Renewable energy technologies such as solar and wind have been gaining momentum due to a growing awareness of climate change and limited resources (e.g., fossil oils) as well as greater support from governments. The success of renewable energy, however, depends significantly on the development of efficient energy storage and conversion systems, including batteries and fuel cells. In general, the performance of energy storage and conversion systems is characterized by their energy density and service lifetime, which are largely determined by the materials used in the system. Therefore, knowledge of the electrochemical and electromechanical behavior of these materials is a critical part of the effort to improve the performance of energy storage and conversion systems. Electrochemical-scanning probe microscopy (EC-SPM) has proven to be a powerful tool for the study of energy materials at the nanometer scale.
Corrosion is an interfacial phenomenon occurring at solid surfaces that are in contact with gases or liquids. Metals, minerals, and plastics are all subject to the destructive power of corrosion, which adversely affects the service lifetime of equipment and structures and increases the cost of manufacturing and maintenance. Corrosion also plays a large detrimental role in the safety and reliability of system operation. Public and private sectors combat corrosion by pouring billions of dollars into research aimed at developing advanced corrosion-resistant materials, corrosion-resistant paints and coatings, and corrosion inhibitors.
Although it often has macroscopic consequences, corrosion typically begins at the atomic level. This makes corrosion processes ideal candidates for study via in situ EC-SPM. The versatility of in situ EC-SPM enables direct, real-time observation of corrosion processes in a wide range of corrosive environments with atomic or near-atomic resolution, often providing valuable kinetic information as well as important structural data.
The invention of scanning tunneling microscopy (STM) in 19811 marked the starting point of the development of scanning probe microscopy, a family of imaging and spectroscopy techniques based on the scanning of a physical probe over the surface of the sample under test. The core of SPM is the measurement and control of current (as in STM) and force interactions (as in atomic force microscopy, or AFM) between a minute probe and a sample surface. As a matter of fact, AFM,2 which is based on probe–sample force detection and is not limited by sample conductivity, became the leading SPM method. The truly exciting features of these methods are their applicability to a broad range of materials and their ability to operate in diverse environments: in vacuum, in air, in gases, in liquid, and at different temperatures. The ability to utilize SPM in liquid and the desire of electrochemists to study electrode surface structure under real electrochemical conditions led to the development of in situ EC-SPM.3,4
EC-SPM essentially combines two independent units: an electrochemical unit and an SPM unit. A simplified block diagram for EC-SPM is shown in Figure 1a. The electrochemical unit includes a potentiostat and a three-electrode cell that controls the electrochemical state of the working electrode (usually the sample). The scanning probe microscope characterizes the surface of the solid electrode via either a passive probe or an active probe. With a passive probe, like that used in EC-AFM, the potential of the probe is not controlled. The AFM cantilever acts as an inert probe that monitors the topographic changes of the electrode surface caused by electrochemical processes, using standard AFM imaging modes.
Figure 1 – Simple block diagram of EC-SPM (a) and an Agilent 5500 AFM (Agilent Technologies, Chandler, AZ) with integrated environmental chamber and glove box for in situ SPM experiments (b).
On the other hand, with an active probe, like that used in EC-STM, a bipotentiostat is utilized to control both the potential of the sample and the potential of the probe versus the same reference electrode. As in regular STM, the tunneling current between the tip and the sample depends on the potential difference and the distance between the two, and is used as the control signal for STM imaging. With EC-STM, the morphological information of the electrode surface under potential control can be imaged in constant current mode, and the changes in the localized electronic state of the electrode surface with electrochemical potential can be studied via current-versus-voltage spectroscopy.
EC-SPM is capable of providing nanometer-scale resolution to study the electrode surface in liquid. The true merit of EC-SPM, however, is its ability to control experimental conditions such as temperature and humidity in order to mimic the real-world environment of the sample under test. This is particularly important for energy and corrosion researchers. A scanning probe microscope with a specially designed dry glove box for energy and corrosion research under controlled environmental conditions is shown in Figure 1b.
Oxygen-free and controlled environments
Environmental control is critical for many EC-SPM experiments. For example, it has long been recognized that electrochemical processes are significantly affected by the amount of oxygen existing in the experimental environment. Therefore, the electrolyte and the cell in regular electrochemical experiments are often deoxygenated by purging inert gases such as N2 and Ar either before or during the experiment. Due to the unique requirements of EC-SPM, special cells and environmental control systems, such as the one shown in Figure 1b, have had to be designed. With the help of environmental control systems, clean and oxygen-free conditions can be achieved to ensure proper electrochemical experiments. Figure 2 shows a set of cyclic voltammograms recorded at different oxygen levels in the environmental control system. The oxygen level was reduced from ambient (~21%) to less than 1% in about 5 min by N2 purging.5
Figure 2 – Effects of oxygen level in the environment. Oxygen level and the local atmosphere of the EC-SPM experiment can be controlled by purging desired gases into the control chamber.
