Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy

Coupled Electron- and Ion-Transfer Processes at a Liquid/Liquid Interface Decorated with Photoactive Nanomaterials

The influence of the ion transfer on photoinduced electron transfer (ET) reactions was studied on the surface of hyperbranched semiconducting BiVO4 particles spontaneously adsorbed at the liquid-liquid (L/L) interface between an aqueous LiCl solution and bis(triphenylphosphoranylidene) ammonium tetrakis(pentaflurophenyl)borate (BATB) in 1,2-dichlorethane. The organic electrolyte was supplemented with [Co(bpy)3](PF6)3 to accept photoexcited electrons from BiVO4 under formation of the corresponding Co(II) complex. The L/L interface was stabilized at the orifice of a micropipette (MP) and allowed to record ion transfer cyclic voltammetry (ITCV) by applying a Galvani potential differenceΔ o w ϕ ${{\rm{\Delta }}_o^w \varphi }$ between two reference electrodes in the electrolyte solutions with intermittent illumination by visible light (λ>420 nm). The photogenerated holes caused oxidation of water to O2. Co(II) and O2 were detected at constantΔ o w ϕ ${{\rm{\Delta }}_o^w \varphi }$ at an amperometric microelectrode (ME) facing the orifice of the MP in either the organic or the aqueous electrolyte. The overall current exhibits a photocurrent only in theΔ o w ϕ ${{\rm{\Delta }}_o^w \varphi }$ -range, in which the IT of PF6 - is kinetically limited. The amperometric detection of photogenerated products followed the same pattern as the photocurrent in the total current.

Enabling artificial photosynthesis systems with molecular recycling: A review of photo- and electrochemical methods for regenerating organic sacrificial electron donors

This review surveys advances in the literature that impact organic sacrificial electron donor recycling in artificial photosynthesis. Systems for photocatalytic carbon dioxide reduction are optimized using sacrificial electron donors. One strategy for coupling carbon dioxide reduction and water oxidation to achieve artificial photosynthesis is to use a redox mediator, or recyclable electron donor. This review highlights photo- and electrochemical methods for recycling amines and NADH analogues that can be used as electron donors in artificial photosynthesis. Important properties of sacrificial donors and recycling strategies are also discussed. Compounds from other fields, such as redox flow batteries and decoupled water splitting research, are introduced as alternative recyclable sacrificial electron donors and their oxidation potentials are compared to the redox potentials of some model photosensitizers. The aim of this review is to act as a reference for researchers developing photocatalytic systems with sacrificial electron donors, and for researchers interested in designing new redox mediator and recyclable electron donor species.

Potential-Modulated Ion Distributions in the Back-to-Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O2 reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O2. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential differenceΔ o w φ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single-step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the appliedΔ o w φ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

Electrodeless Synthesis of Low Dispersity Au Nanoparticles and Nanoclusters at an Immiscible Micro Water/Ionic Liquid Interface

Owing to their biocompatibility, optical, and catalytic properties, Au nanoparticles (NPs) have been the subject of much research. Since smaller NPs have enhanced catalytic properties and NP morphology greatly impacts their effectiveness, controlled and reproducible methods of generating Au NPs are still being sought. Herein, Au NPs were electrochemically generated at a water|ionic liquid (w|IL) immiscible micro-interface, 25 µm in diameter, using a redox active IL and compared to results at a water|oil (w|o) one. The liquid|liquid interface is advantageous as it is pristine and highly reproducible, as well as an excellent means of species and charge separation. In this system, KAuCl4 dissolved in the aqueous phase reacts under external potential control at the water|P8888TB (tetraoctylphosphonium tetrakis(pentafluorophenyl)borate) with trioctyl(ferrocenylhexanoyl)phosphonium tetrakis(pentafluorophenyl)borate (FcIL), an electron donor and redox active IL. FcIL was prepared with a common anion to P8888TB, which greatly enhances its solubility in the bulk IL. Simple ion transfer of AuCl4− and AuCl(4−γ)(OH)γ− at the w|P8888TB micro-interface were characterized voltammetrically as well as their heterogeneous electron transfer reaction with FcIL. This interfacial reaction generates Au NPs whose size can be thermodynamically controlled by modifying the pH of the aqueous phase. Critically, at low pH, nanoclusters, <1.7 nm in diameter, were generated owing to inhibited thermodynamics in combination with the supramolecular fluidic nature of the IL microenvironment that was observed surrounding the as-prepared NPs.

