More efficient electrochemical energy conversion through near-wall flow control

由于世界人口的增加和新兴经济体的工业化，对能源的需求不断增长。这种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，高效的储能显得尤为重要。尤其是燃料电池或电解槽中的电化学能量转换起着重要的作用。由于这些装置中的反应发生在电极表面，微流控溶液似乎是有利的。通过保持必要的体积较小，可以极大地增加表面。大量这样的微流控燃料电池的串联可以用来建立必要的功率。拟议项目的目的是通过应用近壁流动控制来提高电化学能量转换的效率，并了解潜在的物理现象，以便改进未来设备的设计。因此，高时空分辨率的速度场和温度、pH、压力标量分布的测量将被应用于单相和多相微流中。该项目的重点是在多相系统中应用电磁体积力控制近壁对流，在单相系统中通过被动二次流控制流量。其中一个目标是开发一种燃料和氧化剂在其中相邻流动而不发生对流混合的微流控燃料电池。因此，传统燃料电池昂贵的膜是不必要的。这种微流控燃料电池的效率应利用二次流引起的对流来提高，二次流从电极上去除耗尽层并刷新溶液。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气泡的流动。因此，对速度和温度的高分辨率测量将在施加和不施加洛伦兹力的情况下进行。此外，在微电极上的水电解过程中首次观察到Marangoni对流，在单个氢泡上对宏观电极的进一步测量将有助于研究这一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.