Solar water splitting to generate hydrogen fuel—a photothermal electrochemical analysis
Introduction
Hydrogen fuel to drive transportation and other societal energy needs is receiving increasing attention [1], but what is to be the H2 source? Solar water splitting can provide clean, renewable sources of hydrogen fuel [2]. A variety of approaches have been studied to achieve this important goal including indirect or direct or thermochemical [3], [4], photosynthetic [5] or photoelectrochemical [2], [6], [7], [8], [9], [10] solar water splitting. Each of these previous approaches has limitations, and as summarized in Scheme 1 has exhibited a limited conversion of solar energy to H2. The highest efficiencies had been observed with multiple band-gap semiconductor electrolysis cells sustaining over 18% solar energy conversion efficiency to H2 [10]. This paper combines a novel derivation of band-gap-restricted thermal enhanced solar water splitting efficiencies [11], along with recent experimental evidence of thermal electrochemical solar water splitting processes by semiconductor materials support of that model [12]. The model provides a theoretical basis for solar energy to hydrogen conversion efficiencies in the 50% range, by combing excess sub-band-gap insolation with efficient solar driven water electrolysis at elevated temperatures.
The UV and visible energy rich portion of the solar spectrum is transmitted through H2O. Therefore sensitization, such as via semiconductors, is required to drive the water-splitting process. Solar photoelectrochemical attempts to split water utilized TiO2 [6], [7] and InP [8] and also multiple band-gap semiconductors [9], [10]. Photoelectrochemical water splitting studies have generally focused on diminishing the high band gap apparently required for solar water splitting [6], [7]. Semiconductors, such as TiO2 can split water, but their wide band-gap limits the photo response to a small fraction of the incident solar energy. Studies sought to improve the solar water splitting by tuning (decreasing) the band-gap of the photosensitizers, Eg, to better match the water splitting potential, EH2O. Here, we take an alternate approach: instead of tuning Eg to fit EH2O, we tune EH2O to fit Eg. Early photoelectrochemical models had incorrectly predicted only low solar water splitting conversion efficiencies, with a maximum of ∼15%, would be attainable [9]. This was recently shown to be improved to ∼30% solar water splitting modeled conversion efficiency by eliminating (i) the linkage of photo to electrolysis surface area, (ii) non-ideal matching of photo and electrolysis potentials, and incorporating the effectiveness of contemporary (iii) electrolysis catalysts and (iv) efficient multiple band-gap photosensitizers [10]. However, both the early and improved models either did not incorporate heat effects on (semiconductor) charge utilization, or semiconductor effects on heat utilization.
At high temperatures water (≫2000°C) chemically disproportionates without electrolysis. However, catalysis, gas recombination and containment materials limitations above 2000°C have led to very low efficiencies [4]. Electrochemical water splitting, generating H2 and O2 at separate electrodes, largely circumvents the gas recombination limitations, and a hybrid of photothermal electrochemistry will be shown to provide a pathway for efficient solar energy utilization. Utilizing heat to facilitate water electrolysis had been suggested [13], although no rigorous analysis from the fundamentals of solar energy, thermodynamics and electrochemical processes had been developed. One schematic representation for this solar thermal water electrolysis assisted (photothermal electrochemical water splitting) is presented in Scheme 2, and rather than a field of concentrators, similar systems may use individual solar concentrators. Thermally assisted solar electrolysis consists of (i) light harvesting, (ii) spectral resolution of thermal (sub-band-gap) and electronic (super-band-gap) radiation, the latter of which (iiia) drives photovoltaic or photoelectrochemical charge transfer V(iH2O), while the former (iiib) elevates water to temperature T, and pressure, p; finally (iv) V(iH2O) driven electrolysis of H2O(T,p) as schematized in Scheme 2.
Section snippets
Elevated temperature results
Fletcher, repeating the fascinating suggestion of Brown that saturated aqueous NaOH will never boil, hypothesised that a useful medium for water electrolysis might be very high temperature NaOH saturated, aqueous solutions. These do not reach a temperature at which they boil at due to the high salt solubility, binding solvent, and changing saturation vapour pressure, as reflected in their phase diagram [14]. We measure this domain, and also electrolysis in an even higher temperature domain
Theory of electrochemical thermal solar/H2 energy conversion
Photodriven charge transfer through a semiconductor junction does not utilize photons which have energy below the semiconductor band gap. Hence a silicon photovoltaic device does not utilize radiation below its band gap of , while a AlGaAs/GaAs multiple band-gap photovoltaic does not utilize radiation of energy less than the band gap of GaAs. As will be shown, this unutilized, available long wavelength insolation represents a significant fraction of the solar spectrum. This long
Solar/H2 electrochemical thermal conversion efficiencies
Representative results from Fig. 11 for solar water splitting to from AM1.5 insolation include a 50% solar energy conversion for a photoelectrolysis system at 638°C with ; and ηphot=0.32. However, this high H2O partial pressure system requires separation of a low partial pressure of H2. Efficient photoelectrolysis is also determined for high relative H2, such as for systems of , ηheat=0.7, and with a , in which efficiencies
Available solar/H2 electrochemical thermal components
Without inclusion of high temperature effects, we had already experimentally achieved ηsolar>0.18, using an system [10]. Existing, higher ηphot (=0.28–0.33) systems should achieve proportionally higher results, and inclusion of heat effects and the elevated temperature decrease of the water electrolysis potential will lead to even higher values of solar energy to H2 fuel conversion efficiencies.
Experimental components, for example as described in Scheme 2, of efficient solar
Summary
The energy source (sun) and reactive media (water) for solar water splitting are readily available and are renewable, and the resultant fuel (generated H2) and its discharge product (water) are each environmentally benign. The model presented here provides theoretical evidence that the combination of contemporary efficient multiple band-gap photovoltaics and concentrated excess sub-band-gap heat will combine into highly efficient elevated temperature solar electrolysis of H2 fuel. Efficiency
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From a similar paper reprinted with permission from S. Licht, J. Phys. Chem., B 107 (2003) 4253. Copyright 2003 American Chemical Society.
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From a similar paper reprinted with permission from S. Licht, Halperin, L. Kalina, M. Halperin, N. Chem. Commun. 2003, 3006—Reproduced by permission of The Royal Society of Chemistry.