Plasma Electrochemistry for Carbon-Carbon Bond Formation via Pinacol Coupling

Exploring the Balance between Faradaic and Non-Faradaic Processes in Organic Chemical Reactions at Plasma-Liquid Interfaces

Electrochemistry can enable sustainable chemical manufacturing but is limited by the reactions possible with conventional metal electrodes. Plasma electrochemistry, which replaces a conventional solid electrode with plasma in electrochemical cells, opens new avenues for chemical synthesis by combining Faradaic and non-Faradaic processes at the plasma-liquid interface. To understand how plasma electrochemistry differs from conventional electrochemistry, we investigated plasma reactions with acrylonitrile, an industrially relevant molecule used as the precursor in the well-characterized electrosynthesis of adiponitrile. We demonstrate that non-Faradaic processes dominate plasma-driven chemistry through systematic variation of plasma polarity, current, and reactant concentration, combined with comprehensive quantitative analysis of solid, liquid, and gas products. Most notably, we observed no adiponitrile formation (the desired electrochemical product), while total product yields exceeded the theoretical charge-transfer maximum by up to 32-fold. Substantial polyacrylonitrile formation occurred under all conditions, a product not typically seen in conventional electrochemistry. The plasma anode produced consistently higher yields than the plasma cathode, generating hydrogen and propionitrile at 21 and 2 times the charge-transfer maximum, respectively. Electron scavenger experiments confirmed these transformations occurred primarily through non-Faradaic processes rather than charge transfer. These results demonstrate that plasma electrochemistry with acrylonitrile is primarily driven by non-Faradaic processes at plasma-electrolyte interfaces, providing fundamental insights for harnessing these interactions in chemical synthesis.

Picosecond imaging of dynamics of solvated electrons during femtosecond laser-induced plasma generation in water

The dynamics of solvated electrons were visualized using absorption imaging with sub-picosecond time resolution based on a pump-probe measurement during the early stages of femtosecond laser-induced plasma generation in water. The solvated electrons were generated by the propagation of a femtosecond laser pump pulse. In the area with a pump laser intensity over 2 × 1013 W/cm2, where a high density of free electrons was produced, solvated electrons exhibited an additional rapid increase in optical density (OD) at 800 nm, 7-9 ps after the pump pulse excitation. In contrast, no two-step increase in OD was observed when probed at 400 nm, suggesting that the absorption coefficient of the solvated electrons rapidly changed around 800 nm after femtosecond laser excitation for a few picoseconds. This observation might indicate the structural and electronic modulation of solvated electrons owing to the high density of free electrons in water, accompanied by femtosecond-laser-induced plasma generation.

Advances in plasma-driven solution electrochemistry

Energetic species produced by gas-phase plasmas that impinge on a liquid surface can initiate physicochemical processes at the gas/liquid interface and in the liquid phase. The interaction of these energetic species with the liquid phase can initiate chemical reaction pathways referred to as plasma-driven solution electrochemistry (PDSE). There are several processing opportunities and challenges presented by PDSE. These include the potential use of PDSE to activate chemical pathways that are difficult to activate with other approaches as well as the use of renewable electricity to generate plasmas that could make these liquid-phase chemical conversion processes more sustainable and environmentally friendly. In this review, we focus on PDSE as an approach for controlled and selective chemical conversion including the synthesis of nanoparticles and polymers with desired but currently uncontrollable or unattainable properties as the next step in the use of PDSE. The underpinning redox chemistry and transport processes of PDSE are reviewed as many PDSE-driven processes are transport-limited due to the many short-lived highly reactive species involved.

