Scanning gel electrochemical microscopy (SGECM): The potentiometric measurements

Charge-induced deformation of scanning electrolyte before contact

The recent developments in scanning electrochemical probe techniques focus on the strategy of scanning the electrolyte. For example, scanning electrochemical cell microscopy (SECCM) is based on holding the electrolyte in a glass capillary, while scanning gel electrochemical microscopy (SGECM) immobilizes the gel electrolyte on micro-disk electrodes or etched metal wires. In both SECCM and SGECM, the first and essential step is to bring the electrolyte probe into contact with the sample, which is very often achieved by current feedback with a constant applied potential between the probe and the sample. This work attempts to theoretically analyse the deformation of the electrolyte during this approaching process. For a liquid electrolyte in SECCM, surface tension is considered to counterbalance the gravity and electrostatic force in 2D cylindrical coordinates with axial symmetry. The deformation at equilibrium is solved under certain conditions. For a gel electrolyte, a viscoelastic gel is analysed with a simplified 1D geometry. Both equilibrium and dynamic approaching are considered. The results suggest that for both liquid and gel electrolytes, critical conditions exist for breaking the equilibrium. When the applied potential is higher or the distance is lower than the threshold, the force will not equilibrate and the electrolyte will deform until contact. The critical condition depends on the properties (surface tension for a liquid, elastic and viscous moduli for a gel) and geometry (radius of the capillary for a liquid, thickness for a gel) of the electrolyte. Prospects of further extending the work closer to real experimental scenarios, especially SGECM, are also discussed.

Micro Reference Electrode with an Ultrathin Ionic Path

Reference electrode (RE) plays the core role in accurate potential control in electrochemistry. However, nanoresolved electrochemical characterization techniques still suffer from unstable potential control of pseudo-REs, because the commercial RE is too large to be used in the tiny electrochemical cell, and thus only pseudo-RE can be used. Therefore, microsized RE with a stable potential is urgently required to push the nanoresolved electrochemical measurements to a new level of accuracy and precision, but it is quite challenging to reproducibly fabricate such a micro RE until now. Here, we revisited the working mechanism of the metal-junction RE and clearly revealed the role of the ionic path between the metal wire and the borosilicate glass capillary to maintain a stable potential of RE. Based on this understanding, we developed a method to fabricate micro ultrastable-RE, where a reproducible ultrathin ionic path can form by dissolving a sandwiched sacrificial layer between the Pt wire and the capillary for the ion transfer. The potential of this new micro RE was almost the same as that of the commercial Ag/AgCl electrode, while the size is much smaller. Different from commercial REs that must be stored in the inner electrolyte, the new RE could be directly stored in air for more than one year without potential drift. Eventually, we successfully applied the micro RE in the electrochemical tip-enhanced Raman spectroscopy (EC-TERS) measurement to precisely control the potential of the working electrode, which makes it possible to compare the results from different laboratories and techniques to better understand the electrochemical interface at the nanoscale.

Nanoscale kinetic imaging of lithium ion secondary battery materials using scanning electrochemical cell microscopy

To visualize the electrochemical reactivity and obtain the diffusion coefficient of the anode of lithium-ion batteries, we used scanning electrochemical cell microscopy (SECCM) in a glovebox. SECCM provided the facet-dependent diffusion coefficient on a Li₄Ti₅O₁₂ (LTO) thin-film electrode and detected the metastable crystal phase of Li_xFePO₄.

Scanning Gel Electrochemical Microscopy (SGECM): Lateral Physical Resolution by Current and Shear Force Feedback

Scanning gel electrochemical microscopy (SGECM) is a novel technique measuring local electrochemistry based on a gel probe. The gel probe, which is fabricated by electrodeposition of hydrogel on a microdisk electrode, immobilizes the electrolyte, and constitutes a two-electrode system upon contact with the sample. The contact area determines the lateral physical resolution of the measurement, and considering the soft nature of the gel it is essential to be well analyzed. In this work, the lateral physical resolution of SGECM is quantitatively studied from two aspects: (1) marking single sampling points by locally oxidizing Ag to AgCl and measuring their size; (2) line scan over reference samples with periodic topography and composition. The gel probe is approached to the sample by either current or shear force feedback, and the physical resolution of them is compared. For the optimal gel probe based on 25 μm diameter Pt disk electrode of R_g ≈ 2, the lateral physical resolution of SGECM at contact position is ca. 50 μm for current feedback and ca. 63 μm for shear force feedback. More importantly, the lateral physical resolution of SGECM can be flexibly tuned in the range of 14-78 μm by pulling or pressing the gel probe after touching the sample. In general, current feedback is more sensitive to gel-sample contact than shear force feedback. But the latter is more versatile, which is also applicable to nonconductive samples.