Microfluidic Fuel Cell for Off‑The‑Grid Applications
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This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Microfluidic fuel cell for off‑the‑grid applications Seyed Ali Mousavi Shaegh 2012 Seyed, A. M. S. (2012). Microfluidic fuel cell for off‑the‑grid applications. Doctoral thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/50777 https://doi.org/10.32657/10356/50777 Downloaded on 25 Sep 2021 01:14:44 SGT Microfluidic Fuel Cell for off-the-grid Applications Seyed Ali Mousavi Shaegh School of Mechanical and Aerospace Engineering A thesis submitted to Nanyang Technological University in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2012 Abstract The present doctoral thesis studies air-breathing microfluidic fuel cells with separated fuel and electrolyte streams as well as a membraneless fuel cell with selective electrodes. In order to gain more insight into the physio-chemical reactions, numerical simulation of the in-house developed air-breathing microfluidic fuel cell is formulated and solved using COMSOL Multiphysics. The results from the simulation show that fuel stream at the anode side and its interaction with the electrolyte stream has significant impact on the total fuel cell performance. As the first step for improving the hydrodynamic manipulation of the fuel stream, a flow-through porous anode is introduced. The effects of flow architecture on fuel utilization and the whole cell performance are investigated. Experimental results show that the flow-through porous anode improves the cell current in a long-term performance test as compared to the conventional design with flow-over planar anode. Because of the improved current generation, the rate of carbon dioxide generation in the cell increases. At high current densities, carbon dioxide produced in the channel emerges as bubbles that block and hinders reactant transport to the active sites of the anode. Consequently, a new fuel cell design with fuel reservoir above the anode is introduced to address the issue of bubble formation. This design shows a higher and more stable long-term performance compared to the flow-through design. In addition, the air-breathing microfluidic fuel cell with fuel reservoir can use fuels with a high concentration. This new design provides a platform for stacking multiple cells. ii Although the overall performance of air-breathing fuel cells has been improved significantly with the development route mentioned above, fluidic handlings of fuel and electrolyte streams in microfluidic fuel cells are still the main challenges from the perspective of practical applications. Consequently, a paradigm-changing membraneless fuel cell with a high open circuit potential of 0.60 V using hydrogen peroxide as both the fuel and the oxidant is introduced. In this design, hydrogen peroxide is both the electron acceptor and the electron donor over selective electrodes. Moreover, oxygen and water are the environmentally friendly products of the electrochemical reactions within the cell. Since a separating electrolyte medium is removed from the fuel cell, the whole system has the advantage of a very simple design of flow architecture which facilitates the potential integration of such device with actual applications. Mixing of anolyte and catholyte is not an issue and fuel recycling can be possible due to a single fuel stream. Design challenges remain only with the synthesis of high performance selective electrocatalysts. Further development of this membraneless hydrogen peroxide based fuel cell would open a new avenue for the field of micro power generation in lab-on-chip devices, point-of-care applications, and off-the-grid sensing. iii Acknowledgments I would like to express my sincere appreciation to several people without whom this work would have not been possible. Firstly, I am grateful to my supervisor Associate Professor Nguyen Nam- Trung for his continuous motivation and support. He introduced me to the beautiful and inspiring world of microfluidics, His attitude towards doing research taught me how to set goals and seek success. Also, I am deeply thankful to Professor Chan Siew Hwa, my co-supervisor, for his patience and his inspiration to think big to address the problems in the world of engineering and in my social life. Many thanks go to my colleagues in the Fuel Cell Laboratory. I am thankful to Dr. Weijiang Zhou for his valuable comments on electrolcatalysis and performance degradation. I appreciate all the support from Dr. Ge Xioming, Dr. Liu Qinglin, and Dr. Zhang Lan, Dr. Ding Ovi Lian, Dr. Li Aidan, Mr. Zhang Chaizi, Mr. Li Baosheng, Mr. Raj Kamal S/O Abdul Rasheed and Mr. Seyyed Mohsen Mousavi Ehteshami for characterization activities and productive discussions. In