Scale-Up of the Carbon Dioxide Removal by Ionic Liquid Sorbent (CDRILS) System

The Carbon Dioxide Removal by Ionic Liquid Sorbent (CDRILS) system is designed for efficient, safe and reliable carbon dioxide (CO2) removal from cabin air on long-duration missions to the Moon, deep space, and Mars. CDRILS integrates an ionic liquid sorbent with hollow fiber membrane contactors for rapid CO2 removal and recovery. The liquid-based system provides continuous CO2 delivery, which avoids complicated valve networks to switch between absorbing and desorbing beds and enables simpler integration to the Sabatier without the need for the CO2 Management System (CMS). Ionic liquids are particularly desirable as liquid absorbents for space applications since they are non-volatile, non-odorous, and have high oxidative stability. The hollow fiber membrane contactors offer both high contact area and rigorous containment between the gas and liquid phases in a microgravity environment. Scale-up of the CDRILS technology has presented a series of fascinating challenges, since the interaction between hollow fiber properties, ionic liquid properties and performance is complex. Properties measured with lab-scale hollow fiber contactors are used to estimate the performance of contactors that are similar in scale to flight-scale demonstrations. To accomplish this, component and system models have been built to relate the key scrubber and stripper design and operating variables with performance, and experiments directed to validate the models have been performed. System size, weight and power are determined by component selection, arrangement, and operating conditions. Reliability will be extremely important for any long-range mission and depends on the stability of the ionic liquids and hollow fiber contactors. We report on our continuing long term stability experiments for the ionic liquid and contactor materials and our investigation of the physical properties of additional ionic liquids

Excimer Formation and Symmetry-Breaking Charge Transfer in Cofacial Perylene Dimers

The use of multiple chromophores as photosensitizers for catalysts involved in energy-demanding redox reactions is often complicated by electronic interactions between the chromophores. These interchromophore interactions can lead to processes, such as excimer formation and symmetry-breaking charge separation (SB-CS), that compete with efficient electron transfer to or from the catalyst. Here, two dimers of perylene bound either directly or through a xylyl spacer to a xanthene backbone were synthesized to probe the effects of interchromophore electronic coupling on excimer formation and SB-CS using ultrafast transient absorption spectroscopy. Two time constants for excimer formation in the 1–25 ps range were observed in each dimer due to the presence of rotational isomers having different degrees of interchromophore coupling. In highly polar acetonitrile, SB-CS competes with excimer formation in the more weakly coupled isomers followed by charge recombination with τ_CR = 72–85 ps to yield the excimer. The results of this study of perylene molecular dimers can inform the design of chromophore–catalyst systems for solar fuel production that utilize multiple perylene chromophores

Photodriven Oxidation of Surface-Bound Iridium-Based Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO₂ Electrodes Protected by an Al₂O₃ Layer

Improving stability and slowing charge recombination are some of the greatest challenges in the development of dye-sensitized photoelectrochemical cells (DSPECs) for solar fuels production. We have investigated the effect of encasing dye molecules in varying thicknesses of Al₂O₃ deposited by atomic layer deposition (ALD) before catalyst loading on both the stability and the charge transfer dynamics in organic dye-sensitized TiO₂ photoanodes containing iridium-based molecular water-oxidation catalysts. In the TiO₂|dye|Al₂O₃|catalyst electrodes, a sufficiently thick ALD layer protects the perylene-3,4-dicarboximide (PMI) chromophores from degradation over several weeks of exposure to light. The insulating capacity of the layer allows a higher photocurrent in the presence of ALD while initial charge injection is slowed by only 1.6 times, as observed by femtosecond transient absorption spectroscopy. Rapid picosecond-scale catalyst oxidation is observed in the presence of a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical experiments demonstrate higher photocurrents with IrSil compared to IrIr, which show that recombination is slower for IrSil, while higher photocurrents with IrIr upon addition of ALD layers confirm that ALD successfully slows charge recombination. These findings demonstrate that, beyond stability improvements, ALD can contribute to tuning charge transfer dynamics in photoanodes for solar fuels production and may be particularly useful for slowing charge recombination and accounting for varying charge transfer rates based on the molecular structures of incorporated catalysts