Integrated environmental control systems offer many functions, including temperature control, humidity control, and liquid exchange, as well as sample preparation and transportation capabilities, which are often necessary for battery studies.
Corrosion and energy applications
Applications of EC-SPM in the field of corrosion have been focused on understanding the mechanism of corrosion initiation and the process of inhibition, including pitting initiation, surface dissolution, passive film formation, and the effect of inhibitors.6 EC-SPM has been utilized to study a variety of materials, including Cu, Ni, and stainless steel, in many different corrosion environments.
Morphological changes during the process of corrosion are also of particular interest to many researchers. As an example, the in situ study of pitting and corrosion of Cu using EC-AFM is presented in Figure 3. The sample is a smooth Cu film deposited on mica in 0.10 M NaHCO3 solution. The potential was stepped from open circuit potential (OCP) to 0.60 V versus Ag/AgCl for 1.5 min and then held at 0.20 V versus Ag/AgCl. The pitting and the changes in surface morphology were monitored via in situ EC-AFM.
Figure 3 – EC-AFM study of Cu corrosion in 0.10
M NaHCO3 solution, showing the topography image of a fresh Cu film (a) and the corroded surface after stepping the potential to 0.60 V/Ag/AgCl for 1.5 min, then holding at 0.20 V. The profile line in (b) is drawn over a large pit that is about 100 nm deep.
In the research field of energy storage and conversion, EC-SPM in particular is used to gain a better understanding of electrode degradation, mechanical and morphological changes of the electrode during charge/discharge cycling, the process of Li ion intercalation, and so on. An example of zinc electrode surface roughening due to cycling is shown in Figure 4. The zinc foil electrode is freshly polished and cycled in KOH aqueous electrolyte. The roughening of the electrode surface due to stripping and redeposition cycles is observed with in situ EC-AFM.
Figure 4 – Zinc foil in KOH aqueous electrolyte showing surface roughening after two stripping and redeposition cycles (b), in comparison to the fresh surface (a).
Figure 5 shows the changes in morphology of a highly oriented pyrolytic graphite (HOPG) electrode in LiClO4/EC/DEC electrolyte. The electrode changed dramatically after one cycle of potential sweep. The changes in surface morphology revealed by EC-AFM were attributed to the formation of solid electrolyte interface (SEI) in conjunction with Li ion insertion.7 SEI film is formed on the electrode surface due to solvent decomposition. Its thermal stability has also been studied with in situ EC-AFM.
Figure 5 – EC-AFM topography image (1 × 1 μm) of HOPG before and after CV in LiClO4/EC/DEC electrolyte showing the effect of Li ion intercalation and surface film formation.
EC-SPM has become an exceptional tool for energy and corrosion studies. As research rapidly advances in these fields, EC-SPM technology will continue to evolve and offer valuable insight into the electrochemical–mechanical behavior of these materials at the nanometer scale.
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- Gewirth, A.A.; Bard, A.J. In situ scanning tunneling microscopy of the anodic oxidation of highly oriented pyrolytic graphite surfaces. J. Phys. Chem. 1988, 92, 5563–6.
- Manne, S.; Hansma, P.K. et al. Atomic-resolution electrochemistry with the atomic force microscope: copper deposition on gold. Science 1991, 251, 183–6.
- Han, W. Application Note, Agilent Technologies, Inc., 2008, 5989-7700EN.
- Mudali, U.K.; Padhy, N. Electrochemical scanning probe microscope (EC-SPM) for the in situ corrosion study of materials: an overview with examples. Corros. Rev. 2011, 29, 73–103.
- Jeong, S.-K.; Inaba, M. et al. Surface film formation on a graphite negative electrode in lithium-ion batteries: AFM study on the effects of co-solvents in ethylene carbonate-based solutions. Electrochim. Acta 2002, 47, 1975–82.
Shijie Wu, Ph.D., is Sr. Application Scientist, Agilent Technologies, Inc., 4330 W. Chandler Blvd., Chandler, AZ 85226, U.S.A.; tel.: 480-753-4311; e-mail: AFM-Info@agilent.com.