Research On The Controlling Of Mass Transfer And The Enhancement Of Energy Conversion In Microfluidic Electrochemical Energy System

Microfluidic electrochemical system is a highly efficient and environmental friendly energy conversion system which holds the potential to be the next generation electricity supply and energy storage device. In the microfluidic electrochemical systems, microfluidics is integrated with electrochemistry, which accelerates the mass transfer and electrochemical reaction within system. By harnessing laminar flow characteristics of microfluidics, anolyte and catholyte in the system is separated naturely. Therefore, proton exchange membrane is eliminated along with the membrane-related problems, like high cost and complex auxiliary system. Although many advantages are possessed by microfluidic electrochemical system, several crucial problems must be resolved before its commercialization, which are trade-off between fuel utilization and fuel crossover, low energy density, fuel applicability.In the presented research, the experimental studies and theoretical analyses were carried out to investigate the becovered working mechanisms and to develop a microfluidic electrochemical system for practical use.(1) In order to give a clear sight on the mass transfer and energy conversion involved in microfluidic electrochemical system, a numerical model fully coupled with Computational Fluid Dynamics (CFD), mass transfer and electrochemical reaction was developed. Exergy efficiency of the system was analyzed comprehensively. It is found that low fuel utilization, parasitic effect induced by fuel crossover and auxiliary energy consumption for pumping the microfluidic are main causes hindering the energy and exergy efficiency of the system.(2) For resolving the puzzle between reactant crossover and utilization, a microfluidic electrochemical system based on counter-flow microchannel was proposed and studied experimentally and numerically. The results revealed that counter-flow structure is a promising design which restricts reactant crossover by utilizing convection transfer thus enables high fuel utilization. Ohmic resistance is found the only main limiting factor in this system which can be resolved by optimizing the operation concentration of electrolyte. By applying counter-flow design, as high as 91.4% fuel utilization is achieved using 1M formic acid as fuel.(3) Self breathing design of electrode on microfluidic electrochemical system was invented, which enables the system breathe air in at cathode and breathe carbon dioxide out at anode. This novel design eliminates the harmful effect of carbon dioxide bubbles accumulation in microchannel on cell performance. By intergrating self breathing electrode design with counter-flow structure, fuel utilization as high as 76.4% was achieved using 6M formic acid as fuel, which indicates that high fuel utilization and high energy density can be achieved at the same time in microfluidic electrochemical system.(4) In order to improve the applicability of fuel in microfluidic electrochemical system, photocatalytic microfluidic electrochemical system was developed by applying photocatalyst as anode in microfluidic electrochemical system. In this novel system, catalysied by photoelectrochemical anode, organics with complex molecular structure can be used as fuel. Therefore, fuel applicability of microfluidic electrochemical system is extended. Theoretical study was carried out to analyse the working mechanism of the system. Mass and charge transfer is enhanced in the system by applying microfluidics. It is found that the performance of photoanode in microfluidic electrochemical system is both affected by charge transfer within photoande and at the interface of photoanode/electrolyte affacts the performance of photoanode. Each of two charge transfer processes can be the limiting step under different circumstances.