Aggregation of Electrochemically Active Conjugated Organic Molecules and Its Impact on Aqueous Organic Redox Flow Batteries.

Molecule aggregation in solution is acknowledged to be universal and can regulate the molecule's physiochemical properties, which however has been rarely investigated in electrochemistry. Herein, an electrochemical method is developed to quantitatively study the aggregation behavior of the target molecule methyl viologen dichloride. It is found that the oxidation state dicationic ions stay discrete, while the singly-reduced state monoradicals yield a concentration-dependent aggregation behavior. As a result, the molecule's energy level and its redox potential can be effectively regulated. This work does not only provide a method to investigate the molecular aggregation, but also demonstrates the feasibility to tune redox flow battery's performance by regulating the aggregation behavior.

Faradaic Counter for Liposomes Loaded with Potassium, Sodium Ions, or Protonated Dopamine.

Collisional electrochemistry between single particles and a biomimetic polarized micro-liquid/liquid interface has emerged as a novel and powerful analytical method for measurements of single particles. Using this platform, rapid detection of liposomes at the single particle level is reported herein. Individual potassium, sodium, or protonated dopamine-encapsulated (pristine or protein-decorated) liposomes collide and fuse with the polarized micro-liquid/liquid interface accompanying the release of ions, which are recorded as spike-like current transients of stochastic nature. The sizing and concentration of the liposomes can be readily estimated by quantifying the amount of encapsulated ions in individual liposomes via integrating each current spike versus time and the spike frequency, respectively. We call this type of nanosensing technology "Faradaic counter". The estimated liposome size distribution by this method is in line with the dynamic light scattering (DLS) measurements, implying that the quantized current spikes are indeed caused by the collisions of individual liposomes. The reported electrochemical sensing technology may become a viable alternative to DLS and other commercial nanoparticle analysis systems, for example, nanoparticle tracking analysis.

Radical Stabilization of a Tripyridinium-Triazine Molecule Enables Reversible Storage of Multiple Electrons.

A novel organic molecule, 2,4,6-tris[1-(trimethylamonium)propyl-4-pyridiniumyl]-1,3,5-triazine hexachloride, was developed as a reversible six-electron storage electrolyte for use in an aqueous redox flow battery (ARFB). Physicochemical characterization reveals that the molecule evolves from a radical to a biradical and finally to a quinoid structure upon accepting four electrons. Both the diffusion coefficient and the rate constant were sufficiently high to run a flow battery with low concentration and kinetics polarization losses. In a demonstration unit, the assembled flow battery affords a high specific capacity of 33.0 Ah L-1 and a peak power density of 273 mW cm-2 . This work highlights the rational design of electroactive organics that can manipulate multi-electron transfer in a reversible way, which will pave the way to development of energy-dense, manageable and low-cost ARFBs.

Electrochemical Dynamics of a Single Platinum Nanoparticle Collision Event for the Hydrogen Evolution Reaction.

Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mm) under a helium (He) atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under a He atmosphere shows that each discrete current profile of the collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen-containing atmospheres. The particle size distribution estimated from the diffusion-controlled current in He agrees well with electron microscopy and DLS observations. These results shed light on the interfacial dynamics of the single nanoparticle collision electrochemistry.

Observing Discrete Blocking Events at a Polarized Micro- or Submicro-Liquid/Liquid Interface.

Single-entity collisional electrochemistry (SECE), a subfield of single-entity electrochemistry, enables directly characterizing entities and particles in the electrolyte solution at the single-entity resolution. Blockade SECE at the traditional solid ultramicroelectrode (UME)/electrolyte interface suffers from a limitation: only redox-inactive particles can be studied. The wide application of the classical Coulter counter is restricted by the rapid translocation of entities through the orifice, which results in a remarkable proportion of undetected signals. In response, the blocking effect of single charged conductive or insulating nanoparticles (NPs) at low concentrations for ion transfer (IT) at a miniaturized polarized liquid/liquid interface was successfully observed. Since the particles are adsorbed at the liquid/liquid interface, our method also solves the problem of the Coulter counter having a too-fast orifice translocation rate. The decreasing quantal staircase/step current transients are from landings (controlled by electromigration) of either conductive or insulating NPs onto the interface. This interfacial NP assembly shields the IT flux. The size of each NP can be calculated by the step height. The particle size measured by dynamic light scattering (DLS) is used for comparison with that calculated from electrochemical blocking events, which is in fairly good agreement. In short, the blocking effect of IT by single entities at micro- or submicro-liquid/liquid interface has been proven experimentally and is of great reference in single-entity detection.

