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Gold nanoparticle (Au NP) mirrors, which exhibit both high reflectance and electrical conductance, were self-assembled at a [heptane + 1,2-dichloroethane]/water liquid/liquid interface. The highest reflectance, as observed experimentally and confirmed by finite difference time domain calculations, occurred for Au NP films consisting of 60 nm diameter NPs and approximate monolayer surface coverage. Scanning electrochemical microscopy approach curves over the interfacial metallic NP films revealed a transition from an insulating to a conducting electrical material on reaching a surface coverage at least equivalent to the formation of a single monolayer. Reflectance and conductance transitions were interpreted as critical junctures corresponding to a surface coverage that exceeded the percolation threshold of the Au NP films at the [heptane + 1,2-dichloroethane]/water interface.

Redox flow batteries (RFBs) are particularly well suited for storing the intermittent excess supply of renewable electricity, so-called "junk" electricity. Conventional RFBs are charged and discharged electrochemically, with electricity stored as chemical energy in the electrolytes. In the RFB system reported here, the electrolytes are conventionally charged but are then chemically discharged over catalytic beds in separate external circuits. The catalytic reaction of particular interest generates hydrogen gas as secondary energy storage. For demonstration, indirect water electrolysis was performed generating hydrogen and oxygen in separate catalytic reactions. The electrolyte containing V(II) was chemically discharged through proton reduction to hydrogen on a molybdenum carbide catalyst, whereas the electrolyte comprising Ce(IV) was similarly discharged in the oxidation of water to oxygen on a ruthenium dioxide catalyst. This approach is designed to complement electrochemical energy storage and may circumvent the low energy density of RFBs especially as hydrogen can be produced continuously whilst the RFB is charging. Broader contextRenewable energy technologies have evolved to deliver hundreds of terawatt hours of electricity, yet without its direct utilization in the grid part of that energy could be lost. In order to establish a thriving renewable energy economy it is of paramount importance that intermediate energy storage systems be developed. Mediating electricity production and usage will overcome the issues relating to intermittency, which presently limits widespread dependence on wind and photovoltaic power. Various approaches are under development, but no single approach is liable to address the issue as a whole. Combining technologies and hybridizing storage systems to adapt to a multifaceted energy future is the more viable option. This paper discusses one such hybrid system, in which electrochemical energy storage is combined with renewable hydrogen production, delivering a dual platform for energy storage as an electrochemical and chemical medium. † Electronic supplementary information (ESI) available: Charging and discharging curves, cyclic voltammetry, UV-vis spectra of V(II) and V(III), a picture of the cell, and details on the kinetics measurement and calculations. See

Keywords:redox flow battery redox targeting redox mediator solid charge storage materials polyaniline A B S T R A C TIn this work, the viability of polyaniline as a solid charge storage material in an aqueous redox-mediated flow battery is investigated. Fe(III/II) and V(IV/III) were identified as potential redox mediators to target the emeraldine-pernigraniline and leucoemeraldine-emeraldine redox transitions of the polymer. An indirect chemical cycling method was developed and used to investigate the charging/discharging of the polymer by the selected redox mediators. With Fe(III/II) as a redox mediator, respectable specific capacity and cycling stability were demonstrated over 25 cycles. V(IV/III) was deemed unsuitable as a redox mediator, due to rather poor kinetics. When added to the electrolyte tanks of a complete flow battery, a conductive composite of polyaniline and carbon black provided a significant improvement in capacity, exhibiting a specific capacity of 64.8 mA h g À1 at a current density of 38.5 mA cm À2. This represents a three-fold improvement in volumetric capacity, compared with the electrolyte alone. Moreover, the addition of the polyaniline composite was observed to lower the average potential at the positive electrode, providing a considerable improvement in voltage efficiency. This work demonstrates the potential of utilizing redox mediators to enable bulk solid-phase charge storage in the tanks of aqueous redox flow batteries.