Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions.

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy.

Anatase TiO2 is a promising material for Li-ion (Li+ ) batteries with fast charging capability. However, Li+ (de)intercalation dynamics in TiO2 remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO2 with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li+ storage in TiO2 particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

Surface Nanostructure Effects on Dopamine Adsorption and Electrochemistry on Glassy Carbon Electrodes.

Dopamine (DA) adsorption and electron-transfer kinetics are strongly sensitive to the structure and composition of carbon electrodes. Activation of carbon surfaces is a popular method to improve DA detection, but the role of carbon structural features on DA behavior remains uncertain. Herein, we use scanning electrochemical cell microscopy (SECCM) for local anodization of glassy carbon (GC) electrodes in acid media followed by electrochemical imaging of DA adsorption and electrochemistry covering both unmodified and anodized GC regions of the same electrode. Electrochemical measurements of adsorbed DA involve the delivery of DA from the SECCM meniscus (30 µM) for 1 s periods followed by voltammetric analysis at a reasonable sweep rate (47 V s-1). This general approach reduces effects from interelectrode variability and allows for considerable numbers of measurements and statistical analysis of electrochemical data sets. Localized electrode activity is correlated to surface structure and chemistry by a range of characterization techniques. Anodization enhances DA electron-transfer kinetics and provides more sites for adsorption (higher specific surface area). A consequence is that adsorption takes longer to approach completion on the anodized surface. In fact, normalizing DA surface coverage by the electrochemical surface area (ECSA) reveals that adsorption is less extensive on anodized surfaces compared to as-prepared GC on the same time scale. Thus, ECSA, which has often been overlooked when calculating DA surface coverage on carbon electrodes, even where different activation methods would be expected to result in different surface roughness and nanostructure, is an important consideration. Lower graphitic and higher oxygen content on anodized GC also suggest that oxygen-containing functional groups do not necessarily enhance DA adsorption and may have the opposite effect. This work further demonstrates SECCM as a powerful technique for revealing surface structure-function relationships and correlations at heterogeneous electrodes.

Dynamics of Solid-Electrolyte Interphase Formation on Silicon Electrodes Revealed by Combinatorial Electrochemical Screening.

Revealing how formation protocols influence the properties of the solid-electrolyte interphase (SEI) on Si electrodes is key to developing the next generation of Li-ion batteries. SEI understanding is, however, limited by the low-throughput nature of conventional characterisation techniques. Herein, correlative scanning electrochemical cell microscopy (SECCM) and shell-isolated nanoparticles for enhanced Raman spectroscopy (SHINERS) are used for combinatorial screening of the SEI formation under a broad experimental space (20 sets of different conditions with several repeats). This novel approach reveals the heterogeneous nature and dynamics of the SEI electrochemical properties and chemical composition on Si electrodes, which evolve in a characteristic manner as a function of cycle number. Correlative SECCM/SHINERS has the potential to screen thousands of candidate experiments on a variety of battery materials to accelerate the optimization of SEI formation methods, a key bottleneck in battery manufacturing.

Structure-Reactivity Effects of Biomass-based Hydroxyacids for Sustainable Electrochemical Hydrogen Production.

Biomass electro-oxidation is a promising approach for the sustainable generation of H2 by electrolysis with simultaneous synthesis of value-added chemicals. In this work, the electro-oxidation of two structurally different organic hydroxyacids, lactic acid and gluconic acid, was studied comparatively to understand how the chemical structure of the hydroxyacid affects the electrochemical reactivity under various conditions. It was concluded that hydroxyacids such as gluconic acid, with a considerable density of C-OH groups, are highly reactive and promising for the sustainable generation of H2 by electrolysis at low potentials and high conversion rates (less than -0.15 V vs. Hg/HgO at 400 mA cm-2 ) but with low selectivity to specific final products. In contrast, the lower reactivity of lactic acid did not enable H2 generation at very high conversion rates (<100 mA cm-2 ), but the reaction was significantly more selective (64 % to pyruvic acid). This work shows the potential of biomass-based organic hydroxyacids for sustainable generation of H2 and highlights the importance of the chemical structure on the reactivity and selectivity of the electro-oxidation reactions.

In situ catalyst reactivation for enhancing alcohol electro-oxidation and coupled hydrogen generation.

A novel method exploiting the in situ reactivation of a PdNi catalyst to enhance the electro-oxidation of alcohols is reported. The periodic regeneration of the catalyst surface leads to significant gains in terms of conversion rate, energy requirements and stability compared to the conventional potentiostatic method.

