Spatially resolved electrochemistry in ionic liquids : surface structure effects on triiodide reduction at platinum electrodes

Understanding the relationship between electrochemical activity and electrode structure is vital for improving the efficiency of dye-sensitized solar cells. Here, the reduction of triiodide to iodide in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) room temperature ionic liquid (RTIL) is investigated on polycrystalline platinum using scanning electrochemical cell microscopy (SECCM) and correlated to the crystallographic orientation from electron backscatter diffraction (EBSD). Although the rate determining step in all grains was the first electron transfer, significant grain-dependent variations in activity were revealed, with grains with a dominant (110) crystallographic character exhibiting higher catalytic activity compared to those with a major (100) orientation. The SECCM technique is demonstrated to resolve heterogeneity in activity, highlighting that methods incorporating polycrystalline electrodes miss vital details for understanding and optimizing electrocatalysts. An additional advantage of the SECCM over single-crystal techniques is its ability to probe high index facets

High resolution electrochemical imaging for energy conversion and storage applications

The work presented herein involves the development of the scanning electrochemical cell microscopy (SECCM) platform for visualizing electrochemical and (photo)electrochemical activity of processes at electrode surfaces relevant to energy applications. The use of complementary microscopy characterization techniques such as: field emission-scanning electron microscopy (FE-SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM) and Raman microscopy provides a correlation between the localized (photo)electrochemical activity (obtained by SECCM) and physical properties of the investigated surfaces. SECCM studies of a polycrystalline platinum surface highlight the significant variations in electrochemical activity that can be measured at electrode surfaces due to variations in localized crystallographic orientation and the presence of grain boundaries. An ostensibly simple redox couple (Fe2+/3+) in two different acidic media on a polycrystalline platinum foil is utilized as a model system and the localized crystallographic orientation of the surface is determined by EBSD analysis. The approach is then extended to room temperature ionic liquids (RTILs) to study the reduction of triiodide (I3-) to iodide (I-) on polycrystalline platinum for the application of dye sensitized solar cells (DSSCs) as a counter electrode. The coupling of illumination with high sensitivity current followers and external lock-in amplifiers to the SECCM setup is described and the resulting platform is demonstrated to allow investigation of (photo)electrochemical systems. Two examples are provided: imaging photo-anodes in DSSCs and electrodeposition and characterization of conjugated polymers on a transparent electrode for organic photovoltaic devices. Finally, photo-SECCM is used for determining structure-activity relationships for (photo)electrocatalysts of conjugated organic polymers by coupling the technique with AFM and Raman spectroscopy, suggesting the technique as a potential high throughput screening platform. The approach is exemplified by investigating poly(3-hexylthiophene) and provides not only a correlation of film morphology and photo-activity but also extracts important information on film growth and aging

Ionic diodes based on regenerated α-cellulose films deposited asymmetrically onto a microhole

Cellulose films of approximately 5 mm thickness, reconstituted from ionic liquid media onto a poly-ethylene-terephthalate (PET) film with a 5, 10, 20, or 40 mm diameter microhole, show current rectification when immersed in aqueous NaCl. For “asymmetric cellulose deposits” this rectification, or ionic diode behaviour, is then investigated as a function of ionic strength and microhole diameter. Future applications are envisaged in sustainable cellulose-based desalination, sensing, or energy harvesting processes<br/

Ionic diodes based on regenerated α-cellulose films deposited asymmetrically onto a microhole

Cellulose films of approximately 5 mm thickness, reconstituted from ionic liquid media onto a poly-ethylene-terephthalate (PET) film with a 5, 10, 20, or 40 mm diameter microhole, show current rectification when immersed in aqueous NaCl. For “asymmetric cellulose deposits” this rectification, or ionic diode behaviour, is then investigated as a function of ionic strength and microhole diameter. Future applications are envisaged in sustainable cellulose-based desalination, sensing, or energy harvesting processes<br/

A Cationic Diode Based on Asymmetric Nafion® Film Deposits

A thin film of Nafion®, of approximately 5 microm thickness, asymmetrically deposited onto a 6 microm thick film of poly(ethylene terephthalate) (PET) fabricated with a 5, 10, 20, or 40 microm microhole, is shown to exhibit prominent ionic diode behaviour involving cation charge carrier ("cationic diode"). The phenomenon is characterized via voltammetric, chronoamperometric, and impedance methods. Phenomenologically, current rectification effects are comparable to those observed in nano-cone devices where space-charge layer effects dominate. However, for microhole diodes a resistive, a limiting, and an over-limiting potential domain can be identified and concentration polarization in solution is shown to dominate in the closed state

Pseudo-single crystal electrochemistry on polycrystalline electrodes : visualizing activity at grains and grain boundaries on platinum for the Fe2+/Fe3+ redox reaction

The influence of electrode surface structure on electrochemical reaction rates and mechanisms is a major theme in electrochemical research, especially as electrodes with inherent structural heterogeneities are used ubiquitously. Yet, probing local electrochemistry and surface structure at complex surfaces is challenging. In this paper, high spatial resolution scanning electrochemical cell microscopy (SECCM) complemented with electron backscatter diffraction (EBSD) is demonstrated as a means of performing ‘pseudo-single-crystal’ electrochemical measurements at individual grains of a polycrystalline platinum electrode, while also allowing grain boundaries to be probed. Using the Fe2+/3+ couple as an illustrative case, a strong correlation is found between local surface structure and electrochemical activity. Variations in electrochemical activity for individual high index grains, visualized in a weakly adsorbing perchlorate medium, show that there is higher activity on grains with a significant (101) orientation contribution, compared to those with (001) and (111) contribution, consistent with findings on single-crystal electrodes. Interestingly, for Fe2+ oxidation in a sulfate medium a different pattern of activity emerges. Here, SECCM reveals only minor variations in activity between individual grains, again consistent with single-crystal studies, with a greatly enhanced activity at grain boundaries. This suggests that these sites may contribute significantly to the overall electrochemical behavior measured on the macroscale

Electrodeposition and Screening of Photoelectrochemical Activity in Conjugated Polymers Using Scanning Electrochemical Cell Microscopy

A number of renewable energy systems require an understanding and correlation of material properties and photoelectrochemical activity on the micro to nanoscale. Among these, conducting polymer electrodes continue to be important materials. In this contribution, an ultrasensitive scanning electrochemical cell microscopy (SECCM) platform is used to electrodeposit microscale thin films of poly­(3-hexylthiophene) (P3HT) on an optically transparent gold electrode and to correlate the morphology (film thickness and structural order) with photoactivity. The electrochemical growth of P3HT begins with a thin ordered film up to 10 nm thick, after which a second more disordered film is deposited, as revealed by micro-Raman spectroscopy. A decrease in photoactivity for the thicker films, measured in situ immediately following film deposition, is attributed to an increase in bulk film disorder that limits charge transport. Higher resolution ex situ SECCM phototransient measurements, using a smaller diameter probe, show local variations in photoactivity within a given deposit. Even after aging, thinner, more ordered regions within a deposit exhibit sustained enhanced photocurrent densities compared to areas where the film is thicker and more disordered. The platform opens up new possibilities for high-throughput combinatorial correlation studies, by allowing materials fabrication and high spatial resolution probing of processes in photoelectrochemical materials