Thermally-Enhanced Generation of Solar Fuels

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1 Thermally-Enhanced Generation of Solar Fuels Xiaofei Ye, Liming Zhang, Madhur Boloor, Nick Melosh, William Chueh Materials Science & Engineering, Precourt Institute for Energy Stanford University

2 Fundamentals Bridging Technology hν e - Molecular pathways Interfacial structures H +, O 2-, Li + In-situ characterization Establish design rules Rational engineering materials & devices Fuel cells Solar fuels Batteries 2

3 Carbon-neutral energy when & where it s needed Sunita Williams, NASA 3

4 Enhance solar utilization via PECs 5 % Ultraviolet 43 % Visible 52 % Infrared Dionne Photo-electrochemical cell - + Anode bias O 2 H 2 Aqueous electrolyte Hydrogen Oxygen Cathode Power (W m -2 nm -1 ) eV Wavelength (nm) 2H 2 O 2H 2 +O 2 4

5 Can thermal energy make existing materials better? CB s/p s/p d localized states s/p VB s/p Low mobility Small polarons or Carrier trapping A. Walsh, et al, Chem. Mater. 2009, 21, Morin, F. J. Phys. Rev. 1954, 93,

6 Start with an earth abundant material: Ti:Fe 2 O 3 J / ma cm M NaOH 72 o C 48 o C 25 o C 7 o C light dark current J / ma cm E redox =1.23 V dark E / V vs RHE (Reversible Hydrogen Electrode) 1.19 V 1.24 V E / V vs RHE 0.0 Ye, Melosh, Chueh et al. J. Mater. Chem. 3 (2015)

7 Slow kinetics mask thermal enhancement SO 3 2- J / ma cm o C 48 o C 25 o C 7 o C Sulfite Oxidation light E / V vs RHE dark sun Water Oxidation Sulfite Oxidation J/J(25 o C) % K L p W T / o C Ye, Melosh, Chueh et al. J. Mater. Chem. 3 (2015)

8 Another promising semiconductor: Mo:BiVO 4 Minority Carrier Diffusion Length (nm) Fe 2 O BiVO CoPi/Mo:BiVO 4 5 μm 200 nm 42 o C 4 J / ma cm o C 10 o C 3.8 % K -1 2 mv K -1 dark current E / V vs RHE J / ma cm -2 3 Mo:BiVO Ti-Fe 2 O T / o C Zhang, Ye, Melosh, Chueh et al. EES 9 (2016)

9 Thermally-activated monolithic BiVO 4 /SnO 2 /Si 5 (1) Mo:BiVO 4 SnO 2 n-si J / ma cm Mo:BiVO 4 on Si at 55 o C (2) (3) (4) (5) 1 (6) E / V vs RHE 0.5 M phosphate buffer Zhang, Ye, Melosh, Chueh et al. EES 9 (2016)

10 Validating model using rutile TiO 2 nanowires 0. 7 T L D Active Inactive % K - 1 Wire Diameter Average = 38 nm 53 nm 151 nm Zhang, Sun, Melosh, Chueh et al. In preparation. 10

11 Validating model using rutile TiO 2 NWs ev W = 8.7 ± 1.2 nm Fraction of photons collected in the minority carrier diffusion region determines thermal enhancement Zhang, Sun, Melosh, Chueh et al. In preparation. 11

12 Unified view of temperature enhancement Space Charge Layer Length (nm) dj/dt TiO2 Fe 2 O 3 BiVO μa K -1 Minority Carrier Diffusion Length (nm) Zhang, Sun, Melosh, Chueh et al. In preparation. 12

13 Going > 100 C: an all-oxide approach Air Gas Bubbles Light Absorber Liquid Electrolyte Light Absorber Proton-conducting Oxide < 100 C C Ye, Melosh, Chueh et al. PCCP 15 (2013)

14 Going > 100 C: an all-oxide approach (Y,Zr)O 2 Si 3 N 4 Si O 2 SEM BiVO 4 e - h + BiCuVOx O 2- Pt (Y,Zr)O nm SEM Bi Cu 1 μm Boloor, Ye, Zhang, Melosh, Chueh. In preparation. 14

