Taurine Electrografting onto Porous Electrodes Improves Redox Flow Battery Performance

Conductive Polymer Coatings Control Reaction Selectivity in All-Iron Redox Flow Batteries

Aqueous all-iron redox flow batteries are an attractive and economic technology for grid-scale energy storage owing to their use of abundant and environmentally benign iron as the redox active material and water as solvent. However, the battery operation is challenged by the plating/stripping reactions of iron and the competing hydrogen evolution reaction at the negative electrode, which hinder performance and durability. Here, the reaction selectivity of the negative electrode is tailored by introducing conductive polymer coatings onto porous carbonaceous electrodes. Two conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(pyrrole) (PPy) are conformally coated with the dopant poly(4-styrenesulfonate) (PSS) and the resulting electrochemistry is studied on model electroanalytical platforms and redox flow batteries. Both polymers decrease the hydrogen evolution current on rotating disc electrodes, with PPy/PSS strongly inhibiting the reaction at high overpotentials. In full all-iron redox flow cells, PPy/PSS coating extends cyclability and significantly reduces hydrogen evolution, while PEDOT/PSS coating improves the round-trip efficiency, possibly acting as a redox shuttle for the iron stripping reaction. These findings motivate broader investigation and implementation of conductive polymers to engineer reaction selectivity for flow batteries and other electrochemical technologies.

Poly-taurine/poly-L-glutamic acid double-layer coating as potential candidates for surface modification of carbon felt electrode for discrimination and simultaneous detection of morphine and tramadol

An ultrasensitive and reliable electrochemical scaffold was designed for the individual and simultaneous measurement of morphine (Mor) and tramadol (Trm) addictive and illegal drugs, utilizing a cost-effective and flexible carbon felt electrode modified with double-layer poly-taurine/poly-L-glutamic acid (P(Tau)/P(Glu)/CF). It is worth noting that drugs have now become a part of daily life in all societies, and the consumption of tranquilizers and opiates such as Mor and Trm has also increased. Given the frequent co-use of Mor and Trm, accurate and reliable methods for their simultaneous measurement are crucial. Simultaneous diagnostics make the determination more efficient and cost-effective by reducing the need for multiple sensors. Surface modification of CFE was carried out by a green approach, facile and straightforward route by layer-by-layer electropolymerization, forming a thin polymeric film with abundant functional groups responsible for anchoring narcotic drugs. The P(Tau)/P(Glu)/CFE composite showed an exceptionally high rate of active site exposure and proper electrochemical activity, attributed to the synergistic effects of the constituent materials. P(Tau)/P(Glu)/CFE was successfully used to detect saliva, urine, plasma, and body sweat samples with satisfactory recoveries.

Single-Entity Electrochemistry of N-Doped Graphene Oxide Nanostructures for Improved Kinetics of Vanadyl Oxidation

N-doped graphene oxides (GO) are nanomaterials of interest as building blocks for 3D electrode architectures for vanadium redox flow battery applications. N- and O-functionalities have been reported to increase charge transfer rates for vanadium redox couples. However, GO synthesis typically yields heterogeneous nanomaterials, making it challenging to understand whether the electrochemical activity of conventional GO electrodes results from a sub-population of GO entities or sub-domains. Herein, single-entity voltammetry studies of vanadyl oxidation at N-doped GO using scanning electrochemical cell microscopy (SECCM) are reported. The electrochemical response is mapped at sub-domains within isolated flakes and found to display significant heterogeneity: small active sites are interspersed between relatively large inert sub-domains. Correlative Raman-SECCM analysis suggests that defect densities are not useful predictors of activity, while the specific chemical nature of defects might be a more important factor for understanding oxidation rates. Finite element simulations of the electrochemical response suggest that active sub-domains/sites are smaller than the mean inter-defect distance estimated from Raman spectra but can display very fast heterogeneous rate constants >1 cm s-1. These results indicate that N-doped GO electrodes can deliver on intrinsic activity requirements set out for the viable performance of vanadium redox flow battery devices.

