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Photoelectrolysis of water: Solar hydrogen–achievements and perspectives

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Abstract

Thermodynamic analysis of energy conversion from light-to-chemical, light-to-electric and electric-to-chemical is presented by the case study of water photoelectrolysis on TiO2 surface. It is demonstrated that at the current state-of-the-art energy conversion efficiency of water photoelectrolysis can be increased ∼17 times by separating the processes of solar-to-electric and electric-to-chemical energy conversion and optimizing them independently. This allows to mitigate a high overvoltage of oxygen evolution reaction with respect to thermodynamic EO2/H2O0=1.23V potential as well as spectrally narrow absorbtivity of solar light by TiO2 which determine the low efficiency (∼ 1.0%) of direct light-to-chemical energy conversion. Numerical estimates are provided illustrating practical principles for optimization of the solar energy conversion and storage processes.

© 2010 Optical Society of America

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Figures (4)

Fig. 1
Fig. 1 (a) Schematics of water photoelectrolysis cell with external power source and equipment for the measurement of electric parameters. (b) The equivalent scheme of the electric circuit. WE, RE, CE - working, reference and counter electrodes, respectively, GC - gas chromatograph, ΔEext - voltage of external power source, ΔEph - voltage of photocurrent source, Iph - photocurrent, hν- photon energy.
Fig. 2
Fig. 2 (a) Schematics of Si solar cell with electric scheme for cell voltage Vph and photocurrent Iph measurement. (b) Typical photocurrent vs. photovoltage response when electric circuit resistance R varies from R = ∞ (open-circuit) to R = 0 (short circuit). Definition of the fill factor, FF, is schematically shown.
Fig. 3
Fig. 3 (a) Photo-processes on an illuminated TiO2 electrode at anodic bias. (b) Schematic potential diagram of TiO2 surface state under UV illumination, open-circuit (Iph = 0) and photoelectrolysis (Iph ≠ 0) conditions. ΔEext - voltage of external power source, ΔEph - voltage of photocurrent source, p E F h ν and n E F h ν - potential values corresponding to Fermi levels of holes and photoelectrons in UV-illuminated TiO2 surface under open-circuit conditions; Ea and Ec - potentials of anode and cathode under water photoelectrolysis conditions in PEC; ηa and ηc - overvoltage or polarization of anodic and cathodic processes: ηa,c = EiEi=0; ECB and EVB - potential values corresponding to TiO2 conductive and valence bands, E B G T i O 2 - voltage corresponding to TiO2 band gap, HER and OER - hydrogen and oxygen evolution reactions, respectively; voltammograms: 1 - HER on Pt electrode, 2 - real OER on TiO2 surface, 3 - visually observed OER on Ti/TiO2 electrode (see text for details). Iph can vary from ∼ 0.01 to ∼ 10 mA cm−2 depending on the experimental conditions.
Fig. 4
Fig. 4 (a) Schematic representation of water electrolyzer using solar cells connected in series and in parallel as source of photocurrent. (b) Optimal voltammograms of anodic and cathodic processes of water electrolysis in alkaline electrolyte. (c) Comparison of average ECE in visible spectral region for light energy conversion using Ti/TiO2 photoelectrode, DSSC, Si [43] and tandem GaInP/GaAs solar cells.

Equations (19)

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H 2 O h ν H 2 + 1 / 2 O 2
I P C E = Q E = F l u x o f e l e c t r o n s F l u x o f p h o t o n s = I p h / q P / h ν = I p h h ν P q = I p h E h ν P 100 ( % ) ,
E C E = P o u t P 100 ( % ) .
E C E = I p h m p p V m p p P = I p h s c V p h o c F F P 100 ( % ) ,
E C E = ( I p h E O 2 / H 2 O 0 I p h Δ E e x t ) P 100 ( % ) ,
E C E = I p h E O 2 / H 2 O 0 P 100 ( % ) .
θ = E O 2 / H 2 O 0 E a E c 100 ( % ) ,
V H 2 = 22.4 I t 53.6 = 0.418 I t ,
2 TiO 2 h ν TiO 2 + + TiO 2
2 H 2 O 4 e O 2 + 5 H +
H 2 O e OH + H + .
TiO 2 h ν , E a TiO 2 + + e , ( h ν 3.2 eV )
TiO 2 + + H 2 O TiO 2 ( OH ) ads + H + , ( E OH / H 2 O 0 = 2.8 V )
TiO 2 ( OH ) ads TiO 3 + H + + e , ( E O / H 2 O 0 = 2.4 V )
TiO 3 TiOO 2 ( Ti ( 4 + ) peroxide )
TiOO 2 + H 2 O TiO 2 + H 2 O 2
H 2 O 2 O 2 + 2 H + + 2 e ( E O 2 / H 2 O 2 0 = 0.68 V )
2 H 2 O 2 2 H 2 O + O 2 ( disproportionation ) .
2 H 2 O + 4 h + h ν = 3.2 e V E 2.8 V O 2 + 4 H + .
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