National Physical Laboratory

Multi-Scale Electrochemical Imaging

The grand challenge in electrochemistry is to link electronic properties, surface structure, chemistry and topography, interfacial properties and exposure conditions to predict the electrochemical properties of materials. Such fundamental research underpins materials development in fuel cells and battery technology, electrocatalysis, electrochemical sensors, solar energy, colloid science and electrodeposition, and involves the integration of measurement and modelling at different length scales.

Whilst measurement techniques to characterise surface morphology, mechanical properties and chemistry at the micro to nano-scale are comparatively well established, measurement of surface reactivity, kinetics of dynamic processes and mass transport on a highly localised scale remain challenging. Scanning electrochemical microscopy (SECM) offers a unique and powerful approach to such local characterisation and is becoming increasingly prevalent as a chemical imaging tool. At NPL we are developing related state-of-the-art electrochemical scanning probe techniques for a variety of materials-based applications, including:


Scanning Electrochemical Microscopy

Multi-Scale Electrochemical Imaging - Fig 1

In SECM, the sample of interest is immersed in an electrolyte solution and a microelectrode probe is positioned in close proximity to the surface, allowing the chemical and electrochemical properties of the surface to be probed by measurement of the probe current or potential. Scanning the probe laterally above the surface allows one to generate an activity map, yielding detailed information about chemical and electrochemical processes at a localised scale.

At NPL we have used the remarkable chemical sensitivity of SECM to address a range of problems in industry and academia. We are also developing innovative approaches to catalyst, electrocatalyst and photoelectrochemical screening using related techniques.

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Imaging at the Microscale

Traditional SECM probes are 10-25 µm in size allowing relatively large areas (mm to cm) to be mapped with ~10 µm resolution. At NPL we have applied SECM at this scale to electrochemical imaging of model fuel cell catalyst films, corrosion mapping of weld specimens, and characterisation of emerging electrode materials. We have also developed a capability for microarray production and screening, allowing the performance of multiple electroactive or photoactive materials to be assessed on the same surface within a single scan.

Multi-Scale Electrochemical Imaging - Fig 2   Multi-Scale Electrochemical Imaging - Fig 3

Electrocatalyst screening for oxygen reduction (left);
and hydrogen peroxide oxidation (right) at gold nanoparticle microarrays

Publications:


Imaging at the Nanoscale

Nanoelectrochemistry is an area at the frontier of materials research. A key challenge in electrochemical imaging therefore is to drive the spatial resolution down to the nanoscale. To make this possible nanoelectrode probes integrated with a mechanism of accurate topographical positioning are necessary. At NPL we have adopted two main approaches: combining SECM with scanning on conductance microscopy (SICM) or atomic force microscopy (AFM).

Our focus has been in the development and implementation of innovative probes for SECM-SICM and SECM-AFM with the aim to undertake novel, nanoscale measurement of chemical and electrochemical activity of nanostructured materials. Target applications include fuel cell electrocatalysis, photoelectrochemical phenomenon and heterogeneous catalysis.


SECM-SICM

Schematic of SECM-SICM set up (left) and SEM image of typical probe apex (right)
Schematic of SECM-SICM set up (left) and SEM image of typical probe apex (right)


In SICM a pulled glass capillary probe is filled with electrolyte solution and an ion migration current is measured between an electrode inside the capillary and one in bulk solution. This current is sensitive to the proximity to a surface, thus providing a convenient topographical feedback signal. Capillary probes are produced trivially using a pipette puller and can have apertures as small as tens of nm. The integration of a second solid electrode allows a Faradaic current to be measured simultaneously with topographical scanning (SECM-SICM).

At NPL we develop and implement our own probes for combined SECM-SICM using double-barrelled capillaries in which one channel is filled with a solid carbon electrode. These can be freely modified with a range of electrochemically active species. The focus of our work is on imaging catalytic nanoparticles and other electrode materials.

Advantages:

  • High quality electrochemistry (disk-like electrodes)
  • Broad range of electrochemical species and detection modes (amperometric, potentiometric)
  • High topographical and spatial resolution
  • Very high sensitivity current measurement (<100 fA)
SECM-SICM images of gold disks, Pt NPs and Au NPs showing electrochemical feedback, competitive oxygen reduction and hydrogen peroxide generation-collection respectively
SECM-SICM images of gold disks, Pt NPs and Au NPs showing electrochemical feedback,
competitive oxygen reduction and hydrogen peroxide generation-collection respectively


Publications:


SECM-AFM

Schematic of SECM-AFM set up (left) and SEM image of typical probe apex (right)
Schematic of SECM-AFM set up (left) and SEM image of typical probe apex (right)


Another elegant approach is to combine SECM with atomic force microscopy (SECM-AFM) through the fabrication of dual function cantilever probes. Functional modes of AFM are widespread and take advantage of the nm dimensions of etched silicon probes, giving extremely high topographical resolution.

Advantages:

  • Exceptionally high topographical/spatial resolution
  • Various electrode geometries
  • Particularly suited to 2D materials, e.g. graphene
  • Integrate with other AFM-based techniques
SECM-AFM image of exfoliated graphene flake with topography (left) and activity (right)
SECM-AFM image of exfoliated graphene flake with topography (left) and activity (right)


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Last Updated: 7 Oct 2015
Created: 2 Aug 2007

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