Powering the Planet: Chemical Challenges in Solar Energy Utilization

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 11/2006; 103(43):15729-35. DOI: 10.1073/pnas.0603395103
Source: PubMed


Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO(2) emissions in the atmosphere demands that holding atmospheric CO(2) levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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    • "Solar-driven hydrogen production via water splitting using sunlight as the only energy input holds great promise as a sustainable energy source which allows for the direct conversion and storage of energy in the form of a chemical bond, namely H 2 . It addresses the need to produce storable fuels from fluctuating renewable energy sources and may pave the way for future commercial applications [1] [2] [3]. The technical and commercial viability, however, is closely linked to the efficiency and cost-effectiveness of such energy concepts. "
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    ABSTRACT: Direct solar-to-hydrogen conversion via water splitting was demonstrated in an integrated photovoltaic–electrochemical (PV–EC) device using a hydrogenated amorphous silicon thin film tandem junction (a-Si:H/a-Si:H) solar cell as photocathode. The solar cell was adapted to provide sufficient photovoltage to drive both the hydrogen and oxygen evolution reactions. The best results, in terms of photoelectrochemical stability and performance, were obtained with an Ag/Pt layer stack as H2 evolving photocathode back contact and with a RuO2 counter electrode for O2 evolution. Under irradiation by simulated sunlight (AM 1.5 spectrum with 100 mW/cm2), we achieved 6.8% solar-to-hydrogen efficiency at 0 V applied bias in a two-electrode set-up. This sets a fresh benchmark for integrated thin film silicon tandem based photoelectrochemical devices. In addition, the photovoltage at constant current (−3 mA/cm2) was measured over a prolonged period of time and revealed an excellent chemical stability (operation over 50 h) of the photocathode. Furthermore, we present an empirical serial circuit model of the PV–EC device, in which the corresponding photovoltaic and electrochemical components are decoupled. This allows for a detailed comparison between the solar cell and the PV–EC cell characteristics, from which the relevant loss processes in the overall system could be identified. The model was further used to compare calculated and measured photocurrent–voltage characteristics of the investigated PV–EC device which showed excellent agreement.
    Solar Energy Materials and Solar Cells 09/2015; 140. DOI:10.1016/j.solmat.2015.04.013 · 5.34 Impact Factor
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    • "Photocatalytic hydrogen production from water through aritificial photosynthesis holds great promise for addressing environmental problem and energy crisis [1] [2] [3]. Among various inorganic semiconductor photocatalysts, TiO 2 is a promising candidate owing to its high efficiency, low cost, high stability, and non-toxicity [4] [5] [6] [7] [8]. "
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    ABSTRACT: Titanium dioxide is an excellent photocatalyst for photocatalytic hydrogen production but its application is limited by its poor visible light harvesting capability. In this work, a visible-light-responsive photocatalytic hydrogen production system was developed using Zn(II)-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (ZnTCPP) sensitized MoS2/TiO2 (ZnTCPP-MoS2/TiO2) as photocatalysts. The as-prepared composite photocatalysts have apparent adsorption in visible light region, making them active for visible-light-induced hydrogen evolution. Even without noble metals, ZnTCPP-MoS2/TiO2 photocatalyst loaded with 1.00 wt% MoS2 shows the highest H2 production rate of 10.2 μmol h−1, and the turnover number (TON) with respect to ZnTCPP dye reaches 261 after visible light irradiation for 12 h. The highest H2 production rate of ZnTCPP-MoS2/TiO2 is much higher than that of ZnTCPP-Pt/TiO2, suggesting that MoS2 can act as a more effective cocatalyst than the commonly used platinum. This study presents a noble-metal-free and visible-light-responsive TiO2-based photocatalyst for solar-to-hydrogen conversion.
    Chemical Engineering Journal 09/2015; 275. DOI:10.1016/j.cej.2015.04.015 · 4.32 Impact Factor
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    • "Photoinduced water splitting reaction to produce hydrogen is regarded as one of the cleanest and the most promising ways for capturing and storing the inexhaustible energy from the sun [1] [2] [3]. "
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    ABSTRACT: Optically transparent cocatalyst film materials is very desirable for improved photoelectrochemical (PEC) oxygen evolution reaction (OER) over light harvesting photoelectrodes which require the exciting light to irradiate through the cocatalyst side, i.e., front-side illumination. In view of the reaction overpotential at electrode/electrolyte interface, the OER electrocatalysts have been extensively used as cocatalysts for PEC water oxidation on photoanode. In this work, the feasibility of a one-step fabrication of the transparent thin film catalyst for efficient electrochemical OER is investigated. The Ni-Fe bimetal oxide films, 200 nm in thickness, are used for study. Using a reactive magnetron co-sputtering technique, transparent (> 50% in wavelength range 500-2000 nm) Ni-Fe oxide films with high electrocatalytic activities were successfully prepared at room temperature. Upon optimization, the as-prepared bimetal oxide film with atomic ratio of Fe/Ni = 3:7 demonstrates the lowest overpotential for the OER in aqueous KOH solution, as low as 329 mV at current density of 2 mA cm�2, which is 135 and 108 mV lower than that of as-sputtered FeOx and NiOx thin films, respectively. It appears that this fabrication strategy is very promising to deposit optically transparent cocatalyst films on photoabsorbers for efficient PEC water splitting.
    Electrochimica Acta 07/2015; 169:402-408. DOI:10.1016/j.electacta.2015.04.101 · 4.50 Impact Factor
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