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 • Solar to hydrogen efficiency 0.7 % • Estimated to be improved to ~10 % with thicker oxide layer 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. 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) 2) 3) 4) Glass sheet Aqueous electrolyte Meseporous oxide film TCO (transparent conducting oxide) 5) Electrical connection 6) Dye sensitized meseoporous TiO2 7) Electrolyte 8) CE 9) Same as 2) 10) Catalytic cathode 11) 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.