Title: In Situ Scanning Probe Techniques for Evaluating Electrochemical Systems
Description: Falling technology costs are allowing renewable sources of energy to become increasingly more competitive with fossil fuel-based sources. However, challenges still remain in the widespread deployment of sources like wind and solar due to their intermittent nature and cost-prohibitive storage options. An attractive solution to address these issues is by using renewably derived energy to drive electrolysis reactions that generate useful chemicals and fuels. In order to do this effectively and economically, efficient and durable electrocatalysts are needed for the reactions of interest, such as hydrogen production from water electrolysis. Presently, the best catalysts for this process are noble metals such as platinum, which are expensive and in limited supply. The discovery and mechanistic understanding of earth abundant materials that can also efficiently catalyze these reactions remains a current research focus. Scanning probe microscopy (SPM) techniques can be used to aid in the discovery of these materials, as they are able to investigate catalyst surfaces in situ and at a higher resolution than conventional 3-electrode electroanalytical methods.

This dissertation explores the use of two in situ SPM techniques, scanning electrochemical microscopy (SECM) and scanning photocurrent microscopy (SPCM), for evaluating both photocatalytic and electrocatalytic electrochemical systems. Three different studies that use these two techniques were carried out over the duration of my thesis work and are presented in Chapters 2 through 4. After providing an overview of solar fuels and SPM techniques in Chapter 1, Chapter 2 describes the design considerations, implementation and demonstration of a home-built SECM instrument for use with nonlocal continuous line probes (CLPs) that can achieve high areal imaging rates with compressed sensing (CS) image reconstruction. The CLP consists of an electroactive band electrode sandwiched between two insulating layers, where one of the insulating layers needs to be on the same length scale as the band electrode because it determines the average separation distance from the band electrode to the substrate.

Similarly, the spatial resolution of the CLP is determined by the thickness of the band and the realizable imaging rate is determined by its width and linear scan rate. Like conventional SECM systems, a combination of linear motors and a bipotentiostat is needed. However, for the CLP-SECM system both linear and rotational motors are needed to scan at different substrate angles to obtain the necessary raw signal to reconstruct the target electrochemical image with CS algorithms. Detailed descriptions of the microscope design, CLP fabrication, and the procedures necessary to carry out the CLP-SECM imaging are given in this chapter. Measurements with this novel CLP-SECM microscope are done with flat platinum disk electrode samples of varying sizes. A substrate-generation-probe-collection mode is used during the SECM linescan measurements to illustrate procedures for position calibration of the system, CLP and substrate cleaning, as well as verifying the sensitivity along the length of the CLP.

Finally, linescans over a three disk platinum sample were taken and CS image reconstruction was done, with as few as three linescans, to demonstrate the order of magnitude time advantage of this approach over conventional SECM scanning methods. In Chapter 3, colorimetric imaging studies are done using a pH dye indicator to visualize the plume of electroactive species that is generated during in situ SECM measurements for both conventional and CLP-SECM systems. In SECM, the signal recorded by the probe is facilitated by transport of electroactive species and not by direct contact between the probe and the substrate, which is typical of many scanning probe microscopy (SPM) techniques. One of the complexities with SECM is being able to fully understand the interaction between the electroactive species generated at the substrate and the probe. Thus in order to understand this further, a pH indicator dye is used to visualize pH gradients associated with the hydrogen product plume generated by water electrolysis during in situ SECM measurements.

The in situ colorimetric experiments are then used to inform assumptions about the system and validate simulations using finite element modeling software. From this study, we are able to develop quantitative relationships to describe how the plume of electroactive species influences the recorded current at the probe for different probe geometries. Finally, we use this initial study as groundwork for investigating the influence of higher probe scan speeds where convection starts to play a role on the distortion of the signal and plume dynamics, and how it can be corrected using CS post-processing methods. Lastly, SPCM is employed in Chapter 4 to study the optical efficiency losses due to varying size bubbles on a photoelectrode surface. Individual single hydrogen bubbles ranging from 100 µm to 1000 µm were generated on a photoelectrode surface and a laser was used to scan over single isolated bubbles to create localized optical efficiency maps based on photocurrent and external quantum efficiency (EQE).

Moreover, a ray-tracing model based on Snell’s law was also constructed to compare to experimental SPCM linescans. This model showed very good agreement to the experimental SPCM linescan results. This investigation showed that larger bubbles lead to higher optical efficiency losses, not only due to higher inactive electrochemically active surface areas (ECSAs) but also due to a larger region of total internal reflection of light from the edge regions of bubbles. A macroscale study over a large photoelectrode surface was also done where the images of the surface were taken while the “sawtooth” was measured under AM1.5 illumination. Consequently, a predictive current−time profile was generated from the single bubble SPCM empirical relationship between bubble size and optical losses and was compared to the experimental measurement. Understanding how bubbles can impact the efficiency of the overall system is important, as bubbles in a system and on an electrode surface increases ohmic resistances, optical losses, and kinetic losses.

Overall, this study can be used as a starting point for designing systems, electrolyte, and catalyst surfaces to improve one or more of the aforementioned losses.