Boosting performance of membraneless microfluidic fuel cells via cell architecture optimization and flow management

Abstract

Fuel cells have been developed as one of the most promising clean energy devices. Yet, their integration into portable devices is limited due to the usage of an expensive polymer membrane. Microfluidic fuel cells (MFCs) offer a feasible solution to remove the physical membrane. MFCs employ laminar flows to restrain the mixing within a narrow region at the flow interface, which could serve as a virtual membrane. This approach significantly reduces the fuel cell cost and complexity, and it addresses a series of membrane-associated problems such as swelling and mechanical failure. Despite these benefits, improvement of the electrochemical performance, fuel utilization, and long-term durability of the reported MFCs remains a challenge. In the absence of a membrane, the MFC performance is primarily influenced by the quality of the laminar flows and the degree of co-flow. Based on the utilization of a pumping system, MFCs could be classified into self-pumping and pump-assisted categories. For self-pumping MFCs, the flows are driven by capillary action through porous media such as cellulose paper. Hence, the MFC performance is dominated by certain MFC architectures. For pump-assisted MFCs, the flows are well controlled by the pump but the high power output is usually on the sacrifice of low fuel utilization. In this thesis, novel MFC architecture designs have been designed to effectively and economically improve the performance of all MFC categories. An additional flow management approach has been proposed to break the trade-off between high electrochemical performance and low fuel utilization. To simplify the structure and the operation of MFCs, an anode-immersed architecture with a single-flow configuration was developed on a paper substrate. A fuel-tolerated cathodic catalyst was synthesized to grant this design. To allow scalability at a low cost for this paper MFC, fabrication by printing technology was studied. Using potassium formate as the fuel, the electrochemical performance of this paper-based MFC surpasses existing literature benchmarks by an order of magnitude. To boost the open-circuit voltage of a single cell, a gel-aided architecture was designed for self-pumping MFCs with dual electrolytes. Co-flow was generated through two independent paper channels, separated by a hydrogel which allows ion exchange and prevents electrolyte neutralization. The reaction mechanism with H2O2 as both the fuel and oxidant revealed that radicals generated at the electrodes helped to facilitate the reaction kinetics and sustain a large potential difference. To achieve high performance and fuel utilization simultaneously, the physical properties of the electrolytes were adjusted. The improvement of this flow management was characterized by a pump-assisted MFC using methanol fuel, which provided high power density at a flow rate of one order of magnitude lower while its fuel utilization was remarkably increased by 7.5 times. In addition, the MFC robustness and resilience to ambient disturbances were notably fortified. The abovementioned innovative designs and flow management strategy are adaptable for MFCs employing various liquid fuels to fulfill diversified requirements. Finally, this research demonstrates the potential of MFCs for practical applications and paves the way for future investigations in this evolving field. (497 words)