Unraveling the Link between Catalytic Activity and Agglomeration State with Scanning Electrochemical Microscopy and Atomic Force Microscopy

In this article, we set up a methodology to investigate the relationship between the catalytic activity and the agglomeration state of platinum group metal-free ORR catalysts. To this end, we have developed a statistical approach based on scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). Two catalysts are investigated at very low loadings in order to access their intrinsic activity. Differences in terms of dispersion, stability of the inks, and adherence on the substrate are observed, highlighting the importance of measuring the exact amount and agglomeration state of the materials under study. The agglomeration state of the deposits measured by AFM explains the differences in activity measured by SECM. The performances of the catalysts are compared, and the contributions of the intrinsic activity and the agglomeration state are identified. This work paves the way toward various applications ranging from the benchmarking of new catalysts to the optimization of an ink formulation, for ORR and beyond.

Ionosomes: Observation of Ionic Bilayer Water Clusters

Emulsification of immiscible two-phase fluids, i.e., one condensed phase dispersed homogeneously as tiny droplets in an outer continuous medium, plays a key role in medicine, food, chemical separations, cosmetics, fabrication of micro- and nanoparticles and capsules, and dynamic optics. Herein, we demonstrate that water clusters/droplets can be formed in an organic phase via the spontaneous assembling of ionic bilayers. We term these clusters ionosomes, by analogy with liposomes where water clusters are encapsulated in a bilayer of lipid molecules. The driving force for the generation of ionosomes is a unique asymmetrical electrostatic attraction at the water/oil interface: small and more mobile hydrated ions reside in the inner aqueous side, which correlate tightly with the lipophilic bulky counterions in the adjacent outer oil side. These ionosomes can be formed through electrochemical (using an external power source) or chemical (by salt distribution) polarization at the liquid-liquid interface. The charge density of the cations, the organic solvent, and the synergistic effects between tetraethylammonium and lithium cations, all affecting the formation of ionosomes, were investigated. These results clearly prove that a new emulsification strategy is developed providing an alternative and generic platform, besides the canonical emulsification procedure with either ionic or nonionic surfactants as emulsifiers. Finally, we also demonstrate the detection of individual ionosomes via single-entity electrochemistry.

A Review: Scanning Electrochemical Microscopy (SECM) for Visualizing the Real-Time Local Catalytic Activity

Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique for measuring the in situ electrochemical reactions occurring at various sample interfaces, such as the liquid-liquid, solid-liquid, and liquid-gas. The tip/probe of SECM is usually an ultramicroelectrode (UME) or a nanoelectrode that can move towards or over the sample of interest controlled by a precise motor positioning system. Remarkably, electrocatalysts play a crucial role in addressing the surge in global energy consumption by providing sustainable alternative energy sources. Therefore, the precise measurement of catalytic reactions offers profound insights for designing novel catalysts as well as for enhancing their performance. SECM proves to be an excellent tool for characterization and screening catalysts as the probe can rapidly scan along one direction over the sample array containing a large number of different compositions. These features make SECM more appealing than other conventional methodologies for assessing bulk solutions. SECM can be employed for investigating numerous catalytic reactions including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), water oxidation, glucose oxidation reaction (GOR), and CO2 reduction reaction (CO2RR) with high spatial resolution. Moreover, for improving the catalyst design, several SECM modes can be applied based on the catalytic reactions under evaluation. This review aims to present a brief overview of the recent applications of electrocatalysts and their kinetics as well as catalytic sites in electrochemical reactions, such as oxygen reduction, water oxidation, and methanol oxidation.