Critical Roles of pH and Activated Carbon on the Speciation and Performance of an Archetypal Organometallic Complex for Aqueous Redox Flow Batteries.

A lack of suitable high-potential catholytes hinders the development of aqueous redox flow batteries (RFBs) for large-scale energy storage. Hydrolysis of the charged (oxidized) catholyte typically occurs when its redox potential approaches that of water, with a negative impact on battery performance. Here, we elucidate and address such behavior for a representative iron-based organometallic complex, showing that the associated voltage and capacity losses can be curtailed by several simple means. We discovered that addition of activated carbon cloth (ACC) to the reservoir of low-cost, high-potential [Fe(bpy)3]2+/3+ catholyte-limited aqueous redox flow batteries extends their lifetime and boosts discharge voltage─two typically orthogonal performance metrics. Similar effects are observed when the catholyte's graphite felt electrode is electrochemically oxidized (overcharged) and by modifying the catholyte solution's pH, which was monitored in situ for all flow batteries. Modulation of solution pH alters hydrolytic speciation of the charged catholyte from the typical dimeric species µ-O-[FeIII(bpy)2(H2O)]24+, converting it to a higher-potential µ-dihydroxo form, µ-[FeIII(bpy)2(H2O)(OH)]24+, at lower pH. The existence of free bpyH22+ at low pH is found to strongly correlate with battery degradation. Near-neutral-pH RFBs employing a viologen anolyte, (SPr)2V, in excess with the [Fe(bpy)3]2+/3+ catholyte containing ACC exhibited high-voltage discharge for up to 600 cycles (41 days) with no discernible capacity fade. Correlating pH and voltage data offers powerful fundamental insight into organometallic (electro)chemistry with potential utility beyond battery applications. The findings, with implications toward a host of other "near-neutral" active species, illuminate the critical and underappreciated role of electrolyte pH on intracycle and long-term aqueous flow battery performance.

Dimerization of [FeIII(bpy)3]3+ in Aqueous Solutions: Elucidating a Mechanism Based on Historical Proposals, Electrochemical Data, and Computational Free Energy Analysis.

Iron(II) tris-bipyridine, [FeII(bpy)3]2+, is a historically significant organometallic coordination complex with attractive redox and photophysical properties. With respect to energy storage, it is a low-cost, high-redox potential complex and thus attractive for use as a catholyte in aqueous redox flow batteries. Despite these favorable characteristics, its oxidized Fe(III) form undergoes dimerization to form µ-O-[FeIII(bpy)2(H2O)]24+, leading to a dramatic ∼0.7 V decrease during battery discharge. To date, the energetics and complete mechanism of this slow, sequential electrochemical-chemical (EC) process, which includes electron transfer, nucleophilic attack, ligand cleavage, µ-oxo bond formation, and spin state transition, have not been elucidated. Using cyclic voltammetry, redox flow battery data, and density functional theory calculations guided by previously proposed mechanisms, we modeled more than 100 complexes and performed more than 50 geometry scans to resolve the key steps dictating these complex chemical processes. Quantitative free energy surfaces are developed to model the mechanism of dimerization accounting for the spins and identities of any possible Fe(II), Fe(III), or Fe(IV) intermediates. Electrochemical reduction of the dimer regenerates [FeII(bpy)3]2+ in an overall reversible process. Computational electrochemistry interrogates the influence of spin state, coordination environment, and molecular conformation at the electrode-electrolyte interface through a proposed stepwise dimer reduction process. Experimentally, we show that the considerable overpotential associated with this event can be catalytically mitigated with disparate materials, including platinum, copper hexacyanoferrate, and activated carbon. The findings are of fundamental and applied significance and could elevate [FeII(bpy)3]2+ and its derivatives to play a vital role in the burgeoning renewable energy economy.

Electroanalysis and Spectroelectrochemistry of Nonaromatic Explosives in Acetonitrile Containing Dissolved Oxygen.

In search of a rapid, low-cost, and solution-phase detection technique for explosives, the (spectro-)electrochemistry of compounds from two major nonaromatic classes, namely nitramines (RDX and HMX) and nitrate esters (pentaerythritol tetranitrate (PETN) and the plastic explosive composite Semtex 1A) in acetonitrile (AN) is reported. In electrochemical screening, 5 µg of explosive material was detectable in 10 s by multicomponent cyclic voltammetric (CV) analysis on unmodified glassy carbon under ubiquitous environmental influences (i.e., trace water and dissolved oxygen). The explosives were identified with high recoveries under a battery of proof-of-concept testing scenarios in various matrices. In AN containing naturally dissolved oxygen (approx. 2 mM), the superoxide radical is co-electrogenerated during analyte reduction. Free superoxide yields prominent signals that the explosives attenuate quantitatively. To gain further insight into the electrochemical transformation mechanism, spectroelectrochemistry was employed to monitor changes in ultraviolet (UV) absorbance during CV and identify transient intermediates and product species, which could be targeted by future chemical sensors. Overlapping UV spectra of multiple species are deconvoluted using a new strategy, spectral regional baselining, for time- and potential-resolved spectroelectrochemical (SEC) analysis. This study shows that dissolved oxygen, hitherto an interferent purposefully removed from the solution, can be exploited advantageously in electrochemical sensing. The work expands our understanding of high-explosive solution-phase chemistry and offers a novel route to signal transduction for the sensing of energetic materials.

Spray-Coated Multiwalled Carbon Nanotube Composite Electrodes for Thermal Energy Scavenging Electrochemical Cells.

Spray-coated multiwalled carbon nanotube/poly(vinylidene fluoride) (MWCNT/PVDF) composite electrodes, scCNTs, with varying CNT compositions (2 to 70 wt %) are presented for use in a simple thermal energy-scavenging cell (thermocell) based on the ferro/ferricyanide redox couple. Their utility for direct thermal-to-electrical energy conversion is explored at various temperature differentials and cell orientations. Performance is compared to that of buckypaper, a 100% CNT sheet material used as a benchmark electrode in thermocell research. The 30 to 70 wt % scCNT composites give the highest power output by electrode area-seven times greater than buckypaper at ΔT = 50 °C. CNT utilization is drastically enhanced in our electrodes, reaching 1 W gCNT(-1) compared to 0.036 W gCNT(-1) for buckypaper. Superior performance of our spray-coated electrodes is attributed to both wettability with better use of a large portion of electrochemically active CNTs and minimization of ohmic and thermal contact resistances. Even composites with as low as 2 wt % CNTs are still competitive with prior art. The MWCNT/PVDF composites developed herein are inexpensive, scalable, and serve a general need for CNT electrode optimization in next-generation devices.