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Solar thermal water-splitting (STWS) cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight. Two-step, metal oxide-based STWS cycles generate H2 by sequential high-temperature reduction and water reoxidation of a metal oxide. The temperature swings between reduction and oxidation steps long thought necessary for STWS have stifled STWS's overall efficiency because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. We show that these temperature swings are unnecessary and that isothermal water splitting (ITWS) at 1350°C using the "hercynite cycle" exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350°C and reoxidized at 1000°C.

The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Despite this fundamental and ubiquitous role, G has been tabulated for only a small fraction of known inorganic compounds, impeding a comprehensive perspective on the effects of temperature and composition on materials stability and synthesizability. Here, we use the SISSO (sure independence screening and sparsifying operator) approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV atom−1 (~1 kcal mol−1) resolution, and with minimal computational cost, for temperatures ranging from 300–1800 K. We then apply this descriptor to ~30,000 known materials curated from the Inorganic Crystal Structure Database (ICSD). Using the resulting predicted thermochemical data, we generate thousands of temperature-dependent phase diagrams to provide insights into the effects of temperature and composition on materials synthesizability and stability and to establish the temperature-dependent scale of metastability for inorganic compounds.

Sequential exposures of Al(CH 3 ) 3 and H 2 O at 77 °C were used to encapsulate low-density polyethylene (LDPE) particles with an ultrathin Al 2 O 3 film. FTIR studies revealed that the nucleation of Al 2 O 3 atomic layer deposition (ALD) on the LDPE particles occurred primarily via adsorption of Al(CH 3 ) 3 onto the LDPE surface or absorption of Al(CH 3 ) 3 into the LDPE particle followed by the reaction with H 2 O. The FTIR spectra then revealed the progressive switching between AlCH 3 * and AlOH* species with alternating exposure to Al(CH 3 ) 3 and H 2 O. This nucleation of Al 2 O 3 ALD did not require the existence of specific chemical functional groups on the polymer. The FTIR spectra also demonstrated that the sequential exposures of Al(CH 3 ) 3 and H 2 O led to an increase in Al 2 O 3 bulk vibrational modes. The increase of the absorbance for the Al 2 O 3 bulk vibrational modes was linear with the number of AB cycles. The presence of an Al 2 O 3 film on the LDPE particles was confirmed using transmission electron microscopy (TEM). The TEM images revealed that the Al 2 O 3 coating was very conformal to the LDPE particles. The Al 2 O 3 coating was also thicker than expected from typical Al 2 O 3 ALD growth rates. This thicker Al 2 O 3 coating was explained by the presence of hydrogen-bonded H 2 O on the Al 2 O 3 surface that increases the Al 2 O 3 growth rate during Al(CH 3 ) 3 exposures. On the basis of these results and additional investigations, a model is proposed for Al 2 O 3 ALD on polymers. Al 2 O 3 ALD should provide an effective gas diffusion barrier on temperature-sensitive polymeric materials such as LDPE.

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