Simulation of electrochemical processes during oxygen evolution on Pb-MnO2 composite electrodes

The geometric properties of Pb-MnO2 composite electrodes are studied, and a general formula ispresented for the length of the triple phase boundary (TPB) on two dimensional (2D) compositeelectrodes using sphere packing and cutting simulations. The difference in the geometrical properties of2D (or compact) and 3D (or porous) electrodes is discussed. It is found that the length of the TPB is theonly reasonable property of a 2D electrode that follows a 1/r particle radius relationship. Subsequently,sphere packing cuts are used to derive a statistical electrode surface that is the basis for the earlierproposed simulations of different electrochemical mechanisms. It is shown that two of the proposedmechanisms (conductivity and a two-step-two-material kinetic mechanism) can explain the currentincrease at Pb-MnO2 anodes compared to standard lead anodes.The results show that although MnO2 has low conductivity, when combined with Pb as the metal matrix,the behaviour of the composite is not purely ohmic but is also affected by activation overpotentials,increasing the current density close to the TPB. Current density is inversely proportional to the radius ofthe catalyst particles, matching with earlier experimental results. Contrary to earlier SECM experiments,mass transport of sulphuric acid is not likely to have any influence, as confirmed with simulations.A hypothetical two-step-two-material mechanism with intermediate H2O2 that reacts on both the Pbmatrix and MnO2 catalyst is studied. It was found that assuming quasi-reversible generation of H2O2followed by its chemical decomposition on MnO2, results are obtained that agree with the experiments.If the quasi-reversible formation of H2O2 occurs near the peroxide decomposition catalyst, currentincreases, leading to an active TPB and to the current density that scales with 1/r. It is furtheremphasised that both the Pb matrix and MnO2 catalyst are necessary and their optimum ratio dependson the used current density. Yet, additional experimental evidence is needed to support the postulatedmechanism.Peer reviewe

Determination of physical emulsion stabilization mechanisms of wood hemicelluloses via rheological and interfacial characterization

Materials manufacturing industries seek efficient, economic, and sustainable compounds for stabilizing dispersed systems such as emulsions. In this study, novel, abundant biobased hydrocolloids spruce galactoglucomannans (GGM) and birch glucuronoxylans (GX) were obtained from a forestry biorefining process and characterized as versatile stabilizers of rapeseed oil-in-water emulsions. For the first time, GGM and GX isolated by pressurized hot water extraction (PHWE) of spruce and birch saw meal, respectively, were studied in emulsions. The PHWE wood hemicelluloses—polysaccharides with relatively low molar mass—facilitated the formation of emulsions with small average droplet size and efficiently prevented droplet coalescence. GGM and GX lowered the surface tension of emulsions’ oil–water interface and increased the viscosity of the continuous phase. However, viscosity of the wood hemicellulose-based systems was low compared to that of commercial polymeric stabilizers. GGMstabilized emulsions with varying oil volume fractions were characterized in terms of their rheological properties, including large amplitude oscillation shear (LAOS) measurements, and compared to emulsions prepared with a classical small-molecular surfactant, Tween20. The physical emulsion stabilization mechanisms of GGM and GX are suggested as steric repulsion assisted by Pickering-type stabilization. Wood hemicelluloses have potential as highly promising future bioproducts for versatile industrial applications involving colloidal systems and soft materials.Peer reviewe

Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel

We show that composite hydrogels comprising methyl cellulose (MC) and cellulose nanocrystal (CNC) colloidal rods display a reversible and enhanced rheological storage modulus and optical birefringence upon heating, i.e., inverse thermoreversibility. Dynamic rheology, quantitative polarized optical microscopy, isothermal titration calorimetry (ITC), circular dichroism (CD), and scanning and transmission electron microscopy (SEM and TEM) were used for characterization. The concentration of CNCs in aqueous media was varied up to 3.5 wt % (i.e, keeping the concentration below the critical aq concentration) while maintaining the MC aq concentration at 1.0 wt %. At 20 degrees C, MC/CNC underwent gelation upon passing the CNC concentration of 1.5 wt %. At this point, the storage modulus (G') reached a plateau, and the birefringence underwent a stepwise increase, thus suggesting a percolative phenomenon. The storage modulus (G') of the composite gels was an order of magnitude higher at 60 degrees C compared to that at 20 degrees C. ITC results suggested that, at 60 degrees C, the CNC rods were entropically driven to interact with MC chains, which according to recent studies collapse at this temperature into ring-like, colloidal-scale persistent fibrils with hollow cross-sections. Consequently, the tendency of the MC to form more persistent aggregates promotes the interactions between the CNC chiral aggregates towards enhanced storage modulus and birefringence. At room temperature, ITC shows enthalpic binding between CNCs and MC with the latter comprising aqueous, molecularly dispersed polymer chains that lead to looser and less birefringent material. TEM, SEM, and CD indicate CNC chiral fragments within a MC/CNC composite gel. Thus, MC/CNC hybrid networks offer materials with tunable rheological properties and access to liquid crystalline properties at low CNC concentrations.Peer reviewe

Electrochemically Controlled Proton-Transfer-Catalyzed Reactions at Liquid-Liquid Interfaces: Nucleophilic Substitution on Ferrocene Methanol

The generation of α-ferrocenyl carbocations from ferrocenyl alcohols for SN1 substitution at the water–organic solvent interface is initiated by the transfer of protons into the organic phase. The proton flux, and hence the reaction rate, can be controlled by addition of a suitable “phase-transfer catalyst” anion or by external polarization with a potentiostat, providing a new method for the synthesis of ferrocene derivatives

Kinetics of Cu2+ Reduction and Nanoparticle Nucleation at Micro-scale 1,2-dichlorobenzene-water Interface Studied by Cyclic Voltammetry and Square-wave Voltammetry

Funding Information: Steel and Metal Producers′ Fund of the Technology Industries of Finland Centennial Foundation is acknowledged for funding this research. Publisher Copyright: © 2021 The Authors. Electroanalysis published by Wiley-VCH GmbH.Reduction and nanoparticle nucleation of Cu2+ by decamethylferrocene was studied with cyclic and square-wave voltammetry at a microscale liquid–liquid interface established at the tip of a micropipette. With square-wave voltammetry, two reduction steps could be distinguished as two separate current waves. Comparing the experimental results of cyclic voltammetry with finite element method simulations, particle growth could be observed as an increasing limiting current. Furthermore, kinetic parameters could be estimated with square-wave voltammetry simulations following Butler-Volmer kinetics.Peer reviewe