Operando time and space-resolved liquid-phase diagnostics reveal the plasma selective synthesis of nanographenes

Coupling atmospheric-pressure low-temperature plasmas to electrochemical reactors enables the generation of highly reactive species at plasma-liquid interfaces. This type of plasma electrochemical reactor (PEC) has been used to synthesize fluorescent nitrogen-doped graphene quantum dots (NGQDs), which are usable for multifunctional applications in a facile, simple, and sustainable way. However, the synthesis mechanism remains poorly understood, as well as the location of synthesis. To research these issues, we present an in situ diagnostics study on liquid phase chemistry during the PEC synthesis of NGQDs from chitosan. Monitoring of the photoluminescence and UV-VIS absorption at different depths in the reaction medium during plasma treatment reveals that the NGQD production initiates at the plasma-liquid interface but its completion and/or accumulation occurs at a few millimetres depth below the interface, where the liquid ceases to flow convectively, as determined by particle image velocimetry. Our study provides insights into the plasma synthesis of fluorescent GQDs/NGQDs from carbon precursors that may prove useful for achieving the scalability of PEC processes up to continuous-flow or array reactors.

Evidence for Superoxide-Initiated Oxidation of Aniline in Water by Pulsed, Atmospheric Pressure Plasma

Plasma-driven solution electrochemistry (PDSE) uses plasma-generated reactive species to drive redox reactions in solution. Nonthermal, atmospheric pressure plasmas, when irradiating water, produce many redox species. While PDSE is a promising chemical tool, there is limited insight into the mechanisms of the reactions due to the variety of short-lived reagents produced. In this study, we use aniline as a model system for studying redox mechanisms of PDSE. We show that the plasma irradiation of aqueous aniline solutions drives the formation of polyaniline oligomer, which is suppressed under acidic starting conditions. The addition of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), a radical scavenger, decreases the formation of oligomer by 80%, and the addition of superoxide dismutase fully hinders oligomerization. These results lead us to conclude that the oligomerization of aniline by plasma irradiation is initiated by superoxide. This discovery provides novel insights into PDSE mechanisms and illustrates a potential method of harnessing superoxide for chemical reactions.

Selective Electrochemical Modification and Degradation of Polymers

We demonstrate that electrochemical-induced decarboxylation enables reliable post-polymerization modification and degradation of polymers. Polymers containing N-(acryloxy)phthalimides were subjected to electrochemical decarboxylation under mild conditions, which led to the formation of transient alkyl radicals. By installing these redox-active units, we systematically modified the pendent groups and chain ends of polyacrylates. This approach enabled the production of poly(ethylene-co-methyl acrylate) and poly(propylene-co-methyl acrylate) copolymers, which are difficult to synthesize by direct polymerization. Spectroscopic and chromatographic techniques reveal these transformations are near-quantitative on several polymer systems. Electrochemical decarboxylation also enables the degradation of all-methacrylate poly(N-(methacryloxy)phthalimide-co-methyl methacrylate) copolymers with a degradation efficiency of >95 %. Chain cleavage is achieved through the decarboxylation of the N-hydroxyphthalimide ester and subsequent β-scission of the backbone radical. Electrochemistry is thus shown to be a powerful tool in selective polymer transformations and controlled macromolecular degradation.

Plasma-Enabled Graphene Quantum Dot Hydrogel-Magnesium Composites as Bioactive Scaffolds for In Vivo Bone Defect Repair

Bioactive and mechanically stable metal-based scaffolds are commonly used for bone defect repair. However, conventional metal-based scaffolds induce nonuniform cell growth, limiting damaged tissue restoration. Here, we develop a plasma nanotechnology-enhanced graphene quantum dot (GQD) hydrogel-magnesium (Mg) composite scaffold for functional bone defect repair by integrating a bioresource-derived nitrogen-doped GQD (NGQD) hydrogel into the Mg ZK60 alloy. Each scaffold component brings major synergistic advantages over the current alloy-based state of the art, including (1) mechanical support of the cortical bone and calcium deposition by the released Mg2+ during degradation; (2) enhanced uptake, migration, and distribution of osteoblasts by the porous hydrogel; and (3) improved osteoblast adhesion and proliferation, osteogenesis, and mineralization by the NGQDs in the hydrogel. Through an in vivo study, the hybrid scaffold with the much enhanced osteogenic ability induced by the above synergy promotes a more rapid, uniform, and directional bone growth across the hydrogel channel, compared with the control Mg-based scaffold. This work provides insights into the design of multifunctional hybrid scaffolds, which can be applied in other areas well beyond the demonstrated bone defect repair.