15 Going > 100 C: an all-oxide approach O 2 BiVO 4 BiCuVOx e - h + O 2- Si 3 N 4 (Y,Zr)O 2 Pt Si (Y,Zr)O 2 Photovoltage (mv) 1 sun 1 sun Current Density (ma cm -2 ) Light Dark Temperature ( C) Voltage (V) 15

16 Thermally-enhanced generation of solar fuels PEC / Solar cells cooling 16

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18 BiVO 4 / BiCuVO x Photovoltage dependence on heterojunction uv-diode YSZ sub Porous Pt Photovoltages indicate that the heterojunction interface promotes charge separation

19 Combining heat & light: what s possible? Solar-to-Fuel Efficiency Unreachable Temp. 10% Ye, Melosh, Chueh et al. PCCP 15 (2013)

20 Low mobility, high stability semiconductor: Fe 2 O 3 Ti doped α-fe 2 O 3 Pt Al 2 O 3 (0001) 30 nm 200 nm TEM SEM AFM Pulsed-Laser Deposition Ye, Melosh, Chueh et al. J. Mater. Chem. 3 (2015)

21 Thermally-activated monolithic BiVO 4 /SnO 2 /Si M phosphate buffer (1) Mo:BiVO 4 SnO 2 n-si J / ma cm Mo:BiVO 4 on Si at 55 o C E / V vs RHE (2) (3) (4) (5) (6) Zhang, Ye, Melosh, Chueh et al. EES 9 (2016)

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24 Semiconductor/Mixed Conductor Heterojunction A new class of solid state PEC for concentrated sunlight Compatible with elevated temperature Single device, isothermal 24

25 Semiconductor/Mixed Conductor Heterojunction Photon absorption Electron/hole pairs excitation Carrier diffusion Paper submitted 25

26 Semiconductor/Mixed Conductor Heterojunction Light absorber/miec interface: Electrons: thermionic emission Holes: mostly reflected Paper submitted 26

27 Semiconductor/Mixed Conductor Heterojunction MIEC/gas interface Electron transfer, HER Paper submitted 27

28 Semiconductor/Mixed Conductor Heterojunction Gas diffusion (stagnation layer) H 2 O: continuously supplied, diffuse to the surface H 2 : diffuse away from surface, then removed Paper submitted 28

29 Semiconductor/Mixed Conductor Heterojunction Oxygen ions transport to the air side and react with holes Paper submitted 29

30 Efficiency Simulation Efficiency T (K) o C Potential (V) o C µ abs abs /q µ MIEC MIEC /q T (K) E rxn E 0 rxn Broad maximum at ~750 K, 17 % Below 700 K: slow thermionic emission Above 700 K: insufficient photovoltage Paper submitted 30

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32 Figure 1 b Intensity (a.u.) (101) * (011) (-121) * * (004) (200) (002) * (211) (015) (240) (042) * * * * * ** ** ** * (161) (321) (123) * c theta (degree) 5 µm 200 nm

33 Figure 2 Raman intensity (a.u.) pure BiVO 4 0.3% Mo doped BiVO 4 1% Mo doped BiVO 4 3% Mo doped BiVO Raman shift (cm -1 ) Current density (ma/cm 2 ) dark current pure BiVO 4 0.3% Mo doped BiVO 4 1% Mo doped BiVO 4 3% Mo doped BiVO E (V) vs. RHE Current density (ma/cm 2 ) front illumination back illumination Deposition time (min) c

34 Figure 5 a b 2 µm 200 nm c Current density (ma/cm 2 ) dark current macroporous BiVO 4 nanoporous BiVO E (V) vs. RHE

35 Figure 6 a Current density (ma/cm 2 ) b E (V) vs. RHE c Current density (ma/cm 2 ) Current density (ma/cm 2 ) at 0.80 V vs. RHE E (V) vs. RHE Small BiVO 4 NPs Large BiVO 4 NPs

36 Figure 8 Current density (ma/cm 2 ) E (V) vs. RHE log ( j (ma/cm 2 ) ) E (V) vs. RHE