Correlative Voltammetric Microscopy: Structure-Activity Relationships in the Microscopic Electrochemical Behavior of Screen Printed Carbon Electrodes.

Screen-printed carbon electrodes (SPCEs) are widely used for electrochemical sensors. However, little is known about their electrochemical behavior at the microscopic level. In this work, we use voltammetric scanning electrochemical cell microscopy (SECCM), with dual-channel probes, to determine the microscopic factors governing the electrochemical response of SPCEs. SECCM cyclic voltammetry (CV) measurements are performed directly in hundreds of different locations of SPCEs, with high spatial resolution, using a submicrometer sized probe. Further, the localized electrode activity is spatially correlated to colocated surface structure information from scanning electron microscopy and micro-Raman spectroscopy. This approach is applied to two model electrochemical processes: hexaammineruthenium(III/II) ([Ru(NH3)6]3+/2+), a well-known outer-sphere redox couple, and dopamine (DA), which undergoes a more complex electron-proton coupled electro-oxidation, with complications from adsorption of both DA and side-products. The electrochemical reduction of [Ru(NH3)6]3+ proceeds fairly uniformly across the surface of SPCEs on the submicrometer scale. In contrast, DA electro-oxidation shows a strong dependence on the microstructure of the SPCE. By studying this process at different concentrations of DA, the relative contributions of (i) intrinsic electrode kinetics and (ii) adsorption of DA are elucidated in detail, as a function of local electrode character and surface structure. These studies provide major new insights on the electrochemical activity of SPCEs and further position voltammetric SECCM as a powerful technique for the electrochemical imaging of complex, heterogeneous, and topographically rough electrode surfaces.

Electrochemical Detection and Characterization of Nanoparticles with Printed Devices.

Innovative methods to achieve the user-friendly, quick, and highly sensitive detection of nanomaterials are urgently needed. Nanomaterials have increased importance in commercial products, and there are concerns about the potential risk that they entail for the environment. In addition, detection of nanomaterials can be a highly valuable tool in many applications, such as biosensing. Electrochemical methods using disposable, low-cost, printed electrodes provide excellent analytical performance for the detection of a wide set of nanomaterials. In this review, the foundations and latest advances of several electrochemical strategies for the detection of nanoparticles using cost-effective printed devices are introduced. These strategies will equip the experimentalist with an extensive toolbox for the detection of nanoparticles of different chemical nature and possible applications ranging from quality control to environmental analysis and biosensing.

Advanced Nanoscale Approaches to Single-(Bio)entity Sensing and Imaging.

Individual (bio)chemical entities could show a very heterogeneous behaviour under the same conditions that could be relevant in many biological processes of significance in the life sciences. Conventional detection approaches are only able to detect the average response of an ensemble of entities and assume that all entities are identical. From this perspective, important information about the heterogeneities or rare (stochastic) events happening in individual entities would remain unseen. Some nanoscale tools present interesting physicochemical properties that enable the possibility to detect systems at the single-entity level, acquiring richer information than conventional methods. In this review, we introduce the foundations and the latest advances of several nanoscale approaches to sensing and imaging individual (bio)entities using nanoprobes, nanopores, nanoimpacts, nanoplasmonics and nanomachines. Several (bio)entities such as cells, proteins, nucleic acids, vesicles and viruses are specifically considered. These nanoscale approaches provide a wide and complete toolbox for the study of many biological systems at the single-entity level.

Towards single-molecule in situ electrochemical SERS detection with disposable substrates.

Dynamic time-resolved Raman spectroelectrochemistry demonstrates the strong influence of nanostructuring and surface charge of in situ activated disposable substrates for SERS detection. Under specific conditions, a large enhancement factor and estimated calculations agree with the feasible detection of only a few molecules, approaching the limit of single-entity detection.

In Situ Spectroelectrochemical Monitoring of Dye Bleaching after Electrogeneration of Chlorine-Based Species: Application to Chloride Detection.

Spectroelectrochemical techniques are becoming increasingly versatile tools to solve a diverse range of analytical problems. Herein, the use of in situ real-time luminescence spectroelectrochemistry to quantify chloride ions is demonstrated. Utilizing the bleaching effect of chlorine-based electrogenerated products after chloride oxidation, it is shown that the fluorescence of the rhodamine 6G dye decreases proportionally to the initial chloride concentration in solution. A strong decrease of fluorescence is observed in acidic media compared to a lower decrease in alkaline media, which suggests that Cl2, favorably generated at low pH, could be the main species responsible for the fluorescence loss. This fact is corroborated with chronoamperometric measurements where the complete loss of fluorescence for the bulk solution is achieved. A fast mass transfer is needed to explain this behavior, in agreement with the generation of gaseous species such as Cl2. Chloride detection was performed in artificial sweat samples in less than 30 s with great accuracy. This electrochemical/optical combined approach allows us to quantify species that are difficult to measure by electrochemistry due to the inadequate resolution of their redox processes or being without significant optical properties.

