Enhanced sensitivity of scanning bipolar electrochemical microscopy for O2 detection.

The Scanning Bipolar Electrochemical Microscope (SBECM) allows precise positioning of an electrochemical micro-probe serving as bipolar electrode that can be wirelessly interrogated by coupling the electrochemical detection reaction with an electrochemiluminescent reporting process. As a result, the spatially heterogeneous concentrations of an analyte of interest can be converted in real time into a map of sample reactivity. However, this can only be achieved upon optimization of the analytical performance ensuring adequate sensitivity. Here, we present the evaluation and optimized operation of the SBECM for the detection of small changes in local O2 concentrations. Parameters for achieving an improved sensitivity as well as possibilities for improving the signal-to-noise ratio in the optical signal readout are evaluated. The capability of the SBECM for O2 detection is shown at controlled conditions by recording the topography of a patterned sample and monitoring O2 evolution from a photoelectrocatalyst material.

Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon Dioxide Reduction Using Operando Surface-Enhanced Raman Spectroscopy.

Surface-enhanced Raman spectroscopy is a powerful analytical tool and a strongly surface structure-dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time-consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag void structures in order to find the void size providing highest signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface-adsorbed Raman probe, and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy. High-throughput scanning droplet cell experiments were utilized to determine suitable conditions for the electrodeposition of the found highly active structure in a three-electrode electrochemical cell. This structure was subsequently employed as the working electrode in operando surface-enhanced Raman measurements to verify its viability as the signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide.

Scanning Bipolar Electrochemical Microscopy.

Electrochemical techniques offer high temporal resolution for studying the dynamics of electroactive species at samples of interest. To monitor fastest concentration changes, a micro- or nanoelectrode is accurately positioned in the vicinity of a sample surface. Using a microelectrode array, it is even possible to investigate several sites simultaneously and to obtain an instantaneous image of local dynamics. However, the spatial resolution is limited by the minimal electrode size required in order to contact the electrodes. To provide a remedy, we introduce the concept of scanning bipolar electrochemical microscopy and the corresponding experimental system. This technique allows precise positioning of a wireless scanning bipolar electrode to convert spatially heterogeneous concentrations of the analyte of interest into an electrochemiluminescence map of the sample reactivity. After elucidating the working principle by recording bipolar line and array scans, a bipolar electrode array is positioned at the site of interest to record an electrochemical image of the localized release of analyte molecules.

Wireless Light-Emitting Electrochemical Rotors.

Bipolar electrochemistry has been shown to enable and control various kinds of propulsion of nonwired conducting objects: translation, rotation, and levitation. There is a very rapid development in the field of controlled motion combined with other functionalities. Here we integrate two different concepts in one system to generate wireless electrochemical motion of a specifically designed rotor and track its polarization simultaneously by electrochemical light emission. Locally produced hydrogen bubbles at the cathodic pole of the bipolar rotor are the driving force of the motion, whereas [Ru(bpy)3]Cl2 and tripropylamine react at the anodic extremity, thus generating an electrochemiluminescence signal with an intensity directly correlated with the orientation of the rotor arms. This allows in a straightforward way the qualitative visualization of the changing interfacial potential differences during rotation and shows for the first time that light emission can be coupled to autonomously rotating bipolar electrodes.

In Operando Investigation of Electrical Coupling of Photosystem 1 and Photosystem 2 by Means of Bipolar Electrochemistry.

Electrochemical communication between two photobioelectrochemical half-cells based on photosystem 1 and photosystem 2 is investigated in operando. The driving force for the electron-transfer reactions is applied in a wireless mode using bipolar electrochemistry with the actual electrode potentials being self-regulated by the redox processes. Four parameters are assessed to understand the overall performance and elucidate the limiting reactions of the photobioelectrochemical cell. In addition to the potential differences for oxidation and reduction reactions, the current flowing between the half-cells as well as in situ collection of locally evolved O2 by photosystem 2 using a positioned scanning electrochemical microscopy tip are evaluated. In this way, changes in the enzymatic performances as a result of inactivation of either of the protein complexes or variations in the external conditions are monitored.

Bipolar Electrochemistry for Concurrently Evaluating the Stability of Anode and Cathode Electrocatalysts and the Overall Cell Performance during Long-Term Water Electrolysis.

Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst.