Splitting CO 2 to produce syngas and hydrocarbon fuels: PEC and STC
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Energy Sector Analysis
Though very high efficiencies are within reach theoretically, demonstrated efficiencies are currently in the range of 5%. Efficiencies are limited by both engineering (reactor design) and materials aspects, and the interplay between the two.
Splitting CO2 to produce syngas and hydrocarbon fuels: PEC and STC By Eva Karatairi Feature Editor James E. Miller
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hough we live in the Anthropocene era, growing awareness of the potentially damaging global consequences of human dominance is prompting efforts to achieve and retain a favorable global concentration and dynamic equilibrium for CO2, wherein production and consumption balance one another. Large-scale production of sustainable hydrocarbon fuels is a crucial piece of this balancing act, and to this end, scientists and engineers are manipulating three widely available ingredients—sunlight, water, and CO2—through electrochemical, photoelectrochemical, and solar thermochemical means. The focus of much of this work is the initial production of syngas, a versatile precursor of synthetic fuels, consisting of CO and H2, which leverages established commercial processes for syngas conversion to hydrocarbon fuels. In the previous issue of EQ, global efforts on the most technologically ready approach of the three, electrolysis, were examined. Herein, we take a closer look at photoelectrochemistry (PEC) and solar thermochemistry (STC) in this context. PEC, like electrolysis, exploits energetic electrons to provide the energy and to drive the chemical conversions. The difference is the more direct coupling and integration of the electron source, electrode/catalysts, and other components in PEC. For example, since October 2015, the Joint Center for Artificial Photosynthesis (JCAP, a US DOE Energy Innovation Hub) has conducted the mesoscale design project, which is focused on creating a fully integrated PEC device with improved efficiency for the production of renewable fuels. The device comprises a combined PV and electrolyzer, with a catalyst deposited directly on top of solar photon absorbers. It requires the assembly of photoactive semiconductors, cathodic and anodic catalysts, and an ion-conducting membrane. David Tiede, member of the Argonne-Northwestern Solar Energy Research Center, explained the approach: “In classical electrolysis, the electrode is agnostic; it doesn’t know where it gets potential from.… PEC involves semiconductors which, when you shine light on them, move electrons up from the valence to the conduction band. These become mobile electrons, with a reducing power generated by light.” The energetic mobile electrons (and/or holes) can move to the surface or interface regions of the device where they are available for reaction. Tiede added that the initial light absorption pushes electrons to highly exited orbitals, yielding the so-called hot carriers. “If captured in less than picoseconds, before they lose their energy
in the form of heat and end up in the lowest excited states, then one has very reactive hot electrons,” he said
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