Energizing fuel cells with an electrically rechargeable liquid fuel

Abstract

Direct liquid fuel cells, attributed to their high energy density and ease of fuel handling, have been regarded as a promising technology for power generation and thus attracted world-wide attentions in the last decades. However, due to the sluggish reaction kinetics of the conventional liquid fuels even with the use of noble metal catalysts, the commercialization progress of the direct liquid fuel cells is still being greatly hampered. To address this issue, a novel system using an electrically rechargeable liquid fuel (e-fuel) for energy storage and power generation has been recently proposed and demonstrated. The liquid e-fuel is stated to be attainable from diverse kinds of materials such as inorganic materials, organic materials, and suspensions of particles. Furthermore, it is reported that, in comparison to conventional alcoholic liquid fuels, the liquid e-fuel possesses a lot of advantages such as: i) superior reactivity even on carbon-based materials, which thereby eliminates the usage of any noble metal catalysts; ii) low freezing point, which hence allows the operation of the cell at wide temperature range; and iii) rechargeability, which therefore enables the e-fuel to be used for more than 100 cycles, greatly reducing the fuel production cost. Hence, attracted by the superiorities of the e-fuel, the objective of this thesis is aimed at examining the performance of a direct liquid fuel cell using an e-fuel containing vanadium ions through experimental and numerical approaches. To begin with, an active liquid e-fuel cell was developed to experimentally study the effects of various structural parameters and operating conditions on the cell performance. It is found that the direct liquid fuel cell using e-fuel demonstrated an impressive performance with an open-circuit voltage of 1.15 V, a maximum current density of 1980 mA cm-2, a peak power density of 857.0 mW cm-2 and an energy efficiency of 41.6 %. Secondly, to enable the application of this liquid fuel cell in mobile devices, a passive fuel cell using the e-fuel, free from any auxiliary equipment, is designed and fabricated. The passive cell was refuelled 25 times and achieved a stable operation for over 350 hours, presenting its capability for long-term operation. Furthermore, using a transparent cell, a side reaction, found at the anode side of cell, was also examined and proved to be hydrogen evolution reaction. Thirdly, to demonstrate the wide applicability of the e-fuel under diverse kinds of environment, an all-climate liquid fuel cell is designed, fabricated, and tested, which demonstrated the operation capability of the liquid e-fuel under sub-zero environment. Fourthly, apart from the experimental works, a numerical study is also carried out to provide a deep understanding to the complex physical and chemical processes within this passive fuel cell. Lastly, to present the capability of this fuel cell for large-scale application, a liquid fuel cell stack is designed, fabricated, and studied, which is also demonstrated to be capable of powering a toy car stably and thus justifies its great potential for achieving commercial applications in the future.