Many unit operations required in microfluidics can be realised by electrokinetic phenomena. Electrokinetic phenomena are related to the presence of electrical surface charges of microfluidic substrates in contact with a liquid. As surface charges cannot be directly measured, the zeta potential is considered as the relevant parameter instead. PMMA is an attractive microfluidic substrate since micron-sized features can be manufactured at low costs. However, the existence of PMMA surface charges is not well understood and the zeta potential data found in the literature show significant disagreement. In this article, we present a thorough investigation on the zeta potential of PMMA. We use computations of the potential distribution in the electrical double layer to predict the influence of various electrolyte parameters. The generated knowledge is compared to extensive experiments where we investigate the influence of ionic strength, pH, temperature and the nature of the electrolyte. Our findings imply that two different mechanisms influence the zeta potential depending on the pH value. We propose pure shielding in the acidic and neutral milieus while adsorption of co-ions occurs along with shielding in the alkaline milieu.

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In the present work, we report on the microfabrication of metal hydride thin-film electrodes which can be utilized for rechargeable microbatteries or sensor applications. A multi-layer deposition technique is developed based on physical vapor deposition to fabricate the thin-film electrodes on a glass substrate. The morphology and the structure of the thin-film electrodes are studied by using Field Emission Scanning Electron Microscopy coupled with an Energy Dispersive Spectroscopy module. The surface composition of the thin-film electrodes are determined using X-ray Photoelectron Spectroscopy. Cyclic Voltammetry and galvanostatic charge-discharge measurements are performed to obtain insights into the electrochemical performance of the electrodes. Finally, a semi-empirical model is derived which allows for the determination of the equilibrium potential of the electrode as a function of its hydrogen content.

The utilization of micropower sources is attractive in portable microfluidic devices where only low-power densities and energy contents are required. In this work, we report on the microfabrication of patterned α-Ni(OH)2 films on glass substrates which can be used for rechargeable microbatteries as well as for microcapacitors. A multilayer deposition technique is developed based on e-beam evaporation, ultraviolet lithography, and electroplating/electrodeposition which creates thin-film electrodes that are patterned with arrays of micropillars. The morphology and the structure of the patterned electrode films are characterized by employing field emission scanning electron microscopy. The chemical (elemental) composition is investigated by using X-ray diffraction and X-ray photoelectron spectroscopy. Finally, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge measurements are used to evaluate the electrochemical performance of the patterned thin film electrodes compared to patternless electrodes. We observe that patterning of the electrodes results in significantly improved stability and, thus, longer endurance while good electrochemical performance is maintained.

INTRODUCTION Micro power sources are very attractive for nano- and micro-scale devices, such as Point-of-Care medical diagnostics and Micro Electro Mechanical Systems, where only low power densities and capacities are required. In the present work, we report on the micro-fabrication of patterned α-Ni(OH)2 films which are suitable for rechargeable nickel metal hydride (Ni-MH) and nickel-zinc (Ni-Zn) micro-batteries as well as for micro-capacitors. The main objective of the current research is to investigate the influence of three-dimensional (3D) patterned structures on the mechanical stability as well as on the electrochemical performance of the electrode. ELECTRODE FABRICATION The patterned electrode is fabricated through multi-layered films which are deposited by employing e-beam evaporation, UV photolithography (using a negative KMPR photoresist), and electro-deposition techniques. In detail, thin films of chromium and nickel with a thickness of 40nm are deposited on glass and used as adhesion (seed) and current collector layers, respectively. NiOx/Ni(OH)2 is electro-deposited from NiCl2 solution followed by electro-precipitation of Ni(OH)2 films from Ni(NO3)2.6H2O aqueous solution to form a grid-like patterned layer. Figure 1 schematically depicts the fabrication process. MATERIALS & ELECTROCHEMICAL CHARACTERIZATION The chemical composition of the micro fabricated electrodes is characterized by employing X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS). The feature size of the grid-like pattern is determined to be roughly 20 microns in height by using a mechanical profilometry technique. Figure 2 illustrates the XPS spectrum of the Ni(OH)2 electrode material based on the Ni2p3/2 spectrum. The electrochemical characteristics are studied using cyclic voltammetry (CV) in a 1M KOH electrolyte. Measurements are performed for unpatterned as well as patterned electrodes to investigate the influence of the 3D structure as shown in Figure 3. CONCLUSIONS We observe that electrode patterning considerably improves the adhesion of the α-Ni(OH)2 film to the nickel current collector layer. It is also found that the micro fabricated α-Ni(OH)2 electrode is reversible and diffusion limited according to the Randles-Sevcik linearity. Furthermore, the electrolyte concentration dependency is studied via CV which shows minimum concentration of around 0.2M required for good electrode REDOX reactions.

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