Modulating p-Orbital of Bismuth Nanosheet by Nickel Doping for Electrocatalytic Carbon Dioxide Reduction Reaction.

Electrochemical reduction of CO2 (CO2 RR) to value-added chemicals is an effective way to harvest renewable energy and utilize carbon dioxide. However, the electrocatalysts for CO2 RR suffer from insufficient activity and selectivity due to the limitation of CO2 activation. In this work, a Ni-doped Bi nanosheet (Ni@Bi-NS) electrocatalyst is synthesized for the electrochemical reduction of CO2 to HCOOH. Physicochemical characterization methods are extensively used to investigate the composition and structure of the materials. Electrochemical results reveal that for the production of HCOOH, the obtained Ni@Bi-NS exhibits an equivalent current density of 51.12 mA cm-2 at -1.10 V, which is much higher than the pure Bi-NS (18.00 mA cm-2 at -1.10 V). A high Faradaic efficiency over 92.0 % for HCOOH is achieved in a wide potential range from -0.80 to -1.10 V, and particularly, the highest efficiency of 98.4 % is achieved at -0.90 V. Both experimental and theoretical results reveal that the superior activity and selectivity are attributed to the doping effect of Ni on the Bi nanosheet. The density functional theory calculation reveals that upon doping, the charge is transferred from Ni to the adjacent Bi atoms, which shifts the p-orbital electronic density states towards the Fermi level. The resultant strong orbital hybridization between Bi and the π* orbitals of CO2 facilitates the formation of *OCHO intermediates and favors its activation. This work provides an effective strategy to develop active and selective electrocatalysts for CO2 RR by modulating the electronic density state.

Quantized Collision/Fusion Events of Anionic Ionosomes at a Polarized Soft Micro-Interface.

Single-entity collisional electrochemistry (SECE) can capture physicochemical information at the single entity level. In the present work, we systematically studied in-situ generation and detection of single anionic ionosomes via SECE combined with a miniaturized interface between two immiscible electrolyte solutions (ITIES). Ionosome is an ionic-bilayer encapsulated nanoscopic water cluster/droplet that carries a net charge. Discrete spiky ionic currents were observed upon collisions/fusions of individual F- or Cl- -ionosomes with a positively polarized micro-ITIES. This fusion process was proved to follow the bulk electrolysis model. With this method, some essential factors such as concentration and charge density of the hydrated anions, and the interfacial area, were revealed. It demonstrates that anionic ionosomes share a common theoretical framework with their counterparts (i. e., cationic ionosomes, like Li+ -ionosomes). This work will spur the advancements in a myriad of fields, including such as the colloid and interface science, micro- and/or nanoscale electrochemistry, and electrophysiology and brain sciences.

A highly sensitive electrochemical sensor containing nitrogen-doped ordered mesoporous carbon (NOMC) for voltammetric determination of l-tryptophan.

This study developed a novel electrochemical sensor containing nitrogen-doped ordered mesoporous carbon (NOMC) for the sensitive and selective quantification of l-tryptophan (Trp). The electro-oxidation mechanism of Trp on the NOMC/Nafion/glass carbon electrode (GCE) was first investigated, and was found to follow a two-electron/two-proton transfer mechanism. Subsequently, the analytical operation conditions were optimized. Under the optimum testing conditions, the oxidation current was found to increase linearly with Trp concentration in the ranges 0.5-70.0 µM and 70.0-200.0 µM (different slopes in each range), with the limit of detection determined to be 35.0 nM (S/N = 3). In addition, the sensor was highly selective for Trp and showed good repeatability and long-term stability. Studies of Trp in real world systems, such as an 18 amino acid mixture and an enzymatic protein hydrolysate, showed excellent recoveries (99.30-103.60%). Results suggest that NOMC/Nafion/GCE sensor has excellent performance characteristics for routine Trp analysis.