addition, I am deeply indebted to Mr. Nick Chan, technician-in- charge of the Fuel Cell Laboratory for his patience and continuous support. Indeed their contribution shaped much of this work. Also, I am grateful to members of Nguyen’s Microfluidic Group, particularly Song Chaolong, Huang Yuli and Luong Trung Dung . In addition, thanks go to Mr. Yuan Kee Hock from Thermal and Fluid Engineering Lab, and staff members of Micro Machine Centre, Mr. Hoong Sin Poh, Mr. Pek Soo Sion, Mr. Nordin Bin Adbul Kassim and Mr. Ho Kar Kiat. Also, my thanks go to staff members of NTU Library, specifically Mr. iv Ramaravikumar Ramakrishnan, for their continuous support and the updated databases. I appreciate the generous financial support provided by the Agency for Science, Technology and Research (A*STAR) and Nanyang Technological University. Finally, I would like to show my sincere gratefulness to my family. They have always supported me through my studies and inspired me for higher education. I thank all my friends as well for sharing their enthusiasm and happiness. v Dedicated to my parents and all teachers who nurtured me with their inspirations for learning vi Table of Contents Abstract ii Acknowledgment .................................................................................................... iv Table of Contents ................................................................................................... vii List of Publications .................................................................................................. x List of Figures ........................................................................................................ xii List of Tables ......................................................................................................... xii List of Abbreviations ............................................................................................. xx Chapter 1 Introduction* ........................................................................................ 1 1.1 Background and Motivation 2 1.2 Objectives and Scopes 7 Chapter 2 Microfluidic Fuel Cells: Literature Review and Potential Opportunities*.......................................................................................................... 9 2.1 Operation of a Microfluidic Fuel Cell 10 2.2 Membraneless LFFCs Designs and Flow Architectures 12 2.3 Flow-over Electrodes 14 2.3.1 Electrode Location ......................................................................... 14 2.3.2 Carrier Substrate ............................................................................ 15 2.4 Fuel and Oxidant Delivery Schemes 18 2.4.1 Flow-Through Designs .................................................................. 20 2.4.2 Air-breathing Designs .................................................................... 22 2.5 Performance Limiting Factors 27 2.5.1 Inter-diffusion mixing zone and fuel crossover ............................. 28 2.6 Optimization of Channel Geometry 30 2.7 Gas Bubble Formation 35 2.8 Fuel, Oxidant and Electrolytes 37 2.8.1 Fuel Types ...................................................................................... 37 2.8.2 Oxidant Types ................................................................................ 39 2.8.3 Electrolytes .................................................................................... 41 2.9 Challenges and Opportunities for Further Developments 44 Chapter 3 Experimental Methods ....................................................................... 47 3.1 Electrochemical Reactions 48 3.2 Electrode Fabrication 49 3.3 Experimental Conditions 51 Chapter 4 Numerical Simulation of an Air-breathing Microfluidic Fuel Cell* . 53 vii 4.1 Fuel Cell Design and Fabrication 54 4.2 Model Formulation and Theory 56 4.3 Hydrodynamic Equations 57 4.4 Mass Transport and Electrochemical Model 59 4.5 Boundary Conditions 61 4.5.1 Cathode and Anode Overpotentials ............................................... 63 4.6 Numerical Results and Conclusions 67 4.6.1 Polarization Losses in the Fuel Cell............................................... 67 4.6.2 Concentration Distribution of Fuel ................................................ 69 4.6.3 Concentration Distribution of Oxidant through the Fuel Cell ....... 74 4.6.4 Fuel Utilization and Power Density ............................................... 76 4.7 Conclusions 79 Chapter 5 Air-breathing Microfluidic Fuel Cell with Flow-Through Anode* ... 81 5.1 Introduction 82 5.2 Fabrication and Characterization 85 5.3 Results and Discussions 88 5.3.1 Fuel cell Performance Running on 1 M Formic Acid.................... 89 5.3.2 Fuel Cell Performance Running on 0.5 M Formic Acid ................ 94 5.4 Long-term Performance