Evaluation of electrochemical, UV/VIS and Raman spectroelectrochemical detection of Naratriptan with screen-printed electrodes.

Naratriptan, active pharmaceutical ingredient with antimigraine activity was electrochemically detected in untreated screen-printed carbon electrodes (SPCEs). Cyclic voltammetry and differential pulse voltammetry were used to carry out quantitative analysis of this molecule (in a Britton-Robinson buffer solution at pH 3.0) through its irreversible oxidation (diffusion controlled) at a potential of +0.75V (vs. Ag pseudoreference electrode). Naratriptan oxidation product is an indole based dimer with a yellowish colour (maximum absorption at 320nm) so UV-VIS spectroelectrochemistry technique was used for the very first time as an in situ characterization and quantification technique for this molecule. A reflection configuration approach allowed its measurement over the untreated carbon based electrode. Finally, time resolved Raman Spectroelectrochemistry is used as a powerful technique to carry out qualitative and quantitative analysis of Naratriptan. Electrochemically treated silver screen-printed electrodes are shown as easy to use and cost-effective SERS substrates for the analysis of Naratriptan.

Time-Resolved Luminescence Spectroelectrochemistry at Screen-Printed Electrodes: Following the Redox-Dependent Fluorescence of [Ru(bpy)3]2.

In this work, a compact instrument for time-resolved luminescence spectroelectrochemistry using low-cost disposable electrodes is reported. This instrument can be coupled with screen-printed electrodes via a specific cell and a reflection probe, which allows one to observe changes occurring at the electrode/solution interface. This approach allowed one to follow the fluorescence variation of electrofluorochromic species such as [Ru(bpy)3]2+ at screen-printed carbon electrodes. A strong correlation between the electrochemical processes and the fluorescence was found during potentiostatic or multipulsed amperometric measurements. A decrease of the fluorescence was observed when the [Ru(bpy)3]2+ was oxidized to [Ru(bpy)3]3+ and part of this fluorescence is recovered when [Ru(bpy)3]3+ was reduced to the initial species. Moreover, a significant increment of the fluorescence was found when the oxygen reduction reaction takes place, which also confirms its quenching effect. Finally, multipulsed amperometric detection was employed in order to obtain more information about the redox-dependent luminescence of [Ru(bpy)3]2+ finding a continuous quenching over time attributed to bleaching chlorine-based species.

Electroluminescence of [Ru(bpy)3]2+ at gold and silver screen-printed electrodes followed by real-time spectroelectrochemistry.

Real-time spectroelectrochemistry of [Ru(bpy)3]2+ electroluminescence showed a strong correlation with electrochemical processes occurring at metal screen-printed electrodes. Luminescence was quenched when the metal oxidation takes place, but it behaved differently when gold or silver were reduced, which suggests that changes in the structural characteristics of metallic electrodes play a decisive role in luminescence spectroelectrochemistry.

Galvanostatic electrodeposition of copper nanoparticles on screen-printed carbon electrodes and their application for reducing sugars determination.

In this work, a novel method for the galvanostatic electrodeposition of copper nanoparticles on screen-printed carbon electrodes was developed. Nanoparticles of spherical morphology with sizes between 60 and 280nm were obtained. The electrocatalytic effect of these copper nanospheres towards the oxidation of different sugars was studied. Excellent analytical performance was obtained with the nanostructured sensor: low detection limits and wide linear ranges (1-10,000µM) were achieving for the different reducing sugars evaluated (glucose, fructose, arabinose, galactose, mannose, xylose) with very similar calibration slopes, which demonstrates the possibility of total sugar detection. The reproducibility of these sensors was 4.4% (intra-electrode) and 7.2% (inter-electrode). The stability of the nanostructured electrodes was at least 30 days, even using the same device on different days. Several real samples (honey, orange juice and normal and sugar-free soft drinks) were evaluated to study the reliability of the nanostructured sensor.

Point-of-need simultaneous electrochemical detection of lead and cadmium using low-cost stencil-printed transparency electrodes.

