Valeria Mozzetti Page 1/15 11.11.2013 UFSP Solar Light to Chemical Energy Conversion www.lightchec.uzh.ch LightChEC Résumé Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry B. A. Pinaud et. al., Energy Environ. Sci., 2013, 6, 1983 Main findings This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on the photoelectrochemical water splitting technology: 1) single bed particle suspension system 2) dual bed particle suspension system 3) fixed panel array 3) tracking concentrator array A key finding is that the production costs are consistent with the Department of Energy’s targeted threshold cost of $2.00–$4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met. --------------------------------------------------------------------------------------------------------------------------- →Overall particles beds reactors cheaper than panel based but uncertainty in their technical feasibility →Overall costs of panel based system can be reduced by increasing efficiency and by solar concentration. --------------------------------------------------------------------------------------------------------------------------- lower costs but technical feasibility and safety concerns? higher costs but technique more mature, costs can be reduced by increasing efficiency and solar concentration
Transcript
Valeria Mozzetti Page 1/15
11.11.2013
UFSP Solar Light to Chemical Energy Conversion www.lightchec.uzh.ch
LightChEC
Résumé Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry
B. A. Pinaud et. al., Energy Environ. Sci., 2013, 6, 1983
Main findings This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on the photoelectrochemical water splitting technology:
1) single bed particle suspension system
2) dual bed particle suspension system
3) fixed panel array
3) tracking concentrator array
A key finding is that the production costs are consistent with the Department of Energy’s targeted threshold cost of $2.00–$4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met.
Introduction Actual production of H2 is 50 million tonnes / year worldwide (wikipedia 57 million in 2004 with 10% yearly increase):
• for petroleum refining • for ammonia synthesis for fertilizers (50 % wikipedia) 95% from fossil fuels via steam methane reforming. 5%1
High temperature electrolysis of alkalyne solution commercially exploited by Sable Chemicals (Zimbawe, however it seems from a short search on the internet that they need a lot of electricity that they buy under a subsidized form from the government causing shortage in the country, at least seems that the electricity comes from hydropower).
from (partial oxidation, plasma reforming ( Kvaerner-process), from coal (products: syngas (hydrogen and Carbon monoxide) + methane). from water splitting (until 2007 not in use).
Renewable H2 production: a) Thermal processes: reforming bio-derived fuels b) Electrolytic processes: renewable electricity source + electrolyzer c) Photolytic processes: molecular chromophores or semiconductor absorber
Photoelectrochemical (PEC) water splitting 1972 Fujishima and Honda PEC water splitting on TiO2 photoelectrode. PEC cells with iii-iv group semiconductor STH2 12-18%3
PEC cells with multi junction silicon STH 4.7-7.8%
4
Challenges for H2 PEC production:
Durability Efficiency Costs (material and manufacturing)
1 from wikipedia (http://en.wikipedia.org/wiki/Hydrogen_production)
2 STH Solar to Hydrogen Efficiency
3 O. Khaselev and J. A. Turner, Science, 1998, 280, 425–427. and S. Licht, B. Wang, S. Mukerji, T. Soga, M. Umeno and H. Tributsch, J. Phys. Chem. B, 2000, 104, 8920–8924.
4 S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers and D. G. Nocera, Science, 2011, 334, 645–648. and R. E. Rocheleau, E. L. Miller and A. Misra, Energy Fuels, 1998, 12, 3–10.
Basic principles for PEC water splitting Photon Excited electron-hole pair
Band bending occurs:
• at the semiconductor/ electrolyte interface • at p-n junction
excited holes surface or electrode oxygen evolution reaction (OER)
electrons surface or electrode hydrogen evolution reaction (HER)
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PEC hydrogen production via: • single photoanode • single photocatode • multiple absorber (Tandem) • suspensions of photocatalyst particles
Figure 1 Schematic band diagram showing the phenomena of photon absorption, band bending, charge separation, as well as hydrogen and oxygen evolution on semiconductor photoanode and photocatode surfaces. The external circuit could also be replace by a redox mediator to shuttle charges between the two photoelectrodes.
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Key requirements for semiconducting materials • Suitable band gap for light absorption • Proper band gap edge alignment (for redox reactions) • Long term stability in aqueous environment (in light and dark) • Cost • Material availability
Band gap • At least 1.23 eV • Large enough to split water (more than 1.23 eV calculating losses) • Small as possible to absorb the greatest portion of solar spectrum (high band gap = UV
not useful in terrestrial applications) • Possible solution: small band gap materials in parallel
Energy levels of electrons and holes • Higher electrochemical redox potential for HER and OER • Conduction band < 0.0 eV vs. RHE • Valence band > 1.23 eV vs. RHE
Catalysis • To reduce the overpotential and so the total voltage required
Charge transport • In absorber material • At electrode/electrolyte interface
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PEC system efficiency's calculation SOLAR TO HYDROGEN EFFICIENCY: STH EFFICIENCY*
Chemical Energy (H2)
Solar Energy Input
* OER must occur (solar energy and water must be the only inputs, no sacrifical reagents, no half reactions)
STH Efficiency depends on: • Total solar irradiance • Entropic losses due to blackbody radiation and recombination • Kinetic overpotential for the two half reactions
• Non-ideal band edge alignment • Series resistances from the solution or wiring
Thermodynamic limits = upper bound for STH Efficiency • Single PEC 29-31%5
• Tandem PEC 40-41%
6
• With multiple excition generation • With solar concentration
5 R. T. Ross and T. L. Hsiao, J. Appl. Phys., 1977, 48, 4783–4785.
6 M. C. Hanna and A. J. Nozik, J. Appl. Phys., 2006, 100, 074510-1–074510-8.
→ current-voltage relationship for each band gap → STH efficiency*
*7
Maximum photocurrents calculated by summing the absorbable photons over the solar spectrum for materials of varying band gaps assuming all photons with energy greater than the band gap are absorbed.
