The direct and scalable electroreduction of triphenylphosphine oxide (TPPO)the stoichiometric byproduct of some of the most common synthetic organic reactionsto triphenylphosphine (TPP) remains an unmet challenge that would dramatically reduce the cost and waste associated with performing desirable reactions that are mediated by TPP on a large scale. This report details an electrochemical methodology for the single-step reduction of TPPO to TPP using an aluminum anode in combination with a supporting electrolyte that continuously regenerates a Lewis acid from the products of anodic oxidation. The resulting Lewis acid activates TPPO for reduction at mild potentials and promotes P−O over P−C bond cleavage to selectively form TPP over other byproducts. Finally, this robust methodology is applied to (i) the reduction of synthetically useful classes of phosphine oxides, (ii) the one-pot recycling of TPPO generated from a Wittig reaction, and (iii) the gram-scale reduction of TPPO at high concentration (1 M) with continuous product extraction and in flow at high current density.

Large-scale integration of renewable energies into the electrical grid will remain limited until high-capacity storage systems are developed to modulate the intermittency of harvestable sources. This work details the bottom-up design of all-organic materials and shuttles for application in high-capacity storage systems using redox chemistries of both heterogeneous and homogeneous compounds. Tailored heterogeneous polymers are cycled through redox-targeting reactions with a solvated redox-active compound. This hybrid battery, termed a redox-targeting flow battery (RTFB), merges the scalability and tunability of organic flow batteries with the energy density of solid-state batteries. Tuning steric and electronic properties of organic shuttles and solids for high capacities and voltaic efficiencies required the development of an accessible technique to monitor the state of charge (SOC) of the polymer as a function of time. Guided by these studies, we demonstrate RTFB cycling with high SOCs (>85%), high polymer utilization (>90%), and high voltaic efficiencies (>75%).

Redox-targeting flow batteries (RTFBs) merge solution and solid-state redox chemistries to create scalable storage systems with the potential for high energy densities. This study systematically evaluates ferrocenyl derivatives as shuttles that charge/discharge an insoluble ferrocenyl polymer. The polymer utilization, voltaic efficiency, and discharge voltage of cells cycled by each shuttle alone (single system) or by combinations of shuttles (dual system) are directly compared. Here, the performance of a dual system exceeded that of any other system tested, and stable cycling was achieved with added polymer that quintupled the system’s capacity. Most importantly, experimental and computational mechanistic studies provide insights into the interplay between the shuttle’s redox potential (thermodynamic parameter) and the shuttle’s rate of redox targeting reaction (kinetic parameter) with the solid. These data reveal that the rate of redox targeting dictates a system’s performance and depth of charge once the potentials of shuttles and solids are comparable.

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