Elevated Temperature Solar Hydrogen Processes and Components

Fletcher, repeating the fascinating suggestion of Brown that saturated aqueous NaOH will never boil, hypothesized that a useful medium for water electrolysis might be saturated, aqueous solutions of NaOH at very high temperatures.98 These do not reach boiling point at 1 atm due to the high salt solubility, binding solvent, and changing saturation vapor pressure, as reflected in their phase diagram.98 This domain is considered here, and also electrolysis in an even higher temperature domain

Fig. 14. Measured Vh2o (30 mA cm-2) in aq. saturated or molten NaOH compared to thermodynamic Eh2o values.90

above which NaOH melts (318 °C) creating a molten electrolyte with dissolved water, resulting in unexpected VH20.

Figure 13 summarizes measured VH20(7) in aqueous saturated and molten NaOH electrolytes. As seen in the inset, Pt exhibits low over potentials to H2 evolution, and is used as a convenient quasi-reference electrode in the measurements which follow. As also seen in the inset, Pt exhibits a known large overpotential to O2 evolution as compared to a Ni electrode or to £°mo (25 °C) = 1.23 V. This overpotential loss diminishes at moderately elevated temperatures, and as seen in the main portion of the figure, at 125 °C there is a 0.4 V decrease in the O2 activation potential at a Pt surface. Through 300 °C in Fig. 13, measured VH20 remains greater than the calculated thermodynamic rest potential. Unexpectedly, Vh2o at 400 °C and 500 °C in molten NaOH occurs at values substantially smaller than that predicted. These measured values include voltage increases due to IR and hydrogen overpotentials, and hence provide an upper bound to the unusually small electrochemical potential.

This phenomenon is summarized in Fig. 14, in which even at relatively large rates of water splitting (30 mA cm-2) at 1 atm, a measured Vh2o below that predicted by theory is observed at temperatures above the NaOH melting point. Theoretical calculations over an expanded temperature range are presented in Fig. 8, with calculations described in that Section. As seen in Fig. 14, the observed value at high temperature of Vmo approaches that calculated for a thermodynamic system of 500 bar, rather than 1 bar, H2O.

A source of this anomaly is described in Fig. 15. Shown on the left hand is the single compartment cell utilized here. Cathodically generated H2 is in close proximity to the anode, while anodic O2 is generated near the cathode. Their presence will facilitate the water forming back reaction, and at the electrodes this recombination will diminish the potential. In addition to the observed low potentials, two observations support this recombination effect. The generated H2 and O2 is collected, but is consistent with a Coulombic efficiency of ® 50% (varying with T, j, and interelec-trode separation.) Consistent with the right hand side of Fig. 15, when conducted in separated anode / cathode compartments, this observed efficiency is 98%-100%. Here however, all cell open circuit potentials increase to beyond the thermodynamic potential, and at j = 100 mA cm-2 yields measured VH2O values of 1.45V, 1.60V, 1.78 V at 500°, 400°, and 300°, which are approximately 450 mV higher than the equivalent Fig. 13 values for the single configuration cell.

The recombination phenomenon offers advantages (low VH2O), but also disadvantages (H2 losses), requiring study to balance these competing effects to optimize energy efficiency. In molten NaOH, the effects of temperature variation of AG0f (H2O) and the recombination of the water splitting products can have a pronounced effect on solar driven electrolysis. As compared to 25 °C, in Fig. 13 only half the potential is required to split water at 500 °C over a wide range of current densities.

The unused thermal photons which are not required in semiconductor photodri-ven charge generation, can contribute to heating water to facilitate electrolysis at an elevated temperature. The characteristics of one, two, or three series interconnected

Fig. 15. Interelectrode recombination can diminish Vh2o and occurs in open (left) but not in isolated (right) configurations; such as those examined with or without a ZrcO mix fiber separator (ZYK-5H, from Zircar Zirconia, FL, NY) situated between the Pt anode and cathodes.90

Fig. 16. Photovoltaic and electrolysis charge transfer for thermal electrochemical solar driven water splitting.90 Photocurrent is shown for one, two or three 1.561 cm2 HECO 335 Sunpower Si photovoltaics in series at 50 suns. Photovoltaics drive 500-°C molten NaOH steam electrolysis using Pt gauze anode and cathodes. Inset: electrolysis current stability.

