Redox Solid Energy Boosters for Flow Batteries: Polyaniline as a Case Study

In 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−1 at a current density of 38.5 mA cm−2. 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

Electrocatalysis induced by surface redox activities on conductive metal oxide electrodes

Stéphane Fierro

Iridium dioxide electrodes form part of the dimensionally stable anodes (DSA®) and this electrode material is widely used in many industrial processes namely water electrolysis, metal electro-winning, cathodic protection and electro-organic synthesis due to the high electrochemical activity and stability of this electrode material. IrO2-based electrodes can be prepared using different techniques but the most common is the thermal decomposition of H2IrCl6 precursor solution on an inert substrate like titanium. Within the water stability potential domain, the charging/discharging process is attributed to the slow diffusion of protons within the IrO2 coating together with the electrical double layer capacitance. In fact the valence state of the Ir surface atoms of the coating varies from +IV to +VI in the potential domain between the on-set potentials of H2 and O2 evolution. Concerning the oxygen evolution reaction, direct evidence was found that the IrO2 coating participates actively in the reaction using an electrolyte solution containing isotopically labeled H218O. In fact, measurements of the relative amounts of electrogenerated 16O2 and 18O16O have demonstrated that the hydroxyl radicals coming from water discharge interact strongly with IrO2 resulting in the formation of the higher oxide (IrO3) and the decomposition of that oxide produces oxygen. IrO3 is thus the intermediate involved in the OER on these electrodes. During the oxidation of organic compounds, direct evidence was found by marking the IrO2 electrode with 18O that the higher valence state oxide IrO3 participates effectively also in this process. In fact, the oxidation of a solution of formic acid on a marked IrO2 coating containing 18O has shown that C16O18O is evolved proving that the oxidation of organic compounds occurs on IrO3 with competing side-reaction of oxygen evolution. The low overpotential of the OER allows performing selective electro-oxidation of a wide variety of organic compounds. In fact, the competing side reaction of oxygen evolution 'buffers' the potential around values where the oxidation products are not further oxidized showing that the IrO2 electrode is particularly suited for electro-organic synthesis. However, the current efficiency of the oxidation process remains low due to the competing side reaction of oxygen evolution.

EPFL2010

Solid electrochemical energy storage for aqueous redox flow batteries: The case of copper hexacyanoferrate

Christopher Raymond Dennison, Solène Cindy Gentil, Hubert Girault, Grégoire Clément Gschwend, Pekka Eero Peljo, Danick Reynard, Evgeny Smirnov, Elena Zanzola

All redox flow batteries suffer from low energy storage density in comparison with conventional Li-ion batteries. However, this issue can be mitigated by utilization of solid energy storage materials to enhance the energy storage capacity. In this paper we demonstrate the utilization of copper hexacyanoferrate (CuHCF) Prussian blue analogue for this purpose, coupled with N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TEMPTMA) as a soluble redox mediator to target the redox transitions of the solid material. In this case, indirect charging and discharging of CuHCF suspended in the electrolyte by electrochemically oxidized/reduced TEMPTMA was observed by chronoamperometry. Secondly, electrochemistry of different CuHCF composites with carbon black and multi-walled carbon nanotubes were investigated, highlighting that the high conductivity of the solid energy storage materials is crucial to access the maximal charge storage capacity. Finally, a CuHCF-TEMPTMA/Zn aqueous redox flow battery achieved stable cycling performances with high coulombic efficiency of 95% and volumetric capacity of 350 C mL−1.

2019

Anodic oxidation of organics on thermally prepared oxide electrodes

Christos Comninellis, György Foti

Electrochem. oxidn. of orgs. at thermally prepd. dimensionally stable metal oxide anodes (DSA) has been studied. In reactions involving only simple electron transfer, oxide electrodes exhibit an electrocatalytic activity similar to that of noble metal electrodes. In oxidn. reactions of more complex mechanism, oxide anodes show no electrocatalytic activity below the potential of oxygen evolution and oxidn. of orgs. can take place only under conditions of simultaneous oxygen evolution. A simplified mechanism of orgs. oxidn. catalyzed by intermediates of oxygen evolution has been proposed distinguishing between two limiting electrode behaviors: (i) At "non-active" anodes (typically fully oxidized metal oxides) oxidn. of orgs. occurs at a high potential and leads to their combustion through physisorbed hydroxyl radicals. (ii) At "active" electrodes (IrO2, RuO2) the reaction takes place at lower potentials characteristic of the metal oxide and results in a selective oxidn. of the org. compd. at higher oxidized metal oxide surface sites. A kinetic model of orgs. oxidn. competed by oxygen evolution at "active" type anodes is also proposed and confirmed by preparative electrolysis. [on SciFinder (R)]

1997

Electrocatalysis induced by surface redox activities on conductive metal oxide electrodes

Stéphane Fierro

EPFL2010