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
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
Carbon-neutral energy when & where it s needed Sunita Williams, NASA 3
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 ) 1.6 1.2 0.8 0.4 0.0 1.8eV 500 1000 1500 2000 2500 Wavelength (nm) 2H 2 O 2H 2 +O 2 4
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, 547 551 Morin, F. J. Phys. Rev. 1954, 93, 1195. 5
Start with an earth abundant material: Ti:Fe 2 O 3 J / ma cm -2 1.0 0.5 0.1M NaOH 72 o C 48 o C 25 o C 7 o C light dark current 0.2 0.1 J / ma cm -2 0.0 E redox =1.23 V dark 1.0 1.2 1.4 1.6 1.8 E / V vs RHE (Reversible Hydrogen Electrode) 1.19 V 1.24 V 1.2 1.4 1.6 1.8 E / V vs RHE 0.0 Ye, Melosh, Chueh et al. J. Mater. Chem. 3 (2015) 10801. 6
Slow kinetics mask thermal enhancement SO 3 2- J / ma cm -2 0.8 0.6 0.4 72 o C 48 o C 25 o C 7 o C Sulfite Oxidation light 0.2 0.0 0.8 0.9 1.0 1.1 1.2 E / V vs RHE dark 1.6 1.4 1 sun Water Oxidation Sulfite Oxidation J/J(25 o C) 1.2 1.0 1.0 % K -1 0.8 L p W 0 0 10 20 30 40 50 60 70 80 T / o C Ye, Melosh, Chueh et al. J. Mater. Chem. 3 (2015) 10801. 7
Another promising semiconductor: Mo:BiVO 4 Minority Carrier Diffusion Length (nm) Fe 2 O 3 2-4 BiVO 4 70-100 CoPi/Mo:BiVO 4 5 μm 200 nm 42 o C 4 J / ma cm -2 3 2 1 0 25 o C 10 o C 3.8 % K -1 2 mv K -1 dark current 0.4 0.6 0.8 1.0 1.2 1.4 E / V vs RHE J / ma cm -2 3 Mo:BiVO 4 2 1 Ti-Fe 2 O 3 0 0 10 20 30 40 50 60 70 80 T / o C Zhang, Ye, Melosh, Chueh et al. EES 9 (2016) 2044. 8
Thermally-activated monolithic BiVO 4 /SnO 2 /Si 5 (1) Mo:BiVO 4 SnO 2 n-si J / ma cm -2 4 3 2 Mo:BiVO 4 on Si at 55 o C (2) (3) (4) (5) 1 (6) 0 0.0 0.2 0.4 0.6 0.8 1.0 E / V vs RHE 0.5 M phosphate buffer Zhang, Ye, Melosh, Chueh et al. EES 9 (2016) 2044. 9
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
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
Unified view of temperature enhancement Space Charge Layer Length (nm) dj/dt TiO2 Fe 2 O 3 BiVO 4 10-2 μa K -1 Minority Carrier Diffusion Length (nm) Zhang, Sun, Melosh, Chueh et al. In preparation. 12
Going > 100 C: an all-oxide approach Air Gas Bubbles Light Absorber Liquid Electrolyte Light Absorber Proton-conducting Oxide < 100 C 300-700 C Ye, Melosh, Chueh et al. PCCP 15 (2013) 15459. 13
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 2 250 nm SEM Bi Cu 1 μm Boloor, Ye, Zhang, Melosh, Chueh. In preparation. 14
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
Thermally-enhanced generation of solar fuels PEC / Solar cells cooling 16
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BiVO 4 / BiCuVO x Photovoltage dependence on heterojunction uv-diode YSZ sub Porous Pt Photovoltages indicate that the heterojunction interface promotes charge separation
Combining heat & light: what s possible? Solar-to-Fuel Efficiency Unreachable Temp. 10% Ye, Melosh, Chueh et al. PCCP 15 (2013) 15459. 19
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) 10801. 20
Thermally-activated monolithic BiVO 4 /SnO 2 /Si 5 0.5 M phosphate buffer (1) Mo:BiVO 4 SnO 2 n-si J / ma cm -2 4 3 2 1 Mo:BiVO 4 on Si at 55 o C 0 0.0 0.2 0.4 0.6 0.8 1.0 E / V vs RHE (2) (3) (4) (5) (6) Zhang, Ye, Melosh, Chueh et al. EES 9 (2016) 2044. 21
Semiconductor/Mixed Conductor Heterojunction A new class of solid state PEC for concentrated sunlight Compatible with elevated temperature Single device, isothermal 24
Semiconductor/Mixed Conductor Heterojunction Photon absorption Electron/hole pairs excitation Carrier diffusion Paper submitted 25
Semiconductor/Mixed Conductor Heterojunction Light absorber/miec interface: Electrons: thermionic emission Holes: mostly reflected Paper submitted 26
Semiconductor/Mixed Conductor Heterojunction MIEC/gas interface Electron transfer, HER Paper submitted 27
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
Semiconductor/Mixed Conductor Heterojunction Oxygen ions transport to the air side and react with holes Paper submitted 29
Efficiency Simulation Efficiency 0.20 0.15 0.10 0.05 200 300 400 500 600 0.00 400 500 600 700 800 900 T (K) o C Potential (V) 1.6 1.4 1.2 1.0 200 300 400 500 600 o C µ abs abs /q µ MIEC MIEC /q 0.8 400 500 600 700 800 900 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
Figure 1 b Intensity (a.u.) (101) * (011) (-121) * * (004) (200) (002) * (211) (015) (240) (042) * * * * * ** ** ** * (161) (321) (123) * c 20 30 40 50 60 2 theta (degree) 5 µm 200 nm
Figure 2 Raman intensity (a.u.) pure BiVO 4 0.3% Mo doped BiVO 4 1% Mo doped BiVO 4 3% Mo doped BiVO 4 780 800 820 840 860 Raman shift (cm -1 ) Current density (ma/cm 2 ) 1.2 0.8 0.4 0.0 dark current pure BiVO 4 0.3% Mo doped BiVO 4 1% Mo doped BiVO 4 3% Mo doped BiVO 4 0.4 0.6 0.8 1.0 1.2 1.4 E (V) vs. RHE Current density (ma/cm 2 ) 1.2 0.9 0.6 0.3 front illumination back illumination 0.0 0 1 2 3 4 Deposition time (min) c
Figure 5 a b 2 µm 200 nm c Current density (ma/cm 2 ) 3 2 1 0 dark current macroporous BiVO 4 nanoporous BiVO 4 0.2 0.4 0.6 0.8 1.0 E (V) vs. RHE
Figure 6 a Current density (ma/cm 2 ) b 4 3 2 10 25 1 35 45 55 0 0.2 0.4 0.6 0.8 1.0 E (V) vs. RHE c Current density (ma/cm 2 ) Current density (ma/cm 2 ) at 0.80 V vs. RHE 2 1 0 4 3 2 1 10 25 35 45 55 0.2 0.4 0.6 0.8 1.0 E (V) vs. RHE Small BiVO 4 NPs Large BiVO 4 NPs 10 20 30 40 50 60
Figure 8 Current density (ma/cm 2 ) 30 20 10 0 9 25 42 61 80 1.4 1.6 1.8 E (V) vs. RHE log ( j (ma/cm 2 ) ) 1.5 1.0 0.5 0.0 80 61 42 25 9 1.80 1.84 1.88 E (V) vs. RHE