Open circuit voltages were used to estimate the usable photovoltage
were calculated for each band gap taking into account entropic losses modeled after single crystal silicon according to Ross8
7 Eqn (1) is one defnition for STH efficiency, using the product of voltage and short-circuit current to calculate the chemical power output of the PEC water splitting cell under standard-state conditions relative to the power input to the cell by 1 sun AM 1.5 G illumination, assuming 100% Faradaic conversion of water to H2 and O2.
8 R. T. Ross, J. Chem. Phys., 1967, 46, 4590–4593.
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Kinetic overpotentials • for all possible currents • calculated assuming Butler-Volmer kinetics • for H2 and O2 evolution on platinum and ruthenium oxide, respectively.
Shunt losses • neglected (device dependant)
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STH calculation's results for different PEC configurations Configuaration Info Maximum
STH* Band Gap
for Max STH
Single photoabsorber
- 11.2% 2.26 eV
Dual stacked photoabsorber
The top photoabsorber is assumed to be placed above the bottom photoabsorber, thus only photons with energy less than the band gap of the former are transmitted to the latter.
22.8% 1.23 eV and 1.84 eV
Dual side by side photoabsorber
The two electrodes are assumed to be placed next to each other and can each access the full solar spectrum.
15.5% 1.59
Table 1 STH Efficiency for different PEC configurations
* large losses due to reaction overpotential → need for better catalysts
Maximum theoretical STH graphs by different PEC configurations
Single Dual stacked Dual side by side
Figure 2 Maximum theoretical solar-to-hydrogen efficiency
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Demonstrated STH
Panel based PEC configurations • PEC system alone
o Single absorber: disadvantage need wide band gap (ultraviolet > 2.1) for STH> 5% (Figure 2), additionally often materials with band edge potentials for both half reactions have a valence band edge significantly more positive than the water oxidation potential (SrTiO3 and KTaO4 oxides)9
o Dual PEC (2 coupled low band gap)
10
• PEC junction + external phtovoltaic (PV) device : STH > 8% but stability is an issue
o until 12.4% STH but stability is an issue11
• PV + electrolyzer
→Overall 15% STH efficiency is achievable but availability and costs remain an issue
Particle based PEC configurations →These configurations have much lower efficiencies and STH were not calculated an alternative measure of efficiency is used (EQY*) which does not take into consideration the voltage of a particular reaction or product: Overall energy efficiency is not accounted for.
• Single particle / single photon water splitting o highest EQY with wider band gap systems (but these are UV) o EQY of 2.5% at 420 nm with Rh2-yCryO3-loaded(Ga1-xZnx)(N1-xOx)12
• 2 particles / 2 photons water splitting (Z-scheme): need twice as many photons but higher voltage and more material choiches (
in paper many examples p. 6) • half reaction water splitting with sacrifical reagent/non water photocatalyst (may be
useful for photocatalytic decomposition of organic pollutants).
9 J. G. Mavroides, J. A. Kafalas and D. F. Kolesar, Appl. Phys. Lett., 1976, 28, 241–243. and A. B. Ellis, S. W. Kaiser and M. S. Wrighton, J. Phys. Chem., 1976, 80, 1325–1328.
10 R. C. Kainthla, B. Zelenay and J. O. Bockris, J. Electrochem.Soc., 1987, 134, 841–845.
11 O. Khaselev and J. A. Turner, Science, 1998, 280, 425–427.
12 K. Maeda, K. Teramura, L. Daling, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295.
where n is the number of electrons transferred per product
molecule.
Cost calculation for 4 hypothetical reactor types Cost was calculated for a working plant with a net 1 Tonne per day hydrogen output including all costs (material and operating).
Aqueous reactor beds with suspended particles
1 Single bed particle suspension 1.60$
2 Dual bed particle suspension 3.20$
Multilayer absorber planar arrays in aqueous electrolyte
3 Fixed panel array 10.40$
4 Tracking concentrator array 4.00$
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Figure 3 Four reactor types
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Sensitivity analysis results for the different hypothetical reactors
Figure 4 Effect of efficiency, particle or panel cost, and component lifetime on the cost of hydrogen from each reactor design. Each calculation represents the variation of a single parameter from the base case to a higher and lower value as indicated on the left axis.
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Aqueous reactor beds with suspended particles Type 1 and 2
• Cheaper but uncertain technological feasibility • Efficiency has strong impact on costs • Technical feasibility?
o particles with photovoltage for H2O splitting & use of a wide portion of the solar spectrum needed
o compression of H2 and O2 gases needed
Type 1
• Separation of H2 and O2 gases needed most probably technically feasible but nevertheless a huge safety concern
Type 2
• Technical feasibility ? of: o membrane bridges o circulation system o effective use of redox mediator o development of separate particles for O2 and H2 evolution
Type 3 and 4
• All parameters have an influence on costs: o efficiency o cell cost o durability
Type 4
• More cost effectiveness from solar concentration (1:10) • Important to research into the effects of increased light intensity on photoabsorber
materials • Efficiency is a key parameter:
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o with a theoretical efficiency of 25% the costs will drop to 2.90$ (a 22% efficiency is achievable in a tandem structure (Table 1)
