Electrocatalysis under the atomic force microscope

The newly developed method was used to scan the surface of a bimetallic catalyst material in an aqueous medium. The figure shows an overlay of the current signal on a three-dimensional representation of the height image. This clearly shows island-like regions.

The newly developed method was used to scan the surface of a bimetallic catalyst material in an aqueous medium. The figure shows an overlay of the current signal on a three-dimensional representation of the height image. This clearly shows island-like regions. © M. Munz/HZB

The principle of correlative atomic force microscopy in contact mode: a fine tip at the end of a cantilever scans the surface. This allows force interactions between the tip and the sample surface to be measured, including frictional forces. If a voltage is also applied, the electric current flowing through the contact can also be measured.

The principle of correlative atomic force microscopy in contact mode: a fine tip at the end of a cantilever scans the surface. This allows force interactions between the tip and the sample surface to be measured, including frictional forces. If a voltage is also applied, the electric current flowing through the contact can also be measured.

A further development in atomic force microscopy now makes it possible to simultaneously image the height profile of nanometre-fine structures as well as the electric current and the frictional force at solid-liquid interfaces. A team from the Helmholtz-Zentrum Berlin (HZB) and the Fritz Haber Institute (FHI) of the Max Planck Society has succeeded in analysing electrocatalytically active materials and gaining insights that will help optimise catalysts. The method is also potentially suitable for studying processes on battery electrodes, in photocatalysis or on active biomaterials.

To manage the energy transition, it will also be important to rapidly develop cheap and efficient materials that can be used to split water or CO2 by electrocatalysis. In this process, part of the electrical energy is stored in the chemical reaction products. The efficiency of such electrocatalysts depends largely on the nature of the electrode-electrolyte interfaces, i.e. the interfaces between the solid electrodes and the typically aqueous electrolyte. However, spatially resolved physical studies of such solid-liquid interfaces are still relatively scarce.

More insights with AFM

Dr Christopher S. Kley and his team have now developed a new approach to correlative atomic force microscopy (AFM). An extremely sharp tip is scanned across the surface and its height profile is recorded. By attaching the tip to the end of a miniaturised cantilever, the force interactions between the tip and the sample surface, including frictional forces, can be measured with high sensitivity. In addition, the electrical current flowing through the mechanical contact can be measured, provided a voltage is applied. "This allowed us to simultaneously determine the electrical conductivity, the mechanical-chemical friction and the morphological properties in situ (i.e. under the relevant liquid-phase conditions rather than in vacuum or in air)," emphasises Kley.

Copper-gold electrocatalyst

Using this method, the scientists now studied a nanostructured and bimetallic copper-gold electrocatalyst, in collaboration with Prof. Beatriz Roldán Cuenya from the Fritz-Haber-Institute (FHI). Among others, such materials are used in the electrocatalytic conversion of CO2 into energy carriers. "We were able to clearly identify islands of copper oxide with higher electrical resistance, but also grain boundaries and low-conductivity regions in the hydration layer where the catalyst surface comes into contact with the aqueous electrolyte," says Dr Martin Munz, first author of the study.

Such results on catalyst-electrolyte interfaces help to optimise them in a targeted manner. "We can now observe how local electrochemical environments influence charge transfer at the interface," says Kley.

Focus on solid-liquid interfaces

"However, our results are also of general interest to energy research, especially for the study of electrochemical conversion processes, which also play a role in battery systems." Insights into solid-liquid interfaces can also be useful in completely different areas of research, such as understanding corrosion processes, nanosensor systems, and possibly addressing scientific queries in fluidics and environmental sciences, such as dissolution or deposition processes on metal surfaces exposed to water.

Note: This work was carried out within the framework of the CatLab project, where researchers from the HZB and the FHI of the MPG are working together, to develop thin-film catalysts for the energy transition.

arö


You might also be interested in

  • Unconventional piezoelectricity in ferroelectric hafnia
    Science Highlight
    26.02.2024
    Unconventional piezoelectricity in ferroelectric hafnia
    Hafnium oxide thin films are a fascinating class of materials with robust ferroelectric properties in the nanometre range. While the ferroelectric behaviour is extensively studied, results on piezoelectric effects have so far remained mysterious. A new study now shows that the piezoelectricity in ferroelectric Hf0.5Zr0.5O2 thin films can be dynamically changed by electric field cycling. Another ground-breaking result is a possible occurrence of an intrinsic non-piezoelectric ferroelectric compound. These unconventional features in hafnia offer new options for use in microelectronics and information technology.
  • 14 parameters in one go: New instrument for optoelectronics
    Science Highlight
    21.02.2024
    14 parameters in one go: New instrument for optoelectronics
    An HZB physicist has developed a new method for the comprehensive characterisation of semiconductors in a single measurement. The "Constant Light-Induced Magneto-Transport (CLIMAT)" is based on the Hall effect and allows to record 14 different parameters of transport properties of negative and positive charge carriers. The method was tested now on twelve different semiconductor materials and will save valuable time in assessing new materials for optoelectronic applications such as solar cells.
  • Sodium-ion batteries: How doping works
    Science Highlight
    20.02.2024
    Sodium-ion batteries: How doping works
    Sodium-ion batteries still have a number of weaknesses that could be remedied by optimising the battery materials. One possibility is to dope the cathode material with foreign elements. A team from HZB and Humboldt-Universität zu Berlin has now investigated the effects of doping with Scandium and Magnesium. The scientists collected data at the X-ray sources BESSY II, PETRA III, and SOLARIS to get a complete picture and uncovered two competing mechanisms that determine the stability of the cathodes.