In this work, we report a simple and yet efficient stencil-printed electrochemical platform that can be integrated into the caps of sample containers and thus, allows in-field quantification of Cd(II) and Pb(II) in river water samples. The device exploits the low-cost features of carbon (as electrode material) and paper/polyester transparency sheets (as substrate). Electrochemical analysis of the working electrodes prepared on different substrates (polyester transparency sheets, chromatographic, tracing and office papers) with hexaammineruthenium(III) showed that their electroactive area and electron transfer kinetics are highly affected by the porosity of the material. Electrodes prepared on transparency substrates showed the best electroanalytical performance for the simultaneous determination of Cd(II) and Pb(II) by square-wave anodic stripping voltammetry. Interestingly, the temperature and time at which the carbon ink was cured had significant effect on the electrochemical response, especially the capacitive current. The amount of Cd and Pb on the electrode surface can be increased about 20% by in situ electrodeposition of bismuth. The electrochemical platform showed a linear range comprised between 1 and 200 µg/L for both metals, sensitivity of analysis of 0.22 and 0.087 µA/ppb and limits of detection of 0.2 and 0.3 µg/L for Cd(II) and Pb(II), respectively. The analysis of river water samples was done directly in the container where the sample was collected, which simplifies the procedure and approaches field analysis. The developed point-of-need detection system allowed simultaneous determination of Cd(II) and Pb(II) in those samples using the standard addition method with precise and accurate results.

Sequential incorporation of metallic cations (Cd2+ and Hg2+) and N-octylamine into titanium phosphate nanoparticles and their subsequent release in acid media.

Titanium phosphate nanoparticles, TPNP, consisting of a NaTi2(PO4)3 core and a shell of hydrogen phosphate and dihydrogen phosphate of titanium, undergo fast hydrolysis in water releasing phosphoric acid. This reaction is inhibited in the presence of metallic ions like Cd2+ or Hg2+, which are able to replace the protons of the shell acid phosphates. The amount of the adsorbed metallic cations could be regulated using counterions of different basicity. The resulting nanoparticles also incorporate NH2(CH2)7CH3 (N-octylamine) at room temperature forming N-octylammonium/phosphate ion pairs, but it was found that at higher cation concentration inside the nanoparticle, a lower amount of amine was adsorbed. The metallic cations and N-octylamine are released in acid media, but the starting material is not fully recovered.

Enhanced detection of quantum dots by the magnetohydrodynamic effect for electrochemical biosensing.

In this work, we describe the use of a magnetoelectrochemical support for screen-printed electrodes to improve the anodic stripping voltammetry of cadmium due to the generated magnetohydrodynamic (MHD) effect. To create a significant MHD effect, Fe(iii) was added at mM concentrations to the solution. The reduction of Fe(iii) simultaneously with the cadmium deposition on the electrode surface allowed the production of a high cathodic current, which generated a large Lorentz force capable of exerting a convective effect on the solution in the presence of the magnetic field. This convective effect allowed the increase in the mass transfer in the quiescent solution, enhancing the deposition of cadmium as observed by an increased stripping peak current. The optimized method was applied to the detection of CdSe/ZnS quantum dots (QDs) in solution. Using the magnetoelectrochemical support, we were able to detect extremely low concentrations of QDs, with a detection limit of 100 amol of QDs (in particle number). The great performance shown by this system was evaluated in biosensing applications. Firstly, detection of biotin was carried out using a competitive bioassay between biotin and QD-labelled biotin, obtaining good analytical results (0.6 × 10-10 M as the limit of detection). Then, the magnetoelectrochemical support was tested in a more complex biosensor for the determination of anti-transglutaminase IgA antibodies, a celiac disease biomarker. This work shows that the improvement in the metal electrodeposition caused by the MHD effect can be used successfully for the development of disposable electrochemical biosensors with great performance using screen-printed electrodes.

Stabilization of electrogenerated copper species on electrodes modified with quantum dots.

Quantum dots (QDs) have special optical, surface, and electronic properties that make them useful for electrochemical applications. In this work, the electrochemical behavior of copper in ammonia medium is described using bare screen-printed carbon electrodes and the same modified with CdSe/ZnS QDs. At the bare electrodes, the electrogenerated Cu(i) and Cu(0) species are oxidized by dissolved oxygen in a fast coupled chemical reaction, while at the QDs-modified electrode, the re-oxidation of Cu(i) and Cu(0) species can be observed, which indicates that they are stabilized by the nanocrystals present on the electrode surface. A weak adsorption is proposed as the main cause for this stabilization. The electrodeposition on electrodes modified with QDs allows the generation of random nanostructures with copper nanoparticles, avoiding the preferential nucleation onto the most active electrode areas.