More efficient electrochemical energy conversion through near-wall flow control

由于世界人口的增长和新兴经济体的工业化，对能源的需求不断增长。这一需求中越来越多的部分必须使用可再生能源来生产。其中一些能源，如风能和阳光，并不是持续可用的。因此，高效的能量存储是非常重要的。特别是电化学能量转换在燃料电池或电解电池中起着重要的作用。由于这些装置中的反应发生在电极表面，微流体溶液似乎是有利的。通过保持必要的体积小，表面可以大大增加。将大量的微流体燃料电池串联起来，就可以建立所需的动力。本项目旨在通过应用近壁流动控制来提高电化学能量转换的效率，并了解其潜在的物理现象，以改进未来设备的设计。因此，具有高时空分辨率的温度、ph值和压力的速度场和标量分布测量将应用于单相和多相微流中。本课题主要研究多相系统中电磁体积力对近壁对流的控制以及单相系统中被动二次流的流动控制。其中一个目标是开发一种微流体燃料电池，在这种电池中，燃料和氧化剂彼此相邻流动，而不会发生对流混合。因此，不需要昂贵的传统燃料电池膜。这种微流体燃料电池的效率应该通过对流来提高，因为二次流会从电极上去除耗尽层并使溶液刷新。另一个目标是研究在模拟真实电解槽的宏观电极上产生的单个氢泡周围的流动。因此，无论是否应用洛伦兹力，都可以进行速度和温度的高分辨率测量。此外，首次在微电极上观察到电解过程中的马兰戈尼对流，进一步在宏观电极上对单个氢泡的测量将有助于研究这种现象的起源是否与温度或浓度梯度有关。

The demand for energy is growing continuously due to the rising world population and the industrialization of emerging economies. An increasing part of this demand has to be produced using regenerative energy sources. Some of these sources, such as wind power and sunlight are not continuously available. Thus, the efficient energy storage is of great importance. Especially the electro chemical energy conversion in fuel cells or cells for electrolysis plays an important role. Since the reaction in these devices takes place at the electrodes surface, microfluidic solutions seem to be favourable. The surface can be largely increased by keeping the necessary volume small. The connection in series of plenty such microfluidic fuel cells can be used to establish the necessary power. The aim of the proposed project is to increase the efficiency of the electro chemical energy conversion by the application of near wall flow control and to understand the underlying physical phenomena in order to improve the design of future devices. Therefore measurements of the velocity field and scalar distributions of temperature, pH-value and pressure with high spatial and temporal resolution will be applied in single and multiphase micro flows. The proposed project focusses on the control of the near wall convection by the application of electromagnetic volume forces in multiphase systems and the flow control via passive secondary flows for single phase systems.One goal is the development of a microfluidic fuels cell in which the fuel and the oxidant flow next to each other without convective mixing. Therefore the expensive membrane of conventional fuel cell is not necessary. The efficiency of such a microfluidic fuel cell shall be enhanced using convection due to secondary flows which removes the depletion layer from the electrodes and refreshes the solution. Another goal is the investigation of the flow around single hydrogen bubbles produced on a macroscopic electrode mimicking a real electrolyser. Therefore highly resolved measurements of the velocity and the temperature will be performed with and without the application of Lorentz forces. Furthermore, a Marangoni convection was seen for the first time during water electrolysis on microelectrodes and further measurements on macroscopic electrodes on single hydrogen bubbles shall help to investigate if the origin of this phenomenon is related to temperature or concentration gradients.