Solar Fuels From Light & Heat Xiaofei Ye, Liming Zhang, Madhur Boloor, Nick Melosh Will Chueh Materials Science & Engineering, Precourt Institute for Energy Stanford University
Sunita Williams, NASA 2
D. Diliff London R. Laddish Tokyo C.-Y. Yu Chicago 3
Enhance solar utilization 5 % Ultraviolet 43 % Visible 52 % Infrared Dionne Photo-electrochemical cell 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) 4
Combining heat & light: what s possible? Solar-to-Fuel Efficiency Unreachable Temp. 10% X. Ye, J. Melas-Kyriazi, A. Feng, N. A. Melosh, W. C. Chueh. PCCP 15 (2013) 15459. 5
Can thermal energy make existing materials better? Low mobility Morin, F. J. Phys. Rev. 1954, 93, 1195. 6
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 7
Enhancement with temperature & light intensity 5%Ti-Fe 2 O 3 TOP VIEW RHE Thermocouple CE W E Solar simulator Nitrogen Water bath J [ma cm -2 ] J [ma cm -2 ] 4 2 0 0.2 0.1 72 o C 48 o C 25 o C 7 o C T I Δη~ 70 mv 1.19 V 1.24 V 9 suns T 1 sun 0.1 M NaOH ph = 13 Stirrer 0.0 0.8 1.0 1.2 1.4 1.6 1.8 E vs RHE [V] 8
Enhancement with temperature & light intensity 9
Thermally-enhanced fill factor 0.4 0.45 --> 4.5 ma cm -2 0.1% Ti-doped Fe 2 O 3 E [V] 0.3 0.2 160 mv 0.1 10
Another low-mobility semiconductor: BiVO 4 5 μm 200 nm Current density (ma/cm 2 ) 1.2 0.8 0.4 Effect of doping dark current pure BiVO 4 0.3% Mo doped BiVO 4 1% Mo doped BiVO 4 3% Mo doped BiVO 4 0.0 0 0.4 0.6 0.8 1.0 1.2 1.4 0.4 0.6 0.8 1.0 1.2 1.4 E (V) vs. RHE E (V) vs. RHE 0.5 M K 3 PO 4 buffered ph = 7 Electrolyte 11 Current density (ma/cm 2 ) 4 3 2 1 Effect of catalysts dark current dark current with SO 3 2- without CoPi with CoPi without CoPi with SO 3 2-
Thermally-enhanced saturation current Current density (ma/cm 2 ) 3 2 1 0 1 sun dark 9 C 25 C 42 C T T 0.4 0.6 0.8 1.0 1.2 1.4 E (V) vs. RHE Significant enhancement in photocurrent without significant decrease with photovoltage E (V) vs. RHE Open Circuit Voltage (mv) 1.2 1.0 0.8 0.6 0.4 800 750 700 650 light on 1 sun light off light on 3 suns BiVO 4 Pt 0 500 1000 1500 2000 2500 3000 Time (s) 1mM [IrCl 6 ] 4- /0.1 mm [IrCl 6 ] 3-600 10 20 30 40 50 Temperature ( C) ( ) 12
Stability E (V vs. RHE) E [V vs RHE] 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.1% Ti-doped 9 suns, 70 o C 2 ma cm -2 Fe 2 O 3 9 suns, I = 2 ma cm -2 70 C 0 1 2 3 4 Time [hour] Time (h) Current density (ma/cm 2 ) 0.8 0.6 0.4 0.2 42 C 25 C 9 C BiVO 4 1 sun E = 0.6V vs. RHE 0 1 2 3 4 5 Time (h) 13
Thermally-enhanced PEC PEC / Solar cells cooling 14
Going > 100 C: an all-oxide approach Air Gas Bubbles Light Absorber Liquid Electrolyte Light Absorber Proton-conducting Oxide < 100 C 300-700 C esolar 15
16
Semiconductor/Mixed Conductor Heterojunction A new class of solid state PEC for concentrated sunlight Compatible with elevated temperature Single device, isothermal 19
Semiconductor/Mixed Conductor Heterojunction Photon absorption Electron/hole pairs excitation Carrier diffusion Paper submitted 20
Semiconductor/Mixed Conductor Heterojunction Light absorber/miec interface: Electrons: thermionic emission Holes: mostly reflected Paper submitted 21
Semiconductor/Mixed Conductor Heterojunction MIEC/gas interface Electron transfer, HER Paper submitted 22
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 23
Semiconductor/Mixed Conductor Heterojunction Oxygen ions transport to the air side and react with holes Paper submitted 24
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 25
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