Thomas Doneux

In investigations of stepwise proton coupled electron transfer (PCET) reactions, it is often assumed, explicitly or implicitly, that the proton transfer (PT) step is at equilibrium, meaning that the buffering conditions are examined from a strictly thermodynamic perspective. In the present work, the implications of the Brönsted acid–base catalysis on the voltammetric responses of reversible (fast electron transfer) electroactive species are examined in the context of a 1 e⁻, 1 H⁺ square scheme, by numerical simulations accounting explicitly for the PT kinetics. Depending on the pK_a’s of the electroactive species and those of the buffer components, PT kinetics can be slowed down to such an extent that voltammetric features akin to non-buffered conditions can appear even at high buffering capacities. This kinetic buffering is intrinsically asymmetric, the PT rate being affected only in one direction as a natural consequence of the Brönsted catalysis mechanism.

Redox gradients across an Fe-NiS precipitate drive CO ₂ reduction in a microfluidic analogue of hydrothermal vents. Electron flow through the mineral couples spatially separated reactions, mimicking a short circuited electrochemical cell.

Coupling electrochemistry with optical techniques gives in-depth insights into the interfacial processes in action. In that context, fluorescence confocal laser scanning microscopy (F-CLSM) enables an electrode surface characterization with spatial resolution in the lateral plane (xy) as well as in the axial direction (z), perpendicular to the electrode surface. However, like most optical techniques, fluorescence microscopy has intrinsic limitations, notably in terms of resolution and sensitivity, which are investigated in this contribution by conducting F-CLSM experiments with two disk electrodes of different sizes: a large microelectrode (LME, Ø = 250 μm) and a much smaller so-called ultramicroelectrode (UME, Ø = 18 μm). We demonstrated that the diffusion layers of both microelectrodes can be imaged with sufficient resolution and sensitivity to be quantitatively compared with the simulated concentration profiles. This work highlights the intrinsic technical challenges associated with this kind of coupled experiments, and it discusses the conditions that should be fulfilled to obtain reliable results at the microscale. These results pave the way toward reaction layer imaging down to micrometric resolution and could help decipher complex electrochemical reactions possibly involving transient species.

Light-driven enzymatic degradation of lignocellulose could help boosting the economy of biorefineries by either shortening the bioprocessing time, or accelerating enzymatic kinetics and so possibly spare some precious enzymes. The redox Lytic Polysaccharide Monooxygenases - LPMOs, a key enzyme for cellulose degradation, had been shown to benefit from photobiocatalysis settings thanks to the in-situ photocatalytic reduction of O2 into H2O2, a co-substrate of the enzyme and perhaps through a faster reduction of its catalytically active metal Cu-II to Cu-I. Despite the successful photoactivation of LPMO, a clear gain in saccharification yield during enzymatic hydrolysis of industrial-relevant lignocellulosic substrate was never achieved. The previous attempts had seen the use of externally added photosensitizers i.e. chlorophyllin or melanin, or settings where the lignin already present in the biomass was exploited as photosensitizer, in both cases undergoing an uncontrolled photocatalytic production of the H2O2. Eventually, it appears that the choice of the lignocellulosic pretreatment technology - PT, the ratio between glycoside hydrolases - GHs and LPMO in the enzymatic cocktail, and the light irradiation could all be key to set a yield-gaining light-driven biorefinery. In this work, we successfully found that the reductive catalytic fractionation – RCF, a lignin-first PT, is suitable to fractionate birchwood lignin so to provide lignin-oil, and a remaining lignocellulose consisting of a photoactivable lignin and highly digestible cellulose. Up to 20% yield gain was obtained when conducting the enzymatic hydrolysis under intermitted light irradiation using a lab-made cocktail containing GHs and LPMOs. Moreover, the exchange of photo-excited electrons among the various components had been electrochemically characterized. Eventually, confocal laser scanning microscopy had been extensively used to characterize the oxidative status of the lignin during photo-driven catalysis in the presence of LPMO.

Deep eutectic solvents (DESs) are attracting considerable attention as non-conventional media for electrodeposition processes. This opinion contribution discusses the debated nature and definition of these solvents as well as some practical considerations of relevance when performing electrodeposition studies in DESs. Using a few illustrative case studies, it is shown that speciation is a key factor determining the electrochemical behaviour of chemical elements in different DESs, and that accounting for the speciation strong similarities can often be found with more conventional or more documented solvents. The need for thermodynamic data is emphasised and it is suggested to expand the composition range of these solvents beyond fixed ratios between the components to exploit the full potentialities of DESs.