Cell Schematic Choose OER and HER catalysts layers and protection layers that permit facile hole transport to the photoanode surface and electron transport to the photocathode surface. 23

24 Amorphous Si Photoelectrodes TiO2 protection layers do not inhibit charge transport and PV performance AM1.5 IV 10 mm FFC solution 24

25 Conclusions Hydrogen generated from sunlight can help accommodate intermittency of solar power and is a key component of renewable chemical synthesis. Water is the most convenient source of electrons for solar fuel synthesis at large scale, but water oxidation is a kinetically difficult reaction that requires oxidation-stable and reactive surfaces. Atomic layer deposited metal oxides can protect Si and, potentially, other high-quality semiconductor absorbers so they can be used in efficient solar-driven water splitting. Atomic layer deposition of ultrathin TiO 2 /SiO 2 produces a Schottky tunnel junction coupling a high quality semiconductor to an OER catalyst layer. Thicker oxide protection layers achieve record photovoltages with p+nsi buried junction. ALD is a promising approach for OER catalyst deposition. ALD protection approach may enable stable tandem devices. 25

26 Thank You! Y.W. Chen, J. Prange, S. Duehnen, Y. Park, M. Gunji, C.E.D. Chidsey, P. C. McIntyre