Microelectromechanical system (MEMS) is being introduced at the various research fields. In particular, MEMS technology is very useful in portable applications that is sensitive to weight and volume of the system. Analysis, fabrication and testing of MEMS methanol reforming system for micro fuel cell power sources are presented in this thesis. The reforming system has been a key issue for successful development of polymer electrolyte membrane fuel cell (PEMFC) that has grown a great attention as an ideal alternative for micro power source. This thesis presents the development of MEMS methanol reformer complete with a heat source. Three prototypes were fabricated using MEMS technologies integrated with catalyst coating processes that include precipitation method, wet impregnation, and slurry injection method. The performance of the fabricated reformers was measured in the various conditions and the optimal performance was sought. Membrane separation and preferential oxidation were performed to remove carbon monoxide from the reformate gas. Integrated tests of the MEMS reformer with a micro fuel cell were carried out.
First, a catalytic microreactor for hydrogen production was fabricated on photosensitive glasses. Microchannels with high tight tolerance and high aspect ratio were realized by anisotropic wet etching process of the glass. Cu/ZnO was selected as a catalyst for methanol-steam reforming and was prepared by co-precipitation method. The prepared catalyst particles were coated on the surface of microchannels using a precipitation method, resulting in the robust and uniform catalyst layer with the thickness of 30 ㎛. Overall microfabrication process was established for a MEMS-based catalytic microreactor. The fabricated reactor has the volume of 1.8 $cm^3$ including the volume of reaction chamber of $0.3 cm^3$, and produced dry reformate with high hydrogen content of 73%. The hydrogen flow was 4.16 ml/min which can generate power output of 350 m$W_e$ for a PEM fuel cell.
Second, one-chip integrated methanol reformer was developed. The micro device consists of methanol steam reformer, heating element, and heat-exchanger in-between. The all components are integrated into one-chip. As a heating element, two kinds of heat source were used for methanol steam reforming; one is catalytic combustion of hydrogen, and the other is decomposition of hydrogen peroxide. Porous ceramic materials were used as a novel catalyst support that provides ease of catalyst loading, large surface area, and thermal stability. Cu/ZnO was selected as a catalyst for methanol steam reforming and Pt for catalytic combustion of hydrogen. Wet impregnation was used to load catalysts on a porous support. A photosensitive glass wafer was selected as a structural material. Catalyst loaded supports were inserted in the cavity made on the glass wafer. The membrane heat-exchanger was manufactured to increase the heat transfer between the reformer and the combustor. Considering energy balance of reformer/combustor model, the off-gas of fuel cell can be recycled as a feed of the combustor. The micro reformer was successfully fabricated and operated, which generated 67.2 ml/min hydrogen for 4.5 W PEM fuel cell. Catalytic decomposition of hydrogen peroxide is used as a process to supply heat to the reforming reactor. The decomposition process of hydrogen peroxide produces water vapor and oxygen as a product that can be used efficiently to operate the reformer/PEMFC system. At the optimum condition, the hydrogen selectivity was 86.4% and the thermal efficiency was 44.8%. The total volume production rate was 23.5 ml/min. This amount of hydrogen can produce 1.5W of power on a typical PEMFC.
Third, 3-dimentional CFD analysis of the fabricated micro methanol reformer was carried out in order to predict an accurate reforming performance. The flow via the catalyst bed was calculated using Ergun equation. The methanol steam reforming rate was computed with the model that consists of two overall reactions; one for steam reforming of methanol, and the reminder for decomposition of methanol. Conservation equations for a steady-state reaction flow were solved using a commercial FLUNT code with user defined function (UDF) that is a subroutine to calculate the reaction rate. The flow, heat transfer, and reforming rate via the micro reformer were studied with the simulation of 3-dimensional model.
Fourth, silicon-based methanol steam reformer combined with a catalytic combustor. Microchannel was fabricated on a silicon wafer using DRIE process. The channels were coated with Cu/ZnO/$Al_2O_3$ catalysts using slurry injection methanol. Novel design for integration of reformer and combustor was proposed. Silicon wafer was used as a material of the reformer layer to enhance the heat-exchange between reformer and combustor. Glass materials were used for a combustor chamber and a cover of reformer layer. The catalytic combustor was used to generate sufficient amount of heat to sustain the steam reforming of methanol. The improved model that consists of overall methanol-steam reforming reaction model and mass transport model in a porous catalyst was developed. The three-dimensional simulation of the reformer shows good agreement with experiment results.
Finally, palladium membrane supported on an anodic porous alumina using electroless plating. However, the membrane is distorted by different thermal expansion between palladium and support. Microreactor was used for preferential oxidation of CO, which was fabricated using a photosensitive glass process. A Pt/Ru/$\gamma-Al_2O_3$ catalyst was coated on the surface of microchannel. Micro fuel cell was fabricated by assembling GDL, MEA, and flow channel layers. Pt/Ru/C and Pt/C were used as catalysts and Ag/Ti layer was sputtered for electrodes. Integrated performance of the MEMS methanol reformer, PROX, and fuel cell was measured. The power density was 184 mW/㎠ when the potential was 0.64V. The performance was low compared to the result for pure hydrogen because the feed at the fuel cell included undesired CO, $CO_2$, and $N_2$.
본 논문에서는 초소형 연료전지 동력원을 위한 MEMS 메탄올 개질 시스템의 해석, 제작 및 성능평가를 수행하였다. 개질 시스템은 수소를 생산하는 장치로 PEM 연료전지를 초소형 동력원으로 개발하기 위한 핵심 기술로 많은 연구가 수행되고 있다. 본 연구에서는 MEMS 가공기술과 촉매 공정을 통합하여 3가지 시작품을 제작하였다. 각각의 MEMS 개질기는 다른 재료와 가공 방법, 촉매 코팅 방법이 사용되었고, 다양한 반응조건에서 성능평가가 수행되었다. 먼저, 감광성 유리를 이용한 마이크로 촉매 반응기를 비등방성 습식 식각을 통해 제작하였고, 공침법으로 제조된 Cu/ZnO 촉매를 침전-코팅법을 이용하여 코팅하였다. 다음으로, 개질기, 발열장치, 열교환기가 통합된 개질 시스템을 제작하였고, 발열 메커니즘으로는 수소 촉매 연소와 과산화수소 분해반응이 사용되었다. 다공성 세라믹에 촉매를 함침하였고, 다양한 반응조건에서의 실험을 통해 개질기의 통합 성능을 평가하였다. 마지막으로 실리콘 기반의 개질기를 제작하였다. 개질기 층은 열전달이 우수한 실리콘 기판이 사용되었고 연소기 층은 열손실을 최소화하기 위해 유리 기판을 사용하였다. 실리콘 기판에 DRIE 공정을 이용하여 마이크로 채널을 제작하고 슬러리 주입법을 이용하여 촉매를 코팅하였다. CFD 해석을 통해 MEMS 메탄올 개질기의 성능 예측을 수행하고 실험 결과와 비교하여 반응 모델이 적합함을 확인하였다. 다음으로 CO 제거를 위해 막분리 및 선택적 산화반응에 대한 연구가 수행 되었고, 개질된 수소를 이용하여 제작한 MEMS 연료전지를 구동하였다.