Quantifying concentration distributions in redox flow batteries with neutron radiography

The continued advancement of electrochemical technologies requires an increasingly detailed understanding of the microscopic processes that control their performance, inspiring the development of new multi-modal diagnostic techniques. Here, we introduce a neutron imaging approach to enable the quantification of spatial and temporal variations in species concentrations within an operating redox flow cell. Specifically, we leverage the high attenuation of redox-active organic materials (high hydrogen content) and supporting electrolytes (boron-containing) in solution and perform subtractive neutron imaging of active species and supporting electrolyte. To resolve the concentration profiles across the electrodes, we employ an in-plane imaging configuration and correlate the concentration profiles to cell performance with polarization experiments under different operating conditions. Finally, we use time-of-flight neutron imaging to deconvolute concentrations of active species and supporting electrolyte during operation. Using this approach, we evaluate the influence of cell polarity, voltage bias and flow rate on the concentration distribution within the flow cell and correlate these with the macroscopic performance, thus obtaining an unprecedented level of insight into reactive mass transport. Ultimately, this diagnostic technique can be applied to a range of (electro)chemical technologies and may accelerate the development of new materials and reactor designs.

Engineering Lung-Inspired Flow Field Geometries for Electrochemical Flow Cells with Stereolithography 3D Printing

Electrochemical flow reactors are increasingly relevant platforms in emerging sustainable energy conversion and storage technologies. As a prominent example, redox flow batteries, a well-suited technology for large energy storage if the costs can be significantly reduced, leverage electrochemical reactors as power converting units. Within the reactor, the flow field geometry determines the electrolyte pumping power required, mass transport rates, and overall cell performance. However, current designs are inspired by fuel cell technologies but have not been engineered for redox flow battery applications, where liquid-phase electrochemistry is sustained. Here, we leverage stereolithography 3D printing to manufacture lung-inspired flow field geometries and compare their performance to conventional flow field designs. A versatile two-step process based on stereolithography 3D printing followed by a coating procedure to form a conductive structure is developed to manufacture lung-inspired flow field geometries. We employ a suite of fluid dynamics, electrochemical diagnostics, and finite element simulations to correlate the flow field geometry with performance in symmetric flow cells. We find that the lung-inspired structural pattern homogenizes the reactant distribution throughout the porous electrode and improves the electrolyte accessibility to the electrode reaction area. In addition, the results reveal that these novel flow field geometries can outperform conventional interdigitated flow field designs, as these patterns exhibit a more favorable balance of electrical and pumping power, achieving superior current densities at lower pressure loss. Although at its nascent stage, additive manufacturing offers a versatile design space for manufacturing engineered flow field geometries for advanced flow reactors in emerging electrochemical energy storage technologies.

Surface Modification of PAN-Derived Commercial Graphite Felts Using Deep Eutectic Solvents for their Application as Electrodes in All-Vanadium Redox Flow Batteries

All-vanadium redox flow batteries are promising large-scale energy storage solutions to support intermittent power generation. Commercial graphite felts are among the most used materials as electrodes for these batteries due to their cheap price, high conductivity, and large surface area. However, these materials exhibit poor wettability and electrochemical activity towards vanadium redox reactions, which translates into overpotentials and lower efficiencies. Deep eutectic solvents (DES) are mixtures of Lewis acids and bases that exhibit lower melting points than their original components. Here, a DES composed of choline chloride and urea, and a DES composed of FeCl₃ and NH₄ Cl have been employed to modify the surface of graphite felts alongside a series of re-carbonization steps. The resulting materials were compared against pristine, thermally activated, and oxidatively activated graphite felts. Our results indicated that the treatments introduced new oxygen and nitrogen functionalities to the carbonaceous surface and increased the surface area, the degree of disorder and defects in the graphitic layers of the fibres. Cyclic voltammetry studies demonstrated higher electrochemical activity towards vanadium redox reactions and electrochemical impedance spectroscopy experiments showed the modified materials exhibited significantly lower charge transfer resistances. When tested in full cell configuration the electrode modified with the urea-based DES exhibited comparable coulombic efficiencies and superior energy storage capacity retention than the thermally oxidized felt used as benchmark, suggesting that the introduction of oxygen- and nitrogen-rich functional groups had a positive effect on the overall electrochemical performance of graphite felts.