Multiple band gap devices for solar water splitting

Tfy-56.5141 Special Course in Advanced Energy Technologies

Priit Jaanson

Contents

• Direct PV (photovoltaic) electrolysis vs PEC (photo electrochemical cell) electrolysis

• PEC challenges• Biasing the PEC• Examples

Direct PV electrolysis vs PEC electrolysis

• Direct PV electrolysis– Expensive?– High current densities ->

overpotential -> lower efficiency

– Not compact?

• PEC electrolysis– Lower current densities

-> higher electrolysis efficency

– All in one package: cheap, compact.

L. Minggu et al. An overview of photocells and photoreactors for photoelectrochemical water splitting, International Journal of Hydrogen Energy, Vol. 35, 11, 2010 5233-5244.

PEC challenges

• Visible light absorption• Stability against photocorrosion– Metal oxides: Charge transfer kinetics > anodic

decomposition rate– Non-oxides: thin oxide layer formation, dissolving.

• Suitable band gap and band edge energies.

Bandgap matching

• Oxidation covered

• Reduction challenging

• Need for external bias

Solar hydrogen production with nanostructured metal oxides, Roel van de Krol , Yongqi Liang and Joop Schoonman, J. Mater. Chem., 2008,18, 2311-2320

Biasing

• Grid biased– Fossil fuels

• pH biased– More

consumables• PV or DSSC

biased• Internal biased

L. Minggu et al. An overview of photocells and photoreactors for photoelectrochemical water splitting, International Journal of Hydrogen Energy, Vol. 35, 11, 2010 5233-5244.

Internal-biased systems - PV/PEC

Eric L. Miller, Daniela Paluselli, Bjorn Marsen, Richard E. Rocheleau, Development of reactively sputtered metal oxide films for hydrogen-producing hybrid multijunction photoelectrodes, Solar Energy Materials and Solar Cells, Volume 88, Issue 2, 15 July 2005, Pages 131-144.

• Solar to hydrogen efficiency 0.7 %

• Estimated to be improved to ~10 % with thicker oxide layer

Internal-biased systems - PV/PV

• STH efficiency 16.5%

• STE efficiency 28.5%

O. Khaselev, A. Bansal, J.A. Turner, High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production, International Journal of Hydrogen Energy, Volume 26, Issue 2, February 2001, Pages 127-132,

Internal-biased systems - PV/PV

• STH efficiency 7.8%

• STE efficiency 9%

Internal-biased systems - PEC/PEC• Efficiency 5%

1) Glass sheet2) Aqueous electrolyte3) Meseporous oxide film4) TCO (transparent conducting

oxide)5) Electrical connection6) Dye sensitized meseoporous TiO27) Electrolyte8) CE9) Same as 2)10) Catalytic cathode11) Glass frit

Gratzel, M. and Augustynski, J. 2005. Tandem cell for water cleavage by visible light. Patent no. US 6936143.

Conclusions

• Highest STH efficiency achieved is 18.3 % with a multi bandgap PV/PV PEC

• Theoretically over 30% possible.

Future:• Emerging hybrid thermal electrical systems

utilizing wider range of solar spectrum promise up to 50% efficiency.