Fig. 16. Photovoltaic and electrolysis charge transfer for thermal electrochemical solar driven water splitting.90 Photocurrent is shown for one, two or three 1.561 cm2 HECO 335 Sunpower Si photovoltaics in series at 50 suns. Photovoltaics drive 500-°C molten NaOH steam electrolysis using Pt gauze anode and cathodes. Inset: electrolysis current stability.

solar visible efficient photosensitizers, in accord with the manufacturer's calibrated standards, are presented in Fig. 16. These silicon photovoltaics are designed for efficient photoconversion under concentrated insolation (nsolar = 26.3% at 50 sun). Superimposed on the photovoltaic response curves in the figure are the water electrolysis current densities for one, or two series interconnected, 500 °C molten NaOH single compartment cell configuration electrolyzers.

Constant illumination generates for the three series-connected cells, a constant photopotential for stability measurements at sufficient power to drive two series molten NaOH electrolyzers. At this constant power, and as presented in the lower portion of Fig. 16, the rate of water splitting appears fully stable over an extended period. In addition, as measured and summarized in the upper portion of the figure, for the overlapping region between the solid triangle and open square curves, a single Si photovoltaic can drive 500 °C water splitting, albeit at an energy beyond the maximum power point voltage, and therefore at diminished efficiency. This appears to be the first case in which an external, single, small bandgap photosentizer can cleave water, and is accomplished by tuning the water splitting electrochemical potential to decrease to a point below the Si open circuit photovoltage. Vh2o -tunned is accomplished by two phenomena:

1. the thermodynamic decrease of Eh2o with increasing temperature, and

2. a partial recombination of the water splitting products.

VH2O -tunned can drive system efficiency advances, e.g., AlGaAs/GaAs, transmits more insolation, Eir < 1.4 eV, than Si to heat water, and with nphoto over 30%, prior to system engineering losses, calculates to over 50% nsolar to H2.

Without inclusion of high temperature effects, we had already experimentally achieved nsolar > 0.18, using an nphot = 0.20 AlGaAs/Si system.5 Our use of more efficient, nsolar = 26.3% at 50 suns, and inclusion of heat effects and the elevated temperature decrease of the water electrolysis potential, substantially enhances nsolar.90 Existing, higher nphot (= 0.28 to 0.33) systems should achieve proportionally higher results. Experimental components, for example as described in Fig. 2, for solar driven generation of H2 fuel at 40-50% conversion efficiencies appear to be technologically available. In the high efficiency range, photoelectrochemical cells tend to be unstable, which is likely to be exacerbated at elevated temperatures, and the model system will be particularly conducive to photovoltaic, rather than photoe-lectrochemical, driven electrolysis.

As has already been elaborated in Chapter 4, the photovoltaic component is used for photodriven charge into the electrolysis component and does not contact the heated electrolyte. Stable photovoltaics are commonly driven with concentrated insolation97 and specific to the system model here, heat will be purposely filtered from the insolation prior to incidence on the photovoltaic component. Dielectric filters used in laser optics split insolation without absorption losses (Chapter 4). For example, in a system based on a parabolic concentrator, a casegrain configuration may be used, with a mirror made from fused silica glass with a dielectric coating acting as band pass filter. The system will form two focal spots with different spectral configuration, one at the focus of the parabola and the other at focus of the case-grain.99 The thermodynamic limit of concentration is 46000 suns, the brightness of the surface of the sun. In a medium with refractive index greater than one, the upper limit is increased by two times the refractive index, although this value is reduced by reflective losses and surface errors of the reflective surfaces, the tracking errors of the mirrors and dilution of the mirror field. Specifically designed optical absorbers, such as parabolic concentrators or solar towers, can efficiently generate a solar flux with concentrations of ~2000 suns, generating temperatures in excess of 1000 °C.100'101

Commercial alkaline electrolysis occurs at temperatures up to 150 °C and pressures to 30 bar,96 and super-critical electrolysis to 350 °C and 250 bar.102 Although less developed than their fuel cell counterparts which have 100 kW systems in operation and developed from the same oxides,103 zirconia and related solid oxide based electrolytes for high temperature steam electrolysis can operate efficiently at 1000 °C,104 1 05 and approach the operational parameters necessary for efficient solar driven water splitting. Efficient multiple bandgap solar cells absorb light up to the bandgap of the smallest bandgap component. Thermal radiation is assumed to be split off (removed and utilized for water heating) prior to incidence on the semiconductor and hence will not substantially effect the bandgap. Highly efficient photovol-taics have been demonstrated at a solar flux with a concentration of several hundred suns. AlGaAs/GaAs has yielded a nphot efficiency of 27.6% and a GalnP/GaAs cell 30.3% at 180 suns concentration, while GaAs/Si has reached 29.6% at 350 suns, InP/GalnAs 31.8%, and GaAs/GaSb 32.6% with concentrated insolation.97

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